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Title:Fabrication and Structural Design of Micro Pressure Sensors for Tire Pressure Measurement Systems (TPMS)

Abstract：

In this paper we describe the design and testing of a micro piezoresistive pressure sensor for a Tire Pressure Measurement System (TPMS) which has the advantages of a minimized structure, high sensitivity, linearity and accuracy. Through analysis of the stress distribution of the diaphragm using the ANSYS software, a model of the structure was established. The fabrication on a single silicon substrate utilizes the technologies of anisotropic chemical etching and packaging through glass anodic bonding. The performance of this type of piezoresistive sensor, including size, sensitivity, and long-term stability, were investigated. The results indicate that the accuracy is 0.5% FS, therefore this design meets the requirements for a TPMS, and not only has a smaller size and simplicity of preparation, but also has high sensitivity and accuracy.70606

Keywords:

Piezoresistive; TPMS; Pressure sensor; FEM

Nowadays, there is growing interest in Tire Pressure Measurement Systems (TPMS), which has led to the development of sundry pressure sensors. Despite the abundant evidence that tires are definitely one of the most vital safety components on a vehicle, but a tire can lose up to half of its air pressure without appearing to be underinflated and most people ignore the state of their tires so data shows that nearly 250,000 accidents per year occur in the United States alone due to low tire pressure. The place where the rubber meets the road affects traction, handling, steering, stability and braking. Because of this, a sudden tire failure can induce serious consequences, more so if it occurs when driving at highway speeds. To avoid the above, the concept of a TPMS was introduced. Direct TPMS contain small sensors for pressure and acceleration that are installed inside the tires. These sensors monitor tire pressure and continually feedback this information, via wireless signals, to the vehicle’s electronic control unit (ECU). If the tire pressure falls below required limits, a warning signal is sent to the instrument panel that alerts the driver about the problem. Since the discovery of the piezoresistive effect, piezoresistive sensors have been widely employed in mechanical signal sensing, which plays a very important role in TPMS. The piezoresistive sensor has the advantages of simple fabrication and stable performance compared with the capacitive sensor, which has a complicated fabrication process and signal circuits, and moreover, the performance of this last type of sensor is easily affected by circumstantial impurities. The SOI wafer is another choice for the fabrication of pressure sensors, but the expensive cost of SOI materials and the complicated process for implanting the resistance and down-lead have restricted its applications in autos. The piezoresistive pressure sensor is formed by diffusing impurities onto a semiconductor and has the advantages such as easy fabrication, superior linearity and high sensitivity. The measurement range of a TPMS should be 0 to 1 MPa, and it is mounted and used in the tire under circumstances where it may experience changes of temperature from about 0 to 85°C. By applying a simplified processing circuit in the piezoresistive pressure sensor, a correction and compensation circuit is usually added to overcome the nonlinearity caused by any poor temperature characteristics thereof.论文网

Merits of the Micro Electro Mechanical systems (MEMS) technology are proven in the manufacture of sensing elements of small and definite size. The desired measurement range, bandwidth, and sensitivity can be easily achieved by adjusting the size of the sensing elements. In previous work, a structural model of a sensor is built by the finite element method (FEM) using the ANSYS software to calculate the structure stress and to define the size of the sensor. The FEM is widely adopted for stress analysis, thermal effect reduction, packaging design and reliability of enhancements to piezoresistive sensors. In a structural simulation, the FEM is very effective in producing the visualized stiffness, the strength and also in the mathematical minimization of the weight, the materials and the costs. FEM expresses the visualization details of where the structures deform or twist, and indicates the distribution of stresses and the displacements. FEM software possesses an abundance of simulation options for controlling the complex system of both modeling and analysis. In the development of this pressure sensor, bulk micromachining was applied to the fabrication of the sensor. This method is cheap and easily realized. For the bulk micromachining technique, a KOH solution is usually used to etch the silicon substrate to form cavities in a trapezoid shape with a 54.7° inclined wall, which allows for diaphragm deformation. Since the silicon substrate is relatively thick and the bottom region etched is much larger than the top region where the pressure diaphragm defines. In addition, investigation shows that decreasing the thickness of diaphragm and widening the bottom region can give a smaller size to satisfy the measurement requirement of sensors. We have used a 4 inch silicon wafer and a Pyrex7740# glass wafer for silicon-glass anodic bonding, applying a silicon-glass anodic bonding technique instead of Si-Si bonding which is operated in 1000°C due to the destruction of the Al wire. Ti is placed between the Al and the resistance for decreasing the contact resistance. The packaging adhesive is stuffed with impurities for lowing residual stress. In the end, the sensor characteristics were listed its and sensitivity, range, and linearization were also analyzed in this paper.