Analysis Of Line Edge Roughness Research Articles

Analysis of Line-Edge Roughness Using EUV Scatterometry

Smaller and more complex three-dimensional periodic nanostructures are part of the next generation of integrated electronic circuits. Additionally, decreasing the dimensions of nanostructures increases the effect of line-edge roughness on the performance of the nanostructures. Efficient methods for characterizing three-dimensional nanostructures are required for process control. Here, extreme-ultraviolet (EUV) scatterometry is exploited for the analysis of line-edge roughness from periodic nanostructures. In line with previous observations, differences are observed between line edge and line width roughness. The angular distribution of the diffuse scattering is an interplay of the line shape, the height of the structure, the roughness along the line, and the correlation between the lines. Unfortunately, existing theoretical methods for characterizing nanostructures using scatterometry do not cover all these aspects. Examples are shown here and the demands for future development of theoretical approaches for computing the angular distribution of the scattered X-rays are discussed.

Line edge roughness metrology software

A line edge roughness analysis software is developed based on the Canny edge detection algorithm with a double threshold, where threshold values are obtained by Otsu’s method. The performance of the software is demonstrated on features with a 200-nm nominal pitch generated by current-controlled, field-emission scanning probe lithography. Two lithographic modes are applied: (a) direct self-development positive mode and (b) image reversal mode. Atomic force imaging is used to analyze the line edge roughness. This is followed by a benchmarking study, where findings are compared to those provided by metroler software (Fractilia, LLC). This work is the first report on both line edge roughness involving imaging using the same exposure setup and latent image line edge roughness—made possible thanks to the resolving power of imaging through noncontact AFM. The authors are presenting a comparison of patterning through image reversal of the calixarene molecular glass resist from negative-tone to positive-tone as well as direct-write. In image reversal, a close match was observed between the proposed analysis and metroler software for line edge roughness and linewidth.

Quantitative analysis and modeling of line edge roughness in near-field lithography: toward high pattern quality in nanofabrication

Abstract Quantitative analysis of line edge roughness (LER) is very important for understanding the root causes of LER and thereby improving the pattern quality in near-field lithography (NFL), because LER has become the main limiter of critical dimension (CD) control as the feature size of nanostructures is scaled down. To address this challenge, the photoresist point-spread function of NFL with a contact plasmonic ridge nanoaperture can be employed to account for the physical and chemical effects involved in the LER-generation mechanism. Our theoretical and experimental results show that the sources of LER in NFL mainly come from the aerial image, material chemistry, and process. Importantly, the complicated decay characteristics of surface plasmon waves are demonstrated to be the main optical contributor. Because the evanescent mode of surface plasmon polaritons (SPPs) and quasi-spherical waves (QSWs) decay in the lateral direction, they can induce a small image log-slope and low photoresist contrast, leading to a large LER. We introduce an analytical model and demonstrate the relationship between LER and CD to estimate the pattern quality in NFL. We expect that these results can provide alternative approaches to further improve pattern uniformity and resolution, which can lead to advanced nanopatterning results in NFL.

Line edge roughness metrology: recent challenges and advances toward more complete and accurate measurements

Two fundamental challenges of line edge roughness (LER) metrology are to provide complete and accurate measurement of LER. We focus on recent advances concerning both challenges inspired by mathematical and computational methods. Regarding the challenge of completeness: (a) we elaborate on the multifractal analysis of LER, which decomposes the scaling behavior of edge undulations into a spectrum of fractal dimensions similarly to what a power spectral density (PSD) does in the frequency domain. Emphasis is given on the physical meaning of the multifractal spectrum and its sensitivity to pattern transfer and etching; (b) we present metrics and methods for the quantification of cross-line (interfeature) correlations between the roughness of edges belonging to the same and nearby lines. We will apply these metrics to quantify the correlations in a self-aligned quadruple patterning lithography. Regarding the challenge of accuracy, we present a PSD-based method for a noise-reduced (sometimes called unbiased) LER metrology and validate it through the analysis of synthesized SEM images. Furthermore, the method is extended to the use of the height–height correlation functions to deliver noise-reduced estimation of the correlation length and the roughness exponent of LER.

