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Force measurement using zero-compliance mechanism is proposed and studied experimentally. A zero-compliance mechanism is composed of two suspensions connected in series; one of them is operated to ...

In the next generation SAOP (Self-aligned Octuplet patterning) process, the line width and LWR (Line Width Roughness) measurement of lines with sub-10 nm is required. We have already proposed a novel method of line width and LWR measurement with sub-nanometer uncertainty by using FIB (Focused Ion Beam) processing, and TEM (Transmission Electron Microscope) images as reference metrology. In the previous report, we applied the method to SAQP features. A specimen of the SAQP FinFET line and space device is coated with carbon and cut horizontally into a thin planar sample by the FIB sampling system. The thin sample is observed by using planar-TEM. The average PSD of LWR of SAQP FinFET sample at the upper and lower positions calculated from planar-TEM images. In this article, we apply the methodology to line width and roughness measurement of advanced features by SAOP process. The features are vertically or horizontally sliced as a thin specimen by FIB micro sampling system. LWR is calculated from the edges positions, and PSD (Power Spectrum Density) is analyzed for the reference metrology.

We have previously demonstrated our method to obtain unbias dense line’s LER (Line Edge Roughness) processed by EUV (Extreme Ultra-Violet) lithography. No special edge-detection, simulation modeling, beam-scan, nor image processing is required, except optimization of beam-dose parameters. For instance, threshold method that is a conventional edge-detection is used on SEM (Scanning Electron Microscopy) image taken by a conventional beam-scanning with a conventionally used beam-conditions. However, especially for EUV-lithographed features we must determine carefully landing voltage and beam-dose that cause artificial reduction of LER and LWR (Line Width Roughness) due to shrink of photo resist materials. Note that we made edge-detection interval at every 5 nm, that is as small as probe-beam diameter. For instance, we set the vertical magnification, the pixel-density, and the sum-line-per-point parameter at 52.7k, 1024, and 2, respectively. Although we can make the interval as small as sub-nanometer, it is not reliable to measure the sub-nanometer object that is only one-tenth of the probe-beam diameter. Then, we verified accuracy of our unbiased LWR by using two independent experimental methods: TEM and FIB-SEM. We obtained spectra of PSD (Power Spectrum Density) from the TEM, the FIB-SEM and our unbiassing CDSEM-method. We found the three PSDs agreed very well to each others. This result strongly implies that three independent methods measured an identical PSD, of true LWR. By our unbiassing CDSEM-method we can measure the true LWR which is calculated from the true PSD. In this work, by using the similar experimental-methods we verify accuracy of our wiggling measurement. Wiggling is a hot issue in production yield of post dry-etch process. Especially about 20 nm or less in the line-width, wiggling starts appearing due to decreased elastic-stiffness of the line feature. A reason why we study the accuracy of LER, LWR, and wiggling is inline monitor of stochastic failures. As they increase significantly in EUV lithography process, production yield may not be promising. Bisshop et al. [1] investigated pitch and/or space-width correlate to incidence of stochastic-failures, such as breaking-line and bridging-line, down to ppm (parts per million). However, the incidence of the stochastic failures should be reduced less to ppb (parts per billion) or ppt (parts per trillion) which is as small as tolerable particle contamination on a wafer. This implies significance of accurate LWR and/or wiggling monitor because in narrow line or space a-few-nanometer-LWR narrows locally them the more and causes the more stochastic failures. In this work, in order to control ppb-stochastic failures we figure out a correlation between LWR and the failures’ incidence. We need to measure billions of features, that requires too long measurement time for practical process-monitor. To shorten the measurement time we test an effective method to estimate such very few number of failures. Reference [1] Stochastic effects in EUV lithography: random, local CD variability, and printing failures, Peter De Bisschop, J. of Micro/Nanolithography, MEMS, and MOEMS, 16(4), 041013 (2017).

LER (Line Edge Roughness) and LWR (Line Width Roughness) of the semiconductor device are an important evaluation scale of the performance of the device. Conventionally, LER and LWR is evaluated from CD-SEM (Critical Dimension Scanning Electron Microscope) images. However, CD-SEM measurement has a problem that high frequency random noise is large, and resolution is not sufficiently high. For random noise of CD-SEM measurement, some techniques are proposed. In these methods, it is necessary to set parameters for model and processing, and it is necessary to verify the correctness of these parameters using reference metrology. We have already proposed a novel reference metrology using FIB (Focused Ion Beam) process and planar-TEM (Transmission Electron Microscope) method. In this study, we applied the proposed method to three new samples such as SAQP (Self-Aligned Quadruple Patterning) FinFET device, EUV (Extreme Ultraviolet Lithography) conventional resist, and EUV new material resist. LWR and PSD (Power Spectral Density) of LWR are calculated from the edge positions on planar-TEM images. We confirmed that LWR and PSD of LWR can be measured with high accuracy and evaluated the difference by the proposed method. Furthermore, from comparisons with PSD of the same sample by CD-SEM, the validity of measurement of PSD and LWR by CD-SEM can be verified.

