PZT Capacitors Research Articles

ELECTRICAL PROPERTIES OF Pb(Zr,Ti)O3 FERROELECTRIC CAPACITOR ON TiN/W PLUG STRUCTURE

ABSTRACT We investigated the electrical properties of ferroelectric Pb(Zr,Ti)O3(PZT) capacitor on TiN/W storage node contact plug that is capacitor over bitline(COB) cell structure for a high density ferroelectric memory. In order to protecting TiN/W contact plug from oxidation failure during furnace annealing of PZT capacitor, the oxidation barrier of Ti1 − x Al x N (x∼ 0.25) film was interposed at between Pt/IrO2/Ir bottom electrode and TiN/W plug of the COB cell, resulting in the robust contact resistance property of 27 ohm per contact plug after furnace annealing performed at 600°C in O2 ambient for 90 min. The remanent polarization value and leakage current density of the fully integrated PZT capacitor with a top electrode area of 1.8 × 1.8 um2 on TiN/W contact plug showed 32 μ C/cm2 and 2.5 × 10− 6A/cm2, respectively.

MASS PRODUCTIVE METALLIZATION PROCESS FOR 1.6 V OPERATIONAL FRAM USING AL-1 PLATE LINE WITH NEW TECHNOLOGY

ABSTRACT We have developed new BEOL technologies for mass-productive FRAM embedded smart card from which better device productivity was obtained. We have changed previous Al-2 plate line to new Al-1 plate line to improve bad step coverage of Al-2 plate line arising from deep via contact. Additional integration related damage on PZT capacitor by using Al-1 plate line scheme has been prevented by introducing new inter metallic dielectric (IMD) layer and new anneal process. As a result, a mass productive 1.6 V operational FRAM embedded smart card has been successfully demonstrated by the new Al-1 plate line BEOL technology.

Study on oxidization of Ru and its application as electrode of PZT capacitor for FeRAM

Oxidization for Ru through anneal with plenteous oxygen atmosphere and its application as the top electrode of sol–gel PZT capacitor are investigated in this study. PZT capacitor with RuO 2 or oxygen-doped Ru as top electrode can be obtained from Ru/PZT/Pt capacitor through slow-rate anneal at 650 °C for 20 min in cannulation furnace. It has larger remanent polarization, better rectangle shape, better fatigue properties and lower leakage current than the other capacitors with PZT film prepared by the same process and different top electrodes in this study. Plenteous oxygen atmosphere and 650 °C in cannulation furnace are important conditions for the oxidation of Ru and renewed crystallization of PZT in this capacitor. Plenteous oxygen at interface can compensate the oxygen vacancies at PZT/electrode interface, which results in the above good characteristics.

Fabrication of epitaxial and transparent Pb(Zr0.52Ti0.48)O3 ferroelectric capacitors with La0.07Sr0.93SnO3 electrodes

Conductive and transparent La0.07Sr0.93SnO3 (LSSO) films were employed as electrodes for the fabrication of epitaxial Pb(Zr0.52Ti0.48)O3 (PZT) ferroelectric capacitors. Due to a negligible lattice mismatch between LSSO and PZT, the LSSO/PZT/LSSO heterostructures grown on SrTiO3(001) substrates show excellent single-crystalline quality as revealed by high-resolution x-ray diffraction and are transparent with transmittance comparable to that of pure SrTiO3(001) substrates at the wavelength of 400–2500nm. These capacitors show square polarization–electric field hysteresis loops but smaller polarization, larger coercive field, and especially poor fatigue resistance, in sharp contrast with those observed for the conventional epitaxial all-oxide PZT capacitors.

