Extreme Ultraviolet Lithography System Research Articles

Simulation of extreme ultraviolet radiation of laser induced discharge plasma

Extreme ultraviolet (EUV) light source is an important part of EUV lithography system in semiconductor manufacturing. The EUV light source requires that the 4p<sup>6</sup>4d<sup><i>n</i></sup>-4p<sup>5</sup>4d<sup><i>n</i>+1</sup> + 4d<sup><i>n</i>-1</sup>4f transitions of Sn<sup>8+~13+</sup> ions emit thousands of lines which form unresolved transition arrays near 13.5 nm. Laser-induced discharge plasma is one of the important technical means to excite target into an appropriate plasma condition. Laser-induced discharge plasma has a simple structure and a low cost. It also has important applications in mask inspection, microscopic imaging, and spectral metrology. In the design and production process, there are many factors that can influence the conversion efficiency, such as current, electrode shape, and laser power density. The simulation method is a convenient way to provide guidance for optimizing the parameters. In this paper, a completed radiation magneto-hydrodynamic model is used to explore the dynamic characteristics of laser-induced discharge plasma and its EUV radiation characteristics. To improve the accuracy, a more detailed global equation of state model, an atomic structure calculation model including relativistic effect and a collision radiation model are proposed simultaneously. The simulation reconstructs the discharge process effectively, which is divided into five stages in the first half cycle of current, including expansion of laser plasma, column formation of discharge plasma, diffusion of discharge plasma, contraction of discharge plasma, and re-diffusion of discharge plasma. It is revealed that the pinch effect during the current rising time exerts a significant influence on the generation of EUV radiation. The conversion efficiency of EUV radiation is still low under our existing conditions, and hopefully a higher rising rate of current can improve the conversion efficiency in the future work.

Photoresist Fabrication for High-NA EUV Lithography: Material and Mechanism

Extreme ultraviolet (EUV) lithography is critical for the semiconductor industry nowadays. However, high-numerical aperture (NA) EUV systems face challenges in many areas, especially in photoresists. This paper reviews requirements and recent progress in EUV photoresists for high-NA EUV lithography. The key needs for photoresists include high resolution, low stochastic defects, minimal pattern collapse, and high sensitivity. Proposed solutions involve new photoresist materials and mechanisms, such as metal-containing photoresists, non-chemically amplified photoresists and tin-oxo cages photoresists. Basic fabrication and processing procedures are also discussed. Other than photoresists, the weak EUV light source also heavily restrains the development of photoresists. In the future, improvements in EUV light sources and patterning techniques will further help high-NA EUV photoresist development. In summary, the realization of high-NA EUV lithography system depends critically on the breakthrough of photoresists, while solutions are emerging to meet demanding requirements from various directions. Overall, these results shed light on guiding further exploration of next generation lithography.

Fast diffraction model of an EUV mask based on asymmetric patch data fitting.

Calculating the diffraction near field (DNF) of a three-dimensional (3D) mask is a key problem in the extreme ultraviolet (EUV) lithography imaging modeling. This paper proposes a fast DNF model of an EUV mask based on the asymmetric patch data fitting method. Due to the asymmetric imaging characteristics of the EUV lithography system, a DNF library is built up including the training mask patches posed in different orientations and their rigorous DNF results. These training patches include some representative local mask features such as the convex corners, concave corners, and edge segments in four directions. Then, a convolution-based compact model is developed to rapidly simulate the DNFs of 3D masks, where the convolution kernels are inversely calculated to fit all of the training data. Finally, the proposed model is verified by simulation experiments. Compared to a state-of-the-art EUV mask model based on machine learning, the proposed method can further reduce the computation time by 60%-70% and roughly obtain the same simulation accuracy.

Fast diffraction model of lithography mask based on improved pixel-to-pixel generative adversarial network.

Mask three-dimensional (3D) effect is a vital influence factor of imaging performance in the advanced extreme ultraviolet (EUV) lithography system. However, the rigorous 3D mask diffraction model is very time-consuming and brings a great computational burden. This paper develops a fast and accurate method to calculate the mask diffraction near-field (DNF) based on an improved pixel-to-pixel generative adversarial network, where the deformable convolution is introduced for fitting the crosstalk effect between mask feature edges. The long short-term memory model is added to the generator network to fuse and exchange information between the real parts and imaginary parts of DNF matrices. In addition, the simulation accuracy of DNF is enhanced by using the subpixel super-resolution method in the up-sampling step. The calculation accuracy is improved by more than 50% compared to the traditional network, and the calculational efficiency is improved by 128-folds compared to the rigorous electromagnetic field simulation method.

