Dual magnification camera assembly for a bond test system

Abstract

A bond test apparatus 100 includes a stage 18, test tool 48 and camera assembly 104. The stage is for supporting a bond for testing. The test tool has a test tool tip which contacts the bond during th

Description

Field of the Invention The present invention relates to bond test apparatus for testing the strength of electrical bonds on semiconductor devices. In particular, the invention relates to a bond test apparatus comprising a dual-magnification camera assembly for inspecting the electrical bonds on semiconductor devices before and / or after testing the strength of the electrical bonds. 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 ball / bump or copper pillar or wire. 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 the bond 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 are moved in order to perform the test, which comprises measuring the force needed to break the bond. Each bond test tool is mounted to a load cell provided with a force transducer and associated electronics. While there are a variety of different test tools, the part of the machine used to position the test tool and move the test tool or bond during a test may be the same for each test tool. Accordingly, test tools have been designed to be removable from the rest of the machine so that they can be replaced with a different type of test tool or a test tool suitable for a different range of measurements or a different type of bond or test. It may be desirable to use several different test tools on a single substrate under test and so an operator may be required to perform replacement of test tools frequently. When performing a bond test, it is beneficial to illuminate the bond under test with sufficient light to permit accurate pre-alignment and set-up of the test, video recording and visual inspection of the test as it occurs, and subsequent visible inspection of the bond and test tool once it is complete. Current bond test apparatuses are typically fitted with cameras to permit accurate pre-alignment and set-up of the bond test and subsequent visible inspection of the bond and test tool once it is complete. Current bond test apparatuses also rely on various cameras positioned in various locations with or without interchangeable lenses for these various imaging applications. For example, set-up of the bond test includes locating features or bonds to be tested on a semiconductor substrate, which requires a low magnification and large field-of-view (FOV) to assist in locating bonds on the semiconductor substrate. In contrast, a high magnification and small FOV is required to inspect a test site with a suitable level of detail after a bond test has been performed. The current camera arrangements which meet these requirements are achieved by either changing objective lenses depending on the FOV or magnification required, or translating from one camera to another depending on the FOV or magnification required. Both of these alternatives require operator inputs, which take time and skill to perform. Additionally, with dedicated cameras for each of the various imaging applications, separate calibrations to calculate tool to camera offset vectors are required for each camera. Such offset vectors represent a vector between the camera and the tip of the test tool, and allow for partial automation of a bond test. Once a bond to be tested has been selected by an operator using one of the cameras, the bond test apparatus can subsequently translate both the camera and test tool by the offset vector, such that the test tool is adjacent to the selected bond. Similarly, once the bond has been tested, the bond test apparatus can subsequently translate a second camera and the test tool by a second offset vector, such that the second camera is adjacent to the selected bond. However, the calibrations required to obtain an offset vector per camera may also require operator inputs, which take time and skill to perform. The current camera arrangements therefore can slow down the operation of performing and inspecting a bond test. Furthermore, the current camera arrangements do not allow for simultaneous high and low magnification inspection of a test site. It would therefore be beneficial to provide a bond test apparatus which reduces the time taken to prepare, perform and inspect a bond test. Summary of Invention According to a first aspect of the present invention, there is provided a bond test apparatus comprising: a stage for supporting a bond for testing; a test tool having a test tool tip configured to contact the bond during a bond test; a camera assembly configured to image the bond before and / or after the bond test, the camera assembly comprising: an aperture for receiving a light beam therethrough; a beam splitter for splitting the light beam into a least a first beam along a first optical path and a second beam along a second optical path; a first camera arranged at an end of the first optical path and configured to detect the first beam, and a first lens arranged along the first optical path and configured to focus the first beam onto the first camera. A second camera is arranged at an end of the second optical path and configured to detect the second beam, and a second lens is arranged along the second optical path and configured to focus the second beam onto the second camera. The first camera and the second camera are configured to simultaneously image the same point at different magnifications; and the bond test apparatus further comprises a drive mechanism which is configured to provide relative movement between the stage and the camera assembly. In use, the camera assembly may therefore image a bond to be tested or a tested bond using the camera assembly, such that the first camera images the bond at a first magnification and the second camera simultaneously images the bond at a second magnification. Preferably the first camera is configured to image a first field of view (FOV) and the second camera is configured to simultaneously image a second field of view. Both the first and second FOVs contain the same imaged point, which is imaged by the two cameras at different magnifications. The FOV of the lower-magnification camera thus contains the FOV of the higher-magnification camera. As the first lens may have a different FOV or magnification to the second lens, advantageously the operator therefore has a choice of simultaneous FOVs or magnifications without the need to change lenses. As outlined above, different FOVs or magnifications may be particularly beneficial dependent on whether the bond is being imaged either before or after a bond test. Furthermore, the present invention also has the additional benefit that only a singular calibration offset vector between the camera assembly and the test tool needs to be determined to allow accurate use of the bond test apparatus with multiple cameras. As is described in more detail below, the calibration process determines an offset vector between the camera assembly and the test tool. The offset vector then permits automated operation of the test tool once a bond for testing has been located using the camera assembly. As the camera assembly comprises a singular optical axis which passes through the aperture, only a singular offset vector between the camera assembly and the test tool is required to be determined. In contrast, if two separate cameras with two different optical axes were provided, at least two offset vectors between the two respective cameras and the test tool need to be determined. The present invention therefore reduces the time and complexity of the calibration process. The drive mechanism may be connected between the stage and the rest of the bond test apparatus including the camera assembly and