Electroless techniques

Simple electroless techniques have been used for the formation of CdTe layers following an anodic or a cathodic route of deposition. For instance, spontaneous cathodic formation of CdTe was observed on Ti or glass electrodes short circuited with a corroding A1 contact (electron source) in a solution of Cd " " and HTe02 ions [96]. After thermal treatment and subsequent growth of an a-Pb02 layer on them, the as-obtained CdTe thin films were found to exhibit n-type behavior in alkaline polysulfide PEC cells. 

There is a basic difference between the damascene and through-mask plating processes in the way the trenches and vias are filled with electrochemically deposited Cu, through either an eiectrodeposition or an electroless technique. In multilevel metal structures, vias provide a path for connecting two conductive regions separated... 

Porous silicon layers have also been made using electroless techniques. The technique is used with thin films of microcrystalline silicon produced by PECVD and leads to PL intensities (efficiency 1-10 %) comparable with that obtained from anodized crystalline silicon (Solomon et al. 2008). 

Electroless techniques 7 F-band 5 Nanocrystals 2 Photo-excitation process 3 Photoluminescence (PL) 1 Quantum efficiency 2 S-band 5 UV band 3... 

Fig. 19 Optical micrographs of polished Ni-YSZ cermets prepared by electroless technique having nickel content 15 vol% (left) and 20 vol% (right). Magnification used is lOx. The white patches surrounding the individual YSZ (black) is nickel. A clear indication of enhancement of the quantity of the nickel is seen. No pore former was used in these cermet samples.

Although deposition of tin is also possible through immersion and electroless techniques, these deposition methods are not considered here for component finishing due to the relatively high thickness requirements (typically > 8 pm). Also, high productivity is required for component finishing which makes immersion and electroless plating methods not feasible for this application. 

Electrochonical processes for metal deposition are broadly classified in electrolytic and electroless techniques. The electrolytic method implies the use of an external current source, whereas electroless methods—though relying on an electrochemical mechanism—only require the immersion of the substrate into the plating solution. Electroless methods comprise two essentially different processes, displacement or immersion plating and autocatalytic deposition. In Figure 11.2 the basic functioning of these processes is schematically represented. 

The plating bath consists of a palladium ion source [Pd(NH3)4Cl2], a complexant (ethylene-di-amine tefra acetic acid (EDTA)) and a pH controller (ammonia). Deposition is carried out at basic pH (10-12) and at a controlled temperature ranging between 40 and 60°C. In more detail, the electroless technique is based on the following redox reactions, which occur simultaneously in the solution (Bottino et al., 2006) ... 

J. I. Duffy, ed.. Electroless and OtherNon-electroljtic Plating Techniques, Noyes Data Corp., Park Ridge, N.J., 1980. 

Adequate ventilation is necessary for aH process lines to ensure worker safety. Electroless copper baths must have good ventilation to remove toxic formaldehyde vapors and caustic mist generated by the hydrogen evolution reactions and air sparging. Electroless nickels need adequate ventilation to remove nickel and ammonia vapors. Some states and municipalities requite the removal of ammonia from wastewaters. A discussion of printed circuit board environmental issues and some sludge reduction techniques is avaHable (25). 

Numerous variations exist in the electroless plating solutions, processes, and techniques employed both in laboratory and commercial form, to create a great variety of products (39). AH produce a layer of highly conductive copper in specified areas. Modem electroless copper films have a ductiHty and conductivity identical to that of electrolytic copper (40). The three basic classes of copper baths are... 

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. 

Fig. 10. Time dependence of corrosion rate of Co8P and Co films, electrolessly plated and sputter-deposited onto NiP substrates, in a droplet of DI water. The results were obtained by a repeated application of the polarization resistance technique with the potential scanned at 1 mV/sec in a potential range 15 mV above and below the corrosion potential [125]. (Reprinted by permission of The Electrochemical Society). 

Despite H/D kinetic isotope studies, application of modern techniques such as atomic force microscopy (AFM), electrochemical mass spectrometry (EMS) [60], and electrochemical quartz microbalance (EQCM), the mechanism of electroless nickel and cobalt, whatever reducing agent is involved, continues to be the subject of much discussion and varying opinions. 

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