EEE529:Microsystems

RF MEMS

Mamady Kebe

Introduction:

Radio frequency microelectromechanical system refers to Electronic components at micro size scale; Mechanical functionality.e.g:swiches; Radio frequency IC applications.Traditional MEMS has two classes:MEMS actuators.e.g:micromotors;MEMS sensors.e.g:pressure sensors(MPX series

pressure sensors).

RF MEMS are the later development of the traditional MEMS technology.

They have many applications: Sensing; Actuation; Mechanical switching and micro relays; Capacitors and inductors; Filters; Phase-shifters

Introduction contd.

Materials for RF MEMS

Silicon materials are the most common materials in MEMS fabrication.Other materials are involved such as:

Metals and metal alloys:• Metals are being used for long time in IC chips;• Thick-film metal structures are implemented for

MEMS;• Nickel,copper and gold can be electroplated to form

thick-film;

Materials for RF MEMS

Metals and metal alloys contd.• Metal alloys have been also developped for MEMS

structures.• CoNiMn is used for magnetic actuation;• NiFe is electroplated onto silicon for magnetic

microelectrochemical devices like micromotors,microsensors,integrated power converter.It gives the possibility of new micropower magnetics prodution on ICs.

Materials for RF MEMS

Polymers:• Can be used as structural material:elasticity, optical

properties, biocompatibility.• Can be used as functional materials as well.• Microdevices can be made with thin or thick films.• Example of thin polymer:Polyimide (elastic).• Examples of thick polymers: PMMA (elastic, optical);

polysulfone (mechanical and chemical resistant).• As functional polymers we can list: PVDF (polyvinilidene

fluoride)(piezoelectricity) used as actuator or sensor; polypyrrole (conductivity)

Materials for RF MEMS

Other materials are used in RF MEMS fabrication:• Ceramics are used in thin or thick version:ceramic

pressure microsensors.• SiO2, Si3N4 are also frequent in silicon MEMS

fabrication

RF MEMS switches

Switches are vital for all automated systems.

RF MEMS switches

Some parameters have to be taken into account for better swiching capability:

Transition time: time required to raise from 10% to 90 % of the final signal in an on-to-off direction or vice-versa.

Switching rate: time required for the switch to respond after change in control voltage.

Switching tansients: they are decaying voltages at the output due to change in control voltage.

RF MEMS switches

RF Power handling: indcates power efficiency of the switch from one direction to the other.

Impedance matching: good input and output matching is required to avoid the signal reflexion.

Available bandwidth: Bandwidth is determined by the operating frequency range. Upper frequency is limited by resistances and parasite reactances.

Insertion loss: It is determined by the transmitivity of the switching device.

RF MEMS switches

Series resistance: the connection of the switch to the transmission path can offer some series resistance.

Actuation voltage: control voltage for operating the switch.

Life cycle: the time within the switch operates properly.

Intercept points: determines the linearity of the rf signals.

RF MEMS switches

There are two main types of RF MEMS types :• The Cantilever structure MEMS switch;• The Bridge structure MEMS switch.They both operate under the principles of electrostatic force between the upper and lower electrodes.

RF MEMS switches examplesCantilever MEMS Switch

RF MEMS switches examplesCantilever MEMS Switch

RF MEMS switches examplesBridge MEMS Switch

RF MEMS switches examplesBridge MEMS Switch

RF MEMS switches examplesBridge MEMS Switch

RF MEMS Inductors

An inductor is an electronic component capable of inducing a voltage with time varying current.

