Techniques based on laser scanning microscopes for nanoprocessing of periodic structures on silicon with ultra-short laser pulses have been developed. Ripples of 800–900 nm spacing were obtained after laser irradiation at a wavelength of 1040 nm, a repetition rate of 10 kHz and a fluence of 2 J/cm2 in air. Smaller features of 70–100nm spacing were achieved in oil at a wavelength of 800 nm, a repetition rate of 90 MHz and a fluence of 200–300 mJ/cm2 by using a high numerical focusing objective.
©2005 Optical Society of America
Nanostructuring of a variety of materials is gaining widespread importance owing to ever-increasing applications of nanostructures in numerous fields. Laser-induced surface structuring in silicon has been extensively studied [1–7]. Ultra-short pulsed laser radiation has been shown to be highly effective for precision material processing and surface micro-modification because of minimal thermal and mechanical damage in various materials. Silicon, as the most important material for the semi-conductor industry, presents some specific characteristics when irradiating with ultra-short laser pulses at low fluence. Spontaneous periodic surface structures or ripples are observed. We present in this paper results of such periodic nanostructures obtained in air at a wavelength of 1040 nm, a repetition rate of 10 kHz and a fluence of 2 J/cm2 (periodicity of about 800–900 nm) compared with structures obtained in oil at a wavelength of 800 nm, a repetition rate of 90 MHz and a fluence of 200–300 mJ/cm2 (periodicity of 70–100 nm). Sub-100 nm features are observed after infra-red laser irradiation. 2D regular self-assembly arrays nanostructuring with line spacing of 0.1 μm are also shown. All experiments were performed using laser scanning microscopy.
2. Experimental setup
Two experimental setups have been used. A global scheme is depicted in fig. 1. The first one is based on a regenerative amplifier laser chain based on Ytterbium technology, femtoREGEN IC-1040–400 fs (HighQLaser), which provide 350 fs pulses at 10 kHz, at 1040 nm and a maximum mean power of 117 mW. The laser beam was introduced into the optical system FemtOcut (JenLab GmbH). The system is based on a fluorescence microscope with a compact scanning module attached to one microscope port. The beam scanning module consists of beam deflection mirrors, scan optics including an 1:6 beam expander, a motorized beam attenuator, a beam shutter and a fast x,y galvo scanner (GSI Lumonics, USA). The samples were mounted on a two-motorized-axes system controlled with 0.3 μm accuracy. 20x focusing objective with a numerical aperture of 0.5 has been employed.
The second setup use a tuneable mode-locked Titan:sapphire oscillator laser with 170 fs pulse duration, a repetition rate of 90 MHz and a maximum mean power of 1750 mW at 800 nm (Coherent Chameleon). The laser was coupled into an inverted laser scanning microscope for experiments (Zeiss LSM 510 NLO META). A fully adjustable attenuator was also used to control the mean laser power experiments. In order to reach the ablation threshold of silicon, a focusing objective with high numerical aperture was used under oil environment (Zeiss Plan Neofluar 40x / 1.3 Oil) .The laser beam was expanded by a 1:4 telescope. All experiments were carried out with an excitation wavelength of 800nm and a laser power of 270 mW at the focal plane after the objective.
Silicon wafers (Poliertech) with respectively 100 and 600 nm thick SI3N4 and SIO2 layer have been used.
SEM images of the structures obtained in air with the regenerative amplifier and with a 20x focusing objective with a numerical aperture of 0.5 are shown in Fig. 2. One line scan was performed at a scan speed of 2 mm/s and an energy/pulse of 1 μJ. A fluence of approximately 2 J/cm2 was achieved. Images have been taken directly after processing without further cleaning. Some recast or melted matter is observed on the sides of the channel. Periodic structures known as ripples were formed after laser irradiation. Details are magnified in Fig. 2 (b). The periodicity of these structures are found to be of around 800–900 nm which is a little bit less than the laser wavelength of 1040 nm. Sub-μm structures are allowed in air with amplified laser pulses at wavelength even higher than 1 μm.
Results obtained with the tuneable mode-locked oscillator and with a 40x oil microscope objective with a numerical aperture of 1.3 are depicted in Fig. 3. One line scan was also performed as above at a scan speed of approximately 1 mm/s but at very low energy/pulse of 3.5 nJ which represent a fluence close to the ablation threshold (100–300 mJ/cm2 depending on the number of pulses, [8,9]). SEM images have been taken on silicon samples after etching with ammonium fluoride to remove recast, melted matter and nanoparticles at the border and in the structures. Most interestingly, the period of the surface ripples are found to be in the order of 70 to 100 nm. That’s about 10 times less than the wavelength of the oscillator.
