Fabrication of high frequency SAW devices using tri-layer lift-off photolithography

A tri-layer lift-off photolithography technique is presented to enable the fabrication of surface acoustic wave (SAW) devices with near GHz fundamental operating frequencies. SAW devices require high-quality micron-scale features for high frequency operation, a requirement that challenges traditional UV photolithography techniques. The presented process is a modified version of a previously reported tri-layer photolithography process intended for Si and SiO2 substrates which allows for compatibility with materials that are piezoelectric and pyroelectric, often used as the substrate in SAW devices. The process uses a lithographic tri-layer consisting of layers of lift-off resist (LOR) on the bottom, back anti-reflection coating (BARC) in the middle, and photoresist (PR) on top. The addition of the BARC layer prevents back reflection of exposure light, improves the structural integrity of the lithographic stack, and decouples the PR and LOR development, improving resolution by a factor of two over traditional lift-off photolithography techniques. We demonstrate the fabrication of a SAW device with an interdigital transducer (IDT) pitch of 4 μm (minimum feature size of 1 μm) on 128° Y-X cut lithium niobate. The device produces a dip in the measured S11 spectrum at 994.5 MHz, corresponding to the fundamental operating frequency of the device. The fundamental operating frequency of the SAW device is also determined theoretically via numerical modelling and found to be 995.5 MHz. The process thus enables the reliable fabrication of high frequency SAW devices that exhibit excellent agreement with numerical results.