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Innovative Switching Technologies for Future

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Innovative Switching Technologies for Future INNOVATIVE RF SWITCH TECHNOLOGIES P. Blondy, A. Mennai, K. Nadeau, and A. Crunteanu XLIM, Universite de Limoges, CNRS 123, avenue Albert Thomas, 87060 Limoges - FRANCE ABSTRACT of-merit for RF switches, the lower values being the best. By putting several devices in parallel, it is possible to reduce The presentation will highlight new technologies developed the overall insertion loss, by dividing the on state resistance, at the for wideband, low loss and linear RF switch technologies. New expense of degraded isolation. Similarly, putting several devices in switch structures, like phase transition and phase change materials series decreases the series capacitance, and improves isolation, but will be described, along with emerging MEMS zero-level degrades insertion loss. packaged capacitive switch devices. Through several examples, we RF switches are dominated by semi-conductor technology, will show the advantages and drawbacks of these innovative and most devices are currently using CMOS-derived technologies, technologies. using insulating substrates and stacked-gate low voltage transistors in order to obtain low loss, high power switches. Current state-of- INTRODUCTION the-art CMOS RF switches have FoM around 100 fs, can handle 2 Watts of RF power, and have very low DC power consumption. Another important aspect is that CMOS technology is widely The ever-growing demand for wideband, wireless internet available and allows having secure, low cost technology sources access is pushing the demand for reconfigurable, multi standard for RF system providers. RF-front-ends, as shown in Fig. 1. Most RF architectures will However, the specific transistor gate-stacking technique make heavy use of RF switches, to configure antennas, filters and developed to improve power handling makes increasing the multiband amplifiers and antennas. operating frequency difficult, and other technologies have to be sought for future 5G broadband systems. For instance, there is a very limited number of options for high power switch technologies above 5 GHz, while 5G communication systems will permit commercial use of frequency band as high as 24 GHz. Innovative material-based switches are among the possible candidates for building low cost, low loss broadband microwave switches. These materials can be relatively easily integrated into planar RF transmission lines and the resulting in-line planar switches have excellent performances [2]. A planar in-line RF switches cross section and layout is shown below. In the example shown below, the material based on phase change materials, but the structure is essentially the same for phase transition materials like VO2 devices that will be discussed later in the paper. Bias%current%ib% RF6in% RF6out% Figure 1: Typical 4G cellular phone architecture. The tunable components and switches are shown in the red squares. RF-switches will be key element for the fabrication of multi Met1% Heater% Dielectric% standard reconfigurable and tunable front ends. PCM% RF-switch performances stem from their on and off state characteristics like on-state resistance, and off state capacitance. In Figure 2: Layout and cross section of a planar inline RF switch 50 Ω radiofrequency systems, the on-state resistance, Ron needs to based on PCM and PTM materials. be in the 1-10 Ω range in order to obtain reasonable loss (~0.1- 2 dB). The isolation at microwave frequencies is dominated by the As shown in Fig. 2, a heater is integrated in the device and off state capacitance of the switch, Coff, and practical values are allows the material reaching its transition temperature in the case between 10-100 fF. The off-state resistance is usually high enough of VO2 switches. The same structure allows applying long or short so that it can be neglected at high frequencies, but it may limit the temperature pulses to switch a PCM material to its crystalline, isolation is lower end of the band. conductive state, or to set it back to the off state. The typical For comparison purpose, the Ron.Coff product is a usual figure- device length, ananlog to a transistor gate length, is between 1 and 978-1-940470-02-3/HH2016/$25©2016TRF 1 Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, South Carolina, June 5-9, 2016 10 µm, while typical widths are between 20-100 µm. The planar nature of the switches allows obtaining very high isolation, with an off state capacitance less than a few tens of femtoFarads. )(dB)) PHASE TRANSITION RF SWITCHES Phase change materials, like Vanadium Dioxyde (VO ), are 2 Parameters excellent candidates for the fabrication of small size, low loss RF S0 switches. VO2 is a phase change material, with a Metal Insulating Transition (MIT) temperature at 68°C [1-6]. In practice, the resistivity of the material decreases by several orders of magnitude when the material temperature is higher that its transition