Suk-Sang Chang

Microfabrication of millimeter wave vacuum electron devices by two-step deep-etch x-ray lithography

The circuits for millimeter wave vacuum electron devices with all circuit elements including an electron beam tunnel are microfabricated by two-step deep-etch x-ray lithography (x-ray LIGA). The discrepancies of eigenfrequency between experiment and simulation are within 1.1% in a coupled-cavity structure and 1.4% in a folded waveguide structure when the operating frequency is about 100GHz. Furthermore, a measured tolerance of below 2μm, and a measured surface roughness of 20–70nm, of LIGA-fabricated circuits implies the two-step LIGA microfabrication has potential applications up to the submillimeter wave region.

Low-voltage operation of Ka-band folded waveguide traveling-wave tube

Low-voltage operation of millimeter-wave folded waveguide traveling-wave tube (TWT) was investigated using a 12 kV linear electron beam. Backward wave oscillation operated at second space harmonic was observed with output power of 20 W, linear tunability of 6% within 0.01 dB/MHz, and voltage-frequency stability of 0.56 MHz/V. The measured frequency and output power are in a good agreement with the predicted values using a particle-in-cell (PIC) code. In addition, backward-wave interaction at second-space harmonic as an amplifier was observed with a measured linear gain of 15 dB and a bandwidth of 0.3%. For forward wave interaction, a linear gain of 25 dB, bandwidth of 17%, and efficiency of 7% were predicted for fundamental space harmonic using a PIC code. Improvement in efficiency was predicted for the forward-wave amplifier where it was operated as an oscillator employing a delayed feedback. Output power was increased by 10 dB in the delayed feedback oscillator comparing with the backward-wave oscillator. The effect of period doubling due to potential fabrication inaccuracies on the stopband was studied experimentally. The folded waveguide TWT fabricated using the Lighographie, Galvanoformung, Abformung (LIGA) process operated at much higher frequencies is discussed.

Investigations on a microfabricated FWTWT oscillator

A vacuum tube oscillator fabricated by the deep etch X-ray lithography: lithographie, galvanoformung, abformung (LIGA) process was successfully developed for the first time. For the proof-of-concept experiment involving a delayed feedback oscillator, a folding-waveguide traveling-wave tube (FWTWT) was fabricated by computer numerical control milling in advance. With a 12.4-kV and 150-mA electron beam, a Ka-band FWTWT amplifier shows a linear gain of 25 dB and a bandwidth of 10%. Applying a delayed feedback scheme, the threshold feedback strengths for the onset of oscillation and for self-modulation were measured to be -30 dB and -16 dB, respectively. The optimum value of the feedback strength for the single-frequency oscillation at 32.5 GHz was about -18 dB with a net electronic efficiency of 6%. The LIGA-fabricated FWTWT circuit was constructed by a lithographic process using the synchrotron X-ray source at the Pohang Light Source. The resulting accuracy and average surface roughness were less than 10 and 1 /spl mu/m, respectively. The LIGA-fabricated Ka-band amplifier shows a linear gain of 15 dB and bandwidth of 1.7% with a 12.4-kV, 47-mA electron beam. The threshold for the onset of oscillation was about -11 dB and the optimum value of feedback strength for a single frequency oscillation at 35 GHz with a net electronic efficiency of 3.5% was about -8 dB.

Experimental investigations on miniaturized high-frequency vacuum electron devices

We investigated the foundations for high-frequency vacuum electron devices experimentally, with emphasis on deep etch X-ray lithography: lithographie, galvanoformung, abformung (LIGA) to fabricate a miniaturized interaction circuit and a photonic crystal (PC) resonator to excite a stable high-order mode. The successful operation of a LIGA-fabricated folded-waveguide traveling-wave tube was reported. From such physical considerations as Debye length and photonic band gap, we proposed a reflex klystron adopting a cold cathode and a PC resonator.

