Ultrathin Al 2 O 3 films (1.3 nm or 2.5 nm) were deposited by plasma enhanced atomic layer deposition (PEALD) onto the low k (k = 2.5) material. By ALD deposition of an ultrathin Al 2 O 3 film onto the low k surface, the thermal stability and electrical performance for the Cu/Ta/Al 2 O 3 /low k/Si system were significantly improved in comparison to those with even thicker Ta and ALD Ru/TaN layers as diffusion barrier. The effective k value of the system was not degraded by adding the ultrathin Al 2 O 3 layer.With the continuing scaling down of the line pitch in very large scale integrated circuits, ultrathin Cu diffusion barriers are necessary to mitigate the increase in the effective resistivity of the Cu interconnects due to grain boundary scattering and interface scattering. Conformal deposition of thin films in high aspect ratio trenches using traditional physical vapor deposition (PVD) technique is becoming difficult. 1 Atomic layer deposition (ALD) is considered as a promising method to get excellent conformal films with thickness control at atomic level owing to its intrinsic self-limited growth feature. 2 ALD grown metals or nitrides, such as Ru 3,4 and TaN films, 5,6 have been widely studied as Cu diffusion barriers or adhesion layers. ALD grown oxides, such as TiO 2 , 7,8 Ta 2 O 5 7,9,10 and Al 2 O 3 11,12 have also been reported as Cu diffusion barriers due to their excellent uniformity and thermal stability. P. Alen 7 and Qi Xie 8 reported the excellent thermal stability of ALD TiO 2 . P. Majumder et al. reported that the ultrathin Al 2 O 3 layer deposited by thermal ALD as diffusion barrier between Cu and Si substrate had excellent thermal stability. 11,12 However, these oxide layers were all directly deposited onto Si and the very primary properties of diffusion barriers to Cu were measured. A systematic study of ALD oxide films as Cu diffusion barrier onto a low k dielectric has been rarely reported. 7-12 In this work, ultrathin Al 2 O 3 films were deposited by PEALD together with a Ta adhesion layer, and their properties as a Cu diffusion barrier on a low k dielectric (k = 2.5) were studied.Low k films (Novellus CORAL, k = 2.5) with a thickness of 220 nm were deposited on the 12" p-type Si wafers. The wafers were cut into small pieces and then were cleaned in an ultrasonic cleaner with 2-propanol solution before loading into the loadlock chamber. After in-situ annealing at 250 • C for 20 minutes in the high vacuum ALD chamber, 11 or 21 cycles (∼1.2 Å/cycle) Al 2 O 3 films were grown by PEALD using trimethylaluminium (TMA) and remote O 2 plasma (200W) as precursors. The growth temperature was 250 • C. After deposition, samples were taken out and transferred into a PVD chamber. The details of the ALD growth can be seen elsewhere. 7 Then the Cu(50 nm)/Ta (5 nm) film stacks were deposited by DC magnetron sputtering. The thin Ta film was used as a glue layer to promote adhesion between the Cu and Al 2 O 3 barrier layers. To form the patterned metal-insulatorsemiconductor (MIS) structures, a shadow mask...