Charged particle detection for energy dispersive techniques

Abstract

A method and apparatus for detecting charged particles in energy dispersive spectroscopy is disclosed. Charged particles received from an energy dispersive spectroscopy analyzer in the form of a charged particle beam are accelerated toward a detector. The accelerated charged particles are received at a detection pixel array, which forms the detector. The charged particles that reach the detector are spread out in an energy dispersive direction.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method and apparatus for detecting and counting charged particles in a spectroscopy apparatus. The invention further relates to an apparatus for X-ray photoelectron spectroscopy. BACKGROUND

[0002] Many types of spectroscopy require the detection of charged particles, such as ions or electrons. In particular, many spectroscopy techniques, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, time-of-flight spectroscopy, ultraviolet photoelectron spectroscopy, reflection electron energy loss spectroscopy, and ion scattering spectroscopy, require the detection and counting of charged particles incident on a detector. Typically, in such techniques, the detector is used with an energy analyser which disperses the particles according to their energy, such that the energy of the particles can be determined from the position of the particles exiting the analyser. Therefore, a detector comprising multiple channels in at least one direction is typically required to provide the most efficient detection.

[0003] A challenge of such detection techniques is the small signal generated by a single charged particle incident at the detector. Examples of conventional detectors used include microchannel plates or electron multiplier devices. Both types of detectors have the effect of converting a single electron incident at the detector into a signal of multiple electrons (e.g. more than 10 6 electrons from each particle), which in turn can be easily counted by conventional electronics. The microchannel plate or electron multiplier device can then be coupled to a phosphor screen, to a lens and CCD, to a resistive anode, to a dual-land anode, or to a delay line anode to encode the position of the received charged particle. Such electron-multiplier-type detectors can be used with an energy analyser in certain spectroscopy techniques (e.g. XPS), whereby the position of the particle hitting the detector indicates the degree of deflection of the particle in the electrostatic field and therefore provides information about the binding energy of the particle.

[0004] These types of detectors have been perfected. However, they have many drawbacks. For example, the lifetime of such detectors is limited (the lifetime is determined by the amount of charge received (for microchannel plates, this can be 10-20 mC / cm 2 ), the count rate is limited (up to 40 million counts per second) and they are relatively expensive. Furthermore, microchannel plates suffer from pulse pile-up and overlapping pulses at high count rates. Finally, such techniques rely on transmitting an analogue signal from the detector inside the vacuum of the spectroscopy apparatus to the electronics mounted outside the vacuum through a vacuum feedthrough. Avoiding reflections and losses in such a setup is both difficult and expensive.

[0005] In the field of high-energy particle physics, active pixel detectors have recently been developed that can directly detect individual particles incident at the detector without the need to use prior electron multiplication. These detectors utilize an array of semiconductor diodes and complementary metal-oxide-semiconductor (CMOS) technology. These devices consist of a pixelated detector array (e.g., typically 256 x 256 pixels, although other sizes are possible) tightly coupled to an array of CMOS preamplifiers. The small capacitance of each pixel (typically only about 55 pm x 55 pm in size) and the tight coupling of the preamplifiers to each individual pixel allows the detector to pulse count individual particles (or X-rays) that arrive at each pixel. The large number of independent pixels (e.g., 64,000) in the detector array allows for extremely high counting rates to be achieved. Examples of such detectors include the MEDIPIX TM and TIMEPIX TM series of detectors developed for use in high-energy physics experiments conducted in collaboration with CERN in Geneva (e.g., the TIMEPIX3 TM described in U.S. Patent Publication No. US 2012 / 012613). Currently, the smallest particle energy that such detectors can detect is about 3 keV, and they are not designed to detect low-energy particles. Future designs of such detectors are expected to be able to detect lower minimum particle energies.

[0006] These types of active pixel detectors have proven to have potential in a range of applications, particularly in the detection of high-energy particles. In one example, electron backscatter pattern (EBSP) imaging uses an accelerated primary beam incident at a sample to generate backscattered electrons that are then detected. U.S. Patent Publication No. US 2016 / 054240 describes generating EBSP images using MEDIPIX TM or TIMEPIX3 TM detectors. This results in the use of the detectors to receive electrons with a primary beam energy of up to 30 keV, although in other high-energy applications, the detectors can be used to detect higher energy (up to about 1 MeV) particles.

[0007] However, the active pixel detectors described do not appear to be good candidates for use in energy dispersive spectroscopy. The energy dispersive spectroscopy techniques described above must accomplish accurate detection of low-energy particles. For example, in XPS, photoelectrons from a sample are decelerated to an energy of typically 100 eV or less before entering the analyzer. Thus, the energy of the particles leaving the analyzer for detection is relatively low and is not easily detected at the “active pixel” detectors described above.

[0008] However, Jungmann et al. (Int. J. Mass Spectrom. (2013), Vol. 341, pp. 34-44) describe an option for using the described detector in mass spectrometry. Here, particle multiplication is applied before being received by the pixelated detector, and therefore the pixelated detector is used only for readout. Specifically, Jungmann et al. describe the following immediately after TIMEPIX TM Mass spectrometry imaging was performed using a V-shaped microchannel plate (MCP) in front of the detector. Charged particles exiting the mass analyzer were incident on the microchannel plate, producing a single electron, which was multiplied by the MCP to approximately 10-10. 7 Each electron is then received at the detector. Compared to ion microscopy that emits ions, MCP / TIMEPIX... TM The detector assembly is kept at a high voltage. This is to overcome the limitation of kinetic energy emission and is typical in mass spectrometry. In imaging schemes, Vallance et al. (Physical Chemistry, Chemical Physics, 2014, Vol. 16, pp. 383-395) considered using a scintillator, one or more microchannel plates, and / or a phosphorescent screen to multiply the charged particle signal before pixelating the detector. In yet another example, Scienta Omnicron... TM Argus CU TM The detector uses a 128-channel strip anode detector and MEDIPIX. TM A series of readout electronic devices. In this case, MEDIPIX... TM The chip does not appear to be used as a detector, but rather as part of the readout electronics.

[0009] Zha et al., IEEE Transactions on Electron Devices. Another option using a pixelated detector is described in (2012), Vol. 59, p. 3594. This document describes electron detection using a specially back-side thinned CMOS active pixel sensor. The sensor is back-side thinned by a method involving grinding, reactive ion etching, and laser annealing to remove the substrate of the diode device and expose the epitaxial layer. Electrons with energies from 500 eV to 2 keV are received on the back side of the CMOS detector (specifically the epitaxial layer). However, such processing steps for the sensor are complex and can result in low yields, and cannot be directly applied to commercially available detectors. Furthermore, the lowest energy electrons detected using this detector design are approximately 500 eV, which is significantly higher than the typical pass energy of photoelectrons passing through an XPS hemispherical analyzer (approximately 100 eV).

[0010] It is therefore an object of the present invention to provide a method and apparatus for detecting charged particles in spectroscopy techniques that overcomes the shortcomings of the detectors and detection techniques described above. SUMMARY

[0011] The present invention relates to a method and apparatus for detecting charged particles in spectroscopy techniques. The method entails accelerating charged particles exiting a spectroscopy analyzer so as to be received directly at a detection pixel array. There is no need for multiplication of the charged particles, such as an MCP or phosphor plate. Rather, the method and apparatus are configured to accelerate the charged particles to an optimal energy for detection at a pixelated detector.

[0012] Due to the application of acceleration, the pixelated detector can be used without modification (such as thinning or removal of the device substrate), but still be able to detect charged particles of about 100 eV (or even less) low pass energy. The pixelated detector also allows for adaptation and optimization of the system, depending on the energy of the particles to be detected, compared to systems that physically modify the detector chip. Furthermore, using a detection pixel array as a detector, rather than just for readout as shown in some prior art systems, improves the resolution of the measured data and avoids the use of components with limited service life, such as an MCP. The described invention further allows for adjustment of the spectroscopy analyzer image plane to match the detector size, thereby improving detection efficiency. Thus, the described invention provides a viable and improved system for detecting low energy charged particles in spectroscopy analysis techniques (such as XPS) using active pixel sensors.

[0013] The presently described inventive technique entails accelerating the charged particles prior to detection (e.g., by applying a force to the charged particles as they move through an electrostatic field). For this reason, the described technique is particularly useful for elemental particle spectroscopy analysis (such as XPS, Auger, or other spectroscopy using photoelectrons), given the low mass of such particles.

[0014] In a first example, a method for detecting charged particles in spectroscopy analysis is described, the method comprising:

[0015] accelerating the charged particles received from the spectroscopy analyzer towards a detector; and

[0016] receiving the accelerated charged particles at a detection pixel array, the detection pixel array forming the detector.

[0017] In particular, the method can be a method for detecting charged particles in spectroscopy analysis, wherein the charged particles are analyzed according to their energy.

[0018] In the described method, the charged particles exit the energy spectrometer (or energy analyser) and are then accelerated before being incident at the detector. In particular, the charged particles are received directly at the detection pixels of the detector (i.e. without any intermediate component such as a microchannel plate or phosphor screen etc.). The receiving surface of the array of detection pixels forms the detection surface or image plane of the detector.

[0019] The energy spectrometer is an energy dispersive energy spectrometer in which the charged particles passing through the analyser (and subsequently to the detector) disperse in the energy dispersion direction in proportion to their energy.

[0020] The array of detection pixels comprises a plurality of detection pixels in an ordered arrangement or grid configuration. The array of detection pixels can be formed using an arrangement of more than one detector chip (each having a plurality of active pixels). For example, the detector chips can be "tiled", e.g. comprising 1 x 2 detector chips, or 2 x 2 chips, or other arrangements. In one example, the array of detection pixels can be formed from an arrangement of four Medipix chips (2 x 2) each comprising 256 x 256 pixels, thereby providing a detector array of 512 x 512 detection pixels. In an array using several independent detectors, or in an array using a single detector with a large number of pixels, a large number of energy channels in the energy dispersion direction can be recorded simultaneously (and hence a larger range of particle energies can be measured simultaneously). This in turn can result in a reduction in the overall number of data acquisitions. Preferably, each detection pixel is an active detection pixel comprising a semiconductor diode or more specifically a p-n junction. The active detection pixels can be CMOS devices. As such, the detector comprises a two-dimensional array of p-n junctions, each p-n junction in the array being comprised within a different pixel.

[0021] Preferably, the accelerated charged particles are received at the active layer of the semiconductor diode. In other words, the charged particles are first received at a dielectric layer comprising an active region (in conjunction with a p-n junction or other active band structure of a diode). The electrons impinge on the detector diode and generate electron-hole pairs in the depletion layer, which are then collected by an applied bias voltage. The charged particles are not received directly at the epitaxial layer or thinned substrate of the semiconductor diode.

