Nanoporous silicone resins having low dielectric constants and method for preparation

[0015] The present invention is nanoporous silicone resin compositions having low dielectric constants, substrates coated with such compositions, and methods for making such nanoporous silicone resins compositions.

[0016] Semiconductor devices often have one or more arrays of patterned interconnect levels that serve to electrically couple the individual circuit elements forming an integrated circuit (IC). These interconnect levels are typically separated by an insulating or dielectric film. Previously, a silicon oxide film formed using chemical vapor deposition (CVD) or plasma enhanced techniques (PECVD) was the most commonly used material for such dielectric films. However, as the size of circuit elements and the spaces between such elements decreases, the relatively high dielectric constant of such silicon oxide films is inadequate to provide adequate electrical insulation.

[0017] In order to provide a lower dielectric constant than that of silicon oxide, dielectric films formed from siloxane-based resins have found use. An example of such films are those formed from poly(hydrogen)silsesquioxane resins as described for example in Collins et al., U.S. Pat. No. 3,615,272 and Haluska et al. U.S. Pat. No. 4,756,977. While such films provide lower dielectric constants than CVD or PECVD silicon oxide films and also provide other benefits such as enhanced gap filling and surface planarization, typically the dielectric constants of such films are limited to approximately 3 or greater.

[0018] It is well known that the dielectric constant of the above discussed insulating films is an important factor where IC's with low power consumption, cross-talk, and signal delay are required. As IC dimensions continue to shrink, this factor increases in importance. As a result, siloxane based resin materials and methods for making such materials that can provide electrically insulating films with dielectric constants below 3 are desirable. In addition, it is desirable to have siloxane-based resins and method for making such resins that provide low dielectric constant films which have a high resistance to cracking. Also, it is desirable for such siloxane-based resins to provide low dielectric constant films by standard processing techniques.

[0019] It is known that the dielectric constant of solid films decrease with a decrease in density of the film material. Therefore considerable work is being conduct to develop microporous insulating films for use on semiconductor devices.

[0020] Kapoor, U.S. Pat. No. 5,494,859, describes a low dielectric constant insulating layer for an integrated circuit structure and a method of making the layer. A porous layer is formed by depositing on a structure a composite layer comprising an insulating matrix material and a material which can be converted to a gas upon subjection to a converting process. Release of the gas leaves behind a porous matrix of the insulating material which has a lower dielectric constant than the composite layer. The matrix forming material is typically silicon oxide and the material which can be converted to a gas upon subjection to a converting process is exemplified by carbon.

[0021] Hedrick et al., U.S. Pat. No. 5,776,990, describe an insulating foamed polymer having a pore size less than about 100 nm made from a copolymer comprising a matrix polymer and a thermally decomposable polymer by heating the copolymer above the decomposition temperature of the decomposable polymer. The copolymers described are organic polymers that do not contain silicon atoms.

[0022] Smith et al., WO 98/49721, describe a process for forming a nanoporous dielectric coating on a substrate. The process comprises the steps of blending an alkoxysilane with a solvent composition and optional water; depositing the mixture onto a substrate while evaporating at least a portion of the solvent; placing the substrate in a sealed chamber and evacuating the chamber to a pressure below atmospheric pressure; exposing the substrate to water vapor at a pressure below atmospheric pressure and then exposing the substrate to base vapor.

[0023] Mikoshiba et al., Japanese Laid-Open Patent (HEI) 10-287746, describe the preparation of porous films from siloxane-based resins having organic substituents which are oxidized at a temperature of 250° C. or higher. The useful organic substituents which can be oxidized at a temperature of 250° C. or higher given in this document include substituted and unsubstituted groups as exemplified by 3,3,3-trifluoropropyl, β-phenethyl group, t-butyl group, 2-cyanoethyl group, benzyl group, and vinyl group.

