RF Mems Switch 2011

A project report on ELECTROSTATICALLY ACTUATED CAPACITIVE RF MEMS

SWITCH

Submitted to Birla Institute of Technology & Science

In Partial fulfilment of

INTRODUCTION TO MEMS BY

Arun Mathew(2011H131089P)Ashok Kumar(2011H131090P)

Bidyut Prabha Sahu(2011H131094P)Dheeraj Suri(2011H131097P)S.Sri Ram(2011H131093P)Under esteemed guidance of

Dr. N.N Sharma Associate Professor

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CERTIFICATE

This is to certify that the project entitled ‘Electrostatically actuated

capacitive RF MEMS Switch’ contains the work done by Arun Mathews,

Ashok Kumar , Bidyut sahu, Dheeraj Suri, S.SriRam students of First

semester in Master of Electrical Engineering(Specialization in power

electronics & drives) under the guidance of Dr. N.N Sharma.

Project Guide Dr. N.N SharmaAssociate Professor Mechanical Department

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ACKNOWLEDGEMENT

I extend my sincere thanks to Prof. N.N Sharma, Assoc. Professor

for providing me with the guidance and facilities for the Seminar.

I express my sincere gratitude to Seminar coordinators

Mr. Sachin Belgamwar, Mr. Vijay Kumar & Staff in charge, for their co-

operation and guidance for preparing and presenting this seminar.

I also extend my sincere thanks to all other faculty members of

Electronics and Electrical Department and my friends for their support and

encouragement.

Arun Mathew(2011H131089P)Ashok Kumar(2011H131090P)

Bidyut Prabha Sahu(2011H131094P)Dheeraj Suri(2011H131097P)

S.Sri Ram(2011H131093P)

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INDEX

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S.NO DESCRIPTION PAGE NO

0 Abstract 5

1 RF MEMS Technology 6-12

2 Non Linear Electro-Mechanical Model of MEMS Switch

13-17

3 Applications, Advantages, and

disadvantages of RF MEMS Switches

18-22

4

SWITCH DESIGN AND OPERATION

23-30

5 Results 31-36

6 Conclusion 37

7 Bibliography 38

RF Mems Switch 2011

Abstract:

A RF Capacitive MEMS switch designed using INTELLISUITETM is investigated in this

report. The applications which employ RF MEMS switches require low insertion loss, high

linearity, moderate switching speeds, and low to moderate power. The RF MEMS switch

promises integration onto a variety of substrates, including Semiconductor substrates. This

switch includes a vertically movable bridge membrane attached to folded hinges. It is

electrostatically actuated, allowing for fast response, low power consumption, and easy

system integration. Analysis of the switch is done for bridge membrane with different mass

and different air gaps. Different device parameters like hinge thickness and material

parameters were optimized to get a lower actuation voltage for a given air gap. The effect of

the beam thickness, and the air gap were studied and device failure conditions were analyzed.

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1. RF MEMS (Micro Electromechanical System) Technology

The Radio frequency micro electro mechanical system (RF MEMS) acronym refers to electronics components of which moving sub-millimeter-sized parts provide RF functionality. The RF functionality can be implemented by using variety of RF technologies. Besides RF MEMS technology, III-V compound semiconductor (GaAs, GaN, InP, InSb), ferrite, ferroelectric, silicon-based semiconductor (RF CMOS, SiC and SiGe), and vacuum tube 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 handling, power consumption, reliability, ruggedness, size, supply voltage, switching time and weight. In this section our interest lies in the trade off obtained by using MEMS devices.

1.1 Basic Construction of Capacitive RF MEMS Switch

Figure 1(a) shows a capacitive fixed-fixed beam RF MEMS switch which in essence is a micro-machined capacitor with a moving top electrode ,which is the beam .

It is generally connected in shunt with a transmission line and used in X to W- band (77Ghz and 94Ghz) RF MEMS components. An ohmic cantilever RF MEMS switch, as shown in

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Fig. 1(b), is capacitive in the up-state, but makes an ohmic contact in the down-state. It is generally connected in series with the transmission line and is used in DC to the Ka-band components.

