CURRENT STATUS OF RF MEMS DEVICES FOR

WIRELESS COMMUNICATION SYSTEMS8.1: INTRODUCTION

Recently the focus of RF-MEMS research has changed its direction towards

system integration, reliability and the development of RF-MEMS devices with novel

configurations to meet needs of wireless communication systems. These systems have

gained worldwide attention in recent years, particularly due to the proliferation in MEMS

technology which has facilitated the development of radio frequency micro-electro-

mechanical (RF MEMS) devices with novel configurations.

RF-MEMS has emerged as a potential technology for wireless, mobile and

satellite communication and defense applications. Extensive research has been carried out

to identify and overcome the limitations of RF MEMS technology for replacing PIN or

FET based switches for low- loss applications. The key benefit of this technology is that

the devices can be manufactured by processes as of VLSI that has helped in the

realization of many sub millimeter- sized parts to provide RF functionality. RF-MEMS

components include resonators, oscillators, tunable filters, switches, switched capacitors,

varactors and inductors. Among these the RF-MEMS switch has emerged as a favorite

for its high RF performance, ultra-low-power dissipation, very high isolation, very low

insertion loss, very low cost and large-scale integration.

The evolution of RF-MEMS devices is needed for the expanding broadband

wireless radio communication. Radio frequency, semiconductors technologies and IC-

compatible MEMS technologies are improving day by day and have an important role in

the fast growing market of wireless communication systems. This chapter aims to present

the current status of the RF-MEMS devices briefly for the wireless and the satellite

communication systems. This chapter briefly discusses novel RF-MEMS Switches,

vibrating micromechanical diamond disc resonator, RF-MEMS variable capacitors and

MEMS tunable inductor.

Wireless communication systems are utilizing wireless sense and control

technology to bridge the gap between the physical world of humans and virtual world of

physics and electronics. The dream is to automatically monitor and respond to forest fire

143

avalanches, faults in satellites, traffics, hospitals etc. Novel RF-MEMS devices have

better high frequency performance extending beyond 100 GHZ and high Q factor solve

many problems of high-frequency technology for wireless communications. RF-MEMS

finds application in phased arrays and reconfigurable apertures switching networks, phase

shifters and single-pole N-throw switches.

Now there exist many different cellular standards, ultra wide band, wireless

sensor networks and many private systems representing Bluetooth, public mobile

services, WLAN etc. Also there is coming up 4G system requiring new frequency

spectrum. It will be multi network system and will be carried out by the unification of

different networks to enable the ubiquitous connectivity. RF-MEMS is strong contender

as a complimentary technology allowing “More than Moore” where we can find out the

expectation in Figure 8.1 as below.

Figure 8.1: More than Moore’s Law

RF-MEMS are manufactured using conventional 3D structuring technologies, like

bulk micro-machining ,surface micro machining , fusion bonding ,LIGA etc. The

material used includes Silicon, GaAs, SiC or SOI substrates. RF-MEMS have great

potential for integration and miniaturization. They provide lower weight, lesser power

consumption, lower insertion losses, improved linearity, superior performance and higher

144

quality factors than conventional communication components. RF-MEMS are no longer

laboratory toys.

The MEMS technology has improved significantly many RF-MEMS devices like

micro-switches, micro-machined inductors tunable capacitors, resonators, oscillators,

micro-transmission lines, filters, surface acoustic wave (SAW) devices etc. These RF-

MEMS devices are worldwide used recently in mobiles, communication and satellite

systems. The literature and several books have demonstrated and discussed a large

number of RF-MEMS devices for wireless communication systems. Now, focus of RF-

MEMS research is to develop reliable RF-MEMS devices with new configurations. We

are here presenting some of them with new configurations like RF-MEMS switch,

vibrating micromechanical diamond disc resonator, RF-MEMS variable capacitor and

MEMS tunable inductor.

8.2: RF-MEMS Drawbacks and Novel Research Solutions

RF-MEMS technology has its own share of problems. Besides the drawbacks like

high actuation voltage and slow switching speed, there are two main problems associated

with standard MEMS capacitive devices which are temperature sensitivity of the movable

membrane, and dielectric charging problems in the isolator layers (leading to

stiction).Possible improvements in the micromechanical aspect (such as in actuation

voltage, in switching speed ), improvement in the dielectric layer, improvement in the

power handling capability and improvement in the RF performance by reduction in

parasitic are presented below.

1. Improvements in the micromechanical aspect:

(a) Improvement in actuation voltage:

In practice, low spring constant designs like meandering suspensions or thin

springs (to achieve lower actuation voltage) are used. But such designs have issues like

the reliability of the device and the switching speed. The use of push-pull concepts

requires a relatively high actuation voltage. Also, the use of low- height bridges has been

reported to lower the actuation voltage but at the cost of a reduction in the capacitance

ratio.

