Direct Quantification of Bonding Strength Inside Actual Semiconductor Devices, Previously Unmeasurable

[Summary]
Toray Research Center, Inc. (Headquarters: 1-7-2 Nihonbashi Honcho, Chuo-ku, Tokyo; President: Yoshiki Makabe; hereinafter "TRC") has launched an analysis service that directly quantifies the "bonding strength inside actual devices," which has been a challenge in semiconductor hybrid bonding (*1).
As semiconductors become higher performance and more highly integrated, ensuring the reliability of bonding interfaces has become a critical issue. This technology is based on an interface strength evaluation method using the nanoindentation method (*2) systematized by the Inoue Laboratory at Yokohama National University. To apply this to actual devices, TRC has developed a pretreatment technology (polishing and etching) that selectively exposes the bonding interface. This makes it possible to directly evaluate the bonding interface inside laminated structures, which was previously difficult to assess, contributing to improved reliability of hybrid bonding, elucidation of defect causes, and enhanced yield improvement.

[Background]
In recent years, with the advancement of AI and high-performance computing, semiconductors have become higher performance and denser, increasing the importance of 3D packaging technology that stacks and connects chips. Hybrid bonding, a core technology, enables high-density and low-resistance connections by simultaneously connecting metal wiring and bonding insulating layers. However, the bonding interface is extremely fine, on the nano to micrometer scale, and is buried inside the device, making it difficult to evaluate bonding strength in an actual device state.
Conventional bonding strength evaluation methods (e.g., the DCB method (*3)) are limited to evaluations on simplified model samples or wafer edges, making it difficult to reflect the actual device structure including multilayer structures and buffer layers, and thus to grasp the reliability of the bonding interface in a realistic manner. Therefore, a bonding strength evaluation technology applicable to actual devices has been required.

[Technology and Analysis Examples]
In response, TRC has developed a proprietary technology to apply this evaluation method to actual devices, based on knowledge of interface strength evaluation using the nanoindentation method. Specifically, they have established a pretreatment technology (a combination of polishing and etching) that selectively exposes the bonding interface to be evaluated, according to the laminated structure and material properties of the sample. This enables the evaluation of bonding interfaces inside actual devices, which were previously buried and difficult to evaluate directly.
As shown in Figure 1, when a nanoindentation test is performed on the exposed bonding interface, delamination occurs at the interface as the indenter is pressed in. By observing and quantifying the shape and size of this delamination area, the bonding strength is evaluated. If the bonding strength is low, the interface delaminates easily, resulting in a larger delamination area; conversely, if the strength is high, delamination is suppressed, and the delamination area is smaller. This relationship allows for an intuitive understanding of differences in strength within actual device structures.

[Future Outlook]
This service will enable the following:
- Quantification of bonding strength variation
- Identification of bonding defect locations and mechanisms
- Direct feedback for optimizing bonding conditions
Through this technology, TRC will support the development of advanced packaging technologies at semiconductor manufacturers and material manufacturers, and promote the advancement of interface evaluation technology.

[Glossary]
(*1) Hybrid bonding: A semiconductor connection technology that simultaneously achieves electrical connection between metal wiring (mainly Cu electrodes) and bonding of surrounding insulating layers (such as SiO₂). It enables high-density and low-resistance connections by directly connecting fine electrodes, contributing to higher performance in 3D packaging.
(*2) Nanoindentation method: A method in which a diamond indenter is pressed into the sample surface, and the elastic modulus (Young's modulus) and hardness are calculated from the relationship between load and displacement. It is widely used for evaluating the mechanical properties of thin films and micro-regions.
(*3) DCB method (Double Cantilever Beam method): A method in which a sample is wedge-opened to cause delamination, and the bonding strength is evaluated from the propagation behavior. It is mainly applied to simple laminated structures or model samples.