Polymerization_2

r1=K11/K12, r2=K22/K21

1. A blend of two incompatible homo-polymer separates into distinct phases on a large scale (left), whereas block copolymers micro-phase separate into periodic domains (right).
2. Basic morphologies obtained by different block copolymer compositions.

Phase separation of block copolymers

1. A mixture of PMMA (M_w=93.9 kg/mol) and polystyrene (PS, M_w=194.9 kg/mol) (PS/PMMA=70/30, w/w) was dissolved in tetrahydrofuran (THF) to form a 5 wt% solution.
2. Polymer film was made by spin-cast the solution on glass slide.
3. Exposure to cyclohexane at 70^oC to dissolve PS.

Phase separation of a blend of PMMA and PS homo-polymer

Typical self-assembly behavior for linear block copolymers

Block copolymer thin films

TEM micrographs of polystyrene-polybutadiene diblock copolymer film masks (a,c) and lithographically patterned silicon nitride (b,d).

Self-assembly of PS-PB di-block copolymer

The most attractive feature of block copolymer self assembly is the extremely high resolution, easily get features down to 10nm.

PS: polystyrene

PB: polybutadiene

Block copolymer thin films: effect of substrate wetting

Block A is shorter than B

Arranged to minimize surface (interface) energy

Micro-phase separated block copolymer can be directed/aligned by:

• Electric field
• Shearing force
• Surface control of wettability
• Chemical pattern on surface
• Nano-structured surface
• Spatial confinement by surface relief pattern in substrate and mold
• Void in a range of porous host

Guided block copolymer self assembly for long range ordering and periodicity

Alignment by pre-patterning the substrate

Spherical domains assembled from PS–PFS (polystyrene-polyferrocenyldimethylsilane) block copolymer inside patterned SiO₂ grooves.

The 1.5 wt.% PS-PFS block copolymer in toluene solution was spin-coated onto the grooved substrate and then annealed at 140^oC for 48h to obtain a monolayer of spherical PFS domains in a PS matrix within the substrate grooves.

Alignment by shear force

(here for silicon nano-wire fabrication)

Here the etch contrast is increased by staining the block copolymer by 2 min exposure to the vapor from 0.5% aqueous RuO₄, which selectively reacts with the PS block and increases its etch resistance, thus permitting Si nanowires of greater aspect ratio to be fabricated.

1. Fabrication process for a Si nano-wire grid polarizer using block copolymer lithography.
2. SEM image of the finished Si nano-wire grid on fused silica.

Alignment by shear force (for silicon nano-wire fabrication)

Pitch=30nm

Tapping mode atomic force microscopy (TM-AFM) phase images of PS–PHMA thin films on top of an α-Si layer on a fused silica substrate:

1. Quiescently annealed
2. Shear aligned.

Glassy PS cylinders are shown as light in a dark rubbery PHMA matrix.

Polystyrene-b-poly(n-hexyl methacrylate) (PS–PHMA) diblock copolymer with a molar mass of 21 and 64 kg/mol for the respective blocks.

Chaikin, “Silicon nanowire grid polarizer for very deep ultraviolet fabricated from a shear-aligned diblock copolymer template”, Optics letters, 32(21), 3125-3127 (2007).

Templated self-assembly of block copolymers

Polystyrene “brushes” by EUV-IL (interference lithography) and surface initiated nitroxide mediated living free radical polymerization

The PS brush pitch should match that of PS-PMMA self assembly pitch.

Polymerization of block-copolymers on

chemically pre-patterned substrates

P. F. Nealey, H. H. Solak et al. Nature 424 (2003)

Polystyrene-block-methyl meth acrylate (PS-b-PMMA), L₀ = 48nm

Thermodynamics dominates interface widths and domain sizes.

When L_s=47.5nm≈L_o=48nm, block copolymer is almost defect free.

Block copolymer materials that naturally form simple periodic structures were directed to assemble into non-regular device oriented patterns (here an elbow) on chemically nano-patterned substrates.

Mark P. Stoykovich, Marcus Müller,Sang Ouk Kim, Harun H. Solak, Paul F. Nealey, Science, 308, 1442-1446 (2005).

Directed assembly of block copolymer blends into non-regular device oriented structure

Block copolymer lithography (i.e. with pattern transfer)

PS: polystyrene

PB: polybutadiene

• Ozone breaks down PB’s C=C double bond.
• OsO₄ vapor reacts with PB’s double bond.

Degrade with ozone

Stain with OsO₄

Synthesis of nanowires by wetting

1. Gold metal vapor-deposited onto a preformed PS-b-PMMA template.
2. After annealing at 180°C for 1 min., gold nanoparticles segregate selectively to the PS domains and form chains.
3. Repeated deposition and short-time annealing increases the metal loading, forming continuous conductive nanowires.

Wettability masks:

Au and Ag to PS phase

In, Pb, Sn to PMMA phase

Block copolymer lithography

1. After deep UV-exposure, polymer chain of PMMA is cut (PMMA is a positive deep UV lithography resist), making it more soluble in solvent.
2. Whereas the polystyrene (PS) chain is cross-linked, making it hard to dissolve by solvent.
3. Therefore, PMMA can be selectively removed by solvents like acetic acid afterwards.

(PMMA chain can also be broken by UV light at λ=365nm, but need very long time exposure, ~1 h at 40mW/cm² intensity)

Nanofabrication of vertical nanowires by electroplating

• Aligned by electric field during annealing.
• Styrene 71%, to obtain 14nm PMMA cylinder.
• Deep UV simultaneously degrades PMMA and cross-link PS.
• Acetic acid dissolve PMMA but not cross-linked PS.
• Methanol is added to aqueous plating solution to better wet hydrophobic PS membrane.

Science, 290, 2126 (2000)

Electric field for vertical alignment

Density multiplication (here by 9×) lithography

1. Top-down and side-view schematics showing the arrangement of PS-b-PDMS block copolymer molecules in the region surrounding a single post made from cross-linked HSQ resist (by e-beam lithography). The post and substrate surfaces have been chemically functionalized by a monolayer of short-chain PDMS brush.
2. A poorly ordered monolayer of BCP (block co-polymer) spherical domains formed on a flat surface, that is, without templating. The boundaries between different grain orientations are indicated with dashed lines. The inset is a 2D Fourier transform of the domain positions that shows the absence of long-range order.

C-D. SEM images of ordered BCP spheres formed within a sparse 2D lattice of HSQ

Ross, “Graphoepitaxy of self-assembled block copolymers on two-dimensional periodic patterned templates”, Science, 321, 939-943 (2008).

For the moment, this is considered as the most promising route for bit-patterned magnetic recording media fabrication (make the mold for nanoimprint lithography), up to 10Tbits/in² for pitch ~8nm.