temperature, compared to room temperature. Frequency)(GHz)) Vanadium dioxide layers can be obtained on sapphire substrates using E-beam evaporation of vanadium at high temperature in oxygen ambient. After several steps of thermal annealing, the layers present a sharp transition in resistivity when the temperature is increased. )(dB)) Measured VO2 resistivity versus temperature is shown in Fig. 3. Parameters S0 Frequency)(GHz)) Figure 5: Measured performances of VO2 switches at 25°C and 90°C. The measured transmission parameters are shown in Fig. 5, for temperatures below (25°C) and above (90°C) VO2 transition temperature. It can be seen that the insertion loss are extremely small, with Figure 3: Measured electrical resistivity of a 200nm thick an equivalent series resistance of 2 and an off state capacitance vanadium dioxide layer versus temperature. Ω of 7.7 fF. The figure of merit of this switch is 15.4 fsec. From this resistivity diagram, it is possible to design microwave switches with a specific on-state resistance and very PHASE CHANGE MATERIAL (PCM) RF SWITCHES much like transistor gate, the device width and length can be tailored for a given application. A practical example of a VO2 Very recently, phase change materials (PCM), like GeTe switch is shown below in Fig. 4, with a 3x20 µm material length binary chalcogenide alloys have been evaluated to develop RF and width. switches and similarly to VO2, they presents a high resistance in the amorphous off-state and low resistance in the crystalline on- state [5]. These alloys are very well known for the fabrication of DVD-RW discs, where their reflectivity is changed by the application of short/long optical pulses. For RF applications, these alloys have enough contrast to be used in low impedance systems, and can be used in similar circuit configurations like VO2 Ternary alloys like Ge2Sb2Te5 (GST) phase change material can also be used for the fabrication of RF/microwave switches. In a similar fashion as GeTe, GST presents reversible structural amorphous (a-GST) to crystalline (c-GST) thermally induced phase change [7]. This phase change is accompanied by intrinsic properties change of material, in particular a drastic resistivity drop when it passes from amorphous to crystalline state (up to 5 order of magnitude). For several years, GST and GeTe have been Figure 4: Picture of a fabricated Vo2 microwave switch. developed for non-volatile phase change memories (PCRAM) devices with a number of cycle between the two states greater than 105 cycles and a transition time of about 50 ns. For these properties, GST and GeTe are excellent candidates for RF 2 switching applications because of large Roff / Ron ratio with a good compromise in terms of stability and transition time [6]. Compared to currently used RF technologies, the main advantage of all PCM-based switches is that they do not require permanent bias to hold a given state. This is a key feature compared to any other electronic switching technology. Indeed, bi- stable elements are common in high impedance logic circuits, but mechanical relays still dominate 50 Ω RF systems. For instance, space-borne switching matrices have to be configured on ground, before a satellite payload is launched, and such application requires the system to be working without running a remote configuration procedure. Figure 6: Optical picture of a PCM switch. Therefore, these RF switch matrices are still using bulky arrays of mechanical relays, and this part of satellite payloads would greatly benefit from PCM switch technology. The bi-stable nature can be understood by looking at the resistance-temperature diagram below. Upon relatively slow heating, the material goes from amorphous to crystalline state, with a sharp conductivity increase. When a short, high-temperature pulse is applied, the crystal order is broken, and the material resistivity goes up again, and permits to switch the device back to its off state. Figure 7: Typical measured S parameters of a GST alloy PCM switch. Typical measured insertion loss of the devices is shown in Fig. 7 [ref], with reasonable insertion loss, and very high isolation. The off state capacitance is only 7 fF while typical on-state Figure 5: Resistance-Temperature diagram of a PCM switch, and resistance is 15 Ω, leading to a figure of merit of 105 fs. typical melting (Tm) and crystallization (Tc) pulses. DISCUSSION AND OTHER TECHNOLOGIES The crystallization temperature is between 200°C and 300°C for Other technologies like RF-MEMS, have made significant GST and GeTe alloys, with a tradeoff between heating duration progress over the past few years, and finally made their way into and temperature. consumer electronics applications like cellular phones. The The melting temperature, taking the material back to its amorphous application of this technology is tunable capacitors for antennas, state, is above 700°C, for a very short amount of time, typically where very low loss and low power consumption are of prime less than 1 µsec.
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