PROCESS FOR THE PREPARATION OF MAGNESIUM OXIDE FILMS USING ORGANOMAGNESIUM COMPOUNDS

Abstract: A process for coating the surface of a single crystal with a magnesium oxide film which comprises contacting an organomagnesium compound having an oxygen to magnesium atomic ratio of 1:1 with the crystal heated to a temperature ranging from 300 DEG C to 450 DEG C in the absence of oxygen. The magnesium oxide film thus produced has a negligible amount of residual carbon.

Description: PROCESS FOR THE PREPARATION OF MAGNESIUM OXIDE FILMS USING ORGANOMAGNESIUM COMPOUNDS Field of the Invention The present invention relates to a process for coating a substrate with a magnesium oxide film using an organo¬ magnesium compound having an oxygen to magnesium atomic ratio of 1:1. Background of the Invention Magnesium oxide is a transparent and chemically stable material having good electric insulation property and it does not undergo a phase transition even at a high temperature up to its melting temperature of 2852 °C. Magnesium oxide haε been used as a substrate for preparing thereon films of a number of inorganic compounds, i.e., various oxides such as cuprate-based high-T superconductors, lithium niobate, barium titanate, and nitrides such as gallium nitride, niobium nitride, and the like. Although large single crystals having uniform properties have been successfully grown for quartz, silicon, gallium arsenide(GaAs) , sapphire and the like, a process for preparing large single crystals of high-temperature superconductors has not yet been developed. Also, attempts to prepare a high-temperature superconducting film directly on the surface of quartz, silicon, gallium arsenide or sapphire crystal have not been successful. However, it is known that a high-temperature superconductor film having excellent property can be prepared via coating a magnesium oxide film on the surface of a single crystal substrate. For example, a copper oxide high-temperature superconductor may be deposited and grown on a magnesium oxide film coated on the surface of a silicon single crystal [D. K. Fork, F. A. Ponce, J. C. Tramontane, and T. H. Geballe, Applied Physics Letters. 58, 2294 (1991)]. It is also known that magnesium oxide can be used as a diffusion barrier which inhibits the reaction between silicon and barium titanate, and the resulting dielectric material may find use in semiconductor memory devices of the next generation. Hitherto, there have been reported a number of chemical vapor deposition(CVD) methods for the preparation of a magnesium oxide film at a relatively low temperature. For example, Kwak et al. reported that a crystalline film of magnesium oxide may be prepared on the surface of a silicon single crystal or quartz by heating bis(2,2,6,6- tetramethyl-3,5-heptanedionato)magnesium of formula 1 to 196 °C and carrying the vapor thereof in an argon stream containing oxygen to the substrate heated above 650 °C [B. S. Kwak, E. P. Boyd, K. Zhang, A Erbil, and B. ilkins, Applied Physics Letters, 54, 2542 (1989)]. Lu et al. disclosed that a crystalline film of magnesium oxide can be prepared on the surface of sapphire or strontium titanate at a temperature below 600 °C by carrying bis(2,2,6, 6-tetramethyl-3,5-heptanedionato)- magnesium in a helium or argon stream to the substrate and then adding oxygen thereto [Z. Lu, R. S. Feigelson, R. K. Route, S. A. DiCarolis, R. Hiskes, and R. D. Jacowitz, Journal of Crystal Growth, 128, 788 (1993)]. Using a plasma-assisted chemical vapor deposition method, Zhao and Suhr prepared crystalline magnesium oxide films on the surfaces of glass, quartz, silicon single crystal and stainless steel by carrying bis(2,2,6,6- tetramethyl-3,5-heptanedionato)magnesium heated at 200 °C in argon stream to the substrate heated above 400 °C and adding oxygen thereto [Y. . Zhao and H. Suhr, Applied Physics A. 54, 451 (1992)]. Maruyama et al., on the other