U.S. patent application number 09/943451 was filed with the patent office on 2003-03-06 for high-speed mems switch with high-resonance-frequency beam.
This patent application is currently assigned to Intel Corporation. Invention is credited to Ma, Qing.
Application Number | 20030042117 09/943451 |
Document ID | / |
Family ID | 25479690 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030042117 |
Kind Code |
A1 |
Ma, Qing |
March 6, 2003 |
HIGH-SPEED MEMS SWITCH WITH HIGH-RESONANCE-FREQUENCY BEAM
Abstract
A microelectromechanical system (MEMS) switch having a
high-resonance-frequency beam is disclosed. The MEMS switch
includes first and second spaced apart electrical contacts, and an
actuating electrode. The beam is adapted to establish contact
between the electrodes via electrostatic deflection of the beam as
induced by the actuating electrode. The beam may have a cantilever
or bridge structure, and may be hollow or otherwise shaped to have
a high resonant frequency. Methods of forming the high-speed MEMS
switch are also disclosed.
Inventors: |
Ma, Qing; (San Jose,
CA) |
Correspondence
Address: |
Schwegan, Lundberg, Woessner & Kluth, P.A.
P.O. Box 2938
Minneapolis
MN
55402
US
|
Assignee: |
Intel Corporation
|
Family ID: |
25479690 |
Appl. No.: |
09/943451 |
Filed: |
August 30, 2001 |
Current U.S.
Class: |
200/181 |
Current CPC
Class: |
B81B 3/007 20130101;
B81B 2201/014 20130101; B81B 2201/0271 20130101; B81B 2203/0118
20130101; H01H 2059/0036 20130101; H01H 2001/0084 20130101; Y10T
29/49105 20150115; Y10T 29/49002 20150115; B81B 2203/04 20130101;
Y10T 29/49128 20150115; B81C 2201/056 20130101; H01H 59/0009
20130101; B81B 2203/0353 20130101; H01H 2059/0063 20130101; Y10T
29/49082 20150115; Y10T 29/49016 20150115; H01H 1/20 20130101; Y10T
29/49147 20150115 |
Class at
Publication: |
200/181 |
International
Class: |
H01H 057/00 |
Claims
What is claimed is:
1. A microelectromechanical system (MEMS) switch apparatus,
comprising: first and second spaced apart electrical contacts
formed on an upper surface of a substrate; an actuation electrode
formed on the upper surface of the substrate adjacent the first and
second electrical contacts; and a hollow beam to be
electrostatically deflected by activation of the actuation
electrode so as to establish electrical contact between the first
and second electrical contacts.
2. The apparatus according to claim 1, wherein the actuation
electrode is arranged between the first and second electrical
contacts.
3. The apparatus according to claim 1, wherein the hollow beam is
fixed at one end to the first electrical contact.
4. The apparatus according to claim 1, wherein the beam includes
upper and lower walls separated by a plurality of internal support
members.
5. The apparatus according to claim 4, wherein the beam includes a
plurality of internal cavities defined by the internal support
members and the upper and lower walls.
6. The apparatus of claim 1, wherein the beam is fixed to the
substrate at first and second opposite ends of the beam by
respective first and second anchors depending therefrom, and
wherein the first and second electrical contacts and the actuation
electrode are arranged between the first and second anchors.
7. The apparatus of claim 1, further including a voltage source
controller electrically connected to the actuation electrode.
8. A microelectromechanical system (MEMS) switch apparatus
comprising: first and second spaced apart electrical contacts
formed on an upper surface of a substrate; an actuation electrode
formed on the upper surface of the substrate adjacent the first and
second electrical contacts; and a beam in electrostatic
communication with the actuation electrode and to be
electrostatically deflected to establish electrical contact between
the first and second electrical contacts, the beam having a first
tapered section that is wide at a first beam end and that
terminates at a first narrow end.
9. The apparatus of claim 8, wherein the first beam end includes an
anchor fixed to the first electrical contact, the beam further
including a rectangular section that adjoins the first narrow end
of the first tapered section and that is suspended over at least a
portion of the second electrical contact so as to contact the
second electrical contact when the beam is electrostatically
deflected.
10. The apparatus of claim 9, wherein the rectangular section has a
width greater than the width of the first narrow end.
11. The apparatus of claim 8, wherein the first end of the beam and
a second end of the beam opposite the first end include respective
anchors fixed to the substrate, the beam having a second tapered
section that is wide at the second beam end and that terminates at
a second narrow end, wherein the first and second narrow ends
adjoin a rectangular section that at least partially overlaps the
first and second electrical contacts and the actuation
electrode.
