Improving chip density by a factor of 100
According to the semiconductor industry, maskless nanolithography is a flexible nanofabrication technique which suffers from low throughput. But now, engineers at the University of California at Berkeley have developed a new approach that involves 'flying' an array of plasmonic lenses just 20 nanometers above a rotating surface. With this approach, it is possible to increase throughput by several orders of magnitude. The 'flying head' they've created looks like the stylus on the arm of an old-fashioned LP turntable. With this technique, the researchers were able to create line patterns only 80 nanometers wide at speeds up to 12 meters per second. The lead researcher said that by using 'this plasmonic nanolithography, we will be able to make current microprocessors more than 10 times smaller, but far more powerful' and that 'it could lead to ultra-high density disks that can hold 10 to 100 times more data than today's disks.' But read more...
The figure on the left describes this new "high-throughput maskless nanolithography using plasmonic lens arrays. a, Schematic showing the lens array focusing ultraviolet (365 nm) laser pulses onto the rotating substrate to concentrate surface plasmons into sub-100 nm spots. However, sub-100 nm spots are only produced in the near field of the lens, so a process control system is needed to maintain the gap between the lens and the substrate at 20 nm. b, Cross-section schematic of the plasmonic head flying 20 nm above the rotating substrate which is covered with photoresist. c, Schematic of process control system. The laser pulses are controlled by a high-speed optical modulator according to the signals from a pattern generator. The writing position is referred to the angular position of the disk from the spindle encoder and the position of a nano-stage along the radial direction." (Credit: UC Berkeley)
This plasmonic nanolithography process has been developed under the supervision of Xiang Zhang, UC Berkeley professor of mechanical engineering, and several members of research lab. He also worked with David Bogy, another UC Berkeley professor of mechanical engineering and member of the Computer Mechanics Laboratory.
Here are additional details provided by Liang Pan, a UC Berkeley graduate student working on this project. "With optical lithography, or photolithography, you can instantly project a complex circuit design onto a silicon wafer. However, the resolution possible with this technique is limited by the fundamental nature of light. To get a smaller feature size, you must use shorter and shorter light wavelengths, which dramatically increases the cost of manufacturing. Also, light has a diffraction limit restricting how small it can be focused. Currently, the minimum feature size with conventional photolithography is about 35 nanometers, but our technique is capable of a much higher resolution at a relatively low cost."
So how did UC Berkeley researchers overcome this diffraction limit of light? They chose a different approach. "They took advantage of a well-known property of metals: the presence at the surface of free electrons that oscillate when exposed to light. These oscillations, which absorb and generate light, are known as evanescent waves and are much smaller than the wavelength of light. The engineers designed a silver plasmonic lens with concentric rings that concentrate the light to a hole in the center where it exits on the other side. In the experiment, the hole was less than 100 nanometers in diameter, but it can theoretically be as small as 5 to 10 nanometers. The researchers packed the lenses into a flying plasmonic head, so-called because it would "fly" above the photoresist surface during the lithography process."
Already, similar flying heads have been developed in Bogy's Computer Mechanics Laboratory. "'Flying heads support the phenomenal advances in data storage in hard disk drives,' said Bogy. 'They enable the fast speeds and nanometer accuracy required in this potentially new approach to semiconductor manufacturing.' The researchers said the flying head design could potentially hold as many as 100,000 lenses, enabling parallel writing for even faster production."
This research work is available from Nature Nanotechnology as an advance online publication under the name "Flying plasmonic lens in the near field for high-speed nanolithography" (October 12, 2008).
Here is the beginning of the abstract. "The commercialization of nanoscale devices requires the development of high-throughput nanofabrication technologies that allow frequent design changes. Maskless nanolithography, including electron-beam and scanning-probe lithography, offers the desired flexibility but is limited by low throughput. Here, we report a new low-cost, high-throughput approach to maskless nanolithography that uses an array of plasmonic lenses that 'flies' above the surface to be patterned, concentrating short-wavelength surface plasmons into sub-100 nm spots.
For more information, here is another link to the full article (PDF format, 5 pages, 552 KB), from which the above figure and its caption have been extracted.
Finally Zhang expects that an industrial implementation of this technology should appear in the next five years. And if the numbers shown above don't talk to you, here is analogy provided by Zhang. "The speed and distances we're talking about here are equivalent to a Boeing 747 flying 2 millimeters above the ground. Moreover, this distance is kept constant, even when the surface is not perfectly flat." Really impressive...
Sources: University of California at Berkeley news release, October 22, 2008; and various websites
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