[0022] Preferably, each detection pixel is associated with an amplifier. In contrast to, for example, a CCD, each active pixel comprises both a p-n diode (or photodetector) and an active amplifier. In contrast to a CCD pixel which only acquires and transports charge, each pixel comprises electronics which operate independently to detect and count the particles.

[0023] Preferably, an amplifier of each detection pixel of the detector is associated with a comparator and a counter. The amplifier (charge sensitive preamplifier, operational transimpedance amplifier, or operational amplifier) can produce a voltage proportional to the total charge deposited by the incoming charged particle at the semiconductor diode. The comparator is used to measure the analog voltage output from the amplifier and digitize it (in other words, the comparator acts as an analog-to-digital converter). The counter can provide a count of the number of received charged particles based on the output from the comparator. The amplifier, comparator, and counter can be connected in series to the semiconductor diode. The small capacitance of each pixel (typically only 55 pm x 55 pm in size, and a capacitance of about 100 fF) and the close coupling with the amplifier allow the detector to pulse count single charged particles (or X-rays) that arrive at each pixel. The large number of independent pixels in the detector array (typically 64,000 pixels) allows for extremely high count rates to be achieved.

[0024] Preferably, the charged particles are accelerated to at least a detection energy threshold, the detection energy threshold being a minimum energy of the charged particles for detection of the charged particles incident at the detection pixels of the detector. In other words, relatively low energy charged particles exiting the energy spectrometer are accelerated to a higher energy so as to be detected at the detection pixels with a sufficiently high signal-to-noise ratio. Ideally, the charged particles are accelerated to an energy that provides optimal detection efficiency for the array of detection pixels.

[0025] Optionally, the detection energy threshold is greater than 3 keV, or can be greater than 5 keV. The detection energy threshold depends on the type of detector chip used (and the detection pixels included by the detector chip). In certain examples, the detection energy threshold can be greater than 2 keV, greater than 3 keV, greater than 5 keV, greater than 7 keV, or greater than 10 keV.

[0026] Optionally, the energy spectrometer is configured to have a pass energy of less than 500 eV. The pass energy is the energy dependent on the electrostatic field within the energy spectrometer, where only electrons close to a given energy (the pass energy) are able to pass through and exit the energy spectrometer (and thus move forward towards the detector). The pass energy can also be considered to represent the energy of the particles passing out of the exit of the energy spectrometer. The pass energy will typically be much less than the detection energy threshold (e.g. less than 2 keV, or less than 1 keV). In a particular example, the energy of the charged particles received from the energy spectrometer will be no greater than 1.5 keV, no greater than 1 keV, no greater than 750 eV, no greater than 400 eV, no greater than 300 eV, no greater than 200 eV, or no greater than 150 eV. The energy of the charged particles received from the energy spectrometer can be in the range 1 eV to 500 eV, or more preferably in the range 10 eV to 200 eV. Typically, for XPS, the pass energy of the energy spectrometer is in the range 3-150 eV, although other types of energy spectrometry (such as Auger electron spectrometry) can result in a higher energy range.

[0027] Preferably, the detection energy threshold is at least ten times the pass energy of the energy spectrometer. In other words, the acceleration step can increase the energy of the charged particles by at least a factor of ten (one order of magnitude) before the charged particles are received at the detection pixel array.

[0028] Preferably, the charged particles received from the energy spectrometer comprise a charged particle beam, and before the accelerated particles are received at the detector, the method further comprises focusing the charged particle beam to change a magnification of the beam at an image plane of the detector. The image plane is the receiving surface of the detector (or more specifically, the receiving surface at the plurality of detection pixels that make up the array).

[0029] Preferably, focusing the charged particle beam to change the magnification of the charged particle beam comprises changing a cross-sectional area of the charged particle beam at the focal plane of the detector so as to substantially match at least one corresponding dimension of the detection pixel array at the detector.

[0030] Optionally, the energy spectrometer is an energy dispersive energy spectrometer, and wherein the cross-sectional area of the charged particle beam at the focal plane is changed so that a width of the charged particle beam in the energy dispersion direction substantially matches a corresponding dimension of the detection pixel array at the detector. In other words, the magnification can be changed so that the beam in the energy dispersion direction is received across substantially the entire width of the array in a direction corresponding to the energy dispersion direction. Ideally, in the same direction, the width of the beam will not be greater than the width of the detector array to avoid loss (i.e. no detection) of charged particles passing through the spectrometer. In one example, the magnification of the beam will be changed so that the width of the charged particle beam in the energy dispersion direction matches the dimension of the detector array in the same direction.

[0031] Preferably, a charged particle acceleration arrangement, which can be a charged particle optical device or can be comprised within a charged particle optical device, is arranged between the energy spectrum analyzer and the detector, accelerating the charged particles. More specifically, a charged particle optical device is arranged between the exit of the energy spectrum analyzer and the detection pixel array, and is configured to perform the acceleration step. The charged particle optical device is an arrangement of components configured to accelerate the charged particles (in other words, a charged particle accelerator). The charged particle optical device can comprise electrostatic lenses, or can be an arrangement in which voltages are applied between existing components to achieve acceleration.

[0032] Preferably, the charged particle optical device focuses the charged particles (in addition to accelerating the charged particles). In other words, the charged particle optical device is configured to reimage the output plane of the energy spectrum analyzer to match the receiving surface of the detection pixel array, as well as to accelerate the charged particles.

[0033] Preferably, the charged particle optical device comprises at least two lens elements arranged to perform the focusing step and the acceleration step.

[0034] In an alternative, the charged particle optical device can comprise a grid element to prevent penetration of the acceleration field and to provide uniformity of the field. In this case, the acceleration of the charged particles is achieved by maintaining a suitably high electric potential at the detection pixel array compared to the region at the exit of the energy spectrum analyzer. The high electric potential is suitable to create an acceleration field for the charged particles towards the detection pixel array.

[0035] Optionally, a second lens element of the at least two lens elements is arranged downstream of the charged particle beam compared to a first lens element of the at least two lens elements, the applied voltage of the second lens element being at least ten times the applied voltage of the first lens element. In this way, the charged particles travel accelerated between the first lens element and the second lens element. The second lens element can be at the same electric potential as the detector.

[0036] Optionally, the first lens element can be used to provide focusing of the charged particle beam.

[0037] Optionally, the first lens element has an applied voltage V 1 = 1400+ E pass - E k wherein E pass is the pass energy of the energy spectrum analyzer, and E k is the electron kinetic energy.

[0038] Optionally, the second lens element has an applied voltage V 2 = 15000+ E pass - E k wherein E pass is the pass energy of the energy spectrum analyzer, and E k is the kinetic energy of the electron.

[0039] Preferably, the method further comprises applying an extraction field at the exit of the energy spectrum analyzer, wherein the charged particle optics further comprises a third lens element upstream of the first lens element, the third lens element being arranged to apply the extraction field. The extraction field is an electrostatic field at the exit of the energy spectrum analyzer which helps and guides the charged particles to exit the exit aperture of the analyzer. The voltage applied to the third lens is scaled with respect to the pass energy of the analyzer (typically 5-150 eV). In one example, the extraction field provided by the third lens is between 10-20 V / mm.

[0040] Preferably, the first lens element and the second lens element have an applied voltage of opposite polarity to the third lens element. Optionally, when the charged particles are photoelectrons, the third lens element has a negative applied voltage and the first lens element and the second lens element have a positive applied voltage. Optionally, the first lens element and the third lens element have applied voltages of the same order of magnitude.

[0041] Optionally, the third lens element has an applied voltage V 3 = 100– 0.5E pass - E k wherein E pass is the pass energy of the energy spectrum analyzer, and E k is the kinetic energy of the electron.

[0042] Preferably, the detector comprises an array of N rows of detection pixels and M columns of detection pixels, wherein the method further comprises determining the number of charged particles received at each of the N x M detection pixels. In the imaging mode of the energy spectrum analyzer, the charged particle counts from the N x M pixels can be represented as a 2-dimensional image of the sample surface of the analyzer. For example, the described method (and apparatus) can be used in a Thermo Scientific TMTheta Probe angle-resolved X-ray photoelectron spectroscopy (ARXPS) system or Thermo Scientific TM This method is used to provide images in the microprobe of the ESCALAB™ Xi+ X-ray Photoelectron Spectrometer (XPS).

[0043] Preferably, the energy dispersive spectrometer operates in energy dispersive mode, and the method further includes, for each N The number of charged particles received at each detected pixel in the detection pixel column is summed to determine the number of charged particles included in each pixel. N 1 × the sum of the detected pixel columns M 1 × 1-dimensional data vector M A dimensional data vector represents the spatial dispersion of charged particles across one dimension of the image plane of the detector. In this way, a "snapshot energy spectrum" can be obtained from the energy spectrum analyzer. Specifically, by setting at least appropriate power and other operating parameters of the energy spectrum analyzer, charged particles are dispersed according to their energy as they travel through the analyzer (in a hemispherical analyzer, particles disperse radially). Therefore, charged particles reach the receiving surface of the detector in a manner proportional to their energy in the energy dispersion direction. In some instances, energy diffusion can be as high as 14% of the power. Thus, an energy spectrum over a narrow energy range can be obtained by summing (or binning) the counts of charged particles received at each detection pixel column in a direction perpendicular to the dispersion direction. Further data processing can also be applied. In this way, the detection pixel array can be used to simulate the 128 channels of a one-dimensional position-sensitive detector (such as a conventional microchannel plate). Therefore, an energy spectrum with a narrow energy scan can be obtained without scanning the analyzer's operating parameters, and thus, low-energy resolution data can be obtained relatively quickly. This is very useful for time-resolved experiments or for accelerating the in-depth analysis of samples.

[0044] Preferably, the method further includes: adjusting the operating parameters of the energy spectrum analyzer to analyze charged particles input to the energy spectrum analyzer across a certain energy range, wherein the operating parameters are adjusted at certain time intervals to cover the energy range of the input charged particles;

[0045] By determining, for each time interval, the spatial dispersion of charged particles across the detector's image plane in one dimension, representing the analyzer operating parameters during said time interval, is 1 × 10⁻⁶. M 1× dimensional data vectors determine multiple 1×M dimensional data vector; and for each increment of particle energy, summing data elements of the plurality of 1 x M dimensional data vector. In this way, a survey spectrum can be generated. Preferably, the degree of acceleration of the charged particles can be adjusted in a manner corresponding to adjustments in the operating parameters of the energy spectrometer, to maintain a constant relationship between particles exiting the analyser and particles incident at the detector. M dimensional data vector associated with an increment of particle energy. In this way, a survey spectrum can be generated. Preferably, the degree of acceleration of the charged particles can be adjusted in a manner corresponding to adjustments in the operating parameters of the energy spectrometer, to maintain a constant relationship between particles exiting the analyser and particles incident at the detector.

