U.S. patent application number 11/399077 was filed with the patent office on 2006-08-17 for stress bimorph mems switches and methods of making same.
This patent application is currently assigned to HRL Laboratories, LLC. Invention is credited to Tsung-Yuan Hsu, Robert Y. Loo, James H. Schaffner, Adele E. Schmitz, Robert N. Schwartz, Gregory L. Tangonan, Ming Wu.
Application Number | 20060181379 11/399077 |
Document ID | / |
Family ID | 23262729 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060181379 |
Kind Code |
A1 |
Schwartz; Robert N. ; et
al. |
August 17, 2006 |
Stress bimorph MEMS switches and methods of making same
Abstract
A micro-electromechanical system (MEMS) switch formed on a
substrate, the switch comprising a transmission line formed on the
substrate, a substrate electrostatic plate formed on the substrate,
and an actuating portion. The actuating portion comprises a
cantilever anchor formed on the substrate and a cantilevered
actuator arm extending from the cantilever anchor. Attraction of
the actuator arm toward the substrate brings an electrical contact
into engagement with the portions of the transmission line
separated by a gap, thus bridging the transmission line gap and
closing the circuit. In order to maximize electrical isolation
between the transmission line and the electrical contact in an
OFF-state while maintaining a low actuation voltage, the actuator
arm is bent such that the minimum separation distance between the
transmission line and the electrical contact is equal to or greater
than the maximum separation distance between the substrate
electrostatic plate and arm electrostatic plate.
Inventors: |
Schwartz; Robert N.; (Costa
Mesa, CA) ; Wu; Ming; (Los Angeles, CA) ; Hsu;
Tsung-Yuan; (Westlake Village, CA) ; Schmitz; Adele
E.; (Newbury Park, CA) ; Loo; Robert Y.;
(Agoura Hills, CA) ; Schaffner; James H.;
(Chatsworth, CA) ; Tangonan; Gregory L.; (Oxnard,
CA) |
Correspondence
Address: |
Robert Popa;c/o LADAS & PARRY
5670 Wilshire Boulevard, Suite 2100
Los Angeles
CA
90036-5679
US
|
Assignee: |
HRL Laboratories, LLC
The Regents of the University of California
|
Family ID: |
23262729 |
Appl. No.: |
11/399077 |
Filed: |
April 5, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10251677 |
Sep 19, 2002 |
7053737 |
|
|
11399077 |
Apr 5, 2006 |
|
|
|
60324244 |
Sep 21, 2001 |
|
|
|
Current U.S.
Class: |
335/78 |
Current CPC
Class: |
H01G 5/18 20130101; H01H
1/20 20130101; H01H 59/0009 20130101; H01H 2059/0072 20130101; H01G
5/40 20130101; H01H 2059/0018 20130101; H01H 2059/0081
20130101 |
Class at
Publication: |
335/078 |
International
Class: |
H01H 51/22 20060101
H01H051/22 |
Claims
1-19. (canceled)
20. A method for minimizing capacitive coupling between elements of
a micro-electromechanical switch formed on a substrate, said method
comprising: forming a substrate electrostatic plate on said
substrate; forming a transmission line on said substrate, said
transmission line having a gap forming an open circuit; forming an
actuating portion, said actuating portion comprising: a cantilever
anchor formed on said substrate; and a cantilevered actuator arm
extending from said cantilever anchor above said substrate, said
actuator arm having an arm electrostatic plate formed thereon, said
actuator arm having an electrical contact facing said transmission
line, wherein when said switch is in an open position said actuator
arm has a bend such that a minimum separation distance between said
transmission line and said electrical contact is greater than or
equal to a maximum separation distance between said substrate
electrostatic plate and said arm electrostatic plate, whereby
capacitive coupling between said transmission line and said
electrical contact is minimized by maximizing the distance between
said transmission line and said electrical contact formed on said
actuator arm.
21. The method of claim 20, wherein said substrate comprises a
semi-insulating substrate.
22. The method of claim 21, wherein said semi-insulating substrate
comprises gallium arsenide (GaAs).
23. The method of claim 20, wherein said actuator arm comprises
polycrystalline silicon.
24. The method of claim 20 wherein said bend in said actuator arm
is produced by inducing a nonuniform level of residual stress in
said arm electrostatic plate formed on said actuator arm.
25. The method of claim 20 wherein said bend in said actuator arm
is produced by inducing a nonuniform level of residual stress in
said actuator arm.
26. The method of claim 20, wherein said substrate electrostatic
plate and said transmission line comprise gold microstrips on said
substrate.
27. The method of claim 20, wherein said electrical contact
comprises a metal selected from the group consisting of gold,
platinum, and gold palladium.
28. A method of fabricating an electromechanical switch on a
substrate comprising the steps of: a) applying electrically
conductive material to said substrate to form a substrate
electrostatic plate region, an input transmission line region, and
an output transmission line region, said regions being electrically
isolated from one another; b) depositing one or more sacrificial
layers over said regions; c) forming an electrical contact region
on said one or more sacrificial layers, said electrical contact
region being applied above said input transmission line region and
said output transmission line region and said electrical contact
region comprising electrically conductive material; d) forming an
actuating arm on said one or more sacrificial layers, said
actuating arm contacting said substrate at a first end and
contacting said second layer of electrically conductive material at
a second end; e) forming an arm electrostatic plate region on said
actuating arm, said arm electrostatic plate region being disposed
above said substrate electrostatic pate plate region and comprising
electrically conductive material; and f) removing said one or more
sacrificial layers, wherein said steps of forming an actuating arm
and/or forming an arm electrostatic plate region are performed to
induce nonuniform stress in said actuating arm so that actuating
arm will bow upwards when the one or more sacrificial layers are
removed.
29. The method of claim 28, wherein the one or more sacrificial
layers comprise silicon dioxide.
30. The method of claim 28, wherein the actuating arm comprises
polycrystalline silicon.
31. The method of claim 28, wherein said arm electrostatic plate
region comprises aluminum.
32. The method of claim 28, wherein said electrical contact region,
said input transmission line region, and said output transmission
line region comprise gold.
33. A micro-electromechanical switch formed on a substrate
comprising: a substrate transmission line formed on said substrate,
said substrate transmission line having a gap forming an open
circuit; a substrate electrostatic plate formed on said substrate;
and an actuating portion, said actuating portion comprising: a
cantilever anchor formed on said substrate; a cantilevered actuator
arm extending from said cantilever anchor, said actuator arm having
an upper side and a lower side; a conducting transmission line
formed on said upper side of said actuator arm, said conducting
transmission line having one or more conducting dimples projecting
through said actuator arm and positioned facing said transmission
line gap; and an arm electrostatic plate formed on said actuator
arm, said arm electrostatic plate having a first portion formed
proximate said cantilever anchor and a second portion extending
from said first portion along said actuator arm, wherein when said
switch is in an open position, said actuator arm has a bend such
that a minimum separation distance between said substrate
transmission line and said one or more conducting dimples is equal
to or greater than a maximum separation distance between said
substrate electrostatic plate and said arm electrostatic plate,
said arm electrostatic plate and a segment of said actuator arm on
which said arm electrostatic plate is formed defining a structure
electrostatically attractable toward said substrate electrostatic
plate upon selective application of a voltage to said arm
electrostatic plate.
34. The micro-electromechanical switch of claim 33, wherein the
electrostatic attraction of said electrostatically attractable
structure toward said substrate electrostatic plate causes said one
or more conducting dimples to contact said substrate transmission
line to bridge said gap.
35. The micro-electromechanical switch of claim 33, wherein said
substrate comprises a semi-insulating substrate.
36. The micro-electromechanical RF switch of claim 35, wherein said
semi-insulating substrate comprises gallium arsenide (GaAs).
37. The micro-electromechanical switch of claim 33, wherein said
actuator arm comprises polycrystalline silicon.
38. The micro-electromechanical switch of claim 33 wherein said
bend in said actuator arm is produced by inducing a nonuniform
level of residual stress in said arm electrostatic plate formed on
said actuator arm.