From an Electro-mechanical perspective, the component behaves like a damped mass-spring system(refer Fig2.a), actuated by an electrostatic force . The spring constant is a function of dimensions of the beam as well as the Young's modulus, the residual stress and the Poisson ratio of the beam material. The electrostatic force is a function of the capacitance and the bias voltage. Knowledge of the spring constant allows for hand calculation of the pull-in voltage, which is the bias voltage necessary to pull-in the beam, whereas knowledge of the spring constant and the mass allows for hand calculation of the switching time.

From an RF perspective, the components behave like a series RLC circuit(Refer fig 2.b) with negligible resistance and inductance. The up- and down-state capacitance is in the order of 50 fF 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.

Fig 2.a Damped mass Spring System Fig 2.b a series RLC Circuit

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1.2 Biasing

RF MEMS components are biased electrostatically using a bipolar NRZ drive voltage, as shown in Fig. 3, in order to avoid dielectric charging and to increase the lifetime of the device. Dielectric charges exert a permanent electrostatic force on the beam. The use of a bipolar NRZ drive voltage instead of a DC drive voltage avoids dielectric charging whereas the electrostatic force exerted on the beam is maintained, because the electrostatic force varies quadratically with the DC drive voltage. Electrostatic biasing implies no current flow, allowing high-resistivity bias lines to be used instead of RF chokes.

Fig3. Electrostatic biasing of a capacitive fixed-fixed beam RF MEMS Switch

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1.3 Packaging

RF MEMS components are fragile and require wafer level packaging or single chip packaging which allow for hermetic cavity sealing. A cavity is required to allow movement, whereas hermeticity is required to prevent cancellation of the spring force by the Van der Waals force exerted by water droplets and other contaminants on the beam. RF MEMS switches, switched capacitors and varactors can be packaged using wafer level packaging. Large monolithic RF MEMS filters, phase shifters, and tunable matching networks require single chip packaging.

Wafer-level packaging is implemented before wafer dicing, as shown in Fig. 4(a), and is based on anodic, metal diffusion, metal eutectic, glass frit, polymer adhesive, and silicon fusion wafer bonding. The selection of a wafer-level packaging technique is based on balancing the thermal expansion coefficients of the material layers of the RF MEMS component and those of the substrates to minimize the wafer bow and the residual stress, as well as on alignment and hermeticity requirements. Figures of merit for wafer-level packaging techniques are chip size, hermeticity, processing temperature, (in) tolerance to

Fig 4(a) Wafer-level packaging 4(b) Single chip packaging of ohmic cantilever RF MEMS Switch

alignment errors and surface roughness. Anodic and silicon fusion bonding do not require an intermediate layer, but do not tolerate surface roughness. Wafer-level packaging techniques based on a bonding technique with a conductive intermediate layer (conductive split ring)

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restrict the bandwidth and isolation of the RF MEMS component. The most common wafer-level packaging techniques are based on anodic and glass frit wafer bonding. Wafer-level packaging techniques, enhanced with vertical interconnects, offer the opportunity of three-dimensional integration.

Single-chip packaging, as shown in Fig. 4(b), is implemented after wafer dicing, using pre-fabricated ceramic or organic packages, such as LCP injection molded packages or LTCC packages. Pre-fabricated packages require hermetic cavity sealing through clogging, shedding, soldering or welding. Figures of merit for single-chip packaging techniques are chip size, hermeticity, and processing temperature.

1.4 Micro fabrication

An RF MEMS fabrication process is based on surface micromachining techniques, and 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: III-V compound semi-insulating, borosilicate glass, fused silica (quartz), LCP, sapphire, and passivated silicon wafers. As shown in Fig. 4, 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 MMIC and RFIC fabrication processes require 13 to 25 lithography steps.