[0024] Mikoskiba et al., J. Mat. Chem., 1999, 9, 591-598, report a method to fabricate angstrom size pores in poly(methylsilsesquioxane)films in order to decrease the density and the dielectric constant of the films. Copolymers bearing methyl(trisiloxysilyl) units and alkyl(trisiloxysilyl) units are spin-coated on to a substrate and heated at 250°C. to provide rigid siloxane matrices. The films are then heated at 450° C. to 500° C. to remove thermally labile groups and holes are left corresponding to the sized of the substituents. Trifluoropropyl, cyanoethyl, phenylethyl, and propyl groups were investigated as the thermally labile substituents.

[0025] Hacker et al., WO 98/47945, teach a method for reacting trichlorosilane and organotrichlorosilane to form organohydridosiloxane polymer having a cage conformation and between approximately 0.1 to 40 mole percent carbon-containing substituents. Resin formed from the polymers are reported to have a dielectric constant of less than about 3.

[0026] The objectives of the present invention is to provide nanoporous silicone resins suitable for forming crack-free electrically insulating films on electronic devices that have low dielectric constants and a method for forming such porous resins. Such low-dielectric constant films can be formed on electrical components such as semiconductor devices by conventional methods to form nanoporous crack-free films having a dielectric constant less than about 2.

[0027] The present invention is nanoporous films having low dielectric constants prepared from soluble silicone resin compositions and a method for preparing such nanoporous films. The silicone resin comprises the reaction product of a mixture comprising

[0028] (A) 15-70 mol % of a tetraalkoxysilane described by formula

[0029] Si(OR¹)₄,

[0030] where each R¹is an independently selected alkyl group comprising 1 to about 6 carbon atoms,

[0031] (B) 12 to 60 mol % of an organosilane described by formula

[0032] R⁴SiX₃,

[0033] where R4 is selected from the group consisting of alkyl groups comprising 1 to about 6 carbon atoms and phenyl and each X is an independently selected hydrolyzable substituent,

[0034] (C) 15 to 70 mole percent of an organotrialkoxysilane described by formula

[0035] R²Si(OR³)₃,

[0036] where R²is a hydrocarbon group comprising about 8 to 24 carbon atoms or a substituted hydrocarbon group comprising a hydrocarbon chain having about 8 to 24 carbon atoms and each R³is an independently selected alkyl group comprising 1 to about 6 carbon atoms; in the presence of

[0037] (D) water,

[0038] (E) hydrolysis catalyst, and

[0039] (F) organic solvent for the reaction product.

[0040] The silicone resin is cured and heated preferably in an inert atmosphere at a temperature sufficient to effect thermolysis of carbon-carbon bonds of the R²groups thereby forming a nanoporous silicone resin.

[0041] The nanoporous silicone resin of the present invention is formed from a silicone resin which is soluble in standard solvents useful for applying dielectric coatings to electrical components. The silicone resin comprises the reaction product of a mixture comprising

[0042] (A) 15-70 mol % of a tetraalkoxysilane described by formula

[0043] Si(OR¹)₄,  (1)

[0044] where each R¹is an independently selected alkyl group comprising 1 to about 6 carbon atoms,

[0045] (B) 12 to 60 mol % of an organosilane described by formula

[0046] R⁴SiX₃,  (2)

[0047] where R⁴is selected from the group consisting of alkyl groups comprising 1 to about 6 carbon atoms and phenyl and each X is an independently selected hydrolyzable substituent,

[0048] (C) 15 to 70 mole percent of an organotrialkoxysilane described by formula

[0049] R²Si(OR³)₃,  (3)

[0050] where R²is a hydrocarbon group comprising about 8 to 24 carbon atoms or a substituted hydrocarbon group comprising a hydrocarbon chain having about 8 to 24 carbon atoms and each R³is an independently selected alkyl group comprising 1 to about 6 carbon atoms; in the presence of

[0051] (D) water,

[0052] (E) hydrolysis catalyst, and

[0053] (F) organic solvent for the reaction product.

