Non-linear absorption of femtosecond laser pulses in a SiNx layer-influence of silicon doping type

In this article, silicon nitride (SiNx) layers deposited on planar silicon wafers with two different doped areas (emitter: highly phosphorous doped and bulk: lightly boron doped) were locally irradiated by laser pulses. Our investigation is focused on the ablation mechanisms. For that purpose, an ultra-short laser source (pulse duration 0.3-12 ps, wavelength 1025 nm, Gaussian profile) was used. For high pulse durations and low fluences the SiNx layer is removed completely by lift-off. However, for lower pulse durations and higher fluences, the SiNx layer is not completely removed, due to direct ablation. By using an emitter, direct ablation of SiNx layers were observed also for higher pulse durations. Thus, the absorption process in the SiNx layer can be described as avalanche ionization with seed electrons from silicon.