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Impurity detection in solid and molten silicon by laser induced breakdown spectroscopy
Spectrochimica Acta Part B 74–75 (2012) 115–118
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Impurity detection in solid and molten silicon by laser induced breakdown spectroscopy Sarah Darwiche a, c,⁎, Rafik Benrabbah a, c, Malek Benmansour d, Daniel Morvan a, b a
Laboratoire de Génie des Procédés Plasmas et Traitement de Surfaces (LGPPTS)-EA3492, 11, rue Pierre et Marie Curie 75005 Paris, France ENSCP, Chimie ParisTech, France UPMC Université Pierre et Marie Curie, France d CEA-DRT-LITEN-DTS-LMPS, Savoie Technolac, BP332 – 73377 Le Bourget Du Lac, France b c
a r t i c l e
i n f o
Article history: Received 15 December 2011 Accepted 18 June 2012 Available online 29 June 2012 Keywords: Boron Silicon LIBS Molten
a b s t r a c t The application of Laser Induced Breakdown Spectroscopy (LIBS) for the analysis of both solid and molten silicon has been developed. This technique provides fast and reliable chemical characterization of silicon. This work will present the investigation of experimental parameters such as buffering gas nature and pressure in order to find the most suitable conditions to quantify boron in solid silicon. These results show that the signal to background ratio (SBR) is improved by both the use of helium and argon instead of air and by reducing the pressure to 500 mbar. Using calibrated samples, calibration curves were prepared for boron and limits of detection of the order of 0.2 ppm were obtained working at a distance of 50 cm from the sample. Additionally, the capabilities of LIBS to analyze molten silicon (1410 °C) was demonstrated, opening the way for LIBS to be used as a process analytical technique. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The photovoltaic silicon industry has need for in-situ diagnostic techniques. At present, silicon is analyzed at the end of the process of growth and crystallization; this analysis process takes several days. Material not conforming to photovoltaic grade criteria is recycled (remelted), leading to the loss of time, material, and energy. It is here proposed that LIBS can be a useful diagnostic tool as it may be used for the analysis of silicon in either solid or liquid phase. LIBS has already been used on molten materials, notably in the steel industry [1–4]. An increasing number of papers have been published in the field of LIBS. This technique, based on atomic emission spectroscopy, presents many advantages compared to classical techniques: it can be used for any sample, solid, liquid or gaseous without any previous preparation, and each spectral analysis takes only a very short time of a few μs. This technique has been used for different silicon materials and for the detection of impurities (aluminum, carbon, phosphorous) [5–7] and in previous work for boron detection [8]. Recent development of LIBS has been made for real time analysis. E. Baril et al. [9] paved the way for a rapid identification and treatment of data from multiple species, using a patented approach [10], in their Zinc bath technique. J. Gruber et al. [11] performed an in-situ real time monitoring of Cr, Cu, Mn and Ni content in molten steel using LIBS. K. Rai et al. [12] also demonstrated the use of LIBS ⁎ Corresponding author at: Laboratoire de Génie des Procédés Plasmas et Traitement de Surfaces (LGPPTS)-EA3492, 11, rue Pierre et Marie Curie 75005 Paris, France. E-mail address: [email protected] (S. Darwiche). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.06.020
for the analysis of the elemental composition of molten aluminum alloy. Further research exists in the literature on LIBS for use on mineral ores, effluents, molten metals and/or glass, etc [13–18]. In this work, LIBS was used to analyze impurities in solid and molten silicon. The effect of several experimental parameters on the signal to background ratio (SBR) was studied in order to find the most suitable parameters (laser energy, pressure, buffering gas) for boron analysis. From a feasibility perspective, LIBS was also applied to analyze molten silicon to investigate the plausibility of a technology transfer for online measurement during industrial silicon production. 2. Experimental setup The device used in this study consisted of a nanosecond Nd:YAG laser (Brio type – Quantel France) operating at the fourth harmonic of 266 nm, with 4 ns pulse duration and a repetition rate of 20 Hz, and a maximum energy of 10 mJ per pulse (spot diameter 100 μm, fluence 127 J/cm 2). The laser was focused on the sample through a UV-Fused silica (Spectrosil 2000) window. The emitted light was transmitted to the spectrometer via a 19 fiber optical bundle. The emission collection mode was made to be collinear with the laser. The plasma emission was imaged onto the entrance slit of a Czerny– Turner spectrometer with a 750 mm focal length (Acton SpectraPro 2750) equipped with three gratings (1200 grooves.mm −1, 300 nm blaze, 2400 and 3600 grooves.mm −1, 240 nm blaze), with a reciprocal linear dispersion of 1.02 nm.mm −1. The detector was a CCD camera (1024 × 1024 pixels) with an intensifier system (Princeton instruments PIMAX). The width of the entrance slit of the
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Fig. 3. Effect of laser energy on boron SBR in different buffer gases; delay time 500 ns, gate width 3400 ns. Fig. 1. Experimental setup for solid silicon analysis.
