Laser based kerf-less technology for exfoliation of thin Si layers and wafering of Si ingots (KerfLessSi)

An international team of scientists at NTNU and an NTNU startup company Atla Lasers AS in close collaboration with scientists at TU Vienna, Stanford University, as well as with participation in the project of SINTEF and other industrial partners have developed a new laser technology to create thin films of silicon for solar cells, Li-batteries and microelectronics industry without creating excess material. The laser can be used to cut slices of material, and to create ultrathin slices, small channels and microstructures under the material surface. The new, ultra-precise laser technology enables fabrication of silicon solar cells and other semiconductor microchip components that will be produced without material waste. It is important for industry to reduce this waste of expensive materials and make the technology 'kerfless'. ?Kerfless? silicon processing is an important economic task in solar cell manufacturing, semiconductor and telecommunications industries, including a future optical quantum computer industry. The project provided theoretical explanation of the physical and nonlinear optical properties that affect buried layers formations, and developed the laser processing technology on industrial level with the help of ATLA Lasers AS. The laser technology capable of producing structural modifications has been developed and tested for the first time. Clear modifications under the surface ? ?buried layers? of up to 25x25 mm ? have been produced and exfoliated using the developed laser system operating above 2 micrometers wavelength. This proved the concept of possibility of producing thin (less than 1-2 microns) buried layers by ultrashort pulsed lasers in the three-photon absorption regime at any depth inside silicon. The developed new laser technology to produce thin exfoliated layers can be used for solar cell, battery and microchip production in microelectronics and IT industry. As a result of the project the developed laser technology will be transferred to Atla Lasers AS that has emerged from NTNU and then to the Norwegian photovoltaics and Li-battery industry that will use the developed technology for silicon processing, silicon cell and battery production.

1. Demonstration of the single-pulse ultrafast laser-induced buried defects inside silicon with subwavelength size opens up the way to direct ultrafast laser writing of micro-structures in silicon, technology of exfoliation of thin silicon layers for solar cell industry. 2. The developed laser source is unique and will likely lead to a number of interdisciplinary scientific projects in different topics of medicine, material technology, and advanced material micro-processing 3. The laser source has opened the way to National (University of Oslo) and international (CERN) collaboration. 4. NTNU is preparing a Large Scale Interdisciplinary project for the 10th of February NFR Call, and an M-ERA NET project on material processing. 5. The transfer of laser technology to the industrial participant ATLA Lasers allows the company to enter a large (>10BEUR) market of laser processing tools, thus contributing to the competitiveness of the Norwegian Photonic Industry in the Energy Sector.

The project aims at developing new and advanced laser technology for the kerf-loss-free cost-effective laser micro-processing of thin silicon layers, wafers and related solar cells with femtosecond mid-infrared (2-3 µm) fiber based lasers developed jointly at NTNU, SINTEF and ATLA lasers (NTNU spin-off), in collaboration with the SME and large material processing laser tool industry (TRUMPF) as well as other partners. Besides using the advanced high energy femtosecond pulses for fine material processing- technique currently establishing itself now in semiconductor industry via the largest in the world laser tool supplier TRUMPF in Germany (with 1 µm sources) - the 2-3 µm wavelength is critical and disruptive because it lies beyond the two photon absorption edge of most semiconductor materials and lies exactly in the three-photon absorption of Si, allowing the use of the same laser sources to efficiently and finely process various Si-based microchip structures, including buried structures. For details please see attached TRUMPF support letter. The proposed technique is based on formation of a buried micro-cavities based layer upon focused laser treatments of silicon wafers/ingots supported by stress-induced lift-off process caused by deposition of a layer with mismatched coefficient of thermal expansion with respect to the Si substrate. Bonding of thin exfoliated layers on any suitable substrate will open an alternative and disruptive way for the cost-effective processing of thin Si based solar cells avoiding usage of high temperature CVD or any other processes currently used for that purpose. Moreover, thin Si wafers with any desirable thickness will be processed without any kerf loss. Processing of solar cells using exfoliated thin Si layers and thin wafers will be performed and results will be compared with those obtained for solar cells based on epi-Si layers or conventional Si wafers. The developed laser technology will be transferred to industrial partner