the test tool. The drive mechanism may thus be configured to provide relative movement between the stage and the rest of the bond test apparatus including the camera assembly and the test tool. In a preferred embodiment, the camera assembly may be rigidly mounted to the bond test apparatus. The drive mechanism may thus be configured to move the stage to control the location on the stage which is imaged by the camera assembly. The stage is preferably configured to support a sample which includes the bond for testing. The camera assembly is preferably configured to image a point on the stage for inspecting the bond for testing. Preferably the camera assembly is configured so that the first camera and the second camera simultaneously image the same point on the sample which includes the bond for testing. The camera assembly may be configured to image a field of view which does not contain the test tool. The camera assembly may thus be usable to image the bond to be tested before a bond test, and then to image the same bond after a bond test, but if the FOVs of the first and second cameras do not contain the test tool, the camera assembly is not usable to image a bond test while it takes place. The drive mechanism may be configured to move the stage in a plane relative to both the camera assembly and the test tool. The plane may contain both the X and Y directions, so may be denoted an XY plane. The drive mechanism may be configured to move both the camera assembly and the test tool in a direction relative to the stage. The direction may be perpendicular to the plane in which the drive mechanism is configured to move the stage. If the plane is an XY plane, the direction therefore may be the Z direction. The camera assembly and the test tool may therefore be fixed in the XY plane. The stage may be fixed in the Z plane. The camera assembly may be fixed relative to the test tool. The camera assembly and the test tool may both be mounted on the bond test apparatus with a fixed offset distance between them, though the test tool may be movable vertically or laterally during a bond test. In this embodiment, any relative movement between the stage and the camera assembly is equal to any relative movement between the stage and the test tool. The drive mechanism may be configured to provide relative movement between the stage and the test tool during the bond test. In other words, the same drive mechanism which is configured to provide relative movement between the stage and the rest of the bond test apparatus including the camera assembly and the test tool such that a bond may be imaged before and after testing, also may be configured to provide the relative movement between the stage and the test tool in order to perform said bond test. The first optical path may be a straight line. The first optical path may be colinear with a combined optical path between the aperture and the beam splitter. The first optical path may be colinear with the light beam received through the aperture. Advantageously, the straight first optical path colinear with a combined optical path between the aperture and the beam splitter may minimise optical disruption of light within the first optical path. A lower portion of the second optical path may not be parallel to the combined optical path between the aperture and the beam splitter. A lower portion of the second optical path may be substantially perpendicular to the combined optical path between the aperture and the beam splitter. When the first optical path is colinear with the combined optical path between the aperture and the beam splitter, the lower portion of the second optical path may be perpendicular to the first optical path. The second optical path may comprise a 90 degree deviation. An upper portion of second optical path may be substantially parallel to the combined optical path between the aperture and the beam splitter. When the first optical path is colinear with the combined optical path between the aperture and the beam splitter, the upper portion of second optical path may be parallel to the first optical path. In such an embodiment, the first lens and the second lens may be arranged parallel to one another. This may result in efficient packing of the first and second lenses in the camera assembly. The magnification of first lens may be different to the magnification of second lens. Advantageously, the operator therefore has a choice of simultaneous magnifications without the need to change lenses, which may be particularly beneficial dependent on whether the bond is being imaged either before or after a bond test. The magnification of first lens may be greater than the magnification of second lens. The magnification of first lens may be between 0.8x and 10x. For example, the magnification of first lens may be 0.8x, 1x, 2x, 3x, 4x, 6x, 8x or 10x. Preferably, the magnification of first lens is between 6x and 10x. The magnification of second lens may be between 0.5x and 8x. For example, the magnification of second lens may be 0.5x, 0.8x, 1x, 2x, 3x, 4x, 6x or 8x. Preferably, the magnification of second lens is between 0.5x and 2x. The first optical path may be different in length to the second optical path. As described herein, the length of each respective optical path is equal to the light path distance between the beam splitter and the respective camera. The working distance of first lens may be different to the working distance of second lens. As defined herein, the working distances of first and second lenses are equal to the optical path length between the front lens of the respective lens, and the focal plane of the respective lens. This may allow the first lens to be positioned such that the first optical path is collinear with the combined optical path between the aperture and the beam splitter, and the second lens to position offset from the first lens such that the first optical path is parallel but not collinear with the combined optical path between the aperture and the beam splitter, such that the first and second lenses have coplanar focal planes. The images from the first camera and second camera will therefore be simultaneously in focus. This ensures that an operator may quickly switch between images from the first camera and the second camera, without the need to adjust the height between the aperture of the camera assembly and the bond. The working distance of first lens may be less than the working distance of second lens. The working distance of first lens may be between 40 and 220 millimetres. For example, the working distance of the second lens may be 40, 65, 110, 220 millimetres. Preferably, the working distance of the second lens is 65 millimetres. The working distance of second lens may be between 65 and 300 millimetres. For example, the working distance of the second lens may be 65,110, 220 or 300 millimetres. Preferably, the working distance of the second lens is 110 millimetres. Alternatively, the working distance of first lens may be equal to the working distance of second lens. This may be advantageous in embodiments in which at least portions of both the first optical path and the second optical path is parallel but not collinear with combined optical path between the aperture and the beam splitter. The working distance of first lens and the second lens may be between 40 and 300 millimetres. For example, the working distance of first lens and the second lens may be 40, 65, 110, 220 or 330 millimetres. In such an embodiment, preferably the working distance of the first lens and the second lens is 65 millimetres. The camera assembly may further comprise a ring lighting arrangement at least partially surrounding the aperture. Advantageously, the ring lighting arrangement may provide illumination to a bond during inspection of a bond before or after a