it solidifies again and adopts the orientation of the seed crystal. Again 100 or 111
orientations are being used. As the molten zone is moved along the polysilicon rod, the
single-crystal ingot is grown as well. Many impurities remain in and move along with the
molten zone. Nowadays, during the process nitrogen is intentionally added in order to
improve the control over microdefects and the mechanical strength of the wafers. One
advantage of the float zone technique is that the molten silicon is not in contact with other
materials like quartz, as is the case when using the Czochralski method. In the float zone
process the molten silicon is only in contact with the inert gas such as argon. The silicon
can be doped by adding doping gasses like diborane (B 2 H 6 ) and phosphine (PH 3 ) to the
inert gas to get p-doped and n-doped silicon, respectively. The diameter of float-zone
processed ingots is generally not larger than 150 mm, as the size is limited by surface
tensions during the growth.
Both monocrystalline silicon ingots and multicrystalline silicon ingots, which consist
of many small crystalline grains, can be fabricated (not shown in Figure 12.7). This can be
made by melting highly purified silicon in a crucible and pouring the molten silicon into a
cubic-shaped growth crucible. There, the molten silicon solidifies into a multicrystalline
ingot in a process called silicon casting. If both melting and solidification is done in the
same crucible it is referred to as directional solidification. Such multicrystalline ingots can
have a front surface area of up to 70 × 70 cm
2
and a height of up to 25 cm.
Now, as we know how to produce monocrystalline and multicrystalline ingots we will
discuss how to make wafers out of them. The process that is used to make the wafers is
sawing, as illustrated in Figure 12.7. Logically, sawing will damage the surface of the
wafers. Therefore, this processing step is followed by a polishing step. The biggest
disadvantage of sawing is that a significant fraction of the silicon is lost as kerf loss, which
is usually determined by the thickness of the wire or saw used for sawing. Generally, it is
in the order of 100 μm. As typical wafers used in modern solar cells have thicknesses in
the order of 150 μm up to 200 μm, the kerf loss is very significant.
A completely different process for making silicon wafers is the silicon ribbon
technique (not shown in Figure 12.7). As this technique does not include any sawing steps,
no kerf loss occurs. In the silicon ribbon technique a string is used that is resistant against
high temperatures. This string is pulled up from a silicon melt. The silicon solidifies on the
string and hence a sheet of crystalline silicon is pulled out of the melt. Then, the ribbon is
cut into wafers. Before the wafers can be processed further in order to make solar cells,
some surface treatments are required. The electronic quality of ribbon silicon is not as
high as that of monocrystalline silicon.
To summarize, we discussed how to make metallurgical silicon out of quartzite and
orientations are being used. As the molten zone is moved along the polysilicon rod, the
single-crystal ingot is grown as well. Many impurities remain in and move along with the
molten zone. Nowadays, during the process nitrogen is intentionally added in order to
improve the control over microdefects and the mechanical strength of the wafers. One
advantage of the float zone technique is that the molten silicon is not in contact with other
materials like quartz, as is the case when using the Czochralski method. In the float zone
process the molten silicon is only in contact with the inert gas such as argon. The silicon
can be doped by adding doping gasses like diborane (B 2 H 6 ) and phosphine (PH 3 ) to the
inert gas to get p-doped and n-doped silicon, respectively. The diameter of float-zone
processed ingots is generally not larger than 150 mm, as the size is limited by surface
tensions during the growth.
Both monocrystalline silicon ingots and multicrystalline silicon ingots, which consist
of many small crystalline grains, can be fabricated (not shown in Figure 12.7). This can be
made by melting highly purified silicon in a crucible and pouring the molten silicon into a
cubic-shaped growth crucible. There, the molten silicon solidifies into a multicrystalline
ingot in a process called silicon casting. If both melting and solidification is done in the
same crucible it is referred to as directional solidification. Such multicrystalline ingots can
have a front surface area of up to 70 × 70 cm
2
and a height of up to 25 cm.
Now, as we know how to produce monocrystalline and multicrystalline ingots we will
discuss how to make wafers out of them. The process that is used to make the wafers is
sawing, as illustrated in Figure 12.7. Logically, sawing will damage the surface of the
wafers. Therefore, this processing step is followed by a polishing step. The biggest
disadvantage of sawing is that a significant fraction of the silicon is lost as kerf loss, which
is usually determined by the thickness of the wire or saw used for sawing. Generally, it is
in the order of 100 μm. As typical wafers used in modern solar cells have thicknesses in
the order of 150 μm up to 200 μm, the kerf loss is very significant.
A completely different process for making silicon wafers is the silicon ribbon
technique (not shown in Figure 12.7). As this technique does not include any sawing steps,
no kerf loss occurs. In the silicon ribbon technique a string is used that is resistant against
high temperatures. This string is pulled up from a silicon melt. The silicon solidifies on the
string and hence a sheet of crystalline silicon is pulled out of the melt. Then, the ribbon is
cut into wafers. Before the wafers can be processed further in order to make solar cells,
some surface treatments are required. The electronic quality of ribbon silicon is not as
high as that of monocrystalline silicon.
To summarize, we discussed how to make metallurgical silicon out of quartzite and