Critical Dimension Small Angle X-ray Research Articles

X-ray-based overlay metrology using reciprocal space slicing analysis

Overlay serves as the pivotal performance indicator for lithography tools, and its prompt and precise measurement significantly underpins the process yield control. At present, diffraction-based overlay metrology employing optical wavelengths encounters constraints in terms of measurement sensitivity. When transitioning to x-ray wavelengths, the critical-dimension small-angle x-ray scattering (CDSAXS) method for nanostructure characterization necessitates reciprocal space mapping (RSM) and inverse problem solving, resulting in substantial throughput constraints. In this work, we propose an x-ray-based overlay metrology using reciprocal space slicing analysis (RSS), yielding high-precision overlay measurement at one single angle of incidence (AOI). Moreover, we examine the robustness of the proposed method against errors stemming from overlay target grating fabrication and measurement processes, substantiating its efficacy as a novel x-ray-based overlay metrology and unveiling the potential application of x-ray-based techniques within the realm of integrated circuit metrology.

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

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

Inline metrology of high aspect ratio hole tilt and center line shift using small-angle x-ray scattering

High aspect ratio (HAR) structures found in three-dimensional nand memory structures have unique process control challenges. The etch used to fabricate channel holes several microns deep with aspect ratios beyond 50:1 is a particularly challenging process that requires exquisitely accurate and precise control. It is critical to carefully analyze multiple aspects of the etch process, such as hole profile, tilt, uniformity, and quality during development and production. X-ray critical dimension (XCD) metrology, which is also known as critical dimension small-angle x-ray scattering, is a powerful technique that can provide valuable insights on the arrangement, shape, and size of periodic arrays of HAR features. XCD is capable of fast, non-destructive measurements in the cell-area of production wafers, making XCD ideal for in-line metrology. Through several case studies, we will show that XCD can be used to accurately and precisely determine key properties of holes etched into hard mask, multilayer oxide/nitride film stacks and slit trenches. We show that the measurement of hole and slit tilt can be achieved without the aid of a structural model using a Fast Tilt methodology that provides sub-nanometer precision. Measurements were performed across several production wafers to determine the etch uniformity and quality. Particular attention was given at the edge of the wafers to account for large variations observed. In addition, we used a detailed physical model to characterize the HAR structures beyond linear tilt. This approach provides a more complete picture of the etch quality.

X-ray Metrology for the Semiconductor Industry Tutorial.

The semiconductor industry is in need of new, in-line dimensional metrology methods with higherspatial resolution for characterizing their next generation nanodevices. The purpose of this short course is to train the semiconductor industry on the NIST-developed critical dimension small angle X-ray scattering (CDSAXS) method. The topics will include both data processing and instrumentation. The short course will also provide an opportunity for discussion of the requirements for CDSAXS and the necessary improvements in X-ray source technology. Expected audience include semiconductor manufacturers, equipment manufacturers, and component manufacturers. The presentations were made at “X-ray Metrology for the Semiconductor Industry” short course at the National Institute of Standards and Technology on Aug. 25, 2016.

Methodology for evaluating the information distribution in small angle scattering from periodic nanostructures

Optimizing the extraction of information from x-ray measurements while minimizing exposure time is an important area of research in a variety of fields. The semiconductor industry is reaching a point where the traditional optical metrologies need to be augmented in order to better resolve the critical dimensions of structures with feature sizes below 10 nm. Critical dimension small angle x-ray scattering (CDSAXS) is one measurement technique that is capable of characterizing detailed features of periodic nanostructures. As currently implemented, the measurement utilizes the combined scattering from up to 60 different angles. Reducing the number of angles would dramatically improve the feasibility of CDSAXS for implementation in a fabrication setting, but currently there are no clear guidelines as to which angles provide the most information to minimize the uncertainty in the shape of the target structure while maximizing the throughput. In order to develop guidelines for optimizing the angle selection, simulation studies were conducted on a wide variety of structures with subsets of the full angular range to identify which angles minimized the overall shape uncertainty. Analyzing sets of two angle pairs (including all combinations between 0 deg and 60 deg) provides guidance on which angles best constrain the samples. For select samples, higher numbers of angles were included to explore the impact of additional information on the model uncertainty. In general, low angles (<3 deg) best contributed to minimizing the line-width uncertainty, while higher angles near high curvature regions of the scattering profile best constrained the height of the structure. The minimum uncertainty was generally achieved with combinations of the two. This simulation approach can be used to minimize the number of angles measured on real samples and significantly reduce the measurement time.

