An in‐situ Low Energy Argon Ion Source for Local Surface Modification
Authors: Mulders Johannes , Trompenaars Piet 
Published: December 2016

A new in‐situ low energy ion source for SEM and DualBeam has been designed. The static beam of low energy gaseous ions such as Ar
+
, O
+
or Xe
+
can be used for a local modification of the sample surface. Typical energies are in the range 5 ‐ 500 V, covering the interaction types from chemical reaction to reactive ion etching and to ion milling, for energies above the milling threshold. The source is based on the following principle: electrons from the SEM's electron beam partially convert an atomic or molecular gas flow into a beam of ions directed towards a biased sample. A schematic set up is shown in figure 1. A small nozzle delivers the gas and the electron beam enters this nozzle through a slotted hole. The beam is scanned in this slotted hole, penetrates the gas flow and generates thermal ions both by direct ionization and by ionization from beam interactions with the wall of the nozzle. The ions are pulled out of the nozzle by the protruding fields from the biased sample which is located at a short distance from the nozzle: the ions are accelerated in this electrostatic fieldand directed towards the sample. The slotted entry hole is roughly located at half the inner nozzle diameter from the edge.

The source produces a static beam of ions with selectable energy. The direction and width of the beam depend on the geometry and not on the applied bias voltage, because the electric fields define both the acceleration and the trajectories. With a typical SEM excitation condition of 2kV, 26nA and a nozzle to sample distance of 100 um, it is possible to generate a 100 eV Ar
+
beam current of 5 nA and a full width half maximum (FWHM) of 8.2 um. This corresponds to a central average ion current density of 0.095 nA/um
2
, which is very similar to the current density at 500 V of a Ga
+
beam produced by a regular FIB column. The FWHM is easily adjustable by changing the nozzle‐to‐sample distance, allowing for example a broader beam with a wider peak. The source is slightly focusing as shown in figure 2, so the beam diameter does not expend too rapidly with the sample to nozzle distance. In this way, the sample area that is affected by the low energy ions can be more or less defined.

Thanks to the low energy, the new source can be used for polishing the top surface of a sample such as the Ga doped layer after FIB operation (removal of Ga and reduction of damage layer thickness) or it can be used to clean a sample from residual hydrocarbons. The first application can be useful for improvement of the quality of a TEM lamella produced by FIB, or improvement of and EBSD surface prepared by FIB. An example of the interaction with the beam is shown in Figure 3, where a native oxide on Si has been removed in 6 seconds, using 200 V Ar
+
ions. Finally it should be noted that in case of using O
2
gas, the source behavior (size, energy, field distribution) is the same, allowing cleaning of a sample chemically, below the milling threshold. Primary gas switching to other noble gases is straightforward, because the primary principle (ionization by electron impact) and ion trajectory formation and acceleration remain the same.