Abstract
The focus of this study is a detailed characterization of hybrid Cu/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wafer-to-wafer bonding interconnects after reliability testing. Hybrid bonding (or direct bond interconnect) is a technology of choice for fine pitch bonding without microbumps. The main challenge of the hybrid bonding technology is the preparation of a clean Cu/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> surface with controlled Cu dishing. The Cu/Cu interface after hybrid bonding and after reliability testing was investigated by electron backscatter diffraction (EBSD) in this study. The Cu interconnects (diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 ~\mu \text{m}$ </tex-math></inline-formula> and pitch <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$18 ~\mu \text{m}$ </tex-math></inline-formula> ) enclosed by SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were formed by wafer-to-wafer bonding. The small diced stacks were used for subsequent reliability tests. Three types of tests were carried out: temperature shock test (TST) at −40 °C/125 °C up to 1000 cycles, isothermal storage at 150 °C, 300 °C, and 400 °C, and multiple bonding cycles. The results include a comprehensive estimation of the changes in grain diameter, grain boundaries, and texture of the interconnects. All samples showed good reliability and stayed intact after all tests. Grain refinement was observed after TST, storage at 150 °C, and multiple bonding cycles compared to the state after bonding. Grain growth was found for the storage at 300 °C/400 °C (up to 6 h). No significant changes in texture was found after the reliability tests. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\langle 111\rangle $ </tex-math></inline-formula> direction with its characteristic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\langle 115\rangle $ </tex-math></inline-formula> twin orientation is the dominating orientations perpendicular to the bonding interface. The migration of 60° twin boundaries has been observed after reliability testing. The results indicate a rotation toward nearby high-angle grain boundaries (HAGBs, 60°–45°).
Highlights
SEMICONDUCTOR technology for high performance applications requires high I/O number, which can be realized by tiny interconnects provided by microbumps or Cu pads at very small pitch
The current study focuses on the investigation of the interface and microstructure of hybrid bond interconnects after the reliability assessment such as isothermal aging and multiple bonding cycles
The brightness of the measurements indicates that the electron backscatter diffraction (EBSD) mapping quality varies for each state and is linked to the individual quality of each crosssection
Summary
SEMICONDUCTOR technology for high performance applications requires high I/O number, which can be realized by tiny interconnects provided by microbumps or Cu pads at very small pitch. Conventional microbumps with solder are limited to a minimum of 10 μm pitch because of bump bridging or solder shorts (squeeze out of solder from the bonding area) [1,2,3,4]. Hybrid bonding appears to be a more promising technology for 3D stacking in terms of increasing the interconnect density and scaling the interconnect size. It avoids the use of solder caps and creates a full Cu/Cu interconnect, which is enclosed by SiO2. The risk of electrical shorts can be completely eliminated with this technology
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