ULTRASONIC MICROSCOPE AND METHOD FOR OPERATING ULTRASONIC MICROSCOPE - Patent application

Abstract

To provide an ultrasonic microscope which can accurately and quickly inspect a target object.SOLUTION: An ultrasonic microscope includes: a target object holder for holding a target object in a target region; a scan head; and a first transducer supported by the scan head. The first transducer discharges a first acoustic pulse along a direction of discharge, makes the first acoustic pulse focus on a focal point, detects a second acoustic pulse from the target object, and outputs a first detection signal which represents the second acoustic pulse. The first transducer includes: a first actuator for moving the first transducer with respect to the scan head along a vertical direction parallel to the discharge direction; and a control unit for generating a control signal so that a vertical distance between the target object and the first transducer will be a constant distance while the target object is being operated, on the basis of the first detection signal.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an ultrasonic microscope for inspecting objects, in particular for inspecting semiconductor structures. Furthermore, the invention relates to a carrier for carrying an acoustic pulse transducer of an ultrasonic microscope. [Background technology]

[0002] Acoustic microscopes are used for non-destructive testing of objects such as materials and devices. Due to their non-destructive testing capabilities, such ultrasonic microscopes are advantageously applied in the fields of quality monitoring, reliability studies, optimization, and failure analysis. For example, in semiconductor manufacturing, ultrasonic microscopes are used for the inspection of finished and partially processed semiconductor devices. By using such ultrasonic microscopes, delaminations, gaps, cracks, or inclusions in objects can be detected and their mechanical properties can be determined.

[0003] Inspection of an object by using an ultrasonic microscope can be performed by directing an acoustic pulse, in particular an ultrasonic pulse, at the object and analyzing the acoustic pulse reflected by and / or transmitted through the object. The basis of this analysis is the physical phenomenon that an acoustic pulse is partially reflected at and partially transmitted through each acoustic interface of the object. Thus, an object containing multiple different material layers will generate multiple reflected pulses from a single acoustic pulse directed at the object. The multiple reflected acoustic pulses are detected by an acoustic transducer that outputs a time domain signal representative of the intensity of the reflected acoustic pulse. Based on the time delay of each of the multiple reflected pulses relative to the pulse directed at the object, the depth of the material layer within the object can be determined. Furthermore, based on the intensity of the reflected acoustic pulse, the type of material can be estimated, thereby determining defects in the object. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to provide an ultrasonic microscope capable of inspecting objects as quickly as possible with high accuracy. The objects to be inspected can be materials and devices, in particular wafers, packages, components or devices made using semiconductor materials. The surface shape of the object can be irregular relative to the size of the object. [Means for solving the problem]

[0005] According to one aspect of the invention, an ultrasonic microscope for inspecting an object comprises an object holder configured to hold the object at a target area, a scan head movable relative to the target area, and a first transducer supported by the scan head, the first transducer configured to emit a first acoustic pulse along an emission direction and focus the first acoustic pulse at a focal point, the first transducer configured to detect a second acoustic pulse emerging from the object and output a first detection signal representative of the second acoustic pulse detected by the first transducer.

[0006] According to this aspect, the scan head supports a first transducer, and by moving the scan head relative to the region of interest, the first transducer also moves relative to the region of interest.

[0007] The first transducer is configured to emit a first acoustic pulse, in particular an ultrasonic pulse, along an emission direction, whereby the first acoustic pulse is directed to an object within a region of interest. The first acoustic pulse is focused at a focal point, and the object can be positioned relative to the focal point such that a location of interest of the object coincides with the focal point.

[0008] The first transducer is further configured to detect a second acoustic pulse emerging from the object when the first acoustic pulse is directed toward the object. The first transducer is further configured to generate a first detection signal from the detected acoustic pulse, the first detection signal being output by the first transducer. For example, the first detection signal can be a time domain signal representative of the time-dependent intensity of the second acoustic pulse.

[0009] According to one embodiment, an ultrasound microscope comprises a first actuator configured to move a first transducer relative to a scan head along a vertical direction essentially parallel to an emission direction, and a controller configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on a first detection signal.

[0010] The first actuator is configured to move the first transducer along a vertical direction relative to the scan head. This also moves the focal point relative to the scan head in a vertical direction. Thus, the focal point can be translated along the vertical direction by the first actuator in that the first actuator moves the first transducer along the vertical direction relative to the scan head. The controller is configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on the first detection signal, i.e., based on a second acoustic pulse emerging from the object when the first acoustic pulse is directed toward the object. Thus, the focal point can be positioned relative to the object and relative to the scan head based on the first detection signal.

