## 5 Conclusions  
There are several comments that can be made regarding the above results:  
1. The significant standard deviation for thickness is due to the large side lobes of the MDR curve. At some pixels, these high-level side lobes are fitted as the second-highest peak, so the error of thickness is larger. The original influence induced in the measurement process can affect the level of the side lobe. Thus, in the absence of the cause of the side lobe, there is no a priori knowledge to reduce this error.
2. The R-square in the experiment was smaller than the theoretical value. There is a considerable error between the experimental value of \(FWHM_{2}\) and the theoretical value of \(FWHM_{1}\), which is set to the start point of \(FWHM_{2}\) in this study. For the same film material, starting from the empirical value obtained by pre-calibration can produce a higher fitting degree.
3. Although the IPS algorithm has a faster computation speed than other iterative algorithms, the proposed approach is more time-consuming than the MP-SIM. Further study on improving the efficiency of the fitting process would enhance the method's performance.  
In conclusion, we proposed and experimentally validated a method, named iterative peak separation structured illumination microscopy (IPS-SIM), to develop the detection threshold of thickness in SIM. When the MDR curve peaks are too overlapping and bent into one, the traditional maximum-point-based technique (MP-SIM) has no ability to recognize each peak and measure the thickness and surface profile. In order to overcome the limitation, this study suggests obtaining the peak location of the modulation depth response (MDR) curve by the optimization algorithm. The mentioned method can provide a lower measurement threshold and high precision if the iterative variable has a good start value. In this paper, the influence of each parameter on the measurement accuracy  
Figure 9: (**a**) The measurement by the commercial stylus Profilometer. (**b**) The 3D reconstruction of the partial sample by the IPS-SIM, where the pixels range from 101 to 400 at the \(x\)-axis and from 51 to 300 at the \(y\)-axis. (**c**) The cross-section profile at X = 70 of Figure 9b. (**d**) The repeatability measurement at the same point by the IPS-SIM.  
is numerically analyzed and the optimal initial value of each parameter is determined. Single-film-layer specimens coated with photoresist are measured to verify the feasibility of the IPS-SIM and its lower detection threshold than the conventional approach.  
Conceptualization, K.Y. and Y.T.; methodology, K.Y. and Z.X.; software, J.F. and C.H.; validation, K.Y., Z.X. and C.H.; formal analysis, K.Y. and Y.T.; investigation, K.Y.; writing--original draft preparation, K.Y.; writing--review and editing, K.Y. and Y.T.; supervision, Y.T. and S.H.; funding acquisition, S.H. and Y.T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 61875201, 61975211, and 62005287; the Sichuan Science and Technology Program, grant number 2019)DRC0064, 2020 JDJQ0005, and 20202 YD020.  
Not applicable.  
Data Availability StatementData is contained within the article.  
The authors gratefully thank [PERSON] for his assistance during the experiment.  
The authors declare no conflict of interest.