**6.2 mDBO LFLE sample**

448 Recent Advances in Nanofabrication Techniques and Applications

**NI\_BW 1 0.091 S (overlay) -0.19 1 0.16 PR\_HT -0.47 0.54 1 0.047 PR\_BW 0.05 -0.68 -0.62 1 0.052**

3 = 1.05nm **Histogram**

**Frequency**

**-1**

**-0.75**

**-0.5**

**-0.25**

**0**

**0.25**

**Deviation of data point from the line shown in (a)**

**0.5**

**0.75**

**1**

**More**

Table 4. Parameter correlation matrix and precision predicted using model shown in

<sup>3</sup> = 1.05nm **(a) (b)**

Fig. 18. (a) Correlation of mDBO and eDBO for LELE sample. (b) Histogram of the deviation of the data points form the straight line shown in (a). Data shown here for X and similar

Fig. 19. (a) Correlation of mDBO and CD-SEM for LELE wafer. The inset shows the histogram of the deviation of the data points from the straight line shown in main plot. (b) Correlation of mDBO and IBO. The inset shows the histogram of the deviation of the data

To further evaluate the accuracy of scatterometry measurement, mDBO results are compared with other metrology techniques, i.e., The CD-SEM data is from the DBO targets. Image based overlay (IBO) measurements are made on standard box-in-box targets nearby. Correlations of mDBO to these two techniques are shown in Fig. 19. A good correlation (R2=0.99) and a slope of 1.03 are observed between eDBO and CD-SEM. The offset between eDBO and CD-SEM measurements is ~1.3nm. A good correlation (R2=0.99) is also observed between eDBO and IBO. However, there is an offset of ~7.9 nm between the two methods.

**Parameter Correlation Matrix:**

**y = 0.9963x - 0.1062 R2 = 0.9952**

Fig. 16 (a).

**mDBO (nm)**

overlay error is observed for Y direction

**Overlay Error Correlation**



**eDBO (nm)**

points from the straight line shown in main plot.

**Precision (3) (nm)** 

> Targets composed of only one pad are desirable because they further reduce total target size. 2D gratings that are sensitive to overlay errors in both X and Y directions may be used [13, 17]. One example is shown in Fig. 20. A 2D lattice (similar to an IBO box-in-box target) is formed with a period on the order of hundreds of nanometers, chosen to maximize diffraction efficiency and overlay sensitivity. For IBO targets , the scale of the boxes is on the order of microns to a few tens of microns. The size of the IBO target is limited by optical resolution.

Fig. 20. (a) mDBO 2D grating target; (b) IBO box-in-box target

Fig. 21. (a) 2D DBO targets for the LFLE sample. Seven parameters are floated: resist1 BCD, SWA and HT, resist2 BCD, SWA and HT (coupled to resist1 HT), X shift and Y shift defined from the center of the grids (resist1) to the center of the squares (resist2). (b) Overlay S/N spectrum. The signal corresponds to 0.5nm change in X and Y overlay.

Diffraction Based Overlay Metrology for Double Patterning Technologies 451

The parameter correlation matrix is shown in table 5. There are no strong correlations between overlay and the other parameters. The accuracy is first verified by measuring a series of five targets with designed shifts increasing by 2nm between two neighboring targets. The correlation of the measurement and the programmed overlay is displayed in Fig. 23a. R2 is 0.996 and slope is 0.996. An offset of -11.36nm is observed. It comes from the local registration error due to scanner alignment errors. This can be corrected by adding an overlay offset between the layers during exposure. IBO measurements on a Blossom target next to the DBO targets show a registration error of -11.18nm, which agrees with the offset within 0.2nm. The DBO accuracy is further verified by measuring 49 fields across the wafer and correlating to Blossom measurements. The correlation plot is shown in Fig. 23b, with R2

(a) (b)

The dynamic repeatability (DYN 3IS mean, and TIS 3 are reported in table 5. The dynamic repeatability is measured from 15 load/unload cycles on multiple fields (9 fields for all 1D targets, and 15 fields for the 2D targets). DYN 3 is reported as the average of the

where *OVL*0 is the overlay measurement result at 0° loading angle, and *OVL*180 is the measurement result at 180° loading angle. The reported TIS mean is measured over 71 sites for 1D target and 49 sites for 2D targets across the wafer. For all DBO targets, TIS mean is

0 180 2

*OVL O <sup>T</sup> VL IS* (6)

Fig. 23. (a) Correlation of mDBO 2D target measurements with programmed shifts.

