Intrinsic Stress Measurements in CVD Diamond Films
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ABSTRACT Diamond films were grown over Si substrate at 1253K by the hot filament chemical vapor deposition method using CH 4/H2 gas mixture, and intrinsic stresses in the film were deduced from the ex-situ curvature measurements. In order to account for the creep deformation of the Si substrate, an elastic/plastic stress and strain analysis were conducted. Results showed that intrinsic stresses were generally several times larger than the average film stresses and always positive increasing with the film thickness. For the film thickness larger than 1Wpm, stress relaxation by creep of the substrate became significant, and must be considered for the accurate assessment of the film stress in diamond. Later, an analysis based on the grain growth accounted for the development of intrinsic stresses reasonably well. I.INTRODUCTION The work reported in this article had two objectives related to the ex-situ measurement of intrinsic stresses in diamond films on Si substrates; (i) determination of the intrinsic stresses when the substrate underwent creep deformation, and (ii) estimation on the origin of the intrinsic stresses in the chemical vapor deposition (CVD) films. The stresses in thin films on substrates can be viewed as arising from the thermal mismatch, lattice mismatch, and growth (intrinsic) strains [1]. The thermal stress is caused by the difference in the thermal expansion coefficient (CTE, (X)between the film and the substrate and must be subtracted when in-situ properties are inferred from the ex-situ measurements. The lattice mismatch stress could be ignored in the present case because formation of SiC layer at the diamond/Si interface precluded the epitaxial growth [2,3]. Finally, as possible sources of the intrinsic stress several mechanisms including grain growth [4] and excess vacancy annihilation [5] were reported. Additional factors which should be considered in the actual measurements of residual stresses are the stress measurement methods and the creep deformation of Si during the CVD process. Residual stresses in diamond films are typically measured by the curvature method, X-ray diffraction (XRD) and Raman peak shift methods. However, measured stress values often vary widely even for the same specimens [69]. The substrate temperature during the CVD is usually around 0.65-0.75Tm, and the substrates undergo creep deformation [10-12]. In the present analysis, diamond films were grown on the p-type Si(100) substrate by the hot filament(HF) CVD method using the CH 4/H2 gas mixtures, and the substrate curvatures were measured ex-situ in air at various stages of film growth 471 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
using parallel laser beams. Then, intrinsic stresses in film were calculated based on the elastic/plastic analysis, and origins of such stresses were discussed. II. EXPERIMENTAL PROCEDURE 11. 1 Elastic/Plastic Analysis Following R611[13], we assume that the stresses in the film cause the substrate and the film deformation and write; o
M.,, (2) 6= -K,..., ... (3
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