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Nondestructive femtosecond laser lithography of Ni nanocavities by controlled thermo-mechanical spallation at the nanoscale

We present a new approach to femtosecond direct laser writing lithography to pattern nanocavities in ferromagnetic thin films. To demonstrate the concept we irradiated 300~nm thin nickel films by single intense femtosecond laser pulses through the glass substrate and created complex surface landscapes at the nickel-air interface. Using a fluence above the ablation threshold the process is destructive and irradiation leads to the formation of 200~nm thin flakes of nickel around the ablation crater as seen by electron microscopy. By progressively lowering the peak laser fluence, slightly below the ablation threshold the formation of closed spallation cavities is demonstrated by interferometric microscopy. Systematic studies by electron and optical interferometric microscopies enabled us to gain an understanding of the thermo-mechanical mechanism leading to spallation at the solid-molten interface, a conclusion supported by molecular dynamics simulations. We achieved a control of the spallation process that enabled the fabrication of closed spallation nanocavities and their periodic arrangements. Due to their topology closed magnetic nanocavities can support unique couplings of multiple excitations (magnetic, optical, acoustic, spintronic). Thereby, they offer a unique physics playground, before unavailable, for magnetism, magneto-photonic and magneto-acoustic applications.


Introduction
Destructive laser-matter interactions play an important role in the processing and structuring of materials. The direct optical excitation of solid surfaces and interfaces by single or multiple laser pulses can be utilized to fabricate a variety of nanostructures. [1][2][3][4][5][6] Nanostructuring occurs as a consequence of laser-induced phase transitions through a sequence of highly non-equilibrium states of matter 7-9 difficult to control. Depending on laser and material parameters, the laser-induced material destruction/modification mechanisms in absorbing solids can be either of thermal nature (melting, ablation, resoldification, photo-chemistry at surfaces and interfaces) or mechanical disruption (spallation) of the material, assisted by large amplitude acoustic/shock pulses.
In most cases the prerequisite of laser nanostructuring is laser-induced melting. Being an intricate phenomenon by itself, laser-induced melting triggers the complex spatio-temporal hydrodynamic and mechanical motion of melted material. Inevitable modification of resolidified surface landscapes may alter the material properties in an unpredictable manner preventing the use of fs-laser nanostructuring for applications in nanophotonics, where both nanometer precision and well-defined material properties are usually required at once.
Our present investigation aims at circumventing the aforementioned complications and 2 has been inspired by older experiments on macroscopically thick aluminum foils. Salzmann et al. 10 demonstrated that nanosecond optical excitation of the front side of 100 µm thick aluminum foil results in the formation of a closed cavity in the vicinity of its back side. The formation of the cavity has been explained by the local mechanical disruption (spallation) in solid aluminum induced by transient spatially inhomogeneous tensile stresses generated upon reflection of large amplitude compressive acoustic/shock pulses from the metal-air interface. Similar observations have been reported by Tamura et at. 11 using picosecond and femtosecond laser pulses on free-standing aluminum foils. However, owing to the macroscopic thickness of the investigated aluminum layers, these studies were not topical from the perspective of modern nanotechnologies.
Technically similar experimental investigations on metallic thin films with the thicknesses in the range between 10 and 400 nm suggested that fs-laser pulses result either in the removal of the entire film from the substrate and the formation of an ablation crater (punching 12 ) or to a local thermoelastic separation of the entire film from the dielectric substrate, 13 sometimes denoted as "blistering" 14 or "bulging". 12 In this paper we introduce a new way to achieve a deterministic femtosecond laser nanostructuring inside a 300 nm nickel thin film which is partially melted by a single femtosecond laser pulse focused through a glass substrate. The concurrent fast resolidification of the melted material and the spallation dynamics, lead to the formation of a dome-like cavity entirely made by nickel and probably enclosing vacuum. Being neither bulging nor punching, this new mechanism of thermo-mechanical spallation manifests itself in the deterministic, i.e. controllable, separation of a 200 nm layer of solid nickel (ceiling of the dome) from a 100 nm layer of laser-melted nickel remaining on the glass substrate (floor of the dome). The deterministic formation of closed spallation cavities with dimensions that can reach the submicron range as well as their periodic arrangements are quantified by optical interferometric microscopy. 15 Such periodic landscapes open the door for numerous technological applications of this novel and potentially disruptive fs-laser-based nanostructuring technology. From the one side, it allows to approach fundamentals questions of modern science related to the physical properties of novel states of matter, such as amorphous spin glasses 16  contrast displayed by this region suggests that the film hasn't been destroyed but rather structurally modified, i.e. lifted from the substrate. This conclusion is confirmed by the results of optical interferometric microscopy of these structures (to be discussed below), which allow us to state that in this area we are above the threshold for the thermo-mechanical spallation process inside the film. We determined both ablation and spallation thresholds using the Liu method 36 assuming the Gaussian distribution of laser fluence. For a 300 nm thick nickel films excited through the substrate the ablation and spallation thresholds were found to be F abl = (950 ± 100) mJ/cm 2 and F spall = (480 ± 50) mJ/cm 2 , respectively.
Scanning electron microscopy (SEM) of a typical damage area induced by at 70 µJ laser pulse in Fig. 2 provides a detailed microscopic view of uncontrolled spallation, which is nevertheless useful to understand key aspects of the process. A complementary optical transmission microscopy on the same structure helps to visualize multiple areas where the film has been removed from the surface: an elliptical ablation crater and micrometer-small round holes outside it.
Outside the elliptical ablation crater the laser fluence decreases towards the periphery of the crater; at the certain distance from the center the absorbed energy is not sufficient to  Fig. 1).

