Electron filter

Abstract

To provide an X-ray detector and corresponding apparatus and method for reducing degradation in performance of the X-ray detector due to backscattered electrons and undesirable photons.SOLUTION: An X-ray detector adapted to reduce transmission of backscattered electrons from a specimen therethrough, the X-ray detector comprises: a detection element; and a filter positioned between the specimen and the detection element in use. The filter includes: a first material layer formed from an electrically insulating material; and a second material layer formed from an electrically conductive material and arrayed on a side of the first material layer closest to the detection element. The X-ray detector is adapted so that the second material layer can be electrically connected, when in use, to a reference potential to shield the detection element from an electric field attributable to the accumulation of charge in the first material layer due to incident electrons in use.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an x-ray detector and corresponding apparatus and methods for reducing x-ray detector performance degradation caused by backscattered electrons and unwanted photons. [Background technology]

[0002] A scanning electron microscope is a type of imaging device that can be used to image a sample. Typically, this is done by scanning a focused electron beam across the sample surface and measuring the signal generated by the interaction of the electrons with the sample to construct an image. In some applications, an X-ray detector is used to measure characteristic X-rays to determine the elemental composition of the sample.

[0003] FIG. 1 presents an exemplary apparatus for X-ray analysis using an electron microscope (EM) 100. The X-ray spectrum is measured by sensing and measuring the energy of individual X-ray photons emitted by the sample 101 when a focused electron beam 102 strikes the sample 101. Each emitted X-ray photon is a particle with an associated energy, which is typically converted into a proportional measurable charge using a solid-state detector 105. The charge associated with the incident X-ray photons is measured so that counts can be recorded, and a histogram of measured value records is generated that represents a digitized X-ray energy spectrum. Characteristic peaks of chemical elements can be identified within the X-ray energy spectrum, and the energies at which the peaks are recorded are used as a basis for determining the elemental composition of the sample material 101 upon exposure to the electron beam 102.

[0004] Typically, the x-ray detector 105, the final pole piece of the electron microscope 104, and the sample 101 are all positioned in the same vacuum chamber. The vacuum is needed primarily so that the electrons can be accelerated to energies of a few keV and focused into a narrow beam without scattering off gas molecules. However, there are alternative configurations in which the electron beam can be focused in a vacuum region, but the sample is in a region of higher pressure. The x-ray detector can be positioned in the same vacuum region as the electron beam or in a region of higher pressure.

[0005] Backscattered electrons (BSEs) are electrons from the incident electron beam that are scattered in the sample and leave the surface in various directions. As a result, a certain percentage of BSEs will strike the X-ray detector 105. Typically, the energy of these BSEs can be as high as the energy of the incident electron beam. Since these BSEs can cause damage to the X-ray sensor and can interfere with the measurement of the X-ray spectral emission from the sample, it is common to provide an electron trap 105a at the entrance of the detector, which comprises a magnet arrangement to deflect the electrons away from the X-ray sensor.

[0006] However, if the X-ray detector is positioned at position 106 below the electron lens, there is very limited space to insert an electron trap between the sample and the detector, and the fringing magnetic field of any nearby magnets would interfere with the electron optical focusing of the incident electron beam. Therefore, an alternative way to prevent BSEs from reaching the sensor is to insert a filter material between the sample and the detector. WO 2010 / 115873 is a publication that describes such an arrangement, where different thicknesses of filter material are used to modify the proportion of BSEs that reach the X-ray sensor.

[0007] Charging of insulating samples in scanning electron microscopes and the resulting effect on imaging performance is a known phenomenon (see, for example, the review by Postek and Vladar, "Does Your SEM Really Tell the Truth?-How Would You Know? Part 4: Charging and its Mitigation," 2015 Proc SPIE Int Soc Opt Eng. 2015, 9636:963605 (October 21, 2015); doi:10.1117 / 12.219534). A similar undesirable effect can occur if any insulating material that is exposed to backscattered electrons is in the vicinity of the focused electron beam. If the filter material used to absorb BSEs is an insulator, it can slow down the incident BSEs and trap them in the filter material. As a result, the filter material will acquire a negative charge, and the magnitude of the potential of the filter material will increase, potentially to the same magnitude as the voltage used to accelerate the electrons in the incident focused beam. This charging effect in the filter material can interfere with the electron optics of a scanning electron microscope (SEM) and cause shifting and distortion of the images formed by the microscope.

[0008] 2 shows a system according to the prior art with an annular X-ray detector underneath an electron lens. Such an arrangement is described in DE 112015000849 T5.

[0009] In FIG. 2, the X-ray detector 1 is positioned between an electron beam lens system 42 and a specimen stage 43 on which a sample 5 is placed. The focused incident electron beam passes through a hole 14 around which a number of X-ray sensor elements 11, typically silicon drift detector (SDD) sensors, are arranged. In front of each sensor is an X-ray transparent window 12. The interior of the X-ray detector module containing the sensors is evacuated or contains a selected gas at a pressure below atmospheric pressure. The X-ray transparent window 12 may be made of a conductive material such as beryllium or an insulating material such as an insulating polymer. If the material is an insulating polymer, the transparent window 12 has a conductive material coating on the sample side of the window that is bonded to a conductive window frame 13. The conductive coating is typically aluminum. The detector side of the X-ray transparent window 12 is electrically isolated from the window frame 13 and may be inside the housing of the X-ray sensor.

[0010] The X-ray detection element 11 is electrically connected to an X-ray detection preamplifier 21 as the first stage of pulse processing electronics 22 used to measure the X-ray photon signal and accumulate an energy histogram representing the X-ray spectrum incident on the sensor 11. The conductive surface and window frame 13 of the X-ray transparent window 12 are electrically connected to an electronic detection preamplifier 23. In addition, a voltage application unit 24 is used to generate a positive bias of up to several hundred volts between the conductive surface and a specimen stage 43 which is connected to ground potential. X-rays from the sample pass through the X-ray transparent window 12 and are detected by the sensor 11.

[0011] In the configuration described in Figure 2, if a conductive material such as beryllium is used for the x-ray transparent window, electrons absorbed in the window will flow to the electron detection preamplifier, reducing charge build-up in the window. Furthermore, even if an insulating material is used for the x-ray transparent window, the conductive layer on the surface is held at a bias voltage that is positive with respect to ground potential due to the electronic circuitry that measures the current. Thus, the problem of charge build-up in the insulating material causing electrostatic fields that would affect the images recorded by the SEM is reduced. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] International Publication No. 2010 / 115873 [Patent Document 2] German Publication No. 112015000849T5 [Non-patent literature]

[0013] [Non-Patent Document 1] Postek and Vladar, “Does Your SEM Really Tell the Truth?-How Would You Know? Part 4: Charging and its Mitigation,” 2015 Proc SPIE Int Soc Opt Eng. 2015, 9636:963605 (October 21, 2015); doi:10.1117 / 12.219534 Summary of the Invention [Problem to be solved by the invention]

[0014] Beryllium is an excellent choice for electron filters as it provides good transparency to X-rays while blocking BSE, it is electrically conductive, and for most applications of X-ray detectors for materials analysis in SEMs, X-ray emissions from beryllium reaching the X-ray sensor are unlikely to be a problem. However, beryllium is not ideal as it is a hazardous material that requires careful handling. Heavier metals than beryllium could be used for electron filtering, but the X-rays generated by electrons striking the filter would be more energetic, and therefore more likely to interfere with the spectral emissions of interest from the sample.

[0015] In addition, while in principle conductive carbon could be used as a filter material, a carbon layer 4 microns thick would be required to block BSEs up to the 20 keV energy typically used in SEMs, and carbon layers of this thickness are not readily available as free-standing films.

