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Rapid micro-analysis of Al-In-Sn-O thin film using laser induced breakdown spectroscopy with picosecond laser pulses
Spectrochimica Acta Part B 160 (2019) 105684
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Analytical note
Rapid micro-analysis of Al-In-Sn-O thin film using laser induced breakdown spectroscopy with picosecond laser pulses
T
⁎
Shiming Liu, Qing Gao, Junshan Xiu , Zhao Li, Yunyan Liu School of Physics and Optoelectronic Engineering, Shandong University of Technology, Shandong, Zibo 255049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Picosecond laser induced breakdown spectroscopy AITO thin film Magnetron sputtering Optical band gaps
Laser induced breakdown spectroscopy (LIBS) based on picosecond laser pulses was used to achieve rapid microanalysis of Al-In-Sn-O (AITO) thin films deposited by radio frequency (RF) magnetron sputtering at various sputtering powers. We calculated the plasma temperature of 5063 ± 150 K and electron density of 4.6 × 1016 cm−3 with single ablation crater diameter of 46 ± 1 μm. The content ratio of Al/(Al+In+Sn) in AITO thin film was closely related to the optical band gap of the film, and the ratio was exhibited by the spectral line intensity of LIBS for each corresponding element. The quantitative analysis of Al/(Al+In+Sn) with LIBS intensity ratio was achieved by plotting calibration curves according to the value of energy dispersive X-ray spectroscopy (EDS). It was found that the optical band gaps calculated were decreasing as the increase of the sputtering power in the exhibition of the transmittance spectra of AITO thin films deposited at various sputtering powers. In addition, we found the consistency of the evolutions of LIBS target line intensity ratio and optical band gap of AITO thin films. Therefore, the utilization of LIBS based on picosecond laser pulses is available and efficient in the rapid micro-analysis of AITO thin film.
1. Introduction Al-In-Sn-O (AITO) thin film is a transparent semiconductor material with a wider optical band gap comparing with known In-Sn-O (ITO) thin film, which is depending on content ratio of Al element in it [1]. It has great potential to be widely used as UV-transparent semiconductor film materials to transparent electronic devices and ultraviolet electric devices [2,3]. In order to obtain good optical performance of AITO thin film, a precise control and measurement of Al, In, and Sn constituting the AITO thin film is essential in the research and production of AITO electronic devices. At present, there are lots of analytical technologies used to determine the elements composition of thin film materials, such as X-ray fluorescence (XRF), glow discharge optical emission spectroscopy (GDOES), and energy dispersive X-ray spectroscopy (EDS). However, their disadvantages of exhibition stem from not only the equipment with high complexity and cost, but also the expertise and long time required in the preparation of reliable sample, which increases the analytical cost of the thin film and decelerates monitoring and measurement. Laser induced breakdown spectroscopy (LIBS) is a rapid developing technology, which is used widely in different fields with many advantages on analysis with high speed, no sample preparing, simultaneous multi-elements detection, and high sensitive measurement [4–9]. ⁎
Moreover, many investigations have been achieved for the detection of nanometre thin film with LIBS, which has been a feasible method for the analysis of nanometre thin film [10–15]. However, the studies primarily focus on the experimental optimization, such as laser wavelength, the quantitative analysis and plasma emission by LIBS. The relationship between the optical properties of AITO thin films and emission intensity ratios of elements in thin films hasn't been involved further. In our previous work, we had achieved a lot of work on the thin film analysis with LIBS, such as TiO2 [16], and Cu(In, Ga)Se2 thin film [17,18]. While these thin films were detected with LIBS based on nanosecond laser pulses, it was not possible to achieve the laser pulse ablation of single thin film layer with given setup. Thus, the LIBS spectra contained spectral lines from the thin film layer and the substrate, which disturbed the analytical element spectral lines emissions. It was the reason that the micro-analysis of thin films could not be achieved with ns laser pulses in our previous work. Furthermore, an available picosecond LIBS was first introduced to the micro-analysis of a new AITO thin film material. The AITO thin film was fabricated on soda-lime glass substrates by a single quaternary target. Sputtering power was one of important parameters for one-step radio frequency (RF) magnetron sputtering process. We obtained the plasma characteristics from AITO thin film with picosecond LIBS, and
Corresponding author. E-mail address: [email protected] (J. Xiu).
https://doi.org/10.1016/j.sab.2019.105684 Received 12 May 2019; Received in revised form 16 August 2019; Accepted 16 August 2019 Available online 19 August 2019 0584-8547/ © 2019 Elsevier B.V. All rights reserved.
