US20050147763A1

US20050147763A1 - Method and apparatus for uniform deposition onto semiconductor wafers

Info

Publication number: US20050147763A1
Application number: US10/745,773
Authority: US
Inventors: Yen-Hao Shih; Kuang Hsieh
Original assignee: Macronix International Co Ltd
Current assignee: Macronix International Co Ltd
Priority date: 2003-12-24
Filing date: 2003-12-24
Publication date: 2005-07-07

Abstract

A thin film on a deposition area of a wafer structure is formed of substantially parallel, substantially straight tracts of film material, adjacent ones of which partly overlap. Each tract has a lengthwise midline, and the thickness of film material and a section across each tract has a Gaussian or near-Gaussian profile with a maximum at the midline. The distance between midlines of adjacent tracts is in a range from 0.1 to 3 times a standard deviation. Also, methods for forming a thin film on a substrate, include steps of mounting the substrate on a fixture so that a surface of the substrate lies in an x-y plane, depositing onto the substrate surface a plurality of substantially parallel, substantially straight tracts of film material, each tract having a lengthwise midline, and the thickness of film material in a transverse section of each tract having a Gaussian or near-Gaussian distribution with a maximum at the midline, the distance between the midlines of adjacent partly overlapping tracts being a distance about 1.5 times a standard deviation. Also, apparatus for depositing a thin film according to the method includes a programmable controller operable to specify a deposition path and rates of relative movement of the deposition apparatus and the substrate.

Description

BACKGROUND

This invention relates to semiconductor wafer processing and, particularly, to deposit materials to achieve uniform thickness on the wafer surface.
A variety of techniques are available for depositing thin films in semiconductor wafer processing. Among these are various methods for vapor deposition of, for example, metals, semiconductors, dielectric materials, organic materials, oxides, and nitrides. One class of vapor deposition techniques is commonly known as chemical vapor deposition (CVD). In a CVD process the target is held in a chamber at certain pressure (at atmospheric pressure for Atmospheric Pressure CVD; and reduced pressure as low as a fraction of 1 torr for Low Pressure CVD), and the target surface is heated and is exposed to precursor molecules. The precursor molecules react at or above the heated target surface to generate the film material in situ. In other vapor deposition techniques, including techniques commonly known as physical vapor deposition (PVD), the target is held in a chamber at a relatively low pressure (typically below 10⁻²torr), and the material to be deposited is provided in bulk in the chamber. The material is vaporized (by evaporation) or sputtered-out (by sputter), and the material travels line-of-sight throughout the chamber and condenses as a thin film on all surfaces in the chamber, including the target surface.
A further vapor deposition technique is known as Jet Vapor Deposition (JVD). In a JVD process, the film forming material is generated at some remove from the target surface, and is directed by gas flow coming out from a nozzle onto the target surface. U.S. Pat. No. 5,356,672, incorporated herein by reference in its entirety, describes gas Jet apparatus in which a supersonic gas jet serving as a carrier gas transports radical species generated by a microwave discharge onto the target surface. U.S. Pat. No. 5,356,672, and also U.S. Pat. No. 6,148,764, incorporated herein by reference in its entirety, describe basic JVD apparatus, including a nozzle supplied by way of an inlet with a flow of an inert carrier gas, which entrains atoms, molecules or radicals and deposits them on the substrate downstream. Under so-called “critical flow” conditions, a collimated jet emerges from a nozzle at supersonic speeds.
On a stationary substrate such a JVD system forms a deposit on a small, generally circular area centered on the intersection of the jet axis and the target surface. The JVD system deposits a Gaussian profile; that is, the thickness of the deposited material shows a Gaussian (normal) or near-Gaussian distribution about the center or midpoint, with the thickness at a maximum on the midpoint and decreasing with distance from the midpoint. In order to obtain deposition over larger areas, it is necessary to move the target in relation to the jet axis. U.S. Pat. No. 5,356,672 suggests that coverage of larger areas can be achieved by way of a two-dimensional relative motion of the jet and the substrate, and suggests several approaches. In one approach, the substrate is mounted on a spinning and oscillating carousel, and the JVD apparatus operates continuously. This results in low deposition efficiency, as significant amounts of material are directed outside the margin of the wafer. In other approaches, several jets are directed at the substrate. These require complicated apparatus.
In another approach a single large wafer is spun about its axis and is “scanned” past the JVD apparatus with variable, computer-controlled speeds along a wafer diameter. This can result in non-uniform deposition, and variation in resulting film thickness, particularly near the center of the wafer. In some such approaches, known as orbiting methods, the deposition route consists of a series of orbits. Deposition of each orbit is followed by transition to an orbit having a different radius (the jet may step from orbit to orbit toward the center of the wafer, for example). Because the film material continues to flow from the jet during the interorbit transition, the resulting film has thickness nonuniformities, and these nonuniformities can be particularly sever near the center of the wafer. Where the film must be very thin and very uniform, as for example in a layer for an ultra-thin (less than about 2 nm thick) gate insulator, a variation of even as little as one angstrom in film thickness can result in an unacceptable increase in gate leakage current.