12. The apparatus according to claim 11, wherein the rectangular
section has a width greater than the width of the first and second
narrow ends.
13. The apparatus of claim 8, further including a voltage source
controller electrically connected to the actuation electrode.
14. A microelectromechanical system (MEMS) switch apparatus
comprising: first and second spaced apart electrical contacts
formed on an upper surface of a substrate; an actuation electrode
formed on the upper surface of the substrate adjacent the first and
second electrical contacts; and a beam to be being
electrostatically deflected by the actuation electrode to establish
electrical contact between the first and second electrical
contacts, the beam having a step formed therein designed to
increase the resonance frequency of the beam as compared to the
beam without the step.
15. The apparatus of claim 14, wherein the first end of the beam
includes an anchor fixed to the first electrical contact, and
wherein the step section is located at a second end of the beam
opposite the first end and is suspended over at least a portion of
the second electrical contact.
16. The apparatus of claim 14, wherein the first end of the beam
and an opposite second end of the beam include respective anchors
fixed to the substrate, wherein the step is formed between the
first and second ends and at least partially overlaps the first and
second electrical contacts and the actuation electrode.
17. The apparatus of claim 14, further including a voltage source
controller electrically connected to the actuation electrode.
18. A microelectromechanical (MEMS) switch apparatus, comprising: a
first MEMS switch having a first actuation electrode and a first
nonrectangular beam electrostatically coupled to the first
actuation electrode; a second MEMS switch having a second actuation
electrode and a second non-rectangular beam electrostatically
coupled to the second actuation electrode; and a voltage source
controller electrically coupled to the first and second actuation
electrodes to selectively activate the first and second MEMS
switches.
19. The apparatus of claim 18, wherein the first and second MEMS
switches are electrically connected to an antenna, and wherein the
first MEMS switch is electrically connected to receiver electronics
to receive and process a first signal received by the antenna and
the second MEMS switch is electrically connected to transmitter
electronics to generate a second signal to be transmitted by the
antenna.
20. The apparatus according to claim 18, wherein the first and
second nonrectangular beams are selected from the group of beams
comprising: a hollow beam, a single-taper beam, a double-taper
beam, an end-step beam and a center-step beam
21. A method of forming a hollow beam for a MEMS switch,
comprising: forming a release layer atop a first conductive layer;
selectively patterning the release layer to form spaces therein;
forming a second conductive layer atop the patterned release layer,
including filling the spaces; forming openings in the second
conductive layer to provide access to the release layer; and
removing the release layer material through the openings.
22. The method according to claim 21, wherein the spaces formed in
the release layer result in the formation of islands so that
removing the release layer material results in the formation of a
plurality of internal cavities.
23. The method according to claim 21, wherein the spaces formed in
the release layer are cylindrical holes so that removing the
release layer material results in forming a single internal cavity
having a plurality of posts that connect the upper and lower
walls.
24. The method according to claim 21, wherein the release layer
comprises an oxide or a polymer.
25. A method of forming a shaped cantilevered beam for a
microelectromechanical system (MEMS) switch, comprising: forming
spaced apart first and second electrical contacts and an actuation
electrode atop a substrate; forming a release layer atop the
substrate upper surface that covers the electrical contacts and the
actuation electrode, the release layer having an opening to the
first electrical contact; forming a conductive layer atop the
release layer, the conductive layer having a tapered or stepped
section being connected at a first end to the first electrical
contact through the opening in the release layer; and removing the
release layer.
26. The method according to claim 25, including selectively forming
the tapered section to include either a single taper.
27. The method according to claim 25, including selectively forming
the step section at a second end of conducting layer opposite the
first end.
28. A method of forming a shaped bridge beam for a
microelectromechanical system (MEMS) switch, comprising: forming
spaced apart first and second electrical contacts and an actuation
electrode atop a substrate; forming a release layer atop the
substrate that covers the electrical contacts and the actuation
electrode, the release layer having first and second openings to
the substrate selectively forming a tapered or a stepped conductive
layer atop the release layer, the conductive layer being fixed to
the substrate at first and second ends through the first and second
openings in the release layer; and removing the release layer.
29. The method of claim 28, including selectively forming the
tapered conductive layer to have first and second tapered sections
that are widest at respective first and second ends of the
conductive layer, the tapered sections joined by a rectangular
section located at or near the center of the conductive layer and
that at least partially overlaps the first and second electrical
contacts and the actuation electrode.
