An x-ray inspection apparatus and method for semiconductor wafers and an x-ray shield for the apparatus

Abstract

An x-ray shield 230 for a semiconductor wafer x-ray inspection system, where the shield comprises, a frame 240 defining an internal area and an x-ray shielding material 250 which is supported within t

Description

The present disclosure relates to an x-ray inspection apparatus and method for nondestructive inspection of semiconductor wafers. In particular, the disclosure relates to an x-ray shield for use in an x-ray inspection apparatus for non-destructive inspection of semiconductor wafers. Fabricating integrated circuits is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconductor, typically silicon. The entire manufacturing process, from start to finish, takes six to eight weeks and is performed in highly specialized facilities referred to as fabrication plants. Fabrication plants require many expensive devices to function. Estimates put the cost of building a new fabrication plant over one billion U.S. dollars, with values as high as $3-4 billion not being uncommon. As a result, processing time in a fabrication plant is very valuable. Any time that a fabrication plant is not operating, for example for maintenance of a machine within the plant, is very undesirable. So there is a need for all wafer processing steps to be extremely reliable with minimal maintenance required, and also for all processing steps to be made a quick as possible and to take a little space as possible. As part of making processes reliable and in order to ensure that the circuits being produced operate properly, it is desirable to be able to test wafers for defects and faults at various stages of production. While optical inspection of surface features can be achieved rapidly and reliably, inspection of internal faults, such as voids, cracks and misalignments in deposited conductive elements (such as through silicon vias, copper pillars and bumps) is more difficult. Inspection using x-rays is one possibility for internal faults. In order to provide high magnification x-ray images it is necessary to bring the x-ray source very close to the surface of the semiconductor wafer. However, increasingly, semiconductor chips include components that are sensitive to and can be damaged by x-rays. One example is the high bandwidth memory that is used to implement artificial intelligence systems. Accordingly, there is a need to be able to inspect semiconductor wafers for internal faults at high magnification in a manner that does not damage internal components of the wafer and in a manner that is compatible with fabrication plant processes. In a first aspect of the disclosure, there is provided an x-ray shield for a semiconductor wafer in an x-ray inspection system, comprising: a frame defining an internal area; x-ray shielding material supported within the frame and only partially covering the internal area. The x-ray shielding material only partially covering the internal areas means that x-ray shielding can be provided to cover regions of a semiconductor wafer that need protection from x-rays while exposing the remainder of the semiconductor wafer to x-rays allow images to be formed of the remainder. Customised x-ray shields can be made to correspond to particular semiconductor wafer designs so that individual components, such as high bandwidth memory (HBM) regions, can be protected from potential damage. Preferably, the x-ray shielding material has a thickness of less than 3mm, and more preferably less than 2mm. Even more preferably, the x-ray shielding material had a thickness of less than 1mm. This is advantageous because in use the x-ray shield is positioned between an x-ray source and the semiconductor wafer and a thin x-ray shield allows for the x-ray source to be positioned close to the wafer to provide for high magnification images. The x-ray shielding material may cover an area greater than the region of the semiconductor wafer that requires shielding. The x-ray shielding material may cover an area of up to 10% greater than the region of the semiconductor wafer that requires shielding. Preferably, the x-ray shielding material covers an area less than 5% greater than the region of the semiconductor wafer that requires shielding. Because x-rays may be incident on regions of the semiconductor wafer at an oblique angle, it is advantageous for the shielding material to have an area a little greater than the region of the semiconductor wafer that requires shielding. The x-ray shielding material advantageously absorbs at least 90% of x-rays at an x-ray tube voltage of 70kVp. For example, 1mm of tungsten X-ray shielding material absorbs more than 99.99% of x-rays at an x-ray tube voltage of 70kVp and reduces the Air Kerma dose by a factor of over 200,000 times at an X-ray tube voltage of 70kVp. Preferably the x-ray shielding material comprises at least one of: Tungsten, Lead, Stainless Steel, Tantalum, Rhenium, Platinum, Silver, Tin, Molybdenum, Copper, Gold and Zinc. Preferably, the frame is configured to be handled by an automated semiconductor wafer handing system. Advantageously, the frame may be the same size and shape as the frames used to handle the semiconductor wafers to be inspected. For example, the frame may have the same footprint features as a 300mm film frame. A standard wafer frame may be used as the frame. This allows the x-ray shield to be handled by the same equipment. For example the automated handling system may comprise a dual end effector, where one end effector loads and unloads the semiconductor wafer and one end effector loads and unloads the x-ray shield. The frame may be a tape frame for a semiconductor wafer. The frame may comprise a plastics material. The frame may comprise stainless steel. Advantageously, a stainless steel frame provides a greater strength for a given thickness. The x-ray shield may comprise a tensioned film connected to the frame and covering at least part of the internal area. The x-ray shielding material may be supported on the tensioned film. A tensioned film can provide structural support to a thin layer of x-ray shielding material while being very thin itself. The tensioned film may be a plastic film. The plastic film may be of the type already used in semiconductor wafer manufacturing to hold semiconductor wafers. An example is the film frame rings available from Entegristm Inc., 129 Concord Road, Billerica, MA 01821 USA. The tensioned film may be adhesive. The x-ray shielding material may be mounted on the tensioned film and be held in place by the adhesive nature of the film. The x-ray shield may comprise a glass substrate or a rigid polymer substrate supported by the frame and covering at least part of the internal area. The x-ray shielding material may be supported on the glass substrate or rigid polymer substrate. A glass substrate or rigid polymer substrate can also provide structural support to a thin layer of x-ray shielding material while being very thin itself. Glass and many polymers are also materials that are proven to be compatible with semiconductor manufacturing in a clean room environment. The glass substrate or rigid polymer substrate may comprise a substrate adhesive. The x-ray shielding material may be mounted