Abstract:
An electron energy loss spectrometer is described having a direct detection sensor, a high speed shutter and a sensor processor wherein the sensor processor combines images from individual sensor read-outs and converts a two dimensional image from said sensor into a one dimensional spectrum and wherein the one dimensional spectrum is output to a computer and operation of the high speed shutter is integrated with timing of imaging the sensor. The shutter is controlled to allow reduction in exposure of images corresponding to the individual sensor readouts. A plurality of images are exposed by imaging less than the full possible exposure and wherein the plurality of images are combined to form a composite image. The plurality of images can be comprised of images created by exposing the sensor for different exposure times.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 based on U.S. Provisional Patent Application No. 62/281,147 filed Jan. 20, 2016, the disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to the field of electron microscopy and spectrometry. 
       BACKGROUND OF THE INVENTION 
       [0003]    In an exemplary electron microcopy system having a direct detection sensor, the sensor runs at a constant speed such as 400 frames or sensor readouts per second (fps). Each sensor readout is processed by dedicated hardware in the camera to determine the location of each electron striking the detector. For this to work, in each of these sensor readouts the count rate in any area must remain below approximately one electron per twenty pixels or miscounting occurs. Read-out noise is eliminated by the electron-counting process. The system is designed such that incident electrons create a spatially-localized signal at a much higher level than the localized noise associated with read-out of the sensor. By applying a threshold to the signal, counting events above the threshold and negating noise below the threshold the read noise is eliminated. Under the above conditions, noise due to errors in counting such as false positives or missed counts is minimal. When used in an imaging mode this noise minimization, together with low spatial spread of the signal orthogonal to the incident beam, yields an excellent detective quantum efficiency. On a conventional direct detection camera system, the 400 fps data rate is far higher than a typical computer could process. One way to solve this problem is for the processing hardware to sum a number (e.g. 40) of counted frames together before sending the data to the host computer, thus yielding an effective summed-frame rate of 10 fps or an effective exposure time of 0.1 s per recorded summed-frame. 
         [0004]    In this example for an electron microscope used in imaging mode, the minimum exposure is 0.1 seconds, the maximum summed-frame rate to the computer is 10 fps and the maximum dose rate is 20 electrons per second per pixel. 
         [0005]    For electron-energy-loss spectroscopy (EELS) the above limitations are problematic. In a conventional, non-counting, scintillator-based detector, the spectrum is spread out in the non-spectral direction (also called the non-dispersive direction) by a few hundred pixel rows. This value is determined by balancing the competing needs of a high readout speed and low noise, requiring a smaller readout area, and the need for dynamic range and detector lifetime, requiring a larger readout area. Spectroscopy experiments often require high dynamic range to simultaneously detect regions of the spectrum that differ widely in intensity. Incident currents in the pico-Amp range on localized detector regions are common. This corresponds to many millions of electrons per second per pixel, which is much higher than the dose rate capacity of approximately 20 electrons per second per pixel shown above for the direct-detection detector when imaging in counting mode. 
         [0006]    Spreading the spectrum out to less than the full height of the detector decreases the sensor read-out time since fewer pixels are read but, for a direct detection device in counting mode, the narrow spectrum reduces the dynamic range by the same amount that it increases the speed so in most cases there is no advantage in limiting the spectrum to smaller areas. Reducing the area does, however, reduce the lifetime of the detector so the optimal use of the detector is when the full detector is used, that is, when the spectrum spread out in both directions as in  FIG. 1 . Such counting-mode operation performs EELS in the absence of read-out noise and, therefore, at very high sensitivity. The invention described herein is designed to address the difficulties described above that can limit the summed frame rate and dose rate for spectroscopy. 
         [0007]    This application is related to and incorporates by reference PCT application serial number PCT/US2015/037712 titled Electron Energy Loss Spectrometer filed by applicant Gatan, Inc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an image of a spectrum on a two dimensional direct electron detector; 
           [0009]      FIG. 2  is a block diagram of the preferred embodiment of the invention; 
           [0010]      FIG. 3  is diagram illustrating a typical EELS spectrum; 
           [0011]      FIG. 4  is a flow chart of an exemplary complex exposure mode as described herein; and 
           [0012]      FIG. 5  is a block diagram of an exemplary system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    A novel Electron Energy Loss Spectroscopy (EELS) system is disclosed using a direct electron detector coupled to a spectrometer aimed at both imaging and spectroscopy. 
