Materials Science

Under pressure: New ptychography technique examines stressed atoms in 3D

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In a small lead hutch-protected chamber at Argonne National Labs is a nano-probe that bombards nano-sized electronic devices, like a transistor, for example, with x-rays. Think of it as a CT scan. But for atoms. It’s building a 3D model of that device’s atomic structure, for which we can examine the amount of strain it can handle.

ANL’s Stephan Hruszkewycz, a co-author of our paper High-resolution three-dimensional structural microscopy by single-angle Bragg ptychography, developed this x-ray ptychography (“pytcho” means “folding” in Greek) technique to create images of these devices to better understand how they handle the real-world strain of manufacturing and usage.

Ptychography was invented by chemist Walter Hoppe in 1969. It uses x-rays to create computer-generated images. As technology has improved over the decades, the image resolution has improved. And now, with Hruszkewyc’s technique, the technology has leapt from two to three dimensions.

His algorithms use data collected by over-sampling the device with x-ray diffraction. The nano-probe moves in a spiral pattern around the device. And after 50 to 100 iterations, a computer is able to produce a 3D model to test for atomic-level strain and deformation – and how these changes impact device performance. The results could mean understanding how to build more resilient nanowires, magnetic memory structures, nanotube connects, and other materials.

Think about this: A 5nm transistor is about 10 silicon atoms wide. So, squashing or stretching just a single atom can dramatically change how current flows through the silicon. This x-ray diffraction technique can non-invasively and in-situ examine when, at the atomic level, the existing mechanical models of a transistor breakdown.

No strain, no gain

My team provides ANL with devices and device features to test with their scientists and their nano-probe. We then study the model devices’ atomic structure under strain and deformation to not just learn about what breaks down and when, but to also learn what can be altered to improve performance, and for example, help semiconductors keep up with Moore’s Law. Devices modeled in 3D also help us better understand how, by applying the right type of strain, an electrical signal’s mobility is enhanced as it passes through the real world transistor, mentioned in the previous paragraph. Or test if one material could be replaced for another to improve durability.

TOP: From Figure 3 | Experimental geometry. a) A schematic of the device architecture is shown featuring the embedded SiGe stressor geometry. For 3DBPP, coherent diffraction patterns were measured at the specular 004 SiGe Bragg peak at a set of beam positions that effectively overlapped in a series of spiral patterns. The diffraction patterns from all 707 positions were phased simultaneously with 3DBPP. BOTTOM: From Figure 4 | 3D Bragg projection ptychography experimental results. Coherent diffraction patterns measured at the 004 SiGe Bragg peak were phased using 300 iterations of the OS/PIE algorithm adapted for 3DBPP. d) which is derived from the phase of the reconstruction.

I’ve been conducting experiments on IBM devices, at ANL’s Advanced Photon Source facility in Illinois, since 2001. Because it’s the only lab in the United States with these sophisticated nano-probes, I submit proposals and receive beam time to study our devices. I then take the collected data back to IBM’s Thomas J. Watson Research Center in Yorktown Heights, NY to experimentally verify the findings. We then share our “strain” results on everything from magnetic memory to FinFET transistors with manufacturing partners and clients. This data helps them better understand how their devices will perform, and how to better manufacture their devices.

Computing power has increased such that we can do this analysis on a laptop. Our next goal is to collect and stream the data from the probe, apply the ptychography algorithms to give us a reasonable image, and obtain the strain data in real time. Doing this modeling and analysis on the fly will greatly increase efficiency during our five days at ANL!

For more about this new technique, read our paper, High-resolution three-dimensional structural microscopy by single-angle Bragg ptychography at Nature Materials.

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