on the glass substrate or rigid polymer and be held in place by the substrate adhesive. The glass substrate or rigid polymer substrate may be mounted on a tensioned film connected to the frame and covering at least part of the internal area. The frame may be in the form of a plate, sheet or disc of material that extends across the internal area and is substantially x-ray transparent. The frame may comprise carbon fibre reinforced polymer (CFRP). The frame may be a CFRP plate or disc. Advantageously, CFRP has a high strength, so the frame can be made thin. Advantageously, CFRP is substantially x-ray transparent and provides little x-ray attenuation. The frame may comprise a CFRP disc having the same footprint features as a standard tape frame. This allows the frame to be easily handled by standard wafer handling equipment. Using proven materials for semiconductor manufacturing allows for the x-ray shield to be brought very close to, or even to touch, an semiconductor wafer. This advantageous for producing high magnification images. If the frame extends across the internal area, the frame may comprise one or more recesses to hold the x-ray shielding material. The x-ray shielding material may be held within the one of more recesses. The one or more recesses may be recessed from a surface of the frame by a distance substantially equal to the x-ray shielding material thickness. For example, if the x-ray shielding material has a thickness of 0.5mm, the recessed portions are recessed from a surface of the frame by approximately 0.5mm. This configuration allows for high magnification images because the overall thickness of the x-ray shield is minimised and so the x-ray source can be brought close to the wafer. The one or more recesses may be formed by machining. In other embodiments, the recessed portions may be formed by etching. X-ray shielding material may be adhered to at least a portion of each of the recesses . An adhesive may be used to adhere the x-ray shielding material to the recesses. The adhesive may be an epoxy adhesive. The adhesive may be Cyanoacrylate. The x-ray shield may comprise one or more apertures. The frame may comprise one or more apertures. The apertures may be configured to allow x-rays from the x-ray source to reach specific regions of the semiconductor wafer for inspection without any attenuation by the shielding material or frame material. For example, the apertures may be positioned over die regions which are to be inspected using x-rays emitted from the x-ray source. The x-ray shielding material may comprise a foil. The foil may comprise a tungsten foil. The x-ray shield may further comprise an x-ray filtering material supported within the frame. For example, the x-ray filtering material may comprise a zinc foil. The x-ray filtering material may cover all of the internal area. The x-ray filtering material may cover regions of the internal area not covered by the x-ray shielding material. The x-ray filtering material may be aligned with the x-ray shielding material. The x-ray filtering material may be bonded to or overlie the x-ray shielding material. The x-ray filtering material may be between 0.01mm and 2.0mm thick. Preferably, the x-ray filtering material is between 0.1 mm and 1.0mm thick. The x-ray filtering material may be supported by the tensioned film. The x-ray filtering material may be held in place by an adhesive on the tensioned film, on the x-ray shielding material, on a rigid substrate, or on a frame that it is in the form of a plate, sheet or disc of material. An x-ray filtering material may allow enough x-rays through to allow for images to be formed but reduce the number of the x-rays passing through the wafer and thereby reduce potential damage to components of the wafer. The x-ray filtering material may be etched. The x-ray filtering material may be wire eroded. Advantageously, the x-ray shielding material, together with any supporting film, substrate or frame, has a thickness of less than 3mm, and more advantageously less than 2mm. Preferably, the x-ray shielding material, together with any supporting film, substrate or frame has a thickness of less than 1 mm. The x-ray shield may comprise at least one alignment datum for allowing the x-ray shield to be oriented relative to a semiconductor wafer by an automated semiconductor wafer handing system. The x-ray shield needs to be correctly oriented in order that the correct region or regions of the semiconductor wafer are shielded. The at least one alignment datum may be provided on the frame. The at least one alignment datum may be a notch on the frame. The at least one alignment datum may be a marking on the frame. The x-ray shield may be configured to cover portions of a semiconductor wafer in a variety of shapes. The semiconductor wafer may be, circular, square, rectangular or any other 2-dimensional shape. The x-ray shield may be configured to cover an array of diced semiconductor wafer portions. In a second aspect, there is provided an x-ray shield for a semiconductor wafer in an x-ray inspection system comprising: a frame in the form of a plate, sheet or disc of material that is substantially x-ray transparent; the frame comprising one or more recesses; and x-ray shielding material held within the one or more recesses. wherein the frame is configured to be handled by an automated semiconductor wafer handing system. The frame may comprises carbon fibre reinforced polymer (CFRP). The frame may be a CFRP disc. The x-ray shielding material may be adhered to the one or more recesses. An adhesive may be used to adhere the x-ray shielding material to the recesses. The adhesive may be Cyanoacrylate. The frame may comprise one or more apertures. The apertures may be configured to allow x-rays from the x-ray source to reach specific regions of the semiconductor wafer for inspection without any attenuation by the shielding material or frame material. For example, the apertures may be positioned over die regions that are to be inspected using x-rays emitted from the x-ray source. Advantageously, the frame has a maximum thickness of less than 3mm, and more advantageously less than 2mm. Preferably, the frame has a thickness of less than 1mm. The x-ray shielding material may comprise any one or more of: Tungsten, Lead, Stainless Steel, Tantalum, Rhenium, Platinum, Silver, Tin, Molybdenum, Copper, Gold and Zinc. The x-ray shield may further comprise an x-ray filtering material supported within the frame. For example, the x-ray filtering material may comprise a zinc foil. The x-ray filtering material may cover all of the internal area. The x-ray filtering material may cover regions of the internal area not covered by the x-ray shielding material. The x-ray filtering material may be aligned with the x-ray shielding material. The x-ray filtering material may be bonded to or overlie the x-ray shielding material. The x-ray filtering material may be between 0.01mm and 2.0mm thick. Preferably, the x-ray filtering material is between 0.1 mm and 1.0mm thick. The x-ray filtering material may be supported by the tensioned film. The x-ray filtering material may be held in place by an adhesive on the tensioned film, on the x-ray shielding material, on a rigid substrate, or on a frame that it is in the form