         [0014]      FIG. 2  shows an exemplary system for performing the inventive process. As shown, the exemplary system includes a Transmission Electron Microscope (TEM) or Scanning Transmission Electron Microscope (STEM)  200  that includes a scan control element  213  and scan control electronics  211  with a scan amplifier output signal  212  connected to scan control element  213 . A sample  214  is shown placed in the microscope  200 . 
         [0015]    Consistent with embodiments described herein, the system further includes a high speed shutter  202 , controlled by a shutter driver  218  via control signal  219 ; an imaging filter  201 , which comprises an energy offset drift tube  208 , controlled by a drift tube driver  221  via control signal  222 ; and a direct detector camera system  203 . 
         [0016]    Direct detector camera system  203  comprises a direct detector module  204  comprising a direct detector sensor  205 , a direct detector camera processing module  206  and a direct detector camera controller module  209 . As shown in  FIG. 2 , image/spectrum data path  215  is provided from direct detector module  204  to direct detector camera processing module  206 .  FIG. 2  further shows the direct detector camera internal synchronization signals  216  between the direct detector camera controller module  209  and the direct detector camera processing module  206 . The direct detector camera controller module  209  is further connected to and controls the scan control electronics  211  via a scanning pixel advance signal  210 , the direct detector module  204  via direct detector camera internal synchronization signals  216 , the drift tube driver  221  via control signal  220  and the shutter driver  218  via control signal  217 . The output image or spectrum data  230  of the direct detector camera module  206  is connected to a host computer system  207 . 
         [0017]    In the past, only imaging was possible on such systems. Using a counting mode with a detector has a number of advantages for spectroscopy. The two biggest advantages are that detector noise is much lower, allowing lower intensity signals to be measured and that the detector is a transmission detector so the point spread function is much tighter, giving sharper spectra. This means the system can be used at lower dispersions, that is more energy-loss (in electron-Volts, eV) per pixel. This increases the number of electrons per channel and hence reduces the shot noise in the spectrum and increases the spectral field of view, or energy range, for a given detector size in pixels. 
         [0018]    The major elements of the system are imaging filter  201  having a high speed shutter and a direct electron detector, such as a K2 camera from Gatan, Inc. This combination is shown in  FIG. 2  and has been used before for imaging but not spectroscopy because of a number of practical difficulties using a direct detector for recording a spectrum. 
         [0019]    The inventive system improves imaging efficiency. Instead of each frame having the same exposure, the inventive system varies the exposure, using the high speed shutter, and only uses the regions in the spectrum that were taken under appropriate counting conditions. This is most clearly seen in an example comparing two methods of acquiring and combining multiple images. 
         [0020]    In a first method, there is a fixed exposure time, such that 100 sensor readouts of 1 microsecond-exposure are summed, with a total time for acquiring data of approximately 0.25 seconds (e.g., 100 full-height frames at 400 fps), total live exposure time is 100 microseconds. 
         [0021]    A second method according to the embodiments described herein uses a varying or stepped exposure times, such as 10 sensor readouts at 1 microsecond-exposure and 9 sensor readouts at 10 microseconds-exposure. These sensor readouts are summed and the two spectra are combined. This second method allows for the same live exposure time of 100 microseconds on the fainter image areas, but only requires 19 sensor readouts to be summed and takes only approximately 0.05 seconds to acquire. This is five times faster than the simple exposure mode of the first method. 
       Summed-Frame Rate Limitation for Spectra-per-Second 
       [0022]    In the imaging mode of operation, the frame rate is limited by the data path and processing power of the computer. However, in a spectroscopy mode of operation, the final spectrum is a projection of such a 2-D image along the non-dispersive direction as shown in  FIG. 3 . In the inventive system, this projection is performed in the data processing module  206  of direct detector camera system  203 , as shown in  FIG. 2 , and a 1-D spectrum is then transferred from data processing module  206  to the host computer  207 , which reduces the data load by a factor of the pixel height of the detector. 
         [0023]    In some implementations, this difference is a factor of approximately 4,000. This dramatic reduction in data load means that the need to sum multiple sensor readout frames in the processing unit and transmit the resulting summed-frames to the host computer is no longer required and the full 400 spectra per second can be transferred to the host computer without being limited by the data connection bandwidth or the computer processing power. In this case the data rate change would be a combination of a reduction of 4,000× for the projection and an increase in 40× since all projected sensor readout frames are sent to the host computer individually, instead of one summed-frame every 40 read-out frames, leading to a total reduction in the data rate versus the imaging mode of 100× with an increase in the spectra-per-second rate to the computer of 40x. 
