WO2001081998A1

WO2001081998A1 - Ceramic supports and methods

Info

Publication number: WO2001081998A1
Application number: PCT/US2001/012825
Authority: WO
Inventors: James A. Crittenden
Original assignee: Coorstek, Inc.
Priority date: 2000-04-20
Filing date: 2001-04-19
Publication date: 2001-11-01
Also published as: AU2001253715A1

Abstract

A method of forming a ceramic support (20) for a mask comprises forming a plurality of ceramic ribs (22), a plurality of ceramic strips (24) and a frame (32). Each ceramic rib (22) has a plurality of spaced-apart grooves for receiving the ceramic strips (24). The ceramic ribs (22) are positioned in a spaced apart parallel array, and the ceramic strips (24) are secured in the grooves to form a rectangular grid (30) that is framed by the frame (32).

Description

CERAMIC SUPPORTS AND METHODS

BACKGROUND OF THE INVENTION The present invention relates to a ceramic support for a mask used in electron beam lithography, and also to a method of forming the ceramic support.

The concept of writing a pattern directly on a semiconductor chip by e-beam lithography is attractive since it represents the limit in resolution capability. However, this approach is severely limited by the time taken to write a single substrate because of the sequential nature of pattern writing. The sequential nature of e-beam writing also makes it extremely difficult to use for pattern transfer. Accordingly, an alternative approach, referred to as e-beam projection printing has been developed. A schematic of a de-magnifying projection system 10 for such printing is shown in Figure 1. A flood electron gun source 11 provides (via condenser lenses 12) a collimatedbeam of electrons 13 which project (via projector lenses 14) an image 15 of a mask 16 direct onto a photoresist-coated wafer 17.

A mask for e-beam projection printing may comprise a thin (1000- 1500A) membrane of SiNx which carries a thin (250-500A) pattern layer. However, such membranes are so thin that they are unable to cover the full mask area unsupported. For this reason, the mask membrane is supported, perhaps by a support sometimes referred to in the art as a grillage of closely-spaced struts. As well as improving overall robustness, the struts help reduce pattern placement errors and prevent heat build-up.

The support is fabricated from a silicon substrate in which slots are produced to form the struts or webs. Typically, the minimum web width in the plane of the silicone substrate is the same as the thickness of the substrate (i.e. webs need to have a square cross-section). Thus, it is only possible to form webs with a width of 0.040" (1016 μm) in a silicon substrate of thickness 0.040" (1016 μm). Given that substrate thickness is governed by a number of factors, including the need to withstand routine handling, web width is not always as small as it might be. Further, fabrication of the grid from a silicone substrate can be expensive. Hence, the fabrication of such grids from other materials, such as ceramics, have been attempted. More specifically, there is a desire to produce a. ceramic support grid for an e-beam projection printing mask comprising struts whose width in the plane of the grid is not determined by the thickness of the grid. Such a grid could open the way for the next generation of integrated circuits to be fabricated.

SUMMARY OF THE INVENTION

The invention provides exemplary ceramic supports and methods for their construction and use in e-beam projection printing for semi-conductor device fabrication. In one aspect, there is provided a method of forming a ceramic support for use in e-beam projection printing. The method comprises forming a plurality of ceramic ribs that each have a plurality of spaced-apart grooves, and forming a plurality of ceramic strips for insertion in the spaced-apart grooves. The ceramic ribs are positioned in a spaced-apart array, and the ceramic strips are then secured in the grooves to form a grid or grating. Construction of the ceramic support in this manner enables the width of ribs or strips in the plane of the grid to be largely independent of the thickness of the grid.