of a plate, sheet or disc of material. An x-ray filtering material may allow enough x-rays through to allow for images to be formed but reduce the number of the x-rays passing through the wafer and thereby reduce potential damage to components of the wafer. The x-ray shielding material may cover an area greater than the region of the semiconductor wafer that requires shielding. The x-ray shielding material may cover an area of up to 10% greater than the region of the semiconductor wafer that requires shielding. Because x-rays may be incident on regions of the semiconductor wafer at an oblique angle, it is advantageous for the shielding material to have an area a little greater than the region of the semiconductor wafer that requires shielding. The x-ray shield may comprise at least one alignment datum for allowing the x-ray shield to be oriented relative to a semiconductor wafer by an automated semiconductor wafer handing system. The x-ray shield needs to be correctly oriented in order that the correct region or regions of the semiconductor wafer are shielded. The at least one alignment datum may be provided on the frame. The at least one alignment datum may be a notch on the frame. The at least one alignment datum may be a marking on the frame. The x-ray shield may be configured to cover portions of a semiconductor wafer in a variety of shapes. The semiconductor wafer may be, circular, square, rectangular or any other 2-dimensional shape. The x-ray shield may be configured to cover a array of diced semiconductor wafer portions. In a third aspect, there is provided an x-ray shield for a semiconductor wafer in an x-ray inspection system, comprising: a frame defining an internal area; a substrate material supported by the frame and covering at least part of the internal area; x-ray shielding material supported on the substrate material and only partially covering the substrate material. The substrate material may comprise a tensioned film connected to the frame and covering at least part of the internal area. The x-ray shielding material may be supported on the tensioned film. A tensioned film can provide structural support to a thin layer of x-ray shielding material while being very thin itself. The tensioned film may be a plastic film. The plastic film may be of the type already used in semiconductor wafer manufacturing to hold semiconductor wafers. An example is the film frame rings available from Entegris,m Inc., 129 Concord Road, Billerica, MA 01821 USA. The substrate material may comprise a glass substrate supported by the frame and covering at least part of the internal area. The x-ray shielding material may be supported on the glass substrate. A glass substrate can also provide structural support to a thin layer of x-ray shielding material while being very thin itself. Glass is also a material that is proven to be compatible with semiconductor manufacturing in a clean room environment. Advantageously, the x-ray shielding material, together with the substrate material, has a thickness of less than 2mm and preferably less than 1 mm. The x-ray shielding material advantageously absorbs at least 90% of x-rays at an x-ray tube voltage of 70kVp. For example, 1mm of tungsten X-ray shielding material absorbs more than 99.99% of x-rays at an x-ray tube voltage of 70kVp and reduces the Air Kerma dose by a factor of over 200,000 times at an X-ray tube voltage of 70kVp. Preferably the x-ray shielding material comprises at least one of: Tungsten, Lead, Stainless Steel, Tantalum, Rhenium, Platinum, Silver, Tin, Molybdenum, Copper, Gold and Zinc. The x-ray shield may further comprise an x-ray filtering material supported within the frame. For example, the x-ray filtering material may comprises a zinc foil. The x-ray filtering material may cover all of the internal area. The x-ray filtering material may cover regions of the internal area not covered by the x-ray shielding material. The x-ray filtering material may be aligned with the x-ray shielding material. The x-ray filtering material may be bonded to or overlie the x-ray shielding material. The x-ray filtering material may be between 0.01mm and 2.0mm thick. Preferably, the x-ray filtering material is between 0.1mm and 1.0mm thick. The x-ray filtering material may be supported by the tensioned film. The x-ray filtering material may be held in place by an adhesive on the tensioned film, on the x-ray shielding material, on a rigid substrate. An x-ray filtering material may allow enough x-ray through to allow for images to be formed but reduce the number of the x-rays passing through the wafer and thereby reduce potential damage to components of the wafer. Preferably, the frame is configured to be handled by an automated semiconductor wafer handing system. Advantageously, the frame may be the same as the frames used to handle the semiconductor wafers to be inspected. A standard wafer frame may be used as the frame. This allows the x-ray shield to be handled by the same equipment, and in particular the same end effector as is used for handling the semiconductor wafers. The frame may be a tape frame for a semiconductor wafer. The x-ray shielding material may cover an area greater than the region of the semiconductor wafer that requires shielding. The x-ray shielding material may cover an area of up to 10% greater than the region of the semiconductor wafer that requires shielding. Because x-rays may be incident on regions of the semiconductor wafer at an oblique angle, it is advantageous for the shielding material to have an area a little greater than the region of the semiconductor wafer that requires shielding. The x-ray shield may comprise at least one alignment datum for allowing the x-ray shield to be oriented relative to a semiconductor wafer by an automated semiconductor wafer handing system. The x-ray shield needs to be correctly oriented in order that the correct region or regions of the semiconductor wafer are shielded. The at least one alignment datum may be provided on the frame. The at least one alignment datum may be a notch on the frame. The at least one alignment datum may be a marking on the frame. The x-ray shield may be configured to cover portions of a semiconductor wafer in a variety of shapes. The semiconductor wafer may be, circular, square, rectangular or any other 2-dimensional shape. The x-ray shield may be configured to cover an array of diced semiconductor wafer portions. In a fourth aspect of the disclosure, there is provided an x-ray inspection system comprising: an x-ray source; a wafer chuck configured to hold a semiconductor wafer and positioned to receive x-rays from the x-ray source; an x-ray detector positioned on an opposite side of the wafer chuck to the x-ray source; an x-ray shield in accordance with the first or second or third aspect; and an automated semiconductor wafer handing system configured to load the x-ray shield and a semiconductor wafer onto the wafer chuck so that the x-ray shield is positioned between the x-ray source and the wafer. In a fifth aspect of the disclosure, there is provided an x-ray inspection system comprising: an x-ray source; a wafer chuck configured to hold a semiconductor wafer and positioned to receive x-rays from the x-ray source; an x-ray detector positioned on