## 4 Discussion and Conclusions  
We have designed and assembled the orthogonal type two-axis Lloyd's mirror interferometer as the exposure system. Apart from removing of the unexpected beams 4 and 5, the grating shape uniformity is theoretically evaluated in terms of an undesired interference between beams 2 and 3 by an index of degree of orthogonality.  
Experimental results reveal that the difference is not significant for the fabricated 2D gratings under the original polarization status (s, s, s) and the optimal combination polarization status (s, \(-33.5^{\circ}\), \(33.5^{\circ}\)) under a specific incident angle corresponding to a 1 \(\upmu\)m grating period in both X and Y directions. The degree of orthogonality of beams 2 and 3 that fall in the same amounts level in the above two cases is the main reason. The degree of orthogonality of beam 2 and 3 are 0.07 and 0.048, respectively. The fabrication result under polarization was consistent with that shown in previous research, where similar initial polarization states (s, \(-45^{\circ}\), \(45^{\circ}\)) were employed [18]. However, the grating area would be greatly enlarged without polarization as shown in Figure 5a compared with all the simulations and experiments shown in [18]; the optical system complexity was greatly reduced compared with the present technique, TBTE [5; 10].  
It is also confirmed that, under the condition of incident linear polarized light, the combination of initial polarization status of beams 2 and 3 corresponding to orthogonal polarization state in the orthogonal two-axis Lloyd's interferometer does not exist. The microstructures of the fabricated gratings are all consistent with the designed value. However the diffraction efficiency uniformity, both in a relative large area and the four diffraction beams on a point, require improvement.  
In summary, polarization modulation technique of the LIL system can bring a good grating shape despite grating area downsizing, while, without polarization, i.e., usage of the initial polarization under some specific angle that can reach a similar grating performance, with a reduced system complexity and an enlarge grating area.  
Figure 8: AFM images of fabricated grating without polarization under different incident angle \(\theta\) shown in Figure 2: (**a**) 60\({}^{\circ}\) and (**b**) 85\({}^{\circ}\).  
**Author Contributions:** Conceptualization, X.L.; Methodology, X.L. and H.L.; Software, H.L.; Validation, X.L., Q.Z. and K.N.; Resources, Q.Z., G.W., and X.W.; Data Curation, H.L.; Writing--original draft preparation, X.L., H.L.; Writing--review and editing, X.L.; Visualization, X.L.; Supervision, X.L.; Project Administration, Q.Z.; Funding Acquisition, G.W. and K.N.  
**Funding:** This research was supported by the National Natural Science Foundation of China under Project No. 51427805, the Youth Funding of Shenzhen Graduate of Tsinghua University with Grant No. QN20180003, Shenzhen Fundamental Research Funding Grant No. JCYJ20170817160808432 and Grant No. JCYJ20160531195459678, and the National Key Research and Development Program under Grant No. 2016 YFF0100704.  
**Conflicts of Interest:** The authors declare no conflict of interest.