SK Hynix has started recruiting design personnel for logic semiconductors such as CPUs and GPUs, reports Joongang.co.kr. The company is apparently looking to stack HBM4 directly on processors, which will not only change the way logic and memory devices are typically interconnected but will also change the way they are made. In fact, if SK Hynix succeeds, this may transform the foundry industry.
Nowadays HBM stacks integrate 8, 12, or 16 memory devices as well as a logic layer that acts like a hub. HBM stacks are placed on the interposer next to CPUs or GPUs and are connected to their processors using a 1,024-bit interface. SK Hynix aims to put HBM4 stacks directly on processors, eliminating interposers altogether. To some degree, this approach resembles AMD's 3D V-Cache, which is placed directly on CPU dies. But HBM will feature considerably higher capacities and will be cheaper (albeit slower).
SK Hynix is reportedly discussing its HBM4 integration design method with several fabless companies, including Nvidia. It's likely that the two companies will jointly design the chip from the beginning and produce it at TSMC, who will also put SK Hynix's HBM4 device on logic chips using a wafer-bonding technology. A joint design is inevitable in order for memory and logic semiconductors to work as one body on the same die.
The HBM4 memory will use a 2,048-bit interface to connect to host processors, so interposers for HBM4 will be extremely complex and expensive. This makes the direct connection of memory and logic economically feasible. But while placing HBM4 stacks directly on logic chips will somewhat simplify chip designs and cut costs, this presents another challenge: thermals.
Modern logic processors, such as Nvidia's H100, consume hundreds of watts of power and dissipate hundreds of watts of thermal energy. HBM memory is also rather power-hungry. So cooling down a package containing both logic and memory could require very sophisticated methods, including liquid cooling and/or submersion.
"If the heating problem is solved two to three generations later than now, HBM and GPU will be able to operate like one body without an interposer," said Kim Jung-ho, a professor in the Department of Electrical and Electronics at KAIST.
But the integration of memory directly on processors will also change how chips are designed and made. Producing DRAM using the same process technology as logic and on the same fab will guarantee ultimate performance, but will increase memory costs dramatically, so this is not an option that is seriously considered right now. Nonetheless, it looks like memory and logic are poised to get closer, both literally and on the process technology level.
"Within 10 years, the 'rules of the game' for semiconductors may change, and the distinction between memory and logic semiconductors may become insignificant," an industry insider told Joongang.co.kr.
