WO1999053380A1

WO1999053380A1 - Scanning of non-circular image field formed by asymmetric lens of photolithographic reduction system

Info

Publication number: WO1999053380A1
Application number: PCT/US1999/005195
Authority: WO
Inventors: John H. Bruning
Original assignee: Tropel Corporation
Priority date: 1998-04-09
Filing date: 1999-03-10
Publication date: 1999-10-21
Also published as: WO1999053380A8

Abstract

A reduction photolithographic system (10) uses an asymmetric reduction lens (11) forming a non-circular image field at a plane of a substrate (13) to be imaged from a reticle (12). An illuminating system (15) shapes the image field to a configuration for scanning a microcircuit pattern of the reticle onto the substrate in an integer number of substantially equal scans. The image field (20) is configured to provide a single exposure region (20a, 20b, 20c) that is variable in a width transverse to the scan direction and a pair of overlappable double exposure regions (23, 24, 26, 27) at opposite ends of the single exposure region. Integrated illumination intensity along any line parallel with the scan direction in the overlapped double exposure regions equals the integrated illumination intensity along any scan line in the single exposure region.

Description

SCANNING OF NON-CIRCULAR IMAGE FIELD FORMED BY ASYMMETRIC LENS OF PHOTOLITHOGRAPHIC REDUCTION

SYSTEM

Technical Field

A reduction system of photolithography using a reticle bearing a microcircuit pattern for scanning a reduced size image of the microcircuit pattern on a substrate.

Background

The evolution of efforts to achieve finer dimensions for elements of microcircuit devices while also making such devices larger has led to a variety of expedients involving both step and repeat and scanning processes. U.S. Patent No. 5,281 ,996, for example, suggests exposure on a substrate of juxtaposed images of microcircuit patterns so that a lens having an image field smaller than the total pattern to be imaged can complete the imaging in two or more steps. This patent also suggests scanning an image with a long and narrow bar of actinic radiation spanning the image field.

U.S. Patent No. 4,924,257 suggests raster scanning a full substrate with image patterns exposed within the shape of a regular hexagon sweeping repeatedly across the substrate in an overlapping pattern that exposes the substrate uniformly. Either a large pattern filling the whole substrate or a series of smaller patterns fitting within the hexagonal shape can be scanned this way. U.S. Patents Nos. 5,285,236 and 5,291 ,240 offer similar suggestions.

Summary of the Invention

This invention advances scanning or raster scanning in photolithographic reduction systems by devising a way that image patterns too large for the image field of a reduction lens can be scanned in an integer number of sweeps of a variably shaped area of actinic radiation moved across an image area on a substrate. The variable shape is accomplished by variably illuminating an image field of a reduction lens to achieve scanning shapes that differ in full or single exposure scan width and possibly also differ in the width of scan overlap dimensions. This can be done by masking or otherwise distributing illumination intensity within the image field regions to be scanned. From the ranges of available variation, scan width and overlap dimensions are selected so that an integer number of substantially equal scanning sweeps across a reticle pattern uniformly exposes the full reticle pattern on a substrate.

Configuring an illuminating system or mask to accomplish full image scanning in an integer number of scans optimizes the efficiency of the scanning process to maximize the output rate for scanning images onto substrates. When the substrate is a wafer, for example, which can be 200 or 300 millimeters or more in diameter, image patterns to be scanned onto the substrate can be rectangular with heights and widths in which either or both exceed the diameter of the image field of the reduction lens. Under such circumstances, a scanning shape can be selected so that each image pattern can be scanned in an integer number of substantially equal scans to take full advantage of the image field potential of the lens and to complete the scanning of all the required image patterns onto the . wafer in a minimum amount of time.

Because the present invention deals with asymmetric reduction lenses having non-circular image fields, a preferred shape for a variably illuminated image field for scanning purposes for this invention differs from the irregular hexagon shape described in my copending U.S. Application No. 08/941 ,120, filed 30 September 1997, entitled Raster-scan Photolithographic Reduction System, the disclosure of which is hereby incorporated by reference.