39. The micro-electromechanical switch of claim 33 wherein said
bend in said actuator arm is produced by inducing a nonuniform
level of residual stress in said actuator arm.
40. The micro-electromechanical switch of claim 33, wherein said
substrate electrostatic plate and said transmission line comprise
gold microstrips on said substrate.
41. The micro-electromechanical switch of claim 33, wherein said
one or more conducting dimples comprise a metal selected from the
group consisting of gold, platinum, and gold palladium.
43. A method of fabricating an electromechanical switch on a
substrate comprising the steps of: a) forming a first layer of
electrically conductive material over a surface of said substrate,
said first layer comprising an arm plate contact region, a
substrate electrostatic plate region, an input transmission line
region, and an output transmission line region, wherein each of
said regions is electrically isolated from other said regions; b)
depositing a sacrificial layer over said first layer of
electrically conductive material; c) forming an arm structural
layer, said arm structural layer providing a cantilever with a
proximate end and a distal end, said distal end located above said
input transmission line region and above said output transmission
line region, said cantilever having a cantilever anchor and a
cantilever arm, said cantilever anchor formed at said proximate end
and located above said arm plate contact region, said cantilever
arm projecting from said cantilever anchor to said distal end and
located above said substrate electrostatic plate region; d)
removing a portion of said arm structural layer at said distal end
to create a dimple area above said input transmission line region
and said output transmission line region; e) removing a portion of
said arm structural layer at said cantilever anchor to expose said
arm plate contact region; f) depositing a layer of metal on said
arm structural layer, said layer of metal filling said dimple area
and said layer of metal providing an arm electrostatic plate
electrically connected to said arm plate contact region, said arm
electrostatic plate located generally above said substrate
electrostatic plate region; and g) removing said sacrificial layer,
wherein the step of forming the arm structural layer is performed
to induce stress in the arm structural layer that varies from the
stress of the layer of metal deposited on the arm structural layer
so that the cantilever arm will bow upwards when the sacrificial
layer is removed.
44. The method of claim 43, wherein said sacrificial layer
comprises a layer of silicon dioxide.
45. The method of claim 43 wherein said arm structural layer
comprises silicon nitride.
46. The method of claim 43, wherein said step of depositing a layer
of metal on said arm support layer comprises the steps of: sputter
depositing a thin film of titanium; and depositing a layer of
gold.
47. The method of claim 43, wherein the step of removing the
sacrificial layer is performed by wet etching the sacrificial
layer.
48. A micro-electromechanical switch formed on a substrate
comprising: a substrate transmission line formed on said substrate,
said substrate transmission line having a gap forming an open
circuit; a substrate electrostatic plate formed on said substrate;
and an actuating portion, said actuating portion comprising: a
cantilever anchor formed on said substrate; a cantilevered actuator
arm having a proximate end and a distal end, said cantilevered
actuator arm attached to the cantilever anchor at the proximate end
of the cantilevered actuator arm a cantilever insulating layer
attached at the distal end of the cantilevered actuator arm; an
electrical contact formed on said cantilever insulating layer and
positioned facing said transmission line gap; and an arm
electrostatic plate formed on said actuator arm, said arm
electrostatic plate having a first portion formed proximate said
cantilever anchor and a second portion extending from said first
portion along said actuator arm, wherein when said switch is an
open position, said actuator arm has a bend such that a minimum
separation distance between said transmission line and said
electrical contact is equal to or greater than a maximum separation
distance between said substrate electrostatic plate and said arm
electrostatic plate, said arm electrostatic plate and a segment of
said actuator arm on which said arm electrostatic plate is formed
defining a structure electrostatically attractable toward said
substrate electrostatic plate upon selective application of a
voltage to said arm electrostatic plate.
49. The micro-electromechanical switch of claim 48, said switch
further comprising one or more mechanical stops formed on said
substrate, said mechanical stops disposed adjacent said substrate
electrostatic plate and having a height greater than a height of
the substrate electrostatic plate
50. The micro electromechanical switch of claim 48, wherein the
electrostatic attraction of said electrostatically attractable
structure toward said substrate electrostatic plate causes said
electrical contact on said actuator arm to bridge said transmission
line gap.
51. The micro electromechanical switch of claim 48, wherein said
substrate comprises a silicon substrate with a layer of silicon
nitride.
52. The micro electromechanical switch of claim 48, wherein said
actuator arm comprises polycrystalline silicon.
53. The micro electromechanical switch of claim 48 wherein said
bend in said actuator arm is produced by inducing a nonuniform
level of residual stress in said arm electrostatic plate formed on
said actuator arm.
54. The micro-electromechanical switch of claim 48 wherein said
bend in said actuator arm is produced by inducing a nonuniform
level of residual stress in said actuator arm.
55. The micro-electromechanical switch of claim 48, wherein said
substrate electrostatic plate comprises polysilicon.
56. The micro-electromechanical switch of claim 48, wherein said
electrical contact comprises a metal selected from the group
consisting of gold, platinum, and gold palladium.
57. The micro-electromechanical switch of claim 48, wherein said
arm electrostatic plate comprises a metal selected from the group
consisting of gold, platinum, and gold palladium.
58. The micro electromechanical switch of claim 48, wherein said
cantilever insulating layer comprises silicon nitride.
59. A method of fabricating an electromechanical switch on a
substrate comprising the steps of: a) depositing a first layer of
electrically conductive material over a surface of said substrate;
b) depositing a sacrificial cantilever support layer over said
first layer of electrically conductive material; c) forming a arm
structural layer of electrically conductive material over said
sacrificial cantilever support layer, said arm structural layer
having a proximate end and a distal end, said arm structural layer
having a cantilever anchor disposed at said proximate end, said
cantilever anchor formed on said surface of said substrate, and
said arm structural layer having a cantilever arm projecting from
said cantilever anchor; d) depositing a first metal layer on said
cantilever arm, said first metal layer located generally above said
first layer of electrically conductive material; e) depositing a
second metal layer on or above said substrate to form a
transmission line with a gap in the middle, said second metal layer
located adjacent said distal end; g) depositing a conductor
sacrificial layer on top of said second metal layer; h) depositing
a third metal layer on top of said conductor sacrificial layer,
said third metal layer positioned above said second metal layer and
extending across said gap; i) forming an insulating layer above
said third metal layer and located adjacent said distal end, said
insulating layer attaching to said distal end and to said third
metal layer; and j) removing said sacrificial layers, wherein the
step of forming the arm structural layer is performed to induce
stress in the arm structural layer that varies from the stress of
the first layer of metal so that the cantilever arm will bow
upwards when said sacrificial layers are removed.
60. The method of claim 59 further comprising the step of: forming
a mechanical stop layer over said surface of said substrate, said
mechanical stop layer disposed proximate to said first layer of
electrically conductive material and having a height greater than a
height of said first layer of electrically conductive material,
wherein the step of forming a mechanical stop layer being performed
prior to the step of depositing a sacrificial cantilever support
layer.
61. The method of claim 59, wherein said first layer of
electrically conductive material comprises polysilicon.
62. The method of claim 60, wherein said step of forming a
mechanical stop layer comprises the steps of: depositing a
mechanical stop sacrificial layer on top of said first layer of
electrically conductive material; etching said mechanical stop
sacrificial layer to form areas for the mechanical stops; and
filling the areas for the mechanical stops with poly silicon to
form said mechanical stops.
63. The method of claim 59, wherein said arm structural layer
comprises polysilicon.
64. The method of claim 59, wherein said insulating layer comprises
silicon nitride.
65. The method of claim 20, wherein said cantilevered actuator arm
comprises an insulating material and said arm electrostatic plate
is disposed on top of said cantilevered actuator arm and said
cantilevered actuator arm insulates said arm electrostatic plate
from said substrate electrostatic plate when said cantilevered
actuator arm rests against said substrate electrostatic plate.
