Pressure sensors made of piezoresistive thin films

Abstract

Micro-pressure sensors made of piezoresistive thin films were fabricated. The device has a basic structure of a silicon (Si) diaphragm, on the top surface of which some piezoresistive thin films components are deposited with a pattern. When a pressure difference is established across the diaphragm, the diaphragm would be deformed such that the resistance of the thin film components would be changed. If the piezoresistors are connected in the form of a Wheatstone bridge with a proper voltage bias, an electrical signal would be generated in proportion to the magnitude of the pressure difference built up across the diaphragm. Our design is different from a conventional Si-based one where some local regions on the Si diaphragm are doped to serve as the piezoresistive sensing components. Instead, in our design, thin film piezoresistors are used as the sensing elements, such that the structure is potentially useful to evaluate the applicability of any new piezoresistive thin film materials for the use of making pressure sensors of this type. In particular, we selected indium tin oxide (ITO) films to serve as the sensing elements in this study, where the design and fabrication processes were evaluated, and the performance of the devices thus produced were investigated. We started from working out the design by estimating theoretically the magnitude of the electrical signal available from the structure and mechanism. This was accomplished by simulating the mechanical deformation, stresses and strains of the hypothetical structure (a silicon diaphragm) by a finite element analysis package (ANSYS), and compared the results with those obtained from the analytic calculations. The next step was to fabricate sensors according to the selected design. Indium tin oxide (ITO) was chosen as the piezoresistive thin film material, because of its prominent piezoresistive behavior and chemical stability in air. This was accomplished first by fabricating some ITO films with a radio frequency (RF) magnetron sputtering system, and observing their properties according to the deposition conditions. The substrate temperature, O2/Ar flow ratio, ambient pressure and post-annealing conditions etc, were varied as deposition parameters. X-ray diffraction (XRD) experiments were done to monitor the film structure. It was found that post-annealing at 400C greatly reduced the resistivity of the samples, with accompanying prominent crystallization of the film structure. The piezoresistive coefficients of thin films under different sputtering conditions at substrate temperature around 400C were found to be similar to those of other films deposited at lower temperatures but post-annealed at around 400C. With the use of ITO films, pressure sensors were fabricated by means of some micro-machining processes. The performance of the devices was investigated to evaluate the validity of the design and the feasibility of the fabrication processes.