Modeling and laboratory scale proof of concept of the horizontal ribbon growth process: Application to silicon wafer manufacturing
In this work we focus on the development of the Horizontal Ribbon Growth technique to produce highly pure silicon wafers for use in solar cells. We divide this preliminary work in two parts: a small scale experiment and the development of mathematical models to describe the crystallization phenomena. We begin by constructing a laboratory scale experiment to demonstrate the HRG concept using water as the working fluid. We made use of the fact that ice floats on water just like silicon would float on top of its melt. Therefore it is possible to conveniently test and analyze the operability of this technique. We find that the initial seeding process needs to be carefully controlled in order to extract a uniform wafer. Appropriate surface cooling and wall heating are necessary to guarantee the continuous formation of an ice film. We then develop simple mathematical models to predict the crystallization rate and thermal profiles of the system. We state that the driving force for crystallization is convective cooling and the film grows in one direction (downwards). Finally we propose how to validate the model using experimental data and how to extend the work to grow silicon ribbons.
Improvements in the Horizontal Ribbon Growth technique for single crystal silicon
Abstract An improved Horizontal Ribbon Growth (HRG) technique for growing single crystal ribbons of silicon is described. Starting with the essential concept of the Bleil process, namely the “growth wedge”, improvements were achieved in the process by introducing gas cooling and developing techniques for obtaining a “large wedge factor” (or wedge length to ribbon ratio). These refinements have led to demonstrations of high growth rates (maximum of 415 mm/min for single crystal and 850 mm/min for polycrystal ribbons). Various other improvements in the practice of HRG, such as a specially designed crucible, pulling in an inclined direction, and generation of desired melt flow patterns, are also described. They have led to a more stabilized growth operation from the start of seeding through steady-state growth.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.