Apparatus and method for automatically calibrating the position of a test tool of a bond tester

Abstract

A bond testing apparatus 100 comprises sample support 10 and calibration system 6 for determining the offset between a test tool 48 and sample inspection camera 4. The calibration system comprises: a

Description

Field of the Invention The present invention relates to an apparatus and method for automatically calibrating the position of a test tool of a bond tester. In particular, the invention relates to an apparatus and a method for automatically measuring the distance between a camera and a test tool. Background to the Invention Semiconductor devices are very small, typically from 5mm x 5mm square to 50mm x 50mm square, and typically comprise numerous sites for the bonding of electrical conductors to a semiconductor substrate. Each bond consists of a solder or gold ball deposit, known as a “bump”, a copper pillar, or a wire adhered to the substrate. It is necessary to test the bond strength of the bonds, in order to be confident that a particular bonding method is adequate. Because of the very small size of the bonds, tools used to test the bond strength of these bonds must be able to measure very small forces and deflections accurately. There are several different types of bond tests that are used to test bond strength. For example, shear testing tests the shear strength of a bond by applying a shear force to the side of the bond and shearing the bond off the substrate. Pull testing tests the pull strength of the bond by pulling a ball deposit, or a wire embedded in a ball deposit, away from the substrate. In a push test, a force, or load, is applied in the vertical plane directly downward onto a bond. Such tests may be destructive or non-destructive. Machines that perform these tests typically comprise a bond test tool, be it a shear test tool, push test tool or a pull test tool, that can be positioned relative to the bond under test and then either the bond or the tool is moved in order to perform the test by measuring the force needed to break the bond. Existing bond testers comprise a sample inspection camera which allows a user to initially identify the bond to be tested. However, the operator is required to manually position the tool relative to the bond using a stereomicroscope to align the test tool with the chosen bond. This is because the actual offset of the test tool assembly from the sample inspection camera differs from the designed offset as a result of manufacturing and build tolerances. Many bond testers also have interchangeable test tool so that different types of test can be carried out. The offset between the tip of the test tool and the inspection camera will typically be different for each different test tool. The process of manually positioning the test tool is time consuming and requires highly skilled operators. It would be desirable to fully automate the bond testing process. At present this is not possible because there is no way to accurately determine the offset between the sample inspection camera and the test tool assembly. The present inventions seeks to overcome these deficiencies, and to provide a bond testing apparatus and method to calculate the position of a test tool relative to an inspection camera automatically. Summary of Invention According to a first aspect of the disclosure, there is provided a bond testing apparatus comprising: a sample support configured to support a sample to be tested; a test tool assembly comprising a least one test tool, the test tool positioned on a first side of the sample support and configured to move relative to the sample support to interact with a sample on the sample support; a sample inspection camera positioned on the first side of the sample support and configured to capture images of a sample on the sample support, wherein the sample inspection camera is fixed relative to the test tool at a fixed offset; a positioning mechanism configured to move the test tool relative to the sample support; control circuitry configured to operate the positioning mechanism to perform a test on a sample on the sample support, based on one or more images captured by the sample inspection camera; and a calibration system configured to calculate the fixed offset between the sample inspection camera and the test tool, the calibration system comprising: a calibration camera positioned on a second side of the sample support, opposite the first side; a reference mask arranged coplanar with the sample support, the reference mask comprising an aperture and a plurality of mask features that allows for the calculation of a virtual target datum within the aperture; and one or more processors configured to calculate a virtual target datum in the central aperture, using the plurality of mask features upon receiving an image or images from the calibration camera or the sample inspection camera; wherein the one or more processors is configured to determine that the sample inspection camera is aligned with the virtual target datum based on a position of the virtual target datum within an image or images from the sample inspection camera; wherein the one or more processors is configured to determine when the test tool is aligned with the virtual target datum based on an image or images of the test tool from the calibration camera; and wherein the one or more processors is configured to calculate the offset between the sample inspection camera and the test tool based on a movement of the positioning mechanism required to move from a first position in which the sample inspection camera is aligned with the virtual target datum and a second position in which the test tool is aligned with the virtual target datum. In use, the bond testing apparatus can automatically calculate the position of a test tool without the need of manual input from an operator. Advantageously, automated calculation of the test tool position is more accurate than a manual calculation because it negates any parallax error that may be introduced by an operator manually positioning a test tool with a bond. Advantageously, automated calculation of the test tool position is more time and cost efficient over a manual calculation. Advantageously, automated calculation of the test tool position is more precise than a manual calculation by an operator due to the higher processing power of computers over humans. The reference mask features may comprise two or more apertures positioned outside of the central aperture. Advantageously, this configuration of apertures allows the one or more processors to calculate a virtual target datum within the aperture. The virtual target datum is preferably at a centre of the central aperture. The one or more processors may perform a geometric calculation based on a centre point of each aperture outside of the central aperture. For example, a virtual crosshair may be calculated from lines passing through the centre point of each aperture. Advantageously, using apertures as reference mask features may allow for the one or more processors to calculate the virtual target datum in a variety of lighting conditions. The apertures may be circular. The apertures may be rectangular. The apertures may be triangular. The apertures may be a polygonal shape that is most readily identified the one or more processors. Each of the three or more apertures may be a different shape to the others. The two or more apertures may be formed as part of, or may merge with, the central aperture. For example the two or more apertures could be shaped as slits on the edge of the central aperture, wherein each of the slits is angled towards a point that can be defined by the one or more processors as the virtual target datum. This configuration may allow for easier manufacturing of the reference mask because the manufacturing of the central aperture and the three or more apertures can be combined in a single step. The central aperture may be circular. The two or more apertures circumferentially surround the central aperture. The central aperture may be a polygonal shape. The mask features may be vertexes of the polygon. This configuration is advantageous because no additional manufacturing is required to form the plurality of reference apertures beyond formation of the central aperture. The central aperture may be a regular polygonal shape. The central aperture may be a square shape. The central aperture may be a rectangular shape. The central aperture may be a symmetrical shape. The central aperture may be a polygon where the