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Controlling Residual Hydrogen Gas in Mass Spectra During Pulsed Laser Atom Probe Tomography R

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Controlling Residual Hydrogen Gas in Mass Spectra During Pulsed Laser Atom Probe Tomography R Kolli Adv Struct Chem Imag (2017) 3:10 DOI 10.1186/s40679-017-0043-4 RESEARCH Open Access Controlling residual hydrogen gas in mass spectra during pulsed laser atom probe tomography R. Prakash Kolli* Abstract Residual hydrogen (H2) gas in the analysis chamber of an atom probe instrument limits the ability to measure H con- centration in metals and alloys. Measuring H concentration would permit quantification of important physical phe- nomena, such as hydrogen embrittlement, corrosion, hydrogen trapping, and grain boundary segregation. Increased insight into the behavior of residual H2 gas on the specimen tip surface in atom probe instruments could help reduce these limitations. The influence of user-selected experimental parameters on the field adsorption and desorption of residual H2 gas on nominally pure copper (Cu) was studied during ultraviolet pulsed laser atom probe tomography. The results indicate that the total residual hydrogen concentration, HTOT, in the mass spectra exhibits a generally decreasing trend with increasing laser pulse energy and increasing laser pulse frequency. Second-order interaction effects are also important. The pulse energy has the greatest influence on the quantity HTOT, which is consistently less than 0.1 at.% at a value of 80 pJ. Keywords: Atom probe tomography (APT), Copper, Hydrogen, Laser pulsing Background quantitative analysis of elemental concentrations in the Residual hydrogen (H2) gas that is present in the analy- mass spectra [1, 2]. Several recent studies have employed sis chamber of an atom probe instrument is typically H charging where deuterium is introduced into the atom detected in the mass spectrum of a specimen tip during probe tomography (APT) specimen tip as a substitute for analysis [1, 2]. The gas directly adsorbs to the specimen the H to be measured in an iron/vanadium (Fe/V) multi- tip surface between each voltage or laser pulse and can layer film [4], steels [5–7], silver (Ag) [7], zirconium (Zr) then be desorbed during a subsequent pulse. The gas mol- alloys [8], and silicon (Si) [9]. The quantity of residual H2 ecules impact the specimen tip and rebound thereby los- gas must, however, still be accounted for due to overlap ing a small amount of kinetic energy during each bounce in the mass spectra in order to estimate the H concentra- and then bind to a protruding surface atom [3]. The tion. Hence, improved insight and understanding into the presence of field adsorbed residual H2 gas can limit the influence of experimental parameters on the behavior of ability to quantify H concentration in metals and alloys residual H2 gas on the specimen tip surface in state-of- during solid-state phenomena, such as hydrogen embrit- the-art atom probe instruments could help reduce these tlement, corrosion, hydrogen trapping, and grain bound- limitations. Such an understanding may permit minimi- ary segregation since it is difficult to distinguish between zation of the quantity of field adsorbed residual 2H gas the two sources of H. It also promotes the formation of and thereby allow more accurate quantification of the H metal hydride complex ions, MxHy, further complicating concentration in metals and alloys. Several authors have investigated the relationship between H2 gas adsorption and desorption and electric *Correspondence: [email protected] Department of Materials Science and Engineering, University of Maryland, field during pulsed voltage one-dimensional (1D) atom 2144 Chemical and Nuclear Engineering Bldg., #090, College Park, MD probe field ion microscope (APFIM) analysis of materi- 20742‑2115, USA als including a nickel-rich (Ni-rich) alloy [10], tungsten © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kolli Adv Struct Chem Imag (2017) 3:10 Page 2 of 10 (W) [11, 12], aluminum (Al) [13], and copper (Cu) [13]. may permit minimization of the gas in mass spectra. Iron (Fe), Ni, Cu, palladium (Pd), molybdenum (Mo), This study evaluates the synergistic laser pulse energy, W, and platinum (Pt) were also investigated as part of laser pulse frequency, and specimen tip base temperature two broader studies [14, 15]. Some of the aforemen- effects on the quantity HTOT that is detected during UV- tioned studies illustrated that the H signal decreases pulsed laser APT of nominally pure Cu. The results are with increasing field strength [11], or that a critical DC discussed in context of a constant evaporation rate theo- field strength exists depending on the metal or alloy, retical model in order to evaluate the relative influence above which the H signal decreases with increasing field of the three aforesaid