KEYWORDS: Scanning electron microscopy, Model-based design, Monte Carlo methods, Calibration, Cadmium, Mathematical modeling, 3D metrology, Scatterometry, Semiconducting wafers, Metrology
In order to accurately measure narrow-space patterns, we propose an improved secondary-electron extraction efficiency model for the model-based library method. In the conventional model, the same extraction efficiency is applied to all electrons, regardless of from where the electrons are emitted. This is a simplified model that assumes a uniform extraction electric-field strength. In the improved model, the extraction efficiency is calculated as a function of the pattern shape and the emission position of the electrons. The function is based on simulation results for the electric-field strength of critical-dimension scanning electron microscopy (SEM) optics. We verify the effectiveness of the improved extraction model by applying this model to measurements of actual patterns with space widths in the (20 to 30) nm range. The measurement bias of the sidewall angle (SWA) is evaluated through comparison to cross-sectional SEM measurements. We demonstrate that the average SWA bias is improved from 0.8 deg for the conventional model to 0.04 deg for the improved model.
KEYWORDS: Scanning electron microscopy, Calibration, Monte Carlo methods, Model-based design, 3D metrology, Mathematical modeling, Silicon, 3D image processing, Scatterometry, Semiconducting wafers
In order to accurately measure narrow space patterns, we propose an improved secondary-electron extraction efficiency
model for the model-based library (MBL) method. In the conventional model, the same extraction efficiency is applied
to all electrons, regardless of where they are emitted from. This is a simplified model assuming a uniform extraction
electric field strength. In the improved model, the extraction efficiency is calculated as a function of the pattern shape
and the emission position of the electrons. The function is based on simulation results for the electric field strength of
critical-dimension scanning electron microscopy (CD-SEM) optics. We verify the effectiveness of the improved
extraction model by applying it to actual patterns with space widths in the range 20 to 30 nm. The measurement bias of
the sidewall angle (SWA) is evaluated through comparison with cross-sectional SEM measurements. We show that the
average SWA bias is improved from 0.8° for the conventional model to 0.04° for the improved model.
In order to achieve pattern shape measurement with CD-SEM, the Model Based Library (MBL) technique is in the
process of development. In this study, several libraries which consisted by double trapezoid model placed in optimum
layout, were used to measure the various layout patterns. In order to verify the accuracy of the MBL photoresist pattern
shape measurement, CDAFM measurements were carried out as a reference metrology. Both results were compared to
each other, and we confirmed that there is a linear correlation between them. After that, to expand the application field of
the MBL technique, it was applied to end-of-line (EOL) shape measurement to show the capability. Finally, we
confirmed the possibility that the MBL could be applied to more local area shape measurement like hot-spot analysis.
Our purpose is to reduce the critical dimension (CD) bias for very small patterns with line widths of <15 nm. The model-based library (MBL) method, which estimates the dimensions and shape of a target pattern by comparing a measured scanning electron microscopy image waveform with a library of simulated waveforms, was modified in two ways. The first modification was the introduction of line-width variation into the library to overcome problems caused by significant changes in waveform due to changes in both sidewall shape and line width. The second modification was the fixation of MBL tool parameters to overcome problems caused by the reduction in pattern shape information due to merging of right and left white bands. We verified the effectiveness of the modified MBL method by applying it to actual silicon patterns with line widths in the range 10-30 nm. The CD bias measured by MBL method for three heights (20, 50, and 80%) was consistent with the atomic force microscopy results. The CD biases at all heights were <0.5 nm, and the slopes of the CD biases with respect to the CD were <3%.
The purpose of this study is to reduce the critical-dimension (CD) bias (i.e., the difference between actual and measured CD values) for very small line patterns with line widths smaller than 15 nm. The model-based library (MBL) matching technique, which estimates the dimensions and shape of a target pattern by comparing a measured SEM image waveform with a library of simulated waveforms, was modified in two ways to enable it to accurately measure very small patterns. The first modification was the introduction of line-width variation into the library to overcome problems caused by
significant changes in waveform due to changes in both sidewall shape and line width. This modification improved the measurement accuracy. The second modification was the fixation of MBL tool parameters that relate to signal-intensity conversion to overcome problems caused by the reduction in pattern shape information due to merging of right and left white bands. This modification reduced the solution space and improved the measurement stability. We confirmed the effectiveness of the modification by using simulated images. We then verified the effectiveness of the modified MBL matching by applying it to actual SEM images. Silicon line patterns with line widths in the range 10-30 nm were used in this experiment, and the CD bias was evaluated by one-to-one comparison with atomic force microscopy (AFM) measurements. The CD bias measured by MBL matching for three heights (20, 50, and 80%) was consistent with the AFM results. The CD biases at all heights were smaller than 0.5 nm and the slopes of the CD biases with respect to the CD were smaller than 3%.
