miércoles, 10 de febrero de 2010


The RF MEMS acronym stands for radio frequency
micro-electromechanical system, and refers to
components of which moving sub-millimeter-sized parts
provide RF functionality. RF functionality can be
implemented using a variety of RF technologies.
Besides RF MEMS technology, ferrite, ferroelectric,
GaAs, GaN, InP, RF CMOS, SiC, and SiGe technology
are available to the RF designer. Each of the RF
technologies offers a distinct trade-off between cost,
frequency, gain, large scale integration, lifetime, linearity,
noise figure, packaging, power consumption, power
handling, reliability, repeatability, ruggedness, size, supply
voltage, switching time and weight.

There are various types of RF MEMS components, such as
RF MEMS resonators and self-sustained oscillators with low
phase noise, RF MEMS tunable inductors, and RF MEMS
switches, switched capacitors and varactors.
Switches, switched capacitors and varactors
RF MEMS switches, switched capacitors and varactors,
which can replace field effect transistor (FET) switches and
PIN diodes, are classified by actuation method (electrostatic,
electrothermal, magnetic, piezoelectric), by axis of deflection
(laterally, vertically), by circuit configuration (series, shunt),
by clamp configuration (cantilever, fixed-fixed beam), or
by contact interface (capacitive, ohmic).
Electrostatically-actuated RF MEMS components offer
low insertion loss and high isolation, high linearity, high
power handling and high Q factor, do not consume
power, but require a high supply voltage and hermetic
wafer level packaging (WLP) (anodic or glas frit wafer
bonding) or single chip packaging (SCP) (thin film capping,
liquid crystal polymer (LCP) or low temperature co-fired
ceramic (LTCC) packaging).
RF MEMS switches were pioneered by Hughes Research
Laboratories, Malibu, Raytheon, Dallas, TX, and Rockwell
Science, Thousand Oaks, CA, during the nineties. The
component shown in Fig. 1, is a center-pulled capacitive
fixed-fixed beam RF MEMS switch, developed and
patented by Raytheon in 1994. A capacitive fixed-fixed
beam RF MEMS switch is in essence a micro-machined
capacitor with a moving top electrode - i.e. the beam.

MEMS switch
From an electromechanical perspective, the components
behave like a mass-spring system, actuated by an
electrostatic force. The spring constant is a function of the
dimensions of the beam, of the Young's modulus, of the
residual stress and of the Poisson ratio of its material. The
electrostatic force is a function of the capacitance and the
bias voltage. Knowledge of spring constant and mass
allows for calculation of the pull-in voltage, which is the bias
voltage necessary to pull-in the beam, and of the switching time.
From an RF perspective, the components behave like a
series RLC circuit with negligible resistance and inductance.
The up- and down-state capacitance are in the order of 50fF
and 1.2 pF, which are functional values for millimeter-wave
circuit design. Switches typically have a capacitance ratio of 30
or higher, while switched capacitors and varactors have a
capacitance ratio of about 1.2 to 10. The loaded Q factor is
between 20 and 50 in the X-, Ku- and Ka-band.
RF MEMS switched capacitors are capacitive fixed-fixed
beam switches with a low capacitance ratio. RF MEMS
varactors are capacitive fixed-fixed beam switches which
are biased below pull-in voltage. Other examples of
RF MEMS switches are ohmic cantilever switches, and
capacitive single pole N throw (SPNT) switches based on
the axial gap wobble motor.
An RF MEMS fabrication process allows for
integration of SiCr or TaN thin film resistors (TFR),
metal-air-metal (MAM) capacitors, metal-insulator-metal
(MIM) capacitors, and RF MEMS components. An
RF MEMS fabrication process can be realized on a variety
of wafers: fused silica (quartz), borosilicate glass, LCP,
sapphire, and passivated silicon and III-V compound
semiconducting wafers. As shown in Fig. 2, RF MEMS
components can be fabricated in class 100 clean rooms using
6 to 8 optical lithography steps with a 5 μm contact
alignment error, whereas state-of-the-art monolithic microwave
integrated circuit (MMIC) and radio frequency integrated
circuit (RFIC) fabrication processes require 13 to 25 lithography
steps. The essential microfabrication steps are:

