Observations on interdiffusion in planar copper/tin-nickel/gold tricouples

Many of the basic properties of the electrodeposited tin-nickel (Sn-Ni) alloy have been investigated, but two previously unreported aspects are interdiffusion behavior in the Cu/Sn-Ni/Au system and the thermal stability of Sn-Ni when associated with gold. Planar couples were aged at temperatures between 100 and 500°C for times to one year and evaluated by scanning electron microscopy, electron probe microanalysis, and Auger electron spectroscopy. Results show that above 250°C the layer disintegrates into fine particles and the gold and copper interdiffuse as if the Sn-Ni layer were never present. At lower temperatures, the disintegration of the layer is not detectable by the techniques used or within the time period studied. However, tin is still released from the layer and diffuses very rapidly, even through thick gold, to the surface where it is oxidized. The thicker the tin-nickel layer, the greater the amount of surface oxide which is produced for a fixed aging condition. Thus some quantities of tin-oxide can be expected to form on the surface of gold in times of just a few years for application temperatures between 100°C and room temperature.     

Zigzag diffusion paths in multiphase diffusion couples

The diffusion path in a multiphase diffusion couple is predicted to follow a zigzag course when the following conditions are met: the concentration differences between the diffusion couple alloys are small, the alloys contain the same phases, there is one more component in the alloys than the number of phases, and all diffusion takes place in one continuous phase that has a constant diffusivity. An analytical model for the general case of n components and p phases is given that consists of both Heaviside and error functions. The model is consistent with a proposal made by Roper and Whittle concerning “composite diffusion paths” in two-phase ternary systems.     

An electrochemical study on the dissolution of gold and copper from gold/copper alloys

The dissolution behavior of gold and copper from their elemental states and from gold/copper alloys in cyanide solutions has been investigated using a rotating-disc electrode. The anodic and cathodic reactions were studied separately, and the resulting polarization curves were combined to examine the overall dissolution reactions. The dissolution rate of pure gold in the aerated cyanide solutions was inhibited by the anodic passivation on the gold surface, whereas the dissolution rate of pure copper was found to be mass-transfer controlled. On the other hand, the dissolution of gold and copper from the alloys was partially controlled by electrochemical reaction and largely by mass transfer.     

Cyclic strain hardening in polycrystalline copper

Polycrystals of pure copper were cyclically deformed, at room temperature under symmetric tension-compression fatigue at constant total strain amplitude control with an approximate constant plastic strain rate of 10⁻⁴. The relationship between the saturation stress amplitude and strain amplitude over a range of plastic strain from 2 × 10⁻⁷ to 10⁻² reveals three regions of cyclic hardening. A quasi-plateau, where the stresses show a slow constant increase, was observed in the intermediate region extending in the plastic strain range γ_pl, of 1.5 × 10⁻⁵ to 7.5 × 10⁻⁴. In this region, persistent slip bands (PSBs) which consist of “ladder” structures, similar to the case of single crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be Δσ_s/2 = 73 MPa which corresponds to the plastic strain Δϵ_pl/2 = 1.5 × 10⁻⁵ where PSBs do not form.     

Dislocation structures in fatigued polycrystalline copper

The dislocation structures in fatigued polycrystalline copper with small average grain size were investigated over a plastic strain range from 1.5 x 10⁻⁵ to 10⁻². It was found that the dislocation structures are arranged into three types of configurations, which correspond to the three regions in the cyclic stress-strain curve. Cylidirical loop patch structure are present in region A for low strain amplitudes, similar to those observed previously in coarse grained polycrystals. Moreover, irregular loop patches are also formed in this region for small grains polycrystals rather thanin region B at intermediate strain amplitudes for coarse grained polycrystals. In region B, persistent slip band (PSB) structures are formed but with a low volume content compared with the coarse grained polycrystals. In region C, at high plastic strains, the dislocation structures are dominated by dipolar walls. In addition, labyrinth structures are developed in region C instead of region B for coarse grained polycrystals. All the dislocation structures observed are viewed as forms of dipolized structures. A dipolized dislocation arrangement model is proposed to describe the formation process of dislocation structures. It is shown that all the dislocation configurations formed in cycled polycrystalline copper are low energy structures.     

