U.S. patent application number 09/767321 was filed with the patent office on 2002-07-25 for monolithic single pole double throw rf mems switch.
This patent application is currently assigned to HRL Laboratories, LLC. Invention is credited to Dolezal, Franklin A., Hsu, Tsung-Yuan, Loo, Robert Y., Schaffner, James H., Schmitz, Adele E., Tangonan, Gregory L..
Application Number | 20020098613 09/767321 |
Document ID | / |
Family ID | 25079125 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020098613 |
Kind Code |
A1 |
Loo, Robert Y. ; et
al. |
July 25, 2002 |
MONOLITHIC SINGLE POLE DOUBLE THROW RF MEMS SWITCH
Abstract
Apparatus for a micro-electro-mechanical switch that provides
single pole, double throw switching action. The switch comprises a
single RF input line and two RF output lines. The switch
additionally comprises two armatures, each mechanically connected
to a substrate at one end and having a conducting transmission line
at the other end with a suspended biasing electrode located on top
of or within a structural layer of the armature. Each conducting
transmission line has conducting dimples that protrude beyond the
bottom of the armature carrying the conducting transmission line.
Closure of an armature causes the dimples of the corresponding
conducting transmission line to mechanically and electrically
engage the RF input line and the corresponding RF output line, thus
directing RF energy from the RF input line to the selected RF
output line.
Inventors: |
Loo, Robert Y.; (Agoura
Hills, CA) ; Schaffner, James H.; (Chatsworth,
CA) ; Schmitz, Adele E.; (Newbury Park, CA) ;
Hsu, Tsung-Yuan; (Westlake Village, CA) ; Dolezal,
Franklin A.; (Reseda, CA) ; Tangonan, Gregory L.;
(Oxnard, CA) |
Correspondence
Address: |
Ross A. Schmitt, Esq.
c/o LADAS & PARRY
Suite 2100
5670 Wilshire Boulevard
Los Angeles
CA
90036-5679
US
|
Assignee: |
HRL Laboratories, LLC
|
Family ID: |
25079125 |
Appl. No.: |
09/767321 |
Filed: |
January 23, 2001 |
Current U.S.
Class: |
438/73 |
Current CPC
Class: |
H01H 59/0009
20130101 |
Class at
Publication: |
438/73 |
International
Class: |
H01L 021/00 |
Claims
What is claimed is:
1. A micro-electro-mechanical switch, comprising a) a substrate; b)
an input line on top of the substrate; c) a first output line on
top of the substrate and separated from the input line; d) a first
substrate electrode on top of the substrate, located adjacent to
but separated from the input line and the first output line; e) a
second output line on top of the substrate and separated from the
input line; f) a second substrate electrode on top of the
substrate, located adjacent to but separated from the input line
and the second output line; g) a first armature comprising: 1) a
first armature lower structural layer having a first end
mechanically connected to the substrate and a second end positioned
over the input line and first output line; 2) a first conducting
transmission line located at the second end of the first armature
structural layer and suspended above the input line and the first
output line; and 3) a first suspended armature electrode disposed
above and in contact with the first armature lower structural layer
and suspended above the first substrate electrode; and h) a second
armature comprising: 1) a second armature lower structural layer
having a first end mechanically connected to the substrate and a
second end positioned over the input line and the second output
line; 2) a second conducting transmission line located at the
second end of the second armature structural layer and suspended
above the input line and the second output line; and 3) a second
suspended armature electrode disposed above and in contact with the
second armature lower structural layer and suspended above the
second substrate electrode.
2. The micro-electro-mechanical switch of claim 1 wherein the first
conducting transmission line is suspended above the input line and
the first output line when the first armature is in an open
position, and mechanically and electrically contacts the input line
and the first output line when the first armature is in a closed
position and the second conducting transmission line is suspended
above the input line and the second output line when the second
armature is in an open position, and mechanically and electrically
contacts the input line and the second output line when the second
armature is in a closed position.
3. The micro-electro-mechanical switch of claim 2 wherein the first
armature is in a closed position when the second armature is in an
open position and the first armature is in an open position when
the second armature is in a closed position.
4. The micro-electro -mechanical switch of claim 1 wherein the
first suspended armature electrode and the second suspended
armature electrode are electrically connected to an armature
electrode bias pad.
5. The micro-electro-mechanical switch of claim 1 wherein the first
suspended armature electrode is connected to a first armature
electrode bias pad and the second suspended armature electrode is
electrically connected to a second armature electrode bias pad, and
the first and second armature electrode bias pads are electrically
isolated from each other.
6. The micro-electro-mechanical switch of claim 1 wherein the first
transmission line further comprises a first set of one or more
contact dimples that project below a bottom surface of the first
armature and the second transmission line further comprises a
second set of one or more contact dimples that project below a
bottom surface of the second armature.
7. The micro-electro-mechanical switch of claim 6 wherein a gap
between the first set of one or more contact dimples and a plane
defined by the top of the input line and the first output line is
less than a gap between the first armature lower structural layer
and the substrate, and wherein the first set of one or more contact
dimples mechanically and electrically contact the input line and
the first output line when the first armature is in the closed
position and a gap between the second set of one or more contact
dimples and a plane defined by the top of the input line and the
second output line is less than a gap between the second armature
lower structural layer and the substrate, and wherein the second
set of one or more contact dimples mechanically and electrically
contact the input line and the second output line when the second
armature is in the closed position.
