# Towards ultra-low detection limits of lithium by LIBS in LiF atomic layers

Matthias Audren-Paul, Frédéric Le Cras, Vincent Motto-Ros, Alexandre Cherrier, Gunay Yildirim, Bruno Bousquet, Maxime Legallais

► To cite this version:

Matthias Audren-Paul, Frédéric Le Cras, Vincent Motto-Ros, Alexandre Cherrier, Gunay Yildirim, et al. Towards ultra-low detection limits of lithium by LIBS in LiF atomic layers. Spectrochimica Acta Part B: Atomic Spectroscopy, 2026, 238, pp.107467. 10.1016/j.sab.2026.107467. cea-05506912

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# Towards ultra-low detection limits of lithium by LIBS in LiF atomic layers

Matthias Audren-Paul<sup>a,b</sup>, Frédéric Le Cras<sup>a,c,d</sup>, Vincent Motto-Ros<sup>e</sup>, Alexandre Cherrier<sup>d</sup>, Gunay Yildirim<sup>a</sup>, Bruno Bousquet<sup>d,*</sup>, Maxime Legallais<sup>a,*</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Lithium fluoride

Atomic layer deposition

Laser induced breakdown spectroscopy

Limit of detection

## ABSTRACT

The growing number of industrial applications requiring thin-film materials is prompting the development of new characterization methods. We demonstrate that laser-induced breakdown spectroscopy (LIBS) enables the verification of the spatial homogeneity of LiF films deposited by atomic layer deposition (ALD), and the quantification of the lithium amount in LiF thin films. In our experimental conditions, we obtained a limit of detection (LOD) of 0.7–4.1 fg.ng⁻¹ for the Li I line at 670.8 nm and for single-shot LIBS analysis. Then, an estimate of LOD in the range 0.17–0.93 fg.ng⁻¹ (i.e. 170–930 ppb) was obtained with other acquisition parameters. Given the 100 Hz repetition rate of the laser shots, the LOD decreases by a factor of 10 in one second leading to 0.012–0.065 fg/√Hz. Such level of detection limit paves the way for future utilization of LIBS for the chemical characterization of thin films.

## 1. Introduction

Lithium-containing thin-film materials are a key element in several leading technologies, in particular in the fields of microdevices, photovoltaics, Organic Light Emitting Diodes (OLED) and nano-photonics. Among them LiNbO₃, and to a lesser extent its LiTaO₃ counterpart, are currently the most widely represented ones due to a unique combination of ferroelectric, piezoelectric, electrooptic, thermooptic, acoustooptic, and pyroelectric properties [9,15]. LiF layers are also widely used as a buffer layer in OLED and solar cells to improve the carrier injection/ extraction and power conversion efficiencies [3,6,10]. Among additional applications, one can cite microbatteries and microsupercapacitor using Li⁺-conducting materials like LiPON as well as coating of batteries materials also based on Li⁺ conducting materials [21], electro-chromic materials [20] or the modification of semiconducting properties of organic materials by Li-doping [17]. These materials are typically prepared by ion-slicing of a single crystal or by vacuum deposition techniques such as evaporation, magnetron sputtering or atomic layer deposition (ALD), with thicknesses generally ranging from 0.001 to 1 µm. Besides, the introduction of these lithium-containing materials and their manufacturing processes into a conventional microelectronics

environment can also lead to contamination of other manufacturing lines, with detrimental effects [19]. Determining the lithium content in these thin layers and within devices throughout the manufacturing process is particularly important but challenging, especially if the aim is to use an affordable and rapid technique. More precisely, spatial distribution of lithium is essential to assess the thin films homogeneity. Common techniques giving access to quantitative analysis and spatial distribution of elements are: EPMA (electron probe micro-analysis) [18], SEM-EDS (Scanning Electron Microscopy / Energy Dispersive Spectroscopy) [12], LA-ICP-MS (Laser ablation / Inductively Coupled Plasma Mass Spectrometry) [13], and synchrotron XFM (X-Ray Fluorescence Microscopy Imaging micro X-ray fluorescence synchrotron) [14]. Another relevant technique for quantitative analysis and spatial distribution of elements is micro-LIBS (laser-induced breakdown spectroscopy) imaging with its main advantages being table-top and easy operating systems. Moreover, the Li I emission line at 670.78 nm exhibits the strongest emissivity in LIBS [8] which makes it the perfect candidate to assess the limit of detection of the technique in the case of ultra-thin films. However, the corresponding electronic transition (actually a doublet) involving the fundamental energy level of the lithium atom, implies this emission line will likely exhibit self-

