Embedded absorber X-ray mask and method for making same

A mask which is especially useful in X-ray lithography wherein the X-ray absorber material is embedded in the mask membrane.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed to X-ray lithography masks, in general, and to a 
mask having the X-ray absorber material embedded in the mask, per se. 
2. Prior Art 
There are many known masks which have been produced for use with X-ray 
lithography techniques. The known masks generally comprise a thin 
supporting membrane which is transparent to X-rays, a patterned absorber 
layer of dense material which is substantially opaque to soft X-rays 
supported on the membrane, and mechanical support means such as a 
peripheral ring bonded to the membrane. 
The known masks noted above are typically fabricated in a sequence wherein 
a membrane is bonded to a support ring, the membrane is coated with an 
absorber layer (for example heavy metal), and the absorber is patterned 
using suitable lithography and/or etching steps. 
However, the known masks and methods of making same are not totally 
satisfactory. That is, in the known masks the patterned absorber typically 
protrudes above the membrane surface leading to problems with mechanical 
damage and/or wear as well as significant problems with adhesion of 
contamination and particulate matter. More importantly, however, the known 
masks and method require that the absorber material be deposited in such a 
manner that, when patterned, non uniform strain in the thin membrane 
occurs. 
PRIOR ART 
1. E. Bassous, R. Feder, E. Spiller, and J. Topalian, Solid State 
Technology, September 1976, p. 55. 
2. Suzuki et al., Japan J. Appl. Phys., 17, 1978, p. 1,447. 
3. P. L. Spears, H. I. Smith, and R. Stern, U.S. Pat. No. 3,742,230, June 
28, 1973. 
4. Coquin et al., U.S. Pat. No. 3,892,973. 
SUMMARY OF THE INVENTION 
This invention is directed to a mask and method of making same. The mask 
is, typically, useful in the X-ray lithography, or charged particle 
lithography fields. The mask uses a temporary substrate as a template and 
support for the mask membrane, an X-ray absorber layer and a supporting 
membrane layer. A separate support structure can be provided for the mask. 
In addition, an adhesion promoting layer may be optionally utilized 
between the absorber layer and the membrane layer, and a capping layer 
provided to completely seal the absorber pattern, either an etch stop 
layer or a parting agent layer can be disposed between the substrate and 
the absorber layer or capping layer depending upon the ultimate structural 
arrangement of the mask as may be defined by the optional support 
structure.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown a cross-sectional view of an X-ray 
lithography mask known in the art. In FIG. 1, a suitable support ring 10 
such as quartz or other suitable material is provided. A thin supporting 
membrane 11 is stretched cross support structure 10 and bonded thereto in 
a suitable manner. Membrane 11 may be formed of any suitable material 
substantially transparent to X-rays such as thin metals, covalently bonded 
materials, polymers or the like. Patterned absorber layer 12 is disposed 
on the surface of supporting membrane 11. Pattern 12, which is 
substantially opaque to soft X-rays, may be achieved by initially placing 
the layer of opaque material on membrane 11 and then patterning this 
material using a lithography step or an etching step. 
As noted above, it is clear that the prior art mask includes a patterned 
absorber layer 12 which protrudes above the surface of membrane 11 therein 
subjecting the absorber layer to both excessive wear and the possibility 
of trapping contaminants therebetween wherein the mask becomes less useful 
and less accurate. Moreover, by producing the mask in the standard manner, 
membrane 11 is usually subjected to non-uniform stresses as a result of 
the operation of the lithographic and/or etching step which patterns the 
absorber layer to form elements 12. 
Referring now to FIGS. 2a through 2e, there is shown and described a method 
of forming a mask and the mask produced by the method. In FIG. 2a there is 
shown composite 20 which includes a polished planar substrate 21 which 
serves to act as a temporary support for a mask membrane during 
fabrication. Typically, substrate 21 may be formed of polished glass, 
silicon or any other suitable material to serve as a substrate as will 
become evident hereinafter. In the embodiment shown in FIG. 2a, layer 22 
is formed on the planar surface of substrate 21. Layer 22 can be either a 
parting compound or an etch stop as will appear hereinafter. Typically the 
parting compound may be a sugar film or a suitable highly soluble salt 
such as NaCl, CsI, BaCl.sub.2, or the like. Alternatively, the parting 
compound may be a soap or detergent film or soluble organic film as is 
known in the art. Conversely, if layer 22 is an etch stop layer, a 
substance which is not soluble in the etchant to be used on the substrate 
21 may be used. The material of the etch stop layer and the substrate as 
well as the etchant will be mutually defined. The etch stop layer 22 
should be substantially pinhole-free to prevent etching of capping layer 
23, adhesion layer 24 or 26, or absorber layer 25, as noted hereinafter. 
Absorber layer 25 is a layer which is utilized to absorb the X-rays and to 
form a pattern. Layer 25 is typically 0.2-1.0 .mu.m thick and formed of a 
heavy metal such as gold, tungsten, platinum or the like which is 
substantially opaque to X-ray radiation. Typically, layer 25 may be 
deposited by suitable vacuum depositon or other plating process either 
directly on the substrate 21, on etch step or parting layer 22, on capping 
layer 23, on an adhesion layer 24 which may be desirable to promote 
adhesion of the absorber pattern. Use of layers 22, 23, and 24 is 
optional--one layer may perform the function of two or more. Alternatively 
one or more of these intermediate layers may be omitted. 
