Introducing Two-Photon Grayscale Lithography

Two-Photon Grayscale Lithography (2GL®) is a new additive manufacturing technology to generate 2.5D topographies. With its inherent design freedom, this technology targets the needs of modern industrial microfabrication to overcome challenging precision manufacturing demands, e.g. in the sensors, mobile devices, data and telecommunications sectors. The resulting printed objects stand out with submicrometer feature sizes and can contain more than 4,000 gray levels. Topographies with multiple levels are printed while scanning only one layer and thus benefit from drastically reduced printing times. 2GL is a new family member of grayscale lithography technologies that uses power-modulated laser light to shape the height profile of micro- and nanostructured functional devices.

How does Two-Photon Grayscale Lithography work?

Two-Photon Grayscale Lithography is based on Two-Photon Polymerization (2PP), which itself is fundamentally rooted in twophoton absorption. In short, two-photon absorption only occurs in a narrow focal volume of laser light and triggers 2PP within a photoresin, causing the material to solidify. This creates the smallest single-volume pixel, the so-called voxel. When the exposure dose is modulated during the printing process, the polymerization in the photoresin varies across the scanning plane, which in turn varies the size of the voxel. For this purpose, a grayscale image is converted into a spatial variation of the exposure levels, resulting in different voxel heights in one plane and thus a defined topography is printed.

From 2D to 3D exposure dose control

In binary lithography, an exposed positive tone photoresist is completely washed away during development, producing features with rectangular cross-sections of equal height and steep walls. To go one step further, the term grayscale lithography is usually associated with spatial modulation of the exposure dose in linearly absorbing media such as photoresists. A gradient of the exposure dose is translated into a 2.5D photoresist topography during development. In practice this works only for finite-contrast photoresists, which leads to different development rates when the exposure dose is varied, for instance by modulating the laser intensity.

According to Beer’s law, propagation of short-wavelength UV light through a volume of photoresist results in attenuated exposure intensity. In one-photon grayscale lithography, intensity modulation can only occur within a fairly thin layer as far as the light can penetrate the photoresist. Large aspect ratios become increasingly difficult due to the light absorbed more and more in the photoresist volume.

In contrast, the new 2GL technology allows exposure modulation not only in a plane directly beneath the surface of a photosensitive material, but also far below the surface in the volume. This advantage is inherited from 2PP, where the exposure volume is strongly confined to the vicinity of the laser focus. This enables a three-dimensional dose control with very high spatial accuracy at any point within a resin volume. 2GL works very well with highcontrast photoresins compared to the finite contrast resists used in traditional grayscale lithography methods and enables highresolution patterning with feature sizes in the submicrometer range.

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Two-Photon Grayscale Lithography (2GL®) is a microfabrication technology that combines the extraordinary performance of grayscale lithography with the high precision and flexibility of Two-Photon Polymerization (2PP). Thus 2GL inherits the enormous design freedom of the additive technology 2PP to produce almost any micro- and nanostructured surfaces with excellent shape accuracy. The technology enables cost-effective microfabrication of 2.5D topographies for industrial R&D and master production.

Different microfabrication technologies are used for the realization of 2.5D topographies, including maskless grayscale lithography based on one-photon UV light exposure, mask-based grayscale lithography and 2D-based UV lithography followed by reflow techniques. These methods use light to cure photosensitive materials and are established for various microfabrication tasks. However, the limitations become apparent upon considering geometrical design freedom, resolution, workflow and design optimization simplicity. In this chapter, Two-Photon Grayscale Lithography is compared with these microstructuring technologies along with other mastering technologies by mechanical tools.

Design freedom beyond 2D lithography

Typical mask-based photolithography technologies employ one-photon absorption. These use light to transfer a geometric pattern from a photomask to a photosensitive material. In contrast, maskless techniques use direct laser writing to structure the patterns directly onto a wafer without using a mask.

A common fabrication method for microlens arrays is a 2D-based UV lithography approach combined with photoresist reflow. Here, a photoresist is exposed to UV light to pattern 2D microstructures. After exposure and development, a photoresist reflow step is performed. In this phase, the photoresist microstructures deform in a controlled way induced by heat which results in hemispheres or aspheres as surface topographies. The process is intrinsically limited to a range of designs with reduced area fill factor. Design freedom can be further increased by combining reactive ion etching (RIE) with pattern transfer into the underlying substrate. However, this post-processing step increases manufacturing complexity and costs.

Grayscale and maskless lithography for complex topographies

Going beyond binary lithography, one-photon or UV-lithography-based grayscale exposure enables the production of 2.5D topographies with high accuracy. Grayscale photolithography uses very complex photomasks instead of binary 2D masks to achieve the desired modulation of the exposure dose required to pattern the resin topography. These 2.5D masks are complex to fabricate, making it expensive to achieve the desired modulation of the laser dose at the target position. The mask must be designed for many exposures to keep costs per part reasonable. This microfabrication route limits the applications for such grayscale masks, as such costs are not amenable to small- and medium-sized enterprises (SMEs).

