The infrared laser scan microscope is especially suitable for applications in material science and in the semiconductor industry. The infrared laser beam is deflected in x and y directions by a mirror system and the scanning beam is focussed with the help of a light microscope onto the sample. Four different IR (infrared) lasers can be used; the semiconductor lasers AlGaAs/GaAs with a wavelength of about 820nm and GaInAsP/InP with a wavelength of about 1.3μm and the helium-neon-lasers emitting at 1.152μm and 1.523pμm. The lasers are situated outside the optics and are connected by a special monomode glass fiber. The He-Ne laser, with a wavelength of 1.152μm, is of special significance in the investigation of silicon and ICs because its wavelength exactly corresponds to the energy gap of silicon at room temperature ( 1.07eV ). It is therefore possible to image silicon at different depths, to test devices from the back and, moreover, to produce an OBIC ( Optical Beam Induced Current ) signal from the back of the device.