Purity
Purity, in semiconductor manufacturing, is the fraction of a material that is the intended element, and it is quoted in nines: 99.9999999 percent, nine nines, usually written 9N, means one foreign atom in every billion. Electronic-grade polysilicon, the feedstock for wafers, is made by the Siemens process at about nine nines purity or better, and the most demanding grades reach eleven nines. A semiconductor is a material whose electrical behavior is controlled by tiny amounts of added atoms, so the starting point has to be cleaner than the dopants that will later be added on purpose.
The nines scale hides how extreme the requirement is. Silicon has about 5 × 10^22 atoms per cubic centimeter, so one part per billion of impurity is still about 5 × 10^13 foreign atoms in each cubic centimeter, a number comparable to the doping of the most lightly doped devices, which is why even nine nines is only the beginning. Metallurgical-grade silicon, made by heating quartz with carbon in an arc furnace, is roughly 98 to 99 percent pure and unusable for chips; solar-grade material, around six nines, is adequate for many photovoltaic cells; and semiconductor-grade silicon is purified further. Unwanted donors or acceptors at the part-per-billion level therefore set a floor on how lightly doped and how resistive the material can be made.
The purification chain starts with chemistry, because chemistry can pick one compound out of many. In the Siemens process, developed at Siemens in the 1950s, metallurgical silicon reacts with hydrogen chloride to form trichlorosilane, a liquid that boils near 32 degrees Celsius. It is distilled repeatedly, as petroleum is, until boron, phosphorus, metals and carbon compounds are mostly removed, and then it is reacted with hydrogen on thin silicon rods heated to about 1100 degrees Celsius, where silicon deposits as polysilicon. Zone refining, which Pfann introduced in 1951, was the first route to the purity needed for germanium and silicon, and now serves niche materials, while the distilled-chemical route supplies tonnage.
Contamination matters because one stray atom can ruin a transistor. Heavy metals such as iron, copper and gold create recombination centers in the middle of the band gap, which shortens carrier lifetime and increases leakage; sodium ions in a gate oxide shift transistor thresholds, a problem that plagued early MOS devices until it was identified and controlled in the 1960s. Particles are equally harmful, since a speck larger than a fraction of the feature size can short or break a wire. For these reasons fabs run in cleanrooms with air filtered far more finely than in a hospital operating room, and workers wear full-body suits so that skin, hair and clothing do not become the dominant source of dirt in the room.
Ultrapure water and process chemicals extend the same discipline to everything that touches the wafer. A fab rinses wafers many times, so its water is deionized, filtered and treated to reach a resistivity of 18.2 megohm-centimeters at 25 degrees Celsius, the theoretical limit for pure water, with metals at parts per trillion levels and tiny numbers of particles and bacteria. Cleaning sequences descend from the RCA clean developed by Werner Kern at RCA in 1965 and published in 1970, which uses alternating solutions to strip organics, particles and metals. Process gases and chemicals are bought in electronic grades with their own nines, and purifiers sit at the tool.
A common misconception is that purity and crystal perfection are the same thing, or that purer is always better. Nine nines says nothing about whether the atoms form one lattice; polysilicon rods are very pure but are not single crystals, which is why they must be melted and regrown. The goal is also not zero impurity: doping deliberately introduces boron, phosphorus or arsenic at concentrations of parts per million to parts per thousand, and some contaminants, such as oxygen in Czochralski silicon, are tolerated or even exploited. The skill lies in removing every unwanted atom first, and then adding the wanted ones in precisely controlled amounts.
Read purity as the clean slate on which semiconductors are written: nine nines of silicon means one stray atom in a billion, and that near-absence is what allows a few intentional dopant atoms, and not accidental ones, to determine how every transistor behaves.