Direct laser writing is a viable option that overcomes the limitations of mask-based lithography. The maskless grayscale approach based on one-photon lithography transfers the spatially modulated exposure dose typically into a positive-tone photoresist breaking up chemical bonds. Beer’s law causes an exponential decay of the UV dose within the photoresist. The lateral change of the exposure dose converts into a 2.5D topography during development. In contrast to one-photon grayscale lithography, 2GL overcomes height limitations imposed by Beer’s law. The use of liquid negative-tone resins with 2GL can thus achieve structure heights beyond 60 µm as a typical maximum value of thick-film positive-tone resists. Instead, vertically stitching-free object heights of up to 350 µm can be achieved with 2GL. In one-photon grayscale lithography, the photoresist chemistry is very sensitive to varying ambient conditions, such as humidity and temperature. These variations dramatically affect process stability, requiring constant monitoring and adjustment of process conditions to produce consistent results. 2GL is a very robust process in terms of ambient conditions during preparation, printing and development. In most cases, neither spin-coating, nor pre- and post-baking steps are required.

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Quantum X litho is the world’s first maskless lithography system based on Two-Photon Grayscale Lithography (2GL). To ensure that all printing processes are reliable and easy to perform, Nanoscribe developed a straightforward and self-contained workflow, starting from a grayscale source image file. The software offers a complete solution for the preparation and execution of the print projects and enables the printing of high-precision topographies in just a few steps.

Print project preparation

The GrayScribeX software tool prepares print projects for transfer to Quantum X litho. The process starts with the import of a grayscale image containing the topographic design. PNG and BMP formats with 16-bit resolution are recommended to achieve the best print quality. It is also possible to load STL designs that are converted into grayscale images, and to combine multiple images or STL files into one print project. The software converts the image input into control instructions for the printer to direct the laser along defined trajectories that form the topography.

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The choice of resins and substrates is decisive for the quality of the final printed structures. The selection of a photoresin determines the intended final qualities of the printed object, such as resolution, surface roughness, specific chemical and mechanical properties and, last but not least, the printing time. Appropriate substrates in turn ensure robust and reliable printing workflows.

Substrates are the basis for additively manufactured topographies

Substrates play an important role in the maskless process of Two-Photon Grayscale Lithography (2GL) ensuring that a stable connection is made between the substrate and the printed part during the entire printing process. The substrates are chosen according to the preferred printing configuration and the desired resin-objective combination.

During the printing process, the substrate and the printed structure are precisely aligned and firmly attached. It is therefore crucial to use substrates that are perfectly matched to the intended applications and post-processing. Transparent and opaque substrates, such as silicon and fused silica wafers with a diameter of up to 6 inches, are the recommended standard substrates.

Especially in industrial workflows, wafer scale manufacturing is followed by subsequent post-processing, e.g., replication processes, which are performed on wafers. The capability of 2GL to print high-precision topographies at the wafer level is an advantage for the seamless integration of 2GL into industrial production processes.

Robust photoresins as printing materials

Thermoset polymers are the dominant material class for 2GL, a printing technology based on Two-Photon Polymerization (2PP). The polymer chains in a thermoset are strongly cross-linked and thus form an infusible (non-melting) and insoluble network. The most common chemical mechanism for thermoset curing in 2GL is light-stimulated radical polymerization.

Nanoscribe’s IP Photoresins are negative-tone resins suitable for use in 2GL processes. Unlike typical positive-tone grayscale photoresists used in one-photon grayscale lithography, photoresins in 2GL can overcome height limitations or film thicknesses of 60 µm and even reach several hundred microns. Additional preand postprocessing steps such as spin-coating of single and double layers as well as pre- or post-baking are not necessary. This significantly reduces processing time and costs during the preparation and post-processing phase. Moreover, the resins are robust after exposure and less sensitive to the development conditions and environmental influences than typical one-photon grayscale photoresists.

Printing Polymers

Thermoset polymers are the dominant material class for Two-Photon Polymerization (2PP). The polymer chains in a thermoset are strongly cross-linked and thus form an infusible and insoluble network. The most common chemical mechanism for thermoset curing in 2PP-based 3D printing is light-stimulated radical polymerization.

The induced chemical reaction in brief: In the laser focus two photons are absorbed at once, each with near-infrared energy to effectively excite a polymerization reaction that requires energy equivalent to UV light. As a result of this reaction a solid, insoluble thermoset polymer is formed within the exposed resin volume.

Post-processing options for 3D-printed structures

Besides the development of the printed structure, which simply means removing the unpolymerized material, there are some post-processing procedures. Two common methods are thermal or UV post-treatments to ensure optical and mechanical material properties as well as the preparation of the printed object for SEM analysis by sputter coating the structure with a thin conductive layer.

