Coverage metal deposit

While the above XPS results give the impression, that the electrochemical interface and the metal vacuum interface behave similarly, fundamental differences become evident when work function changes during metal deposition are considered. During metal deposition at the metal vacuum interface the work function of the sample surface usually shifts from that of the bare substrate to that of the bulk deposit. In the case of Cu deposition onto Pt(l 11) a work function reduction from 5.5 eV to 4.3 eV is observed during deposition of one monolayer of copper [96], Although a reduction of work function with UPD metal coverage is also observed at the electrochemical interface, the absolute values are totally different. For Ag deposition on Pt (see Fig. 31)... 

A shadow-mask technique has been applied for the local metal deposition to exclude metal residues on other designs processed on the same wafer (Fig. 4.2b). Such metal residues may be caused by imperfections in the patterned resist due to topographical features on the processed CMOS wafers or dust particles. The metal film is only deposited in those areas on the wafer, where it is needed for electrode coverage on the microhotplates. This also renders the lift-off process easier since no closed metal film is formed on the wafer, so that the acetone has a large surface to attack the photoresist. Another advantage of the local metal lift-off process is its full compatibility with the fabrication sequence of chemical sensors based on other transducer principles [20]. 

Figure 2.3 Step coverage of deposited metal. Poor step covoage leads to thin spots in the metal, resulting in poor electromigration performance.

The morphology of metal deposition has a profound effect on catalyst activity. If m represents the number of monolayers of coverage, then the catalyst activity under... 

Figure 6.3 shows the effect of different metal deposits (as islands equivalent to a few monolayers with about 90% surface coverage) on the photocurrent ofThe shift of the i-V curve from that of bare material is due to the catalytic effect of the metal on hydrogen evolution. For a metal deposit the photocurrent is parallel to the exchange current density for the dark evolution of H2 Pt, as a catalyst, has the highest exchange current whereas Pb, as an inhibitor, has a very low exchange current. 

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. 

It has been pointed out that the coke formation on the metallic surface occurs mainly on the flat planes (large particles) [10]. If we assume that Au at high coverage is deposited on the dense planes of the platinum particles, we can propose that Au acts as a diluent of the platinum ensembles, and hence that coke formation is inhibited. The level of coke and its nature are under study, and the results will be reported in a subsequent paper. 

In conventional additive-based electroplating, competitive adsorption for surface sites occurs coincident with substrate immersion and the onset of metal deposition. Additive adsorbate coverage evolves with metal deposition and is often accompanied, to some extent, by incorporation into the growing solid with associated influence on the microstructure and resulting physical properties. The adsorbates can have an accelerating or inhibitory effect on the metal deposition process. Inhibition manifests as an increase in electrode polarization with additive concentration while accelerating or depolarizing additives exhibit the opposite trend. Recent use of... 

Figure 2.5 (a) Schematic of some possible outcomes during metal deposition on well organized monolayer films (adapted from Ref. [105]). (b) Many plating additives form less well ordered films, (c)In some instances the relevant adsorbates are most active at dilute coverage (adapted from Ref. [112]). 

A variety of consumption rate laws have been proposed. One of the simplest, and perhaps most intuitive, is that adsorbate consumption is proportional to coverage, 0y, and metal deposition rate, i, according to [139] ... 

For dilute additive solutions, consumption can easily become limited by diffusion of the adsorbate to the interface. A particularly tractable situation occurs at the dilute limit of the Langmuir isotherm where C(- is proportional to the surface coverage, and in the limit of diffusion controlled adsorption CsoHd is directly proportional to the additive flux and thus the bulk electrolyte concentration. Such transport limited incorporation was reported in some radiotracer studies of thiourea incorporation in nickel and copper plating in the 1950-1960s [1-4, 17, 130, 131, 141-146], Consistently, a common observation was that the additive concentration in the solid was proportional to the additive concentration at the interface and inversely proportional to metal deposition rate, i, [1-4, 130, 131, 141, 142] such that ... 

An assessment of a variety of expressions for the steady-state coverage of single additive systems under both potentiostatic or galvanostatic metal deposition is available [140]. Perhaps the most interesting among these is the possibility of... 

Figure 2.7 Voltammetric curve for metal deposition in the presence of a dilute inhibitor. The hysteresis reflects the possibility of a potential regime where multiple steady-state values for inhibitor surface coverage are possible. This arises from the competition between transport of the blocking species to the interface and its coverage- and rate-dependent consumption (source Ref. [140]).

The effect of the inhibitor coverage on the deposition kinetics can also be mapped by referencing the additive-perturbed deposition kinetics to the additive-free case, in accord with Equation 2.1 [58]. Recent results analyzed in this manner are shown in Figure 2.12, where the coverage is a distinct function of the additive concentration in the bulk electrolyte and the hydrodynamic boundary layer thickness [58], The dotted lines are simulations based on a steady-state model [57] that considers metal deposition to proceed through an adsorbed intermediate M+ads, which competes with coumarin for available surface sites ... 

The dimensionless group X reflects the relative rate of the two serial elementary reactions that describe metal deposition. Where X —> 1 the second reaction is fast, resulting in minimal surface concentration of the intermediate metal species, 0M, whereas for X > 0 the surface coverage of the intermediate, 0M, can become substantial if there are sufficient sites that are not blocked by the inhibitor. 

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