At the 2024 IEEE Symposium on VLSI Technology & Circuits, imec, a renowned research hub in nanoelectronics, unveiled electrically functional CMOS CFET devices with stacked bottom and top source/drain contacts for the first time. The results were achieved with both contacts patterned from the frontside, demonstrating the feasibility of relocating bottom contact formation to the wafer backside, significantly boosting top device survival rate from 11% to 79%.

Imec's logic technology roadmap includes the introduction of complementary FETs (CFETs) in A7 node device architectures, aiming to reduce standard cell track heights without performance degradation. Among various approaches, monolithic integration is considered the least disruptive method for integrating n- and pMOS vertically stacked structures. At the symposium, imec showcased functional monolithic CMOS CFET devices with stacked top and bottom contacts at 18nm gate length, 60nm gate pitch, and 50nm vertical separation between n and p devices. These devices demonstrated electrical functionality, utilizing a common gate and connecting top and bottom contacts from the frontside.

Imec's proposed process flow includes the middle-dielectric isolation (MDI) and the stacked bottom and top contacts. The MDI module, a pioneering initiative by imec, isolates top and bottom gates and facilitates differentiation in threshold voltage settings between n and p devices. The innovative source/drain recess etch with 'in-situ capping' allows MDI-first approach, contributing to superior process control. Another pivotal module is the formation of stacked source/drain bottom and top contacts, separated by dielectric isolation.

Challenges encountered during bottom contact development from the frontside led imec to explore moving bottom contact formation to the wafer backside, leading to a substantial increase in top device survival rates. Ongoing research aims to identify the optimal contact routing approach, with backside bottom contact formation emerging as an attractive option for the industry.