U.S. patent application number 13/540101 was filed with the patent office on 2012-10-25 for low temperature bi-cmos compatible process for mems rf resonators and filters.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Leena Paivikki BUCHWALTER, Kevin Kok CHAN, Timothy Joseph DALTON, Christopher Vincent JAHNES, Jennifer Louise LUND, Kevin Shawn PETRARCA, James Louis SPEIDELL, James Francis ZIEGLER.
Application Number | 20120270351 13/540101 |
Document ID | / |
Family ID | 40581752 |
Filed Date | 2012-10-25 |
United States Patent
Application |
20120270351 |
Kind Code |
A1 |
BUCHWALTER; Leena Paivikki ;
et al. |
October 25, 2012 |
LOW TEMPERATURE BI-CMOS COMPATIBLE PROCESS FOR MEMS RF RESONATORS
AND FILTERS
Abstract
A method of removal of a first and second sacrificial layer
wherein an O.sub.2 plasma or an O.sub.2-containing environment is
introduced to a cavity and a gap region through a plurality of via
holes in a cavity capping material.
Inventors: |
BUCHWALTER; Leena Paivikki;
(Hopewell Junction, NY) ; CHAN; Kevin Kok; (Staten
Island, NY) ; DALTON; Timothy Joseph; (Ridgefield,
CT) ; JAHNES; Christopher Vincent; (Upper Saddle
River, NJ) ; LUND; Jennifer Louise; (Brookville,
MD) ; PETRARCA; Kevin Shawn; (Newburgh, NY) ;
SPEIDELL; James Louis; (Poughguag, NY) ; ZIEGLER;
James Francis; (Edgewater, MD) |
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
40581752 |
Appl. No.: |
13/540101 |
Filed: |
July 2, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13007130 |
Jan 14, 2011 |
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13540101 |
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10316254 |
Dec 10, 2002 |
7943412 |
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13007130 |
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60339089 |
Dec 10, 2001 |
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Current U.S.
Class: |
438/49 ;
257/E21.002 |
Current CPC
Class: |
B81B 2201/0271 20130101;
B81C 1/0023 20130101; B81C 2203/0735 20130101; H03H 3/0072
20130101; H03H 9/2405 20130101; B81C 1/00246 20130101 |
Class at
Publication: |
438/49 ;
257/E21.002 |
International
Class: |
H01L 21/02 20060101
H01L021/02 |
Claims
1. A method of removal of a first and second sacrificial layer
wherein the sacrificial material is exposed to an oxygen
plasma.
2. A method of removal of a first and second sacrificial layer
wherein the sacrificial material is exposed to an anneal in an
oxygen containing environment.
3. The method of removal of a first and second sacrificial layer of
claim 2, wherein the first and second sacrificial layers are
exposed to the anneal in the oxygen containing environment at a
temperature less than 400 degrees C.
4. A method of removal of a first and second sacrificial layer
wherein an O.sub.2 plasma or an O.sub.2-containing environment is
introduced to a cavity and a gap region through a plurality of via
holes in a cavity capping material.
5. The method of claim 4, wherein the first and second sacrificial
layers are a carbon-based material.
6. The method of claim 4, wherein the removal of the first and
second sacrificial layers is performed without a subsequent rinsing
of the cavity.
7. The method of claim 4, further comprising capping the cavity
with a second material having a resistance to etching greater than
that of the first and second sacrificial layers.
8. The method of claim 4, further comprising forming one of a
microelectromechanical resonator and a microelectromechanical
filter in the cavity before the removal of the first and second
sacrificial layers.
9. The method of claim 4, further comprising sealing the plurality
of holes, wherein the plurality of holes have an aspect ratio to
substantially prevent a sealing material from entering the cavity.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is Continuation Application of U.S.
application Ser. No. 13/007,130, filed Jan. 14, 2011, which is a
Divisional Application of U.S. Pat. No. 7,943,412, filed on Dec.
10, 2002, which claims priority to Provisional Application Ser. No.
60/339,089, filed Dec. 10, 2001, the disclosures of which are
herein incorporated by reference in its entirety.
BACKGROUND
[0002] 1. Technical Field
[0003] The present invention relates to microelectromechanical
system (MEMS) resonators and filters, and, more particularly, to
the fabrication of such devices in a manner which allows
integration with other integrated circuit technologies, such as
Bi-CMOS, while maintaining the desired properties of these devices
such as high resonant frequency (f.sub.0) and very high quality
factor (Q).