30. The method of claim 28, including selectively forming the
stepped conductive layer to have a step located between the first
and second ends, the step at least partially overlapping the first
and second electrical contacts and the actuation electrode.
31. The method according to claim 28, wherein the first and second
electrical contacts and the actuation electrode are arranged
parallel to the long axis of the beam.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to microelectromechanical
systems (MEMS), and in particular to MEMS switches that have a
connecting beam with a high resonance frequency to provide for
high-speed switching.
BACKGROUND OF THE INVENTION
[0002] A microelectromechanical system (MEMS) is a microdevice that
integrates mechanical and electrical elements on a common substrate
using microfabrication technology. The electrical elements are
typically formed using known integrated circuit fabrication
techniques. The mechanical elements are typically fabricated using
lithographic and other related processes to perform micromachining,
wherein portions of a substrate (e.g., silicon wafer) are
selectively etched away or added to with new materials and
structural layers. MEMS devices include actuators, sensors,
switches, accelerometers, and modulators.
[0003] MEMS switches (i.e., contacts, relays, shunts, etc.) have
intrinsic advantages over their conventional solid-state
counterparts (e.g., field-effect transistor (FET) switches),
including superior power efficiency, low insertion loss and
excellent isolation. However, MEMS switches are generally much
slower than solid-state switches. This limitation precludes
applying MEMS switches in certain technologies, such as wireless
communications, where sub-microsecond switching is required.
[0004] MEMS switches typically include a suspended connecting
member called a "beam" that is electrostatically deflected by
activating an actuation electrode. The deflected beam touches one
or more electrical contacts, thereby establishing an electrical
connection between the contacts. When the beam is anchored to one
contact while being suspended over another other in cantilever
fashion, it is called a "cantilevered beam." When the beam is
anchored to the substrate at opposite ends and is suspended over
one or more electrical contacts, it is called a "bridge beam."
[0005] The key feature of a MEMS switch that dictates its switching
speed is the form of the beam. In particular, the highest switching
speed is defined by the resonance frequency of the beam, which is a
function of the beam geometry. Conventional beams in MEMS switches
have essentially a solid rectangular structure. While such a
structure is relatively easy to fabricate, is strong, and is
suitable for many switching applications, the resonance frequency
of the beam is generally too low to perform high-speed
switching.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1A is a plan view of an example embodiment of a MEMS
switch of the present invention having a hollow cantilevered beam,
showing the electrical contacts, actuation electrode and the
associated bond pads;
[0007] FIG. 1B is a close-up cross-sectional view of the MEMS
switch of FIG. 1A taken along the line 1B-1B;
[0008] FIG. 1C is a plan view of an example embodiment of a MEMS
switch similar to that illustrated in FIG. 1A but with the two
electrical contacts arranged beneath the suspended end of the
cantilevered beam;
[0009] FIG. 2A is a plan view of an example embodiment of a MEMS
switch of the present invention similar to FIG. 1B but having a
hollow bridge beam, showing the electrical contacts, actuation
electrode and associated bonding pad;
[0010] FIG. 2B is a close-up cross-sectional view of the MEMS
switch of FIG. 2A taken along the line 2B-2B;
[0011] FIG. 3A is a plan view of an example embodiment of a MEMS
switch of the present invention having a cantilevered beam with a
tapered geometry that includes a rectangular end section;
[0012] FIG. 3B is a cross-sectional view of the MEMS switch of FIG.
3A taken along the line 3B-3B;
[0013] FIG. 4A is a plan view of an example embodiment of a MEMS
switch of the present invention having a cantilevered beam with an
end step;
[0014] FIG. 4B is a cross-sectional view of the MEMS switch of FIG.
4A taken along the line 4B-4B;
[0015] FIG. 5A is a plan view of an example embodiment of a MEMS
switch of the present invention having a double-tapered bridge beam
with a center rectangular section, with the electrical contacts and
the actuation electrode arranged beneath the rectangular center
section;
[0016] FIG. 5B is a plan view of an alternate example embodiment of
the double-tapered bridge beam of FIG. 5A, wherein the center
rectangular section is wider than the narrowest end of the tapered
sections;
[0017] FIG. 5C is a cross-sectional view of the MEMS switch of FIG.
5A taken along the line 5C-5C;
[0018] FIG. 6A is a plan view of an example embodiment of a MEMS
switch of the present invention having a rectangular cantilever
bridge beam with a center step section, with the electrical
contacts and the actuation electrode arranged beneath the center
step section;
[0019] FIG. 6B is a cross-sectional view of the MEMS switch of FIG.