8. The micro-electro-mechanical switch of claim 7 wherein the first
suspended armature electrode, the second suspended armature
electrode, the first set of one or more contact dimples, and the
second set of one or more contact dimples each comprise layers of
gold and titanium.
9. The micro-electro-mechanical switch of claim 1 wherein the input
pad, the first output pad, the second output pad, the first
substrate electrode, the second substrate electrode, the first
suspended armature electrode and the second suspended armature
electrode each comprise layers of gold, nickel, and gold
germanium.
10. The micro-electro-mechanical switch of claim 1 further
comprising: a first armature structural layer disposed above and in
contact with the first armature lower structural layer and the
first suspended armature electrode; and a second armature
structural layer disposed above and in contact with the second
armature lower structural layer and the second suspended armature
electrode.
11. The micro-electro-mechanical switch of claim 10, wherein the
structural layers comprise silicon nitride.
12. A method of switching an RF signal applied at an input port to
one of two output ports, comprising the steps of: providing a
monolithic SPDT RF MEMS switch comprising: a substrate; an input
line on top of the substrate; a first output line on top of the
substrate and separated from the input line; a first substrate
electrode on top of the substrate, located adjacent to but
separated from the input line and the first output line; a second
output line on top of the substrate and separated from the input
line; a second substrate electrode on top of the substrate, located
adjacent to but separated from the input line and the second output
line; a first armature comprising: a first armature lower
structural layer having a first end mechanically connected to the
substrate and a second end positioned over the input line and first
output line; a first conducting transmission line located at the
second end of the first armature structural layer and suspended
above the input line and the first output line; and a first
suspended armature electrode disposed above and in contact with the
first armature lower structural layer and suspended above the first
substrate electrode; and a second armature comprising: a second
armature lower structural layer having a first end mechanically
connected to the substrate and a second end positioned over the
input line and the second output line; a second conducting
transmission line located at the second end of the second armature
structural layer and suspended above the input line and the second
output line; and a second suspended armature electrode disposed
above and in contact with the second armature lower structural
layer and suspended above the second substrate electrode; and
connecting the input port to the input line; connecting one of the
output ports to the first output line and the other output port to
the second output line; and applying a voltage between a selected
one of the two substrate electrodes and the armature electrode
suspended above the substrate electrode so as to cause the armature
suspended above the selected substrate electrode to close.
13. The method of claim 12, wherein the first suspended armature
electrode and the second suspended armature electrode are
electrically connected to a common armature electrode pad and the
voltage is applied between one of the two substrate electrodes and
the common armature electrode pad.
14. The method of claim 12, wherein the armature suspended above
the non-selected substrate electrode is in an open position.
15. The method of claim 12, wherein the first transmission line
further comprises a first set of one or more contact dimples that
project below a bottom surface of the first armature and the second
transmission line further comprises a second set of one or more
contact dimples that project below a bottom surface of the second
armature.
16. The method of claim 15, wherein a gap between the first set of
one or more contact dimples and a plane defined by the top of the
input line and the first output line is less than a gap between the
first armature lower structural layer and the substrate, and
wherein the first set of one or more contact dimples mechanically
and electrically contact the input line and the first output line
when the first armature is in the closed position and a gap between
the second set of one or more contact dimples and a plane defined
by the top of the input line and the second output line is less
than a gap between the second armature lower structural layer and
the substrate, and wherein the second set of one or more contact
dimples mechanically and electrically contact the input line and
the second output line when the second armature is in the closed
position.
17. The method of claim 16 wherein the first suspended armature
electrode, the second suspended armature electrode, the first set
of one or more contact dimples, and the second set of one or more
contact dimples each comprise layers of gold and titanium.
18. The method of claim 12 wherein the input pad, the first output
pad, the second output pad, the first substrate electrode, the
second substrate electrode, the first suspended armature electrode
and the second suspended armature electrode each comprise layers of
gold, nickel, and gold germanium.
19. The method of claim 12, wherein the monolithic SPDT RF MEMS
switch further comprises: a first armature structural layer
disposed above and in contact with the first armature lower
structural layer and the first suspended armature electrode; and a
second armature structural layer disposed above and in contact with
the second armature lower structural layer and the second suspended
armature electrode.
20. The method of claim 19, wherein the structural layers comprise
silicon nitride.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field of the Invention
[0002] The present invention relates generally to switches. More
particularly, it relates to the design and fabrication of
microfabricated electromechanical switches having a single pole
double throw configuration.
[0003] 2. Description of Related Art
[0004] In communications applications, switches are often designed
with semiconductor elements such as transistors or pin diodes. At
microwave frequencies, however, these devices suffer from several
shortcomings. PIN diodes and transistors typically have an
insertion loss greater than 1 dB, which is the loss across the
switch when the switch is closed. Transistors operating at
microwave frequencies tend to have an isolation value of under 20
dB. This allows a signal to `bleed` across the switch even when the
switch is open. PIN diodes and transistors have a limited frequency
response and typically only respond to frequencies under 20 GHz. In
addition, the insertion losses and isolation values for these
switches varies depending on the frequency of the signal passing
through the switches. These characteristics make semiconductor
transistors and pin diodes a poor choice for switches in microwave
applications.
[0005] U.S. Pat. No. 5,121,089 issued Jun. 9, 1992 to Larson
discloses a microwave micro-electro-mechanical systems (MEMS)
switch. The Larson MEMS switch utilizes an armature design. One end
of a metal armature is affixed to an output line, and the other end
of the armature rests above an input line. The armature is
electrically isolated from the input line when the switch is in an
open position. When a voltage is applied to an electrode below the
armature, the armature is pulled downward and contacts the input
line. This creates a conducting path between the input line and the
output line through the metal armature. This switch also provides
only a Single Pole, Single Throw (SPST) function, that is, the
switch is either open or closed.
