Femtosecond laser-induced breakdown spectroscopy: Elemental imaging of thin films with high spatial resolution

Accepted Manuscript Femtosecond laser-induced breakdown spectroscopy: Elemental imaging of thin films with high spatial resolution

Christoph M. Ahamer, Kevin M. Riepl, Norbert Huber, Johannes D. Pedarnig PII: DOI: Reference:

S0584-8547(17)30263-X doi: 10.1016/j.sab.2017.08.005 SAB 5286

To appear in:

Spectrochimica Acta Part B: Atomic Spectroscopy

Received date: Revised date: Accepted date:

9 June 2017 6 August 2017 11 August 2017

Please cite this article as: Christoph M. Ahamer, Kevin M. Riepl, Norbert Huber, Johannes D. Pedarnig , Femtosecond laser-induced breakdown spectroscopy: Elemental imaging of thin films with high spatial resolution, Spectrochimica Acta Part B: Atomic Spectroscopy (2017), doi: 10.1016/j.sab.2017.08.005

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ACCEPTED MANUSCRIPT

Femtosecond laser-induced breakdown spectroscopy: Elemental imaging of thin films with high spatial resolution

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Christoph M. Ahamer *, Kevin M. Riepl, Norbert Huber, Johannes D. Pedarnig *

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Institute of Applied Physics, Johannes Kepler University Linz, A-4040 Linz, Austria

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ACCEPTED MANUSCRIPT Abstract

We investigate femtosecond laser-induced breakdown spectroscopy (fs-LIBS) for the spectrochemical imaging of thin films with high spatial resolution. Chemical images are obtained by recording LIBS spectra at each site of 2D raster-scans across the samples employing one fs-laser pulse per site. The diffraction images of the Echelle spectrometer

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are binned to reduce the read-out time of the intensified CCD detector and to increase the stability of the emission signals against peak drifts in the echellograms. For copper thin films on glass the intensities of Cu I emission lines and the size of ablation craters vary non-monotonously with the film thickness hCu = 5-500 nm. The emission efficiency, defined as the Cu I line intensity per ablated volume, strongly decreases for films thicker than the optical penetration depth. The Na I line intensity from glass increases exponentially with

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decreasing Cu film thickness. For yttrium barium copper oxide (YBCO) thin films on MgO various atomic and molecular emission lines of the laser-induced plasma are measured

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(film thickness hYBCO = 200-1000 nm). The obtained element (Y, Ba, Cu, Mg) and molecular (Y-O) fs-LIBS images match the structure of the micro-patterned YBCO films very well. The achieved lateral resolution r = 6 µm is among the best values reported for

KEY WORDS:

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spectrochemical LIBS imaging.

Femtosecond laser ablation, Laser induced breakdown spectroscopy

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(LIBS), Spectrochemical imaging, YBa2Cu3O7 (YBCO), Thin film

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* Corresponding authors:

[email protected] [email protected] http://www.jku.at/applphys

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1. Introduction

Laser-induced breakdown spectroscopy (LIBS) is a well-known technique for chemical element analysis of solid, liquid, gaseous, and particulate matter that is applied in a wide range of fields. LIBS offers several advantages due to its simplicity and versatility, little or negligible sample preparation, the possibility to analyze material in gas or liquid

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background (e.g., in air, vacuum, sea water, etc.), the feasibility of stand-off measurements, and its robustness which enables field applications in harsh environment. Sampling of the target material to be analyzed is performed by pulsed-laser ablation which produces craters on the surface of solid samples. The size of ablation craters can be reduced by using nanosecond (ns) laser pulses of low energy (<1 mJ) that are focused to small spots on the sample surface (diameter Ø ≤ 10 µm) [1]. With femtosecond (fs) laser

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pulses much smaller ablation craters can be produced [2, 3] and the LIBS analysis can be performed on sub-micrometer spots (Ø < 1 µm) [4, 5]. The crater depth reduces due to the

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smaller thermal penetration depth of fs-pulses as compared to ns-pulses [6, 7]. Fs-LIBS depth profiling with depth resolution in the nanometer range has been demonstrated for

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different materials [8, 9, 10].

Chemical imaging with element-specific spatial contrast (“element imaging”) is used in

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many applications ranging from materials science to industrial quality control. Various techniques like scanning electron microscopy with energy dispersive X-ray analysis

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(SEM/EDX) [11], X-ray fluorescence (XRF) [12], particle-induced X-ray emission (PIXE) [13], and mass spectrometry [14] are used for spatially resolved element analysis.

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Restrictions on sample properties (e.g., conductivity), sample preparation (e.g., timeconsuming pre-treatment), measurement conditions (e.g., vacuum), and large operating costs are drawbacks of some of the techniques. LIBS has been applied to

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imaging and mapping of various materials [15] including biological samples [16, 17], ceramics [18], metal coatings [19], metals [20, 21, 22, 23, 24], nanoparticles [25, 26], and paper [27]. Other related analytical techniques like laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) [e.g., 28, 29, 30], matrix-assisted laser desorption ionization (MALDI) [31], and some combination of LA-ICP-MS, Raman spectroscopy, and LIBS have been applied for element imaging also [32, 33, 34].

