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Long Term Stability

 

Long term stability requirement of a pellicle on a photomask depends on the expected usage life time of the photomask and the sensitivity of the lithography process. Most of us like the life time of pellicle on a photomask to be infinite. Unfortunately, most of material in a pellicle and photomask should be considered only have a limited life time.

Outgassing and Crystallization
Long-term usage and storage of a pellicle on a photomask provides a real challenge for pellicle design and material selection. Ideally, a pellicle on a photomask will seal off all outside particle and vapor contamination. Unfortunately, the organic components in a pellicle can lead to outgassing and may contaminate even the area it is supposed to protect. The most susceptible outgassing components of the pellicle are the mounting adhesive, inside coating, and glue which are a mixture of polymers and low molecular weight organic compounds such as residue solvent, plasticizer, anti-oxidant and UV stabilizer. The chrome surface on the photomask is known as an active surface which can adsorb many organic chemicals on it. Unprotected chrome surface made from sputtering can adsorb material from photomask container outgassing and make it impossible to coat a positive photoresist on its surface after only one day. Outgassing and crystallization can also come from the box or storage container and easily contaminate area that is not under pellicle protection. Environmental outgassing or vapor can also contaminate the pellicle, reduce its transmission or form crystal even underneath the pellicle.12 Outgassing or vapor can easily get into the pellicle protected area through the vent hole with a filter or through the film directly when the film is thin and permeability to the vapor is high.

For example, in 1986, crystals of 2,5-di (t-amyl) quinone were found on a pellicle protected photomask surface. In one extreme case, even within a day of mounting pellicle on a photomask, crystals could be found on the chrome pattern edge and the photomask had become useless. The origin of the crystals was identified by us and it came from the outgassing of anti-oxidant stabilizer of 3M447 - a rubber type double-sided pressure sensitive tape used for pellicle mounting adhesive.13 This stabilizer was 2,5-di (t-amyl) hydroquinone. Recently in 2001, a crystal was formed on the pattern surface and identified as a quinone. The quinone comes from the dimerization of the antioxidant as a stabilizer in the hot melt pressure-sensitive adhesive.
An outgassing test should be performed for any new pellicle qualification. In addition, a lifetime test for particle generation on the photomask under the pellicle should be completed in a real production environment as well.

Material Stability
All the material used for a pellicle has to be subjected to some photodegradation from UV light and oxidative degradation from air and outgassing. In addition, film thickness can change due to the residue solvent evaporation out of film, polymer molecular rearrangement and humidity change. Film for pellicle is always chosen to resist UV light radiation which may degrade the film transmission or mechanical strength. However, other components such as film glue, mounting adhesive or frame coating are not necessary going through a rigorous checking. For example, one of our early mounting adhesive, i.e., a polyethylene vinyl acetate hot melt pressure sensitive adhesive without anti-oxidant, lost its pressure sensitivity in 6 months. In our laboratory, a silicone adhesive used to mount the film to frame lost its adhesion on a perfluoropolymer film only after a few days when it is fully cured. Some mounting adhesive using hot melt pressure sensitive adhesive such as SEBS or SES polymer was found not DUV stable and not oxidation stable without anti-oxidant.

As discussed above, the material of container, i.e., the box, used for pellicle and photomask has to be screened for outgassing because some of them do contain possible outgassing low molecular organic compounds in the plastics. Depending on the functional requirement, not all the degradation will result in the defect generation for a photomask with a pellicle. However, depending on the usage life time, sometimes it may take a long time to find if a pellicle is suitable for the process.