Piezoresistive effect of p-type single crystalline 3C-SiC on (111) plane

This paper presents for the first time the effect of strain on the electrical conductivity of p-type single crystalline 3C-SiC grown on a Si (111) substrate. 3C-SiC thin film was epitaxially formed on a Si (111) substrate using the low pressure chemical vapor deposition process. The piezoresistive effect of the grown film was investigated using the bending beam method. The average longitudinal gauge factor of the p-type single crystalline 3C-SiC was found to be around 11 and isotropic in the (111) plane. This gauge factor is 3 times smaller than that in a p-type 3C-SiC (100) plane. This reduction of the gauge factor was attributed to the high density of defects in the grown 3C-SiC (111) film. Nevertheless, the gauge factor of the p-type 3C-SiC (111) film is still approximately 5 times higher than that in most metals, indicating its potential for niche mechanical sensing applications

Development of the Telemetrical Intraoperative Soft Tissue Tension Monitoring System in Total Knee Replacement with MEMS and ASIC Technologies

The alignment of the femoral and tibial components of the Total Knee Arthoplasty (TKA) is one of the most important factors to implant survivorship. Hence, numerous ligament balancing techniques and devices have been developed in order to accurately balance the knee intra-operatively. Spacer block, tensioner and tram adapter are instruments that allow surgeons to qualitatively balance the flexion and extension gaps during TKA. However, even with these instruments, the surgical procedure still relies on the skill and experience of the surgeon. The objective of this thesis is to develop a computerized surgical instrument that can acquire intra-operative data telemetrically for surgeons and engineers. Microcantilever is chosen to be used as the strain sensing elements. Even though many high end off-the-shelf data acquisition components and integrated circuit (IC) chips exist on the market, yet multiple components are required to process the entire array of microcantilevers and achieve the desired functions. Due to the size limitation of the off-chip components, an Application Specific Integrated Circuit (ASIC) chip is designed and fabricated. Using a spacer block as a base, sensors, a data acquisition system as well as the transmitter and antenna are embedded into it. The electronics are sealed with medical grade epoxy

Integration of electronic and optical techniques in the design and fabrication of pressure sensors

Since the introduction of micro-electro-mechanical systems fabrication methods, piezoresistive pressure sensors have become the more popular pressure transducers. They dominate pressure sensor commercialization due to their high performance, stability and repeatability. However, increasing demand for harsh environment sensing devices has made sensors based on Fabry-Perot interferometry the more promising optical pressure sensors due to their high degree of sensitivity, small size, high temperature performance, versatility, and improved immunity to environmental noise and interference. The work presented in this dissertation comprises the design, fabrication, and testing of sensors that fuse these two pressure sensing technologies into one integrated unit. A key innovation is introduction of a silicon diaphragm with a center rigid body (or boss), denoted as an embossed diaphragm, that acts as the sensing element for both the electronic and optical parts of the sensor. Physical principles of piezoresistivity and Fabry-Perot interferometry were applied in designing an integrated sensor and in determining analytic models for the respective electronic and optical outputs. Several test pressure sensors were produced and their performance was evaluated by collecting response and noise data. Diaphragm deflection under applied pressure was detected electronically using the principle of piezoresistivity and optically using Fabry-Perot interferometry. The electronic part of the sensor contained four p-type silicon piezoresistors that were set into the diaphragm. They were connected in a Wheatstone bridge configuration for detecting strain-dependent changes in resistance induced by diaphragm deflection. In the optical part of the sensor, an optical cavity was formed between the embossed surface of the diaphragm and the end face of a single mode optical fiber. An infrared laser operating at 1.55 was used for optical excitation. Deflection of the diaphragm, which causes the length of the optical cavity to change, was detected by Fabry-Perot interference in the reflected light. Data collected on several sensors fabricated for this dissertation were shown to validate the theoretical models. In particular, the principle of operation of a Fabry-Perot interferometer as a mechanism for pressure sensing was demonstrated. The physical characteristics and behavior of the embossed diaphragm facilitated the integration of the electronic and optical approaches because the embossed diaphragm remained flat under diaphragm deflection. Consequently, it made the electronic sensor respond more linearly to applied pressure. Further, it eliminated a fundamental deficiency of previous applications of Fabry-Perot methods, which suffered from non-parallelism between the two cavity surfaces (diaphragm and fiber), owing to diaphragm curvature after pressure was applied. It also permitted the sensor to be less sensitive to lateral misalignment during the fabrication process and considerably reduced back pressure, which otherwise reduced the sensitivity of the sensor. As an integrated sensor, it offered two independent outputs in one sensor and therefore the capability for measurements of: (a) static and dynamic pressures simultaneously, and (b) two different physical quantities such as temperature and pressure

