Mirkin group unveils 55k-pen DPN array


New array suggests throughput could eventually meet commercial needs

By Charles Q. Choi

Dip-pen nanolithography can literally draw structures only nanometers in scale, but was always limited in throughput by how many pens it could write with at the same time. Now the technique’s inventor has devised an array with hundreds of times more pens than before.

Dip-pen nanolithography (DPN) uses atomic force microscope (AFM) tips as pens and dips them into inks containing anything from DNA to semiconductors. The new array from Chad Mirkin’s group at Northwestern University in Evanston, Ill., has 55,000 pens - far more than the previous largest array, which had 250 pens.

The new array can draw 55,000 likenesses of Thomas Jefferson in a space the size of a nickel - images consisting in total of some 470 million 80-nanometer-wide dots - in less than 30 minutes.

“The throughput barrier is a grand challenge for the nanotechnology field, as lithography is the foundation for all that we do. And in this paper Mirkin and coworkers drive a truck through this challenging barrier,” said Joseph DeSimone, a chemist at the University of North Carolina at Chapel Hill and director of the new UNC Institute for Advanced Materials, Nanoscience and Technology. The researchers reported their new DPN array in the November 6 issue of Angewandte Chemie.

“One of the gating aspects of the technology is that, with a single-pen tool, you’re extremely limited in throughput. Seeing they’re able to take their research to a 55,000-tip array instrument would definitely address the issue of throughput,” said Vahe Mamikunian, an analyst with Lux Research.

NanoInk, the Chicago company Mirkin founded in 2002 which is commercializing dip-pen lithography, has licensed the new array and has exclusive rights. “It will be on the research market early next year,” Mirkin said.

The schematic shows the fabrication process for 2D cantilever arrays developed by the Mirkin group at Northwestern University and licensed by NanoInk. Image courtesy of Chad Mirkin
Click here to enlarge image

The array was made by modifying a microfabrication process developed for single AFM probes. Oxidized silicon wafers had 10-micron square openings lithographically patterned into them, and pyramidal pits were etched in those openings. These pits served as molds for pyramidal probes when films of silicon nitride were deposited. The silicon nitride on the front side of the wafers was then lithographically patterned to form arrays of cantilevers.

The probes are roughly 7.6 microns high, with tips about 60 nanometers across. “The tall tips keep the arm holding the tips from running into the surface,” Mirkin said.

The researchers also bend the cantilevers by coating them with gold and then annealing them. The resulting curvature is due to how gold and silicon nitride layers expand differently in heat, and how the different layers in the cantilever restructure due to annealing.

“The bent cantilevers give us more play in the vertical direction to get any misaligned tips in contact with the substrate. In other words, the tips in the array do not have to be perfectly in one plane to bring them all in contact with the surface to be patterned,” Mirkin explained.

The result is an array that can bring all the pens in contact with their substrate using merely gravity, as opposed to a complex set of feedback systems. “This makes the approach innovative, straightforward, inexpensive and extremely useful and versatile,” Mirkin said. Once all the tips of the array are in position, the array is locked in place by a rapidly curing epoxy resin on the tip holder.

“What is most surprising to me is the degree of fidelity achieved,” said Jim De Yoreo, a member of the scientific staff at Lawrence Livermore National Laboratory who conducts research into scanned probe nanolithography techniques. “Highly multiplexed cantilever arrays have been fabricated by a number of groups. But overcoming the challenges of adequately uniform inking of all the tips and obtaining registry between the substrate and all tips has never been achieved. I would have expected these to be daunting tasks, but Professor Mirkin’s group has dealt with both quite handily.

The most important implication of having overcome these two challenges is that high-throughput constructive patterning of a nearly unlimited set of functional chemistries at sub-100-nanometer length scales is now possible. This puts commercial use of constructive scanned probe lithography within our grasp.”

In initial experiments, Mirkin and his colleagues could generate 88 million dot features with their new array, each pen generating 1,600 dots in a 40 by 40 array, where the distance between each 80-to-120-nanometer-wide dot was 400 nanometers. In other experiments, they generated protein nanoarrays. “I was really excited to see 55,000 cantilevers working in unison - really impressive work,” said Thomas Thundat at Oak Ridge National Laboratory in Tennessee, an expert on nanomechanical devices who leads a nanoscale science and devices group at the lab.

The massively parallel DPN approach heralded by the new array “really opens the technique up for many applications, especially in the life sciences,” Mirkin said, “and perhaps some in other areas like integrated electronics and photonics.”

Nanoarrays that DPN can build can “allow us to study cells and the factors that control their behavior - adhesion, growth, motility, differentiation and apoptosis - in a way one could never achieve with conventional technologies. It will allow us to study how viruses work at the single particle level - how they bind and infect cells.”

Moreover, “they will provide important insight into many areas of cancer research - the factors that lead to metastasis events,” Mirkin added. The nanoarrays DPN can build “will lead to many new screening procedures for new therapeutics for a variety of diseases, including many forms of cancer.”

“I suspect the market opportunities for high-resolution, massively parallel DPN will be as diverse as semiconductors, large-area displays, pharmaceutical/packaging tagging for anti-counterfeiting and research tools,” DeSimone said.

The new 55,000-pen DPN array was used to draw 55,000 likenesses of Thomas Jefferson in a space the size of a nickel in less than half an hour. Image courtesy of Chad Mirkin
Click here to enlarge image

Still, it seems this array “is looking at some of the applications that nano-imprint lithography is targeting, and I’m not sure that’s a good place to be targeting,” Mamikunian said. “The reason for that is, with dip-pen nanolithography, you effectively have one supplier right now in the form of NanoInk. So there’s a limited supply base. With nano-imprint, five or six companies are really pushing the technology, from Molecular Imprints to Obducat to Nanonex.”

Mamikunian noted that some of the applications NanoInk is targeting for DPN are different from those of nano-imprint, such as pharmaceutical anti-counterfeiting and photomask and circuit line repair. And, in their paper, Mirkin and his colleagues noted that the advantages their massively parallel DPN approach has over nano-stamping include the distortion effects that plague nano-stamping and nano-stamping’s need to fabricate a mask each time a new design is required.

Another question Mamikunian had with the new array was how manufacturable these were on a large scale.

“If you were ordered to produce 50 or 60 a quarter, what would your ability be to produce such large array instruments predictably and reliably?” Mamikunian said. “It’s really great work, and there will be some applications coming out of it, but until they show how to make such a large array tool on a mass scale, (there are) more questions than answers.”

“These are made by conventional microfabrication processes,” Mirkin responded, adding that mass production was not a big hurdle. Integration is the real issue, he contended, including getting different inks to different tips on the fly. “Maintaining registration with the underlying substrate during the entire process needs to be worked out as well,” he added. “All doable, but it will take some time.”