Dynamic Random Access Memory Applications Research Articles

Deposition of extremely thin (Ba,Sr)TiO3 thin films for ultra-large-scale integrated dynamic random access memory application

(Ba,Sr)TiO3 (BST) thin films with thicknesses ranging from 15 to 50 nm are prepared by a rf magnetron sputtering on Pt/SiO2/Si substrates. The dielectric constants of BST thin films increase with increasing deposition temperature and thicknesses. The leakage current increases with increasing deposition temperature and this prevents the deposition temperature of the 20 nm thick BST thin film from being increased to a value more than 640 °C. The leakage current is also critically dependent upon the postannealing temperature and atmosphere after the top electrode fabrication. The dielectric constant increases with increasing postannealing temperature which further reduces the SiO2 equivalent thicknesses of the BST thin films. A 20 nm thick BST thin film deposited at 640 °C and postannealed at 750 °C under N2 atmosphere for 30 min, shows a SiO2 equivalent thickness of 0.24 nm, dielectric dissipation factor less than 1%, and leakage current of about 40 nA/cm2 at ∓1.5 V.

Deposition and Electrical Characterization of Very Thin SrTiO3 Films for Ultra Large Scale Integrated Dynamic Random Access Memory Application

SrTiO3 thin films are deposited on Pt/SiO2/Si substrates using RF magnetron sputtering in a temperature range from 200° C to 600° C. The film deposited at 600° C shows the best dielectric property and leakage current characteristics due to its good crystallinity and stoichiometric composition. Dielectric constant of the film deposited at 600° C decreases with decreasing thickness from 235 at 120 nm to 145 at 30 nm. Leakage current shows a constant value of about 30 nA/cm2 at 1.6 V in a thickness range from 50 nm to 120 nm but increases rapidly to 5 µ A/cm2 at 30 nm. The electrical properties of the films are explained by a model of the Pt/SrTiO3/Pt capacitor based on the band structure.

Plasma enhanced chemical vapor deposition of TiO2 in microwave-radio frequency hybrid plasma reactor

A plasma tool and accompanying process have been developed for plasma enhanced chemical vapor deposition of TiO2 thin films for dynamic random access memory application. The microwave-rf (radio frequency) hybrid reactor consists of microwave power (2.45 GHz) for electron cyclotron resonance, a hollow cathode (0.1–30 MHz) for control of the plasma potential, and an additional rf (13.56 MHz) power to the substrate electrode for ion extraction and ion energy control. X-ray diffraction showed the anatase phase at a low deposition temperature below 400 °C, but the rutile phase dominanted above 400 °C. Low energy ion bombardment densifies the TiO2 films and suppresses rutile formation. A dielectric constant of 60(±5) at 1 MHz was obtained, independent of film thickness in a range from 20 to 200 nm. A SiO2 buffer layer was found to be important in order to prevent silicide formation and to keep the leakage current below 10−10 A/cm2.

Characterization of Defect States Responsible for Leakage Current in Tantalum Pentoxide Films for Very-High-Density Dynamic Random Access Memory (DRAM) Applications

Defect states in tantalum pentoxide films grown by low-pressure metal-organic chemical vapor deposition on silicon wafers have been studied with Al/Ta2O5/p+-Si and Al/Ta2O5/n+-Si capacitor structures by the zero-bias thermally stimulated current technique. It was demonstrated that a shallow band of defect states is responsible for leakage current. The shallow band of defect states can be suppressed by low-temperature post metallization annealing, resulting in a reduction of leakage current for both positive gate bias and negative gate bias. However, the reduction in leakage current for positive gate bias is much stronger than that for negative gate bias.

