J. Eur. Opt. Society-Rapid Publ. 22, 9( 2026) 87
can be determined precisely. To enable real-time phase calculation, photo detectors are often used in combination with polarization optics [ 14 ]. However, phase errors are not the only challenge in SBIL systems. Precise adjustment and alignment of the optical components are also essential. Reference gratings and dedicated setups are commonly used for pre-alignment [ 15 – 18 ].
Polarization cameras have a pixelated polarizer array in front of the sensor. This offers area-based real-time interferometry. We will use this approach to implement a suitable fringe locking system to a SBIL writing head. Compared to commonly used photo diodes, the area-based fringe locking system provides not only the phase offset error between the interference beams, but also the rotational movements respectively the rotation of the wavefronts. In contrast to the Fourier-transform method [ 19 ], carrier frequencies are not needed to evaluate wavefronts. This provides a dynamic scope for this approach. In this paper, we present a compact design for an SBIL writing head, which is integrated into the nano-positioning and measuring machine NPMM-200 [ 20 ]. We evaluate the experimental performance of the fringe locking system using the polarization phase-shifting interferometry and demonstrate in situ angular tracking of the fringe pattern. In addition, for all performance tests, we verify the measurements with an independent fringe observation system.
2 Effects of fringe perturbation
The quality and accuracy of a diffractive grating crucially depends on the stability and orientation of the interference fringes. Disturbances lead to incorrectly exposed areas in the photoresist and thus directly influence the wavefront quality of the diffractive grating. An SBIL system consists of two main components: the writing head, which generates a small stationary interference pattern, and the stage positioning system, which scans the substrate beneath this pattern to produce extended linear gratings stripe by stripe. In this work, we present a solution that ensures the interference pattern remains stationary even in the presence of drift effects. This system will be integrated in the future into the overall control architecture illustrated in Figure 1, where it will also enable deliberate adjustments of the interference pattern to compensate for stage positioning errors. In contrast to the temporally high-frequency corrections that will be corrected in an 8333 Hz control-loop, the drift compensations of the writing head do require only control-loop frequency of a few ten Hertz due to the relatively slow drift variations.
Typical disturbances in the writing head are phase shifts, fringe pattern rotations and fringe period drifts. The different disturbances are simulated in Figure 2.
2.1 Phase shift
A phase shift occurs when the optical path lengths of the interfering beams change slightly, for instance due to thermal fluctuations or mechanical vibrations. This results in a shift of the fringes in the interference pattern perpendicular to the scanning direction. The period and the fringe
Figure 1. Control architecture of the SBIL writing head setup. The control systems of the lithography writing head and the positioning system NPMM-200 are separated. The dashed components are the scanning system into which the writing head will be included to form an SBIL system. For the performance evaluation presented in this work, a fringe observation system is mounted beneath the writing head on the NPMM-200 stage to monitor the resulting interference pattern.
orientation remain unchanged. Mechanical vibrations are high-frequency errors, but can be minimized by suitable damping of the system. Thermal fluctuations in the environment are low-frequency disturbances that are difficult to control. Therefore, the optical path lengths of the exposure beams are adjusted to compensate for fluctuations. In this paper, the phase shift is measured in an SBIL system and will be compensated by piezo-actuated mirror.
2.2 Period drift
The period drift is a change in the spatial period of the interference pattern. It is caused by drifts in the angle of incidence of the writing beams and also induces a phase shift. The change in the angle of incidence is usually caused by thermal movement of a mirror. Measurements on our system have shown that the mechanical and thermal stability of the writing head setup keeps period drifts( less than 150 ppm / h) in a region where it does not need to be considered.
2.3
Fringe pattern rotation
The rotation of the pattern is caused by a slight change in the angle between the interfering beams, which can be caused by insufficient pre-alignment or thermal expansion of the optical components. The result is a fringe pattern that rotates with respect to the intended scanning direction, which leads to poor dose contrast during exposure. While this is not a problem for our experimental system due to the low thermal drift( see Sect. 2.2), it still might be interesting to be controlled to follow yaw errors of the scanning stage.
2.4 Photoresist exposure and dose contrast loss
Perturbations in the interference pattern affect the photoresist exposure process itself. A stable and well-defined interference field is essential to produce a high-contrast intensity modulation in the resist. When phase shifts, period drifts