Asymmetric lenses do not form image fields around an axis of symmetry, and the image fields they form are not circular. Asymmetric lens systems often involve mirrors, which have the advantage of avoiding errors introduced by refractive elements. Mirrors become especially advantageous as reduction photolithography uses shorter wavelengths of actinic radiation in the deep UV, or extreme UV, or soft X-ray regions of the spectrum, where refractive elements become unusable or unavailable. Examples of asymmetric photolithographic reduction lenses appear in U.S. Patents 5,220,590 and 5,353,322.

As photolithography pursues ever finer resolution, using ever shorter wavelengths of actinic radiation, asymmetric lens systems involving mirrors become increasingly important. Such lens systems are also likely to form off-axis image fields having a variety of shapes other than circular. A common shape for an image field formed by an asymmetric lens system is arcuate or a sector of an annulus, but many other possibilities are feasible.

This invention involves a recognition that something other than a hexagon is required for optimally scanning a non-circular image field to image a full reticle pattern on a substrate in an integer number of substantially equal scans. The inventive way of optimizing a non-circular image field for such scanning purposes requires selecting a region of the image field having desirable image fidelity and illuminating and orienting this region relative to a scan direction for moving the image field across a substrate. Leading and trailing edges of the selected region of the image field are preferably substantially transverse to the scan direction, and a central region of the image field is made variable in a width transverse to the scanning direction. This is preferably done so that integrated illumination intensity or exposure dose along any line parallel with the scan direction within a single exposure region of the image field is uniform. Then a pair of end regions on opposite sides of the central region is configured for overlapping double exposures between successive scans. The overlappable end regions are preferably configured in triangular or near triangular shapes to have equal areas and to produce uniform integrated illumination intensity equal to that of the single exposure region along any overlapped scan line. Depending on the shape of the available image field, the overlappable end regions can also be varied in size to provide different widths of scan overlaps between single exposure scan regions. It is also possible, and in some cases simpler, to vary only the width of the single exposure central region of the image field and use end regions that are invariable in width to form overlapped double exposure scans of a constant width.

Drawi ngs

Figure 1 schematically illustrates an asymmetric reduction lens having a non-circular image field that can be optimized for scanning in my photolithographic reduction system.

Figures 2-5 schematically show several of a multitude of possible variations in variable width scanning shapes formed within non-circular image fields for scanning purposes.

Figure 6 schematically shows a scanning pattern for fully imaging a reticle pattern on a wafer in an integer number of scanning sweeps.

Detailed Description

This invention optimizes scanning of a non-circular image field formed by an asymmetric reduction lens system 10 that is schematically illustrated in FIG. 1. System 10 includes an asymmetric reduction lens 11 that can include any number of optical components. Lens 1 1 can exploit mirrors only (catoptric), mirrors and refractive lenses (catadioptric), or refractive lenses only (dioptric).

Lens 11 images a microcircuit pattern or portion of a reticle 12 on a substrate 13, which can be a wafer on which copies of the microcircuit pattern are imaged in a repetitive fashion. Lens 1 1 reduces the size of the microcircuit pattern at the object field plane of reticle 12 to form a smaller image of the microcircuit pattern at the image field plane of substrate 13. The object field plane and the image field plane are illustrated in FIG. 1 as offset from each other, which is typical, but not essential, for asymmetric lens systems, which do not form circular image field boundaries centered around an axis of symmetry. Such imaging systems may, however, have a plane of symmetry. Although imaging a microcircuit pattern onto a wafer is a typical use for a photolithographic reduction lens, the invention is not limited to this use. For example, a reduced image of a microcircuit pattern can be made on a substrate 13 that serves as a reticle pattern for subsequent photolithographic steps that can include further reduction or simply replication at unit magnification by imaging or shadow printing.

Light for lens 1 1 originates from a source 14 of actinic radiation that is preferably deep UV such as attainable from an excimer laser producing light at wavelengths of about 193 or 157 nanometers. Shorter wavelength deep UV or soft X-ray sources such as 126 nm or 13.4 nm, respectively, may also prove workable.

Light from source 14 is transmitted to asymmetric lens 1 1 by illuminator 15, which is designed in generally known ways for directing radiation over a viewing field of lens 1 1 at the object plane of reticle 12. Illuminator 15 includes a variable system for illuminating the image field at substrate 13, preferably by concentrating the available illumination in a selected region within the image field. This can be done in several ways and, for purposes of illustration, is accomplished by a mask 16 that is schematically arranged within illuminator 15. Mask 16 is preferably formed of movable mechanical elements, as schematically illustrated, for masking out unselected portions of the image field of lens 1 1. Illuminator 15 preferably images mask 16 at the object field plane of reticle 12 so that light outside of regions within masking elements 16 is excluded during scanning.

