Mechanical Characterization of Wafers and Solar Cells

© Fraunhofer CSP

The fracture and strength behavior of wafers can be determined in the four-point bending test.

Material defects such as cracks in the silicon, which are induced during the wafer manufacturing or in solar cell processing can bear negative effects on the strength and reliability over the entire value chain all the way to the module. The characterization of these defects in the wafer or in the solar cell is thus an important contribution to optimize the lifetime of the modules. Especially an improved understanding of the multi-wire sawing process and the characterization of the process components (e.g. structured sawing wires) contribute to the cost optimization in the manufacturing process of silicon wafers.

Services

  • Determination of the fracture and strength behavior of wafers, partially processed or finished solar cells by use of fracture tests (4-point bending test or ball-ring test) including statistical evaluation (Weibull + fractography)
  • Determination of the depth of cracks in brittle materials
  • Analysis of silicon removal processes
  • Quantification of the damage potential of handling processes and equipment
  • Determination of the structure-defining geometrical parameters (period, amplitude, etc.) of structured sawing wires
  • Determination of the strength of sawing wires (straight and structured)
  • Nonlinear multi-physical finite-element simulation of wafers, solar cells or sawing wires under loading in tests or in manufacturing
  • Fracture mechanical simulations

Examples

© Fraunhofer CSP

Fig. 1: (a) 3D model of structured wire (scaled structure 100:1); (b) geometrical characterization by determining periodic bending parameters (bending amplitude ai, bending period Ti).

Structured Sawing Wires

The application of structured sawing wires in slurry-based wire-sawing leads to a distinct process improvement which, however, strongly depends on the structure of the wires. The Fraunhofer CSP has developed a special procedure to characterize the structure and analyze the structural behavior.

© Fraunhofer CSP

Bending of a monocrystalline silicon wafer in the 4-point bending test.

The Strength of Wafers and Solar Cells

Slurry and diamond wire sawed wafers in the »as cut« state possess a very variable and direction-dependent strength which can be improved by applying etching processes. Due to the metallization defects on the back of the cell, however, the strength will decline again in the course of cell manufacturing.