Post by QuantumDiamonds

"𝗬𝗼𝘂𝗿 𝗿𝗲𝘀𝗼𝗹𝘂𝘁𝗶𝗼𝗻 𝗶𝘀 𝟭 μ𝗺 𝗯𝘂𝘁 𝗼𝘂𝗿 𝗳𝗮𝗶𝗹𝘂𝗿𝗲𝘀 𝗮𝗿𝗲 𝟭𝟬𝟬 𝗻𝗺" It sounds obvious: if your imaging resolution is 1 μm, how could you detect a defect ten times smaller? The physics works in your favor here, because we measure 𝘁𝗵𝗲 𝗺𝗮𝗴𝗻𝗲𝘁𝗶𝗰 𝗳𝗶𝗲𝗹𝗱 𝗼𝗳 𝘁𝗵𝗲 𝗰𝘂𝗿𝗿𝗲𝗻𝘁, 𝗻𝗼𝘁 𝘁𝗵𝗲 𝘀𝘁𝗿𝘂𝗰𝘁𝘂𝗿𝗲 𝗶𝘁𝘀𝗲𝗹𝗳. Apply a current, and it follows every path it can. A short adds a path that should not exist. An open stops the current where the connection breaks. Both show up as a clear anomaly in the magnetic map, so the same measurement 𝗳𝗶𝗻𝗱𝘀 𝘀𝗵𝗼𝗿𝘁𝘀 𝗮𝗻𝗱 𝗼𝗽𝗲𝗻𝘀 𝗮𝗹𝗶𝗸𝗲. And the signal does not stay small. As the field travels up through the package, it spreads out: a 100 nm feature under a 10 μm stack leaves a magnetic imprint several microns wide. But 𝗮 𝘄𝗶𝗱𝗲𝗿 𝘀𝗶𝗴𝗻𝗮𝗹 𝗶𝘀 𝗻𝗼𝘁 𝗮 𝘃𝗮𝗴𝘂𝗲𝗿 𝗼𝗻𝗲. Where it peaks, how it falls off, which way the current turns, all of it is 𝗮 𝗳𝗶𝗻𝗴𝗲𝗿𝗽𝗿𝗶𝗻𝘁 𝗼𝗳 𝘁𝗵𝗮𝘁 𝗳𝗮𝗶𝗹𝘂𝗿𝗲 𝘁𝘆𝗽𝗲 𝗮𝘁 𝘁𝗵𝗮𝘁 𝘀𝗽𝗲𝗰𝗶𝗳𝗶𝗰 𝗹𝗼𝗰𝗮𝘁𝗶𝗼𝗻. We read that fingerprint and reconstruct the current path to a precise position in X, Y, and Z. That is what the QDm.1 puts to use: 𝗮 𝟭 μ𝗺 𝗿𝗲𝘀𝗼𝗹𝘂𝘁𝗶𝗼𝗻 𝘀𝘆𝘀𝘁𝗲𝗺 𝗹𝗼𝗰𝗮𝗹𝗶𝘇𝗲𝘀 𝗮 𝟭𝟬𝟬 𝗻𝗺 𝗱𝗲𝗳𝗲𝗰𝘁, 𝗻𝗼𝗻-𝗱𝗲𝘀𝘁𝗿𝘂𝗰𝘁𝗶𝘃𝗲𝗹𝘆, deep inside an optically opaque 2.5D or 3D package. Resolution tells you how sharply you see the field. It is not the limit on the feature size you can find. More on the method, and the measurements behind it, via the links in the comments.