Fig. 5: RF MEMS switch, switched capacitor, or varactor fabrication process

As outlined in Fig. 5, the essential microfabrication steps are:

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Deposition of the bias lines (Fig. 5, step 1) Deposition of the electrode layer (Fig. 5, step 2) Deposition of the dielectric layer (Fig. 5, step 3) Deposition of the sacrificial spacer (Fig. 5, step 4) Deposition of seed layer and subsequent electroplating (Fig. 5, step 5) Beam patterning, release and critical point drying (Fig. 5, step 6)

With the exception of the removal of the sacrificial spacer, which requires critical point drying, the fabrication steps are similar to CMOS fabrication process steps. RF MEMS fabrication processes, unlike BST or PZT ferroelectric and MMIC fabrication processes, do not require electron beam lithography, MBE, or MOCVD.

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2. Non-Linear Electro-Mechanical Model of MEMS Switches

A MEMS DC contact series switch is accurately modeled by a capacitance in the up-state position and a resistor in the down state position . A MEMS shunt capacitive switch is accurately modeled by a capacitance in the upstate position and a CLR model in the down state position. The mechanical analysis of MEMS switches has not followed a parallel approach. Thus an accurate electro-mechanical model which predicts virtually everything about the switching mechanism of RF MEMS Switch is discussed here.

I. Simple Mechanical analysis

MEMS switches are modeled using a simple fixed-fixed beam or a cantilever design. The pull-down voltage is derived from the static calculation and is:

Vp =( 8kg3/27€0Ww)1/2 --(1)

Where k is the spring constant (or modal stiffness) of MEMS Bridge(or cantilever), g is the height of MEMS bridge over pull-down electrode , w is the width of MEMS bridge , and W is the size of pull-down electrode .

Fig 7. Co-ordinate system and simplified mechanical model of a MEMS fixed-fixed beam switch

The spring constant is dependent on the bridge geometry and, for a fixed-fixed beam design with a force distributed over the centre third of bridge length, k is given by :

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K= (32Ewt3 /l3 )(27/49) + 8σ(1-ѵ)wt 27/49l --(2)

Where E is the young’s modulus, ν is the Poisson’s ratio, σ is the residual stress in the fixed-fixed beam, l, w and t are the length ,width and thickness of the beam, respectively. Similar equations can be derived for cantilever switches. The switching time is derived using Newtonian dynamic equation of motion and neglecting the damping underneath the cantilever. Also the applied force is assumed to be constant and is given by the electrostatic force due to a voltage V on the MEMS bridge, and is :

Fe= €0wWѵ2/2g 2 --(3)

The dynamic equation of motion becomes:

md2t/dt2 +kz = F = €0wWѵ2/2g 2 --(4)

Where m is the mass of the bridge, and z is the displacement from the upstate position. The initial conditions are z=0 and dz/dt=0 at t=0 (switch is at rest ), and switching time is calculated for z = g

And is

ts = (Vp/w0vs)√27/2 --(5)

Where w0 = √k/m is the mechanical resonant frequency of the bridge, Vs is is the source voltage and Vp is the pull down voltage given in equation 1. The switching time is dependent on the applied voltage. The higher the applied voltage, the faster the switch . This analysis is accurate if the applied voltage is larger than 2-3VP.

The energy consumed in the switching process can be calculated as the sum of electrical and mechanical energy . The mechanical energy is the energy The mechanical energy is the energystored in the bridge spring and is given by Em = kg2/2.The electrical energy is the energy stored in the MEMS capacitor, and is E, = c d K 2 / 2 (assuming c d >> Cu).Using these values, the energy required for a MEMS bridge with k = 6 N/m, g = 2.5 pm, c d = 2 pF and V, = 50 V is E = E, E, = 2.5 nJ and E, > E,. We will see later that this estimate of the switching energy is inaccurate, and that the damping mechanism must be included inthe switching energy computations.

