Immersion Lithography Research Articles

Source and mask optimization for stability of reticle and wafer stages

The relative motion of the reticle stage and wafer stage caused by vibration during the scanning exposure process of the lithography machine is an important factor that affects the imaging quality under limited lithography ability. In this paper, the influence of the vibration of the lithography stage system on the lithography imaging quality is studied. The lithography model including the vibration error of the stage is taken into account in the framework of source and mask optimization (SMO). For the 193-nm immersion lithography machine, combined with different design patterns with a line width of 40 nm and different vibration states of the stage when the lithography machine is exposed, the SMO joint optimization research is carried out, and the robustness of the stage under the limit process after SMO optimization is explored. The research results show that the contrast and relative light intensity change of the limit design figure under the influence of moving standard deviation (MSD) will greatly affect the linewidth at the light intensity threshold and then affect the process window (PW). The SMO including MSD allows the MSD to change in the range of 0-6 nm without changing the PW.

Super-resolution Microscopy and Photo-lithography: How can one inspire the other

The principles of microscopy and lithography projection techniques are similar. To improve the resolution of the microscope or further reduce the image projection, both techniques face similar limitations, diffraction limits. The diffraction limit is a limitation of physical optics. In the development process of microscopes, to improve optical microscopy technology, researchers have proposed the idea of reducing the wavelength of light sources and increasing the numerical aperture based on the principle of diffraction limit generation. Modern scientists have proposed techniques to break through the diffraction limit and gradually increase the resolution of microscopes to further improve resolution. These breakthroughs also provide ideas for lithography technology, and researchers have invented techniques such as immersion lithography, multi-exposure, and two-photon direct writing. This article introduces the development process of super-resolution microscopy and lithography technology and analyzes the inspiration and influence of microscopy technology on lithography technology, that is, providing ideas for improving the accuracy of lithography technology.

193 nm immersion photodetector with an ultra-high EQE of 83.7%

With the development of chip to high-density integration, the requirement of lithography precision is higher and higher. In this process, controlling the power of the lithography exposure light source is very important to improve the quality of the chip. However, most detectors are unable to conduct light detection in liquid environments, especially in the aspects of real-time detection and calibration of light power in immersion deep ultraviolet lithography. In this paper, the AlN film epitaxial on n-SiC substrate by MOCVD is used as the photoelectrode, based on which an immersion detector with a quick response to 193 nm vacuum ultraviolet (VUV) light is fabricated. Impressively, under 0 V bias, the detector achieves a high responsiveness (130 mA/W) and an EQE up to 83.7% under an excitation with lower light power (45 nW), illustrating its ability to perform highly efficient detection of light signals and underwater imaging in weak-light environment. In addition, the detector also has the shortest detection wavelength when compared with the immersion photodetectors reported so far. At the same time, the simulated immersion lithography monitoring experiment shows that the detector has good real-time monitoring ability. In a word, this work provides a novel method for detecting underwater light power and can work as reference for realizing the real-time monitoring of immersive lithography light sources in future applications.

Electrical evaluation of copper damascene interconnects based on nanoimprint lithography compared with ArF immersion lithography for back-end-of-line process

Nanoimprint lithography (NIL) is promising for the processing of dual damascene structures fabricated in back-end-of-line layers, and initial development began with a simple single-level process to evaluate NIL’s suitability. In this work, a test element group (TEG) pattern with a 70 nm half-pitch was selected, and copper (Cu) filling and chemical-mechanical polishing were performed after NIL pattern transfer. The results were compared with those obtained from the same TEG layout and processes but using ArF immersion lithography instead of NIL. Those obtained by NIL showed high pattern fidelity for all the designed layouts, whereas the resist patterns varied from the designed shape for ArF immersion lithography. The line resistances of Cu interconnects patterned by NIL showed good cumulative distributions at line widths ranging from 60 nm to 78 nm in 2 nm increments, without line breaks or space narrowing of SiO2. NIL showed potential for interconnect patterning with high-precision line width control.

