Additives electroless deposition

Alloys of CoSnP [17] and CoZnP [16] have been prepared by electroless deposition. Both Zn and Sn increase the Hc, and Sn also improves the corrosion resistance and the appearance. For Sn alloys, X-ray diffraction measurements suggest that the Sn addition orients the c axis parallel to the plane of the film and decreases the grain size. 

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. 

Since electroless deposition involves complexants, reducing agents, and solution stabilizers, all of which adsorb to some degree or another on the deposit surface, it is unreasonable to not expect incorporation of minute amounts of elements such as C, or S from a S containing additive, in the deposit. 

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. 

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... 

Not all additives function as inhibitors in electroless deposition solutions. A few additives function to increase the rate of electroless deposition these tend to be... 

Electroless Deposition in the Presence of Interfering Reactions. According to the mixed-potential theory, the total current density, is a result of simple addition of current densities of the two partial reactions, 4 and However, in the presence of interfering (or side) reactions, 4 and/or may be composed of two or more components themselves, and verification of the mixed-potential theory in this case would involve superposition of current-potential curves for the electroless process investigated with those of the interfering reactions in order to correctly interpret the total i-E curve. Two important examples are discussed here. 

Induction Period. The induction period is defined as the time necessary to reach the mixed potential at which steady-state metal deposition occurs. It is determined in a simple experiment in which a piece of metal is immersed in a solution for electroless deposition of a metal and the potential of the metal is recorded from the time of immersion (or the time of addition of the reducing agent, i.e., time zero) until the steady-state mixed potential is established. A typical recorded curve for the electroless deposition of copper on copper substrate is shown in Figure 8.11. 

After discussing adsorption, we discuss the effects of additives on the kinetic parameters of the deposition process and on the elementary processes of crystal growth. The general effect of additives on electroless deposition is discussed in Section 8.4. 

We present here three specific examples of the effect of additives on the properties of electrodeposited Ni. Table 10.1 shows the effect of additives on tensile strength and elongation of Ni produced by three different Ni electrodeposition solutions (40, 41). For additional examples the reader is referred to Refs. 40 and 41. In general, it may be stated that additives which increase the tensile strength of Ni depKJsits will decrease the elongation (ductility) (42). The effect of additives on the properties of electroless deposits is discussed in Section 8.9. 

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... 

Real-time monitoring of deposition or dissolution from liquids can be performed using TSM, APM, or FPW devices. For example, Kanazawa and Doss [179] demonstrated the use of TSM devices as real-time rate monitors of electroless nickel deposition, while Ricco and Martin [180] demonstrated a similar use of APM devices for electroless deposition of copper directly onto the quartz substrate. TSM devices have also been used to monitor surface accumulation of thermal degradation products from jet fuels [181]. The ability to rapidly quantify the rate of accumulation can be used in designing jet fuels with improved thermal stability [182,183]. Turning to material removal, monitoring the dissolution of metals in aqueous environments has also been demonstrated [184]. In addition, by intentionally contaminating a TSM with a surface layer similar to the anticipated contamination on a component to be cleaned, a real-time moni-... 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

The possibility of using electroless deposition to superfill sub-micrometer features has also been explored[347-349] successful filling was recently reported for an alkaline EDTA-complexed electrolyte containing SPS and PEG as additives [349]. However, the tilted sidewalls in the lower half of the features, combined with the absence of kinetic data, make a mechanistic assessment of feature filling difficult. 

In general terms, for the electroless deposition no external current is required. Coatings produced by electroless deposition are uniform and continuous, which makes this process very attractive for different applications. Applications of electroless deposition are related to electronics, energy conversion, aerospace, and biomedical and automotive industries. In addition, new applications in the area of metallization of polymers, ceramics, and fabrics, production of... 

For the media preparation, 25-nm-thick CoZrNb soft magnetic film was sputtered onto Ni-P SUL and then a thick CoPtCr-SiC>2 granular recording layer. For comparison, a media with 200-nm-thick sputtered CoZrNb SUL was used. The results showed that an electroless-deposited ferromagnetic Ni-P layer (low phosphorus content), which is suitable for mass production as a SUL for a double-layered perpendicular recording media, exhibits almost the same magnetic properties and recording performances as a 200-nm-thick sputtered SUL. In addition, the spike noise which is commonly observed from SUL was not found for the media with Ni-P SUL. 

Copper was electrolessly deposited on suspended AI2O3 nanoparticles by the addition of Cu(II)-EDTA alkaline solution and formaldehyde as a reducing agent.97 This approach can be used for various ceramic or polymeric nanoparticles and different metals. 

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