Diffusion barrier performance of amorphous W-Ti-N films in Cu metallization

W-Ti-N films were prepared on a Si wafer by reactive sputter-deposition, followed by a deposition of a Cu thin film by DC magnetron sputtering. The Cu/W-Ti-N/Si samples prepared were annealed at different temperatures under vacuum and then characterized using X-ray diffraction, scanning electron microscopy and auger electron microscopy. The sheet resistivity was determined by four point probe analysis. The results show that the amorphous W-Ti-N film is mainly composed of TiN and W and the crystallization temperature is above 800 °C. W-Ti-N thin films prepared have good thermal stability at 700 °C, but the Cu film tends to agglomerate when the temperature is above 700 °C. A failure mechanism of the diffusion barrier is proposed based on the thermal stress and interface reaction.     

Silicon nitride-stainless steel braze joining with an active filler metal

Hot-pressed Si₃N₄ was brazed to 410-stainless steel using a Ag-Cu-Ti alloy foil in a vacuum. The occurrence of cracking due to processing was examined by systematically varying the brazing temperature and time between 840 and 900 °C and 6 and 60 min, respectively. Cracks were found in Si₃N₄ parallel to the bonding interface when the braze joints were processed at the lower temperatures (for all processing times at 840 °C and for times of 6 and 12 min at 860 °C). A reaction layer was observed to develop in the filler metal adjacent to Si₃N₄, rich in Ti and containing some Si. The thickness of this layer depended on brazing temperature and time. Microcracks were found in the reaction layer normal to the bonding interface in the joints processed at higher brazing temperatures (880 °C for 60 min and at 900 °C for 30 and 60 min). The low temperature cracks occurred, apparently, as a result of the incomplete relaxation of thermal stresses due to the presence of a hard continuous titanium strip in the filler metal; the high temperature microcracks seemed to be affected by the increase in thickness of the reaction layer and by the precipitation of intermetallic compounds. The compressive shear strength of the braze joints were evaluated and correlated with the cracking behaviour and microstructure changes in the joint. A strong braze joint was obtained when the reaction layer was relatively thin and no cracks were present in either the reaction layer or the Si₃N₄.     

Influence of growth parameters on the microstructures of erbium films deposited on Si(111) substrates

Erbium films were grown on single crystal Si(111) substrates by electron beam vapor deposition. The microstructures of the erbium films were systematically investigated by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Results indicate that the surface morphologies and microstructures of the erbium films with Si as substrates are susceptible to the substrate temperatures when the deposition rates are fixed. The pure erbium films with columnar grains were obtained at temperatures below 200 °C, but in the films grown at temperatures higher than 350 °C, some pinholes that are composed of erbium silicides were found. The pinholes have triangular shapes which is in accordance with the geometry of the underlying Si(111) substrate. The films grown at a substrate temperature equal or greater than 450 °C have cracks which would be formed due to the different shrinkage degree of erbium and silicon when the substrate temperature was cooled down to room temperature. The films grown at 200 °C show the (002) preferred orientation, which is consistent to the prediction by the theory of surface energy minimization. The deposition rate and deposition time are considered as factors to affect the reaction of the erbium film and the silicon substrate.     

Mechanical properties of amorphous and microcrystalline silicon films

Hydrogen-free amorphous silicon (a-Si) films with thickness of 4.5-6.5 μm were prepared by magnetron sputtering of pure silicon. Mechanical properties (hardness, intrinsic stress, elastic modulus), and film structure (Raman spectra, electron diffraction) were investigated in dependence on the substrate bias and temperature. The increasing negative substrate bias or Ar pressure results in simultaneous reducing compressive stress, the film hardness and elastic modulus. Vacuum annealing or deposition of a-Si films at temperatures up to 600 °C saving amorphous character of the films, results in reducing compressive stress and increasing the hardness and elastic modulus. The latter value was always lower than that for monocrystalline Si(111). The crystalline structure (c-Si) starts to be formed at deposition temperature of ∼ 700 °C. The hardness and elastic modulus of c-Si films were very close to monocrystalline Si(111). Phase transformations observed in the samples at indentation depend not only on the load and loading rate but also on the initial phase of silicon. However, the film hardness is not too sensitive to the presence of phase transformations.     

