Fabrication and Characterization of ZnO Films for Biological Sensor Application of FPW Device

Flexural plate wave (FPW) device has great potential for biological sensor application due to (1) isolation of its electric circuit from the medium being investigated, (2) low acoustic energy loss in liquid medium, and (3) simpler oscillator circuit design. Since FPW device can be fabricated onto silicon-based substrates by micromachining technology, it offers batch processing for economic sensor fabrication. In this study, ZnO was chosen as a piezoelectric material due to non-toxicity, and chemical and thermal stability for biological sensor application. RF magnetron sputtering and chemical solution deposition (CSD) were investigated for achieving strong c-axis orientation of ZnO films required to launch the acoustic wave in the device. Process parameters such as gas ratio, substrate types, and temperature, were varied for sputtering, and heat treatment and substrate types for CSD. Results showed that process parameters have a strong influence on the preferred orientation and microstructure of ZnO films. Uniform and dense microstructures of ZnO films were obtained by both fabrication methods. CSD method showed, however, stronger dependence of the preferred orientation on substrate types while less dependence on the substrates for sputtering due to energetic sputtered species. Mechanism for ZnO thin film growth will be discussed. FPW devices have been successfully integrated onto 4 inch Si-wafer with 22 different interdigitated electrodes. Optimized device demonstrates the capability to detect biological quantity of 446.13 cm²/gram of sensitivity.