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2019 Vol. 1, No. 3

Reviews
At wavelength coherent scatterometry microscope using high-order harmonics for EUV mask inspection
Yutaka Nagata, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita, Katsumi Midorikawa
2019, 1(3) doi: 10.1088/2631-7990/ab3b4e
Abstract:
Extreme ultraviolet (EUV) lithography with reflective photomasks is currently being refined for high-volume manufacturing of chips with dimensions of 7 nm or less. EUV scanners can replace the most critical layers and provide lithography capabilities complementary to ArF technology. However, the fabrication and inspection of defect-free masks still remain one of the most critical issues facing EUV technology. In this review, we describe our research on the development of the 13.5-nm coherent scatterometry microscope (CMS) with high-order harmonic generation (HHG) for the mask inspection of EUV lithography. Using the HHG-CSM system, we observed programmed pattern defects in a periodic patterned mask. In the diffraction pattern from the EUV mask, a 2-nm wide line defect in an 88-nm line-and-space pattern as well as sub-100 nm sized absorber defects in a 112 nm hole pattern were both detected. By further improving the system, we demonstrated the successful reconstructions of an-88 nm periodic L/S pattern and a cross-pattern with a quantitative phase contrast. These results signify that the standalone HHG-CSM system has tremendous potential.
Near-field acoustic levitation and applications to bearings: a critical review
Minghui Shi, Kai Feng, Junhui Hu Jiang Zhu, Hailong Cui
2019, 1(3) doi: 10.1088/2631-7990/ab3e54
Abstract:
The importance to industry of non-contact bearings is growing rapidly as the demand for high-speed and high-precision manufacturing equipment increases. As a recently developed non-contact technology, near-field acoustic levitation (NFAL) has drawn much attention for the advantages it offers, including no requirement for an external pressurized air supply, its compact structure, and its ability to adapt to its environment. In this paper, the working mechanism of NFAL is introduced in detail and compared to all existing non-contact technologies to demonstrate its versatility and potential for practical applications in industry. The fundamental theory of NFAL, including gas film lubrication theory and acoustic radiation pressure theory is presented. Then, the current state-of-the-art of the design and development of squeeze film air bearings (SFABs) based on NFAL is reviewed. Finally, future trends and obstacles to more widespread use are discussed.
Advances in micro cutting tool design and fabrication
John O'Hara, Fengzhou Fang
2019, 1(3) doi: 10.1088/2631-7990/ab3e7f
Abstract:
Microcutting is a precision technology that offers flexible fabrication of microfeatures or complex 3D components with a high machining accuracy and superior surface quality. The technology may offer great potential as well as advantageous process capabilities for the machining of hard-to-cut materials. The geometrical design and dimension of the tool cutting edge is a key factor that determines the size and form accuracy possible in the machined workpiece. Currently, the majority of commercial microtools are scaled-down versions of conventional macrotool design. This approach does not impart optimal performance due to size effects and associated phenomena. Consequently, in-depth analysis and implementation of microcutting mechanics and fundamentals are required to enable successful industrial adaptation in microtool design and fabrication methods. This paper serves as a review of recent microtool designs, materials, and fabrication methods. Analysis of tool performance is discussed, and new approaches and techniques are examined. Of particular focus is tool wear suppression in the machining of hard materials and associated process parameters, including internal cooling and surface patterning techniques. The review concludes with suggestions for an integrated design and fabrication process chain which can aid industrial microtool manufacture.
