S. H. Rou

Electrodes for ferroelectric thin films

Abstract Platinum and ruthenium oxide (RuO2) deposited by ion beam sputter-deposition are evaluated for use as electrodes for PZT thin film capacitors. The effect of deposition temperature, film thickness, and the presence of oxygen on hillock formation in platinum is discussed. It is shown that the hillock density in Pt/Ti/SiO2/Si films can be significantly reduced by properly controlling the processing conditions and film thickness. Stress measurements correlate with the experimental observation that depositing thinner platinum films (<800 A) is an effective means of reducing hillock formation. The use of an intermediate deposition temperature 200–250°C also helps minimize hillock formation. Diffusion of the Ti adhesion layer into and/or through the platinum was significantly reduced by replacing the Ti with a TiOx adhesion layer. RuO2 electrodes are compared to Pt in terms of resistivity, surface morphology, microstructure and film orientation.

Electro‐optic characterization of ion beam sputter‐deposited KNbO3 thin films

The Electro‐optic properties of potassium niobate thin films deposited using a computer‐controlled ion beam sputtering technique have been studied for the first time. Epitaxial and polycrystalline films were deposited on single crystal magnesium oxide and highly (111) oriented films were deposited on sapphire for the study. All films exhibited a quadratic‐like dependence of birefringence shift on the applied electric field. The microstructure of the films and its relation to the observed electro‐optic properties is discussed.

Bottom electrodes for integrated Pb(Zr, Ti)O3 films

Lower electrodes for Pb(Zr, Ti)O3 (PZT) used in ferroelectric random access memories must have good electrical conductivity and must interact as little as possible with the PZT film. We have evaluated a number of bottom electrodes for use with PZT films deposited by ion beam sputter deposition These electrodes include Pt, Pt/Ti, RuO2, ReO3, and CoSi23N4, all on SiO2/Si; and TiN and Pt on MgO. Films were studied by XTEM, Auger depth profiling, X-ray microanalysis, and XRD. Important issues for platinum include: a) microstructure (porous for magnetron sputtered Pt); b) rapid lead diffusion through porous Pt; c) adhesion (improved by raising deposition temperature or by adding a titanium layer); and d) hillock formation (related to compressive stress in platinum). RuO2 has good conductivity and has no apparent interfacial layer with PZT. Each of the remaining substrates has drawbacks: CoSi2 forms a surface oxide; ReO3 has poor phase stability; TiN oxidizes and loses conductivity.

YBa2Cu3O7−δ films deposited by a novel ion beam sputtering technique

Superconducting films of YBa2Cu3O7−δ have been synthesized in a novel ion beam sputter deposition system which features a rotating target holder with BaO2, CuO, and Y2O3 as the sputtering targets. The dwell time of the ion beam on each oxide target is determined by a computer‐controlled feedback loop using the signal from a programmable quartz crystal resonator. The sputtered fluxes of all film components originate from the same spatial location, ensuring homogeneous film composition. The results presented demonstrate for the first time an automated ion beam sputter deposition system with the capability of producing high Tc superconducting films by controlled sputtering of either elemental metallic components or oxide precursors. The concept may be extended to include processes such as patterning, production of layered structures (junctions), and film encapsulation necessary for microcircuit manufacturing based on high Tc superconducting films.

Computer-controlled ion beam deposition systems for high Tc superconductor and other multi-component oxide thin films and layered structures

Abstract A single beam, multiple target (SBMT) deposition system which features a rotating target holder with either elemental or simple compound targets has been developed for the production of layered thin film structures and multicomponent oxide, silicade or other compound thin films. We are employing the SBMT ion beam sputtering system for the deposition of high temperature superconducting, electro-optic and ferroelectric thin films and multilayered structures. The beam-target-substrate geometry and ion beam characteristics are designed to minimize scattering of the ion beam from the target (which results in resputtering of, and inert gas incorporation into the film) while maintaining high deposition rates. The amount of energy which is deposited into the film may be controlled by means of a secondary reactive or inert ion beam impacting on the growing film. This secondary beam may provide enough energy to promote activated processes, such as the in-situ formation of oriented crystal structures of high temperature superconducting materials. All parameters necessary to control the film properties are under computer control. A deposition cycle, defined as a number of sequential steps, may be easily modified or added to previously existing deposition cycles, thereby permitting the creation of complicated deposition procedures suitable for the production of films with highly reproducible properties for research purposes, and the in-situ fabrication of complex devices for technological applications. Examples are given of the capabilities of the technique as they apply to the production of high T c superconducting devices.

