Soft X-ray Mask Alignment System

Abstract

An alignment system for a soft X-ray lithographic system for aligning a mask with a substrate to be printed with a pattern including a first registration means on the mask and second registration means on the substrate; one of the registration means being a first soft X-ray absorber means of a predetermined form; the other registration means being a second soft X-ray absorber means having a space in it with the same predetermined form, and one of the registration means being carried over a soft X-ray transparent registration window on the mask, the other registration means being carried over a soft X-ray transparent registration window on the substrate.

Description

The invention herein described was made in the course of work performed under a contract with the Department of Air Force, U. S. Department of Defense.

FIELD OF INVENTION

This invention relates to an alignment system for ensuring that each soft X-ray mask in a set of such masks is properly registered with a substrate to be exposed and that each pattern on each mask is properly positioned on that mask relative to the corresponding patterns on the other masks of the set.

BACKGROUND OF INVENTION

Recently soft X-ray lithography has been proposed as a technique for replicating submicron resolution planar patterns, Soft X-Ray Lithographic Apparatus and Process, Smith et al., Ser. No. 217,902 filed Jan. 14, 1972. Soft X-ray masks have been fabricated with acoustic surface wave transducer patterns with 1.3 micron electrode spacing and have been successfully replicated. Thus soft X-ray lithography has shown a resolution capability greater than that of ordinary photolighography and comparable to the scanning electron beam techniques. The simplicity and low cost of soft X-ray lithography indicate that it would have a significant impact in ultra-high resolution device fabrication in the future. However, in many fabrication procedures several masking steps may be required and the patterns produced by one mask must be precisely superimposed with respect to patterns produced by the other masks. However, the precision required is a great deal more than before; for the previous mask replication techniques did not reproduce patterns having elements in the submicron range and the scanning electron beam technique which did create patterns having elements in the submicron range did so by tracing each pattern individually, not by a process of mask replication, so that no mask registration was necessary. Each mask in a set of masks required to fabricate a complete circuit or device on a substrate must have its pattern properly positioned with submicron precision related to the corresponding pattern on each of the other masks of the set. Otherwise although the mask and substrate may be properly registered the pattern may be out of alignment with a pattern or patterns previously replicated in the same pattern area.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an alignment system for a soft X-ray lithographic system which is capable of submicron precision compatible with that of the soft X-ray lithographic replication and which is simple, inexpensive and readily makes use of the soft X-rays available from the soft X-ray lithographic operations.

It is a further object of this invention to provide such an alignment system which provides for precise registration of each mask in a set of masks with a substrate to be exposed.

It is a further object of this invention to provide such an alignment system which ensures that each pattern on each mask in a set of masks is properly positioned on that mask relative to the corresponding patterns on the other masks of the set.

The invention results from the realization that for effective use of soft X-ray lithographic techniques an alignment system is absolutely essential and that because of the high precision i.e. very small pattern elements in the micron range, obtained with such techniques a high precision alignment system is also essential and more importantly can best be made using the soft X-rays available from the soft X-ray lithographic operation.

The invention features an alignment system for a soft X-ray lithographic system for aligning a mask with a substrate to be printed with a pattern. There is a first registration means on the mask and second registration means on the substrate. One of the registration means is a first soft X-ray absorber means having a predetermined form; the other of said registration means is a second soft X-ray absorber means having a space in it of the same predetermined form. One of the registration means is carried over a soft X-ray transparent registration window on the mask and the other registration means is carried over a soft X-ray transparent registration window on the substrate.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 is a diagrammatic plan view of a master mask having registration means and bench mark means according to the alignment system of this invention;

FIG. 2 is a diagrammatic, elevational, cross-sectional view taken along lines 2--2 of FIG. 1;

FIG. 3 is a diagrammatic cross-sectional view illustrating exposure of the electron sensitive resist material using a scanning electron beam microscope to produce the registration means and bench mark means on the master mask;

FIG. 4 is a diagrammatic, elevational view of the master mask shown in FIG. 3 after the exposed resistive material has been developed;

