U.S. patent application number 09/858583 was filed with the patent office on 2002-11-21 for inductor-capacitor resonant rf switch.
This patent application is currently assigned to Institute of Microelectronics. Invention is credited to Guo, Lihui, Xie, Joseph.
Application Number | 20020171517 09/858583 |
Document ID | / |
Family ID | 25328640 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020171517 |
Kind Code |
A1 |
Guo, Lihui ; et al. |
November 21, 2002 |
INDUCTOR-CAPACITOR RESONANT RF SWITCH
Abstract
A new inductor-capacitor resonance RF (LCR-RF) switching device
is achieved. The device comprises a microelectronic mechanical
switch and a spiral inductor. The microelectronic mechanical switch
comprises, first, a first dielectric layer overlying a substrate. A
down electrode overlies the first dielectric layer. A second
dielectric layer overlies the down electrode. An up electrode
overlies the down electrode with the second dielectric layer
therebetween. A bridge post overlies the first dielectric layer and
does not contact the down electrode or the up electrode. Multiple
bridge posts may be used. Finally, a membrane is suspended over
said down electrode. One end of the membrane is fixed to the top of
the bridge post. An electrostatic potential between the membrane
and the down electrode will cause the membrane to flex down toward
the down electrode. This flexing of the membrane will cause the
capacitance of the switching device to vary. The spiral inductor
comprises a metal line configured in a spiraling pattern with a
first end connected to the bridge post and a second end forming an
output node. A dual damascene method to form the LCR-RF switch is
also achieved.
Inventors: |
Guo, Lihui; (Singapore,
SG) ; Xie, Joseph; (Singapore, SG) |
Correspondence
Address: |
STEPHEN B. ACKERMAN
20 MCINTOSH DRIVE
POUGHKEEPSIE
NY
12603
US
|
Assignee: |
Institute of
Microelectronics
|
Family ID: |
25328640 |
Appl. No.: |
09/858583 |
Filed: |
May 17, 2001 |
Current U.S.
Class: |
333/262 |
Current CPC
Class: |
H01P 1/127 20130101;
H01H 59/0009 20130101; H01G 5/16 20130101; Y10T 29/49105
20150115 |
Class at
Publication: |
333/262 |
International
Class: |
H01P 001/10 |
Claims
What is claimed is:
1. A inductor-capacitor resonance RF switching device comprising: a
microelectronic mechanical switch comprising: a first dielectric
layer overlying substrate; a down electrode overlying said first
dielectric layer; a second dielectric layer overlying said down
electrode; a bridge post overlying said first dielectric layer but
not overlying said down electrode; and a membrane suspended over
said down electrode wherein one end of said membrane is fixed to
the top of said bridge post, wherein an electrostatic potential
between said membrane and said down electrode will cause said
membrane to flex down toward said down electrode, and wherein said
flexing of said membrane will cause the capacitance of said
switching device to vary; and a spiral inductor comprising a metal
line configured in a spiraling pattern with a first end connected
to said bridge post and a second end forming an output node.
2. The device according to claim 1 further comprising: an input
signal line connected to said down electrode; and a ground
reference connected to said output node to thereby form a shunt
configuration.
3. The device according to claim 1 further comprising: an input
signal line connected to said down electrode; and an output signal
line connected to said output node to thereby form a series
configuration.
4. The device according to claim 1 further comprising an up
electrode wherein said up electrode overlies said down electrode
with said second dielectric layer therebetween and wherein said
flexing downward causes said membrane to contact said up
electrode.
5. The device according to claim 1 wherein said membrane and said
bridge post comprise metal.
6. The device according to claim 1 wherein said membrane has a
thickness of between about 5,000 Angstroms and 25,000
Angstroms.
7. The device according to claim 1 wherein said membrane and said
bridge post are formed by a dual damascene process.
8. The device according to claim 1 further comprising a second
bridge post wherein said second bridge post is opposite said bridge
post and wherein a second end of said membrane is fixed to the top
of said second bridge post such that the combination of said bridge
post, said second bridge post, and said membrane forms a bridge
profile.
9. The device according to claim 1 wherein said capacitance value
in DOWN state is between about 10 fF and 30 pF.
10. The device according to claim 1 wherein said spiral inductor
value is between about 1 nH and 30 nH.
11. The device according to claim 1 wherein said capacitance and
said spiral inductor create a resonant frequency in DOWN state of
between about 1 GHz and 100 GHz.
12. A inductor-capacitor resonance RF switching device comprising:
a microelectronic mechanical switch comprising: a first dielectric
layer overlying a substrate; a down electrode overlying said first
dielectric layer; a second dielectric layer overlying said down
electrode; an up electrode overlying said down electrode with said
second dielectric layer therebetween; a bridge post overlying said
first dielectric layer but not overlying said down electrode and
said up electrode; and a membrane suspended over said up electrode
wherein one end of said membrane is fixed to top of said bridge
post, wherein an electrostatic potential between said membrane and
said down electrode will cause said membrane to flex down toward
said down electrode, wherein said flexing downward causes said
membrane to contact said up electrode, and wherein said flexing of
said membrane will cause the capacitance of said LCR-RF switching
device to vary; and a spiral inductor comprising a metal line
configured in a spiraling pattern with a first end connected to
said bridge post and a second end forming an output node.
13. The device according to claim 12 further comprising: an input
signal line connected to said down electrode; and a ground
reference connected to said output node to thereby form a shunt
configuration.
14. The device according to claim 12 further comprising: an input
signal line connected to said down electrode; and an output signal
line connected to said output node to thereby form a series
configuration.
15. The device according to claim 12 wherein said membrane and said
bridge post comprise metal.
16. The device according to claim 12 wherein said membrane has a
thickness of between about 5,000 Angstroms and 25,000
Angstroms.
17. The device according to claim 12 wherein said membrane and said
bridge post are formed by a dual damascene process.
18. The device according to claim 12 further comprising a second
bridge post wherein said second bridge post is opposite said bridge
post and wherein a second end of said membrane is fixed to the top
of said second bridge post such that the combination of said bridge
post, said second bridge post, and said membrane forms a bridge
profile.
