Scanning electron microscopy (SEM) is generally used for line edge roughness (LER) measurement; however, it is difficult to achieve high precision LER measurement of photoresist due to shrinkage caused by electron-beam (EB) exposure. We have developed a metrological tilting-atomic force microscopy (AFM), which has a tip-tilting mechanism to measure vertical sidewall. In the last conference, SPIE-AL-2023, we demonstrated quantitative evaluation of shrinkage of ArF photoresist due to EB exposure by measuring the pattern before and after EB exposure [Kizu et al., Proc. SPIE 12496, 1249605 (2023)]. In this study, we will demonstrate quantitative evaluation of shrinkage of EUV photoresist due to EB exposure by the AFM technology.
KEYWORDS: Shrinkage, Photoresist materials, Line edge roughness, Scanning electron microscopy, Atomic force microscopy, 3D metrology, Lithography, Metrology, Fractal analysis, Electron beams
BackgroundScanning electron microscopy (SEM) is commonly employed for line edge roughness (LER) measurements; however, achieving high-precision LER measurement of photoresists is difficult through this approach because electron beam (EB) exposure causes shrinkage of materials. Moreover, the differences in the 3D sidewall shape before and after shrinkage have not been investigated in detail.AimEvaluation of the impact of photoresist shrinkage induced by EB exposure on the sidewall roughness of a pattern.ApproachThe shrinkage was observed by measuring a photoresist pattern before and after EB exposure using atomic force microscopy with a tip-tilting technique (tilting-AFM).ResultsEB exposure smoothed the surface roughness, rounded the top corners, and reduced the pattern height. Roughness parameters evaluated via LER analysis showed that with shrinkage, the standard deviation (σ) and roughness exponent (α) decreased, while the correlation length (ξ) increased.ConclusionsThe results show that SEM-based LER measurements may lead to underestimation of σ and α, and the overestimation of ξ because of the effect of EB-induced shrinkage. Overall, we establish a tilting-AFM technique to evaluate the 3D shape of photoresist patterns without EB damage and with high resolution and low noise.
Scanning electron microscopy (SEM) is commonly used for line edge roughness (LER) measurement; however, it is difficult to achieve high-precision LER measurement of photoresists because exposure to an electron beam (EB) causes shrinkage of the materials. The differences in the 3D sidewall shape before and after shrinkage have not been investigated in detail. In this study, EB-induced photoresist shrinkage was observed by employing the atomic force microscopy with tip-tilting technique (tilting-AFM), which enables high-precision observation of the vertical sidewall of the pattern. In the experiment, the shrinkage deformation was observed by measuring the same photoresist pattern with the tilting-AFM before and after EB exposure (by SEM observation) on the pattern. The results show that the sidewall was smoothed by EB exposure. Further, the tendency of changes in LER (roughness parameters) was observed. This measurement technique can be used to better understand photoresist materials and to improve the LER measurement by SEM.
KEYWORDS: Photoresist materials, Line edge roughness, Metrology, Atomic force microscopy, Scanning electron microscopy, Lithography, Anisotropy, Silicon, 3D metrology, Reliability
Sidewall roughness of a photoresist pattern is important information to consider such as a relation between LERs (or LWRs) of the resist and etched pattern. It is well known that sidewall of a dry-etched Si pattern shows vertical striations (anisotropic roughness) [Kizu et al., JM3 19, 014003 (2020)]. On the other hand, in the case of photoresist, although there are several studies about photoresist sidewall roughness, neither high-resolution sidewall measurement nor roughness evaluation techniques has been established. In this study, we measured photoresist sidewall with high-resolution using a metrological tilting-AFM, which is able to measure vertical sidewall of a line pattern owing to tilted AFM-probe. The result showed the photoresist sidewall roughness has height dependency in contrast to that of a Si line pattern. Further result and discussion will be presented in the conference.
Background: Scanning electron microscopy (SEM)-based line edge roughness (LER) measurement suffers from an error due to noise in SEM image. Noise correction methods have been developed to obtain unbiased roughness results, however, there are still concerns in the viewpoint of measurement precision.
Aim: To develop an unbiased roughness analysis for highly precise LER measurement.
Approach: Combining the conventional unbiased roughness analysis with a profile-averaging method, where a line pattern is repeatedly measured and then the obtained profiles are aligned and averaged. The experimental result measured by SEM was verified using atomic force microscopy (AFM)-based LER metrology, which has higher reliability than SEM.
Results: The experimental evaluation showed that the proposed method can obtain roughness parameters more precisely than the conventional method.
Conclusions: When the noise in the line edge profile by SEM is too large, it is necessary to reduce the noise beforehand and then perform roughness analysis in order to obtain precise roughness results. The proposed method enables to measure LER with the highest precision using SEM. Additionally, the AFM-based LER metrology was demonstrated as a feasible technique to evaluate the performance of SEM-based LER metrology.
