Wire saws

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As wire saws generally the making is wafers for the photovoltaic - and semiconductor industry referred. This is a mechanical separation process for silicon , which is technically correctly referred to as a separation lapping process ( lapping ) with unbound cutting grain and an undirected cutting edge.

Another form of wire sawing that is not discussed in this article is sawing with diamond wire, where the diamond grain is bound to the wire. This means that no lapping agent is required any more.

process description

Because of its similarity to diamond, silicon carbide is used almost exclusively as the cutting grain . Glycols or oils serve as a carrier medium for the cutting grain . Due to their high viscosity, these operating materials delay rapid deposition ( sedimentation ) of the cutting grains . The suspension of carrier medium and cutting grain, which is used as a lapping agent , is also known as a slurry .

Wire sawing process sketch

As shown in the process sketch on the right, a thin wire with a diameter of about 100 to 140 µm, serving as a tool, dips into the slurry suspension and pulls the slurry that is stuck to the wire surface and the silicon carbide grains (4) it contains into the saw gap of the silicon block (2). This is where metal cutting takes place. The cutting grains (4) are pulled through the saw gap with the help of the wire (1) and a defined processing speed and tear small silicon particles from the solid body (2). The removal volume is very much dependent on the grain shape and the grain size distribution of the silicon carbide grain (4). In addition, the silicon carbide grains (3) wear very heavily and become smaller and smaller within the saw gap. The wire wears just as much and loses approx. 5–10% of its diameter during the process. Due to the high mechanical stress, the wire cannot be used again. The term “wire sawing” does not mean sawing wire, but sawing silicon or other semiconductor materials, such as gallium arsenide or indium phosphide , using the wire and the unbound cutting grain of the slurry.

technology

Wire sawing process sketch

The unwinding reel (1) unwinds the wire at the wire inlet via wire guide rollers (2) and a defined wire speed (3) until it is finally rewound via the winding reel (4) at the wire outlet. The silicon column (5) dips into the wire field at a corresponding feed rate (6). Slurry is also applied to the wire field in a defined manner via a nozzle (7). The wire with the corresponding wire speed pulls the adhering slurry into the saw gap of the silicon column and cuts many thin wafers that are close to one another, which, depending on the industrial application, can be between 100 and 350 µm thick. The technique described is used by wire saw machine manufacturers around the world, such as Meyer Burger Technology or Applied Materials .

Process metrics

While in the semiconductor industry the cut wafers are subjected to a grinding or polishing process in order to guarantee correspondingly smooth surfaces, this process step is not possible with the thin wafers of photovoltaics. This makes it all the more important to produce a smooth wafer surface without saw marks with a constant wafer thickness using the sawing process.

Important key figures in the wafer production process are the feed speed of the table with which the piece of silicon column dips into the wire field, and the wire speed with which the wire is pulled through the silicon column. The table feed is usually specified in micrometers per minute (µm / min) or millimeters per minute (mm / min) and is between approx. 200 and 600 µm / min, depending on the application. The wire speed is specified in meters per second (m / s) and varies between 5 and 20 m / s. For an optimal wafer surface with the smallest possible groove depth, an optimum between table feed and wire speed must be set.

In general, the higher the table feed and wire speed selected, the higher the forces that act on the wire and thus also influence the wafer surface quality. The decisive factor here is the force on the wire in the feed direction, which increases with higher feed. Lower table feed speeds reduce the forces in the feed and wire direction and thus reduce the risk of wire cracks and increase the quality of the wafer surface, but mean longer cutting times and thus higher production costs. To accommodate this fact, a higher wire speed can reduce the force in the feed direction, but also means increased force in the wire direction, which also increases the resulting force. The close relationship between these two variables becomes clear.

Another key figure is the wire consumption related to the cut wafer area, also referred to as the specific wire consumption per area. A minimum consumption of wire is required in order to obtain the cut wafer surface with optimal surface quality.

In addition, the SiC particle size in the slurry has a significant influence on the forces in the feed direction. Smaller particles increase the force and energy absorption, which also leads to a higher risk of wire cracks, but ensures a lower surface roughness, which has a significant influence on the susceptibility of the wafer to breakage. The higher forces can be counteracted by lowering the particle concentration in the slurry, but a minimum concentration in the slurry is necessary to maintain the cutting ability.

Individual evidence

  1. Use of used slurry for wire sawing without reprocessing . IP.com website, May 13, 2006, accessed July 23, 2020
  2. Jörn Iken: Pulling or sawing - a system comparison ( memento of October 17, 2007 in the Internet Archive ) solarenergie.com. December 4, 2006, accessed August 16, 2011.
  3. Crystec Technology Trading GmbH, Systems for the Semiconductor Industry : Wire saws for silicon and sapphire ( Memento of July 29, 2014 in the Internet Archive ) , accessed on July 24, 2014

See also