KR20230150944A - Discrimination of dislocation types and densities in semiconductor materials using cathodoluminescence measurements - Google Patents

Abstract

Cathodoluminescence microscopy and methods are used to identify and classify dislocations in semiconductor samples. At least two CL polarized images are acquired simultaneously from the sample. The images are added together to obtain the total intensity image. The normalized difference of the images is taken to obtain the degree of polarization (DOP) image. Total intensity and DOP images are compared to distinguish between edge dislocations and screw dislocations within the sample. Next, the edge dislocation density and screw dislocation density can be calculated.

Description

Distinguishing dislocation types and densities in semiconductor materials using cathodoluminescence measurements

Related Applications

This application claims the benefit of U.S. Provisional Application No. 63/121,752, filed December 4, 2020, and U.S. Provisional Application No. 17/537,422, filed November 29, 2021, the disclosures of which are incorporated herein by reference in their entirety. It is used.

Technology field

The present disclosure relates to scanning cathodoluminescence microscopes and, more specifically, to methods for scanning cathodoluminescence microscopes to enable discovering and classifying different defect types in hardware and semiconductor materials.

The applicant has previously disclosed new scanning cathodoluminescence microscopes, for example in PCT/EP2020/063093, the disclosure of which is incorporated herein by reference in its entirety. Microscopy discovers that when the beam of a scanning electron microscope (SEM) scans a sample, electrons interact with the sample, producing a variety of signals that can be detected and contain information about the sample's surface topography, structure, and composition. It operates based on The types of signals produced by SEM include secondary electrons (SE), backscattered electrons (BSE), characteristic or bremsstrahlung X-rays, optical, absorbed/induced current (EBAC/EBIC) and transmitted electrons. (TEM). The light emitted by the sample upon electron collision (defined as photons with energies in the range of approximately 0.1 to 10 eV) is called cathodoluminescence (CL). Cathodoluminescence measurements can be performed in a scanning electron microscope by scanning the electron microscope's highly focused electron beam probe across the surface of a sample and recording the cathodoluminescence signal intensity as a function of the electron beam position on the sample. Cathodoluminescence maps (also referred to herein as images) can be generated, which provide higher resolution spectroscopic information than wide field optical optical images acquired by optical microscopy. For purposes of this disclosure, the reader is assumed to be familiar with the disclosure cited above. For other disclosures of CL microscopes, the reader is referred to US Patent No. 3,845,305, US Publication Nos. 2013/0335817 and 2019/0103248, and French Patent No. 2173436.

The following summary of the disclosure is included to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such is not intended to specifically identify important or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below.

The aim of the present disclosure is to enable identification and classification of dislocations in a semiconductor sample according to type, for example screw, edge or mixed. Dislocations typically appear as contrasted spots in CL images, and the contrast is usually darker than the surrounding, dislocation-less material, but sometimes, especially in the case of alloys, for example InGaN alloys, brighter than the surrounding material ( See, for example, F. Massabuau et al, ‘Optical and structural properties of dislocations in InGaN’, J. Appl. Phys. 125, 165701 (2019). For simplicity, the spots induced by dislocations will henceforth be referred to as “dark spots”, but the present embodiments also show dark spots referred to as control spots, rather features visible in the CL that indicate the presence of dislocations. is not limited to According to disclosed aspects, observed defects are classified into edge dislocations and screw dislocations by observing strains in the sample at the locations of the defects. In practice, the strains around edge dislocations have mostly isobaric components, in contrast to screw dislocations, which have only shear components. Strain is observed by polarizing the CL emission and generating a degree of polarization (DOP) image using the normalized difference between images formed with vertically and horizontally polarized beams. The normalized difference can be obtained by calculating the ratio of the difference between the two polarized images to the sum of the two polarized images.

According to disclosed aspects, a CL microscope is fitted with a polarizing beam splitter cube and at least two photo detectors. The arrangement makes it possible to capture two polarization complementary images of the scanned area of the samples, the two images being uniquely registered both spatially and temporally, thus avoiding the need to perform alignment of the images and Eliminates artifacts introduced by time shifts or delays in the acquisition of dog images.

In disclosed embodiments, a cathodoluminescence microscope is provided for simultaneously generating two polarization complementary images of a scanned area of a semiconductor sample, the two images being uniquely registered both spatially and temporally, and the microscope comprising an electron source. , an electron beam column having a magnetic lens for focusing the electrons emitted from the electron source and thereby forming the electron beam, and a scanner for scanning the electron beam across the sample; an optical objective that collects CL light emitted from the sample in response to scanning of the electron beam and forms a light beam; a focusing lens, a polarizing beam splitter splitting the light beam into a first polarized beam and a second polarized beam, a first light detector receiving the first polarized beam and a second light detector receiving the second polarized beam. Imaging section containing; and a controller that forms two polarization complementary images from a first signal received from the first photo detector and a second signal received from the second photo detector.

