Simulation and microfabrication of MEMS vacuum electronic devices for terahertz technology
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Numerous applications explore the scarcely used terahertz (THz) band in the electromagnetic spectrum. However, widespread utilization of the THz band is technically challenging, due to the lack of power sources. The power level of the signal generators drastically drops as the frequency approaches the THz frequencies. Moreover, at this frequency range, the attenuation level of electromagnetic waves in the earth atmosphere increases. In vacuum electronics family, traveling wave tube (TWT) has gained particular interest for millimetre-wave and THz signal amplification for high-power and wideband applications. The characteristics of the TWT are mainly represented by the performance of a slow-wave structure (SWS). The SWS in TWT is used to facilitate the input THz signal which gets amplified by the process of the beam-wave interaction. Therefore, an efficient and reliable SWS is an essential aspect of a powerful TWT design. This thesis proposes a novel SWS, which consists of metal corrugations in the Hplane and E-plane of the rectangular waveguide operated in the fundamental mode. The concept of H-plane and E-plane load shows potential benefits to the THz TWTs, such as excellent linear characteristics, an increased interaction impedance, ultrawide bandwidth, and high-power output. Furthermore, the geometry of the SWS made the fabrication easier by using the available microfabrication techniques. The thesis involves all elements of development from device design, simulations, through material growth and interactive device fabrication to extensive characterization, along with the prototype cold-testing of the scaled-up device. The proposed SWS is investigated for TWT working at 400 GHz central frequency. The design and simulation of TWT involve three stages, which are high frequency electromagnetic study of a unit cell of the SWS using Eigenmode simulations, scattering parameters or S-parameters analysis of coupler design with the SWS usingtime-domain transient simulations, and beam-wave interaction study using particlein- cell (PiC). The PiC simulations found that this tube can produce an excess of 25 dB small-signal gain with 60 GHz 3-dB bandwidth at 400 GHz central frequency. The LIGA technique was employed to fabricate the 400-GHz E-plane and H-plane loaded SWS. In particular, the KMPR negative tone photoresist was used in the fabrication process. The KMPR mold produced the nearly perfect features of the mask designs, tolerance within the ±2.5 μm of the lateral features and maximum deviation of 20 in the vertical profiles. The experiments for surface roughness study revealed that the interior sidewall roughness of the LIGA-fabricated metal parts was achieved about 20 nm. A diffusion bonding method was employed to attach the two LIGA-fabricated copper parts of the SWS, producing a bonding strength of 25 MPa at 400 0C bonding temperature and with a void-free bonding interface. A scale-up model of the H-plane and E-plane SWS together with input and output couplers at W-band (85-110 GHz) was designed, manufactured using CNC milling, and verified the electromagnetic characteristics experimentally. The test results of S-parameters agreed with the simulations, S11<-15 dB over a frequency range of 85 GHz - 98 GHz. PiC simulations were also performed to the 93.5-mm length SWS, demonstrating a gain about 24 dB over 89 GHz – 101 GHz. To mitigate the diocotron instability in the sheet-beam transmission, a modified geometry of the sheet beam was employed and analysed through PiC simulations. The robust design and high performance from the H-plane and E-plane loaded SWS suggests that this is a promising candidate for high-power and wideband TWT development in the millimetre and THz frequency ranges.
Has partsArticle1: H-plane and E-plane loaded rectangular slow-wave structure for terahertz TWT amplifier, Laxma Reddy Billa, Muhammad Nadeem Akram and Xuyuan Chen, IEEE Transactions on Electron Devices, Vol. 63, 2016.
Article 2: Improved design and microfabrication of H-plane and E-plane loaded rectangular slow-wave structure for THz amplifier Laxma Reddy Billa, Xianbao Shi, Muhammad Nadeem Akram and Xuyuan Chen, IEEE Transactions on Electron Devices, Vol. 64, 2017
Article 3: UV-LIGA microfabricated THz slow-wave structures for vacuum electronics devices using KMPR photoresist, Laxma Reddy Billa, Muhammad Nadeem Akram and Xuyuan Chen, IOP Journal of Micromechanics and Microengineering. (manuscript submitted).
Article 4: H- and E-plane loaded slow-wave structure slow-wave structure for W-band TWT, Laxma Reddy Billa, Muhammad Nadeem Akram, Claudio Paoloni, and Xuyuan Chen, IEEE Transactions on Electron Devices. (manuscript accepted)