Chapter-1

Introduction1.1 Basic Overview on RF MEMS:

MEMS is the acronym for Micro Electro Mechanical Systems, an engineering discipline concerned with devices whose critical device dimension is in the micrometer range. MEMS technology combines many diverse fields within engineering and science to develop devices and systems to perform highly precise functions in a variety of systems. Micro electromechanical Systems (MEMS) have been developed since the 1970s for pressure and temperature sensors, accelerometers, gas chromatographs, and other sensor devices .

The term RF MEMS refers to the design and fabrication of MEMS for RF integrated circuits. It should not be interpreted as the traditional MEMS devices operating at RF frequencies. The first MEM switch designed specifically for microwave applications was reported in 1990. RF MEMS has seen an amazing growth in the past 10 years due to its immense commercial and defense potential. The reason is that while there were tremendous advances in GaAs HEMT devices (high-electron mobility transistor) and in silicon CMOS (complementary metal-oxide-semiconductor) transistors; there was barely an advance in semiconductor switching diodes from 1985 to 2000. In 1980, the cutoff frequency of silicon CMOS transistors was around 500 MHz and is currently around 100 GHz. Also in 1980, the cutoff frequency of GaAs -HEMT devices was 10-20 GHz and is now above 800 GHz. However, the cutoff frequency of GaAs or InP p-i-n diodes improved from around 500 GHz in 1985 to only 2000 GHz in 2001. Clearly, a radical new technology was needed to push the cut-off frequency of switching devices to 40,000 GHz for low-loss applications, and this was achieved with RF MEMS devices

MEMS devices in RF MEMS are used for actuation or adjustment of a separate RF device or component, such as variable capacitors, switches, and filters. The term MEMS refers to a88

collection of micro sensors and actuators which can sense its environment and have the ability

to react to changes in that environment with the use of a microcircuit control. They include, in addition to the conventional microelectronics packaging, integrating antenna structures for command signals into micro electromechanical structures for desired sensing and actuating functions. The system also may need micro power supply, micro relay and micro signal processing units. Micro components make the system faster, more reliable, cheaper and capable of incorporating more complex functions.

It is seen that, for microwave and millimeter wave systems, these actuating forces are sufficient to change the properties of overall system. Passive devices include bulk micro machined transmission lines, filters and couplers. Active MEMS devices include switches, tuners and variable capacitors. The electromotive force used to move the structures on the wafer surface is typically electrostatic attraction, although magnetic, thermal or even gas-based micro actuator structures have been developed.

RF MEMS are similar to VLSI circuits in a way that it allows the execution of complex functions on a size scale orders of magnitude lower and at far less power than discrete circuits. However, MEM enables this miniaturization on a class of sensors and transducers that traditionally were constructed on the model of a large, often cumbersome transducer or sensor coupled to a highly integrated VLSI readout circuit or processor.In the largest class of RF MEMS devices and components, the micro electromechanical operation is used simply for the actuation or adjustment of a separate RF device or component, the basic example of which consists of a variable capacitor.

1.2Classification of RF MEMS Components:

Although it is still early for a time-tested categorization of RF-MEMS devices, the development to date tends to place them into different classes depending on whether one takes an RF or MEMS viewpoint. From the RF viewpoint, the MEMS devices are simply classified by the RF-circuit component they are contained in, be it reactive elements, switches, filters, or

something else. From the MEMS viewpoint, there are three distinct classes depending on where

and how the MEMS actuation is carried out relative to the RF circuit . The three classes are: 1) the MEMS structure is located outside the RF circuit, but actuates or controls other devices (usually micromechanical ones) in the circuit; 2) the MEMS structure is located inside the RF circuit and has the dual, but decoupled, roles of actuation and RF-circuit function; and 3) the MEMS structure is located inside the circuit where it has an RF function that is coupled to the actuation. These classes are referred as:

RF extrinsic RF intrinsic RF reactive.Each of the MEMS classes has produced compelling examples, like the tunable

micromachined transmission line in the RF-extrinsic class, shunt electrostatic microswitch and comb capacitors in the RF-intrinsic class, and capacitively coupled micromechanical resonator in the RF-reactive class. A collection of these devices is shown in the RF MEMS technology diagram of Figure 1.A.

Figure1.A: Classes of RF MEMS device

Thus RF MEMS research leads to three distinct areas.

Micro-machined transmission lines, high-Q resonators, filter, and antennas that are suitable for 12-200 GHz. They are generally integrated on thin- dielectric membranes or use bulk micromachining of silicon, but are static and do not move. (RF extrinsic)

RF MEMS switches, varactors, and inductors that have been demonstrated from DC-120

GHz and are now a relatively mature technology. Except for the micromachined inductors, MEMS switches and varactors move several micrometers when actuated. (RF intrinsic)

RF micromechanical resonators and filters that use the mechanical vibrations of extremely small beams to achieve high-Q resonance at 0.01-200 MHz in vacuum. In this case, the mechanical movements are of the order of tens of angstroms. Very-high-Q resonators (>8000) have been fabricated using this technology up to 200 MHz, but two-pole filters have only been demonstrated up to 10 MHz. This technology still needs a lot of work before it is ready for commercial applications in miniature 0.1- 3 GHz filters. (RF reactive).

The richest class is clearly the RF-intrinsic, which already boasts three promising devices. Here, we have tunable capacitors and inductors that are expected to operate up to at least a few gigahertz in frequency, and RF embedded switches that operate well from a few gigahertz up to at least 100 GHz.