bond test. In particular, such a ring light provides illumination for darkfield imaging. The ring lighting arrangement may comprise a plurality of LEDs. The bond test apparatus may further comprise a controller configured to control to the power supplied to each LED of the plurality of LEDs. Each LED of the plurality of LEDs may be individually controllable. Therefore, the controller may be configured to control to the power supplied to each LED such that each LED may be individually and selectively powered. A subset of LEDs may be selectively powered in order to provide illumination on a bond from a chosen direction. For example, one half of the plurality of LEDs may be selectively powered in a continuous 180 degree arc. Illumination from the chosen direction may provide a specific contrast which may be particularly advantageous for inspecting bonds before or after a bond test. For example, if a bond under inspection has a high reflectance from a first side of the bond, the half of the plurality of LEDs configured to illuminate the first side of the bond may be unpowered, or powered such that brightness is reduced relative to the other half of the plurality of LEDs. This would reduce the brightness and / or contrast of the image of the bond under inspection. The plurality of LEDs may comprise at least a first subset of LEDs of a first colour and a second subset of LEDs of a second colour different from the first colour. The controller may be configured to control the power supplied to the first subset of LEDs separately from the power supplied to the second subset of LEDs. Advantageously, illuminating a bond with a selected colour may provide more contrast dependent on the material composition of the bond to be inspected. For example, blue light provides enhanced contrast when a bond to be inspected comprises gold. Each LED of the plurality of LEDs may be a colour-controllable LED, herein referred to as an RGB LED. The term RGB LED as used herein includes LED assemblies comprising a red LED, a green LED and a blue LED. Such LED assemblies may further include white or warm white LEDs. The controller may be configured to individually control the colour emitted by each RGB LED of the plurality of RGB LEDs. Each RGB LED may comprise individual red, green, blue &white light outputs, and an interface chip configured to individually control the brightness of the red, green, blue &white light outputs. The colour and brightness of the light emitted from the red, green, blue &white light outputs may therefore be controlled by supplying a brightness value ranging from 0 to 255 from the interface chip to the respective red, green, blue &white light outputs. The brightness values may be assigned to a current provided from the interface chip to the respective red, green, blue &white light outputs. The controller may therefore be configured to supply 4 to 8 bit values to the interface chip to provide full colour and brightness control of each RGB LED. Such an arrangement realises the same advantages as those described above, but allows for far greater customisation options of illumination colour. The controller may be configured such that an operator can configure the colour and / or brightness of each RGB LED either grouped or individually. For example, the controller may be configured to receive a control signal from an external software source, wherein the external software source is configured to display a diagram of the RGB LEDs, and allow an operator to select a colour and / or brightness of each RGB LED either individually or grouped. The camera assembly may further comprise a coaxial lighting arrangement. The coaxial lighting arrangement may be configured to provide illumination through the aperture to illuminate a bond during the bond test. The coaxial lighting arrangement may be configured to provide illumination coaxially with the first optical path or the second optical path. The coaxial lighting arrangement may comprise a coaxial lighting source and a coaxial lighting beam splitter. The coaxial lighting beam splitter may be arranged in either the first optical path or the second optical path for directing light from the coaxial lighting source along the first optical path or the second optical path towards the aperture. Advantageously, the coaxial lighting arrangement may provide illumination to a bond during inspection of a bond before or after a bond test. In particular, such a ring light provides illumination for brightfield imaging. The coaxial lighting beam splitter may be arranged in the first optical path. Advantageously, when the first optical path is a straight line colinear with the combined optical path between the aperture and the beam splitter, the first optical path has a shorter length than the second optical path. As a result, beam expansion of a coaxial light beam provided by the coaxial lighting source is reduced, which minimises an on target spot size where the coaxial light beam meets a bond. According to a second aspect of the present invention, there is provided a method of performing a bond test using the apparatus of the first aspect, the method comprising the steps of: positioning a bond to be tested on the stage; imaging the bond using the camera assembly, such that the first camera images the bond at a first magnification, and the second camera simultaneously images the bond at a second magnification; providing relative movement between the test tool and the stage using the drive mechanism such that the test tool is adjacent to the bond to perform a bond test on the bond; performing a bond test on the bond using the test tool; and imaging the tested bond using the camera assembly, such that the first camera images the tested bond at a first magnification, and the second camera simultaneously images the tested bond at a second magnification. The advantages realised by the method according to the second aspect of the present invention are equivalent to those realised by the apparatus according to the first aspect of the present invention. That is, the operator therefore may have a choice of simultaneous FOVs or magnifications without the need to change lenses, and only a singular calibration offset vector between the camera assembly and the test tool needs to be determined to allow accurate use of the bond test apparatus with multiple cameras. The step of providing relative movement between the test tool and the stage using the drive mechanism such that the test tool is adjacent to the bond may further comprise providing relative movement between the camera assembly and the stage using the drive mechanism, such that the camera assembly is located away from the bond to be tested. The step of providing relative movement between the test tool and the stage and between the camera assembly and the stage may further comprise providing said relative movement dependent on an offset vector stored in a memory in the bond test apparatus. These steps may occur after an operator has located a bond to be tested using the camera assembly, and imaged the bond. These steps therefore allow the camera assembly to be replaced with the test tool in the space adjacent to the selected bond. The step of imaging the tested bond using the camera assembly may further comprise providing relative movement between the test tool and the stage using the drive mechanism such that the test tool is located away from the tested bond, and providing relative movement between the camera assembly and the stage using the drive mechanism, such that the camera assembly is adjacent to the tested bond. Said movements may be dependent on the offset vector stored in the memory in the bond test apparatus. These steps may occur after performing the test on the bond. These steps therefore allow the test tool to be replaced with the camera