High temperature surface Brillouin scattering study of mechanical properties of boron-doped epitaxial polysilicon

B A Mathe,Jd Comins,L W Hobbs,A G Every

doi:10.1063/1.4976977

B A Mathe, Jd Comins + Show 2 more

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https://doi.org/10.1063/1.4976977

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Abstract

A study of the mechanical properties of a boron-doped epitaxial polysilicon layer deposited on a Si (100) substrate specimen has been carried out by surface Brillouin scattering at high temperatures. This type of specimen is widely used in micro-electro-mechanical systems (MEMS). By accumulating spectra with the Rayleigh mode and the Lamb continuum the isotropic elastic constants C44 and C11 were obtained, from which the values of the bulk, shear and Young’s moduli and Poisson’s ratio for the layer were determined over a range of temperatures from 20 °C to 110 °C. By contrast, an examination of the literature on polycrystalline silicon shows that other methods each provide a limited range of the above properties and thus additional experiments and techniques were needed. The SBS method is applicable to other polycrystalline materials such as silicon carbide, silicon nitride, silicon germanium and amorphous diamond that have also been used for MEMS applications.

Highlights

The determination of the mechanical properties of thin polycrystalline films or layers has been actively pursued for several materials
In the present work the non-destructive laser-based technique of surface Brillouin scattering (SBS)[18,19,20,21] is employed to investigate the elastic constants and the engineering moduli of a thin p-type polysilicon layer with 3000 ppm boron-doping deposited on a Si (100) substrate designed for mechanical systems (MEMS) applications and establish their temperature dependence over the range 20 to 110 ◦C
By considering the Rayleigh mode at 16.9 GHz and the longitudinal dip observed at 33.8 GHz in Figure 3, it was possible to determine the unknown transverse elastic constant CT and the Lameconstant, μ. using the Landau and Lifshitz relation[26] for an isotropic medium, which can be experimentally realised by a polycrystal, namely

Summary

INTRODUCTION

The determination of the mechanical properties of thin polycrystalline films or layers has been actively pursued for several materials. Sharpe et al.[1] have conducted a major review of the mechanical properties of MEMS materials providing comparisons of Young’s modulus measured for four different polysilicon specimens with numerous tests being employed. In the present work the non-destructive laser-based technique of surface Brillouin scattering (SBS)[18,19,20,21] is employed to investigate the elastic constants and the engineering moduli of a thin p-type polysilicon layer with 3000 ppm boron-doping deposited on a Si (100) substrate designed for MEMS applications and establish their temperature dependence over the range 20 to 110 ◦C. The work was originally motivated and the samples created by the Draper Laboratory, Cambridge, Massachusetts

CHARACTERIZATION OF SAMPLES

BRILLOUIN SCATTERING THEORY

SBS RESULTS AND DISCUSSION

DISCUSSION AND CONCLUSIONS