JP7643753B2 - Identifying dislocation types and densities in semiconductor materials using cathodoluminescence measurements - Google Patents

Description

(Related Applications)
This application claims the benefit of priority from 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 by reference in their entireties herein.

(Technical field)
The present disclosure relates to scanning cathodoluminescence microscopy, and more particularly to hardware and methods for scanning cathodoluminescence microscopy that enable the discovery and classification of different defect types in semiconductor materials.

The applicant has previously disclosed a novel scanning cathodoluminescence microscope, for example in PCT/EP2020/063093, the disclosure of which is incorporated herein by reference in its entirety. This microscope operates on the observation that as the beam of a scanning electron microscope (SEM) scans a sample, electrons interact with the sample to produce various detectable signals that contain information about the sample's surface topography, structure and composition. The types of signals produced by SEM include secondary electrons (SE), backscattered electrons (BSE), characteristic or Bremsstrahlung X-rays, light, absorbed/induced current (EBAC/EBIC) and transmitted electrons (TEM). The light emitted by the specimen upon electron bombardment (defined as photons having energies ranging from about 0.1 to 10 eV) is called cathodoluminescence (CL). Cathodoluminescence measurements can be performed in a scanning electron microscope by scanning a highly focused electron beam probe of the electron microscope over the surface of the specimen and recording the cathodoluminescence signal intensity as a function of the position of the electron beam on the specimen. A cathodoluminescence map (also referred to herein as an image) is generated, which provides higher resolution spectroscopic information than the wide-field light optical images obtained by light microscopes. For the purposes of this disclosure, it is presumed that the reader is familiar with the above-cited disclosures. For other disclosures of CL microscopes, the reader is referred to U.S. Pat. No. 3,845,305, U.S. Patent 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 it is not intended to particularly identify key 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 objective of the present disclosure is to enable the identification and classification of dislocations in semiconductor samples according to the type of dislocation, e.g., screw, edge or mixed. Dislocations usually appear in CL images as spots of contrast that are usually darker than the surrounding material without dislocations, but sometimes brighter than the surrounding material, especially in the case of alloys, e.g., InGaN alloys (see, for example, F. Massabau et al, 'Optical and structural properties of dislocations in InGaN', J. Appl. Phys. 125, 165701 (2019)). For convenience, the spots induced by dislocations are referred to hereafter as "dark spots", but the present embodiment is not limited to dark spots, but rather features visible in CL that indicate the presence of dislocations, also referred to as contrast spots. According to the disclosed aspects, by observing the strain in the sample at the location of the defect, the observed defect is classified as an edge dislocation and a screw dislocation. Indeed, the strain around an edge dislocation has a predominantly isotropic component, as opposed to a screw dislocation, which has only a shear component. The strain is observed by polarizing the CL emission and generating a degree of polarization (DOP) image using the normalized difference between the images formed with the 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.

In accordance with the disclosed aspects, the CL microscope comprises a polarizing beam splitter cube and at least two photodetectors. This arrangement allows for the capture of two polarized complementary images of a scanned area of a sample, which are essentially aligned in space and time, thus avoiding the need to perform image alignment and eliminating artifacts introduced by a time shift or delay in the acquisition of the two images.

In a disclosed embodiment, a cathodoluminescence microscope is provided for simultaneously generating two polarization-complementary images of a scanned area of a semiconductor sample, the two images being essentially aligned in space and time, the microscope comprising 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 over the sample, an optical objective lens for collecting CL light emitted from the sample in response to the scanning of the electron beam and forming a light beam, an imaging unit comprising a focusing lens, a polarizing beam splitter for splitting the light beam into a first polarized beam and a second polarized beam, a first photodetector for receiving the first polarized beam, and a second photodetector for receiving the second polarized beam, and a controller for forming the two polarization-complementary images from a first signal received from the first photodetector and a second signal received from the second photodetector.

