Laser precession machining of cross-shaped terahertz bandpass filters

Abstract

The laser micro-machining technology is an attractive alternative to conventional photoresist-based technologies for manufacturing terahertz (THz) cross-shaped mesh filters. It can address some limitations related to filter's quality, process complexity, unit cost and available materials. However, there are also open issues associated with the laser micro-machining technology that have to be addressed. In particular, some intrinsic characteristics of laser micro-machined structures, i.e. the side-wall tapering, that impact both achievable geometrical and dimensional accuracy and the filters’ performance. This research proposes a novel fabrication process, called laser precession machining, that addresses some of the key laser micro-machining limitations in producing THz mesh filters. It employs an ultrafast laser and a “precess” module to vary the beam incident angle and thus to minimise the taper angle on the sidewalls of the filters’ cross-shaped through holes. A significant reduction of this negative tapering effect was achieved on micro-structures produced with this new method that led to a significant improvement of filters’ performance. The filters’ performance was compared with the simulation results and they were in good agreement. X-ray photoelectron spectroscopy (XPS) analysis was carried out to analyze the effects of laser precession machining on the composition of copper substrates as a potential factor affecting the filters’ performance.