Physical model-based ArF photoresist formulation development

Since the logic 28 nm technology and beyond, ArF immersion lithography has been widely used in manufacturing. To fully utilize the potential of the lithographic resolution, process simulation has been used since the lithography process setup step. The accuracy of the model prediction can be a key factor in the process stability, defectivity, and yield of the final product. To get better model prediction, it is very important to develop a process model with as many parameters as possible with physical meanings. Since the analysis of exposure data with a physical model can provide insights into the process and the photoresist material, it may greatly accelerate the photoresist formulation development or improvement process. From another aspect, as the function of the photoresist is to record the aerial image information on wafer, theoretically, the photoresist image should not deviate much from the aerial image or it will inevitably reduce the information content of the aerial image, i.e., through various kinds of averaging, mixing, etc. Since the emergence of Optical Proximity Correction (OPC), the time that takes to finalize a photolithography process has been significantly lengthened and made more complicated. To make the OPC model more reliable and less dependent on patches, a more physical OPC model is necessary. Therefore, it is clear that if we can develop a photoresist formulation to realize a more physical photoresist image, or a photoresist image closer to the aerial image, we can greatly improve the photolithography process performance and the time consumed for the process setup including OPC. In this paper, we focus on several physical model parameters for photoresist, especially the effective photoacid diffusion length and photo decomposable base parameters. We will show that these two parameters can provide a good description of photoresist imaging behavior, and the resist formulation can be improved to match the physical model prediction with more accuracy.

Angle monitor of micromirror array for freeform illumination in lithography systems

Source and mask optimization is a critical technique for further resolution enhancement in immersion lithography systems, wherein the optimal illumination source shape is widely generated by the micromirror array. Accordingly, the accurate achievement of the allocated angles of micromirrors is a key prerequisite for the implementation of arbitrary illumination patterns. In this paper, we propose an angle monitor to ensure the high-precision tilting of thousands of biaxial micromirrors. As one of the critical modules for closed-loop control of the two-dimensional micromirror array, the online monitor feeds the monitored high-precision tilt angle back to the processing system. The angle monitor mainly consists of the spot scanning module and the angle detection module. Among them, an f-θ lens and a Fourier transform lens with satisfied performances are designed and evaluated by CODE V. Furthermore, a galvanometer and the designed f-θ lens are adopted for the generation of the spot array irradiated on the electrostatic actuated mirrors. Meanwhile, the designed Fourier transform lenses are employed to detect the corresponding tilt angles. In addition, the performance of the proposed system is identified by simulations and experiments individually. It is demonstrated that the monitorable biaxial tilt range of the system is (−2.5°, +2.5°) with a repeatability of better than 0.005°. The large-format micromirror array can be monitored completely without optical cross-talk. Through the device, a 16 × 16 micromirror array is monitored, where the initial angle with no bias voltage applied is captured and the voltage–angle relation for individual micromirrors is obtained. In general, the proposed system can be utilized in the illumination system, providing an efficient and reliable method for complex source shape generation.

Diamond Immersion Photodetector for 193 nm Lithography

AbstractAt present, most photodetectors work in atmospheric or vacuum environments, which leaves a large gap for detection under liquid conditions. However, this kind of research is of great significance, especially for the monitoring of 193 nm vacuum UV (VUV) light in the immersion lithography system underwater. Here, the p‐type diamond single crystal (PDSC) is applied as the photoelectrode for the construction of a VUV immersion photodetector with a PDSC‐ultra‐pure water‐Pt structure and a high response to 193 nm light. Under 193 nm excitation (@8 µW) and at 0 V, the PDSC immersion photodetector demonstrates a favorable characteristic of having a high responsivity of 5 mA·W−1. Due to its exceptional performance, this device has been proven to function as a highly‐sensitive and self‐powered VUV detector, with significant potential for utilization in the monitoring of 193 nm light in immersion lithography systems.

