Using multiple sources/detectors for high-throughput X-ray topography measurement

An apparatus for X-ray topography includes a source assembly, a detector assembly, a scanning assembly and a processor. The source assembly is configured to direct multiple X-ray beams so as to irradiate multiple respective regions on a sample, wherein the regions partially overlap one another along a first axis of the sample and are offset relative to one another along a second axis of the sample that is orthogonal to the first axis. The detector assembly is configured to detect the X-ray beams diffracted from the sample and to produce respective electrical signals in response to the detected X-ray beams. The scanning assembly is configured to move the sample relative to the source assembly and the detector assembly along the second axis. The processor is configured to identify defects in the sample by processing the electrical signals, which are produced by the detector assembly while the sample is moved.

FIELD OF THE INVENTION

The present invention relates generally to X-ray analysis, and particularly to methods and systems for X-ray analysis using multiple X-ray beams.

BACKGROUND OF THE INVENTION

Microelectronic samples, such as silicon wafers, may be damaged during shipping, handling or production. For example, mechanical damage may cause defects in the crystalline structure of the wafer. Various methods have been developed for detecting crystalline defects.

For example, U.S. Pat. No. 6,782,076, whose disclosure is incorporated herein by reference, describes an X-ray topographic system, comprising an X-ray generator producing a beam of X-rays impinging on a limited area of a sample such as a silicon wafer. A solid-state detector is positioned to intercept the diffracted beam after transmission through or reflection from the sample and produces a digital image of the area on which the X-rays impinge. Relative stepping motion between the sample and the X-ray generator produces a series of digital images, which are combined together. In optional embodiments, an X-ray optic is interposed to produce a parallel beam to avoid image doubling, or the effect of image doubling is removed by software.

U.S. Pat. No. 8,503,611, whose disclosure is incorporated herein by reference, describes an X-ray topography apparatus in which X-rays diffracted from a sample which is scanned with a linear X-ray, are detected by an X-ray detector to obtain a planar diffraction image. The X-ray detector is an imaging plate shaped as a cylinder and provided with a surface area that is larger than the sample, and the imaging plate is made to undergo α-rotation about the center axis of the cylindrical shape in coordination with scanning movement of the linear X-rays. The center axis of the cylindrical shape extends in a direction at a right angle with respect to the direction of the scanning movement of the linear X-rays.

U.S. Pat. No. 8,781,070, whose disclosure is incorporated herein by reference, describes an apparatus for inspection of a disk, which includes a crystalline material and has first and second sides. The apparatus includes an X-ray source, which is configured to direct a beam of X-rays to impinge on an area of the first side of the disk. An X-ray detector is positioned to receive and form input images of the X-rays that are diffracted from the area of the first side of the disk in a reflective mode. A motion assembly is configured to rotate the disk relative to the X-ray source and detector so that the area scans over a circumferential path in proximity to an edge of the disk. A processor is configured to process the input images formed by the X-ray detector along the circumferential path so as to generate a composite output image indicative of defects along the edge of the disk.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein provides an apparatus for X-ray topography including a source assembly, a detector assembly, a scanning assembly and a processor. The source assembly is configured to direct multiple X-ray beams so as to irradiate multiple respective regions on a sample, wherein the regions partially overlap one another along a first axis of the sample and are offset relative to one another along a second axis of the sample that is orthogonal to the first axis. The detector assembly is configured to detect the X-ray beams diffracted from the sample and to produce respective electrical signals in response to the detected X-ray beams. The scanning assembly is configured to move the sample relative to the source assembly and the detector assembly along the second axis. The processor is configured to identify defects in the sample by processing the electrical signals, which are produced by the detector assembly while the sample is moved.

In some embodiments, the apparatus includes respective slits that are located between the source assembly and the sample and are configured to form the multiple X-ray beams to have rectangular cross sections, such that the irradiated regions are rectangular. In other embodiments, the detector assembly includes an X-ray shield that is configured to prevent interference between detection of different X-ray beams. In yet other embodiments, the scanning assembly includes one or more clamps that are configured to support the sample mechanically during scanning, and the clamps include respective shields that are configured to block X-rays from being scattered from the clamps toward the detector assembly.

