Patent Description:
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. dollars, with values as high as $<NUM>-<NUM> 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. Current methods for detecting these faults require taking a wafer out of the fabrication plant and testing using a focussed ion beam, scanning electron microscope or x-rays. However, as soon as a wafer is taken out of the clean environment of the fabrication plant it is effectively destroyed and can no longer be used.

<CIT> discloses a system for imaging a semiconductor wafer in order to detect defects. <CIT> discloses apparatus for measuring the film thickness of a thin film formed on a substrate, such as a semiconductor wafer.

<CIT> discloses an x-ray inspection method and apparatus for performing an inspection of semiconductor wafers based on an x-ray image. The x-ray inspection apparatus comprises an x-ray imaging apparatus <NUM> comprising an x-ray source <NUM>, a stage <NUM> and an x-ray camera <NUM> and is connected to an x-ray image processing apparatus <NUM>. The x-ray source <NUM> is provided on the opposite side of the stage <NUM> to the x-ray camera <NUM>. The x-ray source irradiates a silicon wafer placed on the stage <NUM> and an x-ray image of the silicon wafer is captured by the x-ray camera <NUM>. The output of the x-ray camera <NUM> is input to an imaging control unit <NUM> of the image processing apparatus <NUM>. The x-ray source <NUM> is positioned above the stage <NUM>.

It would be desirable to be able to accurately test semiconductor wafers for voids, cracks and misalignments in deposited conductive elements, in a more efficient and non-destructive manner. It would also be desirable to be able to test semiconductor wafers for voids, cracks and misalignments in deposited conductive elements quickly, in a manner that does not lead to significantly increased processing time for semiconductor wafers.

The invention is defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are defined in dependent claims. Aspects, embodiments or examples falling outside the scope of the appended independent claims are not part of the invention, and are merely included for illustrative or explanatory purposes.

In a first aspect of the disclosure there is provided an x-ray inspection system comprising:.

The cabinet may be constructed to provide an x-ray shield, and may be lead lined for this purpose. It is desirable from a safety perspective to minimise the escape of x-rays from the system.

An x-ray system of this type can be used in a clean room environment to inspect items such as semiconductor wafers. By maintaining a flow of air from above the sample support, past the sample support to an outlet below the sample support, the sample can be protected from any dust and debris. The system is configured such that the flow of air is maintained throughout the operation of the system, i.e. while samples are loaded, imaged, moved and unloaded from the system. The system is configured to provide at least a Class <NUM> ISO <NUM>-<NUM> cleanroom environment.

The x-ray source is preferably a sealed x-ray tube. Sealed x-ray tubes require much less maintenance than open x-ray tubes and so are better suited to use in a semiconductor fabrication plant where any time that operation of the plant is stopped for maintenance is very expensive.

The x-ray source preferably comprises a transmission target. The use of a transmission target allows for a very small spot size x-ray source and allows for high magnification within a compact system because the sample being imaged can be brought close to the target. A sealed, transmission target, x-ray tube is particularly advantageous for semiconductor wafer inspection, as this type of x-ray tube can provide high resolution images, good reliability and long maintenance cycles.

The system is configured to perform transmission microscopy. The x-ray detector may be configured to measure attenuation of a primary incident x-ray beam from the x-ray source, to provide a two dimensional image of the sample or a region of the sample.

The high performance air filter may be a high-efficiency particulate absorption (HEPA) filter or ultra-low penetration air (ULPA) filter. An air filter of this type ensures that the air flow entering the cabinet through the inlet is free from dust.

The air mover is positioned within the cabinet. In this embodiment, the air filter may be positioned between the air mover and the sample support.

Providing the air mover within the cabinet allows for the production of a compact system. However, in examples not forming part of the claimed invention, it is possible to place the air mover outside of the cabinet. The system may comprise a plurality of air movers and a plurality of air filters. Ideally the system is configured so that air flow within the cabinet is uniform and laminar and in a downward direction, without any recirculation of air. The number of air movers used can be chosen to suit the geometry of the system components within the cabinet to achieve laminar flow. In a preferred embodiment, the system comprises two air movers and two associated air filters.

The fan filter unit may comprise an enclosure having an air inlet, a fan within the enclosure and configured to draw air in through the air inlet, an air outlet and a filter plate spanning the air outlet so that air exiting through the air outlet is forced through the filter plate. The fan filter unit may be configured to provide a higher pressure within the enclosure than external to the enclosure. Providing a higher pressure within the fan filter unit improves the uniformity of the air flow through the air filter, which is desirable to prevent any air recirculation within the cabinet.

The baffle plate is advantageously configured to provide for a uniform air flow through the filter plate. The fan may be located at a centre of the enclosure and the baffle plate may be configured to direct air from the fan to the extremities of the enclosure. The enclosure may have external walls, and the baffle plate may be configured to direct air from the fan towards the external walls.

Placing the x-ray source above the support allows a sample on the top of the support, and in particular a top surface of the sample, to be brought close to the x-ray source. This is advantageous for providing high magnification images in a compact system. Since the x-ray source is located above the sample it is stationary during operation to prevent the generation of any dust or debris from any movement mechanism that might damage the sample. Another reason to keep the x-ray source stationary is that it is a relatively bulky and massive component. It also typically requires very large power cables which are relatively inflexible and difficult to move.

The x-ray inspection system may comprise a controller including an image processor. The image processor may be connected to the x-ray detector to receive data from the x-ray detector.

The x-ray inspection system may comprise a first positioning assembly that allows for relative movement between the sample support and the x-ray source, wherein the positioning mechanism is located below the sample support. The first positioning assembly may comprise a first horizontal sample positioning mechanism for moving the sample support in a first horizontal direction, a second horizontal sample positioning mechanism for moving the sample support in a second horizontal direction, and a vertical sample positioning mechanism for moving the sample support in a vertical direction.

In a preferred embodiment, the first positioning assembly is configured so that the first horizontal positioning mechanism moves the sample support and the vertical positioning mechanism in a first horizontal direction. The vertical positioning mechanism may be configured to move both the second horizontal positioning mechanism and the sample support in the vertical direction. The first horizontal positioning mechanism is preferably mounted directly to a supporting frame. This arrangement is advantageous for a system that performs a raster scan of the sample in the horizontal plane. The scan lines of the raster scan extend in the second horizontal direction so the second horizontal positioning mechanism is required to operate over the longest distance, frequently and fast. Accordingly the second horizontal positioning mechanism is configured to move only the sample support and not the mass of any of the other positioning mechanisms. The first horizontal positioning mechanism is also required to move fast and frequently compared with the vertical positioning mechanism. By mounting the first horizontal positioning mechanism directly to a supporting frame, movement in the first horizontal direction can be made fast and accurate. The vertical positioning mechanism alters the image magnification and is required to move relatively infrequently, over a relatively shorter distance that the horizontal positioning mechanisms, and typically not at all during a raster scanning operation. The vertical positioning assembly can be made relatively less massive than the horizontal positioning mechanisms.

The supporting frame, to which the first horizontal positioning mechanism is fixed, may be mounted to the floor. In a preferred embodiment, the supporting frame comprises a first rigid sub-frame configured to be fixed to the floor, and a second rigid sub-frame supported on the first sub-frame through a damping mechanism, with the first horizontal positioning mechanism fixed to the second rigid sub-frame.

