U.S. patent application number 09/997671 was filed with the patent office on 2002-07-25 for mems device with integral packaging.
Invention is credited to Cohn, Michael B., Xu, Ji-Hai.
Application Number | 20020096421 09/997671 |
Document ID | / |
Family ID | 26943623 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020096421 |
Kind Code |
A1 |
Cohn, Michael B. ; et
al. |
July 25, 2002 |
MEMS device with integral packaging
Abstract
A MEMS device and method of making same is disclosed. In one
embodiment, a micro-switch includes a base assembly comprising a
movable structure bearing a contact pad. The base assembly is
wafer-scale bonded to a lid assembly comprising an activator and a
signal path. The movable structure moves within a sealed cavity
formed during the bonding process. The signal path includes an
input line and an output line separated by a gap, which prevents
signals from propagating through the micro-switch when the switch
is deactivated. In operation, a signal is launched into the signal
path. When the micro-switch is activated, a force is established by
the actuator, which pulls a portion of the movable structure
upwards towards the gap in the signal path, until the contact pad
bridges the gap between the input line and output line, allowing
the signal to propagate through the micro-switch. Prior to bonding,
the MEMS structures are annealed on a first wafer and the
conductive traces and other metals are annealed on a second wafer
to allow each wafer to be processed separately using different
processes, e.g., different annealing temperatures.
Inventors: |
Cohn, Michael B.; (Berkeley,
CA) ; Xu, Ji-Hai; (El Sobrante, CA) |
Correspondence
Address: |
FENWICK & WEST LLP
TWO PALO ALTO SQUARE
PALO ALTO
CA
94306
US
|
Family ID: |
26943623 |
Appl. No.: |
09/997671 |
Filed: |
November 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60253851 |
Nov 29, 2000 |
|
|
|
Current U.S.
Class: |
200/181 |
Current CPC
Class: |
B81C 3/001 20130101;
B81B 2201/018 20130101; B81C 1/00301 20130101; H01H 59/0009
20130101; B81C 2201/019 20130101; B81C 2203/019 20130101; B81B
2203/0118 20130101; B81B 7/0077 20130101; B81C 1/00357 20130101;
B81C 2203/0118 20130101; B81B 2207/093 20130101; H01H 2059/0072
20130101; B81C 2203/0109 20130101; H01H 2059/0018 20130101 |
Class at
Publication: |
200/181 |
International
Class: |
H01H 057/00 |
Claims
What is claimed is:
1. A micro-switch, comprising: a first substrate having a signal
path, the signal path including a gap portion for preventing signal
propagation; and a second substrate bonded to the first substrate
to form a sealed cavity, the sealed cavity adapted to house a
movable structure, the movable structure having a contact pad for
bridging the gap portion of the signal path when the movable
structure is pulled toward the first substrate in response to a
force provided by an actuator.
2. The micro-switch of claim 1, further including: a seal ring,
forming a seal around the movable structure.
3. The micro-switch of claim 1, further including: an insulation
ring disposed on the first substrate; and a metal seal ring
substantially aligned with and overlying the insulation ring, the
metal seal ring forming a seal around the movable structure, the
insulating ring electrically insulating the metal seal ring from
the signal path.
4. The micro-switch of claim 1, wherein the movable structure is
made of polysilicon.
5. The micro-switch of claim 1, wherein the movable structure is a
cantilever beam.
6. The micro-switch of claim 1, further comprising: a passive
element disposed on one of the first and second substrates and
coupled to the signal path.
7. The micro-switch of claim 1, further comprising: an active
element disposed on one of the first and second substrates and
coupled to the signal path.
8. The micro-switch in claim 1, further comprising: a plurality of
spacers formed in an insulating layer disposed between the contact
pad and the movable structure.
9. The micro-switch of claim 1, further comprising: a passive
element disposed on the first substrate and electrically coupled to
the signal path.
10. The micro-switch of claim 1, wherein at least one of the first
and second substrates includes a mechanical stop.
11. The micro-switch of claim 1, further including an isolation pad
disposed between the contact pad and the movable structure.
12. The micro-switch of claim 11, wherein the isolation pad is made
of silicon nitride.
13. The micro-switch of claim 2, wherein the seal ring is made of a
deformable metal.
14. The micro-switch of claim 2, wherein the deformable metal
comprises gold.
15. The micro-switch of claim 2, further comprising: a seal-landing
ring disposed on the second substrate, the seal-landing ring for
mating with the metal seal ring to form a hermetic seal.
16. The micro-switch of claim 2, wherein the signal path runs under
the insulation ring to connect to an external contact point outside
the insulation ring.
17. The micro-switch of claim 2, wherein the contact point is a
wire bond pad.
18. A lid assembly for a micro-switch having a lid assembly and a
base assembly, comprising: a substrate; a first insulating layer
formed on the substrate; a first conductive layer formed on the
first insulating layer; a signal path formed in the conductive
layer and having a gap for preventing signals from propagating on
the signal path; an actuator formed in the first conductive layer
for providing a force on a movable structure in the base assembly
when the actuator is activated; a second insulting layer formed on
the first conductive layer; an insulating ring formed in the second
insulting layer; a second conductive layer formed on the second
insulating layer; and a conductive ring formed in the second
conductive layer, the conductive ring substantially aligned with
and overlying the insulating ring, the conductive ring forming a
seal around the movable structure when the lid assembly is bonded
to the base assembly.
19. A base assembly for a micro-switch having a lid assembly and a
base assembly, comprising: a substrate; a sacrificial layer formed
on the substrate; a structural layer formed on the sacrificial
layer; an insulating layer formed on the structural layer; an
insulating pad formed in the insulating layer; a movable structure
formed in the structural layer and the sacrificial layer, the
movable structure bearing the insulating pad; and a contact pad
formed on the insulating pad, the contact pad for bridging a gap in
a signal path on the lid assembly, when the movable structure is
pulled towards the lid assembly in response to a force provided by
an actuator in the lid assembly.
