Humidity Control In Lithography

Self-assembling DNA structures carve out a niche

Researchers have used DNA nanostructures to create raised ridges and tiny trenches in silicon dioxide using an etching technique. The new approach cuts down on the number of steps needed and makes the process compatible with current silicon processing methods. This technique could one day be used to pattern computer chips.  

DNA origami is an attractive technique for creating nanometre-scale structures, but the molecules are far more fragile than the materials with useful electrical and optical properties that they would pattern. Etching hard surfaces like silicon, without destroying the DNA in the process, is challenging. But now, Sumedh Surwade at the University of Pittsburgh and colleagues report a simple solution. 

The key to the technique is the reaction between solid silicon dioxide and gas-phase hydrofluoric acid. These two molecules prefer to recombine as silicon tetrafluoride gas and water vapour, with water serving as a catalyst. 'The rate of this etching reaction critically depends on the amount of water on the surface,' says author Haitao Liu. DNA and silicon dioxide both adsorb water, and their affinities depend on the humidity. In drier environments, silicon dioxide retains water better, but if the air is damp, more water clings to DNA.  

Liu's team took advantage of this quirk to create raised and etched patterns in silicon dioxide using DNA. They first set folded triangles of DNA on the silicon dioxide. In the wetter environment, with a relative humidity of 50 per cent at 25°C, the hydrofluoric acid preferred to react near the DNA, digging channels underneath it. After washing off the DNA, the team used atomic force microscopy to reveal trenches about 17nm wide - roughly the width of the arms of the DNA triangle. Yet with a lower humidity of 30 per cent and at 30°C, the hydrofluoric acid etched the bare silicon dioxide faster. In this case, the triangular DNA protected the silicon, resulting in raised triangles. 

'The method the authors have worked out is essentially kitchen chemistry,' says Paul Rothemund of the California Institute of Technology in Pasadena, inventor of DNA origami. 

Albert Hung, a nano-patterning researcher at the University of California, San Diego, calls the new technique 'elegant', but he notes further challenges of DNA-based lithography remain, 'such as quality and consistency of the fabricated structures and reliable control of DNA deposition'.

Video: Plasma Lithography Surface Patterning for Creation of Cell ...

Systematic manipulation of a cell microenvironment with micro- and nanoscale resolution is often required for deciphering various cellular and molecular phenomena. To address this requirement, we have developed a plasma lithography technique to manipulate the cellular microenvironment by creating a patterned surface with feature sizes ranging from 100 nm to millimeters. The goal of this technique is to be able to study, in a controlled way, the behaviors of individual cells as well as groups of cells and their interactions.

This plasma lithography method is based on selective modification of the surface chemistry on a substrate by means of shielding the contact of low-temperature plasma with a physical mold. This selective shielding leaves a chemical pattern which can guide cell attachment and movement. This pattern, or surface template, can then be used to create networks of cells whose structure can mimic that found in nature and produces a controllable environment for experimental investigations. The technique is well suited to studying biological phenomenon as it produces stable surface patterns on transparent polymeric substrates in a biocompatible manner. The surface patterns last for weeks to months and can thus guide interaction with cells for long time periods which facilitates the study of long-term cellular processes, such as differentiation and adaption. The modification to the surface is primarily chemical in nature and thus does not introduce topographical or physical interference for interpretation of results. It also does not involve any harsh or toxic substances to achieve patterning and is compatible for tissue culture. Furthermore, it can be applied to modify various types of polymeric substrates, which due to the ability to tune their properties are ideal for and are widely used in biological applications. The resolution achievable is also beneficial, as isolation of specific processes such as migration, adhesion, or binding allows for discrete, clear observations at the single to multicell level.

This method has been employed to form diverse networks of different cell types for investigations involving migration, signaling, tissue formation, and the behavior and interactions of neurons arraigned in a network.

1. Creation of molds used for patterning

Lithography process control

G.A.L.A.

Alternative lithography, unleashing the potentials of nanotechnology

Technical Association papers

Fundamental principles of optical lithography, the science of microfabrication