Realization and performance evaluation of high speed autofocusing for direct laser lithography

Single-lens dynamic [Formula: see text]-scanning for simultaneous in situ position detection and laser processing focus control

Existing auto-focusing methods in laser processing typically include two independent modules, one for surface detection and another for [Formula: see text]-axis adjustment. The latter is mostly implemented by mechanical [Formula: see text] stage motion, which is up to three orders of magnitude slower than the lateral processing speed. To alleviate this processing bottleneck, we developed a single-lens approach, using only one high-speed [Formula: see text]-scanning optical element, to accomplish both in situ surface detection and focus control quasi-simultaneously in a dual-beam setup. The probing beam scans the surface along the [Formula: see text]-axis continuously, and its reflection is detected by a set of confocal optics. Based on the temporal response of the detected signal, we have developed and experimentally demonstrated a dynamic surface detection method at 140-350 kHz, with a controlled detection range, high repeatability, and minimum linearity error of 1.10%. Sequentially, by synchronizing at a corresponding oscillation phase of the [Formula: see text]-scanning lens, the fabrication beam is directed to the probed [Formula: see text] position for precise focus alignment. Overall, our approach provides instantaneous surface tracking by collecting position information and executing focal control both at 140-350 kHz, which significantly accelerates the axial alignment process and offers great potential for enhancing the speed of advanced manufacturing processes in three-dimensional space.

High accuracy astigmatic-focusing system for laser targets

An accurate location of the focal position with respect to a solid target is a key task for different applications, for instance, in laser driven plasma acceleration for x-ray generation where minimum required intensities are above 10¹⁴W/cm². For such practical applications, new approaches for focus location and target delivery techniques are needed to achieve the required intensity, repeatability, and stability. There are different techniques to accomplish the focusing and target positioning task such as interferometry-, microscopy-, astigmatism-, and nonlinear-optics-based techniques, with their respective advantages and limitations. We present improvements of a focusing technique based on an astigmatic method with potential applications where maximum intensity at the target position is necessary. The presented technique demonstrates high accuracy up to 5 µm, below the Rayleigh range, and also its capability to work in rough surfaces targets and tilt tolerance of the target, with respect to the normal of the target surface.

In-Situ Depth Measurement of Laser Micromachining

Precision laser micromachining plays an important role in the biomedical, electronics, and material processing industries. During laser drilling, precision depth detection with micrometer-level resolution is required, particularly with blind-hole or heterogeneous structures. We present an optical detection system utilizing an optical confocal structure, experimentally confirmed to achieve a >95% accuracy for micron-diameter holes that are tens-of-microns deep. This system can be easily integrated into commercial laser micromachining processes, and can be employed in laser drilling and three-dimensional active-feedback laser printing.

Implementation and Optimization of a Dual-confocal Autofocusing System

This paper describes the implementation and optimization of a dual-confocal autofocusing system that can easily describe a real-time position by measuring the response signal (i.e., intensity) of the front and the rear focal points of the system. This is a new and systematic design strategy that would make it possible to use this system for other applications while retrieving their characteristic curves experimentally; there is even a good chance of this technique becoming the gold standard for optimizing these dual-confocal configurations. We adopt two indexes to predict our system performance and discover that the rear focal position and its physical design are major factors. A laboratory-built prototype was constructed and demonstrated to ensure that its optimization was valid. The experimental results showed that a total optical difference from 150 to 400 mm significantly affected the effective volume of our designed autofocusing system. The results also showed that the sensitivity of the dual-confocal autofocusing system is affected more by the position of the rear focal point than the position of the front focal point. The final optimizing setup indicated that the rear focal length and the front focal length should be set at 200 and 100 mm, respectively. In addition, the characteristic curve between the focus error signal and its position could successfully define the exact position by a polynomial equation of the sixth order, meaning that the system can be straightforwardly applied to an accurate micro-optical auto-focusing system.

