ZnO Etching and Microtunnel Fabrication for High-Reliability MEMS Acoustic Sensor

This paper describes a technique for uniform step coverage of aluminum metal (Al) on ZnO film in the fabrication of MEMS acoustic sensor. The MEMS acoustic sensors were fabricated by etching ZnO layer in three different etchants: HCl, NH₄Cl with electrolytically added copper ions, and NH₄OH with electrolytically added copper ions. For the first time, a technique is reported, which uses aqueous NH4OH solution with electrolytically added copper ions for etching of ZnO layer. For reliable operation of the device, the electrical testing of Al step coverage on ZnO layer was performed. The maximum currents that can be drawn across Al-deposited ZnO edge etched by HCl, Cu-added NH₄Cl, and Cu-added NH₄OH were 40 mA, 2.5 A, and 3.0 A respectively, without any damage to the structures. The investigations show that uniform Al step coverage on ZnO layer is obtained in case of NH₄OH with electrolytically added copper ions. During fabrication of the device, a novel technique for building a microtunnel for pressure compensation was also developed. This microtunnel is used to compensate the pressure applied on the silicon diaphragm by connecting the cavity to the atmosphere. To realize the smooth inlet of microtunnel in the cavity, photoresist SU8 was used for patterning the cavity after microtunnel etching. The developed technique for microtunnel fabrication reduces the process complexity, providing improved yield of the device. The packaged device performed satisfactorily in the sound pressure level (SPL) of 120-160 dB over a wide frequency range of 30-8000 Hz. The maximum sensitivity of the sensor was measured as 380 μV/Pa.