Showing papers in "Vacuum in 1992"

Bandgap and optical transitions in thin films from reflectance measurements

Abstract: A new formulation and method are presented for evaluating bandgap, optical transitions and optical constants from the reflectance data for films deposited onto a non-absorbing substrate.

Electrical, thermal and optical properties of Se75Ge7Sb18

Abstract: Electrical, thermal and optical properties of Se 75 Ge 7 Sb 18 in bulk and thin film form were studied. X-ray diffraction patterns indicate that the bulk sample and the prepared films at room temperature had amorphous structure. The dc electrical conductivity and the thermal conductivity of Se 75 Ge 7 Sb 18 were measured at room temperature and at elevated temperatures up to the glass transition temperature ( T g ). The conduction activation energy ΔE was determined. Switching and memory effects have been studied. The threshold switching voltage, V th , was found to increase linearly with the film thickness. Moreover, V th decreases exponentially with temperature. The obtained data were analysed using the thermal model for switching processes. The optical constants (refractive index n and absorption index k ) of Se 75 Ge 7 Sb 18 thin films were determined in the spectral range of 0.4–1.2 μm. The absorption coefficient (α) of this system was also determined. Analysis of the absorption spectrum reveals indirect optical transitions. The corresponding forbidden-band width was determined. Its value is approximately twice the conduction activation energy.

Some aspects of surface roughness in polycrystalline thin films: optical constants and grain distribution

Abstract: A modified formulation and new method have been suggested to derive meaningful information on the optical constants, grain size and grain distribution of polycrystalline thin films where surface roughness plays an important part through the contributions arising out of scattered transmission and diffuse reflection. It was observed that this method successfully described the experimental observations in polycrystalline CuInSe2 and CdSe films.

Influence of substrate temperature, deposition rate, surface texture and material on the structure of uhv deposited erbium films

Abstract: Structure of erbium films of 600 nm thickness deposited onto molybdenum substrates of different surface roughness (Rq=151, 500 and 850 nm), glass substrates, and amourphous carbon substrates, for substrate temperatures between 300 and 860 K, at 0.55 and 2.5 nm s−1 deposition rates, are investigated by electron microscopy and X-ray diffraction. The microstructures of the films broadly follow the trends of the Movchan and Demchishin and Thornton zone models, but show some departures in all zones, particularly prominent growth steps and layering. The grain size at a 2.5 nm s−1 deposition rate is found to be between two and five times larger, depending on substrate temperature, than at 0.55 nm s−1 under corresponding conditions, and the variation of grain size with temperature is found to be consistent with the activation energy model of Grovenor et al. X-ray diffraction analysis shows a (002) preferred orientation which reaches a maximum at a 575–775 K substrate temperature, and is stronger for the higher deposition rate. The microstructures and preferred orientations of erbium films are found to be influenced by the surface roughness and material of the substrate.

A new material for optical, electrical and electronic thin film memories

Abstract: A new category of erasable information storage media, M 1−β (TCNQ) films, is described. The photochromatic effect of this organometallic charge-transfer complex can give high-contrast patterns produced directly on a fine-grain film by pulsed photon beams generated from a gas or semiconductor laser. The patterns are erasable. For a radiation with 632.8 nm, the number of write-erase cycles N > 1000 has been obtained with a coefficient of contrast K > 50%. Moreover, the non-linear I–V characteristics of the fine-grain M 1−β (TCNQ) film can be used as a bistable electric switching. An organometallic memory (OMM) with a storage density of 1 Mb cm −2 can be proposed and is compatible with silicon IC chips. To avoid the decoder, an electron-beam operated ‘erasable access storage tube (EAST)’ with a multi-layered M 1−β (TCNQ) target was designed. Its storage density can approach that of a laser disc and the access time is as short as a semiconductor memory. Since no single crystal or p-n junction is needed, the technology is relatively simple and a lower price per bit of memory can be expected.

