Imprint Failure Research Articles

Hydrogen-Induced Degradation in Ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 and PbZr 0.4 Ti 0.6 O 3

Effects of forming gas annealing (FGA) on ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 (BLT) and PbZr 0.4 Ti 0.6 O 3 (PZT) capacitors were investigated as a function of FGA temperature. For low temperature FGA below 350°C, BLT capacitors showed a quite high resistance against hydrogen-induced damages compared to PZT capacitor. For poled capacitors before FGA, PZT capacitors showed a severe imprint failure while BLT capacitors did not show significant change. The difference in degradation at the low temperature FGA seemed to be due to the degree of polarization pinning. On the other hand, for high temperature FGA, both BLT and PZT capacitors showed severe degradation behaviors. Changes in the shape of hysteresis loops, as the degradation proceeded, were thought to reveal quite different degradation mechanisms.

Hydrogen-Induced Degradation in Ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 and PbZr 0.4 Ti 0.6 O 3

Effects of forming gas annealing (FGA) on ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 (BLT) and PbZr 0.4 Ti 0.6 O 3 (PZT) capacitors were investigated as a function of FGA temperature. For low temperature FGA below 350°C, BLT capacitors showed a quite high resistance against hydrogen-induced damages compared to PZT capacitor. For poled capacitors before FGA, PZT capacitors showed a severe imprint failure while BLT capacitors did not show significant change. The difference in degradation at the low temperature FGA seemed to be due to the degree of polarization pinning. On the other hand, for high temperature FGA, both BLT and PZT capacitors showed severe degradation behaviors. Changes in the shape of hysteresis loops, as the degradation proceeded, were thought to reveal quite different degradation mechanisms.

Fatigue-free samarium-modified bismuth titanate (Bi4−xSmxTi3O12) film capacitors having large spontaneous polarizations

Fatigue-free and highly c-axis oriented Bi3.15Sm0.85Ti3O12 (BSmT) thin films were grown on Pt/TiO2/SiO2/Si(100) substrates using the method of metalorganic sol decomposition. The BSmT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2Pr) and the nonvolatile charge as compared to those of the Bi4−xLaxTi3O12 (x=0.75) film capacitor, recently known as the most promising candidate for nonvolatile memories. The 2Pr value of the BSmT capacitor was 49 μC/cm2 at an applied voltage of 10 V while the net nonvolatile switching charge was as high as 20 μC/cm2 and remained essentially constant up to 4.5×1010 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge–retention characteristics with its sensing margin of 17 μC/cm2 and a strong resistance against the imprinting failure.

The reliability of FeRAMS with 2T/2C or 1T/1C type cell structure

Ferroelectric Random Access Memory (FeRAM) provides some advantages such as non-volatility, high endurance on write/read cycles, high radiation hardness and low power consumption compared to conventional memories. However, in order to realize a commercial FeRAM, it is important to overcome the limitation in various reliability items such as retention, imprint, and fatigue failure at high operation temperature. In this paper, the retention characteristic of FeRAM related with imprint degradation of ferroelectric capacitors at high temperature is evaluated on the level of 8” wafer. Both cell configurations of 2T/2C and 1T/1C are discussed in terms of imprint reliability. General reliability items of the FeRAM after TSOP-I type package, such as fatigue endurance, early failure rate (EFR), humidity acceleration stress test (HAST), and temperature cycle (TC), are also evaluated.

Voltage shift of hysteresis loops of SrBi 2 Ta 2 O 9 thin films under unipolar stress

Voltage shifts of hysteresis loops of metalorganic decomposition (MOD)-derived SrBi2Ta2O9 (SBT) thin films, known as imprint, have been observed after exposing the thin-film capacitors to unipolar pulses. The voltage shift changes with cumulative total time at maximum voltage, following a relationship with no pulse-width dependence. The origin of the voltage shift is briefly discussed in terms of an internal bias field induced by injected electrons trapped at positive polarity. The pulse-measurement responses are greatly affected by the internal bias field, even though no imprint failure was observed up to 1010 unipolar pulses. The voltage shift and asymmetric properties can be removed easily by applying bipolar pulses of saturation amplitude.

