Metrology for characterization of wafer thickness uniformity during 3DS-IC processing

There is a constant desire to increase substrate size in order to improve cost effectiveness of semiconductor processes. As the wafer diameter has increased from 2” to 12”, the thickness has remained largely the same, resulting in a wafer form factor with inherently low stiffness. Gravity induced deformation becomes important when using traditional metrology tools and mounting strategies to characterize a wafer with such low stiffness. While there are strategies used to try to reduce the effects of deformation, gravitational sag provides a large source of error in measurements. Furthermore, glass is becoming an important material for substrates in semiconductor applications and metrology tools developed for use for characterizing silicon are inherently less suitable for glass. Using a novel mounting strategy and a measurement technique based on optical interference provides an opportunity to improve on the methodologies utilized to characterize wafer flatness (warp, bow) and total thickness variation (TTV). Not only can the accuracy of the measurement be improved, using an interference based technique allows for full wafer characterization with spatial resolution better than 1 mm, providing substantially more complete wafer characterization.