Spectrochimica Acta Part B 160 (2019) 105684
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sample surface with a shift of about 0.5 mm. The initial plasma emissions were collected at 45-degree angle over the plasma by a combination of two lenses with focal lengths of 35 mm (L2) and 16 mm (L3). The whole plasma could be detected due to the smaller plasma plume induced by a ps laser pulse with a laser pulse energy of 122 μJ and a ablation carter of about 46 ± 1 μm. The output of fiber was connected to the entrance of a portable optical fiber spectrometer (AvaSpecMini2048CL-SOT8, Avantes Technology) in a spectral bandwidth from 240 nm to 415 nm with a spectral resolution of 0.01 nm. In the experiment, the 3-D displacement stage and spectrometer were triggered synchronously by a photodiode (PD shown in Fig. 1). The integration time of the spectrometer was set as 500 ms, and the horizontal moving speed and single moving distance of displacement stage were set as 25 mm/s and 12.5 mm respectively. Note that 250 laser pulses were focused uniformly on the surface of AITO thin film layer with a distance of 12.5 mm. All experimental conditions were kept constant for all AITO thin films samples. Moreover, in order to obtain higher spectral intensity in low plasma temperature (about 5063 K shown in Section 3.2), the detection delay time of plasma emissions was set as the minimum, and the plasma emissions induced from 250 laser pulses were accumulated once under the detection width of 500 ms with the spectrometer. Fig. 2 showed the photomicrographs of ablation craters with amplification factor of 1000 (a) and 40 (b) with a metallographic microscope (4XB, Shanghai CSOIF Co., Ltd.). In Fig. 2a, we could see that the diameter of the single ablation crater in our experiment was measured to be 46 ± 1 μm. It indicated that ps laser pulses used in our experiment could provide micro-ablation with a small ablation crater and achieve the micro analysis for AITO thin film. Fig. 2b showed the successive single pulse ablation on the surface of AITO thin film. We could observe that the ablation craters were roughly uniform in size with good ablation repeatability. In the process of laser ablation, the possible thermal effect of laser ablation could be observed around the ablation crater of the sample of AITO thin film (the red parts in Fig. 2b).
evaluated the feasibility of the micro-analysis with ps LIBS. The content ratio of analytical elements in AITO thin film was determined by the quantitative analysis, and the relationship between optical band gap of AITO thin film and the LIBS analysis was discussed. At last, we evalated the feasibility and availability of picosecond LIBS to fabricate high performance AITO thin film by the one-step radio frequency (RF) magnetron sputtering method. 2. Experimental 2.1. AITO thin film sampling AITO thin films were deposited at room temperature, using RF magnetron sputtering from an indium oxide target (purity, 99.99 wt%, Zhongnuo New Material, China) with a size of 75 mm × 5 mm which was doped with tin oxide and aluminum oxide. And the content ratio of Al, In and Sn in the indium oxide target was 1:7:2 at.%. Normal sodalime glasses (SLG), as substrates, were put into the vacuum chamber. The distance from target to substrate was 150 mm, and the flow rate of the sputtering medium Ar (purity, 99.99 wt%) was 20 sccm, with the 3.0 Pa work pressure. In our work, a series of AITO thin films samples were deposited at different sputtering work powers varied from 50 W to 110 W. The pre-sputtering process was finished before the deposition, the time of which was half an hour, and the film thickness was about 400 nm measured with a film thickness gauge. 2.2. Experimental setup A LIBS setup with picosecond laser pulses was used in this experiment as shown in Fig. 1. The laser pulse was produced by a picosecond microchip laser (RealLight) operating at 1064 nm with a repetition rate of 500 Hz, pulse duration of 350 ps, and pulse energy of 122 μJ. The picosecond laser pulses were focused on the surface of AITO thin film by a combined lens (L1) with beam expanding and beam focusing (focal length of 35 mm). The diameter of the single ablation crater was estimated to be 46 ± 1 μm as shown in Fig. 2a, which corresponded to the mean fluence of about 7.3 J/cm2 with a Gaussian profile of laser pulse delivered to the sample. The AITO thin film sample was placed on a motoring 3-D displacement stages. The distance between lens L1 and film surface was set according to the calcium emissions (Ca I 393.36 nm and 396.85 nm [19]) from the glass substrate under the AITO thin film. In order to avoid the direct ablation of the glass substrate under the AITO thin film, the focus point of the laser beam was set over the
3. Results and discussion 3.1. LIBS spectra and spectral lines selection Fig. 3 shows LIBS spectra of an AITO thin film with a sputtering power of 110 W and glass substrate ranging from 240 nm to 415 nm with an accumulation of 250 laser shots in our experiment. It was obvious to observe several spectral lines of aluminum, indium and tin
Fig. 1. LIBS setup for AITO thin film analysis in ambient pressure. 2
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Fig. 2. Photomicrographs of ablation craters with amplification factor of 100 × of objective lens and 10 × of eyepiece (a) and 40× of objective lens (b) for AITO thin film with sputtering power of 110 W. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
elements in AITO thin film. All of signal-to-background ratios of the spectral lines were > 3:1, and the Ca, Mg and Si elements emissions from glass substrate were not observed in this spectrum of the AITO thin film, compared with the spectrum of glass substrate shown in Fig. 3. This indicated that the AITO thin film layer was only ablated and the glass substrate was not damaged. It demonstrated that the distance was suitable between lens L1 and the surface of AITO thin film samples. In order to enhance micro-analysis performance of AITO thin film, it was important to select suitable spectral lines of three elements. In our interesting spectral region from 240 nm to 415 nm, 10 Sn I lines, 4 In I lines, 2 In II lines and 4 Al I lines were observed, as listed in Table 1, in which the wavelengths of each element represented the peak positions shown in Fig. 3, while the small deviation between the experimental values of the wavelength and theoretical values from the NIST database [19] might be produced because of plasma emission fluctuations and the grating response of the spectrometer. Significant overlaps between these spectral lines, such as In I 325.61 nm and Sn I 326.23 nm shown in Fig. 3, could be observed in the spectrum. A suitable emission line was thus selected for each of the three
Table 1 List of spectral lines observed of AITO thin film. Element
Number of lines
Wavelength (nm) at the peak positions
Sn I
10
In I In II Al I
4 2 4
242.16, 317.51, 275.38, 242.85, 308.21,
270.65, 333.06, 303.93, 289.02 309.27,
277.98, 283.99, 286.33, 300.91, 380.10, 326.23 325.61, 410.17 394.40, 396.15
metals analyzed in AITO thin film, and the lines were first relatively intense for the corresponding elements in the given conditions of ablation and detection, which were chosen to be as little interference as possible with other spectral features. Another important criterion was that the line would not be affected by self-absorption. Thus, the line shape was first inspected to avoid strong self-absorption and self-reversing, such as Sn I 317.51 nm shown in Fig. 5. Finally, the selected lines for Al, In, and Sn from AITO thin film were set as 396.15 nm,
Fig. 3. LIBS spectra of AITO thin film with sputtering power of 110 W and glass substrate. 3
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Fig. 5. Lorentz fitting plot of Sn I 317.51 nm for calculating the electron density.
