Precision Grinding of Ultra-Thin Quartz Wafers

For bulk acoustic wave quartz resonators, the central resonant frequency is inversely proportional to the wafer thickness. The tolerance of the resonant frequency is directly proportional to the total thickness variation of the quartz wafer. To increase the operating frequency while preserving a high tolerance on frequency, thinner quartz wafers with better thickness tolerances are needed. This paper describes the design and implementation of a precision grinding apparatus capable of producing ultra-thin quartz wafers to better thickness tolerances than previously achieved. A distributed-stress fixturing method that permits machining of ultra-thin, brittle substrates is described. The machine’s precision has been achieved through a high stiffness structural loop and real-time position feedback control. Optical interferometry is used in a new technique to measure thickness variation over the entire wafer. This research will enable production of quartz crystal oscillators with higher frequencies and better quality for the resonator industry.