Sidewall structure estimation from CD-SEM for lithographic process control

Automated critical dimension (CD) metrology has long been known to have certain limitations. As a top down imaging technique, retrograde profiles, resist thickness loss and other process issues are difficult to detect with the standard production CD SEMs used throughout the industry. Tilting capability has recently added much needed degrees of freedom to CD SEMs, potentially opening the door to three dimensional metrology. Various methodologies can be used to interpret tilt image information. This paper investigates one particular technique used to extract three dimensional information. Using cross section and/or atomic force microscopy (AFM), one can evaluate the potential benefits and validity of CD SEM tilt capability. Sidewall angle, film thickness and possibly other parameters could possibly become standard calculations the top down CD SEM could perform on a routine basis. Data is reported comparing 3D information generated from top down CD SEM to AFM. Potentially, a couple of different approaches to evaluating 3D information will be compared. Lastly, recommendations for future applications will be discussed.

The importance of line-width roughness (LWR)/line-edge roughness (LER) in relation with leakage current has made it a critical process parameter, especially when device downsizes to 32nm era. Critical dimension scanning electron microscope (SEM) is still the most popular tool to characterize LWR in semiconductor manufacturing. However, true LWR metrology by SEM has been a challenge because of metrology noise and induced damage. Method of repeating multi-shot or long scanning time [5-8] on the same field of view have been proposed to effectively eliminate metrology noise caused by SEM tool variation and image processing. By such method, however, line damage (non-conformal critical dimension enlargement) caused by charging is found and damage level depends on the property of surface material, e-beam energy, and total scanning time. In this article, the impact of damage on LWR metrology and optimal metrology condition are studied. Following the proposed method [8], polysilicon lines both with and without oxide mask are investigated under different e-beam energy and number of shot. LWR metrology noise decreases along with e-beam energy and saturates at 1400eV while damage level is proportional to beam energy. In similar way, the larger number of repeating shot, the more effective of noise rejection and the more damage of line will be at the same time. The damage not only increases metrology noise, but also degrades the LWR, that are verified in both simulation and experiment. In the final, the optimal conditions of true LWR metrology by SEM for both polysilicon line with and without oxide mask are proposed.

As photomask critical dimensions extend well into the submicron range, optical measurement techniques are approaching the end of their useful life. While offering advantages of being nondestructive, relatively fast, and very precise, optical measurement tools may have accuracy problems due to diffraction. Linearity between measured and actual values is typically lost due to interference, resonance, and shadowing effects even before the diffraction limit is reached. In addition, standards for submicron features do not exist for optical tools. These limitations have given rise to using a Scanning Electron Microscope (SEM) for reticle dimension measurement. Another complication is that no NIST standard exists for chromium photomask dimensions. A CD SEM must therefore be calibrated to some other trusted standard or measurement method. A Stylus NanoProfilometer (SNP) has better inherent resolution than a SEM and was chosen as the standard measurement instrument for both CD SEM and optical tool metrology. This paper describes the cross-calibration of the Leica LWM-250 (white light) optical metrology tool and the KLA-Tencor 8100XP-R CD SEM, with reference to a Surface Interface SNP9000. The primary motivation was to define the usable ranges of both SEM and optical CD measurement systems, allowing for flexible implementation into a photomask manufacturing environment.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Defect detection in patterned wafers using multichannel Scanning Electron Microscope

This paper describes the application of a low-voltage scanning electron microscope (SEM) with nanometer-level accuracy for measurement in ultra-large-scale integration (ULSI). Minimum feature sizes of integrated circuits are expected to reach the 100-nm level and below (the nanometer region) in the near future. For the lithography process under that regime, precise critical dimension (CD) control and high resolution of resist patterns will be quite important for device fabrication, because variations in pattern sizes will degrade circuit performance. Therefore, metrology with nanometer-level accuracy is required for device fabrication under the regime. Here, we report on a CD-SEM that operates at 500 V to measure patterns at the 1 Gbit level. We used the S-8840 (Hitachi) to measure holes, lines/spaces, and the calibration standard (Micro-Scale). Several voltages from 500 V to 1000 V were used for the measurements. Static variation of less than 3 nm (3(sigma) ) was obtained in the pitch measurement of the Micro- Scale regardless of the acceleration voltages. For the holes, a lower voltage provided higher accuracy in static measurements. In the nanometer region, resist-pattern sizes microscopically fluctuate to the level of 10 nm due to the polymer characteristics of the resists (nano edge roughness). We could also characterize resist-pattern fluctuations with high accuracy. We compared our measurements with those from an atomic force microscope (AFM) for nanometer-level metrology, and conclude that at present CD-SEMs are more advantageous because of their higher accuracy and throughput.