[0011] According to an embodiment, the control unit is further configured to control the emission of the first acoustic pulse by the first transducer, determine a distance value representative of a distance between the first transducer and the object based on the first detection signal, and control a vertical movement of the first transducer relative to the scan head provided by the first actuator based on the determined distance value. For example, the distance between the first transducer and the object can be calculated by the control unit based on the flight time of one of the first acoustic pulses from the first transducer to the object and the flight time of a second acoustic pulse generated by the one first acoustic pulse from the object to the first transducer. The calculation can utilize parameters such as the speed of sound in air or immersion liquid.

[0012] According to this embodiment, the control unit is configured to cause the first transducer to emit a first acoustic pulse. Thus, the control unit controls the timing of the first transducer to emit the first acoustic pulse. Furthermore, the control unit is configured to obtain a first detection signal from the first transducer after the first acoustic pulse is directed to the object. Thus, the control unit can be configured to determine the timing of the second acoustic pulse emerging from the object to reach the first transducer. Based on the timing of the emission of the first acoustic pulse and the reception of the second acoustic pulse, the control unit can be configured to calculate the distance between the first transducer and the object, and in particular to determine the distance between the first transducer and the multiple material layers of the semiconductor structure. Based on the determined distance value, the control unit can control the vertical movement of the first transducer relative to the scan head provided by the first actuator such that the focal spot is located at a desired position relative to the object.

[0013] For example, the control unit can be configured to control vertical movement of the first transducer relative to the scan head provided by the first actuator based on the determined distance value so that the distance between the first transducer and the object approaches a predetermined working distance.

[0014] For example, the controller can be configured to control vertical movement of the first transducer relative to the scan head provided by the first actuator such that a predetermined working distance is maintained between the first transducer and the object while the scan head moves the first transducer relative to the region of interest. As a particular example, the controller can be configured to control vertical movement of the first transducer relative to the scan head provided by the first actuator such that a predetermined working distance is maintained between the first transducer and a surface of the object while the scan head moves the first transducer relative to the region of interest.

[0015] According to another embodiment, the control unit can be configured to determine a defocus value representing a distance between the focal point and a location of interest on the object based on the determined distance value, and to control a vertical movement of the first transducer relative to the scanning head provided by the first actuator based on the defocus value.

[0016] For example, multiple locations of interest of the object can be defined by a user to be located at a predefined distance below the surface of the object. As mentioned above, the distance between the first transducer and the surface of the object can be determined by the control unit. Thus, the control unit can be configured to determine the distance of the location of interest of the object from the first transducer based on the user-defined predefined distance and distance values. Furthermore, the focal length, i.e. the distance between the focal point generated by the first transducer and the first transducer itself, is also available. Based on these data, the control unit can be configured to determine a defocus value representing the distance between the focal point and the location of interest of the object. Thus, the control unit can control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on the defocus value such that the focal point approaches / matches the location of interest.

[0017] According to a further embodiment, the ultrasound microscope further comprises a scan head positioner configured to move the scan head relative to the object holder in a lateral plane essentially perpendicular to the vertical direction.

[0018] According to this embodiment, the scan head can be moved relative to the region of interest in a lateral plane by the scan head positioner, such that the controller can scan the object by moving the scan head relative to the region of interest while analyzing the object using the first transducer.

[0019] According to another embodiment, the controller comprises an analog circuit configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on the first detection signal. The vertical movement of the first transducer relative to the scan head can thereby be performed at high speed with minimal delay due to signal processing. Thus, by controlling the vertical movement of the first transducer relative to the scan head at high speed, the object can be scanned at high speed (i.e., the scan head is moved to multiple positions relative to the object while the first transducer outputs the first detection signal) while the first acoustic pulse remains well focused.

[0020] According to another embodiment, the controller comprises digital circuitry for performing the various operations disclosed herein.

[0021] According to another embodiment, the control unit is further configured to determine a height profile of the object based on the first detection signal, the "height profile" being indicative of the range of the object in a vertical direction dependent on one or more lateral directions. For example, in an ultrasound microscope configured to determine a distance value representative of a distance between the first transducer and the object based on the first detection signal, the control unit can be configured to determine a height profile in a reference system of the first transducer based on the distance value. The height profile in the reference system of the first transducer can be further converted to a height profile in another reference system. For example, the height profile in the reference system of the first transducer can be converted to the reference system of the scan head by further using information about the vertical position of the first transducer relative to the scan head, i.e., information about the vertical position of the first transducer in the reference system of the scan head. The information about the vertical position of the first transducer relative to the scan head can be obtained from a control signal provided by the control unit to the first actuator, the control signal being representative of a vertical movement performed by the first actuator. Alternatively or additionally, information regarding the vertical position of the first transducer relative to the scan head may be obtained by a measurement unit which measures the vertical position of the first transducer relative to the scan head and outputs a corresponding signal.