[0054] Component (A) is a tetraalkoxysilane as described by formula (1). The present inventors have unexpectedly discovered that the presence of component (A) in the range of 15 mol % to 70 mol %, based upon the total moles of components (A)+(B)+(C) is critical to the solubility and stability of the silicone resin in organic solvents. If the mol % of component (A) is outside the described range, the silicone resin will be at least partially insoluble in typical organic solvents used to form solutions of such resins for applications as coatings. It is preferred that the mol % of component (A) be within a range of about 25 mol % to 50 mol %. In formula (1), each R¹is an independently selected alkyl group comprising 1 to about 6 carbon atoms. R¹can be, for example, methyl, ethyl, butyl, tert-butyl, and hexyl. It is preferred that component (A) be tetramethoxysilane or tetraethoxysilane, because of their easy availability.

[0055] Component (B) is an organosilane described by formula (2). Component (B) is added to the mixture in an amount of 12 mol % to 60 mol % based upon the total moles of components (A)+(B)+(C). Addition of component (B) in an amount outside the described range can limit the solubility of the resulting silicone resin in organic solvents. It is preferred that component (B) be added to the mixture in an amount of about 30 mol % to 50 mol %. In formula (2), R⁴is selected from the group consisting of alkyl groups comprising 1 to about 6 carbon atoms and phenyl. Preferred is when R⁴is selected from the group consisting of ethyl, methyl, propyl, and phenyl. In formula (2), X is a hydrolyzable substituent. X can be any substituent capable of hydrolyzing from the silicon atom in the presence of water under conditions of the described process and which groups when hydrolyzed do not adversely impact the solubility or end use of the silicone resin. Examples of hydrolyzable substituents include halogen, alkoxy groups, and acyloxy groups. Preferred is when X is an alkoxy group comprising 1 to about 6 carbon atoms. Component (B) can be, for example, methyltrichlorosilane, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, methylchlorodimethoxysilane, ethyltrichlorosilane, ethyltriethoxysilane, ethyltrimethoxysilane, propyltrichlorosilane, phenyltrichlorosilane, and phenyltrimethoxysilane.

[0056] Component (C) is an organotrialkoxysilane described by formula (3). Component (C) is added to the mixture in an amount of 15 mol % to 70 mol % based upon the total moles of components (A)+(B)+(C). Component (C) is important to providing a mechanism for providing nanoporosity to films formed from the silicone resin. Specifically, Component (C) comprises an unsubstituted or substituted hydrocarbon group, R², which can be removed from the silicon atom by thermolysis during a heating process thereby creating nanopores in the resulting silicone resin coating. Therefore, the amount of component (C) added to the mixture is used to control the degree of porosity of the resulting silicone resin after heating to cure and remove the R²substituents by thermolysis. Generally, an amount of component (C) below about 15 mol % will result in silicone resin coatings having a porosity too little to impart optimal dielectric properties to the material and poor solubility in solvent, while an amount of component (C) above 70 mol % will result in a resin which may have limited solubility in organic solvents and inadequate physical properties such as crack resistance when used as a porous coating on a substrate. Preferred is when component (C) is added to the mixture in a range of about 15 mol % to 40 mol %.

[0057] In formula (3), R²is a hydrocarbon group comprising about 8 to 24 carbon atoms or a substituted hydrocarbon group comprising a hydrocarbon chain having about 8 to 24 carbon atoms. R²can be a linear, branched, or cyclic hydrocarbon group. The substituted hydrocarbon group can be substituted with such substituents as halogen, poly(oxyalkylene) groups described by formula —(O—(CH₂)_m)_x—CH₃where m and x are both positive integers and preferably a positive integer of 1 to 6, alkoxy, acyloxy, acyl, alkoxycarbonyl, and trialkylsiloxy groups. Preferred is when R²is a linear alkyl group comprising about 8 to 24 carbon atoms. Even more preferred is when R²is a straight chained alkyl group comprising about 16 to 20 carbon atoms. Examples of R²include octyl, chlorooctyl, trimethylsiloxyoctyl, methoxyoctyl, ethoxyoctyl, nonyl, decyl, dodecyl, hexadecyl, trimethylsiloxyhexadecyl, octadecyl, and docosyl.

[0058] In formula (3), each R³is an independently selected alkyl group comprising 1 to about 6 carbon atoms. R³can, be for example, methyl, ethyl, propyl, isopropyl, butyl, heptyl, and hexyl. Preferred is when R³is either methyl or ethyl.

