Technology and Characterization

Multicrystalline silicon blocks are crystallized by directional solidification using the "Vertical Gradient Freeze" (VGF) process with active cooling. The aim is to ensure rapid solidification with improved crystal quality thanks to larger grain structures and a reduced number of microstructural defects. In addition to a process for 250 kg of silicon in an industrial G4 quartz crucible, processes with good columnar solidification have been established for crystallization of 80 kg in a medium-sized crucible and of 10 to 20 kg in a small crucible.

The blocks are further processed with a highly versatile band saw which can be used to prepare any column shapes desired as well as smaller samples for characterization purposes. In preparation for wafering, the columns are beveled and the damaged surface of the sides is ground away. We are researching the interface with the industrial solar cell process using an industrial "DS265" wire saw and the investigation of various abrasives and slurry mixtures in combination with downstream wafer cleaning.

Research priorities:

• Crucible systems for planar solidification of silicon blocks
• Interaction of crucible material and coatings with silicon
• Crystallization of different grades of UMG silicon
• Analysis of the potential of UMG feedstock for InertCell and EpiCell solar cell concepts
• Fracture behavior of UMG silicon wafers
• Influence of wafering and cleaning on solar cell properties

Laboratories and equipment:

• Spray chamber and firing kiln for crucible coatings
• Ellipsoid mirror furnace for coating tests
• VGF 632 near-industrial crystallizer for mc-Si (PVA Tepla)
• Band saw (Mössner) for processing silicon
• Grinding and beveling machine (Arnold GmbH)
• Multiwire saw (Meyer-Burger DS265)
• Batch wafer cleaning (Decker)

Characterization of blocks and wafers:

• Surface profilometer
• Light microscope
• Scanning electron microscope
• Breakage tester (ring-on-ring)
• Resistance topology on columns (eddy current)
• Minority charge carrier lifetime measurement on columns (MW-PCD, QSSPC)