Abstract:
Low dielectric constant porous materials with improved elastic modulus and film hardness. The process of making such porous materials involves providing a porous dielectric material and plasma curing the porous dielectric material with a fluorine-free plasma gas to produce a fluorine-free plasma cured porous dielectric material. Fluorine-free plasma curing of the porous dielectric material yields a material with improved modulus and hardness, but with a higher dielectric constant. The improvement in elastic modulus is typically greater than or about 100%, and more typically greater than or about 200%. The improvement in film hardness is typically greater than or about 50%. The fluorine-free plasma cured porous dielectric material can optionally be post-plasma treated. The post-plasma treatment of the fluorine-free plasma cured porous dielectric material reduces the dielectric constant of the material while maintaining an improved elastic modulus and film hardness as compared to the fluorine-free plasma cured porous dielectric material. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR §1.72(b).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/952,649, filed Sep. 14, 2001 and entitled “PLASMA CURING PROCESS FOR POROUS LOW-K MATERIALS”, which is a continuation-in-part of U.S. patent application Ser. No. 09/528,835, filed Mar. 20, 2000 and entitled “HIGH MODULUS, LOW DIELECTRIC CONSTANT COATINGS” (allowed Nov. 5, 2002) and U.S. patent application Ser. No. 09/681,332, filed Mar. 19, 2001 and entitled “PLASMA CURING PROCESS FOR POROUS SILICA THIN FILM” (allowed Dec. 11, 2002), the disclosures of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to a process which is employed in manufacturing semiconductor chips. More particularly, the invention relates to a process for improving the structural properties of certain porous materials that are utilized as integrated circuit (IC) dielectrics.  
           [0003]    New materials with low dielectric constants (known in the art as “low-k dielectrics”) are being investigated for their potential use as insulators in semiconductor chip designs. A low dielectric constant material aids in enabling further reductions in integrated circuit feature dimensions. The substance with the lowest dielectric constant is air (k=1.0). Therefore, porous dielectrics are very promising candidates since they have the potential to provide very low dielectric constants. Unfortunately, however, such porous low-k dielectrics typically have the problem of insufficient mechanical strength.  
           [0004]    Thin film dielectric coatings on electric devices are known in the art. For instance, U.S. Pat. Nos. 4,749,631 and 4,756,977, to Haluska et al., disclose silica based coatings produced by applying solutions of silicon alkoxides or hydrogen silsesquioxane, respectively, to substrates and then heating the coated substrates to a temperature between 200 and 1000° C. The dielectric constant of these coatings is often too high for certain electronic devices and circuits.  
           [0005]    U.S. Pat. Nos. 4,847,162 and 4,842,888, to Haluska et al., teach the formation of nitrided silica coatings by heating hydrogen silsesquioxane resin and silicate esters, respectively, to a temperature between 200 and 1000° C. in the presence of ammonia.  
           [0006]    Glasser et al., Journal of Non-Crystalline Solids, 64 (1984) pp. 209-221, teaches the formation of ceramic coatings by heating tetraethoxysilane in the presence of ammonia. This reference teaches the use of anhydrous ammonia and that the resulting silica coatings are nitrided.  
           [0007]    U.S. Pat. No. 4,636,440, to Jada, 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. Jada requires that the coating be dried prior to heating. It is specifically limited to hydrolyzed or partially hydrolyzed silicon alkoxides.  
           [0008]    U.S. Pat. Nos. 5,262,201, to Chandra et al., and 5,116,637, to Baney et al., teach the use of basic catalysts to lower the temperature necessary for the conversion of various preceramic materials, all involving hydrogen silsesquioxane, to ceramic coatings. These references teach the removal of solvent before the coating is exposed to the basic catalysts.  
           [0009]    U.S. Pat. No. 5,547,703, to Camilletti et al., teaches 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. This reference teaches the removal of solvent before converting the coating to a ceramic.  
           [0010]    U.S. Pat. No. 5,523,163, to Balance et al., teaches 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. The reference teaches the removal of solvent before converting the coating to a ceramic.  
           [0011]    U.S. Pat. No. 5,618,878, to Syktich et al., discloses 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 does not teach exposure of the coated substrates to basic catalysts before solvent removal.  
           [0012]    U.S. Pat. No. 6,231,989, to Chung et al., entitled “METHOD OF FORMING COATINGS” discloses a method of making porous network coatings with low dielectric constants. 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 cured by heating to form a ceramic. Films made by this process have dielectric constants in the range of 1.5 to 2.4 with an elastic modulus between about 2 and about 3 GPa.  
           [0013]    Porous low-k dielectric materials produced by spin-on and chemical vapor deposition processes typically require a curing process subsequent to the deposition. Typical process conditions for curing these low-k films include nitrogen purged furnace anneals at temperatures between about 350° C. and about 450° C. for 30 to 180 minutes. As was described in U.S. patent application Ser. Nos. 09/681,332, 09/952,649, 09/906,276 and 09/952,398, the disclosures of which are incorporated herein by reference, instead of thermally curing and plasma treating, porous network coatings can be plasma or UV cured, eliminating the need for prior furnace curing.  
