C. Sudhama

University of Texas at Austin, Texas City, TX, United States

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Publications (19)19.84 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of stress parameters (e,g,, stress magnitude, frequency, and pulse shape) on the loss of DRAM polarization during unipolar pulse stressing in sol-gel P(L)ZT thin film capacitors have been studied. The results indicate that there is a strong correlation between the fatigue rate and the DRAM polarization for the fresh device. It has been found that contrary to what one might expect, the fatigue rate does not increase indefinitely with increasing stress voltage but saturates at the same voltage at which the polarization-voltage loop saturates. Though the effect of the rise time of the stress pulses on the fatigue rate is negligible, the fatigue rate is strongly dependent on the pulse width of the trapezoidal stress pulses.< >
    IEEE Electron Device Letters 05/1995; · 2.79 Impact Factor
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    ABSTRACT: The effects of the nonlinearity and polarization relaxation of storage capacitor on DRAM R/W operations are explained using a simple model and PSPICE simulation. For a given line-voltage design and pass device, the voltage signal and the READ/WRITE times depend on the exact shape of the Q-V curve and on the amount of polarization relaxation. It has been found that typical high dielectric constant paraelectric materials provide smaller voltage signals than the linear capacitor, and exhibit slower READ but faster WRITE compared to the linear capacitor.
    Integrated Ferroelectrics 02/1995; 7(1-4):319-328. · 0.38 Impact Factor
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    ABSTRACT: The exposure of PLZT thin films to post deposition processes may have a detrimental effect on the properties of the films. Hence for the proper integration of these films into CMOS technology, these effects need to be studied and improved upon. Fresh devices have been exposed to x-rays, plasma and forming gas anneals. Each of these processes is seen to cause a significant change in the DRAM polarization, leakage current and the reliability properties. An annealing technique has been proposed for a complete recovery in these properties
    Electron Devices Meeting, 1994. IEDM '94. Technical Digest., International; 01/1995
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    ABSTRACT: In this paper, we present electrical and material properties of thin films (100 to 400 nm) of magnetron-sputtered ferroelectric PZT for memory applications. The optimal lead-compensation power (and the resulting film composition) is independent of film-thickness. Reduction of film-thickness leads to a reduction in the crystallization temperature (from 700°C for 400 nm films to 575°C for 100 nm films), and yields evidence for a two-step growth of perovskite rosettes. An optimized 100 nm film yields 12 μC/cm2 for 1.5V operation and fatigues by 25% after 1010 unipolar stress cycles.
    Journal of Electronic Materials 11/1994; 23(12):1261-1268. · 1.64 Impact Factor
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    ABSTRACT: Very low leakage current density (5×10<sup>-7</sup> A/cm<sup>2</sup> even at 125°C) and high charge storage density (100 fC/μm<sup>2</sup>) under V<sub>DD</sub>/2=1 V conditions have been achieved using 5% La doped PZT films for gigabit DRAM capacitor dielectrics. In addition, the fatigue and TDDB measurements indicate good reliability of this capacitor
    VLSI Technology, 1994. Digest of Technical Papers. 1994 Symposium on; 07/1994
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    ABSTRACT: A new technique has been developed for the characterization of dielectric voltage- and charge-decay times (T/sub V/ and T/sub Q/) in sol-gel derived ferroelectric films under open-circuit conditions. Voltage decay potentially poses a limitation on the maximum allowable refresh-time in the DRAM capacitor. This direct measurement of T/sub V/ is preferred to its conventional estimation based on the current-voltage relationship, because of the implicit inclusion of nonlinearities and stress-induced leakage-reduction effects. Undoped and lanthanum-doped lead-zirconate-titanate (PZT) films have also been characterized for frequency-dispersion in large-signal polarization, bit "0" relaxation and a new bit "1" relaxation mechanism using techniques developed recently. It is proposed that any evaluation of high-permittivity dielectrics for DRAM applications must include these measurements.< >
    Reliability Physics Symposium, 1994. 32nd Annual Proceedings., IEEE International; 05/1994
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    ABSTRACT: Important aspects of nonlinear storage capacitor switching and their impact on DRAM READ/WRITE operations are explained using a simple model and PSpice simulation. The voltage signal and charge-transfer rate are found to be dependent not only on the total charged stored, but also on the exact shape of the storage capacitor Q-V curve. Typical paraelectric capacitors are shown to deliver a smaller voltage signal than a linear capacitor that has the same stored charge at the operating voltage. Further, typical paraelectric capacitors have slower READ but faster WRITE compared to the linear capacitor.< >
    IEEE Electron Device Letters 05/1994; · 2.79 Impact Factor
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    ABSTRACT: Time‐zero current‐voltage characteristics and time‐dependent current behavior of metal‐ferroelectric‐metal (Pt‐PZT‐Pt) capacitor structures have been studied. Under constant‐voltage stressing, the current density through the 1500‐Å‐thick lead‐zirconate‐titanate (PZT) film exhibits a power‐law dependence on time, with the exponent (∼0.33) independent of temperature and voltage. Electrode material dependence of current density indicates that the conventional model of trap‐limited single‐carrier injection over nonblocking contacts is inadequate to explain the time‐zero current. A change in top electrode material from Pt to In leads to the observation of work‐function‐driven Schottky contacts between the metal and ferroelectric. The current‐voltage characteristics fit a two‐carrier injection metal‐semiconductor‐metal model incorporating blocking contacts, with distinct low‐ and high‐current regimes (PZT is assumed to be p‐type and trap‐free in this model). Temperature‐dependent I‐V measurements indicate a Pt‐PZT barrier height of 0.6 eV and an acceptor doping level of ∼10<sup>18</sup> cm<sup>-3</sup> in PZT. The implications of this model on the optimization of ferroelectric capacitors for dynamic random access memory applications are discussed.
