Observation of Glass Transition Temperatures in Polymeric Thin Films by Wafer Curvature Measurements
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OBSERVATION OF GLASS TRANSITION TEMPERATURES IN POLYMERIC THIN FILMS BY WAFER CURVATURE MEASUREMENTS P. H. TOWNSEND and B. S. HUBER
The Dow Chemical Company, M. E. Pruitt Research Center,1712 Building, Midland. Ml 48674 D. S. WANG
The Dow Chemical Company, CASAL, B-i1811 Building, Freeport, TX 77541. ABSTRACT Wafer bending measurements have been used to study the glass transition temperature, Tg, of thin coatings of polystyrene and polycarbonate on Si wafers. The observed values of Tg agree with DSC and TMA measurements on bulk samples. The evolution of the substrate curvature has been used to examine the behavior of Tg in thin epoxy films and coatings derived from divinylsiloxane bisbenzocyclobutene, mixed stereo and positional isomers of 1 ,3-bis(2bicyclo[4.2.0]octa-l ,3,5-trien-3-ylethenyl)-l ,1,3,3-tetramethyl disiloxane (CAS 117732-87-3). The dependence of the Tg of the epoxide coatings is studied as a function of the cross-linking. The evolution of the Tg in the benzocyclobutene coating is found to be a monotonic function of the level of conversion of the polymer network. INTRODUCTION The use of wafer curvature measurements to infer the stresses in thin films has been a versatile method for studying the mechanical behavior of inorganic and organic coatings. This technique has the advantage that it is readily adapted to insitu measurements at elevated temperatures in conditions simulating actual processing environments. Suggestions of the observation of Tg by wafer curvature have been previously reported[I,2]. In this work, the observation of Tg by wafer curvature is validated by comparison to accepted methods for Tg measurement with polystyrene and polycarbonate standards. Epoxide coatings are used for a wide variety of applications where protective films are required. In some cases, control of the flow properties of the coating is essential to maintaining the protective integrity. This paper examines the dependence of Tg in epoxide coatings as a function of the formulation with the crosslinking agent. Monomeric divinylsiloxane bisbenzocyclobutene (BCB) polymerizes into a three dimensional network to form a high temperature thermoset useful for multichip module (MCM) applications[3]. Because of the network character of this material, the mechanical behavior has fundamentally different features than in linear polymeric systems, particularly within the context of the processing sequence. This paper examines the mechanical interactions between BCB films and silicon substrates during thermal cycles used to cure the polymeric coatings.
Mat. Res. Soc. Symp. Proc. Vol. 239. ©1992 Materials Research Society
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EXPERIMENTAL Substrate Curvature Measurement Substrate curvatures were measured in this work with an optically levered beam. The instrument was constructed using the approach described by Flinn[4]. The ultimate accuracy of the present system is ca. 20 km for a 4" substrate and ca. 5 km for a 1" sample used for high temperature measurements. The experimental accuracy is slightly less, owing to errors incur
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