Combining model-based measurement results of critical dimensions from multiple tools

Model-based measurement techniques use experimental data and simulations of the underlying physics to extract quantitative estimates of the measurands of a specimen based upon a parametric model of that specimen. The uncertainties of these estimates are based upon not only the uncertainties in the experimental data, but also the sensitivity of that data to the model parameters, and parametric correlations among those parameters. The combination of two or more model-based techniques as well as the Bayesian approach are shown to be optimal for obtaining the lowest possible uncertainties. As an example, using this form of hybrid metrology, state-of-the-art sub-14 nm wide lines from semiconductor manufacturing are measured using a combined regression from critical-dimension small-angle x-ray scattering and scanning electron microscopy that leads to lower uncertainties.

X-ray scattering critical dimensional metrology using a compact x-ray source for next generation semiconductor devices

Semiconductor devices continue to shrink in size with every generation. These ever smaller structures are challenging the resolution limits of current analytical and inline metrology tools. We will discuss the results of a study of critical dimension small angle x-ray scattering (CDSAXS) comparing the measured intensity from a laboratory source and a synchrotron to determine the improvements in compact x-ray source technology necessary to make CDSAXS a high throughput metrology method. We investigated finFET test structures with and without a high-k gate dielectric coating. The HfO2-based high-k gate dielectric substantially increased the scattering intensity. We found that single-angle laboratory source measurements of 15 min from HfO2-coated finFETs had sufficient scattering intensity to measure the higher order peaks necessary for obtaining high-resolution dimensional fits. Identical bare silicon finFETs required at least 2 h of exposure time for equivalent data quality. Using these structures, we measured the scattering efficiency and determined the required photon flux for next generation x-ray sources to make an inline CDSAXS tool high throughput.

Advancing X-ray scattering metrology using inverse genetic algorithms.

We compare the speed and effectiveness of two genetic optimization algorithms to the results of statistical sampling via a Markov chain Monte Carlo algorithm to find which is the most robust method for determining real space structure in periodic gratings measured using critical dimension small angle X-ray scattering. Both a covariance matrix adaptation evolutionary strategy and differential evolution algorithm are implemented and compared using various objective functions. The algorithms and objective functions are used to minimize differences between diffraction simulations and measured diffraction data. These simulations are parameterized with an electron density model known to roughly correspond to the real space structure of our nanogratings. The study shows that for X-ray scattering data, the covariance matrix adaptation coupled with a mean-absolute error log objective function is the most efficient combination of algorithm and goodness of fit criterion for finding structures with little foreknowledge about the underlying fine scale structure features of the nanograting.

Evaluation of the effect of data quality on the profile uncertainty of critical dimension small angle x-ray scattering

A line grating prepared via a self-aligned quadruple patterning method was measured using critical dimension small angle x-ray scattering. A Monte Carlo Markov chain algorithm was used to analyze the uncertainty of the model fit over subsets of the full angular range and for a time series with decreasing signal-to-noise in order to determine the effect of the data quality on the final profile shape uncertainty. These results show how the total measurement time can be reduced while maintaining satisfactory profile shape uncertainty. We found that the typical measurement conditions are highly oversampled and can be reduced considerably with only marginal effect on the shape uncertainty. A comparison is made between the synchrotron measurements and a laboratory system, demonstrating that both measurements result in similar structures.

