Shogo Matsubara

Barrier layers for realization of high capacitance density in SrTiO3 thin‐film capacitor on silicon

High dielectric constant SrTiO3 thin films were sputter deposited on barrier layers/Si substrate to fabricate a capacitor for dynamic random access memories. Dielectric constant (er) values of 140–210 were achieved for the 150‐nm‐thick SrTiO3 films using a Pt/Ti or Pt/Ta double‐layer barrier. In the Pt(50 nm)/Ti(10 nm), Pt(50 nm)/Ti(50 nm), and Pt(50 nm)/Ta(10 nm) barrier, effective er decreased by annealing in the temperature range between 450 and 550 °C, where the interdiffusion of Pt and Si was confirmed by x‐ray diffraction analysis and cross‐sectional transmission electron microscopy. In the Pt(50 nm)/Ta barrier, increase of the Ta thickness from 10 to 50 nm brought out a remarkable improvement of endurance to high‐temperature annealing. That is, in the Pt(50 nm)/Ta(50 nm) barrier, large er value (∼200) was maintained even with annealing at up to 700 °C.

High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers

As‐deposited superconducting thin films (∼0.1 μm) of YBa2Cu3O7−x have been prepared by pulsed laser deposition on (100) Si with buffer layers of BaTiO3/MgAl2O4. X‐ray diffraction studies reveal that the films grow epitaxially with the c axis preferentially oriented normal to the substrate surface. This is confirmed by ion channeling measurements along the (100) (normal to the surface) and (110) directions of the Si substrate showing a minimum yield of 54% along the (100), and 78% along the (110) axes using 2.8 MeV He++. Preliminary transmission electron microscopy study also supports these results. The as‐deposited films have zero resistance temperatures of 86–87 K, and critical current densities of 6×104 A/cm2 at 77 K and 1.2×105 A/cm2 at 73 K. Our results indicate that the superconducting properties of the films are limited primarily by the quality and degree of epitaxal growth of the buffer layers on the silicon substrate.

Epitaxial Y-Ba-Cu-O Films on Si with Intermediate Layer by RF Magnetron Sputtering

Epitaxial films of Y-Ba-Cu-O were obtained on Si substrate using epitaxial intermediate layer consisting of SrTiO3(or BaTiO3)/MgAl2O4. MgAl2O4 was epitaxially grown on Si(100) substrate by chemical vapor deposition, and then SrTiO3 or BaTiO3 was also epitaxially grown on MgAl2O4 layer by means of RF magnetron sputtering. Y-Ba-Cu-O films were prepared on SrTiO3(BaTiO3)/MgAl2O4/Si substrates by RF magnetron sputtering and their epitaxial growth was confirmed by RHEED observation and X-ray diffraction measurements. Epitaxial orientations of Y-Ba-Cu-O films varied in dependence on RF input power; lower RF power resulted in c-axis oriented film and higher RF power resulted in a-axis oriented film. Preparation of Y-Ba-Cu-O directly on MgAl2O4/Si was also studied, but only randomly oriented polycrystal film has been obtained so far. In sputter Auger depth measurement, any notable diffusion between Y-Ba-CuO film and the substrates was not observed. Resistive superconducting transitions with zero resistance at 65K on SrTiO3/MgAl2O4/Si and at 70K on BaTiO3/MgAl2O4/Si were observed.

Preparation of epitaxial ABO3 perovskite-type oxide thin films on a (100)MgAl2O4/Si substrate

Epitaxial thin films of ABO3 perovskite‐type oxides, including PbTiO3, (Pb0.90La0.10) (Zr0.65Ti0.35)0.975O3,BaTiO3, and SrTiO3, have been successfully obtained by rf magnetron sputtering on (100)Si substrate with an intermediate epitaxial layer of MgAl2O4. Only PbTiO3 grew in the tetragonal crystal structure and other materials grew in the cubic structure. The unit axis direction of the perovskite‐type oxide films was coincident with that of the underlying MgAl2O4 films. The tetragonal PbTiO3 films were a mixture of c and a domain. The preferred orientation of the tetragonal PbTiO3 film, that is c to a domain volume ratio, could be controlled by the conditions of sample cooling after the deposition. Highly c‐axis oriented films, which consisted of more than 90% c domains, were produced by cooling the sample at a high cooling rate, typically 30 °C/min, and by maintaining an rf plasma during cooling. The mechanism of the preferred orientation of PbTiO3 film has been explained by a balance of compressive str...