[0016] As a result, polymers, which consist of low atomic number compositions such as carbon, nitrogen, and oxygen and can be fabricated as free-standing films with thicknesses up to a few microns, have been the most common choice for both the x-ray transparent window and the electron blocking filter. The thickness of these films is chosen to provide an optimal compromise between preventing backscattered electrons from the specimen from transmitting through the film and allowing x-rays in the energy band useful for elemental analysis to transmit through the film. In general, the BSEs emitted from the sample toward the detector are much more numerous (~1,000x) than the x-rays, and it is believed that by selecting a material and geometry capable of blocking all the BSEs, the number of x-rays reaching the sensor, especially toward the lower end of the useful energy band, will be greatly reduced. As a result, the material and geometry of the filter are chosen to be as transparent as possible to x-rays in the energy band useful for elemental analysis while still providing that the fraction of the BSEs that still transmit through the filter does not significantly affect the x-ray spectrum recording.

[0017] However, as explained above, if an electrical insulator such as a polymer or mylar is used as an x-ray transparent window and that window is used as an electronic filter to block the transmission of BSE, charge will still build up in the filter due to electrons stopping in the filter. A conductive coating on the sample side of the filter material, held at a fixed voltage, can prevent interference with the operation of the electron optics, but charge buildup in the filter can lead to electrostatic breakdown from the high potential in or on the filter to other surfaces in the vicinity or to the detector sensor itself. Depending on how close the filter is to the sensor and what other surfaces are close to the sensor, these breakdown events can cause spurious signals at the output of the x-ray sensor and false counts in the energy spectrum recording.

[0018] Another problem with existing electronic filter implementations is that insulating materials such as Mylar can produce the generation of optical photons when bombarded by electrons, an effect called cathodoluminescence. If the filter is close to the x-ray sensor, which is desirable to allow a large solid angle, these optical photons can penetrate the surface of the sensor and, although too low in energy to be detected as individual pulses, contribute to the electron flow in the sensor, thereby using up bandwidth and increasing the effective noise and degrading the resolution of the spectral recording.

[0019] Furthermore, if a layer of material is used as a filter to block the transmission of BSEs, and there is any gap between the material layer and the sensor, and there is an inert gas (e.g., nitrogen, argon, neon, xenon) at a pressure lower than the atmospheric pressure, or if there is some residual gas remaining in the interior space region of the X-ray detector after an attempt to evacuate it, positive ions may be generated by collisions of X-ray photons or electrons with gas atoms in the space between the sensor and the electron filter. Typically, silicon drift detector (SDD) sensors have a negative voltage applied to their entrance surface (the surface through which the X-rays enter the SDD and are detected), and any positive ions will be accelerated toward this entrance surface. The accumulated gas molecules may affect the performance of the detector entrance surface, changing the trajectory of the transmitted BSEs that are attracted toward the more positively charged detector surface, and overall leading to a degradation of the spectral resolution, especially for low energy X-rays.

[0020] Therefore, in light of the above problems, there is a need for an improved BSE electronic filter that reduces undesirable side effects that degrade the performance of x-ray detectors. [Means for solving the problem]

[0021] In a first aspect of the present invention there is provided an X-ray detector adapted to reduce internal transmission of backscattered electrons from a specimen, the X-ray detector comprising a detection element and a filter positioned between the specimen and the detection element in use, the filter comprising a first material layer formed from an electrically insulating material and a second material layer formed from a conductive material and arranged on a side of the first material layer nearest the detection element, the X-ray detector being adapted such that in use the second material layer is electrically connectable to a reference potential to shield the detection element from an electric field resulting from charge build-up in the first material layer caused by incident electrons in use.

[0022] The inventors have discovered that by specifically positioning the second material layer on the side of the first material layer facing the detection element of the X-ray detector, the second material layer additionally serves as an electrostatic shield against electric fields. In particular, this serves to reduce so-called "self-counting" effects on the detector due to electrostatic discharges and / or electrostatic breakdowns that may exist, for example, within the first material layer, from the first material layer to other surfaces in its vicinity, or from the first material layer to the detection element itself. Electrostatic discharges may be observed as fast transients that cause electromagnetic interference. Such electrostatic discharges may generate rapidly changing electric fields that may couple into nearby electrical circuits. Thus, if a discharge occurs and is located in the vicinity of the detection element, the resulting electric field change may temporarily affect the potential distribution in the detection element and / or induce charges in the detection element. This may cause step changes in the output of the X-ray detector that are erroneously interpreted as incident X-ray photons. Such discharges may therefore cause spurious X-ray counts in the spectral recording. The second layer of material serves to prevent voltage transients from a discharge event caused by charge built up in the first layer of material from being sensed by the x-ray detector and thereby interfering with the measurement.

[0023] Shielding of the detection element may be understood as electrically and / or electromagnetically shielding the detection element, in other words, the second material layer mitigates undesirable effects on the X-ray detector due to electric fields that arise as a result of electron accumulation in the first material layer when the detector is in use and electrons are incident on the first material layer.

[0024] The second material layer is adapted to be electrically connectable to a reference potential during use. Thus, the second material layer is typically electrically connected to a reference potential during use. The electrical connection is typically a conductive connection. Advantageously, when the second material is electrically connected to a reference potential, such as a system ground during use, shielding of the detection element against undesired electric fields is provided.

[0025] The term "incident electrons" may be understood as electrons that are generated by interaction between the electron beam and the sample when the X-ray detector is in use, that travel along a path between the sample and the filter, and thus irradiate the filter, e.g. backscattered electrons. The second layer of material may shield at least a portion or preferably all of the detection element and / or the incidence surface of the detection element.

[0026] In some examples, the X-ray detector may be adapted to have a filter positioned on or within the X-ray detector. In general, the positioning of the filter may be such that a portion of the second material layer forms an electrical contact with an object suitable for acting as a current sink during use, such as a portion of the X-ray detector. For example, the second material layer may form an electrical contact with a portion of the housing of the X-ray detector, which may be conductively connectable to a reference potential, for example through an electrical connector and / or terminal. In these examples, the portion of the X-ray detector that is in electrical contact with the second material layer may be additionally galvanically isolated from the detection element of the X-ray detector. In some examples, a further portion of conductive material may be disposed in electrical contact with the second material layer, which portion forms a portion of an electrical contact and / or conductive path to the second material layer.

[0027] The second material layer of the filter can also serve to reduce charge build-up in the first material layer. In particular, adapting the X-ray detector such that the second material layer is electrically connectable to a reference potential can also be for reducing charge build-up in the first material layer due to incident electrons. In addition to shielding the detection element from problematic electric fields, if the second material layer is held at one of a reference potential, typically a fixed voltage, a reference ground, or a reference earth, an electrical path is provided between the second material layer and a current sink, allowing electrons migrating from the first material layer into the second material layer to pass through the second material layer and out of the filter. Thus, the particular arrangement of material layers described by the implementation of the first aspect above reduces several undesirable effects arising from charge build-up in the BSE filter so that these effects can be prevented from interfering with the X-ray spectrum recording.

[0028] Electrons may be trapped in a localized state inside the first material layer and unable to move freely, but random thermal fluctuations may give them enough energy to escape this localized state, move for a short time, and then become trapped in a new location. Thus, electrons move slowly through the insulating first material layer, and when they collide with the first material layer, charge accumulates inside the material. However, when the moving electrons reach a conductive surface that can sink current, the electrons, and therefore the charge, will escape. Thus, the presence of a conductive second material layer connected to a reference potential that acts as a current sink, helps to reduce the charge accumulation in the first material layer, especially in the region close to the conductive second material layer. As a result, electrons that escape from the first material layer and reach the second material layer will flow away from the filter, thus reducing the charge accumulation in the first material layer.

[0029] The term "in use" when referring to an X-ray detector should be understood to mean using the X-ray detector as part of an electron beam device to detect X-rays emitted from a sample being irradiated with a focused beam of electrons.

[0030] The X-ray detector may be provided as part of a detector module comprising one or more X-ray detectors, and optionally one or more other types of detectors, such as a BSE detector. The term "detector module" may be understood to refer to a device or set of components adapted for the purpose of detecting particles by means of a detection element, particularly in an electron beam device. The detector module may be adapted to be positioned below a pole piece of an electronic assembly of an analytical device, which may be understood as the detector module being suitable for being placed in such a position during use. In other words, the detector module generally has a size, shape and detection element configuration that allows the sensor to receive X-rays generated by a sample in the device during use.

