Industrial upscaling of nano- and microfabrication

One of today’s challenges in manufacturing microstructures is the scalability of the production chain for high-precision parts. The first step is to identify the mastering technology suitable to produce a microstructure master. The master must fit into standard processes for industrial manufacturing of micro-optics. Mastering technologies are consequently combined with established replication processes like injection molding, roll-to-roll hot embossing or nanoimprint lithography which paves the way for mass production.

Mastering technologies

The choice of mastering process depends on the advantages and disadvantages that affect subsequent processes, as well as structure fidelity. Various technologies are used to fabricate microstructures, including ultra-precision machining, laser ablation, one-photon maskless lithography and high-resolution alternatives such as electron-beam lithography and Two-Photon Polymerization (2PP). Two-Photon Grayscale Lithography (2GL ^®) is an emerging novel technology based on Two-Photon Polymerization. It is capable of fabricating complex micro- and mesoscale topographies with submicrometer resolution in polymers. The 2GL-printed structures are either used as masters for replication or as prototypes for direct validation of the performance of the functional part. For a comparison of the main microstructure mastering technologies, see the table below

Industrial scaling-up technologies

Many technologies are available for the industrial-scale replication of micro- and nanostructures. The key is always to provide costeffective replication without compromising the high structure fidelity. The choice of the most suited replication technology is based on the required product type, structure fidelity, prototyping setup costs and product yield. A general overview of the available replication processes is described here. Each of them should be considered as a family with some specialized subtypes:

Nanoimprint lithography (NIL): Patterns are produced by mechanically deforming an imprint resist using a flat stamp. The resist is cured by UV light during the imprinting.

Roll-to-roll imprinting: The working principle resembles NIL but a roller stamp, such as a sleeve or a cylinder, is used instead of a flat stamp. The resist is cured by UV light or by heat during the imprinting.

Injection molding: A molten polymer is injected into a cavity by pressure and at elevated temperature. Once it has cooled, the resulting part is demolded from the working tool.

Hot embossing: A mold is pressed into a polymer, which is heated to a flexible, soft state. The polymer cools down and is demolded

Materials

In most mastering methods, the master structure is made from a different base material than the final part. Materials used to make master templates include metals, alloys such as brass, metalloids like silicon, different epoxies, photopolymers and specially designed polymers. Any of these materials can later be transformed into a nickel shim through electroforming. Electroforming is a galvanic process in which a metal is deposited on a conductive object and then separated, thus making a faithful replica. At this stage, it is possible to create multiple copies from one nickel shim, creating shim families for long and safe storage.

Photopolymers are suitable for functional prototypes when combined with transparent substrates, such as fused silica glass. The photopolymer structure can be used as a prototype, enabling the direct investigation and validation of the performance of, e.g., functional micro-optics, as clear resins on glass substrates allow for direct testing of optical quality and design. The same functional prototypes are then transferred to mass manufacturing as masters for replication technologies

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The technology behind 2GL derives from the additive manufacturing principle of 2PP where a laser is tightly focused into a photosensitive resin. Once a resin-dependent intensity threshold is reached, the material polymerizes inside the small volume in the focal point, forming an ellipsoidal, vertically elongated voxel.

The laser focus scans through the resin, building up micron-sized 3D objects line by line and layer by layer. Usually, this discretization of the printed object into individual layers leads to a staircase effect visible on sloped object surfaces. This effect is particularly pronounced for low gradients, e.g. in the center zones of optical lenses, as it results in deviations from the design and deteriorates surface quality of the printed object.

2GL technology prevents the staircase effect by dynamic voxel size control: An increased laser intensity leads to voxel expansion, mainly in vertical direction. Thus, dynamic laser power control can be used to modulate the voxel height quasi-continuously within one layer. The laser power modulation is synchronized with the high-speed galvo scanning of the laser focus with an extremely fine 100 nm grid and with the accurate lateral stage movement. Continuous voxel modulation in the printing process drastically reduces the number of slices and thus the printing time.

The 2GL-based microfabrication workfl ow starts with a design defi ned by a 16-bit grayscale image fi le in standard PNG or BMP input formats. Optionally, STL fi les can easily be converted to grayscale image fi les. The print job preparation software GrayScribeX generates a print project according to the design file and well-proven presets for the printing parameters, such as laser power and scan speed. The software converts the image information into control instructions for scanning the laser along defi ned trajectories to form the designed topography by curing the typically liquid photoresin. The actual printing is performed by the Quantum X system and requires a substrate onto which the photoresin is applied.

Rapid design iteration loop
After printing a microstructure with 2GL, the shape, dimensions and surface quality are measured using a surface analysis tool such as a confocal microscope. Based on the comparison of the original design with the results of the confocal microscope analysis, the design is adjusted in GrayScribeX with the necessary compensations. Design changes are easily implemented and reprinted using the software. This approach speeds up the iteration cycles, enabling costeffi cient microfabrication of 2.5D topographies in prototyping and mastering for subsequent industrial-scale replication processes.

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From the original design of a microstructure to high-volume production, each part of the multi-stage process chain affects the structural fidelity of the original design. And only a comprehensive and integrated approach from the beginning can lead to successful industrial replication. But even if all the rules are followed, it is advisable to assess the quality of each intermediate result and correct it, if necessary, starting from mastering, through electroforming and upscaling to the final product. This prevents mass production from being ramped up with a potentially suboptimal structure. These processes are shown schematically in the graphic below with the structure undergoing quality control at various stages.

