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Introduction:-
The Holy Grail of MEMS devices is the micromechanical switch. For more than a decade, researchers have endeavored to perfect the development of microminiature relays using micromachining techniques. With the recent boom in wireless communications, research has intensified in the quest to develop low cost, ultra-low loss switches. The goal is to have these switches replace traditional FETS for reduced loss and improved linearity in key components. There are fundamentally two types of switch contact mechanisms – ohmic contact and capacitive contact. With ohmic switches, two metal electrodes are brought into contact to create a low-resistance connection. In capacitive switches, a metal membrane is pulled down onto a dielectric layer, usually by electrostatic means, to form a capacitive sandwich. At high frequencies, the capacitive suseptance of this sandwich acts like a short circuit. In either case, the mechanical action of the switch causes the switch to efficiently change from high impedance to short circuit.
Micromechanical switches can utilize one of many actuation mechanisms, including magnetic, piezoelectric, thermal, and most commonly electrostatic forces. Switches that operate electrostatic ally require very little energy, usually on the order of tens of nanojoules per switch cycle. The Achilles Heel of all MEMS switches is their switching speed, which is determined by their mechanical resonant frequency. Actuation is typically accomplished in microseconds to 10s of microseconds for electrostatic ally operated devices, and 100s of microseconds to milliseconds for thermal actuators. Despite the fact that MEMS switches operate slower than their electronic counterparts, they are still useful in many applications. One important advantage of MEMS switches is their linearity. Unlike electronic switches made with metal-semiconductor or p-n junctions, the contact area for MEMS switches is perfectly linear. This means that well-designed MEMS switches do not create nonlinearities or distortion such as harmonics or intermediation products. In many cases, these nonlinearities are immeasurable.
History
1948 Invention of the Germanium transistor at Bell Labs (William Shockley)
1954 Piezoresistive effect in Germanium and Silicon (C.S. Smith)
1958 First integrated circuit (IC) (J.S. Kilby 1958 / Robert Noyce 1959)
1959 "There’s Plenty of Room at the Bottom" (R. Feynman)
1959 First silicon pressure sensor demonstrated (Kulite)
1967 Anisotropic deep silicon etching (H.A. Waggener et al.)
1968 Resonant Gate Transistor Patented (Surface Micromachining Process) (H. Nathanson, et.al.)
1970’s Bulk etched silicon wafers used as pressure sensors (Bulk Micromaching Process)
1971 The microprocessor is invented
1979 HP micromachined ink-jet nozzle
1982 "Silicon as a Structural Material," K. Petersen
1982 LIGA process (KfK, Germany)
1982 Disposable blood pressure transducer (Honeywell)
1983 Integrated pressure sensor (Honeywell)
1983 "Infinitesimal Machinery," R. Feynman
1985 Sensonor Crash sensor (Airbag)
1985 The "Buckyball" is discovered
1986 The atomic force microscope is invented
1986 Silicon wafer bonding (M. Shimbo)
1988 Batch fabricated pressure sensors via wafer bonding (Nova Sensor)
1988 Rotary electrostatic side drive motors (Fan, Tai, Muller)
1991 Polysilicon hinge (Pister, Judy, Burgett, Fearing)
1991 The carbon nanotube is discovered
1992 Grating light modulator (Solgaard, Sandejas, Bloom)
1992 Bulk micromachining (SCREAM process, Cornell)
1993 Digital mirror display (Texas Instruments)
1993 MCNC creates MUMPS foundry service
1993 First surface micromachined accelerometer in high volume production (Analog Devices)
1994 Bosch process for Deep Reactive Ion Etching is patented
1996 Richard Smalley develops a technique for producing carbon nanotubes of uniform diameter
1999 Optical network switch (Lucent)
2000s Optical MEMS boom
2000s BioMEMS proliferate
2000s The number of MEMS devices and applications continually increases
2000s NEMS applications and technology grows
Earlier bi-MEMS switch
MEMS Switch characteristics
Actuation Mechanisms:
The actuation forces required for the mechanical movement can be obtained using electrostatic, magneto-static, piezoelectric or thermal designs. To date, only electrostatic-typeswitches have been demonstrated at 0.1-100GHz with high reliability at low RF powers for metal contact and medium power levels for capacitive contacts (100Million to 50 Billion cycles depending on the manufacturer) and wafer-scale manufacturing techniques.Other switches which have demonstrated excellent performance are the Microlab Latching switch (up to 100 Million cycles) using magnetic actuation, and the thermal switches developed independently by Cronos Microsystems and the Univ. of California, Davis. It is hard to test thermal switches for long cycle times due to their slow switching response (1-3ms).
