Depth–Depth of Focus Moiré Fringe Alignment via Broad-Spectrum Modulation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Alignment accuracy is one of the key techniques to determine the overlay accuracy of lithography machine. In this paper, the problem of gap limitation between wafer and mask caused by Talbot effect in Moiré fringe alignment method is improved by using broadband light source. Also, the GA-PSO optimization algorithm is used to obtain the different wavelength ratios of the light source under the longest DOF. This work is significant. The comments of this paper are as follows:

1. The first letter of the word “Moiré” should be capitalized and spelled correctly, and the entire manuscript should be double-checked.

2. The “DOF” should be labeled with its full name and abbreviation when it first appears.

3. In INTRODUCTION part, it is suggested that the structure should be rearranged to point out the problems existing in the existing studies.

4. What’s the simulation conditions of Fig.2 and Fig.4? It is not clear.

5. Line 207, DOF is expressed by the variable Z_c, but it becomes to be I in eq.9. Also, the optimization objective function eq. 10 is not clear. The optimization goal should be the maximum, not the minimum.

6. Line 234-239, it seems to be optimizing all the components of white light, but only 405nm, 532nm, 633nm and 780nm were showed in Fig.7. The optimized variable is only the proportion of these four wavelengths? 

7. In section 2.3, more details should be given about the GA-PSO optimization algorithm, such as parameter settings? The optimization results were compared and analyzed.

8. What’s your considerations on frequency doubling effect? It is suggested that relevant content be added.

9. The simulation is not deep and comprehensive enough, and the fringe contrast has not been analyzed in this manuscript. It is recommended to add the simulation that the fringe contrast calculation of single wavelength and optimized source in the same gap region (full coverage of the DOF for two source condition).

10. Line 361, Figure 8 (b) and (c) should give the units of the horizontal and vertical coordinates.

11. At the end of the CONCLUSIONS part, the limitations or deficiencies of your work should be noted.

12. In REFERENCES part, some references are not properly formatted and cited.

Comments on the Quality of English Language

Minor editing of English language required.

Author Response

Please see the attachment.Thank you

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Alignment technology is the key factor to determine the overlay accuracy which is one of the three indicators of lithography machine. The alignment method based on moiré fringes is an important alignment method. This paper calculate the optimal proportion of each wavelength and using white light incoherent illumination in combination with this proportion to achieve the optimal DOF range. Reducing the limitation of the Talbot effect has certain significance for the expansion of the use of this method. The comments are as follows:

1. In this paper, author put forward the optimization algorithm to obtain the best light source components, but how to realize the adjustment in the system. It should be illustrated in Figure 1.

2. The font in the figure is too small, such as Figure2 and Figure 4.

3. The optimized available focal depth should be marked in the figure. At the same time, it is suggested to compare the results before and after optimization. After optimization, it is recommended to give the moire fringe image with the worst contrast in the whole focal depth range, which is convenient to judge the extreme situation and ensure that the signals in the whole range meet the signal requirements for alignment.

4. Simulation conditions of Table 2 need to be supplemented, such gap between wafer and mask.

5. The initial conditions of optimization need to be supplemented. Only 405nm, 532 nm, 633 nm and 780 nm are used?

Author Response

Please see the attachment.Thank you

Author Response File: Author Response.pdf