Akustik kitle dalga esasına dayanan rezonatörlerin analizi ve tasarımı

Abstract

Bu çalışmada çok katlı kitle dalgası rezonatörleri bilgisayar yardımıyla incelenmiştir. Kitle dalgası rezonatörlerinde rezonans frekansını bir çok parametresi yanında kalınlığı belirler. Rezonans frekansının yüksek olması için kristal malzeme çok ince seçilmelidir. Bunun için piezoelekt rik kristal malzeme belirli yöntemlerle çok ince olarak bir taban üzerine yerleştirilir. Böylelikle akustik dalga taban malzemesinden dönüştürücüye geri yansır ve duran dalga oluşur. Dönüştürücünün çok ince seçilmesiyle rezonans frekansı GHz'ler mertebesine kadar arttırılabilir. Fakat dönüştürücünün yerleştirildiği taban malzemesinin etkisiyle enerji kaybı artar. Bu etkiyi minimize etmek amacıyla çok katlı kitle dalgası rezonatör modeli geliştirilmiştir. Taban malzemesiyle dönüştürücü arasına tabakalar yerleştirilerek iletilen dalganın kendi üzerine birçok kez yansıtılması sağlannmışt ı r. Böylece tek katlı kitle dalgası rezonatöründe dönüştürücünün yerleştirildiği taban malzemesinin etkisi minimize edilmiştir. Dönüştürücü ile taban malzemesi arasına yerleştirilen tabakaların sayısına ve karakteristik empedansları nı n oranına bağlı olarak yansıma katsayısının frekansla değişimi incelenmiştir. Bu değişim gözönünde bulundurularak tam yansımann olduğu tabaka sayısı ve empedans oranlarına sahip yapı oluşturulmuştur. Dönüştürücülerin analizinde kullanmak üzere söz konusu devre modelini esas alan analiz programı gaiıştirılmişti r. Analız yöntemleri filtre analizi için de uygundur.

Acoustic bulk wave transducers which are widely used in high sensitive microscope, optic and elect rooptic can be used as a resonator in microwave frequencies, if they are properly designed. Besides many parameters of piezoelectric crystal, the thickness of transducer determines resonance frequency in the acoustic bulk wave resonators. Transducer should be thinner to realize a resonator in the microwave frequencies. For this purpose, piezoelectric material used as a transducer is deposited on the substrate by sputtering technique. Acoustic wave is reflected back to transducer from the substrate so that acoustic standing wave occours. But energy loss increases because of the effect of substrate that transducer placed on. To minimize this effect, the design of multilayer acoustic bulk wave resonators have been developed. During the recent years, work has been progressing toward the development UHF acoustic resonators that can be fabricated and utilized as stable, high Q, monolithic microwave integrated circuit elements. The aim of this effort is three fold: 1- Circuit simplification and performance improvement associated with stable signal generation and narrow band signal sorting directly at UHF 2- Resonator volume reduction 3- Fabrication in % 100 monolithic form (VI )Krishnoswany, (Josenbaum, Horwitz, Vale and Moore review recent development in miniature monolithic filters based on film bulk, acoustic resonators (FBAR). Thin Film Acoustic Resonators formed by sputter «.lı.-pı.j;* i l 1.1 >n «.it pi.t'soolûc 1. 1 lc film;; mcli a1: ZnO or Al N on substrates offer unique possibility for complete monoiithic integration of high Q circuit elements with semiconductor devices leading to small size and low cost. In their work, they have been developed FBAR filters that, have 30MHz of bandwidth at 1GHz and about IdB of insertion loss. In this device the region of substrate where transducer placed on is etched. Therefore the effect oi substrate is minimized. This device is shown in Figure 1. JH Piezoelectric Transducer Substrate Figure 1. The model of FBAR For very high frquency operation in which a high Q is required, a low loss nonpiezoelect ric crystal is attached to the piezoelectric layer as shown Figure 2. The structure utilizes standing waves, which are always present in a low-loss substrate. The function of the transducer is to couple a small amount of energy in to high Q cavity without introducing appreciable acoustic loss, and the transducer is tVII>usually an extremely small part ot" the total acoustic path lenght. The device Q is thus nearly equal to the metarial Q of the acoustic cavity. Furthermore because the acoustic cavity is v&ry large compared with the acoustic wavelenght, the device operates at a very high harmonic or" its fundamental. Such devices are called high overtone bulk acoustic resonators. Driscall, Jelen, Bally have been studied about HEAR (High overtone Bulk Acoustic Resonators). Their worfc has indicated the results of phase noise measurements i or HBAR& operating at. 640MHz with insertion losses of lû-15dB and unmatched Q* greater than 1101c. Piezoelectri c Transducer Figure 2. The model of HBAR In design of the resonators, it is neccassary that transferred acoustic wave is to be reflected on itself so many times. For this many layers are placed bet-ween transducer and substrate. This structure is shown in Figure 3. Therefore the effect of substrate have neen minimi2ed. For various numbers of layers and the ratio of characteristic impedances of layers that placed between transducer and substrate, reflection coefficients are investigated. The exact reflection is provided by changing the numbers of layers and the ratio ot characteristic impedances ot the layers. tVIII)ı-o- V -o- -> I M x..-.< V ///. //??'/,'/// '/// Figure 3. The model of miltilayer acoustik bulk ware resanators The design of circuit belong to acoustic bulk wave resonators has been developed by Butter Worth-Van Dyke. The resonators have been represented by a constant "clamped" capacitance in parallel with an acoustic or "motional" arm, which can be inductive or çapacı tive. An equivalent circuit must also reflect the existence of series and parallel resonances. A resistor which provides for energy absorption in device, us added to the motional arm. The ratio of the clamped capacitance to the motional capacitance is called the C ratio (Cr) ıs a FOM (Figure of Merit) of resonator. A resonator with small Cr is inductive for a large frequency band. This feature facilitates the deign of widebannd filters and allows oscillators to be pulled by external tuning elements over the induktive frequency band.