Proc. SPIE, Vol. 7131, p. 71311W
Laser-induced Nanopatterning, Ablation, and Plasma Spectroscopy in the Near-Field of an Optical Fiber Tip Johannes Heitza,b1, Sergey Yakunina, Thomas Stehrera,b, Gerard Wysockic, and Dieter Bäuerlea a Institute of Applied Physics, b Christian Doppler Laboratory for Laser-Assisted Diagnostics Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria c Dept. of Electrical Engineering, Princeton University, Engineering Quadrangle, Olden Street, Princeton, NJ 08544, USA ABSTRACT We combine laser processing and the technique of a scanning near-field optical microscope (SNOM) for realization of laser-patterning on a nanoscale, laser ablation near the fiber tip, and micro-analysis of solid surface samples by laserinduced breakdown spectroscopy (LIBS). We describe an universal SNOM-like setup allowing to produce near-field laser patterns by laser heating and laser ablation, laser-induced breakdown spectroscopy, and atomic force microscope (AFM) topography investigation with the same optical fiber tip, which is used as near-field emitter or as probe. With solid Si and Al samples, three laser processing regimes were observed with increasing laser pulse energy: (1) cone formation (only for Si, smallest features with 500 nm width and 100 nm height), (2) formation of small craters (smallest features with 450 nm width and 250 nm depth), and (3) crater formation with a width of more than 2 μm with emission of evaluable plasma emission line spectra. Keywords: SNOM, LIBS, laser ablation, patterning
1. INTRODUCTION Laser micro-and nano-processing is a field of increasing practical relevance [1]. In our previous papers [2-4], we demonstrated that the combination of laser processing and SNOM technique can be used for the realization of laserinduced chemical reactions on a nanoscale, for laser ablation near the fiber tip, and for micro-analysis of solid surface samples. As in a SNOM, we controlled the distance between a fiber tip and the sample surface by means of a shear-force sensor. The laser radiation was coupled into an uncoated optical glass fiber with a tip at one end, which had been produced by wet chemical etching. The tapered fiber was glued onto a quartz tuning fork acting as the shear-force sensor [2]. Nanoscale photochemical and photophysical etching of Si in Cl2 atmosphere was demonstrated with this set-up [3]. With continuous wave (cw) 351 nm Ar+-laser radiation and low intensities, the etching mechanism was purely photochemical. In this regime, the width of patterns, which was 115 nm at full width at half maximum (FWHM), corresponded, approximately, to the diameter of the fiber tip. With 514.5 nm Ar+-laser light etching was observed only at significantly higher laser-light intensities, but patterns with width down to about 30 nm had been achieved. Here, the lateral resolution corresponded to about 1/18 of the laser wavelength employed. Laser ablation could be observed in front of the approached fiber by employing pulsed laser sources, i.e., 532 nm frequency-doubled Nd:YAG laser pulses with a pulse length of 6 ns and an energy of several 10 μJ. This enabled microanalysis of the chemical composition of the irradiated surface by analyzing the emitted light of the ablation plasma plume. The technique is known as laser-induced breakdown spectroscopy (LIBS). We demonstrated micro-LIBS of solid Al alloy samples with a lateral resolution below 10 μm [4]. In the current article, we show our recent results with Si and Al samples using an improved setup. The special focus of this work is on nano- and micro-pattern formation and on the resolution limit for LIBS analysis with approached and retracted fiber tips.
