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.
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.
We have been developing a resist loss measurement function which is based on quantified pattern top roughness. In
order to use practically the resist loss detection function, the PTR index must be calibrated to amount of resist loss.
Furthermore, the evaluation of different chemical formulation and different film thicknesses of the resist is also
required. In this study, we explore the calibration technique of resist loss detection. In order to convert measured PTR
index into amount of resist loss, a reference measurement to pattern height is required. Techniques that can measure
local pattern height are limited to off-line techniques such as AFM or cross-sectional SEM with current technology.
These techniques have a very long Turnaround Time (TAT), and also highly skilled engineer is required, it cannot be
used for in-line processing. Then, we examined the reasonable calibration method by short TAT. At first, the calibration
wafer with changed resist film thickness is exposed using an "open frame" condition. It is measured by an optical film
thickness metrology (FTM) tool and CD-SEM, a conversion factor is determined and converted PTR index of measured
target patterns into resist loss amount. The validity of converted resist loss amount by this method has been proven by
comparing to the resist height obtained by AFM and cross-sectional SEM images. The calibration technique using PTR
index of un-patterned resist allowed us to understand the relationship between un-patterned resist thickness and resist
surface roughness. We have demonstrated a simple and easy way to calibrate pattern resist loss using CD-SEM top-down
images.
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.
With semiconductor technology moving to smaller patterns after the 45nm hp node, introduction of high-NA immersion
lithography progresses, and with it, the challenge of decreasing process latitude. The decreasing lithography tool focus
margin is mentioned as one of the key problems of a high-NA immersion lithography process. Tool focus fluctuation
has an impact on resist pattern shape and not only does CD change, pattern height also decreases. As a result of
previous studies [1][2], it is understood that the resist loss influences pattern formation after etch, and it was confirmed
that resist loss is important for CD control. We observe correlation between the resist top roughness and the resist loss,
and evaluate the resist loss measurement function by quantifying the resist top roughness. This principle of resist loss
detection by measuring roughness is that a changing roughness of resist pattern top is detected as a fluctuation in image
brightness on the CD-SEM. A measurement idea was proposed and performance evaluation has already been performed
by using one kind of sample. In this study, we demonstrate the validity of resist loss detection by investigating various
wafer conditions which contain the dependency by looking at two types of resist and different exposure tool
illumination settings. Furthermore, we have confirmed the sensitivity limit of resist loss detection which is
approximately above 10nm. Finally, we have discussed improving the resist loss detection sensitivity and considered the applicability of resist loss detection for the litho process monitor.
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 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 the previous study, we reported on the CD measurement of multi gate field effect transistors (MuGFETs) by using CD-SEM. We focused on the etching residue at the fin-gate intersection, which causes gate length variation and affects the device performance. Therefore we proposed a technique to quantify the amount of etching residues from CD-SEM top-down images. The increment of the gate linewidth at the fin sidewall was introduced as the "residue index". In this
study, to validate the residue index measurement technique, experiments were carried out. First, the actual shape of the
etching residue was verified in detail by high-resolution experimental-SEM and STEM cross-sectional imaging techniques. Next, the measurement capability of CD-SEM image was confirmed by comparing with the high-resolution experimental-SEM measurement results. Finally, the proposed technique was applied to the layout dependency
evaluation of the residue index, and it was confirmed that the residue index has enough sensitivity to quantify the systematic residue size variation related to fin A/R. Then, we confirmed the reliability of the proposed technique. The residue index measurement technique is expected to be useful for the evaluation of the gate etching process of the MuGFET.
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.
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: Etching, Semiconducting wafers, Transistors, Field effect transistors, Scanning electron microscopy, Scatterometry, 3D metrology, Edge roughness, Metrology, Process control
Multiple Gate Field Effect Transistors (MuGFETs) have been proposed to enable downsizing, when scaling
the transistors to the 32nm technology node. The dimension of the gate on the surface of fin determines the
effective channel length of the device. So, the characterization of the gate profiles at fin sidewalls becomes
extremely critical. It is especially important to quantify the rounded intersection (etch residual) at the
intersection of the fin and gate.
In this report, we show top down images of a MuGFET taken with critical-dimension scanning electron
microscopy (CD-SEM) and the results that were measured and characterized by measuring various portions of
the pattern which will impact the MuGFET performance i.e. gate length, fin width. We will introduce a
quantified relation between fin length and "its effect on the etch residue at the intersection of fin and gate".
Next we discuss our approaches to analyze the variation of the shape of the gate at the fin sidewall.
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.
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.