assembly in the space adjacent to the tested bond. The steps of providing relative movement between the test tool and the stage using the drive mechanism, both such that the test tool is adjacent to the bond and such that the test tool is located away from the tested bond, may further comprise moving the stage in an XY plane relative to both the camera assembly and the test tool. The steps of providing relative movement between the test tool and the stage using the drive mechanism may further comprise moving both the camera assembly and the test tool in a Z direction relative to the stage. The step of performing a bond test on the bond using the test tool may comprise the drive mechanism providing relative movement between the stage and the test tool during the bond test. In other words, the same drive mechanism which is configured to provide relative movement between the stage and the camera assembly and between the stage and the test tool such that a bond may be imaged before and after testing, also is configured to provide the relative movement between the stage and the test tool in order to perform said bond test. The first magnification may be different to the second magnification. Advantageously, the operator therefore has a choice of simultaneous magnifications without the need to change lenses, which may be particularly beneficial dependent on whether the bond is being imaged either before or after a bond test. The first magnification may be greater than the second magnification. The first magnification may be between 6x and 10x. For example, the first magnification may be 6x, 8x or 10x. The second magnification may be between 0.5x and 4x. For example, the second magnification may be 0.5x, 0.8x, 1x, 2x, 3x or4x. In embodiments in which the camera assembly comprises a ring lighting arrangement at least partially surrounding the aperture, the steps of imaging the bond using the camera assembly and / or imaging the tested bond using the camera assembly may comprise illuminating the bond and / or tested bond using the ring lighting arrangement. The step of illuminating the bond and / or tested bond using the ring lighting arrangement may comprise illuminating the bond and / or tested bond from a chosen direction using a subset of LEDs of the ring lighting arrangement. Illumination from the chosen direction may provide a specific contrast which may be particularly advantageous for inspecting bonds before or after a bond test. The step of illuminating the bond and / or tested bond using the ring lighting arrangement may comprise illuminating the bond and / or tested bond with a chosen colour. Advantageously, illuminating a bond with a selected colour may provide more contrast dependent on the material composition of the bond to be inspected. For example, blue light provides enhanced contrast when a bond to be inspected comprises gold. For example, the step of illuminating the bond and / or tested bond using the ring lighting arrangement may comprise individually controlling the brightness of the red, green, blue &white light outputs of each RGB LED of the ring lighting arrangement. This may be achieved by supplying a brightness value ranging from 0 to 255 from the interface chip to the respective red, green, blue &white light outputs. This may be achieved by supplying 4 to 8 bit values to the interface chip to provide full colour and brightness control of each RGB LED. Such an arrangement realises the same advantages as those described above, but allows for far greater customisation options of illumination colour. In embodiments in which the camera assembly comprises a coaxial lighting arrangement, the steps of imaging the bond using the camera assembly and / or imaging the tested bond using the camera assembly may comprise illuminating the bond and / or tested bond using the coaxial lighting arrangement. Advantageously, the coaxial lighting arrangement may provide illumination for brightfield imaging. Brief Description of the Drawings Figure 1 is an isometric view of a first embodiment of a bond testing apparatus; Figure 2 is an isometric view of a section of the bond testing apparatus of Figure 1; Figure 3 is a cross-sectional view of a first embodiment of the camera assembly of the bond testing apparatus; Figure 4 is a isometric view of the camera assembly of Figure 3; Figure 5 is a cross-sectional view of a second embodiment of the camera assembly of the bond testing apparatus; Figure 6 is a isometric view of the camera assembly of Figure 5; Figure 7A is an isometric view of the section of the bond testing apparatus of Figure 2 with the camera assembly aligned with a calibration system; Figure 7B is an isometric view of the section of the bond testing apparatus of Figure 2 performing a touchdown; Figure 7C is an isometric view of the section of the bond testing apparatus of Figure 2 with a test tool aligned with the calibration system; Figure 8 is a schematic illustration of a method of performing a bond test according to a second aspect of the present disclosure; Detailed Description Figure 1 is an isometric view of a bond testing apparatus 100 according to a first aspect of the present disclosure. The bond testing apparatus 100 comprises a test rig 2 mounted to a frame 14. The test rig 2 comprises a camera assembly 104 and a cartridge 16. The cartridge 16 comprises a test tool assembly, and 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 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 camera assembly 104. Located beneath the test rig 2 and mounted to the frame 14 is a sample support 10 and a calibration system 6. The sample support 10 comprises a sample stage 18 for supporting thereon a semiconductor substrate sample to be tested. The semiconductor substrate sample comprises at least one bond to be tested. 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 move are configured to move along the X-guide rails 34 in the X-axis and be fixed in place once in position. 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 X-stage table 54 is configured to move along the Y-guide rails 44 in the Y-axis and be fixed in place once in position. Movement of the X-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 may 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 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 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 118 based on one or more images from the camera assembly 104. 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 camera assembly 4. The processor is configured move the test rig 2 through space by controlling the motor of the Z-screw mechanism. The processor is configured to record the XY coordinates of the positioning system / sample stage when the camera assembly 104 is aligned with the virtual target datum and when the test tool is aligned with the virtual target datum based on an image or images from the camera assembly 104 and / or the calibration camera. The offset between the sample inspection camera and the test tool can then be calculated from the difference between the saved coordinates. The bond testing apparatus 100 comprises control circuitry 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 104. The control circuitry is configured to calculate the Cartesian coordinates of the test tool based on input from the positioning mechanism. Figure 2 is an isometric view of a section of the bond testing apparatus 100 shown in Figure 1. In particular, Figure 2 illustrates the test rig 2. The test rig 2 comprises the camera assembly 104. The cartridge 16 comprises a test tool assembly 62 and a test tool 48. The test tool tip 58 is configured to contact a bond of the sample to be tested during a bond test. The camera assembly 104 has an optical axis A such that samples positioned below the camera assembly 104 and coinciding