Simulation and Microfabrication Development of Folded-Waveguide Slow-Wave Circuit for THz Traveling-Wave Tubes

摘要

Folded waveguide is considered as a robust slow-wave circuit for traveling-wave tubes, particularly for millimeter wave and terahertz wave applications. The relatively simple full-metallic structure of folded waveguide facilitates its fabrication process to be compatible with microfabrication technology, and it is structural robust and thermally stable, especially when the circuit is downsized into microscale dimensions. In this thesis, a folded-waveguide slow-wave circuit working on 220GHz central frequency is targeted for investigation. The thesis includes two major parts, i.e. the theoretical study and electromagnetic field simulations, and an ultra-thick SU-8 process development based upon microfabrication technology. Our cold-circuit analysis reveals that the pass-band of the 220GHz central frequency folded waveguide is ~80GHz, which is between the cut-off frequency and the first stop-band. Parametric cold-circuit study provides better knowledge about how the varying structural parameters can influence the cold-circuit parameters. Optimization of cold-circuit properties via simulation also indicates a ~20GHz 3-dB bandwidth of the circuit, and this is used as the basis of further beam interaction circuit simulations. Following the cold-circuit analysis, we carried out the beam interaction circuit simulations and optimizations with the loss-free particle-in-cell (PIC) simulation method. Our PIC simulations reveal that the transverse dimension and shape of the electron beam tunnel have considerable impact on the beam-wave interaction. The model with circular-cross-section beam tunnel exhibits similar bandwidth, higher efficiency and gain, comparing to that with square-cross-section tunnel. It is also indicated that phase velocity taper of electromagnetic wave on the rear half of circuit can greatly improve the output power and increase the efficiency, up to 70 %. The peak loss-free output power and efficiency predicted by the PIC simulations are 70.5W and 8%, respectively. Experimental study of microfabrication for the folded-waveguide slow-wave circuit was also conducted with the ultra-thick SU-8 process. With the help of confocal laser scanning microscopy, we quantitatively analyzed the sidewall surface roughness of the SU-8 mold.The vertical striation along the sidewall surface was eliminated successfully by proper improvement on the post-exposure-bake conditions, and the RMS (Root Mean Square) line roughness on the SU-8 mold sidewall was greatly reduced from ~1 μm to ~70 nm. AFM analysis was also applied to examine the sidewall surface roughness, and the RMS surface roughness can be as low as 2.6 nm on the optimized samples.A novel micromachining process for fabricating the folded waveguide was developed in our study basing on fiber embedment SU-8 process. Our preliminary results indicate that the fiber can be mounted properly in the SU-8 serpentine mold.