(b) Correlation of mDBO 2D targets with IBO blossom measurements.

3-precisions from the measurement sites. TIS is defined as in eq. (6),

of ~ 0.98 and slope of ~0.97.

**6.3 mDBO LFLE performance** 

The overlay errors are extracted from the DBO target using the modeling approach. The sample structure and modeling details are shown in Fig. 21a. The pitch is 480nm and the nominal values of X shift and Y shift are 42% of the pitch. The design is symmetric in X and Y so that resist side wall angles (SWA) and bottom critical dimensions (BCD) can be coupled between X and Y directions. Seven parameters are floated: resist1 BCD, SWA and height (HT), resist2 BCD, SWA and HT (coupled to resist1 HT), X shift and Y shift defined from the center of the grids (resist1) to the center of the squares (resist2). Sensitivity analysis shows that TE spectra are more sensitive to X overlay while TM spectra are more sensitive to Y overlay (Fig. 21b). Therefore, both TE and TM spectra are used in the measurement.

The experimental spectra and RCWA fits are shown in Fig. 22. In fig. 21b, the overlay S/N corresponds to 0.5nm change in overlay. Sensitivity to overlay of the 2D targets (Fig. 11), is about half of that of 1D targets for the most sensitive wavelength, if both are normalized to 1nm. This is reasonably understood considering the reduction in the target size.

#### Experimental spectrum (sample) and fit (model)

Fig. 22. Experimental TE and TM spectra and theoretical fits for the structure in Fig. 21a.


Table 5. Parameter correlation matrix for the model shown in Fig. 21(a).

The overlay errors are extracted from the DBO target using the modeling approach. The sample structure and modeling details are shown in Fig. 21a. The pitch is 480nm and the nominal values of X shift and Y shift are 42% of the pitch. The design is symmetric in X and Y so that resist side wall angles (SWA) and bottom critical dimensions (BCD) can be coupled between X and Y directions. Seven parameters are floated: resist1 BCD, SWA and height (HT), resist2 BCD, SWA and HT (coupled to resist1 HT), X shift and Y shift defined from the center of the grids (resist1) to the center of the squares (resist2). Sensitivity analysis shows that TE spectra are more sensitive to X overlay while TM spectra are more sensitive to Y overlay (Fig. 21b). Therefore, both TE and TM spectra are used in the

The experimental spectra and RCWA fits are shown in Fig. 22. In fig. 21b, the overlay S/N corresponds to 0.5nm change in overlay. Sensitivity to overlay of the 2D targets (Fig. 11), is about half of that of 1D targets for the most sensitive wavelength, if both are normalized to

Experimental spectrum (sample) and fit (model)

Fig. 22. Experimental TE and TM spectra and theoretical fits for the structure in Fig. 21a.

**Parameters Resist1 SWA Resist Ht Resist1 BCD Y Shift Resist2 SWA X Shift Resist2 BCD**

1nm. This is reasonably understood considering the reduction in the target size.

measurement.

Resist1 SWA -1

Resist Ht -0.79 -1

Resist1 BCD 0.95 -0.85 -1

Y Shift -0.03 -0.34 -0.04 -1

Resist2 SWA -0.62 0.28 -0.6 0.19 -1

X Shift -0.03 -0.37 -0.05 0.43 0.18 -1

Table 5. Parameter correlation matrix for the model shown in Fig. 21(a).

Resist 2 BCD -0.5 0.001 -0.64 0.29 0.83 0.32 -1

The parameter correlation matrix is shown in table 5. There are no strong correlations between overlay and the other parameters. The accuracy is first verified by measuring a series of five targets with designed shifts increasing by 2nm between two neighboring targets. The correlation of the measurement and the programmed overlay is displayed in Fig. 23a. R2 is 0.996 and slope is 0.996. An offset of -11.36nm is observed. It comes from the local registration error due to scanner alignment errors. This can be corrected by adding an overlay offset between the layers during exposure. IBO measurements on a Blossom target next to the DBO targets show a registration error of -11.18nm, which agrees with the offset within 0.2nm. The DBO accuracy is further verified by measuring 49 fields across the wafer and correlating to Blossom measurements. The correlation plot is shown in Fig. 23b, with R2 of ~ 0.98 and slope of ~0.97.

Fig. 23. (a) Correlation of mDBO 2D target measurements with programmed shifts. (b) Correlation of mDBO 2D targets with IBO blossom measurements.

