Most surface spectroscopic techniques involve probing the surface by exposing it to a flux of "particles" (hν , e- , A+ ....) and simultaneously monitoring the response to this stimulation by, for example, measuring the energy distribution of emitted electrons. In their most basic form, these techniques collect information from a relatively large area of surface (∼ mm2 ). In most cases, however, there are variations of these techniques which permit either ,
- information to be collected only from a specific region of the surface - that is ....
- the spatial distribution of a property (eg. simple topography or elemental concentration ) to be mapped - that is ....
Surface Imaging (Surface Microscopy)
The requirement in both cases is for spatial localisation of the spectroscopic technique. This may be achieved in one of two ways ,
A. Localization of the Probe
Spatial localisation of a spectroscopic technique can be achieved by focusing the probe so that it interacts with only a small region of the surface.
The ease with which this can be accomplished and the spatial resolution (degree of localisation) attainable depends primarily upon the nature of the probe - charged particles (e.g. electrons and ions) are easily focused ; neutral particles (e.g. x-ray photons and neutral atoms) are not readily focused.
With this localisation of the probe into a small area, it is clearly advantageous to collect as many as possible of the outgoing particles whose detection completes the spectroscopic process in order to maximize the signal-to-noise.
In the diagram above, the probe is stationary and incident upon a particular small region of the surface in a manner suitable for localised surface spectroscopy. In order to carry out surface imaging or mapping, either
- the probe must be scanned (rastered) across the surface , or
- the surface must be moved under the probe beam.
B. Spatially-localized Detection
The alternative approach is to allow a large area of the sample to be exposed to the probe but only to collect signal (i.e. emitted particles) from a small region of the sample. Thus, in this instance, the spatial localisation is occurring on the detection side of the technique.
In principle, this is easy - it simply requires an appropriate lens system to collect as much emission as possible from the desired part of the surface and focus it onto a detector as illustrated in a highly schematic fashion below.
In practice, this approach has only really been successfully employed in surface analysis when the emitted particles are electrons.
In order to get an image of the surface using a single detector it is necessary to either
- scan the sample position underneath the detection system.
- make use of additional scanning plates in the electon-optical focusing system.
An alternative approach to obtaining images using this same basic arrangement involves employing a linear array or two-dimensional array of detecting elements. In this type of system, the emission from different parts of the surface is effectively focussed onto different parts of the array detector as schematically illustrated below.
Some additional scanning mechanism is still required to build-up an image or map of anything other than the directly imaged region of surface, but the use of multiple (array) detectors greatly enhances the speed of image acquisition and ultimately the image quality.
In summary, spatial localisation of a surface spectroscopic technique may be obtained by either focusing the probe or imaging only a small part of the sample surface onto a detector using a lens system. Images and compositional maps are then usually obtained by scanning the focused probe in the first case, and either translating the sample or employing sophisticated optical/detector systems in the latter case.
Roger Nix (Queen Mary, University of London)