U.S. patent application number 10/620968 was filed with the patent office on 2005-01-20 for fluidic mems device.
Invention is credited to Boucher, William R., Haluzak, Charles C., Smith, Mark A..
Application Number | 20050012197 10/620968 |
Document ID | / |
Family ID | 33477105 |
Filed Date | 2005-01-20 |
United States Patent
Application |
20050012197 |
Kind Code |
A1 |
Smith, Mark A. ; et
al. |
January 20, 2005 |
Fluidic MEMS device
Abstract
A MEMS package comprises a substrate and a cover plate. A MEMS
structure is fabricated on a surface of the substrate. The cover
plate may be bonded to the substrate by a bond ring. The cover
plate, the bond ring and the substrate may define an inner cavity.
The cover plate, the substrate and a breach in the bond ring may
define a fill port.
Inventors: |
Smith, Mark A.; (Corvallis,
OR) ; Boucher, William R.; (Corvallis, OR) ;
Haluzak, Charles C.; (Corvallis, OR) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD
INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Family ID: |
33477105 |
Appl. No.: |
10/620968 |
Filed: |
July 15, 2003 |
Current U.S.
Class: |
257/687 |
Current CPC
Class: |
B81B 7/0061
20130101 |
Class at
Publication: |
257/687 |
International
Class: |
H01L 023/24 |
Claims
What is claimed is:
1. A MEMS package, comprising: a substrate with a MEMS structure
fabricated on a surface of the substrate; a cover plate bonded to
the surface of the substrate by a bond ring; an inner cavity
defined by the substrate, the cover plate and the bond ring; and a
fill port defined by the substrate, the cover plate and a breach in
the bond ring.
2. The MEMS package of claim 1, further comprising: fluid sealed
within the inner cavity.
3. The MEMS package of claim 2, further comprising: a seal disposed
in the fill port.
4. The MEMS package of claim 1, wherein the bond ring comprises at
least one of a glass frit, adhesive, eutectic solder, solder mask
material, anodic bond, covalent bond, laser weld or Sol-gel
material.
5. The MEMS package of claim 3, wherein the seal comprises at least
one of an adhesive, organic adhesive, epoxy, solder or glass-based
sealant.
6. The MEMS package of claim 3, wherein the seal comprises a
curable adhesive.
7. The MEMS package of claim 1, further comprising: bond pads for
making electrical connections to the MEMS package arranged in an
exposed portion of the substrate.
8. A MEMS package adapted for use in a range of operating
temperatures comprising: a substrate with MEMS circuitry fabricated
on a surface of the substrate; a cover plate bonded to the surface
of the substrate by a bond ring; a fill port defined by the
substrate, the cover plate and a breach in the bond ring; an inner
cavity defined by the substrate, the cover plate and the bond ring;
and fluid sealed within the inner cavity, the fluid having a
coefficient of thermal expansion, wherein the inner cavity has a
volume which is small enough so that expansion of the fluid
throughout the range of operating temperatures is accommodated by
deflections of at least the cover plate, substrate and bond
ring.
9. The MEMS package of claim 8, further comprising: a seal disposed
in the fill port.
10. The MEMS package of claim 8, wherein the bond ring comprises
one of a glass frit, adhesive, eutectic solder, solder mask
material, anodic bond, covalent bond, laser weld or Sol-gel
material.
11. The MEMS package of claim 9, wherein the seal comprises at
least one of an adhesive, organic adhesive, epoxy, solder or
glass-based sealant.
12. The MEMS package of claim 9, wherein the seal comprises a
curable adhesive.
13. The MEMS package of claim 8, further comprising: bond pads
arranged in an exposed portion of the surface of the substrate.
14. A MEMS assembly comprising: a substrate with a plurality of
MEMS structures fabricated at a plurality of respective die
locations on a surface of the substrate; a cover plate bonded to
the surface of the substrate by a plurality of bond rings; a
plurality of inner cavities associated with respective die
locations, each being defined by the substrate, the cover plate and
one of the plurality of bond rings; and a plurality of fill ports,
each being defined by the substrate, the cover plate and a breach
in the one of the plurality of bond rings.