The imaging section may further include an optical filter inserted upstream of the polarizing beam splitter. The microscope may also include a half mirror arranged upstream of the polarizing beam splitter, where the focusing lens may be arranged upstream or downstream of the half mirror; a second polarizing beam splitter oriented to receive light reflected by the half mirror to form a third polarized beam oriented at a polarization rotation angle of 45 degrees relative to the first polarized beam and a fourth polarized beam oriented at a polarization rotation angle of 45 degrees relative to the second polarized beam; a third photodetector receiving the third polarized beam; and a fourth photodetector receiving the fourth polarized beam. The microscope may also include a housing that orients the polarizing beam splitter, the first photodetector, and the second photodetector relative to one another; and a rotation mechanism that rotates the housing around an axis aligned with the light beam. The disclosed embodiment is advantageous because the simultaneous acquisition of two polarized cathodoluminescence emissions inherently incorporates spatial and temporal alignment of the images. The embodiment also makes it possible to distinguish between edge and screw dislocations.

According to the disclosed aspect, a computer program stored in a storage device is provided that, when executed by a computer, causes the computer to perform the following steps: receiving a first electrical signal corresponding to a polarized cathodoluminescence beam and a second electrical signal corresponding to the polarized cathodoluminescence beam and having a polarization rotation of 90 degrees relative to the first electrical signal; summing the first and second electrical signals to generate an intensity image of a 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 area; determining coordinates of a center point of each contrast spot appearing in the intensity image; at each coordinate, inspecting the zone in the DOP image to determine whether the stress pattern in the corresponding zone meets a suitable shape or intensity criterion; and classifying the stress pattern as an edge or screw dislocation via a procedure selected from a plurality of available procedures. The aforementioned procedure could be a simple intensity threshold since in some materials screw dislocations do not appear at all in DOP images, or it could be a local variance calculation, a pattern matching technique, machine learning, or other AI-inspired pattern recognition technique.

According to a further aspect, a method of operating a cathodoluminescence microscope for detecting defects in a semiconductor sample is disclosed, the method comprising the steps of scanning an area of the sample with an electron beam, collecting cathodoluminescence light emitted from the area during scanning and forming a light beam from the cathodoluminescence light, passing the light beam through a polarizer beam splitter to obtain two polarized beams having a polarization rotation of 90 degrees from each other, using two photodetectors to simultaneously generate two electrical signals corresponding to the two polarized beams, adding the two signals together to form an intensity image of the area, taking a normalized difference of the two signals to form a degree of polarization (DOP) image of the area, and for each contrast spot appearing in the intensity image, inspecting a corresponding zone in the DOP image, and classifying the corresponding contrast spot as an edge dislocation when indicia of an appropriate strain field appear in that zone in the DOP image.

The step of obtaining the normalized difference may include calculating a ratio of the difference between the first and second electrical signals to a sum of the first and second electrical signals. The step of inspecting the DOP image may include calculating a representative value of the DOP image within the zone and comparing the representative value against a preset threshold.

The accompanying drawings, which are incorporated in and constitute a part of the 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 show major features of exemplary embodiments diagrammatically. The drawings are not intended to depict every feature of an actual embodiment, nor are they intended to show relative dimensions of the depicted elements, and are not drawn to scale.

Other characteristics and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments with reference to the attached drawings, in which:
FIG. 1 is a schematic cross-sectional view of the lower portion of a cathodoluminescence scanning electron microscope for implementing embodiments disclosed herein. FIG. 2 is a simplified schematic diagram of an image acquisition section of a cathodoluminescence microscope according to one embodiment. FIG. 3A is an intensity image showing potential defects in a sample, and FIG. 3B is a DOP image corresponding to the area shown in FIG. 3A. FIG. 4 is a flow chart illustrating steps for classifying defects according to one embodiment. FIG. 5 is a simplified schematic diagram of an image acquisition section of a cathodoluminescence microscope according to another embodiment. FIG. 6 is a schematic diagram of an image acquisition section of a cathodoluminescence microscope according to yet another embodiment. FIG. 7 is a schematic diagram of an image acquisition section of a cathodoluminescence microscope according to a further embodiment.

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

Next, embodiments of the cathodoluminescence scanning electron microscope and the operating method of the present invention will 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 results to be achieved, different features disclosed herein may be utilized partially or to the fullest extent, alone or in combination with other features, balancing requirements and constraints and advantages. Thus, certain features, elements or advantages are emphasized with reference to different embodiments, but are not limited to the disclosed embodiments. That is, the features, elements and advantages disclosed herein are not limited to the embodiments in which they are described, but may be combined with other features ("mixed and matched") or incorporated into other embodiments, even if not explicitly described herein.