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.

A study of hydrogen plasma-induced charging effect in EUV lithography systems

In the extreme ultraviolet lithography system, EUV-induced hydrogen plasma charging effect is observed by in situ embedded micro-detector array. The 4k-pixel on-wafer array can detect and store the distributions of H2 plasma in each in-pixel floating gate for non-destructive off-line read. The local uniformity of H2 plasma intensity extracted by the threshold voltages on an array and its distributions across a wafer by the average bit cell current of MDAs provide insights into the detailed conditions inside advanced EUV lithography chambers.

Simulation of extreme ultraviolet radiation of laser induced discharge plasma

Extreme ultraviolet (EUV) light source is an important part of EUV lithography system in semiconductor manufacturing. The EUV light source requires that the 4p<sup>6</sup>4d<sup><i>n</i></sup>-4p<sup>5</sup>4d<sup><i>n</i>+1</sup> + 4d<sup><i>n</i>–1</sup>4f transitions of Sn<sup>8+~13+</sup> ions emit thousands of lines which form unresolved transition arrays near 13.5 nm. Laser-induced discharge plasma is one of the important technical means to excite target into an appropriate plasma condition. Laser-induced discharge plasma has a simple structure and a low cost. It also has important applications in mask inspection, microscopic imaging, and spectral metrology. In the design and production process, there are many factors that can influence the conversion efficiency, such as current, electrode shape, and laser power density. The simulation method is a convenient way to provide guidance for optimizing the parameters. In this paper, a completed radiation magneto-hydrodynamic model is used to explore the dynamic characteristics of laser-induced discharge plasma and its EUV radiation characteristics. To improve the accuracy, a more detailed global equation of state model, an atomic structure calculation model including relativistic effect and a collision radiation model are proposed simultaneously. The simulation reconstructs the discharge process effectively, which is divided into five stages in the first half cycle of current, including expansion of laser plasma, column formation of discharge plasma, diffusion of discharge plasma, contraction of discharge plasma, and re-diffusion of discharge plasma. It is revealed that the pinch effect during the current rising time exerts a significant influence on the generation of EUV radiation. The conversion efficiency of EUV radiation is still low under our existing conditions, and hopefully a higher rising rate of current can improve the conversion efficiency in the future work.

Radiation of extreme ultraviolet source and out-of-band from laser-irradiated low-density SnO<sub>2</sub> target

The extreme ultraviolet (EUV) lithography technology required for high-end chip manufacturing is the first of 35 “bottleneck” key technologies that China is facing currently. The high conversion efficiency EUV source and low out-of-band radiation play a significant role in the application of the EUV lithography system. In this work, the EUV source and out-of-band radiation are studied by using laser irradiated solid Sn target and low-density SnO<sub>2</sub> target. The result shows that a strong EUV radiation at a wavelength of 13.5 nm is generated when the laser irradiates the two forms of Sn targets. Owing to the self-absorption effect of the solid Sn target plasma, the maximum intensity of the wavelength is not located at the position of 13.5 nm, which is working wavelength of EUV lithography system. However, the peak radiation spectrum is located at the position of 13.5 nm with low-density SnO<sub>2</sub> target due to its weaker plasma self-absorption effect. In addition, the satellite lines are weaker in low-density SnO<sub>2</sub> target than in the solid Sn target, so that the spectrum efficiency of the EUV at 13.5 nm (2% bandwidth) is increased by about 20%. On the other hand, the experimental study of the out-of-band radiation is carried out. The out-of-band radiation spectral results show that the out-of-band radiation is mainly dominated by the continuum spectrum. Compared with the solid Sn target, the low-density SnO<sub>2</sub> target contains a part of the low <i>Z</i> element O (<i>Z</i> = 8), resulting in a low-intensity continuum spectrum. In addition, the collision probability of ion-ion and electron-ion both become low when the laser irradiates the low-density SnO<sub>2</sub> target, resulting in a short out-of-band radiation duration time. Therefore, the out-of-band radiation generated by the laser irradiated on the low-density SnO<sub>2</sub> target is weak based on the above reasons. The angular distribution of out-of-band radiation measurement results shows that the intensity of out-of-band radiation decreases with the angle increasing. A cosine function <inline-formula><tex-math id="M2">\begin{document}$A \cos ^\alpha \theta$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20222385_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20222385_M2.png"/></alternatives></inline-formula> can fit the angular distribution of the total radiation.

