Determination of wavelength and excitation voltage for X-ray lithography

The exposure time of an X-ray lithography system is minimized by the appropriate choice of X-ray wavelength and target excitation voltage, within the constraints of a specified resolution and contrast in the exposed resist pattern. The factors that must be considered in making this choice are the X-ray source brightness of various target materials, the continuum emission spectrum of the target, the wavelength-dependent transmission of the X-ray mask and the vacuum window, and the wavelength-dependent absorption in the resist. The relative exposure time, as a function of wavelength, is predicted for a system using a 25-µm-thick beryllium window and PMMA resist with three choices of mask substrate: 12.5- µm-thick Mylar, 4.0-µm-thick silicon, and 8.5-µm-thick beryllium. A new mask substrate, 0.2-µm-thick aluminum oxide, is presented and shown to be suitable for exposure in vacuum with a 2.5-µm-thick aluminum filter at 13.3-Å wavelength (copper target). X-ray emission spectra from an aluminum target were measured at electron energies of 4.5, 7.9, 10.4, 12.5, 15.5, 19.5, and 28.5 keV. These spectra showed that the continuum radiation contributes little to the degradation of contrast with a gold-on-silicon X-ray mask. Thus a 20-kV electron beam may be used for maximum X-ray production efficiency.