U.S. patent application number 09/981014 was filed with the patent office on 2002-02-28 for mems variable capacitor with stabilized electrostatic drive and method therefor.
Invention is credited to Huang, Jenn-Hwa, Parsey, John Michael JR., Xu, Ji-Hai.
Application Number | 20020025595 09/981014 |
Document ID | / |
Family ID | 23974767 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020025595 |
Kind Code |
A1 |
Xu, Ji-Hai ; et al. |
February 28, 2002 |
MEMS variable capacitor with stabilized electrostatic drive and
method therefor
Abstract
A micro electromechanical systems device having variable
capacitance is controllable over the full dynamic range and not
subject to the "snap effect" common in the prior art. The device
features an electrostatic driver (120) having a driver capacitor of
fixed capacitance (121) in series with a second driver capacitor of
variable capacitance (126). A MEMS variable capacitor (130) is
controlled by applying an actuation voltage potential to the
electrostatic driver (120). The electrostatic driver (120) and MEMS
variable capacitor (130) are integrated in a single, monolithic
device.
Inventors: |
Xu, Ji-Hai; (Gilbert,
AZ) ; Huang, Jenn-Hwa; (Gilbert, AZ) ; Parsey,
John Michael JR.; (Phoenix, AZ) |
Correspondence
Address: |
MOTOROLA, INC.
CORPORATE LAW DEPARTMENT - #56-238
3102 NORTH 56TH STREET
PHOENIX
AZ
85018
US
|
Family ID: |
23974767 |
Appl. No.: |
09/981014 |
Filed: |
October 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09981014 |
Oct 16, 2001 |
|
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09496930 |
Feb 2, 2000 |
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Current U.S.
Class: |
438/48 ; 257/414;
257/415; 257/417; 438/50; 438/52 |
Current CPC
Class: |
H01G 5/16 20130101; H01H
59/0009 20130101 |
Class at
Publication: |
438/48 ; 438/50;
438/52; 257/414; 257/415; 257/417 |
International
Class: |
H01L 021/00; H01L
027/14; H01L 029/82; H01L 029/84 |
Claims
1. A Micro Electro-Mechanical system (MEMS) device, comprising: an
electrostatic MEMS driver, comprising: a first driver capacitor
having a fixed capacitance; and a second driver capacitor having a
variable capacitance, wherein the first driver capacitor is in
series with the second driver capacitor; and a MEMS variable
capacitor, wherein the MEMS variable capacitor is controlled by the
electrostatic MEMS driver.
2. The MEMS device of claim 1, wherein the MEMS variable capacitor
comprises: a first plate coupled to a substrate; a portion of a
dielectric membrane; and a second plate coupled to the dielectric
membrane.
3. The MEMS device of claim 2, wherein the MEMS variable capacitor
is controlled by applying an actuation voltage potential to the
electrostatic MEMS driver.
4. The MEMS device of claim 1, wherein the first driver capacitor
comprises: a first electrode coupled to a substrate; and a dual-use
electrode wherein the dual-use electrode is separated from the
first electrode by an isolation layer.
5. The MEMS device of claim 4, wherein the second driver capacitor
comprises: the dual-use electrode; and a second electrode coupled
to the dielectric membrane.
6. The MEMS device of claim 1, wherein the electrostatic MEMS
driver provides a continuous dynamic response of the MEMS variable
capacitor over an effective displacement range of the dielectric
membrane.
7. The MEMS device of claim 6, wherein the continuous dynamic
response for a forward bias is virtually identical to a reverse
bias of the device.
8. A Micro Electro-Mechanical system (MEMS) device, comprising: a
driver capacitor pair, comprising: a first driver capacitor having
a fixed capacitance; and a second driver capacitor having a
variable capacitance, wherein the first driver capacitor is in
series with the second driver capacitor; and a MEMS variable
capacitor, wherein the MEMS variable capacitor is controlled by the
driver capacitor pair.
9. The MEMS device of claim 8, wherein the MEMS variable capacitor
comprises: a lower plate coupled to a substrate; a dielectric
membrane; and an upper plate coupled to the dielectric
membrane.
10. The MEMS device of claim 8, wherein the MEMS variable capacitor
is controlled by applying an actuation voltage potential to the
driver capacitor pair.