FIG. 5 is a diagrammatic, elevational, cross-sectional view of the master mask after a soft X-ray absorber material has been deposited in the developed areas and the remaining portions of the electron sensitive resist material have been dissolved away;

FIG. 6 is a diagrammatic, cross-sectional, elevational view showing the use of soft X-rays to create a pattern mask from the master mask;

FIG. 7 is a diagrammatic, cross-sectional, elevational view of the pattern mask of FIG. 6 after the exposed portions of the resistive material have been developed;

FIG. 8 is a diagrammatic, cross-sectional, elevational view of the pattern mask of FIG. 7 after a soft X-ray sbsorber has been deposited in the developed areas and the remaining soft X-ray resistive material has been dissolved away;

FIG. 9 is a diagrammatic plan view of the pattern mask of FIG. 8;

FIG. 10 is a diagrammatic, cross-sectional, elevational view of the pattern mask taken along line 10--10 of FIG. 9 after patterns have been fabricated on it;

FIG. 11 is a diagrammatic, cross-sectional, elevational view of the pattern mask of FIG. 10 after windows have been etched beneath each of the registration means and each of the patterns;

FIG. 12 is a diagrammatic, cross-sectional, elevational view showing the use of soft X-rays and the pattern mask to create registration means on a substrate;

FIG. 13 is a diagrammatic, cross-sectional, elevational view of the substrate of FIG. 12 after the exposed portion has been developed and replaced by a soft X-ray absorber material and the unexposed portion has been dissolved;

FIG. 14 is a view of the substrate of FIG. 13 after windows have been etched in it beneath the registration means;

FIG. 15 is a plan view of the substrate of FIG. 14;

FIG. 16 is a diagrammatic, cross-sectional, elevational view of a soft X-ray lithographic system using an alignment system according to this invention to obtain proper registration between a pattern mask and a substrate to be printed; and

FIG. 17 is a diagrammatic, axonometric view of a pattern mask having a multiplicity of pattern areas which uses registration means and bench mark means according to the alignment system of this invention.

The invention may be accomplished using a master mask which carries on it alignment means including at least two registration means and a bench mark means for each pattern which is to be produced on a substrate. The registration means and bench mark means are produced on the master mask using scanning electron beam microscope techniques and each of the alignment means has a predetermined form defined in a soft X-ray absorber material located on a transparent portion of the master mask. Similarly each of the bench mark means has a predetermined form defined in soft X-ray absorber material carried on soft X-ray transparent portions of the master mask. Using soft X-ray lithographic techniques a set of pattern masks is created by exposure to soft X-rays through the master mask. Each of the resulting pattern masks contains one or more registration means which are similar to the registration means on the master mask. Each of these registration means may be either a positive or a negative of the predetermined form that the registration means takes on the master mask. Similarly each of the pattern masks contains a number of bench mark means corresponding to the bench mark means on the master mask and each of these bench marks may also be either a positive or negative of the respective bench mark means on the master mask. Associated with each of these bench mark means on the pattern mask is a pattern area. A pattern is fabricated in each of these areas on each of the pattern masks using scanning electron beam microscope technology. By the use of the bench marks as a starting point the scanning electron beam microscope can precisely lay down a pattern on a mask and lay down compatible patterns at the same relative position on each of the other pattern masks in the set. Thus each of the plurality of the patterns which must be printed on a pattern location on a substrate in order to produce the final whole pattern is precisely aligned with each of the other patterns on each of the other pattern masks which must be successively applied to that substrate. The registration means created on the substrate will be of the same form as those on the master mask and on each of the pattern masks but they will be the obverse of those on the pattern mask so that when the registration means on any particular pattern mask are in proper registration with the registration means on the substrate no soft X-rays can penetrate that area of the pattern mask and substrate. Thus soft X-ray sensors may be used to sense any soft X-rays coming through the registration means and a null detector responsive to the sensor means may be used to indicate when precise registration has occureed. A servo-mechanism using one or more piezoelectric drive units may be used to move the substrate and pattern masks relative to one another to obtain a null indication which represents precise registration. When registration has been achieved the pattern carried on each of the pattern locations on the pattern mask aligned with the substrate is then printed on the substrate using soft X-ray techniques. After the printing with the first pattern mask has been accomplished that mask may be removed and a second pattern mask put in its place and brought into proper registration with the substrate and the second set of patterns is applied to the pattern locations on the substrate. Operation continues in this way until each of the individual patterns carried by each of the pattern masks has been applied to each of the corresponding locations on the substrate and an entire pattern has been fabricated in each of the pattern locations on the substrate. The details of soft X-ray lithography techniques may be obtained from co-pending application Soft X-Ray Lithographic Apparatus and Process, Ser. No. 217,902, filed Jan. 14, 1972 by Henry I. Smith, David L. Spears and Ernest Stern.