with the optical axis A may be imaged. The test tool tip 58 is distanced from the optical axis A of the camera assembly 104 by a set distance vector in the XY plane. The distance vector may differ depending on the cartridge and / or test tool received in the test rig 2. The camera assembly 104 is configured to image a sample before and after a bond test, as is described in more detail below. Figure 3 is a cross-sectional view of a first embodiment of the camera assembly 104 of the bond testing apparatus 100 of the present disclosure. The camera assembly 104 comprises a first camera 181 and a second camera 182. The first and second cameras 181, 182 each comprise respective camera apertures for receiving light therethrough, and respective photodetectors for detecting said light. The first and second cameras 181, 182 are also each electronically connected to the processor of the bond testing apparatus 100 such that the first and second cameras 181, 182 are configured to send images to the processor. The first camera 181 is arranged at an end of a first optical path 185 partially formed by a first lens 183. The first lens 183 comprises an arrangement of at least one optical lenses configured to focus light along the first optical path 185 and onto the first camera 181. The first lens 183 comprises a front lens 197 defining a front portion of the first lens 183. Similarly, the second camera 182 is arranged at an end of a second optical path 186 partially formed by a second lens 184. The second lens 184 comprises an arrangement of at least one optical lenses configured to focus light along part of the second optical path 186 and onto the second camera 182. The second lens 184 comprises a front lens 198 defining a front portion of the second lens 184. The camera assembly 104 comprises a main aperture 196 for receiving a light beam 189 therethrough. Positioned within the camera assembly 104 adjacent to the aperture 196 is a beam splitter 188. The beam splitter 188 is configured to split the light beam 189 received through the aperture 196 into a first beam and a second beam. The first beam is directed by the beam splitter 188 along the first optical path 185 to the first camera 181, such that the first optical path 185 extends between the beam splitter 188 and the first camera 181. The second beam is directed by the beam splitter 188 along the second optical path 186 to the second camera 182, such that the second optical path 186 extends between the beam splitter 188 and the second camera 182. The first optical path 185 extends in a straight line which is substantially colinear with the light beam 189 received through the main aperture 196. In contrast, the second optical path 186 comprises a lower portion which is substantially perpendicular to the light beam 189 received through the main aperture 196, and substantially perpendicular to the first optical path 185. The second optical path 186 further comprises an upper portion which is substantially parallel to the light beam 189 received through the main aperture 196, and substantially parallel to the first optical path 185. A mirror 187 contained within the camera assembly 104 comprises a surface at 45 degrees to the direction of the lower portion of the second optical path 186. The mirror 187 is located within the second optical path 186 such that light reflected from the lower portion of the second optical path 186 continues along the upper portion of the second optical path 186. It can be understood that different physical arrangements of first and second optical paths may be possible. For example, the second optical path 186 need not contain upper and lower portions perpendicular to one another. Instead, for example, the entirety second optical path 186 may extend in a straight line which is substantially perpendicular with the light beam 189 received through the main aperture 196 such that no mirror 187 is necessary. When a sample 21 is positioned under the main aperture 196 for observation, the total optical path length between the front lens 197 of the first lens 183 and the sample 21 is different to the total optical path length between the front lens 198 of the second lens 184 and the sample 21. This is illustrated in Figure 3 by the length of the respective optical paths between the sample and the front lenses 197, 198. Therefore, in order for both lens to be in focus at the same time, the first lens 183 has a different working distance to the second lens 184. In the present embodiment, the first lens 183 has a working distance of 65 mm and the second lens 184 has a working distance of 110 mm. In the present embodiment, the first lens 183 has a magnification of 8x and the second lens 184 has a magnification of 1x. The camera assembly 104 may therefore be referred to as a dual-magnification camera. Therefore, as will be described in more detail below, the first camera 181 may be used for high magnification, small FOV applications, such as inspection of a test site after a bond test has been conducted. The second camera 182 may be used for low magnification, high FOV applications, such as location of a test site prior to testing. As also illustrated in Figure 3, the camera assembly 104 further comprises a coaxial lighting unit 191 comprising a coaxial lighting source 192. The coaxial lighting source 192 may comprise at least one LED, for example. Alternatively, the coaxial lighting source 192 may comprise an RGB LED, the RGB LED comprising a plurality of LEDs. The RGB LED may comprise a diffuser at least partially covering the plurality of LEDs to prevent vignetting and ensure mixing and a good spread of light from each LED source in the RGB LED. The coaxial lighting unit 191 is attached to the first lens 183, and is configured to provide light from the coaxial lighting source 192 to a coaxial lighting beam splitter 193 arranged on the first optical path 185. The coaxial lighting beam splitter 193 is configured to direct light from the coaxial lighting source 192 to the front lens 197 of the first lens 183, and on to the sample 21. The light from the coaxial lighting source 192 may therefore be utilised for brightfield imaging of the sample 21. The coaxial lighting beam splitter 193 is further configured to allow light from the front lens 197 of the first lens 183 to pass through the coaxial lighting beam splitter 193 when travelling along the first optical path 185 to the first camera 181. Although in this embodiment, the coaxial lighting unit 191 and coaxial lighting beam splitter 193 are arranged on the first lens 183 and first optical path 185, it can be understood that the coaxial lighting unit 191 and coaxial lighting beam splitter 193 may instead be located on the second lens 184 and second optical path 186. The coaxial lighting unit 191 is electronically connected to the processor of the bond test apparatus 100, so that the coaxial lighting unit 191 may be controlled via the processor. For example, an operator may control whether the coaxial lighting is turned on or off via the processor. As also illustrated in Figure 3, the camera assembly 104 further comprises a ring lighting unit 194 arranged about the main aperture 196. The ring lighting unit 194 comprises a ring lighting source 195, which consists of a plurality of LEDs. The ring lighting source 195 is shown in more detail in Figure 4, which is a isometric view of the camera assembly of Figure 3. The plurality of LEDs surround the main aperture 196 in a circle, and are configured to illuminate the sample 21 below the main aperture 196. The ring lighting unit 194 is also electronically connected to the processor of the bond test apparatus 100, so that the ring lighting unit 194 may be controlled via the processor. Each LED of the plurality of LEDs are individually controllable and are individually