UV-LIGA microfabrication process for sub-terahertz waveguides utilizing multiple layered SU-8 photoresist

Abstract

A microfabrication process based on UV LIGA (German acronym of lithography, electroplating and molding) is proposed for the fabrication of relatively high aspect ratio sub-terahertz (100 -1000 GHz) metal waveguides, to be used as slow wave structure (SWS) in sub-THz vacuum electron devices. The high accuracy and tight tolerances required to properly support frequencies in the sub-THz range can be only achieved by a stable process with full parameter control. The proposed process, based on SU-8 photoresist, has been developed to satisfy high planar surface requirements for metal sub-THz waveguides. It will be demonstrated that, for a given thickness, it is more effective the stacking of a number of layers of SU-8 with lower thickness rather than using a single thick layer obtained at lower spin rate. The multiple layer approach provides the planarity and the surface quality required for electroforming of ground planes or assembly surfaces and for assuring low ohmic losses of waveguides. A systematic procedure is provided to calculate soft and post-bake times to produce high homogeneity SU-8 multiple layer coating as mold for very high quality metal waveguides. A Double Corrugated Waveguide (DCW) designed for 0.3 THz operating frequency, to be used in vacuum electronic devices, was fabricated as test structure. The proposed process based on UV LIGA will enable low cost production of high accuracy sub-THz threedimensional waveguides. This is fundamental for producing a new generation of affordable sub-THz vacuum electron devices, to fill the technological gap that still prevents a wide diffusion of numerous applications based on THz radiation.

Millimeter wave traveling wave tubes for the 21st Century

“…Currently, the maximum operating frequency for space TWTs is about 70 GHz. However, it is already envisaged the use of E-band (71)(72)(73)(74)(75)(76)(77)(78)(79)(80)(81)(82)(83)(84)(85)(86) for very high data rate satellite communications [12].…”

Multiple-beam and double-mode staggered double vane travelling wave tube with ultra-wide band

“…And, different from the structures made of silicon and coated with a thin metal layer, it provides better thermal robustness due to superior thermal and electrical properties. Moreover, compared to additive manufacturing developing recently, it can obtain lower surface roughness [22][23][24][25] . Although there are some drawbacks of CNC milling like only one structure can be fabricated at one time and the geometry design is limited 26 , we still chose this CNC milling technique to manufacture our W-band SDV-TWT.…”

UV-LIGA microfabrication of 0.3 THz double corrugated waveguide

“…The SU-8 is spin deposited with a proper spin speed to achive the desired thickness given the viscosity of the SU-8. It has previously been shown [8] that for thick layers (>100 μm) a multi-layer process helps in avoiding an uneven surface and reduces the size of edge beads after spin coating. In this work two layers, with a thickness of 80 μm each, were deposited to create the mold.…”

“…This paper describes progress towards multi-layer UV-LIGA fabrication, using SU-8 photoresist [7], of millimeter waves and THz waveguides for use as slow wave structures (SWS) in VEDs [8].…”