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The impact of wafering on organic and inorganic surface contaminations
Applied Surface Science 378 (2016) 384–387
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The impact of wafering on organic and inorganic surface contaminations S. Meyer a,∗ , S. Wahl a , S. Timmel a , R. Köpge a , B.-Y. Jang b a b
Fraunhofer Center for Silicon Photovoltaics CSP, Otto-Eissfeld-Str. 12, 06120 Halle (Saale), Germany Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon, South Korea
a r t i c l e
i n f o
Article history: Received 20 October 2015 Received in revised form 29 March 2016 Accepted 30 March 2016 Available online 1 April 2016 Keywords: Wafer surface Organic and inorganic impurities Quantitative determination
a b s t r a c t Beside the silicon feedstock material, the crystallization process and the cell processing itself, the wafer sawing process can strongly determine the final solar cell quality. Especially surface contamination is introduced in this process step because impurities from sawing meet with a virgin silicon surface which is highly reactive until the oxide layer is formed. In this paper we quantitatively analysed both, the organic and inorganic contamination on wafer surfaces and show that changes of process parameters during wafering may cause dramatic changes in surface purity. We present powerful techniques for the monitoring of wafer surface quality which is essential for the production of high efficiency and high quality solar cells. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Approaching the theoretical limits in solar cell efficiency, new cell concepts are intensively developed and steadily improved. These high efficiency cell processes not only require very pure silicon material but also increasingly high quality wafer surfaces. Chemical and structural characteristics of the wafer surfaces influence the texturing and passivation process for both concepts, PERC and HIT cells [1,2]. The wafering process (sawing and subsequent cleaning) plays a key role in determining the purity of the silicon material and the surface contamination on substrates for high efficiency solar cells. In the wafering step, the silicon surface is bare of any oxide layer for the first time. This virgin silicon surface is decorated with metal ions, which can be subsequently incorporated into the freshly formed oxide layer. This can lead to a higher metal contamination in all subsequent processes and may finally result in lower minority carrier lifetimes [3]. Moreover, it could be shown that not only metallic impurities but also organic residues on the wafer surface may impair down-stream processes such as texturing [4,5,6] In this work we demonstrate methods for sensitive and reliable measurement of both, organic and inorganic surface contamination on solar silicon wafers. We show that a sub-optimal wafering may
∗ Corresponding author. E-mail address: [email protected] (S. Meyer). http://dx.doi.org/10.1016/j.apsusc.2016.03.227 0169-4332/© 2016 Elsevier B.V. All rights reserved.
introduce more than 4-fold the organic impurities and more than 10-fold the metal contamination compared to a reference process. 2. Material and methods 2.1. Wafer samples Multi-crystalline silicon wafers (180 m thickness) as well as thin mono-crystalline wafers (110 m thickness) were sawn using a slurry process followed by batch pre-cleaning and in-line final cleaning. Only hot and cold water with detergents was used as cleaning agent. To improve the removal of sawing debris, a modification of the pre-cleaning step was tested for the thin wafers. They were dipped in a cleaning bath for 30 min at 25 ◦ C and flushed with a cleaning liquid. The cleaning liquid consisted of 10 vol.% mixture of NaOH and IPA (volume ratio 1:1) in distilled water (Resistance: 15 M cm) for batch #2. Batch #1 wafers, instead, were pre-cleaned with distilled water only. 2.2. Surface-extraction total organic carbon (TOC) analysis A two-step process was developed for the rapid and quantitative measurement of surface organics: First, the wafer surface was extracted using the sandwich method [5] and second, the TOC content of the extraction solution was measured using a TOC instrument (Multi N/C UV HS BU, Analytik Jena, Germany). The inorganic carbon is removed from the sample by acidification and purging. Following that, the organic carbon is quantitatively converted into
S. Meyer et al. / Applied Surface Science 378 (2016) 384–387
385
Fig. 1. Schematic workflow of surface extraction and subsequent element or organic carbon analysis.
Fig. 2. 6” Wafer from batch A displaying visible stains (left) and wafer from batch B without stains (right).