The x-ray inspection system may comprise a sample support position detection assembly comprising non-contact position measuring device, such as a laser interferometer, positioned adjacent to the sample support and configured to detect a position or change in position of the sample support. This is particularly advantageous for determining precisely the position of the sample within the horizontal plane, which is required when producing very high magnification images and using them to produce a three dimensional model. In a preferred embodiment, the system comprises two non-contact position measuring devices, preferably laser interferometers. A first non-contact position measuring device for detecting changes in position of the sample support in the first horizontal direction and a second non-contact position measuring device for detecting changes in position of the sample support in the second horizontal direction. Of course, a pair of non-contact position measuring devices could be arranged to detect changes in position in different directions within a horizontal plane than the first and second horizontal directions. Other possible non-contact position measuring devices include optical linear encoders, magnetic encoders, capacitive sensors and sonar distance measuring devices.

The position information provided by the non-contact position measuring device or devices may be used by the image processor. In particular, the change of position of the sample from image to image provided by the non-contact position measuring device or devices can be used in a tomosynthesis calculation. Precise positional information is required when producing a three dimensional model of very small features such as voids in a semiconductor wafer, at very high magnification. The more precise the positional information for the sample, the better the image resolution.

The x-ray inspection system may comprise a proximity sensor fixed to the x-ray source configured to provide a measurement of distance between the x-ray source and a surface of a sample on the sample support. The proximity sensor may be a laser position sensor or a confocal sensor. The proximity sensor may be connected to the image processor to provide distance data to the image processor. The image processor may use the measurement of distance from the proximity sensor in an image processing calculation, such as a tomosynthesis calculation.

The controller may be connected to the sample positioning assembly and may control the sample positioning assembly based on the measurement of distance provided by the proximity sensor. The proximity sensor provides an accurate distance measurement between the x-ray source and a top surface of a sample, which can be used both in an image processing calculation, such as a magnification calculation, and to prevent any collision of the sample and the x-ray source. To provide a useful inspection of small features in a semiconductor wafer in a compact system, the sample is brought very close to the x-ray source, but any collision between the sample and the x-ray source would likely damage both the sample and the x-ray source. Avoiding such collisions, while bringing the sample very close to the x-ray source, is therefore necessary.

The sample positioning assembly may comprise a linear encoder. The controller may be configured to calibrate the linear encoder based on the measurement of distance provided by the proximity sensor.

The x-ray inspection system may further comprise a second positioning assembly that allows for relative movement between the x-ray detector and the x-ray source, wherein the second positioning mechanism is located below sample support. The second positioning assembly may comprise a horizontal detector positioning mechanism for moving the detector in at least two non-parallel directions within a horizontal plane. The detector positioning mechanism may comprise a detector tilting mechanism configured to allow the detector to be tilted from the horizontal plane about at least two non-parallel axes. The detector can then be tilted so that an imaging surface of the detector is always normal to a line between a centre of the detector and the output spot of the x-ray source, wherever the detector is positioned. The controller may be connected to the second positioning assembly.

The first and second positioning assemblies are advantageously positioned below the sample support. The ability to move the sample and detector relative to the x-ray source allows for different portions of the sample to be imaged and different angles of inspection to be used. A series of images of the same sample, or the same portion of a sample, can be used in a tomosynthesis system to generate three dimensional models and images of the sample and accurately locate and measure cracks, voids and other defects.

By locating a fixed x-ray source above the sample, and the positioning assemblies beneath the sample, all the moving components of the system can be located underneath the sample. With a downwardly directed airflow, this reduces the likelihood of any debris generated by the moving parts reaching and damaging the sample.

The system may comprise a perforated deck positioned below the x-ray source but above the first and second positioning assemblies. The perforated deck and air mover are configured to provide a first air pressure above the deck and a second air pressure below the deck, wherein the second air pressure is lower than the first air pressure. The perforated deck is preferably positioned level with the height of the sample support when the sample support is in its uppermost position (which corresponds to maximum magnification). Even a small pressure difference between the space above the sample and the space below the sample prevents any significant flow of air from below the sample to above the sample.

The air inlet advantageously comprises a labyrinthine air flow path. This ensures that x-rays from the x-ray source cannot escape from the cabinet through the air inlet. The air outlet is preferably large to minimise any recirculation of air.

The sample support is configured to support the sample during x-ray inspection. In one embodiment, the sample support is configured to support a semiconductor wafer.

The ability to inspect semiconductor wafers for internal features in a non-destructive fashion during wafer processing is highly desirable. By providing a system in which air flow is directed through the cabinet from above the wafer to below the wafer, while the cabinet still provides the required x-ray shielding, this becomes possible.

The use of air filters, such as those found in fan filter units, a perforated deck level with the sample support, and the positioning of the x-ray source in a fixed position above the sample support, ensure that clean room standards can be met. Further advantageous features, such as the use of a sealed transmission target x-ray tube, provide the required reliability and image quality for a commercially attractive system.

In a second aspect of the disclosure, there is provided a method of inspecting a semiconductor wafer using an x-ray inspection system, the x-ray inspection system comprising a cabinet, the cabinet containing a sample support for supporting the semiconductor wafer, an x-ray source configured to direct x-rays at the semiconductor wafer on the sample support, and an x-ray detector for detecting x-rays that have passed through the semiconductor wafer to provide a two-dimensional image of the semiconductor wafer or a region of the semiconductor wafer, the method comprising:.

The air flow preferably comprises HEPA or ULPA filtered air.

By providing a continuous flow of clean air past the wafer, with no recirculation of air that might have picked up dust or debris from mechanisms within the system, the inspection system can meet clean room standards and the risk of contamination of or damage to the semiconductor wafer is minimised.

Features described in relation to the first aspect of the disclosure may be applied to the second aspect of the disclosure.

Embodiments of the disclosure will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:.

<FIG> is a schematic illustration of the basic elements of an x-ray imaging system. The system shown in <FIG> comprises an x-ray source <NUM>, which in this system is held stationary, a moveable sample support <NUM> and a moveable detector <NUM>. X-rays from the x-ray source <NUM> pass through the support and any sample mounted on the sample support, and impinge on the detector <NUM>. <FIG> illustrates the areas <NUM> on the sample support corresponding to the field of view of the detector <NUM>. The field of view of the detector is selected by a user by the relative positioning of the detector <NUM>, sample support <NUM> and x-ray source <NUM> so that the sample, or an area of the sample, is within the field of view of the detector. The detector can move to different imaging positions so that different projections can be taken through samples on the sample support. In this context, a different projection means that the x-rays pass through the sample on the support in a different direction.

Typically there are <NUM> operation modes. In a first mode the detector stays stationary and the sample support is moved to different locations to acquire different fields of view. In a second mode, the detector and sample support are moved in a co-ordinated manner to get a different angular projection through the same field of view. This coordinated motion enables three dimensional reconstructions to be generated using tomosynthesis.

The support <NUM> is moveable in the XY plane in order that the sample on the support can be moved to a position between the x-ray source and the detector. In the example shown in <FIG>, the support <NUM> is also moveable in a vertical or Z-direction. This allows the magnification of the detected image at the detector to be adjusted. In other words, larger or smaller areas of the support can be made to fall within the field of view of the detector depending on the relative distance between the x-ray source <NUM> and the support <NUM> and the x-ray source <NUM> and the detector <NUM>. As explained, the area of the sample to be imaged must fall within the field of view.

Generally, the x-ray source <NUM> includes a tube that generates the beam of x-rays by accelerating electrons from an electron gun and causing the energetic electrons to collide with a metal target. The x-rays contained in the beam are sufficiently energetic to penetrate through the thickness of target objects on the sample support <NUM> so that attenuated x-rays reach the detector <NUM>. The differential levels of x-ray attenuation by the materials of different density in the sample and their different thicknesses, within the region being imaged produces contrast in the resulting image captured by the detector.

