Silicon Nitride Fibers Using Micro Fabrication Methods

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SILICON NITRIDE FIBERS USING MICRO FABRICATION METHODS J. KOSKINEN AND H. H. JOHNSON Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. ABSTRACT Amorphous LPCVD and PECVD Si 3N 4 films have been patterned into fibers of various shapes using integrated circuit methods. A tensile test apparatus was developed which allows a simultaneous testing of up to 20 fibers. The average tensile strength of LPCVD Si 3 N 4 fibers with a 30 g.m gauge section was 7.0 ± 0.4 GPa. The corresponding value for PECVD Si3 N 4 was 2.4 ± 0.2 GPa. The following properties of the materials were measured: atomic composition (RBS, FRES), density, refractive index, internal stress, elastic modulus and micro hardness. A Weibull analysis of the tensile strength values was performed. INTRODUCTION Research on high strength ceramic fibers is essential to the development of high performance composite materials. Fibers of refractory materials are difficult to manufacture to a specific geometric shape. This work presents methods to manufacture fibers from deposited Si3 N 4 films using integrated circuit methods. 1 The films were deposited using low pressure chemical vapour deposition (LPCVD) and plasma enhanced chemical vapour deposition (PECVD). The tensile strength of the fibers was measured using a specifically designed multi-fiber measuring device. This method allows a unique evaluation of the mechanical properties of the film without interference from the substrate. EXPERIMENTAL The Si3 N 4 films were deposited on two kinds of samples; pure Si (100) wafers and Si wafers with a 1 gm thick film of thermal oxide in LPCVD and in PECVD a 1.2 gm thick film of polyimide ( ProbimideO 287 baked at 250 oC). The deposition parameters are in Table I. The fiber forms were patterned on the films using an optical projection aligner and reactive ion etching (CHF 3 ). Self supporting fibers were then obtained by dissolving the intermediate film. Hydro fluoric acid buffered with ammonium nitride (6:1) was used to etch the thermal oxide and dichloromethane to dissolve the polyimide layer. The etch rate of the oxide was 90 nm/min as compared to the < 1 nm/min etch rate of LPCVD Si 3N 4 . The photoresist layer survived the 70 min etch protecting the top side of the fiber. Finally the photoresist was removed by reactive ion etching (02). Rutherford back scattering (RBS) and forward recoil spectroscopy (FRES) were used to determine the atomic compositions of the films. Films deposited on pure Si wafers were used to determine the following properties of the materials: density, refractive index, residual stress and micro hardness. The wafers were weighed and the curvature was measured before and after the deposition. Since the LPCVD Si 3 N 4 coating was grown on both sides of the

Mat.Res. Soc. Symp. Proc. Vol.130. '1989 Materials Research Society

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Table I Deposition parameters of Si 3 N4 films

PEVD

VD

SiH 4 /NH 3 2060 280 5 3) Si; Si/Polyimide 400 Radial flow 2 30 kHz; 0.12 W/cm

SiH 2 CI 2 /NH 3 30MO 80D 230 4.3 Si; Si/SiO 2 (th