Patent EP-2028292-A2 (EP)
Inventor: MUELLER MATTHIAS
IPC: C30B11/14
Filed: 2008-07-28
Abstract: The method for producing monocrystalline silicon bodies e.g. silicon ingots in the vertical gradient freeze (VGF) procedure, comprises melting a semimetallic or metallic raw material in a melting pot (2, 4) to form a melt (3) and solidifying the melt under effect of a temperature gradient in a vertical direction and from lower end of the melting pot to its upper end to form monocrystalline metallic or semimetallic body. Before introducing the semimetallic or metallic raw material into the pot, the base of the melting pot is covered with a thin monocrystalline priming plate. The method for producing monocrystalline silicon bodies e.g. silicon ingots in the vertical gradient freeze (VGF) procedure, comprises melting a semimetallic or metallic raw material in a melting pot (2, 4) to form a melt (3) and solidifying the melt under effect of a temperature gradient in a vertical direction and from lower end of the melting pot to its upper end to form monocrystalline metallic or semimetallic body. Before introducing the semimetallic or metallic raw material into the pot, the base of the melting pot is covered with a thin monocrystalline priming plate having a crystal orientation parallel to the vertical direction of the melting pot. The temperature of the base of the melting pot is held at a temperature below the melting temperature of the raw material to prevent melting of the priming plate. The priming plate completely covers the base of the pot. The priming plate comprises inner monocrystalline priming plates, which are directly arranged together to completely cover the base of the melting pot. The priming plates have a constant thickness or different thicknesses. The priming plate is separated from the metallic or semimetallic body of a pre-load. The temperature gradient maintains an even, horizontal phase boundary between liquid and solid aggregate condition of the semimetal or metal. A small average section of the base of the melting pot is covered with the priming plate to begin the production of the pre-load. The priming plate for the load is separated from the upper end of the metallic or semimetallic body of the pre-load. The priming plate is manufactured through zone melting after Czochralski method to form the metallic or semimetallic body and the priming plates with rectangular or square base form are separated. The metallic or semimetallic body is partitioned through saws along the sawing lines extending parallel to the crystal orientation into multiple monocrystalline metallic or semimetallic bodies and the priming plates are arranged on the base of the melting pots in such a manner that edges of the priming plates define the beginning of the later sawing lines. The direction of the temperature gradient is constant during the entire procedure. The temperature of the base of the melting pot is 1380[deg] C. The melting pot has a rectangular or square cross section. A heater surrounding the melting pot mechanism comprises a top heating element (6) and a mantle heater (7) surrounding the side walls of the melting pot. The amount of heat of the mantle heater decreases during solidifying from the upper end to the lower end of the melting pot in correspondence to the temperature gradient in the center of the melting pot. The mantle heater maintains uniform isotherms perpendicularly to the longitudinal direction and comprises heating elements, which have meander-shaped flows in the longitudinal direction of the melting pot or perpendicular to the melting pot. The heating elements are perpendicular to the longitudinal direction of extending bars, where the conductor cross sections of the bars increases from the upper end to the lower end in continuous or discrete steps. The bars extend in equidistant and parallel manner and are provided with a conductor cross section, which is narrowed at reversal areas of the meander-shaped flow in diagonal direction so that the conductor cross section equals the conductor cross section of an assigned bar before or after the respective reversal areas. The narrowings of the conductor cross section are formed by punchings or recesses in and/or from the bar materials, which are formed transverse to the conductor cross section in distributed manner. The bars are provided with segments, which are detachably interconnected by connecting elements or interconnected in a material fit. The thermal insulation is not intended between the side wall of the melting pot and the mantle heater. The metallic or semimetallic raw material is lumpy, granular silicon, which is melted by the top edge of the melting pot so that melted, liquid silicon flows downward through the silicon filling. For filling the pot with the raw material, average or fine silicon granulates are applied on the base covered with the priming plate. The silicon granulate is brought in a thin layer and then large silicon plates are brought in horizontal orientation. The large silicon plates are covered by further silicon granulates. The silicon filling is covered by smaller pieces of silicon. The smaller pieces of silicon are pieces of silicon, which originate from a recycling process and/or purification process of waste material, silicon fraction from pre-loads or silicon-wafer fraction. An independent claim is included for monocrystalline silicon wafer.