Line-edge roughness (LER) is often measured from top-down critical dimension scanning electron microscope (CD-SEM) images. The true three-dimensional roughness profile of the sidewall is typically ignored in such analyses. We study the response of a CD-SEM to sidewall roughness (SWR) by simulation. We generate random rough lines and spaces, where the SWR is modelled by a known power spectral density. We then obtain corresponding CD-SEM images using a Monte Carlo electron scattering simulator. We find the measured LER from these images, and compare it to the known input roughness. We find that, for isolated lines, the SEM measures the outermost extrusion of the rough sidewall. The result is that the measured LER is up to a factor 2 less than the true on-wafer roughness. The effect can be accurately modelled by making a top-down projection of the rough edge. Our model for isolated lines works fairly well for a dense grating of lines and spaces, as long as the trench width exceeds the line height.
Dip-pen nanolithography (DPN) is a low-cost, versatile, bench-top technology for direct patterning of materials over surfaces. Our study reports on the production of two-dimensional optical grating nanostructures based on polymers, using DPN. The influence of both the ink composition and the dwell time were investigated. Prototypes of phase masks were manufactured, and their main characteristics were analyzed. The results in our work may contribute to improving the fabrication process of optical structures, including the production of microlenses with controlled focal length. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
Background: In-line metrology for three-dimensional (3D) profiling high-aspect-ratio (HAR) features is highly important for manufacturing semiconductor devices, particularly for memory devices, such as 3D NAND and DRAM. Aim: Our purpose was to obtain the cross-sectional profiles of the HAR features from top-view critical dimension scanning electron microscopy (CD-SEM) images. Approach: Based on Monte Carlo simulation results, we proposed a method for 3D profiling of HAR features using backscattered electron (BSE) signal intensities. Several kinds of HAR holes with different taper angles and bowing geometries were fabricated. High-voltage CD-SEM was used for experiments to determine the feasibility of our approach. Results: Using the BSE line-profile, we constructed cross sections of the taper holes and estimated sidewall angles (SWAs), which were approximately the same as those observed using field-emission scanning electron microscopy (FE-SEM). The constructed cross sections of the bowing holes and the trends of the geometric variance, which were estimated by the middle CD and its depth, were consistent with the cross sections observed by FE-SEM. Conclusions: The results demonstrate that the variation in the HAR holes, such as SWA and bowing geometry, can be measured and monitored using the BSE images. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.
In extreme ultraviolet (EUV) lithography, chemistry is driven by secondary electrons. A deeper understanding of these processes is needed. However, electron-driven processes are inherently difficult to experimentally characterize for EUV materials, impeding targeted material engineering. A computational framework is needed to provide information for rational material engineering and identification at a molecular level. We demonstrate that density functional theory calculations can fulfill this purpose. We first demonstrate that primary electron energy spectrum can be predicted accurately. Second, the dynamics of a photoacid generator upon excitation or electron attachment are studied with ab-initio molecular dynamics calculations. Third, we demonstrate that electron attachment affinity is a good predictor of reduction potential and dose to clear. The correlation between such calculations and experiments suggests that these methods can be applied to computationally screen and design molecular components of EUV material and speed up the development process. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
The Journal of Micro/Nanopatterning, Materials, and Metrology (JM3) publishes peer-reviewed papers on the core enabling technologies that address the patterning needs of the electronics industry.
