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 particu- lar, 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 reso-

nance frequency usually must be done serially, and thus, has low throughput and high cost.

With the advent of frequency specific applica- tions 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 applica- tions 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 applica- tions 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.

Summarv 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 resona- tor 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, poly- silicon structural material may be deposited amorphous at a low temperature then locally annealed into poly- crystalline material which has better material charac- teristics.

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 predeter- mined 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 sub- strate.

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 accom- plished 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 monitor- ing the resonance frequency. The step of controllably heating heats the resonator until the resonator has a desired resonance frequency.

In one embodiment, the micromechanical resona- tor 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 sub- strate electrically coupled to the predetermined micro- structure 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 semiconduc- tor substrate.

The predetermined microstructure may be a semiconductor microstructure such as a silicon semicon- ductor microstructure. In one embodiment, the prede- termined microstructure initially has an amorphous silicon microstructure and wherein the step of control- lably heating changes the amorphous silicon microstruc- ture to a polycrystalline or crystalline silicon micro- structure.

The microstructure may form part of a micro- electromechanical device.

Further in carrying out the above objects and other objects of the present invention, a system is provided for locally annealing a predetermined micro- structure 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 predeter- mined microstructure to receive an electrical signal.

The electrical signal causes an electrical current to flow through the predetermined microstructure to con- trollably and directly heat the predetermined micro- structure 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 micro- structure 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 micro- structure 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 micro- mechanical resonators can be increased or decreased for electronic filtering, oscillator, and gyroscopic appli- cations.

The method and system achieve both controlla- ble frequency trimming and quality factor enhancement in micro-scale mechanical devices. The method and system are electrically-activated and can be implemented on a massive scale, in a similar fashion to programming an EEPROM. The method and system both allow for post- fabrication frequency trimming of microresonator devic- es, and through its Q-enhancement capability, enable low temperature fabrication of high-Q micro-scale resona- tors, which may in turn enable merged circuit+micro- structures processes where structures are fabricated after MEMS.

The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.

Brief Description Of The Drawings FIGURE 1 is a schematic diagram of a relative- ly simple micromechanical resonator and a circuit for applying a voltage across anchors of the resonator to directly heat the resonator in accordance with a first embodiment of the present invention; FIGURE 2 is a graph which illustrates how a frequency characteristic of a device formed through the

method and system of the present invention changes by having a different frequency and a higher Q; FIGURE 3 is a schematic diagram of another embodiment of the method and system of the present invention wherein the micromechanical resonator is indi- rectly heated by a circuit; FIGURE 4 is a schematic diagram of the embodi- ment of Figure 3 with the addition of an isolating microplatform; FIGURE 5 is a perspective schematic diagram illustrating the method and system of the first embodi- ment of the present invention with respect to a comb- driven, folded-beam micromechanical resonator wherein the frequency trimming is done in situ while the resona- tor is being operated; and FIGURE 6 is a side schematic diagram of a microelectromechanical system wherein the invention is employed in situ without affecting on-chip circuits.

Best Mode For Carrving Out The Invention Referring now to the drawing figures, there is illustrated in Figure 1 one embodiment of the method and system of the present invention. In particular, Figure 1 presents one possible embodiment of the localized annealing method and system of the present invention.

Here, a voltage Vaimeai is applied between anchors 10 of a conductive micromechanical device or structure (in this case, a micromechanical resonator) , generally indicated at 12 in Figures 1, 3 and 4, which has a resistance 1?struck

between its terminals. Application of this voltage sources a current 1anneal which flows through a beam 14 of the structure 12 and dissipates a power given by: Anneal = Iannea2Rstruct With sufficient voltage, Vanneai, the structure 12 can be heated to a temperature where annealing occurs, upon which its frequency and quality factor (i.e. Q factor) will change as material and microstructural properties, such as defect density, change. Due to the microscopic size of the structure 12, very little power (e.g., 20 mW) is required to reach temperatures in excess of 1000 Kelvin.

A drive electrode 16 causes the beam 14 to oscillate upon the application of an electrical signal thereto. A sense electrode 18 senses the oscillations of the beam 14.

Referring now to Figure 2, there is illustrat- ed the result of localized annealing in accordance with the present invention. Here, an initial resonator at frequency fob is locally annealed, resulting in a new frequency characteristic, with new resonance frequency foa and a much higher quality factor (i.e., a sharper peak).

The resonance frequency of the micromechanical resonator 12 changes upon annealing. One possible mechanism for this resonance frequency change is because internal stresses are modified, and resonance frequency is generally a function of stress. The quality factor of the micromechanical resonator 12 is a function of defects within the structure, at both molecular and

microstructural levels. The larger the concentration of defects, the lower the quality factor. Thus, since annealing of a structure removes these defects, it is also possible that annealing raises the quality factor.