20. A method of making a lid assembly for a micro-switch having a
lid assembly and a base assembly, the method comprising: forming a
first insulating layer on a substrate; forming a first conductive
layer on the first insulating layer; forming a signal path and an
actuator in the first conductive layer; the signal path having a
gap for preventing signals from propagating on the signal path, the
actuator for providing a force on a movable structure in the base
assembly when the actuator is activated; forming a second
insulating layer on the first conductive layer; forming an
insulating ring in the second insulating layer; forming a second
conductive layer on the second insulating layer; and forming a
conductive ring in the second conductive layer, the conductive ring
substantially aligned with and overlying the insulating ring, the
conductive ring for making a seal between the lid assembly and the
base assembly.
21. A method of making a base assembly for a micro-switch having a
lid assembly and a base assembly, the method comprising: forming a
sacrificial layer on a substrate; forming a structural layer on the
sacrificial layer; forming an insulating layer on the structural
layer; forming an insulating pad in the insulating layer; forming
an intermediate movable structure in the structural layer and the
sacrificial layer; forming a conductive layer on the structural
layer and sacrificial layer; forming a contact pad on the
conductive layer, wherein the contact pad overlies the intermediate
structure; and removing a portion of the intermediate structure to
form a movable structure, the movable structure bearing the contact
pad, the contact pad for bridging a gap in a signal path in the lid
assembly when the movable structure is pulled towards the lid
assembly in response to a force provided by an actuator in the lid
assembly.
22. The lid assembly in claim 18, wherein the signal path extends
under the insulation ring to connect with a contact point external
to the insulation ring.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/253,851, filed Nov. 29, 2000, which is
incorporated by reference herein.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to the packaging of
semiconductor devices and, more particularly, to the use of
wafer-scale bonding techniques in the packaging of
microelectromechanical systems (MEMS).
[0004] 2. Background
[0005] MEMS devices have been manufactured in the past using
integrated circuit (IC) fabrication techniques. Although MEMS
devices are generally fabricated on silicon wafers, MEMS devices
are different from most ICs in that they possess moving elements.
These moving elements make MEMS devices vulnerable to contamination
and moisture because they cannot be encapsulated in a typical
injection-molded plastic package, i.e., at least one element of
every MEMS device must remain free to move.
[0006] Several specialized methods, which are directed to different
stages of the fabrication process, have been employed for the
sealing and packaging of MEMS. One early method directed to the
final packaging stage employs a ceramic package with a co-fired
lead frame, a cavity to contain a complete MEMS device and a lid to
cover the package and cavity. With this chip-scale packaging
method, the MEMS device is placed inside the cavity and the lid is
soldered in place to form a hermetic seal. One problem with this
chip-scale packaging approach is the expense associated with
handling the individual components (e.g., the packages, lids, etc.)
and assembling them into a single package. An additional problem
with this chip-scale packaging approach is that the moving elements
in MEMS are extremely fragile and tend to be damaged by any
handling, especially near the end of the fabrication process.
Therefore, packaging yields are typically low. A further problem
with this packaging approach is that the solder seal on the lid
tends to outgas when re-flowed during the final sealing process. If
a vacuum is needed inside the package, a getter must be added, at
additional cost. In such cases, packaging costs can constitute 80%
of the total device cost. In sum, the use of ceramic packages to
seal and package MEMS devices is inordinately expensive due to the
need to individually handle components, the need to use getters and
the considerable opportunity for damage and contamination of the
MEMS devices.
[0007] A wafer-scale pre-packaging technique of bonding two wafers
together has been used to bond a wafer of silicon or glass onto a
MEMS wafer. In this back-end process, a non-conductive glass frit
is screen-printed onto a lid wafer in the shape of rings or
gaskets. The wafers are then placed together to form cavities
containing MEMS devices and bonded by the application of heat in
excess of 400.degree. C. After bonding, the two-wafer stack is
diced and the resulting hybrid structures are packaged in the
standard manner: by encapsulation in plastic packages such as DIPs,
SOICs, etc. Although this process has been marginally successful,
it suffers from the deficiency that the glass frit cannot be
patterned into fine features. Typically, gasket pattern line widths
are between 100-200 microns. Given that MEMS are currently made
with dimensions or features in the one-micron size range,
significant real estate must be sacrificed, thereby significantly
reducing the number of devices that can be fabricated on a single
wafer. Attempts have been made to improve the glass frit patterning
resolution, using lithographic methods, but these have not shown
success. Further, gasket patterns thinner than 100-200 microns in
width do not appear to provide reliable hermetic seals, possibly
because of the inherently porous nature of glass frits. Thus, it is
currently believed that glass frit techniques simply cannot be
applied on a smaller scale where hermetic seals are desired.
[0008] Glass frit techniques also require high temperatures of
between about 450-600 Celsius for sealing to occur. One drawback of
using such a high firing temperatures is that it effectively
precludes the incorporation of integrated circuitry on either of
the wafers bonded together. These temperatures also preclude the
use of most anti-stiction coatings and hydrophobic coatings, which
are desirable for improving manufacturing yields and the MEMS's
tolerance to moisture and shock. Such high temperatures also damage
the MEMS themselves and, particularly, certain structural films
deposited at lower temperatures and structures in which the film
stress must be carefully controlled.
[0009] Another deficiency of glass frit techniques is that the
glass frit itself employs organic binders that outgas into the
sealed cavity during firing. Since getters are not available to
combat this problem, this fact precludes the use of glass frits for
vacuum applications. Still another limitation of the glass frit
techniques is that the sealing materials, being non-conductive,
cannot be used to establish electrical coupling between the devices
disposed on the bonded substrates.
[0010] Front-end wafer-scale bonding techniques which can be used
for sealing MEMS have also been developed, including anodic
bonding, silicon fusion bonding, and other wafer-bonding methods
employing combinations of silicon, silicon dioxide, and silicon
nitride. These techniques can provide vacuum and hermetic seals and
can do so with improved use of device real estate. These
techniques, however, have low throughput and require high
temperatures for high-quality bonds because their bonding
mechanisms involve solid diffusion across the bond interface.
