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Physics of Thermal Wave NDE of Semiconductor Materials and Devices
Published online by Cambridge University Press: 29 November 2013
Extract
Thermal wave physics is playing an ever increasing role in the on-line characterization of semiconductor materials and devices. This is especially true for thermal wave methods that employ laser beams for both the generation and detection of thermal waves. For the modulated reflectance method discussed here, the pump and probe beams are focused on the same spot. They therefore achieve the noncontact advantage of optical methods in addition to the optimum condition for high spatial resolution, a necessary condition for thermal wave measurements on product wafers.
When a material is excited with an intensity-modulated laser beam or pump, a thermal wave is generated in the material and in the air above the sample. The material within this heated region will undergo a thermal expansion which can be detected with a probe beam interferometer or by deflecting the probe beam from the thermoelastic deformation of the surface. Since the complex refractive index of most materials depends on temperature, a modulated temperature will also induce a corresponding modulation in the refractive index and consequently a modulation on a probe beam passing anywhere near the thermal wave. A probe beam directed along the heated surface of the sample, for example, will be deflected as it passes through the heated region above the surface. This mirage effect can also be observed within the sample by directing a transmitting probe through the heated region beneath the surface. Likewise, using a probe beam directed onto the sample surface one can observe a modulation in reflection, transmission, or scattering. A related noncontact method is the photothermal measurement of infrared radiation emitted from the material's heated region. Note that with all these detection methods, thermal wave measurements can be, and most often are, done in air and at room temperature.
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- On-Line Nondestructive Evaluation
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- Copyright © Materials Research Society 1988
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