Silicon carbide film for X-ray masks and vacuum windows

A layered structure for use in an X-ray membrane (pellicle) mask or a vacuum window is provided in which an intermediate amorphous layer such as silicon dioxide is grown on a silicon substrate which provides a stress relief medium and surface properties which enhance and improve subsequent process layers by breaking the epitaxial nature of these later deposited layers. Upon subsequent deposition of an inorganic overcoat, such as SiC, on the intermediate amorphous layer, the overcoat produces a nearly defect-free layer with a substantially reduced stress of suitable quality for X-ray lithography mask fabrication. Furthermore, additional alternating layers of a silicon carbide film and an intermediate inorganic layer, such as silicon nitride, can be deposited to obtain an even smoother silicon carbide surface and stronger structure.

BACKGROUND OF THE INVENTION 
This invention relates to a method of making and a structure for smooth 
silicon carbide films and particularly to silicon carbide films used in 
X-ray masks and vacuum windows. 
The X-ray mask is a critical structure in X-ray lithographic proximity 
printing. In general, x-rays from a point source of soft x-rays are 
shadowed by a heavy element mask consisting of a pattern absorber layer 
supported on a flat membrane, or pellicle, which is relatively transparent 
to the radiation. At the present time, the pellicle supporting the pattern 
absorber layer is made of a thin, inorganic material which minimizes X-ray 
attenuation, and yet remains mechanically stable to minimize distortion 
caused by stresses in the patterned absorber layer. Typically the pellicle 
is stretched across a stiff, flat ring whose expansion coefficient closely 
matches that of silicon. This stretching process flattens the substrate 
and stiffens it against bending and breaking. Factors important to mask 
fabrication are dimensional stability, absorber line-edge profile, and 
defect density. Also x-ray mask fabrication requires many steps which are 
similar to those used in wafer processing. As in wafer processing, these 
processing steps contribute to the defect density in the X-ray mask. 
Hence, defect density remains a highly critical problem. 
Presently available pellicles are fabricated by the deposition of 
sequential layers of boron nitride and polyimide onto a sacrificial 
silicon substrate. The boron nitride is generally formed by the reaction 
of ammonia and diborane in a suitable chemical vapor deposition process. 
Typically a layer of filtered polyimide is spun on top of the boron 
nitride from a liquid source in order to cover small defects. However, 
many defects still remain at the polyimide surface. 
In addition to boron nitride, silicon carbide has recently been deposited 
by chemical vapor deposition (CVD) onto silicon to form X-ray pellicles. 
This film is chemically inert and has excellent mechanical stability and 
strength, much stronger than boron nitride. Furthermore, the coefficient 
of expansion of silicon carbide can be closely matched to that of silicon. 
It is these properties of silicon carbide that make it an ideal mask 
support material. However, CVD of silicon carbide also has some inherent 
disadvantages. If the silicon carbide is deposited directly onto the wafer 
surface, the film tends to have a large number of defects. Also, for 
direct deposition on silicon, the deposition parameters required for 
optimum film smoothness do not necessarily coincide with the conditions 
required for optimum stress in the membrane. 
Furthermore, vacuum windows used, for example, in electron beam addressed 
liquid crystal displays require deposition of a silicon carbide film 
directly onto a silicon wafer, thus also requiring a smooth silicon 
carbide surface. Additionally, the silicon carbide film must be thick 
enough to stop electrons produced from electron beam bombardment and thin 
enough to have low lateral heat loss. A silicon carbide film typically two 
microns thick is required to stop electrons in the range of 15 KEV to 20 
KEV. However, when deposited to this thickness the surface roughness of 
the silicon carbide film is increased and correct alignment of the 
molecules of the liquid crystal on such a rough silicon carbide surface is 
difficult. 
Therefore, to alleviate the present disadvantages of using silicon carbide 
films for the fabrication of X-ray masks and vacuum windows, a new mask 
structure has been developed. The present invention presents this 
structure and a method for its construction. 