19. The device according to claim 12 wherein said capacitance in
DOWN state is between about 10 fF and 30 pF.
20. The device according to claim 12 wherein said spiral inductor
value is between about 1 nH and 30 nH.
21. The device according to claim 12 wherein said capacitance and
said spiral inductor create a resonant frequency in DOWN state of
between about 1 GHz and 100 GHz.
22. A multiple channel, series-configured, switching circuit
comprising a plurality of MEMS capacitor and spiral inductor pairs,
wherein in each said pair, a first end of each said spiral inductor
is connected to a bridge post of each said MEMS capacitor, wherein
down electrodes of all said MEMS capacitors are connected to a
single input signal, wherein a second end of each said spiral
inductor forms a plurality of output signals, and wherein
activation of any said MEMS capacitor in any said pair causes said
input signal to flow to said output signal for said pair.
23. The circuit according to claim 22 wherein each said MEMS
capacitor has an up electrode.
24. A method to form a microelectronic mechanical switch device in
the manufacture of an integrated circuit device comprising:
providing a down electrode overlying a substrate with a first
dielectric layer therebetween; providing a second dielectric layer
overlying said down electrode layer; forming an up electrode
overlying said second dielectric layer; depositing a first silicon
dioxide layer overlying said up electrode and said second
dielectric layer; depositing a silicon nitride layer overlying said
first silicon oxide layer; depositing a second silicon dioxide
layer overlying said silicon nitride layer; patterning said first
silicon dioxide layer, said silicon nitride layer, said first
silicon dioxide layer, and said second dielectric layer to form
deep trenches for planned bridge posts; patterning said second
silicon dioxide layer and said silicon nitride layer to form
shallow trenches for planned membrane wherein said second silicon
dioxide layer and said silicon nitride layer are etched through to
said first silicon dioxide layer and wherein said shallow trenches
connect to said deep trenches; depositing a metal layer overlying
said second silicon dioxide layer, said silicon nitride layer, said
first silicon dioxide layer, and said second dielectric layer to
fill said deep trenches and said shallow trenches; polishing down
said metal layer to said second silicon dioxide layer to complete
said bridge posts and said membrane; and etching away said second
silicon dioxide layer, said silicon nitride layer and said first
silicon dioxide layer to release said membrane and to complete said
microelectronic mechanical switch device in the manufacture of said
integrated circuit device.
25. The method according to claim 24 wherein said first silicon
dioxide layer is deposited to a thickness of between about 5,000
Angstroms and 45,000 Angstroms.
26. The method according to claim 24 wherein said silicon nitride
layer is deposited to a thickness of between about 300 Angstroms
and 2,000 Angstroms.
27. The method according to claim 24 wherein said polishing down is
by a chemical mechanical polish.
28. The method according to claim 24 wherein said membrane has a
thickness of between about 5,000 Angstroms and 25,000
Angstroms.
29. The method according to claim 24 wherein said step of
patterning said second silicon dioxide layer, said silicon nitride
layer, said first silicon dioxide layer, and said second dielectric
layer to form deep trenches for planned bridge posts further
comprises forming deep trenches for planned microstrip lines,
wherein said step of depositing a metal layer further comprises
filling said deep trenches for planned microstrip lines, and
wherein said step of polishing down said metal layer further
comprises completing said microstrip lines.
30. The method according to claim 24 wherein said step of
patterning said second silicon dioxide layer, said silicon nitride
layer, said first silicon dioxide layer, and said second dielectric
layer to form deep trenches for planned bridge posts further
comprises forming deep trenches for planned spiral inductors,
wherein said step of depositing a metal layer further comprises
filling said deep trenches for planned spiral inductors, and
wherein said step of polishing down said metal layer further
comprises completing said spiral inductors.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The invention relates to a microelectronic mechanical switch
(MEMS) device and method of fabrication, and more particularly, to
an inductor-capacitor resonant radio frequency (LCR RF) switch and
a method of fabricating a LCR RF switch.
[0003] (2) Description of the Prior Art
[0004] Generally, RF switches, consisting of solid state devices,
such as diodes and field-effect transistors (FET), are used in
communication systems applications. For very high frequencies of
about 1 GHz, these diode and FET devices are typically fabricated
using expensive GaAs technology. However, RF switches fabricated
using diodes and FET devices demonstrate high insertion loss and
low isolation when the working frequency exceeds 1 GHz. In
addition, the value of the isolation decreases with frequency.
[0005] Recently, microelectronic mechanical (MEMS) technology has
been used for the fabrication of RF switches. A MEMS switch
features significant advantages in its small size as measured in
the operating wavelength. MEMS has potentially lower costs since IC
batch processing can be used. As an example, a microelectronic
mechanical switch (MEMS) may be constructed which uses
electrostatic force to flex a thin membrane and thereby cause the
switch to be opened or closed. Such devices are fabricated with
dimensions in the range of 100's of microns and can be integrated
onto an integrated circuit device. Since an electrostatic force is
used, the switch can be controlled using only a voltage and very
little, or no, current. Therefore, it consumes virtually no power.
This is an important advantage for portable communication systems,
such as hand-held mobile phones or other wireless communication
devices, where power consumption is recognized as a significant
operating limitation.
[0006] Referring now to FIG. 1, an MEMS device is illustrated in
cross section. There is shown, in highly simplified form, a MEMS
switch over which the present invention is an improvement. It is to
be understood in this regard that no portion of FIG. 1 is admitted
to be prior art as to the present invention. Rather, this highly
simplified diagram is provided in an effort to provide an improved
understanding of the problems which are overcome by the
invention.
[0007] In this example, the device is fabricated on a substrate 10.