Nitridation enhancing effect of ZrO2 on silicon powder

The nitridation behavior of silicon powder with added Zr compounds was studied in order to assess the catalytic effect of zirconium on the formation of reaction bonded silicon nitride, using high purity silicon powder and monoclinic zirconia as starting materials. Thermogravimetric analysis revealed that the addition of ZrO₂ to Si powder reduced the temperature of the main nitridation reaction, and increased the amount of silicon converted to silicon nitride at a given temperature. On the other hand, the nitridation rate at higher temperatures (1380-1400 °C) indicated similar values for both pure Si and Si with ZrO₂ additions.     

High temperature reactions between SiC and platinum

Solid state reactions between SiC and platinum have been studied at temperatures between 900 and 1100 °C. In the reaction zones, alternating layers of Pt₃Si and carbon, and Pt₂Si and carbon were formed at 900 and 1000 °C, respectively. Both the Pt₃Si and Pt₂Si phases were stable at respective temperatures. Annealings at 1100 °C, however, produced alternating layers of mixed Pt-silicides and carbon. The formation of platinum silicides gave rise to interfacial melting between SiC and platinum at all the temperature regimes. Laser Raman microprobe indicates that SiC decomposes into carbon and silicon at all the temperatures. The silicon reacts with platinum and forms platinum silicides, while the carbon forms clusters and stays unreacted. Based on the Raman results, the carbon exists in two different crystalline states depending upon its location from the SiC reaction interface. The reaction kinetics between SiC and platinum and the formation of periodic structure, respectively, are discussed based on the decomposition of the SiC and the phase separation of carbon from platinum silicides.     

Reactive brazing of ceria to an ODS ferritic stainless steel

This research study shows that a ceria ceramic can be bonded to an ODS ferritic stainless steel (MA956) by reactive brazing using a Ag₆₈-Cu_27.5-Ti_4.5 interlayer. The ability to join these materials provides an alternative to the current ceramic interconnects used in the development of solid oxide fuel cells. Initial results show that the ceramic-metal bonds survived the bonding process irrespective of the degree of porosity within the ceria ceramic. Metallographic analyses indicate that a reaction zone formed along the ceria/braze interface, which was not only titanium rich, but also consisted of a mixture of copper oxides. When the ceramic-metal bonds were exposed to high bonding temperatures or when subjected to thermal cycling at 700°C, this reaction layer increased in thickness and had a detrimental affect on the mechanical strength of the final joints.     

Hot-wire chemical vapor deposition of epitaxial film crystal silicon for photovoltaics

We have demonstrated that hot-wire chemical vapor deposition (HWCVD) is an excellent technique to produce high-quality epitaxial silicon at high rates, at substrate temperatures from 620 to 800 °C. Fast, scalable, inexpensive epitaxy of high-quality crystalline Si (c-Si) in this temperature range is a key element in creating cost-competitive film Si PV devices on crystalline seed layers on inexpensive substrates such as display glass and metal foil. We have improved both the quality and rate of our HWCVD Si epitaxy in this display-glass-compatible T range. We understand factors critical to high-quality epitaxial growth and obtain dislocation densities down to 6 × 10⁴ cm⁻² by techniques that reduce the surface oxygen contamination at the moment growth is initiated. We have also developed and validated a model of the HWCVD silicon growth rate, based on fundamentals of reaction chemistry and ideal gas physics. This model enables us to predict growth rates and calculate the sticking coefficient of the Si radicals contributing to film formation between 300 and 800 °C. We obtain efficiencies up to 6.7% with a 2.5-micron absorber layer grown on heavily-doped ‘dead’ Si wafers although these cells still lack hydrogenation and light trapping. Open-circuit voltages up to 0.57 V are obtained on 2-μm cells. Efficient film crystal silicon photovoltaics will require dislocation spacing more than 6 times the cell thickness, or else effective H passivation of the dislocations.     