In an embodiment, the source assembly includes two or more X-ray sources. In another embodiment, the two or more X-ray sources are mounted in a staggered configuration, so as to produce the regions that partially overlap along the first axis of the sample and are offset relative to one another along the second axis of the sample. In yet another embodiment, the source assembly includes at least an X-ray source configured to produce an X-ray beam, and the apparatus further includes a beam splitter that is configured to split the X-ray beam into the multiple X-ray beams.

In some embodiments, the detector assembly includes two or more detectors that are mounted in a staggered configuration, so as to detect the X-ray beams diffracted from the regions that partially overlap along the first axis of the sample and are offset relative to one another along the second axis of the sample. In other embodiments, the detector assembly includes at least one detector, and the processor is configured to define on the at least one detector regions-of-interest corresponding to the X-ray beams diffracted from the respective regions on the sample.

There is additionally provided, in accordance with an embodiment of the present invention, a method for X-ray topography, including directing multiple X-ray beams, using a source assembly, so as to irradiate multiple respective regions on a sample, wherein the regions partially overlap one another along a first axis of the sample and are offset relative to one another along a second axis of the sample that is orthogonal to the first axis. The X-ray beams diffracted from the sample are detected by a detector assembly, and respective electrical signals are produced by the detector assembly in response to detecting the X-ray beams. The sample is moved, using a scanning assembly, relative to the source assembly and the detector assembly along the second axis. Defects in the sample are identified by processing the electrical signals, which are produced by the detector assembly while the sample is moved.

DETAILED DESCRIPTION OF EMBODIMENTS

Overview

X-ray diffraction imaging (XRDI), also known as X-ray topography, may be used to detect defects in a crystalline wafer based on analyzing the intensity of the X-rays diffracted from the wafer. In a conventional XRDI system, an X-ray source directs a beam of X-rays to impinge on an area of one side of the wafer. An X-ray detector, which is positioned at an appropriate angle on the other side of the wafer, receives the X-rays that pass through the wafer and are diffracted from the irradiated area so as to form a diffraction image of the area. Typically, the source and detector are positioned symmetrically, at equal elevation angles with respect to the wafer that are chosen so that the detector receives the Laue diffraction from vertical crystal planes (substantially perpendicular to the surface of the wafer). Alternatively, other angular arrangements can be used to image Laue diffractions from other planes of the crystal. When the irradiated area is defect-free, the X-rays diffract uniformly from the desired crystal plane. Defects in the irradiated area typically appear as a changes in the diffracted intensity, either an increase or decrease as compared to the intensity from the defect-free crystal, due to distortions in the crystal planes in the area under test.

Such distortions are typically caused by defects of different types, such as (micro) cracks, dislocations, precipitates, and slip-bands that may be present in the bulk of the wafer. Optical inspection systems are typically configured to probe the surface of the sample, and are typically unable to detect and measure defects of this sort. XRDI techniques, on the other hand, provide high resolution imaging and more thorough check the bulk of the wafer. X-ray topography, however, has seen little adoption by the semiconductor industry for crystal defect inspection due to typical slow measurement and the large size of the apparatus required for measuring large wafers.

Embodiments of the present invention that are described herein provide improved methods and systems for high throughput measurements of X-ray topography using multiple X-ray beams. In the disclosed embodiments, an XRDI system inspects a large area of a moving sample (e.g., wafer) using multiple simultaneous X-ray beams. The system comprises a source assembly that irradiates the sample with the multiple X-ray beams, and a detector assembly that detects the diffracted beams.

In some embodiments, the source assembly may comprise a single X-ray source wherein the beam from the source is divided into multiple beams. In other embodiments, the source assembly may comprise multiple X-ray sources, each source producing one or more beams.

In the description that follows, the axis along which the sample is moved relative to the source and detector assemblies is referred to as the X-axis. The axis of the sample plane that is perpendicular to the X-axis is referred to as the Y-axis. In the disclosed embodiments, the source assembly is configured to direct the X-ray beams to impinge on different (e.g., adjacent) regions of the sample, wherein adjacent X-ray beams are positioned so that the irradiated regions are offset from one another along the X-axis, and overlap one another along the Y-axis.