Optimization of Nano-Grating Pitch Evaluation Method Based on Line Edge Roughness Analysis

Abstract Pitch uncertainty and line edge roughness are among the critical quality attributes of a pitch standard and normally the analyses of these two parameters are separate. The analysis of self-traceable Cr atom lithography nano-gratings shows a positive relevance and sensitivity between LER and evaluated standard deviation of pitch. Therefore, LER can be used as an aided pre-evaluation parameter for the pitch calculation method, such as the gravity center method or the zero-crossing points method. The optimization of the nano-grating evaluation method helps to obtain the accurate pitch value with fewer measurements and provide a comprehensive characterization of pitch standards.

Study of shot noise in photoresists for extreme ultraviolet lithography through comparative analysis of line edge roughness in electron beam and extreme ultraviolet lithography

In this paper, the influence of the absorption shot noise on line edge roughness (LER) of photoresists for extreme ultraviolet (EUV) lithography is studied experimentally through the comparative analysis of LER obtained by EUV (92 eV photons) and 100 keV e-beam lithography. Techniques for performing EUV and e-beam lithography with a matched image log slope for a fair comparison of LER values are described. Measurements of absorption of 100 keV electrons estimated through a transmissive electron energy loss spectroscopy measurement with a 120 keV electron beam showed that despite having access to core levels in the material (e.g., 284 eV edge in carbon), these electrons mostly just excite the energy levels less than 100 eV in the resist, with a mean deposited energy of 35 eV. By combining the incident flux and the absorption probabilities, the absorbed quanta for patterning of 50 nm half-pitch line/space features was found to be similar between the two patterning technologies.

Analysis of line-edge roughness due to the stick/slip motion of a contact-mode scanning probe in plasmonic lithography

In plasmonic lithography, when using a scanning probe in contact mode, degradation of the line pattern quality occasionally occurs due to variations of probe tip velocity or a stick/slip motion. To avoid poor pattern quality caused by such variations in probe tip motion, we analyzed the motion of the probe tip by using a frictional model based on conventional contact mechanics. The motion of the probe tip was numerically analyzed in terms of adhesion force and probe velocity, which are the dominant factors in micro/nanoscale motion. It was found that stick/slip spacing has a roughly positive relationship with the maximum adhesion force between the substrate and probe tip, and a negative relationship with the probe velocity. Combining the probe tip motion with the exposure model of a near-field wave, we analyzed the quality of line patterns that resulted from various stick/slip spacing in terms of line-edge roughness.

Dependency analysis of line edge roughness in electron-beam lithography

The line edge roughness (LER) has become one of the critical issues which affect the minimum feature size and the maximum circuit density realizable in most lithographic processes. Since the LER does not scale with the feature size, it needs to be minimized as the feature size is reduced well below 100nm. One of the main factors contributing to the LER is the stochastic fluctuation of exposure. In the past, most of the LER researches were based on a 2-D model without considering the resist depth dimension. In this study, the dependency of the LER, caused by the stochastic fluctuation of exposure due to electron scattering in the resist and the shot noise due to variation of electron influx, on lithographic parameters such as shot noise, beam energy, exposing interval, dose, etc., has been investigated with a 3-D model, as the first step toward developing an effective method for minimizing the LER. In the case of CAR, the effect of developing process on LER is also considered. In this paper, the results from an extensive simulation are reported with a detailed discussion.