In this study, we developed a methodology to evaluate scanning electron microscopy (SEM)-based line edge roughness (LER) metrology. In particular, we used a metrological tilting atomic force microscopy (tilting-mAFM) as LER reference metrology. We analyzed the height-height correlation function (HHCF) of SEM line-edge profiles combining averaging and unbiased correction methods. The direct comparison of our method with tilting-mAFM enabled a precise evaluation of the SEM-based LER metrology. We demonstrated that a combination of unbiased HHCF and averaging methods with appropriate condition enabled relatively precise measurement of three roughness parameters. We observed that, for precise roughness evaluation, reducing noise in the line-edge profiles is important before performing the HHCF analysis and unbiased correction.
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.
KEYWORDS: Line edge roughness, Metrology, Atomic force microscopy, Clouds, Scanning electron microscopy, Atomic force microscope, Servomechanisms, 3D metrology, Transmission electron microscopy, Semiconductors
Line edge roughness (LER) measurement is one of the metrology challenges for three-dimensional device structures, and LER reference metrology is important for reliable LER measurements. For the purpose of LER reference metrology, we developed an LER measurement technique that can analyze LER distribution along the height of a line pattern, with high resolution and repeatability. A high-resolution atomic force microscopy (AFM) image of a vertical sidewall of a line pattern was obtained using a metrological tilting-AFM, which offers SI-traceable dimensional measurements. The tilting-tip was controlled with an inclined servo axis, and it scans the vertical sidewall along a line pattern with a high sampling density to enable an analysis of the LER height distribution at the sidewall. A horizontal cross-section of the sidewall shows sidewall roughness with sub-nm resolution. Power spectral density (PSD) analysis of the sidewall profile showed that the PSD noise in the high-frequency region was several orders of magnitude lower than the noise of typical scanning electron microscopy methods. AFM measurements were sequentially repeated three times to evaluate the repeatability of the LER measurement; results indicated a high repeatability of 0.07 nm evaluated as a standard deviation of LER at each height.
KEYWORDS: Scanning electron microscopy, Line edge roughness, 3D metrology, Metrology, Spatial resolution, Edge roughness, Atomic force microscopy, 3D image processing, Error analysis, Lithography
Line edge roughness (LER) measurement of a nanoscale line pattern is a metrology challenge in the inspection of semiconductor devices. Conventional scanning electron microscopy (SEM), a classical LER measurement technique, is a top-view (2D) metrology and incapable of accurately measuring 3D structures. 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. In this study, we used atomic force microscopy (AFM) with the tip-tilting technique to measure the 3D sidewall roughness, as an LER reference metrology. An identical location of a line pattern measured by SEM and AFM was compared to evaluate the SEM’s performance. The line edge profile from the AFM measurement exhibited lower noise than that from SEM. The measured line edge profiles were analyzed using the power spectral density (PSD), height-height correlation function (HHCF), and autocorrelation function. The results demonstrate that the standard deviation (σ) and correlation length (ξ) are overestimated while the roughness exponent (α) is underestimated by SEM, considering the AFM results as reference values.
Line edge roughness (LER) measurement is one of the metrology challenges for 3D device structures, and LER reference metrology is important for reliable LER measurements. We developed an LER measurement technique, which is able to analyze LER distribution along height of a line pattern, with high accuracy, resolution, and reproducibility. Highly accurate atomic force microscopy (AFM) image of a vertical sidewall of a line pattern was obtained using a metrological tilting- AFM, which offers SI-traceable dimensional measurements. The tilting-tip was controlled with an inclined servo axis and scans the vertical sidewall along a line pattern with a high sampling density to enable an analysis of the LER height distribution at the sidewall. A horizontal cross-section of the sidewall shows sidewall roughness with sub-nm resolution. Power spectral density (PSD) analysis of the sidewall profile showed that the PSD noise in the high-frequency region was several orders of magnitude lower than the noise of typical scanning electron microscopy methods. AFM measurements were sequentially repeated three times to evaluate the reproducibility of the sidewall measurement and LER analysis; results indicated a high reproducibility of 0.07 nm evaluated as a standard deviation of LER at each height.
Nanoparticle sizing is the most fundamental measurement for producing nanomaterials, evaluation of nanostructure, and the risk assessment of nanomaterials for human health. Dynamic light scattering (DLS) is widely used as a useful and convenient technique for determining nanoparticle size in liquid; however, the precision of this technique has been unclear. Some international comparisons are now in progress to verify the measurement accuracy of nanoparticle sizing, as a typical example of Asia Pacific Metrology Programme Supplementary Comparison. In this study, we evaluated the precision of DLS technique for nanoparticle sizing and estimated the uncertainty of the DLS data for polystyrene latex suspensions. The extrapolations of apparent diffusion coefficients to infinite dilution and to lower angles yielded more precise values than those obtained at one angle and one concentration. The extrapolated particle size measured by DLS was compared to the size determined by differential mobility analyzer (DMA), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Before the comparison, the intensity-averaged size measured by DLS was recalculated to the number-averaged size, and the thickness of water layer attaching on the surface of particles were added into uncertainty of particle sizing by DLS. After the recalculation, the consistent values of mean particle diameter were obtained between those by DLS and by DMA, AFM, and SEM within the estimated uncertainties.
