Metrology system and method for metrology system

Abstract

This document describes a method and system for locating a sample and characterizing an X-ray beam incident on the sample in a transmission small-angle X-ray scattering (T-SAXS) metrology system. A sample positioning system vertically positions a wafer and actively positions the wafer relative to the X-ray illumination beam in six degrees of freedom without attenuating transmitted radiation. In some embodiments, a cylindrical shielding element is scanned across the illumination beam to precisely locate the beam center while measuring the detected intensity of the transmission flux. In some other embodiments, a periodic calibration target is used to precisely locate the beam center. The periodic calibration target includes one or more spatially defined regions having different periodic structures that diffract the X-ray illumination light into dissimilar, measurable diffraction patterns.

Description

[Technical Field] The described embodiments relate to X-ray metrology systems and methods, and more specifically to methods and systems for improving measurement accuracy. [Previous Technology] Semiconductor devices (such as logic and memory devices) are typically manufactured through a series of processing steps applied to a sample. These processing steps form the various features and multiple structural layers of the semiconductor device. For example, lithography is a semiconductor process that involves creating a pattern on a semiconductor wafer. Additional examples of semiconductor processes include (but are not limited to) chemical mechanical polishing, etching, deposition, and ion implantation. Multiple semiconductor devices can be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices. Metrology procedures are used at various steps during a semiconductor manufacturing process to detect defects on the wafer to improve yield. Several metrology-based techniques, including scattering and reflection measurement implementations and associated analytical algorithms, are typically used to characterize the critical dimensions, film thickness, composition, and other parameters of nanoscale structures. Traditionally, scattering measurements are performed on targets composed of thin films and / or repeating periodic structures to determine critical size. 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 towards smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometries and materials with diverse physical properties pose characterization challenges. For example, modern memory structures are often high-aspect-ratio three-dimensional structures that make it difficult for optical radiation to penetrate to the underlying layers. Optical metrology tools using infrared to visible light can penetrate many translucent material layers, but the longer wavelengths that provide good penetration depth do not offer sufficient sensitivity to small anomalies. Furthermore, the increased number of parameters required to characterize complex structures (e.g., FinFETs) leads to increased parameter correlation. Therefore, the parameters of a characterized target often cannot reliably escape from available measurements. In one instance, an attempt was made to use longer wavelengths (e.g., near-infrared) to overcome the penetration problem in 3D flash devices that utilize polycrystalline silicon as one of the alternating materials in a stack. However, as illumination propagates deeper into the film stack, the mirror-like structure of the 3D flash inherently reduces light intensity. This leads to a loss of sensitivity and correlation issues at greater depths. In this case, SCD was only able to successfully extract the metrological dimensions of a simplified group exhibiting either high sensitivity or low correlation. In another example, opaque high-k materials are increasingly being used in modern semiconductor structures. Optical radiation typically cannot penetrate layers constructed from such materials. Therefore, measurements using thin-film scattering measurement tools such as ellipsometers or reflectometers are becoming increasingly challenging. In response to these challenges, more sophisticated optical metrology tools have been developed. For example, tools with multiple illumination angles, shorter illumination wavelengths, wider illumination wavelength ranges, and more complete information acquisition of self-reflection signals have been developed (e.g., measuring multiple Mueller matrix elements in addition to more familiar reflectivity or ellipsometric measurements). However, these approaches have not reliably overcome the fundamental challenges associated with the measurement and metrological applications of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures using opaque materials) (e.g., line edge roughness and linewidth roughness measurements). Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) can achieve atomic resolution, but they can only probe the surface of the sample. Furthermore, AFM and STM require long scan times. Scanning electron microscopy (SEM) achieves intermediate resolution but cannot penetrate structures to sufficient depth. Therefore, it does not effectively characterize high aspect ratio holes. Additionally, the charging required for the sample negatively impacts imaging performance. X-ray reflectometers also suffer from penetration problems that limit their effectiveness when measuring high aspect ratio structures. To overcome the penetration depth problem, traditional imaging techniques (such as TEM and SEM) are combined with destructive sample preparation techniques (such as focused ion beam (FIB) processing, ion milling, blanket etching, or selective etching). For example, transmission electron microscopy (TEM) achieves high-resolution alignment and can probe arbitrary depths, but TEM techniques require destructive segmentation of the sample. Several iterations of material removal and measurement typically provide the information needed for key metrological parameters across three-dimensional structures. However, these techniques require sample destruction and lengthy procedures. The complexity and time required to complete these types of measurements are attributed to the significant inaccuracies introduced by the drift of etching and metrological steps. In addition, these techniques require several iterations to introduce alignment errors. The transmission small-angle X-ray scattering (T-SAXS) system using photons at a hard X-ray energy level (>15 keV) has shown promise for solving challenging measurement applications. The following cases describe various applications of SAXS technology to critical dimension (CD-SAXS) and overlay (OVL-SAXS) measurements: 1) U.S. Patent No. 7,929,667, entitled "High-brightness X-ray metrology," by Zhuang and Fielden; 2) U.S. Patent Publication No. 2014 / 0019097, entitled "Model Building And Analysis Engine For Combined X-Ray And Optical Metrology," by Bakeman, Shchegrov, Zhao, and Tan; 3) U.S. Patent Publication No. 2015 / 0117610, entitled "Methods and Apparatus For Measuring Semiconductor Device Overlay Using X-Ray Metrology," by Veldman, Bakeman, Shchegrov, and Mieher; and 4) U.S. Patent Publication No. 2015 / 0117610, entitled "Measurement System Optimization For X-Ray Based," by Hench, Shchegrov, and Bakeman. The aforementioned patent documents are: 5) U.S. Patent Publication No. 2016 / 0202193 entitled "X-ray Metrology For High Aspect Ratio Structures" by Dziura, Gellineau, and Shchegrov; and 6) U.S. Patent Publication No. 2017 / 0167862 entitled "X-ray Metrology For High Aspect Ratio Structures" by Gellineau, Dziura, Hench, Veldman, and Zalubovsky; and 7) U.S. Patent Publication No. 2018 / 0106735 entitled "Full Beam Metrology for X-Ray Scatterometry Systems" by Gellineau, Dziura, Hench, Veldman, and Zalubovsky. These patent documents are assigned to KLA-Tencor Corporation of Milpitas, California, USA. SAXS has also been applied to material characterization 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). Studies of CD-SAXS metrology for semiconductor structures are also described in the scientific literature. Most research teams have adopted high-brightness X-ray synchrotron sources, which are unsuitable for use in a semiconductor manufacturing facility due to their enormous size and cost. An example of such a system is described in the article "Intercomparison between optical and x-ray scatterometry measurements of FinFET structures" by Lemaillet, Germer, Kline, et al., Proc. SPIE, Vol. 8681, p. 86810Q (2013). Recently, a team at the National Institute of Standards and Technology (NIST) has begun research using a compact and bright X-ray source similar to that described in U.S. Patent No. 7,929,667. This research is described in the article "X-ray scattering critical dimensional metrology using a compact x-ray source for next generation semiconductor devices" in J. Micro / Nanolith.MEMS MOEMS 16(1), 014001 (January–March 2017). A metrology system must be used to calibrate and align the interaction between the X-ray beam and the target to ensure effective measurement. Illustrative characterization includes: precisely positioning the peak intensity of the X-ray beam onto the target; measuring the X-ray beam intensity distribution; and identifying the boundaries of the X-ray beam such that a specific percentage of the beam flux lies outside the boundaries. Illustrative alignment includes the alignment of the X-ray beam with an optical vision system, and the alignment of the X-ray beam with specific mechanical features of an instrument (e.g., a wafer rotation axis). Generally speaking, a wafer is navigated in the path of an X-ray beam by optical measurement of alignment marks placed across the wafer using an optical microscope. To ensure accurate navigation of a specific target relative to the X-ray beam, the beam profile needs to be measured in the coordinates of the optical microscope used to measure the alignment marks. In some instances, an optical microscope is aligned with a blade, and the blade is aligned with an X-ray beam. The characteristic features of an X-ray beam with a conventional blade are due to the complexity arising from the translucency of the blade material illuminated by X-ray radiation near the edge of the blade. For example, tungsten has a beam attenuation length of approximately 8.4 micrometers when illuminated by photons at an energy level of 20 keV. At this length, transmittance decreases by ~1 / e (e=2.718). For a blade shaped at an angle of 30 degrees, the length of the wedge corresponding to a height of 8.4 micrometers is approximately 14.5 micrometers. This simple estimate of the uncertainty in the position of a blade during an X-ray beam scan illustrates that the translucency of the blade is limited when the required alignment accuracy is less than a few micrometers (e.g., less than 10 micrometers). In some other