2. 2
Outline
• What are the MEMS?
• Function of MEMS
• Motivation
• RF MEMS
• MEMS characterization
• Market Potential And Applications
• RF MEMS Components
• Recent RF MEMS development
• RF MEMS Limitations
• RF MEMS Challenges
• Summary
• References
RF MEMS BASIC
3. 3
What are the Mems?
1) MEMS refer to miniature mechatronic
systems
2) Fabricated using VLSI
technology
3) Techniques and processes to design and
create miniature systems Miniature
embedded system.[3]
Micro electro mechanical Systems (MEMS) have been developed since the 1970s
for pressure and temperature sensors, accelerometers and other sensor devices
The one main criterion of MEMS:
Sort of mechanical functionality
whether or not these elements can
move. [2]
[2]
RF MEMS BASIC
4. 4
Function of MEMS
1. Converts physical
stimuli, events and
parameters to
Electrical, Mechanical,
and optical signals and
vice versa
2. Performs actuation,
sensing and other
functions
RF MEMS BASIC
5. 5
MEMS benefits:
Micro size sensors and actuators: Integration with
electronics on single chip (system or lab on chip)
Decreased cost of production: bulk processing
Many new features and products previously
unthought can be possible
Combination of MEMS with other branches:
Example optical MEMS, Bio-MEMS Æ futuristic
devices
Motivation
[3]
RF MEMS BASIC
6. 6Microwave Couplers
MEMS Fabrication Process
[4]
Silicon-On-Insulator wafers.
This type of wafer consists of a three
layers:
1. Silicon substrate layer (~400 mm)
2. Silicon dioxide buried layer (~1 mm)
3. Silicon device layer (~50 mm)
Five steps:
1) Photo resist spinning and mask contact
photolithography
2. Photo resist development
3. Anisotropic DRIE etching
4. Resist removal
5. Isotropic vapour HF etching
7. 7Microwave Couplers
Mostly used material is silicon
Other materials
ƒ 1) Polycrystalline silicon (poly silicon)
„ 2) Silicon dioxide (SiO2)
„ 3) Silicon nitride (Si3N4)
„ 4) Aluminum (thin film)
„ 5) Gold (thin film)
„ 7) Many more including polymers now a days
„ 8) Doping of silicon [3]
MEMS Materials
8. 8
Micro-electromechanical systems with electronic components comprising moving sub-
millimeter-sized parts that provide radio frequency functionality.
RF MEMS
• Micro system for radio and
millimeter wave applications
• Micro switches
•Micro-machined inductors
•
•capacitors
•Antennas
•Resonators
•filters
RF MEMS BASIC
9. 9
MEMS characterization
Technologies for MEMS characterization:
• Scanning Probe Microscopy (SPM)
„
• Atomic Force Microscopy (AFM)
„
• Scanning Tunneling microscopy (STM)
„
• Magnetic Force Microscopy (MFM)
„
•Scanning Electron Microscope (SEM)
„
•Laser Doppler Vibro meter
„
•Electronic Speckle Interference Pattern technology
(ESPI)
RF MEMS BASIC
11. 11
Market Potential And
Applications
[6]
Cell phones
dominate the
growth
led by the
successes of
Apple's iPhone
and other smart
phones
Apple's iPad will
propel the MEMS
chip market to
over $3 billion by
2014
RF MEMS BASIC
12. 12
RF MEMS Components
Switching networks are used in virtually every communication system
for filter or amplifier selection.
Fig: Schematic diagram of the RF MEMS switch at (a) on-state and (b) off-state. [ 7]
One of the proposed RF MEMS switch that
exploits in-plane, bi-stable, electromagnetic
actuators
• It is similar to a typical coplanar
waveguide
•Impedance mismatch, RF signal is
reflected , off-state. Fig(b)
•, the actuators, original positions, on
state Figure(a)
The switch consumes no standing power due to bi-stability of the actuators
The switch operates at very low voltage
Fabrication process is much simpler than that of the conventional devices
Since the actuators do not face the signal line, they are less susceptible to high power RF signal
RF MEMS BASIC
13. 13
RF MEMS Component (Ohmic switch)
• G voltage 40 to 120 V depends on desig
• Operate up to 60Ghz
• operated under 200 µN
• At 10 GHz: Insertion loss of 0.27 dB
isolation of 12 dB for an 8 contact switc
• on resistance is less than 1 Ω
SEM image of Ohmic switch by Radant
[8]
• Electro statically actuated MEMS
micro switch for both DC and RF
applications
•The micro switch is a 3-terminal device
•The beam is deflected by applying a
voltage between the
G and S
Schematic representation of the ohmic switch
RF MEMS BASIC
14. 14
Components Capacitive switch
[9]
That employ the movement of a mechanical component to place a variable capacitance in the
path of a radio-frequency transmission line so either block transmit RF signal
Applications :
1)Telecommunications,
2)Aerospace and defense
3) Automotive and
4)low-voltage capacitive
shunt micro switch
5)Cmos compatible
•Switch consists of a metallic electrode
•Voltage is applied between the suspended
electrode
RF MEMS BASIC
15. 15
Advantages and Application of RF MEMS Switches
Advantages of MEMS switches over pin diode and FET switch:
Very low Power Consumption
Very High Isolation
Very Low Insertion Loss
High Linearity
Better stability with temperature
Multiple frequency range
Miniaturization (<100um²)
Potential for Low Cost
Applications :
Cellular phones
Transmitters
Receivers
Antennas and transmission lines
Phase shifters
Low frequency applications:
Automatic test equipment
Industrial and medical instrumenta
RF MEMS BASIC
16. 16Microwave Couplers
[10]
Passive Components Tunable
Inductor
Very flexible analog integrated circuits can be implemented by Tunable Inductor
Proposed structure of a variable inductor.