In some embodiments, the detector assembly is positioned on the opposite side of the wafer relative to the source assembly, and is configured to detect the X-ray beams that are diffracted from the wafer, typically in a transmission geometry. In an embodiment, the assembly detector comprises a single detector. In other embodiments, the detector assembly may comprise multiple detectors that are typically arranged in a symmetrical configuration with respect to their corresponding sources.

In an embodiment, each beam passes through a respective slit located between the X-ray source assembly and the wafer, so as to generate a linear-shaped beam (e.g., a beam having a cross-section of a long and narrow rectangle) impinging on the wafer. A scanning assembly (e.g., a scanning stage) is configured to support the wafer and to move the wafer with respect to the X-ray source and detector assemblies, so as to scan the wafer with the linear X-ray beams, and thus, to generate a stripe of the X-rays to be detected by each of the corresponding detectors.

By applying this scanning scheme and configurations of source and detector assemblies, adjacent X-ray beams will typically cover overlapping stripes along the scanning direction using a staggered pattern arrangement of the detectors so as not to interfere with one another. In an embodiment, a processor is configured to receive electrical signals from the detectors, to analyze the signals so as to stitch the scanned stripes, and to generate a composite output image that is indicative of possible defects in the wafer.

In some cases, stray X-ray beams from an X-ray source may be scattered from materials comprised in the system and may interfere with signals of interest diffracted from the wafer. The use of multiple X-ray beams may increase the amount of such undesired X-ray radiation. In case of multiple detectors in the detector assembly, the detector assembly may comprise a shield, which is configured to pass each diffracted X-ray to the appropriate detector and to block stray scattered X-ray radiation.

Typically, the scanning stage comprises clamps that mechanically support the sample during scanning. In an embodiment, the clamps are shielded so as to block X-rays that may be scattered from the clamps toward the detectors.

The disclosed techniques provide high-throughput XRDI inspection using multiple X-ray beams that cover overlapping stripes, and at the same time eliminate cross-talk (i.e., unwanted scattering of X-rays into a detector from a source intended for another detector) between adjacent beams. The disclosed techniques may allow efficient scanning of the entire wafer, and thus may be used to improve the quality and reliability of semiconductor devices by detecting and possibly eliminating hidden defects, such as crystalline defects, from the bulk of the wafer.

System Description

FIG. 1is a schematic side view of a system20for X-ray diffraction imaging (XRDI), in accordance with embodiments of the present invention. Aspects of system20are described in detail in the above-mentioned U.S. Pat. No. 8,781,070. System20is arranged to inspect a semiconductor wafer22(or any other suitable sample), for example in order to identify faults in a crystalline structure of the wafer, possibly created during fabrication, using methods described hereinbelow. The terms “sample” and “wafer” are used interchangeably in the present disclosure.

System20typically comprises a source assembly24that may comprise a single excitation source or multiple excitation sources, such as X-ray tubes25A-25D (shown inFIG. 2), driven by one or more high-voltage power supply units (PSU)26, for example one PSU26powers one tube (e.g., tube25A), or any other suitable configuration as is known in the art. For example, four tubes25A-25D may be used to illuminate an entire 300 mm wafer when the distance between assembly24and wafer22is less than 1 meter. When inspecting a 450 mm wafer, the number of tubes in assembly24may be increased, for example, to six.

In some embodiments, the type of tubes in assembly24may be a low-power microfocus type (less than 100 μm spot-size at a less than 100 W) or mid-power normal focus tubes (typically 1 mm spot-size at 2-3 kW). In other embodiments, assembly24may comprise one or more high-power microfocus tubes (typically 50-100 μm spot-size at 2-3 kW) such as a liquid metal jet tube, or any alternative suitable tube. Each tube in assembly24comprises an anode, which is typically made of molybdenum and operates at 50 kV to produce X-rays capable of penetrating wafer22. Alternatively, other anode materials such as silver may also be used, depending on the application. Each tube in assembly24emits X-rays having a suitable energy range and power flux into X-ray optics (not shown). For each of the tubes, an associated motorized slit30is adjusted so as to shape beam28from the tube to have a cross-section of a long and narrow rectangle. Slit30is made of an X-ray opaque material. The position and size of slit30can be adjusted so as to adapt the X-ray beam divergence and spatial extent as appropriate.

