MEMS switches are surface-micromachined devices which 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 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 (2-4 fF) resultingin excellent isolation at 0.1-60GHz. Also, capacitive switches with a capacitance ratio of 60-160 provide excellent isolation from 8-100Hz. Very Low Insertion Loss: RF MEMS metal-contact and capacitive switches have an insertion loss of 0.1dB up to 100GHz. Linearity and Intermodulation Products: MEMS switches are extremely linear devices and therefore re-
sult 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 switches are fabricated using surface micromachining techniques and can be built on quartz, Pyrex, LTCC, mechanicalgrade high-resistivity silicon or GaAs substrates. RF MEMS switches also have their share of problems, and these are: Relatively Low Speeds: The switching speed of most electrostatic MEMS switches is 2-40 μs, and
is hard to test thermal switches for long cycle times
The near-ideal electrical response of RF MEMS witches (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.
Fig. 4 presents a 4-bit miniature RF MEMS phase shifter developed jointly by the Univ. of Michigan and Rockwell Scientific. It is based on the Rockwell metalcontact 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. Fig. 5 presents 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 metalinsulator- 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). Fig. 6 presents a W-band 3-bit phase shifter developed at the Univ. of Michigan using MEMS capacitive switches . This is the highest frequency MEMS phase shifter to-date and results in an average loss of 2.7-2.9 dB at 77-94GHz with an associated phase error of ±3◦. The results are about 8 dB better than GaAs designs.
Other circuits, which are not shown due to space constraints, are very wideband SP4T switches, highisolation series/shunt switches covering 0.1-50GHz, double-pole double-throw transfer switches, and a whole range of phase shifters from 8GHz to 120GHz. Also, tunable filters covering 200MHz to 23GHz have been developed by various groups. In general, RF MEMS circuits outperform GaAs FET and PIN diode circuits by a large margin at all frequencies of interest
circuits developed in the world can be found in .
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 author believes that RF MEMS switches and varactors will play an essential role in future high-value commercial and defense systems.