US3614423A

US3614423A - Charged particle pattern imaging and exposure system

Info

Publication number: US3614423A
Application number: US73898A
Authority: US
Inventors: Louis N Heynick; Eugene R Westerberg
Original assignee: Stanford Research Institute
Current assignee: SRI International Inc
Priority date: 1970-09-21
Filing date: 1970-09-21
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19
Also published as: GB1328320A; DE2146941A1; FR2106298A5; NL7112975A

Abstract

A charged-particle beam-pattern forming and imaging system is disclosed in which charged particles from one or more sources impinge upon an imaging plate. A high-voltage electrical source is connected between the imaging plate and a target to produce a strong electrical field therebetween. The imaging plate contains one or more long and narrow slits which may be straight or curved to any desired configuration. Each slit functions as a lens to yield one image of itself for each particle source, each image being converged only along the width of the slit (one-dimension convergence). Proper choices of the pattern of the particle sources and the slit arrangement in the imaging plate yield a variety of patterns useful in many applications, such as closely spaced parallel lines for diffraction gratings, interdigital patterns for microwave devices, meander lines, and interconnections for integrated circuits, by formation of the entire patterns at once rather than by the scanning or other sequential techniques used heretofore.

Description

United States Patent CHARGED PARTICLE PATTERN IMAGING AND EXPOSURE SYSTEM 15 Claims, 13 Drawing Figs.

u.s.c1 250/495 c, 29/25.3,14s/1.5,219/1211213,250/495 TE, 250/495 T, 313/80, 315/31 Int. Cl ..H0lj 37/12, H01 j 37/30 Field of Search .1 29/253;

148/1.5; 219/121 EB; 313/80, 85, 86; 315/30, 31; 340/173 CR; 346/74 CR, 74 ER; 250/495 T, 49.5 R, 49.5 TE. 49.5 C

[561 References Cited UNITED STATES PATENTS 3,491,236 1/1970 Newberry 250/495 C Primary ExaminerAnthony L. Birch Attorney-Flehr, Hohbach, Test, Albritton and Herbert ABSTRACT: A charged-particle beam-pattern forming and imaging system is disclosed in which charged particles from one or more sources impinge upon an imaging plate. A highvoltage electrical source is connected between the imaging plate and a target to produce a strong electrical field therebetween. The imaging plate contains one or more long and narrow slits which may be straight or curved to any desired configuration. Each slit functions as a lens to yield one image of itself for each particle source, each image being converged only along the width of the slit (one-dimension convergence). Proper choices of the pattern of the particle sources and the slit arrangement in the imaging plate yield a variety of patterns useful in many applications, such as closely spaced parallel lines for diffraction gratings, interdigital patterns for microwave devices, meander lines, and interconnections for integrated circuits, by formation of the entire patterns at once rather than by the scanning or other sequential techniques used heretofore.

LOW VOLTAGE POWER SUPPL Y may VOLTAGE paws/e suppz. Y

CHARGED PARTICLE PATTERN IMAGING AND EXPOSURE SYSTEM CROSS REFERENCES TO RELATED APPLICATIONS The present invention involves a novel extension of the principle of an invention described in application entitled Multiple Imaging Exposure System, Ser. No. 847,326, filed Aug. 4, [969, and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION This invention relates to electron or ion beam-forming systems and more particularly to such systems for producing patterns of long and very narrow closely spaced parallel lines or grooves, interdigital lines, meander lines, and other more complex patterns on appropriate substrates in nonserial fashion analogous to photography but with higher resolution than can be obtained by photolithographic techniques.

A common method of producing sharply defined configurations on a substrate is to use light to expose the desired configuration to photosensitive resist material deposited on the substrate. The resist is then developed and the areas of the underlying' material uncovered thereby are removed with an etchant that does not attach the resist. The production of submicron patterns, however, requires the use of charged particles rather than light as an exposure means. For example, the useful range of photon wavelength limits practical resolution of photoresists to the order of a micron, whereas the wavelength of electrons used in resist exposure is IOtimes smaller, thus enabling a corresponding increase in resolution.

In principle, such high resolution patterns can be produced serially on electron sensitive resists by using an appropriately driven scanning electron probe for exposure. In practice however, the serial process is very slow in many cases because of beam-current limitations at the required spot size and because of limitations in the sensitivities of resist materials. Also, instabilities of typical scanning electron probes can introduce spurious variations in the patterns produced.

