Focus Monitoring Arrangement and Inspection Apparatus Including such an Arrangement

An inspection apparatus comprising:

• an inspection illumination system configured to deliver inspection radiation to the target;
• and
• an inspection detecting system configured to collect the inspection radiation after being scattered by the target,
• wherein an optical system that forms part of one or both of the inspection illumination system and inspection detection system is provided with a focus monitoring arrangement comprising:
• a focusing beam delivery system configured to deliver focusing radiation for delivering to said optical system focusing radiation, the optical system being arranged to deliver the focusing radiation to a target;
• a focusing beam collection system for collecting configured to collect said focusing radiation after reflection from the target;
• and
• a focus detection system for receiving configured to receive the collected focusing radiation and to process processing the collected focusing radiation to determine whether the optical system is in focus with respect to the target;
• wherein the focus monitoring arrangement further comprises a reference beam delivery system for delivering configured to deliver reference radiation to said focus detection system reference radiation, the reference radiation being arranged to interfere with the collected focusing radiation in said focus detection system,
• wherein the reference radiation and focusing radiation are provided with a relative frequency shift so that the interfering radiation detected in the detection system includes a time-varying component having a characteristic frequency corresponding to the relative frequency shift, and
• wherein the focus detection system comprises one or more lock-in detectors operable with reference to said characteristic frequency.