           [0014]    However, there remains a need for a process of making a porous low-k material with improved structural properties, such as an improved elastic modulus, without compromising or deteriorating its electrical properties.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention meets that need by providing a fluorine-free plasma curing process for porous low-k materials.  
           [0016]    Although the present invention is not limited to specific advantages or functionality, it is noted that the process produces materials having a low dielectric constant and an improved elastic modulus and material hardness. The process significantly reduces cure process times and enables curing at low wafer temperatures as compared to conventional heat curing techniques, in addition to the advantages associated with eliminating the exposure of the dielectric to fluorine plasma species.  
           [0017]    In accordance with one embodiment of the present invention, a process is provided for making a fluorine-free plasma cured material comprising providing a porous dielectric material having a first dielectric constant, having a first elastic modulus, and having a first film hardness. The porous dielectric material is plasma cured with a fluorine-free plasma gas to produce a fluorine-free plasma cured porous dielectric material having a second dielectric constant which is comparable to or greater than the first dielectric constant, having a second elastic modulus which is greater than the first elastic modulus, and having a second film hardness which is greater than the first film hardness. By “comparable to” we mean both equal to and slightly less than, such as a second dielectric constant which is 0.05 less than the first dielectric constant. The increase in elastic modulus is typically greater than or about 100%, and more typically greater than or about 200%.  
           [0018]    The fluorine-free plasma cured porous dielectric material can optionally be post-plasma treated to provide a post-plasma treated, fluorine-free plasma cured porous dielectric material having a third dielectric constant, having a third elastic modulus, and having a third film hardness. Post-plasma treatment of the fluorine-free plasma cured porous dielectric material in some cases reduces the dielectric constant of the material while maintaining the increase in the elastic modulus and film hardness, as compared to the elastic modulus and film hardness before the post-plasma treatment.  
           [0019]    Accordingly, it is an object of the present invention to produce porous dielectric materials having improved elastic modulus and material hardness, and a low dielectric constant.  
           [0020]    These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention. It is noted that the scope of the claims is defined by the recitations therein and not by the specific discussion of features and advantages set forth in the present description.  
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The present invention is based on the discovery that plasma curing virtually any porous dielectric material, without the necessity of thermally curing the material, increases the elastic modulus (Young&#39;s modulus) and material hardness of the material while maintaining its low dielectric constant properties. The porous dielectric material can include, but is not limited to, hydrogen silsesquioxane (HSQ) dielectric materials, methylsilsesquioxane (MSQ) dielectric materials, organic dielectric materials, inorganic dielectric materials, and combinations thereof, which can be produced by spin-on or chemical vapor deposition (CVD) processes. The porous dielectric materials can have porogen-generated, solvent-based, or molecular engineered pores, which may be interconnected or closed, and which may be distributed random or ordered, such as vertical pores.  
         [0022]    Plasma curing can for some materials generate a notable amount of polar species in the porous dielectric material, which can be undesirable in some applications. The present invention is also based on the discovery that applying a thermal treatment can remove these generated polar species resulting in a material with a low dielectric constant, and of equal or further improved elastic modulus and film hardness.  
         [0023]    The process of the present invention is 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 pressure used in the present invention. As such, the porous dielectric materials of the present invention can be used on substrates such as plastics including, for example, polyimides, epoxies, polytetrafluoroethylene and copolymers thereof, polycarbonates, acrylics and polyesters, ceramics, leather, textiles, metals, and the like.  
         [0024]    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. The expressions “electronic device” or “electronic circuit” include, but are not limited to, silica-based devices, gallium arsenide based devices, silicon carbide based devices, focal plane arrays, opto-electronic devices, photovoltaic cells, and optical devices.  
         [0025]    A porous dielectric material is needed as a starting material for the present invention. Typical HSQ-based dielectric materials for use with the present invention include FOx HSQ-based dielectric material and XLK porous HSQ-based dielectric material available from Dow Corning Corporation (Midland, Mich.). In addition, typical ultra low-k porous dielectric MSQ-based materials, made by spin-on processing, for use with the present invention are available from Chemat Technology, Inc. (Northridge, Calif.) and JSR Corporation (Tokyo, Japan).  
         [0026]    The production of typical porous dielectric materials for use with the present invention is well known in the art. One method of making such a porous dielectric material is the porous network coating disclosed in U.S. Pat. No. 6,231,989, which is incorporated herein by reference for its teaching on how to produce porous dielectric materials having ultra low dielectric constants. The patent describes the manufacture of ultra low dielectric constant coatings having a dielectric constant of between about 1.5 and about 2.4, in which pores are introduced into HSQ-based films. HSQ-based films produced according to the method taught in U.S. Pat. No. 6,231,989, which have been cured under thermal conditions, contain about 20 to about 60% Si—H bonds density. When the dielectric constant of the coating is about 2.0, the coating has an elastic modulus of between about 2 and about 3 GPa.  
