Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
1999-05-06
2001-06-19
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S052000
Reexamination Certificate
active
06248610
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the detection and measurement of forces and more particularly to an improved accelerometer incorporating one or more vibrating force transducers for measuring the force applied to a proof mass. The present invention also relates to a method for manufacturing the accelerometer.
A widely used technique for force detection and measurement employs a mechanical resonator having a frequency of vibration proportional to the force applied. In one such mechanical resonator, one or more elongate beams are coupled between an instrument frame and a proof mass suspended by a flexure. An electrostatic, electromagnetic or piezoelectric force is applied to the beams to cause them to vibrate transversely at a resonant frequency. The mechanical resonator is designed so that force applied to the proof mass along a fixed axis will cause tension or compression of the beams, which varies the frequency of the vibrating beams. The force applied to the proof mass is quantified by measuring the change in vibration frequency of the beams.
Recently, vibratory force transducers have been fabricated from a body of semiconductor material, such as silicon, by micromachining techniques. For example, one micromachining technique involves masking a body of silicon in a desired pattern and then deep etching the silicon to remove portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical resonator device, such as a rate gyroscope or an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, “Method of Making an Electrostatic Silicon Accelerometer” and U.S. Pat. No. 4,945,765 “Silicon Micromachined Accelerometer”, the complete disclosures of which are incorporated herein by reference.
In one method of fabricating force detecting devices, a thin layer of silicon, on the order of about 20 micrometers thick, is epitaxially grown on a planar surface of a silicon substrate. The epitaxial layer is etched in a suitable plasma, to form the vibrating components of one or more vibratory force transducers (i.e., vibrating beams and electrodes). The opposite surface of the substrate is etched to form a proof mass suspended from a stationary frame by one or more flexure hinges. While the opposite surface of the substrate is being etched, the epitaxial layer is typically held at an electric potential to prevent undesirable etching of the epitaxial layer. The beams and the electrodes of the transducer are electrically isolated from the substrate by back biasing a diode junction between the epitaxial layer and the substrate. The transducer may then be coupled to a suitable electrical circuit to provide the electrical signals required for operation. In silicon, electrostatically driven, vibrating beam accelerometers, for example, the beams are capacitively coupled to an oscillating circuit.
The above-described method of manufacturing force detection devices suffers from a number of drawbacks. One such drawback is that the beams and electrodes of the vibratory force transducer(s) are often not sufficiently electrically isolated from the underlying substrate. At high operating temperatures, for example, electric charge or current may leak across the diode junction between the substrate and the epitaxial layer, thereby degrading the performance of the transducer(s). Another drawback with this method is that it is difficult to etch the substrate without etching the epitaxial layer (even when the epitaxial layer is held at an electric potential). This undesirable etching of the epitaxial layer may reduce the accuracy of the transducer.
Another drawback with existing force detection devices, such as accelerometers, is that they have often an asymmetrical design, which may reduce the accuracy of these devices, particularly in high performance applications. For example, the proof mass flexure hinge is typically etched on the opposite surface of the substrate to the transducers. This produces an asymmetrical device because the input axis of the accelerometer (i.e., the axis about which the proof mass rotates) is skewed relative to the center of the proof mass. In addition, the transducers are both typically formed on a surface of the active layer, thereby locating both transducers on one side of the proof mass hinge. This asymmetrical transducer design often creates non-linear response characteristics, which may be difficult to correct during high performance applications, such as aircraft and missile guidance.
What is needed, therefore, are improved apparatus and methods for detecting and measuring forces, such as the force resulting from the acceleration of a proof mass, and improved methods for manufacturing these force detecting apparatus. These methods and apparatus should effectively electrically isolate the vibratory force transducers from the proof mass and instrument frame to improve transducer performance at high operating temperatures. In addition, the force detecting apparatus should be designed more symmetrically to increase the accuracy of the transducers, particularly in high performance applications.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for detecting and measuring forces with mechanical resonators and improved methods of manufacturing these force detecting apparatus. The methods and apparatus of the present invention are useful in a variety of applications, such as angular rate sensors and gyroscopes, and particularly useful for measuring acceleration, such as the acceleration of a miniature proof mass in a micromachined accelerometer.
The apparatus of the present invention includes a semiconducting substrate and first and second active layers coupled to the opposite surfaces of the substrate. The substrate has a frame and a proof mass suspended from the frame by one or more flexures for rotation about an input axis in response to an applied force. The active layers include symmetrical vibratory force transducers mechanically coupled to the proof mass for detecting a force applied to the proof mass. With this configuration, the transducers are located on either side of the substrate (i.e., on either side of the input axis). Fabricating the transducers on opposite sides of the input axis improves the differential design symmetry of the force detecting apparatus. When signals are combined, this differential design symmetry reduces non-linear response characteristics, particularly in high performance applications where high vibration is present.
In a specific configuration, an insulating layer is formed between the substrate and the active layers to insulate the active layers from the substrate. Providing separate insulating layers between the substrate and active layers improves the electrical insulation between the proof mass and the transducers, which allows for effective operation over a wide range of temperatures. Preferably, the substrate and active layers are made from a silicon or polysilicon material, and the insulating layers comprise a thin layer (e.g., about 0.1 to 10 micrometers) of oxide, such as silicon oxide. The silicon oxide layers retain their insulating properties over a wide temperature range to ensure effective transducer performance, for example, at high operating temperatures on the order of above about 70° C. In addition, the insulating layers inhibit undesirable etching of the active layers while the substrate is being etched, which improves the accuracy of the apparatus. In one embodiment, the substrate comprises a single, double-sided Silicon-On-Insulator (SOI) substrate having oxide layers and active layers on either side of the substrate. In another embodiment, a pair of single-sided SOI substrates (i.e., each substrate having a single oxide layer and active layer on one side) are bonded together, preferably by molecular level thermal bonding techniques, such as anionic bonding.
In a preferred configuration, the flexure hinges of t
Leonardson Ronald B.
Woodruff James R.
Allied-Signal Inc.
Picardat Kevin M.
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