66. The method of claim 20, wherein said cantilevered arm is
configured such that after a voltage is applied to attract said
cantilevered arm towards said substrate, said cantilevered arm
closes against said substrate electrostatic plate in a continuous
manner wherein portions of said cantilevered arm closet to a first
end of said cantilevered arm close against said substrate
electrostatic plate before portions of said cantilevered arm
closest to a second end of said cantilevered arm close against said
substrate electrostatic plate.
67. The method of claim 28, wherein said actuating arm comprises an
insulating material and said arm electrostatic plate region is
disposed on top of said actuating arm and said actuating arm
insulates said arm electrostatic plate region from said substrate
electrostatic plate region when said actuating arm rests against
said substrate electrostatic plate region.
68. The method of claim 28, wherein said actuating arm is
configured such that after a voltage is applied to attract said
actuating arm towards said substrate, said actuating arm closes
against said substrate electrostatic plate region in a continuous
manner wherein portions of said actuating arm closest to a first
end of said actuating arm close against said substrate
electrostatic plate region before portions of said actuating arm
closest to a second end of said actuating arm close against said
substrate electrostatic plate region.
Description
BACKGROUND
[0001] 1. Field
[0002] The present invention relates to micro-electromechanical
systems (MEMS) and, in particular, to a micromachined
electromechanical radio frequency (RF) switch that can preferably
function over a range of signal frequencies from 0 Hz to
approximately 100 GHz.
[0003] 2. Description of Related Art
[0004] MEMS (micro-electromechanical system) switches have a wide
variety of uses in both military and commercial applications. For
example, electrostatically actuated micro-electromechanical
switches can conduct RF current in applications involving the use
of antenna phase shifters, in the tuning of reconfigurable antenna
elements, and in the fabrication of tunable filters.
[0005] A representative example of a prior art MEMS switch is
disclosed in Yao, U.S. Pat. No. 5,578,976, issued Nov. 26, 1996.
Typically, this type of MEMS switch is fabricated on a
semi-insulating substrate with a suspended micro-beam element as a
cantilevered actuator arm. The cantilever arm is coupled to the
substrate and extends parallel to the substrate, projecting over a
ground line and a gapped signal line formed by metal microstrips on
the substrate. A metal contact, preferably comprising a metal that
does not easily oxidize, such as platinum, gold, or gold palladium,
is formed on the bottom of the cantilever arm remote from the fixed
end of the beam and positioned above and facing the gap in the
signal line. A portion of the cantilever arm and an arm
electrostatic plate located thereon reside above the ground line on
the substrate. When a voltage is applied to the arm electrostatic
plate, electrostatic forces attract the arm electrostatic plate,
and thus the cantilever arm, toward the ground line on the
substrate, bringing the metal contact into engagement with the
separate portions of the gapped signal line, and thereby bridging
the gap in the signal line.
[0006] Another example of an RF MEMS switch utilizing a cantilever
actuator arm is disclosed in Loo et al., U.S. Pat. No. 6,046,659,
issued Apr. 4, 2000. In Loo et al., the cantilever actuator arm
comprises a multiple layer structure containing the arm
electrostatic plate surrounded by insulating layers. As in Yao, the
RF MEMS switch disclosed by Loo et al. provides a metal contact
that bridges a gap between two portions of an RF signal line, when
the switch is closed. Both Yao and Loo et al. disclose that the
cantilever actuator arm is generally disposed parallel to the
surface of the substrate when the RF MEMS switch is in the open
position. Thus, the distance between the metal contact and the RF
signal line when the RF MEMS switch is in the open position is
limited to the distance between the cantilever actuator arm and the
substrate along nearly the entire length of the cantilever actuator
arm.
[0007] RF MEMS switches provide several advantages over
conventional RF switches which use transistors. These advantages
include lower insertion loss, improved electrical isolation over a
broad frequency range, and lower power consumption. Since this type
of switch is fabricated using existing integrated circuit (IC)
processing technologies, production costs are relatively low. Thus,
RF MEMS switches manufactured using micromachining techniques have
advantages over conventional transistor-based RF switches because
the MEMS switches function like macroscopic mechanical switches,
but without the associated bulk and relatively high cost.
[0008] However, integrated RF MEMS switches are difficult to
implement. Due to the proximity of the electrical contact formed on
the cantilever arm to the signal line formed on the substrate,
these switches tend to exhibit poor electrical isolation at high
frequencies. In the RF regime, close proximity of the electrical
contact and the signal line allows parasitic capacitive coupling
between the contact and signal line when the switch is in the
OFF-state, creating an AC leakage path for high frequency signals.
These losses, which increase with signal frequency, limit the use
of MEMS switches in high frequency applications.
[0009] Capacitive coupling may be reduced by increasing the
separation distance between the signal line formed on the substrate
and the metal contact formed on the cantilever arm. However, in the
MEMS switch described above, there is a design tradeoff between the
OFF-state capacitance and the switch actuation voltage. This
tradeoff can be expressed mathematically. The OFF-state capacitance
of the switch is given by the relation: C OFF = .times. .times. 0
.times. A d ( 1 ) ##EQU1## where A is the area of overlap between
the contact and the signal line, d is the distance between the
contact and the signal line, e.sub.0 is the permittivity of free
space and e is the dielectric constant of the material between the
contact and the signal line.
[0010] The actuation voltage of a cantilever beam in a switch as
described above can be approximated by: V S .apprxeq. 18 .times.
EId 3 5 .times. .times. .times. 0 .times. L 4 .times. w ( 2 )
##EQU2## where E is Young's modulus of the beam material, I is the
moment of inertia of the beam cross-section, and L and w are the
length and width of the cantilever beam, respectively. For a
cantilever beam with a uniform width w, and a thickness t, the
moment of inertia is given by: I = t 3 .times. w 12 ( 3 ) ##EQU3##
and V.sub.S can be simplified to: V S = 3 .times. E .function. ( dt
) 3 10 .times. .times. .times. .times. 0 .times. L 4 ( 4 )
##EQU4##
[0011] Combining the above expressions (1) and (4) yields
C.sub.OFF.varies.V.sub.S.sup.-2/3 (5)
[0012] Thus, in the RF MEMS switches of the type described above,
increasing the separation distance between the signal line formed
on the substrate and the electrical contact formed on the
cantilever arm also increases the voltage required to affect
electrostatic actuation of the switch, because the separation
distance between the signal line and the contact is also the
separation distance between the arm electrostatic plate and the
ground line. The energy that must be moved through the switch
control in order to activate the switch, and thus the energy
dissipated by the switch, is a function of the actuation voltage.
Therefore, in order to minimize the energy dissipated by the RF
MEMS switch, it is desirable to minimize the actuation voltage of
the switch.
[0013] Another problem with the conventional cantilever switch
described above stems from the methods used to manufacture the
switch. A polycrystalline silicon (or polysilicon) cantilever beam
can be fabricated by first oxidizing a silicon substrate to provide
a sacrificial layer, then depositing and patterning a layer of
polysilicon into a long, narrow bar directly over the silicon
dioxide. The beam is then separated from the sacrificial silicon
dioxide layer by application of a release agent comprising a
hydrofluoric acid solution, which dissolves the sacrificial layer
and results in a free-standing polysilicon beam spaced apart from
the substrate. The substrate is immersed in the release agent for a
duration sufficient to result in release of the beam. One problem
with the use of this release process for a beam in relatively close
proximity to the substrate is that surface tension forces exerted
by the release agent tend to pull the beam toward the substrate as
the device is immersed in and pulled out of the solutions. This can
cause the beam to stick to the substrate during drying, a
phenomenon known as stiction.
[0014] In view of the foregoing, there is a need for a
micro-electromechanical switch having improved electrical isolation
and improved manufacturability, without requiring a corresponding
increase in actuation voltage.
