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Projects Nanometer Scale Dimensional Metrology Wafer Level AFM Metrology for Critical Dimension Measurements Wafer Level SEM Metrology for Critical Dimension Measurements Photomask Dimensional Metrology Overlay Instrument and Wafer Target Designs Date created: |
Precision Engineering Division Program
Nanomanufacturing Metrology Program Meeting Today’s Industry Needs Program Manager: Richard Silver Annual FTEs: 11 NIST Staff 6.5 Guest Researchers/Contractors 17.5 total FTEs Challenge: The increasing pace of technological change in nanomanufacturing makes accurate dimensional metrology critical to innovation and realization of quality products. Industries currently manufacturing at the nanoscale are challenged by the need for infrastructural metrology and standards to enable process control, enhance product quality and to bring innovative new products to market. The goal is to advance U.S. leadership in nanomanufacturing through the development of physical standards traceable to the International System of Units (SI) and through the development of physics-based models and calibration techniques which enable accurate determination of dimensional information. Overview The Nanomanufacturing Metrology (NanoMet) Program focuses on the development of solutions to dimensional metrology needs of the nanomanufacturing industry by providing dimensional standards, calibrations and infrastructural metrology for measurements in the nanoscale having subnanometer precision. Semiconductor manufacturing, data storage, and photonics industries are currently the main customers. The semiconductor manufacturing industry is a major current focus with critical dimensions of mass produced product in the <50 nm region today. The program remains at the forefront of rapidly evolving metrology needs through active interaction with industry leaders and industry consortia (such as SEMATECH); and by strong participation and leadership in industrial roadmaps (such as the International Technology Roadmap for Semiconductors). The program also serves other industries (e.g., flat panel display or microelectromechanical (MEMS) fabrication), government agencies (e.g., DARPA or Department of Energy Labs), and academic researchers that need traceable dimensional metrology at the sub-micrometer scale. The goal is to provide the calibration techniques and artifacts which realize the Système International d'unités (SI) definition of the meter to meet the most demanding nanoscale industrial needs. The NanoMet Program implements its strategy through a series of objectives, each containing a set of time-sequenced projects. The projects in these objectives will create and deliver solutions to significant dimensional metrology problems, typically leading to new calibrations or standard reference materials. The NanoMet Program continually evaluates new metrology opportunities to assist industry and reviews existing projects for consistency with program objectives. An ongoing goal is to refine calibration methods and standard artifacts in order to reduce the uncertainty in the realization of the SI definition of the meter as it is supplied to industry. This project relies on continual improvement in instrument performance and consistency of measurements between instruments and methods. In addition the work defined within this program addresses several of the identified and validated U. S. Measurement System Measurement Needs (MN). The specific MNs and page numbers from Appendix B of the Report are found at the end of this document. Why NIST? Nanomanufacturing represents one of the crucial markets for advanced manufacturing and innovative product development. Accuracy and precision have become essential to nanotechnology manufacturing. Requirements for measurement standards, instrumentation and standardized calibration techniques are acute when manufacturing in the nanometer domain. NIST has extensive expertise in dimensional metrology at the nanometer scale. NIST has been working with nanomanufacturing industries, such as leading edge semiconductor or MEMs device manufacturers, and has been integrally involved in the development of standards, instrumentation, calibration methods, and measurement science for the advancement of nanotechnology manufacturing. This is a manufacturing arena in which NIST uniquely possesses the expertise and instrumentation to facilitate U.S. leadership. Program Objectives Objective 1: Provide industry with accurate and timely dimensional scale metrology at the nanoscale to enhance U.S. productivity and innovation. Fundamental to this objective is meeting the current and anticipated one and two dimensional scale calibration and the SIdimensional traceability requirements for improved yield and product qualification for the constantly evolving nanomanufacturing industry. The fundamental basis for length metrology is traceability to the definition of the meter in the International System of Units (SI). The challenge is to realize this definition in practice through qualified metrology instruments and calibrated standards and reference materials that form the basis for dissemination. The fundamental goals for this objective are to provide accessibility to the SI unit of length to the nanomanufacturing industries, such as semiconductor manufacturing, data storage, and photonics, by means of reference metrology instruments and calibration standards that are compatible with industry metrology instrumentation. This is required for the industry to remain on the technology roadmaps as device structures are reduced to less than 18 nm within the next 10 years. These nanometer scale manufacturing requirements drive push the limits of length scale measurement. This program identifies the current limits to length scale measurement as shown by uncertainty budgets and develops new technology to reduce them. Projects Nanomanfacturing Metrology Program Project 1.1: Optical Linescale Metrology Project Overview The cornerstone of traceability for the SI Unit of length, the meter is the Linescale Interferometer (LSI) which provides World-class one-dimensional scale calibrations traceable to the SI as verified by international intercomparisons. The Linescale Interferometer (LSI) provides one-dimensional scale calibrations up to one meter in length. Its performance and accuracy is validated through international comparisons, most recently the “NANO 3: Line Scale Standards” comparison, performed under the auspices of the International Bureau of Weights and Measures (BIPM). The LSI provides calibrations for customers in the nanomanufacturing industry, such as major metrology tool suppliers. It is also the traceability link for most of the other metrology instruments within NIST and the NanoMet Program. NanoMet work will be focused on further increasing the reliability of the system, and further minimizing measurement uncertainties. A current LSI research goal is aimed at reducing its measurement uncertainties from (5.2 nm + 5x10-8 * length) to one half that value to obtain higher accuracy measurements specifically to meet those calibration requirements for advanced semiconductor product specifications. The NanoMet Program will develop two-dimensional scale measurements and standard wafer and photomask artifacts that provide two dimensional accuracy of nominally 20 nm over a 15 cm field. Currently the program has calibrated photomask grids and future work will be focused on developing a two-dimensional wafer calibration capability and reducing the uncertainty of the photomask grid measurements. Two-dimensional scale measurements are provided by a Nikon 5i wafer and photomask metrology instrument which has been highly characterized by the NanoMet Program. This instrument operates over a 150 mm by 150 mm range. This instrument uses an optical microscope to locate sample fiducials and has laser-interferometer guided motion stages. For qualification, the scale calibration is traceable to a reference artifact measured on the LSI. Uniformity and orthogonality of the two axes are characterized through self-calibration methods, e.g., repeated measurements of the same artifact with rotations, translations, and reversal. Deliverables and Intermediate Milestones:
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Nanomanfacturing Metrology Program Project 1.2: Nanometer Scale Dimensional Metrology Project Overview Nanometer scale metrology for instrument calibration applications such as atomic force microscopy or scanning electron microscope calibrations will be focused on 50 μm by 50 μm range scale measurements using the NIST-developed calibrated atomic force microscope (C-AFM) will provide accurate height metrology for surface roughness and dimension metrology with picometer accuracy. The C-AFM will be used especially for the smaller length scales needed for AFM and SEM two-dimensional scale calibrations. The C-AFM is a custom-built instrument that has built-in metrology on all three axes of motion traceable to the SI meter through the 633 nm wavelength of a He-Ne laser. The C-AFM also allows vertical scale calibrations, i.e., step height. The instrument, now in its fourth generation, will provide reference measurements for commercial standards suppliers and will be used to participate in international comparisons for nanometrology. Deliverables and Intermediate Milestones:
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Nanomanfacturing Metrology Program Objective 2: Provide accurate critical dimension (linewidth metrology) traceable to the meter. Significant manufacturing metrology challenges exist beyond scale calibration for the accurate determination of the size width and length of physical features. Measurement of linewidth or critical dimension (CD) continues to be one of the most fundamental dimensional metrology needs in the semiconductor and nanomanufacturing industries. Semiconductor manufacturers refer to this continually decreasing measurement limit as critical dimension (CD) metrology. The critical dimension size and tolerance decreases as technology progresses. The demand is so complex and ubiquitous that no single metrology technique can provide the entire solution. Three major techniques will be used within the linewidth metrology objective: 1) critical-dimension AFM (CD-AFM), 2) SEM, and 3) optical microscopy. At this scale, the largest component of measurement uncertainty is usually associated with the interaction of the probe (e.g., mechanical stylus, photons, or a beam of charged particles) with the specimen. The principal challenge in linewidth metrology is to accurately define the position of the physical edge of a feature within the metrology instrument response profile. Each instrument exhibits its own characteristic response profile due to the bandwidth of the electronics, signal collected or probe used. The measurand also contributes to the response as well. The result is an increased linewidth measurement uncertainty. This uncertainty can easily become an undesirable value when dealing with nanometer structures and NIST is continually striving to decrease this measurement limitation. In order to reduce linewidth measurement uncertainty, new measurement techniques producing sharper edge profiles will be developed and adopted. But, ultimately no method has been able to fully achieve the required resolution, and it becomes necessary to assign the physical edge to a definite position within the broadened signal. This in turn requires understanding and modeling the physical process that produce the broadening so that it can be compensated, and as a result electron beam, scanned probe and optical modeling have become major components of this project. NIST expertise will focus on improving the effectiveness of the instrument response profile by using shorter-wavelength light in optical microscopy (OM); smaller beam focus spots in