OPTICAL PRODUCT High-precision grayscale lithography
replication processes.
ALTERNATIVE MICROFABRICATION TECHNOLOGIES Various microfabrication technologies are used to fabricate 2D and 2.5D micro-optical structures. This section presents an overview of lithography-based methods, both mask-based and maskless, and compares their capabilities and limitations with those of Two-Photon Grayscale Lithography.
2D UV LITHOGRAPHY One-photon absorption underpins conventional photolithography, where light transfers geometric patterns from a photomask to a photosensitive material. In contrast, maskless technologies use direct laser writing to pattern structures without the need for masks. Microlens arrays are commonly fabricated using a UV lithography combined with photoresist reflow. In this process, a photoresist is exposed to UV light to fabricate 2D microstructures, which are then thermally reflowed to create hemispherical or aspherical surface profiles. While this process is well-suited for large-scale production of simple microlens designs, it is limited in design complexity and in achieving high fill factors up to 100 %. The design flexibility can be enhanced by transferring patterns into the substrate using reactive ion etching( RIE), although this adds process complexity and cost.
GRAYSCALE LITHOGRAPHY Grayscale lithography extends binary photolithography by enabling fabrication of complex 2.5D microstructures with continuous height profiles. This capability is essential for producing advanced micro-optics such as those used in mobile devices, diffractive optical elements, hologram optics and structured surfaces. Two main approaches exist in grayscale lithography: photomask-based lithography and direct laser writing, each suited to different applications.
Figure 2: High aspect ratio microlens array fabricated using Two-Photon Grayscale Lithography, showcasing submicron shape accuracy and optical-grade surface quality.
While photomask-based lithography offers high throughput for large-area fabrication and high-volume production, direct laser writing enables fast iteration cycles for rapid prototyping.
MASK-BASED 2.5D GRAYSCALE UV LITHOGRAPHY Photomask-based grayscale lithography uses specific masks to modulate exposure doses across a photosensitive resist, creating the desired topography after UV light exposure and development. The resulting 2.5D structures can then be transferred into the substrate via etching or replicated by thermal or UV molding. Although this method allows rapid exposure of large substrates for high-volume production, it requires costly grayscale masks precisely tailored to resist characteristics. These masks are time-consuming and expensive to produce and iterate, which can take days or weeks. This limits their use in projects requiring rapid design iteration cycles.
MASKLESS 2.5D GRAYSCALE UV LITHOGRAPHY Maskless grayscale lithography systems, such as those offered by Heidelberg Instruments, provide flexible fabrication of intricate 2.5D microstructures without the need for expensive masks. In this direct laser writing approach, spatially modulated
UV light directly writes into a positive photoresist, with exposure dose variations encoding the desired structure depths. After development, the resulting 2.5D surface topographies are revealed. These patterns can then be transferred into optically relevant materials using replication techniques such as UV molding. Unlike mask-based grayscale lithography, the maskless approach offers precise exposure control to compensate for nonlinear resist responses and batch-to-batch variations. However, achievable structure heights remain limited by the resist thickness, typically up to 60 µ m. Recent studies using newly developed photoresists have demonstrated structure heights exceeding 160 µ m, expanding the range of possible applications.
THE ROLE OF MATERIALS IN NANO- AND MICROFABRICATION Printing material selection influences resolution, surface roughness, transparency, mechanical stability and long-term durability under varying environmental conditions. These properties are particularly critical in photonics and optics manufacturing, where optical performance is paramount. Recent material innovations have introduced photopolymers with high transmission across the visible
Figure 3: Freeform diffractive optical elements with highest shape accuracy for high optical performance. Design by LightTrans International, print by Nanoscribe.
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