All signal processing requires filters that evaluate signals and remove undesirable frequencies while preserving desirable frequencies. Modern smartphones are required to filter, transmit, and receive paths for 2G, 3G, and 4G in up to 15 bands, as well as support Bluetooth, Wi-Fi, and other wireless communications. Phones such as these could require up to 40+ filters. The revolution of communication technology is driving a dramatic increase in the number of radio frequency (RF) bands that smartphones and other mobile devices must support which significantly increases the number of RF filters. Not surprisingly, with the move to higher frequencies and 5G, the complexity of the devices is expected to increase as well as the performance requirements. At these higher frequencies, Surface Acoustic Wave (SAW) filters require smaller width and pitch of the interdigital transducers, which limits their performance. Bulk Acoustic Wave (BAW) filters is the primary technology employed above 2.5GHz.
Such advances in filter technologies will place stringent demands on manufacturing which in turn will require very accurate metrology techniques.
The thickness for the full stack can shift the center frequency and affect the device performance, and piezoelectric layer thickness control is the key for SAW and BAW devices. The frequency accuracy (3σ) of 0.1% requires film thickness control within the same accuracy or better. Thin film deposition systems with wafer uniformity of (3σ <2%) cannot meet these standards. To overcome this limitation, semiconductor equipment manufacturers have developed a trimming process. Monitor wafer thickness measurements are helpful for characterizing deposition chambers and process qualification but doesn’t help with device-level process control. RF filter manufacturers require the ability to not only measure the thickness but would like to be able to adjust the thickness via a trimming process as thickness is directly correlated to the filter characteristics. It becomes important to accurately measure thickness on multiple sites on production wafers. Typically, the measured thickness is forwarded to a trimming tool to adjust the thickness profile across the wafer and improve the thickness uniformity to enhance device performance and yield.
Another parameter that is also helpful for process control is the ability to monitor the acoustic velocity. Velocity variations can also shift center frequency and affect RF device performance.
We have previously demonstrated the application of the Picosecond ultrasonic technology (PULSE™) for characterizing the inter-digital transducer (IDT) thickness and sound velocity of SiO2. IDT thickness needs to be controlled at the Angstrom level to achieve the desired filter frequency in the final product. The excellent repeatability (3 sigma < 0.15Å) and accuracy of the technique enabled measurements on device wafers and met the tight process control requirements.