Auger electron spectroscopy (AES) is a common secondary surface analytical technique added to our AXIS photoelectron spectrometers. As outlined above, the technique involves the irradiation of the sample with a high energy (3 – 10 keV) focused electron beam. Described in simple terms, the excitation of the atom by the primary electron beam causes ejection of a core electron, leaving an electron hole. As the atom is in an unstable state, the core hole is filled by an outer shell electron. The transition energy of this electron relaxation event can be coupled to a second outer shell electron, which is ejected from the atom if the transferred energy is greater than its orbital binding energy. The kinetic energy, E kin, of the Auger electron defined by the simplified equation: E kin = E core – E b – E’c Where E core, E b and E’c are the binding energies of the core level, first outer shell and second outer shell respectively. The Auger electron is identified using the x-ray notation for the electronic levels. The figure shows ejection of a KLL Auger electron, where K denotes the core level hole, L 1 the relaxing electron’s initial state and L 2,3 the ejected electron’s initial state. It is noted that E kin of the Auger electron is independent of the energy of the excitation source. An Auger spectrum is acquired by collecting the Auger electrons as a function of their kinetic energy. As with XPS, the surface sensitivity of Auger electron spectroscopy derives from the relatively short mean free path of these electrons in a solid. Strengths & capabilities of AES, SAM & SEM The real strength of Auger surface analysis over and above XPS is the significantly better spatial resolution. This makes the technique extremely useful for elemental analysis of small features. The main capabilities of Auger electron spectroscopy are: - High spatial resolution, 100 nm minimum beam diameter - Surface sensitive, 3 – 10 nm information depth - Quantitative identification of all elements (except H & He) - Topographical information from SEM These strengths and capabilities mean that the technique is ideally suited to: - Microelectronic defect and failure analysis - Thin film composition analysis - Corrosion studies - Particle analysis The excitation source for AES on a Kratos spectrometer is a high-performance Schottky field emission 10kV electron source. The mu-metal shielded source is mounted perpendicular to the sample stage’s tilt axis. The electron source region of the field emission gun (FEG) is differentially pumped by a dedicated ion pump, ensuring an optimal working environment for the emitter. To protect the field emission tip, the system incorporates safety interlocks, including those related to cable and vacuum level. Software control of the electron source includes geometry correction, rotation, shear, and scaling. These functions can be used as transform controls. Standard operating parameters are easily stored and retrieved in the ESCApe data system. Moreover, the system provides the flexibility to manually adjust the electron source and secondary electron detector parameters through the software. Direct N(E) Auger spectra and auger maps are acquired with the lens/analyser operating in fixed retard ration (FRR) mode. The spectrometer has a number of pre-set retard ratios for acquiring AES at different energy resolutions. To aid in the interpretation of results, the ESCApe software incorporates specific Auger data processing features, including peak identification. The AES accessory includes a secondary electron detector allowing collection of SEM images from the sample. The detector can be operated with an extraction potential for conventional SEM imaging, or in back-scattered mode, to increase image contrast between atomic numbers. As the technique is capable of working at much higher spatial resolution than XPS, it is important to minimise mechanical vibrational interference. This is achieved by isolating the whole analysis chamber from the frame using air cushioned anti-vibration mounts. The complete hardware package for AES enables the system to deliver exceptional secondary electron microscopy (SEM), Auger electron spectroscopy (AES), and scanning Auger microscopy (SAM) capabilities.
