Ar permeability through densified single-walled carbon nanotube-based membranes

Abstract

Single-walled carbon nanotube (SWCNT)-based membranes (pellicles) and gas counterflows are used as a debris mitigation system in extreme ultraviolet (EUV) lithography. Densification with volatile liquids is a well-known approach to tune the performance of SWCNT membranes. However, densification can change the gas permeability through SWCNT membranes, violating the circulation of buffer gas counterflows. In the present work, we investigate the Ar permeability through SWCNT membranes before and after densification with isopropyl alcohol in the pressure drop test. The effective diffusivity of Ar through pristine and densified SWCNT membranes is, respectively, calculated to be Deffpristine = 330 × 109 m2 s−1 and Deffdensified = 4 × 109 m2 s−1. We developed a probabilistic model of gas penetration through SWCNT membranes. On the basis of the experimental data, the probabilities of Ar atoms penetrating through pristine and densified SWCNT membranes are estimated to be 8% and 0.1%, respectively. Structural changes in SWCNT membranes after densification with scanning electron microscopy and the Brunauer–Emmett–Teller technique are demonstrated. The bulk density of SWCNT membranes, measured using x-ray reflectometry, is found to be 0.38 and 0.89 g/cm3 before and after densification, respectively. The temporal dynamic of isopropyl alcohol evaporation from the volume of SWCNT membranes is analyzed using Fourier-transform infrared spectroscopy (FTIR). Results obtained using EUV and FTIR spectroscopy show that isopropyl alcohol is present in the volume of the SWCNT membrane even after the membrane has been left in vacuum (P < 0.01 Pa) for 22 h.