<footnote><sup>a</sup> CEA, CEA Tech Nouvelle Aquitaine, Pessac F-33600, France</footnote>

<footnote><sup>b</sup> Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, Pau 64000, France</footnote>

<footnote><sup>c</sup> Univ. Grenoble Alpes, CEA, LITEN, F-38000 Grenoble, France</footnote>

<footnote><sup>d</sup> Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, UMR 5026, F-33600 Pessac, France</footnote>

<footnote><sup>e</sup> Institut Lumière Matière, UMR5306, Univ. Lyon 1-CNRS, Université de Lyon, 69622, Villeurbanne, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: bruno.bousquet@u-bordeaux.fr (B. Bousquet), maxime.legallais@cea.fr (M. Legallais).</footnote>
absorption and self-reversal within the laser-induced plasma [11]. For this reason, despite of its strong signal intensity, it appears to be necessary to examine the behavior of this emission line in order to assess its applicability to the chemical characterization of thin films containing lithium as a major component.

In this context, the objective of this work is to assess the performance of LIBS for the analysis of LiF thin films prepared by ALD (atomic layer deposition) that exploits self-limiting surface reactions, and enables the monolayer-by-monolayer deposition of materials with good control over uniformity and conformity on the substrate. Lithium being the easiest element to detect by LIBS, this study investigates whether a film obtained after a single ALD cycle could be analyzed, bearing in mind that the analytical ability of LIBS depends largely on the experimental setup and operating parameters. It should also be noted that, in the present case, lithium originates from a bilayer material constituted by an ultrathin LiF film deposited onto a thick single-crystalline silicon wafer, and not from a bulk material in which lithium is randomly distributed. The use of an absolute limit of detection (LOD), which represents the minimum mass detected, therefore makes more sense than a relative LOD [5] in the context of thin films characterization. Thus, in this work, we report the absolute value of LOD as the mass of lithium being detected at each single-shot analysis. And, in order to further compare our work with other LIBS results and other techniques, we finally simply compare the minimum detected mass of lithium to the mass of the ablation crater.

## 2. Experimental methods

### 2.1. Preparation of LiF layers by ALD

Lithium fluoride thin films were deposited by atomic layer deposition using the process recently reported elsewhere [1], using a Picosun R-200 advanced reactor connected to an argon-filled glovebox (MBraun) via a load-lock. Argon was employed for both purging and as a carrier gas. Lithium bis(trimethylsilyl)-amide (LiHMDS, Sigma-Aldrich, 97%) and titanium tetrafluoride (TiF₄, Epivalence, research grade) were used as the lithium and fluorine precursors, respectively. The temperature within the reactor was set at 250 °C and the pressure was maintained below 10 hPa. Depositions were carried out on polished single-crystal silicon wafers (100) with a native oxide layer on the surface. One ALD cycle of LiF consists of the injection of LiHMDs for 1 s, followed by an 8 s purge, then the injection of TiF₄ for 5 s and a final 8 s purge. In these process conditions, thin films feature a total impurity content in the bulk of less than 2% [1].

### 2.2. Ellipsometry

An InSE-1000 ellipsometer from Semilab was used to accurately measure the LiF thickness. All measurements were carried out at room temperature and were acquired with a xenon lamp over a wavelength range of 360–800 nm using a constant angle of incidence of 60.3°. Data were recorded and analyzed with the SAM/SEA software from Semilab. The measured amplitude and phase (ψ, Δ) were fitted using a model composed of (i) a (100) silicon substrate, (ii) a 3.21 nm-thick layer of native oxide (SiO₂ with refractive index *n* = 1.457 at 632.8 nm) estimated from the measurement of three wafers, and (iii) a LiF layer. The refractive index (*n*) and extinction (*k*) coefficient from the Semilab database was used for LiF, which displays a refractive index of 1.39 at 632.8 nm (and an extinction coefficient of 0).