RELIABLE PZT CMP TECHNOLOGY FOR NOVEL FRAM CAPACITORS

ABSTRACT In this paper, we report novel (TE-free) FRAM capacitors using reliable PZT CMP process in order to improve ferroelectric properties and enhance mass productivity by simplifying integration of processes. FRAM capacitors were prepared using a PZT film for a stopping layer in ILD CMP using a high selective slurry having a very low PZT removal rate relative to oxide removal rate. Noble metal electrode (TE) and contact (TEC) of conventional (TE/FE/BE) capacitors are not desirable for integration of high-density FRAM device and mass production due to contact etch damage and poor etch and deposition properties of Ir. We investigated the structural and electrical characteristics of MOCVD PZT films grown on Ir bottom electrode before and after PZT CMP process. Even though some degradation in polarization hysteresis of polished PZT capacitors was observed compared to as-deposited capacitors due to interface effect between PZT film and top electrode, electrical properties such as leakage current and fatigue were not sensitive to CMP process variables including down pressure, table speed and polishing time, indicating enough process margin in application of PZT CMP technology. In addition, PZT film roughness was dramatically reduced to result in decrease of leakage current due to the reduction of valley area. In this study, we claimed that new CMP technology with high selective slurry to PZT film was a useful approach in achieving novel FRAM capacitors by simplifying integration processes of FRAM device.

Fabrication of 3-Dimensional PbZr1−x TixO3 Nanoscale Thin Film Capacitors for High Density Ferroelectric Random Access Memory Devices

PbZr(x)Ti(1-x)O3 (PZT) thin films were deposited on 3-dimensional (3D) nano-scale trench structures for use in giga-bit density ferroelectric random access memories. PZT thin films were deposited by liquid delivery metalorganic chemical vapor deposition using Pb(thd)2, Zr(MMP)4, and Ti(MMP)4 precursors dissolved in ethyl cyclohexane. Iridium thin films were deposited by atomic layer deposition, and they exhibited excellent properties for capacitor electrodes even at a thickness of 20 nm. The trench capacitor was composed of three layers, viz. Ir/PZT/lr (20/60/20 nm), and had a diameter of 250 nm and a height of 400 nm. Almost 100% step coverage was obtained at a deposition temperature of 530 degrees C. The PZT thin film capacitors with a thickness of 60 nm on a planar structure exhibited a remnant polarization (Pr) of 28 microC/cm2, but the Pr value of the 3D PZT capacitors decreased slightly with decreasing 3D trench pattern size. The transmission electron microscope analysis indicated that the PZT thin films had compositional uniformity in the 3D trench region. Both columnar and granular grains were formed on the sidewalls of the trench capacitors, and their relative proportion exhibited strong size dependence. The trench capacitors with more columnar PZT grains showed good switching behavior under an external bias of 2.1 V and had a remnant polarization of 19-24 microC/cm2.

Characteristics of Polycrystalline SrRuO3 Thin-Film Bottom Electrodes for Metallorganic Chemical-Vapor-Deposited Pb ( Zr0.2Ti0.8 ) O3 Thin Films

In situ and ex situ crystallized polycrystalline (SRO) thin-film electrodes are fabricated by dc magnetron sputtering at a substrate temperature of 550 and 350°C, respectively, followed by postannealing for application as bottom electrodes of metallorganic chemical-vapor-deposited (PZT) thin films. The in situ crystallized SRO electrode shows a negligible change in film composition during the subsequent annealing and works as a good electrode for the ferroelectric PZT films. However, the ex situ crystallization by postannealing largely decreases the Ru content in the SRO film and consequently the PZT film grown on top has a poor ferroelectric performance. In addition, the Zr component in the PZT film initially reacts with the excessive in the electrode, resulting in a deposition of at the initial stage of the PZT deposition which largely deteriorates the ferroelectric performance. Therefore, it is crucial to have in situ crystallized SRO for a reliable electrode of a PZT capacitor.

Integration of lead zirconium titanate thin films for high density ferroelectric random access memory

