2.3 Patterned Organic and Polymeric Films for Tailored (Bio)Interfaces
37
cylinders, and spheres (see below). The sizes of these microdomain structures are
governed by the chain dimensions and are typically on the order of 10–30 nm. For a
given diblock molecular mass, the more incompatible the species, the more stretched
the chains and therefore the larger the spacing of the structure. In the case of strongly
segregated monodisperse diblock copolymers, Semenov et al. [55] derived a relation
between the structure spacing d, the number of monomers per chain N, and the Flory
parameter χ : d 0 ∝ N
2/3
χ
1/6 . While the dependence with N can be easily verified through controlling degree of polymerization, the dependence with χ has been
checked in very few cases due to the very limited variations obtained through temperature modulation, a method usually chosen to change the value of χ. The variations
of structure spacing and object size accessible through the tuning of χ are thus quite
limited, which makes the independent choice of molecular weight and microstructure
sizes almost impossible, once the chemical natures of the blocks are imposed [56].
Structures smaller than 10 nm are also obtainable if one chooses appropriate blocks
with a high Flory-Huggins interaction parameter and decreases the block lengths.
However, the synthesis of block copolymers with very large molecular mass is not
possible because of the high viscosity of the reaction medium, so there were no
limitations to obtain structures with very larger spacing.
Each block of the copolymer can be chosen for a specific application and selective
processing of one block relative to the other is possible by use of chemical or physical
dissimilarities between the two blocks. According to this idea, certain patterns with
a periodicity less than 50 nm have been obtained successfully. In contrast, a similar
periodic patterning by electron beam lithography would be limited in three aspects.
Firstly, routine production of patterns with feature sizes below 30 nm is difficult to
achieve with commercial electron beam systems. Secondly, the obtainable minimum
periodicity of features is still not much below 100 nm. Thirdly, as with any serial technique, large area lithography would be very time-consuming because the processing
is serial. This has led to the prospect that diblock copolymers will become promising
materials in nanotechnology in the future.
An artificial topographic pattern on a substrate has been used to orient the growth
of a thin film in a process known as graphoepitaxy. Recently, similar schemes
have been used for orienting block copolymer systems. Long-range alignment of
cylindrical domains in polystyrene-block-polyisoprene thin films has been observed
following directional solidification of a polymer solution on a patterned substrate
[57]. Patterned substrates were also employed to improve the in-plane order of
polystyrene-block-poly(2-vinylpyridine) (PS/PVP) thin films by floating precast, 1monolayer-thick polymer films onto patterned substrates, followed by annealing.
The PS/PVP block copolymer formed single grains with very low defect levels over
lengths of up to 5 μm adjacent to substrate steps. By combining block copolymer
self-assembly with long range ordering methods would allow nanostructures to
be lithographically fabricated in precise positions on a substrate. Cheng et al.
presented a graphoepitaxy method for orienting self-assembled block copolymers
using substrates patterned by interference lithography over areas of several cm
2 . They
transfer the resulting patterns into an underlying layer of silica to form an ordered
37
cylinders, and spheres (see below). The sizes of these microdomain structures are
governed by the chain dimensions and are typically on the order of 10–30 nm. For a
given diblock molecular mass, the more incompatible the species, the more stretched
the chains and therefore the larger the spacing of the structure. In the case of strongly
segregated monodisperse diblock copolymers, Semenov et al. [55] derived a relation
between the structure spacing d, the number of monomers per chain N, and the Flory
parameter χ : d 0 ∝ N
2/3
χ
1/6 . While the dependence with N can be easily verified through controlling degree of polymerization, the dependence with χ has been
checked in very few cases due to the very limited variations obtained through temperature modulation, a method usually chosen to change the value of χ. The variations
of structure spacing and object size accessible through the tuning of χ are thus quite
limited, which makes the independent choice of molecular weight and microstructure
sizes almost impossible, once the chemical natures of the blocks are imposed [56].
Structures smaller than 10 nm are also obtainable if one chooses appropriate blocks
with a high Flory-Huggins interaction parameter and decreases the block lengths.
However, the synthesis of block copolymers with very large molecular mass is not
possible because of the high viscosity of the reaction medium, so there were no
limitations to obtain structures with very larger spacing.
Each block of the copolymer can be chosen for a specific application and selective
processing of one block relative to the other is possible by use of chemical or physical
dissimilarities between the two blocks. According to this idea, certain patterns with
a periodicity less than 50 nm have been obtained successfully. In contrast, a similar
periodic patterning by electron beam lithography would be limited in three aspects.
Firstly, routine production of patterns with feature sizes below 30 nm is difficult to
achieve with commercial electron beam systems. Secondly, the obtainable minimum
periodicity of features is still not much below 100 nm. Thirdly, as with any serial technique, large area lithography would be very time-consuming because the processing
is serial. This has led to the prospect that diblock copolymers will become promising
materials in nanotechnology in the future.
An artificial topographic pattern on a substrate has been used to orient the growth
of a thin film in a process known as graphoepitaxy. Recently, similar schemes
have been used for orienting block copolymer systems. Long-range alignment of
cylindrical domains in polystyrene-block-polyisoprene thin films has been observed
following directional solidification of a polymer solution on a patterned substrate
[57]. Patterned substrates were also employed to improve the in-plane order of
polystyrene-block-poly(2-vinylpyridine) (PS/PVP) thin films by floating precast, 1monolayer-thick polymer films onto patterned substrates, followed by annealing.
The PS/PVP block copolymer formed single grains with very low defect levels over
lengths of up to 5 μm adjacent to substrate steps. By combining block copolymer
self-assembly with long range ordering methods would allow nanostructures to
be lithographically fabricated in precise positions on a substrate. Cheng et al.
presented a graphoepitaxy method for orienting self-assembled block copolymers
using substrates patterned by interference lithography over areas of several cm
2 . They
transfer the resulting patterns into an underlying layer of silica to form an ordered