    Journal of Applied Physics 02/1994; · 2.21 Impact Factor
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    ABSTRACT: The effects of deposition temperature on the properties of thin films of sputtered lead-zirconate-titanate (PZT) have been studied for ULSI DRAM storage capacitor dielectric applications. The films were deposited by reactive dc magnetron sputtering from a multi-component target. The grain size for the films deposited at 400°C was found to be less than 1000 Å, while it was ∼ 10–30 μm for films deposited at 200°C. Small grain-sized material is desirable since it leads to better cell-to-cell uniformity in terms of charge storage capacity and other electrical and reliability properties. The optimum lead compensation was found to increase as the deposition temperature (Tdep) increased. Leakage current density stays fairly constant as Tdep is varied. As-deposited films, with a deposition temperature of 500°C, were rich in the perovskite phase and showed a high charge storage density of 11.2 μC/cm and a low leakage current density of 5.1 × 10 A/cm (both at 1.5 V). This implies the possibility of eliminating the high temperature crystallization-annealing step.
    Integrated Ferroelectrics - INTEGR FERROELECTRICS. 01/1994; 5(3):169-176.
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    ABSTRACT: Thin films of lead–zirconate–titanate [(PZT) Pb(Zr 0.5 Ti 0.5 )O 3 ] possess demonstrably adequate charge storage densities and endurance to read/write cycling for ultra‐large‐scale integration dynamic random access memory (DRAM) applications. Lanthanum (donor) doping is expected to reduce the (p‐type) conductivity to acceptable levels (≪10<sup>-6</sup> A/cm<sup>2</sup>). In this study, six thin films of 200 nm sol‐gel derived lanthanum‐doped PZT, with the [La]/([La]+[Pb]) concentration ratio varying from 0 to 0.23, have been examined for electrical and reliability properties. The difference between the maximum polarization attained (P max , bit ‘‘1’’) and remanent polarization (P r , bit ‘‘0’’) is denoted as Q c <sup>’</sup>, and is the relevant charge storage density in the DRAM cell. Q c <sup>’</sup> is seen to decrease with an increase in La concentration, contrary to predictions based on an assumption of stoichiometric compositions. A 5% La content results in more than a decade drop in leakage current density (J L ) in comparison with undoped PZT, and after refresh annealing (at 450 °C for 1/2 h in O 2 ), the film exhibits a Q c <sup>’</sup> value of 7.2 μC/cm<sup>2</sup> at 3 V, which is believed to satisfy the 64 Mb DRAM requirements. J L increases almost exponentially with the La concentration beyond 5%, indicating that the increasing volatility of Pb in the oxygen‐rich ambient overwhelms the effect of the enhanced La doping concentration, probably due to a departure from the equilibrium composition Pb (1-1.5x) La x V x/2 (Zr 0.5 Ti 0.5 )O 3 , (where V represents a Pb vacancy). Constant voltage stressing indicates an operating li- - fetime (at room temperature) of about ten years (at 3 V) for the 5/50/50 film. Good fatigue properties are also observed.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1993; · 1.36 Impact Factor
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    ABSTRACT: A low thermal budget (with 550°C annealing) process with Ti-compensation for sputtered ferroelectric PZT thin films has been developed. PZT films with a composition near the morphotrophic phase boundary (Zr/Ti = 53/47) annealed at 550°C for 1 hr in a N2 ambient exhibits high charge storage density and low leakage current density, which are the important requirements of dielectric materials for ULSI DRAM cells. It was also found that Ti compensated films show good fatigue endurance in comparison with non-Ti compensated films.
