Fraunhofer Institute for Solar Energy Systems ISEOne alternative to manufacturing silicon wafers by sawing is found in silicon foil technology. In contrast to conventional sawing technology, the Si foil technology enables significant reductions in the consumption of high-purity silicon, as thinner wafers or films can be produced and, compared to sawing, virtually no material is removed.
We are currently developing processes and technologies for manufacturing even thinner silicon films. Electrochemical porosification of Czochralski silicon crystals followed by the thermal reorganization of porous silicon layers is used to detach silicon films with thicknesses ranging from one to a few dozen micrometers. Such reorganized porous layers can either be detached directly or can serve as a base for subsequent epitaxial silicon deposition. The surface of the porous layer closes during the reorganization process and can then be used as a template for epitaxial silicon growth. Because the silicon layer is porous, the deposited layers can be detached and then processed as silicon films. These films provide an excellent starting material for silicon thin-film solar cells with an efficiency potential of more than 20 %.
Our developments cover the entire process cycle: substrate selection, porosification or implantation, annealing and reorganization of porous layers, epitaxial deposition on reorganized porous layers, detachment of films, and various procedures for affixing or bonding films onto cheap substrates. All these processes are developed with high throughputs, low costs and high material quality in mind, and with the aim of being able to manufacture all samples in high throughput systems.
Main areas of research focus:
- Porosification and implantation
- Reorganization of porous layers
- Si epitaxy on porous silicon layers
- Detaching and transferring silicon films
- Crystallographic and electrical analysis in all process stages
Labs and equipment:
- Semi-automated lasers
- Lift-off equipment
- Si-CVD and epitaxy