Keiko Kushida

Stress induced shift of the Curie point in epitaxial PbTiO3 thin films

A 50 °C shift in Curie temperature has been observed for c‐axis oriented PbTiO3 thin films using x‐ray diffraction. An analysis of the electrostrictive strain based on the Devonshire thermodynamic formalism showed that the shift in the Curie point for these films can be plausibly explained by an effective two‐dimensional compressive stress of ≊400 MPa. The single‐domain, single‐crystal dielectric susceptibility (η33) and piezoelectric coefficient (d33) were calculated and found to be relatively unaffected, at room temperature, by a compressive stress of this magnitude.

Effect of external stress on polarization in ferroelectric thin films

The polarization changes caused by applying mechanical stresses to a lead zirconate titanate (PZT) thin film were investigated. Both the remnant and spontaneous polarizations decreased when the PZT film was loaded with tensile stress. For compressive stresses, the remnant polarization increased, but spontaneous polarization did not change. In fatigue with tensile stress state, the polarization decreased earlier than when there was no stress, which depend on whether or not the initial polarization value was high. Conversely, in fatigue with compressive stress, the initial higher remnant polarization value was maintained compared with the polarization in the unstress condition.

A 100-MHz ultrasonic transducer array using ZnO thin films

A 100-MHz ultrasonic linear transducer array made from a piezoelectric zinc oxide thin film on a sapphire substrate was developed and evaluated. Epitaxial, high-acoustic quality 10-/spl mu/m-thick ZnO film layers were produced by rf-magnetron sputter deposition onto a (111)-oriented gold film (with a chromium adhesion layer) that was vacuum-evaporated onto a (0001) sapphire surface. We found that, in well-oriented growth of gold, it is important to control the chromium sublayer thickness (less than 5 nm). An array was constructed by photolithography with an appropriate etch. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCl and HNO/sub 3/-based) that preferentially etched the c-plane of ZnO. Typical array elements were 90 /spl mu/m wide, 3.2 mm long, and 10 /spl mu/m thick, and the pitch of an array was typically 100 /spl mu/m. Our fine uniform array resulted in uniform ultrasonic response of individual elements throughout the array. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 /spl mu/m at the focal depth, agreeing well with theoretical predictions. Besides the use demonstrated with this present transducer, piezoelectric thin films should also lead to fabrication of various other kinds of ultrasonic transducers that can operate at high frequencies and should provide opportunities for miniaturizing transducers and making integrated ultrasonic devices.<<ETX>>

Piezoelectricity of c‐axis oriented PbTiO3 thin films

Through seeded lateral overgrowth, c‐axis oriented PbTiO3 films are formed on patterned Pt electrode films that are on a SrTiO3 single crystal. The impedance characteristics are then measured for this sample as if it were a composite bulk wave resonator. A series of resonance and antiresonance characteristics is observed. Using a conventional circuit model, the electrical impedance is analyzed to evaluate the electromechanical coupling constant kt of the PbTiO3 films. It is found that kt is as large as 0.8.

Epitaxial growth of PbTiO/sub 3/ films on SrTiO/sub 3/ by RF magnetron sputtering

Epitaxial growth of PbTiO/sub 3/ films on

Single-Target Sputtring Process for Lead Zirconate Titanate Thin Films with Precise Composition Control

The influence of growth conditions on rf-magnetron sputtered lead zirconate titanate (or PZT) thin films has been investigated, and the structural and electrical properties of PZT thin films have been examined. It was found that the Pb content in a sputtered film is proportional to rf power, thereby making it possible to control the film composition precisely. A well-crystallized perovskite structure was obtained by postdeposition annealing at 590°C. A stoichiometric film (i.e., Pb/Zr+Ti=1) had a dielectric constant value of 1180.

Ferroelectric properties of c-axis oriented PbTiO3 films Thin films, surfaces, and small particles

Abstract Through seeded lateral overgrowth, c-axis oriented PbTiO3 films are prepared on patterned Pt electrodes on a (100) SrTiO3 single crystal plate. Large piezoelectricity in terms of thickness vibration mode had been characterized by analyzing the impedance characteristics. In this paper, film growth is discussed to clarify the growth mode. Also, to characterize the film quality, ferroelectric properties are investigated in detail. Structural phase transition and dielectric phase transition are observed to show the similarity of the films’ characteristics to those of single crystals.

Solid-Phase Epitaxy of PbTiO3 Thin Films on SrTiO3 Single Crystal

PbTiO3 films were prepared by rf magnetron sputtering at relatively low temperature (200–300°C) and subsequent annealhig The composition of the sputtered films was investigated in detail and was found to strongly depend on the sputtering gas (Ar-10%O2) pressure. The films fabricated at about 4 Pa had stoichiometry and a perovskite structure under a wide annealing temperature range. Under these conditions, epitaxial growth of PbTiO3 film on SrTiO3 single crystal was attempted. From RHEED patterns and transmission electron micrographs, it was confirmed that epitaxial films with high crystalline quality were obtained.

Process and properties of Pt/Pb(Zr, Ti)O3/Pt integrated ferroelectric capacitors

Abstract A one-mask-patterned ferroelectric capacitor memory cell structures designed with a 0.5-μm feature size were fabricated. Oxygen plasma treatment after dry etching decreased the leakage current to as low as as-deposited film. The one-mask-patterned ferroelectric capacitors with switching charge almost equal to as-deposited film were achieved. Ferroelectric memories as dense as dynamic random access memories will become possible with this technology.

TfP210. Thin-film ZnO ultrasonic transducer arrays for operation at 100 MHz

Abstract Thin-film ZnO ultrasonic transducer arrays for operation at 100 MHz were developed and evaluated. Epitaxial, high acoustic quality ZnO film layers of 10 μm thickness could be produced by sputter deposition on well (111)-oriented gold films with chromium sublayers evaporated on (0001) sapphire. Using a photoresist etching mask of the desired pattern, the upper Au/Cr electrode and subsequent ZnO film were etched to form grooves separating the multi-layered film into array elements with a 100 μm pitch. It was found that the ultrasound beam in the azimuth plane for a 32-element array could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 μm at the focal depth in water. This value reasonably agrees with the theoretically calculated value.