This collection of works is an effort toward finding new solutions to challenges in electromagnetic devices, all connected by the implementation of vanadium dioxide (VO2). VO2 is a phase change material (PCM) that exhibits a reversible phase transition at 68 °C between monoclinic insulating and tetragonal conducting states. We designed, simulated, fabricated, and tested various devices in mmWave and optical wavelength domains. As an alternative to complex solid-state switching networks, we chose VO2 for its ability to react to various stimuli as a PCM, low phase transition temperature, and the freedom to design arbitrary geometries for reconfigurable and smart reactive devices.
First, we experimented with the growth and characterization of VO2 thin films on sapphire and silicon substrates with Al2O3 buffer layers. Traditionally, VO2 has been deposited on sapphire substrates because of the lattice match between the two. This produces films with high resistivity contrast. However, sapphire is not as versatile a substrate material as silicon, being dielectric rather than semiconductor and extremely difficult to etch. To expand the realm of substrates useful for sputtering high quality VO2, we grew and compared such films on C-plane sapphire and silicon wafers with atomic layer deposited (ALD) alumina (Al2O3) films. Silicon has poor lattice match with VO2, and the alumina eliminates that interface. Furthermore, rapid thermal annealing (RTA) the alumina films before sputtering VO2 provides a basis for quasi-epitaxial films that have similar properties to those on the C-plane sapphire substrates. The figure of merit (FOM) resistivity contrast ratios for these variations are 9.76×104, 3.66×103, and 1.46×104 for C-plane sapphire, as- deposited amorphous ALD alumina on Si, and RTA ALD alumina on Si, respectively. We also characterized the films using X-Ray diffraction, atomic force microscopy, and scanning electron microscopy.
In the next step, we examined the material reliability of VO2 in the context of mmWave systems. Previous studies have been performed on the lifetime of VO2 under voltage and current actuation, but literature on its reliability under thermal cycling alone is limited. We designed a coplanar waveguide (CPW) shunt switch with Cu signal and ground and VO2 spanning the gap between the center signal line in the ground planes. The switch was fabricated on a 2” C-plane sapphire substrate with a two-layer process. The VO2 was activated with Cu Joule heaters next to the switch for a total of 100 million thermal cycles at 100 Hz. The switch was designed, simulated, and measured from 35–45 GHz with maximum switching at 38 GHz of inactivated state with S11 = –23.7 dB and S21 = –2.4 dB switching to activated state with S11 = –4.1 dB and S21 = –38 dB. Using a paired t-test with 1% p-value, we compared the S-parameters of four switches at frequencies 35, 38, 40, 43, and 45 GHz after 0, 105, 106, 107, and 108 thermal cycles, finding no statistically significant changes in performance over the course of experimentation.
We also designed, simulated, and fabricated a reconfigurable linear to circular polarization (LCP) converter at Ka-band using VO2. Present literature focuses on single-state LCP converters that exhibit dual band operation with one sense of circular polarization (CP) assigned to each operating band. Our device takes linearly polarized (LP) input and outputs right hand circularly polarized (RHCP) in one configuration and left hand circularly polarized (LHCP) waves in the other. Polarization diversity within a single aperture and frequency band (27.5–31 GHz) performance has not yet been published. The advantages of such performance include increased versatility in satellite communications (satcom) and reducing interference in crowded wireless mobile networks. Based on the classic meanderline polarizer, our device consists of a 3.3 mm square unit cell with Ag meanderline traces, VO2 and NiCr Joule heaters at interruptions in these traces. This meanderline structure is duplicated and rotated 90° within the unit cell with an alumina isolation layer and Ag bridge layer to isolate the configurations in DC. The substrate is 100 μm thick high resistivity float zone Si, and the full polarizer consists of four of these layers separated at a pitch of 600 μm. The device was simulated in ANSYS HFSS as an infinite array with Floquet ports and periodic boundary conditions, and the fabricated device was an 8×8 finite array so that all four layers could be manufactured on one wafer. Focusing lens horn antennas were used due to the small size of the metasurface. The simulation results show FOM axial ratio (AR) ≤0.82 and insertion loss (IL) ≤ 2.59 dB. An AR value under 1 dB is excellent CP performance, and IL under 3 dB is sub-optimal but determined to be an acceptable trade-off for reconfigurability at this stage of technology development. Fabrication results and preliminary testing results are enclosed. Additional rounds of fabrication and testing are currently being planned.
We present lastly the design and Multiphysics simulation of a mid-infrared thermo-optic 4-port router with self-activating VO2. The router has a Si core and SiO2 substrate, with air as the upper cladding. It consists of two bus waveguides and two ring resonators: one Si and one Si with VO2 on top. The device functions by starting with 75% transmittance from input to output, then at the VO2 transition temperature, the initial output transmittance is limited dramatically and rerouted to half input power at an alternate output port. Having VO2 in only one ring resonator creates an asymmetric heating activation effect. From one direction of input, the VO2 activation power threshold is 7.1 mW, at which point the output power is limited by 64 dB and rerouted at 48%; from the other direction, the VO2 activation power threshold is 7.2 mW, where the output power is limited by 56.5 dB and rerouted at 53%. There is also a large bi-stability effect observed that is characterized by VO2 maintaining its metallic state at input powers below the activation threshold, down to 2.2 mW.