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ACCEPTED MANUSCRIPT In most studies on LIBS imaging, nanosecond laser pulses are employed for material sampling. However, femtosecond laser pulses offer several advantages for LIBS imaging compared to conventional nanosecond laser pulses. The much smaller heat affected zone reduces thermal loading of the target material and produces ablation craters of

well

defined size and geometry which is essential for imaging with high spatial resolution. Another advantage is the very low thermal background radiation at early stages of the fsLIBS plasma which allows for non-gated optical detection. Furthermore, non-linear optical

materials and to reduce the matrix effect in LIBS.

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effects (e.g., multi-photon absorption) can be exploited to investigate optically transparent

The LIBS imaging of thin film materials is challenging and rarely investigated. Larger spots may be required to increase the mass of ablated material and to attain sufficient optical emission intensities thus compromising the achievable spatial resolution. Signal

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accumulation by oversampling to improve signal/noise ratios becomes impossible if the single pulse ablation depth exceeds the thickness of film samples. Furthermore, the

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ablation process may be modified if the excitation energy spreads from the thin film into the supporting substrate.

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In this work we report on femtosecond laser ablation and laser-induced breakdown spectroscopy of thin films employing one, two, or three fs-pulses per site on the sample.

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We investigate the size of ablation craters and the intensity of emission lines for Cu thin films on glass and determine the emission efficiency. Micro-patterned YBa2Cu3O7 thin

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films on MgO and YSZ substrates are fs-LIBS imaged by measuring several atomic and molecular emission lines. Binning of diffraction images is employed to increase the scan

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speed and the stability of the LIBS signals.

2. Experimental

2.1 Experimental setup

The LIBS setup used in the experiments is shown schematically in Fig. 1. The Ti:Sapphire amplifier system (Spectra Physics Hurricane i) produced femtosecond laser pulses at wavelength λ = 800 nm, repetition frequency f r = 1 kHz, and pulse energy EL = 200 µJ. A frequency doubling beta barium borate (BBO) crystal generated pulses with λ = 400 nm. A short pass filter (Thorlabs FGB39) after the BBO crystal blocked the fundamental 4

ACCEPTED MANUSCRIPT wavelength. The pulse length tL was adjusted to maximize the pulse energy of the second harmonic radiation at EL = 40 µJ (tL = 175 fs). The pulse energy refers to the average energy of a single pulse measured with a power meter (Spectra-Physics Model 407A). A beam aperture of variable diameter dA was placed after the collimating cylindrical lens (CL) to block non-paraxial beams and stray light. The laser beam profile was close to Gaussian with a beam diameter 2w0 = 2.0 mm as measured by a CCD camera (WinCam D, pixel size 4.65 4.65 µm2) placed after the open aperture (dA > 10 mm). A beamsplitter

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(reflective for 400 nm, Thorlabs DMLP 425) directed the laser beam to one focusing lens (FL) and the sample. We performed experiments using two different biconvex lenses for focusing. The nominal focal lengths were fFL = 9.0 and 15.0 mm and the open diameters of FL were 9.0 and 12.7 mm, respectively. Focusing of the incident laser radiation on the sample was adjusted by translating the FL along the z-direction. The samples were

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mounted on a 2D translation stage (x-y direction) equipped with stepper motors.

The optical emission of the laser-induced plasma was collected in backward direction. The radiation was focused by an off-axis parabolic mirror (Edmund Optics, f/3) into an optical

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fiber (Ocean Optics QP 450-2-XSR, f/2.27, fiber core diameter 450 µm). The fiber guided the light to an Echelle type spectrometer (Multichannel Instruments Mechelle 7500, f/5.8,

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spectral resolution 7500) with an attached intensified charge-coupled device (ICCD, Andor iStar DH734). The ICCD delay time and gate width were set to td = 30 ns and tg = 1000 ns, respectively. The actual delay time was about 60 ns. For the imaging experiments few

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laser pulses (NL = 1 to 3) were applied at a given sample position and the LIBS signal was recorded by averaging over the pulses. Afterwards the sample was stepped in x-y direction

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and the LIBS measurement was repeated. The step width (Δx ≥ 1 µm) defined the centerto-center distance of adjacent laser-irradiated spots. The number of laser pulses NL was

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set by a hardware system which controlled the trigger pulses entering the laser amplifier. Laser, spectrometer, and the xyz-stage were controlled by a computer enabling to perform 2D and 3D LIBS sample scans. A flow of Ar gas was directed to the plasma to increase the detected emission intensity [35].

