Atom Probe Tomography

A Combination of Ion Projection Microscopy and TOF-MS

  • Fig. 1: steel specimen in a LEAP specimen holderFig. 1: steel specimen in a LEAP specimen holder
  • Fig. 1: steel specimen in a LEAP specimen holder
  • Fig. 2: A schematic drawing of a local electrode atom probe (LEAP).
  • Fig. 3: APT measurement of a Si phase in a Sr- modified Al-7wt % Si hypoeutectic alloy. (a) 3D reconstruction (b) A local reconstruction showing the segregated region. (c) 1-D concentration profile along the segregation. The direction of measurement is indicated by an arrow in the illustration.
  • Fig.4: APT measurement showing Fe diffusion in RuAl single-phase. (a) Isoconcentration surfaces are used to highlight the Fe segregations. (b) An ROI illustrates the Fe distribution along a grain boundary. (c) 1-D concentration profile showing Fe, Al and Ru behaviour.
  • A local reconstruction along a {111} pole in a pure Al specimen.
  • MSc.-Ing. Hisham Aboulfadl, Manager Atom Probe Tomography, Saarland University
  • Prof. Dr.-Ing. Frank Mücklich, Chair Functional Materials, Saarland University Director (CEO), Material Engineering Center Saarland

Atom Probe Tomography (APT) today is a widely established technique used to analyze different materials with near-atomic resolution in three-dimensions. It offers the highest spatial resolution in comparizon to any other characterization tool with close to part-per-million analytical sensitivity. The evolution and the basic principles of the technique are presented here with applications illustrating its capabilities.

Introduction

Although atom probe field-ion microscopy (AP-FIM) [1] was developed 45 years ago, it was practiced only in few academic laboratories worldwide. This was due to three main reasons: Firstly, the sample preparation was difficult, especially for specific regions of interest. Secondly, the detected field of view (FOV) of the specimens was small (~200 nm²) with very slow data collection rates (~10³ atoms/hr). Thirdly, the technique was limited to materials with a high electrical conductivity. Only a few years ago these limitations were resolve. This was possible by using advanced focused-ion beam (FIB) instruments with few nanometer resolution for sample preparation, introduction of local electrode technology, and the development of femtosecond-to-picosecond pulse lasers with high repetition rates. Atom probe tomography (APT) [2] today is a a well established characterization technique. Local electrode atom probes (LEAP) are now commercially available and reliable instruments for nano-scale analysis of different materials in three-dimensions.

The Principle

APT is a combination of ion projection microscopy and time of flight mass spectrometry. The technique is based on the field evaporation (FEV) of surface atoms from a sharp (<100 nm radius) needle-shaped specimen by application of high voltage fields at cryogenic temperatures under ultra-high vacuum conditions (< 10-13bar) (Fig.1). The applied electric fields on the specimen are in the range of 10-50 nmV-1 and the specimen base temperature is in 20-60 K range, depending on the specimen material, respectively. FEV is understood as a two step process: First is a thermally activated step, extracting a (+1) charged ion from the surface of the specimen.

In this process an energy barrier is resolved by reducing the FEV activation barrier of the material. The specimen surface electronic structure is temporarily manipulated by applying voltage pulses or by increasing the temperature on the end tip by laser pulsing [3]. The second step is a quantum mechanical electron tunnelling process in which a (+1) charged ion is further ionized (+2 up to +5) until it reaches the detector, known as post-ionization [4]. The extracted ions travel a constant distance until they hit a crossed delay line position-sensitive detector. The time of flight (TOF) is calculated between the evaporation pulse time and the time of an ion impact on the delay line, causing a signal (an ion count), thus the mass-to-charge ratio is identified for each ion count. The X and Y coordinates of each impact on the position sensitive detector are recorded during the measurement and used for creating a 3D reconstruction of the collected data. The reconstruction therefore represents the number of atoms extracted from the sample. The radius evolution of the reconstruction should be as the taper angle of the needle-shaped specimen. The radius r generally increases with the voltage V applied, by r=V/kF, where k is a geometrical factor and F is the evaporation field (material dependent).

Applying the voltage field between a specimen and a local electrode added three major advantages: First, it lowereds the amplitude of the applied voltage pulses which allow the use of voltage pulse generators with higher pulse repetition rates (200 kHz) reaching faster detection rates (> 107 atoms/hr). Second, it localize the field on the specimen which improved the mass resolution, and the FOV enhanced to ~10000 nm². Third, the ability to use planar microtips, where several specimens fitted on a microtip array can be measured simultaneously by aligning each specimen in front of the local electrode aperture (30-50 µm diameter). In 1980, Kellogg et al. [5] discovered that the application of short laser pulses (<1 ns) directed to a specimen apex can initiate FEV of any material, regardless of its electrical conductivity, later understood as a laser absorption process [6]. The complexity of the additional instrumental design was a limitation for an efficient implementation of the technique. LEAP instruments are now fitted with highly engineered laser pulsing equipment thereby widening the possibilities to analyze various materials such as semiconductors, thin-films, and even biological materials. Finally, FIB instruments have facilitated site-specific sample preparation. Microtips are used as substrates for APT samples prepared by Ga ion-beam in the FIB, known as the ‘lift-out' method [7].

Examples

Segregation Along Interfaces
Sr is used as a modifier element to enhance the mechanical properties of Al-Si hypoeutectic alloys through a chemical modification process of the eutectic phase. The mechanism of this process is a matter of debate; however, most of the investigations carried out previously [8, 9] agree that the critical role in this modification is in the Si phase where Sr is primarily segregated. An Al-7 wt % Si alloy with 150 ppm Sr is investigated with a LEAP instrument. A measurement from the eutectic Si phase shows Sr segregations in a linear morphology and Al was also found to segregate (Fig. 3).

Grain Boundary Analysis
RuAl is a highly stable intermetallic, used for high temperature coatings [10]. An experiment was carried out to investigate Fe diffusion in RuAl at high temperatures (900 °C - 20 hrs). An APT measurement shows Fe diffuses strongly from a stainless steel substrate and segregates along RuAl grain boundaries (Fig. 4).

Crystallographic Analysis
Lattice planes can be identified using APT for pure elements or low alloyed metals. By measuring the lattice spacing and crystallographic pole angles the crystallography of the reconstruction can be identified (Fig. 5).

Conclusion

A brief overview on APT was presented showing its capability to reveal buried information in different materials at the nano-scale.

References
[1] Miller M.K. et al.: Atom probe field-ion microscopy, Oxford University Press, Oxford (1996)
[2] Miller M.K.: Atom probe tomography, Plenum Press, New York, (2000)
[3] Miller M.K. et al.: Mat. Char. 60, 461-469 (2009)
[4] Kingham D.R.: Surf. Sci. 116, 273-301 (1982)
[5] Kellogg G.L.: J. Appl. Phys. 51, 1184 (1980)
[6] Houard J. et al.: Phys. Rev. B 81, 125411 (2010)
[7] Thompson K. et al.: Ultramic. 107, 131-139 (2007)
[8] Simensen C.J. et al.: Metall. Mat. Trans. 38A, 1448 (2007)
[9] Lu S. et al.: Metall. Trans. 18A, 1721 (1987)
[10] Mücklich F. et al: Intermet. 13, 5-21 (2005)

 

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Universität des Saarlandes
Campus A5.1
66123 Saarbrücken
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Phone: +49 681 302 3799
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