Stress corrosion cracking (SCC) in aqueous solution is driven by exothermic reactions of metal oxidation. This stimulus, as well as classical mechanisms of SCC, does not apply to SCC in liquid metals (LMs). In the framework of the dissolution-condensation mechanism (DCM), we analyzed the driving force and crack kinetics for this nonelectrochemical mode of SCC that is loosely called “liquid metal embrittlement” (LME). According to DCM, a stress-induced increase in chemical potential at the crack tip acts as the driving force for out-of-the-tip diffusion mass transfer that is fast because diffusion in LMs is very fast and surface energy at the solid-liquid interface is small. In this article, we review two versions of DCM mechanism, discuss the major physics behind them, and develop DCM further. The refined mechanism is applied then to the experimental data on crack velocity V vs stress intensity factor, the activation energy of LME, and alloying effects. It is concluded that DCM provides a good conceptual framework for analysis of a unified kinetic mechanism of LME and may also contribute to SCC in aqueous solutions.

The experiments show that the alloying liquid In with only (0.1–0.5) at% Ti dramatically reduces the equilibrium contact angle Θ ∞ formed by In on the surface of CaF2. The aim of this paper is to clarify whether this practically important and conceptually challenging effect can be explained solely by Ti adsorption at the F-terminated solid–liquid interface without resorting to any other Ti-induced effect. The combination of ab initio calculations and regular solution approximation was proposed for finding the binding energy, ΔE Ti of Ti adatom with the interface “CaF2/liquid solutions In–Ti.” With thus obtained ΔE Ti=1.16 eV, we calculated from the Shishkovsky isotherm the reduction in the solid–liquid interface energy, Δγ SL induced by Ti adsorption from liquid In with various Ti concentration, C. It was found that Δγ SL(C) dependence demonstrated close inverse correspondence with Θ ∞(C) and that the theory fitted very well all available experimental data on the concentration and temperature dependence of Δγ SL. It was concluded that the Ti adsorption effect is large enough to account for the observed wetting improvement. The proposed multiscale modeling approach to the role of adsorption in wetting can be applied also to other nonreactive systems “liquid metal–ceramics” where the substrate determines the surface density of the adsorption sites for the active element.

The synchronized lateral spreading and through-penetration of liquid and solid gallium (Ga) in supported thin polycrystalline films of silver (Ag) were studied. The spreading and penetration kinetics were presumably controlled by a common mechanism. The spreading rate in the 0.5 μm thick film was found to be constant with time. The activation energies of the process responsible for spreading/penetration of liquid and solid Ga were EL ≈ 28.9 ± 4.8 kJ mol−1 and ES ≈ 48.2 ± 9.6 kJ mol−1, respectively. Grain boundary grooving, with Ag diffusion out of the groove either through liquid Ga or through solid Ga, was suggested as a possible mechanism of the spreading and penetration. The model proposed reproduced the observed spreading/penetration rates and gave reasonable estimates of the energies ES and EL.

Self-assembled organic monolayers (SAMs) are good coupling agents and diffusion barriers at Cu/SiO2 and Cu/low-k interfaces and are considered therefore as important elements of future all-wet ULSI metallization with sub-45nm Cu deposited by electroless plating (ELD). We formed SAM of 3-aminopropyltrimethoxy silane (APTMS) onto SiO2/Si substrate, activated the surface of APTMS with 5, 8, 15 and 25nm Au nano-particles (AuNPs), deposited (30–100)nm films of Cu by ELD and measured electrical resistivity ρ of the films in the as-deposited state and after vacuum annealing at 220C. The size of AuNPs was found to be a key factor in getting low resistance sub-100nm Cu films by ELD. The resistivity ρ≈4±0.8μΩ·cm – considerably smaller compared to the previously reported data – was achieved with the use of 5nm AuNPs. XPS and AFM revealed nano-pores, which can contribute to ρ but do not compromise likely the diffusion barrier properties of the SAM.