15. The MEMS assembly of claim 14, wherein the cover plate
comprises a plurality of openings defining a plurality of exposed
portions on the substrate.
16. The MEMS assembly of claim 15, wherein: a first group of
openings define a first group of exposed portions, each of the
first group of exposed portions being adjacent a fill port; and a
second group of openings define a second group of exposed portions
on the substrate.
17. The MEMS assembly of claim 16, wherein the second group of
openings comprise slots.
18. The MEMS assembly of claim 16, further comprising: a plurality
of bond pads on the surface of the substrate arranged in the second
group of exposed portions.
19. The MEMS assembly of claim 14, wherein the bond ring comprises
one of a glass frit, adhesive, eutectic solder, solder mask
material, anodic bond, covalent bond, laser weld or Sol-gel
material.
20. A MEMS assembly comprising: a substrate with a plurality of
MEMS structures fabricated at a plurality of respective die
locations on a surface of the substrate; a plurality of cover
plates and a plurality of bond rings, each plate being bonded to
the substrate by at least one of the bond rings; a plurality of
inner cavities associated with respective die locations, each being
defined by the substrate, a respective cover plate and a respective
bond ring; and a plurality of fill ports, each being defined by the
substrate, the respective cover plate and a breach in the
respective bond ring.
21. The MEMS assembly of claim 20 further comprising: bond pads
arranged in exposed portions on the substrate.
22. The MEMS assembly of claim 14, wherein the bond rings comprise
one of a glass frit, adhesive, eutectic solder, solder mask
material, anodic bond, covalent bond, laser weld or Sol-gel
material.
23. A MEMS package comprising: a substrate with a MEMS structure
fabricated on a surface of the substrate; a cover plate bonded to
the surface of the substrate by a bond ring; an inner cavity
defined by the substrate, the cover plate and the bond ring; and a
fill port defined by the substrate, the cover plate and a breach in
the bond ring, wherein the MEMS assembly was singulated from an
assembly comprising a plurality of inner cavities.
24. The MEMS package of claim 23, further comprising: fluid sealed
within the inner cavity.
25. The MEMS package of claim 24, wherein the inner cavity was
filled with the fluid prior to singulating the MEMS assembly from
the assembly comprising a plurality of inner cavities.
26. A method of manufacturing a fluidic MEMS package comprising:
providing a MEMS assembly with a plurality of inner cavities at a
plurality of respective die locations on a substrate, each inner
cavity being defined by a cover plate, the substrate and a bond
ring and being associated with a fill port defined by the cover
plate, the substrate and a breach in the bond ring; filling the
inner cavities with fluid; sealing the fluid in the inner cavities;
singulating a plurality of MEMS packages from the substrate.
27. The method of claim 26, wherein: filling the inner cavities
with fluid comprises providing a vacuum in the inner cavity,
providing an amount of fluid sufficient to fill the inner cavity at
the fill port and providing pressure on the fluid to cause the
fluid to fill the cavity.
28. The method of claim 26, wherein filling the inner cavities with
fluid is performed prior to singulating a plurality of MEMS
packages from the substrate.
29. The method of claim 26, wherein the bond ring comprises one of
a glass frit, adhesive, eutectic solder, solder mask material,
anodic bond, covalent bond, laser weld or Sol-gel material.
30. A method of manufacturing a fluidic MEMS package comprising:
attaching a cover plate with a plurality of openings to a substrate
with a plurality of bond rings with breaches such that the cover
plate, the substrate and the bond rings define a plurality of
respective inner cavities and the cover plate, the substrate and
the breaches define a plurality of respective fill ports; filling
the inner cavities with fluid; sealing the fluid in the inner
cavities; singulating a plurality of MEMS packages from the
substrate.
31. The method of claim 30, wherein: filling the inner cavities
with fluid comprises providing a vacuum in the inner cavity,
providing an amount of fluid sufficient to fill the inner cavity at
the fill port and providing pressure on the fluid to cause the
fluid to fill the cavity.
32. The method of claim 30, wherein filling the inner cavities with
fluid is performed prior to singulating a plurality of MEMS
packages from the substrate.
33. The method of claim 30, wherein the bond ring comprises one of
a glass frit, adhesive, eutectic solder, solder mask material,
anodic bond, covalent bond, laser weld or Sol-gel material.
