Sputter-deposited Thin Films Research Articles

Stability of thin copper films on mesoporous dielectrics

We report observations of the morphological changes in sputter-deposited thin copper films (<100nm) deposited on solid SiO2 and varying porosity mesoporous silica substrates. Break-up of the copper film into islands (agglomeration) was observed using both an in situ hot-stage scanning electron microscope (SEM) system and an ex situ vacuum-annealing system followed by SEM characterization. The contributions of the underlying substrate to the film and substrate interfacial energy and the film’s elastic strain energy were evaluated. Detailed pore-size and roughness characterization of the mesoporous substrates were performed in order to evaluate the interfacial energy contribution. The intrinsic stress in the copper film was also determined as a function of underlying dielectric porosity. The stress is tensile in as-deposited films and does not dominate the interactions in our system, as the stress energy is two orders of magnitude lower than the surface/interfacial energies. We have observed an enhancement in the film stability up to a limiting porosity and pore-size due to an increase in the effective interfacial area and hence interfacial energy available for the contiguous copper film. The experimentally observed stability behavior is correlated to models that have appeared in literature, but which needed to be adapted for porous substrates.

Highly (0 0 1) oriented FePt ordered alloy thin films fabricated from Pt(1 0 0)/Fe(1 0 0) structure on glass disks without seed layers

Sputter-deposited Fe thin films exhibited (100) preferential orientation when they were deposited at a lower deposition rate below 0.6Å/s. The (100)-oriented Fe layer attained at a lower deposition rate condition induced (100) orientation of Pt layers deposited on it. Fe(100)/Pt(100) thin bilayered films deposited directly on glass disk substrates, without any seed layers, exhibited FePt highly ordered FCT alloy phase with (001) preferential orientation after annealing at 600°C in hydrogen atmosphere. Fe[3nm]/Pt[3nm] thin bilayered films exhibited squareness ratio and coercivity in perpendicular direction of 1.0 and 4.8kOe, respectively, after the annealing process.

Thermoelectric properties and electrical characteristics of sputter-deposited p-CuAlO2 thin films

Thermoelectric and electrical properties of transparent p-type CuAlO2 thin films deposited by dc-sputtering method have been studied in detail. Postdeposition annealing in excess oxygen is a necessary precondition of obtaining enhanced p-type conduction in the films. The effect of postdeposition annealing time in excess oxygen on electrical and thermoelectric properties was studied. Thermoelectric measurements of the films, showed considerable high values of room-temperature Seebeck coefficients ranging from 230 to 120 μV K−1, for annealing times of 90 to 30 min, respectively. Natural layered-structure materials, having an effective two-dimensional carrier density, showed an enhanced thermoelectric figure-of-merit. CuAlO2, having a natural superlattice structure, showed very good thermoelectric properties, and it may become a good thermoelectric material for future applications.

In Situ Modification of RF Sputter-Deposited Lithium Nickel Oxide Thin Films by Plasma Irradiation

Lithium nickel oxide thin films were deposited by sputter deposition. The films were in situ-modified using low-temperature inductively coupled plasma (ICP) irradiation. The technique uses a built-in radio frequency (rf) coil to generate an ICP plasma confined close to the substrate. The ions in the ICP can be drawn to the substrate by negative bias. This enables precise control of both bombarding ion flux (by rf power) and energy (by substrate bias). Varying the flux and energy of bombarding ions modifies the film properties. The crystal structure can be characterized as the layered oxide structure or rhombohedrally distorted NiO-type structure depending on Li content. The films modified by different conditions of plasma irradiation exhibited crystallography changes and texturing. The film composition was measured by ICP atomic emission spectroscopy and X-ray photoelectron spectroscopy. The film crystallography was characterized by X-ray diffraction. The film morphology modified by the plasma irradiation was observed by scanning electron microscope. Half-cells made of the lithium nickel oxide thin films were fabricated and the discharge curves measured to compare the electrochemical properties of the in situ-modified films. The films treated by in situ anneal were also prepared for comparison. © 2004 The Electrochemical Society. All rights reserved.

Local oxidation of metal and metal nitride films

Oxide growth on sputter-deposited thin films is studied on the local scale by atomic force microscope (AFM)-assisted lithography. We investigate the group IV reactive metals Zr, Hf, Ti, and their nitrides. The nitrogen content of the deposition plasma affects the film crystal structure and electrical resistivity, which in turn alter the local oxidation rates. Mass transport plays an important role, producing features with heights ranging from a few nanometers up to hundreds of nanometers. The heights of the largest features are one to two orders of magnitude greater than observed in other material systems, and the growth is well controlled. We use various techniques to investigate the solid-state reaction and transport mechanisms involved in this oxidation driven by a highly localized electric field. Our results demonstrate the potential of AFM lithographic techniques for characterizing oxidation processes across a wide range of time and length scales.

