Silicon: material thought to be brittle proves to be surprisingly stable in the nanometre range

Silicon is considered to be as brittle and fragile as window glass. The material changes its properties in the nanometre range, however. Empa research scientists have demonstrated this by manufacturing tiny silicon pillars. If the diameter of them is small enough, pillars no longer break like larger pieces of silicon under pressure; instead of this, they buckle and deform plastically in a similar way to metallic materials. This finding creates completely new material perspectives for the design of mechanical microsystems and the watch industry.

Empa founder Ludwig von Tetmajer already studied the effects of mechanical pressure on pillars. Following the collapse of the railway bridge in Münchenstein, he was able to demonstrate in laboratory pressure tests that Euler's formula did not always apply to slender bars and needed to be corrected. «We did the same thing 127 years later, except on a nanoscale, and produced surprising results: instead of buckling, fragile silicon nanopillars, we found ones that deform plastically under pressure», explains Johann Michler, head of the «Mechanics of Materials and Nanostructures» department in Thun.

Silicon is the most frequently used basic material in the semiconductor and photovoltaic industry. It acts as the starting material for electronic components such as computer processors as well as for many sensors and micromechanical systems, e.g. for the lever arm in atomic force microscopes. More than 90 per cent of solar cells are manufactured from silicon nowadays too.

The material has its limits, however, because silicon is considered to be brittle: silicon wafers – thin slices of silicon and the starting material for the above-mentioned applications – break into thousands of individual pieces like panes of glass when put under the slightest of pressure. The team headed by Michler has now demonstrated that these properties change in the nanometre range; for this purpose, the physicist Fredrik Oestlund prepared a silicon plate with a focussed ion beam (FIB), an instrument that is used for surface analysis and processing. He removed material from the plate layer by layer in rings with the help of gallium ion beams, so that tiny pillars were created with diameters of between 230 and 940 nanometres.

«Our pillar buckling tests are similar in principle to Tetjmajer's tests. Except that our pillars are about 100 000 times smaller», says Michler. A micro and nano precision tool known as a nanoindenter was used to put pressure on them. Clamped in a scanning electron microscope, the flattened point of a diamond pyramid applied longitudinal pressure to the pillars from above, with constant measurement of the force applied. «Larger» pillars developed cracks under pressure and broke into small pieces, i.e. demonstrated the typically brittle properties.

If the pillars were narrower than 400 nanometres, however, no cracks were formed; the pillars started to deform plastically like metal. The inner structure of the material is the reason for this. The material properties are not determined by the perfect arrangement of the atoms but by faults in this arrangement. If the pillars are smaller than the average distance between certain defects in the regular arrangement, these pillars are suddenly easy to form. Oestlund, Michler and their research partners from the Universities of Uppsala and Minnesota published their results recently in the magazine «Advanced Functional Materials».

«The findings may make it possible for us to use silicon in mechanical applications like metal – provided the silicon is small enough», says Michler. Metallic materials tolerate faults and can, for example, absorb impact via deformation – without breaking. It is in addition difficult to design components with brittle materials, because they fail when the tension intensity level at a place where there is a defect becomes too large. Since it is nearly always the case that the exact location and the size of the critical defect are unknown, the critical load can practically never be determined exactly – this is considerably more simple with a metallic material that deforms at a defined load. These «docile» properties of the plastic deformation of silicon are creating completely new perspectives for the design of mechanical micro- and nanosystems in the watch and semiconductor industry.

Source: Oestlund, F., Rzepiejewska-Malyska, K., Michler, J. et al.: Brittle-to-Ductile Transition in Uniaxial Compression of Silicon Pillars at Room Temperature; Adv. Funct. Mater. 2009, 19, 2439-2444

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