Re: mom cap value per sq um
Please note that separating the total capacitance into area and fringe (and whatever else) components is not a rigorous way to calculate capacitance. This is just a convenient approximation, allowing to estimate capacitance. It does not take into account the environment of the capacitor, other second-order effects (i.e. presence of corners in the layout, or complicated shapes), and so its accuracy is probably not better than several percent (or worse).
Accurate capacitance calculation (to an accuracy better than a few percent) requires solving Laplace equation and integrating electric field or charge, and in general case it's impossible to meaningfully separate the total capacitance into partial components.