Thin films of Bi–Sr–Ca–Cu–O were co-deposited on MgO by ion beam sputtering at substrate temperatures ( T s) of 400°C–700°C with supply of either oxygen molecules or plasma at around 1 mTorr. High-quality Ca-doped Bi2201 phases can be grown with c-orientation having extremely small XRD linewidths ( Δ=0.1°–0.2°) in the region of 550< T s<650°C for both supplies. The Δs of both films increase with decreasing T s for T s<550°C, however they are considerably reduced by the plasma then a quasi high-quality film can be grown at a ultralow T s of 450°C. The c-parameters of all the films are extremely short, indicating elongated a/ b lattice constants due to the ultralow growth rate of 10 −2 Å/s. Bi compositions of both films decrease drastically with increasing T s for T s>550°C, due to extensive re-evaporation of Bi 2O 3 formed in the gas phase. This result supports the proposal of self-limited Bi sticking coefficient. Film thickness increases with increasing T s in T s<550°C for the plasma and in T s<600°C for the molecular oxygen supply, and then decreases rapidly because of the Bi 2O 3 re-evaporation. SEM images show 3D grain-like surface morphologies for T s>550°C while 2D-like smooth morphologies for T s<550°C. The rms surface roughness is reduced by the plasma, and it is 128 Å for the film with the thickness of 1730 Å deposited at 450°C whereas it is only 27 Å for the film with the thickness of 600 Å deposited at 500°C. These films show 1–2 unit cell steps, suggesting the 2D growth mode. All the characteristics indicate that the growth is governed by the thermal process for T s>550°C and the plasma effects are prominent for T s<550°C. The proposed origins for the high-quality film growth are: (1) moderate thermal energy, (2) the ultralow growth rate, (3) collision-induced moderate energy of the sputtered particles and (4) suitably excited oxygen, (5) plasma-induced multiple nucleation, and (6) ordering of Sr and Ca, and reduction of the surface energy of the growing film by the plasma. The last one is proposed to be an important mechanism for the enhancement of film growth and the improvement of crystallinity by the plasma at the low T s.