Wafer Slicing
Wafer slicing is the step that cuts a silicon ingot into thin discs, using either an inner-diameter saw, whose cutting edge is a ring of diamond around a hole in a thin blade, or, more commonly today, a wire saw that draws a web of moving wire through the crystal. It is the first mechanical process the crystal meets after growth, and it sets the thickness, the flatness and the amount of silicon lost before polishing begins. The entry on wafers covers the finished product; this entry follows how it is cut and why the cut leaves damage that must be removed.
The inner-diameter saw was the classic tool for slicing silicon, and it works by tension and a diamond edge. A thin stainless-steel blade with a large central hole is stretched taut across a drum, and the edge of the hole is coated with diamond grit. The ingot passes through the hole and the rotating blade cuts one wafer per stroke, which is well suited to a smaller crystal and gives good flatness, but it cuts one slice at a time and the blade is much thicker than the cut itself needs to be. As ingot diameters grew, the blade had to grow with them and became harder to build and tension, which helped drive the industry toward another method for large wafers.
The wire saw replaced the inner-diameter saw for large wafers by cutting the whole ingot at once. A single long, thin steel wire is wound around grooved guide rollers to form a parallel web of many strands, which the ingot is pushed against as the wire moves back and forth or in one direction. In the classic form, a slurry of abrasive grains, typically silicon carbide in a carrier liquid, is carried by the wire, and the grains do the cutting; the wire only holds the grit against the silicon. A wire web can cut several hundred wafers in one run, with uniform thickness across the batch. In the solar industry diamond-coated wire has largely replaced slurry, because it cuts faster and avoids handling the used abrasive.
Kerf loss is the silicon turned to powder by the cut, and it is a significant fraction of the ingot. The width of the cut, the kerf, is the wire diameter plus the extra space the abrasive carves out, and every slice spends one kerf of silicon. A cut on the order of a couple of hundred micrometres or less is typical for a wire saw, a sizable fraction of the thickness of the wafer it makes. For solar cells, which are cut thinner, the kerf can approach the wafer thickness, which is why thinner wires and diamond wire were a major cost lever. Kerf silicon, mixed with the abrasive and wire debris in slurry sawing, has historically been hard to recycle.
Thickness and flatness are specified by standards, and the cut surface carries damage that has to be removed. A 300 mm wafer is generally about 775 micrometres thick at this stage, thick enough to resist breakage during handling and the thermal steps of processing, and the thickness is held to tight tolerances. The saw leaves a layer of cracks and strained crystal below the surface, as well as saw marks and bow or warp. Lapping or grinding evens the wafer, a chemical etch then dissolves the damaged layer, and polishing produces the mirror finish. A common misconception is that slicing is trivial because it is a cut; in practice the saw's wire tension, feed rate, temperature and abrasive determine how much silicon is lost and how much work lapping, etching and polishing must do.
Read wafer slicing as the point where a grown crystal turns into a product: a controlled act of sawing that costs a measurable fraction of the silicon, fixes the starting thickness, and leaves damage that the later grinding, etching and polishing steps exist to erase.