Ferroelectric Properties of Crystalline-Oriented Lead-Zirconate-Titanates Formed by Sol-Gel Deposition Technique

Ferroelectric properties of lead-zirconate-titanate (PZT) films formed by the sol-gel deposition technique have been discussed from the standpoint of ferroelectric random-access memory (FRAM) and dynamic random-access memory (DRAM) applications. PZT films with <111>-preferred orientation and random orientation were formed on Pt/Ti/SiO2/Si substrates by optimizing fabrication conditions. Remanent polarization of the <111>-preferred film was 23.5 µ C/cm2 which is somewhat higher than that of the randomly oriented one. An abrupt saturation of remanent polarization was observed only for the <111>-preferred film. The voltages which provide saturations of remanent polarization densities for the <111>-preferred films are almost constant between 3.5 V to 4 V independent of thickness. The dielectric constant of the randomly oriented film was 730 which is larger than that of the <111>-preferred one and gradually decreased depending on the applied electric field. The <111>-preferred PZT film is appropriate for FRAM application, and the randomly oriented one is preferred for DRAM application.

Structural and Ferroelectric Properties of Epitaxial La0.5Sr0.5CoO3/Ba0.4Sr0.6TiO3/La0.5Sr0.5CoO3 Thin-Film Capacitors on Silicon for Dram Applications

AbstractEpitaxial La0.5Sr0.5CoO3/Ba0.4Sr0.6Tio3/La0.5Sr0.5CoO3 heterostructures were grown (650- 750 °C) on yttria-stabilized zirconia (YSZ)-buffered silicon (100) by pulsed laser (Nd:YAG) deposition. A 15-nm-thick YBa2Cu3O7 film was deposited on the YSZ to act as a template layer for epitaxial growth of the subsequent conductive oxide and ferroelectric film layers. The Ba0.4Sr0.6TiO3 film was (h00) oriented normal to the Si-wafer surface. The temperaturedependent resistance curve measured on the La0.5Sr0.5CoO3 top electrode layer was found to indicate to some extent the film quality of the underlying layers. Hundreds of capacitors, fabricated by wet chemical etching from the as-deposited films, exhibited dc leakage current densities in the range of 0.05–0.1 μA/cm2 at 1-volt operation. By increasing the thickness of the dielectric layer, some of them were able to be tested at up to 20 volts before resistive leakage became dominant. To reduce leakage current densities and increase dielectric constants in these film capacitors for dynamic random access memory (DRAM) applications, we have made efforts to incorporate hybrid Pt/La0.5Sr0.5CoO3 bottom electrodes into capacitors. This results in improved dielectric constants (451 at 10 kHz) and P-E hysteresis characteristics (Ec = 58 kV/cm, Pr = 7.5 μC/cm2, and Pg = 25 μC/cm2).

Dielectric Properties of (BaxSr1-x)TiO3 Thin Films Prepared by RF Sputtering for Dynamic Random Access Memory Application

Dielectric properties of (Ba0.5Sr0.5)TiO3 and (Ba0.75Sr0.25)TiO3 thin films have been investigated, focusing on the effects of film structure and Ba/Sr compositions on the dielectric properties. Dielectric constant of the films increased with increasing grain size and with improvement in film crystallinity. For the 50-nm-thick films deposited at 660° C, the dielectric constant of 400 for the (Ba0.5Sr0.5)TiO3 film is larger than that of 320 for the (Ba0.75Sr0.25)TiO3 film. This indicated that dielectric constant is affected by the ratio of Ba/Sr composition. Leakage current density of less than 1×10-7 A/cm2 at 1 V and SiO2 equivalent thickness of 0.38 nm are measured at 120° C for 660° C-deposited (Ba0.5Sr0.5)TiO3 film of 30 nm in thickness. The (Ba0.5Sr0.5)TiO3 film has the possibility for application to generations beyond 256 Mbit dynamic random access memories.

Pulsed laser deposition of strontium titanate thin films for dynamic random access memory applications

Polycrystalline strontium titanate (SrTiO 3) films were prepared by a pulsed laser deposition technique on p-type silicon and platinum-coated silicon substrates. The films exhibited good structural and dielectric properties which were sensitive to the processing conditions. The small signal dielectric constant dissipation factor at a frequency of 100 kHz were about 225 and 0.03 respectively. The capacitance-voltage (C-V) characteristics in metal-insulator- semiconductor structures exhibited anomalous frequency dispersion behavior and a hysteresis effect. The hysteresis in the C-V curve was found to be about 1 V and of a charge injection type. The density of interface states was about 1.79 × 10 12 cm -2. The charge storage density was found to be 40 fC μm -2 at an applied electric field of 200 kV cm -1. Studies on current-voltage characteristics indicated an ohmic nature at lower voltages and space charge conduction at higher voltages. The films also exhibited excellent time-dependent dielectric breakdown behavior.

