Apparatus and method for detecting overlay errors using scatterometry

A system for determining overlay between a plurality of first structures in a first layer of a sample and a plurality of second structures in a second layer of the sample, comprising:

• a scatterometry module for illuminating the targets A, B, C and D with electromagnetic radiation to obtain spectra SA, SB, SC, and SD from targets A, B, C, and D, respectively;
• and
• a processor operable for determining any overlay error between the first structures and the second structures using a linear approximation based on the obtained spectra SA, SB, SC, and SD,
• wherein targets A, B, C and D each include a portion of the first and second structures,
• wherein the target A is designed to have an offset Xa between its first and second structures portions,
• wherein the target B is designed to have an offset Xb between its first and second structures portions,
• wherein the target C is designed to have an offset Xc between its first and second structures portions,
• wherein the target D is designed to have an offset Xd between its first and second structures portions, and
• wherein each of the offsets Xa, Xb, Xc and Xd is different from zero, Xa is an opposite sign and differ from Xb, and Xc is an opposite sign and differs from Xd.

A target arrangement comprising a first layer having a plurality of first structures and a second layer having a plurality of second structures, the target arrangement further comprising:

• targets A, B, C and D that each include a portion of the first and second structures,
• wherein the target A is designed to have an offset Xa between its first and second structures portions,
• wherein the target B is designed to have an offset Xb between its first and second structures portions,
• wherein the target C is designed to have an offset Xc between its first and second structures portions,
• wherein the target D is designed to have an offset Xd between its first and second structures portions,
• wherein each of the offsets Xa, Xb, Xc and Xd is different from zero, Xa is an opposite sign and differ from Xb, and Xc is an opposite sign and differs from Xd, and
• wherein the offsets Xa, Xb, Xc, and Xd are selected so that when the targets A, B, C, and D are illuminated with electromagnetic radiation, the targets A, B, C, and D produce a corresponding spectra SA, SB, SC, and SD, respectively which are indicative of any overlay error existing between the first layer and the second layer structures;
• and
• an imaging overlay measurement type target E from which a second overlay error may be determined using imaging overlay metrology.