The work presented here will concentrate on RF MEMS switches, which are essential devices for RF reconfigurability. In doing so, it will become apparent that the mapping between RF device and MEMS class is not unique. It simply means that the switching function, or any RF function for that matter, can often be achieved by different MEMS configurations. This is one of the many reasons as to why RF MEMS have recently become interesting to many RF component and circuit engineers.

RF MEMS Switches:

The micro switch is arguably the paradigm RF-MEMS device. In essence, it is a miniaturized version of the venerable toggle switch. In addition to the three classes based on MEMS actuation, the switches can be categorized by the following three characteristics 1) RF circuit configuration

2) Mechanical structure

3) Form of contact

4) Movement.

The different configurations are summarized as in Table 1.1.This means that one can build at least 32 (2 x 2 x 2 x 4) different type of MEMS switches using different actuation mechanisms, contact, and circuit implementations.

ActuationMechanismVoltage(V)Current(mA)Power(mW)SizeSwitchingTime (s)ContactForce(N)

Electrostatic20-80V00Small1- 20050 - 1000

Thermal3-55-1000-200Large300-10,000500-4000

Magnetostatic3-520-1500-100Medium300-1,00050-200

Piezoelectric3-2000Medium50 - 50050-200

Movement

VerticalLateral

Typically results in small size devicesTypically results in large size devices

Contact Type (Switches Only)

Metal-to-MetalCapacitive

DC 60GHz10-120 GHz

Circuit Configuration

SeriesShunt

DC-50GHz with metal-to-metal contactand low up-state capacitance10-50 GHz with capacitive contact and low up state capacitanceDC-60GHz with metal-to-metal contactand low inductance to ground10-200 GHz with capacitive contact and low inductance to ground

Table 1: Different Configuration of MEMS Devices.

Electrostatic actuation is the most prevalent technique in use today due to its virtually zero power consumption, small electrode size, thin layers used, relatively short switching time

(2-24

s ), 50-200

N of achievable contact forces, and the possibility of biasing the switch

using high-resistance bias lines. In many cases, a thermal actuation is coupled with an

electrostatic (voltage) hold, or a magnetostatic actuation (current in a coil) is coupled with a permanent magnetic field.

1.3 MEMS Switches v/s GaAs PIN Diode & FET Switches:

MEMS switches enjoy several advantages over semiconductor switches in the RF

applications :

Near-Zero Power Consumption: Electrostatic actuation requires 20-80 V but does not consume any current, leading to very low power dissipation; (10-100 nJ per switching cycle). Very High Isolation: RF MEMS series switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resulting in excellent isolation at0.1-40 GHz.

Very Low Insertion Loss: RF MEMS series and shunt switches have an insertion loss of -

0.1 dB up to 40 GHz.

Intermodulation Products: MEMS switches are very linear devices and, therefore, result in very low intermodulation products. Their performance is around 30 dB better than p- i-n or FET switches.

Very Low Cost: RF MEMS switches are fabricated using surface micromachining techniques and can be built on quartz, Pyrex; low-temperature cofired ceramic (LTCC), mechanical-grade high-resistivity silicon, or GaAs substrates.

However, RF MEMS switches also have their share of problems, such as:

Relatively Low Speed: The switching speed of most MEMS switches is around 2-40 s.

Certain communication and radar systems require much faster switches.

Power Handling: Most MEMS switches cannot handle more than 20-50mW. MEMS

switches that can handle 0.2-10 W with high reliability simply does not exist today.

High-Voltage Drive: Electrostatic MEMS switches require 20-80 V for reliable operation, and this necessitates a voltage up-converter chip when used in portable telecommunication systems. Reliability: The reliability of mature MEMS switches is 0.1-10 billion cycles. However,

many systems require switches with 20-200 billion cycles. Also, the long-term reliability

(years) has not yet been addressed.

Packaging: MEMS switches need to be packaged in inert atmospheres (nitrogen, argon, etc.) and in very low humidity, resulting in hermetic or near-hermetic seals. Packaging costs are currently high, and the packaging technique itself may adversely affect the reliability of the MEMS switch. Cost: While MEMS switches have the potential of very low cost manufacturing, one must add the cost of packaging and the high-voltage drive chip. It is, therefore, hard to beat a $0.30-0.60 single-pole double-throw 3-V p-i-n or FET switch, tested, packaged, and delivered.

Table 1.2 shows a comparison between electrostatic MEMS switches and GaAS PIN diode and transistor switches . It is hard to make an accurate comparison over a wide range of RF power levels since the size of diode and transistor switches can be easily increased for high power applications. This, in turn, has a substantial effect on the switch isolation, insertion loss, switching speed, and power consumption. Still, it is evident that MEMS switches,

with their extremely low up-state capacitance (series switches) and their very high capacitanceratio (capacitance contact switches), offer a far superior performance compared to solid-state switches for low to medium power applications (Fig. B).

ParameterRF MEMSPINFET

Voltage (V)20-80 3-53-5

Current (mA)03-200

Power consumption (mW)0.05-0.15-1000.05-0.1

Switching time1-300s1-100ns1-100ns

Cup (series) (fF)1-640-8070-140

Rs (series) ()0.5-22-44-6

Capacitance ratio40-50010n/a

Cutoff frequency (THz)20-801-40.5-2

Isolation (1- 10GHz)Very highHighMedium

Isolation (10- 40GHz)Very HighMediumLow

Isolation (60- 100GHz)HighMediumNone

Power handling (W)