angle of the vertexes are congruent. The central aperture may be a polygon where all of the sides are congruent. The reference mask may comprise a non-reflective coating. This feature is advantageous because it increases contrast in the image or images of the central aperture received by the one or more processors based on an input from the sample inspection camera or the calibration camera. The reference mask may comprise an opaque material. The calibration system may further comprise a touchdown pad arranged coplanar with the reference mask. The positioning mechanism may perform a touchdown of the test tool on the touchdown pad. The one or more processors then stores the vertical position of the positioning mechanism at touchdown as a first vertical position prior to alignment of the test tool with the virtual target datum. The test tool may be configured to descend to the first vertical position during alignment with the virtual target datum point. The test tool may be lowered to the height at which a test is performed. The offset between the test tool and the sample inspection camera may change as the test tool is lowered, it is therefore advantageous to calculate the offset at the height in which a test is performed. The touchdown pad may be coplanar with the sample. Performing a test tool touchdown may allow the test tool to be swiftly lowered to the reference mask without risking collision with other potentially fragile components of the calibration system. The one or more processors may comprise a software configured to perform calculations upon receiving an image or images from the test inspection camera or the sample inspection camera. The reference mask may be coplanar with the focal plane of the calibration camera. This configuration may allow the calibration camera to capture the sharpest image of the test tool and in particular the tip of the test tool. A sharper image of the tip of the test tool may allow the one or more processors to calculate the position of the tip of the test tool more precisely. The calibration system of the bond testing apparatus may further comprise a calibration light system configured to illuminate the test tool. Advantageously, when the test tool is illuminated, the one or more processors receives a clear image of the test tool from the calibration camera. A clear image of the test tool may allow the program in the one or more processors to precisely and accurately determine the test tool position. The calibration light system may be a ring shape. Advantageously, a ring shape light may provide uniform illumination of the test tool. The calibration light system may be a disc shape. The disc shape calibration light may be located between the reference mask plane and the test tool. The calibration light system may comprise a plurality of discrete light sources. The calibration light system may comprise a metallic substrate configured to dissipate heat. The calibration light system may comprise an aluminium substrate. The calibration light system may comprise a calibration light located between the reference mask and the calibration camera. The calibration light may illuminate the plurality of mask features. Illuminated mask features may make it easier for the one or more processors to calculate a virtual target datum. The test tool assembly may form part of a removable cartridge. A different removable cartridge may comprise a different test tool assembly, wherein the different test tool assembly is configured to perform a different test on a sample. For example, a cartridge comprising a test tool assembly configured to perform a shear test may be replaced by a cartridge comprising a test tool assembly configured to perform a pull test. A removable cartridge is advantageous because it increases the variety of tests that a bond testing apparatus can perform. The test tool assembly may comprise a rotatable test tool, the rotatable test tool comprising a tip, wherein the one or more processors stores a plurality of positions of the test tool tip during rotation of the test tool. It is useful to store a plurality of positions during test tool rotation so that tool eccentricities can be calculated and accounted for. For example, a test tool that performs a wire pull test may be a hook-type shape and in order to perform a test on the correct wire, the position of the hook tip must be known throughout the rotation. Storing a plurality of positions of the test tool tip may allow the bond testing apparatus to calculate the test tool tip position throughout a rotation. The calibration system may further comprise an XY translation stage configured to position the reference mask relative to a field of view of the calibration camera. An XY translation stage allows for greater build tolerances as the calibration camera and reference mask do not have to initially be aligned. In a preferred embodiment, the sample inspection camera and the test tool are moveable relative to the calibration camera and the sample support, which are fixed. The positioning mechanism may comprise an XY stage to which the sample support is fixed and a Z stage to which the test tool assembly and the sample inspection camera are fixed. For example, the positioning mechanism may be configured to move the sample support in a plane relative to the sample inspection camera and the test tool. The plane may contain both the X and Y directions, so may be denoted an XY plane. The positioning mechanism may be configured to move both the sample inspection camera and the test tool in a direction perpendicular to the plane in which the sample support moves. So, if the sample support is moveable in the XY plane, the sample inspection camera and the test tool may be moveable in the Z direction. The one or more processors may be configured to calculate a virtual target datum in the central aperture, using the plurality of mask features upon receiving an image or images from the sample inspection camera. The one or more processors may be configured to calculate the same virtual target datum in the central aperture using the plurality of mask features upon receiving an image or images from the calibration camera. The sample inspection camera may comprise a sample lens. The magnification of the lens may be between 0.8x and 10x. The sample inspection camera may comprise a removeable lens. The removeable lens may be replaced with a lens with a different magnification. Samples that are tested range in size, it is useful for a bond tester to comprise a sample inspection camera capable of varying its magnification. A sample inspection camera may comprise a lens that does not fit the plurality of mask features in a single field of view and captures a plurality of images, the plurality of images are received by the one or more processors to construct an image the plurality of mask features. The calibration camera may comprise a calibration lens. The magnification of the calibration lens may be between 0.8x and 10x. The one or more processors may be configured to compare the image or images of the test tool from the calibration camera with a reference image or images of the test tool to determine if the test tool requires cleaning or maintenance. According to a second aspect of the disclosure, there is provided a.calibration system for calculating an offset between a sample inspection camera and a test tool of a bond testing apparatus, the calibration system comprising: a calibration assembly comprising: a calibration camera; a reference mask comprising a central aperture and a plurality of mask features that allows for the calculation of a virtual target datum within the aperture, wherein the calibration camera is positioned to capture an image or images of the central aperture and the plurality of mask features; and a computer program (or computer program product) comprising instructions which, when executed by a processor, causes the processor to: calculate a virtual target datum in the central aperture, using the mask features upon receiving an image or images from the calibration camera or the sample inspection camera of the bond tester; determine when the sample inspection camera is aligned with the virtual target datum based on a position of the virtual target datum within an image or images from the sample inspection camera; determine when the test tool is aligned with the virtual target datum based on an image or images from the calibration camera; and calculate the offset between the sample inspection camera and the test tool based on the relative movement required