SUMMARY

According to the invention a thin layer on a deposition area of a wafer structure is formed of substantially parallel, substantially straight tracts of film material, adjacent ones of which partly overlap. Each tract has a lengthwise midline, and the thickness of film material in a transverse section of each tract has a Gaussian (normal) or near-Gaussian distribution with a maximum at the midline. Near-Gaussian distribution means a distribution with minor discrepancies from a Gaussian one. The distance between the midlines of adjacent partly overlapping tracts is about 1.5 times a fixed standard deviation, and the sum of the thicknesses of adjacent partly overlapping tracts is substantially uniform over the entirety of the deposition area.
In one general aspect the invention features a method for forming a thin film on a substrate, by: mounting the substrate on a fixture in a vacuum chamber so that a surface of the substrate lies generally within an x-y plane; providing deposition apparatus operable within the chamber to direct a jet of film forming material along a jet axis onto a surface of the substrate, the deposition apparatus being of a type that deposits material in a Gaussian profile; translating the substrate in an x direction relative to the deposition apparatus and operating the deposition apparatus to form a first tract of deposited material on the substrate surface, the tract being substantially straight and having a lengthwise midline defined by the locus of points of intersection of the jet axis with the x-y plane of the substrate and the thickness of film material in a transverse section of the tract having a Gaussian (normal) or near-Gaussian distribution with a maximum at the midline; translating the substrate in a y direction in relation to the deposition apparatus so that the jet axis passes through the x-y plane at a distance about 1.5 times a fixed standard deviation; and translating the substrate in an x direction to form a second tract of material on the substrate surface, the second tract being substantially straight and substantially parallel to the first tract. In some embodiments, providing, the deposition apparatus includes providing jet vapor deposition apparatus.
In another general aspect the invention features a method for forming a thin film on a substrate, by mounting the substrate on a fixture so that a surface of the substrate lies in an x-y plane, depositing onto the substrate surface a plurality of substantially parallel, substantially straight tracts of film material, each tract having a lengthwise midline, and the thickness of film material in a transverse section of each tract having a Gaussian or near-Gaussian distribution with a maximum at the midline, the distance between the midlines of adjacent partly overlapping tracts being about 1.5 times a fixed standard deviation, so that the tracts partly overlap. The cumulative film material is made up of the sum of the thicknesses of adjacent partly overlapping tracts, so that the resulting film is substantially uniform over the entirety of the deposition area.
In some embodiments depositing the tracts of film material includes directing a jet of film forming material along a jet axis onto a surface of the substrate, and translating the substrate in relation to the jet in an x direction, the lengthwise midline of each tract being defined by the locus of points of intersection of the jet axis with the x-y plane of the substrate surface. In some embodiments, depositing the tracts of film material includes employing deposition apparatus of a type that deposits a Gaussian profile (normal distribution).
In some embodiments the distance between the midlines of adjacent tracts is in a range 0.1 times a standard deviation to 3 times a standard deviation, usually at least about 0.5 times a standard deviation and preferably at least about 1 times a standard deviation, usually no greater than about 2.5 times a standard deviation and preferably no greater than about 2 times a standard deviation; in particular embodiments the distance between midlines of adjacent tracts is 1.5 times a standard deviation.
In some embodiments the distance between the midlines of adjacent tracts is determined from a specified tolerance for variation of the film thickness, according to the relation: $ThicknessVariation (%) = 14.169 * \exp (\frac{- {(d / σ - 3.3)}^{2}}{2 * {(0.58)}^{2}}), when d / σ \leq 3.$
Any of a variety of film materials may be deposited according to the invention, to form any of a variety of films. Film materials include: metals, such as for example Cu, Au, Ag, Sn, Pd, Ni, Ti, Ta and others, and alloys of these; mixtures of various species with nitrogen for nitride formation, such as mixtures of silane and nitrogen (to form silicon nitride films) or mixtures of metal and nitrogen (to form metal nitride films); mixtures of various species with oxygen for oxide formation, such as mixtures of silane and oxygen (to form silicon oxide films); organic materials, such as light emitting materials used in Organic Light Emit Diode (OLED) or Polymer Light Emit Diode (PLED). Films include: metals and metal alloys, semiconductors, dielectric materials, organic materials, oxides, and nitrides, among others.
In another general aspect, the invention features a thin film, formed according to the invention, and patterned to form integrated circuit features on a wafer; wafers including such patterned thin films, and integrated circuits including such patterned thin films.
In another general aspect, the invention features apparatus for forming a thin film on a surface of a substrate, including a fixture in a chamber onto which the substrate can be mounted, deposition apparatus operable within the chamber to direct film forming material onto the substrate in a Gaussian or near-Gaussian thickness profile, apparatus operable to move the fixture relative to the deposition apparatus in a direction parallel to an x-y plane of the substrate surface, and a controller operable to control the motors and the relative movement of the fixture and the deposition apparatus. Stepping motors are preferred motors that allow higher precision and accuracy of the x-y plane. The controller can be a common micro-controller or a commercial PC with adequate interface; use of a PC to control the stepping motors may be preferred. The controller is programmable to direct relative movement of the deposition apparatus and the substrate in an x direction to deposit substantially straight and substantially parallel partly overlapping tracts of film material onto the substrate, each tract having a lengthwise midline and a thickness profile across the tract having a Gaussian or near-Gaussian profile, and to direct displacement of the deposition apparatus in relation to the substrate in a y direction, so that the midlines of adjacent tracts are separated by a distance in a range about 1.5 times a standard deviation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic plot of a JVD deposition profile.
FIG. 2 is a diagrammatic plot of tract deposition profiles and the cumulative film deposition according to the invention.
FIG. 3 is a sketch in a diagrammatic plan view showing a wafer during processing to form a film according to an embodiment of the invention.
FIG. 4 is a sketch in a diagrammatic plan view showing a wafer during processing to form a film according to an alternative embodiment of the invention.
FIG. 5 is a sketch in an isometric view showing translation of the deposition apparatus in relation to a fixed wafer to deposit a tract of film material on the wafer surface according to an embodiment of the invention.
FIG. 6 is a sketch in an isometric view showing translation of the wafer in relation to a fixed deposition apparatus to deposit a tract of film material on the wafer surface according to an alternative embodiment of the invention.
FIG. 7 is a diagrammatic plot of film thickness variation as a function of distance between adjacent tracts.

