A Facile Approach for Surface Quality Improvement of Mm-Band Planar Electromagnetic Structures Fabricated by Laser Ablation

Travelling-wave-tube (TWT) power amplifiers with planar microstrip slow wave structures (SWS) on dielectric substrates are very promising for operation at high frequencies due to their low-voltage operation, small size and relatively high output power. Recently, the interest to the planar SWSs has increased since their using can significantly decrease the complexity of microfabrication process. In our previous work, we reported successful fabrication of millimeter-band microstrip planar SWSs using the scalable and cost-effective laser ablation process with nanosecond pulse duration. However, the SWS samples fabricated by nanosecond laser ablation suffer from surface distortions and debris on the borders of the ablated zone. In order to improve the quality of resulting structures, here we propose deposition of a supplementary metallic (Al or Ti) layer on top of copper film prior to laser treatment. Such coating is aimed to prevent defects formation in the course of laser irradiation. In the proposed system, debris and distortions from Cu layer irradiation, naturally sediment on the sacrificial layer instead of sticking to the copper layer itself. Sacrificial aluminium coating can later be easily chemically removed together with sedimented debris leaving a clean underlying patterned Cu surface. In contrast, titanium layer prevents sticking of copper debris due to the higher melting temperature of Ti, thus preventing attachment of those to the surface in the first place.