CA2061622C - Sub-micron device fabrication - Google Patents
Sub-micron device fabricationInfo
- Publication number
- CA2061622C CA2061622C CA002061622A CA2061622A CA2061622C CA 2061622 C CA2061622 C CA 2061622C CA 002061622 A CA002061622 A CA 002061622A CA 2061622 A CA2061622 A CA 2061622A CA 2061622 C CA2061622 C CA 2061622C
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- CA
- Canada
- Prior art keywords
- layer
- regions
- pattern
- mask
- transfer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
- G03F1/26—Phase shift masks [PSM]; PSM blanks; Preparation thereof
- G03F1/28—Phase shift masks [PSM]; PSM blanks; Preparation thereof with three or more diverse phases on the same PSM; Preparation thereof
Abstract
Expedient fabrication of phase masks providing for many values of phase delay for exiting pattern delineating radiation depends upon a two-step procedure. Such masks offer improvement in ultimate device fabrication relative to that offered by prior binary-valued masks. In the first step, which may be carried out coincident with introduction of device feature information, apertures of appropriate size and distribution are produced in the relevant mask layer; in the second step material surrounding such apertures is heated to result in backflow-filling. Theconsequential layer thinning is such as to introduce the desired local change in phase delay.
Description
SUB-MICRON DEVICE FABRICATION
Back~round of the Invention 1. Technical Field The invention relates to the fabrication of small-dimensioned devices, 5 e.g. integrated circuits using sub-micron design rules and to apparatus/tools used in such fabrication. While implications are broad, a major thrust concerns lithographic delinç~tion - the use of phase masks to improve image quality. Whether based on presently used deline~ting energy, e.g. in the near ultraviolet spectrum, or on shorter wavelength, e.g. in the deep ultraviolet or x-ray spectrum, improvement in 10 lithographic delineation extends the range to permit further mini~tnrization.Fabrication of Very Large Scale Integrated circuits - electronic as well as optical and hybrid - built to sub-micron design rules - is contemplated.
Back~round of the Invention 1. Technical Field The invention relates to the fabrication of small-dimensioned devices, 5 e.g. integrated circuits using sub-micron design rules and to apparatus/tools used in such fabrication. While implications are broad, a major thrust concerns lithographic delinç~tion - the use of phase masks to improve image quality. Whether based on presently used deline~ting energy, e.g. in the near ultraviolet spectrum, or on shorter wavelength, e.g. in the deep ultraviolet or x-ray spectrum, improvement in 10 lithographic delineation extends the range to permit further mini~tnrization.Fabrication of Very Large Scale Integrated circuits - electronic as well as optical and hybrid - built to sub-micron design rules - is contemplated.
2. Description of the Prior Art The saga of Large Scale Integration from inception to the present time is 15 well-known. Evolution of the present 1-2 megabit chip, built to design rules at or slightly below 1 ~m, does not represent the llltim~te product. Lithographic definition has played, and will continue to play, a signifi~nt role. Fabrication of state-of-the-art devices depends on use of near-ultraviolet radiation (e.g. of wavelength, ~ = 3650 A - the Illel~ y I line). Intensive effort directed toward next generation devices is 20 expected to depend on radiation of still shorter wavelength (radiation within the "deep UV" spectrum, e.g. of wavelength, ~ = 2480 A - the krypton fluoride excimer laser line). Forward-looking work directed toward still smaller design rules contemplates electromagnetic energy in the x-ray spectrum or, alternatively, accelerated electron radiation of equivalent decreased wavelength.
A co~ g effort seeks to extend the capability of presently used UV
deline~ting radiation. As described by M. D. Levenson et al, in IEEE Trans.
Electron Devices, vol. ED-29 (12), p. 1828 (1982) and as reviewed in a New York Times article dated December 12, 1990, design rule-limiting edge resolution is lessened by use of "phase masks" - that is, masks designedly providing for relative 30 phase shifting of r~ tilm as tr~n~mitted through selected mask areas. Impact is two-fold: (1) as applied to usual device fabrication entailing opaque featured masks (e.g. chrome on glass); and (2) as applied to such fabrication entailing clear masks, dispensing with opaque mask features, in which use is made of dark-line imaging resulting from interference as between transparent mask regions of differing phase 35 delay. In either event, use of phase masks perrnits extension to design rules 206162~
generally thought beyond the capability of the particular wavelength used with extension due to phase cancellation of diffraction-scattered radiation at feature edges. In both instances, provision is made for 180~ phase shift regions - either adjacent, or as an integral part of edge-defining mask areas.
Phase masking is considered promising in accordance with traditional business considerations. It permits fabrication of next-generation devices usingpresent equipment and processing. Avoidance of cost of replacement equipment (inany event not yet comlllel.;ially available) as well as of retraining of personnel, assures continlling effort in this direction.
A widespread view serves as basis for expected extension of UV-based plocessing to design rules below 0.3-0.25 ,um by use of phase m~king - likely to the 0.2 ~m and below range commonly thought beyond the effective capability of UV
deline~tion To the extent that this proves to be correct, device fabrication by use of X-My (whether pl~o~dmiLy or projectionj as well as by use of accelerated electron 15 ra li~tion (whether by beam writing or masking), is likely to be deferred to the turn of the century.
Limiting lithographic resolution varies in accordance with the classical relationship:
K
Resolution = NA
20 in which:
= wavelength of deline~ting radiation in al)p~opliate units, e.g. ~lm NA is the nllm~rical apelLule of the optical system Resolution is on the basis of desired feature-space contrast and K 1 is a constant which depends upon details of the imaging system and characteristics of the delineating process, e.g. of the development process - a value of 0.7-0.8 is representative of state-of-the-art fabrication (of 0.8-1.0 ~Lm design rule LSI) 180~ phase mask processing for given wavelength/etch contrast may be described in terms of reduction of K 1 to the ~ 0.5 level (permitting fabrication of 35 devices to design rule of ~ 0.4 !lm), and in some instances to the K 1 ~ = 0.3 level tO
A co~ g effort seeks to extend the capability of presently used UV
deline~ting radiation. As described by M. D. Levenson et al, in IEEE Trans.
Electron Devices, vol. ED-29 (12), p. 1828 (1982) and as reviewed in a New York Times article dated December 12, 1990, design rule-limiting edge resolution is lessened by use of "phase masks" - that is, masks designedly providing for relative 30 phase shifting of r~ tilm as tr~n~mitted through selected mask areas. Impact is two-fold: (1) as applied to usual device fabrication entailing opaque featured masks (e.g. chrome on glass); and (2) as applied to such fabrication entailing clear masks, dispensing with opaque mask features, in which use is made of dark-line imaging resulting from interference as between transparent mask regions of differing phase 35 delay. In either event, use of phase masks perrnits extension to design rules 206162~
generally thought beyond the capability of the particular wavelength used with extension due to phase cancellation of diffraction-scattered radiation at feature edges. In both instances, provision is made for 180~ phase shift regions - either adjacent, or as an integral part of edge-defining mask areas.
Phase masking is considered promising in accordance with traditional business considerations. It permits fabrication of next-generation devices usingpresent equipment and processing. Avoidance of cost of replacement equipment (inany event not yet comlllel.;ially available) as well as of retraining of personnel, assures continlling effort in this direction.
A widespread view serves as basis for expected extension of UV-based plocessing to design rules below 0.3-0.25 ,um by use of phase m~king - likely to the 0.2 ~m and below range commonly thought beyond the effective capability of UV
deline~tion To the extent that this proves to be correct, device fabrication by use of X-My (whether pl~o~dmiLy or projectionj as well as by use of accelerated electron 15 ra li~tion (whether by beam writing or masking), is likely to be deferred to the turn of the century.
Limiting lithographic resolution varies in accordance with the classical relationship:
K
Resolution = NA
20 in which:
= wavelength of deline~ting radiation in al)p~opliate units, e.g. ~lm NA is the nllm~rical apelLule of the optical system Resolution is on the basis of desired feature-space contrast and K 1 is a constant which depends upon details of the imaging system and characteristics of the delineating process, e.g. of the development process - a value of 0.7-0.8 is representative of state-of-the-art fabrication (of 0.8-1.0 ~Lm design rule LSI) 180~ phase mask processing for given wavelength/etch contrast may be described in terms of reduction of K 1 to the ~ 0.5 level (permitting fabrication of 35 devices to design rule of ~ 0.4 !lm), and in some instances to the K 1 ~ = 0.3 level tO
yield quarter micron features.
Summary of the Invention The invention is concerned with device fabrication involving at least one level of 5 pattern delineation dependent on image transfer from mask to device in fabrication. The fundamental thrust entails use of a mask designedly providing for a plurality of values of phase delay and amplitude of delineation radiation with the object of improving image quality - e.g. improving edge resolution of imaged features. While edge resolution is improved independentof feature size, the advance is of particular consequencefor small 10 features - particularly for closely spaced small features. Accordingly, a primary purpose of the invention entails fabrication of submicron design rule features, generally as contained in Integrated Circuits. Relevant approaches are directed to use of device design rules smaller than usually contemplated as expediently ~ in~ble for particular delineating radiation. Design approaches, generic to the present invention, are described in copending Canadian Patent Application S.N. 2,061,624 filed February 21, 1992. The co-filed application describes circuit design approaches of particular significance as implemented in accordance with this invention. While not so limited, examples ofparticular consequence concern interference-generated image features. Such examples include line termination as well as line branching in which facility for many-valued 20 magnitudes of phase delay are utilized. In addition, feature spacing, sometimes compromised by the phase mask approach, is significantly improved by generation of closely spaced adjacent features by different phase delay values of paired defining radiation.
It is observed in the co-filed applicationthat provision for 180~ phase masking as 25 described in the literature (see preceding section), while offering significant improvement in resolution, may be further improved. This literature-described approach is based upon phase cancellation on the assumption that energy to be cancelled is of uniform phase. This assumption does not take into account phase delay which is introduced in the course of im~ging, e.g. through proximity effects of closely spaced 30 features. Such variation from the nominal constant-phase image is of increasing significance for circuitry of decreasing design rules. Actual pattern wave fronts of contemplated sub-micron design rule devices are characterized by a variety - often a continuum - of phase variation. Since unwanted radiation em~n~ting from the mask, e.g.
diffraction-edge scattered radiation, may reflect such phase variation, accommodation 35 in terms of phase cancellation should take this into account. Total cancellation continues to require energy which is 180~
Summary of the Invention The invention is concerned with device fabrication involving at least one level of 5 pattern delineation dependent on image transfer from mask to device in fabrication. The fundamental thrust entails use of a mask designedly providing for a plurality of values of phase delay and amplitude of delineation radiation with the object of improving image quality - e.g. improving edge resolution of imaged features. While edge resolution is improved independentof feature size, the advance is of particular consequencefor small 10 features - particularly for closely spaced small features. Accordingly, a primary purpose of the invention entails fabrication of submicron design rule features, generally as contained in Integrated Circuits. Relevant approaches are directed to use of device design rules smaller than usually contemplated as expediently ~ in~ble for particular delineating radiation. Design approaches, generic to the present invention, are described in copending Canadian Patent Application S.N. 2,061,624 filed February 21, 1992. The co-filed application describes circuit design approaches of particular significance as implemented in accordance with this invention. While not so limited, examples ofparticular consequence concern interference-generated image features. Such examples include line termination as well as line branching in which facility for many-valued 20 magnitudes of phase delay are utilized. In addition, feature spacing, sometimes compromised by the phase mask approach, is significantly improved by generation of closely spaced adjacent features by different phase delay values of paired defining radiation.
