Self-aligned structures by a single-step through-membrane 100-keV electron beam lithography

Patterning of aligned structures on supporting thin membranes is required in many applications, such as fabrication of stacked zone plates and realization of observation windows in nanofluidic channels. Here, the authors present a process flow to fabricate self-aligned structures on both sides of a membrane stack by means of a single-step through-membrane 100-keV electron beam exposure. High energy of the electron beam ensures that the electrons are able to propagate through the membrane stack that is coated with resist on both sides, while the single-step exposure ensures that subsequently etched structures in the supporting silicon nitride membranes are perfectly aligned. Simulations of electron scattering indicate that for micrometer-sized structures the exposed region (containing 90% of the deposited dose) broadening is ∼220 nm for a thick ∼6-μm membrane stack (500 nm of SiO₂ sandwiched between two 250 nm-thick SiN_x layers, with the membrane stack coated with 1 μm of PMMA resist on each side). This corresponds well with the broadening measured from the imaged cross sections of structures etched into the top and bottom SiN_x encapsulating membranes. The broadening can be further optimized by adjusting the process parameters and reducing the thicknesses of the membranes and of the PMMA resist. The relatively small feature-broadening at the bottom of the membrane stack indicates that perfectly aligned submicrometer features are feasible.