Analysis of stress in spherically bent thinned image sensors

Thinning of silicon ship allows bending of the chip during assembly. A curved surface of an imager chip allows reducing the complexity of an objective, i.e., the number of lenses, and by that further miniaturization of optic modules is possible. This paper reports the results of investigations to analyze the process induced stress within thinned image sensors using 50 µm thick silicon chips. At first, the stress generated in the bending process of the chip is evaluated by finite element (FE) simulations. Mechanical simulations show that compressive stress is concentrated in the center and on the edges of the chip. These results are validated by experimental investigation: the thinned silicon chips, are bonded on a spherically curved glass substrate, of radius 60 mm using an adhesive interconnect. The resulting stress is measured by micro-Raman spectroscopy (µRaman). The stress distribution on the chip obtained by the FE simulation is qualitatively equal to the distribution measured by the µ-Raman. However, a difference in the measured value of stress generated on the surface of the chip from the simulative approach are observed, which reveals that the process sequence and the bonding parameter in the adhesive bonding play an important role.