The effects of electrodes on the deposition characteristics and electrical properties of lead zirconate titanate (PZT) films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) are studied. Electrical properties measured in this study are mainly fatigue degradation (decrease in polarization values with polarization reversals) and leakage current for the application to FRAM devices. In this study, $Pt/RuO_2$ hybrid electrode made by coating Pt on $RuO_2$ substrates and Ir metal electrode are intensively investigated.
Stoichiometric PZT films with pure perovskite phase can be obtained in the wider range of deposition conditions such as $Pb(DPM)_2$ flow rate and deposition temperature on $Pt/RuO_2$ hybrid electrodes than on $RuO_2$ electrodes. This means that the Pt template layer of the $Pt/RuO_2$ hybrid electrode is thought to play a decisive role in promoting the nucleation of PZT perovskite phases and making PZT films stoichiometric despite of its small thickness of 10 nm. The PZT capacitors fabricated on $Pt/RuO_2$ hybrid electrodes show lower leakage current and superior polarization characteristics because Pt template layer prevents $RuO_2$ substrate from reacting with PZT films and suppresses the formation of microcrystalline second phases in the interior of PZT films. The high leakage current density of the capacitors using the $RuO_2$ top electrode is associated with the formation of conductive phases at the interface between the $RuO_2$ top electrode and the PZT film during the post-RTA process. The PZT capacitor with the $Pt/RuO_2$ hybrid bottom electrode is not fatigue-free but shows improved fatigue characteristics compared to that on Pt electrode. This indicates that the bottom $RuO_2$ layer of the $Pt/RuO_2$ hybrid electrode plays a role as a sink for oxygen vacancies through the 10-nm-thick Pt template layer. The introduction of $PbTiO_3$ buffer layer prior to the PZT film deposition on $Pt/RuO_2$ improves the fatigue characteristics by suppressing the formation of non-stoichiometric interfacial layer. Electrical properties of the PZT capacitors with three kinds of hybrid electrode configuration ($RuO_2$/PZT/$Pt/RuO_2$, $RuO_2$/Pt/PZT/$RuO_2$, and $RuO_2$/Pt/PZT/$Pt/RuO_2$) are studied. Of them, $RuO_2$/Pt/PZT/$Pt/RuO_2$ capacitor with symmetric electrode configuration has the best fatigue endurance of only 9.0% decrease in $P^*$-P^ after $4×10^9$ cycles and the lowest leakage current of $5.8×10^{-7}$ A/㎠ under an electric field of 200 kV/cm. Accordingly, this electrode structure is thought to be promising for a capacitor of FRAM devices.
Deposition and electrical characteristics of $PbTiO_3$ and PZT film prepared on various Ir-based electrodes (Ir, $IrO_2$, and $Pt/IrO_2$) are investigated. On Ir substrates, $PbTiO_3$ and PZT films with pure perovskite phases can be obtained in very wide processing window of $Pb(DPM)_2$ or ZrTB flow rates. On the contrary, on $IrO_2$ electrodes, PZT films with pure perovskite phases cannot be obtained due to the preferential formation of lead oxide phases. The deposition characteristics of PZT films are strongly dependent on the behavior of lead oxide molecules on substrate materials.
The electrical properties of PZT capacitors prepared on Ir electrodes depend on top electrode materials. Ir, $IrO_2$, Pt and Ru are used as top electrodes. The PZT capacitors prepared on Ir substrates show the similar leakage current level of about $10^{-5}$ A/㎠ under 200 kV/cm for the use of Pt, Ir, and $IrO_2$ as top electrodes, whereas very high leakage current of about 1 A/㎠ under same electric field for Ru top electrode. This result is ascibed to the good diffusion barrier property and low reactivity with PZT films of Ir-based top electrode. Fatigue degradation of the PZT capacitors prepared on Ir substrates is investigated. The decrease in $P^*$-P^ of PZT capacitors for Pt, Ir, and $IrO_2$ top electrodes after $4×10^9$ fatigue cycles are 13%, 28%, and 79%, respectively. These results mean that oxygen vacancies play a decisive roles in fatigue degradations as reported by various research groups. Better fatigue endurance of the capacitor using Ir top electrode than that using Pt top electrode is attributed to ultra-thin $IrO_2$ layer formed at the interface between Ir substrate and PZT film. Fatigued PZT capacitor can recover its initial polarization values by the removal of accumulated oxygen vacancies through annealing in $O_2$-contating enviroment. Considering the fatigue characterization by unipolar pulses and fatigue-induced recovery from plasma damage, the fatigue degradation of CVD PZT films is thought to be initiated by injected charges (mainly electrons) toward PZT films from electrodes. The unipolar pulse does not degrade the $P^*$-P^ value, indicating that domain switching process is necessary for the polarization degradation. Fatigue degradation progresses faster at higher temperature because of the increased ionic mobility that is related to the accumulation of oxygen vacancies toward the PZT/electrode interfaces. Fatigue rate is also increased by decreasing the pulse frequency (lower pulse frequency means longer duration time that is applying time of DC field in one cycle), indicating that DC component as well as cumulative switching number can contribute the fatigue phenomenon.
