Development of SMD 32.768 kHz tuning fork-type crystals using photolithography and selective etching process - Part I: Selective etching of an array of quartz tuning fork resonators

Negative photoresist photolithography was found superior to previously used positive photoresist photolithography to etch array of quartz tuning forks for use in Qualcomm(R) mobile station modem (MSM), 3000(TM) central processing unit (CPU), chips of code division multiple access (CDMA), personal communication system (PCS), and global system for mobile communication (GSM) units. Optimum processing condition was devised for reproducible precision etching of Z-cut quartz wafer into array of tuning forks. Tuning fork pattern was transferred via ordinary photolithographical chromium/quartz glass template using a standard single-sided aligner and subsequent negative photoresist development. A tightly adhering and pinhole-free 600/2000 Angstrom chromium/gold mask is coated over the developed photoresist pattern which was subsequently stripped in acetone. This procedure was repeated on the backside of the wafer. With protective metallization area of tuning fork geometry thus formed, etching through quartz wafer was done at 80 degreesC in a +/- 1.5 degreesC controlled bath containing concentrated solution of ammonium bifluoride to remove unwanted area of the quartz wafer. The quality of quartz wafer surface finish after quartz etching depended primarily on the surface finish of quartz wafer prior to etching and the quality of quartz crystals used. At 80 degreesC, selective etching of 100 mum quartz wafer could be effected within 90 min. Reproducible precision selective etching method has thus been established and enables mass production of miniature tuning fork resonators with electrode patterns on them photolithographic ally.