In a copending application entitled Multiple Imaging Exposure System," Ser. No. 847,326, filed Aug. 4, 1969, and assigned to the assignee of the present invention there is disclosed and claimed an exposure system which utilizes a mesh screen as an array of electron-optical lenses, one per hole in the screen. More specifically, an electron source illuminates a pattern mask having a desired aperture pattern therein. Electrons passing through the pattern mask impinge upon a mesh screen.

A high-voltage electrical source is connected between the mesh screen and an electron-sensitive resist-coated substrate to produce a strong electrical field therebetween. Each hole in the mesh screen acts as a lens for producing an image of the pattern mask on the resist, resulting in an array of exposed images on the electron-sensitive resist. A basic principle of this prior invention is that the converging electrical field at each screen hole has radial symmetry about the axis of the screen hole so that the image produced on the resist-coated substrate is a uniformly demagnified replica of the object pattern.

In the present invention, a plate having one or more long and narrow slits therethrough is used instead of a mesh screen, the principle being that the strong electrical field between the plate and the substrate produces convergence only in the directions corresponding to the narrow dimension of each slit at each location along its length (one-dimensional convergence), s distinguished from the uniform convergence in all directions produced by each screen hole. The slits in the plate may be straight or curved segments, or of any desired combinations or configurations thereof. If such a plate is illuminated with a parallel beam of electrons or a distant or a small source of electrons, then the electron image produced on the substrate by the plate is of the same size and configuration as the slit pattern therethrough but the line width of the image pattern is smaller than the slit width by the one-dimensional convergence factor. If two or more distant or small sources of electrons are used to illuminate the plate then each such source will produce an image of the slit pattern. One convenient method for providing one or more such electron sources is to use an electron emitter (cathode) to properly illuminate a plate having as many apertures therethrough as the number of sources desired, each such aperture thereby constituting an effective source. Combinations of electron sources and slit patterns are selected to produce any desired image patterns, as exemplified later herein.

In the copending application entitled Multiple Imaging Exposure System, Ser. No. 847,326, there is also disclosed and claimed a system for etching patterns into substrates without requiring any resist material thereon. In this system, an ion source is used instead of an electron source and the demagnified ion-beam patterns produced by the mesh screen are used directly to remove material from the substrate by sputter etching or chemical reaction with the substrate. Since the desired patterns are formed in the ion beams themselves, no resists or other forms of substrate masking are necessary.

In the present invention, the use of plates having long and narrow slits in conjunction with ion sources for etching patterns having one dimensional convergence into substrates or doping semiconductor materials by ion implantation without requiring any masking is disclosed SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide charged-particle beam-forming systems based on the use of a plate having long and narrow slits of any desired size and configuration and having high electrical fields therein to produce beam convergence only in the directions corresponding to the narrow dimensions of the slits.

It is another object of this invention to provide such charged-particle beam-forming systems in which the patterns of charged-particle sources and the slit patterns in the beamforming plate are selected to provide any desired one-dimensionally convergent beam configurations.

It is still another object of this invention to provide such charged-particle beam-forming systems in which the charged particles are electrons and the one-dimensionally convergent beams thereof are used for exposing electron-sensitive resist to the corresponding patterns.

It is still another object of this invention to provide such charged-particle beam-forming systems in which the charged I convergent beams thereof are used for doping insulators or semiconductors by ion implantation therein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall system diagram of a pattern-imaging and exposure system.

FIG. 2 is an isometric view of a portion of the system of FIG. 1 showing the relationship between the target surface, imaging plate and source mask.

FIG. 3 is a cross-sectional view of a slit in the imaging plate and target surface illustrating the electrical field therebetween.

FIG. 4a illustrates another embodiment of an imaging plate.

FIG. 4b illustrates the image produced with the imaging plate of FIG. 4a and a source mask having a linear array of holes.

FIG. 5a shows another form of imaging plate.

FIG. 5b shows the image which can be produced with the imaging plate of FIG. 5a.

FIG. 6 shows another form of source mask having two linear arrays of holes which staggered with respect to each other.

FIGS. 7 and 8 illustrate other embodiments of a pattern imaging system for use with the source mask of FIG. 6 for producing with one exposure the image shown in FIG. b.

FIGS. 9a, 9b and 90 show other embodiments of imaging plates for producing images of meander lines, curved lines and intersecting lines, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown one of the preferred embodiments of the invention in which the charged particles used are electrons. A low-voltage power supply 11 is connected between a cathode 12 and a control electrode 13 which are all disposed within a vacuum chamber 14. The vacuum chamber M may be any of the well-known vacuum enclosures which may be evacuated, for example, to Torr and the cathode is typically circular, rfainch in diameter. The cathode 12 produces electrons which pass through control electrode 13 and are focused by a suitable electron lens 15, illustrated in FIG. 1 as a three-electrode unipotential or cinzel lens. Einzel lenses and other suitable electron lenses are well known in the art. A source mask 17 which contains one or more suitably located apertures is placed at the focal point of the lens 15. The intensity distribution of the electrons at the source mask 17 is usually of a Gaussian shape but by using only that portion of the electron stream near the peak of the distribution, the illumination is nearly constant across the apertured portion of the source mask 17. Electrons passing through the source mask 17 impinge on an imaging plate 18.

