Isofocal dose based proximity effect correction tolerance to the effective process blur

The isofocal dose in electron beam lithography (EBL) is defined as the dose that results in the same feature size independent of the effective blur (blur eff ), which is the result of a combination of resist processing, spot size, beam focus, forward scattering, etc., that contributes to the final resist image. In other words, as blur eff changes the same feature size is still obtained while using the same dose. This phenomenon is clearly demonstrated in EBL simulation when varying the blur eff . In this work, the authors identify the isofocal dose for a given resist process consisting of 200 nm of ZEP520A from ZEON Chemicals atop a Si substrate using 300 nm line-space tower patterns with pattern densities ranging from 0% to 100% on an Elionix ELS-7500EX 50 keV EBL tool with a fixed 20 MHz clock at 200 pA with a 30 μm final aperture and a 20 nm beam step size. In this experiment, the dominant component of the blur eff is the electron beam focus. By comparing line width measurements from tower patterns exposed with a focused beam with those from a defocused beam, the resulting blur eff manifest themselves in the exposure latitude as a change in slope for each pattern density. Superimposing the exposure latitudes from each blur eff at their specific pattern density, the intersection of said curves indicates the pattern density dependent isofocal dose of the resist process. Despite the difference in the blur eff , the response to the correction remains invariant when the density dependent isofocal doses are aligned properly using a tunable proximity effect correction (PEC) algorithm. This means if a PEC yields the appropriate pattern density dependent isofocal doses, the same feature sizes will be consistently attainable across all pattern densities regardless of the beam focus accuracy. The text that follows demonstrates the technique used to empirically identify the pattern density isofocal doses for a given resist processes and its application using a commercially available PEC algorithm.