METHOD FOR FORMING A METAL COATING ON A SUBSTRATE

Abstract: A method for producing a uniform metal coating on a substrate consisting of (1) selecting one or more metals to be coated onto the substrate wherein the metals are soluble in a molten salt when heated, (2) preparing a bath consisting of the dissolved metal-molten salt, (3) inserting a substrate into the bath, and (4) decreasing the temperature at the substrate to a level at which the metal will precipitate out of solution onto the substrate. Multiple coatings can be made by repeating the method using the same or different metal-molten salt systems. Deposition can be enhanced by adding more salt to the bath to increase precipitation of the metal onto the substrate or by introducing additional metal into the bath by electrolysis, electrowinning or electrorefining. The metal coating can be patterned by localized heating of the metal coating in the metal-molten salt bath to reverse deposition or by masking areas on the substrate prior to deposition. The method can also be used to purify the metal being precipitated or to precipitate alloys or a metal containing impurities, such as doped GaAs, by controlling the composition and processing parameters of the metal-molten salt bath.

Description: METHOD FOR FORMING A METAL COATING ON A SUBSTRATE Background of the Invention This invention is generally in the field of metal coatings and is in particular a method for creating a metal coating on electrically conductive and non-conductive substrates. There are many instances in which it is de¬ sirable to form a metal coating on either a non- conductive substrate or a metal substrate. A number of methods have been used with a variety of ma¬ terials for this purpose. One commonly used tech¬ nique is electroplating. Others include chemical vapor deposition. Deposition technologies are reviewed in detail in Deposition Technologies for Films and Coatings by R. F. Bunshah et al. (Noyes Publications, Park Ridge, NJ 19-82) . Electroplating is a process generally used to coat an object with a thin layer of a metal. The object to be plated is charged as the cathode and is immersed in an electrolyte bath which, among other things, contains a salt of the metal being plated. The anode can be made of the same metal or some chemically unaffected conductor. A low-voltage current is passed through the bath to electrolyze it and plate the cathode article with the metal to the desired thickness. A firm bond between the metal being deposited and the article is best obtained when using two metals that tend to alloy, such as silver and copper. Disadvantages to electroplating include stringent substrate requirements, such as the type of metal that can be plated, the necessity for absolute cleanliness, and difficulties in plating uneven surfaces. Other disadvantages include the necessity of uniform temperature control and replenishment of the electrolyte bath. Chemical vapor deposition is frequently used to produce a very thin metal coating on a substrate, particularly a substrate formed of a non-conductive material such as silicon. In this method, the metal is vaporized using high temperatures (e.g. 800 to 900°C) and deposited onto the substrate. Unfortu¬ nately, at these high processing temperatures the metal coating diffuses into the underlying substrate at the same time as the deposit is forming. The result can be a diffuse rather than sharp interface between substrate and coating. Sharp interfaces improve device performance in most applications. In general, many of the methods in use are limited to conductive metal substrates or substrates that can tolerate very high temperatures. Some of these methods may also be hazardous due to vaporiza¬ tion of toxic chemicals. It is therefore an object of the present invention to provide a method for applying a thin metal coating to either an electrically conductive or non-conductive substrate. It is a further object of the invention to provide a safe method for coating a substrate with a uniform metal layer which does not require the use of either an electrical current nor excessively high temperatures. It is another object of the invention to provide a method for coating a substrate in such a way that there is little or no diffusion of the metal coating into the substrate. It is still another object of the invention to provide a method for coating a substrate with multiple metal layers. It is another object of the invention to provide a mathod for patterning a metal coating on a substrate. It is yet another object of the invention to provide a method for purifying a metal, selectively doping a pure metal, and depositing an alloy at a specified ratio. Summary of the Invention The preferred embodiment of the present inven¬ tion is a method for producing a uniform metal coating on a substrate consisting of (1) selecting a metal to be coated onto the substrate, where the metal is soluble in a molten salt when heated, (2) preparing a bath by dissolving the metal in its molten salt, (3) immersing one or more substrates into the metal-molten salt bath and (4) decreasing the temperature of the bath at the substrate so that the metal will precipitate out onto the substrate. The rate or extent of deposition using the dissolved method can be enhanced. For example, the rate of deposition is enhanced by the introduction SUBSTITUTE SHEET of additional metal into the dissolved metal-molten salt bath by electrolysis, electrowinning or electro- refining. In a second example, precipitation of the metal is enhanced by addition of other salts to the metal-molten salt solution. The method can also be used to deposit alloys of two or more metals onto a substrate, to purify the metal being deposited and to deposit a metal containing