Environmentally Benign Development of Photoresists in Supercritical Carbon Dioxide

Environmentally benign processes involving supercritical fluids have been investigated as alternatives to conventional solvents due to the increasing awareness of environmental issues. Carbon dioxide is considered to be a “green” solvent as it is nontoxic, nonflammable and non-reactive under most conditions. Supercritical carbon dioxide (scCO2) is a widely used solvent to enhance processing performance in photolithography, especially in the rinsing/cleaning step. We demonstrate using scCO2 as a green developing solvent for low molecular weight resists and conventional polymer photoresist (with CO2 compatible additives). Summary of Research: Molecular glass materials (defined as non-polymeric glass-forming macromolecules) are considered to have the advantages over linear polymers as photoresists. Higher resolution is expected due to the smaller molecular size of the molecular photoresists compared with that of linear polymers. Most of the molecular glass resists explored so far are based on aromatic rings [1], which have strong absorption at deep UV regime and thus cannot extend beyond extreme UV (EUV) and e-beam lithography to 193-nm lithography. Molecular glass compounds based on naturally occurring cores are reported here as scCO2-developable resists for 193-nm lithography [2]. Cyclodextrins (cyclic oligomers of amylase) have drawn much interest since their discovery in 1891. The geometry of cyclodextrins gives the hydrophilic molecules hydrophobic inner cavities so that various hydrophobic molecules can be fitted inside to form supramolecular inclusion complexes. Obtained from a renewable source, starch, cyclodextrins have low toxicity and are biodegradable. Methylated b-cyclodextrin is explored as the core of a molecular glass resist because chemical modification can be easily accomplished on it due to its improved solubility in common organic solvents—the reduced number of hydroxyl groups can be protected more easily and give better contrast when the protection ratio remains the same. Figure 1, left: SEM of tBOC-protected tert-butyl cholate patterned by e-beam lithography and developed in scCO2. Figure 2, right: SEM of tert-butyl ester of methylated b-cyclodextrin patterned by e-beam lithography and developed in scCO2. Another example of alicyclic natural materials is cholic acid. Cholic acid is well known to have strong intermolecular interactions which can contribute to the high glass transition temperatures of itself and cholates as well. Tert-butyl cholate (tBOC) is employed because it has good etch resistance and transparency at 193 nm due to its alicyclic structure. These core molecules are protected with acid-labile and scCO2-solubile tBOC or tert-butyl ester groups. Photoacids generated during exposure can cause catalytic deprotection of tBOC groups and render the molecular glass resists insoluble in scCO2 as negative-tone 193-nm resists. Lithographic evaluation of these materials was done using the Leica VB6. scCO2 development after electron-beam patterning was performed at 40°C and