Protective Electronic Coatings Using Filled Polysilazanes

Europaisches Patentamt 19 European Patent Office

Office europeen des brevets © Publication number : 0 675 537 A2

12 EUROPEAN PATENT APPLICATION

@ Date of filing : 27.03.95

(30) Priority: 01.04.94 US 221597 © Inventor : Haluska, Loren Andrew 4510 James Street Midland, Michigan (US) @ Date of publication of application Inventor : Michael, Keith Winton 04.10.95 Bulletin 95/40 2715 Siebert Midland, Michigan (US)

@ Designated Contracting States : DE FR GB IT NL © Representative : Vandamme, Luc Johan Roger Dow Corning Limited, Cardiff Road © Applicant : DOW CORNING CORPORATION Barry, South Glamorgan CF63 2YL, Wales (GB) Midland, Michigan 48686-0994 (US)

© The present invention relates to a method of forming protective coatings on electronic substrates and to the substrates coated thereby. The method comprises applying a coating comprising a polysilazane and a filler on a substrate and then heating the coated substrate at a temperature sufficient to convert the polysilazane to a ceramic.

CM < co If) If) h-

LU Jouve, 18, rue Saint-Denis, 75001 PARIS 1 EP 0 675 537 A2 2

The present invention relates to a method of filler materials. The expression "filler" describes a forming protective coatings using compositions com- finely divided solid phase which is distributed within prising polysilazanes and fillers. These coatings are the polysilazane and the final ceramic coating. The useful on a variety of electronic substrates. expression "electronic substrate" includes electronic The use of polysilazanes to form ceramic coat- 5 devices or electronic circuits such as silicon based ings on electronic devices is known in the art. For in- devices, gallium arsenide based devices, focal plane stance, WO 93/02472 discloses such a process arrays, opto-electronic devices, photovoltaic cells wherein a solution of a polysilazane resin is applied and optical devices. to an electronic substrate followed by heating the In our process, a protective ceramic coating is coated substrate in air at a temperature in the range 10 formed on a substrate by a process which comprises of 150-800°C. This publication, however, does not de- applying a coating composition comprising a polysi- scribe the use of fillers within the coating. lazane and a f illeronto the substrate and then heating Similarly, the use of fillers within ceramic coat- the coated substrate at a temperature sufficient to ings derived from polysilazanes is also known in the convert the polysilazane to a ceramic. art (JP-A(s) 52287 and 221466). These documents, 15 The polysilazanes (or silazane polymers) useful however, do not describe the use of such materials as in this invention are well known in the art and their protective coatings on electronic substrates. structure is not particularly critical. These polysila- JP-A 3250082 also describes the incorporation of zanes generally contain units of the type [R2SiNH], an electroconductive powder within a ceramic de- [RSi(NH)1.5] and/or [R3Si(NH)1/2] wherein each R is in- rived from a polysilazane. However, it does not de- 20 dependency selected from hydrogen atoms, alkyl scribe protective coatings on electronic substrates. radicals containing 1 to 20 carbon atoms, aryl radicals We have now found that useful coatings for the and alkenyl radicals. Naturally, the polysilazanes protection of electronic devices can be formed from useful in this invention may contain other silazane compositions comprising polysilazanes and fillers. units. Examples include [MeSi(NH)15], [Me2SiNH), The present invention relates to a method of 25 [ViSi(NH)1.5], [Vi2SiNH], [PhMeSiNH], [PhViSiNH], forming a protective coating on the surface of an elec- [MeViSiNH], [HSi(NH)1.5] and [H2SiNH]. As used here- tronic substrate and to the substrates coated thereby. in, Me is a methyl radical, Vi is a vinyl radical and Ph The method comprises first applying a composition is a phenyl radical. Mixtures of polysilazanes may comprising a polysilazane and a filler on the surface also be employed in the practice of this invention. of the electronic substrate. The coated substrate is 30 The polysilazanes of this invention can be pre- then heated at a temperature sufficient to convert the pared by techniques well known in the art. The actual composition to a ceramic coating. method used is not critical. Suitable preceramic sila- Desirable protective coatings can be formed on zane polymers or polysilazanes may be prepared by electronic substrates from a composition comprising the methods in U.S. Patents 4,540,803 and a polysilazane and a filler. Coatings derived there- 35 4,543,344. Other polysilazanes suitable for this in- from are thicker (e.g., > 40 micrometers) than those vention can be prepared by the methods of U.S. Pa- derived from polysilazanes alone (e.g., < 2 microme- tents 4,312,970; 4,340,619; 4,395,460 and ters). We have found a variety of coatings can result 4,404,153. Suitable polysilazanes also include those with electrical properties depending on the filler prepared by U.S. Patents 4,482,689 and 4,397,828. which are crack and pore-free. 40 Still other polysilazanes include those of European Because of these advantages, our coatings are Patent 351,747; U.S. Patent 4,543,344; European particularly valuable as protective coatings on elec- Patent 332,374; U.S. Patents 4,656,300 or 4,689,252 tronic substrates. Such coatings could serve as pas- and U.S. Patent No. 5,030,744. sivation or dielectric coatings, interlevel dielectric lay- Especially preferred polysilazanes are those ers, doped dielectric layers to produce transistor like 45 which have no carbon in the repeating units of the devices, pigment loaded binder systems containing polymer since the resultant coatings have little car- silicon to produce capacitor and capacitor like devic- bon contamination. Endblocking groups such as es, multilayer devices, 3-D devices, silicon on insula- Me3Si(NH)1/2 are acceptable in such polymers since tor devices, coatings for superconductors, super lat- they will be removed during the subsequent pyrolysis tice devices, tamperproof coatings and the like. so steps. As used herein, the expression "ceramic coating" The most preferred polymers are those of U.S. describes the hard coating obtained after heating the Patents 4,340,619 and 4,540,803. The former sila- polysilazane - filler composition. This coating con- zane polymers are prepared by contacting and react- tains both amorphous silica (Si02) materials as well ing in an inert, essentially anhydrous atmosphere a as amorphous silica-like materials that are not fully 55 chlorine containing disilane or mixture of chlorine free of residual carbon (e.g., Si-C), nitrogen (e.g., Si- containing disilanes of the general formula (ClxRySi)2 N), silanol (Si-OH) and/or hydrogen (e.g., Si-H) which with a disilazane having the general formula are obtained upon heating the polysilazane and the (R3'Si)2NH at a temperature in the range of 25°C. to 2 3 EP 0 675 537 A2 4

