Fundamental characteristics of DRAM capacitor application of PLZT ultrathin films prepared by MOCVD

Lanthanum-modified lead zirconate titanate (PLZT) thin films (50 nm to 200 nm) were deposited on Pt/SiO₂/Si substrate by metal-organic chemical vapor deposition (MOCVD). The electrical properties of the films were investigated as a function of the La content or the substrate temperature. Ferroelectric PZT(0/50/50) films were obtained at substrate temperatures as low as 500 °C; their electrical characteristics improved with increasing substrate temperature. La exhibited adequate solid solution in the PZT above 650 °C. PLZT(15/45/55) films with a thickness of 100 nm were found to have good properties for application to the capacitors of dynamic random access memory (DRAM), namely, an effective charge density of 80 fF/μm², a permittivity of 1000, an SiO₂ equivalent thickness of 0.4 nm, and a leakage current density of 5 × 10⁻⁸ A/cm. Addition of La to PZT was effective in reducing the leakage current with an increase in the registration rate. RuO₂ and/or IrO₂ bottom electrodes for ferroelectric PLZT films were also investigated. The RuO₂ films were found effective as diffusion barriers for PLZT and MgO. Significant interdiffusion at RuO₂/Si and RuO₂/SiO₂ interfaces occurred during the deposition of PLZT films. Annealing of the RuO₂ film considerably depressed interface reactions. © 1998 Scripta Technica. Electr Eng Jpn, 122(1): 25–36, 1998     

Integration and characterization of MFISFET using Pb5Ge3O11

Abstract

The first MFIS FETs PMOS using Pt/Pb₅Ge₃O₁₁/ZrO₂/n-Si structure has been successfully fabricated. The PGO thin film was deposited by spin on method. Single phase PGO with strong c-axis orientation and low leakage current was obtained on ZrO₂ substrate. Pt was used as top electrode and the gate stack was dry etched using chlorine chemistry. Using CMOS compatible process, the integration of MFIS FETs is simple and reliable. I_D-V_G and I_D-V_D were characterized on 10 × 10 μrn (L × W) devices. The memory window obtained is about 1.3V with 200nm PGO and 13nm ZrO₂. It is also found that memory window is less dependent on device sizes.     

Deterioration of device characteristics of MFSFET due to fatigue

Abstract

Change of device characteristics of the metal-ferroelectric-semiconductor FET (MFSFET) with the progress of fatigue of the ferroelectric thin film are simulated in this study. The field-dependent polarization model and the square-law FET model are employed in our simulation. C-V_G curves exhibit the accumulation, the depletion and inversion regions clearly. They also exhibit the memory window of 2V. I_D-V_D curves are composed of the triode and the saturation regions. The difference of saturation drain currents of the MFSFET device at the dual threshold voltages in I_D-V_D curve is 6mA/cm², which decreases as much as 50% after fatigue. Our model is expected to be very useful in the estimation of the behaviour of MFSFET devices with the progress of fatigue.     

Fabrication and Characterization of Gate-Connected 1T2C-Type Ferroelectric Memory with Paired Bi4 − xLaxTi3O12 Capacitors

A gate-connected 1T2C-type ferroelectric memory, in which the bottom electrodes of paired ferroelectric capacitors are connected to the gate electrode of an underlying FET (field effect transistor) on the field oxide region, was fabricated using a Bi_{4 ? x}La_xTi₃O₁₂ (BLT) film and its electrical properties were characterized. The I_D-V_G (drain current-gate voltage) characteristics of a FET combined with a single ferroelectric capacitor showed that the paired capacitors had almost the same ferroelectric property. It was found in the readout operation that there existed an optimum voltage to maximize the drain current on/off ratio between datum ‘1’ and datum ‘0,’ and that the maximum ratio was as large as 6 × 10⁴. It was also found that the drain current level remained constant, even if the readout operation was repeatedly conducted. It was concluded from these results that the 1T2C-type memory was successfully fabricated using the proposed process and operated properly.     

