Within the development of 5th generation wireless systems (5G) the quest for high performance duplexers is driving the development of the latest FBAR devices with resonant frequencies of several GHz. At this range, relatively small in-wafer deviations of the membrane bow can lead to substantial frequency shifts as well as significant variations of the coupling coefficient. For this reason, the strict requirements in terms of stress uniformity are no longer confined to the piezoelectric layer, but are becoming more and more important also for the electrodes.
The FBAR electrode material has to show a good balance between low specific resistivity and high acoustic impedance in order to minimise the resistive losses and to maximise the fraction of mechanical energy confined in the piezoelectric layer. The large majority of designs therefore employ Molybdenum (Mo) but recently Ruthenium (Ru) is also gaining more and more popularity.
We have therefore concentrated our efforts on bringing our Mo and Ru process solutions towards the same outstanding stress control and uniformity levels as we achieve for Al1-xScxN. Our accumulated know-how and experience from developing the piezo-layers themselves represented a valuable base on which we could further design specific process kits for the deposition of Mo and Ru with enhanced stress uniformities on our CLUSTERLINE® 200 II.