[0046] In other words, during the accelerating and receiving steps of the method, the operating parameters of the energy spectrometer can be adjusted or stepped through, in order to "scan" a range of charged particle energies. The operating parameters can be stepped through such that, at each set of operating parameters during an interval or dwell time, a 1 x M dimensional data vector is obtained which represents the spatial dispersion of charged particles in one dimension on the detector image plane. The process is repeated for each set of operating parameters until a plurality of 1 x M dimensional data vectors are obtained. Each data element of the 1 x M dimensional data vector is associated with a particle energy. Accordingly, the charged particle counts of data elements associated with the same particle energy (or within the same particle energy increment) across the plurality of 1 x M dimensional data vectors can be binned (or summed) in order to provide a survey spectrum across the range of particle energies scanned by the analyser. In this way, the survey spectrum can be considered to be a sum of many energy offset "snapshot" spectra, as described above.

[0047] Typically, a survey spectrum will be obtained at a relatively low throughput at the energy spectrometer. The step size of the operating parameters of the energy spectrometer can be set to provide sufficient energy spectral resolution. A survey spectrum can be obtained in a similar manner to the survey spectrum, but in this case the measurement is performed at the energy spectrometer using a high pass energy (providing low resolution and high throughput) and a relatively large step size of the operating parameters of the analyser. Accordingly, the survey spectrum can provide a low resolution spectrum covering the entire possible energy range of emitted photoelectrons.

[0048] Preferably, the energy spectrometer operates in an imaging mode or an energy dispersive mode. In one example, the energy spectrometer operates in an energy dispersive mode to disperse the charged particles such that each charged particle is incident at the detector at a column of pixels representing the energy of the charged particle. In an alternative example, the energy spectrometer operates in an imaging mode such that each charged particle is incident at the detector at a pixel representing the location in the sample from which the charged particle originated. In the imaging mode, electrons will pass through the energy spectrometer and so some separation based on charged particle energy can be observed. As such, there can be energy selection in the imaging and in some cases both energy dispersion and angular mapping.

[0049] Optionally, the energy spectrometer is one of: an electrostatic hemispherical analyzer; a cylindrical mirror analyzer; a time-of-flight analyzer; a parallel plate analyzer, a retarding field analyzer. Any type of energy spectrometer that disperses the charged particles according to their energy (in other words, an energy dispersive energy spectrometer) can be used.

[0050] Preferably, the method further comprises generating a digital signal for each detector pixel of the detector, the digital signal representing the number of charged particles received at the detector pixel. Preferably, the method comprises transferring the digital signal of each detector pixel to the processor through a vacuum feedthrough. The digital signal is produced by a diode, a preamplifier, a comparator and a counter. Advantageously, this means that digital signals rather than analog signals need to be transferred out of the vacuum chamber in which the detector resides. It is extremely difficult to transfer analog signals from any device in a vacuum and through a vacuum feedthrough, especially at high speeds. In order to avoid reflections and losses, each analog signal must be carried by a coaxial cable and must pass through a coaxial vacuum feedthrough. This is expensive and takes up a lot of space. With the described detector, which requires a channel per pixel, the complexity of the number of coaxial cables would be prohibitive. Therefore, providing digital signals overcomes many such problems.

[0051] Optionally, the detector is mounted on a printed circuit board (PCB) and the amplifier, comparator and particle counter are arranged on the PCB. As such, the detector can be mounted in a simple and compact manner.

[0052] Optionally, the PCB provides a vacuum seal for an analysis chamber for housing at least the energy spectrometer and the detector.

[0053] Optionally, the PCB further provides a vacuum feedthrough for the digital signal. As such, the PCB can provide a vacuum seal and an electrical feedthrough for signals to and from the detector.

[0054] Optionally, the interface for passing the digital signal to the processor is a bidirectional fiber optic link between the PCB and the processor. Advantageously, the fiber optic link allows the data to be transmitted from the detector (which can float at the energy detection threshold, e.g. about 10 keV) to the processor. Beneficially, the fiber optic link enables USB3 data transfer rates.

[0055] In a second example, a device for detecting charged particles in energy dispersive analysis is described, the device comprising:

[0056] a charged particle optical device; and

[0057] a detection pixel array forming a detector;

[0058] wherein the arrangement of charged particle optical devices is configured to accelerate charged particles received from an energy dispersive analyser towards the detector for receipt at the detection pixel array.

[0059] In particular, a device for detecting charged particles in energy dispersive analysis is described, wherein the charged particles are analysed according to their energy.

[0060] The charged particle optical device (which can be an arrangement of charged particle optical devices comprising electrostatic lenses or, less preferably, magnetic lenses, or can be considered to be a charged particle accelerator) can accelerate and manipulate the charged particles emitted from the energy dispersive analyser. In particular, the charged particle optical device can be used to accelerate the charged particles to an optimal energy for detection at the detection pixel array. The charged particle optical device is arranged between the energy dispersive analyser and the detector.

[0061] The array can comprise at least 1,000 detection pixels. In some examples, the array can comprise 128 x 128 pixels to match the energy resolution of existing detectors. However, much larger arrays can be used, comprising 256 x 256 (or more) pixel arrays, making it possible to achieve much higher energy resolution of the energy dispersive map.

[0062] The energy dispersive analyser is an energy dispersive energy dispersive analyser, wherein the charged particles passing through the analyser (and subsequently to the detector) disperse in the energy dispersive direction in proportion to their energy.

[0063] Preferably, each detection pixel is an active detection pixel comprising a semiconductor diode, and more particularly, a p-n junction. The arriving charged particles generate electron / hole pairs in the semiconductor diode of each receiving pixel. This in turn generates a measurable current at the diode. Thus, the charged particles are detected without using any charged particle (photoelectron) multiplication.

[0064] Preferably, the accelerated charged particles are received at an active layer of the semiconductor diode. In other words, the charged particles are received at a layer that forms part of the diode function, including a dielectric layer, or a layer that forms part of a p-n junction. The charged particles are not received at a substrate or epitaxial layer of the semiconductor diode device. The apparatus according to the present invention avoids any complex semiconductor processing steps, such as backside thinning of the diode device. The counts per second of the detector formed from the array of detection pixels is significantly higher than a typical detector including a microchannel plate or similar.

[0065] Preferably, each detection pixel is connected to an amplifier. The amplifier is tightly coupled to the semiconductor diode. Preferably, each detection pixel, more specifically each amplifier associated with a detection pixel, is connected to a comparator and a counter. In other words, the amplifier is connected to an analog-to-digital converter. The entire assembly of detection pixels, amplifiers, comparators and counters is mounted in vacuum, such that only digital signals need to pass through a vacuum feedthrough. This also increases the readout speed of the detection pixels.

[0066] Preferably, the charged particle optics arrangement is configured to accelerate the charged particles to at least a detection energy threshold, the detection energy threshold being a minimum energy of a charged particle for detection of the charged particle incident at a detection pixel of the detector.

[0067] Preferably, the detection energy threshold is greater than 3 keV, or can be greater than 5 keV. The detection energy threshold can be set to a percentage of the particle energy required for optimal detection efficiency of a given detection pixel. The particle energy required for optimal detection efficiency can be determined by the thickness of the semiconductor diode active layer and the extent to which a particle needs to penetrate into the active layer to generate a current at the device. The detection energy threshold can be greater than 1.5 keV, greater than 2 keV, greater than 3 keV, greater than 4 keV, greater than 5 keV, greater than 7 keV, greater than 10 keV.

[0068] Preferably, the energy spectrometer is configured to have a pass energy of less than 500 eV. In other examples, the pass energy can be less than 1.5 keV, less than 1 keV, less than 750 eV, less than 400 eV, less than 300 eV, less than 200 eV, or less than 150 eV. Typically, for XPS, the pass energy of the energy spectrometer is in the range 50-150 eV.

[0069] Optionally, the detection energy threshold is at least ten times (or an order of magnitude greater than) the pass energy of the energy spectrometer.

[0070] Preferably, the received charged particles from the energy spectrum analyser comprise a charged particle beam, and wherein the charged particle optics arrangement is configured to focus said charged particle beam to change a magnification of said beam at an image plane at the detector. In other words, the charged particle optics are configured to change a cross-sectional area of the charged particle beam at the receiving surface of the detector pixel array. Preferably, the charged particle optics arrangement is configured to change the magnification of the charged particle beam to substantially match a cross-sectional area of the charged particle beam at the detector focal plane to at least one corresponding dimension of the detection pixel array at the detector. Advantageously, this allows the charged particle beam to be received across the maximum possible surface area of the detection pixel array, and thereby optimises the number of received charged particles.

[0071] Optionally, the energy spectrum analyser is an energy dispersive energy spectrum analyser, and the charged particle optics arrangement is configured to change the magnification of said charged particle beam such that a width of said charged particle beam in an energy dispersion direction substantially matches a corresponding dimension of said detection pixel array at said detector. Advantageously, the width of the energy dispersed charged particle beam is matched to be incident across the total number of columns of detection pixels at the detector array, thereby maximising the efficiency of the detector.

[0072] Preferably, said charged particle optics arrangement comprises at least two lens elements arranged to focus said charged particles and to accelerate said charged particles. In particular, at least one of the lens elements can perform a focusing function to change the size of the cross-section of the charged particle beam at the image plane of the detector. The acceleration function can be achieved by applying an electric potential between the two lens elements or between one of the lens elements and the exit of the energy spectrum analyser or between one of the lens elements and the detector. In other words, the charged particle optics arrangement first accelerates the charged particles and second re-images the output plane of the analyser to match the dimensions of the receiving surface of the detection pixel array. The lens elements can be electrostatic lenses, or less preferably magnetic lenses.

[0073] In an alternative, the acceleration can be applied by applying an electric potential between the exit of the energy spectrum analyser and the detection pixel array, for example by holding the detector at a relatively high electric potential. In this case, no lens elements are used, and the charged particle optics form part of the otherwise described apparatus.

[0074] In a further alternative, the charged particle optics comprise only a single lens element.

[0075] Optionally, a second lens element of the at least two lens elements is arranged downstream of the charged particle beam compared to a first lens element of the at least two lens elements. Optionally, an applied voltage of the second lens element is at least ten times an applied voltage of the first lens element.

[0076] Optionally, the first lens element has an applied voltage V 1 = 1400+ E pass - E k wherein E pass is the pass energy of the energy spectrum analyzer, and E k is the electron kinetic energy.

[0077] Optionally, the second lens element has an applied voltage V 2 = 15000+ E pass - E k wherein E pass is the pass energy of the energy spectrum analyzer, and E k is the electron kinetic energy.

[0078] Preferably, the charged particle optics arrangement further comprises a third lens element configured to apply an extraction field at the exit to the energy spectrum analyzer.

[0079] Preferably, the first and second lens elements have applied voltages of opposite polarity to the third lens element. Optionally, when the charged particles are photoelectrons, the third lens element has a negative applied voltage and the first and second lens elements have positive applied voltages. Optionally, the first and third lens elements have applied voltages of the same order of magnitude.

