BST Capacitor Research Articles

Influence of CCTO Doped Amount on Property of BST Capacitor Ceramics Sintered at Low Temperature

(1-x) Ba0.65Sr0.35TiO3-xCaCu3Ti4O12(BST-CCTO) ceramics was prepared by traditional solid-state reaction method, the effect of CaCu3Ti4O12 (CCTO) doped on microstructure and phase composition and dielectric properties of BST ceramics were investigated by X-ray diffraction(XRD), scanning electron microscope(SEM) and other analytical methods. The results show that the microstructure and the dielectric properties and the optimum sintered temperature of BST ceramic materials would be greatly affected by the CCTO doped amount. Appropriate doping amount can be fully incorporated into the perovskite lattice of BST and had the excellent performance. While excessive CCTO doping could lead to the appearance of second phase, and the second phase was coexisted with BST in the ceramics. BST ceramics had the optimum properties, ϵr of 1992, tan δ of 0.006, Eb of 5.5 kVmm−1 (Alternating Current, AC) and the value of ΔC/C ranges was 13.6% and −14.1% in −55°C∼+125°C temperature range and its capacitance temperature property was suited for X7R, the curie temperature was 15°C and the optimum sintered temperature was 1260°C when CCTO doped amount was 1 wt.%.

Microstructure and electrical characteristics of Ba 0.65Sr 0.35TiO 3 thin films etched in CF 4/Ar/O 2 plasma

Radio frequency magnetron sputtered Ba 0.65Sr 0.35TiO 3 (BST) thin films were etched in CF 4/Ar/O 2 plasma by magnetically enhanced reactive ion etching technique. The etching characteristics of BST films were characterized in terms of microstructure and electrical properties. Atomic force microscopy and X-ray diffraction results indicate that the microstructure of the etched BST film is degraded because of the rugged surface and lowered intensities of BST (1 0 0), (1 1 0), (1 1 1) and (2 0 0) peaks compared to the unetched counterparts. Dielectric constant and dielectric dissipation of the unetched, etched and postannealed-after-etched BST film capacitors are 419, 346, 371, 0.018, 0.039 and 0.031 at 100 kHz, respectively. The corresponding dielectric tunability, figure of merit and remnant polarization are 19.57%, 11.56%, 17.25%, 10.87, 2.96, 5.56, 3.62 μC/cm 2, 2.32 and 2.81 μC/cm 2 at 25 V, respectively. The leakage current density of 1.75 × 10 −4 A/cm 2 at 15 V for the etched BST capacitor is over two orders of magnitude higher than 1.28 × 10 −6 A/cm 2 for the unetched capacitor, while leakage current density of the postannealed-after-etched capacitor decreases slightly. It means that the electrical properties of the etched BST film are deteriorated due to the CF 4/Ar/O 2 plasma-induced damage. Furthermore, the damage is alleviated, and the degraded microstructure and electrical properties are partially recovered after the etched BST film is postannealed at 923 K for 20 min under a flowing O 2 ambience.

Improvement of Electrical Properties of $\hbox{Ba}_{{\bf 0.7}}\hbox{Sr}_{\bf 0.3}\hbox{TiO}_{\bf 3}$ Capacitors With an Inserted Nano-Cr Interlayer

The metal-insulator-metal (MIM) capacitors were prepared with Ba0.7Sr0.3TiO3/Cr/Ba0.7Sr0.3TiO3 (BST/Cr/BST) dielectric and Pt electrode. The multilayer BST/Cr/BST was sputtered onto Pt/Ti/SiO2/Si substrate. The presence of nano-Cr interlayer affects the electrical properties of the capacitors. The temperature coefficient of capacitance (TCC) of capacitors with 2 nm Cr is about 69% of that of capacitors without Cr. In a previous work, the formation of the TiO2 secondary phase was found after the BST/Cr/BST dielectrics were annealed at 1023 K in O2 atmosphere for 1 h. It is suggested that the nano-Cr interlayer as a catalyst leads to the TiO2 formation during the annealing in O2 atmosphere. The negative value of TCC of BST can be compensated by the positive TCC of TiO2, and the temperature stability in the dielectric constant can be realized for capacitors with nano-Cr interlayer. The voltage stability of BST is also improved with the insertion of nano-Cr interlayer, and the quadratic coefficient in voltage coefficient of capacitance (VCC) of Pt/BST/Cr(2 nm)/BST/Pt is about 30% of that of the BST capacitor without Cr. The effects of Cr thickness on TCC, VCC, dissipation factor, and leakage current density of Pt/BST/Cr/BST/Pt parallel plate capacitors are investigated.

