MicroTech Systems, Inc. Engineering Wet Process Solutions

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Abstract

Photovoltaic manufacturing is based on the ability to manufacture large volumes of high efficiency solar cells at low costs. Wet surface conditioning processes yield the necessary texturing of monocrystalline or multicrystalline silicon (i) and are cost effective. The ECN process is a common texturing process. The optimized T2 process uses a two-step texturing bath to achieve the optimum efficiency of approximately 17% for single emitter photodiodes (ii). The ECN T1 texturing process can also be used. Both ECN processes include a HF/HNO3 acid bath that also removes the saw damage. Additionally, KOH with IPA or a surfactant can also be used as the texturing bath, alone or sequential after the HF/HNO3 acid bath. An HF/HCl bath is typically performed after the alkaline steps to facilitate removal of the mobile ions and to strip the chemical oxide. The final step creates a uniform hydrophilic surface to facilitate uniform phosphorus doping to create the emitter.

This paper presents a method for cost reduction and green processing by replacing cleaning baths with simplified rinsing processes. The post-texturing HF/HCl bath is eliminating and replaced with a HF/HCl rinse. The final SC-1 or oxidizing bath is replaced with a proprietary rinse. The proprietary rinse forms a high-quality chemical oxide layer, which is advantageous for uniform emitter formation with wet-phosphorus doping (iii).

Both rinses adequately remove the post-texturing contaminants and leave the wafer with a uniform hydrophilic surface. The proprietary rinse water can be used after the HF/HCl rinse and is also recycled to be used again after the HF/HNO3 bath. Figure 1(a) shows the ECN-based texturing process and Figure 1(b) shows the proposed simplified process. A batch immersion processing tool is used, the MTS Orca, that provides a throughput of over 1500 wafer per hour.

The rinse cycles have been optimized to minimize the amount of water. The proposed rinsing processes, with cascade overflow rinses coupled with the reduction of rinsing cycles, uses 45% less water than the spray rinses. The cascade rinse also adequately distributes the rinsing solution over the immersed tightly-packed wafers. The rinse cycle throughput is designed to be compatible with the texturing bath to optimize throughput.

Results show that there is no degradation of reflectance with the simplified process. The reflectance at 1000.4 nm is less than 20% with the ECN-based process and approximately the same with the simplified process.

Photovoltaic Manufacturing - MicroTech Simplified Process

Results show that the oxide formed after the proprietary rinse give a hydrophilic film. Result for film thickness and quality will be compared. Results for phosphorus doping, with the simplified process, will be compared.

Proposed Simplified Process for Photovoltaic Manufacturing

Table 1 shows the time reduction in the processes by being able to reduce the number of tanks and process steps. A greater than 25% improvement in processing time is realized.

In addition to reduction of time and chemical, the footprint of the tool is reduced to approximately 20% less than the full ECN-based process tool; from approximately 560– to less than 440– in length.

In summary; the ECN-based process has been simplified by eliminating the HF/HCl and basic bath and replacing with rinsing steps. Results are equivalent to the ECN-based process and also achieve a faster throughput, lower cost, and smaller footprint.


References

  • D. H. Macdonald, A. Cuevas, M. J. Kerr, C. Samundsett, D. Ruby, S. Winderbaum, and A. Leo, Solar Energy, 76:277-283 (2004) and ISES Solar World Congress (2001).
  • C. J. J. Tool, G. Coletti, F. J. Granek, J. Hoornstra, M. Koppes, E.J. Kossen, H.C. Rieffe, I. G. Romijn, A. W. Weeber, 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, Spain, (6-10 June 2005).
  • P. Papet, O. Nichiporuka, A. Kaminskia, Y. Roziera, J. Kraiema, J.-F. Lelievrea, A. Chaumartina, A. Favea, and M. Lemitia, Solar Energy Materials and Solar Cells, 90(15):2319 (2006) and Solar Cells and Solar Energy Materials Symposium -IMRC (2005).

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