CMOS 65nm technology node requires the introduction of advanced materials for critical patterning operations. The study is focused on the multilayer Anti Reflective Coating (ARC) stack, used in photolithography, for the gate patterning such as Advanced Patterning Film (APF). The interest on this new and complex ARC stack lies in the benefit to guarantee low CD dispersion thanks to a better reflectivity control and resist budget which leads to a larger lithographic process window. However, it implies numerous metrology challenges. The paper deals with the challenges of monitoring the gate Critical Dimension (CD) on this stack. The validation of the scatterometry model versus stack thicknesses and indexes variations, through experiments, is also described. The final result is the complete characterization of the materials for thickness and scatterometry CD control, for photo feedback and for etch feed-forward deployment in an industrial mode. The analysis shows that scatterometry measurements on a standard 65 nm gate process ensure a better effectiveness than the CD Scanning Electron Microscopy (SEM) ones when injected in the Advanced Process Control (APC) system from photo to etch.

As the design rule of lithography becomes smaller, accuracy and precision in Critical Dimension (CD) and controllability of pattern-shape are required in semiconductor production. Critical Dimension Scanning Electron Microscope (CD SEM) is an essential tool to confirm the quality of the mask such as CD control, CD uniformity and CD mean to target (MTT). Unfortunately, in the case of extremely rounded region of arbitrary enclosed patterns, CD fluctuation depending on Region of Interest (ROI) is very serious problem in Mask CD control, so that it decreases the yield. In order to overcome this situation, we have been developing 2-dimensonal (2D) method with system makers and comparing CD performance between mask and wafer using enclosed arbitrary patterns. In this paper, we summarized the results of our evaluation that compare error budget between 1-dimensonal (1D) and 2D data using CD SEM and other optical metrology systems.

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.

Scatterometry is gaining acceptance as a technique for critical dimension (CD) metrology that complements the more established scanning electron microscopy (SEM) techniques. Scatterometry determines the dimensions of the submicron structures by inverse diffraction calculations. SEM and scatterometry are complementary in many respects. Therefore, they are likely to coexist in the foreseeable future. Scatterometry and CD-SEM instruments can be integrated to take advantage of the complementary nature of the two techniques. To explore the joint use of scatterometry and SEM measurements, we measured a set of photoresist grating samples with CDs ranging from 240 nm through 40 nm by scatterometer (Sensys CD-i) and cross-section SEM (Hitachi S-4700). Although a cross-section SEM was used as an absolute standard for comparison of profiles and CDs, our conclusions range to include CD-SEM techniques. It was found that for measurements of profiles that were patterned with high uniformity within the measurement area, scatterometry was very effective, and correlated best with SEM measurements. However, in cases of substantial line-to-line profile variations, or for isolated or non-periodic lines, SEM is the more appropriate measurement method.

The need to characterize line edge and line width roughness in patterns with sub‐50 nm critical dimensions challenges existing platforms based on electron microscopy and optical scatterometry. The development of x‐ray based metrology platforms provides a potential route to characterize a variety of parameters related to line edge roughness by analyzing the diffracted intensity from a periodic array of test patterns. In this study, data from a series of photoresist line/space patterns featuring programmed line width roughness are measured by critical dimension small angle x‐ray scattering (CD‐SAXS). For samples with designed periodic roughness, CD‐SAXS provides the wavelength and amplitude of the periodic roughness through satellite diffraction peaks. For real world applications, the rate of decay of intensity, termed an effective “Debye‐Waller” factor in CD‐SAXS, provides an overall measure of the defects of the patterns. CD‐SAXS data are compared to values obtained from critical dimension scanning electron microscopy (CD‐SEM). Correlations between the techniques exist, however significant differences are observed for the current samples. A tapered cross sectional profile provides a likely explanation for the observed differences between CD‐SEM and CD‐SAXS measurements.

Studies on photomask Cr and MoSi etch processes were carried out and etch kinetics and modeling were performed. The photomasks were etched using two different etch tools and metrology to support theoretical suppositions and process characterization was performed using profilometry and critical dimension (CD) scanning electron microscopy. A 50 keV electron-beam writing system employing positive chemically amplified resist was used for pattern creation. Loading-etch rate equations were theoretically proposed and experimentally tested. It was found that the calculated Cr and MoSi etch rates agreed well with experimental results. Local etch rates versus local loading on one photomask were studied and kinetic equations were proposed showing excellent agreement with experimental results. Cr and MoSi etch CD movements versus local load on one photomask were also investigated. It was found that load effects on Cr and MoSi etch CD movements could be controlled in opposite directions leading to a compensation strategy proposal for MoSi optimization, instead of using a point-to-point 3σ as the optimization parameter (objective function). By using this compensation method, the final MoSi CD uniformity of 100–110 nm technology node photomasks (∼30% load) was in the range of 8.5–10.1 nm (3σ). This final CD uniformity was obtained on both of the photomask etch tools tested.

