Femtosecond laser structuring and plasma polishing of AlN ceramics

Femtosecond laser structuring is applied to produce linear ion traps of AlN ceramics. Such ion trap can serve as an optical clock with exceptionally high long term stability. The selection of techniques, applicable to the high precision microstructuring of AlN ceramics, is limited due to high hardness and fragility of the material. Laser structuring is very attractive for this application. However, long laser pulses cause thermal decomposition and surface metallization of AlN ceramics. A femtosecond laser allows material removal without adverse heat effects. At the same time high quality surface finish together with aspect ratios and precision as required for optical ion trap clocks are ensured. Here, a high process throughput of up to 400 μm³/pulse and a sub-pristine surface roughness down to 350 nm Ra were achieved. The novelty of this structuring technique lies in the separation of processing stages and the application of two distinctive femtosecond laser induced phenomena: ablation and plasma etching. Rapid material removal for the rough contouring is achieved by material ablation with a focused laser beam. The remaining rough surface is further refined to the required specifications by plasma polishing. The laser beam then acts indirectly and serves as a source of energy to sustain a plasma plume in the near vicinity of the substrate.