Cost-effective probing of μLEDs in mass production

Abstract

A wafer with μLEDs is inspected using a cathodoluminescence microscope. Splitting the CL beam into several beams and detecting the beams with point detectors allows fast scanning. Optical filters are inserted in the optical path upstream of the detectors so that each detector detects a different frequency band. The signals are ratioed and compared against an expected standard. Areas of extreme values ​​are identified and, if necessary, high-resolution scans are performed on the area or a sample of the area. A viability score is calculated for each identified area.

Description

[Technical Field]

[0001] (Related Applications) This application claims the benefit of priority from U.S. Provisional Application No. 63 / 138,863, filed January 19, 2021, the disclosures of which are incorporated herein by reference in their entirety.

[0002] (Technical field) The present disclosure relates to cathodoluminescence microscopes, and more particularly to methods for probing μLEDs using cathodoluminescence microscopes to effectively inspect mass-produced μLEDs. [Background technology]

[0003] MicroLED, or μLED, is a new flat-panel display technology composed of an array of tiny LEDs that form individual red / blue / green pixel elements. A typical 4K display has 3840 x 2160 image pixels, or over 8 million pixels, each consisting of three red / blue / green pixels, equivalent to approximately 25 million individual microLEDs. Because the human eye is highly sensitive to color variations, the light emission of each of these LEDs must be uniform in intensity and color, as well as in response to an activation potential. Therefore, it is important to test these LEDs during production to ensure proper operation.

[0004] 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 electron beam of a scanning electron microscope (SEM) scans a sample, the electrons interact with the sample and produce various detectable signals that contain information about the sample's surface topography, structure, and composition. Types of signals produced by SEMs 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 a specimen upon electron bombardment (defined as photons with energies ranging from approximately 0.1 to 10 eV) is called cathodoluminescence (CL). Cathodoluminescence measurements can be performed with a scanning electron microscope by scanning the microscope's highly focused electron beam probe over the surface of a specimen and recording the cathodoluminescence signal intensity as a function of the electron beam position on the specimen. A cathodoluminescence map (also referred to herein as an image or CL image) is generated, which provides higher resolution spectroscopic information than the wide-field optical images obtained by light microscopes. For purposes of this disclosure, it is assumed that the reader is familiar with the above-cited disclosures. For other disclosures on CL microscopy, the reader is referred to U.S. Pat. No. 3,845,305, U.S. Publication Nos. 2013 / 0335817 and 2019 / 0103248, and French Patent No. 2173436.

[0005] LEDs are fabricated on semiconductor wafers, and as LED size decreases, the number of LEDs on a wafer increases. The present inventors investigated the feasibility of using CL microscopy as a non-contact method for testing LEDs during front-end production, i.e., while the LEDs are still on the wafer. The present inventors noted that as LED size decreases, the time it takes to perform traditional electro-optical testing, which requires physical electrical contact, on a single wafer increases significantly. For example, electro-optical testing of a wafer with a 1000 micron chip size takes approximately one hour, whereas testing a wafer with a 50 micron chip size takes two weeks, testing a wafer with a 10 micron chip size takes approximately one year, and testing a wafer with a 5 micron chip size takes approximately four years. Therefore, the present inventors aimed to develop new hardware and new test techniques to make non-contact testing of full wafers with micro LEDs (i.e., chip sizes of 100 microns or less) feasible. Summary of the Invention

[0006] The following summary of the disclosure is included to provide a basic understanding of some aspects and features of the present 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.

[0007] The objective of the present disclosure is to enable identification and classification of micro LED chips within a semiconductor wafer. The hardware and methods disclosed herein may be used to develop and test front-end manufacturing of micro LEDs. The apparatus and methods disclosed herein can be used, for example, to determine changes in process steps that affect manufactured micro LEDs, to detect deviations in the manufacturing process, to detect defects in one or more steps of the manufacturing process, to identify unsalvageable wafers that should be discarded without further processing, and to classify / bin LED chips according to uniformity / performance.

[0008] According to the disclosed embodiment, the CL microscope includes a beam splitter to split the incident CL beam into several beams. Various filters are inserted into the optical paths of the split beams, and separate point detectors are positioned to detect the light passing through the filters. The signals from the point detectors are used to analyze the characteristics of the micro LEDs fabricated in the inspected wafer. Each of the point detectors may be a biased photodiode, an avalanche photodiode, an amplified photodiode, or a photomultiplier tube (PMT).

[0009] In a disclosed embodiment, a cathodoluminescence microscope is provided for inspecting a wafer on which a plurality of micro LEDs are formed, the microscope comprising: an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source to form an electron beam, and a scanner for scanning the electron beam over the wafer; an optical objective lens for collecting CL light emitted from the sample in response to the scanning of the electron beam and forming a CL light beam; a beam splitting arrangement for simultaneously generating multiple split beams from the CL light beam that are essentially aligned in space and time; a filter arrangement arranged to intersect the optical path of at least one of the CL light beams or the optical path of at least one of the split beams; and multiple point detectors arranged to respectively detect one of the split beams. In one embodiment, a different filter is arranged in the optical path of each of the split beams other than one split beam, such that one of the point detectors generates a panchromatic image and the remaining detectors generate filtered images. Furthermore, in other embodiments, a filter can be used in the other unfiltered channel to separate the radiation corresponding to the LEDs from the GaN bulk radiation generated between the chips. A filter that blocks radiation below 400 nm is useful for passing the full spectrum of the mesa emission in addition to removing the GaN bulk emission. In further embodiments, one or more dichroic beam splitters may be used to perform the beam splitting part simultaneously with the filtering part, but still generate at least one panchromatic reference without a filter or with a 400 nm filter to block the GaN bulk emission. The microscope may include a controller that integrates the signal received from each of the point detectors to generate a plurality of integrated emissions and analyzes the integrated emissions to determine structural and optical parameters of the micro-LEDs.