Thickness effect on the formation of SiC nanoparticles in sandwiched Si/C/Si and C/Si multilayers

The effect of carbon (C) and amorphous silicon (a-Si) thicknesses on the formation of SiC nanoparticles (np-SiC) in sandwiched Si/C/Si and C/Si multilayers on Si(100) substrates were investigated using ultra-high-vacuum ion beam sputtering system and vacuum thermal annealing at 500, 700, 900 °C for 1.0 h. Three-layer a-Si/C/a-Si structures with thicknesses of 50/200/50 nm and 75/150/75 nm and a two-layer C/a-Si structure of 200/50 nm were examined in this study. The size and density of np-SiC were strongly influenced by the annealing temperature, a-Si thickness and layer number. Many np-SiC appeared at 900 °C at a density order about 10⁸ cm^− 2 in both three-layer structures while no particles formed in the two-layer structure. The thick a-Si structure (75/150/75 nm) produces a particle density approximately 1.8 times higher than thin structure (50/200/50 nm). This implies that thick a-Si structure had a lower activation energy of SiC formation compared to the thin a-Si structure. Few particles were found at 700 °C and no particles at 500 °C in both three-layer structures. The np-SiC formation is a thermally activated reaction. The higher temperature leads to higher particle density. A mechanism of np-SiC formation in thermodynamic and kinetic viewpoints is proposed.     

Microstructural evolution of brazing 422 stainless steel using the BNi-3 braze alloy

The 422 stainless steel (422SS) is one of the typical martensitic stainless steels with both excellent creep strength and corrosion resistance up to 650°C. Its application includes steam turbine blades, high temperatures bolting ... etc. Repair welding of 422SS is one of the most common methods to fix the turbine blade. However, repair brazing of surface shallow cracks, e.g., less than 1 mm in depth, is an alternative way to fix such blades. The microstructural evolution of brazing 422SS with BNi-3 braze alloy using both infrared and furnace brazing was performed in the study. Based on the experimental results, BNi-3 cannot effectively wet 422SS substrate below 1025°C. As the brazing temperature increases above 1050°C, comprehensive wetting can be obtained in 1200 sec. For the infrared brazed specimen with a short brazing time, the cooling path starts from the formation of a BNi₃ phase in the molten braze, subsequently forms a Ni-rich phase, and finally a eutectic phase is solidified from the residual eutectic liquid. The microstructure of the furnace-brazed specimen is similar to that of infrared brazed specimen, but the interfacial reaction zone is significantly increased in furnace brazing. There are Kirkendall voids in the braze close to the interface between BNi-3 and 422SS, and the size of Kirkendall porosity is increased with increment of the brazing time and/or temperature. The homogenization treatment of the brazed joint at 900°C results in growth of both the interfacial reaction zone and porosity.     

The contamination of nickel surfaces during oxidation in fused silica reaction vessels

In many experiments involving the high temperature oxidation of nickel in fused silica reaction vessels the final oxidation products have been found to include silica as -cristobalite. Prior to oxidation the nickel was decarburized by exposure to oxygen followed by hydrogen reduction of the remaining oxide film. The silicon of the silica was transferred from the vessel walls to the specimen as silicon monoxide formed by the reduction of the silica by the carbon monoxide produced during the decarburization step. The silicon monoxide was reduced at the metal specimen by residual carbon to form silicon which then dissolved in the nickel. Detectable contamination of the surface took place during subsequent oxidation when the silicon was preferentially oxidized at the surface. The reaction of carbon monoxide with the silica tube was enhanced by devitrification caused by previous high temperature (> 700° C) treatment. Contamination could be avoided if the tube was never heated above 700° C and was minimized at 900° C by more complete decarburization of the specimen.     

Thermal expansion of siliconated pyrolytic carbon

The lattice and bulk thermal expansions perpendicular to the layer plane of siliconated pyrolytic carbon, PC(Si), produced by pyrolyzing a mixture of propane gas and silicon tetrachloride vapour at the deposition temperatures of 1440 to 2025° C, have been measured over the temperature ranges 20 to 550° C and 20 to 960° C, respectively. The expansion behaviours of PC(Si) are related to the density and the degree of preferred orientation of crystallites, as is the case for pyrolytic carbon without silicon PC. At a deposition temperature of about 1700° C, the bulk thermal expansion coefficient of PC(Si) is about three times as large as that of PC.     

