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IBM researchers invent nanoprinting technique

Posted: 24 Sep 2007     Print Version  Bookmark and Share

Keywords:nanoscale printing technique  direct deposition  nanoimprint lithography 

Metals, semiconductors and oxides can now be directly deposited onto substrates with single nanoparticle accuracy using a nanoscale printing technique invented at IBM Research GmbH (Zurich). IBM illustrated its nanoprinting technique by placing 60-nanometer diameter gold nanoparticles-20,000 of them-into a precisely rendered 80-micron print of the famous 17th-century image of the alchemists' symbol for gold: the sun. Besides patterning semiconductor chips, IBM claims the printing technique could also be used to craft nanoscale biosensors, to lay down subwavelength metamaterials, and to arrange biomarker molecules into arrays for medical tests that work with very tiny samples.

"We did the image of the sun to demonstrate our single-particle accuracy," said Heiko Wolf, a research staff member at IBM Research (Yorktown Heights, N.Y.) "But we also did a semiconductor example, where we placed catalytic seed nanoparticles for growing semiconducting nanowires."

The direct deposition technique could also be useful for crafting arrays of split-ring resonators tuned to specific wavelengths of light, thereby crafting metamaterials with properties not possible for natural materials, such as a negative index of refraction.

Similarly, in the field of medicine, arrays of bioassay nanoparticles could be printed onto medical test chips for simultaneous screening for thousands of maladies from a single drop of blood, since each 60-nm nanoparticle is 100 times smaller than a human red blood cell.

Since the plates used for depositing nanoparticles are reusable, the technique lends itself to easy mass production by merely duplicating the plates and refilling them with nanoparticles after each use. The process would be similar to re-inking the plates of a printing press, but with the added ability to deposit any material with a super-fine resolution of 100,000 dots per inch, compared with the 1,500 dots per inch typical of printing presses today.

Moulds instead of masks
Today lithographic printing techniques are used to make masks. The masks expose patterns onto semiconductor wafers, which are then etched into the circuits. Even nanoimprint lithography uses this subtractive method, etching away what is unwanted and leaving what is wanted. But instead of masks, nanoimprint lithography crafts its patterns into nanoscale moulds that are dipped into resist, then stamped onto wafers.

Now IBM claims to be perfecting an alternative "additive" approach. It uses moulds like nanoimprint; but unlike all forms of lithography, instead of depositing resist then etching away what's not wanted, IBM makes printing plates from its moulds that can directly deposit the actual materials with single nanoparticle accuracy.

"Depositing individual particles, such as seed molecules from which nanowires can be grown in place, is difficult to impossible using traditional, subtractive lithography," said Wolf. "Instead, we use additive gravure printing with nanofabricated plates, a self-assembly process to ink them, and controlled adhesion for the direct transfer of nanoparticles onto wafers."

Gravure printing uses an intaglio process to ink a printing plate with desired material-gold nanoparticles in IBM's demonstration seeding application, which are then transferred to a substrate by pressing against it.

Traditional gravure printing today reaches resolutions of 10 microns, but IBM's nanofabricated plates, self-assembling inking process and controlled adhesion technique have already demonstrated resolutions down to individual 60-nm particles.

"We have single -nanoparticle resolution, which today is about 60 nanometres," said Wolf. "And we can go much smaller; other groups have reported using experimental techniques to deposit nanoparticles as small as 2 nanometres."

The technique works by first preparing a mould using traditional semiconductor fabrication tools, such as electron-beam lithography. After the pattern has been defined, a 100-micron thin film of rubber is deposited and cured atop the mould. After cooling, the rubber can be separated from the mould and mounted on a plate for printing.

In this way, as many printing plates as necessary can be made from the single master mould, enabling mass production from a single original.

Next, a self-assembly process fills the rubber printing plate with nanoparticles from a colloidal suspension solution by passing the meniscus of the solution over the rubber plate, whereupon capillary forces perfectly guide the gold nanoparticles into every designated location for a particle in the plate. The last step is stamping the filled rubber plate onto a hydrophilic substrate, whereupon the particles naturally adhere to the substrate, whose surface has a stronger force of attraction than the low-energy surface of the rubber printing plate.

The team wants to advance to even smaller nanoparticles—with a short-term goal of 10 nanometers—and to experiment with different types of metals, as well as with nonmetallic nanoparticles, ranging from polymers to semiconductors to oxides. They also hope to increase the accuracy of the printed structures from 12 parts per million today to about 1 ppm, which is what would be required for large-scale integration of microelectronic devices using the technique.

IBM worked with the Swiss Federal Institute of Technology (ETH Zurich) on this research.

R. Colin Johnson
EE Times

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