hand, reported that a crystalline magnesium oxide film may be formed on the surface of glass, quartz or silicon single crystal by treating the substrate heated above 450 °C with an air stream containing magnesium 2-ethylhexanoate of formula 2 [T. Maruyama and J. Shionoya, Japanese Journal of Applied Physics, 29, L810 (1990)]. It was reported therein that a magnesium oxide film did not form when the carrier gas was nitrogen instead of air. formula 2 According to DeSisto and Henry, an amorphous magnesium oxide film was deposited on the surface of a silicon single crystal, quartz or sapphire by ultrasonic spraying of an aqueous or alcoholic solution of bis(2,4-pentanedionato)- magnesium of formula 3 to the substrate heated at 400-550 °C [W. J. DeSisto and R. L. Henry, Applied Physics Letters, 56, 2522 (1990); W. J. DeSisto and R. L. Henry, Journal of Crystal Growth, 109, 314 (1991)]. The amorphous magnesium oxide film thus obtained was subsequently converted to a crystalline form by annealing at 700 °C under an oxygen atmosphere. In the above-mentioned CVD methods, the use of oxygen iε essential for the formation of a magnesium oxide film. The magnesium compounds cited above have a magnesium to oxygen atomic ratio of 1:4. As the corresponding ratio in magnesium oxide is 1:1, the formation of a magnesium oxide film must be accompanied by the removal of three equivalent amount of oxygen together with all of the carbon and hydrogen atoms that constitute the organic moieties. It is not well understood at thiε time exactly how such organomagnesium compounds convert to form a magnesium oxide film in the presence of oxygen, while eliminating the extra oxygen as well as the carbon and hydrogen atoms. However, the magnesium oxide films produced by the prior art methodε tend to be contaminated by a significant amount of residual carbon, the residual carbon imparting undesirable effects to the property of the magnesium oxide film. By using the above-mentioned plasma-assisted CVD method of Zhao and Suhr, a crystalline magnesium oxide film containing little carbon may be obtained below 400 °C, the lowest temperature reported in the prior art methods. However, the plasma-aεεiεted CVD method requireε a high- power radiofrequency wave generator to produce a plasma as well as sophisticated techniques to generate a uniform plasma over the entire surface of the substrate. Moreover, the plasma CVD method haε a serious disadvantage in that the deposition of magnesium oxide occurε only on the εurface exposed to the plasma, in contrast to a thermal CVD procesε wherein film depoεition occurε on all surfaces of the substrate. Accordingly, the throughput of plasma-aεεisted CVD method is much lower than that of thermal CVD method, thuε leεs suitable for use in a large-scale production. Recently, Auld et al. have reported that a zinc oxide film containing little residual carbon can be coated on the εurface of glaεs heated at 250-400 °C by a chemical vapor deposition method using alkylzinc alkoxideε in the abεence of oxygen [J. Auld, D. J. Houlton, A. C. Jones, S. A. Rushworth, M. A. Malik, P. O'Brien, and G. . Critchlow, Journal of Materials Chemistry, 4, 1249 (1994)]. Thiε is in line with the results obtained by Aεhby et al. that zinc oxide or magneεium oxide iε obtained aε a by-product when alkylzinc alkoxide or alkylmagneεium alkoxide iε pyrolyzed, in accordance with the following reaction pathε [ E. C. Aεhby, G. F. illard, and A. B. Goel, Journal of Organic Chemiεtrv, 44, 1221 (1979)]. R 1 MgOC(CHR 2 R 3 )R 4 R 5 > RH + R 2 R 3 C=CR 4 R 5 + MgO RZnOC(CHR 2 R 3 )R A R 5 > R 1 H + R 2 R 3 C=CR 4 R 5 + ZnO A pyrolytic decomposition reaction iε propoεed to proceed via a uni olecular mechaniεm involving a six-membered ring tranεition εtate, as described below. Ashby et al. reported that εublimation alεo occurε aε the pyrolytic decompoεition proceedε when methylmagnesium t-butoxide is heated; Mg-0 / \ H 3 C C(CH 3 ): -> CH* + MgO + H 2 C=C(CH 3 ) 2 / Such alkylmagnesium alkoxide haε not been used in the prior art CVD method for the preparation of a magnesium oxide film, presumably because the reaction waε unknown to materialε εcientiεts of this field until J. Auld mentioned E.C. Ashby et al.'