A rapid thermal process for obtaining silica coatings

Abstract: The present invention is based on the discovery that rapid thermal processing (RTP) can convert hydrogen silsesquioxane resin coatings to ceramic silica coatings. This technique is especially valuable for the application of protective and dielectric layers on electronic devices.

Description: The present invention is based on the discovery that rapid thermal processing (RTP) can convert hydrogen silsesquioxane resin coatings to ceramic silica coatings. This technique is especially valuable for forming protective and dielectric silica layers on electronic devices. The use of thin film ceramic silica coatings as protective and dielectric layers for electronic devices is known in the art. For instance, U.S. Patent No. 4,756,977 describes the use of hydrogen silsesquioxane resin derived silica coatings on various substrates including electronic devices. This patent teaches that such substrates may be coated by applying a hydrogen silsesquioxane resin solution to the substrate, evaporating the solvent to deposit a preceramic coating and then heating to temperatures of 150-1000°C. in a conventional furnace. The coatings formed thereby provide excellent environmental protection to the underlying substrate and effectively inhibit electrical conduction. In spite of their efficacy, these conventional methods require the use of a high thermal budget (temperature x time) for the oxidation and densification of hydrogen silsesquioxane resin. Such high thermal budgets are not acceptable in many temperature sensitive applications as they may damage or destroy the substrate. This is especially true in the electronic industry where recent advances to reduce device dimensions and increase the number of circuits per wafer put severe constraints on the thermal processing to which a wafer may be subjected. In an attempt to decrease the thermal budget for hydrogen silsesquioxane resin, researchers have developed various low temperature conversion methods, by which hydrogen silsesquioxane is converted to silica at temperatures as low as 40°C. Rapid thermal processing (RTP) is a technique for decreasing the thermal budget of a substrate by decreasing the time the substrate is at the elevated temperature. This technique uses high intensity radiation to rapidly heat (50-300°C./sec) the thin coatings to an elevated temperature for a time which allows the desired physical or chemical processes to be completed but not allow the substrate to be adversely affected. As such, this technique has found applications in the electronics industry including, for example, improving the crystalline quality of semiconductors, annealing ion-implanted single-crystal silicon, annealing ion-implanted polycrystalline silicon, annealing ion-implanted compound semiconductors, annealing metal-semiconductor contacts, forming silicides, forming and annealing dielectrics and reflowing passivating glasses. Singh, in J. Appl. Phys. 63 (8), 15 April 1988, pages R59-114, provides a thorough review of rapid isothermal processing (a synonym for RTP), its mechanism and various applications. Included therein, is its use in the formation of thermally grown SiO₂ layers. This reference, however, fails to describe hydrogen silsesquioxane resin as the source of silica. Similarly, Lassig et al. of Peak Systems, Inc. have described Curing of Spin-on Glass Films by Rapid Thermal Anneal. S.E. Lassig, T. DeBolske and J.L. Crowley, Peak Systems Inc., 3550 W. Warren Ave., Fremont, CA 94538 and K.G. Lubic, T.A. Baldwin, and S.K. Gupta, Allied-Signal, Inc., Planarization and Diffusion Products, 1090 S. Milpitas Blvd. Milpitas, CA 95035, pages 1-9, the conversion of various spin-on glass films to SiO₂ coatings by the use of RTP. As with the Singh reference, Lassig et al. fail to describe the use of hydrogen silsesquioxane resin as the source of silica. EP-A-0270263 describes the application to electronic devices of preceramic mixtures comprising organic solvents, metal oxide precursors such as aluminium, titanium and zirconium alkoxides and hydrogen silsesquioxane resin (H-resin). The preceramic coating is then ceramified by heating to a temperature between 200 and 1000°C to produce a ceramic or ceramic-like coating containing silicon dioxide and metal oxide. One or more additional such ceramic coatings may be formed on the initial ceramic coating to provide the required protective or electrical property. EP-A-0386969, published after the priority date of the present application but of earlier priority, describes protective coatings for copper oxide superconductors. The coatings are formed by applying solutions of silica precursor to the superconductor. The silica precursor is chosen to form continuous film and the organic solvent is chosen to volatilize at temperatures below 400°C and also below any temperature at which the solvent would significantly react with the cuprate superconductor. In the coating method, the solvent is evaporated off the coated superconductor and the resulting coating is pyrolyzed to silica in an atmosphere containing oxygen at elevated temperatures but below 400°C, whereby a continuous silica coating is formed. Precursor materials such as