spectrometer was fixed at 50 μm. Signal-to-background ratio (SBR) was used to represent spectral data in order to reduce the effect of background noise due to the matrix effect; in addition, in some cases, the SBR of 244.34 nm silicon emission line was used as an internal standard The signal emission is acquired by averaging 10 laser shots to compensate for laser beam energy fluctuations. The spectrum acquisition was typically made with a delay time of between 100–1000 ns and gate width of 500–4000 ns and will be indicated in captions for each case; the determination of these parameters is discussed in more detail in a previous publication [8]. A 3000 cm 3 analysis chamber with quartz windows was used in order to be able to change the nature of the buffering gas (air, argon, helium), and for pressure control. The pressure was controlled by using a two stage oil sealed rotary vane pump (Edwards E2M1.5). The pressure was measured with a standard vacuum gauge which was not changed for different gases. The analysis chamber was placed on a motorized X–Y–Z table for positioning (Fig. 1). For the experiments using molten silicon, a Flexitune induction generator delivering a maximum of 10 kW was used to melt the solid silicon in a graphite crucible under an argon atmosphere. A concentric tube cooling jacket with flowing water was used to maintain a moderate temperature in the system (Fig. 2). Melting of the granular silicon starts at 8 kW when the bath reaches 1410 °C. To accommodate this experimental setup, the distance from the laser to the sample was increased from 11 to 50 cm in order to avoid damage by projectile liquid silicon.
3. Results In previous work, the effect of the buffering gas composition and pressure on the boron SBR was studied using a Nd:YAG laser operating at 1064 nm with the maximum energy of 95 mJ. An improvement of the boron SBR was observed with helium atmosphere working at a reduced pressure of 60 mbar [8]. A similar study will be done in this work using the shorter wavelength laser (266 nm) and by varying different parameters; in addition, measurements will be made on molten silicon with controlled impurity concentrations. 3.1. Effect of laser energy and buffering gas Laser energy was varied between 4 and 10 mJ and LIBS measurements have been made in different atmospheres (air, argon and helium). It is important to note that with the current laser (266 nm), unlike the 1064 nm laser, boron emission lines have been detected because of the decreasing of the breakdown threshold value. Fig. 3 shows the boron signal to background ratio (SBR) as a function of laser energy with three different atmospheres of air, argon, and helium used for the buffering gas in the sample compartment. The corresponding spectral aquisition parameters are shown in Table 1. It was observed that the SBR increases with increasing laser pulse energy, regardless the atmosphere used in the analysis. However, it appears that the signal quality obtained in an atmosphere of rare gas is better than that for air. The best signal is obtained under an atmosphere of rare gas (argon or helium) with laser pulse energy of 10 mJ. Therefore, for the experiments to be discussed further on in this work, the buffering gas will be either argon or helium, and the maximum laser energy will be used. 3.2. Effect of gas pressure Fig. 4 shows the variation of boron SBR with gas pressure. This analysis was made using the optimized parameters as discussed above, meaning an atmosphere of argon and the higher laser energy value of 10 mJ per pulse. Two trends are observed: a better SBR was obtained for intermediate pressures of either 100 mbar or 500 mbar. Table 1 Summary of temporal parameters for different gases and laser energies.
Fig. 2. Experimental setup used to melt silicon.
Gas
Air
Energy (mJ)
Delay (ns)
Gate (ns)
Delay (ns)
Argon Gate (ns)
Delay (ns)
Helium Gate (ns)
10 8 6 4
700 400 600 100
1200 1000 1200 600
500 400 400 250
3400 3000 1600 1000
400 300 250 100
1800 1600 1200 800
S. Darwiche et al. / Spectrochimica Acta Part B 74–75 (2012) 115–118
Fig. 4. Effect of gas pressure on boron SBR; delay time 400 ns, gate width 1000 ns.
