Sol-Gel Processing of Low Dielectric Constant Nanoporous Silica Thin Films
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Sol-Gel Processing of Low Dielectric Constant Nanoporous Silica Thin Films Deok-Yang Kim1, Henry Du1, Suhas Bhandarkar2, and David W. Johnson, Jr.3 1 Department of Chemical, Biochemical, and Materials Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, U.S.A. 2 Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, U.S.A. 3 Agere Systems, Murray Hill, NJ 07974, U.S.A.
ABSTRACT Tetramethyl ammonium silicate (TMAS) is known as a structuring agent in zeolite synthesis. We report its first use to prepare porous silica films for low k dielectric applications in microelectronics. A solution of TMAS 18.7 wt. % was spin coated on silicon substrates with a 3000 Å thick thermal oxide. The spin coated films were subsequently heattreated at 450oC to obtain porous silica. The use of TMAS solution without gelation led to films of only moderate porosity value of 10%. The addition of methyl lactate, a gelling agent, significantly increased film porosity and improved the pore size distribution. For example, 50% porosity and uniform pore size distribution (average pore size ~ 40 Å) has been achieved. Dielectric constants (k) of our porous films are as low as 2.5.
INTRODUCTION There has been intensive research on low dielectric constant materials for metalinsulator interconnects in integrated circuits. A variety of materials have been explored so far, including polymers [1], inorganic-organic hybrid [2] and porous silica [3-6]. Among those candidates, porous silica is the most promising because it has high tunability of porosity and thermal stability. Tetramethylammonium (TMA) cation salts have been used as common structure-directing agents in the synthesis of zeolite [7]. This quartenary ammonium ion is known to form hydrates in aqueous solution. Water associated with the TMA ion is partially displaced by silicate in the synthesis of zeolite to form precursors for nucleation centers, allowing for the structure-direction to occur. TMA ions have strong affinity to silicate ions, which allows effective screening and dispersing of silicate ions in the solution. Nuclear magnetic resonance study of tetramethyl ammonium silicate (TMAS) has shown that it forms oligomers at high pH [8]. The sol-gel chemistry of TMAS system is much simpler than that of typical TEOS- based recipe because it does not need to use co-solvent (i.e. methanol). The unique feature of TMAS sol-gel process is kinetically controllable gelation with hydrolyzable ester [9]. The addition of ester reduces the pH value down to a level below 10 where gelation takes place in this system. Introduction of positive charge dissipates the space charge on silicate and allows the silica monomers to coalesce under the influence of attractive van der Waals forces. Spin coating has been used to deposit the porous thin silica films on desired substrates. This technique can be easily adapted to a manufacturable process.
EXPERIMENTS An aqueous solution consisting of 18.7 wt.% TMAS in water (Aldrich, 99.99+% purity, LOT 06301HU) was used. The ratio between TMA cation
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