Apparatus and methods for detecting overlay errors using scatterometry

A target structure for measuring an overlay error between a first periodic structure and a second periodic structure both disposed above a third periodic structure, wherein the third periodic structure is oriented in a first direction and the first and second periodic structures are oriented in a second direction, the second direction being substantially orthogonal to the first direction, wherein the third periodic structure is arranged to form a barrier over lower structures beneath the third periodic structures so that such lower structures do not significantly affect scattered radiation from the first and second periodic structures for measuring the overlay error.

A metrology system for measuring an overlay error, critical dimension (CD), or profile of a target, comprising:

• a scatterometry module for directing radiation towards a plurality of targets and detecting scattered signals from the targets;
• and
• a processor configured to analyze the detected scattered signals to thereby determine an overlay error, critical dimension (CD), or profile of such targets, wherein the targets comprise a first periodic structure and a second periodic structure both disposed above a third periodic structure, wherein the third periodic structure is oriented in a first direction and the first and second periodic structures are oriented in a second direction, the second direction being substantially orthogonal to the first direction, wherein the third periodic structure is arranged to form a barrier over lower structures beneath the third periodic structures so that such lower structures do not significantly affect the scattered signals from the first and second periodic structures and analyzed by the processor.