PZT capacitor with Ir/IrO2/Ir electrode fabricated by RTA

Abstract

PZT capacitor with direct contact between Si substrate and bottom electrode of the capacitor was obtained with Ir/IrO₂/Ir/Ti electrode, by crystallizing sol-gel PZT thin film using RTA (650°CC 30 sec.). Contact resistance for hole diameter of 0.72 μm was 19 Ω. It was observed by cross-sectional TEM that Ti silicide was formed at the interface, but there was not oxygen diffusion from PZT thin film. Fatigue property of the PZT thin film was improved by RTA compared with furnace annealed film (600°CC 60 min.). The absolute value of the remnant polarization was 13 μC/cm² for both films, but it did not degrade until 10⁸ cycles of switching for the film by RTA, while it degraded before 10⁵ cycles for furnace annealed film.     

PZT capacitors using IrO2 electrode by RF magnetron sputtering

Abstract

RF magnetron sputtered Pb(Zr_x, Ti_1-x)O₃ [PZT] films were prepared on IrO₂/SiO₂/Si and Pt/IrO₂/SiO₂/Si substrates using the ceramic PZT target with Pb_1.1(Zr_0.52Ti_0.48)O₃ composition. In order to obtain single perovskite phase, PZT film was sputtered at room temperature under Ar plasma and followed by high temperature annealing under oxygen atmosphere. In case of Pt/PZT/IrO₂ capacitor, Δ P (=P*-P∧) was decreased with oxygen annealing temperature. However, it was increased in Pt/PZT/Pt/IrO₂ capacitor. Leakage current density of Pt/PZT/Pt/IrO₂ capacitor, which was used for improving leakage characteristics, was about 10^-2A/cm^?2 order lower than that of Pt/PZT/IrO₂ capacitor. Leakage current density of Pt/PZT/Pt/IrO₂ capacitor annealed at 700°C was 6.6x10^-6A/cm². From the fatigue test, Pt/PZT/IrO₂ capacitor annealed at 650°C and Pt/PZT/Pt/IrO₂ capacitor annealed at 700°C showed 3% and 12% degradation of Δ P after 5×10¹⁰ fatigue cycles, respectively.     

Laser ablation-deposited PZT thin films for piezoelectric microsensors and microactuators

Abstract

We report on growth and deposition process optimization of laser ablation-deposited Pb(Zr_0.53Ti_0.47)O₃ thin films for application in piezoelectric microdevices. Films were grown on three different substrates: (100) cut LaAlO₃ single crystals, Pt/Ti/SiO₂/Si, and Pt/Ti/Si_3+xN₄/Si. On all three substrates, a deposition temperature of 620°C yielded perovskite films with good ferroelectric hysteresis properties. La_0.5Sr_0.5CoO₃ bottom and top electrodes were used for all films. PZT films on Pt/Ti/SiO₂/Si showed a variation of texture with film thickness. Thin membranes of PZT on Si_3+xN₄ were also fabricated.     

Improvement of the electrical properties of PZT thin films using TiO2 buffer layer

Abstract

Thin TiO₂ layers were sputter-deposited on Pt/Ti/SiO₂/Si wafers, as buffer layers for PZT thin film capacitors. It was found that TiO2 buffers of less than 4-nm-thickness could assist in obtaining highly uniform PZT thin films with no second phase. The leakage current behaviors of the PZT based capacitor are improved, while retaining the ferroelectric properties of PZT thin films such as remanent polarization and coercive field. In addition, the uniform distribution of oxygen in PZT on TiO₂/Pt indicates that the TiO₂ buffer layer act as a barrier for lead-platinum reaction, as well as for oxygen diffusion.     

