Method of identifying film stacks based upon optical properties

A method of determining the composition of a film stack, comprising:

• providing a library of optical characteristic traces, each of which correspond to a film stack combination comprised of a known combination of a plurality of process layers formed above a known grating structure;
• providing a wafer having a film stack formed above said known grating structure, said film stack being comprised of at least one of a layer of metal, a layer of polysilicon, a layer of silicon dioxide, a layer of silicon nitride, a layer of silicon oxynitride and a layer of photoresist material;
• illuminating said film stack and said known grating structure;
• measuring light reflected off the film stack and the known grating structure to generate an optical characteristic trace for said film stack and said known grating structure;
• and
• determining the composition of the film stack formed above said known grating structure by correlating the generated optical characteristic trace for said film stack and said known grating structure to an optical characteristic trace from said library, the optical characteristic trace from said library having an associated film stack composition comprised of a known combination of process layers formed above said known grating structure.