**5.2 Prediction of spacer 2nd layer precision**

444 Recent Advances in Nanofabrication Techniques and Applications

without calibration or adjustment of the tools. This is possible because the method of equation (4) is self-calibrating. Absolute spectral matching between tools is not necessary.

**TECHNOLOGY CD:Pitch Tool DYNP (nm) TIS Avg (nm) TIS 3 (nm) Tool Match TMU**

Table 3. Performance summary of DBO on LFLE stack.

agreement between the modeled and actual spectrum.

spacer width at the bottom is 42.5 1nm and SWA is 72.3 0.2 deg.

(a) (b) (c)

**4.5 Prediction vs. observation** 

**5. Spacer double patterning** 

**5.1 Spacer 1st layer patterning** 

CD-SEM Image of spacer L1.

IBO\* Tool4 0.71 0.59 0.18 -0.13 0.56 0.31 0.9 0.67

The dynamic precision (3σ) of ~0.20nm is slightly better than the predictions made in

While LFLE minimizes the number of process steps needed and thus reduces cost it still requires very tight overlay control. Spacer (SADP) forms lines around pre-patterned features, relaxing the requirement for overlay control and potentially allowing the indefinite

In this section we discuss some results from first layer patterning by SADP. Experimental TE and TM spectral data obtained for an SADP stack (Fig. 13a) is fitted to modeled spectra (Fig. 13b). Although the model has not been optimized to improve the fit there is good

The CD-SEM image in Fig. 13c shows the spacer pattern. The measured bottom CD is around 48nm for layer 1 spacers. Although the spacer structures are expected to be identical for both layer 1 and layer2, the CD may vary depending on the application. The model fitting data (Fig. 13a) is consistent with uniform spacer formation across the wafer. The

Fig. 13. (a) STI based spacer Layer 1 stack, (b) Modeled and experimental spectra, and (c)

section 4.2 of 0.24nm (U&SA method) and 0.30nm (noise induced method).

**DBO**

IBO

pitch halving.

65:390 Tool1 0.19 0.17 0.06 -0.01 0.42 0.44 0.09 0.14 0.47 0.49 65:390 Tool2 0.18 0.18 -0.07 0 0.36 0.37 0.09 0.14 0.41 0.44 110:660 Tool2 0.21 0.18 0.07 0.02 0.28 0.32 0.06 0.07 0.36 0.38 110:660 Tool3 0.21 0.19 -0.08 -0.08 0.21 0.19 0.06 0.07 0.3 0.28

**XYXYXYXYXY**

The second spacer layer patterning characteristics were predicted using simulation. Since the static precision predicted for LFLE (in sections 4.2 and 4.4) was consistent with the experimental data in table 3, predictions for the double layer SADP case should also be valid. The good fit between modeled and experimental TETM spectra in Fig. 13b further supports the validity of this approach.

The final SADP stack after completion of the patterning steps is shown in Fig. 14. For LELE and LFLE, both patterning steps are done on the same layer. In the spacer case, the first patterning step is done on L1 (STI+DPT) and second patterning step is done on L2 (WL + DPT) with a programmed shift. The CD of the two spacers can be different.

Fig. 14. The spacer stack DPT steps shown for pitch of 390nm and bottom CD ~65nm

The simulated TETM spectra for the spacer stack in Fig. 15(a) shows sensitivity across the spectral region. The TETM spectral response to overlay shift in Fig. 15(b) is linear for overlay around 25% of the grating pitch.

Diffraction Based Overlay Metrology for Double Patterning Technologies 447

corresponding thickness and CDs are the same for these two pads due to proximity. *D* is fixed to the designed value of 244nm. Fig. 18(b) displays the experimental spectrum and theoretical calculation at best fit for one of the pads. The agreement is excellent. The shape of

> **0 0.05 0.1 0.15 0.2 0.25 0.3 0.35**

**TE reflectance**

(a) (b) Fig. 16. (a) DPT structure with alternative photo resist and nitride lines with silicon over etch. Four parameters are floated: nitride bottom CD (NI\_BW), resist bottom CD (PR\_BW), resist height (PR\_HT) and the shift (*S*) of resist from nitride lines, measured from center to

To check the stability and performance of the model, uncertainty and sensitivity analysis (U&SA) was performed using NanoDiffractTM software [16]. Fig. 17 shows the signal to noise ratio corresponding to a 2 nm change in the overlay error. Reasonable sensitivity is observed. The parameter correlation matrix and predicted static precision (3) are summarized in table 4. No strong correlation is found between overlay and other parameters. The predicted static precision for overlay is 0.16nm (3), which compares well

Fig. 17. Overlay signal/noise ratio. The signal corresponds to 2nm change in overlay.

Fig. 18a compares two-pad mDBO measurements with 4-pad eDBO results. Both data sets are from ~140 dies across the wafer. Excellent correlation (R2 ~0.99) and a slope of 1.00 are achieved. The offset is about 0.1nm. Fig. 18b shows the histogram of the deviation of the data points from the correlation curve shown in Fig. 18a. The distribution follows a normal distribution, indicating the absence of systematic error between these two analysis methods. Standard deviation (3 is 1.05nm, which contains measurement uncertainties from both

center. (b) Experimental spectrum and theoretical calculation.

with the eDBO result of 0.25nm in section 3.3.

measurement methods.

**200 300 400 500 600 700 800 wavelength (nm)**

Theoretical Calc. Exp. Spectrum

the spectrum and fit quality for the second pad is very similar to the first one.

Fig. 15. (a) RCWA simulated spectra; (b) DBO sensitivity over the shift range
