The implementation of photovoltaic solar energy based on silicon is being slowed down by the shortage of raw material. In this context, the use of thinner wafers arises as a solution reducing the amount of silicon in the photovoltaic modules. On the other hand, the manufacturing process with thinner wafers can become complicated with traditional tools. The high number of damaged wafers reduces the global yield. It’s known that edge and surface cracks and defects determine the mechanical strength of wafers. There are several ways of removing these defects e. g. subjecting wafers to a mechanical polishing or to a chemical etching. This paper shows a comparison between different surface treatments and their influence on the mechanical strength

Alarcón Álvarez, Enrique

Barredo Egusquiza, Josu

Fraile de Lerma, Alberto

Hermanns, Lutz Karl Heinz

Jimeno, Juan Carlos

Archivo Digital UPM

The Influence of Different Surface Treatments on the Mechanical Strength of Silicon Wafers

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

 THE INFLUENCE OF DIFFERENT SURFACE TREATMENTS ON THE 
MECHANICAL STRENGTH OF SILICON WAFERS  
 
 
J. Barredo1, L. Hermanns1, A. Fraile1, J. C. Jimeno2 and E. Alarcón1 
Author for correspondence: Josu Barredo 
1Department of Structural Mechanics and Industrial Constructions, Polytechnical University of Madrid, c/ José Gutiérrez 
Abascal 2, 28006 Madrid, Spain. Tel: +34-91-336-53-46, email: jbarredo@etsii.upm.es 
2Microelectronic Technology Institute, Basque Country University-Engineering School, Alameda de Urquijo s/n, 48013 
Bilbao, Spain, email: jcjimeno@jet.es 
 
 
ABSTRACT: The implementation of photovoltaic solar energy based on silicon is being slowed down by the 
shortage of raw material. In this context, the use of thinner wafers arises as a solution reducing the amount of silicon 
in the photovoltaic modules. On the other hand, the manufacturing process with thinner wafers can become 
complicated with traditional tools. The high number of damaged wafers reduces the global yield. It’s known that 
edge and surface cracks and defects determine the mechanical strength of wafers. There are several ways of 
removing these defects e. g. subjecting wafers to a mechanical polishing or to a chemical etching. This paper shows a 
comparison between different surface treatments and their influence on the mechanical strength.  
Keywords: Cost reduction, Manufacturing and Processing, Reliability 
 
 
1 INTRODUCTION 
 
The most widely used material in photovoltaic 
industry is monocrystalline silicon [5]. The cost 
increment of PV modules caused by recent lacks of raw 
material may stop a bigger growth of the photovoltaic 
industry [2, 3]. One way of reducing the cost comes with 
the use of thinner wafers. The main problem of the 
thinner wafers is that the manipulation with traditional 
tools increases the number of broken wafers reducing the 
global yield. Different studies about the mechanical 
properties of silicon wafers have been carried out last 
years [1, 2, 3, 4].  
It’s known that the mechanical strength of wafers is 
determined by the cracks and defects present in the wafer 
[1, 2, 3, 4, 6]. There are several ways of removing these 
defects. This paper presents the results of a study of the 
mechanical properties of monocrystalline silicon wafers. 
First, the steps necessary to get the strength are explained 
in detail. Then, the effect of reducing the thickness of 
wafers and the influence in the strength of different steps 
in the manufacturing process has been analyzed. Finally, 
a comparison between different methods of removing 
surface cracks has been carried out. 
 
 
2 DIFFERENT STEPS INVOLVED IN THE STUDY 
 
The study of the mechanical properties of wafers 
implies to carry out a lot of tests. Wafers with different 
cleavage plane orientations and with different surface 
treatments have been tested making possible a wide 
analysis of several variables. The main steps of the study 
are explained in detail below. 
 
2.1 Test description 
The test chosen for this study has been the four line 
bending test (Figure 1). The force is applied to the two 
supports on top of the wafer while the other two are 
fixed. The displacement of the supports and the force 
applied is recorded.  
With this fracture test both surface and edge cracks 
and defects are contribute to failure [1, 2, 3, 4]. 
 