[0080] Optionally, the third lens element has an applied voltage V 3 = 100– 0.5E pass - E k wherein E pass is the pass energy of the energy spectrum analyzer, and E k is the electron kinetic energy.

[0081] Optionally, the apparatus further comprises an energy spectrum analyzer from which the charged particles exit towards the charged particle optics arrangement.

[0082] Optionally, the energy spectrum analyzer is one of: an electrostatic hemispherical analyzer; a cylindrical mirror analyzer; a time-of-flight analyzer; a parallel plate analyzer, a retarding field analyzer.

[0083] The device may include a controller for controlling an energy dispersive spectroscopy analyzer (including an electrostatic field applied at the analyzer) and a controller for reading out and processing data from the detected pixels. The controllers may be a single controller or two separate controllers. The controllers may be linked to a computer processor or form part of a computer processor.

[0084] Preferably, the device includes a controller configured to adjust the operating parameters of an energy spectrum analyzer to analyze charged particles input to the analyzer having a certain energy range, wherein the operating parameters are adjusted to scan the energy range of the input charged particles. In some cases, the acceleration of the charged particles can be adjusted to be parallel to the adjustment of the operating parameters of the energy spectrum analyzer to maintain a constant relationship.

[0085] Preferably, the detector comprises a... N Each detection pixel row and M An array of detection pixel columns, wherein the controller is further configured to determine in N x M The number of charged particles received at each of the detection pixels. For example, the controller can receive the number of charged particles received from the array of detectors, each coupled to a corresponding amplifier, comparator, and counter. N x M The digital signal output by each semiconductor diode in the detection pixel's semiconductor diodes. In "imaging mode," the controller can convert the digital signal from the... N x M The count date received by each detected pixel is plotted as a two-dimensional image.

[0086] Optionally, the energy dispersive spectrometer operates in energy dispersive mode, wherein the controller is further configured to, for each N The detection pixel column is determined by summing the number of charged particles received at each detection pixel in the detection pixel column to determine the number of charged particles included in each pixel. N 1 × the sum of the detected pixel columns M 1 × 1-dimensional data vector M A 1×12-dimensional data vector represents the spatial dispersion of the charged particles across one dimension of the image plane of the detector. In other words, the controller can be used to process the digital signal received from each individual detection pixel to sum the counts of charged particles read from each detection pixel in a column of the pixel array. In this way, the dispersion of the charged particle beam in the energy dispersion direction can be measured, and an energy spectrum can be obtained. In a specific example, a 1×12-dimensional data vector is obtained for a specific set of operating parameters of the energy spectrum analyzer during a predetermined residence time. M 1 × 1-dimensional data vector M 3D data vectors can be plotted as "snapshot" energy spectra.

[0087] Preferably, the controller is configured to adjust the operating parameters at time intervals to span the energy range of the input charged particles. As with a typical energy spectrum measurement, the electrostatic fields, pass energy, and other device variables of the energy spectrum analyzer can be adjusted to deliver the full range of charged particle energies within the sample through the energy spectrum analyzer to the detector.

[0088] Preferably, the controller is further configured to determine a plurality of 1 x M dimensional data vectors by determining, for each time interval, a 1 x M dimensional data vector representing the spatial dispersion of charged particles across one dimension of the image plane of the detector under the analyzer operating parameters during the time interval; and binning data elements of the plurality of 1 x M dimensional data vectors for increments of particle energy, the binning including summing all elements of the plurality of 1 x M dimensional data vectors associated with an increment of particle energy. In other words, each set of operating parameters of the energy spectrum analyzer is held for a certain dwell time, and a 1 x M dimensional data vector is generated for each set of operating parameters, the 1 x M dimensional data vector representing the charged particle counts in each column of pixels during the dwell time. Data elements in the generated 1 x M dimensional data vectors representing a given increment of particle energy can be binned or summed to generate a complete energy spectrum plot.

[0089] Preferably, the amplifier, comparator, and particle counter are configured to generate a digital signal for each detector pixel, the digital signal representing the number of charged particles received at the detector pixel.

[0090] Preferably, the device further includes a printed circuit board (PCB) on which the detector is mounted, and on which the amplifier, comparator, and particle counter are disposed.

[0091] Optionally, the device further includes a bidirectional fiber optic link for carrying the digital signals from the PCB to the processor.

[0092] Preferably, the PCB is configured to provide a vacuum seal for the analysis chamber, the analysis chamber being used to house at least the energy spectrum analyzer and the detector, the analysis chamber being sealable to contain a vacuum, and the PCB board is further configured to provide a vacuum feedthrough for the digital signals, the vacuum feedthrough being used to deliver the digital signals from inside the analysis chamber to outside the analysis chamber.

[0093] In a third aspect, an X-ray photoelectron spectrometer is described, the X-ray photoelectron spectrometer comprising an apparatus as described above and an electrostatic hemispherical analyzer, wherein the arrangement of charged-particle optics is configured to accelerate photoelectrons received from the electrostatic hemispherical analyzer toward the detection pixel array forming the detector.

[0094] The following clause numbered clauses describe further illustrative examples:

[0095] 1. A method for detecting charged particles in energy dispersive analysis, the method comprising:

[0096] accelerating charged particles received from an energy dispersive analyzer toward a detector; and

[0097] receiving the accelerated charged particles at a detection pixel array, the detection pixel array forming the detector.

[0098] 2. The method of clause 1, wherein each detection pixel is an active detection pixel comprising a semiconductor diode.

[0099] 3. The method of clause 2, wherein each detection pixel is associated with an amplifier, a comparator, and a counter.

[0100] 4. The method of any of the preceding clauses, wherein the charged particles are accelerated to at least a detection energy threshold, the detection energy threshold being a minimum energy of a charged particle for detection of the charged particle incident at a detection pixel of the detector.

[0101] 5. The method of any of the preceding clauses, wherein the charged particles received from the energy dispersive analyzer comprise a charged particle beam, and, prior to receiving the accelerated particles at the detector, the method further comprises:

[0102] focusing the charged particle beam to change a magnification of the beam at an image plane of the detector.

[0103] 6. The method of clause 5, wherein the energy dispersive analyzer is an energy dispersive spectrometer, and wherein focusing the charged particle beam to change a magnification of the charged particle beam comprises changing a cross-sectional area of the charged particle beam at a focal plane such that a width of the charged particle beam in an energy dispersion direction substantially matches a corresponding dimension of the detection pixel array at the detector.

[0104] 7. The method of any of the preceding clauses, wherein charged-particle optics arranged between the energy dispersive analyzer and the detector accelerate the charged particles.

[0105] 8. The method of clause 7, when dependent on clause 5 or 6, wherein the charged particle optics are further configured to focus the charged particles.

[0106] 9. The method of clause 8, wherein the charged particle optics comprise at least two lens elements arranged to perform the focusing step and the accelerating step.

[0107] 10. The method of any one of the preceding clauses, further comprising applying an extraction field at an exit of the energy spectrometer.

[0108] 11. The method of clause 10, when dependent on clause 9, wherein the charged particle optics further comprise a third lens element upstream of the first lens element, the third lens element arranged to apply the extraction field.

[0109] 12. The method of clause 11, the first and second lens elements having an applied voltage of opposite polarity to the third lens element.

[0110] 13. The method of any one of clauses 1 to 5 or clauses 7 to 12, wherein the energy spectrometer is operated in an imaging mode such that each charged particle is incident at the detector at a detection pixel representing a location in the sample from which the charged particle originated.

[0111] 14. The method of any one of clauses 1 to 12, wherein the energy spectrometer is operated in an energy dispersive mode that disperses the charged particles such that each charged particle is incident at the detector at a column of detection pixels representing an energy of the charged particle.

[0112] 15. The method of any one of the preceding clauses, wherein the detector comprises an array of N rows of detection pixels and M columns of detection pixels, wherein the method further comprises:

[0113] determining a number of charged particles received at each of the N x M detection pixels.

[0114] 16. The method of clause 15, when dependent on clause 14, wherein the method further comprises:

[0115] for each of the N columns of detection pixels, summing the determined number of charged particles received at each of the detection pixels in the column of detection pixels to determine a sum comprising each of the N columns of detection pixels.M 1 x M-dimensional data vectors, the 1 x M The 1 x M-dimensional data vectors represent a spatial dispersion of the charged particles across one dimension of the image plane of the detector.

[0116] 17. The method of clause 16, further comprising:

[0117] adjusting operating parameters of the energy spectrum analyser to analyse charged particles input to the energy spectrum analyser across an energy range, wherein the operating parameters are adjusted at time intervals to cover the energy range of the input charged particles;

[0118] determining a plurality of 1 x M-dimensional data vectors by determining, for each time interval, a 1 x M-dimensional data vector for the time interval; M

[0119] binning data elements of the plurality of 1 x M-dimensional data vectors, the binning comprising summing all elements of the plurality of 1 x M-dimensional data vectors associated with a same increment of particle energy. M M

[0120] 18. An apparatus for detecting charged particles in energy spectrum analysis, the apparatus comprising:

[0121] a charged particle optical arrangement; and

[0122] a detection pixel array, the detection pixel array forming a detector;

[0123] wherein the charged particle optical arrangement is configured to accelerate charged particles received from the energy spectrum analyser towards the detector to be received at the detection pixel array.

[0124] 19. The apparatus of clause 18, wherein each detection pixel is an active detection pixel comprising a semiconductor diode.

[0125] 20. The apparatus of clause 19, wherein each detection pixel is connected to an amplifier, a comparator and a counter.

[0126] 21. The apparatus of any one of clauses 18 to 20, wherein the charged particle optical arrangement is configured to accelerate the charged particles to at least a detection energy threshold, the detection energy threshold being a minimum energy of a charged particle for detection of the charged particle incident at a detection pixel of the detector.

[0127] ​​​22. The apparatus of any one of clauses 18-21, wherein the charged particles received from the energy spectrometer comprise a charged particle beam, and wherein the charged particle optics are further configured to focus the charged particle beam to change a magnification of the beam at an image plane of the detector.

[0128] 23. The apparatus of clause 22, wherein the energy spectrometer is an energy dispersive energy spectrometer, and wherein charged particle optics are configured to change the magnification of the charged particle beam such that a width of the charged particle beam in an energy dispersion direction substantially matches a corresponding dimension of the detection pixel array at the detector.

[0129] 24. The apparatus of clauses 22 or 23, wherein the charged particle optics comprise at least two lens elements arranged to focus the charged particles and accelerate the charged particles.

[0130] 25. The apparatus of clause 24, wherein the charged particle optics further comprise a third lens element configured to apply an extraction field at an exit of the energy spectrometer.

