C-Axis Oriented ZnO Film by RF Sputtering and its Integration with MEMS Processing
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1052-DD03-11
C-Axis Oriented ZnO Film by RF Sputtering and its Integration with MEMS Processing Sudhir Chandra, and Ravindra Singh Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India ABSTRACT In the present work, we report a new fabrication process to integrate the “c-axis oriented” ZnO films with bulk-micromachined silicon diaphragms. ZnO films are very sensitive to the chemicals used in the micro-electro-mechanical systems (MEMS) fabrication process which include acids, bases and etchants of different material layers (e.g. SiO2, chromium, gold etc.). A Si3N4 layer is incorporated to protect the ZnO film from the etchants of chromium and gold used for patterning the electrodes. A mechanical jig is used for protecting the front side (ZnO film side) of the wafer from ethylenediamine pyrocatechol water (EPW) during the anisotropic etching of silicon. The resistivity measurement performed on the ZnO film integrated with micro-diaphragm shows the reliability of the fabrication process proposed in this work. INTRODUCTION During last several years, an extensive research has been carried out on zinc oxide (ZnO) thin films because of their applications in surface acoustic wave (SAW) devices, bulk acoustic wave (BAW) resonators, optical waveguides, transparent conducting coatings, light emitting diodes (LED), photodetectors and electroluminescence devices [1-5]. These films can also be used in MEMS as a sensing material because of their piezoelectric properties [6]. More recently, piezoelectric based MEMS have shown great potential for bio-sensor applications [7]. For applications of ZnO films based on its piezoelectric properties, it is a requirement that the films should be “c-axis oriented” and have high resistivity. Furthermore, ZnO films deposition at comparatively low temperature is of great interest for realization of MEMS in post-CMOS processing steps. Among the various deposition techniques, sputtering is the preferred method as oriented and uniform ZnO films can be obtained even at relatively low substrate temperatures. The integration of ZnO films with MEMS processing is a very challenging task because these films are very sensitive to most of the chemicals used in MEMS fabrication process such as H2SO4, hydrofluoric acid (HF), buffered HF (BHF), silicon anisotropic etchants (e.g. KOH, EPW), metal (e.g. chromium, gold, aluminum) etchants etc. [6, 8-9]. The subsequent fabrication processes after ZnO deposition could degrade or damage ZnO film if it is not protected properly. The present work focuses on the techniques to protect the ZnO film against degradation in subsequent micromachining processes. A new fabrication process has been demonstrated to integrate the ZnO films with bulk-micromachined diaphragms in which ZnO film is safely protected from the different chemicals during the etching processes.
EXPERIMENTAL DETAILS Preparation of c-axis oriented ZnO films The c-axis oriented ZnO films have been prepared on Pt (150 nm) / Ti (20 nm) / SiO2 (
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