Metal deposition process

During metal deposition processes the addition of adsorbable species has been found to cause an increase in the deposition overpotential [71 Lou]. Evaluation of the data results in the calculation of an adsorption isotherm. (Data obtained with this method are labelled CT.)... 

The important role of surface defects as nucleation centers in metal deposition processes is well-established. At low overpotentials, i.e., low supersaturation, metal de-... 

O Sullivan describes the fundamental theory, mechanistic aspects and practical issues associated with autocatalytic electroless metal deposition processes. Current approaches for gaining fundamental understanding of this complex process are described, along with results for copper, nickel and various alloys. Emphasis is placed on microelectronic applications that include formation of structures that are smaller than the diffusion layer thickness which influences structure formation. 

Metal deposition processes, early,. 9 760-761. See also Electroless deposition Metal deposition systems, in compound semiconductor processing, 22 188-189 Metal-dielectric composite-metal tandem solar absorbing surface, 23 11 Metal dithionates, 23 677-678 uses for, 23 677-678... 

Exploration of the scope of NPS in electrochemical science and engineering has so far been rather limited. The estimation of confidence intervals of population mean and median, permutation-based approaches and elementary explorations of trends and association involving metal deposition, corrosion inhibition, transition time in electrolytic metal deposition processes, current efficiency, etc.[8] provides a general framework for basic applications. Two-by-two contingency tables [9], and the analysis of variance via the NPS approach [10] illustrate two specific areas of potential interest to electrochemical process analysts. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

Fig. 7.123. The final stage in a metal deposition process, in which the deposited atom becomes incoiporated into the lattice.

This chapter shows that eutectic-based ionic liquids can be made in a variety of ways. The above description of liquids falling into three types is by no means exclusive and will certainly expand over the coming years. While there are disadvantages in terms of viscosity and conductivity these are outweighed for many metal deposition processes by issues such as cost, ease of manufacture, decreased toxicity and insensitivity to moisture. The high viscosity of some of these liquids could be ameliorated in many circumstances by the addition of inert diluents. 

The electrochemistry of ionic liquids is different in some essential features from the electrochemistry of aqueous electrolytes. Particularly for electrodeposition, which involves charge transfer from the electrolyte to the electrode, the double layer on the electrode is of great importance. In general the cathode is negatively charged for the electrodeposition of metals and therefore coated with a (Helmholtz-) layer of cations at least 0.5 nm thick but the metal species in most ionic liquids is anionic (for instance AlCh ). This makes the metal deposition process complicated, for more details we refer to Chapter 2. 

A completely different approach might be the use of radio frequency plasma instead of a DC plasma. The ignition and sustainment of the plasma is decoupled from the application of voltages to the electrodes that are now used only for electrochemical reactions. Another method which has been proven to be quite successful is the application of an U-shaped tube in order to avoid an IR-drop over the ionic liquid (see Figure 10.2). Unfortunately, this set-up led to a large size distribution of the obtained particles but it showed that RF plasma could further improve the stability of the ionic liquids during the metal deposition process. 

Although the deposition of metals from ionic liquids has been possible for over 50 years, to date no processes have been developed to a commercial scale. There are numerous technical and economic reasons for this, many of which will be apparent from the preceding chapters. Notwithstanding, the tantalizing prospect of wide potential windows, high solubility of metal salts, avoidance of water and metal/water chemistry and high conductivity compared to non-aqueous solvents means that, for some metal deposition processes, ionic liquids must be a viable proposition. 

Figure 6 shows the influence of the bulk diffusion coefficient, Dhi on the metal deposition profiles. Obviously, by decreasing the diffusivity the metal deposition process becomes more diffusion rate-determined. With decreasing diffusivity the transport of reactant and intermediates is decreased resulting in a less deep penetration into the catalyst pellet. Therefore, the metal deposition maximum is shifted further to the exterior of the catalyst pellet. 

A Bethe network is a branching structure defined by nodes and bonds, as shown in Figure 4b. The metal deposition process can be envisaged as a uniform deposition of metals in the pores of the network. The dependence of and on the total void space e during the metal deposition process can be determined by analytical relations (i). 

A disadvantage of the Bethe network is that it lacks physical reality, e.g. the presence of closed loops, whereas closed loops are present in the tessellation models. The presence of closed loops is considered to be an important aspect in describing the metal deposition process in catalyst pellets. Therefore, the tessellation approach is favoured over the Bethe network. 

Comparison of HDM catalyst deactivation simulations and experimental deposition profiles in catalyst pellets shows that the metal deposition process can be reproduced. 

It is interesting to consider the metal deposition process from a... 

Water-free inorganic solvents, such as ammonia, sulfur dioxide, and hydrazine, have been tested in terms of their suitability for electrolytic metal deposition. Liquid ammonia is used for a series of electrolytic metal deposition processes. Besides the low boiling point (- 33 °C) of this solvent its toxicity is disadvantageous. It has been reported that group lA and IIA metals, such as hthium, sodium, magnesium, and beryllium can be deposited from solutions based on ammonia as a solvent [45]. However, only thin or incoherent layers are thus produced [43, 44]. Because it is possible to form anions of molybdenum, lead, selenium, and tellmium in anunonia, these elements can be anodically deposited. Thus, deposition of the semiconductor lead selenide has also been achieved with ammonia as a solvent. 

Y. Okinaka and T. Osaka describe the fundamental aspects and technological applications of autocatalytic metal deposition processes. In view of that electroless deposition has found important applications in the manufacture of microelectronic devices, a review of the pertaning electrochemical fundamentals has been long overdue. 

Figure 2.14 Equivalent electric circuit simulating the metal deposition process on a stepped surface according to [2.321. Cdi, double layer capacitance Cads, adatom pseudo-capacitance Ret. adatom charge transfer resistance ggd. adatom surface diffusion resistance R e, adatom incorporation resistance Rdt, resistance of the direct transfer reaction 4tep. step half-distance.

The direct transfer of metal ions to kink sites and/or to step edges, unambiguously found first in the deposition of silver, seems to be a general phenomenon in electrochemical metal deposition and dissolution. From a theoretical point of view, Gerischer [5.98] was the first to recognize the role of direct transfer in metal deposition processes. 

---