S.J. Gillespie's research while affiliated with Texas Research Institute Austin, Inc and other places

The correlation between the dielectric constant and dispersion is investigated for barium strontium titanate (BST) capacitors with platinum and iridium electrodes. For platinum electrode capacitors, dispersion decreases with increasing dielectric constant. In contrast, capacitors with iridium electrodes exhibit the reverse correlation. A simple model is proposed to provide a quantitative explanation for the observed correlation for both platinum and iridium electrodes. In addition, the dependence of the dielectric constant and dispersion on varying ratios of (Ba+Sr)/Ti is also reported.

Due to its low resistivity and excellent thermal stability, IrO2 has attracted attention as an alternative for electrode material in ferroelectric integrated circuit applications. In this work, IrO2 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 IrO2 thin films are suggested.

Highlights and solutions to some of the challenges involved in integrating SrBi 2 Ta 2 O 9 (SBT) capacitors with Pt electrodes on silicon wafers for non-volatile memory applications are discussed. These include the diffusion of Bi through the Pt bottom electrode during firing, capacitor patterning, and process damage that results from hydrogen containing atmospheres. Next, studies of the temperature dependence of many of the important electrical properties of SBT are presented. These include the remanent polarization (2P r ), the non-volatile polarization (P nv ), and the coercive field (E c ) all of which are studied as functions of the pulse amplitude; fatigue resistance of 2P r and P nv ; the retention; the small signal capacitance versus voltage behavior; and the current versus voltage behavior. These studies demonstrate that SBT looks very promising for ferroelectric non-volatile memories over the consumer application range (0 to 70 °C).

The dielectric constant and dispersion of sputtered barium strontium titanate (BST) thin films deposited on Ir electrodes have been measured as a function of frequency and dielectric film thickness. Based on the measured variation in capacitance density with BST film thickness, an interfacial capacitance and thin film capacitance have been extracted. The variation of the interfacial capacitance density and the thin film capacitance density with frequency indicates that the majority of dispersion measured for BST deposited on Ir electrodes is due to the interfacial capacitance, in contrast to results found for Pt electrodes [1]. The temperature dependence of the interfacial capacitance and thin film capacitance has also been measured for these electrodes.

Experimental and modeling results for resistance degradation in thin Ba0.5Sr0.5TiO3 (BST) film capacitors with platinum (Pt) electrodes are reported. The main experimental results are as follows. Under a constant applied voltage, the current density is observed to increase with time until it reaches a maximum value. Once the maximum value is reached, the current density becomes constant with time. The barrier height at the BST/Pt (cathode) interface is observed to decrease after prolonged electrical stressing. The resistance degradation effect is observed to be reversible, particularly at elevated temperatures. Based on the experimental results, a quantitative model for resistance degradation is proposed. In this model, the increase in the current density is attributed to a decrease in the barrier height at the cathode and this decrease is assumed to have a stretched exponential dependence on time. Using experimentally determined parameters, the model calculates the current density as a function of time at various temperatures. The calculated results are verified and the model is shown to be self-consistent. Hence the model provides an accelerated method for determining the lifetime of thin BST films at the operating conditions for advanced memory applications. © 1999 American Institute of Physics.

In the literature, the Schottky emission equation is widely used to describe the conduction mechanism in perovskite-type titanate thin films. Though the equation provides a good fit to the leakage current data, the extracted values of the Richardson and dielectric constants are inconsistent with their experimental values. In this work, a modified Schottky equation is applied. This equation resolves the difficulties associated with the standard Schottky equation. Also, the electronic mobility in thin films of barium strontium titanate is reported. © 1998 American Institute of Physics.

The current density is measured as a function of time for thin (⩽1000 Å) barium strontium titanate (BST) capacitors with platinum electrodes. The current density curve shows a peak prior to the onset of resistance degradation. The peak position on the time axis varies with applied voltage and temperature. The data are explained by the theory for space-charge-limited (SCL) current transients, and the measured current is identified as ionic current associated with oxygen vacancies. Using the SCL analysis, the mobility of the oxygen vacancies is measured as a function of temperature. The mobility obtained from current measurements is shown to be compatible with the Einstein relation for mobility and diffusivity. In summary, the ionic current associated with oxygen vacancies is shown to be an important component of the measured current in thin BST films. © 1998 American Institute of Physics.

High dielectric constant SrTiO₃ films (ε=237 or C=70 fF/μm² measured at 100 kHz) have been fabricated and the dynamic stressing characteristics of these dielectrics has been studied for the first time. Time-dependent dielectric breakdown (TDDB) of SrTiO₃ is shown to be strongly dependent on frequency and duty cycle under dynamic stressing. On the other hand, dielectric dispersion does not exhibit significant dependence on pulse width or on-time of the stressing signal

Long recognized as the best potential solution to the continued scaling of the onetransistor/one-capacitor standalone dynamic random access memory (DRAM) beyond a gigabit, barium strontium titanate (BST) and other high permittivity dielectrics are fast becoming enablers to embedding large amounts of memory into a high performance logic process. System requirements such as granularity, bandwidth, fill frequency, and power pose major challenges to the use of high density standalone DRAM, leading to the current push for embedded solutions where very wide buses are possible. As a result, projected embedded memory sizes are rapidly approaching that of the standalone products, and with the high wafer cost of the combined logic plus memory process, bit cell scaling is critical. The BST memory cell, with its low thermal budget processing, very high charge storage density, and high conductivity metal electrodes has the potential to be efficiently embedded with traditional logic flows if the materials and integration challenges of the required three dimensional (3D) bit cell capacitors can be solved. BST materials properties such as dielectric relaxation, interface capacitance, and resistance degradation and their impact on capacitor scaling will be reviewed along with the electrode materials issues associated with certain 3D capacitor designs. The scaling limits of BST bit cells in the deep sub-micron regime will be discussed.

The effect of hydrogen on strontium bismuth tantalate (SrBi2Ta2O9; SBT) ferroelectric capacitors is investigated. Using several analytical techniques such as x-ray diffraction, electron diffraction, Auger electron, scanning and transmission electron microscopies, the structural and compositional changes in the ferroelectric film are studied as a function of annealing gas and temperature. The mechanism for hydrogen induced damage to the capacitor is identified. Measurements show that the hydrogen induces both structural and compositional changes in the ferroelectric film. Hydrogen reacts with the bismuth oxide to form bismuth and the reduced bismuth diffuses out of the SBT film causing the electrodes to peel.