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J. Eur. Opt. Society-Rapid Publ. 22, 36( 2026)
Figure
6. Scheme of the experimental setup. The fringe observation system( FrObSy) is placed on the stage of the NPMM-200. The lithography head is mounted in a high stable and damped frame made of Zerodur. The two exposure beams interfere in the focal plane of the observation system instead of a substrate.
Figure 7. Left: Mechanical design of the SBIL lithography head. Right: Picture of the experimental setup, inserted in the mechanical mount of the positioning stage.
u stitch ¼ k y stitch: ð11Þ
For a grating aperture composed of N stitched scan lines, the resulting wavefront error can be expressed as
W stitch ðyÞ ¼ k 2p
X N�1
i¼1
u stitch; i Hðy � y i Þ; ð12Þ
where H( y � y i) denotes the Heaviside function marking the location y i of each stitching boundary. These phase discontinuities introduce local phase steps or tilts in the diffracted wavefront, which reduce the coherent diffraction efficiency and increase the overall wavefront error of the optical element.
3 Experimental setup
The experimental setup is divided into three primary parts( Fig. 6): the lithography head, the FrObSy and the nanopositioning machine NPMM-200 [ 7 ]. The lithography head is mounted on a high stable and damped frame made of Zerodur and is decoupled of the movable stage itself. The FrObSy is placed and fixed on the movable stage of the NPMM-200.
3.1 Lithography setup
The exposure laser is a fiber coupled Toptica Topmode 405 with a wavelength of 405 nm and a fiber output power adjustable up to 65 mW. The laser beam has a Gaussian beam profile( TEM 00) with approx. 0.5 mm diameter. Behind the fiber output, the laser beam is collimated and then reflected by a 45 ° mirror. G 1 and G 2 are binary phase grating made of SiO 2 with a period of p = 1lm anda diffraction efficiency of approx. 40 % for the ± first orders. The distance dl between G 1 and G 2 can be set manually from l 1 = 2.5mmtol 2 = 12 mm with a maximum range of dl max = 9.5 mm. With the Olympus microscope objective lens( UMPlanFI) with a numerical aperture of 0.8( MO) and a focal length f MO = 3.6 mm, the two beams interfere in the focal plane, where the FrObSy is placed. Due to the electrical sliding contacts, the entire mount of the gratings can be rotated by a rotary drive up to 360 ° to adjust the orientation c of the fringe pattern in the focal plane of MO. The shifts dx and dy can be performed by lateral actuators( see Fig. 7).
3.2
The NPMM-200
The nanopositioning and measuring machine NPMM-200 [ 10 ] is a stage with nanometer positioning accuracy and a travel range of 200 200 25 mm 3( x, y, z). Six interferometers measure the six degrees of freedom. Except for the rotation around the z-axis, the axis in x, y, z direction and the rotational degrees of freedom r x and r y of the movable platform are controlled by actuators in the single digit nanometer range. The positioning stage is located in a large aluminum chamber, shown in Figure 8, left. This prevents the system from environmental disturbances, such as air and temperature fluctuations and dust particles. The mounting frame for the lithography head is made of Zerodur and is decoupled from the movable platform, as illustrated in Figure 8, right. This machine is a mandatory part of this work. As a reference system for FrObSy, the combination enables the high performance of the calibration and alignment procedures.
3.3 Fringe observation system
The FrObSy is the key tool to calibrate and align the described SBIL lithography head from Section 3.1. The mathematical and physical principle of the experimental evaluation with FrObSy is based on the carrier-frequency method [ 11 ] and the detour phase principle [ 12 ].