34. A method of manufacturing a fluidic MEMS package comprising:
attaching a plurality of cover plates to a plurality of die
locations on a surface of a substrate, the attaching being done
with a plurality of respective bond rings, the bond rings having
breaches, the respective cover plates, bond rings and the substrate
defining a plurality of respective inner cavities and the
respective cover plates, breaches and the substrate defining a
plurality of respective fill ports; filling the inner cavities with
fluid; sealing the fluid in the inner cavities; singulating a
plurality of MEMS packages from the substrate.
35. The method of claim 34, wherein: filling the inner cavities
with fluid comprises providing a vacuum in the inner cavity,
providing an amount of fluid sufficient to fill the inner cavity at
the fill port and providing pressure on the fluid to cause the
fluid to fill the cavity.
36. The method of claim 34, wherein filling the inner cavities with
fluid is performed prior to singulating a plurality of MEMS
packages from the substrate.
37. The method of claim 34, wherein the bond ring comprises one of
a glass frit, adhesive, eutectic solder, solder mask material,
anodic bond, covalent bond, laser weld or Sol-gel material.
38. A method of filling a fluidic MEMS package comprising:
providing the MEMS package with an inner cavity and a fill port;
providing a vacuum in the inner cavity; at an entrance to the fill
port, providing an amount of fluid at least sufficient to fill the
inner cavity; and providing pressure on the fluid causing the fluid
to fill the inner cavity.
39. The method of claim 38, further comprising: purging air from
the inner cavity using a purge gas.
40. The method of claim 39, wherein the purge gas has high
solubility.
41. The method of claim 39, wherein the purge gas is one of carbon
dioxide or helium.
42. The method of claim 38, further comprising: providing a vacuum
environment around the MEMS package.
43. The method of claim 42, wherein providing a vacuum environment
comprises placing the MEMS package in a chamber and evacuating air
from the chamber.
44. The method of claim 42, wherein providing a vacuum environment
comprises assembling the MEMS package in a vacuum environment.
45. The method of claim 38, wherein providing an amount of fluid
comprises submerging the MEMS package in fluid.
46. The method of claim 38, wherein providing an amount of fluid
comprises touching a fluid carrier with fluid to the fill port.
47. A method of filling a fluidic MEMS package comprising:
providing the MEMS package with an inner cavity defined by a cover
plate, a substrate and a bond ring; providing the MEMS package with
a fill port and an evacuate port, each being defined by the cover
plate, the substrate and a respective one of a plurality of
breaches in the bond ring; at an entrance to the fill port,
providing an amount of fluid at least sufficient to fill the inner
cavity; and selecting the fluid, substrate, bond ring and cover
plate such that capillary forces draw the fluid into the inner
cavity causing air within the inner cavity to evacuate through the
evacuate port.
48. A spatial light modulator, comprising: a substrate with a MEMS
mirror array fabricated on a surface of the substrate; a cover
plate bonded to the surface of the substrate by a bond ring; an
inner cavity defined by the substrate, the cover plate and the bond
ring; and a fill port defined by the substrate, the cover plate and
a breach in the bond ring.
49. The spatial light modulator of claim 48, further comprising:
fluid sealed within the inner cavity.
50. The spatial light modulator of claim 49, further comprising: a
seal disposed in the fill port.
51. The spatial light modulator of claim 48, wherein the bond ring
comprises at least one of a glass frit, adhesive, eutectic solder,
solder mask material, anodic bond, covalent bond, laser weld or
Sol-gel material.
52. The spatial light modulator of claim 50, wherein the seal
comprises at least one of an adhesive, organic adhesive, epoxy,
solder or glass-based sealant.
53. A method of assembling a spatial light modulator comprising:
providing the spatial light modulator with a MEMS mirror array
fabricated on a substrate and in a package comprising an inner
cavity and a fill port; providing a vacuum in the inner cavity; at
an entrance to the fill port, providing an amount of fluid at least
sufficient to fill the inner cavity; and providing pressure on the
fluid causing the fluid to fill the inner cavity.