With post-print processes, such as 3D casting, 2.5D replication, atomic layer deposition (ALD), chemical vapor deposition (CVD) or pyrolysis, 3D-printed structures are modified and transferred to other materials as plastics, ceramics, dielectrics, metals or carbon:

• 2.5D Replication: 2PP printed 2.5D polymer masters perfectly support standard mass replication processes, such as injection molding, (direct) hot embossing and nanoimprint lithography, which even opens the way to step and repeat processes for wafer-scale manufacturing.
• 3D Casting: The casting of 3D-printed structures creates a mold for micro injection molding. This post-processing step is essential for single-unit replication of positive or negative molds based on a lost cast.
• Atomic layer deposition: Printed foams are coated with ALD and the selective etching of small perforations allows to remove the polymer to create ceramic foams.
• Chemical vapor deposition: Materials can be deposited uniformly by CVD in order to provide access to further materials, such as silicon; this often is performed in combination with ALD.
• Pyrolysis: The combination of 2PP and subsequent pyrolysis in a vacuum at 900 °C is used to fabricate nanostructured carbon materials that are lightweight and ultra-strong.

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Two-Photon Grayscale Lithography (2GL) redefines the design freedom for industrial applications of numerous 2.5D microstructures. This is especially useful for engineers who are developing innovative refractive and diffractive microoptics. 2GL combines an additive manufacturing approach with maximum precision, quickly transforming digital models into 2.5D microoptics as prototypes and masters within industrial production processes.

Integrating Two-Photon Grayscale Lithography into established replication processes also increases the scale-up capabilities for micro- or nanostructured polymer master production. Such masters can be directly used for the bulk fabrication of microoptical elements. This leads to the transfer of unique microoptics into production chains and leverages the economic impact of innovative products in strongly growing markets. With 2GL, the fabrication of ultra-precise refractive and diffractive microoptics as polymer masters is the starting point for state-of-the-art series production.

Multilevel diffractive optical elements

3D facial recognition is a technology for biometric purposes and a well-known microoptical application that uses diffractive optical elements (DOEs) to generate structured light. This application is challenging owing to the complexity of the DOEs required. DOEs are extremely precise microstructured surfaces that shape light by diffraction and generate patterns of dots, grids, or even complex images in the far field. Common techniques for DOE microfabrication include electron beam lithography and mask-based or maskless photolithography processes. However, these techniques have their limitations. The fabrication of multi-level DOEs involves long production times, expensive mask costs or time-consuming and costly processes. For example, mask-based fabrication consists of several lithography steps with N masks to generate a 2N-level DOE. This process necessitates precise and skillful alignment procedures between adjacent levels.

The 2GL technology is precisely designed for the fabrication of 2.5D topographies such as multi-level DOEs. Multi-level DOEs are produced in one printing step with 2GL and are therefore an application that can be ideally realized with 2GL. By modulating the exposure dose and the associated voxel height, the DOE topography is easily generated along the scanned surface. The number of levels imparts little economic impact on production time and processing costs, as printing time and workload are typically independent of the number of levels. The technology also provides the lateral and axial resolution to meet the required feature sizes of diffraction patterns down to the submicron scale.

Discrete multi-level DOEs with 2, 4, 8 or more levels can be fabricated with 2GL. DOE designs that rather look like continuous topographies can even have of up to 4,096 levels. The latter has implications for a radical shift in optical designs with not yet realized opportunities as compared to classical discrete levels with square pixels. 2GL also meets new trends in optical design engineering and achieves high diffraction efficiency for phase I am really satisfied with the repeatability of the blazed grating structure as well as the accordance with the design. This structure presents difficult challenges, including tight tolerances for the grating angles, low surface roughness and accurate height for the sawtooth structure. 2GL is one of the best methods we know for the microfabrication of multisegmented gratings that are capable of processing different wavelength ranges in a broadband spectrometer. I am convinced that 2GL can handle highly complex microstructures with ease. Artem Shcheglov, Vrije Universiteit Brussel – Brussels Photonics 20 nanoscribe.com modulations > 2π and topographies of several micrometers height for visible and NIR applications. As an example, blazed DOEs can be realized, which spatially separate the undesired zeroth order from the desired diffraction pattern. Further examples are multisegmented blazed gratings, Fresnel lenses and metalenses, which are also strongly supported by 2GL, and enable spectroscopy, beam shaping and structured light applications

What’s next? – 3D printing by 2GL^®

For high-resolution 3D printing of complex designs or curved shapes, the print must be sliced and hatched into a large number of horizontal and vertical layers. This can significantly increase printing time for smooth, curved or filigree structures.

Two-Photon Grayscale Lithography (2GL^®) enables voxel tuning, resulting in a significant reduction of layers to be printed. This is achieved by fast laser modulation in the scan pass. However, this technology was first only available for 2.5D structures.

3D printing by 2GL^® takes Nanoscribe's grayscale technology to the third dimension. The high-resolution 3D printing process is based on dynamic modulation of laser power in real time while scanning at highest speed. This leads to highly precise sizing of the polymerizing voxel to perfectly match the contours of any 3D shape. The proprietary 3D nanofabrication technology produces flawless, smooth surfaces without any slicing steps or voxel-related shape distortions, accurately representing the actual shape of any high-resolution 3D design and is the fastest 2PP-based 3D Microfabrication technology available on the market today. The increase in throughput depends on the structure design, but is 10 to 60 times the throughput of any current 2-photon lithography system while maintaining the same demanding print quality requirements.

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