Methods and test structures for measuring overlay in multilayer devices

A method for calibrating an imaging instrument relative to distance measurements between features of a test structure formed on a substrate, said method comprising the steps of:

• forming a first conductive pattern overlying said substrate and comprising first and second spaced elongate conductors;
• forming a second conductive pattern on a surface of an insulating layer overlying said first conductive pattern, said second conductive pattern comprising a plurality of longitudinally spaced conductive elements overlying said elongate conductors;
• forming a like plurality of conductive connectors in vias extending through said insulating layer, said connectors each being connected to a respective conductive element being disposed at successively varying distances relative to said elongate conductors, at least a first one of said connectors interconnecting said first elongate conductor and one of said conductive elements, and at least a further one of said connectors interconnecting said second elongate conductor and a further conductive element;
• performing a series of electrical measurements between said conductive elements and said first and second elongate conductors so as to provide measured values corresponding to the degree of conduction provided by said connectors between each of the conductive elements and said first conductor and between each of said conductive elements and said second conductor;
• and
• calibrating said imaging instrument using said measured values.