[0004] 2. Description of Related Art
[0005] Microelectromechanical system (MEMS) devices have the
potential for great impact on the communications industry. MEMS RF
switches, oscillators (resonators), filters, varactors, and
inductors are a few of the devices that could replace large and
relatively expensive off-chip passive components. It is even
possible that the introduction of these types of MEMS devices,
particularly resonators and filters, into analog and mixed-signal
integrated circuits could dramatically alter the architecture of
current wireless communication devices. Key to such advancements is
the ability to monolithically integrate MEMS RF components with
integrated circuit technologies to realize cost, size, power, and
performance benefits.
[0006] MEMS resonators and filters have been under development for
some time. For resonators and filters aimed at RF communications
applications, the key design factors are ability to reach the
frequencies of interest (approx. 900 MHz-2 GHz), low voltage
operation, small size, and very high quality factor (Q). Resonators
and filters developed to date have demonstrated high Q's and
reasonably small sizes, but have not achieved the frequency or bias
voltage targets required for incorporation with analog and mixed
signal circuits. Other drawbacks of current MEMS resonators and
filters include incompatibility of materials, processes, and
processing temperatures for integration with other IC processes,
inability to scale the devices to the desired sizes because of
grain size limitations of the materials used and inability to form
very small gaps between electrodes, and failure to provide
protection for the MEMS devices from subsequent processing steps
and ambient conditions and contamination.
[0007] Typical designs of prior art MEMS resonators and filters are
illustrated in FIGS. 1A-1C. FIG. 1 A shows a comb-drive type MEMS
filter. Stationary combs 1 and 7 are connected via anchors 2 and 8
to input and output electrodes 3 and 9, respectively. Moving comb 4
is connected via anchors 5 to ground plane 6. The fingers of all
three comb structures are suspended above the underlying substrate,
the ground plane, and the input and output electrodes except at the
anchor points. All three combs are comprised of a conductive
material, typically heavily doped polysilicon. The ground plane and
input and output electrodes are also conductors typically made from
heavily doped polysilicon. During operation, ground plane 6 is
electrically contacted to the ground potential. The potential of
moving comb 4 is also at ground. An AC excitation, superimposed on
a DC bias, is applied to input electrode 3 and thus, via anchor 2,
to stationary comb 1. The same DC bias is applied to output
electrode 9 and thus, via anchor 8, to stationary comb 7. Because
of the potential difference between the fingers of stationary comb
1 and moving comb 4, moving comb 4 is attracted laterally toward
stationary comb 1. The magnitude of this potential difference, and
thus the distance which moving comb 4 travels, is modulated by the
AC excitation. When the frequency of the exciting AC voltage
closely matches the mechanical resonant frequency f.sub.0 of moving
comb 4, the amplitude of vibration of moving comb 4 reaches a
maximum that is dependent on the quality factor Q of the system.
Simultaneously, the fingers of moving comb 4 and stationary comb 7
comprise a time-varying capacitor as the amount of overlap between
the fingers of the combs changes with the movement of moving comb
4. Thus, through the relationship I=d(CV)/dt, there will also be a
time-varying current which can be sensed electrically at output
electrode 9. The magnitude of this current will also be greatest
when the frequency of the exciting AC voltage at input electrode 3
closely matches the f.sub.0. Thus, the device provides
electromechanical filtering of the input signal around f.sub.0.
[0008] FIG. 1B shows another example of a prior art MEMS filter
which is aimed at achieving higher-frequency operation. Two beams
11 and 15 are connected to ground electrodes 13 via anchors 12.
Beams 11 and 15 are also connected to one another by bridge 14.
Taken alone, either beam 1 for beam 15 comprises a MEMS resonator.