300°C. while distilling volatile byproducts, wherein R (materials which render heat on oxidation) such as is a vinyl group, an alkyl group of 1-3 carbon atoms magnesium, iron, tin, silicon, zinc, precipitated diato- or a phenyl group; R' is a vinyl group, hydrogen atom, mite, aluminum silicate or other silicates, pigments, an alkyl group of 1-3 carbon atoms ora phenyl group; phosphors, wollastonite, mica, kaolin, clay and talc. x has a value of 0.5-3; y has a value of 0-2.5 and the 5 Also, some organic materials such as cellulose, poly- sum of x+y equals 3. An especially preferred embodi- amides and phenol resins may be used. ment of the former polymers involves the reaction of The preferred fillers used herein depend on the methylchlorodisilanes with hexamethyldisilazane to intended use for the coating. For instance, if the coat- produce methylpolydisilylazane. The product sila- ing is used as an interlevel dielectric, a material such zane polymers may have a relatively high chloride ion 10 as silica or alumina may be desirable so that the coat- content and it is preferred that it be lowered before ing has a low dielectric constant (DK) of less than 8. use in the present invention. A method for such re- Similarly, if a coating having a high DK of greater than moval is described in U.S. Patent 4,772,516 which 12 is desired, a material such as barium titantate or comprises treating the product polymers with ammo- lead niobate may be desirable. Similarly, if an opaque nia for a time sufficient to remove the chlorine. 15 coating is desired, an optically or radiopaque material The latter silazane polymers above are prepared may be desired. by a method which comprises contacting and reacting The number average particle size and shape of in an inert essentially anhydrous atmosphere, tri- the above fillers can vary over a wide range depend- chlorosilane with a disilazane at a temperature in the ing on factors such as the type of filler, the desired range of 25°C. to 300°C. while distilling volatile by- 20 coating thickness or the like. Since the coatings are products. The disilazane used in the process has the generally less than 500 micrometers thick, particle si- formula (R3Si)2NH where R is selected from vinyl rad- zes less than this are generally used. Preferred num- ical, hydrogen atom, phenyl radical and alkyl radicals ber average particle sizes are in the range of less than containing 1 to 3 carbon atoms. An especially prefer- 50 micrometers and the most preferred number aver- red embodiment of the latter polymers involves the 25 age particle size is in the submicrometer range to 10 reaction of trichlorosilane with hexamethyldisilazane micrometers. to produce hydridopolysilazane. The amount of filler used in the present invention Although only several polymers are described can also be varied over a wide range depending, for above, nearly any polysilazane may be used herein. example, on the quality and electrical characteristics The fillers used herein are known in the art for 30 desired in the final coating. Generally, the fillers are use in coatings with other polymers. These include used in an amount less than 90 weight percent based various inorganic and organic fillers, especially inor- on the weight of the polysilazane to insure that ganic fillers, in a variety of morphologies including enough resin is present to bind the filler. Obviously, powders, particles, filaments, flakes and microbal- smaller amounts of fillers (e.g., 1-5 wt%) can also be loons. Examples of inorganic fillers include synthetic 35 used. and natural materials such as the oxides, nitrides, If desired, other materials may also be present in borides and carbides of various metals and non-met- our coating composition. For instance, a material als such as glass, alumina, silica, silicon monoxide, which modifies the surface of the filler for better ad- zirconium oxide, titanium dioxide, tantalum.oxide, hesion may be used. Such materials can include, for niobium oxide, zinc oxide, tungsten oxide, ruthenium 40 example, silanes such as glycidoxypropyltrimethoxy- oxide, silicon nitride, aluminum nitride, titanium dibor- silane, mercaptopropyltrimethoxysilane and vinyl- ide, silicon carbide, boron carbide or boron nitride, triacetoxysilane. Similarly, suspending agents such calcium carbonate; high dielectric constant fillers as cellulose, clay, fumed silica, stearates and the like such as the titanate, niobate ortungstate salts of met- may be included in the coating composition. These als such as strontium, zirconium, barium, lead, lan- 45 and other optional components are known to those thanium, iron, zinc and magnesium, including barium skilled in the art. titanate, potassium titanate, lead niobate, lithium ti- According to our process, the polysilazane, filler tanate, strontium titanate, barium strontium titanate, and any optional components are applied to the sur- lead lanthanium zirconium titanate, lead zirconium ti- face of a substrate. This can be accomplished in any tanate and lead tungstate; radiopaque materials (ma- so manner, but a preferred method involves dissolving terials which inhibit penetration by radiation) such as the polysilazane in a solvent and then dispersing the the insoluble salts of barium, lead, silver, gold, cad- filler with any optional components therein. This dis- mium, antimony, tin, palladium, strontium, tungsten persion is subsequently applied to the surface of the and bismuth, including salts such as carbonates, sul- substrate. Various facilitating measures such as stir- fates and oxides (e.g., barium sulfate); optically opa- 55 ring and/or