Critical Dimension Measurement Research Articles

Application of a machine learning method to model-based library approach for critical dimension measurements using CD-SEM

A model-based library (MBL) method has already been established for the accurate measurement of the critical dimension (CD) of semiconductor linewidth using critical dimension scanning electron microscope (CD-SEM) images. In this work, the MBL method is further investigated by combining CD-SEM image simulation and a neural network algorithm. The secondary electron linescan profiles were first calculated using a Monte Carlo simulation method, enabling us to obtain the dependence of linescan profiles on the selected values of various geometrical parameters (e.g. top CD, sidewall angle and height) for Si and Au trapezoidal line structures. Machine learning methods have then been applied to predict the linescan profiles from a randomly selected training set of the calculated profiles. The predicted results agree very well with the calculated profiles with the standard deviations of 0.1% and 6% for the relative error distributions of Si and Au line structures, respectively. The findings show that the machine learning methods can be practically applied to the MBL method for reducing the library size, accelerating the construction of the MBL database and enriching the content of the available MBL database.

Pattern deformation mitigation for EUV photoresists using wafer backside cleaning techniques

This study delves into the effects of wafer backside particles on pattern defocus in EUV lithography (EUVL). Additionally, the potential mitigation of this pattern defocus was examined through wafer backside cleaning. Comparative analyses were conducted between EUVL patterning tests performed using wafers with contaminated and cleaned wafer backside. Defocus points and abnormal critical dimensions (CDs) on the photoresist patterns at the wafer frontside were detected using scanning electron microscopy and optical CD measurement, respectively. The specific locations of these defocus or abnormal CD points were compared and confirmed to be correlated with particles that were present on the same locations on the backside of the wafer. Furthermore, it was found that an effective backside cleaning process can mitigate pattern defocus caused by these wafer backside particles.

Small random library method combined with local searching in optical critical dimension measurements.

The critical dimensions (CDs) of gratings significantly influence their optical performances and require high-resolution measurements. To avoid damaging the gratings, a model-based optical critical dimension (OCD) measurement method utilizing ellipsometry or scatterometry was applied by matching the simulated and experimental values. However, online CD measurements during grating fabrication require a bulky presimulated library containing the condition points with various CDs, making it time consuming and resource intensive to build with large dimension ranges to account for grating fabrication errors. In this study, we proposed a smaller random library with an unevenly distributed resolution, offering finer resolution when the grating to be measured is close to the reference grating. This approach, validated using a home-constructed spectroscopic ellipsometer, resulted in better results. Finally, a local search algorithm based on a random library was applied to further improve the measurement accuracy. This approach extraordinarily reduced the preparation time for OCD measurements and achieved better performance, significantly improving the efficiency of grating development and fabrication inspection.

Study of interferometric signal correction methods in ultra-precision displacement measurement

The measurement of critical dimensions in the field of integrated circuits has moved from 7 nm to 5 nm. The existing chromium atomic lithography grating has a pitch period of 4700 l mm−1 and uniformity of picometer, and the interferometric signal period based on the above grating is as small as 106.4 nm, which brings new problems and challenges to the accurate processing of the signal. This paper investigates the error characteristics of ultra-high precision grating interferometric signals, establishes a Heydemann correction mathematical model for high inscribed line density grating interferometric signals, corrects the grating interferometer signals based on the random sample consensus (RANSAC), and verifies the effectiveness of the algorithm through simulation. By comparing the repeatability and linearity of the original algorithm and the self-traceable grating interferometric displacement measurement data processed by RANSAC, the conclusion that the standard deviation of the self-traceable grating interferometer repeat measurement after RANSAC is 1.60 nm in a 10 000 nm travel is obtained, and the purpose of improving the stability and uniformity of the signal solution with the algorithm of this paper is achieved, which is important for the study of laser interferometer and grating interferometer The results show that the stability and uniformity of the signal solution can be improved by the algorithm of this paper, which is of great significance for the study of the displacement solution of laser and grating interferometers.