Coupling two or more MEMS resonators together creates the MEMS
filter. Beams 11, 15, and bridge 14 are suspended above the
underlying substrate. Ground electrodes 13, and input and output
electrodes 16 and 17 (respectively) are also suspended above the
underlying substrate except at anchor points 12. Beams 11 and 15,
bridge 14, and all electrodes 13, 16 and 17 are composed of a
conductive material, typically heavily doped polysilicon. During
operation, ground electrodes 13 are electrically contacted to the
ground potential; thus, via anchors 12, the potential of beams 11
and 15 and bridge 14 are also at ground. An AC excitation,
superimposed on a DC bias, is applied to input electrode 16. The
same DC bias is applied to output electrode 17. Because of the
potential difference between them, beam 11 is attracted downward
toward electrode 16. The magnitude of this potential difference,
and thus the distance which beam 11 travels, is modulated by the AC
excitation. When the frequency of the exciting AC voltage closely
matches the mechanical resonant frequency f.sub.0 of beam 11, the
amplitude of vibration of beam 11 reaches a maximum that is
dependent on the quality factor Q of the system. The mechanical
energy of vibration of beam 11 is transmitted via bridge 14 to beam
15. Beam 15 and output electrode 17 comprise a time-varying
capacitor as the distance between the two structures changes with
the movement of beam 15. Thus, through the relationship I=d(CV)/dt,
there will also be a time-varying current which can be sensed
electrically at output electrode 17. The magnitude of this current
will also be greatest when the frequency of the exciting AC voltage
at input electrode 16 closely matches the f.sub.0. Thus, the device
provides electromechanical filtering of the input signal around
f.sub.0.
[0009] FIG. 1C shows cross section A-A' of prior art MEMS resonator
11 as seen in FIG. 1B. This cross section also shows the substrate
21 upon which the MEMS resonator or filter is constructed. This
substrate is typically silicon (Si), although other substrates such
as glass, quartz, or gallium arsenide (GaAs) have also been used.
Also shown is insulating layer 22, typically silicon dioxide
(SiO.sub.2), used to electrically isolate the MEMS device from the
substrate and other devices. Air gap 23 can be seen in the cross
section, demonstrating that beam 11 is freestanding except at
anchor points 12. Not shown here is the sacrificial material that
occupied gap 23 during the construction of this device, and was
later removed so that beam 11 would be free to vibrate.
[0010] One of the drawbacks of the prior art is the deposition
temperature of the materials commonly used for construction of the
MEMS device. Although various conductive materials have been used
to form MEMS resonators and filters, polysilicon is the most
common. Polysilicon is frequently chosen because of its relatively
high ratio of elastic modulus (E) to density (.rho.). This ratio is
one of the most important factors in determining the resonant
frequency of the device, and since high frequencies are sought for
RF communications applications, high ratios of E/.rho. are
desirable. However, polysilicon must be deposited at temperatures
in excess of 600.degree. C. Furthermore, the dopant atoms, such as
phosphorus, which are added to the polysilicon to make it
sufficiently conductive, frequently must be annealed at
temperatures near 900.degree. C. in order to activate them. These
temperatures are well above the temperatures used in fabrication of
the metal interconnect levels of integrated circuit processes. This
means that prior art MEMS resonators and filters, if they were to
be integrated in an IC process, would have to be fabricated at the
same time as the transistor devices (which permit higher processing
temperatures). This type of process integration is much more
difficult to achieve and is very specific to the particular IC
process. Thus, the process steps for formation of the MEMS device
would likely need to be altered each time there was a change to the
IC process, or whenever it was desired to integrate the MEMS device
with a different IC process. A much simpler and more modular
approach is to integrate the MEMS device after all circuit
processing, including interconnect levels, has been completed.
However, this cannot be done with prior art MEMS resonators and
filters.
[0011] Another serious issue with prior art MEMS resonators and
filters is the process by which the devices are released from the
surrounding layers and substrate. The most commonly used
sacrificial material (i.e., the material which temporarily occupies
the gap region and is later removed to create the freestanding MEMS
structure) in the prior art is SiO.sub.2. This material is removed
by means of etching in an aqueous buffered hydrofluoric acid
(buffer-HF) solution. This solution will also remove silicon
nitride (SiN), although at a slower rate, and causes etching of or
damage too many metals. Because SiO.sub.2 and SiN are used as
insulating layers in integrated circuits, this release method also
makes it very difficult to integrate prior art MEMS resonators and
filters with IC processes. Another problem with the use of aqueous
buffer-HF as a release method is the occurrence of a phenomenon
known as stiction. After the sacrificial SiO.sub.2 has been
removed, the buffer-HF is rinsed away. As the water is then removed
during the subsequent drying step, the freestanding MEMS parts have
a tendency to stick to the substrate or surrounding materials
because of the high surface tension of the water. Prior art MEMS
devices frequently have to be subjected to an alternative drying
method such as the use of supercritical carbon dioxide (CO.sub.2).
