Electroless deposition mechanism

The film deposition was carried out at room temperature from an aqueous solution of plumbous acetate, ammonium acetate, and ammonium persulphate, using NH4OH to bring the pH to 6. A trace of AgNOs was added as a catalyst for reaction 7.4 [29]. A film of PbOi ca. 50 nm thick was formed in an hour. Once this initial film was deposited, thicker films could be built up, usually at a somewhat higher pH, in the absence of the AgNOs. The initial film formation appears to be a pure CD reaction. However, electrochemical studies of further film buildup showed that an electroless deposition mechanism, involving two partial electrochemical reactions, was responsible for film formation. 

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 ... 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

A discussion of the applicability of the MPT model to a particular electroless system ideally presumes knowledge of the kinetics and mechanisms of the anodic and cathodic partial reactions, and experimental verification of the interdependence or otherwise of these reactions. However, the study of the kinetics, catalysis, and mechanistic aspects of electroless deposition is an involved subject and is discussed separately. 

Donahue [37] was one of the first to discuss interactions between partial reactions in electroless systems, specifically electroless Ni with NaH2PC>2 reducing agent, where mention was made of an interaction between H2PO2 ions and the cathodic Ni2+ reduction reaction with a calculated reaction order of 0.7. Donahue also derived some general relationships that may be used as diagnostic criteria in determining if interactions exist between the partial reactions in an electroless solution. Many electroless deposition systems have been reported to not follow the MPT model. However, mention of these solutions may be best left to a discussion of the kinetics and mechanism of electroless deposition, since a study of the latter is usually necessary to understand the adherence or otherwise of an electroless solution to the MPT model. 

The shapes of the polarization curves shown in Fig. 3, including those generally observed for other reducing agents, are invariably complex, and not representative of simple electrochemical reactions. From the inception of modern electroless deposition practice, a number of mechanisms have been advanced to describe the electroless deposition process, many of which dealt with Ni-P. These classical mechanisms include the following ... 

Mechanism 4, an apparently simple and rather obvious mechanism that invokes a strictly electrochemical approach to electroless deposition, continues to elicit support in some contemporary reports on the mechanism of electroless deposition. It... 

The coevolution of H2 gas in electroless deposition processes is a phenomenon that needs to be understood not only to elucidate the mechanism of deposition, but also since it impacts the properties of deposits by H inclusion. Van den Meerakker [51] first proposed a correlation between simultaneous hydrogen evolution in electroless deposition and the heat of adsorption of hydrogen. In this useful endeavor, however, he has been criticized for erroneously calculating the heats of adsorption of H at Cu by Gottesfeld et al. [52], and Group I (or SP type) metals in general by Bindra and Tweedie [53]. 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

For practical as well as fundamental reasons, there has been considerable interest in the deposition of alloys containing metals such as W, Mo, and Sn. In their pure forms, these metals do not catalyze the oxidation of the usual electroless reducing agents. Therefore, their mechanism of codeposition is intriguing, and developing an understanding of it should help to better understand the mechanism of electroless deposition as whole. Obvious questions in ternary and quaternary alloy deposition include the effect of the third or fourth element containing ions in solution on the rate of electroless deposition, as well as on the P and B contents in the case of alloys such as Ni-P and Ni-B. 

Although electroless deposition seems to offer greater prospects for deposit thickness and composition uniformity than electrodeposition, the achievement of such uniformity is a challenge. An understanding of catalysis and deposition mechanisms, as in Section 3, is inadequate to describe the operation of a practical electroless solution. Solution factors, such as the presence of stabilizers, dissolved O2 gas, and partially-diffusion-controlled, metal ion reduction reactions, often can strongly influence deposit uniformity. In the field of microelectronics, backend-of-line (BEOL) linewidths are approaching 0.1 pm, which is much less than the diffusion layer thickness for a... 

Adsorbed additives also tend to undergo reduction during the electroless process, and become incorporated as impurities into deposits, most likely via a mechanism similar to that involved in ternary alloy deposition. In a manner similar to that discussed below in greater detail for dissolved 02, electroless deposition rates will be lower for features smaller than the stabilizer diffusion layer thickness. The edges of larger features, which experience higher stabilizer levels due to enhanced nonplanar... 

There is compelling evidence that reducing agent oxidation and metal ion reduction are, more often than not, interdependent reactions. Nonetheless, virtually all established mechanisms of the electroless deposition fail to take into account this reaction interdependence. An alternative explanation is that the potentials applied in the partial solution cell studies are different to those measured in the full electroless solution studies. Notwithstanding some differences in the actual potentials at the inner Helmholtz plane in the full solution relative to the partial solutions, it is hard to see how this could be a universal reason for the difference in rates of deposition measured in both types of solution. 

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). 

In this section we show that some electroless deposits have unique properties compared to electrodeposited, evaporated, or sputtered metal deposits. Our discussion is limited to mechanical and diffusion barrier properties. 

Although the concept of phase is well defined thermodynamically, here phase refers to a mechanically separable homogeneous part of an otherwise heterogeneous system. The concept of phase change refers here to a change in the number present or in the nature of a phase or phases as a result of an imposed condition such as temperature or pressure. To clarify and illustrate the topic at hand, we use the specific cases of electrolessly deposited nickel and electrodeposited cobalt. 

Fig. 7.188 Electroless deposition on a conducting but noncatalytic substrate if the deposition is based on an electrochemical mechanism. (Reprinted from B. J. Hwang and S. H. Lin, J. EteCtro-chem. Soc. 142 3749, 1995, Fig. 2. Reproduced by permission of The Electrochemical Society, Inc.)...

There are four types of fundamental subjects involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M a[tice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering the basic components of an electrochemical cell for deposition in the first three chapters. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface. Chapter 6 contains presentation of the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. Chapter 10 contains discussion on the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which... 

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