Decreasing the pressure to 1–10 mbar was not observed to be beneficial for the boron SBR. This improvement can be explained by the fact that at intermediate pressure, an increase in the mean free path of the plasma species in an argon atmosphere reduces collisions and recombination of the excited species. This results in an extension of the lifetime of the excited species emissions. At lower pressures, the plasma goes into a non-thermal regime where the excitation of heavy species is less pronounced, which explains the decrease in emissions.
3.3. Boron calibration curve Fig. 5 shows the calibration curve which was made for boron in silicon using standard samples in the range of 1 to 100 ppm determined by resistivity measurements and certified by the company Siltronix ®. For each measurement, 200 shots were performed with 2400 grooves/mm grating centered at 247 nm, enabling the detection of the three emission lines of silicon around 244–245 nm and boron at 249.77 nm. The ratio of the boron to silicon SBR is plotted, using the silicon line as an internal standard which helps to account for variations in material ablation. The boron to silicon SBR ratio at exactly zero boron concentration was not available, but in principle should be a value between zero and unity. The value of boron SBR at 1 ppm, which indicated a strong non-linearity for low boron concentration, was used to calculate a limit of detection for boron in solid silicon of about 0.2 ppm, defined as a boron peak with a height 3 times the value of the standard deviation of the background noise.
Fig. 5. Calibration curve for boron in silicon prepared from standardized samples; delay time 500 ns, gate width 3400 ns. Pure argon, 500 mbar, pulse energy = 10 mJ.
117
Fig. 6. Spectrum showing carbon and boron emission lines for molten silicon; delay time 1000 ns, gate width 4000 ns.
3.4. Analysis of molten silicon by LIBS Once the optimized parameters had been determined, a feasibility study was made to investigate the possibility of the use of LIBS on silicon in the liquid phase with the view of process control applications. Measurements were made on molten silicon which was prepared by melting granular silicon in a graphite crucible heated by an RF induction generator at 8 kW under argon atmosphere. In a recent study, it was shown that for a temperature change of 25° to 70° above the melting point of a given metal, the LIBS intensity of a given emission line changes linearly. Control of the temperature of the molten silicon is therefore necessary, and is now under investigation [19]. In order to test the response to different impurity levels, two types of silicon, with known compositions of 1.6 and 210 ppm of aluminum, were analyzed. The first sample also contained 6 ppm of boron. Figs. 6–8 show spectra which were obtained by using LIBS on molten silicon with different levels of impurities as indicated in the captions. As can be seen in Fig. 6, boron and carbon are detected. Notably, it is here demonstrated that boron may be detected in molten silicon at a concentration of 6 ppm. By comparing Figs. 7 and 8, showing the signal of aluminum emission lines, the difference in signal intensity between 1 and 200 ppm Al can be observed. The ability to analyze the composition of molten silicon could be very useful for the online process control of various production methods for the industrial production of silicon.
Fig. 7. Spectrum showing aluminum emission lines at 1.6 ppm for molten silicon; delay time 1000 ns, gate width 4000 ns.
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S. Darwiche et al. / Spectrochimica Acta Part B 74–75 (2012) 115–118
Fig. 8. Spectrum showing aluminum emission lines at 210 ppm for molten silicon; delay time 1000 ns, gate width 4000 ns.
4. Conclusion It has been demonstrated that the detection of boron in silicon can be improved by the use of either argon and/or helium at pressures in the range of 100 mbar– 500 mbar. By comparing these results to the boron SBR obtained with the 1064 nm laser [8], it can be observed that the use of the 266 nm Nd:YAG laser doesn't provide any notable improvement to the boron SBR. A calibration curve was made for boron in silicon using standard samples which indicates a limit of detection as low as 0.2 ppm. The use of LIBS was demonstrated on molten silicon, showing that aluminum, calcium, carbon and boron may all be detected. Boron, a very important element for photovoltaic cells, was detected at a level of 6 ppm. At this stage, the analysis of the molten silicon is only qualitative, and so additional work should be done to reach quantitative and reliable performance of the technology in order to facilitate its introduction into industrial use. To simplify the quantification of molten silicon, standard silicon samples (prepared by mixing pure granular silicon with precise amounts of boron and/or aluminum powder) shall be prepared and further experiments will be done in order to quantify the impurities contained in the analyzed silicon, and to further refine the technology for use on both solid and molten silicon. References [1] S. Palanco, S. Conesa, J.J. Laserna, Analytical control of liquid steel in an induction melting furnace using a reme laser-induced plasma spectrometer, J. Anal. At. Spectrom. 19 (2004) 462–467.