The diffraction images (echellograms) on the ICCD were binned to speed up the hyperspectral LIBS imaging. Faster image acquisition and less readout noise can be achieved by binning n n pixels to 1 superpixel since the number of readouts reduces by n² [33, 36, 37, 38, 39]. The calculation of 1D spectra from binned 2D echellograms was not possible with the available software and, therefore, the echellograms have been evaluated 5

ACCEPTED MANUSCRIPT directly. The relevant atomic and molecular emission peaks were located and assigned in the 2D diffraction images. The corresponding signals were calculated by summation of the detected intensity in a small 2D window around each peak in the echellogram and by subtracting the background intensity of an equally sized window with no emission features. If the 2D evaluation window is taken large enough (as was the case in our experiments), then the typical peak drifts in the echellogram do not influence the summed signal. Thus, an initial wavelength calibration prior to the measurement is sufficient and LIBS scans can

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be performed without interruption and time consuming re-calibration. For comparison, imaging was performed also by evaluating line intensities in LIBS spectra that were calculated after read-out of the complete echellograms (1024  1024 pixels). The acquisition rates were 0.5 Hz and 1.8 Hz for the complete and for the 8x8 binned echellograms,

respectively.

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emission

were

identified

using

available

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spectroscopic databases [40, 41, 42].

lines

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2.2 Thin film samples

Thin films of metallic Cu and of the high-temperature superconductor YBa2Cu3O7 (YBCO) were investigated. Cu layers were thermally evaporated onto glass substrates. The Cu film

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thickness hCu was between 5 and 500 nm. Epitaxial YBCO films were grown on (001) MgO single crystal substrates by pulsed-laser deposition (PLD) technique [43]. YBCO films on

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yttria-stabilized zirconia (YSZ) substrate were produced by electron beam evaporation [44]. The thickness of YBCO films hYBCO was in the range 200 to 1000 nm. The oxide films

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were micro-patterned by UV photolithography and wet-chemical etching. The strong lateral chemical contrast of patterned films and the sharp interface of film and substrate make such samples ideally suited for LIBS imaging experiments. For comparison, also thicker

3. Results

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Cu foils (SEM grids) on metal support have been measured.

3.1 Femtosecond laser ablation of Cu thin films

In a first set of experiments we ablated the Cu/glass samples and measured the fs-LIBS spectra as function of the Cu film thickness. The diameter of the beam aperture was dA = 3 mm and the laser pulse energy at the sample was EL ≈ 10 µJ. The spectra were averaged over 14 laser pulses delivered to 14 sites (single shot per site). The fs-LIBS spectra of all 6

ACCEPTED MANUSCRIPT samples revealed intense neutral Cu lines around 510-525 nm and very low background intensity (Fig. 2a). The signal-to-noise ratio (SNR) of the Cu I peak intensity at 510.55 nm was SNR > 75 for all samples. Noise was determined as standard deviation of the background intensity in the range 509-510 nm. Intense lines of Na at 589-590 nm from the glass substrate were observed for the thinnest Cu films. Weaker lines of Na and Ca from the glass were detected also.

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Figures 2b and 2c shows the variation of Cu and Na emission signals with the thickness of Cu films. The spectral intensities of several Cu I and Na I lines (Table 1) were fitted by Lorentz profiles and the peak areas determined. The Cu signals revealed a nonmonotonous dependence on film thickness with small variation in the range h Cu = 50-200 nm and a maximum at hCu = 10-20 nm (Fig. 2b). For thinner films the Cu signals rapidly decreased. The same behavior was observed for all neutral Cu lines investigated (Table

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1). The Na signals increased strongly with decreasing Cu film thickness (Fig. 2c). Exponential fits to the measured Na intensities gave decay lengths of 13.8 1.8 nm (Na,

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588 nm) and 14.2 1.3 nm (Na, 589 nm).

The laser irradiated Cu films were examined by optical microscopy and energy dispersive

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x-ray (EDX) analysis. The size and shape of ablation craters varied with film thickness as revealed by optical microscopy (Fig. 3, upper part). For the 500 nm thick film, small spots

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in the center of ablation craters were recognized by light transmitted through the sample (top right photo in Fig. 3, taken with backside and front side illumination). In these spots

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the Cu layer was entirely removed from the glass substrate as was confirmed by EDX analysis (concentration of Cu, CCu = 1.3 at.%). About half of the craters produced on this sample were ablated in full depth in the crater center. For the thinner films, the spots were

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seen in the center of all craters. The area of these spots, Ap, was determined from 40 spots per sample using the software ImageJ. The measured spot area A p increased with decreasing film thickness, had a maximum of 658 µm 2 at hCu = 35 nm, and then decreased for the thinnest Cu films with hCu < 35 nm (Fig. 3, lower part). The central spot was surrounded by a larger non-transparent region that appeared modified compared to the non-irradiated regions of films. The area of this modified region, Am, was around 350 µm2 for films with hCu > 100 nm (Fig. 3, lower part). The areas Ap and Am were indistinguishable by optical microscopy for films thinner than 75 nm. The inset in lower part of Fig. 3 shows four ablation spots on a 35 nm thick Cu film with a center-to-center distance of 80 µm. In the center of craters tiny ablation spot in the glass (Ø ~ 1.5 µm) were observed. 7

ACCEPTED MANUSCRIPT The application of the fs-LIBS system to the imaging of a structured Cu/Mn sample is shown in Figure 4. A Cu square grid (Athena network, web width 50 µm, pitch 160 µm, thickness of Cu foil 15 µm) was fixed on a bulk manganese substrate and scanned applying a single laser pulse per site (NL = 1). The obtained element images for Cu (Fig. 4a) and Mn (Fig. 4b) correspond to the structure, size, and orientation of the Cu grid on the support (image size 42 x 42 points, step width Δx = 6 µm). The ablation craters of adjacent

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sites slightly overlapped. Laser-induced periodic surface structures (LIPSS) were observed at the outer regions of the craters of bulk samples (electron microscopy).