Experimental investigation of millimeter wave folded-waveguide TWT

Folded-waveguide traveling-wave tube (FWTWT) is one of the candidates for high power, broad-band robust millimeter wave sources. The FWTWT has no periodic reactive load. Therefore, the dispersion relation of space harmonics is estimated by simple transform of that of rectangular waveguide. The folded waveguide circuits are designed and fabricated using 12 kV, 150 mA linear electron beam for Ka-band amplifier and oscillator based on the synthesis process. Solenoid focusing system and pulsed electron beam are used to investigate basic properties of FWTWT in the laboratory. Non-stationary beam-wave interactions performance are investigated by MAGIC3D, and one-dimensional code. From initial folded-waveguide BWO experiment, about 20-watt output power at 35 GHz is obtained with 6% linear tunability. The LIGA fabrication is in process for the operation at the higher frequency.

Experimental investigations on miniaturized vacuum electron devices

We investigated foundations for high frequency vacuum electron devices, experimentally with emphasis on LIGA (deep etch X-ray lithography: lithographie, galvanoformung, abformung; German acronyms) to fabricate a miniaturized interaction circuit and a photonic crystal (PC) resonator to excite a stable high order mode. The successful operation of a LIGA-fabricated folded-waveguide traveling-wave tube was reported. From the physical considerations such as Debye length and photonic band gap, we proposed a reflex klystron adopting a cold cathode and a PC resonator.

100GHz LIGA-fabricated coupled-cavity device

This abstract describes the design, fabrication and testing of microfabricated 100GHz coupled-cavity backward wave oscillator (CCBWO), which is operated in space harmonics −1 at 12 kV and 50 mA with round-beam radius 0.18 mm. The RF performance has studied using 3-D electromagnetic field simulator and particle-in-cell code. The cold-test assembly has been fabricated. The complete device assembly for hot-test is under fabrication. Experimental results will be presented.

THz Multi-frequency resonance filter

The transmission characteristic of a two-dimensional array of rectangular holes was examined theoretically and experimentally in the THz frequency range. The structure under consideration was found to act as a multi-frequency resonance filter whose frequency response of transmission depends on the periodicities and filling material of the structure.

Experimental Investigation of 95GHz Folded Waveguide Backward Wave Oscillator Fabricated by Two-Step LIGA

Most recently, 95GHz three-dimensional folded waveguide structure including the electron beam-tunnel is completely microfabricated by two-step deep-etch X-ray lithography (X-ray LIGA). The precision and positional tolerance of the circuit elements are respectively 2mum and 2-3 mum. We experimentally measured its dispersive and attenuative properties through scattering matrices and compare them to analytical and numerical calculations. The MAGIC3D simulation numerically predicted the folded waveguide BWO (FWBWO) has start-oscillation current of 30mA and produces output power of 5 Watts at the beam condition of about 12kV and 50mA. Currently, we prepare experiment of the FWBWO equipped with the electron gun

Feasibility Study of Two‐Step LIGA‐Fabricated Circuits Applicable to Millimeter/Submillimeter Wave Sources

Some baffling technical problems such as miniaturization and threshold reduction have impeded micro‐vacuum electronic devices from proper application in the millimeter/submillimeter wave region. Two‐step LIGA, proposed to construct three‐dimensional circuit elements within minimum tolerances on the micron scale, is applied to a traditional vacuum electron device — the folded waveguide backward wave oscillator (FWBWO) — and to novel counter‐streaming electron beam oscillators (CSEBOs) for the first time. Circuit characteristics of these devices, in particular those related to excellent higher frequency adoptability, are discussed with design optimization and numerical analyses provided by MAGIC3D simulations. FWBWO and CSEBO circuits were fabricated by the two‐step LIGA process and cold‐tested. The discrepancy between measured and simulated dispersion and phase‐velocity characteristics is about 1.4%. Also, the surface roughness of the LIGA‐fabricated circuit was measured to be 70 nm by AFM. Cold testing of...