Structure of an improved heat expansion
actuator.
When current flows and heat is generated in the lower metal,
the tip portion goes up.
17. 17
Components (Tunable Capacitor)
Top plate : Electroplating nickel (20um)
covered by a gold layer(2um) thickness
Bottom plate :Poly silicon covered by a
nitride layer of 0.35um thickness
[11]
Two dimensional RF MEMS capacitor structure
configuration
• It consists of two parallel plates
• Four T shaped beams
• Air is between the top plate and bottom
plate
•Two plates are flexible
•300% capacitance increase
RF MEMS BASIC
18. 18
Mems resonator
[12]
•A fixed micro disk is sandwiched
between two suspended deformable
waveguides
•The device is fabricated on a
silicon-on-insulator (SOI) wafer
•The radius of the micro disk is 10 m and initial
gap spacing is 1.4 micro meter.
•To avoid the pull-in instability in gap-closing
actuators, the actuator spacing is designed to
be three times larger than the disk-waveguide
spacing
•Using this design, the coupling gap can be
continuously reduced until physical contact
without experiencing instability
RF MEMS BASIC
19. 19
[13]
Single-stage phase shifter
•High-resistivity mono
crystalline-silicon block
Functional drawing of a single-stage dielectric-block phase shifter with actuation contact
points for electrostatic actuation and its cross-sections
The relative phase shift is achieved by vertically pulling the
mono crystalline-silicon block down above the CPW by
electrostatic actuation.
RF MEMS BASIC
20. 20
Linear-coded phase shifters
[13]
•Multi-stage linear-coded phase shifter with total phase-shift of
360° (left), and its phase resolution of 45°
•A multi-stage linear-coded phase shifter with 45° phase-
shift steps can be achieved by placing seven single 45°
stages in series.
•The distance between
each stage is 10 µm
RF MEMS BASIC
21. 21
Today's smart phones antennas have gotten smaller hey are being asked
to cover lower-frequency bands that would normally require larger antenna
form factors.
professor Gabriel Rebeiz, holding a dual-
band frequency tunable antenna
[14]
Recent RF MEMS development
•Incorporating RF MEMS into smart phone
antennas yields "tunable" antennas
• It work efficiently across one or two frequency
bands at a time
RF MEMS serves:
•low-loss switched variable capacitor
• Capable of changing the antenna's resonant
frequency
RF MEMS BASIC
22. 22
15
Metal contact RF MEMS switches
•These metal switches the width of a human hair
Insertion loss: <0.3db at 3Ghz.
Ultra-high linearity
• 50um x 50um which directly impacts cost.
•Used in 4G phones to provide much
faster speeds.
Metal Switch
Metal-contact and
capacitive switches could
turn out to be extremely
important for tunable RF
front ends of next-
generation communication
systems
RF MEMS BASIC
23. 23
Relatively low switching speed
&
Lifetime reliability
Switching speed :
• Most electrostatic MEMS switches is 2-40 μs
•Thermal/magnetic switches are 200-3, 000 μs
Lifetime :
•Mature MEMS switches is 0.1-40 Billion cycles
•Many systems require switches with 20-200 Billion cycles
RF MEMS Limitations
RF MEMS BASIC
24. 24
RF MEMS Limitations
Temperature changes
•Problems in metal packaging
• coefficient of expansion of metals
> 10 times of silicon
• special isolation techniques
required to prevent package
expansion
[16]
RF MEMS BASIC
25. 25
Humidity effects
•Humidity can be considered a serious issue for MEMS structure.
•Surface micro machined devices are extremely hydrophilic for reasons related
to processing and, for this reason, the surfaces in a humidity atmosphere will
experience both condensation
• Which will create bending moment in structures, and capillarity forces.
• Which will create stronger adhesive bonds than Vander Waals forces alone.
[16]
RF MEMS Limitations
RF MEMS BASIC
26. 26
Vibrations and shocks:
[16]
•Vibration is a large reliability concern in MEMS.
• Due to the sensitivity and fragile nature of many MEMS
•External vibrations can have disastrous implications.
•External vibration can cause failure.
•Long-term vibration will also contribute to fatigue.
•For space applications, vibration considerations are
important, because devices are subjected to large
vibrations in the launch process
Fig: Crack in single crystal
silicon support beams caused
by vibrations from a launch test
RF MEMS Limitations
RF MEMS BASIC
27. 27
RF MEMS Challenges
Solution of several challenges:
Environment
Modeling reliability
Integration
Most important for
commercialization
packaging
[17]
RF MEMS BASIC
28. 28
summary
•RF MEMS is a promising Technology
• A lot of applications
• Better performance than convention RF components
• Packaging is a challenging factor
RF MEMS BASIC