6A taken along the line 6B-6B;
[0020] FIGS. 7A, 7B, 7C, 7D, and 7E are cross-sectional views of a
substrate during the various processing steps for forming a MEMS
switch with a cantilever beam designed to have a high resonance
frequency;
[0021] FIGS. 7F and 7G are cross-sectional diagrams similar to that
of FIGS. 7C and 7D but illustrating the formation of two openings
in the release layer used to form the anchors for a bridge-type
beam;
[0022] FIGS. 8A, 8B, 8C, 8E, 8F and 81 are cross-sectional views
illustrating the various structures formed when fabricating the
hollow rectangular beam of the MEMS switches of FIGS. 1B and
2B;
[0023] FIG. 8D is a plan view of islands of release material atop
formed a first conductive layer in forming a hollow beam;
[0024] FIGS. 8G and 8H are plan views of the structure formed after
covering the patterned release layer of FIG. 8D with a second
conductive layer and then forming openings in the conductive layer
to provide access to the release material, for the cases where the
support structures are walls (FIG. 8G) and posts (FIG. 8H); and
[0025] FIG. 9 is a schematic circuit diagram illustrating the use
of the high-speed MEMS switches of the present invention in an
example wireless communication application.
[0026] In the Figures, like reference numbers refer to like
elements.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The present invention relates to microelectromechanical
systems (MEMS), and in particular to MEMS switches that have a
connecting beam with a high-resonance-frequency to provide for
high-speed switching. The connecting beam can be used for MEMS
contact switches, relays, shunt switches and any other type of MEMS
switch.
[0028] In the following detailed description of the embodiments of
the invention, reference is made to the accompanying drawings that
form a part hereof, and in which is shown by way of illustration
specific embodiments in which the invention 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 changes
may be made without departing from the scope of the present
invention. The following detailed description is, therefore, not to
be taken in a limiting sense, and the scope of the present
invention is defined only by the appended claims.
[0029] MEMS Switch with Hollow Cantilever Beam
[0030] FIGS. 1A and 1B show an example embodiment of a MEMS switch
20 according to the present invention. MEMS switch 20 includes a
substrate 22 with an upper surface 24 upon which is formed a first
electrical contact 26 electrically connected to first bond pad 32.
Also formed on upper surface 24 is a second electrical contact 36
spaced apart from the first contact and electrically connected to a
second bond pad 42. An actuation electrode 46 is also formed on
upper surface 24 between first and second electrical contacts 26
and 36, and is electrically connected to a third bond pad 52. Bond
pads 32, 42 and 52 are for establishing electrical contact with
MEMS switch 20 via probes or wires. Contacts 26 and 36, and
actuation electrode 46 are preferably formed from metal, such as
for example gold or aluminum.
[0031] MEMS switch 20 further includes a hollow conductive
cantilever beam 60 having a first end 62, an opposite second end
64, an upper wall 65 having an upper surface 66 and a lower wall 67
having a lower surface 68. Beam 60 has a height H and a length L.
In an example embodiment, beam 60 has a rectangular cross-section.
Beam 60 includes one or more internal cavities 74 defined by upper
and lower walls 65 and 67 and internal support members 76. In FIG.
2B, four cavities 74 are shown with respective widths W1, W2, W3
and W4. In an example embodiment, W1=W2=W3=W4. When internal
support members 76 are sidewalls, multiple internal cavities 74 are
formed. When internal support members 76 are posts, a single
internal cavity results. Cavities 74 located adjacent ends 62 and
64 are also respectively defined by end walls 82 and 84 at
respective ends 62 and 64. Methods of forming hollow beam 60 are
discussed in detail further below.
[0032] In an example embodiment of the present invention, the
length L of beam 60 is on the order of about 100 microns, and
height H is anywhere from about 2 microns to about 5 microns.
Further, cavities 74 may have a width W1 anywhere from about 5
microns to about 10 microns, depending on the height H and the
length L of beam 60. Further, electrical contact 36 may have a
length anywhere from about 20 microns to about 100 microns.