### 2.3. Confocal microscopy

Topographic images of the samples were acquired using an LSM 900 confocal microscope from Zeiss, equipped with an apochromat 50 × objective (Numerical Aperture = 0.95) and a 405 nm laser. The confocal scan was performed over a 10.1 µm depth, with a vertical resolution of

210 nm and a 50% overlap between successive images. Crater profiles and crater volumes were then obtained after image de-noising using the Zeiss Confomap software.

### 2.4. LIBS instrumentation and protocol

The LIBS setup and the general experimental procedure employed in this work are based on previously published configurations [2,4,7,16]. In short, the system is operating a 1064 nm Nd:YAG laser source (Centurion GRM, Lumibird), with a pulse duration of 7 ns and a repetition rate of 100 Hz. The laser beam is focused down to a 10 µm diameter spot onto the sample surface using a × 15 objective lens (LMM-15×-PO1, Thorlabs), while the sample is raster-scanned with 35 µm-steps using a motorized XYZ translation stage. The laser pulse energy was set to 500 µJ using a computer-controlled attenuator. A continuous argon flow (0.8 L/min) was applied during plasma generation to reduce redeposition of ablated material and enhance detection sensitivity through improved plasma confinement. All measurements were conducted at room temperature and under ambient pressure conditions. The system is controlled via a custom software developed in LabVIEW, enabling real-time control of laser parameters, sample positioning, plasma stability, and the execution of automated scan sequences. Plasma emission is collected through two separate fiber-coupled optical paths, each one being connected to a Czerny-Turner spectrometer equipped with an ICCD camera (Istar, Andor Technology). The first one (Shamrock 303, Andor Technology) was fitted with a 1200 lines/mm grating (blazed at 300 nm) and gave access to the 245–294 nm spectral range. The second one (Kymera 328i, Andor Technology), also using a 1200 lines/mm grating (blazed at 700 nm), gave access to the 608–693 nm spectral window. Both spectrometers provided a spectral resolution of approximately 0.15 nm. The ICCD cameras were time-gated and synchronized with the laser Q-switch to ensure optimal signal acquisition. The LIBS spectrum recorded by each spectrometer was the result of the collection of spatially integrated plasma emission.

The Shamrock 303 spectrometer coupled with a Gen II ICCD camera, was optimal for the detection in the UV range and thus selected for silicon (Si) detection, as several Si emission lines are present in this spectral range. Similarly, the Kymera 328i spectrometer coupled with a Gen III ICCD was optimal for the detection in the red side of the visible range and thus selected for the detection of the most intense lithium (Li) emission line at 670.8 nm and the second line of moderate intensity at 610.4 nm. For instance, at 670 nm, the quantum efficiency of the Gen III ICCD reaches 26%, compared to only 3% for the Gen II model. Thus, despite being less commonly reported than the Gen II ICCD in the LIBS literature, the Gen III ICCD was selected to maximize the signal detection of these lithium lines.

The two different optical paths dedicated to the collection of plasma emission were at approximately 45° from the incident laser beam direction. The two collection systems share the following characteristics. Emission light from plasma was coupled inside an optical fiber bundle composed of 19 fibers (NA = 0.2, 50 µm-diameter each) via a 1-inch diameter converging lens of 30 mm focal length and located at a distance of 60 mm from the plasma (4f optical configuration). The plasma image was thus formed at the entrance of the optical fiber bundle such as there is no spatial truncation. The slit width was set to 30 µm for each spectrometer.

Two series of measurements are reported in this study. The first one was conducted with an integration time (gate) set to 1 µs for the Gen II ICCD and 2 µs for the Gen III ICCD, without amplification (gain = 0) and with a 1 µs delay for both systems. This set of parameters was selected in order to build a regression model from the non-saturated LIBS spectra of our selection of LiF thin-film samples related to various numbers of ALD cycles. Then a second set of experimental parameters were used to demonstrate that the signal-to-noise ratio (SNR) of the Li I emission line at 670.8 nm could be significantly increased for the sample obtained after 1-cycle of ALD. In this case, on the Gen III ICCD, the delay was kept
at 1 µs, but the gate width was extended to 10 µs and the gain was set to 1000, to obtain better SNR.

