vapor deposition or CVD or the membrane is used for reactive ion etching to
transfer the mask to the substrate. In this case the simple alumina mask acts as a
simple nanometer level lithography mask.
Highly ordered arrays of quantum dots of GaAs and InGaAs with narrow dotsize distribution, have been grown on GaAs substrates using AM templates. This
method can grow dots for latticed-matched systems as well as strained heterostructures [18]. The templates are prepared by the two-step anodic oxidation described previously. A lift-off process is used to separate thin templates from the
aluminum substrates by etching away the unoxidized aluminum foil in a saturated
HgCl 2 solution. An etch with H 3 PO 4 was used to remove the oxide barrier at the
end as well as to widen the pores to around 80 nm. The templates were bonded
directly onto the GaAs substrate using van der Waals bonding. These ordered
nanopores were then used directly as a mask for epitaxial growth of Q-dots by the
standard MBE method. Q-dots with diameter as low as 20 nm were obtained.
60 nm size magnetic Fe nanodots were fabricated by e-beam evaporation over a 1
cm
2 area of MgO substrate using an alumina membrane (thickness 300 nm, pore
density 10
12 m
À2 ) as a shadow mask on a MgO substrate [19]. The membrane
wass subsequently removed in a 10% NaOH solution.
An alumina membrane mask has been used to generate a 100 nm period hexagonal pattern on Si by using SF 6 reactive ion etching [20]. This also adjusted the
hole diameter to around 60 nm. This mask was then used for ion etching. After the
pattern formation the mask was etched out with H 2 SO 4 and H 2 O 2 (1:1) solution.
Bi nanowires have been grown from direct vapor into AM templates with diameters in the range 7 to 200 nm and their galvanomagnetic properties were studied
[21]. The grown wires were single crystals. The method of growth is rather simple,
the membrane is placed at the mouth of a crucible, which is kept in vacuum and
contains molten Bi.
21.2.4
Templates Made in Block Copolymers
Spontaneous pattern formation in diblock copolymers has been used to produce a
high density (@10
13 m
À2 ) of uniform pores with diameter (@10–30 nm) as wide
area templates for deposition or synthesis of nanowires, nanoparticles or even a
mask for ion milling or reactive ion etching (RIE) [22, 23]. In diblock copolymers
different chains or blocks of polymers are covalently bonded. However, incompatibility between two blocks leads to spontaneous self-assembly when they are heated
above the glass transition temperature of both. The self-assembled structures consist of separated monomers or segmented polymers. These patterns can exhibit
highly ordered morphology and the pattern of the minority phase in the matrix
of a majority phase can range from spherical to cylindrical or lamella. Template
formation by the block copolymer route has certain advantages over the alumina
templates. It is easier to achieve a large density, close to 10
13 –10
14 m
À2 which in
the case of the AM is around 10
12 m
À2 in most cases. Also AM is a two-step oxidation process whereas this is more or less a single-step process. A lower pore size
(@10 nm) is achievable. In the case of AM the process of opening up the oxide
21 Nanolithography and Nanomanipulation
696
transfer the mask to the substrate. In this case the simple alumina mask acts as a
simple nanometer level lithography mask.
Highly ordered arrays of quantum dots of GaAs and InGaAs with narrow dotsize distribution, have been grown on GaAs substrates using AM templates. This
method can grow dots for latticed-matched systems as well as strained heterostructures [18]. The templates are prepared by the two-step anodic oxidation described previously. A lift-off process is used to separate thin templates from the
aluminum substrates by etching away the unoxidized aluminum foil in a saturated
HgCl 2 solution. An etch with H 3 PO 4 was used to remove the oxide barrier at the
end as well as to widen the pores to around 80 nm. The templates were bonded
directly onto the GaAs substrate using van der Waals bonding. These ordered
nanopores were then used directly as a mask for epitaxial growth of Q-dots by the
standard MBE method. Q-dots with diameter as low as 20 nm were obtained.
60 nm size magnetic Fe nanodots were fabricated by e-beam evaporation over a 1
cm
2 area of MgO substrate using an alumina membrane (thickness 300 nm, pore
density 10
12 m
À2 ) as a shadow mask on a MgO substrate [19]. The membrane
wass subsequently removed in a 10% NaOH solution.
An alumina membrane mask has been used to generate a 100 nm period hexagonal pattern on Si by using SF 6 reactive ion etching [20]. This also adjusted the
hole diameter to around 60 nm. This mask was then used for ion etching. After the
pattern formation the mask was etched out with H 2 SO 4 and H 2 O 2 (1:1) solution.
Bi nanowires have been grown from direct vapor into AM templates with diameters in the range 7 to 200 nm and their galvanomagnetic properties were studied
[21]. The grown wires were single crystals. The method of growth is rather simple,
the membrane is placed at the mouth of a crucible, which is kept in vacuum and
contains molten Bi.
21.2.4
Templates Made in Block Copolymers
Spontaneous pattern formation in diblock copolymers has been used to produce a
high density (@10
13 m
À2 ) of uniform pores with diameter (@10–30 nm) as wide
area templates for deposition or synthesis of nanowires, nanoparticles or even a
mask for ion milling or reactive ion etching (RIE) [22, 23]. In diblock copolymers
different chains or blocks of polymers are covalently bonded. However, incompatibility between two blocks leads to spontaneous self-assembly when they are heated
above the glass transition temperature of both. The self-assembled structures consist of separated monomers or segmented polymers. These patterns can exhibit
highly ordered morphology and the pattern of the minority phase in the matrix
of a majority phase can range from spherical to cylindrical or lamella. Template
formation by the block copolymer route has certain advantages over the alumina
templates. It is easier to achieve a large density, close to 10
13 –10
14 m
À2 which in
the case of the AM is around 10
12 m
À2 in most cases. Also AM is a two-step oxidation process whereas this is more or less a single-step process. A lower pore size
(@10 nm) is achievable. In the case of AM the process of opening up the oxide
21 Nanolithography and Nanomanipulation
696