The etching techniques used to produce relatively transparent portions or windows in the various masks and substrates is explained in more detail in the co-pending application entitled Soft X-Ray Mask Support Substrate filed on even date herewith by David L. Spears, Henry I. Smith and Ernest Stern.

There is shown in FIG. 1 a master mask 10 having alignment means including two registration means 12 and 14 and two bench mark means 16 and 18. Typically each mask has two registration means and each pattern area has associated with it one bench mark means. Thus in FIG. 1 where master mask 10 is shown designed to carry two pattern areas 20, 22 there are two bench mark means 16 and 18. Each of registration means 12 and 14 includes a layer of soft X-ray absorber material 24 which has an open space 26 in it that defines the form 28 of the registration mark 25. In this case, the form 28 of registration mark takes the shape of an object with four salient points.

Similarly bench mark means 16 and 18 each includes a layer 30 of soft X-ray absorber material. The bench mark 32 is formed by four spaces 34, which define the form 36 of a Greek cross without the center portion. Each of registration means 12 and 14 and each of bench mark means 16 and 18 is carried on transparent portions of master mask 10 over windows 40, 42 and 44 and 46, respectively, FIG. 2. The membrane 48, 50, 52 and 54 in each of windows 40, 42, 44 and 46 is quite thin and thus transparent to soft X-rays; whereas the remaining areas of master mask 10, which are much thicker, are relatively opaque to soft X-rays.

Master mask 10 may be made beginning with a wafer 60, FIG. 3, of silicon which has been doped to a depth of a few microns with boron to form a boron diffusion layer 62. Wafer 60 is then covered on one surface by a protective layer 64 of, for example, silicon dioxide and on its other surface with a layer 66 of polymethyl methacrylate which is sensitive to electron beam exposure. Wafer 60 is then submitted to a scanning electron beam microscope 68 which traces the required patterns on layer 66 to provide registration means 12 and 14 and bench mark means 16 and 18. Wafer 60 is then subjected to a developer such as a solution of 40 percent methyl isobutyl ketone and 60 percent isopropyl alcohol to remove the exposed portions 67 of layer 66 that now define registration means 12 and 14 and bench mark means 16 and 18.

The various holes 70, FIG. 4, remaining in layer 66 are then filled with a soft X-ray absorber material such as by evaporation coating with gold. The remaining portions of layer 66 are then removed by dissolving them with a solution of trichloroethylene. The remaining portions of soft X-ray absorber material 24, FIG. 5, now define the registration mark 25 of the registration means 12 and 14, and the soft X-ray absorber material 30 defines the bench mark 32 in bench mark means 16 and 18. Openings 72, 74, 76 and 78 are etched in layer 64 using an etch such as buffered hydrofluoric acid which attacks the silicon dioxide of layer 64 but not the silicon of wafer 60. Wafer 10 is now prepared to have windows 40, 42, 44 and 46, FIG. 2, etched in it. An etchant which does not attack the remaining portions of the silicon dioxide layer 62 but does attack the open areas of silicon wafer 60 at openings 72, 74, 76 and 78 may be used. This may be accomplished using a 115.degree. C. solution of 68 ml ethylene diamene, 12g pyrocatechol, and 32 ml water for about 11/2 hours. This etchant performs well on silicon wafer 60 but does not attack the boron diffused layer 62. Thus, as shown in FIG. 2, windows 40, 42, 44 and 46 are created with thin membranes 48, 50, 52 and 54, respectively, covering them. This technique is described in detail in the application Soft X-Ray Mask Support Substrate by David L. Spears, Henry I. Smith and Ernest Stern filed on even date herewith.

