J. Eur. Opt. Society-Rapid Publ. 22, 36( 2026) 357
Figure 1. Left: Scheme of the SBIL lithography head. The binary beam splitter gratings G 1 and G 2 generate two collimated and coherent beams. The green marked area can rotate around the z-axis by angle c. The mechanical setup above the microscope objective lens MO can be laterally shifted in the x – y-plane. Right: Principal scan-and-stitch process of scanning beam interference lithography.
SBIL requires precise calibration of the beam intersection angle as well as wavelength stability [ 1 ]. Wavefront tilts between the beams cause local variations in contrast, and they degrade pattern fidelity during scanning. Residual tilt can be a primary source of systematic error in SBIL grating fabrication [ 1 ]. Accurate wavefront matching can be achieved through interferometric techniques such as reference-grating metrology [ 5 ]. Furthermore, global-alignment strategies developed for large-area interference lithography [ 6 ], provide additional tools with using reference grating for mapping and optimizing contrast across extended substrates, as required for SBIL system calibration.
Although numerous alignment and calibration strategies for interference lithography have been proposed, they typically require complex experimental setups and multiple independent measurement systems to characterize fringe orientation, period, wavefront matching, and contrast. Each subsystem introduces its own alignment steps resulting in workflows that are technically demanding and often difficult to integrate into a single coherent calibration procedure. Our approach is to observe the writing pattern in the substrate plane directly prior to grating fabrication. In combination with a nanopositioning machine [ 7 ], a fringe observation system( FrObSy) is placed instead of a substrate to quantify the movement of the fringe pattern and determine the relevant parameters for the scanning process. Our goal was to develop one tool to quantify all necessary degrees of freedom for an SBIL system. This work demonstrates the alignment and calibration process of a complex lithography system.
2 Degrees of freedom in an SBIL system
The scheme of the writing head is shown in Figure 1, left. The laser beam is reflected by a 45 ° mirror. At a first phase grating G 1 the beam is diffracted to ± first orders. Subsequently, both beams are diffracted at a second phase grating G 2 again and are parallelized. The distance dl between
G 1 and G 2 can be adjusted manually and defines the distance between the beams. Depending on the beam distance, the beams are focused at different angles after passing through the microscope objective( MO), which is why the period can be adjusted via the grating distance dl. Additional orders are neglected in the scheme because of the insignificant low intensity. With the MO lens, the two beams interfere in the focal plane, where the resist coated substrate is placed. The degrees of freedom of the lithography head, which define the features of the fringe pattern, are the fringe period p, the pattern orientation c and the pattern tilts b x, y, respectively the lateral shift dx and dy in relation to the position of the MO lens. The green marked area is a rotational motion unit and rotates the linear gratings G 1 and G 2 around the z-axis. The entire mechanical setup above the MO can be laterally shifted in the x – y-plane to perform adjustments of the pattern tilts b x and b y.
A general scan-and-stitch process of SBIL is shown in Figure 1, right. A writing spot with a few fringes scans along the fringe orientation. After every scan, the spot is shifted perpendicular to the scan direction. The shift is a multiple of the fringe period so that the next scan line is stitched to the already exposed area. This process is repeated until the entire substrate area is exposed. The high-precision fabrication of diffractive elements with SBIL requires precise adjustment of the lithography system. The various degrees of freedom of an SBIL lithography head are discussed below and the effects for the fabrication process in photo resist are explained.
2.1 Fringe period
The function and therefore the application range of a diffractive optical element depends largely on the grating period and is directly defined by the period of the fringe pattern of the writing spot. Therefore, it is essential to know the exact period before the exposure process. The period p is defined by the wavelength k and incident angles h 1 and h 2 of the two interference beams as