DETAILED DESCRIPTION

The invention will now be described in further detail by reference to the drawings, which illustrate alternative embodiments of the invention. The drawings are diagrammatic, showing features of the invention and their relation to other features and structures, and are not made to scale. For improved clarity of presentation, in the FIGS. illustrating embodiments of the invention, elements corresponding to elements shown in other drawings are not all particularly renumbered, although they are all readily identifiable in all the FIGS.
Referring now to FIG. 1, a JVD deposition profile is shown in an idealized diagrammatic plot 2 of thickness of deposited material versus position on the substrate surface. The JVD jet axis is presumed to be generally perpendicular to the x-y plane of the substrate surface. The intersection of the jet axis with the plane of the substrate surface is at zero on the horizontal axis in the plot, and the thickness maximum 4 is at the midpoint of the deposit.
A mathematical equation for this plot is: $y = A ⅇ^{\frac{- x^{2}}{2 σ^{2}}},$
where:

- y is film thickness,
- A is amplifier constant for accurate fitting,
- x is distance to the jet axis, and
- σ is the standard deviation.
  As will be appreciated, the variables x and y in FIG. 1 (and also in FIG. 2) are not the same as, and do not refer to, the x and y directions in the x-y plane of the substrate surface.

The half width (full width at half maximum) 6 of the plot is defined as:
FWHM={square root}{square root over (2ln(2))}σ.
The height of the thickness maximum at a given point of intersection of the jet axis with the plane of the substrate depends principally upon the distance of the jet nozzle from the substrate surface, the rate of flow of material from the jet, and the length of time the jet is directed at the given point on the surface. To limit the thickness the jet must be moved in relation to the substrate surface. If the jet is moved at a constant rate in a straight line parallel to the substrate surface, and the flow of material from the jet is kept constant, the resulting deposit has the form of a straight tract having a lengthwise midline defined by the moving intersection of the jet axis with the substrate surface. The tract has the form of a ridge, with a thickness maximum over the midline and a transverse Gaussian thickness profile. The thickness of the tract depends upon the distance of the jet nozzle from the substrate surface, the rate of flow of material from the jet, and the rate of movement of the jet in relation to the surface. Accordingly, a required thickness maximum of the tract can be obtained by controlling the deposition flow rate and the rate of movement of the jet in relation to the surface, and these parameters can be derived without undue experimentation on the basis of the known or observed deposition properties of the jet apparatus.
To obtain a film of uniform thickness according to the invention, parallel straight tracts formed as described above are deposited onto the substrate, with the lengthwise midlines of adjacent tracts being separated by a distance about 1.5 times a standard deviation (1.5 σ), so that the tracts are partly overlapping. According to the invention, selecting about 1.5 σ as a distance between adjacent tracts provides for an acceptably uniform thickness over substantially the entire deposition area.
FIG. 2 shows idealized diagrammatic plots of deposition profiles of seven adjacent tracts of film material deposited according to the invention, and plots of cumulative film thickness for three, five, and seven such tracts. The plots are taken in a transverse section across the tracts, and the midlines pass above and below, and generally perpendicular to, the plane of the plots. The midlines of adjacent tracts are separated by a distance about 1.5 σ, so that the thickness maxima of the respective plots are regularly spaced at intervals of about 1.5 σ. Particularly, for example, plots 22, 23, 24 show the profiles across three adjacent tracts whose midlines pass through the plane of the plot at −1.5 σ, 0, and +1.5 σ, and plot 27 shows the resulting cumulative thickness; plots 21, 22, 23, 24, 25 show the profiles across five adjacent tracts, and plot 28 shows the resulting cumulative thickness; and plots 20, 21, 22, 23, 24, 25, 26 show the profiles across seven adjacent tracts, and plot 29 shows the resulting cumulative thickness.
The thickness in the plot 29, indicated by a broken line T, is substantially flat over an included range of tracts. As FIG. 2 illustrates, deposited material in tracts to both sides of a given tract contribute to the overall film thickness over the given tract midline. Accordingly, to ensure that the film thickness is uniform over an entire target deposition area, the deposition path should include tracts whose midlines are outside the target deposition area, and the tracts should extend so that their respective midlines reach outside the target deposition area. Particularly, for example, with reference to tract 23 in FIG. 2, material in tracts 24 and 25 to one side of tract 23, and material in tracts 22 and 21 to the other side of tract 23, contribute to the film thickness T above the midline of tract 23. Generally, the cumulative film thickness over the midline of a given tract includes contributions from tracts whose midlines are within a distance of about 3 σ from the given tract midline and, accordingly, to ensure uniform thickness at the edge of the deposition area, the deposition path should include tracts whose midlines are at least about 3 σ from the edge, and each tract should extend a distance at least about 3 σ beyond the edge, of the deposition area.