Fast aerial image model for EUV lithography using the adjoint fully convolutional network

The effects of thick-mask and oblique incidence in extreme ultraviolet (EUV) lithography system make the aerial image calculation a challenging task. This paper develops a fast EUV lithography aerial image model based on a new kind of deep learning framework called adjoint fully convolutional network (AFCN). The AFCN consists of two adjoint data paths to respectively recover the real part and imaginary part of the complex mask diffraction-near-field (DNF). The feature-swapping technique is introduced to exchange the information between the real and imaginary data paths. The AFCN is trained based on a pre-calculated rigorous thick-mask DNF dataset. Given the estimated thick-mask DNF, the Abbe's method is used to calculate the aerial image of the partially coherent lithography system. Compared to the traditional non-parametric kernel regression method, the proposed model reduces the error by more than 80% and achieves 25-fold improvement in computational efficiency.

Characteristics of extreme ultraviolet emission from Gd plasma produced by dual pulse laser

The extreme ultraviolet (EUV) lithography technology, which is required for high-end chip manufacturing, is the first of 35 “neck stuck” key core technologies that China is facing currently. The EUV source with high conversion efficiency is an important part of EUV lithography system. The experiment on dual-pulse irradiated Gd target is carried out to realize the stronger 6.7 nm EUV emission output. Firstly, we compute the contribution of transition arrays of the form 4p-4d and 4d-4f from their open 4d subshell in charge states Gd<sup>18+</sup>−Gd<sup>27+</sup>, and transition arrays of the form 4d-4f from their open 4d subshell in charge states Gd<sup>14+</sup>−Gd<sup>17+</sup> on the near 6.7 nm EUV source. Subsequently, the experimental results of the dual pulse laser irradiated Gd target show that the intensity of 6.7 nm peak EUV emission decreases first, then increases and drops again due to the plasma density decreasing gradually when the delay time between the pre-pulse and main-pulse increases from 0−500 ns. The strongest intensity of 6.7 nm peak EUV emission is generated when the delay time is 100 ns. At the same time, the spectrum efficiency is higher when the delay time is 100 ns, which is 33% higher than that of single pulse laser. In addition, the experimental results show that the half width of EUV spectrum produced by dual pulse in the delay between 10−500 ns is narrower than that of signal laser pulse due to the fact that the method of dual pulse can suppress the self-absorption effect. The half width is the narrowest when the delay is 30 ns, which is about 1/3 time of EUV spectrum width generated by a single pulse. At the same time, the narrowing of Gd EUV spectrum improves the spectral utilization efficiency near 6.7 nm wavelength (within 0.6% bandwidth).

Identifying extreme ultraviolet lithography attenuated phase shifting mask absorber materials using effective media approximation modeling

Pattern transfer in an extreme ultraviolet lithography (EUVL) system requires reflective optical elements illuminated at oblique illumination angles. This, in combination with the three-dimensional effects at the mask, is the source of the so-called mask 3D (M3D) effects that include shadowing, best focus shifts, and contrast fading. Alternative mask absorbers at lower thickness possess the ability to extenuate the M3D effects and improve overall imaging performance. An approach to recognize candidate material combinations as alternative EUVL mask absorbers through dielectric constant modeling by using methods involving Wiener bounds and effective media approximation (EMA) is presented. Using EMA, several stoichiometrically stable binary alloys of refractory metals that may serve as attenuated phase shifting mask absorbers have been studied. The optical properties and thickness of each absorber candidate alloy in a standard EUV mask stack are optimized for imaging performance. The best performing alloys are characterized through rigorous 3D image modeling of near-field intensity and phase at varied illumination angles.