A novel method of sub-nanometer uncertainty for the 3D-profile measurement and LWR (Line Width Roughness) measurement by using FIB (Focused Ion Beam) processing, and TEM (Transmission Electron Microscope) and CD-SEM (Critical Dimension Scanning Electron Microscope) images measurement is proposed to standardize 3D-profile measurement through reference metrology. In this article, we apply the methodology to line profile measurements and roughness measurement of advanced FinFET (Fin-shaped Field-Effect Transistor) features. The FinFET features are horizontally sliced as a thin specimen by FIB micro sampling system. Horizontally images of the specimens are obtained then by a planar TEM. LWR is calculated from the edges positions on TEM images. Moreover, we already have demonstrated the novel on-wafer 3D-profile metrology as "FIB-to-CDSEM method" with FIB slope cut and CD-SEM measuring. Using the method, a few micrometers wide on a wafer is coated and cut by 45-degree slope using FIB tool. Then, the wafer is transferred to CD-SEM to measure the cross section image by top down CD-SEM measurement. We applied FIB-to-CDSEM method to a CMOS image sensor feature. The 45-degree slope cut surface is observed using AFM. The surface profile of slope cut surface and line profiles are analyzed for improving the accuracy of FIB-to-CDSEM method.

A method of sub-nanometer uncertainty for the 3D-profile measurement using TEM (Transmission Electron Microscope) images is proposed to standardize 3D-profile measurement through reference metrology. The proposed method has been validated for profiles of Si lines, photoresist features and advanced-FinFET (Fin-shaped Field-Effect Transistor) features in our previous investigations. However, efficiency of 3D-profile measurement using TEM is limited by measurement time including processing of the sample. In this article, we demonstrate a novel on-wafer 3D-profile metrology as "FIB-to-CDSEM method" with FIB (Focused Ion Beam) slope cut and CD-SEM (Critical Dimension Secondary Electron Microscope) measuring. Using the method, a few micrometer wide on a wafer is coated and cut by 45 degree slope using FIB tool. Then, the wafer is transferred to CD-SEM to measure the cross section image by top down CD-SEM measurement. We apply FIB-to-CDSEM method to CMOS sensor device. 3D-profile and 3D-profile parameters such as top line width and side wall angles of CMOS sensor device are evaluated. The 3D-profile parameters also are measured by TEM images as reference metrology. We compare the 3D-profile parameters by TEM method and FIB-to-CDSEM method. The average values and correlations on the wafer are agreed well between TEM and FIB-to- CDSEM methods.

For yield improvement of 3D-device manufacturing, metrology for the variability of individual device-features is on hot issue. Transmission Electron Microscope (TEM) can be used for monitoring the individual cross-section. However, efficiency of process monitoring is limited by the speed of measurement including preparation of lamella sample. In this work we demonstrate speedy 3D-profile measurement of individual line-features without the lamella sampling. For instance, we make a-few-micrometer-wide and 45-degree-descending slope in dense line-features by using Focused Ion Beam (FIB) tool capable of 300mm-wafer. On the descending slope, obliquely cut cross-section of the line features appears. Then, we transfer the wafer to Critical-Dimension Secondary Electron Microscope (CDSEM) to measure the oblique cross-section in normal top-down view. As the descending angle is 45 degrees, the oblique cross-section looks like a cross-section normal to the wafer surface. For every single line-features the 3D dimensions are measured. To the reference metrology of the Scanning TEM (STEM), nanometric linearity and precision are confirmed for the height and the width under the hard mask of the line features. Without cleaving wafer the 60 cells on the wafer can be measured in 3 hours, which allows us of near-line process monitor of in-wafer uniformity.

Line edge roughness (LER) and linewidth roughness (LWR) of a semiconductor device are important measures for evaluating its performance. Conventionally, LER and LWR have been evaluated from critical dimension scanning electron microscope (CD-SEM) images. However, the problem with CD-SEM measurement is that the high-frequency image noise is large, and the resolution is not sufficiently high. In order to overcome the problem of image noise in CD-SEM measurement, some techniques have been proposed. In these methods, it is necessary to set the parameters for the model and processing, and it is required to verify the correctness of these parameters using reference metrology. We have already proposed a reference metrology using the focused ion beam process and planar transmission electron microscope (planar-TEM) method. In this study, we apply the proposed method to three new samples, namely self-aligned quadruple patterning fin-shaped field-effect transistor device, extreme ultraviolet lithography (EUV) conventional resist, and EUV new material resist. The LWR and the power spectral density (PSD) of LWR are calculated from the edge positions on planar-TEM images. We confirm that the LWR and the PSD of LWR can be measured with high accuracy and evaluate the difference between PSD by the proposed method and that by CD-SEM images. Furthermore, from comparisons with the PSD of the same sample obtained using CD-SEM, the validity of measurement of PSD and LWR by CD-SEM is verified.

Measurement of line edge roughness (LER) is discussed from four aspects: edge detection, power spectrum densities (PSD) prediction, sampling strategy, and noise mitigation. General guidelines and practical solutions for LER measurement today are introduced. Advanced edge detection algorithms such as the wave-matching method are shown to be effective for robustly detecting edges from low SNR images, whereas a conventional algorithm with weak filtering is still effective in suppressing SEM noise and aliasing. An advanced PSD prediction method such as the multitaper method is effective in suppressing sampling noise within a line edge to analyze, whereas a number of lines are still required for suppressing line-to-line variation. Two types of SEM noise mitigation methods, such as the “apparent noise floor” subtraction method and LER-noise decomposition using regression analysis, are verified to successfully mitigate SEM noise from PSD curves. These results are extended to local critical-dimension uniformity (LCDU) measurement to clarify the impact of SEM noise and sampling noise on LCDU.

