SIMS2015 Session DG1-ThM: Depth Profiling/General

Permethrin is used worldwide as a mosquito insecticide for netting and fabric. Permethrin is a contact insecticide so only the permethrin on the surface can directly impact the mosquito. Therefore knowledge of the surface concentration is essential to measure the effectiveness of treated material. TOF-SIMS analysis of the insecticide showed Cl^- as the predominant species in the negative ion mass spectrum. Ion implantation was then used to place a known amount of chlorine into netting, fabric, and silicon samples. The netting material for this study is composed of high density polyethylene (HDPE) and pieces from two thicknesses of sheet HDPE were also implanted. Depth profile analysis of the implanted samples showed distinct chlorine implant profiles, with the silicon used as a standard. Quantification and detection limit for chlorine have been obtained for HDPE and fabric. The chlorine detection limit in HDPE is approximately 2x10¹⁸ atoms/cm³ and the chlorine concentration in netting fibers approximately 1x10²⁰ atoms/cm³. The results make possible the study of insecticide content in netting and fabric. Investigation is in progress for the effect of washing of mosquito nets on chlorine surface concentration.

The safety assessment of spent nuclear fuel final repository is aiming to understand to which extent the ceramic UO₂ matrix will be able to contain the highly radiotoxic minor actinides and fission products over hundreds of thousands of years. The stability of the ceramic matrix will be strongly undermined by the groundwater contacting the fuel itself. The presence of water and the intense alpha radiation emission, will produce oxidizing species allowing U(IV) going to U(VI) and will boost the matrix dissolution. Moreover the physical diffusion of water and oxygen in the fuel matrix will increase the so-called wet surface allowing the dissolution to proceed even faster.

Water and oxygen diffusion in UO₂ have been already studied by our research using ¹⁸O as a tracer and measuring its concentration with SIMS depth profiles after room temperature water contact experiments. The results obtained are confirming that while diffusing, oxygen is also oxidizing the matrix and the obtained effective diffusion coefficients are the average of the real diffusion coefficients for each layer of UO_2+x crossed.

To reduce the uncertainty on the measured parameters, along with the oxygen-18 concentration profile, also the local oxidation level should be measured possibly using the same analytical technique. To evaluate the possibility to measure level of oxidation with SIMS depth profile analysis, specific reference samples with layers characterized by different oxidation state have been prepared and measured both with SIMS and XPS.

The sample preparation procedures and the preliminary results are presented.

The advantages of Magnetic Sector SIMS (Secondary Ion Mass Spectrometry) are well established: extreme sensitivity, high depth and lateral resolution together with high throughput. With magnetic sector SIMS, it is possible to detect very low concentrations (down to ppb) of dopants and impurities in solid samples and to measure the in-depth distribution of trace elements over a large depth range from a few Angstroms to tens of microns. SIMS imaging capabilities can also be used to investigate local non-uniformity of trace elements at sub-micrometer scale.

CAMECA has developed different high-end Magnetic Sector SIMS instruments in order to achieve the highest specific performance for different applications. The IMS 7f-Auto is the most versatile magnetic sector SIMS offering 2D and 3D imaging with sub-µm resolution, high sensitivity depth profiling including atmospheric elements with benchmark throughput, high depth resolution and automation.

Excellent detection limits on light elements measurements are achieved thanks to:

• Continuous ion beam sputtering and magnetic sector mass spectrometer design providing extreme sensitivity;
• UHV analysis chamber with optimized vacuum conditions, minimizing the background level created by residual gases. The chamber is also equipped with a liquid nitrogen trap to lower the partial pressure of water vapor;
• Fully automated six-holder storage chamber with sample outgassing capabilities. Compared to the conventional two holder airlock, the storage chamber provides a much higher throughput, as multiple samples can be pumped overnight in order to achieve optimized detection limits;
• High density Cs primary ion beam allowing high sputtering rates that significantly improve the detection limits.

We will show different applications covered by CAMECA IMS 7f-Auto, including bulk materials, multi-layered structures, thin-film technology as well as different types of materials.

During the last few years a revolution has occurred in the SIMS community with the introduction of sputtering by large argon clusters. With these ion beams, retrieving molecular information while profiling has become possible. It quickly emerged however that these beams were not the most suitable for inorganic materials profiling and, as a consequence, only limited studies have been performed for these applications. Next to organic materials, it is nonetheless crucial to investigate the sputtering of inorganic materials by these large argon clusters as this can not be avoided in many practical applications. This is, among others, the case for heterogeneous organic/inorganic systems or for organic photovoltaic materials where organic/inorganic interfaces are present at both side of the active layers.

