Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Passive components in ics
Utility Patent
1999-05-05
2001-01-02
Hardy, David (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Passive components in ics
C257S528000, C257S075000
Utility Patent
active
06169321
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and systems for annealing a microstructure formed on a substrate and, in particular, to methods and systems for locally annealing a microstructure formed on a substrate and devices formed thereby.
BACKGROUND ART
Many devices require post-fabrication trimming in order to operate within specifications. In particular, sensors and references (e.g., frequency references) require such trimming. For the case of macroscopic devices that are manufactured serially, trimming often does not comprise an overwhelming percentage of the total device cost. For the case of microscopic devices fabricated in batches (e.g., integrated circuits or micromechanical devices), trimming or programming can constitute a dominant percentage of device cost if it must be done serially. For example, laser trimming of micromechanical resonators to achieve a specific resonance frequency usually must be done serially, and thus, has low throughput and high cost.
With the advent of frequency specific applications for micromechanical resonators, such as oscillator references and highly selective bandpass filters, techniques for post-fabrication trimming of resonance frequencies are becoming increasingly important. This is especially true for recent communications applications of micromechanical resonators, in which large numbers of such resonators with precisely located center frequencies must realize parallel filter banks and multiple oscillator references. Since these applications will likely be batch-fabricated using planar technologies, high throughput trimming is desirable.
It is known that rapid thermal annealing (RTA) can change stress profiles in polysilicon thin films.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a device and a method and system for locally annealing a microstructure such as a micromechanical device in situ on a substrate of the resulting device without affecting any other microstructure formed on the substrate.
Another object of the present invention is to provide a method and system for locally annealing a microstructure such as a micromechanical device in situ on a substrate which are especially useful in a batch-mode processing of such devices.
Yet another object of the present invention is to provide a method and system for locally annealing a micromechanical device such as a micromechanical resonator while the resonator is operating on a substrate to achieve a specific resonance frequency and Q factor enhancement at a relatively high throughput and low cost due largely to smaller thermal time constants for the microscale structure.
Yet still another object of the present invention is to provide an electronically-based method and system for locally annealing a micromechanical device such as a resonator in situ on a substrate to allow fabrication at relatively low temperatures which may, in turn, enable merged circuit and microstructure technologies and also allow a wide frequency trimming range without large DC voltages. For example, polysilicon structural material may be deposited amorphous at a low temperature then locally annealed into polycrystalline material which has better material characteristics.
In carrying out the above objects and other objects of the present invention, a method is provided for locally annealing a predetermined microstructure formed on a substrate. The method includes the step of controllably raising the energy state of the predetermined microstructure over a period of time sufficient to change material and/or microstructural properties of the predetermined microstructure without substantially affecting any other microstructure formed on the substrate.
The predetermined microstructure is preferably a micromechanical device such as a micromechanical resonator having a resonance frequency and a Q factor. The step of controllably raising is preferably accomplished by controllably heating the resonator sufficient to cause not only the resonance frequency but also the Q factor of the resonator to change.
Preferably, the method further includes the steps of causing the resonator to oscillate and monitoring the resonance frequency. The step of controllably heating heats the resonator until the resonator has a desired resonance frequency.
In one embodiment, the micromechanical resonator includes a beam.
In another embodiment, the micromechanical resonator is a folded-beam micromechanical resonator. In both embodiments, the micromechanical resonator is typically a polysilicon resonator.
The microstructure typically has a resistance and wherein the step of controllably heating includes the step of causing an electrical current to flow through the predetermined microstructure to heat the predetermined microstructure.
Also, preferably, the method further includes the steps of forming a pair of electrodes on the substrate electrically coupled to the predetermined microstructure and applying an electrical signal at the electrodes. The electrical signal may be a DC signal, but is preferably a time-varying signal such as a signal having one or more pulses.
Preferably, the substrate is a semiconductor substrate, but it also may be glass or other substrates. The semiconductor substrate may be a silicon semiconductor substrate.
The predetermined microstructure may be a semiconductor microstructure such as a silicon semiconductor microstructure. In one embodiment, the predetermined microstructure initially has an amorphous silicon microstructure and wherein the step of controllably heating changes the amorphous silicon microstructure to a polycrystalline or crystalline silicon microstructure.
The microstructure may form part of a microelectromechanical device.
Further in carrying out the above objects and other objects of the present invention, a system is provided for locally annealing a predetermined microstructure formed on a substrate of a device. The system includes an annealing power supply and means adapted to be coupled to a microscopic part of the device for transferring power from the annealing power supply to the microscopic part of the device in the form of an electrical signal so that microscopic part of the device converts the transferred power to a controlled amount of heat over a period of time sufficient to change material and/or microstructural properties of the predetermined microstructure without significantly affecting any other microstructure formed on the substrate.
Still further in carrying out the above objects and other objects of the present invention, a device is provided having at least one microstructure formed on a substrate. The device has electrodes formed on the substrate and electrically coupled to a predetermined microstructure to receive an electrical signal. The electrical signal causes an electrical current to flow through the predetermined microstructure to controllably and directly heat the predetermined microstructure over a period of time sufficient to change material and/or microstructural properties of the predetermined microstructure without affecting any other microstructure formed on the substrate.
Yet still further in carrying out the above objects and other objects of the present invention, a device is provided having at least one microstructure formed on a substrate. The device includes a resistive heating element formed on the substrate immediately adjacent a predetermined microstructure and adapted to receive a signal. The signal causes the element to indirectly heat the predetermined microstructure over a period of time sufficient to change material and/or microstructural properties of the predetermined microstructure without affecting any other microstructure on the substrate.
Preferably, the device further includes a microplatform thermally isolated from the substrate and wherein the resistive heating element and the microstructure are formed on the microplatform.
The method of the invention is a batch-mode trimming technique for micromechanical devices such as resonators by which the quality factor of such micromechanical resonators can be i
Nguyen Clark T.-C
Wang Kun
Brooks & Kushman P.C.
Hardy David
The Regents of the University of Michigan
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