INFLUENCE OF SAW DAMAGE ETCHING ON THE MECHANICAL STABILITY OF MULTI-CRYSTALLINE WAFERS
H. Behnken+, R. Tiefers+, W. Krühler*, A. Froitzheim*, G. Hahn**, D. Franke+
*Shell Solar GmbH, Otto-Hahn-Ring 6, D-81739 München, Germany
**Universität Konstanz, P.O.Box X916, D-78457 Konstanz, Germany
+ACCESS e.V., Intzestr. 5, D-52072 Aachen, Germany
ABSTRACT: In solar cell industry it is well known, that etching off the saw damage from multi-crystalline wafers enhances their mechanical stability. In our work we study this effect and show various influences on wafer stability, concerning the etching process itself and preparing steps before etching. First we measure thickness variations at the edges of multi-crystalline ingot wafers before and after etching. We found a good uniformity but a clear v-formation, explainable by horizontal vibrations of the sawing wire. Secondly, we performed experiments concerning the influence of sorting the wafers into a plastic hoard, as it is in use for a batch-wise etching process. After sorting we found no pre-damage of the wafers that can reduce their mechanical stability. Thirdly, we performed three-line and point bending tests. The measured fracture stresses are clearly different by this two test geometries, although we take parallel wafers from one ingot column which are assumed to have same mechanical properties. Explanations for this effect are given by numerical simulations of wafer internal bending stress distributions. Fourth, we show the change of fracture stresses of multi-crystalline wafers after a saw damage etching with CP4 and NaOH. Wafer stability can raise by more than a factor of 3 by using CP4 with a 15 μm etch removal on each wafer side. Otherwise, a reduction of mechanical stability can be observed by using NaOH in some cases. Fifth, we show exemplary, that 5 μm etch removal in a continuous process is enough to reach high solar cell efficiencies, but is not enough to improve mechanical stability clearly.
Keywords: Multi-Crystalline, Etching, Stability
1 INTRODUCTION
Mechanical stability is one of the main criteria for cost effective mass production. A modern solar cell production of 500 MW needs about 400 Million wafers per year. Taking this into account, production failure by wafer breakage of only 1 % means 4 Million wafers lost per year. Additionally, high financial losses must be calculated if wafers break inside a production furnace or solar cells show cracks inside the finished module. In the near future mechanical properties of wafers will become of much higher importance because industry heads for larger and/or thinner wafers.
In our work we focus on various aspects of mechanical stability of multi-crystalline wafers. We measured wafer thickness variations of wire cut wafers before and after the etching process. Then we analysed the sorting procedure of wafers into a plastic hoard. We assumed that a pre-damage, caused by the drop of the wafers into the hoard, can reduce their mechanical stability.
To perform breakage force measurements various test assemblies are in use. We selected a three-line test and a point test to compare test results. To separate the influence of test assemblies from the original mechanical wafer behaviour, we selected nearby parallel wafers from one single column that are assumed to have the same mechanical properties. We support the experiments by numerical simulations of the internal bending stress distribution in the wafers [1].
Because the effect of saw damages is known in industry, the first step in solar cell production is etching off the sawing induced micro cracks [2]. Therefore, a batch-wise or continuous transport of wafers through the acid bath is in use. We performed experiments using both processes, selecting an etch removal of 15 μm and 5 μm and using CP4 and NaOH etches for our examinations.
Afterwards, we performed measurements of fracture stress with three-line and point bending test assemblies.
2 THICKNESS MEASUREMENTS OF AS SAWN AND ETCHED WAFERS
For cutting off multi-crystalline wafers from ingot columns, wire saws are used by the wafer producers.
Although, this high throughput technique shows good quality wafers, high effort is carried out to optimise this process.
2.1 Experimental method
Thickness variations at the wafers edges before and after etching the saw damage were investigated by micrometer screw measurements. A charge of 12 wafers each was selected, positioned in one ingot column. By optical analysis of the crystal structure, the wafers were attached to each others and to their orientation due to the vertical movement of the sawing wire through the silicon column.
2.2 Results
In Figure 1 results of thickness measurements of as sawn and etched wafers are shown. It is obvious, that one side was the thinnest, relative to the other edges of the wafer. This side was assumed to be the vertical entrance side of the wire into the ingot column at the beginning of the sawing process. The reason of this higher material removal is expected to be higher wire vibrations at the beginning of the sawing process, because the wire is not yet fixed inside the column volume.
The maximum absolute variation of as sawn wafer thickness is lower than 20 μm, which is clearly within wafers specification. After etching we find a uniform etch removal of about 30 μm at the wafers edges. We assume that those low variations in thickness do not influence the mechanical stability of the wafers, but it is in discussion, if higher vibrations of the sawing wire could induce a larger number of micro cracks per wafer area, or maybe deeper micro cracks into the wafer volume. This question
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is under investigation yet by microscopic analyses of crack structures at larger wafer areas.
