SIMS2015 Session CT2-TuA: Complementary Techniques and Multi-Technique Approaches

SIMS provides extraordinary chemical sensitivity and high dynamic range, but, offers limited lateral resolution. On the other hand, Transmission Electron Microscopy (TEM) offers superior spatial resolution, but, the traditional analytical capabilities associated with electron microscopy such as Energy Dispersive Spectroscopy (EDS) or Electron Energy-Loss Spectroscopy (EELS) are inadequate for characterizing samples containing trace elements (at best 0.1 at. %) or for mapping isotopic distributions. Another limitation is that investigations of light elements (such as hydrogen and lithium) are particularly difficult or even impossible using these analytical methods. To tackle modern problems in physical and biological sciences, the capability to simultaneously obtain high spatial resolution and high chemical sensitivity is of increasing importance. An ex-situ combination of TEM and SIMS in an attempt to overcome the limitations of the individual techniques is prone to sample modifications and other artefacts. To overcome the intrinsic instrumental limitations, we have made an in-situ combination to complement the high-sensitivity of SIMS with the exceptional spatial resolution offered by TEM, by developing the correlative TEM-SIMS technique.

To demonstrate the applications of the TEM-SIMS technique, we have developed a prototype instrument for TEM-SIMS based correlative microscopy (Fig. 1). A commercial FEI Magnum Ga⁺ FIB was attached to a modified FEI Tecnai F20 TEM column to act as the primary ion column. The secondary ion extraction optics (extraction efficiency 90%) and a compact high-performance magnetic-sector mass spectrometer were designed and developed in-house. A special sample holder which can be biased to high-voltages (up to 5 kV) was also developed.

To enhance the low intrinsic yield of secondary ions for non-reactive primary ion beams such as Ga⁺ we use reactive gas flooding. Specifically, the enhancement of negative secondary ion yields with Cs flooding was found to be up to four orders-of-magnitude. This enhancement of secondary ion yields leads to detection limits varying from 10^-3 to 10^-6 for a SIMS lateral resolution between 10 nm and 100 nm, respectively (Fig. 2). Sensitivities in the ppm range are possible, but at the cost of spatial resolution due to the inherent physical limit of SIMS. Nevertheless, it is possible to recover the structural details that were thus lost by overlaying high-resolution TEM image over the high-sensitivity (but, poorer resolution) SIMS image.

The challenges and opportunities of the TEM-SIMS correlative microscopy method will be highlighted with a focus on applications in materials science and biology.

For over 60 years XPS has been at the forefront of surface analysis due to its ability to yield quantitative composition and chemical state information on a wide range of materials from the top 10 nm of the sample surface. XP imaging has also allowed the lateral distribution of species to be determined and with the introduction of ion sources, XPS depth profiling can now probe deep into the bulk of a sample. Until the recent development of gas cluster ion sources depth profile studies have been mostly limited to monatomic noble gasses (Ar⁺, Ne⁺) as the primary ion for sputter erosion. While this technique has been used extensively across a broad range of applications, profiling delicate materials has remained out of reach due to the damage caused by impinging ions on the structure of the analysis material. Carbon based materials. E.g. polymers, have been shown to be particularly susceptible to damage with graphitisation occurring even with low energy ions (<250 Ev). Ion induced damage is not restricted to delicate materials but also occurs with many inorganics with elements preferentially sputtered from a surface resulting in changes in composition and oxidation states. More recently massive Argon gas cluster ion sources have been employed by several manufacturers to further reduce chemical damage and extend the range of materials amenable to this type of analysis. Here we show that with Ar_250-2000⁺, where the energy per atom can be 2.5-40 Ev, it is possible to significantly reduce bulk damage and preferential sputtering. We will discuss the use of clusters for depth profiling a range of materials including polymers and binary and tertiary inorganics. We will demonstrate how the analyst can now depth profile through complex multi-layer samples and gain accurate quantification of the species present with confidence

There has been a revival in interest of UPS in recent times due to the desire to understand the nature of novel electronic materials. UPS yields useful information regarding monolayer adsorption and electronic work function and band structure of a material however it is very sensitive to surface contamination. Conventional ion sources are commonly used to sputter clean however the use of high-energy ions can significantly change the properties of the surface. Cluster ions will be discussed as a simple method to remove surface contaminates exposing the pristine surface below.

A detailed understanding of the interface dipole formation between metal electrode and thin organic film and at organic/organic heterojunctions plays a key role for the fabrication of organic electronic devices. Energy level alignment across organic interfaces related to e.g. charge carrier injection, is of utmost importance for organic light-emitting diodes (OLEDs), photovoltaics (OPVs) or chemical sensors¹. The interface energetics determines device performance such as driving voltage and stability of electron and hole injection in OLED², or open circuit voltages in OPV devices³. Most of these devices are thin-film based systems, thus interfaces are present within a few nanometers of any active layers and where overall device properties, integrity and stability are determined. Contacts to molecular and polymer films are complex and often difficult to optimize, thus the self-assembled monolayers (SAMs) are often placed at the interface.

