Christian L. Arrington

Large-area metamaterials on thin membranes for multilayer and curved applications at terahertz and higher frequencies

A possible path for fabricating three-dimensional metamaterials with curved geometries at optical and infrared frequencies is to stack flexible metamaterial layers. We have fabricated highly uniform metamaterials at terahertz frequencies on large-area, low-stress, free-standing 1 μm thick silicon nitride membranes. Their response remains comparable to that of similar structures on thick substrates as measured by the quality factor of the resonances. Transmission measurements with a Fourier transform infrared spectrometer highlight the advantage of fabricating high frequency metamaterials on thin membranes as etalon effects are eliminated. Releasing the membranes enables layering schemes and placement onto curved surfaces in order to create three-dimensional structures.

Design, fabrication, and characterization of metal micromachined rectangular waveguides at 3 THz

Single-mode 75 mum x 37 mum rectangular waveguide components, including horn antennas, couplers, and bends, for operation at 3 THz have been designed and fabricated using thick gold micromachining. THz transmission through these waveguides has been quasi-optically measured at 2.92 THz. This technology offers the potential for realizing miniature integrated systems operating in the 3 THz frequency range.

Properties of Surface Metal Micromachined Rectangular Waveguide Operating Near 3 THz

Single-mode TE10 rectangular waveguides operating near 3 THz have been demonstrated. The waveguides have internal dimensions of 75 μm × 37 μm (WR-0.3) and are fabricated using an additive gold electroplating process on a silicon substrate. The impact of photoresist removal holes was minimized by full-wave design of the hole and matching structures. Waveguides were measured at three frequencies from 2.56 to 3.11 THz and demonstrated loss as low as 1.3 dB/mm at 3.11 THz, corresponding to a loss per wavelength of 0.12 dB/λ. This paper summarizes the design, fabrication, and measurement of these micromachined waveguides operating near 3 THz.

Terahertz quantum cascade laser integration with on-chip micromachined rectangular waveguides

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.

Integrated chip-scale THz technology

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.

Fabrication and characterization of large-area 3D photonic crystals

Full bandgap (3D) photonic crystal materials offer a means to precisely engineer the electromagnetic reflection, transmission, and emission properties of surfaces over wide angular and spectral ranges. However, very few 3D photonic crystals have been successfully demonstrated with areas larger than 1 cm2. Large sheets of photonic bandgap (PBG) structures would be useful, for example, as hot or cold mirrors for passively controlling the temperature of satellites. For example, an omni-directional 3D PBG structure emitting only at wavelengths shorter than 8 microns radiates only 7% of what a black body would at 200degK while radiating more than 40% at 400degK. 3D PBG materials may also find application in thermophotovoltaic energy generation and scavenging, as well as in wide field of view spectral filtering. Sandia National Laboratory is investigating a variety of methods for the design, fabrication, and characterization of PBG materials, and three methods are being pursued to fabricate large areas of PBG material. These methods typically fabricate a mold and then fill it with metal to provide a high refractive index contrast, enabling a full 3D bandgap to be formed. The most mature scheme uses silicon MEMS lithographic fabrication means to create a mold which if filled by a novel tungsten deposition method. A second method uses LIGA to create a mold in PMMA, which is filled by electro-deposition of gold, copper, or other materials. A third approach uses nano-imprinting to define the mold, which is filled using evaporative deposition or atomic layer deposition of metals or other materials. Details of the design and fabrication processes and experimental measurements of the structures are presented at the conference

Micro-fabricated stylus ion trap

An electroformed, three-dimensional stylus Paul trap was designed to confine a single atomic ion for use as a sensor to probe the electric-field noise of proximate surfaces. The trap was microfabricated with the UV-LIGA technique to reduce the distance of the ion from the surface of interest. We detail the fabrication process used to produce a 150 μm tall stylus trap with feature sizes of 40 μm. We confined single, laser-cooled, (25)Mg(+) ions with lifetimes greater than 2 h above the stylus trap in an ultra-high-vacuum environment. After cooling a motional mode of the ion at 4 MHz close to its ground state ( = 0.34 ± 0.07), the heating rate of the trap was measured with Raman sideband spectroscopy to be 387 ± 15 quanta/s at an ion height of 62 μm above the stylus electrodes.

Multilayer metal micromachining for THz waveguide fabrication

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 traditionally. 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.

Optimizing galvanic pulse plating parameters to improve indium bump to bump bonding

The plating characteristics of a commercially available indium plating solution are examined and optimized to help meet the increasing performance demands of integrated circuits requiring substantial numbers of electrical interconnections over large areas. Current fabrication techniques rely on evaporation of soft metals, such as indium, into lift-off resist profiles. This becomes increasingly difficult to accomplish as pitches decrease and aspect ratios increase. To minimize pixel dimensions and maximize the number of pixels per unit area, lithography and electrochemical deposition (ECD) of indium has been investigated. Pulse ECD offers the capability of improving large area uniformity ideal for large area device hybridization. Electrochemical experimentation into lithographically patterned molds allow for large areas of bumps to be fabricated for low temperature indium to indium bonds. The galvanic pulse profile, in conjunction with the bath configuration, determines the uniformity of the plated array. This pulse is manipulated to produce optimal properties for hybridizing arrays of aligned and bonded indium bumps. The physical properties of the indium bump arrays are examined using a white light interferometer, a SEM and tensile pull testing. This paper provides details from the electroplating processes as well as conclusions leading to optimized plating conditions.

Terahertz metamaterials on thin silicon nitride membranes

The terahertz (THz) region of the electromagnetic spectrum holds promise for spectroscopic imaging of illicit and hazardous materials, and chemical fingerprinting using moment of inertia vibrational transitions. Passive and active devices operating at THz frequencies are currently a challenge, and a promising emerging technology for such devices is optical metamaterials. In particular, a chem/bio sensing scheme based on the sensitivity of metamaterials to their dielectric environment has been proposed but may be limited due to the large concentration of electric flux in the substrate. In addition, there is an interest in fabricating 3D metamaterials, which is a challenge at these and shorter wavelengths due to fabrication constraints. In order to address both of these problems, we have developed a process to fabricate THz metamaterials on free-standing, 1 micron thick silicon nitride membranes. We will present THz transmission spectra and the corresponding simulation results for these metamaterials, comparing their performance with previously fabricated metamaterials on various thick substrates. Finally, we will present a scheme for implementing a 3D THz metamaterial based on stacking and possibly liftoff of these silicon nitride membranes.