Abstract:
A charged particle device is provided comprising a charged particle source configured to direct charged particles in the direction of a specimen under examination and an imaging device configured to convert charged particles to an image representing the specimen. The imaging device comprises a detector defining a pixel array. The detector is configured to generate electric charges for individual pixels of the pixel array such that the electric charges collectively define the image. The imaging device is configured such that a portion of the pixel array can be transitioned between a partially masked state and a substantially unmasked state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/492,138 filed Aug. 1, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to charged particle beam devices and, more particularly, to charged particle beam devices used to generate images of specimens under examination.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     Although the present invention relates generally to the use of charged particles in generating an image that represents one or more characteristics of a specimen under examination, the present invention will be described in the context of transmission electron microscopes.  
         [0004]     Transmission electron microscopes use a beam of accelerated electrons that pass through a specimen to provide an image representing the specimen. The image may be a single or multi-dimensional image, a diffraction pattern, a spectrum, or any other suitable representation of the specimen. To provide a record of these images, electrons are converted into light images using a scintillator and a camera then captures the light images. While photographic film has long been used to capture such images, charge coupled devices (CCDs) have found increasing use in this field. Such CCD cameras offer excellent resolution, sensitivity, and linearity, are reusable, and make the image available for viewing within seconds of recording.  
         [0005]     With the advent of digital image processing in transmission electron microscopy, such CCD cameras have been used to transfer images directly from the microscope to a computer. Typically, the electron image is converted to a light image by means of a scintillator such as a phosphor screen mounted inside the transmission microscope. The light image is then transferred to a CCD pixel array, also located inside of the microscope, using a lens or fiber optic coupling. The image collected by the camera is then digitized electronically and stored in a computer where it can be manipulated and viewed with the aid of appropriate software.  
         [0006]     CCD cameras and other imaging devices according to the present invention may operate in either a full frame mode or frame transfer mode. In the full frame mode, the image is exposed onto a relatively large active area of the detector, and the electron beam is then blanked (such as by using a shutter to remove the illumination). The pixels on the detector are then read out and stored. In full frame mode, substantially all of the available detector pixels are used for imaging, which allows for a high quality image. When this cycle of exposure and blanking is repeated in a continuous mode at low exposure times, i.e. high frame rates, the active duty cycle for the camera becomes relatively low. That is, the duty cycle of the camera is the fraction of time that imaging can take place with respect to the total time required for the imaging and read out cycle to take place.  
         [0007]     In the frame transfer mode, a mask is placed on the CCD such that the imaging detector of the camera has a portion (e.g., approximately  50 %) of the area masked from electron or photon illumination. The camera is operated in a continuous mode where the image is always being exposed in the unmasked imaging area. After exposure, the image is shifted by rapidly moving it to the area on the CCD behind the mask and is read out and stored.  
         [0008]     Typically, shifting the image is many times faster than reading it out. In frame transfer mode, the image detector has only half the pixels available for imaging but has a higher duty cycle. The only non-imaging time in the cycle is the transfer of electrical charges to and from the imaging area to the memory area of the detector. Shuttering the beam during image shifting under the mask may not be needed if the shifting process is rapid enough.  
         [0009]     The present invention addresses the trade-off with respect to the high quality images and low duty cycles of the full frame mode and the high duty cycles and lower resolutions of the frame transfer mode. Charged particle imaging devices according to the present invention are not limited to a single mode of operation and there is no need to change cameras to account for the needs of the particular project because imaging devices according to the present invention are capable of operation in a plurality of modes and can be converted from operation in one mode to another quickly.  
         [0010]     The present invention meets those needs by providing a camera that can be operated in at least two different modes and that can be quickly converted from operation in one mode to another. In accordance with one aspect of the present invention, a camera is provided having an imaging detector with an imaging area. In a preferred mode, the imaging detector comprises a charge-coupled device (CCD) having a pixel-receiving array. In other embodiments of the invention, the imaging detector may comprise a photodiode array, a CMOS detector, or other imaging device that can convert photons into electrical charges.  
         [0011]     The camera includes a mask that is capable of being manipulated to permit an image to be received by the full surface area of the imaging detector and then further manipulated to cover or block at least a portion of the area of the image receiving detector from receiving an image. The mask may be either a mechanical mask, a virtual mask implemented using suitable software to manipulate the optics of the system to prevent the image from reaching a portion of the imaging detector, or any other suitable means for restricting an area of the imaging detector from receiving an image.  
