Island Coalescence during Film Growth: An Underestimated Limitation of Cu ALD

The processes involved in the low-temperature (30-60 degrees C) growth of ultrathin copper films such as those required in complementary metal oxide semiconductor (CMOS) processing are studied using atomic layer deposition (ALD) combined with simple modeling. The ALD growth is performed using the copper precursor Cu(II)-bisaminoalkoxide CTA-1 together with a hydrogen plasma. It is found that the growth processes differ significantly between the substrate materials used. On substrates such as Si and TaN, the formation of isolated islands is observed. The coalescence of smaller islands, which were initially formed in very high density on the substrate, is found to be the main reason for this interesting growth mode. The general limitations imposed on the use of ALD as a method for the deposition of ultrathin Cu films resulting from the findings presented here are described.