In this work, we concentrate purely on the investigation and understanding of the inorganic materials profiling. For that purpose, we investigated a simple system consisting of aluminum delta layer(s) buried in a silicon matrix. This allows to define the most favorable beam conditions for practical analysis. We show that, unlike monoatomic ion sputtering, the information depth obtained with large cluster ions is very large (up to 10 nm), which can be attributed to both a large roughness development at early stage of the sputtering process and a large mixing zone. These effects have been investigated using the MRI model in order to disentangle the two contributions. Using this model, the experimental profiles could reasonably be reproduced for a large range of sputter condition. Further, we also show that for some conditions (mostly low Ev/at) ripple formation is observed. As a consequence, at low Ev/at conditions, sample rotation during sputtering is a must. It allows to significantly improve the depth resolution while sample temperature has no effect on the depth resolution. We also show that as a consequence of the roughness development and the large mixing zone, using the maximum of the Al intensity as a depth marker leads to large depth calibration error.

The best depth resolution on this system is obtained for a beam of 2.5 keV, 5000 at/cluster using sample rotation, with a decay length of ~1nm. However, the pratical use of this beam condition in the dual beam Bi/Ar-cluster experiment is limited at present due to the low sputer rate achieved.

In the 80's and early 90's, ToF-SIMS using a pulsed analysis beam with a low duty cycle was considered to be the ideal instrument for static SIMS applications with high sensitivity and high mass resolution. The dynamic SIMS mode was the domain of quadrupole or magnetic sector mass analyzers using DC analysis beams. This changed in 1993 with the introduction of the dual beam depth profiling mode for ToF-SIMS [1]. The pulsed high energy analysis beam is combined with a low energy sputter beam that is switched on while the secondary ions travel through the ToF analyzer (interlaced mode). Analysis and sputter beam parameters can be optimized independently and therefore, the dual beam mode offers very high flexibility. In the following years the technique was optimized for shallow depth profiling with high depth resolution using low energy oxygen or cesium sputter beams. Today, this dynamic SIMS mode is widely applied in most modern ToF-SIMS instruments for the characterization of thin inorganic films with thicknesses from nm to several microns. With the advent of large argon gas cluster beams, organic materials can be sputtered without the accumulation of radiation damage [2]. The same dual beam approach became very popular for the depth profiling of organic materials with very high depth resolution [3] and for the three-dimensional analysis of organic devices with high lateral and high depth resolution [4]. Hybrid sample systems with inorganic and organic layers remain challenging.

In this paper we will discuss recent developments in ToF-SIMS depth profiling of complex sample systems using the high flexibility of the dual beam approach for improvements in sensitivity, depth resolution and quantification. We will also present the latest developments for the measurement of topography, sputter rates and surface roughening using the in-situ AFM technique.

[1] H.-G. Cramer, U. Jürgens, E. Niehuis, M. Terhorst, Z. Zhang, A. Benninghoven, in SIMS IX,

Eds. A. Benninghoven et al., Wiley, Chichester, New York, p. 449 (1994).

[2] S. Ninomiya, K. Ichiki, H. Yamada, Y. Nakata, T. Seki, T. Aoki and J. Matsuo,

Rapid Commun. Mass Spectrom. 23; 1601-1606 (2009)

[3] D. Rading, R. Moellers, H.-G. Cramer and E. Niehuis, Surf. Interface Anal. 45 (2013) 171

[4] E. Niehuis, R. Moellers, D. Rading, H.-G. Cramer, R. Kersting, Surf. Interface Anal. 45 (2013) 158

Low energy Cesium ions have been used for decades to depth profile inorganic materials like semiconductors and metals, with a high sensitivity and a high depth resolution. More recently, we have shown that organic materials, including polymers, sugars or amino acids, were amenable to molecular depth profiling with low energy (~500 eV) Cs⁺, mostly due to the implanted Cs atoms reactivity. Cs⁺ thus appears as a versatile beam for depth profiling both organics and inorganics, which could be a strong asset when it comes to analyze hybrid materials. On the other hand, Gas Cluster Ion Beams (GCIBs) have emerged as the “gold standard” for organic depth profiling, thanks to their huge sputtering yield combined with low surface damage. However, inorganic sputtering is very limited with GCIB so that depth profiling of hybrid materials remains virtually impossible.

