

I 



1210 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



and similar problems occurring, for example, in the evaluation of con- 

 trolled etching and of evaporated layers. 



PRINCIPLE^ 



A half-silvered mirror is brought into contact with a reflecting surface. 

 If this combination is illuminated with monochromatic light, one ob- 

 serves interference fringes. Dark lines appear where the distance between 

 mirror and surface is n X X/2, where n is an integer. Between two points 

 on adjacent fringes the difference in this distance is therefore X/2. Hence j 

 the fringes can be regarded as contour lines for the distance between 

 the mirror and the surface under consideration. Since the mirror is op- 

 tically flat, one can deduce the profile of the surface. Equidistant and 

 parallel fringes, for example, prove the surface to be flat. By taking the 

 profile across a bevel or a step, one is able to measure the depth of one 

 part of the surface with respect to another optically flat part of the sur- 

 face. The reflectivity of the crystal surface should be as high as possible, 

 and that of the mirror should be of the same order. The fringes are then 

 produced by the interference of several wave trains which make the 

 fringes very sharp, and one can measure fractions of X/2. With the equip- 

 ment described here, one is able to interpolate to 3^o of X/2 or less. 



Since small linear dimensions are involved, this principle was adapted 

 for use under a microscope. Hence, it is possible to measure small linear 

 dimensions and the correlated depth simultaneously. 



The measurement of small steps, or steps not too steep in an other- 

 wise flat surface, can be done without altering the sample. For measure- 

 ment on steep and high steps a bevel must be lapped on the sample. 



For the measurement of p-type or n-type surface layers on semicon- 

 ductors, it is essential to lap a bevel on the original surface. The p-n 

 junction is thus exposed and can be found by an electrical method. After 

 marking its position within the bevel, it is then possible to measure its 

 depth with respect to the original surface by taking the profile across 

 the bevel. The marking has to be such that it will be visible in the fringe 

 pattern. By a proper adjustment of the optical flat, a fringe pattern can 

 be produced in which the profile is easily interpreted and the depth 

 measurement amounts to a counting of fringes. 



PREPARATION OF THE SAMPLES 1 



The method requires the use of samples with optically flat and highly 

 reflecting surfaces with respect to which a depth can be measured. It is 



' S. Tolansky, Multiple-Beam Interferometry of Surfaces and Films, Oxford, at 

 at the Clarendon Press, 1948. 