ε resultε in their paper and the alkylmagnesium alkoxides were not readily available. The present inventors nonetheleεε conεidered it attractive to use an alkylmagnesium alkoxide aε a precursor for a clean, pure magnesium oxide film depositable on a εingle cryεtal εubεtrate by a CVD method. Summary of the Invention Accordingly, it iε an object of the preεent invention to provide a new, εimple proceεs for the preparation of a magnesium oxide film coated on a εingle crystal substrate, εaid film having a negligible amount of residual carbon. It iε another object of thiε invention to provide a single cryεtal coated with a magneεium oxide film, said magnesium oxide film being characterized by a negligible residual carbon content. In accordance with one aspect of the present invention, there is provided a procesε for coating a substrate with a magnesium oxide film, which comprises contacting an organomagnesium compound having an oxygen to magnesium atomic ratio of 1:1 with said subεtrate heated to a temperature above 250 °C, preferably. Detailed Description of the Invention The organomagnesium compound suitable for use in the present invention contains magnesium and oxygen atoms in a ratio of 1:1, and is preferably sublimed at 150 °C or below. A particularly preferable organomagnesium compound in practicing the present invention is an alkylmagnesium alkoxide of the formula of R-Mg-OR' wherein R is a C._ A alkyl group and R' is a C 2 _ 8 alkyl group, more preferably, R iε methyl or ethyl and R' iε a C 2 _ 4 alkyl group, and most preferably, R iε methyl and R' is t-butyl group. Alkylmagneεium alkoxide derivatives suitable for use in the present invention may be synthesized according to the known methodε e.g., the method discloεed by Aεhby [E. C. Aεhby, J. Nackaεhi, and G. E. Parriε, Journal of American Chemical Society, 97, 3162 (1975); S. Gupta, S. Sharma, and A. K. Narula, Journal of Organometallic Chemiεtry, 452, 1 (1993)] . In accordance with one aεpect of the present invention, an organomagnesium compound may be sublimed at 60-150 °C and the vapor thereof may be contacted with the surface of a εubεtrate heated at a temperature above 250 °C, preferably at a temperature ranging from 300 to 450 °C to deposit a magneεium oxide film thereon. The temperature of the organomagnesium compound vapor is preferably maintained below 150 °C, preferably below 100 °C, before it comes in contact with the substrate. The pressure may be maintained in the range 10 _5 -10 ~4 mbar initially, but depending on the equipment used, the internal pressure may rise to a level which is 2-10 times the initial value as the magnesium oxide deposition reaction proceedε. A εubstrate which may be uεed in practicing the preεent invention iε any inorganic εolid that iε εtable above the film depoεition temperature, e.g., 350 °C, and exampleε thereof include glaεε, quartz, εilicon, gallium arεenide, εapphire, alkali metal niobate, alkaline earth metal titanate, gallium nitride, niobium nitride and the like, among which εingle cryεtalε of εilicon and gallium arεenide are preferred when the coated εubstrate iε intended for uεe in electronic applicationε. The magneεium oxide film depoεited on a εingle cryεtal εubstrate in accordance with the process of this invention iε characterized by its low residual carbon content; much lower than those found in magnesium oxide films prepared by prior art methods. This beneficial effect of the inventive proceεε iε likely due to the clean, facile nature of the alkylmagneεium alkoxide decompoεition reaction that occurε at a relatively low temperature. The depoεition