Good Filters, Poor Resists

Shrinking feature sizes and more complex lithography schemes are increasing the pressure on all aspects of the lithography process, including resists and resist filtration.

As Clint Haris, vice president and general manager for liquid micro contamination control at Entegris explained, fabs are pushing resist manufacturers toward more stringent control of both contaminants and “soft particles” such as gels.

EUV resist in particular is much more expensive than ArF resist. While the cost is likely to come down as EUV lithography reaches manufacturing and production volumes increase, the value of the resist is rising as a share of the total process. Increasing value brings increasing scrutiny of resist quality, or resist handling and storage practices, and of resist delivery to the process tool.

Tighter specifications are forcing fabs to reconsider many standard resist handling procedures. For example, Haris said that fabs don’t usually think of air as a contaminant, but microbubbles can impede uniform resist development, cause optical artifacts, and contribute to feature roughness, among other things. Entegris is examining filter architectures to minimize bubbles. Resist packages like BrightPak can help squeeze out residual air and reduce bubble formation.

Beyond bubbles, a number of common lithography defects can be traced directly to the photoresist. Particles can lead to blocked or missing contact holes. Insoluble gels can bridge lines that should be distinct. The filter can remove these defect precursors in a number of ways.

Hybrid filtration
A “sieving” filter simply catches larger particles by using pores that don’t allow them to pass through. Smaller membrane pores reduce flow rates, though, and pores alone cannot remove all contaminants. Instead, most filter designs combine sieving action with mechanisms based on the surface energy of the membrane material. For instance, nylon is a polar material, and as such its surface adsorbs many common contaminants. Other surface treatments can capture metals or facilitate the capture of cationic or anionic molecules.

Use of adsorption techniques requires close collaboration with resist suppliers in order to understand the effect of filtration on the underlying chemistry. This collaboration is especially critical as resist formulations evolve. For instance, some JSR resists use a more hydrophilic resist surface to improve defectivity without requiring a topcoat. Changes to the polymer backbone improve resist contrast and line width roughness. However, this new composition could affect the resist’s interaction with filters or chemical dispense lines.

While particles and contaminants may have chemical affinity for the membrane materials, resist components can, too. For example, amine quenching compounds can be adsorbed onto nylon membranes. Another common filtration medium, PTFE, has a high affinity for the non-ionic fluoro-chemical surfactants used as leveling agents in some resists.

Because these agents are present at low, parts-per-billion concentrations, any adsorption by the filter will affect the behavior of the resist. Typically, a fab will discard the first few liters of resist to pass through a new filter in order to allow the filter surfaces to reach equilibrium with the resist and to remove any contamination or bubbles introduced during the filter change. As resist cost rises, though, fabs are trying to minimize this source of waste.

Raul Ramirez, head of the lithography filtration business at Entegris, noted that fabs are paying more attention to how quickly a filter gets primed, wet, and ready to run process wafers, and what volume of resist the installation procedure consumes. Filters have to be very clean out of the box, but there’s still the potential for defects due to the chemistry being filtered.

Negative tone resists are drawing some extra attention lately, particularly for EUV. Being able to choose between negative and positive resists allows manufacturers to choose between a bright field and a dark field mask for any particular layer, compensating for lens flare and other optical anomalies. While the role of filtration in reducing bridge defects in positive tone resists has been studied before, less attention has been paid to negative tone resists.

Bridge defects arise due to exposed resist material that fails to dissolve in developer. In negative tone resists, which use organic developer, bridge defect precursor particles should be hydrophilic and can be captured by a hydrophilic nylon membrane. In positive tone resists, the opposite scenario occurs. Hydrophobic components fail to dissolve in alkaline developer, but can be trapped by hydrophobic HDPE membranes.

It’s possible to overstate the importance of filtration, though. While a particular membrane type or design might offer superior retention of a particular defect source, the resist itself remains the biggest contributor to pattern quality.

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