Wet etching of platinum (Pt) electrodes for piezoelectric transducers using a thick photoresist mask

Platinum (Pt) is widely used in MEMS applications due to its inert nature and high temperature stability. In general, Pt is patterned using dry etching methods which require expensive machinery. In this study, we propose wet etching of Pt electrodes of piezoelectric transducers in hot Aqua Regia at 60 °C using a thick photoresist as a masking material. We showed that this method eliminates the need for metal hard mask and subsequent metal stripping, which may not be compatible with other structures on the process wafer. A stack of Si/Ti/Pt was used as a test sample to verify the effectiveness and chemical stability of AZ® 9260 and SPR™ 220-7 type photoresists in hot Aqua Regia. The optimized process was then successfully applied on a wafer with a pre-patterned pulsed laser deposited lead zirconate titanate (PLD-PZT) using SPR™ 220-7 photoresist. The suitability of the etching process was verified using optical imaging and SEM-EDS analysis. An etch resolution of 3.5 μm was achieved for 100 nm thick Pt thin films after 15 min immersion in hot Aqua Regia at 60 °C without any plasma cleaning. Using descum process with Ar plasma beforehand decreased the etching time down to 3:45 min and improved the minimum feature size down to 1 μm.