[0031] It should be understood that in the context of the present disclosure, an electron beam device or assembly may be considered as configured to generate a focused electron beam, and a detector module may be described as a module for the detection of particles, where the term "particles" is used to refer to both electrons and X-ray photons.

[0032] Typically, the detector module is adapted to operate positioned between the pole piece and the sample, in particular closer to the pole piece than to the sample. Typically, the detector module is also shaped to allow passage of the electron beam from the pole piece to the sample during use. In a preferred example, this configuration involves the shape of the detector module defining a space that allows the electron beam to pass unimpeded. Typically, the detector module is shaped to partially or fully surround this space, and therefore also surround the electron beam during use. In a typical example, the detector module being positioned below the pole piece may be understood as the detector module being in a position below the pole piece to allow the electron beam to pass through to the sample.

[0033] Typically, in instances where a space is defined between the pole piece and the detector module during use, this space is as small as is practical using a mechanical arrangement to hold the module. Typically, the detector module is isolated from the sample chamber environment and may be evacuated, partially evacuated, or contain a selected gas (such as nitrogen, argon, or xenon) at a pressure less than atmospheric pressure. Alternatively, an interface element may be provided that connects the environment of the x-ray detector to the sample chamber environment maintained by the electron microscope pumping system.

[0034] The detector element of an X-ray detector may be considered the "active" part of the X-ray detector, which senses incident X-rays or outputs a signal in response to incident X-rays. The output signal may include data representative of the detection or energy of received X-ray photons. The detector element may have an associated "active area" or "active volume region", which may generally be understood as a sensor region where the detector element responds to or senses particles incident on its surface or a portion thereof. The detector element may, in some examples, comprise multiple detector subelements, each configured to sense incident X-rays or output a signal in response to incident X-rays. The detector element, or any subelement contained therein, may be configured to detect individual received X-ray photons and monitor the energy of each received X-ray photon.

[0035] Typically, a space or spatial region is defined between an exterior surface of the filter and the detecting element of the x-ray detector, and in some instances, the space or spatial region may be evacuated, partially evacuated, or contain a selected gas at a pressure less than atmospheric pressure.

[0036] The material composition of a material layer can be understood in the context of the present disclosure as the material layer being partially or completely formed from one or more materials. When describing illustrative materials or material categories, such as materials characterized by electrical properties, particularly electrical conductivity, the material layer can be formed entirely, primarily, or partially from these materials or material categories. When the material composition includes a metal, it can include an elemental metal or a metal alloy.

[0037] The first material layer of the filter is typically formed from a material that is electrically insulating under standard operating conditions of the detector, which typically refer to standard temperature and pressure conditions. For example, the first material layer may be primarily formed from an electrically insulating material.

[0038] Those skilled in the art can understand suitable materials for the electrically insulating and conductive materials of the first and second material layers, respectively, according to the usual meaning of these material classifications. The term "electrically insulating" as used in this disclosure may refer to the property of the material in which electrons cannot flow freely within the material. In the context of this disclosure, an electrical insulator used to form the first material layer can be functionally defined as a material that is prone to charge accumulation due to incident electrons being stopped within the material. The term "electrically insulating" may refer to a material that has bound electrons and / or only a small proportion of mobile charge carriers. Based on the energy band theory of solids, an electrical insulator can be considered as a material in which the valence band is separated from the conduction band by a large band gap, thus preventing the free movement of electrons within the material. Examples of electrically insulating materials according to the present disclosure include insulating polymers and polyimides, silicon nitride, and silicon dioxide. Electrical insulators can be considered as materials that have a large band gap, such as 10 8 Ωcm to 10 18 For example, Mylar, an insulating polymer material, has a room temperature resistivity of approximately 10 18 It has a room temperature resistivity of Ωcm.

[0039] Typically, the first material layer is further adapted, by its material of construction and / or geometry, to allow partial or complete transmission of X-rays having energies within a first energy range or band, or a relatively large percentage of X-rays, while partially or completely blocking the transmission of electrons. The first range may correspond to X-ray energies useful for elemental analysis. This may be a pre-set energy range. It is understood that the numerical limits of this range may not necessarily be known. However, preferably, the filter is configured to admit X-rays at least within a desired energy band or sub-band thereof. The first energy band may correspond to a range of X-ray photon energies identified or determined to be useful for analysis. Typically, the composition or material of the first material layer, and / or the thickness along a direction perpendicular to the major surface, is configured to have the requisite X-ray transparency and / or requisite opacity to undesired radiation or particles.

[0040] The second material layer of the filter is formed from a material that is electrically conductive under at least standard operating conditions of the detector, For example, the second material layer may be formed primarily from an electrically conductive material.

[0041] The term "conductive" in the present disclosure may refer to a material property that indicates that electrons can flow freely. Functionally, the material may be considered as a material in which current can flow freely in the material when connected to a reference potential of a different potential than the potential of the material. Thus, when a second material layer formed from a conductive material is arranged on a first material layer, electrons that escape from the first material layer will flow away through the second material layer, thus reducing electrostatic charge buildup in the material. The term "conductive" may refer to a material that has free or delocalized electrons and / or a large proportion of mobile charge carriers. Based on the energy band theory of solids, a conductor may be considered as a material in which the valence band overlaps with the conduction band, thus allowing the free movement of electrons in the material. Examples of conductive materials according to the present disclosure include metals, certain carbon allotropes such as graphene, graphite, and amorphous carbon arrays, as well as semiconductors such as silicon when heavily doped to increase electrical conductivity. A conductor may be a material that has a high electrical conductivity of 10 -8 Ωcm to 10 -3 They can have room temperature resistivities on the order of Ωcm. For example, the metals silver, copper, gold, and aluminum all have resistivities on the order of 1×10 -6 Ωcm to 3×10 -6 In addition to being formed from a conductive material, the geometry of the second material layer can be adapted such that the second material layer acts as an electrical conductor.

[0042] In some examples, the second material layer can have at least 1,000 times higher electrical conductivity than the first material layer. However, this factor can be higher in other examples. It should be understood that the second material layer often has orders of magnitude higher electrical conductivity than the first material layer, in accordance with the description of electrical conductors and insulators. The material of the second material layer can have several orders of magnitude higher electrical conductivity than the material of the first material layer. This order of magnitude can be, for example, 9, 14, 15, 20, and 21 or more, and can be, for example, 14, 15, 20, 21, and 26 or less.

[0043] The resistivity and related electrical conductivity discussed in this disclosure will be understood as volume resistivity and volume conductivity, in other words, the resistivity and electrical conductivity that relate to the bulk of a given material and describe the ease or resistance to electrical current that can pass through a spatial region of that material.

[0044] In some examples, the second material layer can further aid in reducing the transmission of BSE by being adapted to partially or completely block the transmission of electrons while allowing partial or complete transmission of x-rays having energies within a first energy range or band. In these examples, the material composition or material and / or thickness of the second material layer is configured to be complementary to the material composition or material and / or thickness of the first material layer such that the overall filter has the requisite x-ray transparency and / or requisite opacity to undesirable radiation or particles.

[0045] A "reference potential" in the context of this disclosure may be generally understood as a potential that is any one of a voltage (usually a fixed or substantially fixed voltage), a reference ground or system ground, and earth potential. A reference ground or system ground may refer to a ground reference or a zero volt reference used for the X-ray detector and / or other components of the electron beam assembly in which the X-ray detector is used. The purpose of a reference potential source is to allow current to flow freely in and out of the reference potential source while minimizing any voltage fluctuations in the reference potential source that would affect any other purpose related to that reference potential. In other words, the reference potential must have a sufficiently low impedance and / or stability of its potential or voltage to act, and preferably continue to act, as an adequate current sink during use. In some instances, a feedback control mechanism may be provided to improve the voltage stability of the reference potential.

[0046] The second material layer being arranged on the side of the first material layer closest to the detection element can be understood to mean that when a space is provided between the detection element and the filter, the outer surface of the second material layer faces the surface of the detection element. When no space is provided and the filter is in contact with the detection element, the surface of the second material layer contacts the surface of the detection element. In other words, the second material layer can be considered as the final material layer through which the particle can travel on its path from the sample to the detection element.

