Standard SIMS measurements of 2D materials may provide vital but ambiguous results - while the excellent sensitivity of the technique may enable the detection of various contaminants, the depth resolution may not be sufficient to localize them. However, in this case, it is not enough to just improve the resolution. Given that many 2D materials are or consist of atomically thin layers it is essential to reach the ultimate, i.e. atomic depth resolution. As a consequence, factors that limit the resolution like the mixing effect could not be only minimized but completely eliminated. At the same time, the established procedures should maintain sufficient secondary ion yield to ensure a reasonable signal-to-noise ratio. To achieve such accuracy it is essential to establish dedicated measurement procedures which are tailored for specific samples. Even though such procedures are time-consuming to set up and execute, the quality of the results justifies all difficulties:

• It is possible to not only detect but also precise localize various contaminants in graphene;
• SIMS instrument can be used to both, implant molybdenum sulfide with oxygen and then observe the temperature-induced stabilization of the MoS₂/MoO₃ heterostructure;
• Depth profiling of MXenes has revealed that these advanced materials are actually transition metal oxycarbides and not carbides as commonly assumed.

Since their invention in 1987 by Tang and Van Slyke, understanding the degradation mechanisms after environmental and/or electrical aging is essential to improve the performances of organic light emitting diodes (OLEDs). However, the study of thin aged organic multilayers (~100 nm thick) requires characterization techniques capable to probe small chemical changes with a high depth-resolution. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) are suitable surface-sensitive techniques providing complementary information: access to molecular information and the elemental chemical environment respectively. Nowadays, ToF-SIMS and XPS depth profiling are routinely used to investigate aged devices. However, the analysis and sputter beams used in these techniques may induce degradations on organic layers [1] and cause bond scission [2] that accumulate in burried layers. We developed a correlative depth-resolved protocol that aims to minimise damage related to analysis beams by analysing the surface of a bevel crater. This shallow angle bevel crater is formed using a continuous, linear variation of ion fluence across a rastered area of 800 × 800 µm². Typically, argon clusters of around 7500 atoms are used at an accelerating voltage of 5 kV. A comparison between an electrically-aged and fresh OLEDs (structure for green light emission) has been performed using this protocol. Electrical ageing consisted of maintaining a stable courant of 4 mA for 430h in order to produce sufficient degradation that would be visible by TOF-SIMS and XPS techniques. The greater degree of fragmentation of the characteristic molecular ions originating from the emissive layers as well as the electron transport and hole blocking layers is observed in ToF-SIMS spectra. XPS measurements confirm that the chemical environment of the elements present in those layers appears to change after ageing, in particular in the hole blocking layer.

1. Morgan, D.J., XPS insights: Sample degradation in X-ray photoelectron spectroscopy. Surface and Interface Analysis, 2023: p. 1-5.

2. Postawa, Z., Sputtering simulations of organic overlayers on metal substrates by monoatomic and clusters projectiles. Applied Surface Science, 2004. 231-232: p. 22-28.

Contamination detection and control play a critical role in achieving the semiconductor industry’s roadmap of zero failures. Compared to metal and particle contamination, knowledge of organic contamination and its detrimental effects is still limited. As devices are continuously scaled down, organic contamination is increasingly becoming a major yield-affecting factor. Previous studies of organic contamination on silicon oxide surfaces have focused on total carbon contamination and its effects on gate-oxide integrity. During wafer fabrication process, there are many possibilities for organic contamination to occur. Wet cleaning-induced organic contamination is one of them.

In recent years the focus of contamination reduction has changed from frontend of line to the backend (assembly). Here especially the packaging of a semiconductor device including the wire bonding process has become more important. During the assembly of a finished device the process where the wire is attached to the bond pad is critical as a good contact of the wire (connection to the outside world) is crucial for reliable functioning of the finished product.