A high-voltage power supply 19 (on the order of 1.5 to 3 RV) is connected between the imaging plate 18 and a substrate 21. The substrate 21 has, in a first embodiment, a target surface or coating 22 of electron-sensitive resist, examples of which are well known in the art. As is explained in more details hereinafter, the imaging plate 18 has a pattern of long and narrow slits which, because of the high-electric field produced thereat by power supply 19, act as one-dimensionally convergent electron lenses for forming images of the slit pattern on the target surface 22 having much narrower line widths than he slits in imaging plate 18. If the source mask 17 has only one aperture, then only one image of the slit pattern in imaging plate 18 is formed on the target surface 22. If the source mask 17 has two or more apertures, then each aperture produces an image of the slit pattern in imaging plate 18 on the target surface 22, and the images are located relative to one another in the same arrangement as the disposition of apertures in the source mask 17, but at correspondingly smaller distances. Alternative arrangements are possible in the system of FIG. 1. For example, if source 12 is made to be an ion rather than an electron source, then one-dimensionally converged ion images of the slit pattern in imaging plate 18 is produced on the coating 22, one such image for each aperture in source mask 17. Since the system of FIG. 1 is wholly electrostatic, the paths taken by the ions are identical to those traversed by the electrons; the ions merely traverse those trajectories at lower velocities. Positive ions, of course, require that the singleness of he voltages supplied by low-voltage power supply 11 and high-voltage power supply 19 be reversed.

If desired, patterns can be etched into the substrate material 21 directly, thereby eliminating the use of a resist layer 22, which requires exposure and development. This is done by using the slit patterns formed in the ion beam to sputter away or chemically remove the substrate material. In such chemical removal the ion beam chemically reacts with the substrate material to form a volatile compound. Hence, pattern micromachining of substrates or thin films thereon can be done directly. Further, such an arrangement can be used for producing arrays of ion-implanted semiconductor devices in semiconductor or insulator substrates by applying from highvoltage power supply 19 a voltage on the order of 20 to I00 KV in order to obtain deep penetration of the ions in the substrate 21. It should be understood that the following discussion is equally applicable to resist exposure, direct micromachining or ion implantation. 5 FIG. 2 is another view showing the relationship between the source mask 17, the imaging plate 18 and the substrate 21 and target surface 22. In one embodiment the imaging plate 18 has a single long and very narrow slit 23 therein. The source mask 17 is spaced from the imaging plate 18 and in this embodiment may have a single hole 24, equivalent to a single source.

As shown in FIG. 3, the electric field which is produced between the imaging plate 18 and the substrate 21 and target surface 22 is convergent only in the direction parallel to the narrow dimension or width w of the slit 23. This convergent electrical field is present along the entire length 1 of the slit 23. Thus, if the source mask 17 has a single hole 24, as shown in FIG. 2, then the image produced on the target surface 22 is a line parallel to and as long as the length 1 of the slit 23, and this line image has a width much smaller than the slit width w or the size of the aperture 24 in the source mask 17. The demagnification factor of the system is adjustable by varying the relative spacing between the target surface 22 and the imaging plate 18 and the relative spacing between the imaging plate 18 and the source mask 17.

Typically, the width of he line image produced on the target surface 22 will be the width of the slit w demagnified by a factor of 150. Thus, very accurately defined submicron width lines may be produced or imaged on the target surface 22.

In more generalized extensions, the imaging screen can have straight or curved slit segments of various lengths, relative orientations, and locations (including junctions and intersections) which, together with appropiatc aperture patterns in the source mask can be selected to provide a great variety of image patterns. For example, turning to FIG. 4a, consider an imaging plate 26 which contains an ell-shaped slit 27. Assuming that the aperture pattern in the source mask is a single aperture, then the image produced on a target surface will be an ell of the same dimensions but of demagnified line width. If the aperture pattern is a linear array of holes, say of 75-micron diameter and spaced on I50-micron centers along a line perpendicular to the long arm of the ell, and the parameters of the system are adjusted to yield a demagnification of 150 times, then the image produced on the target 28 as shown in FIG. 4b consists of a corresponding array 29 of zmicron wide lines, one per aperture in the source mask, spaced on I micron centers and joined together at one end. This array 29 is produced with one exposure and no indexing or scanning of the electron beam is necessary.

