Electroless metallization process

Brenner and Riddell are credited with developing the first successful electroless deposition process, that for Ni-P, in the 1940 s [1]. However, it should be indicated that they were probably the first to develop an example of a controlled electroless deposition process, since a host of metallization processes existed prior to this that bear similarities to modern-day electroless methods. For example, there existed the well-... 

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]. 

O Sullivan describes the fundamental theory, mechanistic aspects and practical issues associated with autocatalytic electroless metal deposition processes. Current approaches for gaining fundamental understanding of this complex process are described, along with results for copper, nickel and various alloys. Emphasis is placed on microelectronic applications that include formation of structures that are smaller than the diffusion layer thickness which influences structure formation. 

A series of nucleation and growth models was developed by, for example, Bewick et al. (11), Armstrong and Harrison (16), and Scharifker and Hills (17). Amblart et al. (18) have shown that nickel epitaxial growth starts with the formation of three-dimensional epitaxial crystallites. An electrochemical model for the process of electroless metal depositions (mixed-potential theory) was suggested by Paunovic (14) and Saito (14b). 

In this process z electrons are supplied by an external power supply (Fig. 2.1). The overall reaction of electroless metal deposition is... 

An electrochemical model for the process of electroless metal deposition was suggested by Paunovic (10) and Saito (8) on the basis of the Wagner-Traud (1) mixed-potential theory of corrosion processes. According to the mixed-potential theory of electroless deposition, the overall reaction given by Eq. (8.2) can be decomposed into one reduction reaction, the cathodic partial reaction. 

Kinetic Scheme. Generally, metal ions in a solution for electroless metal deposition have to be complexed with a ligand. Complexing is necessary to prevent formation of metal hydroxide, such as Cu(OH)2, in electroless copper deposition. One of the fundamental problems in electrochemical deposition of metals from complexed ions is the presence of electroactive (charged) species. The electroactive species may be complexed or noncomplexed metal ion. In the first case, the kinetic scheme for the process of metal deposition is one of simple charge transfer. In the second case the kinetic scheme is that of charge transfer preceded by dissociation of the complex. The mechanism of the second case involves a sequence of at least two basic elementary steps ... 

A variation of the selective Cu deposition process, limited to electroless Cu deposition, is the lift-ojfprocess, a planarized metallization process (16). Figure 19.5 shows a stepwise process sequence for this technology. 

This paper describes a process for activating polyimide surfaces for electroless metal plating. A thin surface region of a polyimide film can be electro-chemically reduced when contacted with certain reducing agent solutions. The electroactivity of polyimides is used to mediate electron transfer for depositing catalytic metal (e.g., Pd, Pt, Ni, Cu) seeds onto the polymer surface. The proposed metal deposition mechanism presented is based on results obtained from cyclic voltammetric, UV-visible, and Rutherford backscattering analysis of reduced and metallized polyimide films. This process allows blanket and full-additive metallization of polymeric materials for electronic device fabrication. 

This paper describes a new seeding process for electroless metallization of polyimides and other electroactive polymers. Polyimide films can be reduced electrochemically at an electrode surface or by contact with an appropriate reducing agent in an electrolyte solution. In the latter case, only the outer surface of the film undergoes reduction. Once the polyimide surface is reduced it then can mediate electron transfer to metal ions or metal complexes in solution causing metal to be deposited at the surface with concurrent reoxidation of the polyimide. 

The deposition of metals such as Pd, Pt, Ni, and Cu renders the surface active towards further metal deposition from conventional electroless metal plating baths. Well-adhering metal films can be formed on polyimides by this method. The main process steps for blanket metallization of a polyimide film, illustrated in Scheme I, involve polymer reduction, metal seeding, and electroless metal plating. Specific details of each process step are provided in the discussion below. 

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