3.1.

Investigation of ${\mathsf{B}}_4\mathsf{C}$ Capped ML

Figure 2(a) shows the experimentally obtained SEECs of Ta-based absorber, Ru-capped ML, and ${\mathrm{B}}_4\mathrm{C}$-capped ML. The yield curves of Ru and ${\mathrm{B}}_4\mathrm{C}$-capped ML showed the peaks at 300 and 200 eV, respectively. The overall SEECs of the ${\mathrm{B}}_4\mathrm{C}$ sample are lower than that of the Ru one. Figure 2(b) shows the experimental SEEC difference (which is calculated by subtracting the SEEC of these MLs from that of the absorber layer) as a function of incident beam energy. In the case of Ru-capped ML, the SEEC difference has a peak near 1,000 eV. We had already reported that the incident beam energy with 1,000 eV was the most sensitive condition for extrusion defect detection, because the contrast is the highest at this condition. On the other hand, in the case of the ${\mathrm{B}}_4\mathrm{C}$-capped ML, the SEEC difference has a peak near 500 eV. Furthermore, the difference is 1.5 times greater than that with the case of the Ru sample. In order to examine the influence of the charging effect on the MLs, effect of incident electron current on the SEEC was investigated using three types of layers as shown in Fig. 3. The SEECs of ${\mathrm{B}}_4\mathrm{C}$-capped ML remain almost constant as the incident electron beam current increases, and this effect is similar to the Ru-capped ML. The SEEC of the nondoped Si layer, which had been used as the capping layer, shows a significant decrease with the increasing beam current, because the emitted SEs return back to the sample surface due to the strong positive charges involved.^16,17 These results indicate that the charging effect attributed to the ${\mathrm{B}}_4\mathrm{C}$-capped ML is negligibly small as is the case with Ru-capped ML; the SE signal from the ${\mathrm{B}}_4\mathrm{C}$-capped ML is not changed regardless of the electron dosage. Figure 4 shows the difference image between simulated PEM image with extrusion defects and that without defects. In the case of 500 eV, the 16-nm-sized defect was detected with a signal intensity more than 10 times higher than that of the standard deviation of the background intensity level ($10\sigma$). Figure 5 shows the peak intensity of the extrusion defect signal as a function of incident beam energy. This signal intensity of 16-nm defect with 500 eV is 1.3 times higher than that of the Ru sample with 1,000 eV, which is the most sensitive condition on Ru sample. The defect signal reached a maximum value at the incident beam energy of 500 eV in each size of the defect. This tendency is in good agreement with the experimentally obtained SEEC difference between the absorber layer and ML as shown in Fig. 2(b). Figure 6 shows the difference image in the case of intrusion defects. The 16-nm-sized defect was detected with $>10\sigma$ in the cases of 50, 250, 500, and 1,000 eV. These results indicate that both extrusion and intrusion defects 16 nm in size can be detected without any false defect at the incident beam energy of 500 eV. Although a false defect was observed at 1,000 eV, this can be removed by elevating the threshold ($18\sigma$), while 16-nm-sized defect remains detected as shown in Fig. 6(f). Figure 7 shows the intrusion defect signal intensity as a function of incident beam energy. The signal intensities of the intrusion defect with 16 nm size are much higher than in the case of the extrusion defect. The incident beam energy with a maximum value of the defect signal was shifted to a lower energy as the defect size decreased. The defect signal reached a maximum value at 500 eV in 64-nm-sized defect, whereas in the 16-nm-sized defect, it reached a maximum value at 250 eV. The authors had already reported that a similar phenomenon was observed in the case of Ru-capped ML.⁹ In the case of the intrusion defect, because the aspect ratio of the smaller defect becomes higher, the elevation angle of the defect becomes narrower as defect size gets smaller. Therefore, the signal from the bottom of the intrusion defect becomes weaker as shown in Fig. 8. This is the same mechanism as that in the case where the SE signals from the bottom of trench pattern decrease as their aspect ratios become high.¹⁸ On the other hand, the signal around the defect generated from the absorber layer does not change even if the defect size becomes smaller. As a result, the defect signal curve corresponds to the yield curve of the absorber layer as shown in Fig. 2(a) when the defect size is comparatively small. And when the defect size is comparatively large, this behavior then corresponds to the curve of SEEC difference as shown in Fig. 2(b). The detail explanation had been described elswere.^9,18 Figure 9 shows the double logarithmic plots of the defect signal intensity as functions of defect size. By fitting these plotted data with a power function, the dependence of defect signal intensity on the defect size can be analyzed as a linear regression. The detectability limits can be extrapolated from the intersection value with $\sigma =10$. The slope of the plot for the intrusion defect is gentler than that of the extrusion defect. This result reflects the fact that the sensitivity of an intrusion defect degrades relatively less than that of an extrusion defect as the size of the defect becomes small as discussed above. Table 1 shows the extrapolated detectability limit using PEM technique. It is clearly shown that the ${\mathrm{B}}_4\mathrm{C}$-capped ML has an advantage due to its high sensitivity of defect detection when using the PEM inspection technique and has a potential to meet the requirements beyond hp 16-nm node from the standpoint of the patterned mask inspection. We have already established that 11-nm-sized defect on the Ru capped ML also can be detected with $10\sigma$ in the inspection condition with 10,000 electrons/pixel.⁵