Uncertainty evaluation of Monte Carlo simulated line scan profiles of a critical dimension scanning electron microscope (CD-SEM)

In recent years, precision and accuracy for a more precise critical dimension (CD) control have been required in CD measurement technology. CD distortion between the measurement by a critical dimension scanning electron microscope (CD-SEM) and a reference tool is the most important factor for a more accurate CD measurement. CD bias varies by a CD-SEM and a pattern condition. Therefore, it is urgently needed to identify, characterize, and quantify those parameters that may or may not affect the CD measurement by a CD-SEM. The sensitivity of the Monte Carlo simulated CD-SEM images with multiple physical modeling components has been studied previously. In this study, we demonstrate that the work function and elastic scattering potential models have a significant impact on secondary electron emission intensity, but their influence on the shape of the linescan profile is small, and other factors like the optical energy loss function and dielectric function models have even smaller effects. We have evaluated the uncertainty in the linescan profiles of Si line structures with different sidewall angles due to several different physical factors. It is found that when the CD is evaluated by a peak/valley method, the uncertainty of the CD is negligible. Therefore, it is concluded that the CD value and its related uncertainty are not critically related to the physical factors of the present Monte Carlo simulation model but rely dominantly on the line structure and electron beam parameters.

The intensity enhancement of transmission small angle x-ray scattering from nanostructures with a high aspect ratio

Transmission small angle x-ray scattering (tSAXS) has been developed as a metrology for the critical dimension (CD) measurements to facilitate integrated circuit (IC) chip fabrications. Synchrotron x-ray sources were used for their high brilliance and a wavelength less than one tenth of a nanometer was chosen for its high penetration power to enable transmission measurements through a silicon wafer with a nominal thickness of 0.7 mm. A major hold back preventing tSAXS from reaching wide applications in IC fabrication is the lack of high brilliance laboratory-based x-ray sources. Within the last few years, even without any major breakthrough in x-ray source technology, this tSAXS metrology has finally been used for 3D NAND and DRAM, i.e., memory chips with tall or high aspect ratio (HAR) architectures. The scattering intensities from HAR structures will be discussed quantitatively in terms of the sample height and the effective longitudinal coherence length of the incident x-ray.

Review of the key milestones in the development of critical dimension small angle x-ray scattering at National Institute of Standards and Technology

An x-ray scattering based metrology was conceived over 20 years ago as part of a collaboration between National Institute of Standards and Technology (NIST) and International Business Machines Corporation (IBM) to evaluate the performance of lithographic patterning materials for the semiconductor industry. This methodology treated a periodic array of lithographic structures as a diffraction grating and focused on extracting the physical dimensions of the structures in the grating by analyzing the diffraction patterns. In the early stages of the work the focus was on developing the transmission small-angle x-ray scattering (tSAXS) as a metrology tool to measure the critical dimensions (CD) of the lithographic features vital to the integrated circuit chip fabrication. Later, the focus shifted to include grazing incident small-angle x-ray scattering and x-ray reflectivity as parts of the CD metrology tool due to their unique capabilities. Frequently the term critical dimension small-angle x-ray scattering (CDSAXS) has been used as a synonym for the metrology of using tSAXS for CD measurements without mentioning transmission. Various milestones in the CDSAXS development are reviewed in this article together with some prospects regarding the future growth of x-ray-based metrology for complex three-dimensional nanostructures important to semiconductor industries.

Optical Metrology of Critical Dimensions in Large-Area Nanostructure Arrays With Complex Patterns

AbstractIt was recently demonstrated that scatterometry-based metrology has the capability to perform high-throughput metrology on large-area nanopatterned surfaces. However, the way this approach is currently pursued requires an a priori generated library of reflectance spectra to be simulated for an exhaustive set of possible underlying critical dimensions (CDs) characterizing the measured nanopatterns. Generating this library is time consuming and can be infeasible for complex patterns characterized by a large number of CDs. This article addresses the aforementioned drawback of optical inspection of CDs of nanopatterned surfaces through the use of an inverse problem-based optimization methodology coupled with a recently introduced approach for efficient organization of the library of previously simulated reflectance spectra. Specifically, for each physically measured reflectance spectrum, the best matching simulated spectrum is sought in the initial incomplete library in order to serve as the initial guess for the inverse problem optimization process. Through that optimization process, further refinements of the best matching simulated spectra are conducted to obtain sufficiently accurate estimates of the CDs characterizing the inspected nanopattern geometries. Capabilities of the newly proposed approach are evaluated through inspection of semiconductor wafer samples with hourglass patterns characterized by eight CDs. It was observed that one can obtain significantly faster measurements of CDs compared to inspection times associated with scanning electron microscopy, while at the same time not deteriorating the corresponding Gage Repeatability and Reproducibility. In conclusion, this method enables real-time, accurate, and repeatable metrology of CDs of large-area nanostructured surfaces with complex nanopatterns.

