LIGA-fabricated compact mm-wave linear accelerator cavities

Millimeter-wave rf cavities for use in linear accelerators, free-electron lasers, and mm-wave undulators are under development at Argonne National Laboratory. Typical cavity dimensions are in the 1000 μm range, and the overall length of the accelerator structure, which consists of 30–100 cavities, is about 50–100 mm. An accuracy of 0.2% in the cavity dimensions is necessary in order to achieve a high Q-factor of the cavity. To achieve this, these structures are being fabricated using deep X-ray lithography, electroforming, and assembly (LIGA). The first prototype cavity structures are designed for 108 GHz and 2π/3-mode operation. Input and output couplers are integrated with the cavity structures. The cavities are fabricated on copper substrates by electroforming copper into 1 mm-thick PMMA resists patterned by deep x-ray lithography and polishing the copper down to the desired thickness. These are fabricated separately and subsequently assembled with precision spacing and alignment using microspheres, optical fibers, or microfabricated spacers/alignment pieces. Details of the fabrication process, alignment, and assembly work are presented in here.