Additionally, these techniques place strict constraints on the
materials and processes used to form the wafers to be bonded
together. Wafer-scale bonding techniques are exceptionally
sensitive to the presence of small particulate contaminants, wafer
warpage, scratches, and other imperfections commonly encountered
during semiconductor fabrication because such contaminants can
cause large defects in the bond and reduce yield substantially.
Moreover, normal variations in surface topology due to the presence
of circuitry can preclude proper bonding and/or sealing using such
techniques. All of these factors make it difficult to employ
wafer-bonding techniques with wafers that have undergone a
significant amount of processing (e.g., wafers bearing finished
microstructures).
[0011] One particular MEMS device, the fabrication of which is
disclosed herein, is a micro-switch. The micro-switch can be used
in, for example, mobile phones for switching RF signals between
transmit and receive modes and other related functions. Additional
applications include micro-relays, which can be used in, for
example, automated test equipment (ATE). As will be described below
with respect to FIGS. 1a and 1b, fabricating a hermetically sealed
micro-switch using materials having good mechanical qualities is
difficult to achieve using prior art front-end, wafer-scale bonding
techniques.
[0012] Referring to FIGS. 1a and 2b, there are shown
cross-sectional views of a prior art MEMS device 100 configured to
operate as a micro-switch (hereinafter also referred to as
"micro-switch 100"). FIG. 1a is a cross-sectional view of the
micro-switch 100 in its "deactivated" state (i.e., no electric
field 118 present) and FIG. 1b is a cross-sectional view of the
micro-switch 100 in its "activated" state (i.e., electric field 118
present).
[0013] The micro-switch 100 includes substrate 102, pedestal 104,
actuator 106, cantilever 108, contact pad 110, and signal path 112.
The substrate 102 is typically made of silicon and provides a
support base for the pedestal 102 and the cantilever 108. The
pedestal 102 and the cantilever 108 are formed in different layers
upon the substrate 102 using known IC fabrication processes (e.g.,
CMOS). The pedestal 104 is disposed on the substrate 102. It
provides support for the cantilever 108 and also provides a
sidewall 114, which together with the cantilever 108 and the
substrate 102 defines a gap 116. The gap 116 is typically very
small (e.g., on the order of 2 microns in the vertical dimension).
It provides the cantilever 108 with at least one degree of
freedom.
[0014] The contact pad 110 is disposed on a free end (distal end)
of the cantilever 108 and is typically made of metal. The location
of the contact pad 110 on the cantilever 108 is selected to ensure
that it makes electrical contact with the signal path 112, when the
actuator 106 is activated and the cantilever 108 is pulled downward
towards the substrate 102 in response to an electric or magnetic
field 118, as shown in FIG. 1b.
[0015] The signal path 112 is disposed on the substrate 100 in
aligned opposition with the contact pad 110. It is typically a
metal trace or strip, which serves as a path for signals
propagating through the micro-switch 100. The signal path 112
includes a gap (not shown) in a location corresponding to the
location of the contact pad 110 on the cantilever 108. If the
actuator 106 is inactive, a signal launched into the signal path
112 cannot propagate through the micro-switch 100 due to the
gap.
[0016] The actuator 106 is disposed on the substrate 100. It
establishes a field 118 (shown in FIG. 1b) when supplied with a
drive voltage. The field 118 provides a force that pulls the
cantilever 108 downward toward the substrate 102, until the contact
pad 110 bridges the gap in the signal path 112. In this "activated"
state, a signal launched into the signal path 112 will propagate
through the micro-switch 100 due to the bridged gap 116 provided by
the contact pad 110. FIG. 1b shows the cantilever 108 pulled into
electrical contact with the signal path 112, thereby bridging the
gap to allow signals to propagate through the micro-switch 100.
[0017] It would be desirable to fabricate the prior art cantilever
108 from materials that exhibit good mechanical qualities. One
example of such material is polycrystalline silicon
("polysilicon`), which has been extensively studied and is known to
be a desirable material for fabricating micromechanical devices.
Polysilicon, however, is not generally used to fabricate prior art
switches (e.g., micro-switch 100), because the cantilever 108 is
designed to lie over the contact pad 110, and therefore must be
deposited on the substrate 102 after the contact pad 110 in the
fabrication process flow. The high temperature of the polysilicon
anneal (e.g., above 400.degree. C.) may cause the contact pad 110
to melt, delaminate, or interdiffuse with other material layers,
thus making polysilicon an unsuitable material for the cantilever
108.
[0018] This problem was addressed in the past by using low
temperature deposited materials, such as PECVD silicon nitride. The
disadvantage with this prior art technique is that a high
temperature anneal is typically needed to control warping of the
cantilever 108 due to stress gradients. The degree of warping
(curling of the cantilever 108 up or down with respect to the
substrate 102) is not easily controlled and leads to wide
variations in the voltage required to actuate the cantilever 108.
This is typically not acceptable in a mass-produced device.
[0019] Accordingly, there is a need for a novel wafer-scale bonding
technique that can employ materials having desirable mechanical
qualities. Such a technique should enable massive, parallel
assembly of hermetically sealed, low-cost MEMS devices, and thus
should provide high yields. To realize these advantages, the
technique should not require extreme wafer planarity or
cleanliness. Additionally, there is a need for an improved
micro-switch/relay device, fabricated using the novel wafer-scale
bonding technique from materials having desirable mechanical
qualities, to provide improved performance and reliability over
prior art MEMS devices.
SUMMARY
[0020] The present invention eliminates the foregoing difficulties
through the use of a novel wafer-scale bonding technique that
enables massively parallel assembly of sealed, low-cost, reliable
MEMS devices using materials having desirable mechanical qualities,
which technique does not require extreme wafer planarity or
cleanliness.
[0021] In one embodiment of the present invention, a micro-switch
includes a base assembly having a movable structure bearing a
contact pad, and a lid assembly having an actuator and a signal
path. A plurality of lid assemblies are fabricated in parallel on a
first wafer and a plurality of base assemblies are fabricated in
parallel on a second wafer, thus allowing each wafer to be
processed separately using different processes (e.g., different
annealing temperatures). The first wafer is then bonded to the
second wafer causing each lid/base assembly pair to form a sealed
cavity. The sealed cavity is adapted to hold the movable structure,
which structure can move in at least one degree of freedom within
the sealed cavity.
