June-mo Koo

Improvement of retention loss in Pb(Zr,Ti)O3 capacitors using Ir∕SrRuO3 top electrodes

We investigated the retention behavior of Pb(Zr,Ti)O3 (PZT) capacitors with Ir∕SrRuO3 (SRO) top electrodes. The capacitors with Pt and Ir∕IrO2 top electrodes were also prepared for comparison. The opposite state retention characteristic of the PZT capacitor was significantly improved by using an Ir∕SRO top electrode structure. The nonvolatile polarization of the opposite state retention was kept at 96% of its initial value even after a 100h baking test at 150°C, while those of Pt and Ir∕IrO2 were 36 and 59%, respectively. The retention enhancement is attributed to the effective inhibition of defect generation on the interface between PZT and SRO.

Fabrication of 3D trench PZT capacitors for 256Mbit FRAM device application

We fabricated trench PbZrxTi1-xO3 (PZT) capacitors that can be used in 256Mbit 1T-1C FRAM devices. The capacitor has 0.25mum diameter and 0.4mum depth. Three layers, Ir(20nm)/PZT(60nm)/Ir(20nm), were deposited in SiO2 trench holes by ALD and MOCVD. Both columnar and granular grains were formed on the sidewalls of the trench capacitors, and their relative portion had strong size dependence. The trench capacitors with more columnar PZT grains showed good switching behavior under 2.1V external bias and 19 to 24 muC/cm2 remnant polarization

Auger spectroscopy and properties of Ir(Pt)/PZT(PZT/PT)/Ir nanosized thin-film structures

Studies of the elemental distribution profiles through the thickness of thin-film ferroelectric capacitor structures using Auger spectroscopy made it possible to establish the relation between the elemental and phase composition of the structures, on the one hand, and their electrophysical properties, on the other. Special features were observed in the behavior of the lead titanate underlayer both in the as-fabricated and aged structures. It is shown that the variation in the characteristics of the capacitor structures during aging is caused by diffusion of the elements at the interfaces and in the PZT film against a background of a significant increase in the oxygen concentration. As a result, interface-modifying oxide layers form and the trap density on the upper and lower interfaces decreases in all samples.

Vertical Structure NAND flash array integration with paired FinFET multi-bit scheme for high-density NAND flash memory application

Multi-bit vertical structure NAND (VsNAND) flash memories with 32-paired FinFET cell string have been successfully integrated for the first time. Its array integration issues regarding the sub-10 nm vertical structure fin could be solved by proper choices of isolation material, ion implantation, and word line patterning. VsNAND flash array cells with TANOS (TaN/Al2O3/SiN/SiOx/Si) charge trap structure show possibilities of acceptable program/erase properties and cell Vth distribution characteristics for multi-level NAND flash application.

Role of IrO2 Electrode in Reducing the Retention Loss of Ir/IrO2/Pb(Zr,Ti)O3/Ir Capacitors

Oxide electrodes are known to significantly improve reliability problems, such as fatigue and retention, in Pb(Zr, Ti)O3(PZT)-based ferroelectric capacitors. To understand the roles of the oxide electrodes on the opposite-state retention, we investigated the isotope tracer experiments and hysteresis measurements on Ir/IrO2/PZT/Ir and Ir/PZT/Ir capacitors before and after the retention tests. The depth profile of isotopic 18O in the Ir/IrO2/PZT/Ir capacitor, measured by the second ion mass spectroscopy, shows little changes in the 18O and 16O distributions at the IrO2/PZT interface. In addition, the hysteresis measurements showed that the internal field created by the retention tests should be nearly the same for both capacitors, indicating that the IrO2 layer should not play an important role in compensating the interface defect charges (possibly, the oxygen vacancies).

Trap Charge Density at Interfaces of MOCVD Pt(Ir)/PZT/Ir(Ti/SiO 2 /Si) Structures

A method providing estimation of the trap density at metal/ferroelectric interfaces of a depleted ferroelectric film located between back-to-back Schottky barriers has been developed. The method is based on the recharge of interface traps induced by external bias pulse applied to the metal/ferroelectric/metal structure. It is shown that the transient current under bias pulse can be controlled by the traps recharge on the reverse-biased interface. Using the method, the trap charge density on interfaces of MOCVD Pt/PZT/Ir(Ti/SiO 2 /Si) and Ir/PZT/Ir(Ti/SiO 2 /Si) capacitors were found from transient current measurements.