2. Mechanical analysis with intermediate accuracy

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An intermediate mechanical analysis takes into account the varying force versus position (or time) as the MEMS bridge is being pulled down to the bottom electrode. The analysis also takes into account the damping factor of the air layer underneath the MEMS bridge. The dynamic equation of motion becomes:

md2z/dt2 + bdz/dt + kz = Fe + Fb --(6)

Fe = ½ €0wWV2/(g +td/€r – z)2 --(7)

where td is the dielectric thickness with a dielectric constant of €r, b is the damping coefficient and is dominated by the squeeze-film damping under the bridge, Fc is the contact force at the dielectric/metal interface. For two parallel plates, b is given by [7]:

b = k/w0Q =~ √2 μairl(w/g)3 --(8)

where w, is the natural resonant frequency of the switch,Q is the quality factor of the oscillating bridge, and pair is the viscosity of air ( 1.8 x 10-5kg/m3). The damping coefficient (b) for an arbitrary switch can be calculated from Eq. (8) and direct measurements of the spring constant (using an atomic force microscope) and the quality factor (extracted from the small displacement frequency response).Equation 6 is a non-linear differential equation, and its solution can be obtained using a non-linear solver such as Mathematica [SI. Again, the boundary conditions are z =0 and d z / d t = 0 at t = 0. Figure 2 presents the time-domain solution for a MEMS gold bridge with t = 0.8 µm, w = 60 µm, l = 300 µm, W = 100 µm, k = 6 N/m, g = 2.5µm, Vp = 25 V and Vs = 35-50 V. The solution is valid until the contact is achieved, and this is for x = td .The switching time is very similar to the simple mechanical solution presented above for V, = 50 V. The MEMS bridge speed ( d z /d t ) is 2-3 m/s, for Vs

= 35-50 V, just before hitting the bottom electrode.

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Fig 8 Simulated (a) Bridge height and (b) current Vs time for a step voltage of 35V and 50V

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The solution of MEMS bridge position Vs time can be used to extract the switching current . The current is given by:

I = dq/dt = CdV/dt + VdC/dt --(9)

C= €0wW/(g +td/€r – z) --(10)

and z versus t is given in Figure 8a. Notice that the peak switching current occurs just near the end of the switching cycle where the bridge speed is the highest. The switching current is shown in Figure 8b. for the cases outlined above, and is 4 mA (Vs = 35 V) to 8 mA (Vs= 50 V), which is significant. The energy consumed in the switching process can be calculated as

E = V∫idt = Ec + Em + Ek + Ed --(11)

where Vs is the source voltage (30 or 50 V), I ( t ) is the switching current, Ee, Em are the mechanical and electrical energy defined above, Ek = ½mv2 and is the kinetic energy of the MEMS bridge, and Ed is the energy dissipated in the damping mechanism. For Vs = 30 V and 50 V, the switching energy is calculated to be 1.98 nJ and 4.1 nJ respectively. The energy stored in the capacitor, Ec is 1.0 nJ and 2.1 nJ for 35 V and 50 V, respectively, showing that the kinetic energy and the damping energy account for 50% of the total switching energy at the point of contact.

GENERAL RELIABILITY CONCERNS

Metal Contact Resistance (Series Contact Switches )

Series contact switches tend to fail in the open circuit state with wear. Even though the bridge is collapsing and making contact with the transmission line, the conductivity of the contact

metallization area decreases until unacceptable levels of power loss are achieved. These out-of-spec increases in resistivity of the metal contact layer over cycling time may be attributed to frictional wear, pitting, hardening, non-conductive skin formation, and/or contamination of the metal. Pitting and hardening can be reduced by decreasing the contact force during actuation. But tailoring the design to minimize the effect involves balancing operational conditions (contact force, current, and temperature), plastic deformation properties, metal

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deposition method, and switch mechanical design. In other cases, the resistivity of the contact increases with use due to the formation of a thin dielectric layer on the surface of the metal. While this has been documented, the underlying physical mechanisms are not currently well understood. As the RF power level is raised above 100 mW, the aforementioned failures are exacerbated by the increased temperature at the contact area and, under hot-switching conditions, arcing and microwelding between the metal layers.