Description
BACKGROUND OF THE DISCLOSURE
[0001] Certain fluidic micro-electro-mechanical systems (MEMS)
applications include fluid in a hermetically sealed inner cavity of
a MEMS package. Such hermetic MEMS packages may comprise rigid
and/or brittle materials. The volumetric expansion rate of fluids
hermetically sealed in MEMS packaging, upon increases in
temperature, may be as much as 20 to 100 times greater, for
example, than the expansion rate of the inner cavity of the package
based on the linear expansion rate of the packaging materials. The
fluid may also be incompressible or have a very low degree of
compressibility. As a result, thermal excursions may result in an
increase of fluid pressure in the inner cavity which may lead to
fluid leakage and/or fracture of the packaging materials.
SUMMARY OF THE DISCLOSURE
[0002] An exemplary embodiment of a MEMS package comprises a
substrate and a cover plate. A MEMS structure is fabricated on the
substrate. The cover plate may be bonded to the substrate by a bond
ring. The cover plate, the bond ring and the substrate may define
an inner cavity. The cover plate, the substrate and a breach in the
bond ring may define a fill port.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] These and other features and advantages of the invention
will readily be appreciated by persons skilled in the art from the
following detailed description of exemplary embodiments thereof, as
illustrated in the accompanying drawings, in which:
[0004] FIG. 1 illustrates an exemplary embodiment of a MEMS
assembly capable of being assembled using a Awafer-scale@
method.
[0005] FIG. 2 illustrates an exemplary embodiment of a MEMS
assembly capable of being assembled using a Apick-and-place@
method.
[0006] FIG. 3 illustrates a cross-sectional view of an exemplary
embodiment of a fluidic MEMS assembly.
[0007] FIG. 4 illustrates a plan view of an exemplary embodiment of
a MEMS assembly.
[0008] FIG. 5 illustrates a plan view of a wafer-level MEMS
assembly used in an exemplary pick-and-place method of
manufacturing exemplary fluidic MEMS assemblies.
[0009] FIG. 6A, 6B and 6C illustrate a cover plate, wafer and
wafer-level MEMS assembly, respectively, each used in an exemplary,
wafer-scale method of manufacturing exemplary embodiments of
fluidic MEMS assemblies.
[0010] FIGS. 7A, 7B and 7C illustrate cross-sectional views of an
exemplary embodiment of a fluidic MEMS assembly at various stages
of fluid fill in an exemplary method of manufacturing a fluidic
MEMS assembly.
[0011] FIGS. 8A, 8B and 8C illustrate a front view of the exemplary
embodiment of a fluidic MEMS assembly shown in FIGS. 7A, 7B and
7C.
[0012] FIGS. 9A, 9B and 9C illustrate alternate exemplary
embodiments of methods of filling fluidic MEMS devices.
[0013] FIG. 10 illustrates an exemplary embodiment of a fluidic
MEMS assembly with a fill port and an evacuate port.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0014] In the following detailed description and in the several
figures of the drawing, like elements are identified with like
reference numerals.
[0015] FIGS. 1 through 4 illustrate exemplary embodiments of MEMS
package assemblies suitable for use as fluidic MEMS devices. A MEMS
assembly or package 1 includes a cover plate 2, a substrate 3 and a
bond ring 4. A primary face 21 (FIG. 3) of the cover plate 2 is
attached to a primary surface 31 of the substrate by a bond ring 4.
The cover plate 2 may be an optical window or aperture and may
comprise silicon, glass, plastic, metal or metal alloys, such as
Kovar (TM), or other suitable material. The substrate 3 may be a
silicon substrate and may have a MEMS structure 32 fabricated on
the primary surface 31. The bond ring 4 may be an inorganic bond
ring. The cover plate may be smaller than the substrate and may
define exposed portions 33 on the substrate which are not covered
by the cover plate. Electrical bonding pads 34 for making
electrical connections to the MEMS may be arranged in the exposed
portions 33 on the substrate.
[0016] The exposed portions 33 may be defined in an opening 22, for
example a slot or hole in the cover plate, or may be defined in a
partial slot or hole which, for example, may remain in a cover
plate after singulation from a wafer-scale assembly (FIGS. 1 and 4)
as described further below. The bonding pads 34 may be electrically
connected, for example, to a printed circuit board (PCB) 5 (FIG.