[0010] According to a disclosed aspect, there is provided a computer program stored on a storage device, which, when executed by a computer, causes the computer to perform steps including receiving a first electrical signal corresponding to a first filtered cathodoluminescence beam and a second electrical signal corresponding to a second filtered cathodoluminescence beam having a different filtering wavelength than the first filtered cathodoluminescence beam, integrating the first and second electrical signals to generate a first integrated signal and a second integrated signal, using the first and second integrated signals to reconstruct an emission spectral curve, and analyzing the emission spectral curve to determine characteristics of the micro LED. Analyzing the emission spectral curve may include determining a peak wavelength shift, an emission intensity change, a FWHM (full width at half maximum), etc. The processor may further assign a viability score to each region of the sample according to the emission spectrum curve, the viability score indicating high viability regions where the LEDs in the region are likely to be within acceptable specifications, low viability regions where the LEDs in the region are likely to be outside acceptable specifications and are not worthy of further investigation, and borderline viability regions where the LEDs in the region are worthy of further detailed investigation.

[0011] According to a further aspect, a method of operating a cathodoluminescence microscope to determine physical and optical properties of micro LEDs within a semiconductor sample is disclosed, the method including the steps of scanning the sample with an electron beam at a first resolution, collecting cathodoluminescence light emitted from an area during the scan and forming a light beam from the cathodoluminescence light, splitting the light beam into a plurality of split beams, filtering at least a portion of the plurality of split beams, using a plurality of point detectors to simultaneously detect the split beams and generate a plurality of electrical signals, each corresponding to the intensity of one of the split beams, and detecting a suspect area within the sample. The method includes analyzing the plurality of signals to identify suspected regions, scanning each of the suspected regions with an electron beam at a second resolution higher than the first resolution, collecting cathodoluminescence light emitted from the suspected regions during the scan and forming a second light beam from the cathodoluminescence light, splitting the second light beam into a plurality of second split beams, inserting a plurality of filters into the optical paths of portions of the second split beams, using a plurality of point detectors to simultaneously detect the second split beams and generate a plurality of second electrical signals, each corresponding to one of the second split beams, and analyzing the plurality of second signals to assign a viability score to each of the suspected regions. The method may further include detecting secondary electrons emitted from the suspected regions and analyzing an image generated by the secondary electrons to determine the shape of the LED within the suspected regions and / or to determine whether a foreign object is present within the suspected regions. The method may further include generating a full-spectrum CL image using a spectrometer and detector array assembly to detect the second light beam. [Brief explanation of the drawings]

[0012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain and illustrate the principles of the invention. The drawings are intended to diagrammatically show major features of exemplary embodiments. The drawings are not intended to depict every feature of an actual embodiment, nor to indicate relative dimensions of the depicted elements, and are not drawn to scale.

[0013] Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, which refer to the accompanying drawings. [Figure 1] FIG. 1 is a schematic cross-sectional view of the bottom portion of a cathodoluminescence scanning electron microscope for practicing embodiments disclosed herein. [Figures 2A-2C] 2A-2C are simplified schematic diagrams of an image acquisition section of a cathodoluminescence microscope according to an illustrated embodiment. [Figure 3] FIG. 3 is a flowchart illustrating a process for evaluating the characteristics and performance of micro LEDs and classifying defects, according to one embodiment. [Figure 4] FIG. 4 is a plot of CL intensity for the total population and individual regions, according to one embodiment. [Figure 5] FIG. 5 is a map of region survival scores according to one embodiment. [Figure 6] FIG. 6 is a map of shape uniformity scores according to one embodiment. [Figure 7] FIG. 7 is a map of out-of-specification scores according to one embodiment. [Figure 8] FIG. 8 is a CL hyperspectral image of a mesa of a micro-LED according to one embodiment. [Figure 9] FIG. 9 is the peak luminous energy (wavelength) per pixel minus the peak luminous energy of the mesa of the micro LED according to one embodiment. [Figure 10] 1A-1D are schematic diagrams illustrating a process flow of a micro LED wafer inspection method according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Some embodiments of the present invention are described in detail below with reference to the accompanying drawings, in which similar functional and structural elements appearing in different drawings may be given the same reference numerals.

[0015] Next, embodiments of the novel cathodoluminescence scanning electron microscope and operating method 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 their fullest extent, alone or in combination with other features, balancing requirements, constraints, and advantages. Accordingly, certain features, elements, or advantages may be 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 ("mixed and matched") with other features or incorporated into other embodiments, even if not explicitly described herein.

[0016] FIG. 1 shows a cross-sectional view of the lower portion of a CL microscope, which is described in more detail in the above-cited PCT / EP2020 / 063093. As shown in FIG. 1, the microscope generally includes an electron column 41 housed within a vacuum enclosure 10 and an imaging section 42 in an atmospheric environment. The integrated microscope shown in FIG. 1 can generate electron beam images, light beam images, cathodoluminescence (CL) images, and CL spectroscopy images. The imaged CL emission can be correlated to the structure and quality of the sample's material at the nanoscale. CL data can reveal material stress, impurities, crystalline structure, and subsurface defects that are not visible using other imaging modes. Importantly, CL imaging is a nondestructive method of examining samples.

[0017] The electron column includes an electron source 1, such as a thermionic or field emission source, which emits electrons. The emitted electrons are shaped into an electron beam 9 by various particle-optical elements, such as an electromagnetic lens 5', an electromagnetic objective lens 5, and aperture disks (sometimes called stops) 6, any of which may function as electrostatic lenses by application of an electric potential. In well-known manner, coils 11 are provided to generate a magnetic field. Most of the magnetic field may be located at the level of the output or exit aperture 13, or alternatively, may be outside the region between the lens and the specimen.

[0018] 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 may be changed by a condenser arrangement such as a lens 5' so that it can be diverged, collimated, or converged. The condenser may be located below the electron emitter. The electron beam typically has a width of a few millimeters, for example, in the range of 2-3 mm.

[0019] The lens 5 has a hollow interior along its optical axis, which allows the electron beam 9 to pass through. The hollow (passage 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 keep the exit 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 in such a way that the working distance remains small.

