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MEMS SWITCHES
INTRODUCTION
Compound solid state switches such as GaAs MESFETs and PIN diodes are widely used in microwave and millimeter wave integrated circuits (MMICs) for telecommunications applications including signal routing, impedance matching networks, and adjustable gain amplifiers. However, these solid-state switches have a large insertion loss (typically 1 dB) in the on state and poor electrical isolation in the off state. The recent developments of micro-electro-mechanical systems (MEMS) have been continuously providing new and improved paradigms in the field of microwave applications. Different configured micro machined miniature switches have been reported. Among these switches, capacitive membrane microwave switching devices present lower insertion loss, higher isolation, better nonlinearity and zero static power consumption. In this presentation, we describe the design, fabrication and performance of a surface micro machined capacitive microwave switch on glass substrate using electroplating techniques.
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 glass 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. With no applied actuation potential, the residual tensile stress of the membrane keeps it suspended above the RF path. 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 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.
The switch is built on coplanar wave-guide (CPW) transmission lines, which have an impedance of 50 that matches the impedance of the system. The width of the transmission line is 160 m and the gap between the ground line and signal line is 30 m. 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. To achieve a reasonable actuation voltage, a 4-m-thick copper is used as the transmission line. 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. The smaller the thickness and the smoother the surface of the dielectric layer, the better isolation of the switch is. But there is another trade-off here. When the membrane is pulled down, the biased voltage is directly applied across the dielectric layer. Since this layer is very thin, the electric field within the dielectric layer is very high. The thickness of the dielectric layer should be chosen such that the electric field will never exceed the breakdown electric field of the dielectric material. The silicon nitride film has breakdown electric field as high as several mega-volts per centimeter and can be utilized as dc block dielectric layer. In this project, the thickness of the silicon nitride layer is chosen as 0.2 m to accomplish the dc block and RF coupling purpose.
The switches were fabricated by surface micro-machining techniques with a total of four masking level. No critical overlay alignment was required. Fig. 2 shows the essential process steps:
1. Ti/Cu seed layer deposition: The starting substrate was a 2-inch glass wafer. A layer of titanium (0.05 m) and copper (0.15m) was sputtered on the substrate as seed layer for electroplating.
2. Silicon nitride deposition: A layer of silicon nitride (0.2m) was deposited and patterned as DC block by using PECVD and reactive ion etch (RIE).
3. Copper electroplating: A photo resist layer was spin coated and patterned to define the electroplating area. Then, a 4-m-thick copper layer was electroplated to define the coplanar wave-guide and the posts for the membranes.
4. Aluminum deposition: A layer of aluminum (0.4m) was deposited by using electron beam evaporation and patterned to form the top electrode in the actuation capacitor structure.
5. Release: The photo resist sacrificial layer was removed to finalize the switch structure.
TEST RESULTS AND DISCUSSIONS
The probe station and network analyzer (HP 8510C) were used to characterize the capacitive MEMS switch. Fig. 3 shows the micrograph of a switch under test. When the switch is unactuated 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 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.
As shown in Fig. 4, in the off-state the switch has insertion loss of approximately 0.1 dB at 10 GHz and 0.4 dB at 25 dB. Compared with typical FET or PIN diode switches, which have about 1 dB insertion loss, the MEMS switches have considerable advantages. For a multiswitch system, the total loss is significantly lower when mechanical switches are utilized. The return loss is better than 20 dB up to 25 GHz, which means the MEMS switch has an excellent impedance match to 50.
The isolation and return loss of the switch in the on-state is shown in Fig. 5. Due to the geometry of the capacitive switch, the signal cannot be coupled to ground perfectly at the low frequency. As the frequency becomes high, the coupling between the signal line and ground lines makes the isolation of the switch approximately 15 dB at 20 GHz, which is sufficient for switching RF signals.
