**3.2.1 Correlation with IBO data**

Fig. 8(a) shows the comparison between DBO and IBO measurements on all 143 fields on the wafer. There is a very good correlation between DBO and IBO measurements (R2=0.99) with an offset of ~7 nm. The correlation is good for the subset of measurements less than ±3 nm (Fig. 8b). The inset histogram in Fig. 8a shows the difference in IBO and DBO measurements after removing the ~7 nm constant offset. The distribution is approximately normal with standard deviation of 1.8nm.

Diffraction Based Overlay Metrology for Double Patterning Technologies 441

DPT application [10]. It is possible that this offset might be coming from differences in mask writing errors between the DBO and IBO targets. The CD-SEM results are closer to the DBO values (Fig. 9), which suggests that the accuracy of the DBO technique is better than that of IBO. The observed DBO repeatability is better than CD-SEM. DBO is also non-destructive,

While scatterometry offers precise and accurate overlay measurements for DPT, the number of reference and sample pads required for such measurements are still a concern. In this section the possibility of reducing the number of pads required without sacrificing the performance is explored using two target types – 2x4 and 2x3 targets (see definitions of target types in Section 2). Fig. 10 shows excellent correlation between overlay measured from these 2 target types. Table 1 shows the root mean square dynamic precision on two 2x3 targets and a 2x4 target. The precision on the 2x3 targets is a factor of ~1.4 (= 2 ) higher than that on the 2x4 target because the number of difference spectra used in the 2x4-target algorithm is twice the number used for the 2x3-target algorithm. The figure shows excellent correlation, suggesting that a 3-pad target is sufficiently accurate for this application. The

> -15 -10 -5 0 5 10

> > **RMS Precision 3 (nm)**

**4-PAD OVL (nm)**

Fig. 10. Correlation between DBO targets with 4 pads/direction and 3 pads/direction.

2x3 Dynamic, Target-II 0.31 2x3 Dynamic, Target-II 0.32 2x4 Dynamic, Target II 0.20 2x4 Static, Target II 0.11

Table 1. Shows the dynamic precision for the 2x3 target is higher by a factor of ~ 2

DBO capability was further assessed by calculating the measurement uncertainty (TMU) from the measurement data using equation (5). Short-term (dynamic) precision (*DYNP*) is


as the sample is not subjected to the charging effects that occur in a CD-SEM.

dynamic (load-unload) and static (no unload) precision is excellent.

y = 1.01x + 0.18 R2 = 0.99

**Target/Measurement Type**

**3.4 DBO performance: precision, TIS, matching, and TMU** 

**3.3 DBO target types – 4 vs. 3 pads** 

**3-PAD OVL (nm)** 

Fig. 8. DBO vs. IBO correlations: (a) shows correlations on 143 targets throughout the wafer, and (b) shows a subset of the same data in a narrow range of ~±3 nm.

#### **3.2.2 Correlation with CD-SEM**

Fig. 9a shows a CD-SEM image of the nitride (dark) and photo resist (gray) lines. Fig. 9(b) and 9(c) show the correlation between DBO and CD-SEM measurements. In Fig. 9b, the overlay errors are calculated from the CD-SEM data for the top of the lines, while in Fig. 9c, the bottom CD data is used. The inset histograms in Fig. 9b and 9c show the distributions of the difference between IBO and CD-SEM measurements after subtracting offsets.

Fig. 9. (a) CD-SEM image of the DPT structure. Dark lines are nitride and the light gray lines are the photo resist, (b) correlations between DBO and top CD-SEM measurements from all 143 fields. The inset histogram shows the histogram of differences between the CD-SEM and DBO results. (c) as (b) but using the bottom CD data.

The slope of the CD-SEM/DBO correlation is 0.95 (for top CD) and 0.97 (bottom CD) compared to 0.9 for the DBO-IBO correlation. The offset between DBO and CD-SEM measurements is in the order of 1 to 2 nm. Concerns of accuracy of IBO measurements have been raised before. Dusa *et al.* reported an offset of ~5 nm between CD-SEM and IBO for a DPT application [10]. It is possible that this offset might be coming from differences in mask writing errors between the DBO and IBO targets. The CD-SEM results are closer to the DBO values (Fig. 9), which suggests that the accuracy of the DBO technique is better than that of IBO. The observed DBO repeatability is better than CD-SEM. DBO is also non-destructive, as the sample is not subjected to the charging effects that occur in a CD-SEM.

#### **3.3 DBO target types – 4 vs. 3 pads**

440 Recent Advances in Nanofabrication Techniques and Applications

Fig. 8. DBO vs. IBO correlations: (a) shows correlations on 143 targets throughout the wafer,

**IBO - Box in Box Targets** 

**.**

y = 0.92x + 7.22 R2 = 0.97

R<sup>2</sup> = 0.97

y = 0.97x + 2.15 R2 = 0.97

**Frequency** 

**-3**

**-2**

**-1**

**0**

**CD-SEM - (DBO+Offset)**

**1**

**2**

**3**


**DBO (nm)**

(b) (c)


(a) (b)


Fig. 9a shows a CD-SEM image of the nitride (dark) and photo resist (gray) lines. Fig. 9(b) and 9(c) show the correlation between DBO and CD-SEM measurements. In Fig. 9b, the overlay errors are calculated from the CD-SEM data for the top of the lines, while in Fig. 9c, the bottom CD data is used. The inset histograms in Fig. 9b and 9c show the distributions of

**Programmed = 228 nm**

**Overlay using CD-SEM (nm)** 

Nitride

**(a)**

the difference between IBO and CD-SEM measurements after subtracting offsets.

