Electrolyzed Water for Selective Etching of Metals
Authors: Laboureur Patricia , Neelsen Ann-Kristine , Zwicker Gerfried , Fryda Matthias 
Published: February 2020

Electrolyzed water (EW) is a promising tool for wet cleaning of semiconductor devices. At the anode, hydroxyl radicals exhibiting a very high oxidation reduction potential (ORP) are produced, generated by electrodes made of materials with high overpotential. Thus, water with high ORP is obtained, which can be higher than the ORP of thermodynamically stable water. In comparison, cleaning solutions made of a mixture of chemicals, such as SC2-solutions, are always within the thermodynamic boundaries. The high ORP of electrolyzed anodic water (AW) is advantageous concerning some issues in IC fabrication, such as its stability in contrast to the change of dilution in SC2-solutions by decomposition of H2O2. Furthermore, an enhanced ORP is beneficial for the removal of metal contamination. In this concern the choice of the electrode material is crucial. For example, platinum electrodes tend to dissolve in the AW and cause contamination during the wet cleaning process. However, electrodes coated with boron doped diamond (BDD) are protected by the BDD layer and thus, there is no contamination of the EW. Indeed, one option for wet cleaning during semiconductor production is particle removal from silicon wafers. For this application, electrolyzed cathodic water (CW) has been applied for already several years to improve the removal of particles smaller than 50 nm [1, 2, 3].
Additionally to CW, Red Ox® cell technology is able to provide various types of EW for different process specifications and applications in semiconductor industry. Water can be processed through either the cathodic side or the anodic side of the cell producing CW, hydrogenated water (HW), ozonized water (OW), anodic water fluoride (AW-F), anodic water chloride (AW-Cl) or anodic water sulphate (AW-SO4), depending on the feed solution. For photo resist removal electrolyzed sulphuric acid (ESA) can be used. In this study, the application of AW in the selective etching of metals is shown. The pH value and the corresponding ORP values of the AW can easily be controlled by adjusting the current and reaction time of the electrolysis. Metal etching rates of AW were determined for four different metals with respect to pH.
Based on the results it becomes clear, that the etching rates for different metals vary significantly on the pH and corresponding ORP of the AW. This means, that by a careful selection of the pH/ORP value, the etching rate of the metal can be adjusted. Furthermore, the etching selectivity for different metals allows for defined etching of one material without the loss of another material. These results indicate that the electrolyzed water chemistry with a dedicated pH/ORP value is a promising tool for wet chemical cleaning of metal contaminations.

Tab. 1: Various types of electrolyzed water (EW), which can be produced for different kinds of wet cleaning applications.

[1] K. Ryoo and B. Kang: Electrolyzed water as an alternative for environmentally-benign semiconductor cleaning chemicals. CLEAN TECHNOLOGY, 7 , p. 215-223(2001).
[2] G. Choi: Necessity of cleaning and its Application in Future Memory Devices. Solid State Phenomena, 219, p. 3-10 (2015).
[3] M. Fryda, Th. Matthée, J. Oshinowo, D.H. Baek, H.-J. Förster and G. Heinze: Operation of a new electrolyzed cell using Boron Doped Diamond electrodes. Solid State Phenomena, 2192, p. 40-43 (2015).

Figure 1