11. The MEMS device of claim 8, wherein the first driver capacitor
comprises: a lower electrode coupled to a substrate; and a dual-use
electrode, wherein the dual-use electrode is separated from the
lower electrode by an isolation layer.
12. The MEMS device of claim 11, wherein the second driver
capacitor comprises: the dual-use electrode; and an upper electrode
coupled to the dielectric membrane.
13. The MEMS device of claim 8, wherein the driver capacitor pair
provides a continuous dynamic response of the MEMS variable
capacitor over an effective displacement range of the dielectric
membrane.
14. The MEMS device of claim 13, wherein the continuous dynamic
response for a forward bias is virtually identical to a reverse
bias of the device.
15. The MEMS device of claim 8, wherein the device further
comprises a second driver capacitor pair, comprising: a third
driver capacitor having a fixed capacitance; and a fourth driver
capacitor having a variable capacitance, wherein the third
capacitor is in series with the fourth capacitor.
16. The MEMS device of claim 15, wherein the second driver
capacitor pair is in parallel with the first driver capacitor
pair.
17. A micro electro-mechanical system device, comprising: a
substrate; an isolation layer fabricated over the substrate; a MEMS
variable capacitor, comprising: a lower plate coupled to the
substrate; a dielectric membrane; and an upper plate coupled to the
dielectric membrane; a first driver capacitor having a fixed
capacitance comprising: a lower electrode coupled to the substrate;
and a dual-use electrode wherein the dual-use electrode is
separated from the lower electrode by the isolation layer; and a
second driver capacitor having a variable capacitance, in series
with the first driver capacitor, the second driver capacitor
comprising: the dual-use plate; and an upper electrode coupled to
the dielectric membrane; wherein the MEMS variable capacitor, the
first driver capacitor and the second driver capacitor are
fabricated on a monolithic, integrated device.
18. The device of claim 17, further comprising: a third driver
capacitor having a fixed capacitance comprising: a second lower
electrode coupled to the substrate; and a second dual-use electrode
wherein the second dual-use electrode is separated from the second
lower electrode by the isolation layer; and a fourth driver
capacitor having a variable capacitance, in series with the third
driver capacitor, the fourth driver capacitor comprising: the
second dual-use plate; and a second upper electrode coupled to the
dielectric membrane.
19. A method for fabricating a micro electro-mechanical system
device, comprising the steps of: providing a substrate; depositing
a first electrically conductive layer on an upper surface of the
substrate to form a lower plate of a first driver capacitor;
forming an isolation layer of non-conductive material over the
lower plate of the first driver capacitor; depositing a second
electrically conductive layer on an upper surface of the isolation
layer to form a lower plate of a MEMS variable capacitor and a
dual-use electrode; forming a sacrificial layer over the second
electrically conductive layer; forming at least one anchor post
coupled to the substrate; forming a dielectric membrane over the
sacrificial layer and coupled to the at least one anchor post;
depositing a third electrically conductive layer on an upper
surface of the dielectric membrane to form an upper plate of the
variable capacitor and an upper electrode of a second driver
capacitor, wherein the first driver capacitor is in series with the
second driver capacitor; removing the sacrificial layer so that the
dielectric membrane is free to displace vertically in response to
an actuation voltage applied to the first driver capacitor and the
second driver capacitor.
20. The method of claim 19, wherein depositing first, second, and
third electrically conductive layers includes depositing metal.
Description
FIELD OF THE INVENTION
[0001] This invention relates, in general, to micro
electro-mechanical system (MEMS) devices and, more particularly, to
a high quality (high-Q) variable capacitor fabricated using MEMS
technology.
BACKGROUND OF THE INVENTION
[0002] One prior art type of variable capacitor, known as the
thermal drive variable capacitor 10, is illustrated in FIG. 1. In
this prior art version, a dielectric gap 11 between two capacitor
plates 12 and 13 is controlled or altered by means of thermal
expansion of lateral components. As shown in FIG. 1, each end of an
upper plate 12 of a capacitor is connected to one end of a movable,
hinged diagonal component 15. The other end of the diagonal
component is connected to a lateral component 14. Movement of
lateral component 14, which is controlled by thermal devices,
causes the hinged diagonal components 15 to translate the lateral
movement to vertical movement of upper plate 12. The vertical
movement of upper plate 12 varies the capacitance of the
device.
