Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-08-03
2001-05-15
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06230567
ABSTRACT:
FIELD OF INVENTION
This invention relates to a low thermal strain flexure support for a micromechanical device.
BACKGROUND OF THE INVENTION
Flexure supports are used to suspend a micromechanical device from a substrate. In accelerometer and gyro applications the micromechanical device contains a proof mass. In pendulous accelerometers the proof mass is suspended from a substrate by flexures which extend beyond the longitudinal edges of the proof mass to anchors mounted on the substrate. A strain relief beam is formed in the flexure support region and this serves to minimize the fabrication stresses that are transferred to the proof mass from the substrate. In order to make the device both sensitive at the milli-g level and manufacturable with a single silicon thickness the flexure is narrow in width and has a high aspect ratio.
Although the strain relief beam used gives an acceptable level of stress on the proof mass, after fabrication the structure is not thermally stable: the anchor regions are spaced relatively far apart and there is a thermal stress imparted by the glass substrate to the flexures. The strain relief beam mitigates the situation from the standpoint of structural fracture but the concern is that small imperfections in the structure result in a rotational stress which tilts the proof mass under thermal load. Any tilt of this nature is differential and will show up as a bias drift. Such a tilt will likely be very small. However, the accelerometer is extremely sensitive.
Because of the inherent stability of silicon a millimeter sized device can routinely sense accelerations in the milli-g level. With the typical device dimensions used this means sensing an acceleration induced tilt of about 1 Angstrom per milli-g. In other words the device bias is extraordinarily sensitive to thermally induced rotation of the proof mass.
An attempt was made recently to support a micromechanical device or proof mass on a single anchor. In theory a single point mount will decouple all stress between the glass substrate and the silicon device. The single point mount is acceptable in a device which is very stiff and measures acceleration in the 10 to 100 g level, for example. However, when the flexures must be made weaker to sense acceleration at the milli-g level a problem develops with rotational stiffness. If a single thickness of silicon is used the flexure has high aspect ratio and a narrow width; the device becomes very weak in rotation, and has a low yield in fabrication due to breakage.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an improved low thermal strain flexure support for a micromechanical device.
It is a further object of this invention to provide such an improved low thermal strain flexure support for a micromechanical device which can be made with a single layer etch resistant silicon process.
It is a further object of this invention to provide such an improved low thermal strain flexure support for a micromechanical device which can be made in a single layer process compatible with other low cost processes.
It is a further object of this invention to provide such an improved low thermal strain flexure support for a micromechanical device which provides the rotational stiffness comparable to that of “outboard” anchor designs but has approximately an order of magnitude lower thermal stress as induced by the thermal mismatch between substrate and micromechanical device.
It is a further object of this invention to provide such an improved low thermal strain flexure support for a micromechanical device which permits the distance between anchors to be substantially reduced.
It is a further object of this invention to provide such an improved low thermal strain flexure support for a micromechanical device which has lower noise, better bias stability, greater ruggedness, lower cost and can be applied to other designs and micromechanical devices.
The invention results from the realization that a truly reliable and improved low thermal strain flexure support for a micromechanical device such as a proof mass can be effected by folding the anchors inboard of the periphery of the proof mass so that the distance between the anchors is reduced thereby reducing differential thermal effects occurring between the substrate and proof mass but preserving the sensitivity and reducing rotational errors by employing support members which extend outwardly from the anchors to mount flexures which extend inwardly to suspend the proof mass from substrate.
This invention features a low thermal strain flexure support for a micromechanical device including a substrate and a micromechanical device having a rotational axis and a longitudinal axis. An anchor structure is disposed on the substrate proximate the longitudinal axis of the micromechanical device and there are first and second support members extending outwardly oppositely from the anchor structure. First and second flexures extend inwardly in the direction of the axis of rotation of the micromechanical device from the first and second support members, respectively, to the micromechanical device for suspending the micromechanical device from the substrate.
In a preferred embodiment the anchor structure may include at least two anchor device disposed on either side of the longitudinal axis and they may be symmetrically disposed about it. The anchor structure may include at least two anchor devices disposed on either side of the rotational axis and they may be symmetrical about that axis. Each anchor device may include a pair of anchor elements, the anchor elements within each pair being symmetrically disposed about the rotational axis and the pairs of anchor elements being symmetrically disposed about the longitudinal axis. The substrate may include glass and the micromechanical device may include silicon. The support members may extend approximately to the longitudinal edge of the micromechanical device. The micromechanical device may be a pendulous accelerometer with a proof mass.
REFERENCES:
patent: 5126812 (1992-06-01), Greiff
patent: 5581035 (1996-12-01), Greiff
patent: 5635739 (1997-06-01), Greiff et al.
patent: 5646348 (1997-07-01), Greiff et al.
patent: 5969250 (1999-10-01), Greiff
Iandiorio & Teska
Moller Richard A.
The Charles Stark Draper Laboratory Inc.
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