n a collaboration with EPFL Lausanne, ETH Zurich and the University of Southern California researchers at the Paul Scherrer Institute PSI have used X-rays to look inside a microchip with higher precision than ever before. The image resolution of 4 nanometres marks a new world record. The high-resolution three-dimensional images of the type they produced will enable advances in both information technology and the life sciences. The researchers are reporting their findings in the current issue of the journal Nature.
Since 2010, the scientists at the Laboratory of Macromolecules and Bioimaging at PSI have been developing microscopy methods with the goal of producing three-dimensional images in the nanometre range. In their current research, a collaboration with the EPFL and the ETHZ, the Swiss Federal Institutes of Technology in Lausanne and Zürich, and the University of Southern California, they have succeeded for the first time in taking pictures of state-of-the-art computer chips microchips with a resolution of 4 nanometres, i.e. 4 millionths of a millimetre – a world record. Instead of using lenses, with which images in this range are not currently possible, the scientists resort to a technique known as ptychography, in which a computer combines many individual images to create a single, high-resolution picture. Shorter exposure times and an optimised algorithm were key to significantly improving upon the world record they themselves set in 2017. For their experiments, the researchers used X-rays from the Swiss Light Source SLS at PSI.
Between conventional X-ray tomography and electron microscopy
Microchips are marvels of technology. Nowadays, it is possible to pack more than 100 million transistors per square millimetre into advanced integrated circuits – a trend that continues to increase. Highly automated optical systems are used to etch the nanometre-sized circuit traces into silicon blanks in clean rooms. Layer after layer is added and removed until the finished chip, the brains of our smartphones and computers, can be cut out and installed. The manufacturing process is elaborate and complicated, and characterising and mapping the resulting structures proves to be just as difficult.
While scanning electron microscopes have a resolution of a few nanometres and are therefore well suited to imaging the tiny transistors and metal interconnects that make up circuits, they can only produce two-dimensional images of the surface. “The electrons don’t travel far enough into the material,” explains Mirko Holler, a physicist at SLS. “To construct three-dimensional images with this technique, the chip has to be examined layer by layer, removing individual layers at the nanometre level – a very complex and delicate process which also destroys the chip.”
However, three-dimensional and non-destructive images can be produced using X-ray tomography, because X-rays can penetrate materials much further. This procedure is similar to a CT scan in a hospital. The sample is rotated and X-rayed from different angles. The way the radiation is absorbed and scattered varies, depending on the internal structure of the sample. A detector records the light leaving the sample and an algorithm reconstructs the final 3D image from it. “Here we have a problem with the resolution,” explains Mirko Holler. “None of the X-ray lenses currently available can focus this radiation in a way that allows such tiny structures to be resolved.”
Read more om PSI website
Image: The sample, an extract from a commercial computer chip, is supported by the gold-coloured pin in the centre of the picture. Less than 0.000 005 metres in diameter (about 20 times smaller than the width of a human hair), it was cut out of the chip using a focused ion beam and placed on the pin.
Credit: Paul Scherrer Institute PSI/Mahir Dzambegovic