FIG. 3 illustrates diagrammatically in a plan view a deposition path taken to form partly overlapping parallel tracts on a substrate according to the invention. Particularly, a circular wafer 100 is shown mounted on a fixture 110, which may be a standard wafer holder in a deposition chamber. The plane of the wafer surface is indicated by x-y coordinates. The deposition path is illustrated as a shaded area beginning at a point 300 and forming a first tract 301 having a lengthwise midline 311, then forming a short transition having a midline 321, then forming a second tract 302, parallel to the first tract, having a lengthwise midline 322, then forming a short transition having a midline 322, followed successively by third, fourth and fifth parallel tracts 303, 304 and 305, and being joined serially by short transitions 322, 323 and 324, respectively. In this embodiment, the path direction is opposite for adjacent tracts, as shown by arrows 333, 334 and 335, for example, and the transitions are alternately at one side and at the other, as shown by the arrows at 323 and 324, for example.
The deposition path continues on a serpentine track in this manner, maintaining a continuous flow of material from the jet and a constant relative movement of the jet in relation to the wafer, until the entire wafer has been covered. Accordingly, the first tract is formed by relative movement of the jet and the substrate in an x direction; the first transition is formed by relative movement of the jet and the substrate in a y direction; the second tract is formed by relative movement of the jet and the substrate in an x direction opposite from that of the first tract (that is, in an antiparallel direction); the next transition is made in a y direction, and so on until the deposition is complete.
As shown above, the thickness profile across each tract displays a Gaussian or near-Gaussian profile, with a thickness maximum at the lengthwise tract midline; according to the invention the midlines of adjacent tracts (for example 311 and 312) are separated by a distance D_Yabout 1.5 times a standard deviation (about 1.5 σ) determined from the tract thickness profile.
Also, as explained above, to ensure that the thickness is uniform over the entire wafer surface, that is, to ensure that the thickness does not decrease at the wafer edge 102, the midline 311 of the initial tract 301, as well as that of the final tract (not shown), which are the tract farthest from the center of the wafer, passes through a marginal region M at a distance at least about 3 σ outside the wafer edge 102. Also, to ensure that the thickness does not decrease at the wafer edge, as explained above, the tracts extend into a marginal region M to a distance at least about 3 σ beyond the wafer edge 102. Accordingly, in the embodiment of FIG. 3, in which the tracts are all of the same length D_X, the tract length is at least about 6 σ greater than the diameter of the wafer 102.
In the deposition path of FIG. 3, a significant amount of transit time and film material is expended well beyond the marginal area outside the wafer edge. The efficiency can be improved by varying the tract lengths according to the length of their respective transits of the wafer (plus margin). This can be accomplished by varying the points at which the respective transitions are made, as shown diagrammatically in FIG. 4, by way of example. As in the example of FIG. 3, the deposition path begins at a point outside the edge of the wafer, and follows a serpentine track, maintaining a continuous flow of material from the jet and a constant relative movement of the jet in relation to the wafer, until the entire wafer has been covered: the first tract is formed by relative movement of the jet and the substrate in an x direction; the first transition is formed by relative movement of the jet and the substrate in a y direction; the second tract is formed by relative movement of the jet and the substrate in an x direction opposite from that of the first tract (that is, in an antiparallel direction); the next transition is made in a y direction, and so on until the deposition is complete. Here, however, the deposition path (illustrated as a shaded area) begins at a point 400 and forms a first tract by relative movement of the jet and the substrate in an x direction (arrow 431), then forms a short transition in a y direction (arrow 421), then forms a second tract, parallel to the first tract, by relative movement of the jet and the substrate in an x direction (arrow 432) opposite that of the first tract, then forms a short transition 422, then forms a third parallel tract in an x direction (arrow 433) opposite that of the second tract, followed successively by additional parallel tracts joined