MEMS device annealing

Abstract

A method of increasing a quality factor for a micromechanical resonator uses a laser beam to anneal the micromechanical resonator. In one embodiment, the micromechanical oscillator is formed by fabricating a mushroom shaped silicon oscillator supported by a substrate via a pillar. The laser beam is focused on a periphery of the mushroom shaped silicon oscillator to modify the surface of the mushroom shaped silicon oscillator. In a further embodiment, the mushroom shaped oscillator is a silicon disk formed on a sacrificial layer. Portions of the sacrificial layer are removed to free the periphery of the disk and leave a supporting pillar at the center of the disk. In further embodiments, different type resonators may be used.

Description

BACKGROUND

Micro- and nano-scale resonators may have far reaching applications in RF circuits as filters and frequency standards, in microscopy as force detectors and as mass sensors. The selectivity of the filter or accuracy of the detector depends greatly on the quality factor (Q) of the oscillator. As device dimensions shrink to increase the resonant frequencies of micro-mechanical (MEMS) oscillators, it has been seen that quality factor decreases.

SUMMARY

A method of increasing a quality factor for a micromechanical resonator uses a laser beam to anneal the micromechanical resonator. In one embodiment, the micromechanical oscillator is formed by fabricating a mushroom shaped silicon oscillator supported by a substrate via a pillar. The laser beam is focused on a periphery of the mushroom shaped silicon oscillator to modify the surface of the mushroom shaped silicon oscillator. In a further embodiment, the mushroom shaped oscillator is a silicon disk formed on a sacrificial layer. Portions of the sacrificial layer are removed to free the periphery of the disk and leave a supporting pillar at the center of the disk. In further embodiments, the resonator may take different shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a MEMS structure that is selectively annealed according to an example embodiment.

FIGS. 2A, 2B, and 2C illustrate a process of forming the MEMS structure of FIG. 1 according to an example embodiment.

FIG. 3 is a schematic block diagram of a system for annealing a MEMS structure according to an example embodiment.

FIG. 4 is a graph showing temperature distribution of a MEMS device that has been heated by a focused laser beam according to an example embodiment.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.

FIG. 1 is a side view representation of a micromechanical (MEMS) oscillator 100. It oscillates in the radio frequency (RF) range and is fabricated in the form of a silicon disc 110 supported by a SiO₂ pillar 120 at the disc center. Other shapes, such as oval or polygons may also be used, and are included in the use of the term disc.

As illustrated in FIG. 2A, commercially available silicon-on-insulator (SOI) wafers 210 with a 250 nm thick silicon layer 215 on top of a 1 micron silicon oxide layer 220 are used in one embodiment for microfabrication. Other thicknesses of the layers are used in various embodiments to produce oscillators that have different resonant frequencies. Discs of radius R from 5 to 20 microns are defined by electron-beam lithography in, defining a pattern in a polymethylmethacrylate (PMMA) mask. Evaporation and lift-off of chrome provided a mask for a CF₄-H₂ reactive ion etch that exposed the oxide layer in unmasked areas as shown in FIG. 2B. The chrome was then removed using a wet chrome etch. The structures were released using a wet oxide etch that removed the oxide from beneath the silicon disk as shown in FIG. 2C.

The radius of the discs affects the resonant frequency. The wet oxide etch, such as dipping the resulting structure into hydrofluoric acid undercuts the silicon oxide starting from the disc's periphery toward the center as shown in FIG. 2C. By timing this wet etch, the diameter of the remaining column of the silicon oxide 230, which supports the released silicon disc 235, is varied.

The resulting oscillators consist of an approximately 0.25 μm thick single crystal silicon disk attached to a silicon substrate by a 1 μm thick silicon-oxide pillar at the disk's center, the cross-section of which resembled a very flat mushroom. The pillar dimensions were controlled by timing the oxide wet etch.

The pillar in one embodiment is found to be conical in shape with a minimum radius of approximately 0.32 μm and a maximum radius of approximately 1.5 μm. One mode observed for this oscillator is γ₀₀, which has no radial or circular nodes. The frequency of vibration of this mode is approximately 3.1 MHz, for a disk radius of 10 μm.

In one embodiment, the oscillators are annealed by the use of one or more lasers. FIG. 3 shows one potential system for both annealing and measuring vibrations of an oscillator generally at 300. An oscillator 305 in one embodiment is mounted on top of a flat piezo-electric transducer represented at 310 and placed in a vacuum chamber 315 (10⁻⁷ Torr) with the top of the oscillator facing a transparent window 320. Very low power (˜250 μW) HeNe (λ=633 nm) laser light 325 is focused by a lens 330 on the periphery of an oscillator disc 305 for the purpose of measurement. While specific wavelengths of radiation are described with respect to embodiments, other laser wavelengths such as visual, infrared, ultraviolet or wavelengths that provide sufficient annealing heat may be used in further embodiments.

