Thin Metallic Interlayer Research Articles

The role of a metallic interlayer on the mechanical response of SiC coatings on stainless steel substrates

The improvement of the mechanical response of silicon carbide (SiC) coatings on steel substrates when a thin metallic interlayer is included was analyzed. The analysis considered the interfacial reactions and the experimental results obtained from bending experiments performed in a scanning electron microscope (SEM). During bending, in situ observations demonstrated the evolution of cracking in the coatings, with or without debonding. The critical experimental conditions associated with damage mechanisms were determined. The mechanical behavior was related to the deposition parameters and to the interlayer and was compared with analytical calculations. A comparison of the calculated and the experimental results elucidated the interfacial mechanical behavior.

Mechanisms for Enhanced Formation of the C54 Phase of Titanium Silicide Ultra-Large-Scale Integration Contacts

▪ Abstract Three methods have recently been developed to enhance the formation of the low-resistivity C54 phase of TiSi2, the most widely used silicide contact in ultra-large-scale integration devices. These methods are (a) ion implantation of a transition metal into the Si before Ti deposition; (b) deposition of a thin transition metal interlayer between the Si and Ti; and (c) codeposition of Ti alloyed with a transition metal. Each of these methods decreases the C49-to-C54 transformation temperature by >100°C and improves the probability of phase formation in narrow lines by increasing the nucleation site density. In this paper, we identify the aspects of phase formation that are shared by these three methods, review the methodology by which they were developed, and summarize the applications to silicon devices. Mechanisms that are responsible for the enhanced formation of C54 TiSi2 are reviewed, based on a combination of temperature-controlled in situ measurements of resistance, X-ray diffraction, and optical scattering, coupled with ex situ studies of phase formation and morphology. The main mechanisms are identified as enhanced nucleation of the C54 phase by a reduction of grain size in the C49 phase and the creation of crystallographic templates of the C40 disilicide phase and the metal-rich Ti5Si3 phase.

Finite Element Simulation of Thermal Residual Stresses in Joining Ceramics with Thin Metal Interlayers

Finite element analysis was used to estimate thermal residual stresses developed in silicon nitride bodies bonded by metallic interlayers. Stresses were calculated for various characteristic metals, namely, Ni, Al, and Si, assuming elastic and elastic‐plastic behavior. The relative importance of the metal properties, such as thermal expansion coefficient, stiffness and ductility, has been evaluated. Two different joint geometries, butt and lap, have been used in stress calculations, and special care was taken in the mesh generation, to obtain comparable results. The yield stress of the interlayer material rather than thermal expansion mismatch is the critical factor in thermal residual tensile stress buildup inside ceramic adherents.

THE ROLE OF SCARF ANGLE IN THE PERFORMANCE OF ALUMINUM MATRIX COMPOSITE JOINTS

The role of geometry on the mechanical performance of scarf joints in Al-matrix composites reinforced with continuous polycrystalline alumina fibers was investigated. Model joints consisting of thin metal interlayers at varying scarf angles between composite sub-elements were designed, manufactured and tested to study the relevant deformation and failure phenomena. Specimens were produced by pressurized infiltration of molten Al-4.5%Mg into fiber preforms containing prescribed discontinuities. In this way, matrix continuity through the interlayer was ensured and defects normally introduced in actual joining operations were minimized. Reference composite specimens were produced in the same manner and exhibited tensile strengths of the order of 700 MPa, compared with a matrix yield strength, σ0, of approximately 100 MPa. The strength of the scarf joints with aspect ratios of 17–48 and angles below 45° was relatively constant, ∼310 ± 10 MPa, but increased at higher angles, reaching over 485 MPa at 75°. Failure of these joints was typically by debonding at the composite-interlayer interface. A simple analytical model suggests that failure occurs when the stress normal to the interlayer reaches a critical value, although the details of the debonding mechanism are still unclear. Conditions leading to a transition from debond failure to plastic collapse in joints of low aspect ratios are also discussed. © 1997 Acta Metallurgica Inc.