[0003] The drawbacks to thermal drive variable capacitor 10 are
several. The mechanical design of the device is complicated
resulting in a costly and inefficient manufacturing process. Also,
the complicated design of the thermal drive negatively impacts the
reliability of the device. The translation of lateral movement to
vertical movement intrinsic to thermal drive devices has the
negative effect of increasing the size of the device. Also, the
thermal expansion and contraction operation of the device is
inefficient, thus resulting in slow speed in varying the
capacitance. Furthermore, thermal operation requires significantly
more power consumption than electrostatically driven MEMS devices
of similar capability.
[0004] Another prior art variable capacitor 20 is illustrated in
FIG. 2. MEMS variable capacitor 20 has at least one driver 21,
itself a simple variable capacitor, for determining the
displacement of a dielectric membrane 22 and a variable capacitor
region for employment with an external circuit (not shown). The
displacement of dielectric membrane 22 is determined by the
application of a voltage potential across drivers 21.
[0005] The application of a voltage to the drivers causes an
electrostatic attraction between the driver electrodes. This
electrostatic attraction results in a downward movement of
dielectric membrane 22, thereby causing a downward displacement.
This reduction in the gap between the upper 23 and lower 24
capacitor plates results in a corresponding variance in
capacitance.
[0006] Hence, a need exists for a high-Q capacitor that is
reliable, cost efficient, and has continuous dynamic response over
the full displacement of the dielectric membrane.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a simplified cross-sectional view of a prior art
thermal drive variable capacitor;
[0008] FIG. 2 is a cross-sectional view of another prior art MEMS
variable capacitor;
[0009] FIG. 3 is a chart comparing actuation voltage potential with
displacement of a membrane for forward and reverse bias of prior
art MEMS variable capacitors;
[0010] FIG. 4 is a schematic diagram of the driver portion of a
MEMS variable capacitor in accordance with an embodiment of the
present invention;
[0011] FIGS. 5-8 are charts which illustrate the dynamic response
of the MEMS variable capacitor of FIG. 4;
[0012] FIG. 9 is a cross-sectional view of a MEMS variable
capacitor in accordance with another embodiment of the present
invention;
[0013] FIG. 10 is a cross-sectional view of a MEMS variable
capacitor in accordance with yet another embodiment of the present
invention; and
[0014] FIG. 11 is a chart comparing actuation voltage potential
with displacement of the membrane for forward and reverse bias for
a MEMS capacitor in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0015] One of the forces intrinsic to MEMS variable capacitors with
electrostatic drive is the restoring force. The restoring force is
a mechanical force that tends to return the dielectric membrane to
its initial or rest position, i.e., the position of the membrane
with no voltage potential across the control electrodes. When the
voltage potential across the driver electrodes is lessened or
removed, the restoring force causes the dielectric membrane
displacement to increase as the dielectric membrane returns to its
initial position, thereby varying the capacitance of the MEMS
variable capacitor.
[0016] One of the limitations of the prior art MEMS variable
capacitor shown in FIG. 2, is reflected in the relationship between
the restoring force F.sub.R and the electrostatic force F.sub.ES.
The restoring force F.sub.R for the dielectric membrane is linear
with respect to the displacement. However, the electrostatic force
F.sub.ES is inversely proportional to the square the difference
between the original gap and the displacement. Therefore, as the
actuation voltage increases, causing further displacement of the
dielectric membrane, F.sub.ES dominates over F.sub.R which results
in the dielectric membrane clamping in the closed or fully
displaced position. This is known as the "pull-in" or "snap effect"
and as the graph in FIG. 3 illustrates, the result is a two-state
or bi-stable capacitor. Furthermore, when the actuation voltage is
reduced, the membrane will remain fully deflected until F.sub.R can
dominate F.sub.ES. Thus, an abrupt form of hysteresis is evident in
the prior art MEMS variable capacitor as shown in FIG. 3. In
addition, the prior art MEMS variable capacitor exhibits poor noise
margins because of the above limitations.