The deposition of highly supersaturated metastable aluminium-magnesium alloys by unbalanced magnetron sputtering from composite targets

Abstract: Aluminium-magnesium alloy coatings over the full composition range have been obtained by unbalanced magnetron sputtering from composite targets. Highly supersaturated single phase solid solutions have been formed over the ranges 0–20% magnesium in aluminium and 0–30% aluminium in magnesium. In the approximate range 45–50% magnesium a single phase amorphous or micro-crystalline glassy metal is formed, and at intermediate compositions the alloys consist of mixtures of the glassy phase and the appropriate solid solution. All the coatings have dense, non-columnar structures and are much harder than conventional alloys, having hardnesses up to 4400 MPa.

Interaction of O2 with SnO2(110)1 × 1 and 4 × 1

Abstract: TPD, photoemission, work function and band-bending measurements have explored the interaction of O2 with two clean surface reconstructions of SnO2(110). While the photoemission spectra are little modified by adsorption, the TPD and work function results suggest the formation of superoxo, O2--type species in both cases.

Microstructural investigation of IN 939 superalloy

Abstract: The vacuum cast nickel base IN 939 superalloy was investigated in the as-cast condition, as well as in the fully heat treated condition, i.e. after solution treatment followed by ageing. The alloy was characterized by optical microscopy, electron probe microanalysis and X-ray diffraction analysis. Volume fraction of the γ'-strengthening phase was determined by electrolytic phase extraction. The room and high temperature tensile properties and the Vickers hardness were determined for the as-cast and heat treated condition of the alloy. It was determined that the structure of the alloy consists of the γ-solid solution matrix, the γ'-intermetallic precipitate and various carbide phases. The simple heat treatment employed was shown to be effective in raising the γ' volume fraction and the yield and tensile stress.

Electron energy distribution function in a dc magnetron sputtering discharge

Abstract: The electron energy distribution function (EEDF) of a dc magnetron sputtering discharge ignited in either pure argon gas or a mixture of Ar and 4.7% methane is measured in the substrate vicinity (7 cm from the cathode) using ac differentiation of a cylindrical Langmuir probe characteristic. The results show the presence of two energy groups at a small discharge current l = 0.1 A for two values of argon gas pressure p = 0.4 and 1.07 Pa. The high-energy electron group is more pronounced at the lower pressure p of the sputtering gas and disappears with increasing l. The small amount of methane gas in the argon does not affect significantly EEDF at the location of measurement, if compared with that obtained in pure argon. Fitting the measured EEDF to the equation f(E) = a·E0.5exp (−b·Ex) we find values of x in the range 1.43–1.70, which correspondsto a distribution between those of the Maxwellian (x = 1) and Druyvesteyn (x = 2) ones. The mean electron energies 〈E〉M calculated from the measured EEDF are in the range of 4.8–7.1 eV, depending on the discharge conditions. For our conditions of operation, the comparison of 〈E〉M values with those obtained by means of the standard Langmuir technique [〈E〉L = 1.5(kTe)] shows a reasonable agreement (〈E〉L = 4.4–6.0 eV).

Residual stress and adhesion of molybdenum coatings produced by magnetron sputtering

Abstract: The non-isotropic mechanical properties of planar magnetron sputtered molybdenum coatings have been studied by scanning electron microscopy, interferometry and X-ray diffraction as a function of the sputtering gas pressure over a range of 2–40×10 −3 mbar. Coating failures like buckles and cracks have been analysed and the mechanical properties, Young's modulus and tensile strength in the plane of the coating have been determined from these failures. At sputtering gas pressures below 1×10 −2 mbar, the coatings are bulk-like with Young's modulus being 2.7×10 11 N m −2 and tensile strength being about 9×10 8 N m −2 . The intrinsic stress as determined by interferometry and diffraction measurements suggest a Young's modulus E f in the direction of the surface normal close to the corresponding single crystal value ( E f = 3.4×10 11 N m −2 ).

Arc deposition of TiC and TiCN using acetylene as a reactive gas

Abstract: Thin hard coatings on cutting tools and metal-forming dies now contribute substantially to the reduction of tool wear. They permit cost-effective machining processes and open the way to new tool applications. Titanium carbonitride physical vapour deposition (PVD) coatings have demonstrated their high performance potential in interrupted cut machining. Hitherto, however, production applications have been limited, due to technical difficulties. On the basis of detailed research into the stability of the PVD arc evaporation process, the paper demonstrates that TiC and TiCN coatings can be deposited successfully and reproducibly on cemented carbides, using acetylene as the carbon-containing gas. The structures and properties of the coatings are described as a function of dominant process parameters. It can be shown that substrates temperatures of approximately 500°C are required for titanium to react with carbon. Arc-PVD coatings produced with acetylene consequently attain high hardness values. Due to high internal compressive stress, however, pure TiC coatings tend to spall. Conversely, very little internal stress occurs in TiCN coatings, resulting in good coating-substrate adhesion.

