Development of an Ion Milling System for Figure Error Correction of a Soft X-ray Multilayer Mirror

Abstract

  We have developed an ion beam milling system enabling effective, gentle and uniform milling of EUV multilayer surface of an imaging mirror for accurate compensation of the reflection phase error. An Ar ion beam of a large 150 mm in diameter accelerated at a low 500 V to avoid surface roughening is irradiated through a template selecting the areas of milling. The Ar beam profile has a quadratic distribution within 100 mm in diameter with an ion current of 0.1 mA/cm² at the center (r=0 mm) and of 40% less 0.06 mA/cm² at the peripheral (r=50 mm) as estimated by trench milling depth measured by an optical surface profiler (WYKO). This was also confirmed by a new detection system composed of an electrode array. A correction mask has been designed for even millig of the selected area of the sample. The milling rate of a Mo/Si multilayer with the uniform ion beam was found to be 2 min/period.