Degradation Of Resolution Research Articles

Characterization of Defocused Coherent Imaging Systems with Periodic Objects.

Recent advancements in quantum and quantum-inspired imaging techniques have enabled high-resolution 3D imaging through photon correlations. These techniques exhibit reduced degradation of image resolution for out-of-focus samples compared to conventional methods (i.e., intensity-based incoherent imaging). A key advantage of these correlation-based approaches is their independence from the system numerical aperture (NA). Interestingly, both improved resolution of defocused images and NA-independent scaling are linked to the spatial coherence of light. This suggests that while correlation measurements exploit spatial coherence, they are not essential for achieving this imaging advantage. This discovery has led to the development of optical systems that achieve similar performance by using spatially coherent illumination and relying on intensity measurements: direct 3D imaging with NA-independent resolution was recently demonstrated in a correlation-free setup using LED light. Here, we explore the physics behind the enhanced performance of defocused coherent imaging, showing that it arises from the modification of the sample's spatial harmonic content due to diffraction, unlike the blurring seen in conventional imaging. The results we present are crucial for understanding the implications of the physical differences between coherent and incoherent imaging, and are expected to pave the way for the practical application of the discovered phenomena.

Design and Evaluation of a Weighted Periodic Sparse Array for Low-Complexity 1-D Phased Array Ultrasound Imaging Systems.

A sparse array offers a significant reduction in the complexity of ultrasonic imaging systems by decreasing the number of active elements and associated electrical circuits needed to form a focused beam. Consequently, for 1-D arrays, it has been adopted in the development of miniaturized systems such as portable, handheld, or smartphone-based systems. Previously, we developed an analytic method that can design a pair of 1-D periodic sparse arrays (PSAs) satisfying three specific constraints, which are the array size, desired grating lobe level, and sparseness factor (SF). In this study, we further developed our method by incorporating aperture weighting functions, which take the form of tapered rectangular functions to introduce null points on the beam pattern. These null points effectively suppress grating lobes generated by a matching pair of arrays. The design process commences with determining transmit and receive PSA patterns, followed by deriving corresponding aperture weighting functions. First, aperture functions of a base and weighting arrays are convolved, which is then upsampled to the targeted array size. Finally, the upsampled aperture is convolved to an aperture function of a subarray, resulting in weighted PSAs (wPSAs). Pulsed wave (PW) simulation confirmed improved grating lobe suppression with wPSAs compared to PSAs. Phantom imaging experiments using a 1-D phased array validated the enhanced contrast due to suppressed grating lobes but at the cost of small degradation in lateral resolution. The signal-to-noise ratio (SNR) also gradually declined with the greater SFs, but no significant difference in SNR was observed between wPSAs and PSAs. Finally, in vivo echocardiography imaging highlighted the clinical potential of wPSAs, particularly with high SFs. Overall, these results suggest that wPSAs can effectively enhance contrast compared to PSAs under the given SF or, alternatively, wPSA with greater SFs can achieve comparable image quality to PSAs with lower SFs. In conclusion, the wPSA approach holds promise for further reducing the complexity of ultrasound imaging systems.

Multi-Grained Point Cloud Geometry Compression via Dual-Model Prediction with Extended Octree

The state-of-the-art geometry-based point cloud compression (G-PCC) (Octree) is the fine-grained approach, which uses the octree to partition point clouds into voxels and predicts them based on neighbor occupancy in narrower spaces. However, G-PCC (Octree) is less effective at compressing dense point clouds than multi-grained approaches (such as G-PCC (Trisoup)), which exploit the continuous point distribution in nodes partitioned by the pruned octree over larger spaces. Therefore, we propose a lossy multi-grained compression with extended octree and dual-model prediction. The extended octree, where each partitioned node contains intra-block and extra-block points, is applied to address poor prediction (such as overfitting) at the node edges of the octree partition. For the points of each multi-grained node, dual-model prediction fits surfaces and projects residuals onto the surfaces, reducing projection residuals for efficient 2D compression and fitting complexity. In addition, a hybrid DWT-DCT transform for 2D projection residuals mitigates the resolution degradation of DWT and the blocking effect of DCT during high compression. Experimental results demonstrate the superior performance of our method over advanced G-PCC (Octree), achieving BD-Rate gains of 55.9% and 45.3% for point-to-point ( D1 ) and point-to-plane ( D2 ) distortions, respectively. Our approach also outperforms G-PCC (Octree) and G-PCC (Trisoup) in subjective evaluation.