Further experiments have been made using this interesting property of spontaneous formation of periodic ripples of sub-100 nm on the surface to create regular self-assembly nanostructured 2D arrays (250 μm × 250 μm). The results of these preliminary studies are depicted in Fig. 4 (in insert the whole array). The array was structured by a succession of several scans accurately shifted for an optimal overlapping. By a mean periodicity of 0.1 μm, more than 2500 lines can be written in this array.
The spontaneous formation of periodic ripples on the surface of a material illuminated with polarized pulsed laser light has been reported in many different materials [2,10–15]. The period and orientation of these surface ripples, known in the literature as laser–induced periodic surface structures (LIPSS), mainly depend on the following aspects of the laser beam: angle of incidence, polarization, frequency, and energy. Previous reports refer to LIPSS spacings on the order of micron close to the laser wavelength. Pedraza et al , have obtained nanochannels spacing in a range of 200–300nm with UV femtosecond laser pulses at a wavelength of 248 nm. The rippling has been attributed to scattering from surface roughness and to reradiation from surface defect sizes. The ripples appear most strongly at intensities close to the melting threshold. Interference between all incident, radiated, or scattered electromagnetic field causes periodic variation in substrate heating and ultimately periodic surface structure. LIPSS could be very simply interpreted as being produced by the interference between the refracted radiation and two scattered waves parallel to the surface propagating in opposite direction, giving the following dependence of fringe spacings, Λ = λ/(1±sin θ) or Λ = λ/ cos θ depending on the direction of the polarisation where λ is the wavelength of the laser and θ the angle of incidence.
If this theory can be applied in our case for results obtained with the regenerative amplifier at a wavelength of 1040 nm, a repetition rate of 10 kHz and a fluence of 2 J/cm2 which is low enough to produce sub- μm ripples (periodicity of about 800–900 nm), it is not still available with results of sub-100 nm nanochannels spacing we obtained with the oscillator at a wavelength of 800 nm, a repetition rate of 90 MHz and a fluence close to the ablation threshold of silicon in oil environment.
The use of high numerical aperture objectives alone can not explain the sub-100 nm features even if it is feasible to break-through the limitation by controlling irradiation profile so that only the central part of the focusing spot is above the breakdown threshold. Other physical phenomena are to be taken into account. Several works on the generation of periodic nanostructures under water confinement have been reported [9, 16–17]. Submicrometer-sized spikes of about 200 nm were obtained in water confinement with femtosecond laser pulses but at a wavelength of 400 nm and relative high energy per pulse . Kautek et al [9,17], have also shown ripples formed under water confinement with typical spacing of also about 100 nm with 800 nm amplified femtosecond laser pulses. The 100 nm periodicity structures were observed at low fluence and with a relative significant number of pulses. It may be plausible that the presence of water influence the interface and the stabilization of the ripples formation and as said it is expected that fluid contact decreases the surface energy and surface tension so that the driving force of the surface near regions to relax and self-organise is drastically reduced. It has to be noted that they have not obtained regular reproducible structures. Under oil confinement, with a higher refractive index as water, it was possible as shown in this paper to perform sub-100 nm (down to 70 nm) regular self-assembly and reproducible structures, allowing to process large areas arrays. If liquid confinement seems to play a major role in femtosecond laser interaction with silicon, we believe also that effects of the accumulation of multiple laser pulses in the MHz range have to be taken into account First the accumulation allow to reach ablation of silicon with very few energy per pulse in the nJ regime as mentioned for other materials in the literature , and as said above, the use of low intensities drastically change the scale of the surface relief, promoting the formation of nanostructures. Second, interaction between roughness and light evolves with an increasing number of pulses contributing certainly to rapid nanochannels formation with very small spacing.
In summary we have performed sub-100 nm nanostructuring of silicon by the use of nJ pulses at a repetition rate of 90 MHz in oil environment. 2D arrays were also written by accurate processing control. These results are of prime interest for potential applications as nanolithography, controlled nanodeposition, high-density data storage, nanooptoelectronics, as well as various biotechnology related applications. These phenomena are not well understood yet and require further investigations.
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