SUMMARY
[0015] Embodiments of electromechanical switches according to the
present invention minimize the OFF-state capacitance of the
electrostatically actuatable micro-electromechanical switch formed
on a substrate, without a corresponding increase in the voltage
required to actuate the switch. Embodiments of the present
invention achieve minimization of the OFF-state capacitance by
utilizing an actuator arm bent such that the minimum separation
distance between an electrical contact formed on the actuator arm
and a transmission line formed on the substrate is equal to or
greater than the maximum separation distance between a substrate
electrostatic plate formed on the substrate and an arm
electrostatic plate formed on the actuator arm. The bilaminar
cantilever structure of the preferred embodiments enable a large
separation (up to approximately 300 micrometers) to be achieved
between the transmission line formed on the substrate and the
electrical contact formed on the actuator arm, while maintaining a
very low actuation voltage (approximately 20 V). This large
separation can be used to reduce the capacitance of the RF MEMS
switch in the OFF state, thus providing high isolation at high
frequencies.
[0016] The desired minimization of the OFF-state capacitance is
achieved without a corresponding increase in the actuation voltage
by forming the arm electrostatic plate at a point on the actuator
arm that allows the distance between the arm electrostatic plate,
formed on the actuator arm, and the substrate electrostatic plate,
formed on the substrate, to be precisely and repeatably controlled,
thus allowing the actuation voltage to be correspondingly
controlled.
[0017] The tendency of the beam to stick to the substrate during
drying is reduced by forming the bend in the actuator arm through
the generation of unbalanced residual stresses in either the
polycrystalline silicon comprising the actuator arm or the metallic
layer formed on the actuator arm, this metallic layer comprising
the arm electrostatic plate. The unbalanced residual stresses can
be generated by manipulation of deposition process parameters
during formation of the actuator arm structure. Due to these
residual stresses in the actuator arm structure, the actuator arm
is in a stressed condition prior to release from the sacrificial
layer and will tend to bend away from the substrate when released.
This counters the tendency of the arm to deflect toward the
substrate in response to surface tension forces exerted by the
release solution.
[0018] A general embodiment of the electromechanical switches
according to the present invention has a cantilevered actuator arm
which has an electrostatic plate disposed above an electrostatic
plate positioned on a substrate. The switch is open and closed by
the electrostatic attraction between the plates. In the open
position, the cantilevered arm curves away from the substrate.
Switching is provided by a gapped transmission line positioned on
the substrate at one end of the cantilevered arm. The arm carries
an electrical contact that bridges the gap when the switch is in
the closed position. The electrical contact may simply be a region
of metal or other electrically conducting material attached to the
arm. The electrical contact may also comprise electrically
conducting material that projects through the arm to contact the
gapped transmission line when the switch is closed. The electrical
contact may also be electrically isolated from the arm by a layer
of insulating material disposed at the end of the arm. The arm may
also be electrically isolated from the electrostatic plate on the
substrate when the switch is closed by mechanical stops disposed
next to the electrostatic plate that prevent the arm from
contacting the plate.
[0019] Embodiments of the switches according to the present
invention may be fabricated by well-known integrated circuit
fabrication processes. Generally, The processes also involve
applying one or more layers of sacrificial material. These layers
of sacrificial material support the fabrication of the desired
structures for the switch. Other processes involve applying one or
more layers of electrically conductive material to form the
electrically conductive elements, such as the electrostatic plates
and electrical contact. As briefly noted above, it is desired that
the actuating arm of the cantilever structure according to the
present invention be fabricated such that it curls or curves
upwards when the switch is open. Processes used to obtain this
result are described below.
[0020] An aspect of the present invention comprises: a substrate; a
first electrical contact formed on the substrate; a substrate
electrostatic plate formed on the substrate; a cantilever actuator
arm anchored to the substrate at a first end of the actuator arm; a
second electrical contact disposed at a second end of the actuator
arm, the second electrical contact being in electrically contact
with the first electrical contact when the switch is in a closed
position; the substrate electrostatic plate disposed beneath the
actuator arm and between the first end and the second end of the
actuator arm; and an arm electrostatic plate formed on the actuator
arm and positioned above the substrate electrostatic plate when the
switch is in a closed position, wherein when the switch is in an
open position, the actuator arm curves away from the substrate.
[0021] Another embodiment of the present invention also provides a
method for switching electrical energy comprising providing an
electrostatically actuated cantilevered arm on a substrate;
applying a voltage to attract the electrostatically actuated
cantilevered arm towards the substrate, the cantilevered arm having
an electrical contact that electrically connects the input to the
output when the voltage is applied; and removing the voltage or
applying a second voltage to cause the cantilevered arm to move
away from the substrate, the electrical contact no longer
electrically connecting the input to the output when the voltage is
removed or the second voltage is applied, such that the
cantilevered arm curves away from the substrate when the voltage is
removed or the second voltage is applied.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a side view of the cantilever beam and contact
arrangement of an RF MEMS switch according to a preferred
embodiment of the present invention, showing the switch in the open
position.
[0023] FIG. 2 is a side view similar to FIG. 1 showing the switch
in the closed position.
[0024] FIG. 3 is a plan view of the switch of FIG. 1 in the open
position.
[0025] FIGS. 4A-N are side views of the switch of FIG. 1
illustrating the steps in fabricating the switch.
[0026] FIG. 5A is a side view of a second embodiment of an RF MEMS
switch according to the present invention, showing the switch in
the open position.
[0027] FIG. 5B is a side view of the switch of FIG. 5A, showing the
switch in the closed position.
[0028] FIG. 6 is a plan view of the switch of FIGS. 5A and 5B,
showing the general size, shape, and orientation of the various
layers of the switch.
[0029] FIGS. 7A-F are side views of the switch of FIGS. 5A, 5B, and
6, illustrating the steps in fabricating the switch.
[0030] FIG. 8 is a side view of a third embodiment of an RF MEMS
switch according to the present invention, showing the switch in
the open position.
[0031] FIG. 9 is a side view of the switch of FIG. 8, showing the
switch in the closed position.
[0032] FIGS. 10A-10T are side views of the switch of FIGS. 8 and 9,
illustrating the steps in fabricating the switch.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] The present disclosure describes a miniature RF switch
designed for applications over a frequency range from DC to
approximately 100 GHz. The following disclosure describes an RF
MEMS switch according to the present invention fabricated on a
silicon-based substrate. However, RF MEMS switches according to the
present invention may also be fabricated from various other
substrate materials, such as gallium arsenide (GaAs), glass, and
other dielectrics.
[0034] In a preferred embodiment, a micro-electromechanical switch,
generally designated 124 and best illustrated in FIGS. 1, 2 and 3,
is fabricated on a substrate 110 using generally known
microfabrication techniques, such as masking, etching, deposition,
and lift-off. In a preferred embodiment, the RF MEMS switch 124 is
directly formed on the substrate 110 and monolithically integrated
with a transmission line 114. Alternatively, the RF MEMS switch 124
may be discreetly formed and then bonded to the substrate 110. The
switch 124 comprises the transmission line 114, a substrate
electrostatic plate 120, an actuating portion 126, and an
electrical contact 134. The substrate electrostatic plate 120
(typically connected to ground) and the transmission line 114 are
formed on the substrate 110. An insulating layer 111 may be used to
electrically isolate the transmission line 114, the substrate
electrostatic plate 120, and the actuating portion 126 from the
substrate 110. The substrate electrostatic plate 120 and the
transmission line 114 preferably comprise microstrips of a metal
not easily oxidized, e.g., gold, deposited or otherwise formed on
the substrate 110. The transmission line 114 includes a gap 118
(shown in FIG. 3) that is opened and closed by operation of the
switch 124, in a manner explained below.
[0035] The actuating portion 126 of the switch 124 comprises a
cantilever anchor 128 formed on the substrate 110, and a
cantilevered actuator arm 130 extending from cantilever anchor 128.
The actuator arm 130 forms a suspended micro-beam projecting from
one end at the cantilever anchor 128 and extending over and above
the substrate electrostatic plate 120 and the transmission line 114
on the substrate 110.