Determining the shape and periodicity of nanostructures using small-angle X-ray scattering

The semiconductor industry is exploring new metrology techniques capable of meeting the future requirement to characterize three-dimensional structure where the critical dimensions are less than 10 nm. X-ray scattering techniques are one candidate owing to the sub-Å wavelengths which are sensitive to internal changes in electron density. Critical-dimension small-angle X-ray scattering (CDSAXS) has been shown to be capable of determining the average shape of a line grating. Here it is used to study a set of line gratings patternedviaa self-aligned multiple patterning process, which resulted in a set of mirrored lines, where the individual line shapes were asymmetric. The spacing between lines was systematically varied by sub-nm shifts. The model used to simulate the scattering was developed in stages of increasing complexity in order to justify the large number of parameters included. Comparisons between the models at different stages of development demonstrate that the measurement can determine differences in line shapes within the superlattice. The shape and spacing between lines within a given set were determined to sub-nm accuracy. This demonstrates the potential for CDSAXS as a high-resolution nanostructure metrology tool.

Scanning electron microscope measurement of width and shape of 10nm patterned lines using a JMONSEL-modeled library.

The width and shape of 10nm to 12 nm wide lithographically patterned SiO2 lines were measured in the scanning electron microscope by fitting the measured intensity vs. position to a physics-based model in which the lines' widths and shapes are parameters. The approximately 32 nm pitch sample was patterned at Intel using a state-of-the-art pitch quartering process. Their narrow widths and asymmetrical shapes are representative of near-future generation transistor gates. These pose a challenge: the narrowness because electrons landing near one edge may scatter out of the other, so that the intensity profile at each edge becomes width-dependent, and the asymmetry because the shape requires more parameters to describe and measure. Modeling was performed by JMONSEL (Java Monte Carlo Simulation of Secondary Electrons), which produces a predicted yield vs. position for a given sample shape and composition. The simulator produces a library of predicted profiles for varying sample geometry. Shape parameter values are adjusted until interpolation of the library with those values best matches the measured image. Profiles thereby determined agreed with those determined by transmission electron microscopy and critical dimension small-angle x-ray scattering to better than 1 nm.

Assessment of critical dimension small-angle x-ray scattering measurement approaches for FinFET fabrication process monitoring

We have used synchrotron-based critical dimension small-angle x-ray scattering (CD-SAXS) to monitor the impact of hydrogen annealing on the structural characteristics of silicon FinFET structures fabricated using self-aligned double patterning on both bulk silicon and silicon-on-insulator (SOI) substrates. H2 annealing under different conditions of temperature and gas pressure allowed us to vary the sidewall roughness and observe the response in the two metrology approaches. In the case of the simpler bulk Si FinFET structures, the CD-SAXS measurements of the critical dimensions are in substantive agreement with the top–down critical dimension scanning electron microscopy metrology. Corresponding characterizations on SOI-based FinFET structures showed less agreement, which is attributed to the more complex structural model required for SOI FinFET CD-SAXS modeling. Because sidewall roughness is an important factor in the performance characteristics of Si FinFETs, we have compared the results of roughness measurements using both critical dimension atomic force microscopy (CD-AFM) and CD-SAXS. The measurements yield similar estimates of sidewall roughness, although the CD-AFM values were typically larger than those generated by CD-SAXS. The reasons for these differences will be discussed.

Determination of the Internal Morphology of Nanostructures Patterned by Directed Self Assembly

The directed self-assembly (DSA) of block copolymers (BCP) is an emerging resolution enhancement tool that can multiply or subdivide the pitch of a lithographically defined chemical or topological pattern and is a resolution enhancement candidate to augment conventional lithography for patterning sub-20 nm features. Continuing the development of this technology will require an improved understanding of the polymer physics involved as well as experimental confirmation of the simulations used to guide the design process. Both of these endeavors would be greatly facilitated by a metrology, which is capable of probing the internal morphology of a DSA film. We have developed a new measurement technique, resonant critical-dimension small-angle X-ray scattering (res-CDSAXS), to evaluate the 3D buried features inside the film. This is an X-ray scattering measurement where the sample angle is varied to probe the 3D structure of the film, while resonant soft X-rays are used to enhance the scattering contrast. By measuring the same sample with both res-CDSAXS and traditional CDSAXS (with hard X-rays), we are able to demonstrate the dramatic improvement in scattering obtained through the use of resonant soft X-rays. Analysis of the reciprocal space map constructed from the res-CDSAXS measurements allowed us to reconstruct the complex buried features in DSA BCP films. We studied a series of DSA BCP films with varying template widths, and the internal morphologies for these samples were compared to the results of single chain in mean-field simulations. The measurements revealed a range of morphologies that occur with changing template width, including results that suggest the presence of mixed morphologies composed of both whole and necking lamella. The development of res-CDSAXS will enable a better understanding of the fundamental physics behind the formation of buried features in DSA BCP films.