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

The electron column includes an electron source 1, such as a thermionic or field emission source, which emits electrons. The emitted electrons are brought into an electron beam 9 by various particle-optical elements such as an electromagnetic lens 5', an electromagnetic objective lens 5 and an aperture disk (sometimes called a stop) 6, any of which may function as an electrostatic lens by application of an electric potential. In a known manner, a coil 11 is provided to generate a magnetic field, which in this figure is a substantially horizontal magnetic field at the level of the optical axis z of the electromagnetic objective lens 5. Most of the magnetic field may be located at the level of the output or exit aperture 13, or alternatively outside the region between the lens and the specimen.

The purpose of the magnetic field is to generate a converging electron beam 9 that can be focused on the surface of the sample 7. In this example, the electron beam 9 generated by the electron emitter 1 propagates downwards from the top of the figure. The electron beam span may be altered by the placement of a condenser, such as a lens 5', so that it can be diverged, collimated or focused. The condenser may be placed below the electron emitter. The electron beam typically has a width in the range of a few millimeters, for example between 2-3 mm.

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

As can be seen in the figure, a reflective objective is provided in the electromagnetic objective 5 to image the surface of the sample 7. In this example, a Schwarzschild reflective objective is used. The Schwarzschild objective is a two-mirror reflective objective, rotationally symmetric about the optical axis z (essentially coinciding with the path of the electron beam), aplanatic and infinity-corrected. The electromagnetic objective 5 and the reflective objective may have the same focal plane. The reflective objective in the electromagnetic objective 5 comprises a first mirror M1, also called the primary mirror, in this example spherical and concave, and a second mirror M2, also called the secondary mirror, in this example spherical and convex. 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 is arranged to reflect light coming from the sample 7 as a result of the electron beam 9 hitting the surface of the sample 7 and to direct the light to the second mirror M2, which is arranged between the sample and the first mirror M1. The second mirror M2 is arranged to redirect the light along the optical axis of the electromagnetic objective lens (i.e. upwards), and the third mirror M3, which is planar in this example, is arranged to redirect the light beam to the output. In this example, the third mirror M3 has an angle of 45° to the axis of the electron beam 9 and is used to redirect the light out of the vacuum enclosure 10. Mirrors M1, M2 and M3 all have apertures or openings along the path of the electron beam so that the electron beam is not blocked.

In the optical imaging section 42, the light reflected by the mirror M3 is focused by the lens 22 into the 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 the mirror 24 is a half mirror, both imagers may be operated simultaneously. Alternatively, the mirror 24 may be a flip mirror, allowing one imager to be operated at a time. In this arrangement, the detector 46 may be used to detect the light intensity at a specific wavelength, and the CCD camera may be used to detect the light intensity at multiple wavelengths simultaneously.

CL microscopes have been used to inspect semiconductors to detect defects. However, in many cases the number of defects is large and includes a variety of different defect types. To aid in the inspection of materials, it is useful to be able to identify and possibly classify the different defect types that appear in CL images, since some types of defects do not affect the performance of the equipment, while others can be fatal. Determining the defect type is therefore crucial in determining the quality of the sample being inspected.

It is also observed that hydrostatic pressure in a sample produces a shift in the energy spectrum of the CL emission, while non-hydrostatic strains can cause spectral broadening 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 the sample and the light emitted from the sample is collected and analyzed. The degree of polarization (DOP) technique has also been used to measure strain in a sample by inserting a polarizer in the light collection path and rotating the polarizer to obtain CL images in horizontal and vertical polarizations. The DOP is then given by DOP = (I _┴ -I _II )/(I _┴ +I _II ). Since strain in a sample affects the polarization of the emitted light, the DOP images identify the location of strain in 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 significant impact on device performance, while other defect types may not be detrimental to device performance. For example, one of the main differences between screw and edge dislocations is that the screw type has a Burgers vector parallel to the dislocation line, i.e., it imparts pure shear to the surrounding material, so that the only change in stress is perpendicular to the surface of the material, while the edge type has a Burgers vector perpendicular to its axis, i.e., it creates a strain field around its core that is perpendicular to the dislocation line and parallel to the surface of the material.

