SEGMENTED DETECTOR FOR A CHARGED PARTICLE BEAM DEVICE

A detector for a charged particle beam device includes a substrate, a number of first sensor devices provided on the substrate, wherein the first sensor devices are structured to be sensitive to and generate a first signal in response to electrons ejected by a specimen, and a number of second sensor devices provided on the substrate, wherein the second sensor devices are structured to be sensitive to and generate a second signal in response to photons emitted by the specimen. Also, a photon detector wherein each of the photon sensor devices is structured to be sensitive to and generate a signal in response to photons emitted by the specimen, and wherein each of the photon sensor devices comprises a MultiPixel Photon Counter device. Further, a method of imaging a specimen using a charged particle beam device uses beam blanking and determination of estimated a decay time constants.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to imaging using a charged particle beam device, such as an electron microscope, and, in particular, to a segmented detector for a charged particle beam device including one or more sensors sensitive to electrons and one or more sensors sensitive to photons, and to a charged particle beam device employing such a segmented detector. The present invention also relates to a segmented photon detector employing MultiPixel Photon Counter technology, and to a method of obtaining an image of decay time constants in order to improve cathodoluminescence (CL) imaging.

2. Description of the Related Art

An electron microscope (EM) is a type of microscope that uses a particle beam of electrons to illuminate a specimen and produce a magnified image of the specimen. One common type of EM is known as a scanning electron microscope (SEM). An SEM creates images of a specimen by scanning the specimen with a finely focused beam of electrons in a pattern across an area of the specimen, known as a raster pattern. The electrons interact with the atoms that make up the specimen, producing signals that contain information about the specimen's surface topography, composition, and other properties such as crystal orientation and electrical conductivity.

In a typical SEM, electrons are generated by an electron gun assembly that is positioned at the beginning of a series of focusing optics and deflection coils, called an electron column or simply “column” because its axis is typically vertical. The column is followed by a sample chamber or simply “chamber” housing the specimen and accommodating a variety of detectors, probes and manipulators. Because electrons are readily absorbed in air, both the column and the chamber are typically evacuated, although in some cases the chamber may be back-filled to a partial pressure of dry nitrogen or some other gas. After being generated by the electron gun assembly, the electrons follow a path through the column and are caused thereby to form a finely focused beam of electrons (on the order of 1-10 nanometers) that is made to scan the specimen in the chamber in a raster fashion as described above.

When the electron beam hits the specimen, some of the beam electrons (primary electrons) are reflected/ejected back out of the specimen by elastic scattering resulting from collisions between the primary electrons and the nuclei of the atoms of the specimen. These electrons are known as backscattered electrons (BSEs) and provide both atomic number and topographical information about the specimen. Some other primary electrons will undergo inelastic scattering causing secondary electrons (SEs) to be ejected from a region of the specimen very close to the surface, providing an image with detailed topographical information at the highest resolution. If the specimen is sufficiently thin and the incident beam energy sufficiently high, some electrons will pass through the sample (transmitted electrons or TEs). Backscattered and secondary electrons are collected by one or more detectors, which are respectively called a backscattered electron detector (BSED) and a secondary electron detector (SED), which each convert the electrons to an electrical signal used to generate images of the specimen.

Cathodoluminescence (CL) is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material cause the emission of photons. It is known in the art to fit an SEM as just described with a separate CL detector. In such a configuration, the focused beam of electrons of the SEM impinges the specimen and induces it to emit photons. Those photons are collected by the CL detector and may be used to analyze the internal structure of the specimen in order to get information on the composition, crystal growth and quality of the material.

