Overlay target design method to minimize impact of lens aberrations

A method for determining a pitch of a second layer alignment target comprising the steps of:

• determining light diffraction patterns of a first layer alignment target generated by a first illumination setting;
• determining a 0th order light diffraction pattern generated by a second illumination setting;
• determining a first distance between a 0th order light diffraction of the second illumination setting and a 0th order light diffraction pattern of the first layer alignment target;
• and
• determining a pitch for said second layer alignment target, wherein a second distance between the 0th order light diffraction patterns of the second illumination setting and a 1st order diffraction pattern for the second layer alignment target generated by said pitch equals said first distance.

A method of determining a pitch of a second layer feature of a target comprising the steps of:

• determining a light diffraction pattern of a first layer feature for a first illumination setting;
• determining an nth order light diffraction pattern for a second illumination setting;
• determining a first distance between the nth order light diffraction for the second illumination setting and an mth order light diffraction pattern of the first layer feature;
• and
• determining a pitch for the second layer feature, wherein a second distance between the nth order light diffraction pattern for the second illumination setting and the mth order diffraction pattern for the second layer feature equals said first distance.

A method of forming two targets on at least two different layers of a substrate, said method comprising:

• forming a first layer over the substrate;
• forming a first target in said first layer;
• forming a second layer over said first layer;
• and
• forming a second target in said second layer, wherein a pitch of said second target is determined by:
• determining light diffraction patterns of the first target and selecting the pitch for the second layer target, wherein at least a 1st order diffraction pattern of said second target overlaps at least some portion of a 0th order diffraction pattern of said first target.

A target system comprising:

• a first target formed in a first layer;
• and
• a second target formed in a second layer, wherein a pitch of said second target is formed by:
• determining light diffraction patterns of the first target and selecting a pitch for the second layer target and wherein at least a 1st order diffraction pattern of said second target overlaps at least some portion of a 0th order diffraction pattern of said first target.

A projection imaging system comprising:

• an illumination source;
• a mask, said mask comprising at least one target, wherein said target is formed by:
• determining light diffraction patterns of a lower layer target and selecting a pitch for the mask target wherein at least a 1st order diffraction pattern of said mask target overlaps at least some portion of a 0th order diffraction pattern of said lower target;.
• a projection lens;
• and
• a projection surface.