Thickness Effects on Charge and Discharge Characteristics of CeOx Mim Capacitors

Abstract

Thickness effects on charge and discharge characteristics of a metal-insulator-metal (MIM) capacitor with ionic-oxygen-conductive CeOx layer was characterized. Transient response in the charging and discharging current and frequency dispersion suggested the movement of oxygen ions in the CeOx layer. The MIM capacitor with a thinner CeOx layer showed larger charging and discharging current and shorter time constant and suggested relation of CeOx layer thickness and ionic conductivity in the CeOx layer. Introduction On-chip decoupling capacitors with MIM structures have been used for suppress the high-frequency noise and support high-speed switching in the power supply line [1]. MIM capacitors are demanded to increase the capacitance density along with the technology node [2]. As an approach to increase capacitance density by thinner films may be limited by the leakage current through the electrodes, scaling in the insulators requires higher dielectric constant must be explored. We thought that sold electric-double-layer (EDL) capacitors with CeOx can be achieved. CeOx has a large value of dielectric constant (high-k) and suggest high-ionic conductivity at room temperature. Recently research, EDL transistors with Gd-doped CeO2 has shown that a large capacitance density of 14μF/cm2 can be achieved by the ionic conductivity in the insulators [3]. In this work, thickness effects on the charge and discharge properties of EDL capacitors with a CeOx layer are characterized. Experiments We fabricated MIM capacitors has a 10nm-thickness CeOx layer or 20nm-thickness CeOx layer. An e-beam deposited CeOx layer is sandwiched by atomic layer deposited SiO2 layers. The top and bottom electrodes are both sputter-deposited TiN. After patterning the top electrode, the capacitors are annealed at 420oC, for 30 min. The characteristics of both MIM capacitors were measured by LCR meter and the charge/discharging transient response and frequency-dependent capacitance were characterized. Results and discussions The time responses of the current after a step voltage of 4 V application (t=30s) and back to 0 V (t=150s) are shown in Fig. 1. When a voltage was applied to the top electrode, exponential decrease in the charging current was observed at both MIM capacitors. The exponential decrease was also observed in the discharging current and back to the steady-state current value at both capacitors. Those transient responses with a long time constant suggests movement of oxygen ions in the CeOx layer. Fig. 1 also showed that larger amount of charging and discharge current can be observed at the MIM capacitor with a 10nm-thickness CeOx layer than the MIM capacitor with a 20nm-thickness CeOx layer, although larger amount of leakage current was observed. Moreover, we calculated time constant from product of discharging current. We observed the short time constant at the MIM capacitor with thinner film. Those results may suggest relation of CeOx layer thickness and ionic conductivity in the CeOx layer and concluded that thinner films are suitable for large capacitance density capacitors. Fig. 2 showed the frequency-dependent capacitance under 0V. Frequency dispersion can be observed at 0.1 to 104 Hz. This behavior suggests the response of oxygen ions the CeOx layer. The obtained capacitance values are larger than the calculated value of 0.14 μF/cm2 by the dielectric constants of CeOx and SiO2. Conclusion We characterized thickness effects on charge and discharge properties of the CeOx MIM capacitors for realization of large capacitance density capacitors. Both MIM capacitors with a difference thickness layer (10nm and 20nm) showed transient responses and the frequency-dependent capacitance of the MIM capacitor with a 10nm-thickness CeOx layer showed frequency dispersion. Those results suggested ionic conductivity in the CeOx layer. Moreover, larger charging/discharge current and shorter time constant were observed at the MIM capacitor with a thinner film. This result suggests relation of CeOx layer thickness and ionic conductivity in the CeOx layer and concluded that thinner films are suitable for large capacitance density capacitors.