Larry D. McMillan

Negative differential resistivity in ferroelectric thin-film current-voltage relationships

Abstract The current-voltage relationship I(V) for several different ferroelectric thin films often exhibits an apparent negative differential resistivity. In the present paper we show that in lead zirconate-titanate (PZT) this is a spurious artifact arising from rapid (parametric) measurement techniques and that it disappears for d.c. measurements made with measuring times ≥300 s. However, in other ferroelectric materials (selected samples with unusually high trap densities) it appears to be a genuine effect characterized by an I = aV3 current-voltage relationship at high current densities (≥100 nA/cm2), I(V) hysteresis above a “pseudo-breakdown voltage,” and distinct V 2-to-V 3 cross-over threshold.

TdI10: Ferroelectric thin films in integrated microelectronic devices

Abstract We present results on four integrated ferroelectric projects: 1) An integrated barium strontium titanate thin-film on GaAs MMIC (monolithic microwave integrated circuit) with dielectric constant of 450 and loss tangent of 10−3 at 2.2 GHz operation, done as a Symetrix/Matsushita Electronics Corp. joint development; 2) A low-density (1 Kb) ferroelectric RAM (random access memory) done with PZT (lead zirconate titanate) and other ferroelectrics on CMOS (complementary metal oxide semiconductor) Si, carried out as a joint development with Olympus Optical Co.; 3) A liquid-source CVD (chemical vapor deposition) machine and its deposition of strontium titanate and barium strontium titanate films of exceptionally low d.c. leakage current (1 nA/cm2 at 120 nm thickness and 3V operation) for DRAM (dynamic RAM) applications; and 4) performance parameters of a properietary material for RAM application which is totally fatigue-free up to at least 5 × 1011 cycles.

D.C. leakage currents in ferroelectric memories

Abstract Experimental data on d.c. leakage currents in lead zirconate titanate thin films (210 nm) are presented. The data show a flat, ohmic response up to a thresh-hold of approximately 2.0 V, above which they satisfy a modified Childls Law, with a perfectly quadratic dependence: I = aV2. This suggests that at 5 V operation the film behaves as a fully depleted device. The difficulty in distinguishing between Frenkel-Poole, Schottky emission, and true space-charge limited currents is briefly discussed.

Switching kinetics of lead zirconate titanate sub-micron thin-film memories

Abstract We have determined the coercive voltage Ec as a function of thickness d for PZT ferroelectric memories. For thick films (≫ 1μm), Ec ∼ d−1/3 , in accord with the theory of Kay and Dunn [Phil Mag. 7, 2027 (1962)]; for submicron films, Ec ∼ d−4/3 , in accord with Neels theory for coercive magnetic field Hc in thin magnetic films [J. Phys Radium 17, 250 (1956)]. The switching times and shape i(t) of the displacement current transient were measured vs. temperature and field. They satisfy the theory of Ishibashi and Takagi [J. Phys. Soc. Jpn. 31, 506 (1971)] and show that domain nucleation is approximately two dimensional. The times τ are very fast (≪100ns at 1.0V drive) and depend upon field E and reduced temperature t = (Tc - T)/TC according to a scaled relationship {Orihara and Ishibashi [Jpn. J. Appl. Phys. 24, 902 (1985)]} where x is given by t/E and α is an activation field equivalent to 120 kV/cm at 300K.

Second generation liquid source misted chemical deposition (LSMCD) technology for ferroelectric thin films

Abstract This paper discusses the second generation liquid source misted chemical deposition (LSMCD) technology currently being developed for ferroelectric thin film deposition. A SubMicron Systems (SMS) Model Primaxx-2F cluster tool was used to deposit the films. The developments that have been made since the first generation machine are discussed. The process chamber schematics along with the characteristics of the aerosol generator are explained in detail. The electrical properties obtained from the films deposited by the tool are found to be similar to that of spin-on films. The step coverage obtained on patterned wafers are also shown. The LSMCD technique combines the advantages of spin-on such as simplicity, good stoichiometry control and superior electrical properties with the advantages of CVD such as superior step coverage, manufacturability, etc.

Ferroelectric memory applications

Recent applications of ferroelectric thin-film memory devices are discussed. A brief description of processing alternatives and device physics is followed by some prototype architecture for Si MOSFET and GaAs JFET structures. Specific applications include radiation-hardened satellite memories, automobile nonvolatile dashboard memories, electronic camera memories, and smart credit cards and inventory devices. Shadow-RAMs, SRAMs, and DRAMs are described. Replacement of NMOS, core, EEPROM (electrically erasable programmable read-only memory), magnetic bubble memory, plated wire, and CMOS with battery backup seems possible within a few years.<<ETX>>

Conformal LSMCD deposition of SrBi2(Ta1-xNbx)2O9

Abstract This paper presents new results of Liquid Source Misted Chemical Deposition (LSMCD) of SrBi2(Ta1-xNbx)2O9 thin films showing good step coverage of 150 nm thick films into square openings approximately 1 μm deep by 0.6 μm wide. A SAMCO model HDF-6000 LSMCD machine was used for the ferroelectric deposition. Prior to the deposition of the ferroelectric film, a Pt bottom electrode of approximately 200 nm thickness was sputtered into an initial opening 1μm deep by 1μm wide. The LSMCD and new chemistry methodologies by which these results were obtained will be described in this paper. The electrical characteristics of the LSMCD films will also be reported. LSMCD films resulted in higher switched charge (2Pr) than the spin-on films. The coercive fields (2Ec) and maximum leakage currents were comparable for the two deposition methods. These results show that conformal LSMCD thin films can be used for sub-micron circuits containing ferroelectric memory.

Anomalous switching kinetics in ferroelectric thin (≤ 200 nm) films

Abstract It has been established empirically for several years that in several different ferroelectric materials (e.g., PZT, KNO3) the coercive field dependence upon thickness d becomes anomalously large below d = 200 nm, changing abruptly from the exp(−1/3) Kay-Dunn dependence to an unexplained exp(−4/3) dependence. Reasons for this are discussed.

Anomalous fatigue behavior in Zn doped PZT

Abstract Doping lead zirconate titanate (PZT) has often been examined in order to investigate changes in the electrical behavior of thin film ferroelectric capacitors.1 In this study, PZT has been doped with Hf, Nb, and Zn. After reviewing the results of the initial studies, Zn doping of PZT was further investigated. Physical and electrical studies of Zn doped PZT have been conducted. These studies include x-ray diffraction analysis, hysteresis, pulsed switching, fatigue and retention measurements. Endurance measurements indicate an anomalous fatigue behavior of some Zn doped PZT thin film capacitors.

Advanced simulation tool for FeRAM design

Abstract A ferroelectric capacitor model was derived, and a ferroelectric device library was implemented into SPICE (both PSPICE and HSPICE) simulation tool. With this SPICE model, 1T-1C/2T-2C, or any other ferroelectric circuit, such as FeFET, chain cell, link cell can be simulated accurately and in real-time. Simulations of hysteresis loops, TVS, switching characteristics are shown in this paper.