The focusing conditions were ensured by the excellent surface quality of the silicon wafers used as samples. These wafers exhibit very low surface roughness and high planarity, which strongly limits local variations of the focal distance during mapping. Prior to each analysis, the planarity of the sample with respect to the instrument optical axis was carefully adjusted using a goniometer. This procedure allowed us to accurately set the sample horizontality and to guarantee a constant lens-to-sample distance over the entire analyzed area. Under these conditions, the use of an active focusing or alignment system was not required. The LIBS images given later in Fig. 2 clearly demonstrate that this protocol was appropriate.

For all LIBS experiments presented in this study, the LIBS signal is the net intensity, meaning peak maximum intensity after baseline subtraction. The baseline was determined as follows. For the Li I emission line at 610.4 nm, it was the average signal value in the 619-620 nm range; for the Li I emission line at 670.8 nm, it was the average signal value in the 680-681 nm range; for the Si I emission line at 288.1 nm it was the average signal value in the 291-292 nm range; and for the Si I emission line at 263.1 nm it was the average signal value in the 266-267 nm range. LIBS imaging acquisitions were performed using 300 × 100 maps, with a 35 µm step between each laser shot (center-to-center). For each sample, an average spectrum and its standard deviation spectrum were computed from a subset of 23,400 spectra (260 × 90 pixels), deliberately excluding regions where the plasma stability was not optimal, specifically, the first scan lines and the beginning and end of each line.

### 3. Results

Fig. 1 depicts the evolution of LiF film thickness (t) as a function of ALD cycle number (N). A very good linear correlation between thickness and number of ALD cycles is visible in the thickness range of 1.3–14.5 nm. This is typical for a self-limiting growth process, and demonstrates a good control of the film growth. It should be noted that the thickness of the thinnest films, corresponding to 1 and 5 ALD cycles, could not be measured because it was below the resolution of the ellipsometer measurement. As shown in Fig. 1, setting the intercept at zero, the slope of the best linear regression model (solid line) was equal to 0.33 Å/cycle, which corresponds to the growth rate per cycle of LiF films.

Fig. 2a and b depict the evolution of the LIBS spectra of the LiF films as a function of the number of cycles, with high SNR. The signal of lithium (a) was detected for the emission lines of Li I at 610.4 nm and 670.8 nm down to one ALD cycle. In the latter case, it is worth mentioning that the SNR for the line at 670.8 nm and for a single-shot LIBS spectrum was still >100. The signal of silicon was found to be

<figure><img src="image_4.png" /><figcaption>Fig. 1. LiF film thickness measured by ellipsometry as a function of the number of ALD cycles, and the best linear regression model (solid line) built from the range 30–450 cycles and considering intercept at zero.</figcaption></figure>

unchanged for the different numbers of cycles (b). This observation indicates that the plasma was mainly composed of silicon, thereby meaning that the contribution of the thin film was negligible in regard to the total amount of ablated matter involved in the formation of the plasma. Fig. 2c shows good spatial homogeneity of the LIBS signal over the entire surface analyzed. It illustrates the excellent thickness uniformity for each ALD layer, which is the main characteristics of the ALD technique. Furthermore, the detection of lithium even after a single ALD cycle indicates that LIBS is sensitive enough to analyze such samples, despite the acquisition parameters not being optimized. Moreover, the effect of detector saturation is clearly visible at 670.8 nm on the average LIBS signals (Fig. 2a) and images (Fig. 2c) related to 300 and 450 cycles. Also, the dark zone in the image related to Li I at 670.8 nm for 0-cycle reveals that this emission line is so sensitive that it can detect surface contamination in lithium of the silicon wafer (not detected via the Li I line at 610.4 nm). Note that the $H_\alpha$ line at 656.28 nm was also present and its intensity was almost constant (Fig. 2a), as expected, since hydrogen comes from the humidity of the air in which the samples are manipulated.

Now considering the laser-induced crater caused during the LIBS analysis, it is possible to estimate the volume and the corresponding mass of lithium involved in each thin film sample analysis.

