K. R. Bellur

Influence of platinum interlayers on the electrical properties of RuO2/Pb(Zr0.53Ti0.47)O3/RuO2 capacitor heterostructures

Lead zirconate titanate, Pb(ZrxTi1−x)O3 or PZT, thin films grown on RuO2 electrodes by the sol‐gel process have excellent resistance to polarization fatigue, but they generally have two drawbacks. The films have high leakage currents and large property variation. In this letter we show that the use of a thin Pt layer (100 A) deposited on the bottom RuO2 electrode to fabricate RuO2/PZT/Pt/RuO2/(100)MgO capacitors has two important effects. It reduces capacitor leakage by two to four orders of magnitude and it significantly reduces the large property variation. In addition, these capacitors retain their excellent resistance to polarization fatigue which is characteristic of the RuO2/PZT/RuO2 heterostructure.

Contribution of electrodes and microstructures to the electrical properties of Pb(Zr0.53Ti0.47)O3 thin film capacitors

Pb(Zr 0.53 Ti 0.47 )O 3 (PZT) thin film capacitors have been fabricated with four electrode combinations: Pt /PZT/ Pt /SiO 2 Si, RuO 2 /PZT/ Pt /SiO 2 /Si, RuO 2 /PZT/ RuO 2 /SiO 2 /Si, and Pt /PZT/ RuO 2 /SiO 2 /Si. It is shown that polarization fatigue is determined largely by the electrode type (Pt vs RuO 2 ), and microstructure has only a second-order effect on fatigue. If either the top or bottom electrode is platinum, significant polarization fatigue occurs. Fatigue-free capacitors are obtained only when both electrodes are RuO 2 . In contrast, the bottom electrode is found to have a major effect on the leakage characteristics of the PZT capacitors, presumably via microstructural modifications. Capacitors with bottom RuO 2 electrodes show high leakage currents ( J = 10 −3 -10 −5 A/cm 2 at 1 V) irrespective of the top electrode material. Capacitors with Pt bottom electrodes have much lower leakage currents ( J = 10 −8 A/cm 2 at 1 V) irrespective of the top electrode material. At low voltage, the I-V curves show ohmic behavior and negligible polarity dependence for all capacitor types. At higher voltages, the leakage current is probably Schottky emission controlled for the capacitors with Pt bottom electrodes.

Low parasitic resistance contacts for scaled ULSI devices

Abstract Analysis of the components of parasitic series resistance in ULSI devices shows that interfacial contact resistivities less than 10 −7 Ω cm 2 will be required for sub 100-nm ULSI devices in order to stay on the historical performance trend. With dimensional scaling, the series resistance–width product decreases because channel lengths are scaled, while it increases in contacts because the contact length is decreased. Unless the contact resistivity is also reduced, the contact resistance ultimately becomes higher than the channel resistance, and no performance advantage will be obtained by making the device smaller. The challenge in meeting the contacting requirements in the 1997 National Technology Roadmap for Semiconductors is especially difficult in light of the desire to simultaneously contact both n + and p + junctions with a single material and given the trend towards lower processing temperatures, in which the equilibrium dopant electrical activity is lower. Several techniques, such as dielectric capping during junction annealing, are effective in reducing contact resistivity by maximizing interfacial dopant concentrations and minimizing contact barrier heights. Higher saturated drive currents, due to lowered parasitic series resistance, are observed in deep submicron devices made using silicides as diffusion sources (SADS); this technique eliminates the interfacial dopant segregation that is associated with conventional silicidation. The use of elevated source drains (ESD) also allows the use of thicker silicides while minimizing the consumption-induced increase in contact resistivity that normally accompanies silicidation; as a result, ESD devices give higher drive currents. The recrystallization of amorphous layers has been observed to result in non-equilibrium dopant activation which can be many times the equilibrium value. Finally, the use of heterojunction contacts using Si–Ge in the context of elevated source/drain devices presents another way to achieve lower contact resistance.

Phase evolution and annealing effects on the electrical properties of Pb(Zr0.53Ti0.47)O3 thin films with RuO2 electrodes

Abstract The electrical properties and crystallization of Pb(Zr 0.53 Ti 0.47 )O 3 (PZT) thin films grown on RuO 2 electrodes by the sol-gel process have been studied. It was found that the amorphous as-deposited thin film first transforms to a pyrochlore phase at 500 °C. On further annealing, perovskite PZT begins to crystallize at about 600 °C. TEM anlysis reveals that a pyrochlore-type second phase still exists in the films even after annealing to temperatures of 750 °C for 10 min. These PZT films are fatigue-free, but they show large property variation and high leakage currents ( J = 10 −3 Acm −2 at 1 V). An 800 °C annealing treatment, for 10 min in air of the RuO 2 bottom electrode prior to film deposition enhanced perovskite PZT nucleation, thereby eliminating the pyrochlore-type second phase. In addition, the leakage currents of PZT films grown on annealed RuO 2 electrodes are about two orders of magnitude lower than those of PZT films grown on unannealed RuO 2 . It is also observed that annealing the entire capacitor stack after the top electrode deposition improved capacitor properties.