The model-based library (MBL) matching technique was applied to measurements of photoresist patterns exposed with a
leading-edge ArF immersion lithography tool. This technique estimates the dimensions and shape of a target pattern by
comparing a measured SEM image profile to a library of simulated line scans. In this study, a double trapezoid model
was introduced into MBL library, which was suitable for precise approximation of a photoresist profile. To evaluate
variously-shaped patterns, focus-exposure matrix wafers were exposed under three-illuminations. The geometric
parameters such as bottom critical dimension (CD), top and bottom sidewall angles were estimated by MBL matching.
Lithography simulation results were employed as a reference data in this evaluation. As a result, the trends of the
estimated sidewall angles are consistent with the litho-simulation results. MBL bottom CD and threshold method 50%
CD are also in a very good agreement. MBL detected wide-SWA variation in a focus series which were determined as in
a process window by CD values. The trend of SWA variation, which is potentiality to undergo CD shift at later-etch step,
agreed with litho-simulation results. These results suggest that MBL approach can achieve the efficient measurements for process development and control in advanced lithography.
The measurement accuracy of critical-dimension scanning electron microscopy (CD-SEM) at feature sizes of 10 nm and
below is investigated and methods for improving accuracy and reducing CD bias (the difference between true and
measured CD values) are proposed. Simulations indicate that CD bias varies with feature size (CD) when the electron
scatter range exceeds the CD. As the change in the CD-SEM waveform with decreasing CD is non-uniform, the CD bias
in the results is strongly dependent on the algorithm employed to process the CD-SEM data. Use of the threshold method
with a threshold level equal to 50% (Th = 50%) is shown to be effective for suppressing the dependence of CD bias on
CD. Through comparison of experimental CD-SEM measurements of silicon line patterns (7-40 nm) with atomic force
microscopy (AFM) measurements, it is confirmed that the threshold method (Th = 50%) is a effective as predicted,
affording a largely invariant CD bias. The model-based library (MBL) method, which is theoretically capable of
eliminating CD bias, is demonstrated to reduce the CD bias to near-zero levels. These experiments demonstrate the
feasibility of next-generation CD-SEM for the measurement of feature sizes of the order of 10 nm and smaller.
Measurement uncertainty requirement 0.37 nm has been set for the Critical Dimension (CD) metrology tool in 32 nm
technology generation, according to the ITRS[1]. The continual development in the fundamental performance of Critical
Dimension Scanning Electron Microscope (CD-SEM) is essential, as in the past, and for this generation, a highly precise
tool management technology that monitors and corrects the tool-to-tool CD matching will also be indispensable.
The potential factor that strongly influences tool-to-tool matching is the slight difference in the electron beam
resolution, and its determination by visual confirmation is not possible from the SEM images. Thus, a method for
quantitative evaluation of the resolution variation was investigated and Profile Gradient (PG) method was developed. In
its development, considerations were given to its sensitivity against CD variation and its data sampling efficiency to
achieve a sufficient precision, speed and practicality for a monitoring function that would be applicable to mass
semiconductor production line. The evaluation of image sharpness difference was confirmed using this method.
Furthermore, regarding the CD matching management requirements, this method has high sensitivity against CD
variation and is anticipated as a realistic monitoring method that is more practical than monitoring the actual CD
variation in mass semiconductor production line.
In this study, the principle of the resist loss measurement method proposed in our previous paper[1] was verified. The technique proposes the detection of resist loss variation using the pattern top roughness (PTR) index determined by scanning electron microscope images. By measuring resist loss with atomic force microscope, we confirmed that the PTR showed a good correlation with the resist loss and was capable of detecting variations within an accuracy of 20 nm for the evaluated sample. Furthermore, the effect of PTR monitoring on line width control was evaluated by comparing the error in line width control after eliminating undesirable resist loss patterns to that of conventional line width monitoring. The error of line width control was defined as the deviation range in post-etch line widths from post-litho values. Using PTR monitoring, the error in line width control decreased from 10 nm to less than 3 nm, thus confirming
the effectiveness of this method.
