Speed-controlled particle injection into a low-pressure system

Protection of extreme ultraviolet lithography (EUVL) masks from any particles (existing inside the manufacturing process or particles generated by equipment motion) is a major concern for the success of deploying the EUVL technology. Under the low-pressure levels (e.g., ≤50 mTorr) in EUVL scanners, thermophoresis is considered as the most promising noncontact method to protect mask surfaces from any possible particle contamination during vacuum exposure. In order to better test the protection capability of thermophoresis we developed a method to supply particles with known speed into a low-pressure chamber (equipped with thermophoresis testing capability) by incorporating a dual stage particle injection system. The particle injection speed could be controlled by the volumetric flow rate through a critical orifice, the cross-sectional area of an injection tube, and the pressure ratio between the two stages. In addition, the length of the particle injection tube into the low-pressure chamber must be long enough to accurately predict the particle speed. The required tube length can be estimated by means of the particle stopping distance and a particle speed adjustment distance. Finally, the use of an axial jet arrangement for the critical orifice delivered particles through the center of the injection tube at known speed.