Elsevier

Ultramicroscopy

Volume 207, December 2019, 112840
Ultramicroscopy

Plan-view sample preparation of a buried nanodots array by FIB with accurate EDS positioning in thickness direction

https://doi.org/10.1016/j.ultramic.2019.112840Get rights and content

Highlights

  • •

    An on-line positioning method in thickness direction is introduced in a dual beam FIB for plan-view TEM sample preparation of a buried thin film.

  • •

    A high quality plane-view TEM specimen of buried BiFeO3 nanodots grown on a 20 nm thick SrRuO3 bottom electrode and SrTiO3 substrate is obtained.

  • •

    This method can be applied to prepare TEM specimens of other complicated structures in FIB.

Abstract

Recently, there are growing demands on focus ion beam (FIB) sample preparation technique in plan-view geometry because it can provide the in-plane microstructure information of thin film and allows direct correlations of the atomic structure via transmission electron microscopy with micrometer-scale property measurements. However, one main technical difficulty is to position the buried thin film accurately in a sandwich structure. In this paper, an on-line positioning method based on the thickness monitoring by EDS is introduced, where the intensities of the characteristic X-ray peaks from different layers are proportional to the relative thickness of them at the same acquisition conditions. A high density array of ∼100 nm squares BiFeO3 nanodots with ∼ 25 nm thickness grown on a 20 nm thick SrRuO3 bottom electrode and (001)-oriented SrTiO3 substrate is selected for demonstration. By monitoring the intensities of Pt-M, Sr-L, Ti-K, Ru-L, Fe-K and Bi-M peaks, the relative thickness of Pt protection layer, the BiFeO3, SrRuO3 and SrTiO3 can be obtained, which provide accurate position of the BFO nanodots array in the thickness direction. With these information, the cutting parameters are optimized and a high quality plan-view specimen of BFO nanodots array is prepared, which is confirmed by high resolution transmission electron microscopy. This positioning method should have a wide application for material science.

Section snippets

Introduction and motivation

Focused ion beam (FIB) milling techniques has been widely used to prepare thin specimens for transmission electron microscopy (TEM) observation for more than two decades due to its incomparable advantages than the conventional mechanically polishing and argon ion milling methods, such as tiny sample consumption, precise selection of target areas, suitable for most solid materials, high efficiency and reliability [1], [2], [3]. Nowadays, the dual beam system with FIB and scanning electron

Method

It is well known that the intensities of EDS signals are proportional to the thickness of a thin specimen at a fixed beam current [16], [17], which has been used to monitor sample thickness [18]. In the present study, the accurate positioning in thickness direction can be achieved by monitoring the relative thicknesses of the top and bottom layers.

Fig. 1 schematically shows the principle of the method. A very thin layer B is buried in-between the top layer A and the bottom layer C. In order to

Materials and equipment

In order to demonstrate our method, a high-density array of BiFeO3 (BFO) nanodots sample was selected for plan-view specimen preparation with FIB lift-out techniques. The structure of the sample is schematically shown in Fig. 2. The BFO nanodots are ∼100 nm squares with 25 nm thickness, which were grown on a 20 nm thick SrRuO3 (SRO) bottom electrode on a (001)-oriented SrTiO3 (STO) substrate by pulsed laser deposition (PLD). More details about the film growth can be found in the literatures [21]

Milling channels and lift-out

In order to make the lamella transfer easier [15], [26], a 38° slice pre-tilt stage was used. The sample rotation was abandoned and the tilt was reduced to one time. Firstly, the sample was stuck on the platform of the pre-tilt stage, as illustrated in Fig. 3a. Secondly, the C layer and Pt layer were deposited on the region of interest (ROI) in FIB to protect the surface further. Then the channels around the ROI were made with a bridge, as shown in Fig. 3b. In the following steps, standard

Conclusions

It is a challenge to position the buried thin film accurately in a sandwich structure when preparing a plan-view specimen of the middle layer by FIB lift-out technique. Here we proposed a positioning method based on the thickness monitoring by EDS, where the intensities of the characteristic X-ray peaks from different layers are proportional to the relative thickness of them at the same acquisition conditions. A plan-view specimen of the high-density array of ∼100 nm squares BiFeO3 nanodots

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2017YFA0303403), the National Natural Science Foundation of China (Grant No. 61974042) and Natural Science Foundation of Shanghai (16ZR1409500).

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