Growing Metal Film Research Articles

A New Approach to the Formation of Nanosized Gold and Beryllium Films by Ion-Beam Sputtering Deposition.

Thin films of beryllium and gold that are several tens of nanometers thick were obtained, for the first time, on silicon and quartz substrates by the ion-beam method with tenfold alternation of deposition and partial sputtering of the nanosized metal layer. Scanning electron and atomic force microscopy indicate the predominant lateral growth of nanosized metal layers along the substrate surface. Optical spectra indicate the suppression of the localized plasmon resonance. The growth of the film occurs under the influence of the high-energy component of the sputtered metal atoms’ flux. The main role in the formation of the nanosized metal film is played by the processes of the elastic collision of incident metal atoms with the atoms of a substrate and a growing metal film. Metal films that are obtained by the tenfold application of the deposition–sputtering of a nanoscale metal layer are characterized by stronger adhesion to the substrate and have better morphological, electrical, and optical characteristics than those that are obtained by means of direct single deposition.

Thermal boundary conductance across epitaxial metal/sapphire interfaces

As electronic devices shrink down to their ultimate limit, the fundamental understanding of interfacial thermal transport becomes essential in thermal management. However, a comprehensive understanding of phonon transport mechanisms that drive interfacial thermal transport is still under development. The thermal transport across interfaces can be strongly affected by factors such as crystalline structure, surface roughness, chemical diffusion, etc. These complications lead to a significant quantitative uncertainty between experimentally measured thermal boundary conductance (TBC) across real material interfaces and theoretically calculated TBCs that are often predicted on structurally and/or chemically ideal interfaces. In this paper, we report on the thermal conductance across interfaces between various epitaxially grown metal films (Co, Ru, and Al) and $c$-plane sapphire substrates via time-domain thermoreflectance over the temperature range of \ensuremath{\sim}80 to \ensuremath{\sim}500 K. The room-temperature interface conductances of Al/sapphire, Co/sapphire, and Ru/sapphire are all $\ensuremath{\sim}350\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ despite the phonon spectra differences among the metals. We compare our results to predictions of TBC using atomistic Green's function calculations and the modal nonequilibrium Landauer method with transmission from the diffuse mismatch model. We found a consistent quantitative agreement between the experimentally measured TBCs and the predictions using the modal nonequilibrium Landauer model for the $\mathrm{Al}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and $\mathrm{Ru}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interfaces. This result suggests that interfacial elastic phonon thermal transport dominates TBC for the various epitaxial metal/sapphire combinations of interest in this work, while other mechanisms are negligible.

Palladium Chloride as Seeding and Surfactant Layer to Mediate the Formation of Top Metal Films on Self-Assembled Monolayers

Using self-assembled monolayers (SAMs) of [1,1′;4′,1″-terphenyl]-4,4″-dimethanethiol (TPDMT) on Au as a test system and nickel as a test adsorbate, we demonstrate that the penetration of deposited metal atoms into a SAM can be completely inhibited by the preliminary formation of palladium-chloride seeding layer at the SAM–ambience interface. This layer is formed by a simple dipping procedure that takes only a few minutes. The palladium atoms in the seeding layer serve as nucleation centers for the growing metal film, staying at its bottom during the growth. In contrast, the chlorine atoms are transferred from palladium to the deposited metal, staying on the top of the growing metal film and serving as surfactants. The above approach is a perspective way to solve the well-known top contact problem at the formation of metal–SAM–metal assemblies that are of potential importance for future electronic and spintronic devices.

Energy transferred to the substrate surface during reactive magnetron sputtering of aluminum in Ar/O2 atmosphere

A study of the reactive sputtering of aluminum was carried out by coupling energy flux measurements at the substrate location with conventional diagnostics of the gas phase and analyses of the deposited films. The main purpose was to get some insight into the elementary mechanisms involved at the substrate surface during the film growth in the well known metal and oxide regimes and at the transitions from one to another. Measurements were carried out in front of a 10cm Al target at a power of 400W (i.e. 5W/cm2) and a total pressure of 0.6Pa. The flow rate ratio (O2/O2+Ar) was varied in the range 0 to 50%. Different kinetics and values of energy transfer, denoting different involved mechanisms, were evidenced at metal-oxide (increasing flow rate) and oxide-metal (decreasing flow rate) transitions. The metal-oxide transition was found to be a progressive process, in agreement with optical emission spectroscopy and deposit analysis, characterized by an increase of the energy flux that could be due to the oxidation of the growing metal film. On the contrary, oxide-metal transition is abrupt, and a high energy is released at the beginning that could not be attributed to a chemical reaction. The possible effect of O− ions at this step was discussed.

