Soft X-ray Mask Support Substrate

Abstract

A soft X-ray mask support substrate including a thick peripheral support structure; a thin, taut membrane, transparent to soft X-rays and carried by the support structure covering the area within the periphery; and a soft X-ray absorber layer arranged in a predetermined pattern on the membrane within the periphery of the support structure.

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 a soft X-ray support substrate, and more particularly to such a substrate having a taut membrane transparent to soft X-rays for supporting a pattern in soft X-ray absorber material.

BACKGROUND OF INVENTION

Soft X-ray printing has been proposed as a technique for replicating sub-micron planar patterns, see Soft X-ray Lithographic Apparatus and Process, filed Jan. 15, 1972, Ser. No. 217,902; typically the pattern is generated by a scanning electron microscope. Soft X-ray exposure masks have been made for 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 photolithography and comparable to the highly sophisticated scanning electron microscope techniques. The simplicity and low cost of soft X-ray lithography indicate that it could have a significant impact on ultra-high resolution device fabrication in the future. However wide scale use of the soft X-ray technique especially as a commercial manufacturing approach will depend in part upon the ease with which large area masks can be made and can be aligned with the substrate or wafer to be exposed. Because of the high absorption coefficient of all solid materials to soft X-rays the supporting portion of the mask must be made quite thin in order to have reasonable transparency.

Beryllium the solid material most transparent to soft X-rays, appears well suited for the supporting portion of the mask. However, the thinnest foil of beryllium commercially available was approximately 12 microns thick. The surface of this foil was irregular, having numerous pits of one micron depth, and was not suitable as a substrate on which high resolution, submicron patterns must be constructed in an absorber layer. In addition, beryllium is attacked by most acids (weak as well as strong) and by alkaline solutions, so a very serious corrosion and chemical compatibility problem exists with this material. Also beryllium dust is very toxic, so elaborate safety precautions must be taken if the material is cut or machined.

The problems encountered with beryllium suggest that a better material might be available. But any other material would be less transparent to soft X-rays and would require a much thinner layer of that material, which would tend to sag of its own weight. Some sort of support was therefore necessary to hold this thinner membrane.

Initial attempts in making a support involved bonding aluminum foil across a metal washer, and evaporating aluminum onto silicon and etching a hole in the silicon up to the aluminum. In all instances the membranes were not flat, but exhibited a large degree of warping. The amount of sag was dependent upon the temperature, due to the difference in thermal expansion of the aluminum and the support frame. The aluminum was also attacked by some of the chemicals to be used in later processing steps.

Another attempt involved using a wafer of silicon doped with phosphorus on which an epitaxial layer of pure silicon was grown, and etching away part of the phosphorus doped substrate up to the epitaxial layer. This membrane structure was chemically resistant, quite rugged and stable with temperature changes. However, the membrane was badly dimpled. Strain in the epitaxial layer was released when the phosphorus doped silicon was etched away resulting in a concave or convex structure. In some cases the membrane was dimpled inward by as much as ten microns. This was not suitable for a high resolution mask.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide a soft X-ray mask substrate which is thick enough in parts to be fully supporting yet thin enough in other parts to be relatively transparent to soft X-rays.

It is a further object of this invention to provide a soft X-ray mask substrate in which the thin area or membrane, which is relatively transparent to soft X-rays and on which a pattern is constructed in soft X-ray absorbent material, is taut and not disposed to sag or droop.

The invention results from the realization that the dimpling or sagging problem would be solved by producing a tension in the membrane that would keep it taut and that such a tension could be produced in a thin silicon membrane supported on a thicker silicon support structure by doping the membrane with boron or phosphorous which has a smaller covalent bond radius than silicon and therefore causes the membrane to shrink relative to the surrounding support structure of undoped silicon and further that such a tension could also be produced in thin silicon membrane supported on a thicker silicon support structure doped with arsenic, gallium, antimony or aluminum each of which has a larger covalent bond radius than silicon and therefore causes the support structure to expand relative to the undoped silicon membrane and stretch the membrane.

The invention features a soft X-ray mask substrate including a wafer of a first material of which a thin layer is doped with a small percentage of a second material which contracts that thin layer of the first material slightly and reduces the attack rate of an etchant on the first material enough to create a thin, taut membrane of the doped material, transparent to soft X-rays to function as a pattern window and yet leave sufficient amounts of the first material in the remaining portion of the first material about the pattern window to act as a support structure. Alternatively, a soft X-ray mask substrate can be constructed using a thick layer of a first material doped with a small percentage of a second material which expands the first material slightly and on which is deposited a thin layer of the first material such that an etchant may selectively remove enough of the doped first material to create a thin, taut membrane of the undoped first material, transparent to soft X-rays to function as a pattern window, and yet leave sufficient amounts of the doped first material about the pattern window to act as a support structure.