[0006] U.S. Pat. No. 6,046,659 of Loo et al. discloses methods for
the design and fabrication of SPST MEMS switches. Each MEMS switch
has a multiple-layer armature with a suspended biasing electrode
and a conducting transmission line affixed to the structural layer
of the armature. A conducting dimple is connected to the conducting
line to provide a reliable region of contact for the switch. The
switch is fabricated using silicon nitride as the armature
structural layer and silicon dioxide as a sacrificial layer
supporting the armature during fabrication. Hydrofluoric acid is
used to remove the silicon dioxide layer with post-processing in a
critical point dryer to increase yield.
[0007] A MEMS switch has a very low insertion loss (less than 0.2
dB at 45 GHz) and a high isolation when open (greater than 30 dB)
over a large bandwidth when compared to semiconductor transistors
and pin diodes. These characteristics give the MEMS switch the
potential to not only replace traditional narrow-bandwidth PIN
diodes and transistor switches in microwave circuits, but to create
a whole new class of high performance and compact microwave switch
circuits.
[0008] A common feature of the MEMS switches described above is
that they all disclose a single pole, single throw (SPST)
configuration, that is, they can only switch an RF signal on or
off. However, RF signals often must be switched between two
destinations, such as when switching an RF signal between a first
antenna array and a second antenna array. Switches that support
this configuration are classified as single pole, double throw
(SPDT) switches.
[0009] SPDT switches known in the art are either solid-state
devices or mechanical relays. Solid-state SPDT RF switches, such as
PIN diodes and FETs, suffer from the limited frequency response,
insertion loss, and isolation problems described above. Isolation
between the two output ports of the SPDT switch is of particular
concern, since coupling of the signal from one output port to the
other output port limits the effectiveness of the switch as a dual
output port device. Mechanical relays are also available in SPDT
configurations, but they are generally quite large, compared to
other RF components, and consume significant amounts of power.
[0010] Therefore, there is a need in the art for a SPDT switch that
provides low insertion loss and high isolation at its output ports.
There is a further need to provide such a switch with a size near
to that of other RF components and consumes little power.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a method of design and
fabrication of a micro-electromechanical single pole double throw
(SPDT) switch. The switch is preferably designed with a pair of
bi-layer or tri-layer armatures which give the switch superior
mechanical qualities. The switch is arranged such that one armature
of the pair of armatures is normally closed while the other
armature is normally open due to the application of an
electrostatic potential which operates on one of the two armatures.
In addition, the switch preferably has conducting dimples with
defined contact areas to provide improved contact
characteristics.
[0012] One embodiment of the invention is a
micro-electro-mechanical switch comprising an input line, two
output lines, and a pair of armatures. The input line and the
output lines are located on top of a substrate. The armatures are
each made of at least one structural layer, a conducting
transmission line on top of, below, or between the structural
layers, and a suspended armature bias electrode similarly placed of
each armature. One end of the structural layer is connected to the
substrate, and a substrate bias electrode is located on top of the
substrate below the suspended armature bias electrode on the
armatures.
[0013] The input line is coupled to a pair of input contacts, each
contact of the pair of contacts being associated with one of the
armatures of the pair of armatures. The output lines are each
coupled to an output contact, each output contact being associated
with one of the armatures of the pair of armatures. A first end of
the conducting transmission line in each armature rests above each
of the input contacts and a second end rests above each of the
output contacts when the switch is in an open position. Each
conducting transmission line also contains a conducting dimple at
both the first end and the second end such that the distance
between the conducting dimple and the input and output contacts is
less than the distance between the conducting transmission line and
the input and output contacts so that the conducting dimples
contact the input and output contacts when the switch is in the
closed position. The structural layer may be formed below, above,
or both above and below the conducting transmission line. The input
line, output lines, input contacts, output contacts, armature bias
pad, substrate bias pad, and substrate bias electrode are comprised
of a stack of films referred to as the first metal layer which is
preferably comprised of a 1500 angstrom film of gold on top of a
100 angstrom film of nickel on top of a 900 angstrom film of gold
germanium. The armature bias electrodes, conducting transmission
lines, and contact dimples are made of a film stack referred to as
the second metal layer, which is preferably comprised of a 1000
angstrom film of deposited or evaporated gold on top of a 200
angstrom layer of titanium. The first and second metal layers have
different compositions since the first layer is deposited on the
substrate while the second layer is deposited on a dielectric, such
as silicon nitride.
[0014] The present invention may also be embodied in a process for
making a micro-electromechanical switch. The process comprises a
first step of depositing a first metal layer onto a substrate to
form an input line, a pair of input contacts, a pair of output
lines, a pair of output contacts, substrate bias electrodes,
substrate bias pads, and armature bias pads. A support layer, also
known as a sacrificial layer, is deposited on top of the first
metal layer and the substrate, and a beam structural layer is
deposited on top of the sacrificial layer. The beam structural
layer forms the armature pair with one end of each armature affixed
to the substrate opposite its corresponding input contact. The
process further comprises the steps of removing a portion of the
structural layer and a portion of the support layer to create a
dimple mold. Conducting dimples are formed in the dimple mold when
the conducting transmission line and suspended armature bias
electrodes are fabricated by depositing a second metal layer, such
that the suspended armature bias electrode is electrically
connected to the armature bias pad. A second structural layer may
or may not be deposited on top of the second metal layer for stress
matching and thermal stability of the switch. Finally, the
sacrificial layer is removed from beneath the armatures to release
the armatures and allow the switch to open and close.