Thin Cu films of microelectronic quality and low electrical resistivity were created by electroless deposition (ELD) onto SiO2 surface modified first with self-assembled monolayer (SAM) of 3-aminopropyltrimethoxysilane (APTMS) and activated then by 5nm gold nano-particles (AuNPs). The presence of highly oriented amino-terminated SAM was revealed by XPS and ToF-SIMS analyses. The Cu films were deposited in boron- and phosphorous-free tartrate/formaldehyde electrolyte. Controlling the deposition rate via the solution pH permitted a minimum value in resistivity ρ. XPS depth profile revealed that diffusion of Cu into SiO2 modified by APTMS did not take place after annealing at 220°C, 4h. Moreover, annealing resulted in the drop of electrical resistivity to ρ=4±0.4μΩcm for the films with the thickness of 35–100nm. This value of ρ is several times smaller than those reported in literature for sub-100nm Cu films deposited by electroless on different SAMs. It is speculated that nano-scale porosity and corrugated structure observed by HRTEM and AFM in the ELD Cu films contribute to the resistivity. The obtained results demonstrate a viable route for formation of low resistivity, sub-100nm Cu films on dielectrics for microelectronic application.

Abstract not Available.

Self-assembled organic monolayers (SAMs) are good coupling agents and diffusion barriers at Cu/SiO2 and Cu/low-k interfaces and are considered therefore as important element of future all-wet ULSI metallization with sub-45 mn Cu deposited by electroless (ELD). We formed SAM of 3-aminopropyltrimethoxy silane (APTMS) onto SiO2/Si substrate, activated the surface of APTMS with 5, 15 and 25 nm An nano-particles (AuNPs), deposited (30-100) nm films of Cu by ELD, and measured electrical resistivity p of the films in the as-deposited state and after vacuum annealing at 220 C. The size of AuNPs was found to be a key factor in getting low resistance sub-100 nm Cu films by ELD. The p approximate to 4 +/- 0.4 mu Omega.cm - considerably smaller compared to the previously reported data - was achieved with the use of 5 nm AuNPs. XPS and AFM revealed nano-pores which can contribute to p but do not compromise likely the diffusion barrier properties of the SAM.

The models of liquid –metal embrittlement (LME) which explore the concept of fast stress/strain induced liquid phase diffusion as the major factor responsible for acceleration of subcritical cracks are overviewed and refined. The models are used for further analysis of several focus issues in the LME kinetics.

Self-assembled organic monolayers (SAMs) of silanes with –SH, –NH2 and –C5H4N functional groups have been shown recently to act as ultra-thin, robust diffusion barriers at the Cu/SiO2 and Cu/ultra low-k dielectric interfaces. More generally, SAMs with their tunable surface chemistry are essential elements of future all-wet ULSI metallization with Cu deposited by electroless (ELD) over SAM-functionalized dielectrics. Far too small is known however on the electrical properties of thin metal films formed onto SAM/dielectric substrates. In this paper, we give first a brief literature survey of what is known about Cu films deposited by electroless over dielectrics modified by SAMs. Second, we present our observations of electrical resistivity ρ of sub-100nm ELD Cu films deposited over the surface of amino-silane SAM/SiO2 activated by Au monodispersed nano-particles and show that this techniques helps to obtain considerably smaller ρ compared to the previously reported data.

50nm films Ag-(0.3-1at.% W)-oxygen deposited by electroless onto Pd activated SiO"2/Si show fast, low activation energy decay of their electrical resistivity @r after annealing above the threshold temperature T^*=90+/-10^oC. The decrease in @r was attributed in our previous works to surface diffusion controlled sintering. In attempt to understand the mechanism of this interesting effect we studied further the origin of open porosity, W segregation, evolution of the tungstate phase Ag"2W"2O"7 at the internal interfaces and temporal evolution of surface roughness in the course of annealing. A possible role of the tungstate phase transition to super ion conductive state in the diffusion mobility of Ag is speculated.