[0036] The actuator arm 130 has a bilaminar cantilever (or bimorph)
structure, that is, the structure comprises two dissimilar
materials, preferably with different residual stresses, layered
together. Due to its mechanical properties, the bimorph structure
exhibits a very high ratio of displacement to actuation voltage.
That is, a relatively large displacement (approximately 300
micrometers) can be produced in the bimorph cantilever in response
to a relatively low switching voltage (approximately 20 V). A first
layer 136 of the actuator arm 130 preferably comprises a
semi-insulating or insulating material, such as polycrystalline
silicon. A second layer 132 of the actuating arm 130 preferably
comprises a metal film (typically aluminum or gold) deposited atop
first layer 136.
[0037] As shown in FIG. 1, the second layer 132 comprises a first
portion 138 formed proximate the cantilever anchor 128 and a second
portion 140 extending from the first portion 138 toward the
position on the actuator arm 130 at which the electrical contact
134 is formed. The second portion 140 typically acts as an arm
electrostatic plate 140 during operation of the switch 124. In the
remainder of the description, the terms "second layer" and "arm
electrostatic plate" will be used interchangeably. A third portion
139 of the second layer 132 may be formed within or on the
cantilever anchor 128 to provide an electrical connection to the
second layer 132. At least a portion of the arm electrostatic plate
140 and a corresponding portion of the actuator arm 130 on which
the arm electrostatic plate 140 is formed are positioned above the
substrate electrostatic plate 120 to form an electrostatically
actuatable structure. The height of the cantilever anchor 128 and
the first layer 136 above the substrate 110 can be tightly
controlled using known fabrication methods. Forming the arm
electrostatic plate 140 on top of the first layer 136 allows a
correspondingly high degree of control over the height of the arm
electrostatic plate 140 above the substrate electrostatic plate
120. As the switch actuation voltage is dependent upon the distance
between the substrate electrostatic plate 120 and the arm
electrostatic plate 140, a high degree of control over the spacing
between the electrostatic plates 120, 140 is preferred in order to
repeatably achieve a desired actuation voltage.
[0038] The electrical contact 134, typically comprising a metal
that does not easily oxidize, e.g., gold, platinum, or gold
palladium, is formed on the actuator arm 130 and positioned on the
arm 130 so as to face the gap 118 formed in the transmission line
114. When the switch 124 is in the closed position, the electrical
contact 134 bridges the gap 118 and provides an electrical
connection between the two portions of the transmission line
114.
[0039] To achieve a low actuation voltage without sacrificing
electrical isolation in the OFF-state (or open switch state), the
actuator arm 130 is formed so it bends or curls upwards and away
from the substrate 110. Preferably, the upwards curl in the
actuator arm 130 is such that the minimum separation distance
between the transmission line 114 and the electrical contact 134
formed on the actuator arm 130 is equal to or greater than the
maximum separation distance between the substrate electrostatic
plate 120 and the arm electrostatic plate 140 when the switch 124
is in the open position. The upwards curl in the actuator arm 130
is caused by nonuniform residual stresses induced in the material
comprising the first layer 136 of the actuator arm 130, the second
layer 132, or both layers 132, 136 during the fabrication of those
layers. Alternatively, the curve in the actuator arm may be induced
by using materials with different residual stresses for the first
layer 136 and the second layer 132 of the actuator arm 130. The
different residual stresses may result from different properties of
the first layer 136 and the second layer 132, such as different
coefficients of thermal expansion.
[0040] The operation of the preferred embodiment will now be
discussed with reference to FIGS. 1-3. In operation, the switch 124
is normally in an open or "OFF" position as shown in FIG. 1. With
the switch 124 in the OFF-state, the transmission line 114 is an
open circuit due to both the gap 118 and the separation of the
electrical contact 134 from the transmission line 114.
[0041] The switch 124 is actuated to the closed or "ON" position by
application of a voltage between the arm electrostatic plate 140
and the substrate electrostatic plate 120. When the voltage is
applied, the arm electrostatic plate 140 is electrostatically
attracted toward the substrate electrostatic plate 120, forcing the
actuator arm 130 to deflect toward the substrate 110. Deflection of
the actuator arm 130 toward the substrate electrostatic plate 120,
as indicated by double-headed arrow 11 in FIG. 1, causes the
electrical contact 134 to come into contact with the transmission
line 114, thereby bridging the gap 118 and placing the transmission
line 114 in an ON-state (i.e., closing the circuit). As previously
explained, the arm electrostatic plate 140 is formed at a point on
the actuator arm 130, (for example, adjacent the cantilever anchor
128) which allows the distance between the arm electrostatic plate
140 and the substrate electrostatic plate 120 formed on the
substrate 110 to be precisely and repeatably controlled using
standard photolithographic processes.
[0042] Furthermore, in the "OFF" or open state, the actuator arm
130 curls upwards so that the minimum separation distance between
the transmission line 114 and the electrical contact 134 formed on
the actuator arm 130 is preferably equal to or greater than the
maximum separation distance between the substrate electrostatic
plate 120 and the arm electrostatic plate 140. Thus, the distance
between the electrical contact 134 and the transmission line 114
formed on the substrate 110 is greater than the corresponding
spacing characteristic of conventional MEMS cantilever-type
switches (such as those disclosed by Yao and Loo et al., as
discussed previously). As a result, the OFF-state capacitance of
the switch is greatly reduced.
[0043] The actuation voltage required to close the switch 124 is
primarily determined by the distance between the substrate
electrostatic plate 120 and the portion of the arm electrostatic
plate 140 disposed closest to the substrate electrostatic plate
120. Since the distance separating the arm electrostatic plate 140
and the substrate electrostatic plate 120 formed on the substrate
110 is precisely and repeatably controlled, the voltage required to
cause the actuator arm 130 to snap down can be correspondingly
controlled and minimized to maintain a relatively low actuation
voltage. Further, due to the curl of the actuator arm 130, a
zipper-like actuation motion is produced upon application of the
actuation voltage. That is, the end of the arm electrostatic plate
140 closest to the cantilever anchor 128 will initially be
attracted towards the substrate electrostatic plate 120. The motion
of this end towards the substrate electrostatic plate 120 will
decrease the distance of the remainder of the arm electrostatic
plate 140 from the substrate electrostatic plate 120, which further
decreases the voltage required to close the switch 124. Therefore,
the overall motion of the actuator arm 130 as it moves towards the
substrate electrostatic plate 120 appears much like the motion of a
zipper as it is closed.
[0044] Embodiments of the present invention provide the important
advantage of reduced OFF-state capacitance without a corresponding
increase in actuation voltage. Thus, the actuation voltage and the
RF performance of the switches according to the present invention
can be separately optimized.
[0045] One possible method of fabricating the switch 124 will now
be described. The switch 124 may be manufactured using generally
known microfabrication techniques, such as masking, etching,
deposition, and lift-off. For example, the switch 124 may be
fabricated using a foundry-based polysilicon surface-micromachining
process, or a metal/insulator surface-micromachining process. The
substrate 110 for one preferred embodiment may be a semi-insulating
GaAs wafer, although other materials such as InP, ceramics, quartz
or silicon may be used. Polycrystalline silicon deposited using
plasma enhanced chemical vapor deposition may be used as the
preferred structural material for cantilever anchor 128 and
actuator arm 130, and silicon dioxide may be used as sacrificial
material, as described below.
[0046] FIGS. 4A-N are side view schematic illustrations of a
process sequence that may be used to fabricate the switch 124
illustrated in FIGS. 1-3. Note that the insulating layer 111 is not
shown in FIGS. 4A-N, but alternative fabrication processes may
include this feature. Note also that the switch 124 may be
fabricated by processes other than those depicted in FIGS. 4A-N.
Further, while FIGS. 4A-N depict multiple separate fabrication
steps, alternative fabrication processes may allow several separate
steps to be combined into fewer steps. Finally, alternative
fabrication processes may use a different sequence of steps.
