By DALE M. ASHBY
Manager, Material Technology/R&D
Parker Hannifin Corp.
Edited by Leland E. Teschler
The sealing industry has feverishly tried to keep up with the fast pace of developments in semiconductor manufacturing. Advanced analytical techniques have yielded novel seal geometries and rubber compounds with improved thermal, chemical, and cleanliness properties.
There are a vast number of polymer choices available today but the ones used most frequently in semiconductor applications are fluorocarbon and perfluoroelastomer. Fluorocarbon is typically a copolymer of vinylidene fluoride and hexafluoropropylene. Fluoroelastomers, as they are commonly called, are the most widely used elastomer seals in the semiconductor industry because they have a balance of mechanical, chemical, and thermal properties that suit numerous applications. Their limitations emerge in the presence of polar solvents or amines. Fluorocarbon seals work in almost every facet of semiconductor manufacture including LPCVD/oxidation, resist stripping, ion implantation, as well as in physical and chemical vapor deposition.
Suppliers have developed numerous specialty fluorocarbon elastomers. Each exhibits special mechanical, chemical or thermal properties. As the applications for elastomers become more demanding, specialty grades will increasingly replace standard fluoroelastomers.
Perfluoroelastomers are typically a copolymer of tetrafluoroethylene and perfluoromethel vinyl ether with a unique perfluorinated cure site monomer added. They possess excellent chemical resistance and the highest thermal stability of any elastomer available today. Both attributes are essential for sealing aggressive semiconductor environments.
The only limitations are an inability to seal molten alkali metals and fully halogenated fluids. The elastomer works in many facets of semiconductor manufacturing including wet and dry etch, physical vapor deposition, LPCVD/oxidation, plasma environments, and resist stripping.
There are a multitude of commercially available recipes within each polymer family, and each is likely to have diverse properties. Probably the most important example is the relative resistance of various elastomers to plasma environments. Various polymers respond quite differently to the deleterious effects of plasma. There are even differences in how plasma affects specific recipes within a polymer family. This is why it is a good idea to contact a seal supplier before selecting a specific polymer recipe.
Seals for vacuums have their own special considerations. Some of the most critical silicon wafer processes take place in high (10-9) or even ultrahigh (10-10 and greater) vacuums. Hitting these extreme vacuum levels is tough to say the least. Of prime concern is relative cleanliness of the elastomer recipe and minimizing pump-down time. Specific recipes within a polymer family lose a different amount of weight in a vacuum. The higher the weight loss, the greater the potential for contamination in the chamber.
The elastomer industry is trying to develop recipes that exhibit minimum permeability rates so pump-down times and contamination potentials are both low. As a point of interest, semiconductor OEMs historically have used vacuum greases to aid in sealing. Unfortunately, the vacuum grease contributed to contamination. The vast majority of semiconductor manufacturers have now eliminated it. The onus today is on suppliers to develop seal recipes that perform well without relying on vacuum grease.
There is an additional challenge in selecting a seal geometry that fits the application. The O-ring is by far the most common sealing geometry in the semiconductor industry. O-rings come in literally any size imaginable and are typically economical. Their use spans all process steps including slit valve doors, ISO KF flange fittings, chamber lid seals, and gas transfer tubes, to name a few.
Other seal configurations are beginning to see more use, especially in dynamic or pseudo-dynamic applications. These O-ring alternatives include rubberto-metal bonded seals for slit-valve doors, centering rings, or vacuum fork blades; resilient metal seals for ultrahigh vacuum, corrosive chemistries or extremely high temperatures; metal spring-energized lip-type seals of PTFE or some other engineering composite for window seals, CMP chemistries or lid seals; and spliced profiles for low-closure-force door or lid seals.
All these have their features, advantages, and benefits. Seal manufacturers can employ finite-element-analysis techniques to decide whether one configuration makes more sense than another for specific applications.
The challenge for the elastomer industry is to develop recipes that exhibit the lowest permeability rates so they will not increase the total vacuum pump down nor contribute to excessive contamination. Permeability rates here are for standard elastomers tested with the indicated gases and temperatures.
Some rubber compounds contain small quantities of oil or other ingredients that become mobile under high vacuum and may deposit a thin film on surrounding surfaces. There is a corresponding weight loss in the elastomer. Low-weight-loss materials should be the choice when sensitive surfaces are involved. The accompanying table gives figures for typical elastomers. Varying weight loss figures within the same polymer family represent different formulations or compounds made with that base polymer.