Carbon nanotube growth from Cu–Co alloys for field emission applications

We have developed a method of catalyst preparation for carbon nanotube (CNT) growth that uses solid-state diffusion to precipitate Co particles in a Cu matrix and control the size and size distribution of catalyst particles. Copper is an excellent electrical and thermal conductor, and has a small or negligible solubility for Co. Cu–Co alloys of approximately 5 atm% Co were formed in bulk by splat quenching the liquid alloy and on Si substrates by e-beam evaporation of the alloy, sputtering of the alloy, and sputter deposition of thin films of pure Cu and Co. The Co easily precipitates from dilute solutions of Cu–Co with heat treatment. These alloys and films were used to generate uniform distributions of Co precipitates, and CNTs were successfully grown by plasma enhanced chemical vapor deposition (PE-CVD) and thermal CVD on the materials. These alloys can be patterned by photolithography and sputtering in order to grow CNTs in defined locations. Therefore, the technique is amenable to very large scale silicon integration for field emission applications. The results of this work and data from field emission studies are presented herein.

Surface morphology of sputtered NiTi-based shape memory alloy thin films

The effect of substrate condition and annealing treatment on the surface morphology of sputter-deposited NiTi-based thin films was studied by means of atomic force microscope. It is found that the surface of the film deposited at 450 °C on a (100) Si wafer is composed of large island groups, consisting of islands of ∼150–300 nm in diameter. Annealing treatment at 400 °C results in a more homogeneous distribution of the island size. However, for the film deposited at 450 °C on a (111) Si wafer, its surface consists of more homogeneous islands, being about 200–250 nm in diameter. For the film deposited at 450 °C on a SiO 2 buffer layer on top of the Si-substrate, the surface islands have ideal spherical shape. After annealed at 650 °C, the islands have grown to about 300 nm in width and 550 nm in length. The surface roughness of the deposited film is related not only to the island sizes but also to the island distributions.

X-Ray photoelectron spectroscopy characterization of reactively sputtered Ti–B–N thin films

Sputter-deposited thin films of Ti–B–N with different B contents were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and nanoindentation measurements in order to investigate the microstructure transformation process upon increasing B content and to find the optimal microstructure corresponding to the highest hardness. The results indicated that part or all of the four phases, TiB, TiB 2, TiN and BN, existing in the reactively sputtered Ti–B–N thin films, depending on the B content. The films with a small amount of B addition were essentially cubic B1–NaCl TiN with (111) preferred orientation to the substrate surface, plus small amounts of TiB and BN phases. As the B content increased, the TiB phase gradually transformed to the TiB 2 phase. After TiB vanished, the BN phase increased sharply at the expense of the TiN phase. The maximum hardness was achieved by the formation of a multiphase microstructure comprising of a mixture of three phases with the following mole fractions ∼73 mol.% TiN, 17 mol.% TiB 2, 10 mol.% BN. The effect of B on stabilizing the multiphase microstructure of TiN, TiB 2 and BN was elucidated and explained on the basis of structural and thermodynamic stability.

The effects of Xe on an rf plasma and growth of ZnO films by rf sputtering

Sputter deposition of thin films using Xe gas instead of Ar is believed to suppress damage to the films caused by ion bombardment. To investigate the effects of Xe on ionic species in the rf plasma, and on the growth of ZnO thin films, rf plasmas generated using Xe and Ar sputtering gases were analyzed using Langmuir probe methods and optical emission spectroscopy (OES). The Langmuir probe results showed that the electron temperature of the Xe plasma was nearly half of that of the Ar plasma. In addition, the ion density in the Xe plasma was found to be greater than the ion density in the Ar plasma. OES data showed that a significant fraction of the ionic species in the Ar plasma was oxygen, which is thought to be the primary cause of damage to growing films. Ionic molecular oxygen (O2+) was observed only in the Ar plasma, and the concentration of excited atomic oxygen (O*) in the Ar plasma was ten times as high as that in the Xe plasma, for an oxygen partial pressure of 20%. In addition, ions in the Xe plasma were found to have smaller kinetic energies than for the ions in the Ar plasma, and the main ionic species were different. Using Xe as the sputtering gas therefore resulted in higher quality ZnO thin films with smoother surfaces.

