Information
-
Patent Grant
-
6639488
-
Patent Number
6,639,488
-
Date Filed
Friday, September 7, 200123 years ago
-
Date Issued
Tuesday, October 28, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Pascal; Robert
- Takaoka; Dean
Agents
-
CPC
-
US Classifications
Field of Search
US
- 333 262
- 333 105
- 333 101
- 200 181
- 342 372
- 257 528
-
International Classifications
-
Abstract
Disclosed is a capacitive electrostatic MEMS RF switch comprised of a lower electrode that acts as both a transmission line and as an actuation electrode. Also, there is an array of one or more fixed beams above the lower electrode that is connected to ground. The lower electrode transmits the RF signal when the top beam or beams are up and when the upper beams are actuated and bent down, the transmission line is shunted to ground ending the RF transmission. A high dielectric constant material is used in the capacitive portion of the switch to achieve a high capacitance per unit area thus reducing the required chip area and enhancing the insertion loss characteristics in the non-actuated state. A gap between beam and lower electrode of less than 1 μm is incorporated in order to minimize the electrostatic potential (pull-in voltage) required to actuate the switch.
Description
FIELD OF THE INVENTION
The present invention relates generally to a micro-electromechanical (MEMS) radio frequency (RF) switch, and more specifically, to a MEMS switch that operates with a low actuation voltage, has a very low insertion loss, and good isolation.
BACKGROUND OF THE INVENTION
A radio-frequency (RF) switch is a device that controls the flow of an RF signal, or it may be a device that controls a component or device in an RF circuit or system in which an RF signal is conveyed. As is contemplated herein, an RF signal is one which encompasses low and high RF frequencies over the entire spectrum of the electromagnetic waves, from a few Hertz to microwave and millimeter-wave frequencies. A micro-electromechanical system (MEMS) is a device or system fabricated using semiconductor integrated circuit (IC) fabrication technology. A MEMS switch is such a device that controls the flow of an RF signal. MEMS devices are small in size, and feature significant advantages in that their small size translates into a high electrical performance, since stray capacitance and inductance are virtually eliminated in such an electrically small structure as measured in wavelengths. In addition, a MEMS switch may be produced at a low-cost due to the IC manufacturing process employed in its fabrication. MEMS switches are termed electrostatic MEMS switches if they are actuated or controlled using electrostatic force which turns such switches on and off. Electrostatic MEMS switches are advantageous due to low power-consumption because they can be actuated using electrostatic force induced by the application of a voltage with virtually no current. This advantage is of paramount importance for portable systems, which are operated by small batteries with very limited stored energy. Such portable systems might include hand-held cellular phones and laptop personal computers, for which power-consumption is recognized as a significant operating limitation. Even for systems that have a sufficient AC or DC power supply such as those operating in a building with AC power outlets or in a car with a large DC battery and a generator, low power-consumption is still a desirable feature because power dissipation creates heat which can be a problem in a circuit loaded with many IC's. However, a major disadvantage exists in prior art MEMS switches, which require a large voltage to actuate the MEMS switch. Such a voltage is typically termed a “pull-down” voltage, and, in the prior art may be anywhere from 20 to 40 volts or more in magnitude and therefore not compatible with modem portable communications systems, which typically operate at 3 volts or less. To explain further, a typical MEMS switch uses electrostatic force to cause mechanical movement that results in electrically bridging a gap between two contacts such as in the bending of a cantilever. In general this gap is relatively large in order to achieve a large impedance during the “off” state of the MEMS switch. Consequently, the aforementioned large pull-down voltage of anywhere from 20 to 40 volts or more is usually required in these designs to electrically bridge the large gap. Also, a typical MEMS switch has a useful life of approximately 10
8
to 10
9
cycles. Thus, in addition to the above concerns, there is an interest in increasing the lifetime of such MEMS switches. Thus there is a need for an electrostatic MEMS switch that is actuated by a low pull-down or actuating voltage and has low power consumption with increased cycle life.
SUMMARY
It is, therefore, an object of the present invention to provide a micro-electromechanical (MEMS) switch that operates with a low actuation voltage, and has a very low insertion loss and good isolation.
It is another object of the present invention to provide a fabrication process that is fully compatible with CMOS, BiCMOS, and SiGe processing, and can be monolithically integrated at the upper levels of chip wiring.
To achieve the above objects, there is provided a capacitive electrostatic MEMS RF switch comprised of a lower electrode that acts as both a transmission line and as an actuation electrode. Also, there is an array of fixed beams that is connected to ground above the lower electrode. The lower electrode transmits the RF signal when the upper beams are up, and when the upper beams are actuated and bent down, the transmission line is shunted to ground.