The strength of metal-matrix composite joints

The mechanical performance of model butt joints between Al-matrix composites reinforced with continuous alumina fibers has been investigated. The joints consisted of thin metal interlayers of varying geometries and thicknesses, normal to the fiber direction in the composites. Materials were produced by pressurized metal infiltration into fiber preforms with prescribed discontinuities in them, thus ensuring matrix continuity through the interlayer and minimization of processing defects normally introduced in actual joining operations. Tensile strengths for reference composite specimens with volume fractions in the 60 ± 5 vol.% range were on the order of 700 MPa, compared with a matrix yield strength of σ 0 ≈ 100 MPa. Measured joint strengths were ∼250 MPa for interlayers of uniform thickness (60 and 300 μm) wherein plane strain conditions predominated, and ∼170 MPa for interlayers of variable thickness (⩽ 6.4 mm) wherein conditions changed from plane strain to plane stress. The predominant mode of failure was interfacial debonding at an estimated local stress of ∼260 MPa. An upper limit to the joint strength was identified as arising from unstable cavitation in the metal interlayer, expected at a stress of ∼4 σ 0. Possible avenues to increase the joint strength to a level consistent with that of the composite are discussed in light of the understanding of failure mechanisms emerging from these model configurations.

The effects of deposition temperature and interlayer thickness on the adhesion of ion-assisted titanium nitride films produced with yttrium metal interlayers

The use of thin metal interlayers to promote the adhesion of coatings is documented. For instance a thin titanium interlayer has been found to improve the adhesion of titanium nitride coatings deposited by a range of physical vapour deposition (PVD) processes. However, much less has been reported about coatings produced by ion beam assisted deposition (IBAD) techniques which have the advantage of low deposition temperatures (<200°C) compared to other PVD processes. This paper describes preliminary work on the development of yttrium interlayers for IBAD titanium nitride coatings. The effects of interlayer thickness and the temperature of deposition of both interlayer and coating on the adhesion of the coatings have been determined. The reactive yttrium layer getters contamination more effectively at low temperatures than titanium and leads to improvements in adhesion over coatings with titanium interlayers deposited at the same temperature. However, the adhesion of the interlayer to the substrate is the critical factor in determining adhesion performance.

Calculation of the stresses in thin interlayers

An analysis is made of the thermal stresses which develop in thin metal gaskets (interlayers) after welding. The case of diffusion welding is discussed in particular. The problem is solved within the range of applicability of the linear theory of elasticity for dissimilar circular plates arranged symmetrically relative to the middle plane. Relations for the stresses and displacements in the plates are derived without the use of the hypothesis of straight normals or certain other assumptions traditionally used for thin plates. Simple formulas are presented for thin metal interlayers.

Formation and properties of (Au, Al, Ag, In) (InP-GaAs) Schottky diodes; contribution of the semiconductor surface to the diode characteristics

The interaction of Au, Ag, Al, In with etched or cleaved surfaces and with surfaces pertubed by oxygen exposure or by the deposition of a thin metal interlayer is studied. By using work function and surface photovoltage topographies, interface formation and Fermi-level pinning is studied. The contribution of both the initial surface properties and the nature of the metal to the features of the interfaces and of the diodes is analysed.

A multilayer, high resolution, ion-bombardment-tolerant electron resist system

A multilayer, high resolution electron resist system, which withstands ion bombardment, has been developed. This system consists of four layers which are, from top to bottom: AZ1350B, a thin metal interlayer, PMMA, and a copolymer of PMMA. The bottom two layers define the actual pattern dimensions. Two independent developers have been chosen for these two layers in order to obtain controllably undercut resist profiles ideal for liftoff applications, while maintaining high resolution in the upper PMMA layer. The top two layers of the four-level system serve to provide a protective metal coating which prevents crosslinking of the underlying polymer layer. This allows processing involving ion bombardment, such as ion milling or reactive ion etching. Without this protective metal layer, difficulty is often encountered in liftoff processing after ion bombardment, due to the presence of a thin crosslinked polymer layer which resists solvent penetration. This resist system has been used in conjunction with reactive ion beam oxidation to fabricate high quality, small area, niobium–lead alloy tunnel junctions in an edge geometry. Using a standard Cambridge EBMF-2 microfabricator, junctions with linewidths as small as 0.25 μm have been produced. With the edge geometry, this corresponds to junction areas smaller than 4×10−10 cm2.

Strength of thin metal interlayers in composites

Results of investigations of mechanical properties of Cu-stainless steel and Cu-Nb composites are presented. Data and information on tensile strength, yield stresses, hardness, lattice parameters, and deformation properties are included. (JRD)