Multi-Scale Analysis of Line Edge Roughness Based on Wavelet Transform

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Metrology Method of Line Edge Roughness with Nanometer Scale Precision Using Atomic Force Microscopes

This paper proposes a methodology method for line edge roughness (LER) measurement and characterization using atomic force microscope. The definition and origins of LER are discussed firstly. A LER quantificational method using image processing and threshold method is presented, which is used to analyze AFM images of Silicon lines and extract LER characteristics. Then the energy distribution of LER is determined by the multi-scale analysis based on wavelet transform and the parameters of multi-scale characterization were given. The experiment data shows that this method can offer an effective quantitative analysis of LER.

Roughness reduction in submicron waveguides by low-molecular weight development

Roughness reduction of a submicron waveguide profile in chemically amplified negative resist is here performed by proper selection of an alkali-based developer, taking into account that its smaller molecules lead to smoother resist surface by altering the developing mechanism of aggregate extraction performed with standard quaternary ammonium hydroxide. Roughness is then analyzed by means of classical Atomic Force Microscope inspection; furthermore, a non-invasive line edge roughness analysis approach based on top-down scanning electron microscope acquisition gives comparable results, in terms of standard deviation and molecular aggregate periodicity.

Depth profile and line‐edge roughness of partially O‐1‐ethoxyethylated low molecular weight amorphous polyphenol and poly(p‐hydroxystyrene) base resists for electron‐beam lithography

AbstractThe line‐edge roughness (LER) of the chemically amplified positive‐tone electron‐beam (EB) resist based on low molecular weight polyphenol, partially O‐1‐ethoxyethylated 4,4′‐methylenebis [2‐[di(2‐methyl‐4‐hydroxy‐5‐cyclohexylphenyl)]methyl]phenol (3M6C‐MBSA‐EE), and poly(p‐hydroxystyrene) (PHS‐EE) as reference were compared. Phenol groups of these materials were partially protected by 1‐ethoxyethyl (EE) groups to control the dissolution rate for an aqueous alkaline developer. In order to confirm the film homogeneity, the film depth profiles by time‐of‐flight secondary ion mass spectometry (TOS‐SIMS) measurement were investigated. From these results, the films of 3M6C‐MBSA‐EE and PHS‐EE base resist showed relatively flat depth profiles for species originating from the resist components and were suggested to be homogeneous. For the LER analysis aspect, the LER values estimated by using the Fourier amplitude spectra were 5.1 and 7.4 nm for 3M6C‐MBSA‐EE and PHS‐EE base resist, respectively. The former resist also represented the suppressed amplitude A(f) value of the Fourier amplitude spectra in the lower frequency region below 10 µm−1. 3M6C‐MBSA‐EE with suppressed polydispersity and dissolution rate distribution would be a hopeful material to control the LER. Also, it was confirmed that the formulation‐modified resist with 3M6C‐MBSA‐EE showed the lower LER value and remarkably depressed A(f) value than the high‐resolution EB resist which was formulated with p‐hydroxystyrene and acrylate derivative copolymer. Copyright © 2006 John Wiley & Sons, Ltd.

Resist effects at small pitches

The ITRS roadmap and Moore’s law are driving us to print ever smaller features and ever tighter pitches. For these ultrasmall features and pitches, resist effects are expected to play a dominant role in limiting the overall lithographic process capability. To study the impact of resist parameters such as acid diffusion and quencher level on the lithographic performance, we have designed and formulated a matrix of 14 extreme ultraviolet (EUV) resists. In this article, we discuss results that we have obtained from EUV exposures of those resists on the 0.3 numerical aperture EUV Micro Exposure Tool at the Advanced Light Source at Lawrence Berkeley National Laboratories. In addition to exposure latitude and line edge roughness, acid diffusion lengths were characterized using a modification of the extended Nijboer–Zernike theory for each photoresist. A simple theory for the drop in resist contrast as function of the diffusion length is tested and leads to a verified correlation between exposure latitude and diffusion length. This theory gives guidelines to predict what acid diffusion lengths should be used for a certain lithography node. Line edge roughness analysis showed that shot noise statistics are present, and also points out that acid diffusion effects come into play. From the experimentally observed tendencies in this unique matrix of EUV photoresists, fundamental understanding was gained about prominent resist effects that will determine the lithographic performance of resist processes in future lithography nodes.

