Revolution under the Hood
Today’s driver of a motor vehicle is master of a whole legion of tiny, helpful “servants” that are busily collecting data: Up to 100 sensors do their duty in today’s typical vehicle. They are incessantly taking measurements, for instance under the engine hood or in the exhaust stream, and they ensure the safe, clean and economical operation of the vehicle.
Airbags, seatbelt tensioners, an antilock brake system (ABS) and an electronic stability program (ESP) are all part of the standard equipment in today’s automobiles. And none of these various systems can operate without using sensors. Sensors inform the control electronics at which speed or with which acceleration the vehicle is moving. The intelligent control system then uses these raw data to decide whether the airbag must be triggered or the ESP must intervene by applying braking force if the car threatens to go into a skid. As their name implies, sensors are the vehicle’s sense organs. They measure pressure, temperature, exhaust emission values, and a multitude of other physical variables.
Microsystems technology (MST) has emerged as a key technology in the production of automotive sensors at Bosch. MST sensors are small (comparable in size to a pinhead), economical, easy to mass-produce, extremely precise, and reliable. These attributes are achieved by using the established methods of microelectronics – such as photolithography and different deposition processes – to simultaneously process hundreds of sensors on a silicon wafer. Additional processing steps that are typical of MST are added in order to structure the wafer. These include processes such as wet and dry etching to remove material – for instance to create a paper-thin membrane for a pressure sensor. Acceleration sensors, on the other hand, consist of freely vibrating elements: The sensitive element is created at the surface, while the layer underneath – known as the sacrificial layer – is removed by an ingenious process.
Integrating the processing electronics on the MST chip makes it possible to produce sensors that are especially compact. However, as an alternative Bosch researchers also use two-chip arrangements. This approach means that the structuring processes that have been improved by Bosch can be used just as advantageously as can the most advanced circuit integration methods in microelectronics.
Microsystems technology (MST) has emerged as a key technology in the production of automotive sensors at Bosch. MST sensors are small (comparable in size to a pinhead), economical, easy to mass-produce, extremely precise, and reliable. These attributes are achieved by using the established methods of microelectronics – such as photolithography and different deposition processes – to simultaneously process hundreds of sensors on a silicon wafer. Additional processing steps that are typical of MST are added in order to structure the wafer. These include processes such as wet and dry etching to remove material – for instance to create a paper-thin membrane for a pressure sensor. Acceleration sensors, on the other hand, consist of freely vibrating elements: The sensitive element is created at the surface, while the layer underneath – known as the sacrificial layer – is removed by an ingenious process.
Integrating the processing electronics on the MST chip makes it possible to produce sensors that are especially compact. However, as an alternative Bosch researchers also use two-chip arrangements. This approach means that the structuring processes that have been improved by Bosch can be used just as advantageously as can the most advanced circuit integration methods in microelectronics.
A two-chip arrangement with the sensor housing cut away: On the top, a signal processor IC; and below, an MST angular rate sensor. The housing is shown standing on top of a wafer in which hundreds of sensor chips have already been structured.