Dislocations in a sample often act as recombination sites and therefore as non-radiative points in a semiconductor sample. As a result, each defect appears as a dark spot in the CL intensity image, except when it appears as a bright spot. On the other hand, as confirmed by the inventors in various experiments, the DOP image shows a change in gray level (usually dark on one side of the dislocation core and bright on the other side) only at the location of the dislocation that induces the strain in the sample, which is seen in the DOP image, i.e., the edge dislocation in the example shown in Figures 3A and 3B. Therefore, the inventors have discovered that the CL DOP image can be used to map edge dislocations separately from other dislocations (e.g., screw dislocations) that generate different strain fields to which the DOP has no sensitivity or that are clearly distinguishable from edge dislocations. In the example of Figures 3A and 3B, the pure screw dislocation is not visible at all in the DOP image.

Therefore, in accordance with a disclosed aspect, a method for determining the density of edge dislocations is provided, comprising the steps of 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 the light beam through a polarizer and directing the light beam to a detector to generate 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 forming a degree of polarization (DOP) image, and calculating the density of edge dislocations by determining the density of corresponding features appearing in the DOP image. The normalized difference may be obtained by calculating the ratio of the difference between the two polarized images to the sum of the two polarized images.

Furthermore, the inventors have found from their experiments that a comparison of the CL intensity image and the DOP image provides further information about the defects. Such a comparison allows for the determination of defect densities differentiated by defect type. That is, the total defect density can be obtained by calculating the density of dark spots in the intensity image, the density of edge dislocations can be obtained by calculating the density of these same dark spots with corresponding features in the DOP image, and the defect density of screw dislocations can be obtained by taking the difference between the total and edge dislocation densities. In other words, the differences in features between the intensity image and the DOP image provide a map of screw dislocations that can be used to determine the density of screw dislocations.

Therefore, a method for determining the density of edge dislocations and the density of screw dislocations is provided, comprising the steps of scanning an area of a semiconductor sample with an electron beam, collecting CL light emitted from the sample and forming an unpolarized beam from the collected light, directing at least a portion of the unpolarized 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 unpolarized beam through a polarizer 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, obtaining a normalized difference of the horizontally polarized image and the vertically polarized image, thereby forming a degree of polarization (DOP) image, calculating the density of edge dislocations by determining the density of dark spots in the intensity image, which also appear in the DOP image, and calculating the density of screw dislocations by subtracting the density of edge dislocations from the overall defect density.

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

A key part of the analysis described above is the accurate alignment of the horizontal and vertical polarized images in both space and time. Alignment in space is important to be able to identify defects according to the intensity levels of the pixels in the images. Also, the time difference between the images should be minimized, or preferably eliminated, to avoid potential problems such as stage drift and the effect of a charging sample on the intensity of the emitted light. Therefore, the image acquisition section 42 of the microscope shown in Figure 1 is modified as detailed below.

Figure 2 is a simplified schematic diagram showing one embodiment of an apparatus for obtaining CL DOP images with intrinsic alignment of horizontally and vertically polarized images. That is, the embodiment shown in Figure 2 simultaneously generates two polarized complementary images of a scanned area of a sample, and these two images are intrinsically aligned both spatially and temporally. Elements in Figure 2 that are the same as in Figure 1 have the same reference letters. The sample 7 is illuminated by scanning an electron beam e. The CL emission is collected by mirrors M1-M3 and directed to the imaging section 42. After passing through the focusing lens 22, the beam is split into horizontally and vertically polarized beams by a polarizing beam splitter (PBS) cube 51. In the embodiment shown, the PBS is designed to split the unpolarized beam in a 50/50 ratio into a reflected S-polarized beam and a transmitted P-polarized beam. Each of the photodetectors 46a and 46b is a point detector, such as a photomultiplier tube (PMT), photodiode, or the like, that detects one of the S-polarized and P-polarized beams. The signal from each detector is sent to the controller 52 (in one example, the signal is input to a scan card in the controller). In this way, the P- and S-polarized signals at each pixel are recorded simultaneously by the controller, thereby essentially aligned in time and space. The controller 52 may be operative to process the signals according to any of the methods disclosed herein. In particular, the controller is operative to add together the signals from both detectors 46a and 46b to generate a sum image indicative of the overall CL intensity image, and the controller is operative to take a normalized difference of the two signals, thereby generating the DOP image.