  • Deposition of the bias lines (Fig. 2, step 3)
  • Deposition of the electrode layer (Fig. 2, step 4)
  • Deposition of the dielectric layer (Fig. 2, step 5)
  • Deposition of the sacrificial spacer (Fig. 2, step 6)
  • Deposition of seed layer and subsequent electroplating
 (Fig. 2, step 7)
  • Beam definition, release and critical point drying
(Fig. 2, step 8)
RF MEMS fabrication processes, unlike barium strontium
titanate (BST) or lead zirconate titanate (PZT) ferroelectric
and MMIC fabrication processes, do not require electron
beam lithography, molecular beam epitaxy (MBE), or metal
organic chemical vapor deposition (MOCVD). With the
exception of the removal of the sacrificial spacer, the
fabrication steps are compatible with a CMOS fabrication
Applications of RF MEMS resonators and switches
include oscillators and routing networks. RF MEMS
components are also applied in radar sensors (passive
electronically scanned (sub)arrays and T/R modules)
and software-defined radio (reconfigurable antennas,
tunable band-pass filters).
Polarization and radiation pattern reconfigurability, and
frequency tunability, are usually achieved by
incorporation of lumped components based on III-V
semiconductor technology, such as single pole single
throw (SPST) switches or varactor diodes. However,
these components can be readily replaced by RF MEMS
switches and varactors in order to take advantage of
the low insertion loss and high Q factor offered by
RF MEMS technology. In addition, RF MEMS
components can be integrated monolithically on low-loss
dielectric substrates, such as borosilicate glass, fused silica or
LCP, whereas III-V semiconducting substrates are generally l
ossy and have a high dielectric constant. A low loss tangent and
low dielectric constant are of importance for the efficiency and
the bandwidth of the antenna.
The prior art includes an RF MEMS frequency tunable
fractal antenna for the 0.1–6 GHz frequency range, and the
actual integration of RF-MEMS on a self-similar Sierpinski
gasket antenna to increase its number of resonant frequencies,
extending its range to 5GHz, 14GHz and 30GHz, an
RF MEMS radiation pattern recongurable spiral antenna for
6 and 10 GHz, an RF MEMS radiation pattern recongurable
spiral antenna for the 6–7 GHz frequency band based
on packaged Radant MEMS SPST-RMSW100 switches,
an RF MEMS multiband Sierpinski fractal antenna, again
with integrated RF MEMS switches, functioning at different
bands from 2.4 to 18 GHz, and a 2-bit Ka-band
RF MEMS frequency tunable slot antenna.
RF bandpass filters are used to increase out-of-band
rejection, if the antenna fails to provide sufficient selectivity.
Out-of-band rejection eases the dynamic range requirement
of low noise amplifier LNA and mixer in the light of
interference. Off-chip RF bandpass filters based on
lumped bulk acoustic wave (BAW), ceramic, surface
acoustic wave (SAW), quartz crystal, and thin film bulk
acoustic resonator (FBAR) resonators have superseded
distributed RF bandpass filters based on transmission line
resonators, printed on substrates with low loss tangent,
or based on waveguide cavities. RF MEMS resonators
offer the potential of on-chip integration of high-Q
resonators and low-loss bandpass filters. The Q factor
of RF MEMS resonators is in the order of 1000-1000.
Tunable RF bandpass filters offer a significant size
reduction over switched RF bandpass filter banks.
They can be implemented using III-V semiconducting
varactors, BST or PZT ferroelectric and RF MEMS
switches, switched capacitors and varactors, and
yttrium iron garnet (YIG) ferrites. RF MEMS
technology offers the tunable filter designer a
compelling trade-off between insertion loss, linearity,
power consumption, power handling, size, and
switching time.
Phase shifters
RF MEMS phase shifters have enabled wide-angle
passive electronically scanned arrays, such as lenses,
reflect arrays, subarrays and switched beamforming
networks, with high effective isotropically radiated
power (EIRP), also referred to as the power-aperture
product, and high Gr/T. EIRP is the product of the
transmit gain, Gt, and the transmit power, Pt. Gr/T is
the quotient of the receive gain and the antenna noise
temperature. A high EIRP and Gr/T are a prerequisite
for long-range detection. The EIRP and Gr/T are a
function of the number of antenna elements per subarray
and of the maximum scanning angle. The number of
antenna elements per subarray should be chosen to
optimize the EIRP or the EIRP x Gr/T product, as
shown in Fig. 3 and Fig. 4.