An insulating layer 12 overlies the substrate 10 to isolate the
switch from the substrate 10. A metal microstrip 14 overlies the
dielectric layer 12. The metal microstrip 14 may be designed to
carry a microwave signal, for example. A dielectric layer 18
overlies the metal microstrip 14. A bridge structure is formed by
the combination of the bridge posts 22 and the membrane 26. The
bridge posts 22 are formed straddling the metal microstrip 14. The
membrane 26 is fixed to the bridge posts 22 at each end. The bridge
posts 22 and membrane 26 may comprise metallic materials. The
membrane 26 is very thin such that an electrostatic force can cause
it to flex. The distance between the membrane 26 and the dielectric
layer 18 is an air gap.
[0008] This MEMS device has two states of operation. In the UP
state, the membrane 26 is suspended above the dielectric layer 18
as shown. In this state, there is very little capacitive coupling
between the bridge structure and the metal microstrip 14. At
microwave frequencies, the small capacitor between the bridge
structure 22 and 26 and the metal microstrip 14 forms a large
impedance value. Therefore, very little of the microwave energy is
transferred into the bridge structure 22 and 26.
[0009] Referring now to FIG. 2, the DOWN state of operation of the
MEMS device is shown. If a sufficiently large, DC bias voltage
exists between the membrane 26 and the metal microstrip 14, the
electrostatic force will cause the thin membrane 26 to flex toward
the microstrip 14. At maximum deflection, the membrane 26 contacts
the dielectric layer 18 as shown. In this state, the capacitive
coupling between the microstrip 14 and the bridge structure 22 and
26 is much higher than in the non-flexed state. The large
capacitance forms a much smaller impedance value for the microwave
signal. Therefore, much of the microwave energy is conducted into
the bridge structure 22 and 26.
[0010] As can be seen, the MEMS device functions as a variable
capacitor on the microstrip 14 node of the circuit. When the
membrane is in the UP state, the switch is OFF. The signal flowing
on the microstrip 14 continues to flow along the microstrip 14.
When the membrane is down, due to the DC bias, the switch is ON.
The signal is redirected through the capacitor and into the bridge
membrane 26 and posts 22.
[0011] The figure of merit for the MEMS device is the ratio of the
insertion loss in the DOWN state and the isolation during the UP
state. The MEMS exhibits very low insertion loss and very high
isolation. The resonant frequency of the MEMS device determines the
particular frequency at which the high isolation can be achieved.
The resonant frequency depends upon the capacitance in the DOWN
state and the small inductance of the bridge structure. Note that
the area of the capacitor formed between the membrane 26 and the
microstrip 14 in the DOWN state is proportional to the area of the
bridge contacting the dielectric layer 18, which is, in turn,
proportional to the contact length L1.
[0012] Referring now to FIG. 3, an equivalent circuit model for the
MEMS device is shown. In this model, the MEMS device is configured
as a shunt switch. The bridge posts are connected to ground. The
microstrip is modeled as the lumped impedance elements Z.sub.0 48.
The MEMS bridge is modeled as a variable capacitor C.sub.b 52, a
series inductance L.sub.b 56 and a series resistance R.sub.s 60.
The variable capacitor C.sub.b 52 represents the aforementioned
variable capacitive coupling due the deflection of the membrane.
The series inductance L.sub.b 56 and series resistance R.sub.s 60
are due to the physical characteristics of the membrane and bridge
posts. When the MEMS switch is in the UP state, C.sub.b 52 is
small, and most of the microwave energy is conducted past the
switch. When the MEMS switch is in the DOWN state, C.sub.b 52 is
large, and most of the microwave energy is conducted through the
switch to ground.
[0013] Note that, in the DOWN state, the series capacitance C.sub.b
52 and the series inductance L.sub.b 56 result in a series resonant
frequency given by:
.omega.=1/(L.sub.bC.sub.b).sup.1/2.
[0014] Typically, the MEMS device can be optimized for useful
operating frequencies of greater than about 5 GHz. However, for
frequency bands below 5 GHz, this MEMS device exhibits too low of
an isolation. This is because the bridge inductance L.sub.b is
usually very small and is not adjustable.
[0015] Finally, the fabrication technique for this MEMS capacitor
RF switch is difficult to control. One fabrication technique is to
spin on a photoresist layer prior to the deposition of the thin
membrane layer. The photoresist layer is then removed to form the
deflection gap. Unfortunately, it is very difficult to uniformly
control the thickness of spun on photoresist. The yield of
qualified MEMS devices in a wafer will therefore be limited.
[0016] Several prior art approaches disclose MEMS devices and
methods to form MEMS devices. Z. J. Yao et al, "Micromachined
Low-Loss Microwave Switches," IEEE Journal of
Microelectromechanical Systems, Vol. 8, No. 2, June 1999, pp.
129-134, discloses an MEMS device for microwave applications. A
capacitively-coupled switch is formed where a dielectric layer
separates a bottom electrode from a suspended membrane. J. B.
Muldavin et al, "High-Isolation Inductively-Tuned X-Band MEMS Shunt
Switches," 2000 IEEE MTT-S International Symposium Digest, June
2000, pp. 169-172, discloses an inductively-tuned MEMS device.
Straight transmission lines are used to add inductance to the
shunt-configured, MEMS switch circuit between the bridge and
ground. U.S. Pat. No. 5,619,061 to Goldsmith et al teaches various
configurations of micromechanical microwave switches. Dielectric,
metallic, and combination membranes are disclosed. Both direct
coupling and capacitive coupling are taught. U.S. Pat. No.
6,069,540 to Berenz et al discloses a MEMS device with a pivot pin
structure. A rigid beam is used to improve reliability. U.S. Pat.
No. 5,880,921 to Tham et al teaches a switched capacitor bank
formed using MEMS technology. U.S. Pat. No. 6,074,890 to Yao et al
discloses a MEMS device where the motion of a signal device is
coupled to the motion of a slave device. The preferred fabrication
uses a backside dry etch to release the suspended MEMS devices and
mechanical couplers. U.S. Pat. No. 6,020,564 to Wang et al teaches
a MEMS device where the actuation voltage is reduced by leveraging
a small actuating beam movement into a large longitudinal beam
movement. U.S. Pat. No. 5,578,976 to Yao teaches a MEMS device with
a cantilevered beam fabricated from dielectric material.