To lower the actuation voltage the use of new materials like AlSi0.04 or Pt as

membrane for the MEMS switch, use of electromagnetic actuation along with

145

electrostatic forces and exploiting buckling and bending effects due to residual stress has

been reported. But, AlSi0.04 has a much higher RF transmission loss. In recent times, a

totally free flexible membrane supported over three pillars has been proposed to lower

the actuation voltage but it needs a double sacrificial layer system and causes low

switching speed.

(b) Improvement in switching speed:

High switching speed of RF-MEMS devices is a main limitation, and much work

has not been done to improve the speed. Usually, the increase in the switching speed of a

RF-MEMS switch affects badly the switching voltage. A quicker switching can be

obtained with improved stiffness, but it certainly leads to an increase in the actuation

voltage.

Mercier et al. proposed the miniaturization of the switches to obtain high speed

and reliability. Further, they also demonstrated sub-microsecond switching times using

dielectric membrane switches with built in tensile stress. Lacroix et al. have reported that

the spring constant of the beam increases by adding simple bent sides on miniaturized

beam edges that increases sub-microsecond switching time.

2. Improvement in the dielectric layer:

The surface roughness of the dielectric layer badly affects the capacitance ratio of

switches and precise explanation of the roughness using the statistical approach is

reported. Various models are proposed to know how contact resistance responds to the

variations in the contact area, the number of asperities in contact and the temperature and

the resistivity profiles at the contact locations.

The RF-MEMS reliability chiefly depends on the dielectric charging

phenomenon. There are many reports on impact of the dielectric material, distributed

dielectric charging and the modeling of dielectric charging. Many materials like ZnO or

Al2O3 alloys, PZT, PZT or HfO2 multi layers, polymer-ceramic composites, BST, TiO2,

amorphous diamond, etc. are now under consideration as possible replacements of

SiO2/Si3N4 as the dielectric material for RF-MEMS technology.

The required properties of dielectric material as an insulation layer points that the

key for selecting a substitute of SiO2/Si3N4 dielectric are:

(a) Dielectric constant,

146

(b) Dielectric strength,

(c) Resistivity,

(d) Leakage current,

(e) Surface roughness,

(f) Ferroelectric properties and

(g) Charge trapping density.

Very few materials are capable with respect to all of these guidelines. Much

research is still required to find any material that can substitute SiO2/ Si3N4 as the

dielectric.

3. Improvement in the power handling capability:

Power handling capability of the RF-MEMS devices is primarily restricted by two

factors like:

(a) Joule heating for high power that results in melting and welding of the contact,

(b) self biasing and RF latching.

Electro-thermal models have been reported to predict the power handling

capability of RF- MEMS devices. The use of two bridge level topology has been

investigated till 8W of RF power.

4. Improvement in the RF performance by reduction in parasitic:

Various reports show that the parasitic has a significant role in the quality factor

of the devices. Due to its attenuation for RF signals, the CMOS grade low-resistivity

silicon substrate is not appropriate for high-frequency applications. For the high

resistivity substrates, a frequency of 10 GHz is big sufficient to drive the silicon substrate

into its dissipative mode of dielectric.

Hence, the use of substrates and a suitable passivation layer have a vital role in

the RF performance of the device. At 40 GHz, by using polymers like Kapton, polyimide

resin, BCB resin etc as the passivation layer on the low resistivity silicon substrate, the

insertion loss could be decreased to 3dB/cm.

8.3: NOVEL RF-MEMS SWITCHES

RF-MEMS switches play an important role in the application of the multiband

and multichannel wireless system for reconfiguration integrated circuits. Figure 8.3

illustrates the RF micromechanical switch by Z. J. Yao et al. This particular RF device

147

consists of a beam fixed at both ends and suspended over a metal electrode. This could be

the central conductor of a coplanar waveguide. When sufficient amount of voltage is

applied between beam and its underlying electrode, shorting the ensuing electrostatic

force pulls the beam down to the electrode.

Figure 8.2: Radio Frequency Micro-Mechanical Switches.

As a result shorting between beam and electrode takes place (for the case of a

direct contact switch), or the electrode to beam capacitance gets increased which affects

an AC short (in case of capacitive switch, where a dielectric film atop the electrode).In

both the cases the switch is effectively closed. In case of electrostatic actuation the

voltage levels greater than 20volts are generally used for radar applications. Although,

this actuation level is very high for the integrated transistor circuits used in wireless

handset applications. Therefore, either the actuation voltage level is reduced or

accommodated at the system-level via charge pumping etc.

