Extra tall target metrology

A metrology system with air bearings comprising:

• an objective lens;
• one or more detectors;
• an objective positioning stage structurally coupled to the objective lens and configured to adjust a focal plane of at least one of the one or more detectors via movement along an optical axis of the metrology system;
• a coarse stage of the objective positioning stage, the coarse stage structurally supported radially about the optical axis by one or more air bearings configured to allow for a longitudinal movement of the coarse stage along the optical axis, and constrain a lateral translation and a tilt of the coarse stage relative to the optical axis when adjusting the focal plane of the one or more detectors;
• and
• one or more proximity sensors configured to measure lateral positions of the coarse stage relative to the optical axis and in directions orthogonal to the optical axis when the objective positioning stage is at each of two or more objective locations along the optical axis,
• wherein the metrology system is configured to capture images of two or more focal planes via movement of the objective positioning stage to two or more objective locations along the optical axis, each focal plane of the two or more focal planes at an axial position along the optical axis of the metrology system and orthogonal to the optical axis,
• wherein at least two of the two or more focal planes are separated by an axial focal distance apart that is greater than a depth of field provided by the objective lens,
• wherein the metrology system is configured to determine a metrology measurement associated with a target on a sample using the images and the measured lateral positions of the coarse stage when the objective positioning stage is at the two or more objective locations when implementing a metrology recipe, the target including features on the two or more focal planes according to the metrology recipe, wherein determining the metrology measurement comprises compensating for at least one of a lateral shift or a tilt of the coarse stage between the two or more objective locations using the measured lateral positions.

A method for overlay metrology, the method comprising:

• receiving lateral positions of a stage element of a metrology system obtained from one or more proximity sensors, the lateral positions measured relative to an optical axis and in directions orthogonal to the optical axis when the objective positioning stage is at each of two or more objective locations,
• wherein the metrology system is configured to adjust a focal position of at least one of one or more detectors of the metrology system, wherein the metrology system is configured to capture images of two or more focal planes via movement of an objective positioning stage to two or more objective locations along the optical axis, each focal plane of the two or more focal planes at an axial position along the optical axis of the metrology system and orthogonal to the optical axis, wherein at least two of the two or more focal planes are separated by an axial focal distance apart that is greater than a depth of field provided by an objective lens;
• receiving the images of the two or more focal planes via the movement of the objective positioning stage to the two or more objective locations along the optical axis;
• and
• determining a metrology measurement associated with a target on a sample using the images and the lateral positions of the stage element when the objective positioning stage is at the two or more objective locations, the target including features on the two or more focal planes, wherein determining the metrology measurement comprises compensating for at least one of lateral shift or tilt of the objective positioning stage between the two or more objective locations using the measured lateral positions.