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 an elevational, schematic view of an initial step in the fabrication of a substrate according to this invention for supporting a soft X-ray mask.

FIG. 2 is an elevational schematic view similar to that shown in FIG. 1 of a subsequent step in the fabrication of a soft X-ray mask support substrate according to this invention.

FIG. 3 is an elevational schematic view similar to that of FIG. 2 of a further step in the fabricating of a soft X-ray mask support substrate according to this invention.

FIG. 4 is an elevational schematic view similar to that of FIG. 3 of a completed soft X-ray mask support substrate according to this invention.

FIG. 5 is a elevational, schematic view of an initial step in the fabrication of an alternative substrate according to this invention for supporting a soft X-ray mask.

FIG. 6 is an elevational, schematic view similar to that of FIG. 5 of a subsequent step in the fabrication of an alternative substrate for supporting a soft X-ray mask.

FIG. 7 is an elevational, schematic view similar to that of FIG. 6 of a further step in the fabrication of an alternative substrate according to this invention for supporting a soft X-ray mask.

FIG. 8 is an elevational, schematic view similar to that of FIG. 7 of a completed soft X-ray mask support substrate according to this invention.

FIG. 9 is a schematic, axonometric view of a soft X-ray mask support substrate with a plurality of pattern windows and membranes.

In one specific embodiment a support substrate 6 for a soft X-ray mask 8 according to this invention may be constructed, FIG. 1, using a single silicon wafer 10 of the N type or lightly doped P type approximately 200 microns in thickness. Wafer 10 is heavily diffused with boron such that a concentration of about 2 .times. 10.sup.19 cm.sup..sup.-3 exists at a depth of 3 microns from the surface forming boron diffusion layer 12.

Silicon dioxide layers 14 and 16 each about 0.1 micron thick, FIG. 2, are then grown on the top and bottom of wafer 10. Silicon dioxide layers 14 and 16 function to provide a protective layer which prevents undesired chemical attack to the silicon. Following this the desired soft X-ray mask pattern 18, FIG. 3, is fabricated in pattern area 19 on silicon dioxide layer 16 using state of the art methods such as scanning electron beam techno ogy or photolithographic techniques. Mask pattern 18 may be formed of gold or any other good soft X-ray absorber. In FIG. 3 a gold layer 20 0.3 micron thick is used and an intermediate layer 22 of chrominum 0.03 micron thick is used to improve the adherence between the gold layer 20 and the silicon dioxide layer 16. Opposite the mask pattern 18 an opening 24 is etched in silicon dioxide layer 14 using an etchant such as buffered hydrofluoric acid which attacks the silicon dioxide layer 14 but not the silicon of wafer 10.

Subsequently the wafer 10 is placed in a 115.degree.C solution of 68 ml ethylene diamine, 12g pyrocatechol, and 32 ml water for about 1 1/2 hours. The solution etches away the bulk of the silicon beneath opening 24 in silicon dioxide layer 14 and creates a pattern window 26, FIG. 4, in the silicon wafer 10 which corresponds to the pattern area 18 which has been created on the opposite side of wafer 10 in the soft X-ray absorbing gold layer 20. This etchant only etches as far as the boron diffused layer 12 and no farther. In addition this etchant also does not attack the chromium or the gold portions. As a result of this etching a membrane 28 is formed from the portion of the boron diffused layer 12 which extends across pattern window 26. Membrane 28 is relatively thin i.e. approximately 3 microns in thickness corresponding in thickness to the boron diffused layer 12. As a result, membrane 28 is quite transparent to the soft X-rays which will be used to expose substrates through pattern area 18 of mask 8.

In addition to providing a support structure which at least in the critical area is sufficiently thin to be relatively transparent to soft X-rays this fabrication technique also introduces a tautness in membrane 28 because of the action of the boron doping in the medium of the silicon: a tension arises because of the slight decrease of the lattice constant produced by boron doping because boron has a smaller covalent bond radius than that of silicon. As a result the membrane 28 shrinks and becomes taut relative to the rest of the silicon substrate which is not doped with boron. Because of this tautness or tension membrane 28 provides a very flat, rigid substrate for the gold absorber layer 20 and its intermediate layer 22; an added advantage of this technique is that the etchant which is used etches much more rapidly in the crystal direction <100> than it does in the direction <111> and so the etching process moves much more quickly in the direction from silicon dioxide layer 14 towards boron diffused layer 12 than it does in the lateral direction transverse to that path so that window 26 is produced in an area beneath opening 24 without any serious undercutting at the sides of window 26 beneath the remaining portions of silicon dioxide layer 14. In fact the etching process provides sloping walls 32 and 34 which slant inwardly in window 26. A mask approximately 1 inch square having 49, 5 um thick pattern windows each 60 mils square has been made with no sagging of the membrane.

