In examination of ir-transmitting materials, internal irregularities and inclusions of foreign material can be detected by their effect upon the ir transmittance of the material. A scanning laser ir microscope has been constructed which produces, on an oscilloscope display, a shadowgraph picture of the ir transmittance of the material under examination. The 3.39-μ emission of a He–Ne laser serves conveniently as the ir source since many materials of interest transmit at this wavelength. The scan consists of a raster of 400 lines, and is completed in 1 sec. The detector is a room temperature operated indium arsenide photovoltaic cell, with a time constant of 2 μsec. A sample area of 1.2 cm × 1.2 cm is scanned with a focused spot having a nominal diameter of approximately 0.003 cm. The optical and electromechanical features of the microscope are described, and its application to examination of semiconductor materials is illustrated by several typical examples.
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