Harnessing forward multiple scattering for optical imaging deep inside an opaque medium

As light travels through a disordered medium such as biological tissues, it undergoes multiple scattering events. This phenomenon is detrimental to in-depth optical microscopy, as it causes a drastic degradation of contrast, resolution and brightness of the resulting image beyond a few scattering mean free paths. However, the information about the inner reflectivity of the sample is not lost; only scrambled. To recover this information, a matrix approach of optical imaging can be fruitful. Here, we report on a de-scanned measurement of a high-dimension reflection matrix R via low coherence interferometry. Then, we show how a set of independent focusing laws can be extracted for each medium voxel through an iterative multi-scale analysis of wave distortions contained in R. It enables an optimal and local compensation of forward multiple scattering paths and provides a three-dimensional confocal image of the sample as the latter one had become digitally transparent. The proof-of-concept experiment is performed on a human opaque cornea and an extension of the penetration depth by a factor five is demonstrated compared to the state-of-the-art.

Research on the degradation of resolution in cameras based on CMOS image sensor under γ- and X-rays irradiation

Remote control engineering in industry usually requires cameras to assist target identification and detection, but the cameras working in nuclear environments face the threat of radiation-induced degradation. Both γ- and X-rays in nuclear environments will cause ionizing radiation damage, but the difference in degradation of camera’s performance induced by ionizing radiation damage with equivalent total ionizing dose (TID) of these two rays is unclear, which is crucial for the evaluation of camera performance in a nuclear radiation environment. In order to figure out the degradation of CMOS cameras by ionization radiation damage under different rays, this study conducted irradiation experiments on CMOS image sensors under X-ray with 10 keV and 60Co-γ ray with 1.25 MeV, respectively, and then tested and calculated the resolution of camera based on this CMOS image sensor. After comparing and analyzing the results, it was found that the dark current and full well capacity of the CIS, as well as the resolution of the camera, were more severely degraded after X-ray irradiation. Further analysis found that the interface dose enhancement effect of X-ray, as well as the generation of high-density oxide trapped charges in the gate oxide, aggravated the degradation of full well capacity on the CIS after X-ray irradiation, resulting in more severe degradation in the resolution of the CMOS camera. This study provides theoretical basis for the safety and reliability evaluation of cameras in strong nuclear radiation environments.

A novel 4H–SiC thermal neutron detector based on a metal-oxide-semiconductor structure

Energy resolution and leakage current are two key parameters for semiconductor neutron detectors. Metal-Oxide-Semiconductor (MOS) detectors introduce an additional oxide layer compared to Schottky Barrier Diodes (SBDs) detectors in order to reduce leakage current. However, this also leads to a degradation in energy resolution due to the introduction of an additional dead layer. Therefore, optimizing the thickness of the oxide layer is important for MOS detectors. In this study, a 4H–SiC-based MOS thermal neutron detector was designed with comprehensive consideration of the oxide layer. Subsequently, a prototype of the detector was fabricated and tested. The prototype of the MOS detector achieved an nA-level leakage current under a reverse bias of −200 V. Additionally, it demonstrated good energy resolution performance in a moderated D-T neutron source test. The MOS detector was able to clearly distinguish between the two reaction channels of 10B converter events, which is consistent with the theoretical branching ratio. This work highlights the potential application of 4H–SiC-based MOS detectors for thermal neutron detection.

A triangular-trapezoidal dual-channel shaping algorithm for resistive anode readout systems and its FPGA implementation.

This paper introduces a novel digital triangular-trapezoidal double-channel shaping algorithm to enhance the counting rate of resistive anode detectors. The algorithm is based on the trapezoidal shaping algorithm and improves it. At the extreme counting rate, the trapezoidal shaping algorithm cannot alleviate the pulse pileup, so the counting rate cannot meet the requirements of a high performance detector. The triangular-trapezoidal double-channel shaping algorithm is introduced in the resistance anode detector, which can replace the trapezoidal shaping filtering algorithm to process the output signal of the resistance anode detector and obtain the single photon position information. This improvement improves the counting rate of the resistor anode detector and reduces the resolution degradation caused by pulse pileup. The algorithm is simulated by System Generator software and implemented on FPGA (field programmable gate array). The triangular-trapezoidal double-channel shaping algorithm presented in this paper plays an important role in reducing electronic noise and pulse pileup. The algorithm is subjected to simulation testing, and it can recognize signals with a minimum pulse interval of 1 µs and counting rate up to 1000 kcps.