scanning electron microscopy (SEM); or sharper, better characterized tips in CD atomic force microscopy (CD-AFM). Such improvements are required to advance this metrology. This contribution to the measurement uncertainty is a greater fraction of the total as feature sizes decrease below 50 nm. Consequently, physics-based modeling is required to understand and overcome this challenge. Validated models for each of the measurement techniques are critical to accurate measurements. In addition, experimental intercomparisons between the various measurement methods are required to further validate the models. To provide traceability, NanoMet will develop custom reference measurement instruments (SEM, SPM and OM) which have been highly engineered to provide traceability through the incorporation of the most accurate laser interferometry. Standard Reference Material (SRM) standards will be certified with these reference measurement instruments to accurately calibrate production instrumentation. Project 2.1: Wafer Level AFM Metrology for Critical Dimension Measurements
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Q4/FY09 |
Develop accurate measurement protocols and an uncertainty statement for prototype polysilicon linewidth standards, and perform new linewidth measurements of RM 2059 and the chrome on quartz photomask that will be used for the BIPM intercomparison. |
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Q4/FY08 |
Measure the uniformity of new SCCDRM prototype specimens during |
Q3/FY11 |
Extend the NIST-developed tip modeling methods for blind reconstruction of tip dimensions. This is important due to the difficulty of obtaining accurately known tip characterizers. (Completion by 3rd Quarter 2011) |
Begin by 4th Quarter 2008; Completion by 4th Quarter 2011 |
Serve as the pilot laboratory for a preliminary key comparison of the nanoscale linewidth measurement capabilities of national measurement institutes around the world, sponsored by the Bureau International des Poids et Mesures (BIPM) in Paris. |
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Nanomanfacturing Metrology Program
Project Overview
In the semiconductor manufacturing industry, feature linewidths for process monitoring and control are routinely measured using CD-SEMs. NIST will develop a reference SEM for calibration of transfer artifacts that are measured using the same imaging physics. The metrology for this reference instrument will be based on a laser interferometer stage with sub-nanometer resolution. This work will be supported by accurate Monte Carlo simulation modeling (MONSEL) of the incident measurement beam-specimen interactions to determine the instrument response profile from various line-edge shapes. MONSEL supports secondary electron generation in vacuum (standard SEM mode) and gaseous environment (Variable Pressure SEM Mode) and utilizes a “model-based library (MBL)” method pioneered by this project to provide edge detection. NIST will use the MBL to quickly determine the line-edge shape from the image intensity profile through the use of a library of images that have been modeled previously and can be quickly scanned and interpolated to determine the best match.
Deliverables and Intermediate Milestones:
Q2/FY09 |
Demonstrate traceable linewidth measurement on amorphous silicon patterns on 300 mm wafers using the new reference metrology SEM with the nominal linewidth of be less than 100 nm with standard uncertainties at or better than 2 nm. |
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Q4/FY11 |
Improve the NIST model of secondary electron generation including many-body effects (e.g., screening) and exhibit better agreement with NIST’s database of inelastic mean free paths (standard reference database 71). |
Q2/FY10 |
Incorporate a gas-scattering model into MONSEL that will allow the modeling of imaging in variable pressure SEMs and enable the measurement of insulating samples with improved accuracy. |
Q3/FY10 |
Collaborate with researchers at the Illinois Institute of Technology to develop methods to speed up MONSEL calculations through hardware acceleration, using the computer’s graphics processor, to implement higher speed, reduced time library generation. |
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Nanomanfacturing Metrology Program
Project 2.3: Photomask Dimensional Metrology
Anticipated Completion Date Q4/2011
Project Overview
The ideal technique for photomask linewidth measurements is the transmission optical UV microscope because optical transmission imaging produces relatively simple high contrast images with a well-defined baseline. Also, transmission optical microscopy emulates the way the photomask is used during the wafer exposure process, thus enhancing the effectiveness of the standard. The NIST scanning UV microscope uses on-axis sampling to reduce aberration and distortion effects. The position of the scanning stage is measured by a laser interferometer. NIST will develop accurate physics-based modeling to deduce the object dimensions from the microscope image. Image modeling will also extend the limits of optical metrology to feature sizes well below the wavelength of light used, as demonstrated by Dianna Nyyssonen at NIST in the 1980s when critical dimensions were micrometer in size. The challenge is to measure state of the art chromium photomasks and phase shifting masks with sub-0.25 micrometer structures. NIST will also work on a next generation photomask standard. This standard will likely contain, in addition to isolated lines, spaces, and pitch patterns from ~100nm wide, a variable set line/space arrays as well as large scale 2-dimensional features in response to requests from the machine vision industry. This project will also develop an essential suite of techniques to calibrate and optimize the optical system errors. This new area of research is critical to improving model to experiment agreement in model-based linewidth measurements.