[0020] As can be seen in the figure, a reflective objective is provided within 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 that is rotationally symmetric about the optical axis z (essentially coinciding with the path of the electron beam), aberration-free, and infinity-corrected. The electromagnetic objective 5 and the reflective objective may have the same focal plane. The reflective objective within the electromagnetic objective 5 comprises a first mirror M1, also called the primary mirror, which in this example is spherical and concave, and a second mirror M2, also called the secondary mirror, which in this example is spherical and convex. The diameter of the first mirror M1 is larger than the diameter of the second mirror M2. A first mirror M1 is positioned above the second mirror M2 and is arranged to reflect light incident from the sample 7 as a result of the electron beam 9 striking 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 a 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 is at a 45° angle to the axis of the electron beam 9 and is used to redirect the light out of the vacuum enclosure 10. All three mirrors M1, M2, and M3 have apertures or openings along the path of the electron beam to prevent the electron beam from being obstructed.

[0021] In the optical imaging section 42, light reflected by mirror M3 is focused by lens 22 onto imaging monochromator 43. In this example, two imagers are provided: a detector array, e.g., a CCD camera 45, and a point detector 46, such as an InGaAs or PMT detector. If mirror 24 is a half mirror, both imagers may be operated simultaneously. Conversely, mirror 24 may be a flip mirror, allowing one imager to be operated at a time. In this arrangement, detector 46 can be used to detect light intensity at a specific wavelength, and a CCD camera can be used to detect light intensity at several wavelengths simultaneously.

[0022] CL microscopes have been used to inspect semiconductors to detect defects. In this disclosure, CL microscopes are adapted for inspecting micro-LEDs, specifically wafers containing multiple micro-LEDs, before they are cut into separate dies or chips. The apparatus and processes described herein may be used both during the development of a micro-LED production process and during actual mass production of micro-LEDs. In either case, it is beneficial to perform inspection of the structured wafer, for example, at the early stage of front-end post-mesa etching or at the end of front-end pre-singulation. Unlike semiconductor manufacturing, in LED manufacturing, a chip is typically a single LED, but hereafter, mesas on structured wafers may be referred to interchangeably as chips. Conversely, in micro-LED manufacturing, singulating each micro-LED on a single chip would be difficult and costly. Therefore, when a wafer is manufactured with multi-color micro-LEDs, each singulated chip tends to contain three micro-LEDs for RGB rendering. Thus, when the wafer is singulated into individual chips, each chip may have a single or several micro LEDs formed thereon, but each micro LED is formed on a single mesa. Thus, in terms of area on the wafer, the terms mesa and LED are interchangeable, while the term chip can refer to an area containing one or several mesas.

[0023] As mentioned above, the primary problem that must be addressed when attempting to inspect micro LED wafers is the sheer number of micro LEDs on each wafer, which significantly lengthens inspection times to an unacceptable extent. Therefore, a new approach was developed to overcome this problem. This new approach is somewhat modular in that it involves several sub-processes that can be utilized, rearranged, or modified as needed to enable testing at different stages of LED manufacturing process development and production maturity.

[0024] As described with respect to FIG. 1, a CL microscope can generate a histogram of light emitted from a sample, which can be used to obtain detailed information about the sample. In contrast, a point detector, such as a photodiode or PMT, generates an intensity signal of all the light it receives and therefore conveys less information than a CCD. However, the advantage of a single-point detector is that it is much faster than a CCD. Thus, a CL microscope has been modified to allow for high-speed scanning at the speed of a point detector, but still provides more meaningful information than that provided by a single-point detector.

[0025] 2A-2C show a modified CL microscope embodiment in which the point detector portion of the CL microscope of FIG. 1 has been enhanced. That is, the CCD, SE, and other detection and imaging elements of FIG. 1 have not been removed from the embodiment of FIGS. 2A-2C, but are not shown because the following description relates only to a single point detector portion. In FIG. 2A, a main CL beam MB is split into multiple split beams SB by a beam splitter arrangement, e.g., beam splitter 51. While three split beams are shown, any number of split beams may be used, and in most cases, at least three split beams are used. A corresponding number of point detectors 46a-46f are arranged to each receive one of the split beams. Each of the point detectors 46a-46f may be a biased photodiode, an avalanche photodiode, an amplified photodiode, or a photomultiplier tube (PMT). Additionally, a filter arrangement, e.g., filters 49a and 49b, is inserted in the optical path of at least one or a portion of the split beams. Useful results can be obtained if one of the point detectors (here 46a) captures the full spectrum, i.e., if the number of filters is equal to the number of split beams minus one. Optionally, a background filter BF may be inserted in the optical path of the main beam or upstream of the point detector (here 46a) capturing the full spectrum. The background filter is designed to remove radiation generated from the wafer region around the mesa, e.g., an optical filter that blocks wavelengths below 400 nm.

[0026] In FIG. 2B, the beam splitter arrangement consists of a beam splitter 51 that splits the main beam MB into two split beams, one directed to detector 46a for full-spectrum detection and the other directed to dichroic mirror 49d. Dichroic mirror 49d has different reflection and transmission characteristics so that the reflected and transmitted light are at two different wavelengths. In this respect, dichroic mirror 49d functions as a combined beam splitter and filter. The split beam reflected toward detector 46e has a different wavelength and / or bandwidth than the split beam transmitted to detector 46d. In FIG. 2C, one or more filters can be replaced with a thin piece of LED epiwafer with a reverse bias voltage applied between its front and back sides. In this configuration, the biased LED epiwafer simultaneously functions as both a custom absorbing optical filter with a complex transmission profile and as a photodetector.

[0027] Each of the bubbles in Figures 2A-2C shows the CL spectrum received by each of the point detectors 46a-46f. The curved lines indicate the actual CL spectrum of the split beams upstream of the filters, and the rectangles indicate the spectrum passing through the filters, i.e., the transmission range of the filters. As shown by the dashed bubble, no filter or only a background filter is applied to the split beam of point detector 46a, so point detector 46a receives the entire spectrum or the spectrum above 400 nm. Therefore, the signal output by point detector 46a corresponds to the full-spectrum (panchromatic) integrated intensity of the CL beam corresponding to the mesa. This is similar to the panchromatic signal (i.e., covering the entire spectrum sensitive to the detector) of the unfiltered point detector 46 in Figure 1. In contrast, as shown by the dotted bubble, filter 49c is a high-pass filter with a narrow bandwidth, so the signal output by point detector 46c corresponds to the high-energy spectrum, narrow-band integrated intensity, of the CL beam. Also, as shown in the dashed-dotted bubble, filter 49b is a broadband low-pass filter, so that the signal output by point detector 46b corresponds to the low-energy spectrum, broadband integrated intensity of the CL beam. Similarly, in Figure 2B, dichroic mirror 49d reflects light in the low-frequency band while transmitting light in the high-frequency band.