Dielectric Properties of (111) and (100) Lead-Zirconate-Titanate Films Prepared by Sol-Gel Technique

Dielectric properties of <111>- and <100>-preferred lead-zirconate-titanate (PZT) films with thickness of 400 nm prepared by the sol-gel deposition technique have been investigated from ferroelectric random-access memory (FRAM) and dynamic random-access memory (DRAM) application standpoints. Remanent polarization of <111>-preferred film is 23.5 µ C/cm2, which is somewhat higher than that of the <100>-preferred one. In contrast, the dielectric constant of <100>-preferred film is 730 which is larger than that of the <111>-preferred one. Thus, dielectric properties of PZT films are strongly dependent on their crystalline orientations. The <111>-preferred PZT film is appropriate for FRAM application and the <100>-preferred one is better for DRAM.

Thermal stability of thin poly-Si/Ta2O5/TiN capacitors for dynamic random access memory applications

Capacitors made up of thin films of Ta2O5/TiN on polysilicon have been studied for application to 256 Mbytes dynamic random access memories. In this study, the thermal stability of poly-Si/Ta2O5 films with TiN electrode film deposited by (i) chemical vapor deposition (CVD) with TiCl4 and NH3, (ii) sputtering, and (iii) metalorganic CVD (MOCVD) was compared. These samples were annealed under vacuum at 700, 800, and 900 °C for 30 min. The as-deposited and the annealed samples were analyzed by Rutherford backscattering with 2 MeV He+ ions. The CVD TiN films, which contained a small amount of Cl, provided pathways for the underlying Ta2O5 to out-diffuse during annealing at all temperatures between 700 and 900 °C. The sputtered films were resistant to the diffusion of Ta2O5 up to 800 °C, but relented at 900 °C, with measurable outdiffusion of Ta. There was no noticeable interaction of the Ta2O5 with the TiN layers, deposited by the MOCVD process, at temperatures up to 900 °C. However, the MOCVD TiN films were found to contain light elements such as carbon and oxygen, which were slightly reduced on annealing. Nuclear reaction analysis and secondary ion mass spectroscopy were used to detect the concentrations and depth profiles of light elements. These indicate presence of C and O contents up to at least 20 at. % in these films.

Barrier mechanism of Pt/Ta and Pt/Ti layers for SrTiO3 thin film capacitors on Si

Abstract The barrier effect of Pt/Ta and Pt/Ti has been investigated, when used as bottom electrodes for SrTiO3 thin film capacitors on Si. The Pt/Ta/Si stacks were more stable than the Pt/Ti/Si, both in vacuum and in oxygen annealing. Though the Pt/Ta bilayer was suitable for the SrTiO3 deposition at 400[ddot]C, its resistivity became slightly higher after the deposition at 600[ddot]C, due to Ta layer oxidation during the SrTiO3 deposition. This would result in a contact resistance problem for high density dynamic random access memory application.

Characteristics of ferroelectric 60/40 PZT films deposited by metallo-organic decomposition technology for memory applications