This paper will present preliminary data obtained with low energy Cs⁺ ions on multilayers containing organic layers and metallic layers. The organic layers (amino acids, cholesterol, polymers) were evaporated in vacuum or spin coated, while the metallic layers (Au) were deposited by sputtering. The layer thicknesses typically ranged from 40 to 200 nm. As anticipated, both the inorganic and organic layers were easily sputtered by the Cs⁺ beam. The molecular signals from organics were detected beyond the metallic layers with intensities comparable to single organic layers. Likewise, metallic layers were easily sputtered beyond the organic layers, down to the Si substrate. This was also demonstrated on organic/inorganic/organic systems, proving the feasibility of the method for hybrid devices. The interface sharpness was in general excellent while going from the organic compounds to the metal, whereas the metal/organic interfaces were broader. This could be due to the large differences in sputtering yields occurring at the interfaces. However, issues with the sample preparation, such as metal diffusion into the cholesterol or amino acid layers were also revealed. The quality of the interfaces will be discussed with a multi-scale imaging approach (AFM, SEM, optical microscopy, ToF-SIMS imaging).

The detection and characterization of nanoparticles in complex matrices is one of the hot topics in the field of nanotechnology. This is because the behavior of nanoparticles present in matrices used in food, cosmetics and their effects on health and environment are far to be understood and represent a great concern.

The availability of the high energy cluster sources open new possibilities in the use of surface analysis techniques such XPS and ToF-SIMS to characterize organic films which are important in different application fields ranging from energy to food and medicine (1, 2). In particular the use of molecular depth profiling allows the investigation of the distribution of molecules within the different layers of organic films and nanostructured materials (3).

Plasma polymerization (PP) is a well establish method to deposit well controlled and pin-hole free thin films on different substrates and it is applied in many industrial and research activities (4, 5). This makes the PP technique quite attractive for the production of films that can be used as model to investigate complex matrices. However, because of the energy involved during the polymerization process the interfacial chemistry may undergo to different phenomena such us species interdiffusion, chemical reactions and contamination. For this a careful characterization of film interfaces is needed.

In this work we have used XPS and ToF-SIMS to investigate multilayers plasma polymerized polyacrylic acid (ppAA) and Teflon-like (PTFE) films as possible model system for the chemical analysis of complex matrices. The depth profiles have been obtained by means of Ar_n⁺ (n=500-5000) and Bi_n⁺ (n=1-7) polyatomic sources, whilst XPS was used to study the surface and interface composition

Our results indicate that the sputtering Yield is proportional to the Ar cluster size and the depth resolution is dependent both from the Ar cluster size and the fragment considered. Moreover, the ppAA profile can be fitted with an erosion model (7) and the In situ XPS analysis reveals some intermixing at the polymer/Si interface and damage to the ppAA and PTFE chemical structure during the profile process.

1) Mahoney C, Mass Spectrom Rev., 2010, 29(2):247

2) Fletcher J.S. Vickerman J.C., Anal. Chem., 2013, 85 (2), 610

3) A. Shard, I. Gilmore, A. Wucher, In, ToF-SIMS: Materials Analysis by Mass Spectrometry Di John C. Vickerman and David Briggs (Eds), 2013, p311

4) K. S. Siow et al., Plasma Proc. Polym., 2006, 3, 392

5) D. B. Haddow at al., Plasma Proc. Polym., 2006, 3, 419

6) J. Brison et al, J. Phys. Chem. C, 2010, 114, 5565

7) A. Wucher, SIA, 40, 1545, (2008)

To properly reconstruct 3D ToF-SIMS data from systems such as multi-component polymers, drug delivery scaffolds, cells, and tissues, it is important to understand the sputtering behavior of the sample and its components. Modern cluster sources enable efficient and stable sputtering of many organics materials. However, not all materials sputter at the same rate and few studies have explored how different sputter rates may distort reconstructed depth profiles of multi-component materials.

In this study we used spun-cast bilayer polymer films of polystyrene and PMMA on a silicon substrate as a model system exhibiting different sputter rates between components. Depth profiling was performed using an Ar1000+ sputter source and Bi3+ analysis beam on an ION-TOF V ToF-SIMS instrument. PMMA sputtered at a significantly higher rate than polystyrene, whilst sputtering of silicon can be considered negligible.

We aimed to optimize methods to reconstruct depth profiles of the bilayer model system. Transforming the z-axis from sputter time to depth using a single sputter rate fails to account for sputter rate variations during the profile. This leads to inaccurate apparent layer thicknesses and interfacial positions, as well as the appearance of continued sputtering into the substrate. Applying measured single component sputter rates with a step change at the interfaces yields more accurate film thickness and interface positions. This transformation can be further improved by applying a linear sputter rate transition across the interface, thus more closely reflecting the expected sputtering behavior seen in blended films.

Reconstructing a depth profile that is accurate and representative requires confidence in the parameters used to transform the data set, in this case the component sputter rates. We examined how errors in depth profile reconstruction and interpretation may arise from inaccuracies in sputter rate determination, highlighting specific cases of erroneous reconstructions.

This study demonstrates that, if accurate 3D reconstructions of complex multi-component organic and biological samples are to be achieved, both accurate evaluation of component sputter rates and careful conversion of sputter time to depth are required.