Described below is a micromechanical substrate including a membrane, and a method for producing the substrate, wherein the membrane is, in particular, a carrier for elements of a sensor.
Gas sensors are increasingly being used in modern technical apparatuses. One example thereof is a leak sensor that detects the occurrence of a single gas. A further example is an exhaust gas sensor that analyzes a mixture of a plurality of gas components. A third example is a gas sensor for fire detection. A further field of use for gas sensors is in respiratory gas analysis, where, by way of example, the presence of alcohols can be detected, or the presence of nitrogen monoxide (NO) for asthma patients.
Gas sensors based on semiconducting metal oxides are of major importance on account of their simple construction and long lifetime. The metal oxide traditionally used is tin dioxide (SnO2), sometimes also WO3. Often, however, other metal oxides and mixtures of different metal oxides and supported catalyst dispersions are also used for improving the sensor properties.
Metal oxide sensors generally have to be heated for operation. The first sensor constructions used for this purpose a small ceramic tube with an inner heating winding as carrier. Modern planar constructions use a usually ceramic substrate, on which an electrical heating structure, for example a platinum heating meander, is fitted on the rear side, and an electrode structure for measuring the resistance of the semiconducting sensor layer is fitted on the front side. The ceramic substrate is then suspended in a housing by small wires (usually composed of platinum).
However, this construction technology is cost-intensive and complex, since in part it involves manual work. In order to reduce the cost and complexity, micromechanical constructions in which the functional elements of the sensor are situated on a membrane having a thickness in the micrometers range, that is to say a very thin membrane, have been developed for the SnO2 sensors. This construction can then be adhesively bonded cost-effectively directly onto a housing or a PCB since the membrane has a high thermal insulation relative to the surroundings and the silicon carrier is therefore practically at room temperature despite the heated membrane. Contact-making is implemented cost-effectively by Al or Au wire bonding.
The membrane used can be realized for example as a silicon nitride membrane. For production purposes, to put it in simplified terms, a silicon wafer is coated with the silicon nitride and a part of the silicon wafer is then removed by a volume etch. EP 0 953 152 B1 discloses the production of a membrane substrate using a silicon-on-insulator wafer (SOI).
The metal oxide SnO2 can be used well on the known membranes, but has a whole series of disadvantages such as, for example:
high variation in manufacturing tolerance of the sensitive properties,
long run-up behavior after every switch-on,
lack of stability in the exhaust gas.
Significant improvements in all these properties are exhibited by more recent metal oxides such as gallium oxide (Ga2O3), for example described in M. Fleischer, “Advances in Application Potential of Solid State Gas Sensors: High-Temperature Semiconducting Oxides and Ambient Temperature GasFET Devices”, Measurement Science and Technology 19 (2008), 1-18. However, this material has to be operated at temperatures of 650-900° C. The known micromechanical constructions having a membrane are not suitable for this purpose on account of a lack of thermal stability, since the continuous conversion of tension into compressive stresses leads to fracture of the membrane. By way of example, in EP 0 953 152 B1, the substrate is heated using a CMOS-compatible heater structure that is likewise not suitable for such temperatures.