Source mask and polarization optimization (SMPO) is a promising extension of the widely used resolution enhancement technology, source mask optimization (SMO), to further enhance chip manufacturability beyond 28-nm node. Our work is aimed to develop an efficient gradient-based SMPO method by employing the hybrid Hopkins-Abbe imaging model to fulfill the goal. In addition to source and mask variables, the model is adapted to also include polarization variables to realize the optimization. Compact formulas for forward and backward model application are derived. The computation benefits from precomputed transmission cross coefficients and features high efficiency. Validity of the method is confirmed by case studies. For dense array pattern case, the optimal source and polarization can be found analytically. SMPO optimized results match well with the theoretical expectations. In addition, process window, mask error enhancement factor, and normalized image log-slope for the studied cases all get improved over the counterpart SMO results, which employ commonly used polarization. Runtime analysis shows the method is computationally efficient. Our work provides a valid way to optimize polarization together with source and mask. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
Background: Design of microelectromechanical system based Bennet's doubler kinetic energy harvester (KEH) is tricky as it has to satisfy the operating criteria of doubler circuit along with the harvester's design constraints for its operation. Aim: Design guidelines for an electrostatic KEH using Bennet's doubler circuit along with its experimental validation are presented. Approach: Bennet's doubler circuit can work as a KEH only for a specific range of capacitance ratio across interdigitated electrodes of the harvester. The constraints on the resonant frequency of Bennet's doubler harvester have been deduced to achieve operational capacitance ratio at both low and high vibrational frequencies. Finally, a test structure is fabricated, using silicon-on-insulator multiuser MEMS processes, and tested for capacitance ratio eta greater than 1 366, a prerequisite for the operation of Bennet's doubler circuit. Results: Resonant operation of the test structure achieves capacitance ratio of 1.39 with a capability of harvesting energy density of 4.63 mu J/cm(3). Further, an improved harvester design is also presented for eta = 1.5, based on the discussed guidelines that increase the energy density to 19.6 mu J/cm(3). Conclusions: We will present an insight into the design of Bennet's doubler harvester for different vibrational frequencies, which is being widely explored for electrostatic energy harvesting. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
Background: To increase the resolution and depth of focus (DOF) of flat panel display (FPD) exposure systems, off-axis illumination (OAI) conditions are used extensively. OAI using narrowband wavelength illumination has been studied sufficiently. In contrast, new techniques that consider broadband wavelength illumination are needed because the effects of OAI differ between broadband and narrowband illumination. Aim: This paper presents a divided spectrum illumination (DSI), a new design concept that achieves both high resolution and a large DOF. Approach: The source wavelength is optimized according to the illumination angle. Results: Experimental imaging results for line and space patterns with a line width of 1.0 and a pitch of 2.0 mu m demonstrate that the DSI design provides improved resolution. Exposure results also indicate that resist profiles using DSI are sufficiently sharp to retain pattern fidelity at the top of the resist. The DOF with DSI is also improved by 21% compared to that obtained with traditional OAI. Conclusions: DSI achieves both high resolution and a large DOF while maintaining high productivity. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
We review the history in connection with the resolution formula of microlithography and argue that it was Abbe rather than Rayleigh who definitively stated the 0.5λNA resolution limit for the minimum pitch first, using an approach more relevant to projection imaging, and hence, this expression should be more appropriately referred to as the Abbe formula for the resolution of a projection imaging system.
Background: Conventional scanning electron microscopy (SEM) that is used for 2D top-view metrology, a classical line edge roughness (LER) measurement technique, is incapable of measuring 3D structures of a nanoscale line pattern. For LER measurements, SEM measurement generates a single line-edge profile for the 3D sidewall roughness, although the line-edge profile differs at each height in the 3D sidewall. Aim: To develop an evaluation method of SEM-based LER measurement techniques and to verify how the 3D sidewall shape is reflected in the SEM's 2D result. Approach: Direct comparison by measuring an identical location of a line pattern by SEM and an atomic force microscopy (AFM) with the tip-tilting technique that is capable of measuring the 3D sidewall. The line pattern has vertical stripes on the sidewall due to its fabrication process. Measured line edge profiles were analyzed using power spectral density, height-height correlation function, and autocorrelation function. Results: Line edge profiles measured by SEM and AFM were well matched except for noise level. Frequency and scaling analyses showed that SEM profile contained high noise and had lost a property of self-affine fractals in contrast to AFM. Conclusions: In the case of the line pattern with vertical stripes on the sidewall, SEM profile is generally consistent with 3D sidewall shape. The AFM-based LER measurement technique is useful as LER reference metrology to evaluate other LER measurement techniques. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.