The ablation crater resulting from our LIBS analysis (single-shot) is of conical shape of approximately 15-µm in diameter and 5-µm in depth, as characterized by confocal microscopy measurements. Thus, considering that all the thickness values of our LiF films are smaller than 15 nm (Fig. 1), it clearly appears that the ablated matter is mainly composed of silicon and that the contribution of LiF is negligible. More precisely, while the ablation crater volume is always $2.94 \times 10^{-10} \text{ cm}^3$, the volume corresponding of the LiF depends on the film thickness and thus the number of ALD cycles. Thus, for 30 ALD cycles (9.9 Å thickness) the volume of the corresponding LiF thin film was estimated at $17.5 \times 10^{-14} \text{ cm}^3$. Considering the linear model presented in Fig. 1 still valid for a number of cycles smaller that 30, we obtained a thickness value of 0.33 Å for a single ALD cycle. Consequently, the corresponding volume of the LiF thin film was estimated at $0.58 \times 10^{-14} \text{ cm}^3$. This extrapolation means that the LiF growth rate is considered as the same between 1 and 30 ALD cycles and after 30 ALD cycles, which is in agreement with the evolution of the LIBS signal of the Li I line at 610.4 nm in Fig. 3.b. It is worth mentioning that the measured thickness at one ALD cycle derived from this treatment is much thinner than the theoretical one of an ideal LiF single layer. This is directly related to the steric hindrance process, caused by the presence of large ALD precursors ligands (HMDS in the case of Li), which prevents chemisorption on all available binding sites and results in the deposition of an incomplete LiF monolayer. Even though such thickness value derived from the linear model does not correspond to a perfect atom arrangement in a single layer, it provides a realistic estimate of the ablated LiF mass as calculated below.

The matter corresponding to the ablation crater ($2.94 \times 10^{-10} \text{ cm}^3$) is almost fully composed of silicon, which is characterized by a density of $2.33 \text{ g.cm}^{-3}$. Thus, the total ablated mass for each single-shot LIBS analysis is estimated at 0.7 ng.

To estimate the mass of lithium in the LiF thin films, we consider that LiF crystallizes in the NaCl structure ($Fm\bar{3}m$ S.G., $Z=4$) with a cubic cell parameter of 4.03 Å [1]. Thus, the Li concentration in LiF is $6.11 \times 10^{22} \text{ at.cm}^{-3}$. This gives $10.7 \times 10^9$ Li atoms, for the 9.9 Å thickness – 30 ALD cycles – $17.5 \times 10^{-14} \text{ cm}^3$ LiF film, considering a dense LiF material as confirmed by X-ray reflectivity measurements [1]. Then, assuming a linear growth of film over the entire ALD cycle range (which is confirmed by the LIBS data presented in Fig. 3), one can obtain, via simple extrapolation, that the 0.33 Å thickness – 1 ALD cycle – $0.58 \times 10^{-14} \text{ cm}^3$ LiF film corresponds to $0.35 \times 10^9$ Li atoms. Finally, considering the atomic mass of Li (6.941 u = $1.15 \times 10^{-23}$ Kg), the mass of Li for one single ALD cycle of LiF film, can be estimated at 4 fg. This value should be compared to the 0.7 ng mass of the ablated crater, hence
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Average LIBS spectra showing (a) emission lines in log-scale attributed to Li I, Si II, H I and Ar I (inset is a close view of Li I line at 610.4 nm); (b) emission line of Si I in linear scale with vertical offset; Intensity (a. u.) means raw intensity values; (c) LIBS images in log scale for the emission lines Si I at 288.1 nm, Si I at 263.1 nm, Li I at 610.4 nm, and Li I at 670.8 nm. The label “not detected” means that the image corresponds to the noise and the label “saturation” means detector saturation. LIBS images were obtained with laser spot size ~10 µm, crater diameter ~ 15 µm, and distance between two consecutive craters (center-to-center) being 35 µm. The same color code is used for all images. Here, Intensity (a. u.) means background subtracted intensity value (see the text for details).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Evolution of the LIBS averaged intensity with the number of ALD cycles for the Li I line at 670.8 nm (a), Li I line at 610.4 nm (b), and Si I lines at 288.1 nm and 263.1 nm (c). For each number of cycles, both the average and standard deviation (SD) values are calculated over the 23,400 pixels of the LIBS image. Error bars correspond to the SD values. The numerical values given in a. u. correspond to background corrected values (see the text for details).</figcaption></figure>

yielding a relative value of concentration of 6 fg.ng⁻¹. The corresponding relative abundance of lithium within the LIBS plasma is thus 6 ppm for 1 ALD cycle.