Pattern 18 may be any type of micron-miniature circuit or system such as electronic circuits or micro-sound circuits. Substrate 6 has a number of advantages: silicon is highly resistant to corrosion and since silicon technology is well developed high quality materials meeting precise specifications may be easily obtained. Further, since the entire substrate 6 including the support structure 30 and membrane 28 are made from the same single crystal, no adhesion problem exists and temperature changes will not distort membrane 28.

Alternatively a support substrate 6' for a soft X-ray mask 8' may be constructed, FIG. 5, using a single crystal silicon wafer 10' approximately 200 microns in thickness. Wafer 10' is heavily diffused with arsenic to a concentration of about 10.sup.19 atoms/cm.sup.3.

A three micron epitaxial layer 40, FIG. 6, of pure silicon is grown on one surface and layers 14' and 16' of silicon nitride about one tenth of a micron thick are provided on both surfaces. Silicon nitride layers 14', 16' function to provide a protective layer which prevents undesired chemical attack to the silicon. Following this the desired soft X-ray mask pattern 18', FIG. 7, is fabricated in pattern area 19' of silicon nitride layer 16' using state of the art methods such as scanning electron beam lithograph or photolithographic techniques. Mask pattern 18' may be formed of gold or any other good soft X-ray absorber. In FIG. 7 a gold layer 20' three tenths of a micron thick is used. Opposite the mask pattern 18' an opening 24' is etched in the silicon nitride layer 14' using an etchant such as concentrated hydrofluoric acid which attacks the silicon nitride layer 14' but not the silicon of wafer 10'.

Subsequently the wafer 10' is placed in a solution of one part hydrofluoric acid, three parts nitric acid and 10 parts acetic acid for about one and one half hours. This solution etches away the bulk of the arsenic doped silicon beneath opening 24' in silicon nitride layer 14' and creates a pattern window 26', FIG. 8, in the silicon wafer 10' which corresponds to the pattern area 18' which has been created on the opposite side of wafer 10' in the soft X-ray absorbing gold layer 20'. This solution does not attack the gold layer 20', silicon nitride layer 14', 16' or the silicon layer 40. Thus the etching process leaves a membrane 28' formed from the silicon layer 40 which extends across pattern window 26'. Membrane 28' is relatively thin i.e. approximately 3 microns in thickness corresponding in thickness to the silicon layer 40. As a result, membrane 28' is quite transparent to the soft X-rays which will be used to expose substrates through pattern area 18' of mask 8'.

In addition to providing a support structure which at least in the critical area is sufficiently thin to be relatively transparent to soft X-rays this fabrication technique also introduces a tautness in membrane 28' because of the action of the arsenic doping in the medium of the silicon: a tension arises because of the slight increase of the lattice constant produced by arsenic doping because arsenic has a larger covalent bond radius than that of silicon. As a result the support structure 30' expands relative to the silicon layer 40 which is not doped with arsenic and causes membrane 28' to become taut. Because of this tautness or tension membrane 28' provides a very flat, rigid substrate for the gold absorber layer 20'.

Typically there are many more than one mask pattern and one pattern window on a substrate. Thus, in FIG. 9, substrate 6" roughly 1 inch square may contain 40 membranes 28" and forty pattern windows 26" each of which is roughly 65 mils square.

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

Claims

What is claimed is:

1. A soft X-ray support substrate comprising: a thick silicon peripheral support structure; and a thin, taut, silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said membrane being doped with a material having a smaller covalent bond radius than silicon to shrink said membrane to produce tension in it and make it taut.

2. The substrate of claim 1 in which said membrane is formed integrally with said support structure.

3. The substrate of claim 1 in which said absorber layer includes an absorber element and an intermediate element for improving adherence between said membrane and said absorber element.

4. A soft X-ray support substrate comprising: a thick silicon peripheral support structure; and a thin, taut, silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said membrane being doped with boron having a smaller covalent bond radius than silicon to shrink said membrane to produce tension in it and make it taut.

5. A soft X-ray support substrate comprising: a thick silicon peripheral support structure; and a thin, taut, silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said membrane being doped with phosphorous having a smaller covalent bond radius than silicon to shrink said membrane to produce tension in it and make it taut.

6. A soft X-ray support substrate comprising: a thick silicon peripheral support structure, and a thin, taut silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said support structure being doped with a material having a larger covalent bond radius than silicon to expand said support structure and produce tension in said membrane making it taut.

7. A soft X-ray support substrate comprising: a thick silicon peripheral support structure, and a thin, taut silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray abosrber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said support structure being doped with anitmony having a larger covalent bond radius than silicon to expand said support structure and produce tension in said membrane making it taut.