The use of ell-shaped imaging screens can be adapted to the formation of interdigital line images. For example, referring to FIG. 5a, an imaging plate 31 has a U-shaped slit 32 therein. Shutters are provided for selectively obstructing

regions

33 and 34 of the slit 32 to selectively prevent these regions from forming images. In a first exposure, the shutter-obstructing region 33 is opened and the shutter-obstructing region 34 is closed, thus yielding an image pattern similar to FIG. 4b. In a second exposure, the beams from the source mask are displaced a distance equivalent to half the distance between the lines on the target surface and the shutter-obstructing region MI is opened while the shutter-obstructing region 33 is closed. The resultant composite image of the two exposures is the interdigital pattern shown in FIG. 5b in which the target surface 36 has

interdigital line arrays

37 and 38. The requisite displacement can be done by electrostatic or magnetic deflections of the charged particles or by physical displacement of the imaging plate 31 relative to the source mask. Alternatively, the displacement can be obtained by using a source mask 42 having twice as many holes and half the spacing required. These holes may be on the same line, or may be arranged in two rows, 39 and 41, of alternative holes as shown in FIG. 6. Shutters are then used to cover alternate holes for the first exposure. The holes that were uncovered are then covered, and the alternate holes which were covered are opened for the second exposure.

It is also possible to use a U-shaped imaging plate 31 to form an interdigitated patten without the use of double exposures and without requiring that movable shutters be used inside the vacuum system. In this embodiment the aperture pattern in the source mask 42 consists of the two

rows

39 and 41 of alternative holes spaced an appropriate distance apart shown in FIG. 6. An auxiliary set of masks is used between the source mask 42 and the U-shaped imaging plate 31 to automatically provide blanking of the illumination through each of the rows of

holes

39 and 41 toa corresponding leg of the U-shaped pattern. Referring to FIGS. 5a, 6 and 7, the source mask 42 has the two rows of

staggered holes

39 and 41. The imaging plate 31 has a U-shaped slit which has

arm areas

33 and 34. An auxiliary mask 43 is provided between the source mask 42 and the imaging plate 31. Auxiliary auxiliary mask 43 in FIG. 7 is positioned such that the charged particles passing through the row of holes 41 is prevented from illuminating any of the area 33. Similarly, charged particles passing through any of the holes 39 are prevented by the auxiliary mask 43 from illuminating any of the area 34. Using this arrangement it is possible to produce an interdigitated image pattern such as shown in FIG. 5b with a single exposure.

FIG. 8 is similar to FIG. 7, except that a different configuration is utilized for auxiliary mask 44. Auxiliary mask 44 functions in the same manner as auxiliary mask 43 in FIG. 7 and prevents any of the charged particles passing through the holes 39 from illuminating area 33 and prevents any of the charged particles passing through the holes 41 from illuminating any of area 34. The arrangement of FIG. 8 likewise functions to produce an interdigitated pattern such as that shown in FIG. 5!).

Image plates having two or more U-shaped slits can be used for simultaneously producing a corresponding number of interdigital patterns by any of the methods cited above. Meander lines can be produced, by using an imaging plate 46 as shown in FIG. 9a which has a slit pattern 46 therein to be used in conjunction with a source mask having one or more holes as an aperture pattern. An imaging plate 48 having a curved slit 49 as shown in FIG. 9b may be used with a source mask having one or more holes to form corresponding images of the curved slit, but having narrow line widths. Likewise,

referring to FIG. 90, an imaging plate 51 may have a slit 52 comprising intersecting slits for forming intersecting line images of narrow width.

In many applications it may be desirable or necessary to obtain uniform electron or ion arrival rates per unit area over the entire image on the target surface or to otherwise control the relative electron or ion intensities produced by each slit segment. To provide for such control and variation ofintensities, the widths of the various slit segments may be selected or varied appropriately relative to one another.

It is evident that in addition to the specific source mask patterns and imaging plates described above, many other source mask and imaging plate combinations can be used to produce a variety of image patterns.

Although the invention has been described with reference to specific embodiments thereof, it should be apparent to those skilled in the art that minor modifications and changes may be made to the embodiment disclosed herein without departing from the true spirit and scope of the invention.

We claim:

1. A beam-forming and imaging system comprising: charged particle source means for directing charged particles along a path, a source mask disposed in said path and having at least one aperture therein, a target disposed in said path, an imaging plate disposed in said path between said target and said.

converged image of said slit on said target for each aperture in said source mask.