Switching Time:
Electrostatic switches can be made small and with a very fast switching time (2-30 μs) while thermal/magnetic actuation requires around 100-2, 000 μs of switching time. An excellent metal-contact switch developed by LETI using thermalactuation but with an electrostatic hold, thereby requiring very little switching energy and virtually zero hold-down power. However, its switching time is still relatively slow (300 μs). The LETI switch has been tested to more than 100 million cycles.
Contact Type:
There are two different contacts in RF MEMS switches, a capacitive contact and a metalto-metal (or DC) contact. The capacitive contact is characterized by the capacitance ratio between the up-state (open circuit) and down-state (short-circuit)positions, and this is typically 80-160 depending on the design. The down-state capacitance is typically 2-3 pF, and is suitable for 8-100GHz applications. In general, it is hard to obtain a large down-state capacitance using nitride or oxide layers, and this limits the low-frequency operation of the device. On the other hand, DC-contact switches with small up-state capacitances (open circuit) can operate from 0.01 to 40GHz, and in some cases, to 60GHz (for example, the Rockwell Scientific switch has an up-state capacitance of only 1.75 fF and an isolation of 23 dB at60GHz). In the down-state position (short-circuit), the DC-contact switch becomes a series resistor with a resistance of 0.5-2 Ω, depending on the contact metal used.
Circuit and Substrate Configurations:
As is the case with all two-terminal devices, the switches can be placed in series or in shunt across a transmission line. Typically, capacitive switches have been used in a shunt configuration, while DC-contact switches are placed in series. The reason is that it is easier to get a good isolation with a limited impedance ratio (such as the capacitive switch) in a shunt-circuit than in a series circuit. Also, MEMS switches are compatible with both microstrip and CPW lines on glass, silicon and GaAs substrates, and have been used in these configurations all the way to 100GHz. For low loss applications at microwave frequencies, it is important to use high-resistivity substrates.
Circuits with RF MEMS switches
The near-ideal electrical response of RF MEMS switches (both metal-contact and capacitive) have allowed many designers to build state-of-the-art switching circuits from 0.1GHz all the way to 120GHz.In the past 4 years, these applications concentrated on the replacement of GaAs phase shifters which are commonly used in phased arrays by the thousands of units. A comparison between 3-bit GaAs phase shifters and MEMS phase shifters is shown in Table I and it is seen that MEMS switches provide an immense performance benefit especially at Ka-Band to W-band applications
It is based on the Rockwell metal contact switch and on CLC delay lines for miniaturization. The phase shifter results in an average loss of 1.4dB at 10GHz, a ±3◦ phase error, and is matched to −13 dB at the input and output ports from 6-16GHz. This phase shifter represents the smaller design using RF MEMS to-date, and with excellent response. an 885-986MHz 5-pole tunable filter using switched MEMS capacitors developed by Raytheon Systems Co. In this case, capacitive switches are used to switch fixed-value metal insulator- metal capacitors in the transmission line. The filter employs 18 switches and is a very complicated circuit with variable resonators and impedance inverters. Its measured response is nearly ideal, with excellent frequency tuning capabilities, very high linearity (in terms of measured IIP3) and a loss of 5-6 dB due to the finite Q of the planar inductors used (Q = 30 at 0.9GHz).
Design and Fabrication of the RF Switch
Design Optimization
The main purpose of this optimization scheme is to maximize the deflection for a constant applied
voltage to the actuator (5 volts). ANSYS software includes a parametric solver that was used to
perform the optimization based on the following criteria:
Design Variables:
o Length of beam (150 μm<L< 400 μm)
o Modulus of elasticity of the cantilever material (50 GPa<E<270 GPa)
_ State Variables
o Width of the cantilever (90 μm <W<100 μm)
_ Objective Function
o Maximizing the deflection of the cantilever beam: (Max. (Xtip))
Simulation and Results
For the optimization parameters stated earlier and for an input voltage of 5 volts, the following results
are obtained.