Structure and optical properties of TiN films prepared by dc sputtering and by ion beam assisted deposition

Abstract: Titanium nitride films have been deposited at 573 K on silicon substrates by dc magnetron sputtering, selecting different bias voltage values, in the 100–800 V range. Additional TiN layers on silicon have been obtained also by ion beam assisted deposition (IBAD) in a machine designed for ion implantation and physical vapour deposition (e-gun). TIN was formed by bombarding at room temperature with a nitrogen ion beam, of energy 30 keV, a growing Ti film, evaporated on silicon in the presence of a nitrogen atmosphere. The films obtained by the two techniques have been characterized with respect to composition, structure, microstructure and optical properties. The effect of ions of different energies impinging on the film during its growth has been found to influence the surface characteristics, among them the optical reflectivity. The importance of process cleanliness is emphasized.

An overview of ion beam lithography for nanofabrication

Abstract: The technology of Ion Beam Lithography (IBL)—a term often meant to include the entire range of process steps possible by harnessing the ion beams such as patterning, etching, implantation and deposition—has been under development at various institutions for about two decades. The success of optical lithography for patterns as fine as 0.75 micron and non-availability of high brightness ion sources have been responsible for the delay in the development of IBL systems. This review describes the development, limitation and applicability of different types of ion sources, viz. liquid metal ion source, gas filled ion source and duoplasmatron. Various organic and inorganic resists used in IBL and their sensitivity to protons and electrons are discussed too. Different building blocks, such as mass separators, beam blankers and deflectors, sample stage and ion optics etc. constituting serial writing systems as well as parallel writing systems have been described. The technology for the development of different types of masks for the IBL system has also been explained. Novel applications of some commercial and R&D systems used for ion beam processing and results obtained therefrom are presented.

Effect of oxidized rhodium on oxygen adsorption on cerium oxide

Abstract: The oxygen adsorption on CeO 2 and Rh/CeO 2 has been studied by ESR. Several types of O 2 − species adsorbed on different sites on cerium oxide and rhodium have been found depending on the treatments. The results indicate that the incorporation of rhodium removes some adsorption sites on CeO 2 , but generates new ones at the rhodium-cerium oxide interphase, because it favours the formation of oxygen vacancies in CeO 2 .

Characterization of fullerenes by laser-based mass spectrometry

Abstract: Mass characterization of C60 related high-mass carbon clusters, called fullerenes, and their compounds is of increasing interest as the unique properties of these materials become known. We are using laser desorption time-of-flight mass spectrometers to investigate direct emission of positive and negative ions and also neutral desorption. We show results on the laser desorption process, namely, desorption thresholds for positive ion, negative ion and neutral desorption and velocity distributions of desorbed neutral particles. Furthermore, post-ionization with the wavelengths 118, 212.8, 266, 355, and 532 nm is investigated.

Collision dominated preferential sputtering of tantalum oxide

Abstract: Tantalum oxide was used as a model system in order to identify the mechanisms responsible for preferential sputtering. After bombarding Ta 2 O 5 with low energy ions (1.5 keV He + and 1 keV Ar + ) at different angles of incidence, the surface composition was analysed by means of AES (Auger Electron Spectroscopy) and ISS (Ion Scattering Spectroscopy). The experiments were simulated with the binary collision code TRIDYN. The good agreement between the results obtained by the two experimental techniques (AES and ISS) and between experiment and computer simulation confirms that in this mass and energy range preferential sputtering of tantalum oxide is governed by coliision processes. The preferential sputtering effects (e.g. steady-state surface composition and characteristic fluence) caused by the different component sputtering yields can be explained qualitatively and quantitatively in the frame of a simple sputtering model. Individual mechanisms of sputtering are discussed in detail. Compositional depth profiles obtained both experimentally and by computer simulation are related to the distribution of deposited energy.

