Influence of etching profile on pattern collapse in complementary metal oxide semiconductor image sensor pixel with deep trench isolation

With the trend toward smaller pixel size for CMOS image sensors, increasingly high aspect ratios are needed for deep trench isolation features, which can lead to pattern collapse during the postetch wet cleaning process. The focus of this study is on a critical structure in a CMOS pixel for which such adhesion events were encountered, consisting of two adjacent silicon pillars. Finite element analysis was used to estimate the equilibrium adhered length after drying. We find that, assuming a vertical trench profile, no computed stable equilibrium position exists that is compatible with the measured adhered length. A proposed explanation for this discrepancy is an influence of the etching profile, as a slight tapering angle is typically introduced at the top to facilitate trench filling. We show that this may increase the risk of pattern collapse by allowing more conformal contact. As higher trench aspect ratios are introduced for image sensors, such issues could become an even more important detractor, leading to an increasing need for numerical models to anticipate potential issues and secure process integration.