Improving manufacturability of FBAR filters on 200mm wafers

Abstract

FBAR (thin film bulk acoustic resonator) filters are widely used in making filters for wireless applications [1]. Until recently, most of the FBAR filters were manufactured on 150mm wafers. In the last couple of years, manufacturing of the FBAR devices started moving onto 200mm wafers. Main reason to move from 150mm wafers to 200mm wafer is that area increases by a factor of about 1.8X. Unfortunately, if the edge exclusion has to be increased in order to avoid bad stress or thickness non-uniformity near the edge of the wafer, it reduces the advantages of the increased area. For example, if the edge exclusion is increased from 3mm to 10mm, it causes 14% yield reduction. This problem has inspired the investigation in the paper. One of the critical parameters for FBAR manufacture is AlN film stress, both, average and across a wafer. If stress varies too much, membranes can develop cracks or peel off. In BAW applications, that don't require stress control for structural reason, it is desirable to keep stress uniform across wafer in order to maintain tight distribution of coupling coefficient. Another critical parameter is film thickness. AlN (aluminum nitride) thickness control of +/-0.2% wafer-to-wafer and across wafer are important in order to obtain high yielding wafers. In this paper we will propose a way to obtain both stress control and film thickness uniformity. Independent stress control is obtained by carefully designing sputtering magnetron with variable magnetic field. Thickness uniformity is independently controlled by ion mill trimming module. By adjusting magnetic field and the length of the plasma discharge (positive plasma column), stress uniformity across 200mm wafer was maintained at less than +/-75MPa. Thickness control on 200mm wafers with only 3mm edge exclusion was demonstrated at <;0.2%.