Water-Drop Migration

and Cu Fiims. Evaporated Ag, Pb, andCu land patterns on ceramic substrates were tested in water-drop experiments under 10 volt bias as-evaporated, after being oxidized, sulphized and exposed to HCI dilute solution to form metal chlorides. Five samples for each metal and surface condition were stressed. The reaction surface layers were 50 nm thick and rather spotty (discontinuous) as determined by electron microprobe. The oxide and sulphide layers either halted migration or reduced its occurrence dramatically. It was obvious that dendrites, whenever they occurred, were associated with incomplete passivation and, in general, the surface treatment was very effective. 

Metal chlorides did not improve the migration resistance. Actually the chlorides made it worse, which can be explained by their high solubility in water. Table I summarizes the results in terms of the time to grow the dendrite and the current recorded during the process. The results show how important the surface condition is and explains why metals that grow stubborn insoluble oxide (e g., Cu) are much more resistant to metal migration than those whose oxides are water-soluble (e.g., Ag). These differences reflect in the pre-exponential factor A which is a function of the material and process. These affect the critical current requirement and 

Metal Untreated Chloride Oxide Sulfide Current for Migration 

It appears that as long as the metal is protected by an insoluble layer of oxide, sulphide, etc., metals will not be able to migrate as the exchanging of ions and electrons across the anode and cathode interfaces (essential for the oxidation-reduction process) will cease. 

Therefore, for very low ion concentration, one requires a correspondingly higher voltage to produce the current density necessary for dendritic growth. 

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The gelatinization temperature appears related to the melting temperature of the amylopectin portion of the starch, which drops rapidly with increasing moisture content. Under normal atmospheric conditions most starches contain 10% to 17% moisture. In particular, starch with 11% water content melts at 65°C (12). Thus, on heating, the water is free to swell the starch after it melts. Much of the water then migrates from the minor protein phase into the major starch phase. 

Water drop tests are not very revealing and do not necessarily predict the migration susceptibility of metals in microelectronics under field conditions (1). The reason is that metal migration and dendritic formation depend on the state of the metal surface, whether it is oxidized or not, and on whether the oxide passivates the surface or it dissolves in water. For instance, copper will rarely produce dendrites when it is... 

Table 1. Migration Across Evaporated Lands (Water-Drop) (i),...

In addition, when the experiment is conducted with acidic solutions, e.g., 10 M H2SO4 solution, the Cu will readily migrate all over, across the electrodes. Figure 12 shows a number of dendrites across parallel Cu wires (0.005 cm diameter) in water drops having a pH of 3.2. No dendrites were obtained with water having pH 7. Therefore, acidic environments that can dissoive the oxide wiil promote metal migration. 

Nl Films. This section deals with nickel migration and how it compares with the migration of Ag, Cu, and Pb. Ni films were evaporated on ceramic blank substrates leaving a 15 mil gap between two halves of the film. The gap was covered with a water drop ( 1 mm diameter) and biased at various voltages ranging from 0.5 to 15 volts. Ni dendrites formed under these conditions. Dendrite growth rates were calculated from the time that each dendrite took to bridge the gap. The ionic current density was 0.1 A/cm . 

Wire samples were also tested in a standard migration cell, and l-V curves were measure to determine the Tafel slope during anodic dissolution in various aqueous media. The results of the migration cell tests in Oj + HjO and 350 ppm HCL showed no dendrite formation however, the anode was roughened (dissolution) and there was Cu in solution. No silver was detected. The Tafel slope was identical to that of Cu, though there was a slight shilft in the noble metal direction. These results support the observations in the water-drop experiment that only Cu dendrites form and that the Cu-15%Ag-2.5%P wire behaves like pure Cu. 

Some of the water-drop experiments reported here have illustrated the usefulness of such data in terms of combating migration under specific environmental conditions and state of surface oxidation. Other important factors are process residues, pH, etc., which likewise must be studied to determine their ievel of tolerance for the given situation. It is no wonder that B, as an experimentai parameter, varies from metal to metal and from process to process. The key is to identify and quantify the variables responsible for the change. 

In practice, spontaneous emulsification can be combined with emulsion inversion. For instance, if a water phase is poured little by little into an oil phase containing a dissolved hydrophilic surfactant and/or alcohol, the first dispersion to occur is a W/O emulsion because there is very little water. As the number of water drops increases, the surfactant migrates from the oil to the water phase and the dynamic interfacial tension can be close to zero. A multiple emulsion often occurs as an intermediate situation. Then an O/W emulsion appears after some time when the kinetic phenomena finally prevail. This caimot be interpreted straightforwardly from the bidimensional map unless the formulation is assumed to change as the surfactant migrates from oil to water. In such a view, the trajectory of change moves from to A , crossing the inversion line somewhere. 

Residue levels of carbon-14 in water were low, especially in the case of carbon-14 DDT, indicating that DDT-related residues remain bound to the soil (Figure 4). After an initial concentration of 0.009 ppb at Day 1, the concentration of carbon-14 DDT residues reached an equilibrium of about 0.02 ppb by the third day and then dropped off slightly to 0.012 to 0.013 ppb at the time the mosquito larvae and fish were introduced. Carbon-14 residues in water derived from carbon-14 robenidine showed an initial concentration of 0.344 ppb and then remained fairly constant at about 1 ppb throughout the study, indicating that robenidine-related residues are polar in nature and readily migrate into the water phase and reach equilibrium very rapidly. 

The term B in equation (1.27) is related to the longitudinal molecular diffusion in the column. It is especially important when the mobile phase is a gas. This term is a consequence of entropy, telling us that a system will tend towards the maximum degrees of freedom as demonstrated by a drop of ink that diffuses after falling into a glass of water. Hence, if the flow rate is too slow, compounds being separated will mix faster than they will migrate. This is why one must never interrupt the separation process, even momentarily. 

The United Nation s report—prepared by over 1,000 scientists—predicts cultural and social disruptions, loss of wetlands, flooding of river deltas, bleaching of coral reefs, permafrost thawing, acidification of oceans, drop in crop output, widespread water shortage, and even starvation in parts of southern Europe, the Middle East, Africa, Mexico, Southern Asia, and the American Southwest. Deforestation, soil erosion, storms, droughts, and devastation of agriculture are likely to result as temperatures exceed the heat tolerance of crops. These trends can combine to cause migration, ethnic strife, social destabilization, and wars. 

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