The imaging section may further include an optical filter inserted upstream of the polarizing beam splitter. Additionally, the microscope may include a half mirror positioned upstream of a polarizing beam splitter, wherein the focusing lens may be positioned upstream or downstream of the half mirror; a third polarized beam that receives light reflected by the half mirror and is oriented at a polarization rotation angle of 45 degrees with respect to the first polarized beam, and a fourth polarization beam is oriented at a polarization rotation angle of 45 degrees with respect to the second polarized beam. a second polarizing beam splitter oriented to form a polarized beam; a third photodetector receiving a third polarized beam; and a fourth photo detector receiving the fourth polarized beam. The microscope may also include a housing attaching the polarizing beam splitter to the first and second photo detectors in fixed orientations relative to each other; and a rotation mechanism that rotates the housing along an axis aligned with the light beam. The disclosed embodiments are advantageous in that the simultaneous acquisition of two polarized cathodoluminescent emissions uniquely integrates the spatial and temporal alignment of the images. Embodiments also make it possible to distinguish between edge and screw dislocations.

According to disclosed aspects, there is provided a computer program stored on a storage device that, when executed by the computer, causes the computer to perform steps, the steps comprising: a first electrical signal corresponding to a polarized cathodoluminescent beam; and Receiving a second electrical signal corresponding to the polarized cathodoluminescent beam having a 90 degree polarization rotation; adding first and second electrical signals to generate an intensity image of the scanned area of the sample; Taking the normalized difference of the first and second electrical signals to generate a degree of polarization (DOP) image; determining the coordinates of the center point of each contrast spot appearing in the intensity image; For each of the coordinates, the corresponding region within the DOP image is inspected to determine the stress pattern in this region that fulfills the appropriate shape or intensity criteria, and the stress pattern is then edge compressed through a selected procedure from several available procedures. or classifying it as a screw. The above described procedure may be a simple intensity threshold for some materials as the screw dislocation simply does not appear at all in DOP images, partition variance calculations, pattern matching methods, or machine learning or any other AI inspired pattern recognition method.

According to further aspects, a method of operating a cathodoluminescence microscope to detect defects in a semiconductor sample is disclosed, the method comprising: scanning an area of the sample with an electron beam; collecting cathodoluminescent light emitted from the area during scanning and forming a light beam from the cathodoluminescent light; Passing the light beam through a polarizer beam splitter to obtain two polarized beams having a 90 degree polarization rotation relative to each other; using two photodetectors to simultaneously generate two electrical signals corresponding to two polarized beams; adding the two signals to form an intensity image of the region; Taking the normalized difference of the two signals to form a degree of polarization (DOP) image; For each contrast spot appearing in the intensity image, examining a corresponding region within the DOP image and, whenever an indication of an appropriate strain field appears within the region within the DOP image, classifying the corresponding contrast spot as an edge dislocation.

Taking the normalized difference may include calculating a ratio of the difference between the first and second electrical signals to the sum of the first and second electrical signals. Inspecting the DOP image may include calculating a representative value of the DOP image within the area and comparing the representative value to a preset threshold.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain and illustrate the principles of the invention. The drawings are intended to illustrate key features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict all features of actual embodiments nor the relative dimensions of the elements shown, and are not drawn to scale.
Other features and advantages of the present invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the accompanying drawings.
- Figure 1 is a schematic cross-sectional view of the lower part of a cathodoluminescence scanning electron microscope for implementing the embodiments disclosed herein.
- Figure 2 is a simplified schematic diagram of the image acquisition section of a cathodoluminescence microscope according to one embodiment.
- Figure 3a is an intensity image showing latent defects in the sample, while Figure 3b is a DOP image corresponding to the area imaged in Figure 3a.
- Figure 4 is a flow chart showing a process for classifying defects according to one embodiment.
- Figure 5 is a simplified schematic diagram of the image acquisition section of a cathodoluminescence microscope according to another embodiment.
- Figure 6 is a schematic diagram of the image acquisition section of a cathodoluminescence microscope according to another embodiment.
- Figure 7 is a schematic diagram of the image acquisition section of a cathodoluminescence microscope according to a further embodiment.

Some embodiments of the invention are described in more detail below with reference to the accompanying drawings. Identical functional and structural elements appearing in different drawings may be assigned identical reference numerals.

Embodiments of the cathodoluminescence scanning electron microscope and operating methods of the present invention will now be described with reference to the drawings. Different embodiments or combinations thereof may be used for different applications or to achieve different benefits. Depending on the result achieved, the different features disclosed herein may be used partially or fully, alone or in combination with other features, balancing the advantages with the requirements and constraints. Accordingly, certain features, elements or benefits will be emphasized with reference to different embodiments, but are not limited to the disclosed embodiments. That is, the features, elements, and benefits disclosed herein are not limited to the embodiment in which they are described, but may be “mixed and matched” with other features and incorporated into other embodiments, although such features are not explicitly stated herein. is not explained by

Figure 1 shows the lower part of a CL microscope in cross-section, which is more fully described in PCT/EP2020/063093 cited above. As shown in Figure 1, the microscope includes an electron column 41 housed within a vacuum enclosure 10, and an imaging section 42 that is typically in an atmospheric environment. The integrated microscope shown in FIG. 1 can generate electron beam images, light beam images, cathodoluminescence (CL) images, and CL spectrophotometric images. The imaged CL emissions can be correlated to the structure and quality of the sample's materials at the nanoscale. CL data can reveal material stresses, impurities, crystallographic, and subsurface defects that are not visible using other imaging modes. Importantly, CL imaging is a non-destructive method of examining samples.