         [0027]    The following method of producing a porous network coating is provided as an example of the production of a typical porous dielectric material. It is not the inventors&#39; intent to limit their invention to only HSQ-based films. The process of the present invention is applicable to virtually any porous dielectric material.  
         [0028]    The method of producing the HSQ-based porous network coating starts with depositing a coating on a substrate with a solution comprising a resin containing at least 2 Si—H groups and a solvent. The resins containing at least 2 Si—H groups are not particularly limited, as long as the Si—H bonds can be hydrolyzed and at least partially condensed by the basic catalyst and water to form a cross-linked network that 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   
         [0029]    wherein each R is independently selected from the group consisting of 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 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 carbons being typical. Examples of alkenyl groups include vinyl, allyl, and hexenyl. Examples of aryls include phenyl. Examples of substituted groups include CF 3 (CF 2 ) n CH 2 CH 2 , where n=0-6.  
         [0030]    Useful in the present invention are various hydridosiloxane resins, known as hydrogen silsesquioxane resins, comprising units of the formula HSi(OH) x (OR′) y O z/2 . In this formula, each R′ is independently selected from the group consisting of 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. Examples of alkyl groups are methyl, ethyl, propyl, butyl, and the like, with alkyls of 1-6 carbons being typical. Examples of alkenyl groups include vinyl, allyl, and hexenyl. Examples of aryls include phenyl. Examples of substituted groups include CF 3 (CF 2 ) n CH 2 CH 2 , where n=0-6. When these R′ groups are bonded to silicon through the oxygen atom, they form a hydrolyzable substituent. In the above formula, x=0 to 2, y=0 to 2, z=1 to 3, and x+y+z=3. These resins may be essentially fully condensed (HSiO 3/2 ) n  where n is 8 or greater, or they may be only partially hydrolyzed (i.e., containing some Si—OR′), and/or partially condensed (i.e., containing some Si—OH).  
         [0031]    The structure of the resin containing at least 2 Si—H groups is not limited. The structure may be what is generally known as ladder-type, cage-type, or mixtures thereof. The HSQ resins may contain endgroups such as hydroxyl groups, triorganosiloxy groups, diorganohydrogensiloxy groups, trialkoxy groups, dialkoxy groups, and others. The HSQ resin may also contain a small number (e.g., less than 10%) of the silicon atoms, which have either 0 or 2 hydrogen atoms attached thereto and/or a small number of Si—C groups, such as CH 3 SiO 3/2  or HCH 3 SiO 2/2  groups.  
         [0032]    The resins containing at least 2 Si—H groups and methods for their production are known in the art. For example, U.S. Pat. No. 3,615,272, to Collins et al., teaches the production of an essentially fully condensed hydrogen silsesquioxane resin (which may contain up to 100-300 ppm silanol) by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium, and then washing the resulting resin with water or aqueous sulfuric acid. Similarly, U.S. Pat. No. 5,010,159, to Bank et al., teaches a method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent.  
         [0033]    Other hydridosiloxane resins, such as those described in U.S. Pat. No. 4,999,397, to Weiss et al., and U.S. Pat. No. 5,210,160, to Saive et al., those produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium, those described in Japanese Kokai Patent Nos. 59-178749, 60-86017, and 63-107122, or any other equivalent hydridosiloxanes, will also function herein.  
         [0034]    Specific molecular weight fractions of the Si—H containing resins may also be used. Such fractions and methods for their preparation are taught in U.S. Pat. No. 5,063,267, to Hanneman et al., and U.S. Pat. No. 5,416,190, to Mine et al. A typical fraction comprises material wherein at least 75% of the polymeric species have a number average molecular weight above about 1200, and a more typical fraction comprises material wherein at least 75% of the polymeric species have a number average molecular weight between about 1200 and about 100,000.  
         [0035]    The Si—H containing resins may contain other components as long as these components do not interfere with the integrity of the coating. It should be noted, however, that certain materials may increase the dielectric constant of the coating.  
         [0036]    Ceramic oxide precursors may also be used in combination with the Si—H containing resins. The ceramic oxide precursors useful 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 phosphorus, which may be dissolved in solution, hydrolyzed, and subsequently pyrolyzed at relatively low temperature to form ceramic oxides. Ceramic oxide precursors useful herein are described in U.S. Pat. Nos. 4,808,653 and 5,008,320 to Haluska et al., and 5,290,394 to Sasaki.  