In one embodiment, the ceramic ribs are formed by cutting elongate slices from a substrate. For example, slices may be cut with a precision dicing saw with a diamond wheel. In this way, the thickness of the substrate may match the thickness of the grid, while the width of the elongate slices is chosen to match the width of the ribs in the plane of the grid. Conveniently, the ceramic ribs may have a width in the range of about 0.020" to about 0.25", more preferably from about 0.020" to about 0.1", and more preferably 0.05" to about 0.08" (1.27xl0-3m to 2.03xl0-3m). The ribs may be spaced apart by a distance of about 0.5" to about 3". The grooves in the ceramic ribs may be provided by forming an array of grooves in the substrate and cutting the substrate so that the elongate slices traverse grooves in the array. Conveniently, the grooves may have a depth in the range of about 0.02" to about 0.06", and more preferably at about 0.03" (7.62x10-4m), and may have a width in the range of about 0.005" to about 0.040", and more preferably about 0.008" (2.03x10- 4m) to about 0.010" (2.54x10-4m). Conveniently, the ceramic strips may have a width in the range from about 0.005" to about 0.040", and more preferably from about 0.008" (2.03x10-4m) to about 0.010" (2.54x10-4m), and may have a height in the range of about 0.02" to about 0.06" and more preferably at about 0.03" (7.62x10-4m). Typically, the width of the strips will be significantly smaller than the ribs. Further, the size of the strip width is more critical that than of the rib width. Since the ribs may be significantly larger than the strips, a variety of techniques known in the art may be used to form the ribs without damaging the ribs, including for example, saw cutting, laser cutting, and the like. One way to make the strips is to lap down a ceramic substrate to the desired width, e.g., 0.008" and then to saw cut strips from the substrate. In this way, relatively thin strips are formed without cracking or otherwise damaging the strips. The thin strips may then be coupled to the ribs. The ceramic strips may be secured in the grooves of the ceramic ribs with an adhesive. The adhesive may be a low melting temperature glass. Such a glass adhesive may be particularly suited to applications which demand a high level of cleanliness.

In one embodiment, the ceramic strips have a height which matches groove depth in the ceramic ribs so that the ceramic ribs and strips have a flush fit on one side of the grid. If desired, a portion of the ceramic ribs on one side of the grid which protrude from the ceramic strips may be removed (e.g. by grinding or lapping) after the ceramic strips have been secured in the grooves. In this way, the ceramic ribs and strips may be flush on both sides of the grid. In one embodiment, the ceramic ribs are positioned in the spaced-apart array with the spaced-apart grooves aligned parallel to a common direction. Conveniently, the ceramic ribs may be positioned parallel to each other. Also, the common direction may be perpendicular to the ceramic ribs, enabling a square or rectangular grid to be formed. In another embodiment, the grid may be surrounded by a frame or border that provide a convenient way to hold and mount the grid. The frame may be coupled to the grid in a variety of ways. For example, a ceramic substrate may be laser cut to create a frame. The grid may then be bonded to the frame. For instance, the grooves may be cut into the frame, and the ribs bonded to the frame. The thin strips may be coupled to the ribs as previously described. However, it will be appreciated that other techniques known in the art may be used to form the frame and ribs. In another aspect, there is provided a ceramic support for a mask used in e-beam projection printing. The ceramic support comprises a plurality of spaced- apart ceramic strips and a plurality of ceramic spacing elements. The ceramic strips and ceramic spacing elements are arranged into a grid, with ceramic spacing elements disposed between and bonded to adjacent pairs of ceramic strips. The ceramic strips and spacing elements may be flush on both sides of the grid.

In another embodiment, the ceramic spacing elements are formed from a plurality of ceramic ribs that each have a plurality of spaced-apart grooves into which the ceramic strips are secured. In this way, the ceramic spacing elements are disposed between pairs of grooves. In such an embodiment, the ceramic ribs and strips may be flush on one side of the grid. For example, the ceramic strips may have a height which matches groove depth. Furthermore, the ceramic ribs and strips may be flush on both sides of the grid.

In one embodiment, the ceramic strips and ceramic spacing elements may be bonded together with an adhesive. The adhesive may be a low melting temperature glass.

In one form of ceramic support, the ceramic strips are parallel in the grid. Also, the ceramic spacing elements may be aligned in parallel rows in the grid. The parallel rows may be perpendicular to the parallel ceramic strips in the grid. In one embodiment, the ceramic strips have a width of about 0.008"

(2.03x10-4m), the width being the shortest dimension in the plane of the grid. The ceramic strips may have a height of about 0.03" (7.62x10-4m), the height being measured normal to the plane of the grid. Further the ceramic spacing elements may space the ceramic strips by about 0.02" to about 0.1", and more preferably about 0.05" to about 0.08" (1.27x10-3m to 2.03x10-3m).

