Liquid Hydrogen Temperature Cryostage for Ice-Assisted Electron-Beam Lithography

Abstract

Liquid nitrogen (LN2) typically acts as a coolant in ice-assisted electron-beam lithography (iEBL) systems, so that the cryostage temperature cannot be lower than 77 K. To condense more gaseous precursors, such as carbon dioxide (CO2) in a high vacuum environment, a cooling system that does not rely on LN2 is necessary. In this article, we integrate a Gifford-McMahon (GM) cryocooler into the iEBL system, which can cool down samples from room temperature to 21 K in 2.25 h. The cold head and sample holder reach minimum temperatures of $5.37~\pm ~0.012$ K and $19.14~\pm ~0.009$ K, respectively, which lies within the temperature zone of liquid hydrogen. Furthermore, a gas-gap isolation system and discrete rotary valve are employed to minimize the vibration effects on the scanning electron microscope (SEM), with the vibration being limited to about 30 nm. Finally, CO2 has been investigated as the precursor, revealing itself as the second positive resist in iEBL, with a critical dose one order of magnitude less than water ice. Gold nanostructures are also successfully fabricated using such a resist. Our system achieves the lowest temperature in iEBL system to date, substantially expanding the range of precursors that can be used in iEBL.