Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
2001-07-16
2004-07-13
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
C073S504020
Reexamination Certificate
active
06761068
ABSTRACT:
The present invention relates to sensors and in particular to micromechanical rotation rate sensors and methods for producing the same.
Micromechanical rotation rate sensors have been known for considerable period of time. They comprise one or several micromechanically structured seismic oscillating masses which are subjected to a controlled periodic movement (excitation movement) in a plane (excitation oscillation plane). These seismic oscillating masses are structured and fastened such that they or parts of them are suspended so as to be movable also in a plane at right angles to the excitation oscillation plane. This plane is determined as detection plane. These rotation rate sensors also comprise a detection unit which detects a deflection of the oscillating mass or of the oscillating mass or parts thereof in the detection plane.
The deflection in the detection plane either results from the Coriolis force F
C
on the moved oscillating masses in the case of linear oscillators, or it results from the angular momentum conservation in the case of rotational oscillators. The deflection depends on the rotational speed &OHgr;, which is also referred to as rotation rate, and on the speed of the excitation movement (v or &ohgr;). The deflection is directed perpendicular to the original excitation movement. Hence, the detection unit can convert the detection of the movement in the detection plane into a rotational speed or rotation rate of the sensor:
F
C
=2 mv×&OHgr;
M=&THgr;&ohgr;×&OHgr;
The peculiarity of sensors used for detecting rotation rates e.g. in comparison with accelerations sensors is that an excitation movement of the seismic oscillating masses is required, such an excitation movement being, of course, not necessary in the case of acceleration sensors. The rotation rate can be measured only indirectly via the speed or rotational speed of the additional excitation movement, which should, however, not have any further influence on the detection. This necessitates a large number of degrees of freedom of movement.
Various production technologies and their boundary conditions limit the possibilities of realizing the structures so that the arrangement must be adapted to the production technologies used. The production technology must therefore be compatible with the complex structure of the whole rotation rate sensor.
Various realization principles in combination with different production techniques are known in the prior art.
DE 195 00 800, for example, shows rotation rate sensors comprising two masses which oscillate linearly relative to one another. The masses are formed by structuring polysilicon that has been grown epitactically on a substrate.
DE 195 23 895 shows rotation rate sensors as rotational oscillators utilizing the angular momentum conservation. These rotation rate sensors are produced in a manner which is similar to that described in DE 195 00 800.
DE 195 28 961 shows rotation rate sensors in the form of a tuning fork, the two prongs being structured from different wafers an subsequently combined so as to obtain the sensor.
WO 98/15799 discloses rotation rate sensors with decoupled orthogonal primary and secondary oscillators which can be produced by means of micromechanical processes, micromechanical surfaces technologies or in a way similar to that described in DE 195 00 800.
The known production methods show a plurality of disadvantages. When surface micromechanical techniques are used for structuring on grown polysilicon the oscillating masses, the torsion bars and torsion springs and the suspensions, stresses will occur in the micromechanical element which are caused by the epitactically grown polysilicon. These stresses result in a change in the mechanical behaviour.
Furthermore, epitactically grown polysilicon can only be produced up to a certain height, whereby the sensor element will be limited to a certain height. It follows that sufficient freedom for determining an optimum aspect ratio of structures does not exist; this may have a disadvantageous effect on the dimensioning, the sensor resolution and the interference susceptibility of the micromechanical rotation rate sensor.
Epitactically grown polysilicon is additionally subjected to ageing processes, in particular when it is constantly acted upon by a mechanical load, whereby the properties of the micromechanical rotation rate sensor may deteriorate with time. This kind of micromechanical rotation rate sensor will age.
DE 195 28 961 discloses with regard to the production of rotation rate sensors in the form of a tuning fork that the oscillating masses of the rotation rate sensor are produced from a plurality of SOI wafers, which have been preprocessed by bulk-micromechanical techniques, these oscillating masses being then connected so as to form sensors. A substantial disadvantage of this method is that the prongs of the fork are not adjusted in a sufficiently precise manner relative to one another and that it is difficult and expensive to match the mechanical properties. A further disadvantage is to be seen in the use of bulk-micromechanical technologies for structuring the prongs, whereby the possibilities of dimensioning the structures are strongly limited.
DE 195 26 903 A1 discloses a rotation rate sensor consisting of a multilayer substrate having a lower silicon layer and an upper silicon layer. The two si!icon layers have provided between them an insulating sacrificial layer. The lower silicon layer is implemented as a silicon wafer having a silicon oxide layer, a silicon nitride layer or glass applied thereto as a sacrificial layer. The upper silicon layer is produced by deposition from a plasma or by bonding a further silicon wafer to the sacrificial layer. The upper silicon layer may comprise polycrystalline silicon material, monocrystalline silicon material or a mixture of poly- and monocrystalline silicon materials.
The technical publication by A. Benitez, et al, which is entitled “Bulk Silicon Microelectromechanical Devices Fabricated from Commercial Bonded and Etched-Back Silicon-on-Insulator Substrates”, Sensors and Actuators, A 50 (1995), pp. 99-103, discloses so-called BESOI substrates. BESOI stands for “bonded and etched-back silicon-on-insulator”. A BESOI substrate is produced as follows. A starting substrate consisting of silicon is provided with an intermediate silicon dioxide layer. This intermediate silicon dioxide layer has applied thereto a further silicon wafer by means of wafer bonding or by means of wafer fusion, whereupon one of the two silicon wafers is etched back to the desired thickness. The silicon dioxide layer provided between the two wafers can be used as a sacrificial layer so as to produce electrostatic micromotors, microturbines and electrostatically operated microrelays.
It is the object of the present invention to provide a concept for micromechanical rotation rate sensors that can be produced at a moderate price, this concept permitting in addition a great freedom of design.
This object is achieved by a micromechanical rotation rate sensor according to claim
1
and by a method for producing a micromechanical rotation rate sensor according to claim
14
.
One advantage of the present invention resides in the fact that it provides micromechanical rotation rate sensors which consist of a suitable material and in the case of which a very great freedom of design exists, without any necessity of matching the structures.
The present invention is based on the finding that the concept of a one-part rotation rate sensor, which is produced from a single wafer with epitactically grown polysilicon or with an SOI structure, must be departed from so as to achieve an optimum freedom of design and an economy-priced producibility. Hence, a micromechanical rotation rate sensor according to the present invention comprises a substrate wafer arrangement, a structural wafer arrangement in which there are defined at least one seismic mass, the suspension of this seismic mass and at least one spring means for connecting the suspension to the sei
Fraunhofer-Gesellschaft zur Foerderung der Angewandten Forschung
Glenn Michael A.
Glenn Patent Group
Kwok Helen
LandOfFree
Micromechanical rotation rate sensor and method for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Micromechanical rotation rate sensor and method for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Micromechanical rotation rate sensor and method for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3212581