Hillock Formation in Platinum Films

Hillocks, surface protrusions from thin metal films, have been observed in Al, Al/Cu, Pb, and other materials. Platinum films are widely used as substrates for the deposition of ferroelectric thin films because of their superior oxidation resistance. However, hillock formation in platinum films has not been reported in the literature. In this work, we report the appearance of hillocks in platinum in Pt/Ti bilayers on oxidized silicon wafers. Platinum films 250 to 300 nra were deposited by ion beam sputter deposition at 25°C and 300°C onto a 70 nm Ti film on oxidized Si wafers. The wafers were then heated in flowing argon to 600°C, held 1 hr at 600°C, and cooled to room temperature while the wafer curvature (and hence the film stress) was measured with a laser beam deflection technique. At 600°C, compressive stresses of 0.1 to 0.4 GPa, due to thermal expansion mismatch, developed in the metal films. The platinum surface, initially flat, showed strong hillocking after the anneal. Cross-sectional TEM revealed that severe Ti/Pt interdiffusion occurred, in one case leading to a Ti layer on the top surface.

Characterization of KNbO3 Thin Films Deposited by Ion Beam Sputtering using a Computer-Controlled Rotating Target Holder

KNbO 3 is a strong candidate as a material for use as channel waveguides due to a high electrooptic figure of merit. High quality single crystals are difficult to obtain due to incongruent melting of the compound. Control of cation concentration and oxygen incorporation are problems encountered in current thin film processing routes. In order to overcome the problems discussed above, an ion beam deposition system featuring a computer-controlled rotatable target holder and quartz crystal resonator (QCR) feedback loop has been developed. Multicomponent films are produced via sputtering from elemental or compound targets sequentially exposed to an ion beam. Initial results are presented on the use of this new technique for the deposition of KNbO 3 . Pressed KNbO 3 , Nb 2 O 5 , and KO 2 powders were used as sputtering targets. By varying the programmed thickness of deposited film from each target being sputtered, the ratio of K:Nb could be reproducibly controlled. The variation in sticking coefficients due to substrate temperature was also compensated for in this manner. Thin films were analyzed by X-ray diffraction and TEM to determine phases present and film microstructure. Film morphology and composition has been studied as a function of substrate temperature, layer thickness, and ion beam process parameters. The relation between deposition parameters and film characteristics are discussed.

Sputter deposition of ferroelectric thin films

Ferroelectric films are typically deposited by a variety of techniques, the two most common being chemical methods (sol-gel, metalorganic decomposition) and sputtering. In this paper we briefly review the sputtering techniques, and then discuss ion beam sputter deposition in greater detail. In particular, ion beam sputter deposition of epitaxial lead zirconate titanate (PZT) films is described. It is shown that the films with compositions close to the morphotropic boundary typically show well-developed ferroelectric hysteresis loops, Pmax around 45 μC/cm2, and Pr around 20 μC/cm2. In comparison with typical polycrystalline sol-gel PZT films, however, coercive fields of thin epitaxial films are large (120-200 kV/cm for 95 nm films). The pulsed fatigue behavior is remarkably similar to a polycrystalline non-oriented sol-gel PZT film investigated for comparison. The similarities suggest that the aging behavior may be dominated by the electrodes, which were Pt in both systems.

Development of an automated dual ion beam assisted process for superconducting oxide film deposition

Abstract An automated dual ion beam sputter deposition system has been developed, in which individual targets of Y, Cu, and BaF2 are sequentially sputtered to produce Y-Ba-Cu-O films with controlled composition. The dwell time of the ion beam on each target is determined by a computer controlled feedback loop using the signal of a quartz crystal resonator. Each individual target is exposed to the ion beam at the same spatial location, which optimizes compositional and thickness homogeneity across the substrate surface. As deposited superconducting Y-Ba-Cu-O films have been successfully grown at temperatures in the range 700–720°C using either an oxygen ion beam or an ozone jet directed at the substrate. Oxygenation effects by the two sources on film stoichiometry and microstructure are discussed.

Synthesis and characterization of epitaxial YBa2Cu3O7−δ/KNbO3 thin‐film heterostructures

YBa2Cu3O7−δ/KNbO3 thin‐film bilayers have been deposited on (100) MgO using an in situ ion‐beam sputter‐deposition technique. X‐ray diffraction and transmission electron microscopy indicate that both films grow epitaxially. The KNbO3 grows with the [00l] direction perpendicular to the MgO substrate surface, and the YBa2Cu3O7−δ (YBCO) grows c‐axis oriented (00l) on the (00l) KNbO3 film. The YBCO films have transition temperatures as high as 85 K. Rutherford backscattering spectrometry indicates that there is no noticeable potassium diffusion into the YBCO film or MgO substrate. Transmission electron microscopy has been used to study the microstructural details of the interface. Despite the epitaxy of YBCO on KNbO3, a thin reaction layer exists at the interface. The reacted layer may possibly be eliminated by optimizing deposition parameters.