Unsupervised deep learning method for single image super-resolution of the thick pinhole imaging system using deep image prior

Thick pinhole imaging system is widely used for diagnosing intense pulsed radiation sources. However, owing to the trade-off among spatial resolution, field of view (FOV) and signal-to-noise ratio (SNR), the imaging system normally falls short in achieving high-precision spatial diagnosis. In this paper, we propose an unsupervised deep learning method for single image super-resolution (SISR) of the thick pinhole imaging system. The point spread function (PSF) of the imaging system is obtained by analytical calculation and Monte Carlo simulation methods, and the mathematical model of the imaging system is established using a linear equation. To solve the ill-posed inverse problem, we adopt randomly initialized deep convolutional neural networks (DCNNs) as an image prior without pre-training, which is named deep image prior (DIP). The results demonstrate that, by utilizing the SISR technique to increase the number of pixels in reconstructed images, the proposed DIP algorithm can mitigate the spatial resolution degradation caused by an insufficient spatial sampling frequency of the camera. Compared with various classical algorithms, the proposed DIP algorithm exhibits superior capabilities in recovering high-frequency signals and suppressing ringing artifacts. Furthermore, the convergence and robustness of the proposed DIP algorithm under different random seeds and SNR conditions are also verified.

An Algorithm for Ship Detection in Complex Observation Scenarios Based on Mooring Buoys

Marine anchor buoys, as fixed-point profile observation platforms, are highly susceptible to the threat of ship collisions. Installing cameras on buoys can effectively monitor and collect evidence from ships. However, when using a camera to capture images, it is often affected by the continuous shaking of buoys and rainy and foggy weather, resulting in problems such as blurred images and rain and fog occlusion. To address these problems, this paper proposes an improved YOLOv8 algorithm. Firstly, the polarized self-attention (PSA) mechanism is introduced to preserve the high-resolution features of the original deep convolutional neural network and solve the problem of image spatial resolution degradation caused by shaking. Secondly, by introducing the multi-head self-attention (MHSA) mechanism in the neck network, the interference of rain and fog background is weakened, and the feature fusion ability of the network is improved. Finally, in the head network, this model combines additional small object detection heads to improve the accuracy of small object detection. Additionally, to enhance the algorithm’s adaptability to camera detection scenarios, this paper simulates scenarios, including shaking blur, rain, and foggy conditions. In the end, numerous comparative experiments on a self-made dataset show that the algorithm proposed in this study achieved 94.2% mAP50 and 73.2% mAP50:95 in various complex environments, which is superior to other advanced object detection algorithms.

Design and performance of a low-emittance electron gun for electron beam probe

An Electron Beam Probe (EBP) is preferable to provide a non-interceptive measurement of the transverse distribution of a dense bunched ion beam. During EBP operation, it was observed that the electron beam spot subjects to expansion when scanned across the ion beam due to the strong space charge field of the ion bunch, leading to significant degradation of spatial resolution. To mitigate this effect, a small electron beam spot is beneficial as demonstrated in previous research. This paper presents design details of a low-emittance, small-spot, and long-work-distance electron gun. Geometry optimization of the gun and focus lens is conducted using a multiobjective genetic algorithm (MOGA) interfaced with space charge simulation codes to maintain normalized thermal emittance within an acceptable increase and achieve a small spot at a long work distance. The emittance is measured using a slit-screen technique, resulting in a norm. rms emittance of 0.046(+-0.005) um*rad under low beam current conditions (50 nA). In high-resolution mode, a beam spot size of 0.6 mm (rms) is achieved at a work distance of 1.2 m with the combination of Einzel lens and solenoid. The gun has been successfully operated as part of EBP experiments for accurate and high-resolution measurement of ion beam width, demonstrating excellent agreement with wire scanner measurements with relative deviation not exceeding 1%.

A hardware/firmware-based switching gate multiplexing method for pulse mode radiation detectors

We present a hardware/firmware-based switching gate multiplexing method for pulse mode radiation detectors that can combine many detector signals into two readout channels. One readout channel passes the signal of the multiplexed detector that “fired” first, and the other channel provides a variable-width logic pulse, i.e., a pulse width modulation (PWM) signal, that identifies the active detector. The multiplexed output pulse is produced by passing the first active detector’s signal to a fan-in circuit by gating on the corresponding channel for a fixed duration while blocking all other detector signals. It does this using individual analog switches for all the detector signals. Each switch is controlled by a fixed width logic pulse that is triggered by the arrival of the first active detector pulse. Both the fixed width logic pulse and the PWM signal are generated using a field-programmable gate array (FPGA). To demonstrate the proposed multiplexing method, a prototype four-channel multiplexer was developed for use with four NaI(Tl) detectors. The performance of the multiplexer was evaluated in terms of its ability to retain energy resolution, timing resolution, and original pulse shape. The proposed multiplexing method showed very little degradation in energy resolution and timing resolution or alteration of pulse shape. The switching gate feature of the proposed method enables the multiplexer output to have very low noise contribution from the inactive channels. This multiplexing technique also has the unique capability of isolating and recovering the first active detector’s output pulse in cases where there is overlap between pulses from different detectors in a single digitized record. These features make the proposed hardware/firmware-based switching gate multiplexing method very promising for application to large radiation detector networks.

