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Master Thesis: Manufacturing of Solar Cells - Mechanisms of ductile cutting of monocrystalline Silicon with Diamond tools
180 µm thick silicon wafers for solar cells are sliced with diamond coated wires as thin as 60 µm which is a demanding process; improvement requires fundamental understanding. This research project focuses on the material removal mechanisms and includes experimental analysis and modelling.
Keywords: manufacturing engineering, diamond wire sawing, silicon, solar cells, photovoltaic, material modelling, experimental investigation, SPH modelling, meshless methods, force modeling
Motivation
The competitiveness of solar cells depends heavily on the manufacturing process: 65% of the cost of a photovoltaic module is allocated to the wafers, of which roughly a third is attributed to the wafer sawing process and two thirds are material cost. Wafers are sawn using diamond-coated wires: in order to reduce material loss, the thickness of the wafers has been reduced to 180 µm, the thickness of the wires to 60 µm. Further reduction of the wafer thickness is only possible by reducing the damage that is induced by sawing with the goal of achieving an efficient wafer with high strength. At the Institute of Machine Tools and Manufacturing, we are looking into the fundamentals of the wire sawing process: the kinetics of the process and the mechanics of the material removal. The proposed thesis topic focuses on the material removal of monocrystalline silicon with diamonds.
Challenge and research question
Silicon is a hard and brittle material, meaning that upon exposure to mechanical stress, it cracks and breaks with very little elastic and plastic deformation. However, under certain condition, ductile cutting of silicon is possible, in which case chips are formed plastically with no fracture involved. Leading to a fault-free surface, this ductile cutting mode is favored for the surface generation of solar wafers. Achieving this cutting mode is difficult, since it is attributed to cutting depth typically smaller than 200 nm and a good fundamental understanding of the material removal process is necessary in order to make it applicable technically. This thesis focuses on identifying the dependency of the ductile to brittle transition on the shape of the cutting edge by means of experiments and modelling.
Motivation The competitiveness of solar cells depends heavily on the manufacturing process: 65% of the cost of a photovoltaic module is allocated to the wafers, of which roughly a third is attributed to the wafer sawing process and two thirds are material cost. Wafers are sawn using diamond-coated wires: in order to reduce material loss, the thickness of the wafers has been reduced to 180 µm, the thickness of the wires to 60 µm. Further reduction of the wafer thickness is only possible by reducing the damage that is induced by sawing with the goal of achieving an efficient wafer with high strength. At the Institute of Machine Tools and Manufacturing, we are looking into the fundamentals of the wire sawing process: the kinetics of the process and the mechanics of the material removal. The proposed thesis topic focuses on the material removal of monocrystalline silicon with diamonds. Challenge and research question Silicon is a hard and brittle material, meaning that upon exposure to mechanical stress, it cracks and breaks with very little elastic and plastic deformation. However, under certain condition, ductile cutting of silicon is possible, in which case chips are formed plastically with no fracture involved. Leading to a fault-free surface, this ductile cutting mode is favored for the surface generation of solar wafers. Achieving this cutting mode is difficult, since it is attributed to cutting depth typically smaller than 200 nm and a good fundamental understanding of the material removal process is necessary in order to make it applicable technically. This thesis focuses on identifying the dependency of the ductile to brittle transition on the shape of the cutting edge by means of experiments and modelling.
Since we have made good progress on all aspects of the investigation, you can focus your research according to your interest and chose a topic with emphasis on experimental work or on process modelling:
The experiments are done using a 5-axis milling machine. Individual diamond grains are measured and used to cut the silicon. The material removal and forces are analysed afterwards.
Modelling includes force and/or damage modelling. A force model that is based on the depth of cut has been verified and is to be expanded to include the effect of the grain shape. Dam-age modelling can be done empirically or applying meshless simulation methods.
The thesis offers a hands-on approach with direct impact on the manufacturing of solar cells.
Since we have made good progress on all aspects of the investigation, you can focus your research according to your interest and chose a topic with emphasis on experimental work or on process modelling: The experiments are done using a 5-axis milling machine. Individual diamond grains are measured and used to cut the silicon. The material removal and forces are analysed afterwards. Modelling includes force and/or damage modelling. A force model that is based on the depth of cut has been verified and is to be expanded to include the effect of the grain shape. Dam-age modelling can be done empirically or applying meshless simulation methods. The thesis offers a hands-on approach with direct impact on the manufacturing of solar cells.
Stefan Süssmaier
044 632 91 36
suessmaier@iwf.mavt.ethz.ch
Stefan Süssmaier 044 632 91 36 suessmaier@iwf.mavt.ethz.ch