Formation of closed spallation cavities below the ablation threshold
In the following we demonstrate that controlled spallation can be indeed achieved tuning the pulse energy in the range between spallation and abalation onsets. Figure 3 shows the interferograms of nickel sample excited below the ablation threshold and slightly above the spallation onset, namely playing with the laser pulse energy in the neighbourhood of 35 µJ.
Interferograms from the front side display pronounced shift of optical interference fringes.
Comparison with reference interferogram of unexcited surface can be used to reconstruct the phase landscape ∆Ψ(x, y) by applying, for example, the 2D-Fourier transform algorithm. 15 For fluences below the ablation threshold the optical properties of the nickel-air interface are not affected by laser-induced phase transitions. Therefore, the phase shift can used to unambiguously determine the surface displacement d(x, y) = (λ/4π)∆Ψ(x, y), where λ=650 nm is optical wavelength used for interferometric imaging.
Laser pulses with energy 35 µJ (peak fluence F = 1.17F spall ) result in a flat-top surface profile ( Fig. 3(a,c)), where a spalled 200 nm-thin solid layer of nickel lifted up to a height of 270 nm from the surface, without ripping the film. Indeed, the preserved continuity of the film is the mechanism that decelerates and eventually stops the lifting up of the spalled layer thanks to the mechanical forces exerted by the un-irradiated film on the sides,resulting in the formation of a flat-top surface profile. Excitation with a slightly higher energy of 36 µJ (peak fluence F = 1.20F spall ) generated a characteristic M-shaped surface profile ( Fig. 3(b,d)). In this case the mechanical forces were sufficiently strong to induce the plastic

Molecular dynamics simulations
In order to deeply understand the mechanism of the spallation process observed in the experiments, we performed the atomistic-continuum modeling of fs-laser induced dynamics for Ni thin films irradiated through glass substrate. The model is based on the Molecular Dynamics (MD) approach, but, due to integrated Two Temperature Model (TTM), 32

Conclusions and outlook
We demonstrated that the controlled thermo-mechanical spallation of nickel films can be induced and controlled by irradiation with a single femtosecond laser pulses. Molecular dynamics simulations shade light at the complex physics of the spallation process occuring in inside the nickel film at an interface between solid and (laser-molten) nickel. By tuning the irradiation parameters, closed spallation cavities with two distinct flat and "M" shaped top have been produced and characterized by high-resolution interferometric microscopy. Using slit projection technique periodic arrays of elongated bell-shaped spallation cavities with periodicities in the range 3 -30 µm and heights up to 200 nm have been produced demonstrating that the process can be finely controlled to induce desired shapes and parallelized for the fast fabrication of metasurfaces and gratings for visible and IR, potentially also for THz, radiation. The flexible nature of the quasi-free-standing outer nickel membrane forming the spallation cavities will allow for substantial and fast modulation of light diffraction by exciting the acoustic eigenmodes of the cavities. The curved nature of the nickel membrane is expected to result in unique topology-induced magnetic states, which are the focus of intense studies. 17 More in general, such magnetic nanocavities can support a variety of excitations: from magnetic to optical and acoustic ones, thereby providing a unique multifunctional metasurface for studying their properties and the physics arising from their coupling. Our results are encouraging and indicate that such back-side femtosecond laser lithography can be further advanced through the use of (i) more complex spatial patterns of pump radiation to create sophisticated surface landscapes and (ii) magnetic materials with tailored physical properties. We envision that future investigations of acoustic, magnetic, magnetophotonic, magneto-acoustic and 'vacuum' properties of spallation cavities could open a new avenue in modern nanoscience. 15