So, how does Chipworks spot a dummy? Simple, says Dick James, an engineer at the Canadian firm. He and his colleagues reverse engineer by peeling away each layer of a chip’s interconnects one at a time until they reach the silicon beneath. When delayering a chip, if they find a line with no contacts to the layer above it—that is, with no way to touch a transistor—it’s a fake.

Dummies are good indications of design for manufacturability, but there are other tricks to DFM that aren’t always that apparent. Some DFM steps create temporary structures, which are then destroyed during subsequent steps. Reverse engineers can’t see these so-called sacrificial layers that might be used for specific fabrication needs. For example, one layer of material might be put in place to protect another layer of circuitry during chemical mechanical polishing, the process that makes a wafer extremely flat. The layer is either destroyed in the process or deliberately removed later.

With sacrificial layers, all the engineers can do is look at the features on the chip and infer what steps went into their production. Occasionally, chemical analysis reveals traces of processing materials. And sometimes, reverse engineers just scratch their heads and wonder what the chipmakers were thinking. —A.-M.C.