Nearly pin-hole-free ferroelectric films of Pb(Zr0.6Ti0.4)O3 (PZT) with uniform compositions and thicknesses were synthesized using metallo-organic decomposition (MOD) technology. The metallo-organic precursors used for these PZT films were lead diethylhexanoate, (Pb(C7H15COO)2), titanium dimethoxy-dineodecanoate (Ti(OCH3)2 (C9 H19 COO)2), and zirconium tetraethylhexanoate (Zr(C7H15COO)4). The film thickness, structural development and electrical properties of these films were characterized using a surface profilometer, X-ray diffraction (XRD) and a standardized RT 66 ferroelectric test system. The effect of the processing conditions on the properties of ferroelectric 60/40 PZT films were investigated. The important processing parameters studied included the substrate history, the spinning speed and time, the film thickness, the pyrolysing temperature and the annealing conditions. The experimental results show that the structural development and electrical properties of these 60/40 PZT films are closely correlated with these processing parameters. The polarization-reversal characteristics and fatigue behaviour, which are important for non-volatile memory applications, were also studied. By optimizing the annealing conditions, a 60/40 PZT film exhibited a dielectric strength of 1 MVcm−1 and a sensed remanent polarization value of 9.3 ΜC cm2 after cycling 1010 times. This demonstrates the excellent potential of PZT films for non-volatile memory applications. The measured dielectric-constant values for these 60/40 PZT films were in the range of 300 to 900, which is about 75 to 220 times larger than that of the currently used dielectric, SiO2, in dynamic random-access memory (DRAM). This fact suggests that PZT film is an excellent alternative for the new generation, extremely high-density DRAM applications.

A model for electrical conduction in metal-ferroelectric-metal thin-film capacitors

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 ∼1018 cm−3 in PZT. The implications of this model on the optimization of ferroelectric capacitors for dynamic random access memory applications are discussed.

Advanced Dram Cell Dielectric Films using Rapid Thermal Processing

ABSTRACTThe reliable operation of a dynamic random access memory (DRAM) device requires a minimum level of charge to be stored in the capacitor. The nonlinear dependence between the scaling of the minimum charge and the cell area for higher DRAM densities is the driving force in the development of exotic capacitor structures and advanced cell dielectric materials. The conventional option of reducing the thickness of the silicon nitride dielectric films for high density DRAM applications will eventually be constrained by the increase in the leakage current due to direct carrier tunneling or by the decrease in the oxidation resistance of the films.In this paper we discuss the use of rapid thermal processing to modify the interface between the polysilicon storage node of the capacitor and the silicon nitride to improve the electrical and structural characteristics without any loss in capacitance. The influence of electrode roughness on the electrical behavior will also be discussed for the various dielectric stack combinations.

Nanocrystalline barium titanate: Evidence for the absence of ferroelectricity in sol-gel derived thin-layer capacitors

Nanocrystalline BaTiO3 thin-layer capacitors were integrated onto silicon by sol-gel processing using alkoxide precursor solutions. 200-nm-thick layers were formed and found to be comprised of 25-nm grains in the cubic perovskite structure. Electrical measurements indicated a lack of polarization-reversal hysteresis characteristics and Curie–Weiss behavior. The dielectric constant was 230 at room temperature. Properties are discussed in terms of a crystallite size effect. Stable dielectric properties with respect to temperature and voltage suggest that nanocrystalline BaTiO3 could find possible use as an integrated capacitor in decoupling and dynamic random access memory applications.

Etched-quartz fabrication issues for a 0.25 μm phase-shifted dynamic random access memory application

A 0.25 μm pattern from a dynamic random access memory application is used to assess the impact of an etched-quartz fabrication process on the lithographic performance of an actual phase-shifted circuit design. A new measurement tool called the aerial image measurement system is used to quantify the effects of an isotropic wet etch following the quartz reactive-ion etching step on the printability of the phase-shifted pattern at two values of partial coherence. The presence of significant transmission and phase errors, in part a result of optical scattering from the edges and/or corners of the etched-quartz trench, reduced the potential benefits obtained from phase shifting. At the optimum postetch conditions for a partial coherence of 0.36, only 65% of the ideal depth-of-focus was obtained. Transmission errors were reduced from greater than 25% with no postetch treatment to less than 5% with wet-etch depths in excess of 1100 Å. However, large phase errors, between 20° and 60°, were present for all postetch conditions, and were strongly dependent on the partial coherence. Optimization of depth-of-focus required a nominal wet-etch depth of 1200 Å for σ=0.6, but only a 900 Å depth at σ=0.36. Large variations in the phase error as a function of the postetch treatment conditions were attributed to electromagnetic resonance phenomena between the sidewalls of the etched-quartz trench.