SUMMARY OF THE INVENTION
[0017] A principal object of the present invention is to provide an
effective and very manufacturable MEMS-based inductor-capacitor
resonant RF switch and method of fabrication.
[0018] A further object of the present invention is to provide a
MEMS-based inductor-capacitor resonant RF switch by combining a
MEMS variable capacitor and a spiral inductor.
[0019] A yet further object of the present invention is to combine
a MEMS variable capacitor and a spiral inductor to thereby create
an inductor-capacitor resonant RF switch with improved operating
characteristics below 5 GHz.
[0020] Another further object of the present invention is to
improve the operating characteristics of a MEMS variable capacitor
by adding an upper electrode to increase capacitive coupling
between the membrane and the microstrip.
[0021] Another further object of the present invention is to apply
the inductor-capacitor resonant RF switch in a shunting
configuration.
[0022] Another further object of the present invention is to apply
the inductor-capacitor resonant RF switch in a series
configuration.
[0023] Another further object of the present invention is to apply
the inductor-capacitor resonant RF switch in a multiple-channel,
series configuration.
[0024] Another further object of the present invention is to
provide a method of fabricating a microelectronic mechanical switch
device with improved uniformity.
[0025] Another yet further object of the present invention is to
provide a method of fabricating a microelectronic mechanical switch
device using a dual damascene process.
[0026] In accordance with the objects of this invention, a new
inductor-capacitor resonance RF switching device is achieved. The
device comprises a microelectronic mechanical switch and a spiral
inductor. The microelectronic mechanical switch comprises, first, a
first dielectric layer overlying a substrate. A down electrode
overlies the first dielectric layer. A second dielectric layer
overlies the down electrode. An up electrode may overlie the down
electrode with the second dielectric layer therebetween. A bridge
post overlies the first dielectric layer and does not contact the
down electrode or the up electrode. Multiple bridge posts may be
used. Finally, a membrane is suspended over said down electrode. At
least one end of the membrane is fixed to the top of a bridge post.
An electrostatic potential between the membrane and the down
electrode will cause the membrane to flex down toward the down
electrode. This flexing of the membrane will cause the capacitance
of the switching device to vary. The spiral inductor comprises a
metal line configured in a spiraling pattern with a first end
connected to a bridge post and a second end forming an output
node.
[0027] Also in accordance with the objects of this invention, a new
multiple channel, series configured LCR-RF switching circuit is
achieved. The circuit comprises a plurality of MEMS capacitor and
spiral inductor pairs. In each pair, a first end of each spiral
inductor is connected to a bridge post of each MEMS capacitor. The
down electrodes of all the MEMS capacitors are connected to a
single input signal. A second end of each spiral inductor forms a
plurality of output signals. Activation of any MEMS capacitor in
any pair causes the input signal to flow to the output signal for
the pair.
[0028] Also in accordance with the objects of this invention, a new
method to fabricate a microelectronic mechanical switch device is
achieved. A down electrode is provided overlying a substrate with a
first dielectric layer therebetween. A second dielectric layer is
provided overlying the down electrode layer. A first silicon
dioxide layer is deposited overlying the second dielectric layer. A
silicon nitride layer is deposited overlying the first silicon
dioxide layer. A second silicon dioxide layer is deposited
overlying the silicon nitride layer. The second silicon dioxide
layer, the silicon nitride layer, the first silicon dioxide layer,
and the second dielectric layer are patterned to form deep trenches
for planned bridge posts. The second silicon dioxide layer and the
silicon nitride layer are patterned to form shallow trenches for
the planned membrane. The shallow trenches connect to the deep
trenches. A metal layer is deposited overlying the second silicon
dioxide layer, the silicon nitride layer, the first silicon dioxide
layer, and the second dielectric layer to fill the deep trenches
and the shallow trenches. The metal layer is polished down to the
silicon nitride layer to complete the bridge posts and the
membrane. The second silicon dioxide layer, the silicon nitride
layer and the first silicon dioxide layer are etched away to
release the membrane and to complete the microelectronic mechanical
switch device in the manufacture of the integrated circuit
device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] In the accompanying drawings forming a material part of this
description, there is shown:
[0030] FIGS. 1 through 3 schematically illustrate a prior art MEMS
device.
[0031] FIGS. 4 and 5 schematically illustrate, in cross sectional
representation, a first preferred embodiment of the MEMS capacitor
of the present invention.
[0032] FIG. 6 illustrates in top view a first preferred embodiment
of the LCR-RF switch of the present invention.
[0033] FIG. 7 illustrates schematically the equivalent circuit
model of LCR-RF device of the present invention applied in a shunt
configuration.
[0034] FIG. 8 illustrates schematically the equivalent circuit
model of LCR-RF device of the present invention applied in a series
configuration.
[0035] FIG. 9 illustrates schematically the equivalent circuit
model of a plurality of LCR-RF devices of the present invention
applied in a multiple channel, series configuration.
[0036] FIGS. 10 and 11 illustrates a second preferred embodiment of
the LCR-RF switch of the present invention wherein a single bridge
post is used.
[0037] FIG. 12 illustrates a top view of the MEMS capacitor of the
LCR-RF switch of the present invention wherein alternative cross
sections are labeled.
[0038] FIG. 13 illustrates a general cross sectional representation
of the MEMS capacitor at the first stage of the preferred
embodiment of the method of fabrication of the present
invention.
[0039] FIGS. 14a, 15a, 16a, 17a, 18a, 19a, 20a, and 21a illustrate
the A-A cross section of the MEMS device at stages in the preferred
method of fabrication of the present invention.
[0040] FIGS. 16b, 18b, 20b, and 21b illustrate the B-B cross
section of the MEMS device at stages in the preferred method of
fabrication of the present invention.