It is observed in the co-filed applicationthat provision for 180~ phase masking as 25 described in the literature (see preceding section), while offering significant improvement in resolution, may be further improved. This literature-described approach is based upon phase cancellation on the assumption that energy to be cancelled is of uniform phase. This assumption does not take into account phase delay which is introduced in the course of im~ging, e.g. through proximity effects of closely spaced 30 features. Such variation from the nominal constant-phase image is of increasing significance for circuitry of decreasing design rules. Actual pattern wave fronts of contemplated sub-micron design rule devices are characterized by a variety - often a continuum - of phase variation. Since unwanted radiation em~n~ting from the mask, e.g.
diffraction-edge scattered radiation, may reflect such phase variation, accommodation 35 in terms of phase cancellation should take this into account. Total cancellation continues to require energy which is 180~
out of phase - but which is out of phase with regard to the locally scattered energy to be lessened (rather than at the uniform fixed 180~ phase delay relative to the nominal phase of the hypothetical unifolm pattern wave front which ignores proximity andother pe,lu,bing effects).
In terms of mask construction, the above objectives require provision for phase change at at least two levels - ideally facility for provision of a phase continuum or near-continuum (e.g. with successive values separated by 5~ or 10~).
Variation may take the form of change in thickness and/or refractive index of mask layer material through which patterning energy is tran~misteA, or, alternatively, of 10 the equivalent for masks operating in surface reflection mode. Phase delay of the invention may be provided in reflection masks in a number of ways either separately or in combination. Required local variation in the length of the phase path may take the form of: relief changes of the free reflecting surface; of variation in penetration depth in a Ll~lsp~nt layer to a reflecting surface for incident radiation; of variation 15 in refractive index in such a layer. Such masks, whether tran~mi~ion or reflection mode, designedly providing for three or more levels of phase delay for cancellation/lessening of unwanted radiation are here referred to as Multi _hasedelay Masks.
Construction of MPMs by extension of procedures used for selective 20 removaVretention of mask layer regions of unaltered (of total) thickness as practiced in literature-described 180~ phase ma~l~ing is avoided in accordance with the invention. Provision for regions of an additional value of phase delay by this means would require an a-l/lifion~l mask layer as well as an atten~ant additional lithographic step - costly in terrns of statistically reduced device yield.
The invention provides for device fabrication by use of expediently prep~cd masks evidencing a multiplicity - even a continuum of phase delays -without need for ad~litional lithographic delin~ation steps. Such masks, whetherop~Ling in the tran~mi~sion or reflection mode, are fabricated by what might be regarded as a two-step process: 1. quantum or "digital" alteration of defined localized 30 mask regions and 2. physical averaging of such alteration. In embodimellts in which phase delay is the consequence of layer thinning alone, step 1 consists of localremoval of layered matçrial to yield ap~ uled material and step 2 consists of bar~filling of ape~lul~s (referred to as "expendable a~elLures") by in~luced flow of surrounding layered material. The result is a layer characterized by layer thickness 35 variation yielding desired phase variation (in addition to pattern information entailed at that level). AperLures may be of a variety of shapes and sizes - in one embodiment -take the form of circular holes of uniform thickness with distribution/density providing for desired m~trri~l removal. Other apellule shapes, perhaps as suggested by Illtim~tr device features, include slits, areas defining pillars or mesas, etc.
Variations may provide for added rather than removed m~ten~l - always followed by 5 an "averaging" step in which, in this instance, height of patterned added material is caused to flow.
A particularly significant aspect of the invention concerns the ability with which expendable features may be backfilled to the extent required (for avoiding meaningful, device-functional retention in the final pattern) while retaining 10 device pattern-delineation features as well as non-device features included on the mask as compen~ting information. A particular example of the latter is a gratingfor lessening of unwanted brightness variation in the final pattern. The convenience of ~ fourth power dependence of flow rate of filling material as related to distance to be traversed expedites separation of the two (separation of features to be retained 15 from those to be filled) - permits differentiation as between "large" dimensioned feaLures to be retained and "small" dimensioned features to be filled. Nevertheless, some degree of filling, to result in lessened surface relief amplitude, is to be expected for retained features. The inventive teaching provides for minimi7~tion of this effect, in some in~t~nr,es, by over-designing a feature be retained to the extent 20 necessary to result in desired relief ~im~n~ions following flow. Termination of the flow step may be by simple timing, or, in critical instances, by realtime monitoring.
As an example of the latter, a diffraction grating of designedly excessive peak-to-trough height may be monitored to identify the processing end point in terms of desired diffraction amplitude. Other variations are described in the Detailed 25 Description.
Rheological understanding leads to mask design rules. Briefly, barL filling may be regarded intellectually as also consisting of two steps. Initial flow is l~isponsive to the high free surface energy of substrate revealed during pattern ~elinf~tion. For appr~pliately chosen materials - yielding requisite small wetting 30 angle - this first re~luilemellt is satisfied by a thin layer of flowing m~teri~l (a layer thickness of ~ 100 A or less). Subsequent flow, responsive to surface tension considerations, is strongly dependent on void dimensions - a dependency sufficient to permit bacl~Slling of expendable apel luies without significant optical effect on voids to be retained (on non-expendable ape~ures). The two considerations give rise 35 to basis for both materials selection and design geometry.
In terms of mask construction, the above objectives require provision for phase change at at least two levels - ideally facility for provision of a phase continuum or near-continuum (e.g. with successive values separated by 5~ or 10~).
Variation may take the form of change in thickness and/or refractive index of mask layer material through which patterning energy is tran~misteA, or, alternatively, of 10 the equivalent for masks operating in surface reflection mode. Phase delay of the invention may be provided in reflection masks in a number of ways either separately or in combination. Required local variation in the length of the phase path may take the form of: relief changes of the free reflecting surface; of variation in penetration depth in a Ll~lsp~nt layer to a reflecting surface for incident radiation; of variation 15 in refractive index in such a layer. Such masks, whether tran~mi~ion or reflection mode, designedly providing for three or more levels of phase delay for cancellation/lessening of unwanted radiation are here referred to as Multi _hasedelay Masks.
Construction of MPMs by extension of procedures used for selective 20 removaVretention of mask layer regions of unaltered (of total) thickness as practiced in literature-described 180~ phase ma~l~ing is avoided in accordance with the invention. Provision for regions of an additional value of phase delay by this means would require an a-l/lifion~l mask layer as well as an atten~ant additional lithographic step - costly in terrns of statistically reduced device yield.
The invention provides for device fabrication by use of expediently prep~cd masks evidencing a multiplicity - even a continuum of phase delays -without need for ad~litional lithographic delin~ation steps. Such masks, whetherop~Ling in the tran~mi~sion or reflection mode, are fabricated by what might be regarded as a two-step process: 1. quantum or "digital" alteration of defined localized 30 mask regions and 2. physical averaging of such alteration. In embodimellts in which phase delay is the consequence of layer thinning alone, step 1 consists of localremoval of layered matçrial to yield ap~ uled material and step 2 consists of bar~filling of ape~lul~s (referred to as "expendable a~elLures") by in~luced flow of surrounding layered material. The result is a layer characterized by layer thickness 35 variation yielding desired phase variation (in addition to pattern information entailed at that level). AperLures may be of a variety of shapes and sizes - in one embodiment -take the form of circular holes of uniform thickness with distribution/density providing for desired m~trri~l removal. Other apellule shapes, perhaps as suggested by Illtim~tr device features, include slits, areas defining pillars or mesas, etc.
Variations may provide for added rather than removed m~ten~l - always followed by 5 an "averaging" step in which, in this instance, height of patterned added material is caused to flow.
A particularly significant aspect of the invention concerns the ability with which expendable features may be backfilled to the extent required (for avoiding meaningful, device-functional retention in the final pattern) while retaining 10 device pattern-delineation features as well as non-device features included on the mask as compen~ting information. A particular example of the latter is a gratingfor lessening of unwanted brightness variation in the final pattern. The convenience of ~ fourth power dependence of flow rate of filling material as related to distance to be traversed expedites separation of the two (separation of features to be retained 15 from those to be filled) - permits differentiation as between "large" dimensioned feaLures to be retained and "small" dimensioned features to be filled. Nevertheless, some degree of filling, to result in lessened surface relief amplitude, is to be expected for retained features. The inventive teaching provides for minimi7~tion of this effect, in some in~t~nr,es, by over-designing a feature be retained to the extent 20 necessary to result in desired relief ~im~n~ions following flow. Termination of the flow step may be by simple timing, or, in critical instances, by realtime monitoring.
As an example of the latter, a diffraction grating of designedly excessive peak-to-trough height may be monitored to identify the processing end point in terms of desired diffraction amplitude. Other variations are described in the Detailed 25 Description.
Rheological understanding leads to mask design rules. Briefly, barL filling may be regarded intellectually as also consisting of two steps. Initial flow is l~isponsive to the high free surface energy of substrate revealed during pattern ~elinf~tion. For appr~pliately chosen materials - yielding requisite small wetting 30 angle - this first re~luilemellt is satisfied by a thin layer of flowing m~teri~l (a layer thickness of ~ 100 A or less). Subsequent flow, responsive to surface tension considerations, is strongly dependent on void dimensions - a dependency sufficient to permit bacl~Slling of expendable apel luies without significant optical effect on voids to be retained (on non-expendable ape~ures). The two considerations give rise 35 to basis for both materials selection and design geometry.
Initial implications of the inventive teaching are expected to be in terms of projection-reduction im~ging using UV delineating radiation. The principles are,however, applicable to contact (proximity) as well as 1:1 projection printing, and also to use of shorter wavelength electromagnetic radiation (e.g. radiation in the x-ray spectrum).
In accordance with one aspect of the present invention there is provided mask fabrication ent~iling a m~king layer having a masking pattern defined in terms of delineation regions of differing properties to enable transfer of a transfer pattern to a transfer surface, to ultimately result in an ultimate pattern by means of transfer radiation in which such ultimate pattern has a least dimension smaller than 1 ~lm, in which the resolution of such transfer pattern is enhanced by provision of phase shifting characteristics in order to lessen resolution-decreasing effects, said characteristics designedly providing for relative phase shift for a portion of said transfer radiation as between different regions of said transfer surface, in which such m~cking pattern contains two types of information: (a) ultimate pattern information to be included in the ultimate pattern and (b) auxiliary transfer information not necessarily included in the ultimate pattern, CHARACTERIZED in that the said masking layer is fabricated from a precursor m~cking layer which is apertured to produce a digital pattern defined by precursor aperture regions extending through the thickness of such precursor m~king layer to reveal underlying material, said precursor regions being of least dimension no greater than 30% of the least dimension of ultimate pattern information, and in which such digital pattern is processed by a method including at least partially filling apertures by physical material flow.
