In Moiré fringe lithography alignment technology, alignment is realized by monitoring the grating interference fringe image in real-time. The technique exhibits excellent sensitivity to displacement changes and is not easily affected by the gap changes between the mask and silicon wafer. Therefore, this technique is widely used in conventional proximity and contact lithography and new-generation micro- and nanolithography systems. The rapid development of semiconductor and integrated circuit industries, as well as the increasing requirements for the resolution of various nanodevices and systems, have posed new challenges in Moiré fringe lithography alignment technology, which are mainly reflected in alignment accuracy, alignment range, and scheme complexity. In this study, the development history, alignment principle, and overall process of Moiré fringe lithography alignment technology are reviewed; the main factors affecting alignment accuracy are analyzed, and corresponding optimization schemes are provided; and finally, the development trend and research focus of Moiré fringe lithography alignment technology are predicted from the marking structure, alignment scheme, and algorithm processing.

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Lithography Alignment Techniques Based on Moiré Fringe

We explore the feasibility of a controllable and easy-to-implement moire-based composite circular-line gratings imaging scheme for optical alignment in proximity lithography. One circular grating and four line gratings located on both the mask alignment mark and wafer alignment mark are used to realize the coarse alignment and fine alignment respectively. The fundamental derivation of coarse alignment employing circular gratings and fine alignment employing line gratings are given. Any displacement of misalignment that occurs at the surface of two overlapped gratings can be sensed and determined through subsequent fringe phase analysis without the influence of the gap between the mask and the wafer or wafer process. The design and manufacture process of the alignment marks are presented. The experimental results validate and demonstrate the feasibility of the proposed approach.

Moiré fringe alignment using composite circular-line gratings for proximity lithography

Photonics and optoelectronics are at the foundations of widespread technologies, from high-speed internet to systems for artificial intelligence, LiDAR, and optical quantum computing. Light enables ultrafast speeds and low energy for all-optical information processing and transport, especially when confined at the nanoscale level, at which the interactions of light with matter unveil new phenomena, and the role of local symmetries becomes crucial. This Perspective discusses how symmetry violations provide unique opportunities for nanophotonics, tailoring wave interactions in nanostructures for a wide range of functionalities. In particular, we discuss broken geometrical symmetries for localized surface polaritons, the physics of moiré photonics, in-plane inversion symmetry breaking for valleytronics and nonradiative state control, time-reversal symmetry breaking for optical nonreciprocity, and parity-time symmetry breaking. Overall, our Perspective aims to present the role of low symmetry and broken symmetry in controlling nanoscale light and its widespread applications for optical technologies. 

Low-Symmetry Nanophotonics

We explore an easy-to-implement moire-based measurement scheme for the mask-wafer misalignment in nanoimprint lithography. By introducing the beat signal of moire fringes, the measurement range increase by dozens or even hundreds of times, while the measurement accuracy doesn’t get affected and still kept in nanoscale. Moreover, the alignment signal, collected throughout the whole imprint process, is independent of the wafer-mask gap and beam fluctuation, which makes it very suitable for the misalignment measurement in NIL. The experiment shows that sub-10 nm alignment could be obtained within a measurement range of 500µm, which is expected to be improved after the parameter optimization.

Misalignment measurement with dual-frequency moiré fringe in nanoimprint lithography.

The mechanical behavior of deep rock masses differs from that of shallow rock masses. Deep rock masses are classified into alternating fractured and intact zones. This unique geological phenomenon is known as zonal disintegration. It is difficult to explain the formation mechanism of such phenomenon under the framework of traditional rock mechanics. The fractured shape and forming conditions of this occurrence also remain unclear. In our present study, we conducted an exploration in deep tunnels of the Dingji coal mine in China to investigate the forming mechanism of zonal disintegration. This phenomenon was observed using a borescope TV. A geomechanical model test was then constructed based on the results of the exploration. An analogical material referred to as barites–iron–sand cementation analogical and optical scale multi-point displacement meters were developed independently for the model test. The model test was then used to validate zonal disintegration and monitor the formation process. The strain and displacement laws of the surrounding rocks during zonal disintegration were obtained and found to be non-monotonic. The fractured shape of zonal disintegration was determined, and the radii of the fractured zones were found to fulfill the relationship of geometric progression. The experimental results were in accordance with the in situ exploration findings. The mechanism of the zonal disintegration was revealed by theoretical analysis based on fracture mechanics. The fractured zones are reportedly circular and concentric to the cavern. Each fracture zone ruptured at the elastic–plastic boundary of the surrounding rocks and then coalesced into the circular form. The geometric progression ratio was found to be related to the mechanical parameters and the ground stress of the surrounding rocks.

In Situ Observation and Model Test on Zonal Disintegration in Deep Tunnels

This paper aims to deal with a four-quadrant gratings alignment method benefiting from phase demodulation for proximity lithography, which combines the advantages of interferometry with image processing. Both the mask alignment mark and the wafer alignment mark consist of four sets of gratings, which bring the convenience and simplification of realization for coarse alignment and fine alignment. Four sets of moire fringes created by superposing the mask alignment mark and the wafer alignment mark are highly sensitive to the misalignment between them. And the misalignment can be easily determined through demodulating the phase of moire fringe without any external reference. Especially, the period and phase distribution of moire fringes are unaffected by the gap between the mask and the wafer, not excepting the wavelength of alignment illumination. Disturbance from the illumination can also be negligible, which enhances the technological adaptability. The experimental results bear out the feasibility and rationality of our designed approach.

Four-quadrant gratings moiré fringe alignment measurement in proximity lithography

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Four-quadrant gratings moiré fringe alignment measurement in proximity lithography