Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1997-06-18
2001-05-01
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514380
Reexamination Certificate
active
06223598
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates in general to a suspension arrangement for a semiconductor accelerometer, and more specifically to such an arrangement which provides complete symmetry to the entire device.
Surface micromachining is an IC-compatible technology in which a thin film microstructure material (typically polysilicon) is deposited and patterned on a sacrificial spacer layer (typically silicon dioxide), and the spacer is subsequently removed to release the free-standing microstructures. This technology has been used by Analog Devices, Inc. to fabricate integrated accelerometers. The devices typically include a moving mass and fixed cantilever electrodes that are interdigitated within electrodes associated with the moving mass. When the structure experiences an acceleration in the sensitive axis the capacitance between the fixed and moving fingers is changed, and a measurable signal results. Suspension tethers are designed to resist motion in the non-sensitive in-plane axis.
Due to the very small scale dimensions of the elements of a semiconductor accelerometer, particularly the moving mass, several problems can occur. Since the materials used, e.g., polysilicon, result in a uniform stress gradient through the thickness of elements such as the mass, the elements tend to bow or warp. In addition the tensile stress of the mass tends to cause shrinkage. If portions of the mass are too low, thus closer to the substrate surface, there is the possibility of contact and sticking. If the mass contacts or sticks to the underlying substrate, the acceleration measurements become inaccurate. Also, the bowing or warping of elements can result in the electrode fingers not aligning well, thus affecting measurement performance. In all, such problems can be optimized by trimming the responses, however, such techniques are time consuming and expensive.
SUMMARY OF THE INVENTION
Accordingly, in one exemplary embodiment of the invention there is provided a semiconductor device including a substrate; a layer suspended above and spaced apart from the surface of the substrate; and a plurality of suspension members coupled between the substrate and the layer to allow limited movement of the layer relative to a plane parallel to the substrate. Each of the suspension members includes a plurality of symmetrically configured flexible arms.
In an alternative exemplary embodiment of the invention there is provided a semiconductor accelerometer including a semiconductor substrate; a layer of conductive material suspended above and spaced apart from the surface of the substrate, the layer including at least one first capacitive element; at least one second capacitive element associated with the substrate positioned in parallel juxtaposition with the at least one first capacitive element, the first and second capacitive elements forming variable capacitances which vary in accordance with acceleration of movement of the first layer; and a plurality of suspension members coupled between the substrate and the layer to allow limited movement of the layer relative to a plane parallel to the substrate. Each of said suspension members includes a plurality of symmetrically configured flexible arms.
In another exemplary embodiment of the invention there is provided a mounting arrangement for a suspended layer in a semiconductor accelerometer, the layer being suspended above and spaced apart from a substrate. The arrangement includes a common anchor point associated with the substrate; first and second anchor points associated with the suspended layer; a first flexible serpentine arm coupled between the first anchor point and the common anchor point; and a second flexible serpentine arm coupled between the second anchor point and the common anchor point. The first and second flexible serpentine arms are symmetrically configured with respect to one another.
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Ahmad et al., “A Two-Dimensional Micromachined Accelerometer,”IEEE Transactions Instruments and Measurement,vol. 46, No. (Feb. 1997), pp. 18-26.
Analog Devices Inc.
Kwok Helen
Samuels , Gauthier & Stevens, LLP
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