Inverse heat conduction method (IHCM) is one of the most effective approaches to obtaining the boiling heat transfer coefficient from measured results. This paper focuses on its application in cryogenic boiling heat transfer. Experiments were conducted on the heat transfer of a stainless steel block in a liquid nitrogen bath, with the assumption of a 1D conduction condition to realize fast acquisition of the temperature of the test points inside the block. With the inverse-heat conduction theory and the explicit finite difference model, a solving program was developed to calculate the heat flux and the boiling heat transfer coefficient of a stainless steel block in liquid nitrogen bath based on the temperature acquisition data. Considering the oscillating data and some unsmooth transition points in the inverse-heat-conduction calculation result of the heat-transfer coefficient, a two-step data-fitting procedure was proposed to obtain the expression for the boiling heat transfer coefficients. The coefficient was then verified for accuracy by a comparison between the simulation results using this expression and the verifying experimental results of a stainless steel block. The maximum error with a revised segment fitting is around 6%, which verifies the feasibility of using IHCM to measure the boiling heat transfer coefficient in liquid nitrogen bath.

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 in a high vacuum environment, a cooling system which does not rely on LN2 is necessary. In this paper, we integrate a Gifford-McMahon cryocooler into the iEBL system, which can cool down samples from room temperature to 21 K in 2.25 hours. The cold head and sample holder reach minimum temperatures of 5.37 ± 0.012 K and 19.14 ± 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, with the vibration being limited to about 30 nm. Finally, carbon dioxide 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.