Critical Dimension Scanning Electron Microscope Research Articles

Device-correlated metrology for overlay measurements

One of the main issues with accuracy is the bias between the overlay (OVL) target and actual device OVL. In this study, we introduce the concept of device-correlated metrology (DCM), which is a systematic approach to quantify and overcome the bias between target-based OVL results and device OVL values. In order to systematically quantify the bias components between target and device, we introduce a new hybrid target integrating an optical OVL target with a device mimicking critical dimension scanning electron microscope (CD-SEM) target. The hybrid OVL target is designed to accurately represent the process influence on the actual device. In the general case, the CD-SEM can measure the bias between the target and device on the same layer after etch inspection (AEI) for all layers, the OVL between layers at AEI for most cases and after develop inspection for limited cases such as double-patterning layers. The results have shown that for the innovative process compatible hybrid targets the bias between the target and device is small, within the order of CD-SEM noise. Direct OVL measurements by CD-SEM show excellent correlation between CD-SEM and optical OVL measurements at certain conditions. This correlation helps verify the accuracy of the optical measurement results and is applicable for the imaging base OVL method using several target types advance imaging metrology, advance imaging metrology in die OVL, and the scatterometrybase OVL method. Future plans include broadening the hybrid target design to better mimic each layer process conditions such as pattern density. Additionally, for memory devices we are developing hybrid targets which enable other methods of accuracy verification.

Addressing FinFET metrology challenges in 1× node using tilt-beam critical dimension scanning electron microscope

At 1× node, a three-dimensional (3-D) FinFET process raises a number of new metrology challenges for process control, including gate height and fin height. At present, there is a metrology gap in inline in-die measurement of these parameters. To fill this metrology gap, in-column beam tilt has been implemented on Applied Materials V4i+ critical dimension scanning electron microscope for height measurement. Low-tilt (5 deg) and high-tilt (14 deg) beam angles have been calibrated to obtain the height and the sidewall angle information. Evaluation of its feasibility and production worthiness is done with applications in both gate height and fin height measurements. Transmission electron microscope correlation with an R2 equal to 0.89 and a precision of 0.81 nm have been achieved on various in-die features in a gate height application. The initial fin height measurement shows less accuracy (R2 being 0.77) and precision (1.49 nm) due to greater challenges brought by the fin profile, yet it is promising for the first attempt. Sensitivity to design of experiment offset die-to-die and in-die variations is demonstrated in both gate height and fin height. The process defect is successfully captured with inline gate height measurement. This is the first successful demonstration of inline in-die gate height measurement for a 14-nm FinFET process control.

Real-time inspection system utilizing scatterometry pupil data

Scatterometry-based critical dimension (CD), also known as optical CD (OCD), significantly matches CD scanning electron microscopy in accuracy and precision, in addition to offering superior full-profile reconstruction. OCD, however, is computationally intensive. We construct an extremely fast screening tool that determines whether a sample should or should not proceed to subsequent manufacturing steps. To this end, we examine the diffraction signals of the grating in order to determine whether a sample is in or out of its specification limits. This allows us to allocate traditional metrology resources only for samples that show unusual behavior. Support vector machines (SVMs) are trained to classify each incoming sample as in-spec or out-of-spec. The constructed classifier is applied to gratings exposed with a focus-exposure matrix for a rectangular silicon-bottom anti-reflective coating-photoresist stack, which include erroneous samples with under–over exposure, necking, and bridging problems. The misclassification rates as well as false and missed alarm rates are analyzed. Results show that our prototype screening system has misclassification errors on the order of 5% to 10%, while the computation time is on the order of one vector dot product.

Actinic critical dimension measurement of contaminated extreme ultraviolet mask using coherent scattering microscopy

The authors evaluated the feasibility of using coherent scattering microscopy (CSM) as an actinic metrology tool by employing it to determine the critical dimension (CD) and normalized image log-slope (NILS) values of contaminated extreme ultraviolet (EUV) masks. CSM was as effective as CD scanning electron microscopy (CD-SEM) in measuring the CD values of clean EUV masks in the case of vertical patterns (nonshadowing effect); however, only the CSM could detect shadowing effect for horizontal patterns resulting in smaller clear mask CD values. Owing to weak interaction between the low-density contaminant layer and EUV radiation, the CSM-based CD measurements were not as affected by contamination as were those made using CD-SEM. Furthermore, CSM could be used to determine the NILS values under illumination conditions corresponding to a high-volume manufacturing tool.