3.2 Spectrochemical imaging of YBCO thin films

For the fs-LIBS imaging experiments micro-patterned thin films of the multi-element

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material YBa2Cu3O7 have been used. Spectrochemical images of different elements and emission lines can be measured at the same sample position and the spatial resolution of

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images can be estimated by comparison to the microstructure of the samples. The fs-LIBS spectra of YBCO films on MgO and YSZ substrates were similar and they revealed various emission lines of all major elements with very low background intensity (Table 1). The

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emission signals of YBCO films were obtaining either from the binned diffraction images or from the LIBS spectra. Figure 5 compares the Mg and Ba images for a YBCO film on MgO

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(hYBCO = 1000 nm) obtained from LIBS spectra (Fig. 5, top row) and from echellograms (Fig. 5, bottom row). The fs-LIBS images for Mg I at 517.27 nm (Fig. 5, left) and for Ba II at

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493.41 nm (Fig. 5, right) exhibited the same features and relative intensities for both methods. The signal to noise ratio SNR of element images was calculated from the measured element signal divided by the standard deviation of noise (average over 5  5

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image pixels). SNR was 8.3 for Mg and 8.1 for Ba (maps constructed from LIBS spectra) and 5.1 for Mg and 7.3 for Ba (maps constructed from binned echellograms). Similar results were obtained for YBCO films on YSZ substrates.

On the same YBCO film different regions were imaged applying different number of laser pulses at each position (NL = 1, Fig. 6a. NL = 2, Fig. 6b. NL = 3, Fig. 6c). The LIBS spectra were accumulated over NL on the ICCD detector and element images were constructed from 8  8 binned echellograms. High signals of Ba from the YBCO (shown in red color) and of Mg from the MgO (in blue) were measured in very good agreement to the microstructure of the film as seen from the overlay of LIBS images with the optical micrographs. 8

ACCEPTED MANUSCRIPT The ablation spots were well separated from each other and on the substrate the single pulse ablation craters had a diameter of 4.1 ± 0.3 µm (Fig. 6a). The SNR for Mg and Ba was >4.3 and >21.4 for all images, respectively. Occasionally, Ba or Mg signals were detected on the MgO substrate or on the YBCO film, respectively. This was due to contaminations by sample handling and due to false detection of strongly amplified noise peaks at high ICCD gain. With 3 laser pulses per site the YBCO film was ablated in full depth and a strong Mg signal from MgO beneath the YBCO was detected (Fig. 6c). This

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result was confirmed by a depth profiling experiment where the same sample area was imaged three times applying one pulse per site (NL = 1) for each scan. The Ba signals were strong and stable for the first and second scan (SNR > 34) and very low in the third scan as the YBCO was removed by the first two pulses. On the other hand, the Mg signals from sample regions covered by YBCO increased with the number of scans. The image acquisition time for scanning the 32  32 points and measuring the 1024 echellograms

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was 11.5 minutes. The measurement frequency (≈1.5 points/s) was limited by the read-out speed of the ICCD camera. Similar frequencies were reported for elemental mapping by

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LIBS [e.g., 1, 22] and by LA-ICP-MS [e.g., 30]. Much higher LIBS imaging frequency (1 kHz) was achieved with high-repetition rate lasers in combination with fast detectors and

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electronics [21].

Element maps of a thinner YBCO film on MgO (h YBCO ≈ 250 nm) are shown in Figure 7.

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The micro-pattern of this sample included structures like letters, numbers, circles, and lines. The fs-LIBS scanned area was 704  704 µm2 (Δx = 8 µm) and each site was

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exposed to one laser pulse. Element images were reconstructed from binned echellograms using different areas for the atomic emissions of Y I (467.49 nm), Ba II (455.40 nm), Cu I (510.55 nm), and Mg I (518.36 nm), and the molecular emission of

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diatomic Y-O (616.51 nm). The Y, Ba, and Cu element images and the Y-O molecular image revealed all features of the patterned film (optical micrograph “mic” in Fig. 7). The element image for Mg had an inverted contrast compared to YBCO as the Mg signals were low in regions covered by the layer. The molecular signal originated from Y-O molecules that were ablated from the oxide film or formed in the early stage of the plasma. Formation of Y-O at the plume contact front was unlikely as measurements were performed in Ar gas flush.

In a further experiment we have fs-LIBS imaged another YBCO film on MgO through a 1 mm thick glass slide that was placed between the focusing (and collecting) lens and the 9

ACCEPTED MANUSCRIPT YBCO film. The obtained element images for Ba and Mg were very similar to the other images presented here. This result demonstrated that our experimental fs-LIBS setup was stable against modifications of the optical beam paths introduced by the glass slide between lens and sample. From this we conclude that femtosecond-LIBS microanalysis is possible also through a transparent cover and without direct mechanical contact to the sample under investigation.