Brief Description of the Drawin~
The four-figure series presented are descriptive of successive steps in the fabrication of exemplary mask forms satisfying the inventive requirements.
FIGS. lA, lB, lC pertain to a structural approach in which a two-layer body region consisting of substrate and resist is directly shaped by deletion (by aperturing), so producing the expendable image, which is followed by flow to produce the final mask.
FIGS. 2A - 2D are somewhat similar to FIGS. lA - lC but provide for transfer of the developed expendable image from the resist to an underlying material layer, again followed by material flow.
- 6a-FIGS. 3A - 3C depict fabrication of a mask in which the final structure is made up of added material as deposited on an already-pattern delineated layer, followed by liftoff to leave a mask pattern primarily or solely dependent on such deposited material.
S FIGS. 4A - 4B are illustrative of fabrication of a reflective mode mask which depends upon e.g. metal vapor which, following deposition on an already defined/backflowed layer, is caused to diffuse into and saturate such layer material, following which the host layer material is removed so that only metal remains.
Detailed Description Fundamental Considerations Fortunately, prior studies have considered many of the parameters needed for implementation of the inventive processes. Attention is directed, for example, to "Fundamentals of Topographic Substrate Leveling", authored by L.E. Stillwagon and R.G. Larson, as appearing at p. 5251, in J. Appl. Phys., vol. 63(11), (1 June 1988).
Information there contained as supplemented by other studies furnishes basis for various of the parameters usefully employed in accordance with the inventive teaching.
An experimentally-verified observation set forth in that reference is worthy of note -to some extent underlies detailed consideration of all aspects of the invention. Reference is made to the forth power dependence on distance to be traversed for flowing material.
As translated for the inventive purposes, the B
2061~2 dependence serves as basis for retention of larger dimensioned features - e.g. the amount of m~tçri~l transported to the center of a 1011m least dimensioned feature to be retained is lo-4 that for an expendable 1 ~m feature. The resulting variation may itself be sufficient for process-dirrelcn~iation, or, ~l~ern:~tively, may be supplemented 5 by additional processing - e.g. by uniform exposure to result in selective etch-removal.
Material Selection Fabrication in accordance with the invention entails b~ckfilling of expendable apellules. Involved m~teri~l compositions, e.g. of the material of the 10 layer to be apellufed as well as that of "substrate" m~teri~l bared during apellul~lg, must first meet usual ~lem~nc1s based on patterning. All such characteristics are well understood, and for most materials, well documented. While fundamental texts arewell-known to the artisan, directions suggested by the Stillwagon et al article cited above, reflect one satisfactory direction with regard to resist choice.
Signifi~nt material characterization concerns back flow. The simplest approach is that discussed in conjunction with FIG. lA in which the resist layer itself serves that function as well. Process simplification, inevitably desirable in terms of yield, may well be the coll~ ial choice. Resist m~tçri~l~ are well characterizedand many have flow p~pel lies which are pro,llisillg. Separation of functions, with 20 resulting relaxed requilements, may yield to use of a second layer to which the pattern is transferred, to be flowed and yield the ~lltim~te phase-dependent layer tthirkness and/or surface). There are a variety of materials of insufficient actinic plup~,llies (relative insensitivity to delineating radiation) but of advantageous pl~ellies, e.g. with regard to stability in the presence of subsequent irradiation as 25 well as mechanical plùl,cllies. One group of m~teri~l~ are the "spin-on" glasses -co.. ~ .~;ially available low melting glasses that can be used to form layers by convçntion~l spinning processes.
In general terms, desirable characteristics with regard to the material tobe "thickness averaged" are apparent. Such characteristics include: chemical 30 characteristics e.g. to result in requisite durability/reactivity both in the course of fabrication and subsequent use; physical characteristics assuring requisite ~lim~n~ional stability e.g. as temperature/envirùllrl-ellt-dependent; optical ~lupel lies to fulfill usual mask requirements, etc.
Additional requirements are imposed by fabrication in accordance with 35 the inventive teaching. It is convenient to discuss such requirements in terms of (a) minim~ for ~tt~inm~nt of parameters encountered in fabrication of the important 2061~22 category of sub-micron LSI, e.g. design rules 0.5 ~m or less and (b) ranges with a view to including both lesser and greater dimensions and taking into account such processing and/or operating considerations as: permitted tempelature rise, either as tolerable or desired; interfacial energy values as between substrate and backfill 5 material to assure sufficiently low contact wetting angle to meet both the fi-n(1:~mtontal requirement and economic considerations.
A caveat - discussion here is largely in terms of the mask. While cost of mask fabrication is certainly of concern, it is far less important than such considerations entailing llltim~te product. Increased mask cost is easily justified by 10 even minor norm~l;7~d product cost improvement.
In the major embodiment in which patterned material is made to flow, e.g. due to enh~nçed fluidity - perhaps even chemical phase change - a prime re luilcllRnt entails flow characteristics. Viscosity, perhaps as decreased through temp~ldtule increase, must be sufficiently low to meet process time objectives.
15 Surface tension of backfilling m~teri~l in the fluid state must be sufficient again to meet time re~luile~ nt in terms of degree of surface planarity to be attained. Both characteristics are lempelatulc-dependent.
Initial wetting of surface bared in lithographic processing (either directlv due to exposure as by evaporation, or indirectly by following development) is aided 20 by a small wetting angle. Under general cil.;ulll~lances, this is no problem. Usual organic resist materials may be heated to lclllpcl~ture levels without adverse consequence such as to result in near-~ro angles. In general, angles of 10~ or more may suffice if other chcu~ .lances do dictate. It may be desirable to include anadditional layer chosen with primary regard to providing a low wetting angle - such 25 additional layer lm(3~rne~th and in contact with the resist or other layer to be back flowed.
As in~ tçd, energetic rç(luil~ ellts are generally s~ti~fi~l by backflo~
to result in a wetting layer less than 100 A in thi~knçss - a consideration of minor optical consequence, or one that can be readily compensated for in mask design.
30 ~ltern~tively, layered material of such minim~l thickness, may be easily rçmoved without significant effect elsewhere on the mask, as by overall exposure to applu~liate etchant.
With regard to b~c'l~filling following the initial wetting, it is fortunate that flow rate dependence on apcllulc size serves as effective basis for controlled 35 processing. "Fnnd~mt-nt~lc of Topographic Substrate Leveling" cited above present~
experimental verification for a concluded fourth power dependency on distance for g flowing m~teri~l in the course of backfilling. The specific conditions entail comparison as between two apertures, both baring underlying substrates of the same free energy and both surrounded by source material (layer material) of the same thickness and defining an interfacial boundary of the same configuration with respect 5 to the aperture. There are a number of factors to be considered in a rigorous mathematical solution, e.g. including variation in the drive force responsible for flow - interfacial slope and layer thicknçss as dependent on difference in apellulc lim~n~ion. Nevertheless, the order of m~gnitucle of flow-transport during a significant part of the filling operation is as dictated by the fourth power dependency.
Above con~ erations give rise to prescription of expendable apel~ulc size relative to non-expendable ap~ ulc size (e.g. relative to feature apel~ures to be retained, in contradistinction to expendable apelluies to be bac~fillç-3). The matenal pal~llc~er controlling backflow is the ratio of the surface tension to the viscosity.
Contemplated m~teri~l~ to be backflowed are likely glassy in the classical sense.
15 Whether or not glassy from this standpoint, suitable m~teri~l~ desirably show a continuous dependence of pl~el~ies of consequence with regard to varying lell~pel~ture within the l~lllp~,.dture range of consequence (within the ~empcldture range within which flow is made to occur). Such m~tPri~l~, whether inorganic or organic, exhibit viscosity which decreases rapidly with lelllpcl~ture within a range 20 suitable to the objective - from a minimum somewhat above the softening ~tilllp~,~ature to a maximum corresponding with retention of viscosity to the extent required for control.
De~e. .,~ tion of appropriate backflow conditions - primarily telllp.,.dture and time - may be pragm~tic. In accordance with this approach, the 25 mask at this stage is heated to a tC;lllp~,ld~ and for a time sufficient to result in the desired backflow, following which the mask is cooled to terminate flow. A variation serves as well with respect to diffraction gratings. Monitoring is on the basis of the above while illtlminating and sensing the intensity of diffracted radiation.
Tllllmin~tion is convenienlly by a laser-generated beam, and detection may make use 30 of a photo diode. It is unlikely that such results will need adjustment by virtue of any difference in wavelength relative to intended delineating radiation. If necessary, this may be readily calculated.
The Stillwagon et al reference (cited above) at p. 5252 sets forth a "dimensionless time" formula which may serve to determine end point on the basis~5 of calculation. The forrnula is set forth:
h3 ~y T = t W4 ~
20~1(;22 in which:
T is a dimensionless constant that may be obtained, for example, from FIG. 13 of the same reference t = time of backflow (while on the assumption of constant temperature, real conditions characterizing the process approximate the assumption sufficiently - backflow time, ordinarily of the range of minutes for these processes together with termination by quenching (to essentially termin~te flow within seconds) support this thesis ho = the physical drive force for flow in terms of height difference at the border between material to be flowed and bared substrate to be filled w = least dimension of the substrate region to be filled (in the instance addressed - that of equal line/space gratings - w = the breadth of a line or space within the concerned region ~= surface tension rl = viscosity As an example, values are: h = 0.5 x 1o-4 cm; w = 0.25 x 10-4 cm; T
(as determined from FIG. 13) = 500 x 1o-4; t = 3 min. Substitution in the formula yields a ratio: 'Y = 0.087 cm/sec. A representative surface tension value of 30 dynes/cm (a value characteristic of suitable resist materials) yields a viscosity of 345 polses.
Mask Design/Fabrication Considerations The inventive approach places certain clem~nll~ on the mask.
30 Considerations detçrmin~tive of the functioning device pattern are retained - in fact assured - by the invention. However, they must be taken into account in the design of fea~ules which are not to be part of the functioning device. Such non-device fea~ s ("colllpellsating features") include both the expendable apc.lules to be flow-filled and any grating/checkerboard structures for meeting gray scale 35 re4uirelllents (apellules which, while not filled, and while serving their intended ~ 2061622 function during image transfer, are of such dimensions and conformation as likely (and preferably) not to be represented in the final structure).
Creating a mask consists of two phases - designing a mask that will produce a particular image on the wafer by specifying the phase shift and the 5 amplitude of the wavefront of the light coming through each point on the mask; and making a mask that has a particular phase shift and amplitude at each point.