[0131] 26. The apparatus of clause 25, wherein the first and second lens elements have applied voltages of opposite polarity to an applied voltage at the third lens element.

[0132] 27. The apparatus of any one of clauses 18-26, further comprising:

[0133] the energy spectrometer from which charged particles exit toward the charged particle optics.

[0134] 28. The apparatus of any one of clauses 18-27, wherein the energy spectrometer is operated in an imaging mode such that each charged particle is incident at the detector at a pixel representing a location in a sample from which the charged particle originated.

[0135] 29. The apparatus of any one of clauses 18-27, wherein the energy spectrometer is operated in an energy dispersive mode that disperses the charged particles such that each charged particle is incident at the detector at a column of pixels representing an energy of the charged particle.

[0136] 30. The apparatus of any one of clauses 18-29, wherein the detector comprises an array of N rows of detection pixels and M columns of detection pixels, wherein the apparatus further comprises a controller configured to:

[0137] Determined at N x M The number of charged particles received at each of the detection pixels.

[0138] 31. The device according to Clause 30, when subordinate to Clause 29, wherein said controller is further configured to:

[0139] For each N The number of charged particles received at each detected pixel in the detection pixel column is summed to determine the number of charged particles included in each pixel. N 1 × the sum of the detected pixel columns M 1 × 1-dimensional data vector M The dimensional data vector represents the spatial dispersion of the charged particles across one dimension of the image plane of the detector.

[0140] 32. The device according to clause 30 or 31, wherein the controller is further configured to:

[0141] The operating parameters of the energy spectrum analyzer are adjusted to analyze charged particles input to the analyzer across a certain energy range, wherein the operating parameters are adjusted at certain time intervals to cover the energy range of the input charged particles.

[0142] 33. The device according to Clause 32, when subordinate to Clause 31, wherein the controller is further configured to:

[0143] By determining a 1 × 10⁻⁶ Ω·cm in one dimension of the spatial dispersion of charged particles in the detector's image plane for each time interval, a Ω·cm in one dimension of the spatial dispersion of charged particles in the detector's image plane is obtained. M 1× dimensional data vectors determine multiple 1× M A 3D data vector, used for the analyzer's operating parameters during operation, and:

[0144] For the plurality of 1 × M The data elements of the 1×10⁻⁸ data vector are binned, and the binning includes the plurality of 1×10⁻⁸ bins associated with the same increment of particle energy. M Summing all elements of a 3D data vector.

[0145] 34. An X-ray photoelectron spectrometer, comprising:

[0146] The equipment pursuant to any one of Clauses 18 to 33; and

[0147] Electrostatic hemispherical analyzer;

[0148] The arrangement of the charged particle optics is configured to accelerate photoelectrons received from the electrostatic hemispherical analyzer toward the detection pixel array forming the detector. BRIEF DESCRIPTION OF DRAWINGS

[0149] Embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0150] Figure 1 is a schematic of an apparatus for detecting charged particles in energy dispersive analysis;

[0151] Figure 2 is a schematic of a detector having an array of detector pixels;

[0152] Figure 3 is a schematic showing the pixel electronics associated with each pixel of the detector;

[0153] Figure 4 is a schematic of an X-ray photoelectron spectrometer;

[0154] Figure 5A is a cross-section of an apparatus for detecting charged particles in energy dispersive analysis;

[0155] Figure 5B is Figure 5A different cross-sectional views of the apparatus for detecting charged particles in energy dispersive analysis shown in

[0156] Figure 6 is Figure 5A and 5B plan views of the apparatus of

[0157] Figure 7 is a schematic of a bidirectional fiber optic USB interface;

[0158] Figure 8 is a graph showing example “snapshot” energy spectra using the described apparatus;

[0159] Figure 9 is a graph showing example “scan” energy spectra using the described apparatus;

[0160] Figure 10 is a plot showing example “full spectral analysis” energy spectra obtained using the described apparatus;

[0161] Figure 11 is a schematic of an array of detector pixels at a detector; and

[0162] Figure 12 is a graph showing example reflection electron energy loss spectra.

[0163] In the drawings, like parts are referred to by like reference numerals. The drawings are not to scale. DETAILED DESCRIPTION

[0164] Reference is made toFigure 1 An apparatus for detecting charged particles in energy dispersive analysis is shown. The apparatus can be applied to a number of different types of energy dispersive analysis or elementary particle spectroscopy techniques, including X-ray photoelectron energy dispersive analysis, Auger electron energy dispersive analysis, ultraviolet photoelectron energy dispersive analysis, reflection electron energy loss energy dispersive analysis, time-of-flight energy dispersive analysis and ion scattering energy dispersive analysis. The apparatus and associated detection method can be used in energy dispersive analysis mode (particularly suitable for analysis leading to temporal or spatial separation of particles having different energy, mass or charge characteristics) or imaging mode, as described below.

[0165] Charged particles (here represented by a spatially dispersed charged particle beam 12) exit the energy dispersive analyser 14 and are received directly at the detector 16. The detector 16 comprises N × M an array of detection pixels. In a particular example, the detector comprises an array of 256 x 256 detection pixels and thus contains over 64,000 pixels in total. In a specific example, the detector is a MEDIPIX TM or TIMEPIX TM detector array, although any detector comprising an array of active detection pixels can be used (including any detector in the X TM / TIMEPIX TM series). Further description of the detector can be found below. Figure 2

[0166] A configuration of lens elements is arranged between the exit of the energy dispersive analyser 14 and the detector 16, which together provide charged particle optics, which in this example is an electrostatic lens arrangement 18. The electrostatic lens arrangement 18 is configured to accelerate the charged particles of the charged particle beam 12. As such, the velocity of the charged particles incident at the detection pixels of the detector 16 is greater than the velocity of the charged particles exiting the energy dispersive analyser. This in turn means that the kinetic energy of the charged particles is increased (uniformly for all particles) due to the acceleration of the electrostatic lens arrangement 18. Therefore, the charged particles are incident at the detection pixels of the detector at an energy closer to the optimum energy for detection at the detection pixels. The specific energy will be determined by the design of the detection pixels.

[0167] However, there is no amplification (or magnification) of the charged particles between the analyser 14 and the detector 16. The inventors have recognised that such amplification is not required when the charged particles incident at the detection pixels have sufficient energy. For example, in Figure 1 ​In the apparatus of Figure 1, charged particles received from the energy analyser are not converted into secondary particles, such as secondary electrons, before reaching the detection pixel array. Thus, for example, there is no electron multiplier (e.g. microchannel plate (MCP)) between the analyser 14 and the detector 16. This is advantageous because such electron multipliers typically have a limited useful lifetime. Furthermore, use of an MCP (or similar) typically requires the application of an additional voltage, thereby increasing the complexity of the detector circuitry.

[0168] In use, the electrostatic lens arrangement 18 is configured to accelerate charged particles to at least a minimum or threshold energy when the charged particles are incident at a detection pixel of the detector 16. The minimum or threshold energy is the minimum energy at which reliable detection at the detection pixel is possible. In a MEDIPIX TM or TIMEPIX TM In a particular example of a detector array, the threshold detection energy that must be exceeded for detection of a charged particle is 5 keV, and more preferably 10 keV or even 15 keV, to obtain optimal detection efficiency. In some current examples, the threshold detection energy is between 5 keV and 15 keV, but in future designs of pixelated detectors, the threshold energy can be much lower (e.g. 2 keV).

[0169] Figure 1 The accelerating and focusing electrostatic fields, mathematically modelled by the electrostatic lens arrangement 18, applied to the charged particles (in this case electrons) are shown in Figure 2. Figure 1 The accelerating and focusing electrostatic fields, mathematically modelled by the electrostatic lens arrangement 18, applied to the charged particles (in this case electrons) are shown in Figure 2. Figure 1 The particular electrostatic lens arrangement of Figure 1 utilises three electrostatic lens elements 20, 22, 24, although other arrangements with different numbers of lenses can be used.

[0170] As described above, the electrostatic lens arrangement is configured to accelerate the charged particles to at least a threshold detection energy required for detection of the charged particles at the active pixels of the detector. In addition, the electrostatic lens arrangement is configured to focus the charged particle beam 12 at the point of incidence of the charged particle beam 12 at the receiving detection surface of the detector 16. Figure 1 The fields generated by the electrostatic lens elements 18 of Figure 1 are used to provide focusing and magnification of the charged particle beam. More specifically, the focusing fields are used to change the width of the charged particle beam 12 at the point at which it is incident at the receiving detection surface of the detector 16, referred to as the image plane of the detector. Ideally, the width of the charged particle beam 12 is selected to match at least one dimension of the receiving detection surface of the detector 16, so as to optimise (and / or maximise) the number of detection pixels at the detector that can receive the charged particles. In some cases, the focusing fields will thus be magnifying fields (to increase the width of the charged particle beam W at the point of incidence at the receiving detection surface of the detector 16), but can also be demagnifying fields (to decrease the width of the charged particle beam W at the point of incidence at the receiving detection surface of the detector 16), but can also be demagnifying fields (to decrease the width of the charged particle beam W). When the analyser 14 is used in imaging mode, the magnification can be modified to ensure that the entire charged particle beam falls within the image plane of the detector 16. When the analyser 14 is used in energy spectrum analysis mode, where the charged particles are dispersed primarily in one dimension according to their energy, the magnification will be modified to ensure that the width of the charged particle beam in the energy dispersion direction W matches the image plane of the detector 16 in the corresponding dimension.

[0171] Thus, a particular configuration of the electrostatic lens arrangement 18 is selected to perform the described acceleration and focusing functions. In the particular example described below, the first lens element 22 is a cylindrical lens element and the second lens element 20 is a conical lens element. Figure 1 In the particular example, the cylindrical first lens element 22 is arranged in alignment with the exit of the energy spectrum analyser 14, such that the charged particle beam 12 exiting the analyser 14 passes through the cavity of the cylindrical first lens element 22. The second lens element 20 is arranged adjacent to and spaced apart from the first lens element 22. The second lens element 20 has a cylindrical portion 20a and a conical portion 20b, with the holes through the cylindrical portion 20a and the conical portion 20b forming an open cavity through which the charged particle beam 12 passes. The second lens element 20 is arranged such that the cylindrical portion 20a is downstream of the charged particle beam 12 from the first lens element 22, with the conical portion 20b of the second lens element 20 protruding into the cavity of the cylindrical first lens element 22.

[0172] The first lens element 22 and the second lens element 20 together perform most of the acceleration and focusing functions described above. The first lens element 22 is primarily used to focus the charged particle beam 12, and to change the width of the charged particle beam 12 at the focal plane of the detector 16 W . The second lens element 20 is primarily used to accelerate the charged particles towards the detector 16 by applying a greater potential at the second lens element 20 compared to the first lens element 22 and other parts of the system, including the third lens element 24 and the analyser 14. Typically, the detector chip 16 will be held at the same potential as the second lens element 20. In one example, the second lens element 20 has an applied potential that is at least one order of magnitude greater, preferably two orders of magnitude greater, than the applied potential at the first lens element 22, in order to provide the required acceleration of the charged particles.

