Selective etching of AI/AIN structures for metallization of surface acoustic wave devices

Metallization is a critical step in the fabrication of high frequency AlN thin film based surface acoustic wave devices. Both state-of-the-art lithography as well as high selectivity and anisotropy during etching of Al with respect to AlN are required for low loss and high performance devices. In this work, the etch rates of reactively sputtered AlN, sputtered Al, thermal SiO2, and Shipley 1813 photoresist as well as the selectivity among Al/AlN, Al/SiO2, and resist/Al have been systematically studied during inductively coupled plasma (ICP) etching. Emphasis is focused on obtaining high Al etch rates, while at the same time keeping the etch rate of AlN and that of the resist sufficiently low, High anisotropy is obtained by passivating the sidewalls by the addition of oxygen. The recipe developed is based on a modified Al etch using a mixture of BCl3, Cl-2, O-2. The parameters varied were gas composition, and substrate bias. Generally it is found that the Al etch rate exhibits a maximum with the O-2 flow, while the AlN etch rate decreases monotonically. Substrate bias is found also to be an important parameter with respect to both etch rate and selectivity. At optimized conditions with respect to process selectivity (500 W ICP power, 83 V bias, 50 sccm BCl3, 25 sccm Cl-2, 10 sccm O-2, pressure 10 mTorr) the Al etch rate is 1000 nm/min with a selectivity of 1000 toward AlN and 9 toward the resist. The same recipe, slightly modified, has also shown similar Al etch rates when etching Al over SiO2 with a selectivity of up to 200. High anisotropy of the Al etch rate is observed with increasing O-2 flow. The former has been determined from cross-sectional scanning electron microscope observations. (C) 2002 American Vacuum Society.