It is also possible to consider and alternative approach to estimate the mass of lithium. In fact, LiF deposition as a 1 0 0 plane corresponds to 1.2 × 10¹⁵ atom/cm², giving 2.1 × 10⁹ Li atoms within the ablation crater area. This second approach gives an estimate of 24 fg for the mass of lithium but, as previously explained, the actual LiF film is not an ideal

single crystal [1] with perfect arrangement of atoms so that the value of 2.1 × 10⁹ Li atoms is overestimated.

In summary, one single ALD cycle of LiF film corresponds to an ablated mass of lithium falling between 4 and 24 fg, for a 0.7 ng total ablated mass. This can be expressed as 6–34 fg.ng⁻¹ when considering the ablated mass, and 6–34 ppm considering the amount of lithium within the LIBS plasma. It is worth noticing that the LIBS signal of the Li I line at 670.8 nm detected for 1 ALD cycle still exhibits a high SNR value
indicating that it is possible to build a quantitative model for lithium, even for such low amounts of matter, i.e. few femtograms.

Fig. 3 shows the evolution of the LIBS averaged intensity of selected emission lines with the number of ALD cycles. For silicon, Fig. 3c confirms that the signal is unchanged whatever the number of cycles for the Si I emission lines at 288.16 nm and 263.13 nm, as illustrated by the relative position of the data points against the horizontal dashed lines. The data points corresponding to the seven numbers of cycles exhibit a RSD of approximately 5–6%. This result indicates that the plasma is almost identical for all the samples and that there is no matrix effect. Thus, a direct comparison between the LIBS spectra can be made without any additional precaution. For lithium, the Li I signal at 670.8 nm (Fig. 3a) exhibits two regimes of saturation: the first one is observed for the points at 100 and 200 numbers of cycles that reveal self-absorption within the LIBS plasma; the second one is observed for the points at 300 and 450 cycles that reveal detector saturation. The inset in Fig. 3a shows a linear trend in the range 1–30 ALD cycles for this emission line. Conversely, the Li I signal at 610.4 nm (Fig. 3b) exhibits a linear trend over the whole range of film thickness. This linear fit for both line intensities from 1 to 30 ALD cycles confirms that the LiF growth is linear in this range.

Based on the results presented in Fig. 3, we recommend to select the Li I line at 610.4 nm for quantifying lithium in LiF thin films over the 1–450 ALD cycles range. However, since the Li I line at 670.8 nm exhibits a much higher sensitivity than the one at 610.4 nm, it is preferred to quantify lithium amounts in ultrathin films. Based on the standard approach of a calibration curve, one can estimate the limit of detection (LOD). The LOD is commonly defined after a linear calibration model as:

$$LOD = 3 \frac{\sigma_B}{a}$$

Where $\sigma_B$ is the baseline noise (expressed in a. u.) evaluated as the standard deviation of the background signal measured in a spectral region without emission lines and a is the sensitivity (slope of the calibration model). The slope was found to be a = 41.77 a. u./cycle for the regression model of the Li I line at 610.4 nm ($R^2$ = 0.997), and a = 464.70 a. u./cycle for the regression model of the Li I line at 670.8 nm ($R^2$ = 0.993; for the range 1–30 ALD cycles). Then, considering a similar noise for the two lines, since they are recorded under the same conditions, the LOD is expected to be at least ten times lower for the Li I line at 670.8 nm. To calculate the LOD, it was required to measure the noise $\sigma_B$. However, due to the averaging performed on a large number of spectra, $\sigma_B$ becomes too low to be measured on the LIBS average spectra, thus preventing the calculation of the LOD. For this reason, we decided to examine the single-shot spectrum. More precisely, we measured the noise as the standard deviation of the signal in the range 644–653 nm for each of the 23,400 spectra constituting the LIBS image for a given sample. Actually, the spectral range used to evaluate the baseline noise does not exhibit any significant emission lines, even when extensive averaging is performed over all pixels of the LIBS image. As clearly illustrated by the averaged spectra shown in Fig. 2a, no emission features are observed in this spectral range, and the signal is exclusively related to the continuum emission. This spectral range therefore constitutes an appropriate and reliable window for evaluating the baseline noise. The resulting distribution of the noise values was found to be perfectly fitted by a Gaussian function, with a peak at approximately 18 a. u. and a broadening (half-width at half maximum) at approximately 3 a. u. For a LiF film resulting from 1 ALD cycle, the signal to noise ratio (SNR) for the Li I line at 670.8 nm is approximately 116 for a single-shot LIBS spectrum. Since the SNR is increasing with the square root of the number of spectra in the LIBS image, for a 23,400-pixel image, the SNR for the average spectrum is thus multiplied by 153 compared to the one of a single pixel, i.e. single shot spectrum. Thus, the value of $\sigma_B$ falls below 1 a. u. for the average spectrum and cannot be extracted, meaning the LOD value cannot be calculated. Consequently, the actual ability of detecting lithium in LiF films by LIBS must be assessed at the scale of a

