The Use of an Interference Microscope 



for Measurement of Extremely 



Thin Surface Layers 



By. W. L. BOND and F. M. SMITS 



(Manuscript received March 15, 1956) 



A method is given for the thickness measurement of p-type or n-type sur- 

 face layers on semiconductors. This method requires the use of samples with 

 optically flat and reflecting surfaces. The surface is lapped at a small angle 

 in order to expose the p-n junction. After detecting and marking the p-n 

 junction, the thickness is measured by an interference microscope. Another 

 application of the equipment is the measurement of steps in a surface. The 

 thickness range ineasurahle is from 5 X 10^^ cm to 10~^ cm. 



INTRODUCTION 



Extremely thin p-type or n-type surface layers can be obtained on 

 semiconductors by recently developed diffusion techniques.^- - Layer 

 thicknesses of the order of 10~^ cm are currently used for making diffused 

 base transistors.^' ^ 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 thick- 

 ness 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~* cm. To avoid this diffi- 

 culty, 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 C. S. Fuller, Phys. Rev., 86, p. 136, 1952. 



2 J. S. Saby and W. C. Dunlap, Jr., Phys. Rev., 90, p. 630, 1953. 

 ' C. A. Lee, B.S.T.J., 35, p. 23, 1956. 



* M. Tanenbaum and D. E. Thomas, B.S.T.J., 35, p. 1, 1956. 



1209 