of a magneεium oxide film is accompanied by the formation of well-defined by-products, an alkane and an olefin which are easily removable from the substrate and do not easily convert to a carbonaceous residue. The following Examples and Comparative Examples are provided for the purposeε of illuεtrating certain aεpectε of the present invention only; they are not to be construed as limiting the scope of the present invention in any way. In each of the Examples and Comparative Exampleε, the coated substrate obtained after the organomagnesium compound treatment was immediately transferred into an X-ray photoelectron spectroscope in order to minimize the exposure thereof to air. Example 1 Methylmagnesium t-butoxide was evaporated at 60 °C and decomposed on a Si(100) substrate at 400 °C without using a carrier gaε. An interference color of violet appeared on the εurface after 1.3 hourε, and at thiε point, the chemical vapor depoεition waε εtopped. The X-ray photoelectron(XP) spectrum of the deposited film showed peaks correεponding to oxygen and magneεium aε well aε carbon, but not the peakε for εilicon. Thiε obεervation εhowε that the εurface of the εilicon substrate is completely covered by a magnesium oxide film. The elemental composition of the film surface measured by comparing the photoelectron peak areas corresponded to an atomic ratio of Mg:0:C = 1.00:1.17:0.38. The X-ray diffraction pattern of the depoεited film εhowed peakε at 2θ = 36.96, 42.98, 62.36°, suggesting that the magnesium oxide film is crystalline. Example 2 Methylmagnesium t-butoxide was evaporated at 60 °C and decompoεed on a Si(100) εubεtrate at 350 °C for 18.5 hours. In this case, the peaks for silicon(the subεtrate) besideε thoεe for magnesium, oxygen and carbon were observed in the XP εpectrum of the depoεited film. The ratio of the elementε in the film surface was Mg:0:C:Si = 1.00:0.92:0.28:0.43. This result suggeεts that either the magnesium oxide film obtained in this example is very thin or the εubεtrate iε not completely coated by magneεium oxide. Example 3 Methylmagneεiu t-butoxide waε evaporated at 60 °C and decompoεed on a Si(100) substrate at 300 °C for 19 hourε. The peakε for εilicon beεideε those for magnesium, oxygen and carbon were obεerved in the XP εpectrum of the deposited film. The ratio of the elements in the film surface waε Mg:0:C:Si = 1.00:1.28:0.62:1.15. Example 4 Methylmagneεium t-butoxide waε evaporated at 60 °C and decompoεed on a GaAε(lOO) εubstrate at 400 °C for 5 hours. The peaks for gallium and arsenic were not observed in the XP εpectrum, which shows that the subεtrate iε completely covered by a magneεium oxide film. The meaεured ratio of the elementε in the film waε Mg:0:C = 1.00:1.12:0.38. Example 5 Methylmagneεium iεopropoxide was evaporated at 60 °C and decomposed on a Si(100) substrate at 400 °C for 3 hours. The ratio of the elements in the film εurface waε measured to be Mg:0:C:Si = 1.00:0.91:0.31:0.02. This result suggeεtε that the εurface of the εilicon εubstrate iε almoεt completely covered by a magneεium oxide film. Example 6 Ethylmagneεium t-butoxide was evaporated at 110 °C and decomposed on a Si(100) subεtrate at 400 °C for 3 hours. The peaks for silicon were not observed in the XP spectrum, which showε that the εurface of the εilicon εubεtrate iε completely covered by a magneεium oxide film. The ratio of the elementε in the film εurface waε measured to be Mg:0:C = 1.00:1.13:0.56. Comparative Example 1 Bis(2,2,6, 6-tetramethyl-3,5-heptanedionato)magnesium was evaporated at 90 °C and attempts were made to decompose it on a Si(100) substrate at 400 °C for 9 hourε. No viεual change of the εubεtrate εurface waε observed during the 9- hour period. Only a trace peak for magnesium waε observed in the XP spectrum (Si:Mg - 1.00:0.01), which shows that a magneεium oxide film is not formed under the conditionε of thiε Comparative Example. Comparative