Feedback controlled electromigration in four-terminal nanojunctions

Abstract

We have developed a fast, yet highly reproducible method to fabricate metallic electrodes with nanometer separation using electromigration (EM). We employ four-terminal instead of two-terminal devices in combination with an analog feedback to maintain the voltage U over the junction constant. After the initialization phase (U 0.2 V), during which the temperature T increases by 80 − 150 o C, EM sets in shrinking the wire locally. This quickly leads to a transition from the diffusive to a quasiballistic regime (0.2 V U 0.6 V). At the end of this second regime, a gap forms (U 0.6 V). Remarkably, controlled electromigration is still possible in the quasi-ballistic regime.

Feedback-controlled electromigration for the fabrication of point contacts

“…The hardware portion of the hybrid feedback scheme involves a four-wire setup similar to that by Wu et al 15 which removes the effect of the relatively large lead resistance of our 1-300 K cryostat (see Figure 2). The voltage source is a PID controller which, when combined with the four-wire measurement, allows a target voltage to be set across the device itself.…”

“…It might not be possible to minimize lead resistance in some cases: low-temperature cryostats, for example, often have resistive wiring to reduce heat transfer. An alternative technique is to remove series resistance effects by lithographically patterning four-terminal devices and adding analog feedback to control the voltage directly across the wire 15 . In this way, the IV relation is no longer double-valued and the instability in the EM process is reduced.…”

Molecular Junctions: From Tunneling to Function

“…[32,36] We have shown that adding an electronic feedback in a four-terminal geometry enables an accurate control of EM progression. [30] The technique is fast, on the scale of seconds, and enables the fabrication of nanogaps in Au electrodes with a yield approaching 90% or better. An example of a gap produced with this technique is shown in Fig.…”

“…2b. [30] By maintaining a large current density in the wire, a diffusion of the metal atoms can be initiated to thin the wire until it finally breaks open. The process was first applied to prepare molecular junctions by Park et al, [31] and further exploited and refined by others.…”

Controlled electromigration protocol revised

“…Substantial efforts have been made in order to improve the reliability of EM by adding an analogous proportional-integral-derivative (PID) feedback loop into the EM controlling scheme 10,11 or by drastically increasing the reaction speed of the control system. 12 Here we demonstrate that an improved performance a) Author to whom correspondence should be addressed: joris.vandevondel@ kuleuven.be of EM-based fabrication can be achieved by a successful revision of the EM protocol, instead of hardware redesign.…”