Enabling Next Generation MEMS with Porous Anodic Alumina
Literature Review
J. Tyler Preston
prestonj@colorado.edu
Advisor: Prof. Conrad Stoldt
Introduction and Scope of Review
In recent years, advances in the engineering of nanostructures have created the
demand for a practical means of applying these structures in order to create working
components. Consequently, the desire to create uniform arrays of nanostructures has
been an emerging area of research. Several techniques have shown the ability to produce
uniform fields of nanostructures, but require extensive processing. Although these
techniques can produce highly uniform arrays, they still do not provide an effective
means of transferring the nanostructures to create applicable components and/or devices.
Research addressing the aforementioned topics has focused on a ceramic material
known as Porous Anodic Alumina (PAA). Alumina (Al2O3) is formed by the oxidation
of aluminum and occurs naturally on any aluminum surface that has been exposed to air.
Nanoporous alumina is formed thin film aluminum (1-100 μm) is completely oxidized
using electrochemical techniques. Full oxidation of the aluminum material results in
nanocolumns (10-100 nm dia.) that run through the alumina material. In 1995, Masuda et
a1 [1] discovered the self-ordering nature of the nanoscale pores that make this material
unique.
This review will focus on the fabrication processes involved in making PAA, pore
growth mechanisms, and applications. Since our research will focus on the beginning-toend fabrication of MEMS devices using PAA, it is important to examine all steps and
processes currently used. The review of pore growth mechanisms will not simply focus
on ordered arrays of pores, but will examine the use of imprinting as a means of
controlling pore uniformity.
Fabrication
Generally speaking, research concerning PAA uses one of two techniques to
obtain an aluminum substrate, on which the porous alumina material may be grown. The
first method involves anodizing high-purity, thin aluminum foils in electrolytic solutions.
Masuda et a1 [1,3] are considered to be pioneers in the fabrication of uniform PAA using
aluminum foils as a substrate. Since the majority of published research regarding PAA
concerns pore uniformity and size, most researchers are not concerned with growing
aluminum films on semiconductor substrates and prepare aluminum foils in a manner
similar to that outlined in Masuda et a1 [1,3]. Small variations in preparation do exist, for
example, Li et a1 [4] annealed the foils in nitrogen at 400˚C to remove mechanical
stresses and recrystallize. The second method for obtaining thin film aluminum
substrates involves depositing aluminum material onto a semiconductor substrate,
typically silicon. The driving method for using aluminum on silicon substrates as a For
means of fabricating PAA, is that nanostructures can be easily fabricated on the substrate
using the PAA layer as a template [5-7]. The aluminum films are typically depostited
using evaporative deposition techniques, as in Shiraki et a1 [8].
The anodizing process is a critical step in the fabrication of PAA. The applied
voltages, as well as the electrolytes used greatly affect the pore quality and size [9].
Throughout the literature there are three different electrolytes used in a given anodizing
process: oxalic, phosphoric, or sulfuric acid. Li et a1 [4] published a study in 1999 that
compared the pore structures produced by the three different electrolytes. Their study
showed that iteratively anodizing in aqueous solutions of oxalic acid produced perfect
order pore arrays within domains of a few micrometers. Another way of controlling pore
uniformity and size is by using an appropriate anodic bias [9]. The necessary voltage is
somewhat dependent on the particular setup and the quality of the connections, but
voltages can range from 20 V [10] to 120 V [11]. It is interesting to note that both
Shiraki et a1[8] and Li et a1 [4] use anodizing voltages of 40 V to obtain very good
uniformity, even though one uses Al-Si-Au layered samples [8] and the other Al foils [4].
Although I have yet to find a study that directly compares the two different Al substrates,
it appears that the difference does not affect the voltage needed, as long as they are
approximately the same thickness.
Pore Growth Mechanisms
Early research aimed at creating uniform pore arrays in alumina used an iterative
anodizing-etching technique, as outlined in Masuda et a1 [1]. This technique works by
creating hexagonally ordered pores through anodizing, then etching away this initial
oxide layer. After etching, periodic patterns of “pits” are left on the surface of Al
substrate and are the sites of pore nucleation during a second round of anodizing [4]. In
1997, Masuda et a1 [3] suggested that imprinting the surface of the Al substrate would be
an effective and less process-intensive means of creating the same periodic patterns in the
surface of the Al substrate.
In recent years, nanoindentation has become a popular technique for determining
mechanical properties of nanomaterials. Alcala et a1 [12] were able to characterize PAA
films formed on Al substrates and found them to have hardness and elastic modulus of
~8.5 and ~140 GPa, respectively. Although these mechanical properties are important to
know it is important to study the effects of nanoindentation prior to anodizing [13]. Choi
et a1 [13] demonstrated the ability to create beautiful hexagonal arrays of nanopores
using nanoindentation to dictate the sites where the pores nucleated. Although they used
complicated Moire patterns of indentations on their films, they demonstrated the general
success of creating a longer-range order of uniform pore arrays in alumina material.
Applications
The interest in developing highly ordered PAA is great, due to the wide range of
devices where it may be implemented. Humidity sensors are one of many applications
where PAA could be applied to produce reliable, highly sensitive detectors [14]. These
sensors work on the principle of ionic conductivity, whereby the presence of water on the
surface of the semiconductive PAA lowers the impedance, thus increasing the overall
conductivity [14]. The thermal properties of ceramic nanomaterials including PAA make
it a desirable material in microheating applications [15]. Govyadinov et a1 [15] showed
that PAA microheaters could reduce power consumption, increase response time, as well
as increase the lifetime of microheaters. The third and most prevalent application for
PAA is its use as a template for growing uniform arrays of nanotubes and nanowires.
Suh et a1 [16] demonstrated the ability to grow highly uniform arrays of carbon
nanotubes (CNT) in the pores of the PAA material. These CNT arrays, as well as other
types of nanowires grown in PAA show promising evidence that implementing nanotubes
and nanowires in MEMS devices is possible. This would have great implications in the
field of Bio-MEMS, where these integrated components could be used in ultra-sensitive
bio-molecule detection. In summary, the applications of PAA are numerous and
furthering research in the area of PAA applications should prove to be very important and
worthwhile.
References
[1]
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