U.S. Pat. No. 8,410,443 describes a system for collecting both electron and CL images simultaneously. However, the method described therein requires reflection of the visible light away from the electron detector to a separate optical detector. The cover figure of the patent shows the light detectors mounted below the BSE (backscattered electron) detector whose outer surface is mirrored. This arrangement considerably lengthens the minimum working distance (the distance between the pole piece and the sample). Also, mirroring of the BSE detector surface necessarily reduces sensitivity to low-energy electrons, which are absorbed by the mirror coating. Furthermore, the extra optical detector consumes a lot of space around the sample. It is now commonly desirable for other types of detectors to be in close proximity to the sample, so space is at a premium. Space is particularly critical for the dual-beam instruments referenced elsewhere herein. The extra optical detector will also reduce the signal reaching a secondary electron detector, which is a standard imaging mode for electron microscopy.

Thus, there is room for improvement in the field of detectors structured for collection of electron and CL images.

SUMMARY OF THE INVENTION

In one embodiment, a detector for a charged particle beam device is provided that includes a substrate structured to be mounted within the charged particle beam device, a number of first sensor devices provided on the substrate, wherein each of the first sensor devices is structured to be sensitive to and generate a first signal in response to electrons ejected by a specimen, and a number of second sensor devices provided on the substrate, wherein each of the second sensor devices is structured to be sensitive to and generate a second signal in response to photons emitted by the specimen.

In another embodiment, a photon detector for a charged particle beam device is provided that includes a substrate structured to be mounted within the charged particle beam device, wherein the substrate includes a pass-through extending through the substrate for allowing a beam of the charged particle beam device to pass through the photon detector, and a plurality of photon sensor devices provided on the substrate spaced about the pass-through, wherein each of the photon sensor devices is structured to be sensitive to and generate a signal in response to photons emitted by the specimen, and wherein each of the photon sensor devices comprises a MultiPixel Photon Counter device.

In another embodiment, a method of imaging a specimen using a charged particle beam device is provided. The method includes directing an electron beam of the charged particle beam device to a first pixel position of the specimen for a first period of time, deflecting the electron beam away from the first pixel position for a second period of time, measuring a plurality of light intensity levels emitted from the first pixel position during the second period of time using a detector having a number of MultiPixel Photon Counter sensors, and using the plurality of light intensity levels to estimate a decay time constant for the first pixel position.

In still another embodiment, a charged particle beam device is provided that includes an electron source structured to generate an electron beam, a beam blanker, a photon detector including a number of MultiPixel Photon Counter sensors, and a control system. The control system is structured to cause the electron beam to be directed to a first pixel position of the specimen for a first period of time, cause the beam blanker to deflect the beam away from the first pixel position for a second period of time, cause the detector to measure a plurality of light intensity levels emitted from the first pixel position during the second period of time, and use the plurality of light intensity levels to estimate a decay time constant for the first pixel position.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As used herein, the singular form of “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. As used herein, the statement that two or more parts or components are “coupled” shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs.

As used herein, “directly coupled” means that two elements are directly in contact with each other.

As used herein, “fixedly coupled” or “fixed” means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other.

As used herein, the word “unitary” means a component is created as a single piece or unit. That is, a component that includes pieces that are created separately and then coupled together as a unit is not a “unitary” component or body.

As used herein, the statement that two or more parts or components “engage” one another shall mean that the parts exert a force against one another either directly or through one or more intermediate parts or components.

As used herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality).

As used herein, the term “segmented” in connection with a detector shall mean that the detector includes multiple discrete sensor devices (e.g., on a single substrate) to enable imaging from different viewpoints (elevation and azimuth), wherein the sensor devices have different sensing/detecting characteristics (e.g., one or more sensor devices have a first sensing/detecting characteristic such as the ability to detect electrons or detect light of a first spectral region, and one or more different sensor devices have a second sensing/detecting characteristic such as the ability to detect photons or detect light of a second, different spectral region), and wherein each sensor or type of sensor can be accessed (read out) independently.