Deliverables and Intermediate Milestones:
Q2/FY09 |
Collaborate directly with representatives of the photomask industry and the SEMATECH Mask Advisory Steering Council to determine their requirements in the next generation NIST photomask linewidth standard. |
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Q3/FY09 |
Complete the assembly and recalibration of the optical transmission photomask instrument. Characterize all optical components to ensure adequate instrument performance. |
Q2/FY09 |
Complete a bilateral intercomparison on photomask linewidth standards between NIST and Physicalishe Technicshe Bundesanstalt to ensure agreement between the leading two international suppliers of photomask calibration standards. |
Q4/FY11 |
Improve optical image modeling capability to better agree with actual AFM images for confirmation of optical measurements. Date?? |
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Incorporate the new specimen stage into the UV. |
Q4/FY11 |
Complete Nano 1, the international intercomparison on photomask linewidth standards that will be piloted by NIST, to ensure measurement agreement between those international standards labs engaged in photomask linewidth metrology. |
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Nanomanfacturing Metrology Program
Objective 3: Provide accurate overlay and registration metrology with subnanometer accuracy to enable manufacturing of the most advanced, fastest semiconductor devices. Developing advanced position metrology, techniques and standards for overlay and registration are critical to the semiconductor industry and will become more important for other nanomanufacturing in the future. The relative overlay of features from different manufacturing process levels is considered one of the most demanding measurement requirements due to the direct effects on device performance. NIST will develop infrastructural metrology for the accurate placement (registration) of multiple layers with sub-nanometer accuracy. Since optical techniques are often best suited to these tasks, research into novel optical overlay and registration instrumentation and target structures is essential. In semiconductor applications, development of new instrument innovations and target designs will be jointly developed with the leading industrial consortium, SEMATECH. These structures enable improved overlay resolution in a smaller, in-chip format of sub-resolution features. It is critical to develop high-resolution optical overlay techniques that are extensible for several manufacturing generations. The progression of technology demands that new generations of SRMs meeting new production challenges must continually be developed to ensure the progress of the industry.
NIST will work jointly with the leading industrial consortium, SEMATECH, to provide improved overlay instrument resolution in a smaller, in-chip format of sub-resolution features that are extensible for a number of manufacturing generations. SRMs for overlay calibration will also be designed and fabricated. The collaborative implementation of new target designs, instrument optimization and modeling, and calibration techniques includes new reticle design and wafer fabrication in the continuing effort undertaken jointly with key industrial partners.
The overlay metrology project is an internationally recognized effort with the goal of developing techniques and targets for improved overlay metrology, primarily for the semiconductor industry. The project has relied on close collaboration with industry leaders in optical tool development and users of overlay metrology tool sets as well as with SEMATECH. NIST will continue to work with the industrial partners to ensure the progress of the industry
Project Overview
A significant challenge for the semiconductor manufacturing industry is to develop advanced metrology techniques for overlay and registration to enable the continued long term advance of device performance the stringent ITRS industry guidelines. The recent advance of double patterning techniques, which directly couple overlay and line measurements resulting in reduced critical feature dimensions, has made this metrology more urgent still. Currently, optical techniques are most widely used for this kind of metrology, and recent advances in these high resolution optical techniques have made an entire class of new overlay target designs possible. NIST will work in a technical leadership position with SEMATECH to patent new overlay structures. The NanoMet Program will produce calibrated overlay SRMs wafers designed jointly with the industry. NIST will also continue the development of new target designs, instrument optimization and modeling, and calibration techniques.
The overlay and registration methods developed within this project have been adopted by leading industrial users as well as the SEMATECH overlay metrology advisory group, and have led to the development of both hardware and analysis methods adopted by leading commercial overlay metrology systems. As we enter the next generation of semiconductor manufacturing, the overlay metrology project will work closely with the industry to enable the adoption of these new techniques as they continue to be advanced.
Deliverables and Intermediate Milestones:
Q1/FY11 |
Develop a new metrology instrument with state-of-the-art uncertainties for overlay measurements and work with SEMATECH to evaluate alternative techniques for overlay metrology. |
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Q4/FY11 |
Implement and overlay reference metrology system and validate optical overlay measurements using optical, particle beam, and AFM probes.. |
Q4/FY10 |
Design, fabricate and calibrate the next generation of overlay wafers standards. |
Q3/FY10 |
Develop physics-based overlay models which accurately and comprehensively model the sample probe interactions and the instrumentation and model the image forming process accurately for uncertainty determination. |
Q3/FY09 |
Model and evaluate the new super-target overlay designs in collaboration with SEMATECH and fabricate test wafers for use by the industry and test target performance. |
Q1/FY10 |
Transfer to industry optical overlay techniques including optical alignment, instrument characterization techniques and improved optical configurations. |
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