Fabrication of ferroelectric thin film specimen for in-situ electrical biasing TEM studies by FIB.

Session

PST.6 - In-situ and in-operando microscopy

Authors

Alexander Vogel (2, 1), Martin Sarott (1), Prof. Dr. Manfred Fiebig (1), Dr. Morgan Trassin (1), Dr. Marta Rossell (2)

Affiliations

1. ETH Zürich
2. Swiss Federal Laboratories for Materials Science and Technology, Empa

Keywords

electrical biasing, ferroelectrics, FIB, in-situ TEM, MEMS chip

Abstract text

Ferroelectrics have a large variety of applications in modern life. These include piezoelectric actuators, sensors, dielectric capacitors or memory devices. In data storage applications, ferroelectricity presents a unique alternative to ferromagnetism as it allows for more energy efficient and faster devices. However, the commercialization of ferroelectric memory devices has been hindered by major reliability issues [1,2] such as retention loss [3], imprint [4], and fatigue [5]. To overcome these issues, a deeper understanding of the microscopic mechanisms of domain wall motion, nucleation, and the role of various types of defects, such as oxygen vacancies, in this context is required. Even so, at present, there is a lack of dynamical data on the ferroelectric switching process at the atomic scale, which impedes further development in this field.

In this work, we report on the fabrication of ferroelectric thin film specimens for in-situ electrical biasing experiments inside the transmission electron microscope (TEM) by focused ion beam (FIB) to fill this gap in knowledge. The specimens were fabricated on commercial micro-electro-mechanical system (MEMS)-chips manufactured by Protochips that allow for electrical and thermal stimuli without applying external strain on the thin films. A FEI Helios 660 SEM/FIB equipped with an EasyLift Nanomanipulator, operating a Ga ion source with acceleration voltages up to 30 kV, was used to machine the electron transparent specimens. A plate capacitor geometry was adopted to mimic the environment of industrial applications for ferroelectric capacitors. The bottom electrode of the capacitor consists of a conductive substrate, such as Nb-doped SrTiO₃, while sputtered Pt and Pt deposited by ion-beam-induced chemical vapor deposition were used as top electrode. The investigated ferroelectric thin films include Pb(Zr_0.2Ti_0.8)O₃ (PZT) and BiFeO₃ (BFO) grown by pulsed laser deposition. 

The specimen geometry was optimized to prevent the mechanical failure of the electrodes caused by the strain induced by the piezoelectric response of the ferroelectric. This allowed for stable PZT thin film specimens, with a film thickness of 100 nm, up to a biasing voltage of 20 V. The investigation of the currents passing through the capacitor in response to the applied voltage revealed a Schottky-diode like behaviour. Typical currents in the reverse-bias direction were on the order of 100 nA at a biasing voltage of 5 V. A thermal breakdown of the lamella occurred once the leakage currents exceed 1 µA, with both the Pt electrode and ferroelectric thin films melting at this point. The initial characterization of the ferroelectric thin films by conventional dark-field (DF) TEM imaging and high-resolution high-angle annular dark-field (HAADF) scanning TEM (STEM) was performed using a FEI Titan Themis microscope equipped with a probe CEOS DCOR spherical aberration corrector operated at 300 kV. In DF-TEM, the correct choice of diffraction spots used for image formation allows for tracking the in-plane or out-of-plane domain distribution in the ferroelectric film. In HAADF-STEM imaging, the ferroelectric polarization is mapped on the atomic scale by a combination of time series averaging [6], probe deconvolution and atomic column fitting [7]. 

Our results provide the path to prepare highly stable ferroelectric thin film capacitor specimens for in-situ S/TEM observations. This shall enable future studies on the dynamic processes in ferroelectric thin films under an applied bias voltage, which are crucial for the practical implementation of these materials in data storage devices [8].

References

[1] O. Auciello, J. F. Scott, and R. Ramesh, Physics Today 51 (1998), p. 22-27.

[2] W. L. Warren, D. Dimos, and R. M. Waser, MRS Bulletin 21 (1996), p. 40-45.

[3] A. Gruverman and M. Tanaka, Journal of Applied Physics 89 (2001), p. 1836.

[4] A. K. Tagantsev et al., Journal of Applied Physics 96 (2004), p. 6616-6623.

[5] A. K. Tagantsev et al., Journal of Applied Physics 90 (2001), p. 1387-1402.

[6] L. Jones et al., Advanced Structural and Chemical Imaging 1 (2015), p. 8.

[7] M. Nord et al., Advanced Structural and Chemical Imaging 3 (2017), p. 9.

[8] The authors gratefully acknowledge funding from the Swiss National Science Foundation under project number 175926.