The completed master mask 10, FIG. 2, may now be used as in FIG. 6 to create a set of pattern masks one of which, pattern mask 80, includes a wafer 82 of silicon containing a boron diffusion layer 84 similar to that discussed, supra. Wafer 82 carried a layer 86 of soft X-ray sensitive material such as polymethyl methracrylate on one surface and a protective layer 88 of material such as silicon dioxide on its other surface. Layer 86 is exposed to soft X-rays 91 from soft X-ray source 90 through windows 40, 42, 44 and 46 in master mask 10 so that the registration means 12 and 14 and the bench mark means 16 and 18 are defined on layer 86. The exposed portions 92 of layer 86 are then subjected to a developer such as a solution of 40 percent methyl isobutyl ketone and 60 percent isopropyl alcohol to remove the exposed portions 92 of layer 86 leaving holes 94, FIG. 7, in their place. Holes 94 are filled with a soft X-ray absorber material such as gold and the remaining portions of layer 86 are then removed by dissolving them with a solution of trichloroethylene leaving a layer of soft X-ray absorbing material 96, FIG. 8, which constitutes a second set of registration means 12' and 14' which are similar to the original registration means 12 and 14 on master mask 10, FIG. 1.

Registration means 12' and 14', FIG. 9, on pattern mask 80 have the same form 28' as registration means 12 and 14 except that in this case the registration mark 25' is defined by the soft X-ray absorber material 24' itself, not, as in the case of registration means 12 and 14, by the space 26 surrounded by the soft X-ray absorber layer 24. Similarly, bench mark means 16' and 18' include bench marks 32' whose form is defined by soft X-ray absorber layer 30' which actually fills spaces 34' to define the form 36' of bench mark 32' as compared with bench mark 32 wherein the form 36 was defined by the soft X-ray absorber layer 30 surrounding spaces 34. Although in FIGS. 1-9 it is indicated that master mask 10 has one variation of the registration marks and bench marks which may arbitrally be designated as positives and the pattern mask 80 has registration marks and bench marks which may be designated negatives, i.e. marks 25', 32' are the negatives or the obverse of marks 25 and 32, this is not a necessary limitation of the invention. For example, if a negative resist were used in place of the polymethyl methacrylate layer 86 then marks 25' and 32' would be positives of marks 25 and 32.

Pattern areas 20' and 22' on pattern mask 80 may now be imprinted with patterns 100 and 102, FIG. 10, using a scanning electron beam microscope guided by bench mark means 16' and 18' in a similar manner as discussed with reference to FIGS. 3-5 and as explained in more detail in co-pending application Soft X-Ray Lithographic Apparatus and Process, Ser. No. 217,902, filed Jan. 14, 1972 by Henry I. Smith, David L. Spears and Ernest Stern. Holes 104, 106, 108 and 110 may now be created in silicon dioxide layer 88 in the same manner as discussed previously with relation to layer 64. Windows 112, 114, 116 and 118, FIG. 11, are then etched in wafer 82 as discussed supra, leaving soft X-ray transparent membranes 120, 122, 124 and 126.

