Lithography Past Light's Limits

A new optical etching technique could lead to faster microchips.

The laws of physics dictate that traditional lenses can't focus light onto a spot narrower than half the wavelength of the light. But converting the light into waves called plasmons can get around this limitation. Plasmonic lithography, which uses plasmon-generated radiation to carve physical features into a substrate, promises to revolutionize optical storage and computing, enabling ultradense DVDs and powerful microprocessors. Now, researchers at the University of California, Berkeley, have surmounted the biggest obstacle to plasmonic lithography by building a prototype that brings a plasmonic lens very close to the substrate.

As the lens flies: This simulation shows how air moves around a microscale bearing that’s the key component of a prototype device for a new kind of high-resolution optical lithography. The red lines show the flow of air around the device; color gradations from dark blue to red indicate air pressure, from low to high. Buoyed by air, the bearing keeps an array of lenses within 20 nanometers of a spinning disc coated with a light-sensitive chemical. Credit: Xiang Zhang

Led by Berkeley mechanical-engineering professors Xiang Zhang and David Bogy, the researchers created what they call a flying plasmonic lens, an array of light concentrators that passes over a surface at a height of only 20 nanometers. The light concentrators are concentric circles patterned onto a thin film of silver; illuminating them with a laser causes electrons on their surfaces to oscillate. The oscillating electrons in turn emit a type of radiation that's more tightly focused than light passing through conventional optics would be, but it can travel only about 100 nanometers from the lens surface. So the Berkeley researchers mounted the lenses on a device that uses so-called air bearings: the shape of the device causes a cushion of air to form under it, holding the lenses about 20 nanometers from a surface. In the researchers' prototype,described in a paper in Nature Nanotechnology, the bearing moves the lens array over a disc spinning at speeds of 4 to 12 meters per second, much as the arm on a turntable holds the needle over a record.

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Kenneth Crozier, a professor of natural sciences at Harvard University, says that the Berkeley researchers' use of the air bearing overcomes "one of the key technological challenges in plasmonics." Over the past few years, Crozier and others have used plasmonics to concentrate light onto ever smaller spots, but they haven't successfully addressed the practical issue of distance control. The Berkeley device, Crozier adds, also offers far faster scanning speeds than other devices do.

The speed and precision of the system is equivalent to flying a Boeing 747 two millimeters above the ground, says Zhang. Indeed, the design of the air bearing is in some ways analogous to the design of an airplane. A pair of pads on the bearing control roll; another pair control pitch, the equivalent of moving a plane's nose up or down.