Frank Chilese

Protection of extreme ultraviolet lithography masks. I. Thermophoretic protection factors at low pressure for diffusing nanoscale particles

Model calculations are presented for thermophoretic protection of an extreme ultraviolet (EUV) mask placed face down in an EUV mask inspection tool. The protection factors, defined as the ratio of challenge particles to deposited particles, are calculated for a variety of test conditions (temperature gradient, gas type, particle density, and particle position) for a reticle bathed in clean gas from a facing showerhead. Thermophoretic protection (in combination with gravity) provides robust protection for particle sizes greater than ∼20 nm. However, for particle sizes less than ∼20 nm, protection falters quickly and is severely degraded for highly diffusing 10 nm particles that are of concern for mask contamination. Estimates are made for the required level of particle protection in both EUV mask inspection and EUV projection lithography. When compared with these estimates for the required protection, it is clear that thermophoresis alone cannot successfully defend against particles smaller than ∼20 nm, an...

Protection of extreme ultraviolet lithography masks. II. Showerhead flow mitigation of nanoscale particulate contamination

An analysis is presented of a method to protect the reticle (mask) in an extreme ultraviolet (EUV) mask inspection tool using a showerhead plenum to provide a continuous flow of clean gas over the surface of a reticle. The reticle is suspended in an inverted fashion (face down) within a stage/holder that moves back and forth over the showerhead plenum as the reticle is inspected. It is essential that no particles of 10-nm diameter or larger be deposited on the reticle during inspection. Particles can originate from multiple sources in the system, and mask protection from each source is explicitly analyzed. The showerhead plate has an internal plenum with a solid conical wall isolating the aperture. The upper and lower surfaces of the plate are thin flat sheets of porous-metal material. These porous sheets form the top and bottom showerheads that supply the region between the showerhead plate and the reticle and the region between the conical aperture and the Optics Zone box with continuous flows of clean ...