[0059] Specific examples of organotrialkoxysilanes represented by formula (3) include octyltriethoxysilane, octyltrimethoxysilane, octadecyltrimethoxysilane, hexadecyltrimethoxysilane, and dodecyltriethoxysilane. Preferred is when the organotrialkoxysilane is selected from the group consisting of octyltriethoxysilane, octadecyltrimethoxysilane, and hexadecyltrimethoxysilane. Further examples of organotrialkoxysilanes include those described by the following formulas:

[0060] (CH₃(CH₂)₁₇—O—CH₂CH₂)Si(OMe)₃, (CH₃(CH₂)₆C(═O)—(CH₂)₈CH₂)Si(OMe)₃, (CH₃(CH₂)₁₇—O—C(═O)—CH₂CH₂)Si(OMe)₃, and (CH₃(CH₂)₁₆—C(═O)—O—CH₂CH₂)Si(OMe)₃, where Me is methyl.

[0061] Component (D) is water. It is preferred that component (D) be added in an amount sufficient to effect essentially complete hydrolysis of hydrolyzable groups bonded to the silicon atoms of components (A), (B), and (C) without an excess so great as to cause a two-phase mixture, which can slow down the reaction. Generally, it is preferred that the amount of water added be about 1.4 to 6 moles per mole of components (A), (B), and (C). Even more preferred is when the water is added in an amount of about 2.5 to 4.5 moles, on the same basis.

[0062] Component (E) is a hydrolysis catalyst and can be any of those organic or inorganic acids and bases known in the art to catalyze the hydrolysis of substituents from silicon atoms in the present of water. The hydrolysis catalyst can be an inorganic base such as potassium hydroxide or sodium hydroxide. The hydrolysis catalyst can be an inorganic acid such as hydrogen chloride, sulfuric acid, and nitric acid. The hydrolysis catalyst can be added separately to the reaction mixture, or in the case where component (B) is an organotrihalosilane may be at least partially generated in situ. A preferred hydrolysis catalyst is hydrogen chloride, at least a portion of which may be generated in situ when component (B) is an organotrichlorosilane.

[0063] The amount of hydrolysis catalyst (catalytic amount) added to the reaction mixture can be any amount that facilitates the hydrolysis of the silicon-bonded hydrolytic groups of components (A), (B), and (C) and the optimal amount will depend upon the chemical composition of the catalyst as well as the temperature at which the hydrolysis reaction occurs. Generally, the amount of hydrolysis catalyst can be within a range of about 0.02 to 0.5 mole per mole of components (A), (B), and (C). Preferred is when the amount of hydrolysis catalyst is within a range of about 0.1 to 0.3 mole, on the same basis.

[0064] Component (F) is an organic solvent for the reaction product. Component (F) can be any organic solvent or mixture of organic solvents in which the reaction product forms a homogeneous solution. Examples, of useful solvents include ketones such as methylisobutylketone, aromatic hydrocarbon solvents such as toluene, xylene, and mesitylene, isobutyl isobutyrate, benzotrifluoride, propylbenzene, isobutyl propionate, propyl butyrate, parachlorobenzotrifluoride, and n-octane. The amount of organic solvent can be any amount sufficient to effect a homogeneous solution of the reaction product. In general it is preferred that the organic solvent comprise about 70 to 95 weight percent of the total weight of components (A) through (F), and preferably 85 to 95 weight percent.

[0065] In a preferred process for making the reaction product comprising the silicone resin, components (A), (B), (C), and (F) are combined to form a first mixture. Then components (D) and (E) are added, either separately or as a mixture, to the first mixture along with mixing to effect formation of the reaction product. The formation of the reaction product can be effected at any temperature within a range of about 15° C. to 100° C., with ambient temperature being preferred. In the preferred process after the resulting reaction is completed, volatiles are removed from the reaction product under reduced pressure to isolate a resin solution. Such volatiles include alcohol by-products, excess water, catalyst, and solvents. If desired all solvent can be removed from the resin solution to form a solid resin. When removing all solvents to isolate a solid resin, the temperature of the resin solution should be maintained below about 60° C. and preferably within a range of about 30° C. to 50° C. Excess heat can lead to the formation of insoluble resins. If desired the catalyst and alcoholic by-products may be separated from the reaction product by washing with or more washes of water with interim phase separation to recovered a solution of the silicone resin in solvent.