[0047] Within the X-ray detector, a filter may be provided with a first layer of material, a second layer of material, and / or any additional layers of material that obscure at least a portion or all of the active area of ​​the detection element. In combination with positioning the filter between the sample and the detection element during use, BSEs emanating from the sample on the path between the sample and the detection element are incident on the filter before reaching the detection element.

[0048] The second material layer can partially cover or more preferably cover the entire surface of the first material layer closest to the detection element. The partial or discontinuous coverage of the first material layer closest to the detection element can include the second material layer comprising one or more holes, preferably apertures, defined by one or more continuous conductive material portions and / or the absence of conductive material between them. The second material layer can thus comprise an array structure and / or a grid structure partially covering the surface of the first material layer closest to the detection element. The partial coverage of the side of the first material layer by the second material layer can in some cases have the form of a filament network or grid formed of conductive material. When the geometry of the second material layer comprises one or more discontinuities, either due to the selected geometry or as a result of a particular material deposition method, any one or more of the thickness, coverage and structure of the second material layer can be adapted to shield the detection element from an electric field resulting from the accumulation of charge in the first material layer by incident electrons during use. The minimum thickness when referring to a layer with discontinuities may refer to the minimum thickness of one or more continuous portions.

[0049] In some examples, the filter may comprise additional layers of material in addition to the first and second layers of material, with example arrangements comprising a third layer of material being described in more detail below, however, in further examples, the filter may comprise any number of layers to provide the requisite x-ray transparency, the requisite impermeability to unwanted radiation or particles, and the requisite mitigation of the problems discussed with respect to charge build-up within the filter.

[0050] In some examples, for each additional layer of material provided in the filter that is electrically insulating, an additional adjacent layer of material formed from a material that is electrically conductive is also provided in the filter. In these examples, the filter may comprise a multi-layer structure generally arranged as an alternating pattern of electrically insulating and conductive layers. However, in other examples, different arrangements of electrically insulating and conductive layers may be provided. More generally, the multi-layer structure may form a stack in various arrangements, and optionally the stack may be periodic, repetitive, or partial. For example, two directly adjacent sets of layers in such a stack may comprise one conductive layer and one insulating layer, and in various embodiments, the set may be directly adjacent to a further layer that may be composed of further such sets on one or two major surfaces, or may be a separate layer of this type, or may be of further or different types or classifications of layers. It should be understood that preferably the overall thickness of the filter is configured to optimize the transparency of the filter to the radiation of interest in conjunction with other requirements. If the filter is constructed of multiple layers of material, for any material layer that is an electrical insulator, any adjacent layers that are conductive, in electrical contact with an adjacent insulating layer, and connected to a reference potential can help reduce charge buildup in the insulating layer.

[0051] Preferably, the number of material layers having an electrically conductive material composition or formed from an electrically insulating material is greater than or equal to the number of material layers having an electrically insulating material composition or formed from an electrically insulating material. As explained above, the final material layer through which a particle travels on its path from the sample to the detection element is required to have the properties of the second material layer described herein, but the material layer closest to the sample in use (i.e. the first surface on which a particle from the sample would be incident) may be either an electrically insulating layer or a conductive layer. However, it is preferred that the material layer furthest from the detection element is electrically conductive and / or blocks undesired radiation.

[0052] Any additional material layer of the filter described as being "electrically insulating" may have any of the characteristics, structures, or functions of a first material layer described herein. Similarly, any additional material layer described as being "electrically conductive" may have any of the characteristics, structures, or functions of a second material layer described herein.

[0053] In general, the electric field resulting from charge accumulation within the first material layer is at least one of a static electric field and a transient electric field. The static electric field can include, for example, an electrostatic field that occurs as a result of incident electrons becoming trapped within the first material layer. The transient electric field can include an electric field that occurs during an electrostatic breakdown or discharge and can occur within the first material layer and / or between the first material layer and one or more other surfaces in the vicinity of the sensing element.

[0054] In some examples, the second material layer is formed from or has a material composition that includes at least one of a metal and carbon. As explained above, metallic materials in general and certain carbon allotropes / arrangements have the electrical conductivity required to flush electrons out of the filter. Alternatively or additionally, the second material layer is formed from or has a material composition that includes a semiconductor, such as silicon, that is heavily doped to increase its electrical conductivity.

[0055] In examples where the second material layer is formed from a metallic material or generally has a material composition that primarily includes a metal, the metal may include at least one of aluminum and titanium. Although heavier metals such as chromium, platinum, silver, gold, and copper may also be suitable source materials that allow for the formation of the second material layer, lighter metals are preferred because the x-rays resulting from electrons incident on the lighter metals generally have lower energy and are therefore less likely to interfere with the spectral emissions of interest from the sample.

[0056] In examples where the second material layer is formed from a material or has a material composition that includes carbon, the carbon may be provided in the form of at least one of graphite and amorphous carbon. Graphite may also be a suitable carbon allotrope for the second material layer due to its electrical conductivity.

[0057] The materials described above, when provided in the proper geometry, have been found to have the necessary electrical conductivity to provide an electrical path and reduce charge build-up within the first material layer. Additionally, the metallic materials described above, when provided in the proper geometry, help reduce the light reaching the detector due to cathodoluminescence within the filter, while still allowing the transmission of x-rays having energies useful for elemental analysis.

[0058] In some examples, the first material layer is formed from a material or has a material composition that includes a polymeric material. In a typical example, the first material layer is formed from at least one of a polyester film derived from polyethylene terephthalate (commonly known under the trade name Mylar) and a polyester film derived from polyimide. These materials generally provide a good compromise between opacity to electrons and other types of radiation and transparency to X-rays with energies useful for elemental analysis when arranged in appropriate geometries. Furthermore, this type of material is readily available in the form of a free-standing film with a thickness perpendicular to the main surface that is suitable for application as a BSE filter for SEM applications with typical beam energies. These thicknesses are typically on the order of micrometers.

[0059] Preferably, the material of the second material layer is selected and / or the geometry of the second material layer, in particular the minimum or average thickness, in particular the intermediate thickness, is adapted to reduce the transmission of photons generated from cathodoluminescence of the first material layer through it. This reduction can be achieved by absorption or reflection of photons having wavelengths in the near infrared, visible or ultraviolet ranges, which are generally the ranges in which photons from cathodoluminescence are emitted. The "reduction" of transmission can be relative to free space or a medium that is transparent to photons having wavelengths in these ranges, or it can be such that a major proportion of photons are prevented from transmitting through the layer. The proportion of photons transmitted through the second material layer depends on both the material and the geometrical properties of this layer. In some examples, the second material layer can exclude 99% of the transmission of cathodoluminescence photons, i.e. it can be adapted to reduce the incidence of photons generated by cathodoluminescence of the first material layer on the detector or detector element by substantially 99%. However, in other examples, the transmission fraction can be significantly less than 1%. In some examples, the second material layer has an average thickness of at least 40 nm, and it has been found that a 60 nm aluminum layer blocks a sufficient amount of cathodoluminescence photons.

[0060] The inventors have found that typical materials used to form the first material layer and adhesives used to attach the filter to the detector can emit light due to cathodoluminescence caused by electrons impinging on these materials. By providing a second material layer facing the detector element and forming the second material layer from a material adapted to reduce the transmission of cathodoluminescence light therethrough, photons generated by this cathodoluminescence in the filter or any adhesive are prevented from reaching the detector element. A typical configuration for the second material layer to provide this effect is an aluminum layer at least 40 nm thick, preferably at least 60 nm thick.

[0061] "Thickness" when referring to any layer of material in this disclosure refers to the thickness in a direction perpendicular to one or more major surfaces of that layer of material. Alternatively, the thickness of a layer of material can be defined as the shortest straight line path between any two outer surfaces of that layer of material. It should be understood that an outer surface can include a surface of a layer of material that is in contact with another layer of material of the filter.

[0062] Typically, the first material layer has an average (mean) thickness on the order of micrometers. However, the thickness may alternatively refer to the maximum or minimum thickness of the first material layer. Typically, a first material layer of this thickness has been found to provide the requisite opacity to electrons and other types of radiation, and transparency to X-rays with energies useful for elemental analysis. Furthermore, since only electrons trapped in the first material layer in a region close to the surface of the first material layer in contact with the second material layer can be swept away from the filter, it is beneficial to minimize the thickness of the first material layer and / or the distance between any portion of the first material layer and the surface of the second material layer while ensuring that the first material layer prevents the transmission of a significant proportion of BSE.

