Takeshi Shioga

Investigation of the importance of interface and bulk limited transport mechanisms on the leakage current of high dielectric constant thin film capacitors

The importance of interface and bulk transport mechanisms on the leakage current of high dielectric constant thin film capacitors is examined by deriving an equation for the J–VA characteristic of a capacitor that includes the transport mechanisms of thermionic emission (TE), thermionic field emission (TFE), and carrier drift–diffusion (DD). The current is controlled by the slowest of three effective velocity parameters v1md, vD, and ṽ2dm characterizing electron injection into the dielectric at the cathode by TE and TFE, carrier DD in the film bulk, and electron ejection from the dielectric at the anode by TE and TFE, respectively. The effective velocity parameters are evaluated for a Pt/BST/Pt thin film capacitor that has been exposed to forming gas and it is shown that the dominant transport mechanism is interface limited TFE from the cathode with negligible influence of carrier transport by DD in the film bulk. Implications of these results on existing transport calculations for high dielectric constan...

Surface core-level shifts of strontium observed in photoemission of barium strontium titanate thin films

Angle resolved x-ray photoelectron spectroscopy was used to investigate the surface electronic structure of barium strontium titanate (BST) films. In contrast to previous photoemission studies which identified two chemical states associated with only Ba in the near surface region, the authors have resolved core-level features from surface Sr atoms which provide new insight into the surface electronic structure of BST thin films. The surface Sr 3d features are found to lie approximately 1eV higher in binding energy than the bulk derived peaks. The effects of aqueous and annealing surface treatments and the origin of the Sr surface core-level shifts are discussed.

Effect of Y doping and composition-dependent elastic strain on the electrical properties of (Ba,Sr)TiO3 thin films deposited at 520 °C

We demonstrate that large and simultaneous improvements in permittivity, tunability, and leakage current density of (Ba,Sr)TiO3 (BST)-based thin-film capacitors can be achieved by yttrium doping. We have found that, for a low deposition temperature (520 °C) sputtering process, Y-doped BST capacitors exhibit tenfold lower leakage current density (<10−9A∕cm2 at 100KV∕cm) and 70% higher permittivity than nominally undoped BST-based capacitors. Furthermore, this work suggests an intriguing correlation between dopant concentration-dependent elastic strain in the films and their enhanced dielectric properties.

A study of current transport in (BaxSr1−x)Ti1+yO3+z thin-film capacitors containing a voltage-dependent interface state charge distribution

J–V–T characteristics, measured over a very wide temperature (50–441 K) range, are analyzed simultaneously with C–V measurements to investigate conduction mechanisms and the state of charge of Pt/BaxSr1−xTi1+yO3+z(BST)/Pt thin-film capacitors during an applied voltage or current stress. A time- and voltage-dependent state of charge of the Pt/BST/Pt capacitors is inferred from stress-induced voltage shifts in the C–V curves. The voltage and temperature dependence of the C–V curve shifts is shown to be consistent with a voltage-dependent charge in interface states resulting from a change in potential across interfacial dipole layers. An intimate contact Schottky barrier model incorporating a voltage-dependent charge in interface states at both cathode and anode contacts is used to investigate conduction mechanisms in Pt/BST/Pt thin-film capacitors. The basic transport mechanisms of drift-diffusion, thermionic field emission, and Fowler–Nordeim tunneling are shown to dominate leakage in limiting voltage and ...

Microstructural and Electrical Properties of (Ba x Sr 1 − x )Ti 1 + y O 3 + z Thin Films Prepared by RF Magnetron Sputtering

The electrical and microstructural properties of (Ba x Sr 1 m x )Ti 1+y O 3+z (BST) thin films prepared by RF magnetron sputtering were investigated as a function of deposition temperature over the range of 100 C to 650 C. Films deposited above approximately 350 C on Pt/TiO 2 /SiO 2 /Si substrates were polycrystalline with relative permittivites of 100 nm thick BST thin films varying from 100 at 350 C to 600 at 650 C. For deposition temperatures below approximately 350 C, the electrical properties were strongly influenced by the presence of a less crystalline BST layer. Films deposited at 250 C were comprised of a multilayer polycrystalline/less crystalline BST structure. The less crystalline BST layer strongly affected both measured dielectric permittivity and leakage properties. Leakage characteristics of Pt/BST(250C)/Pt capacitors exhibited a power law dependence on voltage or an exponential dependence on square root of the applied voltage depending on whether the top Pt electrode adjacent ot the polycrystalline BST layer was biased at a high or low electric potential, respectively. Mechanisms for the observed leakage behavior are discussed.