Brillouin-based fiber loss measurement for PON using ERA-BOTDA with differential width pairs of frequency-swept pump pulses.

We demonstrate Brillouin-based loss distribution measurement for passive optical networks with differential width pairs of frequency-swept pump pulses that can suppress the spatial resolution degradation, especially those created by large loss events typical of an 8-branch splitter. Based on the acoustic wave simulation of a frequency-swept pump pulse, we investigate the arrangement of the modulation frequency within the optical pulse. A preliminary experiment is conducted on a 5-dB loss event near an 8-branch splitter. The results show that the proposed technique can accurately measure loss events with a spatial resolution of better than 3 m.

Strain-Sensor-In-Pixel Technology for Resolution-Sustainable Stretchable Displays.

One of the limitations of stretchable displays is the severe degradation of resolution or the decrease in the number of pixels per unit area when stretched. Hence, we suggest a strain-sensor-in-pixel (S-SIP) system through the adoption of hidden pixels that are activated only during the stretch mode for maintaining the density of on-state pixels. For the S-SIP system, the gate and source electrodes of InGaZnO thin-film transistors (TFTs) in an existing pixel are connected to a resistive strain sensor through the facile and selective deposition of silver nanowires (AgNWs) via electrohydrodynamic-jet-printing. With this approach, the strain sensor integrated TFT functions as a strain-triggered switch, which responds only to stretching along the designated axes by finely tuning the orientation and cycles of AgNW printing. The strain sensor-integrated TFT remains in an off-state when unstretched and switches to an on-state when stretched, exhibiting a large negative gauge factor of -1.1 × 1010 and a superior mechanical stability enduring 6000 cycles, which enables the efficient structure to operate hidden pixels without requiring additional signal processing. Furthermore, the stable operation of the S-SIP in a 5 × 5-pixel array is demonstrated via circuit simulation, implying the outstanding applicability and process compatibility to the conventional active-matrix display backplanes.

Enhancing the radiation hardness of vertical 4H–SiC detectors by the metal-oxide-semiconductor structure

In this work, 4H–SiC α-particle detectors with metal/semiconductor (MS) and metal/oxide/semiconductor (MOS) structures have been fabricated. The effects of introducing a SiO2 dielectric layer to form a MOS structure on the electrical properties, the performances, and the radiation hardness of the detectors have been evaluated systematically by comparing those with the MS structure. It is found that the reverse leakage current and the signal-to-noise ratio of α-particle detectors at high voltages can be improved by the MOS structure. Furthermore, the MOS structure exhibits much higher radiation hardness, as the energy resolution degradation of α particle detectors with MOS and MS structures after the γ irradiation is 0.59% and 2.87%, respectively. This work may provide a way to enhance the radiation hardness of 4H–SiC α-particle detectors.

SPFS: SNR peak-based frequency selection method to alleviate resolution degradation in MPI real-time imaging

Objective. The primary objective of this study is to address the reconstruction time challenge in magnetic particle imaging (MPI) by introducing a novel approach named SNR-peak-based frequency selection (SPFS). The focus is on improving spatial resolution without compromising reconstruction speed, thereby enhancing the clinical potential of MPI for real-time imaging. Approach. To overcome the trade-off between reconstruction time and spatial resolution in MPI, the researchers propose SPFS as an innovative frequency selection method. Unlike conventional SNR-based selection, SPFS prioritizes frequencies with signal-to-noise ratio (SNR) peaks that capture crucial system matrix information. This adaptability to varying quantities of selected frequencies enhances versatility in the reconstruction process. The study compares the spatial resolution of MPI reconstruction using both SNR-based and SPFS frequency selection methods, utilizing simulated and real device data. Main results. The research findings demonstrate that the SPFS approach substantially improves image resolution in MPI, especially when dealing with a limited number of frequency components. By focusing on SNR peaks associated with critical system matrix information, SPFS mitigates the spatial resolution degradation observed in conventional SNR-based selection methods. The study validates the effectiveness of SPFS through the assessment of MPI reconstruction spatial resolution using both simulated and real device data, highlighting its potential to address a critical limitation in the field. Significance. The introduction of SPFS represents a significant breakthrough in MPI technology. The method not only accelerates reconstruction time but also enhances spatial resolution, thus expanding the clinical potential of MPI for various applications. The improved real-time imaging capabilities of MPI, facilitated by SPFS, hold promise for advancements in drug delivery, plaque assessment, tumor treatment, cerebral perfusion evaluation, immunotherapy guidance, and in vivo cell tracking.

