Formation of ferroelectric thin films on ruthenium electrodes

Abstract

Sr_0.8Bi_2.4Ta₂O₉ (SBT) and Bi_3.25+xLa_0.75Ti₃O₁₂ (BLT) films were prepared on Ru electrodes by a sol-gel spin-coating method. It was found in the case of SBT/Ru that the oxidation of Ru was suppressed by annealing at temperatures lower than 650°C in O₂ atmosphere or by annealing lower than 700°C in N₂. However, the remanent polarization values of the films were as low as 2.3 μC/cm2 (2Pr) for the former case and 4.4 μC/cm2 for the latter case. On the other hand, the BLT/Ru samples crystallized at 650°C showed good crystallinity without oxidation of the Ru electrode. It was found that the ferroelectric properties depended on the amount of excess-Bi sensitively, and the film prepared under the optimum Bi composition showed an excellent P-V hysteresis loop with 2 Pr of 25 μC/cm2.     

Supression of size effects in ferroelectric films

Abstract

It is widely reported that the dielectric permittivity of ferroelectric films decreases with decreasing film thickness, and understanding and controlling these size effects are very important for charge storage application of these films. By combining phenomenological theory with careful experimental work, we have shown that the form of the boundary condition for the polarization plays a decisive role in the manifestation of size effects in ferroelectric films. We have taken two extreme boundary conditions to prove our point. For the case normal electrodes, it is assumed that the boundary condition for the component of polarization vector at the ferroelectric/electrode interface is P = 0. This case corresponds to the presence of a strong edge field, resulting in “freezing out” of the ferroelectric polarization at the interface and thus exhibiting severe size effects. However, if one utilizes conductive oxide electrodes that are ferroelectric in nature the polarization would not vanish at the ferroelectric/electrode interface and therefore the size effects are largely suppressed. To prove our point and to eliminate grain size, stress, and compositional effects, epitaxial SrTiO₃ thin films with stoichiometric composition on SrTiO₃ single crystal substrates were investigated. In fact, the experimental data also indicates that the use of ferroelectric electrodes indeed suppresses the size effects.     

Mocvd process model for deposition of complex oxide ferroelectric thin films

Abstract

Thin complex oxide films, such as ferroelectrics, pyroelectrics, waveguides, superconductors, MEMS/MOEMS and piezoelectrics are experiencing rapid growth in a wide variety of commercial markets. In particular, ferroelectrics as used in IC cards, embedded memories with micro-controllers, stand-alone memories, and other ASIC applications, require deposition by a technique that is compatible with deep sub-micron advanced geometry IC integration. Metal Organic Chemical Vapor Deposition (MOCVD) is the deposition method of choice for achieving conformal uniform (composition and thickness) contiguous pin hole free thin films over the challenging sub 0.2 micron geometry topology necessary for implementing advanced devices. MOCVD is free of the physical limitations that produce film defects associated with other liquid source and physical vapor deposition techniques. Of the different MOCVD approaches to complex oxide film deposition, we have found rotating disk reactor (RDR) MOCVD to be the most versatile and capable of producing uniform films efficiently. Using computational fluid dynamic methods, a process model for optimizing the multiple process parameters of RDR-MOCVD, single and multiple wafer production tools, has been developed. The models developed are compatible with plasma-assist, single or multiple precursor and graded composition deposition of films. The developed hardware and model have been combined to maximize thin oxide film properties. SMI modeled RDR MOCVD methodology and film results are presented.     

UHV processing of ferroelectric barium magnesium fluoride films and devices

Barium magnesium fluoride (BaMgF4) has recently emerged as a strong candidate for application as the gate dielectric in ferroelectric random access memory (FERRAM) devices with nondestructive readout (NDRO). In earlier papers we reported the successful growth of oriented BaMgF4 films on Si(100) and other substrates in a ultrahigh vacuum (UHV) system, as well as the results of the structural and electrical characterization of these ferroelectric films. In the present paper, we review some of the earlier results, and also examine the effect of variations in the growth temperature and various post-growth anneals on the stoichiometry, crystallinity, orientation, and electrical characteristics of the BaMgF4 films. Initial attempts at integrating the ferroelectric field-effect transistor (FEMFET) with the standard CMOS VLSIC processing, as well as the effect of adding a thin capping layer of SiO2 on the BaMgF4 will also be described.     