The Effect of La-Doped Bi4Ti3O12 Buffer Layer on Crystallinity and Ferroelectric Properties of PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 Multilayered Thin Films

PbZr_0.58Ti_0.42O₃ (PZT) ferroelectric thin films with Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer of various thickness were fabricated on Pt/TiO₂/SiO₂/p-Si(100) substrates by rf-magnetron sputtering method. The pure PZT film showed (111) preferential orientation in the XRD patterns, and the PZT/BLT films showed (110) preferential orientation with increasing thickness of the BLT layer. There were no obvious diffraction peaks for the BLT buffer layer, for its thin thickness in PZT/BLT multilayered films. There were the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples from the surface images of FESEM. The growth direction and grain size had significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics suggested that 30-nm-thick BLT was just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

EFFECTS OF PZT BUFFER LAYERS ON PZFNT FERROELECTRIC THIN FILMS

ABSTRACT

PZFNT thin films were fabricated on 5-inch Pt/Ti/SiO₂/Si and PZT/Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering method and rapid thermal annealing process. By investigating the two thin films at various annealing temperatures, the results show that the annealing temperature of PZFNT thin films without PZT buffer layers is about 730°C, which is higher than that of PZFNT films with PZT buffer layers. By use of PZT buffer layers, the annealing temperature of PZFNT films is decreased greatly, and the dielectric and ferroelectric properties are improved. In the optimum process, the thin films with PZT buffer layers have a dielectric constant of 1199 and dielectric loss of 3.0% at 1 KHz. The remanent polarization and coercive field of the thin films are 21.1 μC/cm² and 53.5 KV/cm respectively. The films have the significant potential for FRAM and pyroelectric infrared detectors.     

Determination of Cross Sectional Variation of Ferroelectric Properties for Thin Film (Ca. 500 nm) PZT (30/70) via PFM

There is much speculation about the origin of fatigue in ferroelectric thin films that have been grown by sol-gel on various substrates. One of the most important substrates for growing thin film ferroelectric materials is native SiO₂. After the deposition of suitable electrode, most common are Ti-Pt, a thin film of a ferroelectric material can be grown on the substrate. Procedures to grow thin film lead zirconate titanate (PZT 30-70) have been well publicized and it is now routine that almost 100% [111] orientated PZT with a columnar structure of ca. 100 nm is grown on SiO₂ substrates. Recent studies using PFM have shown that it is possible to determine a variation in hysteresis loops with a spatial accuracy of 50 nm on the surface of a ferroelectric material. This study was to determine the variation in d₃₃ for a thin film sample of ferroelectric in terms of a depth profile. It was not suitable to just turn the sample on it's side and analyze the d₃₃ in terms of depth as the continuous back electrode would mean that the resulting vibration of the ferroelectric surface would be a combination of d₃₃ and d₃₁. The de-convolution of this signal was outside the scope of the current investigation. In order to determine the d₃₃ of the material in terms of depth the sample was machined (with an accuracy of 10 nm) to release the ferroelectric from the underlying electrode and electroded in the correct orientation for d₃₃ to be investigated in cross section. Samples of perovskite PZT(30/70) have been produced via sol-gel using a spin coating technique that are ca. 500 nm in cross sectional thickness on Pt/Ti/SiO₂ back electrodes. The PZT is shown to be highly [111] orientated and has in the past been well characterized both at the macro and nanoscale using a variety of techniques. Using focused ion beam (FIB) milling the PZT layer has been released from the SiO₂/Ti/Pt substrate and electroded in such a way as to determine the δ₃₃ properties of the film in terms of thickness of the as-deposited film. Figure 1 indicates the machining process used to generate the free standing PZT block. In Fig. 2, the results of the FIB machining process are shown. The ‘bridge’ structure has the dimensions of 500 nm in cross section and is long enough to ensure that electrode cross talk between the d₃₃ and d₃₁ modes is reduced. Hysteresis loops have been generated using the technique of piezo force microscopy (PFM) at discrete locations across the surface of the film corresponding to a depth profile of the as-deposited PZT. Variations in the shape of the hysteresis loops, calculated δ₃₃ and coercive field for the film have been related to the variation in oxygen defect density in the film. This study shows that the PZT thin film has a variety of fundamental piezoelectric constants that are associated with the variation in local crystal chemistry.     