We developed a calibration method using a pattern matching method for the SEM equipped with laser
interferometer units at an X-Y sample stage. By comparing two images captured before and after the stage movement, an
each of moving pixel number to X and Y direction were analyzed using the image processing technique. Then the pixel
length was calibrated using stage position data and the pixel data.
The developed calibration methods were applied to nano-particle measurements. The sample particle sizes were
nominal diameter of 100 nm and 300 nm. Measurement uncertainty evaluation was done and quantitatively reliable
results were obtained.
In the particle diameter calibration using metrological AFM, the distance between center points of neighboring two
particles is referred to as "lateral diameter" when a single-layer close-packed structure of particles is successfully
formed. The distance between an apex of a particle and a substrate is referred to as "vertical diameter." In the previous
studies, lateral diameter was calculated by manually selecting and extracting a line profile from metrological AFM data
and directly applying a method to calculate a pitch of one-dimensional grating. As the manual line profile extraction
depends on who does it, however, there is a possibility that calculated lateral diameter is varied from person to person.
We developed a technology to calibrate diameter of polystyrene latex (PSL) particles by using our metrological AFM. In
this study, the gravity center method is extended to three dimensions to calculate position of and the center of gravity in
each particle. Lateral diameter, which was defined as distance between gravity centers of neighboring two particles, is
calibrated and uncertainty in the lateral diameter calibration is evaluated. Deformation of particles was also estimated by
using Young's modulus of thin film PSL and bulk PSL.
We developed a measurement method for linewidth patterns of photomasks, and started a calibration service of the
photomask linewidth measurement. For the photomask standards, high-quality of chromium film patterns, typical
thickness of about 80 nm, sharp edges (edge angles more than 85 degree) and smooth side walls on a quart glass
substrate were used. Two kinds of microscopes, an atomic force microscope (AFM) and a scanning electron microscope
(SEM), were employed to calibrate the linewidth.
At the first, the surface profile of line structures were inspected using the AFM, so that the distance between the left and
the right side walls at the edge positions were geometrically-determined. Before the each measurement of photomask
patterns by the AFM tips, each of the tip shape was checked using a needle artifact. Then AFM profiles of the photomask
patterns were corrected using the tip shape data. The linewidth was calculated using the corrected profiles under a
definition of the edge positions, a 10 % level from the top film surface. At the next, the linewidth bias between SEM and
AFM were evaluated using the AFM data. Using this method, an uncertainty of the linewidth measurement was
evaluated at 60 nm for a linewidth range of 0.5 μm-10 μm.
KEYWORDS: Atomic force microscopy, Interferometers, Calibration, Atomic force microscope, Temperature metrology, Solids, Error analysis, Metrology, Data processing, Standards development
A tapping mode nanometrological Atomic Force Microscope system with an ultra-high resolution three-axis laser
interferometer was introduced. In the nano-metrological AFM, laser interferometers are used to measure and control the
relative movement between probe tip and sample, and the AFM can reach very high accuracy. By the tapping Mode
AFM, the pith of a grating standard was calibrated. The sources of uncertainty were analyzed and the corresponding
uncertainty components were given. According to the analysis and calculation, the most reliable value of the grating
pitch with nominal value of 240 nm and its combined standard uncertainty are 240.024 ± 0.157 nm.
By present, the calibration of dimensions in nano scale is being paid more and more attentions. One-dimensional
grating standard with pitches in nano-scale is being proposed by the CCL-WGDM 7 to be one of the five key
comparison parameters in the emerging field of nanometrology. In the pitch calibration of grating standard, Gravity
Center Method and Zero-Cross Points Method are proposed. The two methods are analyzed and simulated under
different conditions. Based on the actual measurement data obtained by AFM, the two methods are used and the best
value is determined. The results in the paper are useful to pitch calibration in nano scale.
We have developed a new measurement techniques employing digital probing with AFM (Atomic Force
Microscope) that can examine sidewalls of fine patterns. This new technique employs digital probing operations, such
sample-tilt step-in operation, tilt-step-in operation with a sharpened tilted tip, and multi-angle step-in operation with a
flared tip. First, we examined the validity of digital probing operation using a carbon nanotube (CNT) tip, showing the
measurement repeatability of approximately 1 nm (3σ) on a fine trench pattern with 50 nm width and 300 nm depth.