[0041] FIGS. 16c, 18c, 20c, and 21c illustrate the C-C cross
section of the MEMS device at stages in the preferred method of
fabrication of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] The LCR-RF switch device of the present invention is applied
in a shunt configuration, a series configuration, and a multiple
channel, series configuration. Two preferred embodiments of the
device are disclosed. In the first preferred embodiment, a bridge
structure with two bridge posts is described. In the second
embodiment, a single post bridge structure is described. A
preferred embodiment of the method of forming the LCR-RF switch is
described. It should be clear to those experienced in the art that
the present invention can be applied and extended without deviating
from the scope of the present invention.
[0043] The LCR-RF switch of the present invention comprises two
components: a novel MEMS capacitor device and a spiral inductor.
The novel combination of the MEMS capacitor device and the inductor
facilitates the formation of an LCR-RF switch with improved
frequency response characteristics. The unique features of the
device of the present invention, in its two embodiments, will be
described. The unique features of the method of fabricating the
device will be described in a section following.
[0044] Referring now more particularly to FIG. 4, there is
illustrated a cross-section of a partially completed integrated
circuit device of the first preferred embodiment. More
particularly, a cross section of the first embodiment of the unique
MEMS capacitor device of the LCRRF switch is illustrated. Several
important features of the present invention are shown. The MEMS
capacitor is a switching device that uses the membrane flexing
principle of operation as outlined in the prior art device. A
substrate 80 may comprise a semiconductor wafer and may further
comprise monocrystalline silicon. This substrate 80 may also
further comprise a thick dielectric material, such as silicon
dioxide, or a low-k material. For simplicity of illustration, the
substrate 80 is shown as a monolithic layer. In practice, a
plurality of active and passive devices, layers, and connective
lines may be formed in the substrate 80 prior to the formation of
the LCR-RF device.
[0045] A first dielectric layer 84 overlies the substrate 80. The
first dielectric layer 84 electrically isolates the substrate from
the rest of the MEMS structure. A down electrode 92 overlies the
first dielectric layer 84. The down electrode 92 acts as the signal
input line for the LCR-RF device. The down electrode 92 preferably
comprises a metal layer. An interlevel dielectric layer 88 overlies
the first dielectric layer 84 and isolates the down electrode 92.
This configuration of the interlevel dielectric layer 88
facilitates the formation of the down electrode 92 by a damascene
process. The dielectric layers 84 and 88 are chosen to have
different etching rates with respect to each other. Dielectric
layers 84 and 88 preferably comprise silicon nitride and silicon
dioxide, respectively. A second dielectric layer 96 overlies the
down electrode 92. The second dielectric layer 96 is the capacitive
coupling dielectric for the MEMS capacitor. The second dielectric
layer 96 preferably comprises silicon nitride that has a thickness
of between about 300 Angstroms and 2,000 Angstroms.
[0046] A particularly important feature is the up electrode 100.
The up electrode 100 overlies the down electrode 92 with the second
dielectric layer 96 therebetween. The up electrode 100 provides a
conductive plate to increase the effective capacitance of the MEMS
capacitor in the flexed state. Further, the capacitance of the MEMS
capacitor in the flexed state can be carefully designed and
controlled by the area of the up electrode 100 to thereby cause
resonance at a specific frequency. The area of the up electrode is
proportional to length L2. Finally, the up electrode 100 preferably
comprises a metal layer.
[0047] Another important feature of the present invention is the
bridge structure 120. In the bridge structure 120 of the present
invention, a single metal layer 120 forms the two bridge posts 118
and the membrane 122. The novel bridge structure 120 may be formed
using a dual damascene process as will be described in the method
of fabrication. In addition, the thin membrane 122 has a preferred
thickness of between about 5,000 Angstroms and 25,000
Angstroms.
[0048] If the voltage potential between the down electrode 92 and
the membrane 122 is less than the minimum activation voltage, then
the MEMS capacitor device is in the UP state as shown in FIG. 4. In
the UP state, the capacitive coupling between the membrane 22 and
the down electrode 92 is at a minimum due to the large air gap 123
between the membrane 122 and the up electrode 100. At microwave
frequencies, the small capacitance between the bridge structure 120
and the down electrode forms a large impedance value. Therefore,
very little of the microwave energy is transferred into the bridge
structure 120 from the down electrode.
[0049] Referring now to FIG. 5, the DOWN state of the MEMS
capacitor is shown. To achieve DOWN state, a sufficiently large DC
bias is created between the down electrode 92 and the membrane 122.
This DC bias may be applied in addition to a high frequency signal
flowing on the down electrode 92. The DC voltage causes the thin
membrane 122 to flex toward the down electrode 92 due to
electrostatic force.
[0050] At full flexure, the membrane 122 contacts the up electrode
100. This is a particularly important feature of the present
invention. As can be seen, the contacting surface area between the
membrane 122 and the up electrode 100 is rather small. This was
also true for the MEMS capacitor of FIG. 2. However, the unique up
electrode 100 of the present invention creates a large top plate to
greatly increase the length L2, and, therefore, the area of
capacitive coupling between the membrane 122 and the down electrode
92. The DOWN state capacitance of the MEMS capacitor of the present
invention is therefore much greater than that of a comparable
device of FIG. 2.
[0051] Referring now to FIG. 6, a top view of the LCR-RF switch
device is illustrated. The LCR-RF switch in the present invention
is formed by the novel combination of the MEMS capacitor of FIG. 4
with a spiral inductor. In this illustration, the LCR-RF switch is
in a shunt configuration. The membrane 122, supported on the posts
118, crosses over the up electrode 100. The down electrode, not
shown, underlies the up electrode 100. The microstrip line 125 is
connected to the down electrode. The microstrip line 125 carries
the high frequency signal.
[0052] One of the posts 118 is connected to the spiral inductor
124. The spiral inductor 124 comprises a metal line configured in a
spiraling pattern with a first end connected to the bridge post 118
and a second end forming an output node. In this case, the second
end of the spiral inductor is connected to ground 128.
