94
J. Eur. Opt. Society-Rapid Publ. 22, 9( 2026)
Figure
13. Piezo calibration measurement. The different amplitude of the sinusoidal curves gives the transfer factor between the piezo movement of mirror M1 and the fringe movement in the substrate plane.
Figure 14. Phase fluctuation measurement with enabled fringe locking. The fringe locking signal is controlled to zero by moving the piezo. The fringe observation system shows a greatly reduced movement of the fringes in the substrate plane.
Figure 15. AFM measurement of a linear grating in photoresist. The grating depth is approx. 192 nm ± 3 nm.
the piezo movement( orange line) and the fringe movement in the substrate plane( red line) are visualized. The fringe locking signal is controlled to zero by moving the piezo. The RMS value of the fringe locking signal in closed-loop is 1.48 nm. The fringe movement is significantly reduced. Between 75 s and 110 s the fringe locking was more influenced by the heat turbulence of the observation camera, so that the peak-to-valley movement is around 25 nm with an RMS value of 5.29 nm. For the lithography process with photo resist, the heat of the observation camera would not disturb the fringe locking. During closed loop control, the movement of the fringes with a period p = 270 nm was mainly reduced to ± 5 nm. This translates to a PV-wavefront reconstruction quality of approx. at such small
k 25
periods. The mean intensity contrast of the fringes is 0.839. The dose contrast is 0.827. With fringe locking control enabled, the contrast loss was reduced from 7.4 % to 1.2 %. For first lithography tests, the fringe observation system was replaced by a substrate with positive photoresist
Figure 16. SEM image of the linear grating in photoresist, sputtered with gold to avoid charging effects. The determined period of the structures is p = 270 nm(± 2 nm).