Heterogeneous Integration of BEOL Logic and Memory in a Commercial Foundry: Multi-Tier Complementary Carbon Nanotube Logic and Resistive RAM at a 130 nm node

The inevitable slowing of two-dimensional scaling is motivating efforts to continue scaling along a new physical axis: the 3rd dimension. Here we report back-end-of-line (BEOL) integration of multi-tier logic and memory established within a commercial foundry. This is enabled by a low-temperature BEOL-compatible complementary carbon nanotube (CNT) field-effect transistor (CNFET) logic technology, alongside a BEOL-compatible Resistive RAM (RRAM) technology. All vertical layers are fabricated sequentially over the same starting substrate, using conventional BEOL nano-scale inter-layer vias (ILVs) as vertical interconnects (e.g., monolithic 3D integration, rather than chip-stacking and bonding). In addition, we develop the entire VLSI design infrastructure required for a foundry technology offering, including an industry-practice monolithic 3D process design kit (PDK) as well as a complete monolithic 3D standard cell library. The initial foundry process integrates 4 device tiers (2 tiers of complementary CNFET logic and 2 tiers of RRAM memory) with 15 metal layers at a ~130 nm technology node. We fabricate and experimentally validate the standard cell library across all monolithic 3D tiers, as well as a range of sub-systems including memories (BEOL SRAM, 1T1R memory arrays) as well as logic (including the compute core of a 16-bit microprocessor) - all of which is fabricated in the foundry within the BEOL interconnect stack. All fabrication is VLSI-compatible and leverages existing silicon CMOS infrastructure, and the entire design flow is compatible with existing commercial electronic design automation tools.