[0033] Beam 60 is fixed ("anchored") to first electrical contact 26
by an anchor 90 that extends downwardly from lower wall 67 at end
62. Beam 60 is arranged such that its second end 64 is suspended in
cantilever fashion over at least a portion of second electrical
contact 36, forming a gap 96 between lower surface 68 and the
second electrical contact. In an example embodiment, gap 96 is
anywhere from about 0.5 microns to about 2 microns. Likewise, lower
surface 68 of beam 60 is suspended over actuation electrode 46 and
is separated therefrom by a gap 98 that is sized so that the
actuation electrode is in electrostatic communication with the
central portion of the beam.
[0034] With continuing reference to FIGS. 1A and 1B, MEMS switch 20
operates as follows. A voltage is provided to actuation electrode
46 through bond pad 52 from a voltage source (not shown). This
voltage sets up an attractive electrostatic force between actuation
electrode 46 and beam 60, which deflects the beam toward the
actuation electrode. This deflection causes the portion of lower
surface 68 at end 64 of beam 60 to touch second electrical contact
36. This establishes an electrical connection between electrical
contacts 26 and 36 via the conducting beam.
[0035] FIG. 1C shows an alternate example embodiment of MEMS switch
20 of FIG. 1A. MEMS switch 20 of FIG. 1C includes, in place of
electrical contacts 26 and 36 and associated bond pads 32 and 42,
two adjacent (but spaced apart) electrical contacts 116A and 116B
located at least partially beneath beam end 64. Contacts 116A and
116B are electrically connected to respective bond pads 122A and
122B. Thus, switching occurs by electrostatically deflecting beam
60 to establish an electrical connection between contacts 116A and
116B via end 64 of beam 60.
[0036] The frequency at which beam 60 can be electrostatically
deflected establishes the switching speed of MEMS switch 20. The
switching speed is thus a function of the resonance frequency F of
beam 60, which depends on the physical structure of the beam. More
specifically, the beam resonance frequency F varies in proportion
to the square root of the ratio of the beam stiffness S to the beam
density D, i.e., F.about.(S/D).sup.1/2. The beam stiffness is
proportional to H.sup.3 (FIG. 1B) As described above in connection
with FIG. 1B, beam 60 has one or more internal cavities 74. The
hollow structure of beam 60 increases the beam stiffness S while
also reducing the beam density D. This results in a significant
increase in the beam resonance frequency F. Thus, whereas a
conventional solid beam may have a resonance frequency F on the
order of 0.5 MHz, beam 60 can have a resonance frequency F on the
order of several MHz. By increasing the beam height H, the
resonance frequency F of beam 60 can be further increased (e.g. up
to 5 MHz), leading to a concomitant increase in switching
speed.
[0037] MEMS Switch with Hollow Bridge Beam
[0038] FIGS. 2A and 2B show an example embodiment of a MEMS switch
150 of the present invention similar to MEMS switch 20 described
above. MEMS switch 150 includes hollow beam 60 as described above,
but the beam is now fixed directly to surface 24 of substrate 22
and both ends 62 and 64 with anchors 90. Further, electrical
contacts 116A and 116B are arranged on substrate upper surface 24
between anchors 90 and on either side of actuation electrode 46. In
operation, beam 60 is electrostatically deflected by the activation
of actuation electrode 46 so that lower surface 68 touches contacts
116A and 116B, thereby establishing an electrical connection
between the contacts. For the same reasons discussed above in
connection with MEMS switch 20, MEMS switch 150 is also capable of
high-speed switching.
[0039] MEMS Switch with Tapered Cantilever Beam
[0040] FIGS. 3A and 3B show an example embodiment of a MEMS switch
200 according to the present invention. MEMS switch 200 is similar
to MEMS switch 20 discussed above in connection with FIGS. 1A-1C.
However, MEMS switch 200 includes a solid tapered cantilever beam
220. The latter has a first end 222 fixed to electrical contact 26
by anchor 90, and a free second end 224 opposite the first end.
Beam 220 includes a tapered section 230 that is widest at first end
222 and that becomes increasingly narrow progressing towards second
end 224. Tapered section terminates before end 224 at a narrow end
234 that adjoins a rectangular section 250, which includes end 224.
Rectangular section 250 is suspended above electrical contact 36 in
cantilever fashion so as to at least partially overlap the
electrical contact. In an example embodiment, rectangular section
250 is wider than narrow end 234 of tapered section 230, so that a
sufficient area of contact can be established between beam 60 and
electrical contact 36 when the beam is deflected.
[0041] The operation of MEMS switch 200 is analogous to that of
MEMS switch 20 of FIGS. 1A-1C in that electrostatic actuation from
actuation electrode 46 causes the beam to deflect and establish an
electrical connection between rectangular section 250 of beam 60
and contact 36.