Other expedients can also be involved in varying the shape and extent of the image field of lens 1 1 at substrate 13. Elements of lens 1 1 can become involved in shaping the image field for scanning purposes, and any such expedients are preferably designed to work in cooperation with illuminator 15. For example, if mirrors are involved in lens 1 1 , slightly tilting mirror surfaces can alter distribution of light intensity within the image field, provided that such movements or tilts do not disrupt precise imaging characteristics of lens 1 1 which must remain stable during exposure.

Reduction lens systems for photolithography are usually designed to make the illumination intensity uniform at the image field of the lens, but this is not required for scanning purposes. In fact, illumination intensity can be deliberately made non-uniform to effect or cooperate with varying dimensions of the image field for scanning purposes. What is important, though, is that integrated illumination intensity or exposure dose along any scan line parallel with the scan direction be equal. Within this requirement, it is possible to have an image field that is wider in a scan direction in a region where illumination is less intense and is narrower in a scan direction in a region where illumination is more intense. Variation in illumination intensity complicates the selection of an optimum scanning shape, but is nevertheless a variable that can be considered, along with the dimensions that are possible within a particular non-circular image field.

In scanning with system 10, reticle 12 and substrate 13 are moved simultaneously at rates of speed that correspond to the reduction achieved by lens 1 1 . Assuming, for example, a 4 to 1 reduction by lens 11 , reticle 12 is moved 4 times as fast and 4 times as far as the corresponding movement of substrate 13. These movements, of course, are made uniform and smooth and are carefully controlled by sophisticated servos and interferometric metrology so that each scanning sweep of a region of substrate 13 is accurately positioned and produces a uniform exposure aimed at error-free imaging of each microcircuit pattern.

Substrate 13 can be a wafer to be imaged with a repeated number of image patterns, as is typical in producing microcircuit chips. Substrate 13 can also take other forms, such as a reticle to be used for patterning of microcircuit devices. A reticle scanned as I propose can later be deployed for contact or proximity printing or for projection imaging, possibly with further image reduction.

FIGS. 2-4 schematically illustrate a few of a multitude of possibilities for optimizing scanning shapes for non-circular image fields produced by asymmetric lenses. Since a sector of an annulus is a clearly possible configuration for such an image field, a few variations on optimizing an annular sector image field 20 are shown in FIG. 2. Field 20 is configured for illustration purposes by masking or by distributing illumination through cooperation of illuminator 15 and lens 1 1 to have a pair of edges 21 and 22 formed as sectors of circles. A length or arcuate extent of sectors 21 and 22 can be varied for optimizing a scanning shape. Edges 21 and 22 are also configured for illustration purposes as having equal radii 21 r and 22r, which provides equal length distances between edges 21 and 22 along any scan line parallel with a scan direction 25 indicated by a double arrow-headed broken line. If lens 1 1 has an axis of symmetry, boundaries 21 or 22 would naturally be circles. If lens 1 1 is asymmetric, boundaries 21 or 22 might naturally be elliptical, particularly if lens 1 1 derives from or is a perturbation or modification of a symmetric lens. Generally, whatever their arcuate form, edges 21 and 22 extend transversely of scan direction 25, and a central region of field 20 between edges 21 and 22 is variable in a width transverse to scan direction 25 to form a single or full exposure region during a scan.

A relatively wide image field region 20a formed within image field region 20 can be moved along scan direction 25 to form a relatively wide single or full exposure scan path. In contrast, a narrower image field region 20b can also be formed within image field 20 to scan a narrower single exposure path along scan direction 25.

A pair of triangular end regions 23 and 24, formed by masking or other distribution of illumination intensity, is preferably arranged at opposite end regions of edges 21 and 22, forming boundaries of variable width image fields 20a and 20b. Triangular regions 23 and 24 can be congruent, as illustrated, so that when regions 23 and 24 are overlapped to form double exposures on successive scans, they form constant width overlap regions between variable width single exposure scan regions. The width variation is then confined to the difference between central, single exposure regions 20a and 20b; and selecting proper widths for these regions can result in a full area of a reticle pattern being imaged in an integer number of scans.

