Femtosecond Laser Ablation and Delamination of Functional Magnetic Multilayers at the Nanoscale

Abstract

Laser nanostructuring of thin films with ultrashort laser pulses is widely used for nanofabrication across various fields. A crucial parameter for optimizing and understanding the processes underlying laser processing is the absorbed laser fluence, which is essential for all damage phenomena such as melting, ablation, spallation, and delamination. While threshold fluences have been extensively studied for single compound thin films, advancements in ultrafast acoustics, magneto-acoustics, and acousto-magneto-plasmonics necessitate understanding the laser nanofabrication processes for functional multilayer films. In this work, we investigated the thickness dependence of ablation and delamination thresholds in Ni/Au bilayers by varying the thickness of the Ni layer. The results were compared with experimental data on Ni thin films. Additionally, we performed femtosecond time-resolved pump-probe measurements of transient reflectivity in Ni to determine the heat penetration depth and evaluate the melting threshold. Delamination thresholds for Ni films were found to exceed the surface melting threshold suggesting the thermal mechanism in a liquid phase. Damage thresholds for Ni/Au bilayers were found to be significantly lower than those for Ni and fingerprint the non-thermal mechanism without Ni melting, which we attribute to the much weaker mechanical adhesion at the Au/glass interface. This finding suggests the potential of femtosecond laser delamination for nondestructive, energy-efficient nanostructuring, enabling the creation of high-quality acoustic resonators and other functional nanostructures for applications in nanosciences.

Keywords: Ni thin films; Ni/Au bilayers; fs-laser pulses; heat penetration depth; laser ablation; laser delamination; thresholds.

Figure 1

Optical microscopy of the structures obtained by a single fs-laser pulse on the Ni films in reflection. The upper set of photos demonstrates the structures for the different thicknesses of Ni film produced by the laser pulse coming from air at the same pulse energy (E_p = 16 μJ). The lower set shows the same for the case of irradiation from the substrate side. The red quarter circles indicate the diameters of ablation craters, where the film was completely removed. The yellow quarter circle for air side ablation of 100 nm Ni indicates the ablation crater due to surface ablation. The scale, indicated for the 20 nm Ni ablation in air at 125 μm, applies to all other micrographs.

Figure 2

Optical microscopy of the structures obtained by a single fs-laser pulse on the Ni/Au films in reflection. The upper set of photos demonstrates the structures for the different thicknesses of Ni/Au film produced by the laser pulse coming from air at a specified energy pulse (E_p = 16 μJ). The lower panel shows the same for the case of irradiation from the substrate side. The red quarter circles indicate the diameters of ablation holes, where the film was completely removed (ablated). The thickness mentioned for NiAu refers to the thickness of the Ni layer deposited on a 5 nm Au layer, which is situated on the glass substrate. The scale, indicated for the 10 nm NiAu ablation in air at 125 μm, applies to all other micrographs.

Figure 3

Optical microscopy of the structures obtained by a single fs-laser pulse on Ni and Ni/Au films on glass in reflection. The set of photos demonstrates the structures for the different thicknesses of Ni and Ni/Au film produced by the laser pulse coming from the glass side at specified energy pulses. The red quarter circles indicate the diameters of structures, obtained with delamination. The thickness mentioned for NiAu refers to the thickness of the Ni layer deposited on a 5 nm Au layer, which was situated on the glass substrate. The scale, indicated for the 80 nm Ni delamination at 50 μm, applies to all other micrographs.

Figure 4

Surface displacement of Ni (a) and Ni/Au (b) delaminated films on both air and glass sides, as measured by interferometry.

Figure 5

(a) Time-resolved reflectivity measurements of acoustic pulses in a Ni (240 nm)/sapphire sample, obtained by pumping at the nickel–air interface and probing at the nickel–sapphire interface, alongside the modeled reflectivity. (b) Acoustic spectrum derived from the experimental reflectivity data and the sensitivity function used for modeling the acoustic spectrum. (c) Simulated acoustic pulse and the sensitivity function employed in the modeling process. (d) Reconstructed strain obtained from experimental data and the corresponding modeled strain. The reflectivity data were well-approximated using a model strain with an exponential decay characterized by a heating depth h = 24 nm. Both the acoustic spectra and pulses were well-reproduced using the two-temperature (TTM) model by Saito et al. [44]. The Fourier reconstructed strain was derived using the algorithm developed by Manke et al. [30]. Time-resolved reflectivity measurements enable precise evaluation of the heating depth h up to the damage threshold.

Figure 6

Ablation and delamination thresholds as a function of thickness for Ni (a) and Ni/Au (b) thin films, subjected to irradiation from both the air and substrate sides. The individual dots denote the thresholds evaluated using Liu’s method (for ease of visual interpretation, these dots are interconnected by lines). The black crossed point in (a) is a threshold value for incomplete Ni ablation. The green lines and the shaded area represent the calculated nickel/substrate interface melting threshold given by Equation (5) for h in a range from 20 nm to 30 nm. The purple lines/shaded area represent the calculated fluence required to heat the nickel at the Ni/Au interface to the melting temperature of gold (1377 K). Yellow squares represent the fs-laser-induced melting thresholds at the nickel–air surface, experimentally obtained by Wellershoff et al. [26]. The thickness mentioned for NiAu refers to the thickness of the Ni layer deposited on a 5 nm Au layer, which is situated on the glass substrate.