Investigation of thermophoretic protection with speed-controlled particles at 100, 50, and 25 mTorr

Thermophoresis is considered as a candidate for protection of extreme ultraviolet lithography masks from particle contamination during vacuum exposures. A thermophoretic force is exerted on a particle by surrounding gas molecules within a temperature gradient. Gas molecules on the "warm side" of the particle provide more momentum than on the "cool side," so particles move from the warm to the cool region. In this study, thermophoretic protection of a critical surface from particles injected with known initial speeds into a quiescent gas has been investigated at 100, 50, and 25 mTorr. Initial particle speed was varied from 10 to 31 ms depending on the gap distances (1, 2, and 3 cm), particle sizes (125 and 220 nm), and system pressures. A pinhole plate is used to supply speed-controlled particles with almost no accompanying gas flow. The results demonstrate that the window of protection offered by thermophoresis is very narrow for inertial particles, and that thermophoresis offers the greatest protection for low-velocity particles subject to diffusional motion in the vacuum system. Furthermore, the experimental results verify the results of an analytical model, developed by Asbach [Appl. Phys. Lett. 87, 234111 (2005)]. The analytical model can be used to predict the particle stopping distance under any thermophoretic gradient.