Optical Critical Dimension Measurement for AEI Structures at Sub 65 Nm Node

In this work, Optical Critical Dimension (OCD) spectroscopy was used to monitor the critical dimension for After-Etch Inspection (AEI) structures at sub 65 nm node, such as Poly Gate, Shallow Trench Isolation, Offset Spacer, and Oxide-Nitride Spacer. The dynamic repeatability of the measurement was obtained through a 10-cycle measurement process with loading and unloading wafers. The long-term stability was examined with more than 3 day’s measurements. The good correlation results between the OCD spectroscopic tool and reference tools, such as, CD Scanning Electron Microscope and Transmission Electron Microscopy, were obtained. The results demonstrated that the OCD spectroscopic tool has advantages of high precision and high stability for 65 nm node and beyond.

Plasma treatments to improve line-width roughness during gate patterning

The major issue related to line width roughness (LWR) is the significant LWR of the photoresist patterns printed by 193-nm lithography that is partially transferred into the gate stack during the subsequent plasma etching steps. The strategy used today to overcome this issue is to apply postlithography treatments to reduce photoresist pattern LWR before transfer. In this article, we investigate the impact of various plasma treatments (HBr, H 2 , He, Ar) on the minimization of the LWR of dense and isolated photoresist patterns and its transfer during gate patterning. To do so, we use critical dimension scanning electron microscopy measurements combined with power spectrum density fitting method to extract unbiased LWR values and provide a spectral analysis of the LWR. We show that plasma treatments that lead to carbon redeposition from the gas phase on the resist pattern sidewalls are less efficient to reduce LWR than plasma treatments where the redeposition is limited. Among all plasma chemistries, H 2 plasmas seem very promising to decrease resist LWR in the whole spectral range, while maintaining square resist profiles. In addition, we show that all frequency roughness components are not equally transferred during gate patterning, and more particularly that the high frequency roughness components are lost.

An atomic force microscopy-based method for line edge roughness measurement

With the constant decrease of semiconductor device dimensions, line edge roughness (LER) becomes one of the most important sources of device variability and needs to be controlled below 2 nm for the future technological nodes of the semiconductor roadmap. LER control at the nanometer scale requires accurate measurements. We introduce a technique for LER measurement based upon the atomic force microscope (AFM). In this technique, the sample is tilted at about 45° and feature sidewalls are scanned along their length with the AFM tip to obtain three-dimensional images. The small radius of curvature of the tip together with the low noise level of a laboratory AFM result in high resolution images. Half profiles and LER values on all the height of the sidewalls are extracted from the 3D images using a procedure that we developed. The influence of sample angle variations on the measurements is shown to be small. The technique is applied to the study of a full pattern transfer into a simplified gate stack. The images obtained are qualitatively consistent with cross-section scanning electron microscopy images and the average LER values agree with that obtained by critical dimension scanning electron microscopy. In addition to its high resolution, this technique presents several advantages such as the ability to image the foot of photoresist lines, complex multi-layer stacks regardless of the materials, and deep re-entrant profiles.

Benefits of plasma treatments on critical dimension control and line width roughness transfer during gate patterning

The present work focuses on the line width roughness (LWR) transfer and the critical dimension control during a typical gate stack patterning and shows the benefits of introducing 193 nm photoresist treatments before pattern transfer into the gate stack to improve process performance. The two investigated treatments (HBr plasma and vacuum ultra violet (VUV) plasma radiation) have been tested on both blanket photoresist films and resist patterns to highlight the etching and roughening mechanisms of cured resists. Both treatments reinforce the etch resistance of the photoresist exposed to fluorocarbon plasma etching process used to open the Si-ARC (silicon antireflective coating) layer. The etch resistance improvement of cured resists is attributed to both the decrease in oxygen content within the resist and the crosslinking phenomena caused by VUV radiation during the treatment. As the magnitude of the surface roughness is directly correlated to the etched thickness, cured resists, which are etched less rapidly, will develop a lower surface roughness for the same processing time compared to reference resists. The LWR evolution along the pattern sidewalls has been studied by critical dimension atomic force microscopy during the Si-ARC plasma etching step. The study shows that the LWR is degraded at the top of the resist pattern and propagates along the pattern sidewalls. However, as long as the degradation does not reach the interface between resist and Si-ARC, the LWR decreases during the Si-ARC etching step. As resist pretreatments reinforce the resist etch resistance during Si-ARC etching, the LWR degradation along the sidewalls is limited leading to minimized LWR transfer. The LWR decrease observed after plasma etching has been explained thanks to a spectral analysis of the LWR performed by critical dimension scanning electron microscopy combined with the power spectral density fitting method. The study shows that the high and medium frequency components of the roughness (periodicity below 200 nm) are not totally transferred during the gate patterning allowing a LWR decrease at each plasma step.