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E-mail:
[email protected]
2. EXPERIMENTAL Figure 1 shows the improved SNOM-like universal setup that offers the possibility of laser micro-/nano- processing, of recording plasma emission spectra as well as of AFM surface characterization. Additionally, it provides the possibility of large area scanning up to a field of 1.5 x 1.5 mm. The setup consists of a pulsed laser source, a beam coupling and delivery fiber system that is connected to a tapered fiber tip, an AFM closed-loop distance controller, a XYZ positioning system, and a plasma emission collection and spectrum recording system. The setup control and data recording are automatized via a PCI interface card with a personal computer (PC) under LabView software. As excitation source, a Q-switched Nd:YAG laser (Continuum Surelite I-20) is employed with a frequency-doubling unit based on a KDP crystal (wavelengths O = 532 nm, length of laser pulses 6 ns). For light transport in the laboratory, the laser beam is coupled into a several meter long silica multimode fiber (3 in Fig. 1) by a glass lens with a focal length f of 70 mm. The tapered fiber tip with a short piece of fiber (2) is connected to the transport fiber (3) by a commercial SMA fiber connector. The optical fiber with the tip is a fused silica step-index fiber and has a numerical aperture (NA) of 0.22. The energy passing through the fiber tip was about 8 % of the energy at the entrance of the short fiber (2) as measured by a NOVA pyroelectric energy meter. This measurement is done in the far field of the tip apex. Therefore, the measured pulse energies also include leakage upstream of the probe tip. Because these values can be taken only as an estimation for the energies in the near-field of the tip, the energies were in practice often measured behind the SMA connector and then recalculated to the values emitted by the tip. The fiber tips are produced by protection layer etching in 30 % hydrofluoric acid with refined sunflower oil as protection layer, similar as in ref. [5]. The etching procedure is performed under control of a digital microscope for exact determination of process end. Thus, we obtain tips with cone angle up to 400. The fiber tips sharpness is controlled by an optical microscope. The obtained tip radii are considerably below 500 nm.
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Fig. 1 : SNOM-like setup for laser micro-/nano- processing and recording of plasma emission spectra
The fiber end with the tapered tip is in mechanical contact with the quartz tuning fork (1), which has a free oscillation frequency of 32.768 kHz. The fiber and the oscillating tuning fork form together an electro-mechanical resonance sensor. If the tip is approached to a surface in a range of few nm, the oscillation frequency is damped and shifted due to shear and friction forces [6].
In the AFM mode, we use a lock-in amplifier and a homemade proportional-integral controller to produce a negative feedback for the high-voltage piezo controller of the piezo nano-positioning stage (7) in Z-direction (Physik Instrumente, P-622.1CD, scan range 250 μm) to maintain a certain distance of a few nm between the fiber tip and the sample surface (6). The feedback signal is recorded and evaluated for the AFM image. The frequency and amplitude feedback parameters are optimized for a good compromise between stability and speed of response. The XY nano-stages (7) (Physik Instrumente P-629.2CD, scan range 1.5 mm in both directions) are also connected to the PC by high-voltage controllers. Both Z stage and XY stage have capacitor distance sensors that allow to measure the coordinates with an accuracy of about 10 nm. The setup is equipped with two digital microscopes (5) (Celestron 44300) for orthogonal observation of the sample. This allows to determine the mutual sample and tip position with an accuracy of about 3 μm. Figure 2 shows an example of images of the two cameras. For rough positioning, we use additionally a 3D step motor system (8 on Fig. 1).
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b) Fig. 2 : Images of the approaching procedure: a) side and b) top digital microscope camera. The fiber with the tapered tip is the near field emitter and AFM probe; the thicker optical fiber with the plane end is the plasma emission collector with high NA.
The surface patterning and surface analysis experiments are performed with two different tip positions: x
fiber tip approached to the sample surface, where a small distance is kept constant by the closed feedback loop (similar as in the AFM mode), and
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fiber retracted from the sample, where the distance is controlled by the capacitor sensor relative to the last approached Z value.