The detector <NUM> may be a digital detector and have a construction as is well known in the art. Generally, the detector includes an active area, a sensor that converts the incoming x-rays over the active area into another signal type that can be measured or imaged, and an amplifier used to boost the amplitude of the signals. The signals are converted from an analogue form to a digital form within the detector <NUM> and a digital image is output from the detector. An exemplary digital detector is a complementary metal oxide semiconductor (CMOS) flat panel detector that includes a two dimensional pixel array of silicon photodiodes constituting the active area.

In <FIG>, the detector <NUM> is shown in four different positions and there are four corresponding regions <NUM> on the support. It should be understood that many more positions are possible. In a tomosynthesis system, a three dimensional model of the area of the sample being imaged may be constructed from any number of projections, and anything between <NUM> and <NUM> projections is used in practice.

The resulting three dimensional model allows a user to inspect any plane through the imaged area, and to review a three dimensional image to find defects such as voids.

Various tomosynthesis algorithms and processing techniques are known in the art, such as the ReconPro reconstruction solution offered by Prexion Inc. of <NUM> Borel Avenue, Suite <NUM>, San Mateo, CA <NUM>, USA.

A requirement for generating a three dimensional model using a plurality of images is a knowledge of the precise spatial relationship between x-ray source, region of interest and detector for each image. The way in which the two-dimensional images are combined in tomosynthesis relies on this geometric information, as it is required in the mathematical formulas that are used.

In order to use a system as described above to inspect and generated models of samples that are produced in a clean room environment, such as semiconductor wafers, during production, it is necessary that the x-ray inspection system itself meets clean room standards.

<FIG> is a schematic cross section of an x-ray inspection system in accordance with one embodiment of the disclosure. <FIG> is a second cross section of the system shown in <FIG>, taken perpendicular to the cross section of <FIG> along plane A-A. <FIG> is a partially cut-away perspective view of the system of <FIG> and <FIG>.

The system illustrated in <FIG>, <FIG> and <FIG> comprises an x-ray tube <NUM>, a sample support <NUM> and a detector <NUM>, within a cabinet <NUM>. The cabinet <NUM> is lead lined to provide a shield from x-rays generated by the x-ray tube <NUM>.

Within the cabinet <NUM> there is a supporting frame <NUM>, on which the x-ray tube <NUM>, sample support <NUM> and detector are all mounted. The sample support <NUM> is configured to hold a semiconductor wafer (not shown in <FIG>, <FIG> or <FIG>).

The frame comprises a detector positioning assembly <NUM> (not visible in <FIG> or <FIG>) that allows the detector <NUM> to move to in a horizontal plane. The detailed components of the detector positioning stage <NUM> are not shown in the <FIG> and <FIG> and any suitable arrangement may be used for a clean room compatible system. However, one inventive and advantageous arrangement is described in detail with reference to <FIG>. In this embodiment the detector positioning mechanism comprises a first beam <NUM>, extending along the X-axis, on which the detector is supported and the supporting frame <NUM> having a rail <NUM> extending in the Y-direction, on which the first rail <NUM> is supported. Linear motors are provided to allow the detector to be moved along the beam <NUM> and to allow the beam <NUM> to be moved along the rail <NUM>. In this manner the detector can be moved to any position within a horizontal plane (within the frame). A tilting mechanism is also provided that allows the active area of the detector to be oriented to face the x-ray source in all imaging positions. The tilting mechanism is shown in <FIG> and will be described in more detail later on in the specification. The detector in this embodiment is a CMOS flat panel detector, such as the Dexela <NUM> NDT, available from Dexela Limited, Wenlock Business Centre, <NUM>-<NUM> Wharf Road, London, N1 7EU, United Kingdom.

A sample positioning stage <NUM> is provided on the supporting frame above the detector positioning stage <NUM>. The detailed components of the sample positioning stage are shown in and described with reference to <FIG> and <NUM>. However, sample support positioning arrangements having a different geometry or layout may be used to provide a clean room compatible system. In this embodiment, the sample positioning stage comprises a similar arrangement of motors and rails as the detector positioning stage for moving the sample support <NUM> in a horizontal plane, with the addition of a further motor to move the sample support in a vertical direction, towards and away from the x-ray tube. This allows for the magnification of the images to be selected. The sample positioning stage is described in more detail below with reference to <FIG> and <NUM>.

A sample input shutter <NUM> as shown in <FIG>, is provided to allow a sample, such as a semiconductor wafer, to be loaded onto the sample support <NUM>. The shutter is provided in the cabinet and has a pneumatically operated mechanism. The shutter is lead lined and sealed by steel labyrinth bars to prevent escape of x-rays. The shutter is automatically opened when inspection of a wafer is complete to enable unloading of the completed wafer and reloading of a new wafer. The wafer load/unload is handled by an Equipment Front End Module (EFEM) unit to provide a continuous clean environment for the wafer as it is moved from a Front Opening Unified Pod (FOUP) to the sample support. A FOUP is a special plastic enclosure designed to hold silicon wafers securely and safely in a clean room environment, and to allow the wafers to be removed for processing or measurement by tools equipped with appropriate load ports and robotic handling systems. The EFEM used in this embodiment is a Brooks JET™ Atmospheric Transport System unit, available from Brooks Automation, Inc. ,<NUM> Elizabeth Drive, Chelmsford, MA <NUM>, U.

The x-ray tube <NUM> is fixed to an x-ray tube bracket <NUM> and is positioned above the sample support <NUM> and the detector <NUM>. The x-ray tube bracket <NUM> is provided on the frame <NUM> above the sample positioning stage. The x-ray tube cannot move relative to the frame <NUM>.

In this embodiment, the x-ray tube100 is a sealed-transmissive type of x-ray tube, such as the NT x-ray tube from Dage Holdings Limited, <NUM> 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 <NUM> 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 with small spot size and allows the sample being imaged to be brought close to the x-ray source. This means that high magnification and high resolution images can be obtained. By arranging the x-ray tube <NUM> above the sample support <NUM> and configuring the sample support to support a semiconductor wafer between the sample support and the x-ray tube, the surface of the semiconductor wafer can be brought very close to the x-ray source, allowing for high magnification images to be obtained within a compact system.

A pair of fan filter units (FFUs) <NUM> is mounted to the cabinet above the sample support. The FFUs are configured to draw air in through respective air inlets <NUM> in the ceiling of the cabinet and drive the air through a HEPA filter plate <NUM> in each FFU, downward past the sample support <NUM> to an air outlet <NUM> in the floor of the cabinet. The direction of the airflow is indicated by the arrows in <FIG> and <FIG>.

In this example, each FFU <NUM> comprises an enclosure <NUM>, a fan <NUM> configured to draw air into the enclosure through an air inlet <NUM> on one face of the enclosure and out through an outlet covered by a HEPA filter plate <NUM>. Each FFU is constructed so that when the fan <NUM> is running, the air pressure within the enclosure <NUM> is higher than outside the enclosure. This helps to provide a uniform flow of air through the filter plate <NUM>, and minimises local flow rate variations.

In this embodiment each FFU <NUM> also comprises an internal shield <NUM> positioned between the fan and the filter plate, which is not a feature of standard FFUs. This shield <NUM> has two functions. It is both an x-ray absorber and an airflow baffle. However, two separate components could be used, one for each of these functions. The shield <NUM> is a lead lined steel tray that is larger than and spans the air inlet <NUM> of the FFU <NUM> so that x-rays from the x-ray tube <NUM> cannot escape through the air inlet <NUM>. The airflow path past the shield <NUM> and out of the FFU <NUM> is made labyrinthine. The shield also forces air from the fan <NUM> to the outer edges of the enclosure <NUM>, as is clearly illustrated in <FIG> and <FIG>. This promotes a uniform air flow through the filter plate <NUM>.