4.1Fs-laser ablation threshold fluence of thin films

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4. Discussion

In femtosecond laser ablation of bulk metals, different regimes of “gentle ablation” (GA) and “strong ablation” (SA) have been observed at low and high laser fluence F [45],

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respectively. The threshold fluences and the ablation rates are low and high in the GA and SA regimes. The single pulse ablation depth L depends on a characteristic length and on

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the ablation threshold fluence Fth

.

(1)

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:  , F H v  / A L  ln F F th  GA SA :   , Fth H  /A  he th  v he HF 

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In the GA regime at low fluence the characteristic length is given by the optical penetration depth  = 1/ with  the absorption coefficient. The threshold fluence can be estimated

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from the evaporation enthalpy ΔHv and the optical absorption A of the metal. For bulk Cu the estimated threshold is Fth ≈ 0.12 J/cm2 using ΔHv = 42 kJ/cm3 [46, 6] and the optical

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parameters  = 14.5 nm and A = 50.3% calculated from Fresnel equations for normal incidence at the laser wavelength λ = 400 nm. The reported experimental threshold fluence was close to this estimate (0.14-0.18 J/cm2 [45, 35, 47]). In the SA regime at high fluence the diffusion of highly excited electrons determines the characteristic length. The hot electron penetration depth in Cu is he ≈ 60-80 nm as calculated with the twotemperature model (TTM, [48]). The estimated threshold fluence is Fth ≈ 0.5-0.65 J/cm2 assuming that the optical absorption at high fluence is A HF ≈ A. The measured thresholds are in the range 0.4-1.0 J/cm2 with mean value at 0.6 J/cm2 [45, 35, 44].

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ACCEPTED MANUSCRIPT For thin metal films on optically transparent and thermally insulating substrates the fs laser ablation properties deviate from the corresponding properties of bulk metals. Au films on glass revealed a fs-laser ablation threshold fluence that linearly decreased with thickness in the range hAu = 31-180 nm [49]. For Al thin films a decreasing ablation threshold was observed for hAl = 30-300 nm and attributed to thermal accumulation and transition from 3D to 2D heat diffusion [50]. With Au films on fused silica a decrease of the melting threshold fluence was measured for hAu < 500 nm and a critical hot electron diffusion

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length c ≈ 440 nm has been calculated by TTM [51]. The fluence of a focused Gaussian laser beam is F(r) = F0  exp(-2 r2/w2), with the peak fluence on the optical axis F0 = 2 Ep / w2, and Ep the pulse energy, w the radius (1/e2) of the beam at the sample position, and r the distance from the optical axis. For a beam with peak fluence F0 > Fth, the ablation depth is highest on the optical axis and L(r) shows a

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parabolic profile according to equ. (1). A thin film of thickness h is ablated in full depth from the substrate for positions r ≤ rp where L(r) ≥ h. The corresponding area A p at the

p

p

w 2  ln F F 0

2

 h 

th

.

(2)

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A r 2 

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substrate-film interface is given by

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The diameter ds and area As of the crater at the film surface are calculated from the condition L(rs) = 0 and are given by

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w 2 ln F0 Fth  2

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(3)

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As d s2  / 4 

At large distance from the optical axis, the fluence F(r) was reduced and the film material was gently ablated. In our experiments, the Cu films were modified in the GA regime and the area of modified surface was Am ≈ 300 µm2 for the 500 nm thick film sample (Fig. 3). From equ. (3) the laser beam diameter at the sample 2w = 12.5 µm (A L = w2 ≈ 120 µm2) and the peak fluence F0 = 16.3 J/cm2 were calculated, assuming that the physical properties of Cu bulk can be applied for this rather thick metal film. The calculated laser spot size AL agreed to the size estimated from measurements with the WinCam CCD camera. For the thinner Cu films the area Am increased which can be explained by a linear decrease of the threshold fluence Fth with thickness similar to the earlier observations [49-

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ACCEPTED MANUSCRIPT 51]. However, for films thinner than 35 nm the area Am rapidly decreased which has not been reported so far. An exponential increase of the threshold fluence Fth was required to compensate for the decreasing optical absorption of very thin layers and to describe the experimental results (solid curve in Fig. 3). Damage and ablation of thin film material by shockwaves could possibly contribute to the variation of ablation crater size with film thickness. The observed strongly non-monotonous variation of crater size with film thickness and the distinct maximum size at a certain thickness indicate that shockwaves

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are probably not the dominating mechanism of material removal. High-resolution timeresolved optical microscopy would have been required to clarify this point but such experiments were beyond the scope of this study.