Optimization Parameters
• E = 75 GPa
• W=90 μm
• L=150 μm
Results
Switching time = 1 ms
Maximum tip deflection = 0.4 μm
Maximum von-miss stress = 28 MPa
Switch fabrication
The summary of the steps proposed for the fabrication are as follows (figure 8) [7]:
Begin with a Silicon substrate -1-
Deposit the silicon nitride (SixNy) as an insulating layer using chemical vapor
Deposition (CVD)
Create a silicon dioxide (SiO2) sacrificial layer by CVD
Using positive photo-resist, the sacrificial layer is exposed to ultra violet rays through a mask
The whole substrate is developed in a developer solution (H2SO4) to remove area of SiO2 exposed to UV
The first layer of metal (Au) is deposited using sputter deposition -2-
Pattern the layer of Au by deep reactive ion etching (DRIE)
Repeat above processes to deposit the heat sink metal -3-
Deposit an SiO2 layer
Use lithography to pattern the SiO2 then deposit the first Nitinol TiNi alloy metal by sputter deposition and repeat same process to deposit the second Au metal contact –see 5-
Pattern the SiO2 layer to open a window in the sacrificial layer
Deposit the polyimide to form the cantilever beam -4-
_ The last sacrificial layer is deposited by CVD
_ Pattern the SiO2 layer to deposit the second TiNi alloy metal
_ Use DRIE to pattern the TiNi in the desired form
Selective etch the SiO2 layer in hydrofluoric acid (HF) leaving a free standing micro structure SiO2 SixNy
ELECTROMECHANICAL MODELING OF RF MEMS SWITCHES
A typical capacitive RF MEMS switch consists of a fixed-fixed thin metallic membrane which is suspended over a bottom electrode insulated by a dielectric film. When the switch is not actuated, there is low capacitance between the membrane and the bottom electrode, and the device is in the OFF state. When voltage is applied between the movable structure and the fixed bottom electrode, electrostatic charges are induced on both the movable structure and the bottom electrode. The electrostatic charges cause a distributed electrostatic force, which deforms the movable structure. In turn, such deformation leads to storage of elastic energy, which tries to restore the structure to its original shape. The structure deformation also results in the reorganization of all surface charges on the device. This reorganization of charges causes further
Structural deformation; hence, the device exhibits a highly nonlinear, coupled electromechanical behavior. Until a certain voltage is applied, the so-called pull-in voltage or actuation voltage, an equilibrium position exists through a balance between the elastic restoring force and electrostatic force. After pull-in, the device is in the ON state and its capacitance is much larger than that in the OFF state.
The switch actuation is therefore a coupled-field problem of electrostatics and structural response. In order to accurately describe the switch deformation and predict the pull-in voltage, an effort to realize modeling has to be made. In the following section, we will discuss a simple 1D parallel-plate actuator model [7, 8], a 2D distributed model [9, 10], and a 3D fully coupled model [11, 12]. The analyses and simulations are dedicated to capacitive MEMS switches, although they are also applicable to other types of electrostatic devices.
Classification:
The RF mems switch mainly classified into two type
1)series(Resistive type)
1. Rockwell’s Resistive (Series) Type MEMS Switch
Problems
• Micro-welding between metal and metal
• Hard to have good isolation over 10 GHz
Specifications
Typical Device Specifications
1. All RF measurements were made in a 50 Ω system.
2. Measurements include bond-wires from die to test-board.
The RMSW240™ is a Single Pole Four Throw (SP4T) Reflective RF Switch utilizing Radant’s breakthrough MEMS technology that delivers high linearity, high isolation and low insertion loss in a chipscale package configuration. This device is ideally suited for use in many applications such as RF and microwave multi-throw switching, radar beam steering antennas, phase shifters, RF test instrumentation, ATE, cellular, and broadband wireless access.
Table . Target specifications of the RF MEMS switch.
