Quartz is a mineral that is commonly found in and on the earth’s crust, a combination of silica and oxygen (SiO2). Despite being a commonly occurring substance, pure quartz has a number of extraordinary properties that make it ideal for Semiconductor and other ultra-pure processes.
Unlike glass, which contains some reactive metallic elements, pure quartz is almost inert and does not react with most other substances. Even at moderately high temperatures, it is very chemically stable. (Some notable exceptions are hydrofluoric acid, HF, or alkaline substances such as KOH).
Along with its low reactivity, quartz has a Mohs hardness of 7 and a lack of cleavage that make it desirable for grueling, yet clean, applications. However, quartz’s durability also makes it exceptionally difficult to mold and weld. The fabrication and machining of quartz vessels and parts require the skills of an experienced and talented craftsman.
PVDF – Polyvinylidene Fluoride, is a highly non-reactive and pure thermoplastic fluoropolymer. It is generally used in high purity applications that require strength and resistance to solvents, acids, bases, and heat. Its low melting point (around 177°C) make it easier to melt than other fluoropolymers and has a relatively low density (1.78) and low cost. It is commonly available as wire insulator, piping, sheet, tubing, films, or plate.
Stainless Steel, also known as inox steel or inox, is defined as a steel alloy with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel. Alloys with a suitable purity for Semiconductor applications are determined by SEMI. Common applications include flammable solvents or detergents or other applications where metallic contamination isn’t an issue.
PTFE – polytetrafluoroethylene (PTFE) is a chemical compound that is very non-reactive, partly because of the strength of carbon–fluorine bonds, and so it is often used in containers and pipework for reactive and corrosive chemicals. PTFE is a white solid at room temperature, with a density of about 2.2 g/cm³. According to DuPont its melting point is 327°C (620.6°F), but its properties degrade above 260°C (500°F).
Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no “memory,” and is subject to creep, also known as “cold flow” and “compression set”. A little bit of creep allows PTFE seals to conform to mating surfaces better than most other plastic seals. Too much creep, however, and the seal can be compromised.
PFA – type of fluoropolymer with properties similar to PTFE. PFA shares PTFE’s useful properties of low coefficient of friction and non-reactivity, and is of a higher purity than PTFE. It can also be molded and welded, making it better suited for bath fittings. PFA is an excellent choice for HF applications, since HF cannot be used with quartz, and it is critical to have no metallic contamination.
PEEK is a semicrystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained in high temperatures. It melts around 343°C (662°F) and is highly resistant to thermal degradation as well as attack by both organic and aqueous environments.
While not the best material for a direct contact with heated chemicals, PEEK’s robustness and machinability make it ideal for bath lids, hinges, screws, etc. Even in a harsh chemical environment, PEEK provides the durability of metal without compromising purity.