Effect of oxygen stoichiometry on the ferroelectric property of epitaxial all-oxide La0.7Sr0.3MnO3/Pb(Zr0.52Ti0.48)O3/La0.7Sr0.3MnO3 thin-film capacitors

Epitaxial all-oxide ferroelectric thin-film capacitors, La0.7Sr0.3MnO3/Pb(Zr0.52Ti0.48)O3/La0.7Sr0.3MnO3 (LSMO/PZT/LSMO), have been grown on LaAlO3(001) substrates by pulsed-laser deposition. By changing the deposition oxygen pressure, we varied the oxygen content in the oxide electrodes and ferroelectric film, respectively, and studied their effect on switching and fatigue behavior of the epitaxial capacitors. The capacitors were also cooled at various oxygen pressures in the range of 5×10−6–10 Torr to examine their tendency to process-induced imprint. It is found that all the capacitors show almost square hysteresis loops with remnant polarization of 35–40 μC/cm2 and a coercive field of 40–50 kV/cm, irrespective of the oxygen concentration in the ferroelectric and electrode films. Contrary to the capacitors using La0.5Sr0.5CoO3 as electrodes, when cooled at reduced oxygen pressures, the LSMO/PZT/LSMO capacitors show a strong resistance to the process-induced imprint failure, which is ascribed to the high thermal stability of the LSMO films. On the other hand, the oxygen content in the electrodes is shown to be an important factor that controls the fatigue behavior of the epitaxial all-oxide ferroelectric capacitors.

Lifetime estimation due to imprint failure in ferroelectric SrBi2Ta2O9 thin films

Two different failure modes for a ferroelectric memory cell caused by imprint, the read failure due to the loss of polarization, and the write failure due to the shift of the hysteresis loop are investigated. The quasistatic hysteresis loop allows us to distinguish which failure mode is dominating in a ferroelectric random access memory application and, hence, it can also be used as a powerful tool for lifetime estimation of ferroelectric thin films limited by imprint failure under operating conditions. The experimental results show that the write failure is only decisive for very low voltage operation (Vp<1.25 V), whereas for the Pt/SrBi2Ta2O9/Pt under investigation the read failure is the dominant failure mode for operating voltages exceeding 1.25 V.

Asymmetric polarization of defect dipoles inBi4Ti3O12andBi2Ti4O11ceramics withNa+andLa3+modifications

Asymmetric polarization of defect dipoles around phase transitions develops the resistivity hysteretic behavior and imprint failure of ${\mathrm{Bi}}_{4}{\mathrm{Ti}}_{3}{\mathrm{O}}_{12}$ and ${\mathrm{Bi}}_{2}{\mathrm{Ti}}_{4}{\mathrm{O}}_{11}$ ceramics under various electric fields. The polarization was determined qualitatively by $P\ensuremath{-}E$ hysteresis loop measurements along with the detection of current transient with time. A reduction in defect-dipole polarization occurs at high-field strength due to the defect-dipole decomposition and/or oxygen vacancy migration.

Influences on imprint failure of SrBi2Ta2O9 thin film capacitors

The imprint behavior of chemical solution deposited SrBi2Ta2O9(SBT) thin films has been investigated. The imprint properties were evaluated by hysteresis loop and pulse polarization measurements. Several parametric variables such as voltage, temperature, thickness and electrode stack structure were used to investigate possible imprint mechanisms. The domain configuration prior to imprinting appears to be an influential factor. If a film is not saturated, imprint rates are more rapid and the voltage shift is accompanied by polarization loss.

Asymmetric properties of Pb(Zr,Ti)O3 thin film capacitors with conducting oxides

Asymmetric properties of Pb(Zr,Ti)O 3 (PZT) thin films has been studied in (La,Sr)CoO 3 /Pb(Zr, Ti)O 3 /(La, Sr)CoO 3 (LSCO) heterostructures in which the conducting oxide (La,Sr)CoO 3 (LSCO) and/or LaCoO 3 (LCO) have been used as an electrode. LCO/PZT/LSCO showed a largely asymmetric P-V hysteresis loop and large relaxation of the remanent polarization at the negatively poled state, eventually leading to an imprint failure. Corresponding to P-V hysteresis loop, leakage current behavior and capacitance - voltage characteristic of the LCO/PZT/LSCO were asymmetric. Post annealing treatment at reducing atmosphere further increased the relaxation of the unstable remanenet polarization.

Built-in voltages and asymmetric polarization switching in Pb(Zr,Ti)O3 thin film capacitors

Asymmetric polarization switching of Pb(Zr,Ti)O3 (PZT) thin films with different electrode configuration has been studied in (La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 (LSCO) heterostructures in which the conducting oxide (La,Sr)CoO3 and/or LaCoO3 (LCO) have been used as an electrode. Polarization-voltage (P-V) hysteresis loop of LSCO/PZT/LSCO was symmetric. However, LCO/PZT/LSCO showed a largely asymmetric P-V hysteresis loop and large relaxation of the remanent polarization at the negatively poled state, eventually leading to an imprint failure. On the other hand, LSCO/PZT/LCO exhibited large relaxation of the positively poled state. The asymmetric behavior of the polarized states implies the presence of an internal electric field inside the PZT layer. It is suggested that the internal electric field is caused by built-in voltages at LCO/PZT and LSCO/PZT interfaces. The built-in voltages at LCO/PZT and LSCO/PZT interfaces were 0.6 V and 0.12 V, respectively.

