Product Development, fluoroelastomers
Tom Blong, Product Development, fluoroplastics
Paula Hubbard, Business Manager, fluoroplastics
The notable high-temperature and chemical-resistance properties of fluoropolymers make them candidates for semiconductor wet-bench processing, ultrahigh purity (UHP) chemical transport, dry-etching, gas-deposition, and thermal applications. High-purity materials used in these processes are vulnerable to contamination through contact with materials and surfaces. But fluoropolymer components can effectively isolate and protect microelectronics materials from debris.
There are many grades and variations of fluoropolymers. So it is important to have a basic understanding of the material to pick physical attributes of the various formulations that match specific needs of each application.
In microelectronics manufacturing it's critical that processing materials don't contribute particulate or contaminants. Surfaces must also not disturb the fluid stream or create turbulence. In addition, processing materials must withstand high temperatures and harsh chemicals. Environmental issues affecting fluoropolymer selection include operating temperatures, chemical environment, pressure, component movement or flex, associated liquid flow rate, mechanical impact, and expected lifetime.
There are nine primary fluoropolymer categories suitable for semiconductor manufacturing, each with its own particular strengths and limitations.
Polytetraf luoroethylene (PTFE), the basic fluoropolymer, is formed by polymerizing the monomer units of tetrafluoroethylene to create a polymer structure completely surrounded with carbon-fluorine bonds. The result is almost universal chemical resistance along with functional performance up to approximately 260°C (500°F).
Chains of PTFE molecules form smooth helical rodlike crystal structures that tend to slip past one another. Thus it is necessary to form very-long (high-molecular-weight) chains to create sufficient interaction between crystals for sufficient strength.
High-molecular-weight materials can't be formed by melting, but rather must be mold processed under heat and pressure. The resulting polymers can be machined to shape, or mixed with a solvent and extruded, followed by heating to remove the solvent and fuse the chains.
PTFE particles can't flow during fusing and, in turn, leave voids in the material. This microporosity results in flow or creep under conditions of pressure and temperature. It also lowers the polymer's permeation resistance because fluids can move through the voids.
Modified PTFEs or TFM PTFEs address the limitations of conventional PTFEs. The modified formulation consists of tetrafluoroethylene monomer polymerized with a trace amount of perfluoropropylvinyl ether (PPVE) — a vinyl ether comonomer that integrates with the backbone of the polymer chain to produce side chain branching. This branching increases polymer-chain entanglement, slightly lowers molecular weight, and reduces voids as well as warpage of the fluoropolymer under pressure.
Permeation resistance and surface smoothness values of TFM PTFE are better than that of PTFE. TFM PTFE components have the same chemical and temperature resistance as PTFE. They can also be welded to one another, or to PFA parts such as tubing to reduce machining waste.
Perfluoralkoxy (PFA) is a fully fluorinated polymer with the same chemical resistance and high temperature performance (up to 260°C) as PTFE. Like TFM PTFE, the PFA fluoropolymer is made from tetrafluoroethylene (TFE) and PPVE monomer units. However, it is polymerized with a much larger percentage of the PPVE comonomer — as much as 3 to 4% compared to <1% for TFM PTFE. This increases polymer-chain entanglement at lower molecular weight levels and results in a fluoropolymer that can be melt processed. PFA can be continuously extruded to form piping or tubing, or injection molded for efficient high-volume part production.
PFAs can be formulated to set values within the molecular weight range that allows for melt processing. This makes it possible to formulate a particular PFA fluoropolymer grade that satisfies specific application requirements. Longer chain variations have excellent physical properties (impact and flex resistance and tensile strength) that are desirable for tubing, but may be less suitable for injection molding because of flow properties.
Ultrahigh purity PFAs (UHP PFAs): Conventional PFAs are processed with agents that control polymerization and help get the desired polymer chain length. During polymerization, these agents attach themselves to the ends of the polymer chains and replace the normal carbon-fluorine bond with lessstable end groups that can react with metals in production equipment to create contaminating metal ions. UHP PFAs undergo a secondary processing step that converts the chain ends to stable fluorinated polymer chain groups. This eliminates the potential for contamination during high-temperature molding or extruding operations.
PFA-FLEX and other "modified" PFAs: While PFAs typically consist of approximately 3 to 5% PPVE, both the PPVE concentration and comonomer type can be altered. Fluoropolymer properties can be adjusted on the basis of the perfluorinated vinyl ether monomer selected for TFE polymerization, and by the ratio of comonomer in the formulation. For example, boosting comonomer content improves the stress crack resistance and flexural properties of the polymerized material, and also improves surface smoothness. PFA-FLEX fluoropolymers have a reduced melt temperature compared to conventional PFAs.