BRIEF DESCRIPTION OF THE FIGURES
The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying figures, in which:
FIG. 1
is a diagram illustrating a cross-section of a metal-dielectric-metal MEMS switch using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material;
FIG. 2
a
is a diagram illustrating a top view of a metal-dielectric-metal MEMS switch with fixed beams connected at both ends to ground;
FIG. 2
b
is a diagram illustrating a top view of a metal-dielectric-metal MEMS switch showing yet another embodiment of the present invention;
FIG. 2
c
is a diagram illustrating a top view of a metal-dielectric-metal MEMS switch showing another embodiment of the present invention;
FIG. 3
is a diagram illustrating a cross-section of a metal-dielectric-metal MEMS switch using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material, and a top actuation (or pull-up) electrode in a cavity;
FIG. 4
is a diagram illustrating a cross-section of a metal-dielectric-metal MEMS switch with two separate actuation electrodes, using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material;
FIG. 5
is a diagram illustrating a top view of the metal-dielectric-metal MEMS switch of
FIG. 4
;
FIG. 6
a
is a diagram illustrating a cross-section of another embodiment of a metal-dielectric-metal MEMS switch with two separate actuation electrodes using CMOS metal levels and a Ta
2
O
5
(Tantalum Pentoxide) dielectric material;
FIG. 6
b
is a diagram illustrating a cross-section of yet another metal-dielectric-metal MEMS switch with two separate actuation electrodes using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material;
FIG. 7
is a diagram illustrating a cantilever metal-dielectric-metal switch;
FIG. 8
is a diagram illustrating another embodiment of a cantilever metal-dielectric-metal switch; and
FIGS. 9-11
are charts illustrating performance characteristics of switches according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.
A diagram illustrating a cross-section of a metal-dielectric-metal MEMS switch
100
using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material is shown in FIG.
1
. The switch comprises a single lower electrode
110
(or first electrode), attached to a substrate, that acts both as a transmission line and as an actuation electrode. Also, there is an array of fixed upper beams
120
acting as support elements that are connected to ground
130
above the lower electrode
110
. Beams
120
are attached to supports
170
fixed to the substrate, creating a space
150
. Attached to the upper beams
120
is an upper electrode
160
(or second electrode). This upper electrode
160
can be comprised of, for example, copper (Cu), tungsten (W), Aluminum (Al), gold (Au), nickel (Ni) and alloys thereof. The lower electrode
110
transmits an RF signal when the upper beams
120
are up and the switch is in the open position. The lower electrode
110
consists of copper back-end layers encapsulated on three sides by TaN/Ta (Tantalum Nitride/Tantalum) barrier material. The top copper surface of the lower electrode is protected by Ta (Tantalum), TaN (Tantalum Nitride), Ta/TaN (Tantalum/Tantalum Nitride), or TaN/Ta (Tantalum Nitride/Tantalum). This protective layer is either fully or partially anodized to yield a thin Ta
2
O
5
(Tantalum Pentoxide) (100-2000 Angstroms) layer
140
, a dielectric material with a dielectric constant of 22. It is possible to use another dielectric material but it is preferred that the dielectric constant be above 10. Some available alternatives are barium strontium titanate, hafnium oxide, hafnium silicate, zirconium oxide, zirconium silicate, lead zirconium titanate, lead silicate, and titanium oxide. It is possible to use methods other than anodization to deposit the high dielectric constant material, such as sputtering or CVD (chemical vapor deposition). When a voltage is applied to the lower electrode
110
, the upper beams
120
are bent down and the upper electrode
160
comes in contact with the lower electrode
110
. At this point, a conducting path is created though the lower electrode
110
and the upper beams
120
shunting the RF signal to ground.
When the upper beams
120
, fabricated using a copper Damascene approach are actuated and bent down (placing the switch in the closed position), the upper electrode
160
touches the anodized Ta
2
O
5
(Tantalum Pentoxide) layer
140
on the lower electrode
110
, and the transmission line is shunted to ground
130
through the resulting capacitance. The release of the upper beams
120
(creating the space
150
between the electrode
110
and the beams
120
) is performed by etching, with an oxygen containing plasma, leaving the space
150
between the lower electrode
110
and the beams
120
. The material removed during the etch can be selected from a group consisting of: SiLK (an example of a class of highly aromatic arylene ethers), BCB (benzocyclobutane), polyimides, unzipping polymers such as PMMA (polymethyhnethacrylate), suitable organic polymers, a-C:H (e.g. Diamond Like Carbon) or a-C:HF (e.g. Fluorinated Diamond Like Carbon. Typical dimensions for the space
150
between the lower electrodes
110
and the beams
120
are 500-1000 Angstroms requiring actuation voltages of less than 3 Volts. Length of the beams
120
vary from 35-100 μm and the lower actuation electrode area is on the order of 2000-3000 μm
2
(i.e. 50×50, 60×40, 70×40 etc.). The thickness of the beams
120
is 1-5 μm and the individual beam width varies from 5-20 μm.