[0053] When the membrane is the UP state, the signal flowing
through the microstrip 125 is unaffected by the switch due to the
minor capacitive coupling of the UP state. However, in the DOWN
state, the increased capacitive coupling channels the microwave
signal into the membrane 122 and the bridge posts 118. The signal
flows through the spiral inductor 124 to ground 128. The switch
effectively shunts the signal to ground. In the preferred
embodiment, the DOWN state capacitance varies between about 10 fF
and 30 pF. In the preferred embodiment, the spiral inductor value
is between about 1 nH and 30 nH.
[0054] Referring now to FIG. 7, an equivalent circuit model of the
LCR-RF switch of the present invention configured as a shunt
circuit is illustrated. The microstrip is modeled as two lumped
impedance elements Z.sub.0 148. The MEMS capacitor is modeled as a
variable capacitor C.sub.b 152 in series with a bridge inductor
L.sub.b 156 and a bridge resistance R.sub.b 160. The spiral
inductor is modeled as an inductor Ls 164 and an inductor
resistance R.sub.s 168. The first end of the spiral inductor is
tied to the MEMS capacitor bridge post, and the second end of the
spiral inductor is tied to ground 144.
[0055] The novel features of the present invention LCR-RF switch
are demonstrated by the model. First, the MEMS capacitor achieves a
higher DOWN state capacitance C.sub.b 152 due to the up electrode.
Second, the spiral inductor adds a much larger inductor L.sub.s 164
to the shunting path. Therefore, the resonant frequency of the
LCR-RF switch can be reduced to below the 5 GHz range. This makes
the LCR-RF switch extremely useful for communications applications.
Further, the inductor L.sub.s 164 can be carefully designed to a
specific size and tightly controlled in the fabrication process.
Therefore, the optimum resonant frequency can be targeted
precisely. The preferred resonant frequency in DOWN state of
between about 1 GHz and 100 GHz.
[0056] Referring now to FIG. 8, an equivalent circuit model of the
LCR-RF switch of the present invention configured as a series
circuit is illustrated. In this case, only a first segment of the
microstrip line is tied the MEMS capacitor. A second microstrip
line 190 is tied to the output of the spiral inductor. The
microstrips are modeled as two lumped impedance elements Z.sub.0
188 and 190. The MEMS capacitor is modeled as a variable capacitor
C.sub.b 192 in series with a bridge inductor L.sub.b 196 and a
bridge resistance R.sub.b 200. The spiral inductor is modeled as an
inductor L.sub.s 204 and an inductor resistance R.sub.s 208. The
first end of the spiral inductor is again tied to the MEMS
capacitor bridge post. However, the second end of the spiral
inductor is tied to the second segment of microstrip 190.
[0057] In the series configuration, the LCR-RF switch permits the
signal to pass from the first segment 188 to the second segment 190
only when the MEMS capacitor is in the DOWN state. In the UP state,
the small capacitive coupling of the MEMS capacitor does not permit
the signal to flow.
[0058] Referring now to FIG. 9, an equivalent circuit model of a
plurality of LCR-RF switches of the present invention configured in
a multiple channel, series circuit is illustrated. A separate
LCR-RF switch is used for each channel of a selector circuit of
size "n." The input segment of the microstrip line 188 is tied all
of the MEMS capacitors. The output microstrip lines 260, 261, 262,
and 263 are tied to the outputs of separate spiral inductors. The
microstrips are modeled as lumped impedance elements Z.sub.0 188,
260, 261, 262, and 263. The first channel MEMS capacitor is modeled
as a variable capacitor C.sub.b1 192 in series with a bridge
inductor L.sub.b1 196 and a bridge resistance R.sub.b1 200. The
first channel spiral inductor is modeled as an inductor L.sub.s1
204 and an inductor resistance R.sub.s1 208. The second channel
MEMS capacitor is modeled as a variable capacitor C.sub.b2 212 in
series with a bridge inductor L.sub.b2 216 and a bridge resistance
R.sub.b2 220. The second channel spiral inductor is modeled as an
inductor L.sub.s2 224 and an inductor resistance R.sub.s2 228. The
"nth" channel MEMS capacitor is modeled as a variable capacitor
C.sub.bn 232 in series with a bridge inductor L.sub.bn 236 and a
bridge resistance R.sub.bn 240. The "nth" channel spiral inductor
is modeled as an inductor L.sub.sn 244 and an inductor resistance
R.sub.sn 248.
[0059] In the multiple channel, series configuration, the LCR-RF
switches permit the input signal to pass through to any output
segment when the MEMS capacitor for that channel is in the DOWN
state. In the UP state, the small capacitive coupling of the MEMS
capacitors do not permit the signal to flow to that channel.
[0060] Referring now to FIG. 10, a second preferred embodiment of
the present invention is illustrated. In this embodiment, the
membrane 441 is supported upon a single bridge post 442. Once
again, the device is fabricated overlying a substrate 400. A first
dielectric layer 404, preferably comprising silicon nitride,
overlies the substrate 400. A down electrode 412 overlies the first
dielectric layer 404. The down electrode 412 preferably comprises a
metal layer. The down electrode 412 may additionally comprise a
microstrip line useful for carrying a microwave signal. An
interlevel dielectric layer 408 overlies the first dielectric layer
404. The interlevel dielectric layer 408 facilitates the formation
of the down electrode 412 using a damascene process. A second
dielectric layer 416 overlies the down electrode 412. The second
dielectric layer 416 preferably comprises silicon nitride. An
important feature is the up electrode 420. The up electrode 420
overlies the down electrode 412 with the second dielectric layer
416 therebetween.
[0061] Another important feature of this embodiment is the bridge
structure 440. In the bridge structure 441 of the present
invention, a single metal layer 440 forms the single bridge post
442 and the membrane 441. The novel bridge structure 440 may be
formed using a dual damascene process as will be described in the
method of fabrication. In addition, the thin membrane 441 has a
preferred thickness of between about 5,000 Angstroms and 25,000
Angstroms.
