Fabrication of SMD 32.768 kHz tuning fork-type crystals: photolithography and selective etching of an array of quartz tuning fork resonators

摘要

Negative photoresist photolithography was used to etch array of quartz tuning forks for use in Qualcomm mobile station modem (MSM)-3000 central processing unit (CPU) chips of code division multiple access (CDMA), personal communication system (PCS), and global system for mobile communication (GSM) units. It was found superior to positive photoresist photolithography. Quartz tuning fork blanks with optimum shock-resistant characteristics were designed using finite element method (FEM) and 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. Tightly adhering and pinhole-free 600/2000 A 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 ℃ in a ±1.5 ℃ controlled bath containing concentrated solution of ammonium bifluoride to remove unwanted area of the quartz wafer. Surface finish of quartz wafer prior to etching and the quality of quartz crystals used primarily affected the quality of quartz wafer surface finish after quartz etching. At 80 ℃, selective etching of 100 μm 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 photolithographically.