Optionally, a frame or border may be provided around the grid to facilitate handling and/or mounting of the grid. The frame may be constructed of ceramic and be flush with one or both sides of the grid. The grid may be bonded to the frame or otherwise secured using techniques known within the art. In accordance with yet another aspect of the invention, there is provided a ceramic support comprising a plurality of ceramic strips forming a planar grid, the planar grid having a thickness in the range of about 0.02" to about 0.06", and more preferably about 0.03" to about 0.05" (7.62xl0-4m to 1.27xl0-3m). Further, the width of the ceramic strips in the plane of the grid may be less than the thickness of the grid. For example, the width of the ceramic strips may be between 10% and 99%, and more preferably between 25% and 75% of the thickness of the grid. In accordance with yet another aspect of the invention, there is provided a system for e-beam projection printing a pattern onto a substrate. The system comprises an electron source, a lens for focusing electrons into a collimated beam, a projection mask, and a lens for focusing a projected electron image of the mask onto the substrate. The projection mask may comprise a pattern supported by a ceramic support as previously described.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a prior art de-magnifying electron projection system. Figure 2 is a schematic partial plan view of a ceramic support embodying the present invention.

Figure 3 is a more detailed partial view of one of the vertical ribs in the ceramic support of Figure 2.

Figure 4 is a more detailed partial view of one of the ceramic strips in the ceramic support of Figure 2.

Figure 5 is a more detailed partial perspective view of the ceramic support of Figure 2.

Figure 6 is a flow chart illustrating one method of producing the ceramic rib and strip of Figures 3 and 4, respectively. Figure 7 is a flow chart illustrating one method of producing a ceramic support according to the invention.

Figure 8A is a schematic representation of a ceramic substrate from which a frame may be constructed according to the invention.

Figure 8B illustrates the substrate of Figure 8A after removal of a center section to form a frame.

Figure 9A illustrates the frame of Figure 8A having a set of cut grooves according to the invention. Figure 9B is a partial side view of the frame of Figure 9A. Figure 10 illustrates the frame of Figure 9A after ceramic strips have been inserted into the grooves according to the invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The invention provides exemplary ceramic supports and methods for their construction and use. The ceramic supports of the invention may be used to support an e-beam projection printing mask of the kind comprising a thin membrane of SiNx which carries a thin pattern layer. Examples of ceramic materials that may be used to construct the supports include alumina, zirconia, and the like. The ceramic supports may have a planar grid structure that is constructed of a set of ceramic strips. The width of each strip in the plane of the grid is less than the thickness of the grid measured normal to the grid. The width may be in the range of 10% to 99%, and more preferably of 25% to 75% of the thickness of the grid. The invention may thus be employed to provide a grid support of thickness in the range of about 0.0.2" to about 0.06", and more preferably from about 0.03" to about 0.05" (7.62xl0-4m to 1.27xl0-3m) using ceramic strips of width in the range from about 0.005" to about 0.04", and more preferably about 0.008" (2.03x10-4m).

The strips may conveniently be spaced apart by use of spacers or ribs. Since the function on the ribs is simply to space the strips, the thickness of the ribs is not nearly as critical as the strips. As such, the ribs may be formed using a variety of techniques known within the art. Because the ceramic strips are relatively thin, care must be taken to not crack or damage the strips during fabrication. One technique for forming such thin strips is to lap a ceramic substrate to the desired thiclαiess and then to cut the strips from the lapped substrate.