As used herein, the terms “solid state photomultiplier” and “MultiPixel Photon Counter (MPPC)” shall mean an array of Geiger mode avalanche photodiodes on a common semiconductor substrate which outputs a current that is proportional to the flux of incident radiation. Current MPPCs are sensitive to photons in the visible (RGB) and near ultraviolet (NUV) regions of the spectrum. In the future, however, there may be MPPCs applicable to infrared or other regions of the spectrum, and it is contemplated that such future MPPCs may be employed in connection with the disclosed concept.

As used herein, the term “silicon photomultiplier (SiPM)” shall mean an MPPC wherein the Geiger mode avalanche photodiodes are formed on a common single silicon substrate.

As used herein, the term “Scintillator-on-photoMultiplier (SoM)” or “SoM sensor” shall mean a device in which a scintillator is intimately coupled to the active surface of an MPPC, such as an SiPM. SoM sensors work in the following way. Electrons reflected or emitted from the sample strike the scintillator, producing multiple photons, the number of which is proportional to the number of electrons of a given energy striking the scintillator. In practice, the electrons hitting the scintillator are predominantly BSEs having energy equal to the SEM accelerating voltage and having intensity strongly related to the local average atomic number (Z) in the region of the sample being impacted by the electron beam at any given time. In turn, the photons generated toward the underlying appropriately-biased MPPC generate a current in the MPPC proportional to their intensity. Thus, at each point in the raster scanned by the incident electron beam, the output from the SoM sensor is proportional to the BSE intensity, and, using appropriate electronics, a BSE image may be produced.

As used herein, the term “bare MPPC” shall mean an MPPC which does not have a scintillator coupled to the active surface thereof (although it may include a non-scintillating coating).

As used herein, the term “bare SiPM” shall mean an SiPM which does not have a scintillator coupled to the active surface thereof (although it may include a non-scintillating coating).

The present invention will now be described, for purposes of explanation, in connection with numerous specific details in order to provide a thorough understanding of the subject invention. It will be evident, however, that the present invention can be practiced without these specific details without departing from the spirit and scope of this innovation.

The disclosed concept provides a charged particle beam device that is able to image both electrons and photons, or measure their intensity, utilizing a single detecting device. As described in greater detail herein, the single detecting device is able to separately and simultaneously detect and image electrons and photons emitted from a sample or target. Examples of charged particle beam devices that may employ the disclosed concept include Electron Microscopes (EMs) as described above, Focused Ion Beam Instruments (FIBs), dual beam instruments, and electron and/or ion beam sample preparation tools.

As described in greater detail herein, a salient characteristic of the disclosed concept is the use of separate and multiple photon and electron sensors in a single, segmented, detector. In the exemplary embodiment described herein, the detector is roughly the same size and thickness as a conventional solid-state backscattered electron detector. In particular, it has a length and width that make it slightly larger than the dimensions of the pole piece of a typical electron microscope, and it has a thickness of between 3 and 6 mm (e.g., between 2 and 5 mm or between 2.5 and 3 mm), which allows a sample to be examined in an SEM at a working distance as small as 8 to 10 mm. Such a detector could use any solid state sensors, provided that one type is sensitive or made sensitive to electrons, while another type is sensitive or made sensitive to photons. Such a detector would allow measurement of electron and photon radiation simultaneously. One particularly advantageous implementation of the detector described herein employs solid MPPC technology, for both the electron and photon segments.

As described below in connection with the exemplary embodiment ofFIG. 1, the most common application of the detector according to the disclosed concept is a single annular detector for electron microscopes that is positioned between the exit point of the electron beam in the electron column (the pole piece of the objective lens, for example, in an SEM) and the sample, such that the primary electron beam passes through a hole in the annular detector and the surrounding discrete electron sensors detect electrons, usually but not limited to BSEs, and adjacent discrete photon sensors detect photons emitted from the sample resulting from CL. It should be noted, however, that the light sensors in the detector according to the disclosed concept can detect the presence of any light, regardless of its origin.

