At Fraunhofer IKTS, X-ray microscopes, SEM, and TEM are used not only for imaging tiny microchip structures, but also for nanomechanical in-situ experiments.

May I introduce myself? I am a processed microchip, fresh from production, so to speak. But my path did not lead to a microelectronics package, as is usually the case, nor did I end up in a smartphone or a similar device to spend my working life there. Rather, it is fair to say that my life journey is extraordinary – and extremely beneficial for my manufacturers. Because I help to perfect my counterparts. In short, I was securely packed in a sample box and sent to the Fraunhofer Institute for Ceramic Technologies and Systems IKTS.

Here, I am to be put through my paces, so to speak – or rather, the IKTS researchers are looking at my reliability, what my materials look like at the micro level, and how I cope with mechanical stresses or thermal loads. Why all this? Well, on the one hand, the results provide information about where the production of my counterparts can be optimized to eliminate sources of error and make us even better, more powerful, more stable, and more durable. On the other hand, the data obtained is fed into design tools that my manufacturers use to develop new chips and assemble them according to the desired functionality. The software ensures that all necessary rules are followed. However, this requires precise data on my functionality as well as on the path lengths of the signals that flow through me. In addition, accurate material data is required: a centimeter-sized piece of copper behaves completely differently than the copper in my nanometer-thin conductive tracks. 

Why Fraunhofer IKTS?

How fortunate that my path led me to Fraunhofer IKTS! Here, I can get the most out of my little microchip life. Why? Not only are there various nanoanalytical microscopes here that can be used to view and examine me down to the atomic level, but all of the microscopes are equipped with small microtesters that can apply the various loads that could affect me—whether thermal, mechanical, electrical, or even chemical—in situ in the microscope. For example, researchers can use micromanipulators to pull on me from two sides, causing a crack to form, while observing the process live through the microscope. At which force does the crack-stopping structure fail, and how? Where is the weak point? What are the properties of the interfaces, and how can they be improved? Various thermal loads can also be applied: In X-ray microscopy, I can be cooled down to minus 190 degrees Celsius or heated up to 600 degrees Celsius. In transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the temperature range is from minus 150 to 400 degrees Celsius.

This means that, at IKTS,virtually all materials can be thermomechanically characterized on a small scale. This data is then incorporated into complex material models for further simulations of the complete chip system.

X-ray microscopy: a look at the larger metallization layers

Well, let's get started! First, a researcher places me in an X-ray microscope. Unlike a normal X-ray tomograph, the beam here does not become wider and wider, but is focused by X-ray lenses – a technology that was long considered technically impossible. It achieves resolutions of 30 to 40 nanometers, which are significantly higher than those of micro-computed tomography: sufficient to view my larger metallization layers. Most people are probably familiar with the advantage of X-rays from medicine: whether it's the human body or a processed microchip, the rays pass through the material being examined without destroying it. A vacuum is also unnecessary, so the examinations can be carried out at normal room atmosphere. This offers a major advantage for the various stress experiments under tension, pressure, or temperature, because they are much easier to carry out at normal air pressure than in an ultra-high vacuum. X-ray microscopy imaging can be used, for example, to check whether my through-hole connections are filled and therefore intact – in three-dimensional tomography mode. In addition, stress tests, in which I am pressed, pulled, and jerked, heated and cooled, reveal how resilient I am. Researchers can follow this on screen while subjecting me to various stresses. 

Scanning electron microscopy

After undergoing this procedure, I continue on to the scanning electron microscope. Although the X-ray microscope has many advantages due to its non-destructive measurement at room atmosphere, scanning electron microscopy scores points with its significantly higher resolution of one to two nanometers. This allows microstructures to be detected that remain hidden from X-rays. It is a very flexible tool for viewing structures with nanometer resolution and characterizing them chemically and crystallographically. In this way, delaminations can be detected, as can unwanted diffusion processes that cause atoms to end up in places where they should not be. Where in me is aluminum, where is copper, where is tungsten? Crystallographic questions can also be answered: unfavorable crystal constellations can lead to reliability problems in the medium term, as crystals have different properties in different directions. Which crystals occur? Are they randomly or uniformly oriented? And this is where the expertise of Fraunhofer IKTS comes into play again: SEM can also be used to perform in-situ experiments at the micro level. For example, a mechanical tester presses down on me with a force of a few nanonewtons, or an interface is removed from me and analyzed to prevent delamination. It is also possible to cut small columns out of me and crush them, or to cut out structures with an ion beam and examine their relaxation. This allows conclusions to be drawn about the internal stress prevailing in my material.

The researchers use a focused electron beam for imaging and an ion beam of xenon or gallium for cutting. The ion beam can be used not only to cut wafer-thin samples out of me for examination in a transmission electron microscope, but also to remove layers of me one by one so that I can be examined in three dimensions.

Transmission electron microscopy

The researchers then take an even deeper look inside me, down to the atomic resolution level, using transmission electron microscopy. In my front-end-of-line, the transistor level, the researchers examine the transistor channels at the atomic level, as well as thin films in the transistor and elsewhere. How do atomic structures merge into one another? Are the diffusion barrier layers around a conductor track closed? What is the chemical composition of layers that are only a few angstroms thick? In short, TEM allows the researchers to look at structures that consist of only a few atoms – the resolution is 0.1 to 0.2 nanometers. Band and electron structures of materials can also be analyzed in TEM. Of course, such investigations require high-vacuum technology, as otherwise the imaging electrons would be absorbed by the air instead of by me. In-situ experiments are possible despite the vacuum: for example, the researchers were able to observe a "time-dependent dielectric breakdown," i.e., a microscopic short circuit, live in another microchip – a unique achievement to date. This type of malfunction is a critical problem in modern electronics. The only traction experiment ever performed on atomically single-layer graphene with simultaneous observation of the band gap also took place at Fraunhofer IKTS.

Comprehensive in-situ analyses

As a microchip, I can undergo comprehensive in-situ analyses at Fraunhofer IKTS, which examine my properties and performance capabilities in detail. The wide range of analysis tools available here enables researchers to gain valuable insights into optimizing my materials and designs. These research approaches contribute decisively to making me even more reliable, powerful, and durable—let's go down this path together!