Investigation of thermal stability, phase formation, electrical, and microstructural properties of sputter-deposited titanium aluminide thin films

Titanium aluminide thin films are being considered as coating materials for high temperature applications due to their high melting points and high oxidation resistance. In this study, Ti 37Al 63 and Ti 53Al 47 thin films are deposited onto SiO 2 substrates by RF magnetron sputtering using compound targets and then annealed in vacuum to investigate the properties of the films. Rutherford backscattering spectrometry, X-ray diffractometry, transmission electron microscopy, and four-point probe measurements are used to analyze the characteristics of Ti 37Al 63 and Ti 53Al 47 thin films for high temperature electronics applications. The films show good thermal stability up to 700 °C for 1 h in vacuum. Reasonable resistivity is obtained when appropriate compositions and anneal conditions are used.

Grain growth in sputtered nanoscale PdIn thin films

Using transmission electron microscopy we investigated grain growth in 30-nm thick sputter-deposited PdIn thin films that were annealed in a differential scanning calorimeter for 5–60 min at temperatures from 300 to 550 °C. The sequence of grain growth was found to begin with normal grain growth with monomodal and lognormal grain size distributions. Secondary grain growth was observed in late stages with a bimodal grain size distribution. A grain growth exponent n=2.3±0.2 was obtained by fitting the measured grain sizes at all stages of growth to the grain growth law R ̄ t n− R ̄ 0 n=αt at three temperatures 500, 525 and 550 °C. At 420 °C, however, a significantly higher value of n (∼4.5) was obtained. The activation energy for grain growth within the temperature range of 500–550 °C was found to be 54±3 kJ/mol.

Microstructural characterization of sputter-deposited Pt thin film electrode

Pt thin films of various thicknesses (30 nm ∼ 200 nm) were deposited on Si wafers with SiO2, Ti, TiO2, or IrO2 buffer layers at various temperatures (room temperature ∼200 °C) by a direct current magnetron sputtering process. The Pt films showed a strong (111)-preferred texture irrespective of the thickness, under-layer, and growth temperature. The authors previously reported [J-E. Lim, D-Y. Park, J.K. Jeong, G. Darlinski, H.J. Kim, and C.S. Hwang, Appl. Phys. Lett. 81, 3224 (2002)] that the films were composed of three kinds of grains with slightly different (111) lattice parameters (bulklike, 1.0% and 2.1% larger). This study details the microstructural variations of the Pt films according to the variations of experimental parameters. The different deposition conditions produced slightly different crystalline structures, but the three different (111) lattice parameters were always found. Epitaxial (200) Pt films on a (200) MgO substrate and a highly (111) textured Au thin film on a SiO2/Si did not show the same splitting in the lattice parameter. The grains with 1.0% and 2.1% larger (111) lattice parameter almost disappeared after postannealing at 1000 °C. However, surface chemical binding of the Pt film before and after annealing was unchanged. Therefore, it is believed that the lattice parameter splitting in the (111) textured Pt film originated from the interfacial grains with the distorted crystal structure due probably to growth stress.

Microstructural characterization of sputter-deposited Pt thin film electrode

Microstructural characterization of sputter-deposited Pt thin film electrode

Structure and Hardness of the Sputtered Al-Cu Thin Films System

The phase diagram of magnetron sputter-deposited Al-Cu thin films is simpler than the equilibrium diagram. Above 86.17 at.%Cu, the films are single αCu phase. Below, the microstructure consists of a mixture of the solid solution αAl phase and an intermetallic compound phase, the previous θ(Al2Cu) phase for the 1.8 to 45.99 at.%Cu films and an unexpected (Cu3Al) phase for 49.07 to 66.64 at.%Cu films. We note the appearance of a phase separation (αAl + αCu + Cu3 Al) in 66.64 at.%Cu films. The microhardness and the young modulus of the sputtered films increase regularly with Cu concentration reaching a maximum (H ≈ 8000 MPa and E ≈ 200 GPa). This phenomenon of strengthening of aluminium by means of copper is essentially due to a combination of solid solution effects and grain size refinement.

Effect of aluminium ion implantation on the oxidation resistance of DC magnetron sputter-deposited TiB2 thin films

This paper presents a novel approach to providing high-temperature oxidation protection for titanium diboride thin films via aluminium ion implantation . Through ion beam modification, it was anticipated that metastable phases, not otherwise observed in stoichiometric TiB 2 , would form, giving rise to a stable oxidation barrier layer upon oxidation. Films, deposited onto single-crystal Si (1 0 0) substrates, were subjected to metal vapour vacuum arc aluminium ion implantation to doses of between 1×10 16 and 3×10 17 ions/cm 2 using extraction voltages of up to 35 kV, vacuum annealed at 1100 °C, then oxidised in air at 650 °C for durations between 1 and 60 min. Analysis was performed using Rutherford backscattering spectroscopy , heavy ion elastic recoil detection analysis, X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. Aluminium ion implantation resulted in the formation of various Ti–Al and Al–B phases. Upon oxidation, the preferential oxidation of aluminium was observed, giving rise to a protective Al 2 O 3 barrier layer. Aluminium ion implantation was thus observed to reduce the oxidation rate of titanium diboride thin films.