10.1007/s11451-008-1031-2

The processes accompanying the formation of ytterbium films on the Si(111) surface at room temperature are investigated by the contact potential difference method, Auger electron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. It is shown that the grown metal films are uniform in thickness and that Si atoms virtually do not dissolve in the films. The atoms of the silicon substrate can diffuse in limited amounts into the Yb metal film only when the surface is bombarded by high-energy primary electron beams employed in Auger electron spectroscopy. The results obtained permit the conclusion that the previously observed oscillations of the work function in Yb-Si(111) thin-film structures cannot originate from dissolution of silicon atoms in the ytterbium film.

Quantification of the incorporation coefficient of a reactive gas on a metallic film during magnetron sputtering: The method and results

Reactive Magnetron Sputtering is a complex process and huge efforts are made addressing the understanding of its fundamental phenomena and the simulation of the deposition process by e.g. Particle in Cell/Monte Carlo (PIC/MC). One of the most uncertain parameters in this reactive sputtering process is the incorporation coefficient of the reactive gas in the growing layer, i.e. the real-time sticking coefficient during deposition. In this work, mass spectrometry is used to deliver more insights on this complex matter.Earlier, a method was developed to determine the incorporation coefficient of the reactive gas molecules in the growing metal film, using mass spectrometry combined with thin film analysis techniques (electron probe microanalysis and x-ray photoelectron spectroscopy). This method delivers a global, realistic incorporation coefficient which can be used in models for the reactive sputtering process. In this work, new insights have been added to the classical method. As previously only molecular reactive gas particles were considered, the here described method extends to determine the global incorporation coefficient of all reactive gas particles, i.e. both molecular as atomic. The incorporation coefficient of oxygen gas during the reactive magnetron sputtering of Cu, Al, Mg and Y, is determined. It can be concluded that for the first three metal systems, the obtained incorporation coefficient is mainly determined by the molecular oxygen, while for Y both atomic and molecular oxygen are important. Furthermore, a trend between the difference in electronegativity of the metal and the reactive gas, and the incorporation coefficient can be observed.

Specific features of the formation of ytterbium films on the Si(111) surface at room temperature

The processes accompanying the formation of ytterbium films on the Si(111) surface at room temperature are investigated by the contact potential difference method, Auger electron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. It is shown that the grown metal films are uniform in thickness and that Si atoms virtually do not dissolve in the films. The atoms of the silicon substrate can diffuse in limited amounts into the Yb metal film only when the surface is bombarded by high-energy primary electron beams employed in Auger electron spectroscopy. The results obtained permit the conclusion that the previously observed oscillations of the work function in Yb-Si(111) thin-film structures cannot originate from dissolution of silicon atoms in the ytterbium film.

Development of Oil/Water/Surfactant Microemulsions as Templates for Micro and Nanostructured Metal Foams

ABSTRACTThere are a range of applications for bicontinuous metal micro and nanostructures including as porous electrodes for supercapacitors, in fuel cells and in advanced batteries. We have previously demonstrated a number of methods of generating metal nanostructures by electrodeposition into various templates including hexagonal surfactant phases and mesoporous silica. More recently we have been exploring the application of oil-in-water microemulsions as templates. We have found empirically that the formation of microstructure is strongly effected by the viscosity of the oil phase, the rate of metal electroreduction and the surface roughness of the substrate. We interpret these observations as reflecting a competition between growth of the metal around the oil droplet and the redistribution of the oil in front of the growing metal film. We are currently exploring bicontinuous and water-in-oil phases with the expectation that such templates will yield bicontinuous metal films with larger accessible surface areas. Preliminary results with water-in-oil phases appear to show the formation of bicontinuous nanostructured films on the 20 nm length scale.

Electrochemical deposition: the concept of residence times in structure development

The concept of the residence time of an atom in a kink position was introduced to predict the structure of electrochemically deposited surface coatings. The residence time is obtained from the reciprocal value of the rate constant for separation from the kink position. Residence times and deposition rates determine the density of kink positions [ksp] on the growing metal film. For equilibrium conditions (Nernst potential), the density of kink positions [ksp]0 depends on the exchange current density i0 and on the residence time τ by the equation: $$ \left[ {ksp} \right]_0 = \frac{{i_0 \tau N_{\text{L}} }} {{zF}} $$ , where NL is Loschmidt’s or Avogadro’s number, F is Faraday’s constant and z is the charge of the metal ions in the electrolyte. Calculated values of residence times are presented for pure metals as well as for silver–lead and nickel–aluminium alloys.

Stress oscillations in a growing metal film

The stress in Co monolayers has been measured during epitaxial growth on Cu(0 0 1). The Co-induced stress is found to oscillate with a period of one atomic layer. Simultaneous stress and medium energy electron diffraction identify maximum stress for filled Co layers. Strain relaxation in Co islands leads to a reduced stress contribution of 2.9 GPa in the partially filled top layer as compared to 3.4 GPa for the filled layers. The corresponding variation of the elastic energy is 1 meV per Co atom. Atomic scale calculations reveal that the size-dependent mesoscopic mismatch is the driving force for stress relaxation in Co islands.