Fig. 3. Surface TOC content of impaired and normal wafer (batch A vs. batch B) as measured by surface-extraction TOC analysis.
CO2 by wet chemical and UV oxidation and analyzed by a NDIR detector. The etching solution was prepared by mixing HF (48%), HNO3 (69%) and water in a 1:1:23 ratio. The workflow is shown schematically in Fig. 1. The limit of detection for organic carbon was measured as 0.25 ng/cm2 which provides a sufficient sensitivity for state-of-the-art solar silicon wafer (see results below). The recovery rate was determined using several organic substances common to the wafering process like polyethylene glycol or cleaning detergents. Values between 90% (PEG) and 100% (detergents) were found up to a concentration range of 200 ng/cm2 (data not shown). 2.3. Surface-extraction and inductively coupled plasma mass spectrometry (ICPMS) An analogous procedure was applied as for the TOC measurement. The wafers were extracted using the sandwich method and
subsequently the extraction solution was diluted and measured with ICPMS. A high resolution ICP mass spectrometer was used (Element XR, Thermo Scientific, Germany). Ultrapure chemicals were used to ensure a low background. 3. Results and discussion 3.1. Organic contamination On two batches of multi-crystalline wafers the amount of organic surface contamination was determined. After the precleaning process the wafers partially contained visible stains (Fig. 2) because for batch A a reduced process time during the first pre-cleaning step was applied. This contamination could not be removed by the final cleaning step. As a measure for organics the total content of organic carbon TOC was used. To determine the
386
S. Meyer et al. / Applied Surface Science 378 (2016) 384–387
Fig. 4. 6” thin wafer from batch #1 displaying visible stains (left) and wafer from batch #2 without stains (right).
Fig. 5. Inorganic surface impurities of two different wafer batches as determined by surface-extraction ICPMS. Upper panel: logarithmic scale, the limit of detection (LOD) is given for each element.
surface TOC the wafers were etched with a solution containing HF/HNO3 which was then analysed using a TOC analyser. A significantly higher content of organic contamination was found on the stained batch A wafer compared to batch B as can be seen in Fig. 3. The stained wafer contained on average 100 ng/cm2 total organic carbon compared to 24 ng/cm2 for the reference wafer. Furthermore, within the impaired wafer batch a significantly higher variation in TOC content was found, correlating with a heterogeneous stain density. A TOC content of ∼25 ng/cm2 was found for many other tested wafer batches from different manufacturers and seems to be a typical organic surface content for solar silicon wafers (data not shown). The newly developed method of surface-extraction TOC analysis was found to be a sensitive and rapid tool for characterisation of wafer surface quality. Beyond the demonstrated extreme example of visible stained wafers, the method can easily detect also invisible organic impurities. Thus, it may help to quantitatively specify the surface quality, providing a basis for further process optimizations towards higher quality wafers. 3.2. Metal contamination It is known that metal abrasion increases with decreasing wafer thickness because the wire experiences a longer distance to saw more wafers from the same sized ingot. For example, to saw an ingot with a length of 400 mm, the distance of wire moving for 120 mwafers is 223 m while that for 180 m-wafers is only 184 m. The higher wire abrasion induces larger amount of metal debris within
the slurry and increases the slurry’s viscosity. The debris in the slurry having a higher viscosity easily adheres to the wafers due to the interaction energy between silicon surface and particles [7]. As a consequence, metal contamination occurs on the wafer surface. The contamination was efficiently reduced just by adding coolant to keep the slurry’s viscosity constant during the sawing process. Details on mechanism and removal of the metal contaminations are given in our previous research [8]. Two different batches (batch #1 and #2) of the metal contaminated wafers from the same sawing process with different pre-cleaning were analysed for their inorganic surface contamination. The two batches differ in their pre-cleaning procedure. Batch #1 was pre-cleaned with only distilled water whereas the pre-cleaning of batch #2 was carried out with