Optical measurement of exposure depth and refractive index in positive photoresists

In this article, we present guided mode techniques based on the m-lines setup for measuring the penetration depth of light into a positive photoresist layer. The specific example of Shipley S1813 photoresist will be studied. A method allowing for the determination of the penetration depth in partially exposed layers without developing them is presented, and the results obtained are compared with those for developed films subjected to the same exposure doses. A comparison of the refractive index of a bleached photoresist film measured by m-lines and by spectroscopic ellipsometry is also given. A model describing the optical changes in photoresist layers during exposure to UV light is proposed and subsequently validated by experimental results.