Quasi-seamless stitching for large-area micropatterned surfaces enabled by Fourier spectral analysis of moiré patterns

Abstract

The main challenge in preparing a flexible mold stamp using roll-to-roll nanoimprint lithography is to simultaneously increase the imprintable area with a minimized perceptible seam. However, the current methods for stitching multiple small molds to fabricate large-area molds and functional surfaces typically rely on the alignment mark, which inevitably produces a clear alignment mark and stitched seam. In this study, we propose a mark-less alignment by the pattern itself method inspired by moiré technique, which uses the Fourier spectral analysis of moiré patterns formed by superposed identical patterns for alignment. This method is capable of fabricating scalable functional surfaces and imprint molds with quasi-seamless and alignment mark-free patterning. By harnessing the rotational invariance property in the Fourier transform, our approach is confirmed to be a simple and efficient method for extracting the rotational and translational offsets in overlapped periodic or nonperiodic patterns with a minimized stitched region, thereby allowing for the large-area and quasi-seamless fabrication of imprinting molds and functional surfaces, such as liquid-repellent film and micro-optical sheets, that surpass the conventional alignment and stitching limits and potentially expand their application in producing large-area metasurfaces.

Fig. 1. Overview of the MAPI method.

a Overall schematic of the R2R NIL process using a quasi-seamless flexible mold. b Schematic of the MAPI method in the overlapped area between the imprinted pattern (p_n) and imprint mold pattern (p_n+1). (i) Inevitable rotational and translation offsets occurs in the overlapped area due to an initial random alignment state; (ii) remaining translational offset after rotational alignment based on the moiré pattern analysis; and (iii) the quasi-seamless tiled patterns after compensating for the translational offset. Scale bar: 200 µm.

Fig. 2. Simulated results for the extraction of the rotational offset using a Fourier spectral analysis of the moiré lattices formed by two identical overlapped patterns.

a Simulated images of base and moiré lattices (insets show the corresponding Fourier-transformed images of the overlapped patterns (p_n + p_n+1) for the periodic structures). b The measured rotation angle (θ_M) as a function of the input rotation angle (θ_R).

Fig. 3. Sequential explanation of the proposed MAPI process.

a Real-time monitoring of the moiré patterns in the superposed area. b Extracted image for FT. c Analysis and compensation for the rotational offset (∆θ). d,e Analysis and compensation for the translational offset (∆xy). f Quasi-seamless micropatterned surfaces after completion of the MAPI process. Scale bar: 200 µm.

Fig. 4. Quasi-seamless large-area microstructured surfaces using step-and-repeat stitching based on the MAPI process (stitching sequence: 1 → 2 → 3 → 4).

Photographs of the MAPI-based stitched microstructured surfaces: a interconnected hexagonal microcavity and f MLA film. Representative 3D profile or optical images for translational (seam and edge) and height offsets: b–d interconnected hexagonal microcavity and g–i MLA films. Statistical analysis (n = 5) for the rotational, translational, and height offsets in stitched microstructured surfaces (w/: with MAPI process, w/o: without MAPI process): e interconnected hexagonal microcavity and j MLA films. Scale bars are 20 mm (a, f), 20 µm (b, g), and 100 µm (c, h). Error bars represent the standard deviations of five independent data points.

Fig. 5. R2R imprinting process for the continuous fabrication of large-area functional films with flexible molds formed using the MAPI and manual stitching processes.

a R2R-NIL system using a wrapped flexible mold around the roller. b, c The measured contact angle on the initial flexible mold and their replicated films fabricated using a multiple-stitched flexible mold (n = 3 in each area). d The wetting and pinning property of sliding droplets in the seam areas. e The light focusing characteristic of the replicated MLA films in the seam areas. Scale bar: 100 µm.