NaOH based detergent. Batch #1 wafers displayed visible stains at the edges whereas batch #2 wafers were free of any macroscopic defects as shown in Fig. 4. To analyse the metal impurities, the wafer surfaces were etched with a HF/HNO3 containing solution. The etching solution was then measured with ICPMS. As shown in Fig. 5, a significant variation of the contamination levels between both batches was found. All measured contamination levels were orders of magnitude above the detection limit as can be judged from the third bar shown in Fig. 5. The highest values were found for iron and copper on wafers from batch #1 (2 × 1014 and 7 × 1014 atoms/cm2 , respectively). The wafers from batch #1 (having visible stains) contain significant more V, Fe, Cu, Zn and Ag than those from batch #2. Thus, the pre-cleaning using NaOH/IPA efficiently removes a lot of metallic impurities from the wafer surface. On the other hand, batch #2
S. Meyer et al. / Applied Surface Science 378 (2016) 384–387
wafers display higher surface contents for alkali and earth alkali elements compared to batch #1, with potassium being the most prevalent contamination of batch #2 wafers. This may result from the alkaline cleaning solution and should be improved by optimizing the final in-line cleaning step. Overall, our results show, that the NaOH/IPA pre-cleaning is an efficient method not only to yield a good optical wafer quality but also to significantly reduce the surface load of metals like iron, copper and nickel. For all measured elements, the detection limit was well below the sample value. Thus, one can state that the surface extraction combined with high resolution ICPMS provides sufficient sensitivity for reliable determination of surface impurities.
387
Acknowledgements The authors wish to thank Dominik Lausch for critically reading the manuscript. This work was funded by the German Federal Ministry of Education and Research (BMBF, contract number 03IPT607A) and by industry partners of the R&D project “DiaCell”. B.-Y J. would like to thank for support by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20148520120040). References
4. Conclusion
A quantitative determination of organic and inorganic impurities on wafer surfaces released from the sawing process revealed significant heterogeneities in wafer surface quality. Our data show that variations during wafering may introduce more than 4-fold the organic impurities and more than 10-fold the metal contamination compared to an optimal process. Also, especially for thin wafers, batch pre-cleaning subsequently to sawing can have dramatic impact on contamination with metals and organic compounds. With respect to the increasing influence of surface impurities on the quality of high efficiency cells, a thorough monitoring of wafer surface characteristics may help to improve yield and overall efficiency of solar cell production. Within this contribution, adequate methods for controlling the surface contamination level were presented.
[1] I. Kashkoush, G. Chen, D. Nemeth, J. Rieker, Wafer surface preparation for high-efficiency solar cells, IEEE Photovolt. Spec. Conf. (2014) 1211–1213. [2] C. Kranz, S. Wyczanowski, U. Baumann, S. Dorn, S. Queisser, J. Schweckendiek, D. Pysch, T. Dullweber, Industrial cleaning sequences for Al2 O3 -passivated PERC solar cells, Energy Procedia 55 (2014) 211–218. [3] D. Meißner, S. Meyer, O. Anspach, Enrichment of metal ions in virgin Si-surfaces, Energy Procedia 27 (2012) 27–32. [4] M. Al-Amin, A. Assi, in: A. Méndez-Vilas (Ed.), Efficiency Improvement of Crystalline Silicon Solar Cells, Materials and Processes for Energy: Communicating Current Research and Technological Developments, 2013, pp. 22–31. [5] F. Buchholz, E. Wefringhaus, S. Eisert, H. Egenolf, A. Groß, Measurement and impact of surface transition metal contamination of textured multicrystalline silicon wafers, EU PVSEC Proc. (2011) 1187–1190. [6] C. Sonner, A. Oltersdorf, M. Zimmer, J. Rentsch, Influence of contaminations and cleaning sequences on alkaline texturization, in: Proceeding of EU-PVSEC, Hamburg, 2011, pp. 1666–1670. [7] K. Uemura, Y. Mori, T. Haibara, Cleaning technology of silicon wafers, Nippon Steel Tech. Rep. 83 (2001) 61–68. [8] S. Choi, B.-Y. Jang, J. Kim, H. Song, M. Han, Cu-contamination of single crystalline silicon wafers with thickness of 100 lm during multi-wire sawing process, Sol. Energy 125 (2016) 198–206.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The impact of wafering on organic and inorganic surface contaminations S. Meyer a,∗ , S. Wahl a , S. Timmel a , R. Köpge a , B.-Y. Jang b a b