Interests are being focused on types of nonvolatile memories such as ferroelectric random access memory (FRAM), phase change random access memory, or magnetoresistance random access memory due to their distinct memory properties such as excellent write performance which conventional nonvolatile memories do not possess. Among these types of nonvolatile memories, FRAM whose cell structure and operation are almost identical to dynamic random access memory (DRAM) can ideally realize cell size and speed of DRAM. Thus FRAM is the most appropriate candidate for future universal memory where all memory functions are performed with a single chip solution. Due to the poor ferroelectric properties of downscaled ultrathin lead zirconium titanate (PZT) capacitors as well as technical issues such as hydrogen and plasma related degradation arising from embedding ferroelectric metal-insulator-metal capacitors into conventional complementary metal oxide semiconductor processes, current FRAM still falls far below its ideally attainable cell size and performance. In this paper, based upon PZT capacitor, current mass-productive one pass transistor and one storage capacitor (1T1C), capacitor over bit line (COB) cell technologies are introduced upon which cell size of 0.937μm2 at 250nm minimum feature size technology node has been realized. And then, most recent 1T1C, COB cell technologies are discussed from which cell size of 0.27μm2 at 150nm minimum feature size technology node has been realized, and finally future three dimensional capacitor technologies for the FRAM with cell size of less than 0.08μm2 beyond 100nm minimum feature size technology node are suggested.

TRIAL FOR MAKING THREE DIMENSIONAL PZT CAPACITOR FOR HIGH DENSITY FERROELECTRIC RANDOM ACCESS MEMORY

ABSTRACT Three dimensional capacitors of Pb(Zr,Ti)O3 [PZT] together with the Ru electrodes were tried to deposit by metal organic chemical vapor deposition for realizing high density ferroelectric random access memory (FeRAM). Good conformability was obtained for both of CVD-Ru and PZT layers on the trenched substrates by optimizing the deposition conditions. In addition, PZT films deposited on Ru bottom electrode showed the good compositional conformability. Finally we demonstrated the Ru/PZT/Ru three dimensional capacitor. These data accelerate the development of three dimensional PZT capacitors.

CHARACTERIZATION OF NOVEL ELECTRODE FOR HIGH-DENSITY Pb(Zr, Ti)O3 CAPACITOR APPLICATION

ABSTRACT For high-density FRAM application, we found novel electrodes of IrTi family having a sufficiently low resistance. In addition, IrTi show good adhesion and better surface roughness than Ir. From FWHM results, PZT crystallinity is improved by applying IrTi. IrTi enhanced PZT capacitor properties of Fatigue and Retention. In 60 nm PZT, IrTi and IrTiOx/IrTi bottom electrode were effected increasing remnant polarization of 37 uC/cm2 and 47 uC/cm2 at 1.8 V. We successfully adopted IrTi family bottom electrode for making ultra thin PZT:40 nm PZT have good electric results such as [2Pr = 37 μ C/cm2, at 2 V]. Results of 40 nm PZT using IrTiOx/IrTi electrode is possibly given a challengeable clue for high-density FRAM.

Fatigue Properties of Epitaxial Pb(Zr0.35Ti0.65)O3 Films with Various Volume Fractions of 90° Domains Grown on (100)cSrRuO3/(100)SrTiO3 Substrates

(001)/(100)-oriented tetragonal epitaxial Pb(Zr0.35,Ti0.65)O3 [PZT] thin films with various volume fractions of 90° domains were grown on (100)cSrRuO3∥(100)SrTiO3 substrates by metalorganic chemical vapor deposition. The polarization-electric field hysteresis curve and fatigue behavior of a PZT capacitor with Pt and SrRuO3 top electrodes were systematically investigated. In a Pt/PZT/SrRuO3 capacitor, the degradation of switching polarization occurred between 104 and 107 cycles. In contrast, a PZT capacitor with SrRuO3 top electrodes revealed almost fatigue-free behavior up to 109 switching cycles. It was found that the fatigue properties of PZT films is strongly dependent on the type of top electrode, irrespective of the volume fraction of the (001) orientation. Moreover, it was also found that the a-domains switching of PZT thin films is independent of the fatigue properties.