Step Height Oscillations during Layer-by-Layer Growth of Pb on Ge(001)

Heteroepitaxial growth of Pb on the Ge(001) surface has been studied by He atom scattering. For low substrate temperatures, Pb is found to grow layer by layer with (111) orientation. A detailed analysis of the specular peak profile as a function of the He wave vector reveals that the step height of the growing monatomic terraces oscillates with the film thickness. This variation, initially as large as $\ifmmode\pm\else\textpm\fi{}15%$ around the value of the Pb(111) bulk interlayer spacing, gradually dampens out after the deposition of a dozen monolayers. This is direct evidence of quantum size effects affecting the interlayer distance of a growing metal film.

Quenching of metal sticking by photo-oxidation of an amorphous semiconductor: Zn on GeS2.

The sticking probability of Zn on amorphous ${\mathrm{GeS}}_{2}$ is greatly reduced when the UHV-prepared semiconductor film is photo-oxidized in the presence of band-gap radiation. The phenomena underlying this interesting effect have been elucidated in a kinetic and structural study using electron spectroscopy and x-ray-absorption spectroscopy. Selective photo-oxidation of Ge sites strongly suppresses the formation of Ge-Zn bonds at the interface: these act as nucleation sites for the growing metal film. A consistent picture emerges that also accounts for the very different behavior encountered during deposition of Ag films with and without photo-oxidation in the closely related Ag/${\mathrm{GeS}}_{2}$ system.

Copper Film Growth by Chemical Vapor Deposition: Electrical and Optical Measurements in Real Time, and Studies of Morphology

The deposition of copper by low pressure chemical vapor deposition (CVD) from copper bis‐hexafluoroacetylacetonate is monitored in real time and in situ by the measurement of the optical reflectivity and electrical resistance of the growing metal film. Changes of the deposit morphology during growth were analyzed by interrupting the CVD process at different stages and observing the samples by transmission electron microscopy. Pure copper is deposited at a temperature of 400°C and 1 mbar total reactor pressure of helium, precursor, and water vapor. Two successive regimes are distinguished in the deposition: island formation and continuous film growth. The transition between these two regimes is visible in the real time specular reflectance measurement. The copper deposition rate is twice higher during the island growth than during the continuous film growth at the applied conditions. The influence of a metal seeding layer (from 0.001 monolayer to 1 monolayer) on the copper deposition is shown both in the real time measurements and in the ex situ analysis of films.

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.

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the ( 14 1 1 5 ) reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

LBL-12071C Preprint Submitted to the Journal of Surface Science THE STRUCTURE OF EPITAXIALLY GROWN METAL FILMS ON SINGLE CRYSTAL SURFACES OF OTHER METALS: GOLD ON Pt(100) AND PLATINUM ON Au(100) J.W.A. Sachtler, M.A. Van Hove, J.P. Biberian, and G.A. Somorjai January 1981 TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copyy call Tech. Info. Divisiony Ext. 6782. Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48

LEED detection of twin formation in epitaxially grown metal films

Twin faults in evaporated films of (111) oriented fcc metals cause a characteristic dependence of the LEED intensity on electron energy. By a quantitative analysis of the shape of the IV- curves twin formation in the films can be detected. Experimental data on the Ag/Au(111) and Cu/Au(111) systems are reported.

Surface studies with epitaxially grown metal films

Single-crystal films of Pd, Ag and Cu were prepared by evaporation in UHV onto air-cleaved rocksalt and mica substrates. LEED, Auger electron spectroscopy and electrical resistivity measurements served as means for studying the initial stages of epitaxial growth in situ. These measurements were supplemented by high energy electron diffraction and electron micrograph observations. In all cases pronounced island structures precede the formation of uniform and continuous metallic films, which may contain lattice defects such as microtwinning and stacking faults. The lattice constants normal to the surfaces were derived from intensity/voltage ( I V ) curves of the specularly reflected LEED beams using the kinematic approximation and were found to agree in all cases with the corresponding bulk values. The surfaces were either free of detectable impurities or could easily be cleaned by mild noble gas ion sputtering. They are thus well suited for subsequent adsorption studies.

The Epitaxial Growth and Oxidation of Nickel, Cobalt and Iron on Rocksalt

The structure and orientations of films of iron, nickel and cobalt, deposited by thermal evaporation on rocksalt, have been examined by electron diffraction. The oxides, forming spontaneously on the epitaxially grown metal films, have also been examined. The results are considered in the light of current theories of epitaxy and it is shown that the lattice misfit between substrate and deposit materials is generally less important than the maintenance of correct co-ordination of atoms in the first layer. Generally no evidence of a strained deposit lattice was obtained except in the case of iron oxide films on iron.