Accurate measurement of the beam diameter for a finely focused ion beam is difficult using conventional methods which have been applied in electron beam systems. Generally, the beam is scanned across an abrupt edge while measuring beam current or secondary electron intensity. For ions, the sputtering of the sample degrades the shape of the edge and thus the accuracy of the measurement. Furthermore, implantation can change the secondary electron contrast during the measurement. In this paper, a new method is suggested that avoids these difficulties. The ion beam diameter is determined by measuring the ratio of ion flux in a single pixel wide feature to that of a feature which is much larger than the beam diameter. This ratio can be measured using a threshold process. For this paper, we have used sputtering of a thin film as the threshold process and secondary ion mass spectrometry as the end-point determinant.

Harriott, L. R. (1990). Beam-size measurements in focused ion beam systems. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 8(2), 899–901.