an opposite side of the wafer chuck to the x-ray source; an x-ray shield in accordance with the first or second or third aspect fixed to the wafer chuck; and an automated semiconductor wafer handing system configured to load the semiconductor wafer onto the wafer chuck so that the x-ray shield is positioned between the x-ray source and the semiconductor wafer. The x-ray shield may be fixed to the wafer chuck. For example, the x-ray shield may be fixed to the wafer chuck using screws and can be raised and lowered using lift pads which are comprised within the wafer chuck. When the lift pads raise the x-ray shield, the semiconductor wafer can be positioned between the wafer chuck and the x-ray shield. The x-ray shield can then be lowered for inspection. In a sixth aspect, there is provided a method of imaging a semiconductor wafer using x-rays, comprising: loading a semiconductor wafer onto a wafer chuck in a x-ray inspection machine comprising an x-ray source; providing an x-ray shield on the wafer chuck so that it is positioned between the semiconductor wafer and the x-ray source, wherein the x-ray shield is an x-ray shield according to the first or second aspect; and recording an x-ray image of the semiconductor wafer using the x-ray inspection machine. The step of providing the x-ray shield may comprise loading the x-ray shield onto the wafer chuck. Loading the x-ray shield may comprise orienting the x-ray shield relative to the semiconductor wafer so that the x-ray shielding material covers a portion of the semiconductor wafer that can be damaged by x-rays. The wafer chuck may be a vacuum chuck and loading the x-ray shield comprises clamping the x-ray shield to the semiconductor wafer using the vacuum chuck. The x-ray shield may be loaded onto the wafer chuck after the semiconductor wafer is loaded. The x-ray shield may be fixed to the wafer chuck prior to loading of the semiconductor wafer. The wafer chuck may comprise lift pads configured to raise the x-ray shield before the semiconductor wafer is loaded. The x-ray shield may comprise an alignment datum window. An operator may load the semiconductor wafer to the wafer chuck. The operator may use the alignment datum window to manually the align the alignment datum of the x-ray shield to a wafer loaded on the wafer chuck. The x-ray shield may be screwed into position. As an alternative, an automated loading system can use the alignment datum window to align the alignment datum with the wafer chuck, for example to alignment features on the lift pads, or to an orientation notch on a pre-loaded wafer. The x-ray shield can then be fixed to the lift pads using screws, or using vacuum suction or automated clamps. Embodiments of the disclosure will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an x-ray inspection system for semiconductor wafers; Figure 2 is a close-up perspective view of an x-ray inspection being performed on a semiconductor wafer; Figure 3a shows an x-ray shield according to a first embodiment above a semiconductor wafer; Figure 3b shows an x-ray shield according to a first embodiment after being loaded on a semiconductor wafer; Figure 4 illustrates of a load sequence of the semiconductor wafer and x-ray shield according to a first embodiment; Figure 5 shows an x-ray shield according to a second embodiment; Figure 6 illustrates a load sequence of the semiconductor wafer and x-ray shield according to a second embodiment; Figure 7 shows the relative positions of the x-ray source, shielding material and the shielded component on the sample assembly being inspected; Figure 8a shows an x-ray shield fixed to a wafer chuck according to a third embodiment; Figure 8b is a close up of a section of the x-ray shield and wafer chuck of Figure 8a; Figure 9 illustrates a load sequence of the semiconductor wafer and x-ray shield according to a third embodiment; and Figure 10 shows an x-ray shield according to a fourth embodiment above a semiconductor tray. Figure 11 shows an x-ray shield according to a fifth embodiment. Figure 1 is a perspective view of an x-ray inspection system 10 for semiconductor wafer. The x-ray inspection system 10 comprises an x-ray tube 100, a wafer chuck 200 and a detector 300. The x-ray inspection system 10 is typically located within a cabinet (not shown) that is configured to prevent x-rays generated in the x-ray tube from escaping the system. The wafer chuck 200 is configured to releasably retain a semiconductor wafer. The wafer chuck 200 is mounted to a sample positioning stage 210. The detector 300 is mounted to a detector positioning stage 310 beneath the sample positioning stage. The x-ray tube 100 is fixed to an x-ray tube bracket 115 and is positioned above the wafer chuck 200 and the detector 300. The x-ray tube bracket 115 is provided on a supporting frame 120 above the sample positioning stage 210. In this system, the x-ray tube 100 cannot move relative to the supporting frame 120. Others systems are possible in which the x-ray tube can move relative to the frame, for example in the vertical direction to change magnification. The sample positioning stage 210 and the detector positioning stage 310 are mounted on the supporting frame 120. The sample positioning stage 210 and the detector positioning stage are connected to control circuitry of the x-ray inspection system 10. The x-ray detector is positioned below the wafer chuck to capture x-rays that have passed through the sample. The detector is a flat panel detector that includes a two dimensional pixel array of silicon photodiodes. In order to record different projections through a sample, the detector and sample must be moved accurately to different imaging positions relative to the x-ray source. Plural projections can then be combined using a tomosynthesis algorithm to generate a three-dimensional model of the sample or of a region of the sample. The detector positioning stage 310 is configured to move the detector 300 in the horizontal plane. The detector positioning mechanism 310 comprises a first beam (not shown), extending along the X-axis, on which the detector 300 is supported. The detector positioning stage 310 comprises a detector rail (not shown) extending in the Y-direction, on which the first beam is supported. Motors, such as a servo or stepper motors, are provided to allow the detector 300 to be moved along the first beam and to allow the first beam to be moved along the detector rail. In this manner the detector 300 can be moved to any position within a horizontal plane within the confines of the supporting frame 120. The detector positioning stage 310 provides feedback to the control circuitry of the position of the detector 300, for example using a linear encoder. A tilting mechanism may be connected to the detector 310, the tilting mechanism being configured to allow the active area of the detector to be oriented to face the x-ray source 50 in all imaging positions. The detector in this embodiment is a CMOS flat panel detector, such as the Varex1512 -Varex Imaging West LLC, 1678 S.Pioneer Road, Salt Lake City, UT84104 and the Onyx &Aspire XL from Dage Holdings Limited, 25 Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckingham HP19 8RY, United Kingdom . The sample positioning assembly is used to position