hydrogen silsesquioxide, ethyl polysilicate and C-resin are described. The inventors herein have now unexpectedly found that RTP can be used to rapidly convert hydrogen silsesquioxane resin to silica coatings with desirable characteristics such as high density and low film stress. This invention relates to a method of applying a silica coating to a substrate with a solution comprising hydrogen silsesquioxane resin and a solvent. The solvent is evaporated to deposit a hydrogen silsesquioxane resin coating. The coated substrate is then exposed to high intensity radiation comprising incoherent light in a spectral region selected from the group consisting of visible, infrared, ultraviolet and combinations thereof, in order to heat said coated substrate sufficiently to facilitate conversion of said hydrogen silsesquioxane resin coating to a silica coating. This invention is based on the discovery that rapid thermal processing (RTP) can be used to rapidly convert hydrogen silsesquioxane resin (H-resin) coatings to silica coatings at a low thermal budget. This rapid conversion was particularly unexpected, especially since the art teaches that extended periods of time (i.e., generally >1 hour) are required for the conversion of Si-H bonds to Si-OH and the condensation of this material to silica. Moreover, nothing in the art suggests that RTP could reduce the Si-H oxidation time to the order of seconds or minutes. The fundamental concept behind RTP is that radiative heating and cooling of coatings on thermally isolated substrates allows for rapid and uniform temperature changes such that the thermal budget is decreased to a point where desired reactions occur in the coating without undesired harmful reactions occurring in the substrate. To achieve this, the substrate and coatings are heated to and maintained at a desired temperature with high intensity radiation comprising incoherent light in the visible, infrared and/or ultraviolet region (wavelength about 0.1 to 4 micrometers). The high intensity radiation is discontinued after the desired heating time has elapsed and the coated substrate is allowed to cool by radiation into the environment. The above heating and cooling occur in a controlled environment (temperature, pressure and atmosphere) wherein the substrate is thermally isolated by a means which has a low thermal mass and very low thermal conductivity. It has now been found that this concept can be used for the application of H-resin-derived silica coatings. In this process, a high intensity radiation source is used to heat a preceramic H-resin coating (at 30 to 500°C per second) to a temperature sufficient for silica conversion (above 200°C), and to maintain the temperature for a time sufficient for silica conversion (greater than about 1 second) yet insufficient to damage the substrate. The radiation is then discontinued and the coated substrate allowed to cool. By this method, a planarizing, crack-free silica coating is rapidly formed without adversely affecting the substrate. Moreover, since a controlled environment is used, reactive gases such as oxygen, wet or dry ammonia or amines may be introduced to assist in silica conversion. The invention provides a method of applying a silica coating to a substrate other than a cuprate substrate comprising: coating said substrate with a solution comprising hydrogen silsesquioxane resin (H-resin) and a solvent; evaporating said solvent to deposit an H-resin coating on said substrate; and converting said H-resin coating to a silica coating, characterised in that said substrate coated with said H-resin coating is subjected to rapid thermal processing (RTP) by means of high density radiation comprising incoherent light in a spectral region selected from visible, infrared and ultraviolet light regions, and combinations thereof, whereby said H-resin is rapidly heated at 30-500°C/sec to an elevated temperature from above 200°C to 1000°C for a period of time from more than 1 second to not more than 1 hour sufficient to convert said H-resin coating to a silica coating, said substrate being thermally isolated during said heating step whereby damage to said substrate is avoided. The rapid thermal process also has numerous other advantages over conventional furnace pyrolysis. For instance, 1) the process is relatively inexpensive. The cost of the RTP systems described herein can be modest compared with other more conventional pyrolysis equipment. 2) The process is relatively energy efficient. This effect results from the relatively efficient conversion of electricity to high intensity radiation and the direct coupling of the thermal energy to the substrate. 3) The process provides a means for high throughput of substrates. This advantage results from the short time necessary for processing and, therefore, allows for the adaptation of this process to mass manufacturing. Likewise, the coatings produced by this technology have many desirable characteristics over conventional furnace fired coatings. 1) The amount of film stress produced by this process is significantly reduced over that of conventional furnace processing. As discussed herein, this effect results from the coating heating from the substrate outward, thereby allowing volatiles to escape. 