The first transistors were known as “planar” transistors because all elements of the transistor, (gate, source, drain) were situated on a 2D plane. For many following generations, the performances of the planar transistors could be increased by shrinking the gate length. As the transistor’s dimensions continued to decrease, the space between the source and drain decreased to the point where the gate was not able to avoid leakage current issues. Because of this, the industry has shifted from planar to “3D” transistors, known as FinFETs. Another evolution of the transistor is currently underway in the industry. These next-generation transistors are known as “Gate-All-Around” (GAA) transistors using stacked horizontal nanosheets. A more recent evolution of the GAA devices sees the n and p channels moving closer together or stacked vertically in both forksheet or 3D complementary FET (CFET). The evolution of the device architecture from 2D (planar) to 3D (CFET) has always of course been accompanied by an improvement of the patterning capabilities, which turned to be more and more challenging nowadays with the reduced dimensions and the increased complexity of the devices to produce. Together with the change of architecture, the industry has implemented and is still investigating the use of new materials, such as high-k materials replacing the gate SiO2, strained-Ge and SiGe thanks to their excellent hole and electron mobilities and even more recently 2D materials such as MoS₂ or WS₂ as new channel materials.

All these evolutions (change from 2D to 3D architecture, size reduction of the devices with dimensions smaller than the SIMS beam spot size, complexity of the mass spectra, with more and more mass interferences due to increased number of elements present in the devices, ...) have made the SIMS analysis more complex to perform in order to retrieve accurate information about bulk and layer composition, dopant quantification, layer uniformity, .... In this paper, we will review how SIMS has evolved during the past years to remain relevant to the microelectronic industry. We will show how the use of concepts such as Self Focusing SIMS (SF-SIMS) [1] and the introduction of new developments in the SIMS technique such as the Orbitrap^TM mass analyzer and the combination of SIMS with other complementary techniques such as AFM, allow to extend the application of SIMS in the semiconductor industry for the next decades. Several examples will be discussed.

References

[1] A. Franquet et al., Appl. Surf. Sci. 365 143 (2016).

Fuel injectors and filters are critical engines components which are affected by deposit formation, causing decreased fuel efficiency and increased harmful emissions. The industry has sought to understand deposit formation by employing a range of analytical techniques on these challenging samples. SIMS provided a new perspective on deposit analysis, initially with ToF-SIMS enabling extraction of spatial and depth information to reveal a chemically layered deposit structure, but only small fragment ions could be identified. More recently, the debut of OrbiSIMS brought a combination of spatial information with high-resolution mass spectrometry, yielding a molecular characterisation in three dimensions which indicates the origin of various deposit components. The application of OrbiSIMS to both diesel and gasoline deposits suggested chemistry of lubricant oil origin, inorganic salt contamination, and carbonaceous material found deep within the deposit. Complementary X-ray photoelectron spectroscopy (XPS) depth profiling provided validation with elemental quantification, notably confirming a high sodium salt concentration in a diesel injector sample. Further work by Edney et al. evidenced a progressive carbonisation of deposits over time, focusing on polyaromatic precursors which were corroborated by the softer ionisation technique of AP-MALDI Orbitrap^TM mass spectrometry. Our recent work applies OrbiSIMS with XPS to the analysis of “lab-grown” deposits formed using a laboratory benchtop test serving as a cheap and rapid simulation of injector deposit formation. We investigate the relationship between deposit composition and fuels and contaminants, finding a typically carbonaceous matrix that is influenced by components in the fuel such as biodiesel or sodium. With these new understandings, research can be focused on mitigation strategies for these deposit chemistries and structures to ensure optimal and sustainable use of internal combustion engines during the transition towards decarbonisation.

In recent years, it has been discovered that silanes can effectively safeguard metals from corrosion. By carefully choosing appropriate silane adhesion promoters, a monolayer of silanes firmly bound to the surface forms a protective film on for example aluminium, titanium, and other metals[i]. It was later found that sol-gel chemistry provides a better anti-corrosive strategy compared to separate silane molecules because it allows for the creation of a more robust and homogeneous coating on the surface of the material. The so-deposited sol gel is a uniform, dense, highly crosslinked, and continuous film that is tightly bound to the surface of the substrate, hence giving rise to long-lasting protection against environmental factors such as moisture, oxygen, and salt[ii].

