RECONFIGURABLE RF MEMS DEVICES AND ANTENNAS

RF MEMS

Abstract.-This paper presents the overview of RF MEMS

(Radio Frequency-MicroElectroMechanical Systems)

components and reconfigurable antennas designed and

produced in Middle East Technical University (METU)

using the in-house fabrication process. The design and

measurement results of various components such as

switches, impedance tuners, phase shifters, frequency

tunable antennas, a phased array antenna with RF MEMS

phase shifters, and a reflectarray design are summarized.

It is shown that the fabricated RF MEMS components

satisfy the design specifications and have better performance

and reconfigurability capabilities compared to their

conventional counterparts.

Introduction

MEMS technology is a powerful way of merging the

functions of sensing and actuation with computation

and communication to control physical parameters

at the microscale. Present markets of MEMS technology

are mainly in pressure and inertial sensors, inkjet print

heads, and high-resolution digital displays. Future and

emerging applications include tire pressure sensing,

fiber optical components, fluid management and

processing devices for chemical microanalysis,

medical diagnostics, and drug delivery, and RF

and wireless electronics, namely RF MEMS. The

application of MEMS technology to RF systems

enables production of components with low power

consumption, high linearity, low insertion loss, and

high isolation. RF MEMS components are particularly

attractive for researchers due to their tunable properties.

This technology is used to implement many tunable

circuits and systems in a miniaturized way that has

never been implemented before using any other

technology.

This paper presents some of the RF MEMS

components and reconfigurable antennas designed

and produced in METU using the in-house fabrication

process developed at the Microelectronics Facilities

(METU MET). Section II gives the RF MEMS switch

structures, which are the key element of technology.

Section III and Section IV present the impedance tuner

and reconfigurable antennas, respectively. Section V

gives the monolithic phased array structure implemented

with MEMS technology and the proposed reflectarray

design.

RF MEMS Switches

Metal-to-Metal Contact RF MEMS

Shunt Switch RF MEMS

Switches are the key elements in the design of the

complete systems hence the improvement in one

switch can affect the whole system performance.

We have designed a metal-to-metal contact switch

operating at 1-6 GHz band to obtain a better performance

than the existing ones. The switch, shown in Figure 1,

consists of two cantilevers located on ground planes

of a coplanar waveguide (CPW) transmission l

ine. With the help of actuation electrodes beneath

the cantilevers, the switching of the RF signal is maintained.

According to the measurement results,

the switch has isolation better than 20 dB in the 1-6 GHz

band and better than 10 dB in the 1-20 GHz band.

Insertion loss of the structure is better than the 0.3 dB

in the 1-20 GHz band. The switch has a measured actuation

voltage of 7 V.

Capacitive ContactRF MEMS

Shunt Switch

A capacitive contact RF MEMS shunt switch is designed

as shown in Figure 3 (a). The switch has recessed sections

on the ground plane of coplanar waveguide and the

meanders supporting the bridge increase the series

inductance of the bridge, which is used to tune the

switch to operate at Ku-band [2]. The switch has a measured

down-state capacitance of 2.08 pF which results in better

than 20 dB measured isolation and better than 0.2 dB insertion

loss for the 11-17 GHz band as shown in Figure 3 (b).

The measured actuation voltage of the switch is 13 V.

Impedance Tuners

Reconfigurable double-stub and triple stub impedance

tuners are designed and produced for X and Ku-band

applications. These can be used for LNA matching, antenna

matching, noise parameter, and load-pull measurements.

One of the designs has a double-stub structure which has

10 CPW based MEMS switches distributed evenly on the

two stubs forming a distributed structure [3]. Figure 4 (a)

shows the schematic view of this design. The capacitive

MEMS switch used has a specific design which uses two

variable capacitors between the signal and the ground of

the CPW for discrete biasing requirements for each MEMS

switch of the matcher.

The design is capable of matching 25 different points on the

Smith Chart with a maximum measured VSWR of 5.27 at

18 GHz when a single MEMS switch is actuated from each

stub. The design can also be operated by actuating more

than one MEMS switch resulting with 1024 (210) combinations

which has a maximum VSWR of 50.8 at 18 GHz. Smith

Chart shows distribution of all possible states. It should

be noted here that external SMT resistors were used for

the DC biasing of the MEMS switches.

