The behavior of {f111g}-textured Pb(Zr(0.53Ti0.47O3) (PZT) deposited by the sol-gel technique in thin film bulk acoustic resonators (TFBAR's) was investigated at a resonance frequency of about 1 GHz. The resonators were fabricated on Si wafers using deep silicon etching to create a membrane structure and using platinum as top and bottom electrodes. The best response of the resonators was observed at a bias voltage of -15 kV/cm with values of about 10% for the coupling constant and about 50 for the quality factor. This voltage corresponds to optimal values of piezoelectric constant d33 and dielectric constant measured as a function of the electric field. The influence of a bias voltage on the resonance frequency, antiresonance frequency, and coupling constant were observed. Both the resonance and antiresonance frequency show a hysteretic change with applied bias. This effect can be used to shift the whole band of a filter by applying a voltage. The TFBAR structure also allowed us to extract values for materials parameters of the PZT film. Dielectric, piezoelectric, and elastic properties of the f111g-textured PZT film are reported and compared to direct measurements and to literature values.
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...
Pb(Zr0.53, Ti0.47)O3 (PZT) thin films are potentially interesting as piezoelectric layer in bulk acoustic wave (BAW) resonators. We investigated properties and performance of {111} and {100} textured, dense films deposited by sol-gel techniques in the frequency range of 1 to 2 GHz. The resonators were fabricated on Si wafers using deep silicon etching to create a membrane structure and using platinum as top and bottom electrodes. The best response of the resonators was observed at a bias voltage of −15kV/cm with values of around 10% for the coupling constant and around 50 for the quality factor. This voltage corresponds to the maximal value of the piezoelectric constant d33 and minimal value of the dielectric permittivity measured as a function of the electric field. Resonance and antiresonance frequencies were strongly influenced by a bias voltage, showing a hysteretic behaviour as expected for ferroelectrics. Both of these frequencies shifted in the same direction. As a consequence, the dc voltage can be potentially used to shift the whole band of a filter. In unipolar operation, the coupling constant could be varied from 6 to 10 %. Materials parameters were extracted from the admittance as a function of frequency. Dielectric, piezoelectric and elastic properties of textured PZT films are reported and compared to direct (low frequency) measurements and to literature values. It was found that PZT thin films have lower stiffness than the one of PZT bulk ceramics and it was observed that {111}-textured films are stiffer than {100}-textured films.
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