Preparation of BaPbO3 Electrode Thin Films by RF Magnetron Sputtering

For electrode materials of Pb(Zr,Ti)O₃ (PZT) thin films in ferroelectric random access memory (FeRAM), various materials have been studied. As new electrode material with which the polarization and fatigue properties are improved, we take notice of barium metaplumbate BaPbO₃ (BPO). Because the BPO contained lead (Pb) and oxygen is conductor that adopted same perovskite structure as PZT. BPO thin films were prepared by rf magnetron sputtering on various substrates. (SiO₂/Si, MgO, Al₂O₃ and Pt-coated substrates), and influence of growth conditions (sputtering gas, rf power, the substrate-heating temperature and post anneals) on crystallization and conductivity were investigated. In case of post anneal after sputtering at room temperature, perovskite single phase was obtained above 400°C. In case of substrate heating while sputtering, without post anneal, perovskite single phase was obtained at 350–500°C on SiO₂/Si substrates (110) preferred orientation BPO films obtained at low temperature, and resistivity of the films decreased at decreasing sputtering temperature. Resistivity of the film at substrate temperature 350°C was 3 × 10^?3 Ω cm. In the case of single crystal substrate, the BPO films were epitaxially grown. Orientation of the films was varied with the sputtering condition. The epitaxial PZT thin films were also grown on the BPO, revealing that PZT(111)[011] //BPO(111)[011] //Pt(100)[011] //MgO(100)[011] and PZT(111)[011] //BPO(111)[011] //Pt(111)[011] //Al₂O₃(001)[100] structures were obtained, and their ferroelectric properties were also evaluated.     

MFIS and MFMIS structures using Pb(Zr,Ti)O3 films for nonvolatile memory devices

Abstract

In this work, metal / ferroelectric / insulator / semiconductor (MFIS) and metal / ferroelectric / metal / insulator / semiconductor (MFMIS) structures using Pb(Zr, Ti)O₃ (PZT) films were fabricated and characterized for nonvolatile NDRO memory device. 300nm-thick PZT films were deposited by reactive RF magnetron sputtering method on ZrTiO₄(ZT)/Si and Pt/ZT/Si substrates. C-V hysteresis were measured in both MFIS and MFMIS structures. By using a small-size MFM capacitor on a large-size MIS structure, it was found that the memory window of MFMIS structure was larger than that of the MFIS structure. There is a critical area ratio (S_MIS/S_MFM) in MFMIS structure. When an area ratio in MFMIS structure is below 12, the memory window increased with increasing area ratio. We could obtain that the memory window of MFMIS structure with a S_MIS/S_MFM of 11.8 was 2.1 V and 3.2 V with an applied voltage at 3 V and 5 V.     

FERROELECTRIC GATE FET MEMORY BASED ON CONDUCTION OF FERROELECTRIC-INSULATOR INTERFACE

ABSTRACT

A new type of ferroelectric gate field effect transistor (FET) using ferroelectric-insulator interface conduction has been proposed. Drain current flows along the interface between ferroelectric and insulator layers and needs no semiconductor. This FET consists of source and drain electrodes on ferroelectric film (Pb(Zr_0.52Ti_0.48)O₃(PZT)) prepared on Pt/TiO₂/SiO₂/Si substrate, and gate electrode on HfO₂ insulator film on the PZT film between the source and the drain electrodes. Drain current flows through the interface of the ferroelectric and the insulator. Drain current versus gate voltage characteristics shows clockwise hysteresis loop similarly to the conventional p-channel FET with ferroelectric gate. The FET shows that the On/Off ratio of the conduction current is about 10⁵ and the Off state current is about 10^{? 10}A.     

