Using in situ scanning transmission electron microscopy (STEM) (FEI Titan microscope operating at 300 keV), a microelectromechanical system (MEMS) chip and a dedicated biasing and heating sample holders, built in-house, we investigated electrical and thermal properties of 15-nm-thick Ni nanobridges. These techniques allow to visualize nanobridge morphology transformations down to atomic scale while electrical current is passed. If thin metallic wire is subjected to high current density, the material transfer can start which results in the wire break. This phenomenon is called electromigration ¹.

Ni nanobridges with a length of 500 – 1000 nm and a width 200 – 500 nm were produced by e-beam metal evaporation onto a 100-nm-thick freestanding silicon nitride membrane and patterned using electron beam lithography (Fig. 1a). Contacts to the nanobridges were made with a 100-nm-thick layer of Au and a 3-nm-thick adhesion layer of Cr. Initial resistance of the structures, including bridge, contact pads and leads, is in the range of 160 – 250 Ohm. More details of the sample preparation can be found elsewhere ². Using electrical setup ³, I–V measurements were performed in bias-ramping mode. Voltage is gradually increased (with a speed of 15 mV/s) from 0 V to a predefined value of 500–600 mV, followed by a decrease back to 0 V, after which a new cycle with higher maximum voltage was performed (Fig. 1f).

Fig.1 shows STEM images of Ni nanobridge with 10-nm-thick Al₂O₃ oxidation-protective layer on top taken before electromigration and after each bias-ramping cycle with maximum voltages 500 mV, 520 mV, 540 mV and 580 mV. Fig. 2f shows corresponding I–V curves for four voltage cycles applied in a row. Sample temperature prior to voltage apply was 100 K. During electromigration experiments in Ni material transfer was shown to be voltage polarity dependent: Voids initially form near the cathode contact pad of the bridge, as in the majority of metals due to electron-wind force, but at the end bridge breaks near the anode side.

Also, we visualised morphological transformations in polycrystalline Ni film (deposited on top of the heater with 20-nm-thick windows in Si₃N₄ membrane) during substrate heating up to 400°C (Fig. 2) and estimated the bridge temperature achieved in electromigration experiments due to the Joule heating to be around the Curie point. In order to enhance the contrast between grains, annular dark-field STEM imaging was used ⁴.

Enriched with oxygen bubbles formation was found due to Ni nanobridges oxidation after a month of their storage at atmospheric pressure. In order to prevent samples oxidation, 10-nm-thick Al₂O₃ layer was used as a protective layer. The place of bridge break near the anode side was shown to be independent on the ambient pressure and substrate temperature.

Acknowledgement: The authors gratefully acknowledge STW UPON and ERC project 267922 for support.

1.         Ho, P. S.; Kwok, T. Rep Prog Phys 1989, 52, (3), 301-348.

2.         Kozlova, T.; Rudneva, M.; Zandbergen, H. Nanotechnology 2013, 24, 505708.

3.         Martin, C. A., et al. Rev Sci Instrum 2011, 82, 053907.

4.         Rudneva, M., Kozlova, T. & Zandbergen, H. Ultramicroscopy 2013, 134, 155-159. 

Figures:

Fig. 1. STEM images of Ni nanobridge with Al2O3 protective layer showing material transfer during electromigration at 100 K. (a–c) During electromigration (white arrows indicate the direction of electrons), voids form at the cathode contact pad (indicated with yellow arrows). (d–e) At higher electrical current (approx. 1.2 mA) the bridge starts to shrink due to a neck formation near the anode side (indicated with green arrows) and eventually breaks. (f) I–V curves for four loops with gradually increased maximum voltage.

Fig. 2. STEM image series of the grain growth in 15-nm-thick Ni film during heating; the temperature is indicated in each image.

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