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Electron Backscattering Patterns and Ion Blocking Patterns

Automated Crystal Orientation Microscopy (ACOM), or in short Orientation Microscopy (OM), on bulk surfaces is usually based on Backscatter Kikuchi Diffraction (BKD) in the SEM. A commercial acronym for this technique is EBSD. Spatial resolution is usually in the range of 0.05 μm, accuracy is better than 0.5°, and speed of on-line measurement is actually more than 1000 patterns per second with Fast EBSD. An alternative technique yielding higher spatial resolution is microbeam electron diffraction in the TEM or SEM using spot (SAD) or transmission Kikuchi patterns (TKD). Thin foils, however, are difficult to prepare for observation in transmission and, even worse, are sources of orientation errors due to wrinkling which is frequently manifested by bend contours. The study of fine grain and heavily deformed materials in general, of nanomaterials, of recrystallization, grain growth and the characterization of grain boundaries demand for a substantially higher spatial resolution than is achieved with EBSD in the SEM.


1. Introduction to Ion Blocking Patterns

Diffraction patterns can be produced not only by electrons, but as well by ions of some ten keV kinetic energy when impinging on a crystalline surface. The first Ion Blocking Patterns (IBP) have been published by A.F. Tulinov in 1965 [1, 2]. The patterns have been produced by proton beams of 150, 200, and 500 keV steered onto a single crystal of W respectively Mo at an angle of incidence of 30°. They have been recorded on special photographic plates, placed parallel with the surface of the crystal, to cover a large solid angle. Patterns have been indexed in these papers and in further papers published in the same year by A.F. Tulinov and co-workers, so providing evidence of the crystallographic nature of the patterns and of the mechanism of their generation by channeling the ions along low-index lattice planes and crystallographic directions.

BKP from copper at 20 keV
Backscatter Kikuchi Pattern from Cu at 20 keV
Proton Blocking Pattern
Proton blocking pattern from W at 200 keV [1].
The permission for reproduction is gratefully acknowledged to Prof. A.F. Tulinov, Lomonosow State University Moscow, and Uspekhi Fizicheskikh Nauk, Moscow.

IBP have, at a first glimpse, an appearance substantially different from Backscatter Kikuchi Patterns (BKP):
1. Instead of broad Kikuchi bands, they show narrow straight bands of high contrast, nearly black lines, due to the shorter deBroglie's wavelength of ions. Background is more even and continuous. Band profiles are described by almost symmetrical cusps.
2. The band width is given by the critical channeling angle rather than by the Bragg angle.
3. The band widths also depend on the atomic number of the target atoms of the crystal. This is not the case in electron diffraction.
4. No high-order lines are seen. High-order Kikuchi lines are very useful in the precise determination of orientation and lattice parameters.
5. Dark patches mark low-index zone axes.
6. Intensity is high also in backward scattering direction whereas BKP have a strong intensity maximum in forward scattering direction.

Channeling and interaction volumes

Simple models of generation of BKP by channeling and blocking

Interaction volumes of TKP, BKP and IBP
The probe size is marked in red, and spatial resolution in blue.

Kikuchi patterns are clearly generated by diffraction, whereas IBP could perhaps be better interpreted by the classical ballistic model of channeling. Precise experimental measurements are still missing so as to decide whether Bragg's equation is valid here. The background is produced by blocked ions according to Rutherford scattering. There is no significant difference in backscattering from an amorphous and from a crystalline solid for ions that impinge at directions off the critical angles (~ Bragg angles). At kinetic energies in the range of several ten keV, their mean free paths are less than a few atomic layers. Only the channeled ions are missing in case of a crystal, they penetrate deep in the crystal and form sharp cusps of low intensity on the background. So the background intensity can be used as signal. This information about the crystal structure comes from the upmost atomic layers, like Auger electrons. As a consequence, spatial resolution in IBP is only limited by the diameter of the primary beam probe. On the other hand, BKP are formed by electrons that are backscattered from the interaction volume that extends beneath the surface to a depth of the mean free path of energetic electrons. Hence spatial resolution in EBSD is limited to some ten nanometer, despite that the electron beam can easily be focused to a significantly smaller probe size.

The geometrical basics of IBP and BKP, however, are quite similar. The center line of a band corresponds to the section line of the - imaginarily extended - diffracting lattice plane with the recording plane. The crossing points of bands, named poles, represent the positions of zone axes of the crystal. So the crystal structure and crystal orientation of the diffracting volume can be determined from the intensity distribution and positions of the bands in an IBP in quite a similar way as in a BKP. As an application, C.S. Barrett and co-workers [3] have used proton blocking patterns at 100 keV for phase differentiation. Crystallographic aspects of ion beam scattering have been discussed in a review article [4]. A commercial instrument for the identification and rapid alignment of single crystals was manufactured by Edwards High Vacuum Intern. [5].