Image region 20c, which is also variable in width and derivable from image field 20, illustrates the possibility of using a pair of end triangles 26 and 27 that differ in height from triangles 23 and 24. This difference produces a wider overlapped double exposure region between successive single exposure scans and affords another variable that can be used to dimension a scanning region from image field 20 that will accomplish scanning of a full reticle pattern in an integer number of scans. In some circumstances, it may even be possible to hold constant the width of a central scanning region 20a, b, or c and accomplish the necessary scan width variations by adjusting the height of triangles 26 and 27. It is also possible to make first and final scans that respectively overlap a top and bottom of reticle 12 to achieve an integer number of scans by wasting the light that overlaps the upper and lower boundaries of the reticle. This reduces illumination efficiency somewhat, but has the advantage of simplicity.

For optimum efficiency in using the available illumination to make the scanning advance as rapidly as possible, it is better to have a variable width scan that is adjustable for an integer number of scans of the image area. It is less efficient to make wider overlapped double exposure regions than to keep the double exposure scan width to a comfortable minimum and vary the width of single exposure central regions. For the sake of simplicity, though, and with some reduction in efficiency, it is possible to fix both the scanning width and the overlap width and achieve an integral number of scans by overlapping the upper and lower borders of the reticle. The efficiency reduction from wasting light over the reticle boundaries is greater for image areas sized to require only a few scans, and larger image areas requiring many scans would not be noticeably inefficient if scan widths and overlaps widths were invariable.

FIG. 3 illustrates the fact that an image field of lens 1 1 need not be annular or circular. Edges 31 and 32 of an image field 30 are curved and are preferably equally spaced apart in scan direction 25, although such spacing is not essential. Field edge lines 31 and 32 can be spaced apart at variable distances, providing that an exposure dose along any line between edges 31 and 32 parallel with scan direction 25 is equal. Field edges 31 and 32, like edges 21 and 22, form leading and trailing edges as an image field configuration is moved in either of the two possible directions along scan path 25.

An image area 30a, derived from image field 30, illustrates a relatively wide scanning area extending transversely of scan direction 25. End triangles 33 and 34 extending between ends of edges 31 and 32 provide double exposure overlap regions between successive scans, while central area 30a provides a full or single exposure region of each scan. A scan region 30b illustrates a narrower single exposure scan shape capped at its ends by the same pair of triangles 33 and 34. A range of width variation illustrated by areas 30a and 30b affords a selection that can scan a full reticle pattern in an integer number of scans.

A scan region 30c, which is also derived from image field 30 and can be variable in width, illustrates the possibility of different height end triangles 36 and 37, for varying a double exposure overlapped scan width between single exposure scan regions. Variation in the heights of triangles 36 and 37 can also be used to configure scan regions for exposing a full image in an integer number of scans.

An image field 40 of FIG. 4, with its curved edges 41 and 42, illustrates another way of optimizing a scanning region from a field shape. Within image field 40, a variable width scanning field 40a can be formed within multiple straight line segments 41 a and 42a. Distances between lines 41 a and 42a are preferably equal along scanning direction 25, assuming uniform illumination distribution within image field 40a. Edges 41 a and 42a become leading or trailing, depending upon a direction of movement along scan path 25. Triangular regions 43 and 44 connect end regions of segmented lines 41 a and 42a to provide overlappable double exposure regions between single exposure scan widths formed by central region 40a. A region 40b, also derived from image field 40, is somewhat narrower in scanning direction 25 than central region 40a. Triangular end regions 46 and 47 connecting ends of segmented lines 41 b and 42b illustrate the fact that such triangular regions need not be isosceles. It is also possible to form overlappable double exposure regions that are not precisely triangular. Nearly triangular regions with complementary convex and concave edges are possible, for example, and may be desirable for certain configurations of image fields formed by an asymmetric lens.

