We report high-quality yttrium-iron-garnet (YIG; Y3Fe5O12) ultrathin films grown on {111} gadolinium-gallium-garnet (GGG; Gd3Ga5O12) substrates using RF sputtering deposition on an off-stoichiometric target and optimized thermal treatments. We measured a narrow peak-to-peak ferromagnetic resonance linewidth (Delta H) whose minimum value was 1.9 Oe at 9.43 GHz for a 60-nm-thick YIG film. This value is comparable to the most recently published value for a YIG thin film grown by pulsed laser deposition. The temperature dependence of the Delta H was investigated systematically, the optimal annealing condition for our growing condition was 875 degrees C. Structural analysis revealed that surface roughness and crystallinity played an important role in the observed Delta H broadening. Furthermore, the thickness dependence of the Delta H, which indicated that 60 nm thickness was optimal to obtain narrow Delta H YIG films, was also investigated. The thickness dependence of Delta H was understood on the basis of contributions of surface-associated magnon scattering and magnetic inhomogeneities to the Delta H broadening. Other techniques such as transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were used to study the crystalline structure of the YIG films. The high quality of the films in terms of their magnetic properties was expressed through a very low coercivity and high saturation magnetization measured using a vibration sample magnetometer. (C) 2017 Elsevier B.V. All rights reserved.