Electroless copper deposition

In terms of kinetics and mechanisms, electroless deposition processes have many similarities. In an attempt to analyze the electroless deposition, several mechanisms such as atomic hydrogen, hydride ion, metal hydroxide, electrochemical, and universal have been proposed.1-3 It is important to note that these mechanisms were developed for cases of nickel and copper electroless deposition, which were the most widely studied metals in this respect. Based on the proposed mechanisms, most of the features of electroless deposition can be explained. However, there are some characteristics of electroless deposition, which cannot be explained using these mechanisms. The major problems arise when attempting to generalize the proposed models explaining the mechanistic aspects. 

Kim JW, Ryu JH, Lee KT, Oh SM (2005) Improvement of silicon powder negative electrodes by copper electroless deposition for lithium secondary batteries. J Power Sourc 147 227-233... 

Electroless Deposition of Copper. The basic ideas of the mixed-potential theory were tested by Paunovic (10) for the case of electroless copper deposition from a cupric sulfate solution containing ethylenediaminetetraacetic acid (EDTA) as a complexing agent and formaldehyde (HCHO) as the reducing agent (Red). The test involved a comparison between direct experimental values for and the rate of deposition with those derived theoretically from the current-potential curves for partial reactions on the basis of the mixed-potential theory. 

The second example is electroless deposition of copper from solutions containing dissolved oxygen (49,53). In this case the interfering reaction is the reduction of the oxygen, and the cathodic partial current density is the sum of two components ... 

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

For example, in electroless deposition of copper, when the reducing agent is formaldehyde and the snbstrate is Cu, Hads desorbs in the chemical reaction (8.17). If the substrate is Pd or Pt, hydrogen desorbs in the electrochemical reaction (8.18). 

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. 

Ohno (54) used ac polarization data and Ricco and Martin (55) used an acoustic wave device for in situ determination and monitoring of the rate of deposition. Various empirical rate equations were derived for electroless deposition of copper (15,33). 

Tfie electroless deposition of copper is usually done in solutions containing EDTA as a complexing agent. Tfie stability constant for the CuEDTA complex is... 

The value of the transfer coefficient a is usually 0.50. In the electrochemical oxidation of an organic molecule the transfer coefficient a may be considerably less than 0.50. One example is the oxidation of formaldehyde in electroless deposition of copper. 

Diffusion barriers are coatings that serve in that role specifically, protection against undesirable diffusion. One of the best examples is that of a 100- tm-thick electrode-posited copper layer that serves as an effective barrier against the diffusion of carbon. Another example is that of nickel and nickel alloys (notably, electrolessly deposited Ni-P) that block diffusion of copper into and through gold overplate. This is achieved by the deposition of a relatively thin Ni-P layer (less than 1 /mm) between the copper and its overlayer. Naturally, the effectiveness of the diffusion barrier increases with its thickness. Other factors in the effectiveness of a diffusion barrier... 

Zehner RW, Sita LR. Electroless deposition of nanoscale copper patterns via microphase-separated diblock copolymer templated self-assembly. Langmuir 1999 15 6139-6141. 

Schlesinger and Marton (15) studied the nucleation and growth of electrolessly deposited thin nickel (Ni-P) films. These studies were later extended and complemented by the studies performed by Cortijo and Schlesinger (19, 20) on radial distribution functions (RDFs). RDF curves were derived from electron diffraction data obtained from similar types of films as well as electrolessly deposited copper ones. Those studies, taken together, have elucidated the process of crystallization in the electroless deposition of thin metal films. 

Figure 13.13 The aCP of Pd nanoparticles on an amino-functionalized substrate and the subsequent electroless deposition of copper.80...

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