# Patent

## Title
Low metal porous silica dielectric for integral circuit applications

## Abstract

The invention relates to the production of nanoporous silica dielectric films and to semiconductor devices and integrated circuits comprising these improved films. The nanoporous films of the invention are prepared using silicon containing pre-polymers and are prepared by a process that allows crosslinking at lowered gel temperatures by means of a metal-ion-free onium or nucleophile catalyst.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the production of nanoporous silica dielectric films and to semiconductor devices and integrated circuits comprising these improved films. The nanoporous films of the invention are prepared using silicon containing pre-polymers and are prepared by a process that allows crosslinking at lowered gel temperatures by means of a metal-ion-free onium or nucleophile catalyst.
2. Description of the Related Art
As feature sizes in integrated circuits are reduced to below 0.15 μm and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials used in the manufacture of integrated circuits.
One type of material with a low dielectric constant is nanoporous silica films prepared from silicon containing pre-polymers by a spin-on sol-gel technique. Air has a dielectric constant of 1, and when air is introduced into a suitable silica material having a nanometer-scale pore structure, such films can be prepared with relatively low dielectric constants (“k”). Nanoporous silica materials are attractive because similar precursors, including organic-substituted silanes, such as tetraacetoxysilane (TAS)/methyltriacetoxysilane (MTAS)-derived silicon polymers are used as the base matrix and are used for the currently employed spin-on-glasses (“S.O.G.”) and chemical vapor deposition (“CVD”) of silica SiO2. Such materials have demonstrated high mechanical strength as indicated by modulus and stud pull data. Mechanical properties can be optimized by controlling the pore size distribution of the porous film. Nanoporous silica materials are attractive because it is possible to control the pore size, and hence the density, mechanical strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous films offer other advantages including thermal stability to 900° C.; substantially small pore size, i.e., at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit; preparation from materials such as silica and tetraethoxysilane (TEOS) that are widely used in semiconductors; the ability to “tune” the dielectric constant of nanoporous silica over a wide range; and deposition of a nanoporous film can be achieved using tools similar to those employed for conventional S.O.G. processing.
Thus, high porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. An additional advantage, is that additional compositions and processes may be employed to produce nanoporous films while varying the relative density of the material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous films that controls the dielectric constant of the material, and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the degree of porosity decreases, and vice versa. This suggests that the density range of nanoporous films must be optimally balanced between the desired range of low dielectric constant and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate suitable for the purpose. Usually, a precursor in the form of, e.g., a spin-on-glass composition is applied to a substrate, and then polymerized in such a way as to form a dielectric film comprising nanometer-scale voids.
When forming such nanoporous films, e.g., by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and water to cause polymerization/gelation (“aging”) during an initial heating step. In order to achieve maximum strength through pore size selection, a low molecular weight porogen is used.
U.S. Pat. No. 5,895,263 describes forming a nanoporous silica dielectric film on a substrate, e.g., a wafer, by applying a composition comprising decomposable polymer and organic polysilica i.e., including condensed or polymerized silicon polymer, heating the composition to further condense the polysilica, and decomposing the decomposable polymer to form a porous dielectric layer. This process, like many of the previously employed methods of forming nanoporous films on semiconductors, has the disadvantage of requiring heating for both the aging or condensing process, and for the removal of a polymer to form the nanoporous film. Furthermore, there is a disadvantage that organic polysilica, contained in a precursor solution, tends to increase in molecular weight after the solution is prepared; consequently, the viscosity of such precursor solutions increases during storage, and the thickness of films made from stored solutions will increase as the age of the solution increases. The instability of organic polysilica thus requires short shelf life, cold storage, and fine tuning of the coating parameters to achieve consistent film properties in a microelectronics/integrated circuit manufacturing process.
Formation of a stable porous structure relies on the condition that the porogen removal temperature is higher than the crosslinking temperature (or the gel temperature) of the matrix material. It was found that a stable nanoporous structure of less than 10 nm pore size cannot be produced when the concentration of the alkali cation such as sodium is below 200-300 ppb level in the spin-on solution. However, stringent requirement for low metal concentration must be met for IC applications. The general practice is to have metal concentration below 50 ppb in the spin-on solution. Therefore, there is a need to develop a low metal nanoporous silica film that can consistently give dielectric constant less than 2.5 and pore size less than about 10 nm in diameter. It has now been found that by the use of onium ions or nucleophiles the formation of a porous silica network at lower temperature in a low metal spin-on formulation can be facilitated. The effect of the onium ions or nucleophiles is to lower the gel temperature so that the rigid network is set in before the removal of the porogen, thus producing a nanoporous film without requiring the presence of an undesirable alkali ion.
SUMMARY OF THE INVENTION
The invention provides a method of producing a nanoporous silica dielectric film comprising
(a) preparing a composition comprising a silicon containing pre-polymer, a porogen, and a metal-ion-free catalyst selected from the group consisting of onium compounds and nucleophiles;
(b) coating a substrate with the composition to form a film,
(c) crosslinking the composition to produce a gelled film, and
(d) heating the gelled film at a temperature and for a duration effective to remove substantially all of said porogen.
The invention also provides a composition comprising silicon containing pre-polymer, a porogen, and a catalyst selected from the group consisting of onium compounds and nucleophiles.
The invention further provides a method of lowering the temperature at which a porous silica film forms comprising the step of adding onium ions or nucleophiles to a silicon-containing prepolymer and porogen.



BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows FTIR Spectra for the films of Example 8 wherein the silanol content is in the decreasing order: Post Bake Entry 1>>>Post Bake Entry 2>Post Cure, Entry 1≠Post Cure Entry 2.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Accordingly, nanoporous silica dielectric films having a dielectric constant, or k value, ranging from about 3 or below, can be produced by the methods of the invention. Typically, silicon-based dielectric films, including nanoporous silica dielectric films, are prepared from a composition comprising a suitable silicon containing pre-polymer, blended with a porogen and a metal-ion-free catalyst which may be an onium compound or a nucleophile. One or more optional solvents and/or other components may also be included. The dielectric precursor composition is applied to a substrate suitable, e.g., for production of a semiconductor device, such as an integrated circuit (“IC”), by any art-known method to form a film. The composition is then crosslinked, such as by heating to produce a gelled film. The gelled film is then heated at a higher temperature to remove substantially all of the porogen.
The films produced by the processes of the invention have a number of advantages over those previously known to the art, including improved mechanical strength, that enables the produced film to withstand the further processing steps required to prepare a semiconductor device on the treated substrate, and a low and stable dielectric constant. The property of a stable dielectric constant is advantageously achieved without the need for further surface modification steps to render the film surface hydrophobic, as was formerly required by a number of processes for forming nanoporous silica dielectric films. Instead, the silica dielectric films as produced by the processes of the invention are sufficiently hydrophobic as initially formed.
Further, the processes of the invention advantageously require a relatively low temperature for the initial polymerization (i.e., gelling or aging) of an applied prepolymer composition. The processes of the invention provided for a nanometer scale diameter pore size, which is also uniform in size distribution. The resulting nanoporous silica film typically has a dielectric constant of about 3 or below, more typically in the range of from about 1.3 to about 3.0, and most typically from about 1.7 to about 2.5. The film typically has an average pore diameter ranging from about 1 nm to about 30 nm, or more preferably from about 1 nm to about 10 nm and typically from about 1 nm to about 5 nm. The film typically has a void volume of from about 5% to about 80% based on the total volume of the film.
It should be understood that the term nanoporous dielectric films, is intended to refer to dielectric films prepared by the inventive methods from an organic or inorganic glass base material, e.g., any suitable silicon-based material. Additionally, the term “aging” refers to gelling, condensing, or polymerization, of the combined silica-based precursor composition on the substrate after deposition. The term “curing” refers to the removal of residual silanol (Si—OH) groups, removal of residual water, and the process of making the film more stable during subsequent processes of the microelectronic manufacturing process. The curing process is performed after gelling, typically by the application of heat, although any other art-known form of curing may be employed, e.g., by the application of energy in the form of an electron beam, ultraviolet radiation, and the like.
Dielectric films, e.g., interlevel dielectric coatings, are prepared from suitable compositions applied to a substrate. Art-known methods for applying the dielectric precursor composition, include, but are not limited to, spin-coating, dip coating, brushing, rolling, spraying and/or by chemical vapor deposition. Prior to application of the base materials to form the dielectric film, the substrate surface is optionally prepared for coating by standard, art-known cleaning methods. The coating is then processed to achieve the desired type and consistency of dielectric coating, wherein the processing steps are selected to be appropriate for the selected precursor and the desired final product. Further details of the inventive methods and compositions are provided below.
A “substrate” as used herein includes any suitable composition formed before a nanoporous silica film of the invention is applied to and/or formed on that composition. For example, a substrate is typically a silicon wafer suitable for producing an integrated circuit, and the base material from which the nanoporous silica film is formed is applied onto the substrate by conventional methods, including, but not limited to, the art-known methods of spin-coating, dip coating, brushing, rolling, and/or spraying. Prior to application of the base materials to form the nanoporous silica film, the substrate surface is optionally prepared for coating by standard, art-known cleaning methods.
Suitable substrates for the present invention non-exclusively include semiconductor materials such as gallium arsenide (“GaAs”), silicon and compositions containing silicon such as crystalline silicon, polysilicon, amorphous silicon, epitaxial silicon, and silicon dioxide (“SiO2”) and mixtures thereof.
On the surface of the substrate is an optional pattern of raised lines, such as metal, oxide, nitride or oxynitride lines which are formed by well known lithographic techniques. Suitable materials for the lines include silica, silicon nitride, titanium nitride, tantalum nitride, aluminum, aluminum alloys, copper, copper alloys, tantalum, tungsten and silicon oxynitride. Useful metallic targets for making these lines are taught in commonly assigned U.S. Pat. Nos. 5,780,755; 6,238,494; 6,331,233B1; and 6,348,139B1 and are commercially available from Honeywell International Inc. These lines form the conductors or insulators of an integrated circuit. Such are typically closely separated from one another at distances of about 20 micrometers or less, preferably 1 micrometer or less, and more preferably from about 0.05 to about 1 micrometer. Other optional features of the surface of a suitable substrate include an oxide layer, such as an oxide layer formed by heating a silicon wafer in air, or more preferably, an SiO2 oxide layer formed by chemical vapor deposition of such art-recognized materials as, e.g., plasma enhanced tetraethoxysilane oxide (“PETEOS”), plasma enhanced silane oxide (“PE silane”) and combinations thereof, as well as one or more previously formed nanoporous silica dielectric films.
The nanoporous silica film of the invention can be applied so as to cover and/or lie between such optional electronic surface features, e.g., circuit elements and/or conduction pathways that may have been previously formed features of the substrate. Such optional substrate features can also be applied above the nanoporous silica film of the invention in at least one additional layer, so that the low dielectric film serves to insulate one or more, or a plurality of electrically and/or electronically functional layers of the resulting integrated circuit. Thus, a substrate according to the invention optionally includes a silicon material that is formed over or adjacent to a nanoporous silica film of the invention, during the manufacture of a multilayer and/or multicomponent integrated circuit. In a further option, a substrate bearing a nanoporous silica film or films according to the invention can be further covered with any art known non-porous insulation layer, e.g., a glass cap layer.
The crosslinkable composition employed for forming nanoporous silica dielectric films according to the invention includes one or more silicon containing prepolymers that are readily condensed. It should have at least 20 two reactive groups that can be hydrolyzed. Such reactive groups include, alkoxy (RO), acetoxy (AcO), etc. Without being bound by any theory or hypothesis as to how the methods and compositions of the invention are achieved, it is believed that water hydrolyzes the reactive groups on the silicon monomers to form Si—OH groups (silanols). The latter will undergo condensation reactions with other silanols or with other reactive groups, as illustrated by the following formulas:

Si—OH+HO—Si→Si—O—Si+H2O

Si—OH+RO—Si→Si—O—Si+ROH

Si—OH+AcO—Si→Si—O—Si+AcOH

Si—OAc+AcO—Si→Si—O—Si+Ac2O

    
    
        R=alkyl or aryl
        Ac=acyl (CH3CO)
    
    


These condensation reactions lead to formation of silicon containing polymers. In one embodiment of the invention, the prepolymer includes a compound, or any combination of compounds, denoted by Formula I:

Rx-Si-Ly  (Formula I)

wherein x is an integer ranging from 0 to about 2 and y is 4−x, an integer ranging from about 2 to about 4),

R is independently alkyl, aryl, hydrogen, alkylene, arylene and/or combinations of these,

L is independently selected and is an electronegative group, e.g., alkoxy, carboxyl, amino, amido, halide, isocyanato and/or combinations of these.

Particularly useful prepolymers are those provided by Formula I when x ranges from about 0 to about 2, y ranges from about 2 to about 4, R is alkyl or aryl or H, and L is an electronegative group, and wherein the rate of hydrolysis of the Si-L bond is greater than the rate of hydrolysis of the Si—OCH2CH3 bond. Thus, for the following reactions designated as (a) and (b):

Si-L+H2O→Si—OH+HL  (a)

Si—OCH2CH3+H2O→Si—OH+HOCH2CH3  (b)

The rate of (a) is greater than rate of (b).
Examples of suitable compounds according to Formula I include, but are not limited to:

    
    
        Si(OCH2CF3)4 tetrakis(2,2,2-trifluoroethoxy)silane,
        Si(OCOCF3)4 tetrakis(trifluoroacetoxy)silane*,
        Si(OCN)4 tetraisocyanatosilane,
        CH3Si(OCH2CF3)3 tris(2,2,2-trifluoroethoxy)methylsilane,
        CH3Si(OCOCF3)3 tris(trifluoroacetoxy)methylsilane*,
        CH3Si(OCN)3 methyltriisocyanatosilane,

[* These generate acid catalyst upon exposure to water]and or combinations of any of the above.