Abstract. Background: Although the wet cleaning process has been widely used in semiconductor device manufacturing due to its convenience, it faces theoretical limits. That is, when the size of the objected particle is smaller than 100 nm, it is buried in the stagnant layer where there is substantially no fluid flow. Aim: Only small particles below the stagnant layer (<100 nm) is removed without any damage to the fine patterns or substrate: pattern collapse, critical dimension shift, and optical property shift. Approach: Utilizing unique characteristics of water: volume expansion when freezing, solid (ice) is lighter than liquid (water), and particles adhered the substrate is peeled off from the substrate and rise to the water surface along with the surrounding ice. Results: By repeating the cycle of cooling, thawing, and rinsing, polystyrene sphere particle of 80 nm in diameter can be removed with high particle removal efficiency (PRE >90 % ) and no negative influences on the pattern or substrate. Conclusions: A new cleaning method for very small (<100 nm) particles is proposed with high PRE and low damage. This method is thought to be applied to every process if water can infiltrate into the gap between the particles and the substrate.
Next-generation extreme ultraviolet (EUV) systems with numerical apertures of 0.55 have the potential to provide sub-8-nm half-pitch resolution. The increased importance of stochastic effects at smaller feature sizes places further demands on scanner and mask to provide high contrast images. We use rigorous mask diffraction and imaging simulation to understand the impact of the EUV mask absorber and to identify the most appropriate optical parameters for high NA EUV imaging. Simulations of various use cases and material options indicate two main types of solutions: high extinction materials, especially for lines spaces, and low refractive index materials that can provide phase shift mask solutions. EUV phase masks behave very different from phase shift masks for DUV. Carefully designed low refractive index materials and masks can open up a new path toward high contrast edge printing. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.
With the introduction of the NXE:3400B scanner, ASML has brought EUV to High-Volume Manufacturing (HVM). The high EUV power of >200W being realized with this system satisfies the throughput requirements of HVM, but also requires reconsideration of the imaging aspects of spectral purity, both from the details of the EUV emission spectrum and from the DUV emission. This paper will present simulation and experimental results for the spectral purity of high-power EUV systems, and the imaging impact of this, both for the case of with and without a pellicle. Also, possible controls for spectral purity will be discussed, and a novel method will be described to measure imaging impact of varying CE and DUV. It will be shown that CE optimization towards higher source power leads to reduction in relative DUV content, that the small deltas in EUV source spectrum for higher power do not influence imaging. It will also be shown that resulting variations in DUV do not affect imaging performance significantly, provided that a suitable reticle black border is used. In short, spectral purity performance is not a bottleneck for increasing power of EUV systems to well above 250W.
Abstract. Background: Molecular logic circuits have great potential applications. DNA logic circuit is an important research direction of DNA computing in nanotechnology. DNA self-assembly has become a powerful tool for building nanoscale structures. The combination of different self-assembly methods is an interesting topic. Aim: Two different self-assembly methods are combined to realize large-scale logic circuit. A basic logical unit is extended to complex logic circuits by self-assembly. Approach: The complex logic circuit is solved by combining nanoparticles. One DNA strand attached to nanoparticle maps to a logical unit. Just as the combination between logical units can form logic circuits, the combination between nanoparticles can be used to structure logic circuits. On a larger-scale logic circuits, this is done by attaching the assembled nanoparticles to an origami template. Different logical values are mapped into different DNA initiators. Results: After the reaction is over, the nanoparticles are dynamically separated from the DNA origami template, indicating that the result is true. The nanoparticles remain on the DNA origami template, indicating that the result is false. The simulation results show that this self-assembly model is highly feasible for complex logic circuits. Conclusions: The model combines two different self-assembly methods to realize large-scale logic circuits. Compared with previous models, this model implements a larger logic circuit on one origami template. This method can be used to construct more complex nanosystems and may have potential applications in molecular engineering.
