1. A method of controlling a manufacturing process, the method comprising the steps of: a) providing a testing area with a periodic structure having a period d, wherein within each period d the periodic structure includes a plurality of alternating lines and spaces therebetween, and wherein said lines and spaces within said each period d have a monotonically increasing width; b) illuminating the periodic structure with a light, thereby producing a plurality of non-zero order diffraction signals; c) collecting the diffraction signals to produce a test signature; d) matching the test signature with a reference signature; and e) controlling a manufacturing process using a control setting set associated with the matching reference signature.