    
    


In another embodiment of the invention, the composition includes a polymer synthesized from compounds denoted by Formula I by way of hydrolysis and condensation reactions, wherein the number average molecular weight ranges from about 150 to about 300,000 amu, or more typically from about 150 to about 10,000 amu.
In a further embodiment of the invention, silicon-containing prepolymers useful according to the invention include organosilanes, including, for example, alkoxysilanes according to Formula II:




Optionally, Formula II is an alkoxysilane wherein at least 2 of the R groups are independently C1 to C4 alkoxy groups, and the balance, if any, are independently selected from the group consisting of hydrogen, alkyl, phenyl, halogen, substituted phenyl. For purposes of this invention, the term alkoxy includes any other organic groups which can be readily cleaved from silicon at temperatures near room temperature by hydrolysis. R groups can be ethylene glycoxy or propylene glycoxy or the like, but preferably all four R groups are methoxy, ethoxy, propoxy or butoxy. The most preferred alkoxysilanes nonexclusively include tetraethoxysilane (TEOS) and tetramethoxysilane.
In a further option, for instance, the prepolymer can also be an alkylalkoxysilane as described by Formula II, but instead, at least 2 of the R groups are independently C1 to C4 alkylalkoxy groups wherein the alkyl moiety is C1 to C4 alkyl and the alkoxy moiety is C1 to C6 alkoxy, or ether-alkoxy groups; and the balance, if any, are independently selected from the group consisting of hydrogen, alkyl, phenyl, halogen, substituted phenyl. In one preferred embodiment each R is methoxy, ethoxy or propoxy. In another preferred embodiment at least two R groups are alkylalkoxy groups wherein the alkyl moiety is C1 to C4 alkyl and the alkoxy moiety is C1 to C6 alkoxy. In yet another preferred embodiment for a vapor phase precursor, at least two R groups are ether-alkoxy groups of the formula (C1 to C6 alkoxy)n wherein n is 2 to 6.
Preferred silicon containing prepolymers include, for example, any or a combination of alkoxysilanes such as tetraethoxysilane, tetrapropoxysilane, tetraisopropoxysilane, tetra(methoxyethoxy)silane, tetra(methoxyethoxyethoxy)silane which have four groups which may be hydrolyzed and than condensed to produce silica, alkylalkoxysilanes such as methyltriethoxysilane silane, arylalkoxysilanes such as phenyltriethoxysilane and precursors such as triethoxysilane which yield SiH functionality to the film. Tetrakis(methoxyethoxyethoxy)silane, tetrakis(ethoxyethoxy)silane, tetrakis(butoxyethoxyethoxy)silane, tetrakis(2-ethylthoxy)silane, tetrakis(methoxyethoxy)silane, and tetrakis(methoxypropoxy)silane are particularly useful for the invention.
In a still further embodiment of the invention, the alkoxysilane compounds described above may be replaced, in whole or in part, by compounds with acetoxy and/or halogen-based leaving groups. For example, the prepolymer may be an acetoxy (CH3—CO—O—) such as an acetoxy-silane compound and/or a halogenated compound, e.g., a halogenated silane compound and/or combinations thereof. For the halogenated prepolymers the halogen is, e.g., Cl, Br, I and in certain aspects, will optionally include F. Preferred acetoxy-derived prepolymers include, e.g., tetraacetoxysilane, methyltriacetoxysilane and/or combinations thereof.
In one particular embodiment of the invention, the silicon containing prepolymer includes a monomer or polymer precursor, for example, acetoxysilane, an ethoxysilane, methoxysilane and/or combinations thereof.
In a more particular embodiment of the invention, the silicon containing prepolymer includes a tetraacetoxysilane, a C1 to about C6 alkyl or aryl-triacetoxysilane and combinations thereof In particular, as exemplified below, the triacetoxysilane is a methyltriacetoxysilane.
The silicon containing prepolymer is preferably present in the overall composition in an amount of from about 10 weight percent to about 80 weight percent, preferably present in the overall composition in an amount of from about 20 weight percent to about 60 weight percent.
For non-microelectronic applications, the onium or nucleophile catalyst may contain metal ions. Examples include sodium hydroxide, sodium sulfate, potassium hydroxide, lithium hydroxide, and zirconium containing catalysts.
For microelectronic applications, preferably, the composition then contains at least one metal-ion-free catalyst which is an onium compound or a nucleophile. The catalyst may be, for example an ammonium compound, an amine, a phosphonium compound or a phosphine compound. Non-exclusive examples of such include tetraorganoammonium compounds and tetraorganophosphonium compounds including tetramethylammonium acetate, tetramethylammonium hydroxide, tetrabutylammonium acetate, triphenylamine, trioctylamine, tridodecylamine, triethanolamine, tetramethylphosphonium acetate, tetramethylphosphonium hydroxide, triphenylphosphine, trimethylphosphine, trioctylphosphine, and combinations thereof. The composition may comprise a non-metallic, nucleophilic additive which accelerates the crosslinking of the composition.
These include dimethyl sulfone, dimethyl formamide, hexamethylphosphorous triamide (HMPT), amines and combinations thereof. The catalyst is preferably present in the overall composition in an amount of from about 1 ppm by weight to about 1000 ppm, preferably present in the overall composition in an amount of from about 6 ppm to about 200 ppm.
The composition then contains at least one porogen. A porogen may be a compound or oligomer or polymer and is selected so that, when it is removed, e.g., by the application of heat, a silica dielectric film is produced that has a nanometer scale porous structure. The scale of the pores produced by porogen removal is proportional to the effective steric diameters of the selected porogen component. The need for any particular pore size range (i.e., diameter) is defined by the scale of the semiconductor device in which the film is employed. Furthermore, the porogen should not be so small as to result in the collapse of the produced pores, e.g., by capillary action within such a small diameter structure, resulting in the formation of a non-porous (dense) film. Further still, there should be minimal variation in diameters of all pores in the pore population of a given film. It is preferred that porogen is a compound that has a substantially homogeneous molecular weight and molecular dimension, and not a statistical distribution or range of molecular weights,and/or molecular dimensions, in a given sample. The avoidance of any significant variance in the molecular weight distribution allows for a substantially uniform distribution of pore diameters in the film treated by the inventive processes. If the produced film has a wide-distribution of pore sizes, the likelihood is increased of forming one or more large pores, i.e., bubbles, that could interfere with the production of reliable semiconductor devices.
Furthermore, the porogen should have a molecular weight and structure such that it is readily and selectively removed from the film without interfering with film formation. This is based on the nature of semiconductor devices, which typically have an upper limit to processing temperatures. Broadly, a porogen should be removable from the newly formed film at temperatures below, e.g., about 450° C. In particular embodiments, depending on the desired post film formation fabrication process and materials, the porogen is selected to be readily removed at temperatures ranging from about 150° C. to about 450° C. during a time period ranging, e.g., from about 30 seconds to about 60 minutes. The removal of the porogen may be induced by heating the film at or above atmospheric pressure or under a vacuum, or by exposing the film to radiation, or both.
Porogens which meet the above characteristics include those compounds and polymers which have a boiling point, sublimation temperature, and/or decomposition temperature (at atmospheric pressure) range, for example, from about 150° C. to about 450° C. In addition, porogens suitable for use according to the invention include those having a molecular weight ranging, for example, from about 100 to about 50,000 amu, and more preferably in the range of from about 100 to about 3,000 amu.
Porogens suitable for use in the processes and compositions of the invention include polymers, preferably those which contain one or more reactive groups, such as hydroxyl or amino. Within these general parameters, a suitable polymer porogen for use in the compositions and methods of the invention is, e.g., a polyalkylene oxide, a monoether of a polyalkylene oxide, a diether of a polyalkylene oxide, bisether of a polyalkylene oxide, an aliphatic polyester, an acrylic polymer, an acetal polymer, a poly(caprolactone), a poly(valeractone), a poly(methyl methacrylate), a poly (vinylbutyral) and/or combinations thereof. When the porogen is a polyalkylene oxide monoether, one particular embodiment is a C1 to about C6 alkyl chain between oxygen atoms and a C1 to about C6 alkyl ether moiety, and wherein the alkyl chain is substituted or unsubstituted, e.g., polyethylene glycol monomethyl ether, polyethylene glycol dimethyl ether, or polypropylene glycol monomethyl ether.
Other useful porogens are disclosed in commonly assigned patent application Ser. No. 10/507,411 filed on the same day as this application, are porogens that do not bond to the silicon containing pre-polymer, and include a poly(alkylene) diether, a poly(arylene) diether, poly(cyclic glycol) diether, Crown ethers, polycaprolactone, fully end-capped polyalkylene oxides, fully end-capped polyarylene oxides, polynorbene, and combinations thereof. Preferred porogens which do not bond to the silicon containing pre-polymer include poly(ethylene glycol) dimethyl ethers, poly(ethylene glycol) bis(carboxymethyl) ethers, poly(ethylene glycol) dibenzoates, poly(ethylene glycol) diglycidyl ethers, a poly(propylene glycol) dibenzoates, poly(propylene glycol) diglycidyl ethers, poly(propylene glycol) dimethyl ether, 15-Crown 5, 18-Crown-6, dibenzo-18-Crown-6, dicyclohexyl-18-Crown-6, dibenzo-15-Crown-5 and combinations thereof.