This method and the associated tools are also not part of any
current IC process flow. Another drawback to using aqueous
buffer-HF to remove the sacrificial layer is that it restricts the
aspect ratios and gap dimensions that can be achieved in MEMS
devices. Very small gaps (tens--few hundred nanometers) cannot be
formed because of limited transport of the etchant and etch
products in and out of the gap region. Small gaps are desirable in
MEMS devices because they allow the use of lower actuation
voltages. Typical RF IC's use supply voltages of 3V; most prior art
MEMS resonators and filters require biases of 20V and up.
[0012] Another concern with prior art MEMS resonators and filters
is the lack of adequate encapsulation of the devices for protection
during subsequent processing steps, and from ambient contamination,
humidity, and pressure when fabrication is complete. Once the MEMS
device has been released, additional processing steps create the
risk of re-filling the gap area with deposited material and
re-connecting the device to the substrate, causing failures due to
stiction, or adversely affecting yield or performance via the
introduction of particulates to the gap region or the device
itself. Even after all fabrication is complete, MEMS resonators and
filters are quite sensitive to ambient conditions. For example, a
particulate adhering to the resonator beam could change the mass
(and thus the resonant frequency) of a small beam by several
hundred percent. A particulate lodged in the gap region would damp
or completely prevent resonance.
[0013] Finally, it has been established that the quality factor of
MEMS resonators and filters is directly related to ambient
pressure, and in order to maximize Q, MEMS resonators and filters
must be operated at pressures below about 0.1 Torr. Several
encapsulation schemes have been proposed in the prior art. The most
common methods involve bonding a second substrate with an etched
cavity over the MEMS device by various means (e.g. anodic bonding,
eutectic bonding, etc.). However, to date these methods have not
been adequately demonstrated at wafer scale. Each individual device
must be capped. This method is not compatible with reasonable
manufacturing processes. This method also causes difficulties with
integrated circuit designs intended for packaging via flip-chip
(solder bump) die attach. Furthermore, this method assumes that the
MEMS resonator or filter is the last device fabricated (i.e., it is
exposed on the top surface of the chip), and it has already been
seen that prior-art MEMS resonators and filters are not compatible
with fabrication after the completion of IC processing. Another
encapsulation method that has been proposed in the prior art is to
cover the MEMS resonator with additional SiO.sub.2, then to cap the
entire structure with a shell of porous polysilicon. The device is
then exposed again to aqueous buffer-HF, which is transported
through the porous polysilicon, removes the covering SiO.sub.2, and
diffuses back out through the porous polysilicon. This method is
unsatisfactory for many reasons, several of which (deposition
temperature of polysilicon and stiction) have already been
discussed.
BRIEF SUMMARY
[0014] Accordingly, it is an object of the present application to
provide a MEMS resonator or MEMS filter having electrodes energized
by an applied DC potential and excited by an applied AC potential,
causing a moveable structure to vibrate at its mechanical resonant
frequency, thereby providing a frequency reference or filtering a
signal of interest around the resonant frequency.
[0015] It is another object to provide a method for the
construction of these devices which allows them to be fabricated at
temperatures low enough to be compatible with the metal
interconnect levels of any analog, digital, or mixed signal
integrated circuit process.
[0016] It is yet another object to provide a method for releasing
the freestanding portions of these devices from the surrounding
substrate and materials in a manner which is also compatible with
the metal interconnect levels of any analog, digital, or mixed
signal integrated circuit process, which eliminates stiction during
processing, and which allows for the construction of ultra-small
gaps between electrodes in these devices.
[0017] It is a further object of the invention to provide a method
of encapsulation of these devices which protects them from
subsequent processing steps, contamination, and ambient conditions
such as humidity, pressure, and the like.
[0018] The present invention describes a process for the
fabrication of MEMS resonators or filters at temperatures low
enough to permit integration with analog, digital, or mixed-signal
IC processes after or concurrently with the formation of metal
interconnects in those processes.
[0019] According to the present invention, the performance
requirements of MEMS resonators and filters, in particular high
frequency operation and high Q, can be achieved through the
selection of materials that are compatible both with IC processes
and with these specifications.
[0020] The invention also includes methods for the clean removal of
sacrificial material without yield loss due to stiction, without
need for non-standard processing techniques, and permitting the
construction of very small gaps that allow operation of the MEMS
resonators and filters using actuation voltages compatible with
analog IC supply voltages.
[0021] It is a principal object of the present invention to provide
a method of vacuum encapsulation of these devices that will protect
them from subsequent processing steps (such as dicing, die attach,
bonding, packaging, and the like) and ambient contamination and
humidity which may cause device failure, and that will provide low
enough pressure for maximum device performance (high Q). This
method of encapsulation is wafer-scale; that is, all MEMS devices
on a wafer are protected simultaneously during processing.
