Deformable nanosilicon enables smaller sensors
Since the invention of the MOSFET transistor sixty years ago, which is based on the semiconductor silicon, this chemical element has been an integral part of modern life. Silicon is readily available, cheap and has ideal electrical properties, but it also has an important disadvantage: it is very brittle and therefore breaks easily. This can become a problem if you want to use silicon to make microsystems, i.e. mechanical devices only a few micrometers in size, such as acceleration sensors in modern mobile phones.
At ETH Zurich, a team led by Jeffrey Wheeler, Senior Scientist in the Laboratory of Nanometallurgy, together with colleagues from the Laboratory for the Mechanics of Materials and Nanostructures at the Empa Materials Research Institute, has shown that under certain conditions silicon can be much more resistant and ductile than previously thought. In order to understand how the smallest structures of silicon can deform, Wheeler took a closer look at a widely used manufacturing method: the focused ion beam. Such a beam of charged particles can very effectively mill desired shapes into a silicon wafer, but it also leaves behind distinct traces in the form of surface damage and defects that make the material more easily broken.
The researchers had the idea of trying out a special form of lithography as an alternative to the ion beam method. “First, we created the desired structures – in our case tiny columns – by etching away the material not covered by a mask from a silicon surface with a gas plasma,” explains Ming Chen, a former doctoral student in Wheeler’s research group. In a further step, the surface of the columns, some of which are less than a hundred nanometers wide, is first oxidized and then cleaned by removing the oxide layer completely with an acid.
Chen then used an electron microscope to examine the strength and plastic deformability of silicon columns of different widths and compared the two production methods. To do this, he pressed a tiny diamond punch into the columns and observed their deformation behavior under the electron microscope.
The results were astounding: the columns milled by ion beam broke down at a width of less than half a micrometer. In contrast, the columns produced by lithography only broke at widths of over four micrometers, but thinner specimens were largely able to withstand the stress. “These lithographic silicon columns are still deformable even at dimensions ten times larger than those observed with plasma-milled silicon with the same crystal direction – and with twice the strength,” Wheeler summarizes the results of his experiments.
The strength of the lithographically produced columns even reached values that one would actually only theoretically expect for ideal crystals. According to Wheeler, the highlight is the absolute purity of the column surfaces, which is achieved with the final cleaning. This leaves far fewer surface defects that could cause the material to break. With the support of Alla Sologubenko, a researcher at the ScopeM microscopy centre at ETH Zurich, the researchers were also able to observe a striking change in the deformation mechanisms at small dimensions thanks to this additional deformability. This brought to light new details about the deformation of silicon.
The results of the ETH researchers could have a direct impact on the production of silicon microsystems, and gyroscopes used in mobile phones that detect the rotation of the device could become even smaller and more robust. This should not be too difficult to achieve, as industry already uses the combined etching and cleaning method that Wheeler and colleagues are investigating.
This should also be applicable to other materials with a similar crystal structure to that of silicon, the researchers suspect. In addition, more elastic silicon could also be used to further improve its electrical properties for certain applications. Strong tension in the semiconductor can increase the mobility of its electrons, which can lead to shorter switching times, for example. While this used to require the manufacture of nanowires, it could now be achieved directly with structures integrated into the semiconductor chip.
The results of the Swiss researchers were recently published in the journal Nature Communications.
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