[0017] Other limitations of the prior art MEMS variable capacitor
follow from the snap effect. For example, in most applications any
capacitor variability is limited to approximately one third of the
sum of the available dielectric membrane deflection plus the
thickness of the dielectric membrane divided by the dielectric
constant, as shown in the following equation:
d=(g.sub.0+g.sub.1/.di-elect cons.)/3 (1)
[0018] where:
[0019] d is the dielectric membrane displacement;
[0020] g.sub.0 is the effective gap between the lower surface of
the dielectric membrane and the upper surface of the lower
electrode;
[0021] g.sub.1 is the thickness of the dielectric membrane; and
[0022] .di-elect cons. is the dielectric constant.
[0023] Therefore, precise actuation voltage control is required,
particularly near the pull-in point, i.e., the point on the curve
where the membrane will clamp, which is shown in FIG. 3 as
approximately one third of the available displacement.
[0024] Furthermore, the snap effect results in a device with a poor
noise margin. That is, voltage spikes will cause the membrane to
clamp. Thus, as a practical matter, the prior art MEMS variable
capacitor is typically employed as a bi-stable device, rather than
as a true variable capacitor that is controllable over a continuum
of capacitances.
[0025] Now referring to FIG. 4, a schematic diagram of an
electrostatic driver portion 30 of a MEMS variable capacitor in
accordance with an embodiment of the present invention is shown.
The electrostatic driver essentially combines a fixed capacitor
C.sub.1 in series with a variable capacitor C.sub.v. Both C.sub.1
and C.sub.v are fabricated in the same monolithic, integrated
device. The addition of C.sub.1 alters equation (1) by introducing
the ratio of C.sub.v (which equals C.sub.0, the initial drive
capacitance at zero voltage) to C.sub.1 and the stable condition
becomes:
d=(g.sub.0+g.sub.1/.di-elect cons.)*(1+C.sub.0/C.sub.1)/3 (2)
[0026] The inclusion of the C.sub.1, and thus the term
(1+C.sub.0/C.sub.1), increases the effective displacement range of
the dielectric membrane for controlling the variable capacitance.
Furthermore, the system is stable in the full deflection range if
the following condition is satisfied:
C.sub.0/C.sub.1.gtoreq.(2-g.sub.1/.di-elect
cons.*g.sub.0)/(1+g.sub.1/.di-- elect cons.g.sub.0) (3)
[0027] Thus, the device is stable over the entire operating range
if C.sub.0/C.sub.1 is greater than or equal to approximately 2.
FIGS. 5-8 illustrate the dynamic response of the circuit at several
different values of C.sub.0/C.sub.1. The exemplary values for the
other relevant parameters (g.sub.0, g.sub.1, and .di-elect cons.)
for the purposes of FIGS. 5-8 are g.sub.0=2 .mu.m, g.sub.1=1 .mu.m,
and .di-elect cons.=3.9. Note that for the ratio of
C.sub.0/C.sub.1=0 (FIG. 5), i.e., only one capacitor, the snap
effect occurs at approximately 10 volts and only approximately one
third of the displacement is useable. When C.sub.0/C.sub.1=1 (FIG.
6), approximately 60 percent (1.2/2) of the displacement is
useable. For C.sub.0/C.sub.1=1.5 (FIG. 7), approximately 90 percent
(1.8/2) of the displacement is useable. And for C.sub.0/C.sub.1=2
(FIG. 8), virtually 100 percent of the displacement is useable.
[0028] Now referring to FIG. 9, the structural aspects of a MEMS
variable capacitor in accordance with an embodiment of the present
invention are disclosed. MEMS variable capacitor 99 with stabilized
electrostatic drive 100 is comprised of at least one driver
capacitor pair 120 having a driver fixed capacitor 121 in series
with a driver variable capacitor 126. FIG. 6 illustrates MEMS
variable capacitor 99 with an electrostatic driver comprised of a
single driver capacitor pair 120.
[0029] MEMS variable capacitor 100 also has a variable capacitor
130 that connects to the remainder of the circuit (not shown).