8. A soft X-ray support substrate comprising: a thick silicon peripheral support structure, and a thin, taut silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said support structure being doped with gallium having a larger covalent bond radius than silicon to expand said support structure and produce tension in said membrane making it taut.

9. A soft X-ray support substrate comprising: a thick silicon peripheral support structure, and a thin, taut silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said support structure being doped with arsenic having a larger covalent bond radius than silicon to expand said support structure and produce tension in said membrane making it taut.

10. A soft X-ray support substrate comprising: a thick silicon peripheral support structure, and a thin, taut silicon membrane transparent to soft X-rays and carried by said support structure covering the area within the periphery of said support structure for carrying a soft X-ray absorber layer arranged in a predetermined pattern on said membrane within said periphery of said support structure; said support structure being doped with aluminum having a larger covalent bond radius than silicon to expand said support structure and produce tension in said membrane making it taut.

11. A method of making a soft X-ray mask support substrate comprising: doping a first surface of a silicon substrate with boron to a predetermined thickness to shrink that doped portion of said silicon substrate; providing an etch resistant layer on the second surface of said substrate except in the area corresponding to the area on the first surface for carrying a predetermined pattern in soft X-ray absorber material; and subjecting the exposed said area of said second surface to an etchant which attacks the silicon of the support substrate and which is ineffective in etching the boron doped silicon and the etch resistant layer to create a window in the silicon substrate covered by a membrane formed from the boron doped silicon and kept taut by the shrinking of the boron doped silicon.

12. A method of making a soft X-ray mask support substrate comprising: doping a first surface of a silicon substrate with phosphorous to a predetermined thickness to shrink that doped portion of said silicon substrate; providing an etch resistant layer on the second surface of said substrate except in the area corresponding to the area on the first surface for carrying a predetermined pattern in soft X-ray absorber material; and subjecting the exposed said area of said second surface to an etchant which attacks the silicon of the support substrate and which is ineffective in etching the phosphorous doped silicon and the etch resistant layer to create a window in the silicon substrate covered by a membrane formed from the phosphorous doped silicon and kept taut by the shrinking of the phosphorous doped silicon.

13. A method of making a soft X-ray support substrate comprising:

doping a wafer of silicon with antimony to expand said wafer;

depositing a layer of silicon on a first surface of said wafer;

providing an etch resistant layer on the second surface of said wafer except in the area corresponding to the area on the first surface for carrying a predetermined pattern in soft X-ray absorber material; and

subjecting said exposed area of said second surface to an etchant which attacks said silicon wafer doped with antimony and which is ineffective in etching the silicon layer and the etch resistant layer to create a window in the antimony doped silicon wafer covered by a membrane formed from the silicon layer and kept taut by the expansion of the antimony doped silicon wafer.

14. A method of making a soft X-ray support substrate comprising:

doping a wafer of silicon with gallium to expand said wafer;

depositing a layer of silicon on a first surface of said wafer;

providing an etch resistant layer on the second surface of said wafer except in the area corresponding to the area on the first surface for carrying a predetermined pattern in soft X-ray absorber material; and

subjecting said exposed area of said second surface to an etchant which attacks said silicon wafer doped with gallium and which is ineffective in etching the silicon layer and the etch resistant layer to create a window in the gallium doped silicon wafer covered by a membrane formed from the silicon layer and kept taut by the expansion of the gallium doped silicon wafer.

15. A method of making a soft X-ray support substrate comprising:

doping a wafer of silicon with arsenic to expand said wafer;

depositing a layer of silicon on a first surface of said wafer;

providing an etch resistant layer on the second surface of said wafer except in the area corresponding to the area on the first surface for carrying a predetermined pattern in soft X-ray absorber material; and

subjecting said exposed area of said second surface to an etchant which attacks said silicon wafer doped with arsenic and which is ineffective in etching the silicon layer and the etch resistant layer to create a window in the arsenic doped silicon wafer covered by a membrane formed from the silicon layer and kept taut by the expansion of the arsenic doped silicon wafer.

16. A method of making a soft X-ray support substrate comprising:

doping a wafer of silicon with aluminum to expand said wafer;

depositing a layer of silicon on a first surface of said wafer;

providing an etch resistant layer on the second surface of said wafer except in the area corresponding to the area on the first surface for carrying a predetermined pattern in soft X-ray absorber material; and

subjecting said exposed area of said second surface to an etchant which attacks said silicon wafer doped with aluminum and which is ineffective in etching the silicon layer and the etch resistant layer to create a window in the aluminum doped silicon wafer covered by a membrane formed from the silicon layer and kept taut by the expansion of the aluminum doped silicon wafer.