Reconfigurable antennas

RF MEMS components have tunable characteristics;

thus the integration of these components with radiators

may yield several advantages such as reconfigurability

in polarization, frequency, and radiation pattern. One

of the reconfigurable antenna structure designed and

fabricated is a tunable frequency CPW fed rectangular

slot antenna [4]. In order to achieve a shift in the

resonant frequency, a short circuited stub with

RF-MEMS capacitors is inserted opposite to the

feeding transmission line as shown in Figure 6 (a).

Measured reflection coefficient characteristics for

different actuation voltages are shown in Figure 6 (b).

It is observed that the resonant frequencies can be

shifted from 8.7 GHz to 7.7 GHz, and from 10.57 GHz to

10.22 GHz by changing the actuation voltage, -hence

the height of the MEMS capacitors-, from 0 to 16 volts.

The measurement results are in a good agreement with

the simulations. The antenna radiates broadside for all

of the four resonances and increasing the capacitance

by lowering down the bridges does not cause any

adverse effect on the radiation patterns. The other

reconfigurable antenna structure employs the idea

of loading a microstrip patch antenna with a stub

on which MEMS capacitors are placed periodically

as shown in Figure 7 (a) [5]. MEMS capacitors are

electrostatically actuated with a low tuning voltage

in the range of 0-11.9 V. The antenna resonant

frequency can continuously be shifted from

16.05 GHz down to 15.75 GHz as the actuation

voltage is increased from 0 to 11.9 V.

0

Phased arrays

This section presents the phased array system

designed at 15 GHz employing 3-bit DMTL type

phase shifters which are monolithically integrated

with the feed network of the system and the

radiating elements on the same substrate [6]. The

phase shifter can give 0°-360° phase shift with 45°

steps at 15 GHz which is used to obtain various

combinations of progressive phase shift in the

excitation of radiating elements. The phased array

is composed of four linearly placed microstrip

patch antennas. Figure 8 (a) shows the

photograph of the phased array fabricated on

a glass substrate. The digital phase shifter

used in the system is composed of a periodically

loaded high-impedance transmission line (> 50 _)

with MEMS bridges in series with lumped

capacitors, forming a DMTL. Figure 9 (a) and

(b) shows the unit cell of the fabricated DMTL

phase shifter and its circuit model [ ]. Measured

inserted phase shifts for different states are shown

in Figure 9 (c). Figure 8 (b) shows the measured

radiation pattern results for different progressive

phase shift. The main beam can be steered by

4°, 14° as the phase shifter states are adjusted

accordingly.

Another application of the MEMS technology

is to use MEMS components in reflectarrays to

scan the beam. In this work, in order to control

the phase of the wave reflected from each element

of reflectarray, the lengths of the microstrip lines

are changed using RF MEMS series switches [7].

Thus, progressive phase shift between the elements

are changed and reflectarray with reconfigurable main

beam is obtained by the help of MEMS technology.

In the proposed design, the patch antenna at

26.5GHz is printed on a glass substrate bonded to

another glass substrate which contains the

transmission lines with MEMS switches.

Transmission line with series RF-MEMS switch

is shown in Figure 10 (a). Transmission lines are

coupled to the patch antenna by means of a slot

in the ground layer between the two glass

substrates as shown in Figure 10 (a) and (b).

Control of the reflection phase from each unit element

is achieved by the help of phase design curve

obtained from infinite array simulations in HFSS.

The phase design curve shown in Figure 10 (c) indicates

the dependence of the reflection phase on the

transmission line length. Required transmission line

lengths are chosen from this phase design curve.

To switch the beam from broadside to 40°, two sets

of lengths have been chosen by the ON-OFF states

of RF MEMS switch. HFSS simulations have also

been performed for transmission line with RF MEMS

switch for further tuning of the phase.

Conclusions

This paper reviews some examples of RF MEMS

components and antennas designed and fabricated

in the facilities of METU. It is shown by the

measurement results that the fabricated components

satisfy the design specifications. More components,

as well as production, modeling and packaging studies

will be presented and discussed during the presentation

in the conference.

Página: www. radantmems.com

Realizado: Franco A Rivera C.

Asignatura: CRF