Overlay measurement using multiple wavelengths
A method of determining overlay (“OVL”) in a pattern in a semiconductor wafer manufacturing process comprises capturing images from a cell in a metrology target formed in at least two different layers in the wafer with parts of the target offset in opposing directions with respect to corresponding parts in a different layer. The images may be captured using radiation of multiple different wavelengths, each image including +1 and −1 diffraction patterns. A first and second differential signal may be determined for respective pixels in each image by subtracting opposing pixels from the +1 and −1 diffraction orders for each of the multiple wavelengths. An OVL for the respective pixels may be determined based on analyzing the differential signals from multiple wavelengths simultaneously. Then an OVL for the pattern may be determined as a weighted average of the OVL of the respective pixels.
1. A method of determining overlay (“OVL”) in a pattern in a semiconductor wafer manufacturing process comprising:
receiving images captured from a metrology target formed in at least two different layers in a wafer with parts of the metrology target offset in opposing directions with respect to corresponding parts in a different layer, using radiation of multiple different wavelengths, each of the images captured from the metrology target including +1 and −1 diffraction patterns;
determining a first and second differential signal for respective parts of the metrology target by subtracting opposing pixels from the +1 and −1 diffraction patterns in each of the images for each of the multiple different wavelengths;
determining an OVL for each of the opposing pixels based on first and second differential signals from the multiple different wavelengths; and
determining an overall OVL as a weighted average of the OVL of each of the opposing pixels, wherein the weighted average is weighted according to a variation of a sensitivity of the OVL due to variations in wavelength.
2. The method of claim 1 wherein determining the OVL for one of the opposing pixels comprises:
determining an asymmetry of the differential signals by using an average of the differential signals (K sig ) and a sensitivity of the differential signals to the OVL using a difference of the differential signals (G sig );
determining a first matrix comprising values of sensitivity at different wavelengths;
determining a second matrix comprising values of average differential signals at the different wavelengths; and
determining the overall OVL by a formula that includes the first and second matrices.
3. The method of claim 2 wherein the first matrix is defined as:
G sig (λ,pix)≡½( D 1 (λ,pix)− D 2 (λ,pix))
where D 1 and D 2 represent the differential signals.
4. The method of claim 2 wherein the second matrix is defined as:
K sig (λ,pix)≡½( D 1 (λ,pix)+ D 2 (λ,pix))
where D 1 and D 2 represent the differential signals.
5. The method of claim 2 wherein;
determining the OVL for one of the opposing pixels comprises determining a resonant wavelength at which a sensitivity approximates zero; and
said different wavelengths comprise different wavelengths on either side of the resonant wavelength.
6. The method of claim 2 wherein the different wavelengths are selected from flat regions, or a combination of flat and resonant regions, in a signal spectral landscape.
7. The method of claim 2 wherein the OVL for the opposing pixels is determined as
OVL
pp
(
pix
)
=
f
0
〈
K
·
G
〉
-
〈
K
〉
·
〈
G
〉
〈
G
·
G
〉
-
〈
G
〉
·
〈
G
〉
where G is the first matrix and K is the second matrix.
8. The method of claim 7 wherein the overall OVL is determined as
overlay
=
∑
pix
OVL
pp
(
pix
)
·
W
pp
(
pix
)
∑
pix
W
pp
(
pix
)
.
9. The method of claim 1 comprising determining a per pixel inaccuracy δA(pix) from the overall overlay, and detecting a variation in the semiconductor wafer manufacturing process by monitoring variations in said per pixel inaccuracy.
10. The method of claim 9 wherein the per pixel inaccuracy is determined as:
δ
A
(
pix
)
=
〈
K
〉
-
overlay
f
0
〈
G
〉
,
where f 0 is the metrology target offset.
11. A computer readable medium comprising instructions which, when implemented in a processor in an analysis unit of a metrology system, cause the system to implement the method of claim 1 .