The critical-dimension scanning electron microscope (CD-SEM) is an essential tool for semiconductor fabrication process control because of its high resolution and high precision. However, in ArF lithography, the CD of resist changes during CD-SEM measurement due to shrinkage caused by the electron beam irradiation. This shrinkage can be reduced by measurement parameters; however, there is a trade-off relationship between shrinkage and precision. Thus, measuring the CD of an ArF resist pattern precisely with small shrinkage is difficult. The authors propose an optimization method using the Taguchi method. Four measurement parameters were chosen as control factors for an L18 orthogonal array: probe current, acceleration voltage, horizontal length of field-of-view, and number of image acquisitions. As a result, high prediction accuracy was obtained that is smaller than 0.2 nm for shrinkage and 0.1 nm for precision. Moreover, an optimum measurement condition that achieves 0.28 nm shrinkage and 0.37 nm precision was also obtained. Thus, the proposed method was demonstrated as a promising method to optimize CD measurement parameters.

To establish a method of measuring interconnect line-edge roughness (LER), low-k line patterns were observed and electric field concentration was simulated on the basis of observation results. Wedge-shaped LERs were observed at the edges of low-k lines, and the bottom and the top widths of the average wedge feature were 60 and 7 nm (or smaller), respectively. Simulation showed that LER causes serious electric field concentration, which may cause the degradation of time-dependent dielectric breakdown (TDDB) lifetime at 100-nm-pitch Cu/low-k interconnects. The maximum electric field strength depends on the conventional LER metric 3Rq, but depends more strongly on the wedge angle, the curvature of the tip. That is, other metrics such as wedge angle can predict fatal electric field concentration caused by LER than the conventional metric 3Rq.

A metrological definition and a target value for linewidth roughness (LWR) in a gate pattern of MOSFETs are proposed. The effects of sampling interval gate-LWR measurements by critical-dimension scanning electron microscopy on measurement accuracy were examined by both experiment and simulation. It was found that a 10-nm interval is sufficiently small to fully characterize roughness in a typically chosen 2-mum-long line. Random image noise and intrinsic LWR variations are found to have larger effects on the measured LWR value than the finiteness of the sampling interval. A practical procedure for improving the measurement accuracy is also devised. Moreover, a methodology for establishing the gate-LWR target is proposed. Threshold-voltage shift caused by gate-LWR is determined from the LWR spectrum and the I-V curves of a transistor without LWR (i.e., ideal I-V curves).

A novel method of sub-nanometer uncertainty for the line profile measurement using TEM (Transmission Electron Microscope) images is proposed to calibrate CD-SEM (Critical Dimension Scanning Electron Microscope) line width measurement and to standardize line profile measurement through reference metrology. The proposed method has been validated for profile of Si line and photoresist features in our previous investigations. In this article, we apply the methodology to line profile measurements of advanced-FinFET (Fin-shaped Field-Effect Transistor) features. The FinFET features are sliced as thin specimens of 100 nm thickness by FIB (Focused Ion Beam) micro sampling system. Cross-sectional images of the specimens are obtained then by TEM. The profiles of fin, hardmask and dummy gate of FinFET features are evaluated using TEM images. The width of fin, the length of hardmask, and the length of dummy gate of FinFET features are measured and compared to CD-SEM measurement. The TEM results will be used to implement CD-SEM and CD-AFM reference metrology.

The design rule of semiconductor devices in 1984s, when optical microscope was used for critical dimension measurement of device patterns, was around 1.3 µm. Instead of the optical microscope which did not provide enough resolution for the further device shrinkage, CD-SEM (Critical-Dimension Scanning Electron Microscope) was used for 1 µm or smaller design rule. This report introduces recent CD-SEM technologies and future prospect for such smaller design rule. Furthermore, evaluation of CD-measurement accuracy is introduced. It consists of short-term and long-term repeatabilities and tool-to-tool matching. A new linewidth-measurement algorithm is implemented for better short-term repeatability. A method of contrast-gradient is used for tool-to-tool matching. A traceable standard-sample for magnification calibration is also used.

General property of line-edge roughness (LER) in resist pattern and its transferability to under-lying layer were investigated. Longer-period components were found to have larger amplitudes in a resist pattern and to remain after dry etching. In addition, long-period LER strongly affects a transistor performance. Long-period LER in resist patterns, therefore, is as important as short-period LER. Metrology of LER was reconsidered to evaluate the both LER properly, and a guideline for choosing measurement parameters was proposed from a viewpoint of device performance estimation.

We have proposed a new method for correcting electron beam (EB)-induced photoresist shrinkage in two-dimensional pattern contours extracted from a scanning electron microscope image. This method restores the original shrinkage-free contour from the experimentally determined “shrunk contour”, based on a shrinkage model which takes into account of the elastic nature of the shrinkage phenomena caused by the photoresist-volume reduction. Verification of this shrinkage model was demonstrated by using ArF resist patterns as follows. First, the model was calibrated with the shrinkage data of several line patters with different linewidth prior to the contour correction. Next, the amount of shrinkage of elbow patterns was measured by comparing its contours obtained with small and sufficiently large EB dosages. It was found that the shrinkage of the inner edge of the elbow corner was smaller than that of the outer edge, which can be interpreted as a result of the elastic deformation. Finally, validity of shrinkage correction was examined. The model calculation correctly reproduced the observed shrinkage including its dependence on the location in the pattern. The restored contour showed a good consistency with the experimental results and the total root-mean-square error of the shrinkage correction was 0.5 nm. This result confirmed that our shrinkage model adequately describes the shrinkage of two dimensional patterns. Consequently, proposed shrinkage correction method is expected to improve the accuracy of contour measurements by a critical dimension-scanning electron microscope.

