RF MEMS 2

RF MEMS

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.

Components

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.

Microfabrication

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

process.

Applications

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).

Antennas

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 reconfigurable spiral antenna for

6 and 10 GHz, an RF MEMS radiation pattern reconfigurable

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.

Filters

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 G_r/T. EIRP is the product of the

transmit gain, G_t, and the transmit power, P_t. G_r/T is

the quotient of the receive gain and the antenna noise

temperature. A high EIRP and G_r/T are a prerequisite

for long-range detection. The EIRP and G_r/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 G_r/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,

G_r = G_t = 10 dBi, BW = 2 GHz, P_t = 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 G_r/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 G_r/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 k_B 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)
Sphere	0.0314	10
Rear of Car	20	51
Dihedral Corner Reflector	60.9	67
Fighter Jet	400	107

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 figure (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.

Página: www.wikipedia.com

Realizado: Franco A Rivera C.

Asignatura: CRF