Decay of remanent polarization in ferroelectric films using polarization-dependent photovoltages

Abstract

Following polarization reversal, the strength of photovoltaic currents from lead-based perovskite film capacitors shows a dependence on time. An initial photovoltaic current decays rapidly to an almost steady-state current, which then decays slowly. We use a nonswitching surface layer model to explain remanent-polarization-dependent photovoltaic outputs in terms of photocarrier-generated screening of an incompletely compensated depolarization field within the film bulk. The rapid decrease in the initial photovoltaic current (photovoltaic transient) is explained as resulting from a change in the potential distribution within the film. The initial distribution results when charges compensating polarization are confined to the electrodes; the final distribution results when compensating charges occupy defect states within the volume of the surface layers. The slow decay in the steady photovoltaic current is explained as the result of changes in the potential distribution resulting from a slow decay in the remanent polarization. The log time rate of decay of the photovoltaic current is then equal to that of the remanent polarization, its determination serves as a measure of the log time decay rate of remanent polarization.     

The bulk photovoltaic effect in ferroelectric Pb(Zr,Ti)O3 thin films

Abstract

The bulk photovoltaic effect (BPE) has been investigated in lead zirconate titanate (PZT) thin films. Measurements of the kinetics, spectral distribution and photocurrent hysteresis loops have been made. In the extrinsic spectral region, the steady-state photocurrent is primarily due to the BPE, where the photovoltaic tensor component has been determined to be G₃₁ = 10_?9 cm/V. However, in the intrinsic region, the BPE has not been determined due to the strong contribution from photoinjection currents. Finally, it is shown that the BPE may be the driving force for photoinduced hysteresis changes in PZT thin films, particularly in the extrinsic spectral region.     

Mobile defect contribution to the dielectric non-linearity of PZT ferroelectric thin films

Abstract

Effect of the speed of bias voltage variation v on the dielectric non-linearity of metal-PZT-metal thin film capacitors has been studied. A distance ΔV between two maxima of C-V dependence on the voltage scale, characteristic for ferroelectric phase, as a function of the v value was investigated. It was established that decreasing vvalue led to ΔV decrease: ΔV = 1.8–2.0 V when v = 1.6x10⁴ V/s, and ΔV ? 1.0 V when v = 6.5x10^?2 V/s. The ΔV(v) dependence can be explained by the decreasing of the coercive field of the film due to the migration of charged mobile defects such as the doubly ionized oxygen vacancies, and the formation of space charge regions near the electrodes. Using the experimental data some parameters of the migration process were evaluated: the concentration of oxygen vacancies and their mobility were found to be about 10²⁴ m^?3 and 10^?11 m²/Vs, respectively. These values are close to the data published in the literature obtained using the alternative methods of investigation.     

Thermal stability of metal electrodes (Pt,Ru, & Ir) on polycrystalline silicon in ferroelectric capacitors

Abstract

Electrode materials such as Pt, Ru, and Ir on polycrystalline silicon were annealed and the thermal stability was investigated to check the feasibility of the structure. Sputtered Pt reacted with silicon to form PtSi at a low temperature of 400 °C and the top layer of silicide was oxidized in oxygen ambient. Ru and Ir films by sputtering were also silicidized above 550 °C and it made voids beneath silicide layers. All of the films are found to allow oxygen to diffuse through them and to get the underlying layers oxidized. However, Ir deposited by e-beam evaporation did not form silicide up to 700 °C and did not get oxidized up to 550 °C. Ir also allowed oxygen diffusion when annealed at 550 °C or lower temperature, but it was prevented when annealed at 700 °C.     

The dependence of ferroelectric and fatigue behaviors of PZT films on annealing conditions

Films of PZT about 0.2 μm thick with the composition PbZr_0.53Ti_0.47O₃ were prepared using the metalloorganic decomposition (MOD) process. The amorphous films produced by pyrolysis at 350°C were annealed at 550, 575, 600 and 650°C for 10 minutes, 1 hour or 4 hours. Films annealed at temperatures below 550°C showed no ferroelectric behavior while others annealed above 650°C showed signs of loss of ferroelectric behavior. Most films demonstrated satisfactory ferroelectric properties such as low switching voltage and high polarization values. Some PZT films also demonstrated fatigue life-time of more than 10⁹ switching reversals. The performance of the films was dependent on the annealing time and temperature. It was found that films with better initial polarization values did not necessarily demonstrate better fatigue behavior. The causes of film degradation as a result of switching based on the pinning of domains at grain boundaries triggered by the migration of pores is discussed.     

