Geometry of nanopore devices fabricated by electron beam lithography: Simulations and experimental comparisons

Abstract

Fabrication of solid-state nanopore sensors for individual biomolecule analysis is a growing topic of interest. A number of recent studies demonstrate that engineered nanopore devices (ENDs) can be fabricated by electron beam lithography (EBL) with high density (on the order of 10 devices per cm2). The internal pore geometry of ENDs is a critical characteristic of the devices, and often obtained by some combination of SEM, TEM, AFM, and conductance measurements. However, experimental data alone is not sufficient to understand the nanopore geometry under a broad set of fabrication conditions. It is necessary to also examine the physical basis underlying the EBL-based fabrication of ENDs. In this work, the internal pore geometry of ENDs is calculated from electron energy distributions in EBL while investigating the effects of dose, operating blur, substrate, and dosing pattern. The photoresist is ZEP-520 on silicon or silicon nitride substrates. It is found that higher beam blur and lower dose cause a greater degree of pore tapering, with the most prominent tapering observed in sub-10nm pores. Nanopores in silicon nitride tapered more than those in silicon. The results also demonstrate that a combination of blur and dose can be chosen to achieve a target tapering angle and pore size at a given depth in the substrate. Because the pore tapering angle is non-uniform, the ICP (inductively coupled plasma) etch depth may also be used to tune pore size and geometry following EBL. The resist sensitivity is shown to increase with beam blur for pore sizes larger than 10nm. By comparing to our experimental data, it is found that beam intensity measured by the EBL instrument may not translate to the operating blur, as is often assumed in EBL simulations. Secondary electrons were found to be responsible for pore tapering and beam broadening in the resist.