INTERFERENCE MEASUREMENT OF THIN SURFACE LAYERS 1211 



also advisable to use plane-parallel samples to facilitate the lapping of a 

 l)Ovel at a small angle. 



For lapping, the sample is waxed with its back side to the face of a 

 short steel cylinder. The face is cut at a small angle. Angles of 0.5° or 

 1.0° are practical. The cylinder is placed in a jig, in svich a position that 

 approximately half of the sample surface projects above the plane of 

 'the jig (Fig. 1). A short grind on a slightly rough glass plate using a 

 line abrasive with water gives usable bevels. For a shiny finish just the 

 light degree of roughness of the glass is important. The use of a lapping 

 machine with a vulcanized fiber plate and fine abrasive gives a better 

 surface finish, but the ridge is not as sharp. A 0.5° bevel could be obtained 

 only on a glass plate. 



Fig. 1 — Jig for lapping a bevel. 



MARKING OF p-TYPE OR n-TYPE SURFACE LAYERS 



In a sample with a p-type or n-type surface layer the junction is ex- 

 posed within the bevel. The next step is to detect and mark the junc- 

 tion. 



The sample is fixed on a microscope stage which allows a micrometer 

 controlled movement in two directions (Fig. 2 shows a Wilder microm- 

 eter cross slide). The sample is oriented in such a way that the ridge 

 is parallel to one direction of movement (y-direction). One or two lines 

 of aquadag are applied to the surface of the sample, perpendicular to 

 the ridge. The acjuadag should be diluted with water in such a proportion 

 as to achieve a thin film w^hich is non reflecting. 



A needle is fixed to the base of the stage with a suitable linkage leav- 

 ing a vertical degree of freedom. The needle is brought into contact with 

 the surface of the sample outside the acjuadag. Thus, the sample can be 

 moved under the needle while the needle maintains contact. In a suitable 

 electrical circuit, the needle serves as detector of the junction. The sam- 

 ple is moved in the direction perpendicular to the ridge (x-direction) 