2) Electronic devices coated in this manner have a) minimal bond strength loss at the bond welds (bond pull), b) decreased development of Hillock growths and c) decreased formation of intermetallics at the bond welds (purple plague). 3) The performance of electronic devices coated in this manner is not altered. Conventional processing with high thermal budgets, on the other hand, can adversely effect device function. 4) The coatings produced using RTP are generally more dense than those generated using conventional techniques. As such, the coatings provide superior protection and dielectric effects. Because of the above processing and product-related advantages, the methods of this invention are particularly valuable for the deposition of protective or dielectric coatings on electronic devices, electronic circuits or plastics including, for example, polyimides, epoxides, polytetrafluoroethylene and copolymers thereof, polycarbonates, acrylics and polyesters. However, the choice of substrates and devices to be coated by the instant invention is limited only by the need for thermal and chemical stability of the substrate under the conditions used for RTP. The coatings taught herein may also serve as interlevel dielectric layers, doped dielectric layers to produce transistor-like devices and pigment-loaded binder systems containing silicon to produce devices such as, for example, capacitor and capacitor-like devices, multilayer devices, 3-D devices, silicon-on-insulator devices, and, super-lattice devices. As used in the present invention, the expression "silica" (SiO₂) includes both amorphous silica and amorphous silica-like materials which are not fully free of residual carbon, silanol (Si-OH) and/or hydrogen. In addition, the "silica" may contain up to 30 weight percent of a modifying ceramic oxide as described infra . Such silica or silica-like materials may have variable densities depending on the temperature and time used for silica conversion; the expressions "hydrogen silsesquioxane resin" or "H-resin" are meant to include those hydridosilane resins which are fully condensed (HSiO 3/2 ) n as well as those which are only partially hydrolyzed and/or partially condensed and, thereby, may contain residual SiOR and/or SiOH substituents; and the expressions "electronic device" or "electronic circuit" include, but are not limited to, silicon-based devices, gallium arsenide-based devices, focal plane arrays, opto-electronic devices, photovoltaic cells and optical devices. As defined above, the hydrogen silsesquioxane resin used in this invention includes those which are fully condensed as well as those which are only partially hydrolyzed and/or condensed. Exemplary of fully condensed H-resins are those formed by the process of Frye et al. in U.S. Patent No. 3,615,272. This polymeric material has units of the formula (HSiO 3/2 ) n in which n is generally 8-1000. The resin has a number average molecular weight of from about 800-2900 and a weight average molecular weight of between about 8000-28,000. When heated sufficiently, this material yields a ceramic silica coating essentially free of SiH bonds. Though this material is described as fully condensed, recent analysis has confirmed that up to 100-200 ppm silanol may be present. Exemplary H-resins which may not be fully condensed (polymers containing units of the formula HSi(OH) a O (3-a)/2 wherein a = 0-2) are those of Bank et al. in U.S. Patent No. 5,010,159 or those of Frye et al. in U.S. Patent No. 4,999,397. Bank et al. describe a process which comprises hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin, which is then contacted with a neutralizing agent. Recent experimentation has shown that an especially preferred H-resin which forms substantially crack-free coatings may be prepared by this method in which the acid/silane ratio is greater than about 2.67:1, preferably about 6/1. Frye et al. describe a process which comprises hydrolyzing trichlorosilane in a non-sulfur containing polar organic solvent by the addition of water or HCl and a metal oxide. The metal oxide therein acts as a HCl scavenger and, thereby, serves as a continuous source of water. Exemplary H-resin which is not fully hydrolyzed or condensed is that of Baney et al. in EP-A2 0443760. This latter European application describes soluble polymers having units of the formula HSi(OH) x (OR) y O z /2, in which each R is independently an organic group which, when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent, x = 0-2, y = 0-2, z = 1-3, x + y + z = 3 and the average value of y over all of the units of the polymer is greater than 0. Examples of R groups in the above equation include alkyls of 1-6 carbon atoms such as methyl, ethyl, propyl etc., aryls such as phenyl and alkenyls such as vinyl. As described in the European application (EP-A2 0443760), these resins may be formed by a process which comprises hydrolyzing a hydrocarbonoxy hydridosilane with water in an acidified oxygen-containing polar organic solvent. The above H-resin is dissolved in a solvent to form a solution for application. Various facilitating measures such as stirring and/or heating may be used in the dissolution so as more effectively