Abstract:
The present invention provides a method of forming a free standing shape made of a material containing refractory metal, which entails providing a mandrel in a CVD enclosure, injecting a refractory halide gas and a reducing gas in the enclosure, reacting the gases in the enclosure to generate a material containing refractory metal, growing a layer of the material containing refractory metal on the mandrel and removing the mandrel to obtain the free standing shape, wherein the reducing gases is a silicon hydride gas or a mixture thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing free standing shapes of refractory metals and their silicides through a Chemical Vapor Deposition process, and particularly to a method of forming high quality refractory parts with high corrosion resistance. 
     2. Description of the prior art 
     Refractory metal parts are usually made through powder metallurgy processes. Die pressing and sintering of powders is a well established process route. More recently, injection moulding has extended the processing technology of metal powders to the production of more complex, near net shape components at density levels much higher than before. (For more details about this technology, it can be referred to the article of Robert W. MESSLER entitled &#34;Powder injection moulding, research at Rensselear&#34;: an update Metal Powder Report, vol. 44, May 5, 1989-Pages 362-368, or to the article of P.J. JAMES entitled &#34;Injection molding of metal powder&#34;, Metal Powder Report, vol. 44, May 5, 1989, pages 369-372). 
     However, although metal powder injection moulding is a potential and promising method, several problems and drawbacks remain. Such injection molding process requires large size sophisticated equipment, high investment cost and high production rates. The process itself is complicated (the debinding step requires a careful heating in a controlled atmosphere to optimize said step which makes it critical to achieve high quality products), time consuming (up to a few days for the overall process), energy consuming (sintering temperature is at least 1500° C. for refractory metals such as W, Mo or Ta). Finally, it is a poor yield process wherein shrinkage occurs during the sintering steps making it difficult to obtain tight dimensional tolerance without a post-machining step of workpiece. 
     In order to avoid those drawbacks, it has been recently suggested to use a chemical vapor deposition (CVD) process for small shaped and/or thin parts : it is suggested in &#34;Advanced P.M. Materials at Fulmer Research&#34; Metal Powder Report, Vol 44, Sep. 9, 1989 pages 607-611 to carry out Tungsten deposition on a support by reduction of Tungsten fluoride with hydrogen H 2  under CVD conditions. The same has also been suggested in Japanese patent application JP 63/286574 of SASAKI et al. 
     However, the use of refractory metal containing parts, either obtained by powder metallurgy or CVD routes, is limited under high temperature oxidizing environments under which oxidation of parts becomes then a real problem. In order to limit such phenomena, it is known to deposit and additional corrosion resistive layer typically refractory metal silicide such as MoSi2, well-known for its anti-corrosion properties. Such additional layers are deposited by usual techniques such as pack cementation, slip pack or slurry methods. In this case, refractory metal parts withstanding to corrosive atmosphere are manufactured through a multi-step process. 
     SUMMARY OF THE INVENTION 
     The present invention aims at improving the elaboration of refractory metal parts by CVD processes. Another object of the present invention is to manufacture directly products through Chemical Vapor Deposition Process, such products having better resistivity to corrosion. 
     The present invention relates to a method of forming a free standing shape made of a material containing refractory metal comprising the steps of providing a mandrel in a CVD enclosure, injecting a refractory metal halide gas and a reducing gas in said enclosure, reacting said gases in said enclosure to generate a material containing refractory metal, growing a layer of said material containing refractory metal on said mandrel, then removing the mandrel to obtain said free standing shape, wherein the reducing gas is selected among the silicon hydride gases and their mixtures. 
     It has been unexpectedly found that, by using Silicon Hydride gases (SinH2n+2) instead of hydrogen gas to reduce metal halides, free standing shapes of substantially pure refractory metals or/and highly corrosion resistant free standing shapes refractory metal silicides can be formed at low temperature, with high speed and high quality. 
     According to the invention, it is possible to make a piece of composition MSiz, z being selected between about 0.01 and about 3. Z can be determined according to the reaction conditions in the CVD enclosure, and more precisely to the metal halides and silicon hydrides gases flowrates. When z is between about 0.01 and 0.1, the product is considered as a substantially pure refractory metal product, while when z is between about 0.1 and 3, the piece is a refractory silicide metal one, with corrosion resistance properties. In the latest case, such a product is more particularly adapted to be used under high temperature oxidizing environments. 
     Moreover, it is also possible according to the present invention, to vary at will the z value during the manufacture of these free standing shape parts, and thus to combine refractory and anti-corrosion properties. It is also within the scope of the present invention to prepare in the same enclosure and continuously a composite free standing shape part with high silicon content on one or both its surfaces (z&gt;0.1) and with low silicon content in the bulk (z&lt;0.1) confering to a substantially pure refractory metal part with corrosion resistance properties on one or both surfaces (or vice-versa in certain cases). 
     In order to obtain a refractory metal silicide MSiz (with z&gt;0.1) offering corrosion protection, ti has been found that the ratio silicon hydride (SiH 2  n+2)/metal halide (MX 6 ) must be chosen in the upper part of the selected range of flowrates according to the invention while substantially pure metal pieces can be obtained by decreasing the ratio silicon hydride (Si n  H 2n+2 )/metal halide (MX 6 ). 
     By comparison, Sasaki et al does not teach nor suggest such corrosion resistant material while they only teach to use hydrogen gas. 