The electron column comprises an electron source 1, such as a thermionic or field emission source, which emits electrons. The emitted electrons are formed into an electron beam 9 by means of various particle optical elements, such as an electromagnetic lens 5', an electromagnetic objective 5, and aperture disks (sometimes referred to as stops) 6. . Note that any of the aperture disks 6 can function as an electrostatic lens by application of a potential. In a known manner, the coils 11 are provided for generating a magnetic field, in this figure a substantially horizontal magnetic field, at the level of the optical axis z of the electromagnetic objective 5 . The majority of the magnetic field may be located at the level of the output or exit aperture 13, or may instead be outside that area between the lens and the sample.

The purpose of the magnetic field is to generate a converging electron beam 9 that can be focused onto the surface of the sample 7. In this example, the electron beam 9 generated by the electron emitter 1 propagates downward from the top of the figure. The electron beam span can be modified by an array of condensers, such as lenses 5', so that it can be divergent, collimated or convergent. A condenser may be placed below the electron emitter. The electron beam typically has a width of several millimeters, for example in the range of 2 to 3 mm.

The lens 5 has a hollow interior along its optical axis, so that the electron beam 9 can pass through. The hollow portion (passage or gap) is wide enough so that the light emitted by or reflected from the sample 7 can also pass through without much obstruction. Since it is desirable to keep the output aperture 13 of the electromagnetic objective 5 as small as possible to maintain good electro-optical performance, it is desirable to build the system so that the working distance is kept small.

As can be seen, a reflective objective is provided within the electromagnetic objective 5 to image the surface of the sample 7. In this example, a Schwarzschild reflection objective is used. The Schwarzschild objective is a two-mirror reflection objective that is rotationally symmetric about the optical axis (z) (coincident with the path of the original electron beam), is aberration-free, and is infinitely corrected. The electromagnetic objective 5 and the reflective objective may have the same focal plane. The reflective objective in the electromagnetic objective 5 consists of a first mirror (M1), also referred to as the primary mirror, in this example spherical and concave, and a second mirror (M2), also referred to as the secondary mirror, in this example spherical and convex. ) includes. The diameter of the first mirror (M1) is larger than the diameter of the second mirror (M2). The first mirror (M1) is positioned above the second mirror (M2) and reflects the light coming from the sample as a result of the electron beam (9) hitting the surface of the sample (7), between the sample and the first mirror (M1). It is arranged to direct light toward the second mirror (M2) disposed in . The second mirror (M2) is arranged to redirect the light along (i.e. upward) the optical axis of the electromagnetic objective, and the third mirror (M3), which in this example is planar, is arranged to redirect the light beam towards the output. do. In this example, the third mirror (M3) has a 45° angle to the electron beam (9) axis and is used to redirect the light from the vacuum enclosure (10). All three mirrors (M1, M2 and M3) have apertures or openings along the electron beam path to ensure that the electron beam is unobstructed.

In the optical imaging section 42, the light reflected by mirror M3 is focused by lens 22 onto imaging monochromator 43. In this example, two imagers are provided, a CCD camera 45 and a detector 46, such as an InGaAs or PMT detector. If mirror 24 is a half mirror, then both imagers can be operated simultaneously. Conversely, mirror 24 may be a flip mirror that allows operating one imager at a time. In this arrangement, detector 46 can be used to detect light intensity at a specified wavelength, while the CCD camera can be used to detect light intensity at several wavelengths simultaneously.

CL microscopy was used to inspect semiconductors to detect defects. However, in many cases, the number of defects is large and includes a variety of different types of defects. To aid the inspection of materials, it is possible to identify and possibly identify the different types of defects that appear in CL images, since some types of defects may be catastrophic to the device, while other types may not affect device performance. It would be beneficial to be able to classify. Therefore, determining the types of defects is critical to determining the quality of the inspected sample.

It has also been observed that while hydrostatic pressure within the sample will produce a shift in the CL emission energy spectrum, non-hydrostatic strain may cause a broadening of the spectrum, but not a shift in the peak of the energy spectrum. These phenomena have been studied using photoluminescence, where a light beam (e.g., a laser beam) is scanned over a sample and the light emitted from the sample is collected and analyzed. Additionally, the degree of polarization (DOP) technique was used to measure the strain in the sample by inserting a polarizer within the light collection path and rotating the polarizer to take CL images in horizontal and vertical polarizations. Then, the DOP is It is acquired by. Because strain within the sample will affect the polarization of the emitted light, the DOP image will identify the locations of strain within the sample.

In some semiconductor materials, such as gallium nitride (GaN), silicon carbide (SiC), gallium arsenide (GaAs) and indium phosphide (InP), dislocations are linear defects that have a major impact on the performance of devices. Conversely, some other types of defects may not be detrimental to the performance of the device. For example, one of the key differences between screw and edge dislocations is that the screw type has a Burgers vector parallel to the dislocation line, meaning that the screw type exerts a net shear on the surrounding material, resulting in only a stress change. This means that the edge type has a Burgers vector perpendicular to its axis, while perpendicular to the surface of the material, creating a strain field perpendicular to the dislocation lines around its core and parallel to the surface of the material.

