A growing number of wet chemical processes are moving to filtration systems with fluoropolymer membranes to achieve higher levels of purity. High-performance hydrophobic membranes in critical aqueous chemical applications present challenges in prewetting filters during installation. This article examines difficulties and solutions for handling prewet filters in ultrahigh-purity chemical and water applications.
Robert Gieger, Tom Gutowski, Anthony Shucosky, Pall Microelectronics, East Hills, New York
In today’s semiconductor fabrication processes, filter cartridges constructed from all-fluorocarbon components are used to filter high-purity chemicals. The strict requirements for clean materials, as well as compatibility issues, often necessitate the use of fluorocarbon materials. Polytetrafluoroethylene (PTFE) is an excellent material for filter membranes and is widely used for bulk, distribution, and point-of-use chemical filtration applications.
Many semiconductor applications involve the filtration of aqueous chemicals, which include materials for etching, cleaning, and photoresist development. Since PTFE membranes are hydrophobic, prewetting with low surface-tension fluids such as isopropanol (IPA) is required. Prewetting can be best accomplished by vacuum-drawing or pumping the fluid through the filter. This is especially recommended for sub-0.1µm filters, which are more difficult to wet because of smaller pores. While the prewetting procedure is well established, safety and environmental regulations limiting the amount of organic vapor in the workplace are forcing users to reduce the use of IPA and other solvents for prewetting purposes.
Attempts at prewetting the hydrophobic filters by submerging them in water pressurized above the bubble point of the membrane have seen limited success. And, until now, packaging, fluid shipment, and purity issues have hampered efforts to provide prewet hydrophobic filters for direct installation on tools and distribution systems.
Prewet filter cartridges
In order to minimize installation difficulties and reduce environmental exposure to organic prewetting fluids, prewet PTFE membrane filters have been developed for use in ultrahigh-purity (UHP) semiconductor chemical and water applications. The prewet manufacturing method yields filters that are packaged in sterile UHP DI water, without any additives, and can be used in chemicals without the need to prewet with a low surface-tension fluid.
Following manufacture, prewet filters are tested for integrity with an alcohol-based fluid by measuring diffusional flow characteristics. The testing of every filter ensures 100% integrity of filters from the manufacturing facility. The filters are then rinsed with filtered (0.04µm) UHP DI water ≥18MΩ-cm, placed in a fluorocarbon bag filled with 0.04µm filtered UHP water, and then delivered in a prewet state in a sealed, sterile, fluorocarbon package to the customer. There are no materials in the package other than fluorocarbon (of the filter and bag) and UHP water.
Sterility of water in packaged filters
Prewet filters run the risk of contamination due to microbial growth if the packaging fluid water is not properly prepared. In order to assess shelf life, packaged and prewet all-fluoropolymer filters with PTFE membranes were kept in storage for 24 months. The water was sampled for viable bacteria at three-month intervals from 3 months to 2 years. The water samples were incubated in the growth media for five days at 25°C, and examined for growth of colonies. No colonies were found.
Table 1 confirms the manufacturer’s guaranteed shelf life of 12 months with respect to sterility. The sterility of the samples during shelf life means that microbial contamination will not occur as long as the package remains integral and the filter is stored properly. This eliminates the need for preflush or sanitization of the filters prior to being used on-line.
Stability of prewet condition
To determine the effect of shelf life on prewet condition, the filters from the shelf-life testing noted previously were subjected to differential pressure measurement. If the filters were not completely prewet, high differential pressures would be observed.
Differential pressure was measured by flowing the UHP water through the filters at varying flow rates, both ascending and descending, and measuring differential pressure using a pressure transducer. Flow was monitored with a rotameter.
By measuring the differential pressure in UHP water directly out of the package, stability of the prewet condition was confirmed through one-year shelf life. Summarized in the table, the differential pressure data also confirmed that the new prewet process had no effect compared to conventionally prewet cartridges. The differential pressures measured indicate that dewetting of the hydrophobic membrane did not occur over the 24-month shelf-life testing.
Precautions to prevent dewetting
Fluoropolymer filters are available either dry or prewet. If the filter is received in the dry form, it must be prewet with a low surface-tension fluid and flushed to completely remove that fluid so it does not contaminate the fluid being filtered. This flushing needs to be extensive; if the filter effluent is not monitored for purity, users run the risk of contaminating the fluid that is being filtered, and, ultimately, the processed product.
There are some precautions that must be undertaken when fluorocarbon membrane filters are used. These precautions are critical when filtering fluids with a surface tension >28 dynes/cm. The water-wet hydrophobic PTFE membrane will easily accept gas into the pores of the membrane, resulting in dewetting. Dewetting is defined as the previously fluid-wet PTFE membrane returning to a hydrophobic state coupled with the inability of the membrane to pass aqueous liquid. Once the membrane dewets, it is necessary to rewet with the low surface-tension fluid and flush. In some instances, the filter may have to be thoroughly dried at an elevated temperature for 24 hr and then re-wet - a process that costs time and money.
Dewetting can be caused by gas from the atmosphere or long air-filled lines leading to the housing in which the filter is installed. It could also be caused by the degassing of water used to flush the cartridge, or chemicals that give off gas, either from trapped microbubbles (sulfuric acid) or as a breakdown product of the chemical (hydrogen peroxide).
