Colloidal Lithography and Nanopatterning

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To date there is a technological demand for the fabrication of structures where both the feature size and separation can be controlled at the nanoscale. Typically such structures are obtained by direct writing using electron beam lithography, but the method is sequential and thus slow and not cost-effective. An alternative strategy exploits the deposition of colloidal particles as masks for lithography. Colloidal lithography is a large-area, robust, parallel and cheap method, but conventional approaches have little control on the inter-particle separation, typically yielding close-packed particle arrays. For instance, in biosensing applications, large separations compared to the feature size are necessary to avoid cross-talk between neighboring sensing spots. As another example, nanofabricated arrays of silicon nanowires have significant potential as platforms for cell transfection or as materials for optics and energy applications, but their diameter, height and lateral separation need to be finely tuned to achieve the desired final properties.

In our group we harness and exploit the self-assembly of colloidal particles at a water/oil interface to meet these challenges. One strategy makes use of charged nanoparticles. The presence of a discontinuity in the dielectric properties at the interface induces an asymmetric charge distribution on the particle surfaces, leading to the formation of electrostatic dipoles. Such interacting dipoles can be used to direct the self-assembly of non-close packed particle arrays, with separations up to 10 particle diameters. After self-assembly at the liquid interface, the NP arrays can be deposited on a solid substrate for lithography. The technique allows in a single step to produce 2D patterns where the size of the features and their separation can be controlled independently and has been used to produce nanopore arrays, biosensing structures, nanopatterned hydrogels, porous polymer membranes and nanowire arrays out of different materials

SEM images of latex colloids deposited on silicon wafers after self-assembly at a water/n-hexane interface to be used as lithography masks. Particle size: a. 500nm; b. 200nm; c. 90 nm; d. 40nm.
SEM images of latex colloids deposited on silicon wafers after self-assembly at a water/n-hexane interface to be used as lithography masks. Particle size: a. 500nm; b. 200nm; c. 90 nm; d. 40nm.

The deposited particles can be used a direct etching masks (e.g. for the etching of pillars in semiconductor materials) or as the mask for the deposition of a metal layer that is used as the etching mask after lift-off of the colloids (e.g. for the etching of nanopore arrays).

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Left - GaAs nanopillars etched from a colloidal mask obtained by self-assembly of 200nm latex spheres at a water/n-hexane interface.
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Right - Nanopore arrays obtained after patterning a Cr etching mask using the deposition of self-assembled 200nm latex spheres at a water/n-hexane interface.

As a second strategy we started using core-shell hydrogel particles, which show a rich phase diagram under compression at the water-oil interface and afford more robust transfer during deposition. In particular, we can deposit particle monolayers with gradients of spacing by simultaneous compression and deposition. The colloidal patterns showed in the figure below can then be turned into lithography masks for the fabrication of controlled arrays of vertically-aligned silicon nanowires.

2D phase diagram of microgels at an oil-water interface, showing the transition between two hexagonal crystals of different lattice constants
2D phase diagram of microgels at an oil-water interface, showing the transition between two hexagonal crystals of different lattice constants.
Example of a vertically aligned silicon nanowire array. The top part of the figure shows that achieving gradients of pitch of the nanowires makes it possible to produce materials with smoothly varying structural colors covering the whole visible range over one single substrate.
Example of a vertically aligned silicon nanowire array. The top part of the figure shows that achieving gradients of pitch of the nanowires makes it possible to produce materials with smoothly varying structural colors covering the whole visible range over one single substrate.
 
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23.07.2017
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