Fig. 4. Boltzmann plot obtained from Sn I spectral lines of Table 1.
410.17 nm, and 317.51 nm, respectively.
dominate radiative processes. The lowest value of the electron density of the plasma could be calculated by using the McWhirter criterion
3.2. Plasma characteristics of AITO thin film using picosecond LIBS
Ne (cm−3) ≥ 1.6 × 1012T1/2ΔE 3
The plasma temperature (T) and electron density (Ne) are two important parameters, which can describe the plasma state in order to understand directly laser – material interaction and plasma characteristics. The excitation temperature of the plasma plume is in general retrieved using the well known Boltzmann plot method [20]. In our present work, the excitation temperature was calculated by Sn I spectral lines listed in Table 1, and the Sn emissions were induced from the AITO thin film deposited with the sputtering power of 110 W. Fig. 4 showed the Boltzmann plot. The linear fitting equation was:
y = −0.849 − 2.29x According to the Boltzmann equation [20], y is the value of ln
T was the plasma temperature in K and ΔE was the energy gap in eV between upper and lower level of the spectral line. In our work the energy gap ΔE of Sn I 317.50 nm was 3.91 eV and plasma temperature T = 5063 K was considered, the minimal electron density value of Ne = 6.805 × 1015 cm−3 was obtained according to the Eq. (3). According to the results above, plasma from AITO thin film could satisfy the McWhirter criterion, which was important to quantitative analysis for AITO thin film.
(1)
3.3. LIBS intensity ratio evolution of analytical elements in AITO thin films deposited at various sputtering powers
Iλ , gk Aki
and x is the excitation energy of the upper level (Ek). Iλ represents the measured line intensity. Aki is the transition probability(s−1), and gk (dimensionless) is the statistical weight of the upper level. The slope of the linear fitting equation is −1 , in which kB is the Boltzmann constant kB T (J K−1). Thus the calculated temperature of plasma was 5063 ± 150 K, which exhibited low plasma temperature under picosecond excitation for AITO thin film. The plasma temperature obtained in our experiment was relatively lower than that from common nanosecond excitation (> 7000 K with a pulse width of 5 ns [21] and > 10,000 K with a pulse width of 20 ns [22]), although the laser used had different excited wavelength and energy. The electron density (Ne) of plasma was calculated from
Ne = (Δλs /2ωref ) Nref
(3)
Since the concentration ratios of Al in AITO thin film were known to be important to control the optical band gap and carrier concentration [25], these ratios could be predicted by using LIBS intensity of each element. We calculated the spectral lines ratios of Al/(Al+In+Sn) by LIBS spectra produced from AITO thin films deposited at various sputtering powers. Fig. 6 showed that LIBS intensity ratio evolution of spectral line of analytical elements in different samples of AITO thin film. In Fig. 6, each data point represented the mean value of the 7 replicate measurements, and the error bars represented their associated standard deviation.
(2)
where, ωref is the electron collision parameter, and the value of ωref is taken from Ref. [23]. Δλs is the full width at half maximum (FWHM) of spectral line. In our work, Sn I 317.51 nm spectral line was selected to calculate the electron density. It was not a resonance line, and its shape was well fitted by a Lorentzian function, indicating that self-absorption was negligible, as shown in Fig. 5. The spectral data of this line were obtained from the Sn emission induced from the AITO thin film deposited with the sputtering power of 110 W. The determined electron density was about 4.6 × 1016 cm−3, and was of the order of 1016 cm−3 for AITO thin film using picosecond laser excitation, which was found slightly lower than the one corresponding to nanosecond laser excitation of ~1017 cm−3 with a pulse width of 20 ns [22]. For further quantitative analysis, the plasma temperature and electron density from the picosecond laser ablation plasma of AITO thin film were used to verify the McWhirter criterion [24]. The electron density had to be high enough to ensure that collision processes
Fig. 6. LIBS intensity ratio of analytical spectral lines and EDS atomic concentration ratio of AITO thin film deposited sputtering power (W). 4
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Table 2 EDS value of different AITO thin film samples. Sputtering power(W)
50 70 90 110
Atomic concentration (at. %) Al
In
Sn
Al/(Al+In+Sn)
4.26 4.07 3.70 3.61
23.93 28.67 28.95 29.67
7.47 3.96 3.65 3.30
0.1213 0.1109 0.1019 0.0987
In Fig. 6, the selected spectral line intensities of Al, In and Sn elements from AITO thin film (396.15 nm, 410.17 nm, and 317.51 nm, respectively) were used to calculate the ratios of Al/(Al+In+Sn). We could observe that the ratio was decreasing as the increase of sputtering power. In order to verify the accuracy of LIBS measurements, energy dispersive X-ray spectroscopy (EDS) was used to detect the composition of the AITO thin films. Table 2 listed the atomic concentration of Al, In and Sn in AITO thin films at different sputtering powers, as well as the atomic ratios of Al/(Al+In+Sn) of the samples. Moreover, we observed that the ratio evolutions of LIBS intensity and atomic concentration were consistent with the increase of sputtering power. Therefore, it was feasible and available of LIBS to obtain accurate measurements for AITO thin films. The ratio evolutions of intensity and atomic concentration of Al, In, Sn and Al/(Al+In+Sn) were caused directly by the changeable elemental composition of AITO thin films deposited at various sputtering powers, the possible reason of which was different mobility of the adatoms caused by the sputtered adatoms obtaining different kinetic energies at various RF sputtering powers, resulting in that the sputtered adatoms had different opportunities to collision with argon atom and loss their energy during the transformation process from target surface to substrate [26].
