Method for fabricating a silicon carbide film to provide improved radiation hardness

A method for radiation hardening by means of a suitable thermal treatment of the membrane material, which in the preferred embodiment is silicon carbide (SiC). The method includes steps of thermally treating a freestanding SiC film (drumlike membrane with a silicon frame) or an unetched film (attached to the silicon wafer throughout its entire area), in an inert atmosphere or vacuum. The temperature for the treatment may range from a couple hundred degrees Celsius to one above the growth temperature. The treatment time, depending on the anneal temperature, can be as short as few minutes or as long as a couple of hours. Optimal anneal times and temperatures will depend on the material and the degree of radiation hardening required. Significant hardening may be achieved even at the lowest temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for fabricating a membrane 
structure and more particularly to a process for fabricating a silicon 
carbide membrane for an X-ray mask having improved radiation hardness. 
2. Description of the Prior Art 
U.S. Pat. No. 4,543,266, issued Sep. 24, 1985 to Matsuo et al entitled 
METHOD OF FABRICATING A MEMBRANE STRUCTURE, discloses a process wherein a 
thin film which becomes a membrane is formed over one major surface of a 
substrate by a plasma deposition process utilizing microwave electron 
cyclotron resonance. The substrate is then removed, other than a portion 
of the substrate which remains as a frame, so as to form a membrane 
structure. A dense and high quality membrane is formed at a low 
temperature and the internal stress of the membrane is controlled by 
varying the conditions under which the plasma deposition process is 
carried out and by heat treating the thin film after its formation. The 
heat treatment is related only to stress control on the films. There is no 
teaching of improvement of radiation hardness and the thermal treatment 
described is not associated with minimizing radiation damage. 
European Patent EP 372645A, issued Jun. 13, 1990 entitled X-RAY MASK 
MEMBRANE OF SILICON-CARBIDE, discloses a structure wherein, at least one 
of the two main surfaces of a single crystalline Si wafer, a SiC layer is 
formed. This is made into a mask-supporting membrane by removal of 
Si-material over the entire surface except the edge-regions. The feature 
is that before or after this selective etch, an anneal takes place at a 
temperature in the range of 200-1350 degrees C. for 2-10 hours in an 
oxidizing ambient. The preferred conditions are four hours at 1100.degree. 
C. This anneal may also be given to selected areas of the SiC film using 
laser-beam heating in an oxidizing ambient. The SiC film may be formed by 
CVD on the wafer substrate. The process can produce layers with a lower 
stress, a higher transparency to visible light used for alignment and a 
lower surface roughness. 
The annealing is used exclusively for stress control and the application of 
an antireflective oxide coating. The heat treatment is done under 
oxidizing atmospheres to apply the oxide coating. There is no mention of 
radiation damage avoidance. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method that consists of 
thermally treating a freestanding SiC film (such as a drumlike membrane 
structure with a silicon frame) or an unetched film (attached to the 
silicon wafer throughout its entire area) in an inert atmosphere or 
vacuum. The temperature for the treatment may range from a couple hundred 
degrees Celsius to one above the growth temperature. The treatment time, 
depending on the anneal temperature, can be as short as a few minutes or 
as long as a couple of hours. Optimal anneal times and temperatures will 
depend on the material and the degree of radiation hardening required. 
Significant hardening may be achieved even at the lower temperatures. The 
process succeeds in thermally exciting mechanisms that lead to the 
improvement of the radiation hardness of the material, and in the case of 
high temperature, chemically vapor deposited optical transparency of the 
membrane increases during the treatment.

DESCRIPTION OF A PREFERRED EMBODIMENT 
X-ray mask materials are required to withstand very large doses of X-rays 
while suffering virtually no radiation induced distortion. Unfortunately, 
many materials in the as-grown state are susceptible to radiation damage. 
In principle, mask material could be radiation hardened during fabrication 
by direct exposure to an appropriate dose of X-rays. Such method would be 
very expensive and rather impractical. This invention describes an 
improved method for radiation hardening by means of a suitable thermal 
treatment of the membrane material, which in the preferred embodiment is 
silicon carbide (SiC). 
The method includes steps of thermally treating a freestanding SiC film 
(drumlike membrane with a silicon frame) or an unetched film (attached to 
the silicon wafer throughout its entire area), in an inert atmosphere or 
vacuum. The temperature for the treatment may range from a couple hundred 
degrees Celsius to one degree above the growth temperature. The treatment 
time, depending on the anneal temperature, can be as short as a few 
minutes or as long as a couple of hours. Optimal anneal times and 
temperatures will depend on the material and the degree of radiation 
hardening required. Significant hardening may be achieved even at the 
lowest temperature. 
The process succeeds in thermally exciting mechanisms that lead to the 
improvement of radiation hardness of the material. For SiC film material, 
no known way of increasing the radiation hardness existed previously. 
An additional benefit of the process in the case of high temperature 
chemically vapor deposited SiC, is that the optical transparency of the 
membrane increases during treatment. 
According to the present invention, SiC membranes grown by high temperature 
chemically vapor deposition are heat treated to provide radiation hardness 
using the following sequence of steps and under the indicated conditions 
when the sample is placed in a treatment chamber. 
Step 1. A vacuum of 10.sup.-6 Torr is slowing created around the sample in 
a chamber. 
Step 2. The sample is ramped to a temperature of approximately 1250.degree. 
C. 
Step 3. The temperature is held for approximately 2 hours. 
Step 4. The sample is slowly cooled, brought to atmospheric pressure and 
removed from the chamber. 
The thickness of the SiC membranes employed in the present invention is not 
critical. The membrane need only be thick enough to undergo the thermal 
treatment. A typical range of thickness, for example, is 0.5 to 2.5 
microns. 
The method is performed in either vacuum (10.times.10-6 Torr) or inert gas. 
The chamber or container used in Step 1 to contain the membrane merely has 
to be capable of withstanding the treatment conditions of atmospheric 
pressure and heat load. A simple water cooled quartz bell jar may be used. 
The heating may be carried out using resistive state-of-the-art 
boron-nitride/graphic heaters. The system is a custom made tool. 
The ramp time in Step 2 is unimportant from the radiation point of view of 
the method. It only becomes relevant since a ramp too fast can destroy the 
membrane. Even more important than the ramp-up (heating), is the cooling 
ramp. The thermal stresses and the sign of the mismatch between Si and SiC 
is such that during cooldown can stretch the membrane beyond its ultimate 
strength and break it. Typical ramp times for method Step 2 are 10 minutes 
up and 10 minutes down. These times do not represent the limits of the 
ramping speed and much faster ramps are possible. 
Radiation hardening is produced in samples treated following the process 
steps outlined above. Samples treated according to the described method 
showed no radiation induced distortion at exposure levels where untreated 
samples would have been already distorted. One skilled in the art will 
appreciate that the basic process can be tailored to a specific set of 
material, growth/deposition process, and desired level of hardening. 
The annealing steps can also be performed on other types of SiC material, 
such as ECR-plasma or PECVD grown SiC.