OKI, in collaboration with Nisshinbo Micro Devices, has successfully achieved three-dimensional (3D) integration of thin-film analog ICs using Crystal Film Bonding (CFB) technology. This technology applies to heterogeneous integration, which enables the integration of various types of semiconductor devices, such as analog ICs. The two companies plan to push ahead with product development based on this technology, targeting mass production by 2026.
With a growing demand for enhanced functionality of semiconductor devices driven by the spread of AI and autonomous driving in recent years, attention has focused on chiplet technology. Rather than integrating all functions into a single chip, chiplet technology divides functions into smaller chips, which are then integrated using 2.5D and 3D assembly techniques to achieve large-scale functionality integration at lower cost and with reduced space requirements. This division also improves yield and allows the optimal semiconductor manufacturing processes to be selected for each function, avoiding cost increases.
Applying the conventional chiplet technology to the 3D integration of low-end analog ICs entailed the following two challenges:
The first was to develop a 3D integration technology compatible with legacy processes. 3D integration involves stacking chips vertically, which significantly improves integration density and miniaturization. However, electrical bonding between stacked chips typically requires Through Silicon Via (TSV) technology, which entails significant investment into equipment and advanced process development. Implementing the process using conventional technologies thus previously incurred prohibitively high costs.
The second was to prevent the noise (crosstalk noise) generated by electrical signal interference. Analog ICs process continuous signal variations rather than the simple 0s and 1s of digital signals in digital ICs, and handle higher voltage signals compared to digital ICs, making them more susceptible to crosstalk noise. Stacking ICs also increases the proximity between circuit layers, increasing crosstalk noise.
To address the first challenge, OKI developed a new thin-film chiplet technology, consisting of lifting off and bonding (CFB process) followed by rewiring. It completely protects the analog IC’s functionality and lifts off only the functional layer from the substrate. The thin-film analog IC is then bonded to another analog IC, enabling the successful 3D integration of thin-film analog ICs. Unlike 3D integration based on conventional TSV technology, which results in IC chip thicknesses in the range of tens to hundreds of micrometers, this technology produces thin-film analog ICs with thicknesses on the order of only several micrometers, allowing rewiring using conventional semiconductor lithography. This rewriting enables the use of low-cost legacy processes, which are widely available.
To resolve the second challenge, proprietary localized shielding technology developed by Nisshinbo Micro Devices was applied to analog ICs. This technology shields only specific areas between upper and lower chips affected by interference, not the entire chip, suppressing signal interference without degrading circuit functionality. The technology draws on low-noise analog IC technology accumulated over the years by Nisshinbo Micro Devices for high-quality audio ICs, and has been proven to operate normally with crosstalk noise suppressed even under a high voltage output of 20 Vpp.
The successful 3D integration of thin-film analog ICs in the collaboration between Nisshinbo Micro Devices and OKI makes it possible to offer analog solutions by combining a wide range of different analog ICs. “Applying OKI’s thin-film chiplet technology to the heterogeneous integration of all kinds of digital, analog, optical, power, and sensor semiconductor devices will also contribute to the development of new semiconductor devices,” commented Kei Kato, an Executive Officer and Head of Global Marketing Center at OKI.