         [0037]    The Si—H containing resins are applied to the substrates as solvent dispersions to form a coating on the substrate (“SiH resin coating”). Solvents that may be used include any agent or mixture of agents that will dissolve or disperse the resin to form a homogeneous liquid mixture without affecting the resulting coating or the substrate. These solvents can include alcohols, such as ethyl alcohol or isopropyl alcohol; aromatic hydrocarbons, such as benzene or toluene; branched or linear alkanes, such as n-heptane, dodecane, or nonane; branched or linear alkenes, such as n-heptene, dodecene, or tetradecene; ketones, such as methyl isobutyl ketone; esters; ethers, such as glycol ethers; or linear or cyclic siloxanes, such as hexamethyidisiloxane, octamethyldisiloxane, and mixtures thereof, or cyclic dimethylpolysiloxanes; or mixtures of any of the above solvents. The solvent is generally present in an amount sufficient to dissolve/disperse the resin to the concentration desired for application. Typically, the solvent is present in an amount of about 20 to about 99.9 wt %, and more typically from about 70 to about 95 wt %, based on the weight of the resin and solvent.  
         [0038]    If desired, other materials can be included in the resin dispersion. For instance, the dispersion can include fillers, colorants, adhesion promoters, and the like.  
         [0039]    Specific methods for application of the resin dispersion to the substrate include, but are not limited to, spin coating, dip coating, spray coating, flow coating, screen printing, or others. A typical method is spin coating.  
         [0040]    At least about 5 volume % of the solvent should remain in the SiH resin coating until the resin is contacted with the basic catalyst and water. This solvent forms the pores of the porous network coating as the Si—H bonds are hydrolyzed and condensed. In some embodiments, it may be typical that at least about 10 volume % solvent remains, while in others, it may be typical that at least about 15 volume % solvent remains, and in still others, it may be typical that at least about 25 volume % solvent remains.  
         [0041]    The method of retaining the solvent is not particularly restricted. In a typical embodiment, a high boiling point solvent can be used alone or as a co-solvent with one of the solvents described above. In this manner, processing the resin dispersion as described above under normal conditions allows for at least about 5% residual solvent remaining. Typical high boiling solvents in this embodiment are those with boiling points above about 175° C. including hydrocarbons, aromatic hydrocarbons, esters, ethers, and the like. Examples of specific solvents which can be used in this embodiment include saturated hydrocarbons, such as dodecane, tetradecane, hexadecane, etc., unsaturated hydrocarbons, such as dodecene, tetradecene, etc., xylenes, mesitylene, 1-heptanol, dipentene, d-limonene, tetrahydrofurfuryl alcohol, mineral spirits, 2-octanol, stoddard solvent, Isopar H™, diethyl oxalate, diamyl ether, tetrahydropyran-2-methanol, lactic acid butyl ester, isooctyl alcohol, propylene glycol, dipropylene glycol monomethyl ether, diethylene glycol diethyl ether, dimethyl sulfoxide, 2,5-hexanedione, 2-butoxyethanol acetate, diethylene glycol monomethyl ether, 1-octanol, ethylene glycol, Isopar L™, dipropylene glycol monomethyl ether acetate, diethylene glycol monoethyl ether, N-methylpyrrolidone, ethylene glycol dibutyl ether, gamma-butyrolactone, 1,3-butanediol, diethylene glycol monomethyl ether acetate, trimethylene glycol, triethylene glycol dimethyl ether, diethylene glycol monoethyl ether acetate, alpha-terpineol, n-hexyl ether, kerosene, 2-(2-n-butoxyethoxy)ethanol, dibutyl oxalate, propylene carbonate, propylene glycol monophenyl ether, diethylene glycol, catechol, diethylene glycol monobutyl ether acetate, ethylene glycol monophenyl ether, diethylene glycol dibutyl ether, diphenyl ether, ethylene glycol monobenzyl ether, hydroquinone, sulfolane, and triethylene glycol. Hydrocarbon solvents are particularly preferred.  
         [0042]    The above processing (i.e., primarily deposition of the SiH resin coating solution) can be done in an environment that inhibits solvent evaporation prior to contact with the basic catalyst and water. For example, the spin coating can be performed in a closed environment such that the subsequent steps (i.e., contact with the basic catalyst and water) can occur before the solvent is completely evaporated.  
         [0043]    The SiH resin coating containing at least about 5 volume % solvent is then contacted with a basic catalyst and water. Examples of basic catalysts include ammonia, ammonium hydroxide, as well as amines. The amines useful herein may include primary amines (RNH 2 ), secondary amines (R 2 NH), and/or tertiary amines (R 3 N) in which R is independently a saturated or unsaturated aliphatic, such as methyl, ethyl, propyl, vinyl, allyl, ethynyl, etc.; an alicyclic, such as cyclohexylmethyl; an aromatic, such as phenyl; a substituted hetero atom, such as oxygen, nitrogen, sulfur, etc.; or compounds in which the nitrogen atom is a member of a heterocyclic ring such as quinoline, pyrrolidine, or pyridine. In addition, any of the above amine compounds may be substituted with other hydrocarbon and/or hetero containing groups to form compounds such as diamines, amides, etc. Finally, it is also contemplated that compounds, which are converted to amines under the reactions conditions used, would function in an equivalent manner. For example, a compound such as an ammonium salt that yields an amine upon dissolution would provide the desired catalytic effect.  
