Electrode recession

The pulse sequence with three different potentials that is usually applied for the detection of carbohydrates is shown in Figure 8.12a. However, the cleaning potential E2 being applied for 200 ms leads to an electrode recession that enlarges the cell volume as shown in Figure 8.13. This, in turn, lowers the linear speed of the liquid through the detector cell that consequently results in a decrease of the response factor for a given analyte concentration. 

Electrode recession can be minimized by avoiding a strongly positive potential, although the removal of the top layer is a prerequisite for a clean, catalytically... 

Many electrodes qualify as recessed electrodes. A metal recording electrode linked by a salt bridge to a potential source is a recessed electrode. Recessed skin-surface electrodes are generally cup-shaped. The gap between the skin surface and the metallic cup electrode is filled with electrode paste or some other electrolyte to maintain good electrical contact between the electrode and the surface which it contacts. The advantage of this type of... 

Shaping. Most metal-shaping operations in ECM utilize the same inherent feature of the process whereby one electrode, generally the cathode tool, is driven toward the other at a constant rate when a fixed voltage is appHed between them. Under these conditions, the gap width between the tool and the workpiece becomes constant. The rate of forward movement between the tool and the workpiece becomes constant. The rate of forward movement of the tool is matched by the rate of recession of the workpiece surface resulting from electrochemical dissolution. 

Unlike solid electrodes, the shape of the ITIES can be varied by application of an external pressure to the pipette. The shape of the meniscus formed at the pipette tip was studied in situ by video microscopy under controlled pressure [19]. When a negative pressure was applied, the ITIES shape was concave. As expected from the theory [25a], the diffusion current to a recessed ITIES was lower than in absence of negative external pressure. When a positive pressure was applied to the pipette, the solution meniscus became convex, and the diffusion current increased. The diffusion-limiting current increased with increasing height of the spherical segment (up to the complete sphere), as the theory predicts [25b]. Importantly, with no external pressure applied to the pipette, the micro-ITIES was found to be essentially flat. This observation was corroborated by numerous experiments performed with different concentrations of dissolved species and different pipette radii [19]. The measured diffusion current to such an interface agrees quantitatively with Eq. (6) if the outer pipette wall is silanized (see next section). The effective radius of a pipette can be calculated from Eq. (6) and compared to the value found microscopically [19]. 

Galceran, J., Taylor, S. L. and Bartlett, P. N. (1999). Application of Danckwerts expression to first-order EC reactions. Transient currents at inlaid and recessed microdisc electrodes, J. Electroanal. Chem., 466, 15-25. 

For the sake of completeness, glass microelectrodes [48, 59, 184] will first be mentioned. Two types of these electrodes are used, spear-shaped microelectrodes [59] and recessed-tip microelectrodes [165] (see fig. 4.5). In the former case, the microelectrode is drawn from a capillary of an ion-exchanger glass and is insulated on the outside, except for the tip, by inserting the microelectrode into a micropipette made of an inactive glass. In the latter case, the outer micropipette extends over the microelectrode tip. The two capillaries are sealed together and the ISM is in contact with the liquid between the two capillaries. 

The two major causes of uneven current distribution are diffusion and ohmic resistance. Nonuniformity due to diffusion originates from variations in the effective thickness of the diffusion layer 8 over the electrode surface as shown in Figure 10.13. It is seen that 8 is larger at recesses than at peaks. Thus, if the mass-transport process controls the rate of deposition, the current density at peaks ip is larger than that at recesses since the rate of mass transport by convective diffusion is given by... 

The ability to cover a surface and to reduce pinhole density in the film is affected by addition agents and deposition waveform. Throwing power, the ability of the depositing material to plate inside a deep, narrow recess, depends on a number of factors, including complexing of the ions, electrode polarization, current density, etc. Additives to increase throwing power are usually organic materials. 

If the heights between peaks and recesses on the electrode are small compared with the diffusion-layer thickness 8 (Fig. 7.167), then will be less than 8recess and, therefore, ipeak will be greater than irecess. Hence, there will be greater amounts of deposition on the parts of the substrate that stick out. The nonuniformity increases, and the formation of macrorough deposits can be understood. 

For shallow recessed microdisc electrode arrays, the hemispherical diffusion is larger than that for coplanar microdisc arrays. The minimum interelectrode distance necessary for hemispherical diffusion becomes smaller as recess depth increases [58],... 

The lift-off process is usually employed to fabricate metal electrodes. This method, as opposed to the wet-etch process, allows the dual-composition electrode to be patterned in a single step [747]. In order to achieve well-defined metal electrodes in a channel recess using the lift-off technique, the metal (Pt/Ta) will not be deposited onto the sidewalls of the photoresist structure (see Figure 2.32). This discontinuity of the deposited metal layer around the sidewalls allows metal on the resist to be removed cleanly from the surface without tearing away from the metal on the surface. Thus negative resists were used because they can be easily processed to produce negatively inclined sidewalls. To achieve this, the photoresist is subjected to underexposure, followed by overdevelopment [141]. 

To facilitate leak-free fusion bonding of chips consisting of metal electrodes, the metal layers were deposited in pre-etched recesses on chips [141,276-280,747,1127],... 

An HgCdTe layer is epitaxially grown over the surface and individual HgCdTe detector elements 26 are formed in the recessed portions when the Si02 layer has been removed. The surface of the HgCdTe detector elements are then polished and etched before electrodes 27 are connected to connection pads 25. 

A p-type HgCdTe layer 12 is formed epitaxially on a substrate 11 of CdTe. Recessed regions 13B are formed between protruding regions 13A and 13B. N-type regions 14A-14C are formed to which connections are made by indium electrodes 16 and 17 after an insulating film 15 of ZnS has been formed. 

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