ETCH TECHNIQUE 41 



(fragments of quartz loosened by grinding, and grinding refuse embedded in 

 microscopic surface irregularities), and to remove predetermined small 

 amounts of the surface for frequency adjustment. It is common for these 

 purposes to use weaker etching solutions, since very small amounts of 

 quartz are to be removed. With hydrofluoric acid, weak solutions (less 

 than 20% HF) have an advantage in that their concentrations are little 

 reduced by exposure to the air. In fact with very weak solution the con- 

 centration may increase slightly by exposure, and thus partly compensate 

 for the HF lost by reaction. Weak ammonium bifluoride solutions may also 

 be used, provided no deposit forming material is added. 



5.7 The Effect of Twinning in the Finished Plate 



While it is commonly considered that electrical and optical twinning are 

 not allowable in a finished oscillator plate, it cannot be unconditionally 

 stated that small amounts of twinning will too seriously affect the properties 

 of all types of oscillator plates. The allowance of even small amounts of 

 twinning in the finished plate would save quartz and simplify the processing 

 procedures. Hence, consideration must be given to the factors which would 

 affect the utilization of twinned material, and the effect of twinning on the 

 operating characteristics of the finished plate. Consideration will first be 

 given to the nature and distribution of electrical and optical twins- in the 

 raw quartz. 



The analysis of twinning in raw quartz has been carried out by the ex- 

 amination of numerous, etched Z-cut surfaces. By the method to be de- 

 scribed it is possible to detect the handedness, and the axial orientation and 

 sense, of each homogeneous portion, twin, appearing at the etched surface 

 of a twinned specimen. Both electrical and optical twins may be analyzed 

 by this method. It might be added that electrical twinning boundaries 

 and orientation are only detectable at an etched surface, and that while 

 interior optical twinning may be detected by polarized light, its exact 

 analysis is only possible at an etched surface. 



Figure 5.22 E shows the optical arrangement used for examining twinning 

 in etched Z-cut sections. The sections (prepared with a fine grind and weak 

 etch) were mounted on a turntable, illuminated from an elevation of about 

 30° to the horizontal etched surface by a spot lamp, and viewed (or photo- 

 graphed) from vertically above the section according to principles of Section 

 5.41). With the section properly aligned on the table (with the predeter- 

 mined electric axes parallel to the table-lines joining diametrically opposite 

 fiducial marks), the table was successively turned into positions about 12° 

 to the right or left of the plane of illumination and reflection (as indicated 

 by the R and L marks and the index pointer). Four of these positions of 



^^ See footnote 4. 



