Sputtered Metal Films Research Articles

Tensile Testing of Ultra-Thin-Film Materials Deposited on Polyimide for Mems Applications

AbstractA newly developed approach to thin-film tensile testing has been utilized for evaluating ultra-thin-films of about 60-100 nm in thickness. Thin sputtered metal films deposited on 3.5 gm thick polyimide film substrates were tested using a Mesotronix nanotensiometer. Modifications to sample handling and zero strain calibration helped to improve testing reproducibility. The response of a thin-film metal was determined by subtracting the response of the polyimide carrier film from that of the metal-on-polyimide composite. Stress-strain curves for two amorphous alloys sputtered from Al-25 at% Ti and Nb-25 at% Al targets exhibited well-defined elastic regions followed by brittle behavior. The ultimate tensile strengths (UTS) were about 1200 MPa for the Al-Ti and 1700 MPa for the Nb-Al, both of which occurred near the elastic strain limit of about 2%. After recrystallization at 280°C for an hour, the Al-Ti film showed what appears to be an upper and lower yield point, in addition to strain hardening. Initial yielding occurred at a strain of about 1.2% and a stress of about 800 MPa, while the UTS of about 1000-1060 MPa occurs at strains as high as 9%. Stress relaxation measurements performed on both the amorphous and the recrystallized films showed lower values of stress relaxation at 1% strain compared to low alloy polycrystalline Al films.

Auger depth profiles and stud pull adhesion of metal films on graphite–epoxy composites

Ion beam depth profiling and Auger electron spectroscopy (AES) have been used to characterize sputtered metal films on graphite epoxy composite substrates, with emphasis on elemental analysis of the interface between film and substrate. The AES data are correlated with adhesion strength values, obtained by the stud pull method. Films studied include chromium, nickel, titanium, tantalum, platinum, and vanadium, deposited by RF planar diode sputtering in thicknesses from 0.5 micrometers to 1.5 micrometers on substrates of graphite epoxy composite (Fiberite 934). The AES/ion beam etch profiling technique was used to determine the atomic concentration of oxygen, carbon, and the deposited metal, from the outer surface through the depth of the film, into the substrate. Atomic concentration profiles are presented for each metal and correlated with adhesion strengths, substrate preparation, and sputtering conditions. High adhesion strengths for Cr and Ti films are associated with elevated oxygen concentrations in the films, while the strong adherence of Ni to graphite–epoxy is associated with very low oxygen content. Lack of adhesion of Pt and Ta to graphite–epoxy is also associated with relatively low oxygen content in the metal/graphite–epoxy interface.

Investigation of the elastic modulus of thin films using simple biaxial bending techniques

The increasing use of thin films in numerous applications has raised the interest in the mechanical behavior of thin films; particularly the hardness and the elastic modulus of these films. The aim of the present study is to evaluate a simple biaxial bending technique to measure the elastic modulus of films with thicknesses ranging from 0.1 to 2 μm. In this range, it can be difficult to use simple indentation techniques owing to the influence of the substrate, and in order to determine the performance of these systems it is often essential to be able to determine the elastic modulus unambiguously. Tests have been carried out on glass substrates, and magnetron sputtered metal films (copper and aluminium) have been deposited on glass discs to evaluate the film properties. The samples were loaded in biaxial flexure using a point load on the disc supported by three point supports. The bending apparatus uses a commercially available micromechanical probe (UMIS 2000) that enables the measurement of very low loads and small displacements. Comparison is made between the force-displacement response of the disc with and without the sputtered film. Analysis of the bending response gave values of the elastic modulus for the glass discs of E=54.7±0.4 GPa, while E=116±8 GPa for the copper films and E=84±5 GPa for the aluminium films.

The study of island growth of ion beam sputtered metal films by digital image processing

The island growth of ultra-thin metal films of Au, Cu and Pt prepared by ion beam sputtering on an amorphous carbon substrate has been analysed using digital image processing methods. The transmission electron microscopy pictures revealing topological evolution of the metallic films with an increase in the film equivalent thickness, have been fed into the computer and processed to determine the area coverage of the discrete metallic particles for various thicknesses. The average particle size has also been measured at a small area coverage. The behaviour has been analysed using a nucleation and growth model of metallic films. It is found that the power exponent α of the function ⨍ = Atα, where ⨍ is the fractional substrate area coverage and t the equivalent film thickness, for Pt is close to unity whilst those for Au and Cu are close to 0.64 and 0.71 respectively. The results are considerably more accurate than those published earlier without using such image processing methods, hence more reliable details of the island growth of metallic films are revealed. It is also found that at a small area coverage the average particle size of Au and Cu is proportional to the equivalent thickness. These findings, based on the theoretical model, give strong confirmation of a two-dimensional growth mechanism for Pt and three-dimensional ones with cluster (island) dissociation up to a critical cluster size of m atoms for Au and Cu. It is indicated that the dissociation value m for Cu is greater than that of Au.

