Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2002-01-15
2004-01-20
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C427S387000, C427S226000, C428S447000, C525S100000
Reexamination Certificate
active
06680107
ABSTRACT:
This invention relates to a film forming composition for forming a porous film having good dielectric properties, adhesion, uniformity and mechanical strength and a method for preparing the same, and a method for forming a porous film as well as the porous film thus formed.
BACKGROUND OF THE INVENTION
As the integration degree of semiconductor integrated circuits increases, an increase of wiring delay time due to an increase of the wiring capacitance which is a parasitic capacitance between metal wiring strips now becomes one factor of preventing the semiconductor integrated circuits from further performance enhancement. The wiring delay time is in proportion to the resistance of metal wiring multiplied by the wiring capacitance and thus also known as RC delay.
To reduce the wiring delay time, the resistance of metal wiring or the wiring capacitance must be reduced.
Once the wiring capacitance is reduced, semiconductor devices can be designed to a higher integration degree without inviting any wiring delay. This enables higher speed operation and minimizes power consumption.
One approach contemplated for reducing the wiring capacitance is to reduce the relative permittivity of an interlayer insulating film formed between metal wiring layers. For the insulating film having a low relative permittivity, the use of porous film instead of conventional silicon oxide film is under study. This is because the porous film is substantially the only one film that can attain a relative permittivity of less than 2.0 and be formed in a practically acceptable manner. Then a number of methods for forming porous films have been proposed.
A first method for forming porous film involves synthesizing a solution of a siloxane polymer precursor containing a thermally unstable organic component, applying the precursor solution to a substrate, and heat treating the coating for decomposing and vaporizing the organic component. After the organic component is volatilized off, a number of pores are left.
In a second method for forming porous film, a wet gel is formed on a substrate by applying a silica sol solution thereto or carrying out chemical vapor deposition (CVD). While volumetric shrinkage is restrained by controlling the evaporation rate of the solvent from the wet gel, condensation reaction is effected on the silica sol, yielding a porous film.
A third method for forming porous film is by applying a solution of silica microparticulates to a substrate, and firing the coating for consolidation, thereby forming a number of pores among silica microparticulates.
A fourth method is given by JP-A 2000-38509 disclosing a porous film-forming composition comprising (A) R
1
n
Si(OR
2
)
4-n
wherein R
1
is a monovalent organic group and n is an integer of 0 to 2, (B) a metal chelate compound and (C) a polymer resulting from polymerization of a monomer in the form of an acrylate or methacrylate.
All these methods have serious drawbacks.
The first porous film forming method adds to the cost because a siloxane polymer precursor solution must be synthesized. When the precursor solution is applied to form a coating, a considerable amount of silanol groups is left in the coating, which gives rise to the degassing phenomenon that moisture will evaporate during the subsequent heat treatment step and the problem that the porous film degrades due to moisture absorption.
The second porous film forming method adds to the cost because a special applicator is needed in order to control the evaporation rate of the solvent from the wet gel. A number of silanol groups are left on surfaces of pores in the film. Without any treatment, the film is so hygroscopic that it may undergo a substantial degradation of quality. It is then necessary to silylate silanol groups on the surface, and so, the process becomes complex. When the wet gel is formed by CVD, a special CVD system different from a plasma CVD system commonly used in the semiconductor technology is needed, which also adds to the cost.
In the third porous film forming method, the pores defined among silica microparticulates have a very large diameter because the pore diameter is geometrically determined by the deposition structure of deposited silica microparticulates. It is then very difficult to form a porous film having a relative permittivity of less than 2.
In the fourth method using a composition comprising components (A), (B) and (C), the metal chelate compound (B) is essential for improving the compatibility between components (A) and (C) and making the thickness of the cured coating uniform, but is undesirable because it invites complication of the composition, complication of the manufacturing process and a cost increase. It would be desirable to have a composition which can form a uniform solution in the absence of a chelate component and afford a flat coating after curing.
SUMMARY OF THE INVENTION
An object of the invention is to provide a film-forming composition which solves the above-mentioned problems and can form a porous film having a relative permittivity of 2.0 or lower through simple steps and at a low cost, and a method for preparing the same. Another object of the invention is to provide a method for forming a porous film and the porous film formed thereby.
Our study on the prior art porous film forming technology has led the knowledge that a silicone component is generally less compatible with an acrylic resin and without any treatment, the silicone is difficult to uniformly mix with the resin.
We made further investigations on such a mixed solution. Even though the solution is apparently clear and uniform, a thin film thereof applied by spin coating becomes whitened or noticeably striated if compatibility is insufficient only a little. In order that a composite material be used in the application contemplated herein, a very high degree of compatibility between components is required.
In the case of a mixed system of silicone resin and acrylic polymer, the prior art failed to prepare a uniform solution using only a silicone resin component and an acrylic resin component. For example, the approach of the above-referred JP-A 2000-38509 uses a metal chelate compound an essential component in addition to the silicone and acrylic components. The disclosure that the chelate component is desirably reacted with both silicone resin and acrylic resin components suggests that a uniform solution can only be formed with the aid of the chelate component, though not definitely described.
The chelate component used to solve the problem are exemplified therein by titanium, zirconium, aluminum, tin, antimony, tantalum, lead, etc. Such a component can trigger an unexpected problem when the related material is applied to semiconductor devices, and leaves some concern about moisture absorption and film degradation.
We have found that the problem can be overcome by controlling the structure of the silicone component. Specifically, a silanol group-bearing silicone resin comprising 30 to 100 mol % of structural units (T units) represented by the formula: R
1
—SiZ
3
and among the entire T units, 30 to 80 mol % of structural units (T-2 units) containing only one silanol group represented by the formula: R
1
—Si(OH)Z′
2
wherein R
1
, Z and Z′ are defined below, and having a number average molecular weight of at least 100 is used and mixed with an acrylic polymer, the resulting mixed system forms a very uniform solution. When the solution is applied and heat cured, a uniform flat coating is formed which is not spotted, whitened or striated.
Furthermore, when the thus formed coating is heated at the decomposition temperature of the acrylic resin component, the curing of the silicone resin component proceeds and at the same time, the acrylic resin component is decomposed whereupon decomposed products vaporize off, leaving pores in the coating. A porous film is eventually obtained. Upon measurement of relative permittivity, this film exhibits a very small value as compared with the relative permittivity inherent to silicone resins. By adjusting the content of th
Iwabuchi Motoaki
Yagihashi Fujio
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S
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