The focused ion beam (FIB) method is widely used to prepare specimens for observation by transmission electron microscopy (TEM), which offers a wide variety of imaging and analytical techniques. TEM has played a significant role in material investigation. However, the FIB method induces amorphization due to bombardment with the high-energy gallium (Ga⁺) ion beam. To solve this problem, electron beam induced deposition (EBID) is used to form a protective layer to prevent damage to the specimen surface. In this study, we introduce an optimized TEM specimen preparation procedure by comparing the EBID of carbon and tungsten as protective layers in FIB. The selection of appropriate EBID conditions for preparing specimens for TEM analysis is described in detail.

Two-dimensional (2-D) materials, including graphene, MoS₂, WS₂, and h-BN, are crystalline films consisting of a single layer of atoms. Since these materials exhibit a variety of interesting properties from high-band gap insulators to semiconductors and semi-metals, they are widely used in electronic and photonic applications in batteries and electrochemical solar cells. The microstructural, chemical, and electrical properties of 2-D materials are typically characterized by analytical tools such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Among these methods, TEM equipped with additional analytical attachments is a powerful technique to obtain accurate information regarding the microstructure, interfaces, thickness, and chemical composition of the material at the nanoscale. Because of these useful functions, TEM analysis has been widely used in the fields of materials science and engineering. However, to obtain accurate data using TEM analysis, the specimen should be made very thin so the electron beam can easily penetrate the specimen. In general, ion milling (John & Robert, 1984), mechanical flat and wedge polishing (Cha et al., 2016; Mkhoyan et al., 2006; Okuno et al., 2008), and focused ion beam (FIB) (Giannuzzi & Stevie, 1999; Stevie et al., 1995) are used as methods for preparing TEM specimens (Williams & Carter, 2009).

The ion milling method, a commonly used preparation method for TEM specimens, can be used to strip off the surface layer by firing argon (Ar⁺) or xenon (Xe⁺) ions at a particular angle after mechanical polishing. However, it is difficult to prepare patterned samples such as actual electronic devices and materials with poor adhesion (e.g. interface between a 2-D material and SiO₂).

On the other hand, FIB has been extensively used in the semiconductor industry and field of materials science because the specimen preparation is facile without any of the restrictions mentioned above. However, the FIB method forms a damaged layer as a result of bombardment by highly accelerated gallium (Ga⁺) ions. This damage typically appears in the form of amorphization on the surface on the TEM specimen. To solve this problem, low-energy Ga⁺ or Ar⁺ ion milling as final step is used to minimize the damage caused by the high-energy Ga⁺ ion beam. In addition, electron beam induced deposition (EBID) can be used to gently deposit a protective layer on the surface before forming the lamellar structure (Kato, 2004). Carbon, platinum, and tungsten materials are generally used as a depositing source in EBID. The representative platinum-EBID protective layer is known to prevent damage caused by collisions between Ga⁺ ions and the surface but it is very expensive. Alternatively, carbon and tungsten-EBID can be used as inexpensive protective layers. However, studies on inexpensive C and W-based protective layers have not been reported to date.

Therefore, in this study, we introduce an optimized TEM specimen preparation procedure using FIB by comparing carbon and tungsten as a depositing source in EBID for the generation of inexpensive protective layers.

A thin molybdenum film was deposited on a transferred CVD-grown graphene substrate by e-beam evaporation. MoS₂ was synthesized by sulfurizing at 300°C under a H₂S and Ar plasma atmosphere for 1.5 h in the plasma-enhanced chemical vapor deposition(PECVD) (Ahn et al., 2015). The crystallinity of the PECVD-grown MoS₂ was characterized by Raman spectroscopy at 532 nm (Alpha300 M+; WI-Tec Gmbh Inc., Germany). The plan-view TEM specimens were prepared by rinsing with deionized water (D.I. water) and floating the MoS₂ on SiO₂ substrate on a diluted hydrofluoric acid (HF) solution using a conventional transfer method to characterize the crystallinity of the PECVD-grown MoS₂. The SiO₂ layer was etched using a diluted HF solution, detaching the MoS₂ from the underlying SiO₂ substrate. Finally, the detached MoS₂ films were transferred to a TEM grid.