FIG. 2
a
is a diagram illustrating a top view of a metal-dielectric-metal MEMS switch showing fixed beams connected at both ends to ground. The top electrode consists of a set of beams
220
either connected together at both ends or individually connected to the lower ground electrodes
230
. An advantage of this configuration is that by having multiple beams, a large overlap area is created with the lower electrode
210
that results in effective grounding of the RF signal when the top beams
220
are pulled down, contacting the upper electrode to the high dielectric constant material of the lower electrode
210
. Another advantage of this multiple beam configuration is the ability of single beams to achieve higher switching frequencies than a flat rectangular plate. Also, single beams are less likely to deform with multiple actuation, a common problem encountered when using a flat rectangular plate. The beam width can also be variable along its length. In a preferred embodiment, the set of beams are covered by a layer selected from a group consisting of silicon nitride and silicone dioxide.
FIG. 2
b
is a diagram illustrating a top view of a metal-dielectric-metal MEMS switch showing another embodiment of the present invention. The top electrode beams
320
are connected together at the center where they form an overlap area
340
on top of the RF signal electrode (or lower actuation electrode)
310
. The top beams
320
are all connected to ground
330
at both ends but they could also be connected with each other at their fixed ends or in different locations along their length.
FIG. 2
c
is a diagram illustrating a top view of a metal-dielectric-metal MEMS switch showing yet another embodiment of the present invention. The shape of the middle upper beams
420
is modified to yield a lower actuation voltage.
FIG. 3
is a diagram illustrating a cross-sectional view of a metal-dielectric-metal MEMS switch using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material, and a top actuation (or pull-up) electrode in a cavity. In this embodiment, lower space
550
preferably defines a distance (d) from the beams
520
to bottom electrode
510
. Upper space
580
, from surface
585
to the top electrode
590
, preferably defines a distance (
2
d
), although it is contemplated that the distance between surface
585
and top electrode
590
may be equal to distance (d), so that the distance is in the range of d to
2
d
. When actuated, this electrode
590
assists in releasing the beams
520
from the bottom electrode
510
by pulling up on the beams
520
. The top surface of the upper space
580
may have small access holes through which release of the structure can be achieved. As a result, the top actuation electrode
590
may be perforated. Materials that can be used for this electrode are Titanium Nitride (TiN), Tungsten (W), Tantalum (Ta), Tantalum Nitride (TaN), or copper (Cu) cladded by Tantalum Nitride/Tantalum (TaN/Ta).
FIG. 4
is a diagram illustrating a cross-section of a metal-dielectric-metal MEMS switch using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material, but with two separate actuation electrodes
670
. By utilizing two separate actuation electrodes
670
, it is possible to separate the DC voltage in the actuation electrodes
670
from the RF potential of the RF signal electrode, creating circuit design advantages to those skilled in the art. In the case of multiple lower electrodes
670
and
610
, a beam
620
length of 100 mm can be used with two lower actuation electrodes
670
that are 25 μm long and an RF signal electrode
610
that is 50 μm long. A top view of this embodiment of the switch is illustrated in FIG.
5
.
FIG. 6
a
is a diagram illustrating a cross-section of another embodiment of a metal-dielectric-metal MEMS switch with two separate actuation electrodes using CMOS metal levels and a Ta
2
O
5
(Tantalum Pentoxide) dielectric material.
FIG. 6
a
shows a continuous Ta
2
O
5
(Tantalum Pentoxide) layer
840
across all three lower electrodes
870
and the transmission line
810
. This increases the effective dielectric constant of the coplanar wave (CPW) guide structure consisting of the center transmission line
810
and the actuation electrodes
870
on either side. The increased dielectric constant will yield a transmission line
810
with a lower characteristic impedance, making it useful for impedance matching to low impedance active elements. Additionally, the wavelength will be reduced due to the increased dielectric constant allowing distributed elements (i.e. quarter wavelength transmission lines) to be shorter, taking up less space. Finally, the increased dielectric constant will tend to guide the fringing fields of the CPW structure away from the substrate cutting down on power loss in the substrate. A key advantage to using a CPW transmission line lies in the wide range of characteristic impedance values achievable by varying the signal to ground spacing (here, signal to actuation electrode
870
spacing). This design freedom is not as easily achievable with a standard microstrip line configuration, especially in a standard silicon back end, where the signal to ground plane spacing is quite small (on the order of a few microns).