parameters. A statistical analysis strength [10, 12]. was also performed to ascertain the significance of each Some studies on the effect of experimental parameters parameter and their interactions on the quantity HTOT. on H2 gas field adsorption and desorption using laser- Other H sources that can influence the quantity HTOT pulsed mode in an atom probe instrument have been are discussed including the possibility of impingement performed [16–18]. More specifically, field adsorption and dissociation of H2O on the specimen tip and surface and desorption of H2 gas on molybdenum (Mo) dur- diffusion of residual 2H gas molecules. The short-range ing 1D-pulsed laser atom probe (PLAP) experiments binding energy of a H2 molecule on a Cu metal surface equipped with a nitrogen laser (337 nm wavelength) was atom is estimated based on the model presented in+ Refs. previously investigated [16]. This study evaluated the H [21, 22], and the mechanisms to form the H+ and H3 ions signal intensity at substrate (base) temperatures between detected in the mass spectra are discussed in context of 100 and 150 K and at slow pulse rates between 1 and the binding energy. We discuss how the residual H2 gas 60 Hz. An investigation on the field adsorption of 2H gas in the mass spectra can be minimized and the broader on W during 1D PLAP experiments was also performed applicability of the results to other materials. [17] and evaluated the H signal intensity at temperatures between ~80 and 250 K. The pulse rates accessible by the Experimental methods instruments used in these studies are significantly slower A 0.5 mm diameter 99.999 wt.% polycrystalline Cu wire than that attainable in today’s state-of-the-art instru- taken from Standard Reference Material (SRM) 482 ments, and the base temperatures studied were greater of the National Institute of Standards and Technology than used today. More recently, the influence of laser (NIST) was used as the source material. Specimens with pulse energy on field adsorption of residual H2 gas was a needle-shaped geometry necessary for APT analysis evaluated on a nickel–chromium (Ni–Cr) alloy when were fabricated using a FEI Nova 600 dual-beam scan- using a Local-Electrode Atom-Probe (LEAP®) instrument ning electron microscope/focused ion beam (SEM/ equipped with a green laser (532 nm wavelength) [18]. FIB) instrument following standard lift-out and annular The high mass resolving power, m/Δm, and high ele- milling procedures [23–25]. A platinum (Pt) protective mental sensitivity in state-of-the-art atom probe instru- layer was deposited over a region of interest (ROI) using ments [19, 20] make performing APT measurements of established procedures. Annular milling was performed H concentration in metals and alloys feasible. However, employing a 30 kV gallium ion (Ga+) beam and sequen- no studies have been performed to study the influence of tially decreasing probe current following standard pro- user-selected experimental parameters in modern atom cedures after transfer of the ROI to silicon (Si) microtip probe instruments equipped with a UV (355 nm wave- posts with an Omniprobe Autoprobe 200 Micromanipu- length) laser on the quantity of residual H2 gas detected lator. A low kV ion beam of 5 kV was allowed to raster in mass spectra. Furthermore, limited earlier studies over the specimen tip to remove material that had been using laser-pulsed mode were reported in the extant liter- damaged by the 30 kV Ga+ ion beam annular milling ature, and few have evaluated low temperatures <100 K, operation [26]. or fast pulse rates ≥100 kHz, or the combined effect of Pulsed laser APT was performed employing a user-selected experimental parameters. Typically stud- CAMECA LEAP® 4000X instrument in the Si configura- ies were performed to evaluate a single parameter. Addi- tion with a 355 nm UV-pulsed laser under ultra-high vac- −8 −11 tionally, Cu is an important electrical and structural uum (UHV) conditions of 1.2 × 10 Pa (9 × 10 Torr). material that experiences the aforementioned solid-state A full factorial 3 × 3 × 3 experimental matrix was used phenomena and studies on controlling residual H2 gas where the laser pulse energy was set at 40, 60, or 80 pJ; field adsorption and desorption on Cu have not been the laser pulse frequency at 100, 250, or 500 kHz; and the reported in the literature. Improved insight and under- specimen tip base temperature at 20, 50, or 80 K leading standing into the influence of experimental parameters to 27 individual experiments on different specimen tips. in state-of-the-art atom probe instruments on the total The nominal detection rate was maintained at a constant residual hydrogen concentration, HTOT, that is detected 0.01 ions per pulse or 1%. Datasets of greater than two Kolli Adv Struct Chem Imag (2017) 3:10 Page 3 of 10 + million ions were collected for each experiment.
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