Line width control using a defocused low voltage electron beam

We present a method for line width control in electron beam lithography, which is especially suited for the manufacture of diffractive optical elements. By defocusing the spot of our "gaussian beam" electron beam writer, the line width can be controlled continuously from the 100 nm range to many microns. Test exposures at a beam energy of 2.5 keV with varying defocus and line dose values have been evaluated. It is shown that a simple analytical expression can predict the line widths with high precision down to 300 nm. By an empirical modification, we were able to extend our model down to 100 nm line width. As an example of the method's usefulness, the fabrication of Fresnel zone plates for x-ray microfocussing applications is described.