Using high-contrast salty development of hydrogen silsesquioxane for sub-10-nm half-pitch lithography

When used as a negative-tone electron-beam resist, hydrogen silsesquioxane (HSQ) is typically developed in an aqueous alkali solution such as tetramethyl ammonium hydroxide. This development process results in low contrast. In this work, the authors instead used a mixture of salt and alkali to significantly increase the contrast of HSQ. Contrast values as high as 10 in a 115-nm-thick resist were achieved by developing HSQ in an aqueous mixture of NaOH alkali and NaCl salt. Remarkably, this salty developer resulted in contrast enhancement without significant decrease in resist sensitivity. The improved contrast of HSQ enabled the fabrication of 7nm half-pitch nested-“L” structures in a 35-nm-thick resist with minimal loss in thickness using a 30kV electron-beam acceleration voltage. They noticed a strong dependence of contrast enhancement on the concentration and type of cations and anions in the aqueous developer solution.