to move from a first position in which the sample inspection camera is aligned with the virtual target datum and a second position in which the test tool is aligned with the virtual target. The calibration system according to the second aspect of the present disclosure may include any of the features described with reference to the calibration system of the first aspect. The computer program may be configured to cause the processor to compare the image or images of the test tool from the calibration camera with a reference image or images of the test tool to determine if the test tool requires cleaning or maintenance. The calibration system according to the second aspect of the present disclosure may be retrofittable to existing bond testers. The computer program may be downloadable. The computer program may be downloadable onto existing bond testers. According to a third aspect of the disclosure, there is provided a method for calculating an offset between a sample inspection camera and a test tool of a bond testing apparatus, the bond testing apparatus comprising a calibration system, the calibration system comprising: a reference mask and a calibration camera, the reference mask comprising a central aperture and a plurality of mask features that allow for the calculation of a virtual target datum within the aperture, wherein the calibration camera is positioned to capture an image or images of the central aperture and the plurality of mask features; and one or more processors configured to receiving an image or images from the calibration camera and the sample inspection camera of the bond testing apparatus; wherein the sample inspection camera and the test tool are positioned on a first side of the reference mask and the calibration camera is positioned on a second side opposite the first side of the reference mask; wherein the sample inspection camera is at a fixed offset from the test tool; the method comprising: calculating a virtual target datum within the central aperture using the one or more processors based on an image or images received from the sample inspection camera or the calibration camera; aligning the sample inspection camera with the virtual target datum; aligning the test tool with the virtual target datum based on an image or images from the calibration camera; and calculating the offset between the sample inspection camera and the test tool using the one or more processors based on the relative movement required to move from a first position in which the sample inspection camera is aligned with the virtual target datum and a second position in which the test tool is aligned with the virtual target datum. The advantages realised by the method according to the third aspect of the disclosure are the same as those realised by the apparatus according to the first aspect of the disclosure and the system according to the second aspect of the disclosure. That is, automation of the process of calculating the test tool position provides an increase in time efficiency, accuracy, and precision over a method of manual calculation by an operator. The step of aligning the test tool with the virtual target datum further may further comprise determining a vertical test tool position by performing a test tool touchdown on a touchdown datum arranged coplanar with the reference mask. The test tool may be lowered so it can be captured more clearly by the calibration camera. Performing a test tool touchdown may allow the test tool to be swiftly lowered to the reference mask without risking collision with other potentially fragile components of the calibration system. The method may further comprise a step of comparing an image or images of the test tool from the calibration camera with a reference image or images of the test tool to determine if the test tool requires cleaning or maintenance. During use, a test tool may become damaged or pick up debris. A defective test tool may not perform tests correctly, so it is advantageous to have a method that increases the likelihood that a correctly functioning test tool is being used. It is particularly advantageous to automate the process of assessing if a test tool requires cleaning because computers are typically capable of identifying defects with increased accuracy compared to humans. The method may further comprise a step of rotating the test tool about its longitudinal axis through a plurality of rotational positions, and recording an image or images of the test tool in each of the plurality of different rotational positions. For example, a test tool that performs a wire pull test may be a hook shape. In order to perform a test on the correct wire, the position of the hook tip must be known throughout the rotation. Storing a plurality of positions of the test tool tip may allow the bond testing apparatus to calculate the test tool tip position throughout a rotation. The calibration system according to the third aspect of the present disclosure may be the calibration system of the first or second aspect and may include any of the features described with reference to the first or second aspect. In a fourth aspect of the disclosure, there is provided a bond testing apparatus comprising: a sample support configured to support a sample to be tested; a test tool assembly comprising a least one test tool, the test tool positioned on a first side of the sample support and configured to move relative to the sample support to interact with a sample on the sample support; a positioning mechanism configured to move the test tool relative to the sample support; control circuitry configured to operate the positioning mechanism to perform a test on a sample on the sample support; and a test tool inspection system configured to inspect the test tool for damage or debris, the test tool inspection system comprising: a tool inspection camera positioned on a second side of the sample support, opposite the first side, the tool inspection camera configured to capture one or more images of the test tool; and one or more processors configured to compare the one or more images of the test tool from the tool inspection camera with a reference image or images of the test tool to determine if the test tool requires cleaning or maintenance. Brief Description of the Drawings Adetailed description of embodiments of the disclosure will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a bond testing apparatus according to a first embodiment of the present disclosure; Figure 2A is an isometric view of a section of the bond testing apparatus of Figure 1 with the image camera aligned with the calibration camera; Figure 2B is an isometric view of a section of the bond testing apparatus of Figure 1 performing a touchdown; Figure 2C is an isometric view of a section of the bond testing apparatus of Figure 1 with the test tool aligned with the calibration camera; Figure 3 is an exploded view of components of the calibration assembly; Figure 4 is a close up of a test tool during alignment with the virtual target datum; Figure 5 is schematic illustration of a reference mask; Figure 6 is a flow chart of a method of calculating the offset between the sample inspection camera and the test tool of a bond testing apparatus; Figure 7 is a flow chart of a method of determining if the test tool requires cleaning or maintenance. Detailed Description Figure 1 is an isometric view of a bond testing apparatus 100 according to a first embodiment of the present disclosure. The bond testing apparatus 100 comprises a sample inspection camera 4 and a test tool cartridge 16, both comprised within a test rig 2. Beneath the test rig 2 is a sample support 10 on which a sample to be tested using a test tool is fixed. A sample stage 18 is mounted onto the sample support 10. A calibration system 6 is fixed to the sample support. The sample support 10 and the calibration assembly 6 are fixed relative to one another on a X-stage table 54. The X-stage table 54 comprises a X-guide rails 34, which are positioned on the top surface of the X-stage table 54. The sample support 10 and the calibration assembly 6 are configured to move along the X-guide rails 34 in the X-axis. Movement of the sample support 10 and the calibration assembly 6 along the X-guide rails 34 is achieved using an actuator, such as a stepper or servo motor. The X-stage table 54 is mounted onto an Y-stage table 56. The Y-stage table 56 forms part of the support structure 38. The Y-stage table comprises Y-guide rails 44, which are positioned on the top surface of the Y-stage table 56. The