to dissolve the H-resin and to create a more uniform solution. The solvent can be any agent or mixture of agents which will dissolve the H-resin to form a homogenous solution without affecting the ceramic coating. These solvents can include, for example, alcohols such as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanes such as n-heptane or dodecane, ketones, esters, glycol ethers or cyclic dimethylsiloxanes, in an amount sufficient to dissolve the above materials to low solids. Generally, enough of the above solvent is used to form a 0.1-50 weight percent solution. Besides the H-resin, the coating solution may also include a modifying ceramic oxide precursor. The modifying ceramic oxide precursors that can be used herein include compounds of various metals such as aluminum, titanium, zirconium, tantalum, niobium and/or vanadium as well as various non-metallic compounds such as those of boron or phosphorous. The expression "modifying ceramic oxide precursor", therefore, includes such metal and non-metal compounds having one or more hydrolyzable groups bonded to the above metal or non-metal. Examples of hydrolyzable groups include, but are not limited to, alkoxy, such as methoxy, ethoxy, propoxy etc., acyloxy, such as acetoxy or other organic groups bonded to said metal or non-metal through an oxygen. These compositions must form soluble solutions when mixed with the silica precursors and must be capable of being hydrolyzed, and subsequently pyrolyzed, at relatively low temperatures and relatively rapid reaction rates to form modifying ceramic oxide coatings. When such a modifying ceramic oxide precursor is used, it is generally present in the preceramic mixture in an amount such that the final ceramic coating contains 0.1 to 30 percent by weight modifying ceramic oxide. Examples of modifying ceramic oxide precursors include tetra n-propoxy zirconium, tetraisobutoxy titanium, aluminum trispentanedionate, pentaethoxy tantalum, tripropoxy vanadium, pentaethoxy niobium, zirconium pentanedionate and titanium dibutoxy bispentanedionate. If modifying ceramic oxide precursors are to be included in an H-resin preceramic solution, they may be simply dissolved in the solution comprising the H-resin and the solvent and allowed to stand at room temperature for a time sufficient to allow the modifying ceramic oxide precursor to react into the structure of the H-resin. Generally, a period of more than about 2 hours is necessary for said reaction to occur. The solution may then be applied to the substrate as discussed infra . Alternatively, the modifying ceramic oxide precursor may be hydrolyzed or partially hydrolyzed, dissolved in the solution comprising the solvent and H-resin and then immediately applied to the substrate. Various facilitating measures such as stirring or agitation may be used as necessary to produce said solutions. A platinum or rhodium catalyst may optionally be included in the solution to increase the rate and extent of silica formation. Generally, any platinum or rhodium compound or complex that can be solubilized in this solution will be functional. For instance, an organoplatinum composition such as platinum acetylacetonate or rhodium catalyst RhCl₃[S(CH₂CH₂CH₂CH₃)₂]₃, obtained from Dow Corning Corporation, Midland, Mich. is within the scope of this invention. These catalysts are generally added to the solution in an amount of from about 5 to 500 ppm platinum or rhodium based on the weight of H-resin in solution. The solution comprising the H-resin, solvent and, optionally, a modifying ceramic oxide precursor and/or a platinum or rhodium catalyst is then coated onto the substrate. The method of coating can be, but is not limited to, spin coating, dip coating, spray coating or flow coating. Other equivalent means, however, are also deemed to be within the scope of this invention. The solvent is allowed to evaporate from the coated substrate to result in the deposition of a preceramic coating. Any suitable means of evaporation may be used such as simple air drying by exposure to an ambient environment or by the application of a vacuum or mild heat. It is to be noted that, when spin coating is used, an additional drying period is generally not necessary as the spinning drives off the solvent. The above coating is then converted to silica using RTP. Though numerous RTP systems are commercially available and functional herein, generally they comprise five basic features: 1) one or more heat sources which cause the coating to heat quickly and uniformly, 2) a means for holding the sample which has a low thermal mass and very low thermal conductivity so that temperature gradients are not produced across the wafer, 3) a means for precisely monitoring temperature, 4) a mechanism for transporting the sample in and out of the processing system, and 5) a controlled environment for pyrolysis and cooling. The heat sources used in RTP systems emit radiation in a wavelength of from about 0.1 to about 4.0 micrometers and, thus, cover the ultraviolet, infrared and visible light regions. This radiation should be of sufficient intensity to rapidly heat the coatings, preferably at a rate of at least about 30-500°C./sec. The usual source of such radiation is