single laser shot (1 spectrum – 1 pixel). This approach is perfectly aligned with other works on LIBS imaging for which the analytical performance is evaluated at each pixel of the image.

Considering valid the extrapolation of our linear model below 1 ALD cycle despite an incomplete LiF layer formation (e.g. islands of single LiF layer randomly spread on wafer surface), the LOD value of single-shot LIBS analysis is 0.12 cycle (LOD = 3 × 18 a. u./464.7 a. u./cycle), presumably 0.5–2.9 fg for 0.7 ng ablation crater. This corresponds to 0.7–4.1 fg·ng⁻¹, i.e. a relative abundance of lithium of 0.7–4.1 ppm inside the LIBS plasma. It is worth mentioning that such values of relative abundance are not related to traces of lithium randomly spread inside a silicon matrix but to incomplete thin LiF film deposited onto a bulky Si wafer. The LOD value of 0.12 ALD cycle does not correspond to real-life condition of LiF thin film deposition. Thus, the way to estimate the actual LOD value might be subject to endless debate. The most important conclusion is that the amount of lithium in LiF thin films has been successfully quantified via single-shot quantitative LIBS analysis and that the Li I emission line at 670.8 nm exhibited a SNR of approximately 116 for a LiF film corresponding to 1 ALD cycle, under the experimental conditions reported here. Keeping in mind that this emission line is the most emissive in LIBS, the next step consisted in demonstrating that this SNR value could still be increased, in order to open the path to the detection of less emissive emission lines from other elements. This work is reported in the following section.

The results presented up to now correspond to the following experimental conditions for the detection of the LIBS spectra: Kymera 328i spectrometer coupled with a Gen III ICCD; gain = 0; delay = 1 µs; gate = 2 µs. Based on this setup, the Li I emission line intensity at 610.4 nm allowed for the creation of a linear regression model covering the 1–450 ALD cycles range and the Li I emission lines at 670.8 nm enabled the quantification of the amount of lithium in ultrathin films of LiF down to 1 ALD cycles range, for which the SNR was 116. We now report the result obtained for the 1 ALD cycle sample after applying a second set of experimental parameters: Kymera 328i spectrometer coupled with a Gen III ICCD; gain = 1000; delay = 1 µs; gate = 10 µs. Actually, a higher value of gain corresponds to higher number of photoelectrons within the amplifier (MCP) of the ICCD camera. The result is an increase of both the signal and the noise with an increase of the signal-to-noise ratio (SNR). Fig. 4 presents the single-shot LIBS spectra recorded under the two experimental conditions described above for 1 ALD cycle LiF layer, namely [gain 0 / gate 2 µs] and [gain 1000 / gate 10 µs]. In this latter case, the signal increases from 2094 to 30,746 a. u. (multiplied by 14.68) while the noise increases from 18 to 61 a. u. (multiplied by 3.39). Consequently, the combined effect of longer gate and higher gain increased the SNR value by a factor of 4.3 (from 116 to 504). This

<figure><img src="image_7.png" /><figcaption>Fig. 4. Evolution with the acquisition parameters of single-shot LIBS signal (Li I line at 670.8 nm) and noise for one-ALD cycle LiF film. Blue: gain 0 / gate 2 µs; Purple: gain 1000 / gate 10 µs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
improvement of SNR by 4.3-fold obviously improves the LOD. Indeed, making the simple assumption that the case “0 ALD cycle” provides 0 a. u., and that the signal linearly increases with the number of ALD cycles, the new LOD value would be divided by this factor of 4.3 giving 0.12–0.65 fg, again for an ablation crater of 0.7 ng, which finally corresponds to 0.17–0.93 fg·ng⁻¹, i.e. 170–930 ppb. Again, these values of LOD will not be discussed in a perspective of analytical performance but the main message is that the LIBS spectrum of a sample of LiF film prepared after one single ALD cycle exhibit a SNR of >500 for the Li I line at 670.8 nm. It demonstrates that the SNR value was actually modified by changing the experimental conditions. Another set of acquisition parameters might still allow to increase the SNR value. Higher is the SNR value of the Li I line at 670.8 nm, better is the detection of other emission lines.

