|vlsitechnology.org /layout_rules /snap grid|
The snap grid is the smallest increment allowed in the layout file. For micron rule sets, it is set by the foundry and for 0.13µm technology is 0.005µm.
For lambda rule sets, the value chosen directly impacts the available lambda scaling factor. The lambda snap grid multiplied by the lambda scaling factor must be a multiple of the micron snap grid.
Let us consider scaling pharosc 2µm layout to generic 0.13µm rules. If the lambda snap grid is 1 with λ=0.055, then scaled it is 0.055µm which is a multiple of 0.005µm.
If the lambda snap grid is 0.5 with λ=0.055, then scaled it is 0.0275µm which is not a multiple of 0.005µm. So λ=0.055 cannot be used; λ=0.06 must be used instead.
Thus, while being able to draw in 2µm rules with a 0.5µm snap grid can give some improvements to the layout density, there can be an overall impact through a bigger lambda scaling factor.
The pharosc rules and the sxlib rules have a snap grid of 1, while both the MOSIS rules use 0.5.