[0022] As a first example, the controller can be configured to control the vertical movement of the first transducer relative to the scan head. Thus, the vertical position of the first transducer relative to the scan head while scanning the object using the scan head positioning device is available to the controller via the control signal. Furthermore, while scanning the object using the scan head positioning device, the vertical distance between the first transducer and the object can be determined as described above. Based on the vertical position of the scan head and the vertical distance between the first transducer and the object, the controller can determine the height profile of the object in the reference system of the scan head and the reference system of the scan head positioning device.

[0023] As a second example, the controller can be configured to control vertical movement of the first transducer relative to the scan head provided by the first actuator such that a predefined working distance is maintained between the first transducer and the surface of the object while the object is scanned laterally. In this example, since the distance between the surface of the object and the first transducer remains essentially constant (at the level of the predefined working distance), the height profile can be derived directly from the control signal.

[0024] According to another embodiment, the controller is further configured to control the scan head positioning device such that the scan head is positioned at a plurality of positions. Each time the scan head is positioned at one of the plurality of positions, the controller causes the first transducer to emit at least one of the first acoustic pulses, detect at least one of the second acoustic pulses, and output a first detection signal. The controller is further configured to store the first detection signals or values ​​derived from the first detection signals in association with the plurality of positions, and control a vertical movement of the first transducer relative to the scan head provided by the first actuator based on the set of stored first detection signals or values ​​derived from the first detection signals.

[0025] According to this embodiment, the controller is configured to control the scan head positioning device to position the scan head at multiple positions. That is, the controller causes the scan head positioning device to move the scan head and thus position the first transducer at multiple positions. For example, the scan head positioning device is moved along a path, such as one or more lines, relative to the region of interest, so that multiple positions of the object can be analyzed along the path.

[0026] At each of the multiple positions of the scan head, the controller causes the first transducer to emit at least one of the first acoustic pulses, detect at least one of the second acoustic pulses, and output a first detection signal. That is, each time the scan head is positioned at one of the multiple positions, the position of the object is analyzed by directing at least one of the first acoustic pulses at the object and detecting a second acoustic pulse emerging from the object, whereby a first detection signal is output at least once at each of the multiple positions of the scan head. Thus, multiple first detection signals can be obtained while moving the scan head along a path to multiple positions.

[0027] A plurality of first detection signals or values ​​derived therefrom, such as the distance values ​​or defocus values, are stored by the controller in relation to a plurality of positions of the scanning head (or in relation to a plurality of positions of the object), such that for each of the plurality of positions of the scanning head (or for each of the plurality of positions of the object), one of the plurality of detection signals or values ​​derived therefrom is available in the memory for controlling the first actuator.

[0028] For example, the first detection signal or a value derived from the first detection signal may be obtained by the above procedure and stored in a memory for a first set of positions of the scanning head (or a first set of positions of the object). Before moving the scanning head to a second position different from each position of the first set of positions of the scanning head (or different from each position of the first set of positions of the object), the control unit may calculate a predicted value for controlling the vertical movement of the first transducer relative to the scanning head provided by the first actuator based on the first detection signal or the value derived from the first detection signal stored in the memory. Thus, even before moving the scanning head to the second position, where the object may not have been analyzed before, the first actuator may already be controlled according to the predicted value. This allows the first transducer to be brought or approached to a predetermined working distance, or the defocus value may already be quite small before or while the scanning head is moved to the second position. This reduces the time required to properly position the first transducer relative to the object when the object is scanned.

[0029] According to a further embodiment of the above embodiment, a subset of the detection signals or values ​​derived from the detection signals stored in the memory can be used to determine a predicted value for controlling the first actuator for a second position of the scanning head. The subset can be selected by the control unit based on a distance between the second position and each of the first positions associated with one of the first detection signals or values ​​derived from the first detection signals. For example, the subset can be defined to include first detection signals or values ​​derived from the first detection signals associated with first positions, each of the first positions having a distance to the second position that is less than a predefined threshold distance value. The predefined threshold distance value can be less than 1 μm, in particular less than 100 nm, or less than 10 nm. According to this embodiment, a first detection signal or a value derived from the first detection signal previously obtained at a position of the scanning head close to the second position where the object is next to be analyzed is used to control the first actuator. To determine the predicted value, the subset can be processed, for example, using interpolation, extrapolation, etc.

[0030] For example, the object may first be coarsely scanned, thereby determining a coarse height profile of the object. The coarse height profile of the object may then be used to control the vertical movement of the first transducer relative to the vertical scanning head provided by the first actuator as the object is finely scanned. For example, a predicted value of the object's height at a particular location on the surface of the object may be determined by the controller based on the coarse height profile, for example using interpolation. Subsequently, when the scanning head is moved to position the first transducer to emit a first acoustic pulse at a particular location, the controller may already move the first transducer to an appropriate position in the vertical direction based on the predicted value for the particular location such that the first acoustic pulse is properly focused when the scanning head arrives at that location. Thus, the object may be scanned quickly.