3). The exposed portions may alternatively be defined by the cover
plate being smaller than the substrate such that exposed portions
of the substrate that extend beyond the edges of the cover plate
(FIGS. 2 and 3).
[0017] In an exemplary embodiment illustrated in FIG. 3, the
primary surface 31 of the substrate 3, the primary face 21 of the
cover plate 2 and the bond ring 4 define an inner cavity 11. The
height 41 of the inner cavity 11 may be about 3 to 10 microns. A
breach 42 between two ends 43, 44 (FIG. 2) in the bond ring 4
defines a fill port 111 and fill path 112 into the inner cavity 11.
The bond ring may comprise, for example, a glass frit, adhesives,
eutectic solders, solder mask materials, anodic bond, covalent
bond, laser weld, Sol-gel materials or other material suitable for
bonding between the substrate and the cover plate. A eutectic
solder may, for example, comprise an AuSn eutectic solder.
[0018] Exemplary fluidic MEMS devices may be assembled using
various techniques. In one exemplary process, a cover plate is
attached by a bond ring to a substrate to define an inner cavity.
The size of the substrate and cover plate may be chosen to permit
access to bond pads in exposed areas of the substrate. For example,
the cover plate may be smaller than the substrate defining exposed
portions of the primary surface of the substrate after the cover
plate is attached. The exemplary embodiment illustrated in FIG. 2
may be assembled using this method. This method is an exemplary
embodiment of a Apick-and-place@ method in which individual cover
plates are picked up and attached individually to a substrate.
[0019] In an alternate pick-and-place embodiment illustrated in
FIG. 5, a plurality of cover plates 2 may attached to a substrate
300 with a plurality of MEMS structures 32 at a plurality of die
locations 35 on the substrate to form a MEMS assembly 100. The
substrate 300 may be a silicon wafer with a plurality of MEMS
structures fabricated on a surface of the substrate. Each MEMS
structure may be located at a die location. Each die location
adapted to form individual MEMS dies when singulated from the
wafer. In FIG. 5, the die locations 35 are indicated by the
rectangles defined by the grid-lines on the wafer.
[0020] Each cover plate 2 is attached to the wafer 300 by a bond
ring 4 at a die location 35. The wafer 300, the cover plates 2 and
the bond rings 4 define a plurality of inner cavities. Bond pads 34
are provided for making electrical connections to the MEMS device
structures. Individual MEMS devices or dies may be singulated from
the wafer after the cover plates are attached. Attaching the cover
plates at the wafer level may provide some protection to the MEMS
structures on the substrate during any subsequent manufacturing,
assembly or handling. The individual MEMS devices could be filled
with fluid at the wafer level, prior to singulation, as discussed
below. The exemplary embodiments illustrated in FIGS. 2 or 3 may be
assembled using this method.
[0021] In an alternate Awafer-scale@ embodiment illustrated in
FIGS. 6A-6C, a cover plate 200 with a plurality of openings 22
(FIG. 6A), which may for example comprise slots and/or holes, may
be attached to a substrate or wafer 300 with a plurality of MEMS
structures 32 and bond pads 34 (FIG. 6B) to form a MEMS assembly
100 (FIG. 6C). The cover plate 200 may be attached to the wafer 300
by a plurality of bond rings 4. The cover plate, the wafer and the
bond rings may define a plurality of inner cavities 11. The
plurality of inner cavities may correspond to a plurality of die
locations 35. A plurality of individual MEMS devices or dies may be
singulated from the wafer level assembly 100. The exemplary
embodiments illustrated in FIGS. 1 or 4, for example, may be
assembled using this method. Although the substrate in the
embodiment illustrated in FIGS. 6A-6C is described as a wafer, it
is understood that the method may be used with any substrate with a
plurality of MEMS structures fabricated at a plurality of die
locations 35 on a surface of the substrate. It is also understood
that the plurality of bond rings 4 may comprise fewer contiguous
segments of bond ring material than the number of inner cavities
defined. A single, contiguous amount of bond ring material, for
example, could define a plurality of bond rings.