a specified concentration and composition of impurities. Bath composition and temperature are varied as required to promote deposition or to alter the stoichiometry of the metals and impurities. In general, the impurities will not be deposited at the same temperature and at the same concentrations as the metal to be purified. Since metal deposited under these conditions will not entrain molten salt, only the metal and the desired impurities, if any, are deposited. When the method is used to deposit an alloy or an element along with a dopant, such as silicon along with boron, onto the substrate, the amount of dopant to be added to the bath in addition to the metal to be deposited is determined empirically from the ratio of dopant to silicon on the substrate at a particular temperature. Multiple coatings can be formed, with the same or different metals by repeating the method, alter¬ ing the bath composition and conditions as required. The metal coating can be patterned by heating areas of the coating, in the metal-molten salt bath, to redissolve the metal or by masking areas to prevent deposition. For example, a metal, such as copper, is deposited onto a silica substrate. Patterns are "etched" into the copper layer by means of a laser which heats the substrate in a specified manner. At points heated by the laser, the metal goes back into solution in the molten salt, leaving a bare pattern on the substrate. Detailed Description of the Invention The present invention is a method for forming a metal coating on a substrate by exploiting the temperature dependence of metal solubility in a metal-molten salt solution. The metal to be de¬ posited is first dissolved in the metal-molten salt solution, then precipitated out of the molten salt bath onto a substrate by lowering the temperature of the bath at the place where it contacts the sub¬ strate. This can be accomplished by cooling the substrate, by cooling the bath or by cooling both the substrate and the bath. In reverse, a metal coating can be selectively removed by immersion of the coated substrate into a metal-molten salt bath and locally heating the areas at which the metal coating is to be removed. It is a well-known fact that metals dissolve in their molten salts, as well as in mixtures contain¬ ing their salts, without the passage of any current. Therefore, any metal that is soluble in a molten salt solution as a function of temperature can be used in the present invention. Examples of metal- molten salt solutions include Si in K λ-SiF o-, in MgCl_, and Cd in CdCl . The extent of dissolution and deposition are usually functions of the composi¬ tion and temperature of the bath. Usually, the higher the concentration of the salt of the metal to be deposited and the higher the temperature of the bath, the greater the solubility of the metal. Dissolution of the metal also occurs when current is passed through the bath during electro¬ plating, electrowinning, or electrorefining. Such chemically dissolved metal can impart a measure of electronic conductivity to the electrolyte allowing it to pass current without electrodepositing metal. This reduces the current efficiency of electrolysis and is referred to as a departure from Faraday's Law. The present invention is based on the observa¬ tion that raising the temperature of the molten salt solution increases the solubility of the metal in the molten salt, and lowering the temperature lowers metal solubility. Since the molten salt solution at a lower temperature is supersaturated with the metal dissolved at a higher temperature, the metal tends to come out of solution. The subsequent introduc¬ tion of a substrate into the supersaturated molten salt induces precipitation of the dissolved metal onto substrate. Since this involves the formation of critical size nuclei of the dissolved metal on the surface which grow to form a coating, the nature of the surface, i.e., crystal structure, roughness, temperature, etc., and the extent of saturation of SUBSTITUTE SHEET the molten salt with the dissolved metal determine the nature and extent of the coating. The extent of dissolution can be enhanced by increasing the salt concentration of the metal- molten salt solution. The solubility of a metal in a molten salt is at a maximum at any temperature when the melt consists of a pure salt of the given metal. Thus, it is possible to enhance metal rejection from a melt by adding other salts to change the bath composition after initial metal dissolution has occurred. The method of the present invention can also be used to purify a metal, to alter the stoichiometric ratio of constituent elements in an alloy or com- pound, and to control the dopant concentration in a material such as silicon by exploiting the differ¬ ential solubilities of various elements in molten salts. To purify a metal, the metal is dissolved in the molten salt. At any temperature there is a solubility limit. Conversely, for any concentration of dissolved metal there is a temperature below which the metal precipitates out of solution. Through judicial choice of elemental concentrations and temperatures it is possible to precipitate the impurities out before the precipitation of the metal or to maintain them in solution while the metal is precipitated. A series of discrete precipitations may be performed or a thermal gradient formed in the metal-molten salt bath with a substrate introduced SUBSTITUTE SHEET at the place where the optimum precipitation of a desired elemental constituent will occur. In a similar fashion, an alloy can be deposited at a desired stoichiometric ratio after first determining the requisite concentrations of elements dissolved in the bath for deposition of the metal alloy or compound at the specified molar