Figure 3A is a CL intensity image obtained by summing the signals from the two detectors 46a and 46b. Figure 3B is a DOP image obtained by taking the normalized difference of the signals from the two detectors. In Figure 3A, each dark spot corresponds to a non-emitting site and therefore indicates a defect. However, from the intensity image of Figure 3A, it is not possible to distinguish the different defect types in the image. The DOP image of Figure 3B shows a grayscale change at the location of each distortion and therefore indicates the defect site that generates the distortion. However, here, the two defects circled in Figure 3A have no corresponding grayscale disturbance or change in the DOP image of Figure 3B. Therefore, these two spots very likely correspond to screw dislocations.

To distinguish screw dislocations from edge dislocations, the following process may be employed, detailed with reference to FIG. 4, which shows the non-sequential steps of the process: In step 400, the intensity image is inspected to identify all dark spots (dislocations) and calculate the coordinates of the center of each spot. In step 405, the coordinates are replaced with those in the DOP image, and in step 410, a zone around each center in the DOP image is determined. The zone identifies an area that is considered relevant for determining a dislocation at the identified center point. The size of the zone may be determined empirically, since a smaller zone provides more discrimination between closely spaced defects, but increases the likelihood of missing a pixel corresponding to a defect. On the other hand, a larger zone is more likely to include pixels belonging to neighboring defects.

In step 415, a threshold is set. However, the threshold may be set at any time and may be used for multiple tests of multiple samples. The threshold depends on the analysis performed in step 420, which is generally referred to as calculating a representative value for each zone in the DOP image. This may be done in various ways. For example, in one embodiment, the local variance (i.e., gray level) of the signal within each defined zone is calculated as the representative value. According to another embodiment, a fitting equation is applied to the gray levels within the zone and the fitted equation is set as the representative value. An example of curve fitting is given in Zwirn, G. & Beeri, Ronen & Gilon, Dan & Akselrod, S., Adaptive Attenuation Correction in Contrast Echo. Computers in Cardiology. 32. 1-4. 10.1109/CIC.2005.1588017 (2005). According to yet another example, the comparison is performed against a stored database using, for example, principal component analysis, or a similar AI-based pattern recognition algorithm.

In step 425, the results of the analysis on each defect are compared against a threshold value to determine whether the defect is a screw or edge dislocation. For example, in step 425, if the variance is below a set threshold value, the defect is determined to be a screw dislocation since this means that there is not enough grayscale variance in the zone to indicate the defect in the DOP image. Similarly, if the fitted equation has a magnitude lower than a set threshold value, the defect is identified as a screw dislocation. When using a comparison against a pre-stored database, if the zone does not correspond to the database beyond the threshold value, the defect is identified as a screw dislocation. Of course, in each of these analyses, if the comparison against the threshold value does not result in a determination of a screw dislocation, the defect is identified as an edge or mixed dislocation. In other embodiments, multiple threshold values can be defined to identify multiple dislocation types, from pure edge characteristics, to mixed, to pure screw. Also, in each of the above analyses, further refinement may be achieved by comparison against a calculated "background" grayscale value. For example, the background grayscale value can enhance the analysis of variance within a zone. If the variance is not large enough from the background value, it may simply represent a screw dislocation or image noise rather than an edge dislocation.

Illustratively, the double-headed arrows in Fig. 3A and 3B point to the dark spots in Fig. 3A and the arrowheads in Fig. 3B point to the zones around the corresponding locations, i.e., corresponding coordinates, in Fig. 3B. The zones in Fig. 3B appear to have dark and light regions. As a result, the gray level variance within the zones is relatively high compared to the regions indicated by the ellipses in Fig. 3B. Such variance exceeds a threshold, and the defect is classified as an edge dislocation. On the other hand, the corresponding dark spots in Fig. 3A are classified as screw dislocations, since the variance in the regions indicated by the ellipses is below the variance.

Therefore, a method for identifying and characterizing defects in a semiconductor sample is provided, comprising the steps of illuminating an area of the sample with an electron beam, collecting light emitted from the area, thereby generating a light beam, passing the light beam through a polarizer, thereby generating a first polarized beam and a second polarized beam, directing the first polarized beam to a first detector and the second polarized beam to a second detector, generating an intensity image by adding together the output signals of the first and second detectors, generating a polarized image by taking a normalized difference of the output signals of the first and second detectors, generating a list of coordinates identifying the center of each dark spot appearing in the intensity image, defining a defect zone in the polarized image at each coordinate, calculating a representative value for each defect zone, comparing the representative value of each defect zone against a preset threshold value and categorizing the zone as an edge or screw dislocation depending on whether the representative value is below or above the preset threshold value.