RF micro-mechanical switches with insertion losses about 0.1dB and low switch

power consumption of order pico-watts have been reported. Hence these can easily

outperform semi-conductor switches (FETs and diodes) in antenna and filter switching

applications. Their micro mechanical structure allows the use of metal materials with

much lower resistivity than semiconductors. These RF micro-switches with microsecond

148

switching speeds are now reported. The microsecond speeds are useful for many

switching applications in wireless systems where low loss and high linearity are

important.

Another novel RF-MEMS switch with two movable electrodes is here mentioned

that is proposed by S K Lahiri et al, in 2009 by using bulk micromachining the substrate

below the CPW central line, typically low-resistivity silicon, under the membrane area

selectively. It facilitates both the CPW central line and the shunt membrane to move in

opposite direction at the same time. RF-MEMS switches with two movable electrodes

have also been reported by Babaei et al., but they mainly deal with the process

technologies to fabricate the switch. The two movable electrodes result in the reduction in

actuation voltage and switching time and the removal of the silicon from beneath the

CPW central line causes a reduction in the parasitic.

Two wafers oxidized on both sides are used in the fabrication of this switch. A

thin Cr/Au layer is deposited on the top surface of bottom high resistivity silicon wafer

and on the lower surface of top wafer. On bottom wafer, CPW structure is defined by

photolithography. The central conductor is covered by a fitting protecting layer. A thin

eutectic gold layer is selectively deposited on the outer conductors. Then, a thin low-loss

dielectric film (0.15μm) is deposited to cover the central conductor over the regions

where capacitors will be formed. Windows etched on the oxide film in the backside (after

oxidation) are used for selective removal of silicon by bulk micromachining to release the

central conductor.

The gold film on top wafer is patterned by photolithography to shape the stripes bridging

between the ground electrodes on either side. Then, a thin gold eutectic film is deposited

and patterned as required for the eutectic bonding. Windows are opened (after oxidation)

on the bottom surface such that the stripes become free to move after bulk

micromachining. The backside alignment should be perfect. Finally, bulk

micromachining is performed in a fitting anisotropic etchant like EDP or TMAH with

oxide mask on both sides of the selected bonded wafers. The eutectic coated surfaces of

the two wafers are held together face to face, aligned correctly and bonded at temperature

about 300oC at a adequate contact pressure. Fit anti-stiction structures and treatment

149

should be included.

Figure 8.3: SEM picture of the fabricated MEMS shunt switch without top

substrate.

An SEM picture of the fabricated MEMS shunt switch without top substrate is

shown in figure 8.3. The thickness of gold electrodes in the diagram is about 1μm and the

gap between the electrodes is estimated to 2.5μm. The maximum and minimum values of

capacitance are found to be 3.5pf and 30fF respectively. These reported novel designs

have been mentioned by keeping in mind the limitations of the RF-MEMS technology

discussed in section 8.2. These have benefits of reducing the switching time, parasitic,

and actuation voltage at the same time.

In the early days, use of RF-MEMS switches was difficult due to reliability

issues. Lifetime of early switches was of the order of only 10 million cycles. Now, due to

combination of contact engineering, adequate packaging and fabrication control, RF-

MEMS switches with switching around 100 billion cycles are available. This clears a way

for more use of these switches into wireless communication systems.

RF-MEMS switches could have applications in band switching and filter

switching because future wireless receivers will need to operate at several bands covering

wide range of frequencies including 1-5 GHz. Still the research issues for RF-MEMS

Switches include their reliability, their switching speeds, their switching voltages etc.

150

8.4: VIBRATING MICRO-MECHANICAL DIAMOND DISC RESONATOR

As vibrating micro-mechanical resonators produces much higher quality factor

than their electrical counterparts, hence, these are essential components in

communication systems. With improving MEMS technology, these devices can be

designed to oscillate over a very wide operating frequency range, varying from <1 KHz

to >1GHz. This much frequency range makes them ideal for ultra stable oscillation and

low loss filter functions.

A lot of vibrating micro-mechanical resonators had been reported. For example,

clamped-clamped beam resonator, free-free beam resonator, wine glass disc resonator,

counter-mode disc resonator, hollow disc ring resonator etc. Many researches had been

performed on the VHF range resonator to use the reference oscillator of the wireless

communication systems.

