Piezoelectric materials parameters for piezoelectric thin films in GHz applications

Abstract

Piezoelectric thin films have existing and promising new applications in microwave filter technologies. The final performance depends on many parameters, and very specifically on the materials properties of each involved material. In this article, materials and properties for thin-film bulk acoustic wave resonators are discussed on some selected issues: the piezoelectric coefficients and acoustic losses of AlN, the relation of the first one with microstructural parameters, the inclusion of parasitic elements, and the merits of and problems with ferroelectric materials. I . I N T R O D U C T I O NThe wurtzite materials AlN and ZnO are currently the only piezoelectrics that are used in thin-film form for microwave applications in the 1-10 GHz range. They combine good piezoelectric properties with excellent acoustic qualities, and grow most easily in the optimal film orientation for RF filter applications. In addition, they can be synthesized by sputter deposition, a plasma method allowing for low processing temperatures. According to Hickernell [1], efforts to grow ZnO thin films go back to around 1965; good-quality films were obtained in the mid-to late 1970s, mostly by sputtering [2,3], but also by CVD [4]. Besides electro-acoustic applications, optical applications [4] were also investigated. The potential of these films for RF filtering with bulk acoustic waves (BAWs) was soon discovered [5][6][7]. Such devices are composed of several electromechanical resonators, commonly called thin-film bulk acoustic resonators (TFBARs), which are based on standing bulk waves trapped in a film slab, as sketched in Fig. 1, whereby film thickness defines the frequency of resonance. In these early years, ZnO was much more frequently investigated than AlN. Insufficient vacuum in the deposition tools at this time is certainly one of the reasons, because nitrides require much better vacuum conditions than oxides. In addition, the target applications had frequencies in the ultra-high frequency range (television), requiring a film thickness of several micrometers. This is not ideal for a material such as AlN that tends to create immense mechanical stresses, and also exhibits a very high sound velocity requiring almost twice as thick layers as with ZnO. Anyhow, the time was not yet ripe for TFBARs. Surface acoustic wave (SAW) devices based on piezoelectric single crystals such as quartz, LiNbO 3 , and LiTaO 3 were and are much more practical in the frequency range below 1 GHz. SAW filter production was much less demanding, once single-crystal wafers of these materials were available. TFBARs had to wait until the mid-1990s for the first industrial activities [8,9]. Sputter sources and industrial-type vacuum systems were considerably improved in the meantime, MEMS technology was already at a suitable development stage, and then the application was ready as well: mobile communication. Especially the second generation with carrier frequencies around 2 GHz was and still is ideal for TFBARs, because the required thin-film thickness is in...

Low temperature aluminum nitride thin films for sensory applications

“…Such textured epitaxial growth was observed for high temperature AlN as well. 33 Hence, the improvement of the FWHM as a result of thickness increase is related to the self-induced bias e.g. adding to the bombardment of the growing surface promoting a better texture quality.…”

“…adding to the bombardment of the growing surface promoting a better texture quality. 3,23,33 Further investigations focus on the piezoelectric and dielectric properties of the deposited films with respect to the film thickness. Clamped (effective) piezoelectric coefficients as a function of the AlN film thickness are depicted in Figure 7(a).…”

“…These values are well below the previously published data. 2,5,8,33 Since dielectric losses mainly originate from structural defects, such low losses can be attributed to a superior crystal quality of the deposited films. This a very promising result for a wide application range, particularly for ME sensors where the noise voltage density is proportional to tan δ of the piezoelectric layer.…”

Piezoelectric aluminum nitride thin films for microelectromechanical systems

“…X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed pure c -axis orientation with a columnar microstructure ( Figure 9 ), similar to that observed for undoped AlN thin fi lms. 58 The favorable microstructure is maintained as long as the Sc concentration is not too large and the growth temperature is not too high (i.e., not above about 400°C).…”

Foreword

“…Of the piezoelectric papers, Al Ahmad and Plana [1] describe the displacement detection of thin-film AlN using capacitance measurements while Menéndez et al [2] report a design methodology for microwave acoustic filters. The third paper, by Muralt et al [3], highlights the impact of material parameters for GHz applications; monolithic thin-film piezoelectric-on-substrate high-frequency filters are addressed by Abdolvand and Ayazi [4] in the fourth paper. The fifth paper (Ikehashi et al [5]) outlines a lithographical bending control method for piezoelectric actuator fabrication for MEMS applications.…”