 
 
Figure 1: The four line bending test 
 
2.2 Numerical model 
In order to determine the stress state of the wafer in 
the fracture moment, it’s necessary to develop a 
numerical model. Wafers in the four line bending test 
present a non-linear behaviour due to large displacement 
and contact between supports and wafer.  
A Finite Element model using the commercial 
software ANSYS has been developed. The large 
displacement formulation and the sliding and friction in 
the contact points have been taken into account. 
 
Wafer 15. Thickness=133.5 μm
0
20
40
60
80
0 0.5 1 1.5 2
Displacement (mm)
Fo
rc
e 
(N
)
Test result
ANSYS simulation
 
 
Figure 2: Fitting of test results and numerical simulation 
 
2.3 Test results 
Simulating the wafer behaviour in the test with the 
numerical model (Figure 2) it’s possible to obtain the 
maximum stress in the wafer in the fracture moment. 
This result has been taken as the mechanical strength of 
the wafer [1]. Finally, wafer with the same properties 
22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
1164
have been grouped and statistical calculus has been 
applied in order to get a global result for each group.  
 
 
3 INFLUENCE OF THE SURFACE TREATMENT 
IN THE MECHANICAL STRENGTH 
 
3.1 Relation decreased thickness-mechanical strength 
It’s known that the mechanical strength is determined 
by the cracks and defects present in the wafers surface 
and edges [1, 2, 3, 4, 6]. Many of these cracks and 
defects are generated in the wire sawing process. So, an 
obvious way to increase the wafers strength is removing 
the work-affected layer. In order to study this influence, 
several wafers have been prepared for being tested. In 
table I a summary is presented with the average values 
for each group of wafers tested.  
 
Table I: Summary of wafer tested 
 
Original thickness Decreased thickness Thickness in 
 (μm) per face (μm) test (μm) 
 355 70 215 
 212 0 212 
 212 0.6 211 
 211 5.6 200 
 210 12 186 
 207 17.5 172 
 209 24.5 160 
 207 28 151 
 209 30 149 
 211 43 125 
 206 53 100 
 141 0.6 140 
 141 5.6 130 
 140 12 116 
 
Several wafers of each group have been tested. The 
numerical model for each wafer has been adjusted for the 
test result and the maximum stress at the fracture moment 
has been obtained. The mean value for each group is 
represented in the figure 3: 
 
Mean values of the maximum bending stress at the fracture 
moment  
0
50
100
150
200
250
300
350
90 110 130 150 170 190 210 230 250
Thickness (μm)
M
ax
im
um
 b
en
di
ng
 s
tr
es
s 
(M
Pa
)
Orig. Thickness=0.35 mm Orig. Thickness=0.215 mm Orig. Thickness=0.14 mm
 
 
Figure 3: Maximum bending stress according to the 
wafer thickness 
 
As can be seen, wafers of each group present a 
similar tendency in the maximum stress with the 
decreased thickness. They have been grouped according 
to the decreased thickness, independently of the thickness 
during the test. In the Figure 4 the bending strength with 
the decreased thickness per face has been represented. 
Mean values of maximum bending strength
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60 70 80
Decreased thickness per face (μm)
B
en
di
ng
 s
tr
en
gt
h 
(M
Pa
)
 
 
Figure 4: Bending strength versus decreased thickness 
 
The bending strength rises linearly with the 
decreased thickness up to a value close to 300 MPa 
which corresponds with a decreased thickness of 30 μm 
per face. Since then, it seems that the bending strength 
becomes constant. So, it could be concluded that for the 
wafers tested the most important surface cracks reside in 
an approximately 30 μm thick top layer [1].  
The wafers tested for this analysis have the cleavage 
plane orientations of 45º from the edge. Their thicknesses 
have been reduced by caustic soda baths of different 
duration.  
These results have been taken as the reference for the 
comparison with those obtained with other methods to 
decrease thickness. 
 
3.2 Influence of different steps in the solar cell 
manufacturing process in the mechanical strength 
In a PV factory, after a wafer is cut from an ingot, 
there are several steps to convert the wafer in a solar cell. 
The influence of some of these steps in the mechanical 
strength has been analyzed.  
The processes chosen for the analysis have been the 
texturing procedure and the phosphorus addition. The 
influence of these processes has been studied for 
different thicknesses of wafers. To this end, wafers 
subjected to caustic soda baths of different duration have 
been prepared previous to the analysis of the different 
steps. These wafers have been divided into three groups. 
The wafers of the first group have been tested to get a 
reference value. Those included in the second group have 
been subjected a texturing process while the wafers on 
the third group have been added phosphorus. Then, the 
last two groups have been tested and the complete 
process explained above has been applied (Figure 5).  
 