Interface structure and adhesion of sputtered metal films on silicon: The influence of Si surface condition

Ni(100–1000 nm)/Ti(250 nm) films were prepared by dc planar magnetron sputtering on Si(100) surfaces. Interface structures between Ti and Si and adhesion of the Ti films to Si after different surface pretreatments have been investigated. Before the film deposition, the Si substrate received an Ar ion bombardment or a chemical etching treatment. In the case of the Ar ion bombardment, we have investigated the effect of the cathodic voltage. A low cathodic voltage (50 V) resulted in high adhesion. The results by Rutherford backscattering spectroscopy showed that the amount of Ar incorporated in the Si surface during the Ar ion bombardment is increased with the cathodic voltage. The existence of Ar at the interface between the Si substrate and the Ti–Si mixed layer seems to lower the adhesion. In the case of the chemical pretreatment, we have investigated the effect of the exposure time in the atmosphere after the chemical etching treatment. A shorter exposure time (within 1 h) has been found to be preferred to a longer exposure time. The results of Auger electron spectroscopy and the peeling test showed that the exposure time is related to the oxide thickness, the Ti–Si alloy thickness, and the adhesion. It is considered that the stable SiO2 formation prevents the Ti–Si mixed layer formation and lowers the adhesion.

Nucleation and growth of ion beam sputtered metal films

Transmission electron microscope observations have been made on ion beam sputtered (IBS) films of Cu, Au, Pt and Ni (of predetermined thicknesst′ ⩾ 0.5 nm) deposited on amorphous carbon substrates. The influence of IBS parameters on particle size distribution and deposition rate has been measured, also the fractional substrate coveragef as a function oft′ which indicates three-dimensional island growth in Au, Cu films and two-dimensional growth in Pt, Ni films. Electron diffraction measurements appear to show that the f.c.c. metal particles grow with (1 1 0) parallel to the substrate with, in the case of Ni, a critical island thickness of ∼ 0.8 nm.

Preparation of cytoskeletons for high-resolution SEM by freeze-drying and cryosputtering

We have installed a magnetron sputter-coater (Ion Tech, model 310) within a freeze-dryer, thereby making it possible to metal coat biological samples directly after drying without breaking the vacuum or increasing the temperature of the specimen. We call this cryosputtering. Theoretically cryosputtering should give better preservation of supramolecular structure because the metal coat replicates the structure of the sample while molecular structure is still preserved by low temperature and before it is changed by the rehydration that results when a dried sample is taken out into a humid atmosphere. The freeze-dryer (Tis-U-Dry Histological Freeze Dryer, model TFD-130, FTS Systems Inc., Stone Ridge, NJ, USA) is connected to a turbo molecular pump (Balzer model TSH 332), backed by a rotary vane pump (Balzer model DUO 016B). This system provides a high, clean vacuum that minimizes contamination of the sample and the sputtered metal film from backstreaming oil or other substances that are present in the low vacuum of an ordinary sputter coater.

Microstructure of sputtered metal films grown in high- and low-pressure discharges

Sputtered metal films grown in different discharge environments are subject to different degrees of energetic particle bombardment during film growth, resulting in different microstructure and subsequent thin-film properties. The effect on film structure of energetic gas neutrals, backscattered from the sputtering target, together with the role of sputtered neutral metal atoms and those Penning ionized in traversing the plasma, are evaluated in different pressure regimes and under different biasing conditions. It will be shown that the film morphology, degree of texturing, and anisotropic lattice distortions change in a systematic way depending on En, the energy delivered to the growing metal film surface per arriving metal atom. En changes predictably in different discharge pressure regimes as do the resultant film structures and properties.

