Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon

TL;DR

This study investigates how dopant type and concentration influence the femtosecond laser ablation threshold and incubation behavior of silicon, revealing that dopant levels affect single-pulse thresholds but not the long-term ablation limit.

Contribution

It provides new insights into the effects of dopant type and concentration on femtosecond laser ablation thresholds and incubation behavior in silicon.

Findings

Single-pulse ablation threshold decreases with higher dopant concentration.

N-type doping results in a greater threshold reduction than P-type.

Infinite-pulse ablation threshold remains constant across doping levels.

Abstract

In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser micromachining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study femtosecond (800 nm, 110 fs, 0.1-1 mJ/pulse) micromachining of a material with well-defined initial defect concentrations: doped Si across a range of dopant types and concentrations. The single-pulse ablation threshold (Fth,1) was observed to decrease with increasing dopant concentration, from a maximum of 0.70 J/cm2 (+/-0.02) for undoped Si to 0.51 J/cm2 (+/-0.01) for highly N-type doped Si. The effect was greater for N-type doped Si than for P-type, consistent with the higher carrier mobility of electrons compared to holes. In contrast, the infinite-pulse ablation threshold (Fth,inf) was the same for all doping levels and types. We attribute this asymptotic behaviour to a maximum defect concentration that is independent of the initial defect concentration and type. These results lend insight into the mechanism of multipulse, femtosecond laser ablation.