[0066] The silicone resin comprising the reaction product as described above contains Si—OH functionality and may contain Si—OR³functionality, where R³is as previously described. It is preferred that the silicone resin comprise about 10 to 30 mol % of SiOH and 0 to 10 mol % Si—OR³.

[0067] A further embodiment of the present invention is a method to increase the molecular weight of and improve the storage stability of the reaction product comprising the silicone resin prepared as described above (hereafter in the alternative referred to as “bodying method”. The bodying method comprises (i) forming a solution of the silicone resin in an organic solvent at about 10 to 60 weight percent in the presence of an optional condensation catalyst, (ii) heating the solution at a temperature sufficient to effect condensation of the silicone resin to a weight average molecular weight of about 100,000 to 400,000, and (iii) neutralizing the solvent solution of silicone resin. It is preferred that in step (i) of this method that the silicone resin be present at about 20 to 30 weight percent in the organic solvent. The organic solvent can be any of those organic solvents described above. The optional condensation catalyst added in step (i) can be any of those acid and bases described above as hydrolysis catalyst for forming the reaction product. A preferred condensation catalyst is hydrogen chloride at a concentration within the range of 5 to 200 weight parts HCl per part of resin solid. More preferred is when the condensation catalyst is hydrogen chloride at a concentration within the range of 10 to 50 weight parts HCl per part of resin solid.

[0068] The temperature at which the solution of silicone resin is heated in step (ii) can be from about 50° C. up to the reflux temperature of the solution. In a preferred method the solution of silicone resin is refluxed to effect the increase in weight average molecular weight. In step (ii) it is preferred that the solution be heated such that the silicone resin after heating has a weight average molecular weight in the range of about 150,000 to 250,000. In step (iii) of the method the solvent solution of silicone resin is neutralized. Neutralization can be effected by washing the solution with one or more portions of water, or by removing the solvent under reduced pressure and redissolving the silicone resin in one or more portions of an organic solvent. The organic solvent used for the neutralization step can be any of the organic solvents described above.

[0069] The solution stability of the neutralized silicone resin can by further improved by dissolving the silicone resin in an organic solvent or organic solvent mixture and adding about 0.05 to 0.4 weight percent water, based upon the total weight of silicone resin, solvent, and water. Preferred is adding about 0.1 to 0.25 weight percent water, on the same basis. The organic solvent can be any of those organic solvents or mixtures thereof described above.

[0070] Specifically, the present invention is a method for forming a nanoporous silicone resin. By the term “nanoporous” it is meant a silicone resin having pores less than about 20 nm in diameter. A preferred embodiment of the present invention is an electronic substrate containing a nanoporous coating of the silicone resin. In the preferred embodiment of the invention, the nanoporous coating has a pore diameter within a range of about 0.3 nm to 2 nm. The reaction product comprising the silicone resin, if a solid, is dissolved and diluted in an organic solvent as described above for forming coating solutions. The concentration of silicone resin in the organic solvent is not particularly critical to the present invention and can be any concentration at which the silicone resin is soluble and which provides for acceptable flow properties for the solution in the coating process. Generally, a concentration of silicone resin in the organic solvent of about 10 to 25 weight percent is preferred. The silicone resin can be coated on the substrate, for example, by standard processes for forming coatings on electronic components such as spin coating, flow coating, dip coating, and spray coating. The substrate having the silicone resin coating is then heated in preferably an inert atmosphere at a temperature sufficient to effect curing of the silicone resin coating and thermolysis of carbon-carbon bond of the R²groups. The heating may be conducted as a single-step process or as a two-step process. In the two-step process the silicon resin is first heated in preferably an inert atmosphere at a temperature sufficient to effect curing without significant thermolysis of R²groups from silicon atoms. Generally, this temperature can be in a range of from about 20° C. to 350° C. Then, the cured silicone resin is further heated preferably in an inert atmosphere at a temperature within a range of greater than 350° C. up to the lessor of the decomposition temperature of the silicone resin polymer backbone or that which causes undesirable effects on the substrate to effect thermolysis of the R²groups from the silicon atoms. Generally, it is preferred that the thermolysis step be conducted at a temperature in a range of greater than 350° C. to about 600° C., with a temperature in a range of about 400° C. to 550° C. being most preferred. In the single-step process the curing of the silicone resin and thermolysis of R²groups from silicon atoms are effected simultaneously by heating the substrate having the silicone resin to a temperature within a range of greater than 350° C. up to the lessor of the decomposition temperature of the silicon polymer backbone or that which causes undesirable effects on the substrate. Generally, it is preferred that the single-step method of heating be conducted at a temperature in a range of greater than 350° C. to about 600° C., with a temperature in a range of about 400° C. to 550° C. being most preferred.