[0173] Figure 1A third lens element 24 is also shown, which is primarily used to provide an extraction field for drawing charged particles from the exit of the energy dispersive spectroscopy (EDS) analyzer 14. The third lens element 24 is cylindrical and aligned with the exit of the EDS analyzer 14 such that charged particles pass through an aperture in the third lens element 24 after exiting the analyzer 14. The third lens element is also arranged circumferentially around (but spaced apart from) a first lens element 22, such that at least a portion of the first lens element 22 extends through a cavity or aperture in the third lens element 24. The cylindrical first lens element 22 extends through a length less than half the length of the cavity in the cylindrical third lens element 24. The voltage applied to the third lens element 24 is scaled relative to the pass-through energy of the EDS analyzer 14 (typically in the range of 5-150 eV). Typically, the third lens element 24 provides an extraction field between 10-20 V / mm.

[0174] The acceleration, focusing, and extraction functions depend on the specific configuration of lens elements 20, 22, and 24, including their shape, spacing, and applied voltage. The voltage applied to lens elements 20, 22, and 24 varies with the kinetic energy of the charged particles. E k And potential energy scaling (in other words, by applying a blockage to the charged particle, and therefore the pass-through of the energy spectrum analyzer). E pass (Scaling). In Figure 1 In a specific instance, the voltage applied to the first lens element E 1 Voltage applied to the second lens element E 2 and the voltage applied to the third lens element E 3 as follows:

[0175] E 1 = 1400+ E pass – E k

[0176] E 2 = 15000+ E pass – E k

[0177] E 3 = 100 - 0.5 E pass - E k

[0178] In a particular example, for a betatronic E pass = 100 eV and photoelectron kinetic energy E k = 1000 eV, the approximate potentials applied to the three lens elements are E 1 = 500 V, E 2 = 14.1 kV and E 3 = -950 V (for focusing, acceleration and extraction, respectively).

[0179] Note that, although cylindrical lens elements are discussed with respect to the example of Figure 1 , any shape of lens element sufficient to perform the described functions can be used (e.g., a set of rods or other lens configuration). Note also that, although a third lens element 24 can be advantageous, it is not required to perform the focusing and acceleration functions of the described system.

[0180] Figure 2 A detector is shown for use within the apparatus described above for detecting charged particles in energy spectrum analysis. The detector incorporates a two-dimensional array of detection pixels (in other words, the detector is pixelated). An example of a suitable detector array is a MEDIPIX TM or TIMEPIX TM detector, as described in US Patent Publication No. US 2012 / 012613. Such detectors can be used to detect or sense radiation, e.g., X-rays, electrons, gamma rays, alpha particles, charged particles, ions or neutrons.

[0181] The detector includes a conversion layer 26 incorporating a doped semiconductor material, so as to provide a plurality of semiconductor diodes (as described in further detail below with reference to Figure 3 The conversion layer 26 is for receiving incident radiation or charged particles and converting the radiation or particles into electrical signals. Each of the semiconductor diodes represents a detector pixel.

[0182] The conversion layer 26 is connected to an electronics layer 30. Electronic components are arranged in the circuitry layer beneath each semiconductor diode. Each detection pixel is connected to its own dedicated pixel electronics (described in further detail below with reference to Figure 3 ) for individual pixel readout of the electrical signals generated by each semiconductor diode. As such, each pixel is considered to be an "active pixel", e.g., unlike the pixels in a charge-coupled device (CCD). The dedicated pixel electronics sets are arranged in a planar array beneath the conversion layer 26, e.g., along Xand Y directionally regularly arranged N × M In the pixel circuitry array, as Figure 2 shown in FIG. 1. The conversion layer and the electronic circuitry layer are connected by flip-chip bonding at each pixel using solder bumps 32. The use of dedicated pixel electronics associated with each pixel (in particular, each pixel includes a diode and an active amplifier) allows single pixel readout and defines the detector as having an "active pixel array."

[0183] Figure 3 is a schematic view of the components that make up and are connected to each detection pixel. Each detection pixel 34 includes a p-n junction in the conversion layer 26 formed by an n-type or p-type semiconductor material diffusion layer 36 on a semiconductor substrate 38, along with an implanted region 40 in the substrate (the implanted region 40 is formed of semiconductor material of the opposite type (p or n) compared to the diffusion layer 36). In the case of a CMOS detector, the diffusion layer 36 includes n-type semiconductor material and the implanted region 40 is p-type. It will be appreciated that the diffusion layer and the substrate layer are generally planar, with the implanted regions arranged in an array in the substrate. Each of the implanted regions 40 represents the area of a detection pixel in the conversion layer. Although a square pixel array is shown, it will be appreciated that the pixels can have any shape or array arrangement. Figure 3 Figure 2

[0184] In use, a charged particle (such as a photoelectron) incident directly at the surface of the diffusion layer 36 diffuses through the diffusion layer 36 to the depletion region 38, causing the creation of an electron-hole pair in a number proportional to the energy of the incident particle. The applied bias across the conversion layer 26 causes the electron-hole pair to move to the adjacent implanted region 40. The change in charge is therefore evident at the circuitry 30 bonded to each implanted region 40.

[0185] Each implanted region 40 is individually bonded (by solder bumps 32 using flip-chip bonding) to dedicated pixel electronics 48 in the electronic circuitry layer 30. Each implanted region 40 is electrically connected to an amplifier 42 so as to produce a voltage pulse proportional in height to the total charge deposited at the associated diode by the incoming charged particle. For example, the pixel electronics can include an amplifier charge-sensitive preamplifier.

[0186] ​​Each amplifier 42 is connected in series to a comparator 44 associated with each detection pixel. The comparator 44 detects whether the voltage output of each amplifier 42 is above or below a certain threshold, which can be arbitrarily set for the whole chip. In doing so, the comparator 44 can be used to digitize the analog voltage output from the amplifier 42. As such, the comparator 44 acts as an analog-to-digital converter. The output of the comparator 44 thus provides a digital signal representative of the charge detected at the associated detection pixel 34. A counter 46 can be connected to the output of each comparator 44 in order to provide a count of the number of charged particles received at each individual pixel.

[0187] Figure 4 A schematic diagram of an apparatus for X-ray photoelectron spectroscopy (XPS) is shown in conjunction with Figures 1 to 3 An X-ray 52 is generated at an X-ray source 50 and passed through suitable optics 54 to an X-ray monochromator 56. The X-ray monochromator 56 is used to select radiation of a defined wavelength (e.g. an aluminium Ka ray). The selected X-ray radiation is directed to impinge on a sample 58 causing photoelectrons to be emitted due to the photoelectric effect. A lens system 60 (comprising an electrostatic lens arrangement) retards the photoelectrons to the pass energy of an electrostatic hemispherical analyser 114 and further directs the photoelectrons to enter the electrostatic hemispherical analyser 114 in the form of a narrow focused beam. Voltages V 1 , V 2 Applied to the concentric hemispherical electrodes of radii R1 and R2 at the electrostatic hemispherical analyser 114 creates an electrostatic field between them with a 1 / r dependence (where r is the radial distance from the centre of the analyser 114). Thus, photoelectrons passing through the analyser 114 are radially dispersed according to their kinetic energy. R 2 The electrostatic field has a dependence on the kinetic energy of the photoelectrons (where Ekin is the kinetic energy of the photoelectron and V is the voltage applied to the hemispherical electrodes). R Thus, photoelectrons passing through the analyser 114 are radially dispersed according to their kinetic energy.

[0188] At the exit of the electrostatic hemispherical analyser 114, the photoelectrons are spatially separated according to their energy. Knowledge of the path radius travelled by a particular photoelectron through the electrostatic hemispherical analyser 114, together with the electric field applied by the analyser, allows the kinetic energy of the particular photoelectron at the exit of the analyser to be determined. From the relationship E b = h v - E k - where Ei is the energy of the radiation incident on the sample, h v E k is the kinetic energy of the emitted photoelectron, and is the work function (dependent on the spectrometer and sample material), the kinetic energy of the photoelectron is related to the binding energy of the photoelectron on the sample E b ​Directly related.

[0189] In Figure 4 , photoelectrons leaving the electrostatic hemispherical analyser 114 are received at a detector 16. The detector 16 is described above in relation to Figures 1 to 3 and comprises an array of detection pixels. Between the exit of the electrostatic hemispherical analyser 114 and the detector 16, the photoelectrons pass through an arrangement of electrostatic lenses 18 as described above in relation to Figure 1 . The electrostatic lenses 18 are arranged to accelerate and focus the charged particles. In particular, the charged particles are accelerated to increase the energy of the particles to exceed a detection energy threshold of the detector 16 whilst maintaining the relative positions of the charged particles. Typically, photoelectrons generated in X-ray photoelectron spectroscopy (XPS) leave the sample with kinetic energies of up to 1486 eV. However, prior to entering the analyser, the photoelectrons are decelerated to the "pass energy" of the analyser. The photoelectrons with the pass energy then move around the analyser and then leave at this energy. In XPS, a typical "pass energy" for an electrostatic hemispherical analyser is between 3 eV and 200 eV. Accordingly, the electrostatic lens arrangement 18 is configured to accelerate photoelectrons leaving the spectroscopy analyser in this energy range to at least the detection energy threshold of the detector. As such, the energy of a charged particle emitted from the sample surface (at point A in Figure 4 ) will be higher than the energy of the charged particle after passing through the lens system 60 and the deceleration grid (at point B in Figure 4 ), at which point the charged particle will be at the pass energy of the spectroscopy analyser. However, after acceleration (at point C in Figure 4 ), the energy of the charged particle will be significantly higher than at point A or B in Figure 4 (e.g. the energy is increased by about two orders of magnitude compared to the pass energy).

[0190] Furthermore, the arrangement of electrostatic lenses 18 changes the width of the photoelectron beam as discussed above in relation to Figure 1 . In particular, the width of the photoelectron beam is magnified or reduced to match the width of the surface of the detector 16 onto which the photoelectrons are incident. Changing the width of the beam in this way maximises the number of detector pixels within the output field of the photoelectron beam and can provide for the full spatial spread of photoelectrons leaving the analyser to be incident onto the detection surface.

[0191] Typically, energy dispersive spectroscopy is aimed at obtaining an energy spectrum of a sample over a certain energy range. In cases where a spectrum over a wide energy range is required, the electric potentials applied at the electrostatic hemispherical analyser 114 are adjusted (or scanned) accordingly. In the arrangement of the present invention, the acceleration applied by the electrostatic lens arrangement 18 can also be adjusted or scanned in order to provide the appropriate acceleration before the charged particles leaving the analyser 114 are received by the detector 16.

