Chie Shishido

Application of model-based library approach to Si3N4 hardmask measurements

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.

Characterizing cross-sectional profile variations by using multiple parameters extracted from top-down SEM images

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.

CD bias reduction in CD-SEM linewidth measurements for advanced lithography

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.

CD-bias evaluation and reduction in CD-SEM linewidth measurements

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.

Influence of electron incident angle distribution on CD-SEM linewidth measurements

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.

Dose and focus estimation using top-down SEM images

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.

Pattern alignment method based on consistency among local registration candidates for LSI wafer pattern inspection

This paper reports an image-processing algorithm for robust inspection of LSI wafer patterns using SEM. In order to detect defects in a regular LSI pattern, a pair of long patterns are compared, blocked images are aligned, and defects are judged using the aligned images. The LSI wafer pattern is defined to consist of blank space, fine repetitive patterns, and unique patterns. Distortion of the SEM image is larger than the repetitive pattern pitch, requiring the system to keep track of the alignment in areas without pattern information or in blank space and mitigate the indeterminacy of repetitive patterns. To satisfy these requirements, a two-layer algorithm is proposed. The lower layer calculates registration candidates in each block, and the upper layer determines the correct registration route, i.e. the chain of the correct registration, using candidate information in all the related blocks. Experimental evaluations confirm that most pattern cases can be inspected correctly using the proposed SEM inspection system.

Accurate measurement of very small line patterns in critical dimension scanning electron microscopy using model-based library matching technique

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%. C 2011 Society of

CD bias reduction in CD-SEM of very small line patterns: sidewall shape measurement using model-based library matching method

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%.

MPPC technique for gate etch process monitoring using CD-SEM images and its validity verification

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.