Volume 8, Issue 3-4 - 3D Lithography / Guest Editors: Robert Kirchner, Jun Taniguchi

Grayscale lithography is used to produce three-dimensional (3D) structures on micro- and nanoscale. During the last decade, micro-optics and other applications were actively pushing the market demand for such structures. Direct-write systems that use lasers and heated scanning probes can be used for high-precision grayscale micro- and nanolithography. They provide solutions for the most demanding applications in research and industrial manufacturing. At both the micro- and nanoscale, though, some challenges remain, mainly related to throughput. Ongoing R&D efforts and emerging new applications drive several companies to join forces in order to meet the market demands for grayscale lithography of today and in the future.

We report on the fabrication of very high-resolution discrete four-resist-level grayscale patterns in poly(methyl methacrylate) with just 6-nm step height and down to 32-nm step width using dose-modulated, grayscale electron beam lithography and a low-contrast resist-developer system. This direct pattern writing is important for replication in high-volume manufacturing of diffractive optics. An innovative concept of unexposed auxiliary spacers helped to enhance the discrete character of the multi-level patterns. For pattern step widths between 100 and 32 nm, a transformation toward blazed gratings with increasingly continuous-slope character was obtained. All high-resolution patterns were prepared in a single exposure and development process from an initially about 30-nm thin film. The pattern roughness due to a relatively large polymer molecular weight was reduced using selective thermal annealing with only minimally affecting the global pattern shape by reflow. The results will enable further approaches toward single-digit vertical and prospectively single-digit lateral resolution grayscale patterns.

Following the demand for three-dimensional (3D) micromachined structures, additive and subtractive processes were developed for fabrication of real 3D shapes in metals, alloys and monocrystalline Si (c-Si). As a primary structuring step for well-defined 3D structuring of the photoresist, grayscale lithography by laser direct writing was used. For additive fabrication of 3D microstructures, structured photoresist was used as molds. They were sputtered and subsequently electroplated by a metal (Cu) and an alloy (NiCo). The derived electroplated structures were demolded from the photoresist using an organic stripper. These metal structures are satisfactory replicas of the photoresist pattern. For subtractive pattern transfer of 3D structures into c-Si, reactive ion etching (RIE) was used to transfer the 3D photoresist structure into c-Si with 1:1 pattern transferability. The process parameters of RIE were optimized to obtain a selectivity of 1 and an anisotropy factor close to 1. Whereas conventional X-ray lithography (LIGA) and nanoimprint lithography result in 2.5D patterns, these techniques allow the fabrication of almost any arbitrary 3D shapes with high accuracy. In many cases, 3D structures (‘free forms’) are required, e.g. for molding of optical components such as spheres (or aspheres), channels for lab-on-a-chip and pillars for biological applications. Moreover, 3D structures on Si could be used as optical gratings and sensors.

The effects of nanomolding characteristics on an antireflective surface fabricated via injection molding were investigated. The optical property of a sub-wavelength structure (SWS) of our own making was also measured. The sizes of nanostructures fabricated on SWS molds were controlled by changing the average particle diameters used as mask and the time of reactive ion etching. The maximum filling ratio of the injected polymer was increased from 51.7% to 90.4% by changing the average particle diameters from 83.8 nm to 111.2 nm. In addition, the filled ratio of the injected polymer was increased from 51.7% to 73.7% under the same processing conditions. The results of the measurements of the optical property indicated that the reflectance of small-sized and large-sized SWSs fabricated with the same process condition was decreased at the wavelengths of 550 nm and 980 nm, respectively. The wavelength showed that the minimum reflectance was varied from the visible range to the near-infrared range by changing the size of the SWS under the same processing condition. This result led us to conclude that we can obtain antireflection surfaces for any wavelength by varying the size of the SWS under the same injection-molding condition.

The applications of freeform optical surfaces in modern optical systems are providing unique solutions over rotationally symmetric surfaces. These surfaces offer higher degrees of freedom to the designer to enhance the high-end performance of the optical system. The precise metrology of freeform optics is one of the major bottlenecks for its use in imaging applications. Modern optical fabrication methods (i.e. fast or slow tool servo configuration) are, in principle, capable to meet the challenges to generate complex freeform surfaces if supported by precise metrology feedback for error compensation. In the present work, we have developed a Shack-Hartmann sensor-based metrology technique that can be used for quantitative in situ measurement of freeform optics. The sensor head is used to measure freeform optics in the reflection mode by following the CNC tool path in the offline mode. The measurements are used as feedback for corrective machining. Quantitative analysis is also performed to estimate the error budget of the metrology system. Further, the proposed in situ metrology scheme is validated by measuring freeform surface using a coherence correlation interferometric optical profiler.

In order to realize the mass production of the large-area flexible transparent film heater (FTFH) at low-cost, this paper presents a novel method which can achieve the direct fabrication of the large-area FTFH with Ag-grid by using an electric-field-driven jet deposition micro-scale 3D printing. The effects of the line width and the pitch of the printed Ag-grids on the optical transmittance and the sheet resistance are revealed. A typical FTFH with area of 80 mm × 60 mm, optical transmittance of 91.5% and sheet resistance of 4.7 Ω sq −1 is fabricated by the nano-silver paste with a high silver content (80 wt.%) and high viscosity (up to 20 000 mPa · s). Temperature-time response profiles and heating temperature distribution show that the heating performance of the FTFH has good thermal and mechanical properties. Furthermore, the adhesive force grade between the Ag-grid and the PET substrate measured to be 4B by 3M scotch tape. Therefore, the FTFH fabricated here is expected to be widely used in industry, such as window defroster of vehicles and display or touch screens owing to its striking characteristics of large area and low cost fabrication.