Density functional theory study on reaction mechanisms of Co(tbu2DAD)2 for area selective-atomic layer deposition of Co films on metal surfaces

As electronic devices scale in size approaching nm scales, the smaller feature sizes become more difficult and expensive to pattern. The most common patterning technique currently used in microelectronics industry, ArF laser immersion lithography, requires more and more steps to pattern one layer, the smaller the pitch becomes. Area selective-atomic layer deposition (AS-ALD) is one possible solution that allows for both fewer patterning steps and smaller feature sizes. However, the fundamental mechanisms of surface selectivity and the role of reducers in ALD growth are not fully understood. This modeling work focuses on the detailed atomic scale processes of AS-ALD deposition of Co metal on various substrate surfaces. Co is of particular interest for its capability in reducing resistance of metal interconnects in back end of lines when replacing Cu lines below 16 nm in critical dimension, and such a small linewidth can be achieved by the AS-ALD growth of Co films. This work shows the mechanisms and properties associated with the growth of Co on various surfaces (Cu, Pt, Co, and SiO2) as well as the role of a reducing agent in facilitating surface reactions during ALD processes. Density functional theory was used to describe the reaction mechanisms and accurately describe the system’s energetic and electronic characteristics during the deposition process. These findings provide insight into the fundamental mechanisms of selective ALD growth on metal surfaces against oxide surfaces and the catalytic role of reducers in facilitating the kinetics of ALD precursor reactions on metal surfaces.

Decomposition-learning-based thick-mask model for partially coherent lithography system.

The simulation of thick-mask diffraction near-field (DNF) is an indispensable process in aerial image calculation of immersion lithography. In practical lithography tools, the partially coherent illumination (PCI) is applied since it can improve the pattern fidelity. Therefore, it is necessary to precisely simulate the DNFs under PCI. In this paper, a learning-based thick-mask model proposed in our previous work is extended from the coherent illumination condition to PCI condition. The training library of DNF under oblique illumination is established based on the rigorous electromagnetic field (EMF) simulator. The simulation accuracy of the proposed model is also analyzed based on the mask patterns with different critical dimensions (CD). The proposed thick-mask model is shown to obtain high-precise DNF simulation results under PCI, and thus is suitable for 14 nm or larger technology nodes. Meanwhile, the computational efficiency of the proposed model is improved up to two orders of magnitude compared to the EMF simulator.

Concurrent Steiner Tree Selection for Global routing with EUVL Flare Reduction

The aggressive scaling down of the feature size in modern ICs has introduced major bottlenecks in printing layouts using a conventional 193nm immersion lithography system. As different mitigation techniques have reached their limitations, Next Generation Lithography (NGL) techniques have been gaining popularity. Extreme Ultra-violet Lithography (EUVL) system which uses light having 13.5nm wavelength is one of the popular NGL for printing features below 20nm. However, EUVL faces the problem of flare caused by irregular reflection from clear-field mask surfaces used. It leads to distortion of critical dimension (CD) during layout printing which in turn degrades the performance of the circuit. In this paper, we have formulated a delay-aware global routing problem in order to minimize flare without sacrificing the delay. Two different solvers, namely ILP and SAT-SMT, are employed and the results are compared. Experimental results show a significant reduction in flare without much compromise on delay.

Cover Image, Volume 140, Issue 21

This cover designed by Xuemei Li and Peng Zhao demonstrates the metallic contamination control of the fittings transporting ultrapure water in immersion lithography. The influence of process parameters on metallic contamination is investigated and optimized through the orthogonal experiment. This paper provides a novel idea for improving the cleanliness of the environment in chip manufacturing and a contribution to the next generation of chips. DOI: 10.1002/app.53940

Scalable manufacturing of high-index atomic layer–polymer hybrid metasurfaces for metaphotonics in the visible

Metalenses are attractive alternatives to conventional bulky refractive lenses owing to their superior light-modulating performance and sub-micrometre-scale thicknesses; however, limitations in existing fabrication techniques, including high cost, low throughput and small patterning area, have hindered their mass production. Here we demonstrate low-cost and high-throughput mass production of large-aperture visible metalenses using deep-ultraviolet argon fluoride immersion lithography and wafer-scale nanoimprint lithography. Once a 12″ master stamp is imprinted, hundreds of centimetre-scale metalenses can be fabricated using a thinly coated high-index film to enhance light confinement, resulting in a substantial increase in conversion efficiency. As a proof of concept, an ultrathin virtual reality device created with the printed metalens demonstrates its potential towards the scalable manufacturing of metaphotonic devices.