[0022] In one embodiment of the present invention, the signal path
in the lid assembly includes an input line and an output line
separated by a gap, which prevents signals from propagating through
the micro-switch. In operation, a force is established by the
actuator, which pulls a portion of the movable structure upwards
towards the gap in the signal path, until the contact pad bridges
the gap between the input line and output line, allowing signals to
propagate through the micro-switch.
[0023] In one embodiment of the present invention, the lid assembly
includes an insulation ring disposed on the first substrate and a
metal seal ring substantially aligned with and overlying the
insulation ring, such that the metal seal ring forms a seal around
the movable structure and the insulating ring electrically
insulates the metal seal ring from the signal path. The signal path
runs under the insulation ring to connect with an external contact
point, such as a wire bond pad.
[0024] An additional advantage of the present invention is that the
improved planarity of the movable structure leads to improved
planarity of the contact pad. This, in turn, is believed to enable
longer lifetime of the micro-switch.
[0025] A further advantage of the present invention is that the
bonding enables hermetic sealing. Hermetic sealing has been shown
to significantly improve device lifetime.
[0026] Another advantage of the present invention is the use of
high-temperature materials in the movable structure, allowing the
use of higher temperatures in the metal deposition steps that form
the contacting surfaces. As a general principle, materials that are
formed or processed at higher temperatures display greater
reliability and wear resistance. These are critical factors in a
micro-switch, because of the metal-to-metal contact, which may be
repeated up to about 10.sup.12 times in the life of the device.
[0027] A further advantage of the present invention is that a wider
choice of materials is available for the metal contacts. This is
because they are the last materials deposited on their respective
wafers.
[0028] An additional advantage is that the gap between the metal
contacts can be controlled directly, by varying the thickness of
the seal ring. This thickness can be controlled to within about
5-10%.
[0029] Yet another advantage of the present invention is improved
yield. The present invention enables all of the microstructures on
a given wafer to be protected immediately after release. Thus, they
are protected during dicing. In the past, MEMS devices were
generally diced before packaging. Particulates and water present
during the dicing process would damage devices, drastically
lowering yield.
[0030] A further advantage of the present invention is that very
small electrical connections can be formed between the two
substrates. This allows, for example, high-frequency or
noise-sensitive signals to be coupled between them with a minimum
of attenuation or degradation. A serious challenge in the design of
MEMS, for example, is coupling signals from a mechanical
microstructure to an integrated circuit, when the two are formed on
separate substrates. Wire bond pads, or solder bumps, with a
typical size of 100.times.100 um, comprise a significant parasitic
capacitance. The interconnection approach of the present invention,
for example, using metal micro-bumps in the 10.times.10 um range,
enable a hundred-fold reduction of parasitic capacitance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] These and other features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
where:
[0032] FIG. 1a is a cross-sectional view of a prior art
micro-switch 100 shown in its "deactivated" state;
[0033] FIG. 1b is a cross-sectional view of the prior art
micro-switch 100 shown in FIG. 1a in its "activated" state;
[0034] FIG. 2a is a cross-sectional view of a lid assembly 200 of a
micro-switch 400, fabricated in accordance with the present
invention;
[0035] FIG. 2b is a cross-sectional view of a base assembly 220 of
a micro-switch 400, fabricated in accordance with the present
invention;
[0036] FIG. 3a is a top plan view of the lid assembly 200 shown in
FIG. 2a, fabricated in accordance with the present invention;
[0037] FIG. 3b is a top plan view of the base assembly 220 shown in
FIG. 2b, fabricated in accordance with the present invention;
[0038] FIG. 4a is a cross-sectional view of an assembled
micro-switch 400, fabricated in accordance with the present
invention;
[0039] FIG. 4b is a cross-sectional view of the micro-switch 400
shown in FIG. 4a in its "activated" state;
[0040] FIG. 5 is a cross-sectional view of the micro-switch 400
shown in FIG. 4a, including dielectric spacers;
[0041] FIG. 6a is a cross-sectional view of the micro-switch 400
shown in FIG. 4a, including upper and lower mechanical stops 317,
328;
[0042] FIG. 6b is a cross-sectional view of the micro-switch 400
shown in FIG. 6a after final bonding;
[0043] FIG. 7 is a flow diagram showing the process flow steps used
in the fabrication of the lid assembly 200 shown in FIG. 2a;
[0044] FIGS. 8a-1 are cross-sectional views showing the process
flow steps used in the fabrication of the lid assembly 200 shown in
FIG. 2a;
[0045] FIG. 9 is flow diagrams showing the process flow steps used
in the fabrication of the base assembly 220 shown in FIG. 2b;
[0046] FIGS. 10a-i are cross-sectional views showing the process
flow steps used in the fabrication of the base assembly 220 shown
in FIG. 2b; and
[0047] FIGS. 11a-b illustrate a wafer-scale bonding technique used
to fabricate the micro-switch 400 shown in FIG. 4a.
DETAILED DESCRIPTION OF EMBODIMENTS
[0048] Although the MEMS device described below is a micro-switch,
it should be understood by the reader that the present invention is
not limited to a micro-switch, but also applies to other types of
MEMS devices, including but not limited to optical switches,
micro-relays, microdisplays, optical attenuators, variable optical
attenuators, variable capacitors, phase shifters, surface acoustic
wave devices, film bulk acoustic resonator (FBAR) devices,
inductors, gyroscopes, accelerometers, pressure sensors, fluidic
channels, chambers, resonators, strain sensors, rotary
accelerometers, fiber optic aligners, optical micro-benches,
scratch drives, and actuators. As used herein, the terms
"micro-switch" and "micro-relay" are interchangeable.
[0049] Although the description below refers to a single
micro-switch, it should be understood that a plurality of such
devices are typically fabricated (as opposed to a single device)
simultaneously by bonding two wafers together in accordance with
the present invention. Such techniques provide massive parallel
assembly of sealed MEMS devices with high device yields.