samples in the XY plane relative to the x-ray source so that images of desired regions of interest within the sample and different projections of regions of interest can be obtained. It is also desirable to move the wafer chuck in the Z-direction, which is the vertical direction, towards and away from the x-ray source, to alter the magnification of the images. In particular, for semiconductor wafers there is a need to bring the wafer very close to the x-ray source so that very high magnification images can be produced while keeping the overall height of the system within a standard ceiling height and allowing the system to be reasonably easily transportable. The sample positioning stage 210 provides feedback to the control circuitry of the position of the wafer chuck 200, for example using a linear encoder. The sample positioning stage 210 comprises a second beam 214 and a third beam 216, onto which the wafer chuck is mounted 200. The sample positioning mechanism 210 comprises a similar arrangement of motors and rails as the detector positioning stage 310 to move the wafer chuck 200 in the horizontal plane, with the addition of a further motor to move the wafer chuck 200 in a vertical direction towards and away from the x-ray tube 100. This allows for the magnification of the images to be selected. Figure 2 is a close-up perspective view of an x-ray inspection being performed on a semiconductor wafer 220. The semiconductor wafer 220 has a front surface which faces the x-ray source 50 and a rear surface which is opposite to the front surface. The wafer chuck 200 holds the semiconductor wafer 220 in position by suction to the rear surface. This is a well-known wafer handling technique that avoids damage to the wafer. The wafer chuck 200 and the semiconductor wafer 220 form part of a sample assembly 280. The load sequence of the sample assembly 280 and x-ray shield 230 will be described in further detail in the description of Figure 4. In this embodiment, the x-ray tube 100 is a sealed-transmissive type of x-ray tube, such as the NT3 and NT4 x-ray tubes from Dage Holdings Limited, 25 Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckingham HP198RY United Kingdom. This type of x-ray tube provides for a very long service lifetime, typically more than 5000 hours of operation before maintenance is required, as well as very high resolution imaging. A sealed-transmissive type of x-ray tube comprises a fully sealed vacuum tube and a transmission target forming a portion of the exterior wall of the tube. The transmission target is constructed so that electrons impinge on a first side of the target facing towards the interior of the tube and at least some of the x-rays generated are emitted through a second side of the target facing outwardly from the tube. This is sometimes referred to as an end window transmission tube. An end window transmission tube allows for the generation of an x-ray source 50 with small spot size and allows the sample being imaged to be brought close to the x-ray source 50. This means that high magnification and high resolution images can be obtained. By arranging the x-ray tube 100 above the wafer chuck 200 and configuring the wafer chuck to support a semiconductor wafer 220 between the wafer chuck 200 and the x-ray tube 100, the surface of the semiconductor wafer 20 can be brought very close to the x-ray source, allowing for high magnification images to be obtained within a compact system. In an x-ray inspection system as described, x-rays from the x-ray tube 100 not only pass through the semiconductor wafer but must also pass through the wafer chuck 200 before reaching the detector 300. The wafer chuck must therefore be made from a material that does not attenuate the x-rays to too great an extent and does not have a crystalline structure that would diffract x-rays. Suitable materials include polyether ether ketone (PEEK), beryllium, and acetal. While x-rays need to pass through the semiconductor wafer in order to generate images, some semiconductor wafer designs may include components that can be damaged by x-rays. In order to protect those components from damage a shielding structure is provided between the them and the x-ray source. Figure 3a is a schematic of an x-ray shield 230 according to a first embodiment shown above a semiconductor wafer 220. The x-ray shield 230 of Figure 3a may be used on the x-ray inspection system 10 of Figures 1 and 2. The x-ray shield 230 comprises a frame 240 that supports a tensioned film 260. The tensioned film in the embodiment shown in Figure 3a is a plastic film 260. The plastic film 260 supports an x-ray shielding material 250. The x-ray shielding material 250 is held in place by the adhesive nature of the plastic film 260. In use, the x-ray shielding material 250 is positioned between the semiconductor wafer 220 and the x-ray source 50.Figure 3b is a plan view of the x-ray shield 230 a positioned on the semiconductor wafer 220. The x-ray shielding material 250 is configured to cover regions of the semiconductor wafer 220 that need protection. In this embodiment, the regions requiring protection are high bandwidth memory (HBM) regions 275. The regions that require x-ray inspection are die regions 285. The x-ray shielding material 250 in this embodiment comprises a tungsten foil. Tungsten has a high x-ray attenuation, so a thin foil can be used as the x-ray shielding material compared to lower attenuating materials like steel. This allows for high magnification of x-ray images because the semiconductor wafer is still able to be brought very close to the x-ray source 50. The tungsten foil has a thickness of between 0.2-1.0mm. The x-ray shield 230comprises apertures 270 which allow x-rays to reach regions of the semiconductor wafer. The apertures in this embodiment are formed by etching of the x-ray shielding material 250. The apertures 270 match the layout of a particular semiconductor wafer to be tested. The x-ray shielding material 250 is supported within the frame on the substrate material 260. In the embodiment of Figures 3a and 3b, the substrate material 260 is a plastic film held under tension by the frame 240. Plastic film is commonly used in semiconductor wafer manufacturing to hold semiconductor wafers. Example plastic films are polyethylene and polyolefin. In other embodiments, the substrate material 260 may be glass or a rigid polymer. The glass or rigid polymer substrate is similarly supported by the wafer frame. In other embodiments, instead of a foil sheet with apertures, the x-ray shielding material may be provided on a substrate, such as tensioned plastic or glass, as one or more patches of material for covering particular regions of an underlying semiconductor wafer. Figure 3b illustrates the x-ray shield 230 overlying the semiconductor wafer 220.. The x-ray shielding material 250 is positioned on the substrate material 260 relative to the frame 240 so the HBM regions 275 of the semiconductor wafer 220 can be shielded. The frame 240 comprises at least one alignment datum 295, in this example in the form of notches. The semiconductor wafer 220 also comprises an orientation notch 265.The orientation notch 265 and the alignment datum 295 are used by an automated wafer handling system, including