Monte Carlo Simulation of Secondary Electron Emission from Dielectric Targets

In modern physics we are interested in systems with many degrees of freedom. The Monte Carlo (MC) method gives us a very accurate way to calculate definite integrals of high dimension: it evaluates the integrand at a random sampling of abscissa. MC is also used for evaluating the many physical quantities necessary to the study of the interactions of particle-beams with solid targets. Letting the particles carry out an artificial random walk and taking into account the effect of the single collisions, it is possible to accurately evaluate the diffusion process.Secondary electron emission is a process where primary incident electrons impinging on a surface induce the emission of secondary electrons. The number of secondary electrons emitted divided by the number of the incident electrons is the so-called secondary electron emission yield. The secondary electron emission yield is conventionally measured as the integral of the secondary electron energy distribution in the emitted electron energy range from 0 to 50eV.The problem of the determination of secondary electron emission from solids irradiated by a particle beam is of crucial importance, especially in connection with the analytical techniques that utilize secondary electrons to investigate chemical and compositional properties of solids in the near surface layers. Secondary electrons are used for imaging in scanning electron microscopes, with applications ranging from secondary electron doping contrast in p-n junctions, line-width measurement in critical-dimension scanning electron microscopy, to the study of biological samples.In this work, the main mechanisms of scattering and energy loss of electrons scattered in dielectric materials are briefly treated. The present MC scheme takes into account all the single energy losses suffered by each electron in the secondary electron cascade, and is rather accurate for the calculation of the secondary electron yield and energy distribution as well.

Dose-focus monitor technique using a critical-dimension scanning electron microscope and its application to local variation analysis

A dose-focus monitoring technique using a critical-dimension scanning electron microscope (CD-SEM) is studied for applications on product wafers. Our technique uses two target structures: one is a dense grating structure for dose determination, and the other is a relatively isolated line grating for focus determination. These targets are less than 6 μm, and they can be inserted across a product chip to monitor dose and focus variation in a chip. Monitoring precision is estimated to be on the order of 1% for dose and 10 nm for focus, and the technique can be applied to dose and focus monitoring on product wafers. The developed technique is used to analyze spatial correlation in dose and focus over a wide range of distances, using a mask with a multitude of these targets. The variation (3σ ) of dose and focus difference between two monitor targets is examined for various separation distances, and the variation of focus difference increases from 10 to 25 nm as the separation distance increases from ∼20 μm to ∼10 mm . The variation of 10 nm observed at the shortest distance reflects focus monitoring precision, and focus variation sources such as wafer thickness variation come into play at longer distances.

Monte Carlo Simulation of CD‐SEM Images for Linewidth and Critical Dimension Metrology

In semiconductor industry, strict critical dimension control by using a critical dimension scanning electron microscope (CD-SEM) is an extremely urgent task in near-term years. A Monte Carlo simulation model for study of CD-SEM image has been established, which is based on using Mott's cross section for electron elastic scattering and the full Penn dielectric function formalism for electron inelastic scattering and the associated secondary electron (SE) production. In this work, a systematic calculation of CD-SEM line-scan profiles and 2D images of trapezoidal Si lines has been performed by taking into account different experimental factors including electron beam condition (primary energy, probe size), line geometry (width, height, foot/corner rounding, sidewall angle, and roughness), material properties, and SE signal detection. The influences of these factors to the critical dimension metrology are investigated, leading to build a future comprehensive model-based library.

Time-dependent electron-beam-induced photoresist shrinkage effects

We explore how photoresist shrinkage behavior due to e-beam measurement by critical dimension-scanning electron microscope (CD-SEM) depends on various time-related factors. This will include an investigation of how the photoresist critical dimension (CD) and CD shrinkage varies with photoresist age and the differences in shrinkage trends between load/unload and static and dynamic repeatability cases, where time between measurements is a key variable. The results for this typical immersion argon flouride photoresist process will show that resist CD and shrinkage variation due to resist age and vacuum-cycling is insignificant, yet the shrinkage is strongly linked to time between consecutive measurements, with a well-defined, high-certainty logarithmic decay with time. These experiments identify a key difference between the shrinkage seen in static versus dynamic measurements, which will be shown to have far-reaching implications for the shrinkage phenomenon in general and for the best-known methods for executing CD-SEM metrology with photoresist samples.

