Foreword At the beginning of the new century, due to quality problems with Firetone tires, numerous tire blowouts and rollovers resulted in over 100 deaths and 400 injuries, drawing the attention of the automotive industry and the US government. The company was forced to recall 6.5 million tires the following August. Statistics show that 260,000 traffic accidents annually in the US are caused by low tire pressure or leaks, accounting for 75% of all traffic accidents. In China, 70% of traffic accidents on highways are caused by tire blowouts. Tire quality experts believe that "maintaining standard tire pressure and timely detection of tire leaks are key to preventing blowouts." Currently prevalent low-cost handheld digital tire pressure gauges cannot guarantee timely detection of tire leaks. The indirect measurement technology currently used in over 2 million GM and Ford vehicles—using anti-lock braking tire speed sensors to measure and compare the rotational speed of each tire to infer whether the tire pressure is insufficient—is both cumbersome and inaccurate. Smart tires, which integrate tire pressure monitoring sensors and radio frequency transmitters into each tire, are attracting significant attention. These smart tires, in conjunction with a receiver and display on the car's dashboard, constitute a tire pressure monitoring system (TPMS), also known as a tire pressure monitoring sensor. A mandatory federal law enacted by the National Highway Traffic Safety Administration (NHTSA) in the mid-1970s spurred the widespread adoption of automatic fuel injection systems and the first major surge in automotive sensor applications. Another federal law passed by the NHTSA in 2002 mandated that U.S. vehicles be equipped with TPMS systems at annual rates of 15%, 35%, 65%, and 100% from 2003 to 2006, respectively, triggering a new wave of automotive pressure sensor applications. The unique operating environment of tires dictates the high requirements for pressure sensors used in real-time tire pressure monitoring. These requirements include a wide temperature range, high overall practical accuracy over a wide power supply voltage range, low power consumption, wireless signal transmission capabilities, resistance to harsh environments, and low cost. Motorola is an active developer of TPMS systems. It uses the MPXY8020A, a silicon-integrated capacitive pressure sensor based on MEMS technology, as its tire pressure monitoring unit, featuring low power consumption and full integration. It employs the 32-pin packaged MC68HC908RF2 as its signal control, processing, and transmission unit; this device integrates an 8-bit microcontroller and a UHF transmitter, and uses the MC33594 as its receiver. GE NovaSensor, a renowned sensor manufacturer recently acquired by General Electric, is another active developer. It uses a silicon piezoresistive pressure sensor based on MEMS technology as its tire pressure monitoring unit, equipped with a dedicated CMOS large-scale integrated circuit capable of control, measurement, signal compensation and adjustment, and transmission. Two chips packaged in a standard 14-pin SOIC package constitute its TPMS, model NPXC01746. Due to its wake-up transient operating mode, its power consumption is only 9.7 microamp-seconds, sufficient for a ten-year battery life. Thanks to digital compensation, its pressure measurement accuracy is better than 1.5%FS within a range of -40℃ to +125℃ and a battery voltage of 2.1V to 3.0V. The greater significance of the Nova mode lies in its ability to integrate with existing ASICs and any integrated Wheatstone full-bridge piezoresistive pressure sensor, resulting in a wider range of TPMS products. The Norwegian company SensoNor specializes in manufacturing dedicated integrated sensors based on piezoresistive pressure, which are used by tire pressure monitoring system suppliers such as TRW Automotive and SmarTire to create TPMS. SensoNor's piezoresistive tire sensors combined with SmarTire's RF generators have successfully created TPMS that have been adopted by Siemens VDO Automotive and Goodyear Tire & Rubber Company. Design and Development of Force-Sensitive Chip for Tire Pressure Sensor To develop a tire pressure sensor, we designed and fabricated a suitable absolute pressure sensor. This is a miniature piezoresistive absolute pressure sensor based on MEMS silicon micromachining technology, with an integrated Wheatstone full-bridge as the sensing element. The force-sensitive resistors are conventionally distributed at the center points of the four edges of a square silicon thin film, arranged in the <110> crystal orientation, with one pair arranged longitudinally and another tangentially, thus forming a Wheatstone strain full-bridge. The resistor strips are 8µm wide and 60µm long, and the average effective stress ensures an output sensitivity of 20mV/V. The resistors are formed using ion implantation doping, providing excellent uniformity and doping accuracy to ensure zero-point and sensitivity stability. The resistor impedance is designed to be 5KΩ. The vacuum reference cavity of the absolute pressure sensor is formed using silicon-silicon bonding technology. This method offers better thermal expansion system matching than using silicon-pyrex glass anode bonding to form the absolute pressure reference cavity, thus improving the product's thermal and temporal stability. Another significant advantage of this design technology is the substantial reduction in unit chip size. The unit chip in this design is 1mm × 1mm, allowing for the fabrication of over 7,000 force-sensitive element units on a single four-inch silicon wafer. In contrast, unit chips using silicon-glass bonding typically range from 1.5 × 1.5mm to 2.2 × 2.2mm, resulting in 3,400 and 1,600 pressure-sensitive element units respectively on a four-inch silicon wafer. Clearly, our design helps reduce unit manufacturing costs. Of course, this design relies on mastering silicon-silicon bonding and thin silicon diaphragm fabrication techniques. Given the large range of tire pressure sensors and the relatively thick silicon diaphragm, precision mechanical thinning or isotropic etching techniques can easily meet the design requirements. Even extending the range of this design to that of automotive MAP and AAP absolute pressure sensors, Smart Cut technology or epitaxial wafer electrochemical selective etching techniques can easily yield silicon films of 15–20µm thickness. Nonlinearity: 0.05–0.1%FS; Hysteresis and repeatability: 0.03%FS; Output sensitivity: 10–20 mV/V; Range: 700 kPa; Overload capacity: 300%; Zero point and sensitivity temperature coefficient: 1–3 × 10⁻⁴ /℃·FS. This chip can also be used in tire pressure sensors and handheld digital tire pressure sensors. Conclusion A tire pressure sensor chip for tire pressure monitoring systems has been successfully fabricated using MEMS silicon micromachining technology. The use of silicon-silicon bonding design and related process technologies has reduced chip size and lowered unit cost, marking a first step in the development of TPMS products.