The quantum cascade laser (QCL) is currently the only solid-state source of coherent THz radiation capable of
delivering more than 1 mW of average power at frequencies above
~ 2 THz. This power level combined with
very good intrinsic frequency definition characteristics make QCLs an extremely appealing solid-state solution
as compact sources for THz applications. I will present results on integrating QCLs with passive rectangular
waveguides for guiding and controlling the radiation emitted by the QCLs and on the performance of a THz
integrated circuit combining a THz QCL with a Schottky diode mixer to form a heterodyne receiver/transceiver.
We describe a monolithically integrated THz transceiver consisting of a Schottky diode embedded into a THz
quantum cascade laser (QCL) waveguide. Besides functioning as a heterodyne receiver for externally incident
radiation, the device is a useful tool for characterizing the performance and dynamics of the QCL. Here we
present an overview of the device, demonstrate receiver operation, and present laser dynamics measurements
especially related to feedback of the QCL's emission due to retroreflections.
Thick multi-layer metal stacking offers the potential for fabrication of rectangular waveguide components,
including horn antennas, couplers, and bends, for operation at terahertz frequencies, which are too small to machine
Air-filled, TE10, rectangular waveguides for 3 THz operation were fabricated using two stacked electroplated
gold layers on both planar and non-planar substrates. The initial layer of lithography and electroplating defined 37
micrometer tall waveguide walls in both straight and meandering geometries. The second layer, processed on top of the
first, defined 33 micrometer thick waveguide lids. Release holes periodically spaced along the center of the lids
improved resist clearing from inside of the electroformed rectangular channels. Processing tests of hollow structures on
optically clear, lithium disilicate substrates allowed confirmation of resist removal by backside inspection.
Integration of THz quantum cascade lasers (QCLs) with single-mode 75 μm x 37 μm rectangular waveguide components, including horn antennas, couplers, and bends, for operation at 3 THz has been designed and fabricated using thick gold micromachining. Measurements on the isolated waveguide components exhibit fairly low loss and integration with THz QCLs has been demonstrated. This technology offers the potential for realizing miniature integrated systems operating in the 3 THz frequency range.
We have fabricated and characterized radio frequency microelectromechanical systems (RF MEMS) ohmic switches for applications in discrete tunable filters and phase shifters over a frequency range of 0 to 20 GHz. Our previously reported cantilever switches have been redesigned for higher isolation and are now achieving 22 dB of isolation at 10 GHz. The measured insertion loss is 0.15 dB at 10 GHz. We have also fabricated and characterized new devices, designated “crab” switches, to increase isolation and contact forces relative to the cantilever design. The measured insertion loss and isolation are 0.1 dB per switch at 20 GHz and 22 dB at 10 GHz, respectively. A simple and accurate equivalent model has been developed, consisting of a transmission line segment and either a series capacitor to represent the blocking state or a series resistor to represent the passing state. Experimental analysis of the switch shows that high contact and substrate capacitive coupling degrades the isolation performance. Simulations indicate that the isolation improves to 30 dB at 10 GHz by reducing these capacitances. The crab switch design has a measured contact force of 120 μN, which represents a factor of four increase over the cantilever switch contact force and results in consistent, low-loss performance.
We have developed radio frequency microelectromechanical systems (RF MEMS) capacitive switches using amorphous diamond (a-D) as a novel tunable dielectric with controlled leakage. The switch is fabricated from sputtered and electroplated metals using surface micromachining techniques. The mechanical stress and resistivity of the a-D dielectric are controlled by the parameters of a high-temperature annealing process. These initial devices exhibit a down-state capacitance of 2.6 pF, giving an isolation of better than 18 dB at 18 GHz, and a predicted static power dissipation of 10 nW. This technology is promising for the development of reliable, low power RF MEMS switches.
Radio frequency microelectromechanical systems (RF MEMS) is an enabling technology for the miniaturization of future radar and communication systems. RF MEMS ohmic and capacitive switch performance and fabrication are discussed. Sandia National Laboratories’ program in RF MEMS is motivated by defense and national security applications not currently being met by the private sector. Examples of fabricated switches and switched circuits under investigation at Sandia are presented.
We have fabricated and tested a surface micromachined, metal-metal contacting radio frequency microelectromechanical systems (RF MEMS) switch. The switch was fabricated out of electroplated metals on semi-insulating GaAs at process temperatures below 300°C. It was anchored by folded springs to one end of a coplanar waveguide (CPW) gap, forming a cantilever. This configuration allowed us to simplify the fabrication process by eliminating mechanical dielectric films that are normally necessary to isolate the switch contact from the actuation metal. The measured insertion loss and isolation at S band were 0.21 dB and 28 dB isolation, respectively. An average switching speed of 83 μs at 55 volts was measured. This switch demonstrated >105 cold switching cycles without sticking, however rapid increase of the contact resistance was observed. A new switch was designed to increase isolation and reduce insertion loss by decreasing the coupling capacitance and increasing the contact force.
Future microwave networks require miniature high-performance tunable elements such as switches, inductors, and capacitors. We report a micro-machined high-performance tunable capacitor suitable for reconfigurable monolithic microwave integrated circuits (MMICs). The capacitor is fabricated on a GaAs substrate using low-temperature processing, making it suitable for post-process integration with MMICs, radio frequency integrated circuits (RFICs) and other miniaturized circuits. Additionally, the insulating substrate and high-conductivity metal provide low-loss operation at frequencies over 20 GHz. The device demonstrates a capacitance of 150 fF at 0 V bias, pull-in at about 15 V to 18 V, and further linear tuning from 290 fF to 350 fF over a voltage range of 7 V to 30 V. Also, the device demonstrates self-resonance frequencies over 50 GHz, and Q’s over 100 at 10 GHz. To enable integration into circuits, a simple equivalent circuit model of the device has been developed, demonstrating a good match to the measured data through 25 GHz. Initial testing to 1 billion cycles indicates that metal fatigue is the primary limitation to reliability and reproducibility, and that dielectric charging does not have a significant impact on the device. This device is promising for high-performance tunable filters, phase shifters, and other reconfigurable networks at frequencies through K-band.