The reflective surfaces of the substrate and the device set up a Fabry-Perot type interferometer. Motion of the disc perpendicular to the laser beam 325 modulates the intensity of the reflected laser light by changing the device-substrate distance. An AC-coupled photodetector 335 and a spectrum analyzer 340 were used to detect this modulation. The spectrum analyzer 340 also provides a sweeping RF voltage to the piezo actuator 310, providing the ability to obtain frequency-amplitude response curves.

In addition to a HeNe measurement laser 325, an Ar³⁰ (λ=450 nm) laser 345 may be used to provide extra power for the purpose of annealing. An adjustable polarizer 350 in the HeNe laser beam path controlled the power of the HeNe laser, while the CW Ar³⁰ laser 345 power is controlled using an electro-optical modulator 355. In one embodiment, the range of laser power that can impinge on the device ranged from 0.03 to 12 mW. Further variation of power may also be used.

One estimate is that the absorption of the laser light by the silicon disk is about 25%. In one embodiment, both lasers were focused on the periphery, but on opposite sides of the oscillator, each having a spot diameter of about 2 μm. In further embodiments, the size of the spot may be varied, and one or more lasers may be used to provide heating. The spots may be moved in a further embodiment during heating, such as by moving the lens to avoid overheating a single spot. The temperatures obtained may be calculated using thermodynamics and finite element methods (FEM).

It can be shown that the temperature of the silicon disk just above the pillar is given by: $T=\frac{L}{k_{\mathrm{ox}}\pi }\left(\frac{P_{\mathrm{las}}-P_{\mathrm{rad}}}{r_{\max }{r}_{\min }}\right)+T_0,P_{\mathrm{rad}}=\mathrm{\varepsilon \sigma }{T}^4\left(2\pi R^2\right).$ where L is the height of the pillar, P_las is the absorbed laser power, k_ox is the thermal conductivity of silicon oxide (1.6 W/m/K), r_max and r_min are the maximum and minimum radius of the conical oxide pillar, T₀ is the temperature of the substrate (assumed to be 300 K), P_rad is the power radiated by the disk, σ is the Stefan-Boltzmann constant, and R is the radius of the disk. The emissivity, ε, was taken to be unity to assume the worst case with respect to radiation losses.

In one embodiment, a maximum obtainable temperature above the oxide pillar is around 1300° K. The temperature of the disk may be 20-40% higher than this at the points where the lasers are focused as observed in a temperature distribution in FIG. 4. Damage may occur after annealing at higher powers, likely due to sublimation. In alternative embodiments, the focal points of the laser or lasers on the disc may be moved to provide a more even distribution of heating and minimize the risk of disc damage.

In one embodiment, argon and HeNe laser powers are increased to the desired levels. The disc is exposed to the beams for 30 seconds. The lasers are then removed. If measurements are desired, the Ar laser is then blocked, and the HeNe laser power is reduced to 250 μW in order to measure the lorentzian response curve of the device. Care may be taken to ensure that the RF voltage applied to the piezo element is low enough so that asymmetries in the response curve due to non-linearity are minimized. The measurement power of the HeNe laser was below the regime where limit-cycle oscillations are possible. Vacuum was not broken during the process of annealing.

Using the method described above, an order-of-magnitude increase (from 7,000 to over 100,000) in quality factor for a 3.105 MHz resonator may be obtained. As the quality factor increases, the resonant frequency of the oscillator also increases from 3.105 to 3.133 MHz. These changes may be attributed to the removal of surface contaminates, such as oxide. The resulting oscillator has a reduced amount of oxide. Surface related losses have been found to be a large factor in determining the quality factor of an oscillator as device dimensions shrink and the surface-to-volume ratio increases. The removal of oxide may be responsible for enhanced quality factor. Other methods of annealing may also remove oxide and enhance quality factor.

Vacuum conditions and estimates of the disc temperature during anneal and are such that the disc would be in the region of active oxidation, where the surface remains free of SiO₂, but is slowly etched by the reaction 2Si +O₂→2SiO. Post-anneal decay of the quality factor and resonant frequency may occur as a function of whether vacuum conditions are maintained. This phenomenon is caused by passive oxidation (the formation of an oxide film) and the acquisition of other contaminates over time. Annealing at UHV pressures may reduce this effect.

Device damage may occur at higher laser powers (˜6 mW), resulting in a lower Q-factor and a much higher frequency increase (˜10%). These frequency increases are likely due to the sublimation of the silicon at the point of laser focus.

The above described methods of annealing provide a very localized heating of a MEMs oscillator. Many different types of oscillators may be used other than those that are somewhat circular in shape, such as beam type oscillators. The use of a laser provides the ability to anneal a device that is already packaged in a modest vacuum following an activating getter, provided a suitably laser transparent cover is employed. The laser provides localized heating that can be used to minimize heating of adjacent circuitry, allowing an integrated MEMs device with circuitry on a single substrate or within a single package. While the mushroom shaped MEMs device provides further thermal isolation from such circuitry, allowing low power annealing (approximately 10 mW in one embodiment) other MEMs devices integrated with CMOS circuitry may also benefit from such localized heating. Conventional annealing might exceed thermal budgets for such circuitry or otherwise damage it.