[0050] Lid Assembly
[0051] Referring to FIG. 2a, there is shown a cross-sectional view
of one embodiment of a lid assembly 200 for a micro-switch 400
shown in FIGS. 4a and 4b, which switch is fabricated in accordance
with the present invention. The lid assembly 200 is fabricated by
depositing layers of materials in a predetermined hierarchy using a
process described with respect to FIG. 7. As will be described
below, the lid assembly 200 is bonded to a base assembly 220 (FIG.
2b) to form a hermetically sealed micro-switch 400, as shown in
FIGS. 4a and 4b. While a hermetic seal is preferred, other seals
can be employed with the present invention, including but not
limited to vacuum seals and gross-leak seals. FIG. 3a shows a top
plan view of the lid assembly 200. The description of the lid
assembly 200 will refer to both FIGS. 2a and 3a.
[0052] In one embodiment of the present invention, the lid assembly
200 includes substrate 202, insulating film 204, seal ring 206
(316), signal path 208 (300, 302), actuator 210 (310), anchor
contact bump 212 (312), and insulating ring 214 (314). The
substrate 202 can be made of any silicon, but is preferably made of
high-resistivity silicon, glass or quartz and provides a common
support structure for the microstructures in lid assembly 200.
Other materials having differing resistivities can be used for the
substrate 202, including but not limited to gallium arsenide,
indium phosphide, indium gallium aresenide, sapphire,
silicon-on-insulator, silicon-on-sapphire, polysilicon, amorphous
silicon, plastic, or any combination of the these materials. The
substrate 202 is coated with the insulating film 204, which is
preferably made of silicon nitride, but other materials can be
used, such as silicon oxide or ceramic. In this particular
embodiment, the insulating film 204 acts as a diffusion barrier
between the substrate 202 and a plurality of conductive traces 300,
302, 304, 306, as described below with respect to FIG. 3a.
[0053] Referring to FIG. 3a, the signal path 208 preferably
comprises an input line 300 and an output line 302 separated by a
gap 308, which serves as a barrier to signals (e.g., RF signals)
transmitted on the input line 300. To improve performance of the
micro-switch 400, a co-planar waveguide (CPW) structure can be used
as the input line 300 and the output line 302. Although only one
signal path 208 is shown in this embodiment, the present invention
can employ one or more signal paths, each having one or more gaps.
The one or more signal paths 208 can be disposed in the lid
assembly 200 or the base assembly 220 or both. The signal path 208
is preferably made of metal, including but not limited to gold,
titanium, titanium nitride, tantalum, tantalum nitride, copper,
aluminum, tungsten, solders, metal amalgams, gallium, mercury, tin,
lead, indium, palladium, thallium, bismuth, or any combination of
these metals.
[0054] Also shown in FIG. 3a, is actuator driveline 304, which
terminates in the actuator 310 (210) and anchor ground line 306,
which terminates in the anchor contact bump 312 (212). The actuator
driveline 304 provides the actuator 310 with an AC or DC actuation
voltage in either positive or negative form. The actuation voltage
causes the actuator 310 to apply a force 402 on the movable
structure 230, as described with respect to FIG. 4b. The anchor
ground line 306 provides an electrical ground for the actuation
voltage. Preferably, the actuator 310 is an electrode capable of
establishing an electric field, but one or more actuators of
various types may be used with the present invention, including but
not limited to electrostatic, capacitive, electromagnetic,
magnetic, scratch drive, membrane capacitive, thermal,
electro-thermal, shape memory alloy, mechanical and any combination
of the above actuators.
[0055] The conductive traces 300, 302, 304, 306, are coupled to one
or more wire bond pads (not shown) located outside the insulating
ring 314 (214) using wire bonding or tab bonding techniques.
Preferably, the connections to the wire bond pads run under the
insulating ring 314 (214), so that they do not short the metal seal
ring 316, as shown in FIG. 3a. The insulating ring 314 (214)
overlies the conductive traces 300, 302, 304, 306, in a ring-shaped
pattern. This pattern is substantially a mirror image of a
seal-landing ring 320, which is described with respect to base
assembly 220. The insulating ring 314 (214) pattern can be any
closed curve, but is preferably a square pattern as shown in FIG.
3a. The insulating ring 314 (214) is preferably formed of silicon
dioxide, since it has a relatively low dielectric constant, but is
thermally and mechanically robust. The insulating ring 314 (214) is
preferably about 2 microns in thickness, to ensure adequate
electrical isolation between the conductive traces 300, 302, 304,
306, and the seal ring 314 (214). In this particular embodiment,
the seal ring 316 (206) overlies and is substantially aligned with
the insulating ring 314 (214), as shown in FIG. 3a. Alternatively,
the insulation ring 314 (214) and the seal ring 316 (206) can be
formed in the base assembly 220 instead of the lid assembly
200.
[0056] Although the seal-ring 316 (206) is preferably made of metal
with little or no surface oxide (e.g., a pure, soft, bright gold),
other electrically conductive materials having differing
resistivities can be used for the other conductive elements 300,
302, 304, 306, 310, 312. These materials include, but are not
limited to metals such as gold, titanium, titanium nitride,
tantalum, tantalum nitride, copper, aluminum, tungsten, solders,
metal amalgams, gallium, mercury, tin, lead, indium, palladium,
thallium, bismuth, or any combination of these metals.
[0057] Although the insulation ring 314 (214) and the seal ring 316
(206) are shown as part of the lid assembly 200, the insulation
ring 314 (214) and the seal ring 316 (206) can be part of the base
assembly 220 or part of both the lid and base assemblies 200,
220.
[0058] Base Assembly
[0059] Referring to FIG. 2b, there is shown a cross-sectional view
of one embodiment of a base assembly 220 for the micro-switch 400,
fabricated in accordance with the present invention. The base
assembly 220 is fabricated by depositing layers of materials in a
predetermined hierarchy using a process described with respect to
FIG. 8. As will be described below, the base assembly 220 is bonded
with the lid assembly 200 (FIG. 2a) to form the hermetically sealed
micro-switch 400 shown in FIGS. 4a and 4b. FIG. 3b shows a top plan
view of the base assembly 220. The description of the base assembly
220 will refer to both FIGS. 2b and 3b.
