As a former employee of SolarWorld, Shell Solar, and Siemens Solar (over 18 Years experience) I thought I might post a little something on the manufacturing process of Silicon Wafers.
In addition: If anyone is interested I will post more on solar and the actual construction of a module.
SILICON WAFER MANUFACTURING:
Silicon wafers are manufactured from Poly silicon. Moreover, Poly silicon is used in almost all integrated circuits in the electronics industry. Made from quartzite, a pure form of sand, the quartzite is treated to create polycrystalline silicon (poly silicon), a very pure silicon with multiple single-crystal regions of different sizes and orientations. The poly silicon is used to "grow" a single-crystal or mono-crystalline ingot that is sliced into wafers.
Here is a description of what is involved to make crystalline silicon wafers.
Poly-silicon and a Boron doping is loaded into a quartz vessel for a crystal grower and heated to its melting point. A piece of polycrystalline silicon with the correct atomic orientation known as a seed is lower into the vessel and slowly rotated and raised forming a crystal ingot. A one thousand millimeter long, one hundred fifty millimeter diameter crystal ingot takes about 24 hours to grow properly.
NOTE: This growing process can take hours to days depending on the diameter and the length of the crystal. In addition, a crystal grower will pull anywhere from a few hundred kilowatts to Megawatts.
After the crystal ingot is formed, the top of the grower is opened the crystal ingot is lifted out using a crane or hoist, and then allowed to slowly cool down. Improper cooling can cause cracks, fractures, and breakage. Even a micro fracture known as a “slip” seen only by a microscope can cause a failure and is unacceptable.
The crystal ingot then is taken to an area where the ends of the crystal ingot are cut off perpendicular to its length and cut into lengths for a saw, then depending if the wafers are to be round or square the ingot will either turned, and ground down the a specific diameter, or mounted in a jig and “squired up” to formed the four sides in the wafer and then moved off to the saw area.
In the saw area, the ingots are mounted (normally on etched glass) using epoxy, allowed to cure for a minimum of twelve hours, and then placed onto the saw.
The saws used today are commonly known as “wire saws” they replaces the old type saws with metal blades years ago but I have operated both. A wire saw takes a spool of very thin wire, one hundred to two hundred kilometers long, only a few microns thick. The wire is threaded through tension pulleys and spindles (Rollers) to form what is known a web, and then taken to the taken to the “take-up real.
These wire saws cut the ingot into wafers in just one pass by use of cutting slurry made up of silicon carbide and cutting oil. The typical wafer thickness between one hundred microns (0.1mm) and two hundred microns (0.200mm) thick and can take six to eighteen hours to complete its cutting cycle.
Some wire saws can cut up to four crystal ingots in one pass with two ingot set-ups being the most common.
After the sawing process, the wafers still mounted to the glass are rinsed with solvent and un-mounted from the saw and placed in to a carrier and sent for additional cleaning.
The sliced ingot (now wafers) are placed into solvent tanks and agitated, then rinsed and placed into acidic acid to make the epoxy pliable and then rinsed more.
The wafers then undergo one hundred percent inspection and placed into polypropylene cassettes (sometimes referred to as boats) and moved off for further cleaning.
The above process is standard in both the semi-conductor and photovoltaic industries.
Feel free to contact me here on the forum or at testbenchtech@gmail.com with questions /comments.
