Fabrication of high-secondary-electron-yield grids for spatial-phase-locked electron-beam lithography

Poor placement accuracy is a major issue and inhibitor in all forms of electron-beam lithography (EBL), a direct result of the open-loop nature of EBL. In scanning-electron-beam lithography (SEBL), the beam position is not continuously monitored during exposure. Instead, one depends upon a laser-interferometer-controlled stage and secondary referencing. In SEBL, the beam can deviate from its intended position causing pattern-placement errors that are often worse than the resolution. Spatial-phase-locked electron-beam lithography (SPLEBL) is being developed at MIT to achieve nanometer-level absolute placement accuracy. In its global-fiducial-grid mode, SPLEBL directly references the beam location to an electron-transparent grid, placed on top of the e-beam resist, during exposure. Recently pattern-placement precision of ∼1nm was demonstrated with secondary electrons as the reference signal [J. T. Hastings, F. Zhang, and H. I. Smith, J. Vac. Sci. Technol. B 21, 2650 (2003)]. However, the fabrication process used for the reference grid was complex and would not be acceptable in manufacturing. In this article, we describe a process for putting down the grid that is user friendly and preserves its long-range spatial-phase coherence. We also investigated a variety of candidate grid materials (various low-Z metals and C60) for their secondary-electron yields and processibility.