The novel method of sub-nanometer uncertainty for the line width measurement and the line profile measurement using STEM (Scanning Transmission Electron Microscope) images is proposed to calibrate CD-SEM line width measurement and the standardization of line profile measurement as reference metrology. In accordance with the proposed method, we already have established the methodology of profile of Si line and photoresist feature for reference metrology. In this article, we applied the proposed method to the sidewall roughness measurement of photoresist features and line profile measurement of finFET features. Using the proposed method, specimens of photoresist feature and finFET feature are sliced as thin specimens of 100 nm thickness by FIB (Focused Ion Beam) micro sampling system. Then the cross-sectional images of the specimens are obtained by STEM and TEM. The sidewall roughness of photoresist features is estimated by the maximum slope of the image intensity graph at the edge. Then, the sidewall roughness is also measured by CD-AFM (Critical Dimension Atomic Force Microscope); we compared the results by STEM image and CD-AFM. Moreover, the line profile of finFET features is defined using TEM images for reference metrology. We compared the line width of fin measured by the proposed method and CD value by CD-SEM measurement.

Metrology of line-edge roughness (LER) or line-width roughness (LWR) reduced less than a few nanometers in recent advanced-process is one of issues because measured LER is strongly dependent on measurement conditions such as magnification and beam dose. It may happen that different organizations measure different LERs on an identical sample. By using an ultra-low LER sample we demonstrate intolerable change of measured LER between with and without necessary key-points in the measurement conditions of critical-dimension secondary electron microscope (CD-SEM).

The mechanism of photoresist shrinkage induced by electron-beam (EB) irradiation was studied. A precise cross-sectional profile of a photoresist pattern was obtained by a scanning transmission electron microscope (STEM) after atomic layer deposition of HfO2 on the sample patterns. Photoresist lines and spaces fabricated with positive-tone development and negative-tone development were exposed to an EB with much higher dose than a practical dose (to accelerate shrinkage intentionally). The obtained STEM images of the patterns before and after EB irradiation show that the shrinkage of the negative-tone-developed patterns is smaller than that of the positive-tone-developed patterns. This observation is explained by the fact that negative-tone-developed photoresist molecules do not contain protection groups, whose volatilization caused by EB irradiation is one of the origins of shrinkage. Another finding is that the EB irradiation causes top-rounding and necking of the pattern profile as well as linewidth slimming. The rounding of the pattern top profile suggests that the pattern’s shape was elastically deformed. In addition, EB irradiation only onto the spaces caused sidewall shrinkage and a necking profile, although no electrons were irradiated directly onto the pattern. These phenomena are considered to be due to the electrons scattered from the spaces to the pattern sidewall. Finally, a Monte Carlo simulation of electron scattering showed that the distribution of the deposited EB energy on the pattern surface corresponds to the above-described change in pattern shape. Consequently, these observations and simulation results clarify the importance of the effect of elastic shape change and the impact of the electrons scattered from the underlying layer onto the sidewall in the mechanism of photoresist shrinkage.

A new method for calculating critical dimension (CDs) at the top and bottom of three-dimensional (3D) pattern profiles from a critical-dimension scanning electron microscope (CD-SEM) image, called as “T-sigma method”, is proposed and evaluated. Without preparing a library of database in advance, T-sigma can estimate a feature of a pattern sidewall. Furthermore, it supplies the optimum edge-definition (i.e., threshold level for determining edge position from a CDSEM signal) to detect the top and bottom of the pattern. This method consists of three steps. First, two components of line-edge roughness (LER); noise-induced bias (i.e., LER bias) and unbiased component (i.e., bias-free LER) are calculated with set threshold level. Second, these components are calculated with various threshold values, and the threshold-dependence of these two components, “T-sigma graph”, is obtained. Finally, the optimum threshold value for the top and the bottom edge detection are given by the analysis of T-sigma graph. T-sigma was applied to CD-SEM images of three kinds of resist-pattern samples. In addition, reference metrology was performed with atomic force microscope (AFM) and scanning transmission electron microscope (STEM). Sensitivity of CD measured by T-sigma to the reference CD was higher than or equal to that measured by the conventional edge definition. Regarding the absolute measurement accuracy, T-sigma showed better results than the conventional definition. Furthermore, T-sigma graphs were calculated from CD-SEM images of two kinds of resist samples and compared with corresponding STEM observation results. Both bias-free LER and LER bias increased as the detected edge point moved from the bottom to the top of the pattern in the case that the pattern had a straight sidewall and a round top. On the other hand, they were almost constant in the case that the pattern had a re-entrant profile. T-sigma will be able to reveal a re-entrant feature. From these results, it is found that T-sigma method can provide rough cross-sectional pattern features and achieve quick, easy and accurate measurements of top and bottom CD.

The novel method of sub-nanometer uncertainty for the line width measurement and line profile measurement using STEM (Scanning Transmission Electron Microscope) images is proposed to calibrate CD-SEM line width measurement and standardization of line profile measurement as reference metrology. In accordance with the proposed method, we already have established the methodology of Si line profile and line width measurement for reference metrology. In this article, we applied the proposed method to edge determination methodology of the photoresist feature. Using the proposed method, a specimen of photoresist feature is coated with metal, and then it is sliced as a thin specimen of 100 nm thickness by FIB (Focused Ion Beam) micro sampling system. Then the dark-field cross-sectional images of the specimen are obtained by STEM. The edge position of the photoresist is defined at the distance of half metal coating thickness from the peak positions of metal coating area. Then the detected line profiles are compared with the measurement results by CD-AFM. From series of analyses, we established the edge detection methodology of photoresist feature for reference metrology.

