Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend

Solar Grade-Silicon, Ingot, Wafer Technology and Market Trend (2008~2012)
3.5.3 Ribbon Wafer (Type Ⅱ)
This method instantly deletes latent heat, which is generated on mushy
interface during solidification of silicon, though cold substrates touching the
silicon. Unlike the type I that deletes latent heat with radiant heat of solidified
ribbon itself, the type II moves heat through large area substrates that it
efficiently deletes latent heat, hence grows ribbon fast. The maximum speed to
grow ribbon, Vp, is as follows.
“a” is a valid heat delivery coefficient, “s” is a mushy interface area in a
direction of ribbon growth, and “ T" is a temperature difference between liquid
phase silicon and substrate. According to the equation, the maximum speed to
grow ribbon is 600cm/sec when T is 160 o C. It rapidly creates ribbon by
efficiently deleting the latent heat and has high productivity. However, it grows
crystal starting from areas touching substrates that it has relatively smaller
crystal structure with chaotic direction compared to the type II method. The
type II method grows crystals at irregular speed which varies upon time. It has
a fast crystal growing speed at initial solidification phases where liquid phase
silicon touch substrates directly, but the speed slows down once ribbons are
created because the latent heat generated from the mushy interface
undergoes the created ribbon thickness and exits towards substrates. It is
expressed in the following equation. “s(t)” is a location of mushy interface upon
time.
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Jan’09