[0015] The materials and fabrication techniques used for the
process comprise standard integrated circuit manufacturing
materials and techniques. The sacrificial layer is made of silicon
dioxide and is removed by wet etching the silicon dioxide with HF
and with post processing in a critical point dryer. The beam
structural layer is comprised of silicon nitride. As discussed
above, the first metal layer is preferably comprised of a film of
gold on top of a film of nickel on top of a film of gold germanium.
The second metal layer is preferably comprised of a film of gold on
top of film of titanium. A second beam structural layer may be
deposited on top of the conducting line such that the conducting
line is encased between the first structural layer and the second
structural layer. In alternative embodiments of the present
invention, the second metal layer is deposited underneath, in
between, or on top of the structural layers. If the second metal
layer is underneath the structural layers, then a dielectric or
insulator is deposited on top of the substrate bias electrodes to
prevent electrical shorting to the armature bias electrodes when
the switch is in the closed position
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] These and other features and advantages will become more
apparent from a detailed consideration of the invention when taken
in conjunction with the drawings in which:
[0017] FIG. 1 is a top overview view of two discrete SPST MEMS
switches connected in a SPDT configuration.
[0018] FIG. 2A shows the isolation achieved with the SPDT switch
depicted in FIG. 1.
[0019] FIG. 2B shows the insertion loss achieved with the SPDT
switch depicted in FIG. 1.
[0020] FIG. 3 is a top overview of the monolithic SPDT MEMS switch
embodying the present invention.
[0021] FIG. 4A is a side view of the monolithic SPDT MEMS switch
depicted in FIG. 3 taken along the section line 3-3' showing one
armature in an open position.
[0022] FIG. 4B is a side view of the monolithic SPDT MEMS switch
depicted in FIG. 3 taken along the section line 3-3' showing one
armature in a closed position.
[0023] FIG. 5A shows the isolation achieved with the monolithic
MEMS SPDT switch according to the present invention
[0024] FIG. 5B shows the insertion loss achieved with the
monolithic MEMS SPDT switch according to the present invention.
[0025] FIGS. 6A-6F are side elevational views of the monolithic
MEMS SPDT switch of FIG. 3 taken along section line 3-3' during
progressive steps of a fabrication process further embodying the
present invention.
[0026] FIG. 7 is a picture of one embodiment of a monolithic SPDT
RF MEMS switch according to the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] FIG. 1 is a general overview of a hybrid SPDT switch 100
constructed from two discrete SPST MEMS switches 10A, 10B. The two
switches 10A, 10B are identical, so the description below refers to
both switches 10A, 10B.
[0028] In the switch 10A, 10B, one end of an armature 16 is affixed
to the substrate 14 near an armature bias pad 34 on the substrate
14. The other end of the armature 16 is positioned over a left RF
contact 21 and a right RF contact 19. A substrate bias electrode 22
is printed on the substrate 14 below the armature 16. The armature
16 contains an armature bias electrode 30 which is electrically
isolated from the substrate bias electrode 22 by an air gap (not
shown in FIG. 1) and a layer of silicon nitride (not shown in FIG.
1), when the switch 10A, 10B is in an open position. When the
switch 10A, 10B is in a closed position, the layer of silicon
nitride still serves to electrically isolate the armature bias
electrode 30 from the substrate bias electrode.
[0029] A right conducting dimple 25 and a left conducting dimple 24
protrude from the armature 16 toward the left RF contact 21 and the
right RF contact 19. A conducting RF line 28 is printed on the
armature 16 and electrically connects the right conducting dimple
25 to the left conducting dimple 24. When the MEMS switch 10A, 10B
is in an open position, the dimples 24, 25 are electrically
isolated from the left RF contact 21 and the right RF contact 19 by
an air gap. The left RF contact 21 and the right RF contact 19 are
isolated from each other by a nonmetallic gap in the substrate 14.
The left RF contact 21 is electrically connected to a left RF line
20 and the right RF contact 19 is electrically connected to a right
RF line 18.
[0030] The armature 16 is comprised of a beam structural layer 26,
the conducting line 28, suspended armature bias electrode 30, and
via hole 32. The armature bias electrode 30 is encapsulated within
the beam structural layer 26 and extends over the majority of the
armature 16. The armature bias electrode 30 connects to the
armature bias pad 34 through metal deposited in the via hole 32.
The substrate bias electrode 22 is in electrical contact with a
substrate bias pad 36. The substrate bias pad 36 and the substrate
bias electrode 22 may comprise a single layer of deposited metal.
When a voltage is applied between the suspended armature bias
electrode 30 and the substrate bias electrode 22, an electrostatic
attractive force will pull the suspended armature bias electrode 30
as well as the attached armature 16 towards the substrate bias
electrode 22, so that the right conducting dimple 25 touches the
right RF contact 19 and the left conducting dimple 24 touches the
left RF contact 21. Since the RF conducting line 28 electrically
connects the right conducting dimple 25 to the left conducting
dimple 24, the conducting line 28 and the dimples 24, 25 bridge the
gap between the right RF contact 19 and the left RF contact 21,
thereby closing the MEMS switch 10A, 10B. The RF conducting line 28
is electrically isolated from the armature bias electrode 30, so
the voltage applied to the armature bias electrode 30 is isolated
from the RF signal carried through the RF conducting line 28.