instances, the X-ray beam profile is characterized by a high-resolution X-ray camera positioned relative to a point on the X-ray beam (e.g., the focal point of a focusing optics). In these instances, the beam profile is measured using the high-resolution X-ray camera, and the measured coordinates of the beam are transmitted to an optical microscope used to navigate the wafer along the path of the X-ray beam. Unfortunately, the errors associated with transmitting the measured coordinates from the X-ray camera to the optical microscope are significant and exceed the required navigation accuracy. Furthermore, characterization of an X-ray beam by means of an X-ray camera or blade is inherently indirect and does not provide quantitative data on the photon flux incident on the target or the photon contamination in adjacent areas. Future metrology applications face metrological challenges due to increasingly smaller resolution requirements, multi-parameter correlations, increasingly complex geometrologies (including high aspect ratio structures), and the growing use of opaque materials. Existing methods for X-ray tool alignment and target navigation are limited to an accuracy of approximately 10 to 20 micrometers. These methods cannot provide sufficient accuracy for locating and measuring small (~50 micrometers) metrological targets within an X-ray beam for semiconductor metrology applications. Therefore, improved alignment and calibration methods and systems for X-ray beams in SAXS systems are desired to meet the placement requirements of advanced manufacturing nodes. [Summary of the Invention] This document describes a method and system for positioning a sample and characterizing an X-ray beam incident on the sample in a transmission small-angle X-ray scattering (T-SAXS) metrology system. Practical T-SAXS measurements in a semiconductor manufacturing environment require measurements over a large angle of incidence and azimuth relative to the surface of a sample (e.g., a semiconductor wafer) having a small beam spot size (e.g., less than 50 micrometers across the effective illumination point). Accurate positioning of the wafer and characterization of the beam size and shape are required to achieve a small measurement cell size. Furthermore, this document presents calibration for accurately positioning the illumination beam on the desired target area on the surface of a semiconductor wafer across the entire range of angles of incidence and azimuth. In one embodiment, a metrology tool includes a sample positioning system configured to vertically position a wafer (i.e., the plane of the wafer surface is approximately aligned with the gravity vector) and actively position the wafer relative to an illumination beam in six degrees of freedom. The sample positioning system supports the wafer at its edges, allowing the illumination beam to penetrate the wafer at any location within its active region without remounting. By vertically supporting the wafer at its edges, gravity-induced indentation of the wafer is effectively mitigated. In a further state, a balancer statically balances the rotating mass of the sample positioning system such that the center of gravity of the rotating mass is approximately aligned with its axis of rotation. In some embodiments, three sensors are mounted on the sample positioning system to measure the distance of the back side of the wafer relative to the sample positioning system. In this way, wafer warpage is measured and compensated by moving the wafer using a tip-tilt Z-stage. In another embodiment, a SAXS metrology system employs at least one beam-blocking calibration target to position an X-ray illumination beam relative to a sample positioning system. The beam-blocking calibration target comprises at least one marker and a cylindrical blocking element. A alignment camera is used to position the marker in the coordinates of the sample positioning system. The position of the marker relative to the cylindrical blocking element is known in advance (e.g., with an accuracy of less than 200 nanometers). Therefore, the position of the cylindrical blocking element in the coordinates of the sample positioning system is easily determined by a direct coordinate transformation. The cylindrical blocking element is scanned across the illumination beam while measuring the detected intensity of the transmission flux. The center of the illumination beam is precisely positioned relative to the cylindrical blocking element based on the measured intensity. Since the position of the cylindrical blocking element in the coordinates of the sample positioning system is known, the center of the illumination beam in the coordinates of the sample positioning system is precisely positioned by a simple coordinate transformation. In some instances, a beam-blocking calibration target is used to calibrate the incident position of the illumination beam relative to the sample positioning system. In other instances, a beam-blocking calibration target is used to align the rotation axis of the stage reference frame relative to the illumination beam at the incident point of the illumination beam and a wafer. In another embodiment, a SAXS metrology system employs at least one periodic calibration target to position an X-ray illumination beam relative to the sample positioning system. Each periodic calibration target comprises one or more spatially defined regions having different periodic structures that diffract