Method of overlay measurement

A method comprising:

• forming a conductive line over a peripheral region of a substrate, wherein the conductive line extends in a first direction;
• depositing an etch stop layer over the substrate and covering the conductive line;
• depositing an inter-metal dielectric (IMD) layer over the etch stop layer;
• forming an opening in the IMD layer to expose a portion of the etch stop layer directly above the conductive line, wherein a width of the opening in a second direction perpendicular to the first direction is larger than a width of the conductive line in the second direction;
• performing an overlay measurement to obtain a spatial distribution of an atomic weight of the conductive line and a spatial distribution of an atomic weight of the etch stop layer;
• and
• after performing the overlay measurement, etching the etch stop layer by using the IMD layer as an etch mask to expose the conductive line.

A method comprising:

• depositing an etch stop layer over a conductive line;
• depositing a low-k dielectric layer over the etch stop layer;
• performing a first etching process to form an opening in the low-k dielectric layer;
• performing a second etching process to the opening to deepen the opening into the etch stop layer, wherein after performing the second etching process, a portion of an edge of the conductive line is covered by the low-k dielectric layer;
• and
• after performing the second etching process, performing an overlay measurement to the low-k dielectric layer and the conductive line to obtain an image of the opening and the conductive line, wherein the image comprises the portion of the edge of the conductive line covered by the low-k dielectric layer.