Imec tackles interconnect heating at 1nm
Scaling down the logic technology roadmap to 1nm and beyond will require the introduction of new conductor materials in the most critical layers of the back-end-of-line.
Researchers at imec have shown stoichiometric AlCu and Al2Cu films with resistivity as low as 9.5µΩcm for new conductors in advanced semi-damascene interconnect integration schemes combined with airgaps.
In this combination, however, Joule heating effects are becoming increasingly important, and the heating effect has been shown by the lab in a 12-layer back-end-of-line (BEOL) structure in a paper at the 2021 International Interconnect Technology Conference (IITC 2021) this week in Kyoto, Japan.
Aluminium and ruthenium (Ru) have lower resistivity than conventional elemental metals such as copper, cobalt or molybdenum. at 1nm. Imec has investigated the resistivity of thin films of aluminides, including AlNi, Al3Sc, AlCu and Al2Cu. At 20nm thickness and above, all PVD-deposited films showed resistivities comparable to or lower than molybdenum. The lowest resistivity of 9.5µΩcm was achieved for 28nm films of AlCu and Al2Cu – a value that goes below that of Cu. The experiments also indicated challenges for the studied aluminides, such as the control of the film stoichiometry and surface oxidation.
With 3nm chips planned for later this year and plans for 2nm technology accelerating, the attention of researchers globally is turning to the 1nm node using ‘intermetallic’ materials such as ruthenium.
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However using these intermetallic compounds in advanced semi-damascene process technologies needs direct etch of a patternable metal to achieve higher aspect ratio lines. Further improvements in RC delay can be obtained by gradually introducing partial or full airgaps in between the metal lines. Replacing conventional low-k dielectrics with electrically isolating airgaps is expected to reduce capacitance at scaled dimensions. But airgaps have an extremely poor thermal conductivity, which raises concerns for Joule heating at operation conditions.
Imec has quantified this challenge by performing Joule heating ‘calibration’ measurements at local 2-layer metal interconnect level and projecting the results to a 12-layer BEOL structure through modeling. The study predicts a 20 percent increase in temperature with airgaps. The density of the metal lines was found to play an important role: higher metal density showed to help reducing the Joule heating.
“These insights are key to improve semi-damascene metallization schemes as an interconnect option for 1nm and beyond,” said Zsolt Tokei, imec fellow and program director of nano-interconnects at imec. “Besides, imec is expanding the interconnect roadmap with other options, including hybrid metallization and new middle-of-line schemes, while solving critical challenges related to process integration and reliability. These and other topics are presented at this year’s IITC in 9 papers, illustrating the impact of our global R&D efforts to create solutions for sub-2nm interconnect scaling.”
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