[0047] The fabrication of the switch 124 may begin with the
fabrication of the substrate electrostatic plate 120. Prior to
forming the substrate electrostatic plate 120, a first layer 142 of
sacrificial material, such as silicon dioxide, is formed on the
substrate 110, as shown in FIG. 4A. A hole 121 is then etched in
the first sacrificial layer 142 to accommodate the substrate
electrostatic plate 120, as shown in FIG. 4B. A first layer of gold
(or other conductor) is preferably deposited using electron beam
evaporation and liftoff to form the substrate electrostatic plate
120, as shown in FIG. 4C.
[0048] The gapped transmission line 114 may then be formed. A
second layer 146 of sacrificial material, such as silicon dioxide,
is deposited, as shown in FIG. 4D. A hole 115 at one side of the
switch 124 is then etched through the first sacrificial layer 142
and the second sacrificial layer 146 for a first portion of the
transmission line 114, as shown in FIG. 4E. A corresponding hole
(not shown in FIGS. 4A-4N) on the other side of the switch 124 is
also etched through the first sacrificial layer 142 and the second
sacrificial layer 146 for a second portion of the transmission line
114. The first and second portions of the transmission line 114 are
separated by the gap 118. A second layer of conductive material,
such as gold, is preferably deposited using electron beam
evaporation and liftoff to form the first and second portions of
the transmission line 114, as shown in FIG. 4F.
[0049] The electrical contact 134 may then be formed. A third
sacrificial layer 148 is deposited on top of the second sacrificial
layer 146 and portions of the transmission line 114, as shown in
FIG. 4G. A hole 135 is then preferably partially etched in the
third sacrificial layer 148, as shown in FIG. 4H. A third layer of
conductive material, such as gold, is preferably deposited using
electron beam evaporation and liftoff to form the electrical
contact 134, as shown in FIG. 41.
[0050] The actuating portion 126 may then be formed. A hole 137 is
etched through the first sacrificial layer 142, the second
sacrificial layer 146, and the third sacrificial layer 148 to form
the location for the cantilever anchor 128, as shown in FIG. 4J. A
layer of polycrystalline silicon 136 is then deposited atop the
sacrificial layers 142, 146, 148 to form the cantilever anchor 128
and the actuator arm 130, as shown in FIG. 4K. Preferably, the
residual stresses in the polycrystalline silicon are used to
control the extent of the upwards curl in the actuator arm 130 when
the switch 124 is in the OFF-state. Process factors affecting the
residual stresses in polycrystalline silicon during the deposition
and release phases include the structure of the deposited layer
(i.e., the degree of crystallinity), the texture of the layer, the
thickness of the layer, the speed at which the layer deposition
process occurs and the presence or absence of doping. Also, during
the release phase, residual stresses in polycrystalline silicon are
affected by time of exposure to release agents. By controlling
these factors, the residual stresses in the polycrystalline silicon
layer 136 may be affected.
[0051] The arm electrostatic plate 140 may then be formed. The
second layer 132, comprising a metal, for example, aluminum, is
deposited using electron beam evaporation and liftoff to form the
arm electrostatic plate 140 on the actuator arm 130, as shown in
FIG. 4L. As previously described, the arm electrostatic plate 140
is formed so that it is substantially above the substrate
electrostatic plate 120 to maximize the electrostatic attraction
between the two plates 120, 140.
[0052] Fabrication of the switch is completed by using chemical
release methods known in the art to remove the sacrificial layers
142, 146, 148. FIG. 4M shows the switch 124 after the sacrificial
layers 142, 146, 148 have been removed, but without the desired
curl in the actuator arm 130. Removal of the sacrificial layers
142, 146, 148 should actually result in the actuator arm 130
curling upwards, as shown in FIG. 4N.
[0053] FIGS. 5A and 5B depict side views of an alternate embodiment
of an RF MEMS switch 224 according to the present invention. In
this embodiment, a cantilever actuator arm 230 comprises both a
first arm structural layer 236 constructed of an insulating
material and a second arm electrostatic plate layer 232 constructed
of a conducting material. FIG. 5A depicts the RF MEMS switch 224 in
the open or OFF-state. FIG. 5B depicts the RF MEMS switch 224 in
the closed or ON-state. FIG. 6 is a top plan view of this
embodiment that shows the orientation of the various elements of
this embodiment, discussed in additional detail below.
[0054] The RF MEMS switch 224 is fabricated upon a substrate 210,
preferably GaAs, although other materials may be used, such as InP,
ceramics, quartz or silicon. The material for the substrate 210 is
chosen primarily based on the technology of the circuitry the RF
MEMS switch 224 is to be connected to so that the switch 224 and
the circuitry may be fabricated simultaneously. For example, InP
can be used for low noise HEMT MMICS (high electron mobility
transistor monolithic microwave integrated circuits) and GaAs is
typically used for PHEMT (pseudomorphic HEMT) power MMICS.
[0055] The switch 224 comprises a transmission line 214, a
substrate electrostatic layer 220, and a cantilever actuator arm
230. The cantilever actuator arm 230 comprises the arm structural
layer 236, the arm electrostatic plate layer 232, and a conducting
transmission line 234 with at least one dimple 235 that preferably
protrudes below the arm structural layer 236. FIG. 6 shows that the
cantilever actuator 230 may have two dimples 235, while alternative
embodiments may have more than two dimples 235 or a single dimple
that is disposed beneath the length of the conducting transmission
line 234. The arm electrostatic plate layer 232 connects to an arm
plate layer contact 228 at the base of the cantilever actuator arm
230 to provide an electrical connection between the arm
electrostatic plate layer 232 and the arm plate layer contact 228.
The substrate electrostatic layer 220 contains a substrate
electrostatic plate 222 located generally beneath the cantilever
actuator arm 230. The arm electrostatic layer 232 contains an arm
electrostatic plate 238 located generally above the substrate
electrostatic plate 222. An application of a voltage between the
arm electrostatic plate 238 and the substrate electrostatic plate
222 will cause the plates to be electrostatically attracted. When
the plates 222, 238 are electrostatically attracted together, the
switch 224 is in the closed position and the dimples 235 are in
electrical contact with the transmission line 214. Since the
dimples 235 are electrically connected together by the conducting
transmission line 234, the electrical contact of the dimples 235
with the transmission line 214 bridges the gap 218 in the
transmission line 214 when the switch 224 is in the closed
position.
[0056] One possible method of fabricating switch 224 is discussed
below. As previously discussed, an advantage of the present
invention is that it can be manufactured using standard integrated
circuit fabrication techniques. The switch 224 can also be
fabricated on wafers that contain other integrated circuit devices.
The flexibility in the fabrication of this and other embodiments of
the present invention allows the present invention to be used in a
variety of circuits. Note also that the same or similar materials
for the layers and thicknesses for the layers discussed below may
also be used in the fabrication of the switch 124 discussed above,
along with the same or similar fabrication steps.
[0057] FIG. 7A shows a profile of the MEMS switch 224 after the
first step of depositing a first metal layer onto the substrate 210
for the arm plate layer contact 228, the transmission line 214, and
the substrate electrostatic layer 220 is complete. The first metal
layer may be deposited lithographically using standard integrated
circuit fabrication technology, such as resist lift-off or resist
definition and metal etch. In the preferred embodiment, gold (Au)
is used as the primary composition of the first metal layer. Au is
preferred in RF applications because of its low resistivity. In
order to ensure the adhesion of the Au to the substrate, a thin
layer (preferably about 250-500 angstroms) of titanium (Ti) is
deposited, followed by preferably about a 1000 angstrom layer of
platinum (Pt), and finally the Au. The Pt acts as a diffusion
barrier to keep the Au from intermixing with the Ti and causing the
metal to lose adhesion strength to the substrate 210. In the case
of a group III-V semiconductor substrate, a thin layer of gold
germanium (AuGe) eutectic metal may be deposited first to ensure
adhesion of the Au by alloying the AuGe into the semiconductor,
similar to a standard ohmic metal process for any group III-V
MESFET or HEMT.
[0058] Next, as shown in FIG. 7B, a sacrificial layer 242 is placed
on top of the first metal layer and etched so that the cantilever
actuator arm 230 may be produced above the sacrificial layer 242.