Without meaning to be bound by any theory or hypothesis as to how the invention might operate, it is believed that porogens that are, “readily removed from the film” undergo one or a combination of the following events: (1) physical evaporation of the porogen during the heating step, (2) degradation of the porogen into more volatile molecular fragments, (3) breaking of the bond(s) between the porogen and the Si containing component, and subsequent evaporation of the porogen from the film, or any combination of modes 1-3. The porogen is heated until a substantial proportion of the porogen is removed, e.g., at least about 50% by weight, or more, of the porogen is removed. More particularly, in certain embodiments, depending upon the selected porogen and film materials, at least about 75% by weight, or more, of the porogen is removed.
Thus, by “substantially” is meant, simply by way of example, removing from about 50% to about 75%, or more, of the original porogen from the applied film.
A porogen is preferably present in the overall composition, in an amount ranging from about 1 to about 50 weight percent, or more. More preferably the porogen is present in the composition, in an amount ranging from about 2 to about 20 weight percent.
The overall composition then optionally includes a solvent composition. Reference herein to a “solvent” should be understood to encompass a single solvent, polar or nonpolar and/or a combination of compatible solvents forming a solvent system selected to solubilize the overall composition components. A solvent is optionally included in the composition to lower its viscosity and promote uniform coating onto a substrate by art-standard methods (e.g., spin coating, spray coating, dip coating, roller coating, and the like).
In order to facilitate solvent removal, the solvent is one which has a relatively low boiling point relative to the boiling point of any selected porogen and the other precursor components. For example, solvents that are useful for the processes of the invention have a boiling point ranging from about 50 to about 250° C. to allow the solvent to evaporate from the applied film and leave the active portion of the precursor composition in place. In order to meet various safety and environmental requirements, the solvent preferably has a high flash point (generally greater than 40° C.) and relatively low levels of toxicity. A suitable solvent includes, for example, hydrocarbons, as well as solvents having the functional groups C—O—C (ethers), —CO—O (esters), —CO— (ketones), —OH (alcohols), and —CO—N-(amides), and solvents which contain a plurality of these functional groups, and combinations thereof.
Without limitation, solvents for the composition include di-n-butyl ether, anisole, acetone, 3-pentanone, 2-heptanone, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl lactate, ethanol, 2-propanol, dimethyl acetamide, propylene glycol methyl ether acetate, and/or combinations thereof. It is preferred that the solvent does not react with the silicon containing prepolymer component.
The solvent component is preferably present in an amount of from about 10% to about 95% by weight of the overall composition. A more preferred range is from about 20% to about 75% and most preferably from about 20% to about 60%. The greater the percentage of solvent employed, the thinner is the resulting film. The greater the percentage of porogen employed, the greater is the resulting porosity.
In another embodiment of the invention the composition may comprises water, either liquid or water vapor. For example, the overall composition may be applied to a substrate and then exposed to an ambient atmosphere that includes water vapor at standard temperatures and standard atmospheric pressure. Optionally, the composition is prepared prior to application to a substrate to include water in a proportion suitable for initiating aging of the precursor composition, without being present in a proportion that results in the precursor composition aging or gelling before it can be applied to a desired substrate. By way of example, when water is mixed into the precursor composition it is present in a proportion wherein the composition comprises water in a molar ratio of water to Si atoms in the silicon containing prepolymer ranging from about 0.1:1 to about 50:1. A more preferred range is from about 0.1:1 to about 10:1 and most preferably from about 0.5:1 to about 1.5:1.
Those skilled in the art will appreciate that specific temperature ranges for crosslinking and porogen removal from the nanoporous dielectric films will depend on the selected materials, substrate and desired nanoscale pore structure, as is readily determined by routine manipulation of these parameters. Generally, the coated substrate is subjected to a treatment such as heating to effect crosslinking of the composition on the substrate to produce a gelled film.
Crosslinking may be done in step (c) by heating the film at a temperature ranging from about 100° C. to about 250° C., for a time period ranging from about 30 seconds to about 10 minutes to gel the film. The artisan will also appreciate that any number of additional art-known curing methods are optionally employed. including the application of sufficient energy to cure the film by exposure of the film to electron beam energy, ultraviolet energy, microwave energy, and the like, according to art-known methods.
Once the film has aged, i.e., once it is is sufficiently condensed to be solid or substantially solid, the porogen can be removed. The latter should be sufficiently non-volatile so that it does not evaporate from the film before the film solidifies. The porogen is removed in a step (d) by heating the gelled film at a temperature ranging from about 150° C. to about 450° C., preferably from about 150° C. to about 350° C. for a time period ranging from about 30 seconds to about 1 hour. An important feature of the invention is that preferably the step (c) crosslinking is conducted at a temperature which is less than the heating temperature of step (d).
Utility:
The present composition may also comprise additional components such as adhesion promoters, antifoam agents, detergents, flame retardants, pigments, plasticizers, stabilizers, and surfactants. The present composition has utility in non-microelectronic applications such as thermal insulation, encapsulant, matrix materials for polymer and ceramic composites, light weight composites, acoustic insulation, anti-corrosive coatings, binders for ceramic powders, and fire retardant coatings.
The present composition is particularly useful in microelectronic applications as a dielectric substrate material in microchips, multichip modules, laminated circuit boards, or printed wiring boards. The present composition may also be used as an etch stop or hardmask.
The present films may be formed by solution techniques such as spraying, 10 rolling, dipping, spin coating, flow coating, or casting, and chemical vapor deposition, with spin coating being preferred for microelectronics. For chemical vapor deposition (CVD), the composition is placed into an CVD apparatus, vaporized, and introduced into a deposition chamber containing the substrate to be coated. Vaporization may be accomplished by heating the composition above its vaporization point, by the use of vacuum, or by a combination of the above. Generally, vaporization is accomplished at temperatures in the range of 50° C.-300° C. under atmospheric pressure or at lower temperature (near room temperature) under vacuum.
Three types of CVD processes exist: atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), and plasma enhanced CVD (PECVD). Each of these approaches had advantages and disadvantages. APCVD devices operate in a mass transport limited reaction mode at temperatures of approximately 400° C. In mass-transport limited deposition, temperature control of the deposition chamber is less critical than in other methods because mass transport processes are only weakly dependent on temperature. As the arrival rate of the reactants is directly proportional to their concentration in the bulk gas, maintaining a homogeneous concentration of reactants in the bulk gas adjacent to the wafers is critical. Thus, to insure films of uniform thickness across a wafer, reactors that are operated in the mass transport limited regime must be designed so that all wafer surfaces are supplied with an equal flux of reactant. The most widely used APCVD reactor designs provide a uniform supply of reactants by horizontally positioning the wafers and moving them under a gas stream.
In contrast to APCVD reactors, LPCVD reactors operate in a reaction rate-limited mode. In processes that are run under reaction rate-limited conditions, the temperature of the process is an important parameter. To maintain a uniform deposition rate throughout a reactor, the reactor temperature must be homogeneous throughout the reactor and at all wafer surfaces. Under reaction rate-limited conditions, the rate at which the deposited species arrive at the surface is not as critical as constant temperature. Thus, LPCVD reactors do not have to be designed to supply an invariant flux of reactants to all locations of a wafer surface.