A substrate 130, FIG. 12, on which a pattern is to be constructed is prepared from a silicon wafer 132 having a layer 134 of soft X-ray sensitive material such as polymethyl methacrylate on one surface and a protective layer 136 such as silicon nitride on the other surface. Layer 134 is then exposed to soft X-rays 91', FIG. 12, through a pattern mask 80. In this operation the pattern areas 100 and 102 are not exposed since it is only required to produce alignment means on the substrate 130. Some means, such as soft X-ray opaque elements 138, 140 may be used to cover windows 116 and 118 in pattern mask 80 to prevent the patterns 100 and 102 from being patterned onto layer 134. However, soft X-rays from soft X-ray source 90' do pass through windows 112 and 114 through the registration means 12' and 14' and expose portions 142 of layer 134. The exposed portions 142 are removed by dissolving them with 40 percent methyl isobutyl ketone and 60 percent isopropyl alcohol and are replaced with soft X-ray absorber material; then the remaining unexposed portions 141 of layer 134 may be dissolved in trichloroethylene leaving only registration means 12" and 14", FIG. 13. Windows 150 and 152, FIG. 14, may now be created in wafer 132 by attacking the silicon with an etchant such as a combination of hydrofluoric and nitric acid or a combination of hydrofluoric acid, nitric acid and acetic acid. With these etchants the etching will be stopped substantially short of the other surface by means of a neutralizing bath. In the case of substrate 130 a less selective etchant may be used because maximum transparency through the use of a very thin membrane is not absolutely necessary as will be seen subsequently. Registration means 12" and 14" on substrate 130, FIG. 15, include a layer 24" of soft X-ray absorber material in which is contained a space 26" having the form 28" of registration mark 25". Since pattern mask 80 is flipped over to print on substrate 130 the arrangement of the negative means on substrate 130 is a mirror image of that on pattern mask 80. Registration means 12" and 14" are the obverse of registration means 12' and 14' and are the same as registration means 12 and 14. But the latter of this particular set of relationships is not essential to the invention. However, it is important that registration means 12" and 14" be the obverse of registration means 12' and 14', whether or not either set of registration means 12', 14' or 12", 14" are the same or the obverse of the original registration means 12 and 14 on master mask 10. Substrate 130, FIGS. 14 and 15, carries two pattern areas 20" and 22" but no bench marks as the final alignment requires that only each successive pattern mask be properly aligned with the substrate 130 during exposure. Previously each of the patterns on each of the pattern areas such as 20' has been created using the bench mark means as a guide for the scanning electron beam microscope so that each pattern on the pattern areas on each of the succeeding pattern masks is properly aligned with respect to each of the patterns in the same location on each of the other pattern masks of the set.

In operation substrate 130, FIG. 16, is coated with a layer 160 of soft X-ray sensitive material such as polymethyl methacrylate and is subjected to soft X-rays 91" from a soft X-ray source 90" through a pattern mask 80. Soft X-rays 91" from soft X-ray source 90" pass through windows 112 and 114 in pattern mask 80 only in the areas not blocked by the soft X-ray absorber material 24'. The soft X-rays which are not blocked by soft X-ray absorber material 24' then pass through substrae 130 in the areas not blocked by soft X-ray absorber material 24". Thus when pattern mask 80 and substrate 130 are aligned the soft X-ray absorber materials 24" and 24' are also precisely aligned so that no soft X-rays will pass through substrate 130 and soft X-ray sensor 170 will sense no soft X-rays. As a result a null point will be detected by null detector 172 and an indication thereof will be submitted to control 174. A duplicate system including soft X-ray sensor 170' and null detector 172' is associated with registration means 14' and 14". Control 174 may be used to develop signals to drive orthogonally oriented piezoelectric drives 175, 175' such as are available from Coherent Optics Inc. (Model 44 Electro-Micrometer) which are capable of moving substrate 130 very small distances, in the order of microns, required for the precise alignment of pattern mask 80 and substrate 130.

In operation the soft X-rays 91" passing through windows 116 and 118 of pattern mask 80 would be blocked prior to the registration operation. Once proper registration is accomplished the blockage would be removed and the soft X-rays passing through windows 116 and 118 about the patterns 100 and 102 would create an exposure pattern in layer 160 which could then be developed and further etched or processed by other means known in the art. After this operation is complete the next pattern mask of the set would be introduced in place of pattern mask 80 and the next pattern would be laid down in areas 20" and 22" until all the patterns required to make a complete pattern in areas 20" and 22" have been reproduced on substrate 130.

Typically the substrate made according to this invention would have a multiplicity of pattern areas on it and would require a pattern mask having a similar multiplicity of pattern areas and bench mark means. Thus in FIG. 17 a typical pattern mask 80' includes 42 separate pattern areas 180 each of which has associated with it a bench mark means 182. Also included are two registration means 184 and 186. Pattern mask 80' is typically 1 inch square and each of the 42 pattern areas 180 is approximately 65 mils square in area.