Referring again to FIG. 2, a wavelength filter 50 (i.e., an optical filter, such as, for example, a bandpass or monochromatic filter) may be optionally inserted in the path of the beam to improve the signal-to-noise ratio. In particular, the emissions of interest in generating the DOP images of the embodiments disclosed herein are band-edge emissions. Thus, a wavelength filter 50 may be used to filter out other emissions, for example, emissions from defect bands.

The embodiments disclosed so far work well when the sample is precisely aligned to produce polarized light whose Burgers vector of the dislocation is aligned with the orientation of the PBS cube. To ensure that other dislocations that are less well aligned are also detected, a second independent measurement can be made by rotating the sample by an angle of, for example, 45°. However, as mentioned above, in CL measurements, even small spatial drifts, such as a shift of one pixel between measurements, can invalidate the results of the entire measurement. Also, the inherent use of rotation means that the images are no longer simultaneous in time, which can also affect the measurement. Figures 5 and 6 show embodiments that avoid these problems.

The embodiment of FIG. 5 utilizes the CL microscope of FIG. 2 with additional optical elements. Specifically, a non-polarizing beam splitter 53 (here a half mirror with 50% transmission and 50% reflection) is inserted in the light beam path upstream of the first PBS cube 51. Half of the light beam passes through the non-polarizing beam splitter 53 and is processed as disclosed in the embodiment of FIG. 2. The other half is reflected toward a second PBS cube 51' that is rotated 45° relative to the first PBS cube 51. This rotation is shown in the callout in FIG. 5, which depicts that detector 45c receives a beam that is rotated 45° polarization angle relative to the beam received by the first detector 46a. In essence, the second PBS cube 51' is rotated 45° about the beam path as illustrated by the curved arrow so that detector 46c is shifted in or out of the page plane. Similarly, the polarization angle of the beam received by detector 46d is rotated by 45° relative to the polarization of the beam received by detector 46b.

In the arrangement of FIG. 5, the four signals are received by the controller 52, which may use these signals as follows: The controller 52 may generate an intensity image by adding together the signals of any two detectors that are paired with the same PBS cube, for example by adding together the signals of detectors 46a and 46b, or by adding together the signals of detectors 46c and 46d. Alternatively, the controller 52 may generate an intensity image by adding together the signals of all four detectors. The controller may then generate two DOP images: one composed of the normalized difference of the signals from detectors 46a and 46b, and the other composed of the normalized difference of the signals from detectors 46c and 46d. The process, for example as outlined in FIG. 4, may then be repeated for each of the two DOP images.

Now, in the embodiment of FIG. 5, two focusing lenses 22 and 22' are utilized, one paired with each PBS cube. Such an arrangement offers design freedom, especially with regard to the placement of the PBS cubes and the detectors. It also allows for a short focal length in each half beam path, and therefore a wide field of view image. However, alternatively, a single focusing lens may be used, inserted upstream of the non-polarizing beam splitter 53, as shown by the dashed double-headed arrow. In such an arrangement, the beam paths to all detectors must be of the same length.

Figure 6 shows a further embodiment in which six detectors are used to generate DOP images at three different polarization rotations. In this embodiment, two non-polarizing beam splitters 53 and 53' are inserted into the light beam path, thereby generating three different light paths. In this embodiment, the first non-polarizing beam splitter 53 may be more transmissive than reflective, e.g., 55%-70% transmissive, i.e., a transmission mirror, rather than a strict half mirror. In this respect, a transmission mirror may refer to a mirror that transmits any amount of light, since a half mirror is a special case of a transmission mirror that transmits half of the light. On the other hand, the second non-polarizing beam splitter 53' may be a 50/50 half mirror. As illustrated in the bubble, the second and third PBS cubes 51' and 51'' are rotated 30° and 60° with respect to the orientation of the first PBS cube 51. As with the embodiment of FIG. 5, the signals from any pair of detectors may be added together to form an overall intensity image, or signals from all detectors may be used to form an intensity image. The controller may also generate three DOP images with normalized differences in the signals of each pair of detectors, i.e., one DOP image from detectors 46a and 46b, another DOP image from detectors 46c and 46d, and yet another DOP image from detectors 46e and 46f. For example, the process outline in FIG. 4 may be repeated for each of the three DOP images.