         [0044]    Examples of the amines that may be used herein include methylamine, ethylamine, butylamine, allylamine, cyclohexylamine, aniline, dimethylamine, diethylamide, dioctylamine, dibutylamine, methylethylamine, saccharin, piperidine, trimethylamine, triethylamine, pyridine, diethyl toluidene ethylmethylpropylamine, imidazole, choline acetate, triphenyl phosphene analine, trimethylsilylimidazole, ethylenediamine, diethylhydroxylamine, triethylenediamine, n-methylpyrolidone, etc.  
         [0045]    The basic catalyst can generally be used at any concentration sufficient to catalyze hydrolysis of the Si—H bonds. Generally, concentrations of the basic catalyst can be from about 1 ppm to about 100 wt % based on the weight of the resin, depending on the basic catalyst.  
         [0046]    The water used can be that present in the ambient environment (e.g., &gt;about 25% relative humidity), the ambient environment can be supplemented with additional water vapor (e.g., relative humidity up to about 100%), water can be used as a liquid, or a compound which generates water under the reaction conditions can be used.  
         [0047]    Contact of the SiH resin coating with the basic catalyst and water can be accomplished by any means practical or desirable. For instance, the SiH resin coating can be contacted with vapors of the basic catalyst and water vapor. Alternatively, the SiH resin coating can be contacted with the basic catalyst and water in the liquid state, such as by immersing the coating in an ammonium hydroxide solution.  
         [0048]    The SiH resin coating is typically exposed to an environment comprising the basic catalyst and water in the vapor state, more typically ammonia and water vapor. For instance, the SiH resin coated substrate may be placed in a container and the appropriate environment introduced therein, or a stream of the basic catalyst and water may be directed at the SiH resin coating.  
         [0049]    The method used to generate the basic catalyst and water environment is generally not significant in the present embodiment. Methods such as bubbling the basic catalyst (e.g., ammonia gas) through water or ammonium hydroxide solutions (to control the amount of water vapor present), heating a basic catalyst and water, or heating water and introducing the basic catalyst gas (e.g., ammonia gas) are all functional herein. It is also contemplated that methods, which generate basic catalyst vapors in situ, such as the addition of water to amine salts, or the addition of water to a silazane, such as hexamethyldisilazane, will also be effective.  
         [0050]    The basic catalyst used may be at any concentration desired. For example, the concentration may be from about 1 ppm up to a saturated atmosphere.  
         [0051]    The exposure can be at any temperature desired from room temperature up to about 300° C. A temperature in the range of from about 20° C. to about 200° C. is typical, with a range of from about 20° C. to about 100° C. being more typical.  
         [0052]    The SiH resin coating should be exposed to the basic catalyst and water environment for the time necessary to hydrolyze the Si—H groups to form silanols (Si—OH) and for the silanols to at least partially condense to form Si—O—Si bonds. Generally, exposures of up to about 20 minutes are typical, with exposures of at least about 1 second up to about 5 minutes being more typical. If the coatings are to be used as a dielectric layer, it is generally typical to have a shorter exposure, as longer exposures tend to increase the dielectric constant of the coating.  
         [0053]    When the coating is exposed to the basic catalyst and water in the liquid state, the exposure is usually conducted by immersing the coated substrate in a solution. Other equivalent methods can be used, such as flushing the coating with a basic catalyst and water solution. In addition, vacuum infiltration may also be used to increase penetration of the basic catalyst and water into the coating.  
         [0054]    The basic catalyst solution used in this embodiment may be at any concentration desired. Generally when ammonium hydroxide is used, a concentrated aqueous solution of between about 28 and about 30% is typical since the duration of exposure is thereby shortened. When dilute solutions are used, the diluent is generally water.  
         [0055]    Exposure to the basic catalyst and water solution in this embodiment may be conducted at any temperature and pressure desired. Temperatures from about room temperature (20-30° C.) up to about the boiling point of the basic catalyst solution, and pressures from below to above atmospheric are all contemplated herein. From a practical standpoint, it is typical that the exposure occur at about room temperature and at about atmospheric pressure.  
         [0056]    The resin coating is exposed to the basic catalyst solution in this embodiment for the time necessary to hydrolyze the Si—H groups to form silanols (Si—OH) and for the silanols to at least partially condense to form Si—O—Si bonds. Generally, exposures of up to about 2 hours are typical, with exposures of at least about 1 second up to about 15 minutes being more typical.  