Fig. 2

(a) Experimentally obtained secondary electron emission coefficients (SEECs) of Ta-based absorber, Ru-capped ML, and ${\mathrm{B}}_4\mathrm{C}$-capped ML. (b) Experimental SEEC difference (which was calculated by subtracting SEEC of MLs from that of the absorber layer) as functions of incident beam energy.

Fig. 3

Experimentally obtained SEECs of Ru-capped ML, ${\mathrm{B}}_4\mathrm{C}$-capped ML, and nondoped Si layer as functions of primary electron current with the energy of 200, 300, and 300 eV (the maximum values of their yield curves), respectively. The SEECs are normalized by the SEEC value for 50 pA. Diameter of the incident beam was estimated to be $\sim 1.2\mu \mathrm{m}$ at 2 nA with 300 eV.

Fig. 4

The difference image for extrusion defects on ${\mathrm{B}}_4\mathrm{C}$-capped ML sample. The intensity values less than the threshold value ($10\sigma$) are set to zero. The incident beam energy was (a) 50 eV, (b) 250 eV, (c) 500 eV, (d) 1,000 eV, and (e) 3,000 eV.

Fig. 5

The signal intensity of the extrusion defect signal as a function of incident beam energy.

Fig. 6

The difference image for the intrusion defects on a ${\mathrm{B}}_4\mathrm{C}$-capped ML sample. The intensity values less than the threshold value ($10\sigma$) are set to zero. The incident beam energy was (a) 50 eV, (b) 250 eV, (c) 500 eV, (d) 1,000 eV, and (e) 3,000 eV. (f) The threshold was elevated to $18\sigma$ in order to remove the false defect observed in (d).

Fig. 7

The signal intensity of the intrusion defect signal as a function of incident beam energy.

Fig. 8

Schematic illustration of the intrusion defect and the secondary electron signals blocked by the side wall of the defect.

Fig. 9

The extrusion and intrusion defects signal intensities as functions of defect size in the case of Ru-capped ML with 1,000 eV and ${\mathrm{B}}_4\mathrm{C}$-capped ML with 500 eV.

Table 1

Extrapolated detectability limit of the defect.

3.2.