A Laplace sensitivity operator enhances the calculation efficiency of OCD metrology.

In integrated circuit manufacturing, optical critical dimension measurement is an efficient and non-destructive metrology method. It is also a model-based metrology in which a numerical model of the target device is formed to simulate the optical spectrum. The result is then reconstructed by fitting the simulated spectrum to the experimentally measured optical spectrum. Normally, the measured optical spectrum contains a great deal of data points that consume the storage space, and increase the fitting time. Therefore, it is worth finding an appropriate approach to downsample these data points without losing much accuracy. To quickly and accurately extract critical data with high sensitivity, we propose a Laplace sensitivity operator that is widely used for feature extraction. Compared with traditional sensitivity calculation, the Laplace sensitivity operator focuses more on the correlation and coupling between multiple parameters. Thus, the sensitivity can be properly analyzed from different dimensions. To test the feasibility and correctness of the proposed method, three basic structures were used for single-parameter verification: thin film, one-dimensional grating, and two-dimensional grating, and a vertical gate-all-around device used for multi-parameter analysis. Using the Laplace sensitivity operator, the extracted data showed better results in most cases than those achieved by the traditional sensitivity calculation method. The data volume was compressed by approximately 70%, the result matching loss was not significantly increase in terms of the root mean square error, and the calculation speed was increased by a factor of 2.4. Compared to the traditional sensitivity operator, the Laplace sensitivity operator was able to reduce the RMSE by up to 50%.

MEMS High Aspect Ratio Trench Three-Dimensional Measurement Using Through-Focus Scanning Optical Microscopy and Deep Learning Method

High-aspect-ratio structures have become increasingly important in MEMS devices. In situ, real-time critical dimension and depth measurement for high-aspect-ratio structures is critical for optimizing the deep etching process. Through-focus scanning optical microscopy (TSOM) is a high-throughput and inexpensive optical measurement method for critical dimension and depth measurement. Thus far, TSOM has only been used to measure targets with dimension of 1 μm or less, which is far from sufficient for MEMS. Deep learning is a powerful tool that improves the TSOM performance by taking advantage of additional intensity information. In this work, we propose a convolutional neural network model-based TSOM method for measuring individual high-aspect-ratio trenches on silicon with width up to 30 μm and depth up to 440 μm. Experimental demonstrations are conducted and the results show that the proposed method is suitable for measuring the width and depth of high-aspect-ratio trenches with a standard deviation and error of approximately a hundred nanometers or less. The proposed method can be applied to the semiconductor field.

Integrated interferometry for in-process monitoring of critical dimension in vertical NAND flash memory dry etch

Integrated interferometry installed on top of dry etch equipment was developed for the in-process measurement of critical dimension (CD) in vertical NAND flash memory dry etch forming word-line steps, also known as word-line processes. The etching process sequentially consists of mask trimming and mold etching (multilayered structure of Si3N4 and SiO2). In-process real-time CD measurements can be made in a trimming step that etches the photoresist (PR) to form a vertical mask shape. The measurement principle is based on the analysis of the interference fringe signal of light reflection to a single point in the center of the wafer. To check the measurement accuracy of the interferometer, the CD measured with interferometry was compared with an in-line SEM (scanning electron microscope). The correlation coefficient between the interferometric measurements measured during the mask trimming step and the in-line SEM measured after the completed process was 86%. On the other hand, when the mask trimming and the mold etching steps including Si3N4 over etching were added, the total measurement results showed that the correlation coefficient was 95%. This is because the CD value was derived based on the phenomenon that the PR thickness change measured by interferometry in the trimming step was a linear relationship with the change in the lateral PR amount, so the lateral PR amount was weakly etched in the mold etching step after the trimming step, which eventually affected the final formed CD. This metrology method can be simply applied to dry etching equipment in real production lines.

True 3D Nanometrology: 3D-Probing with a Cantilever-Based Sensor.

State of the art three-dimensional atomic force microscopes (3D-AFM) cannot measure three spatial dimensions separately from each other. A 3D-AFM-head with true 3D-probing capabilities is presented in this paper. It detects the so-called 3D-Nanoprobes CD-tip displacement with a differential interferometer and an optical lever. The 3D-Nanoprobe was specifically developed for tactile 3D-probing and is applied for critical dimension (CD) measurements. A calibrated 3D-Nanoprobe shows a selectivity ratio of 50:1 on average for each of the spatial directions x, y, and z. Typical stiffness values are N/m, N/m, and N/m resulting in a quasi-isotropic ratio of the stiffness of 1.1:0.9:1.0 in x:y:z, respectively. The probing repeatability of the developed true 3D-AFM shows a standard deviation of 0.18 nm, 0.31 nm, and 0.83 nm for x, y, and z, respectively. Two CD-line samples type IVPS100-PTB, which were perpendicularly mounted to each other, were used to test the performance of the developed true 3D-AFM: repeatability, long-term stability, pitch, and line edge roughness and linewidth roughness (LER/LWR), showing promising results.

