Advanced overlay metrology for CIS bonding applications

On-product overlay (OPO) control is becoming more and more critical to successful 3D heterogeneous process integration which includes wafer-to-wafer (W2W) bonding. In this work, we will present novel overlay (OVL) methods and experimental metrology results on an advanced CMOS Image Sensor (CIS) W2W process. We will discuss metrology challenges such as thick wafer measurement, target design, precision, and accuracy. Different target designs will be presented and evaluated with the irArcher® 007 from KLA. We will demonstrate Total Measurement Uncertainty (TMU) of 1 nm on production wafers, with Tool-Induced Shift (TIS) values comparable to the current best-in-class metrology tool for single wafer OVL measurement and suitable throughput for High-Volume Manufacturing (HVM). We will highlight the importance of such precise OVL metrology tools to address the sub-50 nm OVL challenge in sub-micron pitch, wafer-to-wafer bonding applications. Inter-field and intra-field terms will be presented. Unexpected intra-field signatures will be discussed. Finally, the impact of bonding OVL on back-side lithography steps will be introduced opening interest for Advanced Process Control (APC) loops.