Investigation of S/N Property on Sputtered Metal Films

Investigation of S/N Property on Sputtered Metal Films

Intrinsic stress in A1N prepared by dual-ion-beam sputtering

The intrinsic stress of AIN, prepared by dual-ion-beam sputtering, has been investigated as a function of deposition temperature and nitrogen ion energy. The results show that the stress is compressive over the temperature range 75–700°C with a maximum value of -28×10 9 dyn cm -2 at 75°C. The stress decreases gradually with increasing deposition temperature up to T d/ T m = 0.33 and then decreases sharply for higher temperatures. These results are similar to those reported for evaporated and r.f. sputtered metal films and appears to be a general result independent of the sign of the stress. The temperature dependence is interpreted in terms of the annealing of a strained layer and grain growth models. The results show that the compressive stress is due to the bombardment of the condensing film by energetic particles. Calculations indicate that the most energetic particles are of the order of 160 eV, originating as neutralized back-reflected argon neutrals. The nitrogen ions from the auxiliary gun do not appear to contribute materially to the intrinsic stress. Evaporation of aluminum, while simultaneously bombarding the film with an independent source, indicates that a saturation stress condition is established above a normalized energy of about 50 eV atom −1.

Lattice expansion and contraction in sputtered metal films due to hydrogen charging

Hydrogen charging of Ti-30Mo plates (where the composition is in approximate weight per cent) sputtered with thin films of iron, tantalum or titanium results in an increase or decrease in the film lattice parameter, depending on the circumstances. Films thinner than 200 nm show continuous decreases in lattice parameter during charging, whereas relatively thick films show positive changes in the lattice parameter. The decrease in the lattice parameter of the film metal is a consequence of fine-scale plastic deformation caused by the hydrogen charging. Mesa-like protruberances are formed in the film, which oxidize in air, causing compression of their volumes. At very small film thicknesses the contraction effect prevails because most of the film is converted into protruberances. The charging of relatively thick films causes positive changes in the lattice parameter because the expansion caused by hydrogen dissolved in the film overcomes the contraction experienced by the small volume of film near the free surface.

The influence of deposition temperature on the structure and optical properties of vanadium oxide films

Thin films of various vanadium oxides, including the phase transition material vanadium dioxide, have been deposited onto sapphire substrates by reactive ion-beam sputtering. In this process, an argon ion beam sputters a vanadium target in the presence of an oxygen background. Coating chemistry, structure, surface morphology and electrical properties, together with the optical response at 3.4 μm wavelength, were studied as a function of the variation of film growth temperature Ts. Films deposited in the range 350 °C<Ts<620 °C and with a uniform argon ion beam pressure (2×10−4 Torr) and oxygen background pressure (3×10−4 Torr) show a strong dependence of growth characteristics and film properties on the deposition substrate temperature. In analogy to Thornton’s zone model for the temperature dependence of sputtered metal film growth, these metal oxide films can also be separated into different growth regions based on observed physical properties.

The influence of discharge current on the intrinsic stress in Mo films deposited using cylindrical and planar magnetron sputtering sources

Previous work has shown that the transition pressure, pt, below which large compressive stresses are found in magnetron sputtered metal films deposited at low T/Tm, increases with the discharge current, at least for the case of cylindrical-post magnetrons with magnetic field configurations generated by magnets mounted within the cathode barrels. This paper reports measurements of the internal stress as a function of discharge current for Mo films deposited using a planar magnetron and also a cylindrical-post magnetron having an externally generated magnetic field which permitted the discharge voltage to be varied independent of the current. At each current, coatings were deposited over a sufficient pressure range to identify pt. Transition pressures, determined by stress, resistivity, and optical reflectance measurements, for both the planar and cylindrical magnetrons, exhibited an increase with discharge current, consistent with the previous work. Reducing the cylindrical magnetron magnetic field to increase the discharge voltage at a fixed current consistently yielded higher pt. Strong correlations were seen between pt and the discharge voltage for both the cylindrical and planar magnetrons. It is concluded that the increase in pt with discharge current is significantly influenced by the accompanying increase in discharge voltage and not associated with the concurrent increase in deposition rate per se.

Stress and property control in sputtered metal films without substrate bias

Previous investigations of sputtered metal and silicon films have revealed that deposit characteristics commonly associated with bias sputtering, such as low resistivity and compressive internal stress, can also be obtained without substrate bias under certain conditions such as a low pressure of the working gas. More recently it was found that increased rates of sputtering raise the pressure threshold below which these benefits are attained. The present investigation extends these observations for a variety of discharge geometries, and for other film properties. Measurements of substrate heating rates and inert gas entrapment are also examined to assess the possibility that momentum transfer is the mechanism controlling the modification of properties.