[0060] In one particular embodiment of the present invention, the
base assembly 220 includes substrate 222, seal-landing ring 224
(320), anchor contact 226 (318), pedestal 228, movable structure
230 (326), isolation pad 232, and contact pad 234 (322). The
substrate 222 is preferably formed of single-crystal silicon and
provides a common support structure for the microstructures in the
base assembly 220. Other materials can be used for the substrate
222, including those materials described with respect to substrate
202. The pedestal 228 overlies the substrate 222 and is preferably
made of silicon dioxide but other materials may be used, such as
germanium or graphite. The pedestal 228 provides support for the
movable structure 230 and a sidewall 236, which together with the
movable structure 230 and the substrate 222 defines a gap 238. The
gap 238 is typically very small (e.g., on the order of about 2 um
in the vertical dimension). It provides the movable structure 230
with at least one degree of freedom within a sealed cavity 404, as
shown in FIG. 4b.
[0061] Although the embodiment shown in FIG. 2b employs a single
cantilever as the movable structure 230, one or more movable
structures 230 can be employed, together with one or more signal
paths, to create variety of MEMS switches for switching one or more
signals, including but not limited to single pole single throw,
single pole double throw, single pole multiple throw, and multiple
pole multiple throw configurations. Such configurations may require
one or more additional substrates and movable structures. Moreover,
the movable structure 230 need not be supported at one end by a
pedestal, but can also be supported at several locations by one or
more pedestals or no pedestals (e.g., fixed directly to substrate
222). For example, the cantilever beam 230 shown in FIG. 2a can be
supported in its center by a single pedestal, which acts as a pivot
for the cantilever beam to move in a "see-saw" fashion, each free
end of the cantilever beam bearing one or more contact pads 234 for
bridging one or more gaps 308 in one or more signal paths 208
located on substrate 202 and/or and substrate 222.
[0062] The movable structure 230 (326) is preferably formed of
polysilicon and doped with a dopant so that is it conductive (e.g.,
phosphorus), but can also be formed of monocrystalline silicon. The
anchor contact 226 (318) preferably is deposited at the anchored
end of the movable structure 230 (326) and provides an electrical
ground for the actuation voltage. The free end (distal end) of the
movable structure 230 (326) bears isolation pad 232, which is
preferably composed of a silicon nitride layer, for isolating the
actuation voltage from the signals. The contact pad 234 (322)
overlies the isolation pad 232 and is used to bridge the gap 308
between the input line 300 and the output line 302. The location of
the isolation pad 232 (and therefore the contact pad 234) on the
movable structure 230 is selected to ensure that the contact pad
234 (322) bridges the gap 308 when the actuator 210 (310) is
activated. Preferably, the shape and width of the contact pad 234
(322) is selected to achieve this purpose.
[0063] In this particular embodiment, the movable structure 230
(326) lies under the contact pad 234 (322), and therefore is
deposited on the substrate 222 before the contact pad 234 (322) in
the fabrication process flow, as illustrated in FIGS. 8A and 8B.
This preferred configuration allows the polysilicon to be annealed
at high temperatures (above 400.degree. C.) for the movable
structure 230 (326), without causing the contact pad 234 (322) to
melt, delaminate, or interdiffuse with other material layers.
[0064] The seal-landing ring 224 (320) surrounds the movable
structure 230 (326) and is preferably about 1.5 microns thick in
its vertical dimension and has a wall thickness in the range of
about 5-20 microns. The seal landing ring 224 (320), together with
the seal ring 206 (316) bonds the lid assembly 200 to the base
assembly 220, forming a hermetic seal around the movable structure
230 (326). The movable structure 230 (326) includes etch access
holes 324, which facilitate removal of material underlying the
movable structure 230 (326), as described with respect to FIGS. 8A
and 8B.
[0065] Assembled Micro-Switch
[0066] Referring to FIG. 4a, there is shown cross-sectional views
of one embodiment of an assembled micro-switch 400 fabricated in
accordance with the present invention. Preferably, a plurality of
lid assemblies 200 are processed on a first wafer and a plurality
of base assemblies 220 are processed on a second, separate wafer.
This preferred configuration enables the conductive traces and
other metal elements to be processed on the first wafer using the
appropriate annealing temperatures for such metals (e.g., below
300.degree. C.) and the MEMS structures (e.g., movable structure
230) to be processed on the second wafer using the appropriate
annealing temperatures for structures (e.g., above 400.degree. C.
for polysilicon).
[0067] Although the micro-switch 400 is shown with a cantilever
beam as the movable structure 230, the present invention is not
limited to this preferred configuration. For example, the movable
structure 230 can be any type of structure (e.g., T-shaped
structure, membrane structure). Moreover, the micro-switch 400 can
be configured to operate in an electrically closed state when in
its deactivated state (i.e., normally closed) or in an electrically
opened state when in its deactivated state (i.e., normally open),
as shown in FIGS. 4a and 4b. Alternatively, the movable structure
230 can include a signal path for transmitting signals from the lid
assembly 200 to the base assembly 220 or vice-versa depending on
the configuration of the micro-switch 400. The location of the
actuator 310 on the substrate 202 can vary between the contact pad
234 (322) and the anchor contact 226 (318).
[0068] Operation of Micro-Switch
[0069] Referring to FIG. 4b, there is shown the assembled
micro-switch 400 in its "activated" state. In operation, an RF
signal is launched into the input line 300 of the micro-switch 400.
Because of the gap 308, the signal cannot reach the output line
302. If an actuation voltage is applied to the actuator 210 (310),
a force 402 is established, which pulls the movable structure 230
upward towards the substrate 202 (i.e., the movable structure 230
flexes upwards), until the contact pad 234 (322) bridges the gap
308 between the input line 300 and the output line 302, thereby
allowing the signal to propagate through the micro-switch 400.
[0070] Applications for the assembled micro-switch 400 include but
are not limited to, mobile phones, portable one-way and two-way
radios, satellite phones, base stations, mobile Internet access
devices, fixed location Internet access devices, radar collision
sensing, Bluetooth compliant devices, wireless networks, or any
combination of these applications.