Y-stage table 54 is configured to move along the Y-guide rails 44 in the Y-axis. Movement of the Y-stage table 54 along the Y-guide rails 44 is achieved using an actuator, such as a stepper or servo motor. The X and Y guide rails 34, 44 form part of the top surfaces of the X and Y stage tables 54, 56. The calibration assembly 6 comprises a housing 20 and a reference mask 8 located on a side of the housing 20 facing the test rig 2. The sample stage 18 and the calibration assembly 6 are configured such that the sample stage 18 and reference mask 8 are coplanar with one another on a sample plane 50. The test rig 2 is mounted on the frame 14 and moves in a direction normal to the sample plane 50 in the Z-axis. The bond testing apparatus comprises a Z-screw mechanism 22 which is configured to move the test rig 2 along the Z-axis. The test rig 2 comprises the sample inspection camera 4 and the test tool cartridge 16. The cartridge 16 comprises a test tool assembly, the test tool assembly comprises a test tool 48. The cartridge 16 is configured to be interchangeable and be replaced by another cartridge comprising a different test tool to perform a different bond test. The cartridge may sometimes be replaced with a cartridge comprising the same type of test tool assembly when a test tool is defective or requires cleaning. The test tool may be a shear tool, a push tool, or a pull tool depending on the test to be performed. Once the cartridge 16 is in position, the test tool 48 is at a fixed offset from the sample inspection camera 4. The bond testing apparatus 100 is configured to perform tests on bonds of samples on the sample stage 18. By replacing the cartridge 16, different tests can be performed. A non-exhaustive list of tests types are: a push test, a wire pull test, a shear test, a cold ball pull test. For a shear test, a test tool with a chisel shaped tool tip is placed on the surface of a sample, next to the bond to be tested. The tool tip moves across the surface of the sample to assert a shear force on the bond by pushing the bond in a direction parallel to the sample. The force at which the bond is sheared from the sample is measured. For a wire pull test, a test tool with a hook shaped tool tip is rotated beneath the midpoint of a wire that is connected to two bonds. The tool tip moves away from the sample surface, pulling the wire until it breaks. The force at which the wire breaks is measured. The bond testing apparatus 100 comprises a viewing lens 60 for an operator to look through. The viewing lens 60 is angled so the operator is able to view the test tool 48 interact with bonds on the sample stage 18. The bond testing apparatus 100 comprises a processor configured to operate the test tool assembly to perform a test on the sample stage 18 based on one or more images from the sample inspection camera 4. The processor is configured move the test rig 2 through space by controlling the motor of the Z-screw mechanism. The bond testing apparatus 100 comprises a calibration system to allow the process of locating the position of the test tool relative to a particular identified bond to be automated. The calibration system is configured to calculate an offset vector between the test tool and sample inspection camera 4 of the bond testing apparatus 100 so that once a bond for testing has been identified by the inspection camera the test tool can be correctly positioned to perform a test on it. This is described in further detail with reference to Figures 2A-2C. Figure 2A is an isometric view of a section of the bond testing apparatus showing the sample inspection camera 4 aligned with the calibration assembly 6. When describing Figures 2A-2C, the test rig 2 moves in the Z direction and the calibration assembly moves in the XY plane consistent with the embodiment illustrated in Figure 1. The test tool 48 is offset from the sample inspection camera 4 in a direction parallel to the reference mask 8. The calibration assembly comprises a reference mask 8. The reference mask 8 comprises a central aperture 52 through which an image can be captured by the calibration camera. The calibration mask 8 also includes reference features, in this embodiment satellite apertures around the central aperture, that allow for the calculation of a virtual target datum within the central aperture based on images of the mask captured on one side by the test inspection camera and on the other side by the calibration camera. The reference mask 8 can be seen more clearly in Figure 5. The processor is configured to generate a virtual target datum using the plurality of mask features upon receiving an image or images from the sample inspection camera when the test inspection camera 4 is aligned with the central axis C. The processor (not shown) receives an image or images from the test inspection camera 4. The positioning mechanism of the bond testing apparatus 100 aligns the test inspection camera 4 with a central axis C of the calibration assembly 6, based on an image or images received from the sample inspection camera 4. When a central pixel or pixels of the sample inspection camera 4 is aligned with the virtual target datum, the co-ordinates of the positioning mechanism are stored in a memory connected to the processor. Figure 2B is an isometric view of a section of the bond testing apparatus performing a touchdown. The reference mask 8 is in the focal plane of the calibration camera. In order for the test tool 48 to be swiftly lowered to the same plane, within the central aperture 52 of the reference mask 8, and later to the sample, without risk of damage to components, a test tool 48 touchdown is performed. The calibration assembly 6 comprises a touchdown pad 82 arranged coplanar with the central aperture 52. The touchdown pad 82 is formed as part of the housing 20. To perform a touchdown, the position of the touchdown pad 82 is found using the sample inspection camera 4. The position is stored in the processor. The test tool 48 is then roughly aligned with the touchdown pad 82 and lowered in the Z axis until the test tool tip 58 contacts the touchdown pad 82. The Z position of the positioning mechanism when the test tool 48 contacts the touchdown pad 82 is stored as a first virtual position in the memory connected to the processor. Touchdown can be detected using known techniques using sensors are part of the test tool assembly. The test tool may be mounted on cantilever arms within the cartridge and flexure of those arms when a touchdown occurs can be detected. After the first vertical position is found, the test tool 48 is then aligned with the calibration assembly 6. The test tool 48 is lowered to the first vertical position so the tool tip is in the plane within the central aperture 52. A clearly focussed image of the test tool 48 is generated as the test tool 48 is in the focal plane of the calibration camera. The virtual target datum, the coordinates of which are found using the sample inspection camera 4 and stored in the processor, is created again based on an image or images from the calibration camera. The processor controls the positioning mechanism to align the test tool tip with the virtual target datum based on the image or images received from the calibration camera. Figure 2C is an isometric view of the assembly in this position. The co-ordinates of the positioning mechanism when the test tool tip is aligned with the virtual target datum are stored in a memory connected to the processor. The distance between the test tool and the central pixel or pixels of the sample inspection camera is then calculated based on the stored co-ordinates of the positioning assembly. In particular the difference between the XY co-ordinates when the sample inspection camera is aligned with the virtual target datum and the XY co-ordinates when the test tool tip is aligned with the virtual target datum corresponds to the offset between the sample inspection camera and test tool. Using this offset, the test tool 48 can be accurately positioned when performing sample tests on the sample stage 18. If the test tool 48 is rotatable, the rotatable test tool is first aligned to the virtual datum point and then rotated. The processor records an image or images of the rotatable test tool in a plurality of different positions during rotation. For example, a test tool that performs a wire pull test may be a hook shape. In order to perform a test on the correct wire, the position of the hook tip must be known throughout the rotation in order to rotate underneath the correct wire for