[0192] In particular, in a typical energy dispersive analysis mode, where photoelectrons are spatially dispersed according to their energy at the energy analyser 14, the electrostatic lens arrangement 18 must be configured to accelerate the photoelectrons to at least the detection energy threshold of the detector 16 (where the detection energy threshold is the minimum energy of an incident photoelectron for detection at a detection pixel of the detector 16). In this mode, the photoelectrons can be accelerated to the same energy at the point of their incidence at the detector 16, as the energy analysis of the charged particles is allowed by their incidence at opposite locations of the detector 16 (due to the energy-space dispersion provided by the electrostatic hemispherical analyser 114).

[0193] In an alternative, the apparatus can be used in an imaging mode. In this case, the photoelectrons pass through the energy analyser and are uniformly accelerated to an energy greater than the detection energy threshold. The particles are then received at a detection pixel array, the charged particle count for each pixel across the array can be read out from the array and presented in the form of a two-dimensional image.

[0194] Figure 5A and 5B A cross-sectional view of a particular electrostatic lens 18 and detector 16 arrangement used in an example X-ray photoelectron spectrometer (e.g. as shown in Figure 4 is shown. Figure 6 A plan view of a number of elements of the apparatus shown in Figure 5A and 5B is shown.

[0195] In Figure 5A , 5B and 6, the detector is mounted on a printed circuit board (PCB) which includes the pixel electronics 48 (amplifiers, comparators and counters) described above with reference to Figure 3 . Three electrostatic lens elements 20, 22, 24 (as shown in Figure 1 ) are arranged relative to the detector 16. The second electrostatic lens element 20 is arranged around the detector, and the first electrostatic lens element 22 and the third electrostatic lens element 24 comprise concentric electrodes positioned around and spaced apart from the second electrostatic lens element 20. The third electrostatic lens element, the first electrostatic lens element and the second electrostatic lens element are arranged in series in the direction of travel of the charged particles towards the detector, respectively.

[0196] The third electrostatic lens element 24 is primarily used to extract photoelectrons from the exit of the analyser 114 and to collimate them. The first electrostatic lens element 22 is primarily used to focus the beam of charged particles, setting the magnification to adjust the width of the beam, preferably to fill the detection area of the detector. The second electrostatic lens element 20 is primarily used to provide the necessary acceleration of the photoelectrons before they hit the detector 16.

[0197] The first electrostatic lens element 22 and the third electrostatic lens element 24 are each in the form of a cylindrical electrode. The cylindrical electrode of the third electrostatic lens element 24 has a larger diameter than the cylindrical electrode of the first electrostatic lens element 22 and is at least partially concentric with the first lens element 22. The second electrostatic lens element 20 is configured as an electrode having a cylindrical portion and a conical portion, wherein at least the conical portion is concentric with the first lens element 22. The shape of the conical end of the first electrostatic lens element is used to minimize spherical aberration. In use, photoelectrons pass through the apertures of the third lens element 24, the first lens element 22, and the second lens element 20, respectively. The detector 16 is mounted downstream of the cylindrical end of the second electrostatic lens element 20.

[0198] The detector 16 is mounted in the cavity 64, forming part of the analysis chamber of the energy dispersive spectrometer. The length of the cavity can be adjusted to match the focal length of the lens with the output or image plane of the energy dispersive spectrometer. For example, the analysis chamber can also accommodate the energy dispersive spectrometer and the sample. Figure 5A and 5B (Not shown in the image). For XPS, the analyzer and sample can be as follows: Figure 4 The arrangement is shown in the diagram, but other forms of energy dispersive spectroscopy known in the art may be arranged differently. During energy dispersive spectroscopy measurements, the analytical chamber is maintained under vacuum.

[0199] exist Figure 5A and 5B In a specific example, the PCB forms a vacuum seal between the analysis chamber and the outside atmosphere. Digital signals for each detected pixel, generated by the pixel electronics 48 (described above) on the PCB, can be transmitted from the vacuum chamber to a computer processor (not shown). In this way, the PCB provides a vacuum feedthrough 66 for the digital signals.

[0200] Interface 68, which carries digital signals from the PCB to the processor, is also included. Figure 5B As shown in the diagram. Here, the interface is implemented as a bidirectional fiber optic universal serial bus (USB) interface, such as... Figure 7 As shown in the diagram, the interface provides a fiber optic USB interface 80 to the PCB board (and pixel electronics), which connects to a fiber optic USB interface 84 at the computer processor via a fiber optic connection 82. The fiber optic connection 82 provides 15 kV isolation between the PCB and the processor, which is particularly necessary when the detector 16 and at least the components of the electrostatic lens assembly 18 operate at a “floating” voltage higher than the potential of the surrounding spectrometer equipment. For example, the detector and its associated electronics can be floated at a boost voltage of up to 15 kV (e.g., 1-15 kV or 5-15 kV). A high-voltage power supply can be used to float the detector and electronics.

[0201] Notably, the generation of digital signals offers significant advantages over traditional detectors that generate analog signals, such as microchannel plate detectors and electron multipliers. It is extremely difficult to transfer analog signals from any device in a vacuum through a vacuum feedthrough. In order to avoid reflections and losses, each signal must be carried by a coaxial cable and pass through a coaxial vacuum feedthrough. This is expensive and takes up a lot of space. In contrast, digital signals can be transferred from the vacuum using the PCB and interface described above. This not only provides a high-speed, low-noise interface, but also allows a large number of different signals, such as those obtained by multiple active pixels in a detector array, to be transmitted through a small feedthrough area.

[0202] Further modifications can be made to the apparatus of Figure 5A and 5B , for example, including a heat sink 70 connected to the detector 16, and a high voltage insulator 72 arranged relative to the electrostatic lens arrangement 18. A floating USB power supply 86 can also be connected.

[0203] In XPS, the number of photoelectrons of a specific energy emitted from a sample when illuminated with X-rays needs to be calculated separately. The photoelectrons of interest are selected by a hemispherical analyser and counted by a detector and pulse counting electronics after leaving the analyser. In the energy spectrum analysis mode, the analyser is of the energy dispersive type, such that the position of the photoelectron at the exit of the analyser can be used to determine the energy of the electron. The energy of the electron leaving the analyser is typically linearly dependent on the position of said electron, and thus an energy spectrum plot can be generated by plotting the charged particle counts over the width of the detector pixel array.

[0204] In one particular example, the detector assembly described above in relation to Figures 1 to 3 has been installed on a Thermo Scientific TM Nexsa TM XPS spectrometer. In particular, the detector used comprises a 256 x 256 array of detection pixels. The photoelectron beam is focused by the electrostatic lens arrangement to match the width of the detector surface. Thus, the photoelectron beam spans all 256 pixel columns at the detector.

[0205] Figure 8 A "snapshot" energy spectrum plot obtained using this experimental apparatus is shown, in which the analyser of the XPS spectrometer was set to the photoelectron energy of the Ag 3d doublet (using X-ray radiation of 1486.6 eV). All voltages at the analyser and electrostatic lens arrangement were held constant. The electrostatic lens arrangement 18 was configured to accelerate the photoelectrons to an energy of at least 10 keV.

[0206] A snapshot energy spectrum is the result of fast simultaneous detection of the beam across the 256 pixel columns of the detector. Specifically, a snapshot energy spectrum (plotting binding energy versus counts of photoelectrons per second) represents the total number of photoelectrons detected at the detection pixels in each of the 256 columns of the pixel array at the detector. The x-axis is associated with the spatial and energy dispersion of photoelectrons across the 256 columns of detection pixels.

[0207] Thus, a 1 x M dimensional data vector can be generated, each element of which comprises the sum of the number of photoelectrons detected in each of the M pixels in a column of the detector pixel array (as illustrated in N M N Figure 11 Figure 8 In the specific case of the Thermo Scientific M Nexsa XPS spectrometer, the plot is generated after further binning the 256 columns of the array into representative 128 channels, in order to reproduce the spatial resolution of the prior art detector. However, the potential improvements offered by increasing the number of available channels will be apparent to the skilled person (even though the current resolution is determined by the geometry of the analyser).

[0208] Figure 9 A "scanning" energy spectrum obtained using the described detector assembly installed in a Thermo Scientific TM Nexsa TM XPS spectrometer is illustrated. In order to generate a scanning energy spectrum, a set of operating parameters of the electrostatic hemispherical analyser and the potentials at the electrostatic lens arrangement are adjusted (or stepped) to scan a specific range of electron energies known to be consistent with the energy spectrum position (or energy spectrum peak) of interest. The counts of charged particles received at each detection pixel of the array during the dwell time of each set of operating parameters are then recorded and data processing is performed to generate the plot.

[0209] It should be noted that while the operating parameters of the scanning analyser are adjusted, the voltage applied to each element of the electrostatic lens arrangement is also simultaneously adjusted, in order to provide the appropriate acceleration for the photoelectrons (or charged particles) leaving the analyser. Specifically, the electrostatic lens arrangement is configured to ensure that the photoelectrons (or charged particles) received from the analyser are accelerated to at least a detection energy threshold. The relationship between the adjustment of the operating parameters of the analyser and the adjustment of the electrostatic lens arrangement is predefined and can be constant.

[0210] In order to generate the scanning energy spectrum of Figure 9 , for each set of operating parameters, a 1 x M dimensional data vector is generated, each element of which comprises the sum of the number of photoelectrons detected in each of the M ​​​​A 3D data vector represents the number of detectors during the corresponding dwell time. M The charged particle count is performed at each pixel column. Thus, multiple 1 × 10⁻⁶ cells are generated by repeatedly counting for each set of operating parameters of the analyzer. M Data vectors. Due to the known relationship between energy and the positional dispersion of the particles by the analyzer, each element of each data vector can be associated with the energy increment of the particle. Therefore, each element of each data vector can be binned with other data elements representing the same energy increment. In this way, subsequent generation... Figure 9 The scanned energy spectrum is plotted across the input particle energies. Thus, as described above, the scanned energy spectrum can be considered as the sum of snapshots of the energy spectrum with many energy shifts (and specifically, measured with relatively low pass energies).

[0211] and Figure 8 Compared to the snapshot energy spectrum, Figure 9 The scan spectrum displays a customized energy range. For example, a snapshot spectrum can be approximately 13.6% of the pass energy (e.g., approximately 13 eV for a 100 eV pass energy). In contrast, the energy range of a scan spectrum is customized; for example, if looking at a single peak, the energy range can be only 3-4 eV. The significantly increased count rate of the scan spectrum (up to 100-fold increase) compared to the snapshot spectrum is a result of adding a large number of individual scans, each under different sets of analyzer operating parameters. Therefore, higher resolution is achieved, and much more detail can be seen in the scan spectrum compared to the snapshot spectrum. Typically, Figure 8 The snapshot energy spectrum shown can be used to accelerate the data collection process or in situations where resolution (or chemical information) is not required.