In the strong ablation regime, the film was ablated in full depth and the area Ap where the metal had been entirely removed from the substrate was Ap ≈ 7 µm2 for the 500 nm thick

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Cu film. The characteristic length ≈ 157 nm was calculated from equ. (2) using F = F0 and Fth = 0.6 J/cm2, assuming that the ablation properties for this thick film were the same

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as for bulk Cu. This length was somewhat larger than the values reported for bulk Cu (he ≈ 60-80 nm). The calculated ablation depth at r = 0 was L0 ≈ 518 nm in the SA regime. Comparable single fs-pulse ablation depths were reported for bulk Cu [35]. The area Ap

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rapidly increased for the thinner films with maximum Ap = Am > AL at hCu = 35 nm. For the simulation of Ap an exponential-linear dependence of Fth on hCu was assumed as for the simulation of Am (dashed line in Fig. 3). The volume of ablation craters was estimated from

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crater areas Am and Ap and layer thickness hCu assuming a truncated conical shape of

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craters for the sake of simplicity, Vabl(hCu) = [(√Am + √Ap)2 - √(AmAp)] hCu / 3. The measured emission line intensities of Cu, ICu, did not decay with decreasing layer

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thickness hCu as might have been expected for thin films with hCu ≤ L (Figs. 2a and 2b). The intensities varied only slightly over a wide range of film thicknesses (50-350 nm) and ICu showed a maximum at around h Cu = 10-20 nm. From the measured line intensities ICu(hCu) and the estimated crater volume an emission efficiency η = ICu(hCu) / Vabl(hCu) was calculated. For each of the four Cu lines evaluated, the intensity ICu(hCu) was normalized to the intensity measured for the thickest film and η determined (Fig. 8). The emission efficiency rapidly dropped with increasing film thickness and stayed constant for films with hCu ≥ 50 nm. Interestingly, the relevant length scale for the signal drop was close to the optical penetration depth (approx. 15 nm) and much smaller than the hot electron diffusion length (approx. 150 nm). The ablated volume rapidly increased with film thickness (inset in

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ACCEPTED MANUSCRIPT Fig. 8). From this result we conclude that only a very thin layer at the film surface (thickness ~ 2-3 ) contributed to the LIBS signal. For thicker films the ablated volume strongly increased but most of the material was ablated as dark non-luminous matter that was not in the plasma state (time window of detection 30-1000 ns). The fs-laser ablation of layers in solid/liquid state from thin metal films has been measured recently [52, 53].

The emission line intensities of Na from the glass substrate, INa, decreased exponentially

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with Cu film thickness (Fig. 2c). Also for Cu films on silicon substrate an exponential increase of the Si line intensity with decreasing Cu layer thickness was reported [9]. For Al thin films on silicon an exponential dependence of the threshold fluence for the detection of Si on the Al layer thickness has been observed [54]. In our experiments, the fitted decay length of INa(hCu) was very close to the optical penetration depth in Cu ( = 14.5 nm). The LIBS signal for Na was correlated to the laser fluence FIF at the Cu-glass interface, INa(hCu)

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expected from equ. (1), was not observed.

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 FIF = F0  exp(- hCu/). A logarithmic dependence of INa on FIF, that might have been

4.2Lateral resolution in fs-LIBS imaging of thin films

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For the discussion of results obtained in LIBS imaging, we concentrate on the achievable spatial resolution. Highly conductive materials like Cu have large hot electron diffusion

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lengths and the ablation craters can become substantially larger than the laser spot size, therefore. This limits the achievable lateral resolution in LIBS imaging of such materials. In

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order to demonstrate high resolution LIBS imaging we have performed experiments on the YBCO films. The electrical conductivity of YBCO and Cu scales with σYBCO / σCu 1 / 100

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indicating that the oxide materials has a much shorter electron diffusion length than Cu and that imaging of YBCO with high resolution is possible. The resolution is determined by the parameters of the laser beam, the size of the laser spot AL and of the ablation crater Am, the step width of the scanner Δx, and the material properties and structure of the sample. The effect of a larger spot size is illustrated with another YBCO film (Fig. 9). For imaging, the aperture was removed from the laser beam path and the step width had to be enlarged to Δx = 20 µm to account for the larger laser spot and crater size. The LIBS signal for Ba was calculated from two Ba II emission lines by placing a large integration window around the two peaks in the echellogram. The integrated intensity exhibited a considerably improved stability compared to the integrated intensity for one emission line. The fs-LIBS image for Ba and the overlay of this image with an optical micrograph of the 13

ACCEPTED MANUSCRIPT same position on the film are shown in Figs. 9a and 9b, respectively. The film micropattern with numbers and horizontal and vertical lines was recognized in the LIBS image. However, sample structures with sizes similar to the step width appeared distorted in the LIBS map depending on the exact location and number of sampling laser pulses. This can be seen at the 3 holes in the YBCO film (diameter 40 µm) that are marked with white circles in Fig. 9b. Although the holes were uniform in shape and size their LIBS images were different since either 2 or 4 sampling pulses interrogated the holes producing 2 or 4

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pixels of low Ba signal in the LIBS map. Also the vertical line (marked with white arrow) was not imaged correctly, since the number of laser pulses sampling this line varied between 1 and 2. According to the Nyquist-Shannon sampling theorem the sampling frequency must be at least twice the frequency of the signal to be reconstructed [55, 13]. Hence, the smallest structure that is resolvable by LIBS imaging has a size r ≥ 2 xLIBS,