#### **6.3 mDBO LFLE performance**

The dynamic repeatability (DYN 3IS mean, and TIS 3 are reported in table 5. The dynamic repeatability is measured from 15 load/unload cycles on multiple fields (9 fields for all 1D targets, and 15 fields for the 2D targets). DYN 3 is reported as the average of the 3-precisions from the measurement sites. TIS is defined as in eq. (6),

$$TIS = \frac{OVL\_0 + OVL\_{180}}{2} \tag{6}$$

where *OVL*0 is the overlay measurement result at 0° loading angle, and *OVL*180 is the measurement result at 180° loading angle. The reported TIS mean is measured over 71 sites for 1D target and 49 sites for 2D targets across the wafer. For all DBO targets, TIS mean is

Diffraction Based Overlay Metrology for Double Patterning Technologies 453

and target size. In addition to overlay data, mDBO provides CD measurements and profile data for the target, which is not possible with other methods. The multi-pad DBO approach is a good method of overlay process control, especially if combined with in-chip

The authors thank C. Saravanan and Nagesh Avadhany (*Nanometrics*), for their contribution

[1] C. Ludwig and S. Meyer, "Double Patterning for Memory ICs", "Lithography / Book

[2] ITRS, "http://www.itrs.net/Links/2009Summer/PresentationsPDF/Litho\_7-2009\_SF-

[3] Yi-Sha Ku, Chi-Hong Tung and N. P. Smith, "In-chip overlay measurement by existing

[4] P. Dasari et al., "Diffraction Based Overlay Metrology for Double Patterning

[5] R. Kim et. al., "22nm half-pitch patterning by CVD spacer self alignment double

[6] C. Saravanan et al., "Evaluating Diffraction Based Overlay Metrology for Double

[7] J. Bishoff, R. Brunner, J. Bauer and U. Haak, "Light diffraction based overlay

[8] Chun-Hung Ko et al., "Comparisons of overlay measurement using conventional brightfield microscope and angular scatterometer," Proc. SPIE 5752, 987 (2005) [9] H.-T. Huang et al., "Scatterometry-based Overlay Metrology," Proc SPIE 5038, 126

[10] W. Yang, et al., "Novel Diffraction-based Spectroscopic Method for Overlay Metrology,"

[11] Daniel Kandel et al*.*, "Differential Signal Scatterometry Overlay Metrology: An

[12] Jie Li et al., "Advancements of Diffraction-Based Overlay Metrology for Double

[13] M Dusa et al., "Application of optical CD for characterization of 70mm dense lines,"

[14] N.P. Smith et al., "Overlay metrology at the crossroads," Proc SPIE 6922, 2

[15] Jie Li et al., "Simultaneous Overlay and CD Measurement for Double Patterning:

[16] P Vagos et al., "Uncertainty and sensitivity analysis and its application in OCD

Scatterometry and RCWA Approach," Proc SPIE 7272, 4 (2009)

bright-field imaging optical tools," Proc. SPIE 5752, 43 (2005)

measurements using an alternative technique [3].

2/Chapter IX", ISBN 978-953-307-1356-4.

Technologies," Proc SPIE 7272, 41 (2009)

measurement," Proc SPIE 4344, 222 (2001)

Patterning," Proc SPIE 7971, 70 (2011)

measurements", Proc SPIE 7272, 65 (2009)

patterning (SADP)," Proc SPIE 7973, 22 (2011).

Patterning Technologies," Proc SPIE 6922, 10 (2008)

Accuracy Investigation," Proc SPIE 6616, 0H1 (2007)

**8. Acknowledgement** 

V2.pdf"

(2003)

(2008)

Proc SPIE 5038, 200 (2003)

Proc SPIE 5752, 30 (2005)

to this article.

**9. References** 

nearly zero. TIS 3 reported is from multiple fields (9 fields for all 1D targets, and 15 fields for the 2D targets); with *OVL*0 and *OVL*180 results averaged over 15 load/unload cycles respectively. By removing the contribution from dynamic variations for each loading angle, TIS 3 is very small (on the order of 0.01nm

All three types of standard 1D DBO targets have shown excellent performance with TMU <0.1nm and the 2D 1x1 target has a TMU ~0.2nm (not including tool matching). It is worth mentioning that the mDBO 2x2 target has better TMU, which is a good balance between measurement performance, target size, and measurement time. Similar performance is also observed for Y.


Table 6. Performance summary of eDBO and mDBO targets (\*TMU does not include tool matching)