serially by short transitions 423 and 424, respectively. As in embodiments where the tracts are all of the same length D_X, the longest tracts in the embodiment of FIG. 3, namely those that cross near the center of the wafer, have a length D_Xwhich is about 6 σ greater than the diameter of the wafer 102, to ensure uniform thickness at the edges of the wafer. Tracts that cross only the marginal region M outside the wafer edge 102, however, and that cross nearer the edge of the wafer well away from the center, can be shorter and yet extend a suitable distance (at least about 3 σ) beyond the wafer edge. Particularly, for example, the length D_X1of the first tract, between the starting point 400 and the first short transition 421, can be significantly shorter than the longest tracts; the length D_X2of the second tract, between the first short transition 421 and the second short transition 422, although greater than the length D_X1of the first tract, can also be significantly shorter than the longest tracts; the length D_X3of the third tract, between the second short transition 422 and the third short transition 423, although greater than the length D_X2of the second tract, can also be shorter than the longest tracts; and so on, with the respective tract lengths being progressively longer as the deposition path approaches center of the wafer and then progressively shorter again as the deposition path continues to completion.
The relative movement in x and y directions of the jet and the substrate can be accomplished either by holding the substrate (wafer) in a fixed position in the chamber, and moving the jet; or by holding the jet in a fixed position in the chamber and moving the substrate (wafer). For deposition of some materials the jet and associated deposition equipment is bulky, that is, it is large or massive or both large and massive; or, the deposition equipment is connected to ports in the chamber wall. It may be therefore be impractical or less straightforward to move the jet and, in such instances it is preferred to hold the jet in a fixed position in the chamber and to move the fixture onto which the substrate (wafer) is mounted.
For example, for deposition of a nitride film the deposition apparatus includes, attached to the jet facility, a microwave plasma system for production of nitrogen radicals. The plasma system is too heavy to be readily moved, and it is preferred in such instance to hold the jet and associated plasma system in a fixed position and to move the wafer.
In other applications, movement in an x direction (to deposit tracts) can be accomplished by moving the substrate (wafer), while movement in a y direction (for the short transitions) can be accomplished by moving the jet and associated deposition apparatus.
FIG. 5 illustrates in general a system in which the wafer is fixed and the deposition apparatus is moved. A wafer 500 is mounted onto a fixture 510 in the processing chamber. A jet produces a flow 522 to deposit a tract 530 as the jet is moved in an x direction as shown by arrow 512 from an initial position (shown in phantom at 520) to a subsequent position 520′, while the fixture and wafer are held in place. As may be appreciated, a subsequent short transition in a y direction (to start another tract) can be accomplished either by movement of the jet in a y direction or by movement of the fixture in a y direction (opposite the short transition path direction).
FIG. 6 illustrates in general a system in which the deposition apparatus is fixed and the wafer is moved. A wafer is mounted onto a movable fixture in the processing chamber. A jet 620 is held in a fixed position in the processing chamber, and produces a flow 622 to deposit a tract 630 as the fixture, carrying the wafer, is moved in an x direction (opposite the deposition path direction) as shown by arrow 612 from an initial position (shown in phantom: wafer 600, fixture 610) to a subsequent position, wafer 600′, fixture 610′. Here, too, as may be appreciated, a subsequent short transition in a y direction (to start another tract) can be accomplished either by movement of the jet in a y direction or by movement of the fixture in a y direction (opposite the short transition path direction).
Various apparatus are known for moving substrates, and for controlling the movement of substrates, in processing chambers. A device for moving the substrate (wafer) in an x-y plane according to the invention can be constructed using a precision two-dimensional motor system. The x-y motor system can be controlled by a computer programmed to configure wafer movement sequences to provide desired deposition paths. Design and construction of a suitable wafer moving device, and connection of it with a suitable programmable controller, can be accomplished using general mechanical engineering approaches.
The programmable controller is a micro-controller or a computer, programmed to provide an interface by which an operator can specify various parameters, including at least the rate of relative movement of the deposition apparatus and the substrate and the dimensions and form of the deposition path. In some embodiments the operator can select a flow rate for deposition of the film material.
