SAB-Enabled Room Temperature Hybrid Bonding

3D integration is gaining more and more interest for a large panel of applications including CMOS Image Sensor, High Performance Computing, DRAM including HBM stacks and display. Image sensors based on hybrid bonding 3D stacking are the state-of-the-art in imaging applications[1]. Highest integration densities with micron-size pixels are achieved using copper-oxide hybrid bonding in advanced CMOS Image Sensor (CIS) processes. Recently, functional imagers with sub 1 µm pitch of hybrid bonding interconnection have shown great performances in term of reliability [2]. Hybrid bonding low temperature (< 300°C) is increasingly requested by the industry to reduce thermal budget of sensible chips integration such as Image sensors, displays, or memory. Surface Activated Bonding (SAB) is one of the most promising method to enable very low temperature hybrid bonding as very early proposed by Suga et al [3]. Up to know, they have been mainly performed using modified SAB technology [4] or with Oxide recess [5]. In this study, 200 mm wafers with 2.5 µm copper pads spaced 2.5 µm apart (5 µm pitch) in a silicon oxide matrix are used. SAB uses argon atom bombardment as surface activation. This bombardment is performed in UHV (ultra- high vacuum) at room temperature during several tens of seconds. Surface copper oxide is removed during the activation, leaving dangling metallic bonds on copper pads surface. Under UHV, two identical wafers are then immediately bonded at RT under a low pressure of 0.3 MPa. Noteworthy, no precise alignment is performed for this hybrid bonding. In order to better understand bonding behavior, a first part of the study focuses on the etching effect due to argon bombardment. AFM (Atomic force microscope) inspections are performed on such activated hybrid surfaces. Then the SAB bondings are characterized by SAM (Scanning Acoustic Microscopy) with or without annealing at 150°C and 200°C during 2 hours. Once the top silicon is removed by grinding, further characterizations such as TEM (Transmission Electron Microscopy) and ACOM (Automated Crystal Orientation Mapping) are carried out. For comparison purposes, standard atmospheric hybrid bonding samples are also fabricated at an annealing temperature between 200°C and 300°C. They are characterized in the same way. SAB for hybrid bonding proves its relevance in this study. Indeed, all bonded samples are successfully grinded and thinned out leaving only the damascene structure (which thickness is around 1