Nondestructive Evaluation and Materials Characterization Using Photothermal-Optical-Beam-Deflection Imaging

摘要

Photothermal optical beam deflection (PTOBD) imaging involves use of a focused, modulated laser beam to locally heat a sample and a second laser beam to probe the resulting changes in sample temperature. For opaque samples, the photothermal heating occurs essentially at the surface and temperature changes in the bulk occur via thermal diffusion to a depth below the surface of the order of thermal diffusion length. Since the limiting factor in definition/resolution is not the thermal diffusion length, features of the order of the heating laser beam diameter or smaller can be investigated, even at relatively low modulation frequencies. Lateral spatial resolution for subsurface features much closer to the surface than a diffusion length is independent of the diffusion length and is determined principally by the diameter of the probe beam can affect the resolution. Generally, the limiting factors on spatial resolution in PTOBD imaging are the heating beam diameter, the shape and depth of the subsurface feature being imaged, and the modulation frequency (to a lesser extent). This resolution less than 10 micrometers. Some contrast issues and underlying materials issues are discussed qualitatively. (Reprints)