[0022] The openings 22 may provide access to fill ports 111 for
filling and sealing, as discussed below, and/or access to bond pads
34 for making electrical connections. When the cover plate is
attached to the wafer, the access openings define exposed portions
on the primary surface of the substrate or wafer. In the exemplary
embodiment of FIGS. 6A-6C, a group of openings 22a in the form of
holes define a first group of exposed portions 33a adjacent to fill
ports 111. A second group of openings 22b in the form of slots
define a second group of exposed portions 33b. Bond pads 34 are
arranged on the exposed portions 33b.
[0023] The exposed portions 33a at the fill ports may provide
access to fill the inner cavity through the fill port, may provide
a platform adapted to receive fluid to be provided for use in
filling the inner cavity and may provide a platform for placing a
seal at the fill port after filling the inner cavity. Exposed
portions 33 adjacent fill ports in other exemplary embodiments may
also provide a platform for providing fluid for use in filling the
inner cavity and may provide a platform for placing a seal at the
fill port after filling the inner cavity. The exposed portions 33b
on which the bond pads 34 are arranged provide access to the bond
pads to make electrical connections to the individual MEMS devices
or dies after singulation from the assembly 100. Exposed portions
33 in other exemplary embodiments may also provide access to bond
pads to make electrical connections to MEMS devices.
[0024] In the exemplary embodiment illustrated in FIGS. 6A-6C, the
openings 22a define exposed portions 33a at the fill ports only and
each one defines an exposed portion at one fill port. The openings
22b define exposed portions 34b only where bond pads are arranged.
It is understood that the openings could be arranged to define
exposed portions for more than one fill port and that openings
could be arranged to define exposed portions that are adjacent a
fill port and on which bond pads are arranged. Where certain
openings define such dual purpose exposed portions, such dual
purpose exposed portions could be members of the group of openings
which define exposed portions adjacent fluid ports and members of
the group of openings defining exposed portions on which bond pads
are arranged.
[0025] In an exemplary embodiment of a MEMS device 1, it may be
desirable to fill the inner cavity 11 with fluid 6. Such fluidic
MEMS device applications include without limitation micromirror
arrays, micromotors, microswitches or accelerometers. Fluids used
in these applications may comprise aromatic solvents, such as 1,1,
Diphenylethylene, organosilianes, such as 3-chloropropyl
triethoxysilane, perfluoroethers, such as Galden HT-100 (TM),
silicones and silanes, such as polymethylphenylsiloxane, and
polydimethylsiloxane, water, mixtures of water and water-soluble
organics, ionic materials dissolved in water, pigmented fluids,
colloidal suspensions.
[0026] Fluid may be introduced into an inner cavity by a method
illustrated in FIGS. 7 and 8. A low pressure, vacuum environment is
provided in the inner cavity. The vacuum in the inner cavity may be
provided by providing an environment 7 at a vacuum encompassing the
MEMS assembly 1 which can be accomplished, for example, by placing
the MEMS assembly into a chamber and evacuating air from the
chamber to create a vacuum. The low pressure or vacuum environment
7 encompassing the MEMS assembly causes air located within the
inner cavity to exit the inner cavity through the fill port 111,
thereby providing the vacuum in the inner cavity. The low pressure
or vacuum environment could be provided, alternatively, by
assembling the assembly within a low pressure or vacuum
environment.
[0027] When the inner cavity is provided with a vacuum, an amount
of fluid at least sufficient to fill the inner cavity may be
provided at the feed port 111. In an exemplary embodiment
illustrated in FIG. 9a, the fluid 6 can be provided at the feed
port by submerging the MEMS assembly in fluid 6. In another
exemplary embodiment illustrated in FIG. 9b, the MEMS assembly
could be submerged in the fluid 6 at a and removed, leaving the
amount of fluid 6 at least sufficient to fill the inner cavity at
the feed port at b. In yet another exemplary embodiment illustrated
in FIG. 9c, the fluid 6 could be provided at the feed port using a
fluid carrier 61 which has been dipped into the fluid at a., moved
at b., and touched to the fill port at c., leaving at least the
desired amount of fluid at the fill port. The fluid carrier may
comprise a pin or a capillary tube. A plurality of fluid carriers
may be used to provide a sufficient amount of fluid to a plurality
of fill ports simultaneously.