the X-ray illumination beam into dissimilar diffraction patterns measurable by one of the SAXS metrology systems described herein. Additionally, each periodic calibration target includes one or more markers that can be read by an optical microscope to position the periodic calibration target relative to the sample positioning system with high alignment accuracy (e.g., 0.5 micrometers or less). Each spatially defined region has spatially well-defined boundary lines. The positions of these boundary lines relative to the markers are known to have high accuracy (e.g., 0.2 micrometers or less) in one or more dimensions. In another scenario, the precise alignment of the illumination beam with the plane of the wafer surface is determined based on the interaction between the illumination beam and two or more beam-blocking calibration targets, such as those measured by an X-ray detector. In another embodiment, the precise alignment of the rotation axis with a calibration target mark in the plane of the wafer's surface is determined based on an image of a mark collected by an alignment camera mounted on a lateral alignment stage. In another embodiment, the shape of the wafer surface in the Z direction is mapped using an alignment camera, an optical proximity sensor, a capacitive proximity sensor, an interferometric sensor, or any other suitable proximity sensor. In some instances, the wafer surface is mapped onto the front side (i.e., the patterned side) of the wafer. In other instances, if the wafer thickness is sufficiently uniform, well-modeled, or measured in situ or pre-measured, the wafer surface is mapped onto the back side (i.e., the unpatterned side) of the wafer. The foregoing description is the subject of the invention and therefore must contain simplifications, generalizations, and omissions of details; thus, those skilled in the art will understand that the description is illustrative only and not limiting. Other variations, features, and advantages of the apparatus and / or procedures described herein will become apparent from the non-limiting detailed description set forth herein. [Simplified Explanation of the Diagram] Figure 1 is a diagram illustrating a metrology system 100 configured to perform calibration of various system parameters according to the methods described herein. Figure 2 depicts an end view of a beam-shaping slit mechanism 120 in a configuration. Figure 3 depicts one end view of the beam-shaping slit mechanism 120 in another configuration. Figure 4 depicts an X-ray illumination beam 116 incident on wafer 101 with a specific orientation, described by angles Φ and θ. Figure 5 is a schematic diagram of a sample positioning system 140, wherein the wafer stage is moved to a position where the illumination beam 116 is incident on the wafer 101. Figure 6 is a diagram of a sample positioning system 140 with additional details. Figure 7 depicts a beam blocking calibration target 190 in one embodiment. Figure 8A depicts a top view of an illumination beam 116 incident on wafer 101 as shown in Figure 5, wherein the rotation axis 153 intersects the illumination beam 116 at the point of incidence of the illumination beam 116 and wafer 101. Figure 8B depicts a top view of an illumination beam 116 incident on wafer 101 as depicted in Figure 5, wherein the rotation axis 153 is misaligned with the surface of wafer 101 in the Z direction. Figure 8C depicts a top view of an illumination beam 116 incident on wafer 101 as depicted in Figure 5, wherein the rotation axis 153 is offset from the illumination beam 116 in the X direction. Figure 9 illustrates a sample positioning system 140, in which the wafer stage is moved to a position where the illumination beam 116 is blocked by a cylindrical pin element 151. Figure 10 depicts a curve 170 illustrating the measured flux as a function of the relative position of a cylindrical pin with respect to the illumination beam 116. Figure 11 illustrates another diagram of a sample positioning system 140 that includes a periodic calibration target 171 positioned on wafer 101. Figure 12 depicts one embodiment of a periodic calibration target 210. Figure 13 depicts one embodiment of a periodic calibration target 220. Figure 14 depicts one embodiment of a periodic calibration target 230. Figure 15 depicts one embodiment of a periodic calibration target 240. Figure 16 depicts one embodiment of a periodic calibration target 250. Figure 17 depicts one embodiment of a periodic calibration target 260. Figure 18 depicts one embodiment of a periodic calibration target 270. Figures 19A and 19B depict a set of periodic calibration targets 290 and 295, each adapted to position an illumination beam relative to the periodic calibration target in one direction. Figure 20 depicts a periodic calibration target 280 containing markers 288 and 289 and one of seven different periodic regions 281 to 287 configured as a hexagonal pattern. Figure 21 is a diagram showing one of the components of the metering system 100 contained in a vacuum environment separated from the sample 101. Figure 22 is a diagram illustrating a model establishment and analysis engine 180 configured to analyze sample parameter values ​​based on T-SAXS data according to the method described herein. Figure 23 depicts a flowchart illustrating an exemplary method 300 for calibrating an angle of incidence offset value based on T-SAXS measurements of multiple angles of incidence and azimuth as described herein.