[0063] In some examples, the first layer of material and / or the filter is provided as a free-standing membrane, it being understood that the first layer of material or filter has the ability to maintain or substantially maintain its shape during use without the use of any additional support members or structures that may degrade the performance of the filter with respect to x-ray transmission, in particular, has sufficient rigidity or structural integrity. In this manner, the first layer of material may be mounted on the x-ray detector and supported only by its edges.

[0064] In some examples, the second layer of material is arranged as a coating on the surface of the first layer of material closest to the detection element. Since the second layer of material is generally thinner than the first layer of material, forming the second layer of material as a coating on the surface of the first layer of material is a convenient way to provide the second layer of material in the filter without requiring the second layer of material to be freestanding. The second layer of material can be produced as a coating by any material deposition method suitable for the source material or sources from which it is formed. The material deposition method can also be selected to be suitable for application onto the material contained in the first layer of material and / or onto the incident surface of the detection element, if appropriate. For example, the coating can be produced by physical vapor deposition (PVD) techniques such as sputtering, evaporation (e.g., electron beam assisted, thermal assisted, ion assisted), or pulsed laser deposition. Alternatively, chemical vapor deposition (CVD) techniques, atomic layer deposition, or plasma enhanced CVD in various pressure environments can be utilized. The deposition technique selected to produce the second material layer may depend on the ability of the first material layer to withstand the conditions under which the deposition is performed and / or the thickness of the layer to be produced. These deposition techniques can make it possible to produce the second material layer with a thickness on the order of single digit nanometers.

[0065] In another example, the second layer of material can be created by altering a portion of the first layer of material to be conductive, i.e., the material or material composition of the first layer of material near the surface closest to the detector can be altered to create a conductive layer.

[0066] Typically, the second material layer has an average thickness smaller than the first material layer. Typically, the second material layer has a minimum or average thickness on the order of nanometers or tens of nanometers, preferably at least 10 nm. It has been found that a material layer of this geometry, when formed from or having a material of the appropriate material composition, has the requisite electrical conductivity to provide an electrical path that reduces charge accumulation in the first material layer, while still allowing the transmission of X-rays with energies useful for elemental analysis. However, if the second material layer further significantly reduces the incidence of photons generated by cathodoluminescence in or from the first material layer onto the detector or detector element, the second material layer preferably has an average thickness on the order of tens of nanometers. As explained above, in some examples, the second material layer is formed from aluminum and can have an average thickness of at least 40 nm, preferably at least 60 nm.

[0067] Typically, the detection element of the X-ray detector is one of a silicon drift detector (SDD), a PIN diode detector, a gas proportional counter, a charge-coupled device (CCD) array, and a complementary metal oxide semiconductor (CMOS) array.

[0068] Furthermore, the inventors have found that by providing the second material layer with a negative voltage equal to or greater than the voltage with respect to system ground applied to the incident surface of the detector element, degradation of the performance of the X-ray detector over time can be reduced, because the resulting electric field in the region between the filter and the detector element prevents any nearby positive ions from being attracted towards the incident surface of the X-ray detector. In other words, the positive ions will not be attracted towards the detector element, and the more negative voltage will repel them away from the incident surface.

[0069] In some examples, the filter is arranged such that the side of the second layer of material closest to the sensing element is in contact with the incident surface of the sensing element. This contact can be full contact (i.e., all of the surface of the second layer of material closest to the sensing element is in contact with the incident surface of the sensing element) or substantially full contact. By providing the filter directly on the surface of the sensing element, the filter does not have to be formed from a layer of material available as a free-standing film, thus allowing a variety of filter materials or filter material compositions to be used.

[0070] Typically, the X-ray detector further comprises a housing, and the filter is arranged to cover an opening in the housing. This covering can be a partial covering or a complete (or substantially complete) covering of the opening in the housing. In such an example where the filter is arranged to completely or substantially completely cover the opening, the filter can effectively seal the detector housing. However, this sealing is not required, and the filter can be positioned across the opening in front of the X-ray detector to block BSE from reaching the detector elements.

[0071] Preferably, the filter comprises a third layer of material arranged on an opposite side of the first layer of material to the second layer of material and formed from a conductive material, and the X-ray detector is adapted such that, during use, the third layer of material is electrically connectable to a reference potential.

[0072] Advantageously, the third layer of material further acts as an additional electrostatic shield to prevent electric fields due to charge build-up in the first layer of material from interfering with the electron optics and deflection fields of the scanning electron microscope during use.

[0073] Shielding of the electron optics and deflection fields can be understood as electrically and / or electromagnetically shielding the electron optics and deflection fields. In other words, the third material layer mitigates problematic effects on the electron beam device in use due to electric fields that arise as a result of electron accumulation in the first material layer when the detector is in use and electrons are incident on the first material layer.

[0074] The third material layer is adapted to be electrically connectable to a reference potential during use. Thus, the third material layer is typically electrically connected to a reference potential during use. This connection can be made in a similar or equivalent manner to the second material layer. The third material layer of the filter can also serve to reduce charge build-up in the first material layer. In particular, adapting the X-ray detector such that the third material layer is electrically connectable to a reference potential can reduce charge build-up in the first material layer due to incident electrons in a generally similar or equivalent manner as described in relation to the second material layer.

[0075] In some examples, the X-ray detector may be adapted such that the filter is positioned within the X-ray detector such that a portion of the third material layer forms electrical contact with a portion of the X-ray detector in the same manner as described above in relation to the second material layer.

[0076] When the third material layer is held at a reference potential during use, typically a fixed voltage, a reference ground, or a reference earth, an additional electrical path to the current sink is provided for the electrons to pass through. This electrical path provided by the third material layer is in addition to the electrical path provided by the second material layer. As a result, electrons escaping from the first material layer to reach the third material layer will flow away from the filter through the electrical path provided by the third material layer and the reference potential. In other words, by providing both the second material layer and the third material layer as described above, the charge built up in the first material layer is further reduced by the multiple electrical paths through which the electrons in the first material layer can escape.

[0077] In other words, by requiring the third material layer to be electrically connectable to a reference potential, when the third material layer is connected to such reference potential when the x-ray detector is in use, some of the electrons that would otherwise be trapped in the first material layer can reach the third material layer, and the electrical path and potential gradient created by the third material layer and the reference potential allows the electrons that reach the third material layer to flow out of the filter. In this manner, electrostatic charge build-up within the first material layer is further reduced during operation of the x-ray detector in the electron beam device.

[0078] The third material layer is formed from a material that is conductive under standard operating conditions of the detector. In some examples, the third material layer can be at least 1,000 times more conductive than the first material layer. The third material layer often has orders of magnitude more conductive than the first material layer. The material of the third material layer can have several orders of magnitude more conductive than the material of the first material layer. The orders of magnitude can be, for example, 9, 14, 15, 20, and 21 or more, and can be, for example, 14, 15, 20, 21, and 26 or less.

[0079] The third material being provided on the opposite side of the first material layer from the second material layer can be understood to mean that the third material layer is provided on the side of the first material layer closer to the sample when the detector is in use. In an example where only the first, second and third material layers are provided in the filter, the third material layer can be considered as the first material layer through which particles can travel on their path from the sample to the detection element. The third material layer of the filters described herein can have any of the features, structures or functions of the second material layer described herein.

[0080] In some examples, the third material layer is formed from or has a material composition that includes at least one of a metal and carbon. As explained above, metallic materials in general and certain carbon allotropes / arrangements have the electrical conductivity required to flush electrons out of the filter. Alternatively or additionally, the third material layer can be formed from or has a material composition that includes a semiconductor, such as silicon, that is heavily doped to increase its electrical conductivity.

[0081] In examples where the third material layer is formed from a material or has a material composition that includes a metal, the metal can include at least one of aluminum and titanium. As with the second material layer, heavier metals such as chromium, platinum, silver, gold, and copper can also be suitable raw materials that allow for the formation of the third material layer, although lighter metals are preferred for the reasons presented earlier in this disclosure.