Intrinsic stress of polycrystalline and epitaxial Ag, Cu and Au films on mica (001)

Abstract: Thin films of Ag, Cu and Au have been deposited onto single-crystalline mica (001) at different substrate temperatures and characterized in uhv by intrinsic stress measurements and low energy electron diffraction, as well as by scanning tunnelling microscopy in air. At room temperature the films are polycrystalline and exhibit film forces that are typical of the polycrystalline Vollmer-Weber mode, i.e. tensile forces during island, network and channel stages of film growth and compressive forces in the continuous films. At temperatures between 500 and 700 K film growth proceeds by the epitaxial Vollmer-Weber mode and films consisting of large, atomically flat grains are obtained. A huge stress contribution due to the formation of single-crystalline domain walls is observed during the network stage.

Techniques for improving thin film adhesion

Abstract: In most engineering applications the minimum criterion for adequate performance of a coated component is that the coating survive in the operating environment for the component lifetime. In practice this means that good adhesion of the coating to the substrate is of prime importance in dictating the lifetime of the component and a number of techniques have been developed to improve coating/substrate adhesion based on this requirement. Cleaning of the components prior to coating in order to remove protective layers, organic contaminants, adsorbed layers, dust and fingerprints is essential and several multi-stage cleaning processes have been developed to achieve this. Once loaded in the coating system some pre-heating of the components in vacuum or in a reducing plasma can be used to remove friable adsorbates and weak surface oxides. At this stage, sputter cleaning is also an important technique; this is used to break up more tenacious oxides and sputter contaminants from the component surface. However, this process can change the surface chemistry and properties of the components and does tend to lead to redistribution of sputtered material elsewhere in the system which can be a disadvantage. For deposition technologies which operate at high temperature a certain amount of diffusional intermixing at the interface can be achieved once coating starts which can be beneficial for adhesion and similar mixing is possible by the use of ion bombardment during the early stages of film deposition. Finally, for ceramic coatings, the use of metallic interlayers or bond coats can be used to improve adhesion and there is an optimum interlayer thickness to maximise coating/substrate adhesion. In this paper the effects of cleaning pre-treatments, sputter cleaning and interlayers on adhesion (assessed using the scratch adhesion test) will be illustrated with results for titanium nitride coatings on a stainless steel substrate.

Rugate filter fabrication using neutral cluster beam deposition

Abstract: Neutral cluster beam deposition (NCBD) is shown to be memethod directly applicable to rugate filter fabrication. NCBD provides the necesarry stable low deposition rates and control (±0.1 A s −1 ) required for thin rugate layers (typically 80 A). This paper specifically describes use of NCBD for co-evaporation of ZnS and BaF 2 producing digtal rugate filter structures with a range of effective refractive indices. Automated process control aspects to achieve the necessary tolerances for rugates will be described. This is demonstrated via the excellent agreement between theoretical and experimental rugate performance. Process scalability and production issues are addressed.

Correlation of microstructure and properties of hard coatings

Abstract: If the maximum performance benefits are to be obtained for a particular coating/substrate system, it is of fundamental importance that the interrelationship between the microstructure and properties of the coating be understood. In physical vapour deposition (PVD) processes, the application of a substrate bias during deposition is used to control the degree of ion bombardment that the coating experiences during growth. This, in turn, affects the grain size, texture, morphology, composition and density of the coating. The control of coating microstructure is thus possible, to a limited extent, and this is illustrated for PVD titanium nitride films in this paper. The importance of the coating microstructure in dictating many of the physical properties of the coating (for example hardness, residual stress, adhesion, etc.), is highlighted. The implications of these microstructure/property relationships for the selection of optimised coatings for tribological applications is assessed, given the amount of microstructural control that is possible in PVD coating processes.

