Inspection apparatus and methods, methods of manufacturing devices

An inspection apparatus for measuring properties of a target structure, comprising:

• a radiation source;
• an image detector;
• and
• an optical system, the optical system defining the following beam paths:
• an illumination path for receiving radiation from the radiation source, forming a beam of illuminating radiation having a selected illumination profile and focusing said illuminating radiation onto a target on a substrate;
• a collection path for measuring collecting diffracted radiation from said target and delivering a selected portion of the diffracted radiation to the image detector;
• and
• a reference path for receiving radiation from the radiation source and delivering a beam of reference radiation to the image detector so as to interfere with the diffracted radiation,
• wherein the image detector comprises an array of pixels for capturing two-dimensional images,
• wherein at least one of said illumination path and said reference path includes a device for shifting an optical frequency of the reference radiation so that the intensity of radiation at the image detector includes a time-varying component having a characteristic frequency corresponding to a difference between the frequencies of the diffracted radiation and the reference radiation, and
• wherein said image detector comprises a heterodyne lock-in image detector operable with reference to said characteristic frequency to record for each pixel information representing both amplitude and phase of said time-varying component and such that amplitude and phase of multiple angles from the target are measured simultaneously.