Low-energy Ion Scattering - Experimental Setup

Experimental Setup

LEIS systems consist of the following:

1. Ion Gun, used to direct a beam of ions at a target sample. An electron ionization ion source is typically used to ionize noble gas atoms such as He, Ne or Ar, while heating of wafers containing alkali atoms is used to create an alkali ion beam. The ions thus created hold a positive charge, typically +1, due to the ejection of electrons from the atoms. The range of energies used most often in LEIS is 500 eV to 20 keV. In order to attain good experimental resolution it is important to have a narrow energy spread (ΔE/E < 1%) in the outgoing ion beam.
2. Ion beam manipulator, includes the electrostatic lenses of the ion gun for focusing and beam-chopping. Lenses consist of a series of either plate or cylinder geometries and serve to collimate the beam as well as to selectively filter the beam based on mass and velocity. Beam chopping is performed using a pulsed-wave generator when time-of-flight (TOF) experiments are performed. Ions only pass through the chopper when there is no applied voltage.
3. Sample manipulator, allows an operator to change the position and/or angle of the target in order to perform experiments with varying geometries. Using directional controls, azimuthal (rotational) and incident angle adjustments may be made.
4. Drift tube/drift region, used in TOF setup. TOF measurements are used when analysis of particle velocity is required. By pulsing ions towards the sample with a regular frequency, and observing the time to travel a certain distance after surface impact to a detector, it is possible to calculate the velocity of ions and neutrals coming from the surface. An accelerator may also be used in this setup, prior to the drift tube, in order to achieve separation of ions from neutrals when desired.
5. Detector/electrostatic analyzer, used to detect the velocities and/or energies of scattered particles including ions and, in some cases, neutral species. As opposed to TOF analyzers, electrostatic analyzers achieve ion energy resolution using electrostatic deflectors to direct only ions of a particular energy range into a collector, while all other ions are redirected. This type of analyzer can give good energy resolution (and thus, selectivity) but typically suffers from poor sensitivity due to the fact that it only detects ions of a certain energy range and ignores neutral species altogether. Two types of detectors are used: channel electron multiplier (CEM) and microchannel plate (MCP) detectors. CEMs operate in a similar manner to photomultipliers, displaying a cascade of secondary electron emission processes initiated by ion or fast neutral (energy > 1 keV) impact to give a gain in signal current. In this way it is possible to efficiently detect even small ion or neutral particle fluxes. MCP detectors are essentially 2-dimensional arrays of CEMs, and they allow additional information about particle position to be obtained at the cost of sensitivity at any given position.
6. Vacuum pumps; Studies are performed in ultra-high vacuum (UHV) conditions (< 10−10 torr) in order to prevent unwanted interference with the ion beam and/or sample. Common UHV pumps include turbomolecular and ion pumps, with roughing pumping typically performed using a rotary vane pump. Due to the extreme surface (i.e. first-layer) sensitivity of LEIS, samples also need to be rigorously cleaned prior to analysis. Some common processes used to clean samples include sputtering and annealing. Appropriate equipment for cleaning must be contained within the vacuum chamber.
7. Other analysis tools; in many cases it is desirable to perform multiple types of analysis on a sample within the same UHV system, or even at the same time. Some additional tools may include Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). Use of these tools typically requires the presence of additional detectors as well as electron and/or x-ray sources where applicable.