[0028] The fluid provided at the fluid port should be arranged such
that an increase in the pressure of the environment 7 surrounding
the MEMS assembly or in the chamber causes fluid to enter the inner
cavity through the feed port. In an exemplary embodiment
illustrated in FIGS. 7 and 8, for example, the fluid provided at
the feed port spans the entire entrance to the fill port. While
fluid is provided at the fill port, the pressure inside the inner
cavity is initially the same as the pressure in the environment 7
surrounding the MEMS assembly. The pressure of the environment 7
surrounding the MEMS assembly is increased creating a pressure
gradient across the fluid from the exposed portion to the portion
in contact with the inner cavity. If the amount of fluid provided
at the fill port is sufficient to fill the cavity and to provide a
seal during the entire fill process, the fluid will fill the inner
cavity. A residual amount of fluid may remain at the fill port
after filling. The pressure of the environment is raised to a
target pressure which, for example, may be atmospheric pressure or
greater than atmospheric pressure.
[0029] The rate of pressure increase should be selected such that
the differential pressure between the increased pressure in the
environment or chamber and the low pressure or vacuum in the inner
cavity causes the fluid to enter the cavity through the breach in
the bond ring and completely fill the space between the silicon
wafer and the cover plate. For viscous fluids, the fill time may be
dominated by the time it takes the fluid to work its way in through
the fill port. For fluids of lower viscosity, the fill time may be
dominated by the time it takes to fully create the vacuum and
evacuate air from the chamber and/or the inner cavity. A variety of
factors may influence the length of the fill process, including
fluid viscosity, temperature, fill port geometry, gap height,
surface tension between the fluid and the cavity surfaces, and/or
MEMS geometry. The duration of the fill process may be decreased by
using higher than atmospheric pressure to increase the flow rate
into the cavity.
[0030] The pressure of the environment or the chamber may be
increased while the MEMS assembly is completely submerged in fluid
or when the MEMS assembly has been removed from the liquid, leaving
amounts of fluid sufficient for filling the cavities at the fill
ports, or after a sufficient amount of fluid has been placed at the
fill ports by other means. The amount of fluid provided at the fill
port should be sufficient to fill the inner cavity and to prevent
the introduction of air and/or gas into the inner cavity during the
fill process. Where the MEMS assembly is submerged in fluid, the
pressure due to the fluid alone may cause some fluid to enter the
inner cavity before the pressure of the environment is increased.
Capillary forces may also contribute to causing fluid to enter the
inner cavity. The wafer is cleaned of excess fluid by an
appropriate method, for example using a solvent, evaporation or
wiping.
[0031] Certain alternative, optional embodiments may include
several purge cycles with a gas or gasses, for example, carbon
dioxide or helium, to help ensure that all of the air is removed
from the inner cavity. Purge gases suitable for use in purging the
inner cavity may be selected so that the gas or gases have high
solubility in the fluid, the gases are inert with respect to the
fluid and with respect to other materials present in the inner
cavity. Suitable gases may comprise helium or carbon dioxide. In
those embodiments in which a purge gas is used, the use of purge
gases with high solubility in the fluid helps reduce the formation
of residual gas bubbles in the fluid. The fluid used to fill the
inner cavity may also be degassed prior to filling. Degassing the
fluid may prevent absorbed gas from coming out of solution and
nucleating a bubble in the fluid.
[0032] In exemplary embodiments, it may be desirable to remove
adsorbed fluid, which may comprise water, from the surfaces of the
inner cavity. The adsorbed fluid may be removed during the
evacuation step. Elevated temperatures may be used to speed up the
removal of adsorbed fluid.
[0033] In an alternative exemplary embodiment, capillary forces
alone may be sufficient to fill the cavity without using a vacuum.
The MEMS assembly may comprise a bond ring with a plurality of
breaches, for example two breaches. FIG. 10 illustrates an
exemplary embodiment of a fluidic MEMS assembly comprising a
substrate 3, a cover plate 2 and a bond ring 4. The bond ring 4 has
two breaches 42a, 42b which define a fill port 111a and an evacuate
port 111b. An amount of fluid sufficient to fill the inner cavity
is provided at the fill port. The fluid is drawn into the inner
cavity by capillary forces. Air from the inner cavity displaced by
fluid drawn into the cavity is evacuated through the evacuate port.
