Interferometric methods for metrology of surfaces, films and underresolved structures

A method for determining information about a test object, comprising:

• simultaneously acquiring two or more scanning interference signals, wherein each of the two or more scanning interference signals correspond to interference between test light and reference light as an optical path length difference between the test and reference light is scanned, wherein the test and reference light are derived from a common source, and wherein the test light scatters from the test object over a range of angles and each of the two or more scanning interferometry systems corresponds to a different scattering angle or polarization state of the test light;
• combining values of the two or more scanning interference signals for each scan position of the optical path difference to form a combined signal in which a contribution to the combined signal from a feature of the test object is reduced relative to the contribution of the feature to the two or more scanning interferometry signals;
• analyzing the combined signal to determine information about the test object;
• and
• outputting the information about the test object,

An apparatus comprising:

• an interferometer configured to direct test light to a test surface over a range of illumination angles and subsequently combine it with reference light to form an interference pattern, the test and reference light being derived from a common source;
• the common source;
• a multi-element detector;
• one or more optics configured to direct at least a portion of the combined light to the detector so that different elements of the detector correspond to different illumination angles of the test surface by the test light;
• and
• an electronic processing system coupled to the multi-element detector, which during operation receives two or more scanning interference signals from the multi-element detector, combines the two or more scanning interference signals to form a synthetic interference signal and analyzes the synthetic interference signal to determine information about the test object.