Fraunhofer Center for Silicon Photovoltaics CSP, Otto-Eissfeld-Str. 12, 06120 Halle (Saale), Germany Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon, South Korea
a r t i c l e
i n f o
Article history: Received 20 October 2015 Received in revised form 29 March 2016 Accepted 30 March 2016 Available online 1 April 2016 Keywords: Wafer surface Organic and inorganic impurities Quantitative determination
a b s t r a c t Beside the silicon feedstock material, the crystallization process and the cell processing itself, the wafer sawing process can strongly determine the final solar cell quality. Especially surface contamination is introduced in this process step because impurities from sawing meet with a virgin silicon surface which is highly reactive until the oxide layer is formed. In this paper we quantitatively analysed both, the organic and inorganic contamination on wafer surfaces and show that changes of process parameters during wafering may cause dramatic changes in surface purity. We present powerful techniques for the monitoring of wafer surface quality which is essential for the production of high efficiency and high quality solar cells. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Approaching the theoretical limits in solar cell efficiency, new cell concepts are intensively developed and steadily improved. These high efficiency cell processes not only require very pure silicon material but also increasingly high quality wafer surfaces. Chemical and structural characteristics of the wafer surfaces influence the texturing and passivation process for both concepts, PERC and HIT cells [1,2]. The wafering process (sawing and subsequent cleaning) plays a key role in determining the purity of the silicon material and the surface contamination on substrates for high efficiency solar cells. In the wafering step, the silicon surface is bare of any oxide layer for the first time. This virgin silicon surface is decorated with metal ions, which can be subsequently incorporated into the freshly formed oxide layer. This can lead to a higher metal contamination in all subsequent processes and may finally result in lower minority carrier lifetimes [3]. Moreover, it could be shown that not only metallic impurities but also organic residues on the wafer surface may impair down-stream processes such as texturing [4,5,6] In this work we demonstrate methods for sensitive and reliable measurement of both, organic and inorganic surface contamination on solar silicon wafers. We show that a sub-optimal wafering may
∗ Corresponding author. E-mail address: [email protected] (S. Meyer). http://dx.doi.org/10.1016/j.apsusc.2016.03.227 0169-4332/© 2016 Elsevier B.V. All rights reserved.
introduce more than 4-fold the organic impurities and more than 10-fold the metal contamination compared to a reference process. 2. Material and methods 2.1. Wafer samples Multi-crystalline silicon wafers (180 m thickness) as well as thin mono-crystalline wafers (110 m thickness) were sawn using a slurry process followed by batch pre-cleaning and in-line final cleaning. Only hot and cold water with detergents was used as cleaning agent. To improve the removal of sawing debris, a modification of the pre-cleaning step was tested for the thin wafers. They were dipped in a cleaning bath for 30 min at 25 ◦ C and flushed with a cleaning liquid. The cleaning liquid consisted of 10 vol.% mixture of NaOH and IPA (volume ratio 1:1) in distilled water (Resistance: 15 M cm) for batch #2. Batch #1 wafers, instead, were pre-cleaned with distilled water only. 2.2. Surface-extraction total organic carbon (TOC) analysis A two-step process was developed for the rapid and quantitative measurement of surface organics: First, the wafer surface was extracted using the sandwich method [5] and second, the TOC content of the extraction solution was measured using a TOC instrument (Multi N/C UV HS BU, Analytik Jena, Germany). The inorganic carbon is removed from the sample by acidification and purging. Following that, the organic carbon is quantitatively converted into
S. Meyer et al. / Applied Surface Science 378 (2016) 384–387
385
Fig. 1. Schematic workflow of surface extraction and subsequent element or organic carbon analysis.
Fig. 2. 6” Wafer from batch A displaying visible stains (left) and wafer from batch B without stains (right).