The Investigation of Ferroelectric Domain Behavior Affected by Thin Metallic Electrode

The physical and chemical properties of metallic electrodes on Pb(Zr,Ti)O3 (PZT) are investigated by atomic force microscopy. Pt, a catalyst, tends to change itself into a dielectric oxide. Such a passive layer should cause a depolarization field driving the domain structure into the polystate. Therefore, the effective thickness of the Pt electrode becomes smaller than that of Ir electrode. Thus, we observed an opposite domain contrast around a region covered by thin Pt on PZT, which implied the failure of the screening of the depolarizing field. This multidomain configuration can cause a suppression of the remnant polarization of PZT capacitors. Also, Ir is better than Pt in regard to film smoothness, indicating that the adhesion property of Ir on PZT is superior to that of Pt on PZT. We prove it using height–height correlation function. This study is significant on the scaling issues of ferroelectric capacitors, such as the thickness limit of the electrode and the requirement for high surface smoothness of ferroelectric films.

Novel Ir–Ti Alloy Electrodes for High-Density Ferroelectric Memory Applications

An Ir-based Ir–Ti alloy was improved by controlling the interface layer between Pb(Zr,Ti)O3 (PZT) and an electrode for the high-density ferroelectric memory application. Compared with the Ir electrode, the Ir–Ti alloy electrode was resistant to oxygen annealing and the surface did not roughen much. The increase in resistivity after the annealing was also sufficiently low to maintain its feasibility as an electrode. Furthermore, Ti in the Ir–Ti alloy seemed to assist the growth of (111) grains of PZT, thus resulting in a larger 2Pr. The Ir–Ti alloy layer also acted as a seed layer for PZT. This means that we can skip the deposition of the seed layer for the purpose of good quality PZT. Furthermore, the Ir–Ti alloy is still applicable to very thin films. In ultra thin films of PZT and a bottom electrode, IrTiOx (20 nm)/IrTi (10 nm) is a good choice as the bottom electrode to conserve 2Pr as well as protect the reliability of thin PZT. This is a very different result from IrOx. It is believed that these results could provide a possibility for commercializing embedded or stand-alone FeRAM in high-density ferroelectric memory applications.

Effect of Ultra Thin Bottom Electrode on the Ferroelectric Properties of Pb(Zr,Ti)O3 Films Grown by Metal-Organic Chemical Vapor Deposition

ABSTRACT In order to fabricate highly integrated ferroelectric random access memory (FeRAM), three-dimensional (3D) trench structure should be employed. A capacitor stack, two electrodes and ferroelectric layer, fills the shallow trench. Thus the thickness of each layer is critical to realize 3D structure. It has been well known that that the degradation of ferroelectric layer when it is scaled down is very serious problem. In order to minimize such scaling effect of PZT, the thickness of electrode must be as thin as possible. In this study, we investigated the properties of Pb(Zr,Ti)O3 (PZT) deposited by metal organic chemical vapor deposition (MOCVD) on very thin Ir bottom electrode (40 nm). We found that the Ti film used as glue layer has serious effect on the leakage property of PZT thin film. The leakage problem was reduced by lowering the post annealing temperature.ABSTRACT In order to fabricate highly integrated ferroelectric random access memory (FeRAM), three-dimensional (3D) trench structure should be employed. A capacitor stack, two electrodes and ferroelectric layer, fills the shallow trench. Thus the thickness of each layer is critical to realize 3D structure. It has been well known that that the degradation of ferroelectric layer when it is scaled down is very serious problem. In order to minimize such scaling effect of PZT, the thickness of electrode must be as thin as possible. In this study, we investigated the properties of Pb(Zr,Ti)O3 (PZT) deposited by metal organic chemical vapor deposition (MOCVD) on very thin Ir bottom electrode (40 nm). We found that the Ti film used as glue layer has serious effect on the leakage property of PZT thin film. The leakage problem was reduced by lowering the post annealing temperature.

Formation of stoichiometric SrRuO3 electrodes for PZT capacitors by pulsed-MOCVD

SrRuO 3 films were deposited at 300-500°C on Pb(Zr 0.4 , Ti 0.6 )O 3 -covered Ir/SiO 2 /Si substrates by metallorganic chemical vapor deposition (MOCVD). Through the pulsed introduction of a source gas mixture (pulsed-MOCVD), the Ru/(Ru + Sr) ratio became unity above 350°C due to a similar deposition rate for SrO and RuO x components in the film. It became unity at 450°C in spite of the increased input-gas flow rate of the Ru source under the fixed Sr source and we obtained films consisting of crystalline stoichiometric SrRuO 3 . This demonstrated that good crystalline stoichiometric SrRuO 3 film was reproducible by pulsed-MOCVD. Excellent fatigue properties and ferroelectricity were obtained from the SRO/PZT/Ir capacitor.