Barrier metal , electroless deposition

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. 

A comparison was made between Ni and Co diffusion barriers produced by electroless, electro-, and evaporation deposition (64). This comparison shows that only electrolessly deposited metals and alloys, at a thickness of 1000 im, have barrier properties for Cu diffusion. For Co(P) 1000-pm-thick barriers, annealed for 14h, the amount of Cu interdiffused into Co(P) is less than 1 at %. Thicker barriers of Ni(P), Ni(B), and Co(B) are required for the same degree of Cu interdiffusion. The same metals, if electrodeposited, both do and do not have inferior barrier properties. This... 

A comparative smdy of great practical value has been carried out between several Ni-based diffusion barrier properties. Those were produced by means of electroless deposition from nickel sulfate and nickel sulfamate deposition solutions (73). It was concluded on the basis of Auger depth profiling (see Section 13.3) that Ni(P) sulfamate has much better diffusion barrier properties than Ni(P) sulfate. This conclusion is a telling example of the influence of anions on the physical properties of electro-chemically deposited metals. 

In our study, a three-layered Al/Cu/Ti film was employed as the seeding layer for electroless Cu deposition process. These metal films were deposited using the electron-beam evaporation technique and the substrates employed were thermally oxidized <100> silicon wafers. Ti is employed as the first layer, to serve as a barrier/adhesion promotion layer since Ti adheres well to most dielectric substrates and can prevent Cu diffusion into Si02. The second layer, Cu is the best homogenous catalyst for electroless Cu deposition. The last layer, A1 is a sacrificial layer to prevent Cu oxidation before immersing into the electroless deposition solution. 

Tin deposited directly over copper, whether by electrolytic or electroless methods, should be avoided except for low-end/low-cost, short-field-life products. Sn alloys with copper, even at room tenqjerature, form a brittle intermetaUic compound, albeit slowly. It has limited solderable shelf-Ufe at room tenqjerature and the Sn-Cu intermetaUic can inhibit soldering and may result in incomplete solder wetting or solder joints of inferior strength. For tin to be used effectively, an appropriate non-porous barrier metal, such as Ni, should be deposited between the Cu and the Sn. Immersion Sn can be plated only as a very thin layer since its deposition is by a self-limiting replacement reaction. When there is no more exposed Cu on the surface of the PWB during the surface-plating operation, Sn deposition wUl cease.Therefore, it is impossible to plate enough Sn by tins method to extend the solderable sheU-hfe or reduce the brittleness of the solder joint. 

The possibilities afforded by SAM-controlled electrochemical metal deposition were already demonstrated some time ago by Sondag-Huethorst et al. [36] who used patterned SAMs as templates to deposit metal structures with line widths below 100 nm. While this initial work illustrated the potential of SAM-controlled deposition on the nanometer scale further activities towards technological exploitation have been surprisingly moderate and mostly concerned with basic studies on metal deposition on uniform, alkane thiol-based SAMs [37-40] that have been extended in more recent years to aromatic thiols [41-43]. A major reason for the slow development of this area is that electrochemical metal deposition with, in principle, the advantage of better control via the electrochemical potential compared to none-lectrochemical methods such as electroless metal deposition or evaporation, is quite critical in conjunction with SAMs. Relying on their ability to act as barriers for charge transfer and particle diffusion, the minimization of defects in and control of the structural quality of SAMs are key to their performance and set the limits for their nanotechnological applications. 

Electroless nickel coatings can be easily soldered and are used in electronic applications to facilitate soldering of light metals such as aluminum. Electroless nickel is often used as a barrier coating to be effective, the deposit must be free of pores and defects. In the as-deposited amorphous state, the coating corrosion resistance is excellent (Table 12), and in many environments is superior to that of pure nickel or chromium alloys. However, after heat treatment the corrosion resistance can deteriorate. 

Composite membrane fabricated using metal supports can also use an intermediate layer for reducing the thermal mismatching between the support and the Pd-alloy film. For instance, ceramic layers have been positioned in between stainless steel porous supports and Pd-Ag and Pd films [62]. The ceramic interlayer also reduces intermetallic diffusion of the supports into the active Pd-based layers that could contaminate the membrane. Intermetallic film made of a porous Pd-Ag layer obtained through continuous electroless plating of alternating Pd and Ag baths has been used in the fabrication of composite Pd membranes, which consist of porous stainless steel plates with a selective Pd layer [63]. These membranes were successfully tested up to 500°C. Their effectiveness as diffusion barriers for Pd membrane deposited over porous stainless steel supports at 400°C was demonstrated by the presence of a zir-conia layer [64]. 

The preparation procedure, as described by Azzurri et al. (2011), can follow the following subsequent steps (1) oxidation of the porous metal (2) dip-coating deposition of a ceramic barrier layer (3) dip-coating deposition of an activation layer, containing PdCl2 and (4) Pd deposition by electroless plating. 

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