Plasma focus x-ray source for lithography

Abstract

A bright and reliable x-ray source for lithography has been developed using plasma focus. Discharge with constant pressure gas, one of the features of plasma focus, makes the x-ray source system simple and lengthens lifetime. A fine ceramic insulator made of alumina in place of a conventional Pyrex glass insulator improves system reliability. The system operates for more than 105 discharges without maintenance. The lifetime of the system is ten times longer than that of a conventional plasma focus device. The resolution of a pattern printed by multishot exposure depends not only on the diameter of pinched plasma but also on the variation of source position. A new spherical electrode surrounding the plasma-focusing space is added to stabilize the location of the spot on the axis by eddy currents which exert the Lorentz force on the plasma. The spot position deviation has become negligibly small as compared with the pinched plasma diameter. The x-ray source size for neon is 1 mm in diameter and 10 mm in length. Consequently, 0.4-μm fine pattern has been printed with this source. Neon radiates intense x rays in opposite voltage polarity to that of a conventional plasma focus. Polarity inversion enables a very thin beryllium window to be located on the axis with the assistance of magnetic deflector and plasma stop. An x-ray intensity of 5 mJ/cm2/shot 25 cm from the source with an irradiance of 10 mW/cm2 at the 2-Hz repetition rate has been obtained. The plasma focus is a promising x-ray source for lithography from the viewpoint of intensity, resolution, and lifetime.