[0031] To improve the dynamic behavior of the first transducer in the vertical direction (i.e., the movement of the first transducer relative to the scanning head provided by the first actuator), a weight compensation device can be provided to compensate for the weight of the first transducer. For example, a magnetic spring can be provided to compensate for the force acting on the first actuator by the weight of the first transducer. The weight compensation device minimizes the effective mass moved by the first actuator, which results in an improved dynamic behavior of the first transducer. In particular, the first transducer can accelerate and decelerate more quickly compared to when the weight compensation device is not used. Another advantage of the weight compensation device is that the dynamic behavior of the first transducer is essentially the same in both vertical directions (up and down), thus simplifying the control of the dynamic behavior of the first transducer by the first actuator.

[0032] According to another embodiment, the ultrasound microscope further comprises a second transducer supported by the scan head such that the target region is located between the first transducer and the second transducer, the second transducer configured to detect a second acoustic pulse emerging from the target and to output a second detection signal representative of the second acoustic pulse detected by the second transducer.

[0033] According to this embodiment, the first and second transducers are provided in an ultrasonic microscope, so that the second acoustic pulse reflected from the object can be detected by the first transducer, while the second acoustic pulse transmitted through the object can be detected by the second transducer, thereby enabling the object to be inspected more accurately.

[0034] According to another embodiment of the above embodiment, the ultrasonic microscope may further include a second actuator configured to move the first transducer relative to the scan head along a lateral direction essentially perpendicular to the vertical direction, and the control unit is further configured to control the lateral movement of the first transducer relative to the scan head provided by the second actuator based on the second detection signal.

[0035] The first actuator is configured to move the first transducer relative to the scan head along a vertical direction, while the second actuator is configured to move the first transducer relative to the scan head along a lateral direction that is essentially orthogonal to the vertical direction, in particular orthogonal to the vertical direction. By moving the first transducer relative to the scan head along the lateral direction, the first transducer that emits the first acoustic pulse and the second transducer that detects the second acoustic pulse transmitted through the object can be aligned with each other along the lateral direction. For example, the control unit can be configured to control the lateral movement of the first transducer relative to the scan head provided by the second actuator based on the second detection signal such that the intensity of the second detection signal is maximized. When the intensity of the second detection signal is maximized, the focal point of the first acoustic pulse generated by the first transducer can be considered to coincide with the focal point of the second transducer for detecting the second acoustic pulse in the lateral direction. This improves the signal-to-noise ratio of the second detection signal, and therefore improves the accuracy of the inspection of the object.

[0036] Additionally or alternatively, the control unit can be configured to calculate a frequency domain representation of the second detection signal and control the lateral movement of the first transducer relative to the scan head provided by the second actuator based on the frequency domain representation of the second detection signal. In particular, the frequency domain amplitude distribution of the second detection signal and the bandwidth of the second detection signal can be an appropriate amount for alignment, such that the control unit can be configured to control the lateral movement of the first transducer relative to the scan head provided by the second actuator based on the frequency domain amplitude distribution of the second detection signal and / or the bandwidth of the second detection signal. As a specific example, coarse alignment of the first transducer relative to the second transducer in the lateral direction can be achieved by using the intensity of the second detection signal in the time domain, while fine alignment can be achieved by using at least one of the frequency domain representation of the second detection signal, the frequency domain amplitude distribution of the second detection signal, and the bandwidth of the second detection signal.

[0037] According to another embodiment of the above embodiment, the ultrasonic microscope may further comprise a third actuator configured to move the second transducer along a vertical direction relative to the scan head. Thus, the second transducer, and thus the focal point for detecting the second acoustic pulse by the second transducer, may be moved along the vertical direction by the third actuator relative to the scan head, and thus relative to the first transducer and its focal point. Thus, the focal point generated by the first transducer and the focal point generated by the second transducer may be matched along the vertical direction by a control unit that controls the vertical movement of the second transducer relative to the scan head provided by the third actuator. Furthermore, the vertical distance between the first transducer and the second transducer may be controlled by a control unit configured to control the first and third actuators.

