Activation electroless deposition

The electrochemical mechanism was rejected by Salvago and Cavallotti [26] on the basis that it does not explain several features of electroless deposition of ferrous metals it does not account for the isotopic composition of the H2 gas evolved it does not explain the effect of the various solution components on reaction rate and it does not account for the homogeneous decomposition of very active solutions or the fact that they can give deposition on insulating surfaces. These authors put forward a chemical mechanism, involving various hydrolyzed nickel species, which they claim explains the observed behavior of the system ... 

Electrodeposition is more flexible than electroless deposition, in that it is not limited by the requirement of having a catalytically active surface. Electrodeposition allows a wider variation in the alloy composition and in the deposit properties than does electroless deposition. This flexibility has not been widely exploited, however, and most of the electrodeposited alloys have had compositions similar to those obtained by electroless deposition (i.e. CoP or CoNiP). 

M.H. Pournaghi-Azar and H. Razmi-Nerbin, Voltammetric behaviour and electrocatalytic activity of the aluminum electrode modified with nickel and nickel hexacyanoferrate films, prepared by electroless deposition. J. Electroanal. Chem. 456, 83-90 (1998). 

In the mixed potential theory (MPT) model, both partial reactions occur randomly on the surface, both with respect to time and space. However, given the catalytic nature of the reductant oxidation reaction, it may be contended that such a reaction would tend to favor active sites on the surface, especially at the onset of deposition, and especially on an insulator surface catalyzed with Pd nuclei. Since each reaction strives to reach its own equilibrium potential and impose this on the surface, a situation is achieved in which a compromise potential, known as the mixed potential (.Emp), is assumed by the surface. Spiro [27] has argued the mixed potential should more correctly be termed the mixture potential , since it is the potential adopted by the complete electroless solution which comprises a mixture of reducing agent and metal ions, along with other constituents. However, the term mixed potential is deeply entrenched in the literature relating to several systems, not just electroless deposition. 

Figure 3 shows polarization curves for the anodic oxidation of H2CO at various metal electrodes recorded by Ohno et al. [38] in a solution maintained at 25 °C and containing EDTA (a commonly used complexant in electroless Cu solutions) and maintained at a pH = 12.5. After exhibiting exceptional activity at potentials less than -0.8 Y (SCE)2, the activity of Cu decreases at ca. 0.3 Y (SCE) this region of activity is more than adequate for electroless deposition of Cu. Although they... 

The metal ion in electroless solutions may be significantly complexed as discussed earlier. Not all of the metal ion species in solution will be active for electroless deposition, possibly only the uncomplexed, or aquo-ions hexaquo in the case of Ni2+, and perhaps the ML or M2L2 type complexes. Hence, the concentration of active metal ions may be much less than the overall concentration of metal ions. This raises the possibility that diffusion of metal ions active for the reduction reaction could be a significant factor in the electroless reaction in cases where the patterned elements undergoing deposition are smaller than the linear, or planar, diffusion layer thickness of these ions. In such instances, due to nonlinear diffusion, there is more efficient mass transport of metal ion to the smaller features than to large area (relative to the diffusion layer thickness) features. Thus, neglecting for the moment the opposite effects of additives and dissolved 02, the deposit thickness will tend to be greater on the smaller features, and deposit composition may be nonuniform in the case of alloy deposition. 

The O2 reduction reaction affects not only the steady-state deposition kinetics, but also the initiation of deposition, the so-called induction time [126, 127], At the beginning of the deposition process, the open circuit potential (Eoc) of either a uniformly catalytically active substrate, or a catalyst particle on an insulator, will be higher than that required for electroless deposition to occur. This is a consequence of the surface of the catalyst being covered with O or OH species which mask the catalytic activity of the surface the value of would be expected to be in the range of... 

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

Various strategies are used to produce electrode structures within the membrane pores, including sol—gel synthesis, CVD, eiectrodeposition, and electroless deposition. With careful control of the synthetic conditions, the pores are either filled completely or preferentially coated at the pore walls, producing hollow tubes (see Figure 10b). Following infiltration with the desired electrode material, the membrane is subsequently removed under conditions that do not disturb the active material, leaving an array of either solid nanofibers or nanotubes attached to a current collector like the bristles of a brush (Figure 11). In this case there is very limited interconnectedness between the nanofibers, except at the current collector base. 

In this chapter we discuss the electrochemical model of electroless deposition (Sections 8.2 and 8.3), kinetics and mechanism of partial reactions (Sections 8.4 and 8.5), activation of noncatalytic surfaces (Section 8.6), kinetics of electroless deposition (Section 8.7), the mechanism of electroless crystallization (Section 8.8), and unique properties of some deposits (Section 8.9). 

Wagner-Traud Diagram, According to the mixed-potential theory, the overall reaction of the electroless deposition, [Eq. (8.2)] can be described electrochemically in terms of three current-potential i-E) curves, as shown schematically in Eigure 8.2. First, there are two current-potential curves for the partial reactions (solid curves) (1) ic =f(E), the current-potential curve for the reduction of ions, recorded from the rest potential E eq M the absence of the reducing agent Red (when the activity of is equal to 1, eq,M E m) and (2) = f(E), the current-potential... 

Noncatalytic surfaces (e.g., nonconductors, noncatalytic metals, noncatalytic semiconductors) have to be activated (i.e., made catalytic) prior to electroless deposition. This activation is performed by generating catalytic nuclei on the surface of a noncatalytic material. Two major types of processes have been used to produce catalytic nuclei electrochemical and photochemical. 

Activation by Displacement Deposition. Silicon can be made catalytic for electroless deposition of Ni by replacing the surface Si atoms with Ni atoms (58,62) ... 

Activation by Thermal Decomposition of Metallic Oxides. The surface of alumina, AI2O3, may be activated by employing laser or ultraviolet irradiation to decompose AI2O3 (68). Decomposition of AI2O3 results in the generation of aluminum particles that are catalytic for electroless deposition of Cu (the first reaction probably is displacement deposition). 

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