A preferred fixture is adapted for mounting a semiconductor wafer, and a preferred fixture can be an electrostatic chuck or a chuck with mechanical clamps, holding a standard circular wafer with a diameter as small as 2″ or as large as 12″. The fixture may be constructed of stainless steel or brass with ceramic coating, for example.
A preferred two-dimensional motor system includes two motors, one arranged for movement in an x direction and the other arranged for movement in a y direction. Preferred motors include linear stepper motors capable of moving the wafer in relation to the deposition apparatus with a constant velocity at least in an x direction, during deposition of tracts.
Preferably the flow of material is continuous throughout the deposition, including during the transitions as well as during deposition of the tracts. To ensure uniform thickness at the edges of the deposition area (for example, at the edge of the wafer) the short transitions between tracts are routed outside the deposition area and at sufficient distances from the edge of the deposition area so that deposition of film material during the transitions does not contribute undesirably to the thickness within the edge. That is, the transitions preferably are routed to a distance at least 3σ from the edge so that the deposition profiles of the transitions do not overlap the tracts within the deposition area.
As noted above, the thickness maximum of the tracts is determined by, among other factors, the rate of relative movement of the jet and the substrate in the x direction. As will be appreciated, because the transitions are outside the deposition area, the thickness of film material in the transitions need not be the same and, accordingly, the rate of relative movement of the jet and the substrate during the transitions need not be the same as the rate of movement during deposition of the tracts. To save resources and time, it may be desirable to make the transitions at a higher rate of movement.
Depending upon the specified film thickness, and upon the specified tolerance range for thickness irregularity, the distance between midlines of adjacent tracts may differ, within limits, from a preferred distance about 1.5 times a standard deviation. FIG. 7 is a calculated plot showing film thickness variation as a function of the ratio of the distance over the standard deviation, d/σ. At a distance equal to 1.5 σ, the standard deviation of film thickness would be less than 0.5% of the film thickness. At higher ratios the film thickness variation is greater but, for applications where slightly greater film thicknesses are tolerable, greater distances between adjacent tracts can be employed. At a distance about twice a standard deviation, for instance, the film thickness variation can be less than 2%.
Thickness variation can be determined from calculated data by the relation: $ThicknessVariation (%) = 14.169 * \exp (\frac{- {(d / σ - 3.3)}^{2}}{2 * {(0.58)}^{2}}), when d / σ \leq 3,$
and the distances between adjacent tracts can be set according to the desired thickness variation. Usually, for example, a thickness variation of 2% is acceptable. Accordingly, where a 2% thickness variation is acceptable, the distances between adjacent tracts should be kept under 2.25 σ.
Thin films according to the invention will in some instances be subjected to further treatment as part of the wafer fabrication process. Particularly, for example, a film of uniform thickness may be patterned, for example by masking or etching, or by a lift-off process, to form features of integrated circuitry. Films made according to the invention are particularly useful where very thin films of highly uniform thickness are desired or necessary. And the wafer itself may be subjected to further treatment, as the formation (and/or patterning) of the thin film may not constitute an end step in wafer processing. For instance, a silicon nitride thin film constructed according to the invention may be patterned to form part of a gate structure or other feature. Or, for instance, a thin film of a selected material may be patterned to form lines in a memory array or other circuit.
Other embodiments are within the following claims. For example, adjacent tracts need not be deposited sequentially; rather, successive tracts in the deposition path can be deposited in any order, so long as the distances between the tract midlines are some integer multiple of about 1.5 σ. For instance, in a first pass tracts could be deposited at intervals of 3 σ, and then in a second pass tracts could be deposited within the intervals midway between the tracts that were deposited in the first pass.
And, for example, the short transitions need not be perpendicular to the respective tract midlines; instead, they could run more nearly parallel to the wafer edge, at a distance far enough from the edge so that deposition from the short transitions does not contribute in an unwanted way to the film thickness within the deposition target area where uniform thickness is desired.
And, for example, the deposition area of the substrate may have a shape other than circular. For instance, the invention is applicable to forming films of uniform thickness over rectangular or square wafers.

