Electron beam lithography tool for manufacture of X-ray masks

An electron beam lithography system suitable for manufacturing X-ray masks with critical dimensions down to 0.35 µm is described. The system features a 50-kV variable shaped spot (VSS) electron column with a variable axis immersion lens (VAIL). This column is capable of maintaining 0.035-µm edge acuity of the focused spot over a 2.1-mm deflection field. These fields are stitched together over an 84 × 84-mm active pattern area via motion of an xy table. The table position is measured using a laser interferometer. The measurement data are fed back to the magnetic deflection to correct small errors. Maintaining positional accuracy of the beam relative to the writing surface relies on a strategy of measuring and correcting repeatable errors. This is described in detail. Pattern placement accuracy is 0.070 µm (3σ) and image size control is 0.025 µm (3σ), achieved over the entire 84 × 84-mm pattern area. This performance is achieved with yield better than 90%, as confirmed by routine measurements. The system is currently used to manufacture product X-ray masks with 0.35-µm critical dimensions. Typical measurement results on product masks are presented.