The intricate web of interconnects that makes up an integrated circuit is a mind-numbing maze of metal, stretching to all corners of a microchip. These metal traces race from one end of the device to the other, traversing the multiple layers of metal used to route signals from point to point on the IC. Modern IC devices are inherently three-dimensional; most modern devices require as many as nine or ten different metal layers on an integrated circuit, all stacked atop one another, in order to achieve the necessary signal density. The dense stack-up of layers can often make it difficult to determine key information about a given device from top-down; in these cases, an analyst can augment their understanding of a part through an integrated circuit (IC) cross section.
Integrated circuit cross sectioning involves taking a microchip and polishing into it with an ultrafine (submicron) abrasive film. Unlike parallel lapping, where the abrasive is used to polish away one layer of metal on the die at a time, in an IC cross section the device is polished perpendicular to the plane of the device, revealing all metal layers and the transistor circuitry on the silicon substrate simultaneously. This allows an analyst to examine connections between traces on a device, as well as to look for any defects that may exist between metal layers. Oftentimes, this type of information cannot be obtained at all without a cross-section.
For failure analysis, integrated circuit cross sectioning can often unearth defects that would be completely overlooked (or, at the least, very difficult to find) when performing a parallel lap. As an example, consider a case where an open circuit has been reported on one signal on a die. The analyst has performed isolation with electron microscopy using voltage contrast techniques, and has determined that the problem exists between two of the metal layers of a device, most likely in the via connecting the two layers. An attempt to parallel lap carries with it a high level of risk – after all, the defect may only be a several nanometer wide crack in the via, nearly impossible to hit precisely – and, even if the defect was uncovered, there would be little evidence to suggest that it could cause an open circuit, since the metal that it was supposed to connect to is no longer present! Conversely, a precision cross-section into the via of question would prove the issue beyond a shadow of a doubt, providing a clear image of both metal layers in question and the open circuit in the via between them.
Integrated circuit cross-sections have uses beyond the realm of failure analysis, as well. A cross-section is often vital for clients looking to defend or investigate intellectual property claims, as they provide a relatively complete look at the construction of a given device. Whether your business is FA or IP, a good source for integrated circuit cross sections is a must!