The linewidth measurement ability of the Model-Based Library (MBL) matching technique is evaluated by a simulation study, and an improvement in the technique is proposed. In this study, a focused electron beam model is introduced in the MONSEL Monte Carlo simulator to estimate the effects of the electron incident angle distribution on linewidth measurements. By using the focused electron beam model, the images that will be obtained by an actual critical-dimension scanning electron microscope (CD-SEM) were simulated. Measurements were carried out on the images which would be taken with the SEM focus conditions in a range maintained by the auto-focus system. As a result of measurements of simulated images with various sample geometries, it was confirmed that the current MBL matching with a simple Gaussian electron beam model could cause a measurement error of more than 3 nm for the linewidth and 2° for the sidewall angle. Since the incident angle distribution distorts the effective beam shape and image profile at the edge of a pattern, conventional MBL matching with a simple Gaussian beam model cannot give a proper measurement of sample geometry for the image profile formed by the focused electron beam, and this results in measurement errors. To eliminate these measurement errors, another library produced by the focused electron beam model, is employed for the MBL matching. The new library consists of simulated profiles at only the best focus, and it enables the MBL to use a better model and to achieve accurate measurements without increased computational costs. By using the new library, measurement errors are reduced to 0.6nm for the linewidth and to 0.2° for the sidewall angle.
The influence of spatial resolution on linewidth measurements in the critical dimension scanning electron microscope (CD-SEM) was investigated experimentally. Measurement bias variation and measurement repeatabilities of four edge detection algorithms were evaluated with a series of images at varying focus in order to determine the effect of resolution variation. Three of these algorithms, maximum derivative, regression to baseline, and sigmoidal fit, are commonly used on commercial CD-SEMs, and the other is a model-based library (MBL) approach that detects the line edge by comparing CD-SEM line scans to a library of simulated line scans. MBL is able to take into account beam size and other parameters (including sidewall angle of the line structure). These algorithms were applied to images of polycrystalline silicon lines with various sidewall angles taken under different focus conditions. In general, it was observed that repeatability is degraded by defocus, and bias varied with focus and target shape. These results indicate that if two or more tools had different resolutions, measurement results would have different biases depending on target shape. The amounts of errors depend on the algorithms, with MBL the most stable against focus variation. However, it still has some systematic errors and outliers far from best focus. Investigations of electron distributions and the effect of electron incident angle were performed for a better model.
An image processing technique for estimating the incidence angle of an electron beam (beam-tilt angle) of a critical dimension scanning electron microscope (CD-SEM) has been developed. The estimation and correction of the error of the beam-tilt angle are indispensable for high precision measurement of CD and/or three-dimensional profiles of semi-conductor device patterns. In this technique, a pyramidal-shaped crystal sample made by anisotropic etching is used for calibration. From the top-down and tilted views of the sample, x and y directional beam-tilt angles relative to the top-down view are estimated simultaneously, with the geometrical variations of the pyramid ridge lines detected by image processing. Exact positioning of the sample is not required because the inclination and rotation of the sample towards the wafer surface are estimated separately from the beam-tilt angles.
Evaluation of 40 sample images, including 4 directional tilt angles, indicated that deviations of the estimated x and y beam-tilt angles were 0.13 and 0.12 degree respectively (3 sigma). It will also be shown that the technique is robust against characteristic SEM image distortion and low S/N.
This technique has achieved high precision and quantitative estimation for the beam-tilt angles, and will provide a method for high precision measurement of CD and three-dimensional profile for semi-conductor process monitoring and control in the future.
KEYWORDS: 3D image processing, 3D image reconstruction, Scanning electron microscopy, 3D modeling, Error analysis, Reconstruction algorithms, Semiconductors, Edge detection, Analytical research, 3D metrology
This study presents a new and unique method to reconstruct 3D profile from tilt images of SEM for semiconductor device pattern called 'Inverse Stereo Matching'. This method is based on 'the shape from shading' and it’s more stable than the conventional stereo matching method in case of low S/N in sidewall of tilt images, and it is able to reconstruct gradual change of sidewall shape that is difficult for the conventional stereo matching to reconstruct in detail. Additionally, this study presents a new method using 'MPPC Indices' to compensate errors of local shape in reconstruction 3D profile caused by the particular characteristic of secondary electron.
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.
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.
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.
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.
KEYWORDS: Oxides, Electron beams, Resistance, Inspection, Semiconducting wafers, Imaging systems, Scanning electron microscopy, Transmission electron microscopy, Wafer inspection, Analytical research
A new voltage contrast imaging method using single scan of high current electron beam has been developed. This method achieved the automatic inspection system, which detects electrical failures in acceptable amount of time. The sensitivity of the system is evaluated using open failure of via holes. First, the image contrast of poly-Si deposited on defective via holes is measured. Then the cross section of the defects is examined to obtain the correlation between contrast and the thickness of resistive residue at the bottom of the defective via holes. The result shows that this imaging method is capable of detecting 2 nm oxide remaining at the bottom of via.
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