A silicon substrate with the dimensions of 100 × 140 × 0.3 mm was grown directly from liquid silicon with gas pressure. The silicon melt in the sealed melting part was injected into the growth part at applied pressure of 780–850 Torr. The solidified silicon substrate was then transferred by the pull of the cooled dummy bar. A desirable structure with a liquid-solid interface perpendicular to the pulling direction was formed when the mold temperature in the solidification zone of the growth part was much higher than that of the dummy bar, as this technique should be able to overcome thermal loss through the molds and the limited heat flux derived from the very narrow contact area between the silicon melt and the dummy bar. In addition, because the metallic impurities and expansion of volume during solidification are preferably moved to a liquid phase, a high-quality silicon substrate, without defects such as cracks and impurities in the substrate, could be manufactured in the interface structure. The present study reports the experimental findings on a new and direct growth system for obtaining silicon substrates characterized by high quality and productivity, as a candidate for alternate routes for the fabrication of silicon substrates.
Silicon (Si) is one of the important fundamental materials in the modern semiconductor industry, which could be prepared by chemical and metallurgical route as well as recycling of end-of-life photovoltaic module. It is also very useful as an electronic substrate material [
A novel direct growth system for Si substrate was set up, as shown in Figure
Schematic diagram of the novel growth system for directly manufacturing Si substrate.
Figure
(a) Photograph of as-grown Si substrate and (b) optical microstructure on the surface of the substrate. The arrow indicates the pulling direction.
The key role of the dummy bar in this system is to remove the latent heat of molten Si during solidification toward the cooled dummy bar, thereby causing the liquid-solid interface to become perpendicular to the pulling direction. The interface structure, comprising the EFG and SR, is useful in compensating the expansion of volume generated during liquid-solid transformation and in purifying the substrate, because the volume expansion and impurities preferentially move to the liquid phase. Figure
Schematic diagram of the structural formations of the liquid-solid interface depending on the difference between the temperature of the mold (
The metallic impurities can be induced by contamination in the silicon melting from physical contact of the graphite mold. The impurities in molten Si could be concentrated and trapped at the substrate. However, the concentration of impurities in the remaining liquid fraction becomes enriched during solidification, with the result that the solidified portion in the molten Si tends to be purified. This phenomenon can be quantified by the introduction of a segregation coefficient
As regards the high extraction of latent heat through the cooled dummy bar, however, the liquid-solid interface was formed perpendicular to the pulling direction, similar to EFG and SR in the vertical growth system, as shown in Figure
In order to simultaneously derive a high growth rate and a large grain size from the desirable liquid-solid interface structure, the latent heat of the Si should be quickly transported to the dummy bar, thereby preventing the movement of the heat through the molds in the solidification zone. Molten Si should be solidified by conduction at the contact area between the molten Si and the dummy bar, where the dummy bar plays the role of a seed. The temperature of the dummy bar should be much lower than that of the liquid Si, because a lot of latent heat from the melts could not actually be transported toward the dummy bar due to the very narrow contact area (30 mm2) of the melt and the dummy bar. In order to overcome this limited geometrical factor, the heat flux rate of Si should be considered, using the following relation [
Figure
Cross section images of Si substrates as a function of mold temperature in the solidification zone of the growth part: at (a) 1400°C, (b) 1500°C, and (c) 1600°C.
In order to fabricate a Si substrate without kerf-loss, a novel direct system of growing Si substrate from Si melts using gas pressure was developed in order to satisfy demands for high quality and productivity. The system consisted of a sealed part for melting Si feedstock, a growth part for Si substrate of the desired dimensions, and a substrate transfer part for securing continuous growth using the dummy bar in a vacuum chamber. The gas pressure on the surface of the Si melts in the melting part should be in a range of 780–850 Torr when injecting the melt into the growth part, due to the very high surface tension and low viscosity of the silicon melts. In the case of a mold temperature of ≤1500°C, defects such as cracks and impurities were generated at the center of the substrate by the formation of a liquid-solid interface structure running parallel to the pulling direction, such as RGS and CDS, which indicated that nucleation did not occur on the surface of the dummy bar due to thermal loss through the molds and the restricted heat flux toward the dummy bar. On the other hand, a desirable liquid-solid interface structure that was perpendicular to the pulling direction, such as EFG and SR, was formed at the mold temperature of 1600°C in the solidification zone of the growth part, because thermal loss and limited heat flux were overcome at that temperature. Consequently, the temperature of the mold should be much higher than the melting temperature of Si to form a liquid-solid interface structure perpendicular to the pulling direction, with nucleation occurring at the surface of the dummy bar, which was useful in compensating the volume expansion generated during liquid-solid transformation and in purifying the substrate, as the volume expansion and impurities preferentially move into the liquid phase.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Research and Development Program of the Korea Institute of Energy Research (KIER) (B5-2464) and by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) with a grant funded by the Korean Government Ministry of Knowledge Economy (no. 20103020010060).