FIG. 1is a schematic diagram of an SEM1according to one exemplary embodiment of the disclosed concept. SEM1includes an electron column2, normally positioned vertically, coupled to a sample chamber3. Electron column2and sample chamber3may at times herein be referred to collectively as an evacuated housing, being evacuated through a pumping manifold4. In some cases, the sample chamber3may be referred to simply as the “chamber” and the electron column simply as the “column”; when either one is referred to singly, it may also apply to the entire evacuated housing. An electron gun assembly5comprising an electron source6is provided at the top of column2. Electron source6is structured to generate an electron beam7within column2, which beam continues on its path into sample chamber3, directed toward and eventually impinging on the sample (or specimen)13. SEM1further includes one or more condenser lenses9within column2which focus electron beam7of primary electrons, also called the “primary beam”, to a predetermined diameter, such that the beam intensity, i.e., the “probe current”, increases strongly with the beam diameter. The column2of SEM1also includes deflection (scanning) coils10and an objective lens12, represented by its pole piece, which further focuses electron beam7to a small diameter, such that electron beam7is convergent on sample13at the selected working distance11(i.e., the distance between the bottom of the pole piece of the objective lens12and the surface of sample13), such sample13being positionable in several axes (usually X-Y-Z-Tilt-Rotation), by virtue of a sample stage (or specimen holder)14. Scanning coils10deflect electron beam7and create the raster scan in the X-Y axis on the surface of sample13. In the illustrated embodiment, there is also at least one Everhart Thornley (ET) detector, such as SED detector15, entering the sample chamber3or the column2through an access port, such SED detector15providing electrical signals to a control system16(comprising suitable electronic processing circuitry), which in turn produces a secondary electron image on a display system17.

Furthermore, an electron and photon detector (EPD)18according to the disclosed concept is positioned under the pole piece of objective lens12within sample chamber3. EPD18is coupled to control system16by wires34(e.g., bias, signal, and ground wires) which pass through a vacuum feed-through36provided in sample chamber3. EPD18is an annular segmented detector including a central opening and at least one sensor sensitive to photons and at least one sensor sensitive to electrons provided around the central opening. As such, that the primary electron beam of SEM1is able to pass through the central opening and the surrounding discrete electron sensors and the adjacent discrete photon sensors.

As seen inFIG. 1, SEM1also includes an X-ray detector38. The intensity of a BSE signal is strongly related to the atomic number (Z) of the sample13. Thus, in one embodiment, the BSE signal collected by EPD18configured to collect backscattered electrons is used to supplement the X-ray detector38which provides direct elemental analysis.

FIG. 2is a schematic diagram of EPD detector18-1according to one non-limiting, exemplary embodiment. As described below, the sensors of EPD detector18-1employ MPPC technology and SoM technology. In particular, EPD detector18-1includes a printed circuit board (PCB) assembly40that includes a substrate42having a pass-through or opening44provided therein that is structured to allow electron beam7to pass through EPD18-1so that it can reach sample13. In the illustrated embodiment, opening44is circular such that the distal end of PCB assembly40has a generally annular shape, but can also be square or rectangular.

As seen inFIG. 2, PCB assembly40includes four electron sensors46(labeled46A,46B,46C, and46D) positioned on the inner radius of the distal end of PCB assembly40and four photon sensors48(labeled48A,48B,48C, and48D) positioned on the outer radius of the distal end of PCB assembly40. In the illustrated embodiment, each electron sensor46is an SoM sensor, such as an SiPM type SoM sensor, and each photon sensor48is a bare MPPC sensor, such as a bare SiPM sensor. Each electron sensor46and each photon sensor48is coupled to associated conductive traces which in turn are coupled to associated wires50which allow for electrical connections to be made to control system16as described herein such that each electron sensor46and each photon sensor48can be accessed (read-out) independently by control system16.