[0022] Further and still other objects of the present invention
will become more readily apparent from the following detailed
description taken in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0023] FIGS. 1A-1C describe the prior art.
[0024] FIGS. 2A-2B show cross-sectional views of sample substrates
upon which the device could be fabricated.
[0025] FIGS. 3-11B show cross sectional views of the device at
various stages of fabrication.
[0026] FIG. 12 shows an alternative embodiment in which the MEMS
resonator or filter has been incorporated into the interconnect
layers rather than added afterward.
[0027] FIGS. 13A-D show an alternative embodiment in which the MEMS
resonator or filter has been fabricated on a planar surface rather
than in a cavity and the encapsulation provided subsequently.
[0028] FIGS. 14A-C show an alternative embodiment in which the
cavity material and sacrificial encapsulating material are
shared.
DETAILED DESCRIPTION
[0029] FIGS. 3-11B show cross-sectional views according to a
preferred embodiment of the invention for a device such as the one
shown in the prior art in FIG. 1B and in cross-section in FIG. 1C.
However, MEMS resonators and filters can be designed in a wide
variety of configurations including comb-drive resonators, beams
fixed at two ends, beams fixed at one end, beams with suspensions,
coupled beams, tuning forks, beams with bends or turns, curved
beams, disks, and so on. These drawings of FIGS. 3-11B are in no
way intended to exclude other geometries and configurations for
building a MEMS resonator or filter, and all such geometries and
configurations are included herein. The process of manufacture
described is the same for all such configurations, and the design
shown in FIGS. 3-11B is chosen for convenience of description
only.
[0030] FIG. 2A shows a typical starting substrate 31 covered by an
insulating layer 32. Starting substrates 31 may include a variety
of materials such as silicon with or without epitaxial layers, high
resistivity silicon (HRS), silicon-on-insulator (SOI), glass,
quartz, sapphire, GaAs, or other substrates commonly used in
integrated circuit manufacturing. The insulating layer 32 could be
SiO.sub.2, SiN, silicon oxy-nitrite (SiON), or any of a variety of
organic insulators, or some combination of layers of insulating
materials. If the starting substrate 31 is itself an insulator such
as quartz, the insulating layer 32 may be omitted. FIG. 2B shows a
semi-conducting substrate 31 such as silicon, SOI, or the like in
which active devices 33 such as transistors, diodes, varactors,
etc. have already been fabricated, along with any desired local
interconnect layers 34. An insulating layer 32 has covered the
substrate and active devices. The drawings that follow assume that
a substrate such as the one in FIG. 2B has been used, since this
type of substrate offers maximum utility for the MEMS device.
[0031] FIG. 3 shows the device after the completion of the
fabrication of the metal interconnects used to join active and
passive devices in an integrated circuit. Interconnecting wires 41
may be made of Al, AlCu, Cu, W, or any conductive material or
combination of such materials commonly in use in IC fabrication.
Inter-level vias 43 are similarly comprised. Inter- and intra-level
dielectrics (ILD) 42 may be comprised of SiO.sub.2, SiN, SiON, any
of a variety of organic insulators, or other insulating material or
combination of such materials commonly in use in IC fabrication.
The number of layers of interconnecting wires 41, inter-level vias
43, and ILD layers 42 shown in FIG. 3 is arbitrary. Any number of
layers, including none, may be used. These layers may be deposited
and patterned according to any method commonly in use in IC
fabrication.
[0032] A final conducting layer 44 is then deposited and patterned.
Conducting layer 44 will form the input and output electrodes of,
the electrical connection to, and the physical anchor points for
the MEMS resonator or filter. Conducting layer 44 may or may not be
also used as an interconnect level in the integrated circuit.
Conducting layer 44 may be comprised of Al, AlCu, Cu, W, or any
conductive material or combination of such materials commonly in
use in IC fabrication, with the caveat that if a material such as
Cu that oxidizes readily is used, conducting layer 44 should have a
relatively non-reactive conductive material coating its top
surface. As long as the base or coating material will develop less
than a few 10's of nm of oxide in the presence of an oxygen plasma,
it will be satisfactory for the operation of the MEMS resonator or
filter. Otherwise, the coating material should be a noble metal
such Au, Pt, Pd, Ir, Rh, or Ru. Conducting layer(s) 44 may be
deposited by any means commonly used in IC fabrication, including
but not limited to sputter deposition, CVD, PECVD, evaporation, or
electroplating, as long as that deposition method does not exceed
the maximum allowable temperature T.sub.max which existing
interconnect layers 41, vias 43, and ILD 42 can withstand.