Under the low pressure of an LPCVD reactor, for example, operating at medium vacuum (30-250 Pa or 0.25-2.0 torr) and higher temperature (550-600° C.), the diffusivity of the deposited species is increased by a factor of approximately 1000 over the diffusivity at atmospheric pressure. The increased diffusivity is partially offset by the fact that the distance across which the reactants must diffusive increases by less than the square root of the pressure. The net effect is that there is more than an order of magnitude increase in the transport of reactants to the substrate surface and by-products away from the substrate surface.
LPCVD reactors are designed in two primary configurations: (a) horizontal tube reactors; and (b) vertical flow isothermal reactors. Horizontal tube, hot wall reactors are the most widely used LPCVD reactors in VLSI processing. They are employed for depositing poly-Si, silicon nitride, and undoped and doped SiO2 films. They find such broad applicability primarily because of their superior economy, throughput, uniformity, and ability to accommodate large diameter, e.g., 150 mm, wafers.
The vertical flow isothermal LPCVD reactor further extends the distributed gas feed technique so that each wafer receives an identical supply of fresh reactants. Wafers are again stacked side by side, but are placed in perforated-quartz cages. The cages are positioned beneath long, perforated, quartz reaction-gas injector tubes, one tube for each reactant gas. Gas flows vertically from the injector tubes, through the cage perforations, past the wafers, parallel to the wafer surface and into exhaust slots below the cage. The size, number, and location of cage perforations are used to control the flow of reactant gases to the wafer surfaces. By properly optimizing cage perforation design, each wafer may be supplied with identical quantities of fresh reactants from the vertically adjacent injector tubes. Thus, this design may avoid the wafer-to-wafer reactant depletion effects of the end-feed tube reactors, requires no temperature ramping, produces highly uniform depositions, and reportedly achieves low particulate contamination.
The third major CVD deposition method is PECVD. This method is categorized not only by pressure regime, but also by its method of energy input. Rather than relying solely on thermal energy to initiate and sustain chemical reactions, PECVD uses an rf-induced glow discharge to transfer energy into the reactant gases, allowing the substrate to remain at a lower temperature than in APCVD or LPCVD processes. Lower substrate temperature is the major advantages of PECVD, providing film deposition on substrates not having sufficient thermal stability to accept coating by other methods. PECVD may also enhance deposition rates over those achieved using thermal reactions. Moreover, PECVD may produce films having unique compositions and properties. Desirable properties such as good adhesion, low pinpole density, good step coverage, adequate electrical properties, and compatibility with fine-line pattern transfer processes, have led to application of these films in VLSI.
PECVD requires control and optimization of several deposition parameters, including rf power density, frequency, and duty cycle. The deposition process is dependent in a complex and interdependent way on these parameters, as well as on the usual parameters of gas composition, flow rates, temperature, and pressure. Furthermore, as with LPCVD, the PECVD method is surface reaction limited, and adequate substrate temperature control is thus necessary to ensure uniform film thickness.
CVD systems usually contain the following components: gas sources, gas feed lines, mass-flow controllers for metering the gases into the system, a reaction chamber or reactor, a method for heating the wafers onto which the film is to be deposited, and in some types of systems, for adding additional energy by other means, and temperature sensors. LPCVD and PECVD systems also contain pumps for establishing the reduced pressure and exhausting the gases from the chamber.
Preferably, the present composition is dissolved in a solvent. Suitable solvents for use in such solutions of the present compositions include any suitable pure or mixture of organic, organometallic, or inorganic molecules that are volatized at a desired temperature. Suitable solvents include aprotic solvents, for example, cyclic ketones such as cyclopentanone, cyclohexanone, cycloheptanone, and cyclooctanone; cyclic amides such as N-alkylpyrrolidinone wherein the alkyl has from about 1 to 4 carbon atoms; and N-cyclohexylpyrrolidinone and mixtures thereof. A wide variety of other organic solvents may be used herein insofar as they are able to aid dissolution of the adhesion promoter and at the same time effectively control the viscosity of the resulting solution as a coating solution. Various facilitating measures such as stirring and/or heating may be used to aid in the dissolution. Other suitable solvents include methyethylketone, methylisobutylketone, dibutyl ether, cyclic dimethylpolysiloxanes, butyrolactone, γ-butyrolactone, 2-heptanone, ethyl 3-ethoxypropionate, 1-methyl-2-pyrrolidinone, and propylene glycol methyl ether acetate (PGMEA), and hydrocarbon solvents such as mesitylene, xylenes, benzene, and toluene.
The present composition may be used in electrical devices and more specifically, as an interlayer dielectric in an interconnect associated with a single integrated circuit (“IC”) chip. An integrated circuit chip typically has on its surface a plurality of layers of the present composition and multiple layers of metal conductors. It may also include regions of the present composition between discrete metal conductors or regions of conductor in the same layer or level of an integrated circuit.
The present films may be formed on substrates. Substrates contemplated herein may comprise any desirable substantially solid material. Particularly desirable substrate layers comprise films, glass, ceramic, plastic, metal or coated metal, or composite material. In preferred embodiments, the substrate comprises a silicon or gallium arsenide die or wafer surface, a packaging surface such as found in a copper, silver, nickel or gold plated leadframe, a copper surface such as found in a circuit board or package interconnect trace, a via-wall or stiffener interface (“copper” includes considerations of bare copper and its oxides), a polymer-based packaging or board interface such as found in a polyimide-based flex package, lead or other metal alloy solder ball surface, glass and polymers. Useful substrates include silicon, silicon nitride, silicon oxide, silicon oxycarbide, silicon dioxide, silicon carbide, silicon oxynitride, titanium nitride, tantalum nitride, tungsten nitride, aluminum, copper, tantalum, organosiloxanes, organo silicon glass, and fluorinated silicon glass. In other embodiments, the substrate comprises a material common in the packaging and circuit board industries such as silicon, copper, glass, and polymers. The circuit board made up of the present composition will have mounted on its surface patterns for various electrical conductor circuits. The circuit board may include various reinforcements, such as woven non-conducting fibers or glass cloth. Such circuit boards may be single sided, as well as double sided.
The present films may be used in dual damascene (such as copper) processing and substractive metal (such as aluminum or aluminum/tungsten) processing for integrated circuit manufacturing. The present composition may be used in a desirable all spin-on stacked film as taught by Michael E. Thomas, “Spin-On Stacked Films for Low keff Dielectrics”, Solid State Technology (July 2001), incorporated herein in its entirety by reference. The present composition may be used in an all spin-on stacked film having additional dielectrics such as taught by commonly assigned U.S. Pat. Nos. 6,248,457B1; 5,986,045; 6,124,411; and 6,303,733.
Analytical Test Methods:
Dielectric Constant: The dielectric constant was determined by coating a thin film of aluminum on the cured layer and then doing a capacitance-voltage measurement at 1 MHz and calculating the k value based on the layer thickness.
Refractive Index: The refractive index measurements were performed together with the thickness measurements using a J. A. Woollam M-88 spectroscopic ellipsometer. A Cauchy model was used to calculate the best fit for Psi and Delta. Unless noted otherwise, the refractive index was reported at a wavelength of 633 nm (details on Ellipsometry can be found in e.g. “Spectroscopic Ellipsometry and Reflectometry” by H. G. Thompkins and William A. McGahan, John Wiley and Sons, Inc., 1999).
Average Pore Size Diameter: The N2 isotherms of porous samples was measured on a Micromeretics ASAP 2000 automatic isothermal N2 sorption instrument using UHP (ultra high purity industrial gas) N2, with the sample immersed in a sample tube in a liquid N2 bath at 77° K.
For sample preparation, the material was first deposited on silicon wafers using standard processing conditions. For each sample, three wafers were prepared with a film thickness of approximately 6000 Angstroms. The films were then removed from the wafers by scraping with a razor blade. These powder samples were pre-dried at 180° C. in an oven before weighing them, carefully pouring the powder into a 10 mm inner diameter sample tube, then degassing at 180° C. at 0.01 Torr for >3 hours.
The adsorption and desorption N2 sorption was then measured automatically using a 5 second equilibration interval, unless analysis showed that a longer time was required. The time required to measure the isotherm was proportional to the mass of the sample, the pore volume of the sample, the number of data points measured, the equilibration interval, and the P/Po tolerance. (P is the actual pressure of the sample in the sample tube. Po is the ambient pressure outside the instrument.) The instrument measures the N2 isotherm and plots N2 versus P/Po.