With aggressive scaling of single-expose EUV lithography to the sub-7 nm node, stochastic variations play a prominent role in defining the lithographic process window. Fluctuations in photon shot noise, absorption and subsequent chemical reactions can lead to stochastic failure, directly impacting electrical yield. Fundamental characterization of the mode and magnitude of these variations is required to define the threshold for failure. In this work, a complementary series of techniques is enlisted to probe the nature and modulation of stochastic variation in single exposure EUV patterning. Unbiased line edge roughness (LER), local critical dimension uniformity (LCDU) and defect inspection techniques are employed to monitor the frequency of stochastic variations leading to failures in line/space and via patterning. Using this methodology, we explore the modulation of stochastic variations by different photoresists and illuminations, with emphasis on material and process down-selection for improved yield at the sub-7 nm node.
The Journal of Micro/Nanopatterning, Materials, and Metrology (JM3) publishes peer-reviewed papers on the core enabling technologies that address the patterning needs of the electronics industry.
A single-chip microelectromechanical system (MEMS) capacitive microphone is designed and modeled. The mechanical model of the structure is extracted and the mathematical equations for a description of the microphone behavior are obtained. Then the proposed microphone characteristics are considered. In this structure, by adding Z-shape arms around the diaphragm, diaphragm hardness is decreased and diaphragm displacement becomes uniform. The sensitivity and the pull-in voltage are improved despite the decreasing size. The perforated diaphragm of this microphone is supported by Z-shape arms at its four corners. These arms around the diaphragm decrease the stiffness and air damping of the microphone. The behavior of this microphone is also analyzed by the finite element method. The structure has a diaphragm thickness of 2 mu m, a diaphragm size of 0.32 x 0.32 mm(2), an air gap of 2 mu m, and a highly doped monocrystalline silicon wafer as a backplate. The proposed microphone is simulated with IntelliSuite software. According to the results, the new microphone has a sensitivity of 14.245 mV/Pa and a pull-in voltage of 5.83 V. The results show that the proposed MEMS capacitive microphone is one of the best structures in performance. The obtained mathematical equations for description of the microphone's behavior have good agreement with the simulation results. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
Stochastic defects are becoming major concern in the future EUV lithography as their probability Pd exponentially increases with decreasing feature size and is highly sensitive to variations in process/mask conditions. Photon shot noises and discrete/probabilistic nature of materials have been blamed as their causes. We introduce models for relating Pd to photon and resist statistics under various exposure and material conditions and analyze their impact in future EUV lithography. Three-dimensional reaction distributions are calculated by a fully-coupled Monte Carlo simulation including discrete photon, photoelectron scattering, and resist stochastics. Then, probability models predict Pd from statistical data extracted from Monte Carlo results. Stochastic defect generation is enhanced by cascade and/or cluster of correlated reactions among nearby polymers/molecules due to secondary electrons (SE)/acid diffusion and SEs generated along scattered photoelectron trajectories. Pd decreases with increasing reaction density, suppressing effective image blur, and introducing quenchers, where reaction density is limited by SE, PAG and reaction site. Defect probability increases with decreasing target size for the same k1-factor, while strongly dependent on image slope and defocus. Our analyses suggest that applying EUV lithography to smaller target requires careful material choice, extremely precise process control, and further EUV power enhancement.
We continue our work on the physics of mask-topography-induced phase effects in imaging using extreme ultraviolet (EUV) lithography, and specifically how these effects can be mitigated by alternative mask absorbers. We present a semianalytical model to calculate the mask-topography-induced phase offset and study its trend throughout the entire material space at 13.5-nm wavelength. We demonstrate that the model is in good agreement with 3D rigorous simulations. Using the model, we explain why the previously demonstrated phase shift close to 1.2 pi works optimally for EUV imaging. We show a low refractive index mask absorber (n < 0.91) is crucial for good mask 3D mitigation. We demonstrate the importance of mask bias and incident angle for imaging with an optimized attenuated phase-shift mask (PSM), which makes good source-mask optimization indispensable. We present the lithographic performance of alternative mask absorbers including a high-k mask, and a low- and high-transmission attenuated PSM for a few basic use cases, confirming the lithographic gain that can be obtained by mask-absorber optimization. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)
