**4.2 Prediction based on simulation**

The LFLE stack is built on silicon with BARC and two resist lines patterned on top. Simulated spectral response curves (for example from NanoDiffractTM, Nanometrics scatterometry software) are used to predict the static precision of overlay measurements in the range of interest for the LFLE DPT stack.

Static precision for the LFLE model can be determined by two different methods. The first method uses analysis of measurement covariance as described by Vagos *et al* [16], and which is referred to as the "Uncertainty and Sensitivity Analysis" method (U&SA). In the second method random noise is introduced into the simulated DBO spectra for all four pads and static precision determined as if the spectra are obtained experimentally. The predicted static precision for the LFLE stack is 0.24nm (3σ) using the U&SA method and 0.30nm by the noise induced method.

#### **4.3 DBO accuracy (freeze process) 4.3.1 Correlation with IBO**

The CD-SEM image in Fig. 12a shows the resist lines in one of the four pads of a 110/660 target. To test measurement accuracy, the results from two 2x4 DBO targets (65/390, 110/660) are compared with IBO measurements using nearby Blossom targets. Fig. 12b shows the excellent correlation (R2 ~0.99) between the DBO and Blossom data.

Fig. 12. Correlation of DBO vs. IBO: (a) 110/660 pad, (b) Correlation of DBO 2x4 targets vs. blossom

### **4.4 LFLE DBO performance**

Good overlay control requires good measurement capability. Table 3 summarizes the precision, TIS, tool matching and measurement uncertainty (TMU) for DBO of LFLE structures. DBO dynamic precision was less than 0.2nm and TMU less than 0.5nm. Average TIS was under 0.1nm.

Table 3 contains site-by-site (SBS) 3 DBO matching data for this stack from three tools with the same design of reflectometer optical head. Tool matching of 0.14nm or less is achieved

Diffraction Based Overlay Metrology for Double Patterning Technologies 445

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

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

**Shift**

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

supports the validity of this approach.

around 25% of the grating pitch.


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.

Table 3. Performance summary of DBO on LFLE stack.

#### **4.5 Prediction vs. observation**

The dynamic precision (3σ) of ~0.20nm is slightly better than the predictions made in section 4.2 of 0.24nm (U&SA method) and 0.30nm (noise induced method).

#### **5. Spacer double patterning**

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 pitch halving.

#### **5.1 Spacer 1st layer patterning**

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 agreement between the modeled and actual spectrum.

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

Fig. 13. (a) STI based spacer Layer 1 stack, (b) Modeled and experimental spectra, and (c) CD-SEM Image of spacer L1.