2. The beam-forming and imaging system of claim 1 wherein said charged particle source means comprises a source of electrons and wherein said target comprises electrosensitive resist.

3. The beam-forming and imaging system of claim 1, wherein said charged particle source means comprises a source of electrons and wherein said target comprises a substrate having a coating of electron-sensitive resist thereon.

4. The beam-forming and imaging system of claim 1 wherein said charged particle source means comprises a source of ions and wherein said target comprises ion-sensitive resist.

5. The beam-forming and imaging system of claim wherein said charged particle source means comprises a source of ions and wherein said target comprises a substrate having a coating of ion-sensitive resist thereon.

6. The beam-forming and imaging system of claim 1 wherein the apertures in said source mask comprise a linear array of holes and wherein said imaging plate has an ellshaped slit whereby the one dimensionally converged images of said ell-shaped slit formed on said target by said imaging plate comprise an array of lines joined at one end.

7. The beam-forming and imaging system of claim 1 wherein the apertures in said source mask comprise first and second linear arrays of holes, said first and second linear arrays staggered with respect to each other, said imaging plate having a U-shaped slit with first and second legs, an auxiliary mask disposed in said charged particle path between said source mask and said imaging plate, said auxiliary mask adapted to obstruct said first leg of said U-shaped slit from charged particles passing through said first linear array of holes and to obstruct said second leg of said U-shaped slit from charged particles passing through said second linear array of holes whereby two interdigitated arrays of lines are imaged on said target with each array having the lines thereof connected at one end.

8. The beam-forming and imaging system of claim I wherein said slit is curved.

9. The beam-forming and imaging system of claim 1 wherein said slit has portions oriented at an angle with respect to other potions thereof.

it). The beam-forming and imaging system of claim 1 including a plurality of slits of arbitrary sizes, shapes, and configurations.

11. A beam-forming and imaging system comprising: ion source means for directing ions along a path, a source mask disposed in said path and having at least one aperture therein, a target disposed in said path, an imaging plate disposed in said path between said target and said source mask and comprising an opaque member having at least one slit therein, said slit being substantially long in length and relatively narrow in width but of otherwise arbitrary configuration, a voltage source connected between said imaging plate and said target and adapted to produce an electrical field therebetween, said electrical field convergent about said slit only in a direction parallel to said relatively narrow width, said convergent electrical field present along the substantially long length of said slit whereby the ions are formed into a convergent ion beam impinging upon said target.

112. The beam-forming and imaging system of claim 11 wherein said convergent ion beam is used to sputter-etch said target.

13. The beam-forming and imaging system of claim 11 wherein said convergent ion beam is used to remove material from said target by chemical reaction therebetween.

14. The beam-forming and imaging system of claim 11 wherein said target comprises a semiconductor material whereby said convergent ion beam implants ions in said semiconductor material. V, M W [5. The iiam rbniiig and imaging system of claim 11 wherein said target comprises an insulator material whereby said convergent ion beam implants ions in said insulator material.

Claims

1. A beam-forming and imaging system comprising: charged particle source means for directing charged particles along a path, a source mask disposed in said path and having at least one aperture therein, a target disposed in said path, an imaging plate disposed in said path between said target and said source mask and comprising an opaque member having at least one slit therein, said slit being substantially long in length and relatively narrow in width, a voltage source connected between said imaging plate and said target and adapted to produce an electrical field therebetween, said electrical field convergent about said slit only in a direction parallel to said relatively narrow width, said convergent electrical field present along the substantially long length of said slit whereby the charged particles in said path are focused by the convergent electrical field about said slit to form a one-dimensionally converged image of said slit on said target for each aperture in said source mask.

6. The beam-forming and imaging system of claim 1 wherein the apertures in said source mask comprise a linear array of holes and wherein said imaging plate has an ell-shaped slit whereby the one dimensionally converged images of said ell-shaped slit fOrmed on said target by said imaging plate comprise an array of lines joined at one end.

8. The beam-forming and imaging system of claim 1 wherein said slit is curved.

10. The beam-forming and imaging system of claim 1 including a plurality of slits of arbitrary sizes, shapes, and configurations.

12. The beam-forming and imaging system of claim 11 wherein said convergent ion beam is used to sputter-etch said target.

14. The beam-forming and imaging system of claim 11 wherein said target comprises a semiconductor material whereby said convergent ion beam implants ions in said semiconductor material.

15. The beam-forming and imaging system of claim 11 wherein said target comprises an insulator material whereby said convergent ion beam implants ions in said insulator material.