Pressure Sensor for Exhaust Systems

Exhaust gas regulations for motor vehicles are becoming stricter year-after-year, and as internal combustion engines are being made more efficient and their exhaust gas reduced, there is increased demand for pressure sensing in the exhaust systems of diesel engine vehicles. Applying a single-chip semiconductor pressure sensor fabricated by a CMOS process, which has been successfully used for manifold pressure measurement, Fuji Electric has developed an exhaust system pressure sensor capable of withstanding an exhaust gas environment containing corrosive substances. The newly developed sensor, in a DIN standard SO2 gas test, exhibited the ability to withstand corrosion that is more than 2.5 times greater than that of conventional sensors, and is suitable for use in absolute pressure sensing and relative pressure sensing applications. tors and the like are mounted, and the parts had many electrical contacts(2). To enhance the reliability by reducing the number of parts, Fuji Electric developed a pressure sensor in which the sensing unit, amplifier circuit, temperature characteristic compensation circuit and EMC protection devices are integrated into a single chip using a standard semiconductor CMOS (complementary metaloxide-semiconductor) process. This single-chip type of pressure sensor has been deployed in the market mainly as a MAP sensor since 2002. Figure 1 shows an overview of the sensor unit of this pressure sensor, and Fig. 2 shows the cross-sectional structure of this pressure sensor. Diffused piezo-resistors are fabricated on a diaphragm at the same time as the IC process is performed, and a Wheatstone bridge is configured with four diffused piezo-resistors. The diaphragm ensures a high burst pressure and is formed, using Fuji Electric’s proprietary silicon etching process, in a precise round shape. When the application of pressure causes the diaphragm to flex, the resistance values of the diffused piezo-resistors come to change due to the piezo-resistive effect, and the output of the Wheatstone bridge changes. As a result of this principle, the pressure quantity can be converted into a voltage. Moreover, a high precision amplifier for amplifying the signal output from the Wheatstone bridge and an adjustment circuit for compensating the sensor characteristics are built on a single chip. Additionally, protection devices for protecting internal circuits from surges generated by the automobile engine control system, and from static electricity during the assembly process and electromagnetic noise from external sources, and so on, are all provided on a single chip(3). As shown in Fig. 3, the sensor unit is embedded in a resin package, and electrical and mechanical connections are made to the exterior. The sensor cell package is made from PPS (polyphenylene sulfide), which is used for automotive parts and has excellent resistance to acid, and the sensor chip unit, lead frame and wires are coated with fluoro silicone gel, which has excellent chemical resistance and oil resistance. 3. Application to Pressure Sensors for Exhaust System Exhaust emissions regulations for automobiles have been strengthened year after year since the Muskie Act in the 1970s, and in Japan, Post-New Long-Term exhaust emission regulations came into effect in 2009. In Europe, the EURO5 emission regulaFig.2 Cross-sectional structure of pressure sensor unit Fig.3 Structure of pressure sensor cell Sensor chip (silicon) Vacuum chamber Diffused piezo resister Bending stress of diaphragm Glass base