NIL making a big imprint for small devices

Etching microscopic patterns onto pieces of thermoplastics through a technique known as NanoImprint Lithography (NIL), could potentially change the way miniature-sized products, like microchips, are created.

Invented by Dr. Stephen Chou at Princeton University in 1994, NIL is a technique whereby a pattern is etched onto a piece of metal or a mold, then transferred onto a piece of thermoplastic, explained Richard Flynn, communications officer at the National Research Council’s Industrial Materials Institute (NRC-IMI) in Boucherville, Que.

The pattern sizes the NRC-IMI has been working with range from 200 millimetres (mm) to 0.02 nanometres (nm), according to Dr. Neil Cameron, research scientist at the NRC-IMI.

Although there are several techniques associated with NIL, a lot of the NRC-IMI’s work has been with hot-embossing NIL.

“Hot embossing technology is characterized by using a thermoformable polymer that has been heat softened,” he explained. “We press a stamp into [the thermoplastic] that imparts the texture and then we take it apart.”

This technology can be mass-produced, meaning the metal stamps can be re-used. It is relatively cheap in comparison to other nanolithography techniques, and is not limited like photolithography, the standard way of producing silicon computer chips.

With photolithography, patterns are limited to a minimum size of 60 nm, according to Dr. Teodor Veres, research scientist at the NRC-IMI. Microchips are nearing this limit, and chip-makers, like Intel Corp., will have to devise other methods to produce faster microprocessors.

In fact, it was this limitation that Princeton’s Chou was trying to overcome when he developed NIL.

Polymers used in hot-embossing NIL have to flow at temperatures between 18C and 350C, Cameron said.

“Hot embossing is limited to thermoplastic or thermocurable polymers because the viscosity of the polymer has to be quite low, above the glass transition temperature (Tg),” he explained. “With any amorphous polymer, below Tg it gets glassy.”

Amorphous polymers aren’t particularly required for this NIL technique, but they’re easier to work with than crystalline thermoplastics because crystalline plastics have sharper melting points and don’t exist long in the putty-like state needed for hot- embossing NIL.

For nanostructures, the NRC-IMI is using polycyclic olefins, polymethyl methacrylate (PMMA), polystyrene (PS) and polycarbonate (PC), Cameron said.

For nanostructures and thin films, the NRC-IMI uses PMMA, PS and commercially proprietary polymers, he added.

One particular application the NRC-IMI has been developing is disposable devices for diagnosing certain illnesses. In the third world, detecting and determining the causes of fevers is difficult because doctors usually don’t have access to labs to have blood cultures examined, Cameron explained.

In this application, hot-embossing NIL would be used to create detection chambers on a piece of thermoplastic. The detection chambers would be filled with chemicals capable of detecting pathogens. For example, doctors could simply press the diagnostic strip of plastic onto a patient’s tongue, and based upon the chemical reaction i.e., a colour change, a doctor could determine what illness the patient is suffering from.

And because of poverty in the third world, it has to be done cheaply.

“NanoImprint technology gets bandied about for this technology because once you’ve made the stamp you can use it over and over again, and it can be replicated,” Cameron said.

Later this year, Canadian companies will have a new resource to help them develop using NIL.

Back in June, the Canadian NanoBusiness Alliance (CNBA) and the National Research Council plugged a total of $4.55 million into developing a NIL prototyping facility in Canada, which will be located at the NR-IMI’s complex in Boucherville.

When the facility opens later this year, companies will be able to take advantage of the technology to develop their own applications with NIL (for a fee), and to take advantage of the facility’s short-run processing capabilities.

Further details will be announced later this year.