Optionally, a frame may be placed about the strips to facilitate handling and/or mounting of the strips. The frame may also be constructed of ceramic and may conveniently be coupled to the spacers using techniques known within the art. Referring now to Figure 2, one embodiment of a ceramic support 20 will be described. Ceramic support 20 comprises a plurality of ceramic ribs 22 and a plurality of ceramic strips 24. The ceramic ribs 22 are aligned vertically (parallel to the y-axis) and each includes a plurality of spaced-apart grooves 26 (see Figure 3) which define spacing elements 28 therebetween. The ceramic strips 24 are aligned horizontally (parallel to the x-axis) and are secured in the grooves 26 to form a planar grid 30. The ceramic strips 24 measure about 0.008" (2.032x10-4m) in the y-direction, and the ceramic ribs 22 measure about 0.07" (1.78xl0-3m) in the x-direction. In other words, the width of the ceramic strips 24 is significantly less than that of the ceramic ribs 22 in the plane of the grid 30. The length of the ceramic ribs 22 and the strips 24 may be about 5.0" (0.127m), and the grid 30 extends beyond what is shown in Figure 2. However, it will be appreciated that other sizes may be used. The grid 30 is provided with a frame 32 retaining the free ends 34,36 of the ceramic ribs 22 and strips 24, respectively. Merely by way of example, support 20 may be constructed so as to fit within an 8" diameter disk or frame.

Referring to Figures 3 and 4, the geometries of the ceramic rib 22 and ceramic strip 24 become more readily apparent. The ceramic rib 22 is a castellated structure, with the grooves 26 being aligned transversely to the longitudinal axis A-A extending the length of ceramic rib 22. The grooves 26 extend to a depth D, about three-fifths of the way through the thiclαiess or depth of the ceramic rib 22. An adhesive (not shown) such as a low melting temperature glass is applied to the groove walls 38 to secure the ceramic strips 22 when received in the grooves 26. Other adhesives that may be used include epoxy, and the like. The ceramic strip 24 is a plank-like structure, of height H.

Referring to Figure 5, the intersection of ceramic ribs 22 and ceramic strips is illustrated. The depth D of the grooves 26 in the ceramic ribs 22 is chosen to precisely accommodate the full height H of the ceramic strip so that the upper surface 40 of the grid 30 is flat. In other words, the tops of the ceramic ribs 22 are level with the tops of the ceramic strips 24 (i.e. ceramic strips 24 are a flush fit in the grooves 26). On the other hand, the lower surface 42 of the grid 30 is stepped, with the bottoms of the ceramic ribs 22 protruding beyond the bottoms of the ceramic strips 24. The protuberant parts 44 of the ceramic ribs 22 (shown in phantom shading in Figure 5) may be removed by grinding once the ceramic strips have been firmly secured in the grooves 26. By removing the protuberant parts 44, only the spacing elements 28 of the ceramic ribs 22 will remain. Referring now to Figure 6, one method for forming the ceramic ribs 22 will be described. As shown in step 50, a ceramic substrate is provided, which may be of size 5"x5"x0.05" (0.127mx0.127m x 1.27xl0-3m). The ceramic substrate is placed in a precision dicing saw with a diamond wheel and, as shown in step 52, up to about 100 parallel and evenly-spaced grooves are cut into the upper surface of the substrate. At step 54, the substrate is rotated through 90° so that the diamond wheel is perpendicular to the parallel grooves. Finally, at step 56, elongate slices are cut from the substrate and these form the ceramic ribs 22, with the spacing elements 28 disposed between truncated grooves. In one alternative, steps 52-56 may be reversed. In this way the substrate is first cut into elongate slices. The slices are then bundled and the slots cut into the slices.

Also shown in Figure 6, step 58 provides for the fabrication of the ceramic strips 24. Since the ceramic strips 24 are plank-like, they may be cut directly from another substrate by omitting steps 52 and 54. The elongate slices cut for the ceramic strips may have a different width to those cut for the ceramic ribs, simply by altering the cut setting of the precision dicing saw.

Referring now to Figure 7, the assembly of the grid 30 will now be described. At step 60, the ceramic ribs 22 are positioned in a spaced-apart parallel array with the grooved surface of the ceramic ribs uppermost. Next, as shown in step 62, adhesive is applied to the side walls 38 of each groove in each ceramic rib 22. Thereafter, as shown in step 64, the ceramic strips 24 are secured in the grooves 64 by the adhesive to form the grid 30. The next step 66 is optional and involves grinding the underside of the ceramic ribs 22 to remove protuberant parts 44 and leave the spacing elements 28 between the ceramic strips 24.