Dislocations within the sample will most often act as recombination sites and therefore non-radiative sites within the semiconductor sample. Therefore, each defect appears as a dark spot in the CL intensity image, except in some cases where the defects appear as bright spots. Conversely, as confirmed in various experiments by the inventors, DOP images show gray level changes (typically darker on one side of the dislocation core, brighter on the other) in the sample that can be spotted in the DOP image. The positions of dislocations that induce strain in the interior, i.e. edge dislocations in the example shown in FIGS. 3A and 3B, will only be shown. Accordingly, the inventors have determined that the CL DOP images as separated from other dislocations (e.g. screw dislocations) that generate different strain fields that are either insensitive or sensitive in a way that allows the DOP to be clearly distinguished from edge dislocations. It was discovered that they can be used to map edge dislocations. In the examples of FIGS. 3A and 3B , pure screw dislocations are not visible at all in the DOP image.

Accordingly, according to the disclosed aspect, a method is provided for determining edge dislocation density, the method comprising: scanning an area of a semiconductor sample with an electron beam, collecting CL light emitted from the sample and forming a light beam from the collected light. passing a light beam through a polarizer and directing the light beam to a detector and generating a horizontally polarized image of the area and a vertically polarized image of the area, the horizontally polarized image and the vertically polarized image. Obtaining the normalized difference and thereby forming a degree of polarization (DOP) image, and calculating the edge dislocation density by determining the density of corresponding features appearing in the DOP image. The normalized difference can be obtained by calculating the ratio of the difference between the two polarized images to the sum of the two polarized images.

Moreover, by experimentation, the inventors determined that additional information about the defects can be obtained by comparison of the CL intensity image and the DOP image. Such comparison may make it possible to determine defect densities differentiated by type of defects. That is, by calculating the density of dark spots in the intensity image, the overall defect density can be obtained, and by calculating the density of these same dark spots with corresponding features in the DOP image, the density of edge dislocations can be obtained, and the total and By taking the difference between the edge dislocation densities, the defect density of screw dislocations can be obtained. Stated another way, the differences in features between the intensity image and the DOP image provide a map of screw dislocations that can be used to determine screw dislocation density.

Accordingly, a method for determining edge dislocation density and screw dislocation density is provided, the method comprising: scanning an area of a semiconductor sample with an electron beam, collecting CL light emitted from the sample, and generating an unpolarized light beam from the collected light. forming, directing at least a portion of the unpolarized light beam to a detector and generating an intensity image of the area, calculating the overall defect density by determining the density of dark spots appearing in the intensity image, Passing at least a portion of the polarized light beam to generate a polarized beam and directing the polarized beam to a detector and generating a horizontally polarized image of the area and a vertically polarized image of the area, the horizontally polarized image and Obtaining a normalized difference of the vertically polarized image and thereby forming a degree of polarization (DOP) image, further calculating the edge dislocation density by determining the density of dark spots in the intensity image represented in the DOP image, and and calculating the screw dislocation density by subtracting the edge dislocation density from the total defect density.

According to further aspects, a method is provided for generating a screw potential map, comprising scanning an area of a semiconductor sample with an electron beam, collecting CL light emitted from the sample and producing an unpolarized light beam from the collected light. forming, directing at least a portion of the unpolarized light beam to a detector and generating an intensity image of the area, passing at least a portion of the unpolarized light beam through a polarizer to generate a polarized beam, and generating a polarized beam. directing to a detector and generating a horizontally polarized image of the area and a vertically polarized image of the area, obtaining a normalized difference of the horizontally polarized image and the vertically polarized image, thereby producing a polarization (DOP) image. forming a degree of, identifying all spots that appear as contrasts in the corresponding coordinates in the intensity image and the DOP image and thus obtaining a mapping of the edge potentials. Alternatively, the intensity image is generated by simply summing the horizontally polarized image of the area and the vertically polarized image of the area.

An important part of the analysis described above is the proper alignment of the horizontally and vertically polarized images, both spatially and temporally. Alignment in space is important to enable distinguishing defects according to the intensity levels of pixels within the images. Additionally, time differences between images should be minimized, or preferably eliminated, to avoid potential problems such as stage drift and the effect of sample charge on the emitted light intensity. Accordingly, the image acquisition portion 42 of the microscope shown in Figure 1 has been modified, as detailed below.