         [0012]     Where the mask is a mechanical mask, it is preferably constructed of a low Z (i.e., low atomic number) material such as, for example, aluminum. The use of low Z materials for the mechanical mask provides the additional benefit of reducing the amount of hard X-rays emitted from the mask, making X-ray shielding of the imaging detector simpler. This in turn reduces any problems associated with X-ray emissions causing false signals to register on the imaging detector.  
         [0013]     In one embodiment, the mechanical mask comprises a pair of shutters located on either side of the imaging detector. The halves of the mask are driven using suitable mechanical, electrical, or hydraulic motive power. The mask is movable from at least a first position where the mask does not interfere with the imaging detector (e.g., the entire surface of the imaging detector is uncovered) to at least a second position where at least a portion of the imaging detector is covered such that at least a portion of the image does not reach the masked off portion of the detector.  
         [0014]     In the full frame mode, the image is exposed onto the full area of the detector, and the electron beam is then blanked (such as by using a shutter to remove the illumination). The pixels on the detector are then read out and stored. In full frame mode, all of the available pixels on the device are used for imaging, which allows for a high quality image.  
         [0015]     In the frame transfer mode, the imaging detector of the camera has a portion (e.g., approximately 50%) of the area masked from electron or photon illumination. In the embodiment of the invention using a mechanical mask, this is accomplished by moving the mask to cover a predetermined percentage of the surface area of the imaging detector. In the embodiment of the invention using a virtual mask, the optics of the system are manipulated to mask of a predetermined percentage of the surface area of the imaging detector. The camera is operated in a continuous mode where the image is always being exposed in the unmasked imaging area. After exposure, the image is rapidly moved to a memory area on the camera behind the mask and is read out and stored.  
         [0016]     In a pipelined frame transfer mode mode, the image may be an asymmetric spectrum that is exposed on a narrow area of the imaging detector. This mode is typically used in conjunction with a spectrometer. In one embodiment, using a 2048×2048 pixel CCD device, only the middle 200×2048 pixels of the detector are illuminated. The remainder of the surface area of the detector is masked. After the spectrum exposure is complete, the image of the spectrum is transferred along the detector to an area just outside the portion on which the spectrum has been exposed. In this way many spectra, typically 1 to 10, can be exposed on the detector in a “pipeline” before the first spectrum is read out. As moving charge across the full surface area of the detector can take a considerable amount of time, this pipelining allows for much faster frame rates for spectroscopy uses.  
         [0017]     Imaging devices according to the present invention are versatile in operation and can be used in conjunction with a number of electron microscope configurations. For example, the camera may be positioned at the bottom of a transmission electron microscope (TEM) column or in the 35 mm camera port on the side of the microscope column. The camera may also be used in conjunction with an imaging filter or an electron energy loss spectrometer (EELS) to capture images.  
         [0018]     Accordingly, it is a feature of the present invention to provide a camera that can be operated in at least two different modes and that can be quickly converted from operation in one mode to another. This and other features and advantages of the invention will become apparent from the following detailed description, and the accompanying drawings.  
         [0019]     In accordance with one embodiment of the present invention, a charged particle device is provided comprising a charged particle source configured to direct charged particles in the direction of a specimen under examination and an imaging device configured to convert charged particles to an image representing the specimen. The imaging device comprises a detector defining a pixel array. The detector is configured to generate electric charges for individual pixels of the pixel array such that the electric charges collectively define the image. The imaging device is configured such that a portion of the pixel array can be transitioned between a partially masked state and a substantially unmasked state. The partially masked state defines a set of masked readout pixels and a set of unmasked imaging pixels. The set of masked readout pixels occupies a portion of the pixel array that is large enough to accommodate image data held in the unmasked imaging pixels. The imaging device is programmed to operate in a full frame imaging mode when the pixel array is in the substantially unmasked state and in a frame transfer imaging mode when the pixel array is in the partially masked state. The device may also operate in a pipelined frame transfer mode when the pixel array is in the partially masked state.  
         [0020]     In accordance with another embodiment of the present invention, a charged particle device is provided comprising a specimen holder, a charged particle source, an imaging device, and a user interface. The specimen holder defines a specimen position and the charged particle source is configured to direct charged particles in the direction of the specimen position. The imaging device is configured to convert charged particles to an image representing a specimen held in the specimen position. The detector is configured to generate electric charges for individual pixels of the detector pixel array such that the electric charges collectively define the image. The user interface is configured to enable selection from at least two operating modes comprising a frame transfer mode and a full frame mode. The imaging device is configured to transition the pixel array between a partially masked state in the frame transfer mode and a substantially unmasked state in the full frame mode.  