In some embodiments, system20comprises a computer-controlled mechanical assembly, positioned between the tube and sample (not shown), for automatically switching a collimating crystal (not shown) in/out of the X-ray beam so as to increase the system resolution in certain applications. Passing beam28through the collimating crystal produces a substantially parallel and monochromatic beam, but reduces the intensity of the X-ray beam. The usage of the crystal can be switched on and off to allow switching between a high-intensity mode, and a low-noise mode without manual intervention. This capability allows the option for providing high-resolution rocking curves on the same system, which may be used by substrate manufacturers as a quality measure of the sample. The X-ray tubes, slits and optional crystal assemblies are mounted on a motorized rotation stage42(denoted source stage) with the axis of rotation centered at the wafer surface (not shown). Stage42is controlled by a processing unit38, which may comprise a general-purpose computer that runs a suitable control software.

Stage42allows to adjust beam28to a desired angle with respect to the sample surface so as to inspect different diffraction planes of the crystal structure of wafer22. In addition, stage42may be used for fine adjustment of an incidence angle in the vicinity of the selected diffraction plane prior to measurement, so as to compensate for local deformations of the wafer. When the collimating crystals are inserted between assembly24and wafer22, stage42is configured to maintain the diffraction condition of the collimated beams in the event of wafer deformation, which causes a long-range change in the diffraction angle across the wafer.

In an embodiment, wafer22is mounted on a movable platform, such as an X-Y-φ stage40, which enables moving the sample with respect to the X-ray beam in the X and Y directions, as well as applying azimuthal rotation φ about an axis perpendicular to the surface of the sample. Without loss of generality, the terms “X direction” and “X-axis” refer to the scan direction, i.e., the axis along which wafer22is scanned. The terms “Y direction” and “Y-axis” refer to the axis on the plane of the wafer that is orthogonal to the scan direction, as shown in detail inFIG. 3.

One of the linear axes (Y) may be reduced in range compared to the main scanning axis (X) or removed completely since the beam height spans a large fraction of the wafer diameter, or possibly the entire wafer or more. Moving stage40can be controlled by stepper motors, servo motors, or some combination thereof, which may be controlled, for example, by a processor running suitable motion-control software (not shown). The wafer may be moved along the X-axis in a series of small, discrete steps (step scanning) or at a constant speed (continuous scanning).

A detector assembly32is configured to detect the X-rays diffracted from wafer22. A beam-stopper44made from an X-ray opaque material is located between wafer22and detector assembly32, and is configured to occlude the directly transmitted beam from irradiating assembly32. In addition, a first detector of assembly32(e.g., detector33A shown inFIG. 3below) should receive diffracted radiation solely from its corresponding first tube (e.g., tube25A) of assembly24. Beam-stopper44is configured to block undesired diffracted beams from irradiating detector33A as a result of stray radiation from an adjacent second X-ray tube, such as tube25B.

Detectors33A-33D are arranged in a staggered pattern with overlapping regions between the individual detectors (as shown inFIGS. 3 and 5) so as cover the wafer without gaps. Each detector in assembly32is a two-dimensional (2D) position-sensitive X-ray camera that is adapted to measure the X-rays diffracted through wafer22according to Laue geometry, as a function of the detector position with respect to the surface of the wafer. In some embodiments, detector assembly32may comprise one or more charge-coupled device (CCD) or complimentary metal-oxide semiconductor (CMOS) cameras featuring X-ray sensitive scintillator screens in the case of step scanning. In other embodiments, such as during continuous scanning, one or more time delay and integration (TDI) X-ray cameras, such as those manufactured by Hamamatsu Photonics (Japan) or by Teledyne DALSA (Waterloo, Ontario, Canada), may be used to increase signal-to-noise ratio (SNR) at high scanning speeds.

