Electrodiffusion (sweeping) of ions in quartz-a review

Sweeping, a high-temperature process that selectively exchanges monovalent ions in quartz, is reviewed. Sweeping is used commercially to replace interstitial alkalis with hydrogen. Electrodiffusion improves the radiation hardness of quartz oscillator crystals and lowers the etch channel density. Sweeping affects the substitutional aluminum with its associated alkali and the extended dislocation networks with their precipitated impurities. At high temperatures the interstitials are freed and swept down the open Z-axis channels by an applied electric field. These interstitials can then be swept out at the negative electrode, provided replacement ions, usually hydrogen, are brought in at the positive electrode. Thus, the alkali associated with the aluminum is replaced by a hydrogen, and the precipitates in the dislocation networks are modified so that they are less reactive to the etchants. With the field applied at a room temperature, a peak or plateau in the sample current is observed during warm-up near 250-300 degrees C, followed by a decay at the 500 degrees C sweeping temperature. When the current becomes steady, the sweeping is thought to be complete. While infrared and high-temperature resistance measurements are useful, EPR techniques seem to provide the most reliable test for complete ion exchange at the aluminum site.