After the initial prewet and flush, or the removal of the filter from the prewet packaging, the filter must be immediately placed in the housing and the fluid started flowing through the filter. If the filter sits exposed to the atmosphere, the wetting fluid may drain by gravity from the membrane and cause dewetting. This may only partially dewet the membrane, but the reduction of the effective filtration area compromises the performance of the filter in the form of high differential pressure and low flow rates.
Precautions are necessary when introducing the fluid to the filter; this is critical when the filter is in a water-wet state, such as after the prewet step or removal from the prewet packaging. It takes very little pressure to force the water from the pores of a PTFE membrane. Typically, only 2-3psig is sufficient to start the membrane dewetting. If a fluid is introduced rapidly into a housing and venting is not sufficient to evacuate the gas from the housing at the same rate as introduction, the resulting backpressure could easily force air through the PTFE membrane and cause it to dewet.
In automatic chemical delivery systems with diaphragm pumps that are controlled by pneumatic solenoids and pneumatic flow valves, the build-up of backpressure can be rapid; it commonly causes PTFE membrane to dewet in chemical delivery systems. The introduction of the chemical must be controlled in order to avoid any backpressure in the filter housing. Pumps should be started at low pump-stroke cycles. If there is a valve on the outlet of the pump/inlet of the housing, it should be used in a manual mode and slowly opened to let chemical into the housing at a rate equal to the flow of air through the inlet vent. Outlet valves and vents should be open so the fluid will easily flow through the filter.
Once the filter and housing are completely filled with fluid, the inlet valve can be completely opened, the vent valves closed, and the pump brought up to normal operating conditions. In some chemical filtration applications, the inlet vent may need to have a constant bleed. If off-gassing or release of bubbles from the chemical is suspected, then accumulated gas in the housing could blind off the filter or cause dewetting of the membrane.
Nondewetting PTFE membrane filters
Previously used techniques to increase PTFE surface energy rely on chemical coating and grafting methods. In this approach, a perfluoropolymer copolymer is bound to the surface in a solvent wetting, chemical immersion, drying, and flushing process. Such coatings (which may contain solid components) may plug or block pores such that the resulting liquid flow rate through the membrane can be reduced by as much as 10-25%. Coated membranes may contain oxygen, sulfur, or other elements used in the copolymer mixture, and may create unstable surface charges. Only the user can ascertain the effect of these surface changes on each fluid application.
A recent advance in PTFE membrane filters has yielded newer surface-modified PTFE membranes that eliminate the possibility of in-service dewetting. An example of this new technology is Pall’s UltiKleen Excellar filter cartridge. The installation and operating precautions typically associated with standard hydrophobic PTFE membrane filters are no longer a concern. The Excellar filter was developed to overcome the propensity of hydrophobic PTFE to dewet and cause filter performance problems. This was accomplished by modifying the PTFE membrane using a new proprietary technology, called molecular surface tailoring (MST).
The novel PTFE membrane filter with MST is not a coating and does not disrupt the native polymer. These are the only elemental components of native PTFE. The PTFE membrane remains robust and durable and more closely retains the chemical compatibility and purity of the original membrane.
The proper application of hydrophobic PTFE membrane filters in semiconductor manufacturing processes is critical to meeting fluid purity and cleanliness needs. A variety of options are available including dry, prewet, and prewet nondewettable. Guidelines and recommendations have been provided that allow users to select and properly apply each of these options.
- Filterite Electronics, Fluoropolymer Products Technical Performance Guide, p. 15.
- J. O’Sullivan, B. Gotlinsky, “The Latest in Prewetting Technology,” Future Fab International, Issue 5, pp. 125-127, 1998.
- Pall Microelectronics Contamination Control for the Microelectronics Industry, p. 30.
- J. O’Sullivan, B. Gotlinsky, Pall STR-PUF-30, “Eliminating the Need to Prewet Hydrophobic Filters with Low Surface Tension Fluids,” 1994.
- Pall Product Data Sheets: E28A - Emflon Filter; E75A - UltiKleen-CDS; E84A - UltiKleen-Excellar Filter and Kleen-Change Assemblies; E87A - Fluoryte High Flow Filter; E97 - Prewet UltiKleen and Kleen-Change Filters; E98 - UltiKleen-S Filter and UltiKleen-G2 Filter.
For more information, contact Robert Gieger at Pall Corp., 2200 Northern Blvd., East Hills, NY 11548; ph 516/801-9827; e-mail firstname.lastname@example.org.
Guidelines for proper use of PTFE membrane filters
Although unlikely, filters containing hydrophobic PTFE membrane may dewet under certain circumstances. Precautions for the installation and operation of these filters need to be taken to prevent dewetting of the membrane and thereby maintain optimal filter performance.
- Water-wet PTFE membrane filters should not be exposed to any potential drying.
- When starting flow through a newly installed prewet filter in an empty housing, it is necessary to adequately vent it to prevent any backpressure within the housing. This backpressure could dewet the filter.
- Process fluid flow needs to be introduced slowly to the filter for the same reason as in #1.
- Precautions need to be taken when changing chemicals both in chemical distribution systems and point-of-use applications. Introducing air in lines can potentially dewet the PTFE membrane.
- Filter systems and housings that contain chemicals with the potential to outgas should have proper venting so accumulated gas can escape.
- Sizing of filters for proper flow should be addressed so that excessive pressure drops across the filter membrane do not cause degassing and the formation of bubbles.