Wafering

Ingot slicing Machine 1
Figure 1: Photograph of four bricks in a wire-saw machine ready to be sliced (picture courtesy of Trina Solar).

Wafers are produced from slicing a silicon ingot into individual wafers. In this process, the ingot is first ground down to the desired diameter, typically 200 mm. Next, four slices of the ingot are sawn off resulting in a pseudo-square ingot with 156 mm side length. Then, the wafers are sawn using wire with 180 μm thickness of hard steel wire (resulting in a kerf loss of approximately 200 μm). The wire spacing can be adjusted to produce the desired wafer thickness. The wires are wrapped around rotating rollers with equidistant grooves and move at a speed of approximately 10 m/s. Several mono or multicrystalline silicon ingots are glued to a glass plate and a moved through the mesh of wires with a speed of less than 1 mm/s, as shown in Figure 1. During the whole wire sawing process, an abrasive slurry containing silicon carbide powder is fed into the system and hence this process is typically referred to as slurry based wire sawing. The sawing process takes 6-8 hours for a typical 156 mm block of silicon and the end result is shown in Figure 2.

Ingot slicing close shot 1
Figure 2: Photograph of a multicrystalline silicon brick after the wafer sawing process. Picture courtesy of Trina Solar

In recent years, the industry has fully moved from slurry based to diamond-wire based wafer sawing. In this case, wires coated with small diamond particles are used to cut the wafer. Although the diamond-coated wires and the production equipment are more expensive compared to their slurry-based counterparts, the total costs of ownership is significantly lower. This is due to the fact that the process is faster, allows for recycling of the silicon, and has a significantly lower kerf loss. In other words, you can get more wafers out of an ingot in less time! In addition, the saw damage region of the silicon wafer is roughly half compared to slurry based wafers. The transition was quickest for monocrystalline silicon, but now also multicrystalline silicon has fully moved to diamond wire sawing. The surface texture of diamond-wire-sawn wafers is different from slurry-sawn wafer which requires significant changes in both the alkaline and acid texturing step (see Figure 3 and 4). In addition, the transition from slurry to diamond wire sawing also inspired some companies to investigate more advanced surface texturing techniques which are typically referred to as black silicon.

Figure 3: Photograph of a slurry-sawn monocrystalline silicon wafer.
Figure 4: Photograph of a diamond-wire-sawn monocrystalline silicon wafer.

The process from Cz pulling to wafer sawing is shown in the animation below.

[1]        P. J. Verlinden, “Semiconductor materials and their properties,” in Photovoltaic Solar Energy: From Fundamentals to Applications, A. Reinders, P. J. Verlinden, W. Van Der Stark, and A. Freundlich, Eds., 2017.