An Electrostatically Actuated Broadband MEMS Switch

RF MEMS

Radant MEMS, Incorporated

An electrostatically actuated broadband microswitch has

been developed that has applications from DC through

the microwave region. The microswitch is a 3-terminal

device based on a cantilever beam and is fabricated

using an all-metal, surface micromachining process.

It operates in a hermetic environment obtained through

a wafer-bonding process. Characteristics of the

wafer-level packaged switch include DC on-resistance

of less than 1 Ohm with an actuation voltage of 80 V,

lifetime of greater than 1010 cycles with on-resistance

variation of less than 0.2 Ohm and current handling

capability of 1 Ampere. Key RF characteristics at 2 GHz

include an insertion loss of 0.3 dB and isolation of 30 dB.

Preliminary measurements at higher microwave frequencies

are extremely promising with full characterization and

planned product improvements underway.

Introduction

MEMS microswitches are receiving increasing attention,

particularly in the RF community. Low power consumption,

large ratio of off-impedance to on-impedance and the ability

to be integrated with other electronics; makes micro

switches an attractive alternative to other mechanical and

solid-state switches. MEMS switches can be used in a

variety of RF applications including cell phones, phase

shifters and smart antennas and also in lower-frequency

applications such as Automatic Test Equipment

(ATE) and industrial and medical instrumentation.

 We have developed an electrostatically actuated MEMS

microswitch for both DC and RF applications. The

microswitch is a 3-terminal device that employs a

cantilever beam  and is fabricated using an all-metal,

surface micromachining process on high-resistivity silicon.

In operation, the beam is deflected by applying a voltage

between the gate and source electrodes, so that the free

end of the beam contacts the drain  and completes an

electrical path between the drain and the source.

The contact material is a thin layer of a Platinum group

metal deposited on the underside of the beam and

on the drain. The device operates in a hermetic

environment obtained through a waferlevel capping process.

Since 1999 we have employed a commercial MEMS

foundry for switch fabrication and have obtained

fabrication yields in excess of 90%.

Ongoing activities include manufacture of prototypes for

use in a X-band Electronically Steerable Antenna (ESA)

under an US Air Force contract, process and design

modifications to optimize RF performance and contact

resistance, and development of a 4-terminal relay for better

isolation between the actuation and signal paths.

Low Frequency Performance and Applications

Important operational characteristics are as follows.

A gate voltage between 40 and 120 V, depending on

the specific design, actuates the microswitch. Devices

are usually operated at a contact force of  200 μN, and

have a single contact resistance of 3 Ω. Switches typically

have 8 contacts in parallel to yield a total on-resistance,

including interconnects, of less than 1 Ω at DC and low

frequencies.

Device lifetime is generally measured at a current of

10 mA or less with the current applied only during switch

closure (to avoid hot "breaks" and "makes", i.e.

"cold-switched"). Under these conditions, switch lifetime

exceeds 10 cycles . Measurements at higher

currents, up to 500 mA per switch contact, show lifetime

greater than 10 cycles. Currently, lifetime measurements

are limited by test time, rather than any failure mechanism.

A relatively small number of hot-switching lifetime

measurements with 1 V indicates a life of about 107 cycles

with the lifetime limited by erosion of the drain metal.

The on-response time is approximately 5 μs, and is

limited by squeeze-film damping of the cantilever beam.

The off-response time is much smaller, and is limited by

parasitics in our measurement circuit.

Another characteristic, which is important for some

low-frequency applications, is the "standoff" voltage,

or the maximum voltage that can be applied across an

open switch without damaging it. A voltage across the

open microswitch sets up a field in the gap between the

drain and the end of the beam. This can cause the switch

to fail in two ways, either by causing the beam to deflect

and closing the switch, or by causing an electrical breakdown

of the gap. In our current designs, the failure is due to beam

deflection, and occurs at about 150 V.

Microswitches are interesting candidates as replacements

for low-power mechanical relays (generally reed relays),

particularly in applications utilizing a large matrix of relays,

often for multiplexing. Examples of such applications are

pin electronics in Automated Test Equipment, transducer

arrays in medical ultrasound, and sensor and data acquisition

applications. Microswitches can bring significant savings in

size and power consumption in such applications, and may

also provide longer life. For example, a 8x8 array of reed

relays each 2 cm x1 cm in size would occupy 250 cm2, the

same array using microswitches would consume 1 cm2.

Some applications may require an improvement of the

stand-off voltage limit, and the hotswitched power handling

capability. Certain applications may require a true 4-terminal

relay, in which the signal path does not share a terminal with

the actuation circuit. Prototypes of such devices have been

built .

RF Performance and Applications

The benefits and applications for MEMS RF switches as

fundamental building blocks, supplanting PIN diode and

FET RF switches are numerous because MEMS switches

combine the best features of both, having the low control

power requirements of FETs, but having "on" resistances

(and RF insertion losses) lower than PIN diodes. Furthermore,

MEMS switches have lower off-state capacitance and,

as a result, better off-state RF isolation than either

FETs or PIN diodes, and, in addition, have inherently high

RF linearity. Intended applications include microwave

switches that replace PIN diode and FET switches, while

providing lower insertion loss, higher isolation, higher

linearity, higher radiation resistance, superior tolerance for

high temperature environments, and lower prime power

consumption. Examples of such applications include T/R

switches in a variety of products such as cellular handsets

and base stations, phase shifters for Electronically Steerable

Antennas, tunable filters and reconfigurable antennas, to

name a few.

One application of current interest is the cellular telephone

market. Here, the frequency of interest is approximately

2 GHz. Measured performance of our wafer-level capped

MEMS switches are very good at this frequency, with

an insertion loss of 0.3 dB and isolation of 30 dB for a

single element, series, SPST switch in a 50 mil sq. capped

die package similar to that of Figure 3.

This RF insertion loss is several tenths of a dB better than

most current PIN diode, MESFET, and PHEMT switches

with comparable isolation at this frequency. By combining

series and shunt MEMS switches within the same capped

package, substantially higher isolation should be attainable

in the future, with little degradation in insertion loss. Future

plans also include capped SP2T, SP4T, and transfer switches.

This reflective loss will be mostly eliminated in the future

by reducing switch inductance. Efforts are currently underway

to increase the off-state switch isolation by modifying the switch

geometry and to decrease the on-state insertion loss by identifying

and minimizing the various loss mechanisms.

Summary

We have developed a surface micromachined microswitch which

can be used in applications from DC through microwave.

We have also developed a wafer scale package, and reported

results in this package up to 4 GHz. Key characteristics for a

single element, series, SPST switch are; at 4 GHz, insertion loss

of 0.4 dB and isolation of 27 dB.

Results from DC to low frequencies include a total

on-resistance, including interconnects, of less than 1 Ω,

cold switch lifetimes exceeding 1010 cycles, and hot

switch lifetimes exceeding 107 cycles. Additional testing is

underway along with efforts to further increase the lifetime

of these devices. Applications for our microswitch include

T/R switches in a variety of products such as cellular

handsets and base stations, phase shifters for Electronically

Steerable Antennas, tunable filters and reconfigurable

antennas, pin electronics for Automated Test Equipment,

transducer arrays in medical ultrasound, and sensor and

data acquisition applications.

Página: www.ece.uci.edu.com

Realizado: Franco A Rivera C.

Asignatura: CRF