Limiting current density with deposits present

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

The abovementioned phenomenon — encountered rather frequently in the case of a low concentration of ions in the solution — can be observed for instance during the oourse of depositing metals from aqueous solutions. In such solutions also hydrogen ions are present which have been formed by dissociation of water. Should the current density exceed the limit value of the metal ions discharge gaseous hydrogen will be deposited at the oathode with increased rate simultaneously with the metal. 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

In most cases, additional metal ions are present, mainly those with a positive standard potential, which are deposited preferentially at the cathode and have a low overpotential for hydrogen evolution. On the other hand, zinc sponge is deposited even at relatively low current densities during electrolysis of ZnSO solutions in the presence of Cu, As or Sb ions these are nobler than zinc and are deposited at the maximum rate (i.e. at their limiting current) in a powdery form, and the zinc deposited on them "copies" their powdery structure. Powdered metals (Fe, Cu, Mi) are usually deposited at the cathode at current densities higher than the limiting one with simultaneous evolution of hydrogen. 

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