Calibration of high magnification in the measurement of critical dimension by AFM and SEM

In the measurement of critical dimension (CD) by scanning electron microscopy (SEM) and atomic force microscopy (AFM), the measured linewidths of the CD patterns are different from the real values due to the edge effect of secondary electrons and the radius and half cone angle of the tips for AFM. In order to solve this problem, a certified reference material (CRM) with a series of CD lines having different linewidth from 20 nm to 60 nm was designed to calibrate the magnification. The linewidths of the CRM could be traceably certified by high resolution scanning transmission electron microscope from the lattice constant of Si(1 1 1) plane in the Si(1 0 0) substrate. As a result, the magnification of AFM and SEM can be calibrated from the certified linewidths of the CRM. The magnification correction factors of AFM and SEM showed the relative standard uncertainties of 1.3% and 0.9%, respectively. The linewidths of the CD lines in the target samples can be traceably determined from the calibrated magnification after calibrating the instruments using the CRM and a calibration program.

Plasmon-assisted optical critical dimension measurement for three-layer orthogonal metallic gratings

In this work, a 3-layer cross copper grating structure, which is the basic structure in semiconductor processing technology, is simulated by the rigorous coupled wave analysis (RCWA) and the finite element method (FEM). The reflection spectrum shows a reflection dip in the infrared frequencies. The electric field distributions indicate that the dip is caused by the 1st order bonding localized surface plasmon mode excited in the 2nd Cu gratings. Such mode is excited by the part of incident light which evanescently passes through the 1st Cu gratings. Another anti-bonding mode can be observed in the visible light frequencies. As the plasmons excited in the 2nd Cu gratings are screened by the 1st and 3rd Cu gratings, they are very sensitive to the change in the dielectric environment between the 1st and 3rd Cu gratings. Therefore, the plasmonic resonance peaks in the reflection spectrum can be useful for the Optical Critical Dimension (OCD) metrology because the depth and the frequency of the resonance peak are highly sensitive to the structural parameters.

34.4: Applications of Electron Beam Review System for AMOLED Patterning Process Control

As the development of display technologies, much smaller critical dimension design is an inevitable trend and much tighter statistical process control is needed and necessary. Electron Beam Review System can provide much more precise critical dimension measurement without glass to be broken compared with traditional optical metrology tools. Besides, taper CD or taper angle is a critical parameter for display product quality control and Electron Beam Review System is the only tool in display Fabs that can provide such information.

Dynamic analysis of tapping mode atomic force microscope (AFM) for critical dimension measurement

Transient dynamics of tapping mode atomic force microscope (AFM) for critical dimension measurement are analyzed. A simplified nonlinear model of AFM is presented to describe the forced vibration of the micro cantilever-tip system with consideration of both contact and non-contact transient behavior for critical dimension measurement. The governing motion equations of the AFM cantilever system are derived from the developed model. Based on the established dynamic model, motion state of the AFM cantilever system is calculated utilizing the method of averaging with the form of slow flow equations. Further analytical solutions are obtained to reveal the effects of critical parameters on the system dynamic performance. In addition, features of dynamic response of tapping mode AFM in critical dimension measurement are studied, where the effects of equivalent contact stiffness, quality factor and resonance frequency of cantilever on the system dynamic behavior are investigated. Contact behavior between the tip and sample is also analyzed and the frequency drift in contact phase is further explored. Influence of the interaction between the tip and sample on the subsequent non-contact phase is studied with regard to different parameters. The dependence of the minimum amplitude of tip displacement and maximum phase difference on the equivalent contact stiffness, quality factor and resonance frequency are investigated. This study brings further insights into the dynamic characteristics of tapping mode AFM for critical dimension measurement, and thus provides guidelines for the high fidelity tapping mode AFM scanning.