    Integrated Ferroelectrics - INTEGR FERROELECTRICS. 01/1993; 3(2):113-120.
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    ABSTRACT: The fabrication of high dielectric constant lead zirconate titanate thin films for dynamic random access memory application involves post‐deposition annealing to convert the low dielectric constant pyrochlore phase to the desired high dielectric constant perovskite phase. The results of our experiments detailing the dielectric characteristics such as degradation field (the electric field at which a sudden increase in leakage takes place), leakage current density, time‐dependent dielectric breakdown characteristics, charge storage density, coercive field, and charge storage density are reported. The charge storage density was measured using both double‐bipolar pulses and quasistatic capacitance‐voltage techniques. The results of these two measurement techniques are discussed for oxygen and nitrogen annealed films. Additionally, a comparative study of the phase transformation kinetics when the post‐deposition annealing is carried out in a nonoxidizing ambient such as nitrogen, as against an oxidizing ambient (pure oxygen), is detailed.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 08/1992; · 1.43 Impact Factor
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    ABSTRACT: Thin film capacitors of Lead Zirconate Titanate (PZT, 400 nm) of Zr/Ti ratio 65/35, deposited by reactive dc-magnetron sputtering, with low leakage current and high charge storge density(’Q c ) for use as capacitor dielectrics in ultra-large scale integration dynamic random access memory (ULSI DRAM) cells have been fabricated and studied. The equivalent SiO2 thickness for the optimized film is 5.3å for the fresh film and 9.1å after 1010 unipolar stress cycles (0 to -3 V). The leakage current density is 1.32 × 10-7A/cm2 for 3 V operation which is equivalent to an effective SiO2 field of 55 MV/ cm. X-ray diffraction analysis reveals that as-deposited films (T dep . = 200‡ C) contain no detectable perovskite phase. Post-deposition annealing is therefore essential, and critical to the fabrication of high quality capacitors for memory applications. The pyrochlore-to-perovskite phase transformation, the evolution of the microstructure, composition and the presence of different phases in the film have been studied as a function of annealing conditions. There is a purely outward radial growth of the interphase boundary, resulting in the increase in the curved surface area of the cylindrical perovskite aggregates throughout the film thickness with increasing thermal budget. The increase in perovskite phase content with annealing time at a constant annealing temperature indicates that a diffusional phase transformation from the pyrochlore to cubic perovskite phase above the Curie temperature occurs as a first step in the formation of the ferroelectric perovskite phase. The variation of two important dielectric properties, charge storage density and leakage current density is reported as a function of the annealing time and temperature. Furthermore, the variation of the charge storage density due to unipolar dynamic electrical stress is studied. The total area under the large-frequency C-V curve (which is the total reversible polarization) increases under unipolar dynamic stress (0 to -3 V) after 1010 stress cycles. The degradation in charge storage density is found to be primarily due to an increase in remanent polarization caused by the shift in the hysteresis loop as a result of the reduction in the internal bias field under the influence of the unipolar dynamic stress.
    Journal of Electronic Materials 04/1992; 21(5):503-512. · 1.64 Impact Factor
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    ABSTRACT: Lead zirconate titanate [PZT, Pb(Zr <sub>x</sub> Ti <sub>1-x</sub>) O <sub>3</sub>, x=0.65, 400  nm ] thin-film capacitors with very low leakage current and large stored charge densities (Q′<sub>c</sub>=P<sub> max </sub>–P<sub>r</sub>) for ultralarge scale integration dynamic random access memory (ULSI DRAM) memory applications have been fabricated by reactive dc magnetron sputtering followed by an annealing step. In this letter, we present a comparative study of the significant influence of the use of an oxidizing ( O <sub>2</sub>) as against a nonoxidizing annealing ambient ( N <sub>2</sub>) on the evolution of the perovskite phase microstructure and the electrical properties of PZT capacitors. The phase transformation kinetics are faster in a N <sub>2</sub> annealing ambient at low temperatures, yielding capacitors with a large Q′<sub>c</sub> at low annealing thermal budgets. However, N <sub>2</sub> annealed films exhibit a lower breakdown field and time to breakdown which is in agreement with a degradation model based on the field-induced concentration polarization of oxygen vacancies.
