Accelerating social implementation of next-generation 3D stacked semiconductor technology

The Tokyo Institute of Technology WOW Alliance^[1] and NCKU^[2] have agreed on a technical partnership for social implementation of Next-generation 3D integrated technology^[3] based on BBCube (Bumpless Build Cube)^[4]. This is the first Japan-Taiwanese academia alliance for next-generation 3D integration processes. Plans to achieve commercialization will proceed through the BBCube business alliance. NCKU has joined the Tokyo Tech WOW Alliance to participate in research and development of next-generation 3D technology. This includes the construction and testing of a BBCube pilot manufacturing line within the university, as well as developing the necessary human resources for its operation.

A BBCube business alliance was concluded with TEX, a company established by Professor Takayuki Ohba of Tokyo Tech, Institute of Innovative Research. TEX will provide the transfer of WOW Technology^[5], and COW Technology^[6], both based on the BBCube platform, to NCKU. Processes, equipment, and materials will be utilized, based on the research achievements of the Tokyo Tech WOW Alliance.

Currently, many universities and affiliated companies are participating in the Tokyo Tech WOW Alliance, and are conducting research and development of next-generation semiconductor 3D technology. As industry is entering an age when production yields of cutting-edge semiconductor devices become saturated, due to the increase in invisible defects at the atomic scale, COW chiplet integration and WOW wafer stacking technology becomes ever more important. This Japan-Taiwan collaboration has been formed in response to this need, making product and market orientation seamless, It is expected to promote basic development of post-miniaturization 3D integrated technology and social implementation, as well as strengthening the semiconductor supply chain. The plan is to establish a pilot manufacturing line for R&D at NCKU by the end of 2023, and sequentially apply WOW and COW production processes as BBCube platform technologies.

The Tokyo Tech WOW Alliance, whose aim is to help implement research achievements into society (practical application), established TEX as a Tokyo Tech startup company in 2018. TEX has now concluded a business agreement with NCKU's Innovation Headquarters to establish a BBCube business alliance at NCKU (Figure 1). Research achievements, resulting from the Tokyo Tech WOW Alliance, will be provided through its affiliated company, TEX Taiwan.

OSAT (Out-sourced Semiconductor Assembly and Test), device manufacturers, and other companies are planning to participate in the BBCube business alliance, which is expected to further accelerate social implementation.

A trial manufacturing line for WOW/COW technologies, based on BBCube architecture, will be built at NCKU, and R&D will be conducted with actual equipment, starting from the end of 2022 (phase 1). An integrated line will be established in FY2024 (phase 2). Since it will be possible to simultaneously integrate chiplets at the wafer scale, this integrated line will allow for seamless verification at both the front and back ends.

BBCube, developed by the WOW Alliance, makes it possible to minimize the wiring for all constituent devices (memory, CPU, capacitors, etc.) (Figure 2). For example, the distance between capacitors and devices can be reduced from the millimeter to micron level. The WOW Alliance is cooperating on design, process, equipment, and material development, including thermal design, with the aim of miniaturizing beyond "thumb size."

Power consumption with HBM^[7], which is used in high-end devices such as servers, is about one-fifth of conventional DDR5^[8]. For 2.5D and 3D using BBCube, when the transfer bandwidth reaches the terabyte per second level, the transfer energy per bit is one order of magnitude smaller than with HBM, and the system energy consumption is the world's lowest (less than 10 W) (Figure 3).

Up to now, wiring connection requires a protruding bump electrode, formed by plating on the electrode. The resistance of the bump and its high density are an obstacle to speeding up and stacking.

Bumpless technologies such as BBCube are able to avoid such issues. The WOW Alliance has developed technology for thinning wafers to the micron level, and technology for stacking multiple wafers via direct vertical interconnects using TSV^[9] (Figure 4). Applying these technologies to DRAM, large memory capacity can be achieved by increasing the number of stacked wafers. This is especially applicable to AI, which requires a large amounts of memory. Under this alliance, bumpless COW has also been developed that makes it possible to have minimum wiring between chips and wafers, allowing for chiplet integration by combining different devices.

Bumpless interconnects in the BBCube system is a technology that minimizes the vertical connections between chips. Since there are no bumps, the resistance and electrical capacitance are small, and when combined with wafer thinning technology (Si thickness < 4 microns), the TSV wiring length can be the same as the wafer thickness, which improves the electrical properties. Since there are no bump restrictions, the wiring density of TSVs can be increased along with the advancement of lithography, minimizing the space required for TSV on the chip, which had been an issue in the past. This allows better distribution of the TSV inside the chip, and simultaneously minimizing the wiring between chips.

In the case of DRAM chips, if the memory is divided inside the chip and then wired vertically in parallel, typically long-distance signals and power can be transferred in parallel over a short distance instead, via vertical interconnects, proportional to the number of divisions, something that traditionally had to be done on the surface of the chip. This parallelism allows lower power consumption, while maintaining the same transmission bandwidth at lower transmission speeds.

Another advantage of high-density bumpless TSV is better heat dispersion. Conventionally, long TSV wiring and bumps raise the temperature inside the chip as it is stacked, due to long TSV wiring and bumps. The BBC technology greatly relaxes this limit on stacking.