Variable capacitor 130 is comprised of a lower plate 131, an upper
plate 132, and a portion of a dielectric membrane 140. Dielectric
membrane 140 displaces vertically, as shown by the bold arrow, in
response to the application of an actuation voltage potential to
driver capacitor pair 120, thereby varying the capacitance of
variable capacitor 130.
[0030] MEMS variable capacitor 99 with stabilized electrostatic
drive 100 is fabricated beginning with a substrate 110, preferably
a non-conductive substrate. In the preferred embodiment, an
isolation layer of non-conductive material 111, such as silicon
oxide or silicon nitride, is deposited on substrate 110 to
facilitate the manufacturing process. In a subsequent step, a metal
layer is deposited to form a lower electrode 122 of driver fixed
capacitor 121 on the isolation layer. Alternatively the metal layer
may be deposited directly on substrate 110. Lower electrode 122 of
driver fixed capacitor 121 is connected to the remainder of the
actuation circuit by any number of conventional means such as a
metal trace.
[0031] Subsequently, another isolation layer of non-conductive
material 112 is fabricated over isolation layer 111 and lower
electrode 122 of driver fixed capacitor 121. Thus, the lower
electrode 122 of fixed capacitor 121 is electrically isolated from
the remainder of the device. Then, another metal layer is deposited
on an upper surface of second isolation layer 112 to form lower
plate 131 for variable capacitor 130 and a dual-use electrode 125.
Dual-use electrode 125 serves as the upper electrode of driver
fixed capacitor 121 and the lower electrode of the driver variable
capacitor 126.
[0032] Subsequent steps include the formation of a sacrificial
layer (not shown), i.e., a temporary layer used to establish the
effective gap go between dielectric membrane 140 and dual use
electrode 125, the formation of an anchor post 150 from which
dielectric membrane 140 is suspended, the formation of dielectric
membrane 140 and the deposition of another metal layer on the upper
surface of dielectric membrane 140, which forms upper plate 132 of
variable capacitor 130, and an upper electrode 127 of driver
variable capacitor 126.
[0033] Ultimately the sacrificial layer is removed so that
dielectric membrane 140 is free to displace vertically in response
to the application of the actuation voltage. In the embodiment
illustrated in FIG. 9, there is an offset in dielectric membrane
140 to adjust the gap between lower plate 131 and dielectric
membrane 140 to be slightly different from the gap g.sub.0.
However, in alternate embodiments, the offset may be varied or even
eliminated, depending on the application of the device.
[0034] Now referring to FIG. 10, a MEMS variable capacitor 101 with
stabilized electrostatic drive in accordance with another
embodiment is illustrated. MEMS variable capacitor 101 includes a
substrate 110 and isolation layers 111 and 112, as in the previous
embodiment. However, MEMS variable capacitor 101 is comprised of
two driver capacitor pairs 120. Each of the driver capacitor pair
120 is comprised of a driver fixed capacitor 121 and a driver
variable capacitor 126. Driver fixed capacitor 121 is comprised of
a lower electrode 122 and a dual-use electrode 125. Also, the
driver variable capacitor is comprised of dual-use electrode 125
and an upper electrode 127. Thus, as in the previous embodiment,
dual-use electrode 125 serves as the upper electrode of driver
fixed capacitor 121 and the lower electrode of driver variable
capacitor 126.
[0035] Each end of dielectric membrane 140 is connected to an
anchor 150. Variable capacitor 130, comprising a lower plate 131,
an upper plate 132 and a portion of dielectric membrane 140, is
located in the approximate center of dielectric membrane 140. The
actuation voltage is simultaneously applied to each of the driver
capacitor pairs 120 to control the deflection of dielectric
membrane 140.
[0036] FIG. 11 is a chart 200 comparing actuation voltage potential
with displacement of the membrane for forward and reverse bias for
a MEMS capacitor in accordance with an embodiment of the present
invention. Chart 200 is a plot of actuation voltage versus
displacement for a typical device. Note that the forward bias curve
is virtually identical to the reverse bias curve. The snap effect
is effectively eliminated and there is no abrupt hysteresis as was
common in prior art devices.
[0037] Although the invention has been particularly shown and
described with reference to a preferred embodiment thereof, it will
be understood by those skilled in the art that changes in form and
detail may be made therein without departing from the spirit and
scope of the invention.
* * * * *