**6. Model-based overlay measurement (mDBO)**

The success of scatterometry for CD and profile measurement comes from the ability to model the signal formation process. The signature contains enough information that the measurement can be made by finding those parameters that give the closest fit between modeled and experimental signatures. The same approach can be applied to overlay measurement, reducing the number of measurement pads needed and providing profile data as well as overlay.

#### **6.1 mDBO LELE sample**

The first mDBO DPT structure consists of alternate photo resist and nitride lines on a silicon substrate (Fig. 16a). As mentioned in section 3, eDBO measurements are performed on four specially designed pads per direction (Fig. 4) with *D* designed to be around 25-35% of the pitch to ensure maximal overlay sensitivity [6]. The mDBO measurements are performed on two of the pads with shift +*D* and –*D*. For normal incident polarized reflectometry, it is found that the TE spectrum is more sensitive to overlay than TM [6]. To reduce measurement time without compromising sensitivity, only TE spectra are collected and used for data analysis. In mDBO analysis a physical model is first set up using NanoDiffract Software to describe the sample structure. Fig. 16 shows the model of one of the pads with designed shift +*D*. Four parameters, nitride bottom CD (NI\_BW), resist bottom CD (PR\_BW), resist height (PR\_HT) and the distance between the nitride and resist lines (*S*), are floated to optimize the model fit to the measured spectra. When there is an overlay error , the distance between the nitride and resist lines, denoted as *S*(+) with + for positive shift, is given by *D*+. The model for the second pad with designed shift -*D* is identical to the first pad except that the shift denoted by *S*(-) is D-. In the regression it is assumed that the

(a) (b)

The dynamic precision (3σ) observed for a 2x4 65/390 target in the case of LFLE was better than predicted (sections 4.2-4.4). Assuming the same behavior applies to SADP, precision

The success of scatterometry for CD and profile measurement comes from the ability to model the signal formation process. The signature contains enough information that the measurement can be made by finding those parameters that give the closest fit between modeled and experimental signatures. The same approach can be applied to overlay measurement, reducing the number of measurement pads needed and providing profile

The first mDBO DPT structure consists of alternate photo resist and nitride lines on a silicon substrate (Fig. 16a). As mentioned in section 3, eDBO measurements are performed on four specially designed pads per direction (Fig. 4) with *D* designed to be around 25-35% of the pitch to ensure maximal overlay sensitivity [6]. The mDBO measurements are performed on two of the pads with shift +*D* and –*D*. For normal incident polarized reflectometry, it is found that the TE spectrum is more sensitive to overlay than TM [6]. To reduce measurement time without compromising sensitivity, only TE spectra are collected and used for data analysis. In mDBO analysis a physical model is first set up using NanoDiffract Software to describe the sample structure. Fig. 16 shows the model of one of the pads with designed shift +*D*. Four parameters, nitride bottom CD (NI\_BW), resist bottom CD (PR\_BW), resist height (PR\_HT) and the distance between the nitride and resist lines (*S*), are floated to optimize the model fit to the measured spectra. When there is an overlay error

the distance between the nitride and resist lines, denoted as *S*(+) with + for positive shift, is

pad except that the shift denoted by *S*(-) is D-. In the regression it is assumed that the

. The model for the second pad with designed shift -*D* is identical to the first

,

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

**5.3 Spacer DBO prediction vs. expectation** 

**6. Model-based overlay measurement (mDBO)** 

should be of the order of ~0.05nm.

data as well as overlay.

**6.1 mDBO LELE sample** 

given by *D*+

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

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 center. (b) Experimental spectrum and theoretical calculation.

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 with the eDBO result of 0.25nm in section 3.3.

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 measurement methods.

Diffraction Based Overlay Metrology for Double Patterning Technologies 449

The source of the offset is not clear. The deviation of the data points from the linear correlation curve is 1.50nm 3 between mDBO and CD-SEM, and 1.22nm 3 between

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

(a) (b)

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

(a) (b)

spectrum. The signal corresponds to 0.5nm change in X and Y overlay.

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

mDBO and IBO.

resolution.

**6.2 mDBO LFLE sample** 


Table 4. Parameter correlation matrix and precision predicted using model shown in Fig. 16 (a).

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 overlay error is observed for Y direction

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 points from the straight line shown in main plot.

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. The source of the offset is not clear. The deviation of the data points from the linear correlation curve is 1.50nm 3 between mDBO and CD-SEM, and 1.22nm 3 between mDBO and IBO.
