The Use of an Interference Microscope for Measurement of Extremely Thin Surface Layers

Extremely thin p-type or n-type surface layers can be obtained on semiconductors by recently developed diffusion techniques. 1 , 2 Layer thicknesses of the order of 10~4 cm are currently used for making diffused base transistors. 3 , 4 The thickness of the diffused layer is an important parameter for the evaluation of such transistors. Its measurement is facilitated by lapping a bevel on the original surface, thus exposing the p-n junction within the bevel where the thickness appears in an enlarged scale. With a sharp and well defined angle, one would obtain the thickness by the measurement of the angle and of the distance between the vertex and the p-n junction. However, it is extremely difficult to obtain vertices sharp enough for measurements of thicknesses of the order of 10_1 cm. To avoid this difficulty, an interferometric method was developed in which the depth is measured directly by counting interference fringes of monochromatic light. The method can also be used for the measurement of small steps 1 2 3 4