Spectral analysis of line-edge roughness in polyphenol EB-resists and its impact on transistor performance

Resists using polyphenol resin are introduced to reduce line-edge roughness (LER), and the spatial frequency characteristics of LER are evaluated. It is found that the long-period components of LER are suppressed in our low molecular-weight polyphenol resists. Device simulation using the measured LER shows that our polyphenol-based resist can drastically reduce the number of low-threshold-voltage (Vth) transistors compared with a conventional resist due to reduced long-period LER. Because LER impact is more serious as the transistor width shrinks, our results suggest that the use of the polyphenol-type resist will be more effective in improving device performance in future lithography process. In addition, it is shown that spectral analysis is a powerful tool for LER evaluation, especially from the viewpoint of device performance estimation.

Effect of thin-film imaging on line edge roughness transfer to underlayers during etch processes

For the patterning of sub-100 nm features, a clear understanding of the origin and control of line edge roughness (LER) is extremely desirable, from a fundamental as well as a manufacturing perspective. With the migration to thin photoresists coupled with bottom antireflective coating (ARC)-hardmask underlayers, LER analysis of the developed resist structures is perhaps an inaccurate representation of the substrate roughness after the etch process, since those underlayers can play a significant role in increasing/decreasing linewidth variations during the image transfer process and hence can impact the device performance. In this article, atomic force microscopy is used to investigate the contribution of the imaging resist sidewall topography to the sidewall roughness of the final etched feature in thin photoresists, ARC, and hardmasks. Resist systems suitable for 248 and 193 nm lithography as well as fluorine-containing resists were processed using N2-H2 or fluorocarbon plasma etch. It is shown that the interaction of different etch chemistries with existing sidewall profiles can result in loss of the original morphological information and creation of new spatial frequency domains that act as physical templates for subsequent image transfer processes. Excessive roughness transfer into the hardmask layer due to insufficient resist thickness or inadequate etch resistance originates from striation propagation from the resist layer into the hardmask layer. In the case of fluorine-containing materials, a decreased etch resistance and reduced initial film thickness values give rise to critical underlayer roughening during plasma etch. Based on the results shown, it is predicted that advanced resist systems for 157 nm lithography and beyond will require the use of ARC layers with built-in hardmask properties in those particular cases in which patterning of deep trenches is needed, in order to maintain LER values within acceptable levels.

Analysis of Line Edge Roughness Using Probability Process Model for Chemically Amplified Resists

Line edge roughness (LER) in chemically amplified resists (CARs) is understood to be fluctuations in the acid catalyzed reaction that determine molecular solubility and developer percolation. Two probability processes that cause LER are modeled: one is local acid generation with diffusion process, and the other is the main reaction and developer percolation process. A typical fluctuation size in these processes is significantly larger than the molecular size and depends on various parameters, such as acid concentration, diffusion length, molecular size, protection ratio and variation, and image conditions. Careful scaling for these parameters is required to reduce LER with certain shrinking design feature sizes. Our results suggest various factors that dominate LER in several resist systems. Reliable metrology for LER requires a sufficient sample size due to the random nature of origin. High contrast exposure characteristics of the dissolution rate in CARs today are also explained with the model.

Analysis of Line-edge Roughness in Resist Patterns and Its Transferability as Origins of Device Performance Degradation and Variation

General property of line-edge roughness (LER) in resist pattern and its transferability to under-lying layer were investigated. Longer-period components were found to have larger amplitudes in a resist pattern and to remain after dry etching. In addition, long-period LER strongly affects a transistor performance. Long-period LER in resist patterns, therefore, is as important as short-period LER. Metrology of LER was reconsidered to evaluate the both LER properly, and a guideline for choosing measurement parameters was proposed from a viewpoint of device performance estimation.