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

RF MEMS switches examples Cantilever MEMS Switch

RF MEMS switches examples Bridge 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 Inductors Meander Inductor

RF MEMS Inductors Spiral 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 = L 1 + L 2 + … + L i Stray Capacitance due to leads

RF MEMS Inductors Solenoids

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 Capacitors MEMS 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 Capacitors MEMS gap-tuning capacitors

RF MEMS Capacitors MEMS area-tuning capacitors

RF MEMS Capacitors MEMS 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

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 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.