[0031] In the hybrid SPDT switch 100, an electrical connection 101
is used to connect the right RF line 18 of the first MEMS switch
10A to the left RF line 20 of the second MEMS switch 10B. The
electrical connection 101 may comprise a wirebond, a solder line,
or other electrical connecting means known in the art. Thus, in the
SPDT configuration, the right RF line 18 of the first switch 10A
and the left RF line 20 of the second switch 10B comprise the input
port 110 of the SPDT switch 100. RF energy may be provided to the
input port 110 by connecting to either the right RF line 18 of the
first MEMS switch 10A or the left RF line 20 of the second MEMS
switch 10B, or, as shown in FIG. 1, using a "Y" connection 111 to
connect input RF energy to both the right RF line 18 and the left
RF line 20. The left output port 120 of the hybrid SPDT switch 100
is electrically connected to the left RF line 20 of the first MEMS
switch 10A and the right output port 122 is electrically connected
to the right RF line 18 of the second MEMS switch 10B.
[0032] The hybrid SPDT switch 100 operates by either opening the
first switch 10A and simultaneously closing the second 10B, or vice
versa. If the first switch 10A is opened and the second 10B is
closed, RF energy will be directed out of the second output port
122. If the first switch 10A is closed and the second 10B opened,
RF energy will be directed out of the first output port 120.
[0033] FIG. 2A shows the isolation achieved between the input port
110 and an output port 120 of the first switch 10A when the second
switch 10B is in the closed position and the second output port 122
is connected to a matched load. Note at frequencies lower than 14
GHz, the isolation is greater than 30 dB. In RF circuits, it is
usually desirable to have RF isolation exceed 30 dB. FIG. 2B shows
the insertion loss seen with the hybrid SPDT switch 100 described
above. As shown in FIG. 2B, the insertion loss does not exceed 0.2
dB, which is generally acceptable performance.
[0034] Creation of a hybrid MEMS SPDT switch by combining two
discrete MEMS SPST switches has some serious drawbacks. The first
major drawback is the fabrication process for the hybrid MEMS SPDT
switch requires an additional manufacturing step of electrically
connecting together the two discrete MEMS SPDT switches. Another
drawback, as illustrated in FIG. 2A, is that the RF isolation
provided by the switch suffers due to RF coupling between the two
output ports, caused by the wirebond that couples the two switches.
A further drawback is that the size of the switch is essentially
twice the size of the two individual SPST switches.
[0035] A monolithic SPDT switch provides for improved operation
over that provided by the hybrid MEMS switch described above. A
monolithic MEMS SPDT switch is based upon the simultaneous
fabrication of two SPST switches in a side-by-side configuration on
the same substrate. A general overview of a MEMS SPDT switch 300
according to the present invention is shown in FIG. 3. The MEMS
SPDT switch 300 shown in FIG. 3 contains many features similar to
those depicted and described for the hybrid MEMS SPDT switch 100
discussed above. Thus, materials and techniques used for
constructing the hybrid MEMS SPDT switch 100 described above may
also be used be in the construction of the monolithic MEMS SPDT
switch 300 according to the present invention.
[0036] One end of a first armature 316 is affixed to the substrate
314 near an armature bias pad 334 on the substrate 314. Similarly,
one end of a second armature 317 is also affixed to the substrate
314 near the armature bias pad 334 on the substrate 314. The other
end of the first armature 316 is positioned over a left input
contact 356 and a left output contact 321. The other end of the
second armature 317 is positioned over a right input contact 357
and a right output contact 326. The first armature 316 and second
armature 317 may be oriented in a parallel direction to each other
so that they project above the substrate 314 in the same direction.
The left output contact 321 is electrically connected to a left RF
output line 320. The left output contact 321 and the left RF output
line 320 may be constructed as a single metal structure. Similarly,
the right output contact 326 is connected to a right RF output line
325, and may also be a single metal structure. The left input
contact 356 and the right input contact 357 are both electrically
connected to an RF input line 315. The left input contact 356, the
right input contact 357, and the RF input line 315 may also be a
single metal structure.
[0037] A first substrate bias electrode 322 is printed on the
substrate 314 below the first armature 316 and a second substrate
bias electrode 323 is printed on the substrate below the second
armature 317. The first armature 316 contains a first armature bias
electrode 330, preferably encapsulated with a first beam structural
layer 326. Similarly, the second armature 317 contains a second
armature bias electrode 331, preferably encapsulated within a
second beam structural layer 327. Both the first armature bias
electrode 330 and the second armature bias electrode 331 are
electrically isolated from their corresponding substrate bias
electrodes 322, 323 by an air gap (not shown in FIG. 3) and a
dielectric layer (not shown in FIG. 3), preferably silicon nitride,
beneath the armature bias electrodes 330, 331 within the beam
structural layers 326, 327 when the armatures 316, 317 are in an
open position. When the armatures 316, 317 are in a closed
position, the dielectric layer beneath the armature bias electrodes
330, 331, provides electrical isolation from the substrate bias
electrodes 322, 323.
[0038] A first substrate bias electrode pad 336 is electrically
connected to the first substrate bias electrode 322 by a first
metal path 338. Preferably, the first substrate bias electrode pad
336, the first substrate bias electrode 322, and the first metal
path 338 comprise a single metal structure, which may be formed by
depositing a single metal layer on the substrate 314. A second
substrate bias electrode pad 337 is electrically connected to the
second substrate bias electrode 323 by a second metal path 339.
