Part 4:

2 Potential Next Generation Soft-Lithographic Methods For Nanostructuring Surfaces

Micro-Contact Printing Dip Pen Nanolithography

Part 4i: Micro-Contact Printing (

m

CP)

Learning Objectives

After completing PART 4i of this course you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods.

(i) Appreciate the

m

CP methodology, (ii) Factors making a good stamp, (iii) Limitations in stamp design, (iv) Methods of stamp inking, and (v) Methods of how mCP has been utlised in applications/nanostructuring surfaces.

m

-Contact Printing

Infact structures as small as 50 nm can be fabricated.

1 PDMS Monomer 2 PDMS Stamp 3

In order to achieve this the master has to be prepared by e-beam processing.

4 Master PDMS Stamp Inked with Thiols 5

So in fact the process has evolved into…

…Nano-Contact Printing

Gold Surface SAM of Thiol

Post Processing of Printed Features

The Stamp

The stamp must (i) exhibit an exact negative of the relief pattern on the master (ii) have good release properties from the master after curing the stamp, (iii)be robust enough to hold its shape after release, (iv)be aable to adsorb the ink, (v)be able to transfer the ink, (vi)be able to make conformal contact with the substrate, and (vii)be easily removed from the substrate.

The Stamp Material

(PDMS)

O

The most common choice of stamp material is theSylgard 184 (Dow Corning) which is a poly(dimethylsiloxane) (PDMS)

Si Si

n

PDMS has (i) (ii) low surface energy allowing easy release form the master.

has sufficient mechanical strength to hold its shape, (iii) is soft enough to allow conformal contact with the surface of the substrate.

The PDMS monomer (i) is a fluid, allowing it to fill the master relief pattern, (ii) is cured at near ambient temperatures, (iii) gives a cross-linked elastomer, (iv) reproduces a high fidelity negative of the master after curing

The Stamp Limitations

The Stamp Roof Collapse

Roof Collapse occurs due to the roof of the features collapsing under the weight of the stamp itself, or under pressure during the printing process, Conditions r>> h

Langmuir

21 12060 (2005)

Pairing of Features

Pairing occurs due to mechanical instability of small PDMS features with high aspect ratios (h/w) and relatively narrow rooves (r).

The collapsed features can pair up due to interfacial adhesive forces.

Capillary forces, occurring due to the receding solvent meniscus upon drying of the stamp during wet inking, have also been proposed as a mechanism that induces pairing [34].

The pairing process results in poor replication of the desired pattern due to distortions of the pattern on the stamp.

Stamp Printed Feature Printed Feature

Adv. Mater.

9 741 (1997).

Langmuir

20 8646 (2004).

Stamp-Substrate Contact Time

The transfer of ink from the stamp to the substrate is a diffusion process.

Thus, molecules can diffuse outwards from the point of stamp contact with the substrate.

This edge diffusion increases the size of the printed patterns and this compromises their edge definition and lateral dimensions.

The effect of edge diffusion can be reduced by the use of (i) higher molecular weight inks and (ii) by shorter printing times.

The effect of printing time on linewidths and clarity of square features 15s 105s 195s Features created by printing 10mM mercaptohexadecanoic acid on Au then etching

Inking The Stamp

Wet Inking

simply involves covering the entire patterned surface of the stamp with ink solution and then drying the stamp with an inert gas to form a uniform covering of ink on the patterned surface. The inking of the rooves and sidewalls of the stamp features allows ink transfer to the substrate via gas diffusion; this can have a deleterious effect in the printed pattern [38].

Permanently inked stamp

This method involves the use of a 'reservoir' of ink solution that is located at the back of the stamp allowing ink to diffuse through the bulk of the stamp. As the rooves and feature sidewalls of the stamps are inked, the same problems of gas diffusion can occur for a permanently inked stamp as for wet inking.

Contact inking

Contact inking involves the inking and blow drying of a featureless block of PDMS which has been immersed in the ink solution, thus creating an ink pad. The patterned stamp is then left in contact with this pad resulting in the diffusion of ink molecules from the ink pad to the raised features of the stamp. As only the raised features are inked, the problems associated with ink diffusionfrom rooves and sidewalls is eliminated.

Serially Inking the Stamp

J. AM. CHEM. SOC. 2005

, 127, 1106-1107 Flip Stamp And Print AFM cantilever is inked, And ink transferred to a feature on a PDMS stamp.

This process repeated for different inks onto different features of the stamp 10 m M Inks: Three different fluorophores

Inkless Printing

Oxidation TMS OH Acid Labile TMS Rinse Nano Lett., 2003, 3 , 1449 Silicon Oxide TMS SAM Au

Aligning Carbon Nanotubes

J. Phys. Chem. B 2003, 107, 3455-3458

Hierarchical assemblies of mesoporous silica can be fabricated using microcontact printing to define patterns of nucleation sites.

A. S. G. Khalil et al,

Adv. Mater.

18 1055 (2006).

a. Diagram depicting μcp in a microchannel and b. Fluorescence image of TRITC-labelled Abs printed into Au coated microchannels.

Foley et al,

Langmuir

Chemical Society.

21 11296 (2005).

©2005, American

Arrays of human epidermal carcinoma A431 cells formed on arrays of RADSC 14 peptide.

S. Zhang et al,

Biomater.

20 213 (1999).

©1999, Elsevier.

Immobilization of E-Coli bacteria to a MHA-dot array.

S. Rozhok et al,

Small

1 445 (2005).

©2005, Wiley-VCH STM)

Conclusions on

m

CP

m CP is a very facile and versatile route to create nanostructured surfaces, with resolution better than photolithography and almost equalling EBL. Although to produce structures with less than 100 nm is still challenging.

The master formation requires photolithogrphy or EBL for formation but can be designed and purchased from various companies, and is reusable.

It is a parallel process.

Technology is very simple.