[0038] According to one embodiment of the above embodiment, the third actuator is configured to control the vertical movement of the second transducer relative to the scanning head provided by the third actuator based on the first detection signal. Assuming that the control unit is also configured to control the vertical movement of the first transducer relative to the scanning head provided by the first actuator, the control unit can be configured to generate a control signal for controlling the vertical movement of the first transducer relative to the scanning head provided by the first actuator based on the first detection signal. For example, the control signal can be generated such that the vertical distance between the object and the first transducer remains essentially constant during scanning of the object. The first detection signal and / or the control signal derived from the first detection signal can be used by the control unit to control the vertical movement of the second transducer relative to the scanning head provided by the third actuator. For example, the vertical distance between the first transducer and the second transducer can be maintained at a predetermined value during scanning of the object. For example, the control unit can be configured to control the vertical movement of the first and second transducers provided by the first and third actuators, respectively, based on the first detection signal (and / or the control signal) such that the distance between the first transducer and the second transducer remains essentially constant during scanning of the object. This is particularly useful when the object is composed of at least one layer and is slightly curved. While the control unit controls the vertical movement of the first transducer provided by the first actuator such that the first acoustic pulse remains focused on the surface of the object (or another predefined path associated therewith) during scanning of the object, the distance between the first and second transducers is maintained essentially constant (by controlling the third transducer based on the first detection signal or the control signal) such that the focal points of both the first and second transducers are essentially coincident during scanning of the object.

[0039] Additionally or alternatively, the control unit can be configured to control the vertical movement of the second transducer relative to the scan head provided by the third actuator based on the second detection signal. For example, the control unit can be configured to control the third actuator such that the strength of the second detection signal is maximized. When the strength of the second detection signal is maximized, the focal point generated by the first transducer and the focal point generated by the second transducer can be assumed to coincide in the vertical direction. Thus, the signal-to-noise ratio of the second detection signal can be improved, thereby improving the accuracy of the inspection of the object.

[0040] Additionally or alternatively, the control unit can be configured to calculate a frequency domain representation of the second detection signal and control the vertical movement of the second transducer relative to the scan head provided by the third actuator based on the frequency domain representation of the second detection signal. In particular, the frequency domain amplitude distribution of the second detection signal and the bandwidth of the second detection signal can be an appropriate amount for alignment, such that the control unit can be configured to control the vertical movement of the second transducer relative to the scan head provided by the third actuator based on the frequency domain amplitude distribution of the second detection signal and / or the bandwidth of the second detection signal. As a specific example, coarse alignment of the first transducer relative to the second transducer in the vertical direction can be achieved by using the intensity of the second detection signal in the time domain, while fine alignment can be achieved by using at least one of the frequency domain representation of the second detection signal, the frequency domain amplitude distribution of the second detection signal, and the bandwidth of the second detection signal.

[0041] According to another embodiment, the ultrasound microscope further comprises a fourth actuator configured to move the object holder along a vertical direction relative to the scan head, and the controller is further configured to control the fourth actuator such that the object can be variably positioned vertically relative to the focal point generated by the first actuator.

[0042] According to this embodiment, a fourth actuator is provided for moving the object holder relative to the scan head and thus relative to the focal point of the first acoustic pulse generated by the first transducer. While the positions of the first and second transducers relative to each other and thus the positions of the focal points generated by the first and second transducers relative to each other are maintained, the position of the focal point generated by the first transducer relative to the object can be variably set using the fourth actuator, which moves the object holder along the vertical direction relative to the scan head. Thus, while the alignment of the first and second transducers is maintained, the position of the object to be analyzed can be easily changed using the fourth actuator. This provides an effective way to set the position of the object to be analyzed along the vertical direction without additional expenditure on the control of the first and second transducers. It should be noted that the position of the object being analyzed along the lateral direction can also be easily varied while maintaining the relative positions of the first and second transducers by a scan head positioning device configured to move both the first and second transducers relative to the scan head along the lateral direction.

[0043] According to another embodiment, the scanning head positioning device is configured to displace the scanning head by up to a first displacement value and the second actuator is configured to displace the first transducer by up to a second displacement value relative to the scanning head, the ratio between the first displacement value and the second displacement value being at least 10, in particular at least 100 or at least 1000.

[0044] According to this embodiment, both the scan head positioner and the second actuator are configured to perform lateral movements. In particular, the scan head positioner is configured to move the scan head laterally relative to the target area, and the second actuator is configured to move the first transducer laterally relative to the scan head. The first displacement value is much larger than the second displacement value. Thus, the scan head positioner is used for large displacements of the entire scan head relative to the target, while the second actuator is used for small displacements, such as alignment of the first and second transducers.

[0045] According to another embodiment, the ultrasound microscope may further comprise an immersion device configured to provide a continuous connection of immersion fluid between the first transducer and the object. In an embodiment in which the ultrasound microscope also comprises a second transducer, the immersion device may further be configured to provide a continuous connection of immersion fluid between the second transducer and the object.

[0046] The immersion liquid may be a liquid close to room temperature, for example 20° C. The immersion liquid may include at least one of water, oil, a liquid metal, and a liquid metal alloy.

[0047] The constant connection of immersion fluid between the first transducer and the object and between the second transducer and the object makes it possible to obtain a larger numerical aperture for generating the respective focal points and thus improves the resolution.

