A new approach was developed that overcomes the limitation of conventional chip-testing methods on 3D chips, which include many thin horizontal “floors” connected to one another by vertical pathways called through-substrate vias (TSVs). TSVs help 3D chips do three essential things: speed up, shrink down, and cool off. By allowing elements on different floors to communicate with each other, signals no longer need to travel all the way across a comparatively sprawling 2D chip, meaning calculations go faster and electrons heat up far less conducting material as they move.

These “eye” diagrams reveal how much noise is present in a digital signal. As the signal grows noisier, its characteristic shape grows distorted, shrinking the center so it resembles an eye closing. The new 3D chip-testing method passes microwaves through chip material, allowing researchers to quickly detect flaws that would create noise and make the diagram change from the open-eyed clarity of the top image, to the squintier distortion on the bottom. (Credit: Y. Obeng and N. Hanacek/NIST)

Along with these advantages, TSVs also carry one drawback: their reliability is hard to test with the conventional method, which involves passing direct current through the conductor and waiting for its resistance to change. It is very time-consuming, requiring weeks or even months to show results. The chip industry required a new metrology approach that is quick and realistic, and that would reveal the impact on the high-speed signal that actually runs through the conductors.

The new approach to testing multilayered 3D computer chips quickly assesses the reliability of this relatively new chip construction model. With the new testing method, chip designers may have a better way to minimize the effects of electromigration — a perennial cause of chip failure rooted in the wear and tear that relentless streams of flowing electrons inflict upon the fragile circuitry that carries them. The approach could give designers a quicker way to explore the performance of chip materials in advance, thereby providing more — and almost real-time — insight into what materials will best perform in a 3D chip.

The new method sends microwaves through the material, and measures changes in both the amount and quality of the signal. The test setup, which simulates real-world conditions, repeatedly heats and cools the material, causing it to develop flaws; over time, the microwave signal decreases in strength and decays from a clean, square-shaped wave to one that is noticeably distorted.

Using microwaves brings multiple benefits — perhaps chief among them is how rapidly the method provides information about a device’s reliability in the actual device of interest, long before it actually fails. Before failure comes a quiescent period when the beginnings of defects are blowing around through the material, like seeds in the wind; the microwaves show this process happening. Microwaves could reveal information about defects as quickly as three days after testing begins, while conventional tests can take months.

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