

334 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



the localization is so effective that the barrier effects found with n-type 

 semiconductors can be ignored; if not, the barrier can be overcome by 

 light or heat, as will be described below. 



If part of the work is coated with an insulating varnish, electrolytic 

 etching will take place only on the uncoated surfaces. This technique, 

 often called "masking," has the limitation that the etching undercuts 

 the masking if any considerable amount of material is removed. The i 

 same limitation applies to photoengraving, in which the insulating coat- 

 ing is formed by the action of light. 



The cathode of the electrolytic cell may be limited in size and placed 

 close to the work (which is the anode). Then the etching rate will be 

 greatest at parts of the work that are nearest the cathode. Various 

 shapes can be produced by moving the cathode with respect to the I 

 work, or by using a shaped cathode. For example, a cathode in the form | 

 of a wire has been used to slice germanium. 



Instead of a true metallic cathode, a "virtual cathode" may be used 

 to localize electrolysis.^ In this technique, the anode and true cathode 

 are separated from each other by a nonconducting partition, except for 

 a small opening in the partition. As far as localization of current to the 

 anode is concerned, the small opening acts like a cathode of equal size 

 and so is called a virtual cathode. The nonconducting partition may 

 include a glass tube drawn down to a tip as small as one micron diameter 

 but nevertheless open to the flow of electrolytic current. With such a 

 tip as a virtual cathode, micromachining can be conducted on a scale 

 comparable to the wavelength of visible light. A general advantage of 

 the virtual cathode technique is that the cathode reaction (usually 

 hydrogen evolution) does not interfere with the localizing action nor 

 with observation of the process. :| 



In the jet-etching technique, a jet of electrolyte impinges on the 

 work.^'* The free streamlines that bound the flowing electrolyte are 

 governed primarily by momentum and energy considerations. In turn, 

 the shape of the electrolyte stream determines the localization of etch- 

 ing. A stream of electrolyte guided by wires has been used to etch semi- 

 conductor devices.^ Surface tension has an important influence on the 

 free streamlines in this case, 



PROPERTIES OF ELECTROLYTE-SEMICONDUCTOR BARRIERS 



The most distinctive feature of electrolytic etching of semiconductors 

 is the occurrence of rectifying barriers. Barrier effects for germanium 

 will be described; those for silicon are qualitatively similar. 



The voltage-current curves for anodic n-type and p-type germanium 