[0082] In examples where the third material layer is formed from a material or has a material composition that includes carbon, the carbon may be provided in the form of at least one of graphite and amorphous carbon. Graphite may also be a suitable carbon allotrope for the third material layer due to its electrical conductivity.

[0083] The beneficial effects of the materials discussed above include those discussed for these materials in relation to the second material layer.

[0084] In some examples, the third material layer is arranged as a coating on a surface of the first material layer opposite the second material layer. The third material layer can be produced as a coating by any of the material deposition methods described above in connection with the second material layer. Beneficial effects include those described for the surface coating in connection with the second material layer.

[0085] In another example, the third layer of material may be formed by altering a portion of the first layer of material to change its electrical conductivity, in particular to make it more conductive, i.e., to become an electrical conductor, or in other words, to alter the material or material composition of the first layer of material proximate the surface opposite the second layer of material to create a further conductive layer.

[0086] In some examples, the third material layer has an average thickness smaller than the first material layer. Typically, the third material layer has a minimum or average thickness on the order of nanometers or tens of nanometers, preferably at least 10 nm. The beneficial effects are the same as those described for the layer thickness in relation to the second material layer. In some examples, the third material layer can have the same or substantially the same average thickness as the second material layer. In other examples, the third material layer has a minimum or average thickness smaller than the second material layer.

[0087] In a second aspect of the present invention there is provided an apparatus for analysing a sample comprising an electron beam assembly for generating a focused electron beam and an X-ray detector according to the first aspect or any implementation thereof. It will be appreciated that the beneficial effects presented above in relation to the first aspect apply equally to the second aspect.

[0088] The apparatus may further include an accessory device which may comprise or be an additional X-ray detector provided with an electron trap supported on a mechanical assembly on which the X-ray detector or detector module is also supported.

[0089] In a third aspect of the present invention, a method for analyzing a sample is provided. Preferably, the method can be implemented using an X-ray detector according to the example of the first aspect or an apparatus according to the example of the second aspect. Typically, the method is implemented in an electron beam instrument such as an electron microscope. The method includes the steps of generating a focused electron beam using an electron beam assembly, preparing an X-ray detector according to the first aspect or any implementation thereof, the X-ray detector being positioned below a pole piece of the electron beam assembly through which the focused electron beam emerges towards the sample, such that X-rays and backscattered electrons generated by the interaction between the electron beam and the sample are incident on a filter of the X-ray detector, connecting a second material layer of the filter to a reference potential, and monitoring the energy of each received X-ray photon using the X-ray detector. It should be understood that the beneficial effects presented above in relation to the first aspect apply equally to the third aspect.

[0090] In particular, by carrying out the above method with an X-ray detector according to an example of the first embodiment and further connecting the second material layer of the X-ray detector to a reference potential, such as a fixed power supply, a reference ground, or a reference earth, prior to monitoring the energy of each received X-ray photon, an electrical path is provided to a current sink for electrons to pass through the second material layer. As explained in relation to the first embodiment above, providing this electrical path serves to reduce charge build-up due to electrons in the first material layer and shields the detector from electric fields that arise as a result of charge build-up within the first material layer.

[0091] Typically the method includes connecting the second layer of material to a fixed power supply, preferably a voltage applied to the second layer of material that is a negative voltage with respect to system ground that is equal to or greater than the voltage applied to the incident surface of the detection element.

[0092] The inventors have found that by providing the second material layer with a negative voltage equal to or greater than the voltage applied to the incident surface of the detector element, degradation of the performance of the X-ray detector over time can be reduced, because the resulting electric field in the region between the filter and the detector element prevents any nearby positive ions from being attracted towards the incident surface of the X-ray detector. In other words, if the second material layer is held at a negative voltage at least as large as the voltage on the front surface of the radiation sensor, there will be no electric field to attract the positive ions towards the surface of the X-ray detector, and a more negative voltage will repel these positive ions away from the incident surface.

[0093] In some examples, the X-ray detector is an X-ray detector according to an example of the first embodiment that includes a third material layer. Thus, the beneficial effects of the third material layer described in relation to the first embodiment apply to the third embodiment as well. In these examples, the method typically further includes connecting the third material layer to a reference potential. Typically, this includes connecting the third material layer to a fixed voltage.

[0094] Typically, the voltage applied to the third material layer is equal to the voltage applied to the second material layer with respect to the system ground. In this arrangement, a common power source can be used to apply a voltage to the second and third material layers.

[0095] In a fourth aspect of the invention, there is provided a filter adapted to reduce internal transmission of backscattered electrons from a sample. The filter is suitable for use in an X-ray detector. The filter comprises a first material layer formed from an electrically insulating material and a second material layer formed from an electrically conductive material, the second material layer being disposed on one side of the first material layer, the filter adapted to be attached to the X-ray detector, the filter being positioned in use between the sample and the detection element, the second material layer being arranged on a side of the first material layer closest to the detection element of the X-ray detector. The filter is adapted such that the second material layer is electrically connectable in use to a reference potential to shield the detection element from an electric field resulting from charge build-up in the first material layer by incident electrons in use.

[0096] Alternatively or additionally, the filter is adapted such that the second material layer is connectable to a reference potential to reduce charge build-up in the first material layer due to incident electrons.

[0097] The filter according to the fourth aspect of the invention may comprise any of the features, structures or functionality of the filters described as part of the X-ray detector according to the first aspect of the invention.

[0098] Examples of the invention will now be described with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0099] [Figure 1] 1 shows diagrammatically a first arrangement for X-ray analysis in an electron microscope according to the prior art; [Diagram 2] 2 shows diagrammatically a second arrangement for X-ray analysis in an electron microscope according to the prior art; [Diagram 3] 1 is a schematic cross-sectional view of a first exemplary X-ray detector according to the present invention; [Figure 4A] 2A-2C are schematic cross-sectional views of various filter arrangements for an X-ray detector according to the invention; [Figure 4B]2A-2C are schematic cross-sectional views of various filter arrangements for an X-ray detector according to the invention; [Figure 4C] 2A-2C are schematic cross-sectional views of various filter arrangements for an X-ray detector according to the invention; [Diagram 5] 4 is a schematic cross-sectional view of a second exemplary X-ray detector according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0100] In this specification, the electron beam is defined as traveling vertically downward toward the sample, which provides background for terms such as "down" and "up." In practice, the electron beam can be oriented in any direction, including vertically upward. Additionally, dimensions of elements shown in the examples described herein are not drawn to scale.

[0101] 3 shows a first exemplary X-ray detector 300 according to the present invention, and in particular shows a cross section through an X-ray detector in use with an electron beam assembly. In this embodiment, the detector 300 is positioned inside the sample chamber of an electron microscope, which is typically evacuated to a low pressure to reduce scattering of electrons in a focused electron beam by gas molecules.

[0102] In Figure 3, a sample 301 positioned on a sample stage 303 is impinged by electrons traveling in a focused electron beam 302. At the spot where the focused electron beam 302 impinges on the sample 301, X-ray photons 304 are generated from atoms in the sample material. Some of the X-ray photons 304 will travel towards the X-ray detector 300, and any photons that pass through the entrance surface 312 of the detection element 311 of the X-ray detector 300 will generate a stepped pulse signal in the associated electronics that is proportional to the X-ray energy. The resulting energy histogram of the pulse measurements represents the emitted X-ray energy spectrum from the sample 301.

[0103] A percentage of electrons incident on the sample 301 will be backscattered from the surface and emitted as backscattered electrons (BSEs) 305. For X-ray photons 304 and BSEs 305 traveling towards the X-ray detector 300, the X-ray detector 300 is adapted such that these particles must pass through a filter before reaching the detection element 311. The filter is adapted to maximize the transmission of X-rays in the useful energy range while blocking a percentage of the BSEs from reaching the detection element 311.

