Use of cover members when preparing a melt of silicon in a crucible assembly

A method for preparing a silicon melt in a crucible assembly of a crystal pulling system, the crystal pulling system comprising a housing defining a growth chamber, the crucible assembly being disposed within the growth chamber for containing the silicon melt and a heat shield that defines a central passage through which an ingot passes during ingot growth, the crucible assembly comprising a bottom, an outer sidewall, an inner weir that extends upward from the bottom, and a central weir disposed between the outer sidewall and inner weir, a crucible melt zone being disposed between the outer sidewall and the central weir, an intermediate zone being disposed between the central weir and the inner weir, and a growth zone being disposed within the inner weir, the method comprising:

• adding an initial charge of polycrystalline silicon to the crucible assembly;
• lowering a cover member through the central passage defined by the heat shield to cover at least a portion of the initial charge, the cover member comprising one or more plates and a shaft that supports the cover member;
• heating the initial silicon charge to produce the silicon melt in the crucible assembly while the cover member covers at least a portion of the initial charge;
• adding a secondary amount of polycrystalline silicon to the crucible melt zone after the initial silicon charge has melted, the secondary amount of polycrystalline silicon being introduced through a silicon feed tube, the silicon feed tube extending through the housing and into the crucible assembly;
• melting the secondary amount of polycrystalline silicon to cause the silicon melt to flow through a central weir opening to the intermediate zone disposed between the central weir and the inner weir, and to cause silicon melt to flow through an inner weir opening to the growth zone disposed within the inner weir, the cover member covering at least a portion of the silicon melt while adding the secondary amount of polycrystalline silicon to the crucible melt zone;
• and
• raising the cover member after the secondary amount of polycrystalline silicon has melted.