Overlay sampling methodology

A method for overlay sampling, comprising:

• providing a semiconductor wafer surface that includes a plurality of fields having equal areas and being arranged in a pattern so neighboring fields abut one another, wherein each field includes multiple overlay structures each made up of two or more layers that are to be aligned;
• using an imager to measure a first number of overlay conditions at a corresponding first number of overlay structures for an inner subgroup of the plurality of fields, wherein fields of the inner subgroup are arranged in a central region of the wafer surface;
• measuring a second number of overlay conditions at a corresponding second number of overlay structures for an outer subgroup of the plurality of fields, wherein neighboring fields of the outer subgroup are arranged near a circumferential edge of the wafer surface and wherein the second number is greater than the first number;
• based on the measured first number of overlay conditions and the measured second number of overlay conditions, determining an alignment condition for the two or more layers.

A metrology system, comprising:

• a light source configured to provide light towards an optical axis of the metrology system;
• a moveable stage configured to retain a wafer having one or more overlay structures thereon such that light from the light source impinges on the one or more overlay structures;
• and
• an imager configured to measure a first number of overlay conditions at a corresponding first number of overlay structures within an inner field on the wafer, wherein the inner field is arranged in a central region of the wafer;
• wherein the imager is further configured to measure a second number of overlay conditions at a corresponding second number of overlay structures within an outer field, wherein the outer field is included in an outer region near a circumferential edge of the wafer and wherein the second number is greater than the first number.