346 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 | 



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likely oxidation product, silicon dioxide, should react with the hydro-! 

 fluoric acid to give silicon tetrafluoride, which could escape as a gas. In 

 fact, a gas is formed at the anode and the silicon loses weight. But the 

 gas is hydrogen and an effective valence of 2.0 ± 0.2 (individual deter- 

 minations ranged from 1.3 to 2.7) was found instead of the value 4 that i 

 might have been expected. The quantity of hydrogen evolved is con- 

 sistent with the formal reaction 



Si —> Si"*"'" + me (electrochemical oxidation) 



Si+™ + (4-to)H+ -^ Si+' + Vz (4-m)H2 (chemical oxidation) 





where m is about two. The experiments were done in 24 per cent to 48 

 per cent aqueous solutions of HF at current densities up to 0.5 amp/cm^. 



The suggestion that the electrochemical oxidation precedes the chemi- 

 cal oxidation is supported by the appearance and behavior of the etched 

 surfaces. Instead of being shiny, the surfaces have a matte black, brown, 

 or red deposit. 



At 40 X magnification, the deposit appears to consist of flakes of a; 

 resinous material, tentatively supposed to be a silicon suboxide. A re- 

 markable reaction can be demonstrated if the silicon is rinsed briefly in 

 water and alcohol after the electrolytic etch, dried, and stored in air for 

 as long as a year. Upon reimmersing this silicon in water, one can observe 

 the liberation of gas bubbles at its surface. This gas is presumed to be 

 hydrogen. To initiate the reaction it is sometimes necessary to dip the 

 specimen first in alcohol, as water may otherwise not wet it. The speci- 

 mens also liberate hydrogen from alcohol and even from toluene. 



Thus, chemical oxidation can follow electrolytic oxidation. But 

 chemical oxidation does not proceed at a significant rate before thei 

 current is turned on. 



Smooth, shiny electrolytic etching of p-type silicon has been obtained; 

 with mixtures of hydrofluoric acid and organic hydroxyl compounds,; 

 such as alcohols, glycols, and glycerine. These mixtures may be an- 

 hydrous or may contain as much as 90 per cent water. The organic 

 additives tend to minimize the chemical oxidation of the silicon. They; 

 also permit etching at temperatures below the freezing point of aqueous 

 solutions. They lower the conductivity of the electrolyte. 



For a given electrolyte composition, there is a threshold current 

 density, usually between 0.01 and 0.1 amps/cm , for smooth etching.; 

 Lower current densities give black or red surfaces with the same hy- 

 drogen-liberating capabilities as those obtained in aqueous hydrofluoric 

 acid. 