The sacrificial layer 242 is typically comprised of 2 microns of
SiO.sub.2 which may be sputter deposited or deposited using PECVD
(plasma enhanced chemical vapor deposition). A via 243 is etched in
the sacrificial layer 242 so that the metal of the arm plate layer
contact 228 is exposed. The via 243 definition may be performed
using standard resist lithography and etching of the sacrificial
layer 242. Other materials besides SiO.sub.2 may be used as a
sacrificial layer 242. The important characteristics of the
sacrificial layer 242 are a high etch rate, good thickness
uniformity, and conformal coating by the layer 242 of the metal
already on the substrate 210. The thickness of the layer 242
partially determines the initial thickness of the switch opening,
before the arm 230 begins to curve away from the substrate 210 due
to residual stresses. The sacrificial layer 242 will be removed in
the final step to release the cantilever actuator arm 230, as shown
in FIG. 7F.
[0059] Another advantage of using SiO.sub.2 as the sacrificial
layer 242 is that SiO.sub.2 can withstand high temperatures. Other
types of support layers, such as organic polyimides, harden
considerably if exposed to high temperatures. This makes a
polyimide sacrificial layer difficult to later remove. The
sacrificial layer 242 is exposed to high temperatures when the
preferred material of silicon nitride for the arm structural layer
236 is deposited (as shown in FIG. 7C), as a high temperature
deposition is desired when depositing the silicon nitride to give
the silicon nitride a lower buffered oxide etch (BOE) etch rate. A
low BOE etch rate minimizes the amount of the arm structural layer
236 that is lost when the SiO.sub.2 is etched away.
[0060] FIG. 7C shows the fabrication of the arm structural layer
236. The arm structural layer 236 is the supporting mechanism of
the cantilever actuator arm 230 and is preferably made out of
silicon nitride, although other materials besides silicon nitride
may be used. The material used for the arm structural layer 236
should have a low etch rate compared to the sacrificial layer 242
so that the arm structural layer 236 is not etched away when the
sacrificial layer 242 is removed to release the cantilever actuator
arm 230. The arm structural layer 236 is patterned and etched using
standard lithographic and etching processes.
[0061] The arm structural layer 236 is preferably formed below the
arm electrostatic plate layer 232. Since the arm structural layer
230 is fabricated on only one side of the arm electrostatic plate
layer 232, bowing will occur in the cantilever actuator arm 230
when the arm 230 is released if the residual stress in the arm
structural layer 236 differs from the stress in the arm
electrostatic plate layer 232. The materials in the arm structural
layer 236 and the arm electrostatic plate layer 232 are chosen such
that the differing stresses in the materials cause the arm 230 to
bow upwards. The techniques used to deposit the arm electrostatic
plate layer 232 also affect the amount of curvature achieved.
[0062] In FIG. 7D, a dimple receptacle 253 is etched into the arm
structural layer 236 and the sacrificial layer 242. The dimple
receptacle 253 is an opening where the dimple 235 will later be
deposited. The dimple receptacle 253 is created using standard
lithography and a dry etch of the arm structural layer 236,
followed by a partial etch of the sacrificial layer 242. The
opening in the sacrificial layer 242 allows the dimple 235 to
preferably protrude through the sacrificial layer 242. Note that a
plurality of dimple receptacles 253 may be formed to allow a
plurality of dimples 235 to be used to form an electrical contact
with the transmission line 214 when the switch 224 is in the closed
position.
[0063] Next, a second metal layer is deposited onto the arm
structural layer 236. The second metal layer forms the arm
electrostatic plate layer 232, the conducting transmission line
234, and the dimple 235. In a preferred embodiment, the second
metal layer is comprised of a sputter deposition of a thin film,
preferably about 200 angstroms, of Ti, preferably followed by about
a 1000 angstrom deposition of Au. The second metal layer must be
conformal across the wafer and acts as a plating plane for the Au.
The plating is done by using metal lithography to open up the areas
of the switch that are to be plated. The Au may be electroplated by
electrically contacting the membrane metal on the edge of the wafer
and placing the metal patterned wafer in the plating solution. The
plating occurs only where the membrane metal is exposed to the
plating solution to complete the electrical circuit and not where
the electrically insulating resist is left on the wafer. After
about 2 microns of Au is plated, the resist is stripped off of the
wafer and the whole surface is ion milled to remove the membrane
metal. Some Au will also be removed from the top of the plated Au
during the ion milling, but that loss is minimal because the
membrane is preferably only 1200 angstroms thick.
[0064] As shown in FIG. 7E, the result of this process is that the
conducting transmission line 234 and the dimple 235 are created by
the second metal layer, which comprises Au in a preferred
embodiment. In addition, the Au fills the via 251 and connects the
arm electrostatic plate layer 232 to the arm plate layer contact
228. Au is a preferred choice for the second metal layer because of
its low resistivity. When choosing the metal for the arm
electrostatic layer 232 and the material for the arm structural
layer 236, it is important to select the materials such that the
stress of the arm structural layer 236 varies from the stress of
the arm electrostatic layer 232 so that the cantilever actuator arm
230 will bow upwards when the switch 224 is in the OFF-state. This
is done by carefully determining the deposition parameters for the
structural layer 236. Silicon nitride was chosen for this
structural layer 236 not only for its insulating properties, but,
in large part, because of the controllability of these deposition
parameters and the resultant stress levels of the layer.
[0065] The arm structural layer 236 is then lithographically
defined and etched to complete the switch fabrication. Finally, the
sacrificial layer 242 is removed to release the cantilever actuator
arm 230. If the sacrificial layer 242 is comprised of SiO.sub.2,
then it will typically be wet etched away in the final fabrication
sequence by using a hydrofluoric acid (HF) solution. The etch and
rinses are performed with post-processing in a critical point dryer
to ensure that the cantilever actuator arm 230 does not come into
contact with the substrate 210 when the sacrificial layer 242 is
removed. If contact occurs during this process, device sticking and
switch failure are probable. Note, however, that the bimorph
character of the cantilever actuator arm 230 should cause the arm
230 to bow upwards and should also reduce the likelihood of contact
with the substrate 210 upon removal of the sacrificial layer 242.
Contact may be prevented by transferring the switch from a liquid
phase (e.g. HF) environment to a gaseous phase (e.g. air)
environment not directly, but by introducing a supercritical phase
in between the liquid and gaseous phases. The sample is etched in
HF and rinsed with de-ionized (DI) water by dilution, so that the
switch is not removed from a liquid during the process. DI water is
then replaced with methanol. The sample is transferred to the
critical point dryer and the chamber is sealed. High pressure
liquid CO.sub.2 replaces the methanol in the chamber, so that there
is only CO.sub.2 surrounding the sample. The chamber is heated so
that the CO.sub.2 changes into the supercritical phase. Pressure is
then released so that the CO.sub.2 changes into the gaseous phase.
Now that the sample is surrounded only by gas, it may be removed
from the chamber into room air. A side elevational view of the MEMS
switch 224 after the support layer 242 has been removed, but before
the cantilever actuator arm 230 curls upwards, is shown in FIG.
7F.
[0066] FIGS. 8 and 9 depict views of another embodiment of an RF
MEMS switch 324 according to the present invention. In this
embodiment, an insulating layer 340 electrically isolates a
conducting transmission line 334 from an arm structural layer 336
and an arm electrostatic plate 332. FIG. 8 depicts the RF MEMS
switch 324 in the open or OFF-state. FIG. 9 depicts the RF MEMS
switch 324 in the closed or ON-state.
[0067] The switch 324 comprises a transmission line 314, a
substrate electrostatic plate 320, and a cantilever 326. The switch
324 is fabricated upon a substrate 310, preferably comprising GaAs,
although other materials may be used, such as InP, ceramics, quartz
or silicon. The substrate 310 may also be coated with an insulating
layer 311 comprising, for example, silicon nitride. The cantilever
326 comprises a cantilever anchor 328 fabricated on the substrate
310 and a cantilever actuator arm 330. The cantilever actuator arm
330 comprises the arm structural layer 336, the arm electrostatic
plate 332, an insulating layer 340, and the conducting transmission
line 334. Preferably, the cantilever anchor 328 also is integral
with the arm structural layer 336.