Fig. 3. Surface TOC content of impaired and normal wafer (batch A vs. batch B) as measured by surface-extraction TOC analysis.
CO2 by wet chemical and UV oxidation and analyzed by a NDIR detector. The etching solution was prepared by mixing HF (48%), HNO3 (69%) and water in a 1:1:23 ratio. The workflow is shown schematically in Fig. 1. The limit of detection for organic carbon was measured as 0.25 ng/cm2 which provides a sufficient sensitivity for state-of-the-art solar silicon wafer (see results below). The recovery rate was determined using several organic substances common to the wafering process like polyethylene glycol or cleaning detergents. Values between 90% (PEG) and 100% (detergents) were found up to a concentration range of 200 ng/cm2 (data not shown). 2.3. Surface-extraction and inductively coupled plasma mass spectrometry (ICPMS) An analogous procedure was applied as for the TOC measurement. The wafers were extracted using the sandwich method and
subsequently the extraction solution was diluted and measured with ICPMS. A high resolution ICP mass spectrometer was used (Element XR, Thermo Scientific, Germany). Ultrapure chemicals were used to ensure a low background. 3. Results and discussion 3.1. Organic contamination On two batches of multi-crystalline wafers the amount of organic surface contamination was determined. After the precleaning process the wafers partially contained visible stains (Fig. 2) because for batch A a reduced process time during the first pre-cleaning step was applied. This contamination could not be removed by the final cleaning step. As a measure for organics the total content of organic carbon TOC was used. To determine the
386
S. Meyer et al. / Applied Surface Science 378 (2016) 384–387
Fig. 4. 6” thin wafer from batch #1 displaying visible stains (left) and wafer from batch #2 without stains (right).
Fig. 5. Inorganic surface impurities of two different wafer batches as determined by surface-extraction ICPMS. Upper panel: logarithmic scale, the limit of detection (LOD) is given for each element.
surface TOC the wafers were etched with a solution containing HF/HNO3 which was then analysed using a TOC analyser. A significantly higher content of organic contamination was found on the stained batch A wafer compared to batch B as can be seen in Fig. 3. The stained wafer contained on average 100 ng/cm2 total organic carbon compared to 24 ng/cm2 for the reference wafer. Furthermore, within the impaired wafer batch a significantly higher variation in TOC content was found, correlating with a heterogeneous stain density. A TOC content of ∼25 ng/cm2 was found for many other tested wafer batches from different manufacturers and seems to be a typical organic surface content for solar silicon wafers (data not shown). The newly developed method of surface-extraction TOC analysis was found to be a sensitive and rapid tool for characterisation of wafer surface quality. Beyond the demonstrated extreme example of visible stained wafers, the method can easily detect also invisible organic impurities. Thus, it may help to quantitatively specify the surface quality, providing a basis for further process optimizations towards higher quality wafers. 3.2. Metal contamination It is known that metal abrasion increases with decreasing wafer thickness because the wire experiences a longer distance to saw more wafers from the same sized ingot. For example, to saw an ingot with a length of 400 mm, the distance of wire moving for 120 mwafers is 223 m while that for 180 m-wafers is only 184 m. The higher wire abrasion induces larger amount of metal debris within
the slurry and increases the slurry’s viscosity. The debris in the slurry having a higher viscosity easily adheres to the wafers due to the interaction energy between silicon surface and particles [7]. As a consequence, metal contamination occurs on the wafer surface. The contamination was efficiently reduced just by adding coolant to keep the slurry’s viscosity constant during the sawing process. Details on mechanism and removal of the metal contaminations are given in our previous research [8]. Two different batches (batch #1 and #2) of the metal contaminated wafers from the same sawing process with different pre-cleaning were analysed for their inorganic surface contamination. The two batches differ in their pre-cleaning procedure. Batch #1 was pre-cleaned with only distilled water whereas the pre-cleaning