Reduced wetting string for ribbon crystal
A string for use in a string ribbon crystal has a base portion with a refractory material, and an externally exposed layer radially outward of the refractory material. The base portion has a coefficient of thermal expansion that is generally matched with the coefficient of thermal expansion for silicon. The externally exposed layer has a contact angle with silicon of between about 15 and 120 degrees.
1. A string for use in a string ribbon crystal comprising a crystal material, the crystal material being one of silicon, silicon-germanium, gallium arsenide and indium phosphide, the string comprising:
a substrate;
a refractory layer supported on the substrate;
an externally exposed layer having a contact angle with the crystal material of between about 15 and 120 degrees, the externally exposed layer being radially outward of the refractory layer; and
a handling layer radially outward of the refractory layer, the handling layer applying a generally radially inward force to the refractory layer.
2. The string as defined by claim 1 wherein the handling layer includes the externally exposed layer.
3. The string as defined by claim 1 wherein the externally exposed layer is radially outward of the handling layer.
4. The string as defined by claim 1 wherein the externally exposed layer comprises at least one of pyrolytic carbon, oxide, and nitride.
5. The string as defined by claim 1 wherein the externally exposed layer has a contact angle with the crystal material of greater than about 25 degrees.
6. The string as defined by claim 1 wherein the substrate comprises carbon, the refractory layer comprising silicon carbide.
7. The string as defined by claim 1 wherein the crystal material has a material coefficient of thermal expansion, the substrate, refractory layer, and exposed layer having a combined coefficient of thermal expansion that is substantially matched to the material coefficient of thermal expansion.
8. The string as defined by claim 1 wherein the exposed layer is thinner than the refractory layer.
9. The string as defined by claim 1 wherein the string has a coefficient of thermal expansion that is generally matched to the coefficient of thermal expansion of polysilicon.
10. The string as defined by claim 1 , wherein the handling layer has a contact angle with the crystal material of between about 15 and 120 degrees.