# Metrology for Measuring Bumps in a Protection Layer Based on Phase Shifting Fringe Projection
## 1 Introduction
In recent years, the under bump metallurgy (UBM) layer used in conventional wafer-level chip scale packages (WLCSP) has been eliminated in favor of the UBM-free integration (UFI) fan-in WLCSP; this improves the bond between the solder ball and copper redirect pad and reduces the cost and package profile. However, measurement of the bump height has become more challenging with the deposition of a polymer protection layer (PL) in advanced WLCSP processes. The PL, which is usually polyimide (PI) or polybenzoxazole (PBO) is used to secure the solder in place; it is semitransparent and varies in thickness, which introduces errors in measurements of the bump height with the triangulation technique [20]. Current manufacturers have worked around this problem by applying a light interferometer profiler to provide a bump height offset value to the triangulation measurement. The PL thickness is assumed to be obtained by measurement through the film to the substrate. However, when there is more than one semitransparent layer (e.g., an oxide insulation layer between the top PL and bottom silicon substrate), this makes the interference image contrast returned by the interferometric measurements ambiguous along the layer depth. The reflection of the Si substrate has an especially strong influence. Because protection layers are typically a few to tens of microns thick, a vertical scan along a given range is time consuming; this makes it impossible to complete a 100% inspection in time, which limits the accuracy and repeatability of the bump height measurement. This paper presents a major effort towards improving the performance of 3D shape measurement systems based on fringe projection and phase shifting. A novel measurement algorithm was developed that is based on an area scan camera with a flexibly programmable region of interest (ROI) sensor area containing selectable rows to simultaneously acquire three fringe images with 120\({}^{\circ}\) phase shifts. Because only the pixel information within the ROI is stored in the memory, this tremendously reduces the data volume and processing and greatly accelerates the area scan acquisition. Sufficient information is retained to reconstruct a 3D model in a continuous scan mode instead of the conventional approach of taking three full-field fringe images sequentially in pause mode; this improves the measurement frame rate up to several hundredfold. In addition, the hardware system can be flexibly adjusted compared with the previous three-row line scan camera as long as the ROI contains the signal of the three rows with 120\({}^{\circ}\) phase shifting. A reflectometry sensor is implemented for measuring the onsite PL thickness to simultaneously and accurately provide the proper calibration map for measuring the bump height at each site. A spectroscopic reflection model and Fourier transform (FT) analysis algorithm, which are applicable in the case of a silicon substrate covered by a thick semitransparent PI layer on top of another semitransparent insulating film, were developed to accurately determine the thickness of each transparent film and their contributions to the bump height measurement error. Eliminating the strong reflection effect of the silicon substrate in optical measurements is not easy; spectroscopic reflection model fitting may be the best way to clearly identify the multiple interfaces between the air and silicon substrate.