Hello,
Substrate capaciatance should be defined in the process itself as capaciatnce per square meter. To find it you should no the back end metalization of the process(i.e. number of layers and layers materials)
to calculate the capacitance of each layer then add all the capcitance in series to get the total capacitance.
If you're working with the TSMC 0.35um process, then you should have gotten their documentation where various M1, M2, and M3 capacitances are characterized. Go to Mosis site, get an account, and download the process and design documentaion for the TSMC process.