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[1] A.F. Tulinov: On an effect accompanying nuclear reactions in single crystals and its use in various physical investigations. Soviet Physics - Doklady 10 (1965) 463-465 (English translation of the original article of A.F. Tulinov, Doklady Akademii Nauk SSSR 162 (1965) 546-548)

[2] A.F: Tulinov, V.S. Kulikauskas and M.M. Malov: Proton scattering from a tungsten single crystal. Physics Letters 18 (1965) 304-307
[3] C.S. Barrett, R.M. Mueller and W. White: Proton blocking patterns for hcp and wurtzite structures. Transactions MS AIME 245 (1969)427-429
      C.S. Barrett: Line intensities in proton scattering. Transactions MS AIME 245 (1969) 429-430
[4] C.S. Barrett: Ion Beam Scattering applied to crystallography. Naturwissenschaften 57 (1970) 287-295
[5] R.G. Livesey: A 30 keV instrument for ion surface interactions studies. Vacuum 22 (1972) 595-597.


2. Orientation Microscopy in the Ion Scanning Microscope

The working principles of ion scanning microscopes and of the more common Scanning Electron Microscopes (SEM) are very similar, with the exception of an ion gun and electrostatic or quadrupol lenses which replace the electron gun and round magnetic lenses. Ion scanning microscopes are used in the microelectronic industry to repair masks and electronic circuits. Focused Ion Beam (FIB) appliances to the SEM or stand-alone ion scanning microscopes with a Liquid Metal Ion Source (LMIS) ( Ga+ ) are frequently employed in materials science for the preparation of TEM and SEM specimens by ion milling. Bulk surfaces are imaged in scanning mode whereby ion induced electrons, backscattered primary ions, secondary ions which have been released from the sample, neutrals or cathodoluminescense radiation are used as signals.

Sources of image signals in the ion microscope.Automated Crystal Orientation Microscopy with a scanning ion microscope  

 

A similar technique as for BKD has been proposed for the acquisition and indexing of Ion Blocking Patterns (IBP) in an ion scanning microscope [2]. Orientation microscopy with IBP promises the following advantages over EBSD in the SEM:
• The sample is tilted at moderate angles of about 45° to the primary beam direction to accommodate the wide-angle pick-up of IBP. So image distortion and spatial resolution in beam direction are markedly reduced as compared to EBSD where the specimen is steeply tilted to typically 70°.
•  Sample preparation is less difficult since deformation layers or foreign surface layers can be removed in situ at a controlled rate by using a primary beam of heavy ions until clear blocking patterns have developed.
•  3D reconstruction of the volumetric microstructure from planar 2D slices is facilitated by controlled serial ion sectioning since the specimen may remain stationary in this position or is merely tilted to a steeper angle to the beam. The significantly smaller information depth of a few atomic layers favors 3D reconstruction.
•  Specimen charging is less harmful than in the SEM because secondary electrons are released from walls of the specimen chamber by the impact of scattered ions and neutrals. They reduce positive surface charging.

The combination of ACOM/IBP with a Helium Ion Microscope Zeiss Orion® [3] promises particular advantages:
•  Virtually no specimen sputtering is induced by the light-weight Helium ions during orientation measurement nor during imaging the microstructure.
•  The excitation volume is not significantly larger than the minimum spot size. Spatial and in-depth resolution of ACOM with IBP is expected to approach the sub-nanometer range.
• Specimens that adversely react to Ga+ ions, such as aluminum base alloys, can be investigated.

However, beam current in small ion probes is presently very low in the Orion® Ion Scanning Microscope, so that we did not yet succeed in producing sequences of IBP. A more feasible alternative may be a stand-alone FIB ion microscope with a Ga+ LMIS, but to the disadvantage of less spatial resolution.

The blocking-and-channeling effect is manifested in an exceptionally high orientation contrast when a parallel beam of energetic ions is scanned across a polycrystalline surface. Small angular tilts of the specimen lead to strong changes of contrast between the grains [1]. This is an indication of the pronounced anisotropy of ion backscattering from crystals, an effect which is clearly reflected as well in the polar intensity display of the 2D IBP.




Orientation contrast as a function of specimen tilt in the field ion microscope.
Copper specimen imaged with electrons induced by impact of Ga ions at 30 keV in the Ion Scanning Microscope.
I thank Prof. Dr. U. Wendt, University of Magdeburg, Germany, for providing this figure.

Orientation contrast in the ion scvanning microscope

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[1] U. Wendt and G. Nolze: FIB milling and channeling. G.I.T. Imaging & Microscopy 9#3 (2007) 34-36
[2] R.A. Schwarzer: Spatial resolution in ACOM - What will come after EBSD. Microscopy Today 16 #1 (2008) 34-37
[3] L. Scipioni, L. Stern and J. Notte: Applications of the Helium Ion Microscope. Microscopy Today 15 #6 (2007) 12-15