使用MEMS技术的用于射频无线应用的微型SAW器件

A miniature SAW device is designed and  fabricated at 1 GHz for wireless communication system. A 5  m thin film of ZnO is successfully deposited using RF  sputtering technique on PECVD SiO₂ layer of 1µm on top of  Si wafer under various operating conditions. The c-axis oriented ZnO film exhibit a sharp diffraction peak  corresponding to the (002) reflection at 2θ=34.42. The  fabrication process utilizing the MEMS technology of the SAW  device is described. Simulation of the RF SAW filter is  performed. Measurements and experimental work are  presented for the RF SAW device.

The realization of miniature RF-SAW device on CMOS  substrate for mobile and wireless communication system  has become very important in recent years. Acoustic wave  propagation along the surface of a piezoelectric material  provides a means of implementing a variety of signalprocessing devices at frequencies ranging from several MHz  to a few GHz. The IDT provides the cornerstone of  SAW technology. Its function is to convert electrical energy  into mechanical energy and vice versa, for generating and  detecting the SAW. The central frequency of a conventional  SAW filter is determined by the width of the IDT finger d and the phase velocity which excite the SAW on the  piezoelectric materials. Reducing the width of the IDT  finger or choosing the piezoelectric material with a higher  SAW phase velocity can increase frequency of operation  into the GHz range. The SAW wavelength λ (λ =4d) is given by the ratio υ / fo. Where υ the velocity of  a SAW wave on a piezoelectric substrate depends on the  material and fo is the central frequency. The sensing action of such transducers involves any influences that will alter  the acoustic wave velocity υ and, consequently, the  associated properties of the wave, such as frequency and  time to travel between the sensor and the detector.

In order to achieve good SAW performance, the  piezoelectric films should have a smooth surface morphology, sharp interface, and perfect c-axis texture. It is  also important that the SAW filter reveals high phase  velocity and large electromechanical coupling coefficient as  the thickness of piezoelectric film increases. ZnO has a high  piezoelectric coupling coefficient, which can be used in surface and bulk acoustic wave devices. ZnO films have  been deposited on Si and SiO₂/Si, GaAs, and sapphire  (Al2O₃) substrates. Si and GaAs substrates are of interest  for the integration of SAW devices with the main stream  microelectronics technology. So these types of devices  can be implemented with micro-electro-mechanical systems  (MEMS) compatible with CMOS technology.

In this work a miniature SAW filter of 1 µm input/output  IDTs of 30 pairs is designed and fabricated on 5 µm thin  film of ZnO substrate with a center frequency of 1GHz.  ZnO film is grown on SiO₂/Si substrates by RF sputtering  technique. The growth process is optimized to obtain highly  oriented ZnO film with a smooth surface morphology. The  structural properties of the films are investigated using Xray diffraction. High quality ZnO thin film has been  achieved, which are needed for fabrication of low-loss SAW  filter. Characterization of the RF SAW device as a  transmitter and receiver filter is performed. Measurements  and experimental results are presented for the RF SAW  filter.  

Fig1

The measured results show that the velocity and  electromechanical coupling coefficient are strongly  influenced as the thickness of ZnO is increased. There are  many factors that the central frequency of a conventional  SAW filter is determined. Those are the IDT geometry, and  the velocity of piezoelectric substrate material. Recent  advances in MEMS and NEMS make it possible to reduce  the electrode width to submicron and fabricate a multilayer  piezoelectric structure. Also increase the thickness of the  piezoelectric layer in SAW devices.

This work has received primary funding from the NSF  Directorate for Engineering, Division of Electrical and  Communications Systems. Additional funding and support  has been provided by ERI. We would like to thank Prof. Dr.  Ayman El-Dessouki, president of ERI for supporting this  work in the Central Electrical and Electronics Research  Inst.,CEERI , India. We thank Eng. Sherief Saleh for his  assistance in this work, and Dr. S. Ahmed from CEERI. for  providing the MEMS facilities.