Compensation of EUV lithography mask blank defect based on an advanced genetic algorithm.

Mask blank defect is one of the most important factors that degrades the image quality of extreme ultraviolet (EUV) lithography system, and further leads to a yield lose. In order to compensate the amplitude and phase distortions caused by the EUV mask blank defects, this paper proposes an advanced algorithm to optimize the mask absorber pattern based on genetic algorithm. First, a successive approximation correction method is used to roughly compensate the effect of mask blank defect. Then, an advanced genetic algorithm is proposed to obtain higher efficiency and compensation accuracy, which uses an adaptive coding strategy and a fitness function considering normalized image log slope of lithography image. For illustration, the proposed method is verified based on rectangular contact patterns and complex pattern with different defects. The aerial images of optimized masks are evaluated by a commercial lithography simulator. It will show that the proposed method can mitigate the impact of mask defects, and improve the fidelity of lithography print image. The simulation results also demonstrate the higher convergence efficiency and mask manufacturability can be guaranteed by the proposed method.

Patterning nickel for extreme ultraviolet lithography mask application I. Atomic layer etch processing

The stringent requirement for patterning highly absorbing metal thin films as a mask for the next-generation extreme ultra-violet lithography system dictates the development of an atomic layer etching process to tailor the etch rate and the etch profile. A “plasma-thermal” atomic layer etching process is developed where an oxygen plasma is used to convert the metallic Ni layer into NiO, followed by formic acid vapor reacting with NiO to form nickel formate [Ni(COOH)2], thereby removing nickel. The directionality of the oxygen ions is used to create a directional chemical conversion of Ni into NiO, resulting in an anisotropic etch profile. Using the SiO2 patterned Ni thin film, a high etching selectivity to the mask (virtually no etching of SiO2) and a high etching anisotropy (a sidewall angle up to 87°) are achieved.

Synthesis and Property of Tellurium-Containing Molecular Resist Materials for Extreme Ultraviolet Lithography System

Synthesis and Property of Tellurium-Containing Molecular Resist Materials for Extreme Ultraviolet Lithography System

Learning-based compressive sensing method for EUV lithographic source optimization.

Extreme ultraviolet (EUV) lithography emerges as a promising technique to fabricate next-generation integrated circuits. In order to improve the lithography imaging fidelity, source optimization (SO) technique is widely used to compensate for the imaging distortion. This paper develops an efficient learning-based SO approach for EUV lithography under the compressive sensing (CS) framework. The dimensionality of EUV-SO problem is significantly reduced by sparsely sampling the layout pattern. Then, the EUV-SO is formulated as an l1-norm inverse reconstruction problem based on the sparse prior of source patterns. The cost function is established based on a rigorous imaging model to take into account the characteristic effects in EUV lithography systems. In addition, a learning-based method is proposed to jointly optimize the source dictionary and projection matrix according to the sparsity and incoherence conditions in CS theory. The optimal source dictionary and projection matrix can be learned from a set of training samples collected from typical layout features in advance. Then, the optimized dictionary and projection matrix can be repetitively used in the following SO algorithms. Based on a set of simulations, the proposed SO method is proved to achieve good performance in both imaging fidelity and computational efficiency.

Effects of kinematics and groove parameters on the mid-spatial frequency error of optics induced during full aperture polishing

The mid-spatial frequency (MSF) error is a key controlled specification of optical elements for many important applications (e.g., high power laser systems, extreme ultraviolet lithography systems and other high-resolution imaging systems). However, the effective controlling of MSF error is one of the difficulties in high precision optical materials processing, especially in the full aperture polishing process. In this study, a theoretical model of optical element MSF error induced during full aperture polishing process was established. The effects of kinematics and groove parameters on workpiece surface MSF error were studied by numerical simulation. According to the theoretical analysis results, several process optimization measures for reducing workpiece MSF error during full aperture polishing process were proposed. The practical effects of these optimization measures were verified in the following polishing experiments. The theoretical analysis and polishing experimental results suggested that the period of MSF error is obviously influenced by the grooving spacing of the polishing pad, and the MSF error value (PSD-RMS) can be significantly reduced by optimizing the polishing speed and swing parameters.

