Inertial Microsensor System Technical Paper - IMSS

MICROMECHANICAL ANGULAR RATE SENSORS ARE AMONG THE MOST COMPLEX OF ALL SENSOR SYSTEMS FOR THE MASS MARKET. IT IS COMPOSED, IN THE SMALLEST OF SPACES, OF AN INERT MASS IN A VACUUM THAT EXECUTES COMPLICATED MOTION AS WELL AS SOPHISTICATED ELECTRONIC CIRCUITRY TO DRIVE THE DEVICE AND ANALYSE ITS RESPONSE.

Supported by the latest fabrication technologies and design expertise, SensorDynamics has succeeded in creating an angular rate sensor, ready for series manufacture. It satisfies all the demands made by today's applications, especially those of the automotive industry: a small footprint, mechanical ruggedness, high, long-term stability, unrestricted fail-safety and AEC-Q100 certification. The angular rate sensor is suitable for use in motor vehicles for sure detection of dangerous situations, such as skidding or overturning; and enabling blind driving in navigation systems, outside the range of a GPS signal. The small size of the micromechanical angular rate sensor is what makes some applications possible in the first place, such as stabilizing camcorders or electrically powered unicycles for use inside buildings.

The way in which an angular rate sensor works is based on what is called the Coriolis force. This is produced when a moving object is forced into a certain path relative to a rotating reference system. For example, if an aircraft flies from the North Pole to a destination on the Equator, it must constantly correct its course to the left to compensate for the Earth's rotation. Apparently, there is a force acting on the aircraft that causes it to veer to the right. On looking more closely, however, one would find that the axis of rotation of the rotating system, the direction of an object moving within it and the Coriolis force acting on it are always exactly at right angles to one another.

This perception leads to the basic structure, shown in Figure 1, of the micromechanical angular rate sensor presented here.

Sensor fabrication is based on silicon technology like that used for the mass production of electronic integrated circuits on silicon wafers. But manufacturing the angular rate sensor also calls for specially developed key technologies that are not needed for normal electronic circuits. These include: a process to generate mobile silicon structures with very precise dimensions; an operation to hermetically encapsulate the sensor elements; and a method to create the vacuum inside the sensor. The proven PSM-X2 process to generate mobile structures was brought up to series manufacturing status by SensorDynamics at its Itzehoe facility. The nucleus of it is a process by which reactive ions generated in a plasma are bombarded vertically onto the wafer surface to etch the required structures out of the silicon.

Figure 3 shows the etching apparatus used for this purpose. It produces extremely sharp-edged structures with a depth of 11 µm and width of only 1 µm.

Figure 4 shows a section of the sensor with its exact micromechanical structures of polycrystalline silicon. The real-life section is approx. 0.12 mm long.

For encapsulation a second top wafer is deposited on the sensor wafer. This is shaped in a way, that, together with the bottom sensor wafer, precisely defined cavities are formed. The two wafers are welded together at high temperature on the edges of these cavities. The material used is a thin gold layer, the gold alloying with the silicon at 400°C to form a eutectic, resulting in a hermetically tight join.

The vacuum inside the sensor is created by means of a getter, heated to absorb all the gas in the cavity. This getter is a thin film made of a zirconium alloy, which subsequently remains in the sensor (see Fig. 2). The vacuum produced in this way will remain constant for decades.

Independent development of the individual components such as the sensor, ASIC and package is no longer adequate to guarantee the requirements made by the sensor system, e.g. extended operating temperature range, shock and vibration resistance as well as fail-safety throughout. Consequently, in an early project phase, SensorDynamics set up parallel development of a performance model of the overall system using the results of the FEM simulation of the sensor element, for example, and those of the circuit simulation.

Another important aspect in the manufacturing of a sensor system for the automotive market is thorough traceability of all fabrication steps and their continuous improvement. Among other things, this is guaranteed by sole use of TS 16949-certified manufacturing equipment and the installation of statistical process control. SensorDynamics also develops special methods to test quality in mass production. Examples are a leakage test at wafer level to inspect the tightness of the encapsulated sensor elements and a new component handler to calibrate angular rate sensors.