Consideration for Broadband Frequency Conversion Based on Ba0.5Sr0.5TiO3 Films

A frequency converter based on a BaxSr1-xTiO3 (x=0.5) (BST) film was successfully realized using the nonlinearity of the film. Frequency conversion occurred due to the distortion of a signal across the BST film, because the relation between electric charge and external bias is nonlinear in the film. Frequency conversion at low frequency from 2 to 6 MHz and 4 to 12 MHz was realized using a parallel-plate BST capacitor. Frequency conversion at high frequency from 500 MHz to 1 GHz was demonstrated, yielding a conversion loss of 30 dB at an input power of 19 dBm. The conversion loss of the frequency converter can be improved by increasing the input power on a BST film.

A 1-GHz active phase shifter with a ferroelectric varactor

An active phase shifter circuit implemented with discrete components is reported. The tuning element, a ferroelectric varactor, is a parallel plate capacitor with Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3/ (BST) as the dielectric. The circuit consists of two bipolar junction transistors coupled with a feedback network, which contains the varactor and thus produces a transfer function that can be varied with a control voltage. The active nature of the circuit allows for signal gain, while the BST varactor provides a high-Q tuning element. This represents an improvement over strictly passive phase shifters with ferroelectric elements. Circuit simulation results are presented and compared with measured data from the implemented system. The network, even with markedly nonideal transistors, can provide a true all-pass response over the frequency band of interest (200-1100 MHz). The measurement results demonstrate an analog tunability of about 100/spl deg/ with a gain variation of about 0.6 dB at I GHz when using a BST capacitor with a tunability of 2.75:1.

(Ba,Sr)TiO3 (BST) capacitor fabrication using electrochemical deposition (ecd) platinum process

ECD Pt process has been developed as an electrode for sub-0.lμm technology device. Using ECD Pt process, it is possible to fabricate higher. stacked BST capacitor with a lot of advantages such as selective deposition which is storage node patterning without Pt RIE process and low cost of ownership (COO). Physical properties of ECD Pt films such as sheet resistance, surface roughness, and film stress are similar to sputtered Pt film. The leakage current density and Tox of CVD BST capacitor using ECD Pt bottom electrode were less than 10−7A/cm2 at ± 1.0V and 0.5nm, respectively. Using the ECD Pt process, three dimensional Pt stacked BST capacitor with 0.22μm width and 0.45μm height was successfully fabricated. The capacitance and leakage current of Pt stacked BST capacitor with ECD Pt showed 29fF/cell and 10−17A/cell in 256k array cell, respectively. In addition to these, we confirmed that it is possible to use ECD Pt film as top electrode.

Stabilization of ultra-thin Pt bottom electrode for 0.15 μm cup-type BST capacitor

Structural properties and electrical properties of the ultra-thin Pt film on the cup-type structure were studied. We found that Pt film was segregated inside the cup-type structure after oxygen ambient anneal above 400 °C. For the cup-type structure, the segregation was occurred by Pt migration from bottom part of the cup structure, the thinner area, to top part of the cup structure, the thicker area. To overcome this Pt segregation issue, we developed a post-anneal sequences using hydrogen ambient. With the optimized post-anneal sequence, hydrogen anneal above 500 °C, was be able to prevent from the segregation of Pt thin film for bottom electrodes of the cup structure. It was confirmed that the Pt bottom electrode annealed in hydrogen ambient did not degrade the electrical properties of the BST film. In addition, using this novel anneal sequence, 0.15 μm cup-type BST capacitor was demonstrated the leakage current of 1 fA/cell and capacitance density of 12 fF/cell.

Reliability of high dielectric Ba0.5Sr0.5TiO3 capacitors using iridium electrode

In order to clarify the decrease of the dielectric constant of Ba0.5Sr0.5TiO3 capacitor under the electrical stress, the degradation phenomena including resistant degradation were studied in detail. They were well explained by the increases of the dielectric relaxation and dielectric dispersion which are caused by the generation of oxygen vacancies. This explanation was proved by several additional experiments such as the investigation of Schottky barrier with the stress, the observation of some bubbles and O2 plasma annealing. It was also confirmed that the electrical degradation of BST capacitor is effectively suppressed by using iridium (Ir) electrode, especially as a top electrode.