Effective of Ta, Ti and TiN barriers on diffusion and oxidation kinetics of sputtered CoCrAlY coatings

Abstract: The effect of Ta, Ti and TiN barriers on the diffusion of aluminium from sputtered CoCrAlY coatings to Ni-base superalloy substrates and on the oxidation kinetics of the coatings at 1000°C has been studied. The results show that the depth ( h ) of the β+γ growth zone between the coating and the substrate is proportional to the root of the annealing time ( t ), from which a method to evaluate the inhibition degree of aluminium diffusion has been proposed. It was found that all the three barriers could inhibit the diffusion of aluminium from the coatings to the substrates. The inhibition order is as follows: Ta > TiN > Ti. However, the oxidation kinetics was not improved by the additions of the barriers. The TiN barrier has a little influence on the oxidation kinetics of the CoCrAlY coating, while Ta and Ti barriers have a determental effect.

Raman spectroscopy of thermally annealed TiO2 thin films obtained by chemical vapour deposition and the spray method

Abstract: Thin polycrystalline films of TiO 2 deposited by chemical vapour deposition (CVD) and the spray method have been examined by Raman spectroscopy before and after thermal annealing at 100, 200, 300, 500, 800 and 900°C in a non-oxidizing atmosphere of H 2 . According to Parker and Siegel [ J Mater Res, 5, 1246 (1990)] it is possible to calibrate a Raman spectrum to the oxygen non-stoichiometry of TiO 2 . Curves are used as calibrational ones for the peaks of ‘144 cm −1 ’ anatase vibrational mode and ‘443 cm −1 ’ vibrational rutile mode. It can be seen that for CVD samples obtained non-stoichiometry reaches a maximum at 500°C and goes back to stoishiometry at the phase transition temperatures in the range of 800–900°C. For the samples obtained by the spray method nonstoichiometry reaches a maximum at the lower temperature of 200°C. In the whole range from room temperature to 900°C the spray method obtained samples are much more non-stoichiometric than CVD obtained samples. Furthermore, Raman spectra of CVD obtained TiO 2 films annealed at 900°C in hydrogen reveal an amorphous phase immediately before the anatase-rutile phase transition. For samples obtained by the spray method we did not observe the same phenomenon.

Investigation on properties of ceramic coatings of ZrN

Abstract: The ceramic coatings of zirconium nitride were deposited on M2 high speed steel by reactive magnetron sputtering ion plating. The microhardness and the scratch critical load (Lc) of the coatings with different nitrogen partial pressures have been discussed. The adhesion and toughness of the coatings as well as the scratch failure have also been studied by means of the scratch test.

Ultra-high vacuum measurements of the internal stress of PVD titanium films as a function of thickness and its dependence on substrate temperature

Abstract: The internal stress of thin titanium films was measured during, as well as after, their deposition under ultra-high vacuum (uhv) conditions using a cantilever beam technique. Diverse measurements were made at a number of constant deposition temperatures between room temperature and 350°C. At room temperature titanium films exhibit only tensile stress over the whole thickness range. In terms of our model for the origin of internal film stress such a stress curve is characteristic of materials with a low mobility, i.e. high melting point, and indicates columnar grain growth. For deposition at substrate temperatures above about 60°C, tensile, as well as compressive stresses are found with increasing film thickness. This indicates a transition to an island growth mode due to increased metal mobility. In addition, this increased mobility at a higher temperature leads to a more pronounced recrystallization which correlates well with the larger stress changes after the film deposition. Furthermore, results of experiments are presented in which we have used 100 nm thick titanium films as substrates for the deposition of a second titanium film. Variations in the chemical or physical properties (e.g. deposition temperature or oxidation) of this titanium substrate are also reflected in the stress vs thickness curves obtained for the additional titanium films.

The uhv deposition of short-period multilayers for X-ray mirror applications

Abstract: We have used ultra-high vacuum dc magnetron sputtering to deposit a series of multilayers onto silicon substrates at either 50 or −90°C. The multilayers have Si as the light element and one W, Hf, Mo or Pt as the heavy element, with modulation periods between 13 and 40 A. We compare multilayers deposited at the two temperatures and discuss the problems of re-sputtering and intermixing, together with the more general issues of interface profiles and materials considerations for the optimisation of such multilayers.

