1. Field
The present invention relates to microelectromechanical devices and especially to a gyroscope structure and a gyroscope, as defined in the preambles of the independent claims.
2. Description of the Related Art
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronics can be fabricated on the same piece of silicon to enable advanced devices.
MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. For example, a microelectromechanical gyroscope can be applied to quickly and accurately detect very small angular displacements. Motion has six degrees of freedom: translations in three orthogonal directions and rotations around three orthogonal axes. The latter three may be measured by an angular rate sensor, also known as a gyroscope. MEMS gyroscopes use the Coriolis Effect to measure the angular rate. When a mass is moving in one direction and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. The resulting physical displacement caused by the Coriolis force may then be read from, for example, a capacitively, piezoelectrically or piezoresistively sensing structure.
In MEMS gyros the primary motion is typically not continuous rotation as in conventional ones due to lack of adequate bearings. Instead, mechanical oscillation may be used as the primary motion. When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results. This creates a secondary oscillation orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular rate.
Gyroscopes are very complex inertial MEMS sensors. The basic challenge in gyroscope designs is that the Coriolis force is very small and therefore the generated signals tend to be minuscule compared to other electrical signals present in the gyroscope. Spurious responses and susceptibility to vibration plague many MEMS gyro designs.
In an advanced prior art MEMS gyro design, an external applied angular velocity is configured to induce to two parallelly positioned planar seismic masses an opposite phase motion about a common axis of rotation. This motion can be detected with electrodes positioned above the plane of the seismic masses. With the explicit oscillation directions of the specific prior art configuration, the primary mode oscillation and the detection mode oscillation are effectively kept apart so that a robust sensor structure that is highly insensitive to external shocks has been provided.
Typically a cover or cap, fixed to the substrate or to a functional layer, encases the MEMS gyroscope structure, forming a casing that protects the MEMS gyroscope against external conditions. The challenge with MEMS is, however, to provide environmental protection that does not restrict movement of the mobile parts of the structure. For example, in the above prior art structure, the seismic masses and the excitation structures are in a structure wafer that is enclosed between a handle wafer and a cap wafer. Traditional accelerometers and gyroscopes have been considered as one of the easiest MEMS packages because they have no mechanical contact with outside world. However, in the above prior art gyroscope structure, the sensing electrodes have been patterned to the cap wafer. This makes the structure more vulnerable to deviations from the designed dimensions and increases complexity of the sensor packaging, since traditional epoxy overmolding processes cannot be used.