Fig. 8. The transmittance spectra of AITO thin films deposited at various sputtering powers measured by the UV-Vis-NIR spectrophotometer.
linear fitting coefficient R2of the calibration curve was up to be 0.995, which indicated the good agreement between the LIBS intensity ratio and the atomic concentration ratio. When AITO thin films were deposited at different sputtering powers, the element concentration ratio could be calculated rapidly by the calibration curve above with LIBS intensity ratio, which avoided tedious sample preparation and long detect time with EDS. Therefore, we could obtain rapidly the element concentration ratio of Al/(Al+In+Sn) in AITO thin film in order to achieve the rapid analysis.
3.5. The transmittance spectra of AITO thin films deposited at various sputtering powers
3.4. Quantitative analysis of elemental composition ratio in AITO thin films
The optical band gap of AITO thin film was tunable by Al concentration ratio in film changing. In order to obtain this optical band gap of AITO thin films deposited at various sputtering powers, the UVVIS-NIR spectrophotometer (UV-3600 Plus, SHIMADZU) was used to record the corresponding transmittance spectra of AITO thin films. The four AITO thin films were placed in the spectrophotometer testing system to obtain the corresponding transmittance spectra, as shown in Fig. 8. We could observe that the average transmittances of AITO thin films deposited at 50 W, 70 W, 90 W and 110 W were up to be > 80% under visible spectral range. In Fig. 8, a red shift in the absorption edge of the thin film transmission curve was higlighted as the increase of sputtering power, which was resulted from Al element concentration ratio decreasing in the total concentration ratio of Al+In+Sn in AITO thin film. The optical band gap (Eg) can be determined by extrapolating a linear plot of (hν2) versus photon energy curve to the intercept on horizontal photon energy axis [27], which was converted from the circular region of transmission curves shown in Fig. 8. According to this method, these determined Eg values of four AITO thin films deposited at various sputtering power (i.e. 50, 70, 90, and 110 W) were 3.87, 3.79, 3.71, and 3.66 eV, respectively. From the evolutions of LIBS target line intensity ratio and optical band gap of AITO thin films deposited at different sputtering powers above, we could observe that the evolutions were both decreasing with increasing of the sputtering power consistently. It indicated that the optical performance of AITO thin film could be exhibited by LIBS spectral line intensity, and the element emission intensity ratio of Al/ (Al+In+Sn) in AITO thin film by LIBS could determine the optical band gap. In the further work, we would determine the relationship between the electrical properties of AITO and LIBS analysis.
The optical band gap and carrier concentration of AITO thin film could be affected by the concentration ratio of Al/(Al+In+Sn) in the film. Therefore, in order to achieve the rapid quantitative analysis of elemental composition ratio in the films deposited at various sputtering powers, the calibration curve of spectral lines intensity ratios of Al/(Al +In+Sn) by LIBS was plotted according to the atomic concentration ratio of that, as shown in Fig. 7. Each data represented the mean value of 7 replicate measurements by LIBS intensity at the corresponding EDS value, and each error bars were their associated standard deviation. The
Fig. 7. The calibration curve of spectral lines intensity ratios of Al/(Al+In+Sn) by LIBS. 5
Spectrochimica Acta Part B 160 (2019) 105684
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4. Conclusions [7]
AITO thin film had a broad application prospects in the fields of transparent conductive film and ultraviolet electric parts. In our work, AITO thin film was deposited by RF magnetron sputtering at various sputtering powers. Picosecond LIBS was used to achieve the rapid micro-analysis of AITO thin film. Plasma characteristics from AITO thin film was exhibited, and the plasma temperature was 5063 ± 150 K and electron density was 4.6 × 1016 cm−3. The close relationship between the content ratio of Al/(Al+In+Sn) in AITO thin film and optical band gap of film was represented by LIBS spectral line intensity of each corresponding element. Calibration curve of Al/(Al+In+Sn) with LIBS intensity ratio was plotted according to EDS value, exhibiting good agreement. Moreover, the optical band gaps of AITO thin films deposited at various sputtering powers were calculated by the transmittance spectra, and the optical band gap was decreasing as the increase of the sputtering power. The evolutions of LIBS target line intensity ratio were consistent with optical band gap of AITO thin films. It indicated that the element emission intensity ratio of Al/(Al+In+Sn) in AITO thin film by LIBS could determine the optical band gap of AITO thin film. All results indicated that it was feasible and efficient to the rapid micro-analysis of AITO thin film by using LIBS based on picosecond laser pulses.