Reactive Coevaporation Synthesis and Characterization of SrTiO3 Thin Films

SrTiO3 thin films were prepared by the reactive coevaporation method, where the Ti and Sr metals were evaporated in oxygen ambient with an E-gun and K-cell, respectively. A uniform depth profile in composition was achieved by altering the Ti evaporation rate according to the Sr evaporation rate change. A typical dielectric constant of 170 was measured on films of 75 nm in thickness. The in-situ annealing in oxygen plasma reduced the leakage current.

Interface Structure and Dielectric Properties of SrTiO 3 Thin Film Sputter-Deposited onto Si Substrates

SrTiO 3 thin film preparation onto Si substrates using RF magnetron sputtering has been studied for a high capacitance density required for the next generation of LSIs. Structural and chemical analysis on the interface between SrTiO 3 film and Si was carried out with cross-sectional TEM, EDX, and AES. Dielectric properties were measured on AuTi/SrTiO 3 /Si/Ti/Au capacitors. The as-grown dielectric films on Si were analyzed and found to consist of three layers; SiO 2 , amorphous SrTiO 3 and crystalline SrTiO 3 , from interface toward film surface. By annealing at 600 °C, the amorphous SrTiO 3 layer was recrystallized, and consequently the capacitance value increased. A typical specific capacitance was 4.7 fF/μm 2 and the leakage current was in the order of 10 −8 A/cm 2 , for 180 nm thick SrTiO 3 film. The dielectric constant decreased from 147 to 56 with decreasing SrTiO 3 film thickness from 480 nm to 80 nm. This is due to the low dielectric constant SiO 2 layer (e=3.9) at the interface. From the film thickness dependence of the e value, the SiO 2 layer thickness was calculated to be 3.9 nm, which agreed well with the value directly observed in the TEM. To avoid SiO 2 layer formation, barrier layers between SrTiO 3 and Si have been studied. Among various refractory and noble metals, RuSi and a multi-layer of Pt/Ti have been found to be promising candidates for the barrier material. When RuSi film or Pt/Ti film was formed between SrTiO 3 film and Si substrate, dielectric constant of about 190 was obtained in dependent of the SrTiO 3 film thickness in the range of 80–250 nm. Analysis on the barrier layers was performed by means of RBS, XPS and XRD.

Epitaxial Growth of PbTiO3 on MgAl2O4/Si Substrates

Lead titanate (PbTiO3) thin films are successfully sputter-deposited onto (100)MgAl2O4/(100)Si substrates at 480–550°C sputtering temperatures. It is confirmed, from X-ray and electron diffraction analyses, that the PbTiO3 films grow epitaxially on MgAl2O4 films. The crystal properties of PbTiO3 films depend on those of MgAl2O4 films strongly. The PbTiO3 films are composed of c-axis and a-axis oriented domains. The ratio of the c-axis oriented domain to the a-axis oriented domain is controllable by the cooling rate of the substrate. The dielectric constant of the PbTiO3 film is estimated to be about 100, based on C-V measurements with a metal-insulator-semiconductor structure.

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.

Epitaxial relations between in situ superconducting YBa2Cu3O7−x thin films and BaTiO3/MgAl2O4/Si substrates

In situ superconducting YBa2Cu3O7−x films with Tc0 up to 87 K and Jc, 77 K up to 6×104 A/cm2 were prepared on Si substrates with MgAl2O4 and BaTiO3 double‐buffer layers. The epitaxial relations between various layers were established by transmission electron microscopy. The MgAl2O4 layer is heavily faulted. The subsequent BaTiO3 layer stops most of the faults, provides a template for the YBa2Cu3O7−x growth, and partially screens off the stress due to different thermal expansion coefficients. The microstructure of the YBa2Cu3O7−x layer is very similar to that of the films deposited directly on SrTiO3, exhibiting a homogeneous heavily faulted single‐crystal‐like structure free from secondary phases and grain boundaries. The slight degradation of the transport properties is attributed to residual thermal stress.

Ferroelectric Pb(Zr, Ti)O3 Thin Films Prepared by Metal Target Sputtering

Lead zirconate titanate Pb(Zr, Ti)O3 thin films have been deposited by rf magnetron sputtering of a multi-element metal target. The metal target design provided the feasibility for controlling the PZT film composition by adjusting the surface areas of Pb, Zr and Ti at the target. The cubic pyrochlore structure has been obtained on low temperature (100 to 300°C) substrate and a perovskite structure resulted by a post deposition annealing at 600°C. The Pb rich film was found to be a requirement for obtaining the perovskite structure after annealing. The dielectric constants and dielectric losses of perovskite films were from 300 to 500 and from 0.02 to 0.2, respectively. From P–E hysteresis loops, the highest remanent polarization measured was 12.5 µC/cm2 where the corresponding coercive field was 90 kV/cm.