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首页> 外文期刊>Review of Scientific Instruments >Subsurface mapping of thermal properties with optical heterodyne force microscopy
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Subsurface mapping of thermal properties with optical heterodyne force microscopy

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Optical heterodyne force microscopy (OHFM) is a novel local probe microscopy that combines an atomic force microscope (AFM) with a periodically modulated laser source. The amplitude and phase of the vibrations of the sample surface induced by the periodic heating can be imaged with OHFM. The thermal diffusivity, rather that just the thermal conductivity, has a significant effect on the image contrast. Surface and subsurface variations in thermal properties can be mapped to depths of the order of the thermal diffusion length, which decreases as the modulation frequency increases. The use of an incident laser beam chopped at megahertz frequencies typically allows ～1 μm lateral or depth resolution in thermal properties. However, the thermal near field can be imaged to a lateral resolution of that of the AFM, namely a few nanometers. In this study we have detected a buried aluminum film with OHFM and done quantitative comparisons with a one-dimensional theoretical model based on the thermal wave propagation in the sample. We will show the topography and OHFM images for a 2.5×2.5 μm region obtained when scanning across a boundary between the Cr/SiO_(2) structure and the Cr/Al/SiO_(2) structure. The top layer consists of a 90 nm Cr film. For the Cr/Al/SiO_(2) structure only there is a buried intermediate layer, a 180 nm Al film. The films are deposited on a silica substrate. An infrared laser beam of wavelength 830 nm is chopped at ～4 MHz to give an average incident power of 3 mW focused to a ～3 μm diameter spot on the tip. A commercial transparent Si_(3)N_(4) cantilever of spring constant ～0.1 Nm~(-1) and a tip radius ～20 nm is used. Compared with the one-dimensional model of the thermoelastic response of the structure, the experimental contrast between the two regions is somewhat smaller than predicted for the amplitude images and larger than predicted for the phase images. The discrepancy may be due to a thermal boundary resistance or to three-dimensional effects not accounted for in the model.

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