Spreading of liquid and solid Ga over the surface of thin polycrystalline Ag film is accompanied by fast penetration of Ga through the film. The penetration process between +60°C and –10°C has likely a common mechanism with the spreading. The activation energies of the process responsible for spreading/penetration are EL = 0.3±0.05 eV and ES = 0.5±0.1 eV for liquid and solid Ga, respectively. The common mechanism is attributed to grain boundary (GB) grooving with diffusion of Ag out of the groove either via liquid Ga or along GBs in solid Ga. A possible formation of intermetallic compound between Ag and Ga is considered as the secondary process, which does not control the kinetics. The model reproduces the spreading/penetration rates that are observed, and gives reasonable estimates of the experimental activation energies ES and EL.

Thermodynamic conditions for spontaneous grain boundary wetting (GBW) and stress driven liquid metal embrittlement (LME) are related to each other. Kinetic mechanism responsible for fast GB penetration (GBP) under small stress is described. Dissolution–condensation mechanism of LME and linear fracture mechanics for calculation of crack profile are applied to the classical system “Al-liquid Ga”. The results tend to support the idea that the recently observed fast linear penetration of Ga along 150° tilt <110> GB of Al should be considered as propagation of LME crack under small residual stress rather than as spontaneous GBW. With the residual tensile stress σ≈ 0.5MPa acting normal to the GB plane, all major findings reported for this model system are explained in semi quantitative way assuming that GB spreading coefficient is extremely small by its absolute value, i.e. that the system is near the threshold of spontaneous GBW.

We studied electroless 50nm films Ag (0.2–1.2 at.% W) which contain also some oxygen and were deposited onto SiO2/Si substrate. The films showed better corrosion resistance and hardness than pure Ag, but rather high electrical resistivity ρ which decayed fast after annealing at T⩾100°C [Y. Shacham-Diamand, A. Inberg, Y. Sverdlov, N. Croitoru, J. Electrochem. Soc. 147 (2000), 3345–3349; A. Inberg, V. Bogush, N. Croitoru, V. Dubin, Y. Shacham-Diamand, J. Electrochem. Soc. 150 (2003) C285–C291; V. Bogush, E. Ginsburg, A. Inberg, N. Croitoru, V. Dubin, Y. Shacham-Diamand, in: G.W. Ray et al. (Eds.), Advanced Metallization Conference 2003 (AMC 2003), MRS, Warrendale, PA, 2004, pp. 607–611; E.E. Glickman, A. Inberg, V. Bogush, N. Croitoru, Y. Shacham-Diamand, Microelectron. Eng. 70 (2003) 495–500; E.E. Glickman, A. Inberg, V. Bogush, G. Aviram, N. Croitoru, Y. Shacham-Diamand, Microelectron. Eng. 76 (2004) 182–189]. Application of XPS, HRTEM/EELS and HRSEM/EDS made it possible to reveal that open porosity, co-segregation of W and oxygen and formation of non-metallic phases Ag2W2O7 and AgO at the grain/cluster of grains interfaces are the major factors which contributed to high ρ in the as-deposited films. AFM data treated in terms of fractal scaling theory suggest that surface tension driven sintering in the course of annealing at 125°C is responsible for the resistivity decay, while HRTEM shows that grain size does not change.

Our 50 nm Ag(1% W) film deposited by electroless onto Pd activated SiO2 shows rather high resistivity ρ≈35 μΩcm, which is caused most likely by open porosity and weak contacts between the grains [E. Glickman, A. Inberg, V. Bogush, Y. Shacham-Diamand, N. Croitoru, Microelectron. Eng. 70 (2003) 45]. The resistivity does not decrease after long aging at 20 °C or vacuum annealing at 80 °C, but shows a 3-fold drop after 1 h of isothermal vacuum annealing at 100 °C. Assuming the diffusion nature of the process responsible for the resistivity decay, we derived from the annealing kinetics ρ(T,t) at 100–300 °C the diffusion activation enthalpy E=0.19 eV and pre-factor D0∼(10−8–10−12) cm2/s. It is shown that these extraordinary small values are not compatible with any mechanism of bulk- or grain boundary diffusion in Ag, but can be explained in terms of surface diffusion (SD) controlled, surface tension driven sintering. It is assumed that surface of the film is contaminated/oxidized that prevents production of thermal equilibrium adatoms. Formation of non-thermal Ag adatoms, which are considered to mediate SD mass transport, is explicable in terms of decomposition of non-stable silver oxides on the film surface. The AFM, HRSEM, DSC and SIMS observations are in general agreement with the proposed sintering mechanism.