Figure 1: Mean thickness variation at the edges of 12 as sawn multi-crystalline wafers and 12 wafers after CP4 batch-wise etching. The numbering from I to IV marks the four edges of the wafers. Edge number II is assumed to be the vertical entrance side of the sawing wire into the column.
3 SORTING WAFERS INTO A PLASTIC HOARD The manual handling of wafers is often identified as a critical step to cause wafer breakage. One manual handling step is the sorting of wafers into a plastic hoard, preparing them for a batch-wise damage etching process.
3.1 Experimental method
To investigate the influence of the sorting procedure on a possibly induced pre-damaging, we selected 24 wafers, each 2 nearby parallel from one ingot column which have the same mechanical properties. The one half of these wafers were sorted by a maximum careless drop down into the hoard. The other half were taken as reference samples. Both are tested due to their breakage force with a three line bending test.
3.2 Results
Overall, there is not a satisfying statistics so far, but our first test shows no influence of the sorting process on mechanical stability of the wafers. Only one sorted wafer shows a remarkable lower fracture stress, but this is within the normal spread of measured values, compared to additional results not presented here.
Both wafer series, sorted and not sorted, break at a mean fracture stress of about 136 MPa. From Weibull analysis it can be seen, that even the sorted wafers show a slightly higher 63 % breakage probability with 135 MPa than the reference wafers with 126 MPa, but those values are within the normal spreading of measurements. After this first result, we will continue this work to get better statistics.
4 THREE-LINE AND POINT BENDING TESTS For the testing of mechanical stability, mainly of maximum bending resistance of wafers and their fracture force, various test assemblies are in use. To compare measurement results it is necessary to study the special features of these tests.
4.1 Numerical modelling
We selected a three-line test and a point bending test for our investigations. In three-line tests the wafer is
supported by two parallel lines with a distance of 80 mm.
The bending force is induced by a third line, centered between both supports. In the case of point bending, the test force is applied at the centre of the wafers top side.
Three point supports are positioned at the bottom side on a circle of radius 40 mm. We selected the commercial software ABAQUS to perform the simulations.
4.2 Simulation results
Results in Figure 2 show, that there is a large area of high bending stress in the surrounding of the force indenter line in the three-line bending test. This high stress area covers a large wafer plain, including partly its edges. Because of this, the probability of a crack with a critical length being located within this large area is much higher than in other tests with a less expanded stress load.
Therefore the three-line test yields lower fracture stresses in general.
Contrariwise, the stress distribution in the cases of the point bending shows maximum stress located on a very small area under the intender point. This means that the edges of the wafer are not covered by high bending stresses and thus edge influences on the mechanical behaviour is not taken into account by selecting point bending test assemblies.
Figure 2: Sketches of two test assemblies and simulation results of the wafer maximum bending stress.
4.3 Experimental results
Results from bending tests are shown in Figure 4.
Here, Weibull statistics of as sawn and etched wafers are compared by using both test methods. It can be seen that the 63 % fracture probability is clearly different for as sawn wafers in both test methods (black lines and dots in both figures). For the line-bending we measured 150 MPa and for the point bending test 190 MPa. Because both series of wafers were assumed to have the same mechanical properties, differing measurement results follow from the test procedures itself. The explanation for this effect is as given in the previous chapter. The result shows, that the larger tested area and edge effects reduce the measured value of mechanical stability noticeable in
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comparison to the very locally tested wafers by the point bending.
5 CHANGE OF MECHANICAL STABILITY AFTER SAW DAMAGE ETCHING
We selected batch-wise process conditions for our experiments, using CP4 and NaOH to compare the results. Wafer surfaces after etching are shown in Figure 3. Furthermore, we performed etching experiments in a continuous process.
Figure 3: Surface structures of CP4 (left) and NaOH (right) etched wafers.
5.1 Experimental method
To study the effect of saw damage etching, we chose the following procedure: we selected parallel wafers from one single ingot column for all our experiments. By this, we can compare all results from the different measurements of fracture stress. We convert the measured fracture forces into internal acting stresses in the wafers by results of numerical simulations. So we can use the Weibull statistics to compare test results in terms of fracture stress and fracture probability. We use sets of 47 wafers each for batch-wise and continuous etching processes. In batch processes we adjust an etch removal of 15 μm on each wafers sides and of 5 μm for the continuous etching.
5.2 Results of batch-wise CP4 etching
The influence of the saw damage etching on the mechanical stability of the wafers can be seen in Figure 4.
In values of 63 % fracture probability, stability raises by a factor of 1.9 from 150 MPa up to 280 MPa, measured with the tree-line bending. Using the point bending test, a raise in mechanical stability by a factor of 3.2 from 190 MPa up to 600 MPa is observed.
The differences by selecting various test assemblies can be explained as done before, especially it can be seen that the influence of the wafers edge and the larger tested wafer surface is much more drastic after etching.