[0068] The substrate electrostatic plate 320 is formed on the
substrate 310 and is located generally beneath the arm
electrostatic plate 332 on the cantilever actuator arm 330. An
application of a voltage between the arm electrostatic plate 332
and the substrate electrostatic plate 320 will cause the plates
320, 332 to be electrostatically attracted. Preferably, mechanical
stops 316 are formed on the substrate that are electrically
isolated from the substrate electrostatic plate 320 and have a
greater height than the substrate electrostatic plate 320. The
mechanical stops 316 prevent the cantilever actuator arm 330 from
coming into electrical contact with the substrate electrostatic
plate 320. An insulating, semi-conducting, or conducting layer 313
may be located beneath the transmission line 314 to isolate the
transmission line from the substrate 310 or to decrease the amount
of deflection required for the conducting transmission line 334 to
contact the transmission line 314.
[0069] One possible method of fabricating switch 324 is discussed
below. This embodiment of the present invention is particularly
adapted for fabrication by using standard three-polysilicon-layer
surface-micromachining processes, such as that provided by the
Multi-User MEMS Processes (MUMPs.TM.) from Cronos Integrated
Microsystems of Research Triangle Park, N.C. However, other methods
of micromachining fabrication may be used.
[0070] FIG. 10A shows a cross-section of-an embodiment of the
switch 324 at the beginning of the fabrication process. The surface
of the starting n-type silicon substrate 310 is heavily doped with
phosphorus in a standard diffusion furnace using POCl.sub.3 as the
dopant source. Preferably, a blanket layer 311, about 0.5 .mu.m
thick, of low stress silicon nitride is deposited on the substrate
310 as an insulating layer. Then, preferably, a polysilicon layer
320 (POLY0), about 0.5 .mu.m-thick, is deposited for providing the
conducting surfaces for the substrate electrostatic plate 322 and
the conducting layer 313 for the gapped transmission line 314. The
wafer is then coated with an ultraviolet-sensitive photoresist
layer 390.
[0071] The photoresist layer 390 is lithographically patterned by
exposing it to ultraviolet light through a first level mask and
then developing it. The photoresist layer 390 in exposed areas is
removed leaving behind a patterned photoresist mask for etching, as
shown in FIG. 10B. Reactive Ion Etching (RIE) is used to remove any
unwanted polysilicon. After the etch, the photoresist is chemically
stripped in a solvent bath. This method of patterning the wafers
with photoresist, etching and stripping the remaining photoresist
is also used to remove unwanted portions of the additional
polysilicon layers described below.
[0072] After the unwanted polysilicon is removed from the POLY 0
layer 320 and the photoresist is removed, the conducting surfaces
for the substrate electrostatic plate 322 and the conducting layer
313 remain, as shown in FIG. 10C. A blanket layer 371,
approximately 2.0 .mu.m thick, of phosphosilicate glass (PSG) is
deposited on the structure by low pressure chemical vapor
deposition (LPCVD). The deposit of this first sacrificial layer 371
is shown in FIG. 10D. Other materials such as SiO.sub.2 may also be
used for the sacrificial layer 371.
[0073] The structure is then coated with photoresist and the areas
381 for the mechanical stops 316 are lithographically patterned.
These areas, which reach the nitride layer 311, are reactive ion
etched into the first sacrificial layer 371. After the etch, the
photoresist is stripped. The structure after removal of the
photoresist is shown in FIG. 10E.
[0074] A second layer 317 (POLY1), approximately 2.0 .mu.m thick,
of un-doped polysilicon is deposited by LPCVD. This layer serves to
fill in the mechanical stop areas 381. The results of this step are
shown in FIG. 10F. The POLY1 layer 317 is again coated with
photoresist, patterned, and etched. The result is to remove the
bulk of the POLY1 layer 317 on top of the sacrificial layer 371, as
shown in FIG. 10G. A second sacrificial layer 372, preferably
comprising PSG, is then deposited on top of the structure, as shown
in FIG. 10H.
[0075] The structure is again coated with photoresist and the
anchor area 329 for the cantilever anchor 328 is etched into both
the second sacrificial layer 372 and first sacrificial layer 371
down to the nitride layer 311, as shown in FIG. 10I. A third
polysilicon layer 336 (POLY2) is then deposited onto the structure
by LPCVD, as shown in FIG. 10J. The POLY2 layer 336 provides the
cantilever arm structural layer 336 and the cantilever anchor
328.
[0076] The POLY2 layer 336 is then etched to provide access to the
area 341 to be used for the gapped transmission line 314, as shown
in FIG. 10K. An additional mask and etch process is used to remove
portions of the first sacrificial layer 371 and the second
sacrificial layer 372 to expose the conducting layer 313, as shown
in FIG 10L. The structure is then coated with photoresist and a
metal layer is lithographically patterned for the arm electrostatic
plate 332 and the gapped transmission line 314. The metal layer,
preferably gold with a thin adhesion layer, is deposited by
lift-off patterning. The photoresist and any unwanted metal are
then removed in a solvent bath. The structure after removal of
unwanted metal is shown in FIG. 10M.
[0077] A third sacrificial layer 373, also preferably comprising
PSG, is then provided over portions of the arm electrostatic plate
332, the arm structural layer 336, the gapped transmission line
314, the first sacrificial layer 371 and the second sacrificial
layer 372, as shown in FIG. 10N. The third sacrificial layer 373 is
then etched to provide an area 335 for the conducting transmission
line 334. Metal, again preferably gold with a thin adhesion layer,
is deposited to create the conducting transmission line 334 and
then any unwanted portions of metal are removed. The structure
after creation of the conducting transmission line is shown in FIG.
100.
[0078] Additional portions of the third sacrificial layer 373 are
then removed to provide for mechanically coupling the conducting
transmission line 334 to the arm structural layer 336. The
structure after removal of portions of the third sacrificial layer
is shown in FIG. 10P. An insulating material layer 340, such as
silicon nitride, is deposited on top of the conducting transmission
line 334 and proximate to the arm structural layer 336. The
insulating material layer 340 essentially causes the conducting
transmission line 334 to be fixedly attached to the arm structural
layer 336, as shown in FIG. 10Q. Portions of the insulating
material layer 340 may be removed as shown in FIG. 10R. The removal
of portions of the insulating material layer 340 may be done to
decrease the weight of the cantilever actuator arm 330 and to allow
the cantilever actuator arm 330 to curl upwards as desired.
[0079] Finally, the cantilever actuator arm 330 is released by
removing the sacrificial layers 371, 372, 373. The release may be
performed by immersing the structure in a bath of 49% hydrofluoric
acid at room temperature for 1.5 to 2 minutes. Other methods known
in the art may also be used to remove the sacrificial layers 371,
372, 373. This is followed by several minutes in DI water and then
alcohol to reduce the likelihood of stiction. Finally, the
structure is placed in an oven at 1100.degree. C. for at least 10
minutes. The structure after removal of the sacrificial material is
shown in FIG. 10S. Note that due to the bimorph character of the
switch, the cantilever actuator arm 330 should curl upwards as
shown in FIG. 10T.
[0080] Although the present invention has been described with
respect to specific embodiments thereof, various changes and
modifications can be carried out by those skilled in the art
without departing from the scope of the invention. In particular,
the substrate, cantilever anchor, cantilever arm, electrostatic
plates, and metal contacts may be fabricated using any of various
materials appropriate for a given end use design. The cantilever
anchor, cantilever arm, electrostatic plates, and metal contacts
may be formed in various configurations, including multiple anchor
points, cantilever arms, and metal contacts. It is intended,
therefore, that the present invention encompass such changes and
modifications as fall within the scope of the appended claims.
* * * * *