In this research, we improved litho process monitor performance with CD-SEM for hyper-NA lithography. First, by
comparing litho and etch process windows, it was confirmed that litho process monitor performance is insufficient just
by CD measurement because of litho-etch CD bias variation. Then we investigated the impact of the changing resist
profile on litho-etch CD bias variation by cross-sectional observation. As a result, it was determined that resist loss and
footing variation cause litho-etch CD bias variation. Then, we proposed a measurement method to detect the resist loss
variation from top-down SEM image. Proposed resist loss measurement method had good linearity to detect resist loss
variation. At the end, threshold of resist loss index for litho process monitor was determined as to detect litho-etch CD
bias variation. Then we confirmed that with the proposed resist loss measurement method, the litho process monitor
performance was improved by detection of litho-etch CD bias variation in the same throughput as CD measurement.
KEYWORDS: Atomic force microscopy, Scanning electron microscopy, Scanning transmission electron microscopy, Monte Carlo methods, Critical dimension metrology, Metrology, Calibration, Silicon, Cadmium, Model-based design
The model-based library (MBL) matching technique was applied in hardmask linewidth metrology with a criticaldimension
scanning electron microscope (CD-SEM). The MBL matching measures the edge positions and shapes of
samples by comparing simulated images to measured images. To achieve reliable, stable measurements, two important
simulation parameters were determined empirically. One was the beam width, and the other was a material parameter,
the residual energy loss rate. This parameter is especially important for measurement of hardmask patterns, which have
relatively high SEM image contrast. These simulation parameters were estimated so as to fit to actual SEM images, and
then pinned to the estimated values during MBL matching. Hardmask patterns made of Si3N4 were measured by MBL
matching with the estimated parameters. The accuracy of the measurements was evaluated by one-to-one comparison
with atomic force microscope (AFM) results. The pattern profile deduced from only the top-down CD-SEM image with
MBL matching agreed well with the AFM profile and a scanning transmission electron microscope (STEM) crosssectional
image. The average measurement bias between the MBL matching and AFM results was 1.58 nm for the
bottom CD and -0.64 nm for the top CD, with a standard deviation of about 1.3 nm.
The linewidth measurement capability of the model-based library (MBL) matching technique was evaluated
experimentally. This technique estimates the dimensions and shape of a target pattern by comparing a measured SEM
image profile to a library of simulated line scans. The simulation model uses a non-linear least squares method to
estimate pattern geometry parameters. To examine the application of MBL matching in an advanced lithography process,
a focus-exposure matrix wafer was prepared with a leading-edge immersion lithography tool. The evaluation used 36
sites with target structures having various linewidths from 45 to 200 nm. The measurement accuracy was evaluated by
using an atomic force microscope (AFM) as a reference measurement system. The results of a first trial indicated that
two or more solutions could exist in the parameter space in MBL matching. To solve this problem, we obtained a rough
estimation of the scale parameter in SEM imaging, based on experimental results, in order to add a constraint in the
matching process. As a result, the sensitivity to sidewall variation in MBL matching was improved, and the measurement
bias was reduced from 22.1 to 16 nm. These results indicate the possibility of improving the CD measurement capability
by applying this tool parameter appropriately.
KEYWORDS: Line edge roughness, Scanning electron microscopy, Monte Carlo methods, Detection and tracking algorithms, Smoothing, Image processing, Signal to noise ratio, Model-based design, Edge detection, Electron beams
A new image processing algorithm is proposed and applied to model-based library (MBL) matching to achieve precise
and accurate linewidth measurements in critical-dimension scanning electron microscopy (CD-SEM). Image quality is
very important in image-based metrology to obtain reliable measurements. However, CD-SEMs are constrained to use
poor signal-to-noise ratio images to avoid electron-beam-induced damage. The proposed algorithm is a line edge
roughness (LER) compensation averaging algorithm that averages scan lines taking LER into account. The algorithm
preserves the edge-bloom shape, which contains 3-dimensional information on the target pattern, while noise is removed
by averaging. Applying the algorithm to MBL matching is expected to improve the accuracy of measurement, since
MBL matching reduces shape-dependent CD-bias by using the edge-bloom shape. The proposed technique was
evaluated by simulation. Precision, accuracy, and relative accuracy were tested and compared to the conventional
threshold method. Precision using the proposed technique was 0.49 nm (3σ), which was worse than the 0.23 nm obtained
with the conventional method. However, the relative accuracy was 0.5 nm, which was significantly better than the 2.9
nm obtained with the conventional method. As a result, the total measurement error (root mean square of precision and
relative accuracy) was reduced from 2.9 nm to 0.7 nm.