[8]
[9]
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[12]
[13]
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[16]
Acknowledgment
[17]
Financial supports from Shandong Provincial Natural Science Foundation (ZR2016AQ22), and the National Natural Science Foundation of China (11704228, 11404191) are highly acknowledged.
[18]
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Analytical note
Rapid micro-analysis of Al-In-Sn-O thin film using laser induced breakdown spectroscopy with picosecond laser pulses
T
⁎
Shiming Liu, Qing Gao, Junshan Xiu , Zhao Li, Yunyan Liu School of Physics and Optoelectronic Engineering, Shandong University of Technology, Shandong, Zibo 255049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Picosecond laser induced breakdown spectroscopy AITO thin film Magnetron sputtering Optical band gaps
Laser induced breakdown spectroscopy (LIBS) based on picosecond laser pulses was used to achieve rapid microanalysis of Al-In-Sn-O (AITO) thin films deposited by radio frequency (RF) magnetron sputtering at various sputtering powers. We calculated the plasma temperature of 5063 ± 150 K and electron density of 4.6 × 1016 cm−3 with single ablation crater diameter of 46 ± 1 μm. The content ratio of Al/(Al+In+Sn) in AITO thin film was closely related to the optical band gap of the film, and the ratio was exhibited by the spectral line intensity of LIBS for each corresponding element. The quantitative analysis of Al/(Al+In+Sn) with LIBS intensity ratio was achieved by plotting calibration curves according to the value of energy dispersive X-ray spectroscopy (EDS). It was found that the optical band gaps calculated were decreasing as the increase of the sputtering power in the exhibition of the transmittance spectra of AITO thin films deposited at various sputtering powers. In addition, we found the consistency of the evolutions of LIBS target line intensity ratio and optical band gap of AITO thin films. Therefore, the utilization of LIBS based on picosecond laser pulses is available and efficient in the rapid micro-analysis of AITO thin film.
1. Introduction Al-In-Sn-O (AITO) thin film is a transparent semiconductor material with a wider optical band gap comparing with known In-Sn-O (ITO) thin film, which is depending on content ratio of Al element in it [1]. It has great potential to be widely used as UV-transparent semiconductor film materials to transparent electronic devices and ultraviolet electric devices [2,3]. In order to obtain good optical performance of AITO thin film, a precise control and measurement of Al, In, and Sn constituting the AITO thin film is essential in the research and production of AITO electronic devices. At present, there are lots of analytical technologies used to determine the elements composition of thin film materials, such as X-ray fluorescence (XRF), glow discharge optical emission spectroscopy (GDOES), and energy dispersive X-ray spectroscopy (EDS). However, their disadvantages of exhibition stem from not only the equipment with high complexity and cost, but also the expertise and long time required in the preparation of reliable sample, which increases the analytical cost of the thin film and decelerates monitoring and measurement. Laser induced breakdown spectroscopy (LIBS) is a rapid developing technology, which is used widely in different fields with many advantages on analysis with high speed, no sample preparing, simultaneous multi-elements detection, and high sensitive measurement [4–9]. ⁎
Moreover, many investigations have been achieved for the detection of nanometre thin film with LIBS, which has been a feasible method for the analysis of nanometre thin film [10–15]. However, the studies primarily focus on the experimental optimization, such as laser wavelength, the quantitative analysis and plasma emission by LIBS. The relationship between the optical properties of AITO thin films and emission intensity ratios of elements in thin films hasn't been involved further. In our previous work, we had achieved a lot of work on the thin film analysis with LIBS, such as TiO2 [16], and Cu(In, Ga)Se2 thin film [17,18]. While these thin films were detected with LIBS based on nanosecond laser pulses, it was not possible to achieve the laser pulse ablation of single thin film layer with given setup. Thus, the LIBS spectra contained spectral lines from the thin film layer and the substrate, which disturbed the analytical element spectral lines emissions. It was the reason that the micro-analysis of thin films could not be achieved with ns laser pulses in our previous work. Furthermore, an available picosecond LIBS was first introduced to the micro-analysis of a new AITO thin film material. The AITO thin film was fabricated on soda-lime glass substrates by a single quaternary target. Sputtering power was one of important parameters for one-step radio frequency (RF) magnetron sputtering process. We obtained the plasma characteristics from AITO thin film with picosecond LIBS, and
Corresponding author. E-mail address: [email protected] (J. Xiu).
https://doi.org/10.1016/j.sab.2019.105684 Received 12 May 2019; Received in revised form 16 August 2019; Accepted 16 August 2019 Available online 19 August 2019 0584-8547/ © 2019 Elsevier B.V. All rights reserved.