Preferably, the second substrate bias electrode pad 337, the second
substrate bias electrode 323, and the second metal path 339
comprise a single metal structure, which may be formed by
depositing a single metal layer on the substrate 314.
[0039] A left input conducting dimple 342 and a left output
conducting dimple 341 protrude from the first armature 316 toward
the left RF input contact 356 and the left RF output contact 321. A
first conducting transmission line 340 is printed on the first
armature 316 and electrically connects the left input conducting
dimple 342 to the left output conducting dimple 341. When the first
armature 316 is in an open position, the conducting dimples 341,
342 are electrically isolated from the left RF input contact 356
and the left RF output contact 321 by an air gap. The left RF input
contact 356 and the left RF output contact 321 are separated from
each other on the substrate 314 by a nonconducting gap.
[0040] The first armature 316 is comprised of the first beam
structural layer 326, the first conducting transmission line 340,
the first suspended armature bias electrode 330, and a first via
hole 332. The first armature bias electrode 330 may be encapsulated
within the first beam structural layer 326 so that dielectric
material covers both the top and bottom of the first armature bias
electrode 330. The first armature bias electrode 330 extends over
the majority of the first armature 316, but the first armature bias
electrode 330 is electrically isolated from the first conducting
transmission line 340. The first armature bias electrode 330
connects to the armature bias pad 334 through metal deposited in
the first via hole 332. When a voltage is applied between the first
suspended armature bias electrode 330 and the first substrate bias
electrode 322, an electrostatic attractive force will pull the
first suspended armature bias electrode 330 as well as the attached
first armature 316 towards the first substrate bias electrode 322,
such that the left input conducting dimple 342 touches the left
input contact 356 and the left output conducting dimple 341 touches
the left output contact 321. Since the conducting line 340 is
fabricated to electrically connect the left input conducting dimple
342 to the left output conducting dimple 341, the conducting line
340 and the dimples 341, 342 bridge the gap between the RF input
line 315 and the left RF output contact line 320, thereby directing
RF energy applied to the RF input line 315 to the left RF output
line 320.
[0041] Similarly, a right input conducting dimple 346 and a right
output conducting dimple 347 protrude from the second armature 317
toward the right RF input contact 357 and the right RF output
contact 326. A second conducting transmission line 345 is printed
on the second armature 317 and electrically connects the right
input conducting dimple 346 to the right output conducting dimple
347. When the second armature 317 is in an open position, the
conducting dimples 346, 347 are electrically isolated from the
right RF input contact 357 and the right RF output contact 326 by
an air gap. The right RF input contact 357 and the right RF output
contact 326 are separated from each other on the substrate 314 by a
nonconducting gap.
[0042] The second armature 317 is comprised of a second beam
structural layer 327, the second conducting transmission line 345,
a second suspended armature bias electrode 331, and a second via
hole 333. The second armature bias electrode 331 may be
encapsulated within the second beam structural layer 327 so that
dielectric material covers both the top and bottom of the second
armature bias electrode 331. The second armature bias electrode 331
extends over the majority of the second armature 317, but the
second armature bias electrode 331 is electrically isolated from
the second conducting transmission line 345. The second armature
bias electrode 331 connects to the armature bias pad 334 through
metal deposited in the second via hole 333. When a voltage is
applied between the second suspended armature bias electrode 331
and the second substrate bias electrode 323, an electrostatic
attractive force will pull the second suspended armature bias
electrode 331 as well as the attached second armature 317 towards
the second substrate bias electrode 323, such that the right input
conducting dimple 346 touches the right RF input contact 357 and
the right output conducting dimple 347 touches the right RF output
contact 326. Since the second conducting line 345 is fabricated to
electrically connect the right input conducting dimple 347 to the
right output conducting dimple 347, the second conducting line 345
and the dimples 346, 347 bridge the gap between the right RF input
contact 357 and the right RF output contact 326, thereby directing
RF energy applied to the RF input line 315 to the right RF output
line 325.
[0043] The substrate 314 may be comprised of a variety of
materials. If the monolithic MEMS switch 300 is intended for use
with semiconductor devices, it is preferable to use a
semiconducting substance such as gallium arsenide (GaAs) for the
substrate 314. This allows the circuit elements as well as the MEMS
switch 300 to be fabricated simultaneously on the same substrate
using standard integrated circuit fabrication technology such as
metal sputtering and masking. For low-noise HEMT MMIC (high
electron mobility transistor monolithic microwave integrated
circuit) applications, indium phosphide (InP) can be used as the
substrate 314. Other possible substrate materials include high
resistivity silicon, various ceramics, or quartz. The flexibility
in the fabrication of the monolithic MEMS switch 300 allows the
switch 300 to be used in a variety of circuits. This reduces the
cost and complexity of circuits designed using the present MEMS
switch.
[0044] The gaps between the dimples 341, 342, 346, 347 and the
input and output contacts 356, 357, 321, 326 are smaller than the
gap between the armatures 316, 317 and the substrate 314, as shown
in FIG. 4A. When actuated by electrostatic attraction, an armature
316, 317 bends towards the substrate 314. First, the dimples 341,
342, 346, 347 contact their corresponding input and output contacts
356, 357, 321, 326 at which point the armature 316, 317 bends to
allow the suspended armature bias electrode 330, 331 to rest
directly above the substrate bias electrode 322, 323, but isolated
from the substrate bias electrode 322, 323 by dielectric material
in the beam structural layer. This fully closed state is shown in
FIG. 4B. The force of the metallic contact between the dimples 341,
342, 346, 347 and the input and output contacts 356, 357, 321, 326
is thus primarily dependent on the flexibility of the armature 316,
317 and the geometry of the dimples and not on the attractive
forces of the armature electrode 330, 331 to the substrate
electrode 322, 323.