The evaluation of electronic density of states enables to better understand observed physical properties of materials and interfaces. In order to unravel electronic structure the UPS- (Ultraviolet Photoelectron Spectroscopy) and XPS- (X-ray Photoelectron Spectroscopy) based valance band measurements can be applied. Combining above methods with gas cluster ion beam (GCIB) sputtering allows to investigate chemical composition and/or electronic properties of the materials from the surface to hundreds of nanometers or more into the bulk. However, each material requires a separate approach to define the advantages and limitations of the GCIB assisted depth profiling.

Here we describe XPS and UPS depth profiling using GCIB (Ar₂₅₀₀⁺) on multilayer polymer thin film structures. First, to study SAM influence on the electrode surface, we used model systems which were thin films of PEDOT:PSS modified with various organosilane SAMs. In the next step, PEDOT:PSS layer functionalized with SAMs was covered with thin film of P3HT:PCBM blend. These can be recognized as a part of organic solar cell devices. Obtained results showed that combination of XPS or UPS together with GCIB depth profiling can be use to verify the presence and chemical structure of SAM placed at buried organic/organic interfaces and give information about e.g. work function changes through the considered multilayer system.

This work was partially supported by the Polish National Science Centre project no.2013/09/N/ST5/00874.

References

1. Oehzelt M, et al. Nature Communications 2014, 5.

2. Braun S, et al. Advanced Materials 2009, 21: 1450-1472.

3 . Goh C, et al. Journal of Applied Physics 2007, 101.

X-ray photoelectron spectroscopy is a powerful technique widely used in the field of organic electronics. It is intrinsically a surface-sensitive method due to the photoelectron mean free path of around 5 nm in typical experimental conditions. However, information on chemical composition of deeper sample layers can be accessed by sputtering away layers of material with an ion beam. Unfortunately, in case of organic materials, mono-atomic ions typically used in such process penetrate deep below the surface. This results in cascades of displaced atoms and considerable damage that is thus introduced into the sample, which affects the collected XPS spectra and may lead to erroneous interpretation of experiments.

In our studies instead of using mono-atomic ions we have utilized massive ionized Argon gas clusters (approx. 2500 atoms per cluster, GCIB). Both computer simulations and experiments show [1-4] that bulky projectiles of this type deposit their energy very close to the surface of an organic material and hence do not damage deeper regions of the sample. Thus the chemical composition of the material under study can be iteratively determined from a series of XPS spectra.

In this presentation I will show our latest studies of materials used for fabrication of organic photodetector devices, namely thin films of PCBM, PBDTTT-C and their blends deposited on glass/ITO; part of the samples were covered with PEDOT:PSS. I will demonstrate how chemical composition of such films, sandwich structures and interfaces can be precisely determined by means of XPS coupled with GCIB. Furthermore, performance of organic electronics deteriorates with age. This is due to chemical changes in organic materials resulting from extended exposure to light or high temperature. From the experimental point of view, these modifications lead to peak shifts, peak broadening and/or the occurrence of new chemical states in XPS spectra. I will show how such alterations can be pin-pointed in samples subjected to aging.

This work was supported by the Polish National Science Centre, project no. 2013/09/B/ST4/02951.

[1] Z. Postawa et. Al, Anal. Chem. 75 (2003) 4402; Surf. Interface Anal. 43 (2011) 12

[2] M. Tanaka et al., Rapid Commun. Mass Spectrom. 2010; 24: 1405–1410

[3] N. Toyoda et al., Nucl. Instr. And Met. In Phys. Res. B 273 (2012) 11–14

The preparation and analysis of polymeric materials relevant to tissue engineering, especially concerning controlled drug delivery and wound healing, have become important issues with synthetic and analytical significance. Recent efforts have focused on developing hydrogels with properties appropriate for controlling drug acceptance, sequestration, and release. Modifying the surface properties of the hydrogel has been identified as a promising method for accomplishing more efficient drug delivery. In this work, methyacrylic terminated poly(ε-caprolactone)-poly(perfluoropolyether)-poly(ε-caprolactone) (PCL-PFPE-PCL) telechelic triblock co-polymers are synthesized to blend and react with poly(2-hydroxyethyl methacrylate) (HEMA). The blends are photo-polymerized in different mixture ratios followed by cleaning, after which the properties of the materials are characterized by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). Surface segregation of the PFPE was observed at elevated levels with respect to bulk composition in all samples containing 1%-5% of the triblock copolymer in the HEMA blend. Environment-induced surficial reorganization and the potential as a self-healing material is also explored with TOF-SIM depth profile experiments.

​

Validation of a complementary ToF-SIMS and UHPLC-ESI-MS/MS method to study the release of polymer additives from pharmaceutical packaging

C. Pouech, C. Bordes, Y. Clement, P. Lanteri, E. Vuillet, and D. Leonard

Université de Lyon - Institut des Sciences Analytiques, UMR 5280 CNRS, Université Lyon1, ENS-Lyon - 5 rue de la Doua, 69100 Villeurbanne, France.

