Portland, Ore. - A soft-lithography technique that harnesses deoxyribonucleic acid (DNA) as a self-reproducing template is being developed at the Massachusetts Institute of Technology's Supramolecular Nano Materials Group. Researchers in the SuNMag project have demonstrated a self-assembling method, dubbed nanocontact printing, that transfers subnanoscale patterns from a master wafer to any number of production wafers. In doing so, the method sidesteps the problems of both photolithography and nanoimprinting.
"What we have developed is a method that is able to reproduce DNA patterns from one surface to another," said materials scientist Francesco Stellacci, who heads the project. Stellacci, who describes the technique as using "DNA strands as Gutenberg movable type," performed the work with EE professor Henry Smith, EE graduate student Tim Savas and materials science graduate student Amy Yu. Yu described Stellacci's new method as marshaling "nature's most efficient printing technique: the DNA/RNA [ribonucleic acid] information transfer."
The method is based on previous research into self-assembled monolayers (SAMs) that was conducted by George Whitesides at Harvard University. More than a decade ago, Whitesides began developing an alternative to traditional photolithography. His "soft lithography" circumvents the traditional problems encountered with submicron resolution of design features on advanced microchips.
Photolithography is limited by the optical diffraction and high intensity of the extreme-ultraviolet wavelengths necessary to image nanoscale design features. In addition, photolithography can only be applied to two-dimensional planar surfaces and offers no controls over the chemistry of materials.
Whitesides' brand of soft lithography attempts to solve those problems by creating molecular-scale patterns through molecular self-assembly in SAM films. In a nutshell, Stellacci is using DNA as a submicrometer duplication process that offers a means of transferring design features that are beyond the reach of other soft-lithography approaches.
"Nature takes DNA double helixes, separates them into single-stranded DNA, then copies the whole genetic information onto a strand of RNA," said Stellacci. "This is a printing method, if you think about it, that is comparable to photolithography because is takes information from one point to another one, like lithography. But it is much better because it has subnanometer resolution, works at room temperature, does not require special chemicals and transfers a lot more information."
Once the DNA is patterned across a wafer, for instance, there are many methods already available with which to convert the organic material into semiconductors, metals and insulators.
According to Stellacci, it should be possible someday to pattern entire wafers with subnanometer designs in three dimensions using his method.
"There is a huge amount of literature out there that already shows you how to convert DNA into useful devices," Stellacci said. "So once you print the DNA, you don't have to use it as such, because there are known methods to convert it, for instance, into silver wires or to assemble semiconducting nanoparticles on top of DNA for single-electron transistors.
"And because you can print many different sequences simultaneously, you could imagine converting some sequences into a wire, and another sequence could assemble a nanoparticle," Stellacci added. "In the end, you get subnanometer-resolution transistors entirely from self-assembly methods."
Stellacci's method uses widely available genetic-engineering tools to build a particular DNA sequence that will later be used to, say, fabricate a subnanometer transistor channel. First the DNA is patterned on a substrate using SAM techniques. Next, each strand produces a complement strand to which it is attached. The far end of the complement strand is modified chemically so that it will adhere to a substrate.
"We actually start from single strands of DNA; then we hybridize a complement of that DNA. This complement has a sticky end on the top part, so when we place a surface on top of our pattern, our surface binds to the sticky end of the DNA.
"We then do the same thing that nature does: We tell the DNA to de-hybridize, and the two DNA strands separate, giving us a copy of the original pattern on the second substrate," said Stellacci.
Because the original substrate is unmodified by the process, it can be repeated, printing press-style, to churn out endless copies. To control the creation of DNA patterns, the researchers have been using nano Dip-Pen lithography. Developed by Chad Mirkin at Northwestern University, the technique can paint virtually any chemical compounds onto substrates with nanometer control. The demo process used X-ray lithography to define a grating used to transfer an array of DNA wires.
Even though the DNA itself measures in angstroms (one-tenth of a nanometer), today Stellacci's prototypes can only achieve about 40-nm resolution. He has high hopes of improving this resolution in future prototypes but says the fairly clumsy genetic-engineering tools currently available may never match the angstrom resolution commonplace in the natural world.
"Today we have a resolution of 40 nm, which is much better than any other soft-lithography method," said Stellacci. "But we may never be able to match nature."
Even at 40-nm resolution, Stellacci's method offers the advantage of self-assembly. By contrast, other soft-lithography techniques must repeatedly stamp an imprint across a wafer to pattern such elements as transistor channels, gates and interconnection lines.
Eventually, using Stellacci's method, complete three-dimensional cells could be patterned in a single step, instead of requiring the repeated application of different masks (as in photolithography) or the repeated application of stamps (as in nanoimprinting).
"The real beauty of our approach is that the DNA finds its own complement on the surface, so that you can copy as many sequences at the same time as you want. At the end of the day, you end up with a pattern composed of many different sequences, but it only takes one step to print them all at once," said Stellacci.
The technique is still immature, said Stellacci, but he predicts that it could become commonplace for the subnanoscale microchips that will be fabricated about 10 to 15 years in the future.
Stellacci talked about the substrates he plans to use next. "Right now, we are printing on a gold substrate-a conductor-which is not good for electronics. But already in the pipeline we have a method for printing on insulating substrates like glass or silicon dioxide, as well as on polymers," said Stellacci. "Then we want to self-assemble complete single-electron transistors."
The project is being funded by MIT's Deshpande Center, which supports the development and commercialization of biotechnology, information technology and nanotechnology. Now that the process has been demonstrated to work, the center plans to spin out a company based on the technique targeted at the gene-chip array market.
The nanocontact printing process is expected to be 10 times more cost-effective than current processes, the center claims in a statement on its Web site (see web.mit.edu/deshpandecenter).