Plasmons Power Patterning
BERKELEY, Calif., Oct. 23, 2008 -- A new lithography process that resembles a needle playing an album on a traditional LP turntable uses plasmonic lenses as "optical styluses" and could make microprocessors much smaller and more powerful and create disks that hold 10 to 100 times more data.
By combining metal lenses that focus light through the excitation of electrons -- or plasmons -- on the lens' surface with a "flying head" that resembles the stylus on the arm of an old-fashioned LP turntable and is similar to those used in hard disk drives, the researchers at the University of California, Berkeley, were able to create line patterns only 80 nanometers wide at speeds up to 12 meters per second, with the potential for higher resolution detail in the near future.
This new way of creating computer chips could revitalize optical lithography, a patterning technique that dominates modern integrated circuits (ICs) manufacturing, they said.
In this schematic of plasmonic lithography, the plasmonic flying head produces nanoscale patterns onto the spinning disk covered with photosensitive chemicals. Ultraviolet light is delivered through the flying head onto the plasmonic lenses, which are used as optical styluses in this process. The setup resembles a stylus playing a record on traditional LP turntables. (Image: Liang Pan and Cheng Sun, UC Berkeley)
"Utilizing this plasmonic nanolithography, we will be able to make current microprocessors more than 10 times smaller, but far more powerful," said Xiang Zhang, UC Berkeley professor of mechanical engineering and head of the research team. "This technology could also lead to ultrahigh density disks that can hold 10 to 100 times more data than disks today."
Zhang worked jointly on the project with David Bogy, UC Berkeley professor of mechanical engineering. The study now appears online in Nature Nanotechnology, and is scheduled for the journal's December print issue.
The process of optical lithography shares some of the same principles as film photography, which creates pictures by exposing film in a camera to light, and then developing the film using chemical solutions. In the semiconductor industry, optical lithography is a process in which light is transferred through a mask with the desired circuit pattern onto a photosensitive material, or photoresist, that reacts chemically when exposed. The material then goes through a series of chemical baths to etch the circuit design onto a wafer.
"With optical lithography, or photolithography, you can instantly project a complex circuit design onto a silicon wafer," said Liang Pan, a UC Berkeley graduate student working with Zhang and Bogy, and one of three co-lead authors of the paper. "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."
The UC Berkeley researchers chose a different approach to overcome the diffraction limit of light. 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.
Similar flying heads have been developed at UC Berkeley's Computer Mechanics Laboratory, which is directed by Bogy. "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.
A scanning electron image of a 4-by-4 array of plasmonic lenses. Each lens is 4 µm in diameter and can be used as an optical stylus in the pattern writing process. (Image courtesy of Xiang Zhang Lab, UC Berkeley)
The researchers compared this flying plasmonic head to the arm and stylus of an LP turntable, with the photoresist surface spinning like a record. Instead of a needle moving along the grooves of a spinning record, however, the flying plasmonic head contains a nanometer-scale optical stylus that "writes" onto the spinning surface of the photoresist without actually touching it.
Because the light from plasmons decays less than 100 nm from the metal surface, the photoresist material must be placed very close to the lens. To accommodate this limitation, the researchers designed an air bearing that uses the aerodynamic lift force created by the spinning to help keep the two surfaces a mere 20 nm apart.
Air bearings are used to create magnetic tapes and disk drives, but this is the first application for a plasmonic lens.
With this innovative setup, the engineers demonstrated scanning speeds of 4 to 12 meters per second.
"The speed and distances we're talking about here are equivalent to a Boeing 747 flying 2 millimeters above the ground," added Zhang. "Moreover, this distance is kept constant, even when the surface is not perfectly flat."
The researchers pointed out that a typical photolithography tool used for chip manufacturing costs $20 million, and a set of lithography masks can run $1 million. One of the reasons for the great expense is the use of shorter light wavelengths to create higher resolution circuitry. Shorter wavelengths require nontraditional and costly mirrors and lenses.
Their system uses surface plasmons that have much shorter wavelengths than light, yet are excitable by typical ultraviolet light sources with much longer wavelengths. The researchers estimate that a lithography tool based upon their design could be developed at a small fraction of the cost of current lithography tools.
Other alternatives have been developed that can achieve higher resolution than conventional photolithography and without the need for a lithography mask. However, those techniques -- electron beam lithography, scanning probe lithography and focused ion-beam lithography -- work at a snail's pace compared to the flying plasmonic lens system, they said.
Zhang noted that the flying head design is not limited to plasmonic lenses. His lab has been developing metamaterials -- composite materials capable of bending electromagnetic waves in extraordinary ways -- into lenses that can be used for nano-optic imaging and other applications.
"I expect in three to five years we could see industrial implementation of this technology," said Zhang. "This could be used in microelectronics manufacturing or for optical data storage and provide resolution that is 10 to 20 times higher than current Blu-ray technology."
The other co-lead authors of the study are Werayut Srituravanich, a former PhD student in Zhang's lab and currently a lecturer in mechanical engineering at Chulalongkorn University in Thailand; and Yuan Wang, a graduate student in mechanical engineering. The study was also co-authored by Cheng Sun, a former graduate student in Zhang's lab and currently an assistant professor in mechanical engineering at Northwestern University.
The work was supported by the National Science Foundation Center for Scalable and Integrated Nano-Manufacturing.
For more information, visit: www.berkeley.edu
- As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.
- Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
- The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
- A chemical substance rendered insoluble by exposure to light. By means of a photoresist, a selected pattern can be imaged on a metal. The unexposed areas are washed away and are ready for etching by acid or doping to make a microcircuit.
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