[0028] Thus, the system of Figures 2A-2C generates one panchromatic image and multiple filtered images of different energy centers and bands. These are merely examples, and any number, type, and combination of beam splitters, mirrors, and filters may be used. However, for useful results, the resulting split beams should be distinct from one another so that the resulting signal outputs of the point detectors are distinct. Mirrors and filters may include filters with edge-pass, band-pass, or even more complex transmission characteristics. As shown in Figure 2B, a special class of filter 49d, called a dichroic mirror, is used to allow both the filtered and transmitted portions of the signal to be detected by detectors 46d and 46e. This simplifies the design and optimizes overall photon collection efficiency. In Figure 2C, a special custom optical filter 49f is implemented in the form of a reverse-biased LED epiwafer to allow both the transmitted light to be detected by detector 46f and the light absorbed by the filter to be recorded as a photocurrent on the reverse-biased LED epiwafer. In this respect, the reverse-biased LED epiwafer acts as a beam splitter (to pass some of the light and absorb other parts of the light), a filter (to pass light in a different energy band than the light it absorbs), and a detector (to output a current that correlates to the light absorbed).

[0029]

[0006] Accordingly, there is provided a cathodoluminescence microscope for inspecting a wafer having a plurality of micro LEDs formed thereon, the microscope comprising: an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source to form an electron beam, and a scanner for scanning the electron beam over a sample; an optical objective lens for collecting CL light emitted from the sample in response to scanning the electron beam and forming a CL light beam; a beam splitting arrangement for simultaneously generating a plurality of split beams from the CL light beam that are essentially aligned in space and time; a filter arrangement arranged to filter at least one of the split beams; a plurality of point detectors arranged to respectively detect one of the split beams; and a controller for receiving output signals from the plurality of point detectors and generating therefrom a panchromatic image and a plurality of filtered images. The controller may be further operative to integrate a signal received from each of the point detectors to generate a plurality of integrated emissions, and may be further operative to analyze the integrated emissions to determine characteristics of the micro LEDs. The controller may be further operative to generate a plurality of ratios, each ratio comprising a ratio of signals received from two of the plurality of point detectors.

[0030] 3 shows a flow chart of a process utilizing CL microscopy to inspect micro-LED wafers, according to one embodiment. The process disclosed herein may be divided into three modules that may be included or omitted in the workflow depending on the particular testing environment (process development or production) and the maturity of the manufacturing process. For example, only Module 1 may be required as an inspection method when inspecting wafers under a mature production process, while all three modules may be implemented during micro-LED technology development.

[0031] Regardless of the workflow being performed, if the workflow includes inspection, it begins with module 1, fast scan 300 (otherwise, if only wafer sampling is required, it can skip to module 2 or 3). The fast scan is performed in a global area, meaning the entire wafer, or several large areas that together encompass a substantial portion of the wafer to form an adequate representation of the entire wafer. The fast scan is a low-resolution scan, for example, with a spatial resolution of 250 μm to 250 nm, depending on the size of the targeted mesas. For example, the resolution can be adjusted by the sampling rate of the point detector. In one embodiment, the spatial resolution is set so that at least one data point is collected from each mesa (LED) in the global area, and advantageously, multiple data points are obtained from each mesa. For example, the scan resolution can be set to obtain 2×2, 3×3, 4×4, 5×5, 6×6, or up to 10×10 pixels per mesa. For example, for a wafer with a 10 micron mesa size, setting the resolution to 4x4 pixels per mesa results in a spatial resolution of 2.5 microns, and for a wafer with a 1 micron mesa, 4x4 pixels per mesa results in a spatial resolution of 250 nm.

[0032] During the rapid scan, each point detector outputs a signal corresponding to CL emission in a defined energy band (or all energies if no filters are used). In one embodiment, the data obtained from the point detectors is analyzed 305 to identify suspect regions where the optical properties deviate from defined expectations 310. In one embodiment, the point detector data for each spatial scan position is used to roughly reconstruct the spectral emission at that position. For example, the signal from the unfiltered split beam can be used to determine the total emission intensity, and by using differential analysis (e.g., obtaining the ratio of the point detector signals), it is possible to estimate the emission intensity per wavelength and determine parameters related to the emission FWHM, the emission asymmetry, i.e., the ratio of the low-energy half-maximum to the high-energy half-maximum, which ultimately affects the color rendering properties of the LED.

[0033] As an example of differential analysis, if five point detectors are used, one panchromatic and four filtered, the five intensity signals are: panchromatic signal, I p , low-pass narrowband signal, I LNB , low-pass wideband signal, I LWB , high-pass narrowband signal, I HNB , high-pass wideband signal, I HWB Also, the signal can be I LNB / I p , I LWB / I p , I HNB / I p , I HWB / I p , I LNB / I LWB , I LNB / I HNB , I LNB / I HWB , I LWB / I HNB , I LWB / I HWB, etc. These detector signals and ratios may be compared against expected ratios and used to construct a total wafer population, which may be used to compare each mesa / LED ratio against the distribution of the total wafer population, etc. Also, as described above, the detector signals and ratios may be used to estimate the energy distribution curve of each mesa / LED. The estimated distribution may be compared against an expected distribution or against the distribution of the total wafer population. The results of such analysis may be used to inspect the manufacturing line and / or to bin the wafers / LEDs.

[0034] To provide a numerical example, assume the system records two channels. Ch1 integrates all wavelengths (panchromatic), while Ch2 integrates wavelengths from 470 to 600 nm. If the LED target wavelength is 470 nm, the expected ratio for Ch2 / Ch1 is: Therefore, a measured ratio of 0.6 means that the spectrum is redshifted, i.e., the peak wavelength is higher than the desired 470 nm, or that the ratio of the emission widths (longer wavelengths to shorter wavelengths) has changed significantly. By recording additional channels, such as Ch3, which integrates a narrow band below the desired 470 nm, e.g., 450 to 460 nm, and Ch4, which integrates a narrow band above the desired 470 nm, e.g., 480 to 490 nm, it is possible to determine whether the change in the Ch2 / Ch1 ratio is due to a peak shift or a change in the linewidth / symmetry of the emission spectrum.

