Design And Fabrication Of High Frequency Diamond Surface Acoustic Wave (SAW) Filter In Layered Structure

With the research and development of high frequency(3G) communication products, Surface Acoustic Wave (SAW) device based on diamond is attracting interest because diamond has the highest elastic constant of all substrates and the SAW velocity is more than 10, 000 m/s. Although diamond is not piezoelectric, its high acoustic propagation makes it desirable substrate for SAW device when coupled with piezoelectric thin films such as ZnO thin film. SAW characteristics in layered structure of interdigital-transducer (IDT) /ZnO/diamond have been calculated. These calculations predict the velocity and coupling coefficient (K2) value of the SAW to be more than10, 000 m/s and 1% respectively. In this regard, ZnO/diamond layered SAW filters, which have high velocity and high power durability, can be expected for broad applications on future high frequency and low loss communication system as well as meet the increasing demand for large data volume transmission and mobile communication.In this paper, theories of thin film SAW filter in layered structure have been discussed. IDT/ZnO/Diamond structure SAW filter is designed, fabricated and tested. In our experiment, we have also used Si substrate to help explore the techniques.In the design, the section structure and thickness of ZnO films were carefully selected because they determine the filter's working mode, velocity and mechanical-electrical coupling coefficient (K2). Image-impedance connection and portrait coupling IDT structure is adopted in the design, which is proved to be successful to depress the insert loss and improve the stop-band rejection. The piezoelectric ZnO layer is fabricated on diamond and silicon substrates using DC magnetron sputtering, and the sputtering parameters have been extensively investigated here. Substrate heater temperature, sputtering gas pressure, and Ar-to-O2 rate are the most important factors to determine the quality of ZnO thin film. After these parameters have been properly set, grown ZnO layer shows highly oriented, dense and fine-grain, excellent surface flatness and high resistivity. The whole techniques of fabricating SAW filters were discussed in detail. Al IDTs were formed by Electron Beam Photolithography with high accuracy. For the etching of fine Al IDT electrode in high frequency SAW devices, reactive ion etch (RIE) is chosen because the dry process performs fine dimention control of narrow line-width feature with high yield. After RIE, the edges of electrodes were sharp and continuous with width and space of 1μm. The frequency characteristics of the fabricated SAW filters were measured with RF-network analyzer. Diamond SAW filters have rather good performance with centre frequency of 2.5GHz, 2.3% passband, 27 dB insert loss. ZnO/Si structure SAW filter work well too, with operation frequency 790MHz and insert loss 17dB. The result accord with our design objective and are very promising to make the real RF front-end monolithic integration filter.