Two-step in situ annealing effects on sputter-deposited MgB2 thin films

We have investigated the effects of in situ annealing on the superconducting properties of MgB2 thin films for the purpose of obtaining high critical temperature close to bulk value. MgB2 thin films were fabricated by rf magnetron sputtering on C-plane sapphire (Al2O3) substrate. Thin films were produced by simultaneously sputtering pure B and a Mg metal target. Sputtering deposition was followed by in situ annealing in a high vacuum. To prevent the evaporation of Mg from the film surface, two-step annealing was adopted: the first step is the crystallization stage at low-temperature annealing and the next is an improvement of the film's superconducting properties by annealing at a high temperature. In the optimal annealing process (a thin film is first heated at 600 °C for 2 h and then 650 °C for 2 h) we have consistently obtained thin films with a zero resistivity temperature of 29 K.

Characteristics of sputter-deposited Ru thin films on Si substrates

X-ray reflectivity and high-resolution X-ray diffraction measurements were employed to characterize the microstructure and morphology of buried layers and the interfacial structure of Ru films on Si substrates prepared by radio frequency magnetron sputtering technique. Extremely highly c-axis orientated Ru films were obtained at substrate temperatures ranging from 400 to 700 °C. The presence of oscillation fringe in the diffuse scattering indicates that the conformal relationship exists between the Ru surface and the film/substrate interface in high temperature deposition. The films deposited at temperatures ≥600 °C exhibited clear interference fringes on the high-resolution X-ray diffraction measurements around the (0 0 2) diffraction peak. The pronounced interference fringes show not only a highly c-axis orientation of the layers but also a rather smooth surface and interface. The as-deposited films have extremely good texture with very smooth surface when deposited at high temperature. The full-width at half-maximum (FWHM) of ∼1° was obtained for the films deposited at high temperature. High temperature deposited Ru films have a lowest electrical resistivity of ∼11 μΩ cm.

Paramagnetic-to-antiferromagnetic phase transformation in sputter-deposited Ni-Mn thin films

Sputter-deposited, equiatomic Ni-Mn thin films were observed to possess a metastable, nanocrystalline, chemically disordered, fcc (Al) structure. Grain growth and a phase change to a chemically ordered, antiferromagnetic L1 0 structure were identified by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) experiments revealed exothermic signals that correspond to the grain growth and phase transformation reactions. The enthalpy of transformation for the A1 to L1 0 phase change was calculated as -3.5 kJ/mol, which agrees with thermodynamic modeling. An activation energy of 139 kJ/mol was calculated for the phase transformation by the Kissinger method.

Ti-Ni sputter-deposited SMA thin films and their applications to microactuators

The motivation for fabricating sputter-deposited TiNi-base shape memory alloy thin films originates from the great demand for the development of powerful microactuators which can drive micromachines, because actuation force and displacement are greatest in the shape memory alloys among many actuator materials. Besides, even an intrinsic disadvantage of the shape memory alloy such as slow response due to the limitation of cooling rate can be greatly improved in micro size actuators, because the surface/volyme ratio of materials will drastically increase in thin films so that cooling rate becomes efficient. The present paper will give an outline of the recent development and characterization of sputter-deposited Ti-Ni based shape memory alloy thin films and their applications for microactuators. Some prototypes of microstructures consisting if Si warfare and sputter-deposited Ti-Ni thin films have been fabricated by utilizing silicone-based micromachining techniques. The fabrication processes and actuator characteristics of the microactuators will be presented.

Effect of magnetic field configuration on the properties of thin films sputter deposited from a dc magnetron TiNi target

A direct current magnetron target assembly with rotating adjustable magnets arrangement has been used for thin film sputter deposition from a 200 mm diameter TiNi target in a PE 4450 sputtering system. It was found that for a simple circular magnet arrangement placed 20 mm off the rotation axis the films deposited on the stationary pallet have significant variation in M s temperatures as measured from the center of the magnetron. This variation across the 100 mm wafer is as large as -100°C. The gradient in M s for the sputtered film can be significantly lowered by using an appropriate configuration of magnets, which in combination with rotation of the pallet, results in narrowing the difference in M s to less than 10°C. Further adjustment of transformation temperatures of the film can be accomplished by simultaneous sputtering on the rotating pallet from two targets. Depending on which direction the transformation intervals need to be moved, the second target should be either Ti or Ni. The deposition rates for the TiNi and Ni targets were calibrated and calibration results were applied for calculating the power settings to sputter film with A f = ∼15°C. The transformation intervals and stress-strain curves across a 100 mm wide area were measured for this film.