Patent Description:
In an exemplary electron microcopy system having a direct detection sensor, the sensor runs at a constant speed such as <NUM> 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 <NUM> 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. <NUM>) of counted frames together before sending the data to the host computer, thus yielding an effective summed-frame rate of <NUM> fps or an effective exposure time of <NUM> per recorded summed-frame.

In this example for an electron microscope used in imaging mode, the minimum exposure is <NUM> seconds, the maximum summed-frame rate to the computer is <NUM> fps and the maximum dose rate is <NUM> electrons per second per pixel.

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 <NUM> electrons per second per pixel shown above for the direct-detection detector when imaging in counting mode.

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>. 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.

An electron energy loss spectrometer according to the present invention is defined in claim <NUM>. A method of acquiring an electron energy loss spectrum according to the present invention is defined in claim <NUM>. Preferred embodiments are set out in dependent claims <NUM>-<NUM> and <NUM>-<NUM>.

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.

<FIG> 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) <NUM> that includes a scan control element <NUM> and scan control electronics <NUM> with a scan amplifier output signal <NUM> connected to scan control element <NUM>. A sample <NUM> is shown placed in the microscope <NUM>.

Consistent with embodiments described herein, the system further includes a high speed shutter <NUM>, controlled by a shutter driver <NUM> via control signal <NUM>; an imaging filter <NUM>, which comprises an energy offset drift tube <NUM>, controlled by a drift tube driver <NUM> via control signal <NUM>; and a direct detector camera system <NUM>.

Direct detector camera system <NUM> comprises a direct detector module <NUM> comprising a direct detector sensor <NUM>, a direct detector camera processing module <NUM> and a direct detector camera controller module <NUM>. As shown in <FIG>, image/spectrum data path <NUM> is provided from direct detector module <NUM> to direct detector camera processing module <NUM>. <FIG> further shows the direct detector camera internal synchronization signals <NUM> between the direct detector camera controller module <NUM> and the direct detector camera processing module <NUM>. The direct detector camera controller module <NUM> is further connected to and controls the scan control electronics <NUM> via a scanning pixel advance signal <NUM>, the direct detector module <NUM> via direct detector camera internal synchronization signals <NUM>, the drift tube driver <NUM> via control signal <NUM> and the shutter driver <NUM> via control signal <NUM>. The output image or spectrum data <NUM> of the direct detector camera module <NUM> is connected to a host computer system <NUM>.

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.

The major elements of the system are imaging filter <NUM> having a high speed shutter and a direct electron detector, such as a K2 camera from Gatan, Inc. This combination is shown in <FIG> 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.

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.

In a first method, there is a fixed exposure time, such that <NUM> sensor readouts of <NUM> microsecond-exposure are summed, with a total time for acquiring data of approximately <NUM> seconds (e.g., <NUM> full-height frames at 400fps), total live exposure time is <NUM> microseconds.

A second method according to the embodiments described herein uses a varying or stepped exposure times, such as <NUM> sensor readouts at <NUM> microsecond-exposure and <NUM> sensor readouts at <NUM> microseconds-exposure. These sensor readouts are summed and the two spectra are combined. This second method allows for the same live exposure time of <NUM> microseconds on the fainter image areas, but only requires <NUM> sensor readouts to be summed and takes only approximately <NUM> seconds to acquire. This is five times faster than the simple exposure mode of the first method.

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 <NUM>-D image along the non-dispersive direction as shown in <FIG>. In the inventive system, this projection is performed in the data processing module <NUM> of direct detector camera system <NUM>, as shown in <FIG>, and a <NUM>-D spectrum is then transferred from data processing module <NUM> to the host computer <NUM>, which reduces the data load by a factor of the pixel height of the detector.

In some implementations, this difference is a factor of approximately <NUM>,<NUM>. 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 <NUM> 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 <NUM>,000x for the projection and an increase in 40x since all projected sensor readout frames are sent to the host computer individually, instead of one summed-frame every <NUM> read-out frames, leading to a total reduction in the data rate versus the imaging mode of 100x with an increase in the spectra-per-second rate to the computer of 40x.

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 <NUM>%).

For EELS data, however, large dynamic ranges are typically present (on the order of <NUM><NUM>), 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 <NUM> to limit the total number of electrons impinging on the detector. An exemplary spectrometer device based on imaging filter <NUM> has a shutter <NUM> that allows exposures as short as <NUM> microsecond (or <NUM> millionth of a second) to be acquired. This shutter control is integrated with the direct detection camera system <NUM> to allow reduction of the intensity in each <NUM>/<NUM> second sensor frame read-out up to <NUM>,500x.

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 <NUM> microsecond and <NUM> milliseconds. For exposure above <NUM> millisecond the high-speed shutter <NUM> is just left open and sensor readout frames are summed within the data processing module <NUM>. 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.