Reactive Ion Etching of Lead Zirconate Titanate (PZT) Thin Film Capacitors

One of the key processing concerns in the integration of thin film capacitors into the existing VLSI for ferroelectric or dynamic random access memory applications is the patterning of these films and the electrodes. In this work, we have identified a suitable etch gas for dry etching of PZT thin films on electrodes. The etch rate and anisotropy have been studied as a function of etching conditions. The trends in the effect on the etch rate of the gas pressure, RF power and additions to the etch gas have been determined and an etching mechanism has been proposed. It was found that ion bombardment effects are primarily responsible for the etching of both PZT and thin films. Etch rates of the order of 20–30 nm/min were obtained for PZT thin films under low gas pressure and high RF power conditions. The etch residues and the relative etch rates of the components of the PZT solid solution were determined using XPS. The results show that the etching of is the limiting factor in the etch process.

Effect of lanthanum doping on the electrical properties of sol-gel derived ferroelectric lead–zirconate–titanate for ultra-large-scale integration dynamic random access memory applications

Thin films of lead–zirconate–titanate [(PZT) Pb(Zr0.5Ti0.5)O3] 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 (&amp;lt;10−6 A/cm2). 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 (Pmax, bit ‘‘1’’) and remanent polarization (Pr, bit ‘‘0’’) is denoted as Qc′, and is the relevant charge storage density in the DRAM cell. Qc′ 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 (JL) in comparison with undoped PZT, and after refresh annealing (at 450 °C for 1/2 h in O2), the film exhibits a Qc′ value of 7.2 μC/cm2 at 3 V, which is believed to satisfy the 64 Mb DRAM requirements. JL 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)LaxVx/2(Zr0.5Ti0.5)O3, (where V represents a Pb vacancy). Constant voltage stressing indicates an operating lifetime (at room temperature) of about ten years (at 3 V) for the 5/50/50 film. Good fatigue properties are also observed.

Structural and electrical characteristics of SrTiO3 thin films for dynamic random access memory applications

Polycrystalline SrTiO3 thin films having perovskite structure were prepared by the metallo-organic solution deposition technique on platinum coated silicon and bare silicon substrates. Crack free and crystalline films with uniform composition and thickness were fabricated by spinning and post deposition rapid thermal annealing at a low temperature of 550 °C for 60 s. The films exhibited good structural, dielectric, and insulating properties. The dielectric constant was found to depend on film thickness and annealing temperature. The small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 225 and 0.008, respectively, for a 0.8-μm-thick film annealed at 700 °C for 60 s. The frequency dependence of the dielectric constant and the dielectric loss was also studied. The measurement of the current-voltage (I-V) characteristics on films in metal-insulator-metal configuration indicated the conduction process to be bulk limited. The I-V characteristics were ohmic at low fields and space-charge limited at high fields. Room temperature resistivity of 1013 Ω cm and leakage current density of less than 10−8 A/cm2 were obtained for a 0.8-μm-thick film at an applied electric field of 200 kV/cm, establishing good insulating behavior. The interfacial properties of gold-strontium titanate-silicon structures were studied experimentally by measuring the capacitance-voltage (C-V) characteristics. The C-V curves exhibited anomalous frequency dispersion behavior and a hysteresis phenomenon. The hysteresis in the C-V curve was found to be about 0.8 V and of a charge injection type. The density of interface states recharged during the bias cycle in hysteresis measurement was estimated to be of the order of 8.3×1011 cm−2. The density of interface states at the flatband voltage was found to have a value of 1.38×1012 eV−1 cm−2. For a 0.5-μm-thick SrTiO3 film, a unit area capacitance of 3.5 fF/μm2 and a charge storage density of 36.7 fC/μm2 were obtained at an applied electric field of 200 kV/cm. These measured parameters place SrTiO3 as one of the suitable dielectric material for dynamic random access memory applications.

Sputtered ferroelectric thin films for dynamic random access memory applications

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.