A procedure for calibrating a resist model iteratively adjusts appropriate parameters until the simulations of the model match the experimental data. The tunable parameters may include the shape of the illuminator, the geometry and transmittance/phase of the mask, light source and scanner-related parameters that affect imaging quality, resist process control and most importantly the physical/chemical factors in the resist model. The resist model can be accurately calibrated by measuring critical dimensions (CD) of a focus-exposure matrix (FEM) and the technique has been demonstrated to be very successful in predicting lithographic performance. However, resist model calibration is more challenging in the low k1 (<0.3) regime because numerous uncertainties, such as mask and resist CD metrology errors, are becoming too large to be ignored. This study demonstrates a resist model calibration procedure for a 0.29 k1 process using a 6% halftone mask containing 2D brickwall patterns. The influence of different scanning electron microscopes (SEM) and their wafer metrology signal analysis algorithms on the accuracy of the resist model is evaluated. As an example of the metrology issue of the resist pattern, the treatment of a sidewall angle is demonstrated for the resist line ends where the contrast is relatively low. Additionally, the mask optical proximity correction (OPC) and corner rounding are considered in the calibration procedure that is based on captured SEM images. Accordingly, the average root-mean-square (RMS) error, which is the difference between simulated and experimental CDs, can be improved by considering the metrological issues. Moreover, a weighting method and a measured CD tolerance are proposed to handle the different CD variations of the various edge points of the wafer resist pattern. After the weighting method is implemented and the CD selection criteria applied, the RMS error can be further suppressed. Therefore, the resist CD and process window can be confidently evaluated using the accurately calibrated resist model. One of the examples simulates the sensitivity of the mask pattern error, which is helpful to specify the mask CD control.

Double patterning technology overlay errors result in critical dimension (CD) distortions, and CD nonuniformity leads to overlay errors, demanding increased critical dimension uniformity (CDU) and improved overlay control. Scatterometry techniques are used to characterize the CD uniformity, focus, and dose control. We present CDU and profile characterization for spacer double patterning structures by advanced scatterometry methods. Our results include normal incidence spectroscopic reflectometry (NISR) and spectroscopic ellipsometry (SE) characterization of CDU sensitivity in spacer double patterning stacks. We further show the results of spacer DP structures by NISR and SE measurements. Metrology comparisons at various process steps including litho, etch, and spacer, and validation of CDU and profile, are all benchmarked against traditional critical dimension scanning electron microscope measurements.

The measurement of resist image size using a finely focused ion beam (FIB) is described. Comparisons of FIB and scanning electron microscope (SEM) measurements of resist features are presented. The short penetration range of an ion relative to an electron is shown to offer fundamental advantages for critical dimension metrology. Ion beams can induce less dimensional change (damage) than SEMs during the measurement process, potentially yielding improved measurement precision. Resist cross sectioning using FIB is demonstrated, and the factors contributing to quality of the cross section are presented. An H2O gas assisted etching process for polymer resists has been developed and is shown to significantly improve the quality of resist cross sections.

Today, metrology toolsets report out more information than ever. This information applies not only to process performance but also metrology toolset and recipe performance through various diagnostic metrics. This is most evident on the Critical Dimension Scanning Electron Microscope (CD-SEM). Today state of the art CD-SEMs report out over 250 individual data points and several images per measurement. It is typical for a state of the art fab with numerous part numbers to generate at least 20TB of information over the course of a year on the CD-SEM fleet alone pushing metrology toolsets into the big data regime. Most of this comes from improvements in throughput, increased sampling and new data outputs relative to previous generations of tools. Oftentimes, these new data outputs are useful for helping to determine if the process, metrology recipe or tool is deviating from an ideal state. Many issues could be missed by singularly looking at the key process control metric like the bottom critical dimension (CD) or a small subset of this available information. By leveraging the entire data set the mean time to detect and finding the root cause of issues can be significantly reduced. In this paper a new data mining system is presented that achieves this goal. Examples are shown with a focus on the benefits realized using this new system which helps speed up development cycles of learning and reducing manufacturing cycle-time. This paper concludes discussing future directions to make this capability more effective.