Shrinkage behavior caused by a single-line scan of an electron beam over a photoresist line was studied, including shrinkage distribution in the photoresist-line direction. As single-line scan is the minimum unit of controllable electron-beam irradiation during scanning-electron-microscope-image (SEM-image) processing, the minimum amount of shrinkage should be observed in the condition. A new method for evaluating the minute amount shrinkage and the shrinkage distribution caused by a single-line scan was developed. According to the results of evaluations with this method, the shrinkage of a 50-nm-wide photoresist line caused by a single-line scan is less than 0.1 nm under landing energies of 200, 300, and 500 eV and probe current of 8 pA. This shrinkage is more than ten times smaller than the typical amount of shrinkage caused by a standard two-dimensional scan. This result indicates the possibility of a significant reduction of photoresist shrinkage during SEM measurements. The evaluations also show that the shrinkage caused by a single-line scan distributes more than about 30 nm in the photoresist-line direction, which is wider than the simulated electron-scattering range. Moreover, the evaluations show that the shrinkage distribution is narrower at higher position of the photoresist-line. This tendency suggests that the wide shrinkage-distribution does not stem from the distribution of the back-scattered electrons (BSEs) which enter the side wall of the photoresist line from the spaces nearby, because the incidents of BSEs distribute wider at higher position of the photoresist-line. Hence, shrinkage occurs in a wider region of the photoresist line than the region where electrons (including directly incident electrons and BSEs) reach. This result suggests that in order to interpret the photoresist-shrinkage mechanism it is important to clarify how the microscopic volume-reduction caused by electron-molecule interactions is integrated into macroscopic photoresist-pattern deformation. An elastic deformation is a plausible mechanism for this macroscopic photoresist-shrinkage process.

The novel method of sub-nanometer accuracy (uncertainty) for the line width measurement and line edge detection using STEM (Scanning Transmission Electron Microscope) images is proposed to calibrate CD-SEM line width measurement and standardization of line edge detection. In accordance with the proposed method, the traceability and reference metrology are established using Si lattice structures and edge detection of Si line with metal coating. First, the interface of SiO2-Air is defined using image intensity of STEM dark field images after metal coating. Second, an image magnification is calculated using 2D Fourier analysis of the images. Third, the edge positions are detected. Using the proposed method, the expanded uncertainty (3 sigma) less than 0.3 nm for the line width of 50 nm is established.

A method for achieving high repeatability in edge detection considering a three-dimensional (3D) feature of a pattern sidewall was developed. An edge-detection condition is regarded as the metric representing the z-coordinate (height) of the edge position. The dependences of line-edge or linewidth roughness (LER/LWR) and LER/LWR bias on the edgedetection condition were calculated. The calculated dependence is interpreted as the qualitative z-dependence of the roughness metrics. The 3D feature of the sidewall can therefore be roughly estimated from the dependence. The condition that minimizes the LER/LWR bias to achieve high repeatability for local measurements was determined. First, to check the validity of this method, the feasibility of 3D information extraction was examined. Unbiased LER and LER bias of resist and silicon pattern samples were plotted against the edge-detection condition, and it was found that the characteristics of the graphs of the unbiased LER and bias are related to the 3D feature observed by atomic force microscopy. Next, the robustness of the method against a change in the pattern shape caused by exposure defocus was demonstrated. Finally, it was shown that the method actually improves local measurement repeatability. In the case of the linewidth of a 9-nm-long line, repeatability was improved by 40% compared with that under the conventional edgedetection condition.

We fabricated sub-50-nm pitch reference grating with positional identification mark for specifying the location. The address mark of silicon groove was fabricated by EB lithography and dry etching processes. The sub-50-nm pitch multilayer substrate was bonded with the address mark silicon substrate and polished as a flat chip. Next the fine pitch grating reference pattern was fabricated by SiO 2 selective chemical etching. Finally the sub-50-nm pitch grating pattern was set on the flat surface for CD-SEM due to retarding bias system for low voltage inspection. As a result of the fundamental characteristics evaluation using CD-SEM, the uniformity of the pitch size in the reference chip was smaller than 1 nm in 3σ. The positional identification marks are useful for obtaining accurate calibrations by specifying the location of the grating and the number of calibrations. Also, the pitch-size was obtained by diffraction angle measurements with a high-accuracy grazing incidence small-angle x-ray scattering (GI-SAXS). The traceability of calibration is under vertification.

The novel method of sub-nanometer accuracy (uncertainty) for the line width and line profile measurement using STEM (Scanning Transmission Electron Microscope) images is proposed to calibrate CD-SEM line width measurement. In accordance with the proposed method, the traceability and reference metrology of line width measurements are established using Si lattice structures. First, we define two interfaces of Si-SiO2 and Si-Air. The interface of Si-SiO2 is defined as the end of Si lattice structure, and the interface of SiO2-Air is defined using image intensity of STEM image after metal coating. Second, an image magnification is calculated using 2D Fourier analysis of a STEM image. Third, the edge positions of the line are detected by Si lattice patterns and image intensity. Using the proposed method, the estimated accuracy less than 0.5 nm for the line width of 50 nm is established.