Furthermore, sol-gel chemistry allows for the incorporation of various functional groups into the coating, allowing additional benefits such as improved adhesion, durability, and flexibility It also makes possible the occurrence of anchoring groups to which subsequent coating layers can adhere, making sol-gels an ideal replacement for chromium-based conversion coatings[iii],

This paper suggests an innovative characterization approach of silica-based conversion coatings on an Al substrate by, on the one hand, salt spray tests and, on the other hand, time-of-flight secondary ion mass spectrometry (ToF-SIMS) performed in depth profile mode by a gas cluster ion beam source (GCIB). For this study, we considered 3 Al substrates which were coated by films with a composition corresponding to various commercial formulations. Whereas the salt spray tests determine differences in anticorrosion properties between the 3 samples, ToF-SIMS provides molecular information throughout the entire coating and even down to the beginning of the Al substrate. Therefore, both the inner composition of the conversion coating and the interface bond of this one to the Al substrate could be studied. The ToF-SIMS measurements revealed Si₂O₂⁺ and SiOAl⁺ fragments which both contribute to understanding the differences in the anticorrosion performances. Si₂O₂⁺ relates to the crosslinking of the coating and subsequently takes part in the barrier properties. SiOAl⁺ originates from the interface bond and is a clue for the “conversion coating” of the deposited layer. Differences in depth profile of Si₂O₂⁺ and SiOAl⁺ between the 3 samples are interrelated to the respective anti-corrosion performance of these ones.

[i] De Graeve, I., Vereecken, J., Franquet, A., Van Schaftinghen, T., & Terryn, H. (2007). Silane coating of metal substrates: Complementary use of electrochemical, optical and thermal analysis for the evaluation of film properties. Progress in Organic Coatings, 59(3), 224-229.

[ii] Wang, D.; Bierwagen, G. P., Sol–gel coatings on metals for corrosion protection, Progress in Organic Coatings, Volume 64, Issue 4,2009, Pages 327-338.

[iii] WO2010/095146A1: Anti-corrosion sol-gel hybrid coating on zinc and zinc alloy steel sheets and preparing method thereof.

The increasingly complex architectures and diversity of materials used in modern semiconductor devices makes their characterization challenging. TOF-SIMS analysis is particularly well suited to dopant and multilayer analysis in such devices, but standard approaches such as surface imaging or dual-beam depth profiling can be limited when faced with deeply buried interfaces and very heterogeneous samples. This is the case for through-silicon vias and copper pillars. These objects are typically several tens of microns in dimension and are used in 3D integration approaches to connect chips together. To address this type of sample, the capabilities of a TOF-SIMS instrument can be extended by the use of an in situ focused ion beam (FIB) gun. This is often a gallium source FIB, but other sources can be used such as Xe plasma sources to obtain higher etch rates for larger samples. In fact, even in the absence of a dedicated FIB column, the LMIG source (Ga, Bi, Au etc) that is normally used for spectroscopy on the instrument can also be used without pulsing to produce a continuous high current beam for FIB milling. The drawback of this method is that the sample must be rotated between milling and imaging steps and the LMIG parameters will require changing. There may also be milling artefacts arising from the presence of a mix of monoatomic and cluster ions when a bismuth or gold source is used.

The use of a dedicated FIB column opens up the possibility of performing FIB-TOF-SIMS tomography experiments and limits sample drift and re-positioning errors as the sample may be kept in the same position. However, FIB-TOF-SIMS tomography experiments can be time-consuming (overnight analysis to several days) and the investigated volume is limited by the sputter rates obtainable. To overcome these limitations, a prescreening with a non-destructive X-ray imaging technique can help to identify positions of interest for FIB-TOF-SIMS tomography to be performed on. This can be TSVs containing filling defects (voids) that can then be investigated at high resolution and with compositional information by FIB-TOF-SIMS. Another important question is whether copper diffused out from the TSV, through the diffusion barrier (often a thin layer of metal nitride) to the surrounding silicon. The presence of copper is deleterious for electronic properties and once present in silicon can diffuse relatively fast to active areas of the device. The engineering of barrier layers is routinely performed by depth profiling on full wafer samples to maximize layer quality and presence of certain defects may vary between full sheet deposition vs conformal filling of high aspect ratio holes. The use of an in situ FIB allows both tomography experiments and depth profiles to be performed on the side of a FIB-cut parallel to the object of interest.

Part of this work, carried out on the Platform for Nanocharacterisation (PFNC), was supported by the “Recherches Technologiques de Base” of the French National Research Agency (ANR).