Predicting diffusion paths and interface motion in γ/γ + β, Ni-Cr-Al diffusion couples

A simplified model has been developed to predict β recession and diffusion paths in ternary γ/γ + β diffusion couples (γ: fcc, β: NiAl structure). The model was tested by predicting β recession and diffusion paths for four γ/γ + β, Ni-Cr-Al couples annealed for 100 hours at 1200 °C. The model predicted β recession within 20 pct of that measured for each of the couples. The model also predicted shifts in the concentration of the y phase at the γ/γ + β interface within 2 at. pct Al and 6 at. pct Cr of that measured in each of the couples. A qualitative explanation based on simple kinetic and mass balance arguments has been given which demonstrates the necessity for diffusion in the two-phaseregion of certain γ/γ + β, Ni-Cr-Al couples.     

On the formation of the ordered phases CuAu and Cu3Au at a copper/gold planar interface

The formation of the ordered compounds, CusAu and CuAu, has been observed in a study primarily aimed at determining the rate of interdiffusion in the gold-copper system. In the planar diffusion couples aged at temperatures from 50°C to 400°C and for times to 4 years, the compounds were observed to form as layers at the goId/copper interface for temperatures under 400°C. The layers were visible in the scanning electron microscope backscattered image of polished cross-sections, and their compositions were determined by the corresponding plateaus observed in the electron microprobe X-ray intensity profiles. Kirkendall type porosity was also observed to form in the copper-rich zone adjacent to the ordered compound layers. These results are compared and contrasted to the findings of several other studies which have considered this reaction and its practical implications to device performance.     

Room-temperature diffusion in Cu/Ag thin-film couples caused by anodic dissolution

Thin, 100-nm films of first silver and then copper were deposited consecutively onto inert substrates by magnetron sputter deposition. Constant anodic current densities were applied at room temperature to dissolve the outer copper film to varying depths. The 50Cu/50Ag interface, derived from the auger electron spectroscopic concentration-depth profile, initially moved into the copper toward the outer dissolving surface, indicating enhanced diffusion of copper into silver. After longer times at all anodic current densities, the interface reversed and moved back toward the underlying silver-rich layer, indicating that eventually diffusion of silver into copper predominated. The reversal time was inversely proportional to the anodic current density. These effects are explained by anodic formation of subsurface vacancies which migrate as divacancies to the copper/silver interface where they affect interface movements by the well-known Kirkendall mechanism. Calculated diffusivities up to 10⁻¹² cm²/s at maximum anodic current densities of 900 μA/cm² are dramatically above any that are normally observed at room temperature.     

Room-temperature diffusion in Cu/Ag thin-film couples caused by anodic dissolution

Thin, 100-nm films of first silver and then copper were deposited consecutively onto inert substrates by magnetron sputter deposition. Constant anodic current densities were applied at room temperature to dissolve the outer copper film to varying depths. The 50Cu/50Ag interface, derived from the auger electron spectroscopic concentration-depth profile, initially moved into the copper toward the outer dissolving surface, indicating enhanced diffusion of copper into silver. After longer times at all anodic current densities, the interface reversed and moved back toward the underlying silver-rich layer, indicating that eventually diffusion of silver into copper predominated. The reversal time was inversely proportional to the anodic current density. These effects are explained by anodic formation of subsurface vacancies which migrate as divacancies to the copper/silver interface where they affect interface movements by the well-known Kirkendall mechanism. Calculated diffusivities up to 10⁻¹² cm²/s at maximum anodic current densities of 900 μA/cm² are dramatically above any that are normally observed at room temperature.     

Pressure effects in multiphase binary diffusion couples

A systematic study has been carried out of the effect of pressure upon growth kinetics of intermediate phases formed in diffusion couples in the binary systems Ni-Al, U-A1, and U-Cu. Even though applied pressures greater than 100 MPa and long times were investigated little or no pressure effect was observed, in disagreement with previous literature reports. The magnitude of observed pressure effects falls within that expected by closure of Kirkendall porosity.     