FIG. 5 shows another variation of image field 60a and 60b, illustrating the fact that overlappable end regions need not be precisely triangular. Scanning region 60a has overlappable end regions formed by lines 63 and 64 extending between leading and trailing edge lines 61 and 62. Since line 62 is arcuate and is extended into the end regions bounded by lines 63 and 64, such end regions have slightly convex sides and are not precisely triangular. Image field 60b uses end regions bounded by lines 63 and 65 that form an upper area with a convex edge bounded by arcuate line 62 and a lower area having a concave side bounded by arcuate edge 61. The upper area bounded by line 63 slightly exceeds the area of a triangle, and the lower area bounded by line 65 is slightly less than a triangular area so that the two overlappable regions produce an exposure dose in a scan direction equal to the scan direction exposure dose of the central region.

Designers of photolithographic lens systems generally aim for uniform illumination throughout an image area, which is also feasible in practicing this invention. If a lens system produces illumination that departs from uniformity in some regions of a potential image area, then the shape of a scanning region can be selected to compensate. Varying scanning shapes is generally simpler than varying illumination intensity.

FIG. 6 schematically illustrates an example of a typical application of optimized scanning of a microcircuit pattern onto a wafer 50 while using a non-circular image field from an asymmetric reduction lens. An image 51 of a full microcircuit pattern is located appropriately on an area of wafer 50 and is scanned according to the invention with an integer number of scans of a non-circular image field. For purposes of illustration, scan area 20a of FIG. 2 is chosen in a width that scans the full pattern 51 in three substantially equal scans. A first scan along a top edge of image 51 positions top triangle 23 above a preferably masked upper boundary of chip area 51 so that a central, single exposure area 20a scans a path along the upper boundary of chip area 51. Lower triangle 24 then incompletely exposes chip area 51 along an overlap path 53.

The next scan reverses direction as shown by the illustrated scan direction arrows and overlaps upper triangle 23 with the previous path of lower triangle 24 for a double exposure that completes a full exposure along overlap path 53. In a final scan along a lower boundary of chip area 51 , upper triangle 23 again overlaps with a path previously made by lower triangle 24 to complete a double exposure along overlap path 54 while single exposure 20a completes a full exposure extending to the lower boundary of chip area 51 . For the final scan, lower triangle 24 extends below the lower boundary, where its incomplete exposure can be masked.

Masking is also preferably applied along vertical edges of chip area 51 to allow turnaround room for the scan image area to pass beyond the boundary of chip area 51 and move into a position for a subsequent scan overlapping a previous scan. The turnaround room required at the end of each scan path depends partly on the width of the scanning area in the scan direction, which in turn depends on the shape of the image field available from a particular asymmetric lens. Making the scanning area narrow in the scan direction can improve the efficiency of the turnarounds necessary at the end of each scan, providing that the asymmetric lens system 10 can distribute actinic radiation effectively within such a configuration. This represents another of the many factors that must be considered in optimizing the light distribution available from an image field of a particular asymmetric lens.

Claims

I Claim:

1 . A reduction photolithography system comprising: a. an asymmetric reduction lens forming a non-circular image field in a plane of a substrate to be imaged; b. an illumination system shaping the image field to a configuration for scanning a reticle pattern larger than the image field in an integer number of substantially equal scans; c. the illumination system providing the image field configuration with leading and trailing edges that are variable in a length transverse to a scan direction to establish a single exposure scan region having a width that is variable transversely to the scan direction; d. the image field configuration having a pair of incomplete exposure end regions extending between ends of the leading and trailing edges; and e. integrated illumination intensity within the end regions equaling integrated illumination intensity in the single exposure region when the end regions are overlapped in successive scans.

2. The system of claim 1 wherein the end regions are equal in area.

3. The system of claim 1 wherein the leading and trailing edges are non-linear.

4. The system of claim 1 wherein the end regions are triangular and are arranged within the non-circular image field.

5. The system of claim 1 wherein the non-circular image field is arcuate.

6. The system of claim 1 wherein the end regions have a boundary line connecting the leading and trailing edges.

7. The system of claim 1 wherein the end regions have variable sizes that vary in overlapped width of successive scans.

8. A reduction photolithographic system comprising: a. an asymmetric reduction lens forming a non-circular image field in a plane on a substrate to be imaged from a reticle ; b. an illuminating system for shaping the image field to a configuration for scanning a microcircuit pattern of the reticle onto the substrate in an integer number of substantially equal scans; c. the image field being configured to provide a single exposure region that is variable in a width transverse to a scan direction; d. integrated illumination intensity in the single exposure region being uniform along any scan line within the single exposure region extending parallel with the scan direction; e. a pair of overlappable double exposure regions arranged at opposite ends of the single exposure region to be overlapped on successive scans; and f . integrated illumination intensity along any line parallel with the scan direction in the overlapped double exposure regions equaling the integrated illumination intensity in the single exposure region.

