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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).
Introduction contd.
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
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.
𝐶𝑉 2
F=
2𝑑
RF MEMS switches examples
Cantilever MEMS Switch
RF MEMS switches examples
Cantilever MEMS Switch
RF MEMS switches examples
Bridge MEMS Switch
RF MEMS switches examples
Bridge 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
 It is easy to fabricate;
 Low inductance (negative mutual inductance)
 The inductance is : L =
NФ
;
𝐼
 I is current, Ф is magnetic flux, N is the turn number
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 = L1 + L2 + … + Li
Stray Capacitance due to leads
RF MEMS Inductors
Solenoids
 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 : L =
μ0 μ𝑟 N2 A𝑐
𝑙𝑐
 μ0 and μ𝑟 are free space and relative permittivity;
N: turn number; A𝑐 : cross-section area; l𝑐 : core length
RF MEMS Inductors
Solenoids
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 : C =
εA
𝑑
This capacitance can be tuned by changing either area (A), the
distance (d) between plates, or the dielectric constant (ε).
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
 The capacitance is : C =
d
3
εA
𝑋
with x variable.
 At the distance of X = , there is pull in effect where
top plate collapses on bottom plate.
 Hence the tuning range is limited.
RF MEMS Capacitors
MEMS 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. C =
D:distance
Between
Plates.
𝑥𝑤
ε ;
𝑑
w: width; x: occupied length;
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
 Insertion loss: ratio between input and output signal.
 Quality factor: ratio of energy stored to the
𝑓0
dissipated energy. Q =
𝛥𝑓
 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(𝛥𝑓) and the central
frequency (𝑓0 ).
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”.