[0035] The analysis in 305 can be used to identify extremes, i.e., regions / mesas that do not fit the expected distribution. If the expected distribution is known, for example, if the scan is performed on wafers under a mature production process, the distribution can be simply compared to a reference distribution, e.g., a "golden wafer." Conversely, if no reference is available, or if one wishes to determine within wafer distribution, the distribution of the total population (of the scanned wafer, or of the total population of multiple previously scanned wafers) can be used as a reference, and the distribution of each region can be compared against the reference. The intensity distribution within any good region is expected to be comparable to the total population distribution.

[0036] Figure 4 shows the intensity distribution plot of the total population of LEDs within a wafer. Plotted separately (inside the ellipses) are the intensity distributions of two regions within the wafer. As can be seen, one of the plots is slightly shifted to the right, but somewhat resembles the total population distribution plot. This region can be considered within the acceptable range. Conversely, the second plot is significantly shifted to the left, and its distribution does not resemble the total population distribution and therefore may be considered outside the acceptable range.

[0037] The analysis performed at 305 can identify spatial regions that are out of specification. Furthermore, if the resolution is set so that data is collected for each individual mesa / LED, the region may correspond to a specific LED. In this regard, the term region as used herein may encompass a single mesa / LED or a specific mesa / LED and several adjacent mesas / LEDs. For example, subpopulations can be determined from the extremes of the population distribution based on predetermined percentiles (e.g., bottom 0.1%, top 0.1%, and median 1%), and filtering can be performed based on performance characteristics such as the intensity of a given channel (point detector), the ratio of two channels, etc.

[0038] Upon completion of step 310, a determination can be made at 315 whether to proceed to module 2 processing. For example, if module 1 testing is performed to inspect a mature production line, it may be necessary only to identify whether a process deviation has occurred or to verify that the process is stable and production can proceed, and therefore it may not be necessary to proceed to modules 2 or 3. Furthermore, under such conditions, module 1 processing may be used to bin LEDs during production; that is, the emission spectrum of an LED corresponds to the color of light the LED emits during operation. Thus, for example, inspected LEDs may be binned according to the energy of their peak intensity, according to their width at FWHM, according to the asymmetry of their intensity distribution, etc.

[0039] If a decision is made to proceed to module 2 in step 315, e.g., during process development or prior to maturation of the production process, a high-resolution review scan is performed only on selected regions identified as extremes. If there is no "golden" standard, a review scan may be performed on local regions selected from the worst-, best-, and median-performing regions identified in step 310. If there is a golden standard, a review scan may be performed only on the worst-performing local regions to further investigate the cause of the failure.

[0040] During the review scan 320, the point detector is operated at a higher sampling rate than during the fast scan so that additional information can be gathered from each scan area, e.g., each selected mesa. For example, the review scan may be configured to generate more than 16 x 16 pixels per mesa, up to a maximum of 64 x 64 pixels. Additionally, other data, such as full spectrum SE imaging using a CCD, is collected during the review scan. The data collected during the review scan is analyzed in 325. For example, high-resolution CL images can be evaluated for the intensity and uniformity of the light emission and threading dislocation density (see, e.g., U.S. Patent Application No. 17 / 537,422). Additionally, if the local scan area includes a particular mesa and its neighboring mesas, the spectrum of the particular mesa can be compared to the spectrum of the neighboring mesa. The SE images can be evaluated for the presence of mask printing errors, such as mesa shape (e.g., by comparison to a reference "golden" shape or by line tracing the outer edge of the mesa), the presence of particles such as etch deposits, and uniformity. Image processing and machine vision techniques, such as edge tracing and feature recognition, can also be used to assign a shape uniformity factor (SUF). Full-spectrum data, e.g., from a CCD, may be evaluated for high spatial resolution peak emission per wavelength, FWHM, total intensity, etc.

[0041] As mentioned above, for various considerations such as within wafer uniformity, the review scan and analysis can be based on randomly sampling the best, median, and worst performers of the population based on the results of the fast scan. By randomly sampling the best, median, and worst performers of the population and evaluating the variance in the sampled mesas / LEDs, the performance of each region can be evaluated with respect to process control limits.

[0042] The analysis performed in 325 may be used in step 330 to assign viability scores to various regions. In one embodiment, the viability scores range from 10 to 100 and depend on the population distribution for a given process window, e.g., at the region level, wafer level, cassette level, etc. FIG. 5 shows region viability scores calculated for a wafer-level process window. Calculating region viability scores helps identify high- and low-performing regions on a wafer as early as possible. In one embodiment, the viability regions are divided into three levels. High viability regions (HVRs) are regions where even low-performing mesas / LEDs are likely to be within specifications. When further measurements are performed on the wafer, these regions can be skipped, thereby reducing measurement time. Low viability regions (LVRs) are regions where even the best-performing mesas / LEDs are likely to be out of specifications. Even in this case, there is no need to extend further measurements to investigate these regions. The regions of most interest are borderline viability regions (BVRs). These areas are more likely to have mesas / LEDs that are within specification and mesas / LEDs that are out of specification, so metrology efforts should be focused only on those areas.

[0043] Step 330 can be highly beneficial under certain circumstances. For example, early identification of low viability areas supports WIP reduction by reducing feedback time at critical process steps. This also reduces total probing costs by optimizing the time spent in later metrology stages and allocating the probing time budget more effectively. This can also support faster time to market by helping to understand early which process changes affect viability more than others.

[0044] In addition to the area viability score map shown in Figure 5, other maps can be generated to aid in process development and inspection, as well as binning. For example, since the review scan produces high-resolution images of individual mesas, the shape of each mesa can be analyzed for uniformity. Figure 6 shows a shape uniformity score map, which may indicate problems in the photolithography or etching stages.

[0045] Similarly, Figure 7 shows deviation score mapping that can be performed for various parameters of a wafer. The score correlates to how far the results deviate from the defined process window. That is, the score in each region indicates the degree of deviation from specification. This can be done, for example, for wavelength at peak intensity (which correlates to LED color), width at FWHM (which correlates to color sharpness / broadness), etc.