Centroid-position-based autofocusing technique for Raman spectroscopy

In Raman spectroscopy, it is crucial to focus the laser on the sample in order to guarantee the intensity and repeatability of the characteristic peaks, which is known as autofocus. In this paper, we propose a novel low-cost scheme based on the subtle placement of the laser source and the image sensor. We confirm the feasibility of monitoring the focus status through the centroid position of the laser spot's image (CPSI) in theory. Both the simulation and experimental results illustrate that the distance-ordinate function is similar in shape to the logarithm, which not only helps to shorten the autofocus time but also achieves the sub-decimeter measuring range and micrometer resolution near the focal point. Meanwhile, we discuss in detail how to obtain the desired performance by adjusting the extrinsic camera parameters and the way to overcome the disturbance of the noise, ambient light and non-normal incidence. An autofocus-free handheld Raman spectrograph utilizes this method to autofocus the alcohol in the centrifuge tube successfully and the spectral reproducibility is improved. Our results may pave the way to a novel autofocus approach for Raman mapping in vivo.

Facile large-area autofocusing Raman mapping system for 2D material characterization

Two-dimensional (2D) materials have attracted tremendous research interests due to their intriguing properties and promising applications. As one of the most typical 2D material characterization methods, however, the conventional Raman mapping only works within few-hundreds micrometers range at a time due to the focus depth constraint and the non-ideal level of the substrate. To implement wafer-scale Raman scanning, large-area autofocusing Raman mapping (LARM) is highly desirable. Here, we present a modified centroid method to build a facile LARM system in which the Raman excitation laser is employed as the focus laser, reducing the system cost and complexity. Based on identifying the shape of the semicircle laser reflection image, a self-written autofocusing algorithm allows a real-time adjusting the focus position during the large-scale scanning. As a state-of-the-art demonstration, the thickness distribution of both few layer WS₂ triangle domains sparsely located in sub-millimeter range and polycrystalline continuous MoS₂ film up to 2-inch scale can be well-revealed. Our results may shed light on wafer-scale nondestructive optical characterization of 2D materials.

High-Speed Focus Inspection System Using a Position-Sensitive Detector

Precise and rapid focus detection is an essential operation in several manufacturing processes employing high-intensity lasers. However, the detection resolution of existing methods is notably low. This paper proposes a technique that provides a rapid-response, high-precision, and high-resolution focus inspection system on the basis of geometrical optics and advanced optical instruments. An ultrafast interface position detector and a single-slit mask are used in the system to precisely signal the focus position with high resolution. The reflected images on the image sensor are of a high quality, and this quality is maintained persistently when the target surface is shifted along the optical axis. The proposed system developed for focus inspection is simple and inexpensive, and is appropriate for practical use in the industrial production of sophisticated structures such as microcircuits and microchips.

High-speed laser writing of arbitrary patterns in polar coordinate system

In order to realize high-speed laser writing arbitrary patterns, we establish a set of high-speed polar coordinate laser writing system. Although the polar coordinate laser writing system is generally suitable for fabricating circular symmetric patterns, there are challenges when dealing with arbitrary patterns. Here, we propose an effective method to solve this problem by converting the pattern data from Cartesian coordinates to polar coordinates for high-speed laser writing of arbitrary patterns. Several types of arbitrary patterns are written on chalcogenide thin films with a minimum pattern linewidth of 700 ± 70 nm and a maximum writing speed of approximately 10 m/s, which corresponds to more than 600 mm²/min at 1.0 μm linewidth. This writing speed is ten times faster than that of the conventional x-y type Cartesian coordinate laser writing system.

Enhancement of height resolution in direct laser lithography

To address the requirements of multi-level semiconductors, we propose a new technique for overcoming the height limitation of direct laser lithography. In the proposed system, an original source beam is fed into an interference generator that divides the input beam by 50: 50 into two output beams. After going through an imaging lens, these two beams make two focusing spots, which are slightly separated in the axial direction. In the overlapped region, these two spots generate a small interferogram that shortens the depth of focus. By using this phenomenon, we are able to overcome the height limitation of direct laser lithography. The governing equations are also derived in this manuscript by using the Gaussian beam model.

Improvement of linewidth in laser beam lithographed computer generated hologram

We propose a new laser lithographic technique with enhanced resolution. A calcite wave plate is introduced in our system to separate an input lithographic beam into two orthogonally polarized beams. After going through an imaging lens, these two beams meet again on the focal point, and generate a small interferogram that sharpens the shape of the focused beam spot. Using this phenomenon, we can overcome the diffraction limit of the imaging lens and achieve a 486-nm-linewidth.