Fluorine Gas Coinjection as a Solution for Enhancing Spatial Resolution of Time-of-Flight Secondary Ion Mass Spectrometry and Separating Mass Interference

Abstract

Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) detectors have been intensively developed in recent decades due to their unprecedented capability of representing a sample elemental composition in a 3-dimensional space from nano-to submili-scale with high spatial resolution and mass resolution. A compact high-vacuum-compatible version of these detectors can be integrated into a Focused Ion Beam (FIB) system which, assembled with Scanning Electron Microscopy (SEM), is the most popular tool used in nanotechnology and material science. This gives a new opportunity for combining TOF-SIMS analysis with other instruments within the same analytical chamber. In this work we present the results of conducting elemental characterization of a dedicated model multilayer sample composed of 100 nm thick thin films of Cu, Zr and ZrCuAg alloy in a fluorine gas atmosphere provided by an in-situ Gas Injection System (GIS). In general, the secondary ion signals were significantly enhanced by up to three orders of magnitude leading to much higher spatial resolution. The quality of elemental images and depth profiles was improved during a single measurement (which usually cannot be obtained at standard vacuum conditions) at high beam energy of 20 keV. Moreover, fluorine assistance has enabled a mass interference between 107 Ag + and 91 Zr 16 O + ions to be separated. This remarkable finding has never been reported before and is expected to play an important role in the future evolution of TOF-SIMS analytical protocols as currently the mass interference between ions remains one of the main drawbacks of the technique.

The matrix effect in TOF-SIMS analysis of two-element inorganic thin films

“…The sputtering rates of the alloys have varied between 0.65 nm/frame and 1.09 nm/frame at given experimental conditions. The results presented in this work are the second stage of our extended efforts targeted in exploring the potential and limitations of gas-assisted TOF-SIMS 8 which can lead to significant improvement of elemental image quality and separation of mass interference 24 . In order to verify the influence of water vapour and fluorine gas on the ionization process of two-element alloys, first the generation of secondary ions in standard vacuum conditions has to be known.…”

Potential of gas-assisted time-of-flight secondary ion mass spectrometry for improving the elemental characterization of complex metal-based systems

“…The absence of a uniform ionization model capable of explaining the generation of secondary ions implies the necessity of performing systematic empirical studies on the materials of interest. As demonstrated in our previous work 16,19,30 , the ionization efficiency and response to the provided gas is element dependent. Moreover, in the case of complex materials, the sample components can completely invert the influence of co-injected gas (e.g., from secondary ion signal enhancement to its decrease).…”

“…The maximum divergence from the natural isotope abundance of the most prominent 90 Zr + isotope reaches only 0.2 % (whilst the minimum error of estimating total S90Zr+,which originates in the uncertainty of the interface location, is 0.65 %, see Table S1 and Table S2) and maximum 0.7 % in the case of minor isotopes. As we have postulated Please do not adjust margins Please do not adjust margins before 19 , this can be correlated with the higher electron affinity of fluorine (322 kJ/mol) when compared to oxygen electron affinity (141 kJ/mol) 42 , which can result in preferable formation of Zr-F ions and lower probability of generating oxygencontaining ions. Consequently, this can lead to the separation of mass interference.…”

“…As the ionization probability strongly depends on the chemical surface state (so called matrix effect [9][10][11], it can be modified by introducing certain species to an experimental setup. The presence of oxygen 2,9,12 , water [13][14][15][16] and fluorine [16][17][18][19] have been shown to facilitate positive ion production, whilst Cs 9,[20][21][22][23][24][25][26] was found to increase negative ion yields. The development of compact high-vacuum (HV) compatible TOF-SIMS detectors [27][28][29] , which can be integrated within Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) instruments, gives a new opportunity for performing elemental analysis assisted using a supplementary gas delivered by a Gas Injection System (GIS).…”

“…This is an important observation as, under standard vacuum conditions, two separate experiments must be performed, as the highest lateral resolution is achieved at 30 keV and the highest depth resolution is obtained at low beam energies (usually <5 keV). Moreover, it was observed that due to the different responses of Zr and Ag to the presence of fluorine gas as well as the fluorine-induced modification of metal-oxide formation, the mass interference between Ag + and 91 Zr 16 O + was separated 19 . This remarkable finding was observed for the first time and is expected to play an important role in the development of future TOF-SIMS protocols as currently mass interference remains one of the main drawbacks of this technique.…”