Certain techniques of bonding two substrate wafers for sealing caverns for micromechanical devices are conventional. In wafer bonding, one of the two substrate wafers usually includes micromechanical structures, for example sensor structures. The other substrate wafer, the cap wafer, has recesses. The recesses in the cap wafer and the sensor structures in the sensor wafer are situated such that a hollow space, a so-called cavity, forms above each sensor structure when the wafers are joined by a wafer bonding method. The cavity is hermetically sealed against the environment and thus protects the micromechanical sensor structure against environmental effects, such as moisture and particles. To ensure this, the connecting surface between the two wafers must be arranged such that a hermetic bond connection remains around each individual chip even after the wafer has been separated into individual sensor chips.
The equipment used in the wafer bonding process provides a process pressure to be set, among other things. Since the cavities are hermetically sealed during wafer bonding, the process pressure set on the equipment is encapsulated in each individual cavity in the combined wafer at the sealing temperature. At room temperature, this pressure is reduced according to the ideal gas law. The pressure in the cavities is an important parameter for many applications. In micromechanical acceleration sensors, for example, a relatively high pressure must be encapsulated to ensure adequate damping of the sensor element. Conversely, in micromechanical yaw rate sensors, which are operated resonantly, a low internal pressure is usually selected to ensure high quality as well as a low drive voltage.
Sensor elements are becoming increasingly smaller in micromechanics. Increasing miniaturization provides for multiple sensor elements to be integrated on a single sensor chip. Certain sensor chips simultaneously detect, for example, accelerations in all 3 spatial directions. Due to the process management described above for the wafer bonding method, it is currently not possible to easily integrate different sensor elements having different internal pressures (for example, micromechanical acceleration and yaw rate sensors) on a sensor chip.
Certain publications show a micromechanical component which has multiple cavities having different internal pressures. The cavities are sealed using a wafer capping process. One cavity is opened again and subsequently resealed using an oxide layer at a different internal atmospheric pressure.
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