         [0057]    Alternatively, the coating may be exposed to both a liquid basic catalyst and water environment (e.g., ammonium hydroxide) and a gaseous basic catalyst and water vapor environment (ammonia gas and water vapor). The exposures may be either sequential or simultaneous, and are generally under the same conditions as those described above.  
         [0058]    After the resin is exposed to one of the above environments, the solvent is then removed from the coating. This can be accomplished by any desired means, including but not limited to, heating the coating, and by vacuum. When the solvent is removed by heating the coating, condensation of the remaining silanols may be facilitated.  
         [0059]    The coating produced by this process can be used as the starting material (“porous network coating”) in the present invention. In a typical procedure to produce a porous network coating, a substrate is coated with the Si—H containing resin and solvent in a manner which ensures that at least about 5 volume % of the solvent remains in the coating. The coating is then exposed to the basic catalyst and water, and the solvent is evaporated.  
         [0060]    Another method of making such a porous network coating is to thermally cure a siloxane resin containing large alkyl groups and to thermally decompose the alkyl groups to create porosity in the coating. As disclosed in U.S. Pat. Nos. 6,143,360 and 6,184,260, to Zhong, which are hereby incorporated herein by reference, hydridosilicon containing resin was allowed to contact with a 1-alkene comprising about 8 to about 28 carbon atoms in the presence of a platinum group metal-containing hydrosilation catalyst, effecting formation of an alkylhydridosiloxane resin where at least about 5 percent of the silicon atoms are substituted with at least one hydrogen atom, and the resulting resin was heated at a temperature sufficient to effect curing of the resin and thermolysis of alkyl groups from the silicon atoms, thereby forming a nanoporous silicone resin.  
         [0061]    U.S. Pat. Nos. 6,232,424, 6,359,096 and 6,313,045, and U.S. patent application Ser. No. 425,901 to Zhong et al., which are hereby incorporated herein by reference, disclose silicone resins and porous coatings made from the silicone resins. The silicone resins are made from a mixture compromising 15 to 70 mol % of tetraalkoxysilane, 12 to 60 mol % of an organosilane described by formula R′SiX3, where R′ is an hydrogen or alkyl group containing 1 to 6 carbon atoms, and 15 to 70 mol % of an organotrialkyoxysilane described by formula R″Si(OR″′)3, where R″ is a hydrocarbon group compromising about 8 to 24 carbon atoms or a substituted hydrocarbon group compromising a hydrocarcon chain having about 8 to 24 carbon atoms.  
         [0062]    U.S. patent application Ser. No. 09/951,819 entitled “SILICONE RESINS AND POROUS MATERIALS PRODUCED THEREFROM”, to Zhong, filed Sep. 12, 2001 and hereby incorporated herein by reference, discloses porous coatings made from silicone resins having the general formula (R 1 SiO 3/2 ) x (HSiO 3/2 ) y  where R 1  is an alkyl group having 8 to 24 carbon atoms. The coatings produced therein have a dielectric constant between 1.5 and 2.3. The above-referenced patent application further provides the following description of a porous low-k dielectric coating made in two steps from a resin with a formula of (R 1 SiO 3/2 ) x (HSiO 3/2 ) y  where R 1  is 3,7,11,15-tetramethyl-3-hydroxy-hexadecyl.  
         [0063]    U.S. patent application Ser. No. 09/951,899 entitled “SILICONE RESINS AND POROUS MATERIALS PRODUCED THEREFROM”, to Zhong, filed Sep. 12, 2001 and hereby incorporated herein by reference, discloses porous coatings made from silicone resins having the general formula (R 1 SiO 3/2 ) u (HSiO 3/2 ) v (SiO 4/2 ) w (HOSiO 3/2 ) z  where R 1  is a branched alkyl group having 8 to 24 carbon atoms containing at least one electron-withdrawing group in a pendant position on the alkyl chain; u has a value of 0.1 to 0.7; v has a value of 0.12 to 0.6; z≧0.5; w+z has a value of 0.15 to 0.7; and u+v+w+z=1.  
         [0064]    Step 1. A resin sample was prepared by combining components (A), (B), (C), (D), (E), and (F) as described below in the amounts described in Table 1 of the above-referenced U.S. patent application:  
         [0065]    (A) 0.45 mole parts of triethoxysilane,  
         [0066]    (B) 0.25 mole parts of an organotriethoxysilane, RSi(OR′)3 where R is 3,7,11,15-tetramethyl-3-hydroxy-hexadecyl,  
         [0067]    (C) 0.30 mole parts of tetraethoxysilane, and  
         [0068]    (D) a mixture of methyl isobutyl ketone (MIBK) and isobutyl isobutyrate (6:4 weight ratio), enough to make the concentration of the resulting resin 9%.  
