Thin-film of water extraction in immersion lithography
Overview: Lithographic scanner is a critical step in chip making process. Immersion lithography is now used in the top-10 corporations, like AMSL, where use is made of the liquid lens scanner, taking advantage from the fact that water improves resolution and depth of focus, as it has been known since 18th century. But this is a delicate and fast process, requiring printing very small lines of 38nm, and all (30 to 40) layers of 1/1.000 mm thickness are put exactly on top of each other.
The challenge: The challenges for ASML engineers with the immersion lithography are related to the consequences of water being introduced in the system, which indeed could raise various issues, e.g.:
- Contamination by droplets left on surfaces
- Air bubbles in the liquid lens
- Cold spots due to evaporation of water
- Disturbance forces due to two-phase flow phenomena
TransAT Solution: This is a typical microfluidics flow where surfaces forces are dominant and need to be accounted for using an interface tracking method, with a robust contact angle model. TransAT using the Level-Set and VOF (and also Phase Field) is capable to predict the water thin-film extraction mechanism rather well, thanks to its ability to deal with the contact angle effects, which is modelled in the code based on Youngs’ triple-forces based dynamic contact angle. The results shown below testify on the effect of varying the static contact angle; 90 and 60 Deg. The work is extended now to include the effect of possible evaporation of water, thanks to phase-change capabilities of the code.
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Dr Siju Thomas
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