     The terms &#34;mandrel&#34; or &#34;preform&#34; are used throughout the specification and claims with the same meaning, i.e. support means having the desired shape on which the refractory metal containing layer is deposited and which can be further eliminated without substantially changing the integrity of the free standing shape. 
     The mandrel or preform used for deposition can be removed either by chemical or mechanical processes. In the case of chemical removal, the mandrel is chosen among metals easily dissolved in current acids, preferably copper and its alloys. In the case of mechanical removal, the mandrel is chosen among materials easily machinable such as Carbon. 
     Reaction temperature in the CVD enclosure is selected between about 20° C. and about 750° C. 
     Pressure is selected between 0.01 Torr (1,33 Pascal) and atmospheric pressure. 
     The metal halide gas are preferably fluoride gas or chloride gas, such as WF 6 , WC 16 , MoF 6 , MoC 16 , TaF 5 , TaC 15 . The silicon hydride gas Si n  H 22n+2  (n=1, 2, 3. . . ) is preferably SiH 4 . 
     The ratio of flowrates of silicon hydride to metal halide is selected between about 0.2 and about 25. 
     In the case of SiH 4  and WF 6 , the preferred conditions for the formation of the Tungsten pieces are as follows : 
     The ratio of flowrates SiH 4  /WF 6  is between about 0.5 to about 0.8 to obtain almost pure Tungsten pieces. 
     The ratio of flowrates SiH 4  /WF 6  is greater than 0.8 to obtain a silicide having corrosion protection properties. 
     Temperature deposition is between about 150° C. and about 750° C., preferably between about 300° C. and about 600° C. 
     In the case of SiH 4  and MoF 6 , the preferable conditions for the formation of Molybdenum pieces are as follows : 
     The ratio of flowrates SiH 4  /MoF 6  is between about 0.2 and about 2 to obtain almost pure Molybdenum pieces. 
     The ratio of flowrates SiH4 to MoF6 is between about 2 and about 25 in order to obtain a silicide for anticorrosion properties. 
     Temperature deposition is set between about 150° C. and about 750° C., preferably between about 300° C. and 600° C. 
     In addition to resistivity to corrosion, others advantages of the method disclosed herein are numerous, compared with the powder metallurgy process : 
     Thin refractory parts can be obtained at very low temperature in comparison with high temperature of sintering (at least 1500° C.) used in powder metallurgy process. Parts of some microns thick are easily obtainable according to the invention while the thickness lower limit for powder metallurgy is about 100 microns. 
     Considering that debinding and controlled atmosphere difficulties and long time processing problem are removed, cost becomes very attractive. 
     The problem of shrinkage, specific to the powder metallurgy processes, is avoided and the piece size can be adjusted exactly as a function of time deposition. 
     Another advantage of the invention over the prior art technique using H 2  is the lower deposition temperature (preferably down to 300° C.) for still high deposition rates. 
     In order to combine refractory and anti-corrosion properties, it is also in the scope of the present invention to form free standing shapes according to a continuous process comprising at least two steps the first step consists in the formation of an almost pure refractory metal piece allowing refractory property while the second step, consisting in the deposition of a refractory metal silicide layer, confers anti-corrosion properties to the part (or vice-versa in certain cases). For each step of the above process, it is within the scope of the present invention to vary at will the z value during the formation of the free-standing shape as explained herein before. 
     The applications of the present invention are preferably : thin wires and tubes, rings, refractory piping and especially piping for nuclear equipments, crucibles and preferably crucibles for physical evaporation, thin sheets for walls in hot furnace and for radiation shielding against X-Ray radiations, and in defense applications. 
     In order to improve the granular structure, substances in gaseous state at the reaction temperature can be added during the vapor growth. These substances are selected among hydrocarbons, saturated or not, substituted or not, and/or metallic carbonyl containing compounds R n  (CO) m  wherein R is a metallic compound. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a CVD system used to carry out the present invention. 
     FIG. 2 is a graph showing X-Ray diffraction pattern for Tungsten sheets when Silicon amount is very low and considered as an impurity. Here, the ratio SiH 4  /WF 6  =0.50. 
     FIG. 3 is a graph showing the relation between the Si amount in the deposited layer and the ratio SiH 4  /MoF 6  for a specific CVD enclosure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Example 1 
     FIG. 1 is an example of CVD system which can be used for the present invention. It consists of a gas distribution system, a cold wall type chamber, a heating system, and a pumping system. The available gases are SiH 4 , WF 6  (or MoF 6 , WC 16 , MoCL 6 , TaF 5 , TaCl 5 ), He, and Ar. 
     Helium (He) is used as a carrier gas for metal halides. Argon is used to purge the chamber after the deposition is completed. 
     The chamber (1) is a cold wall type water cooled (2) reactor made in stainless steel. A copper mandrel (3), which has been forged under the shape of the desired piece, is set on a quartz plate (4) under which is a radiant heater (5) with three infrared lamps, positioned in the middle of the chamber. 
     The substrate temperature is measured by an infrared pyrometer (6). The chamber is pumped down by a turbomolecular pump (7), which is backed by one master booster pump (8) and a rotary pump (9). A pressure base around 10 -6  Torr can be easily reached. 
     A gate valve (10) isolates the chamber from the turbomolecular pump. In addition a second pumping line bypassing the gate valve (11), a trap for detoxification (12), a master booster pump (13) and a rotary pump (14) is operated during deposition. 
     When the pressure base is reached, typically around 10 -6  Torr, the pumping line is switched on that used for deposition. 
     In this example 1 here described, a Tungsten W crucible is made. After the stabilization of the substrate temperature, gases SiH 4 , WF 6  and He are injected under the following flowrates : 
     
