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Micro-nanostructure devices play an important role in the fields of micro-optics and optical engineering, tribology and surface engineering, biology and biomedical engineering. Precision Glass molding (PGM) technology is the most effective method for manufacturing micro-nanostructure glass devices, and its premise is to have the micro-nanostructure molds that are complementary to the shape of the designed micro-nanostructure of the glass device. Researchers have developed many mold manufacturing methods for manufacturing extremely small and high-quality micro-nanostructures to meet the functional requirements of micro-nanostructure glass devices in various applications. In addition, since the service performance of the mold will greatly affect the accuracy of the molded glass and the production cost, improving the service performance of the micro-nanostructure mold is also the focus of mold manufacturing research.
Prof. Tianfeng Zhou, PhD student Yupeng He, master student Tianxing Wang et.al. from the Advanced Machining Technology Lab of Beijing Institute of Technology wrote " A review of the techniques for the mold manufacturing of micro/nanostructures for precision glass molding" on the "International Journal of Extreme Manufacturing" (IJEM). In this article, the authors comprehensively summarize the extreme requirements of mold materials used for mold manufacturing, the technical principles of varieties of non-mechanical and mechanical for micro-nanostructures manufacturing, their adaptable materials, technical characteristics and application scopes. Furtherly, the author discussed the service performance of nickel-phosphorus (Ni-P) micro-nanostructure molds. Finally, the authors discuss the characteristics and development direction of manufacturing technology of the micro-nanostructure mold in the future research.
Due to their significant functions in hydrophobicity, friction reduction, and optical reflection and diffraction, micro/nanostructures are widely applied in the fields of optical imaging and sensing, biomedicine, etc. Commonly used micro/nanoarrays include lens arrays, columnar arrays, groove arrays, and pyramid arrays, as shown in Figure 1. Micro/nanostructured glass components have the advantages of miniaturization, integration, and being lightweight, which are widely used in the microsystems. The scale reduction and performance improvement of these microsystems are largely determined by the quality of the micro/nanostructured glass components, which requires overcoming the challenge of manufacturing micro/nanostructures with the features of both extremely small size and high quality. Considering the broad application prospects and extreme features of small scale and high quality, ultra-precision manufacturing technology for the fabrication of micro/nanostructured glass components has become a strategic development field in many countries.
Figure 1. Typical micro/nanostructures. (a) Microlens array, (b) groove array, (c) columnar array, and (d) pyramid array.
Micro/nanostructures are produced by generating a series of geometric units on a surface at the micro/nanometer scale via certain methods. Attempts are currently underway to advance the extreme manufacturing ability of these methods in terms of size, accuracy, consistency, and efficiency. The concept of glass molding technologies involves inducing the proper pressure at a high temperature to copy the micro/nanostructure array from a mold onto a glass, which is deemed as the best technology to fabricate micro/nanostructures on glass surfaces. During the molding process, glass is softened via heating and then solidified via annealing. Glass molding has the advantages of high forming accuracy, efficiency, good consistency, and low processing costs, and is therefore suitable for the mass production of micro/nanostructure arrays. It should be noted that the micro/nanostructure generated on the glass surface is completely copied from the mold surface during the molding process, so the precise manufacturing of the micro/nanostructure with extreme features of small size and high quality on the mold is the premise of glass molding. Recently, many methods, including both mechanical and nonmechanical methods, have been rapidly developed to overcome the challenge of manufacturing small, high quality micro/nanostructures.
Figure 2. (a) Photograph of PFLF7-60A molding machine and (b) it’s schematic diagram.
3. Recent advances
The latest developments in micro-nanostructure mold manufacturing technology have been divided into four sections: the new mold material development, the non-mechanical manufacturing methods, the mechanical manufacturing methods, and the mold service performance. In each section, the author discusses its technical principles, classification and latest developments in turn. In addition, the typical micro-nanostructures processed by various technologies, as well as the manufacturing capacity and scope are discussed.
To suppress the forming error resulting from high-temperature deformation, a mold material with low thermal expansion and high-temperature resistance should be selected. The materials used for glass molding must have the following characteristics: (1) high hardness and strength at high temperatures, a low thermal expansion coefficient, and excellent stability of the chemical properties at high temperatures; (2) good material consistency, meaning that it can be processed to meet the requirements of an optical-grade surface; (3) inert adhesion and reaction with glass.