A 1.51GHz nano-crystalline diamond micro-mechanical disc resonator had been

reported from U C Berkeley. It can achieve a quality factor up to 11,555 in vacuum and

10,100 in air (i.e. at atmospheric pressure).Its quality factor is very impressive as

compared to others. The resonator consists of a 2 µm thick diamond disc having diameter

of 20 µm. Its diamond disc has been fabricated by using the CVD process. The disc is

suspended by a doped-polysilicon stem self aligned to be exactly at it center. Then all this

is surrounded by doped-polysilicon electrodes. The spacing between electrodes and disc

perimeter is 80nanometer.

When vibrating in its radial contour mode, the disc expands and contracts around

its perimeter. This amounts to a high stiffness and high kinetic energy. As disc center

corresponds to a node location for the radial contour vibration mode shape, anchor losses

through the supporting stem are very much prevented.

In this way, this design can retain a very high quality factor even at UHF

frequency. Also, the high stiffness of its radial contour mode gives this resonator a very

large total kinetic energy during vibration. Due to this, the energy losses arising from

viscous gas damping are very much reduced in this resonator. Therefore, this helps the

resonator in maintaining quality factors greater than 10,000 even at atmospheric pressure.

This single resonator attains a frequency applicable to the RF front terminals of

many commercial wireless devices. Since its quality factor is 10100 in air. Therefore, to

151

achieve high quality factor, this design removes the requirement for vacuum which

makes it economical.

Figure 8.4: SEM Photograph of the CVD Diamond Micromechanical Disk

Resonator.

This resonator can operate at and beyond gigahertz frequencies when properly

scaled and do so while retaining sufficiently large dimensions to maintain proper power

handling. Figure 8.4 shows the SEM photograph of the diamond disc resonator with self

aligned stem. Also the frequency temperature coefficient of this resonator is

-12ppm/deg.C. It has found practical applications as reference local oscillator, VHF-S-

Band Filter and RF channel select networks.

Micro-mechanical resonators are replacing conventional crystal oscillators used as

frequency reference or time source devices. These resonators have more advantages than

crystal oscillators in terms of money, size reduction, and better integration with silicon

etc. There operating frequency could be increased into GHz range. Higher frequency

operation would also allow new filtering and mixing functions, and completely novel

architectures.

8.5: RF-MEMS VARIABLE TUNABLE CAPACITOR

Variable capacitor, with performance superior to varactor diodes in areas such as

non-linearity and losses, can be feasible with MEMS technology. Initially, MEMS

parallel plate variable capacitors with an electro-static actuator are fabricated. The tuning

152

range of such type of MEMS capacitors is limited up to 50% due to the failure of

capacitor structure when the voltage becomes more than the pull-in-voltage.

Figure 8.5: A Schematic Diagram of MEMS Variable Capacitor.

Then a MEMS variable capacitor with 100% tuning range was proposed in. In this

actuation electrodes are spaced differently from the capacitor plates. But, in practice, this

capacitor should operate over a smaller tuning range to avoid collapse of capacitor.

Therefore, MEMS variable capacitors having a much wider tuning range should be

designed without the collapse of the capacitor structure.

Another MEMS variable capacitor with novel configuration and design was

proposed in. Figure 8.5 shows a schematic diagram of this capacitor. It consists of two

movable parallel plates with an insulating dielectric layer on top of the bottom plate.

Since both plates are flexible, both plates can attract each other. Hence, maximum

spacing between two plates decreases before the pull-in-voltage comes into action. In

addition the capacitor has an extended tuning range even after the two plates touched

each other.

This capacitor is constructed using two structural layers, three sacrificial layer and

two insulating layers of Nitride. The top plate is made of nickel with a thickness of 24

µm. It is covered by a gold layer of thickness 2 µm. The bottom plate is made of poly-

silicon which is covered by a nitride layer of a thickness of 0.35 µm. The different layers

153

use to make this capacitor are prepared using MetalMUMPs process. The two

dimensional layers are generated using CoventorWare.

Figure 8.6: An SEM image of the MEMS Variable Capacitor.

An SEM picture of the fabricated MEMS variable capacitor is shown in figure

8.6. At 1GHz the achievable tuning range of this capacitor is found equal to 280 %. This

value is very much greater than that of traditional parallel plate variable capacitors.

Paired with medium quality factor inductors, this capacitor can enhance the performance

of low noise voltage controlled oscillators (VCOs). Also, when paired with micro-

mechanical inductors it can allow implementation of low noise VOCs with much lower

power consumption than those using IC technology.

8.6: RF-MEMS TUNABLE INDUCTORS

Paired with inductors having quality factor greater than 20, tunable MEMS

capacitors will be useful in communication circuits. Conventional IC technology can only

produce spiral inductors with quality factors less than 7 due to excessive series resistance

and substrate losses. Using MEMS technologies inductors with quality factor as high as

70 at 1GHz have been demonstrated.

