POINT-CONTACT TRANSISTOR SURFACE EFFECTS 807 



before etching could affect the efficiency of etch. There have, however, 

 ])een some indications that this does occur. Certain chemical treatments 

 appear to passivate the surface to any subsequent treatment, for ex- 

 ample, the results in Sections 4.2.3 and 4.2.4. The electro-etched sur- 

 face followed by an HF treatment does not change rapidly with time in 

 room air, while the superoxol-etched .'^urface followed by an HF treat- 

 ment changes quite rapidly. Surfaces which have been etched in CP4 

 and subseciuently treated in HF appear to be as stable as electro-etched 

 surfaces. Subsec^uent treatments in superoxol do not appear to result in 

 significant changes in the surface characteristics. Experiments on un- 

 ctched germanium wafers indicate that none of the components of CP4 

 alone will prevent normal etching, but if an unetched Avafer is treated 

 with a combination of 50 per cent nitric acid plus 48 per cent HF for a 

 few moments, the surface will be stabilized as to retard the formation of 

 the normal pyramidal etch pattern when the eurface is etched in super- 

 oxol etch. Taken together these ol)servations may imply that certain 

 types of oxide surfaces are more stable than others and perhaps may even 

 ])e passivated to subsequent environmental conditions. 



With this background of information it becomes more believable that 

 chemical treatments before etching could affect the surface of the ger- 

 manium resulting from the subseciuent etching. It is not unreasonable 

 to believe that any variation in surface potential resulting from pre-etch 

 treatment might influence the reaction between the etchant and the 

 germanium. An experiment was performed using gold-bonded bases to 

 isolate the contribution of the solder flux normally used in the base- 

 wafer attachment. Twenty wafers from the same slice were divided into 

 four subgroups of five. The groups were treated in such a way that any 

 effects of HF or solder flux soaking before superoxol etching could ])e 

 detected. 



The results of this experiment do indicate that presence of flux before 

 etching significantly affects the collector currents and turn-off time of 

 transistors made on such surfaces. Although there was no apparent dif- 

 ference in forming yield between sub-groups, it is felt that this variation 

 would show up as a difference in forming yield in a process where the 

 forming efficiency is decreased somewhat by the impregnant. 



4. .3 Conclusions 



Treatment of an etched surface with germanium dioxide solvents such 

 as HF or KOH degrades the surface to such an extent that transistor 

 forming efficiency is decreased. A similar effect is produced by corrosive 

 flux and heat. Thus, pre-forming measurements may be used to predict 



