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.
B. Sherman and J. F. Black, "Scanned Laser Infrared Microscope," Appl. Opt. 9, 802-809 (1970)