As will be appreciated, BSEs are more intense as the reflection angle approaches 90°. Thus, the exemplary embodiment shown inFIG. 2employs a configuration wherein the electron sensors46are placed on the inner radius and the photon sensors48are provided on the outer radius. It will be understood, however, that this is meant to be exemplary only, and that other configurations employing different sensor positions are contemplated within the scope of the disclosed concept. Furthermore, in the exemplary embodiment, an optically opaque coating, such as an aluminum coating, is used in EPD detector18-1to prevent the SoM sensors from responding to ambient light or cathodoluminescence.

In the exemplary embodiment, a single technology, such as SiPM technology, is used for both electron sensors46and photons sensors48. SiPM technology provides high sensitivity, wide dynamic range, and fast recovery times (compatible with fast imaging). Although the use of photodiodes or avalanche photodiodes (APDs) instead of SiPMs is contemplated within the scope of the disclosed concept, the resulting device would be significantly slower as compared to a device implemented using SiPM technology. Also, technologies could be mixed, such as incorporating photodiodes or avalanche photodiodes with SiPMs in the device, but such a device would require the electronics to be different for the photon sensor(s)48(if it/they were SiPM based, for example) compared to the electron sensor(s)46(if it/they were APD based, for example), and would therefore likely be more complex and costly. Using SiPMs for all the sensors46and48allows the biasing and imaging electronics to be very similar, possibly identical, for all sensors46,48. Nevertheless, the disclosed concept contemplates the use of any solid state sensors integrated into a single, segmented detector, such that one type of sensor is sensitive to photons, and one type sensitive to electrons.

An advantage of EPD18-1is that it incorporates small sensors close to sample13for high efficiency. This is in contrast to some traditional CL detectors that place large parabolic mirrors inside the chamber. Another advantage of EPD18-1is that its small size minimizes interference with other detectors placed inside chamber3. Still another advantage of EPD is that only one electrical feed-through or chamber access port36is required for both the BSE and CL detectors. Traditional CL detectors require a separate access port and take up valuable and limited space outside the specimen chamber as well as inside the chamber.

Yet another advantage of EPD18-1is that photon sensors48are segmented (as are electron sensors46). This allows the photon emission to be viewed from photon sensors48having different perspectives on sample13, and enables enhanced imaging renditions. For example,FIG. 3is a processed image of an ore particle agglomerate collected with a prototype EPD18-1. The image ofFIG. 3shows a strong “glowing” effect in the light emitting areas that results from the segmentation. More specifically, the processed image ofFIG. 3starts with four independent gray scale images captured by the prototype EPD18-1. Numbering the images from 1 to 4, the source images are as follows: (1) Image 1 is generated from the sum of the outputs of photon sensor48A with one of its nearest neighbors, e.g., photon sensor48B; (2) Image 2 is generated from the sum of the outputs of photon sensors48C and48D; (3) Image 3 is the sum of the outputs of electron sensor46A with one of its nearest neighbors, e.g., electron sensor46B; (4) Image 4 is the sum of the outputs of electron sensors46C and46D. Thus, Images 1 and 2 are collected from diametrically opposite sides of opening44, while Images 3 and 4 are electron images collected from diametrically opposite sides of opening44. False coloring was used to render the BSE images in blue-gray and the CL images in pink. The images are then overlaid to produce the final image ofFIG. 3.

According to another embodiment, shown schematically inFIG. 4, filters52(labeled52A,52B,52C, and52D) can be used over discrete photon sensors48A,48B,48C, and48D to allow specific sensors to be sensitive to a spectral region of interest, with the region of interest being different for different sensors or the same for all sensors. Traditional CL detectors use spectrometers, so that the blue light, for example, can be measured or imaged uniquely from, say, red light. The use of filters52can produce a similar result, albeit with less range, at a much lower cost. Filters52can be applied as separate components, glued or otherwise attached to the surface of the associated photon sensor48, introduced on a mechanical device such as a filter wheel, or applied to the associated photon sensor48as part of or subsequent to the lithography process. Utilizing one or another of these techniques, one or more photon sensors48can be permanently or temporarily “tuned” to specific to regions of the spectrum. For example, one photon sensor48, or set of photon sensors48, could be permanently or temporarily configured to detect blue light, while another detects red, and still another detects green.