Industrial high-precision 3D Printing (HP3DP) via two-photon absorption (TPA) provides freedom in design for the fabrication of novel products that are not feasible with conventional techniques. Up to now, 2PP-fabrication has only been used for structures on the micrometer scale due to limited traveling ranges of the translation stages and the field-of-view (FoV) of microscope objectives (diameters below 0.5 mm). For industrial applications, not only high throughput but also scalability in size is essential. For this purpose, this contribution gives insights into different manufacturing strategies composed of varying exposure modes, fabrication modes, and structuring modes, which enable the generation of large-scale optical elements without relying on stitching. With strategies like stage-only mode or synchronized movement of galvoscanners and translation stages, optical elements with several millimeters in diameter and freeform shape can be fabricated with optical surface quality.

There are a growing number of applications where three-dimensional patterning is needed for the fabrication of micro- and nanostructures. Thus far, grayscale lithography is the main technique for obtaining a thickness gradient in a resist material that is exploited for pattern transfer by anisotropic etch. However, truly three-dimensional structures can only be produced by unconventional lithography methods such as direct laser writing, focused ion beam electrodeposition, colloidal sphere lithography, and tilted multiple-pass projection lithography, but at the cost of remarkable complexity and lengthiness. In this work, the three-dimensional shape of light, which is formed by Talbot effect diffraction, was exploited to produce inherently three-dimensional patterns in a photosensitive polymer. Using light in the soft X-ray wavelength, periodic three-dimensional structures of lateral period 600 nm were obtained. The position at which the sample has to be located to be in the Fresnel regime was simulated using an analytical implementation of the Fresnel integrals approach. Exploiting the light shape forming in diffraction effects thus enables the patterning of high-resolution three-dimensional nanostructures over a large area and with a single exposure pass – which would be otherwise impossible with conventional lithographic methods.

The ever-growing trend of device multifunctionality and miniaturization puts enormous burden on existing manufacturing technologies. The requirements for precision, throughput, and cost become increasingly harder to achieve with minimal room for compromises. Femtosecond lasers, which saw immense development throughout the last few decades, have been proven time and time again to be a superb tool capable of standing up to the challenges posed by modern science and the industry for ultrahigh-precision material processing. Thus, this paper is dedicated to provide an outlook on how femtosecond pulses are revolutionizing modern manufacturing. We will show how they are exploited for various kinds of material processing, including subtractive (ablation, cutting, and etching), additive (lithography and laser-induced forward transfer), or hybrid subtractive-additive cases. The advantages of using femtosecond lasers in such applications, with main focus on how they enable the most precise kinds of material processing, will be highlighted. Future prospects concerning emerging industrial applications and the future of the technology itself will be discussed.

Nanostructures have unique characteristics, such as large specific surface areas, that provide a wide range of engineering applications, such as electronics, optics, biotics, and thermal and fluid dynamics. They can be used to downsize many engineering products; therefore, new nanofabrication techniques are strongly needed to meet this demand. A simple fabrication process with high throughput is necessary for low-cost nanostructures. In recent years, three-dimensional (3D) nanostructures have attracted much attention because they dramatically opened up new fields for applications. However, conventional techniques for fabricating 3D nanostructures contain many complex processes, such as multiple patterning lithography, metal deposition, lift-off, etching, and chemical-mechanical polishing. This paper focuses on controlled-acceleration-voltage electron beam lithography (CAV-EBL), which can fabricate 3D nanostructures in one shot. The applications of 3D nanostructures are introduced, and the conventional 3D patterning technique is compared with CAV-EBL and various 3D patterning techniques using CAV-EBL with nanoimprinting technology. Finally, the outlook for next-generation devices that can be fabricated by CAV-EBL is presented.

Aspheric surfaces, in particular rotationally invariant surfaces, can be described according to the ISO standard 10110 Part 12 as sagitta functions of the surface coordinates. Usually, such functions are standardized as a combination of conic terms and power series or orthogonal polynomials. Similar functions are applied for surface forms, which are not rotationally invariant as cylindric and toric surfaces. In the following, different forms of describing aspheric surfaces as given in the standard as well as other forms will be presented and compared in an overview, and their special features will be discussed.

Recently, freeform optics have been introduced for application adapted beam shaping in laser heat treatment. There, intensity distributions are generated that induce previously defined temporal and spatial temperature profiles. To this end, a two-step simulation strategy is necessary, where in the first step the intensity distribution must be derived for which in the second step the freeform optics is calculated. To provide a design that can successfully be integrated in an experimental setup, the incoming laser beam’s characteristics must be accounted for in the derivation of the adapted intensity distribution as well as in the freeform optics design. Here, the two most relevant quantities are the beam’s maximum output power as well as the divergence angle. In this work, strategies are presented that account for the beam’s maximum output power in the derivation of the adapted intensity distribution. Furthermore, stabilizing methods are introduced to enhance the performance of a previously introduced freeform optics design algorithm that takes into account the laser beam’s finite divergence angle but suffers from numerical noise and oscillation problems. A simulation example that uses both techniques is given for (nano)ceramic thin-film laser processing.

Methods for fabricating micro- and nanoscale three-dimensional (3D) structures such as electron-beam lithography (EBL) attracted attention in various fields. In EBL, an acceleration-voltage modulation method can be used to control the developing depth of the structure. In this study, we fabricated a rose petal structure using acceleration-voltage modulation. Using a rose petal mold, plastic- and silver-duplicated rose petals were prepared using nano-imprint lithography (NIL). We demonstrated that various complex 3D structures and materials can be duplicated using NIL by applying an acceleration-voltage modulation method.