Relaxation Induced by Imprint Phenomena in Low-Temperature (400 °C) Processed Hf_0.5Zr_0.5O₂-Based Metal-Ferroelectric-Metal Capacitors

Abstract

Ferroelectricity in Hf 0.5 Zr 0.5 O 2 (HZO) has garnered increasing interest due to its potential applications in neuromorphic and nonvolatile memory devices. However, with time, the adverse shift in the coercive voltage (V c ) observed in a prepoled HZO ferroelectric capacitor can lead to insufficient polarization switching from one polarity owing to the development of a built-in voltage, causing reliability concerns. Another consequence of developed built-in voltage is polarization relaxation, that is, the rapid loss of stored polarization with time (t < 6 s). In this work, the correlation between the data retention issues in ferroelectric films, namely, imprint (V c shift with time) and polarization relaxation, is studied carefully by pulse measurement techniques in HZO-based metal-ferroelectric-metal capacitors. In a nonaged ferroelectric, there are no significant relaxation effects for a switching time delay up to 6 s, even at higher measurement temperatures until 125 °C. After a baking process was performed at higher temperatures (25−125 °C) to a prepoled ferroelectric capacitor, an apparent systematic imprint was observed with increasing baking time (up to 200 h). Conjunctively, we also see that the imprinted ferroelectric capacitor shows a strong polarization relaxation effect even for short time periods due to the generated built-in voltages. For the 2.0 V operation, based on the delay time between the re-WRITE pulse and READ pulses, the lifetime extracted for a 65 °C bake can show a stable 10-year lifetime using a 10 μs delay time compared to a 0.5 year lifetime when using a delay time of 6 s. This work explores the drastic relaxation effects due to imprint in ferroelectric HZO films and the need to mitigate defects/space charges causing these issues.