       Dose Rate Limit 
       [0024]    In the imaging mode operation of a microscope, the dose rate is limited by the microscope optics and the type of image. The brightest area in any direct-detector sensor readout must be below the counting limit of approximately one electron per twenty detector pixels as described above in the Background, a typical intensity variation of each frame in this mode of operation is modest (typically on the order of 10%). 
         [0025]    For EELS data, however, large dynamic ranges are typically present (on the order of 10 4 ), and it is certain that for some areas of the spectrum, the intensity will be too high to count. This limitation can be overcome by the use of high speed shutter  202  to limit the total number of electrons impinging on the detector. An exemplary spectrometer device based on imaging filter  201  has a shutter  202  that allows exposures as short as 1 microsecond (or 1 millionth of a second) to be acquired. This shutter control is integrated with the direct detection camera system  203  to allow reduction of the intensity in each 1/400 second sensor frame read-out up to 2,500×. 
         [0026]    In this way, the counting regime can be maintained even for very bright areas. The simplest use of this mode is to acquire a single sensor readout frame with an exposure of between 1 microsecond and 2.5 milliseconds. For exposure above 2.5 millisecond the high-speed shutter  202  is just left open and sensor readout frames are summed within the data processing module  206 . Any number of these sensor readout frames can be summed to give the final spectrum, either fully exposed sensor readouts or attenuated dose sensor readouts. 
       Dose Efficiency 
       [0027]    When using the high-speed shutter  202  to maintain all areas on the direct detector  205  in the counting regime, efficiency can be low. If the shutter is running at its minimum exposure time of 1 microsecond, then the system is only detecting 1/2,500 of the possible signal. On a typical system, this would only be needed for the zero-loss region and the low loss region of the spectrum. A typical EELS spectrum is shown in  FIG. 3 , with the zero-loss region  301 , the low loss region  302  and the core loss region  303 . 
         [0028]    In a single sensor readout frame the zero loss may have a reasonable number of counts, but even the low loss (also known as the plasmon) area intensity is so low that shot noise hides the real signal. This can be solved most simply by summing sensor readouts, but again this is inefficient and when taking a spectrum image (that is, acquiring a spectrum at every point in an image in Scanning Transmission Electron Microscopy (STEM) mode), sensor readout frame summing slows the process down to the point the experiment becomes impractical. 
       Dual EELS 
       [0029]    On a non-direct detection version of an EELS spectrometer, a mode called Dual EELS is implemented wherein a single readout two spectra are recorded on two different areas of the detector with both a variation of exposure and a variation of energy between the two spectra. The energy may be varied via use of a drift tube within the spectrometer held at elevated voltage. The use of two separate detector areas has the additional effect of avoiding after-glow from the scintillator response from an intense low-loss signal interfering with the weaker signal from the core-loss region. 
         [0030]    On a direct detection system, as disclosed herein, shifting areas has no advantage as direct detection does not show the afterglow a scintillator shows, and using the full area of the direct detector  205  is the optimal way to use the detector in most use cases. 
         [0031]    An exemplary embodiment of the invention, as shown in  FIG. 2 , enables the varying exposure mode controller of the system  209  to also control the energy offset device of the system. The result is that the camera controller can be programed to acquire multiple energy ranges with varying exposures in each range. 
       Complex Exposure Mode Controller 
       [0032]    An exemplary method  400  for operating an EELS that includes direct detection sensor  205 , is shown in  FIG. 4 , wherein the variable exposure mode is controlled the direct detector camera controller  209 . As described herein, process  400  may be implemented by camera controller  209  or a combination of camera controller  209  with direct detector camera processing module  206 , and/or host computer system  207 . In this implementation, multiple (e.g. 5) exposures can be set up in a sequence. Each separate exposure can have the fast shutter exposure time set per sensor read-out frame. The number of sensor readout frames to sum and the energy offset of the spectrum are programed as separate values for each of the five or fewer exposures. In some embodiments, the direct detector camera controller  209  also controls the scanning pixel advance signal  210  that drives electronics to move the STEM electron probe to the next point for analysis in STEM spectrum imaging, thus fully controlling the EELS acquisition experiment. In this way a spectrum image can be acquired with optimal acquisition time for the entire data set. 