testing without coming into contact with another component on the sample. Storing a plurality of positions of the test tool tip allows the bond testing apparatus to calculate the test tool tip position during a rotation. The rotational offset in each position can be added to the calculated XY offset to ensure that the test tool is correctly positioned relative to a bond to be tested in each rotational position. Figure 3 is an exploded view of components of the calibration assembly 6. The calibration system may be a separable unit, that can be added to any bond testing apparatus that can be configured or reconfigured to accommodate it. A mask holder 72 is mounted to the housing 20 of the calibration assembly 6. The mask holder 72 comprises a central cavity configured to retain the mask mount 58, the reference mask 8, the calibration light 56, and the light diffuser 54. The mask mount 58, the reference mask 8, the calibration light 56, and the light diffuser 54 all comprise through holes 98. The through holes 98 are equidistant from the centre of each component 8, 56, 54. The through holes 98 are aligned and held in position with screws 98. The screws 98 fix the reference mask 8, the calibration light 56, and the light diffuser 54 in position to prevent relative movement and rotation. The mask mount 58 is retained within the central cavity of the mask holder 72 and is configured to provide a flat surface for the reference mask 8. The mask mount 58 is configured to act as a heat sink for the heat emitted from the calibration light 56. The housing 20 is substantially hollow and internally contains the calibration camera 78. The reference mask 8 is in the field of view of the calibration camera 78. The reference mask 8 comprises a central aperture 52 surrounded by four equally spaced circular apertures 92. The circular apertures 92 are circular because image recognition software is configured to recognise circles and calculate a central point of a circle better than other shapes. There may be embodiments with apertures having a different shape that can be recognised by software that is programmed to recognise a different shape e.g. a square.. The plurality of circular apertures 92 are equidistant from the centre of the reference mask 8. A processor (not shown) is configured to create a virtual crosshair that defines a virtual datum point using the plurality of mask features based on an image or images from the calibration camera 78. The cross hair is formed from two lines, each of which pass through the centre of diametrically opposite circular apertures. The central aperture may be any one of a variety of shapes; for example, a square. Any number of mask features above the minimum that are required for the processor to calculate a virtual target datum within the central aperture can be used. The magnification of the lens on either the sample inspection camera or the calibration camera may require the relative geometry of the central aperture 52 and the plurality of mask features 92 to differ. For example, a higher magnification lens may require the central aperture and the plurality of mask features to have a smaller geometry to fit into the field of view. Therefore, a multitude of different reference masks can be provided for use in the calibration system. The mask holder 72 comprises a translation stage 64 which supports the reference mask 8. The translation stage 64 is configured to be controllable to position the reference mask 8 so the central aperture 52 and plurality of mask features 92 are encompassed in the calibration camera 78 field of view. The translation stage 64 is also configured to fix the reference mask 8 in position, once the central aperture 52 and plurality of mask features 92 are in the field of view of the calibration camera 78. The translation stage 64 may be manually adjusted. The mask holder 58, the calibration light 56 and the light diffuser 54 each comprise a central opening, wherein the central opening encompasses the entire the plurality of mask features 92 and central aperture 52 of the reference mask 8. The calibration light 56 is positioned on the opposite side of the reference mask 8 to the calibration camera 78. The calibration light 56 is configured to illuminate a test tool to provide a clear image for the calibration camera 78. The calibration light 56 comprises a plurality of point light sources (not shown). The light diffuser 54 is positioned between the calibration light 56 and the test rig. The light diffuser 54 is configured to diffuse the point light sources in the calibration light 56, to evenly illuminate the test tool. A lens 74 is attached to the calibration camera 78. The lens 78 is telecentric, so the field of view is maintained regardless of distance. A conventional lens may be used instead. The lens 74 magnifies the image of the reference mask 8, but does not exceed a magnification that does not encompass all plurality of mask features 92 in the field of view of the calibration camera 78. In use, the calibration camera 78 is moved axially towards or away from the reference mask 8 until the image of the reference mask 8 is in focus, the calibration camera 78 is then clamped into position. The calibration camera 78 is electrically connected to a camera connector 84. Image data from the calibration camera 78 passes through the camera connector 84. The camera connector 84 is connectable to the processor. The camera connector 84 is connectable to a bond testing apparatus with a USB cable. In other embodiments, different cable connections may be used. The processor may be external to the calibration assembly. Figure 4 is a close up of a test tool 48 being aligned with a virtual datum point. The test tool 48 is in the field of view of the calibration camera through the central aperture 52. The processor calculates a virtual target datum using the plurality of mask features 92 and controls the apparatus to align the test tool 48 with the virtual datum point. Figure 5 is schematic illustration of a reference mask 8. The plurality of mask features 92 are arranged around the central aperture 52. The plurality of mask features 92 are circular satellite apertures in the reference mask 8 that are outside of the central aperture. The central aperture 52 is circular. The plurality of mask features and the central aperture may take many different forms, for example, different shapes. The configuration of the plurality of mask features and the central aperture may be different, for example the central aperture may be a rectangle, where the plurality of reference mask features are the corners of the rectangle. The reference mask 8 has a non-reflective coating to increase contrast in the image or images of the central aperture received by the one or more processors based on an input from the sample inspection camera or the calibration camera. The one or more processors calculate the virtual target datum using the plurality of mask features 92 based on an image or images received from the sample inspection camera. The same calculation is completed based on an image or images received from the calibration camera. Image recognition software is used to calculate the centres of the satellite apertures 210, 220, 230, 240 (or the position of the corners of the central rectangular aperture in an embodiment that uses a rectangular central aperture). The one or more processors calculate the virtual target datum as the point of intersection of virtual lines drawn between the centre points of opposite satellite apertures 92. Different calculations may be used for different reference mask embodiments, for example embodiments with a different number of mask features where a different number of virtual lines are drawn. In the example shown in Figure 5, the processor stores the centre position of each of the reference apertures 210, 220, 230, 240 in space as a Cartesian coordinate. The one or more processors then calculate the point of intersection between a line between the centre of aperture 210 and the centre of aperture 220 and a line between the centre of aperture 230 and the centre of aperture 240. The point of intersection is stored as the virtual target datum. Figure 6 is a flowchart of a method of calculating the offset between the sample inspection camera and the test tool of the bond testing apparatus described. In step 300 the process of calculating the offset between the sample inspection camera and the test tool of a bond testing apparatus is initiated. In step 305 a new test cartridge is fitted to the bond testing