111b. When the inner cavity is filled, the fill port and evacuate
port are sealed. A fluidic MEMS assembly may alternatively be
filled using pressure at the fill port, a vacuum at the evacuate
port and/or capillary forces, alone or in combination.
[0034] An amount of adhesive, which may be curable adhesive, is
applied to the location of the breach or breaches and cured to
complete the containment of the fluid. Suitable adhesives may
comprise organic adhesives, such as epoxies, which are thermally or
UV cured, solders or glass-based sealants. Suitable sealants may be
chemically inert or compatible with the fluid, may have a thermal
expansion coefficient compatible for use with other components, may
have good adhesion to all surfaces, high reliability, hermeticity.
In the exemplary embodiments illustrated in FIGS. 2 and 3, a MEMS
assembly with a cover plate 2, a substrate 3, a bond ring 4 with a
breach and filled with fluid 6 includes a seal 45 at the fill port
111.
[0035] The height 41 of the inner cavity may be selected such that
the volume of the fluid contained within the inner cavity is
sufficiently small so that the change in volume upon expansion is
sufficiently small to be accommodated by a slight deflections of
the cover plate, substrate, bond ring, adhesive seal, thereby
reducing the risk of damage to the cover plate.
[0036] In an exemplary embodiment of the fluidic MEMS device and
method of manufacturing a fluidic MEMS device, a plurality of inner
cavities defined on a wafer may be filled with fluid
simultaneously. In one exemplary embodiment, a plurality of cover
plates may be individually attached to each die location on a
single substrate or wafer, as illustrated in FIG. 5. In a further
exemplary embodiment, a single cover plate may be attached to a
single substrate by a plurality of separate bond rings to define a
plurality of inner cavities, as illustrated in FIGS. 6A-6C. In each
one of these embodiments, the plurality of inner cavities may be
filled at the same time. The entire wafer-level assembly 100 can be
submerged in the fluid to provide at least the desirable amount of
fluid at the plurality of feed ports. Where the fluid is provided
to the feed port using a fluid carrier, for example a pin or
capillary tube, that was dipped into fluid, an array with a
plurality of such pins could be dipped into the fluid and touched
onto the fill ports leaving a dollop of fluid at each of the
plurality of feed ports. Alternatively, an array with a smaller
number of fluid carriers, or even one fluid carrier, could be
dipped into fluid and then successively touched onto fill ports
until fluid is provided at the fill port of each MEMS assembly to
be filled.
[0037] In an exemplary embodiment, a method for filling MEMS
assemblies does not require drilling holes in the substrate or
silicon wafer, which may result in increased simplicity and cost
savings. The fluid containment may be accomplished virtually
entirely by hermetic materials, thereby increasing reliability by
reducing the risks of vapor loss from and/or air ingress into the
inner cavities. The fluid filling process may occur at the wafer
level. Many devices are filled at once, yielding a throughput
gain.
[0038] Attaching the cover plate or plates at the wafer stage may
provide protection to the active silicon MEMS structures during the
manufacturing process. This may be particularly advantageous for
devices where particle sensitivities are high or where the silicon
contains delicate structures. Assembling the MEMS device packages
or assemblies at the wafer stage may also permit fully functional
testing at the wafer level. This may permit faulty parts to be
identified at an early stage of the manufacturing process, thereby
saving further manufacturing costs. The MEMS device and methods of
this disclosure may reduce or eliminate the number of fluid
interconnections and/or flexible diaphragms, resulting in fewer
manufacturing steps and reduced manufacturing costs.
[0039] Providing an inner cavity with sufficiently small volume
will reduce the total volumetric expansion of fluid in the fluidic
MEMS device, thereby reducing the risk of leakage or other
structural damage to the MEMS package due to the fluid expansion.
The thermal expansion of the fluid may be entirely accommodated by
deflections in the cover plate, substrate and bond ring without
fracturing or damaging the packaging materials.
[0040] It is understood that the above-described embodiments are
merely illustrative of the possible specific embodiments which may
represent principles of the present invention. Other arrangements
may readily be devised in accordance with these principles by those
skilled in the art without departing from the scope and spirit of
the invention.
* * * * *