It stores magnetic energy (e.m.f). It is generally coils of wires in spiral or circular shape.The use of wire in macroscopic scale affects the

circuit in the following manner: It has capacitance and resistance along with the

inductance; The signal is delayed due to these parasitic effects;

RF MEMS Inductors

The noise is generated due to the resistance; Insertion loss is increased.Hence a microscale version of an inductor implies

minimized values of parasitic elements

RF MEMS Inductors

The different types of micromachined inductors are:• Meander (a);• Spiral (c);• Solenoid;

RF MEMS InductorsMeander Inductor

It is easy to fabricate; Low inductance (negative mutual inductance) The inductance is : ; I is current, is magnetic flux, N is the turn number

RF MEMS InductorsSpiral Inductor

It is IC compatible; It has a closed magnetic circuit; It has a low resistance; The total inductance is the sum of the individual

inductances of different paths : L = L1 + L2 + … + Li

Stray Capacitance due to leads

RF MEMS InductorsSolenoids

Coils wrapped around a magnetic thin film. Coil ends are connected to substrate via contacts; More turns means larger value of inductance,

however it induces high resistive elements. Electroplating the contacts could reduce the

resistance. The inductance is given by : and are free space and relative permittivity;N: turn number; : cross-section area; : core length

RF MEMS InductorsSolenoids

RF MEMS Capacitors

The capacitors are usually used in RF MEMS application as variable capacitors, although there are also non variable MEMS capacitors.The capacitance is given by :

This capacitance can be tuned by changing either area (A), the distance (d) between plates, or the dielectric constant ().

RF MEMS Capacitors

Therefore RF MEMS tunable capacitors are categorized according to their tuning parameters: MEMS gap-tuning capacitors; MEMS area-tuning capacitors; MEMS dielectric tunable capacitors.

RF MEMS CapacitorsMEMS gap-tuning capacitors

The gap-tuning capacitors can be made with two parallel electrodes. The lower is fixed while the upper is connected to a spring and is movable.A dc voltage is applied to the electrodes. The gap between the electrodes changes with change in applied voltage.This principle can be implemented with three plates, the middle one being movable and the other two being fixed.The bridge switch can be used also as a variable capacitor.

RF MEMS CapacitorsMEMS gap-tuning capacitors

The capacitance is : with x variable. At the distance of , there is pull in effect where top

plate collapses on bottom plate. Hence the tuning range is limited.

RF MEMS CapacitorsMEMS area-tuning capacitors

The most common area-tuning capacitor in MEMS technology is the interdigital comb structure capacitor. A dc Voltage is applied between the combs ( one fixed, the

other movable). The fingers length determines the tuning range. No pull in effect. ; w: width; x: occupied length;D:distanceBetween Plates.

RF MEMS CapacitorsMEMS dielectric tunable capacitors

The interdigital structure can be used also as dielectric tunable capacitor. Both combs can be fixed. The dielectric material (STO), grows in size as the

temperature increases. Hence its dielectric constant changes.

MEMS Filters

Filters are used to suppress the unwanted frequency components and keep the wanted ones.Filters in general implies mechanical waves propagation (vibration) in their operation. However, some micromachining filters are not mechanical waves processors. Based on the frequency band they transmit, MEMS filters can be classified as low pass, high pass, band pass or band stop filters.Band pass filters are the most common ones in communication. To evaluate the performance of a filter, several parameters are considered.

MEMS Filters

Insertion loss: ratio between input and output signal. Quality factor: ratio of energy stored to the

dissipated energy. Roll off: the rate at which the filter passes from pass

band to stop band. Stop band rejection: the signals transmitted through

the filters at frequencies beyond the pass band.Every filter is composed of a resonator circuit, which determines the bandwidth() and the central frequency ().

MEMS Filters

MEMS phase-shifters

A phase-shifter is a two port network which has the ability to control the phase difference between the input and output phase.They are used in phase-arrays where multiple antennas are fed by a single input power.The phase-shifters must have low insertion loss, low cost and lightweight.There are digital phase-shifters (discrete phase values), and analog phase-shifters.

MEMS phase-shifters

MEMS phase-shifters

MEMS switches can be used as phase-shifters by switching between two different signal paths.They have low insertion loss and small foot print.

References

Vijay K.Varadan, K.J.Vinoy, K.A Jose “RF MEMS and their applications”;

John R.Reinke “CMOS-MEMS Variable Capacitors for Reconfigurable RF Circuits” ;

Lei Zhou “RF MEMS DC Contact Switch for Reconfigurable Antennas”;

R.Aigner “MEMS in RF Filter Applications : Thin-film Bulk Acoustic Wave Technology”.