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Comparative study of ion milling techniques for the
preparation of ceramic fibers in transmission electron
microscopy

Clementine Fellah, Berangere Lesaint

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Clementine Fellah, Berangere Lesaint. Comparative study of ion milling techniques for the preparation of ceramic fibers in transmission electron microscopy. Micron, 2024, 180, 10.1016/j.micron.2024.103612. hal-04672856

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# Comparative study of ion milling techniques for the preparation of ceramic fibers in transmission electron microscopy

Clémentine FELLAH<sup>a</sup>, Bérangère LESAINT<sup>b</sup>

<sup>a</sup> Université Claude Bernard Lyon1, LGL-TPE, UMR 5276, ENS Lyon, CNRS, UJM, Villeurbanne, 69100, France.

<sup>b</sup> Université de Lyon, INSA-Lyon, UMR CNRS 5510 MATEIS, Villeurbanne Cedex, France

**Keywords:** TEM, fibers, ion milling

## Abstract

Sample preparation, essential to any analysis, remains an under-documented step. Preparation methods for transmission electron microscopy (TEM) are complex and lead to artifacts that need to be identified to avoid wrong conclusions about the sample's microstructure. Ion milling techniques are increasingly becoming the reference techniques to prepare thin foils. The possibilities of different ion beam processes for milling samples will be shown and compared, using ceramic carbon/carbide fibers, a material applied in many industrial applications, as a test specimen. This overview of ion milling preparation techniques will enable us to identify the advantages, disadvantages and parameters in order to best prepare these thin samples. It will be carried out for the community, and will also highlight a new preparation method for ceramic fibers to minimize the artefacts inherent in these techniques.

## Introduction

Materials characterization by transmission electron microscopy (TEM) requires the use of high-quality samples that are sufficiently thin to be transparent to electrons (less than 100 nm) (1). Sample preparation is therefore an essential step in the observation and analysis processes. There is little documentation in the literature highlighting the different preparation techniques, whose the operating procedure, difficulties and artifacts differ according to sample families (2–5).

Initially, specimens were prepared mainly by mechanical techniques such as tripod (6), concave "Dimpler" grinding (7) and ultramicrotomy (8,9). Their use allowed precise results but involved tedious work requiring a long learning process and some dexterity. These techniques also induced artefacts in specimens such as work hardening, stress, thermal effects and transport of matter.
33 Increasingly, these methods have been replaced by ion milling preparation methods (10–13), which 34 are more up-to-date and easier to implement despite the high cost (14–16). These techniques 35 involve a broad ion beam (BIB) or a focused ion beam (FIB) to mill the sample, by removing surface 36 atoms until the required thinning is achieved. In this work, we wanted to carry out a comparative 37 study of different preparation methods by ion milling to give us an overview of the advantages, 38 disadvantages and artifacts of each technique on a reference sample of ceramic fibers.

39 Ceramic fibers are widely used in composite materials for high-temperature applications in high-tech 40 sectors (aeronautics, nuclear) (17–21). One of the main objectives in considering their application is 41 to optimize their mechanical behavior and their coupling with the composite matrix (22–24). To meet 42 these expectations, information must be obtained on fiber structure and chemical composition by 43 transmission electron microscopy (TEM). However, fibers are complex materials to prepare due to 44 their size (10 µm in diameter), irritant properties and carbon-rich composition. This latter 45 characteristic requires special attention to minimize artifacts and carbon not from the sample 46 composition, such as pollution, organic contamination or from the environment around the samples. 47 It is also important to be able to prepare them in transverse and longitudinal directions to access 48 both the surface and the core.

49 In this study, silicon carbide (SiC) and carbon fibers were prepared by various conventional ion- 50 milling methods. These studied fibers are commercial Hi Nicalon S fibers from NGS and Tyranno SA3 51 from Ube for SiC fibers and T300 fibers from Toray for carbon fibers. We worked with similar 52 parameters for each techniques, for both SiC and carbon fibers. The main parameters of the studied 53 techniques to be adapted to these samples are explained. Artifacts are also investigated using a JEOL 54 FEG 2010F TEM at 200 kV, so that they can be differentiated from the structure of the specimens. 55 Subsequently, we have used combinations of these ion-milling techniques, which are not new but 56 not conventional on fibers. The aim of this study is to propose a new solution for preparing ceramic 57 fibers for transmission electron microscopy, and to mitigate as much as possible the disadvantages 58 and artifacts of their preparation.

## II Ion milling techniques: classic methods

60 The first classic tested technique was the focused ion beam (FIB/SEM). This microscope (NVision 40; 61 Carl Zeiss Microscopy for the study) is equipped with an electron column and a tilted ion column, for 62 milling, imaging and depositing material using generated ions or electrons (10). The sample is milled 63 by scanning the ion beam over a region of interest. The fibers have been cut in both longitudinal and 64 transverse directions. 65
Before beginning the FIB process (Figure 1), the fibers have been aligned correctly side by side. A platinum layer (2 x 20 µm²) was deposited with the *insitu* gas injection system to define the target area and protect the fiber surface from the ion beam. The protective layer was first formed by electron-beam induced deposition at 1.5 kV to prevent ion-beam induced deposition at 30 kV and 300 pA from resulting in ion penetration into the sample below. A coarse trench was milled around the interest region at 30 kV and 13 nA. After cleaning, this region was extracted from the sample using a micromanipulator and mounted on a copper grid. The sample was thinned by the ion beam with parameters ranging from 30 kV and 700 pA to 2 kV and 50 pA. By milling at lower energies, it is possible to obtain fine thin foils with even smaller damage layers.