         [0069]    To this mixture was added a mixture of (E) water and (F) hydrogen chloride in the amounts described in Table 1 of the above-referenced U.S. patent application. The resulting reaction product was stripped of volatiles under reduced pressure at 60° C. until the solid content became 14 to 21%. Isobutyl isobutyrate was added to make the solid content 14%. The solution was then heated to reflux for 2 hours and water produced was removed continuously. The solvent was then changed to cyclohexanone by stripping off isobutyl isobutyrate and adding cyclohexanone.  
         [0070]    Step 2. The resulting resin solution was spin-coated onto silicon wafers suitable for dielectric constant measurements and cured in a nitrogen flow at 440° C. for 1 hour. The dielectric constant was measured as 1.9. Alternatively, the curing of the spin-coated films may be accelerated with plasma and/or UV assisted processes.  
         [0071]    U.S. patent application Ser. No. 09/915,899, to Albaugh et al., which is hereby incorporated herein by reference, discloses porous coatings from resins containing (RSiO 3/2 )(R′SiO 3/2 )(R″SiO 3/2 ) resins wherein R is an alkyl group having 1 to 5 carbon atoms or a hydrogen atom, R′ is a branched alkoxy group and R″ is a substituted or un-substituted linear, branched, or cyclic monovalent organic group having 6 to 30 carbon atoms.  
         [0072]    U.S. patent application Ser. Nos. 09/915,902, to Albaugh et al., and 09/915,903, to Boisvert et al., which are hereby incorporated herein by reference, disclose porous coatings made from resins of the formula TRTR′ where R is either a methyl or hydrogen group and a R′ is a branched alkoxy group.  
         [0073]    Although porous dielectric materials having low dielectric constants are desirable, it would be advantageous to have a porous dielectric material with a higher elastic modulus and film hardness.  
         [0074]    In order to raise the elastic modulus of the porous dielectric material, it is exposed to a plasma cure. In accordance with the present invention, the process utilizes a fluorine-free plasma gas. By “fluorine-free” we mean a plasma gas that does not contain fluorine species. For example, the fluorine-free plasma gas can be a combination of CH 4  and N 2 , or CH 4  and N 2  in combination with H 2  or a noble gas such as, for example, Ar or He.  
         [0075]    By utilizing a fluorine-free plasma gas, there is no opportunity for fluorine species to react with or penetrate into the porous low-k material. Trapping of such fluorine species in the porous network can lead to voiding, corrosion, and other forms of damage to the dielectric material, which can cause the immediate failure of a device containing such material, as well as affect device yield and/or significantly reduce the useful lifetime of the device. Moreover, unbound fluorine species can move through the dielectric and react with other absorbed or trapped residual compounds, or reach interfaces to other materials within the dielectric causing severe damage. Accordingly, the present invention by employing a fluorine-free plasma gas for curing of the porous dielectric material significantly reduces or eliminates the presence of fluorine species in the film.  
         [0076]    The fluorine-free plasma cure can be done by radio frequency (RF), inductive coupled, RF capacitive coupled, helical resinator, microwave downstream, and microwave electron cyclotron resonance (ECR) plasma. The fluorine-free plasma curing process improves the mechanical properties of the porous low-k dielectric material, increasing material hardness while maintaining the dielectric pore, structure, density, and electrical properties.  
         [0077]    In a typical fluorine-free plasma curing process, the wafer is quickly heated in a rapid temperature ramp-up step to the desired temperature, and the wafer is plasma cured with a gas mixture comprising CH 4  and N 2 , which generates the curing plasma chemistry. The plasma gas is devoid of any plasma fluorine.  
         [0078]    The exact conditions for the fluorine-free plasma cure depend upon what type of plasma cure is being used. Typically, the porous dielectric material is plasma cured at a process pressure between about 1.0 Torr and about 5.0 Torr. Examples of typical microwave plasma cure conditions for a 200 mm wafer are illustrated in Table 1 below.  
                             TABLE 1                       Typical Fluorine-Free Plasma Cure Conditions for a 200 mm Wafer                                    Microwave Plasma Power:   1000-2000 W           Wafer Temperature:   250-420° C.           Process Pressure:   1.0-5.0 Torr           Plasma Cure Time:   &lt;120 seconds           Plasma Gasses:   CH 4 /N 2  or CH 4 /N 2 /H 2             CH 4 /N 2  Flow Rate:   2000-3000 sccm           CH 4 :N 2  Gas Ratio:   0.03 to 0.05                      
 
         [0079]    The elastic modulus and film hardness of the fluorine-free plasma cured porous dielectric materials are increased as compared to a furnace (thermally) cured porous dielectric material. The furnace cured elastic modulus is between about 0.5 GPa and about 3.5 GPa when the dielectric constant is between 1.6 and 2.4. This increase in the elastic modulus is typically greater than or about 100%, and more typically greater than or about 200%. Typically, the elastic modulus of the plasma cured porous dielectric material is greater than or about 2.5 GPa, and more typically between about 2.5 GPa and about 10 GPa. The film hardness of the furnace cured porous films is about 0.1 GPa and the increase in the film hardness is typically greater than or about 50%. Typically, the film hardness of the plasma cured porous dielectric material is greater than or about 0.25 GPa, and more typically between about 0.25 GPa and 0.8 GPa.  
