Determination of Mechanical Properties of Porous Silica Low-k Films on Si Substrates Using Orientation Dependence of Surface Acoustic Wave

Abstract

The measurement accuracy of Young's modulus E and Poisson's ratio σ of thin low-dielectric-constant (low-k) films is improved by the simultaneous analysis of the laser-pulse-generated surface-acoustic wave (SAW) propagating along two different crystalline Si orientations. Frequency ( f)-dependent phase velocities v110( f) and v100( f) of SAW propagating along [110] and [100] directions of the Si substrate were obtained by analyzing the SAW waveforms measured using a piezoelectric transducer. The mass density ρ and the thickness d of low-k films were determined by X-ray reflectance and spectroscopic ellipsometry, and were then used to determine the values of E and σ that fit the dispersion curves v110( f) and v100( f) best. Different dependencies of v110( f) and v100( f) on E and σ were the key for the accurate determination of the values. This method was employed to study a series of porous organosilica films in which methyl content was varied. The results showed that E and σ decrease with methyl content from 9.5 to 4.3 GPa and from 0.36 to 0.25, respectively, in the studied range of methyl content. It is concluded that the reported method is an accurate nondestructive technique for the simultaneous determination of E and σ of low-k films on Si.