[0046] In step 335, a decision may be made to proceed to optional Module 3, which involves detailed examination of specific mesas / LEDs within the wafer. In step 340, an ultra-high-resolution review is performed. Ultra-high-resolution review Module 3 is of particular interest for the process development stage and identification of key reductors, as measurements can be performed immediately before and after specific front-end process steps, such as the mesa etch process, to better understand the process interactions with the device's optoelectronic properties. A better understanding of the interactions of epi process steps ultimately leads to better process control through better definition of key performance indicators (KPIs) and discovery of new KPIs.

[0047] In the ultra-high resolution scan 340, the resolution is set to 64x64 pixels or more per mesa. With respect to spatial resolution, ultra-high resolution is set to 100 nm or better, e.g., 20 nm or more and 100 nm or less. In this scan, a high-resolution panchromatic / filtered data set is collected, or hyperspectral data is collected for each mesa, e.g., using a CCD. The acquisition of hyperspectral data in this step can be spatially resolved using the resolution specified herein, but does not need to be spatially resolved. If not spatially resolved, a field of view is defined (e.g., 16x16 pixels), and the area is scanned with an electron beam while the emitted light is collected by the CCD. The CCD buffer is read only after a preset integration time. This mode can be referred to as a time-integration scan mode.

[0048] The first type of collection can be analyzed to identify, for example, threading dislocation density, emission wavelength uniformity, intensity uniformity, mesa edge shape, mesa uniformity, etc., for each mesa. Hyperspectral maps can be analyzed to determine all of the above as well as other metrics, such as peak emission energy or FWHM and other related parameters, as shown in Figure 9. Figure 8 shows a hyperspectral intensity map for a single mesa obtained from an ultra-high-resolution scan. Figure 9 shows a hyperspectral map of the same data as Figure 8, but with the peak emission energy subtracted from the average energy for each mesa, obtained from an ultra-high-resolution scan. In the plot in Figure 9, subtracting the average peak emission energy from the data results in an energy shift. Pixels (in the map) with a positive energy shift indicate that the emission is shifted more toward the blue wavelength range (compared to the average), while pixels with a negative energy shift indicate a shift toward the red wavelength range.

[0049] According to embodiments disclosed herein, various methods for inspecting micro-LEDs using CL microscopy can be tailored to provide appropriate information during process development and device fabrication. The various modules disclosed along with the flowchart in FIG. 3 can be implemented as needed using a CL microscope according to the example shown in FIG. 2. A general flow is shown in FIG. 10, where multiple point detectors receive split beams passing through different filters, and a fast multi-channel scan of the wafer or a large global area thereof is performed in module 1. The signals of the various filtered bands (e.g., red, green, and blue filters) are integrated to generate different ratios of the channels. The results can be compared against expected results or average results obtained from the entire population of wafers.

[0050] Localized regions with significant deviations from expected results are selected for high-resolution scanning in Module 2. Scans are performed on the specific mesa and its nearest neighbors. During the scan, various imaging techniques can be performed, such as SE imaging, CL hyperspectral imaging, and CL filtered / single-spectrum imaging. This imaging information can be analyzed to obtain, for example, intensity, peak emission energy or FWHM, mesa shape, and inter-mesa spacing. In Module 3, specific mesas are selected for ultra-high-resolution review. The ultra-high-resolution scans generate high-spatial resolution images, such as SE images, CL images, CL peak shifts, and CL peak FWHM, of the specifically selected mesas to investigate their characteristics.

[0051] In one embodiment, a method for inspecting micro LEDs using a CL microscope is provided, comprising: performing a first scan of a global area of ​​a wafer at a first resolution using multiple point detectors, i.e., single-channel detectors such as PMTs or biased photodiodes; and inserting multiple distinct optical filters into the optical paths of at least a portion of the multiple point detectors. The optical filters may include filters with edge-pass, band-pass, or more complex transmission characteristics. When several band-pass filters are used, the center frequencies or bandwidths of all the filters are different. The scanned global area can be the entire wafer or a wafer sub-area having a size that is statistically meaningful and representative of the entire wafer. The method then analyzes the population distribution of the measured global area by determining the ratio of the recorded single channels and comparing the ratio to an expected value from a reference spectrum. The reference spectrum may be constructed using data collected for the entire population within the scanned global area. The method then determines subpopulations from the extremes of the population distribution based on predetermined percentiles (e.g., worst 0.1%, best 0.1%, and median 1%) based on performance characteristics such as the strength of a given channel or the ratio of two channels.

[0052] Once the subpopulations are identified, the method may proceed to a review scan, in which the subpopulations are scanned at a second resolution higher than the first resolution. During the scan of the local regions, the method obtains high-resolution panchromatic images, high-resolution SE images, hyperspectral maps, and / or area-averaged hyperspectral acquisition maps. The sampling rate of the subpopulations can be determined by required throughput criteria, and each local region may include a particular mesa and its immediate neighboring mesas. The hyperspectral characteristics of the reviewed mesa are compared against predetermined process control limits or expected results determined from averaging the results of all sampled populations to obtain the criteria. Then, for each subpopulation, associated performance criteria are evaluated to estimate the percentage of the reconstructed population that falls within a predetermined process window. The process criteria may include any combination of total emission intensity, emission wavelength per energy, emission FWHM, emission spectral asymmetry (e.g., the ratio of the high-energy half-maximum to the low-energy half-maximum, which affects color rendering), shape uniformity factor (corresponding to the shape of the mesa relative to the reference shape), threading dislocation density, etc. The scores for each of these performance criteria are combined, optionally with weighting, to generate a region survival score, which may be determined according to the process sensitivity to the individual indicators.

[0053] In one embodiment, a method for inspecting micro LEDs using a CL microscope is provided, comprising: performing a first scan of a global area of ​​a wafer using multiple point detectors, i.e., single-channel detectors such as PMTs or biased photodiodes; and a filter arrangement limiting the bandwidth of light directed to at least a portion of the multiple point detectors. The filter arrangement may include filters with edge-pass, band-pass, dichroic mirrors, biased wafers, or more complex transmission characteristics. The scanned global area can be the entire wafer or a wafer sub-area that is statistically meaningful and representative of the entire wafer. The method then analyzes the population distribution of the measured global area by determining the ratio of the recorded single channels and comparing the ratio to an expected value from a reference spectrum. The reference spectrum may be constructed using data collected for the entire population within the scanned global area.