Claims

1. A method for forming a thin film on a substrate, comprising:

mounting the substrate on a fixture in a vacuum chamber so that a surface of the substrate lies generally within an x-y plane;

providing deposition apparatus operable within the chamber to direct a jet of film forming material along a jet axis onto a surface of the substrate, the deposition apparatus being of a type that deposits material in a Gaussian or near-Gaussian profile;

translating the substrate in an x direction relative to the deposition apparatus and operating the deposition apparatus to form a first tract of deposited material on the substrate surface, the tract being substantially straight and having a lengthwise midline defined by the locus of points of intersection of the jet axis with the x-y plane of the substrate, and the thickness of film material in a transverse section of the tract having a Gaussian or near-Gaussian distribution with a maximum at the midline;

translating the substrate in a y direction in relation to the deposition apparatus so that the jet axis passes through the x-y plane at a distance about 1.5 times a fixed standard deviation; and

translating the substrate in an x direction to form a second tract of material on the substrate surface, the second tract being substantially straight and substantially parallel to the first tract.

2. The method of claim 1 wherein the deposition apparatus includes jet vapor deposition apparatus.

3. The method of claim 1 wherein translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance from 0.1 times a standard deviation to 3 times a standard deviation.

4. The method of claim 3 wherein translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance at least about 0.5 times a standard deviation.

5. The method of claim 4 wherein translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance at least about 1 standard deviation.

6. The method of claim 3 wherein translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance no greater than about 2.5 times a standard deviation.

7. The method of claim 6 wherein translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance no greater than about 2.25 times a standard deviation.

8. The method of claim 1 wherein translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance 1.5 times a standard deviation.

9. The method of claim 1 wherein the film has a specified thickness tolerance, and translating in a y direction comprises translating the substrate so that the jet axis passes through the x-y plane at a distance related to the thickness tolerance according to the equation:

ThicknessVariation (%) = 14.169 * \exp (\frac{- {(d / σ - 3.3)}^{2}}{2 * {(0.58)}^{2}}), when d / σ \leq 3.

10. A method for forming a thin film on a substrate, comprising:

mounting the substrate on a fixture so that a surface of the substrate lies in an x-y plane,

depositing onto the substrate surface a plurality of substantially parallel, substantially straight tracts of film material, each tract having a lengthwise midline, and the thickness of film material in a transverse section of each tract having a Gaussian or near-Gaussian distribution with a maximum at the midline, the distance between the midlines of adjacent partly overlapping tracts being a distance in a range 0.1 times a standard deviation to 3 times a standard deviation.

11. The method of claim 10 wherein the distance between the midlines of adjacent partly overlapping tracts is at least about 0.5 times a standard deviation.

12. The method of claim 11 wherein the distance between the midlines of adjacent partly overlapping tracts is at least about 1 standard deviation.

13. The method of claim 10 wherein the distance between the midlines of adjacent partly overlapping tracts is no greater than about 2.5 times a standard deviation.

14. The method of claim 13 wherein the distance between the midlines of adjacent partly overlapping tracts is no greater than about 22.5 times a standard deviation.