Nanoscale Phase Change Material Array by Sub-Resolution Assist Feature for Storage Class Memory Application

High density phase change memory array requires both minimized critical dimension (CD) and maximized process window for the phase change material layer. High in-wafer uniformity of the nanoscale patterning of chalcogenides material is challenging given the optical proximity effect (OPE) in the lithography process and the micro-loading effect in the etching process. In this study, we demonstrate an approach to fabricate high density phase change material arrays with half-pitch down to around 70 nm by the co-optimization of lithography and plasma etching process. The focused-energy matrix was performed to improve the pattern process window of phase change material on a 12-inch wafer. A variety of patternings from an isolated line to a dense pitch line were investigated using immersion lithography system. The collapse of the edge line is observed due to the OPE induced shrinkage in linewidth, which is deteriorative as the patterning density increases. The sub-resolution assist feature (SRAF) was placed to increase the width of the lines at both edges of each patterning by taking advantage of the optical interference between the main features and the assistant features. The survival of the line at the edges is confirmed with around a 70 nm half-pitch feature in various arrays. A uniform etching profile across the pitch line pattern of phase change material was demonstrated in which the micro-loading effect and the plasma etching damage were significantly suppressed by co-optimizing the etching parameters. The results pave the way to achieve high density device arrays with improved uniformity and reliability for mass storage applications.

Surface property control for 193 nm immersion resist by addition of Si compound

In ArF immersion lithography, the presence of immersion liquid between the resist surface and the lens causes problems, such as the leaching of the photoacid generator into the liquid and the presence of residual liquid on the resist surface, which can result in watermarks and other defects. One method to address such issues is adding an F-based compound with low dry-etch resistance to the resist. In the present study, we developed a novel resist for ArF immersion exposure that replaces the F compound with an Si (dimethylpolysiloxane)-based additive to enhance dry-etch resistance. We experimentally evaluated contact angles with respect to water and developer solution, depth concentration of the additives (segregation), agent dissolution (leaching), dry-etch resistance, and spectral transmittance of the comparative resists. Simulation studies were performed to evaluate pattern profiles. The developed resist with the Si-based additive showed improved properties compared with that with the F-based additive.

Performance measurement technique for 193-nm depolarizer

Herein, a performance measurement technique for 193-nm depolarizers, which is an area that still lacks research, is proposed and demonstrated. Utilizing a reflective fused-quartz Brewster wedge plate as a polarizer, a large beam at 193 nm is effectively polarized with a large separation angle, which is not possible with traditional polarization prisms. A pre-polarizing and beam-splitting structure is used to reduce power drift. A statistical method that makes full use of repeated measurements and priori knowledge (polarization decomposition and Malus' law) is developed to significantly improve measurement accuracy. Verification, functional, and repeatability tests are conducted. The experimental results are in good agreement with the theoretical analysis. This technique exhibits excellent accuracy and precision. It can be used for offline measurement of the depolarizer used in 193-nm immersion lithography, which is of considerable significance. Furthermore, the results provide important insights for related works on high-accuracy polarization measurement at ultra-short wavelengths.

Effect of Drilling Parameters on Machining Performance in Drilling Polytetrafluoroethylene.

Polytetrafluoroethylene (PTFE) plays an important role in semiconductor manufacturing. It is an important processing material for the key sealing components in the field of immersion lithography. The lack of research related to the mechanical processing of PTFE leads to many challenges in producing complex parts. This paper conducted a drilling experiment on PTFE. The effect of cutting parameters on the drilling performance was investigated. Thrust, torque, surface roughness, and drilling temperature were used to evaluate the influence of cutting parameters on drilling performance. In addition, the empirical mathematical models of thrust and torque were developed using analysis of variance (ANOVA). The results indicated that the spindle speed had the most important effect on the thrust and the feed rate had the most significant effect on the torque. The lowest values of thrust and torque were, respectively, 22.64 N and 0.12 Nm, achieved in the case of spindle speed of 5000 rev/min, and feed rate of 50 mm/min. The surface quality is also best at this cutting parameter. Studies have shown that higher spindle speeds with lower feed rates are ideal parameters for improving the drilling performance and machining quality of PTFE. In addition, it was found that the temperature differences due to different drilling depths were related to chip accumulation. Surface roughness inconsistencies at various locations in the inner wall of the hole were influenced by chip adhesion during machining. This paper provides a suggestion for optimizing cutting parameters and hole quality.

