Method of forming coatings

Details Method of forming coatings Method of forming coatings

1998-11-20

2001-05-15

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of silicon containing

C427S226000, C427S337000, C427S340000, C427S341000, C427S343000, C427S344000, C428S209000, C428S210000, C428S446000, C428S901000

Reexamination Certificate

active

06231989

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a method of forming a coating on a substrate by depositing a solution of a resin containing Si—H groups and a solvent in a manner in which solvent remains in the coating followed by exposing the coating to a basic catalyst and water and then evaporating the solvent from the coating. The method of the invention is particularly useful for applying low dielectric constant coatings on electronic devices.
Thin film dielectric coatings on electronic devices are known in the art. For instance, Haluska et al. in U.S. Pat. Nos. 4,749,631 and 4,756,977, which are incorporated herein by reference, disclose silica based coatings produced by applying solutions of silicon alkoxides or hydrogen silsesquioxane, respectively, to substrates and then heating the coated substrates to temperatures of 200-1000° C. The dielectric constant of these coatings, however, are often too high for certain electronic devices and circuits.
Haluska et al. in U.S. Patent Nos. 4,847,162 and 4,842,888 also teach the formation of nitrided silica coatings by heating hydrogen silsesquioxane resin and silicate esters, respectively, to a temperatures of between 200 and 1000° C. in the presence of ammonia. These references teach the use of anhydrous ammonia so that the resulting coating has about 1 to 2 % by weight nitrogen incorporated therein.
Glasser et al. in the Journal of Non-Crystalline Solids, 64 (1984) pp. 209-221 teach the formation of ceramic coatings by heating tetraethoxysilane in the presence of ammonia. As with Haluska '162 above, however, this reference also teaches that the ammonia should be anhydrous and that the resultant silica coatings are nitrided.
Jada in U.S. Pat. No. 4,636,440 discloses a method of reducing the drying time for a sol-gel coated substrate comprising exposing the substrate to aqueous quaternary ammonium hydroxide and/or alkanol amine compounds. The methods of this reference, however, are different than those disclosed herein in that Jada requires the coating to be dried prior to heating. Moreover, Jada is specifically limited to hydrolyzed or partially hydrolyzed silicon alkoxides and fails to teach the utility of the process on coatings having Si—H bonds.
Haluska et al. in U.S. Pat. No 5,262,201 and Baney et al. In U.S. Pat. No. 5,116,637 teach the use of basic catalysts to lower the temperature necessary for the conversion of various preceramic materials, including hydrogen silsesquioxane, to ceramic coatings. These references, however, teach the removal of solvent before the coating is exposed to the basic catalysts.
Camilletti et al. in U.S. Pat. No. 5,547,703 teach a method for forming low dielectric constant Si—O containing coatings on substrates comprising heating a hydrogen silsesquioxane resin successively under wet ammonia, dry ammonia and oxygen. The resultant coatings have dielectric constants as low as 2.42 at 1 MHz. Again, however, this reference teaches the removal of solvent before converting the coating to a ceramic and the dielectric constant of the resultant coatings are not as low as those disclosed herein.
Ballance et al. in U.S. Pat. No. 5,523,163 teach a method for forming Si—O containing coatings on substrates comprising heating a hydrogen silsesquioxane resin to convert it to a Si—O containing ceramic coating and then exposing the coating to an annealing atmosphere containing hydrogen gas. The resultant coatings have dielectric constants as low as 2.773. Again, however, this reference teaches the removal of solvent before converting the coating to a ceramic and the dielectric constant of the resultant coatings are not as low as those disclosed herein.
Syktich et al in U.S. Pat. No. 5,618,878 teach coating compositions containing hydrogen silsesquioxane resin dissolved in saturated alkyl hydrocarbons useful for forming thick ceramic coatings. The alkyl hydrocarbons disclosed are those up to dodecane. The reference, however, fails to teach exposure of coated substrates to basic catalysts before solvent removal.
The present inventors have now discovered that by exposing a Si—H containing resin to basic catalysts before complete solvent removal, porous network coatings with low dielectric constants can be produced.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to a method of forming a coating on a substrate. The method comprises depositing a coating on a substrate with a solution comprising a resin containing at least 2 Si—H groups and a solvent in a manner in which at least 5 volume % of the solvent remains in the coating after deposition. The coating is then exposed to an environment comprising a basic catalyst and water. Finally, the solvent is evaporated from the coating to form a porous network. If desired, the coating can be heated to form a ceramic.
In other aspects, the present invention relates to the coatings produced by the above method as well as the coating solution used in the process.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the discovery that basic catalysts and water can be used to form novel porous network coatings from Si—H containing resins. Such coatings are useful on electronic substrates because of their low dielectric constant.
The methods of this invention are particularly applicable to the deposition of coatings on electronic devices or electronic circuits where they can serve as interlevel dielectric layers, doped dielectric layers to produce transistor like devices, pigment loaded binder systems containing silicon to produce capacitor and capacitor like devices, multilayer devices, 3-D devices, silicon on insulator devices, super lattice devices, and the like. 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 at the temperature and atmosphere used in the present invention. As such, the coatings of the invention can be used on substrates such as plastics including, for example, polyimides, epoxides, polytetrafluoroethylene and copolymers thereof, polycarbonates, acrylics and polyesters, ceramics, leather, textiles, metals, and the like.
As used in the present invention, the expression “ceramic” includes ceramics such as amorphous silica and ceramic-like materials such as amorphous silica-like materials that are not fully free of carbon and/or hydrogen but are otherwise ceramic in character and the expressions “electronic device” or “electronic circuit” include, but are not limited to, silicon based devices, gallium arsenide based devices, silicon carbide based devices, focal plane arrays, opto-electronic devices, photovoltaic cells and optical devices.
The resins containing at least 2 Si—H groups useful in the present invention are not particularly limited so long as the Si—H bonds can be hydrolyzed and at least partially condensed by the basic catalyst and water to form a crosslinked network which serves as the structure for the porous network. Generally, such materials have the formula:
{R
3
SiO
1/2
}
a
{R
2
SiO
2/2
}
b
{RSiO
3/2
}
c
{SiO
4/2
}
d
wherein each R is independently selected from the group consisting or hydrogen, alkyl, alkenyl, or aryl groups or alkyl, alkenyl, or aryl groups substituted with a hetero atom such as a halogen, nitrogen, sulfur, oxygen or silicon, and a, b, c and d are mole fractions of the particular unit and the their total is 1, with the proviso that at least 2 R groups per molecule are hydrogen and the material is sufficiently resinous in structure to form the desired network. Examples of alkyl groups are methyl, ethyl, propyl, butyl, and the like with alkyls of 1-6 carbon atoms preferred. Examples of alkeyl groups include vinyl, allyl and hexenyl. Examples of aryls include phenyl. Examples of substituted groups include CF
3
(CF
2
)nCH
2
CH
2
, (where n=0-6).
Particularly preferred in the present invention are various hydridosiloxane resins, known as hydrogen silsesquioxane resins, comprising units of the formula HSi(OH)
X
(OR)
y

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