Cyclic oxidation resistance of CoCrAl alloys and their sputtered microcrystalline coatings with and without yttrium

Abstract: The cyclic oxidation of the normal-grained CoCrAl alloys and their sputtered microcrystalline coatings with and without yttrium was investigated at 1000°C in air. The results indicated that the normal-grained Co30Cr5Al alloy exhibited very poor resistance against cyclic oxidation. After only one or two cycles, the initially formed Al 2 O 3 scale severely spalled off, and then Cr 2 O 3 and CoCr 2 O 4 spinels were formed as a surface layer, while Al 2 O 3 only existed in the form of internal oxides. In contrast, the normal-grained CoCrAl alloy with yttrium and the two kinds of sputtered microcrystalline coatings with and without yttrium showed excellent cyclic oxidation resistance at 1000°C. Neither scale spallation nor obvious internal oxidation was observed. The scales formed on the latter three kinds of samples were mainly composed of Al 2 O 3 . The reasons for the beneficial effects of reactive elements on the oxidation resistance have been well documented in the literature. In this paper, the mechanisms of the beneficial effect of microcrystallization on the oxidation resistance have been proposed.

Low temperature SiO2 films deposited by plasma enhanced techniques

Abstract: The use of silicon halides to obtain SiO2 films with plasma enhanced chemical vapor deposition (PECVD) techniques at low temperatures is reported. The silicon halides used were SiF4 and SiCl4. SiF4 was used in a direct PECVD system while a remote-PECVD (RPECVD) system was used with SiCl4. The deposition temperature was 250°C in the first case and in the range of 25–200°C in the second case. Optical measurements such as ir%T and ellipsometry and measurements of wet etch rate were used to characterize the films. It was found that the oxides were free of OH even for oxides deposited at the lowest temperatures. The electrical properties of the oxides were measured from metal-oxide-semiconductor structures fabricated with them. Breakdown fields for these oxides were typically of the order of 8 MV cm−1 and the fixed charge density in the range of 1010–1011 charges cm−2.

Mechanically induced desorption

Abstract: In the course of experimentation with uhv systems Groszkowski ( Bull Acad Pol Sci, 9 , 2 (1961), found that there was release of gas from the surface when it was rubbed. The cause of this phenomenon was referred to as mechanically induced desorption (MID). Although many experiments were carried out by Patý and his team the physical mechanism resulting in MID has not yet been discovered. This paper summarizes experimental results and theoretical considerations about the pressure changes in vacuum systems if rubbing of the surface occurs. Some considerations for uhv analysis chambers are presented and possible uses of MID for investigation of the wear of surfaces during friction processes are discussed.

Novel PVD films by unbalanced magnetron sputtering

Abstract: The development of the unbalanced magnetron has greatly increased the versatility of the sputtering process. High quality thin films of many types can be deposited more readily than had been possible previously. The ease with which the energies of the depositing atoms can be altered, from low to very high levels, enables the unbalanced magnetron to serve as a powerful research device, as well as being, in many cases, ideal for production systems. The paper highlights the principles of operation of an unbalanced magnetron, and exhibits its versatility by describing two examples of film deposition. The deposition of OFHC copper has enabled the creation of highly dense and ductile fine grained, ‘bulk’ type films up to 1 mm in thickness, at deposition rates of up to 3 μm min −1 . It has been shown that for copper, a level of substrate power density of about 0.1 W cm −2 is required to ensure a dense, non-columnar microstructure, for any combination of deposition parameters. The deposition of highly orientated reflective aluminium films onto polished silicon wafers has demonstrated how the unbalanced magnetron could be used to emulate existing and more expensive deposition techniques.

Analysis of ion exchanged Me : LiNbO3 and Cu : LiTaO3 waveguides by AES, SAM, EPR and optical methods

Abstract: Optical waveguides of Me : LiNbO 3 (where Me is Ag, TI, Cu, Mn or Cr) and Cu : LiTaO 3 have been studied to establish the specific features and some regularities of the ion exchange processes Me + ⇆ Li + , Me 2+ ⇆ 2Li + and Me 3+ ⇆ 3Li + . The surfaces of the crystals were examined by scanning Auger microscopy (SAM) and Auger electron spectroscopy (AES) together with Ar + sputter depth profiling. In addition we have employed electron paramagnetic resonance (EPR) and optical methods. The results obtained are discussed in detail and give a better understanding of the mechanism of isovalent and non-isovalent ion exchange processes in LiNbO 3 and LiTaO 3 crystals.