Review of Recent Advances in Gas-Assisted Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS)

Abstract

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful chemical characterization technique allowing for the distribution of all material components (including light and heavy elements and molecules) to be analyzed in 3D with nanoscale resolution. Furthermore, the sample's surface can be probed over a wide analytical area range (usually between 1 µm² and 10⁴ µm²) providing insights into local variations in sample composition, as well as giving a general overview of the sample's structure. Finally, as long as the sample's surface is flat and conductive, no additional sample preparation is needed prior to TOF-SIMS measurements. Despite many advantages, TOF-SIMS analysis can be challenging, especially in the case of weakly ionizing elements. Furthermore, mass interference, different component polarity of complex samples, and matrix effect are the main drawbacks of this technique. This implies a strong need for developing new methods, which could help improve TOF-SIMS signal quality and facilitate data interpretation. In this review, we primarily focus on gas-assisted TOF-SIMS, which has proven to have potential for overcoming most of the aforementioned difficulties. In particular, the recently proposed use of XeF₂ during sample bombardment with a Ga⁺ primary ion beam exhibits outstanding properties, which can lead to significant positive secondary ion yield enhancement, separation of mass interference, and inversion of secondary ion charge polarity from negative to positive. The implementation of the presented experimental protocols can be easily achieved by upgrading commonly used focused ion beam/scanning electron microscopes (FIB/SEM) with a high vacuum (HV)-compatible TOF-SIMS detector and a commercial gas injection system (GIS), making it an attractive solution for both academic centers and the industrial sectors.

Keywords: FIB-TOF-SIMS; elemental characterization; gas injection system.

Figure 1

TOF-SIMS data acquisition and representation. Reprinted with permission from A. Priebe et al., “Detection of Au⁺ Ions During Fluorine Gas-Assisted Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) for the Complete Elemental Characterization of Microbatteries”, ACS Applied Materials & Interfaces 2021 13 (34), 41262–41274 [22]. Copyright © 2021 American Chemical Society.

Figure 2

Interior of a FIB/SEM analytical chamber (a) and a SEM image of a 5-line GIS (b).

Figure 3

Gas-assisted FIB-TOF-SIMS with a homemade instrumentation: (a) Schematic of a Cs evaporator and (b) spatial arrangement of the experimental setup in a FIB/SEM analytical chamber (FIB column is located behind the SEM; therefore, it is not visible here) [23]. Reprinted from Ultramicroscopy, Vol 196, A. Priebe and J. Michler, Application of a novel compact Cs evaporator prototype for enhancing negative ion yields during FIB-TOF-SIMS analysis in high vacuum, Pages 10–17 [23], Copyright © 2019, with permission from Elsevier.

Figure 4

Cs-induced enhancement of the Au⁻ secondary ion signal by two orders of magnitude, measured via FIB-TOF-SIMS [23]. Reprinted from Ultramicroscopy, Vol 196, A. Priebe and J. Michler, Application of a novel compact Cs evaporator prototype for enhancing negative ion yields during FIB-TOF-SIMS analysis in high vacuum, Pages 10–17 [23], Copyright © 2019, with permission from Elsevier.

Figure 5

Fluorine gas-induced separation of mass interference during a FIB-TOF-SIMS measurements [5]. The TOF-SIMS depth profiles of the Cu/Zr/ZrCuAg@Si sample acquired without (a) and with fluorine gas (b). Reprinted with permission from A. Priebe et al., “Application of a Gas-Injection System during the FIB-TOF-SIMS Analysis—Influence of Water Vapor and Fluorine Gas on Secondary Ion Signals and Sputtering Rates”, Analytical Chemistry 2019 91 (18), 11712–11722 [21]. Copyright © 2019 American Chemical Society.

Figure 6

The 2D chemical images of the Cu/Zr/ZrCuAg@Si sample obtained without (a1–d1,a2–d2) and with (a3–d3, a4–d4) fluorine gas [5]. The top views (a1–d1,a3–d3) represent lateral images in the x-y plane, and the side views represent the depth images in the x-z plane. The shadowed regions show the data excluded when generating the depth profiles (given in Figure 5). Reprinted with permission from A. Priebe et al., “Application of a Gas-Injection System during the FIB-TOF-SIMS Analysis—Influence of Water Vapor and Fluorine Gas on Secondary Ion Signals and Sputtering Rates”, Analytical Chemistry 2019 91 (18), 11712–11722 [21]. Copyright © 2019 American Chemical Society.

Figure 7

Isobaric mass interference. Comparison of Zr (red dots) and Mo (violet dots) natural isotope abundance (a) and number of detected ions (N_ID) at different mass-to-charge ratios (m/q), which were calculated based on the measured TOF-SIMS signals without (b) and with (c) fluorine gas. Reprinted with permission from A. Priebe et al., “Mechanisms of Fluorine-Induced Separation of Mass Interference during TOF-SIMS Analysis”, Analytical Chemistry 2021 93 (29), 10261–10271 [6]. Copyright © 2021 American Chemical Society.

Figure 8

Diagram representing one of the potential mechanisms of fluorine gas-induced separation of mass interference: Change in secondary ion yield in the presence of fluorine is different and characteristic for various elements [6]. (a) A hypothetical sample is made of two elements M₁ and M₂, whose neutral atoms are shown as red and green dots, respectively. (b) Due to Ga⁺ primary ion beam bombardment, the population of secondary species is ejected from the sample’s surface (dots with yellow halos denote ions). (c) A characteristic response of an element to the presence of fluorine results in different ionization efficiencies and, therefore, a different number of produced secondary ions (∑) of the two elements/isotopes. This can vary by several orders of magnitude. Reprinted with permission from A. Priebe et al., “Mechanisms of Fluorine-Induced Separation of Mass Interference during TOF-SIMS Analysis”, Analytical Chemistry 2021 93 (29), 10261–10271 [6]. Copyright © 2021 American Chemical Society.

