1. Field of the Invention
The present invention relates to a silicon manufacturing method that purifies a metallic silicon material by irradiating an electron beam, and particularly to a silicon purification method where it is possible to purify high-purity silicon with a low content of phosphorous (P), iron (Fe), aluminum (Al), or calcium (Ca), which is suitable for a solar cell material.
2. Background Art
As a conventional silicon purification method, a method is disclosed that continuously purifies impurities, such as phosphorous, above a water-cooled copper hearth by vaporization, drops into a copper-made water-cooled mold, and then solidifies unidirectionally from the bottom by irradiating an electron beam to the molten surface (P. 575 to 582, No. 10, Vol. 67, Journal of the Japan Institute of Metals (October 2003)).
However, this method requires both a purification mechanism for dephosphorization and a solidification purification mechanism, which leads to a complex apparatus.
In addition, when conducting solidification purification, new silicon which has never been subjected to solidification purification is continuously fed into a molten pool and mixed at all times. As a result, the purification effect is inferior to a case where a metal to be solidification-purified is fully melted and then solidified unidirectionally.
Furthermore, in the method of P. 575 to 582, No. 10, Vol. 67, Journal of the Japan Institute of Metals (October 2003), as the height of a solidified layer increases, the temperature gradient in a liquid phase decreases in a direction perpendicular to the solidification interface in the vicinity of the interface (solidification interface) between the liquid phase and a solid phase. Therefore, a constitutional supercooling (described below) is likely to occur.
In practice, according to an analysis of the impurity concentration distribution in an ingot in an industrial sized testing apparatus, a purification effect is lowered remarkably at a position which exceeds 50% or 60% height in the depth direction.
Moreover, in order to solve the above problems, a solidification purification method has been suggested including a mechanism that rotates a water-cooled copper mold (P. 575 to 582, No. 10, Vol. 67, Journal of the Japan Institute of Metals (October 2003) and Japanese Unexamined Patent Application, First Publication No. H10-251008).
However, this method requires the addition of a mechanism that rotates the mold and reverses the rotation direction at appropriate time intervals, which leads to a problem in that the equipment becomes more complicated.
In addition, in order to actually increase purification efficiency, rotation of the mold at a high speed is required, which leads to a problem in that the molten metal scatters away due to the centrifugal force.
Furthermore, in a case where the mold is not rotated, silicon forms a shallow solidified layer, that is, scull on the wall surface of the water-cooled copper mold, but if the mold is rotated at a high speed, scull is not formed, and thus molten silicon metal and the copper mold are brought into direct contact, which necessitates that consideration be given to contamination by copper constituting the mold.
Incidentally, it is known that, particularly in the case of silicon, the equilibrium distribution coefficient (the ratio of the impurity concentration in a liquid phase to the impurity concentration in a solid phase in a case where impurities are fully homogenized in a liquid phase via convection or diffusion) of the impurity elements, such as Fe and Al, which are impurities other than B and P, is extremely small, and these impurities can be efficiently removed by solidification purification.
However, in reality, in the case of solidifying at a limited solidification speed in consideration of productivity, impurities discharged into the liquid phase from the solidification interface are distributed along the solidification interface in a higher concentration without being transported or homogenized via diffusion or convection.
The distribution coefficient of impurities in consideration of such a phenomenon, that is, the effective distribution coefficient is closer to 1 than the equilibrium distribution coefficient, thereby degrading purification efficiency.
Furthermore, in actual solidification, impurities extruded and concentrated along the solidification interface lower the melting point of the liquid phase, and, from the relative relationship between the melting point corresponding to the concentration distribution and the actual temperature distribution, unsolidified portions occur even above the melting point in the vicinity of the solidification interface.
Such a phenomenon is called constitutional supercooling, and, due to this phenomenon, the solidification interface becomes unstable and loses flatness, thereby increasing unevenness (cell growth), and, in an extreme case, the solidification interface grows in a dendrite shape (branch shape).
That is, silicon crystal grows into the liquid phase in a protrusion shape, and impurities are pushed away to both sides.
Therefore, impurity elements are segregated at a micro level, but barely segregated at a macro level, therefore a solidification purification effect is significantly lost.
In particular, such a constitutional supercooling is known to easily occur in the case of (1) a slow temperature gradient in a liquid phase in the vicinity of the solidification interface, (2) a high impurity concentration, and (3) a high solidification speed.
Among smelting methods using an electron beam heating and melting method, which have been conventionally suggested, there are problems in a method that removes impurities with a high vapor pressure, such as phosphorous, and further combines solidification purification therewith in that the equipment used is complicated and the equipment cost becomes high.
In order to solve the above problems, the object has been to develop a method in which it is possible to realize both purifications of solidification purification and dephosphorization by a simple apparatus with a low equipment cost.
As methods to solve the above problems, for example, a method has been considered that fully melts silicon using an electron beam melting method using a water-cooled copper hearth, gradually weakens the output, solidifies the melted silicon unidirectionally from the molten metal bottom toward the molten metal surface side which is irradiated by the electron beam, extrudes a liquid phase in which impurities are concentrated, and thereafter conducts dephosphorization purification.
Alternatively, a method has been considered that conducts dephosphorization purification first, and then conducts solidification unidirectionally.
However, such methods have a problem of low production efficiency since only limited metallic silicon materials can be melted at one time.
The reason is that, if a large amount of metallic silicon material is loaded into a water-cooled copper hearth in an electron beam melting furnace at one time, the metallic silicon material is not melted in the bottom, and thus scull occurs.