The effects of ferroelectric capacitor testing methods on predicted imprint failure points

The imprint mechanism in ferroelectric capacitors is a well known cause for ferroelectric memory cell failure. Various test methodologies for accelerating the imprint effect for failure point due to imprint predictions have been proposed. These methods include testing the sample at elevated temperature, biasing the sample at elevated temperature, and fatigue cycling the sample prior to and during imprint testing. A parametric study was conducted using various test conditions to determine which conditions or combination of conditions had the greatest effect on the measured imprint rate. Data will be presented to show the relationships between the test method used and the measured imprint rate. A figure of merit which can be used for comparisons of the imprint rate of multiple test samples as well as failure point prediction will also be presented.

Effects of interface charges on imprint of epitaxial Bi4Ti3O12 thin films

Using La0.5Sr0.5CoO3 (LSCO) or Pt film as a bottom electrode layer, epitaxial Bi4Ti3O12 (BTO) thin films were grown on MgO(001) substrates by pulsed laser deposition. A symmetric Pt/BTO/Pt capacitor structure shows a surprisingly large asymmetric polarization switching behavior, but a Pt/BTO/LSCO structure has a nearly symmetric P–V hysteresis. The strong asymmetric behavior in the Pt/BTO/Pt was attributed to positive charges resulting from interdiffusion at the bottom BTO/Pt interface. P–V hysteresis studies using numerous top electrode materials and Auger electron spectroscopy depth profile measurement also support formation of interfacial charges. Imprint pulse test shows that such an imprint failure cannot be recovered by applying a dc bias field.

Effects of Interfacial States on Asymmetric Polarization Switchings of Epitaxial Bi4Ti3012 Thin Films

ABSTRACTEpitaxial Bi4Ti3012 (BTO) films with Lao0.5Sr0.5Co03 (LSCO) or Pt bottom electrodes were grown on MgO(OOl) substrates by pulsed laser deposition. Surprisingly, a symmetric Pt/BTO/Pt capacitor showed a highly asymmetric polarization switching and an asymmetric Pt/BTO/LSCO capacitor revealed a nearly symmetric polarization switching. To understand these intriguing phenomena, Auger electron spectroscopy and x-ray photoemission spectroscopy depth-profiles were used. The evidences for interdiffusions at the bottom BTO/Pt interface were found. To get further understanding on the interfacial states, a capacitance-voltage (C-V) measurement was performed on the Pt/BTO/Pt capacitor. By fitting the C-V data with a back-to-back Schottky diode model, built-in voltages at the top and the bottom interfaces were determined to be 1.1 V and 3.2 V, respectively. From the obtained built-in voltages, an asymmetric band diagram for the Pt/BTO/Pt structure was suggested. Therefore, the imprint failure can be explained by existence of asymmetric interracial states.

Imprint of (Pb,La)(Zr,Ti)O3 thin films with various crystalline qualities

(Pb,La)(Zr,Ti)O3 (PLZT) thin films with various crystalline qualities, i.e., epitaxial, oriented, and polycrystalline, have been grow. These PLZT thin films cooled at various oxygen cooling pressures were found to have different asymmetric switching and imprint behaviors, depending on the crystalline quality of the PLZT thin films. It was found that the initial polarization state of as-grown PLZT thin films is an important indicator to determine further imprint behavior. Epitaxial PLZT thin-film capacitors were prepolarized in an as-grown state and further exhibited the large dependence of asymmetric switching and imprint behavior on the oxygen cooling pressure. On the other hand, oriented or polycrystalline PLZT capacitors exhibited very weak polarization and little dependence of the initial prepolarization state on the oxygen cooling pressure. Consequently, the PLZT capacitors containing polycrystalline nature had a strong resistance to the imprint failure.

Imprint testing of ferroelectric capacitors used for non-volatile memories

Abstract A set of criteria is presented, based on the polarization versus electric field hysteresis characteristics, which can be used to reveal a tendency towards imprint failure in ferroelectric capacitors. A series of pulsed voltage waveforms, generated by the RT66A ferroelectric tester, are utilized to validate our established criteria for the tendency towards imprinting. The tests have been performed on hetero-structure Pb(ZrxTi1−x)O3(PZT)-based capacitors produced by a pulsed laser ablation depositon technique (PLAD). The loss of retained charge in the ferroelectric capacitors is considered in light of the imprint test methodology. Rates of retention loss and imprint are extracted from the experimental data and treated as separate processes that lead to device failure. The rate of approach towards imprint failure is found to be influenced by the magnitude of the read pulse.