Interest of using polymers for packaging for pharmaceutical products has been increasing as they offer many specific properties (being lightweight, recyclable and processable materials). However, the pharmaceutical container should not be itself a source of contamination. It is then critical to study the chemical entities that could be extracted from components of a container (extractables) including chemical entities that could migrate into a drug product over the course of its shelf-life (leachables).

In a previous effort to develop a reliable method with high degree of selectivity, sensitivity, and specificity, an original approach was conceived by taking advantage of combining Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) with Ultra-High Performance Liquid Chromatography Tandem Mass Spectrometry (UHPLC-MS/MS) [1]. This method is the first one to take into account a very high number of possible detected molecules, i.e. the total number of authorized polymer additives [2] but also some notably used other additives as well as some of the possible degradation products, corresponding to a set of 25 molecules.

When considering the ToF-SIMS approach, molecules reference spectra and polymer packaging spectra were treated using statistical approaches to possibly define two levels of identification (additive considered as intrinsic to the polymer vs additive considered as a contamination possibly related to the fabrication process). UHPLC-MS/MS data were obtained from water solutions (at various Ph and temperature conditions) in contact with the packaging. Quantification was obtained when the concentration was superior to the method quantification limits of each substance.

To further substantiate the validation of this original method, significant data were acquired on six different polymers used for pharmaceutical packaging applications, two being more specifically intended to be top-layers of flexible pouches. This set of data made possible to discuss potential artifacts of the method but more interestingly to validate the two levels of identification listed above in the context of the release issue.

[1] C. Pouech, C. Bordes, P. Lanteri, C. Cren, and D. Leonard, SIMS XIX conference talk

[2] European Pharmacopoeia 7.8, 7th Edition 2013 (7.8)

High voltage I/O devices traditionally use thermally grown silicon dioxide as dielectric layer in MOSFET structures. While this generates a reliable thick gate oxide, it substantially consumes silicon. In FINFET devices the narrow dimensions of the FIN do not allow significant silicon loss. To address the problem a low temperature ALD process was developed that includes plasma nitridation and rapid thermal anneal steps to create an oxide with equivalent properties when compared to thermal oxide. This paper addresses how to accurately quantify and monitor nitrogen within an oxide surrounding FINFET structures as well as in planar films using a multidisciplinary approach of SIMS, HRBS, TEM and XPS. Precise as well as accurate measurements of nitrogen within the oxide surrounding the FIN are critical to understand changes in voltage breakdown that were observed during different wafer processing steps. HRBS was used to accurately quantify Nitrogen in blanket SiON films. Area corrected SIMS profiles show good agreement between Nitrogen analysis in blanket SiON films and SiON surrounding FINFET structures. TEM analysis yields lateral Nitrogen distribution not accessible with the other techniques.

In the XPS method, the analyzed materials are exposed to an X-ray beam, which depending on the structure of the studied polymer, induces its cross-linking or degradation. It has been shown that damage caused by X-rays leads to additional morphological features in craters sputtered by Ar-GCIB. The origin of these features can be traced to different sputter rates between damaged and non-damaged areas [1].

In this work we examined the influence of the X-ray beam on polymer damage induced by Ar-GCIB. The studies were performed on thin films (ca. 100 nm) of conventional (e.g. polystyrene, poly(methyl methacrylate)) and conjugated (e.g. poly(3-alkyl thiophenes)) polymers. Stability of the polymers was investigated by XPS analyzes. We were able not only to determine the change in sputter rate due to X-ray damage, but also the degree of chemical modification of the polymer material. Experiments were performed for varying x-ray doses. We also examined the effect of sample temperature with respect to glass transition temperature on the stability of selected polymers.

This work was supported by the Polish National Science Centre, project no. 2013/09/B/ST4/02951.

[1] P. J. Cumpson J. F. Portoles, N. Sano, A. J. Barlow, J. Vac. Sci. Technol. B 31(2013) 021208

It is well known in literature that silicone (polydimethylsiloxane) breast implants surface features such as texture, roughness and chemical composition play key roles in cell adhesion and grown. In fact, the utility of mammary prosthesis texturing in the prevention of capsular contracture was equally well established. As a result of γ radiation, silicone undergoes a series of changes due to radiation cross-linking. We decided to study the textured surface of silicone prostheses irradiated with a radiotherapy dose of 50 Gy. The effect of gamma radiation on chemical, physical and mechanical properties of silicone breast implant was studied. The local surface roughness, as determined by high-resolution profilometry measurements is in the range of 20-30 nm. Untreated and irradiated PDMS prostheses were investigated by time-of-flight static secondary ion mass spectrometry (ToF-SIMS) technique and multivariate analysis revealing a polymer main chain scission and a drop in the abundance of high molecular weight fragments.