9. The system of claim 8 wherein the overlappable double exposure regions are triangular.

10. The system of claim 9 wherein the triangular regions are arranged within the non-circular image field.

1 1 . The system of claim 8 wherein the overlappable double exposure regions are equal in area.

12. The system of claim 8 wherein leading and trailing edges of the single exposure region are non-linear.

1 3. The system of claim 8 wherein the non-circular image field is arcuate.

14. The system of claim 8 wherein the overlappable double exposure regions have a boundary line connecting leading and trailing edges of the image field.

15. A reduction photolithographic scanning method comprising: a. variably illuminating a non-circular image field of an asymmetric reduction lens to form within the image field a single exposure region having a width transverse to a scan direction and having overlappable end regions that produce uniform illumination intensity equal to the illumination intensity of the single exposure region when overlapped in the scan direction; and b. dimensioning the single exposure region to form successive scans of the single exposure region and overlapped double exposure end regions that uniformly expose a full area of a microcircuit pattern onto a region of a substrate in an integer number of substantially equal scans.

16. The method of claim 15 including configuring the overlappable end regions as triangular.

17. The method of claim 15 including distributing illumination intensity within the single exposure and double exposure regions so that integrated illumination intensity along any scan line parallel with the scan direction is equal.

1 8. The method of claim 15 employed in imaging a reticle pattern repeatedly and identically on a substrate, regardless of the size of the imaged reticle pattern.

1 9. The method of claim 15 including making the width of the single exposure region variable.

20. The method of claim 15 including varying a width of the overlappable end regions.

21 . A method of scanning a reduced image of a microcircuit pattern onto a substrate for photolithographic purposes, the method using an asymmetric reduction lens having a non-circular image field at a plane of the substrate, and the method comprising: a. confining illumination of the image field to a region within non-linear leading and trailing edges extending transversely of a scan direction to form a single exposure central region having a width in a direction transverse to the scan direction and a pair of double exposure end regions that are overlapped on successive scans; and b. dimensioning the single exposure region relative to the double exposure regions so that an integer number of substantially equal scans uniformly images all of the microcircuit pattern onto the substrate.

22. The method of claim 21 including making the double exposure regions substantially triangular.

23. The method of claim 21 including making the double exposure regions equal in area.

24. The method of claim 21 including varying the width of the single exposure region.

25. The method of claim 24 including varying an overlap width of the double exposure regions.

26. The method of claim 21 including distributing illumination within the single and double exposure regions so that integrated illumination intensity along any scan line parallel with the scan direction is equal.

27. The method of claim 21 including repeatedly and identically imaging the microcircuit pattern onto the substrate.

28. A reduction photolithographic system imaging a reticle pattern of a microcircuit device onto a substrate with an asymmetric lens having a non-circular image field that cannot encompass a full area of the microcircuit device in a single exposure, the system comprising: a. an illumination system confining illumination of the image field to a central region between non-linear leading and trailing edges and a pair of end regions joining the non-linear edges; b. integrated illumination intensity within the central region along any line parallel with a scan direction being uniform; c. the central region having a width in a direction transverse to the scan direction; d. the end regions being overlapped in successive scans so that double exposure of the end regions equals single exposure of the central region; and e. widths of the central region and overlapped end regions being selected so that single exposure portions of the central region and double exposure portions of the overlapped end regions combine for uniformly exposing the substrate to the full area of the microcircuit device in an integer number of substantially equal scans.

29. The system of claim 28 wherein the end regions are triangular.

30. The system of claim 28 wherein the end regions are equal in area.

31 . The system of claim 28 wherein the image field is annular.

32. The system of claim 31 wherein the central region and the end regions are arranged within the annular image field.

33. The system of claim 28 wherein the width of the central region is variable.

34. The system of claim 33 wherein the width of the overlapped end regions is variable.

35. The system of claim 28 wherein the end regions each have a boundary line extending between the non-linear edges.