The present invention generally provides an apparatus and a method for inspecting a substrate in a substrate processing system. In one aspect, a voltage or current source is used in conjunction with a power density receiving device, such as a spectrometer, to inspect a substrate for various noise spectrum signatures. In one embodiment, spectral data collected from a given substrate is used to generate a current or voltage spectral signature. This spectral signature may then be compared to a reference spectral density signature to predict reliability of a feature structure of a substrate in processing and feedback to the substrate processing system for substrate processing control. Embodiments of the invention further include computer-readable media containing instructions for controlling the substrate processing system, and computer program products having computer-readable program code embodied therein for controlling the substrate processing system and inspecting defects on semiconductor features.