Thermal-Wave Microscopy

Abstract

There has been considerable interest lately in the unique imaging capabilities of thermal-wave microscopy. In this new technique, an intensity-modulated optical (laser) or electron beam is focused and scanned across the surface of an opaque sample. Localized periodic heating results from the absorption by the sample of the incident beam, and thermal waves are generated. In spite of their strong damping, thermal waves have several of the characteristics of more conventional propagating waves. In fact, one can, from a formal mathematical viewpoint, treat the interactions of harmonically generated thermal waves with thermal barriers or discontinuities in terms of "scattering" and "reflection" processes. It is through such interactions that surface and subsurface thermal features are imaged in thermal-wave microscopy. In analogy with optics and ultrasonics, the resolution attainable in thermal-wave imaging is set by both the spot size of the scanning beam and by the thermal wavelength, or thermal diffusion length. This wavelength is in turn determined by the frequency at which the optical or electron beam is modulated. Typically, modulation frequencies in the 200 kHz - 20 MHz range provide thermal resolutions in metals of 5-0.5 μm and in thermal insulators of 1-0.1 μm.