Laterally movable gate field effect transistors for...

Details Laterally movable gate field effect transistors for... Laterally movable gate field effect transistors for...

1999-02-12

2001-03-20

Crane, Sara (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Physical deformation

C257S415000, C257S252000, C257S253000

Reexamination Certificate

active

06204544

ABSTRACT:

This invention pertains to transistors with laterally movable gates, and to the application of such transistors to microsensors and microactuators.
There is a continuing, unfilled need for improved microsensors and microactuators. Current microsensor technologies rely on principles such as measuring extremely small changes in capacitance of a moving part. The inherent difficulty of measuring such small changes in capacitance is compounded by large stray capacitances attributable to bonding pads or to relatively long electrical connections. Many currently available microsensors exhibit low sensitivity, low mechanical stability, and nonlinearity.
D. Edmans et al., “Micromachined accelerometer with a movable gate transistor sensing element,”
Proceedings of
1997
SPIE Symposium on Micromachining and Microfabrication, SPIE
vol. 3224, pp. 37 ff (Austin, Tex, Sep. 29-30, 1997); A. Yoshikawa et al., “Properties of a movable-gate-field-effect structure as an electromechanical sensor,”
J. Acoust. Soc. Am.,
vol. 64, pp. 725-730 (1978); and H. Nathanson et al., “The resonant gate transistor,”
IEEE Trans. Electron Devices,
vol. ED-14, pp. 117-133 (1967) each disclose transistors having a moving gate, in which the gate moves perpendicular to the substrate, for example as a cantilever. The gate thus always remains over the active channel area of the device, with the distance to the channel varying. The response of such a device is generally nonlinear. Also, the vertical range of travel is low (on the order of 1 &mgr;m); and a relatively large actuating voltage is required to accomplish this small range of travel.
We have discovered a field effect transistor whose output is modulated by a control gate that is movable in a direction parallel to the transistor substrate surface. The device is capable of lateral motion with large amplitude. With appropriate selection of gate materials and design, the extent of lateral motion may be tens of micrometers—10 &mgr;m, 20 &mgr;m, or even more. Prior techniques for making perpendicularly movable gates are not applicable to the case of a laterally movable gate, particularly due to the need to maintain a gap between the gate and an insulator-coated substrate that is constant within a tolerance on the order of a fraction of a micrometer. The novel transistors may be used in microsensors or microactuators. When used as a microsensor, the gate may be driven parallel to the substrate by the force to be measured (such as an inertial force). When used as a microactuator, the gate may be driven by an actuating drive such as an electrostatic, magnetic, or thermal actuation, and the measured position of the gate provides feedback to control the operation of the actuator.
The novel device allows measurement or control of the displacement of a mechanical structure in a direction parallel to the transistor substrate. The in-plane motion of the mechanical structure, induced by an inertial force, an electrostatic force, or other force is measured by the change in the FET current. The signal from the transistor can be used as feedback to control the in-plane motion of the structure if desired. The gate of the FET is a part of (or is connected to) the structure in motion. The change in the output current of the transistor is a linear function of the in-plane displacement of the gate, allowing direct mechanical displacement-to-current conversion to be achieved. This direct, linear conversion provides improved sensitivity, improved signal-to-noise ratio, and improved signal conditioning and signal processing. It allows precise measurement, actuation, and control of the lateral displacement.
With the movable gate outside the channel, the conduction channel of the transistor may be either normally conducting (depletion type) or normally off (enhancement type). The position of the gate over the channel modulates the channel conductance. By suitable choice of geometries, the device may be designed so that, within a suitable range of applied gate bias voltages, the position of the gate is directly proportional to the transistor source-to-drain current; i.e., the response is linear over this range.
The gate may be formed as a thin film or a high aspect ratio structure, with an initial position either over, partially over, or outside the transistor conduction channel. The channel is coated with an insulator, preferably on the order of tens of nm thick. The insulator may, for example, comprise silicon dioxide, silicon nitride, or a two-layer insulator in which a silicon dioxide layer directly contacts the underlying substrate, and a silicon nitride layer covers the silicon dioxide layer. In a preferred manufacturing technique, the gate is fabricated as a high aspect ratio mechanical structure by a process such as deep X-ray lithography and electroforming, or as a thin film structure made by surface micromachining. The problem of stray capacitance encountered by other devices is avoided. The nature of the novel transistor readily admits feedback capability.
Advantages of the invention include a direct, linear relation between the mechanical position of the gate and the drain current of the transistor, a high sensitivity to force or position, the possibility of efficient closed-loop feedback control, compatibility with very large scale integrated circuit fabrication, high signal-to-noise ratio due to high sensor sensitivity and low parasitic capacitance, and a high quality factor due to low damping of motion.
Applications of the novel device include the following: electromechanical switches, position sensors, actuators, manipulators, frequency-based signal processors, accelerometers, multi-dimensional accelerometers, automotive sensors, sensors for manufacturing process monitoring and control, gyroscopes, and aviation systems.


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Edmans, D. et al., “Micromachined accelerometer with a movable gate transistor sensing element,” Proceedings of 1997 SPIE Symposium on Micromachining and Microfabrication, SPIE vol. 3224, pp. 37 ff Austin, Tx, Sep. 29-30, 1997).
Kloeck, B. et al., “Motion Investigation of Electrostatic Servo-Accelerometers by Means of Transparent ITO Fixed Electrodes,” Hitachi Research Laboratory,IEEEpp. 108-111 (1991).
Nathanson, H. et al., “The resonant gate transistor,”IEEE Trans. Electron Devices, vol. ED-14, pp. 117-133 (1967).
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Stadler, S., “Integration of a post-fabricated accelerometer on a chip with standard CMOS circuitry using deep X-ray lithography,” MS thesis, Louisiana State University, Baton Rouge, LA (Aug. 1997).
Stadler, S. et al., “Integration of LIGA structures with CMOS circuitry,” Proc. SPIE Symp. Smart Structures and Materials, vol. 3046, pp. 230-241, San Diego, CA (Mar. 4-6, 1997).
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Yoshikawa, A. et al., “Properties of a movable-gate-field-effect structure as an electromechanical sensor,”J. Acoust. Soc. Am., vol. 64, pp. 725-730 (1978).

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