Passive subarrays based on RF MEMS phase shifters
may be used to lower the amount of T/R modules in
an active electronically scanned array. The statement
is illustrated with examples in Fig. 3: assume a
one-by-eight passive subarray is used for transmit as
well as receive, with following characteristics: f = 38 GHz,
Gr = Gt = 10 dBi, BW = 2 GHz, Pt = 4 W. The low
loss (6.75 ps/dB) and good power handling (500 mW)
of the RF MEMS phase shifters allow an EIRP of 40W
and a Gr/T of 0.036 1/K. The number of antenna elements
per subarray should be chosen in order to optimize the
EIRP or the EIRP x Gr/T product, as shown in Fig. 3
and Fig. 4. The radar range equation can be used to
calculate the maximum range for which targets can be
detected with 10 dB of SNR at the input of the receiver.

In which kB is the Boltzmann constant, λ is the free-space

wavelength, and σ is the RCS of the target. Range values
are tabulated in Table 1 for following targets: a sphere with
a radius, a, of 10 cm (σ = π a2), a dihedral corner reflector
with facet size, a, of 10 cm (σ = 12 a4/λ2), the rear of a
car (σ = 20 m2) and for a contemporary non-evasive fighter
jet (σ = 400 m2). A Ka-band hybrid ESA capable of
detecting a car 100 m in front and engaging a fighter jet at
10 km can be realized using 2.5 and 422 passive subarrays
(and T/R modules), respectively.
Table 1: Maximum Detectable Range
(SNR = 10 dB)

RCS (m2)
Range (m)
Rear of Car
Dihedral Corner Reflector
Fighter Jet
The usage of true-time-delay TTD phase shifters instead
of RF MEMS phase shifters allows ultra-wideband (UWB)
radar waveforms with associated high range resolution,
and avoids beam squinting or frequency scanning. TTD
phase shifters are designed using the switched-line
principle or the distributed loaded-line principle.
Switched-line TTD phase shifters are superior to
distributed loaded-line TTD phase shifters in terms
of time delay per decibel noise gure (NF),
especially at frequencies up to X-band, but are
inherently digital and require low-loss and
high-isolation SPNT switches. Distributed
loaded-line TTD phase shifters, however, can be
realized analogously or digitally, and in smaller
form factors, which is important at the subarray
level. Analog phase shifters are biased through a
single bias line, whereas multibit digital phase
shifters require a parallel bus along with complex
routing schemes at the subarray level. In addition,
usage of an analog bias voltage avoids large phase
quantization errors, which deteriorate the EIRP and
beam-pointing accuracy, and elevate the sidelobe
level of an electronically scanned array.
The prior art in passive electronically scanned arrays,
shown in Fig. 6, includes an X-band continuous
transverse stub (CTS) array fed by a line source
synthesized by sixteen 5-bit reflect-type RF MEMS
phase shifters based on ohmic cantilever RF MEMS
switches, an X-band 2-D lens consisting of parallel-plate
waveguides and featuring 25,000 ohmic cantilever
RF MEMS switches, and a W-band switched
beamforming network based on an RF MEMS SP4T
switch and a Rotman lens focal plane scanner.

T/R modules
Within a T/R module, as shown in Fig. 7, RF MEMS limiters,
tunable matching networks  and TTD phase shifters can be
used to protect the LNA, load-pull the power amplifier (PA)
and time delay the RF signal, respectively. Whether
RF MEMS T/R switches - i.e. single pole double throw
(SPDT) switches, can be used depends on the duty
cycle and the pulse repetition frequency (PRF) of the
pulse-Doppler radar waveform. To date, RF MEMS
duplexers can only be used in low PRF and medium
PRF radar waveforms for long-range detection, which
use pulse compression and therefore have a duty cycle
in the order of microseconds.

Realizado: Franco A Rivera C.
Asignatura: CRF

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