Dielectric Breakdown (Shunt Capacitive Switches)

Shunt capacitive switches often fail due to charge trapping, both at the surface and in the bulk states of the dielectric. Surface charge transfer from the beam to the dielectric surface results in the bridge getting stuck in the up position (increased actuation voltage). Bulk charge trapping, on the other hand, creates image charges in the bridge metallization and increases the holding force of the bridge to a value above its spring restoring force. There are several actions that can be taken to mitigate dielectric charging in the design phase, including choosing better dielectric material and designing peripheral pull-down electrodes to decouple the actuation from the dielectric behavior at the contact. Unlike series contact switches, capacitive shunt switches do not experience hard failures at RF power levels > 100 mW, as long as the bridge contact metallization is thick enough to handle the high current densities. However, RF power may be limited in some cases by a recoverable failure, self-actuation. While not yet fully understood, it has been observed that a capacitive shunt switch will self-actuate at 4W of RF power (cold-switching failure) and experience latch-up (stuck in down position) in hot-switching mode at 500 mW. Even though these “failures” are recoverable – the switch operates normally if the RF power is decreased below the latch-up value of 500 mW – they still illustrate a lifetime consideration for high power applications.

Radiation and Other Effects

There are some areas of RF MEMS reliability research that have not been investigated in detail and are in need of immediate attention. For example, RF MEMS series contact switches were thought to be immune to radiation effects until JPL’s total dose gamma irradiation experiments on the RSC MEMS contact switch showed design-dependent charge separation effects in the pull-down electrode dielectric material, which noticeably decreased the actuation voltage of the device. This immediately begs the question of how radiation effects will accelerate the dielectric material failure mechanisms of capacitive switches, which have known dielectric failure mechanisms, or other series switches that utilize dielectric material in their electrode structures.

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Power Handling

As outlined above, reliable operation of RF MEMS switches at power levels above 500 mW cannot be guaranteed at this time. Capacitive shunt switches experience recoverable failures at this level, while series contact switches may permanently fail in the short circuit configuration if hot switched above 100 mW. Hot-switching series contact switches at any power is not recommended. Thermal dissipation precautions in packaging are unnecessary, as RF MEMS do not generate sufficient thermal energy.

3. Applications , Advantages and Disadvantages of RF MEMS Switches

Applications of RF MEMS Switches

RF MEMS resonators are applied in filters and reference oscillators. RF MEMS switches, switched capacitors and varactors are applied in electronically scanned (sub)arrays (phase shifters) and software-defined radios (reconfigurable antennas, tunable band-pass filters).

Antennas

Polarization and radiation pattern reconfigurability, and frequency tunability, are usually achieved by incorporation of III-V semiconductor components, such as 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 compound semi-insulating and passivated silicon substrates are generally lossier and have a higher 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 switches 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

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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 can be used to increase out-of-band rejection, in case the antenna fails to provide sufficient selectivity. Out-of-band rejection eases the dynamic range requirement on the LNA and the mixer in the light of interference. Off-chip RF bandpass filters based on lumped bulk acoustic wave (BAW), ceramic, SAW, quartz crystal, and 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.

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 resonators and switches, switched capacitors and varactors, and YIG ferrites. 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 100-1000. RF MEMS switch, switched capacitor and varactor technology, offers the tunable filter designer a compelling trade-off between insertion loss, linearity, power consumption, power handling, size, and switching time.

Phase shifters

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.9: 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. EIRP, also referred to as the power-aperture product, 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 in order to optimize the EIRP or the EIRP x Gr/T product, as shown in Fig. 10 and Fig. 11. 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.

Advantages of MEMS Switches

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MEMS switches are surface micro-machined devices which use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission line . RF MEMS switches are the specific micro mechanical switches which are designed to operate at RF to mm-wave frequencies (0.1 to 100 Ghz). The advantage 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 very low power dissipation (10-100nJ per switching cycle) . On the other hand thermal/magnetic switches consume a lot of power unless they 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 (2-4fF) resulting in excellent isolation at 0.1-60 GHz. Also capacitive switches with a capacitance ratio 60-160 provide excellent isolation at 8-100 GHz.

Very low Insertion loss: RF MEMS metal contact and capacitive switches have an insertion loss of 0.1 dB up to 100 GHz.

Linearity and Intermodulation Products: MEMS devices are extremely linear devices and therefore result in very low intermodulation products in switching and tuning operations. Their performance is 30-50 dB better than PIN or FET switches.

Potential for low cost: RF MEMS devices are fabricated on surface micromachining techniques and can be built on quartz, Pyrex, LTCC, mechanical grade high-resistivity silicon or GaAs substrates.

