Design rulesDesign rules are developed for each separate fabrication sequence. They include the minimum feature size, but essentially they tell you how much overlap you need to leave between two mask layers to ensure that the two features will be coincident when fabricated. They also ensure that there is a suitable gap between two features that should not touch when fabricated. Design rules depend upon the expected alignment error that will be introduced during fabrication. This will be comparatively large when doing double sided alignment. If we take an arbitrary value of +/- 1um alignment error, for example, and we wish to etch a via hole over a metal pad, then it would be wise to leave at least an underlap of at least 2um (try and work in multiples of the minimum feature size, if possible) to ensure the final structure is fabricated as desired: see figure 20.
 Figure 20. Also illustrated in figure 20 is the consequence of alignment sequence, which may be different to fabrication sequence. Alignment errors will be cumulative. For instance, if the metal pattern is aligned to a mark etched into the substrate, and the via pattern is aligned to this same mark, then the metal pattern could end up -1um out of alignment and the via pattern +1um: an error of 2um between metal and via pattern. Here you would want to try and leave at least 4um underlap if possible. Note that if the via were to be aligned to a mark on the metal pattern, it would only be +/- 1um out of registration with the metal, but +/- 2um out of alignment with the mark on the substrate. Many mask design software packages come with design rule checkers, which will automatically check your design against the rules that you enter, and highlight any deviation from the rules. |