Effect on Critical Dimension Performance for Carbon Contamination of Extreme Ultraviolet Mask Using Coherent Scattering Microscopy and In-situ Contamination System

The impact of carbon contamination on imaging performance was analyzed using an in-situ accelerated contamination system (ICS) combined with coherent scattering microscopy (CSM) which was installed at 11B extreme ultraviolet lithography (EUVL) beamline of the Pohang Accelerator Laboratory (PAL). The CSM/ICS is composed of CSM for measuring imaging properties and ICS for implementing acceleration of carbon contamination. The mask critical dimension (CD) and reflectivity were compared before and after carbon contamination through accelerated exposure. The reflectivity degradation was measured as 1.3, 2.0, and 2.8% after 1, 2, and 3 h exposure, respectively, due to carbon contamination of 5, 10, and 20 nm as measured by Zygo interferometer. The mask CD change for 88 nm line and space pattern was analyzed using CSM and a CD scanning electron microscope (SEM), and the result shows CD-SEM and CSM give large difference of 3.8 times in mask ΔCD after carbon contamination. This difference confirms the importance of using actinic inspection technique that employs exactly the same imaging condition as exposure tool.

Effect on Critical Dimension Performance for Carbon Contamination of Extreme Ultraviolet Mask Using Coherent Scattering Microscopy andIn-situContamination System

The impact of carbon contamination on imaging performance was analyzed using an in-situ accelerated contamination system (ICS) combined with coherent scattering microscopy (CSM) which was installed at 11B extreme ultraviolet lithography (EUVL) beamline of the Pohang Accelerator Laboratory (PAL). The CSM/ICS is composed of CSM for measuring imaging properties and ICS for implementing acceleration of carbon contamination. The mask critical dimension (CD) and reflectivity were compared before and after carbon contamination through accelerated exposure. The reflectivity degradation was measured as 1.3, 2.0, and 2.8% after 1, 2, and 3 h exposure, respectively, due to carbon contamination of 5, 10, and 20 nm as measured by Zygo interferometer. The mask CD change for 88 nm line and space pattern was analyzed using CSM and a CD scanning electron microscope (SEM), and the result shows CD-SEM and CSM give large difference of 3.8 times in mask ΔCD after carbon contamination. This difference confirms the importance of using actinic inspection technique that employs exactly the same imaging condition as exposure tool.

Unbiased line width roughness measurements with critical dimension scanning electron microscopy and critical dimension atomic force microscopy

With the constant decrease of semiconductor device dimensions, line width roughness (LWR) becomes one of the most important sources of device variability and thus needs to be controlled below 2 nm for the future technological nodes of the semiconductor roadmap. The LWR control at the nanometer scale requires accurate measurements, which are inevitably impacted by the noise level of the equipment that causes bias from true LWR values. In this article, we compare the capability of two metrology tools, the critical dimension scanning electron microscopy (CD-SEM) and critical dimension atomic force microscopy (CD-AFM) to measure the true line width roughness of silicon and photoresist lines. For this purpose, we propose several methods based on previous works to estimate the noise level of those two equipments and thus extract the true LWR. One of the developed methods for the CD-SEM technique generalizes the power spectral densities (PSD) fitting method proposed by Hiraiwa and Nishida with a more universal autocorrelation function, which includes both correlation length and roughness exponent. However, PSD fitting method could not be used with CD-AFM due to the time consuming character of this technique. Hence, other experimental protocols have been set up for CD-AFM in order to accurately characterize the LWR. Our study shows that the CD-SEM technique combined with our PSD fitting method is much more powerful than CD-AFM to get all roughness information (true LWR, correlation length, and roughness exponent) with a good accuracy and efficiency on hard materials such as silicon. Concerning materials degradable under electron beam exposure such as photoresist, the choice is more disputable, since ultimately they are impacted by the electrons. Fortunately, our PSD fitting method allows working with low number of integration frames, which limits the resist degradation. Besides, we have highlighted some limitations of the CD-AFM technique due to the tip diameter. This technique can underestimate LWR if the roughness presents significant amount of high frequency components, as it is the case for photoresist patterns. So far, there is no universal technique to accurately estimate the LWR on any materials. Nevertheless, the CD-SEM protocol we propose opens a way for a better characterization of the photoresist LWR after lithography and a better understanding of the LWR transfer during the plasma etching steps involved in gate patterning processes.

