Improving coupling coefficient distribution on BAW filters manufactured on 200mm wafers

• Published in: 2017 Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS) pp. 542 - 546
• Main Authors: Mishin, Sergey, Oshmyansky, Yury
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.07.2017
• Subjects: AlN; Aluminum nitride; coupling coefficient; Couplings; film stress; III-V semiconductor materials; Magnetic fields; Magnetomechanical effects; Process control; Stress; stress control
• ISSN: 2327-1949
• DOI: 10.1109/FCS.2017.8088952

In the past, most BAW and FBAR filters were produced on 150mm wafers. Typical yield on 150mm wafers was > 90% [1], [2]. In the last few years, most filter manufacturers started migrating to 200mm wafers in order to reduce the cost of manufacturing. Unfortunately, due to variation of coupling coefficient (kt^2) [3], [4], [5], yields on 200mm wafers were significantly lower than on 150mm wafers. Stress in the piezoelectric material is the largest variable that changes coupling coefficient [6]. Figure 1. Shows how coupling coefficient varies with stress for devices that use piezoelectric aluminum nitride (AlN) and molybdenum (Mo) electrodes. Depending on the exact processing involved in making a filter, the same AlN deposition process will produce a completely different stress/kt^2 variation across the wafer. Figure 2 illustrates a difference coupling coefficient variation across wafer, using tungsten (W) vs. Mo electrode material while leaving everything else the same. In order to be able to obtain the best coupling coefficient with different processing on 200mm wafer, it is critical to have independent control of film stress across wafer. In this paper, we will demonstrate a method of controlling coupling coefficient across wafer within less than +/-1% (total range) of the target.