         [0021]     In accordance with yet another embodiment of the present invention, a method of operating a charged particle device is provided. The device is configured to direct charged particles in the direction of a specimen under examination and convert the charged particles to an image representing the specimen. The charged particle device comprises a user interface and a controller programmed to prompt a user to select one of a plurality of imaging modes via the user interface. A portion of the pixel array is transitioned between a partially masked state and a substantially unmasked state as a function of the imaging mode selected via the user interface.  
         [0022]     Accordingly, it is an object of the present invention to provide an improved charged particle device. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0023]     The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:  
         [0024]      FIG. 1  is an isometric view of an imaging device according to the present invention in a substantially unmasked state;  
         [0025]      FIG. 2  is an isometric view of an imaging device according to the present invention in a partially masked state;  
         [0026]      FIG. 3  is an illustration of a full frame mode according to the present invention;  
         [0027]      FIGS. 4 and 5  are illustrations of a frame transfer mode according to one embodiment of the present invention;  
         [0028]      FIGS. 6 and 7  illustrate a full frame mode and a frame transfer mode according to another embodiment of the present invention;  
         [0029]      FIG. 8  is an illustration of a pipelined frame transfer mode according to the present invention;  
         [0030]      FIG. 9  is a schematic illustration of one embodiment of the imaging device of the present invention mounted at the bottom of a TEM column;  
         [0031]      FIG. 10  is a schematic illustration of one embodiment of the imaging device of the present invention mounted at the end of an imaging filter; and  
         [0032]      FIG. 11  is a schematic illustration of one embodiment of the imaging device of the present invention mounted at the end of an electron energy loss spectrometer.  
     
    
     DETAILED DESCRIPTION  
       [0033]      FIGS. 1 and 2  illustrate one embodiment of the invention in which the imaging device  10 , typically including a CCD imaging detector  15 , is cooled by water from cooling lines  11 . Imaging device  10  includes a mechanical mask  12  selectively movable across the pixel array of the CCD detector  15 . In  FIG. 1 , the mechanical mask  12  is illustrated in an open position Such as would be used when the imaging device  10  is operated in the full frame mode. In  FIG. 2 , the mechanical mask  12  is illustrated in a partially closed position such as would be used when the imaging device  10  is operated in the frame transfer mode.  
         [0034]     As can be seen, the mask includes a movable piston  14  driven by pneumatic pressure supplied from line  16 . Application of pneumatic pressure causes piston  14  to move. A pair of wedge-shaped cams  18  located on either side of piston  14  ride against pins  20 , located on opposing halves  22  and  24  of mask  12 . As piston  14  moves, the cams, in conjunction with the pins overcome the bias of springs  26  and cause the respective mask halves to retract, exposing additional surface area of the detector  15 .  
         [0035]     The mechanical mask  12  may be constructed of a low Z material, i.e., a low atomic weight material. The use of low Z materials for the mechanical mask provides the additional benefit of reducing the amount of hard X-rays emitted from the mask, making X-ray shielding of the imaging detector simpler. This in turn reduces any problems associated with X-ray emissions causing false signals to register on the imaging detector. Aluminum is a suitable material to construct mask  12 . Heavier atomic weight metals such as tungsten do not provide the low X-ray properties of aluminum.  
         [0036]     Referring to  FIG. 3 , in typical full frame operation, the full surface area of the detector  15  is available for sensing incoming photons during exposure. During readout of the image  25 , charge is shifted sequentially across the pixel array of the detector  15  to designated readout cells coupled to one or more signal output amplifiers  30 . In the embodiment of  FIG. 3 , the pixel array of the detector  15  is divided into quadrants, each coupled to an independent signal output amplifier  30 . During readout, a mechanical shutter or other suitable means in the system blocks any incoming photons from reaching the pixel array.  
         [0037]     The full frame mode uses substantially all of the pixels in the array of the detector  15 . The pixels are typically square so that there is no image distortion. In general, and by way of example, not limitation, the array undergoes readout by shifting individual rows of images in sequence in parallel fashion to a serial shift register coupled to an output amplifier  30 . The serial shift register then sequentially shifts each row of image information to the amplifier as a serial data stream. The process is repeated until all rows of image data are transferred to the output amplifier  30  and then to an analog to digital converter circuit. Reconstruction of the image  25  in a digital format yields the final image.  