ratio. The kinetic processes associated with precipitation on a substrate surface make it possible to deposit alloys and compounds in stoichiometric ratios difficult to attain otherwise. For example, the higher volatil¬ ity of the non-metal constituent in gallium arsenide and in mercury cadmium telluride results in non- stoichiometry when these compounds are processed from the vapor phase. The same method applies to a doped material such as silicon. A series of tests may be conducted on the metal-molten salt bath wherein the amount of metal or dopant deposited as a function of temperature and composition is deter- mined. It is then relatively•straightforward for one skilled in the art to extrapolate backward to determine what temperature and what ratio of metal- molten salt bath components are required for the desired precipitate composition. In another embodiment of this invention, multiple coatings of the same or different composi¬ tion are applied to the substrate by repeated processing in baths of the appropriate chemistry. A variety of coatings with respect to thickness and uniformity is also possible using variations in time of deposition, temperatures, bath compositions and SUBSTITUTE SHEET addition to the bath of salt or metal through electrolysis. In another embodiment of the method, the metal coating is continuously applied to one or more substrates by setting up a metal-molten salt bath and then forming and maintaining a thermal gradient such that at one end of the gradient metal is continuously dissolved into the metal-molten salt bath and precipitated out of solution onto the substrate at the other end of the gradient. In a further embodiment of the method, a metal coating is applied to a substrate and then "pat¬ terned". The pattern is developed by heating selected portions of the metal coating on the substrate within the metal-molten salt bath. The heated metal dissolves back into the molten salt. Using a laser, the amount of material which is removed can be controlled and a very fine pattern etched into the metal. The metal coating can also be deposited in a pattern by masking the substrate in the areas not to be coated prior to placing the substrate in the metal-molten salt bath. This invention is further illustrated by the following non-limiting examples demonstrating the deposition of a metal coating on both conductive and non-conductive substrates. Example 1; Formation of an Mn Coating on an Iron Substrate A molten salt bath containing 50 mole percent MnCl -50 mole percent NaCl was prepared by melting SUBSTITUTE SHEET the MnCl_-NaCl in an iron crucible. A Mn rod was lowered into the crucible at 810°C. The temperature was increased to 900 β C. After three hours, the Mn rod was pulled out of the salt. Approximately 20 grams NaCl was added to the melt to decrease Mn solubility and promote precipitation of the Mn from the molten salt solution. The temperature was lowered to 800"C and maintained at that temperature for two hours. The crucible was then cooled to room temperature. An Mn coating was formed uniformly on the surface of the crucible, as determined by energy dispersive analysis by X-ray (EDAX) . Example 2: Formation of an Mn Coating on a Copper Wire Substrate A molten salt bath containing 20 mole percent MnCl -80 mole percent NaCl was prepared in a fused quartz crucible. As in Example 1, an Mn rod was lowered into the crucible, the crucible heated to 810°C, the temperature raised to 870 β C for 20 minutes, and the rod removed. A copper wire, l/8th inch in diameter, was lowered into the salt bath. The salt bath was maintained at 744 β C overnight. The copper wire was then removed from the bath and analyzed by EDAX. Analysis confirmed a uniform deposition of Mn on the wire. SUBSTITUTE SHEET Example 3 : Formation of a Manganese Coating on a Fused Quartz Crucible A 40 mole percent MnCl -60 mole percent NaCl bath was prepared using a procedure similar to those in Examples 1 and 2. A Mn rod was lowered into the salt bath, the temperature of the bath raised to approximately 900"C, the Mn rod withdrawn after 90 minutes and the bath cooled to room temperature. As before, additional NaCl was added to the bath after withdrawal of the Mn rod to decrease the solubility of the Mn in the molten salt. A Mn coating was uniformly formed on the surface of the fused quartz crucible, as determined by EDAX. The coating had a very low electrical resistance, as measured using a digital multimeter. The uncoated side of the crucible had infinite resistance, indicating no deposition of the metallic coating. Example 4 ; Formation of a Molybdenum Coating on a Fused Quartz Surface by Chemical Deposition in the Presence of Electro- deposition A bath containing KCl-K_MoCl_ (93 wt. percent KC1- wt. percent K-MoClg) was heated to 800"C in a graphite crucible. Electrodeposition of Mo was conducted in the crucible using a Mo rod as the anode and a graphite rod as the cathode. A fused quartz bell was placed around the graphite rod cathode. SUBSTITUTE SHEET Following electrolysis, a Mo coating was formed on the inside of the fused quartz bell, on the side facing the cathode. The coating was analyzed by EDAX and confirmed as elemental molybdenum. It is important to note that the fused quartz bell itself was not polarized and could not act as an electrode. The current efficiency was well below 100%. This indicates that non-Faradaic processes were occurring during the electrolysis, and that the Mo coating was produced by chemical, not electro¬ chemical, processes. Although this invention has been described with reference to its preferred embodiments, other variations and modifications will be apparent to those skilled in the art and it is intended to include all such variations and modifications within the scope of the appended claims. We Claim:

