Cracks break the rules of nanofabrication

When it comes to nanofabrication, cracks are usually best avoided. But now researchers in South Korea have discovered that cracks aren’t always bad - if harnessed, they can be used to make controlled patterns. The technique is inexpensive and straightforward, and could have benefits for electronics and microfluidics.

Koo Hyun Nam, the lead author of the study who is based at Ewha Womans University in Seoul, says his group’s technique is similar to the way that ancient Egyptian masons created rocks of just the right size for pyramid construction. Rather than cut rocks by hand, the masons would force a wooden wedge into a small hole in the rock, and soak it with water. The wedge would expand, concentrating force at its tip, neatly breaking the rock apart.

The South Korean group’s technique is more refined, but uses the same principle. They use a process called plasma etching to create tiny notches and steps in a silicon substrate, before depositing a thin layer of silicon nitride. Such deposition would often be accompanied by uncontrolled cracks, but the use of notches and steps guides the cracks’ propagation: the notches concentrate stress to initiate the crack and the steps bring the crack to a halt. Generally, cracks would follow the direction of the silicon substrate’s crystal orientation, but by introducing a sandwiched layer of silicon dioxide between the silicon and silicon nitride, the researchers could force the cracks in other directions.

To demonstrate the effectiveness of their technique, Nam and colleagues fabricated a square box with lettering underneath, with details in the micrometer range. Nam says the resolution of controlled cracking is limited only by the accuracy of the methods used to create the notches and steps and deposit the silicon nitride. ‘I believe the accuracy would be better than 1µm, but can be better when you use better equipments or photomasks,’ he adds.

Normally, nanofabrication relies on lithography and etching, but these techniques can be time-consuming and costly. Controlled cracking might offer a new technique that is quicker and cheaper. It should be compatible with current techniques in semiconductor fabrication, which means it could be used to help make future generations of silicon chips. Another application would be microfluidics, where micro-channels are required to manipulate biomolecules.

Vladimir Antonov, a physicist at Royal Holloway University of London in the UK who uses nanofabrication on a daily basis, points to areas where the technique could be improved. For instance, the silicon nitride must be grown at a fairly high temperature that could affect other semiconductor elements and the depth of the crack ought to be fully controllable so it doesn’t always propagate into the substrate. But he says that practical applications should be considered. ‘Here there is huge potential,’ he adds.