The resonant frequency of 23.4 GHz was observed when the membrane was in the down position. This means that the switch can be equivalently modeled as a capacitor, inductor and resistor connected in series between the signal and ground lines. Since the switch has a better isolation around the resonant frequency, it can be designed such that the desired frequency overlaps with the resonant frequency by adjusting the geometry of the switch, i.e. the width of the membrane and the gap between the membrane and the lower electrode.
The actuation voltage of the MEMS switch is about 50V. 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.
RF MEMS Technology
Currently, both series and shunt RF MEMS switch configurations are under development, the most common being series contact switches and capacitive shunt switches.
RF SERIES CONTACT SWITCH
An RF series switch operates by creating an open or short in the transmission line, as shown in Figure 6. The basic structure of a MEMS contact series switch consists of a conductive beam suspended over a break in the transmission line. Application of dc bias induces an electrostatic force on the beam, which lowers the beam across the gap, shorting together the open ends of the transmission line . Upon removal of the dc bias, the mechanical spring restoring force in the beam returns it to its suspended (up) position. Closed-circuit losses are low (dielectric and I2R losses in the transmission line and dc contacts) and the open-circuit isolation from the ~100 µm gap is very high through 40 GHz. Because it is a direct contact switch, it can be used in low-frequency applications without compromising performance. An example of a series MEMS contact switch, the Rockwell Science Center MEMS relay, is shown in Error! Reference source not found.7.
RF SHUNT CAPACITIVE SWITCH
A circuit representation of a capacitive shunt switch is shown in Figure8. In this case, the RF signal is shorted to ground by a variable capacitor. Specifically, for RF MEMS capacitive shunt switches, a grounded beam is suspended over a dielectric pad on the transmission line (see Figure9). When the beam is in the up position, the capacitance of the line-dielectric-air-beam configuration is on the order of ~50 fF, which translates to a high impedance path to ground through the beam [IC=1/©]. However, when a dc voltage is applied between the transmission line and the electrode, the induced electrostatic force pulls the beam down to be coplanar with the dielectric pad, lowering the capacitance to pF levels, reducing the impedance of the path through the beam for high frequency (RF) signal and shorting the RF to ground . Therefore, opposite to the operation of the series contact switch, the beam in the up position corresponds to a low-loss RF path to the output load, while the beam in the down position results in RF shunted to ground and no RF signal at the output load (see Figure9). While the shunt configuration allows hot-switching and gives better linearity, lower insertion loss than the MEMS series contact switch, the frequency dependence of the capacitive reactance restricts high quality performance to high RF signal frequencies (5-100 GHz) , whereas the contact switch can be used from dc levels.
PRODUCTION AND MANUFACTURING ISSUES
PACKAGING
The primary production issue at this time is the lack of low-cost packaging options. As will be discussed in section Error! Reference source not found., the hermeticity requirement for RF MEMS switch packaging leaves only high-cost, military- or space-grade traditional packaging methods as appropriate for high reliability assurance. Expensive packaging precludes the large-scale production needed for extensive reliability testing and the low risk statistics for widespread commercial sales.
AVAILABLE VENDORS
Significant manufacturing hurdles have the following repercussions for spacecraft systems MEMS technology insertion. First, there are few available vendors (currently one “ Teravicta) and limited in-stock product. Second, and most importantly, much reliability testing remains to be completed and what has been done isn™t widely available due to commercial proprietary concerns. For space flight applications, this means that if one can find switches to purchase, the knowledge of their physics of failure and, consequently, the ability to predict what conditions may trigger them, is severely compromised. In-house performance characterization and reliability testing, and the resulting database of MEMS RF switch failure mechanisms, will enable accelerated MEMS technology insertion.
GENERAL RELIABILITY CONCERNS
Because RF MEMS switches are at such a low maturity level, there are reliability concerns at all levels “ design, fabrication, post-production/packaging, and system insertion/harsh environments. Before addressing the failure modes, it is useful to point out that RF MEMS switches are not subject to structural mechanical failure of the beam: the beams don™t crack or break even after billions of cycles.
For low - medium power operation (<100 mW) the primary design failures are based in materials choice and placement, increased resistance at the metal contact in series switches and dielectric charging in shunt switches.
