Microcharacterisation of implanted charge

40x 40mm KPM images of the surface potential on SiO2 implanted/ sputtered in a Focused Ion Beam system with a 30keV, Ga+ ion beam for (a) 0.5s. (b) 3s and (c) 7.5s. The gallium ions penetrate up to ~40nm into SiO2. The specimen was coated with a thin grounded layer of conductive material prior to implantation/sputtering. These images show the evolution of significant residual surface potentials of several hundred mV from the complex charging effects resulting from focussed ion beam implantation/sputtering. The bright contrast indicates negative potential while the dark contrast indicates positive potential.

The Electron Microscope Unit at UNSW has a suite of ten major microscope facilities including Transmission Electron Microscopes (TEM), Scanning Electron Microscopes (SEM), Scanning Probe Microscopes (SPM) and a Focussed Ion Beam (FIB) system, with a further two microscopes to be installed in 2003. These microscopes are flexibly configured with a range of micro-analytical systems enabling the chemistry, structure and physical properties of materials to be determined with high sensitivity and high spatial resolution. In parallel with projects involving the micro-characterisation of a range of technologically important materials, I have also been working on the development of accurate micro-analytical techniques. It is very important to understand the perturbing influence that an analytical probe has on the specimen under investigation.

A FIB system is the ion beam analog of a SEM and produces a finely focused, energetic beam of gallium ions which may be scanned over the surface of a specimen. The FIB is particularly useful for the examination of materials where subsurface microstructure information is required. It is therefore important to assess the influence of ion implantation and ion milling on the specimen structure. In particular, ion beam irradiation of poorly conducting materials may result in the trapping of charge at either pre-existing or implantation induced defects. Trapped charge produces a highly localized electric field within the ion-irradiated micro-volume of specimen which may influence the local structure, by inducing local electro-migration of charged mobile species.

Kelvin Probe Microscopy (KPM) is a specialized Atomic Force Microscopy technique in which long-range Coulomb forces between a conductive atomic force probe and a specimen enable the electrical potential at the specimen surface to be directly measured at high spatial resolution. Significant localized residual charging has been directly observed for the first time within the gallium implanted micro-volumes of non-conductive materials prior to and following the onset of sputtering. Charge mitigation strategies including coating the specimen with a layer of thin grounded conductive material prior to milling and/or the use of an electron flood gun during milling have been investigated. The degree of charging is influenced by a number of different self regulating dynamic processes including implantation, non-stoichiometric sputtering from compounds, secondary electron emission, secondary electron trapping by irradiation induced defects, secondary ion re-attraction and trapping, etc.

The reproducible characteristic surface potentials associated with the ion implantation induced trapped charge have been successfully modelled using three dimensional conformal Finite Element Analysis. This work has provided insight into the complex charging processes that occur during implantation and sputtering and the resultant spatial distributions of the residual trapped charge.

Marion Stevens-Kalceff




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