         [0038]     Referring to  FIGS. 4 and 5 , in the frame transfer mode, a portion of the pixel array is masked to define a set of masked readout pixels  40  and a set of unmasked imaging pixels  50 . The image  25  is captured on the unmasked area of the array while the masked portion serves as a storage array for a transferred image  35 . Once captured, the charges representing the image  25  are transferred from the unmasked imaging pixels  50  to the storage area defined by the masked readout pixels  40 . Once in the storage area, the charges are transferred off of the pixel array in much the same way as the full frame mode operation described above. Specifically, the detector array undergoes readout by shifting rows of image information in a parallel fashion, one row at a time, to a serial shift register. The serial register then sequentially shifts each row of information to an output amplifier as a serial data stream. During the period in which the readout pixels  40  are being read, the imaging pixels  50  (i.e., the unmasked portion of the detector) is exposed to another image frame. This mode of operation permits faster frame rates and increased duty cycles. It is noted that the multiple modes of operation of the imaging device of the present invention may be controlled by running parallel and serial clock lines in different sequences for the different modes. Such operation can be facilitated by controlling voltages using a digital signal processor (DSP).  
         [0039]     It is also noted that, the embodiment of  FIGS. 4 and 5  illustrates a pixel array that is defined in four quadrants, with each quadrant of the array including masked readout pixels  40  and unmasked imaging pixels  50 . It is contemplated, however, that imaging devices according to the present invention may incorporate pixel arrays configured for readout in a variety of ways. For example,  FIGS. 6 and 7  illustrate the full frame mode and the frame transfer mode of the present invention in the context of a pixel array  15  that merely includes a single area of masked readout pixels  40  and a single area of unmasked imaging pixels  50 , as opposed to respective quadrants of readout and imaging pixels.  
         [0040]     Referring to  FIG. 8 , to operate an imaging device according to the present invention in a pipelined frame transfer mode, the mask  24  is positioned so that the image, which in the illustrated embodiment comprises a spectrum, is exposed onto only a relatively narrow area of unmasked imaging pixels  50  of the imaging detector  15 . This mode is typically used in conjunction with a spectrometer. In one embodiment using a 2048×2048 pixel CCD device, only the middle 200×2048 pixels of the detector are illuminated. The remainder of the surface area of the detector is mechanically masked or virtually masked using software to modify the spectrometer electron optics. After the spectrum exposure is complete, the image of the spectrum is transferred along the detector to an adjacent equivalent area of masked imaging pixels  40  just outside the portion on which the spectrum has been exposed. Subsequent image transfer occurs in a progressive manner until image data is read out by the signal output amplifier  30  coupled to the final area of masked pixels  40  in the pipeline. In this way many spectra, typically 1 to 10, can be exposed on the detector in a “pipeline” before the first spectrum is read out by the signal output amplifier  30 . As moving charge across the full surface area of the detector can take a considerable amount of time, this pipelining allows for much faster frame rates for spectroscopy uses.  
         [0041]     Imaging devices according to the present invention may be used in conjunction with a variety of charged particle beam systems. For example and by way of illustration, not limitation, as shown schematically in  FIG. 9 , an imaging device  10  according to the present invention may be mounted in the bottom of a TEM column  60 , such as, for example, the TEM described in Krivanek, U.S. Pat. No. 5,065,029, the entire disclosure of which is hereby incorporated by reference.  FIG. 9  also illustrates the use of a user interface  62  and a controller  64  programmed to prompt a user to select one of a plurality of imaging modes via the user interface  62 . It is noted that the user interface  62  and the controller  64  may take a variety of suitable forms and may be utilized in a variety of embodiments of the present invention.  
         [0042]     As a further example, referring to the schematic illustration of  FIG. 10 , an imaging device  10  according to the present invention may be mounted to the end of an imaging filter  70 , such as, for example, the energy-selected electron imaging filter described in Krivanek, U.S. Pat. No. 4,851,670, the entire disclosure of which is hereby incorporated by reference.  
         [0043]     In yet another embodiment of the invention schematically illustrated in  FIG. 11 , an imaging device  10  of the present invention may be mounted on the end of an electron energy loss spectrometer (EELS)  80 , such as, for example, the EELS device described in Krivanek, U.S. Pat. No. 5,097,126, the entire disclosure of which is hereby incorporated by reference.  
         [0044]     Embodiments of the imaging device of the present invention are capable of multiple modes of operation. This allows high quality images to be captured and fast viewing of captured images with high duty cycles, thereby providing high sensitivity and fast spectra readout using the same device. To achieve these features, a real or virtual removable mask to adjustably shield a portion of the imaging detector is used. Additionally, multiple methods of moving the charge on the imaging detector are used. In many cases the detector will be a CCD, although other suitable detectors can be used, for example a photodiode array or a CMOS detector.  
         [0045]     It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.  
         [0046]     For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.  
         [0047]     Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.