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Future of Pellicles

 

The Future of Pellicles
The introduction of 157 nm for next-generation optical lithography has created a need for new pellicle materials optimized for this wavelength. Currently, there are four strategies being considered.

Soft Pellicle
First, new fluorocarbon-based polymers have to be developed that are transparent, damage-resistant, and possess mechanical properties to enable their preparation in very thin pellicle form. Initial results from the Massachusetts Institute of Technology's Lincoln Labs demonstrated that commercial fluoropolymers used for pellicles at 248 and 193nm wavelengths, such as Teflon AFR and CytopR, rapidly burst under irradiation with 157 nm light because they lack sufficient mechanical integrity. Consequently, an extensive program was initiated to develop and screen novel fluoropolymer candidates with the desired properties for 157 nm lithography. Although some polymers did show promising transmission, their lifetime is still insufficient due to photochemical darkening. Research is still needed to solve this fundamental problem.

Hard Pellicle
A hard pellicle is simply a thin, quartz glass on a frame. Fluorinated fused silica has a sufficient lifetime for the 157 nm process. Although thickness control and thickness uniformity is a challenge, a good parallelism, i.e. thickness uniformity, has been achieved. However, even thin fused silica is several hundred times thicker than a soft film pellicle. With a typical thickness of 800 µm a hard pellicle would act as an additional optical element and impact the imaging and overlay performances. Developers have achieved good optical homogeneity and surface finish for hard pellicles, but improvements to the mounting frames are required to keep the pellicle bending low in order to avoid significant optical distortion. A circular pellicle with a circular photomask should be used to minimize any distortion.

Removable Pellicle or Cover
The third strategy is to use the soft pellicle as a photomask cover only during transportation and storage. The pellicle would then be removed before exposure and remounted after exposure. There are three mounting options being considered - adhesive, magnetic, and a modified reticle carrier. Although the process can be easily proved in a research line, the long term contamination control and inspection will still be a challenge and the process will have to be proven in the very costly production line.

No Pellicle
The last approach is a pellicle-less solution. However, after 20 years of using pellicles the "no pellicle" proposal will be a challenge because the feature size of IC is much smaller than before - 0.1 £gm vs. 4 £gm. In addition, the photomask and reticle is easier to get contaminated because the feature size is now much smaller. A reliable, constant inspection feedback system is therefore necessary for the success of this method.

At this time of writing, it seems that the hard pellicle will produce enough life time and contamination free protection for 157 nm reticle. The challenge of production of a clean, defect free hard pellicle and mounting without distortion or repetitive distortion will still have some challenges to overcome.
 
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