The apparent BET (Brunauer, Emmett, Teller method for multi-layer gas absorption on a solid surface disclosed in S. Brunauer, P. H. Emmett, E. Teller; J. Am. Chem. Soc. 60, 309-319 (1938)) surface area was calculated from the lower P/Po region of the N2 adsorption isotherm using the BET theory, using the linear section of the BET equation that gives an R2 fit >0.9999.
The pore volume was calculated from the volume of N2 adsorbed at the relative pressure P/Po value, usually P/Po ̃0.95, which is in the flat region of the isotherm where condensation is complete, assuming that the density of the adsorbed N2 is the same as liquid N2 and that all the pores are filled with condensed N2 at this P/Po.
The pore size distribution was calculated from the adsorption arm of the N2 isotherm using the BJH (E. P. Barret, L. G. Joyner, P. P. Halenda; J. Am. Chem. Soc., 73, 373-380 (1951)) pore size distribution from the N2 isotherm using the Kelvin equation theory. This uses the Kelvin equation, which relates curvature to suppression of vapor pressure, and the Halsey equation, which describes the thickness of the adsorbed N2 monolayer versus P/Po, to convert the volume of condensed N2 versus P/Po to the pore volume in a particular range of pore sizes.
The average cylindrical pore diameter D was the diameter of a cylinder that has the same apparent BET surface area Sa (m2/g) and pore volume Vp (cc/g) as the sample, so D (nm)=4000 Vp/Sa.
The following non-limiting examples serve to illustrate the invention.
EXAMPLE 1
This example shows the production of a nanoporous silica with a porogen having a high concentration of sodium. A precursor was prepared by combining, in a 100 ml round bottom flask (containing a magnetic stirring bar), 10 g tetraacetoxysilane, 10 g methyltriacetoxysilane, and 17 g propylene glycol methyl ethyl acetate (PGMEA). These ingredients were combined within an N2-environment (N2 glove bag). The flask was also connected to an N2 environment to prevent environmental moisture from entering the solution (standard temperature and pressure).
The reaction mixture was heated to 80° C. before 1.5 g of water was added to the flask. After the water addition is complete, the reaction mixture was allowed to cool to ambient before 4.26 g of polyethylene glycol monomethylether (“PEO”; MW550 amu) (with >300 ppb Na) was added as a porogen, and stirring continued for another 2 hrs. Thereafter, the resulting solution was filtered through a 0.2 micron filter to provide the precursor solution masterbatch for the next step. The solution is then deposited onto a series of 8-inch silicon wafers, each on a spin chuck and spun at 2500 rpm for 30 seconds. The presence of water in the precursor resulted in the film coating being substantially condensed by the time that the wafer was inserted into the first oven. Insertion into the first oven, as discussed below, takes place within the 10 seconds of the completion of spinning. Each coated wafer was then transferred into a sequential series of ovens preset at specific temperatures, for one minute each. In this example, there are three ovens, and the preset oven temperatures were 80° C., 175° C., and 300° C., respectively. The PEO is driven off by these sequential heating steps as each wafer was moved through each of the three respective ovens. Each wafer is cooled after receiving the three-oven stepped heat treatment, and the produced dielectric film was measured using ellipsometry to determine its thickness and refractive index. Each film-coated wafer is then further cured at 425° C. for one hour under flowing nitrogen. A non-porous film made from the liquid precursor of this invention will have a refractive index of 1.41 and a k de-gas of 3.2. In comparison, air has a refractive index of 1.0. The porosity of a nanoporous film of the invention, is therefore proportional to the percentage of its volume that is air. The film has a bake thickness of 5920 Å, a bake refractive index of 1.234, a cure thickness of 5619 Å and a cure refractive index of 1.231. The cured film produced has a porosity of about 43% (see entry 1 of the following table). In the table, capacitance of the film was measured under ambient conditions (room temperature and humidity). Dielectric constant based on ambient capacitance value is called k ambient. The capacitance of the film was measured again after heating the wafer in a hot plate at 200C for 2 minutes in order to drive off adsorbed moisture. Dielectric constant based on the de-moisture capacitance is called k de-gas.
EXAMPLE 2 (COMPARATIVE)
This example shows the production of a nanoporous silica with a porogen having a low concentration of sodium.
Crude PEO (polyethylene glycol monomethyl ether MW=550) with high concentration of sodium was purified by mixing the crude PEO with water in a 50:50 weight ratio. This mixture was passed through an ion exchange resin to remove metals. The filtrate was collected and subjected to vacuum distillation to remove water to produce neat, low metal PEO(with <100 ppb Na). The procedure of Example 1 was then followed with the low metal PEO substituted for the high metal PEO. It is estimated that from a k de-gas value of 3.03, the film basically collapses and has a porosity of only about 7%, a drop from 43% as compared to Example 1 (Comparative). The film has a bake thickness of 4179 Å, a bake refractive index of 1.353, a cure thickness of 3875 Å and a cure refractive index of 1.331 (see entry 2 of the following table).
EXAMPLE 3
Example 2 is repeated except this example adds sodium cation (sodium hydroxide (see entry 3), or sodium sulfate (see entry 4) to restore the low k. Sodium hydroxide (NaOH, 23 ppm) or sodium sulfate (Na2SO4, 40 ppm) was were added to the ion-exchanged PEO and the precursor masterbatch. Films are deposited onto a wafer by spin coating at 2400 rpm or 3500 rpm. After spin coating, the film is heated in three hot plates at temperatures of 80° C., 175° C. and 300° C., one minute each. After bake, the film is cured under flowing nitrogen at 425° C. for one hour. The results of k and R.I. of the post-cure films are listed in the following table.
EXAMPLE 4
Example 2 is repeated except this example adds tetraorganoammonium (TMAA) (entries 5, 6, and 7), TMAH (entry 8), or TBAA (entry 9)) ion to restore the low k. Various amounts of TMAA were added to the ion-exchanged PEO and the precursor masterbatch. In some cases, small amount of methyltriacetoxysilane (MTAS, 1%, see entry 6) were added to the solution to serve as an in-situ surface modifier to make the surface less hydrophilic. Films were deposited onto a wafer by spin coating at 2400 rpm or 3500 rpm. After spin coating, the film was heated in three hot plates at temperatures of 80° C., 175° C. and 300° C., one minute each. After bake, the film was cured under flowing nitrogen at 425° C. for one hour. For entries 5 and 6, the average pore size diameter was 2.5 nm. The results of k and R.I. of the post-cure films are listed in the following table. It is shown that the k value is below 2.5 when the concentration of the ammonium ion is greater than about 65×10−9 mole/gm of solution, which corresponds to approximately 3 ppm of TMAA by weight.
EXAMPLE 5
This example shows the production of a nanoporous silica prepared from a porogen having a low concentration of sodium, and a commercially available methylsiloxane polymer (Honeywell ACCUGLASS® SPIN-ON GLASS 512B).
Crude PEO (polyethylene glycol monomethyl ether MW=550) with high concentration of sodium is purified by mixing the crude PEO with water in a 50:50 weight ratio. This mixture is passed through an ion exchange resin to remove metals. The filtrate is collected and subjected to vacuum distillation to remove water to produce neat, low metal PEO(with <100 ppb Na). The resulting PEO (4.88 g) and butanol (48 g) are mixed in with ACCUGLASS® SPIN-ON GLASS 512B (43 g). Thereafter, the resulting solution is filtered through a 0.2 micron filter to provide the precursor solution masterbatch for the next step. The solution is then deposited onto a series of 8-inch silicon wafers, each on a spin chuck and spun at 3000 rpm for 30 seconds. The presence of water in the precursor resulted in the film coating being substantially condensed by the time that the wafer was inserted into the first oven. Insertion into the first oven, as discussed below, takes place within the 10 seconds of the completion of spinning. Each coated wafer is then transferred into a sequential series of ovens preset at specific temperatures, for one minute each. In this example, there are three ovens, and the preset oven temperatures were 80° C., 175° C., and 300° C., respectively. The PEO is driven off by these sequential heating steps as each wafer was moved through each of the three respective ovens. Each wafer is cooled after receiving the three-oven stepped heat treatment, and the produced dielectric film was measured using ellipsometry to determine its thickness and refractive index. Each film-coated wafer is then further cured at 425° C. for one hour under flowing Nitrogen. The film collapses and cannot form a porous structure. The film has a bake thickness of 1690 Å, a bake refractive index of 1.395, a cure thickness of 1615 Å and a cure refractive index of 1.367. The cured film produced has a porosity of about 5% (see entry 10 of the following table).
EXAMPLE 6
Example 5 was repeated except this example adds tetraorganoammonium (TMAA (entry 11) ion to restore the low k. TMAA (10 ppm) is added to the ion-exchanged PEO (3.64 g), butanol (13 g) and ACCUGLASS® SPIN-ON GLASS 512B (25 g). Films were deposited onto a wafer by spin coating at 2000 rpm. After spin coating, the film was heated in three hot plates at temperatures of 125° C., 200° C. and 350° C., one minute each. After bake, the film was cured under flowing nitrogen at 425° C. for one hour. The results of k and R.I. of the post-cure films are listed in the following table. The cured film produced has a porosity of about 40%. The average pore size diameter was 2.5 nm.