electronically connected to the processor of the bond test apparatus 100. The operator may therefore selectively power only a subset of the plurality of LEDs via the processor. For example, only half of the plurality of LEDs may be selectively powered. In particular, the half of the plurality of LEDs may be a continuous arc of 180 degrees of the plurality of LEDs. Any sample 21 below the main aperture 196 is therefore illuminated from only a single side, for example, from a left or right hand side. Such a configuration of powered LEDs may advantageously provide a different contrast to the sample 21. This may allow the operator to better inspect a test site post inspection, or identify sites for testing. In addition, each LED of the plurality of LEDs is an RGB LED, with the colour output of each RGB LED being individually controllable via the processor of the bond test apparatus 100. For example, the colour output of each LED, or a subset of LEDs of the plurality of LEDs, may be set to a specific colour or wavelength of light, such as red, green or blue. Any sample 21 below the main aperture 196 is therefore illuminated with this colour light. Such a configuration of individually controllable RGB LEDs may advantageously provide a different contrast to the sample 21. As an example, blue light may provide better contrast when inspecting samples 21 comprising gold. Gold is commonly used in the electronics industry and is often one of the materials used to make bonds such as wire bonds. Gold also absorbs blue light, so when illuminated with blue light, gold bonds appear black. This may allow the operator to better inspect a test site comprising gold post inspection, or identify sites comprising gold for testing. Furthermore, printed circuit boards on which bonds may be located often comprise a green resist layer, which absorbs purple light. By illuminating a bond on such a printed circuit board, the green resist layer would appear black, resulting in greater contrast with the bond under inspection. Figure 5 is a cross-sectional view of a second embodiment of a camera assembly 204 of the bond testing apparatus 100 of the present disclosure. The second embodiment of the camera assembly 204 may replace the first embodiment of the camera assembly 104 in the bond test apparatus 100 illustrated in Figure 1. The second embodiment of the camera assembly 204 is similar to the first embodiment of the camera assembly 104, so will be described with respect to its differences only. Like reference numerals are used to indicate like parts. As illustrated in Figure 5, the first optical path 285 does not extend in a straight line which is substantially colinear with the light beam 289 received through the main aperture 296. Instead, the first optical path 285 comprises three portions: a lower portion, a middle portion, and an upper portion. The lower portion of the first optical path 285 extends in a straight line from the beam splitter 288 to a first mirror 279. The lower portion of the first optical path 285 is substantially colinear with the light beam 289 received through the main aperture 296. The first mirror 279 is contained within the camera assembly 204 and comprises a surface at 45 degrees to the direction of the lower portion of the first optical path 285. The first mirror 279 is located within the first optical path 285 such that light reflected from the lower portion of the first optical path 285 continues along the middle portion of the first optical path 285. The middle portion of the first optical path 285 extends in a straight line from the first mirror 279 to a second mirror 278. The middle portion of the first optical path 285 is substantially perpendicular with the light beam 289 received through the main aperture 296. The second mirror 278 is contained within the camera assembly 204 and comprises a surface at 45 degrees to the direction of the middle portion of the first optical path 285. The second mirror 278 is located within the first optical path 285 such that light reflected from the middle portion of the first optical path 285 continues along the upper portion of the first optical path 285. The upper portion of the first optical path 285 extends in a straight line from the second mirror 278 to the first camera 281. The upper portion of the first optical path 285 is substantially parallel, but not colinear, with the light beam 289 received through the main aperture 296. The second optical path 286 is substantially identical to the first embodiment of the camera assembly 104, and extends from the beam splitter 288 to the mirror 287, and through the second lens 284 to the second camera 282. In this arrangement, the length of the middle portion of the first optical path 285 is equal to the length of the lower portion of the second optical path 286. The front lens 297 of the first lens 283 and the front lens 298 of the second lens 284 are also offset from the main aperture 296 by equal heights. Therefore, the total optical path length between the front lens 297 of the first lens 283 and the sample 21 is equal to the total optical path length between the front lens 298 of the second lens 284 and the sample 21. This is illustrated in Figure 5 by the length of the respective optical paths between the sample and the front lenses 297, 298. Therefore, in order for both lens to be in focus at the same time, the first lens 283 has an equal working distance to the second lens 284. In the present embodiment, the first lens 283 and the second lens 284 both have a working distance of 65 mm. In the present embodiment, the first lens 283 has a magnification of 8x and the second lens 284 has a magnification of 0.8x. Rather than being attached to the first lens 283, the coaxial lighting unit 291 is instead attached to the second lens 284, and is configured to provide light from the coaxial lighting source 292 to a coaxial lighting beam splitter 293 arranged on the second optical path 286. The second embodiment of the camera assembly 204 further comprises a ring lighting unit 294 arranged about the main aperture 296. The ring lighting unit 294 is similar to that described with respect to the first embodiment of the camera assembly 104, in that the ring lighting unit 294 comprises a ring lighting source 295 which consists of a plurality of LEDs. However, as illustrated in Figures 5 and 6, the second embodiment of the camera assembly 204 further comprises a retaining cover 249 secured over the ring lighting unit 294 . The retaining cover 249 is transparent to allow light to pass from the ring lighting source 295 to the sample 21. Figures 7A to 7C illustrate the process of calibrating the bond test apparatus 100 of Figure 1 to determine the offset vector between the test tool tip 58 and the optical axis of the camera assembly 104. As illustrated in Figures 7Ato 7C, the test rig 2 moves in the Z axis and the calibration system 6 moves in the XY axis in the manner of Figure 1. Figure 7A is an isometric view of a section of the bond testing apparatus 100 with the optical axis of the camera assembly 104 aligned with the calibration system 6. The calibration processor (not shown) receives an image or images from the sample inspection camera 104. The calibration processor controls the position of the camera assembly 104 so that it aligns with a central axis C of the calibration system 6 based on an image or images received from the camera assembly 104. XY coordinates of a central pixel or pixels of the camera assembly 104 that are aligned with a virtual target datum are then stored in a memory of the calibration processor. Figure 7B is an isometric view of a section of the bond testing apparatus 100 performing a touchdown. The calibration system 6 comprises a touchdown datum 83 arranged in a coplanar fashion with the central aperture 52. The