Figure 2 is a simplified schematic diagram showing one embodiment of an apparatus for acquiring CL DOP images with a unique alignment of horizontally and vertically polarized images. That is, the embodiment shown in Figure 2 simultaneously generates two polarization complementary images of the scanned area of the sample, with the two images uniquely registered both spatially and temporally. Elements in Figure 2 that are identical to elements in Figure 1 have the same reference characters. Sample 7 is illuminated by a scanning electron beam e. CL emission is collected by mirrors M1 to M3 and directed towards imaging section 42. After the beam passes through the focusing lens 22, the beam is split into a horizontally polarized beam and a vertically polarized beam by a polarizing beam splitter (PBS) cube 51. In the depicted embodiment, the PBS is designed to split the unpolarized beam into reflected S-polarized and transmitted P-polarized beams in a 50/50 ratio. Each of photodetectors 46a and 46b is a point detector, such as a photomultiplier tube (PMT), photodiode, etc., and detects one of the S-polarized and P-polarized beams. Signals from each of the detectors are transmitted to the controller 52 (in one example, the signals are input to a scanning card within the controller). In this way, the P and S-polarized signals for each pixel are simultaneously recorded by the controller, so that they are uniquely assigned in time and space. Controller 52 may then operate to manipulate the signals according to any of the methods disclosed herein. In particular, the controller is operable to add the signals from both detectors 46a and 46b to generate a sum image representing the entire CL intensity image, and the controller obtains the normalized difference of the two signals and thus the DOP It can be operated to generate an image.

Figure 3A is a CL intensity image obtained by summing signals from two detectors 46a and 46b. Figure 3b is a DOP image obtained by taking the normalized difference of signals from two detectors. In Figure 3a, each dark spot corresponds to a non-emitting site and thus indicates a defect. However, from the intensity image in Figure 3A, one cannot distinguish between different types of defects in the image. The DOP image in Figure 3b shows the change in gray scale at each location of strain, thus indicating the defect site generating the strain. However, note that the two defects circled in Figure 3A do not have corresponding gray scale disturbances or changes in the DOP image in Figure 3B. Therefore, it is very likely that these two spots correspond to screw dislocations.

To distinguish screw dislocations from edge dislocations, the following processes can be used as detailed with reference to Figure 4, which shows the unordered steps of the process. In step 400, the intensity image is examined to identify all of the dark spots (dislocations) and calculate the coordinates of the center of each spot. At step 405, the coordinates are transposed to coordinates within the DOP image, and at step 410, an area around each of the centers within the DOP image is determined. The zone identifies the area considered relevant for determining the potential at the identified center. The size of the zone can be determined empirically, as the smaller the zone, the more it distinguishes between closely clustered defects, but also the more likely it is that the zone will miss pixels corresponding to the defect. Conversely, the larger the region, the more likely it is that the region will contain pixels belonging to neighboring defects.

In step 415, a threshold is set; The threshold can be set at any time and used for multiple examinations of multiple samples. The threshold will depend on the analysis performed in step 420, generally referred to as calculating a representative value for each region within the DOP image. This can be done in a variety of ways. For example, in one embodiment, the partition variance of the signal (i.e., gray level) within each defined region is calculated as a representative value. According to another embodiment, the fitting equation is applied to the gray levels within the region and the equation fit is set to a representative value. For examples of curve fitting, see Zwirn, G. & Beeri, Ronen & Gilon, Dan & Akselrod, S., Adaptive Attenuation Correction in Contrast Echo. Computers in Cardiology. 32. 1-4. It can be found at 10.1109/CIC.2005.1588017 (2005). According to another example, comparisons are made to a stored database using, for example, principal component analysis, or similar AI-based pattern recognition algorithms.

At step 425, the results of the analysis for each defect are compared to a threshold to determine whether the defect is a screw or edge dislocation. For example, in step 425, it is determined that the defect is a screw dislocation because if the dispersion is below a set threshold, this means that there is no sufficient gray scale dispersion in the area to indicate the defect in the DOP image. Similarly, if the equation fit has an amplitude lower than the set threshold, the fault is identified as a screw dislocation. When using a comparison against a pre-stored database, if a zone does not correspond to the database to a degree that exceeds a threshold, the defect is identified as a screw dislocation. Of course, for each of these analyses, if comparison to the threshold does not result in the determination of the screw dislocation, then the fault is identified as an edge or mixed dislocation. In another embodiment, several thresholds may be defined to identify several types of dislocations, from pure edge character to mixed to pure screw. Additionally, for each of the above analyses, further refinement can be achieved by comparison to the calculated “background” gray scale values. For example, background gray scale values can improve analysis of variance within a region. If the variance is not large enough from the background value, then it may simply mean screw dislocations or image noise rather than edge dislocations.

For illustration purposes, the double arrows across FIGS. 3A and 3B indicate dark spots in FIG. 3A and the arrowheads in FIG. 3B indicate the corresponding locations in FIG. 3B, i.e., areas for the corresponding coordinates. The area in Figure 3b appears to have dark and stepped areas. Therefore, the gray level variance within the region will be relatively high compared to the region indicated by the oval in Figure 3b. Such dispersion must be above a threshold, so the defect will be classified as an edge dislocation. Conversely, since the dispersion in the area indicated by the ellipse is below dispersion, the corresponding dark spots in Figure 3a would be classified as screw dislocations.

Accordingly, a method is provided for identifying and distinguishing defects in a semiconductor sample, the method comprising: irradiating an area of the sample with an electron beam; collecting light emitted from the area and thereby generating a light beam; Passing the light beam through a polarizer thereby generating a first polarized beam and a second polarized beam; Directing a first polarized beam to a first detector and directing a second polarized beam to a second detector; generating an intensity image by adding the output signals of the first and second detectors; generating a polarization image by obtaining a normalized difference between the output signals of the first and second detectors; generating a list of coordinates, each coordinate identifying the center of each dark spot appearing in the intensity image; For each coordinate, defining a defect zone within the polarization image; For each defect zone, calculating a representative value; Comparing the representative value of each defect zone to a preset threshold and categorizing the zone as an edge dislocation or screw dislocation depending on whether the representative value is below or above the preset threshold.

