Dielectric response of tantalum oxide subject to induced ion bombardment during oblique sputter deposition

摘要

We describe the deposition of insulating tantalum oxide thin films under conditions of controlled ion bombardment, which can be achieved using reactive sputtering on 90° off-axis substrates with an applied substrate bias. Capacitive measurements of Ta_2O_5 deposited on unbiased off-axis substrates indicate low frequency dielectric constants as high as ε_r～300. Low frequency loss tangents are high, tan δ>0.5, and have a pronounced frequency dependence. Deposition of the film off-axis with sufficient applied rf bias to the substrate (negative bias >-70 V) recovers the on-axis properties typical of Ta_2O_5, e.g., ε_r～22 and tan δ～0.02. The recovery of normal dielectric behavior is attributed to the ion bombardment of the growing film under substrate bias, similar to on-axis depositions but absent from depositions on off-axis substrates with no applied substrate bias. We suggest that insufficiently bombarded films develop a Maxwell-Wagner type polarization along columnar voids. The void structure and the associated dielectric response vary with distance from the sputtering source due to variations in ion density and angle from the sputtering source. A similar dielectric response is observed in depositions on on-axis substrates as a function of angle from the central sputter gun axis. Our results suggest that ion bombardment is necessary for good quality sputtered dielectric films but that a controlled Ar~+ flux is essentially equivalent to the uncontrolled O_2~-/O~(2-) flux of on-axis reactive sputtering.