touchdown datum 83 is formed as part of the housing 20. To perform a touchdown, the XY position of the touchdown datum 83 is found using the camera assembly 104. The position is stored in the calibration processor. The test tool 48 is approximately aligned with the touchdown datum 83 and lowered in the Z axis until the test tool tip 58 contacts the touchdown datum 83. The Z position of the test tool 48 is stored as a touchdown height in the memory of the calibration processor. After the touchdown height is found, the test tool 48 is returns to its initial position, then approximately aligned with the central axis C of the calibration system 6. The test tool 48 is lowered by the touchdown height so it is on the same plane within the central aperture 52. Aclearly focussed image of the test tool 48 is generated by the calibration camera as the test tool 48 is now in the focal plane of the calibration camera. The calibration processor controls the system to perform accurate XY moves of the test tool 48 to align it with the virtual target datum based on the clear image or images received from the calibration camera. Figure 7C is an isometric view of the system in this position. Once the test tool 48 is aligned with the virtual target datum, the XY position is stored in the memory of the calibration processor. The offset vector between the stored XY position of the test tool 48 and the central pixel or pixels of the sample inspection camera is determined by the calibration processor and provided to the processor. Therefore, the test tool 48 can be accurately positioned using the offset vector when performing sample tests on the sample stage 18 or when inspecting test sites after sample tests on the sample stage 18, as is described below. Because only one offset vector is calculated for two cameras in the camera assembly 104, the time taken to perform a calibration is reduced compared to if for example the test rig 2 comprised two separate cameras with two different optical axes. This calibration process may be carried out using either the camera assembly 104 or the camera assembly 204 of Figures 5 and 6.Figure 8 illustrates a method of performing a bond test according to a second aspect of the present invention. The method of performing a bond test is described using the bond test apparatus 100 of Figure 1. The method of performing a bond test is performed after the above calibration process has taken place, such that the offset vector between the test tool tip 58 and the optical axis of the camera assembly 104 is already determined. The method comprises the first step 1001 of positioning a sample 21 comprising one or more bonds to be tested on the sample stage 18. The sample 21 is secured relative to the sample stage 18 such that the sample 21 does not move relative to the sample stage 18 during testing. In a second step 1002, the sample 21 is imaged using the camera assembly 104, such that the first camera 181 images the sample 21 at a first magnification, and the second camera 182 simultaneously images the sample 21 at a second magnification. In this example, the drive mechanism provides relative movement between the camera assembly 104 and the sample stage 18 such that the optical axis of the camera assembly 104 is positioned over the sample 21. In the embodiment shown in Figure 1, the sample stage 18 is translated relative to camera assembly 104. The operator may then use images from the first and / or second camera 181,182 in order to select a bond on the sample 21 to be tested. Considering the camera assembly of the first embodiment 104, the second lens 184 has a magnification of 1 x. Images from the second camera 182 therefore provide a wider FOV of the sample 21. The operator may therefore use one or more images from the second camera 182 in order to more easily locate and select a bond to be tested from a wide area of the sample 21. In a third step 1003, the drive mechanism provides relative translation between the test tool 48 and the sample stage 18 such that the test tool 48 is adjacent to the selected bond. To achieve this, the processor sends an electronic drive signal to the drive mechanism dependent on the calculated offset vector. The test tool 48 is therefore accurately positioned adjacent to the selected bond. In the embodiment shown in Figure 1, the sample stage 18 is translated relative to the test tool 48. The height of the test tool 48 in the Z-axis is also adjusted such that the test tool does not strike the surface of the sample 21 during translation of the sample stage 18 relative to the test tool 48 in the XY plane, For example, the test rig 2 comprising the test tool 48 may be retracted a predetermined distance of at least 1 micrometre in the Z-axis prior to translation of the sample stage 18 relative to the test tool 48 in the XY plane. In a fourth step 1004, the bond test apparatus 100 performs a bond test on the bond using the test tool 48. For example, if the test tool 48 is a shear tool, the shear tool applies a force horizontally across the semiconductor substrate to shear a bond off the substrate. If the test tool 48 is a push tool, the push tool applies a vertical compression force on a component on the substrate. The force applied by these tools is measured. If the test tool 48 is a pull tool, the pull tool may, for example, have a hook at the bottom of the tool that is used to hook an electrical lead, which is attached between a component and a semiconductor substrate, with a vertical force being applied to the tool to pull the lead off the semiconductor substrate and measure the force required to pull the lead off the semiconductor substrate. An example of a suitable shear tool is described in US 6,078,387, the contents of which are incorporated herein by reference. An example of a suitable pull tool is described in US 6,310,971, the contents of which are incorporated herein by reference. In a fifth step 1005, the drive mechanism provides relative translation between the test tool 48 and the sample stage 18 such that the test tool 48 is moved away from the tested bond, and instead the optical axis of the camera assembly 104 is positioned over the tested bond. To achieve this, the processor sends an electronic drive signal to the drive mechanism dependent on the calculated offset vector. Again, the height of the test tool 48 in the Z-axis is adjusted such that the test tool does not strike the surface of the sample 21 during translation of the sample stage 18 relative to the test tool 48 in the XY plane, For example, the test rig 2 comprising the test tool 48 may be again retracted a predetermined distance of at least 1 micrometre in the Z-axis prior to translation 5 of the sample stage 18 relative to the test tool 48 in the XY plane. The camera assembly 104 is therefore again accurately positioned adjacent to the tested bond. In the embodiment shown in Figure 1, the sample stage 18 is translated relative to the test tool 48 and camera assembly 104. The tested bond on the sample 21 is imaged using the camera assembly 104, such that the first camera 181 images the tested bond on the sample 21 at a first 10 magnification, and the second camera 182 simultaneously images the tested bond on the sample 21 at a second magnification. The operator may then use images from the first and / or second camera 181,182 in order to inspect the tested bond on the sample 21 to ensure the bond test has been carried out successfully. Considering the camera assembly of the first embodiment 104, the first lens 183 has a 15 magnification of 8x. Images from the first camera 181 therefore provide a narrower FOV of the sample 21. The operator may therefore use one or more images from the first camera 181 in order to inspect the tested bond with a greater magnification, and hence greater level of detail.

