Strain map

Generally you don’t want to put engineering materials under too much strain. But in transistors and other electronic devices just the right amount can improve the mobility of charge carriers, making them work faster with less energy loss. The image above is a two-dimensional map of the strain in a silicon transistor on a substrate. Bluer areas indicate where the silicon lattice has been compressed while yellower areas indicate where it has been stretched. “Without measurements, you can’t know what you’ve done is what you think you’ve done,” explains Martin Hÿtch at the National Centre for Scientific Research (CNRS) in France.

Hÿtch and colleagues at CNRS are able to produce such strain maps because they have successfully combined existing “moiré” technique with electron holography. In moiré technique, a coherent electron beam is sent through a strained sample that has been stacked on top of an unstrained sample. Each sample produces its own diffracted beam, and the interference between the beams creates fringes that reveal the samples’ relative strain.

Normally, moiré technique only gives a modest spatial resolution, and it is difficult to stack nano-sized samples on top of each other. However, Hÿtch’s group places the samples side by side and, as in electron holography, interferes the diffracted beams using a “biprism” (Nature 453 1086). This method produces fringes that give both a high spatial resolution (down to 4 nm) and a wide field of view (up to 1 µm).