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The passage around this formula

What a mask count actually costs is not one extra exposure. It is an extra circuit through the entire patterning loop: another resist coat, another exposure, another develop, in most multi-patterning schemes another etch and clean, and another round of the metrology that has to confirm the result before the next layer can be trusted to sit on top of it. If a critical layer’s target pitch is split across N separately printable exposures — as litho-etch-litho-etch and self-aligned multiple patterning both do, in different ways — and each exposure contributes an independent, zero-mean placement error of standard deviation $\sigma$ , the layer’s overlay-driven placement uncertainty combines in…

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