ELECTROCHEMICAL NANOFABRICATIONS: A GENERAL REVIEW - Electrochemical Nanofabrication: Principles and Applications - page 2

bottom-up processes. An exception is epitaxial processes that create
layers through layer-by-layer growth with registry at the atomic level.
Electrodeposition has traditionally been used to form high-
quality, mostly metallic, thin films for decades. By carefully
controlling the electrons transferred, the weight of material formed
can be determined according to Faraday's law of electrolysis. It
was shown that high-quality copper interconnects for ultra-large-
scale integration chips can be formed electrochemically on Si wafer
[1,2]. Electrodeposition has thus been shown compatible with
state-of-the-art semiconductor manufacturing technology. The
largest semiconductor companies, for example, IBM, Intel, AMD,
and Motorola, are installing wafer-electroplating machines on their
fabrication lines [1]. The electrodeposition of Cu with the line width
250 nm was used in the mass production of the microprocessor
Pentium III in 1998. In 2003, the line width of the CPU was reduced
to 130 nm in Pentium IV. Electrochemistry was largely used in chip
fabrication [3] and the packaging of microelectronics [4]. However,
compared with other nanofabrication techniques, electrochemical
nanofabrication is still a maiden area, which needs further
development and fulfillment.
1.1 ELECTROCHEMICAL ATOMIC LAYER EPITAXY
(EC-ALE)
Electrochemical atomic layer epitaxy (EC-ALE) is the combination
of underpotential deposition (UPD) and ALE. UPD is the formation
of an atomic layer of one element on a second element at a potential
under, or prior to, that needed to deposit the element on itself [5,6].
The shift in potential results from the free energy of the surface
compound formation. Early UPD studies were carried out mostly
on polycrystalline electrode surfaces [7]. This was due, at least in
part, to the difficulty of preparing and maintaining single-crystal
electrodes under well-defined conditions of surface structure and
cleanliness [8]. The definition of epitaxy is variable but focuses on
the formation of single crystal films on single crystal substrates.
This is different from other thin film deposition methods where
polycrystalline or amorphous film deposits are formed even on single
crystal substrates. Homoepitaxy is the formation of a compound on