It is a straightforward problem to coll~ule the image that will be produced by light with a particular distribution of phase and amplitude as it leaves the mask. This colllpu~ation can be performed fairly accurately and is discussed in 10 textbooks on optics (see, for example, Reynolds, DeVelis, Parrent, Thompson, "Physical Optics Notebook", in Tutorials in Fourier Optics, pub. SPE Optical Engineering Press, p. 27 (1989).) However, the reverse problem - you want a particular pattern on the wafer and you want to know what phase delay and amplitude as introduced by the 15 mask will produce that pattern - is a much more difficult problem. There may be several ~lirfelent answers that produce almost the same result. Also, it may be impossible to get the ideal pattern and we must be satisfied with the best colllplv,l~ise and clefining what is a good colllproll~ise is usually a matter of judgment.
An important category of masks depend on a transparent layer made up 20 of regions of varying thickness to produce specific phase shift in tranimitte~ or reflected light at each point. The mask is generally created by using an electron beam to expose an electron resist such that the resist is completely removed at certain points and remains at other points. The pattern in the electron resist is thereafter L-~lsr~lled into another m~teri~l of appf~liate physical pr~ ies to 25 satisfy flow requ.l~men~s (e.g. of a~l,lupliate softening te~ c and viscosity).
It is convenient to discuss relevant mask dimensions. Discussion is conveniently discussed in terms of a future device generation built to design rules of - 0.35 ~m (on the assumption of continued Sx reducti~n projection, this results in 1.75 ~m least ~lim~n~ion on the mask):
1. Expendable apel~ufes of least dimension 0.25 ,um on the mask would be seven times smaller than 1.75 ,~Lm features, and a 2,401 x fill rate relative to device fea~ ;s.
2. Gray scale is produced by diffracting light out of the system. The gratings or checkerboard patterns that produce the gray scale are of diffraction-determinative fiim~n~ions of the order 0.75 to 1 micron (for ~ = 0.365 ~m), and the back fill ratio for 0.25 ~m expendable features is from 81x to 256x.
Minimum features that are to be printed on the wafer, i.e. in accordance with permitted design rules, will have a dimension of the order:
Size (on wafer) = NA
5 in which:
The corresponding size of such feature on the mask is in accordance with the linear reduction factor of the mask-to-wafer transfer system.
Reduction factor of 5 is common for projection systems now in prevalent use.
~ = wavelength of delineating radiation NA = numerical aperture of the optical system Kl is a constant.
Accordingly, for a reduction factor of 5, the minimum feature size on the mask is of dimension, 2 ,um, corresponding with 0.4 ,um feature size on the wafer. Mask features, expendable apertures and grating lines as well as pattern features are often, in effect, larger, than that calculated by the formula, as viewed on the mask. This is a consequence of teachings of C~n~ n Patent Application S.N. 2,061,624 cited above.
Increased size of mask features is the consequence of surrounding regions of varying phase and gray scale as used to increase feature edge acuity, e.g. to reduce fringing and other artifacts of interference.
The initial thickness of the concerned layer is normally one or an integral number of wavelengths to afford maximum versatility - to permit attainment of any of the entire range of phase delay values as corresponding with layer thinning. This initial thickness is:
n - 1 in which:
~ = wavelength of delineating radiation n = index of refraction For example, at the mercury I line (~ = 365 ~m for a layer of glassy material ofn = 1.6,) the initial thickness is about 61 ~Lm. Material removal and flow in accordance with the invention, accommodates any desired phase delay.
The Drawing D
-Figures presented are exemplary of likely commercial versions of the invention. All share the overall objective - that of expeditious fabrication of a mask adapted for precisely controlling the image front as received on an article being fabricated - particularly with regard to phase angle of the delineating radiation within 5 the front so as to improve resolution as detailed above. Fabrication of the entirety or of relevant portions of the mask, in accordance with the invention, invariably involves two filn-l~m~nt~l procedures: 1. that of "digital" removal of material within regions extending through the entirety of the "resist" layer, with removed regions of such cross-sectional dimension and distribution as to assure accomplishment of the 10 objective of; 2. "averaging" of the stepwise variations, resulting directly or indirectly by digital removal, by a processing step consisting of or incl~l-ling material flow. In the simplest embodiment, such flow, of material surrounding removed regions, itself accomplishes the overall objective. Other embodiments provide for flow of material about apertures or within protuberances resulting from resist pattern transfer, 15 perhaps into underlying material; perhaps into overlying subsequently deposited material (as by liftoff); perhaps in the form of solute m~teri~l retained after removal Of p~ttçrnç~l - sometimes flow-filled - solvent material.
FIGS. lA - lC are exemplary of the least complex form of fabrication process of the invention - that in which digital patterning as well as backflow-filling 20 occur within a single, inidally presented "resist" layer. (For these purposes the term "resist" is defined in its most generic sense - that of patternability to result in selective "resisting" of subsequent processing, e.g. a~ -p~ttçrning as by removal or retention (perhaps supplemented by m~t~ri~l addition) to enable transfer of either tone of the resist pattern to the wafer. The term, "resist", is intended to include 25 m~t~ri~l which is pa~le. "~ responsive to the delineating energy itself, e.g. by ev~pol~ion, as well as responsive to subsequent or ~imlllt~neous development.
FIG. lA depicts the starting body consisting of substrate 10 and ~uppo~d resist layer 11. While not intended as limiting, exposition is aided by reference to state-of-the-art fabrication in accordance with which substrate 10 may 30 be of 6"-8" in lateral dimension and of a thickness of a fraction of an inch, all as required for processing expedience and rigidity and other mechanical p~opel lles.
Material both of substrate 10 and resist 11 depend upon a number of familiar considerations. For example, if the mask is to operate in the transmission mode,then the substrate must be sufficiently transparent to be m~kç(l If the mask is to 35 operate in the reflection mode, e.g. to mask-delineate energy in the x-ray spectrum.
substrate mAteri~l is chosen solely with regard to mechanical properties.
The starting body of FIG. lA is then pattern-delineated, likely by electron beam writing (e.g. by use of Electron Beam Exposure System writing - see "Introduction to Microlithography", ed. by L. F. Thompson, C. G. Wilson and M. J.
Bowden, ACS Symposium Series, (1983) at p. 73, for a general description of this5 commonly used patterning approach). An ~ltern~tive makes use of a "master" mask, itself likely fabricated by use of electron beam writing, image transfer from a master entails mask-illllmin~tion by unpatterned radiation, e.g. raster scan or flood electromagnetic radiation in the UV or x-ray spectra.
It is an important aspect of the invention that a single delineation step 10 may result both in device-function p~tterning as well as p~tterning of expendable or other features or other compensation features for which replication in the final device is not required - is ordinarily to be avoided. Stated differently, features concerned not with device function, but with device-fabrication expediency, e.g. expendable apel lures, diffraction gratings for imposing gray scale, may be delineated in the 15 single step in which relevant device-function pattern-delineation results. This aspect of the invention, in pelll~iuing both operations in a single step with a single mask layer avoids further yield loss associated with additional masking steps.
Material of which resist layer 11 is constitllted, again depends upon intended mask mode (reflection or tr~n~mission), and, the likely related nature of 20 device-delineating radiation to be masked. From the standpoint of the inventive mask fabrication, resist m~teri~l, whether pat~c~ncd by the delineating energy itself or by ~ttend~nt development, must have an additional prop~l~y not ordinarily required in a resist. For the embodiment shown, resist, once apcl~ulc-patterned,must have flow plvpel~es, however induced, to result in timely flow to accomplish 25 the inventive filling requirement.
FIG. lB depicts the structure of FIG. lA following pattern delineation.
Selective removal of material of layer 11 has resulted in expendable feature holes 12, pillars 13 and rectangular line protuberances 14, to reveal substrate surface 15.
In FIG. lC flow of resist material as surrounding ap~ Ulc5 12 and as 30 constituting protuberances 13 and 14, perhaps as induced by temperature increase, perhaps by other means, has resulted in now averaged layer thickness. Associatedflow has produced a resist layer (constituting a portion of the functional masking layer 16 of five thi~knesses: that of region 17 as initially deposited, region 18 as resulting from flow to substantially fill equally dimensioned/spaced apertures 12;
35 that of region 19 resulting from flow-filling of rectangular apertures embracing protuberances 14; that of regions 20 and 21, resulting the designedly limited flow, as -20616~
elsewhere to result in local averaging (including that of thinner region 21 as produced from the lesser total m~t~ri~l retained in the form of more widely spaced pillars yielding flow material; and finally, that of region 22 which is essentially unfilled - essentially as bared during the delineation responsible for other patterning.
5 (As elsewhere discussed, region 22 in fact supports a very thin layer of resist material (not shown), perhaps 100 A in thickness as dictated by satisfaction of energetic requirements imposed by choice of substrate 10 to present a bared surface of sufficiently high energy to permit the low angle wetting required by the invention.) For simplicity, FIGS lA-lC depict portions of a mask being fabricated with such portions concerned primarily, if not solely, with the compensation patterning of the invention. As in(lic~te l, it is an advantage of the invention that device-pattern delineation may be yielded by the same mask simlllt~neously with pattern variations of the invention (as directed to e.g. resolution improvement of 15 device pattern). Accordingly, the broken section depicted is likely but a portion of a mask-in-fabrication which also includes such additional device-pattern information.
As shown, FIGS lA-lC likely depicts a tr~n~mission mode mask being fabricated (although a~p~ iate choice of material of resist layer 11 permits use in the reflection mode). While not shown, the surface morphology corresponding with20 flow-imposed variations on resist layer thickness, even for transparent resist, may be utilized in the reflection mode by addition of a metallic or other reflective coating (not shown), as by vapor deposition. A further ~ltern~tive depends upon an underlying reflective layer (not shown), in this instance with variable path length and associated changed phase delay involving round-tlip passage through the processed 25 coating (consisting of regions 16-20).
FIGS. 2A-2D depict a variant in which initial resist patterning is transferred to an underlying layer and in which the now-patterned underlying layer is subsequently caused to flow much in the manner depicted and discussed in FIG. lC.
A purpose is to separate desired actinic pr~el ~ies of the resist from desired mask 30 prupellies of the ~leline~ting material - e.g. to substitute a relatively stable/durable inorganic glassy m~t~n~l for the resist as finally processed.
FIG. 2A shows a substrate 30 supporting, in succession, a layer 31 tO
llltim~tely serve as the mask pattern layer, in turn ~u~l)olli~lg resist layer 32. In FM. 2B pattern delineation and development - again likely by electron beam direct-35 write - has produced features in the form of apellul~,s 33, rectangular protuberances 34 and pillars 35, while revealing substrate surface 36 - in this inst~n~e, surface of layer 31.
