barrier at the end of the oxidized membrane leads to increase in the pore size.
However, in materials fabrication where the processing temperatures go beyond
150
C to 170
C these templates become unusable. In the case of AM a much
higher processing temperature (@500
C) is possible. This is a particular necessity
where complex functional materials are synthesized in the templates.
Typical copolymer blends are PS (polystyrene)–PMMA (polymethylmethacrylate),
PS–PB (polybutadiene) and PS–PI (polyisoprene). Generally the PS is the majority
phase that makes the matrix and the other polymer is the minority phase which
forms the ordered structure. The majority PS phase generally has a large molecular
weight compared to the minority phase. The pattern formed depends on the relative chain lengths and the segmental interactions. As an example [22] the PS–PB
block copolymers produce hexagonally ordered PB cylinders in a PS matrix, while
in PS–PI one obtains spheres of PI. A schematic of the templates produced in
block copolymers is shown in Figure 21.7.
For templates and etching masks that use a spherical particle one may restrict
the film thickness to around the diameter of the particle (monolayer) to get ordered
structures. In the PS–PMMA system one can obtain aligned cylinders of PMMA in
PS if an electric field is applied during the curing process as shown in Figure 21.7.
In this case one can get templates with sizes of the order of a micron.
Generally the block copolymer is spin-coated onto the substrate on which the
pattern has to be grown or transferred. Some times the wetting of the substrate
by the constituent homopolymer components can be controlled by a layer (@5 nm)
of neutral brush layer of hydroxy-terminated random copolymer of the same constituents [24]. For instance PS homopolymer de-wets random copolymer surfaces
of PS–PMMA with styrene fractions ( f ) less than 0.65 while the PMMA de-wets
for f > 0:5. Tuning the interfacial energy can thus control the orientation of the
microphase-separated structures obtained from a diblock copolymer. The curing,
lasting for a few hours to a day, is carried out at a temperature above the glass
transition temperature T g of both polymers. This leads to the formation of the
separated phase. The etching of the pattern generally involves a chemical treatment that either stabilizes the matrix polymer or the separated phases against
chemical or dry etching. As an example, with UV exposure and sonication in glacial acetic acid and deionized water the PMMS phase is degraded and can be removed [25]. Similarly ozonization attacks the carbon double bond in PB which
thus can be degraded for preferential etching.
21.3
Fabrication of Nanostructures in the Templates
Making templates of ordered nanopores is the starting point for template based
nanolithography. The next step involves ‘‘filling the pores’’ of the membrane with
different materials of interest using one or more available methods. The methods
include electrochemical deposition (ED), vapor phase deposition such as chemical
vapor deposition (CVD), pressure injection of molten metals and sol–gel methods.
In this section we describe some of the methods of deposition in the nanopores.
21.3 Fabrication of Nanostructures in the Templates 697
However, in materials fabrication where the processing temperatures go beyond
150
C to 170
C these templates become unusable. In the case of AM a much
higher processing temperature (@500
C) is possible. This is a particular necessity
where complex functional materials are synthesized in the templates.
Typical copolymer blends are PS (polystyrene)–PMMA (polymethylmethacrylate),
PS–PB (polybutadiene) and PS–PI (polyisoprene). Generally the PS is the majority
phase that makes the matrix and the other polymer is the minority phase which
forms the ordered structure. The majority PS phase generally has a large molecular
weight compared to the minority phase. The pattern formed depends on the relative chain lengths and the segmental interactions. As an example [22] the PS–PB
block copolymers produce hexagonally ordered PB cylinders in a PS matrix, while
in PS–PI one obtains spheres of PI. A schematic of the templates produced in
block copolymers is shown in Figure 21.7.
For templates and etching masks that use a spherical particle one may restrict
the film thickness to around the diameter of the particle (monolayer) to get ordered
structures. In the PS–PMMA system one can obtain aligned cylinders of PMMA in
PS if an electric field is applied during the curing process as shown in Figure 21.7.
In this case one can get templates with sizes of the order of a micron.
Generally the block copolymer is spin-coated onto the substrate on which the
pattern has to be grown or transferred. Some times the wetting of the substrate
by the constituent homopolymer components can be controlled by a layer (@5 nm)
of neutral brush layer of hydroxy-terminated random copolymer of the same constituents [24]. For instance PS homopolymer de-wets random copolymer surfaces
of PS–PMMA with styrene fractions ( f ) less than 0.65 while the PMMA de-wets
for f > 0:5. Tuning the interfacial energy can thus control the orientation of the
microphase-separated structures obtained from a diblock copolymer. The curing,
lasting for a few hours to a day, is carried out at a temperature above the glass
transition temperature T g of both polymers. This leads to the formation of the
separated phase. The etching of the pattern generally involves a chemical treatment that either stabilizes the matrix polymer or the separated phases against
chemical or dry etching. As an example, with UV exposure and sonication in glacial acetic acid and deionized water the PMMS phase is degraded and can be removed [25]. Similarly ozonization attacks the carbon double bond in PB which
thus can be degraded for preferential etching.
21.3
Fabrication of Nanostructures in the Templates
Making templates of ordered nanopores is the starting point for template based
nanolithography. The next step involves ‘‘filling the pores’’ of the membrane with
different materials of interest using one or more available methods. The methods
include electrochemical deposition (ED), vapor phase deposition such as chemical
vapor deposition (CVD), pressure injection of molten metals and sol–gel methods.
In this section we describe some of the methods of deposition in the nanopores.
21.3 Fabrication of Nanostructures in the Templates 697