The influence of some steps in the cell manufacturing 
process
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60 70 80
Decreased thickness per face (μm)
B
en
di
ng
 s
tr
en
gt
h 
(M
Pa
)
After NaOH etching After texturing procedure After phosphorus addition
 
 
Figure 5: The influence of some steps in the mechanical 
strength 
22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
1165
The wafers prepared for this study have the cleavage 
planes oriented 22.5º from the edge. The original 
thickness was 330 microns before starting any surface 
treatment.  
It seems that the texturing process and the addition of 
phosphorus don’t affect significantly the bending 
strength of the wafers.  
 
3.3 Influence of the cleavage plane orientation in the 
mechanical strength 
The previous results show that the two steps analyzed 
don’t modify the mechanical strength of the silicon 
wafers. All these points can be grouped in a unique curve 
which corresponds to the relation between decreased 
thickness and the mechanical strength for wafers with 
cleavage planes oriented 22.5º from the edge. The 
influence of the cleavage plane orientation in the 
mechanical strength can be analyzed comparing the 
curves of decreased thickness-mechanical strength for 
each group or wafers (Figure 6).  
 
The influence of the cleavage planes orientation
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60 70 80
Decreased thickness per face (μm)
B
en
di
ng
 s
tr
en
gt
h 
(M
Pa
)
Cleavage planes orientation: 45º Cleavage planes orientation: 22.5º
 
 
Figure 6: The influence of the cleavage planes 
orientation in the mechanical strength 
 
The results show that the mechanical strength is not 
significantly affected by the orientation of the cleavage 
planes. This result confirms the conclusion drawn in [1]. 
 
 
4 COMPARISON BETWEEN METHODS OF 
DECREASING THICKNESS 
 
4.1 Mechanical polishing 
Another different way of decreasing the thickness of 
wafers could be the mechanical polishing. This method 
doesn’t employ any chemical product and decreases the 
thickness through a physical contact.  
For the study of this method, two different groups of 
wafers have been prepared. The original thicknesses in 
both cases were 210 μm approximately. Through a 
mechanical polishing the thickness of these wafers has 
been reduced to 177 μm and to 147 μm. The results are 
presented in Figure 7 with the reference curve shown 
before. 
The results show that wafers subjected to a 
mechanical polishing present a lower mechanical 
strength than wafers bathed in caustic soda. It’s thought 
that the mechanical polishing caused a surface damaged 
in the wafers that justifies the low values of the strength. 
This surface damage in the wafer may be caused by the 
physical contact. 
Comparison with the mechanical polishing
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60 70 80
Decreased thickness per face (μm)
B
en
di
ng
 s
tr
en
gt
h 
(M
Pa
)
Caustic soda bath Mechanical polishing
 
 
Figure 7: Comparison between the caustic soda bath and 
the mechanical polishing 
 
4.2 Acid bath 
The behaviour of wafers subjected to an acid bath has 
also been analyzed. The acid bath is more aggressive 
than the basic one and it’s more difficult to control the 
decreasing thickness process.  
For this study, two different groups of 
monocrystalline wafers with cleavage plane oriented 45º 
from the edge have been employed. Ones were cut with a 
thickness of 350 μm and the others with a thickness of 
210 μm. The wafers of the second group were bathed in a 
solution of HF-HNO3 for different duration getting 
several thicknesses for testing. However, the treatment 
for the thicker wafers was a bit different. First, the 
thickness was reduced through a basic bath with caustic 
soda. And finally, they were put into a solution of HF-
HNO3 during five and eight minutes, getting wafers with 
a decreased thickness of 60 and 70 μm per face 
respectively (the last two points in the right corner of the 
figure 8). The results of the test are plotted together with 
the reference in the figure below. 
 