As already indicated, the accuracy of the process disclosed herein depends on perfect spatial and temporal alignment of the images. Furthermore, accuracy can be improved by accurate calibration of each of the two paired photodetectors. That is, a given light intensity results in an identical electrical signal output from each of the paired photodetectors. Such calibration may be performed electronically by normalizing the signals from the paired detectors. Furthermore, FIG. 7 shows an example that allows for accurate calibration of the paired detectors. Although the features of the embodiment of FIG. 7 are shown with respect to the two paired detectors of FIG. 2, the features are applicable to any two paired detectors in any of the disclosed embodiments.

In FIG. 7, the focusing lens 22, the PBS cube 51, and the paired detectors 46a and 46b are mounted in a housing 54, which is provided so that the spatial relationship between these elements is constant. A rotation mechanism 55 rotates the housing 54 about an axis of rotation that coincides with the optical beam path, as illustrated by the curved arrow. Because the optical elements are mounted in the housing with a fixed spatial orientation, their spatial orientation does not change during rotation. Therefore, the paired detectors can be accurately calibrated by rotating the housing and checking the output signals from the detectors.

As will be appreciated from the above, the disclosed embodiment includes a vacuum enclosure, an electron source disposed at an upper position within the vacuum enclosure, a housing having an entrance aperture at an upper surface thereof and an exit aperture at a lower portion thereof, an electromagnetic objective lens disposed at a lower position within the vacuum enclosure, electromagnetic coils disposed radially within the housing, an optical objective lens disposed within the housing and including a concave mirror having a first axial aperture and a convex mirror having a second axial aperture, a first set of deflectors and a second set of deflectors disposed within the housing and cooperating to scan the electron beam over the specimen. A cathodoluminescence (CL) electron microscope is provided, which includes an electron beam deflector having two deflector sets, a deflector mirror that receives light collected by an optical objective lens and deflects the light outside the vacuum enclosure, an imaging unit that is disposed 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 photodetector that receives the first polarized beam, and a second photodetector that receives the second polarized beam, and a controller that forms two polarized complementary images from a first signal received from the first photodetector and a second signal received from the second photodetector.

Disclosed embodiments provide a method of identifying defects in a semiconductor sample, comprising scanning an area of the sample with an electron beam, collecting cathodoluminescence light emissions 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 the vertically polarized beam and using a second photodetector to generate a second electrical signal corresponding to the horizontally polarized beam, summing the first and second electrical signals to generate an intensity image of the area, taking a 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 in the area. In this method, the normalized difference may be obtained by calculating a ratio of the difference between the first and second electrical signals to the sum of the first and second electrical signals.

Also, a method of operating a cathodoluminescence microscope includes energizing an electron emitter, thereby generating an electron beam; energizing an electromagnetic objective lens, thereby generating a magnetic field for focusing the electron beam at a focal plane of the electromagnetic objective lens; energizing a scanner to scan the electron beam over an area of a semiconductor sample; collecting cathodoluminescence 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; and filtering the first electrical signal corresponding to the vertically polarized beam. The method includes using a photodetector to generate a second electrical signal corresponding to the horizontally polarized beam, adding the first and second electrical signals together to generate an intensity image of the region, taking a normalized difference of the first and second electrical signals to generate a degree of polarization (DOP) image, determining the coordinates of each dark spot center point appearing in the intensity image, inspecting a corresponding zone in the DOP image at each coordinate to determine whether a signature of stress appears in the zone, and classifying the corresponding dark spot as an edge dislocation at each signature of stress.

The methods disclosed herein may be implemented by a controller 52, which may be a special purpose computer or a general computer, such as a PC, that executes a program embodying the methods. The methods disclosed herein may be implemented as a computer program stored in a storage device that, when executed by a computer, causes the computer to execute steps implementing the methods. 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, summing the first and second electrical signals to generate an intensity image of a scanned area of the sample, taking a 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, examining the corresponding zone in the DOP image at each coordinate to determine whether a stress signature appears in the zone, and classifying the corresponding contrast spot at each stress signature as an edge dislocation.

While the present invention has been shown and described in detail in the drawings and the foregoing description, such illustrations and descriptions are intended to be illustrative and not restrictive, and the present invention is not limited to the disclosed embodiments. Other embodiments and modifications can be understood and effected by those skilled in the art upon a study of the drawings, the disclosure and the appended claims when practicing the claimed invention.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The mere fact that different features are recited 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 interpreted as limiting the scope of the invention.

