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The near ideal straight-line resistance curves of RF-MEMS switches offers another important design advantage. Because of this, RF-MEMS don’t introduce the non-linearity's introduced with the curves of solid-state I-V curves. This lowers the overall distortion level of RF signals coming out of the RF-MEMS switch when compared one to one with a FET or PIN diode. This reduces harmonics and distortion and hence lowers the design requirements of filters to obtain the same level of selectivity.
The elimination of the transistor as the switch, hence gives RF-MEMS switches excellent linearity potential with third point intercept levels approaching 70 dBm. Furthermore, the linearity enables signals with power levels way beyond the 1 Watt continuous RF power to be switched. |
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Figure: 3D Mechanical Drawing of Series RF Micro-Electro-Mechanical Switch Illustrates the Relatively Small Size of the Device and the Accumulation of Charges which causes switch actuation.
Source: WiSpry, Inc. |
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Reliability concerns with electrostatic series resistor cantilever switches have traditionally been the wear and tear on the cantilever and contacts as a result of the mechanical movement. However, designers have overcome this problem with the use of new wear and fatigue-resistant materials.
Capacitive Shunt Switch
Another type of MEMS switch is the capacitive shunt switch. This too, employs a micro-cantilever. However the micro-cantilever is shorted between a dielectric material and the plate. For a capacitive shunt switch, the fundamental specifications related to performance are the on and off capacitance.
Capacitive shunt switches, because a capacitor is used as the basis for the design, will not pass low frequency signals. Because of this, pure capacitive shunt based designs are in general used for high frequency applications in the order of 5 to 100 GHz range. However, depending on the frequency response and the design, they can be used at lower operating frequencies.
Another benefit of capacitive shunt resistors is they offer builders of filters a higher Q. This is because they don’t have a resistor associated with the series contact switch. |
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Figure: A Capacitive RF MEMS Device Utilizes the Cantilever Displacement from a Dielectric to Change the Overall Impedance of the Path – To Create a RF Signal Path to Ground
Source: WiSpry, Inc.
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Finally, research has shown that actuation voltages in the order of 1.5 volts can be achieved with capacitive based shunt switches. Specifically, the meander cantilever structure and meander bridge design structures show promise to reduce the actuation voltage enough for integration into SoCs.
Thermal Actuated Switch
The thermal actuated switch takes advantage of the fact that silicon material expands when heated. A thermal actuated switch places two silicon interconnects in close proximity to each other. Next to the interconnects are two silicon heating elements (resistors). Simply put, once a current is applied to the resistors, the resistors heat, which results in the expansion of the interconnect, which in turn shorts the contact bumps on the interconnects together.
An important advantage of the thermal MEMS switch is that it has a low actuation voltage, suitable for SoC integration. However on the down side, it offers a lower bandwidth and has higher power consumption, because of the resistor heating elements.
Hybrid Electrostatic / Thermal Switches
Another approach to RF-MEMS switch design is hybrids based on both electrostatic and thermal actuation. This approach takes the best of both designs to produce a switch that slightly increases power consumption in exchange for a lower actuation voltage and improved switching speed.. Power consumption from the hybrid switch occurs for a short time during activation, when current flows through the heating resistors–but shuts off afterwards, allowing the design to achieve very low levels of power consumption..
Magnetic RF-MEMS Switches
The magnetic RF-MEMS switch, takes advantage of the force applied in a magnetic field, similar to a relay. A current is applied to a coil, which creates a magnetic field, which in turns pulls the cantilever such that it will make required contact. Reported advantages include a lower actuation voltage.
RF MEMS and The Future—A New Moore’s Law
Moore’s Law, which states that the size required to manufacture a transistor decreases by 50 percent every 12 to 18 months, has been a fairly accurate predictor of the future potential of silicon based VLSI chips for the last 20 years. Although a MEMS manufacturing law has not been officially established, trends point to MEMS devices with smaller and smaller dimensions. This is particularly notable in microarrays, another type of MEMS devices, used for DNA analysis. Affymetrix, a leading DNA microarray company, reports that in 1994 its GeneChip microarrays had a feature length of 100 microns. In 2003, that feature length has dropped to 11 microns. |
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This article was produced with assistance from WiSpry, Inc., a venture capital financed, fabless MEMs company located in Irvine. California. WiSpry technical illustrations are published with the permission of WiSpry, Inc. |
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