[0048] Different types of immersion devices can be used. For example, the immersion device can include a jet generator, which can be configured to generate a first constant jet of immersion liquid between the first transducer and the object (area of ​​interest) and / or a second constant jet of immersion liquid between the second transducer and the object (area of ​​interest). The jet generator can be attached to the scan head. Although the jet generator adds complexity to the ultrasound microscope, as is known in the related art, neither the first nor the second transducer should be moved in the bath of immersion liquid. Moving the first and / or second transducer in a bath of immersion liquid generally reduces the speed at which the first and / or second transducer can be moved relative to the object. Furthermore, by avoiding the need to move the first and / or second transducer in a bath of immersion liquid, problems with fluid resistance (drag) can be avoided.

[0049] According to another example, the immersion device may comprise a container for containing an immersion liquid, the container may be filled with the immersion liquid such that the area of ​​interest is partially located within the immersion liquid, i.e. such that objects within the area of ​​interest are partially located within the immersion liquid.

[0050] In this example, assuming that the first transducer is positioned vertically above the area of ​​interest and the second transducer is positioned vertically below the area of ​​interest, the second detection pulse detected by the second transducer propagates in a bath of immersion liquid defined by the container. This example can be improved by providing the above-mentioned jet generator configured to generate a first continuous jet of immersion liquid between the first transducer and the object. A further improvement of this example can be achieved by further providing the above-mentioned jet generator configured to generate a second continuous jet of immersion liquid between the second transducer and the object, thereby removing micro (air) bubbles between the second transducer and the object.

[0051] According to yet another example, the container can be filled with immersion liquid such that the first transducer above the area of ​​interest is at least partially located in the immersion liquid, i.e. in a bath of immersion liquid provided by the container. According to this example, a jet generator is not required, since both the first and second transducers are at least partially located in the immersion liquid contained in the container. However, the immersion device can still comprise the above-mentioned jet generator for removing micro (air) bubbles in the immersion liquid between the first and / or second transducer and the object.

[0052] According to another embodiment, the first transducer is arranged above the target area. In this example embodiment, the first transducer is arranged vertically above the target area and thus above the object that may be placed in the target area. The vertical direction is defined as a direction that is essentially parallel to the emission direction of the first acoustic pulse. However, the vertical direction can also be defined as a direction that is essentially antiparallel to the direction of gravity of the ultrasound microscope.

[0053] According to another embodiment, the first transducer is configured to emit a first acoustic pulse such that the first acoustic pulse has an acoustic frequency that exceeds a threshold frequency, the threshold frequency being selected from a group of a plurality of threshold frequencies, the group of a plurality of threshold frequencies consisting of 10 MHz and 15 MHz.

[0054] According to another aspect of the invention, there is provided a carrier for carrying an acoustic pulse transducer of an ultrasonic microscope within an immersion liquid, the carrier comprising a first part positioned within the immersion liquid and a second part positioned outside the immersion liquid, the first part including a solid structure having openings allowing the immersion liquid to flow through the first part, the first value representing a total area of ​​the openings in a cross-section of the first part and the second value representing a total area of ​​the solid structure in a cross-section of the first part, the ratio of the first value to the second value being greater than 5, in particular greater than 10, more in particular greater than 20.

[0055] Said carrier can be used to carry one acoustic pulse transducer of an ultrasonic microscope defined above. For example, in an embodiment of said ultrasonic microscope, a first transducer is arranged vertically above the target area and a second transducer is arranged vertically below the target area, a container is provided and filled with immersion liquid up to the target area, and when the scanning head moves relative to the target area, the second transducer moves in the immersion liquid. Thus, drag forces act on the second transducer and on the carrier carrying the second transducer. That is, the second transducer is supported by the scanning head via the carrier. Said carrier provides drag forces that are minimized by the openings in the solid structure of the first part of the carrier. These openings allow the immersion liquid to flow through the carrier, in particular through the first part of the carrier. Thus, drag forces acting on the carrier when moving the carrier in the immersion liquid are minimized by said structure of the carrier. Thus, the second transducer and the carrier can move quickly through the immersion liquid, which allows a fast inspection of the object.

[0056] The first part of the carrier may have a length in cross section of at least 10 cm, in particular at least 30 cm, and may have a length in cross section of not more than 1 m, in particular not more than 60 cm. That is to say, a significant long side of the carrier is represented in cross section. In particular, the cross section may be arranged orthogonal to the direction used to move the scanning head relative to the area of ​​interest. For example, assuming that the object is inspected by scanning the object line by line, the cross section is selected to be orthogonal to the direction of the lines. The cross section therefore represents a plane of the carrier on which drag forces act as the carrier moves through the immersion liquid.