[0104] In the embodiment shown in Fig. 3, the filter comprises a first material layer 306, a second material layer 307 and a third material layer 308. The first material layer 306 is formed from an electrically insulating material, and the second material layer 307 and the third material layer 308 are formed from an electrically conductive material. The second material layer 307 and the third material layer 308 are arranged on opposite sides of the first material layer 306 such that on the path from the sample 301 to the detection element 311, an X-ray photon 304 reaching the detection element 311 passes through the third material layer 308, the first material layer 306 and the second material layer 307 in sequence.

[0105] The x-ray detector 300 shown in Figure 3 comprises a detector element 311, which in this embodiment is a silicon drift detector (SDD). The detector element 311 has an entrance surface 312 which forms part of the entrance electrode for the detector element 311. This entrance surface 312 is connected to a power supply 313 which is typically a negative voltage of around -100V. The surface of the detector element 311 opposite the entrance electrode has a number of electrodes (not shown) including an electrode connected to electronics for measuring the pulse signal.

[0106] The detector element 311 of Figure 3 is shown in a detector housing 314, where the interior of the X-ray detector 300 is isolated from the sample chamber environment, and the interior may be evacuated, partially evacuated, or contain a selected gas (e.g., nitrogen, argon, or xenon) at a pressure less than atmospheric pressure. Alternatively, the detector housing 314 may have an interface element that connects the environment of the X-ray detector 300 to the sample chamber environment and is maintained by an electron microscope pumping system (not shown). Although Figure 3 shows a filter that substantially seals the detector housing 314, this sealing is not required, and the filter need only be supported in a position that blocks the BSE 305 before the detector element 311.

[0107] The first material layer 306 and / or the filter as a whole are provided with a suitable material or material composition with a sufficient thickness along a direction perpendicular to its major surface to prevent the BSE 305 from passing through the filter and transmitting towards the detection element 312. In the illustrated embodiment, this direction is vertical on the page. For example, when the energy of the electrons in the focused electron beam 302 is approximately 20 keV, a first material layer 306 formed from mylar or polyimide with an average thickness of 7 microns is generally sufficient to prevent a significant proportion of the backscattered electrons from reaching the detection element 311. At this average thickness, the first material layer can be provided as a free-standing film. Some of the BSE 305 traveling towards the X-ray detector 300 gradually lose energy in the process of being inelastically scattered by atoms in the first material layer 306 and are trapped in the first material layer 306. Because the first material layer 306 is electrically insulating, and comprises, for example, mylar or polyimide, electrons trapped within the first material layer 306 will cause a negative charge to build up in the material over time if there are no electrical paths leading out of the first material layer 306.

[0108] In some cases, the accumulation of negative charges in the first material layer 306 can cause electric fields that can interfere with the x-ray detection and / or electron beam optics, and can also cause localized electrostatic discharge events on surfaces in or near the first material layer 306. These discharge events can cause spurious signals in the x-ray detector electronics that can cause artifacts in the x-ray spectrum recording. To mitigate these effects, the second material layer 307 shown in FIG. 3 is arranged as a coating on the side of the first material layer 306 closest to the detection element 311. The second material layer 307 is further connected to a power supply 310 that provides a fixed or substantially fixed voltage to the second material layer 307. Alternatively, the second material layer 307 can be connected to a reference / system ground or earth. In this way, some electrons inside the first material layer 306 can escape towards and enter the second material layer 307, thereby displacing them from the electrode detector 300 and reducing charge build-up in the first material layer 306. Furthermore, due to its electrical conductivity, the second material layer 307 also acts as an electrostatic shield between the first material layer 306 and the detection element 311, preventing transient electrostatic discharges from interfering with the X-ray detector 300 and / or its electronics, thereby reducing artifacts in the X-ray spectrum recording.

[0109] The second material layer 307 is arranged by implantation or deposition on the surface of the first material layer 306 and may be formed of aluminum, typically having an average thickness of the order of tens of nanometers. However, alternatively, amorphous or crystalline carbon allotropes or other metals such as chromium, platinum, silver, gold, titanium, and copper may be used in the material or material composition of the second material layer 307.

[0110] Furthermore, if the first material layer 306 is formed primarily from a material such as Mylar that exhibits cathodoluminescence when exposed to incident electrons, these photons can be prevented from reaching the detection element 311 by ensuring that the second material layer 307 is of a material and thickness suitable for reflecting or absorbing photons generated within the first material layer 306. A coating of aluminum (Al) having a thickness on the order of tens of nanometers and disposed on the surface of the first material layer 306 closest to the detection element 311 will typically block a significant proportion of photons generated from cathodoluminescence arising from within the first material layer 306.

[0111] In some embodiments, the power supply 310 can supply the second material layer 307 with a negative voltage equal to or greater than the voltage with respect to system ground applied by the power supply 313 to the detector incidence surface 312. As explained above, the x-ray photons 304 and / or BSEs 305 traveling toward the detection element 311 can interact with gas molecules in their path, for example, in the region 315 near the detector incidence surface 312. This interaction can cause ionization, thereby leaving behind positive ions. Any positive gas ions near the x-ray detector incidence surface 312 would normally be attracted thereto due to the negative bias of this surface, but if the voltage of the power supply 310 is set to a negative voltage equal to or greater than the voltage of the power supply 313, the resulting electric field will not attract positive ions toward the detection element 311. Preferably, the application of a negative voltage by the power supply 310 that is more negative than the voltage of the power supply 313 will expel any positive ions from the detection element 311. This application of a voltage to the second material layer 307 addresses the problem that when positive gas ions flow towards the detection element 311, unwanted charge can build up, which can cause some degradation of the detector's performance, especially for low energy photons absorbed near the surface.

[0112] To further prevent electrostatic fields due to charge accumulation in the first material layer 306 from interfering with the electron optics and deflection fields of the scanning electron microscope, a third material layer 308 is arranged as a conductive coating on the side of the first material layer 306 closest to the sample during use. The third material layer 308 is connected to a power supply 309 or alternatively to a reference ground or earth. The third material layer 308 provides an additional path for electrons inside the first material layer 306 to escape the filter to the second material layer 307, such that electrons that reach the third material layer 308 enter the third material layer 308 and flow away. Typically, the power supply 309 is also connected to the detector housing 314 and can be the same power supply as the power supply 310. However, the third material layer 309 and the detector housing 314 can alternatively be connected to a reference ground or earth.

[0113] As with the second material layer 307, the third material layer 308 can be arranged by implantation or deposition on the surface of the first material layer 306. Typically, the third material layer is also formed of aluminum with an average thickness on the order of a few tens of nanometers. However, alternatively, amorphous or crystalline carbon allotropes or other metals such as chromium, platinum, silver, gold, titanium, and copper can be used in the material or material composition of the third material layer 308.

[0114] Providing a conductive layer on the side of the first material layer closest to the sample can help reduce interference with the electron beam optics, but the third material layer 308 is not required to reduce the proportion of photons that reach the detection element 311 resulting from charge accumulation within the first material layer and / or cathodoluminescence.

[0115] 4A-4C show various implementations of a filter as part of an X-ray detector in accordance with the present invention. In each of these embodiments, the filter is shown oriented such that the X-rays and BSEs generated from the sample are assumed to approach the filter from below (i.e., from the bottom of the page).

[0116] In Fig. 4A, the filter is composed of an electrically insulating layer as the first material layer 306 and an electrically conductive layer as the second material layer 307. These material layers have the properties of the first material layer 306 and the second material layer 307 described above in relation to Fig. 3. The material layers of the filter are arranged such that an incident particle from the sample must pass through the first material layer 306 and the second material layer 307 in sequence to penetrate to the detection element 311. In this embodiment, the electrically conductive second material layer 307, when constructed with the appropriate material or material composition and geometry, simultaneously serves as an electrostatic shield to prevent discharge events from interfering with the exposure radiation spectrum, as an electrical path for electrons in the first material layer 306 to escape and flow away, and as a filter to prevent photons generated by cathodoluminescence in the first material layer 306 and some backscattered electrons 305 from reaching the detection element 311.

[0117] 4B shows a filter arrangement that additionally comprises a third material layer 308. As explained above in relation to FIG. 3, the additional conductive layer positioned on the sample side of the first material layer 306, when configured with the appropriate material and geometry, provides a second path for electrons in the first material layer 306 to escape and flow away. This additional layer also shields the electron optics of the electron beam from interference due to electric fields generated by charges in the first material layer 306.