[2] C. Aragon, J.A. Aguilera, J. Campos, Determination of carbon content in molten steel using laser-induced breakdown spectroscopy, Appl. Spectrosc. 47 (1993) 606–608. [3] R. Noll, H. Bette, A. Brysch, M. Kraushaar, I. Mönch, L. Peter, V. Sturm, Laser-induced breakdown spectrometry- applications for production control and quality assurance in the steel industry, Spectrochim. Acta Part B (2001) 637–649. [4] F. Boué-Bigne, Laser-induced breakdown spectroscopy applications in the steel industry: Rapid analysis of segregation and decarburization, Spectrochim. Acta Part B 63 (2008) 1122–1129. [5] D. Romero, J.J. Laserna, Surface and tomographic distribution of carbon impurities in photonic-grade silicon using laser-induced breakdown spectrometry, J. Anal. At. Spectrom. 13 (1998) 557–560. [6] M. Milaan, P. Lucena, L.M. Cabalian, J.J. Laserna, Depth profiling of phosphorus in photonic-grade silicon using laser-induced breakdown spectrometry, Appl. Spectrosc. 52 (1998) 444–448. [7] D. Romero, J.M. Fernandez Romero, J.J. Laserna, Distribution of metal impurities in silicon wafers using imaging-mode multi-elemental laser-induced breakdown spectrometry, J. Anal. At. Spectrom. 14 (1999) 199–204. [8] S. Darwiche, M. Benmansour, N. Eliezer, D. Morvan, Investigation of optimized experimental parameters including laser wavelength for boron measurement in photovoltaic grade silicon using laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 65 (2010) 738–743. [9] E. Baril, L. St-Onge, M. Sabsabi, J. Lucas, Novel Method for On-line Chemical Analysis of Continuous Galvanizing Baths, In: Galvatech '04, Chicago (IL), April 4-7 2004, published in Galvatech'04 Conf. Proceedings Association for Iron & Steel Technology, 2004, pp. 1095–1104. [10] M. Sabsabi, R. Héon, J. Lucas, Method and apparatus for in-process liquid analysis by laser induced plasma Spectroscopy, U.S. Pat. No. 6,700,660 issued Sept. 2003. [11] J. Gruber, J. Heitz, H. Strasser, D. Bauerle, N. Ramaseder, Rapid in-situ analysis of liquid steel by laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 56 (2001) 685–693. [12] A.K. Rai, F.Y. Yueh, J.P. Singh, D.K. Rai, In: in: Jagdish P. Singh, Surya N. Thakur (Eds.), Laser-Induced Breakdown Spectroscopy of Solid and Molten Material, 255, 2007. [13] L. St-Onge, E. Kwong, M. Sabsabi, E.B. Vadas, Rapid analysis of liquid formulations containing sodium chloride using laser-induced breakdown spectroscopy, J. Pharm. Biomed. Anal. 36 (2004) 277–284. [14] L. Barette, S. Turmel, On-line iron-ore slurry monitoring for real-time process control of pellet making processes using laser-induced breakdown spectroscopy: graphitic vs. total carbon detection, Spectrochim. Acta Part B 56 (2001) 715–723. [15] N.K. Rai, A.K. Rai, A. Kumar, S. Thakur, Detection sensitivity of laser-induced breakdown spectroscopy for Cr II in liquid samples, Appl. Opt. 47 (2008) G105–G111. [16] A. Kaminska, M. Sawczak, K. Komar, G. Śliwiński, Application of the laser ablation for conservation of historical paper documents, Appl. Surf. Sci. 253 (2007) 7860–7864. [17] X.K. Shen, H. Wang, Z.Q. Xie, Y. Gao, H. Ling, Y.F. Lu, Detection of trace phosphorus in steel using laser-induced breakdown spectroscopy combined with laser-induced fluorescence, Appl. Opt. 48 (2009) 2551–2558. [18] A.-M. Matiaske, I.B. Gornushkin, U. Panne, Double-pulse laser-induced breakdown spectroscopy for analysis of molten glass, Anal. Bioanal. Chem. 402 (2012) 2597–2606. [19] T. Vaculovic, Z. Zverina, V. Otruba, V. Kanicky, Laser-induced breakdown spectroscopy of molten: influence of sample temperature, Z. Naturforsch. 66a (2011) 643–648.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Impurity detection in solid and molten silicon by laser induced breakdown spectroscopy Sarah Darwiche a, c,⁎, Rafik Benrabbah a, c, Malek Benmansour d, Daniel Morvan a, b a
Laboratoire de Génie des Procédés Plasmas et Traitement de Surfaces (LGPPTS)-EA3492, 11, rue Pierre et Marie Curie 75005 Paris, France ENSCP, Chimie ParisTech, France UPMC Université Pierre et Marie Curie, France d CEA-DRT-LITEN-DTS-LMPS, Savoie Technolac, BP332 – 73377 Le Bourget Du Lac, France b c
a r t i c l e
i n f o
Article history: Received 15 December 2011 Accepted 18 June 2012 Available online 29 June 2012 Keywords: Boron Silicon LIBS Molten
