Patent Number: 052346099
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is described above, silicon carbide membranes prepared by the thermal CVD method are not quite suitable as a material of an X-ray lithographic mask despite their excellent properties in many respects solely due to the poor smoothness of the surface. Accordingly, the inventors have conducted extensive investigations to solve the problem of improving the surface smoothness of a silicon carbide membrane. As a result, the inventors have arrived at a discovery that a silicon carbide membrane having excellent surface smoothness can be obtained by the thermal CVD method when the gaseous atmosphere consisting of a silicon source compound such as dichlorosilane SiH.sub.2 Cl.sub.2 and a carbon source compound such as propane C.sub.3 H.sub.8 is admixed with a nitrogen-containing compound such as ammonia NH.sub.3 so that the membrane has a chemical composition of the formula SiC.sub.x N.sub.y, in which the subscripts x and y are each a positive number. Further, the membrane prepared in this way has a transmissivity to visible light of 75 to 85% when the membrane has a thickness of 2 .mu.m and contains only about 0.5 atomic % or less of hydrogen despite the use of a hydrogen-containing reactant compound or compounds to exhibit high resistance against high-energy beam irradiation not to cause distortion by the irradiation even though the membrane has an amorphous structure. As is mentioned above, the X-ray permeable membrane of the invention for X-ray lithographic mask has a chemical composition expressed by the formula SiC.sub.x N.sub.y and is prepared, preferably, by the thermal CVD method from a gaseous compound comprising the elements of silicon, carbon and nitrogen or a combination of compounds comprising, as a group, these three kinds of the elements. Examples of the silicon-containing compound include silane SiH.sub.4, dichlorosilane SiH.sub.2 Cl.sub.2, trichlorosilane SiHCl.sub.3, tetrachlorosilane SiCl.sub.4 and the like, examples of the carbon-containing compound include methane CH.sub.4, propane C.sub.3 H.sub.8, acetylene C.sub.2 H.sub.2 and the like and examples of the nitrogen-containing compounds include ammonia NH.sub.3, nitrogen N.sub.2 and the like. Although the reactant compounds should desirably be gaseous at room temperature, liquid compounds can also be used if the compound can be readily vaporized by bubbling with hydrogen gas or an inert gas or by decreasing the pressure or increasing the temperature. It is not always necessary that the gaseous atmosphere for the thermal CVD method is formed from a ternary combination of the above mentioned three groups of the compounds but also the atmosphere can be formed by a binary combination of a compound comprising silicon and carbon in a molecule such as methyl trichlorosilane CH.sub.3 SiCl.sub.3 and a nitrogen-containing compound such as ammonia. It is further possible that the gaseous atmosphere is formed from a single compound comprising the three kinds of the elements in a molecule such as hexamethyl disilazane of the formula [Si(CH.sub.3).sub.3 ].sub.2 NH. It is preferable, however, that the gaseous atmosphere is formed from a ternary combination of the three kinds of compounds each containing one of the elements of silicon, carbon and nitrogen or from a binary combination of the compounds providing the three kinds of the elements as a group in respect of the versatility in the control of the atomic proportion of the three kinds of the elements in the gaseous atmosphere. When a single compound containing the three kinds of the elements in a molecule is used alone for the gaseous atmosphere, the atomic proportion of the elements in the membrane is necessarily determined by the stoichiometric proportion of the elements in the starting gaseous compound not to give a possibility of freely modifying the atomic proportion of the elements in the membrane. The procedure of the thermal CVD method, by which the X-ray permeable membrane of the invention is formed, is not particularly limitative and can be conventional. The CVD chamber can be of the normal-pressure or reduced-pressure CVD chamber of the hot-wall type, epitaxial CVD chamber or cold-wall CVD chamber although a reduced-pressure CVD chamber is preferred. The temperature of the substrate plate in conducting the thermal CVD method should be in the range from 800.degree. to 1500.degree. C. When the temperature is too low, the film deposited on the substrate by the method contains a somewhat increased amount of hydrogen to affect the stability of the membrane against high-energy beam irradiation. When the temperature is too high, the film deposited on the substrate would be subject to crystallization so as to decrease the smoothness of the surface in addition to the problem of an increased amount of foreign materials entering the membrane. Needless to say, the temperature cannot exceed the melting point of the material forming the substrate, which is preferably formed from a high-purity silicon wafer or a fused quartz glass plate. As is mentioned above, the atmosphere inside of the CVD chamber is formed preferably from a mixture of at least two kinds of gases containing, as a group, the three elements of silicon, carbon and nitrogen. As to the composition of the gaseous mixture forming the atmosphere, it is preferable that the composition of the gaseous atmosphere is formulated in such a way that the molar ratio of Si:(C+N) is in the range from 0.05 to 2.0 and the molar ratio of C:N is in the range from 0.2 to 5.0 in the membrane. When the molar ratio of Si:(C+N) is too small, the excess of the carbon source gas and/or the nitrogen source gas would be discharged as unutilized in addition to the problem of eventual deposition of free carbon to cause a decrease in the light transmission through the membrane. When this molar ratio is too large, free silicon would eventually be deposited also to cause a decrease in the light transmission through the membrane along with a decrease in the surface smoothness. When the molar ratio of C:N is too small, the membrane thus formed would be less resistant against high-energy beam irradiation. When the molar ratio of C:N is too large, on the other hand, the surface of the membrane would have somewhat increased ruggedness. Further, the content of carbon in the gaseous mixture should be in the range from 5 to 95 atomic % because, when the content of carbon is too small, the membrane thus formed would be poor in the content of SiC so that the radiation resistance of the membrane would be decreased and no improvement can be obtained in the mechanical strengths of the membrane as compared with a membrane of silicon nitride while, when the content of carbon is too large, the properties of the membrane would approximate those of silicon carbide membrane with increased ruggedness of the surface although the membrane would have excellent resistance against high-energy beam irradiation. The chemical composition of the membrane formed in the above described manner can be expressed by the formula SiC.sub.x N.sub.y, in which x is a positive number of 0.25 to 0.86 and y is a positive number of 0.18 to 1.0 provided that the composition of the gaseous mixture for the CVD atmosphere satisfies the above mentioned requirements. When the value of x in the formula is too small or the value of y is too large, the radiation-resistance inherent in a silicon carbide membrane would be lost while, when the value of x is too large of the value of y is too small, the membrane would have increased ruggedness of the surface. The X-ray permeable membrane of the invention should have a thickness in the range from 0.3 to 3.0 .mu.m. When the thickness is too small, the membrane would have insufficient mechanical strengths while, when the thickness is too large, the membrane would have an increased stress or an insufficient transmissivity to visible light. The thickness should be as uniform as possible to have such a standard deviation that three times of the standard deviation .sigma., i.e. 3.sigma., does not exceed 5% since otherwise the uniformity in the transmissivity to visible light of or the internal stress in the X-ray permeable membrane would be noticeably decreased. The membrane of the invention should have surface smoothness as high as possible and the surface roughness Ra should not exceed 0.01 .mu.m or, preferably, 0.005 .mu.m. When the value of Ra is too large, a problem is caused in the accuracy of the X-ray absorbing pattern formed on the surface of the membrane to prepare an X-ray lithographic mask. The content of hydrogen in the membrane should be 1.0 atomic % or lower as determined by the RBS-HFS method because a membrane containing a larger amount of hydrogen would be less resistant against high-energy beam irradiation. The conventional infrared absorption spectrophotometric method for the determination of the hydrogen content in the membrane is not applicable in this case because of the low sensitivity of the method not to give a measure to ensure such a low hydrogen content of 1.0 atomic % or lower. The membrane should desirably have a transmissivity to visible light of at least 60% or, preferably, at least 70% even when the membrane has a thickness sufficient to ensure good mechanical strengths because a membrane less transparent to visible light would cause a problem of decreased accuracy of alignment by using visible light. Further, the internal stress in the membrane should be in the range from 1.times.10.sup.8 to 6.times.10.sup.9 dyn/cm.sup.2. When the internal stress is too small, a difficulty is caused in processing the film deposited on a substrate into a frame-supported membrane while, when the internal stress is too large, the patterning accuracy of the X-ray lithographic mask would be decreased. The membrane should have such a resistance against high-energy beam irradiation that the change in the internal stress of the membrane is 1% or smaller by the irradiation with high-energy electron beams of 10 keV energy in a dose of 500 MJ/cm.sup.3. In the following, the X-ray permeable membrane of the present invention is described in more detail by way of an example. EXAMPLE A 2 mm-thick silicon wafer of 3 inches diameter having a crystallographic plane of (100) was fixedly held by using a carbon-made holder in a fused quartz glass tube as a CVD chamber in a hot-wall type thermal CVD apparatus. The silicon wafer was positioned within the uniform-temperature zone in the furnace which was heated by means of an electric resistance heater. The CVD chamber was connected to a vacuum line and evacuated to have a pressure of 0.01 Torr and then the temperature of the chamber was increased up to 1000.degree. C. taking 1 hour. A gaseous mixture of silane SiH.sub.4, propane C.sub.3 H.sub.8 and ammonia NH.sub.3 at flow rates of 77 ml/minute, 20 ml/minute and 200 ml/minute, respectively, was introduced into the chamber while the pressure therein was controlled at 0.5 Torr to conduct the thermal CVD process for 3 hours. After the end of the above mentioned reaction time, the furnace was cooled to room temperature and the chamber was flushed with nitrogen gas at normal pressure. The silicon wafer was taken out of the chamber and the thin film deposited on the silicon wafer was examined by the RBS method to find that the film had a chemical composition expressed by the formula SiC.sub.0.5 N.sub.0.7. The film had an average thickness of 1.0 .mu.m, of which the variation as given by 3.sigma. was 0.05 .mu.m and the internal stress therein was 1.times.10.sup.9 dyn/cm.sup.2. The content of hydrogen therein was 0.1 atomic % as determined by the RBS-HFS method. The surface of the film was very smooth with roughness Ra of 0.01 .mu.m or smaller. The film deposited on the substrate surface was processed into a frame-supported membrane in a conventional manner. Namely, a coating film of boron nitride was formed on the back surface of the silicon wafer by the plasma CVD method and then a 25 mm by 25 mm square area at the center of the boron nitride film was removed by plasma-induced dry-etching method. With the thus formed boron nitride film with the square opening as an etching resist, the silicon wafer in the 25 mm by 25 mm wide unprotected square area was removed by etching with a hot aqueous potassium hydroxide solution leaving the deposited film as a frame-supported membrane. The membrane thus obtained to serve as a mask blank had 80% of light transmission at a wavelength of 633 nm. Further, the membrane was irradiated with high-energy electron beams of 10 keV energy in a dose of 500 MJ/cm.sup.3 to find that the membrane was excellently resistant against high-energy beam irradiation with only 1% or smaller of a change in the internal stress by the above mentioned irradiation test.