This inductor with quality factor about 70 when paired with a micro-mechanical

capacitor should enhance performance of low noise VCOs with much lower power

consumption than those using traditional IC technology. Also, if inductors with quality

factor about 300 become possible, then tunable RF filters might be achievable that could

find use in RF transceivers circuits like filters, matching networks etc. MEMS tunable

inductors can also be used to build filters with tunable bandwidth.

By coupling a drive coil to the RF inductor, a few variable inductors have become

feasible. The tunability is attained by varying mutual inductance between the two

154

inductors. Based upon the relative phase of the current in the two coils, the mutual

inductance component can be increased or decreased continuously. By this technique, a

100% tuning range has been attained. But an additional driver circuit is used in the drive

coil in this technique.

The tunable inductor circuit proposed in consists of variable capacitors, two fixed

capacitors and two inductors. The parallel plate variable capacitor is shown in the Figure

8.7. It is fabricated as follows: the upper plate is made of poly 2 and lower plate is made

of poly 1. Then a layer of gold is deposited on upper plate. The air gap between these

plates is of 0.75 µm. To ensure the etching of the oxide between these two plates, holes

are made in the plates of the capacitor. Capacitance value can be changed by voltage

applied to the variable capacitor. When top plate moves towards fixed lower plate, the

distance between plates changes. This changing distance changes the value of

capacitance.

Figure 8.7: MEMS Tunable Inductor Chip

The tunable inductor was constructed using Multi-User MEMS processes

(MUMPs) surface ploy silicon micro-machining technology. The MUMPs process has

three layers of poly silicon (poly0, poly1, and poly2), two layers of oxide and one layer

of gold (poly2). The gold layer is deposited on the top ploy silicon layer. The thicknesses

155

of gold layer, poly 2 layer , second oxide layer, poly 1 layer, first oxide layer and ploy0

layer are 0.5 µm, 1.5 µm, 0.75 µm, 2.0 µm, 2.0 µm, 2.0 µm and 0.5 µm respectively.

The design of fixed capacitors is same except that voltage is not applied to the

plates of the fixed capacitors by R. R. Mansour et al. This design uses eight pads: six

pads for the coplanar RF input and output signals and two pads for grounds and a DC

voltage. Here the pads are made of poly 2 layer and gold layers. To ensure the trapping of

oxide and protection from the HF etch, anchors are provided around the edges of the

pads. The following Figure 8.7 shows a fabricated MEMS tunable inductor chip.

8.7 Summary

In recent years, the focus of RF-MEMS research has been shifted to system

integration, reliability and the development of RF-MEMS devices with novel

configurations to meet needs of wireless communication systems. The evolution of RF-

MEMS needed for expanding the broadband wireless communication systems. It should

be progressed in parallel with the miniaturization of the CMOS following diversification

technologies of “More than Moore”. Micromechanical devices attained via MEMS

technologies have been described that can play potentially a key role in removing the

board- level packaging requirements that currently limit the size of communication

transceivers etc.

RF-MEMS devices are the strong candidates that could meet future wireless

communication systems requirements in terms of low power consumption,

reconfiguration, reliability and miniaturization. As there are many different kinds of

technologies, modeling, theories, processes and simulation techniques for the RF-MEMS,

there would be no anxiety for the future because many researchers, groups etc have

extremely high motivation for the RF-MEMS devices.

The literature and several books have demonstrated and discussed numerous RF-

MEMS devices for wireless communication systems. Now, RF-MEMS research is

focusing to develop reliable RF-MEMS devices with new configurations. This very

chapter, here, presents some of them with novel configurations such as RF-MEMS

switches, vibrating micromechanical diamond disc resonator, RF-MEMS variable

capacitor and MEMS tunable inductor. Possible improvements in the micromechanical

aspect (such as in actuation voltage, in switching speed), improvements in the dielectric

156

layer, improvements in the power handling capability and improvements in the RF

performance by reduction in parasitic are also presented in this chapter.

During the last years, RF-MEMS have experienced a tremendous progress in

terms of technology development and RF-applications. RF-MEMS circuits have been

demonstrated with such exceptional properties that they potentially can be part of the

essential building blocks of next generation emerging wireless communication and RF-

sensing applications. RF-MEMS technology has reached a level where MEMS switches

can be successfully integrated into practical RF systems with proven long term reliability.

The integration of MEMS switches and Monolithic Microwave Integrated Circuits

(MMICs) is thus considered as a next logical step in MMIC process growth and

commercialization of RF-MEMS technology.

157