A new method for fabricating high performance polymeric thin films by chemical vapor polymerization
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A new method for fabricating high performance polymeric thin films by chemical vapor polymerization Justin F. Gaynora) Texas Instruments Semiconductor Process & Device Center, 13536 N. Central Expressway, 75243 P.O. Box 655012, MS 944, Dallas, Texas 75265
J. Jay Senkevich and Seshu B. Desu Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, 213 Holden Hall, Blacksburg, Virginia 24601-0237 (Received 3 November 1995; accepted 21 February 1996)
A vacuum deposition method is presented in which copolymer films are grown from a vinylic monomer chosen for desirable properties and paraxylylene. The concentration of paraxylylene in the final copolymer can be negligibly small if proper deposition conditions, presented here for the first time, are employed. Films of paraxylylene with N-phenyl maleimide deposited at 40 ±C, for example, showed thermal stability and FTIR spectra nearly identical with homopolymers of poly(N-phenyl maleimide). Different rate-limiting steps are proposed to explain film composition; paraxylylene is under surface reaction control, while the comonomer obeys mass flow control. This results in a deposition environment extremely rich in comonomer. Growth rates and compositions were consistent with predictions. The initiation reaction did not appear different from homopolymerization of paraxylylene. The general method presented here allows fabrication of vapor-deposited thin films with properties limited primarily by the comonomer employed.
I. INTRODUCTION
The chemical vapor deposition (CVD) of inorganic films has attained great technological and commercial importance. The inherent advantages of CVD, along with the growing importance of thin polymer films for diverse applications, suggest a general scheme for chemical vapor polymerization (CVP) would find rapid acceptance in industry. In this paper, the requirements for chemical vapor polymerization are briefly reviewed, as are polyparaxylylenes, the most widely studied class of vapor-deposited polymers. A method is proposed for chemical vapor copolymerization (CVcP) with a paraxylylene in which the resultant film is composed predominantly of comonomer. The method makes testable predictions, which are confirmed experimentally. CVP has many advantages over other methods of polymer film deposition.1–7 Because of the high surface mobility of monomer compared to high polymer, better penetration into troughs or porous surfaces can be achieved. The absence of solvents allows the deposition of smooth, pinhole free coatings of exceptional purity. There is no liquid intermediate phase between the gas and solid, so problems due to surface tension, such as pulling away from corners, are avoided. Finally, CVP, which is carried out in an isolated vacuum environment, a)
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J. Mater. Res., Vol. 11, No. 7, Jul 1996
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eliminates the environmental hazards associated
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