Comments on breakdown,fatigue, and ultrasonic attenuation in ferroelectric films

Abstract

We compare several models of electrical breakdown, fatigue, and attenuation in ferroelectric thin (ca 100 nm) films.     

Electrodes for ferroelectric thin films

Abstract

Platinum and ruthenium oxide (RuO₂) deposited by ion beam sputter-deposition are evaluated for use as electrodes for PZT thin film capacitors. The effect of deposition temperature, film thickness, and the presence of oxygen on hillock formation in platinum is discussed. It is shown that the hillock density in Pt/Ti/SiO₂/Si films can be significantly reduced by properly controlling the processing conditions and film thickness. Stress measurements correlate with the experimental observation that depositing thinner platinum films (<800 Å) is an effective means of reducing hillock formation. The use of an intermediate deposition temperature 200–250°C also helps minimize hillock formation. Diffusion of the Ti adhesion layer into and/or through the platinum was significantly reduced by replacing the Ti with a TiO_x adhesion layer. RuO₂ electrodes are compared to Pt in terms of resistivity, surface morphology, microstructure and film orientation.     

Influence of platinum-based electrodes on the microstructure of sol-gel and MOD prepared lead zirconate titanate films

Abstract

The microstructure and preferred orientation of PZT films deposited by spin-coating techniques are shown to depend on the morphology of the Ti/Pt bottom electrode. Annealing of the as-deposited Ti/Pt electrodes results in the formation of hillocks. These hillocks serve as nucleation sites for perovskite formation and thus determine the microstructure and preferred orientation of the crystallized PZT films. The microstructure features are investigated by X-ray diffraction analyses and scanning and transmission electron microscopies.     

The dependence of ferroelectric and fatigue behaviors of PZT films on microstructure and orientation

Microstructure of PbZr_0.53Ti_0.47O₃ films about 0.2 μm thick was studied as a function of annealing temperature and time using TEM and SEM. The films were heat treated in the temperature range 550–650°C for times from 10 minutes to 4 hours. The features characterized were crystal structure of individual PZT particles, PZT grain size and shape, porosity, and grain boundaries. The films were prepared by the metallo-organic decomposition (MOD) process which produced films amorphous to x-rays after pyrolysis. A growth mechanism of PZT particles from the amorphous film is discussed. It was found that at mild annealing conditions (i.e., low temperature and short annealing times), agglomerates of microcrystallites of perovskite, and probably pyrochlore, were formed. As the temperature was increased, the microcrystallites grew into porous submicron perovskite single crystals. With excessive heating, intraparticle pores were found to migrate and became pinned at the grain boundaries, causing widening of boundaries which probably was responsible for the degradation in ferroelectric properties observed in such films. The dependence of ferroelectric and fatigue behaviors on grain orientation was also investigated. It was found that PZT films oriented in the [111] direction demonstrated better fatigue behavior but not necessarily higher polarizations.     

Substrate effect on in-plane ferroelectric and dielectric properties of Ba0.7Sr0.3TiO3 thin films

Heteroepitaxial Ba_0.7Sr_0.3TiO₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) (LSAT) and SrTiO₃ (001) (STO) single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction characterization revealed a good crystallinity and a pure perovskite structure for films grown on both LSAT and STO substrates. The in-plane ferroelectric and dielectric properties of the films were studied using interdigital electrodes (IDE). The film grown on LSAT substrate exhibited an enhanced in-plane ferroelectricity, including a well-defined P-E hysteresis loop with the remnant polarization P _r = 10.5 μC/cm² and a butterfly-shaped C-V curve. Nevertheless, only a slim hysteresis loop was observed in the film grown on STO substrate. Curie temperature T _c of the film grown on LSAT substrate was found to be ∼105^∘C, which is nearly 70^∘C higher than that of the bulk Ba_0.7Sr_0.3TiO₃ ceramics. T _c of the film grown on STO substrate has almost no change compared to the bulk Ba_0.7Sr_0.3TiO₃ ceramics. The dielectric tunabilities were found to be 64% and 52% at 1 MHz for the films grown on LSAT and STO substrates, respectively.     