Pt/Pb(Zr, Ti)O3/Pt capacitor with excellent fatigue properties prepared by sol–gel process applied to FeRAM

Lead zirconate titanate (PZT) film is prepared on Pt/Ti/SiO2/Si substrate by proposed processes based on the sol–gel method and rapid thermal anneal (RTA). The ratio of Zr/Ti in the PZT film is 40/60. The PZT film has a mixture of perovskite orientations in which the (110) orientation is dominant. The Pt/PZT/Pt capacitor has remanent polarization of approximately 28.8 µC cm−2 and coercive voltage of approximately 0.76 V at 3 V voltage amplitude. The Pt/PZT/Pt capacitor has excellent fatigue properties. Switch polarizations decrease to 93.1% after 6 × 1012 switch cycles. The excellent fatigue properties result from the ameliorations of PZT/Pt interface conditions, restraining Pb volatilization and the directions of crystal domains in the RTA process. Some electric properties of the PZT capacitor proposed are contrasted with those of PZT capacitors with a different anneal process in the preparation.

Enhancement in Ferroelectric Properties of Pb ( Zr0.4Ti0.6 ) O3 Thin-Film Capacitors with PtOx Electrodes

The characteristics of sputtered electrodes and the ferroelectric reliability of (PZT) thin-film capacitors for memory application in relation to the oxygen content in the electrodes were investigated. The PZT thin films of thickness were deposited on electrodes. Both Pt and PtO phases coexist and a compressive stress is also induced in the electrode after rapid thermal annealing (RTA) at . The polarization-switching characteristics, including polarization value and switching rate of PZT capacitor, are improved by using the as electrodes, especially for the bottom electrode, in comparison to that made with Pt electrodes. In addition to the enhancement in polarization-switching characteristics, the properties of fatigue endurance were tested at room temperature. High fatigue endurance (97% of initial remnant polarization remains after ) can also be achieved for the capacitors having both top and bottom electrodes made with having a high oxygen content.

Highly Reliable 0.15 µm/14 F2 Cell Ferroelectric Random Access Memory Capacitor Using SrRuO3 Buffer Layer

We investigated a novel technique of modifying the interface between a Pb(ZrxTi1-x)O3 (PZT) thin film and electrodes for high density 64 Mbit ferroelectric random access memory (FRAM) device. Using a SrRuO3 buffer layer, we successfully developed highly reliable 0.15 µm/14 F2 cell FRAM capacitors with 75-nm-thick polycrystalline PZT thin films. The SrRuO3 buffer layer greatly enhanced ferroelectric characteristics due to the decrease in interfacial defect density. In PZT capacitors with a total thickness of 180 nm for whole capacitor stack, a remnant polarization of approximately 42 µC/cm2 was measured with a 1.4 V operation. In addition, an opposite state remnant polarization loss of less than 15% was observed after baking at 150 °C for 100 h. In particular, we found that the SrRuO3 buffer layer also played a key role in inhibiting the diffusion of Pb and O from the PZT thin films.

Comparison of Properties of Pt/PZT/Pt and Ru/PZT/PtFerroelectric Capacitors

Pb(Zr0.4Ti0.6)O3 film prepared by sol-gel spin coating ona Pt/Ti/SiO2/Si substrate is applied to ferroelectric capacitorswith Pt or Ru as the top electrode. For the Pt/PZT/Pt and Ru/PZT/Ptferroelectric capacitors, although with the same ferroelectric film,different top electrode materials incur different properties of PZTcapacitors, such as fatigue, leakage, remanent and saturatedpolarization, except the similar crystal orientations of the PZT film.After 1010 switch cycles, the remanent polarizations of theRu/PZT/Pt and Pt/PZT/Pt capacitors decrease to 70% and 84%,respectively. The leakage current density of the latter increasesobviously at positive bias after 108 switch cycles, compared withthe former. Different materials for the top electrode bring differentconditions at the PZT/top electrode interface. The influence ofoxygen-vacancy concentration at the PZT/electrode interface and theinfluence of oxides of the electrode material at the PZT/electrodeinterface to charge injection can explain the difference of propertiesof the PZT capacitors with Pt or Ru as the top electrodes.