[0071] The method for forming the nanoporous coating on a substrate, either the one-step or two-step heating process, is preferably conducted in an inert atmosphere. The inert atmosphere is desirable because the presence of oxygen may oxidize Si—H bonds and cause an increase of residual silanol levels in the films resulting in an increased dielectric constant for the silicone resin. However, if desired, a minor amount of an oxidizing agent such as oxygen may be present in the atmosphere to tailor properties of the resulting nanoporous silicone resin. The inert atmosphere can be any of those known in the art, for example, argon, helium, or nitrogen.

[0072] The nanoporous silicone resins formed by the described method are particularly useful as low dielectric constant films on electronic devices such as integrated chips. The nanoporous silicone resin coatings prepared by the present method preferably can have a dielectric constant less than about 2. Such nanoporous silicone resins may also be made in particulate form by standard methods such as spray drying and heating as described above to make nanoporous and used in such applications as packing in chromatography columns and other such applications where porous materials are used. The following examples are provided to illustrate the present invention. These examples are not intended to limit the scope of the claims herein. In the Examples all parts are expressed as weight parts and mol % is based on the total moles of components (A)+(B)+(C) as described below.

[0073] Example 1. Samples 1-1 through 1-4 were prepared by mixing in a glass container components (A), (B), (C) and (F) as described below in the amounts described in Table 1:

[0074] (A) tetraethoxysilane,

[0075] (B) As indicated in Table 1,

[0076] (c) octadecyltrimethoxysilane, and

[0077] (D) mixture of methyl isobutyl ketone (MIBK) and toluene (85:15 weight ratio).

[0078] To this mixture was added a mixture of (D) water and (E) hydrogen chloride in the amounts described in Table 1. The weight part of component (C) was 1. The mole %'s of (A), (B) and (C) in each sample were 30%, 50% and 20%. The resulting reaction product was stripped of volatiles under reduced pressure at 60° C. The solubility of the resulting solid silicone resin was tested for MIBK solubility by adding 8.3 g of MIBK to 1.7 g of the solid silicone resin 24 hours after stripping was completed. The solid was considered soluble in the solvent if a clear solution was formed and no particles or gels where visually observed. The MIBK solubility is reported in Table 1.

[0079] Samples 1-1, 1-3, and 1-4 were heated to make porous and porosity determined. A sample of the solid resin was placed in a crucible and heated at 500° C. in nitrogen for 0.5 hour. The resulting solid was tested for nitrogen adsorption at 77° K using a Micrometrics ASAP 2000 Accelerated Surface Area and Porosimetry System (Micrometrics Instrument Corporation, Norcross, Ga.). H-K analysis (Horvath, J. Chem. Eng. Jpn., 1983, Vol. 16, p. 476) of the adsorption data was used to determine median pore sizes and micropore volumes. The results are shown in Table 2.

[0080] Samples 1-1, 1-2, and 1-4 were coated on a substrate and physical characteristics on the substrate determined. A sample of the solid silicone resin was dissolved at 17 Wt. % in MIBK and used to spin coat a silicon wafer. The coated silicon wafer was heated in a nitrogen atmosphere at 450° C. for 1 hour. The thin film data is show in Table 3.