Returning to Figure 2, a wavelength filter 50 (i.e., an optical filter, such as a bandpass or monochromatic filter, for example) may be selectively inserted within the path of the beam to improve the signal-to-noise ratio. Specifically, for generating DOP images of the embodiments disclosed herein, the emission of interest is the band edge emission. Accordingly, wavelength filter 50 may be used to reject other emissions, for example emissions from defect bands.

The embodiments disclosed so far work well when the sample is properly aligned, such that the Burgers vector of the dislocations generates polarization that is aligned with the orientation of the PBS cube. To ensure that other dislocations that are not so aligned can also be detected, a second independent measurement can be made by rotating the sample, for example by an angle of 45°. However, as noted above, in CL measurements, even small spatial drifts, such as a shift of one pixel between measurements, can evade the results of the entire measurement. Additionally, the inherent use of rotation means that the images do not temporarily coexist, which can also affect measurements. Figures 5 and 6 show embodiments that avoid these problems.

The embodiment of Figure 5 uses the CL microscope of Figure 2, with additional optical elements. Specifically, a non-polarizing beam splitter 53 (here a half mirror with 50% transmission and 50% reflection) is inserted into the light beam path upstream of the first PBS cube 51 . Half of the light beam then passes through the unpolarizing beam splitter 53 and is handled as described for the embodiment of Figure 2. The other half is reflected toward the second PBS cube (51'), which is rotated 45° with respect to the first PBS cube (51). This rotation is illustrated by the callout in Figure 5, which shows detector 45c receiving a beam with a 45° polarization angle rotated relative to the beam received by first detector 46a. Essentially, the second PBS cube 51' is rotated 45° relative to the beam path, as illustrated by the curved arrow, so that the detector 46c is shifted into or out of the plane of the page. Similarly, the polarization angle of the beam received by detector 46d is rotated by a 45° polarization angle relative to the polarization of the beam received by detector 46b.

In the arrangement of Figure 5, four signals are received by controller 52, which can use these signals as follows: Controller 52 adds the signals of any two detectors paired with the same PBS cube, for example, by adding the signals of detectors 46a and 46b or by adding the signals of detectors 46c and 46d. An intensity image can be generated. Alternatively, controller 52 may generate an intensity image by adding the signals from all four detectors. The controller may then generate two DOP images: one consisting of the normalized difference of the signals from detectors 46a and 46b, and one consisting of the normalized difference of the signals from detectors 46c and 46d. It is made up of differences. For example, the process as outlined in Figure 4 can then be repeated for each of the two DOP images.

Incidentally, in the embodiment of Figure 5, two focus lenses 22 and 22' are used, one paired with each of the PBS cubes. Such an arrangement provides design flexibility, especially for placement of the PBS cubes and detectors. Additionally, it allows for a shorter focal length for each half beam path, thus enabling wide field of view images. However, alternatively, a single focus lens could be used, which is inserted upstream of the unpolarizing beam splitter 53, as shown by the dashed double arrow. In such an arrangement, the beam path to all of the detectors should have the same length.

Figure 6 shows a further embodiment where six detectors are used to generate DOP images with three different polarization rotations. In this embodiment, two non-polarizing beam splitters 53 and 53' are inserted within the optical beam path, thereby generating three different optical paths. In this embodiment, the first unpolarizing beam splitter 53 may not be a strict half mirror, but may be a transmissive mirror that transmits more than reflects, for example, 55% to 70% transmission. In this respect, a transmitting mirror can refer to a mirror that transmits an arbitrary amount of light, while a half mirror is a special case of a transmitting mirror that transmits half of the light. Conversely, the second non-polarizing beam splitter 53' may be a 50/50 half mirror. As illustrated in the callouts, the second and third PBS cubes 51' and 51'' are rotated at angles of 30° and 60° relative to the orientation of the first PBS cube 51. Like the embodiment of Figure 5, signals from any pair of detectors can be added to form an overall intensity image, or signals from all of the detectors can be used to form an intensity image. Additionally, the controller generates three DOP images by the normalized difference of the signals of each pair of detectors, one DOP image from detectors 46a and 46b, one DOP image from detectors 46c and 46d. , and one DOP image from detectors 46e and 46f. For example, the process outline shown in Figure 4 can be repeated for each of the three DOP images.

As already indicated, the accuracy of the processes disclosed herein relies on perfect spatial and temporal alignment of the images. Additionally, accuracy can be improved by accurate calibration of each two pairs of photodetectors. That is, a given light intensity should result in the same electrical signal output from each of the paired photo detectors. Such calibration can be done electronically by normalizing the signals from the paired detectors. Additionally, Figure 7 shows an example that allows accurate calibration of paired detectors. Although the features of the embodiment of Figure 7 are shown for the two pair detector of Figure 2, the features are applicable to any two pair detector within any of the disclosed embodiments.