Figure 9

Diagram representing one of the potential mechanisms of fluorine gas-induced separation of mass interference: Change in complex-ion (such as oxides or hydrides) formation efficiency [6]. (a) A sample is made of two elements M₁ and M₂, whose masses are different (m₁ < m₂). (b) During Ga⁺ primary ion beam bombardment, complex ions are produced (oxygen comes from the oxidized sample surface or residual gas in an analytical chamber). (c) Fluorine affects the efficiency of metal–oxygen bond production, and metal fluorides are generated more favorably than oxides. Reprinted with permission from A. Priebe et al., “Mechanisms of Fluorine-Induced Separation of Mass Interference during TOF-SIMS Analysis”, Analytical Chemistry 2021 93 (29), 10261–10271 [6]. Copyright © 2021 American Chemical Society.

Figure 10

Mass spectra of a Au/Cr/SiO₂/Si sample representing the main isotopes of Au (a–d), Cr (e–h), and Si (i–l) [22]. The data were acquired without (black lines) and with (red lines) fluorine gas in positive (yellow labels) and negative (blue labels) ion detection modes. Fluorine significantly enhances positive secondary ion generation and decreases the generation of negative secondary ions. Reprinted with permission from A. Priebe et al., “Detection of Au⁺ Ions During Fluorine Gas-Assisted Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) for the Complete Elemental Characterization of Microbatteries”, ACS Applied Materials & Interfaces 2021 13 (34), 41262–41274 [22]. Copyright © 2021 American Chemical Society.

Figure 11

The 2D chemical maps of a Au/Cr/SiO₂/Si sample measured without (a,b) and with (c, d) fluorine gas in positive (a,c) and negative (b,d) ion detection modes. The data are shown in depth plane (i.e., x-z plane). Fluorine-induced enhancement of positive ion yields and charge inversion of secondary ions, from negative to positive, allows for obtaining complete chemical information directly from the same volume during a single measurement [22]. Reprinted with permission from A. Priebe et al., “Detection of Au⁺ Ions During Fluorine Gas-Assisted Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) for the Complete Elemental Characterization of Microbatteries”, ACS Applied Materials & Interfaces 2021 13 (34), 41262–41274 [22]. Copyright © 2021 American Chemical Society.

Figure 12

The 2D chemical maps (x-z plane) of a complex Al₂O₃/Ni/Al₂O₃/Au/Al₂O₃/Cu/Al₂O₃@Si multilayer system built of alternating ALD and PVD thin films [24]. Data were measured without (a) and with (b) fluorine gas. Reprinted with permission from A. Priebe et al., “High Sensitivity of Fluorine Gas-Assisted FIB-TOF-SIMS for Chemical Characterization of Buried Sublayers in Thin Films”, ACS Applied Materials & Interfaces 2021 13 (13), 15890–15900 [24]. Copyright © 2021 American Chemical Society.

Figure 13

Elemental images of the Al/Al₂O₃/Al/Al₂O₃/…/Al multilayer deposited with an in situ ALD-PVD system and measured with FIB-TOF-SIMS. Due to the high sensitivity of the FIB-TOF-SIMS technique, the ²⁷Al⁺ signal distribution was sufficient to assess the location of metallic PVD (Al) and ceramic ALD (Al₂O₃) thin films. Furthermore, higher sputtering rates during fluorine gas-assisted Ga⁺ primary ion beam milling allowed more layers to be represented (b) when compared to the experiments conducted under standard vacuum conditions (a). Reprinted with permission from A. Priebe et al., “High Sensitivity of Fluorine Gas-Assisted FIB-TOF-SIMS for Chemical Characterization of Buried Sublayers in Thin Films”, ACS Applied Materials & Interfaces 2021 13 (13), 15890–15900 [24]. Copyright © 2021 American Chemical Society.

Figure 14

Characterization of Li-ion solid-state batteries (blocking lithium dendrite growth in solid-state batteries with an ultrathin amorphous Li-La-Zr-O solid electrolyte): Depth profiles of Au/Li7La₃Zr₂O₁₂/Pt/MgO/Si sample. Under standard vacuum conditions (a), information on Au and Pt distribution is not accessible. However, simultaneous delivery of fluorine gas during Ga⁺ primary ion beam bombardment inverts the polarity of generated secondary ions from negative to positive (b) providing complete and direct information on the sample chemical structure. Reprinted with permission from A. Priebe et al., “Detection of Au⁺ Ions During Fluorine Gas-Assisted Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) for the Complete Elemental Characterization of Microbatteries”, ACS Applied Materials & Interfaces 2021 13 (34), 41262–41274 [22]. Copyright © 2021 American Chemical Society.

Figure 15

Chemical characterization of commercial 2507 super duplex stainless steel: Comparison of data obtained with EDX (a) FIB-TOF-SIMS under standard vacuum conditions (b), and during fluorine gas-assisted FIB-TOF-SIMS (c) [59]. Reprinted with permission from K. Wieczerzak et al., “Practical Aspects of Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry Analysis Enhanced by Fluorine Gas Coinjection”, Chemistry of Materials 2021 33 (5), 1581–1593 [59]. Copyright © 2021 American Chemical Society.