After the slip calculation between the tilted-tip and the sidewall for the new sidewall measurement technique, we
measured a perpendicular reference sidewall with two types of operations, namely, tilt-step-in and multi-angle step-in
operations. We then obtained 3D images of ArF resist patterns that involved measurement of sidewall surface
roughness. Finally, we demonstrated a possibility of extending the technique for measuring denser trench patterns by
using sample-tilt method and a tilted CNT tip.
A silicon grating pattern with a 100-nm pitch size for calibration of electron-beam (EB) metrology systems was formed by EB cell projection writing using a grating stencil mask and dry etching. According to the evaluation results from a critical-dimension scanning electron microscope (CD-SEM), the uniformity of the pitch size in a 1.8 x 1.8-mm chip was smaller than 3 nm within 3 sigma in the x and y directions. The obtained 100-nm pitch size was calibrated by DUV laser diffraction. The difference between designed 100-nm pitch size and the calibrated pitch size by DUV laser diffraction was smaller than 0.1 nm. It is thus concluded that more precise calibration of the CD-SEM using this 100-nm pitch grating is expected compared with conventional calibration using 240-nm pitch reference grating fabricated by laser-interferometer lithography and anisotropic chemical etching.
KEYWORDS: Atomic force microscopy, Interferometers, 3D scanning, Scanners, Digital signal processing, Servomechanisms, Metrology, Mirrors, Signal processing, Precision measurement
A critical-dimension atomic force microscope system equipped with an ultra-high resolution, three-axis laser interferometer was constructed and tested. The MIRAI (Millennium Research for Advanced Information Technology) project has been improving the precision of critical dimension measurements with atomic force microscopy (AFM) by implementing modularized laser interferometers, to meet requirements for dimensional measurement in 45 nm technology node. The stability of the cross-sectional profile of an AFM image for a rectangular cross-section was greatly improved by optimizing interferometer linearity and resolution with DSP signal processing and reducing the angular motion and mechanical vibration of the monolithic three-dimensional probe scanner with a unique parallel spring mechanism. The repeatability of linewidth measurement of a nominal 100 nm linewidth along the same scanned line showed a standard deviation of 0.5-1.0 nm (3-sigma). This shows AFM to be one of the most promising metrological tools for next-generation nanodevice fabrication processes. Instrumentation, measurement results, and precision will be discussed.
A design of a high-resolution homodyne interferometer is presented, modularized, and installed in a prototype, critical-dimension atomic force microscope (CD-AFM). A newly designed symmetrical layout of the optical path of the homodyne interferometers enabled highly stable measurements of the mechanical displacements of a wafer-positioning stage and an AFM scanner. In the performance measurement of the wafer-positioning stage, the mechanical drift after long-stroke travel and unlocking of the servo control was reduced to less than ten nanometers per minute by optimizing the preceding motion before stopping. An AFM scanner with a three-dimensional (3D) parallel spring structure has been implemented for the interferometer modules. Using a DSP-based electronic interpolation technique, displacement of the scanner was resolved and calibrated at better than 50 pm and 200 pm, respectively.
A basic scheme of direct, highly accurate dimensional measurements of nanostructures is presented. We have constructed a scanning tunneling microscope (STM) unit combined with a diode laser-based Michelson interferometer module. The compact size of the STM allows it to be installed in an ultra high vacuum (UHV) chamber and is capable of measuring atomic spacings on a reconstructed single crystal surface. This method aims at direct dimensional calibration of microelectronic structures such as linewidths and line/space features. The calibrated dimensions of these features will be traceable to the international unit of length through the He-Ne laser wavelength and be a reliable standard for next generation nanostructures and nanofabrication.
Critical dimension metrology of silicon integrated circuit features at the sub-micrometer scale is an essential task in state-of-the-art semiconductor manufacturing. Determining the width of a feature or the scale in a pitch measurement with appropriate accuracy is consistently one of the most challenging elements of semiconductor metrology and manufacturing.
We have designed and constructed a cryogenic photoelectron yield spectroscopy (PYS) and demonstrated a high energy resolution of 10 meV and a wide dynamic range of 7 orders of magnitude. This system enabled us to measure the absolute value of work function and analyze the electronic structure around the Fermi level (EF) for a Bi2Sr2CaCu2Ox(BSCCO) single crystal and YBa2Cu3O7-(delta ) (YBCO) epitaxial films. When the photoelectron was collected along <001> axis, semiconductive spectrum having a broad tail was observed for both materials. When the measurement was performed from the edge of CuO2 plane, we observed metallic spectra with sharp threshold, of which slope depended on temperature in a way consistent with the Fermi-Dirac distribution function. For BSCCO, a metallic spectrum superimposed a dip structure at 10 K in the vicinity of EF. The data could be well fitted with a curve numerically simulated from the BCS function with a superconducting gap value ((Delta) ) of 20 meV.
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