[0054] In this embodiment, a review scan is not performed. Instead, the process proceeds to compare the measured population distribution of the mesa against predetermined process control limits. The process then determines a parametric performance index for all relevant performance criteria by calculating the percentage of the population distribution that falls within the process control limits. Process criteria may include any combination of total emission intensity, emission wavelength per energy, emission FWHM (estimated by combining multiple single-channel intensity ratios), and emission spectral asymmetry (e.g., the ratio of high-energy half-maximum to low-energy half-maximum, which affects color rendering). The scores for each of these performance criteria are combined, optionally with weighting, to generate an area-effective score. With respect to the flow of FIG. 3, this means that step 330 is performed after step 310. The weighting may be determined according to the process sensitivity for each index.

[0055] According to this disclosure, there is provided a method for fabricating micro LEDs, the method comprising the steps of forming a plurality of mesas on a wafer, each mesa having a length of 100 microns or less, and transferring the wafer to a CL microscope; performing a first scan of the wafer at a first resolution to generate a plurality of CL beams; using a plurality of single point detectors to simultaneously detect the plurality of CL beams, each point detector being positioned to receive a different partial spectrum from the remaining point detectors; generating a plurality of detector ratios, each detector ratio comprising a ratio of signals received from two of the point detectors; and comparing the detected ratios to expected ratios to determine a characteristic of the mesa.

[0056] Also provided is a method for inspecting a wafer having multiple mesas, each mesa having a length of 100 microns or less, for forming microLEDs in each mesa, the method comprising: placing the wafer in a CL microscope; performing a first scan of the wafer at a first resolution to generate multiple CL beams; using multiple single point detectors to simultaneously detect the multiple CL beams, each point detector being arranged to receive a different partial spectrum from the remaining point detectors; generating multiple detector ratios, each detector ratio comprising a ratio of signals received from two of the point detectors; and comparing the detected ratios to expected ratios to determine a characteristic of the mesa. The first resolution may be set to generate at least one pixel per mesa and up to 10 x 10 pixels per mesa. One of the multiple point detectors may be arranged to receive a spectrum at wavelengths higher than background wavelengths, e.g., greater than 400 nm. The method may further comprise using signals received from the multiple detectors to reconstruct the spectral emission at each scanned mesa. The method may also include using signals received from the multiple detectors to determine CL emission asymmetry at each scanned mesa. The method may include using signals received from the multiple detectors to determine an intensity distribution at each scanned mesa and comparing it to a total population intensity distribution determined from all scanned mesas. The signals received from the multiple detectors may be used to assign the scanned mesas to multiple groups. The signals received from the multiple detectors may be used to identify target areas on the wafer, and then performing a second scan of the target areas at a second resolution higher than the first resolution to generate an inspection signal and using the inspection signal to determine the peak emission per wavelength within each target scan area. A secondary electron detector may also be used during the second scan to form a secondary electron image of each target scan area. Image processing may be used to determine the physical shape of each mesa within the target scan area.The secondary electron images can be examined to identify foreign objects within the target scan areas. A viability score can be assigned to each target scan area.

[0057] As disclosed with reference to FIG. 2 , a cathodoluminescence microscope for inspecting a wafer having a plurality of micro-LED mesas formed thereon is provided, the cathodoluminescence microscope comprising: an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source to form an electron beam, and a scanner for scanning the electron beam over a sample; an optical objective lens for collecting light emitted from the sample in response to the scanning of the electron beam and forming a light beam; an imaging unit including a focusing lens, a splitter arrangement for splitting the light beam into a plurality of split beams, a plurality of point detectors arranged to receive each of the split beams, and a filter arrangement for limiting the bandwidth of at least a portion of the plurality of split beams; and a controller for receiving output signals from the plurality of point detectors and generating a ratio of pairs of output signals of the plurality of point detectors. The point detectors may comprise biased photodiodes, avalanche photodiodes, amplified photodiodes, or photomultiplier tubes (PMTs). The cathodoluminescence microscope may further comprise a background filter for blocking wavelengths less than 400 nanometers. The filter arrangement may include at least one dichroic mirror, multiple bandpass filters with different center frequencies or different bandwidths, or at least one biased LED wafer. The controller may compare the ratio against a reference ratio and identify regions of the wafer having ratios that deviate from the reference ratio. The controller may further assign a survival score to the region.

[0058] While the invention has been shown and described in detail in the drawings and foregoing description, such illustration and description is to be considered as illustrative and exemplary and not restrictive, and the invention is not limited to the disclosed embodiments. Other embodiments and modifications can be understood and effected by those skilled in the art in practicing the claimed invention, upon a study of the drawings, the disclosure and the appended claims.

[0059] 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

1. 1. A cathodoluminescence microscope for inspecting a wafer having a plurality of micro-LED mesas formed thereon, comprising: an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source to thereby form an electron beam, and a scanner for scanning the electron beam over a sample; an optical objective lens that collects light emitted from the sample in response to scanning of the electron beam and forms a light beam; an imaging section comprising a focusing lens, a splitter arrangement for splitting the light beam into a plurality of split beams, a plurality of point detectors arranged to receive each of the split beams, and a filter arrangement for limiting a bandwidth of at least a portion of the plurality of split beams; a controller receiving output signals from the plurality of point detectors and generating a ratio of paired output signals of the plurality of point detectors.

2. 2. The cathodoluminescence microscope of claim 1, wherein the point detector comprises a biased photodiode, an avalanche photodiode, an amplified photodiode or a photomultiplier tube (PMT).

3. 10. The cathodoluminescence microscope of claim 1 further comprising a background filter that blocks wavelengths below 400 nanometers.

4. 2. The cathodoluminescence microscope of claim 1, wherein the filter arrangement comprises at least one dichroic mirror.