a b s t r a c t The application of Laser Induced Breakdown Spectroscopy (LIBS) for the analysis of both solid and molten silicon has been developed. This technique provides fast and reliable chemical characterization of silicon. This work will present the investigation of experimental parameters such as buffering gas nature and pressure in order to find the most suitable conditions to quantify boron in solid silicon. These results show that the signal to background ratio (SBR) is improved by both the use of helium and argon instead of air and by reducing the pressure to 500 mbar. Using calibrated samples, calibration curves were prepared for boron and limits of detection of the order of 0.2 ppm were obtained working at a distance of 50 cm from the sample. Additionally, the capabilities of LIBS to analyze molten silicon (1410 °C) was demonstrated, opening the way for LIBS to be used as a process analytical technique. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The photovoltaic silicon industry has need for in-situ diagnostic techniques. At present, silicon is analyzed at the end of the process of growth and crystallization; this analysis process takes several days. Material not conforming to photovoltaic grade criteria is recycled (remelted), leading to the loss of time, material, and energy. It is here proposed that LIBS can be a useful diagnostic tool as it may be used for the analysis of silicon in either solid or liquid phase. LIBS has already been used on molten materials, notably in the steel industry [1–4]. An increasing number of papers have been published in the field of LIBS. This technique, based on atomic emission spectroscopy, presents many advantages compared to classical techniques: it can be used for any sample, solid, liquid or gaseous without any previous preparation, and each spectral analysis takes only a very short time of a few μs. This technique has been used for different silicon materials and for the detection of impurities (aluminum, carbon, phosphorous) [5–7] and in previous work for boron detection [8]. Recent development of LIBS has been made for real time analysis. E. Baril et al. [9] paved the way for a rapid identification and treatment of data from multiple species, using a patented approach [10], in their Zinc bath technique. J. Gruber et al. [11] performed an in-situ real time monitoring of Cr, Cu, Mn and Ni content in molten steel using LIBS. K. Rai et al. [12] also demonstrated the use of LIBS ⁎ Corresponding author at: Laboratoire de Génie des Procédés Plasmas et Traitement de Surfaces (LGPPTS)-EA3492, 11, rue Pierre et Marie Curie 75005 Paris, France. E-mail address: [email protected] (S. Darwiche). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.06.020
for the analysis of the elemental composition of molten aluminum alloy. Further research exists in the literature on LIBS for use on mineral ores, effluents, molten metals and/or glass, etc [13–18]. In this work, LIBS was used to analyze impurities in solid and molten silicon. The effect of several experimental parameters on the signal to background ratio (SBR) was studied in order to find the most suitable parameters (laser energy, pressure, buffering gas) for boron analysis. From a feasibility perspective, LIBS was also applied to analyze molten silicon to investigate the plausibility of a technology transfer for online measurement during industrial silicon production. 2. Experimental setup The device used in this study consisted of a nanosecond Nd:YAG laser (Brio type – Quantel France) operating at the fourth harmonic of 266 nm, with 4 ns pulse duration and a repetition rate of 20 Hz, and a maximum energy of 10 mJ per pulse (spot diameter 100 μm, fluence 127 J/cm 2). The laser was focused on the sample through a UV-Fused silica (Spectrosil 2000) window. The emitted light was transmitted to the spectrometer via a 19 fiber optical bundle. The emission collection mode was made to be collinear with the laser. The plasma emission was imaged onto the entrance slit of a Czerny– Turner spectrometer with a 750 mm focal length (Acton SpectraPro 2750) equipped with three gratings (1200 grooves.mm −1, 300 nm blaze, 2400 and 3600 grooves.mm −1, 240 nm blaze), with a reciprocal linear dispersion of 1.02 nm.mm −1. The detector was a CCD camera (1024 × 1024 pixels) with an intensifier system (Princeton instruments PIMAX). The width of the entrance slit of the
116
S. Darwiche et al. / Spectrochimica Acta Part B 74–75 (2012) 115–118
Fig. 3. Effect of laser energy on boron SBR in different buffer gases; delay time 500 ns, gate width 3400 ns. Fig. 1. Experimental setup for solid silicon analysis.