In FIG. 2C the pattern has been transferred to layer 31, now designated layer 31a to result in a positive image, again consisting of holes and protuberances 37, 38 and 39, while revealing surface 40, in this instance, surface of 5 substrate 30. Positive replication of features 33, 34, 35 as 37, 38 and 39 may be the consequence of use of positive tone resist constituting layer 32.
In FIG. 2D flow, in this instance of m~t~ri~l of layer 31 - e.g. of low m-~lting glass, perhaps as induced by increasing temperature, has again resulted in six discrete regions in a patterned layer now designated 3 lb. In order of thickness 10 such regions are denoted 41, 42, 43, 44, 45 and 46, the latter devoid of supported material 39 except to the angstroms-m~gnitll-le (e.g., as discussed, perhaps ~ 100 A) thickn~ss required to satisfy energetic re4uiremellts resulting from exposure of the high energy surface.
FIGS. 3A-3C depict yet another variation in which digital removal of 15 resist m~t~ri~l is l~ltim~tely followed by flow-filling of expendable apertures (to result in layer thiskn~ss averaging).
In this example, FM. 3A depicts a mask-in-fabrication at the level of that of FIG. lB or of 2C. As shown, substrate 50 supports a delineated layer of m~teri~l 51 (e.g. of resist material of layer 11 of FIG. lA or of e.g. glassy material of layer 31a of FIG. 2C). Layer 51 as patterned includes holes 52, rectangular protuberances 53 and pillars 54. At this stage in fabrication, deposition conditions, uniform across the latent device portion shown, has resulted in addition of layer regions 55. Choice of m~t~llic or other reflective material serving in layer 55 satisfies con-litions for one form of reflection mask in accordance with the invention.
In FIG. 3B, liftoff of m~t~ri~l 51 has resulted in retention only of regions 55 in contact with substrate 50. Replication brought about by liftoff asshown, is, in me~ningful sense, of negative tone so that holes 52 are now represented by pillars 56, protuberances 53 by troughs 57 and pillars 54 by apertures 58.
In FIG. 3C, flow, of material, of properties and under conditions 30 discussed in conjunction with preceding figures, has resulted in local thickness averaging, thereby producing regions 59, 60, 61, 62 of successively tlimini~hingthiçkness, and finally substantially bared region 63 (always allowing for energetically dictated initial flow, likely of angs~ s thickness range).
FIGS. 4A and 4B, in comrnon with other figures presented, illustrate 35 mask fabrication in which many-leveled, or even continuously varying, phase del~y values, are provided by flow, responsive to digital alteration. As shown, FIG. 4A is 2a61 622 at a stage at which the uppermost layer 65 as supported by substrate 66 has been caused to flow resulting, for example, in the structure shown at the stage of FIG. 1 C. Flow has produced five discrete regions of reducing thickness, here denoted 67, 68, 69, 70 and 71.
5 At this stage, the backflow-filled structure is exposed to vapor 72, e.g. metal vapor under conditions and for a time suff1cient to saturate resist layer 65 throughout its entire thickness.
In FIG. 4B the resist host (or matrix) has been removed, e.g. by dissolution or vol~tili7~tion under conditions to leave metal layer 73 with such layer characterized by 10 regions 74-78 such regions are being of the same relative thicknesses as those of regions 67-71, respectively, to the extent that uniform saturation was attained. Under the processing conditions responsible for the device depicted, vapor 72 is not deposited on free surface 71, but is retained only by solution in host material of layer 65. Under these conditions, region 78 is not coated.
In the instance in which vapor 72 is of metal which is reflective with respect to device fabrication-delineatingenergy, the product - the mask - is reflective and evidences controlled differential path length to produce the desired stepped phase delay values for such energy. In many instances, the approach offers precise control of control of extremely thin layered material - a degree of control not so easily realized by means of physical flow.
Device Delineation It is useful to consider operating characteristics of the mask as constructed inaccordance with the invention. A primary consideration relates to wavelength of the radiation to be used in device fabrication. In this section relevance of this aspect to the invention is discussed.
The purpose of first generation phase masks will be to obtain higher resolution and image quality with presently used delineating energy. This includes energy in the visible spectrum: e.g. the mercury G line at 4360 A; in the near UV spectrum, e.g. the mercury I line at 3650 A; in the deep UV spectrum, e.g. the krypton fluoride excimer laser line at 2480 A or the mercury 2540 A line, and ultimately at still shorter wavelength, perhaps at the argon fluoride excimer laser 1930 ~ line.
The invention is not wavelength limited. It will likely serve for use with delineating radiation in the x-ray spectrum - as used in projection or proximityprinting. Copending Canadian Patent Application S.N. 2,052,734 filed October 3, 1991, describes and claims an effective projection system which may operate in the reduction mode, likely for the longer wavelength ~9 portion of the x-ray spectrum - e.g. within the 40A - 200A range. It may serve also in x-ray pro~ y printing, thereby enabling use over the shorter portion of the spectrum as well (e.g. including the 9angstrom- 18A range). In the new technologies the same principles apply to masks for x-ray projection lithography in 5 the 40 to 200 A range. The techniques of mask making could also be used in x-ray proximity printing in the 9 - 18 A range.
A signific~nt degree of coherence of delineating radiation is required for contemplated projection systems providing for improved feature edge acuity through phase delay cancellation of edge-scattered radiation as well as of other attendant 10 resolution-impairing effects. Illumination coherence concerns both temporal (longit~ in~l) coherence and lateral or spatial coherence. Temporal coherence isrelated to the bandwidth of the radiation and can be expressed in coherence length e.g. as expressed in terms of number of wavelengths. For meaningful interference to occur the phase difference between waves combining at a particular point must be15 less than the coherence length of the radiation. In cases of interest in phase masks, phase differences are seldom more than a few wavelengths. In these terms, coherence length available by use of illustrative sources mentioned, is sufficiently long as not to present a problem.
The other aspect is spatial or lateral coherence. This is characterized by 20 the numerical aperture, NA, and the filling factor ~. If the source is a laser or an incoherent source at a distant point (at sufficient distance to serve as a point source).
cs = O, and there is considerable lateral coherence. For a larger source (e.g., a point source that has been diffused), lateral coherence is less, resulting in larger ~. In ordinary lithography a low ~ produces sharper edges on small features but has 25 ringing and intelrelcnce effects so that a somewhat larger value of ~ is preferred -value of ~s = ~ 0.5 is generally found suitable. In a phase mask ringing is a signific;ln concem, but, again, a finite value of ~ - representing a colllplulllise between ringing and edge definition - is useful.
Currently, phase mask systems are designed to work with existing 30 cameras, e.g. with ~ = ~ 0.5. Masks produced in accordance with the invention rTUv make use of such ~ values. Conditions encountered are likely to result in col~lùll~ise values which are somewhat smaller. Use of partially coherent light (~
of finite value) averages out many effects of interference. Complete control of b~th phase and amplitude elimin~tes need for such averaging and permits optimization ot 35 edge acuity realizable by use of ~ = 0.
In accordance with one aspect of the present invention there is provided mask fabrication ent~iling a m~king layer having a masking pattern defined in terms of delineation regions of differing properties to enable transfer of a transfer pattern to a transfer surface, to ultimately result in an ultimate pattern by means of transfer radiation in which such ultimate pattern has a least dimension smaller than 1 ~lm, in which the resolution of such transfer pattern is enhanced by provision of phase shifting characteristics in order to lessen resolution-decreasing effects, said characteristics designedly providing for relative phase shift for a portion of said transfer radiation as between different regions of said transfer surface, in which such m~cking pattern contains two types of information: (a) ultimate pattern information to be included in the ultimate pattern and (b) auxiliary transfer information not necessarily included in the ultimate pattern, CHARACTERIZED in that the said masking layer is fabricated from a precursor m~cking layer which is apertured to produce a digital pattern defined by precursor aperture regions extending through the thickness of such precursor m~king layer to reveal underlying material, said precursor regions being of least dimension no greater than 30% of the least dimension of ultimate pattern information, and in which such digital pattern is processed by a method including at least partially filling apertures by physical material flow.
Brief Description of the Drawin~
The four-figure series presented are descriptive of successive steps in the fabrication of exemplary mask forms satisfying the inventive requirements.
FIGS. lA, lB, lC pertain to a structural approach in which a two-layer body region consisting of substrate and resist is directly shaped by deletion (by aperturing), so producing the expendable image, which is followed by flow to produce the final mask.
FIGS. 2A - 2D are somewhat similar to FIGS. lA - lC but provide for transfer of the developed expendable image from the resist to an underlying material layer, again followed by material flow.
- 6a-FIGS. 3A - 3C depict fabrication of a mask in which the final structure is made up of added material as deposited on an already-pattern delineated layer, followed by liftoff to leave a mask pattern primarily or solely dependent on such deposited material.
S FIGS. 4A - 4B are illustrative of fabrication of a reflective mode mask which depends upon e.g. metal vapor which, following deposition on an already defined/backflowed layer, is caused to diffuse into and saturate such layer material, following which the host layer material is removed so that only metal remains.
Detailed Description Fundamental Considerations Fortunately, prior studies have considered many of the parameters needed for implementation of the inventive processes. Attention is directed, for example, to "Fundamentals of Topographic Substrate Leveling", authored by L.E. Stillwagon and R.G. Larson, as appearing at p. 5251, in J. Appl. Phys., vol. 63(11), (1 June 1988).
Information there contained as supplemented by other studies furnishes basis for various of the parameters usefully employed in accordance with the inventive teaching.
An experimentally-verified observation set forth in that reference is worthy of note -to some extent underlies detailed consideration of all aspects of the invention. Reference is made to the forth power dependence on distance to be traversed for flowing material.
As translated for the inventive purposes, the B
2061~2 dependence serves as basis for retention of larger dimensioned features - e.g. the amount of m~tçri~l transported to the center of a 1011m least dimensioned feature to be retained is lo-4 that for an expendable 1 ~m feature. The resulting variation may itself be sufficient for process-dirrelcn~iation, or, ~l~ern:~tively, may be supplemented 5 by additional processing - e.g. by uniform exposure to result in selective etch-removal.
Material Selection Fabrication in accordance with the invention entails b~ckfilling of expendable apellules. Involved m~teri~l compositions, e.g. of the material of the 10 layer to be apellufed as well as that of "substrate" m~teri~l bared during apellul~lg, must first meet usual ~lem~nc1s based on patterning. All such characteristics are well understood, and for most materials, well documented. While fundamental texts arewell-known to the artisan, directions suggested by the Stillwagon et al article cited above, reflect one satisfactory direction with regard to resist choice.
Signifi~nt material characterization concerns back flow. The simplest approach is that discussed in conjunction with FIG. lA in which the resist layer itself serves that function as well. Process simplification, inevitably desirable in terms of yield, may well be the coll~ ial choice. Resist m~tçri~l~ are well characterizedand many have flow p~pel lies which are pro,llisillg. Separation of functions, with 20 resulting relaxed requilements, may yield to use of a second layer to which the pattern is transferred, to be flowed and yield the ~lltim~te phase-dependent layer tthirkness and/or surface). There are a variety of materials of insufficient actinic plup~,llies (relative insensitivity to delineating radiation) but of advantageous pl~ellies, e.g. with regard to stability in the presence of subsequent irradiation as 25 well as mechanical plùl,cllies. One group of m~teri~l~ are the "spin-on" glasses -co.. ~ .~;ially available low melting glasses that can be used to form layers by convçntion~l spinning processes.