The ceramic support 20 may be used to support the pattern in a projection mask. The projection mask may comprise a membrane on which a pattern layer is formed. The resulting, supported projection mask may be used in place of the conventional projection mask 16 in the projection system 10 of Figure 1. As previously mentioned, the grids of the invention may conveniently be provided with a frame or border to facilitate handing or mounting, such as within a projection system. Figures 8 A through 10 illustrate one technique for producing such a frame. However, it will be appreciated that other techniques may be used to construct a frame around a grid. The frame is constructed from a ceramic substrate 100 as illustrated in Figure 8 A. Conveniently, substrate 100 may have a thickness of about 0.030" to about 0.250". As shown in Figure 8B, a laser is used to cut a center piece from the substrate to form a frame 102. The width W of frame 102 may be in the range from about 0.25" to about 2" to facilitate handing and mounting as just described.

Frame 102 may then be placed into a precision dicing saw (which may be the same type of saw used to make the ribs and thin struts of the grid) where grooves 104 are cut part way through the substrate as shown in Figures 9A and 9B. Ribs 106 and thin struts 108 (which may be formed using any of the techniques and having any of the dimensions as previously described) may then be assembled into frame 102 to form a grid assembly 110 as illustrated in Figure 10. In figure 10, only one rib 106 is shown for convenience of illustration. After assembly, a bonding agent, such as glass, may be applied to the joints and the assembly fired to secure the pieces together. Hence, a grid array that is similar to any of those previously described may conveniently be produced with a surrounding border to facilitate handling.

The invention has now been described in detail for purposes of clarity of understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of forming a ceramic support for use in beam projection printing, the method comprising: forming a plurality of ceramic ribs that each include a plurality of spaced- apart grooves; forming a plurality of ceramic strips; positioning the ceramic ribs in a spaced-apart array; and securing the ceramic strips in the grooves of the ceramic ribs to form a grid.

2. A method according to claim 1, further comprising providing a frame, with the grid being framed by the frame.

3. A method according to claim 1 or 2, wherein the plurality of ceramic ribs are formed by cutting elongate slices from a ceramic substrate.

4. A method according to claim 3, further comprising securing the ribs within groves in the frame.

5. A method according to claim 3, further comprising forming an array of grooves in the substrate prior to cutting the elongate slices, the array of grooves in the substrate providing the spaced-apart grooves in the ceramic ribs when formed.

6. A method according to claim 3, further comprising forming an array of grooves in the ribs.

7. A method according to claim 1 or 2, wherein the plurality of ceramic strips are formed by cutting elongated slices from a substrate.

8. A method according to claim 1 or 2, wherein the ceramic strips are secured in the grooves with an adhesive comprising a low melting temperature glass.

9. A method according to claim 1 or 2, wherein the ceramic strips have a height which matches a groove depth so that the ceramic ribs and ceramic strips are flush on one side of the grid.

10. A method according to claim 7, further comprising removing any protuberant parts of the ceramic ribs after securing the ceramic strips in the grooves so that the ceramic ribs and ceramic strips are flush on both sides of the grid.

11. A method according to claim 1 or 2, wherein the spaced-apart grooves are aligned parallel to a common direction in the spaced-apart array.

12. A method according to claim 11, wherein the ceramic ribs are parallel to each other in the spaced-apart array.

13. A method according to claim 12, wherein the common direction is perpendicular to the ceramic ribs.

14. A method according to claim 1 or 2, wherein the ceramic strips have a width of about 0.008" (2.03x10^"4m).

15. A method according to claim 14, wherein the ceramic strips have a height of about 0.03" (7.62x10^"4m).

16. A method according to claim 1 or 2, wherein the ceramic ribs have a width in the range of about 0.02" to about 0.1".

17. A method according to claim 1 or 2, wherein the ceramic ribs have a width in the range of about 0.02" to about 0.25".