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sample surface with a shift of about 0.5 mm. The initial plasma emissions were collected at 45-degree angle over the plasma by a combination of two lenses with focal lengths of 35 mm (L2) and 16 mm (L3). The whole plasma could be detected due to the smaller plasma plume induced by a ps laser pulse with a laser pulse energy of 122 μJ and a ablation carter of about 46 ± 1 μm. The output of fiber was connected to the entrance of a portable optical fiber spectrometer (AvaSpecMini2048CL-SOT8, Avantes Technology) in a spectral bandwidth from 240 nm to 415 nm with a spectral resolution of 0.01 nm. In the experiment, the 3-D displacement stage and spectrometer were triggered synchronously by a photodiode (PD shown in Fig. 1). The integration time of the spectrometer was set as 500 ms, and the horizontal moving speed and single moving distance of displacement stage were set as 25 mm/s and 12.5 mm respectively. Note that 250 laser pulses were focused uniformly on the surface of AITO thin film layer with a distance of 12.5 mm. All experimental conditions were kept constant for all AITO thin films samples. Moreover, in order to obtain higher spectral intensity in low plasma temperature (about 5063 K shown in Section 3.2), the detection delay time of plasma emissions was set as the minimum, and the plasma emissions induced from 250 laser pulses were accumulated once under the detection width of 500 ms with the spectrometer. Fig. 2 showed the photomicrographs of ablation craters with amplification factor of 1000 (a) and 40 (b) with a metallographic microscope (4XB, Shanghai CSOIF Co., Ltd.). In Fig. 2a, we could see that the diameter of the single ablation crater in our experiment was measured to be 46 ± 1 μm. It indicated that ps laser pulses used in our experiment could provide micro-ablation with a small ablation crater and achieve the micro analysis for AITO thin film. Fig. 2b showed the successive single pulse ablation on the surface of AITO thin film. We could observe that the ablation craters were roughly uniform in size with good ablation repeatability. In the process of laser ablation, the possible thermal effect of laser ablation could be observed around the ablation crater of the sample of AITO thin film (the red parts in Fig. 2b).
evaluated the feasibility of the micro-analysis with ps LIBS. The content ratio of analytical elements in AITO thin film was determined by the quantitative analysis, and the relationship between optical band gap of AITO thin film and the LIBS analysis was discussed. At last, we evalated the feasibility and availability of picosecond LIBS to fabricate high performance AITO thin film by the one-step radio frequency (RF) magnetron sputtering method. 2. Experimental 2.1. AITO thin film sampling AITO thin films were deposited at room temperature, using RF magnetron sputtering from an indium oxide target (purity, 99.99 wt%, Zhongnuo New Material, China) with a size of 75 mm × 5 mm which was doped with tin oxide and aluminum oxide. And the content ratio of Al, In and Sn in the indium oxide target was 1:7:2 at.%. Normal sodalime glasses (SLG), as substrates, were put into the vacuum chamber. The distance from target to substrate was 150 mm, and the flow rate of the sputtering medium Ar (purity, 99.99 wt%) was 20 sccm, with the 3.0 Pa work pressure. In our work, a series of AITO thin films samples were deposited at different sputtering work powers varied from 50 W to 110 W. The pre-sputtering process was finished before the deposition, the time of which was half an hour, and the film thickness was about 400 nm measured with a film thickness gauge. 2.2. Experimental setup A LIBS setup with picosecond laser pulses was used in this experiment as shown in Fig. 1. The laser pulse was produced by a picosecond microchip laser (RealLight) operating at 1064 nm with a repetition rate of 500 Hz, pulse duration of 350 ps, and pulse energy of 122 μJ. The picosecond laser pulses were focused on the surface of AITO thin film by a combined lens (L1) with beam expanding and beam focusing (focal length of 35 mm). The diameter of the single ablation crater was estimated to be 46 ± 1 μm as shown in Fig. 2a, which corresponded to the mean fluence of about 7.3 J/cm2 with a Gaussian profile of laser pulse delivered to the sample. The AITO thin film sample was placed on a motoring 3-D displacement stages. The distance between lens L1 and film surface was set according to the calcium emissions (Ca I 393.36 nm and 396.85 nm [19]) from the glass substrate under the AITO thin film. In order to avoid the direct ablation of the glass substrate under the AITO thin film, the focus point of the laser beam was set over the
3. Results and discussion 3.1. LIBS spectra and spectral lines selection Fig. 3 shows LIBS spectra of an AITO thin film with a sputtering power of 110 W and glass substrate ranging from 240 nm to 415 nm with an accumulation of 250 laser shots in our experiment. It was obvious to observe several spectral lines of aluminum, indium and tin
Fig. 1. LIBS setup for AITO thin film analysis in ambient pressure. 2
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Fig. 2. Photomicrographs of ablation craters with amplification factor of 100 × of objective lens and 10 × of eyepiece (a) and 40× of objective lens (b) for AITO thin film with sputtering power of 110 W. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
elements in AITO thin film. All of signal-to-background ratios of the spectral lines were > 3:1, and the Ca, Mg and Si elements emissions from glass substrate were not observed in this spectrum of the AITO thin film, compared with the spectrum of glass substrate shown in Fig. 3. This indicated that the AITO thin film layer was only ablated and the glass substrate was not damaged. It demonstrated that the distance was suitable between lens L1 and the surface of AITO thin film samples. In order to enhance micro-analysis performance of AITO thin film, it was important to select suitable spectral lines of three elements. In our interesting spectral region from 240 nm to 415 nm, 10 Sn I lines, 4 In I lines, 2 In II lines and 4 Al I lines were observed, as listed in Table 1, in which the wavelengths of each element represented the peak positions shown in Fig. 3, while the small deviation between the experimental values of the wavelength and theoretical values from the NIST database [19] might be produced because of plasma emission fluctuations and the grating response of the spectrometer. Significant overlaps between these spectral lines, such as In I 325.61 nm and Sn I 326.23 nm shown in Fig. 3, could be observed in the spectrum. A suitable emission line was thus selected for each of the three
Table 1 List of spectral lines observed of AITO thin film. Element
Number of lines
Wavelength (nm) at the peak positions
Sn I
10
In I In II Al I
4 2 4
242.16, 317.51, 275.38, 242.85, 308.21,
270.65, 333.06, 303.93, 289.02 309.27,
277.98, 283.99, 286.33, 300.91, 380.10, 326.23 325.61, 410.17 394.40, 396.15
metals analyzed in AITO thin film, and the lines were first relatively intense for the corresponding elements in the given conditions of ablation and detection, which were chosen to be as little interference as possible with other spectral features. Another important criterion was that the line would not be affected by self-absorption. Thus, the line shape was first inspected to avoid strong self-absorption and self-reversing, such as Sn I 317.51 nm shown in Fig. 5. Finally, the selected lines for Al, In, and Sn from AITO thin film were set as 396.15 nm,
Fig. 3. LIBS spectra of AITO thin film with sputtering power of 110 W and glass substrate. 3
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Fig. 5. Lorentz fitting plot of Sn I 317.51 nm for calculating the electron density.
