Conductive filament shape in HfO₂ electrochemical metallization cells under a range of forming voltages

The development of neuromorphic computing architectures based on two terminal filamentary resistance switching devices is limited in part by the high degree of variability in resistance states and switching voltages. Because of the large role filament shape plays in directing thermal and electric fields around the filament (and thus switching parameters), unambiguous knowledge of filament morphology resulting from direct characterization of filament shape is essential to solve critical ongoing challenges of device switching variability. Here, we have utilized a conductive atomic force microscopy scalpel technique to simultaneously scribe through a polycrystalline dielectric layer in formed Cu/HfO₂/p+Si electrochemical metallization cell devices. Filament tomograms reveal that when conductive filaments are formed at typical bias conditions (4 V, 100 μA), a variety of filament shapes result, which deviate from the inverse conical shape predicted by the phenomenological electrochemical model. Furthermore, the observation of an increasing spectrum of damage which scales with forming voltage (associated with compliance current overshoot), and which is uncorrelated with electric field or oxide microstructure, supports the role of thermal pulses in expanding filaments, leading to irreversible dielectric breakdown structures at the extreme. Overall, these findings suggest that the original conductive filament shape can be highly varied as a result of thermally driven expansion from joule heating during the forming step, which is not explicitly accounted for in the widely accepted electrochemical model.