apparatus. Once a new cartridge is fitted, the test tool position relative to the sample inspection camera needs to be calculated in order to accurately test the a sample. Of course, the step of calculating the position of a test tool may be performed for a cartridge that is not newly fitted. For example the offset between the sample inspection camera and the test tool may be re-calculated after test tool performs multiple tests on a sample. It is also possible for a cartridge to comprise multiple test tools, each of which can be moved into a testing position within the cartridge before performing a test. The step of calculating the position of a test tool may be performed each time a different test tool is moved into a testing position within the cartridge. The method further comprises step 310 of moving the sample inspection camera to the reference mask centre. In this position the sample inspection camera is roughly aligned with the central axis of the reference mask, so the movements can be relatively quick as high precision is not required at this step 310. In this position, the sample inspection camera can capture an image or images of the central aperture and mask features of the reference mask. Step 310 may be carried out by moving the calibration assembly through space using the positioning mechanism shown in Figure 1. In step 315 the processor calculates the position of the virtual target datum using mask features of the reference mask based on an image or images from the sample inspection camera. The processor assesses whether a central pixel or pixels of the sample inspection camera are aligned with the virtual target datum: if YES then move to step 325, if NO then move to step 320. In step 320 the processor controls the positioning mechanism to align the axis of the central pixel or pixels of the sample inspection camera with the virtual target datum. Once the positioning mechanism is in a new position, the coordinates of the positioning mechanism are stored in the processor. The method returns to step 315. In step 325 the test tool is aligned with the touchdown datum. The test tool and sample inspection camera are offset at an approximate distance that is known due to the dimensions of each component. The sample inspection camera aligns with a touchdown pad, the coordinates are stored in the processor as touchdown pad coordinates, the test tool is then approximately aligned with the touchdown pad based on the touchdown pad coordinates. In step 328, once the test tool is aligned with the touchdown datum, the positioning mechanism lowers the test tool until the test tool comes into contact with the touchdown pad. The processor stores the Z axis coordinate of the positioning system as a first vertical position when the test tool touches the touchdown pad. The method further comprises step 330 of using the positioning mechanism to move the test tool in the Z axis to the reference mask. The test tool will be on an axis approximately aligned with in the centre of the central aperture of the reference mask and positioned at the same height as the first vertical position. The height of the first vertical position is coplanar with the reference mask. The reference mask is in the focal plane of the calibration camera. In step 335 the processor re-calculates the position of the virtual target datum based on an image or images from the calibration camera. The processor assesses whether the test tool is aligned with the virtual target datum by measuring the deviance of the test tool from the virtual target datum based on an image or images from the calibration camera: if YES, the tool is aligned, then move to step 345, if NO, the tool is not aligned, then move to step 340. In step 340 the processor controls the positioning mechanism to align the test tool with the virtual target datum by moving the calibration assembly. Once the sample test tool is aligned with the target datum, the coordinates of the positioning system are stored in the processor. The method returns to step 335. The method further comprises step 345 of assessing whether an additional operation is required if the test tool is a rotating type: if YES, move to step 350, if NO, proceed to step 355. The method further comprises step 350 of rotating the test tool about its longitudinal axis through a plurality of rotational positions and recording an image or images of the test tool in each of the plurality of different rotational positions. For example, a hook-type test tool could have its position recorded, then be rotated 180 degrees about its longitudinal axis and the new position recorded, then the hook-type test tool is rotated to its original position. In order to perform a test on the correct wire, the position of the hook tip must be known throughout the rotation in order to rotate underneath the correct wire for testing without coming into contact with another component on the sample. Storing a plurality of positions of the test tool tip allows the bond testing apparatus to calculate the test tool tip position in any rotational position. The method further comprises step 355 of proceeding to a next stage of operation of the bond testing apparatus. The processor will signal to the control circuitry that the test tool position has been calibrated. The method further comprises step 360 of finishing the test tool calibration operation. Figure 7 is a flow diagram of a method of determining if the test tool requires cleaning or maintenance. The method comprises step 400 which initiates the flowchart process of comparing an image or images of a test tool with a database of test tool images to determine if a test tool requires cleaning or maintenance after performing a test on a sample. The process illustrated in Figure 7 occurs after the position of the test tool has been calibrated. The method further comprises step 405 of interrogating a look-up table which indicates the threshold number tests a specific test tool can perform until inspection is due. In step 410 a test is performed on a sample by a test tool. The method further comprises step 415 of assessing if an inspection of the test tool is due. When a test is performed on a sample, the test tool may become damaged or it may become contaminated by materials like solder. The number of tests that a test tool will perform before it is inspected will depend on the test tool type and productivity requirements. When a test is first performed at step 405, the processor will begin a test counter. In step 415 the processor will calculate whether the test counter has reached the threshold integer: if NO, the move to step 420, if YES, move step 425. In step 420 of the test counter is increased by a single integer. The method then returns to step 410. The method further comprises step 425 of moving the test tool to the reference mask. As the method of Figure 7 occurs after calibration of the position of the test tool, the test tool can be automatically moved to the reference mask in step 425. In step 430 the processor compares an image or image received from the calibration camera with images from the look-up table for the specific tool. Based on a result of the comparison, the processor determines if the tool is contaminated with debris or is damaged. If the determination is YES, the test tool is contaminated with debris or damaged, then the method returns to step 410. If the determination is NO, the method continues to step 435. In step 435 the tool is moved to a cleaning station where a cleaning operation is performed. Examples of cleaning options are mechanical wiping, solvent wiping, heated solder reflow, and vacuum-air interaction. At step 435, one or more cleaning operations may be performed. In step 440, the tool is moved to the reference mask. Now the tool is at the reference mask, in step involves assessment the processor compares an image or image received from the calibration camera with images from the look-up table for the specific tool. Based on a result of the comparison, the processor determines if the tool is contaminated with debris or is damaged. If the determination is YES, the test tool is contaminated with debris or damaged, then the method returns to step 410. If the determination is NO, the method continues to step 450. The method further comprises step 450 of calling for maintenance if it is determined that the test tool is contaminated with debris or damaged. For example, an operator may be notified via a prompt like a light or a noise which indicates that the test tool needs replacing or further cleaning.