Dielectric Characteristics of In-Plane Polarized Lead Zirconate Titanate Thin Films on Oxide Layers

Sol-gel derived lead zirconate titanate (PZT) films have been prepared on STO-passivated silicon substrates. The STO buffer layer of thickness about 55 nm is prepared by rf-magnetron sputtering. XRD results reveal that the PZT film has well-crystallized perovskite phase, indicating that the thin STO layer can effectively prevent reaction and inter-diffusion between the PZT film and silicon substrate. Interdigitated electrodes (IDEs) have then been deposited on the PZT film by magnetron sputtering and patterned using the standard photolithography. With the IDEs, the dielectric and ferroelectric properties of the PZT film under transverse or in-plane electric fields have been investigated. By assuming a uniform distribution of electric field (in-plane electric field model), the estimated relative permittivity of the PZT film is about 2100, while the dielectric loss is less than 1%. Good in-plane polarization hysteresis loop is observed, showing an observed remanent polarization value of 21 μC/cm², which is comparable to that of a PZT film with both top and bottom electrodes. The in-plane polarized PZT/STO/SiO₂/Si film can be used to fabricate d₃₃-mode unimorph bending transducers, which will have much better performance than the conventional bending transducers driven electromechanically through the piezoelectric d₃₁ mode.     

An aqueous,low temperature process for synthesizing PZT(53,47) thin films

Abstract

The Deposition by Aqueous Acetate Solution (DAAS) technique has been developed for the preparation of thin films of Pb(Zr_0.53Ti_0.47)O₃[PZT(53,47)] perovskites. This process, which employs titanium acetate, tends to establish a chemically bound network in the pre-annealed phase and facilitates the crystallization of ferroelectric lead perovskites at a relatively low temperature. The addition of surface wetting reagents and oxidants and the action of ultrasonic waves were shown to affect the crystallinity and film quality of PZT(53, 47) perovskites on Pt<111>/Ti/SiO₂/Si<100> substrates. The band structure analysis of the FTIR spectrum is illustrated to be a simple way of monitoring the crystallization of PZT(53, 47) perovskites. Physical and electrical characterization of the resultant thin films were performed. The advantages of the DAAS process for fabricating thin ferroelectric films are examined.     

Bilayered Pb(Zr,Ti)O3/(Bi,Nd)4Ti3O12 thin films

Bilayered ferroelectric thin films consisting of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) have been successfully synthesized on Pt/Ti/SiO₂/Si substrates, via a combined sol–gel and rf-sputtering route. Their ferroelectric and dielectric properties are critically dependent on the phases present, film texture and in particular layer and film thicknesses. Due to the coupling of PZT and BNT bilayers, there requires an optimized thickness combination of the two ferroelectric layers, in order to give rise to the wanted ferroelectric and dielectric properties, while the phenomenon can not be accounted for by the simple series connection model.     

Pb(Zr,Ti)O3 thin film growth on yttrium-treated Si(100)

Abstract

Pulsed laser ablation has been used to deposit ferroelectric Pb(Zr, Ti)O₃ (PZT) thin films on Si(100) and on yttrium-treated Si(100) substrates. The yttrium (Y) treatment of a Si surface followed by oxidation resulted in formation of a very thin, Y-enhanced SiO₂ antidiffusion barrier layer, thereby suppressing the undesirable PZT/Si interdiffusion. The best PZT film grown on Y-treated Si(100) had a breakdown voltage of 0.6 MV/cm, a coercive field of 71 KV/cm, and a remanent polarization of 18 μC/cm².     

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.     

Interaction of Ir and IrO2 thin films with polysilicon,W and WSIx

Abstract

Ir and IrO₂ thin films have been identified as potential electrode materials for ferroelectric capacitors. These electrodes have shown excellent electrical characteristics. The integration of ferroelectric capacitors into memory cell requires the bottom electrode material to be placed directly over a contact plug. This paper studies the interaction of Ir and IrO₂ with commonly used plug materials such as polysilicon, tungsten (W), and tungsten silicide (WSi_x) after a post-deposition annealing at 800°C. Film properties such as composition, resistivity, crystallinity, adhesion, and micro-structure have been examined before and after anneal. The results show that W is a possible plug material for Ir electrode; while polysilicon and WSi_x are potential candidates if IrO₂ electrodes are used.