U.S. patent application number 11/358689 was filed with the patent office on 2007-05-17 for mems device annealing.
Invention is credited to Keith Aubin, Harold G. Craighead, Brian H. Houston, Jeevak M. Parpia, Lidija Sekaric, Maxim Zalalutdinov, Alan T. Zehnder.
Application Number | 20070109656 11/358689 |
Document ID | / |
Family ID | 34222368 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070109656 |
Kind Code |
A1 |
Aubin; Keith ; et
al. |
May 17, 2007 |
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.
Inventors: |
Aubin; Keith; (Freeville,
NY) ; Zalalutdinov; Maxim; (Silver Springs, MD)
; Sekaric; Lidija; (Mount Kisco, NY) ; Houston;
Brian H.; (Fairfax, VA) ; Zehnder; Alan T.;
(Ithaca, NY) ; Parpia; Jeevak M.; (Ithaca, NY)
; Craighead; Harold G.; (Ithaca, NY) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Family ID: |
34222368 |
Appl. No.: |
11/358689 |
Filed: |
February 20, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US04/27226 |
Aug 20, 2004 |
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11358689 |
Feb 20, 2006 |
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60496431 |
Aug 20, 2003 |
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60496421 |
Aug 20, 2003 |
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60496430 |
Aug 20, 2003 |
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Current U.S.
Class: |
359/623 |
Current CPC
Class: |
H03D 3/34 20130101; H03C
1/46 20130101; H03H 9/02259 20130101; H03H 2009/02488 20130101;
H03H 9/2436 20130101; G02B 26/0858 20130101; H03D 1/02 20130101;
H03H 3/0072 20130101; H03H 9/2405 20130101; H03H 2009/02511
20130101; H03B 5/30 20130101; H03D 1/00 20130101 |
Class at
Publication: |
359/623 |
International
Class: |
G02B 27/10 20060101
G02B027/10 |
Goverment Interests
GOVERNMENT FUNDING
[0002] The invention described herein was made with U.S. Government
support under Grant Number DMR-0079992 awarded by National Science
Foundation. The United States Government has certain rights in the
invention.
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.degree. 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.
Description
RELATED APPLICATION
[0001] This application is a Continuation Under 35 U.S.C. .sctn.
1.111(a) of International Application No. PCT/US2004/027226, filed
Aug. 20, 2004 and published in English as WO 2005/035436 on Apr.
21, 2005, which claims priority to U.S. Provisional Application
Ser. No. 60/496,430 (entitled Laser Annealing for MEMS Devices,
filed Aug. 20, 2003) which is incorporated herein by reference.
This application also claims priority to U.S. Provisional
Application Ser. No. 60/496,431 (entitled Method and Apparatus for
Thermal-Mechanical Signal Processing, filed Aug. 20, 2003), which
is incorporated herein by reference. This application also claims
priority to U.S. Provisional Application Ser. No. 60/496,421
(entitled Shell-Type Micromechnical Actuator and Resonator, filed
Aug. 20, 2003) which is incorporated herein by reference. This
application is related to U.S. patent application Ser. No.
10/097,178 (entitled Heat Pumped Parametric MEMS Device, filed Mar.
12, 2002), which is incorporated herein by reference.
BACKGROUND
[0003] 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
[0004] 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
[0005] FIG. 1 is a block diagram of a MEMS structure that is
selectively annealed according to an example embodiment.
[0006] FIGS. 2A, 2B, and 2C illustrate a process of forming the
MEMS structure of FIG. 1 according to an example embodiment.
[0007] FIG. 3 is a schematic block diagram of a system for
annealing a MEMS structure according to an example embodiment.
[0008] 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
[0009] 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.
[0010] 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 SiO.sub.2 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.
[0011] 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 CF.sub.4-H.sub.2 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.
[0012] 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.
[0013] The resulting oscillators consist of an approximately 0.25
.mu.m thick single crystal silicon disk attached to a silicon
substrate by a 1 .mu.m 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.
[0014] The pillar in one embodiment is found to be conical in shape
with a minimum radius of approximately 0.32 .mu.m and a maximum
radius of approximately 1.5 .mu.m. One mode observed for this
oscillator is .gamma..sub.00, 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 .mu.m.
[0015] 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.sup.-7 Torr) with the top of the oscillator
facing a transparent window 320. Very low power (.about.250 .mu.W)
HeNe (.lamda.=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.
[0016] 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.
[0017] In addition to a HeNe measurement laser 325, an Ar.sup.30
(.lamda.=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 Ar.sup.30 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.
[0018] 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 .mu.m. 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).
[0019] It can be shown that the temperature of the silicon disk
just above the pillar is given by: T = L k ox .times. .pi. .times.
( P las - P rad r max .times. r min ) + T 0 , .times. P rad =
.sigma. .times. .times. T 4 .function. ( 2 .times. .pi. .times.
.times. R 2 ) . ##EQU1## where L is the height of the pillar,
P.sub.las is the absorbed laser power, k.sub.ox is the thermal
conductivity of silicon oxide (1.6 W/m/K), r.sub.max and r.sub.min
are the maximum and minimum radius of the conical oxide pillar,
T.sub.0 is the temperature of the substrate (assumed to be 300 K),
P.sub.rad is the power radiated by the disk, .sigma. is the
Stefan-Boltzmann constant, and R is the radius of the disk. The
emissivity, .epsilon., was taken to be unity to assume the worst
case with respect to radiation losses.
[0020] In one embodiment, a maximum obtainable temperature above
the oxide pillar is around 1300.degree. 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.
[0021] 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 .mu.W 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.
[0022] 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.
[0023] 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 SiO.sub.2, but
is slowly etched by the reaction 2Si +O.sub.2.fwdarw.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.
[0024] Device damage may occur at higher laser powers (.about.6
mW), resulting in a lower Q-factor and a much higher frequency
increase (.about.10%). These frequency increases are likely due to
the sublimation of the silicon at the point of laser focus.
[0025] 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
[0026] 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.
[0027] 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.sub.red=4 mW) and a
blue Ar+ ion laser (P.sub.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.
[0028] 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.
[0029] The Abstract is provided to comply with 37 C.F.R.
.sctn.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.
* * * * *