ENOWA II – Development of Highly Efficient and Cost-Effective Silicon Wafers

Duration: January 2015 - July 2016
Contracting Authority/ Sponsors: German Federal Ministry of Economic Affairs and Energy (BMWi)
Project Partners: Solarworld Innovations GmbH, Petrofer, Fraunhofer CSP, Fraunhofer IWM
Project Focus:
REM image
© Fraunhofer ISE

REM image of the cross section of a resin bonded diamond wire

REM/EDX image
© Fraunhofer ISE

REM/EDX image on a cross section of a resin-bonded wire showing SiC content of the resin

ENOWA II - Microscope image
© Fraunhofer ISE

Microscope image of an adhesive film for the quantification of particles on the cleaned wafer; 7.9% of the wafer area show particles.

An important process step for the entire production chain of silicon photovoltaics is cutting the silicon crystals into wafers. The wafer manufacturing costs amount to approx. 30 % of the solar cell. Thus, a cost reduction in this process step would contribute significantly to a reduction of the overall manufacturing costs. One cost reduction lever is the application of the diamond wire cutting process, which enables the use of more cost-effective, water-based cooling lubricants and short process times due to the efficient removal mechanism. A systematic evaluation of the auxiliary and operating materials as well as the further development of the wafer cleaning process contribute to the establishment of the diamond wire cutting technology.

To establish the diamond wire cutting process for silicon, different auxiliary and operating materials, such as diamond wires, cooling lubricants, cleaning chemicals, sacrificial materials, and adhesives were evaluated and the cutting process optimized with respect to these operating materials.
An important aspect is the composition of the embedding material for the diamond wires. In addition to nickel-bound diamond wires, resin-bound wires are available in the market. The cross-section of such a wire is shown in Fig. 1. The diamonds are set very deeply in the embedding material, which impact the cutting parameters to be selected and the cutting efficiency. The cutting efficiency is assessed based on geometrical and surface properties of the manufactured wafers. The wire strength is also influenced by the chemical composition of the coating. For example, a wire with harder resin coating shows lower torsional tensile strength. This is caused by the addition of smallest SiC particles, which can be seen in the REM/EDX image in Fig. 2.
Wafers manufactured with diamond wire cutting do not only differ from slurry-cut wafers in their surface quality, but also in their surface contamination. Within the scope of the “ENOWA II” project, two new, standardizable measuring processes for the quantification of the residual particle amount on the wafer surface were developed, in particular, for wafer cleaning assessment. These processes require precise sampling on the wafer surface that seems to be already optically clean. The subsequent sample preparation and automated image analysis enable a currently one-time quantitative assessment of the degree of contamination of the wafer surface. Fig. 3 shows the microscopic image of an adhesive film used for collecting residual particles on the cleaned wafer. Based on the microscopic image, the residual particle amount can be determined quantitatively (7.9 % in the shown example) as well as the particle size distribution. This enables an assessment of cleaning processes and thus, the optimization of solar cell manufacturing during the structuring step.