Critical Dimension Measurement Using OCD Spectroscopy for Gate and STI AEI Structures

In this work, optical critical dimension (OCD) spectroscopy was used to monitor the critical dimension for gate and STI AEI (After-Etch Inspection) structures. The reference measurements with CD-SEM (CD Scanning Electron Microscope) and TEM (Transmission Electron Microscopy) were also carried out. The OCD results show high precision, high stability and good correlation. This work demonstrates the capability of the OCD as an important metrology technique for IC process control.

Characterization of Si nanowaveguide line edge roughness and its effect on light transmission

Abstract The manufacture of low-loss silicon-on-insulator (SOI) photonic wires for telecommunication wavelengths (∼1.55 μm) is a challenge. Side wall and line edge roughness (LER) are the dominant sources of scattering loss. In this work the characterization of Si nanowaveguide (NW) LER was performed and its effect on light transmission was theoretically analyzed and measured. NW structures with a width of 0.5 μm and height of 0.22 μm were prepared using different thickness of hard-mask and plasma etch process technologies on SOI platforms. LER determination was performed by critical-dimension scanning electron microscope (CD-SEM), conventional atomic force microscope (AFM) and 3D-AFM. The characterization of LER quality was carried out by monitoring of its RMS deviation σ , main frequencies and correlation length L c . The effect of sampling length was evaluated. The data for σ from 0.8 to 6 nm and L c from 20 to 50 nm were found to be dependent mostly on etch process technology. Theoretical simulation of scattering loss due to LER based on the 2D analytical model for planar waveguides was performed. Finally, the correlation between NW optical transmission (scattering) losses (TL) and LER was obtained and can be applied to future technology development. Especially for the case of σ ∼ 0.85 nm and L c ∼ 20 nm the lowest value of TL ∼ 1.2 dB/cm was measured at a wavelength of 1.55 μm. Good qualitative agreement between calculated and measured losses due to LER was found.

25 nm pitch comparison between a traceable x-ray diffractometer and a metrological atomic force microscope

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.

Holistic metrology approach: hybrid metrology utilizing scatterometry, critical dimension-atomic force microscope and critical dimension-scanning electron microscope

Shrinking design rules and reduced process tolerances require tight control of critical dimension (CD) linewidth, feature shape, and profile of the printed geometry. The holistic approach consists of utilizing all available information from different sources such as data from other toolsets, multiple optical channels, multiple targets, etc., to optimize recipe and improve measurement performance. Various in-line CD toolsets such as scatterometry optical CD, CD-SEM, and CD-AFM are typically utilized individually in fabs. Each of these toolsets has its own set of limitations that are intrinsic to specific measurement technique and algorithm. Here we define metrology to be the use of any two or more toolsets in combination to measure the same dataset. We demonstrate the benefits of the hybrid on two test structures: 22-nm-node gate develop inspect and 32-nm-node fin-shaped field effect transistor gate final inspect. We will cover measurement results obtained using typical BKM (nonhybrid, single toolset standard results) as well as those obtained by utilizing the hybrid approach. Measurement performance will be compared using standard metrics; for example, accuracy and precision.

Comparison between Energy Straggling Strategy and Continuous Slowing Down Approximation in Monte Carlo Simulation of Secondary Electron Emission of Insulating Materials

Secondary electrons are commonly used for imaging in scanning electron microscopes, with applications ranging from secondary electron doping contrast in p-n junctions, line-width measurement in critical-dimension scanning electron microscopy and dimensional parameters evaluation in the production of masks and wafers in the semiconductor industry, to the study of biological samples. This paper describes the secondary electron emission yield calculated using two different Monte Carlo approaches. In the first, based on the energy straggling strategy, one takes into account all the single energy losses suffered by each electron in the secondary electron cascade. This method has been demonstrated to be very accurate for the calculation of the secondary electron yield and of the secondary electron energy distribution as well. An alternative way to calculate the secondary electron yield is based on a continuous slowing down approximation and uses as input the electron stopping power of the material being considered. As this work demonstrates that the secondary electron yields calculated using the two approaches are very close and in agreement with the experiment, the much faster continuous slowing down approximation is recommended. On the other hand, if other physical quantities, such as the secondary electron distributions, are required, the energy straggling strategy should be preferred, even if it requires much longer CPU times, due to its stronger physical background.