Method and apparatus for determining surface layer thickness using continuous multi-wavelength surface scanning
An inspection tool includes an illumination element for directing light beams onto a workpiece at differing wavelengths or at differing angles of incidence or combinations thereof. Such beams producing reflected light and scattered light optical signals. A scanning element and optical detector elements are provided. The optical detector elements receive reflected light signals and scattered light signals. Circuitry for receiving the reflected light signals and scattered light signals are used to determine thickness values for partially transmissive layers formed on the workpiece and correct for the effects of the thickness of the partially transmissive layer so that said signals can be used to identify and characterize defects of the workpiece. Moreover, the invention includes descriptions of methods for accomplishing such inspections.
1. A method for conducting surface inspections comprising:
providing a workpiece having a at least one partially transmissive layer formed thereon;
scanning the surface of the workpiece with at least two light beams to generate at least two reflected light signals having light intensity values;
detecting the light intensity of the reflected light signals; and
processing only the detected light intensity values of the reflected light signals to obtain thickness measurements for the at least one partially transmissive layer formed at the surface of the workpiece, wherein said processing comprises:
accessing a parametric reflected intensity curve that associates thickness of the at least one partially transmissive layer with two detected light intensity values for the reflected light signals generated by the at least two light beams; and
comparing the two detected light intensity values with the light intensity values of the parametric curve to obtain thickness measurements for the at least one partially transmissive layer.
2. The method of claim 1 wherein comparing the two detected light intensity values with the light intensity values of the parametric reflected intensity curve to obtain thickness measurements includes disambiguating ambiguous measurements by scanning toward a portion of the workpiece having a non-ambiguous thickness measurement to resolve the ambiguity and then obtaining a thickness measurement for the at least one partially transmissive layer.