Role of SrRuO3 Buffer Layers in Enhancing Dielectric Properties of Ba0.5Sr0.5TiO3 Tunable Capacitors

AbstractBaM0.5Sr0.5TiO3 (BST) is a first candidate material for the development of voltage-tunable microwave devices, such as, filter, phase-shifter and VCO. In this work, crystal structures and dielectric properties of BST film are investigated with and without SrRuO3(SRO) buffer layers. BST and SRO thin films are sequentially prepared by pulsed laser deposition and Au/Ti metal electrodes are fabricated by a DC magnetron sputtering system. The capacitance of the capacitors has been measured as a function of bias voltages at room temperature using a low frequency LCR meter. For the high frequency characteristics (∼2GHz), a microstrip resonator with ∼2GHz resonance frequency and the center coupling design is fabricated. Using flip-chip BST capacitor attached at the position of the center coupling on the microstrip resonator, Its dielectric loss and tunability were obtained. The microwave loss was obviously enhanced in the film with SRO buffer layer..

(Ba,Sr)TiO 3 thin films for ultra large scale dynamic random access memory. : A review on the process integration

Integration processes of the (Ba,Sr)TiO 3 (BST) capacitors for the next-generation dynamic random access memories (DRAMs) are reviewed. Various integration schemes utilizing different processes, different electrode materials and structures are reviewed from the point of view of the mass-production compatible process. Integration issues, mostly related to the electrode and the barrier material and their fabrication techniques, are described. The current status and the problems of the two most viable techniques for the BST deposition, sputtering and metal-organic chemical vapor deposition (MOCVD), are comparatively described. Plate electrode technologies with the back-end processing issues are also briefly described, and some valuable experimental results are presented to show the feasibility of the BST capacitor.

Chemical Vapor Deposition Technology of (Ba,Sr)TiO3 Thin Films for Gbit-Scale Dynamic Random Access Memories

AbstractThe current status of (Ba,Sr)TiO3 [BST] capacitor technology using a liquid source chemical vapor deposition (CVD) method is reviewed, focusing on the CVD techniques and the physical, electrical and process-integration-related properties of Ru/BST/Ru capacitors. The use of a new titanium metalorganic (MO) source, titanium bis(tert-butoxy) bis(dipivaloylmethanato) [Ti(tertBuO)2 (DPM)2] dissolved in tetrahydrofuran (THF) turned out to enable highly conformal deposition of BST films with a coverage ratio of ∼ 70 % for a trench with an aspect ratio of ∼ 5. Electrical properties of a 24-nm-thick BST film, deposited on a Pt substrate at a low substrate temperature of 480 °C, were also confirmed to be equivalent SiO2 thickness (teq) of ∼ 0.5 nm and leakage current of ∼ 1 ×10-7 A/cm2 at 1 V. As for the Ru/BST/Ru capacitors, no deteriorations of Ru electrode and BST/Ru interface were observed after 750 °C post-annealing experiment, showing good thermal stability of Ru as a practical electrode material. Although current leak through Ru/BST/Ru capacitors slightly increased after the H2 annealing, such degradation in the leakage properties was restored by post-annealing in N2 ambience. Integrated Ru/BST/Ru capacitors with a 30-nm-thick CVD-BST film were fabricated by 0.5 μm ULSI technology, and low leakage current was confirmed for the stacked capacitors. Regarding the reproducibility of BST deposition by the liquid source CVD method, the deviation ratio of ∼ ± 2.3 % in film thickness was obtained for ∼ 100 successive depositions, thickness uniformity across the wafers was ∼ ± 1.1 %. The above results imply the potential applicability of BST capacitor technology using a liquid source CVD method for Gbit-scale DRAMs.

High frequency electrical characterization of bst capacitors

We have characterized the dielectric properties of barium strontium titanate (Ba1-xSrxTiO3, x=0.5; BST) capacitors up to 5 GHz employing a simple, single port scattering parameter measurement technique. Metal-insulator-metal type capacitors with areas ranging from 400 to 10,000 μm2 were defined by standard photolithographic and etching techniques. BST films were rf-magnetron sputter-deposited on metallized (111) silicon. The bottom electrode was made by evaporating 100A of Ti for adhesion followed by 1500A of Pt. The top electrode consisted of 3000A of gold with a 100A titanium adhesion layer. The test capacitors were surrounded by a reference capacitor approximately 140 times in area to facilitate high frequency measurements with air coplanar (ACP) probes. The test and the reference capacitors were in series during characterization ensuring that the resulting capacitance was dominated by the smaller test structure. Single port scattering parameters (S11) were obtained using a HP8510B network analyzer. HP Microwave and RF Design System (MDS) was used for data analysis. The BST capacitor structures exhibited a roll-off near 1 GHz where the dielectric constant was reduced by 10–20% of the value at 50 MHz.