A Polarization-based frequency shifting interferometry for inspecting transparent objects in microelectronics manufacturing

The normal FSI system shows good performance about target objects with specular surface such as semiconductor dies or flat panel glasses. But, if there are transparent objects on test surfaces, their optical polarization characteristics usually make the observed interference fringes degraded. When illuminated light reflects or penetrates, the direction of polarization of light rotates depending on the polarization characteristic of objects. The rotation of direction of polarization causes difficulty in measurement. In this paper, a PFSI (Polarization-based Frequency Shifting Interferometer) system is proposed, which applies the polarization analysis method to the conventional FSI system. First, the PFSI system is proposed for robust measurement to object. Low contrast problem of interference fringe due to polarization rotation of acquired fringe image can be solved by using polarization adjustment. In addition, light distribution of object beam and reference beam can be controlled. So, reflected light intensities of the reference beam and object beam can be made similar for conspicuous interference signals. Second, using PFSI system, we can measure the transparent object. For example, the height of flux and the height of die of flux bottom side can be measured in the same system. In case of measuring the height of the flux, the multi-layer reflections are generated in the surface and bottom side of flux. Three interference signals are observed when transparent flux is deposited on the PCB surface. By controlling the polarization of the system, the height of flux and the height of bottom side of flux can be measured simultaneously. Third, the signal processing acceleration method for fast height calculation is proposed for the PFSI, based on parallel processing architecture, which consists of parallel processing hardware and software called GPU(Graphic Processing Unit) and CUDA(Compute Unified Device Architecture). As a result, the processing time reaches into tact time level of real-time processing. Finally, the proposed system is evaluated in terms of accuracy and processing speed through a series of experiment and the obtained results show the effectiveness of the proposed system and method.