Polytetrafluoroethylene (PTFE) is an ideal material for semiconductor fluid handling applications. It is inherently inert because it is composed of only carbon and fluorine atoms. This carbon fluorine bond is one of the strongest known. The strength of these bonds prevents PTFE from being degraded by chemicals or absorbing them. These bonds also permit PTFE to withstand temperatures ranging from - 270°C to 260°C. This article discusses the selection criteria when using PTFE for ultrahigh-purity fluid handling applications.

In semiconductor fluid handling, material purity is as important as compatibility. Since PTFE can be processed without additives that can leach out and contaminate fluids, material purity is controlled by the processes utilized to get it to final component form. When specifying semiconductor fluid handling components, it is important to know how the PTFE resins are being processed. Typical questions asked by purity conscious designers are:

  1. What controls are in place to prevent contamination during manufacturing?
  2. What methods are used to remove contaminates from component surfaces?
  3. How are parts packaged?
  4. What testing and certification processes/procedures are performed to ensure parts meet minimum purity standards?

Two concerns with parts that are used in ultrahigh-purity fluid handling are material inclusions and surface contaminates. Material purity is a function of the cleanliness of the molding processes used to transform the resin into its intermediate shape. Surface cleanliness is related to the machining processes used to transform the part into its final shape, as well as the processes used to clean and package the finished component.

Both material purity and surface cleanliness are influenced by the environment in which PTFE is processed. Clean rooms are commonly found in semiconductor manufacturers to control contaminates. This technology can also be found in facilities of ultrahigh-purity PTFE component manufacturers. Clean rooms are most commonly used to prevent surface contamination from contacting the component as well as in areas where finished PTFE parts are cleaned and packaged. Clean rooms can also prevent contaminating particles from entering the resin during the molding process.

Clean room classifications

Clean rooms are classified by the maximum number and sizes of particles that are present in the environment. Typically, PTFE manufacturing clean room classifications range from ISO Class 7 (Federal Standard Class 10,000) to ISO Class 5 (Federal Standard Class 100). An ISO Class 7 clean room is certified as having less than 2930 5μm particles per m³ and less than 352,000 0.5μm particles per m³. ISO Class 5 clean rooms have less than 29 5μm particles per m³ and less than 3520 0.5μm particles per m³.

Material purity and surface cleanliness are also influenced by people, tooling, machinery and ancillary materials. Control of these factors is typically a prerequisite to establishing clean room environmental controls.

Metallic contaminants. Care must be taken when molding ultrahigh-purity PTFE resins to ensure contaminants are not introduced into the molded shape. Molded PTFE shapes typically have minimum metallic contaminants since the resin is not exposed to metals at high temperatures. Increasing purity requirements have made it necessary to add controls that further minimize contaminants from being introduced during molding. The most common source of metallic contaminants is from degradation of mold components used to mold PTFE shapes. To reduce the potential for contamination, carbon steel press components might be nickel- or PTFE-coated, or all together replaced with stainless steel components. This reduces the potential for metallic flakes or other contaminants to get into the molded shapes and leach into process fluids. Regardless of the mold type, strict cleaning procedures for the molds and mold facility must be established and followed.

Another source for metallic contaminants is the surrounding processes and machinery. To avoid this, ultrahigh-purity PTFE is molded in areas dedicated to PTFE, in clean rooms or under particle control hoods. The greatest potential for environmental contamination occurs during the transfer of resin from its sealed container to the mold. In some cases, only this portion of the process is done under a hood or in a clean room. For extreme requirements, both resin transfer and molding are done in a clean room. When resin purity is of the utmost importance, the clean room environment is monitored with a particle counter.

Organic materials. Another category of undesirable contaminants is organic materials. The first step to controlling these is to mold in a controlled environment utilizing filtered air. As purity requirements increase, the filtration media is improved, room air changes are monitored and the rooms are kept at positive pressure to prevent outside contaminants from flowing in. At some point, the operators and other materials can become the primary source of the particulates. To reduce these sources of contamination, operators are required to wear lint-free garments. Another method used is to eliminate the use of particle releasing materials such as corrugated containers and wooden pallets. These items are typically used in resin transport and can be replaced with non-particle shedding materials. Once particle levels drop significantly, the rooms can then become certified to ISO cleanroom standards by an accredited testing facility.

Machining process. After the molded shape is complete, concerns shift to surface contamination that can be introduced by machining processes. Ultra high-purity PTFE machining is performed on mills and lathes that are dedicated to plastics to avoid the potential for metallic particles to be introduced into the material. These machines are regularly cleaned and inspected. Durable tooling is required to reduce contamination associated with tool wear. Carbon- and diamond-tipped tooling is commonly used to meet this requirement. When possible, part surfaces are kept clean by avoiding the use of coolants.

When the specific application demands it, parts can then be cleaned after machining is complete. Cleaning should be performed in a clean room area that meets ISO standards or other specified requirements. Visible particles are wiped from the surface with IPA prior to bringing components inside a clean room. Once inside, there are several options for cleaning. The simplest of cleaning methods uses a lint free wipe. More complex cleaning processes require the part to be immersed in baths that degrease, clean and rinse the part. Parts are typically cleaned with chemicals or solvents and then rinsed with high purity DI water. In some cases, these baths have strict temperature controls and agitation requirements. Drying is either performed at room temperature or in ovens. CO2 cleaning is an alternative to liquid cleaning. During CO2 cleaning, parts are bombarded with CO2 pellets that loosen contaminants and blow them away from the surface. No rinsing is required when this process is used.

Cleaned parts will then be packaged prior to leaving the clean room environment. Packaging typically consists of double bagging and heat sealing. Vacuum sealing or purging with an inert gas is sometimes used. The packaging material is rated to the same level or higher than the clean room. After pouching, parts are packaged in protective materials to prevent damage to the sealed pouch or finished part during shipping. Protective packaging can be performed inside or outside the clean room.

In addition to the manufacturing controls described, testing can be performed on finished parts to ensure purity. SEMI F57-0301 outlines particle contribution, ionic contamination, metallic contamination, total organic carbon contamination and surface roughness tests that can be used to evaluate finished parts. These tests are typically performed by an outside vendor at predetermined intervals, or after process changes have been made.

Conclusion

PTFE is one material that, when processed correctly, easily meets the increasing fluid handling requirements of the semiconductor industry. Over the past decades, PTFE has continued to withstand ever increasingly stringent semiconductor fluid handling purity requirements and process sensitivities. As semiconductor processes continue to advance, the importance of cleanliness in the processes used to manufacture PTFE will only increase.

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