Implementation Method

Claims

1. A measurement system comprising: An X-ray illumination source configured to generate an X-ray illumination beam incident on a semiconductor wafer; a sample positioning system configured to actively control the position of a semiconductor wafer relative to the X-ray illumination beam in six degrees of freedom, wherein a vector normal to a surface of the wafer is substantially perpendicular to the direction of a gravitational force applied to the semiconductor wafer by gravity during measurement of the semiconductor wafer by the metrology system; an X-ray detector configured to detect a first amount of X-ray radiation from the semiconductor wafer in response to the incident X-ray illumination beam; and a computing system configured to determine a value of a parameter of interest for a structure characterized on the semiconductor wafer.

2. As in the metrology system of request item 1, the sample positioning system includes: A basic framework; A stage reference frame configured to rotate relative to the substrate frame about a rotation axis perpendicular to the illumination beam and generally parallel to the wafer surface; a wafer stage mounted to the stage reference frame, the wafer stage configured to position the wafer relative to the incident illumination beam at any desired location within an active region of the semiconductor wafer; a three-axis stage mounted to the wafer stage, the three-axis stage configured to move the semiconductor wafer in a direction generally aligned with the illumination beam and to rotate the semiconductor wafer about two orthogonal rotation axes also generally perpendicular to the illumination beam; and a rotating stage mounted to the three-axis stage, the rotating stage configured to rotate the wafer about an axis generally normal to the wafer surface.

3. The metrology system of claim 2, wherein the wafer stage and the triaxial stage are mechanically coupled by means of six mechanical contact points configured as a motion coupler.

4. As in the metrology system of request item 1, the sample positioning system includes: One or more sensors configured to measure the position of a back surface of a semiconductor wafer relative to a sample positioning system in a direction generally normal to one of the wafer surfaces; one or more sensors configured to measure the position of a front surface of a semiconductor wafer relative to a sample positioning system in a direction generally normal to one of the wafer surfaces; or a combination thereof.

5. The metrology system of claim 2, wherein the sample positioning system includes one or more edge gripper devices configured to mechanically couple the semiconductor wafer to the rotary stage at the edge of the semiconductor wafer.

6. The metrology system of claim 1, wherein the sample positioning system includes a rotary balancer mounted on the stage reference frame, wherein the center of mass of the stage reference frame configured to rotate about the rotation axis relative to the base frame is substantially aligned with the rotation axis.

7. The metering system of claim 1, further comprising: A first vacuum chamber that encloses a significant portion of an illumination beam path between the X-ray illumination source and the semiconductor wafer.

8. The metering system of claim 1, further comprising: A first vacuum chamber encloses a significant portion of a beam-focusing path between the semiconductor wafer and the X-ray detector.

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