Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1999-07-11
2001-06-26
Dawson, Robert (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C502S162000, C502S167000, C502S172000, C528S020000, C528S021000, C528S023000, C528S043000, C556S458000, C428S304400
Reexamination Certificate
active
06251486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to low dielectric constant materials, in particular materials comprising siloxanes.
2. Discussion of the Related Art
As integrated circuit device integration densities rise and circuit dimensions shrink, certain problems are encountered. For example, the smaller line dimensions increase the resistivity of the metal lines, and the narrower interline spacing increases the capacitance between the lines. This increased resistance and capacitance causes problems in propagation delay, crosstalk noise, and power dissipation. Moreover, as the device speed increases due to smaller feature sizes, the resistance-capacitance (RC) delay caused by the increased resistivity and capacitance will tend to be the major fraction of the total delay (transistor delay+interconnect delay) limiting the overall chip performance. It is therefore desirable to reduce the increased resistance and capacitance in integrated circuit applications.
To address these problems, new materials for use as metal lines and interlayer dielectrics (ILD), as well as alternative architectures, have been proposed to replace the current SiO
2
-based interconnect technology. These alternative architectures will require the introduction of low dielectric constant (&kgr;<3) materials as the interlayer dielectric and/or low resistivity conductors such as copper.
It is desired that new low &kgr; materials exhibit a variety of electrical, chemical, mechanical and thermal properties. These properties include low dielectric constant, high thermal stability, good adhesion, low stress, good mechanical properties, etchability and etch selectivity, low moisture absorption, high thermal conductivity, low leakage current, high breakdown strength, and easy and inexpensive manufacturability.
A variety of low &kgr; materials have been proposed to meet some or all of these criteria. The materials are typically produced by chemical vapor deposition (CVD) or by spin-on coating. Materials produced by CVD include fluorinated SiO
2
glass (&kgr;=3.5), fluorinated amorphous carbon, and polymers such as the parylene and polynaphthalene families, and polytetrafluoroethylene (PTFE) (K=2.7-3.5 for nonfluorinated polymers and 1.8-3.0 for fluorinated polymers). Materials deposited by spin-on coating include organic polymers, morganic polymers, inorganic-organic hybrids, and porous materials such as xerogels or aerogels. Organic materials typically offer lower dielectric constants than inorganic materials but, in some cases, can exhibit undesirably low thermal stability and poor mechanical properties.
One approach to polymeric low &kgr; materials has been the use of porous organic polymers. See, e.g., U.S. Pat. Nos. 5,895,263, 5,773,197, and 5,883,219, the disclosures of which are hereby incorporated by reference. See also J. Remenar et al., “Dendri-Glass-Design of Ultra-low Dielectric Constant Materials Using Specialty Highly Branched Polymers,”
Polymer Preprints
, Vol. 39, No. 1 (March 1998); and R. Miller et al., “Porous Organosilicates as Low-&kgr; Insulators for Dielectric Applications,” Abstract No. 01.2, MRS Spring 99 Meeting. These articles relate to methyl- or methy/phenyl-silsesquioxane resins with organic macromolecular pore generators, referred to as porogens. These porogens are capable of being decomposed after the resin is cured to leave nanoscopic pores. But a careful heating regime is generally required to prevent pore collapse as well as cracking. Also, methylsilsesquioxane-based materials tend to be brittle, and thus prone to cracking due to thermal-mechanical shock. (This problem was orally presented by R. Miller at the MRS Spring 1999 Meeting.) The cracking problem is particularly acute when the materials are deposited in several layers, which is typically the case.
Thus, organic low &kgr; materials are desired, where the materials exhibit a variety of desirable properties, particularly good crack resistance at the elevated temperatures experienced during fabrication of an IC device.
SUMMARY OF THE INVENTION
The invention provides an improved siloxane-based composition for use as a low&kgr; dielectric material in integrated circuit applications. The composition exhibits desirable thermal mechanical stability compared to conventional siloxane-based low&kgr; compositions. (Thermal mechanical stability indicates the temperature at which a material maintains its mechanical integrity after exposure for an extended time, e.g., greater than one month.)
In one embodiment, a modified methylsilsesquioxane composition is provided. This composition is more suitable for high temperature applications than a composition formed from only methylsilsesquioxane (referred to herein as all-methyl). (The term modified methylsilsesquioxane indicates the presence of dimethyl and/or phenyl pendant groups on a methylsilsesquioxane structure). According to this embodiment, onto a substrate is disposed a composition containing a modified methylsilsesquioxane oligomer, a pore generator material, a solvent suitable for depositing the composition onto a substrate, e.g., by spin-deposition, and, optionally, a catalyst. Suitable catalysts include moderately strong protic acids (stronger than a non-perfluorinated carboxylic acid, but weaker than a sulfonic acid), as well as moderately strong bases (pK
a
of at least 9).
The modified oligomer is characterized by the pendant group ratio A:B:C, where A represents the percentage of pendant groups that are methyl and is typically about 13 to about 67, B represents the percentage of pendant groups that are dimethyl and is typically greater than 0 to about 33, and C represents the percentage of pendant groups that are phenyl and is typically greater than 0 to about 67. (A+B+C=100.) (A pendant group is a carbon-based moiety that is attached to a silicon atom in the siloxane backbone, e.g., methyl, dimethyl, phenyl.) Curing rates depend heavily upon the particular pendant group ratio of the modified methylsilsesquioxane and on the particular composition. The presence of dimethyl and phenyl pendant groups provides a molecular structure that has better crack-resistance than all-methyl silsesquioxane, and this embodiment thus exhibits a thermal mechanical stability superior to an all-methyl material.
Advantageously, the modified methylsilsesquioxane oligomer is fabricated by a particular technique of the invention, such that a layer exhibiting even higher thermal mechanical stability is formed. This fabrication technique for the modified methylsilsesquioxane involves mixing methyltriethoxysilane monomer, before hydrolysis and condensation, with dimethyldiethoxysilane monomer that has already been partially hydrolyzed and condensed. Optionally, phenyltriethoxysilane monomer is added to the partially hydrolyzed dimethyldiethoxysilane, and it is possible for the phenyltriethoxysilane to be partially hydrolyzed.
This fabrication technique reduces the possibility of forming large blocks of methylsiloxy groups in the condensation polymer. Such reduction is desirable because extensive cross-inking of methylsiloxy groups leads to a stiff molecular structure with relatively poor thermal mechanical stability, evidenced by a tendency of the material to develop cracks. In the molecular structure formed from this novel fabrication technique, the cross-linking of methylsiloxy groups is reduced, and the dimethylsiloxy groups act as plasticizer segments between the cross-links. Phenyl pendant groups increase shock resistance, thermal stability, and also contribute to the overall flexibility of the structure by reducing the cross-link network density. This flexible molecular structure is able to withstand relatively high temperatures. For example, a composition containing a 20:60:20 oligomer formed by this technique was able to withstand about a month at temperatures over 200° C. without cracking.
REFERENCES:
patent: 3450672 (1969-06-01), Merrill et al.
patent: 4264649 (1981-04-01), Claypoole et al.
patent: 4835057 (1989-05-01), B
Chandross Edwin Arthur
Kuck Valerie Jeanne
Agere Systems Guardian Corp.
Dawson Robert
Peng Kuo-Liang
Rittman Scott
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