Properties of reactively sputtered IrOx for PZT electrode applications

Abstract

The use of IrO_x for electrodes in PbZr_xTi_1?xO₃ (PZT) capacitors for ferroelectric memory applications has proven to be advantageous in several respects. In comparison with Pt, IrO_x often exhibits improved fatigue and provides resistance to hydrogen induced degradation at elevated temperatures. Since IrO_x is often produced by sputtering in an oxygen containing environment, several forms of IrO_x can be produced depending on the process conditions. This work concentrates on an analysis of the DC reactive sputtering of IrO_x from an Ir metal target. As with other oxidizable metals, Ir exhibits a transition between metal and oxide mode sputtering when sputtered in oxygen containing atmospheres. Variations in the Ar/O₂ gas flow ratio were used to produce Ir and IrO_xfilms on both sides of the metal-to-oxide mode reactive sputtering transition. Changes in the IrO_x film properties were quantified by using a combination of metrics including X-ray diffraction, sheet resistance, and stress. It was found that the, IrO_x crystalline structure and other IrO_x properties could be directly related to the ferroelectric switching performance of PZT capacitors with IrO_xtop electrodes. A relationship between IrO_x deposition processes and resistance to etch induced damage of the ferroelectric properties was also observed.     

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(Zr_xTi_1?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.     

Dielectric characteristics investigation of (Ba0.7Sr0.3)(Ti0.9Zr0.1)O3 ferroelectric thin films

By the radio frequency (RF) magnetron sputtering methods, (Ba_0.7Sr_0.3)(Ti_0.9Zr_0.1)O₃ (BSTZ) ferroelectric thin films were deposited on the Pt/Ti/SiO₂/Si(100) substrates. The crystal structural and microstructure of these thin films were analyzed by means of the XRD, SEM, and AFM. Moreover, the dielectric characteristics were also investigated by the C-V and J-E analyses. The optimal deposition parameters for these BSTZ thin films were: RF power is 160 W, oxygen concentration is 25%, substrate temperature is 580°C, and chamber pressure is 0.075 mPa. Under these optimal deposition conditions, the (111) and (110) oriented polycrystalline of the BSTZ thin films grow easily. And under a bias voltage of 0.5 MV/cm, the dielectric constant and leakage current density of the BSTZ thin films are 191 and 3×10^?8 A/cm², respectively. In addition, under various measured temperatures (0 ～ 80°C) and frequencies (100 kHz ～ 1 MHz), all the dielectric constants remain almost unchanged. Compared to BSTZ thin films reported previously, in this study, the deposited thin films have the advantage of lower leakage current and hence are suitable for the applications of dynamic random access memory.     

Ferroelectric properties and reliability of La-Sr-Co-O/Pb-La-Zr-Ti-O/La-Sr-Co-O heterostructures on si for non-volatile memory applications

Abstract

Highly oriented La-Sr-Co-O(LSCO)/Pb-La-Zr-Ti-O(PLZT)/La-Sr-Co-O heterostructures have been successfully grown on a highly oriented Pt film which was grown on a thermally oxidized Si (SiO₂/Si) substrate. The growth of oriented Pt film on the SiO₂/Si substrate was made possible through the use of a thin bismuth titanate template layer which is c-axis oriented on the SiO₂/Si substrate. The hybrid LSCO/Pt structure effectively reduced the sheet resistance of the electrodes by at least 3–5 times compared with a single LSCO electrode. These ferroelectric PLZT capacitors on Si exhibited symmetric hysteresis loops with very desirable ferroelectric properties. The test capacitors showed reliable performance at both room and high (100°C) temperatures with respect to fatigue, retention, aging, and imprint, suggesting that they can be used as reliable, nonvolatile memory elements.     

Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO₂/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO₂ interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ～ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.     

Adaptive-learning neuron circuits using ferroelectric thin films

Abstract

Adaptive-learning neuron circuits are reviewed, in which a pulse frequency modulation (PFM) system is used and the interval of output pulses is changed through the learning process. Key devices of the circuits are MFSFETs (Metal Ferroelectric Semiconductor Field Effect Transistors). They are used for representing the synaptic weights of neurons and the polarity of the films is gradually changed by applying input pulses to the gates. In order to produce PFM signals, circuit using a UJT (unijunction transistor) is discussed. As a preliminary experiment for realization of MFSFETs, the electrical properties of metal ferroelectric metal (MFM) capacitor using sol-gel derived PZT films are discussed.