BS ISO 16531:2013 download.Surface chemical analysis — Depth profiling — Methods for ion beam alignment and the associated measurement of current or current density for depth profiling in AES and XPS.
In surface chemical analysis with AES (Auger electron spectroscopy) and XPS X’ray photoelectron spectroscopy), Ion sputtering has been extensively Incorporated for surface cleaning and for the in- depth characterization of layered structures in many devices and materials. Currently, ultra-thin films oI.c 10 nm thickness are increasingly used in modern devices and so lower energy ions are becoming more important for depth profiling. For reproducible sputtering rates and for good depth resolution, it is important to align the ion beam at the optimal position. This optimization becomes increasingly critical as better and better depth resolutions are required. It is not necessary to conduct a beam alignment routinely but it is necessary to align the beam when instrument parameters change as a result, for example, from replacement of ion-gun filaments or from an instrument bake-out. During the beam alignment, care must be taken not to sputter or otherwise affect specimens for analysis on the sample holder. Instruments have different facilities to conduct alignment and seven methods are described to ensure that mos analysts can conduct at least one method. Two of these methods are also useful for measuring the Ion beam current or the current density — Important when measuring sputtering yields and for measuring sputtenng rate consistency. With commercial Instruments, the manufacturer may provide a method and equipment to conduct the beam alignmenL II this Is adequate, the methods described here may not be necessary but may help to validate that method.
ISO 146061, describes how the depth resolution may be measured from a Layered sample arid used to monitor whether the depth profiling is adequate. properly optimized or behaving as intended. That method, from the instrumental setup to the depth resolution evaluation via in-depth measurement is. however, time-consuming and so the present, quicker procedure is provided to ensure that the ion beam Is properly aligned as the first step to using ISO 14606 or for more routine checking.
BS ISO 16531 specifies methods for the alignment of the Ion beam to ensure good depth resolution in sputter depth profiting and optimal cleaning of surfaces when using inert gas ions in Auger electron spectroscopy and X-ray photoelectron spectroscopy, These methods arc of two types: one involves a Faraday cup to measure the ion current: the other Involves Imaging methods. The Faraday cup method also specifies the measurements of current density and current distributions in ion beams. The methods are applicable for ion guns with beams with a spot size below—I mm in diameter. The methods do not include depth resolution optimization.
2 Normative references
The following documents, In whole or in part, are normatively referenced in BS ISO 16531 and are Indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies
ISO 18115- 1.Surfoce chemical analysis — Vocabulary — Part 1:Generoltermsandtermsusedin spectroscopy
3 Terms, definitions, symbols and abbreviated terms
For the purposes of this documenL the terms and definitions given In ISO 18115-1 and the following symbols and abbreviated terms apply.
A Area of Faraday cup aperture
Ao Area of ion beam raster In sample plane
AR Raster area at a known orientation to the Ion beam
B Ion beam broadening parameter equal to ratio i,uter/Iinnrr
C Current
CD Current density
Ion dose rate at the sample
F’ Ion fluence rate delivered by ion gun
FC Faraday cup
FWIIM Full width at the halimaximum
1 Rastered Ion beam current measured in aperture of Faraday cup
Io Stationary, small diameter ion beam current measured In aperture of Faraday cup
limier Ion current measured at inner electrode of co-axial cup
5.3.2 lIthe Faraday cup aperture size is able to be selected, a size smaller than the ion beam allows the best alignment and current density measurement, but it should still be sufficiently large to accept sufficient current for adequate measurement by the current meter available. Apertures larger than at least twice the FWHM of the ion beam profile may be used to measure the total beam current.
NOTE Parameters of the Faraday cup design and applied voltages on both the outer shield and inner cup. important for accurate measurements, are described by References 151 and 1I. Accuracies better than 1 % may be achievable. For non.nonnally incident ions, care is required in considerations of the aperture edge and the depth and structure of the Faraday cup if measurement errors are to be avoided.
5.3.3 Adjust the Ion beam X and Y position offsets and tune the voltages of the Ion gun objective (final) lens until attaining a maximum current in the Faraday cup as shown In Figure 4.11 a condenser lens Is also available, increasing its voltage usually allows a smaller spot size to be obtained but with a lower beam current. After changing the condenser lens strength, the objective lens may need refocusing. Depending on the equipment available, this sequence may need to be repeated iteratively until final settings for beam size, position and current are attained. This provides the Xo and Yo settings for the X and Y offsets for correct alignment for the gwen setting of the beam energy, lenses, etc
NOTE If the ion beam is not at normal Incidence to the sample or Faraday cup. the X and V deflections may etther give equal angular deflections for equal settings or one deflection may be scaled against the other to give equal deflections on the sample surface. It is useful to check this to understand the equipment behaviour. The apparent width of the Faraday cup aperture In the X and V directions will be affected by these considerations.
5.3.4 II the beam width observed Is greater than required. It may be possible to reduce it by reducing the beam current and checking the focus. Ion beams may exhibit an astigmatic focus. This can be checked by scanning over the Faraday cup in both X and V directions, optimizing the focus each time, with the average focal setting being used. Ensure that the X and Y offsets are tuned last for the final operating condition to set Xoand Yo.
Ion beam bombardment increases the emission of secondary electrons, and it may damage the secondary electron detector or electron energy analyser. Therefore, before switching on the ion beam, check if it is necessary to turn off or decrease both the voltages supplied to the electron multiplier detectors and or any other radiation sensitive detector.