A numerical investigation of grain-boundary (GB) grooving by means of the level set (LS) method is carried out. GB grooving is emerging as a key element of electromigration (EM) drift in polycrystalline microelectronic (ME) interconnects, as evidenced by a number of recent studies. The purpose of the present study is to provide an efficient numerical simulation, allowing a parametric study of the effect of key physical parameters (GB and surface diffusivities, grain size, current density, etc.) on the EM drift velocity as well as on the morphology of the affected regions. An idealized polycrystalline interconnect which consists of grains separated by parallel GBs aligned normal to the average orientation of interconnect's surface is considered. Surface and GB diffusions are the only diffusion mechanisms assumed. The diffusion is driven by surface curvature gradients and by an externally applied electric field. The corresponding mathematical system is an initial boundary value problem for a two-dimensional Hamilton–Jacobi type equation. To solve the electrostatic problem at a given time step, a full model based on the solution of Laplace's equation for the electric potential is employed. The resulting set of linear algebraic equations (from the finite difference discretization of the equation) is solved with an effective multigrid iterative procedure. The details of transient slit and ridge formation processes are presented and compared with theoretical predictions on steady-state grooving [E. Glickman, M. Nathan, J. Appl. Phys. 80 (1996) 3782; L. Klinger, E. Glickman, V. Fradkov, W. Mullins, C. Bauer, J. Appl. Phys. 78(6) (1995) 3833; L. Klinger, X. Chu, W. Mullins, C. Bauer, J. Appl. Phys. 80(12) (1996) 6670]

Electromigration (EM) drift velocity (DV) experiments in polycrystalline pure Cu lines are simulated numerically with the level set method. The simulation is based on a grain boundary (GB) grooving model, incorporating an electric field. The model is distinguished by two key requirements imposed at the triple point where two surfaces and a GB meet: that of GB and surface flux coupling (flux continuity), and that of permanent equilibrium between surface and GB tensions. Surface diffusion exists only at the advancing cathode edge, and is driven both by local curvature gradients and by the local field. Using independent, literature diffusivity values, the simulation yields both the DV prefactor and the EM activation energy in an Arrhenius-type expression. An excellent match is obtained with experimental DV values in the T range of 573–723 K. Some implications regarding the material transport mechanism are discussed.

Modeling supported by experimental observations suggests that in short, near-threshold thin film interconnects, there is a transition to diffusional creep-controlled drift velocity (DV) electromigration (EM). In this regime, the creep viscosity emerges as the major material parameter which determines the short line effect. A model that relates the viscosity to the mechanism of hillock formation is proposed.

A model of creep affected electromigration (CAEM) in near-threshold conductors (‘near-threshold’ denoting length L or current density j close to the corresponding Blech thresholds Lth and jth) is shown to fit electromigration (EM) threshold and kinetics data better than the Blech model. Its most significant prediction is that the overall rate of EM displacement in near-threshold conductors (or short polygranular segments in near-bamboo interconnects) is controlled by a strictly local mechanical process, diffusional creep, most likely by a bulk diffusion Nabarro–Herring process.

Proposed SEM /EDS fractography analysis complemented with microstructural observations on cross -sections has been successfully used for quantitative characterization of the early stage of thermal fatigue kinetics. Application of SEM/EDS fractography technique to EGA and CSP solder joints should be of special practical interest.