Producing smaller feature sizes in microelectronics poses lithographic challenges, in which new techniques must be developed to overcome the problems.  A technique using a carbon dioxide compatible salt (CCS) and supercritical carbon dioxide (scCO₂) solution to develop extreme ultraviolet (EUV) photoresists is being investigated with the aim to reduce line edge roughness and image collapse of high aspect ratio features.  In this work, the kinetics of photoresist dissolution in CCS/scCO₂, the photoresist swelling behavior in CO₂, and the effect CO₂ has on the photoresist glass transition temperature (Tg) will be explored. 

The kinetics and the photoresist dissolution mechanism are being studied using a quartz crystal microbalance (QCM).  QCM measures frequency changes with time, where the frequency change is a product of changes in mass loadings, hydrostatic pressure, temperature, and solution viscosity.   The theory of QCM has been developed for high pressure CO₂ conditions to correct for changes in frequency due to hydrostatic pressure and solution viscosity, allowing for calculations of mass changes on the crystal surface.  The initial photoresist dissolution rates in CCS/scCO₂ will be measured for pressures between 4000-5000 psig and temperatures between 40^oC and 50^oC.  Effects of density, temperature, film thickness, drying techniques, and various commercial EUV photoresists will be reported.

Because CO₂ is a well-known plasticizer for polymers, the swelling behavior and the photoresist Tg in CO₂ are also of interest.  The EUV photoresist used in this study is insoluble in scCO₂ alone, which enables examination of the photoresist swelling behavior in CO₂.  An ellipsometer equip with a high pressure cell, where the polarized light shines through fused silica windows, will be used to measure the film thickness and refractive index of the photoresist for temperatures between 40^oC and 50^oC and at selected pressures ranging from 0 to 1600 psig to obtain photoresist swelling plots for sorption and desorption of CO₂. 

Beyond understanding the photoresist swelling behavior in CO₂, the effect CO₂ has as a plasticizer in lowering the photoresist Tg will also be examined.  Using a high pressure calorimeter, which changes CO₂ pressure at a constant rate with a fixed system temperature, the glass transition pressure (Pg) can be measured.  Knowing the solubility of CO₂ in the photoresist film from ellipsometry and QCM measurements and the Pg from calorimetric measurements, the Chow Theory will be used to predict how the photoresist Tg changes with CO₂ pressure.  

Combining the information pertaining to the photoresist dissolution kinetics, the photoresist swelling behavior, and photoresist Tg depression, the mechanism of photoresist dissolution in CCS/scCO₂ will be better understood and will aid in optimization of the photoresist development process.