Three-dimensional x-ray metrology for block copolymer lithography line-space patterns

We report on the development of a new measurement method, resonant critical-dimension small-angle x-ray scattering (res-CDSAXS), for the characterization of the buried structure of block copolymers (BCP) used in directed self assembly (DSA). We use resonant scattering at the carbon edge to enhance the contrast between the two polymer blocks and allow the determination of the three-dimensional shape of the native lamella in a line-space pattern by CDSAXS. We demonstrate the method by comparing the results from conventional CDSAXS to res- CDSAXS on a 1:1 DSA BCP sample with a nominal 50-nm pitch. The res- CDSAXS method provides substantially improved uncertainty in the fit of the line shape and allows the determination of the buried structure. © 2013 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.JMM.12.3.031103)

Characterization of correlated line edge roughness of nanoscale line gratings using small angle x-ray scattering

To meet the challenges in dimensional metrology as the minimum feature size in semiconductor devices approaches sub-35 nm length scales, we have been developing a nondestructive method using x rays termed critical dimension small angle x-ray scattering (SAXS). Its capacity to quantify the dimension of linewidth, pitch, line height, and sidewall angle of line gratings has been demonstrated. In this work, we have further extended its capabilities to quantify the correlated line-edge roughness (LER) and linewidth roughness (LWR). Model line grating patterns with controlled LER and LWR were prepared and measured using x ray, their results were analyzed with model calculations. The magnitude of LER/LWR deduced from x-ray results compares favorably with the scanning electron microscopy results obtained from the same samples. An apparent Debye–Waller factor, which can be deduced from the SAXS data without any detailed model-based calculations, is found to be a convenient parameter to quantify the amplitude of LER/LWR.

Real-Time Shape Evolution of Nanoimprinted Polymer Structures during Thermal Annealing

The real-time shape evolution of nanoimprinted polymer patterns is measured as a function of annealing time and temperature using critical dimension small-angle X-ray scattering (CD-SAXS). Periodicity, line width, line height, and sidewall angle are reported with nanometer resolution for parallel line/space patterns in poly(methyl methacrylate) (PMMA) both below and above the bulk glass transition temperature (T(G)). Heating these patterns below T(G) does not produce significant thermal expansion, at least to within the resolution of the measurement. However, above T(G) the fast rate of loss in pattern size at early times transitions to a reduced rate in longer time regimes. The time-dependent rate of polymer flow from the pattern into the underlying layer, termed pattern "melting", is consistent with a model of elastic recovery from stresses induced by the molding process.

Pattern fidelity in nanoimprinted films using critical dimension small angle x-ray scattering

The primary measure of process quality in nanoimprint lithography (NIL) is the fidelity of pattern transfer, comparing the dimensions of the imprinted pattern to those of the mold. Routine production of nanoscale patterns will require new metrologies capable of nondestructive dimensional measurements of both the mold and the pattern with subnanometer precision. In this work, a rapid, nondestructive technique termed critical dimension small angle x-ray scattering (CD-SAXS) is used to measure the cross sectional shape of both a pattern master, or mold, and the resulting imprinted films. CD-SAXS data are used to extract periodicity as well as pattern height, width, and sidewall angles. Films of varying materials are molded by thermal embossed NIL at temperatures both near and far from the bulk glass transition (TG). The polymer systems include a photoresist and two homopolymers. Our results indicate that molding at low temperatures (T-TG<40°C) produces small-aspect-ratio patterns that maintain periodicity to within a single nanometer, but feature large sidewall angles. While the observed pattern height does not reach that of the mold until very large imprinting temperatures (T-TG70°C), the pattern width of the mold is accurately transferred for T-TG>30°C.