In approached state, the lateral scanning speed of the sample is limited by the response time of the feedback loop and the sample roughness. The upper limit is typically about 0.5 μm/s. Thus, the scanning of a 5x5 μm area with a 250 nm grid takes from 2 to 5 min. With retracted tip faster scanning is possible. In our experiments, we use the same fiber tip to record the AFM map and to produce laser-induced surface changes. After image recording, we apply 2D Fourier filtering to reduce the noise of the AFM images. For the LIBS experiments, the intensity of the pulsed light in front of the fiber tip has to be high enough to induce ablation and plasma formation. The light emitted from the laser-induced plasma is collected directly by an optical fiber (4 in Fig. 1 and thicker fiber in Fig. 2) with 400 μm core diameter, polished fiber ends and a high NA of 0.37. One end is adjusted with a 3D manual stage to a distance of about 300 μm from the laser-irradiated area at the sample. The second fiber end is connected with a SMA connector to the entrance port of a Czerny Turner spectrometer (SpectraPro 500i, Acton Research Corporation) with a 2400 lines/mm grating blazed at 240 nm as dispersive element. The spectrometer is equipped with a 1024 pixel photo-diode-array (PDA) (RL1024, EG&G RETICON) cooled by a Peltier-element down to -14 0C. The resulting spectral range of the individual spectra is between 16 and 20 nm. The light signal before the PDA is amplified and gated by a double micro channel plate (DMCP). The laser synchronization output is used for time control of the gate of the DMCP. It is even possible to set negative gate delay times ahead of the laser pulse.
3. RESULTS AND DISCUSSION 3.1 Cone formation due to surface melting We detected the formation of elevated cones on silicon (100) wafer samples after single pulse laser irradiation for pulse energies E above about 3 μJ. Figure 3 shows an example of one of the smallest feature obtained so far. The cone has a height of about 100 nm and a width of about 500 nm (FWHM) at the smallest axis. With increasing laser energy, the cones became more extensive and obtuse. With energies higher than about 6 μJ additionally hole formation occurred in the cone center, which at higher energies is the dominating feature.
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b) Fig. 3 Cone formation on Si (100) surface irradiated through an approached fiber tip with a single 532 nm Nd:YAG laser pulse (E ~ 3 μJ): a) AFM image recorded with the same tip, b) height profile.
A similar process of silicon-cone formation was investigated in [7], where an array of features (0.5 μm FWHM) was produced by pulsed UV laser irradiation through a regular lattice of silica microspheres. The cone growth is explained there by Si melting and consequent solidification that leads to an elevation of the central part of the melted zone.
3.2 Small ablated features For aluminum samples (250 μm thick Al foils), we observed hole formation after single pulse laser irradiation for energies E of about 3 μJ without any preliminary cone features at lower energies. Only an elevated rim usually appeared around the hole in the center. Figure 4 shows an example for a hole in Al with a width of about 450 nm (FWHM) and a depth of 250 nm. The diameter of elevated rim around hole is about 1μm with a height about 75 nm above sample surface. An increase of the laser pulse energy resulted in a larger hole depth and hole width and also to a wider elevated area around the hole. Figure 5 shows the dependency of the hole width and depth on E for Al. The solid lines in the figures are guides for the eyes. The hole width is about 2 times lager than the hole depth. The diameter of the elevated rims was typically 1.5 to 3 times wider than the hole width (data not shown).
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b) Fig. 4 Hole formation on Al foil irradiated through an approached fiber tip with a single 532 nm Nd:YAG laser pulse (E ~ 3 μJ): a) AFM image recorded with the same tip, b) height profile.
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Fig. 5 Dependencies of feature size on laser pulse energy for Al foils irradiated through an approached fiber tip with a single 532 nm Nd:YAG laser pulse.
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E >PJ] 3.3 LIBS Spectra with approached tip Figure 6 shows a LIBS spectrum of a Si (100) wafer sample collected from single 532nm Nd:YAG laser pulse irradiation through the approached fiber tip with a laser pulse energy E of 40 μJ. The spectrum consists of several atomic or ionic emission lines, an continuous background and noise. The line at 288.15 nm is one of the most prominent emission lines of Si. The signal to noise (or background) ratio for presented spectrum should be enough to provide qualitative and rough quantitative chemical analysis of the sample. The use of a laser pulse train of 3 to 5 pulses at 20 Hz instead of a single pulse resulted in better stability of spectral signal and a better signal to noise ratio. Following to LIBS spectra recording, the surface modifications were characterized by AFM measurement. The surface topography corresponding to the LIBS spectrum of Fig. 6 is presented in Fig. 7.
