AVS 71 Session AS-ThP: Applied Surface Science Poster Session

The work function is the minimum energy required to remove an electron from a solid surface. This quantity can be directly related to the Fermi level which is of major interest for solid-state physics, material science, and semiconductor applications. Kelvin Probe Force Microscopy (KPFM) combines Scanning Probe Microscopy (SPM) with the electrostatic Kelvin probe method. It can laterally resolve the work function difference between the probing tip and the sample surface together with the corresponding surface topography.

Although the work function describes a macroscopic property of a solid, it may vary locally due to doping, surface contamination or surface oxides. Therefore, clearly defined measurement conditions are required to avoid artifacts and to gain reliable results.

For this work, a combined instrument for time-of-flight secondary ion mass spectrometry (TOF-SIMS) and SPM is used. It enables working under clean UHV conditions, preparation and measurement take place completely in-situ. As mentioned above, the work function of the sample is not measured absolutely but relatively to the probing tip. Once the chemical termination of the tip changes due to e.g. wear or oxidation, the reference is changed, and a general comparison is not possible anymore.

Furthermore, the resulting voltage differences from varying work functions causes additional contributions to the tip-sample force interaction leading also to artifacts in topography measurements. This variation may be sample specific but can also be artificially induced by the ion beam. Separating the work function effect from the topography signal enhances the reliability of the SPM results.

For comparative studies or quality control purposes it is mandatory to control the reference, determined by the termination of the tip. This can be achieved by in-situ tip cleaning, as is performed by the ion sources of the TOF-SIMS. The ion milling removes unknown contaminations and potentially sharpens the tip apex, resulting in a well-defined tip as a reproduceable reference.

As a model system, a silicon wafer is bombarded with different doses of different ions, to compare the effect on the work function. Known tip and sample conditions allow a clear correlation between work function variations and the ion bombardment to characterize the sputter induced effects on the work function.

This work demonstrates the possibilities of KPFM for the investigation of implants, doping or compound semiconductors. In addition, it aims at separating the effects of the work function in topographic measurements and therefore remove artifacts.

Femtosecond laser ablation (fs-LA) is a newly developing XPS depth profiling technique which avoids the chemical damage observed using traditional monatomic and gas cluster ion beam sputtering [1]. The laser pulse duration plays a key role in determining the involvement (or not) of thermal processes in the ablation mechanism. InP is a thermally sensitive compound semiconductor material, as shown by enhanced preferential sputtering effects being observed when profiled using a gas cluster ion beam compared to a monatomic ion beam [1]. As such, it is a useful test material for studying the effects of laser pulse length on chemical composition during profiling. fs-LA XPS depth profiles of bulk InP were recorded using a 1030 nm laser for pulse durations varying between 160 fs and 6 ps. To ensure the true chemical composition could be retained at ultrashort pulse lengths, a multi-shot regime at a laser energy below the ablation threshold was required. The effect of laser pulse duration and variation of the number of shots per ablation level on the chemical composition, ablation threshold energy and crater surface morphology during profiling will be presented and discussed.

[1] M.A.Baker et al, Applied Surface Science 654 (2024) 159405

XPS depth profiling is widely employed to determine the chemical composition and offer chemical state information for thin films, thin film devices, coatings, surface treatments and surface degradation processes. Traditionally, XPS depth profiling has been performed through sputtering, using a monatomic or gas cluster ion beam (GCIB). However, many materials suffer from ion beam induced chemical damage during profiling, resulting in distorted chemical compositions and incorrect chemical state information being recorded during the depth profile. Recently, in a new approach to XPS depth profiling, ion beam sputtering has been replaced by femtosecond laser ablation (fs-LA). This new methodology has been shown to offer significant advantages over sputtering: (i) avoidance of chemical damage; (ii) profiling to much greater depths (several 10s microns); (iii) faster profiling speeds; (iv) ease of varying the ablation rate for different materials [1]. Using a 1030 nm wavelength, 160 fs pulsed laser, fs-LA XPS depth profiles will be shown for selected thin films, coatings, devices, surface treatments and oxidised surfaces, demonstrating the capabilities of this new technique.

[1] M.A.Baker et al, Applied Surface Science 654 (2024) 159405

The quantitative evaluation of the depth-dependent chemical composition of thin films plays a pivotal role in the development of novel technologies across several sectors, from electronics to medicine. While Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) offers exceptional chemical sensitivity and spatial resolution as well as the possibility of acquiring data as a function of depth from the surface (through sputtering), standardless quantification has remained a significant challenge due to matrix effects and the complex physics of secondary ion generation. Here, we demonstrate the first successful implementation of the Full Spectrum Method (FSM) for quantitative concentration depth profiling of inorganic thin films using ToF-SIMS. The FSM approach—though documented in only six publications over two decades—effectively minimizes matrix dependencies by leveraging large ion clusters that incorporate numerous neutral atoms, thereby decreasing the ratio of charged particles per cluster. In this study, we systematically quantified molybdenum, sulfur, and oxygen concentrations in physical vapor deposited MoS₂ thin films with varying stoichiometries. Our ToF-SIMS measurements achieved excellent agreement with complementary Rutherford Backscattering Spectrometry performed on reference samples from identical deposition batches. This validation not only establishes FSM as a viable pathway for standardless, semi-quantitative ToF-SIMS analysis of complex inorganic systems, but also enhances the analytical capabilities of ToF-SIMS for characterizing complex organic specimens, layered structures, and heterogeneous thin films central to emerging technologies in electronics, energy storage, and catalysis.