The air flow through the cabinet from the filter plates <NUM> past the sample support <NUM> is laminar. There is no recirculation of air from below the sample support <NUM> to a position above the sample support. The use of HEPA filters <NUM>, laminar airflow and ensuring that there are no moving parts in the system between the filter plates <NUM> and the sample support <NUM>, means that there is minimal risk of any dust or other particulates from becoming airborne, or falling from above, and landing on, and thereby contaminating or damaging, the semiconductor wafer on the sample support <NUM>.

In order to ensure that there is no recirculation of air from below the sample support <NUM> to above the sample support, a perforated deck <NUM> (best seen in <FIG>) is provided at the level of the sample support. The perforated deck <NUM> is positioned at the same height as an uppermost position of the sample support <NUM>, corresponding to the highest magnification of the system. The perforated deck <NUM> allows air from above the deck to pass to beneath the deck, but creates a small pressure difference across the deck so that there is higher air pressure above the deck than below the deck. The perforated deck restricts the air flow path of the air by approximately <NUM>%. This restriction increases the pressure of the air above the deck. This small pressure difference substantially prevents recirculation of air to above the deck, as air will naturally flow from an area of high pressure to an area of low pressure. A baffle plate <NUM>, as shown in <FIG>, is also provided to promote laminar air flow.

The air outlet <NUM> in the floor of the cabinet is relatively large compared with the air inlets <NUM>, again to promote laminar airflow and reduce any recirculation of air upward. <FIG> shows the top of the cabinet. The air inlets <NUM> can be seen, allowing air to be drawn into the FFUs. <FIG> is a cut-away view of the air outlet in the bottom of the cabinet. The air outlet <NUM> comprises four separate openings. The openings are surrounded by steel blocks <NUM>. The spaces between the four outlet openings are provided to accommodate electrical cabling for providing power and data to and from the cabinet. The air outlet is covered by an outlet shielding plate <NUM> (shown in <FIG> transparent and in dotted outline) to prevent the escape of x-rays from the cabinet. The function of the outlet shielding plate <NUM> is best shown in <FIG>. The outlet shielding plate <NUM> is lead lined to absorb x-rays. Steel blocks <NUM> are also provided on the underside of the shielding plate as x-ray absorbers within the air flow path to the outlet. The steel blocks <NUM> and <NUM> absorb x-rays and are positioned so that the air flow path to the outlet is made labyrinthine. This ensures that no x-rays can escape the cabinet through the air outlet.

In operation the FFUs <NUM> continuously force air through the cabinet to ensure that no dust or debris produced as a result of the operation of the sample positioning assembly and detector positioning assembly can reach samples on the sample support <NUM>. The FFUs provide for at least a class <NUM> ISO <NUM>-<NUM> clean roon environment within the cabinet. The FFUs operate throughout the movement and operation of the sample support <NUM> and detector <NUM>, and as the samples are loaded and unloaded from the system. A system as described with reference to <FIG>, <FIG> and <FIG> should be able to operate for thousands of hours without requiring maintenance.

An exemplary sample positioning assembly will now be described in detail with reference to <FIG> and <NUM>.

The sample positioning assembly is used to position samples 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. The x-ray source is positioned above the sample support and is static. It is therefore desirable to be able to move the sample support in the XY plane, which is the horizontal plane, to provide different projections and images of different regions of the sample. It is also desirable to move the sample support 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.

In operation, when obtaining a set of different projections for a tomosynthesis calculation, the sample needs to be moved in the X- and Y- directions more often than in the Z-direction. Once the image magnification is set for a sample, then the sample need only be moved in the X- and Y- directions to obtain the different projections.

In order that the x-ray inspection process does not become a bottleneck within a wafer processing plant, the inspection process needs to be fast. This means that the mechanism for moving the sample in the X- and Y- directions needs to be fast. It also needs to be accurate, particularly at very high magnification, in order to produce high resolution three dimensional models, as discussed.

In this embodiment, the sample positioning assembly is configured to move in raster scan pattern between imaging positions as a line by line collection of images along parallel lines, with these parallel scan lines extending in the X-direction. The X direction is indicated by the X arrow shown in <FIG> and <NUM>. This means that X-direction movement mechanism will have the highest amount of travel, and therefore should be fast so as to increase the overall speed of the process for collecting the x-ray images that are needed. For this reason, in this embodiment, the X-axis drive mechanism is coupled directly to the sample support, which means that the X-axis drive mechanism moves minimal mass comprising only the sample support and the moving portions of x-axis drive mechanism. Most importantly, the X-axis drive mechanism does not support or carry either the Y-axis drive mechanism or Z-axis drive mechanism and this enables the X-axis drive mechanism to move more quickly. Because the Z-axis drive mechanism is used relatively infrequently and does not need to be as fast as the X-axis and Y-axis drive mechanisms, it can be made with a relatively lower mass than the X-axis and Y-axis mechanisms. The Z-axis drive mechanism is positioned to support the X-axis drive mechanism and moves both the X-axis drive mechanism and the sample support in the Z-axis. The Y-axis drive supports the Z-axis drive mechanism and so moves the Z-axis drive mechanism, the X-axis drive mechanism and the sample support in the Y-axis direction. The Y-axis drive mechanism is mounted to a supporting frame, to which the x-ray source is also mounted.

This arrangement is illustrated schematically in <FIG>. The sample support <NUM> is mounted on a shuttle <NUM>. The shuttle moves in the X-direction on a first frame <NUM>. The shuttle <NUM> and frame together form the X-axis drive mechanism. The first frame <NUM> is mounted on a track <NUM> and can move up and down the track <NUM>. The track <NUM> and first frame <NUM> together form a Z-axis drive mechanism. The track <NUM> is mounted on a second frame <NUM> and can move along the second frame <NUM> in the Y-direction.

Figure <NUM> illustrates one embodiment of this arrangement in more detail, in a system as shown in <FIG>, <FIG> and <FIG>. In Figure <NUM> it can be seen that the sample support <NUM> is configured to support a circular semiconductor wafer. The sample support is mounted to a first pair of linear motors <NUM>. Each linear motor comprises a track of permanent magnets <NUM> extending in the X-direction and a coil assembly <NUM> that travels along the track in response to electrical control signals. Linear motors of this type are available from Aerotech, Inc. , <NUM> Zeta Drive, Pittsburgh, PA <NUM>, USA. The linear motors <NUM> are mounted on a first frame <NUM>. The first pair of linear motors <NUM> on the first frame form part of the X-axis drive mechanism.

The first frame <NUM> is mounted on a pair of lead screws <NUM>, on opposite sides of the first frame <NUM>, only one of which is visible in Figure <NUM>. Each lead screw <NUM> is driven by a rotary motor <NUM> through an angular gearbox <NUM>. The lead screws <NUM> are each mounted to a plate <NUM> and move the first frame <NUM>, together with the X-axis drive mechanism, up and down in the Z-axis direction relative to the plates <NUM>. Four guide rails <NUM> are provided at the corners of the first frame <NUM> to support the first frame <NUM> and keep it stable as the first frame is moved in the Z-axis direction by the lead screws <NUM>. The lead screws <NUM>, associated rotary motors <NUM>, gear boxes <NUM> and guide rails <NUM> form part of the Z-axis drive mechanism. A linear encoder <NUM> is provided between the first frame <NUM> and the plate <NUM> to determine and allow control of the vertical position of the first frame <NUM> and wafer support <NUM>.