Ultrafast dynamics observation during femtosecond laser-material interaction
Baoshan Guo, Jingya Sun, YongFeng Lu, Lan Jiang
2019, 1(3) doi: 10.1088/2631-7990/ab3a24
Abstract:
Femtosecond laser technology has attracted significant attention from the viewpoints of fundamental and application, especially femtosecond laser processing materials presents the unique mechanism of laser-material interaction. Ultrafast lasers can change the states and properties of materials through interactions with them, and they can be used to control the processing of materials from the micrometer scale down to the nanometer scale or across scales. Under the extreme nonequilibrium conditions imposed by femtosecond laser irradiation, many fundamental questions concerning the physical origin of the material removal process remain unanswered. In this review, cutting-edge ultrafast dynamic observation techniques for investigating the fundamental questions, including time-resolved pump-probe shadowgraphy, ultrafast continuous optical imaging, and four-dimensional ultrafast scanning electron microscopy are comprehensively surveyed. Each technique is described in depth, beginning with its basic principle, followed by a description of its representative applications in laser-material interaction and its strengths and limitations. The consideration of temporal and spatial resolutions and panoramic measurement at different scales are two major challenges. To address the challenges, the article outlines the development and prospects for the technical advancement in this field. The multiscale observation system could be used to determine the evolution of the structure and properties from electron ionization (femtosecond-picosecond scale) and material phase transition (picosecond-nanosecond scale) in a manufacturing activity in which the observations of multiscale processes have high spatial-temporal resolution, which would bring about a paradigm shift in femtosecond laser manufacturing.
Research
Formation mechanism of a smooth, defect- free surface of fused silica optics using rapid CO2 laser polishing
Linjie Zhao, Jian Cheng, Mingjun Chen, Xiaodong Yuan, Wei Liao, Qi Liu, Hao Yang, Haijun Wang
2019, 1(3) doi: 10.1088/2631-7990/ab3033
Abstract:
Surface defects introduced by conventional mechanical processing methods can induce irreversible damage and reduce the service life of optics applied in high-power lasers. Compared to mechanical processing, laser polishing with moving beam spot is a noncontact processing method, which is able to form a defect-free surface. This work aims to explore the mechanism of forming a smooth, defect-free fused silica surface by high-power density laser polishing with coupled multiple beams. The underlying mechanisms of laser polishing was revealed by numerical simulations and the theoretical results were verified by experiments. The simulated polishing depth and machined surface morphology were in close agreement with the experimental results. To obtain the optimized polishing quality, the effects of laser polishing parameters (e.g., overlap rate, pulse width and polishing times) on the polishing quality were experimentally investigated. It was found that the processing efficiency of fused silica materials by carbon dioxide (CO2) laser polishing could reach 8.68 mm2/s, and the surface roughness (Ra) was better than 25 nm. Besides, the cracks on pristine fused silica surfaces introduced by initial grinding process were completely removed by laser polishing to achieve a defect-free surface. The maximum laser polishing rate can reach 3.88 μm/s, much higher than that of the traditional mechanical polishing methods. The rapid CO2 laser polishing can effectively achieve smooth, defect-free surface, which is of great significance to improve the surface quality of fused silica optics applied in high-power laser facilities.
3D particle tracking velocimetry for the determination of temporally resolved particle trajectories within laser powder bed fusion of metals
Eric Eschner, Tobias Staudt and Michael Schmidt
2019, 1(3) doi: 10.1088/2631-7990/ab3de9
Abstract:
Laser powder bed fusion (L-PBF) is likely to be found in R&D, small batch production and prototyping as well as in industry enabling the processing of a wide range of materials with high freedom in part design. However, the production of defect free and highly dense parts still requires extensive parameter studies and process knowledge. The reason for this are the complex process dynamics involved - namely originating from evaporation effects inside the interaction zone. They are the main driver of spatter generation and powder movement within the vicinity of the interaction zone, which are therefore of critical interest for a better process understanding. In order to enable quantification of characteristic measures of the particle movement, we present an automated three-dimensional particle tracking velocimetry (3D PTV) measurement approach. It relies on a stereoscopic ultra high-speed camera setup and an optimized image processing approach. This approach enables the 3D measurement of particle trajectories within large datasets with an acceptable time frame. The approach is validated towards synthetic images with known ground truth of particle trajectories.