[0045] The first beam structural layer 326 is the primary support
of the first armature 316 and the second beam structural layer is
the primary support of the second armature 317. The first armature
electrode 330 and the second armature electrode 331 are printed
either on top of the corresponding beam structural layers 326, 327
or are encapsulated within the beam structural layers 326, 327. The
beam structural layer 326, 327 is made from a stress-free material
such as silicon nitride. The multiple layer design of the armature
electrode 330, 331 encapsulated within a resilient structural layer
326, 327 gives each armature 316, 317 enhanced mechanical
properties.
[0046] An embodiment of a monolithic SPDT RF MEMS switch according
to the present invention is pictured in FIG. 7. A monolithic SPDT
switch according to the present invention provides significantly
better performance than the hybrid switch discussed above.
Isolation and insertion loss data for the switch shown in FIG. 7 is
presented in FIGS. 5A and 5B. As shown in FIG. 5A, the isolation
provided by the switch is 40 dB or greater below 15 GHZ. Hence, the
monolithic SPDT switch provides an improvement of up to 10 dB in
isolation over the hybrid SPDT switch. The monolithic switch does
not suffer from increased insertion loss. As shown in FIG. 5D, the
insertion loss is less than 0.3 dB for frequencies below 15
GHz.
[0047] A layer of SiO.sub.2 is used to support the armature 316,
317 during the fabrication of the MEMS switch 300, but it is
removed in the last fabrication step, hence its term "sacrificial
layer." It is necessary to remove this sacrificial SiO.sub.2 layer
in order to free each armature 316, 317 such that they are free to
deflect out of plane of the substrate 314. An HF etchant solution
is typically used, and openings in the beam structural layers 326,
327 allow the HF to etch the sacrificial layer beneath the
armatures 316, 317 in this last fabrication step as discussed below
in conjunction with FIGS. 6E and 6F.
[0048] FIGS. 6A-6F illustrate the manufacturing processes embodying
the present invention used to fabricate the monolithic MEMS switch
300 of FIGS. 3, 4 and 7. FIGS. 6A-6F present a profile of the
switch taken along the section line 3-3' of FIG. 3. Therefore,
FIGS. 6A-6F specifically illustrate the steps required to fabricate
the structures associated with the first armature 316. However, the
structures associated with both the first armature 316 and the
second armature 317 may be fabricated simultaneously in the
monolithic MEMS switch 300. Therefore, the process discussion below
addresses the steps used to fabricate the entire monolithic MEMS
switch 300.
[0049] The process begins with a substrate 314. In a preferred
embodiment, GaAs is used as the substrate. Other materials may be
used, however, such as InP, ceramics, quartz or silicon. The
substrate is chosen primarily based on the technology of the
circuitry the MEMS switch is to be connected to so that the MEMS
switch and the circuit may be fabricated simultaneously. For
example, InP can be used for low noise HEMT MMICS (high electron
mobility transistor monolothic microwave integrated circuits) and
GaAs is typically used for PHEMT (pseudomorphic HEMT) power
MMICS.
[0050] FIG. 6A shows a profile of the MEMS switch 300 after the
first step of depositing a metal 1 layer onto the substrate 314 for
the armature bias pad 334, substrate bias electrode pads 336, 337
(not shown in FIG. 6A), the output lines 320, 325, the input line
315 (not shown in FIG. 6A) and the substrate bias electrodes 322,
323 is complete. The metal 1 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 metal 1 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 900 angstrom layer of gold
germanium is deposited, followed by a 100 angstrom layer of nickel,
and finally a 1500 angstrom layer of gold. The thin layer of gold
germanium (AuGe) eutectic metal is deposited to ensure adhesion of
the Au by alloying the AuGe into the semiconductor similar to a
standard ohmic metal process for any III-V MESFET or HEMT.
[0051] Next, as shown in FIG. 6B, a support layer 372 is placed on
top of the Au and etched so that the armatures 316, 317 may be
produced above the support layer 372. The support layer 372 is
typically comprised of 2 microns of SiO.sub.2 which may be sputter
deposited or deposited using PECVD (plasma enhanced chemical vapor
deposition). Vias 332, 333 are etched in the sacrificial layer 372
so that the metal of the armature bias pad 334 is exposed. The vias
332, 333 definition may be performed using standard resist
lithography and etching of the support layer 372. Other materials
besides SiO.sub.2 may be used as a sacrificial layer 372. The
important characteristics of the sacrificial layer 372 are a high
etch rate, good thickness uniformity, and conformal coating by the
oxide of the metal already on the substrate 314. The thickness of
the oxide partially determines the thickness of the switch opening,
which is critical in determining the voltage necessary to close the
switch as well as the electrical isolation of the switch when the
switch is open. The sacrificial layer 372 will be removed in the
final step to release the armatures 316, 317, as shown in FIG.
6F.
[0052] Another advantage of using SiO.sub.2 as the support layer
372 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 the polyimide
sacrificial layer difficult to later remove. The support layer 372
is exposed to high temperatures when the silicon nitride for the
beam structural layers 326, 327 is deposited, as a high temperature
deposition is desired when depositing the silicon nitride to give
the silicon nitride a lower HF etch rate.