Qualitative observations on the diffusion of copper and gold through a nickel barrier

To retard failure of gold plated copper parts by diffusion of copper to the gold surface, a layer of nickel is frequently used between the copper and gold as a diffusion barrier. To evaluate the mechanisms whereby the nickel retards the motion of copper atoms to the gold surface, planar tri-couples of Cu/Ni/Au were prepared by electroplating nickel and gold layers on OFHC copper coupons. Diffusion anneals were carried out at temperatures from 150 to 750°C. A qualitative evaluation of diffusion behavior was provided by an electron microprobe utilizing X-ray wavelength dispersive analysis on polished cross sections. Results demonstrate that the nickel layer retards but does not block the transport of copper to the gold surface. Possible mechanisms for the anomalous buildup of copper at the gold/nickel interface and gold at thecopper/nickel interface are discussed.     

Qualitative observations on the diffusion of copper and gold through a nickel barrier

To retard failure of gold plated copper parts by diffusion of copper to the gold surface, a layer of nickel is frequently used between the copper and gold as a diffusion barrier. To evaluate the mechanisms whereby the nickel retards the motion of copper atoms to the gold surface, planar tri-couples of Cu/Ni/Au were prepared by electroplating nickel and gold layers on OFHC copper coupons. Diffusion anneals were carried out at temperatures from 150 to 750°C. A qualitative evaluation of diffusion behavior was provided by an electron microprobe utilizing X-ray wavelength dispersive analysis on polished cross sections. Results demonstrate that the nickel layer retards but does not block the transport of copper to the gold surface. Possible mechanisms for the anomalous buildup of copper at the gold/nickel interface and gold at the copper/nickel interface are discussed.     

Lateral diffusion in planar lipid bilayers

Direct measurements by fluorescence correlation spectroscopy of lateral diffusion coefficients of fluorescent lipid analogs in lipid bilaryer membranes indicate self-diffusion coefficients D greater than 10(-7) square centimeters per second for various lipid systems above their reported transition temperatures. Cholesterol in egg lecithin at mole ratio of 1 : 2 reduces D by about twofold, while retained hydrocarbon solvent can increase it by two- to threefold.     

Classification of concentration profiles in quaternary diffusion couples

Concentration profiles that can occur in quaternary diffusion couples have been classified into five types according to the number of extrema, the initial concentration differences, and the concentration gradient at the initial interface. The various profiles that are possible have zero, two or four extrema and are illustrated in this work by plotting appropriate error function solutions to the diffusion equation. The conditions under which the different types occur and the transitions between types, are given as well for quaternary systems. The only assumptions made are that the diffusivity is constant in the reaction zone and that the system is single phase.     

Kinetics of recovery and recrystallization in polycrystalline copper

The kinetics of the restoration processes that take place during the isothermal annealing of polycrystalline copper have been analyzed with the aid of a three component model for softening. The data describing the overall softening were obtained by the method of interrupted hot compression and these were supplemented by a quantitative metallographic study of the recrystallized structures. Prestrainmg was performed at rates between 1.8 × 10⁻³ and 8 × 10⁻² s⁻¹, in the temperature range 450 to 540°C and to true strains between 0.05 and 0.4. The temperature-corrected strain rate was the same for all temperatures.The three components of softening are identified as static recovery, classical recrystallization and metadynamic recrystallization. The paper describes the procedures used to separate the individual components of softening from the total restoration curve. It is shown that the softening due to the two recrystallization processes is faithfully described by an expression of the form x = 1 −exp(−ktⁿ) where x is the fraction of the process complete at a given time t,k is a rate constant and n a constant characteristic of the process. The time exponents for classical and metadynamic recrystallization are independent of the amount of prestrain and temperature and are equal to 2.2 and 1.0 respectively. The static recovery process is shown to be comprised of two sequential mechanisms, the first of which leads to a considerably smaller degree of dislocation annihilation than the second. The first mechanism appears to be primarily anelastic and probably involves the reverse motion of network dislocations from the forward (dynamic recovery) to the backward (static recovery) obstacles. The second mechanism leads to a decrease in the sub-boundary dislocation density as well as in internal stress, but does not reduce the “hardness state” as much as it effects the yield stress.