MIT Team Develops New Way To Create Microscopic Patterns On Surfaces

CAMBRIDGE, Mass.--The microscopic 3-D stripes of material that AssistantProfessor Paula T. Hammond is depositing on thin gold wafers represent anew technique for creating patterns--and structures--on surfaces. Thetechnique, which Professor Hammond pioneered some two years ago, involves"printing" a pattern onto a surface, then taking advantage of a material'selectrical properties to build up layers of that material over the pattern.

Because the technique is relatively easy and inexpensive it couldbecome an alternative to conventional patterning procedures such as thephotolithography used in the manufacture of computer chips. One potentialapplication: printing electronic circuitry on treated paper or plasticsurfaces. "This would be much harder to do with photolithography and wouldalso be more expensive," Professor Hammond explained.

In the December 18 issue of the journal Advanced Materials,Professor Hammond and graduate student Sarah L. Clark, both of theDepartment of Chemical Engineering, report new advances in the work. Theseinclude the automation of the process and the ability to create morecomplex patterns using changes in processing conditions.

HOW IT WORKS

The technique specifically allows the researchers to create layersof polymer--materials made of repeating subunits--over a pattern (such as astripe). They can currently create stripes that are only about 3 1/2micrometers, or millionths of a meter, wide (a human hair is about 75micrometers in diameter). Layers of an inert material are deposited betweenthe stripes.

Professor Hammond's technique is a variation of one calledlayer-by-layer assembly, in which polymers with different charges aresequentially absorbed onto a surface. "For example, a polymer that'spositively charged will attract and absorb one that's negatively charged,"Professor Hammond said.

Layer-by-layer assembly, however, produces continuous polymerfilms. Professor Hammond is the first to create patterns. "Continuous films[like those produced via the original layer-by-layer technique] haveseveral applications, but those applications can be greatly increased ifyou can pattern the surface, tell the [polymer] film where to go,"Professor Hammond said.

Her variation begins by "stamping" a surface with the pattern ofinterest using microcontact printing, a technique developed at Harvard byProfessor Hammond's thesis advisor, Professor George Whitesides. ProfessorHammond took microcontact printing in another direction: she found that"ink" with specific properties would selectively attract certain polymers.Meanwhile the spaces between the stamped patterns, which she fills with theinert material, repel the polymer.

From there, just as with the original layer-by-layer deposition,Professor Hammond coaxes polymers of alternating charges to form layersover the pattern. Layers of inert material continue to fill in the gaps.

UNEXPECTED TWIST

In an unexpected twist, Professor Hammond's team has found thatchanging such things as the salt content or pH of the system can cause thepolymer and the inert material to switch allegiances. Stripes thatpreviously attracted the polymer now attract the inert material, and viceversa.

"This was a surprise," Professor Hammond said. Remembering themoment when her team first discovered the switch, initially with respect tosalt content, she continued: "We had to back up and say, 'wait a minutenow!' We had to check and double-check, until we could say, 'yes, this hashappened.'"

The researchers have since been instigating "reversals" to buildever more complicated structures. For example, as reported in AdvancedMaterials they can now produce "blankets" of polymer over the entiresurface, covering both the layers of polymer and the layers of inertmaterial. (In this case the reversal is incomplete, allowing polymer tospread over both types of material.) "It's really getting fun now,"Professor Hammond said.

The Advanced Materials paper also reports how the researchers havenow automated the process by adapting a machine originally designed forstaining slides used in biological studies. Among other advantages,automation means less time handling samples and better-controlledconditions. This has resulted in more uniform patterns, as well as anincrease in the number of layers that can be deposited. The researchershave also shown that the technique is reproducible (they have created up toten samples at a time).

Other members of Professor Hammond's research team are graduatestudents David M. DeWitt and Xueping Jiang, both of the Department ofChemical Engineering. Martha F. Montague, MIT SB 1998, was also on the team.

The work is funded by the Office of Naval Research and the MITCenter for Materials Science and Engineering.