heating may be used to dissolve or dis- que fillers such as inorganic pigments, silicon nitride perse the polysilazane or filler and to create a more powder, silicon carbide powder, aluminum nitride uniform application material. Solvents may be any powder, silica and alumina; tamperproof materials agent or mixture of agents which will dissolve or dis- 3 5 EP 0 675 537 A2 6 perse the polysilazane and filler to form a liquid mix- cellent adhesive properties and have a variety of ture without affecting the resultant coating. These electrical properties (e.g., low DK or high DK). As solvents can include aromatic hydrocarbons such as such, they are particularly useful for a variety of elec- benzene or toluene; alkanes such as n-heptane ordo- tronic applications such as protective layers (e.g., decane; ketones; esters; glycol ethers or cyclic dime- 5 passivation, dielectric or tamperproof). thylpolysiloxanes, in an amount sufficient to dis- Additional coatings may be applied over these solve/disperse the above materials to the concentra- coatings if desired. These can include coatings of tion desired for application. Generally, enough of the Si02, Si02/ceramic oxide, silicon, silicon carbon, sili- above solvent is used to form a 0.1-80 weight percent con nitrogen, silicon nitrogen oxygen, silicon nitrogen mixture, preferably a 1-50 wt% mixture. 10 carbon and/or diamond-like carbon coatings. Meth- If a liquid method is used, the liquid mixture com- ods for the application of such coatings are known in prising the polysilazane, filler, solvent and, any op- the art and many of these are described in U.S. Patent tional components is then coated onto the substrate. 4,756,977. An especially preferred coating is silicon The method of coating can be by spin coating, dip carbide applied by the chemical vapor deposition of coating, spray coating or flow coating. Similarly, the 15 silacyclobutane. This specific process is described in coating can be selectively deposited by a masking or US Patent 5,011,706. silk screening process. Other equivalent means, how- The following non-limiting examples are included ever, are also possible. so that one skilled in the art may more readily under- The solvent is then allowed to evaporate from the stand the invention. coated substrate resulting in the deposition of the 20 polysilazane and filler coating. Any suitable means of Example 1 evaporation may be used such as simple air drying by exposure to an ambient environment, by the applica- Silica glass microballoons (SDT-60) with a num- tion of a vacuum or mild heat (e.g., less than 50°C.) ber average particle size of 5 micrometers, 0.3 g, or by the early stages of the heat treatment. When 25 were ground in a mortar and pestle for several min- spin coating is used, the additional drying period is utes to decrease the particle size. A coating compo- minimized as the spinning process drives off the sol- sition was then formed by mixing the ground glass, 2 vent. g of hydridopolysilazane (50 wt% solids in xylene) Although the above methods primarily focus on made by the method of U.S. Patent 4,540,803, 0.2 g using a liquid approach, one skilled in the art would 30 of Minusil™ and 0.4 g of glycidoxypropyltrimethoxy- recognize that other equivalent means would also silane. Asonic probe was used for 20 seconds to com- function herein. pletely disperse the materials. The coating composi- The polysilazane and filler coating is then typical- tion was applied to the surface of an 11.4 cm2 alumi- ly converted to the ceramic by heating it to a sufficient num panel using a 50 micrometer drawdown bar. The temperature. Generally, this temperature is in the 35 coating was allowed to dry for 1 .5 hours. The coated range of 50 to 1000°C. depending on the pyrolysis at- panel was then heated at 500°C. for 1 hour. The coat- mosphere. Preferred temperatures are in the range of ing was 12.5 micrometers thick and was crack-free at 100 to 800°C. and, more preferably, 150-500°C. 1000x. Heating is generally conducted for a time sufficient to ceramify, typically up to 6 hours, with less than 3 40 Example 2 hours being preferred. The above heating may be conducted at any ef- 4 g of plasma alumina with a number average fective atmospheric pressure from vacuum to super- particle size of 6 micrometers, 0.3 g, 2 g of hydrido- atmospheric and under any effective oxidizing or non- polysilazane (50 wt% solids in xylene) made by the oxidizing gaseous environment such as those com- 45 method of U.S. Patent 4,540,803, 0.4 g of glycidoxy- prising air, 02, oxygen plasma, an inert gas, nitrogen, propyltrimethoxysilane and 1 g cyclic polydimethylsi- ammonia, amines, moisture and N20. loxane were mixed. A sonic probe was used for 30 Any method of heating such as the use of a con- seconds to completely disperse these materials. The vection oven, rapid thermal processing, hot plate or coating composition was applied to the surface of an radiant or microwave energy is generally functional so 11.4 cm2 aluminum panel using a 50 micrometer herein. The rate of heating, moreover, is also not crit- drawdown bar. The coating was allowed to dry for 5 ical, but it is most practical and preferred to heat as minutes. The coated panel was then heated at 500°C. rapidly as possible. for 1 hour. The coating was 16.1 micrometers thick By the above methods a ceramic coating is pro- and was crack-free at 1000x. duced on the substrate. The thickness of the coating 55 can vary over a wide range (up to 500 micrometers). These coatings smooth the irregular surfaces of various substrates, are relatively defect free, have ex- 4 7 EP 0 675 537 A2 8

