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Edge Placement Error (EPE)

The difference between the intended and the printed features of an IC layout.


Edge placement error (EPE) is the difference between the intended and the printed features of an IC layout. It involves patterning of tiny features in precise locations. For example, a feature could be a line, and that line has right and left edges. But in a device, the line and its edges must be precise and placed in exact locations. Then, a contact may land on that line in the device. If these are not precise and exact, that results in misalignment, or an EPE. And if one or more EPE issues crop up in the production flow, the device is subject to shorts or poor yields, which could cause the entire chip to fail.

It doesn’t matter if a fab uses optical lithography tools with multiple patterning, extreme ultraviolet (EUV) lithography or other technology types. The overall goal is to put the features in the right places and avoid EPEs. This goes for patterning all parts of the device.

EPE may sound trivial, but the challenges are escalating and piling up at each node. The problems are caused by a host of issues in the fab. It’s still possible to pattern the features in exact places at advanced nodes, but the process becomes more expensive and difficult. The margin for error is down to single-digit nanometers with angstrom-level tolerances. Needless to say, the industry needs to keep a close eye on the issues behind EPE and the potential solutions.

Nearly two decades ago, lithographers devised a term called edge placement error, which involves several arcane patterning and metrology concepts. But the topic began to heat up at the 22nm/20nm logic node, when chipmakers moved from single to multiple patterning techniques in the fab.

Patterning is one of the key parts of the IC manufacturing flow. In patterning, a lithography scanner exposes light in select places on a wafer, creating tiny patterns or features that make up a device. Over the years, lithography tool suppliers have developed light sources with shorter wavelengths, which in turn can print smaller features.

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