         [0080]    A comparison of process conditions and material properties for the fluorine-free plasma cure process of the present invention, a fluorine-containing plasma cure process, a hot plate thermal cure process, and a vertical furnace thermal cure process are presented in Table 2 below.  
                                                           TABLE 2                           Process Conditions and Material Properties for Different Cure Conditions                Fluorine-   Fluorine-       Vertical           Free Plasma   Containing   Hot Plate   Furnace           Cure   Plasma Cure   Thermal Cure   Thermal Cure                        Tool   Plasma Asher   Plasma Asher   Hot Plate   Vertical Furnace       Atmosphere   CH 4 /N 2     CF 4  + H 2 /N 2     N 2     N 2         Pressure (kPa)           AP   0.15       Cure Temp. (° C.)   420   420   425   425       Cure Time (min.)   2   2   3   30       R.I.   1.249   1.258   1.248   1.248       Film Shronk (%)   2.5   2.6   2.0   2.9       Dielectric   2.30   2.28   2.34   2.22       Constant (k)       Dielectric   2.16   2.16   2.16   2.12       Constant at       200° C.       Delta k   0.14   0.12 0.18   0.10       Elastic Modulus   4.4   4.1   4.1   4.7       (GPa)       Hardness (GPa)   0.61   0.60   0.55   0.66                  
 
         [0081]    The fluorine-free plasma cured porous dielectric materials of the present invention have improved chemical stability and improved dimensional stability. By improved chemical stability, we mean that the fluorine-free porous dielectric materials are more resistant to chemicals, such as cleaning solutions and chemical polishing solutions, and plasma damaging during photoresist ashing and dry etching processes.  
         [0082]    The fluorine-free plasma cure significantly reduces or eliminates the outgassing of oliomeric polysilica and other substances from the porous films. In addition, unlike plasma cure processes that utilize a plasma gas comprising fluorine, the fluorine-free plasma cure process of the present invention does not generate a notable amount of polar species in the film. Ordinarily, with such fluorine-based plasma cure processes, the initial plasma curing of the film can introduce chemical and electrical changes that are reversed or repaired by employing a second post-cure plasma treatment to condition the film. However, by employing a fluorine-free plasma gas, the plasma cure process of the present invention defines a single-phase process that cures the film without causing unwanted changes therein. Accordingly, an additional post-cure treatment of the film need not be performed.  
         [0083]    The fluorine-free plasma cured porous dielectric materials can optionally be post-plasma treated using any type of thermal exposure to reduce the dielectric constant and/or further increase the elastic modulus and film hardness, if desired. For example, the plasma cured porous dielectric materials can be annealed by placing the materials in a conventional oven, such as at a temperature of between about 400° C. and about 450° C. for between about 30 and about 60 minutes. An alternative process for annealing the materials involves annealing the plasma cured porous dielectric materials in a Rapid Anneal Processing (RAP) chamber in order to reduce the dielectric constant. The fluorine-free plasma cured porous dielectric material is annealed at a typical temperature for a sufficient time, and cooled to about 100° C. However, RAP may not be necessary in some applications.  
         [0084]    Typical operating conditions for the RAP process are shown in Table 3 below.  
                             TABLE 3                       Typical Rapid Anneal Processing Conditions                                    Ramp Rate:    15-150° C.           Wafer Temperature:   300-450° C.           Annealing Time:   &lt;120 seconds           Process Pressure:   atmospheric           Atmosphere:   N 2                        
 
         [0085]    The dielectric constant of the post-plasma treated, fluorine-free plasma cured porous dielectric materials is reduced as compared to the fluorine-free plasma cured porous dielectric materials. The dielectric constant of the post-plasma treated, fluorine-free plasma cured porous dielectric materials is typically between about 1.1 and about 3.5, and more typically between about 1.8 and about 2.4.  
         [0086]    For some applications it is desirable to utilize the fluorine-free plasma for a partial conversion of the porous low-k films. The partial conversion process allows to control the material properties of the porous low-k films, such as Young&#39;s modulus, film hardness, hydrophobicity, and dielectric constant, as well as the Si—H, Si—OH, and/or Si—CH 3  contents of the porous low-k film. Different partial conversion conditions are achieved by utilizing different plasma cure conditions, such as time, pressure, temperature, and plasma gas composition. For HSQ-based porous low-k films, typical partial plasma conversion processes result in films that have a Si—H content of between more than 0% and less than or about 70%, and more typical between about 1% and about 30%.  
         [0087]    While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and methods disclosed herein may be made without departing from the scope of the invention. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that is have the full scope permitted by the language of the following claims.