A sure shot: Fine-tuning focused ion beam placement with critical-dimension localization microscopy

A.C. Madison, C.R. Copeland, R.G. Dixson, B.R. Ilic, J.A. Liddle, S.M. Stavis
National Institute of Standards and Technology,
United States

Keywords: focused ion beam, optical microscopy, placement accuracy


As applications of focused ion beams become more demanding, feature placement accuracy becomes more important. Previously, we identified systematic errors of feature placement across an ultrawide patterning field. Such errors are a general problem and of particular concern for machining standards that provide reference positions, such as aperture arrays for optical microscopy. Conversely, our recent advances of localization traceability present an opportunity to improve placement accuracy. In this study, we quantify feature positions by critical-dimension localization microscopy with ultrahigh throughput, revealing complex errors extending to several micrometers across a submillimeter field. A novel correction reduces scale errors by three orders of magnitude and distortion errors by more than a factor of 40, greatly improving the placement accuracy of a common focused-ion-beam system. We begin by designing a square array of apertures to be machined into a platinum film of sub-micrometer thickness. This initial design has a lateral extent of 200 µm by 200 µm, a nominal pitch of 2502 nm, and apertures with a nominal diameter of 500 nm. This particular value of pitch separates aperture centers by an integer number of pixels across our wide patterning field and separates the aperture images in optical micrographs beyond the resolution limit for optical microscopy and localization analysis. After fabrication, we optically transilluminate the aperture array, localize each aperture, and register the resulting positions with those of the initial design through a rigid transformation. This analysis reveals total errors with magnitudes extending 2 µm, with root-mean-square values of 528.9 nm in the x direction and 1007.7 nm in the y direction. A similarity transformation between the experimental and nominal positions distinguishes errors of uniform scale factor and complex distortion effects, returning the pitch of the experimental array as 2472.01 nm ± 0.27 nm, which is a scale error of 1.20 %. Additional systematic errors of distortion are as large as approximately 1 µm and with root-mean-square values of 399.3 nm in both the x and y directions. We report uncertainties as 68 % coverage intervals. We modify the array design to negate these errors, uniformly increasing the array pitch to account for the 1.20 % scale error, to achieve a nominal pitch of 2500 nm, and modeling the distortion errors by an interpolant that adjusts the design position of each aperture. We machine and measure a new array and apply a similar analysis, registering the localization data with the new design. We measure a pitch of 2500.03 nm ± 0.27 nm, corresponding to a scale error of 0.001 %, and additional distortion errors extending up to approximately 40 nm and with root-mean-square values of 9.0 nm in the x direction and 9.4 nm in the y direction. In this way, we realize new accuracy of a common focused ion beam system, fine-tuning this fabrication method for position-critical applications.