5. 2. The cathodoluminescence microscope of claim 1, wherein the filter arrangement comprises a plurality of bandpass filters having different center frequencies or different bandwidths.

6. 2. The cathodoluminescence microscope of claim 1, wherein the filter arrangement comprises at least one biased LED wafer.

7. The cathodoluminescence microscope of claim 1 , wherein the controller compares the ratio against a reference ratio and identifies areas of the wafer having a ratio that deviates from the reference ratio.

8. The cathodoluminescence microscope of claim 7 , wherein the controller is further configured to assign a viability score to the region.

9. 1. A method for inspecting a wafer having a plurality of mesas for forming a micro LED on each mesa, each mesa having a length of 100 microns or less, comprising: placing the wafer under a CL microscope; performing a first scan of the wafer at a first resolution to generate a plurality of CL beams; using a plurality of single point detectors for simultaneously detecting the plurality of CL beams, each of the single point detectors being positioned to receive a different partial spectrum than the remaining single point detectors; generating a plurality of detector ratios, each of said detector ratios comprising a ratio of signals received from two of said point detectors; comparing the detected ratio against an expected ratio to determine a characteristic of the mesa.

10. 10. The method of claim 9, wherein the first resolution is set to produce at least 1 pixel per mesa and up to 10x10 pixels per mesa.

11. The method of claim 9 , wherein one of the plurality of point detectors is positioned to receive a spectrum of wavelengths above a background wavelength.

12. The method of claim 9 , further comprising using the signals received from the multiple detectors to reconstruct the spectral emission at each of the scanned mesas.

13. The method of claim 9 , further comprising using the signals received from the multiple detectors to determine asymmetry of CL emission in each of the scanned mesas.

14. 10. The method of claim 9, further comprising using the signals received from the multiple detectors to determine an intensity distribution at each of the scanned mesas and comparing against a total population intensity distribution determined from all of the scanned mesas.

15. The method of claim 9 , further comprising using the signals received from the multiple detectors to assign the scanned mesas to multiple groups.

16. using the signals received from the plurality of detectors to identify areas of interest on the wafer; performing a second scan of the area of ​​interest at a second resolution greater than the first resolution to generate an interrogation signal; 10. The method of claim 9, further comprising using the test signal to determine a peak emission per wavelength within each scanned area of ​​interest.

17. 17. The method of claim 16, further comprising using a secondary electron detector to form a secondary electron image of each of the object scan areas.

18. The method of claim 17 , further comprising using image processing to determine the physical shape of each mesa within the target scan area.

19. 20. The method of claim 17, further comprising inspecting the secondary electron image to identify foreign objects within the target scan area.

20. The method of claim 16 , further comprising the step of assigning a viability score to each of the target scan regions.

21. 1. A cathodoluminescence microscope for inspecting a wafer having a plurality of mesas formed thereon, comprising: an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source to thereby form an electron beam, and a scanner for scanning the electron beam over a wafer; an optical objective lens that collects CL light emitted from the wafer in response to scanning of the electron beam and forms a CL light beam; a beam splitting arrangement for simultaneously generating from the CL light beam a plurality of split beams that are essentially aligned in space and time; a filter arrangement arranged to intersect an optical path of at least one of the CL light beams or an optical path of at least one of the split beams; a plurality of point detectors, each positioned to detect one of the split beams.

22. 22. The microscope of claim 21, wherein the filter arrangement includes a background filter that blocks radiation below 400 nm.

23. 22. The microscope of claim 21, further comprising a controller that integrates signals received from each of the point detectors to generate a plurality of integrated emissions.

24. 22. The microscope of claim 21, wherein the filter arrangement comprises any one of a plurality of bandpass filters, at least one dichroic mirror, and a biased LED wafer.

25. 1. A computer program for inspecting a wafer having a plurality of mesas, stored on a storage device and configured to cause the computer, when executed by a computer, to perform the steps of: receiving a first electrical signal corresponding to a first filtered cathodoluminescence beam emitted from one of the mesas and a second electrical signal corresponding to a second filtered cathodoluminescence beam emitted from the mesa and having a different filtering wavelength than the first filtered cathodoluminescence beam; integrating the first and second electrical signals to generate first and second integrated signals; using the first and second integrated signals to reconstruct an emission spectral curve of the mesa; and analyzing the emission spectrum curve to determine characteristics of the mesa.

26. 26. The computer program of claim 25, wherein analyzing the emission spectral curve comprises determining a peak wavelength shift, an emission intensity change, or a full width at half maximum.

27. 27. The computer program of claim 26, further comprising the step of assigning a viability score to each region of the wafer according to the emission spectral curve.

28. 1. A method of operating a cathodoluminescence microscope to determine physical and optical properties of a mesa in a semiconductor wafer for micro-LED fabrication, comprising: scanning an area of ​​the wafer with an electron beam at a first resolution; collecting cathodoluminescent light emitted from said region during scanning and forming a light beam from said cathodoluminescent light; splitting the light beam into a plurality of split beams; filtering at least a portion of the plurality of split beams; using a plurality of point detectors to simultaneously detect the split beams and generate a plurality of electrical signals, each corresponding to an intensity of one of the split beams at a filtering frequency; analyzing the plurality of signals to identify suspect regions within the wafer.

29. scanning each of the suspect regions with the electron beam at a second resolution higher than the first resolution; collecting cathodoluminescent light emitted from the suspect area during scanning and forming a second light beam from the cathodoluminescent light; splitting the second light beam into a plurality of second split beams; filtering at least a portion of the plurality of second split beams; using the plurality of point detectors to simultaneously detect the second split beams and generate a plurality of second electrical signals, each corresponding to one of the second split beams; 30. The method of claim 28, further comprising analyzing the plurality of second electrical signals to assign a viability score to each of the suspicious regions.

30. detecting secondary electrons emitted from the suspect region; 30. The method of claim 29, further comprising: analyzing an image produced by the secondary electrons to determine a shape of a mesa within the suspect area.

31. detecting secondary electrons emitted from the suspect region; 30. The method of claim 29, further comprising the step of determining whether a foreign object is present within the suspect region.

32. 30. The method of claim 29, further comprising generating a full spectrum cathodoluminescence image using a spectrometer and detector array assembly to detect the second light beam.

Images