In general terms, desirable characteristics with regard to the material tobe "thickness averaged" are apparent. Such characteristics include: chemical 30 characteristics e.g. to result in requisite durability/reactivity both in the course of fabrication and subsequent use; physical characteristics assuring requisite ~lim~n~ional stability e.g. as temperature/envirùllrl-ellt-dependent; optical ~lupel lies to fulfill usual mask requirements, etc.
Additional requirements are imposed by fabrication in accordance with 35 the inventive teaching. It is convenient to discuss such requirements in terms of (a) minim~ for ~tt~inm~nt of parameters encountered in fabrication of the important 2061~22 category of sub-micron LSI, e.g. design rules 0.5 ~m or less and (b) ranges with a view to including both lesser and greater dimensions and taking into account such processing and/or operating considerations as: permitted tempelature rise, either as tolerable or desired; interfacial energy values as between substrate and backfill 5 material to assure sufficiently low contact wetting angle to meet both the fi-n(1:~mtontal requirement and economic considerations.
A caveat - discussion here is largely in terms of the mask. While cost of mask fabrication is certainly of concern, it is far less important than such considerations entailing llltim~te product. Increased mask cost is easily justified by 10 even minor norm~l;7~d product cost improvement.
In the major embodiment in which patterned material is made to flow, e.g. due to enh~nçed fluidity - perhaps even chemical phase change - a prime re luilcllRnt entails flow characteristics. Viscosity, perhaps as decreased through temp~ldtule increase, must be sufficiently low to meet process time objectives.
15 Surface tension of backfilling m~teri~l in the fluid state must be sufficient again to meet time re~luile~ nt in terms of degree of surface planarity to be attained. Both characteristics are lempelatulc-dependent.
Initial wetting of surface bared in lithographic processing (either directlv due to exposure as by evaporation, or indirectly by following development) is aided 20 by a small wetting angle. Under general cil.;ulll~lances, this is no problem. Usual organic resist materials may be heated to lclllpcl~ture levels without adverse consequence such as to result in near-~ro angles. In general, angles of 10~ or more may suffice if other chcu~ .lances do dictate. It may be desirable to include anadditional layer chosen with primary regard to providing a low wetting angle - such 25 additional layer lm(3~rne~th and in contact with the resist or other layer to be back flowed.
As in~ tçd, energetic rç(luil~ ellts are generally s~ti~fi~l by backflo~
to result in a wetting layer less than 100 A in thi~knçss - a consideration of minor optical consequence, or one that can be readily compensated for in mask design.
30 ~ltern~tively, layered material of such minim~l thickness, may be easily rçmoved without significant effect elsewhere on the mask, as by overall exposure to applu~liate etchant.
With regard to b~c'l~filling following the initial wetting, it is fortunate that flow rate dependence on apcllulc size serves as effective basis for controlled 35 processing. "Fnnd~mt-nt~lc of Topographic Substrate Leveling" cited above present~
experimental verification for a concluded fourth power dependency on distance for g flowing m~teri~l in the course of backfilling. The specific conditions entail comparison as between two apertures, both baring underlying substrates of the same free energy and both surrounded by source material (layer material) of the same thickness and defining an interfacial boundary of the same configuration with respect 5 to the aperture. There are a number of factors to be considered in a rigorous mathematical solution, e.g. including variation in the drive force responsible for flow - interfacial slope and layer thicknçss as dependent on difference in apellulc lim~n~ion. Nevertheless, the order of m~gnitucle of flow-transport during a significant part of the filling operation is as dictated by the fourth power dependency.
Above con~ erations give rise to prescription of expendable apel~ulc size relative to non-expendable ap~ ulc size (e.g. relative to feature apel~ures to be retained, in contradistinction to expendable apelluies to be bac~fillç-3). The matenal pal~llc~er controlling backflow is the ratio of the surface tension to the viscosity.
Contemplated m~teri~l~ to be backflowed are likely glassy in the classical sense.
15 Whether or not glassy from this standpoint, suitable m~teri~l~ desirably show a continuous dependence of pl~el~ies of consequence with regard to varying lell~pel~ture within the l~lllp~,.dture range of consequence (within the ~empcldture range within which flow is made to occur). Such m~tPri~l~, whether inorganic or organic, exhibit viscosity which decreases rapidly with lelllpcl~ture within a range 20 suitable to the objective - from a minimum somewhat above the softening ~tilllp~,~ature to a maximum corresponding with retention of viscosity to the extent required for control.
De~e. .,~ tion of appropriate backflow conditions - primarily telllp.,.dture and time - may be pragm~tic. In accordance with this approach, the 25 mask at this stage is heated to a tC;lllp~,ld~ and for a time sufficient to result in the desired backflow, following which the mask is cooled to terminate flow. A variation serves as well with respect to diffraction gratings. Monitoring is on the basis of the above while illtlminating and sensing the intensity of diffracted radiation.
Tllllmin~tion is convenienlly by a laser-generated beam, and detection may make use 30 of a photo diode. It is unlikely that such results will need adjustment by virtue of any difference in wavelength relative to intended delineating radiation. If necessary, this may be readily calculated.
The Stillwagon et al reference (cited above) at p. 5252 sets forth a "dimensionless time" formula which may serve to determine end point on the basis~5 of calculation. The forrnula is set forth:
h3 ~y T = t W4 ~
20~1(;22 in which:
T is a dimensionless constant that may be obtained, for example, from FIG. 13 of the same reference t = time of backflow (while on the assumption of constant temperature, real conditions characterizing the process approximate the assumption sufficiently - backflow time, ordinarily of the range of minutes for these processes together with termination by quenching (to essentially termin~te flow within seconds) support this thesis ho = the physical drive force for flow in terms of height difference at the border between material to be flowed and bared substrate to be filled w = least dimension of the substrate region to be filled (in the instance addressed - that of equal line/space gratings - w = the breadth of a line or space within the concerned region ~= surface tension rl = viscosity As an example, values are: h = 0.5 x 1o-4 cm; w = 0.25 x 10-4 cm; T
(as determined from FIG. 13) = 500 x 1o-4; t = 3 min. Substitution in the formula yields a ratio: 'Y = 0.087 cm/sec. A representative surface tension value of 30 dynes/cm (a value characteristic of suitable resist materials) yields a viscosity of 345 polses.
Mask Design/Fabrication Considerations The inventive approach places certain clem~nll~ on the mask.
30 Considerations detçrmin~tive of the functioning device pattern are retained - in fact assured - by the invention. However, they must be taken into account in the design of fea~ules which are not to be part of the functioning device. Such non-device fea~ s ("colllpellsating features") include both the expendable apc.lules to be flow-filled and any grating/checkerboard structures for meeting gray scale 35 re4uirelllents (apellules which, while not filled, and while serving their intended ~ 2061622 function during image transfer, are of such dimensions and conformation as likely (and preferably) not to be represented in the final structure).
Creating a mask consists of two phases - designing a mask that will produce a particular image on the wafer by specifying the phase shift and the 5 amplitude of the wavefront of the light coming through each point on the mask; and making a mask that has a particular phase shift and amplitude at each point.
It is a straightforward problem to coll~ule the image that will be produced by light with a particular distribution of phase and amplitude as it leaves the mask. This colllpu~ation can be performed fairly accurately and is discussed in 10 textbooks on optics (see, for example, Reynolds, DeVelis, Parrent, Thompson, "Physical Optics Notebook", in Tutorials in Fourier Optics, pub. SPE Optical Engineering Press, p. 27 (1989).) However, the reverse problem - you want a particular pattern on the wafer and you want to know what phase delay and amplitude as introduced by the 15 mask will produce that pattern - is a much more difficult problem. There may be several ~lirfelent answers that produce almost the same result. Also, it may be impossible to get the ideal pattern and we must be satisfied with the best colllplv,l~ise and clefining what is a good colllproll~ise is usually a matter of judgment.
An important category of masks depend on a transparent layer made up 20 of regions of varying thickness to produce specific phase shift in tranimitte~ or reflected light at each point. The mask is generally created by using an electron beam to expose an electron resist such that the resist is completely removed at certain points and remains at other points. The pattern in the electron resist is thereafter L-~lsr~lled into another m~teri~l of appf~liate physical pr~ ies to 25 satisfy flow requ.l~men~s (e.g. of a~l,lupliate softening te~ c and viscosity).
It is convenient to discuss relevant mask dimensions. Discussion is conveniently discussed in terms of a future device generation built to design rules of - 0.35 ~m (on the assumption of continued Sx reducti~n projection, this results in 1.75 ~m least ~lim~n~ion on the mask):
1. Expendable apel~ufes of least dimension 0.25 ,um on the mask would be seven times smaller than 1.75 ,~Lm features, and a 2,401 x fill rate relative to device fea~ ;s.
2. Gray scale is produced by diffracting light out of the system. The gratings or checkerboard patterns that produce the gray scale are of diffraction-determinative fiim~n~ions of the order 0.75 to 1 micron (for ~ = 0.365 ~m), and the back fill ratio for 0.25 ~m expendable features is from 81x to 256x.
Minimum features that are to be printed on the wafer, i.e. in accordance with permitted design rules, will have a dimension of the order:
Size (on wafer) = NA
5 in which:
The corresponding size of such feature on the mask is in accordance with the linear reduction factor of the mask-to-wafer transfer system.
Reduction factor of 5 is common for projection systems now in prevalent use.
~ = wavelength of delineating radiation NA = numerical aperture of the optical system Kl is a constant.
Accordingly, for a reduction factor of 5, the minimum feature size on the mask is of dimension, 2 ,um, corresponding with 0.4 ,um feature size on the wafer. Mask features, expendable apertures and grating lines as well as pattern features are often, in effect, larger, than that calculated by the formula, as viewed on the mask. This is a consequence of teachings of C~n~ n Patent Application S.N. 2,061,624 cited above.
Increased size of mask features is the consequence of surrounding regions of varying phase and gray scale as used to increase feature edge acuity, e.g. to reduce fringing and other artifacts of interference.
The initial thickness of the concerned layer is normally one or an integral number of wavelengths to afford maximum versatility - to permit attainment of any of the entire range of phase delay values as corresponding with layer thinning. This initial thickness is:
n - 1 in which:
~ = wavelength of delineating radiation n = index of refraction For example, at the mercury I line (~ = 365 ~m for a layer of glassy material ofn = 1.6,) the initial thickness is about 61 ~Lm. Material removal and flow in accordance with the invention, accommodates any desired phase delay.