optical cameras and an aligner, to align the semiconductor wafer 220 relative to the x-ray shield 230. Once the x-ray shield 230 is aligned with the semiconductor wafer 220 and loaded onto the wafer chuck 200, the HBM regions 275 are shielded by the x-ray shielding material 250. The apertures 270 expose die regions 285 of the semiconductor wafer 220 to the x-ray source 50 for inspection. The design of an x-ray shield 230 is bespoke for each unique semiconductor wafer. Figure 4 illustrates a load sequence of the semiconductor wafer 220 and x-ray shield 230. There are six stages 500, 510, 520, 530, 540, 550 to the load sequence. The wafer chuck 200 is a vacuum chuck 200. The wafer chuck 200 comprises vacuum ports (not shown) to retain components as described below. The wafer chuck 200 further comprises lift pads 225 onto which the x-ray shield 230 is loaded. End effectors 190,191 form part of an automated semiconductor wafer handling system configured to load and unload the semiconductor wafer 220 and x-ray shield 230 from the wafer chuck 200. In the first stage 500, the wafer chuck 200 is empty and ready for loading. In the second stage 510, an end effector 190, holding the semiconductor wafer 220, loads the semiconductor wafer 220 to the wafer chuck 200. Prior to being loaded to the wafer chuck 200, the semiconductor 220 is placed onto an aligner (not shown). The aligner aligns the semiconductor wafer 220 using the orientation notch 265 as a reference point. In the third stage 520, the end effector 190 retracts away from the wafer chuck 200. Vacuum ports underneath the rear surface of the semiconductor wafer are turned on to clamp the semiconductor wafer 220 in place. In the fourth stage 530, the end effector 191 loads the x-ray shield 230 on to lift pads 225. The lift pads 225 extend further from the wafer chuck 200 than the thickness of the semiconductor wafer 220. The x-ray shield 230 is placed above the semiconductor wafer 220. Prior to being loaded to the wafer chuck 200, the x-ray shield 230 is placed onto the aligner. The aligner aligns the x-ray shield 230 using the at least one alignment datum 295 as a reference point. In the fifth stage 540, the end effector 191 retracts away from the wafer chuck 200. In the sixth stage 550, the lift pads 225 actuate down towards the wafer chuck 200, lowering the x-ray shield 230. Vacuum ports underneath the frame 240 of the x-ray shield 230 are turned on to clamp the x-ray shield 230 in place, ready for inspection. The unload sequence of the semiconductor wafer 220 and the x-ray shield 230 follow the six steps 500, 510, 520, 530, 540, 550 in reverse. Figure 5 is a schematic of an x-ray shield 1230 according to a second embodiment. The design of the x-ray shield 1230 is bespoke for each unique semiconductor wafer design. However, it is advantageous that the x-ray shield has the same footprint features as a standard wafer tape frame so that the same standard wafer handling equipment can be used to handle the x-ray shield. The x-ray shield 1230 according to the second embodiment may be configured to shield the same semiconductor wafer design 220 as the first embodiment. The x-ray shield 1230 may be used with the x-ray inspection system 10 of Figures 1 and 2. The x-ray shield 1230 according to the second embodiment has the same function as the x-ray shield 230 according to the first embodiment. That is, to cover regions of a semiconductor that need protection. For example, HBM regions that are sensitive to x-ray radiation require protection from an x-ray shield. The x-ray shield 1230 comprises a frame 1240 that supports an x-ray shielding material 1250. In use, the x-ray shielding material 1250 is positioned between the semiconductor wafer and the x-ray source 50. The x-ray shielding material 1250 comprises a tungsten foil. Tungsten has a high x-ray attenuation, so a thin foil can be used as the x-ray shielding material compared to lower attenuating materials like steel. This allows for high magnification of x-ray images because the semiconductor wafer is able to be brought very close to the x-ray source 50. The tungsten foil has a thickness of between 0.2 and1.0mm. The frame 1240 is a carbon fibre reinforced polymer (CFRP) disc. The frame 1240 comprises one or more recesses 1255 configured to retain the x-ray shielding material 1250. The one or more recesses 1255 are recessed from a surface of the frame 1240 by a distance substantially equal to the x-ray shielding material thickness. For example, if the x-ray shielding material 1250 has a thickness of 0.5mm, the recessed portions 1255 are recessed from the surface of the frame 1240 by approximately 0.5mm. The one or more recesses 1255 are formed by machining. In other embodiments, the recessed portions may be formed by etching. The x-ray shielding material 1250 is adhered to the recessed portions 1255. An adhesive may be used to adhere the x-ray shielding material 1250 to the recessed portions 1255, for example, Cyanoacrylate. The frame 1240 comprises apertures 1270 which allow x-rays to reach regions of the semiconductor wafer. The apertures are formed by etching of the CFRP disc 1240. The apertures 1270 match the layout of a particular semiconductor wafer to be inspected. The CFRP disc 1240 comprises an alignment datum 1295 which is used by an automated handling system to align with an orientation notch 265 on the semiconductor wafer. Once the x-ray shield 230 is aligned with and loaded onto the semiconductor wafer 220, the HBM regions 275 are shielded by the x-ray shielding material 1250. The apertures 1270 expose die regions 285 of the semiconductor wafer to the x-ray source 50 for inspection. Figure 6 illustrates a load sequence of the semiconductor wafer 220 and x-ray shield 1230 according to the second embodiment. The load sequence of the of the semiconductor wafer 220 and x-ray shield 1230 according to the second embodiment is substantially identical to the load sequence of the semiconductor wafer 220 and x-ray shield 230 according to the first embodiment. In the first stage 1500, the wafer chuck 200 is empty and ready for loading. In the second stage 1510, an end effector 1190, holding the semiconductor wafer 220, loads the semiconductor wafer 220 on the wafer chuck 200. Prior to being loaded to the wafer chuck 200, the semiconductor 220 is placed onto an aligner. The aligner aligns the semiconductor wafer 220 using the orientation notch 265 as a reference point. In the third stage 1520, the end effector 1190 retracts away from the wafer chuck 200. Vacuum ports underneath the rear surface of the semiconductor wafer are turned on to clamp the semiconductor wafer 220 in place. In the fourth stage 1530, the end effector 1191 loads the x-ray shield 1230 on to lift pads 225. The lift pads extend further from the wafer chuck 200 than the thickness of the semiconductor wafer 220. The x-ray shield 1230 is placed above the semiconductor wafer 220. Prior to being loaded to the wafer chuck 200, the x-ray shield 1230 is placed onto the aligner. The aligner aligns the x-ray shield 230 using the at least one alignment datum 1295 as a reference point. In the fifth stage 1540, the end effector 1191 retracts away from the wafer chuck 200. In the sixth stage 1550, the lift pads actuate down