Claims: Clai s 1. A method for producing a metal coating on a substrate comprising: a) selecting a first metal to be coated onto the substrate wherein said metal is soluble in a molten salt when heated to a first temperature, b) heating a bath of said metal and molten salt to the first temperature to dissolve said metal, c) placing the substrate to be coated in the bath, and d) decreasing the temperature of the bath at the substrate to a second temperature, wherein the dissolved metal precipitates out of the molten salt onto the substrate at the second temperature. 2. The method of Claim 1 further comprising selecting the substrate from the group con- sisting of electrically conducting materials and electrically insulating materials. 3. The method of claim 1 further comprising selecting the metal and molten salt from the group consisting of Mn-MnCl /NaCl, Ag-AgCl/NaCl, Mo-K 3 MoCl 6 /KCl, Mg-MgCl 2 , Ca-CaCl 2 , Al-Na 3 AlF 6 , Na-NaCl, and Si-K 2 SiF 6 /KF. 4. The method of claim 1 further comprising enhancing precipitation of the dissolved metal out of the molten salt onto the substrate by adding more of said first metal to the bath. 5. The method of claim 4 wherein additional metal is added to the bath by passage of an electric current through said metal in the bath. 6. The method of claim 1 further comprising altering the salt composition after heating the first metal-molten salt solution to the first temperature. 7. The method of claim 6 further comprising cooling said first metal-molten salt solution to the second temperature. 8. The method of claim 7 further comprising altering the salt composition to favor exsolu- tion of said first dissolved metal. 9. The method of claim 1 further comprising selecting a second metal to be coated onto the substrate, wherein said first and second metals are soluble in a molten salt mixture. 10. The method of claim 1 further comprising repeating steps a-c to form additional coatings on said precipitated metal coating. 11. The method of claim 10 further comprising changing the bath chemistries and processing parameters to create a multilayer coating of discrete compositions. 12. The method of claim 1 further comprising removing portions of said metal coating by heating said substrate in the metal-molten salt bath at the sites where the metal coating is to be removed. 13. The method of claim 12 further wherein said substrate is heated using a laser to selec¬ tively redissolve portions of said metal coating. 14. The method of claim 1 further comprising masking areas of the substrate prior to precipi¬ tation of the metal onto the substrate. 15. The method of claim 1 for purifying a metal wherein the temperature of the bath is de¬ creased to a temperature at which the pure metal precipitates out of the molten salt solution and the impurities remain in solution. 16. The method of claim 1 for purifying a metal wherein the temperature of the bath is de¬ creased to a temperature. at which pure metal is in solution and the impurities precipitate out of the molten salt solution. 17. The method of claim 1 further comprising selecting a second metal, wherein said first and second metals are soluble in the molten salt when heated to a first temperature, adding said second metal to the molten salt and decreasing the temperature of the molten- salt bath to precipitate said first and second metals. 18. The method of claim 17 further comprising adding said first metal to the molten salt bath at a ratio of said first metal to said second metal so said first and second metals co- precipitate. 1 . The method of claim 1 further comprising adding impurities to the metal-molten salt bath, wherein said impurities precipitate out of the solution with the metal. • 20. The method of claim 1 further comprising maintaining a thermal gradient in the metal- molten salt bath. 21. The method of claim 20 further comprising continuously replenishing the metal-molten salt bath with dissolved metal. 22. The method of claim 21 further comprising continuously precipitating said metal onto substrate positioned appropriately within the thermal gradient. 23. A metal coating formed by: a) selecting a first metal to be coated onto the substrate wherein said metal is soluble in a molten salt when heated to a first temperature, b) heating a bath of said metal and said molten salt to the first temperature to dissolve said metal, c) placing the substrate to be coated in the bath, and d) decreasing the temperature of the bath at the substrate to a second temperature, wherein the dissolved metal precipitates out of the molten salt onto the substrate at the second temperature. 24. A patterned metal coating formed by removing portions of said metal coating by heating said substrate in the metal-molten salt bath at the sites where the metal coating is to be removed. 25. A multilayer metal coating formed by: a) selecting a first metal to be coated onto the substrate wherein said metal is soluble in a molten salt when heated to a first temperature, b) heating a bath of said metal and molten salt to the first temperature to dissolve said metal, c) placing the substrate to be coated iri the bath, d) decreasing the temperature of the bath at the substrate to a second temperature, wherein the dissolved metal precipitates out of the molten salt onto the substrate at the second temperature, and e) repeating steps a-c to form additional coatings on said precipitated metal coating.

Inventors: SADOWAY DONALD R, SINGH RANA PARTAP, SADOWAY, DONALD, R., SINGH, RANA, PARTAP
Applicants: MASSACHUSETTS INST TECHNOLOGY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Application year: 1987
Main classification: C23C2
IPC codes: C23C2, C25D3, H05K3