Claims

1. A bond testing apparatus comprising:a sample support configured to support a sample to be tested;a test tool assembly comprising a least one test tool, the test tool positioned on a first side of the sample support and configured to move relative to the sample support to interact with a sample on the sample support;a sample inspection camera positioned on the first side of the sample support and configured to capture images of a sample on the sample support, wherein the sample inspection camera is fixed relative to the test tool at a fixed offset;a positioning mechanism configured to move the test tool relative to the sample support; control circuitry configured to operate the positioning mechanism to perform a test on a sample on the sample support, based on one or more images captured by the sample inspection camera; anda calibration system configured to calculate the fixed offset between the sample inspection camera and the test tool, the calibration system comprising:a calibration camera positioned on a second side of the sample support, opposite the first side;a reference mask arranged coplanar with the sample support, the reference mask comprising an aperture and a plurality of mask features that allows for the calculation of a virtual target datum within the aperture; andone or more processors configured to calculate a virtual target datum in the central aperture, using the plurality of mask features upon receiving an image or images from the calibration camera or the sample inspection camera;wherein the one or more processors is configured to determine that the sample inspection camera is aligned with the virtual target datum based on a position of the virtual target datum within an image or images from the sample inspection camera;wherein the one or more processors is configured to determine when the test tool is aligned with the virtual target datum based on an image or images of the test tool from the calibration camera; andwherein the one or more processors is configured to calculate the offset between the sample inspection camera and the test tool based on a movement of the positioning mechanism required to move from a first position in which the sample inspection camera is aligned with the virtual target datum and a second position in which the test tool is aligned with the virtual target datum.

2. A bond testing apparatus according to claim 1, wherein the mask features comprise three or more apertures positioned outside of the central aperture.

3. A bond testing apparatus according to claim 1, wherein the central aperture is a polygon and the mask features are vertexes of the polygon.