The plates <NUM> slide along guides <NUM> formed on the supporting frame <NUM>. A second pair of linear motors <NUM> is connected between the plates <NUM> and the supporting frame <NUM> to move the plates <NUM>, together with the Z-axis drive mechanism and the X-axis drive mechanism, in the Y-direction relative to the supporting frame. The second pair of linear motors may be larger and of a higher power than the first pair of linear motors, as they are required to move a greater mass than the first pair of linear motors. Linear motors of this type are available from Aerotech, Inc. , <NUM> Zeta Drive, Pittsburgh, PA <NUM>. The second pair of linear motors <NUM> on the first frame <NUM> forms part of the Y-axis drive mechanism.

It should be clear that although this arrangement has been described in relation to a system for inspecting semiconductor wafers in a clean room environment, it can also be used in x-ray inspection systems that do not need to operate in a clean room environment and so do not include the air movers and air filters described.

As explained, one of the requirements for producing good quality tomosynthesis models is very accurate knowledge of the relative position of the x-ray source, sample and detector. In particular, it is necessary to know accurately the change in relative positions from one imaging position to the next so that the images can be properly combined.

To provide high magnification images, the distance between the sample and the x-ray source is much smaller than the distance between the detector and the x-ray source. This means that small changes in position of the sample lead to large changes in the image recorded by the detector. This in turn means that the position of the sample needs to be known to a much higher accuracy than the position of the detector.

A non-contact position measuring device may be used to accurately determine the position of the sample support. In one embodiment of the disclosure, an interferometer based system is used to determine the changes in position of the sample support from one imaging position to the next. Figure <NUM> is a schematic illustration of the interferometer based detection arrangement. Two interferometers are provided. A first interferometer <NUM> is used to determine changes in the position of the sample support <NUM> in the X-direction and a second interferometer <NUM> is used to determine changes in the position of the sample support in the Y-direction. The assemblies for the X- and Y- directions are identical. Each assembly comprises a laser light source <NUM>, <NUM> that provides a laser beam to the associated interferometer <NUM>, <NUM>. The interferometer directs a portion of the laser beam to a mirror <NUM>, <NUM> mounted to the sample support <NUM>. Light reflected from the mirror is directed back to the interferometer and then to a detector <NUM>, <NUM>. Changes in the interference between light that has travelled to the sample support and light that has not travelled to the sample support, as the sample support is moved, is detected at the detector to provide a very accurate measure of the change in position of the sample support in the sensed (i.e. x or y) direction. Suitable interferometer systems, including the laser light sources and mirrors, are available from Renishaw plc, New Mills, Wotton-under-Edge, Gloucestershire, GL12 8JR, United Kingdom. Other possible non-contact position measuring devices include optical linear encoders, magnetic encoders, capacitive sensors and sonar distance measuring devices.

The output from the detectors is an accurate measure of the change in position of the sample support in the X- and Y- directions as the sample support moves between imaging positions. These measurements are provided to an image processor, as will be described, and used in a tomosynthesis calculation. The measurements from the detectors may also be used to calibrate the X and Y positioning assemblies.

Figure <NUM> illustrates an arrangement in which a separate laser light source is provided for each interferometer. However, it should be clear that a single laser light source and a beam splitter could be used. Furthermore, it is possible to use an identical arrangement to determine changes in position of the sample support in the Z-direction, although the position of the sample support in the Z-direction typically does not need to be determined with such a high degree of accuracy.

<FIG> illustrates how the arrangement as illustrated in Figure <NUM> is integrated into the sample positioning assembly shown in Figure <NUM>. The first mirror <NUM> is fixed to the sample support <NUM> facing in the X-direction and the second mirror <NUM> is fixed to the sample support facing in the Y-direction. The interferometers <NUM>, <NUM> are fixed to the supporting frame <NUM>. The mirrors <NUM>, <NUM> have sufficient height and width that light from the interferometers <NUM>, <NUM> is incident on the mirrors in all possible imaging positions. In this example, the usable area of each mirror is <NUM> wide and <NUM> high. The mirrors are typically provided as part of the interferometer system as described above. However, suitable mirrors are also available as standalone items from optics manufacturers, such as Gooch and Housego PLC, Dowlish Ford, Ilminster, TA19 0PF, UK.

It should be clear that although this arrangement has been described in relation to a system for inspecting semiconductor wafers in a clean room environment, it can also be used in x-ray inspection systems that do not need to operate in a clean room environment and so do not include the air movers and air filters described. It may also be used in conjunction with a different arrangement for positioning the sample and the detector.

As described, for high magnification images it is necessary to bring the sample very close to the x-ray source. It is therefore necessary to control the position of the sample in the Z-direction reliably. It is also necessary to know the position of the sample in the Z-direction for image processing and data interpretation purposes.

While the position of the sample support <NUM> in the Z-axis can be determined from the Z-axis positioning mechanism or from a linear encoder mounted to the Z-axis positioning mechanism, there is the problem that different samples have different thickness and so the actual distance between the x-ray source and a top surface of the sample cannot be determined accurately from the position of the sample support <NUM>. Accordingly, in one aspect of the disclosure, a proximity sensor is used to provide a direct measurement of the distance between a top surface of the sample and the x-ray source.

Providing a direct measurement of the distance between the x-ray source and the top surface of a sample is beneficial for several reasons, particularly in a high magnification system in which the top surface of the sample, which typically comprises the regions of interest, is brought very close to the x-ray source. First, the distance measurement can be used to calibrate the Z-axis positioning mechanism, so that accurate positioning and subsequent image processing can be achieved. Second, the distance measurement can be used directly in a magnification calculation to provide an accurate measure of magnification. Third, the distance measurement or multiple distance measurements can be used to prevent any collision between the top surface of the sample and the x-ray source, which would likely be very damaging to both.

<FIG> is a schematic illustration of a proximity sensing arrangement in accordance with an aspect of the disclosure. A semiconductor wafer <NUM> is illustrated on a sample support <NUM>, beneath a sealed x-ray tube <NUM>. A laser distance sensor <NUM> is fixed to the x-ray tube <NUM>. A suitable laser distance sensor is available from Keyence Corporation, <NUM>-<NUM>-<NUM>, Higashi-Nakajima, Higashi-Yodogawa-ku, Osaka, <NUM>-<NUM>, Japan. As an alternative, a confocal detector may be used. A laser beam from the distance sensor is indicated by arrow <NUM>. The sample positioning assembly <NUM> is illustrated schematically and includes a linear encoder <NUM> for determining a Z-axis position of the sample support <NUM>. Both the sample positioning assembly and the distance sensor are connected to a controller <NUM>.

<FIG> illustrates the mounting of the laser distance sensor <NUM> to the x-ray tube <NUM> in a system of the type shown in <FIG>, <FIG> and <FIG>.

The laser distance sensor <NUM> provides a direct measurement to the top surface of the sample, in this example a semiconductor wafer. The laser distance sensor <NUM> measures the distance to the sample from its output end, herein referred to as the read head facing the top surface of the sample. The x-ray tube <NUM> produces x-rays from an output spot on the transmission target. The transmission target forms the output window <NUM> of the x-ray tube, so that the output spot lies in the plane of the output window <NUM> of the x-ray tube <NUM>. The read head of the laser distance sensor <NUM> may not be mounted at exactly the same height as the output spot of the x-ray tube <NUM>. In other words, the read head of the laser distance sensor may not be coplanar with the output window of the x-ray tube. But the difference in height between the output window and the output end of the laser distance sensor, known as the offset, can be calculated during system set-up by imaging a feature of known size, or two features of known spacing, on the sample support, in different positions as explained below.

