Focused Low Energy-Argon Ion Milling
A Must-Have Toll for Cs-Corrected TEM
- Focused Low Energy-Argon Ion Milling - A Must-Have Tool for Cs-Corrected TEM
- Fig. 1: Distribution of the Ga and Ar ions in the GaN material in dependence of the material depth calculated by SRIM.
- Fig. 2: Thickness map by energy-filtered TEM. Inset shows thickness profile across ROI marked in the TEM image.
- Fig. 3: HAADF-HRSTEM image of the GaN-SiC interface taken at 80 kV accelerating voltage.
- Fig. 4: HAADF-HRSTEM image of the TiO2-SrTiO3 interface recorded at 300 kV accelerating voltage.
High energy focused ion beam (FIB) milling produces ion-induced damage into TEM samples and a certain amount of Ga ions implantation cannot be avoided. Additional polishing of FIB lamellae at low voltages can damage the sample further. To overcome these disadvantages, a low-energy Ar+-milling of a FIB lamellae can be applied [1,2]. In this work, we focus on TEM sample preparation of different thin films and interface structures using a combination of FIB with a focused low-energy Ar+-polishing.
For high-resolution aberration-corrected (Cs-corrected) scanning transmission electron microscopy (STEM) the quality of prepared TEM samples is crucial. Nowadays, the widely used FIB preparation technique with standard FIB configuration cannot be employed in fabrication of high-quality TEM samples with a thickness of less than 20 nm. Especially, FIB preparation of cross-sections from interfaces for atomic resolution Cs-corrected TEM and EELS studies is often a difficult issue. Noticeable, thin TEM samples with thicknesses of 10 to 20 nm for such TEM investigations can be prepared by using a modified FIB technique [3,4]. However, it requires either a special rotation-tilt holder or a well aligned FIB system at low Ga ion energies (1 and 2 kV) . In addition, damage and amorphization of the specimen surface during the FIB milling process occurs. Figure 1 represents the influence of the type of ion beam (Ga+ or Ar+), ion energy and angle of incidence on penetration depth of Ga and Ar ions into the GaN material. Low-energy Ar ions produce less implantation and surface defects than Ga ions. Thus, to overcome the disadvantages of the FIB preparation, a low-energy Ar+-polishing post FIB lamella treatment has to be applied. In the present work, we focus on TEM sample preparation of GaN (thin film)-SiC (substrate) and TiO2(thin film)-SrTiO3(substrate) interfaces for imaging in Cs-corrected STEM at 80 kV and 300 kV.
For FIB lamellae preparation standard FIB lift-out method in a Zeiss Auriga DualBeam system was used.
Afterwards, the FIB lamellae were further treated by a focused low-energy Ar+-polishing with ion energies of less than 1 kV in a NanoMill (Fischione) system. The last step was plasma treatment of a TEM lamella for 10 min. TEM observations were performed with a probe Cs-corrected Titan3 G2 60-300 microscope equipped with HAADF, BF, DF, ABF and Super-X EDX detectors as well as with a GIF Quantum Gatan imaging filter. The TEM was operated at 80 kV and 300 kV accelerating voltages.
Figure 2 shows thickness measurements by means of energy filtered TEM of a porous GaN thin film grown on a 6H-SiC(0001) substrate [5,6]. The intensity of colors in figure 2 shows different thicknesses in the sample: the brighter the thicker. Since the brightness of the image does not change so much, the thickness of the TEM sample after the focused Ar ion beam milling is rather uniform over a wide area. The thickness of the TEM sample at the SiC-GaN interface is in the range of 9-11 nm.
Figure 3 shows a HAADF-HRSTEM image of the interface between GaN thin film and SiC substrate acquired at 80 kV. The image in figure 3 was recorded from an interface region of the sample shown in figure 2. The atomic structure of the GaN-SiC interface in figure 3 is clearly resolved.
Another example of a post-treated FIB lamella with a focused low-energy Ar ion beam is given in figure 4. The Figure gives atomic resolution Cs-corrected HAADF-STEM image of the TiO2-SrTiO3 interface acquired at 300 kV accelerating voltage. The initial thickness of the TEM sample after the FIB preparation was approximately 80 nm. The thickness of the TEM sample was reduced down to 20 nm over a wide area after polishing with a focused low-energy Ar ion beam. The atomic columns in the TiO2 and SrTiO3 as well as at the TiO2-SrTiO3 interface are clearly resolved in figure 4.
The combination of FIB with post-processing focused low-energy Ar ion milling (LEIM) enables routinely preparation of high quality TEM lamellae with thicknesses down to 10 nm. The TEM samples using this approach are well suited for atomic resolution TEM and STEM studies as well as for atomic EDX and EELS analyses at 80 kV and at 300 kV.
The financial support of the European Union and the Free State of Saxony (LenA project; project no. 100074065) is greatly acknowledged.
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Dr. Andriy Lotnyk
Group Leader Electron Microscopy
Leibniz Institute of Surface Modification (IOM)