Additionally it can be seen, that the spreading of fracture stress is much higher after etching. This can be interpreted as follows: In some wafers micro cracks exist, that reach deeper into the wafers volume than the etched material thickness of 15 μm. Those cracks persist the etch removal and reduce the wafer stability because they act as source for the wafer fracture. A higher etch removal should reduce the spreading, but it must be taken into account, that only some deep cracks could resist and may be enough to prevent a better wafer stability. One alternative in etching technique could be to remove the most micro cracks by a practicable etching depth and to
round the tip region of those cracks reaching deeper into the wafers volume.
Figure 4: Comparison of fracture stresses of as sawn and of etched wafers, tested by three-line and point bending.
5.3 Results of batch-wise NaOH etching
In Figure 3 it can be seen, that the use of NaOH results in a clearly structured wafer surface. Especially at the edges a rough structure with a noticeable number of notches can be observed.
In Figure 4 the results of the bending tests are summarised. For the three-line bending test the 63 % fracture probability raises lightly from 150 MPa to 165 MPa, but about half of the etched wafers show a lower fracture stress than before the etching. Because this effect is not observed in the point bending test, where the fracture probability raise from 190 MPa to 400 MPa, it can be assumed that the reduced stability is caused by the worse wafer edge behaviour.
5.4 Inhomogeneity of fracture stresses in the hoard In Figure 5 two examples are given, how the mechanical stability of wafers can vary after saw damage etching, in dependency on the position of the wafers in the hoard. The selected examples show a clear decrease of fracture stress, if wafers are located in the center of the hoard, compared to those at the hoard side. In some additional experiments it was found, that this effect is symmetric from the hoard center to both sides. These results are found for CP4 and NaOH, but are not reproducible in all the performed experiments.
We assume the following effect to be responsible for those variations: the fluid flow in the acid bath can vary
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in the clearance between the wafers. This effect should depend on the inlet and outlet flow of the acid from the bath, or the technical implementing of acid heating.
Because we found no clear correlation of thickness fluctuations on dependency to the position of the wafers in the hoard we continue our investigations by more etching experiments under various etch bath conditions to be sure of the reasons of the observed effect.
Figure 5: Inhomogeneity of wafer fracture stress in dependency to their position in the hoard during the etching. The selected examples are taken from the NaOH experiments, but results are comparable to the CP4 etch.
5.5 CP4-structure etching in a continuous process To compare various etching techniques, we started investigations regarding a continuous etching process.
Our expectance was to find a higher homogeneity of mechanical stability by using a continuous process in comparison to the batch-wise process. We select a CP4- structure etching at elevated temperature and subsequent cleaning of the wafers. For a first approach, we chose a low material removal of 5 μm on each wafers side. After etching, all wafers are tested due to their mechanical stability by using a three-line and point bending tests.
5.6 Results of continuous CP4-structure etching
Results of mechanical tests are summarised in Figure 6. It can be seen clearly, that the low material removal of only 5 μm on each wafers side could not raise the mechanical stability as shown for the higher etch removal documented in Figure 4.
The 63 % fracture probability was found at 150 MPa for the as sawn reference wafers and at 158 MPa after etching for the three-line bending and from 190 MPa up to 242 MPa in point bending test. It is interesting to mention, that the chosen etching process is well suited for high solar cell efficiency. For a first interpretation of this result it can be assumed, that micro cracks from the wire sawing process are not as critical for the electrical performance of solar cells.
6 OUTLOOK
Our first objective in the ongoing work is to get better statistics for our experiments. Actually we prepare more batch-wise etching experiments under well controlled conditions of fluid flow in the acid bath.
Figure 6: Comparison of fracture stresses of as sawn and etched wafers in a continuous etching process. Because the etch removal was selected to be only 5 μm there is not a as clear improvement in mechanical stability as shown for the higher values in Figure 4. Apart from this, solar cells produced from those wafers show a good electrical efficiency.
As shown, the spreading of mechanical stability of the wafers after saw damage etching covers a wide range.
While the best wafers pass solar cell production without mechanical problems, only some of the worst wafers could cause the problems. We are looking for a test method to identify and separate wafers which show a lower mechanical stability.
At least, we think that the comparison of results from batch-wise and continuous processes will be of interest, especially to show alternatives to the inhomogeneity of mechanical stability in dependency to the position of the wafers in the hoard.
REFERENCES
[1] H. Behnken, M. Apel, D. Franke 2003
"Simulation of Mechanical Stress During Bending Tests for Crystalline Wafers"
In Proc.: 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, May 11-18, pp.
1308-1311.
[2] W. Koch, A. L. Endrös, D. Franke, C. Häßler, J. P.
Kalejs 2003
"Bulk Crystal Crowth and Wafering for PV". In:
Handbook of Photovoltaic Science and Engineering, Ed. A. Luque and S. Hegedus, John Wiley & Sons Ltd, pp. 223-230.
ACKNOWLEDGEMENT
This work is part of the German ASIS project. Within this project 11 university institutes and 3 companies are working together to support the German photovoltaic industry. Our work within this project was carried out with financial support of the German Federal Ministry of Economics and Technology under grant number 0329846C, which is gratefully acknowledged.
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