Other embodiments of the method and system for localized annealing are presented in Figures 3 and 4 with the same micromechanical resonator 12. Here, heating is achieved using a resistive heating device such as a resistor 20 located immediately adjacent or near the structure (i.e. resonator 12) to be annealed.

In Figure 3, the resistor 20 and structure 12 are situated directly on a substrate 22.

In Figure 4, the resistor 20 and the structure 12 are situated on a microplatform 24, which is thermal- ly isolated from the substrate (not shown in Figure 4) via long, thin struts 26. The embodiment in Figure 4 can achieve annealing with much lower power, since it has better thermal insulation.

The voltage applied for heating of the struc- ture 12 using any of the embodiments need not be a DC voltage, as shown. It can be pulsed (as illustrated in Figure 5) or an AC signal for more controlled annealing, or for location-dependent annealing. For example, at very high frequencies, the current flows mainly over the surface of the structure 10 (due to the skin effect), and thus, high frequency voltages can be used to anneal only the structure's surface. Current crowding effects combined with thermal loss topologies can also be harnessed for location specific localized annealing.

Figure 5 presents a schematic depicting details of this filament-like annealing procedure as applied to a properly biased and excited comb-driven, folded-beam Hmechanical resonator 30 with sense elec- tronics including a sense electrode 32 and an amplifier 34. The resonator 30 also includes a comb transducer 36 formed on a sliced ground plane 40 and a drive electrode 38. The resonator design is similar to prior art designs in all respects, except for the provision of distinct leads or annealing electrodes 42 coupled to anchors 44 and 46.

During normal resonator operation, the pulse voltage generator Vanneai is inactive and provides the ground voltage for all components in this resonator system. In this configuration, the anchor leads 42 are tied to a DC-bias voltage Vp, and an AC drive signal is applied to the one or more of the transducer electrodes 38 to induce vibration. Once vibration is established, an output current is generated via a DC-biased, time- varying capacitor at the output electrode 32. This current is then sensed and amplified to a voltage, V,, by the amplifier 34. Figure 5 shows an open loop implemen- tation but it is understood that a closed loop oscilla- tor hookup can also be annealed in situ.

To anneal the resonator 30 during induced vibration, the pulse voltage generator Vanneai is activat- ed. Depending on the requirements, Vaiineai is made to emit one or more voltage pulses of magnitude Vanneai for each annealing cycle. During each pulse, the potentials of the input and output electrodes 46, the ground plane 40, and one of the resonator anchors 44 are raised by Vameal, while the potential at the remaining resonator anchor 44

remains constant at Vp Thus, each pulse effectively applies a voltage across the resonator 30, from anchor- to-anchor, of magnitude Vanneai, which then sources a current 1anneal from anchor-to-anchor. This current flows through the resonator structure, dissipating a power given by: Panneai = Ia%neaiRstruct, <BR> <BR> <BR> <BR> <BR> where Rstruct is the resistance between the anchors 44 and 46 of the resonator 30. Heat is thus generated through- out the resonator structure, raising its overall temper- ature and effectively annealing it.

A controller 48 monitors the voltage V0 and is able to determine the resulting resonant frequency of the annealed microresonator 30. If the resulting resonant frequency is not the desired resonant frequen- cy, the controller 48 sends a control signal to the pulse voltage generator to supply another pulse to further heat the microresonator 30.

Referring now to Figure 6, there is illustrat- ed a MEMS having both a micromechanical device 50 and microelectrical devices 52. The microelectrical device 52 may include a capacitor 54 and layers of PMOS and NMOS The annealing of the device 50 is possible without affecting the on-chip circuits in the merged microcir- cuit plus microstructure technology represented in Figure 6.

The main advantage behind the localized annealing method and system of the present invention is its simplicity. The mere application of a voltage across anchor points is very easy to implement, and this

simplicity makes the technique applicable to massive arrays of resonators fabricated using batch processes.

Just as EEPROM's with millions of transistors are electronically programmable, this localized annealing technique allows the electronic tuning/trimming of micromechanical resonators, perhaps thousands of them on each die of a wafer. Trimming should be possible at the wafer level in batch-mode using localized annealing, and this can substantially lower the cost of micromechanical devices, perhaps making some of them economically feasible for the first time.

Furthermore, these localized annealing proce- dures invite several novel circuits+micromechanical device technologies. For example, the recent MICS technology attempted to modularly merge CMOS and micro- mechanics by first implementing CMOS, then microstruc- tures. The main problem in this process was in deposit- ing microstructural layers above CMOS, since the deposi- tion temperature required to achieve stress-free poly- silicon structures (600"C) is greater than the eutectic temperature of aluminum contacts (540"C). Localized annealing can solve this problem by allowing deposition of silicon at lower temperatures (at which it is depos- ited amorphous), then localized annealing of individual structures (in a batch process) to crystallize the silicon, making it conductive and annealing away the stress. This results in a foundry-compatible merged technology process that can enable sensor and micro- actuator products.

While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.