Jonathan G. Parsons

New concerns with the design of filters for the protection of lithography optics

The optimal medium in which DUV resists are exposed is becoming increasingly under investigation by lithography tool manufacturers. These medium requirements have created even more design restrictions for effective filtration methods. Traditional airborne molecular contamination control in tracks and exposure tools has focused on the removal of weak bases that poison the resist. Newer concerns, including; the degradation of optics, changes in resist sensitivity, the index of refraction and the demands of tighter geometries, all contribute to the need for quantification and control of gas-phase contamination within exposure tools. As a result, filter manufacturers are required not only to remove a broader spectrum of contaminants (e.g. organic and acid gases), but to supply removal efficiency data, under a variety of conditions including; variable challenge concentrations, mixed stream contamination, humidity (dew points), flow rates, and temperatures. The paper will address these new concerns focusing on the efficiency effects of relative humidity for various contaminant streams using a variety of filter media. In addition, the removal of hydrophilic compounds such as ammonia and sulfur dioxide when drying an air stream will also be considered. This ongoing study has contributed to the design of a point-of-use air-shower filter used to protect lithography lenses. Preliminary field tests by a major manufacturer indicate that the removal of the specified bases, acids and total organics is below detection limits of <0.1 ppb.

Are ambient SO2 levels a valid indicator of projected acid gas filter life

The requirement to extend existing lithography equipment working levels and life has required manufacturers and end users to extend their filtration requirements beyond airborne base gases. Due to the effect acid gases can have on optics, masks, reticles, steppers, wafers, and metrology tools, they have become a more important molecular airborne contaminate to remove from inside a fab. SO2 is known to be especially problematic within the cleanroom and exposure tool environment. However, a host of other molecular acids can also be found, some of which are present at considerably higher concentrations; the most prevalent of which are oxides of nitrogen, NOx. Although the removal of NOx contaminants is currently not considered to be as critical as SO2, its presence can have a significant impact on acid gas sensitive applications. Several of these applications are addressed in this work in order to point out that NOx contaminants do pose a problem, especially when considering the performance of acid gas chemical filters. More importantly, this preliminary work puts forth the recommendation that in addition to SO2, the concentration of other acid gases should be taken into account when acid gas chemical filter efficiency and life estimates are being made.

Removal of Low Concentrations of Acid Gases: Issues and Solutions

Part per billion concentrations of acid gases such as SOx and NOx have been detected in both high purity gases and CDA lines. These contaminants can have deleterious effects on a number of high purity applications such as the optics found in lithography equipment steppers, scanners, and inspection tools. In addition, acidic gases have also been shown to reduce the life of masks and reticles, decrease fuel cell output due to catalyst poisoning, and cause hard disk drive crashes due to surface contamination and corrosion. Consequently, acid gas control in these applications has become a critical part of the required filtration system. SOx concentrations are typically used as the baseline for acid gas filter exposure guidelines and performance testing. However, this approach has been shown to provide poor filter life predictions, which has been attributed to the presence of other acidic and organic contaminants that compete with SOx for the available adsorption sites. Equally important, the type of sorbents and methods used to control acid gases can significantly affect the ability to remove SOx. In this work we will compare the performance of various sorbents, structures, and methods for the removal of SOx and NOx.