[0071] Alternate Layouts
[0072] Referring to FIG. 5 (with reference to FIGS. 1a and 4b),
there is shown a cross-sectional view of the micro-switch 400,
including dielectric spacers 500 located between the isolation pad
232 and the anchor contact 226. In the past, a contiguous
dielectric layer (e.g., silicon nitride) was often used as an
isolation layer between the cantilever 108 and the actuator 210.
The two-substrate process flow described with respect to FIGS. 7
and 8, enables such dielectric layer (e.g., the same dielectric
layer used to pattern the isolation pad 232) to be patterned and
etched into one or more discrete islands, hereinafter referred to
as dielectric spacers 500. This is advantageous because it reduces
the total area of dielectric material. Dielectric materials tend to
accumulate charge over time and through use. This accumulated
charge can cause a shift in the required actuation voltage or
sticking of the movable structure 230. Reducing the area of
dielectric material reduces these negative effects due to the
accumulated charge.
[0073] Referring to FIGS. 6a and 6b, there are shown
cross-sectional views of the micro-switch 600. The micro-switch 600
is similar to the micro-switch 400, but includes upper mechanical
stops 317 and lower mechanical stops 328. FIG. 6b shows the
micro-switch 600 after the final bonding step, as described below
with respect to FIGS. 7 and 8. The upper mechanical stops 317 are
part of the lid assembly 200 and the lower mechanical stops 328 are
part of the base assembly 220, as show in FIGS. 3a and 3b,
respectively. The mechanical stops 317, 328, are used to provide a
consistent gap between the movable structure 230 and the substrate
202. These mechanical stops 317, 328, are preferably comprised of
the same layers used to fabricate the micro-switch 400, as shown in
FIG. 6a.
[0074] Integration With Other Devices
[0075] One or more micro-switches can be integrated with one or
more passive and active devices, including but not limited to
capacitors, inductors, filters, low-noise amplifiers, power
amplifiers and phase shifters. For example, one or more
micro-switches can be fabricated as described above with at least
one passive component (e.g., one or more capacitors, inductors
and/or phase shifters) or at least one active component (e.g., one
or more power amplifiers), fabricated on at least one of the two
substrates 202, 222, with at least one electrical interconnection
(e.g., via conductive layers, wire bonding, tab bonding) connecting
at least one of the fabricated components (e.g., anchor contact
226) with one or more passive components. Alternatively, the
passive components do not need to be fabricated on either of the
two substrates 202, 222, but can be electrically interconnected to
one or more micro-switches (e.g., via conductive layers).
[0076] Process Flow for Lid Assembly
[0077] Referring to FIG. 7 (with reference to FIGS. 8a-1 showing
corresponding cross-sectional views of the process flow), there is
shown a flow diagram of one embodiment of a process flow for
fabricating a lid assembly (e.g., lid assembly 200). The process
starts 700 with a glass, quartz, or high-resistivity silicon
substrate (e.g., substrate 202). The substrate is coated 702 with
an insulating film (e.g., insulating film 204) comprising about 1
micron of silicon dioxide (oxide 1.5). Deposited 704 on the
insulating film is about 1000 A of sputtered TiW, about 1-2 um of
gold and about 1000 A of TiW. The TiW acts as a diffusion barrier,
and also provides adhesion between the gold and the adjoining
layers above and below. Hereinafter, this tri-layer is also
referred to as metal2.
[0078] Metal2 is wet etched 706 to define conductive traces (e.g.,
input line 300, output line 302) and wire bond pads. About 2
microns of silicon dioxide (ox2) is deposited 708 over the
conductive traces. Ox2 is patterned 710 to form a insulating ring
(e.g., insulating ring 214). Step 710 can be performed using a wet
etchant, such as 5:1 BOE, or a dry etch, such as CH4/CHF3/He. The
insulating ring can have any desired width, but is preferably in
the range of about 20 to about 50 microns.
[0079] Next, the wafer is coated 712 with a seed layer for
electroplating. The seed layer preferably consists of about 1000A
of TiW, followed by about 1000A of Au, preferably sputter
deposited. A stencil mask is formed 714, preferably using a
planarizing resist such as SJR5740. The stencil mask is formed 714
with an opening in the shape of a ring, nominally overlying the
insulating ring. An opening in the shape of a small square, roughly
10.times.10 um in size, is also provided, nominally overlying the
anchor contact 226. The resist is exposed and developed 716 to
prepare the surface for plating. A deformable metal is plated 718
to a thickness of about 8 microns. This is preferably a pure, soft,
bright gold. Hereinafter, this layer is also referred to as metal3.
Finally, the resist is stripped 720 and the exposed seed layer is
stripped 722 to finish the process flow.
[0080] Process Flow for Base Assembly
[0081] Referring to FIG. 9 (with reference to corresponding FIGS.
10a-i showing cross-sectional views of the process flow), there is
shown a flow diagram of one embodiment of a process flow for
fabricating a base assembly (e.g., base assembly 220). The process
starts 900 with a silicon substrate (e.g., substrate 222). The
substrate is coated 902 with about 2-3 microns of silicon dioxide
(ox1). The silicon dioxide is then doped 904 with about 2 microns
of polycrystalline silicon (poly1), either in-situ, or from
symmetrical (upper and lower) layers of dopant source material,
such as thin layers of doped polysilicon. The poly1 layer, together
with the ox1 layer, provide an intermediate structure from which a
cantilever (e.g., movable structure 230) will be formed in later
process flow steps. The polysilicon is annealed for about 60
minutes at about 1000.degree. C., in nitrogen, to distribute the
dopant and reduce stress. About a 60 second RTA is performed in
nitrogen to further reduce stress. About 0.5 microns of silicon
nitride (nitride1; preferably low-stress) is deposited 906 over the
polysilicon. The nitride2 layer is patterned 908 to form an
insulating pad (e.g., insulating pad 232) at what will become the
free end (distal end) of a cantilever. Additionally, one or more
small pads (e.g., dielectric spacers 500) of nitride2 can be
distributed over the area of the movable structure, to act as
insulating spacers and mechanical stops (e.g., mechanical stops
328) between the cantilever and the actuator, as shown in FIG. 5.