Similarly, the intra-level dielectric surrounding conducting layer
44 should not be etched by an oxygen plasma; if this is the case
then this intra-level dielectric should be coated with another
insulating material such as SiO.sub.2, SiN, SiON, or the like. In
the preferred embodiment, the wafer surface is planar after the
construction of conducting layer 44. This planarization may be
achieved through chemical-mechanical polishing (CMP) or other
method commonly in use for IC manufacturing.
[0033] FIG. 4 shows the formation of a cavity 51 in which the MEMS
resonator or filter will be constructed. The cavity material 52 is
first deposited by a means such as plasma-enhanced chemical vapor
deposition (PECVD) or other method (e.g. sputtering, spin-on, etc.)
that keeps the temperature below T.sub.max. In the preferred
embodiment, the cavity material should be a layer or set of layers
of an insulator which will not be etched by an oxygen plasma, such
as SiO.sub.2, SiN, SiON, or the like. The cavity 51 is patterned by
any typical method, such as reactive ion etching (RIE) or wet
chemical etching.
[0034] In FIG. 5, a layer of sacrificial material 61 is deposited
and patterned to expose anchor points 62. The space occupied by
sacrificial layer 61 will later form the gap between the input or
output electrodes and the resonating member of the MEMS resonator
or filter. As such, sacrificial layer 61 should be made the same
thickness as the desired gap spacing. We propose the use of
carbon-based materials that can be easily removed in an
oxygen-based dry chemistry or alternatively by annealing in the
presence of oxygen (O.sub.2) gas at temperatures less than
400.degree. C. (typical T.sub.max). As carbon is readily removed
with O2 plasma ashing or O.sub.2 annealing, no aqueous solutions
are necessary. Thus, concerns about stiction are alleviated.
Additionally, in the O.sub.2 ashing or annealing environment, most
materials (with the exception of carbon-based materials) do not
exhibit any significant etch rates. Therefore, the use of
carbon-based release layers will allow for a greater flexibility of
material choices for MEMS devices. The carbon-based release layer
61 can be deposited by a variety of methods, including but not
limited to PECVD, evaporation, sputtering, and spin-on techniques.
The choice of deposition technique generally relates to other
structural requirements such as conformality, thickness control,
and thermal stability of the sacrificial layer. The type of
material can be any solid form of C, CH, CHO, or CHON. During the
patterning of sacrificial layer 61 to form anchor points 62, it may
be necessary or desirable to use a secondary hard mask of metal,
silicide, or other dielectric layers.
[0035] Next the materials that comprise the resonating member of
the MEMS resonator or filter are deposited and patterned. In the
preferred embodiment shown in FIG. 6A, the resonating member is
comprised of a thin layer of conductive material 70 followed by a
thick structural layer 71. Conductive layer 70 is used so that
electrical contact to the resonating member may be made, and so
that electrostatic actuation between the input or output electrode
75 and the resonator may be achieved. Because good conductors such
as metals and silicides typically have very high density (.rho.),
and low density materials are more desirable for achieving high
resonant frequencies, the conducting layer 70 is made very thin.
Any material that has good conductivity may be used, with the
caveat that if a material such as Cu that oxidizes readily is used,
conducting layer 70 should have a relatively non-reactive
conductive material coating its lower surface. As long as the base
or coating material will develop less than a few 10's of nm of
oxide in the presence of an oxygen plasma, it will be satisfactory
for the operation of the MEMS resonator or filter. Otherwise, the
coating material should be a noble metal such Au, Pt, Pd, Ir, Rh,
or Ru.
[0036] Because many dielectric materials have an excellent E/.rho.
ratio for achieving high frequency operation of the MEMS resonator
or filter, the bulk structural material 71 of the resonating member
is comprised of dielectric in the preferred embodiment. Any
material that is not etched or significantly altered by O.sub.2
plasma may be used; however aluminum nitride (AN), aluminum oxide
(Al.sub.2O.sub.3), silicon nitride (Si.sub.3N.sub.4), tantalum
silicon nitride (TaSiN), and many piezoelectric materials make
excellent choices. The material in layers 70 and 71 may be
deposited by any typical means whose temperature does not exceed
T.sub.max, for example PECVD, sputtering, evaporation,
electroplating, etc. The resonating member is then patterned,
typically by RIE.