Disadvantages of MEMS Switches

Relatively low speeds: The switching speed of most electrostatic MEMS switches is 2-40µs, and thermal/magnetic switches are 200-3000µs . Certain communication and RADAR systems require must faster switches.

Power Handling: Most switches cannot handle more than 200mW although some switches have shown up to 500mW power handling (Terravicta and Raytheon). MEMS devices that handle 1-10W with high reliability simple don’t exist today.

Reliability: The reliability of mature MEMS switch is 0.1-40 billion cycles. However many systems require reliability of 20-200 billion cycles. Also the long term reliability has not yet been addressed. It is now well known that the capacitive switches are limited by dielectric charging that occurs in the actuation electrode, while the metal contact switches are limited by the interface problems between the contact metals, which could be severe under low contact forces (in electrostatic designs, the contact forces are around 40-100 µN per contact).

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Packaging: MEMS switches need to be packaged in inert atmospheres(Nitrogen, Argon etc.) and in very low humidity, resulting in hermetic or near hermetic seals. Hermetic packaging costs are currently relatively very high, and the packaging technique itself may affect the reliability of MEMS switch.

Fig12 An SPDT switch packaged using a gold-to-gold seal ring(courtesy of Microassembly, inc.) the top cover is taken off so as to show the seal ring

Microassembly (fig 12) and Analog devices have both developed excellent packages for RF MEMS switches. The Microassembly package is based on gold-to-gold thermo-compression at 250C while the Analog devices package is based on glass-to-glass seal at 400-450C.

COMPARISION OF MEMS SWITCHES WITH SOLID STATE SWITCHES

RF switches are used in a wide array of commercial, aerospace, and defense application areas, including satellite communications systems, wireless communications systems, instrumentation, and radar systems. In order to choose an appropriate RF switch for each of the above scenarios, one must first consider the required performance specifications, such as frequency bandwidth, linearity, power handling, power consumption, switching speed, signal level, and allowable losses.

Traditional electromechanical switches, such as waveguide and coaxial switches, show low insertion loss, high isolation, and good power handling capabilities but are power-hungry, slow, and unreliable for long-life applications. Current solid-state RF technologies (PIN diode- and FET- based) are utilized for their high switching speeds, commercial availability, low cost, and ruggedness. Their inherited technology maturity ensures a broad base of expertise across the industry, spanning device design, fabrication, packaging,

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applications system insertion and, consequently, high reliability and well-characterized performance assurance. Some parameters, such as isolation, insertion loss, and power handling, can be adjusted via device design to suit many application needs, but at a performance cost elsewhere. For example, some commercially available RF switches can support high power handling, but require large, massive packages and high power consumption.

In spite of this design flexibility, two major areas of concern with solid-state switches persist: breakdown of linearity and frequency bandwidth upper limits. When operating at high RF power, nonlinear switch behavior leads to spectral regrowth, which smears the energy outside of its allocated frequency band and causes adjacent channel power violations (jamming) as well as signal to noise problems. The other strong driving mechanism for pursuing new RF technologies is the fundamental degradation of insertion loss and isolation at signal frequencies above 1-2 GHz.

By utilizing electromechanical architecture on a miniature- (or micro-) scale, MEMS RF switches combine the advantages of traditional electromechanical switches (low insertion loss, high isolation, extremely high linearity) with those of solid-state switches (low power consumption, low mass, long lifetime). shows a comparison of MEMS, PIN-diode and FET switch parameters. While improvements in insertion loss (<0.2 dB), isolation (>40 dB), linearity (third order intercept point>66 dBm), and frequency bandwidth (dc – 40 GHz) are remarkable, RF MEMS switches are slower and have lower power handling capabilities. All of these advantages, together with the potential for high reliability long lifetime operation make RF MEMS switches a promising solution to existing low-power RF technology limitations