We have developed high-resolution grazing incidence x-ray diffraction (HRGIXD) pitch calibration system with wavelength of 0.1540593 nm (Cu K D 1 line). In order to ensure accuracy of th is calibration system, we measured average pitch of a 100-nm pitch grating reference, which is being used for magnification calibration of a current critical-dimension scanning electron microscope (CD-SEM), and compared with the results of deep ultra violet (DUV) laser diffraction pitch calibration system with wavelength of 193 nm. The average pitch determined by the HRGIXD system agrees with that determined by the DUV laser diffraction system within th e range of uncertainty. We measured average pitch of a fine 25-nm pitch grating, which will be used for magnification reference. In the DUV laser diffraction, wavelength of 193 nm no longer satisfies diffraction condition for the 25-nm pitch grating, becau se wavelength must be shorter than twice of the pitch size. On the other hand, wavele ngth of x-ray is much shorter than the pitch size. We have successfully detected more than ten shar p diffraction peaks corresp onding to the 25-nm period. The average pitch of the grating is measured in very high-accuracy with standard uncertainty of less than 10 pm. Keywords: GIXD, grating reference, 25-nm pitch, calibration system

The novel calibration method of sub-nanometer accuracy for the line width measurement using STEM images is proposed to calibrate CD-SEM line width measurements. In accordance with the proposed method, the traceability and reference metrology of line width standards are established using Si lattice structures. First, we define the edge of a line as the end of Si lattice structure as the interface between Si lattice and oxide film. Second, an image magnification and inclination angles are calculated using 2D Fourier analysis of a STEM image. Third, the edge positions of the line are detected after the novel noise reduction method using averaging by Si lattice patterns. Then, the uncertainty of the line width measurement is evaluated with the uncertainty contributors of pixel size, edge detections and repeatability. Using the proposed method, the expanded uncertainty less than 0.5 nm for the line width of 45 nm is established.

We have developed a 25-nm pitch multilayer grating pattern for CD-SEM magnification calibration instead of the conventional 100-nm pitch grating reference. The 25-nm pitch grating reference was fabricated by multilayer deposition of alternative alternating SiO 2 and Si layers and then the reference chip was fabricated by substrate bonding and polishing process. Finally the 25-nm pitch grating pattern was achieved using the material-selective chemical etching of the polished cross-sectional surface. We evaluated the 25-nm pitch grating reference chip using CD-SEM. A high-contrast secondary electron image of the grating pattern was obtained under 1-kV acceleration voltage. The uniformity of the 25-nm pitch size of the grating was less than 1 nm in 3σ. The line edge roughness of the grating pattern was also less than 1 nm. Such a fine and uniform grating pattern will fulfill the requirements of a magnification calibration reference for next-generation CD-SEM.

To establish the method for evaluating interconnect LER, resist, low-k, and Cu/low-k samples were observed and electric-field enhancement was simulated. Wedge-shape LER was observed in the edges of low-k and Cu/low-k patterns, and simulations showed that the wedge causes serious electric-field enhancement which can degrade TDDB property. To predict the risk of TDDB, inspections of the wedge angle after low-k etching and Cu CMP are required as well as that of the degree of LER.

We have developed a novel multi-layer grating pattern with a sub-50-nm pitch size for CD-SEM magnification calibration instead of the conventional 100-nm pitch grating reference. The sub-50-nm pitch size grating reference was fabricated by multi-layer deposition of alternative two alternating materials and then the material-selective chemical etching of the cleaved cross-sectional surface. A line and space pattern with 10-nm pitch size was easily resolved and a high-contrast secondary electron image of the grating pattern was obtained under 1-kV acceleration voltage using CD-SEM. The uniformity of the 20-nm pitch size of the grating was less than 1 nm in 3σ. The line edge roughness of the grating pattern was also less than 1 nm. Such a fine and uniform grating pattern will fulfill the requirements of a magnification calibration reference for next-generation CD-SEM.

Emerging three-dimensional (3D) transistor structures have increased the demand for an easy and practical method to measure pattern feature metrics (such as CD, line-edge roughness, etc.) as a function of height (z coordinate). We have examined 3D pattern-profile extraction from a top-view image obtained using a critical-dimension scanning electron microscope (CD-SEM). An atomic force microscope (AFM) was used to measure 3D pattern profiles as a reference. In this examination, line-edge positions were firstly obtained from a CD-SEM image at various threshold levels, and the result was compared with the reference profile measured using the AFM. From this comparison, a mapping function from threshold levels of CD-SEM image-processing to z coordinates is obtained. Using this mapping function, 3D pattern profiles were reconstructed from CD-SEM signal profiles, and the obtained profiles were similar to the directly obtained cross-sectional profile. Put simply, a 3D pattern-profile was extracted from a top-view image successfully. Though the results are not sufficient to confirm the validity of our method yet, the method may feasibly be introduced for quick and easy 3D measurement.

Oval defects in GaAs layers are serious impediments to GaAs device fabrication. Of particular importance is the relationship between the droplets deposited on the crucible walls and the number of oval defects. These droplets previously believed to be gallium oxides are shown to consist of GaAs shells covering the Ga particles. By increasing the wall temperature of the crucible orifice, and reducing the arsenic beam intensity, the density of droplets is reduced, thus decreasing oval defects. Using a uniform‐heated Ga effusion cell and adjusting the cell configuration, the droplets can be reduced substantially and the defect density can be reduced from >104 cm−2 to less than 200 cm−2.