The Drawing D
-Figures presented are exemplary of likely commercial versions of the invention. All share the overall objective - that of expeditious fabrication of a mask adapted for precisely controlling the image front as received on an article being fabricated - particularly with regard to phase angle of the delineating radiation within 5 the front so as to improve resolution as detailed above. Fabrication of the entirety or of relevant portions of the mask, in accordance with the invention, invariably involves two filn-l~m~nt~l procedures: 1. that of "digital" removal of material within regions extending through the entirety of the "resist" layer, with removed regions of such cross-sectional dimension and distribution as to assure accomplishment of the 10 objective of; 2. "averaging" of the stepwise variations, resulting directly or indirectly by digital removal, by a processing step consisting of or incl~l-ling material flow. In the simplest embodiment, such flow, of material surrounding removed regions, itself accomplishes the overall objective. Other embodiments provide for flow of material about apertures or within protuberances resulting from resist pattern transfer, 15 perhaps into underlying material; perhaps into overlying subsequently deposited material (as by liftoff); perhaps in the form of solute m~teri~l retained after removal Of p~ttçrnç~l - sometimes flow-filled - solvent material.
FIGS. lA - lC are exemplary of the least complex form of fabrication process of the invention - that in which digital patterning as well as backflow-filling 20 occur within a single, inidally presented "resist" layer. (For these purposes the term "resist" is defined in its most generic sense - that of patternability to result in selective "resisting" of subsequent processing, e.g. a~ -p~ttçrning as by removal or retention (perhaps supplemented by m~t~ri~l addition) to enable transfer of either tone of the resist pattern to the wafer. The term, "resist", is intended to include 25 m~t~ri~l which is pa~le. "~ responsive to the delineating energy itself, e.g. by ev~pol~ion, as well as responsive to subsequent or ~imlllt~neous development.
FIG. lA depicts the starting body consisting of substrate 10 and ~uppo~d resist layer 11. While not intended as limiting, exposition is aided by reference to state-of-the-art fabrication in accordance with which substrate 10 may 30 be of 6"-8" in lateral dimension and of a thickness of a fraction of an inch, all as required for processing expedience and rigidity and other mechanical p~opel lles.
Material both of substrate 10 and resist 11 depend upon a number of familiar considerations. For example, if the mask is to operate in the transmission mode,then the substrate must be sufficiently transparent to be m~kç(l If the mask is to 35 operate in the reflection mode, e.g. to mask-delineate energy in the x-ray spectrum.
substrate mAteri~l is chosen solely with regard to mechanical properties.
The starting body of FIG. lA is then pattern-delineated, likely by electron beam writing (e.g. by use of Electron Beam Exposure System writing - see "Introduction to Microlithography", ed. by L. F. Thompson, C. G. Wilson and M. J.
Bowden, ACS Symposium Series, (1983) at p. 73, for a general description of this5 commonly used patterning approach). An ~ltern~tive makes use of a "master" mask, itself likely fabricated by use of electron beam writing, image transfer from a master entails mask-illllmin~tion by unpatterned radiation, e.g. raster scan or flood electromagnetic radiation in the UV or x-ray spectra.
It is an important aspect of the invention that a single delineation step 10 may result both in device-function p~tterning as well as p~tterning of expendable or other features or other compensation features for which replication in the final device is not required - is ordinarily to be avoided. Stated differently, features concerned not with device function, but with device-fabrication expediency, e.g. expendable apel lures, diffraction gratings for imposing gray scale, may be delineated in the 15 single step in which relevant device-function pattern-delineation results. This aspect of the invention, in pelll~iuing both operations in a single step with a single mask layer avoids further yield loss associated with additional masking steps.
Material of which resist layer 11 is constitllted, again depends upon intended mask mode (reflection or tr~n~mission), and, the likely related nature of 20 device-delineating radiation to be masked. From the standpoint of the inventive mask fabrication, resist m~teri~l, whether pat~c~ncd by the delineating energy itself or by ~ttend~nt development, must have an additional prop~l~y not ordinarily required in a resist. For the embodiment shown, resist, once apcl~ulc-patterned,must have flow plvpel~es, however induced, to result in timely flow to accomplish 25 the inventive filling requirement.
FIG. lB depicts the structure of FIG. lA following pattern delineation.
Selective removal of material of layer 11 has resulted in expendable feature holes 12, pillars 13 and rectangular line protuberances 14, to reveal substrate surface 15.
In FIG. lC flow of resist material as surrounding ap~ Ulc5 12 and as 30 constituting protuberances 13 and 14, perhaps as induced by temperature increase, perhaps by other means, has resulted in now averaged layer thickness. Associatedflow has produced a resist layer (constituting a portion of the functional masking layer 16 of five thi~knesses: that of region 17 as initially deposited, region 18 as resulting from flow to substantially fill equally dimensioned/spaced apertures 12;
35 that of region 19 resulting from flow-filling of rectangular apertures embracing protuberances 14; that of regions 20 and 21, resulting the designedly limited flow, as -20616~
elsewhere to result in local averaging (including that of thinner region 21 as produced from the lesser total m~t~ri~l retained in the form of more widely spaced pillars yielding flow material; and finally, that of region 22 which is essentially unfilled - essentially as bared during the delineation responsible for other patterning.
5 (As elsewhere discussed, region 22 in fact supports a very thin layer of resist material (not shown), perhaps 100 A in thickness as dictated by satisfaction of energetic requirements imposed by choice of substrate 10 to present a bared surface of sufficiently high energy to permit the low angle wetting required by the invention.) For simplicity, FIGS lA-lC depict portions of a mask being fabricated with such portions concerned primarily, if not solely, with the compensation patterning of the invention. As in(lic~te l, it is an advantage of the invention that device-pattern delineation may be yielded by the same mask simlllt~neously with pattern variations of the invention (as directed to e.g. resolution improvement of 15 device pattern). Accordingly, the broken section depicted is likely but a portion of a mask-in-fabrication which also includes such additional device-pattern information.
As shown, FIGS lA-lC likely depicts a tr~n~mission mode mask being fabricated (although a~p~ iate choice of material of resist layer 11 permits use in the reflection mode). While not shown, the surface morphology corresponding with20 flow-imposed variations on resist layer thickness, even for transparent resist, may be utilized in the reflection mode by addition of a metallic or other reflective coating (not shown), as by vapor deposition. A further ~ltern~tive depends upon an underlying reflective layer (not shown), in this instance with variable path length and associated changed phase delay involving round-tlip passage through the processed 25 coating (consisting of regions 16-20).
FIGS. 2A-2D depict a variant in which initial resist patterning is transferred to an underlying layer and in which the now-patterned underlying layer is subsequently caused to flow much in the manner depicted and discussed in FIG. lC.
A purpose is to separate desired actinic pr~el ~ies of the resist from desired mask 30 prupellies of the ~leline~ting material - e.g. to substitute a relatively stable/durable inorganic glassy m~t~n~l for the resist as finally processed.
FIG. 2A shows a substrate 30 supporting, in succession, a layer 31 tO
llltim~tely serve as the mask pattern layer, in turn ~u~l)olli~lg resist layer 32. In FM. 2B pattern delineation and development - again likely by electron beam direct-35 write - has produced features in the form of apellul~,s 33, rectangular protuberances 34 and pillars 35, while revealing substrate surface 36 - in this inst~n~e, surface of layer 31.
In FIG. 2C the pattern has been transferred to layer 31, now designated layer 31a to result in a positive image, again consisting of holes and protuberances 37, 38 and 39, while revealing surface 40, in this instance, surface of 5 substrate 30. Positive replication of features 33, 34, 35 as 37, 38 and 39 may be the consequence of use of positive tone resist constituting layer 32.
In FIG. 2D flow, in this instance of m~t~ri~l of layer 31 - e.g. of low m-~lting glass, perhaps as induced by increasing temperature, has again resulted in six discrete regions in a patterned layer now designated 3 lb. In order of thickness 10 such regions are denoted 41, 42, 43, 44, 45 and 46, the latter devoid of supported material 39 except to the angstroms-m~gnitll-le (e.g., as discussed, perhaps ~ 100 A) thickn~ss required to satisfy energetic re4uiremellts resulting from exposure of the high energy surface.
FIGS. 3A-3C depict yet another variation in which digital removal of 15 resist m~t~ri~l is l~ltim~tely followed by flow-filling of expendable apertures (to result in layer thiskn~ss averaging).
In this example, FM. 3A depicts a mask-in-fabrication at the level of that of FIG. lB or of 2C. As shown, substrate 50 supports a delineated layer of m~teri~l 51 (e.g. of resist material of layer 11 of FIG. lA or of e.g. glassy material of layer 31a of FIG. 2C). Layer 51 as patterned includes holes 52, rectangular protuberances 53 and pillars 54. At this stage in fabrication, deposition conditions, uniform across the latent device portion shown, has resulted in addition of layer regions 55. Choice of m~t~llic or other reflective material serving in layer 55 satisfies con-litions for one form of reflection mask in accordance with the invention.
In FIG. 3B, liftoff of m~t~ri~l 51 has resulted in retention only of regions 55 in contact with substrate 50. Replication brought about by liftoff asshown, is, in me~ningful sense, of negative tone so that holes 52 are now represented by pillars 56, protuberances 53 by troughs 57 and pillars 54 by apertures 58.
In FIG. 3C, flow, of material, of properties and under conditions 30 discussed in conjunction with preceding figures, has resulted in local thickness averaging, thereby producing regions 59, 60, 61, 62 of successively tlimini~hingthiçkness, and finally substantially bared region 63 (always allowing for energetically dictated initial flow, likely of angs~ s thickness range).
FIGS. 4A and 4B, in comrnon with other figures presented, illustrate 35 mask fabrication in which many-leveled, or even continuously varying, phase del~y values, are provided by flow, responsive to digital alteration. As shown, FIG. 4A is 2a61 622 at a stage at which the uppermost layer 65 as supported by substrate 66 has been caused to flow resulting, for example, in the structure shown at the stage of FIG. 1 C. Flow has produced five discrete regions of reducing thickness, here denoted 67, 68, 69, 70 and 71.
5 At this stage, the backflow-filled structure is exposed to vapor 72, e.g. metal vapor under conditions and for a time suff1cient to saturate resist layer 65 throughout its entire thickness.
In FIG. 4B the resist host (or matrix) has been removed, e.g. by dissolution or vol~tili7~tion under conditions to leave metal layer 73 with such layer characterized by 10 regions 74-78 such regions are being of the same relative thicknesses as those of regions 67-71, respectively, to the extent that uniform saturation was attained. Under the processing conditions responsible for the device depicted, vapor 72 is not deposited on free surface 71, but is retained only by solution in host material of layer 65. Under these conditions, region 78 is not coated.
In the instance in which vapor 72 is of metal which is reflective with respect to device fabrication-delineatingenergy, the product - the mask - is reflective and evidences controlled differential path length to produce the desired stepped phase delay values for such energy. In many instances, the approach offers precise control of control of extremely thin layered material - a degree of control not so easily realized by means of physical flow.