CONCLUSION

High frequency and high quality factor, Q, (defined as a half-width of the resonant peak) are the key factors that enable applications of microelectromechanical (MEMS) oscillators for supersensitive force detection or as elements for radio frequency signal processing. By shrinking the dimensions of MEMS resonators to the sub-micron range, the resonant frequency of the devices increases. Shrinking the devices, however, also increases the surface-to-volume ratio leading to a significant degradation of the quality factor (to below 5,000) due to the increased contribution of surface-related losses.

Local annealing performed by focused low-power laser beams can improve the quality factor of MEMS oscillators or resonators by more than an order of magnitude. Quality factors over 100,000 were achieved after laser annealing 3.1 MHz disc-type oscillators (radius R=10 micrometers, thickness h=0.25 micrometer) compared with a Q=6,000 for the as-fabricated device. The mushroom-type design of our resonator (a single-crystal silicon disc supported by a thin silicon dioxide pillar at the center) provides low heat loss. The combined power of a red HeNe laser (P_red=4 mW) and a blue Ar+ ion laser (P_blue=5 mW) focused on the periphery of the mushroom provides enough energy for surface modification. The post-treatment quality factor, exceeding 100,000 for MHz-range resonators, boosts the performance of MEMS to be comparable to that of lower frequency single-crystal quartz devices. The local nature of laser annealing, safe for surrounding electronics, is a crucial element for integration of MEMS resonators into an integrated circuit environment.

While specific values for dimensions of the MEMS oscillators or resonators have been described, a wide range of dimensions may be used. Oscillators having dimensions in the micrometers to nanometer range may benefit from the annealing process described herein. The invention is not meant to be limited to the use of lasers to perform the anneal. Any type of radiation or other means of heating the MEMS structures that accomplishes the desired heating may be used. Laser annealing by a single laser or multiple lasers provides a good local heating without significantly adversely affecting nearby components.

The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Claims

1. A method of increasing a quality factor for a micromechanical resonator, the method comprising: using a laser beam to anneal the micromechanical resonator.

2. The method of claim 1 wherein the laser beam is provided by a red laser and a blue laser.

3. The method of claim 2 wherein the red laser comprises a HeNe laser with a power of approximately 4 mW, and the blue laser comprises an Ar+ ion laser with a power of approximately 5 mW.

4. The method of claim 2 wherein the lasers are focused on opposite sides of the oscillator.

5. The method of claim 1 wherein the laser power ranges from approximately 0.03 mW to 12 mW.

6. The method of claim 1 wherein the laser beam is applied in vacuum.

7. The method of claim 1 wherein the micromechanical resonator is heated by the laser beam to approximately 1300° K.

8. A method of forming a micromechanical oscillator, the method comprising: fabricating a mushroom shaped silicon oscillator supported by a substrate; and focusing a laser beam on a periphery of the mushroom shaped silicon oscillator to modify the surface of the mushroom shaped silicon oscillator.

9. The method of claim 8 wherein the laser beam is provided by a red laser and a blue laser.

10. The method of claim 9 wherein the red laser comprises a HeNe laser with a power of approximately 4 mW, and the blue laser comprises an Ar+ ion laser with a power of approximately 5 mW.

11. The method of claim 9 wherein the lasers are focused on opposite sides of the oscillator.

12. The method of claim 8 wherein fabricating the mushroom shaped silicon oscillator comprises forming a silicon disk on a sacrificial layer and removing portions of the sacrificial layer to free the periphery of the disk and leave a supporting pillar at the center of the disk.

13. The method of claim 8 wherein the laser power ranged from approximately 0.03 mW to 12 mW.

14. The method of claim 8 wherein the laser beam is applied in vacuum.

15. A method comprising: fabricating a micromechanical oscillator supported by a substrate; and annealing the oscillator to remove oxide and significantly enhance the Q of the oscillator.

16. The method of claim 15 wherein the micromechanical oscillator comprises a mushroom shaped oscillator supported by a pillar.

17. The method of claim 16 wherein the micromechanical oscillator comprises silicon.

18. The method of claim 16 wherein annealing comprises focusing a laser beam on a periphery of the mushroom shaped oscillator.

19. The method of claim 16 wherein annealing comprises focusing multiple laser beams on a periphery of the mushroom shaped oscillator.

20. The method of claim 17 wherein two laser beams are focused on opposite sides of the periphery of the mushroom shaped oscillator.

21. The method of claim 15 wherein the micromechanical oscillator comprises a circular disc supported by a pillar such that outer portions of the circular disc are free to oscillate.

22. A micromechanical oscillator comprising: a micromechanical disc supported by a pillar and having edges that are free to oscillate, wherein the disc has a Q of at least 7000.

23. The micromechanical oscillator of claim 22 wherein the disc has a Q of at least 100,000.

24. The micromechanical oscillator of claim 22 wherein the disc has a reduced amount of oxide.

25. A method comprising: fabricating a micromechanical oscillator supported by a substrate; and heating the oscillator in a localized manner to anneal the oscillator.