of batch #2 was carried out with NaOH based detergent. Batch #1 wafers displayed visible stains at the edges whereas batch #2 wafers were free of any macroscopic defects as shown in Fig. 4. To analyse the metal impurities, the wafer surfaces were etched with a HF/HNO3 containing solution. The etching solution was then measured with ICPMS. As shown in Fig. 5, a significant variation of the contamination levels between both batches was found. All measured contamination levels were orders of magnitude above the detection limit as can be judged from the third bar shown in Fig. 5. The highest values were found for iron and copper on wafers from batch #1 (2 × 1014 and 7 × 1014 atoms/cm2 , respectively). The wafers from batch #1 (having visible stains) contain significant more V, Fe, Cu, Zn and Ag than those from batch #2. Thus, the pre-cleaning using NaOH/IPA efficiently removes a lot of metallic impurities from the wafer surface. On the other hand, batch #2
S. Meyer et al. / Applied Surface Science 378 (2016) 384–387
wafers display higher surface contents for alkali and earth alkali elements compared to batch #1, with potassium being the most prevalent contamination of batch #2 wafers. This may result from the alkaline cleaning solution and should be improved by optimizing the final in-line cleaning step. Overall, our results show, that the NaOH/IPA pre-cleaning is an efficient method not only to yield a good optical wafer quality but also to significantly reduce the surface load of metals like iron, copper and nickel. For all measured elements, the detection limit was well below the sample value. Thus, one can state that the surface extraction combined with high resolution ICPMS provides sufficient sensitivity for reliable determination of surface impurities.
387
Acknowledgements The authors wish to thank Dominik Lausch for critically reading the manuscript. This work was funded by the German Federal Ministry of Education and Research (BMBF, contract number 03IPT607A) and by industry partners of the R&D project “DiaCell”. B.-Y J. would like to thank for support by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20148520120040). References
4. Conclusion
A quantitative determination of organic and inorganic impurities on wafer surfaces released from the sawing process revealed significant heterogeneities in wafer surface quality. Our data show that variations during wafering may introduce more than 4-fold the organic impurities and more than 10-fold the metal contamination compared to an optimal process. Also, especially for thin wafers, batch pre-cleaning subsequently to sawing can have dramatic impact on contamination with metals and organic compounds. With respect to the increasing influence of surface impurities on the quality of high efficiency cells, a thorough monitoring of wafer surface characteristics may help to improve yield and overall efficiency of solar cell production. Within this contribution, adequate methods for controlling the surface contamination level were presented.
[1] I. Kashkoush, G. Chen, D. Nemeth, J. Rieker, Wafer surface preparation for high-efficiency solar cells, IEEE Photovolt. Spec. Conf. (2014) 1211–1213. [2] C. Kranz, S. Wyczanowski, U. Baumann, S. Dorn, S. Queisser, J. Schweckendiek, D. Pysch, T. Dullweber, Industrial cleaning sequences for Al2 O3 -passivated PERC solar cells, Energy Procedia 55 (2014) 211–218. [3] D. Meißner, S. Meyer, O. Anspach, Enrichment of metal ions in virgin Si-surfaces, Energy Procedia 27 (2012) 27–32. [4] M. Al-Amin, A. Assi, in: A. Méndez-Vilas (Ed.), Efficiency Improvement of Crystalline Silicon Solar Cells, Materials and Processes for Energy: Communicating Current Research and Technological Developments, 2013, pp. 22–31. [5] F. Buchholz, E. Wefringhaus, S. Eisert, H. Egenolf, A. Groß, Measurement and impact of surface transition metal contamination of textured multicrystalline silicon wafers, EU PVSEC Proc. (2011) 1187–1190. [6] C. Sonner, A. Oltersdorf, M. Zimmer, J. Rentsch, Influence of contaminations and cleaning sequences on alkaline texturization, in: Proceeding of EU-PVSEC, Hamburg, 2011, pp. 1666–1670. [7] K. Uemura, Y. Mori, T. Haibara, Cleaning technology of silicon wafers, Nippon Steel Tech. Rep. 83 (2001) 61–68. [8] S. Choi, B.-Y. Jang, J. Kim, H. Song, M. Han, Cu-contamination of single crystalline silicon wafers with thickness of 100 lm during multi-wire sawing process, Sol. Energy 125 (2016) 198–206.