<figure><img src="image_3.png" /><figcaption>Figure 1. Preparation stages of ceramic fibers FIB sample : a. Protective platinum layer, b. Ion beam milling around the region of interest, c. Needle extraction of the sample to be milled, d. Specimen mounting on copper grid, e. Ion milling, f. Finished thin foil. All the images were taken in high vacuum at 1.5 kV using the FIB's SESI detector (Secondary Electrons Secondary Ions).</figcaption></figure>

The advantage of the FIB is that it is a site-selective technique with nanometer-scale positioning accuracy. The deposit protects the sample surface and is placed across the fibers, i.e. both on top and between, which allows several fibers to be removed at once and in any direction. However, artefacts can alter the fibers structure, including the implantation of incident gallium ions, observable with high-resolution TEM (Figures 2a. and 2b.) (25). The amorphization of the sample surface and the redeposition of grid copper, sputtered by the ion beam, are also artifacts inherent to this preparation technique (26–28). Nevertheless, these can be minimized by cleaning the samples at low voltages and using a molybdenum rather than a copper support.
It is also possible to use FIBs with different ion sources to limit gallium-related artefacts. These new technologies are recent but are increasingly used for sample preparation. Plasma-FIB (Xe-FIB for example) workflows are similar to those applied for Ga-FIB in this study. However, the voltage and current parameters differ and it is more difficult to obtain an ultimate thin foil thickness that allows the sample to be transparent to electrons (29,30). FIB based on a gas field-ionization source allow finer films to be produced (31). However, this type of technology is rarer and less accessible to users.

<figure><img src="image_4.png" /><figcaption>Figure 2. Artifacts from FIB preparation of ceramic fiber: Ion implantation (a. and b.) The arrows show gallium (Ga) ions implanted over the entire surface of the thin foil, which has lost its protective layer due to excessive milling time. Figure 2b. shows a zoomed-in region of the surface of the fiber. The EDX spectrum of the boxed analysis region confirms the presence of gallium ions implanted in the sample.</figcaption></figure>

The second conventional tested method is Broad Ion Beam milling (BIB) with a Precision Ion Polishing System (PIPS, Gatan) (32–34). The system consists of two accelerated argon ion beams generated by two guns in vacuum ($5 \cdot 10^{-6}$ Torr), one with a positive and one with a negative angle, directed at both sample sides to polish them simultaneously and to create two polishing cups that meet at the center of the sample thickness. The energies used for milling are low compared with those used for the FIB, ranging from 0.1 to 10 kV. The ion beams can be tilted from 0 to 10 degrees and the stage rotates to ensure continuous processing. Sample preparation, based on the method developed by M.-H. Berger and A. Bunsell (35), involved stretching and fixing several fibers side by side on a 3 mm diameter copper disk (Figure 3a.). The milling was carried out, without a pre-milling step, at 4 kV for 2 hours then 2 kV for 3 hours with decreasing tilted positive and negative angles of 6 degrees then 2 degrees and a rotation speed of 3 rpm (Figure 3b.).
<figure><img src="image_5.png" /><figcaption>Figure 3. Stretched and fixed fibers to a copper grid (a.) and TEM image of a ceramic fiber after PIPS milling (b.)</figcaption></figure>

Several regions of interest can be studied on the same sample, but only in the fibers longitudinal direction. The ion milling is performed with argon ions and at lower energies than with FIB. However, this technique still relies on ion-matter interactions, and more or less significant amorphization can occur on the fiber surface (Figure 4a.). The angle and incidence energy of the PIPS are modified and reduced to limit this artifact. A low-angled milling has enabled to improve the surface finish of thinned areas. As with FIB, the grid copper can also be milled by the ion beam, leading to sample contamination (Figure 4b.). A final drawback is the preparation time for this technique, which can be several hours or even days for some massive samples.

<figure><img src="image_6.png" /><figcaption>Figure 4. PIPS ceramic fiber preparation artifacts: Sample surface amorphization (a.). The double arrow shows the amorphization extent in the fiber thickness. Redeposition of metal particles (b.) Cu twinning from the TEM grid can be seen on the sample surface. This artefact was confirmed by EDX analysis carried out during the TEM observations. This elemental analysis shows a large Cu peak.</figcaption></figure>
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III Ion milling techniques: combined methods

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To limit FIB artifacts such as Ga ion implantation, a solution is to combine the ion milling techniques mentioned above (36–38). With the new version of PIPS (PIPS II), it is possible to use low energies (8 kV to 100 eV) for finishing and cleaning steps.