generally a tungsten halogen lamp, an arc lamp or a graphite heater , but other sources which function in an equivalent manner are also deemed to be within the scope of this invention. Each of these sources has its own set of advantages/disadvantages which the user must choose between. Of these, however, the wavelength of radiation appears to be most critical. It has been found that sources with a wavelength below about 1.4 micrometers couple most efficiently with silicon, and, therefore, have a high conversion of input power into usable radiation. Heat sources with wavelengths above 1.4 micrometers (more infrared absorption - i.e., closer to conventional furnace heating), on the other hand, are less efficient since they heat predominantly by free carrier absorption. The above differentiation in wavelengths is important from a practical standpoint since sources which couple efficiently with silicon tend to heat the coating from the substrate outwards, thereby allowing volatiles to escape and the film stress to be lowered. Heat sources which rely on free carrier absorption, on the other hand, heat from the surface of the coating inwards, trapping volatiles and increasing film stress. Since film stress appears to play a major role in determining the efficacy of coatings on electronic devices, this factor should be carefully considered. Tungsten lamps have been used for many years in numerous lighting and heating applications. They emit radiation in the 0.4 to 4 micrometer range and have a peak intensity between 0.5 and 1 micrometer. Since these lamps are generally rated at only a few kilowatts each, an array of lamps is usually necessary to uniformly heat an entire wafer at the desired rate. These lamps are especially advantageous in that they are reliable and relatively inexpensive to purchase and operate. Arc lamps are also a popular heat source. They generally have a spectral radiation in the range of less than about 1 micrometer and, therefore, more directly couple with silicon (about 70%). They have a microsecond response because of the low thermal mass and, therefore, have faster ramp-ups (the rate at which the coating is heated). Since these lamps are generally rated at about 35-100 kilowatts each, only one lamp is generally required and a wider steady state temperature range is possible. Arc lamps are, however, generally more expensive and less reliable than tungsten lamps. A graphite heating element can also be used as a heat source for RTP. In a typical graphite system, the heating element is usually preheated to between about 1000 and about 1400°C. (in a vacuum to prevent oxidation of the heater) and the radiation therefrom is intensified by the use of a series of reflectors. The coated substrate is then placed in the pyrolysis chamber which is separated from the heat source by a shutter means. The shutter is opened to allow rapid exposure to the radiation and closed to allow cooling. The radiation from this source is primarily in the infrared region and, therefore, the coupling of radiation is primarily by free carrier absorption. The means for holding the sample is important to ensure uniformity of heating and cooling. If non-uniformity in temperature occurs, the coating and/or underlying substrate may be adversely affected. As such, the holding means should be of low thermal mass to prevent generating thermal gradients and even lower thermal conductivity to inhibit conducting heat away from the substrate. Quartz devices such as pins or holders are generally used for this purpose but any equivalent means would also function herein. A means of reliably measuring temperature is critically important to RTP, especially since small effects on the thermal budget may adversely affect the substrate. Factors such as the rapid temperature changes and the wide variety of steady state temperatures can make such measurements difficult. Generally, most systems use a pyrometer (infrared or optical), a thermocouple or both. Other equivalent methods, however, would function as well. The pyrometer or thermocouple is generally used in a closed loop system in which a computer receives measurements and automatically controls radiation levels, wafer temperatures, time at temperature and ramp rates. The mechanism for transporting the sample into and out of the system can be any practical means. For example, a manual system of placing the substrate on the holding means or an automated system such as a conventional conveyor system or a robotic device may be used herein. Such transporting means should not introduce contamination or detrimentally affect the coatings. The RTP heating and cooling is generally conducted in a controlled environment process chamber which allows for optimized, uniform and reproducible processing. For instance, the temperature of the pyrolysis chamber walls must be carefully regulated to prevent thermal gradients in the wafers,and to avoid a gradual rise in temperature as a series of wafers are heated. Moreover, the process gas (i.e., oxygen, ammonia etc.) and/or atmospheric pressure must be optimized and maintained in the chamber to ensure effective silica formation. In addition, however, the controlled environment also prevents the substrate from