[0212] In addition to the snapshot and scanning spectra described, X-ray photoelectron spectroscopy (XPS) typically aims to provide a "full-spectrum analysis" spectrum across a broader range of binding energies. In most cases, a full-spectrum XPS spectrum yields a spectrum with photoelectron energies ranging from approximately 50 eV to 1486 eV (the X-ray energy range of the most commonly used XPS sources). Such a spectrum can be used to cover the entire possible range of emitted photoelectrons from the sample. The full-spectrum analysis spectrum can be used to identify spectral features of interest within the sample, and then a scanning spectrum with higher resolution can be obtained, pointing to the identified region of interest.

[0213] Figure 10 The image shows the use of equipment installed at Thermo Scientific. TM Nexsa TMA full spectrum analysis spectrum obtained with the described detector assembly in an XPS spectrometer. The spectrometer is programmed to scan a broad energy range corresponding to all available XPS peaks, while the operating parameters of the electrostatic hemispherical analyser and the potentials at the electrostatic lens arrangement are adjusted and / or scanned accordingly. Essentially, the generation method of the full spectrum analysis spectrum is the same as for the generation of a scan spectrum, including the described data processing steps, as described above. However, for the generation of the full spectrum analysis spectrum, a higher pass energy is used at the analyser, resulting in a lower resolution and a larger particle flux. In addition, the step size of the analyser parameters (equivalent to the step size of the analysed particle energies) is increased compared to the scan spectrum. In this way, the full spectrum analysis spectrum provides a scan spectrum with a larger range but lower energy resolution.

[0214] Although the generation of the spectrum by using the described method and apparatus, many factors contribute to the measured energy resolution in spectroscopy techniques like XPS (including the size of the analyser input slit and the hemispherical analyser radius, etc.), it has been demonstrated that a particularly high energy resolution spectrum can be obtained compared to prior art systems. In principle, the described system can be used to decode the position of a charged particle received on the detector surface to a resolution of the number of pixel columns provided by the detector (which can be much higher than the resolution provided by prior art). Furthermore, the described method and apparatus enable very high count rates without the pulse pile-up and overlapping pulses at high count rates observed in conventional microchannel plate detectors. Count rates of more than 500 million counts per second have been demonstrated to be possible. In particular, the higher count rates are enabled due to the large number of pixels (e.g. more than 64,000 pixels in a single typical active pixel array) and dedicated counter at each pixel of the detector and fast signal generation and recovery. Furthermore, using the described detector means that simultaneous arrival of electrons at separate adjacent pixels does not pose a problem compared to conventional detectors used in XPS.

[0215] The above with respect to Figures 1 to 3 The described apparatus can advantageously be applied to a number of different types of spectroscopy analysis. In a further example, Figure 12 A Reflected Electron Energy Loss Spectroscopy (REELS) spectrum obtained using the described detector and electrostatic lens arrangement is shown. The spectrum shows a peak count rate of 571 million counts per second. Beneficially, this count rate is more than 10 times higher than can be achieved using a conventional channel plate detector. In addition, the background count rate is typically less than 1 count per second, thereby giving a dynamic range of about a billion to one.

[0216] It will be appreciated that the described data processing, including generating the described 1 x M dimensional data vectors and binning the data vectors to generate energy spectra, can be conducted at a computer processor in communication with the described apparatus. In particular, the processor can conduct the described data processing during or after data collection at the processor. Figure 5A and 5B , Figure 6 and Figure 7 The fiber optic link shown is connected to the described apparatus. The described data processing can be conducted at the processor during or after data collection.

[0217] Many combinations, modifications or variations of the features of the above embodiments will be apparent to the skilled person and are intended to form part of the present application. Any of the features described in specific relation to one embodiment or example can be used in any other embodiment, by making appropriate alterations.

[0218] For example, in much of the above description, the detector is used in a "spectroscopy mode". The spectroscopy mode assumes that the analysis of the sample relies on the spatial or temporal dispersion of the charged particles in the energy spectrometer according to their energy distribution. For example, the spatial dispersion of photoelectrons in XPS causes photoelectrons of different energies to be incident on the detection surface of the detector at different locations (and more specifically, at different columns of the pixel array). In the alternative, the temporal dispersion of the charged particles by the energy spectrometer (for example, in a time-of-flight spectrometer) causes charged particles of different energies to be decelerated by different amounts in the spectrometer field, and thus exit the spectrometer and subsequently be received at the detection surface of the detector at different times. In the case of temporal dispersion of the particles, the fast readout of the detector described herein is particularly beneficial compared to conventional methods of charged particle detection.

[0219] However, the same detector as described above (with a detection active pixel array) can be used for further applications, such as for "imaging mode" to obtain a two-dimensional data set. Particular examples include the use of the detector within the Thetaprobe TM and Escalab TM systems from Thermo Fisher Scientific TM for XPS imaging or angle-resolved spectroscopy analysis. In these cases, the readout of each of the detection pixels in the detector pixel array can be stored and plotted independently to obtain a two-dimensional image or plot. The image can be shown with very high image resolution, with particularly high count rates. Furthermore, the parallel imaging application benefits from the detector providing near-zero background.

[0220] In still further examples, the detectors described above (with a detection active pixel array) can also be used to directly measure the energy of received charged particles (i.e. in an "energy sensing mode"). In this case, the pixel electronics does not contain a counter, and is arranged to provide a digital signal representative of the energy of a received particle. However, when applying the detector in an energy sensing mode, the other components and methods of data processing described above can be used. In particular, from the analyser, the charged particles should be accelerated before they directly impact on the surface of the detector conversion layer. In particular, all charged particles should be accelerated at least above a threshold energy below which the detection of particles by the detector becomes unreliable.

[0221] In the "energy spectrum analysis mode", the particles must be accelerated by at least one electrostatic lens described above in order to exceed at least one detection threshold energy. However, in the "energy sensing mode", for all particles, the relative speed (or energy) of the charged particles between the exit of the energy analyser and the impact on the surface of the detector needs to remain constant (in other words, a constant relationship) while also exceeding at least one detection threshold energy.

Claims

1. A method for detecting photoelectrons in X-ray photoelectron spectroscopy analysis, the method comprising: The photoelectrons received from the electrostatic energy dispersive spectroscopy analyzer in the form of a photoelectron beam are accelerated toward the detector; as well as Accelerated photoelectrons are received at a detection pixel array that forms the detector, wherein the photoelectrons arriving at the detector diffuse in an energy dispersion direction such that each photoelectron is incident at the detector at a detection pixel column representing the energy of the photoelectron.

2. The method of claim 1, wherein each detection pixel is an active detection pixel comprising a semiconductor diode.

3. The method of claim 2, wherein each detected pixel is associated with an amplifier, a comparator, and a counter.

4. The method according to any one of claims 1-3, wherein the photoelectrons are accelerated to at least a detection energy threshold, the detection energy threshold being the minimum energy of a charged particle for detecting a charged particle incident on a detection pixel of the detector.

5. The method according to any one of claims 1-3, wherein before receiving the accelerated particle at the detector, the method further comprises: The photoelectron beam is focused to change the magnification of the photoelectron beam at the image plane of the detector.

6. The method of claim 5, wherein focusing the photoelectron beam to change the magnification of the photoelectron beam comprises changing the cross-sectional area of ​​the photoelectron beam at the focal plane such that the width of the photoelectron beam in the energy dispersion direction substantially matches the corresponding size of the detection pixel array at the detector.

7. The method of claim 5, wherein a charged particle optics arranged between the electrostatic energy dispersive spectroscopy analyzer and the detector accelerates the photoelectrons.

8. The method of claim 7, wherein the charged particle optics is further configured to focus the photoelectrons.

9. The method according to any one of claims 1-3, wherein the detector comprises a... N Each detection pixel row and M An array consisting of columns of detected pixels, wherein the method further includes: Determined at N × M The number of photoelectrons received at each of the detection pixels.

10. The method of claim 9, wherein the method further comprises: For each N The number of photoelectrons received at each detection pixel in the detection pixel column is summed to determine the number of photoelectrons included in each pixel. N 1 × the sum of the detected pixel columns M 1 × 1-dimensional data vector M The dimensional data vector represents the spatial dispersion of the photoelectrons across one dimension of the image plane of the detector.

11. An apparatus for detecting photoelectrons in X-ray photoelectron spectroscopy analysis, the apparatus comprising: Electrostatic energy dispersive spectroscopy analyzer; Charged particle optical devices; as well as A detection pixel array, wherein the detection pixel array forms a detector; The charged particle optics are arranged such that photoelectrons received in the form of a photoelectron beam from the electrostatic energy dispersive spectroscopy analyzer are accelerated toward the detector for reception at the detection pixel array; and The electrostatic energy dispersive spectroscopy analyzer causes the photoelectron beam to reach the detector in a manner that diffuses in the energy dispersion direction, such that each charged particle is incident on the detector at a pixel column representing the energy of the charged particle.

12. The device of claim 11, wherein each detection pixel is an active detection pixel comprising a semiconductor diode.

13. The device of claim 12, wherein each detection pixel is connected to an amplifier, a comparator, and a counter.

14. The device according to any one of claims 11 to 13, wherein the charged particle optics is configured to accelerate the photoelectrons to at least a detection energy threshold, the detection energy threshold being the minimum energy of the charged particles for detecting the charged particles incident on a detection pixel of the detector.

15. The apparatus according to any one of claims 11-13, wherein the charged particle optics is further configured to focus the photoelectron beam to change the magnification of the beam at the image plane at the detector.

16. The apparatus of claim 15, wherein the charged particle optics are configured to change the magnification of the photoelectron beam such that the width of the photoelectron beam in the energy dispersion direction substantially matches the corresponding size of the detection pixel array at the detector.

17. The device of claim 15, wherein the charged particle optics comprises at least two lens elements arranged to focus the photoelectrons and accelerate them.

18. The device of claim 13, wherein the detector comprises a... N Each detection pixel row and M An array of detection pixel columns, wherein the device further includes a controller configured to: Determined at N × M The number of photoelectrons received at each of the detection pixels.

19. The device of claim 18, wherein the controller is further configured to: For each N The number of photoelectrons received at each detection pixel in the detection pixel column is summed to determine the number of photoelectrons included in each pixel. N 1 × the sum of the detected pixel columns M 1 × 1-dimensional data vector M The dimensional data vector represents the spatial dispersion of the photoelectrons across one dimension of the image plane of the detector.

20. An X-ray photoelectron spectrometer, comprising: The device according to any one of claims 11 to 19; and The electrostatic energy dispersive spectroscopy analyzer mentioned above is an electrostatic hemispherical analyzer; and The arrangement of the charged particle optics is configured to accelerate photoelectrons received from the electrostatic hemispherical analyzer toward the detection pixel array forming the detector.

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