R. Winn Hardin
Microchip and display companies are showing interest in a variation on a laser micromachining technique that creates large-area, low-defect crystalline silicon films.
Liquid crystal displays and microchips use amorphous silicon as the substrate for control chips and microprocessors. The material is relatively easy to make, cheap and plentiful, but crystal defects limit the performance of these devices. A better option would be crystalline or single crystal silicon, but manufacturing processes are slow and expensive and produce small-area yields. Until now, that is.
With funding from the Defense Advanced Research Projects Agency and the US Department of Energy's division of basic science, James Im, Robert Sposili and Mark Crowder of Columbia University have developed a technique that could make crystalline silicon films in the same way paper rolls off a printing press.
The process is similar to laser micromachining an amorphous silicon substrate, Im said. A 308-nm XeCl excimer laser from Lambda Physik Inc. of Fort Lauderdale, Fla., bombards the amorphous silicon with ultraviolet light, delivering less than
1 J in a 100-ns pulse at 30 Hz. The light passes through a chevron mask, which resembles a zigzag line, melting the silicon. At the elbows of the mask, the silicon re-forms to create a uniform crystalline structure.
Trick's in the remelt
By carefully alternating between completely melted and partially melted sections of amorphous silicon, the process uses the first crystal as a seed and, theoretically, could continuously create crystalline silicon from an amorphous silicon substrate, forming a large-area crystalline silicon film.
Im would like to increase the energy per pulse and repetition rates, which should lead to faster manufacturing processes. Previous attempts to create crystalline silicon using lasers required high stability between laser pulses. However, Im said his process forgives energy density variations between pulses of up to several tenths of a percent, ensuring that the material is of uniform quality across the entire film.
Researchers at Lawrence Livermore National Laboratory in California and other institutions have successfully patterned thin-film transistors onto Columbia's crystal film. These films could lead to 3-D microprocessors where conventional chips are patterned one atop the other.