This presentation will focus on examples of organic contamination in the backend process. The first example will show how low IMC coverage (inter metallic compound) formation (<60%) was observed during wire bonding. The hypothesis that the low IMC is caused by bond pad surface contamination is discussed. It will be demonstrated that ToF-SIMS only (compared to AES, XPS, SEM) was feasible to analyze some stain-like carbenoid contamination on the suspected bond pads. The results show that bond pads with low IMC exhibit organic stains and increased surface roughness. The results indicate that the stains are back grind tape residues, which adhere preferentially due to the higher bond pad surface roughness.

In two more examples, the use of ToF-SIMS surface analysis and - imaging analysis is demonstrated to find stains on bond pads and on passivation layers and how to match fingerprint signals helping to identify the source of the contamination.

From the analytical viewpoint, the detection of molecular secondary ions as well as structural fragments with high sensitivity has a big potential for the evaluation of low levels of organic contamination on the sample surface. The identification of molecules and the detection of surface functionalities are commonly applicable to wide variety of materials. These capabilities of ToF-SIMS offer the possibility to identify the root cause of any organic contamination. In the current era of zero failure tolerance, ToF-SIMS can identify the source of organic contamination and therefore prevent reoccurrence which is one important pillar in the today’s quality mind set.

Inkjet 3D printing offers opportunities for additive manufacturing of multi-material electronic devices, such as healthcare sensors, which rely on the interfaces of dissimilar materials to attain their function. We previously demonstrated successful inkjet manufacture of such interfaces and devices with low dimensional materials: graphene/hexagonal boron nitride field effect transistors [1], perovskite (CsPbX₃) nanocrystals / graphene photodetectors [2], and graphene / PEDOT:PSS and silver nanoparticle / PEDOT:PSS heterostructures [3]. However, the performance of these devices strongly relies on the quality of the material interfaces, which are yet to be fully explored and understood; there are few tools and techniques in place to explore these interfaces’ quality, chemistry, and interpenetration, which all define the function on the devices. We explore the interfaces of inkjet-printed heterostructures based on combinations of 2D (graphene, hBN) and 0D (perovskite nanocrystals and metal nanoparticles) materials, and conductive (PEDOT:PSS) and dielectric (PVP, PEG) polymers using complementary Time-of-Flight Secondary Ion Mass Spectrometry and Focused Ion Beam-Scanning Electron Microscopy, revealing infiltration and intermixing between the layers. We examine the effects of the layer composition and deposition/post-deposition parameters on the quality of the interfaces, seeking to establish strategies for control of the interface properties. Polymeric materials, whether as capping agents or as ink formulation components, can strongly influence the interfaces. Our results provide an insight on the composition of inkjet deposited interfaces, which can inform future development of functional heterostructure devices,

References

[1] F. Wang, J.H. Gosling, G.F. Trindade, G.A. Rance, O. Makarovsky, N.D. Cottam, Z. Kudrynskyi, A.G. Balanov, M.T. Greenaway, R.D. Wildman, Inter‐Flake Quantum Transport of Electrons and Holes in Inkjet‐Printed Graphene Devices, Adv. Funct. Mater. (2020) 2007478.

[2] J.S. Austin, N.D. Cottam, C. Zhang, F. Wang, J.H. Gosling, O. Nelson-Dummet, T.S.S. James, P.H. Beton, G.F. Trindade, Y. Zhou, Photosensitisation of inkjet printed graphene with stable all-inorganic perovskite nanocrystals, Nanoscale. 15 (2023) 2134–2142.

[3] G. Rivers, J.S. Austin, Y. He, A. Thompson, N. Gilani, N. Roberts, P. Zhao, C.J. Tuck, R.J.M. Hague, R.D. Wildman, L. Turyanska, Stable large area drop-on-demand deposition of a conductive polymer ink for 3D-printed electronics, enabled by bio-renewable co-solvents, Addit. Manuf. 66 (2023) 103452. https://doi.org/10.1016/J.ADDMA.2023.103452.