Fig. 4. Boltzmann plot obtained from Sn I spectral lines of Table 1.
410.17 nm, and 317.51 nm, respectively.
dominate radiative processes. The lowest value of the electron density of the plasma could be calculated by using the McWhirter criterion
3.2. Plasma characteristics of AITO thin film using picosecond LIBS
Ne (cm−3) ≥ 1.6 × 1012T1/2ΔE 3
The plasma temperature (T) and electron density (Ne) are two important parameters, which can describe the plasma state in order to understand directly laser – material interaction and plasma characteristics. The excitation temperature of the plasma plume is in general retrieved using the well known Boltzmann plot method [20]. In our present work, the excitation temperature was calculated by Sn I spectral lines listed in Table 1, and the Sn emissions were induced from the AITO thin film deposited with the sputtering power of 110 W. Fig. 4 showed the Boltzmann plot. The linear fitting equation was:
y = −0.849 − 2.29x According to the Boltzmann equation [20], y is the value of ln
T was the plasma temperature in K and ΔE was the energy gap in eV between upper and lower level of the spectral line. In our work the energy gap ΔE of Sn I 317.50 nm was 3.91 eV and plasma temperature T = 5063 K was considered, the minimal electron density value of Ne = 6.805 × 1015 cm−3 was obtained according to the Eq. (3). According to the results above, plasma from AITO thin film could satisfy the McWhirter criterion, which was important to quantitative analysis for AITO thin film.
(1)
3.3. LIBS intensity ratio evolution of analytical elements in AITO thin films deposited at various sputtering powers
Iλ , gk Aki
and x is the excitation energy of the upper level (Ek). Iλ represents the measured line intensity. Aki is the transition probability(s−1), and gk (dimensionless) is the statistical weight of the upper level. The slope of the linear fitting equation is −1 , in which kB is the Boltzmann constant kB T (J K−1). Thus the calculated temperature of plasma was 5063 ± 150 K, which exhibited low plasma temperature under picosecond excitation for AITO thin film. The plasma temperature obtained in our experiment was relatively lower than that from common nanosecond excitation (> 7000 K with a pulse width of 5 ns [21] and > 10,000 K with a pulse width of 20 ns [22]), although the laser used had different excited wavelength and energy. The electron density (Ne) of plasma was calculated from
Ne = (Δλs /2ωref ) Nref
(3)
Since the concentration ratios of Al in AITO thin film were known to be important to control the optical band gap and carrier concentration [25], these ratios could be predicted by using LIBS intensity of each element. We calculated the spectral lines ratios of Al/(Al+In+Sn) by LIBS spectra produced from AITO thin films deposited at various sputtering powers. Fig. 6 showed that LIBS intensity ratio evolution of spectral line of analytical elements in different samples of AITO thin film. In Fig. 6, each data point represented the mean value of the 7 replicate measurements, and the error bars represented their associated standard deviation.
(2)
where, ωref is the electron collision parameter, and the value of ωref is taken from Ref. [23]. Δλs is the full width at half maximum (FWHM) of spectral line. In our work, Sn I 317.51 nm spectral line was selected to calculate the electron density. It was not a resonance line, and its shape was well fitted by a Lorentzian function, indicating that self-absorption was negligible, as shown in Fig. 5. The spectral data of this line were obtained from the Sn emission induced from the AITO thin film deposited with the sputtering power of 110 W. The determined electron density was about 4.6 × 1016 cm−3, and was of the order of 1016 cm−3 for AITO thin film using picosecond laser excitation, which was found slightly lower than the one corresponding to nanosecond laser excitation of ~1017 cm−3 with a pulse width of 20 ns [22]. For further quantitative analysis, the plasma temperature and electron density from the picosecond laser ablation plasma of AITO thin film were used to verify the McWhirter criterion [24]. The electron density had to be high enough to ensure that collision processes
Fig. 6. LIBS intensity ratio of analytical spectral lines and EDS atomic concentration ratio of AITO thin film deposited sputtering power (W). 4
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Table 2 EDS value of different AITO thin film samples. Sputtering power(W)
50 70 90 110
Atomic concentration (at. %) Al
In
Sn
Al/(Al+In+Sn)
4.26 4.07 3.70 3.61
23.93 28.67 28.95 29.67
7.47 3.96 3.65 3.30
0.1213 0.1109 0.1019 0.0987
In Fig. 6, the selected spectral line intensities of Al, In and Sn elements from AITO thin film (396.15 nm, 410.17 nm, and 317.51 nm, respectively) were used to calculate the ratios of Al/(Al+In+Sn). We could observe that the ratio was decreasing as the increase of sputtering power. In order to verify the accuracy of LIBS measurements, energy dispersive X-ray spectroscopy (EDS) was used to detect the composition of the AITO thin films. Table 2 listed the atomic concentration of Al, In and Sn in AITO thin films at different sputtering powers, as well as the atomic ratios of Al/(Al+In+Sn) of the samples. Moreover, we observed that the ratio evolutions of LIBS intensity and atomic concentration were consistent with the increase of sputtering power. Therefore, it was feasible and available of LIBS to obtain accurate measurements for AITO thin films. The ratio evolutions of intensity and atomic concentration of Al, In, Sn and Al/(Al+In+Sn) were caused directly by the changeable elemental composition of AITO thin films deposited at various sputtering powers, the possible reason of which was different mobility of the adatoms caused by the sputtered adatoms obtaining different kinetic energies at various RF sputtering powers, resulting in that the sputtered adatoms had different opportunities to collision with argon atom and loss their energy during the transformation process from target surface to substrate [26].
