Control Method for Helium Gas Pressure and Concentration in X-ray Steppers

Abstract

A new method for controlling helium gas (He) pressure and concentration in X-ray steppers has been developed. The mask chamber to which an X-ray mask is attached must be filled with pure He gas to use the X-ray effectively, and the pressure difference between the inner (He) and outer (air) surfaces of the mask must be controlled to a level below 0.005 mmHg to keep the mask deflection within 1μm. The key feature of this control method is the utilization of the density difference between air and He (γ_A-γ_H). He gas regulated at 1 mmHg (gauge pressure) flows into the mask chamber through a small diameter orifice. To compensate for the pressure drop ΔP in the discharge pipe, the gas is discharged to the atmosphere through a pipe kept at a location, that is h= ΔP/ (γ_A-γ_H) higher than the mask center. He concentration of more than 99.95% and the pressure difference of less than 0.002 mmHg were achieved in the X-ray stepper.