1. A global node optimization (GNO) technique for generating a model for a planar multiple layer film stack structure, the GNO technique comprising: designating a design of experiment (DOE); obtaining spectra from the DOE; obtaining target thicknesses from the DOE; identifying a continuous film approximation (CFA) and a grating factor (GF) set using a GF lookup model, wherein the GF lookup model takes into account diffraction variables that vary with thickness, grating dimension or scattering, wherein the CFA is a mixing model, and wherein each grating factor introduces a correction at an interface of a binary layer of the DOE using a processor; and optimizing the model using the CFA and the GF set by simultaneously fitting a plurality of the spectra while minimizing error compared to the target thicknesses.