II 



070 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1950 



thick wafers are necessary since the reverse biased junction space charge 

 region extends rapidly with voltage for almost intrinsic material, and 

 high lifetime is necessary in order to get the conductivity modulation 

 effect in these thick w^afers. Therefore at present it is necessary to com- 

 promise the highest reverse breakdown voltages with the lowest for- 

 ward voltage drops, in a similar manner to that discussed in Section 1. 

 However this is now done at a different order of magnitude of voltage 

 and current density. | 



4.0 FABRICATION OF MODELS 



4.1 It has been pointed out in Section 1.2 that a low series resistance, 

 Rs , is desirable and that it is composed of two parts; the body resistance 

 and the contact resistance. In Section 3 a method for reducing the body 

 resistance was described. The contact resistance can also be made very ! 

 low. It has been found to be very difficult to solder low temperature 

 solders (M.P. up to 325°C) to silicon with any of the standard commer- 

 cial fluxes. However, it is quite easy to plate various metals to a surface 

 of silicon from an electroplating bath or by an electro-less process^ to ! 

 which leads can readily be soldered. Some metals used for plating con- 

 tacts are rhodium, gold, copper, and nickel. This type of contact yields 

 a low contact resistance. Another techniciue that has shown some prom- 

 ise for making the necessary extremely low resistance contact is the 

 hydride fluxing method.'^ 1 



4.2 A wafer which may be about one inch in diameter is ready to be i 

 diced after it is prepared for a soldering operation. Up to this point all ; 

 the material may undergo the same processing. Now it is necessary to ' 

 decide how the prepared material is to be used; whether low current i 

 ('^l amp) devices or medium or high current ('^10-50 amps) devices;. 

 are desired. The common treatment of all material for the entire class i 

 of rectifiers is one reason these devices are highly attractive from a . 

 manufacturing point of view. | 



The dicing process may be one of several techniques; mechanical! 

 cutting with a saw, breaking along preferred directions, etching alonii 

 given paths with chemical or electrical means after suitable maskiiiti 

 methods, etc. In the case of mechanical damage to the exposed junc- 

 tions, the dice should be etched to remove the damaged material. The 

 dice are cleaned by rinses in suitable solvents and are then ready for 



« Brenner, A., and Riddell, Grace E. J., Proc. American Electroplaters' Society, 

 33, p. 16, 1946,34,1). 156, 1947. 



' Sullivan, M. V., Hydrides as Alloying Agents on Silicon, Semiconductor 

 Symposium of the Electrochemical Society, May 2-5, 1955. 



I 



