Silicon pillars yield like metal

8 October 2009

Researchers in Switzerland have demonstrated how minute silicon pillars yield to pressure under load and undergo plastic deformation in the same way as metal.

The findings from Empa (Swiss Federal Laboratories for Materials Testing and Research) are contrary to long-held beliefs about the properties of silicon.

Silicon is usually considered to be as brittle and breakable as window glass. The Empa research team demonstrated that this is not the case if silicon is fashioned into pillars of diameters less than 400nm.

Silicon is the most commonly used raw material in the semiconductor and photovoltaic industries, yet the material’s brittleness presents limits.

Johann Michler, head of Empa’s Mechanics of Materials and Nanostructures laboratory in Thun, and his colleagues showed how this brittleness disappears on the nanometre scale.

The team demonstrated this by treating a silicon plate using a Focused Ion Beam (FIB) instrument, which is used for the analysis and preparation of surfaces.

Using a beam of gallium ions, the team removed ring-shaped zones of material from the plate layer by layer, leaving only tiny pillars of silicon standing. The diameters of the pillars varied between 230 and 940nm.

The pillar-bending tests involved applying a force to the columns with a micro- and nano-precision tool called a nanoindenter. The flattened tip of a pyramid-shaped diamond tool, mounted in a scanning electron microscope, was pressed down along the longitudinal axis of a silicon column. The force exerted by the tip was continuously measured.

‘Our results show that it might be possible to use silicon like a metal in mechanical applications if the dimensions of the silicon structure are small enough,’ said Michler.

The discovery opens the way for completely new design techniques for mechanical microsystems and the watch industry.