In Figure 7, a housing 54 is provided to which the focus lens 22, the PBS cube 51 and the paired detectors 46a and 46b are attached, so that the spatial relationship between these elements is fixed. Rotation mechanism 55 rotates housing 54 about a rotation axis coincident with the light beam path, as illustrated by the curved arrow. Because the optical elements are attached within the housing with a fixed spatial orientation, such spatial orientation does not change during rotation. Therefore, it can be accurately calibrated by rotating the housing and checking the output signals from the detectors and pair detectors.

As can be understood from the above, the disclosed embodiments provide a cathodoluminescence (CL) electron microscope, the electron microscope comprising: a vacuum enclosure; an electron source located at an upper position within the vacuum enclosure; An electromagnetic objective positioned at a bottom position within the vacuum enclosure, the electromagnetic objective comprising a housing having an inlet aperture at the top surface and an outlet aperture at the bottom; an electromagnetic coil radially positioned within the housing; an optical objective positioned within the housing and including a concave mirror with a first axial aperture and a convex mirror with a second axial aperture; an electron beam deflector positioned within the housing and including a first set of deflectors and a second set of deflectors cooperating to scan the electron beam across the sample; a deflector mirror that receives the light collected by the optical objective and deflects the light toward the exterior of the vacuum enclosure; It is located outside the vacuum enclosure and includes a focusing lens, a polarizing beam splitter that splits the light beam into a first polarized beam and a second polarized beam, a first light detector that receives the first polarized beam, and a first polarized beam that receives the second polarized beam. an imaging section including a second photodetector; and a controller that forms two polarization complementary images from a first signal received from the first photo detector and a second signal received from the second photo detector.

Disclosed embodiments provide a method for identifying defects in a semiconductor sample, the method comprising: scanning an area of the sample with an electron beam; collecting cathodoluminescent light emission from the sample and generating a light beam therefrom; Passing the light beam through a polarizing beam splitter (PBS) to generate a vertically polarized beam and a horizontally polarized beam; using a first photodetector to generate a first electrical signal corresponding to a vertically polarized beam and using a second photodetector to generate a second electrical signal corresponding to a horizontally polarized beam; adding first and second electrical signals to generate an intensity image of the region; Taking the normalized difference of the first and second electrical signals to generate a degree of polarization (DOP) image; and comparing the intensity image to the DOP image to identify defects within the area. In the method, the normalized difference may be obtained by calculating the ratio of the difference between the first and second electrical signals to the sum of the first and second electrical signals.

Also provided is a method of operating a cathodoluminescence microscope, comprising applying a voltage to an electron emitter to thereby generate an electron beam; applying a voltage to the electromagnetic objective thereby generating a magnetic field to focus the electron beam on the focal plane of the electromagnetic objective; applying a voltage to the scanner to scan the electron beam across an area of the semiconductor sample; collecting cathodoluminescent light from the sample and forming a light beam therefrom; Passing the light beam through a polarizing beam splitter (PBS) to generate a vertically polarized beam and a horizontally polarized beam; using a first photodetector to generate a first electrical signal corresponding to a vertically polarized beam and using a second photodetector to generate a second electrical signal corresponding to a horizontally polarized beam; adding first and second electrical signals to generate an intensity image of the region; Taking the normalized difference of the first and second electrical signals to generate a degree of polarization (DOP) image; determining the coordinates of the center point of each dark spot appearing in the intensity image; For each of the coordinates, examining the corresponding region within the DOP image to determine whether an indication of stress appears within the region; and for each indication of stress, classifying the corresponding dark spot as an edge dislocation.

The method disclosed herein may be implemented by a controller 52, which may be a special purpose computer or a general purpose computer, such as a PC, that executes a program embodying the method. The disclosed methods may also be implemented as a computer program stored on a storage device that, when executed by a computer, causes the computer to execute steps implementing the method. For example, the steps may include receiving a first electrical signal corresponding to a vertically polarized CL beam and a second electrical signal corresponding to a horizontally polarized CL beam; adding first and second electrical signals to generate an intensity image of the scanned area of the sample; Taking the normalized difference of the first and second electrical signals to generate a degree of polarization (DOP) image; determining the coordinates of the center point of each contrast spot appearing in the intensity image; For each of the coordinates, examining the corresponding region within the DOP image to determine whether an indication of stress appears within the region; and, for each indication of stress, classifying the corresponding control spot as an edge dislocation.

Although the invention has been shown and described in detail in the drawings and the foregoing description, such illustrations and descriptions are to be regarded as illustrative and exemplary and not restrictive, and the invention is not limited to the disclosed embodiments. Other embodiments and variations may be understood and accomplished by those skilled in the art when practicing the claimed invention based on a study of the drawings, disclosure and appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude plurality. The mere fact that different features are listed in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims (20)

