Dr. Vladimir Galindo
Phone: +49 351 260 3391

Ribbon Growth on Substrate (RGS) process

New continuous casting technology

Cooperation with RGS Development BV, Netherlands

Compared to the concept of growing and cutting ingots, the fundamental difference of the Ribbon Growth on Substrate (RGS) process lies in a continuous and direct casting of silicon in near-net-shape. This is realized by means of feeding molten silicon into an open casting frame, from where a solidified foil is extracted side-wise on a sub-cooled moving substrate underneath. It results in both a close to perfect material yield and a low energy consumption due to its continuous nature. Concurrently, this also entails the crucial benefit of fully decoupled solidification and casting velocities. On the other hand, effects like flow instabilities and meniscus oscillations can negatively affect the properties and quality of the silicon sheet.

Scheme of the RGS process (left) and model (right)

  • Excellent material yield
  • Cost efficient and scalable
  • Wide range of applications (photo-voltaic wafer, TEG module)
  • Free-surface instabilities
  • Limited wafer quality
  • Measurement difficult during the production process
  • ! Application of AC magnetic fields
  • ? Numerical investigation and design

Magnetic retention

A tailored AC magnetic field does not only compensate heat losses, but it is also able to stabilize the melt flow as it realizes a magnetic valve. It thus prevents leakage in the slit region and reduces oscillations at the extraction site of the silicon foil solely with the aid of electromagnetic forces. The corresponding Lorentz force field more specifically acts to counter the gravitational forces acting on the melt. Hence, the induction coil must be designed such that the magnetic pressure at the sides of the extracted foil reaches the same order of magnitude as the hydrostatic pressure of the melt at the bottom of the casting frame.

Lorentz force distribution Magnetic pressure along wafer side (hydrostatic pressure pH = 500 N/m2) for different wall thicknesses

Free surface dynamics

Even though the application of oscillating magnetic fields constitutes a sophisticated way of stabilizing the RGS process, their presence will also induce a bulk melt flow. The flow in turn gives rise to a dome-shaping of the melt in the casting frame. It has been found that the Lorentz-force is even dominating the melt-flow and that the deformation of the free-surface may not simply be ignored. A detailed insight into the free-surface dynamics is therefore a key for understanding and optimizing the RGS process, which can e.g. be achieved by different coil geometries or power supply parameters.

Numerical simulation of the RGS process

To comply with this demand, we have developed a novel numerical tool based on OpenFOAM (foam-extend), which is capable of effectively simulating the free-surface dynamics of the melt flow under the influence of an applied alternating magnetic field. Our model thereby resolves the complex interaction of three-dimensional hydrodynamic and magnetodynamic effects in a single computational framework. Based on this solver we were able to reveal dominant flow structures of the RGS process and first conclusions could be drawn for possible design improvements.

Model development for general MHD free-surface flows

Our new software platform and the underlying numerical model has been formulated in a very flexible and general form, which may be used for the investigation of similar multiphysics problems in future projects. The capabilities of this powerful tool have been demonstrated by several different metallurgical applications. This comprises e.g. problems of electromagnetic levitation, the melting of metal within an induction crucible furnace with large surface deformation and also cases where the electromagnetic force is accompanied by thermal convection and other thermal effects.

Levitated drop of silicon Dome-shaping of woods metal
Melt flow in cone shaped crucible