Fig. 8. The transmittance spectra of AITO thin films deposited at various sputtering powers measured by the UV-Vis-NIR spectrophotometer.
linear fitting coefficient R2of the calibration curve was up to be 0.995, which indicated the good agreement between the LIBS intensity ratio and the atomic concentration ratio. When AITO thin films were deposited at different sputtering powers, the element concentration ratio could be calculated rapidly by the calibration curve above with LIBS intensity ratio, which avoided tedious sample preparation and long detect time with EDS. Therefore, we could obtain rapidly the element concentration ratio of Al/(Al+In+Sn) in AITO thin film in order to achieve the rapid analysis.
3.5. The transmittance spectra of AITO thin films deposited at various sputtering powers
3.4. Quantitative analysis of elemental composition ratio in AITO thin films
The optical band gap of AITO thin film was tunable by Al concentration ratio in film changing. In order to obtain this optical band gap of AITO thin films deposited at various sputtering powers, the UVVIS-NIR spectrophotometer (UV-3600 Plus, SHIMADZU) was used to record the corresponding transmittance spectra of AITO thin films. The four AITO thin films were placed in the spectrophotometer testing system to obtain the corresponding transmittance spectra, as shown in Fig. 8. We could observe that the average transmittances of AITO thin films deposited at 50 W, 70 W, 90 W and 110 W were up to be > 80% under visible spectral range. In Fig. 8, a red shift in the absorption edge of the thin film transmission curve was higlighted as the increase of sputtering power, which was resulted from Al element concentration ratio decreasing in the total concentration ratio of Al+In+Sn in AITO thin film. The optical band gap (Eg) can be determined by extrapolating a linear plot of (hν2) versus photon energy curve to the intercept on horizontal photon energy axis [27], which was converted from the circular region of transmission curves shown in Fig. 8. According to this method, these determined Eg values of four AITO thin films deposited at various sputtering power (i.e. 50, 70, 90, and 110 W) were 3.87, 3.79, 3.71, and 3.66 eV, respectively. From the evolutions of LIBS target line intensity ratio and optical band gap of AITO thin films deposited at different sputtering powers above, we could observe that the evolutions were both decreasing with increasing of the sputtering power consistently. It indicated that the optical performance of AITO thin film could be exhibited by LIBS spectral line intensity, and the element emission intensity ratio of Al/ (Al+In+Sn) in AITO thin film by LIBS could determine the optical band gap. In the further work, we would determine the relationship between the electrical properties of AITO and LIBS analysis.
The optical band gap and carrier concentration of AITO thin film could be affected by the concentration ratio of Al/(Al+In+Sn) in the film. Therefore, in order to achieve the rapid quantitative analysis of elemental composition ratio in the films deposited at various sputtering powers, the calibration curve of spectral lines intensity ratios of Al/(Al +In+Sn) by LIBS was plotted according to the atomic concentration ratio of that, as shown in Fig. 7. Each data represented the mean value of 7 replicate measurements by LIBS intensity at the corresponding EDS value, and each error bars were their associated standard deviation. The
Fig. 7. The calibration curve of spectral lines intensity ratios of Al/(Al+In+Sn) by LIBS. 5
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4. Conclusions [7]
AITO thin film had a broad application prospects in the fields of transparent conductive film and ultraviolet electric parts. In our work, AITO thin film was deposited by RF magnetron sputtering at various sputtering powers. Picosecond LIBS was used to achieve the rapid micro-analysis of AITO thin film. Plasma characteristics from AITO thin film was exhibited, and the plasma temperature was 5063 ± 150 K and electron density was 4.6 × 1016 cm−3. The close relationship between the content ratio of Al/(Al+In+Sn) in AITO thin film and optical band gap of film was represented by LIBS spectral line intensity of each corresponding element. Calibration curve of Al/(Al+In+Sn) with LIBS intensity ratio was plotted according to EDS value, exhibiting good agreement. Moreover, the optical band gaps of AITO thin films deposited at various sputtering powers were calculated by the transmittance spectra, and the optical band gap was decreasing as the increase of the sputtering power. The evolutions of LIBS target line intensity ratio were consistent with optical band gap of AITO thin films. It indicated that the element emission intensity ratio of Al/(Al+In+Sn) in AITO thin film by LIBS could determine the optical band gap of AITO thin film. All results indicated that it was feasible and efficient to the rapid micro-analysis of AITO thin film by using LIBS based on picosecond laser pulses.
[8]
[9]
[10] [11]
[12]
[13]
[14] [15]
[16]
Acknowledgment
[17]
Financial supports from Shandong Provincial Natural Science Foundation (ZR2016AQ22), and the National Natural Science Foundation of China (11704228, 11404191) are highly acknowledged.
[18]
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