Electrostatic multipolar probe-forming objective lens for FIB

This paper explores the possibility of using a system of four aligned electrostatic quadrupoles, also known as Russian quadruplet, as an objective lens for a Focused Ion Beam (FIB). The system has been designed compact to be fitted onto standard FIB columns installed in dual beam systems. Each quadrupole has been designed to allow for simple mechanical assembly, whilst retaining the ability to generate an octopole field self-centered with the main quadrupole field by polarizing the ground apertures surrounding the main electrodes. The goal of this additional octopole field will be to compensate for strong aperture aberrations. The performance of this new probe-forming multipole objective lens has been investigated by spot milling a silicon sample's surface using both LMIS and plasma sources. Probe-forming properties and beam shape affected by aperture aberrations of the new Russian quadruplet objective lens are then observed by Scanning Electron Microscope (SEM) images and compared with standard spots observed using conventional rotational symmetric einzel lens. Using an LMIS source, we were able to focus spots with sizes, depending on the current range, at least equivalent to those obtained with conventional optics, while maintaining a working distance more than three times greater. In particular, at low currents in the tens-of-picoampere range, spot diameters of 200 nanometers were achieved, which are half the size of conventional spots. At high currents in the hundred-of-nanoamperes range, spot diameters in the micrometer range were achieved. The results obtained with a plasma source made it possible to generate spots in the tens-of-micrometers range at high currents, while maintaining the advantage of the large working distance. However, for long etching times in the hundred of seconds, a long "evanescent line" is observed along one direction. We will discuss its origin and propose ways to correct this artifact. Finally a first milling application using focused ion lines is also presented at the end of this work.