ETCH TECHNIQUE 37 



six-fold and may not be used to determine electrical sense or twinning. It 

 ma)', however, be used to determine approximately the orientation of the 

 X-axes. Cleavage-figures are seldom strong, but appear to be best with 

 coarse grinding. ^^ 



5.6 The Process of Etching Quartz 



Few factors related to the chemical process of etching quartz have been 

 extensively studied. Much of the information here presented is taken from 

 preliminary' reports of L. Egerton of the Laboratories, who has undertaken 

 an investigation of the etching process. Though the information mainly 

 regards hydrofluoric acid etching, some data is given on etching with hydro- 

 fluoric gas, and bifluoride mixtures. 



The reaction of quartz, which is silicon dioxide (Si02), with hydrofluoric 

 acid (HF) is given by the following equations: 



Si02 + 6HF ^ SiF4 + 2H2O + (2HF) i=± HoSiFe + 2H2O. 



Since the hydrofluoric acid is a solution of HF gas in water, the reaction of 

 the acid with quartz results in a reduction of the concentration of HF. At 

 the same time there is produced silicon tetrafluoride (SiF4) which reacts 

 with more HF to give fluosilicic acid (H2SiF6) in solution. It is common 

 practice to start with about 50% HF acid and to continue etching until the 

 HF concentration is down to 20 or 25%, at which time there should also be 

 a 30% to 35% concentration of H2SiF6, if all the depletion of HF were due to 

 reaction with the quartz. Actually much smaller concentrations of HoSiFe 

 are found, and this discrepancy is mainly due to the large continuous loss 

 of HF from the solution by gassing. Further, the etching powder of this used 

 acid is not the same as w^ould be obtained with a solution of 20%-25% HF 

 alone in water. However, this dift'erence is hardly noticeable except with 

 weak etches. 



Through the useful life of the acid, starting with 50% HF and depleting 

 to about 20% HF, practically identical etch-figures may be obtained by 

 properly adjusting the etching time. Means of testing the etching power of 

 the acid to determine the proper etching time are compHcated by the pro- 

 duction of HoSiFe in the solution, and by the irregular loss of HF by gassing. 

 Further, the power of the acid to produce useable etch-figures is not the 

 same as its power to remove quartz, or to etch glass, or as its concentration 

 of HF or HoSiFe. For these reasons any indirect method of measuring 

 etching-power must be correlated empirically with the etching-time required 

 to give the desired etch-figures. 



An indirect method of testing the etching-power, developed by Dr. W. 

 Hoft" of Western Electric, Hawthorne, involves the etching of sand blasted 



** Scrubbing the surface with soap, water, and brush sometimes improves the figure. 