towards the wafer chuck 200, lowering the x-ray shield 1230. Vacuum ports underneath the frame 1240 of the x-ray shield 1230 are turned on to clamp the x-ray shield 1230 in place, ready for inspection. Figure 7 is a diagram illustrating the relative positions of the shielding material, the x-ray source and the HBM regions in use. The diagram shows a portion of the x-ray shield 1230 according to the second embodiment covering a portion of the semiconductor wafer 220. The x-ray shield 1230 is positioned between the x-ray source 50 and the semiconductor wafer 220. The portion of the semiconductor wafer 220 comprises a HBM region 1275 and a die region 1285. The die region 1285 is inspected using x-rays emitted from the x-ray source 50. The HBM region 1275 needs to be protected from x-rays emitted from the x-ray source 50. The x-ray shield 1230 comprises a recessed portion 1255 that accommodates the x-ray shielding material 1250. The thickness of the x-ray shielding material 1250 matches the depth of the recessed portion 1255. The x-ray shielding material 1250 covers the HBM portion 1275 of the semiconductor wafer 220. The x-ray shielding material 1250 has a width greater than the width of the HBM region 1275. As shown by the angle 0 in the diagram, the rays from the x-ray source 50 may travel at an oblique angle towards the semiconductor wafer 220. Therefore, the x-ray shielding material 1250 is dimensioned to ensure that the HBM regions are not directly exposed to the x-ray beam emitted from the x-ray source 50. This same principle can be applied to the x-ray shield of the first embodiment. Figure 8a shows an x-ray shield 2230 fixed to a wafer chuck 2200 according to a third embodiment. The x-ray shield 2230 according to the third embodiment the same as the x-ray shield 1230 of the second embodiment. Figure 8b is a close up of a section of the x-ray shield 2230 and wafer chuck 2200 of Figure 8a. The x-ray shield 2230 comprises an alignment datum window 2298.An operator loads the semiconductor wafer 220 to the wafer chuck 2200. The operator uses the alignment datum window 2298 to manually align the alignment datum 2295 of the x-ray shield 2230 to the orientation notch 265 of a semiconductor wafer 220 pre-loaded onto the wafer chuck. Once the x-ray shield 2230 and the semiconductor wafer 220 are aligned, the x-ray shield 2230 is screwed into position. The frame 2240 of the x-ray shield include apertures 2215 that align with corresponding apertures and screws 2220 on the lift pads. The screws 2220 pass through the alignment apertures 2215 on the frame to fix the frame to the lift pads of the wafer chuck. The pre-loaded wafer can then be removed. As an alternative, optical cameras and a manipulator use the alignment datum window 2298 to align the alignment datum 2295 with the wafer chuck, and in particular to features on the lift pads 2210, or to an orientation notch on a pre-loaded wafer. The x-ray shield can then be fixed to the lift pads using screws, or using vacuum suction or automated clamps for example. Figure 9 is a schematic of a load sequence of the semiconductor wafer 220 and x-ray shield 2230 according to a third embodiment. The wafer chuck 2200 is a vacuum chuck. The wafer chuck 2200 further comprises lift pads 2210, onto which the x-ray shield 230 is fixed as described with reference to Figures 8a and 8b. In a first stage 2500, the lift pads of the wafer chuck 2200 are raised. The lift pads 2210 hold the x-ray shield 2230 in a raised position. In the second stage 2510, an end effector 2190, holding the semiconductor wafer 220, loads the semiconductor wafer 220 to the wafer chuck 2200 so the semiconductor wafer 200 is positioned between the wafer chuck 2200 and the x-ray shield 2230. In the third stage 2520, the end effector 2190 retracts away from the wafer chuck 2200. Vacuum ports underneath the rear surface of the semiconductor wafer 220 are turned on to clamp the semiconductor wafer 220 in place. In the fourth stage, the lift pads actuate down towards the wafer chuck 2200, lowering the x-ray shield 2230. Vacuum ports underneath the x-ray shield 2230 are turned on to clamp the x-ray shield 2230 in place, ready for inspection. Figure 10 is a schematic of an x-ray shield 3230 according to a fourth embodiment shown above a semiconductor carrier tray 3220. The carrier tray 3220 comprises diced portions of a semiconductor wafer, comprising HBM regions 3275 and die regions 3285. The x-ray shield 3230 is configured to cover the HBM regions 3275 that are sensitive to x-ray radiation. The die regions 3285 of the semiconductor tray 3220 are inspected using x-rays emitted from an x-ray source 50. The x-ray shield 3230 comprises a frame 3240 that supports the x-ray shielding material 3250. The x-ray shielding material 3250 comprises a tungsten foil. The frame 3240 is a CFRP frame. The frame comprises one or more recesses 3255 configured to retain the x-ray shielding material 3250. The one or more recesses 3255 are recessed from a surface of the frame 3240 by a distance substantially equal to the x-ray shielding material 3250 thickness. For example, if the x-ray shielding material 3250 has a thickness of 0.5mm, the recessed portions 3255 are recessed from the surface of the frame 3240 by approximately 0.5mm. The one or more recesses 3255 may be formed by machining. The x-ray shield 3230 comprises apertures 3270 which allow x-rays to reach the die regions on the tray 3230. The apertures are formed by etching or machining of the CFRP frame 3240. The CFRP frame 3240 comprises an alignment datum 3295 which is used to align with an orientation notch 3265 on the carrier tray 3220. Once the x-ray shield 3230 is aligned with and loaded onto the carrier tray 3220, the HBM regions 3275 are shielded by the x-ray shielding material 3250. The apertures 3270 expose die regions 3285 to the x-ray source 50 for inspection. Figure 11 shows an x-ray shield 4230 according to a fifth embodiment. The x-ray shield 4230 according to the fifth embodiment is similar to the x-ray shield 230 of the first embodiment, but it further comprises an x-ray filtering material 4245 overlying the tungsten shielding material and the apertures. The x-ray shield 4230 comprises a frame 4240 that supports a tensioned film 4260. The x-ray shielding material 4250 is adhered to the tensioned film and includes apertures corresponding to areas of the wafer that need to be imaged. The x-ray filtering material 4245 overlies the x-ray shielding material 4250 and the apertures in the x-ray shielding material. The x-ray filtering material 4245 is bonded to the x-ray shielding material 4250. In the embodiment shown in Figure 11, the x-ray filtering material 4245 covers substantially the same area as the x-ray shielding material 4250. There may be embodiments where the x-ray filtering material covers a different area to the x-ray shielding material and includes one or more apertures. The x-ray filtering material 4245 in this embodiment is a zinc foil about 0.15mm thick. The zinc foil allows enough x-ray through to allow for images to be formed in the apertures but reduces the number of the x-rays passing through and thereby reduces potential damage to x-ray sensitive regions of the semiconductor wafer that need to be imaged.

