Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. A number of metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
Traditionally, scatterometry critical dimension (SCR) measurements are performed on targets consisting of thin films and/or repeated periodic structures. During device fabrication, these films and periodic structures typically represent the actual device geometry and material structure or an intermediate design. As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. Optical metrology tools utilizing infrared to visible light can penetrate many layers of translucent materials, but longer wavelengths that provide good depth of penetration do not provide sufficient sensitivity to small anomalies. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements.
In one example, longer wavelengths (e.g. near infrared) have been employed in an attempt to overcome penetration issues for 3D FLASH devices that utilize polysilicon as one of the alternating materials in the stack. However, the mirror like structure of 3D FLASH intrinsically causes decreasing light intensity as the illumination propagates deeper into the film stack. This causes sensitivity loss and correlation issues at depth. In this scenario, SCD is only able to successfully extract a reduced set of metrology dimensions with high sensitivity and low correlation.
In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response to these challenges, more complex optical metrology tools have been developed. For example, tools with multiple angles of illumination, shorter illumination wavelengths, broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM) are able to achieve atomic resolution, but they can only probe the surface of the specimen. In addition, AFM and STM microscopes require long scanning times. Scanning electron microscopes (SEM) achieve intermediate resolution levels, but are unable to penetrate structures to sufficient depth. Thus, high-aspect ratio holes are not characterized well. In addition, the required charging of the specimen has an adverse effect on imaging performance. X-ray reflectometers also suffer from penetration issues that limit their effectiveness when measuring high aspect ratio structures.
To overcome penetration depth issues, traditional imaging techniques such as TEM, SEM etc., are employed with destructive sample preparation techniques such as focused ion beam (FIB) machining, ion milling, blanket or selective etching, etc. For example, transmission electron microscopes (TEM) achieve high resolution levels and are able to probe arbitrary depths, but TEM requires destructive sectioning of the specimen. Several iterations of material removal and measurement generally provide the information required to measure the critical metrology parameters throughout a three dimensional structure. But, these techniques require sample destruction and lengthy process times. The complexity and time to complete these types of measurements introduces large inaccuracies due to drift of etching and metrology steps. In addition, these techniques require numerous iterations which introduce registration errors.
Transmission, Small-Angle X-Ray Scatterometry (T-SAXS) systems employing photon at a hard X-ray energy level (>15 keV) have shown promise to address challenging measurement applications. Various aspects of the application of SAXS technology to the measurement of critical dimensions (CD-SAXS) and overlay (OVL-SAXS) are described in 1) U.S. Pat. No. 7,929,667 to Zhuang and Fielden, entitled “High-brightness X-ray metrology,” 2) U.S. Patent Publication No. 2014/0019097 by Bakeman, Shchegrov, Zhao, and Tan, entitled “Model Building And Analysis Engine For Combined X-Ray And Optical Metrology,” 3) U.S. Patent Publication No. 2015/0117610 by Veldman, Bakeman, Shchegrov, and Mieher, entitled “Methods and Apparatus For Measuring Semiconductor Device Overlay Using X-Ray Metrology,” 4) U.S. Patent Publication No. 2016/0202193 by Hench, Shchegrov, and Bakeman, entitled “Measurement System Optimization For X-Ray Based Metrology,” 5) U.S. Patent Publication No. 2017/0167862 by Dziura, Gellineau, and Shchegrov, entitled “X-ray Metrology For High Aspect Ratio Structures,” and 6) U.S. patent application Ser. No. 15/419,130 by Gellineau, Dziura, Hench, Veldman, and Zalubovsky, entitled “Full Beam Metrology for X-Ray Scatterometry Systems,” the content of each of these documents is incorporated herein by reference in its entirety. The aforementioned patent documents are assigned to KLA-Tencor Corporation, Milpitas, Calif. (USA). In addition, U.S. Pat. No. 9,606,073 to Mazor, et al. entitled “X-ray scatterometry apparatus” describes various aspects of the application of SAXS technology to semiconductor structures, the content of which is incorporated herein by reference in its entirety.
SAXS has also been applied to the characterization of materials and other non-semiconductor related applications. Exemplary systems have been commercialized by several companies, including Xenocs SAS (www.xenocs.com), Bruker Corporation (www.bruker.com), and Rigaku Corporation (www.rigaku.com/en). Both Bruker and Rigaku offer small angle x-ray scatterometry systems and wide angle x-ray scatterometery systems named “Nanostar” and “Nanopix”, respectively. These systems include adjustable sample-to-detector distance.
Research on CD-SAXS metrology of semiconductor structures is also described in scientific literature. Most research groups have employed high-brightness X-ray synchrotron sources which are not suitable for use in a semiconductor fabrication facility due to their immense size, cost, etc. One example of such a system is described in the article entitled “Intercomparison between optical and x-ray scatterometry measurements of FinFET structures” by Lemaillet, Germer, Kline et al., Proc. SPIE, v.8681, p. 86810Q (2013), the content of each of these documents is incorporated herein by reference in its entirety. More recently, a group at the National Institute of Standards and Technology (NIST) has initiated research employing compact and bright X-ray sources similar those described in U.S. Pat. No. 7,929,667. This research is described in an article entitled “X-ray scattering critical dimensional metrology using a compact x-ray source for next generation semiconductor devices,” J. Micro/Nanolith. MEMS MOEMS 16(1), 014001 (January-March 2017), the content of each of these documents is incorporated herein by reference in its entirety.
SAXS systems employ several different types of detectors including hybrid pixel photon counting detectors, charge integrating pixel array detectors, gaseous avalanche detectors, etc. Pixel sizes of available detectors range from about 50 micrometers to approximately 200 micrometers. Prototypes having 25 micrometer pixels are currently under development.
A substantial disadvantage of all prior SAXS architectures is the substantial size of the instrument required to measure typical semiconductor structures. Fine angular resolution is needed to resolve the diffraction image at the detector. Currently, this resolution is achieved by increasing the length of the instrument.
In one example, the “Nanostar” system manufactured by Bruker may be configured with a sample to detector distance of 1070 mm, a detector having a 68 micrometer pixel size, and q-space resolution of 5 e-3 Angstroms-1.
In semiconductor fabrication facilities, metrology and inspection tools must conform to relatively small footprint sizes to maximize the use of expensive cleanroom space and accommodate more tools for higher throughput. Hence, tool length of current SAXS systems must be reduced from current levels to be useful in a practical semiconductor manufacturing setting.
Furthermore, the resolution of hard X-ray detectors also limits the ability of current systems to reconstruct detailed profiles of semiconductor devices. For example, current vertical-NAND (VNAND) structures are fabricated with fundamental pitch lengths on the order of micrometers. Measurement of these structures with hard X-rays creates very fine diffraction patterns that cannot be resolved based on angular resolution alone without resorting to impractical tool lengths (e.g., ˜40m tool length). As such, this measurement application is infeasible with current tools.
In summary, current CD/OVL SAXS systems are not suitable for implementation in a production semiconductor fabrication facility due to excessively large footprint, limited resolution, and reliance on angular order separation. To further improve device performance, the semiconductor industry continues to focus on vertical integration, rather than lateral scaling. Thus, accurate measurement of complex, fully three dimensional structures is crucial to ensure viability and continued scaling improvements. Future metrology applications present challenges for metrology due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures including high aspect ratio structures, and increasing use of opaque materials. Thus, methods and systems for improved x-ray scatterometry measurements, including improved resolution and smaller footprint, are desired.