         [0033]    Initially, a set of ten 0.1 micro second exposures are made and summed with a setting of 1/400s read-out frame duration (block  410 ). Next, the summed frames are converted to a one-dimensional spectrum, this 1D spectrum data  421  is transferred to a buffer or to host computer system  207  (block  420 ). 
         [0034]    A set of ten 1 micro second exposures are made and summed with a setting of 1/400s read-out frame duration (block  430 ). Next, the summed frames are converted to a one-dimensional spectrum, this 1D spectrum data  441  is transferred to a buffer or to computer system  207  (block  440 ). 
         [0035]    At block  450 , a set of 50 10 microsecond exposures are made and summed with a setting of 1/400s read-out frame duration. At block  460  a pixel advance signal pulse  461  is sent. Next, the summed frames are converted to a one-dimensional spectrum (block  470 ), this 1D spectrum data is transferred to a buffer or to host computer system  207  (block  471 ). Next, a determination is made as to whether the spectrum imaging block is complete (block  480 ) and, if not, the process may return to block  410  for a next set of read-out frames. 
         [0036]      FIG. 5  is a diagram illustrating exemplary physical components of a device  500 . Device  500  may correspond to various devices within the above-described system, such as direct detector camera system  203 , host computer  207 , etc. Device  500  may include a bus  510 , a processor  520 , a memory  530 , an input component  540 , an output component  550 , and a communication interface  560 . 
         [0037]    Bus  510  may include a path that permits communication among the components of device  500 . Processor  520  may include a processor, a microprocessor, or processing logic that may interpret and execute instructions. Memory  530  may include any type of dynamic storage device that may store information and instructions, for execution by processor  520 , and/or any type of non-volatile storage device that may store information for use by processor  520 . 
         [0038]    Software  535  includes an application or a program that provides a function and/or a process. Software  535  is also intended to include firmware, middleware, microcode, hardware description language (HDL), and/or other form of instruction. By way of example, with respect to the network elements that include logic to provide proof of work authentication, these network elements may be implemented to include software  535 . Additionally, for example, device  500  may include software  535  to perform tasks as described above with respect to  FIG. 4 . 
         [0039]    Input component  540  may include a mechanism that permits a user to input information to device  500 , such as a keyboard, a keypad, a button, a switch, etc. Output component  550  may include a mechanism that outputs information to the user, such as a display, a speaker, one or more light emitting diodes (LEDs), etc. 
         [0040]    Communication interface  560  may include a transceiver that enables device  500  to communicate with other devices and/or systems via wireless communications, wired communications, or a combination of wireless and wired communications. For example, communication interface  560  may include mechanisms for communicating with another device or system via a network. Communication interface  560  may include an antenna assembly for transmission and/or reception of RF signals. In one implementation, for example, communication interface  560  may communicate with a network and/or devices connected to a network. Alternatively or additionally, communication interface  560  may be a logical component that includes input and output ports, input and output systems, and/or other input and output components that facilitate the transmission of data to other devices. 
         [0041]    Device  500  may perform certain operations in response to processor  520  executing software instructions (e.g., software  535 ) contained in a computer-readable medium, such as memory  530 . A computer-readable medium may be defined as a non-transitory memory device. A non-transitory memory device may include memory space within a single physical memory device or spread across multiple physical memory devices. The software instructions may be read into memory  530  from another computer-readable medium or from another device. The software instructions contained in memory  530  may cause processor  520  to perform processes described herein. Alternatively, hardwired circuitry may be used in place of or in combination with software instructions to implement processes described herein. Thus, implementations described herein are not limited to any specific combination of hardware circuitry and software. 
         [0042]    Device  500  may include fewer components, additional components, different components, and/or differently arranged components than those illustrated in  FIG. 5 . As an example, in some implementations, a display may not be included in device  500 . In these situations, device  500  may be a “headless” device that does not include input component  540 . As another example, device  500  may include one or more switch fabrics instead of, or in addition to, bus  510 . Additionally, or alternatively, one or more components of device  500  may perform one or more tasks described as being performed by one or more other components of device  500 . 
         [0043]    Although different implementations have been described above, it is expressly understood that it will be apparent to persons skilled in the relevant art that the implementations may be modified without departing from the spirit of the invention. Various changes of form, design, or arrangement may be made to the invention without departing from the spirit and scope of the invention. Therefore, the above mentioned description is to be considered exemplary, rather than limiting, and the true scope of the invention is that defined in the following claims. 
         [0044]    The term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. 
         [0045]    No element, act, or instruction used in the present application should be construed as critical or essential to the implementations described herein unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.