Inductor-capacitor resonant RF switch

Details Inductor-capacitor resonant RF switch Inductor-capacitor resonant RF switch

2001-05-17

2002-10-29

Lee, Benny (Department: 2817)

Wave transmission lines and networks

Long line elements and components

Switch

C200S181000, C029S622000

Reexamination Certificate

active

06472962

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to a microelectronic mechanical switch (MEMS) device and method of fabrication, and more particularly, to an inductor-capacitor resonant radio
54
frequency (LCR RF) switch and a method of fabricating a LCR RF switch.
(2) Description of the Prior Art
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.
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.
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.
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.
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
.
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
.
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
.
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 L
1
.
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
0
48
. The MEMS bridge is modeled as a variable capacitor C
b
52
, a series inductance L
b
56
and a series resistance R
s
60
. The variable capacitor C
b
52
represents the aforementioned variable capacitive coupling due the deflection of the membrane. The series inductance L
b
56
and series resistance R
s
60
are due to the physical characteristics of the membrane and bridge posts. When the MEMS switch is in the UP state, C
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
b
52
is large, and most of the microwave energy is conducted through the switch to ground.
Note that, in the DOWN state, the series capacitance C
b
52
and the series inductance L
b
56
result in a series resonant frequency given by:
&ohgr;=1/(
L
b
C
b
)
½
.
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
b
is usually very small and is not adjustable.
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.
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

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