Common used mold materials are usually the super-hard and difficult-to-process materials such as monocrystalline silicon, silicon carbide, and cemented carbide. Figure 3 shows a new type mold material named as graphene-nickel phosphide (G-Ni-P) composite mold material.
Figure 3. (a) G-Ni-P composite plating, (b) comparison of the mechanical properties of the Ni-P plating and G-Ni-P composite plating.
Non-mechanical method for micro-nanostructure mold manufacturing
Non-mechanical manufacturing of the micro-nanostructures mainly refers to the use of chemical, femtosecond laser and micro-EDM etching techniques to remove materials on the mold surface to form a micro-nano unit array. Figure 4 shows the 6H-SiC microlens mold prepared by combining single point turning and ion beam (IBE) etching. Figure 5 shows the nanowire structure fabricated on the silicon surface using a chemical etching assisted femtosecond laser.
Figure 4. The micro lens array mold fabricated by combine the micro cutting and ion beam etching.
Figure 5. The nanowire structure fabricated using a chemical etching assisted femtosecond laser.
Mechanical method for micro-nanostructure mold manufacturing
The nonmechanical methods can achieve the micro/nanoscale manufacturing, but some resistance exists in the conductivity and magnetism of the materials. The cross-sections of the micro/nanostructures cannot be precisely regulated. In contrast, mechanical methods are available for most workpiece materials, and can achieve micro/nanostructures with superior geometric freedom and lower surface roughness. The mechanical methods for machining micro-nano structures are divided into: slow tool servo (STS), fast tool servo (FTS), micro-nano milling, fly-cutting, micro-grinding and ultra-precision grinding technology. Figure 6 shows a rotary ultrasonic vibration texturing (RUT) method that combines ultrasonic vibration, feed motion, and rotational motion and the manufactured micro-nanostructure. Figure 7 shows the principle diagram of the micro-nano groove machining by the axial feed fly-cutting and the two-stage structure machining by the low-frequency vibration-assisted axial feed fly-cutting. Figure 8 shows the lapping process of the micro lens array on the 3D curved surface.
Figure 6. (a) Rotary ultrasonic vibration texturing and (b) the generated micro-nanostructures.
Figure 7. Schematic diagrams of (a) the ARFC method and the corresponding grooves and (b) vibration assisted ARFC and the corresponding two-level structures.
Figure 8. A cross-section sketch of the 3D microlens array lapping process. (a) A lapping ball stuck in the hole on the support and rolled to lap a micro-cavity on a curved substrate; (b) one micro-cavity produced on the curved surface.
Mold service performance
The temperature in the glass molding process is usually above 500 ºC, and the change of mold material at high temperature will affect the accuracy of the micro-nano mold, then affect the morphology accuracy and surface quality of the molded glass components. To improve the shortcomings of low forming accuracy and short mold life due to the crystalline transformation of the amorphous Ni-P mold during the molding process, the amorphous Ni-P is pre-converted to a crystalline material before machining micro-nano structure on it. Figure 9 shows the surface of the micro-nanostructure mold of amorphous Ni-P and crystalline Ni-P after molding.
Figure 9. (a) Amorphous Ni-P and (b) crystalline Ni-P micro-nano structure mold after molding.
In the future, the techniques to manufacture micro/nanostructure molds and the extreme features of micro/nanostructures, including the realization of extremely small sizes and high quality, will remain the study focus. A greater expectation of the component performance makes the large-area processing another extreme feature of the micro/nanostructured molds, which will require high efficiency and low-cost manufacturing. In addition, due to the upgrading of mold materials, research on the most suitable processing methods and techniques will always be a hot spot in micro/nano mold manufacturing. Meanwhile, the compound technology, such as the combination of etching and micro/nano cutting, will receive more attention.
5. About the Authors
Tianfeng Zhou is a professor at the School of mechanical engineering of Beijing Institute of Technology, and a member of the “Thousand Talents Program for Young Talents”. His current research is focused on ultra-precision cutting, molding processing, electrical discharge machining and other mechanical manufacturing technology research and equipment development, mainly including mold material development, micro-nano mold manufacturing, glass aspheric/free-form surface lens manufacturing, micro-nanostructure and micro-lens array manufacturing, etc.