4. A bond testing apparatus according to claim 1 or 2, wherein the reference mask comprises non-reflective coating.

5. A bond testing apparatus according to any preceding claim, wherein the calibration system further comprises a touchdown pad arranged coplanar with the reference mask, wherein the positioning mechanism performs a touchdown of the test tool on the touchdown pad and the processor stores the vertical position of the positioning mechanism at touchdown as a first vertical position prior to alignment of the test tool with the virtual target datum; and wherein the test tool is configured to descend to the first vertical position during alignment with the virtual target datum point.

6. A bond testing apparatus according to any preceding claim, wherein the reference mask is coplanar with a focal plane of the calibration camera.

7. A bond testing apparatus according to any preceding claim, further comprising a calibration light system configured to illuminate the test tool.

8. A bond testing apparatus according to claim 7, wherein the calibration light system comprises a calibration light located between the reference mask and the calibration camera.

9. A bond testing apparatus according to any preceding claim, wherein the test tool assembly is comprised within a removable cartridge.

10. A bond testing apparatus according to any preceding claim, wherein the test tool assembly comprises a rotatable test tool, the rotatable test tool comprising a tip, wherein the one or more processors stores a plurality of positions of the test tool tip during rotation of the test tool.

11. A bond testing apparatus according to any preceding claim, wherein the one or more processors is configured to compare an image or images of the test tool from the calibration camera with a reference image or images of the test tool to determine if the test tool requires cleaning or maintenance.

12. A bond testing apparatus according to any preceding claim, the calibration system further comprising an XY translation stage configured to position the reference mask relative to a field of view of the calibration camera.

13. A bond testing apparatus according to any preceding claim, wherein the positioning mechanism comprises an XY stage to which the sample support is fixed and a Z stage to which the test tool assembly and the sample inspection camera are fixed.

14. A bond testing apparatus according to any preceding claim, wherein the one or more processors is configured to calculate a virtual target datum in the central aperture, using the plurality of mask features upon receiving an image or images from the sample inspection camera, and is configured to calculate the same virtual target datum in thecentral aperture using the plurality of mask features upon receiving an image or images from the calibration camera.

15. A calibration system for calculating an offset between a sample inspection camera and a test tool of a bond testing apparatus, the calibration system comprising: a calibration assembly comprising:a calibration camera;a reference mask comprising a central aperture and a plurality of mask features that allows for the calculation of a virtual target datum within the aperture, wherein the calibration camera is positioned to capture an image or images of the central aperture and the plurality of mask features; anda computer program (or computer program product) comprising instructions which, when executed by a processor, causes the processor to:calculate a virtual target datum in the central aperture, using the mask features upon receiving an image or images from the calibration camera or the sample inspection camera of the bond tester;determine when the sample inspection camera is aligned with the virtual target datum based on a position of the virtual target datum within an image or images from the sample inspection camera;determine when the test tool is aligned with the virtual target datum based on an image or images from the calibration camera; andcalculate the offset between the sample inspection camera and the test tool based on the relative movement required to move from a first position in which the sample inspection camera is aligned with the virtual target datum and a second position in which the test tool is aligned with the virtual target.

16. A calibration system according to claim 15, further comprising a touchdown pad arranged coplanar with the reference mask, wherein the positioning mechanism performs a touchdown of the test tool on the touchdown pad and the processor stores a vertical position of the positioning mechanism at touchdown as a first vertical position prior to alignment of the test tool with the virtual target datum; and wherein the test tool is configured to descend to the first vertical position during alignment with the virtual target datum point.

17. A calibration system according to either claim 15 or 16, further comprising a calibration light system configured to illuminate the test tool.

18. A calibration system according to claim 17, wherein the calibration light system comprises a calibration light located between the reference mask and the calibration camera.

19. A calibration system according to any one of claims 15 to 18, wherein the processor is configured to compare an image or images of the test tool from the calibration camerawith a reference image or images of the test tool to determine if the test tool requires cleaning or maintenance.

20. A calibration system according to any one of claims 15 to 19, further comprising an XY translation stage configured to position the reference mask relative to a field of view of the calibration camera.

21. A method for calculating an offset between a sample inspection camera and a test tool of a bond testing apparatus, the bond testing apparatus comprising a calibration system, the calibration system comprising:a reference mask and a calibration camera, the reference mask comprising a central aperture and a plurality of mask features that allows for the calculation of a virtual target datum within the aperture, wherein the calibration camera is positioned to capture an image or images of the central aperture and the plurality of mask features; andone or more processors configured to receive an image or images from the calibration camera and the sample inspection camera of the bond testing apparatus;wherein the sample inspection camera and the test tool are positioned on a first side of the reference mask and the calibration camera is positioned on a second side opposite the first side of the reference mask;wherein the sample inspection camera is at a fixed offset from the test tool;the method comprising:calculating a virtual target datum within the central aperture using the one or more processors based on an image or images received from the sample inspection camera or the calibration camera;aligning the sample inspection camera with the virtual target datum;aligning the test tool with the virtual target datum based on an image or images from the calibration camera;calculating the offset between the sample inspection camera and the test tool using the one or more processors based on the relative movement required to move from a first position in which the sample inspection camera is aligned with the virtual target datum and a second position in which the test tool is aligned with the virtual target datum.

22. A method according to claim 21, wherein the step of aligning the test tool with the virtual target datum further comprises determining a vertical test tool position by performing a test tool touchdown on a touchdown datum arranged coplanar with the reference mask.

23. A method according to either claim 21 or 22, further comprising comparing an image or images of the test tool from the calibration camera with a reference image or images of the test tool using the one or more processors to determine if the test tool requires cleaning or maintenance.

24. A method according to any one of claims 21 to 23, further comprising rotating the test tool about its longitudinal axis through a plurality of rotational positions, and recording an image or images of the test tool in each of the plurality of different rotational positions.

Images