[0057] According to one embodiment, the solid structure of the carrier is shaped such that essentially no lift and / or drag forces occur when the first part moves in the immersion liquid. In terms of the description of an ultrasonic microscope, the solid structure is shaped such that no forces along the vertical direction act on the carrier and the second transducer due to lift forces when the scanning head moves laterally. [Brief description of the drawings]

[0058] The foregoing and other advantageous features of the present disclosure will become more apparent from the following detailed description of exemplary embodiments, which refer to the accompanying drawings, in which: It is to be noted that not all possible embodiments necessarily exhibit each and every, or any, of the advantages identified herein. [Figure 1] FIG. 1 is a schematic diagram of an ultrasonic microscope according to a first embodiment. [Diagram 2] FIG. 2 shows a schematic of the interaction between an ultrasonic microscope and an object. [Diagram 3] FIG. 3 is a block diagram showing components of the ultrasonic microscope according to the first embodiment. [Figure 4] FIG. 4 shows a schematic of the alignment of the focal spot generated by the components of an ultrasonic microscope. [Diagram 5] FIG. 5 shows examples of time and frequency domain signals used for focal spot alignment. [Figure 6] FIG. 6 shows an example of controlling the first actuator of an ultrasonic microscope. [Figure 7] FIG. 7 is a schematic diagram of an ultrasonic microscope according to the second embodiment. [Figure 8] FIG. 8 is a schematic diagram of an ultrasonic microscope according to the third embodiment. [Figure 9] FIG. 9 shows a perspective view of a carrier for an ultrasonic microscope. [Figure 10] FIG. 10 shows a cross-sectional view of the carrier of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] In the exemplary embodiments described below, components having similar functions and structures are given similar reference numbers as much as possible. Therefore, to understand the characteristics of the individual components of a specific embodiment, it is necessary to refer to the description of other embodiments and the summary of this disclosure.

[0060] Figure 1 shows a first embodiment of an ultrasound microscope 1 for inspecting an object 3. The ultrasound microscope 1 comprises an object holder 5 configured to hold the object 3 in an area of ​​interest 7. The ultrasound microscope 1 further comprises a scan head 9 movable relative to the area of ​​interest 7. The ultrasound microscope 1 further comprises a first transducer 11 supported by the scan head 9 via a first actuator 13 and a second actuator 15. The first transducer 11 will be described in more detail with reference to Figure 2.

[0061] Figure 2 shows diagrammatically the interaction of an ultrasonic microscope 1 with an object 3. A first transducer 11 is configured to emit a first acoustic pulse 17 (schematically shown by an arrow). The first transducer 11 emits the first acoustic pulse 17 along an emission direction represented by the direction of the arrow representing the first acoustic pulse. Furthermore, the first transducer 11 focuses the first acoustic pulse 17 at a focal point 20 indicated by the tip of a triangle.

[0062] When a first acoustic pulse is directed towards the object 3, a second acoustic pulse emerges from the object 3. An example of a second acoustic pulse emerging from the object towards the first transducer is represented by the arrow 21. Such a second acoustic pulse may be (partially) reflected by the object.

[0063] The first transducer 11 is further configured to detect a second acoustic pulse emerging from the object 3. Furthermore, the first transducer 11 is configured to output a first detection signal S1 representative of the second acoustic pulse detected by the first transducer 11. A focal point 20 is also applied to the detection portion of the first transducer.

[0064] 1, the first actuator 13 is configured to move the first transducer 11 relative to the scan head 9 along a vertical direction 23 indicated by the arrow. The vertical direction is essentially parallel to the emission direction of the first acoustic pulse. In particular, in this example, the vertical direction is parallel to the emission direction of the first acoustic pulse.

[0065] The second actuator 15 is configured to move the first transducer 11 relative to the scan head 9 along a lateral direction 25 indicated by the arrow, which is essentially perpendicular to the vertical direction.

[0066] The ultrasound microscope 1 further comprises a second transducer 27 supported by the scan head 9 such that the target area 7 is located between the first transducer 11 and the second transducer 27. Referring again to FIG. 2, the second transducer 27 is configured to detect a second acoustic pulse emerging from the object 3 towards the second transducer 27. An example of a second acoustic pulse transmitted through the object 3 towards the second transducer 27 is represented by an arrow 29. The second transducer 27 is further configured to output a second detection signal S2 representative of the second acoustic pulse detected by the second transducer 27. The second transducer provides a focal point 28 for detection represented by a triangular tip 28.

[0067] 1 , the second transducer 27 is supported by the scan head 9 via a third actuator 31 and a carrier 33. In particular, the second transducer 27 is rigidly connected to the carrier 33 which is itself supported by the scan head 9 via the third actuator 31. The third actuator 31 is configured to move the second transducer 27 along the vertical direction 23 relative to the scan head 9.