1. A cathodoluminescence microscope for simultaneously generating two polarization complementary images of a scanned area of a semiconductor sample, wherein the two images are uniquely registered both spatially and temporally;
an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source and thereby forming an electron beam, and a scanner for scanning the electron beam across the sample;
an optical objective that collects light emitted from the sample in response to scanning of the electron beam and forms an optical beam;
a focusing lens, a polarizing beam splitter splitting the light beam into a first polarized beam and a second polarized beam, a first light detector receiving the first polarized beam and a second light detector receiving the second polarized beam. an imaging section containing a photodetector; and,
and a controller for forming the two polarization complementary images from a first signal received from the first photo detector and a second signal received from the second photo detector. The cathodoluminescence microscope of claim 1, wherein the imaging section further comprises an optical filter inserted upstream of the polarizing beam splitter. The method of claim 1, further comprising: a half mirror located upstream of the polarizing beam splitter; a third polarized beam receiving light reflected by the half mirror and oriented at a 45 degree polarization rotation angle relative to the first polarized beam, and oriented at a 45 degree polarization rotation angle relative to the second polarized beam. a second polarizing beam splitter oriented to form a fourth polarized beam; a third photo detector receiving the third polarized beam; and a fourth light detector receiving the fourth polarized beam. 2. The method of claim 1, further comprising: a first transmitting mirror located upstream of the polarizing beam splitter; a third polarized beam receiving light reflected by the first transmitting mirror and oriented at an angle of 30 degrees relative to the first polarized beam, and a fourth oriented at an angle of 30 degrees relative to the second polarized beam. a second polarizing beam splitter oriented to form a polarized beam; a third photo detector receiving the third polarized beam; and a fourth photo detector receiving the fourth polarized beam; and, a second transmission mirror; a fifth polarized beam receiving light reflected by the second transmitting mirror and oriented at an angle of 60 degrees relative to the first polarized beam, and a sixth polarized beam oriented at an angle of 60 degrees relative to the second polarized beam. a third polarizing beam splitter oriented to form a polarized beam; a fifth photo detector receiving the fifth polarized beam; and a sixth photo detector receiving the sixth polarized beam. 5. The cathodoluminescence microscope of claim 4, wherein the first transmission mirror comprises a 55% to 70% transmission mirror. 2. The apparatus of claim 1, further comprising: a housing attaching the polarizing beam splitter to the first and second photo detectors in a fixed orientation relative to each other; and a rotation mechanism that rotates the housing along an axis aligned with the light beam. 4. The method of claim 3, wherein the focus lens is positioned between the half mirror and the polarizing beam splitter; A cathodoluminescence microscope further comprising a second focusing lens positioned between the half mirror and the second polarizing beam splitter. 4. The cathodoluminescence microscope of claim 3, wherein the focusing lens is located upstream of the half mirror. A computer program stored on a storage device that, when executed by a computer, causes the computer to perform steps implementing the method, the method comprising:
Receiving a first electrical signal corresponding to a polarized cathodoluminescent beam and a second electrical signal corresponding to the polarized cathodoluminescent beam and having a 90 degree polarization rotation with respect to the first electrical signal; adding the first and second electrical signals to generate an intensity image of the scanned area of the sample; taking a normalized difference of the first and second electrical signals to generate a degree of polarization (DOP) image of the region; determining the coordinates of the center point of each contrast spot appearing in the intensity image; For each of the coordinates, examining a corresponding region within the DOP image to determine whether an indication of stress appears within the region; and, for each indication of stress, classifying the corresponding control spot as an edge dislocation. 10. The computer program of claim 9, wherein taking the normalized difference comprises calculating a ratio of the difference between the first and second electrical signals to the sum of the first and second electrical signals. 11. The computer program of claim 10, wherein the examining step includes calculating a representative value of the DOP image within the region and the determining step includes comparing the representative value to a preset threshold. 12. The computer program of claim 11, wherein calculating the representative value comprises calculating the variance of the DOP image within the region and said determining includes comparing the variance to a preset threshold. 12. The computer program of claim 11, wherein calculating the representative value includes applying a fitting equation to the gray level values within the region. A method of operating a cathodoluminescence microscope to detect defects in a semiconductor sample, comprising:
scanning an area of the sample with an electron beam;
collecting cathodoluminescent light emitted from the area during the scanning and forming a light beam from the cathodoluminescent light;
passing the light beam through a polarizer beam splitter to obtain two polarized beams having a 90 degree polarization rotation relative to each other;
using two photodetectors to simultaneously generate two electrical signals corresponding to the two polarized beams;
adding the two signals to form an intensity image of the region;
taking the normalized difference of the two signals to form a degree of polarization (DOP) image of the region;
For each contrast spot appearing in the intensity image, examining a corresponding region in the DOP image and when an indication of stress appears within the region in the DOP image, classifying the corresponding contrast spot as an edge dislocation. , method. 15. The method of claim 14, wherein taking the normalized difference comprises calculating a ratio of the difference of the two signals to the sum of the two signals. 16. The method of claim 15, wherein the inspecting step includes calculating a representative value of a DOP image within the area and comparing the representative value to a preset threshold. 17. The method of claim 16, wherein calculating the representative value comprises calculating the variance of the DOP image within the region. 17. The method of claim 16, wherein calculating the representative value includes applying a fitting equation to the gray level values within the region. 17. The method of claim 16, wherein calculating the representative value comprises using principal component analysis to compare the DOP image to a stored reference image. 15. The method of claim 14, further comprising filtering the light beam prior to passing the light beam through the polarizer beam splitter.