"If the long strings of genomic DNA molecules are not uncoiled gradually, it is like dumping a pot of long spaghetti noodles into the sink and hoping they'll all go down the drain smoothly," Han Cao, a researcher at Princeton, told nanotechweb.org. "It's hard."

Fabricating a micropost array in front of the nanochannels helps to pre-stretch the long DNA molecules, as they must partially unwind to negotiate a path through the posts. The Princeton team reckoned that applying a gradient to the micropost region - so that the pathways become gradually shallower and narrower as they near the nanochannel region - would stretch out the molecules more effectively.

But making the structure easily and cheaply was a challenge. "You could spend hundreds and thousands of dollars and hours to write patterns with e-beam lithography, but it is prohibitively expensive and hard to scale up," said Cao. Conventional photolithography, meanwhile, struggles to produce features below a few hundred nanometres.

Not daunted, Cao and colleagues came up with a modified photolithography technique that they dubbed diffraction gradient lithography. To create the micropost region and gradient structure they applied a photoresist onto a silicon substrate chip containing large arrays of nanochannels made by nanoimprinting lithography. Then they used a photomask containing the post array pattern and, in the crucial step, added an aluminium blocking mask above part of the photomask.

This blocking mask both protected the region where the nanochannels were to remain and caused light diffraction along its edge. The diffraction led to a gradient in light intensity on the photoresist surface. And, following further treatment, such as development of the photoresist and reactive ion etching, this resulted in a gradient in the thickness of the silicon substrate in the area next to the remaining nanochannels.

"The whole industry has spent billions of dollars trying to minimize the feature/resolution-limiting optical light diffraction in making smaller and smaller structures, and I actually use diffraction to make smaller features," explained Cao. "The best part is, it costs nearly nothing and can be compatible with current industry-standard platforms."

The researchers reported their work in Applied Physics Letters.