| ABSTRACT: | RESONANCE ULTRASOUND SPECTROSCOPY FOR EVALUATING ELASTIC CONSTANTS
AND INCOHESIVE BONDS OF THIN FILMS
N. Nakamura, H. Ogi, and M. Hirao
Osaka University, Toyonaka, Osaka, Japan
Recently, understanding of thin film’s mechanical properties is getting more important in developing micro-
and nano-scale devices, because there is always a demand for insuring device reliability and, more
importantly, there are correlations between their mechanical properties and their magnetic, electric, optic
properties. Especially, their elastic constants are indispensable not only for designing acoustic devices but
also evaluating defects (microcracks, incohesive bond, and so on.). Evaluation of defects in thin films is an
important issue because they shorten the device life. It is usually difficult to observe such a defect in a
deposited film with conventional microscopic methods. However, accurate measurement of the elastic
constants solves this problem, because elastic constants are strongly affected by the volume fraction and
shape of defects. In this study, we propose an advanced resonance acoustic spectroscopy (RUS) for
determining the thin film elastic constants. RUS determines the elastic constants of solids from their free-
vibration resonance frequencies. Previously, it has been applied to many bulk materials, but hardly to thin
films because of two difficulties: (i) accurate measurement of resonance frequencies and (ii) correct mode
identification for observed resonance frequencies. We solve these long-running problems by developing a
piezoelectric tripod to measure the frequencies and by incorporate the laser-Doppler interferometer in the
system to scan vibrating specimen’s surface to measure vibration modes. The former establishes high-
accurate frequency measurement and the latter allows us to identify vibration modes unambiguously. We
apply our technique to copper thin films (0.7-9 um) and chemical-vapor-deposition diamond thin films (1.0-
500 um). Determined elastic constants are smaller than those of bulks and elastically anisotropic between
in-plane and out-of-plane directions. We attribute these observations to thin incohesive-bond regions, which
are quantified through micromechanics calculations.
|