PARAMETER RF MEMS PIN-DIODE FET

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Voltage 20 – 80 3 – 5 3 – 5

Current (mA) 0 0 – 20 0

Power Consumption (mW) 0.5 – 1 5 – 100 -.5 – 0.1

Switching 1 – 300 s 1 – 100 ns 1 – 100 ns

Cup (series) (fF) 1 – 6 40 – 80 70 – 140

Rs (series) () 0.5 – 2 2 – 4 4 – 6

Capacitance Ratio 40 – 500 10 n/a

Cutoff Freq. (THz) 20 – 80 1 – 4 0.5 – 2

Isolation (1 – 10 GHz) Very high High Medium

Isolation (10 – 40 GHz) Very high Medium Low

Isolation (60 – 10 GHz) High Medium None

Loss (1 – 100 GHz) (dB) 0.05 – 0.2 0.3 – 1.2 0.4 – 2.5

Power Handling (W) <1 <10 <10

3rd order Int. (dBm) +66 – 80 +27 – 45 +27 - 45

4. SWITCH DESIGN AND OPERATION

The geometry of a capacitive MEMS switch is shown in Fig. 1. The switch consists of a lower

electrode fabricated on the surface of the sillicon wafer and a thin aluminum membrane suspended

over the electrode. The membrane is connected directly to grounds on either side of the electrode

while a thin dielectric layer covers the lower electrode. The air gap between the two conductors

determines the switch off-capacitance.

Application of a DC electrostatic field to the lower electrode causes the formation of positive and

negative charges on the electrode and membrane conductor surfaces. These charges exhibit an

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attractive force, which, when strong enough, causes the suspended metal membrane to snap down

onto the lower electrode and dielectric surface, forming a low impedance RF path to ground.

Figure 1

The switch is built on coplanar wave-guide (CPW) transmission lines. The

insertion loss is dominated by the resistive loss of the signal line and the coupling between

the signal line and the membrane when the membrane is in the up position. To minimize

the resistive loss, a thick layer of metal needs be used to build the transmission line. The

thicker metal layer result in a bigger gap that reduces the coupling between signal and

ground yet also requires higher voltage to actuate the switch.. The glass wafer is chosen for

the RF switch over a semi-conductive silicon substrate since typical silicon wafer is too

lossy for RF signal. When the membrane is in the down position, the electrical isolation of

the switch mainly depends on the capacitive coupling between the signal line and ground

lines. The dielectric layer plays a key role for the electrical isolation.

When the switch is un-actuated and the membrane is on the up position, the switch is

called in off-state. When the switch is actuated and the membrane is pulled down, the

switch is called in on-state. The major characteristics of the switch are the insertion loss

when the signals pass through and the isolation when signals are rejected. In the off-state

the RF signal passes underneath the membrane without much loss. In the on-state, between

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the central signal line and coplanar wave-guide grounds exists a low impedance path

through the bended membrane. The switch will reflect the RF signal.

The Hinge Shape:

Both the number of folds and the size of the hinge affected the structure and performance of the

switch. The results that were considered were the first mode of natural frequency, driving voltage,

and stress. The thickness of the silicon hinge was 1.5 microns, and the air gap was 5 microns.

Models were created in IntelliSuite for each hinge shape and then analysed in its mechanical

analysis module.

The actuation voltage of the MEMS switch is about 25Vin case of 5 micron air gap and 4 volt in

case of 3 micron air gap. The spring constant of the membrane and the distance between the

membrane and the bottom electrode determines the actuation voltage of the switch. The spring

constant of the membrane is mainly determined by the membrane material properties, the membrane

geometry, and the residual stress in the membrane.

There are two methods to create the 3D models of MEMS devices in IntelliSuite™; one is

directly from the fabrication process, the other is through the 3D geometry interactive

builder. Using the fabrication process, the masks for the MEMS device were imported

first, then a process table was generated which included all of the process steps necessary

to create the device and from which the resulting material properties were determined.

During process design, the imported mask set was linked to the process, which provided

the definition of the x-y geometry of the structure. Then the 3D model of the device could

be visualized in the 3D Viewer, and the model exported to an analysis module.

Step 1

Mask Design

Layer 0.

Components of RF switch- Substrate

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Layer 1

Components of RF switch- Bridge membrane support, Bottom Electrode, signal lines

Layer 2

Components of RF switch- Bridge membrane support, signal lines

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Layer 3

Components of RF switch- Bridge membrane support.