<FIG> is a schematic illustration of the arrangement shown in <FIG>, showing how the offset between the read head <NUM> of the laser distance sensor <NUM> and the output window <NUM> of the x-ray tube can be calculated by imaging a pair of features <NUM>, <NUM> of a known spacing from one another on a gauge plate on the sample support <NUM>.

The known distance between the features <NUM> and <NUM> on the gauge plate is D<NUM>. The distance between the images <NUM>, <NUM> of the two features <NUM>, <NUM> on the detector <NUM> is D<NUM>. D<NUM> can be determined from the output of the detector using standard image processing techniques.

It is well known in this field that the ratio D<NUM>/D<NUM> is equal to the ratio A/H. The distance H between the output window of the x-ray source and the imaging surface of the detector <NUM> is known from the system specifications. So A can be calculated using the formula: <MAT>.

The distance B between the read head of the laser distance sensor and the sample support is directly measured by the laser distance sensor <NUM>. Consequently, the offset C between the read head of the laser distance sensor <NUM> and the output window of the x-ray tube is determined by subtraction: <MAT>.

Therefore, the difference in height, or offset C, between the output window and the read head of the laser distance sensor can be calculated from the formula C=B-Hx(D<NUM>/D<NUM>) during system set-up by imaging a feature of known size.

Subsequent measurements of the distance to the sample from the read head of the laser distance sensor can be adjusted by this offset C to get the distance from the output window to the sample, which is used in magnification calculations as explained below.

The laser distance sensor can then be used to calibrate a height sensor within the sample positioning assembly. In this example, the height sensor is the linear encoder <NUM>, which is used in the Z-axis sample positioning mechanism, as shown in <FIG>. This is particularly beneficial if, during inspection, the sample is positioned so that it is obscured from the laser distance sensor by the x-ray tube so that the laser distance sensor measurements cannot be used directly. When the sample is in a position that it is not obscured by the x-ray tube, referred to here as the starting position, the controller <NUM> receives distance measurements from the laser distance sensor <NUM> and at the same time receives an output from the linear encoder <NUM>. The absolute distance between the top plane of the sample and the output window of the x-ray tube can be established using the laser distance sensor <NUM>. This is done by taking the absolute measurement reading from the laser distance sensor <NUM> and subtracting the offset C, calculated as described. This distance between the top plane of the sample and the output window of the x-ray tube is then used as the calibration for the linear encoder, which measures changes in distance from the starting position. This calibration process can be carried out periodically.

In addition, the laser distance sensor measurements can be used to determine image magnification, which is used during image processing. Image magnification (IM) is the ratio of the size of the object as it appears in the image on the detector <NUM> to the actual size of an object. With reference to <FIG>, it is known in this field that the magnification ratio is equal to H/A. A is determined from the measured distance B minus the calculated offset C. H is a known value. So image magnification is given by the formula: <MAT>.

For example, if the distance H is <NUM>, the measured distance B is <NUM> and the offset value C has been calculated as <NUM>, then the image magnification will be: <MAT>.

This means that a distance between features that appears as a <NUM> distance on the detector is an image of a real distance of <NUM>.

This ability to accurately determine magnification has two benefits. Firstly, the size of the features within the sample can be established very accurately, allowing good quantitative assessment of geometric feature sizes such as wafer bump diameter or void area. Secondly, during tomosynthesis, the angle and location of individual projections is well known, so the computed three-dimensional model can be made accurate. An image magnification calculation using measurement from the laser distance sensor is typically carried out as a calibration calculation before a set of images of a particular region or regions of interest are captured.

The proximity sensor, or laser distance sensor, <NUM> illustrated in <FIG>, <FIG> can also be used to prevent any collision between the x-ray source and the sample. A collision between the x-ray source and the sample is likely to irreparably damage the sample and also cause significant damage to the x-ray tube. Because samples, in this example semiconductor wafers, can have different thicknesses, simply relying on an output from the linear encoder <NUM> which only provides information about the position of the sample support, may not be effective at preventing collision when trying to obtain very high magnification images, which require that the sample be placed very close to the x-ray tube, making the distance A in <FIG> very small.

In order to prevent collision, prior to inspection of a semiconductor wafer at high magnification, the wafer is safely raster scanned by the laser distance sensor at a low magnification height, which is known to be safe for all possible wafers, to establish the distance of the top-most feature on the wafer <NUM> from the end of the x-ray tube <NUM>. The raster scan is achieved by operating the sample positioning assembly to move the sample support in the XY plane. This illustrated in <FIG>. The location on the linear encoder scale is recorded for the low magnification height as a baseline value. The shortest distance measured by the laser height sensor, corresponding to top-most feature on the wafer <NUM>, is recorded. The controller <NUM> then creates a virtual reference plane based on the baseline encoder scale reading at the measured shortest distance which dictates how close the wafer can safely be brought to the surface of the x-ray tube. The virtual plane may be located at a predetermined clearance above the top-most feature on the wafer. For example, if the shortest distance to the top of the sample measured by the laser distance sensor at the low magnification height is <NUM> and the known offset between the output end of the laser distance sensor <NUM> and the output window of the x-ray tube, C, is <NUM>, then the shortest distance between the output window of the x-ray tube and the top of the sample during the raster scan is <NUM>. If it is desired that in operation the top surface of the sample should be no closer than <NUM> from the output window of the x-ray tube in order to prevent any possibility of a collision, then the sample support can be moved no closer than <NUM> from the low magnification height recorded as a baseline on the linear encoder. This <NUM> maximum travel can be controlled using the linear encoder readings. The linear encoder scale can be used during inspection of the wafer to ensure that no collision can occur.

Figure <NUM> is a flow diagram illustrating the control process used to prevent collision between the x-ray tube and the wafer. In step <NUM> a semiconductor wafer is loaded into the system. In step <NUM>, the wafer is positioned below the laser distance sensor at a known safe height, i.e. at a height at which even the very thickest sample would be well clear of the x-ray tube. In step <NUM> the laser distance sensor is then activated and the sample support moved in a raster scan pattern in the XY plane by the sample positioning assembly. The distance to the closest feature on the wafer is recorded. In step <NUM> the maximum permitted upward travel of the sample support is calculated corresponding to a minimum safe distance between the sample support and the x-ray tube or a maximum safe height for the sample support. In step <NUM> inspection of the wafer is performed at high magnification but with the sample support at a position lower than or equal to the maximum safe height for the sample support.

This process can be performed quickly and automatically for every new sample that is loaded into the machine. Again, it should be clear that this system and method is applicable not only to semiconductor wafers but to any type of sample that is required to be imaged at high magnification.

As described, the x-ray detector is positioned below the sample support 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, as previously described.

In order to record different projections through a sample, the detector must be moved accurately to different imaging positions. The projections are then combined using a tomosynthesis algorithm to generate a three-dimensional model of the sample or of a region of the sample. As described, it is desirable for the plurality of different projections to be recorded as quickly as possible. And for high magnification images, in which the sample support is positioned very close to the x-ray source, the x-ray detector must move much greater distances between imaging positions than the sample support, and so it is necessary for the detector to be moved at a relatively higher speed than the sample support.