[0062] The theory of operation of the single post device is the
same as for the two post device of FIGS. 5 and 6. If the voltage
potential between the down electrode 412 and the membrane 441 is
less than the minimum activation voltage, then the MEMS capacitor
device is in the UP state. In the UP state, the capacitive coupling
between the membrane 441 and the down electrode 412 is at a minimum
due to the large air gap 443 between the membrane 441 and the up
electrode 420. At microwave frequencies, the small capacitance
between the bridge structure 440 and the down electrode 412 forms a
large impedance value. Therefore, very little of microwave energy
is transferred into the bridge structure 440 from the down
electrode 412. A large DC bias voltage between the down electrode
412 and the membrane 441 will cause the membrane 441 to flex toward
the down electrode 412 due to electrostatic force. The membrane 441
will contact the up electrode 420 at full flexure. The up electrode
420 provides a large top plate for the membrane-to-down electrode
capacitor to thereby increase the DOWN state capacitance.
[0063] Referring now to FIG. 11, the top view of the second
preferred embodiment MEMS capacitor of the LCR-RF switch is shown.
The membrane 441 is supported by the post 442 such that the
membrane is suspended over the up electrode 420. The down electrode
412 and microstrip are combined.
[0064] Referring now to FIGS. 12 through 21c, the preferred method
of fabricating the LCR-RF switch of the present invention is
illustrated. More particularly, a method to form the MEMS capacitor
of the LCR-RF switch is described. The spiral inductor may be
fabricated using any patterned metal layer technique, including
those illustrated in this embodiment. A dual damascene method for
forming the bridge structure, comprising posts and a membrane, is
achieved. The damascene process is also applied to the formation of
the down electrode and up electrode. However, this is not a crucial
aspect of the fabrication method.
[0065] Referring particularly to FIG. 12, a top view of the MEMS
capacitor of the LCR-RF switch of the present invention is
illustrated. The bridge structure comprises two posts 542
supporting a membrane 541. The membrane 541 is suspended over the
up electrode 524. The down electrode, not shown, underlies the up
electrode 524 with a dielectric layer therebetween. A metal
microstrip line 543 connects to the down electrode. In FIG. 12,
three alternative cross sections are labeled as A-A, B-B, and C-C.
In the figures that follow, those figures that bear the suffix "a,"
such as FIG. 16a, show cross section A-A, those bearing "b" show
cross section B-B, and those bearing "c" show cross section
C-C.
[0066] Referring particularly to FIG. 13, a general cross-section
is illustrated. A substrate 500 is provided. Preferably, the
substrate 500 comprises a semiconductor material, such as
monocrystalline silicon. This substrate is preferably covered with
a thick dielectric material and, more preferably, with a silicon
dioxide or low-k material. The fabrication method is intended to
facilitate integration of the LCR-RF switch onto a ultra-large
scale integration (ULSI) circuit. For simplicity, substrate 500 is
shown as a single layer. In practice, the substrate 500 may contain
a plurality of layers, devices, and connective lines formed prior
to the beginning of the fabrication method of the present
invention.
[0067] A first dielectric layer 504 is deposited overlying the
substrate 500. The first dielectric layer 504 is used to separate
and electrically isolate the subsequently formed down electrode
from the substrate 500. The first dielectric layer 504 preferably
comprises silicon nitride. A first interlevel dielectric layer 508
is deposited overlying the first dielectric layer 504. The first
interlevel dielectric layer 508 laterally isolates down electrodes.
In the preferred embodiment, the first interlevel dielectric layer
508 comprises silicon dioxide.
[0068] Referring now to FIG. 14a, cross section A-A is illustrated
for the next step. The first interlevel dielectric layer 508 is
patterned to form trenches for the down electrodes. By choosing
silicon nitride for the first dielectric layer 504 and silicon
dioxide for the first interlevel dielectric layer 508, a selective
etching process may be used. The first dielectric layer 508 can act
as an etching stop.
[0069] Referring now to FIG. 15a, a down electrode metal layer 512
is deposited overlying the first interlevel dielectric layer 508
and filling the trenches. The down electrode metal layer 512 is
then polished down to the top surface of the first interlevel
dielectric layer 508 to thereby complete the down electrodes 512.
The down electrode metal layer 512 preferably comprises copper.
[0070] Referring now to FIG. 16a, the up electrode 524 overlies the
down electrode 512 with the second dielectric layer 516
therebetween. The second dielectric layer 516 is deposited
overlying the first dielectric layer 508 and the down electrode
512. The second dielectric layer 516 preferably comprises silicon
nitride. A metal layer 524 is then deposited overlying the second
dielectric layer 516. This metal layer 524 is then patterned, or
etched, to form the up electrode 524. The up electrode 524
preferably comprises Al, Ta, Ti, or W.
[0071] Referring now to FIG. 16b, the B-B cross section is
illustrated. Note that the down electrode 512 is present but the up
electrode 524 is not present.
[0072] Referring now to FIG. 16c, the C-C cross section is
illustrated. Note that the down electrode 512 is present but the up
electrode 524 is not present.
[0073] Referring now to FIG. 17a, important features of the present
invention are illustrated. As a first important feature, a first
silicon dioxide layer 528 is deposited overlying the up electrode
524 and the second dielectric layer 516. The first silicon dioxide
layer 528 establishes the height of the air space between the
subsequently formed membrane and the up electrode 524. The first
silicon dioxide layer 528 is preferably deposited by chemical vapor
deposition (CVD) to a thickness of between about 5,000 Angstroms
and 45,000 Angstroms. The use of a removable first silicon dioxide
layer 528 is advantageous because it can be deposited to a very
controlled thickness across the wafer and results in uniform die
yield.
[0074] As a second important feature, a silicon nitride layer 532
is deposited overlying the first silicon dioxide layer 528. The
silicon nitride layer 532 will provide a stop layer for etching the
subsequently deposited second silicon dioxide layer 534. The
silicon nitride layer 532 is preferably deposited by a CVD process
to a thickness of between about 300 Angstroms and 2,000
Angstroms.
[0075] As a third important feature, a second silicon dioxide layer
534 is deposited overlying the silicon nitride layer 532. The
second silicon dioxide layer 534 will establish the thickness of
the subsequently formed membrane. The second silicon dioxide layer
534 is preferably deposited by a CVD process to a thickness of
between about 5,000 Angstroms and 45,000 Angstroms.