A multifunctional sensor system for brake system applications

The objective of this thesis is to develop a multifunctional MEMS (Micro Electro Mechanical System) sensor system for the simultaneous measurement of pressure and temperature inside a hydraulic system, more specifically a hydraulic brake system for automotive applications. The multifunctional pressure and temperature sensor system presented in this Thesis was designed to be installed in a new brake-by-wire system that requires the simultaneous reading of pressure and temperature per wheel cylinder. This system needs to control and monitor these parameters at each wheel cylinder to adjust the pressure for optimal braking. Current sensing systems installed in regular brake systems use a single pressure sensor that is positioned in the main cylinder and they do not include a temperature sensor. Moreover, while numerous approaches have been taken to control and monitor the pressure in a brake system real-time, no MEMS sensor system has yet been reported that can carry out real-time measurements of the brake system's pressure and temperature. In a representative automobile hydraulic brake system, the pressure and temperature can reach up to about 4 Mpa and 120 °C, respectively. These conditions are developed in an oily environment with a pH ~ 11. The multifunctional sensor system presented here is based on the two sensors, one for pressure and one for temperature, working within the same packaging. These two sensors are glued on the surface of an adequate Transistor Outline (TO) base using a temperature resistance adhesive. The substrate with the two sensors is covered by a parylene layer for dielectric protection, protection from the corrosive medium and protection from the moisture inherent in the brake fluid. The interface of the sensor system to the hydraulic brake system uses a commercial 1/4 18 NPT fitting customized to serve as an interface as well as a metal shell between the sensor and the hydraulic cylinder. The TO base and the metal shell were joined by micro-brazing to minimize heat-affected areas and ensure that critical components are unharmed. A finite element model to understand the effect of the parylene layer on the performance of the sensors was developed using COMSOL Multiphysics®. The model was validated by testing many prototypes of the developed sensor system using a custom made hydraulic hand pump which pressure is monitored by a digital hydraulic pressure gauge. The sample fitting is covered by a coil heater that includes a type -T Thermocouple positioned close to the sample to monitor the temperature. The complete apparatus allowed characterization of the test sensor from room pressure (13 psi) to 500 psi over a temperature range of 25 to 120°C. The test samples were characterized from atmospheric pressure to 450 psi over a temperature range of 25 to 120°C. The experimental data shows a reduction in pressure sensitivity of 18.2 % due to the parylene layer which closely agrees with the model predictions of a reduction of 21%. In summary, a multifunctional sensor system has been developed that can be used to control and monitor the pressure and temperature of a hydraulic cylinder real-time. The sensor system is novel in that it measures both parameters at a single point real-time with good sensitivity and accuracy making it ideal for applications in brake-by-wire systems

Doctor of Philosophy

dissertationNew hydrogel-based micropressure sensor arrays for use in the fields of chemical sensing, physiological monitoring, and medical diagnostics are developed and demonstrated. This sensor technology provides reliable, linear, and accurate measurements of hydrogel swelling pressures, a function of ambient chemical concentrations. For the first time, perforations were implemented into the pressure sensors piezoresistive diaphragms, used to simultaneously increase sensor sensitivity and permit diffusion of analytes into the hydrogel cavity. It was shown through analytical and numerical (finite element) methods that pore shape, location, and size can be used to modify the diaphragm mechanics and concentrate stress within the piezoresistors, thus improving electrical output (sensitivity). An optimized pore pattern was chosen based on these numerical calculations. Fabrication was performed using a 14-step semiconductor fabrication process implementing a combination of potassium hydroxide (KOH) and deep reactive ion etching (DRIE) to create perforations. The sensor arrays (2×2) measure approximately 3 × 5 mm2 and used to measure full scale pressures of 50, 25, and 5 kPa, respectively. These specifications were defined by the various swelling pressures of ionic strength, pH and glucose specific hydrogels that were targeted in this work. Initial characterization of the sensor arrays was performed using a custom built bulge testing apparatus that simultaneously measured deflection (optical profilometry), pressure, and electrical output. The new perforated diaphragm sensors were found to be fully functional with sensitivities ranging from 23 to 252 μV/V-kPa with full scale output (FSO) ranging from 5 to 80 mV. To demonstrate proof of concept, hydrogels sensitive to changes in ionic strength were synthesized using hydroxypropyl-methacrylate (HPMA), N,N-dimethylaminoethyl-methacrylate (DMA) and a tetra-ethyleneglycol-dimethacrylate (TEGDMA) crosslinker. This hydrogel quickly and reversibly swells when placed environments of physiological buffer solutions (PBS) with ionic strengths ranging from 0.025 to 0.15 M. Chemical testing showed sensors with perforated diaphragms have higher sensitivity than those with solid diaphragms, and sensitivities ranging from 53.3±6.5 to 271.47±27.53 mV/V-M, depending on diaphragm size. Additionally, recent experiments show sensors utilizing Ultra Violet (UV) polymerized glucose sensitive hydrogels respond reversibly to physiologically relevant glucose concentrations from 0 to 20 mM