In order that the detector can be moved accurately but at a high speed it is advantageous that the detector be moved within the XY plane, on rigid axes. The alternative of moving the detector on a pivotable, arcuate track, which has been used in prior systems, does not allow for such accurate movement at high speed because the mechanism is less rigid. This alternative system would also suffer from excessive vibration when starting and stopping at high speed. Movement of both the detector and sample within parallel XY planes, without any Z-axis movement, also has the advantage that the image magnification remains the same for all imaging positions, as the magnification is determined by the formula IM = A/H as previously discussed.

However, movement of a detector having the detector face that lies in the XY plane solely within that XY plane suffers from the disadvantage that the detector is not always facing the x-ray source. At extreme oblique angles between the face of the detector and the point of emission of the x-rays from the x-ray source blurring of the image can occur. In one aspect of the disclosure, a tilting mechanism is provided for the detector, in addition to a mechanism for moving the detector in an XY plane, which allows the detector to be oriented so that it faces the x-ray source in all imaging positions.

<FIG> is a schematic cross-section illustration of an x-ray inspection system in accordance with the disclosure, as illustrated in <FIG> but showing a tilting mechanism <NUM> for the x-ray detector. The detector positioning assembly <NUM> allows the detector <NUM> to move in a horizontal plane.

The remaining features of the system shown in <FIG> are as described with reference to <FIG>. The cabinet and FFUs are configured for clean room operation. A sample positioning stage <NUM> is provided on the supporting frame above the detector positioning stage. The detailed components of the sample positioning stage are shown in, and described with reference to, <FIG> and <NUM>. The x-ray tube <NUM> is fixed to the bracket <NUM> and is positioned above the sample support <NUM> and the detector <NUM>.

In order to provide for high speed movement in the XY plane, first and second linear motors are used to move the detector in the x direction and y direction respectively. As illustrated in <FIG>, a first linear motor <NUM> is mounted between a frame <NUM> of the detector assembly and a horizontally extending beam <NUM>. The beam <NUM> extends in the X-direction and is supported to move along the supporting frame <NUM>. To provide for this movement, a pair of second linear motors <NUM> (only one of which is visible in <FIG>), extending in the Y-direction, is mounted on the supporting frame <NUM> and drive the remote ends <NUM> of the beam <NUM>. Activation of the first linear motor <NUM> moves the detector assembly along the beam <NUM> in the X-direction. Activation of the second linear motors <NUM> moves the beam <NUM> and the detector assembly in the Y-direction, which is perpendicular to beam <NUM>.

This mechanism allows the detector to be moved within a horizontal plane quickly and accurately. The first and second linear motors may be identical and are larger that the linear motors used for the sample support positing assembly. Linear motors of this type are available from Aerotech, Inc. , <NUM> Zeta Drive, Pittsburgh, PA <NUM>.

<FIG> shows the tilting mechanism <NUM> in detail. Two further axes of movement for the detector are provided by this tilting mechanism <NUM> so that the planar imaging surface, or face <NUM>, of detector <NUM> is always able to be faced directly at the x-ray source. The detector <NUM> is mounted to a first gimbal frame <NUM> so that it can be tilted about a first rotary axis <NUM>. A first motor <NUM> is configured to rotate the detector about the first rotary axis. One example of a suitable motor is the APR150DR-<NUM> from Aerotech Inc. , <NUM> Zeta Drive, Pittsburgh, PA <NUM>, USA.

The first gimbal frame <NUM> is rotatably mounted to a motor <NUM>, which is mounted to second gimbal frame <NUM>, to rotate about a second rotary axis <NUM>. The second motor <NUM> is configured to rotate the first gimbal frame <NUM> about the second rotary axis <NUM>. The second gimbal frame <NUM> is mounted to the mover of linear motor <NUM> and the stator of linear motor <NUM> is mounted to the beam <NUM>, as described with reference to <FIG>, so that the detector and tilting mechanism can be move along the beam <NUM>. The first and second motors comprise direct read encoders on an output side to allow for a very accurate determination of the orientation of the detector.

The tilting mechanism is arranged so that the first and second rotary axes <NUM>, <NUM> are coplanar with the active area, or face <NUM>, of the detector. This means, as shown in <FIG>, that both first rotary axis <NUM> and second rotary axis <NUM> run across, or align with, the face <NUM> of the detector <NUM>. This simplifies the image processing calculations required. It also removes the need for any compensatory Z-axis movement of the sample support or detector to address changes in image magnification. This is because the centre of the face of the detector remains at the same Z-axis distance from the x-ray source in all positions and so the image magnification remains constant regardless of the position of the tilting mechanism.

The first and second rotary motors are connected to and controlled by a controller (not shown in <FIG>) that is also connected to the linear motors for the XY movement of the detector. The controller is configured to ensure that in each imaging position in which the detector is stopped to record an x-ray image, the active area <NUM> of the detector is facing the x-ray source. Preferably, the controller will orient the active area, or face <NUM>, of the detector so that it is perpendicular to a line drawn from the centre of the face of the detector to the point at which the x-rays are emitted from the x-ray source.

By constantly facing the active area <NUM> of the detector towards the x-ray source, oblique angles between the face of the detector and the point of emission of the x-rays from the x-ray source, which can cause blurring of the image, are avoided. This improves the quality of the image.

It should be clear that although this arrangement has been described in relation to a system for inspecting semiconductor wafers in a clean room environment, it can also be used in x-ray inspection systems that do not need to operate in a clean room environment and so do not include the air movers and air filters described. It is also possible to use this detector positioning assembly with a different arrangement for the sample support position assembly and without the position detection systems so far described.

The sample support <NUM> for the semiconductor wafers <NUM> holds each semiconductor wafer in position by applying suction to a rear surface of the wafer. This is a well known wafer handling technique that avoids damage to the wafer. <FIG> is a plan view of the sample support and shows a plurality of concentric depressions or grooves <NUM> formed in a planar upper support surface <NUM> of the sample support. The grooves <NUM> are connected to a vacuum port <NUM> by a radial channel <NUM>. When a wafer is placed onto the support surface <NUM> a vacuum is applied to the port <NUM>. This holds the wafer firmly against the support surface <NUM>. The "imaging area" of the sample support is the portion of the sample support that may appear in an x-ray image of a portion of the semiconductor wafer.

In an x-ray inspection system as described, x-rays from the x-ray tube <NUM> not only pass through the semiconductor wafer but must also pass through the sample support <NUM> before reaching the detector <NUM>. The sample support 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.

However, even these materials will attenuate the x-rays to some extent. The amount of attenuation depends on the thickness of the sample support that the x-ray must pass through. The grooves <NUM> result in a local thinning of the sample support and so the pattern of thickness changes caused by the grooves will appear in the resulting x-ray images. The groves of a conventional wafer chuck are rectangular in cross-section with parallel sidewalls and a flat bottom, as shown in <FIG> shows a wafer chuck <NUM> with a rectangular cross-section groove <NUM>. If the x-ray image spans a groove, x-rays that pass a groove on the wafer chuck will be less attenuated than those that pass through the remainder of the wafer chuck. The rectangular cross-section means that, given the vertical sidewalls of the grooves, the change in wafer chuck thickness is very abrupt. This can cause very sudden changes in contrast in the x-ray image that would obscure or confuse the image, particularly of weakly contrasting features of a wafer. This in turn would make automated analysis of the images slow or even impossible in some cases.

To minimise this problem, rather than providing grooves having rectangular cross-sections, the grooves or depressions used in the present disclosure for providing suction to the rear of a wafer are configured to provide only small and gradual changes in wafer chuck thickness to cause less patterning in the x-ray image that would obscure or confuse the image. Thus, the thickness of the wafer chuck is continuously varied rather than having sudden changes in thickness, and the grooves are made comparatively wide and shallow.

