temperatures around 5–17
C for 10 to 24 hours with applied voltage between 30
and 150 V. This leads to a layer of Al 2 O 3 on the Al foil. This alumina layer is then
generally etched with a mixture of orthophosphoric acid and H 2 CrO 4 at around 50
to 60
C for 10 to 12 hours (depending on the oxide thickness). The resulting aluminum has a hexagonal array surface structure. It is then oxidized again in a second step under the same conditions as before. The hexagonal structure that is left
behind on the aluminum acts as the template for further growth of the oxide. This
leads to typical circular tubes of alumina arranged in a hexagonal array and the
bottom of the tube (where it terminates on the aluminum layer) closes on a barrier
layer of Al 2 O 3 which has a U-shaped structure, as shown in Figure 21.5.
In the next step the Al layer at the bottom is generally removed with HgCl 2 . This
results in a honeycomb of porous alumina with one end closed by a U-tube type
structure. The tube runs straight through the thickness of the membrane. The
barrier layer at the bottom can be opened chemically or by using ion beams.
Chemical methods of removing the bottom layer generally consist of an etch with
H 3 PO 4 , the etching time generally depends on the thickness of the end Al 2 O 3 . It
appears that if this etching is not performed properly it can lead to non-uniform
end size. The chemical process of etching the barrier layer also leads to pore radius
broadening. The removal of the bottom Al 2 O 3 and pore broadening is thus a crucial process.
Recently there has been a report of the use of Ar
þ ion milling (500 eV) to open
the bottom cap in the anodized alumina using a normal ion milling machine. The
rate of milling can be controlled by the ion beam current density (generally a1
mA cm
À2 ) and also the incident angle. The nominal milling rate is 100 A ˚ min
À1 at
normal incidence for a current density of 1 mA cm
À2 [12].
There are also reports of opening the barrier layer by anodic dissolution [13]. An
applied voltage less than the anodic oxidation voltage (@10–20 V) can lead to dissolution of the barrier oxide. For this to happen the dissolution rate has to be more
than the anodic oxidation rate. This method is particularly suitable for cases where
the alumina is being grown as a template on a substrate and the membrane is thin
and supported by a substrate.
The applied bias can control the pore size as well as the pore separation. For
growth voltages in the range 30–150 V the pore diameters are in the range 20–150
nm. Typical values are about 1 nm V
À1 for the pore diameter and 2.5–2.7 nm V
À1
for the cell spacing. The growth rate depends on the current density and also the
temperature. Typical current density is in the range 1–10 mA cm
À2 . The growth
process is a result of two processes. The oxidation proceeds at the oxide metal interface and there is an electric field assisted chemical dissolution of oxide at the
base of each pore. The growth rate is an equilibrium between these growth and
dissolution processes.
A typical SEM picture of an AM is shown in Figure 21.6. In general one can get
continuous pores over the complete thickness of the membrane. But occasionally
one may get damaged pores.
Generally the AM films are composed of polycrystalline g-alumina which is expected to have defects. The body of the membrane is more compact and crystalline
21 Nanolithography and Nanomanipulation
694
C for 10 to 24 hours with applied voltage between 30
and 150 V. This leads to a layer of Al 2 O 3 on the Al foil. This alumina layer is then
generally etched with a mixture of orthophosphoric acid and H 2 CrO 4 at around 50
to 60
C for 10 to 12 hours (depending on the oxide thickness). The resulting aluminum has a hexagonal array surface structure. It is then oxidized again in a second step under the same conditions as before. The hexagonal structure that is left
behind on the aluminum acts as the template for further growth of the oxide. This
leads to typical circular tubes of alumina arranged in a hexagonal array and the
bottom of the tube (where it terminates on the aluminum layer) closes on a barrier
layer of Al 2 O 3 which has a U-shaped structure, as shown in Figure 21.5.
In the next step the Al layer at the bottom is generally removed with HgCl 2 . This
results in a honeycomb of porous alumina with one end closed by a U-tube type
structure. The tube runs straight through the thickness of the membrane. The
barrier layer at the bottom can be opened chemically or by using ion beams.
Chemical methods of removing the bottom layer generally consist of an etch with
H 3 PO 4 , the etching time generally depends on the thickness of the end Al 2 O 3 . It
appears that if this etching is not performed properly it can lead to non-uniform
end size. The chemical process of etching the barrier layer also leads to pore radius
broadening. The removal of the bottom Al 2 O 3 and pore broadening is thus a crucial process.
Recently there has been a report of the use of Ar
þ ion milling (500 eV) to open
the bottom cap in the anodized alumina using a normal ion milling machine. The
rate of milling can be controlled by the ion beam current density (generally a1
mA cm
À2 ) and also the incident angle. The nominal milling rate is 100 A ˚ min
À1 at
normal incidence for a current density of 1 mA cm
À2 [12].
There are also reports of opening the barrier layer by anodic dissolution [13]. An
applied voltage less than the anodic oxidation voltage (@10–20 V) can lead to dissolution of the barrier oxide. For this to happen the dissolution rate has to be more
than the anodic oxidation rate. This method is particularly suitable for cases where
the alumina is being grown as a template on a substrate and the membrane is thin
and supported by a substrate.
The applied bias can control the pore size as well as the pore separation. For
growth voltages in the range 30–150 V the pore diameters are in the range 20–150
nm. Typical values are about 1 nm V
À1 for the pore diameter and 2.5–2.7 nm V
À1
for the cell spacing. The growth rate depends on the current density and also the
temperature. Typical current density is in the range 1–10 mA cm
À2 . The growth
process is a result of two processes. The oxidation proceeds at the oxide metal interface and there is an electric field assisted chemical dissolution of oxide at the
base of each pore. The growth rate is an equilibrium between these growth and
dissolution processes.
A typical SEM picture of an AM is shown in Figure 21.6. In general one can get
continuous pores over the complete thickness of the membrane. But occasionally
one may get damaged pores.
Generally the AM films are composed of polycrystalline g-alumina which is expected to have defects. The body of the membrane is more compact and crystalline
21 Nanolithography and Nanomanipulation
694