TABLE I






Additive




Final





Conc.




Na
Cured
Cured


Entry
Ppm
PEO
kambient
Kde-gas
Δk
Ppb
R.I.
Thickness Å























1
 0
Crude
2.60
2.36
0.24
285
1.231
5619


2
 0
Low
3.62
3.03
0.59
<25
1.331
3875




metal


3
 40 (Na2SO4)
Low
2.21
2.10
0.11

1.232
7244




metal


4
 23 (NaOH)
Low
2.24
2.21
0.03

1.246
5479




Metal


5
 6 (TMAA)
Low
2.34
2.23
0.11
43
1.209
6127




metal


6
 6 (TMAA)
Low
2.24
2.13
0.11
43
1.218
6319




metal


7
 3 (TMAA)
Low
2.65
2.38
0.27
95
1.248
6495




metal


8
 22 (TMAH)
Low
2.27
2.10
0.17
<25
1.215
7871




metal


9
100 (TBAA)
Low
2.55
2.27
0.28
65
1.213
7716




metal


10
 0
Low
n/a
N/a
n/a
<25
1.367
1615




metal


11
 10 (TMAA)
Low
2.41
2.24
0.17
<25
1.215
6467




Metal









EXAMPLE 7
The following example (entry 1 of table II) shows the condensation (also known as cross-linking of silanol groups) reaction is impaired in the absence of TMAA at 300° C. For illustrative purpose, porogen is not added. A precursor was prepared by combining, in a 100 ml round bottom flask (containing a magnetic stirring bar), 10 g tetraacetoxysilane, 10 g methyltriacetoxysilane, and 17 g propylene glycol methyl ethyl acetate (PGMEA). These ingredients were combined within an N2-environment (N2 glove bag). The flask was also connected to an N2 environment to prevent environmental moisture from entering the solution (standard temperature and pressure). The reaction mixture was heated to 80° C. before 1.5 g of water was added to the flask. After the water addition is complete, the reaction mixture was allowed to cool to ambient before the resulting solution was filtered through a 0.2 micron filter to provide the precursor solution masterbatch for the next step. The solution is then deposited onto a series of 8-inch silicon wafers, each on a spin chuck and spun at 2500 rpm for 30 seconds. The presence of water in the precursor resulted in the film coating being substantially condensed by the time that the wafer was inserted into the first hot-plate. Insertion into the first hot-plate, as discussed below, takes place within the 10 seconds of the completion of spinning. Each coated wafer was then transferred into a sequential series of hot-plates preset at specific temperatures, for one minute each. In this example, there are three hot-plates, and the preset hot-plate temperatures were 80° C., 175° C., and 300° C., respectively. Each wafer is cooled after receiving the three-hot-plate stepped heat treatment, and the produced dielectric film was measured using ellipsometry to determine its thickness and refractive index, and FTIR to measure the silanol(SiOH, ν: 3100-3800 cm−1)-to-methyl(CH3, ν: 2978 cm−1) area ratio. The observed (Si)OH to CH3 is greater than 20 for the 3 kÅ film with RI of 1.41±0.01. The FTIR spectrum of the baked film illustrates the large amount of silanol. (See FIG. 1) Each film-coated wafer is then further cured at 425° C. for one hour under flowing nitrogen. The resulting film has a (Si)OH to CH3 ratio of 2.
EXAMPLE 8
Example 7 was repeated except this example adds tetraorganoammonium (TMAA) (entry 2). This example illustrates the condensation reaction at 300° C. is catalyzed by the presence of TMAA. Films were deposited onto a wafer by spin coating at 2400 rpm. After spin coating, the film was heated in three hot plates at temperatures of 80° C., 175° C. and 300° C., one minute each. After bake, the film was analyzed by FTIR to determine the silanol(SiOH)-to-methyl(CH3) area ratio. The observed (Si)OH to CH3 ratio was ca. 4 for the 3 kÅ film with RI of 1.41. The decrease in silanol content is better illustrated by the FTIR spectrum of the film. (See FIG. 1). FIG. 1 shows FTIR spectra where the silanol content is in the decreasing order: Post Bake Entry 1>>>Post Bake Entry 2>Post Cure, Entry 1≈Post Cure Entry 2.
Each film-coated wafer was then further cured at 425° C. for one hour under flowing nitrogen. The resulting film has a (Si)OH to CH3 ratio of 2.










TABLE II









Baked
Cured













TMAA Conc.