[0037] FIG. 6B shows an alternative embodiment in which the entire
thickness of the resonating member is made of conductive material
70. This may be done, for example, when creating a MEMS resonator
or filter whose vibration is in the lateral direction, such as a
comb filter. The drawback to this embodiment is the lower E/.rho.
ratio and thus the lower f.sub.0 of the MEMS resonator or
filter.
[0038] FIG. 6C shows an alternative embodiment in which the
resonating member is comprised of more than two layers of material,
for example a lower conductor 70, a structural layer 71, and an
upper conducting layer 72. This may be done to offset performance
effects due to the differing thermal coefficients of expansion of
the different layers in the resonating member. Although only three
layers are shown in FIG. 6C, any number of layers may be used, and
any materials may be used for the different layers, as long as at
least one conducting layer is used, and all materials obey the
processing temperature restrictions and show good resistance to
etching by oxygen plasma.
[0039] Following the deposition and patterning of the resonating
member, the remainder of cavity 51 is filled in with additional
sacrificial material 80 (FIG. 7). This material may be the same as
or different from the sacrificial layer 61 shown in FIG. 5, as long
as it is also a carbon-based material which is easily removed in an
oxygen plasma or by annealing in an oxygen ambient. The same
material and deposition method choices apply. Following deposition
of additional sacrificial material 80, the entire structure is
planarized by a method such as CMP. This step may not be necessary
if the material chosen for sacrificial material 80 has
self-planarizing properties.
[0040] In FIG. 8, the entire structure is then capped with
additional dielectric layer(s) 81. Although a single layer is shown
in FIG. 8, multiple layers may be employed. This layer or layers
may be comprised of SiO.sub.2, SiN, SiON, or the like, as long as
these layers are not etched by an O.sub.2 plasma, and these layers
may be deposited by any typical means such as PECVD. Subsequently
(FIG. 9), very small via holes 90 are etched in this cavity
"ceiling" by RIE, thereby exposing sacrificial material 80.
[0041] Next (FIG. 10), the sacrificial material above, surrounding,
and below the MEMS resonator or filter is removed via an O.sub.2
ashing step, or by annealing in the presence of O.sub.2 gas at
temperatures less than T.sub.max. This procedure again reveals
cavity 51 and creates air gap 91. Now the resonating member of the
MEMS resonator or filter is free to move except at the anchor
locations. Due to the ease of removal of the sacrificial material
with this process, very small gaps on the order of 100 nm can be
achieved. Since no rinsing of reagents or etch by-products is
required, problems with stiction are eliminated.
[0042] The structure is then coated with additional dielectric
layer(s) 92 as shown in FIG. 11A in a two-step process. In the
first phase, non-selective PECVD is used to partially seal off
release vias 90. The poor conformality of this process works in our
favor in this case to rapidly pinch off the release vias while
depositing very little material inside vias 90 or cavity 51 itself.
Both the aspect ratio of release vias 90 and the parameters of the
PECVD process can be optimized to minimize deposition of unwanted
material within cavity 51. In the second phase of the process, via
holes 90 are finally and completely sealed in a physical vapor
deposition process such as evaporation or sputtering, wherein the
ambient pressure is around 10 mT or less. This is an order of
magnitude lower than the pressure required for optimum performance
of MEMS resonators and filters. If pressure this low is not
required for device operation, the entire pinch-off procedure can
be done in a single PECVD step. In the preferred embodiment, the
release process and the pinch-off process are accomplished in the
same manufacturing tool so that the devices do not need to be
exposed to the ambient in between. If necessary, a forming gas
anneal can be performed between these two steps (release and
pinch-off) to reduce any metal oxides formed on the surfaces of the
electrodes or resonating member of the MEMS resonator or filter.
Since the material used for the pinch-off process, typically
SiO.sub.2, SiN, SiON or some combination of these, may not provide
a long-term hermetic seal for the MEMS device, the alternative
embodiment shown in FIG. 11B demonstrates a metal "lid" 93 used to
prevent diffusion, particularly of water vapor, through the cavity
ceiling. In an alternative embodiment, the vacuum encapsulation
process presented here could be further combined with other
techniques such as eutectic bonding to another substrate, etc., to
gain additional protection for the MEMS device.
