In almost all new vehicles, an electronic engine control module teams with sensors and actuators to constitute the engine control system where film resistors play important roles. Piezoresistive transducers implanted in a silicon wafer with bulk micromachined diaphragms have become the dominant technology for producing automotive pressure sensors. Initially an absolute, bulk micromachined, piezoresistive pressure sensor element with a backside constraint for the formation of a vacuum reference cavity--a necessary element for MAP/BAP applications--emerged as the most popular configuration. Lately, however, gauge and differential configurations, as well as configurations where a backside constraint is added for a stress relief only, have evolved to significant production volumes.

Silicon micromachined pressure sensor elements exhibit wide variation of their parameters from part to part, as well as over temperature. To produce a truly interchangeable module for vehicle manufacturers, sensor developers must individually calibrate and temperature-compensate each sensor.


The usual method is to use some sort of adjustment (trimming) within the signal conditioning circuit. The manufacturing and assembly methods and the finished sensor module's electrical performance requirements help determine how the signal conditioning circuit is implemented. Integration technology for this circuit--CMOS, BiCMOS, or bipolar--depends primarily on the selected trimming technique.

At first, automotive piezoresistive pressure sensors contained analog signal conditioning circuits, built using off-the-shelf, packaged, bipolar ICs that were mounted on a PCB. All the necessary adjustments were done using laser trimming of the thick film resistor network that was deposited on a ceramic substrate and then inserted into the PC board in the form of a trim-pack. NRC series and LMK series of Micorhm Electronics are produced in this way. Subsequent advances in hybrid circuit technology permitted both fabrication of the ceramic substrate containing printed thick film resistors and mounting of the signal conditioning ICs in either packaged or bare-die form. These sensor modules became compact, and significant volumes of them are still in production. This mature manufacturing process provides cost-effective sensor modules for applications with moderate size requirements, where thick film resistor stability and accuracy are adequate.


The automotive industry's constant drive for low-cost components, however, forced higher levels of integration to minimize the component count and assembly cost of pressure sensor modules. As a result, analog active circuits were integrated on the same chip with passive, thin film trimmable resistors. By doing this, manufacturers dramatically reduced sensor module sizes and improved sensor performance, primarily because of the thin film resistor's superior stability, as is the case with NAP series and NR series from Microhm Electronics.

 Integration of that type has created two-chip solutions, with the sensing element as one chip and the IC with thin film resistors as the other. Monolithic (single-chip) solutions, in which the sensing element, active circuit, and thin film resistors are integrated on one silicon die, are also used in volume production.

All of the above signal conditioning circuits are analog in nature, and the laser trimming adjustments are assumed to be continuous.