Recording performance and magnetic characteristics of sputtered cobalt-nickel-tungsten films

Sputtered films of various CoNiW compositions were evaluated as to magnetic properties. These evaluations indicate the existence of an optimum tungsten content (25% by wt.) for maximum coercive force of 650 oe at a thickness of 750 Å. The hysteresis squareness (M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> /M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> ) and coercivity squareness (S*) were 0.78 and 0.93, respectively, and were not altered by tungsten additions at or below 25%. A selected CoNiW composition was evaluated relative to basic digital recording performance and compared to current oxide coatings. The CoNiW films exhibit approximately 15 dB SNR improvement relative to gamma Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> coatings. The recording performance of sputtered discs based on areal density capability is at least 3 times that of current oxide coatings; i.e.. 1.02 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> bits per cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 3.66 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> bits per cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for oxide and sputtered metal films, respectively.

Cluster formation in amorphous and polycrystalline thin films

Ions field evaporating from thin films r.f. sputter deposited onto molybdenum and tungsten tips in the presence of hydrogen are identified by employing an energy-focused atom probe field ion microscope. The existence of multiatomic clusters is unequivocally established. While the abundance of clusters for a well-developed tungsten tip is about 0.01%, polycrystalline sputtered metal films (nickel, gold and platinum) yield 2–5% cluster ions. For polycrystalline NiO x films and amorphous germanium and silicon films, the yield of clusters increases up to about 20% and the cluster size also increases. The abundance of cluster ions is thus directly related to the degree of disorder of the materials. Study of cluster ion formation may shed light on the bonding mechanisms of thin film materials.

Effects of substrate orientation and rotation on internal stresses in sputtered metal films

Effects of substrate orientation and rotation on internal stresses in sputtered metal films

Effects of substrate orientation and rotation on internal stresses in sputtered metal films

This paper reports the influence of substrate orientation and rotation on internal stresses in sputtered films of Cr, Mo, Ta, and 304 stainless steel (SS). The sample films were prepared by magnetron sputtering from cylindrical cathodes under conditions (0.13 Pa argon; deposition rate ∠0.4 nm s−1) known to yield compressive stresses at normal incidence with minimal heating of the deposits. The effects of fixed, oblique incidence are found to fall into three regions. At near-normal incidence little effect is observed. The width of this region is much greater for Ta and SS (±45°) than for Cr and Mo (±5°, ±20°). Beyond the first region the stresses change rapidly with increasing angle, becoming sharply tensile in Cr and SS, while in Mo and Ta they remain compressive but become small. In the third, high-angle region the stresses diminish gradually toward zero. The stress dependence on angle of incidence is similar to a dependence on working gas pressure that was observed in previous studies. Stresses in samples rotated continuously through all angles of incidence were always more tensile than the appropriate average of the static results would indicate. Data are also given for the effects of oblique deposition and rotation on electrical resistivity, optical reflectance, and gas incorporation in the deposited metals.

Analytical approximate equations for the resistivity and its temperature coefficient in thin polycrystalline metallic films

In the usual thickness range of sputtered metallic films, analytical linearized approximate expressions of polycrystalline film resistivity and its t.c.r. are deduced from the Mayadas-Shatzkes theoretical equations. A good experimental fit is observed for Al rf sputtered metal films.

The compressive stress transition in Al, V, Zr, Nb and W metal films sputtered at low working pressures

Investigation of internal stresses in thin sputtered films of Al, V, Zr, Nb and W extends the observation of compression at low working pressures that was originally detected with Cr (D. W. Hoffman and J. A. Thornton, Thin Solid Films, 40 (1977) 355–363) and probes the atomic mass dependence of the transition pressure that was subsequently detected with Ti, Ni, Mo and Ta (J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., 14 (1977) 164–168). The observation of compressive stresses at sufficiently low sputtering pressures in all ten of the elements so far examined strongly supports the generality of this phenomenon. The five new metals under study also confirm the overall increase of the transition pressure with atomic mass indicated by the metals examined earlier. However, periodic deviations from the general trend are now evident among the group IVB, VB and VIB elements examined. The electrical conductivity and optical reflectance of the sputtered metal films exhibit abrupt changes in behavior near the transition pressure for compressive stresses. Above the transition pressure the conductivity and reflectance drop off rapidly; below the transition pressure these properties tend to level off at maximum values for the sputtered metal films. The interpretation of these observations in terms of a peening mechanism of energetic particle bombardment is discussed. Data are presented for films up to 0.3 μm thick sputtered onto glass substrates at a nominal deposition rate of 1 nm s −1 over the pressure range 0.067–4.0 Pa of argon.