Physical mechanisms of SiNx layer structuring with ultrafast lasers by direct and confined laser ablation

Abstract

In the production process of silicon microelectronic devices and high efficiency silicon solar cells, local contact openings in thin dielectric layers are required. Instead of photolithography, these openings can be selectively structured with ultra-short laser pulses by confined laser ablation in a fast and efficient lift off production step. Thereby, the ultrafast laser pulse is transmitted by the dielectric layer and absorbed at the substrate surface leading to a selective layer removal in the nanosecond time domain. Thermal damage in the substrate due to absorption is an unwanted side effect. The aim of this work is to obtain a deeper understanding of the physical laser-material interaction with the goal of finding a damage-free ablation mechanism. For this, thin silicon nitride (SiN x ) layers on planar silicon (Si) wafers are processed with infrared fs-laser pulses. Two ablation types can be distinguished: The known confined ablation at fluences below 300 mJ/cm 2 and a combined partial confined and partial direct ablation at higher fluences. The partial direct ablation process is caused by nonlinear absorption in the SiN x layer in the center of the applied Gaussian shaped laser pulses. Pump-probe investigations of the central area show ultra-fast reflectivity changes typical for direct laser ablation. Transmission electron microscopy results demonstrate that the Si surface under the remaining SiN x island is not damaged by the laser ablation process. At optimized process parameters, the method of direct laser ablation could be a good candidate for damage-free selective structuring of dielectric layers on absorbing substrates. V C 2015 AIP Publishing LLC.

Laser amplification in excited dielectrics

“…As this energy stays spatially confined on ultrashort time scales, these laser pulses have become the instrument of choice for dielectric structuring on a micrometre or even nanometre scale [1] as well as for applications in nanosurgery [2]. Depending on the incident intensity, a large range of material modifications can be realized: While optical Kerr effect, self [3]-and cross-phase [4] modulation or field-induced currents [5] are examples of reversible processes, higher intensities can cause permanent refractiveindex changes [6,7] ablation of the material or even the creation of new material phases [8]. It is the transient conduction-band-electron density that plays the fundamental role in determining the optical properties and time-resolved pump-probe experiments are an excellent tool for studying its dynamics, demonstrated in a manifold of experiments.…”

Challenges facing copper‐plated metallisation for silicon photovoltaics: Insights from integrated circuit technology development

“…Another approach which has been successfully used to increase interfacial adhesion is the use of a ps UV laser to ablate the ARC from the Si surface . The short pulse laser results in increased direct ablation of the SiN x with only a small surface region of the Si being thermally‐melted by the laser process . Importantly from the perspective of increased adhesion, the short pulses cause the formation of laser‐induced periodic surface structures (LIPSS) on the exposed Si surface after SiN x ablation (see Figure ) which can act to increase the adhesion of plated metal.…”