The poly layer is patterned 910 to define the cantilever.
Preferably, one or more etch access holes (e.g., etch access holes
324) are provided in the cantilever, except in the area that is to
be supported by an anchor pedestal (e.g., pedestal 228). The wafer
is preferably sputter coated 912 with about 1000 A of TiW, followed
by about 1 um of Au. Hereinafter, this layer is also referred to as
metal1.
[0082] The metal1 layer is etched 912, preferably using a wet
etchant such as KI, to define a contact pad (e.g., contact pad
234), the cantilever's ground contact (e.g., anchor contact 226),
and a metal seal ring (e.g., seal ring 206). Preferably, a timed
wet etch of ox1 is performed in HF. The etch access holes allow the
HF to undercut the main body of the cantilever. The wafer is
removed from the etch and rinsed in DI water, before the pedestal
area is etched 916 away. This results in the finished
cantilever.
[0083] Bonding Procedure
[0084] One embodiment of a bonding procedure is described below in
accordance with the present invention. Other bonding processes,
however, may be employed with the present invention, including but
not limited to, use of different temperatures, different pressures,
different ambients, ultrasonic agitations, megasonic agitation,
surface cleaning or any combination of the above techniques.
[0085] Referring to FIGS. 11a-c, there is shown a wafer-scale
bonding technique in accordance with the present invention. The
first wafer (the wafer bearing the lid assemblies) and the second
wafer (bearing the base assemblies) are aligned 1100 and tack
bonded 1102, using, for example, a Karl Suss MA/BA-6 bond aligner
and SB-6 wafer bonder. Preferably, a gold thermocompression recipe
is used for tack bonding, e.g. about 300.degree. C. in inert gas or
vacuum, approximately a 1 minute bond time, at full pressure (about
2-3 bar). To achieve a final bond 1104, the wafers are preferably
transferred to a hydraulic press apparatus, where approximately
about 100-200 bar is applied, at a temperature of about
270.degree.-300.degree. C., for about 10-20 minutes. This is a
relatively long "soak," which enables the gold to plastically
deform and create a hermetic seal where the two rings bond (e.g.,
seal ring 206 and seal-landing ring 224). The long soak time also
enables a suitably large degree of plastic deformation without
excessive applied pressure. If a shorter soak time is used, higher
pressures are required, which leads to cracking of the wafers. The
wafers are preferably constrained to prevent lateral shearing or
shifting of one wafer relative to the other, while this higher
level of pressure is applied. This may be achieved by placing the
wafer between two metal plates, of diameter larger than the
wafers'. The two plates are bolted or otherwise secured together at
a number of points around their perimeters. Spacers are used to
create a gap between the plates, equal to the combined thickness of
the two wafers, plus about 25-100 um. The thickness and diameter of
the plates is chosen so that they can flex to allow compression of
the wafers, coming into contact against the wafers with relatively
little applied pressure (e.g. about 10% of the full applied
pressure of about 200 bar). In this way, the two metal plates act
to prevent lateral motion of the wafers during the compression
step. The soak time is preferably adjusted to achieve a compression
of about 50% in the electroplated gold layer. This leads to a gap
of about 2 um between the contact pad (e.g., contact pad 234) and
the signal path (e.g., signal path 208).
[0086] An advantage of the techniques described above is that each
lid/base assembly pair is individually bonded together to form a
sealed cavity 404 adapted to house the movable structure 230 (FIG.
4a) so that it can move freely under the influence of the actuator
210. The sealed cavity 404 can significantly increase the lifetime
of the micro-switch 400 by sealing off the microstructures from
harmful contaminants in the environment.
[0087] Although the bonding procedure described above employs
wafer-scale bonding techniques, the present invention is equally
applicable to die-scale bonding techniques, including the bonding
of any subset of devices on a wafer or other substrate (e.g.,
n.times.m array of devices).
[0088] Dicing Procedure
[0089] Once the micro-switches 400 are assembled, a dicing saw is
used to expose the bond pads patterned on the first wafer (the
wafer bearing the lid assemblies). This is achieved by making cuts
in the second wafer (the wafer bearing the base assemblies). The
micro-switches 400 are then singulated by cutting with the dicing
saw completely through the bonded wafers. The micro-switches 400
are protected during dicing by their respective hermetic seals
during the wafer-scale bonding step. In the past, MEMS devices were
generally diced before packaging. Particulates and water present
during the dicing process would damage devices, drastically
lowering yield.
[0090] Second-level Packaging
[0091] The singulated devices are packaged in a conventional manner
using standard IC chip packaging techniques. Preferably, this is
done using a surface-mount plastic package, or a wafer-scale
package, such as used for stacked-chip devices such as DRAMs.
[0092] The above description is included to illustrate the
operation of the preferred embodiments and is not meant to limit
the scope of the invention. The scope of the invention is to be
limited only by the following claims. From the above discussion,
many variations will be apparent to one skilled in the relevant art
that would yet be encompassed by the spirit and scope of the
invention. For example, to ensure that at least about a 2 um gap
between the contact pad and signal path is achieved in a
repeatable, uniform manner, a two-layer structure may be
substituted for the electroplated gold ring. In this case, the
first layer would be formed of nickel or other relatively hard
material. Nickel may be conveniently electroplated in this case.
Subsequently, gold-tin alloy or other solder material would be
electroplated on top of the nickel. The nickel layer here serves as
a well-defined hard stop, determining the final thickness of the
plated layer. In the bonding step, the gold-tin would melt and
reflow, sealing the ring structure and forming electrical contacts,
but taking on a negligible thickness. As an alternative to
gold-tin, pure gold could be used. With adequate pressure and soak
time, it may be expected that the gold layer would be compressed to
negligible thickness. An additional alternative embodiment might
employ polysilicon or other ceramic material as the mechanical
stop. This material could be patterned to lie outside the active
area of the device, but could conceivably occupy a larger surface
area than the nickel in the embodiment above. The larger surface
area would lead to reduced stress, and more reliable establishment
of the desired vertical gap during the bond process.
* * * * *