EMISSA: Exploring millimetre indicators of solar-stellar activity III. Comparison of Ca II indices and millimetre continua in a 3D model atmosphere

Amongst several spectral lines, some of the strongest chromospheric diagnostics are offered by the Ca II H K lines. These lines can be used to gauge the temperature stratification of the atmosphere since the line core and wings are formed in different regions of the solar atmosphere. Furthermore, the Ca II lines act as tracers for the magnetic structure of the solar atmosphere, as the line cores are formed in the upper chromosphere even though they are formed in non-local thermodynamic equilibrium (NLTE). In contrast, the formation of millimetre (mm) continuum radiation occurs under local thermodynamic equilibrium (LTE) conditions. As a result, the brightness temperatures obtained from observations with the Atacama Large Millimetre/Submillimetre Array (ALMA) offer a complementary perspective on the activity and thermal structure of stellar atmospheres. The overall aim is to establish more robust solar/stellar activity indicators using ALMA observations in comparison with classical diagnostics, such as the s index and infrared triplet (IRT) index. We employed the 1.5D radiative transfer codes RH1.5D and advanced radiative transfer (ART) to compute the synthetic spectra for the Ca II lines and the millimetre (mm) continua, respectively. These calculations were performed using an enhanced network atmosphere model, which incorporates non-equilibrium hydrogen ionisation generated by the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) Bifrost code. To account for the limited spatial resolution of ALMA, we simulated the effect using a Gaussian point spread function (PSF). Additionally, we analysed the correlations and slopes of scatter plots between the Ca II indices and mm continuum for the original and degraded resolutions, focusing on the entire simulation box, quiet Sun regions, and enhanced network patches separately. The activity indices generated from these lines could further be used to compare the spectra of Sun-like stars with the solar spectrum. We present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm to 8.5 mm) and the Ca II activity indices; namely, the s index and infrared triplet (IRT) index. The Ca II activity indices and mm brightness temperatures are weakly correlated at the high resolution, with the highest correlation observed at a wavelength of 0.3 mm, corresponding to ALMA band 10. As the resolution decreases, the correlation consistently increases. Conversely, the slopes exhibit a decreasing trend with increasing wavelength, while the degradation of resolution does not noticeably affect the calculated slopes. As the spatial resolution decreases, the standard deviations of the Ca II activity indices and brightness temperatures decrease, while the correlations between them increase. However, the slopes do not exhibit significant changes. Consequently, these relationships could be valuable for calibrating the mm continuum maps obtained through ALMA observations.

Non-telecentric photoacoustic microscopy for multi-scale imaging.

Optical-resolution photoacoustic microscopy (OR-PAM) excels in precisely imaging a biological tissue based on absorption contrast. However, existing OR-PAMs are confined by fixed compromises between spatial resolution and field of view (FOV), preventing the integration of large FOV and local high-resolution within one system. Here, we present a non-telecentric OR-PAM (nTC-PAM) that empowers efficient adaptation of FOV and spatial resolution to match the multi-scale requirement of diverse biological imaging. Our method allows for a large-scale transformation in FOV and even surpassing the nominal FOV of the objective with minimal marginal degradation of the lateral resolution. We demonstrate the advantage of nTC-PAM through multi-scale imaging of the leaf phantom, mouse ear, and cortex. The results reveal that nTC-PAM can switch the FOV and spatial resolution to meet the requirements of different biological tissues, such as large-scale imaging of the whole cerebral cortex and high-resolution imaging of microvascular structures in local brain regions.