Battery devices are complex, multi-layered systems with many surfaces and interfaces that contribute directly to performance. Increased global energy demands and environmental concerns have driven the need for next-generation battery materials with excellent performance and stability, low cost, and improved safety. But the multi-component interfaces and dynamic nature of these systems leads to challenges with their characterization. Developing new materials and technologies to meet energy storage needs requires physicochemical characterization approaches with high-spatial resolution, chemical and morphological information, and correlation of properties.

A review of recent literature has revealed several common flaws and limitations in surface analysis using various methods, particularly XPS, as well as a notable deficiency in reporting critical sample, instrument, and analysis parameters essential for assessment of the reported information by readers and any efforts to replicate the results. The core assumption is that new and casual method users want to do quality research, but with the increasing number of techniques involved in many research activities, it is a challenge to have expertise for each method. Multiple efforts have been made in response to the identified problems, each intended to provide easily accessible and useful information and tools that can assist an analyst in avoiding the limitations and faulty analyses found in the literature. This poster will provide updated information on some of these efforts. Topics to be addressed include: 1) as a reviewer or reader, you do not need to be an XPS expert to recognize many peak fitting and analysis issues in XPS, 2) an overview of topics addressed in the topical paper collections Reproducibility Challenges and Solutions I and II appearing in the Journal of Vacuum Science and Technology A, 3) an overview of the topics discussed in the shorter Notes and Insights papers now appearing in Surface and Interface Analysis, 4) information about a series of detailed instrument papers starting to appear in Surface Science Spectra to help with instrument parameter reporting and providing descriptions of instrument operation modes, 5) overview of two new ISO standards for reporting on the selection, handling, storage and preparation of samples for surface analysis to be recorded and included as part of sample provenance information, and 6) other journal and web-based papers and platforms intended to assist peak fitting, parameter reporting and error identification.

Deuterium and hydrogen plasma exposures were performed on ultra-high temperature ceramics TiB2 and ZrB2 using the PISCES-RF linear plasma device as early screening tests for first wall, plasma facing material applications. These ion plasma exposures were performed using 40 eV ion energies at 240, 525, and 800 ◦C sample temperatures and 90 eV ion energies at 240 ◦C sample temperatures to analyze TiB2 and ZrB2 sputtering and surface morphology evolution behavior. Post-plasma exposure chemistry characterization of the near surface (< 50 nm) region using x-ray photoelectron spectroscopy (XPS) shows transition metal enrichment, indicating boron preferential erosion, and resulting in reduced total sputtering yields compared to predicted assuming stoichiometric sputtering. Transition metal to boron fractions vary with plasma exposure temperature under the 40 eV ion energy exposure at different temperatures; metal enrichment is maximized at 800 ◦C and then minimized at 525 ◦C. Sputtering yield measurements of the 40 eV ion energy plasma exposed samples show that the samples with greater metal surface enrichment have lower sputtering yields, likely due to the rougher surfaces of the more metal-enriched samples leading to higher instances of prompt redeposition processes. XPS data was acquired on the as-exposed TiB2 and ZrB2 samples. Depth profiles were then done to track the amounts of T (or Zr) and B as a function of Ar-ion sputter depth. Data was finally acquired on the well sputtered sample surfaces. This abstract has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy.

Atom probe tomography (APT) utilizes the physics of field evaporation to collect time-of-flight mass spectra from materials [1]. Field evaporation is initiated by either pulsing the electric field (voltage pulse, VP mode) or the specimen temperature via a pico-second laser pulse (LP mode). Fundamental field evaporation characteristics of model materials (mainly metals) have been studied and reported over the years [2], but few recent studies have attempted to expand understanding using modern instrumentation.

Voltage-plus-laser pulsing (VpL) is a new LEAP^® 6000 pulsing mode, capable of initiating field evaporation using a simultaneous combination of field and temperature pulses [3,4]. VpL mode lowers the standing field on the specimen surface between pulses, resulting in a reduction of out-of-time evaporation events. The relative timing of the pulses can also be adjusted to terminate delayed evaporation events emitted during the apex cooling process in laser pulse mode (reduction of thermal tails in the mass spectrum). In this study, VpL mode is used to expand the available combinations of standing field, evaporation field, and apex temperature to better understand the full ion emission properties from a standard specimen.

In this poster presentation, we will discuss the experimental and analytical methods that enable exploration of residual gas emission and complex molecule formation trends as a function of evaporation rate, apex temperature, standing field, and evaporation field via VpL mode.

References:

1. D.J. Larson et al., “Local Electrode Atom Probe Tomography”, (Springer, New York, 2013).

2. T. T. Tsong, “Atom-Probe Field Ion Microscopy: Field Ion Emission and Surfaces and Interfaces at Atomic Resolution”, (Cambridge, Great Britain: Cambridge University Press, 1990).

3. R. Ulfig et al., Microscopy and Microanalysis 28 (2022), p. 3190.

4. D.J. Larson et al., Microscopy and Microanalysis 28 (2022), p. 718.