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with xLIBS the effective size of the LIBS interrogation (crater size or step width). The dependence of the lateral resolution in LIBS imaging on the laser beam parameters is

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summarized in Fig. 10. The data are taken from the review by Piñon et al [15] and some further publications [1, 8, 17, 23, 24, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67]. The blue star symbol is the result of our present study. Most of the experiments reported were

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performed with nanosecond pulses (full symbols), fewer results were reported on fs-LIBS (open symbols). Laser pulses at lower energy produce smaller ablation craters and enable

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to scan samples with reduced step width and to resolve smaller structures. With fs-LIBS, a spatial resolution of 1-2 µm has been shown [23, 67], sub-micron imaging could be

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achieved using highly efficient optical collection and detection systems. In our study, the laser pulses were focused by a simple biconvex lens. With microscope objective lenses used for tight focusing and efficient light collection much smaller sample structures should

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become resolvable. LIBS chemical imaging with super-resolution r < λ may be envisaged considering the sub-wavelength craters that can be produced with ultra-short pulses [2, 4]. The faint emission signals from such tiny sample volumes ~(r)3 could be amplified, for instance, by a second orthogonal laser pulse.

5. Conclusions

We applied femtosecond laser pulses to ablate metal and oxide thin films and to perform elemental and molecular imaging by fs-LIBS. For Cu thin films on glass the ablation craters were larger in size (Am) than the laser spot size (AL). The crater area varied with 14

ACCEPTED MANUSCRIPT the film thickness hCu due to a non-monotonous dependence of the ablation threshold fluence on hCu. The emission efficiency (LIBS signal for Cu / volume of ablation crater) rapidly dropped for films thicker than the optical penetration depth in Cu ( ~ 15 nm). FsLIBS imaging of metal thin films with high lateral and vertical resolution may be achieved at low fluence F0 ≈ Fth in the gentle ablation regime where small (Am ≤ AL) and shallow (L ≈ ) craters can be obtained. With other materials of lower electrical conductivity and thus shorter hot electron diffusion length [45, 46, 51], fs-LIBS imaging with high resolution

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should be easier to achieve. The spectrochemical imaging of YBa2Cu3O7 thin films reproduced all sample features with 6 µm spatial resolution (i.e., Am ≈ AL). Quantitative LIBS is restricted to the analysis of single, non-interfering analyte emission lines. For qualitative spectrochemical imaging also a combination of various analyte lines can be used. In the measured 2D echellograms small arrays of detector pixels containing the analyte peaks can be identified and the corresponding optical signals read-out. This is

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faster than the read-out of the complete ICCD detector array. With even faster detection systems (e.g., Paschen-Runge spectrometers) and more efficient collection optics (e.g.,

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microscope objectives) the spatial resolution in fs-LIBS imaging may be improved and the measurement time reduced.

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Acknowledgements

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The authors would like to thank H. Piglmayer-Brezina for preparing the Cu film samples and for the SEM measurements, and A. Nimmervoll for help in all matters concerning

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electronics. We appreciate the supply with YBCO/YSZ samples by R. Semerad (THEVA Dünnschichttechnik GmbH). We acknowledge gratefully the support by the Austrian industrial research initiative PAC and the financial support by the Austrian Research

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Promotion Agency (project number 843546 imPACts).

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ACCEPTED MANUSCRIPT Figure captions

Figure 1: Schematic of femtosecond-LIBS setup. Laser pulses at wavelength 800 nm are frequencydoubled using a SHG crystal (cylindrical lenses CL, filter F, aperture A). The 400 nm pulses are focused (FL) on the sample and the plasma radiation is guided to the

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spectrograph by off-axis parabolic mirror (OAP) and optical fibre. Samples on x-y stage are flushed with Ar.

Figure 2:

Fs-LIBS spectra of Cu thin films on glass substrates (focusing lens with focal length fFL =

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15 mm). Thickness of Cu films hCu = 5-500 nm. Average over 14 spectra per sample measured by single pulse ablation on 14 different sites. Spectra shown with offset intensity

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for clarity (a). Emission signals for neutral Cu (b) and Na (c) lines for different thickness of Cu thin films on glass. Exponential fits to Na signals (in c). Cu I lines at 510, 515, and 521

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nm and Na I lines at 589 and 590 nm (a).

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Figure 3:

Optical micrographs and sizes of laser ablation craters on Cu films of different thickness

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(focusing lens fFL = 15 mm). Upper part: Optical micrographs; numbers are the film thickness in nm. Lines in micrograph of 200 nm thin film are guides to the eye indicating the area of modified (Am) and pierced (Ap) film. Lower part: Surface ablated area with

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partial removal of Cu film (▼) and pierced area with full-depth ablation of Cu film (▲), as measured by optical microscopy. Simulated areas of surface ablated film (solid line) and pierced film (dashed line). Optical micrograph of 35 nm thick Cu film with four ablation spots (spot-to-spot distance 80 µm, inset in lower part).

