Highly Porous Silicon Disintegrates Like Soapsuds
Bosch research scientists are continually improving and expanding the microsystems technology toolbox. Examples include new methods for structuring the silicon wafer as well as techniques of testing sensors for correct function even during their manufacture.
An impressive array of sensors collects data for the efficient operation of the automobile engine – including a pressure sensor that measures the intake air pressure in the intake manifold: A very thin membrane is more or less deflected into a reference vacuum as a function of changes in external pressure.
In manufacturing such pressure sensors, it is presently necessary to structure both the front and the rear surfaces of the silicon wafer. The rear surface of the silicon is removed by an etching process over an area of about one square millimeter, while a few micrometers (millionths of a meter) of the front surface are left in place to form the membrane. An airtight cavity is formed by encapsulating one side of the membrane to create a reference vacuum. Piezoresistors are produced in the front surface of the membrane by doping. As the membrane is deformed, these resistors are used to measure the resulting mechanical stress. This is a well-established method, and Bosch produces such pressure sensors by the million.
Now Bosch researchers have developed a better, simpler method, in which the reference vacuum can be produced directly within the silicon. The surface of the silicon wafer is treated with hydrofluoric acid as a voltage is applied, so that a highly porous (“nanoporous”) silicon layer is produced. Next, a monocrystalline silicon layer – which will form the membrane – is deposited on top of the porous layer. When the nanoporous silicon is subsequently heated to 1,000 degrees Celsius, it collapses like soapsuds. What remains is a continuous cavity with a reference pressure of less than one millibar.
To achieve a robust sensor design, Bosch researchers optimize the system as a whole by using design and simulation programs. These programs enable them to consider all accessible geometric, materials and process parameters – such as membrane and resistance geometries, doping or deposition variables. Process tolerances that are known from the production side are also taken into account to create a sensor design that meets the functional specifications with the best possible production yield.
The research scientists derive critical parameters from the simulation results. These parameters must be monitored in the production process in order to ensure that the components comply with the specifications. Such testing is most efficiently performed directly on the wafer. But during this production stage, pressure sensors can’t be tested by applying test pressure, nor can acceleration sensors be tested by applying test acceleration. Instead, the sensors must be checked for correct function as swiftly as possible – known in the industry as the Bosch process – enables exactly this type of delicate work. Troughs a few micrometers in width but as much as several hundred micrometers in depth can be trenched in silicon surfaces by applying a sequence of etching and passivation phases. This method allows the creation of extremely precise spring-mass systems for acceleration and angular rate sensors.
In manufacturing such pressure sensors, it is presently necessary to structure both the front and the rear surfaces of the silicon wafer. The rear surface of the silicon is removed by an etching process over an area of about one square millimeter, while a few micrometers (millionths of a meter) of the front surface are left in place to form the membrane. An airtight cavity is formed by encapsulating one side of the membrane to create a reference vacuum. Piezoresistors are produced in the front surface of the membrane by doping. As the membrane is deformed, these resistors are used to measure the resulting mechanical stress. This is a well-established method, and Bosch produces such pressure sensors by the million.
Now Bosch researchers have developed a better, simpler method, in which the reference vacuum can be produced directly within the silicon. The surface of the silicon wafer is treated with hydrofluoric acid as a voltage is applied, so that a highly porous (“nanoporous”) silicon layer is produced. Next, a monocrystalline silicon layer – which will form the membrane – is deposited on top of the porous layer. When the nanoporous silicon is subsequently heated to 1,000 degrees Celsius, it collapses like soapsuds. What remains is a continuous cavity with a reference pressure of less than one millibar.
To achieve a robust sensor design, Bosch researchers optimize the system as a whole by using design and simulation programs. These programs enable them to consider all accessible geometric, materials and process parameters – such as membrane and resistance geometries, doping or deposition variables. Process tolerances that are known from the production side are also taken into account to create a sensor design that meets the functional specifications with the best possible production yield.
The research scientists derive critical parameters from the simulation results. These parameters must be monitored in the production process in order to ensure that the components comply with the specifications. Such testing is most efficiently performed directly on the wafer. But during this production stage, pressure sensors can’t be tested by applying test pressure, nor can acceleration sensors be tested by applying test acceleration. Instead, the sensors must be checked for correct function as swiftly as possible – known in the industry as the Bosch process – enables exactly this type of delicate work. Troughs a few micrometers in width but as much as several hundred micrometers in depth can be trenched in silicon surfaces by applying a sequence of etching and passivation phases. This method allows the creation of extremely precise spring-mass systems for acceleration and angular rate sensors.
