Line width insensitive wafer target detection in two directions

A method of aligning a wafer comprising the steps of:

• projecting electromagnetic radiation onto a wafer, the wafer having spaced alignment marks thereon, the alignment marks having complementary first and second sets of like edges;
• detecting electromagnetic radiation reflected from the alignment marks in a first direction;
• detecting electromagnetic radiation reflected from the alignment marks in a second direction;
• selecting first dominant signals representative of the first set of like edges from the electromagnetic radiation reflected from the alignment marks in the first direction;
• selecting second dominant signals representative of the second set of like edges from the electromagnetic radiation reflected from the alignment marks in the second direction;
• and
• calculating a position of the wafer alignment marks based on the first and second dominant signals,
• whereby the wafer is accurately aligned.

An alignment system for use in photolithography comprising:

• an illumination source providing electromagnetic radiation;
• optic means, positioned to receive the electromagnetic radiation from said illumination source, for illuminating alignment marks on a substrate, said alignment marks having a first set of like edges and a second set of like edges;
• forward detector means, positioned to receive the electromagnetic radiation from the alignment marks, for detecting the first and second set of like edges in a forward direction;
• rearward detector means, positioned to receive the electromagnetic radiation from the alignment marks, for detecting the first and second set of like edges in a rearward direction;
• like edge selector means, coupled to said forward and rearward detector means, for selecting signals from said forward detector means and said rearward detector means representative of the first set of like edges and the second set of like edges;
• and
• signal analyzer means, coupled to said like edge selector means, for calculating the location of the alignment marks.