Wafer-level testing of inverse-designed and adjoint-inspired dual layer Si-SiN vertical grating couplers

Recently, silicon photonics foundries started providing access to new dielectric stacks which can be utilized to reduce optical I/O losses. For example, in a hybrid c-Si/SiN platform, inverse design techniques can be used to create novel dual layer grating coupler (GC) designs which, in simulations, reach state-of-the-art performance. In this paper, we experimentally validate such designs for perfectly vertical single-polarization GCs in the O-band consisting of a single-etch c-Si layer with a patterned SiN overlay, fabricated using a DUV immersion lithography process on wafers. Here, we investigate designs generated by two different design paradigms: inverse design based on the adjoint method and adjoint-inspired design. Using wafer-level testing, we experimentally demonstrate a record low median insertion loss (IL) of (with interquartile range of –) for perfectly vertical coupling in DUV lithography compatible devices which is a improvement over previously demonstrated single-layer, single-etch c-Si GCs.

Volatile vapor knife of immersion lithography hood using solutal Marangoni effect

Water-based immersion lithography has been introduced for achieving O(10 nm) spatial resolution in the semiconductor industry. The major challenges remaining in immersion lithography are to decrease the tail of the main lens and to prevent residual droplet formation after the main lens while increasing the relative speed of the silicon wafer with respect to the main lens. Here, we propose a novel method to control the shape of the immersion lens by applying Marangoni stress using volatile vapor. Furthermore, we experimentally and theoretically observed that the stability and wafer speed of the immersion lens are increased by the vapor-driven solutal Marangoni effect.

Aberration analysis and compensate method of a BP neural network and sparrow search algorithm in deep ultraviolet lithography.

Mass production can be planned by utilizing the multiple patterning technology of 193 nm immersion scanners at the 7 nm technology node. In deep ultraviolet lithography, imaging performance is significantly affected by distortions of projection optics. For 7 nm immersion lithography layer patterns, distortions of the projection optics must be tightly controlled. This paper proposes an optimization method to determine the distribution of Zernike aberration coefficients. First, we build aberration prediction models using the backpropagation (BP) neural network. Then, we propose an aberration optimization method based on the sparrow search algorithm (SSA), using the common indicators of the lithography process window, depth of focus, mask error enhancement factor, and image log slope as the objective function. Some sets of optimized aberration distributions are obtained using the SSA optimization method. Finally, we compare the results of the SSA optimization algorithm with those obtained by rigorous computational simulations. The aberration combination distribution optimized by the SSA method is much more significant than the value under the zero aberration (ideal conditions), a nonoptimal distribution in deep ultraviolet lithography image simulation. Furthermore, the results indicate that the aberration optimization method has a high prediction accuracy.

Design of an objective for a hyper-numerical-aperture immersion lithography tool with a multi-step alternative grouping design method.

An adequate initial structure plays an important role in achieving a high-quality hyper-numerical-aperture (NA) catadioptric objective for an immersion lithography tool. A multi-step alternative grouping design method is innovated to achieve a feasible aspherical initial structure of the hyper-NA objective. The objective is first divided into three groups: the object-side group (G1), the middle group (G2), and the image-side group (G3), and then each group is designed separately. To guarantee the effective connection of these separate groups, G1 and G2 or G2 and G3 are connected as G12 or G32 on the object or image side with an aspherical structure. Then, through ray tracing in the forward path and the reverse path, G2 will be recalculated to a new aspherical structure, and the hybrid iterating process will be adopted to get a better G2 and to better connect the whole system. Finally, G1 and the new G2 and G3 are connected as a feasible aspherical initial structure of the hyper-NA catadioptric objective, which can be considered a good starting point for further optimization. A high-performance catadioptric objective with a hyper-NA of 1.35 was designed by the proposed method.