Towards Flexible Sensors and Actuators : Application Aspect of Piezoelectronic Thin Film
Man’s desire to replicate/mimic the nature’s creation provided an impetus and inspiration to the rapid advancements and progress made in the sensors and actuators technology. A normal human being has five basic sensory organs, which helps and guides him in performing the routine tasks. This underlines the importance of basic sensory organs in a human life. In a similar fashion, sensors and actuators are of paramount importance for most of the science and engineering applications. The aim of the present thesis work is to explore the application of piezoelectric ZnO thin films deposited on a flexible substrate for the development of sensors and actuators. Detailed study was performed on the suitability of three different flexible substrates namely Phynox, Kapton and Mylar. However, Phynox alloy substrate was found to be a suitable substrate material for the above mentioned applications. Sputtering technique was chosen for the deposition of ZnO thin films on to Phynox substrate. The necessary process parameters were optimized to achieve good quality piezoelectric thin films. In the present work, sensors have been developed by utilizing the direct piezoelectric effect of ZnO thin films deposited on Phynox alloy substrate. These includes a flow sensor for gas flow rate measurement, impact sensor for non-destructive material discrimination study and a Thin Film Sensor Array (TFSA) for monitoring the impact events. On the other hand, using the converse piezoelectric effect of ZnO thin films, actuators have also been developed. These include a thin film micro actuator and a Thin Film Micro Vibrator (TFMV) for vibration testing of micro devices. The thesis is divided into following seven chapters. Chapter 1: This chapter gives a general introduction about sensors and actuators, piezoelectric thin films, flexible substrates, thin film deposition processes and characterization techniques. A brief literature survey of different applications of piezoelectric thin films deposited on various flexible substrates in device development is presented. Chapter 2: A novel flexible metal alloy (Phynox) and its properties along with its applications are discussed in this chapter. ZnO thin films were deposited on Phynox substrate by Rf reactive magnetron sputtering technique. The sputtering process parameters such as: Ar:O2 gas ratio, substrate temperature and RF power were optimized for the deposition of good quality piezoelectric ZnO thin films. The deposited ZnO thin films were characterized using XRD, SEM, AFM and d31 coefficient measurement techniques. Chapter 3: It reports on the comparative study of properties of piezoelectric ZnO thin films deposited on three different types of flexible substrates. The substrate materials employed were a metal alloy (Phynox), polyimide (Kapton), and polyester (Mylar). Piezoelectric ZnO thin films deposited on these flexible substrates were characterized by XRD, SEM, AFM and d31 coefficient measurement techniques. A vibration sensing test was also performed for the confirmation of good piezoelectric property. Compared to the polymer flexible substrates, the metal alloy flexible substrate (Phynox) was found to be more suitable for integrating ZnO thin film for sensing applications. Chapter 4: The development of a novel gas flow sensor for the flow rate measurement in the range of L min-1 is presented in this chapter. The sensing element is a Phynox alloy cantilever integrated with piezoelectric ZnO thin film. A detailed theoretical analysis of the experimental set–up showing the relationship between output voltage generated and force at a particular flow rate has been discussed. The flow sensor is calibrated using an in-house developed testing set-up. Chapter 5: This chapter is divided into two sections. Section 5.1 reports on the development of a novel packaged piezoelectric thin film impact sensor and its application in non-destructive material discrimination studies. Different materials (Iron, Glass, Wood and Plastic) were successfully discriminated by using the developed impact sensor. The output response of impact sensor showed good linearity and repeatability. The impact sensor is sensitive, reliable and cost-effective. Section 5.2 reports on the development of a Thin Film Senor Array (TFSA) for monitoring the location and magnitude of the impact force. The fabricated TFSA consists of evenly distributed ZnO thin film sensor array. Chapter 6: It consists of two sections. Section 6.1 reports on the fabrication of micro actuator using piezoelectric ZnO thin film integrated with flexible Phynox substrate. A suitable concave Perspex mounting was designed for the actuator element. The actuator element was excited at different frequencies for the supply voltages of 2V, 5V and 8V. The developed micro actuator has the potential to be used as a micro pump for pumping nano liters to micro liters of fluids. Section 6.2 reports the design and development of a portable ready to use Thin Film Micro Vibrator (TFMV). The TFMV is capable of providing the vibration amplitude in the range of nanometer to micrometer. A thin silicon diaphragm was used as a test specimen for its vibration testing studies using the developed TFMV. The TFMV is light-weight and have internal battery, hence no external power supply is required for its functioning. Chapter 7: The first section summarizes the salient features of the work presented in this thesis. In the second section the scope for carrying out the further work is given.