This paper introduces the Taguchi method for the optimization of measurement conditions in critical dimension-scanning electron microscope (CD-SEM). As a result, four parameter conditions are optimized for low resist shrinkage and highly precise measurement through eighteen experiments. The prediction accuracy is under 0.2 nm. Moreover, high prediction accuracy is obtained for three different ArF resists. Therefore, it is clarified that the proposed method is a promising tool for CD-measurement optimization.

To establish a method for measuring interconnect line-edge roughness (LER), low- k line patterns were observed and electric-field concentration was simulated based on the observation results. Wedges were observed on the edges, and the bottom and the top widths of the average wedge feature were 60 nm and 7 nm (or smaller), respectively. Simulation showed that the LER causes serious degradation of TDDB immunity at 100-nm-pitch Cu/low- k interconnects. The maximum electric-field intensity depends upon the conventional LER metric, 3 R q , but depends more strongly on the wedge angle, the curvature of the tip, and the minimum linewidth.

We fabricated a grating reference with EB cell projection lithography and silicon dry etching, instead of conventional 240-nm pitch grating references fabricated with laser-interferometer lithography and anisotropic chemical wet etching. We developed a novel 100-nm pitch grating reference based on our grating reference for critical dimension-scanning electron microscope (CD-SEM) calibration. We obtained high-contrast secondary electron signals and uniform grating patterns within 3 nm in 3σ during CD-SEM measurements because we eliminated the proximity effect of EB exposure. The reference has an array of 100-nm grating cells in the x-and y-directions. Each cell consists of a 100-nm grating unit, an X-Y coordinate number in the array, and an addressing mark for the CD-SEM to identify the calibration position. These positional identification marks enable accurate calibration by specifying the location of the grating and the number of calibrations. Also, the pitch size of the reference grating can be accurately calibrated by optical diffraction angle measurements with a deep ultraviolet (DUV) laser.

To establish a method of measuring interconnect line-edge roughness (LER), low-k line patterns were observed and electric field concentration was simulated on the basis of observation results. Wedge-shaped LERs were observed at the edges of low-k lines, and the bottom and the top widths of the average wedge feature were 60 and 7 nm (or smaller), respectively. Simulation showed that LER causes serious electric field concentration, which may cause the degradation of time-dependent dielectric breakdown (TDDB) lifetime at 100-nm-pitch Cu/low-k interconnects. The maximum electric field strength depends on the conventional LER metric 3Rq, but depends more strongly on the wedge angle, the curvature of the tip. That is, other metrics such as wedge angle can predict fatal electric field concentration caused by LER than the conventional metric 3Rq.

Change in the cross-sectional profile of a photoresist (PR) pattern due to shrinkage was evaluated to investigate the mechanism of electron beam-induced shrinkage. A scanning transmission electron microscope (STEM) was used to observe the cross-sectional profiles of PR lines after atomic-layer deposition of metal oxide and carbon deposition on the sample surface. A HfO2 thin layer enhanced the profile contrast in the STEM measurements without blurring the edge, which enabled the precise cross-sectional measurement of the PR patterns. We found interesting features associated with shrinkage from the detailed profile change obtained using this method, such as a rounding of the pattern top, a necking of the sidewall profile, a rounding of the foot in the pattern on the organic underlying layer, and voltage-independent sidewall shrinkage under a large electron beam dose. These behaviors along with the results from a Monte Carlo simulation are discussed. Consequently, these observations experimentally clarified that the elastic deformation effect and the impact of the secondary electrons emitted from the spaces around the pattern into the sidewall are important to interpret the change in the shape of the pattern induced by shrinkage.

Line width slimming (LWS) occurring in ArF photo-resist is measured with various threshold levels. The LWS decreases as the threshold level decrease, with electron’s landing voltage (Ve) = 800 V. Contrary to this, with Ve = 300 V the LWS slightly increases as the threshold level decrease. The line edge detected by threshold = 20% locates in the sidewall where the elevation angle is nearly zero, whilst the line edge by threshold = 80% locates in the top corner where the elevation angle is more than 30 degrees. To estimate the electron dose that is sensitive to the elevation angle of the incident electron, we used an in-house made Monte Carlo simulator. The LWS variation with the threshold level can be explained by calculated electron dose ratio that is sensitive to the elevation angle.

ArF-resist-shrinkage and line-edge roughness-induced CD errors are the two main challenges for CD SEM. The requirement of measurement precision for the 65-nm node is less than 0.5 nm. The current CD SEM ADI precision is between 0.7 to 0.9 nm after shrinkage curve correction. Optical CD (OCD) has provided three major advantages. That is more sampling (> 2500:1), insensitivity to line edge roughness, and less resist damage. These advantages facilitate much better measurement precision (< 0.3 nm) than CD SEM and make OCD a potential APC metrology candidate. However, the recipe and library generation of OCD is more complicated and time consuming than CD SEM. Any thin-film variation will disturb the CD accuracy and recipe coverage range of OCD. For different pitches and film combinations, new OCD libraries need to be generated. Matching through all pitches between CD SEM and OCD is also very difficult. We propose a new concept on optical-CD-like CD SEM measurement, i.e. average line width (ALW) and contact hole diameter (ACD) measurement at high resolution and low magnification (HRLM) CD SEM. The resolution chosen is below 2nm and the magnification is 50KX. The low magnification CD measurement can average the e-beam dosage and reduce the ArF shrinkage. Several repeated patterns such as line/space and hole arrays are measured to get an averaged CD under lower magnification condition. These low magnification average CDs increase the sampling size and they are insensitive to the line edge roughness. The CD linearity of ALW/ACD and the CD matching to current CD SEM methodology will be presented. Small step FEM CD by low magnification and high magnification CD measurement will be studied. The difference between low/high magnification SEM and optical CD will be also studied.