To compare TEM samples prepared with carbon- and tungsten-EBID as protective layers to prevent the bombardment by Ga⁺ ions during FIB, cross-sectional TEM specimens were fabricated via FIB (NX2000; Hitachi Inc., Japan) using the lift-out technique, as shown in Fig. 1. Carbon- and tungsten-EBID were deposited using the focused electron beam and gas injection system (GIS) to form a protective layer. The TEM samples were etched using a high-energy Ga⁺ ion beam at 30 keV and 1.5 nA until reaching thickness of ~500 nm and were subsequently thinned to a foil thickness of ~70 nm at 5~10 keV and 40 pA. As the final step, a low-energy Ar⁺ ion beam at 1 keV and 19 nA was used to minimize the damage to the surface layers.

The prepared samples were investigated by analytical TEM (JEM-ARM200F; JEOL, Japan) at 80 keV to avoid electron beam-induced damage to the samples during TEM observation (Garcia et al., 2014). High-resolution (HR) images were recorded using a CCD camera (Oneview; Gatan Inc., USA).

Fig. 2 shows the Raman spectra acquired from MoS₂ excited at 532 nm, plan-view HRTEM images, and fast Fourier transform (FFT). As shown in Fig. 2A, the in-plane (E¹_1g, ~383 cm⁻¹ for bulk MoS₂) and out-of-plane vibrations (A_1g, ~408 cm⁻¹ for bulk MoS₂) modes in the PECVD-grown MoS₂ were observed at 378 and 406 cm⁻¹, respectively. Fig. 2B and C shows the plan-view HRTEM image and the corresponding FFT. In the plan-view HRTEM image, the lattice arrangement of the PECVD-grown MoS₂ adopted a hexagonal structure nanocrystalline grains ~5 nm in size were observed (inset of Fig. 2B). These results confirmed that MoS₂ was fully synthesized by indexing the ring pattern in the FFT.

After characterizing the crystallinity of MoS₂, the cross-sectional TEM specimens fabricated by FIB were examined. Fig. 3 shows the cross-sectional HR TEM images of the specimens with carbon and tungsten protective layers. For the TEM specimen fabricated using carbon-EBID, the MoS₂ layer was amorphized by the accelerated carbon source when the carbon layer was deposited using an electron beam, while graphene was observed without amorphization or damage, as shown in Fig. 3A. When measuring the d-spacing of each layer using the line profile of the HR TEM image, the d-spacing of the MoS₂ layer could not be measured due to amorphization. The d-spacing of the graphene layer was approximately 0.35 nm, which is consistent with that of typical graphite. For the TEM specimen fabricated using tungsten-EBID, the MoS₂ and graphene layers were similar to their respective pristine states shown in Fig. 2 without amorphization or damage from the tungsten source (Fig. 3B). Unlike the large crystalline MoS₂ synthesized by thermal CVD, discontinuous layers were observed because the MoS₂ layer existed in the nanocrystalline state, as observed in the plan-view HR TEM image (Fig. 2). The d-spacing of the MoS₂ and graphene layers measured by the line profile were ~0.625 and ~0.35 nm, respectively. The number of layers and crystallinity of the MoS₂ were confirmed.

From the above results, the importance of selecting appropriate materials for the protective layer deposited by electron beam when fabricating TEM specimens using FIB is clear.

In this study, we developed an optimized TEM specimen preparation method by generating an appropriate protective layer using EBID in the FIB method. By preparing successful TEM specimens using appropriate protective layers, it is possible to precisely analyze 2-D, biological, and organic materials which are especially vulnerable to damage by electron and ion beams.

This study was supported by the Ministry of Trade, Industry & Energy (MOTIE) [project number 10080625], the Korea Semiconductor Research Consortium (KSRC) support program for the development of future semiconductor devices, and NRF grants [NRF-2018R1A5A6075959] funded by the Korean government (MSIP). The authors are grateful for support from the SKKU Advanced Institute of Nanotechnology (SAINT) at Sungkyunkwan University.

No potential conflict of interest relevant to this article was reported.