Surface profile metrology is preferably performed with two-arm interferometers such as Twyman-Green or Fizeau types that compare the test wave front reflected from the object surface under inspection with a reference wave front generated from an ideal model surface. In conducting the optical comparison testing, the deviation of the test wave front from the reference wave front should be bounded so that the spatial frequency of generated interferometric fringes falls within the Nyquist frequency limit of the photo-detector array used. Otherwise, the problem of $2\pie$ ambiguities arises in phase unwrapping with subsequent failure of accurate quantitative evaluation. This is in fact often the case when inspecting rough surfaces whose profile irregularities are of the order of magnitude of the wavelength of light.
In this study, as a new approach for profiling rough surfaces, we present a diffraction interferometer system that has been specially devised for absolute profile metrology. The absolute interferometer system is constituted with multiple sources of two-point diffraction and a CCD camera of two-dimensional photo-detectors array. The diffraction sources are deployed along the circumference of a circle, while the camera is located about at the center of the circle facing the object surface during measurement. Each diffraction source is an independent two-point-diffraction interferometer made of a pair of single-mode optical fibers. Two optical fibers are housed in a ceramic ferrule to emanate two spherical wave fronts by means of diffraction at their free ends. The two spherical wave fronts then interfere with each other and subsequently generate a unique fringe pattern on the test surface. A He-Ne source provides coherent light to the two fibers through a 2 × 1 optical fiber coupler, while one of the fibers is elongated using a piezoelectric PZT tube to produce phase shifting. The camera captures fringe patterns while switching on and off all the diffraction sources one by one in sequence to obtain multiple fringes images projected from different geometrical angles. Measurement has been performed for the warpage inspection of chip scale packages (CSPs) that are tape-mounted on ball grid arrays (BGAs) and backside profile of a silicon wafer in the middle of integrated-circuit fabrication process. when a diagonal profile is measured across the wafer, the maximum discrepancy turns out to be 5.6 micrometers with a standard deviation of 1.5 μm.
In conclusion, the two-point-diffraction interferometer system described so far is capable of profiling rough surfaces with high precision without enlarging the wavelength of light. Perhaps, the diffraction interferometer system may be compared to geometric optical profilometry techniques such as optical triangulation with projection of structured light or moire topography relying on geometrical intensity interference. However, the measurement principles of the diffraction interferometer system are based on wave optics, whose accuracy is therefore far better, to a level of a few micrometers, than the geometric optical techniques.
