Gerald T. Mearini

Applying microfabrication to helical vacuum electron devices for THz applications

A new class of helical THz vacuum electron devices is under development using unconventional applications of microfabrication technology, modern computer modeling, and novel materials. The resulting slow wave circuits consist of a coil of gold wire, smaller in outside diameter than a human hair, supported by a thin diamond sheet and suspended within a diamond box. This configuration will extend the operating range of the helical slow wave circuit into the THz frequency band. Previously, the advantages of the wide bandwidth and high efficiency of the helical slow wave circuit have been available only for operation at frequencies below 50 or 60 GHz because of the difficulty of winding small coils of wire and because it is impossible to transmit a significant beam current through the small aperture offered by the center of the helix. These obstacles are overcome by fabricating the helices lithographically and by passing the electron beam around the outside of the helix. The design and fabrication of a 650 GHz backward wave oscillator (BWO) will be described as well as proposed applications of this technology to traveling wave tubes (TWTs) operating at frequencies as high as 1.0 THz. A THz amplifier, possibly with multioctave bandwidth, would have a wide range of important applications.

95 GHz helical TWT design

The helical slow-wave circuit is an attractive choice for traveling wave tube amplifiers (TWTAs) because of its inherently large bandwidth and relatively high RF efficiency. Unfortunately, as the operational frequency increases beyond Q-or V-band, its use has been limited by conventional fabrication techniques, and by the difficulty of passing enough current through the center of such a small structure. This paper describes the design and fabrication status of a 95 GHz TWT using microfabrication technology to create and assemble the helix. The electron beam propagates as two kidney shaped beamlets between the helix outer diameter and barrel.

Microfabrication of diamond-based slow-wave circuits for mm-wave and THz vacuum electronic sources

Planar and helical slow-wave circuits for THz radiation sources have been made using novel microfabrication and assembly methods. A biplanar slow-wave circuit for a 650 GHz backward wave oscillator (BWO) was fabricated through the growth of diamond into high aspect ratio silicon molds and the selective metallization of the tops and sidewalls of 90 μm tall diamond features using lithographically created shadow masks. Helical slow-wave circuits for a 650 GHz BWO and a 95 GHz traveling wave tube were created through the patterning of trenches in thin film diamond, electroplating of gold half-helices, and high accuracy bonding of helix halves. The development of new techniques for the microfabrication of vacuum electronic components will help to facilitate compact and high-power sources for terahertz range radiation.

Fabrication and testing of the 0.650 THz helical BWO

Teraphysics Corporation has assembled and tested a 0.65 THz backward wave oscillator (BWO) with a helical slow wave circuit. The helix is a coil of gold wire, which is smaller in outside diameter than a human hair. It is supported by a thin diamond sheet and mounted within a micromilled copper block.

A 650 GHz helical BWO

The operational frequency of helical vacuum electron devices has always been limited by the ability of conventional technology to fabricate very small helices, and by the difficulty of passing a meaningful current through the center of such a small structure. Microfabrication technology now offers the possibility of creating helices smaller in outside diameter than a human hair. These helices are not fabricated by wrapping wire or tape around a mandrel, they are grown. We will use such small helices to create a new class of vacuum electron devices that will operate by passing an electron beam around the outside of the helix, rather than through the center. This technology not only offers the opportunity to make meaningful incursions into a previously unused portion of the electromagnetic spectrum, it offers the possibility to do so inexpensively and in large quantities. This paper will discuss the design rationale and fabrication plan to be employed for the first device of this new class, a 650 GHz helical backward wave oscillator (BWO). A companion paper will describe the computational simulation of the 650 GHz BWO.

Diamond-based sub millimeter backward wave oscillator

Summary form only given. The diamond-based backward wave oscillator (BWO) provides a miniature, energy efficient, electronically tunable and mass producible signal source in the sub-mm wavelength regime. Fabricated within a shell of chemically vapor deposited (CVD) diamond for mechanical and thermal robustness, the BWO employs a novel biplanar interdigital slow wave circuit. The electron source for the BWO is a Spindt type field emission cathode. The device has been modeled extensively, and preliminary designs of the slow wave circuit, electron gun and collector for operation at 300 and 600 GHz have been completed. Fabrication of the 300 GHz device is in progress.

Backward Wave Oscillator Development at 300 and 650 GHz

A 300 GHz backward wave oscillator (BWO) is presently in fabrication and the design of a 650 GHz BWO is nearing completion. Both devices make use of the novel biplanar interdigital slow wave circuit and patented GENVAC technology for the fabrication of intricate chemical vapor deposition (CVD) diamond structures

Microfabricated mm-wave TWT platform for wireless backhaul

Teraphysics Corporation is developing a suite of miniature mm-wave TWTs for application to wireless backhaul. These devices are based on the 94 GHz TWT that was presented at IVEC 2014. This TWT, which weighs only .5 kg and has the footprint of a credit card, will serve as a platform for the miniature devices needed to fit into the limited space available on typical backhaul sites, such as telephone poles or street lights. The first application will be in E-Band. However, the sweet spot for the Teraphysics miniature helical amplifiers will be in the atmospheric windows centered at 140 and 240 GHz.

Assembly and preliminary testing of the prototype 650 GHz BWO

Approximately twenty intricate diamond structures have been fabricated, selectively metallized and assembled into prototype 650 GHz backward wave oscillators (BWOs). The chemical vapor deposition (CVD) diamond structure is grown into a silicon mold fabricated using deep reactive ion etching. The lithographic fabrication processes employed produce dozens of devices from each lot. The RF power generated is radiated directly from the BWO for compatibility with a quasi-optical sub-mm system. The interdigital slow wave circuit, the electron gun, the output coupler and the antenna are fabricated as a single piece of diamond to eliminate difficulties with alignment. We will describe the processes of fabrication and assembly, and present available preliminary results of prototype testing.

Diamond-based submillimeter backward wave oscillator

Making use of fabrication technology commonly employed in the manufacture of liquid crystal and semiconductor devices, but not previously applied to vacuum devices, the diamond-based backward wave oscillator (BWO) provides a miniature, energy efficient, electronically tunable and mass producible signal source in the sub mm wavelength regime. Fabricated within a shell of chemical vapor deposited (CVD) diamond for mechanical and thermal robustness, the BWO employs a novel biplanar interdigital slow wave circuit, which will be manufactured by utilizing a process developed at Genvac. Conventional silicon fabrication technology is used to form a negative of the desired structure, which serves as a mold for the deposition of the diamond. The diamond structure is then selectively metallized. The structure is formed in two halves and then accurately positioned and bonded using techniques routinely employed in the fabrication of liquid crystal displays. The device has been modeled extensively, and designs of the slow wave circuit, electron gun and collector for operation at 300 and 600 GHz have been completed. Fabrication of the 300 GHz device is in progress. It is estimated to weigh 29 gm, and, for operation over a 10% tuning range, the minimum output power is predicted to be 18 mW.