    Applied Physics Letters 12/1991; · 3.79 Impact Factor
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    ABSTRACT: The electrical and reliability characteristics of ferroelectric capacitors fabricated using sol-gel derived 50/50 lead-zirconate-titanate (PZT) thin films have been examined for ULSI DRAM (dynamic random access memory) applications. Various electrode materials, film thicknesses (200 nm to 600 nm) and capacitor areas were used. A large stored-energy density (Q/sub c/) of 15 mu C/cm/sup 2/ (at 125 kV/cm) was measured using different methods. The results indicate that PZT thin films exhibit material properties which might satisfy the requirements of ULSI DRAMs.< >
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 12/1991; · 1.82 Impact Factor
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    ABSTRACT: The Pt-Lead Zirconate Titanate (PZT) thin film interface plays a key role in determining the electrical properties and phase transformation kinetics of Pt-PZT-Pt thin film capacitor structures. The results of the electrical and material properties of PZT (65/35) films that vary in thickness between 500 Å to 4000 Å deposited by DC-magnetron sputtering under identical deposition conditions, and subjected to the same post-deposition thermal processing conditions shows that the Pt-PZT interface dominates thin film properties at low thicknesses (≦ 2000 Å). The charge storage density (Qc') and maximum polarization (Pmax) shows an anomalous variation with thickness, showing an initial increase followed by a drastic decrease as the film thickness is scaled down to 500Å. Significant interdiffusion at the PZT film-Pt electrode retards the pyrochlore-to-perovskite phase transformation nucleation and growth rate in PZT films of thickness 2000Å and lower. Gate polarity dependence of the time-tobreakdown and the degradation field is observed for all PZT film thicknesses (including 4000Å). This indicates that the ferroelectric film-electrode interface has an important role to play in determining the electrical reliability properties even in the 4000Å thick PZT film, although Qc' and Pmax are not adversely affected at these thicknesses.
    MRS Proceedings. 12/1990; 243.
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    ABSTRACT: The electrical and reliability characteristics of lead-zirconate-titanate (PZT) ferroelectric capacitors are presented. Thin Pb(Zr<sub>0.5</sub>Ti<sub>0.5</sub>)O<sub>3</sub> films of 350 nm were prepared using a solution-gelatin technique. Hysteresis loop and pulse measurement methods indicate that PZT films exhibit a large charge storage density of 7.3 μC/cm<sup>2</sup> and an equivalent dielectric constant of ~577. In addition, good endurance behavior (no fatigue or aging wearout after >10<sup>13</sup> cycles under accelerated unipolar stressing), reasonable transient characteristics (with estimated write time of ~0.1 ns and refresh time of ~2.5 s), and high effective breakdown field suggest that the PZT ferroelectric material has good potential for applications in advanced DRAMs (dynamic RAMs)
    Electron Devices Meeting, 1989. IEDM '89. Technical Digest., International; 01/1990
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    ABSTRACT: This paper investigates the issues in the scaling of thin film PZT (Lead-Zirconate-Titanate) capacitors for DRAM (Dynamic Random Access Memories) applications. The test structures used were MIM (metal-insulator-metal) capacitors with platinum electrodes and PZT deposited using a sol-gel process. Charge storage density (Q'c), leakage current density (JL), unipolar switching time to 10% decay (ts), time dependent dielectric breakdown (TDDB) and electrical fatigue have been analyzed. Unipolar switching time has been modeled as an RC time constant, where C is electric-field dependent. Q'c at a given electric field appears to remain constant over the range of film thicknesses and electrode areas studied. Leakage current density and time-to-breakdown (tBD) for a given electric field degrade with decreasing film thickness. Unipolar stressing causes considerably less fatigue than bipolar stressing, and after 2 × 1011 cycles, a 400nm film still exhibits sufficient Q'c for DRAM operation.
    MRS Proceedings. 12/1989; 200.
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    ABSTRACT: Oxidation in a circular trench is of relevance in several applications, including trench capacitors in memories. In this work a model for stress-dependent oxidation is presented. The model is based on modifications of the Deal-Grove model, which is used conventionally for planar oxidation. It is seen that even when stress-effects are ignored, the circular trench geometry leads to a drop in the oxidation rate, in comparison to planar oxidation. The oxidation rate drops even further as the initial diameter of the trench is reduced. When the stress due to volume-expansion is included, with silicon dioxide considered as an elastic material, an additional drop is seen in the oxidation rate. Calibration of the model to measured data involves only a few parameters. Experimental data are presented that qualitatively demonstrate some predictions of the theory.

Publication Stats

128 Citations
19.84 Total Impact Points

Institutions

  • 1989–1995
    • University of Texas at Austin
      • • Center for Microelectronics Research
      • • Department of Electrical & Computer Engineering
      Texas City, TX, United States