The influence of an electric current on the following solid state transformations in metals are considered: (1) intermetallic compound formation and growth in diffusion couples, (2) precipitation, (3) crystallization of amorphous alloys and (4) recrystallization and grain growth of cold worked metals. The formation and growth of intermetallic compounds were in qualitative accord with electromigration theory. Regarding precipitation, an electric current can either enhance or retard the precipitation rate, depending on the alloy, the current density and its frequency. Important factors appear to be the effect of current on the quenched-in vacancies and the presence of an internal stress. Both a continuous d.c. current and high current density electropulsing enhanced the crystallization rate of amorphous alloys. The effects are greater than can be explained by simple electromigration theory and suggest the cooperative motion of a larger number of atoms. Electropulsing enhanced the recrystallization rate of cold worked metals, but retarded subsequent grain growth. Enhancement of the recrystallization rate resulted mainly from an increase in the pre-exponential factor of the Arrhenius rate equation, which is considered to refer to the nucleation rate. Retardation of subsequent grain growth resulted from a lower residual dislocation density within the newly-formed grains.

Effects of electric current on solid state phase transformations in metals

This study investigates the effect of electromigration upon the interfacial reactions between the promising lead-free solders, Sn–Cu and Sn–Ag, with Ni substrate. Sandwich-type reaction couples, Sn–0.7 wt% Cu/Ni/Sn–0.7 wt% Cu and Sn–3.5 wt% Ag/Ni/Sn–3.5 wt% Ag, were reacted at 160, 180, and 200 °C for various lengths of time with and without the passage of electric currents. Without passage of electric currents through the couples, only one intermetallic compound Ni3Sn4 with ∼7 at. % Cu solubility was found at both interfaces of the Sn–0.7 wt% Cu/Ni couples. With the passage of an electric current of 500 A/cm2 density, the Cu6Sn5 phase was formed at the solder/Ni interface besides the Ni3Sn4 phase. Similar to those without the passage of electric currents, only the Ni3Sn4 phase was found at the Ni/solder interface. Directions of movement of electrons, Sn, and Cu atoms are the same at the solder/Ni interface, and the growth rates of the intermetallic layers were enhanced. At the Ni/solder interface, the el...

Electromigration effect upon the Sn–0.7 wt% Cu/Ni and Sn–3.5 wt% Ag/Ni interfacial reactions

The influence of high dc currents (up to 1019 A cm−2) on the interaction between thin Au-Al layers was investigated over the temperature range 400-500°C In contrast with earlier diffusion couple results, only four of the five intermetalllic compounds in this system were observed, in both the absence and the presence of a current at all levels The intermetallics Au5 Al2 and Au2 Al were the dominant products while AuAl and AuAl2 were relatively insignificant The latter showed little change, growing only to a total thickness of a few micrometres, regardless of the magnitude of the current density and time of annealing While the sequence of formation of the different intermetallics was not affected by the current, the incubation time and rate of growth of the layers were strongly influenced The incubation time for the appearance of a measurable product layer was markedly reduced by the current, by nearly two orders of magnitude in some cases The effect of the current on the nucleation of the pr

High-flux current effects in interfacial reactions in Au–Al multilayers

Interfacial reactions are important in microelectronic devices and can be accelerated or decelerated by imposing a direct electric current normal to the interface. These effects are analyzed by including electromigration-driven interchange of atomic species in a conventional analysis of reaction layer thickening in a binary system controlled by interdiffusion in the layer and by an interfacial reaction barrier. New types of behavior are predicted. When the electromigration augments interdiffusion, layer growth can accelerate as the layer thickens, in contrast to the usual deceleration. When the electromigration opposes interdiffusion, there is a limiting layer thickness, inversely proportional to the applied current.

Electromigration effects on compound growth at interfaces

This study investigates the electromigration effect upon the Zn/Ni and Bi/Ni interfacial reactions by using reaction couple techniques. Three phases, β1-NiZn, γ-Ni5Zn21, and δ-NiZn8 formed in the Zn/Ni couples reacted at 150°C and 200°C for 4 h to 360 h, and the reaction layers grow thicker with longer reaction time. Passage of a 300 A/cm2 current through the Zn/Ni couples has no significant effect upon the interfacial reaction. There is no noticeable difference in the phase formation and layer thickness of the two kinds of Zn/Ni couples with and without the passage of electric currents. Only NiBi3 phase was found in the Bi/Ni couples reacted at 150, 170, 185 and 200°C. Passage of a 300 A/cm2 electric current through the Bi/Ni couples did not change the phase formation, and growth rates of the NiBi3 phase in the couples reacted at 185°C and 200°C were not affected by passage of electric currents either. However, growth rate of the NiBi3 phase was enhanced in the Bi/Ni couples reacted at 150°C and 170°C with the passage of a 300 A/cm2 electric current. A mathematical model was proposed to describe the electromigration effect upon the growth of the intermetallic compounds. Physical parameters in the models were determined by optimization based on experimental measurements, and the results indicate that the values of the apparent effective charge of Bi and Ni decreased sharply with increasing temperatures.

Electromigration effect upon the Zn/Ni and Bi/Ni interfacial reactions

The concept of recoil implantation is proposed to facilitate fabrication of ultrashallow p+/n junctions. In this method, a thin boron film is first deposited onto the Si wafer surface. Then the boron atoms are knocked into the Si substrate by Ge implantation or Ar plasma source ion implantation. Dopant activation and damage removal are achieved via rapid thermal annealing. Preliminary results show the realization of sub-100 nm deep p+/n junctions with this technique. Monte Carlo simulations were performed to predict the recoiled boron profiles, and agree well with the experimental results.

Ultra-shallow P+/N junctions formed by recoil implantation

An ion-beam mixing technique is used to fabricate ultrashallow p+/n junctions. In this method, a thin boron layer is first sputter deposited onto the Si wafer surface. Then a 10–40 keV Ge ion beam or −3 kV Ar plasma source ion implantation (PSII) is used to knock the boron atoms into the Si substrate by means of ion-beam mixing. For the thin (0.7 nm) boron layers used in this effort, a selective etch for the removal of boron is unnecessary. Sub-100 nm p+/n junctions have been realized with this method. Numerical simulations, performed to predict the recoiled boron profiles, show good agreement between the simulated and measured data.

Wei Wang

Papers

Ultra-shallow P+/N junctions formed by recoil implantation

Recoil implantation of boron into silicon for ultrashallow junction formation: Modeling, fabrication, and characterization

Plasma-assisted Recoil Implantation for Shallow Boron Doping in Silicon