being contaminated and, thereby, avoids undesirable coatings. In a standard procedure, the coated substrate is introduced into the pyrolysis chamber, the incoherent light source is applied in such manner that the temperature of the coating is rapidly ramped up to that desired, the temperature is maintained for a given period of time, the source of radiation is discontinued, the coated substrate is rapidly cooled by radiation and the coated substrate is removed from the pyrolysis chamber. It is also contemplated that RTP can be used as a single step in a coating forming process. For instance, it is contemplated herein that the hydrogen silsesquioxane resin coating may be first hydrolyzed using, for example, a wet ammonia environment and subsequently heated using the RTP process. The temperature to be used in the above process is one which is effective for the conversion of H-resin to silica. Generally, higher temperatures will provide faster results, but the duration of exposure to such temperatures must be carefully monitored to prevent a thermal budget which alters the substrate. Temperatures in the range of from about 50 to about 1000°C. are usually used, and temperatures in the range of from about 300 to about 900°C. are preferred. The time for exposure to the above temperature is one which is sufficient for silica conversion yet insufficient adversely to affect the substrate. As stated above, higher temperatures usually require shorter exposures. Generally, exposure times in the range of from about 1 second up to about 1 hour are adequate for the above temperature range, and temperatures in the range of from about 1 second up to about 15 minutes are preferred. The process gas and atmospheric pressure used in the process chamber are not critical and can correspond to any environment which is conductive to silica formation. Pressures from a vacuum up to superatmospheric pressure are functional herein and reactive gases such as air, oxygen, ammonia, amines etc. may all be used. Although some oxygen is generally necessary for the conversion of H-resin to silica, heating in an oxygen-free environment usually results in the formation of a ceramic coating of undetermined structure which also possesses desirable characteristics. By the above methods, a thin (less than 2 micrometers) ceramic planarizing coating is produced on the substrate. The coating smooths the irregular surfaces of various substrates, has a low defect density, excellent adhesion, high density and minimal stress. As such, the coatings are useful on electronic devices as protective coatings, as corrosion resistant and abrasion resistant coatings, as temperature and moisture resistant coatings, as dielectric layers in, for instance, multilayer devices and as diffusion barriers against ionic impurities such as sodium and chloride. In addition, the planarizing silica coating may be covered by other coatings such as further SiO₂ coatings, SiO₂/modifying ceramic oxide layers, silicon-containing coatings, silicon/carbon-containing coatings, silicon/ nitrogen-containing coatings, diamond-like carbon coatings and/or silicon/nitrogen/carbon-containing coatings. The following non-limiting examples are included so that one skilled in the art may more readily understand the invention. Example 1 Hydrogen silsesquioxane resin made by the method of Collins et al. in U.S. Patent No. 3,615,272 was diluted to 10 weight percent solids in a cyclic dimethylsiloxane solvent. A platinum catalyst comprising platinum acetylacetonate in toluene was added to the solution at a concentration of approximately 100 ppm platinum based on the weight of H-resin. Approximately 100 microliters of the above H-resin solution were applied to the surface of approximately 25mm (1 inch) square silicon wafers, and the wafers were spun at 3000 rpm for 10 seconds. The wafer was then exposed to the radiation from an AG Associates Heat Pulse RTP system. This system comprises a radiation-transparent quartz tube which is used as a protective and isolative pyrolysis chamber,and contains 4 pointed quartz pins used to hold the substrate. The radiation source for this system comprises a number of quartz tungsten halogen lamps, half of which are located above the quartz tube pyrolysis chamber and half of which are located below. The wavelength of the incoherent light emitted from these lamps ranges from about 0.4 to about 4.0 micrometers. A pyrometer is used in this system for temperature sensing. Encasing the above components is a water-cooled reflective enclosure. The coated wafers were heated in the above equipment at temperatures of 400, 500, 600, 700 and 900°C. for 5, 10 and 30 seconds in oxygen at atmospheric pressure. The amount of SiH remaining after the above exposures was then calculated using FTIR by comparing the integrated area under the SiH peak before and after RTP exposure. The following table summarizes the results. TEMP(°C.) TIME(SEC) % SiH REMAINING 400 10 32.4 400 30 32.7 500 10 17.5 500 30 18.2 600 10 3.6 600 30 6.4 700 10 6.3 700 30 11.2 900 5 4.7 900 10 5.5 Example 2 Approximately 25mn (1 inch) square silicon wafers were coated in the same manner as Example 1. These wafers were exposed to the radiation from a Peak systems