Chemistry Reference
In-Depth Information
6
Cathodic Behavior and
Redox Couples
6.1. INTRODUCTION
This chapter concerns the reactions that occur at cathodic potentials and those related
to redox couples. The cathodic reactions of many silicon/electrolyte systems have been
the subject of numerous studies, but they generally have not been investigated to the
level of details comparable to the anodic reactions in fluoride and alkaline solutions.
As a result, cathodic processes such as hydrogen evolution and metal deposition are
not as well characterized and are much less understood than those at anodic potentials.
This had been largely due to the lack of mainstream applications of the cathodic reac-
tions in the silicon technologies. However, the research activities on cathodic reactions,
most notably on metal deposition on silicon, have, significantly increased in recent
years because of the demand for higher quality wafer surface and the industry's drive
to use copper instead of aluminum as the conducting material for microdevices. It can
be expected that in the future, research progress in this area will be fast as many details
of the cathodic reactions on silicon are characterized and deeper systematic under-
standing of the various phenomena is obtained.
6.2. HYDROGEN EVOLUTION
Hydrogen evolution is a particularly important electrode reaction on silicon in
that it is involved in almost all reactions at both anodic and cathodic potentials. Hydro-
gen reaction is a principal process that determines the surface condition of silicon. The
surface of a silicon electrode can be passivated by hydrogen termination of the surface
silicon dangling bonds, which is a technologically very important feature. Hydrogen
evolution is a part of the etching process of silicon in alkaline solutions and occurs
during anodic dissolution in fluoride-containing solutions. Thus, hydrogen reaction
affects all electrode processes on silicon in aqueous electrolytes, such as cleaning, metal
deposition, redox reactions, formation of anodic oxides, and quantum efficiency of
photocurrent.
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