Claims

1. An x-ray shield for a semiconductor wafer in an x-ray inspection system, comprising:a frame defining an internal area;x-ray shielding material supported within the frame and only partially covering the internal area, wherein the x-ray shielding material has a thickness of less than 2mm.

2. An x-ray shield according to claim 1, wherein the frame is configured to be handled by an automated semiconductor wafer handing system.

3. An x-ray shield according to claim 1 or claim 2, comprising a tensioned film connected to the frame and covering at least part of the internal area, wherein the x-ray shielding material is supported on the tensioned film.

4. An x-ray shield according to claim 1, wherein the tensioned film is a plastic film.

5. An x-ray shield according to claim 1 or claim 2, comprising a glass, polymer or composite substrate supported by the frame and covering at least part of the internal area, wherein the x-ray shielding material is supported on the substrate.

6. An x-ray shield according to any preceding claim wherein the frame is a tape frame for a semiconductor wafer.

7. An x-ray shield according to claim 1 or claim 2, wherein the frame is a in the form of a plate, sheet or disc of material that extends across the internal area and is substantially x-ray transparent.

8. An x-ray shield according to claim 7, wherein the frame comprises carbon fibre reinforced polymer.

9. An x-ray shield according to claim 7 or 8, wherein the frame comprises one or more recesses and wherein the x-ray shielding material is held within the one or more recesses.

10. An x-ray shield according to claim 9, wherein the x-ray shielding material is adhered to the one or more recesses.

11. An x-ray shield according to any one of claims 7 to 10, wherein the frame comprises one or more apertures.

12. An x-ray shield according to any preceding claim, wherein the x-ray shielding material comprises a foil.

13. An x-ray shield according to any preceding claim, wherein the x-ray shielding material comprises any one or more of: Tungsten, Lead, Stainless Steel, Tantalum, Rhenium, Platinum, Silver, Tin, Molybdenum, Copper, Gold and Zinc.

14. An x-ray shield according to any preceding claim wherein the frame comprises at least one alignment datum for allowing the x-ray shield to be oriented relative to a semiconductor wafer by an automated semiconductor wafer handing system.

15. An x-ray shield for a semiconductor wafer in an x-ray inspection system comprising:a frame in the form of a plate, sheet or disc of material that is substantially x-ray transparent;the frame comprising one or more recesses; andx-ray shielding material held within the one or more recesses.

16. An x-ray shield according to claim 15, wherein the frame is a configured to be handled by an automated semiconductor wafer handing system.

17. An x-ray shield according to claim 16, wherein the frame comprises carbon fibre reinforced polymer.

18. An x-ray shield according to any one of claims 15 to 17, wherein the x-ray shielding material is adhered to the one or more recesses.

19. An x-ray shield according to any one of claims 15 to 19, wherein the frame has a thickness of less than 3mm.

20. An x-ray shield according to any one of claims 15 to 20, wherein the x-ray shielding material comprises any one or more of: Tungsten, Lead, Stainless Steel, Tantalum, Rhenium, Platinum, Silver, Tin, Molybdenum, Copper, Gold and Zinc.

21. An x-ray inspection system comprising:an x-ray source;a wafer chuck configured to hold a semiconductor wafer and positioned to receive x-rays from the x-ray source;an x-ray detector positioned on an opposite side of the wafer chuck to the x-ray source;an x-ray shield in accordance with any one of claims 1 to 23; andan automated semiconductor wafer handing system configured to load the x-ray shield and a semiconductor wafer onto the wafer chuck so that the x-ray shield is positioned between the x-ray source and the wafer.

22. An x-ray inspection system comprising:an x-ray source;a wafer chuck configured to hold a semiconductor wafer and positioned to receive x-rays from the x-ray source;an x-ray detector positioned on an opposite side of the wafer chuck to the x-ray source;an x-ray shield in accordance with any one of claims 1 to 20 fixed to the wafer chuck; andan automated semiconductor wafer handing system configured to load the semiconductor wafer onto the wafer chuck so that the x-ray shield is positioned between the x-ray source and the semiconductor wafer.

23. A method of imaging a semiconductor wafer using x-rays, comprising:loading a semiconductor wafer onto a wafer chuck in a x-ray inspection machine comprising an x-ray source;providing an x-ray shield on the wafer chuck so that it is positioned between the semiconductor wafer and the x-ray source, wherein the x-ray shield is an x-ray shield according to any one of claims 1 to 20; andrecording an x-ray image of the semiconductor wafer using the x-ray inspection machine.

24. A method of imaging a semiconductor wafer according to claim 23, wherein the step of providing the x-ray shield comprises loading the x-ray shield onto the wafer chuck.

25. A method of imaging a semiconductor wafer according to claim 24, wherein loading the x-ray shield comprises orienting the x-ray shield relative to the semiconductorwafer so that the x-ray shielding material covers a portion of the semiconductor wafer that can be damaged by x-rays.Application No: GB2409727.1Examiner:James HoggClaims searched: 1-25Date of search: 26 November 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-5, 7,21-22 US 2020 / 0303163 Al (MATSUSHITA et al.) See paragraphs [0032], [0044] and figure 2 X 1-5, 7,21-22 WO 2023 / 181700 Al (SUMITOMO HEAVY INDUSTRIES ION TECH CO LTD) See WPI Accession No. 2023-A1861H X 1, 3-5 US 4024405 A (SZOT) See column 2 lines 25-28 and column 3 lines 15-29 X 1 CN 204863251 U (UNIV PLA 3RD MILITARY MEDICAL) See WPI Accession No. 2015-83925D X 1-2 JP 7239285 B2 (SEMES CO LTD) See WPI Accession No. 2020-192811Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of before the filing date of this invention. same category. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB, EP, WO &US patent documents classified in the following areas of the UKCX :International Classification:Subclass Subgroup Valid From G21F 0007 / 06 01 / 01 / 2006 GOIN 0023 / 20 01 / 01 / 2018Application No: GB2409727.1Examiner: James HoggClaims searched: 15-25Date of search: 19 March 2025Patents Act 1977Further Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance v A 15-16, 18, 20-24 US 2015 / 0103974 Al (KIM et al.) See paragraphs [0017],[0066], [0072], [0091], [0120], [0126], fig 1, fig 35 X 15-16, 18, 20-24 KR 20180111429 A (SEMES CO LTD) See WPI abstract accession no. 2018-79562N X 15, 18-20 IP 2001188096 A (SHIMADZU CORP) See WPI abstract accession no. 2001-525623 X 15, 18-20 JP 2001153961 A (SHIMADZU CORP) See WPI abstract accession no. 2002-069063 X 15, 18-20 JP 2001153960 A (SHIMADZU CORP) See WPI abstract accession no. 2001-446794 v A 15, 20 US 2014 / 0334604 Al (TESHIMA et al.) See paragraphs [0036], [0077], fig la, fig 4dCategories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From G21F 0007 / 06 01 / 01 / 2006 GOIN 0023 / 20 01 / 01 / 2018

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