Micromachined strain sensor

Details Micromachined strain sensor Micromachined strain sensor

1999-01-15

2003-08-19

Williams, Hezron (Department: 2855)

Measuring and testing

Specimen stress or strain, or testing by stress or strain...

Specified electrical sensor or system

Reexamination Certificate

active

06606913

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains generally to the field of micromechanical structures and processes for producing such structures.
BACKGROUND OF THE INVENTION
Photolithographic techniques similar to those used in semiconductor processing have been applied to the formation of micromechanical and microelectrical mechanical (MEMS) structures. In the lithographic processes used for producing such structures, a particular challenge is the control of the residual strain in the formed structures. For example, residual compressive strain in freed structures such as bridges, cantilevers and diaphragms can result in the unwanted deflection or distortion of such structures. The residual strain is affected by a number of manufacturing parameters, and small changes in such parameters can significantly impact the performance of the micromechanical devices. For example, a capacitive pressure sensor with a diaphragm that is 2 &mgr;m thick and 2mm on the side will lose sensitivity by a factor of 25 for residual tensile stress of just 30 MPa. Residual stress is affected not only by manufacturing parameters such as deposition temperature, pressure, precursor concentrations, post-anneal conditions, etc., but also by packaging variables such as die attach materials, and deployment conditions such as operating temperature and humidity. A number of micromachined strain sensors have been developed in the past to monitor this important material property with the dimensional resolution of a few hundreds microns. Although some of these require mechanical actuation (See O. Tabata, K. Kawahata, S. Sugiyama, I. Igarashi, “Mechanical property measurements of thin films using load-deflection of composite rectangular membrane,” MEMS '89, pp. 152-156, 1989), most involve passive structures that are designed to deform measurably under the residual stress when they are released from the substrate. The measurement and control of residual strain is particularly significant in the formation of polysilicon microstructures, as carried out, for example, in U.S. Pat. No. 4,897,360, which also discusses strain sensitive structures that may be used to assess the level of residual strain in the formed materials. See also Y. B. Gianchandani and K. Najafi, “Bent-Beam Strain Sensors,” JMEMS 5(1), pp. 52-58, March, 1996; H. Guckel, D. Burns, C. Rutigliano, E. Lovell, B. Choi, “Diagnostic microstructures for the measurement of intrinsic strain in thin films,” J. Micromech. Microeng., 2, pp. 86-95, 1992; M. Mehregany, R. Howe, S. Senturia, “Novel microstructures for the in situ measurement of the mechanical properties of thin films,” J. Appl. Phys. 62 (9), pp. 3579-3584, Nov. 1, 1987; L. Lin, R. Howe, A. Pisano, “A passive in situ micro strain gauge,” MEMS '93, pp. 201-206, 1993; L. B. Wilner, “Strain and strain relief in highly doped silicon,” Hilton Head '92, pp. 76-77, June 1992; J. F. L. Goosen, B. P. van Drieenhuisen, P. J. French, R. F. Wolffenbuttel, “Stress measurement structures for micromachined sensors,” Transducers '93, July 1993. These deformations are measured visually, sometimes using a micromachined vernier. Although convenient in a laboratory setting, this method is not amenable to high volume manufacturing. More importantly, it renders the device useless for post-packaging or post-deployment readout, eliminating many conceivable applications.
If a strain sensor with electronic readout could be co-fabricated or co-packaged with another device such as an accelerometer, gyroscope, or pressure sensor, the system accuracy can be improved by offering real-time or test-mode calibration over the lifetime of the system. A method for electronic readout has been developed in which a micromachined bridge is deflected by applying a voltage bias to an electrode located under it, generally to the point that the suspension collapses. K. Najafi, K. Suzuki, “A novel technique and structure for the measurement of intrinsic stress and Young's modulus of thin films,” MEMS '89, pp. 96-97; P. M. Osterberg and S. D. Senturia, “M-TEST: A test chip for MEMS material property measurement using electrostatically actuated test structures,” JMEMS 6(2), June 1997, pp. 107-118. Its usage is constrained in some cases: (a) the fabrication process must permit the inclusion of the electrode; (b) the vertical deflection (perpendicular to the substrate) might not provide accurate data for devices such as the accelerometers and gyroscopes that are designed to deflect laterally (in-plane), particularly when the structural material is anisotropic, such as single crystal silicon or polysilicon with a preferential grain orientation; (c) stiction forces may prevent recovery from collapse, raising concerns about the lifetime of the device and the repeatability of a measurement; and (d) these structures generally are not suitable for compressive materials since they may buckle and collapse.
SUMMARY OF THE INVENTION
In accordance with the invention, a micromachined strain sensor is provided which can be incorporated with other micromechanical and microelectronic devices on a substrate such as a semiconductor chip. The strain sensor can be incorporated in a sealed package with other microelectrical and micromechanical components with the residual strain monitored electronically from outside the package. The residual strain in the micromechanical structural elements can thus be monitored conveniently and economically in a production environment, and, if desired, can be monitored over the life of the component to account for changes in structural properties of the micromechanical materials due to changes in environmental conditions, such as temperature, as well as effects due to aging. The strain sensors in accordance with the invention are fully compatible with conventional planar micromachining of common micromechanical materials such as polysilicon, without requiring significant additional device forming steps beyond those required for formation of the micromechanical devices.
The micromachined strain sensor of the invention is formed on a substrate, such as a semiconductor wafer, having a top surface. A microstructural beam member of the strain sensor is anchored to the substrate at one position and has a portion which, during formation of the sensor, is freed from the substrate and which extends over the top surface of the substrate. At least one electrically conductive displaceable tine is connected to the microstructural beam member to be displaced by the member as it is freed from the substrate. A mating electrically conductive tine is mounted to the substrate at a position adjacent to the displaceable tine such that a capacitor is formed between the adjacent tines. Preferably, there are a plurality of displaceable tines and a plurality of mating tines, with the sets of displaceable tines and mating tines connected together in parallel to increase the effective overall capacitance. The microstructural member is formed from a microstructural material, such as polysilicon or electroplated metal, which, for example, is deposited on a sacrificial layer on the substrate. Preferably, the intended micromechanical structures are also formed from the same material at the same time. The material from which the microstructural member is formed may have an intrinsic built-in strain, either compressive or tensile, in its as-deposited form on the sacrificial layer. The mating tines are also preferably formed on the sacrificial layer in a position parallel to and adjacent to the displaceable tines at known spaced positions from the displaceable tines. In the typical production of micromechanical devices, the sacrificial layer is etched away, freeing the microstructural member from the substrate except at its anchor position. When so freed, the microstructural member will tend to expand or contract, depending on whether the built-in strain is compressive or tensile, thereby moving the displaceable tines either toward or away from the mating tines and thereby changing the effective capacitance between the sets of displaceab

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