A review detector46is typically an x-ray sensitive CCD or a CMOS detector. Detector46is typically used for selected area imaging of crystalline defects at high-spatial resolution, e.g. 10 μm, that may have been located through the X-ray inspection at high throughput or from some external input, such as the coordinates supplied from an optical defect detection apparatus.

Additionally, the system may comprise an optical microscope (not shown) for visual inspection of surface defects or fiducials and for navigation on the wafer.

The irradiated region of the sample emits diffracted X-rays, which are captured by one of the detectors. Responsively to the captured X-rays, the detectors generate electrical signals, which are conveyed to signal processing unit38. Unit38comprises a processor34(will be described in detail below), which is configured to process the electrical signals, and an interface36for communicating the electrical signals from detector assembly32to processor34.

Processor34is configured to acquire data from the detectors and to determine a diffraction intensity image of the X-ray photons captured by the detectors. Processor34typically comprises a general-purpose computer, which performs these functions under the control of suitable software. The software is configured for detector control, data acquisition and data analysis, and may be downloaded to the processor in electronic form, over a network, for example, or it may alternatively be provided on tangible media, such as optical, magnetic or electronic memory media.

FIG. 2is a schematic side view of system20for XRDI measurement, in accordance with embodiments of the present invention. The side view perspective ofFIG. 2is orthogonal to the side view perspective shown inFIG. 1.

Four tubes25A-25D are arranged in a staggered pattern so as to produce X-ray beams that cover overlapping regions. For example, the four tubes may be arranged in two pairs with an offset between the pairs. Tubes25A and25C represent one pair, which is in front of the plane presented inFIG. 2, and tubes25B and25D represent the second pair, which is located on the rear of theFIG. 2plane. This arrangement can be seen in three-dimensions inFIG. 3. Beams28are formed by slits30to have a rectangular cross-section. The long axis of the rectangular cross-section is shown inFIG. 2and the short axis is shown inFIG. 1.

FIG. 3is a schematic illustration of system20for XRDI measurement, in accordance with an embodiment of the present invention.FIG. 3is a three-dimensional illustration of the system described inFIGS. 1 and 2above. Four tubes25A-25D are arranged with an offset in the X direction and are aligned in the Y direction. In some embodiments, the four corresponding detectors33A-33D are arranged in a similar way, each detector facing its corresponding tube. In alternative embodiments, the detector assembly may comprise any suitable number of detectors that are able to detect all diffracted X-rays from wafer22and to eliminate cross-talk, such as a single detector that comprises virtual regions of interest defined by processor34.

When stage40moves along wafer22in the X direction, the actual overlap between the regions irradiated by adjacent beams28occurs due to the staggered pattern arrangement of the detectors. This property is shown inFIG. 4. Wafer22is typically placed by a handling robot (not shown) on stage40, which comprises three moving tables; for example a lower table for the X axis, a middle plate for the Y axis, and an upper plate for a rotation axis. All three plates are actually frames (no material in the center) so as to allow beam28to imping on the lower surface of wafer22. The upper plate comprises clamps48so as to provide mechanical support to wafer22as will be described inFIG. 6.

Each X-ray tube24emits X-ray beam28, which passes through wafer22and diffracted from a selected crystallographic plane under test (and thus, at a given angle related to the plane), from the upper surface of wafer22, through beam-stopper44, into the corresponding detectors.

FIG. 4is a schematic illustration of a scanning scheme that can be used to scan wafer22, in accordance with an embodiment of the present invention. As explained above, tubes25A-25D are arranged in two pairs with an offset in the X direction and with an overlap in the Y direction. Tubes25A-25D emit respective X-ray beams that are shaped by slits30and thus irradiate rectangular regions31A-31D on the wafer surface. The four X-ray beams are diffracted from wafer22towards respective detectors in assembly32.

This configuration allows an overlap during scanning, but ensures that radiation from a given point on wafer22at a given time is detected only by the appropriate detector. In other words, adjacent detectors do not collect XRDI signals from the same point on the wafer at the same time. For example, when the wafer is scanned along the X-axis, irradiated regions31A and31B form diffraction stripes27A and27B, respectively. An overlap area29is comprised in both stripes27A and27B and thus, scanned twice, but at different times, first by region31B and later by region31A.