Device Delineation It is useful to consider operating characteristics of the mask as constructed inaccordance with the invention. A primary consideration relates to wavelength of the radiation to be used in device fabrication. In this section relevance of this aspect to the invention is discussed.
The purpose of first generation phase masks will be to obtain higher resolution and image quality with presently used delineating energy. This includes energy in the visible spectrum: e.g. the mercury G line at 4360 A; in the near UV spectrum, e.g. the mercury I line at 3650 A; in the deep UV spectrum, e.g. the krypton fluoride excimer laser line at 2480 A or the mercury 2540 A line, and ultimately at still shorter wavelength, perhaps at the argon fluoride excimer laser 1930 ~ line.
The invention is not wavelength limited. It will likely serve for use with delineating radiation in the x-ray spectrum - as used in projection or proximityprinting. Copending Canadian Patent Application S.N. 2,052,734 filed October 3, 1991, describes and claims an effective projection system which may operate in the reduction mode, likely for the longer wavelength ~9 portion of the x-ray spectrum - e.g. within the 40A - 200A range. It may serve also in x-ray pro~ y printing, thereby enabling use over the shorter portion of the spectrum as well (e.g. including the 9angstrom- 18A range). In the new technologies the same principles apply to masks for x-ray projection lithography in 5 the 40 to 200 A range. The techniques of mask making could also be used in x-ray proximity printing in the 9 - 18 A range.
A signific~nt degree of coherence of delineating radiation is required for contemplated projection systems providing for improved feature edge acuity through phase delay cancellation of edge-scattered radiation as well as of other attendant 10 resolution-impairing effects. Illumination coherence concerns both temporal (longit~ in~l) coherence and lateral or spatial coherence. Temporal coherence isrelated to the bandwidth of the radiation and can be expressed in coherence length e.g. as expressed in terms of number of wavelengths. For meaningful interference to occur the phase difference between waves combining at a particular point must be15 less than the coherence length of the radiation. In cases of interest in phase masks, phase differences are seldom more than a few wavelengths. In these terms, coherence length available by use of illustrative sources mentioned, is sufficiently long as not to present a problem.
The other aspect is spatial or lateral coherence. This is characterized by 20 the numerical aperture, NA, and the filling factor ~. If the source is a laser or an incoherent source at a distant point (at sufficient distance to serve as a point source).
cs = O, and there is considerable lateral coherence. For a larger source (e.g., a point source that has been diffused), lateral coherence is less, resulting in larger ~. In ordinary lithography a low ~ produces sharper edges on small features but has 25 ringing and intelrelcnce effects so that a somewhat larger value of ~ is preferred -value of ~s = ~ 0.5 is generally found suitable. In a phase mask ringing is a signific;ln concem, but, again, a finite value of ~ - representing a colllplulllise between ringing and edge definition - is useful.
Currently, phase mask systems are designed to work with existing 30 cameras, e.g. with ~ = ~ 0.5. Masks produced in accordance with the invention rTUv make use of such ~ values. Conditions encountered are likely to result in col~lùll~ise values which are somewhat smaller. Use of partially coherent light (~
of finite value) averages out many effects of interference. Complete control of b~th phase and amplitude elimin~tes need for such averaging and permits optimization ot 35 edge acuity realizable by use of ~ = 0.
Claims (23)
1. Mask fabrication entailing a masking layer having a masking pattern defined in terms of delineation regions of differing properties to enable transfer of a transfer pattern to a transfer surface, to ultimately result in an ultimate pattern by means of transfer radiation in which such ultimate pattern has a least dimension smaller than 1 µm, in which the resolution of such transfer pattern is enhanced by provision of phase shifting characteristics in order to lessen resolution-decreasing effects, said characteristics designedly providing for relative phase shift for a portion of said transfer radiation as between different regions of said transfer surface, in which such masking pattern contains two types of information: (a) ultimate pattern information to be included in the ultimate pattern and (b) auxiliary transfer information not necessarily included in the ultimate pattern, CHARACTERIZED in that the said masking layer is fabricated from a precursor masking layer which is apertured to produce a digital pattern defined by precursor aperture regions extending through the thickness of such precursor masking layer to reveal underlying material, said precursor regions being of least dimension no greater than 30% of the least dimension of ultimate pattern information, and in which such digital pattern is processed by a method including at least partially filling apertures by physical material flow.
2. The process of claim 1 in which resolution-decreasing is at least in part produced by diffraction-scattered radiation and in which the said characteristics designedly provide for 180° relative phase shift for a portion of said transfer radiation in which precursor regions include expendable regions being of least dimension no greater than 15% of the least dimension of ultimate pattern information and in which material flow results in substantial filling of expendable regions.
3. The process of claim 2 in which included precursor regions include non-printing regions larger than such expendable regions but sufficiently small as to be excluded from the final pattern.
4. The process of claim 3 in which such non-printing regions include features of such size and spacing as to diffract final pattern transfer radiation to the extent necessary to avoid its incidence on the transfer surface.
5. The process of claim 3 in which such non-printing regions comprise diffraction gratings.
6. The process of claim 1 in which said final pattern constitutes a functioning part of a discrete mask.
7. The process of claim 1 in which patterning is initiated by electron beam writing in a resist layer so as to selectively remove resist material to produce an aperture image.
8. The process of claim 7 in which the aperture image includes expendable regions and in which resist material proximate to such expendable regions is caused to flow and fill such expendable regions.
9. The process of claim 7 in which said aperture image is transferred to an underlying layer which is processed to result in the said masking layer.
10. The process of claim 9 in which processing of the underlying layer comprises material removal to produce an aperture image including expendable regions which are filled by flow.
11. The process of claim 10 in which the underlying layer consists essentially of a glassy layer which is increased in temperature to cause flow and so fill the expendable regions.
12. The process of claim 7 in which an overlying layer is produced by material deposition on top of the aperture-patterned resist layer.
13. The process of claim 12 in which further processing includes liftoff of deposited material by removal of resist so as to produce an image defined by deposited material selectively retained in apertures.
14. The process of claim 13 in which selectively retained material is caused to flow to fill regions resulting from liftoff.
15. The process of claim 8 in which the resulting reflow-patterned resist layer is exposed to a fluid serving as a source for solute for said resist layer under conditions as to saturate substantially the entirety of such layer.
16. The process of claim 15 in which the said resist is substantially removed toleave a patterned layer of solute.
17. The process of claim 16 in which a Distributed Bragg Reflector is constructed on top of the patterned layer of solute.
18. The process of claim 1 in which backflow produces a masking layer comprising at least three discrete regions of sufficiently differing thickness and associated differing surface morphology to result in a phase difference of at least 10° from each other with regard to transfer radiation.
19. The process of claim 1 in which backflow produces a masking layer comprising a continuum of layer material of varying thickness and associated differing surface morphology to result in a maximum variation in phase difference of at least 10°
with regard to transfer radiation within the continuum.
with regard to transfer radiation within the continuum.
20. The process of claims 18 or 19 including replicating to yield a separate mask of negative tone as replicated.
21. The process of claim 20 in which replicating is by hot-pressing.
22. The process of claim 20 in which replicating is by casting.
23. The process of claims 18 or 19 in which the mask to be replicated is modified by selective addition or subtraction of material to improve the surface to be replicated.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/673,626 US5217831A (en) | 1991-03-22 | 1991-03-22 | Sub-micron device fabrication |
| US673,626 | 1991-03-22 |
Publications (2)
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|---|---|
| CA2061622A1 CA2061622A1 (en) | 1992-09-23 |
| CA2061622C true CA2061622C (en) | 1998-09-22 |
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| CA002061622A Expired - Fee Related CA2061622C (en) | 1991-03-22 | 1992-02-21 | Sub-micron device fabrication |
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| EP (1) | EP0505103B1 (en) |
| JP (1) | JPH0594001A (en) |
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| CA (1) | CA2061622C (en) |
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| JP3179520B2 (en) * | 1991-07-11 | 2001-06-25 | 株式会社日立製作所 | Method for manufacturing semiconductor device |
| US5867363A (en) * | 1992-09-18 | 1999-02-02 | Pinnacle Research Institute, Inc. | Energy storage device |
| US5821033A (en) * | 1992-09-18 | 1998-10-13 | Pinnacle Research Institute, Inc. | Photolithographic production of microprotrusions for use as a space separator in an electrical storage device |
| US5279925A (en) * | 1992-12-16 | 1994-01-18 | At&T Bell Laboratories | Projection electron lithographic procedure |
| JP2546135B2 (en) * | 1993-05-31 | 1996-10-23 | 日本電気株式会社 | Method of forming semiconductor fine shape, method of manufacturing InP diffraction grating, and method of manufacturing distributed feedback laser |
| US5980977A (en) * | 1996-12-09 | 1999-11-09 | Pinnacle Research Institute, Inc. | Method of producing high surface area metal oxynitrides as substrates in electrical energy storage |
| US6368763B2 (en) | 1998-11-23 | 2002-04-09 | U.S. Philips Corporation | Method of detecting aberrations of an optical imaging system |
| US6248486B1 (en) * | 1998-11-23 | 2001-06-19 | U.S. Philips Corporation | Method of detecting aberrations of an optical imaging system |
| US6361911B1 (en) | 2000-04-17 | 2002-03-26 | Taiwan Semiconductor Manufacturing Company | Using a dummy frame pattern to improve CD control of VSB E-beam exposure system and the proximity effect of laser beam exposure system and Gaussian E-beam exposure system |
| EP1869399A2 (en) * | 2005-04-11 | 2007-12-26 | Zetetic Institute | Apparatus and method for in situ and ex situ measurement of spatial impulse response of an optical system using phase-shifting point-diffraction interferometry |
| JP2008541130A (en) * | 2005-05-18 | 2008-11-20 | ゼテテック インスティテュート | Apparatus and method for measuring optical system flare in-situ and other than in-situ |
| US11528971B2 (en) * | 2018-05-13 | 2022-12-20 | Bob Michael Lansdorp | Jewelry image projection and method |
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| JP2710967B2 (en) * | 1988-11-22 | 1998-02-10 | 株式会社日立製作所 | Manufacturing method of integrated circuit device |
| US5091979A (en) * | 1991-03-22 | 1992-02-25 | At&T Bell Laboratories | Sub-micron imaging |
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- 1991-03-22 US US07/673,626 patent/US5217831A/en not_active Expired - Lifetime
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| KR100211624B1 (en) | 1999-08-02 |
| JPH0594001A (en) | 1993-04-16 |
| US5217831A (en) | 1993-06-08 |
| EP0505103B1 (en) | 1997-08-06 |
| DE69221350T2 (en) | 1997-11-20 |
| DE69221350D1 (en) | 1997-09-11 |
| EP0505103A1 (en) | 1992-09-23 |
| HK1001927A1 (en) | 1998-07-17 |
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