Venkatasubramani Balu

(Ba,Sr)TiO3 thin films with conducting perovskite electrodes for dynamic random access memory applications

Interfaces and hence electrodes determine the performance of (Ba,Sr)TiO3 (BST) capacitors for ultralarge scale integration dynamic random access memories. Electrode materials forming a rectifying contact on BST drastically reduce the dielectric constant and hence the capacitance and charge storage density of the capacitor, when the dielectric thickness is reduced. This can limit the role of Pt as an electrode material for gigabit dynamic random access memories (DRAM). The conducting oxide, La0.5Sr0.5CoO3 (LSCO) with its perovskite structure, has structural and chemical compatibility with BST. Our results in LSCO/BST/LSCO capacitor show that the mechanism of conduction is not interface limited but predominantly bulk limited. A 75 nm BST film with LSCO electrodes shows a leakage current density of 1×10−7 A/cm2 at 1 V, 85 °C. The dielectric constant at 1 V, 105 Hz is 350, making LSCO a potential contact electrode for DRAM memories.

Ir-electroded BST thin film capacitors for 1 giga-bit DRAM application

(Ba,Sr)TiO/sub 3/ (BST) thin film capacitors using iridium (Ir) as an electrode material are investigated for high density (1 Gbit-scale and beyond) dynamic random access memories (DRAMs). Excellent electrical characteristics (e.g. high polarization and low leakage) of BST capacitors with Ir top electrodes were obtained. The excellent resistance of these capacitors to hydrogen damage during forming gas anneals is also reported.

Effects of Ir electrodes on barium strontium titanate thin-film capacitors for high-density memory application

Excellent electrical characteristics of RF-sputtered Barium Strontium Titanate (BST) thin-film capacitors with iridium (Ir) electrodes were obtained and the influence of Ir on device properties was investigated. In contrast to conventional Pt-electroded system, BST capacitors with Ir electrodes exhibit higher polarization and slightly higher leakage current. The stronger crystallinity of a thin BST layer (/spl sim/70 /spl Aring/) initially grown on Ir substrate is believed to be the cause for higher charge storage density of the Ir-electroded capacitors. However, this higher polarization is accompanied by higher dielectric dispersion (3.12% per decade for Ir versus 1.98% for Pt electrodes). On the other hand, leakage current appears to be dominated by the Schottky barrier formed by Ir-BST and Pt-BST contacts, respectively, at high field. The analysis from temperature-dependent J-V data indicates a lower barrier height for the Ir-BST contact than Pt-BST contact. The slightly higher leakage current density of the BST capacitors with Ir electrodes can thus be attributed to the lower barrier height.

Effect of niobium doping on the microstructure and electrical properties of strontium titanate thin films for semiconductor memory application

Niobium doped strontium titanate [Sr(Ti1−xNbx)O3] thin films (≅40 nm) were deposited on Ir substrates using rf magnetron sputtering. The effect of Nb content (x=0, 0.001, 0.01, and 0.05) on the microstructure, dielectric constant, dielectric dispersion, and leakage current was studied. It was found that with increasing Nb content the dielectric constant decreased, probably owing to an observed decrease in grain size. The dielectric dispersion of all the Nb-doped ST films was lower than that of undoped ST film deposited at the same temperature and pressure. For the case of x=0.01, dispersion as low as 0.425% per decade was observed. The leakage current was found to increase slightly for x=0.001 and x=0.01, and drastically for the x=0.05 case.

The effects of forming gas anneal on the electrical characteristics of ir-electroded BST thin film capacitors

Abstract The effect of various temperature nitrogen anneals prior to top electrode deposition on the ability of Ba0.7Sr0.3TiO3 (BST) thin-film capacitors with both Ir and Pt top electrodes to withstand hydrogen damage was investigated. Experimental results show that samples that underwent a 750 °C N2 pre-top electrode anneal exhibited the lowest leakage current density at positive bias for both Ir- and Pt-electroded devices after forming gas anneal. It was also found that DRAM polarization values decreased slightly after forming gas anneal. Also, a post-top electrode deposition 550°C O2 anneal improved both electrical characteristics (lowered leakage and increased DRAM polarization) of these devices. Complete recovery of the leakage level prior to hydrogen damage was obtained after a 550°C N2 recovery anneal for some devices independent of the pre-top electrode anneal. Ir- and Pt-electroded BST (40nm) capacitors have been shown to meet the 1 giga-bit DRAM leakage current requirement of 10−8 A/cm2 at 1.7 V...

Stability of reactive DC-sputtered Ir and IrO2 thin films in various ambients

Abstract Ir and IrO2 are potential electrode materials for ferroelectric thin film capacitors. However, some process related issues need to be considered before integrating ferroelectric capacitors into memory cells. This paper presents the effects of post-deposition annealing on the characteristics of DC-magnetron sputtered Ir and IrO2 thin films. Film properties such as composition, resistivity, crystallinity, adhesion, and microstructure were examined before and after annealing.

The niobium doping effects on resistance degradation of strontium titanate thin film capacitors

The rate of resistance degradation of thin (<450 A) niobium-doped strontium titanate polycrystalline films with platinum top electrodes and iridium bottom electrodes was investigated as a function of direct current (dc) voltages, temperature, Nb atomic fractions [Sr(Ti1−xNbx)O3+y, x=0, 0.001, 0.01, and 0.05, respectively], and capacitor areas (from 2.50×10−5 to 2.91×10−3 cm2). It was found that by increasing the amount of niobium, the resistance degradation rates were greatly reduced, but the leakage currents increased. Also, the degradation rates seemed fairly independent of the areas of the devices.

A new electrode technology for high-density nonvolatile ferroelectric (SrBi/sub 2/Ta/sub 2/O/sub 9/) memories

New electrode materials (Ir and IrO/sub 2/) are proposed for high-density nonvolatile ferroelectric random access memories (NVFERAMs). These electrodes are used in order to integrate ferroelectric (SrBi/sub 2/Ta/sub 2/O/sub 9/) capacitors with standard CMOS technology. Excellent electrical characteristics (e.g. low leakage and high polarization), good fatigue resistance and good mechanical properties (i.e. excellent adhesion to SiO/sub 2/ without using a glue layer) were obtained for the new capacitor structures.

Interaction of Ir and IrO2 thin films with polysilicon, W and WSIx

Abstract Ir and IrO2 thin films have been identified as potential electrode materials for ferroelectric capacitors. These electrodes have shown excellent electrical characteristics. The integration of ferroelectric capacitors into memory cell requires the bottom electrode material to be placed directly over a contact plug. This paper studies the interaction of Ir and IrO2 with commonly used plug materials such as polysilicon, tungsten (W), and tungsten silicide (WSix) after a post-deposition annealing at 800°C. Film properties such as composition, resistivity, crystallinity, adhesion, and micro-structure have been examined before and after anneal. The results show that W is a possible plug material for Ir electrode; while polysilicon and WSix are potential candidates if IrO2 electrodes are used.

Electrode Materials for Ferroelectric Capacitors: Properties of Reactive DC Sputtered IrO 2 Thin Films

Due to its low resistivity and excellent thermal stability, IrO 2 has attracted attention as an alternative for electrode material in ferroelectric integrated circuit applications. In this work, IrO 2 films deposited using reactive DC magnetron sputtering were studied. Film properties such as resistivity, crystallinity and morphology were examined as a function of deposition condition. Optimum process parameters to obtain high quality IrO 2 thin films are suggested.