This offset in time and space is managed, for example, by a software program, which is configured to construct the image based on the location in space of the diffraction stripe for each combination of tube and its respective detector. In an embodiment, this image construction can be done after completing the measurement over the entire wafer. In another embodiment, the image from each camera can be constructed as each stripe is collected by the camera using “on-the-fly stitching” techniques. Using this approach, the whole wafer may be imaged with a single sweep across the wafer.

Scattered X-Rays and Interference Between X-Ray Tubes and Detectors

FIG. 5is a schematic illustration of detector assembly32, in accordance with an embodiment of the present invention. Four detectors33A-33D are arranged in two pairs with offset in the X direction and with overlap in the Y direction. One of the pairs comprises detectors33A and33C and the other pair comprises detectors33B and33D. During wafer scanning, the beams are diffracted from wafer22toward their respective detectors so as to form diffraction stripes. The detector output signals are processed by the software running on processor34, which constructs an image from all or some of the stripes. For example, tube25A creates region31A, which is shaped by slit30and impinges on wafer22. Detector33B receives region31B as diffracted from wafer22and processor34creates stripe27B. The same flow is conducted at all tubes and detectors and all the output stripes (and thus overlaps, such as overlap29) are processed by processor so as to create an image, which indicates possible defects on wafer22.

The X-ray beams are typically polychromatic, and thus may diffract from the illuminated portion of wafer22in any direction. A diffracted beam that is addressed to a given detector may be also detected by adjacent detectors as interference that may cause image degradation, including additional “nuisance lines” and intensity variations. In addition, interaction between the beam and the various elements of system20may create scattered X-ray radiation that may interfere with signals of interest scattered from the wafer. The use of multiple X-ray tubes may increase the amount of undesired X-ray radiation emitted from adjacent tubes or scattered from elements of system20. Means for protecting from such cross-talk are described herein.

In some embodiments, shields43,45and47are attached to detector assembly32so as to block cross-talk radiation originated from interfering sources. The shields are typically made from an X-ray opaque material. For example, shield45is located between detectors33A and33C so as to block cross talk between tube25A and detector33C, and between tube25C and detector33A. Shield43is located between the two pairs of detectors so as to block cross-talk radiation, for example, between tube25D and detector33C. Likewise, shield47blocks, for example, possible cross-talk between tube25B and detector33D.

FIG. 6is a schematic illustration of clamping accessories for positioning wafer22, in accordance with an embodiment of the present invention. During the measurement process, in order to accurately detecting defects and their locations, wafer22must be kept at an accurate position with respect to assemblies24and32. The upper plate of stage40typically comprises clamps48, which are configured to hold wafer22stationary on stage40while the stage moves during scanning.

When wafer22is measured in a transmission geometry by multiple beams, one or more the beams may interact with the materials that comprise clamps48. The clamps may scatter stray X-ray radiation that may interfere with signals of interest that are scattered from the wafer, and thus, may degrade the measurement quality.

In some embodiments, a labyrinth arrangement of shielding50made from an X-ray opaque material is provided around clamps48so as to allow exposure of the entire wafer to the incident X-ray radiation, and yet, to prevent scattering from the supporting clamps to appear within the image. In other embodiments, the clamps may comprise an X-ray opaque material, and in yet other embodiments, the mechanical shape and size of the clamps may be chosen to substantially reduce the X-ray radiation scattered from the clamps.

The examples ofFIGS. 1-6refer to a specific configuration of X-ray system20. This configuration, however, is chosen purely for the sake of conceptual clarity. In alternative embodiments, the disclosed techniques can be used, mutatis mutandis, in various other types of XRDI systems or analyzing modules, comprising any suitable excitation source, power supplies, focusing optics and detection system. For example, the source assembly may comprise a single source that emits a single X-ray beam into a beam splitter that splits the beam into multiple X-ray beams. In case of multiple sources and multiple detectors, the number of sources and detectors may be different. Moreover, a single source may be associated with multiple detectors, and a single detector may be associated with multiple sources.