Ellipsometric studies of the absorption of liquid by photo resist

Abstract

In situ spectroscopic ellipsometry, vacuum UV ellipsometry, and imaging ellipsometry were employed to study the absorption of liquid by photoresist (PR) used for 193 nm immersion lithography. When 140 nm thick PR was soaked in water over a period of >70 minutes, ∼7% increase in thickness was observed. From the effective medium approximation analysis of ellipsometric spectra covering from near infrared to deep UV, we could estimates less than 2 vol. % uptake of water by PR after completion of soaking. This resulted in very small decrease in refractive index of PR (∼0.4%). When imaging ellipsometry was used, the absorption of water by PR in much shorter periods could be detectible. In imaging ellipsometry, the microscopic images of (Δ, Ψ) in small area are obtained thanks to two dimensional multi-channel detection systems such as CCD. Using imaging ellipsometry, we could observe the interaction of PR with water even upon 1 s of contact from contrast. Also, we found that the water absorption or interaction was not uniform over surface and the magnitude of absorption was highly dependent on age and treatment of PR such as baking. Similar results were obtained for PR with top antireflective coating. More studies are required for the implication of this observation. Obviously, imaging ellipsometry is a good technique to inspect water mark in immersion lithography. We also repeated similar experiments for high reflective index liquid (JSR HIL-001) but to find negligible absorption by PR.