18. A method according to claim 1 or 2, wherein the grooves have a depth of about 0.03" (7.62x10^"4m).

19. A method according to claim 1 or 2, wherein the grooves have a width in the range from about 0.005" to about 0.04".

20. A method according to claim 19, wherein the grooves have a width in the range from about 0.005" to about 0.04".

21. A method according to claim 1 or 2, wherein the groove spacing is about 0.02" to about 0.1".

22. A method according to claim 21, wherein the ceramic rib spacing in the spaced-apart array is about 0.5" to about 3".

23. A method according to claim 1 or 2, wherein the ceramic rib spacing in the spaced-apart array is about 0.5" (12.7 x 10^"3m).

24. A method according to claim 1 or 2, wherein the ceramic rib spacing in the spaced-apart array is about 0.5" (12.7 x 10^"3m) to about 3".

25. A method according to claim 1 or 2, wherein the beam is an e-beam.

26. A ceramic support for a mask used in beam projection printing, comprising: a plurality of spaced-apart ceramic strips; and a plurality of ceramic spacing elements, the ceramic strips and ceramic spacing elements being arranged into a grid with ceramic spacing elements disposed between and bonded to adjacent pairs of ceramic strips.

27. A ceramic support according to claim 26, further comprising a ceramic frame disposed about the grid.

28. A ceramic support according to claim 26 or 27, wherein the ceramic strips and ceramic spacing elements are flush on both sides of the grid.

29. A ceramic support according to claim 26 or 27, wherein the ceramic spacing elements are provided by a plurality of ceramic ribs, each ceramic rib having a plurality of spaced-apart grooves into which the ceramic strips are secured, the grooves defining the ceramic spacing elements therebetween.

30. A ceramic support according to claim 29, wherein the ceramic ribs and ceramic strips are flush on one side of the grid.

31. A ceramic support according to claim 30, wherein the ceramic ribs and ceramic strips are flush with the frame.

32. A ceramic support according to claim 30, wherein the ceramic ribs and ceramic strips are flush on both sides of the grid.

33. A ceramic support according to claim 32, wherein the ceramic ribs and ceramic strips are flush with the frame.

34. A ceramic support according to claim 26 or 27, wherein the ceramic strips and ceramic spacing elements are bonded together with an adhesive.

35. A ceramic support according to claim 34, wherein the ceramic ribs and ceramic strips are bonded to the frame with an adhesive.

36. A ceramic support according to claim 34, wherein the adhesive is a low melting temperature glass.

37. A ceramic support according to claim 26 or 27, wherein the ceramic strips are parallel in the grid.

38. A ceramic support according to claim 26 or 27, wherein the ceramic spacing elements are aligned in a plurality of rows in the grid.

39. A ceramic support according to claim 37, wherein the ceramic spacing elements are bonded perpendicularly to the ceramic strips.

40. A ceramic support according to claim 26 or 27, wherein the ceramic strips have a width of about 0.008" (2.03x1 O^m), the width being measured in a plane of the grid.

41. A ceramic support according to claim 26 or 27, wherein the ceramic strips have a height of about 0.03" (7.62x10^"4m) the height being measured in a direction normal to one side of the grid.

42. A ceramic support according to claim 26 or 27, wherein the ceramic strips are spaced apart by the ceramic spacing elements by an amount in the range of about 0.05" to about 0.08" (1.27xl0^"3m to 2.03x10^"3m).

43. A ceramic support according to claim 38, wherein adjacent rows are spaced apart by a distance in the range from about 0.05" to about 0.08" (1.27x10^"

³m to 2.03x10^"3m).

44. A ceramic support according to claim 26 or 27, wherein the ceramic support is used in e-beam projection printing.

45. A system for beam projection printing a pattern onto a substrate, the system comprising: a particle source; a lens for focusing particles into a collimated beam; a projection mask; and a lens for focusing a projected image of the mask onto the substrate, wherein the projection mask comprises a pattern supported by a ceramic support according to claim 20 or 21.

46. A system according to claim 45 wherein the particle source is an electron source and the particles are electrons.