2
Y. Wang and Y. Zeng
1.1 Introduction
Micro/sub-micron fabrication techniques have become critical issues as the miniaturization trend in various areas, such as microelectronics, micro electromechanical
system (MEMS), lab-on-a-chip, optics, and bioengineering [1]. The fabrication of
microchannels, micro holes, and controlled microstructures on various components
is attracting much attention in manufacturing discipline in recent years. Lithography
based processes are widely applied to fabricate microstructures on semiconductor
and metal materials. However, the lithography-based micromachining processes are
high in cost and require a vacuum or ultra-clean working environment [2].
Micromachining techniques have been extensively studied in recent years. A
variety of processes have been applied to machining microstructures such as
laser beam machining, micro electric discharge machining, electrochemical micromachining, electrochemical discharge machining, plasma etching. Thermal based
micromachining technique could introduce the recast layer and heat affected zone
to the machined surface. The lithography based micromachining processsed always
suffered from a higher cost. Electrochemistry-based micromachining, referred to as
electrochemical micromachining (EMM), has received much attention in micro and
nano surface patterning and fabrication of metal materials. It is a non-contact, heat
and residual stresses free process, regardless of the mechanical properties of the
anodic materials. Thus, the processed surface by EMM has a higher surface integrity
compared with that processed by other thermal based machining processes. Micro and
even nanostructures could be fabricated by various electrochemistry-based micromachining processes [3]. In electrochemistry-based micromachining processes, materials are removed with redox reaction at the anode/electrolyte interface. In creating
microstructures with high localization, electrochemical reactions should be restricted
in a small and controlled area. Many technologies taking the advantages of electrochemistry principles have been developed for microfabrication, such as LIGA
(Lithography, electroplating, and molding) [4, 5], electrochemical nanolithography
[6], confined etchant layer technique (CELT) [7], three-dimensional electrodeposition [8], and EMM with ultrashort voltage pulses [9], etc. In order to process
microstructures, electrochemical dissolution or deposition should be confined in an
area of a micrometer or sub-micrometer scale. The electrochemical nanolithography
combines the electrochemistry and scanning probe microscope using a nanometerscale tip as the tool electrode, which is capable of fabricating profiles with nanometer
and atomic resolution [10, 11]. CELT could replicate microstructures by confining the
outer boundary of the diffusion layer of the etchant and can retain the 3-dimensional
fine micro-pattern of the mold electrode [12]. High aspect ratio microstructures
obtained by electrochemical deposition could be fabricated with the LIGA technique [13]. Electrochemical nanoimprint lithography provided a nanoscale accuracy method for producing semiconductor microdevices [14]. With the introduction
of ultrashort voltage pulses of nanosecond duration to electrochemical micromachining, the dissolution area of the materials is highly confined in a small area with
sub-micrometer-scale precision [15]. Microstructures of various contours could be
Y. Wang and Y. Zeng
1.1 Introduction
Micro/sub-micron fabrication techniques have become critical issues as the miniaturization trend in various areas, such as microelectronics, micro electromechanical
system (MEMS), lab-on-a-chip, optics, and bioengineering [1]. The fabrication of
microchannels, micro holes, and controlled microstructures on various components
is attracting much attention in manufacturing discipline in recent years. Lithography
based processes are widely applied to fabricate microstructures on semiconductor
and metal materials. However, the lithography-based micromachining processes are
high in cost and require a vacuum or ultra-clean working environment [2].
Micromachining techniques have been extensively studied in recent years. A
variety of processes have been applied to machining microstructures such as
laser beam machining, micro electric discharge machining, electrochemical micromachining, electrochemical discharge machining, plasma etching. Thermal based
micromachining technique could introduce the recast layer and heat affected zone
to the machined surface. The lithography based micromachining processsed always
suffered from a higher cost. Electrochemistry-based micromachining, referred to as
electrochemical micromachining (EMM), has received much attention in micro and
nano surface patterning and fabrication of metal materials. It is a non-contact, heat
and residual stresses free process, regardless of the mechanical properties of the
anodic materials. Thus, the processed surface by EMM has a higher surface integrity
compared with that processed by other thermal based machining processes. Micro and
even nanostructures could be fabricated by various electrochemistry-based micromachining processes [3]. In electrochemistry-based micromachining processes, materials are removed with redox reaction at the anode/electrolyte interface. In creating
microstructures with high localization, electrochemical reactions should be restricted
in a small and controlled area. Many technologies taking the advantages of electrochemistry principles have been developed for microfabrication, such as LIGA
(Lithography, electroplating, and molding) [4, 5], electrochemical nanolithography
[6], confined etchant layer technique (CELT) [7], three-dimensional electrodeposition [8], and EMM with ultrashort voltage pulses [9], etc. In order to process
microstructures, electrochemical dissolution or deposition should be confined in an
area of a micrometer or sub-micrometer scale. The electrochemical nanolithography
combines the electrochemistry and scanning probe microscope using a nanometerscale tip as the tool electrode, which is capable of fabricating profiles with nanometer
and atomic resolution [10, 11]. CELT could replicate microstructures by confining the
outer boundary of the diffusion layer of the etchant and can retain the 3-dimensional
fine micro-pattern of the mold electrode [12]. High aspect ratio microstructures
obtained by electrochemical deposition could be fabricated with the LIGA technique [13]. Electrochemical nanoimprint lithography provided a nanoscale accuracy method for producing semiconductor microdevices [14]. With the introduction
of ultrashort voltage pulses of nanosecond duration to electrochemical micromachining, the dissolution area of the materials is highly confined in a small area with
sub-micrometer-scale precision [15]. Microstructures of various contours could be