Micromachined Waveguide Integrated RF MEMS Phase Sifter Operating between 500-600 GHz

This paper presents a 500-600 GHz submillimeterwave MEMS-reconfigurable phase shifter. It is the first ever RF MEMS component reported to be operating above 220 GHz. The phase shifter is based on a micromachined rectangular waveguide which is loaded by 9 E-plane stubs, which can be individually blocked by using MEMS–reconfigurable surfaces. The phase-shifter is composed of three metallized silicon chips which are assembled in H-plane cuts of the waveguide. Submillimeter-wave or terahertz frequency band have emerging applications in imaging systems, radar, spectral analysis, radio astronomy, and high bandwidth wireless data communications [1]. Phase shifters are, for instance, widely employed for beam steering applications in phased-array antennas used in communications and radar applications. Electrically-controlled phase shifters can replace mechanical scanning stages and thus dramatically increase both the measurement speed and system integration. Phase shifters using GaAs, MESFET, and p-i-n diode switches have high insertion loss, poor linearity performance, and large power consumption, in particular at sub-THz frequencies [2]. Ferrite based phase shifters perform well, but have large size and high cost. Phase shifters using liquid crystals [3] have too slow response time in seconds. Micro-electromechanical systems (MEMS) phase shifter can achieve low loss, high linearity, low power consumption and large bandwidth and have been shown to perform well up to 110 GHz frequency [2]. Micromachined waveguides have shown promising performance even up to 2.7 THz [4]. (Moving) MEMS-based devices have so far been reported only up to a maximum frequency of 220 GHz [5}. Fig. 1. Submillimeter-wave MEMS phase shifter design using 9 E-plane stubs loading a micromachined rectangular waveguide reconfigured by using independently-operated MEMS-reconfigurable surfaces. As shown in Fig. 1, the design is based on an E-plane stub-loaded rectangular waveguide phase shifter concept [6], where each stub is blocked or unblocked by MEMS-switched surfaces [7]. The rectangular waveguide used has non-standard dimensions (04156 mm × 0.2078 mm) to have the phase shifter functioning in the lower WR-1.5 band. The waveguide is loaded with 9 stubs each having a nominal phase shift of 10° resulting in a total phase shift of 90° in 10° steps, i.e., ten different phase states (3.3 bit). MEMS–switched E-plane surfaces are inserted perpendicularly to the wave propagation into the waveguide stubs. Each MEMS surface consists of distributed metallized elements, which can be reconfigured by on-chip MEMS actuators so that they can either block or unblock the TE10 mode wave propagation into the waveguide stubs. This is achieved by a number of vertical columns that are split into smaller sections grouped into a set of fixed and a set of movable cantilevers. The fixed set is anchored and the movable set is mechanically connected via horizontal suspension bars to electrostatic comb-drive MEMS actuators. The MEMS actuators provide synchronous lateral movement of the vertical cantilevers. When the MEMS surface is open (nonblocking), the vertical cantilevers of the fixed and moving elements are not in contact and thus allow the electromagnetic wave to propagate freely through the switched surface. When the MEMS surface is closed, the movable vertical cantilevers are laterally moved into contact with the non-movable vertical cantilevers, to form closed vertical columns which block the wave propagation through the surface. The phase shifter consists of a micromachined waveguide chip, a MEMS switched stub chip with the MEMS-switched surfaces, and a cap chip terminating the stubs. The micromachined waveguide and the MEMS-switched surface chips both have micromachined alignment structures which are used to align the chips to machined metal blocks as well as to each other. The MEMS chip is aligned and placed on a machined metal block with the cap chip underneath. This is followed by aligning and placing the micromachined waveguide chip on top of the MEMS chip. The top metal block is aligned to the micromachined waveguide chip and placed on top. Alignment pins are used to align the metal blocks to each other as well as to the waveguide flanges. Fig. 2. Measured S-parameters and normalized phase shift of the fabricated 500-600 GHz phase shifter. DC biasing is provided by multi-contact DC probes, with a total of 12 contacts and an operation voltage of 32 V. Fig. 2 shows the measured insertion and return loss of the phase shifter for all ten phase states. The phase shifter behaves exceptionally broadband over the whole design frequency range of 500-600 GHz with the return loss better than 15 dB and the insertion loss of less than 5 dB. The phase shift is normalized to the state with all stubs blocked and shows a linear phase shift of 20° in 10 discrete steps (3.3 bit) instead of the 90° expected from simulations. The cause for this discrepancy is currently being investigated and most likely due to the non-optimized sidewall roughness in the stubs. In conclusion, this paper demonstrates the first ever reconfigurable MEMS phase shifter in the 500600 GHz frequency range. A linear phase shift of 20° has been measured (in ten steps) with a return loss of better than 15 dB and insertion loss of better than 5 dB in the designed band. The performance of this component can be further enhanced by making smoother sidewalls and improving tolerances on packaging.