Resist

Fig. 9. (a) CD-SEM image of the DPT structure. Dark lines are nitride and the light gray lines are the photo resist, (b) correlations between DBO and top CD-SEM measurements from all 143 fields. The inset histogram shows the histogram of differences between the CD-

The slope of the CD-SEM/DBO correlation is 0.95 (for top CD) and 0.97 (bottom CD) compared to 0.9 for the DBO-IBO correlation. The offset between DBO and CD-SEM measurements is in the order of 1 to 2 nm. Concerns of accuracy of IBO measurements have been raised before. Dusa *et al.* reported an offset of ~5 nm between CD-SEM and IBO for a

SEM and DBO results. (c) as (b) but using the bottom CD data.


**DBO (nm)**


y = 0.95x + 1.20 R2 = 0.99

and (b) shows a subset of the same data in a narrow range of ~±3 nm.

**DBO (nm)**


y = 0.90x + 7.11 R2 = 0.99

**3.2.2 Correlation with CD-SEM** 


**IBO - Box in Box Targets** 

**Overlay using CD-SEM (nm)** 

**Frequency** 

**-3**

**-2**

**-1**

**0**

**CD-SEM - (DBO+Offset)**

**1**

**2**

**3**

**.**

Frequency

While scatterometry offers precise and accurate overlay measurements for DPT, the number of reference and sample pads required for such measurements are still a concern. In this section the possibility of reducing the number of pads required without sacrificing the performance is explored using two target types – 2x4 and 2x3 targets (see definitions of target types in Section 2). Fig. 10 shows excellent correlation between overlay measured from these 2 target types. Table 1 shows the root mean square dynamic precision on two 2x3 targets and a 2x4 target. The precision on the 2x3 targets is a factor of ~1.4 (= 2 ) higher than that on the 2x4 target because the number of difference spectra used in the 2x4-target algorithm is twice the number used for the 2x3-target algorithm. The figure shows excellent correlation, suggesting that a 3-pad target is sufficiently accurate for this application. The dynamic (load-unload) and static (no unload) precision is excellent.

Fig. 10. Correlation between DBO targets with 4 pads/direction and 3 pads/direction.


Table 1. Shows the dynamic precision for the 2x3 target is higher by a factor of ~ 2

### **3.4 DBO performance: precision, TIS, matching, and TMU**

DBO capability was further assessed by calculating the measurement uncertainty (TMU) from the measurement data using equation (5). Short-term (dynamic) precision (*DYNP*) is

Diffraction Based Overlay Metrology for Double Patterning Technologies 443

An example LFLE DPT structure consists of ~120 nm photoresist lines with silicon over etch

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

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

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

(a) (b)

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

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

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

shows the excellent correlation (R2 ~0.99) between the DBO and Blossom data.

**4.1 DPT Structure** 

noise induced method.

blossom

**4.4 LFLE DBO performance** 

TIS was under 0.1nm.

**4.3.1 Correlation with IBO** 

as shown in Fig. 11(a) and (b).

**4.2 Prediction based on simulation** 

**4.3 DBO accuracy (freeze process)** 

the range of interest for the LFLE DPT stack.

the Root-Mean-Square (RMS) of three times the standard deviation of the 20-cycle measurements at each target, and combines static precision (occurring without any movement within the tool) and the effects of target reacquisition. *TIS3<sup>T</sup>* is three times the standard deviation of the TIS in the measurements at each target. Where multiple tools are available, site-by-site tool matching (TM) is included. TM is three times the standard deviation of the difference in the average measurement from each tool at the same location. Matching data is not included in the DBO to IBO TMU comparison.

$$TMTLI = \sqrt{\left(DYNP\right)^2 + \left(TIS3\sigma\_T\right)^2 + \left(TM\right)^2} \tag{5}$$

The results for both DPT applications are summarized in Table 2. For the gate level DPT the dynamic precision is less than 0.1 nm and the TMU is 0.26 nm Average TIS is under 0.1nm. DBO matching data is between two tools (Atlas and FLX) with similar reflectometer optical heads. For the DPT structure on a silicon surface precision is 0.2nm. TIS and matching data is not available for this structure.


Table 2. Performance summary of DBO on two stacks discussed in Fig. 4.

### **4. Litho-freeze-litho-etch (LFLE)**

While LELE involving two process steps offers an adequate solution for DPT process steps, both are very expensive and slow. The alternative Litho-Freeze-Litho-Etch (LFLE, Fig. 1b) process reduces cost by replacing the intermediate etch step with a process step in the litho track.

Fig. 11. DPT structure that has alternative photo resist lines with silicon over etch: (a) 65/390 Line/Pitch ratio, (b) 110/660 Line/Pitch ratio