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The demand for microscale components is increasing rapidly, requiring large quantities of inexpensive products with high reproducibility. Recently, polymer-based replication processes have attracted extensive attention due to their low cost, material properties and reproducibility. In combination with high-throughput techniques such as micro injection molding, they offer even faster and more cost-effective alternatives to conventional materials such as silicon and glass.

Micro injection molding is a fast, fully automated process with high throughput, which makes it one of the most suitable processes for the mass production of polymer-based microcomponents. The principle of micro injection molding and its basic steps are like those of a conventional injection molding process. The key factor for success in micro injection molding is the very precise control of pressure and heat during the injection molding cycle.

Why micro injection molding?

Micro injection molding makes use of its high throughput to rapidly mold high-precision microstructures with complex geometries. The production cycle time can be less than 30 seconds. The technology is compatible with numerous polymers, allowing to find a suitable polymer for the desired application. These advantages match the increasing demand for innovative microstructures needed in various application fields such as micro-optics, photonics or microfluidics.

2GL mastering for micro injection molding

Two-Photon Grayscale Lithography (2GL) combines the advances of Two-Photon Polymerization with the capacity of grayscale lithography, which signifi cantly increases microfabrication performance and microstructure quality. Although 2GL is slower than one-photon grayscale lithography, 2GL offers more design freedom and higher shape accuracy, enabling microstructures of better optical quality and performance. This is because 2GL enables rapid and precise microfabrication of ultra-smooth surfaces and structures with excellent shape accuracy in 2.5D, which are used as masters for replication with micro injection molding.

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Nanoimprint lithography (NIL) is used to transfer a single-die master structure to multiple wafer-level imprints. In this process, step & repeat NIL is a decisive option for producing a fully populated wafer-level master by replicating the single-die master across the entire area of a large wafer. The resulting wafer-level master is used for the subsequent high-volume replication process to scale up the wafer-level structures to hundreds or thousands of imprints.

Many industries are already utilizing NIL as a flexible and efficient manufacturing method enabling applications ranging from consumer devices like smartphones and wearables to automotive and biomedical applications. These markets integrate photonic and optical components into their products and continue to drive many advancements in photonics, optical sensors, lighting modules or biomedical chips.

Why nanoimprinting lithography?

As one of the most precise replication technologies NIL has shown to be ideally suited to facilitate the patterning of microstructures with challenging geometries, even freeform microstructures, required for emerging devices and applications across the photonics market. NIL is very flexible and can be used to replicate a wide range of shapes and structures, e.g., prisms, spheric and aspheric lenses, micro lens arrays and various diffractive structures.

A key asset of NIL is the straightforward transfer of complex and high-precision structures to high-volume manufacturing as structures can be replicated with high fidelity over a large area in a single step. NIL enables single nanometer resolution and targets structure dimensions up to a lateral extent in the millimeter range. And wafer-level NIL is a particularly effective and cost-efficient lithography process

2GL mastering for nanoimprint lithography

Two-Photon Grayscale Lithography (2GL) offers a high-precision manufacturing process suitable for rapid prototyping and mastering. The 2GL-printed structures are transferred to fully populated wafer-scale masters by step and repeat (S&R) NIL, using die-level UV-NIL replication technology. These large-area masters on 200 mm or 300 mm wafers are then taken for actual mass production. Combining 2GL with NIL significantly simplifies and accelerates the transfer from prototyping, e.g. of a microoptical design, to mass production.

NIL can replicate the high resolution and shape accuracy of the 2GL-printed structures in large numbers. Thus, 2GL combined with NIL supports a direct route to wafer-level fabrication of advanced designs and to scale up production. Previous constraints on design and miniaturization of mass-produced microoptical elements Nanoimprint lithography 05. 2GL master nanoscribe.com 15 can be overcome, resulting in improved performance and new applications. The combination of 2GL and NIL enables master molds to be scaled up to larger substrates, allowing more devices to be produced simultaneously, reducing overall production costs. In this way, novel optical products based on sophisticated optical designs can not only serve niche markets but also be scaled to mass markets.

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Freeform micro-optics are required e.g. for photonic applications which demand high shape accuracy and cost-effective manufacturing on an industrial scale. Nanoimprint lithography can deliver high and reliable imprint quality for mass production. Reproducible pattern height and low surface roughness throughout the process and over many imprints are crucial for high-quality optical devices.

Pattern fidelity
First, the overall height variation along the entire process, including the 2GL master, the S&R master and final imprints, was determined. The height reduction from the initial 2GL master to the final imprint of about 4% is explained by material shrinkage. This factor is already considered when creating the master design, ending up with the desired structure dimensions in the NIL replications.

Surface roughness
Particular emphasis is placed on low surface roughness for such microstructure imprints, which should be preserved from the 2GL master, through the S&R master, to the final imprints.

The versatility of patterns moldable by NIL in combination with the quality of 2GL master templates enables various applications such as regular and random MLAs, freeform microlenses, and prism arrays for photonic applications. Even complex shapes such as hybrid lenses and tetrahedron pile arrays can be replicated, showcasing a new design freedom in terms of dimension, shape, size and material.

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