         ______________________________________   SiH.sub.4    5 sccm   WF.sub.6    10 sccm   He          64 sccm______________________________________ 
    
     The ratio SiH 4  /WF 6  is 0.5. Deposition time is 2 hours. The pressure inside the chamber is set at 0.4 Torr and temperature is 500° C. 
     After the deposition is completed, several purges are carried out with Ar gas. Afterward, the Copper mandrel is dissolved in acids, and then characterization can be carried out. 
     A Scanning Electronic Microscope (SEM) analysis is carried out in order to determine the thickness of the piece which is free-standing. The measured thickness of the piece is 115 microns which have been obtained in 120 minutes, i.e. a deposition rate of about 1 micron/min. 
     X-Ray diffraction results analysis, exemplified on FIG. 2, is carried out on the sample obtained under aforementioned conditions. This graph shows the three characteristic peaks of W Alpha phase. The presence of this Alpha phase indicates that those compounds have a crystalline structure. 
     Example 2 
     With the same conditions as those described in example 1, but with a deposition time of 15 minutes, a layer of 14 micron thick having the same composition is obtained. 
     Example 3 
     In this example, the flowrates ratio has been changed which determines the change of Si amount, while using the same reactor as in example 1, under the following conditions : 
     Various experiments have been done, using SiH 4  and MoF 6  as the reactant gases, and He as the carrier gas. For every experiment, a copper foil substrate is set in the reactor, then heated at 400° C. After temperature stabilization, gases are injected. Deposition time is 45 minutes. The pressure inside the chamber is set at 0.4 Torr. 
     Four identical experiments are done, with the following SiH 4  /MoF 6  ratio : 2, 2.2, 5, 10. For each experiment, adjusting the pressure at 0.4 Torr is realized by controlling the He flowrate. After each experiment, a quantitative and qualitative analysis have been been carried out. 
     The results are shown on FIG. 3. From this graph, it becomes easy to choose the final properties of the part by selecting the right ratio for a desired amount of Si in the layer. 
     For example, after deposition with a ratio of 2.2, a quantitative and qualitative analysis is done, indicating the amount of Si in the formed sheet. In the same way as in example 1, SEM analysis is carried out. In this example, the measured thickness is 180 microns which means a deposition rate of about 4 microns/min. 
     Example 4 
     This example describes the way to obtain a Tungsten (W) containing sheet combining almost pure refractory metal and anti-corrosion properties, in a continuous process, in the same enclosure. 
     A first step consists in forming an almost pure Tungsten sheet with refractory properties. The second step consists in depositing a Tungsten silicide layer providing protection against corrosion. The apparatus is the same as in example 1. A Copper foil substrate is set in the reactor, and then heated at 400° C. 
     After the stabilization of the substrate temperature, the first step consists in injecting SiH 4 , WF 6  and He under the following flowrates : 
     
         ______________________________________   SiH.sub.4    6 sccm   WF.sub.6    10 sccm   He          64 sccm______________________________________ 
    
     The ratio SiH 4  /WF 6  is 0.60. Deposition time is 45 min. The pressure inside the chamber is set at 0.2. Torr and the temperature is about 300° C. 
     The second step consists in depositing a Tungsten silicide layer. The flowrates become : 
     
         ______________________________________   SiH.sub.4   50 sccm   WF.sub.6    10 sccm   He          64 sccm______________________________________ 
    
     The ratio SiH 4  /WF 6  is 5, deposition time is 3 min, pressure inside the chamber remains at 0.2. Torr and temperature at 300° C. 
     Afterward, the Copper foil is dissolved in acids. 
     In this example, measured thickness at SEM is 65 microns about 60 microns for the almost pure Tungsten part, and about 5 microns for the protective Tungsten silicide layer. 65 microns are obtained in 48 min, which means a deposition rate of about 1.4. micron/min.