Kazuo Hirata

Preparation of Pb(Zr, Ti)O3 Thin Films by Multi-Target Sputtering

Lead zirconate titanate (PZT) thin films have been prepared with a multi-target sputtering system onto Pt-coated Si substrates. Structure, composition and dielectric properties have been investigated on films ranging in thickness from 60 nm to 1400 nm. The dominant phase of PZT thin films varied from pyrochlore to perovskite with increasing film thickness. An interface PZT layer with about 100 nm thickness, which was confirmed to mainly consist of pyrochlore phase grains, was observed at the PZT/Pt interface in a cross-sectional transmission electron microscopy (TEM) image of an 850 nm thick film. That is, the pyrochlore grains preferentially grow at the early stage of deposition and do not change their structure during subsequent deposition. Dielectric constant and remnant polarization were degraded with decreasing thickness. This can be explained in view of the existence of the interface layer with a low dielectric constant.

Sputter Synthesis of Ba2YCu3Oy As-Deposited Superconducting Thin Films from Stoichiometric Target–A Mechanism of Compositional Deviation and Its Control

A mechanism of compositional deviation of Ba-Y-Cu-O films during sputtering has been investigated, and a process for obtaining as-deposited superconducting films from a stoichiometric target has been developed. The main factor of the deviation is the radiation of high-energy electrons emitted from the oxide target with a low work function onto the growing surface. By eliminating the electron bombardment, films with the same composition were obtained reproducibly. As-grown films 4000 A thick deposited at 580°C showed Tc ( ρ=0) of 82.5 K and an excellent surface smoothness.

Preparation of Pb(Zr, Ti)O3 Thin Films by Multitarget Sputtering

Thin lead zirconate titanate [PZT] (Zr/Ti=50/50) films (230–430 nm) were deposited by multitarget sputtering with three stoichiometric PZT targets and one PbO target. To control Pb content in film, the rf power for the PbO target was varied. The rf power for each PZT target was also varied to control the deposition rate. PZT films with single perovskite phase or dominant perovskite phase were obtained only when the film composition was self-controlled. Low deposition rate resulted in a wider range of PbO input power in which Pb/(Zr+Ti) ratio was almost constant. Therefore low deposition rate would enhance the self composition control (SCC) mechanism. A 240-nm-thick PZT film deposited under the SCC mechanism and low deposition rate showed remanent polarization of 21.5 µ C/cm2, dielectric constant of 600 and leakage current of 7.0×10-8 A/cm2.

Pb(Zr, Ti)O3 Thin-Film Preparation by Multitarget Magnetron Sputtering

Ferroelectric lead zirconate-titanate (PZT) thin films have been prepared on Pt-coated oxidized Si substrates (Pt/Ti/SiO2/Si) utilizing a multitarget magnetron sputtering system with three Pb(Zr0.5, Ti0.5)O3 targets and one PbO target. Crystal structure and dielectric properties of the films have been studied as functions of substrate temperature (Ts) and Pb content. The Pb content in the film has been precisely controlled by optimizing the rf input power for the PbO target (PPbO). As a result, single-phase perovskite films have been prepared fairly easily with appropriate combinations of Ts and PPbO. A film with atomic ratios of Pb/(Zr+Ti)=1.3 and Zr/(Zr+Ti)=0.5 prepared at Ts=600°C exhibited a dielectric constant of 780 and remanent polarization of 19 µC/cm2.

Undercutting Phenomena in Al Plasma Etching

Undercutting phenomena in Al plasma etching were investigated. Photoresist, SiO2 and Si3N4 were used for etching masks. PSG, Si(100), Si3N4 and Al2O3 were used for underlying layers. In various combinations of etching masks and underlying layers, only photoresist mask exhibits no undercutting. However, even in this case, undercutting occurs according to over-etching time and pattern density. An hypothesis that a film observed on an Al pattern sidewall acts as a protection against undercutting was proposed. Undercutting phenomena were easily explained by this hypothesis.

Filling of Sub-μm Through-holes by Self-sputter Deposition

Filling of sub-µ m holes with a high aspect ratio by self-sputter deposition of copper is investigated. Good bottom coverages of 100% by means of thinner thin-film deposition and 50% by thicker thin-film deposition are attained. It is found that the bottom coverage decreases sharply when the ratio of the distance between the target and the substrate, D st, to the diameter of the erosion center ring is less than 1, and although the bottom coverage by the self-sputter deposition at lower pressures saturates at longer D st, that by the conventional sputter deposition at higher pressures decreases with D st. The bottom coverage also decreases with increases of the pressure, the thin film thickness deposited, and the radial distance of the substrate position from the target center toward the erosion center.

A New Self-Aligned Framed Mask Method for Selective Oxidation

A new selective oxidation method, which realizes fine isolation width for MOS LSIs, has been developed. The oxidation mask in this method has a Si3N4 frame formed at a perimeter of a conventional mask pattern by self-aligned technique. This technique is composed of deposition step of Si3N4 film to cover the conventional mask and dry etching step of this film to form the frame without a photoresist pattern. Birds beak extent is suppressed to less than 300 nm. Dislocation generation in silicon substrate is also suppressed in this framed mask method. Birds beak suppression brings about extention in an effective device area and realizes a transistor with effective channel width 1.0 µm wider than that in the conventional mask method. The p-n junction prepared by this new method exhibits as small leakage current as that by the conventional method.

Overlay Accuracy Evaluation in Step-and-Repeat X-Ray Lithography

Overlay accuracy in step-and-repeat X-ray lithography is evaluated through MOS device fabrication using an X-ray stepper applied to four lithography levels. Though relatively good overlay accuracies of 0.15 µm (σ) between two levels are obtained, these values are not satisfactory for sub-half-micrometer ULSI fabrication. To improve overlay accuracy, error factors are investigated. It is found that overlay errors are strongly affected by mask pattern placement errors. The standard deviation of mask errors is as high as 0.1 µm. These errors are introduced by e-beam writing errors and pattern displacement caused by stress relief of absorber and membrane layers in the etching process. The influence of these error factors on pattern placement accuracy is discussed.

MOS LSI Fabrication Process using Direct Electron Beam Writing

A procedure to construct an MOS LSI fabrication process, using direct electron beam writing technology, has been proposed. Positive resist PMMA and aluminum liftoff technique are used. Proximity effect and resist thickness dependence, as well as line width and undercut profile of resist patterns, play important roles in determination of optimum patterning condition. Electron beam radiation damage can be annealed out by suitable heat-treatment, while the amount of damage depends on processing steps. The process also includes a plasma etching with improved gas composition, a two-step glass flow technique and molybdenum wet etching with newly developed solution. Successful fabrication results, 1-kbit MOS RAM and TEG with 2 µm minimum pattern dimension, demonstrate the validity of these processes.