Method and apparatus for the rapid acquisition of data in coherence scanning interferometry

A method of determining a height of a point on an object surface, comprising the steps of:

• (a) positioning an object surface along an optical axis so that a predetermined point on the object surface is optically aligned with an imaging device;
• (b) producing an interference pattern of the object surface location by means of an interferometer to produce an intensity value at a pixel of the imaging device;
• (c) varying an optical path difference between the object surface and a reference surface of the interferometer through a predetermined range including a position of zero optical path difference for said point;
• (d) receiving at said imaging device pixel, as the optical path difference is varied through said predetermined range, a plurality of intensity values of the interference pattern at randomly-selected consecutive data points spaced along said range by a predetermined phase difference;
• (e) applying said received intensity values to an interference discriminator for identifying in said intensity values a region of high frequency interference data;
• (f) storing a plurality of the received intensity values in the region of high frequency interference identified by said interference discriminator;
• (g) storing concurrently with at least one of said stored intensity values the relative position along said predetermined range of said at least one stored intensity value;
• and
• (h) determining the height of said predetermined object surface location by calculating, using said stored intensity values and said stored relative position, the position along said range of zero optical path difference for said point.

A method of profiling a surface of an object, comprising the steps of:

• (a) positioning an object along an optical axis so that a predetermined feature of the object surface is optically aligned with an imaging device;
• (b) producing an interference pattern of the object surface by means of an interferometer and an illumination source to produce an intensity value on the imaging device for each pixel of an image of the object surface, each said pixel corresponding to a location on the object surface;
• (c) varying an optical path difference between the object surface and a reference surface of the interferometer through a predetermined range including a position of zero optical path difference for each said location;
• (d) receiving at each said pixel intensity data as the optical path difference is varied through said predetermined range;
• (e) applying said received data for each said pixel to an interference discriminator operable for locating the presence of interference in said received data;
• (f) storing for each said pixel a plurality of intensity values of the interference pattern, as located by the interference discriminator, at randomly-selected consecutive data points spaced along said predetermined range by a predetermined phase difference;
• (g) storing, for each said pixel, concurrently with at least one of stored intensity values for said pixel the relative position along said predetermined range of said at least one stored intensity value;
• and
• (h) determining a difference in height between two selected locations on the object surface, said two selected locations corresponding to respective first and second imaging device pixels, by calculating for each of said first and second pixels, using said stored intensity values and stored relative position, the position along said range of zero optical path difference for the corresponding selected location.