Q:  How does purification differ from filtration?
A:  In the simplest terms, filtration is the removal of suspended particles and aerosols.  Purification is the removal of molecular impurities (also called gaseous or volatile impurities). 

Q:  When selecting a purifier, which characteristics should I consider?
A:  When selecting purification media and hardware, users should consider the following parameters:

• Molecular impurity removal efficiency
• Molecular impurity removal capacity / purifier lifetime
• Purifier orientation sensitivity
• Out-of-bag cleanliness
• Purifier flow characteristics
• Particle removal efficiency

Q:  What contaminants do Pall purifiers remove?
A:  Different purification media will remove a variety of molecular contaminants based on the gas being purified.  These impurities may include H₂O, O₂, CO, CO₂, metal carbonyls and others.  Please visit our Gas Purification Section for specific information.

Q:  Which gases can be purified with Pall purifiers?
A: Pall offers technologies to purify many common semiconductor gases. We continue to add capabilities to our extensive purification product line, which covers a wide variety of gases, including inerts, chlorinated, reactive and other specialty gases.

Recently, for example, we developed a technology to purify carbon monoxide (CO), an important gas used in plasma etch processes. The purification technology is based on Pall’s AresKleen™ inorganic purification media, which was first introduced in 2002 to meet the semiconductor industry’s increasingly stringent gas purification needs.

Please visit our Gas Purification Section for specific information.

Q:  How will I know if my Pall purifier is spent?
A:  Select purifier models have optional fiber optic end-point detectors that will notify a user when the purifiers are 75% spent.  For purifiers that do not have this option, Pall has a purifier lifetime calculator that will approximate the life of our purifiers based on specific process conditions.  If a user provides Pall with the process conditions (specific gas, flow rate, operating pressure, duty cycle and inlet impurity challenge), we can calculate the number of service hours the user can expect with our purifiers.  Service hour prediction is dependent on the accuracy of the process conditions supplied, and the user should keep in mind that upsets will affect the life of the purifier, but not the downstream gas purity.

Q: When should I prewet a photoresist filter?
A: Pall Microelectronics significantly reduced the need to prewet photolithography filters with the introduction of our P-Nylon hydrophilic filter eight years ago. Pall's naturally hydrophilic medium does not require any modification techniques to achieve high spontaneous wettability, even in fluids with surface tensions of 72 dynes/cm2.

These hydrophilic filters have eliminated the need for prewetting filters in developer and water applications on the track, resulting in decreased tool downtime and reduced purge times. It is now possible to eliminate cross contamination problems associated with using a low surface tension fluid to prewet PTFE or native polyethylene membranes.

On photoresist and BARC chemistries, prewetting decisions are complicated by the purge time that is needed to remove all air from a membrane pore structure. The naturally hydrophilic media still spontaneously wets, allowing any micro bubbles to pass through without forming microbubble defects. However, a balance needs to be determined. Some customers still prewet with solvents, choosing to monitor the resist coat thickness to determine when the solvent has been completely displaced by the resist. Others have found that spontaneous wettability and fast purge associated with the hydrophilic media has allowed them to eliminate the solvent prewet. In either case, naturally hydrophilic media reduces overall purge times. For more information, contact Michael Mesawich at michael_mesawich@pall.com.

Q: How does filter wettability affect my photoresist performance?
A: Pall has found that adopting a naturally hydrophilic filter media in photoresist can provide several key benefits in lithography applications:

• Naturally hydrophilic filter media can eliminate the need to prewet, as discussed above.
• Naturally hydrophilic filter media will not leach any unwanted contaminants into the lithography process, an event that can occur with modified membranes.
• Naturally hydrophilic filter media will not trap micro bubbles as a conventional hydrophobic media would. Trapped micro bubbles can act as nucleation sites, where they grow into larger bubbles and are subsequently released, showing up as defects in the lithography coatings. In addition, trapped micro bubbles in phobic media can dewet portions of the media, resulting in increased operating and differential pressure. Higher differentials pressures have been shown to increase outgassing of resists and increase the possibility of gel extrusion.  This is especially important as customers are extending the change-out schedule of photoresist filters.

Q: What other products does Pall offer to support my lithography application?
A: Pall offers a wide range of optimized filters for almost all lithography applications, including bulk filtration and point-of-use filters. Check out the new Pall Microelectronics filter catalog, which offers the complete selection of our latest filtration and purification products.