The effect of thin metal overlayers on the electron beam exposure of polymethyl methacrylate

Here, the authors consider the effect of thin metal coatings on the contrast, clearing dose, and resolution of polymethyl methacrylate (PMMA) electron beam resist. They present processes suitable for deposition, exposure through, and removal of 5-nm-thick Al, Cr, and Cu layers. Contrast and clearing dose were determined by exposing large features and resolution was assessed using single pixel lines at beam energies from 2to30keV. In all cases the presence of the metal layer increases the clearing dose. They obtained a minimum linewidth of 40nm from the single pixel patterns. Both the experimental results and Monte Carlo simulations of the point spread function (PSF) support the feasibility of direct writing of sub-40-nm patterns despite of additional forward electron scattering. Applying thin metal coatings on resist provides an alternative to conductive polymers for charge reduction and an excellent foundation for fiducial grids used in spatial-phase locked electron beam lithography.