Process of forming silicon nitride film

A process of forming a passivation film made of silicon nitride (SiN) on a nitride semiconductor layer, the process comprising steps of:

• loading an epitaxial substrate that includes the nitride semiconductor layer within a furnace at a first temperature;
• converting an atmosphere in the furnace from air to nitrogen (N2) and setting a pressure in the furnace to be a first pressure;
• raising a temperature of the furnace to a second temperature higher than 700° C.;
• converting the atmosphere in the furnace to ammonia (NH3);
• and
• supplying di-chloro-silane (SiH2Cl2) within the furnace to begin the deposition of the passivation film on the nitride semiconductor layer,
• wherein the step of raising the temperature and the step of converting the atmosphere to NH3 sets an elapsed time to be shorter than 60 minutes, the elapsed time being measured from an instant when the temperature in the furnace reaches a critical temperature TC defined by an equation:
• 103·A/TC+B=exp(P/C),
• to a beginning of the deposition, where P is the first pressure in a unit of Pascal (Pa), TC is the critical temperature in the unit of Kelvin (K), and A to C are −10.1, 12.5, and 0.434, respectively.