SiO—H/

SiO—H/


Entry
Ppm
RI
SiC—H3
RI
SiC—H3





1
0
1.41
22
1.41
2


2
5
1.41
 4
1.41
2









While the present invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above and all equivalents thereto.

## Claims

1. A method of producing a nanoporous silica dielectric film comprising
(a) preparing a composition comprising a silicon containing pre-polymer, a porogen, and a metal-ion-free catalyst selected from the group consisting of tetramethylammonium acetate, tetrabutylammonium acetate, tetramethylphosphonium acetate, tetramethylphosphonium hydroxide, triphenylphosphine, trimethylphosphine, trioctylphosphine, and combinations thereof;
(b) coating a substrate with the composition to form a film,
(c) crosslinking the composition to produce a gelled film, and
(d) heating the gelled film at a temperature and for a duration effective to remove substantially all of said porogen.

2. The method of claim 1 wherein the nanoporous silica dielectric film has a pore void volume of from about 5% to about 80% based on the volume of the film.

3. The method of claim 1 wherein the resulting nanoporous silica dielectric film has a dielectric constant of about 3 or below.

4. The method of claim 1 wherein the nanoporous silica dielectric film has an average pore diameter in die range of from about 1 nm to about 30 nm.

5. The method of claim 1 wherein the composition further comprises a non-metallic, nucleophilic additive which accelerates the crosslinking of the composition.

6. The method of claim 1 wherein the composition further comprises a nucleophilic additive which accelerates the crosslinking of the composition, which is selected from the group consisting of dimethyl sulfone, dimethyl formamide, hexamethylphosphorous triamide, amines and combinations thereof.

7. The method of claim 1 wherein the composition further comprises water in a molar ratio of water to Si ranging from about 0.1:1 to about 50:1.

8. The method of claim 1 wherein the composition comprises a silicon containing pre-polymer of Formula I:


Rx-Si-Ly  (Formula I)

wherein x is an integer ranging from 0 to about 2, and y is x−4, an integer ranging from about 2 to about 4;
R is independently selected from the group consisting of alkyl, aryl, hydrogen, alkylene, arylene, and combinations thereof;
L is an electronegative moiety, independently selected from the group consisting of alkoxy, carboxyl, acetoxy, amino, amido, halide, isocyanato and combinations thereof.

9. The method of claim 8 wherein the composition comprises a polymer formed by condensing a pre-polymer according to Formula I, wherein the number average molecular weight of said polymer ranges from about 150 to about 300,000 amu.

10. The method of claim 1 wherein the composition comprises a silicon containing pre-polymer selected from the group consisting of an acetoxysilane, an ethoxysilane, a methoxysilane, and combinations thereof.

11. The method of claim 1 wherein the composition comprises a silicon containing pre-polymer selected from the group consisting of tetraacetoxysilane, a C1 to about C6 alkyl or aryl-triacetoxysilane, and combinations thereof.

12. The method of claim 11 wherein said triacetoxysilane is methyltriacetoxysilane.

13. The method of claim 1 wherein the composition comprises a silicon containing pre-polymer selected from the group consisting of tetrakis(2,2,2-trifluoroethoxy)silane, tetrakis(trifluoroacetoxy)silane, tetraisocyanatosilane, tris(2,2,2-trifluoroethoxy)methylsilane, tris(trifluoroacetoxy)methylsilane, methyltriisocyanatosilane and combinations thereof.

14. The method of claim 1 wherein the porogen has a boiling point, sublimation point or decomposition temperature ranging from about 150° C. to about 450° C.

15. The method of claim 1 wherein step (c) comprises a crosslinking which is conducted at a temperature which is less than the heating temperature of step (d).

16. The method of claim 1 wherein step (e) comprises heating the film at a temperature ranging from about 100° C. to about 250° C., for a time period ranging from about 30 seconds to about 10 minutes.

17. The method of claim 1 wherein step (d) comprises heating the film at a temperature ranging from about 150° C. to about 450° C., for a time period ranging from about 30 seconds to about 1 hour.

18. The method of claim 1 wherein the porogen has a molecular weight ranging from about 100 to about 50,000 amu.

19. The method of claim 1 wherein the porogen is selected from the group consisting of a polyalkylene oxide,a monoether of a polyalkylene oxide, a diether of a polyalkylene oxide, bisether of a polyalkylene oxide, an aliphatic polyester, an acrylic polymer, an acetal polymer, a poly(caprolatactone), a poly(valeractone), a poly(methyl methacrylate), a poly (vinylbutyral) and combinations thereof.

20. The method of claim 1 wherein the porogen comprises a polyalkylene oxide monoether which comprises a C3 to about C6 alkyl chain between oxygen atoms and a C1 to about C6 alkyl ether moiety, and wherein the alkyl chain is substituted or unsubstituted.

21. The method of claim 20 wherein the polyalkylene oxide monoether is a polyethylene glycol monomethyl ether or polypropylene glycol monobutyl ether.

22. The method of claim 1 wherein the porogen is present in the composition in an amount of from about 1 to about 50 percent by weight of the composition.

23. The method of claim 1 wherein the composition further comprises a solvent.

24. The method of claim 1 wherein the composition further comprises solvent in an amount ranging from about 10 to about 95 percent by weight of the composition.

25. The method of claim 1 wherein the composition further comprises a solvent having a boiling point ranging from about 50 to about 250° C.

26. The method of claim 1 wherein the composition further comprises a solvent selected from the group consisting of hydrocarbons, esters, ethers, ketones, alcohols, amides and combinations thereof.

27. The method of claim 23 wherein the solvent is selected from the group consisting of di-n-butyl ether, anisole, acetone, 3-pentanone, 2-heptanone, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl lactate, ethanol, 2-propanol, dimethyl acetamide, propylene glycol methyl ether acetate, and combinations thereof.

28. A nanoporous silica dielectric film formed from a composition comprising a silicon containing pre-polymer, a porogen, and a metal-ion-free catalyst selected from the group consisting of tetramethylammonium acetate, tetrabutylammonium acetate, tetramethylphosphonium acetate, tetramethylphosphonium hydroxide, triphenylphosphine, trimethylphosphine, trioctylphosphine, and combinations thereof,
which nanoporous silica dielectric film is produced according to the method of claim 1.

29. A semiconductor device comprising a nanoporous dielectric film of claim 28.

30. The semiconductor device of claim 29 that is an integrated circuit.

31. The nanoporous silica dielectric film of claim 28 wherein the composition additionally comprises a solvent.

32. The nanoporous silica dielectric film of claim 28 wherein said metal-ion-free catalyst is tetramethylammonium acetate.

33. The nanoporous silica dielectric film of claim 28 wherein said silicon containing pre-polymer comprises a combination of acetoxy-based leaving groups.

34. The nanoporous silica dielectric film of claim 33 wherein said silicon containing pre-polymer comprising a combination of acetoxy-based leaving groups comprises tetraacetoxysilane and methyltriacetoxysilane.

35. The nanoporous silica dielectric film of claim 28 wherein said porogen comprises polyethylene glycol monomethylether.

36. The nanoporous silica dielectric film of claim 28 wherein said porogen comprises polypropylene glycol dimethylether.

37. The nanoporous silica dielectric film of claim 28 wherein said porogen comprises polyethylene glycol dimethylether.

38. The nanoporous silica dielectric film of claim 28 wherein said porogen comprises polypropylene glycol monobutyl ether.

39. A precursor for nanoporous silica dielectric film formation comprising said composition of claim 28.

40. A nanoporous silica dielectric film of claim 28 wherein said composition is a spin-on composition.