Disclosed herein is a method for forming a test key system for characterizing wafer processing states, the method comprising forming a plurality of shallow trench isolation structures (STIs) on a substrate of a wafer and in a scribe line of the wafer and forming a test key on the substrate of a wafer and in the scribe line of the wafer. Forming the test key comprises forming at least one test key group having a plurality of test key series, each of the plurality of test key series having a plurality of test pads, each one of the plurality of test key series having a first physical characteristic different from the first physical characteristic of other test key series the at least one first test key group.