To construct the structure illustrated in
FIG. 6
a
, a Ta (Tantalum), TaN (Tantalum Nitride), Ta/TaN (Tantalum/Tantalum Nitride), or TaN/Ta (Tantalum Nitride/Tantalum) layer is deposited on top of the copper electrodes. The copper lower electrodes
810
and
870
are typically recessed after chemical mechanical polishing (CMP). The TaN (Tantalum Nitride) layer at the top surface is continuous on top of the insulator in-between electrodes. Anodization of this layer will convert it to Ta
2
O
5
(Tantalum Pentoxide) so that the oxide is in contact with the insulator material between electrodes.
FIG. 6
b
is a diagram illustrating a cross-section of yet another metal-dielectric-metal MEMS switch with two separate actuation electrodes using CMOS metal levels and Ta
2
O
5
(Tantalum Pentoxide) as dielectric material. The lower copper electrodes
910
and
970
are capped by a thin Ta (Tantalum) layer. The Ta (Tantalum) is removed from the top surface by CMP. A Si
3
N
4
(Silicon Nitride) layer
980
is deposited as a blanket film covering the three lower electrodes
910
and
970
to prevent chemical interaction between the lower electrodes
910
and
970
, and the first layer of dielectric material. On top of the center electrode
910
area, the nitride is etched down to the liner which is subsequently patterned in the center electrode
910
and an AlCu layer
990
is deposited to allow for electrical contact of the TaN (Tantalum Nitride) anodization. Finally, a TaN (Tantalum Nitride) layer
940
is deposited and converted to Ta
2
O
5
(Tantalum Pentoxide) by anodization and subsequently patterned along with the AlCu (Aluminum Copper) layer
990
to result in a protruding center electrode
910
capped by the high dielectric constant material.
FIGS. 7 and 8
are variations of the switch top electrodes using cantilever beams
1010
and
1110
, and copper (
FIG. 7
) or tungsten (
FIG. 8
) as beam materials. The end of the cantilever that does the shorting to ground extends beyond the beam thickness. This is because cantilevers have shown to have instabilities when actuated. The “tip” approach can also be used with fixed beams or plates, but extra fabrication mask levels will be needed.
FIGS. 9-11
are charts illustrating performance characteristics of switches according to the present invention.
FIG. 10
illustrates that excellent isolation (more than 30 dB) and insertion loss (less than 0.2 dB) can be obtained using beams 55 μm long and with a total width of 80 μm (individual beams are 5-20 μm wide). A set of 4-8 beams can be used to realize this switch.
FIG. 11
illustrates the benefits of introducing a dielectric material with higher dielectric constant such as HfO
2
(Hafnium Oxide) (dielectric constant of 40) or sputtered BSTO (Barium Strontium Titanate) (dielectric constant of 30). By implementing dielectric material with a high dielectric constant, improved switch characteristics, especially in terms of isolation, are achieved.
While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims
- 1. A MEMS (micro-electromechanical) RF switch apparatus operable under low actuation voltage, the apparatus comprising:a substrate; a first electrode attached to the substrate; a first layer of dielectric material having a dielectric constant above 10 on the first electrode; a second electrode positioned above the first electrode creating a first space having a height less than 5000 Angstroms between the first layer of dielectric and the second electrode; and a support element for suspending the second electrode when the switch is in an open position and for moving the second electrode when the second electrode is pulled to the layer of dielectric material when the switch is in a closed position in response to a voltage between the first and second electrodes.
- 2. The MEMS RF switch apparatus of claim 1 wherein the first electrode forms a transmission line.
- 3. The capacitive MEMS RF switch apparatus of claim 1 wherein the first electrode is an actuation electrode.
- 4. The MEMS RF switch apparatus of claim 1 wherein a capacitance between the first and second electrodes when the switch is in the closed position creates an RF short between the first and second electrodes.
- 5. The MEMS RF switch apparatus of claim 1 wherein the second electrode forms a transmission line.
- 6. The MEMS RF switch apparatus of claim 1 wherein the support element comprises a plurality of beams, electrically coupled together, between the second electrode and a fixed support attached to the substrate to provide mechanical isolation between the beams.