APL SP35X unit with a 35 KN.m/s (35 kW) high intensity arc lamp in an oxygen atmosphere. The wafers were heated to 300°C. for 5 and 10 minutes, 400°C. for 5 and 10 minutes, 500°C. for 1 minute and 600°C. for 1 minute. The percent SiH remaining was measured using FTIR as in Example 1 (upper limit). In addition, however, a second estimate of percent SiH remaining was calculated correcting for the loss of material by volatilization (lower limit). The correction was made as follows: Area SiH Peak/thickness [nanometres (angstroms)] before RTP x 10000 = X Area SiH Peak/thickness [nanometres (angstroms)] after RTP x 10000 = Y Y/X x 100 = Percent SiH remaining The following table summarizes the results. % SiH REMAINING TEMP(°C.) TIME (min) (upper) (lower) THICKNESS [nm(A)] 300 5 63.0 52.7 180.4 (1804) 300 5 54.6 46.0 188.6 (1886) 300 10 47.1 38.6 181.9 (1819) 300 10 45.0 37.1 176.5 (1765) 400 5 31.3 25.8 184.3 (1843) 400 5 37.6 30.0 171.1 (1711) 400 10 24.5 19.9 179.3 (1793) 400 10 27.3 21.9 179.3 (1793) 500 1 13.9 11.3 181.1 (1811) 500 1 9.8 7.9 176.9 (1769) 600 1 0.0 0.0 166.2 (1662) 600 1 0.0 0.0 174.4 (1744) Example 3 This example demonstrates that atmospheres other than oxygen can be used with RTP. Hydrogen silsesquioxane resin made by the method of Collins et al. in U.S. Patent No. 3,615,272 was diluted to 10 weight percent solids in a solvent comprising 95% by weight heptane and 5% by weight dodecane. A platinum catalyst comprising platinum acetylacetonate in toluene was added to the solution at a concentration of approximately 100 ppm platinum based on the weight of H-resin. Approximately 25 mm (1 inch) square silicon wafers were coated with the above solution in the same manner as Example 1. The coated wafers were exposed to RTP in the same manner as in Example 2 except that the effect of using a dry 100% ammonia atmosphere during RTP was compared with the effect of using an oxygen atmosphere as in Example 2. The percent SiH remaining was calculated in the same manner as in Example 1. The following table summarizes the results. Example 4 This example demonstrates that RTP can cure thicker films for use as interlayer dielectrics. Hydrogen silsesquioxane resin made by the method of Collins et al. in U.S. Patent No. 3,615,272 was diluted to 30 weight percent solids in cyclic dimethyl siloxane solvent. A platinum catalyst comprising platinum acetylacetonate in toluene was added to the solution at a concentration of approximately 100 ppm platinum based on the weight of H-resin. Approximately 1 mL of the above H-resin solution was applied to the surface of 102 mm (4 inch) silicon wafers, and the wafers were spun at 3000 rpm for 10 seconds. The wafers were exposed to RTP in the same manner as in Example 2, and the percentage of SiH was calculated in the same manner as the lower limit of Example 2. The following table summarizes the results. FILM THICKNESS [nm(A)] BEFORE RTP/AFTER RTP TEMP(°C.) TIME(min) % SiH REMAINING 1632.0 (16320)/913.6 (9136) 400 10 1.57 1754.5 (17545)/944.8 (9448) 500 10 0.00 1725.2 (17252)/866.1 (8661) 600 10 0.00 Example 5 This example demonstrates the unexpected improvement in bond strength of CMOS devices when processed using RTP as compared with devices coated with silica by the prior art process of pyrolyzing a H-resin coating in a conventional furnace (i.e., U.S. Patent 4,756,977). Approximately 50 microliters of the H-resin solution prepared in Example 1 were applied to the surface of CMOS devices and the devices were spun at 3000 rpm for 10 seconds. The CMOS devices were then exposed to the RTP of Example 2. The bond pull strength was determined using MIL-STD-883C specification, method 2011. The following table summarizes the bond pull strengths of the 14 lead wires on each device. (minimum acceptable = 2.0) TEMP (°C.) TIME (MIN) AVG BOND STR (grams) RANGE BOND STR (grams) NUMBER OF DEVICES 410 10 and 4.3 3.6 - 4.8 1 500 5 400 10 4.1 3.0 - 5.6 5 500 5 4.4 3.2 - 6.2 5 600 0.5 4.4 3.4 - 5.4 1 400°C. furnace for 1 hour 3.0 2.0 - 4.7 3 unprocessed 5.5 4.4 - 6.2 3 The region where lead wires were detached from the bond pads was also examined under optical microscopy to analyze for 'purple plague'. None was observed. Example 6 The following example demonstrates the unexpected decrease in coating film stress on silicon wafers when processed using RTP as compared with wafers coated with silica by the prior art process of pyrolyzing a H-resin coating in a conventional furnace (i.e., U.S. Patent No. 4,756,977). Approximately 1 mL of the H-resin solution prepared in Example 1 was applied to the surface of 102 mm (4 inch) silicon wafers, and spun at 3000 rpm for 10 seconds. The wafers were then exposed to the RTP of Example 2. Stress measurements were obtained from a FSM 8800 instrument which measures wafer curvature using an optically levered laser beam. The following table summarizes the results (1 dyn = 0,1 Pa). TEMP (°C.) TIME (min) TENSILE STRESS (dynes/cm²) 500 10 1.099 x 10⁻⁹ 500 5 1.207 x 10⁻⁹ 400 10 1.113 x 10⁻⁹ 500 10 0.886 x 10⁻⁹ 600 10 0.806 x 10⁻⁹ O₂ Plasma 250°C. for 3 hours 2.319 x 10⁻⁹ furnace 400°C. for 3 hours 2.077 x 10⁻⁹ Example 7 The following example demonstrates that highly densified silica coatings are obtained using