For 90 nm technology and below, we need to fight for every nanometer to improve the CD uniformity (CDU). New materials, especially for low-k material, bring about not only complicated integration challenges, but also new metrology difficulties such as SEM image focus failure if using low landing energy (300V) on charging wafer (e.g. -300V). The wafer global charging will also distort the CD SEM magnification and result in CD measurement error. CD SEM venders propose that the distortion be corrected by voltage contrast focus. In order to compare and quantify the measurement error correction with and without using retarding voltage focus, ArF resist non-uniform charging wafers (~ -300V) and low charging wafers (~ -7V) were prepared. Low landing energy like 300V is one of the solutions for ArF resist shrinkage. However, as the low landing energy (300V) meets the high global charging wafer (-300V), SEM cannot get sufficient secondary electron signal to construct image. Therefore, two landing voltages 500eV and 800eV were chosen for the evaluation. Three pitches 1600 nm, 460 nm and 230 nm were investigated. Two indexes are used to evaluate the wafer global charging effect on CD and CDU. One is within-wafer pitch uniformity for determining the CD SEM magnification error. The other is ArF-resist-shrinkage amplitude used to estimate the effective landing energy at charging area. The experimental results show that the pitch uniformity difference with and without using retarding focus can be larger than 2.5 nm. Similar phenomenon is also found for the line width uniformity. Resist shrinkage amplitude is significantly reduced at the highly charged area. Both results show that accurate focus procedure, i.e. retarding voltage focus employing first, is the key to reduce the CD metrology tool measurement error and improve CDU.

The influence of line-edge roughness (LER) on transistor performance was investigated experimentally and the preciously proposed guideline for CD and LER measurements was examined. First, regarding the transistor-performance measurements, a shift of roll-off curves caused by LER within a gate pattern was observed. Moreover, the effect of transistor-width fluctuation originating from long-period LER was found to cause a variation in transistor performance. Second, regarding LER and CD metrology, the previously reported guideline was validated by using KrF and ArF resist-pattern samples. It was found that both CD and LER should be evaluated with the 2-μm-long inspection area. Based on this guideline, a comprehensive approach for evaluating LER and CD for transistor fabrication process is presented. The authors consider that this procedure can provide useful information for the 65-nm-node technology and beyond.

A guideline for evaluating LER and total procedure to estimate effects of measured LER on device performance were proposed. Spatial-frequency distributions of LER in various resist materials were investigated and general characteristics of spatial-frequency distribution of LER were obtained. Measurement parameters for accurate LER measurement can be calculated according to the guideline. Measured line-width distribution was used for predicting degradation and variation in MOS transistor performance using the 2D device simulation. Effect of long-period component of LER was clarified as well as short-period component.

A mechanism of the photoresist shrinkage induced by electron-beam (EB) irradiation was studied in detail. A precise cross-sectional profile of a photoresist pattern is obtained by a scanning transmission electron microscope (STEM) after HfO2 atomic layer deposition on a sample. Photoresist lines and spaces formed on either of bottom anti-reflective coating (BARC) layer or spin-on-glass (SOG) layer were exposed to EB at a much higher dose than a practical dose (to accelerate shrinkage intentionally). The obtained STEM images of the patterns before and after EB irradiation showed that EB irradiation causes necking of the pattern profile as well as linewidth slimming. In addition, it was suggested that the BARC layer shrinkage results in the elastic deformation of the pattern profile, whereas the SOG layer does not shrink. Furthermore, EB irradiation only to the lines or spaces was performed. The EB irradiation to spaces caused sidewall shrinkage and necking of the pattern profile, although no electron was irradiated directly into the pattern. This result is considered to be due to the electrons scattered from the spaces to the pattern sidewall. Finally, a Monte Carlo simulation showed that the distribution of the deposited energy on the pattern surface corresponds to the change of the pattern shape. Consequently, our study clarifies the importance of the effect of elastic shape change and the impact of the electrons scattered from the underlying layer to the sidewall on photoresist shrinkage.

A one-dimensional grating (1D grating) is one of the most important reference standards to calibrate nanometrological instruments, such as a critical dimension scanning electron microscope. Recently, the resolutions of nanometrological instruments have become higher, and 1D grating standards with smaller pitches are required. Based on this demand, 1D gratings with 25 nm pitch consisting of Si/SiO2 multilayer thin-film structures have been developed. Furthermore, pitch calibrations using metrological atomic force microscopes (metrological AFMs) and an x-ray diffractometer have been applied. In this study, the results of a 25 nm pitch comparison between a length-and-angle-standards-traceable x-ray diffractometer and a metrological AFM are reported.

We present a novel multilayer grating pattern with a sub-50-nm pitch for critical dimension-scanning electron microscope (CD-SEM) magnification calibration as an advanced version of the conventional 100-nm pitch grating reference. A 25-nm pitch grating reference is fabricated by multilayer deposition of alternating materials and then material-selective chemical etching of the polished cross-sectional surface. A line and space pattern with 25-nm pitch is easily resolved, and a high-contrast secondary electron image of the grating pattern is obtained under 1-kV acceleration voltage using the CD-SEM. The uniformity of the 25-nm pitch of the grating is