Hancock, N.H. - A startup hoping to capitalize on nanofabrication technology has introduced a tool that it says can define features down to 10 nanometers-five times smaller than e-beam lithography equipment can achieve and 10 times smaller than current lithographic equipment can accomplish.
Further, NanoInk Inc.'s Nscriptor system is not limited to defining patterns in photoresist but can lay down virtually any substance on any substrate. The system is also far cheaper than e-beam equipment, the company claims.
The tool is based on dip-pen nanolithography, which was invented just four years ago by Chad Mirkin and his colleagues at Northwestern University (Evanston, Ill.). Mirkin founded the company to commercialize the process, which is adapted from atomic force microscope (AFM) probe technology.
If NanoInk's engineers achieve their goal of an addressable array of millions of AFM tips, the development would go beyond current semiconductor lithography capability both in the size of features and in the complexity of those features. And the ability to deposit virtually any material on any substrate goes far beyond current semiconductor processes in terms of the types of structures that can be created. For example, experiments at NanoInk with the Nscriptor have created complex three-dimensional structures on a substrate by depositing a variety of substances in succession.
In NanoInk's process, an AFM probe tip is dipped in a well containing deposition material and is scanned across a substrate. A tiny droplet of water attached to the tip allows the molecules of the deposition material to slide off the tip and onto the substrate. Using AFM positioning accuracy, the approach can define complex patterns in a variety of materials at nanometer dimensions.
The Nscriptor is a module that can be retrofitted to scanning probe microscope equipment. It includes a graphical interface, called DPNwrite, that accepts high-level pattern descriptions, translating them into computer control commands for the scanning probe control system. The product also includes a computer-aided design package, called InkCAD, that features a design interface, supports multiple pattern levels for different materials and can import patterns in GDS-II standard format.
Sylvain Cruchon-Dupeyrat, a nanotechnologist at NanoInk, described the system and the technology behind it at the NanoEngineering World Forum, held late last month in Marlborough, Mass. In effect, he said, the developers have built a tool kit "that interfaces the inorganic with the organic-the world of microelectronics with the world of biochemistry and polymers."
Cruchon-Dupeyrat and his fellow workers at NanoInk have demonstrated the ability to pattern microelectronic materials such as silicon dioxide or aluminum oxide on silicon and gallium arsenide wafers. In addition, photoresists have been deposited to demonstrate the ability to build up complex, three-dimensional etched patterns selectively.
In literature describing its technique, NanoInk uses the metaphor of the quill pen. A compelling illustration shows a passage from physicist Richard Feymann's prescient 1960 essay on nanotechnology, "There is Plenty of Room at the Bottom," which is rendered at about 60-nm design rules using an Nscriptor.
Just as the microelectronics revolution leveraged photolithographic printing technology, the Nscriptor resembles the more ancient scripting technique of the ink pen. But that metaphor also brings out a fundamental limitation of dip-pen nanolithography: serial one-of-a-kind production. The same limitation has prevented e-beam lithography from becoming a mainstream manufacturing technique.
Photo imaging has the critical advantage of parallel definition :An entire circuit containing hundreds of millions of features is defined in a single snapshot. While e-beam definition can get far below the feature size of photolithography, that means very little if the sequential process slows production to a crawl.
Still, e-beam lithography has become an essential tool in prototyping leading-edge circuits, and NanoInk is promoting the Nscriptor as a critical tool for the nanoengineering revolution. The company is developing near-term markets by developing chemical dip-pen processes for repairing photomasks and circuit defects.
NanoInk is also beginning to engineer parallel definition systems. Cruchon-Dupeyrat described a prototype array of 1.3 million AFM probe tips. All of the fixed "diving board" tips-cantilevers with an AFM tip attached-reproduce the features defined by movements of the whole array. That might be useful for creating very dense arrays of identical devices similar to semiconductor memory arrays, but it is inflexible if more interesting circuit features are required.
The array was built with photolithographic-based etching processes. First, a silicon dioxide layer was deposited on a silicon wafer to act as a medium for molding the levers and tips of the AFM cantilevers. An array of rectangular areas was created by etching away the silicon dioxide, and the molds for the AFM tips were etched in the silicon below. The crystalline pattern of the silicon is oriented so that the etching process produces an inverted pyramid defined by the atomic planes of the crystal.
Next, low-pressure vapor deposition was used to fill the molds with a silicon-rich silicon-nitride compound. The array is then bonded to a Pyrex wafer and the silicon substrate is etched away, leaving an array of silicon-nitride cantilevers attached to the Pyrex substrate.
NanoInk engineers are also working to create addressable arrays. Cruchon-Dupeyrat said individually addressable probe tips have been created by building a heater consisting of coiled gold wire at the base of the cantilever and depositing a second material on the remaining region. A signal to a specified tip will cause the cantilever to bend as it heats up because of the disparity between the two materials' coefficients of thermal expansion. As a result, the tip retracts, offering two essential functions. When writing, retracted tips will not deposit material; when the array is being prepared by dipping the tips in wells, the retracted tips will not be coated. Thus, a series of dipping operations could result in any arbitrary pattern of materials across the array of tips.
Likewise, when writing, different tips could be activated at any point in the scan so that arbitrary patterns of many different materials could be created. Both the movement of the array and the sequence and pattern of tip retractions can be fully automated with computer control.
Massively parallel arrays of tips pose a problem for the dip-pen mode of operation since they will require a corresponding array of wells to hold material. NanoInk engineers are trying some microfluidic-based systems as a solution.
The Nscriptor could also be used with other microcontact printing methods being developed at various labs. Systems are appearing that use e-beam lithography to create a flexible stamp that can then be used in mass production to imprint materials on wafers. And the dip-pen process can be used in silicon circuit fabrication to define features or add exotic materials to conventional circuits.
In another presentation at the forum, James Murday, Chemistry Division superintendent at the Naval Research Laboratories, pointed out a critical difference between the strict rules governing processes based on conventional chemistry and the new flexibility offered by fabrication on the nanometer scale. With conventional chemistry there are only 109 building blocks-the elements and one set of assembly rules based on their grouping in the periodic table. Nanostructures offer an unlimited number of building blocks-quantum dots, nanoclusters, macromolecules, wires, tubes-and assembly techniques from atomic bonding to coulomb forces to molecular recognition.
The potential of such a broad base of building blocks and assembly techniques means that the eminent nanotechnology revolution could be more diverse than the era of VLSI-based electronics. Murday used the example of the diversity of living forms, all created by nanoscale building blocks using the nanofabrication machinery of the cell.
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