Comparison between acid and basic bath
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60 70 80
Decreased thickness per face (μm)
B
en
di
ng
 s
tr
en
gt
h 
(M
Pa
)
Caustic soda bath Acid bath
 
 
Figure 8: Comparison between the caustic soda bath and 
the acid bath 
 
There are some points which have similar behaviour 
as in the basic bath case but there are another which are 
too far of the reference curve. The results obtained don’t 
permit to draw a final conclusion at this moment. More 
experiments have to be carried out and further research is 
necessary in order to know the final state of the wafer 
after an acid bath.  
 
 
 
 
22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
1166
5 SUMMARY AND CONCLUSIONS 
 
It’s well known that the surface damage caused by 
the multi wire sawing process reduces the maximum 
stress that the wafer can support. So, an obvious way of 
increasing the mechanical strength is removing the work-
affected layers of the wafers. This paper shows an 
analysis of the influence of the surface treatments in the 
mechanical strength and a comparative study of different 
methods of decreasing thickness. 
The main conclusions that can be drawn are: 
• Decreasing thickness, the behaviour of wafers is the 
same despite they were cut with different 
thicknesses. So, the most important issue for getting 
a high value of mechanical strength is to eliminate 
the damaged layers, independently of the initial 
thickness.  
• The mechanical strength increases linearly with the 
reduction of the thickness. Decreasing the thickness 
over 30 μm per face, the strength remains constant. 
However these conclusions are only valid when a 
caustic soda bath is used for the removal of the 
surface layers. 
• The texturing procedure and the phosphorus addition 
process don’t seem to affect significantly the 
mechanical properties. 
• The orientation of the cleavage plane is not very 
important in the mechanical strength. 
• The caustic soda bath appears as the most reliable 
way of reducing thickness of the methods analyzed.  
• The mechanical polishing process leads to low values 
of mechanical strength of wafers. It could be justified 
by the damage caused by the physic contact. 
• The acid bath results are very scattered. This way of 
decreasing thickness is more aggressive than the 
basic bath and the final state of the wafer is 
unknown.  
 
Several ways are being studied to confirm the results 
obtained so far. First, the influence of other steps in the 
manufacturing solar cells process is being analyzed, as 
can be the addition of HF. Moreover, it’s interesting to 
study the behaviour of wafers subjected to an acid bath. 
Finally, the same study for multicrystalline silicon wafers 
is going to be carried out. 
 
 
6 ACKNOWLEDGMENTS 
 
This work has been carried out under the sponsorship 
of the University and Investigation State Secretary, 
belonging to Spanish Ministry of Education and Science, 
in its contract ENE2005-09431-C03-03. The presented 
results have been obtained in cooperation with Institute 
of Solar Energy (IES) and Microelectronic Technology 
Institute (TIM). The authors want to thank Isofotón for 
providing wafers. The authors want to thank also to 
Philippe Nasch of HCT Shaping Systems for supplying 
thinner wafers and Stephan Glunz for the supply of 
mechanically polished wafers. 
 
 
 
 
 
7 REFERENCES 
 
[1] J. Barredo, A. Fraile, J. C. Jimeno, E. Alarcón. 
Mechanical characterization of monocrystalline 
silicon wafers. 20th European Photovoltaic Solar 
Energy Conference, Barcelona, Spain, 2005 
[2] C. Funke, O. Sciurova, S. Kaminski, W. Fütterer, H. 
J. Möller. Mechanical properties of thin silicon 
wafers. 21st European Photovoltaic Solar Energy 
Conference, Dresden, Germany, 2006 
[3] E. Cereceda, J. R. Gutiérrez, J. C. Jimeno, J. Barredo, 
A. Fraile, E. Alarcón, S. Ostapenko, A. Martínez, M. 
A. Vázquez. Crack origin and detection in thin 
cristallyne solar cells in a production line. 22nd 
European Photovoltaic Solar Energy Conference, 
Milan, Italy, 2007 
[4] B. Cotterell, Z. Chen, J. B. Han, N. X. Tan. The 
Strength of the Silicon Die in Flip-Chip Assemblies. 
Journal of electronic packaging, ASME, USA, 2003. 
[5] A. Luque, S. Hegedus. Handbook of Photovoltaic 
Science and Engineering. John Wiley & Sons Ltd, 
West Sussex, England, 2003 
[6] D. Broek. Elementary Engineering Fracture 
Mechanics. Martinus Nijhoff Publishers, 1986 
 
22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
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The Influence of Different Surface Treatments on the Mechanical Strength of Silicon Wafers

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