As the design rules for semiconductor devices continue to shrink, layer-to-layer overlay of actual circuit patterns needs to be measured by using a critical dimension scanning electron microscope (CD-SEM). In addition to the overlay, process variations are making not only local size variations but also local position errors in patterns on the same layer increasingly non-negligible compared to pattern size. In cases where upper and lower patterns partially overlap, these variations and errors cause variations to occur in the positions and visible area of the lower layer patterns in SEM images. As a result, the center position of each lower layer pattern has become difficult to accurately measure resulting in errors in the overlay measurement. In response to this problem, we have developed a method for dynamically placing measurement area cursors for overlay metrology using segmentation technique. This method determines a measurement area cursor by recognizing the position and size of each lower layer pattern through segmentation images generated from the target SEM images using unsupervised deep learning. The performance of this method is evaluated by creating imitated images having the programmed overlay, the local size variations, and the local position errors. We compared the measurement results for the imitated images obtained by the proposed method and a conventional method that uses fixed measurement area cursors and found that the proposed method maintains sensitivity and repeatability even for targets where sensitivity and repeatability degrade with the conventional method due to process variations.
Wafer-to-wafer hybrid bonding is a key technology for achieving high-density three-dimensional interconnections in semiconductor devices. This technology directly bonds Cu pads formed on the surface of two wafers, where the surface height of the Cu pad compared to the SiCN surrounding the Cu pad have to be within a few nm. We have developed a method to measure the Cu pad surface height with sub-nm precision by using a top-view scanning electron microscope image. The proposed method is based on the physical principle that the difference in the backscattered electron (BSE) signals of the opposing detectors is dependent on the slope. It estimates the slope of the target with the BSE signal and then calculates the height of the target on the basis of this slope. We compared the Cu pad height measurement results by this method with those by atomic force microscopy and found that ours provided measurement precision on the sub-nm order and demonstrated the capability for evaluation of layout dependency and intra-wafer distribution. Because of its speed and alignment capability, our proposed method is promising for Cu height control in wafer-to-wafer hybrid bonding.
KEYWORDS: Semiconducting wafers, 3D image processing, Scanning electron microscopy, Education and training, Signal detection, Electron beams, Tolerancing, Image analysis, Target detection, Semiconductors
The shrinkage of circuit patterns for improvement of the semiconductor device performance has reduced the tolerances in production. To fit in the tolerances, technologies for improving the uniformity of three-dimensional (3D) shapes of circuit patterns inter- or intra- wafers has been developed. Then, we developed a method for quantifying variations in 3D shapes by critical-dimension scanning electron microscopy (CD-SEM), which can measure widths of circuit patterns with high sensitivity. Since variations in the SEM-image signal are caused by 3D-shape variations, in the method, multiple feature values representing the signal detect shape variations. To compare the effect of the variation in each feature value on the shape variation, the amount of variation in the feature values was normalized by local variations in a reference image. Evaluation on etched wafers showed that several features exhibited independent variation trends that were larger than the local fluctuation. Cross-sectional verification confirmed that one of the feature values correlated with the width at the middle height that cannot be seen in top-view, and variations of 0.24 nm can be detected. It is expected that adjusting processing conditions based on this variation trend will efficiently improve the uniformity of the 3D shape.
KEYWORDS: 3D metrology, 3D image processing, Scanning electron microscopy, 3D acquisition, Electron microscopes, Semiconducting wafers, Optical lithography, Image processing, Critical dimension metrology, Visualization
A depth measurement technique for extremely deep holes (such as channel holes in 3D flash memory devices)—by using back-scattered-electron (BSE) images obtained by a high voltage critical dimension scanning electron microscope (CDSEM)— was developed. A high voltage CD-SEM can detect BSEs that penetrate solids surrounding deep holes. These BSE images include rich information concerning the bottom structures of deep holes. As the BSEs lose their energies according to the distance they travel in solids, it is deduced that the BSE image intensity at hole bottoms depends on hole depth. In a feasibility study on depth measurement using an SEM simulator, it was found that the intensity also depends on hole diameter. The relationship between BSE intensity, hole depth, and hole diameter was modeled by simplifying a backscattering model and approximating the target medium by volume density. Based on this model, a depth measurement technique using only a top-view BSE image is proposed. Measurement error of the technique for channel holes of a 3D flash memory device with depths of a few microns was evaluated by using a high voltage CD-SEM. According to the results of the evaluation, error range was 62 nm and measurement repeatability was ± 18 nm. It is concluded that these values are sufficient for detecting depth defects. This technique achieves fast and non-destructive depth measurement of individual extremely deep holes.
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 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 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%.
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.
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.
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: 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.
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