ETCH TECHNIQUE 13 



determine exactly the twinning boundaty locations. The detection of twin- 

 ning and twinning boundaries by this method has been practiced for years. 

 The determination or orientation and sense of orientation has been exploited 

 only more recently. At present the etch methods play an important and 

 extensive role in the processing of quartz plates, not only in the routine de- 

 termination of orientation, but also in the detection of twinning so that the 

 most economical cutting methods may be practiced.^ 



5.2 Twinning (General) 



Although the problems related to twinning are largely those of determining 

 orientation of the crystal structure, the nature and prevalence of twinning in 

 crj^stal quartz presents a special group of problems that would be absent 

 were the twinning absent, and hence are separately grouped as twinning 

 problems. As pointed out in Chapter I\', there are only two common types 

 of twinning in the commercial quartz used for piezoelectric plates, namely, 

 electrical and optical twinning. A simplifying feature of both these types 

 is that the structure axes (optic axis and electric axes) of all portions of a 

 single crj'stal are parallel each to each. However, they are not of the same 

 sense, or handedness. The difference between the two types is as follows: 



In a cr\'stal which is only ELECTRICALLY TWINNED, the cr}^stal is 

 entirely of one handedness (either right or left), but one portion is of OP- 

 POSITE ELECTRICAL SENSE to another portion, i.e., the electric axes 

 are of opposite sense. 



In a cr>'stal which is only OPTICALLY TWINNED, one portion of the 

 crA'Stal is of OPPOSITE HANDEDNESS, and electrical sense, to another 

 portion. This twinning (but not electrical) is detectable by optical means 

 (polarized light) and is named optical twinning for this reason. 



The extent of twinning that may be present in commercial cr}-stals is seen 

 in Fig. 5.1, which shows both electrical and optical twinning boundaries at 

 the top surface of some Z-cut (basal) sections of quartz (which were cut up 

 for the manufacture of quartz oscillators). Though the cr^'stals are seldom 

 entirely free of twinning, they do not on the average run as badly twinned 

 as here shown. These views, taken by means to be described, correspond to 

 what one sees when examining an etched quartz surface by reflection from a 

 strong light. 



Since untwinned finished plates must be cut entirely from one twin or 

 another (not across a boundary), and since the proper sense of angular orien- 

 tation of the plate is opposite for two adjacent electrical twins, the economic 

 utilization of twinned quartz is a difficult problem.^ It involves cutting the 



' Etching is also used on finished plates for removing grinding debris, and for frequency 

 adjustment. 



■* As herein used, a tuin is one of the homogeneous, untwinned portions of a twinned 

 crystal. 