[0053] FIG. 6C shows the fabrication of the beam structural layers
326, 327. The beam structural layers 326, 327 are the supporting
mechanism of the armatures 316, 317 and are preferably made out of
silicon nitride, although other materials besides silicon nitride
may be used. Silicon nitride is preferred because it can be
deposited so that there is neutral stress in the beam structural
layers 326, 327. Neutral stress fabrication reduces the bowing that
may occur when the switch is actuated. The material used for the
structural layers 326, 327 must have a low etch rate compared to
the support layer 372 so that the structural layers 326, 327 are
not etched away when the sacrificial layer 372 is removed to
release the armatures 316, 317. The structural layers 326, 327 are
patterned and etched using standard lithographic and etching
processes.
[0054] The beam structural layers 326, 327 may be formed only below
the armature bias electrodes 330, 331. If the beam structural layer
326, 327 are fabricated only below the first armature bias
electrodes 330, 331, bowing will occur in the armatures 316, 317
when the switch is actuated, if the stresses in the structural
layers 326, 327 differs from the stresses in the armature bias
electrodes 330, 331. The armatures 316, 317 will bow either upwards
or downwards, depending upon which material has the higher stress.
Bowing can change the voltage required to activate the switch and,
if the bowing is severe enough, can prevent the switch from either
opening (bowed downward) or closing (bowed upward) regardless of
the actuating voltage.
[0055] The beam structural layers 326, 327 may also be formed both
above and below the armature bias electrodes 330, 331 to minimize
the bowing in the armatures 316, 317. By fabricating the beam
structural layers 326, 327 on both sides of the armature bias
electrodes 330, 331, the effect of different material stress is
minimized because the portions of the beam structural layers 326,
327 that are above the armature bias electrodes 330, 331 will flex
in the same manner as the portions of the beam structural layers
326, 327 that are below the armature bias electrodes 330, 331. The
armature bias electrodes 330, 331 are constrained by the structural
layers 326, 327 and will therefore flex with the structural layers
326, 327 so that the bowing in the switch is minimized.
[0056] In FIG. 6D, dimple receptacles 376 are etched into the beam
structural layers 326, 327 and the support layer 372. The dimple
receptacles 376 are openings where the conducting dimples 341, 342,
346, 347 will later be deposited, as shown in FIG. 6E. The dimple
receptacles 376 are created using standard lithography and a dry
etch of the beam structural layers 326, 327, followed by a partial
etch of the support layer 372. The openings in the structural
layers 326, 327 allow the dimples 341, 342, 346, 347 to protrude
through the structural layers 326, 327.
[0057] Next, as shown in FIG. 6E, a metal 2 layer is deposited onto
the beam structural layers 326, 327. The metal 2 layer forms the
suspended armature bias electrodes 330, 331, the conducting
transmission lines 340, 345 (not shown in FIG. 6E), and the dimples
341, 342, 346, 347. In the preferred embodiment, the metal 2 layer
is comprised of a sputter deposition of a thin film (200 angstroms)
of Ti followed by a 1000 angstrom deposition of Au. The metal 2
layer must be conformal across the wafer and acts as a plating
plane for the Au. The plating is done by using metal 2 lithography
to open up the areas of the switch that are to be plated. The Au is
electroplated by electrically contacting the membrane metal on the
edge of the wafer and placing the metal 2 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 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 only 1200 angstroms thick.
[0058] The result of this process is that the conducting
transmission lines 340, 345 and the dimples 341, 342, 346, 347 are
created in the metal 2 layer, primarily Au in the preferred
embodiment. In addition, the Au fills the vias 332, 333 and
connects the armature bias electrodes 330, 331 to the armature bias
pad 334. Au is a preferred choice for metal 2 because of its low
resistivity. When choosing the metal for the metal 2 layer and the
material for the beam structural layers 326, 327, it is important
to select the materials such that the stress of the beam structural
layers 326, 327 such that the armatures 316, 317 will not bow
upwards or downwards when actuating. This is done by carefully
determining the deposition parameters for the structural layer.
Silicon nitride was chosen for this structural layer not only for
its insulating characteristics but in large part because of the
controllability of these deposition parameters and the resultant
stress levels of the film.
[0059] The beam structural layers 326, 327 are then
lithographically defined and etched to complete the switch
fabrication. Finally, the sacrificial layer 372 is removed to
release the armature 316, as shown in FIG. 6F.
[0060] If the sacrificial layer 372 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 armatures 316, 317 do not come into contact with
the substrate 314 when the sacrificial layer 372 is removed. If
contact occurs during this process, device sticking and switch
failure are likely. Contact is 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 DI water by dilution, so
that the switch is not removed from a liquid during the process. DI
water is similarly replaced with ethanol. The sample is transferred
to the critical point dryer and the chamber is sealed. High
pressure liquid CO.sub.2 replaces the ethanol 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 300 after the support layer 372 has been
removed is shown in FIG. 6F.
[0061] As can be surmised by one skilled in the art, there are many
more configurations of the present invention that may be used other
than the ones presented herein. For example, other metals can be
used to form the conducting transmission line layer, the bias
electrodes and pads, and the input and output lines. Also, the beam
structural layer and the sacrificial layer may be fabricated with
materials other than silicon nitride and silicon dioxide. It is
therefore intended that the foregoing detailed description be
regarded as illustrative rather than limiting and that it be
understood that it is the following claims, including all
equivalents, that are intended to define the scope of this
invention.
* * * * *