Selective ablation of thin films using ultra-short laser pulses

Abstract

The micro processing of materials using ultra short pulse (USP) lasers with durations in the low picosecond (ps) and femtosecond (fs) range allows for the possibility of precision material removal on both nanometre and micron scales. Precision material removal can be achieved due to the near diffraction limited focus spot size and ultra-short pulse durations, which provide extremely high peak intensities with minimal thermal impact on the surrounding area. The work presented in this thesis is primarily concerned with the selective ablation of thin films deposited on various surfaces, using lasers with picosecond temporal pulse lengths at 1064 nm. As a result, damage to the substrate is negated through exploitation of the difference in linear absorption coefficients between the thin film and substrate. To elucidate the mechanism of selective processing with USP lasers; absorption, single and multi-pulse ablation effects were investigated in both fixed and variable beam positions. A sample of white float glass vacuum coated with indium tin oxide (ITO) was chosen as the material for this study. Experimental results demonstrate that linear absorption (α (λ)) of the ITO and substrate plays a key role in achieving selective thin film ablation. As a direct consequence of the difference in absorption coefficients at 1064 nm, the single (ϕ_th^1 ) and multi (ϕ_th^n ) pulse ablation thresholds of both materials are altered during the high peak intensity exposure. Selective processing was achieved by exploiting the difference between the ablation thresholds of ITO and glass. When irradiated with multiple pulses the ablation threshold of the substrate was observed to decrease with increasing pulse number. This change in threshold fundamentally limits the selective processing window; therefore incubation (S) effects must be considered when determining the viability of selective processing. For the purpose of practical applications, a series of case studies are also presented which attempt to utilise selective materials processing. These investigations were split into industrial and conservation. Industrial case studies focused on successfully micro processing a small thin film ITO circuit using a Spatial Light Modulator and a new low cost solar cell (F doped SnO2); whilst in conservation, the restoration of a pair of Royal gloves and the removal of unwanted bronze gilding is presented. The application of USP lasers in conservation represents a relatively new field of study where little previous research has been carried out. These case studies not only showcase the wide range of USP applications in which selective processing can be applied but also highlight the limitations of this method.