spectrometer was fixed at 50 μm. Signal-to-background ratio (SBR) was used to represent spectral data in order to reduce the effect of background noise due to the matrix effect; in addition, in some cases, the SBR of 244.34 nm silicon emission line was used as an internal standard The signal emission is acquired by averaging 10 laser shots to compensate for laser beam energy fluctuations. The spectrum acquisition was typically made with a delay time of between 100–1000 ns and gate width of 500–4000 ns and will be indicated in captions for each case; the determination of these parameters is discussed in more detail in a previous publication [8]. A 3000 cm 3 analysis chamber with quartz windows was used in order to be able to change the nature of the buffering gas (air, argon, helium), and for pressure control. The pressure was controlled by using a two stage oil sealed rotary vane pump (Edwards E2M1.5). The pressure was measured with a standard vacuum gauge which was not changed for different gases. The analysis chamber was placed on a motorized X–Y–Z table for positioning (Fig. 1). For the experiments using molten silicon, a Flexitune induction generator delivering a maximum of 10 kW was used to melt the solid silicon in a graphite crucible under an argon atmosphere. A concentric tube cooling jacket with flowing water was used to maintain a moderate temperature in the system (Fig. 2). Melting of the granular silicon starts at 8 kW when the bath reaches 1410 °C. To accommodate this experimental setup, the distance from the laser to the sample was increased from 11 to 50 cm in order to avoid damage by projectile liquid silicon.
3. Results In previous work, the effect of the buffering gas composition and pressure on the boron SBR was studied using a Nd:YAG laser operating at 1064 nm with the maximum energy of 95 mJ. An improvement of the boron SBR was observed with helium atmosphere working at a reduced pressure of 60 mbar [8]. A similar study will be done in this work using the shorter wavelength laser (266 nm) and by varying different parameters; in addition, measurements will be made on molten silicon with controlled impurity concentrations. 3.1. Effect of laser energy and buffering gas Laser energy was varied between 4 and 10 mJ and LIBS measurements have been made in different atmospheres (air, argon and helium). It is important to note that with the current laser (266 nm), unlike the 1064 nm laser, boron emission lines have been detected because of the decreasing of the breakdown threshold value. Fig. 3 shows the boron signal to background ratio (SBR) as a function of laser energy with three different atmospheres of air, argon, and helium used for the buffering gas in the sample compartment. The corresponding spectral aquisition parameters are shown in Table 1. It was observed that the SBR increases with increasing laser pulse energy, regardless the atmosphere used in the analysis. However, it appears that the signal quality obtained in an atmosphere of rare gas is better than that for air. The best signal is obtained under an atmosphere of rare gas (argon or helium) with laser pulse energy of 10 mJ. Therefore, for the experiments to be discussed further on in this work, the buffering gas will be either argon or helium, and the maximum laser energy will be used. 3.2. Effect of gas pressure Fig. 4 shows the variation of boron SBR with gas pressure. This analysis was made using the optimized parameters as discussed above, meaning an atmosphere of argon and the higher laser energy value of 10 mJ per pulse. Two trends are observed: a better SBR was obtained for intermediate pressures of either 100 mbar or 500 mbar. Table 1 Summary of temporal parameters for different gases and laser energies.
Fig. 2. Experimental setup used to melt silicon.
Gas
Air
Energy (mJ)
Delay (ns)
Gate (ns)
Delay (ns)
Argon Gate (ns)
Delay (ns)
Helium Gate (ns)
10 8 6 4
700 400 600 100
1200 1000 1200 600
500 400 400 250
3400 3000 1600 1000
400 300 250 100
1800 1600 1200 800
S. Darwiche et al. / Spectrochimica Acta Part B 74–75 (2012) 115–118
Fig. 4. Effect of gas pressure on boron SBR; delay time 400 ns, gate width 1000 ns.
