Acoustic phase velocity measurements with nanometer resolution by scanning acoustic force microscopy

Abstract

With the increasing interest in nanostructures and thin films, the need for a quantitative measuring method for elastic constants on the nanometer scale has become more evident. The fundamental physical quantity characterizing the elastic constants is the acoustic phase velocity. Due to the strong localization of surface acoustic waves (SAWs) in the near-surface region, SAWs are particularly favored for such investigations. The velocity measurement is commonly performed by time delay and acoustic far-field methods. Therefore the lateral resolution of the velocity measurement is restricted by the wavelength involved to some tens of microns. Recently, we introduced the scanning acoustic force microscope (SAFM) for the measurement of SAW amplitude distributions with nanometer lateral resolution. The key to detecting high-frequency surface oscillations by the slowly responding force microscope cantilever is the nonlinear force curve. This nonlinearity can be exploited in a heterodynetype setup for high-frequency wave mixing of a probe and a reference wave, revealing the phase of the probe wave. The difference frequency can be chosen to be as low as 1 kHz. We present measurements of the phase velocity over a lateral distance of 19.9 nm. The phase velocity dispersion due to Au layers on a quartz substrate was measured over distances as small as 200 nm and compared with calculations.

Transient grating measurements of film thickness in multilayer metal films

“…Acoustic probing of elastic properties has in turn been used to study phase transitions, [1][2][3] surface quality, defects, and interactions with chemical species at a surface. 4 -11 Nondestructive probing through the use of acoustic waves is of particular interest in the microelectronics and coatings industries, and a variety of techniques have evolved including Brillouin scattering, 12 acoustic microscopy, 2,[13][14][15][16][17][18][19][20][21] and techniques which use lasers to generate acoustic waves. [22][23][24][25][26][27][28][29][30][31][32] Lasers generate acoustic responses in optically absorbing samples through sudden electronic excitation and subsequent lattice thermalization and expansion.…”