INTERFERENCE MEASUREMENT OF THIN SURFACE LAYERS 1221 



In the second case the fringes turn liack. With a sUghtly different set- 

 ting of the mirror the fringes could be ahiiost parallel "wdth the turning 

 |)()int outside the field of sight. The fringe pattern then resembles the 

 lirst case, and therefore it might be easily misinterpreted. 



The third case makes the plot unnecessary. The x-axis is chosen in 

 such a way that it coincides with a fringe in the original surface. Thus, 

 in the profile plot, the original surface is horizontal. Hence, the layer 

 thickness can be obtained by counting the number of intersecting fringes 

 between original surface and the p-n junction. This is, therefore, the most 

 convenient setting of the mirror. 



The noted number gives in each case the layer thickness in "fringes." 

 All three cases are in essential agreement. The layer thickness in this 

 : case is 



An X ^ = (16.5 ± 0.5) X ~ X 10"' cm 



= (4.85 ± 0.15) X 10"' cm 



j Fig. 9 gives the fringe pattern obtained with a silicon p-n junction 



marked by the plating procedure. 



The evaluation of steps in a surface is shown for two cases. The very 

 ' shallow step in Fig. 10 is an example in which fractions of X.'2 are to be 



measured. The step here is 



^ X ^ = 0.195 X ^^ X 10"' cm = 5.75 X 10"' cm 



a + 6 2 2 



In Fig. 11 the step is so high and steep that it is impossible to correlate 

 the fringes crossing the step. But with the aid of the bevel, seen in the 

 lower part of Figure 11, a correlation is possible. The height of the step 

 along the drawn line is 



*t) 



(25 - 12) X ^ = 13 X ^ X 10"' cm = 3.8 X 10"' cm 



The accuracy of the method depends mainlj^ on the (juality of the opti- 

 cally flat mirror since it serves as a plane of reference. A thin mirror is 

 likely to be slightly bent under the pressure of the clamp. Therefore, it 

 is advisable not to work with too high a pressure. For the measurement 

 of layer thicknesses the (juality of the original surface is also important. 

 An accuracy of 5 per cent is easily obtained using half-sih'ered micro- 

 scope slides for the mirror. These slides are essentially flat over the small 

 region covered by the microscope. 



