Sculpting a Nano 'World'
A new tool makes etching extremely small features relatively easy and cheap.
IBM researchers have invented a low-cost and relatively simple fabrication tool capable of reliably creating features as small as 15 nanometers. To show off the tool, the researchers at IBM’s Zurich lab made a three-dimensional map of the Earth so small that 1,000 of them would fit onto a single grain of salt.
Existing nano-fabrication techniques like electron beam lithography have difficulty making features much smaller than 30 nanometers and are expensive and complex instruments. In contrast, the IBM researchers say their new fabrication tool sits on a tabletop at one-fifth to one-tenth the cost.
The new instrument is a descendant of the scanning tunneling microscope (STM) invented by IBM Zurich scientists in the early 1980s. That microscope made it possible, for the first time, to image and manipulate atoms. The new instrument uses an extremely small silicon tip that is rapidly scanned across the surface of the substrate. The tip is cantilevered like those used in atomic force microscopy (or AFM: an offshoot of STM that was invented in 1986), enabling it to apply nanonewtons of force to the surface. But unlike AFM, the tip is heated.
Where it touches the substrate, the thermal energy at the tip is sufficient to break weak bonds within the material. “We provide enough thermal energy so these molecules become mobile, crawl along the hot tip and evaporate,” says Urs Duerig, a scientist IBM’s Zurich Research Laboratory, in Switzerland. Together with colleague Armin Knoll and others, Duerig developed the new technique. What’s remarkable about this, he says, is that it removes exactly the same amount of the material each time.
The advantage of the new instrument, compared to techniques such as e-beam lithography that involve removing material by bombarding it with particles, is that the effect is more localized. Although e-beam lithography can create features as small 15 nanometers, at resolutions below 30 nanometers, stray electrons tend to cause interactions with parts of the material neighboring the target area.
One advantage of the new technique is that it can bore down into the substrate at different depths, again at very high resolutions. This was demonstrated by etching into a molecular glass substrate a 25-nanometer-high topographical representation of the Swiss mountain, the Matterhorn, with a scale of 1:5 billion. The 3-D image was made by selectively removing material in 120 different layers.
This ability to create 3-D structures is intriguing, says Zahid Durrani at Imperial College London. “It’s completely novel,” he says. “I’ve never seen anything like this before.” However, as with other probe technologies, extending the process to large numbers of tips operating in parallel is likely to prove challenging, says Durrani.
Karl Berggren, co-director of MIT’s Nanostructures Laboratory, says IBM’s instrument is an incredibly “clever and elegant” solution. “They’ve done something quite creative here,” he says. Researchers have long struggled with thermal methods of probe lithography, but it was slow and resolutions were mediocre, says Berggren. “IBM has changed that,” he says. “So making sub-20-nanometer-scale lithography available to labs that need it at reasonable cost may be the long-term legacy of this work. And it is a very important one.”
In contrast, e-beam lithography requires several steps and tends to be very expensive, with systems costing up to $5 million, says Berggren. The IBM instrument is small enough to sit on a desktop and should cost around $100,000.
It is also relatively fast, says Duerig. Because the tip can write each “pixel” in microseconds, it can be scanned across the substrate very rapidly. The world map, for example, which consists of 500,000 pixels, took just two minutes to draw.
A crucial step in developing this technique involved finding suitable organic substrates. To this end, colleagues at IBM’s Research-Almaden, in California, were brought in to help find hard organic substrates that could be used as so-called “resists,” a sort of mask used in chip fabrication.
The challenge was to find materials that were tough enough to be used as substrates, but which could be thermally decomposed easily, evaporating into nonreactive chunks when brought into contact with the hot tip. In the case of the world map, a polymer called polyphthalaldehyde was found suitable, and for the Matterhorn, the IBM scientists used a form of molecular glass.