Layer 4

Components of RF switch- Bridge membrane support, Metal switching bar

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Layer 5

Components of RF switch- Bridge membrane support, Dielectric

Layer 6

Components of RF switch- Bridge membrane support, Bridge membrane with holes, Hinge

connecting the Bridge membrane to the support.

Complete Mask

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RF switch Dimensions

Length and width of substrate: (200,100)μm

Length and width of support: (15,30)μm

Length and width of two Bottom Electrodes: (35,60)μm

Length of Metal switching bar:10 μm

Length and width of 2 RF signal lines:(10,40)μm

Length of Dielectric: 10 μm

Length and width of Bridge membrane:(140,60) μm

Step 2 :-

Simulation in 3-D Builder

Thickness of substrate: 10μm

Thickness of support: 10μm

Thickness of Bottom Electrode: 1μm

Thickness of RF signal lines: 1μm

Thickness of air gap: 5μm

Thickness of metal switching bar: 2μm

Thickness of dielectric: 0.5μm

Thickness of Bridge membrane and hinge: 1.5μm

Entity definition :

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All components of the RF switch are given different entities.

Models

a) Bridge membrane without holes

b)Bridge membrane with holes

Step3:-

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Material Definition:

Substrate: Silicon

Bridge membrane support: Silicon

Bridge membrane and hinge: Polysilicon

Metal switching bar: Aluminium

Bottom electrode: Aluminium

RF Signal lines: Aluminium

Dielectric layer between Bridge membrane and metal switching bar : Sio2

Boundary Condition:

Bridge membrane support and bottom part of substrate: Fixed

Load condition:

Bottom electrode: 0 V, Bridge membrane: 20 V, 25 V.

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5. Result And Discussion :-

ThermoElectroMechanical Analysis

a) Bridge membrane without holes :-

1) Air gap:5 μm

Voltage applied: 100 V

Bridge membrane and hinge Thickness : 3 micron.

Maximum z displacement obtained=-0.2 μm.

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2) Air gap: 5μm

Voltage applied: 500 V

Bridge membrane and hinge Thickness : 3 micron

Maximum z displacement obtained=-1.8 μm.

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b) Bridge membrane with holes

1) Air gap: 3μm

Bridge membrane and hinge Thickness : 1.5 micron

Voltage applied: (0-2.4 V) with incremental step of 2.4 V.

Maximum z displacement obtained=-3.13 μm

Thus 2.4 V is threshold voltage for the given Bridge membrane with an air gap of 3μm.

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2) Air gap: 5 μm

Bridge membrane and hinge Thickness : 1.5 micron

Voltage applied: 0-25 V with incremental step of 25 V.

Maximum z displacement obtained=-5.12μm

Thus 25 V is threshold voltage for the given Bridge membrane with an air gap of 5μm.

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3) Air gap: 5 μm

Bridge membrane and hinge Thickness : 1.5 micron

Voltage applied: 0-20 V with incremental step of 20 V.

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Maximum z displacement obtained=-3.22μm

6.Conclusion

1) The RF switch with the holes in the Bridge membrane has a much lesser threshold

voltage than the without holes. This is due to the reduction in mass leading to more

displacement.

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2) The threshold voltage decreases with the decrease in air gap and vice-versa.

3) The threshold voltage increases with increase in thickness of the Bridge membrane.

.

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7. BIBLIOGRAPHY:-

1. S.Majumdar,J.lampen,R.Morrison,andJ.Maciel,MEMS SWITCHES,IEE instrumentation and measurement magazine,march 2003.

2. Gabriel M.Rebeiz,Hoboken,NJ,John Wiley &sons,RF MEMS THEORY,DESIGN &TECHNOLOGY,January 2003.

3. Gopinath,A and Ranklin.JB,IEEE Transaction on electronic development ,GaAs FET RF switches ,VOL ED-32.

4.Cavery.R.H,`DISTORTION OF OFF-STATE ARSENIDE MESFET

SWITCHES’,IEEE Transaction,VOL.41,NO.8,august 1993.

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