U.S. patent number 3,742,230 [Application Number 05/267,672] was granted by the patent office on 1973-06-26 for soft x-ray mask support substrate.
Invention is credited to Henry I. Smith, David L. Spears, Ernest Stern.
United States Patent |
3,742,230 |
Spears , et al. |
June 26, 1973 |
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.
Inventors: |
Spears; David L. (Acton,
MA), Smith; Henry I. (Sudbury, MA), Stern; Ernest
(Concord, MA) |
Family
ID: |
23019723 |
Appl.
No.: |
05/267,672 |
Filed: |
June 29, 1972 |
Current U.S.
Class: |
378/35;
250/492.2; 430/967; 430/5; 976/DIG.435; 216/2; 438/705; 216/12 |
Current CPC
Class: |
G03F
1/22 (20130101); G21K 1/10 (20130101); Y10S
430/168 (20130101) |
Current International
Class: |
G21K
1/00 (20060101); G21K 1/10 (20060101); G03F
1/14 (20060101); G01n 021/34 () |
Field of
Search: |
;250/65R,49.5TE
;96/36.2,38.4 ;29/578 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Dixon; Harold A.
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.
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:
* * * * *