Charaterizing multicrystalline silicon ingots

S.Y. Lim1, M. Forster2, D. Macdonald1, 1The Australian National University 2Apollonsolar

PROCEDURE

PROCEDURE (cont.) INTRODUCTION

SOLIDIFICATION INTERFACE SHAPE IMAGES

Fig. 1. Flow chart of the solidification interface derivation process. * Surface limited condition refers to a homogeneous SRV >105 cm/s across wafer surface. This can be achieved by surface abrasion, for example.

One of the main challenges in the production of multicrystalline ingots is the control of crystal quality which is strongly related with the solidification interface shapes. Non-planar solidification interface shapes influence: - non-uniform net dopant density distribution - metallic impurity segregation - columnar grain growth Some conventional measuring methods include X-ray topography, ultrasound imaging or camera attached to a transparent furnace. === 2D determination The goal of this work is to introduce a new method which enables the 3D estimation of solidification interface geometry along the ingot height.

PL MEASUREMENTS & CALIBRATION

Australian National University

ANU College of Engineering & Computer Science

RESULTS

Fig. 4. Evolution of solidification interface shapes along ingot height. Reduction in curvature (hence more homogeneous temperature field) is observed at the upper region of the ingot.

Fig. 3. Net dopant density images (left) are utilized to derive solidification front (right) based on Scheil’s law

Wafer 72

Wafer 252

MATERIAL USED :

Compensated multicrystalline (Image taken from Btimaging)

PROCEDURE :

Fig. 2. Raw PL image is separated to both low frequency and high frequency data. The former contains dopant density information while the latter showing grains morphology.

CONCLUSIONS

ACKNOWLEDGEMENTS

REFERENCES

This work has been supported by the Australian Research Council.

R. Einhaus et al. Innovative crystallization of multi-crystalline silicon, 17th Asia PVSEC, Japan (2007). J. Kraeim et al, Innovative crystalline of multi-crystalline silicon ingots from different types of silicon feedstock, 23rd European PVSEC, Valencia (2008). T. Trupke et al, Photoluminescence imaging of silicon, Applied Physics Letters 89 (2006) 044107 A. Cuevas, SiliconPVconference 1(2011)94-99 S. Y. Lim et al, Journal of Applied Physics 110(2011)113712

A method for estimating solidification interface shapes is presented Steps involves 1. PL imaging 2. Calibration 3. Conversion based on Scheil’s law This method is useful for estimating concave/convex geometry of solid front and homogeneity of temperature field in the system. It is applicable to monocrystalline, compensated or non-compensated multicrystalline ingots.

FURTHER OBSERVATIONS

FASTER GRAINS GROWTH IS OBSERVED IN REGION WHERE SOLID FRONT SHAPE IS MORE CONVEX

Wafer 95

Fig. 5. Grains are observed to grow more rapidly in regions where solid front is strongly convex as shown in Fig. 3 & 4.

Wafer 72

Wafer 122 Wafer 172