Study of HfN as seed layer for next generation of BAW RF filters : synthesis, characterization, and investigation of piezoelectric performance

Abstract: Micro-electro-mechanical systems (MEMS) have become an essential component of a wide range ofelectronic devices over the last decades such as accelerometers, microphones, gas sensors, and filters.During this new millennium, a new radio frequency (RF) technology has been developed to satisfy thetough demands that arose due to the implementation of 5G wireless communication: bulk acoustic wave(BAW) filters.BAW devices use the piezoelectric effect, converting mechanical vibrations to electrical signals, topower wireless devices. BAW filters can operate between 3.5 GHz and 6 GHz, therefore, within therange of the new 5G. BAW technology offers lower insertion loss, higher heat dissipation, andperformances at higher power and frequency which increases the data speed considerably.This thesis will be focused on the study of the materials used in BAW devices. A common BAW filteris made from different layers distributed in a stack, from the bottom to the upper part, the BAW filteris composed of a substrate, a transducer layer made of a piezoelectric layer in between of two electrodes,and intermediate layers that can enhance the addition of the deposited layers on top called buffer layers,or the crystal quality of the films on top called seed layers.The main characteristic that a buffer layer must possess is an intermediate lattice parameter betweenthat of the substrate and the top layer. When these two layers present a high lattice mismatch, theinterface quality is rather poor. By using a buffer layer, and therefore, by adding two different interfaces,the crystal quality is improved by decreasing the internal stress and the crystal distortion. Buffer layermaterials depend on the type of materials that will be in contact with them.A seed layer is usually used to improve the crystal quality of a layer that requires extreme sputteringparameters to be used to be deposited possessing a high crystal quality and a preferred orientation. Seedlayers used in BAW devices, whose piezoelectric layer is made of AlScN or AlN, are usually made ofhighly c-axis oriented and highly crystalline AlN.The objective of this study is to analyze the deposition of AlN and HfN by means of reactive radiofrequency magnetron sputtering and reactive pulsed-direct current magnetron sputtering, respectively.AlN is largely used as a buffer layer and as a seed layer, however, the new approach of this report is tostudy the sputtering of HfN and compare it as a possible candidate to replace AlN as a seed layer.