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. Decreasing the contact force during actuation can reduce pitting and hardening. But tailoring the design to minimize the effect involves balancing operational conditions (contact force, current, and temperature), plastic deformation properties, metal 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 micro welding 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 JPLâ„¢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. These and other issues, such as reconfiguration (does a switch recover from long-duration continuous actuation) and long lifetime ruggedness must be investigated in detail to ensure robust and reliable design of RF MEMS devices.
PACKAGING
Beyond the design and production phases, reliability concerns can be introduced in post-production (such as release stiction fails) and, most importantly, in packaging. Several factors must be considered before choosing a package for RF MEMS switches. First and foremost, RF MEMS performance will quickly degrade in the presence of contaminants and humidity. Therefore, the initial package criterion is hermeticity.
A traditional approach would involve dicing the wafer, releasing the device, attaching the substrate to the package base, and attaching the lid with a hermetic seal, incorporating baking and vacuum conditions as necessary to ensure no out gassing after seal. With the many options available for microelectronics packaging, a suitable hermetic package can be found that minimizes thermal-mismatch induced stresses and provides low-loss RF electrical connections. Although it is possible to successfully package MEMS RF switches in this manner, it is impractical for two reasons: itâ„¢s prohibitively expensive for large-scale production and manipulating released devices is tedious. In response to these difficulties, the current trend is toward wafer-level packaging, which reduces cost and mitigates the structural fragility by bonding the package around the released switch in the production phase, before dicing and subsequent handling. Wafer-level packaging for RF MEMS is a topic of intense study. Work is currently underway to find a suitable bonding method that provides adequate hermetic seal without out gassing contaminants into the body of the package or thermally damaging the delicate MEMS structures.
RELIABILITY AND RADIATION TALL TENT POLES
The failure mechanisms outlined in section Error! Reference source not found. determine the specific reliability concerns for each mission scenario. In general, one must address both the operational and environmental stresses imposed on the device throughout the lifetime of the mission. The only operational stress addressed here will be high RF power, since RF MEMS technology is not yet mature enough to consider system-level behaviors.
OPERATIONAL
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.
TECHNOLOGY EVOLUTION IN NEAR TERM
The fundamental architecture for RF MEMS switches, both contact series and capacitive shunt, is stable and likely to persist through commercial insertion. Design subtleties will be adjusted to optimize performance (i.e. more robust metal contact) and increase reliability, but will likely be considered revisions rather than a new design. Since there is no set packaging method, the end product has yet to be fully realized.
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 wave-guide 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, 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.
ADVANTAGES OF MEMS SWITCHES
(1) Small size :
Semiconductor manufacturing techniques used in the batch fabrication of micro systems, these systems process sizes ranging from micro meters to a few milli meters.
(2) Low Cost :
Mems technology allows complex electromechanical systems to be manufactured using batch fabrication techniques, allowing cost of switches to be put in party with that of integrated circuits. Much of labour involved in packing and assembly of such a system would simply disappear.
(3) Low power consumption :
The mems switches are power efficient. The power losses in data transmission and also the time lag are eliminated because the switches are made next to control circuitory in the same chip.
(4) High isolation :
Isolation of mems switches in the range 1-40GHz is very high than the other switches.
(5) Ability to be integrated with other electronic devices with excellent linearity.
CONCLUSION
MEMS capacitive switches of RF applications show low insertion losses in the OFF state and high isolation in the ON state. The micro machine switches have applications in phased antenna arrays, in MEMS impedance matching networks, and in communication applications.
REFERENCES
1. IEEE Instrumentation and Management magazine, March 2003, MEMS Switches (Page No. 12).
2. Gopinath A and Rankain J B GaAs FET RF Switches, IEEE Trans. On Electron Dev., Vol.ED-32, No.7,July 1985.
3. Cavery,R.H. Distortion of Off-State Arsenide MESFET Switches, IEEE Trans. On Microwave Theory and Tech. Vol.41,No.8, August 1993.
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