RTP. Approximately 1 mL of the H-resin solution prepared in Example 1 was applied to the surface of 102 mm (4 inch) silicon wafers and spun at 3000 rpm for 10 seconds. The wafers were then exposed to the RTP of Example 2. The density calculations were made by obtaining the weight of the coating using a microbalance and the thickness of the film obtained by ellipsometry. The theoretical maximum density for silica is 2.2. The following table summarizes the results. TEMP TIME COATING DENSITY (°C.) (min) ATMOSPHERE THICKNESS [nm(A)] (g/cm³) 400 10 oxygen 130.0 (1300) 1.857 400 10 air 130.0 (1300) 1.810 500 10 oxygen 145.0 (1450) 1.910 500 5 oxygen 310.0 (3100) 1.913 400 10 oxygen 910.0 (9100) 1.896 500 10 oxygen 945.0 (9450) 1.988 600 10 oxygen 870.0 (8700) 1.888 Example 8 The following example compares the amount of SiH remaining after various methods of converting H-resin to silica. Approximately 100 microliters of the H-resin solution prepared in Example 1 were applied to the surface of approximately 25 mm (1 inch) square silicon wafers, and the wafers were spun at 3000 rpn for 10 seconds. The wafers were then exposed to various conditions to facilitate conversion to silica. Percent SiH remaining was calculated by the method of Example 1. The following table summarizes the results. (RTP exposure was by the method of Example 2). TREATMENT % SiH REMAINING THICK FILMS (1.0 MICROMETRE) 400°C. RTP for 10 min 2 400°C. furnace for 3 hours 20-40 250°C. oxygen plasma for 4 hours 50-90 400°C. bottom up heating on a hot plate 1 hr 31-50 THIN FILMS [100-350 nm (1000-3500 A)] 400°C.RTP for 1-10 min 0-10 400°C. furnace for 1 hour 7-25 250°C. oxygen plasma for 2 hours 8-13 400°C. bottom up heating on a hot plate 1 hr 50

Claims: A method of applying a silica coating to a substrate other than a cuprate substrate comprising: coating said substrate with a solution comprising hydrogen silsesquioxane resin (H-resin) and a solvent; evaporating said solvent to deposit an H-resin coating on said substrate; and converting said H-resin coating to a silica coating, characterised in that said substrate coated with said H-resin coating is subjected to rapid thermal processing (RTP) by means of high intensity radiation comprising incoherent light in a spectral region selected from visible, infrared and ultraviolet light regions, and combinations thereof, whereby said H-resin is rapidly heated at 30-500°C/sec to an elevated temperature from above 200°C to 1000°C for a period of time from more than 1 second to not more than 1 hour sufficient to convert said H-resin coating to a silica coating, said substrate being thermally isolated during said heating step whereby damage to said substrate is avoided. A method according to claim 1, wherein said high intensity radiation has a wave-length in the range of from 0.1 - 1.4 micrometres. A method according to claim 1, or 2 wherein said elevated temperature is within the range of from 300 to 900°C. A method according to any of claims 1 to 3, wherein said period of time is in the range of from more than 1 second to 15 minutes. A method according to any of claims 1-4, wherein said H-resin is a condensed resin of the formula (HSiO 3/2 ) n , wherein n is within the range of from 8 to 1000, inclusive. A method according to any of claims 1-4, wherein said H-resin is an hydridosilane which is only partially hydrolysed and/or partially condensed. A method according to any of claims 1-4, wherein the hydrogen silsesquioxane resin is selected from (HSiO 3/2 ) n , a polymer having units of the formula HSi(OH) a O( 3-a)/2 and a polymer having units of the formula HSi(OH) x (OR) y O z/2 , wherein each R is independently an organic group which, when bonded to silicon through the oxygen atom, forms a hydrolysable substituent, a = 0-2, x = 0-2, y =0-2, z =1-3, x + y + z =3, n is an integer greater than 3, and the average value of y over all of the units of the polymer is greater than 0, and wherein said solvent is selected from alcohols, aromatic hydrocarbons, alkanes, cyclic dimethylsiloxanes, ketones, esters and glycol ethers and is present in an amount sufficient to form an 0.1 to 50 weight percent solution of said H-resin. A method according to any of claims 1-7 wherein said solution comprising said H-resin and said solvent also contains a modifying ceramic oxide precursor comprising a compound containing an element selected from titanium, zirconium, aluminium, tantalum, vanadium, niobium, boron and phosphorus and having at least one hydrolysable substituent selected from alkoxy and acyloxy radicals said compound being present in an amount such that said silica coating contains from 0.1 to 30 percent by weight of a modifying ceramic oxide. A method according to any of claims 1-8 wherein said solution comprising said H-resin and said solvent and, optionally, said modifying ceramic oxide precursor, also contains a platinum or rhodium catalyst in an amount of from 5 to 500 ppm platinum or rhodium based on the weight of said H-resin in solution.

Inventors: CHANDRA GRISH, GENTLE THERESA EILEEN, CHANDRA, GRISH, GENTLE, THERESA EILEEN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1991
Main classification: H01L21
IPC codes: C09D183, B05D3, H01L21, C08J7, C23C18