This technology is applicable to heterogeneous integration, which enables the integration of various types of semiconductor devices, such as analogue ICs. The two companies plan to push ahead with product development based on this technology, targeting mass production by 2026.
With growing demand for enhanced functionality of semiconductor devices driven by the spread of AI and autonomous driving in recent years, attention has increasingly focused on chiplet technology where, rather than integrating all functions into a single chip, it divides functions into smaller chips, which are then integrated using 2.5D and 3D assembly techniques. This delivers a combination of large-scale functionality integration at lower cost and with reduced space requirements. It also improves yield and allows the optimal semiconductor manufacturing processes to be selected for each function, avoiding cost increases.
By applying the conventional chiplet technology to the 3D integration of low-end analogue ICs faced a number of significant challenges, according to OKI.
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 miniaturisation. However, electrical bonding between these stacked chips typically requires Through Silicon Via (TSV) technology, which requires significant investment into equipment and advanced process development. Implementing the process using conventional technologies had previously incurred prohibitively high costs.
The second was to prevent the noise (crosstalk noise) generated by electrical signal interference. Analogue 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 analogue IC’s functionality and lifts off only the functional layer from the substrate. The thin-film analogue IC is then bonded to another analogue IC, enabling the successful 3D integration of thin-film analogue ICs.
Unlike 3D integration based on conventional TSV technology, which results in IC chip thicknesses in the range of tens to hundreds of micrometres, this technology produces thin-film analogue ICs with thicknesses on the order of only several micrometres, allowing rewiring using conventional semiconductor lithography. This form or rewriting means that it is possible to use low-cost legacy processes, which are widely available.
To resolve the second challenge, proprietary localised shielding technology developed by Nisshinbo Micro Devices was applied to analogue 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 analogue 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 analogue ICs in the collaboration between Nisshinbo Micro Devices and OKI makes it possible to offer analogue solutions by combining a wide range of different analogue ICs.
“Applying OKI’s thin-film chiplet technology to the heterogeneous integration of all kinds of digital, analogue, 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.
Both OKI and Nisshinbo Micro Devices plan to develop new value-added products based on this technology, targeting mass production by 2026. Both companies will also explore partnerships and licensing opportunities.
