CONDUCTIVITY-MODULATED SILICON RECTIFIER 



997 



20 



15 - 



10 - 



m 



^ 

 z 



5 - 





12 3 4 



[sxlO'S-gxlO'sJ [l0'9-4Xl0'9] [SX 1019 -9X10'9] [>1020-9 x lo2o] 



SURFACE CONCENTRATION IN CM"^ 



Fig. 12 — Distribution of surface concentration of 28 P2O5 diffusions bj^ the 

 open tube deposition technique. 



18 



centrations were calculated assuming an erfc distribution."* All the 

 diffusions are on lapped silicon surfaces in the temperature range of 1 ,200 

 to 1,300° C. 



Fig. 12 shows the distribution of surface concentration of 28 P2O5 

 diffusions by the open tube process. The surface concentrations vary 

 from 10^^ to 5 X 10"° atoms/cm . These values are about a decade lower 

 than the closed tube values of surface concentrations reported by Fuller.^^ 



The measured diffusion depths were in the order of 2 X 10"^ to 

 5 X 10"^ cm. Fig. 13 shows the distribution of diffusion depths normalized 

 with the calculated diffusion depth as unity. The diffusion depths were 

 calculated from the measured surface concentration assuming an erfc'^ 

 distribution. 



The observed variation in diffusion depth is difficult to explain. Some 

 of the possibilities which have been considered are: 



1. The diffusion temperature from lot to lot would have to be from 

 to 50 degrees below the expected value to explain the variations. Dis- 

 crepancies this large have not been observed. 



2. One impurity distribution which may explain some of the results 

 is a modified Gaussian with considerable out diffusion. There are some 

 runs with high sheet resistance and diffusion depths which are consistent 

 with this picture. Generally the sheet resistances are so small that there 

 could not be much out diffusion. 



3. Some workers have suggested the possibility of the diffusion con- 

 stant being a function of the surface concentration. Fig. 13 does not 