Fluorinated ethylene propylene (FEP) is a copolymer of TFE and hexafluoropropylene (HFP) that is chemically inert, resists weather, heat, and impacts, and sports good electrical properties. This material is an essential component in films, linings, tapes, wires, and cables in a variety of industries including telecommunications, semiconductor, food processing, and packaging.
Polyvinylidene fluoride (PVDF) differs from the other polymers in that it is not a copolymer of TFE. Rather, PVDF is produced by polymerizing the VF2 (vinylidene fluoride CH2CF2) monomer — most commonly as a homopolymer of VF2. PVDF differs in two significant ways from the other fluoropolymer resins in that it is not fully fluorinated, and it is a tougher and more dimensionally stable resin.
PVDF has better chemical resistance to strong acids used in semiconductor manufacturing, but doesn't stand up to strong bases (>pH 11) or polar solvents such as ketones. It is an economical fluoropolymer and works well with melt-processing techniques that produce a variety of finished component forms. PVDF is the most dimensionally stable and self-supporting of the fluoropolymers. Parts made from this material can be machined to close tolerances. However, its operating temperature is limited to 150°C (302°F).
Fluoroelastomer (FKM) is a general term for a wide variety of fluorocarbon elastomers. Their performance can vary depending on the monomer composition, the weight percent fluorine (% F), and the viscosity or molecular weight. These materials can be further differentiated by the vulcanization (heating under pressure and in the presence of a curing agent) process used to cure them and by the compound ingredients combined with the FKM to impart specific properties.
The base monomer for the majority of FKM materials is VF2, which is copolymerized with such monomers as HFP, TFE, chlorotrifluoroethylene (CTFE), perfluorovinylethers (PVE), and even monomers such as ethylene or propylene. The combination of these different monomers results in a range of 65 to 71% F. In general, a higher % F means a better resistance to chemicals with the trade-off of a higher glass-transition temperature (Tg). The tradeoff of Tg can sometimes be offset by using PVE comonomers, but this will generally boost cost.
VF2-based FKM can be cured in two ways. One is ionic curing based on bisphenol-AF in tandem with an onium (ion formed by adding a proton to a neutral molecule). The second, the radical curing system, uses peroxides and requires the polymer be copolymerized with a cure-site monomer that reacts to radicals, such as a brominated or iodinated monomer.
In general, bisphenol-AF cure systems have better heat resistance than peroxide systems; however peroxide cure systems can have better chemical resistance.
Compounding ingredients such as carbon black and mineral fillers can impart specific properties to FKM materials including improvements in sealing, wear resistance, and physical properties such as tensile strength and ultimate elongation. The same base polymer and cure system compounded with different filler systems can have a drastic effect on properties, so the selection of filler combination can be a science in itself.
Perfluoroelastomer (FFKM) is a term used for a class of fully fluorinated-fluorocarbon-elastomers based on TFE copolymerized with perfluorovinylethers (PVE) in the presence of a cure-site monomer. The PVE and the cure-site monomer are added to break up the crystallinity of the TFE-based polymer and to include a site for vulcanization. FFKM materials provide the best heat and chemical resistance of any known elastomeric material. They can also be differentiated by the compound ingredients used to impart specific properties.
For FFKM, the cure system that is used depends largely on the cure-site monomer that was copolymerized into the base polymer. There are three basic cure systems that are employed. One is peroxide cure based on the addition of peroxides and coagents. The second is ionic curing based on dinucleophiles. The third is catalyzing incorporated perfluoronitrile cure sites.
In general terms, the three cure systems will be differentiated based on upper use temperature, peroxide systems being in the 220°C (428°F) range, dinucleophiles in the 280°C (536°F) range, and perfluoronitriles being used up to 320°C (608°F).
The three cure systems also affect overall chemical resistance. Peroxide cures have an advantage over the other systems. They better withstand organic acids, bases, and water/steam applications but with the trade-off of upper operating temperatures that are not as high.
Similar to FKM, carbon black and mineral filler compounding ingredients can boost the FFKM material's sealing properties, wear resistance, tensile strength, and ultimate elongation. Unlike FKM, FFKM can have excellent properties without fillers or additives, which can lead to some unique compounds such as semitransparent and optically clear products.
Dyneon LLC, a 3M Company, (800) 810-8499, www.dyneon.com