[0043] FIGS. 3-11 showed the preferred embodiment of a MEMS
resonator or filter that was fabricated after the completion of all
the processing steps required for IC fabrication. An alternative
method is to incorporate the fabrication of the MEMS device into
the process steps used for formation of the interconnect layers.
For example, in FIG. 12, a MEMS device is shown wherein the metal
level that forms the input and output electrodes and electrical
contact/anchors for the resonating member is shared with first
interconnect metal 101. The material that comprises the cavity is
the same as the inter-level dielectrics that insulate interconnect
metals 102 and 103. Interconnect metals 104, 105, and 106 are
formed after the completion of the fabrication and encapsulation of
the MEMS resonator or filter. The number of metal levels shown in
the example in FIG. 12 is arbitrary, as is the placement of the
MEMS device among them. That is, fabrication of the MEMS device
could have just as easily begun with interconnect metals 102 or
103, etc.
[0044] In another alternative embodiment, the MEMS resonator or
filter is not fabricated in a cavity 51 as shown in FIG. 4.
Instead, the MEMS resonator or filter is first constructed on a
planar surface, and then the encapsulation procedure is executed
afterward. In FIG. 13A, sacrificial layer 61 has been deposited and
patterned as in FIG. 5, and resonating structure materials 70 and
71 have been deposited and patterned as in FIG. 6A. The difference
is that cavity 51 is missing. In FIG. 13B, additional sacrificial
material 80 has been deposited and planarized as in FIG. 7. Again,
however, cavity 51 is not present. In FIG. 13C, hard mask 111,
typically consisting of layers of SiO.sub.2, SiN, or SiON, has been
deposited and patterned. The pattern has been transferred to
sacrificial material 80. In FIG. 13D, additional dielectric
material 112, again typically consisting of SiO.sub.2, SiN, SiON,
or some combination of these, has been deposited and the entire
structure planarized by a process such as CMP. The difference
between FIG. 13D and FIG. 7 is that the CMP process of FIG. 13D
planarized an inorganic dielectric, and the CMP process of FIG. 7
planarized the carbon-based sacrificial material. Subsequent to
FIG. 13D, the rest of the process is as from FIG. 8 onward. The
choice of which embodiment to pursue will depend on factors
including the lithography and CMP capabilities of the manufacturing
line.
[0045] FIGS. 14A-C show an alternative embodiment in which the MEMS
device is constructed on a planar surface, and the sacrificial
material is one and the same as the "cavity" material. Assuming
that the process steps which lead up to FIG. 13B are followed, in
FIG. 14A a thick dielectric membrane 120 is deposited over the
entire structure. Via holes 90 are then etched through dielectric
layer 120 (FIG. 14B). When the release process is executed,
"cavity" 55 of FIG. 14C results due to the isotropic nature of the
O.sub.2 plasma etch or the anneal in O.sub.2 containing ambient.
The extent of lateral etch will depend on the etch rate of the
carbon-based sacrificial material and the time required to free the
MEMS structure. The lateral etch will not be problematic as long as
appropriate design ground rules are enforced in laying out the
design.
[0046] Although the process described herein was developed
particularly for the fabrication of MEMS resonators and filters, it
should be noted that the methods, particularly of release and
encapsulation, could be equally well applied to other types of MEMS
devices. For example, device shown in cross section in FIG. 11A
could function as a metal-metal contact switch (MEMS switch) if the
applied DC voltage between the resonating member and the input or
output electrode were sufficiently high.
[0047] The preferred embodiment used carbon-based release layers
and an O.sub.2 plasma release process for reasons stated earlier.
However, the combined release-and-encapsulation process could also
be applied to other material sets, as long as compatibility
requirements are met. The basic process involves forming a cavity
in a material, filling the cavity with a material readily removable
without significant etching of the material surrounding the cavity,
capping the cavity with another material not readily removed when
removing the material inside the cavity, patterning small holes in
the material capping the cavity, removing the material within the
cavity through the holes in the capping material, and finally
sealing the cavity with a vacuum coating process. Another
sacrificial material that could be used with a metal/dielectric
MEMS resonator or filter is sputtered or evaporated silicon,
removed with a plasma containing xenon difluoride (XeF.sub.2).
Other combinations are possible as well.
[0048] While the presented invention has been described in terms of
a preferred embodiment, those skilled in the art will readily
recognize that many changes and modifications are possible, all of
which remain within the spirit and the scope of the present
invention, as defined by the accompanying claims.
* * * * *