Characteristics of highly orientated BiFeO3 thin films on a LaNiO3-coated Si substrate by RF sputtering

BiFeO₃ (BFO) films were grown on LaNiO₃-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 °C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 °C and at 700 °C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 °C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 °C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 °C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present.     

Effect of annealing temperature on ZnO:Al/p-Si heterojunctions

Al-doped, zinc oxide (ZnO:Al) films with a 1.2 at.% Al concentration were deposited on p-type silicon wafers using a sol-gel dip coating technique to produce a ZnO:Al/p-Si heterojunction. Following deposition and subsequent drying processes, the films were annealed in vacuum at five different temperatures between 550 and 900 °C for 1 h. The resistivity of the films decreased with increasing annealing temperature, and an annealing temperature of 700 °C provided controlled current flow through the ZnO:Al/p-Si heterojunction up to 20 V. The ZnO:Al film deposited on a p-type silicon wafer with 1.2 at.% Al concentration was concluded to have the potential for use in electronic devices as a diode after annealing at 700 °C.     

High temperature behaviour of a Ti-6Al-4V/TiCp composite processed by BE-CIP-HIP method

The high temperature behaviour of a Ti-6Al-4V/TiC_p composite (10% Vol. of TiC) was investigated. A composite produced by Dynamet Technology according to the blended-elemental-cold-hot isostatic pressing (BE-CHIP) method was used. The stress-strain properties of the material were tested at 25, 200, 400, 500, 600 and 800°C. Composite specimens were aged in air at 500 and 700°C or under vacuum at 500, 700 and 1050°C, for periods ranging between 100 and 500 hours. The thermal stability of the matrix/ceramic interfaces was studied by using scanning electron microscope, electron probe microanalysis and x-ray diffraction. Carbon diffusion from the ceramic particles towards the composite matrix occurred (very likely already during the composite fabrication) because the metal matrix of all the composite samples (either in the as received or thermally treated conditions) showed a high content of carbon (more than 1% at.). However, the thermal treatments carried out at both 500 and 700°C under vacuum did not result in a ceramic-metal reaction. In spite of this, the formation of an ordered phase of formula Ti₂C can be inferred. Long period aging under vacuum at 700°C (500 h) did not lower the composite tensile strength. On the other hand, above 500°C in air the titanium matrix rapidly underwent oxidation, which gave rise to the formation of a thick surface reaction layer; this confirms that the composite material cannot be used above this temperature. Furthermore, the thermal treatment performed at 1050°C (under vacuum) resulted in a strong composite microstructure modification: the formation of new mixed carbides of Al and Ti was observed.     

Ceramic thin film thermocouples for SiC-based ceramic matrix composites

Conductive ceramic thin film thermocouples were investigated for application to silicon carbide fiber reinforced silicon carbide ceramic matrix composite (SiC/SiC CMC) components. High temperature conductive oxides based on indium and zinc oxides were selected for testing to high temperatures in air. Sample oxide films were first sputtered-deposited on alumina substrates then on SiC/SiC CMC sample disks. Operational issues such as cold junction compensation to a 0 °C reference, resistivity and thermopower variations are discussed. Results show that zinc oxides have an extremely high resistance and thus increased complexity for use as a thermocouple, but thermocouples using indium oxides can achieve a strong, nearly linear response to high temperatures.     

Fabrication of a n-ZnO/p-Si heterojunction diode by ultra-high vacuum magnetron sputtering

Heterojunction diodes of n-type ZnO were fabricated on a p-type Si(100) substrate using an ultra-high vacuum radio frequency magnetron sputtering method at room temperature. A short-time post-annealing process was performed to prevent inter-diffusion of Zn, dopants, and Si atoms. The post-annealing process at 600 °C enhanced the crystallinity of ZnO films and produced a high forward to reverse current ratio of the heterojunction diode with a barrier height of approximately 0.336 eV. A thin SiO_x layer at the interface of the ZnO film and Si substrate appeared distinctly at the 600 °C annealing, however the post-annealing at 700 °C showed an a-(Zn_2xSi_1 − xO₂) structure caused by diffusion of silicon into the ZnO film. In the n-ZnO/p-Si sample annealed at 700 °C, a rapid change in the barrier height was considered due to the effect of the dopant segregation from the substrate and deformation of the a-SiO_x structure.     