Operating frequency 8–12 GHz (X band)
Insertion loss < 0•5 dB
Isolation > 30 dB
Actuation voltage < 50
RF power handling capacity < 500mW
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 ingle 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 orosilicate glass, fused silica or LCP, whereas III-V semiconducting substrates are generally lossy 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 8 GHz, 14 GHz and 25 GHz, 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, subarray and switched beam forming networks, with high effective isotropic ally 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
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 40 W 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.
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.
T/R modules
Within a T/R module, as shown in Fig. 7, RF MEMS limiters, tunable matching networks [31][32] 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.
Advantages
MEMS switches are surface-micromachined deviceswhich use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission-line (Figs. 1-2). RF MEMS switches are the specific micromechanical switches which are designed to operate at RF to mm-wave frequencies (0.1 to 100 GHz). The advantages of MEMS switches over PIN diode or FET switches are
Near-Zero Power Consumption:
Electrostatic actuation requires 30-80 V, but does not consume any current, leading to a very low power dissipation (10- 100 nJ per switching cycles). On the other hand, thermal/magnetic switches consume a lot of current un less theys are made to latch in the down-state position once actuated.
Very High Isolation:
RF MEMS metal-contact switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resulting in excellent isolation at 0.1-60GHz. Also, capacitive switches with a capacitance ratio of 60-160 provide excellent isolation from 8-100GHz.
Very Low Insertion Loss:
RF MEMS metal-contact and capacitive switches have an insertion loss of 0.1dB up to 100GHz.
Linearity and Intermediation Products:
MEMS switches are extremely linear devices and therefore re-sult in very low intermediation products in switching and tuning operations. Their performance is 30-50 dB better than PIN or FET switches.
Potential for Low Cost:
RF MEMS switches are fabricated using surface micromachining techniques and can be built on quartz, Pyrex, LTCC, mechanicalgrade high-resistivity silicon or GaAs substrates
Dis¬¬advantages
Relatively Low Speeds:
The switching speed of most electrostatic MEMS switches is 2-40 μs, and thermal/magnetic switches are 200-3,000 μs. Certain communication and radar systems require much faster switches
11.2High Voltage or High Current Drive:
Electrostatic MEMS switches require 30-80 V for reliable operation, and this requires a voltage up-converter chip when used in portable telecommunication systems. Thermal/magnetic switches can be actuated using 2-5 V,but require 10-100 mA of actuation current.
Power Handling:
Most MEMS switches cannot handle more than 200 mW although some switches have shown up to 500 mW power handling (Terravicta and Raytheon). MEMS switches that handle 1-10 W with high reliability simply do not exist today.
Packaging: MEMS
Switches need to be packagedin inert atmospheres (Nitrogen, Argon, etc..) and in very low humidity, resulting in hermetic or nearhermetic seals. Hermetic packaging costs are currently relatively high, and the packaging technique itself may adversely affect the reliability of the MEMSswitch.
Future scope
It is now clear that we understand RF MEMS switches well, both from the mechanical and electrical/electromagnetic point of view. We can design complicated circuits using MEMS switches or varactors, and we can accurately predict their performance all the way to 120 GHz. They are still not accepted in the commercial and defense arena due to their need of a hermetic package, and their reliability under medium to high-power conditions. There is currently an intense effort to solve these problems, and the we believes that RF MEMS switches and varactors will play an essential role in future high-value commercial and defense systems.
Conclusion
Thus we conclude that RF MEMS technology provides a new implementation approach that promises to reduce the pressure on design of multi radio products. It enables a variety of novel functions and capabilities based on the concept of tunable RF.Due to their ability to move usually under software control,RF-Mems devices allow a single hardware component to emulate the behavior of multiple discrete devices with great performance at comparable size and competitive cost. Although tremendous progress has been made, and remaining adoption barriers should be overcome within the frame of twelve to eight months so that RF MEMS devoices could be deployed on global basis 3G mobiles. Just as semiconductors tuned out to be the savior of electronics five decades ago RF-MEMS would be the key of future generation.
Bibliography
RF MEMS for wireless communications magazine Aug2010
MEMS technology in communications by St.joseph College
IEEE journals RF MEMS Technologies
Web reference;
wikiRFMEMS
googlerfmems
ieeejournalsmems
pdfsearchRFmems
