Photolithography Process Stabilizes Porous Silicon

With the use of a relatively inexpensive photolithography process, researchers at Purdue University (West Lafayette, Ind.) have devised a way to stabilize nanocrystalline silicon (a form of porous silicon), using its light-emitting properties to create microelectronic devices. The procedure has application possibilities in optoelectronics, and could prove advantageous in integrating optical devices into silicon chips.

Studying the material's possible use in biological applications, the researchers note that the material's surface could respond to specific chemical environments or cues, and could be used to deliver drugs. A porous silicon wafer could be made, for example, to survive within blood plasma, said Jillian M. Buriak, an associate professor of chemistry who led the research. Untreated porous silicon would dissolve too quickly in such environments, she added.

In fact, porous silicon also is typically too fragile to withstand microelectronic applications. "Oxygen and water molecules in the air interact with the surface of porous silicon to create a glass-like coating that disrupts its photoluminescent properties," Buriak said. But Buriak and doctoral student Michael P. Stewart found a way to treat the porous silicon so that it can hold up to harsh environments.

When light interacts with nanocrystalline silicon, it causes electrons to jump to a higher energy level. As is common in laser reactions, the energy emits itself as light as the particles move back to their former state. When the particles move to a higher energy state, they create excitons. The researchers used white tungsten light to create excitons, exposing wafers for 30-60 minutes in the presence of alkenes or alkynes. The nanocrystalline silicon reacts with the compounds to create a carbon-silicon bond that makes a stabilizing coat.

Taken a step further, this same procedure can be used to pattern the wafer photolithographically. When a mask is inserted into the process, the reaction can be applied selectively, taking place only where the light is allowed to hit the wafer's surface. It is this use of the reaction that could make the process practical for fabricating optoelectronic devices.

The most recent study was published in the Aug. 15 issue of the Journal of the American Chemical Society . The researchers are further studying ways to exploit the reaction to develop nanoscale structures, integrating light and electronics on silicon.