SUMMARY OF THE INVENTION 
In accordance with the illustrated preferred embodiment, the present 
invention uses an intermediate amorphous layer grown directly onto a 
silicon wafer prior to deposition of a silicon carbide film Unlike the 
pellicle structures of the prior art (which use no intermediate layer), 
the amorphous structure of this intermediate film breaks the epitaxy of 
the silicon carbide and allows it to be very fine grained, thus resulting 
in an extremely smooth surface. The defect density of the pellicle using 
this particular structure is drastically reduced and the layer becomes 
acceptable for quality mask and vacuum window fabrication.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 there is shown a cross-section of a layered structure of the 
preferred embodiment, which can be used in fabricating X-ray masks or 
other elements requiring very smooth silicon carbide surfaces. In 
accordance with aspects of the invention, an intermediate amorphous layer 
B is grown on a silicon substrate A (typically of &lt;100&gt; orientation, 
although other orientations can be used) and then subsequently deposited 
with a silicon carbide overcoat C. The thickness and choice of the 
material used for intermediate amorphous layer B provides the degree of 
freedom which allows for the improvement in the film properties of the 
silicon carbide overcoat C. The intermediate amorphous layer B also acts 
as a stress relief medium between the silicon substrate A and the silicon 
carbide overcoat C. In the preferred embodiment in which the substrate A 
is silicon, layer B is typically SiO.sub.2 grown to a thickness of about 
1000 Angstroms by conventional oxidation techniques, although the 
thickness can range from as little as 200 to as much as 10,000 Angstroms. 
The oxide layer B has a thermal coefficient of expansion significantly 
less than that of silicon substrate A. Hence, to ensure that the silicon 
substrate is under tension when etched, the silicon carbide is deposited, 
as will be explained shortly, with a slightly larger expansion coefficient 
than that of silicon. The preoxidation of the silicon substrate breaks the 
epitaxial nature of the silicon carbide deposition, thus making it 
possible to substantially improving the smoothness of the silicon carbide 
overcoat relative to silicon carbide layers deposited directly on the 
silicon. Other materials which are suitable for intermediate amorphous 
layer B are silicon nitride, boron nitride, and boron carbide, the 
thickness ranging from about 100 Angstroms to about 10,000 Angstroms. 
The silicon carbide overcoat is deposited by conventional chemical vapor 
deposition techniques (see, e.g., W. M. Feist, S. R. Steele, and D. W. 
Ready, "The preparation of films by Chemical Vapor Deposition," Physics of 
Thin Films, Vol. 5, (edited by G. Has and R. E. Thun) Academic Press, New 
York, London (1969) pp. 237-314; J. J. Tietijen, "Chemical Vapor 
Deposition of Electronic Materials," in: A. Rev. Mater. Sci. 3, (edited by 
R. A. Huggins, R. H. Sube, and W. Roberts), published by Annual Reviews 
(1973) pp. 317-326; T. L. Chu and R. K. Smelzer, "Recent Advances in 
Chemical Vapor Growth of Electronic Materials," J. Vac. Sci. Technol. 10, 
1(1973)). Typically, a methane line is added to a standard horizontal 
reactor, and films of silicon carbide are deposited onto the silicon 
substrate at temperatures of 1000-1150 degrees celsius by reaction with 
silane. (Other hydrocarbons could, of course, be used. However, the 
substantial purity of commercially available methane makes it the 
preferable reactant gas.) The residual stress can be adjusted over a wide 
range by varying the silane to methane ratio and deposition temperature. 
In order to obtain an appropriate film for x-ray masks, the ratio of 
methane to silane is generally kept greater than about 10:1. When 
deposited on substrates with approximately 1000 Angstroms of thermal 
oxide, silicon carbide films deposited by the above process are resistant 
to oxidation, cannot be etched by standard plasma methods, are relatively 
pinhole free, and exhibit improved transparency and generally good visual 
quality. Further, silicon carbide films produced by the above process are 
extremely smooth, and have a mean surface roughness which is reduced by at 
least 7:1 as compared with silicon carbide deposited directly onto a 
silicon substrate. Typically, surface smoothnesses as low as 100 Angstroms 
(root mean square) have been obtained using the above process. 