[0076] Referring now to FIG. 18a, an important feature of the
present invention is illustrated. The second silicon dioxide layer
534, the silicon nitride layer 532, the first silicon dioxide layer
528, and the second dielectric layer 516 are patterned to form deep
trenches for the planned bridge posts. A photolithography and etch
sequence may be used to pattern the trenches. For example, a
photoresist layer, not shown, is deposited overlying the second
silicon dioxide layer 534 and then exposed to actinic light through
a mask. The photoresist layer is then developed to remove a part of
the photoresist layer. A dry etch may then be performed to remove
the second silicon dioxide layer 534, the silicon nitride layer
532, the first silicon dioxide layer 528, and the second dielectric
layer where exposed by the photoresist. The remaining photoresist
is then stripped away. Note that the silicon nitride layer 532 and
the second dielectric layer 516 may act etching stops for the
second silicon dioxide layer 534 and the first silicon dioxide
layer 528, respectively. Finally, it is important to note that
these deep trenches may also comprise trenches for planned
microstrip lines or spiral inductor lines.
[0077] Referring now to FIG. 18b, the B-B cross section is
illustrated for this stage. Note that the bridge post trenches are
present.
[0078] Referring now to FIG. 18c, the C-C cross section is
illustrated for this stage. Note first that the bridge post
trenches are also present. In addition, note that a trench 539 for
a planned microstrip has been etched during the bridge post
patterning step. This trench 539 will effectively extend the
microstrip layer 512.
[0079] Referring now to FIG. 19a, an important feature of the
present invention is illustrated. The second silicon dioxide layer
534 and the silicon nitride layer 532 are patterned to form shallow
trenches for the planned membrane. The silicon nitride layer 532
may be used as an etching stop for the dry etching of the second
silicon dioxide layer 534. The first silicon dioxide layer 528 may
be used as an etching stop for the dry etching of the silicon
nitride layer 532. Note that the second silicon dioxide layer 534
and the silicon nitride layer 532 are only etched where the
membrane is planned. In this way, both the membrane and the bridge
posts may be formed by a damascene technique.
[0080] Referring now to FIG. 20a an important feature of the
present invention is illustrated. A metal layer 540 is deposited
overlying the second silicon dioxide layer 534, the silicon nitride
layer 532, and the first silicon dioxide layer 528, and filling the
trenches. The metal layer 540 fills both the shallow membrane
trenches and the deep bridge post trenches. Further, if microstrip
trenches or if spiral inductor trenches are present, these trenches
are filled as well. The metal layer 540 may comprise, for example,
copper or copper alloy.
[0081] The metal layer 540 is then polished down to the top surface
of the second silicon dioxide layer 534 to form the membrane and
the bridge posts from a common metal layer 540. Preferably, a
chemical mechanical polish is used for the polishing down process.
The method is dual damascene since a single metal deposition fills
both types of trenches. Further, this process can be integrated
with up metal processing for an ULSI device. This polishing down
step also completes any microstrip lines or spiral inductors in the
process.
[0082] Referring now to FIG. 20b, the B-B cross section for this
stage is shown. The bridge posts 540 are completed. Because the
second silicon dioxide layer 534 and the silicon nitride layer 532
were not etched in this area, no membrane is formed.
[0083] Referring now to FIG. 20c, the C-C cross section for this
stage is shown. The bridge posts 540 and the microstrip line 540
are formed by the metal deposition and polish down. No membrane is
formed because the second silicon dioxide layer 534 and the silicon
nitride layer were not patterned in this area.
[0084] Referring now to FIG. 21a, another important feature of the
present invention is illustrated. The second silicon dioxide layer
534, the silicon nitride layer 532 and the first silicon dioxide
layer 528 are etched away to complete the MEMS capacitor device.
This forms a void underlying the membrane 541 and adjacent to the
bridge posts 542. The membrane 541 is freed to flex under
electrostatic force. Further, the up electrode 524 is exposed so
that the membrane 541 and the up electrode 524 may contact at full
flexure. The second silicon dioxide layer 534, the silicon nitride
layer 532 and the first silicon dioxide layer 528 may be removed by
a dry etching process that is isotropic, or non-directional, in
character.
[0085] Referring now to FIG. 21b, the B-B cross section is
illustrated. The removal of the second silicon dioxide layer 534,
the silicon nitride layer 532 and the first silicon dioxide layer
528 leaves the free standing bridge posts 542.
[0086] Referring to FIG. 21c, the C-C cross section is illustrated.
The removal of the second silicon dioxide layer 534, the silicon
nitride layer 532 and the first silicon dioxide layer 542 leaves
the bridge posts 542 and the microstrip 543.
[0087] The advantages of the present invention may now be
summarized. First, a novel LCR-RF device is achieved. This device
combines a MEMS capacitor with a spiral inductor to create a
microwave switch with a precise resonant frequency. Second, the
LCR-RF switch can be designed for operation in the regime between
about 1 GHz and 5 GHz where a larger capacitance and inductance is
needed. Third, the LCR-RF switch can be constructed using a single
post or a double post bridge structure. Fourth, the LCR-RF device
can be configured for shunting or for series operation. Fifth, a
plurality of LCR-RF devices can be combined to form a multiple
channel, series circuit. Sixth, a method for fabricating the LCR-RF
switch is described. The method is compatible with dual damascene
upper metal processing and ULSI processing. The dual damascene
process for forming the bridge posts and membrane of the MEMS
capacitor produces devices with better yield and uniformity than
the prior art.
[0088] As shown in the preferred embodiments, the present invention
provides a very manufacturable LCR-RF device and a method of
fabrication in the manufacture of an integrated circuit device.
[0089] While the invention has been particularly shown and
described with reference to the preferred embodiments thereof, it
will be understood by those skilled in the art that various changes
in form and details may be made without departing from the spirit
and scope of the invention.
* * * * *