Referring now to FIG. 2b, absorber layer 25 is shown as having been 
patterned. Patterning of layer 25 is typically performed by a lithography 
step and by a wet or dry etching step as is known in the art. Conversely, 
the patterning layer could be produced by plating through a lithography 
mask or by a lift-off method or any combination of the above. These steps 
are typically known in the art. Moreover as noted above, each of these 
steps tends to produce strained distortions in the supporting substrate 
21. In composite shown in FIG. 2b, the stress variations produced by the 
patterning of layer 25 produce strain distributed through substrate 21 
which is, typically, over 100 times as thick as the X-ray mask membrane 
such as membrane 11 in FIG. 1. Consequently, little or no strain is 
produced and patterning is maintained in a high fidelity and resolution. 
Referring to FIG. 2c, the supporting membrane 27 is deposited. Membrane 27 
can be deposited by means of spinning a polymide film or the like onto the 
patterned surface of the composite. Of course, other techniques such as 
vacuum evaporation, plasma deposition, low vacuum condensation and the 
like can be utilized. Suitable materials for substrate 27 have high 
transparency or transmissivity for soft X-rays and can be formed of 
materials such as Be, B.sub.4 C, organic polymers or the like. 
Incidentally, in some cases it may be desirable to include thin layer 26 of 
a material to promote adhesion between the absorber 25 and the membrane. 
The adhesion promoting layer 26 can be of any suitable material such as 
titanium and can be applied in any typical fashion known in the art. It 
should be noted that membrane 27 is provided only subsequent to the 
patterning of layer 25 wherein no non-uniform stresses or strains are 
applied to this membrane. 
Referring now to FIG. 2d, there is shown one alternative of the final mask 
structure. In FIG. 2d, a separate supporting peripheral ring 28 is 
provided and is bonded to membrane 27 in any suitable fashion. After 
support ring 28 is in place, supporting substrate 21 may be etched away in 
a suitable fashion. Alternatively, in the event that the layer 22 was 
formed of a parting compound, layer 22 may be dissolved in a suitable 
manner wherein the mask comes free from the supporting substrate 21 
thereby leaving a mask with a planar surface and X-ray absorbing pattern 
embedded therein. 
Referring to FIG. 2e, there is shown an alternative mask structure wherein 
a portion of support substrate 21 is etched away thereby leaving a 
peripheral support ring 21A. In this structure, layer 22 is an etch stop 
layer which controls etching through support substrate 21a to prevent 
etching of membrane 25. Thus, the various relationships between membrane 
27, support substrate 21, etch stop layer 22 and the etchant are 
determined as noted above. 
At the time of separation of the mask from support substrate 21, the stress 
of the patterned absorber 25 is freed to distort the thin mask membrane 
which now consists of a composite of layers 23, 24, 26, and 27. If, 
however, care is taken to deposit a stress-free absorber layer 25, or to 
match the tension of the absorber 25 and the membrane film 27, the 
resultant mask will exhibit uniform tension and be free of pattern related 
strains. Also, non-planar layers can be added to both sides of the mask 
membrane composite to adjust the composite membrane tension. 
FIGS. 2d and 2e show a totally encapsulated mask structure, wherein 
frequently the capping layer 23 and membrane 27 are of the same material. 
It has already been indicated that many of the layers used might be 
eliminated by using a single layer with several functions (e.g. a "capping 
layer" that also functions as an etch stop). In fact, useful "partially 
embedded" masks can be made without layers 22, 23, 24, or 26. For example, 
in FIG. 2b, metal absorber pattern 25 can be disposed directly on a 
suitable substrate 21 such as glass, silicon or the like. In FIG. 2c, the 
pattern 25 is coated directly with a suitable membrane material 27. After 
removal of the substrate 21 by an etchant which does not attack absorber 
pattern 25 or mask membrane 27, the finished "partially embedded" mask 
similar to the devices seen in FIGS. 2d or 2e is produced (without layers 
23, 24 and 26). In FIG. 2d, support frame 28 is attached to membrane 27 
and the substrate 21 completely removed. In FIG. 2 e, a portion of 
substrate 21a is retained for mechanical support. 
Thus, there has been shown and described a method of forming an X-ray mask 
wherein a patterned X-ray absorber layer is embedded in the mask 
substrate. No stress or strain are imposed upon the mask supporting 
membrane during the patterning process related to the absorber layer. The 
planar mask layer provides both mechanical and contamination free 
advantages. Moreover, the technique described herein produces masks which 
are relatively distortionless as a result of the patterning process of the 
X-ray absorber layer. The distortionless mask permits high resolution of 
microcircuits and the ready replication thereof. While certain specific 
materials have been recited in the description, other materials can be 
utilized in a satisfactory manner. The description is intended to define 
the best mode of the invention known to date and is not intended to limit 
the scope of the invention. The scope of this invention is limited by the 
claims appended hereto.