Diana Gamzina

Terahertz vacuum electronic circuits fabricated by UV lithographic molding and deep reactive ion etching

The 0.22 THz vacuum electronic circuits fabricated by UV lithography molding and deep reactive ion etching processes are under investigation for submillimeter wave applications. Eigenmode transient simulations show that, accounting for realistic values of our currently achievable fabrication tolerances, the transmission, and dispersion properties of the operation modes of a TE-mode, staggered, double grating circuit are maintained within less than 1 dB and 2% deviation, respectively. Scanning electron microscopy and atomic force microscopy analyses of the fabricated circuit samples demonstrate that both of the microelectromechanical system fabrication approaches produce circuits with ±3–5 μm dimensional tolerance and ∼30 nm surface roughness.

UV Lithography and Molding Fabrication of Ultrathick Micrometallic Structures Using a KMPR Photoresist

By using a novel negative-tone photoresist, KMPR, we have investigated ultraviolet (UV) lithographic microelectroforming fabrication of ultrathick metallic microstructures ( ¿ 400 ¿m). Scanning coating spin speed together with the film thickness and uniformity has been characterized at low spin speed from 1000 to 200 r/min. Based on the film profile characterization, the single-spin lithography conditions for a 400-¿m-thick electroforming mold are optimized by scanning process parameters of UV exposure energy and bake temperature and time. SEM-measured dimensional accuracy and sidewall verticality of the optimized thick KMPR mold are ¿ ¿ 3 ¿m and 90° ±1°, respectively, which are comparable to those of SU8 molds. The SEM analysis of the patterned film and the electroformed structure has shown that the submillimeter-thick KMPR features have ~ 5-10:1 aspect ratio. The sidewall surface roughness of the copper deposition is locally measured to be about ~ 50-100 nm by atomic force microscopy, which is significantly smoother than that resulting from other mechanical machining approaches. This novel photoresist enables the lithographic molding microfabrication process to mass produce plastic and metallic microcomponents for various microelectromechanical systems applications.

Scandate Dispenser Cathode Fabrication for A High-Aspect-Ratio High-Current-Density Sheet Beam Electron Gun

A high-current-density scandate tungsten dispenser cathode was used for the demonstration of a 25 : 1-aspect-ratio 750-A/cm2 -current-density sheet beam for the Defence Advanced Research Project Agency High-Frequency Integrated Vacuum Electronics (HiFIVE) program intended for the realization of a wideband ( ~ 30%) 220-GHz traveling wave tube. The elliptical cathode with homogeneous microstructure was made from 1-2-μm-size tungsten powder added with nanosized Scandia using the sol-gel method; it has a current density of up to 160 A/cm2 at 1050 °C. A sheet beam gun analyzer was built to test the terahertz sheet beam gun and determine the size and current density of a sheet electron beam produced by the impregnated scandate tungsten dispenser cathode. A sheet electron beam with an aspect ratio of 12.5 : 1 with a current density exceeding 375 A/cm2 has been obtained using a BVERI impregnated scandate dispenser cathode without magnetic compression; further magnetic field compression would give the final current density of 750 A/cm2.

Nanoscale Surface Roughness Effects on THz Vacuum Electron Device Performance

Vacuum electron devices are the most promising solution for the generation of watt-level power at millimeter wave and terahertz frequencies. However, the three-dimensional nature of metal structures required to provide an effective interaction between an electron beam and THz signal poses significant fabrication challenges. At increasing frequency, losses present a serious detrimental effect on performance. In particular, the skin depth, on the order of one hundred nanometers or less, constrains the maximum acceptable surface roughness of the metal surfaces to be below those values. Microfabrication techniques have proven, in principle, to achieve values of surface roughness at the nanometer scale; however, the use of different metals and affordable microfabrication techniques requires further investigation for a repeatable quality of the metal surfaces. This paper compares, for the first time, the nanoscale surface roughness of metal THz waveguides realized by the main microfabrication techniques. In particular, two significant examples are considered: a 0.346-THz backward wave tube oscillator and a 0.263-THz traveling wave tube.