[0042] An advantage of single-taper cantilevered beam 220 over a
solid rectangular beam as conventionally used in MEMS switches is
that beam 220 has a higher resonance frequency because it has a
higher effective spring-constant-to-mass ratio. This means beam 220
can respond to higher-frequency electrostatic actuation, which
allows for MEMS switch 200 to perform high-speed switching.
[0043] MEMS Switch with End Step Cantilever Beam
[0044] FIGS. 4A and 4B show an example embodiment of a MEMS switch
300 according to the present invention. MEMS switch 300 is similar
to MEMS switch 200 described immediately above. MEMS switch 300,
however, includes a rectangular cantilever beam 320 with a first
anchored end 322, a substantially uniformly rectangular central
section 323, and a suspended second end 324 opposite end 322. End
324 has a step 330 formed therein, which reduces the mass of beam
320 while allowing the beam to maintain its overall stiffness. As
discussed above, the reduction in mass of the beam increases the
beam resonance frequency F, which allows for MEMS switch 300 to
have a faster switching speed than that possible without step
330.
[0045] MEMS Switch with Double-Tapered Bridge Beam
[0046] FIGS. 5A-5C show an example embodiment of a MEMS switch 400
according to the present invention. MEMS switch 400 has a beam 420
with a first end 422 fixed to substrate surface 24 by a first
anchor 90. Beam 420 also includes a rectangular center section 424,
and a second end 426 opposite the first end that is fixed to the
substrate surface by a second anchor 90. Beam 420 includes first
and second tapered sections 430 and 440 that are wide at respective
first and second ends 422 and 426 and that becoming increasingly
narrow towards center section 424. First and second tapered
sections 430 and 440 terminate at respective narrow ends 442 and
444 that define respective sides of rectangular center section 424.
In an example embodiment, center section 424 has width that matches
that of narrow ends 442 and 444, as illustrated in FIG. 5A. In
another example embodiment illustrated in FIG. 5B, center section
424 has a width that is greater than that of narrow ends 442 and
444. This provides for a relatively large contact area between beam
420 and underlying electrical contacts (described below) when the
beam is deflected.
[0047] MEMS switch 400 further includes first and second electrical
contacts 450A and 450B formed on upper surface 24 of substrate 22
beneath center rectangular section 424. Contacts 450A and 450B are
electrically connected to respective bond pads 452A and 452B.
Actuation electrode 46 is arranged between contacts 450A and 450B
for electrostatically engaging beam 420 and deflecting the beam
downward to touch contacts 450A and 450B to establish an electrical
connection between the contacts.
[0048] In an example embodiment of MEMS switch 400, the height of
actuation electrode 46 is less than that of contacts 450A and 450B
so that beam 420 does not touch the actuation electrode when
deflected. Also, as illustrated in FIGS. 5A and 5C, in an example
embodiment of MEMS switch 400 actuation electrode 46 and contacts
450A and 450B are arranged perpendicular to long axis A1 of beam
420. In another example embodiment, actuation electrode and
contacts 450A and 450B can be arranged parallel to long axis
A1.
[0049] As discussed above in connection with the other example
embodiments of the present invention, the double taper of beam 420
results in a higher resonance frequency F than for a conventional
bridge-type MEMS beam because the former has a larger
effective-spring-constant-to-mass ratio.
[0050] MEMS Switch with Center-Step Bridge Beam
[0051] FIGS. 6A and 6B show an example embodiment of a MEMS switch
500 similar to MEMS switch 400 discussed above in connection with
FIGS. 5A-5C, the only difference being the form of the beam. MEMS
switch 500 includes a rectangular beam 520 having first and second
opposite ends 522 and 524 each anchored to substrate surface 24 via
respective anchors 90. Beam 520 includes a center step 536 formed
between ends 522 and 524 that is suspended over contacts 450A and
450B as well as over actuation electrode 46. Center step 536
reduces the mass of beam 520 as compared with a conventional
rectangular beam of uniform thickness. As discussed above in
connection with the other MEMS switch example embodiments of the
present invention, center step 536 reduces the mass of a uniform
rectangular beam, which results in an increase of the resonance
frequency F of the beam, leading to faster switching speeds for
MEMS switch 500.
[0052] Method of Forming MEMS Switches with Shaped Beams
[0053] FIGS. 7A through 7G describe methods of fabricating the MEMS
switches of the present. Fabrication begins with the formation of a
shaped cantilevered beam such as beam 220 of MEMS switch 200
discussed above in connection with FIGS. 3A and 3B.
