edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 20 THE ADVANTAGES OF SYSTEMATICALLY DEFOCUSING THE ION BEAM FOR FOCUSED ION BEAM WORKFLOWS William M. Mook and Dustin D. Ellis Sandia National Laboratories, Albuquerque, New Mexico wmmook@sandia.gov EDFAAO (2025) 3:20-25 1537-0755/$19.00 ©ASM International® INTRODUCTION Purposefully and systematically defocusing the ion beam during standard focused ion beam (FIB) processing mitigates the most common FIB artifacts. This simple adjustment leads to several positive outcomes. First, it enhances the efficiency of material deposition by moving the deposition regime from precursor-limited to beamlimited as is done in focused electron beam induced deposition (FEBID)[1] and focused ion beam induced deposition (FIBID).[2] Second, in conjunction with sample over-tilt, it optimizes milling rate while minimizing the formation of curtaining by allowing each ion to have a lineof-sight impact on the milling surface. Finally, defocusing decreases the thermal spike overlap from individual ion/ sample impacts, which mitigates thermal damage that is largely unaccounted for, yet ubiquitous in FIB. PROCESS AND WORKFLOW The benefits of defocusing the ion beam during FIB are due to the difference between a single ion/sample interaction compared to when these interactions overlap in time and space as they do at the sample surface within the well-focused ion beam of a FIB. This is where FIB processing differs from most other ion beam applications, where the concept of ion beam dose is used and is defined as the number of ions impacting the sample surface per unit area, generally reported in terms of C/m2. Fig. 1 By systematically defocusing the ion beam of a FIB, it is possible to control the instantaneous current density of the ion beam. (a) Adjusting beam defocus will change the ion beam spot size at the sample’s surface, also known as blur diameter. (b) The instantaneous current density, J, of the ion beam is calculated as a function of the total spot size diameter assuming a circular spot. Here it is graphed only as a function of added beam defocus to highlight what happens to the instantaneous current density without defocus. It is counter-intuitive that with no defocus, the instantaneous current density of all beam conditions remains approximately constant within one order of magnitude. (b) (a)
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