Kinetic and microstructural aspects of the reaction layer at ceramic/metal braze joints

The formation and stability of the reaction layer when brazing non-oxide ceramic materials were studied. Si₃N₄-Si₃N₄, SiC-SiC and Si₃N₄-stainless steel braze joints were produced and investigated. Several filler metals, most Cu- and Ag/Cu-based, containing different amounts of titanium were used to evaluate the effect of titanium on the formation and growth of the reaction layer. Some braze joints were processed using filler metals containing precious metals for high-temperature and oxidation-resistant applications. It was established that the matrix composition of titanium-bearing filler metals affects the ceramic wetting characteristics and the reaction layer kinetics. In the Si₃N₄ braze joints, the reaction layer consisted of TiN and titanium silicides. An activation energy corresponding to the diffusion of nitrogen in TiN was calculated for the growth of the reaction layer. During fabrication of the braze joints with precious-metal-containing filler metals at 1250°C, Si₃N₄ decomposed and a sound joint could not be processed. Premetallizing the Si₃N₄ with an AgCulnTi filler metal resulted in the formation of the reaction layer and permitted the fabrication of sound braze joints at 1250°C. Attempts to produce SiC braze joints with CuTi filler metals were unsuccessful owing to the decomposition of the SiC; a TiCreaction layer had developed, but this did not prevent the diffusion of copper into the ceramic substrate, nor did it slow down the decomposition of the SiC.Visiting Professor at Werkstoffwissenschaften, Aachen, Germany.     

High thermal stability of AlCrTaTiZr nitride film as diffusion barrier for copper metallization

To inhibit rapid Cu diffusion in interconnect structures, an effective diffusion barrier layer with high thermal stability, low electrical resistivity and good interface adhesion is strongly demanded. Thus in this study, an amorphous nitride film of equimolar AlCrTaTiZr alloy with an N content of about 41 at.% was deposited by reactive radio-frequency magnetron sputtering. Thermal stability of the AlCrTaTiZr nitride film and its barrier property to Cu diffusion were investigated under thermal annealing at 700-900 °C. The AlCrTaTiZr nitride film remained an amorphous structure after thermal annealing at 700 °C and then crystallized at 800 °C. However, no interdiffusion between Si substrate and Cu metallization through the AlCrTaTiZr nitride film occurred. The electrical resistivity of the film remained at the low level of as-deposited value, indicating its good thermal stability as an effective diffusion barrier layer. With temperature further increasing to 900 °C, severe interdiffusion occurred, along with the formation of silicides and large pores. The electrical resistivity then significantly increased, implying the failure of the AlCrTaTiZr nitride film.     

Diffusionslöten von Aluminium mittels PVD applizierter Lotzusatzwerkstoffe

Diffusion brazing of aluminium by PVD applied filler metals Diffusion brazing of aluminium and aluminium alloys precoated with filler metal components enables fluxless wetting and obtains braze joints of high strength at moderate brazing temperatures. Previously deposited components of filler metals on the base materials as thin film, using Arc‐PVD‐process lead during a subsequently diffusion brazing process to the formation of a local liquid phase (transient liquid phase). The liquid phase is formed from the deposited thin film material and the base material and is solidified isotherm due to diffusion procedures. In doing so braze joints of higher melting point than brazing temperature can be realised. In this work, vacuum brazing of the two systems, Al‐Cu and Al‐Cu‐Si have been investigated. Cu and Al‐Cu‐Si were deposited on the base material using Arc‐PVD‐process. The base materials were pure aluminum and EN‐AW6060. Metallographic and scanning electron microscope analyses proved that the braze seam area after the completed diffusion brazing process shows similar structure and composition as the base material.