[0054] With reference first to FIGS. 7A and 7B, actuation electrode
46 and electrical contacts 26 and 36 (as example electrical
contacts) are formed by first depositing a conductive layer 600
atop surface 24 of substrate 22 (FIG. 7A). Conductive layer may be,
for example, polysilicon or metal such as gold or aluminum. In an
example embodiment, conductive layer 600 may have a thickness
ranging from a few thousand angstroms up to about a micron.
Conductive layer 600 is then selectively etched to form isolated
contacts 26 and 36 and actuation electrode 46 (FIG. 7B).
[0055] In FIG. 7C, a release layer 610 is first formed atop
substrate upper surface 24, covering contacts 26 and 36 and
actuation electrode 46. Release layer 610 may be, for example, an
oxide such as silicon dioxide, or a polymer such as polyimide or
photoresist. Release layer 610 is then selectively etched (e.g.,
using a dry or a reactive plasma etch) to form a first opening 612
that extends down to contact 26.
[0056] In FIG. 7D, a conductive layer 620 is then selectively
formed atop release layer 610 so that it has a desired shape, such
as a tapered shape (e.g., beam 220 of FIG. 3A) or stepped shape
(e.g., beam 320 of FIGS. 4A). The desired shape is one that will
result in the formation of a high-resonance-frequency beam.
Conductive layer 620 may be of the same material as conductive
layer 600, e.g., polysilicon or metal. The selective deposition of
conductive layer 620 may be accomplished using standard
semiconductor processing techniques, such as for example by forming
a masking layer (not shown) atop release layer 610 prior to
depositing the conductive layer, and then stripping away the
masking layer. In depositing layer 620, a portion of the conductive
material fills first opening 612 down to contact 26, thereby
forming anchor 90.
[0057] In FIG. 7E, release layer 610 is removed by etching,
dissolving or other techniques suitable to the material of the
release layer. The result is the formation of a shaped cantilevered
beam 640 formed from conductive layer 620 that is fixed to
electrode 26 by anchor 90 and that has a relatively high resonance
frequency F as compared with a solid rectangular beam.
[0058] In 7F and 7G, the method described immediately above can
also be used with slight modification to form a shaped bridge beam
650, such as the double-tapered bridge beam 420 of FIGS. 5A and 5B,
or the center-step bridge beam 520 of FIG. 6A. In forming shaped
bridge beam 650, release layer 610 is formed as discussed above in
connection with FIG. 7C. Release layer 610 is then selectively
etched to form first and second openings 662 and 666 on either side
of actuation electrode 46 and the electrical contacts (not shown).
Openings 662 and 666 extend down to surface 24 of substrate 22 and
are used to form corresponding first and second anchors 90 when
depositing conductive layer 620, as illustrated in FIG. 7G.
Actuation electrode 46 is located between anchors 90.
[0059] Method of Forming Hollow Beam for MEMS Switch
[0060] FIGS. 8A-8H describe methods for forming hollow beam 60 of
MEMS switches 20 and 150 as discussed above in connection with
FIGS. 1B and 2B. The method begins with the layered structure
illustrated in FIG. 7D. For the sake of clarity, only the added
structures that ultimately form the hollow beam are shown in FIGS.
8B-8H.
[0061] With reference now to FIG. 8A and as discussed above in
connection with FIG. 7D, conductive layer 620, now a first
conducting layer, is deposited atop release layer 610. In an
example embodiment, layer 620 may be anywhere from 0.5 microns to 2
microns thick. With reference to FIG. 8B, atop conductive layer 620
is formed a release layer 710 similar to layer 610 discussed above.
In an example embodiment, release layer 710 has a thickness
anywhere from about 1 micron to about 2 microns. Release layer 710
is then selectively patterned via etching or dissolving to form, in
one example embodiment, islands 714 with spaces 716 in between, as
illustrated in FIGS. 8C and 8D.
[0062] With reference now to FIG. 8E, a second conductive layer 720
is formed over islands 714 and that connects up with first
conductive layer 620 (dashed line) by filling spaces 716. This
process forms the framework of a hollow conductive beam 60 that
includes upper wall 65 formed from second conductive layer 720, and
lower wall 67 formed from conductive layer 620 (see also FIG. 1B).
Conductive material that fills spaces 716 forms support members 76
in the form of sidewalls or posts that connect the lower and upper
walls 65 and 67 to reinforce the beam and contribute to its
stiffness. Support members 76 are preferably substantially
perpendicular to upper and lower walls 65 and 67.