Synthesis and Property of Tannic Acid Derivatives and Their Application for Extreme Ultraviolet Lithography System

Synthesis and Property of Tannic Acid Derivatives and Their Application for Extreme Ultraviolet Lithography System

High-numerical aperture extreme ultraviolet scanner for 8-nm lithography and beyond

Current extreme ultraviolet (EUV) projection lithography systems exploit a projection lens with a numerical aperture (NA) of 0.33. It is expected that these will be used in mass production in the 2018/2019 timeframe. By then, the most difficult layers at the 7-nm logic and the mid-10-nm DRAM nodes will be exposed. These systems are a more economical alternative to multiple-exposure by 193 argon fluoride immersion scanners. To enable cost-effective shrink by EUV lithography down to 8-nm half pitch, a considerably larger NA is needed. As a result of the increased NA, the incidence angles of the light rays at the mask increase significantly. Consequently, the shadowing and the variation of the multilayer reflectivity deteriorate the aerial image contrast to unacceptably low values at the current 4× magnification. The only solution to reduce the angular range at the mask is to increase the magnification. Simulations show that the magnification has to be doubled to 8× to overcome the shadowing effects. Assuming that the mask infrastructure will not change the mask form factor, this would inevitably lead to a field size that is a quarter of the field size of the current 0.33-NA step and scan systems and reduce the throughput (TPT) of the high-NA scanner to a value below 100 wafers per hour unless additional measures are taken. This paper presents an anamorphic step and scan system capable of printing fields that are half the field size of the current full field. The anamorphic system has the potential to achieve a TPT in excess of 150 wafers per hour by increasing the transmission of the optics, as well as increasing the acceleration of the wafer stage and mask stage. This makes it an economically viable lithography solution.

Novel EUV Photoresist for Sub-7 nm Node

Extreme ultraviolet (EUV) lithography has been recognized as a promising candidate for the manufacturing of semiconductor devices as LS/CH pattern for 7 nm node and beyond. EUV lithography is ready for high volume manufacturing stage. For the high-volume manufacturing of semiconductor devices, significant improvement of sensitivity and line edge roughness (LWR) and Local CD Uniformity (LCDU) is required for EUV resist. It is well-known that the key challenge for EUV resist is the simultaneous requirement of ultrahigh resolution (R), low line edge roughness (L) and faster sensitivity (S). Especially sensitivity and good roughness is important for EUV lithography high volume manufacturing. We are trying to improve sensitivity and LWR/LCDU from many directions. From material side, we found that both sensitivity and LWR/LCDU are simultaneously improved by controlling acid diffusion length and efficiency of acid generation using novel resin and PAG. And optimizing EUV integration is one of the good solution to improve sensitivity and LWR/LCDU. We are challenging to develop new multi-layer materials to improve sensitivity and LWR/LCDU. Our new multi-layer materials are designed for best performance in EUV lithography system. From process side, we found that sensitivity was substantially improved maintaining LWR applying novel type of chemical amplified resist (CAR) and process. EUV lithography evaluation results obtained for new CAR EUV interference lithography. And, metal containing resist is one possibility to break through sensitivity and LWR trade off. In this paper, we will report the recent progress of sensitivity and LWR/LCDU improvement of JSR novel EUV resist and process.

Mechanism and model of atomic hydrogen cleaning for different types of carbon contamination on extreme ultraviolet multilayers

The use of atomic hydrogen to clean carbon contaminants on multilayers in extreme ultraviolet lithography systems has been extensively investigated. Additional knowledge of the cleaning rate would not only provide a better understanding of the reaction mechanism but would also inform the industry's cleaning process. In this paper, which focuses on the atomic-hydrogen-based carbon contamination cleaning process, a possible mechanism for the associated reactions is studied and a cleaning model is established. The calculated results are in good agreement with the existing experimental data in the literature. The influences of the main factors – such as activation energy and types of contamination – on the cleaning rate are addressed by the model. The model shows that the cleaning rate depends on the type of carbon contamination. The rate for a polymer-like carbon layer is higher than the rate for graphitic and diamond-like carbon layers. At 340K, the rate for a polymer-like carbon layer is 10 times higher than for graphitic carbon layers. This model could be used effectively to predict and evaluate the cleaning rates for various carbon contamination types.