Electrical transport and dielectric breakdown in Pb(Zr,Ti)O3 thin films

Abstract The I-V-t characteristics of Pb (Zr0.53Ti0.47)O3 thin films have been studied. We show that there are three regions of importance in the current-time curves. At short times, there is a transitory region, where current decreases with time, and which may last for >103 seconds in the case of high quality samples. This is followed by the region of steady state leakage, which is best described by a Schottky-type model. However, the bottom electrode (especially oxides) can modify the microstructure significantly, leading to grain boundary conduction paths. The third region is resistance degradation and/or time dependent dielectric breakdown.

Effect of composition and annealing conditions on the electrical properties of Pb(ZrxTi1−x)O3 thin films deposited by the sol-gel process

Abstract The effects of annealing temperature, Zr/Ti ratio and film lead content on the dielectric and ferroelectric properties of Pb(Zr x Ti 1− x )O 3 Pt-based thin film capacitors (Pt/PZT/Pt/Ti/SiO 2 /Si) were investigated. The PZT films were grown using a spin-on sol-gel process. It was found that a minimum annealing temperature of 650°C is required to obtain good ferroelectric properties. The polarization fatigue rate was found to increase with increasing annealing temperature and decreasing Zr content. However, as the number of switching cycles increased passed 10 7 -10 8 cycles, the polarization values for the various Zr/Ti ratios and the various annealing temperatures became essentially equal. Changing the lead content of the PZT thin films had a smaller effect on their fatigue and ferroelectric properties than did the Zr/Ti ratio and annealing temperature. The small signal dielectric constant showed a peak near the morphotropic phase boundary with a maximum value of about 600. The remanent and saturation polarization were largest near the morphotropic phase boundary and for the Ti-rich composition.

Leakage and interface engineering in titanate thin films for non-volatile ferroelectric memory and ulsi drams

Abstract The leakage behavior of Pb(Zr,Ti)O3 PZT and (Ba,Sr)TiO3 (BST) thin films has been studied. The leakage behavior is dependent upon the bottom electrode. PZT films on RuO2 bottom electrodes display a large leakage, predominantly ohmic in behavior, which we have shown to be PZT microstructure-controlled. The leakage of PZT and BST films on Pt display Schottky emission characteristics which is controlled by the film/electrode interface. In the case of PZT films, we have shown that several methods can be utilized to successfully lower the RuO2/PZT/RuO2 system leakage, while retaining the long term performance. These methods include pre-annealing of RuO2 bottom electrode prior to PZT film deposition; addition of buffer layer between RuO2 and PZT film; and PZT film growth via in-situ ion beam sputter-deposition.

Electrical characterization of sol-gel derived PZT thin films

Thin films of lead zirconate titanate (PZT) were deposited on a variety of substrates using the sol-gel process. These included Pt/Ti/SiO/sub 2//Si, RuO/sub 2//SiO/sub 2//Si, RuO/sub 2//MgO, Pt/MgO, and Pt/Ti/MgO. Epitaxial, oriented, and polycrystalline films were obtained after annealing at 700 degrees C for 10 min. Polycrystalline PZT films with RuO/sub 2/ as top and bottom electrodes showed superior hysteresis and fatigue behavior as compared to such films on Pt. Epitaxial films on Pt/MgO and Pt/Ti/MgO displayed excellent fatigue behavior and large remnant polarization as compared to their polycrystalline counterparts on Pt/Ti/SiO/sub 2//Si. The results suggest that different fatigue mechanisms may be simultaneously operating.<<ETX>>

Fatigue and retention of Pb(Zr0.53Ti0.47)O3 thin film capacitors with Pt and RuO2 electrodes

Abstract Pb(Zr0.53Ti0.47)O3 (PZT) thin films were deposited on Pt and RuO2 coated Si and MgO substrates using the sol-gel process. Fatigue and retention tests were performed on these samples. The films grown on RuO2 electrodes are fatigue-free up to nearly 1011 cycles. Their retention life-time extrapolates to more than 1010 seconds. The fatigue behavior of films grown on Pt electrodes depends on the PZT film orientation. Highly oriented (001) PZT films maintain 50% of their initial P∗r-P⁁r value after 1011 cycles. The randomly oriented films maintain less than 3% of the initial P∗r-P⁁r value after 1011 cycles. However, the retention life-time of both highly oriented and randomly oriented PZT films grown on Pt electrodes extrapolates to higher than 1011 seconds. It appears that fatigue of films grown on RuO2 is mainly controlled by the film/electrode interface. On the other hand, fatigue of films grown on Pt appears to depend on both the film/electrode interface as well as on bulk effects.

DC leakage and failure of PZT thin film capacitors for non-volatile ferroelectric memory and dram applications

Abstract DC leakge and dielectric breakdown of Pb(ZrxTi1−x)O3 (PZT) thin films have been studied. Three regions in the time spectrum of the DC leakage current are discussed. It is emphasized that it is important to measure the time dependence of current before doing current versus voltage (I–V) measurements. Resistivity degradation is observed after a DC voltage is applied for a certain length of time. It is found that the resistivity degradation is dependent on voltage polarity and humidity of the atmosphere. It is pointed out that the time dependent dielectric breakdown (TDDB) must be distinguished from the phenomenon of resistivity degradation. It is found that breakdown in PZT thin films is defect related. The difficulty of confirming whether the breakdown mechanism is electrical (i.e., intrinsic) or thermal is discussed.