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

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.

Introduction: Gate-all-around (GAA) transistor will replace fin field-effect transistor (FinFET) in technology nodes below 3nm compared with FinFET process[1], GAA process mainly adds three process modules, among which channel release is extremely important. This step requires complete removal of the SiGe sacrificial layer while preserving the Si channel. Although some excellent SiGe selective etching methods have been reported such as gas-phase etching[2] or remote plasma source (RPS) etching [3], However, most of these literatures focus on optimizing etching profile, yet rarely mention the interface state of the Si channel after etching. In this article, we focus on the morphology after different etching depths, obtain interface state density through different schemes, and ultimately develop gate-all-around devices (GAA) to study the effects of different schemes on the electrical characteristics of the devices. This study provides insights and references for the industry in selecting channel release methods.

Results and Discussion: Figure 1 shows the morphology obtained by combining F₂ gas reaction with remote plasma source at different etching depths, and the results show good morphology control from 5nm to 50nm. Figure 2 shows that the scheme exhibits high etching selectivity under high-resolution transmission electron microscopy, and the channel atoms remain intact. Figure 3 shows that adding RPS processing can reduce the interface state. Figure 4 shows the channel of the GAA device released by the combination of F₂ gas and RPS, and the electrical characteristics are better than those released by F₂ alone (with lower subthreshold (ss) characteristics). Conclusion: We propose a new etching method combining F₂ gas and remote plasma, which respectively helps enhance channel mobility and improve subthreshold swing (SS). This offers references and insights for path finding in channel release method and facilitates the volume manufacturing of GAA.

Reference:

[1] Basker V. S., VLSI, (2024).

[2] Yi Chen., J. Vac. Sci. Technol. A 43, 042604 (2025).

[3] Erwine Pargon., et al., J. Vac. Sci. Technol. A 37, 040601 (2019).

Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) is often the instrument of choice when researchers desire rapid high lateral resolution chemical analysis.Time of flight secondary ion mass spectroscopy (ToF-SIMS) instrumentation has matured over the past few decades – state of the art instruments can now perform rapid high lateral resolution chemical analysis of all elements (including H and Li), however its use as a microscopy technique is often overlooked.

BAM-L200, is a certified reference material prepared from a cross-sectioned epitaxially grown layer stack of Al_(0.70)Ga_(0.3)As and In_(0.2) Ga_(0.8)As on a GaAs substrate. The surface of the cross sectional BAM-L200 sample provides a flat pattern with layer widths down to 1 nm. Calibration distances, grating periods and layer widths have been certified by TEM with traceability to the length unit. The combination of gratings, isolated narrow lines and sharp edges of wide lines offers plenty of options for the determination of lateral resolution, sharpness and calibration of length scale for analytical instruments.

In this poster, we will compare line scan traces generated using ToF-SIMS and EDS on a SEM.We will show that the lateral resolution using the 20% 80% rule, is 21 nm for ToF-SIMS and 54 nm for EDS. In addition to being faster and providing higher lateral resolution imaging, ToF-SIMS also allows for the collection of a full mass spectrum (all elements from H to U, and high mass molecular fragments) at every volume element; ToF-SIMS also has much higher chemical sensitivity (detection limit for most elements is in the ppm to ppb range); and ToF-SIMS does not require the deposition of a thin conductive coating.

In this project, we are studying the atomic layer deposition (ALD) of Al₂O₃ on InAs. The interest in this system stems from the intrinsic properties of III-V semiconductors for electronic applications. Specifically, InAs has a narrow, direct band gap and high electron mobility, making it a good candidate to outperform Si-based Metal Oxide Semiconductor Field Effect Transistors (MOSFET) regarding speed and power consumption¹. The detrimental native oxide should thereby be replaced by a so-called high-k oxide such as Al₂O₃, allowing a smaller gate size. However, the performance of this system is limited due to high defect density in the InAs/Al2O3 interface², which is related to incomplete removal of the native oxide layer. The precise deposition of Al₂O₃ on InAs is achieved through the ALD process, enabling the production of thin films with atomic-scale precision. It is characterized by its layer-by-layer mechanism and self-limiting interaction of two gaseous precursors with the substrate³. In our case, two half-cycle depositions were performed using trimethylaluminum (TMA) and water as precursors. A common method to study the chemical composition and interface quality of ALD films is to perform X-ray photoelectron spectroscopy (XPS) measurements after the entire process or after each half-cycle. Here, we went a step further and used ambient pressure XPS (AP-XPS) to observe time-resolved surface chemical reactions occurring during the ALD half-cycles. For this purpose, the ALD process was implemented in the ALD cell of the SPECIES beamline at the MAX IV Laboratory⁴.

During the first half-cycle, i.e. the deposition of TMA, the As-oxide was removed entirely, simultaneously with the formation of Al components and organic TMA ligands on the sample surface. In contrast, the In-oxide decreased, but still a small amount remained at the interface. Also, it was observed that the formation of Al₂O₃ started already during the initial TMA deposition. During the water deposition, a slight increase in In-Oxide was taking place, indicating the reoxidation of the InAs/ Al₂O₃ interface, which is in contrast to the findings from previous AP-XPS studies of HfO₂ ALD on InAs⁵. These results help to understand the reaction dynamics of ALD processes and the role of the metalorganic precursor, as well as to improve the interface in future MOS-based electronics.

1. D’Acuntoet al., Surf. and Interf. 39, 1029272
2. Brennan & Hughes,. J. Appl.Phys 108, 0535163
3. Karnopp et al., Coatings 14, 5784
4. Kokkonen et al., J Synchrotron Rad 28, 5885
5. D’Acunto et al., ACS Appl. Electron. Mater. 2, 3915

Inverse photoemission spectroscopy (IPES) is a powerful tool for investigating unoccupied electronic states where the electron beam is incident upon the sample and the emitted photons are detected. However, its broader application is hindered by inherently low sensitivity, caused by the low probability of photon emission during the transition of free electrons to the unoccupied states. To enhance sensitivity through improved photon collection, an off-axis parabolic (OAP) mirror is designed and fabricated for the low-energy version of the IPES (LEIPS). Optical simulations showed that the OAP mirror increased photon collection efficiency from 3.06% to 63.3%, which is experimentally validated in the lowest unoccupied molecular orbital (LUMO) spectra of C60 thin films. The OAP mirror-LEIPS system is applied to study the energy level alignment (ELA) of pentacene films on substrates with different work functions (WF). By measuring both the highest occupied molecular orbital (HOMO) and LUMO levels, the evolution of transport gaps and the ELA of HOMO and LUMO with the Fermi level are analyzed. Pentacene exhibited n-type behavior on a low WF substrate (Cs₂CO₃) and switched to p-type on a high WF substrate (ITO). The OAP mirror-enhanced LEIPS system significantly reduced the time required for such experiments, enabling efficient and reliable measurements.