Three-dimensional structure recognition of circuit patterns on semiconductor devices using multiple SEM images detected in different electron scattering angles

We propose a method for recognizing the three-dimensional structure of circuit patterns to achieve automatic pattern width measurement of semiconductor devices using a scanning electron microscope (SEM). Pattern measurement requires a pattern recognition technique to identify the measurement position from an SEM image. However, the pattern width and brightness values of an image fluctuate depending on the patterning conditions, type of material, etc., so distinguishing between line and space through image matching based on brightness information is difficult. In view of this problem, we propose a line and space discrimination method that focuses on fundamentally different three-dimensional structures (concavity and convexity), and considering that this difference between concavity and convexity appears as a difference in the scattering angles of secondary electrons, we capture two images corresponding to these two scattering angles. In this way, we are able to design image features that present the difference between line and space from these two images, thus enabling the easy, high-speed capture of concavity and convexity information. Experimental results show that the proposed method achieves a 100% accuracy rate in automatic critical-dimension measurements.

Active Contour Method Based Sub-pixel Critical Dimension Measurement of Thin Film Transistor Liquid Crystal Display (TFT-LCD) Patterns

A novel application-oriented sub-pixel measurement method utilizing an active contour model with image resampling is presented for a critical dimension (CD) measurement of pixel patterns of a flat-panel thin film transistor crystal display (TFT-LCD) and organic light-emitting diode display. In modern manufacturing of flat panel display, optical measurement of each dimension of pixel patterns has a critical role in the process control, and the resolution of a measurement system becomes a limitation of the manufacturing process. Several methods have been developed to overcome this limit at the image post processing stage, but as the level of pixel integration and manufacturing throughput are increasing rapidly, more robust and effective inspection approach is required. In this paper, a novel sub-pixel level edge detection algorithm with active contour method and fast pixel resampling is proposed for micron scale CD measurement, and its advantages on measurement repeatability and noise handling are presented, along with the results of various industrial sample measurements and comparison with conventional critical dimension measurement algorithms.

Automated Optical Inspection (AOI) for FOPLP with Simultaneous Die Placement Metrology

Abstract As the final step of IC fabrication, packaging is the process to encapsulate the chip and provide the interconnections for the I/O of the final form factor. The demand for increasingly hig...

Tilted beam scanning electron microscopy, 3-D metrology for microelectronics industry

In the microelectronics industry, most of the dimensional metrology relies on critical dimension (CD) estimation. These measurements are mainly performed by critical dimension scanning electron microscopy, because it is a very fast, mainly nondestructive method and enables direct measurements on wafers. To measure CDs, the distance is estimated between the edges of the observed pattern on an SEM image. As the CD becomes smaller and smaller, the needs for more reliable metrology techniques emerge. In order to obtain more meaningful and reproducible CD measurements regardless of the pattern type (line, space, contact, hole, etc.), one needs to perform a CD measurement at a known and constant height due to a methodology that determines the topographic shape of the pattern from SEM images. An SEM capable of bending the electron beam (up to 12 deg in our case) allows images to be caught at different angles, giving access to more information. From the analysis of such images, pattern height and sidewall angles can be determined using geometric considerations. Understanding interaction between three-dimensional (3-D) shapes, pattern materials, and the electron beam becomes essential to correlate topography information. A preliminary work based on Monte–Carlo simulations was conducted using JMONSEL, a software developed by the National Institute of Standards and Technology. With this analysis, it is possible to determine theoretical trends for different topographies and beam tilt conditions. Due to the effects highlighted by simulations, the processing of the tilted beam SEM images will be presented, as well as the method used to create a mathematical model allowing topographic reconstruction from these images. Finally some reconstruction using this model will be shown and compared to reference measurements. The overall flow used to process images is presented. First, images are transformed into grayscale profiles. After a smoothing procedure, positional descriptors are computed for specific profile derivatives values. Then, from these descriptors coming from two images of the same pattern taken at different tilt angles, we use a low-complexity linear model in order to obtain the geometrical parameters of the structure. This model is created and initially calibrated using JMONSEL simulations and then recalibrated on real silicon patterns. We demonstrate that the use of real SEM images coming from real silicon patterns with our model leads to results that are coherent with conventional 3-D measurements techniques taken as reference. Moreover, we are able to make reliable reconstructions on patterns of various heights with a single calibrated model. Our batch of experiment shows a three-sigma standard deviation of 10 nm on the estimated height for heights ranging from 50 nm to more than 200 nm. Based on simulations, we are able to reconstruct the corner rounding (CR) from SEM images. However, because our wafer has no CR variability, measurements still need to be assessed on real wafer.