Metal conductors

Soldering materials are alloys that are composed primarily of tin and lead (qv), and have low melting temperatures relative to the conductor metals which are being soldered (see Lead alloys Tin and tin alloys). Welding requires sufficientiy high temperatures for the fusion of metals. 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

The electrical resistance of most conductors, metals in particular, decreases as the temperature of the conductor decreases. For some pure metals and compounds of the metals, the resistance decreases with temperature as usual, but at some critical temperature the resistance drops identically to zero. The resistance remains zero as long as the material is maintained at a temperature below the critical temperature. Such a material is termed a supercon-... 

One other long-term condition that takes place with relatively low level DC fields in the presence of moisture is the migration of the metal of the conductor into the plastic. This was discovered to be a common thing in the past with silver conductors and phenolic insulators. The first instance of field failures were discovered in telephone equipment. The problem can occur with other metals with phenolic and also conceivably with other plastics that are moisture sensitive and can have a solvating action on the conductor metals that they contact. Most of these type plastics should be avoided inside hermetically sealed containers with movable contacts. Vapors released from the organic plastic deposit on the contacts to produce an insulation layer leading to contact failure. 

Interconnect. Three-dimensional structures require interconnections between the various levels. This is achieved by small, high aspect-ratio holes that provide electrical contact. These holes include the contact fills which connect the semiconductor silicon area of the device to the first-level metal, and the via holes which connect the first level metal to the second and subsequent metal levels (see Fig. 13.1). The interconnect presents a major fabrication challenge since these high-aspect holes, which may be as small as 0.25 im across, must be completely filled with a diffusion barrier material (such as CVD titanium nitride) and a conductor metal such as CVD tungsten. The ability to fill the interconnects is a major factor in selecting a thin-film deposition process. 

In principle there are two types of solid state devices (i) pn-photocells and (ii) Schottky type cells. The first one consists simply of a pn-junction whereas the other of a semi-conductor-metal junction. The energy schemes of these cells are given in Fig. 1 a and b. The current-potential dependence of both types of cells is given by (see e.g. ) ... 

At the contact of two electronic conductors (metals or semiconductors— see Fig. 3.3), equilibrium is attained when the Fermi levels (and thus the electrochemical potentials of the electrons) are identical in both phases. The chemical potentials of electrons in metals and semiconductors are constant, as the number of electrons is practically constant (the charge of the phase is the result of a negligible excess of electrons or holes, which is incomparably smaller than the total number of electrons present in the phase). The values of chemical potentials of electrons in various substances are of course different and thus the Galvani potential differences between various metals and semiconductors in contact are non-zero, which follows from Eq. (3.1.6). According to Eq. (3.1.2) the electrochemical potential of an electron in... 

The required degree of understanding of the physical properties of metal thin films used for interconnects on chips is illustrated by the following example. It was found that the performance of conductors on chips, A1 or Cu, depends on the structure of the conductor metal. For example, Vaidya and Sinha (10) reported that the measured median time to failure (MTF) of Al-0.5% Cu thin films is a function of three microstructural variables (attributes) median grain size, statistical variance (cr ) of the grain size distribution, and degree of [111] fiber texture in the film. 

Technically this means rather more than bad conductor . Metals conduct electricity because some of their electrons come free of their parent atoms and are at liberty to roam through the material. Their motion corresponds to an electrical current. A semiconductor also has wandering electrons, but only a few. They are not intrinsically free, but can be shaken loose from their atoms by mild heat some are liberated at room temperature. So a semiconductor becomes a better conductor the hotter it is. Metals, in contrast, become poorer conductors when hot, because they gain no more mobile electrons from a rise in temperature and the dominant effect is simply that hot, vibrating atoms obstruct the movement of the free electrons. 

Electrons transport charge in a metallic conductor. Metallic conductors include metals and the nonmetal graphite. 

Semiconductor An element or compound that can conduct electricity better than an insulator (nonmetal) but not as well as a conductor (metal) the electrical flow in a semiconductor can be changed with a change in temperature or by adding other materials. 

Galvanic cell (or galvanic element) — A galvanic cell is an - electrochemical cell in which reactions occur spontaneously at the -> electrodes when they are connected externally by a conductor. In these cells chemical energy can be converted into electrical energy [i, ii]. The galvanic cell consists of two electrodes, i.e., electron conductors (-> metal, carbon, semiconductor etc.) in contact with one or more ionic conductors (which may be - electrolyte solutions, ionic liquids, electrolyte melts, or - solid electrolytes). 

The band theory of solids provides a clear set of criteria for distinguishing between conductors (metals), insulators and semiconductors. As we have seen, a conductor must posses an upper range of allowed levels that are only partially filled with valence electrons. These levels can be within a single band, or they can be the combination of two overlapping bands. A band structure of this type is known as a conduction band. 

Unfortunately the electromagnetic theory is only valid under a series of limiting suppositions, so that the emissivities calculated from it frequently differ from reality. Despite this, it provides important, qualitative statements that can be used for the extrapolation from measurements or to estimate for missing data. We will not discuss the electromagnetic theory, see for this [5.4], but will use some of its results in the treatment of emissivities of electrical insulators and electrical conductors (metals). These two material groups differ significantly in their radiation behaviour. 

As a result of these factors, the universal paradigm for inorganic solar cells, the p-n junction, cannot be adapted for organic semiconductors. The contrast with inorganic semiconductors is shown schematically in Fig. 7.2. The alternative of a metal-semi-conductor-metal device structure, where photocurrent is directed by the difference in work function between the two metals, also cannot be used because the electric field created by available asymmetric contact materials is insufficient to separate the singlet exciton into electron and hole polarons. Therefore, alternative device architectures are needed. 

Kurita, N., Fukatsu, N., Miyamoto, S., Sato, F., Nakai, H., Irie, K. and Ohashi, T. (1996) The measurement of hydrogen activities in molten copper using oxide protonic conductor. Metall. Mater. Trans. B, -a, 929-35. 

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