15. The method of claim 10 wherein the distance between the midlines of adjacent partly overlapping tracts is 1.5 times a standard deviation.

16. The method of claim 10 wherein the film has a specified thickness tolerance, and wherein the distance between the midlines of adjacent partly overlapping tracts is related to the thickness tolerance according to the equation:

ThicknessVariation (%) = 14.169 * \exp (\frac{- {(d / σ - 3.3)}^{2}}{2 * {(0.58)}^{2}}), when d / σ \leq 3.

17. The method of claim 10 wherein depositing the tracts of film material includes directing a jet of film forming material along a jet axis onto a surface of the substrate, and translating the substrate in relation to the jet in an x direction, the lengthwise midline of each tract being defined by the locus of points of intersection of the jet axis with the x-y plane of the substrate surface.

18. The method of claim 10 wherein depositing the tracts of film material includes employing deposition apparatus of a type that deposits a Gaussian profile.

19. A thin film on a deposition area of a wafer structure, comprising substantially parallel, substantially straight tracts of film material, adjacent ones of which partly overlap.

20. The thin film of claim 19 wherein each tract has a lengthwise midline, and the thickness of film material in a transverse section of each tract has a Gaussian (normal) or near-Gaussian distribution with a maximum at the midline.

21. The thin film of claim 20 wherein the distance between the midlines of adjacent partly overlapping tracts is in a range 0.1 times a standard deviation to 3 times a standard deviation.

22. The thin film of claim 21 wherein the distance between the midlines of adjacent partly overlapping tracts is at least about 0.5 times a standard deviation.

23. The thin film of claim 22 wherein the distance between the midlines of adjacent partly overlapping tracts is at least about 1 times a standard deviation.

24. The thin film of claim 21 wherein the distance between the midlines of adjacent partly overlapping tracts is no greater than about 2.5 times a standard deviation.

25. The thin film of claim 24 wherein the distance between the midlines of adjacent partly overlapping tracts is no greater than about 2.25 times a standard deviation.

26. The thin film of claim 19 wherein the distance between the midlines of adjacent partly overlapping tracts is 1.5 times a standard deviation.

27. The thin film of claim 19 wherein the film has a specified thickness tolerance, and the distance between the midlines of adjacent partly overlapping tracts is related to the thickness tolerance according to the equation:

ThicknessVariation (%) = 14.169 * \exp (\frac{- {(d / σ - 3.3)}^{2}}{2 * {(0.58)}^{2}}), when d / σ \leq 3.

28. The thin film of claim 19, comprising a material selected from the group consisting of metals, metal alloys, semiconductors, dielectric materials, organic materials, oxides, and nitrides.

29. The thin film of claim 19, comprising a metal.

30. The thin film of claim 19, comprising a metal alloy.

31. The thin film of claim 19, comprising a semiconductor.

32. The thin film of claim 19, comprising a dielectric material.

33. The thin film of claim 19, comprising an organic material.

34. The thin film of claim 19, comprising a nitride.

35. The thin film of claim 19, comprising an oxide.

36. The thin film of claim 33, the organic material comprising an OLED material.

37. The thin film of claim 33, the organic material comprising a PLED material.

38. The thin film of claim 34, comprising a metal nitride.

39. The thin film of claim 34, comprising a silicon nitride.

40. The thin film of claim 35, comprising a metal oxide.

41. The thin film of claim 35, comprising a silicon nitride.

42. A thin film as recited in claim 19, patterned to form integrated circuit features on a wafer.

43. In integrated circuit comprising the patterned thin film of claim 42.

44. Apparatus for forming a thin film on a surface of a substrate, comprising

a fixture in a chamber onto which the substrate can be mounted,

deposition apparatus operable within the chamber to direct film forming material onto the substrate in a Gaussian or near-Gaussian thickness profile,

apparatus operable to move the fixture relative to the deposition apparatus in a direction parallel to an x-y plane of the substrate surface, and

a controller operable to control the relative movement of the fixture and the deposition apparatus, the controller being programmable to direct relative movement of the deposition apparatus and the substrate in an x direction to deposit substantially straight and substantially parallel partly overlapping tracts of film material onto the substrate, each tract having a lengthwise midline and a thickness profile across the tract having a Gaussian or near-Gaussian profile, and to direct displacement of the deposition apparatus in relation to the substrate in a y direction, so that the midlines of adjacent tracts are separated by a distance in a range about 0.1 to 3 times a standard deviation.