- 7. The MEMS RF switch apparatus of claim 6 wherein the plurality of beams are covered by a layer selected from a group consisting of silicon nitride and silicone dioxide.
- 8. The MEMS RF switch apparatus of claim 1 wherein the support element comprises a plurality of spaced beams, each beam having a first and second end attached to fixed supports attached to the substrate, and the second electrode is coupled to the beams between the first and second ends.
- 9. The MEMS RF switch apparatus of claim 1 wherein a voltage of three volts or less causes the switch to actuate to a closed position.
- 10. The MEMS RF switch apparatus of claim 1 wherein a top surface of the first electrode is covered with a liner to prevent chemical interaction between the first electrode and the first layer of dielectric material.
- 11. The MEMS RF switch apparatus of claim 1 further comprising actuation electrodes attached to the substrate on opposite sides of the first electrode.
- 12. The MEMS RF switch apparatus of claim 1 wherein the first layer of dielectric material is selected from a group consisting of tantalum oxide, barium strontium titanate, hafnium oxide, hafnium silicate, zirconium oxide, zirconium silicate, lead zirconium titanate, lead silicate, titanium oxide, and other dielectric materials with a dielectric constant greater than 10.
- 13. The MEMS RF switch apparatus of claim 1 wherein the second electrode is selected from a group consisting of copper (Cu), tungsten (W), aluminum (Al), gold (Au), nickel (Ni) and alloys thereof.
- 14. A method for fabricating a MEMS RF switch apparatus operable under a low actuation voltage, the method comprising:selecting a substrate; fixing a first electrode to the substrate; fixing a first layer of dielectric material having a dielectric constant above 10 on the first electrode; attaching a second electrode to a flexible support element positioned above the first electrode creating a first space having a height (d) between the first electrode and the second electrode; and attaching a third electrode to a non-flexible support element positioned above the second electrode creating a space having a height no greater than (2d) between the third electrode and the flexible support element; the third electrode attached above the second electrode to create a second space having a height between 500 and 10000 Angstroms between the second and third electrodes; wherein the flexible support element suspends the second electrode when the switch is in an open position and pulls the second electrode to the layer of dielectric material when the switch is in a closed position in response to a voltage between the first and second electrodes.
- 15. A MEMS (micro-electromechanical) RF switch apparatus operable under low actuation voltage, the apparatus comprising:a substrate; a first electrode attached to the substrate; a first layer of dielectric material having a dielectric constant above 10 on the first electrode; a second electrode positioned above the first electrode creating a first space having a height less than 5000 Angstroms between the first layer of dielectric and the second electrode; a support element for suspending the second electrode when the switch is in an open position and for moving the second electrode when the second electrode is pulled to the layer of dielectric material when the switch is in a closed position in response to a voltage between the first and second electrodes; and a third electrode positioned above the second electrode creating a second space having a height between 500 and 10000 Angstroms between the second and third electrodes.
- 16. The MEMS RF switch apparatus of claim 15 wherein the third electrode forms a transmission line.
- 17. The MEMS RF switch apparatus of claim 15 wherein the third electrode is a pull-up electrode for pulling the second electrode up from the first electrode.
- 18. The MEMS RF switch apparatus of claim 15 further comprising a second layer of dielectric material covering the surface of the second electrode facing the second space.
- 19. The MEMS RF switch apparatus of claim 15 wherein the third electrode further comprises a layer of Si3N4 (Silicon Nitride) on a top surface of the third electrode.
- 20. A MEMS (micro-electromechanical) RF switch apparatus operable under low actuation voltage, the apparatus comprising:a substrate; a first electrode attached to the substrate; a first layer of dielectric material having a dielectric constant above 10 on the first electrode; a second electrode positioned above the first electrode creating a first space having a height less than 5000 Angstroms between the first layer of dielectric and the second electrode; and a support element for suspending the second electrode when the switch is in an open position and for moving the second electrode when the second electrode is pulled to the layer of dielectric material when the switch is in a closed position in response to a voltage between the first and second electrodes, wherein the support element comprises at least one beam having one end attached to the second electrode, and a second end attached to the substrate.
- 21. The MEMS RF switch apparatus of claim 20 wherein the first electrode forms a transmission line.
- 22. The MEMS RF switch apparatus of claim 20 wherein the first electrode is an actuation electrode.
- 23. The MEMS RF switch apparatus of claim 20 wherein a capacitance between the first and second electrodes when the switch is in the closed position creates an RF short between the first and second electrodes.
- 24. The MEMS RF switch apparatus of claim 20 wherein the second electrode forms a transmission line.
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A |
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