Method for producing silicon oxide film and method for producing optical multilayer film

A method for producing an optical multilayer film, comprising:

• providing a sputtering target comprising silicon carbide and silicon such that the spattering target has a ratio in number of atoms of C to Si in a range of 0.5 to 0.95;
• determining conditions for a transition region in which AC sputtering is to be carried out using the sputtering target and alternating current of a frequency in a range of 1 kHz to 1,000 kHz in a sputtering atmosphere including an oxidizing gas substantially without generating hysteresis;
• carrying out AC sputtering in the transition region based on the conditions using the sputtering target and alternating current of a frequency in the range in the sputtering atmosphere such that a transparent silicon oxide film is deposited on a substrate;
• and
• forming on the substrate an optical multilayer film comprising a plurality of layers including the transparent silicon oxide film,
• wherein the transition region is in a voltage region from B to B+(A−B)×0.9 where A is a voltage when the flow rate of the oxidizing gas is 0, and B is a voltage at an intersection of tangent lines of a voltage change curve obtained when a voltage become substantially constant at a low value and when a voltage has a maximum absolute value of inclination during reduction of the flow rate of the oxidizing gas, and the flow rate of the oxidizing gas is controlled to change in a range such that the flow rate of the oxidizing gas is increased toward the voltage of B and that the flow rate of the oxidizing gas is decreased toward the voltage of A.