Example 2 Biε(2,2,6,6-tetramethyl-3,5-heptanedionato)magneεium waε evaporated at 120 °C and carried in an oxygen carrier gas to a Si(100) substrate heated to 500 °C for 2 hours. The peakε for silicon were not observed in the XP spectrum, which εhowε that the surface of the silicon substrate is completely covered by a magnesium oxide film. The ratio of the elements in the film was Mg:0:C = 1.00:1.70:0.69. Although the deposition of the magneεium oxide film was conducted at 500 °C using an oxygen carrier gas, the carbon content thereof waε much higher than those of the films of Examples 1-6 wherein the film depositionε were conducted at 300-400 °C in the abεence of oxygen. Comparative Example 3 A commercial magnesium oxide single crystal was analyzed with X-ray photoelectron εpectroεcopy in order to eεtabliεh a εtandard for compariεon with the magneεium oxide filmε of Exampleε 1-6 and Comparative Exampleε 1-2. The XP spectrum of the magneεium oxide single crystal as received exhibited a large carbon peak. When this magnesium oxide single cryεtal waε waεhed with a hydrochloric acid solution containing nitric acid and hydrogen peroxide, the carbon peak became much smaller, but did not totally disappear. The ratio of the elementε in the εurface of the cleaned cryεtal was Mg:0:C = 1.00:1.15:0.25. This ratio, instead of the theoretical ratio of 1:1:0, should be taken aε the repreεentative value for an eεsentially pure magnesium oxide. Based on this criterion, the filmε produced in Examples 1, 2 , 4 and 5 should be judged to be compoεed of eεsentially pure magneεium oxide having a low carbon content. Table 1 εummarizeε the experimental conditionε and the reεultε of Exampleε and Comparative Exampleε. Table 1. Preparation of the Magnesium Oxide Films in accordance with the Procedures of Examples and Comparative Examples In accordance with the present invention aε deεcribed above, a cryεtalline magnesium oxide film having a much reduced carbon content may be prepared in the abεence of oxygen at a temperature lower than those used in the prior art CVD methods. While the invention haε been described with reεpect to the specific embodiments, it should be recognized that various modifications and changeε may be made by thoεe skilled in the art to the invention which also fall within the scope of the invention as defined by the appended claims.

Claims: What is claimed is: 1. A process for coating a εubεtrate with a magnesium oxide film which comprises contacting an organomagnesium compound having an oxygen to magnesium atomic ratio of 1:1 with the substrate heated to a temperature above 250 °C. 2. The process of claim 1, wherein said subεtrate iε heated at a temperature ranging from 300 to 450 °C. 3. The proceεε of claim 1, wherein the organomagneεium compound iε εublimable at 150 °C or below. 4. The proceεε of claim 1, wherein the organo¬ magnesium compound iε an alkylmagnesium alkoxide of the formula R-Mg-OR', R being a C._ 4 alkyl group and R', a C 2 _ 8 alkyl group. 5. The process of claim 3, wherein the organomagnesium compound is an alkylmagnesium alkoxide of the formula R-Mg-OR', R being methyl or ethyl and R', a C 2 _ alkyl group. 6. The process of claim 5, wherein R is methyl and R' is t-butyl. 7. The procesε of claim 1, wherein the εubεtrate iε a εingle cryεtal of εilicon or gallium arεenide. 8. A εilicon εingle crystal coated with a magnesium oxide film prepared in accordance with the procesε of claim 1. 9. A gallium-arεenide εingle cryεtal coated with a magnesium oxide film prepared in accordance with the process of claim 1.

Inventors: KIM YUNSOO, KOH WONYONG, KU SU JIN, KIM, YUNSOO, KOH, WONYONG, KU, SU, JIN
Applicants: KIM YUNSOO, KOH WONYONG, KOREA RES INST CHEM TECH, KU SU JIN, KIM, YUNSOO, KOH, WONYONG, KOREA RESEARCH INSTITUTE OF CHEMICAL TECHNOLOGY, KU, SU, JIN
Application year: 1996
Main classification: C23C16
IPC codes: C01F5, C23C16, C30B25, C30B29