Pimo: memory-efficient privacy protection in video streaming and analytics

Video streaming from cameras to backend cloud or edge servers for neural-based analytics has gained significant popularity. However, the transmission of data from cameras to a backend raises substantial privacy concerns, particularly regarding sensitive information like facial data. To offer privacy protection, visual processing techniques, such as Generative Adversarial Networks (GANs), have been employed on cameras to blur and safeguard such data intelligently. However, these techniques frequently face memory challenges, particularly when dealing with high-resolution videos. In this paper, we propose PIMO, a memory-efficient visual privacy protection scheme designed to effectively blur video content leveraging adaptive slicing of frames and resolution degradation. Our extensive experimental evaluations validate that PIMO’s adaptive mechanism proficiently navigates fluctuating memory constraints. Furthermore, utilizing a content-based blur scheme, our approach can maintain an impressive mean precision of 95.2%, as compared to the original, non-blurred images.

Depth-encoding using optical photon TOF in a prism-PET detector with tapered crystals.

High-resolution brain positron emission tomography (PET) scanner is emerging as a significant and transformative non-invasive neuroimaging tool to advance neuroscience research as well as improve diagnosis and treatment in neurology and psychiatry. Time-of-flight (TOF) and depth-of-interaction (DOI) information provide markedly higher PET imaging performance by increasing image signal-to-noise ratio and mitigating spatial resolution degradation due to parallax error, respectively. PET detector modules that utilize light sharing can inherently carry DOI information from the multiple timestamps that are generated per gamma event. The difference between two timestamps that are triggered by scintillation photons traveling in opposite directions signifies the event's depth-dependent optical photon TOF (oTOF). However, light leak at the crystal-readout interface substantially degrades the resolution of this oTOF-based depthencoding. We demonstrate the feasibility of oTOF-based depth encoding by mitigating light leak in single-ended-readout Prism-PET detector modules using tapered crystals. Minimizing light leak also improved both energy-based DOI and coincidence timingresolutions. The tapered Prism-PET module consists of a 16 16 array of 1.5 1.5 20 lutetium yttrium oxyorthosillicate (LYSO) crystals, which are tapered down to 1.2 1.2 at the crystal-readout interface. The LYSO array couples 4-to-1 to an 8 8 array of 3 3 silicon photomultiplier (SiPM) pixels on the tapered end and to a segmented prismatoid light guide array on the opposite end. Performance of tapered and non-tapered Prism-PET detectors was experimentally characterized and evaluated by measuring flood histogram, energy resolution, energy-, and oTOF-based DOI resolutions, and coincidence timing resolution. Sensitivities of scanners using different Prism-PET detector designs were simulated using Geant4 application for tomographic emission (GATE). For the tapered (non-tapered) Prism-PET module, the measured full width at half maximum (FWHM) energy, timing, energy-based DOI, and oTOF-based DOI resolutions were 8.88 (11.18)%, 243 (286)ps, 2.35 (3.18)mm, and 5.42 (13.87)mm, respectively. The scanner sensitivities using non-tapered and tapered crystals, and 10 rings of detector modules, were simulated to be 30.9 and 29.5kcps/MBq,respectively. The tapered Prism-PET module with minimized light leak enabled the first experimental report of oTOF-based depth encoding at the detector module level. It also enabled the utilization of thinner (i.e., 0.1mm) inter-crystal spacing with barium sulfate as the reflector while also improving energy-based DOI and timingresolutions.

Overlay Mark Design Using Irregular Grating Patterns

Overlay refers to controlling the vertical alignment and misalignment between circuit patterns manufactured in the previous process and circuit patterns manufactured in the current process during the stacking process of circuit patterns in the semiconductor manufacturing process. A critical factor in ensuring precise overlay measurements is using an optimized overlay mark design. Many semiconductor manufacturers have put considerable effort into optimizing overlay measurement marks to reduce high costs. However, image-based overlay measurement encounters several challenges, including resolution degradation, image distortion, and noise introduction during digital data conversion. These issues can amplify the uncertainty of overlay measurements, impair optical resolution, and heighten the risk of mismeasurement. In this paper, we introduce irregular grating patterns designed to address the principal challenges faced by the commonly utilized Advanced Imaging Metrology (AIM) marks, namely large overlays and image distortion. Furthermore, we propose an overlay mark capable of precisely identifying overlays exceeding half a pitch and incorporating as much data as possible within the limits of optical resolution. This aims to improve overlay measurement performance. The experimental findings demonstrate that the proposed Pulsated Grating Target (PGT) mark enhances overall measurement uncertainty by approximately 17% compared to the AIM mark. The practical applicability of the PGT was validated through experiments, and we achieved enhanced precision and reliability in overlay measurements by resolving the issue of large overlays—the most significant challenge with grating patterns—and simultaneously enhancing mark performance.