Continuously varying offset mark and methods of determining overlay
The present invention relates to overlay marks and methods for determining overlay error. One aspect of the present invention relates to a continuously varying offset mark. The continuously varying offset mark is a single mark that includes over laid periodic structures, which have offsets that vary as a function of position. By way of example, the periodic structures may correspond to gratings with different values of a grating characteristic such as pitch. Another aspect of the present invention relates to methods for determining overlay error from the continuously varying offset mark. The method generally includes determining the center of symmetry of the continuously varying offset mark and comparing it to the geometric center of the mark. If there is zero overlay, the center of symmetry tends to coincide with the geometric center of the mark. If overlay is non zero (e.g., misalignment between two layers), the center of symmetry is displaced from the geometric center of the mark. The displacement in conjunction with the preset gain of the continuously varying offset mark is used to calculate the overlay error.
1. An overlay target comprising:
a first periodic structure located on a first layer, the first periodic structure having a first value of an attribute of a periodic structure; and
a second periodic structure located on a second layer, and positioned over the first periodic structure, the second periodic structure having at least a second value of the attribute of a periodic structure, wherein the first and second values have a substantially small difference and wherein each of the first and second periodic structures is for measuring overlay in two directions and wherein the attribute is rotation or pitch,
wherein the length of the first and second periodic structures is independent from the substantially small difference.
2. The overlay target as recited in claim 1 wherein the second value is larger than the first value.
3. The overlay target as recited in claim 1 wherein the second value is smaller than the first value.
4. The overlay target as recited in claim 1 wherein the first and second periodic structures each consist of a plurality of sub segments oriented in two directions.
5. The overlay target as recited in claim 1 wherein the difference between the first and second values is less than 10%.
6. The overlay target as recited in claim 1 wherein the difference between the first and second values is less than 5%.
7. The overlay target as recited in claim 1 wherein the difference between the first and second values is less than or equal to 1.0%.
8. The overlay target as recited in claim 1 , wherein the small difference is configured to generate a preset gain for the overlay target.
9. The overlay target as recited in claim 1 wherein the difference between the first and second values is between about 0.01% and about 10%.
10. The overlay target as recited in claim 9 wherein the difference between the first and second values is between about 0.10% and about 0.99%.
11. An overlay target comprising:
a first portion located on a first layer, the first portion including a first grating having a first value of a pitch;
a second portion located on a second layer and centered above the first portion, the first and second portions having substantially a same dimensional profile, the second portion including a second grating and third grating that are positioned side by side, the second grating having a second value of a pitch, and the third grating having a third value of a pitch, the second value being slightly larger than the first value, the third value being slightly smaller than the first value,
wherein the second grating and the third grating extend substantially over the first grating.
12. The overlay target as recited in claim 11 wherein a difference between the first value and the second and third values is 1.0%.