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Conventional FIB preparation steps were carried out until a thin foil thickness of around 100 nm was obtained. The final thinning steps were not performed with the FIB, but with the PIPS II. The PIPS II was used as a finishing tool, at an energy of 100 eV for 10 min and with 5 degrees incidence positive and negative angles (figure 5.). The ion beam was applied to the sample at high angles to take into account the grid geometry constraints. Despite more complex sample handling, this first method, which combines FIB and PIPS II techniques, significantly improved the sample quality. Thin foil thickness was rapidly reduced with PIPS II to values below 80 nm. The artifacts due to FIB ion milling (ion implantation and amorphization) were considerably mitigated by using PIPS II at very low energy at the end of the preparation.

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<figure><img src="image_7.png" /><figcaption>Figure 5. Progressive milling of the FIB thin foil after PIPS II for 5 then 10 minutes. The fiber is coated with a protective platinum layer formed during the initial stages of FIB sample preparation.

The arrows show the organized atomic planes on the fiber surface, which become increasingly visible during PIPS II milling.</figcaption></figure>

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One of the main disadvantages of PIPS remains the time required to thin the samples. To retain the advantages of argon-beam ion milling and save time during this stage, the Ilion II system, could be combined with the PIPS II technique. Ilion II (39–42) is a low-energy (0.1-8 kV) surface preparation technique for scanning electron microscope observation. This device polishes samples in two polishing modes using two focused argon ion beams: cross-section and planar milling. Argon ion beams are directed in vacuum ($5 \cdot 10^{-6}$ Torr) towards the surface of the sample, which causes slight erosion of the surface material. This instrument can be used to thin larger samples ($10 \times 10 \times 4 \text{ mm}^3$) than PIPS (2-3 mm).

For this study we used the planar milling mode. This polishing method is highly dependent on the parameters to be set (polishing voltage, duration, temperature, angle of the gun, number of revolutions per minute). Few artifacts are observed on ceramic fibers prepared using this technique (Figure 6a.), except curtaining effects which can be reduced by using low-angle ion beams and a metallic or carbon layer protecting the fiber surface.

Samples were prepared in the same way as for conventional PIPS preparation. The fibers were fixed to a 3 mm diameter copper grid, which in turn is bonded to the Ilion support. This set-up allowed the Ilion to be used in planar mode, so that milling has taken place in the center of the sample. A 5 keV voltage was applied to the sample for one hour at a 5° angle for ion guns at room temperature. After Ilion milling, PIPS II was used as a finishing tool on ceramic fibers at 2 keV for 40 minutes with tilted **positive and negative angles of 2 degrees**. This combined method made it possible to prepare ceramic fibers for TEM study in less time and with few defects or structural modifications due to argon ion implantation (Figure 6 b.).

<figure><img src="image_8.png" /><figcaption>Figure 6. SEM observation of an Ilion II-thinned fiber in planar mode (a.). TEM ceramic fiber surface prepared using the Ilion II-PIPS II method (b.).</figcaption></figure>
However, some artifacts from Ilion persist. A residual topography could be observed on the fibers surface after Ilion milling due to the inhomogeneities of the sample and the incidence angle at which the ion beam is applied (Figure 7a). A shallower angle (i.e. not perpendicular to the surface of the fibers) at this preparation step would further smoothen the surface of the thinned fibers by processing reliefs. An amorphization of the fiber structure was also observed (Figure 7b). These artefacts were gradually reduced after PIPS II treatment until they are significantly reduced as shown in Figure 6b, which is a TEM image of the ceramic fiber after it has been prepared by this method.

<figure><img src="image_9.png" /><figcaption>Figure 7. Ilion + PIPS ceramic fiber preparation artifacts : Surface residual topography (Focus was carried out on the extreme surface of the fiber to better highlight this preparation artefact) (a.) and surface amorphization (b.).</figcaption></figure>

## Conclusion

The comparative study has highlighted the benefits of using ion-milling techniques for the TEM preparation of ceramic fibers. Despite artifacts, the FIB (Focused Ion Beam) remains an advanced instrument for preparing these samples, as the sampling may be highly localized and precise. The simpler BIB (Broad Ion Beam) is ideal as a finishing tool.

By combining these methods, we were able to improve significantly the quality of the samples, by exploiting their strengths, and thus minimizing preparation artifacts. The Ilion, low-focus Argon ion-beam polisher for SEM preparations, has also proved to be an effective tool for the TEM preparation of these samples, by simplifying the process and saving the operator time. This study therefore led to
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an evaluation of an original preparation procedure that could be applied to samples of similar
morphology (plant fibers, asbestos ...) or chemical nature (carbon-rich) to ceramic fibers.

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**Acknowledgments:**

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We would like to thank James Braun and Cédric Sauder for bringing us all these samples for the study, as well as CLYM (Consortium Lyon St-Etienne de Microscopie) for providing access to preparation (PIPS II, Ilion II+, FIB NVision 40) and observation (JEOL 2010F TEM) techniques.

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