When using the high-speed shutter <NUM> to maintain all areas on the direct detector <NUM> in the counting regime, efficiency can be low. If the shutter is running at its minimum exposure time of <NUM> microsecond, then the system is only detecting <NUM>/<NUM>,<NUM> 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>, with the zero-loss region <NUM>, the low loss region <NUM> and the core loss region <NUM>.

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.

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.

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 <NUM> is the optimal way to use the detector in most use cases.

An exemplary embodiment of the invention, as shown in <FIG>, enables the varying exposure mode controller of the system <NUM> 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.

An exemplary method <NUM> for operating an EELS that includes direct detection sensor <NUM>, is shown in <FIG>, wherein the variable exposure mode is controlled the direct detector camera controller <NUM>. As described herein, process <NUM> may be implemented by camera controller <NUM> or a combination of camera controller <NUM> with direct detector camera processing module <NUM>, and/or host computer system <NUM>. In this implementation, multiple (e.g. <NUM>) 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 <NUM> also controls the scanning pixel advance signal <NUM> 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.

Initially, a set of ten <NUM> micro second exposures are made and summed with a setting of <NUM>/<NUM> read-out frame duration (block <NUM>). Next, the summed frames are converted to a one-dimensional spectrum, this 1D spectrum data 421is transferred to a buffer or to host computer system <NUM> (block <NUM>).

A set of ten <NUM> micro second exposures are made and summed with a setting of <NUM>/<NUM> read-out frame duration (block <NUM>). Next, the summed frames are converted to a one-dimensional spectrum, this 1D spectrum data 441is transferred to a buffer or to computer system <NUM> (block <NUM>).

At block <NUM>, a set of <NUM><NUM> microsecond exposures are made and summed with a setting of <NUM>/<NUM> read-out frame duration. At block <NUM> a pixel advance signal pulse <NUM> is sent. Next, the summed frames are converted to a one-dimensional spectrum (block <NUM>), this 1D spectrum data is transferred to a buffer or to host computer system <NUM> (block <NUM>). Next, a determination is made as to whether the spectrum imaging block is complete (block <NUM>) and, if not, the process may return to block <NUM> for a next set of read-out frames.

<FIG> is a diagram illustrating exemplary physical components of a device <NUM>. Device <NUM> may correspond to various devices within the above-described system, such as direct detector camera system <NUM>, host computer <NUM>, etc. Device <NUM> may include a bus <NUM>, a processor <NUM>, a memory <NUM>, an input component <NUM>, an output component <NUM>, and a communication interface <NUM>.

Bus <NUM> may include a path that permits communication among the components of device <NUM>. Processor <NUM> may include a processor, a microprocessor, or processing logic that may interpret and execute instructions. Memory <NUM> may include any type of dynamic storage device that may store information and instructions, for execution by processor <NUM>, and/or any type of non-volatile storage device that may store information for use by processor <NUM>.

Software <NUM> includes an application or a program that provides a function and/or a process. Software <NUM> 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 <NUM>. Additionally, for example, device <NUM> may include software <NUM> to perform tasks as described above with respect to <FIG>.

Input component <NUM> may include a mechanism that permits a user to input information to device <NUM>, such as a keyboard, a keypad, a button, a switch, etc. Output component <NUM> may include a mechanism that outputs information to the user, such as a display, a speaker, one or more light emitting diodes (LEDs), etc..

Communication interface <NUM> may include a transceiver that enables device <NUM> 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 <NUM> may include mechanisms for communicating with another device or system via a network. Communication interface <NUM> may include an antenna assembly for transmission and/or reception of RF signals. In one implementation, for example, communication interface <NUM> may communicate with a network and/or devices connected to a network. Alternatively or additionally, communication interface <NUM> 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.

Device <NUM> may perform certain operations in response to processor <NUM> executing software instructions (e.g., software <NUM>) contained in a computer-readable medium, such as memory <NUM>. 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 <NUM> from another computer-readable medium or from another device. The software instructions contained in memory <NUM> may cause processor <NUM> 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.

Device <NUM> may include fewer components, additional components, different components, and/or differently arranged components than those illustrated in <FIG>. As an example, in some implementations, a display may not be included in device <NUM>. In these situations, device <NUM> may be a "headless" device that does not include input component <NUM>. As another example, device <NUM> may include one or more switch fabrics instead of, or in addition to, bus <NUM>. Additionally, or alternatively, one or more components of device <NUM> may perform one or more tasks described as being performed by one or more other components of device <NUM>.

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 scope of the invention as defined in the appended claims.

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.

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.

Claim 1:
An electron energy loss spectrometer comprising:
a direct detection sensor configured for operating in a counting mode when directly exposed to an electron spectrum spread in two dimensions along dispersive and non-dispersive axes so that the full sensor area is used; and
a sensor processor configured for receiving a sensor readout frame from said direct detection sensor representative of said electron spectrum in two dimensions,
wherein said sensor processor is configured to convert said sensor readout frame into a one-dimensional spectrum, and
wherein said sensor processor is configured to output said one dimensional electron spectrum to a host computer.