High Current Density and Long-Life Nanocomposite Scandate Dispenser Cathode Fabrication

A nano-Sc₂O₃-added W powder for use in high current density thermionic cathodes has been made using a solution-gel method with the following controllable uniform average particle sizes: ~72, 146, 272, and 587 nm. Using these powders, nanostructured Sc₂O₃ -added tungsten matrices with uniform nanosized tungsten grains and homogenous pore distribution were obtained. Nanocomposite Sc₂O₃-added W impregnated cathodes have been prepared from these powders and have shown excellent emission properties. For example, space-charge-limited current densities of 40 ± 1 A · cm^-2 at 850°C and 170 ± 5 A · cm^-2 at 1050°C have been obtained using a 300-nm Sc₂O₃-added (4.77 wt.%) W powder. Life testing is ongoing with 50 ± 2.5 A · cm^-2 current density emission demonstrated at 1050°C after 10 680 h.

THz Backward-Wave Oscillators for Plasma Diagnostic in Nuclear Fusion

Understanding of the anomalous transport attributed to short-scale length microturbulence through collective scattering diagnostics is key to the development of nuclear fusion energy. Signals in the subterahertz (THz) range (0.1-0.8 THz) with adequate power are required to map wider wavenumber regions. The progress of a joint international effort devoted to the design and realization of novel backward-wave oscillators at 0.346 THz and above with output power in the 1 W range is reported herein. The novel sources possess desirable characteristics to replace the bulky, high maintenance, optically pumped far-infrared lasers so far utilized in this plasma collective scattering diagnostic. The formidable fabrication challenges are described. The future availability of the THz source here reported will have a significant impact in the field of THz applications both for scientific and industrial applications, to provide the output power at THz so far not available.

0.22 THz wideband sheet electron beam traveling wave tube amplifier: Cold test measurements and beam wave interaction analysis

In this paper, we describe micro-fabrication, RF measurements, and particle-in-cell (PIC) simulation modeling analysis of the 0.22 THz double-vane half period staggered traveling wave tube amplifier (TWTA) circuit. The TWTA slow wave structure comprised of two sections separated by two sever ports loaded by loss material, with integrated broadband input/output couplers. The micro-metallic structures were fabricated using nano-CNC milling and diffusion bonded in a three layer process. The 3D optical microscopy and SEM analysis showed that the fabrication error was within 2–3 μm and surface roughness was measured within 30–50 nm. The RF measurements were conducted with an Agilent PNA-X network analyzer employing WR5.1 T/R modules with a frequency range of 178-228 GHz. The in-band insertion loss (S21) for both the short section and long section (separated by a sever) was measured as ∼−5 dB while the return loss was generally around ∼−15 dB or better. The measurements matched well with the S-matrix simulation...

Nano CNC milling of two different designs of 0.22 THz TWT circuits

To satisfy the growing interest in high power (∼1–200 W) THz sources, our research group has been working on various technologies to miniaturize vacuum electronic devices while keeping the power levels high and offering wide instantaneous bandwidth operation. We have manufactured two different designs of 0.22 THz circuit structures including input/output couplers to WR4 waveguide. We have used nano CNC milling as our primary approach and we were able to achieve excellent surface finish and meet the high tolerance requirements of the high frequency wide bandwidth traveling wave tube circuit.

Nano CNC milling technology for terahertz vacuum electronic devices

To satisfy the growing interest in high power (∼1–200 W) THz sources, our research group has been working on various technologies to miniaturize vacuum electronic devices while keeping the power levels high and offering wide instantaneous bandwidth operation. We are exploring several methods for manufacturing 0.22 THz circuits including LIGA, silicon DRIE, and nano machining. Manufacturing circuits using micro-processing techniques allows multiple circuits to be made at a time, but the process is complex and requires perfection. On the other hand, nano machining allows for rapid prototyping of any circuit or coupler design. We were able to directly machine and test 0.22 THz circuits in various materials with transmission losses around 5–15 dB and with wide bandwidth.

Nano-CNC Machining of Sub-THz Vacuum Electron Devices

Nano-computer numerical control (CNC) machining technology is employed for the fabrication of sub-THz (100-1000 GHz) vacuum electron devices. Submicron feature tolerances and placement accuracy have been achieved and surface roughness of a few tens of nanometers has been demonstrated providing high-quality radio frequency (RF) transmission and reflection parameters on the tested circuit structures. Details of the manufacturing approach are reported for the following devices: W-band sheet beam (SB) klystron, two designs of a 220-GHz SB double-staggered grating traveling wave tube (TWT), 263-GHz SB TWT amplifier for an electron paramagnetic resonance spectrometer, 346-GHz SB backward wave oscillator for fusion plasma diagnostics, 346-GHz pencil beam backward wave oscillator, and 270-GHz pencil beam folded waveguide TWT self-driving amplifier. Application of the nano-CNC machining to nanocomposite scandate tungsten cathodes as well as to passive RF components is also discussed.