Decreasing the pressure to 1–10 mbar was not observed to be beneficial for the boron SBR. This improvement can be explained by the fact that at intermediate pressure, an increase in the mean free path of the plasma species in an argon atmosphere reduces collisions and recombination of the excited species. This results in an extension of the lifetime of the excited species emissions. At lower pressures, the plasma goes into a non-thermal regime where the excitation of heavy species is less pronounced, which explains the decrease in emissions.
3.3. Boron calibration curve Fig. 5 shows the calibration curve which was made for boron in silicon using standard samples in the range of 1 to 100 ppm determined by resistivity measurements and certified by the company Siltronix ®. For each measurement, 200 shots were performed with 2400 grooves/mm grating centered at 247 nm, enabling the detection of the three emission lines of silicon around 244–245 nm and boron at 249.77 nm. The ratio of the boron to silicon SBR is plotted, using the silicon line as an internal standard which helps to account for variations in material ablation. The boron to silicon SBR ratio at exactly zero boron concentration was not available, but in principle should be a value between zero and unity. The value of boron SBR at 1 ppm, which indicated a strong non-linearity for low boron concentration, was used to calculate a limit of detection for boron in solid silicon of about 0.2 ppm, defined as a boron peak with a height 3 times the value of the standard deviation of the background noise.
Fig. 5. Calibration curve for boron in silicon prepared from standardized samples; delay time 500 ns, gate width 3400 ns. Pure argon, 500 mbar, pulse energy = 10 mJ.
117
Fig. 6. Spectrum showing carbon and boron emission lines for molten silicon; delay time 1000 ns, gate width 4000 ns.
3.4. Analysis of molten silicon by LIBS Once the optimized parameters had been determined, a feasibility study was made to investigate the possibility of the use of LIBS on silicon in the liquid phase with the view of process control applications. Measurements were made on molten silicon which was prepared by melting granular silicon in a graphite crucible heated by an RF induction generator at 8 kW under argon atmosphere. In a recent study, it was shown that for a temperature change of 25° to 70° above the melting point of a given metal, the LIBS intensity of a given emission line changes linearly. Control of the temperature of the molten silicon is therefore necessary, and is now under investigation [19]. In order to test the response to different impurity levels, two types of silicon, with known compositions of 1.6 and 210 ppm of aluminum, were analyzed. The first sample also contained 6 ppm of boron. Figs. 6–8 show spectra which were obtained by using LIBS on molten silicon with different levels of impurities as indicated in the captions. As can be seen in Fig. 6, boron and carbon are detected. Notably, it is here demonstrated that boron may be detected in molten silicon at a concentration of 6 ppm. By comparing Figs. 7 and 8, showing the signal of aluminum emission lines, the difference in signal intensity between 1 and 200 ppm Al can be observed. The ability to analyze the composition of molten silicon could be very useful for the online process control of various production methods for the industrial production of silicon.
Fig. 7. Spectrum showing aluminum emission lines at 1.6 ppm for molten silicon; delay time 1000 ns, gate width 4000 ns.
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S. Darwiche et al. / Spectrochimica Acta Part B 74–75 (2012) 115–118
Fig. 8. Spectrum showing aluminum emission lines at 210 ppm for molten silicon; delay time 1000 ns, gate width 4000 ns.
4. Conclusion It has been demonstrated that the detection of boron in silicon can be improved by the use of either argon and/or helium at pressures in the range of 100 mbar– 500 mbar. By comparing these results to the boron SBR obtained with the 1064 nm laser [8], it can be observed that the use of the 266 nm Nd:YAG laser doesn't provide any notable improvement to the boron SBR. A calibration curve was made for boron in silicon using standard samples which indicates a limit of detection as low as 0.2 ppm. The use of LIBS was demonstrated on molten silicon, showing that aluminum, calcium, carbon and boron may all be detected. Boron, a very important element for photovoltaic cells, was detected at a level of 6 ppm. At this stage, the analysis of the molten silicon is only qualitative, and so additional work should be done to reach quantitative and reliable performance of the technology in order to facilitate its introduction into industrial use. To simplify the quantification of molten silicon, standard silicon samples (prepared by mixing pure granular silicon with precise amounts of boron and/or aluminum powder) shall be prepared and further experiments will be done in order to quantify the impurities contained in the analyzed silicon, and to further refine the technology for use on both solid and molten silicon. References [1] S. Palanco, S. Conesa, J.J. Laserna, Analytical control of liquid steel in an induction melting furnace using a reme laser-induced plasma spectrometer, J. Anal. At. Spectrom. 19 (2004) 462–467.