Claims

1. A bond test apparatus comprising:a stage for supporting a bond fortesting;a test tool having a test tool tip configured to contact the bond during a bond test; anda camera assembly configured to image the bond before and / or after the bond test, the camera assembly comprising:an aperture for receiving a light beam therethrough;a beam splitter for splitting the light beam into a least a first beam along a first optical path and a second beam along a second optical path;a first camera arranged at an end of the first optical path and configured to detect the first beam, and a first lens arranged along the first optical path and configured to focus the first beam onto the first camera; anda second camera arranged at an end of the second optical path and configured to detect the second beam, and a second lens arranged along the second optical path and configured to focus the second beam onto the second camera,wherein the first camera and the second camera are configured to simultaneously image the same point at different magnifications; andwherein the bond test apparatus further comprises a drive mechanism configured to provide relative movement between the stage and the camera assembly.

2. A bond test apparatus according to claim 1, wherein the camera assembly is rigidly mounted to the bond test apparatus, and in which the drive mechanism is configured to move the stage to control the location on the stage which is imaged by the camera assembly.

3. A bond test apparatus according to claim 1 or 2, in which the camera assembly is configured to image a point on the stage for inspecting a sample which includes the bond for testing.

4. A bond test apparatus according to claim 1,2 or 3, in which the camera assembly is configured to image a field of view which does not contain the test tool.

5. A bond test apparatus according to any preceding claim, wherein the camera assembly is fixed relative to the test tool.

6. A bond test apparatus according to any preceding claim, wherein the drive mechanism is configured to provide relative movement between the stage and the test tool during the bond test.

7. A bond test apparatus according to any preceding claim, wherein the magnification of first lens is different to the magnification of second lens.

8. A bond test apparatus according to any preceding claim, wherein the first optical path is different in length to the second optical path.

9. A bond test apparatus according to any preceding claim, wherein the working distance of first lens is different to the working distance of second lens.

10. A bond test apparatus according to any preceding claim, wherein the first optical path is a straight line.

11. A bond test apparatus according to claim 10, wherein the first optical path is colinear with a combined optical path between the aperture and the beam splitter.

12. A bond test apparatus according to any preceding claim, wherein a lower portion of the second optical path is perpendicular to the first optical path.

13. A bond test apparatus according to claim 12, wherein the second optical path comprises a 90 degree deviation, such that an upper portion of second optical path is parallel to the first optical path.

14. A bond test apparatus according to any preceding claim, wherein the camera assembly further comprises a ring lighting arrangement at least partially surrounding the aperture.

15. A bond test apparatus according to claim 14, wherein the ring lighting arrangement comprises a plurality of LEDs.

16. A bond test apparatus according to claim 15, wherein the bond test apparatus further comprises a controller configured to control to the power supplied to each LED of the plurality of LEDs.

17. A bond test apparatus according to claim 15 or 16, wherein the plurality of LEDs comprises at least a first subset of LEDs of a first colour and a second subset of LEDs of a second colour different from the first colour, and wherein the controller is configured to control the power supplied to the first subset of LEDs separately from the power supplied to the second subset of LEDs.

18. A bond test apparatus according to any of claims 15 to 17, wherein each LED of the plurality of LEDs is an RGB LED, and wherein the controller is configured to individually control the colour emitted by each RGB LED of the plurality of RGB LEDs.

19. A bond test apparatus according to any preceding claim, wherein the camera assembly further comprises a coaxial lighting arrangement.

20. A bond test apparatus according to claim 19, wherein the coaxial lighting arrangement comprising a coaxial lighting source and a coaxial lighting beam splitter, and wherein the coaxial lighting beam splitter is arranged in either the first optical path or the second optical path for directing light from the coaxial lighting source along the first optical path or the second optical path towards the aperture.

21. A bond test apparatus according to claim 20, wherein the coaxial lighting beam splitter is arranged in the first optical path.

22. A method of performing a bond test using the bond test apparatus of any preceding claim, the method comprising the steps of:positioning a bond to be tested on the stage;imaging the bond using the camera assembly, such that the first camera images the bond at a first magnification, and the second camera simultaneously images the bond at a second magnification;5providing relative movement between the test tool and the stage using the drive mechanism such that the test tool is adjacent to the bond to perform a bond test on the bond;10 performing a bond test on the bond using the test tool; andimaging the tested bond using the camera assembly, such that the first camera images the tested bond at a first magnification, and the second camera simultaneously images the tested bond at a second magnification.15

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