0.4 Fig. 6 LIBS spectrum of a Si (100) wafer sample irradiated through an approached fiber tip with a single 532 nm Nd:YAG laser pulse (E = 40 μJ).
LIBS Signal [a.u.]
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b) Fig. 7 a) AFM image and b) height profile of the ablation crater of Fig. 6.
For all experiments, we recorded also the spectral signal of plasma emission. But we could only see characteristic LIBS spectral features above a certain threshold of the laser pulse energy. At least for Al and Si, this threshold was significantly higher than the threshold for hole or crater formation. In Fig. 8, the dependency of LIBS signal (line intensity) on the laser pulse energy E is presented for Al. The LIBS spectra emission lines became distinguishable from the noise or background level at an energy of above about 20 μJ. A linear extrapolation of the fit of the measured data gives an intercept with the energy axis at 12 μJ. According Fig. 5, this value corresponds to crater width of about 2 μm. It is not clear however, whether this limit for LIBS is due to insufficient sensitivity of the detection system or whether at low laser energies the hole or crater formation is based mainly on thermal processes without strong plasma formation.
Fig. 8 The dependence of the LIBS signal (amplitude of the Al line at Ȝ=395 nm) vs. laser pulse energy E.
Al foil LIBS signal
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Other groups have reported similar limits. In ref. [8], the investigation of mineralogical samples is discussed by means of 3 ns N2 laser pulses. The smallest craters showing also LIBS signals had a width in the order of 2 μm. Smaller features showed no LIBS spectra (as the authors claimed due to too low spectrometer sensitivity). In ref. [9], near-field and farfield tight focusing LIBS of 200 nm thick chromium films with 532 nm ns Nd:YAG pulses is compared. In both cases the smallest craters showing also evaluable LIBS spectra had a width in the order of 2 to 3 μm, although smaller features were produced with lower laser pulse energies. The same authors have shown [10], that one method to reach better spatial resolution (at least for far-field tight focusing LIBS) is the use of fs laser pulses. They could record LIBS spectra with distinct elemental emission lines from craters with a width down to 650 nm.
3.4 LIBS Spectra with retracted fiber tip The ablated material can interact with the fiber tip. At higher pulse laser energies (larger craters), this leads to etching of the very tip of the tapered fiber. There is also an influence to the area of the ablated spot at the surface, probably due to interaction of ejected liquid material directly with the tip and/or changed plasma dynamics. For these higher energies, the smallest ablation spots are not obtained with approached tips but with tips, which are retracted from the surface by a distance of a few μm. This is demonstrated in Fig. 9 for a fiber, which was used in other LIBS experiment before and had an reduced tip radius of 20 μm probably due to interaction with the laser plasma (etching) or the sample (wear). The smallest continuous ablation spot is observed for a distance of about 20 μm corresponding to the tip radius. We obtained similar results with sharper tips as well. At least with tips with a tip radius of 4 μm and larger the smallest continuous features at Al samples were found with retracted tips at distances, which were about equal to the tip radius. With retracted fiber tip, stable multi pulse ablation and scanning is possible. The probe retraction is very efficient to enhance work life of the fiber tip and LIBS signal stability. But it decreases the lateral resolution at least to the scale of distance between probe and sample surface. However, for many potential applications it seems to be the best method.
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Tip-Sample Distance Z Fig. 9. Micrographs of single pulse ablation spots on an Al surface irradiated through a tapered fiber by a 532 nm Nd:YAG laser with various tip-sample distances Z from 40 μm (left) to 0.1 μm (right) for a fiber tip with radius of 20 μm.
ACKNOWLEGEMENT The Austrian Science Fund FWF under contract no. P17360-N08 and the Christian Doppler Research Society are acknowledged for financial support and provision of measurement apparatus.
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