Illumination source and associated metrology apparatus

An illumination source comprising:

• a gas delivery system configured to operate in a first state and a second state,
• wherein when operating in the first state, the gas delivery system is configured to provide a first gas forming a gas target for generating an emitted radiation at an interaction region of the gas target;
• and
• wherein when operating the second state, the gas delivery system is configured to provide a second gas during an idle state of the illumination source such that a first heat transfer coefficient of the first state matches a second heat transfer coefficient of the second state.

An inspection apparatus, comprising:

• an illumination source comprising:
• a pump radiation source configured to emit pump radiation;
• and
• a gas delivery system configured to operate in a first state and a second state, wherein when operating in the first state, the gas delivery system is configured to provide a first gas forming a gas target for generating an emitted radiation at an interaction region of the gas target when irradiated by the pump radiation, and when operating in the second state, the gas delivery system is configured to provide a second gas during an idle state of the illumination source such that a first heat transfer coefficient of the first state matches a second heat transfer coefficient of the second state;
• an illumination system configured to direct the emitted radiation onto a target structure of a sample;
• a detection system configured to detect reflected radiation from the target structure;
• and
• one or more processors configured to receive a signal from the detection system and calculate a property of the target structure based on the signal.

A method of operating an illumination source, comprising:

• operating a gas delivery system of the illumination source in a first state and a second state;
• wherein operating in the first state comprises providing a first gas by the gas delivery system to form a gas target for generating an emitted radiation at an interaction region of the gas target;
• and
• wherein operating in the second state comprises providing a second gas by the gas delivery system during an idle state of the illumination source such that a first heat transfer coefficient of the first state matches a second heat transfer coefficient of the second state.