Effect of dose stoichiometry on the structure of vapor-deposited polyimide thin films
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PMDA-ODA polyimide thin films can be formed by vapor co-deposition of the precursor molecules PMDA (pyromellitic dianhydride) and ODA (4,4'-oxydianiline) if the resulting polyamic acid film is heated to a 473 to 573 K cure step. We have used laser Raman spectroscopy to study how dose composition, dose rate, and substrate temperature influence the properties of the resulting polyimide films. We find that only doses with excess PMDA produce high quality films. Doses with 1:1 stoichiometry or excess ODA produce thermally unstable films that contain imine bonds; these films decompose below 575 K. Dosing onto substrates below 3f5 K produces the polyamic acid precursor of polyimide. At higher substrate temperatures, films with high defect densities or decomposed films are produced. The equilibrium vapor pressures of ODA and PMDA are reported.
I. INTRODUCTION This paper describes defects that develop during the processing of the aromatic polyimide PMDA-ODA, polyfiV, Af'-bis(phenoxyphenyl) pyromellitimide]. Such defects can adversely alter the dielectric and adhesive properties of polyimide films which are widely used, for example, in microelectronic packaging.1"3 Polyimide films can be cast or spin-coated from solution, or formed by direct vapor deposition of precursor molecules which is the subject of this paper. Vapor deposition of thin polyimide films was first demonstrated by Salem et al.,4 and by Iijima et al5 Its advantages over solvent-based techniques include elimination of deleterious interactions between the substrate and solvent.6 Figure 1 shows the basic condensation reaction for formation of the polyimide PMDA-ODA from the precursor monomers PMDA (1,2,4,5-benzenetetracarboxylic dianhydride or pyromellitic dianhydride) and ODA (4-aminophenyl ether or 4,4'-oxidianiline). This reaction has been extensively studied in solution7 where ODA and PMDA molecules initially interact via formation of a charge transfer complex. Immediately, nucleophilic substitution of an ODA amine group at the PMDA carbonyl group takes place and is accompanied by the opening of the anhydride ring to form a positively ^Present address: Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712. b 'Present address: Department of Chemistry, University of North Carolina at Charlotte, Charlotte, North Carolina 28223. c 'Now with Department of Chemistry, University of Maine, Orono, Maine 04469. d 'Author to whom correspondence should be addressed. 3218 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 12, Dec 1993 Downloaded: 11 Mar 2015
charged amide nitrogen and a negatively charged carboxylate group. The subsequent transfer of a proton
n H2N
Polyamic acid
heat
- 2H-P
Polyimide
FIG. 1. Schematic diagram for the reaction of PMDA with ODA to form polyimide via the intermediate polyamic acid. 1993 Materials Research Society IP address: 157.182.150.22
R. G. Pethe et al.: Effect of dose stoichiometry on the structure of vapor-deposited polyimide thin films
within the molecule leads to formation
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