The disclosed concept may also employ arrays of MPPCs and SoMs rather than single MPPC and SoM chips. This is illustrated inFIG. 5, which is a schematic diagram of an EPD18-2according to an alternative embodiment. As seen inFIG. 5, EPD18-2includes a PCB assembly54having first and second electron sensor arrays56A and56B, and first and second photon sensor arrays58A and58B. First and second electron sensor arrays56A and56B each include an array of individual SoMs60, such as SiPM type SoMs, and first and second photon sensor arrays58A and50B each include an array of individual bare MPPCs62, such as bare SiPMs. In one exemplary embodiment, EPD18-2would have a thickness of between 3 and 6 mm, more preferably between 4 and 5 mm, in order to provide enhanced stiffness and support for the arrays56and58.

FIG. 6is a schematic diagram of a photon detector64according to a further alternative exemplary embodiment. Photon detector64is similar to EPD detector18and may be used in place of EPD detector18inFIG. 1. Photon detector64, however, includes a PCB assembly66wherein all of the sensors are photon sensors48as described herein (labeled48A-48H). As such, photon detector64provides a compact and segmented CL detector. In this embodiment, filters52may be used in connection with one or more of the photon sensors48as described herein.

FIGS. 7A-7Dprovide a comparison of standard SED images to CL images captured using the prototype EPD18. In particular, the images inFIGS. 7A and 7Care secondary electron images captured using a standard SEM detector while the images inFIGS. 7B and 7Dwere captured using the prototype EPD18. Note that in the CL images ofFIGS. 7B and 7D, a faint electron image appears. This is because a bare MPCC was used for photon detection, without any coating to absorb electrons. This is a benefit from the ability of a bare MPPC to produce an electron image. The value of this is that the outline of the regions of the sample which do not emit light provides an exact location of the light emitting areas in the context of the overall sample. If no electron image is wanted, a relatively thick layer of an electrically conductive but optically transparent coating like ITO can be used to eliminate the electron signal.

FIG. 8is a schematic representation of an overlay image of an ore particle agglomerate produced in the manner ofFIG. 3with the prototype EPD18showing BSE and CL images. Energy Dispersive X-ray (EDX) analysis shows that the cluster of bright particles pointed out on the left side of the image is Fe-rich compared to the matrix, which is predominantly silicon, aluminum, sodium and oxygen (spectrum in the lower right ofFIG. 8). Since the Fe-rich cluster is of higher average atomic number compared to the matrix, it appears bright in the image, showing conventional atomic number contrast of BSE imaging. EDX analysis of the bright areas pointed out on the right side of the image shows them to be rich in Ca and F. As calcium fluoride is a known CL emitter, the brightness in this case is due to light emission. Although the image ofFIG. 8was collected in a sequential manner and colorized according to the method explained in connection withFIG. 3for maximum visual effect and information content, a single gray scale image can be collected from the sum of all sensors showing both contrast mechanisms acting simultaneously.

Furthermore, it is a known problem in cathodoluminescence imaging that many cathodoluminescent materials continue to glow after the electron beam is removed. This is known as persistent luminescence or phosphorescence. Known remedies for this problem include very long pixel dwell times, from hundreds of microseconds to a few milliseconds, interpixel delay, which allows the persistent emission to decay between pixels, and using short wavelengths only, which tend to decay faster. Each of these known remedies, however, has a disadvantage associated therewith. Long dwell times result in very slow imaging and contribute to possible charging effects on the electron-imaging side since many minerals are non-conductive. Interpixel delay is often not long enough for complete decay of the persistence. Using only short wavelengths greatly reduces the usable fraction of the information available from the CL technique.