KEYWORDS: Image resolution, Scanning electron microscopy, Monte Carlo methods, Optical simulations, Spatial frequencies, Correlation function, Spatial resolution, Interference (communication), Fourier transforms, Process control
This report presents a technique for quantifying the differences in resolution between tools from the SEM images at sub-nanometer scales. The accuracy of resolution monitoring of SEM images depends on the image noise factor and the sample shape factor. Therefore, a resolution monitoring method that is less dependent on the noise and the sample shape is highly desirable. In this study, the dependence on random noise and changes in sample shape are evaluated for three existing resolution measurement methods: the contrast-to-gradient (CG), fast Fourier transform (FFT) and auto correlation function (ACF) methods. By analyzing simulated and experimental SEM images, it was found that the CG method was the least dependent on noise and the sample, while the other two methods exhibited larger variations between samples. On the basis of these benchmarking results, the CG method appears to exhibit the best performance out of these existing resolution measurement techniques.
KEYWORDS: Semiconducting wafers, Atomic force microscopy, Scanning electron microscopy, Transistors, 3D metrology, Etching, Process control, Image processing, Control systems, Monte Carlo methods
The effectiveness of multiple parameter profile characterization (MPPC) as a three-dimensional measurement technique for etched gates is examined by comparison of shape indices with device performance. The MPPC method derives shape indices from top-down, critical-dimension scanning electron microscopy (CD-SEM) images to characterize the sidewall angle and footing roundness of the gate, which are considered to be the structural features that have a great effect on device performance. The capabilities of the proposed method are evaluated through experiments using processed gate wafers etched under different conditions, comparing the shape indices with the cross-sectional profiles obtained by atomic force microscopy. The relationship between the MPPC indices and threshold voltage is also investigated, confirming that variations in sidewall angle and footing roundness have several times the impact on threshold voltage as line width variation. This study confirms the importance of three-dimensional measurement of gate profiles for process monitoring through the use of a method such as MPPC.
KEYWORDS: Semiconducting wafers, Scanning electron microscopy, Etching, Process control, Algorithm development, Critical dimension metrology, 3D metrology, Monte Carlo methods, Metrology, Control systems
This study presents a method of extracting 3D metrological information for etched gate structures from top-down SEM images for use in critical dimension analysis. The variations in sidewall angle and bottom corner roundness are quantified as feature indices by multiple parameter profile characterization (MPPC), and are used as the main indicators of device performance. A stable algorithm developed based on simulation and experimental results partitions the SEM image signal into the sidewall and footing based on the first derivative of the image signal. The width of the sidwall is used as an index of the sidewall angle, and the width of the footing is used as an index of the footing roundness. The validity of the MPPC method is confirmed through experiments using actual poly-Si gate wafers, and is shown to have a 3σ accuracy of ±0.9° for sidewall angles deviating by mroe than 2°. The sidewall angle index and its distribution map are useful for evaluating the etching process, and are particularly effective for revealing subtle macro variations like asymmetry, while the footing roundness index is useful for screening out bad wafers. As MPPC employs only top-down SEM images, no throughput loss will be incurred in comparison with conventional CD measurements.
As design rules shrink and process windows become smaller, strict process control is becoming increasingly important. The two primary process parameters in the photolithography process, exposure dose and focus, require strict control in order to maintain the photoresist profile. This paper presents the second stage of an approach towards monitoring the semiconductor photolithogprhay process by using critical dimension-scanning electron microscopy. In the former paper, we propsed a method that quantifies the photoresist pattern profile variation caused by dose or focus variation. In this paper, a new method for estimating the variation in exposure dose and focus is presented. Top-down SEM imagse are intrinsically limited in the inability to observe the re-entrant profile. This limitation has been overcome through the use of two tyeps of common patterns: island patterns and window patterns. Island patterns, such as isolated line patterns, have a tapered profile for negative defocus, while window patterns, such as isolated spaces patterns, have an inverse tapered profile for negative defocus. Using both types of patterns allows the focus deviation to be monitored, whether positive or negative defocus. The behavior of the two types of patterns is considered here based on photolithography simulation, and a new algorithm for estimating the exposure dose and focsu variation is proposed.
This paper describes a new approach towards monitoring the semiconductor lithography process using critical dimension scanning electron microscopy (CD-SEM). In the lithography process, there are two important process parameters, exposure dose E and focus F. To monitor both the E and F variation, a new method for characterizing the cross-sectional profile of the photoresist pattern from the secondary electron (SE) waveform has been developed. An innovative feature of this method is that it can quantify the degree of top rounding (TR) and bottom rounding (BR) of the cross-sectional profile separately.
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