Comparison of Improved Bulge and Microtensile Techniques for Mechanical Thin Film Characterization - Application to Polysilicon

This paper reports on recent improvements of the bulge and microtensile techniques for the reliable extraction of material parameters such as the Young´s modulus E, Poisson´s ratio v, plane strain modulus E_ps = E/(1-v²), prestress sigma₀, fracture strength mu and Weibull modulus m, and on the direct comparison of these two methods. The bulge technique is extended to full wafer measurements enabling throughputs of data with statistical relevance whereas key improvements of a previous fabrication process of microtensile specimens lead now to much higher yields, approaching 100%. In view of its importance and wide use in MEMS, polycrystalline silicon is characterized with both methods. Values of E_ps and sigma₀ of 160 GPa and -8 MPa, respectively, are extracted for films deposited at 625degC and fully annealed at 1050degC, with no thickness dependence. Fracture data reveal a decrease of the strength mu, normalized to a reference tensile volume V_E of 32times10³ mum³ , from 2.17 to 1.38 GPa as the film thickness increases from 0.53 to 1.91 mum, with Weibull moduli m of the order of 20.