[0118] FIG. 4C shows a filter comprising an arrangement of conductive and insulating layers formed as a stack of alternating layers. The filter arrangement of FIG. 4C can be conceptually considered as a combination of the filter arrangements of FIG. 4A and FIG. 4B into a single filter arrangement. In this embodiment, three conductive layers and two insulating layers are provided in an alternating pattern of conductive and insulating layers such that each insulating layer contacts two conductive layers. This alternating arrangement provides multiple electrical paths for electrons to escape the insulating layers, maximizing the reduction of charge build-up inside the two insulating layers during use. The conductive layer 407 of FIG. 4C has structural and functional characteristics equivalent to the second material layer 307 of FIG. 4B. Similarly, the conductive layer 408 of FIG. 4C has structural and functional characteristics equivalent to the third material layer 308 of FIG. 4B. The conductive layer 410 is provided as an intermediate conductive layer between the insulating layers 406 and 409. In use, conductive layer 410 is connected to a fixed power supply, typically the same power supply as that of conductive layer 408. Alternatively, both conductive layer 408 and conductive layer 410 may be connected to a reference / system ground or earth.

[0119] In an embodiment such as that shown in Figure 4C, in which the filter comprises multiple layers of electrically insulating electron filtering material, each insulating material layer can be thinner than the corresponding insulating material layer of a filter arrangement comprising only a single insulating material layer, so that the filter as a whole provides sufficient opacity to backscattered electrons while at the same time allowing sufficient transmission of X-rays having energies suitable for elemental analysis. For example, electrically insulating layer 406 and electrically insulating layer 409 can perform the same function and have the same material composition or be formed from the same material as first material layer 306 of Figure 4B, but each is generally thinner than first material layer 306.

[0120] FIG. 5 shows an alternative embodiment of an X-ray detector 500 according to the invention. This embodiment differs from the embodiment shown in FIG. 3 only in that there is no spatial region 315 provided between the filter and the detector element 311, and the filter for backscattered electrons is arranged in direct contact with the entrance surface 312 of the detector element 311. In this embodiment, the side of the second material layer closest to the detector element 311 is in full contact with the detector surface 312 of the detector element 311, and both are connected to a common power supply 313 providing a negative voltage, typically of the order of −100 V. The layers of the filter are arranged such that the filter comprises a stack of material layers in direct contact with the entrance surface 312. Advantageously, by manufacturing the filter layers directly on the entrance surface 312 of the detector element 311, it is not necessary that any of the layers in the filter are available as free-standing films, thus making it possible to use a variety of filter materials and / or filter material compositions. Furthermore, as explained above in relation to FIG. 3, the third material layer 308 is not essential.

[0121] Although the embodiment presented above shows an X-ray detector comprising a single element silicon drift detector suitable for use in a scanning electron microscope, the same principles can be used for any X-ray sensor that is expected to be adversely affected by BSE, electrostatic discharge, cathodoluminescence light, or the accumulation of positive gas ions. Thus, the filter arrays described can be used, for example, with PIN diode semiconductor detectors, gas proportional counters, CCD imaging arrays, or CMOS imaging arrays. [Explanation of symbols]

[0122] 300 X-ray detector 301 Sample 302 Focused Electron Beam 303 Sample stage 304 X-ray photons 305 Backscattered Electrons (BSE) 306 First material layer 307 Second Material Layer 308 Third Material Layer 309 Power supply 310 Power supply 313 Power supply 311 Detection elements 312 Incidence surface 314 Detector housing 315 Spatial domain

Claims

1. 1. An x-ray detector adapted to reduce internal transmission of backscattered electrons from a specimen, comprising: A detection element; a filter positioned in use between the sample and the detection element; Equipped with the filter comprises a first material layer formed from an electrically insulating material and a second material layer formed from an electrically conductive material and disposed on a side of the first material layer closest to the sensing element; The X-ray detector is adapted such that, during use, the second material layer is electrically connectable to a reference potential to shield the detection element from an electric field resulting from charge accumulation in the first material layer caused by incident electrons during use.

2. The x-ray detector of claim 1 , wherein the electric field resulting from charge accumulation in the first layer of material is at least one of a static electric field and a transient electric field.

3. The X-ray detector according to claim 1 , wherein the second material layer is formed from a material containing at least one of a metal and carbon.

4. The x-ray detector of claim 3 , wherein the second material layer is formed from a material including a metal, the metal including at least one of aluminum and titanium.

5. 5. The X-ray detector of claim 3, wherein the second layer of material is formed from a material containing carbon in the form of at least one of graphite and amorphous carbon.

6. The X-ray detector of claim 1 , wherein the first material layer is formed from a material including a polymer material.

7. 7. The X-ray detector of claim 1, wherein a material of the second material layer is selected and / or a geometry of the second material layer is adapted to reduce internal transmission of photons generated from cathodoluminescence of the first material layer.

8. 8. An X-ray detector as claimed in claim 1, wherein the first layer of material is provided as a free-standing film.

9. 9. An X-ray detector according to claim 1, wherein the second layer of material is arranged as a coating on a surface of the first layer of material closest to the detector element.

10. 10. An X-ray detector according to claim 1, wherein the second layer of material has an average thickness less than that of the first layer of material.

11. 11. The x-ray detector of claim 1, wherein the detection element is one of a silicon drift detector, a PIN diode detector, a gas proportional counter, a CCD array, and a CMOS array.

12. 12. An X-ray detector as claimed in claim 1, wherein the filter is arranged such that a side of the second layer of material closest to the detector element is in contact with an incidence surface of the detector element.

13. The X-ray detector of claim 1 , further comprising a housing, the filter being arranged to cover an opening in the housing.

14. 14. An X-ray detector as claimed in any preceding claim, wherein the filter comprises a third layer of material arranged on an opposite side of the first layer of material to the second layer of material and formed from a conductive material, the X-ray detector being adapted such that during use the third layer of material is electrically connectable to a reference potential.

15. The x-ray detector of claim 14 , wherein the third material layer is formed from a material including at least one of a metal and carbon.

16. The x-ray detector of claim 15 , wherein the third layer of material is formed from a material including a metal, the metal including at least one of aluminum and titanium.

17. 17. An X-ray detector as claimed in claim 15 or 16, wherein the third layer of material is formed from a material including carbon in the form of at least one of graphite and amorphous carbon.

18. 18. An X-ray detector as claimed in any one of claims 14 to 17, wherein the third layer of material is arranged as a coating on a surface of the first layer of material, on an opposite side of the first layer of material to the second layer of material.

19. 19. An X-ray detector according to any of claims 14 to 18, wherein the third layer of material has an average thickness less than that of the first layer of material.

20. an electron beam assembly for generating a focused electron beam; An X-ray detector according to any one of claims 1 to 19; An apparatus for analyzing a sample comprising:

21. 1. A method for analyzing a sample, comprising the steps of: generating a focused electron beam with an electron beam assembly; Providing an X-ray detector according to any one of claims 1 to 19; Including, the X-ray detector is positioned below a pole piece of the electron beam assembly through which the focused electron beam is emitted toward the specimen, such that X-rays and backscattered electrons generated by an interaction between the focused electron beam and the specimen are incident on the filter of the X-ray detector; The method comprises: connecting the second material layer of the filter to a reference potential; monitoring the energy of each received X-ray photon with the X-ray detector; The method includes:

22. 22. The method of claim 21, comprising connecting the second layer of material to a fixed power supply, preferably wherein the voltage applied to the second layer of material is a negative voltage with respect to a system ground that is equal to or greater than a voltage applied to the incidence surface of the sensing element.

23. 23. The method according to claim 21 or 22, wherein the X-ray detector is an X-ray detector according to any of claims 14 to 19, and the method further comprises the step of connecting the third layer of material to a reference potential.

24. 24. The method of claim 23, comprising connecting the third layer of material to a fixed voltage.

25. 25. The method of claim 23 or 24, wherein the voltage applied to the third material layer is equal to the voltage applied to the second material layer with respect to a system ground.

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