A further aspect of the disclosed concept provides an improved solution to the persistent luminescence or phosphorescence problem. In particular, in this aspect of the disclosed concept, the high speed imaging afforded by SiPM technology (relative to other solid-state detectors like APDs) is used in conjunction with beam blanking technology to allow measurement and time-lapse imaging of the rate-of-decay of the emissions across the imaged region of a sample. A beam blanker is a well-known device that allows for the temporary deflection (typically in about 50 nS) of the electron beam off the specimen in an SEM. Such timing is a good match to the SiPM recovery time of about 100 nS or so.

FIG. 9is a schematic diagram of an SEM1′ according to an alternative exemplary embodiment in which this further aspect of the disclosed concept may be implemented. SEM1′ includes many of the same parts as SEM1, and like parts are labeled with like reference numerals. SEM1′ further includes a beam blanker68that is operatively coupled to electron column2and control system16. Beam blanker68may be any known or hereafter beam blanking device such as, without limitation, the PCD beam blanker commercially available from Deben UK Limited.

FIG. 10is a flowchart illustrating one particular embodiment of the method of this further aspect of the disclosed concept as implemented in SEM1′. In the exemplary embodiment, control system16includes a non-transitory computer readable medium, such as a non-volatile memory, that stores one or more programs having instructions for implementing the method shown inFIG. 10. As seen inFIG. 10, the method begins at step70, wherein electron beam7is directed at the current pixel position of specimen13for a predetermined period of time. Next, at step72, electron beam7is deflected away from specimen13for a predetermined period of time using beam blanker68. Then, at step74, light from the current pixel position is sampled a plurality of times using any of the detector embodiments (that include one or more photon detectors48) described herein while electron beam7is deflected in order to get a plurality of light intensity measurements while the cathodoluminescence is decaying. In the exemplary embodiment, light is sampled for a few to a few 10's of microseconds after electron beam7is removed. In the present method, it is not necessary to wait for the light to decay entirely. Rather, all that is needed is enough of the decay curve to estimate the exponential time constant of the decay for the current pixel position. The fast response of photon detectors48of (which are MPPC type sensors such as bare SiPMs) allows for the light decay of specimen13to be distinguished from the signal decay of photon detectors48as long as at least 10 or so detector (e.g., SiPM) measurements and associated decay times (a microsecond or so) are obtained. In this aspect of the disclosed concept, the decay image can be collected in roughly the same time as current “fast mapping” X-ray systems, with dwell times of 10 to 100 uS.

Next, the method moves to step76. At step76, the decay time constant for the current pixel position is estimated in control system16using the obtained light intensity measurement values. Then, at step78, electron beam7is moved to the next pixel position and the method returns to step70to repeat the process for the next pixel position. The method ofFIG. 10will be repeated until measurements are made for each pixel position of specimen13.

Once an image of decay time constants per pixel is obtained as just described, the decay time constants per pixel can then be used in subsequent operation of SEM1′ to compute the contribution of previous pixels in a scan to the light detected at the pixel currently illuminated by electron beam7. The sum of contributions from the current pixel and those prior pixels whose contributions are still significant can be deconvolved using any of a number of well-known software image restoration algorithms as a post-image-collection processing step. For example, the iterative Richardson-Lucy (R-L) algorithm was revived when the Hubble Space Telescope was discovered to have spherical aberration. R-L does not require the point spread function (equivalent to the smearing caused by persistent luminescence) to be the same at all pixels, which many Fourier-space methods require. R-L is now commercially available in a number of consumer astrophotography software packages. The deconvolution causes all light emitted by a single pixel to be restored to that pixel, eliminating the blurring effect of fast scanning. Because of the scanned nature of SEM electron imaging, the blurring from persistent luminescence is one-dimensional (along the scan line) rather than two-dimensional as in conventional image restoration.