Lithographic apparatus focus test method and system, and device manufacturing method

A device manufacturing method comprising transferring a pattern from a patterning device onto a substrate and performing a focus test under operational conditions, comprising:

• providing a substrate with a layer of radiation sensitive material;
• illuminating the substrate using a first focus sensitive feature, which provides a measurable shift in a characteristic of the imaged feature when the feature is defocused;
• illuminating the substrate using a second focus feature;
• analyzing the substrate to determine results of the focus test based on the first focus sensitive feature as imaged on the substrate;
• and
• determining a reliability of the focus test based on whether or not predetermined statistic properties associated with the second focus feature as imaged on the substrate are within predetermined limits.

A lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate having a layer of radiation sensitive material thereon, wherein the lithographic apparatus includes a focus test arrangement that conducts a focus test under operational conditions, the focus test arrangement comprising:

• an illuminator that illuminates the substrate in the lithographic apparatus using a first focus sensitive feature, which provides a measurable shift in a characteristic of the imaged feature when the feature is defocused, and a second focus feature;
• an analyzer that analyzes the substrate to determine results of the focus test based on the first focus sensitive feature as imaged on the substrate;
• and
• a reliability determination device that determines reliability of the focus test based on whether or not predetermined statistic properties associated with the second focus feature as imaged on the substrate are within predetermined limits.