Using the above constructed layered structure, an X-ray mask can then be 
formed in a manner similar to conventional techniques used for boron 
nitride X-ray masks. For example, a typical process for preparing a 
silicon carbide x-ray mask structure is shown in FIG. 2. On a top surface 
of a silicon substrate 100, a padding layer 130 is formed, typically 
consisting of either a silicon dioxide (SiO.sub.2) or silicon nitride 
(Si.sub.3 N.sub.4) film using chemical vapor deposited (CVD) techniques. A 
CVD layer of silicon carbide 140 is then formed over padding layer 130. 
The thickness of padding layer 130 is customarily about 1,500 Angstroms 
and the silicon carbide layer 140 is usually in the range of 10,000 to 
30,000 Angstroms. Support plates 143 are bonded to selected portions of a 
bottom surface of silicon substrate 100 using conventional methods. 
Typically, masking plates known under the tradename of "Pyrex" are used. 
Then the silicon substrate 100 is removed in region 145 using wet etching 
techniques. Silicon carbide (SiC) layer 140 is overlaid with a polyimide 
layer 150 using conventional methods, such as spinning and metal layer 160 
is formed over polyimide layer 150. Typically metal layer 160 is made of 
gold or a gold alloy and is formed either by evaporation or sputtering 
techniques. Desired patterned features are formed in metal layer 160 by 
electron beam techniques, leaving surface portions of polyimide layer 150 
exposed. An overcoat layer 170 made from polyimide is formed by means of 
spinning over the now patterned metal layer 160 and exposed surface 
portions of polyimide layer 150. Overcoat layer 170 is primarily a 
protective coating. 
In FIG. 3 there is shown a cross-section of a layered structure of a second 
embodiment, which can be used in fabricating an x-ray mask. By depositing 
additional alternating layers of an intermediate inorganic layer 135 and 
silicon carbide layer 140' over padding layer 130' an even smoother 
silicon carbide surface can be obtained. Structures with five or seven 
alternating layers show dramatic improvement in silicon carbide film 
surface smoothness. The use of an intermediate inorganic layer between 
successive silicon carbide layers terminates the epitaxial growth of the 
silicon carbide layer deposited on the intermediate inorganic layer. For 
best results, intermediate inorganic layer 135 is made of silicon nitride 
typically deposited by CVD to a thickness of about 1000 Angstroms, 
although the thickness can range from as little as 200 Angstroms to as 
much as 0.35 micron. Other materials which are suitable for intermediate 
inorganic layer 135 are silicon dioxide, boron nitride, and boron carbide, 
the thickness ranging from about 500 Angstroms to about 3500 Angstroms. 
Support plates 143' are then bonded to selected portions of a bottom 
surface of silicon substrate 100' using conventional methods. Then the 
silicon substrate 100' is removed in region 145' using wet etching 
techniques. Silicon carbide layer 140' is overlaid with a polyimide layer 
150' using conventional methods, such as spinning and subsequently 
patterned by electron beam techniques to produce patterned layer 160', 
leaving surface portions of polyimide layer 150' exposed. An overcoat 
layer 170' of polyimide is formed by means of spinning over the now 
patterned metal layer 160' and exposed surface portions of polyimide layer 
150'. 
Layered silicon carbide films deposited by the above process are stronger 
than conventional boron nitride films, show significantly reduced crack 
propagation at the grain boundaries, and are extremely smooth. As a 
result, they are also especially good for vacuum windows, which require 
these properties. The SiC has a mean surface roughness which is reduced by 
at least 7:1 as compared with single SiC films deposited directly onto a 
Si substrate. Layered SiC structures with more alternating layers (e.g., 5 
or 7 alternating silicon carbide and silicon nitride layers) show further 
enhanced surface smoothness. 
FIG. 4 shows a layered window structure for use in a vacuum system. This 
structure can be formed in a manner similar to the methods used for 
preparing a layered silicon carbide x-ray mask. Support plates 143" are 
bonded to selected portions of a bottom surface of silicon substrate 100" 
and a silicon nitride layer 130" is deposited over silicon substrate 100". 
Silicon substrate 100" is removed over region 145" using wet etching 
techniques. Silicon nitride layer 130" is overlaid with a silicon carbide 
layer 140" and an intermediate layer of silicon nitride layer 135' is then 
deposited over silicon carbide layer 140". A silicon carbide layer 140" is 
then deposited over silicon nitride layer 135'.