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Figure 4: Femtosecond-LIBS image of Cu square grid on bulk Mn metal. Images for Cu (a) and Mn (b) recorded by a single laser pulse per site, NL = 1. Scan size 42 42 points, scan step width Δx = 6 µm (fFL = 15 mm). Element signals in false colors. Intensity scale: low signals in blue color, high signals in green to red. Cu grid web width 50 µm, thickness 15 µm (inset

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not to scale). Cu I line at 521.82 nm (a) and Mn I line at 482.35 nm (b).

Figure 5:

Element image of micro-patterned YBa2Cu3O7 thin film on MgO substrate by femtosecondLIBS (fFL = 9 mm). Element signals for Mg (left column) and Ba (right column). Images

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constructed from measured LIBS spectra (top row) and echellograms (bottom row). Scan 32 32 points, step width Δx = 8 µm, number of laser pulses per site NL = 1. Mg I line at

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517.27 nm (left) and Ba II line at 493.41 nm (right).

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Figure 6:

Fs-LIBS element images for Ba (in red color) and Mg (in blue) of a micro-patterned YBCO

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thin film (hYBCO = 1000 nm) on MgO (fFL = 9 mm). Element images overlaid with optical micrographs. LIBS measurements performed in different regions on the sample with NL = 1 (a), 2 (b), and 3 (c) laser pulses per site. White dashed lines indicate edges of the YBCO

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film pattern. Scan 32  32 points, step width Δx = 8 µm. Ba II line at 493.41 nm (in red)

Figure 7:

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and Mg I line at 518.36 nm (in blue).

Element images for Y, Ba, Cu, and Mg of a micro-patterned YBCO film (hYBCO ≈ 250 nm) on MgO substrate (fFL = 15 mm). Optical micrograph (mic). Fs-LIBS element images reconstructed from atomic emission of Y, Ba, Cu, and Mg and from molecular emission of diatomic Y-O. Binning 8  8, scan 88  88 points, step width Δx = 8 µm, NL = 1. The signals for Mg and Ba are divided by 1.5 and 5, respectively (intensity scale bar). Y I line at 467.49 nm, Ba II line at 455.40 nm, Cu I line at 510.55 nm, Y-O line at 616.51 nm, and Mg I line at 518.36 nm.

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Figure 8: Emission efficiency η = ICu / Vabl of fs-laser induced Cu plasma depending on Cu film thickness (fFL = 15 mm). Cu emission line intensity ICu normalized to intensity for thickest

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film. Volume Vabl of ablation crater (inset).

Figure 9:

Element map of patterned YBCO film on MgO imaged with low spatial resolution (step width Δx = 20 µm, scanned area 640  640 µm2, fFL = 9 mm). LIBS image for Ba II (a). Overlay of Ba image with optical micrograph of YBCO film (b). White circles mark etched circular holes in YBCO. Film thickness hYBCO = 200 nm. Ba II lines at 455.40 nm and

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493.41 nm.

Figure 10:

Lateral resolution in LIBS imaging versus laser pulse energy, laser wavelength (λ), and

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pulse duration (short sp, ultra-short usp). Data taken from review by Piñon et al [15] and

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references [1, 8, 17, 23, 24, 56-67], the blue star symbol is this study.

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ACCEPTED MANUSCRIPT Table captions

Table 1: Wavelength λ, Einstein coefficient Aik, energies of upper (Ek) and lower (Ei) levels of transition, degeneracy factors of higher (gk) and lower (gi) levels of transition for neutral (I) and singly ionized (II) atomic emission lines of Ba, Cu, Mg, Mn, Na, and Y measured in fs-

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laser induced plasma of Cu and YBCO thin films [40]. Electronic ground state (GS) and excited electronic state (ES) of di-atomic Y-O with the vibronic quantum numbers v´ and

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v´´ of the involved transition [41, 42].

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ACCEPTED MANUSCRIPT Table 1

X2Σ

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gk

gi

4 2 6 8 4 4 6 2 3 5 8 4 2 8 v´

2 2 8 10 6 2 4 4 3 3 10 2 2 6 v´´

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A2Π

Aik (107 s-1) 11.1 9.53 3.2 3.8 0.2 6.0 7.5 0.165 3.37 5.61 4.99 6.16 6.14 1.33

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Mn I Na I

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Mg I

Ei (eV) 0 0 5.10238 5.07209 1.38895 3.78590 3.81669 1.64223 2.71159 2.71664 2.31917 0 0 0.06576 GS

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Cu I

Ek (eV) 2.72175 2.51211 7.80459 7.73703 3.81669 6.19118 6.19203 3.78590 5.10783 5.10783 4.88886 2.10443 2.1030 2.71717 ES

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Ba II

λ (nm) 455.40 493.41 458.70 465.11 510.55 515.32 521.82 578.21 517.27 518.36 482.35 589.00 589.59 467.49 λ (nm) 616.51

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights:  Chemical imaging by femtosecond laser-induced breakdown spectroscopy (fs-LIBS). High lateral resolution in atomic and molecular fs-LIBS images of oxide thin films.

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On metal thin films large ablation craters impede high resolution LIBS imaging.

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Ablation threshold fluence of metal films depends non-monotonously on film thickness.

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