Dielectric constant dispersion of yttrium-doped (Ba,Sr)TiO3 films in the high-frequency (10kHz–67GHz) domain

The frequency dispersion of the dielectric constant of yttrium (Y)-doped (Ba,Sr)TiO3 thin films (Y-BST) in the high-frequency domain (10kHz–67GHz) was investigated. In order to remove the substantial parasitic capacitances, inductances, and resistances from the measured impedance data, test samples, short-circuit standard, and open-circuit standard structures were fabricated and their frequency response was measured. Before removing parasitic components, the measured dielectric response showed a rolloff at approximately 4GHz. However, after circuit calibration, the dielectric constant was almost constant up to 40GHz where another rolloff was observed. However, this rolloff was due to the uncompensated small parasitic components. Therefore, the dielectric constant of the Y-BST films (170 with a film thickness of 30nm) showed small frequency dispersion corresponding to the Curie–von Schweidler dispersion, of which the exponent is −0.0131, up to 40GHz. Furthermore, the decrease of the capacitance was 17% in ...

Electrical properties of low-inductance barium strontium titanate thin film decoupling capacitors

Very low inductance capacitors using barium strontium titanate (BST) based thin film have been developed for use in decoupling applications for GHz LSI operations. Increasing clock frequency and integration density of high performance logic LSI such as CPU requires very low power line impedance over wide frequency ranges up to GHz order. BST thin film capacitors are promising for GHz LSI applications due to excellent electrical properties of low inductance and high capacitance. Low temperature sputter deposited BST thin films show high capacitance and good leakage properties. The fabricated thin film chip capacitors of 150 μm bump pitch show low equivalent series inductance (ESL) of 17 pH and low ESR of 0.05 Ω. Impedance of the chip capacitor at 1 GHz is 100 times lower than conventional multilayered ceramic capacitors (MLC). These results indicate that developed capacitors are suitable for the decoupling applications to GHz LSI operation.

Micro loop heat pipe for mobile electronics applications

This study demonstrates a new cooling solution for the thermal management of mobile electronics. It uses a micro loop heat pipe (μLHP) with a thickness of 0.6 mm for the evaporator and 1.0 mm for the vapor line, and is designed to fit in the casing of a smartphone. We fabricated a prototype model of the μLHP by a chemical-etching and diffusionbonding process for thin copper plates. Heat from the heatgenerating components was successfully transferred by optimizing the pressure drop in the vapor and condenser lines. Thermal resistance between the μLHPs evaporator and condenser of 0.8 K/W was achieved at 5 W with an evaporator temperature of 50.5 °C for horizontal operation. We confirmed that the orientation does not significantly affect the operation of the μLHP, which is capable of transferring a heat load of up to 15 W despite different orientations.

Relaxorlike dielectric behavior in Ba0.7Sr0.3TiO3 thin films

We present the results of a systematic dielectric study for sputter deposited barium strontium titanate thin film planar capacitors measured over a wide temperature range of 20–575K for frequencies between 1kHz and 1MHz. Our observations of dielectric loss peaks in the temperature and frequency domains cannot be understood in the typical framework of intrinsic phonon losses. We find that the accepted phenomenological Curie–von Schweidler dielectric behavior (universal relaxation law) in our barium strontium titanate films is only applicable over a narrow temperature range. An excellent fit to the Vogel-Fulcher expression suggests relaxorlike behavior in these films. The activation energy of the observed phenomenon suggests that oxygen ion motion play a role in the apparent relaxor behavior, although further experimental work is required to test this hypothesis.

Barium Strontium Titanate Thin Film Capacitors for Low Inductance Decoupling Applications

Sputter deposited barium strontium titanate (BST) based thin film capacitors have been developed for use in GHz LSI decoupling applications. The fabricated 1.60×1.85 mm 2 BST chip decoupling capacitors with Pt electrodes and 150 μm bump pitch, have a capacitance density of 1.2 μF/cm 2 , low equivalent series inductance of 15 pH, and a low equivalent series resistance of 0.02 Ω. The impedance of the chip capacitors at 1 GHz is over 1000 times lower than conventional multilayered ceramic capacitors. Fundamental electrical and reliability properties of Pt/BST/Pt thin film capacitor structures were also investigated. Capacitors with 200 nm thick BST thin films deposited at 500 °C by RF magnetron sputtering achieved a C/A of 1.8 μF/cm 2 , leakage current density -9 A/cm 2 at 2 volts, and a breakdown field > 2.5 MV/cm at 20 °C. A fit of the failure data to a Weibull distribution indicated at least two different physical mechanisms causing capacitor failure. The primary failure mechanism for 1.5 volt operation was due to resistance degradation without catastrophic capacitor failure. At higher applied voltages, catastrophic capacitor failure occurred with the breakdown event characterized by a thermal runway process. The physical mechanisms contributing to capacitor failure are interpreted to be due to ionic migration and charge injection, and the contribution of these mechanisms to the degradation process could be partially resolved by bi-polar voltage pulse stressing. The projected mean time to failure for 1.5 volt operation is extrapolated to be in excess of 10 4 years at 75 °C and 126 years at 125 °C. The results indicate that sputter deposited BST thin film capacitors are promising for future GHz LSI decoupling applications.