The rapid growth of mobile devices, automotives and IoT market requires a strong demand for various acoustic filters at different frequencies. While high end communication market is shifting from 4G to 5G, demand for lower frequency SAW filter is still increasing with increased communication through Wi-Fi, LTE, GSM, Bluetooth, etc. Due to narrow bandwidth requirements, process control for each filter’s manufacturing becomes tighter than any other semiconductor process. To meet the very tight requirement of acoustic filter process control, manufacturers need metrology tools to meet the stringent requirements for repeatability and accuracy. Picosecond Ultrasonic (PULSE™) technology is a proven metrology for the measurement of metal film thickness. Capability for measuring sound velocity of transparent and semi-transparent films is also well established. In an earlier paper, we described how the technology was adopted for characterizing single crystal piezoelectric materials.
In this paper, we report how we can measure metal film thickness of IDT (Inter-digital transducer) and sound velocity of silicon dioxide (SiO2) that is surrounding the IDT on product wafer of TC-SAW (Temperature-compensated surface acoustic wave) filter devices. Then we will show that TC-SAW filter frequency is affected by both IDT thickness and sound velocity of SiO2 that is surrounding IDT. This will demonstrate that accurate and repeatable metrology for IDT thickness and SiO2 sound velocity is inevitable for the frequency control of TC-SAW devices and Picosecond ultrasonic technology (PULSE™) meets the requirement of TC-SAW process control.