Other embodiments will occur to those skilled in the art and are within the following claims:

Claims

What is claimed is:

1. An alignment system for a soft X-ray lithographic system including a registration system for registering a mask with a substrate to be printed with a pattern comprising a first registration means on said mask and second registration means on said substrate, one of said registration means being a first soft X-ray absorber means of a predetermined form, the other of said registration means being a second soft X-ray absorber means having a space in it of said predetermined form and one of said registration means being carried over a soft X-ray transparent registration window on said mask, the other registration means being carried over a soft X-ray transparent registration window on said substrate, one of said registration means being the obverse of the other.

2. The alignment system of claim 1 in which said predetermined form includes four salient portions.

3. The alignment system of claim 2 in which said salient portions are arranged in spaced pairs on transverse axes.

4. The alignment system of claim 1 further including a soft X-ray detector for detecting soft X-rays passing about said first soft X-ray absorber means and through the space in said second soft X-ray absorber means, and a first null detector means for determining when said first absorber means blocks the space in said second absorber means indicating that said mask and substrate are in registration.

5. The alignment system of claim 4 further including a piezoelectric positioning device responsive to said first null detector means for moving said mask and substrate relative to one another to provide proper registration of said first registration means.

6. The alignment system of claim 1 in which there are at least two sets of first and second registration means on each mask and substrate respectively.

7. The alignment system of claim 1 further including a benchmark system for properly positioning each pattern, on each mask in a set of masks, relative to the corresponding patterns on the other masks of the set, said benchmark system including a benchmark of predetermined form defined in first material.

8. The alignment system of claim 7 in which said benchmark of predetermined form includes four salient portions.

9. The alignment system of claim 8 in which said salient portions are arranged in spaced pairs on transverse axes.

10. The alignement system of claim 7 in which said predetermined form is defined by a space surrounded by a first material.

11. An alignment system for a soft X-ray lithographic system comprising:

a master mask having at least a first registration means of a first form in soft X-ray absorber material carried on a soft X-ray transparent portion of said master mask;

a number of pattern masks each having on a soft X-ray transparent portion one second registration means for each of said first registration means, said second registration means including a second form similar to said first form in soft X-ray absorber material; and

a substrate to be exposed through said pattern mask and having one third registration means for each of said second registration means, said third registration means including a third form obverse to said second form in soft X-ray absorber material.

12. The alignment system of claim 11 further including a soft X-ray detector for detecting soft X-rays passing about said second and third registration means, and null detector means for determining when said second and third registration means block transmission of soft X-rays indicating that said mask and substrate are in registration.

13. The alignment system of claim 11 further including a benchmark system for properly positioning each pattern, on each mask in a set of masks, relative to the corresponding patterns on the other masks of the set, said benchmark system including a plurality of first benchmark means, one for each pattern to be exposed, carried on said master mask on a soft X-ray transparent portion, and a plurality of second benchmark means, one for each pattern to be exposed, carried on each of said pattern masks.

14. A method of producing a soft X-ray alignment system for alignment of a soft X-ray mask and substrate comprising:

on a soft X-ray transparent portion of a master mask generating a first registration means of a first form in soft X-ray absorber material;

generating from said master mask on each of a plurality of pattern masks a second registration means of a second form similar to said first form in soft X-ray absorber material on a soft X-ray transparent portion; and

generating on a substrate third registration means obverse to said second form for alignment with respective ones of said second forms on successive ones of said pattern masks.

15. A method of producing a soft X-ray alignment system for alignment of a soft X-ray mask and substrate comprising:

generating on a soft X-ray transparent portion of a master mask a first registration means of a first form in soft X-ray absorber material;

generating on a soft X-ray transparent portion of a master mask a first benchmark means of a second form in soft X-ray absorber material;

generating from said master mask on each of a plurality of pattern masks second registration means of a third form similar to said first form in soft X-ray absorber material on a soft X-ray transparent portion;

generating from said master mask on each of said plurality of pattern masks second benchmark means of a fourth form similar to said second form.