[0063] The actual form of support members 76 depends on the nature
of spaces 716. In an alternative example embodiment to the
rectangular spaces 716 illustrated in FIG. 8D, spaces 716 may
instead be cylindrical openings formed within release layer 710,
which are then filled with conductive material to form post-type
support members 76. In this alternative embodiment, release layer
710 remains contiguous rather than divided up into islands 714.
[0064] In FIGS. 8F-8H, openings 730 are then formed in second
conductive layer 720 (now wall 65) of beam 60 Openings 730 extend
at least through to islands 714 and may extend down through to
first conductive layer 620 (now lower wall 65). Openings 730 may be
formed, for example, by masking upper wall 65 with photoresist and
then etching the masked structure. Openings 730 are used to provide
access to islands 714 (or contiguous layer 710 in the alternative
embodiment that uses post support members 76, discussed above) so
that an etchant or solvent solution can be introduced to remove the
release layer material.
[0065] The result is the formation of one or more cavities 74, as
illustrated in FIG. 81. In the alternative embodiment wherein
support members 76 are posts rather than sidewalls, a single
internal cavity 74 is formed.
[0066] Openings 730 also provide a conduit for air to enter and
leave the one or more cavities 74 when beam 60 is deflected, which
reduces the "squeezed air" damping effect that can occurs when such
cavities are sealed. This effect can reduce the resonance frequency
of the beam and thus result in a slower switching speed.
[0067] Application of the MEMS Switches
[0068] FIG. 9 shows a schematic circuit diagram of a MEMS-based
wireless communication system 800. System 800 includes an antenna
810 for receiving a signal 814 and transmitting a signal 820.
System 800 also includes first and second MEMS switches 830 and 840
that are electrically connected to antenna 810 via a branch circuit
844 having a first branch wire 846 and a second branch wire 848.
MEMS switch 830 includes first and second electrical contacts 852
and 854 electrically connected to respective bond pads 862 and 864,
and an actuation electrode 870 electrically connected to a bond pad
872. Likewise, MEMS switch 840 includes first and second electrical
contact 882 and 884 electrically connected to respective bond pads
892 and 894, and an actuation electrode 900 electrically connected
to a bond pad 902. Thus, in an example embodiment, first branch
wire 846 is connected to MEMS switch 830 via bond pad 862, while
second branch wire 848 is connected to MEMS switch 840 via bond pad
892. MEMS switches 830 and 840 may be any one of the MEMS switches
20, 150, 200, 300, 400 or 500 of the present invention discussed in
detail above.
[0069] System 800 further includes a voltage source controller 912
that is electrically connected to MEMS switches 830 and 840 via
respective actuation electrode bond pads 872 and 902. Voltage
source controller 912 includes logic for selectively supplying
voltages to actuation electrodes 870 and 900 to selectively
activate MEMS switches 830 and 840.
[0070] Further included in system 800 are reciever electronics 930
electrically connected to MEMS switch 830 via bond pad 864, and
transmitter electronics 940 electrically connected to MEMS switch
840 via bond pad 894.
[0071] In operation, system 800 receives and transmits wireless
signals 814 and 820. This is accomplished by voltage source
controller 912 selectively activating MEMS switches 830 and 840 so
that received signal 814 can be transmitted from antenna 810 to
receiver electronics 930 for processing, while transmitted signal
820 generated by transmitter electronics 840 can be passed to
antenna 810 for transmission.
[0072] An advantage of using MEMS switches rather than
semiconductor-based switches (e.g., transistors) in the present
application is that leakage of high transmitter power into the
sensitive and fragile reciever circuits is avoided--i.e., the MEMS
switches provide for high isolation. Switching between multiple
frequency bands in a wireless communication device such as system
800 requires switching at frequencies of several megahertz, which
is possible with the MEMS switches of the present invention.
Conclusion
[0073] The present invention is a MEMS switch that includes one of
a number of beams having a high-resonance frequency, along with
methods for forming such beams. An advantage of having a
high-resonance-frequency beam is that the switching speed can be
faster than that of conventional beams, which allows for the MEMS
switches of the present invention to be used in a variety of
applications, such as wireless communications, that require
high-speed switching.
[0074] While the present invention has been described in connection
with preferred embodiments, it will be understood that it is not so
limited. On the contrary, it is intended to cover all alternatives,
modifications and equivalents as may be included within the spirit
and scope of the invention as defined in the appended claims.
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