Claims (15)

1. A cathodoluminescence microscope for simultaneously generating two polarized complementary images of a scanned area of a semiconductor sample, said images being essentially aligned both spatially and temporally, comprising:
an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source thereby forming an electron beam, and a scanner for scanning the electron beam over the semiconductor sample ;
an optical objective lens that collects light emitted from the semiconductor sample in response to scanning of the electron beam and forms a light beam;
an imaging unit including a focusing lens, a polarizing beam splitter that splits the light beam into a first polarized beam and a second polarized beam, a first photodetector that receives the first polarized beam, and a second photodetector that receives the second polarized beam;
and a controller for forming the two polarization complementary images from a first signal received from the first photodetector and a second signal received from the second photodetector. The cathodoluminescence microscope of claim 1, wherein the imaging section further comprises an optical filter inserted upstream of the polarizing beam splitter. The cathodoluminescence microscope of claim 1 further comprising: a half mirror disposed upstream of the polarizing beam splitter; a second polarizing beam splitter oriented to receive light reflected by the half mirror and to form a third polarized beam oriented at a polarization rotation angle of 45 degrees relative to the first polarized beam and a fourth polarized beam oriented at a polarization rotation angle of 45 degrees relative to the second polarized beam; a third photodetector that receives the third polarized beam; and a fourth photodetector that receives the fourth polarized beam. The cathodoluminescence microscope according to claim 1, further comprising: a first transmission mirror disposed upstream of the polarizing beam splitter; a second polarizing beam splitter oriented to receive light reflected by the first transmission mirror and form a third polarized beam oriented at an angle of 30 degrees relative to the first polarized beam and a fourth polarized beam oriented at an angle of 30 degrees relative to the second polarized beam; a third photodetector receiving the third polarized beam; a fourth photodetector receiving the fourth polarized beam; a second transmission mirror; a third polarizing beam splitter oriented to receive light reflected by the second transmission mirror and form a fifth polarized beam 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 fifth photodetector receiving the fifth polarized beam; and a sixth photodetector receiving the sixth polarized beam. The cathodoluminescence microscope of claim 4, wherein the first transmission mirror comprises a 55%-70% transmission mirror. The cathodoluminescence microscope of claim 1, further comprising a housing that orients the polarizing beam splitter, the first photodetector, and the second photodetector relative to one another, and a rotation mechanism that rotates the housing around an axis aligned with the light beam. A second focusing lens is disposed between the half mirror and the second polarizing beam splitter,
4. The cathodoluminescence microscope according to claim 3, wherein the focusing lens is disposed between the half mirror and the polarizing beam splitter. The cathode luminescence microscope according to claim 3, wherein the focusing lens is disposed upstream of the half mirror. 1. A method of operating a cathodoluminescence microscope to detect defects in a semiconductor sample, comprising:
scanning an area of the semiconductor sample with an electron beam;
collecting cathodoluminescent light emitted from said region while scanning and forming a light beam from said cathodoluminescent light;
passing said light beam through a polarizer beam splitter to obtain two polarized beams having a polarization rotation of 90 degrees with respect to each other;
using two photodetectors to simultaneously generate two electrical signals corresponding to the two polarized beams;
summing the two electrical signals to form an intensity image of the region;
taking a normalized difference of the two electrical signals to form a degree of polarization (DOP) image of the area;
For each contrast spot appearing in the intensity image, inspecting a corresponding zone in the DOP image; and classifying the corresponding contrast spot as an edge dislocation when indicia of stress appear within the zone in the DOP image. The method of claim 9 , wherein obtaining the normalized difference comprises calculating a ratio of the difference of the two electrical signals to a sum of the two electrical signals . The method of claim 10 , wherein the step of inspecting comprises the steps of: calculating a representative value of the DOP image within the zone; and comparing the representative value against a preset threshold. The method of claim 11 , wherein the step of calculating the representative value comprises the step of calculating the variance of the DOP image within the zone. The method of claim 11 , wherein calculating the representative value comprises applying a fitting equation to grey level values within the zone. The method of claim 11 , wherein calculating the representative values comprises using principal component analysis to compare the DOP image against a stored reference image. 10. The method of claim 9 , further comprising filtering the light beam prior to passing the light beam through the polarizer beam splitter.