Claims

1. A method of forming a protective coating on an electronic substrate comprising: applying a coating composition compris- 5 ing a polysilazane and a filler onto an electronic substrate; and heating the coated substrate at a temperature in the range of 50 to 1000°C. for up to six hours to convert the coating composition into a 10 ceramic.

2. The method of claim 1 wherein the coating composition is applied to the substrate by a process which comprises coating the substrate with a liq- 15 uid mixture comprising a solvent, the polysilazane and the filler and then evaporating the solvent.

3. The method of claim 2 wherein the coated sub- 20 strate is heated at a temperature in the range of between 50°C. and 800°C. for less than 3 hours.

4. The method of claim 2 wherein the coated substrate is heated in an environment containing one 25 or more compounds selected from air, 02, oxygen plasma, inert gas, nitrogen, ammonia, amines, moisture and N20.

5. The method of claim 1 wherein the filler is in a 30 form selected from powders, particles, filaments, flakes and microballoons.

6. The method of claim 5 wherein the filler has a particle size less than 500 micrometers. 35

7. The method of clai m 1 wherein the filler is present in the coating composition in an amount less than 90 weight percent. 40 8. The method of claim 1 wherein the ceramic coating has a dielectric constant less than 8.

9. The method of claim 1 wherein the ceramic coating has a dielectric constant greater than 12. 45

10. An electronic substrate coated by the method of claim 1.