Femtosecond vs Nanosecond: An Analysis on the Laser Ablation Properties of Dielectric Layers for Solar Cells

This paper reports on the ablation and microstructuring properties of dielectric layers using a high average power femtosecond and nanosecond laser sources. The dielectrics investigated include SiN_X, AlO_X/SiN_X stack and thermal SiO₂/SiN_X stack deposited on planar n-type silicon wafer. Initially, single pulse ablation properties such as threshold fluence and energy penetration depth were determined for both laser sources. In femtosecond ablation, the presence of two different ablation regimes: gentle and strong ablation was identified. An analytical model has been developed to estimate the line width micro-machined at different pulse spacing. The modelled line width is in good agreement with the experimentally measured values for femtosecond ablation due to negligible debris deposition. As such, for line ablation using femtosecond laser, the reduction in the threshold fluence with respect to pulse overlap ratio has been estimated. These ablation properties are very useful in carrying out precise and low damage structuring of dielectrics for ablation intensive architectures such as interdigitated back contact (IBC) solar cells.

I. Introduction

Laser, as a processing tool, has gained significant attention in the PV industry due to its inherent advantages such as high speed processing, versatility and precision that are vital in the fabrication of cost effective and highly efficient solar cells [1]. Being a non-contact process, laser material processing induce far less damage to the substrate than mechanical scribing. Recently, ultrashort pulse laser technology using femtosecond laser source is being adapted into solar cell processing [2]. A femtosecond (fs) laser pulse is shorter than the timescale of photon-electron-lattice interactions and thus the laser pulse ends before the excited electrons could transfer energy to the ions [3]. Consequently, it is possible to obtain highly localized energy deposition with negligible thermal diffusion to the surrounding area. The use of lasers in solar cell processing can only be successful if it does not have detrimental effects on the solar cell performance. As such, it is vital to understand the fundamentals of laser interaction with dielectric layers and silicon. This work focuses mainly on the ablation and micro-structuring properties of three widely used dielectric layers using an industrial laser tool that consists of high average power femtosecond (fs) and nanosecond (ns) laser sources having a Gaussian pulse profile. The nanosecond laser source produces pulses at 38 ns duration and 532 nm central wavelength. The femtosecond laser source produces pulses at 480 fs duration and 1030 nm central wavelength. However, in this study, the fs laser source is used in second harmonic generation mode (SHG) at 515 nm for comparison with ns laser source. The laser beam scanning system has a maximum marking speed of 20 m/s. Furthermore, the parameters such as average output power, pulse repetition rate, marking speed and focus position are controlled via software interface. Fig. 1.

Schematic of the test sample illustrating the process of indirect ablation.

Table I Dielectric layers and their thickness

Dielectric Layer	Thickness (nm)
SiNx	70
AlOx / SiNx stack	10 / 70
SiO2 / SiNx stack	30 / 160