Investigation of ${\mathsf{B}}_4\mathsf{C}$ Buffered Ru Capped ML

Figure 10 shows the difference image in the case of extrusion defects on the ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML. In the case of 1,000 eV, the 16-nm-sized defect was detected with $>10\sigma$. This peak signal intensity is 1.6 times higher than that of an Ru sample with 1,000 eV. Some false defects observed at 1,000 eV can be removed by elevating the threshold to $13.5\sigma$ as shown in Fig. 10(f). We already had reported that the SEEC of the Ru-capped ML was affected by the underlying Si and Mo with lower SEECs than that of Ru.¹⁰ In the case of ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML, the thickness of Ru is two times thinner than that of the Ru-capped ML sample. Furthermore, density and SEECs of ${\mathrm{B}}_4\mathrm{C}$ are much lower than those of Ru. Therefore, the SEECs of ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML are much affected by the underlying layers.^10,19 As shown in Fig. 11(a), the SEECs of ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML were lower than those of Ru-capped ML when the incident beam energy was $>600\mathrm{eV}$, whereas those SEECs are almost the same in the range of 200 to 500 eV. These phenomena affect the sensitivity of defect inspection as shown in Fig. 11(b). The defect signal intensity is almost the same in the case of 250 and 500 eV on both samples, while, on the other hand, in the case of 1,000 eV, the intensity for ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML is 1.6 times higher than that for the Ru sample (intensities are 19.7 and 12.2, respectively) as indicated by the dashed circle in Fig. 11(b). However, in the case of 3,000 eV, the defect signal is lower than in the case of 1,000 eV in spite of the large SEEC difference. This phenomenon can be explained by electron scattering near the edge of a Ta-based absorber layer as shown in Fig. 12.²⁰ In the case of 3,000 eV, electron scattering is severe and affects the opposite side wall and bottom of the pattern as compared with 500 and 1,000 eV. Such severely scattered electrons become a source of noise and degrade the signal-to-noise ratio of the defect. Figures 13(a) and 13(b) show the simulated images with extrusion defects on the ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML at the incident beam energy of 1,000 and 3,000 eV, respectively. The L/S patterns are observed at the center of the images, and dark areas at the top and bottom sides of the image correspond to the capping layer. It is clearly shown that a blurred image was obtained at 3,000 eV as compared with the case at 1,000 eV. Figure 13(c) shows their signal intensity profiles along the arrows indicated in Figs. 13(a) and 13(b). The intensity difference between the two conditions with 1,000 and 3,000 eV for the capping layer indicated as (a) corresponds to its SEEC difference between these conditions. In a similar manner, the intensity difference between 1,000 and 3,000 eV for the absorber layer indicated as (b) corresponds to its SEEC difference between these conditions. However, peak-to-peak intensity swing of the L/S pattern signal at 3,000 eV (c) is smaller than that at 1,000 eV (d) in spite of the large SEEC difference. This is because the severely scattered electrons become the source of noise, and then the effect of the large SEEC difference is blocked by that noise. This result indicates that the sensitivity of defect detection is affected by the SEEC difference and also by electron scattering. Thus, the small incident beam energy is preferable when the SEEC difference is sufficiently high.

Fig. 10

The difference image for extrusion defects on ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML sample. The intensity values less than the threshold value ($10\sigma$) are set to zero. The incident beam energy was (a) 50 eV, (b) 250 eV, (c) 500 eV, (d) 1,000 eV, and (e) 3,000 eV. (f) The threshold was elevated to $13.5\sigma$ in order to remove the false defect observed in (d).

Fig. 11

(a) Experimental SEEC for Ta-based absorber and Ru-capped ML, and simulated SEEC for ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML. (b) The defect signal intensity of the 16-nm-sized extrusion defect and the SEEC difference between Ta-based absorber layer and ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML as functions of incident beam energy.

Fig. 12

Simulated electron trajectories injected near the edge of a Ta based absorber layer. The incident beam energy is 500, 1,000, and 3,000 eV.

Fig. 13

Simulated images with extrusion defects on the ${\mathrm{B}}_4\mathrm{C}$-buffered Ru-capped ML at the incident beam energies of (a) 1,000 eV and (b) 3,000 eV. (c) Their signal intensity profiles along the arrows indicated in (a) and (b). The signal intensity is normalized by the maximum value for 1,000 eV.