Quadrupole lens alignment with improved STIM and secondary electron imaging for Proton Beam Writing

Minimal proximity effect coupled with uniform energy deposition in thin polymer layers make Proton Beam Writing (PBW) an intuitive direct-write lithographic technique. Feature sizes matching the focused beam spot size have been fabricated in photoresists down to 19 nm. Reproducible sub-10 nm beam focusing will make PBW a promising contender for sub-10 nm lithography. In this paper, we present beam size characterization by imaging a PBW resolution standard using transmitted/scattered ions and secondary electrons. Using Scanning Transmission Ion Microscopy (STIM) spectra for 1 and 2 MeV H₂⁺ beams, we experimentally measure the thickness of the resolution standard to be 0.9 ± 0.1 μm, applying two independent calibration methods, which match the original intended thickness during fabrication. Through bias optimization of a Micro-Channel Plate (MCP), we show a tuneable secondary electron detection per proton for imaging with a maximum of 75% e/p for a beam of 1 MeV H₂⁺. Based on STIM mode beam size measurement, we discuss considerations for quadrupole system alignment in order to remove higher order translational and rotational misalignments critical to achieve sub-40 nm spot sizes. A spot size of 13 × 32 nm² (STIM) was achieved using a newly developed interface, capable of autofocusing ion beams and performing PBW.