Electronics copper dissolution

Surface polishing can be achieved under certain conditions of electrochemical dissolution, which is a reverse process of electroplating (EP). A simple electrochemical cell is shown in Fig. 10.1. Two metal (e.g., Cu) bars are immersed in an electrolyte. A voltage is applied between the two bars. The one connected to the positive pole of the power supply is anode. The other one is cathode. The positive potential applied to the anode may pump out electrons from copper atoms on the anode surface. As a result, copper dissolution may occur in certain electrolytes. Conversely, copper deposition may occur on the cathode. That is, copper electroplating results when the working electrode is chosen to be cathode, and copper dissolution is accomplished when the working electrode is chosen to be the anode. 

A salt bridge serves as an ionconducting connection between the two half-cells. When the external circuit is closed, the oxidation reaction starts with the dissolution of the zinc electrode and the formation of zinc ions in half-cell I. In half-cell II copper ions are reduced and metallic copper is deposited. The sulfate ions remain unchanged in the aqueous solution. The overall cell reaction consists of an electron transfer between zinc and copper ions ... 

Because electrons are neither products nor reactants in chemical reactions, the two processes are interdependent and neither can occur alone. The zinc metal dissolution must furnish electrons for the copper metal plating. The reaction of zinc and copper sulfate solution is a spontaneous reaction involving a transfer of electrons, i.e., is a spontaneous redox process. The spontaneity of the reaction is commonly explained by saying that zinc loses electrons more readily than copper or, alternatively, that Cu2+ ions gain electrons more readily than Zn2+ ions. 

This situation cannot persist for long the concentration of copper (II) ions in water will initially be extremely small, unless some other source is also involved, and will quickly be depleted. The important point is that, as soon as electrical contact is made, the zinc becomes an anodic electrode, and the copper a cathode. If another cathodic reaction besides reaction 16.2 is possible, however, then dissolution (i.e., corrosion) of the zinc will continue, while the copper will serve merely as an electrically conducting surface to deliver electrons for the alternative cathodic reaction. In pure water, the obvious alternative reaction is hydrogen evolution (reaction 16.3) for which Eh is —0.414 V at pH 7 ... 

The anodic reaction is an oxidation reaction producing electrons in the anode, while the cathodic reaction is a reduction reaction consuming electrodic electrons at the cathode interface. We shall consider, as an example, an electrochemical cell consisting of a metallic zinc electrode and a metallic copper electrode, in which the anodic reaction of zinc ion transfer (zinc dissolution) is coupled with the cathodic reaction of copper ion transfer (copper deposition) as shown in the following processes ... 

The dissolution of the zinc is no longer inhibited by a buildup of negative charge in the metal, because the excess electrons are removed from the zinc by copper ions that come into contact with it. At the same time, the solution remains electrically neutral, since for each Zn ion introduced to the solution, one Cu ion is removed. The net reaction... 

This relation tells us that a cell potential will change by 59 millivolts per 10-fold change in the concentration of a substance involved in a one-electron oxidation or reduction for two-electron processes, the variation will be 28 millivolts per decade concentration change. Thus for the dissolution of metallic copper... 

The formation of Cu-Sn alloy by galvanic contact deposition in the trimethyl-n-hexylammonium [bis(trifluoromethyl)sulfonyl]amide ([TMHAl TfiN ) ionic liquid at a temperature above 100 °C has been demonstrated by Katase et al. [41] Sn(II) was introduced into the liquid by dissolution of the SnflT N) salt which has a solubility of 0.2 mol dm f In the plating cell, a copper sheet was used as the cathodic substrate, a Sn sheet was used as the anode, and a Sn rod immersed in the same solution was used as a quasi-reference electrode. On short-circuiting, the Sn anode was oxidized to Sn(II) giving two electrons through external circuit to... 

The dissolution of zinc at one electrode and the deposition of copper at the other is, therefore, an inseparably connected process, which is caused by the flow of electrons in the external electric circuit from the zinc to the copper electrode. 

Galvanic corrosion tends to occur when two metals with different electrochemical potentials are electrically connected and exposed in an electrolyte. As a result, the less noble metal will suffer from accelerated corrosion [58]. When excess copper is polished away by copper CMP, copper and barrier metal are exposed to the CMP slurry simultaneously. Copper and barrier metal have different electrochemical potentials and thus trigger galvanic corrosion at the interface between copper and barrier metal at a certain kind of slurry composition. In this galvanic corrosion, electrons are transferred from titanium anode to copper cathode. During overpolishing of the patterned wafer, titanium near the copper structure is recessed owing to dissolution (Ti Ti -I- 2e ) and Cu " ions are preferentially deposited onto... 

Oxidizing agents such as NOj and Fe(CN)/ may be added to the NH40H-based slurries to increase the copper polish rate by increasing the dissolution rate of the abraded material. In order to form the complex ion, the NH3 complexing agent first requires the oxidation of copper to Cu (reaction (7.2)). Reaction (7.2) requires an associated reduction reaction to sink the electrons. Even if the NH3 were to complex the copper metal directly in one step ... 

Deposition of metals on a silicon surface can be either a conduction band process or a valence band process depending on the redox potential of the metal and solution composition. Deposition of Au on p-Si in alkaline solution occurs only under illumination indicating that it is a conduction band process due to the unfavorable position of the redox couple for hole injection. " On the other hand, deposition of platinum on p-Si can occur in the dark by hole injection into the valence band. For Cu, although the deposition proceeds via the conduction band as shown in Fig. 6.9, it can also proceed via the valence band because a large anodic current of n-Si occurs in the dark in copper-containing HF solution as shown in Fig. 6.10. The reduction of copper under this condition is via hole injection. The holes are consumed by silicon dissolution and the silicon reaction intermediates then inject electrons into the conduction band, resulting in the anodic current on n-Si in the dark. 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

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