3D Stacking Process Technologies for Advanced CMOS Image Sensors

Recently, in the field of CMOS Image Sensor (CIS), there has been high demand for new functions that can respond to various photo-taking scenes. To meet the market needs, we have started the mass-production of Back-Illuminated-CIS (BI-CIS) and stacked BI-CIS in 2008 and 2012 respectively[1] [2]. Those CISs have effectively improved the sensitivity and functionality compared with the conventional Front-Illuminated-CIS. In the early type of stacked BI-CIS the Through-Silicon-Via(TSV) technology was used to electrically connect upper CIS chip and lower logic chip[2]. In 2015, to fabricate high-performance and highly productive stacked BI-CISs, we have developed a novel 3D stacking technology called Cu-Cu hybrid bonding [3]. The fine-pitch Cu-Cu hybrid bonding has achieved the wide dynamic range and high-speed processing[4]. In 2019, we also have announced a role out of back-illuminated InGaAs image sensor with 1280×1040 pixels at 5-um pitch by using Cu-Cu hybridization connecting different materials, a III-V InGaAs/InP of photodiode array (PDA), and a silicon readout integrated circuit (ROIC)[5]. This sensor has achieved low dark current and high sensitivity for wavelengths ranging from visible to short-wavelength infrared (SWIR). In 2020, we also have successfully attached the unique logic circuits with edge processing functionality enabling high-speed edge AI processing. Such new intelligent vision sensor is worthy of a special mention as an example of functional expansion[6]. In 2022, by using both CuCu hybrid bonding and TSV technologies, the three-layer stacked device has been proposed [7] [8]. In 2023, we also have announced the over-400mm ² -large chip-on-wafer Cu-Cu hybrid bonding process[9]. Such multi-layer stacked CISs have been expected to append the various functions. As shown in figure, our 3D stacking process technologies have successfully evolved the CISs from viewpoint of performance improvement, performance expansion and functional expansion. In the very near future, our cutting-edge devices will be key not only for mobile gadgets, surveillance, factory automation, autonomous driving, medical imaging but also for robotics and AI system. We are convinced that our CISs will help realize a safer and more convenient society.