<FIG> is a schematic cross-section of one of the grooves <NUM> of <FIG> in accordance with the disclosure. In this example the groove has a width W of around <NUM> and a maximum depth below the support surface of <NUM>. The sidewall of the groove extends in a continuous curve from a first side to a second side of the groove. The curve is substantially circular and has a maximum radius of curvature of around <NUM>. It can be seen that the radius of curvature is substantially two orders of magnitude greater than the maximum depth of the groove. This ensures that the rate of change of thickness of the wafer check is small. The maximum thickness of the sample support in the imaging area D is <NUM>.

The size and shape of the grooves must satisfy two competing requirements. The grooves must be large enough to provide a sufficient suction force on the back of the wafer. But they must also not obscure or confuse images of features of interest in or on a semiconductor wafer.

In this example, the depth of the groove increases to <NUM> over a radial distance of <NUM>, which corresponds to about a <NUM>% change in the thickness of the wafer since <NUM>/<NUM> =. The average rate of change of thickness of the wafer across the groove is <NUM>/<NUM> = <NUM> change in thickness per mm of travel parallel to the planar support surface. The maximum rate of change of thickness of the wafer is at the edge of the depression and is approximately <NUM> per mm of travel parallel to the planar support surface.

The width of the groove W is two orders of magnitude greater than the typical features of interest within a semiconductor wafer. As the groove varies continuously across its width rather than having sharp edges, this means that at a magnification appropriate for inspection, only around <NUM>% of the total thickness variation across the groove is seen as variation in the image background of a feature of interest.

When examining a <NUM> diameter solder bump on a silicon wafer, analysis software may use four points outside of the bump area to determine a baseline for adjusting image contrast. In the worst case, these points will be <NUM> apart from each other. If the solder bumps overlie the edge of a groove, where the rate of change of thickness of the chuck is at its highest, the effective depth of the sample support will vary by about <NUM>% across the imaged area. This is calculated as the maximum rate of change of depth x diameter of region of interest/maximum thickness of chuck <NUM> x. <NUM>/<NUM> ≈. This does not give rise to a significant change in image contrast across the image compared to the contrast provided between the solder bump and its surrounding area and any defects in the solder bump.

It is desirable not to have any sharp edges in the grooves or depressions. In other words, the rate of change in the change of thickness of the sample support should be low. This is to ensure that there are no sharp edges that might be enhanced by edge detection algorithms in the image processing software used. By providing depressions that extend in a continuous curve from one side to another, sharp edges are avoided within the depression themselves. The edges of the depressions should also ideally be smooth. As can be seen in <FIG> the edges <NUM> of the groove <NUM> are rounded to remove any sharp transitions. The radius of curvature of the transition region between the sidewall of the groove and the planar support surface <NUM> has a minimum radius of curvature of around <NUM>.

It should be clear that this is just one example of a wafer support in accordance with the disclosure and that different geometries for the groves can be used that provide a low maximum rate of change of thickness of the support across the imaging area. Clearly the required dimensions of the grooves depend on the size and nature of the regions to be imaged and on the density of the material (which is closely related to how strongly x-rays are attenuated) of the sample support compared to the density of the samples being imaged.

The various aspects of the x-ray inspection system so far described can be controlled to operate automatically and synchronously with each other. In particular the positioning of the sample support and of the detector must be co-ordinated and informed by measurements from the position detection arrangements. The air movers, x-ray tube and wafer handling equipment must also be co-ordinated with the positioning assemblies.

<FIG> is a schematic diagram illustrating the use of a central controller <NUM>, to coordinate the operation of each of the controllable components of the system and to receive and process the data required to provide a tomosynthetic model.

A central controller <NUM>, including an image processor <NUM> is connected to each of the controllable components of the system, as well as to an output <NUM> and a fabrication plant interface <NUM>.

The controller <NUM> controls operation of the x-ray tube <NUM> as well as the FFUs <NUM>. It operates an automated wafer handling mechanism <NUM> that extends through the shutter to place a wafer onto the sample support <NUM> and also removes the wafer from the sample support <NUM> after the wafer has been inspected within the system. It positions the wafer through the sample support positioning assembly <NUM> and correspondingly controls the detector through the detector positioning assembly <NUM>. It receives the output from the detector to build the three-dimensional model. It receives input from the laser distance sensor <NUM> to control the vertical position of the wafer relative to the x-ray source to avoid a collision. Input from the laser distance sensor <NUM> is also used in image magnification calculations. It also receives inputs from the interferometers which indicate the change in position of the sample support <NUM> as it moves to different positions for images to be collected.

The controller coordinates the movement of the sample support and the detector in accordance with a pre-programmed sequence of operation as well as performing initial calibrations as previously described. The controller must both control the sample stage, which has three axes of movement, and the detector, which has four axes of movement, two of them being rotational.

<FIG> is a flow diagram illustrating an exemplary cycle of operation for an x-ray inspection system of the type illustrated in <FIG>. The system is activated in step <NUM>. In this step, the controller is switched on and the FFUs are subsequently switched on to establish a laminar air flow. The FFUs then run continuously during operation of the system. In step <NUM>, the wafer handling assembly <NUM> is operated to load a semiconductor wafer into the system onto the sample support. The wafer handling assembly in this embodiment comprises conventional wafer handling equipment used in semiconductor fabrication plants, in this example the Brooks JET™ Atmospheric Transport System described earlier. The wafer is secured on the sample support using suction, as described. In step <NUM> the maximum height for the sample support is calculated using the process described with reference to <FIG>. In step <NUM> the x-ray tube <NUM> is activated and projections of a plurality of different regions of interest on the wafer are recorded by the detector <NUM>. In this process the sample support <NUM> and detector <NUM> are moved to a plurality of predetermined imaging positions. This process may be repeated for different regions of interest on the wafer as indicated by arrow <NUM>. In step <NUM> the projections of a particular region of interest are processed using a tomosynthesis algorithm to generate a three-dimensional model. The image processor <NUM> uses the recorded images from the detector as well as associated information about the position of the sample, sample support and detector for each image. This model is output to a display and/or memory in step <NUM>, together with any other collected data relating to the image capture process. After all of the required images for a particular wafer have been captured by the detector <NUM>, the wafer is unloaded from the system in step <NUM> using the wafer handling assembly <NUM>, and new wafer can then be loaded in step <NUM>. The unloading and loading of wafers can be carried out simultaneously with the image processing operations.

Claim 1:
An x-ray inspection system comprising:
a cabinet (<NUM>), the cabinet containing a sample support (<NUM>) for supporting a semiconductor wafer to be inspected, an x-ray source (<NUM>) configured to direct x-rays at the semiconductor wafer on the sample support and an x-ray detector (<NUM>) for detecting x-rays that have passed through the semiconductor wafer to provide a two-dimensional image of the semiconductor wafer or a region of the semiconductor wafer; and
a fan filter unit comprising:
an air mover (<NUM>) configured to force air into the cabinet through an air inlet (<NUM>) in the cabinet above the sample support,
a high performance air filter (<NUM>) located above the sample support; and
a baffle plate (<NUM>) coupled to the air mover;
wherein the baffle plate is an x-ray shield arranged to prevent the escape of x-rays from the cabinet through the air mover; and
wherein the air mover and cabinet are configured to force air through the cabinet from the air inlet past the sample support to an air outlet (<NUM>) in the cabinet below the sample support, wherein the air outlet comprises a shielding plate (<NUM>) and a labyrinthine airflow path past the shielding plate to prevent the escape of x-rays through the air outlet, and wherein the x-ray source is located above the sample support and is fixed relative to the cabinet.