The Investigation of Ferroelectric Domain Behavior Affected by Thin Metallic Electrode

The physical and chemical properties of metallic electrodes on Pb(Zr,Ti)O3 (PZT) are investigated by atomic force microscopy. Pt, a catalyst, tends to change itself into a dielectric oxide. Such a passive layer should cause a depolarization field driving the domain structure into the polystate. Therefore, the effective thickness of the Pt electrode becomes smaller than that of Ir electrode. Thus, we observed an opposite domain contrast around a region covered by thin Pt on PZT, which implied the failure of the screening of the depolarizing field. This multidomain configuration can cause a suppression of the remnant polarization of PZT capacitors. Also, Ir is better than Pt in regard to film smoothness, indicating that the adhesion property of Ir on PZT is superior to that of Pt on PZT. We prove it using height–height correlation function. This study is significant on the scaling issues of ferroelectric capacitors, such as the thickness limit of the electrode and the requirement for high surface smoothness of ferroelectric films.

Imprint characteristics of Pt/Pb(Zr,Ti)O3/Ir capacitors

The dynamic and static imprint characteristics of PZT thin film capacitors prepared by a two step reactive sputtering method were studied. The imprint is caused by an internal field generated by the trapping of electronic charges at interfacial layers that are injected, from the electrode. When unipolar pulse stressing is applied to the PZT capacitor, the cumulative time rather than the pulse cycles accounts more for the dynamic imprint characteristics. The amount of voltage shift in the polarization(P)-voltage(V) curve is reduced when the unipolar pulses are applied at elevated temperatures or a high amplitude pulse is applied. For static imprint stressing, the voltage shift increases with the temperature and is readily removed by application of bipolar pulses to the imprinted PZT capacitor at elevated temperatures. A method to estimate the lifetime limitation as a result of imprint failure in 1T/1C FRAMs is proposed.

Formation of Stoichiometric SrRuO[sub 3] Electrodes for PZT Capacitors by Pulsed-MOCVD

SrRuO 3 films were deposited at 300-500°C on Pb(Zr 0.4 , Ti 0.6 )O 3 -covered Ir/SiO 2 /Si substrates by metallorganic chemical vapor deposition (MOCVD). Through the pulsed introduction of a source gas mixture (pulsed-MOCVD), the Ru/(Ru + Sr) ratio became unity above 350°C due to a similar deposition rate for SrO and RuO x components in the film. It became unity at 450°C in spite of the increased input-gas flow rate of the Ru source under the fixed Sr source and we obtained films consisting of crystalline stoichiometric SrRuO 3 . This demonstrated that good crystalline stoichiometric SrRuO 3 film was reproducible by pulsed-MOCVD. Excellent fatigue properties and ferroelectricity were obtained from the SRO/PZT/Ir capacitor.

Fabrication of Nanoscale PtO[sub x]∕PZT∕PtO[sub x] Capacitors by E-beam Lithography and Plasma Etching with Photoresist Mask

Nanoscaling of PtO x /Pb(Zr x Ti 1-x )O 3 (PZT)/PtO x capacitors in well-ordered arrays down to a cell size of 90 X 90 nm was sucessfully achieved by electron-beam lithography and plasma etching with photoresist mask. Fast etch of PtO x and PZT layers was obtained with a rate of 165 nm/min in plasma of Ar/Cl 2 /10%O 2 and 240 nm/min in Ar/CF 4 /30%O 2 , respectively. The selectivity of PtO x to NEB resist was 5.8 and that for PZT was 4.3. Crystallization of PZT film and reduction of PtO x layer were also observed after post-annealing of the etched specimens. Direct electrical measurement reveals that a good polarization switching characteristic can be retained in the nanoscaled PZT capacitors.

Improvement of retention loss in Pb(Zr,Ti)O3 capacitors using Ir∕SrRuO3 top electrodes

We investigated the retention behavior of Pb(Zr,Ti)O3 (PZT) capacitors with Ir∕SrRuO3 (SRO) top electrodes. The capacitors with Pt and Ir∕IrO2 top electrodes were also prepared for comparison. The opposite state retention characteristic of the PZT capacitor was significantly improved by using an Ir∕SRO top electrode structure. The nonvolatile polarization of the opposite state retention was kept at 96% of its initial value even after a 100h baking test at 150°C, while those of Pt and Ir∕IrO2 were 36 and 59%, respectively. The retention enhancement is attributed to the effective inhibition of defect generation on the interface between PZT and SRO.