[2] C. Aragon, J.A. Aguilera, J. Campos, Determination of carbon content in molten steel using laser-induced breakdown spectroscopy, Appl. Spectrosc. 47 (1993) 606–608. [3] R. Noll, H. Bette, A. Brysch, M. Kraushaar, I. Mönch, L. Peter, V. Sturm, Laser-induced breakdown spectrometry- applications for production control and quality assurance in the steel industry, Spectrochim. Acta Part B (2001) 637–649. [4] F. Boué-Bigne, Laser-induced breakdown spectroscopy applications in the steel industry: Rapid analysis of segregation and decarburization, Spectrochim. Acta Part B 63 (2008) 1122–1129. [5] D. Romero, J.J. Laserna, Surface and tomographic distribution of carbon impurities in photonic-grade silicon using laser-induced breakdown spectrometry, J. Anal. At. Spectrom. 13 (1998) 557–560. [6] M. Milaan, P. Lucena, L.M. Cabalian, J.J. Laserna, Depth profiling of phosphorus in photonic-grade silicon using laser-induced breakdown spectrometry, Appl. Spectrosc. 52 (1998) 444–448. [7] D. Romero, J.M. Fernandez Romero, J.J. Laserna, Distribution of metal impurities in silicon wafers using imaging-mode multi-elemental laser-induced breakdown spectrometry, J. Anal. At. Spectrom. 14 (1999) 199–204. [8] S. Darwiche, M. Benmansour, N. Eliezer, D. Morvan, Investigation of optimized experimental parameters including laser wavelength for boron measurement in photovoltaic grade silicon using laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 65 (2010) 738–743. [9] E. Baril, L. St-Onge, M. Sabsabi, J. Lucas, Novel Method for On-line Chemical Analysis of Continuous Galvanizing Baths, In: Galvatech '04, Chicago (IL), April 4-7 2004, published in Galvatech'04 Conf. Proceedings Association for Iron & Steel Technology, 2004, pp. 1095–1104. [10] M. Sabsabi, R. Héon, J. Lucas, Method and apparatus for in-process liquid analysis by laser induced plasma Spectroscopy, U.S. Pat. No. 6,700,660 issued Sept. 2003. [11] J. Gruber, J. Heitz, H. Strasser, D. Bauerle, N. Ramaseder, Rapid in-situ analysis of liquid steel by laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 56 (2001) 685–693. [12] A.K. Rai, F.Y. Yueh, J.P. Singh, D.K. Rai, In: in: Jagdish P. Singh, Surya N. Thakur (Eds.), Laser-Induced Breakdown Spectroscopy of Solid and Molten Material, 255, 2007. [13] L. St-Onge, E. Kwong, M. Sabsabi, E.B. Vadas, Rapid analysis of liquid formulations containing sodium chloride using laser-induced breakdown spectroscopy, J. Pharm. Biomed. Anal. 36 (2004) 277–284. [14] L. Barette, S. Turmel, On-line iron-ore slurry monitoring for real-time process control of pellet making processes using laser-induced breakdown spectroscopy: graphitic vs. total carbon detection, Spectrochim. Acta Part B 56 (2001) 715–723. [15] N.K. Rai, A.K. Rai, A. Kumar, S. Thakur, Detection sensitivity of laser-induced breakdown spectroscopy for Cr II in liquid samples, Appl. Opt. 47 (2008) G105–G111. [16] A. Kaminska, M. Sawczak, K. Komar, G. Śliwiński, Application of the laser ablation for conservation of historical paper documents, Appl. Surf. Sci. 253 (2007) 7860–7864. [17] X.K. Shen, H. Wang, Z.Q. Xie, Y. Gao, H. Ling, Y.F. Lu, Detection of trace phosphorus in steel using laser-induced breakdown spectroscopy combined with laser-induced fluorescence, Appl. Opt. 48 (2009) 2551–2558. [18] A.-M. Matiaske, I.B. Gornushkin, U. Panne, Double-pulse laser-induced breakdown spectroscopy for analysis of molten glass, Anal. Bioanal. Chem. 402 (2012) 2597–2606. [19] T. Vaculovic, Z. Zverina, V. Otruba, V. Kanicky, Laser-induced breakdown spectroscopy of molten: influence of sample temperature, Z. Naturforsch. 66a (2011) 643–648.