## 4. Conclusions

LiF thin-films have been deposited on silicon wafer by atomic layer deposition (ALD). LIBS images have showed the good uniformity of the deposited layers and the stability of the silicon signal from the substrate between the series of samples of increasing thickness together with the crater volume characterization demonstrated that the LIBS plasma was mainly composed of silicon with only little amount of lithium inside. The signal of the Li I line at 610.4 nm showed a good linearity with the number of ALD cycles, demonstrating that this emission line can be used for quantification all along the 1–450 ALD cycles range. Then, the single-shot LIBS signal of the Li I line at 670.8 nm was found to be the most sensitive and adapted to the characterization of a thin film resulting from only 1 ALD cycle, which corresponds to an incomplete LiF single atomic layer due to steric hindrance effect of the ALD precursor ligands. Despite the performance limit of the ellipsometry to estimate the thickness of films from 1 to 30 ALD cycles, the LIBS linear response of both Li I lines in this ALD range demonstrates that the LiF growth is linear from 1 to 30 ALD cycles. Under the last acquisition conditions (gate = 10 µs; gain = 1000) presented in this paper, we did not repeat the full process giving a calibration curve, but instead we simply built a 2-points model, i.e. LIBS signal related to 1-ALD cycle, and the intercept at zero. The resulting limit of detection of lithium, which is thus only an estimate, is about 0.12–0.65 fg, for 0.7 ng ablation crater, corresponding to 0.17–0.93 fg·ng⁻¹ (i.e. 170–930 ppb within the plasma) under the best experimental conditions examined in this work, and although these conditions were not the result of an optimization process. Even if the values of LOD reported here were not supported by a complete metrological study, the method described in this paper gives access to good estimate of LOD.

Finally, given the 100 Hz repetition rate of the laser shots, the LOD decreases by a factor of 10 in one second yielding 0.012–0.065 fg/√Hz. Given the transient regime at the very beginning of the deposition process by ALD, the definition and values of LOD reported here are questionable and subject to debate. This is why we focused our efforts in increasing the SNR for the Li I line at 670.8 nm in order to further transpose the method to the analysis of other elements.

Further investigation on acquisition parameters may permit to increase again the SNR. The results presented here demonstrate that single-shot LIBS is highly sensitive and particularly well suited to the analysis of lithium thin-films. It is anticipated that micro-LIBS may also permit to analyze other elements in thin films with sufficient SNR values to retrieve valuable information about the initial growth of film and its spatial distribution.

### CRediT authorship contribution statement

**Matthias Audren-Paul:** Methodology, Investigation. **Frédéric Le Cras:** Writing – review & editing. **Writing – original draft, Validation, Supervision, Funding acquisition.** **Vincent Motto-Ros:** Writing – review & editing. **Writing – original draft, Validation, Investigation, Formal

analysis. **Alexandre Cherrier:** Methodology, Investigation. **Gunay Yildirim:** Supervision, Funding acquisition. **Bruno Bousquet:** Writing – review & editing. **Writing – original draft, Validation, Methodology, Investigation, Conceptualization.** **Maxime Legallais:** Writing – review & editing. **Writing – original draft, Validation, Supervision, Methodology, Conceptualization.**

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgment

The authors thank the Région Nouvelle-Aquitaine and REACT-EU for the financial support. This work was partially supported by the French region Rhônes Alpes Auvergne (Optolyse, CPER2016), the French government “France 2030” initiative, under the DIADEM program managed by the “Agence Nationale de la Recherche” (ANR-22-PEXD-0014, “Libelul”).

During the preparation of this work, the authors used Chat-GPT 5.0 in order to partially generate the graphical abstract. After using this tool/service, the authors reviewed and edited the content as needed and takes full responsibility for the content of the published article.

### Data availability

Data will be made available on request.

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