JISIS – Diamond-based surface acoustic wave devices: a reverse fabrication design
Key words: surface acoustic filters, vapor chemical vapor deposition, diamond thin film, ID tag
Surface acoustic wave (SAW) devices are critical components for wireless communication systems. In fact, the increasing demand for large volume data transmission requires an increasing number of devices operating at high frequencies. SAW filters can be used as reflective delay lines, resonators, impedance sensors, fixed-code ID tags and a wide variety of sensors (temperature, pressure, torque, current and even chemical sensors). The Telecommunications Institute (IT) has been pursuing research on the use of SAW devices not just for object identification (RFID) but also for sensing applications. In fact, the time delay of the acoustic wave may carry information about many physical magnitudes, such as mass, density, strain, stress and temperature. With proper signal-processing hardware, this could provide for the replacement of many transducers that are at present expensive or hard to use, such as strain gages. These devices will be passive, i.e., they won’t require any internal source of electrical power, and can be remotely interrogated by means of an RF beam at the appropriate frequency. The operating frequency of a SAW device is simply determined by the ratio between the phase velocity of the acoustic wave and the spatial period of the inter-digital transducers (IDT). The increase in the operating frequency implies an increase in the spatial resolution of the lithographic techniques and a constant demand for materials with high phase velocities. Thanks to its increased resonance frequency, high values of the electromechanical coupling coefficient and a high mass sensitivity, chemical vapour deposited (CVD) diamond is one of the most attractive materials for SAW devices . In addition, diamond films can be an effective protective layer, since CVD diamond will survive in harsh radiation environments . As the SAW operation requires an electromechanical conversion, a piezoelectric layer such as ZnO or AlN is required and the metallic IDT are deposited on the diamond surface. Different SAW devices using diamond films have already been reported.
A major disadvantage of the use of CVD diamond films is the large roughness of polycrystalline diamond (PCD). In fact, the surface roughness must be as low as possible in order to decrease the wave scattering and propagation losses. Different approaches have been used: the mechanical polishing of the PCD growth surface deposition of nanocrystalline diamond (NCD)  or the use of the exposed nucleation surface. In any of these cases, the piezo material has to be deposited directly on the diamond surface. This step is not easy, and a lot of effort has been devoted in order to deposit, for instance, ZnO, AlN or SiO2 on the diamond surface with acceptable quality. If an operating device (i.e., the piezo substrate with the proper IDTs) is coated with a diamond film, an increase in the operating frequency can be expected. In addition, frequency multiplication effects will arise, with possible applications to the design of new electronic devices. The diamond film will also behave as a protective layer.
Even though the deposition of diamond on piezo substrates is not a straightforward task, the researchers at TEMA have developed a new nucleation process (NNP) that allows the deposition of high quality diamond films at lower temperatures on a variety of substrates, including piezomaterials such as AlN, SiO2 and LiNbO3. The nucleation surface arising from this technique is extremely smooth and an intimate contact is assured between the piezo substrate and the diamond film, decreasing thus the wave scattering and the propagation losses.
Test devices with simple IDT structures will have to be fabricated in order to carry out this project, and the appropriate analogue interfaces developed. Once the diamond-coated SAW devices have been fabricated, the characteristics of the substrate / diamond interface will be tested in terms of losses and phase velocity for longitudinal waves, both in diamond and in the substrate (most likely SiO2).
The proposed work will be conducted in the IT-Aveiro and in the TEMA. The research team is unequally well qualified in the areas required to successfully achieve the aims of the proposed research. Doctor Joana Catarina Madaleno is highly experienced in the deposition of diamond on foreign substrates for electronic applications and their characterization. She has recently proposed a new field-effect transistor fabrication technique using the nucleation surface. Work at IT in the area of the use of SAW’s, both for electronic components and for identifiers/sensors has been pursued since 2005. Several types of SAW’s have been designed and most of the results are contained in an M. Sc. Thesis.