[0068] The ultrasound microscope 1 further comprises an immersion device 35 configured to provide a constant connection of an immersion liquid 37 between the first transducer 11 and the object 3. Furthermore, the immersion device 35 is configured to provide a constant connection of an immersion liquid 37 between the second transducer 27 and the object 3.

[0069] The immersion device 35 comprises a container 39 for containing an immersion liquid 37. The container 39 provides a volume in which the immersion liquid 37 is held. Furthermore, the container 39 is constructed and arranged such that the object area 7 and the second transducer 27 are arranged in said volume and the first transducer 11 is arranged at least partially in said volume. Since said volume can be filled with the immersion liquid 37, the container 39 provides a constant connection between the object 3 and the first transducer 11 and a constant connection between the object 3 and the second transducer 27.

[0070] The ultrasound microscope 1 further comprises a fourth actuator 41 configured to move the object holder 5 along the vertical direction 23 relative to the scan head 9. The fourth actuator 41 allows the object 3 to be displaced simultaneously relative to the first transducer 11 and the second transducer 27, thereby allowing the focus within the object to be easily set / changed in the vertical direction 23.

[0071] Fig. 3 shows a block diagram of the components of the ultrasonic microscope 1. In particular, Fig. 3 shows a first actuator 13, a second actuator 15, a third actuator 31, and a fourth actuator 41 connected to a control unit 43 of the ultrasonic microscope 1. Furthermore, the first transducer 11 and the second transducer 27 are connected to the control unit 43. The control unit comprises a memory 51.

[0072] The ultrasound microscope 1 further comprises a scan head positioning device 45 connected to the control unit 43. The scan head positioning device is configured to move the scan head 9 relative to the object holder 5 and thus relative to the object 3 in a transverse plane essentially perpendicular to the vertical direction 23. The transverse plane is therefore also indicated by the numeral 25 in Fig. 1. To simplify the drawing in Fig. 1, the scan head positioning device 45 is not shown. However, the scan head positioning device 45 is configured to move the scan head 9 and thus the first transducer 11 and the second transducer 27 in the transverse plane 25.

[0073] 1, the scan head positioning device 45 is configured to displace the scan head 9 by a first displacement value at most indicated by a distance D1. The second actuator 15 is configured to displace the first transducer 11 by a second displacement value at most indicated by a distance D2 relative to the scan head 9. The ratio of the first displacement value D1 to the second displacement value D2 may be, for example, at least 10. That is, the scan head positioning device 45 may be used to displace the first transducer 11 significantly relative to the object 3, while the second actuator 15 may be used to displace the first transducer 11 slightly relative to the object 3.

[0074] The configuration of the control unit 43 will be described in more detail below. The control unit 43 is configured to control the first actuator 13 based on the first detection signal S1. In particular, referring to FIG. 2, the control unit 43 can be configured to cause the first transducer 11 to emit a first acoustic pulse 17. A distance H between the first transducer 11 and the object 3, in particular a surface 47 of the object 3, can be determined based on a propagation time of the first acoustic pulse from the first transducer 11 to the surface 47 and a propagation time of a (reflected) second acoustic pulse from the surface 47 to the first transducer 11. That is, the distance H can be determined using the time of flight of the acoustic pulse emitted from the first transducer 11, reflected by the surface 47, and detected by the first transducer 11. Further parameters such as the speed of sound, parameters describing the influence of the medium between the first transducer 11 and the object 3 (e.g. air or immersion liquid 37) on the speed of sound, parameters describing the influence of the material of the object 3 on the speed of sound etc. can be stored in the memory 51 and used to determine the distance H. The control unit 43 can thus be configured to determine a distance value representative of the distance H and to control the first actuator 13 based on the determined distance value.

[0075] In particular, the control unit 43 can be configured to control the first actuator 13 such that the distance H between the first transducer 11 and the object 3, in particular the distance between the first transducer 11 and the surface 47 of the object 3, approaches a predefined working distance H0. In the example shown in Fig. 2, the predefined working distance H0 is equal to the focal length of the first transducer 11, i.e. equal to the distance between the first transducer 11 and the focal point 20. This setting can be used when the surface 47 of the object 3 is to be inspected, where a particular analysis target 49 may be located.

[0076] Since the control unit 43 is configured to determine a distance value representative of the distance H, the control unit can also be configured to control the first actuator 13 to move the first transducer 11 relative to the scanning head 9 and thus relative to the object 3, such that the distance H approaches a predefined working distance H0.

[0077] Instead of controlling the distance H to approach a predefined working distance H0, the control unit can be configured to determine a defocus value representing a distance O between the focal point 20 and a location of interest on the object 3 based on the determined distance value H. In the example shown in Fig. 2, the location of interest coincides with the surface 47 of the object 3. However, other locations within the object 3 may also be locations of interest.

[0078] For example, the control unit 43 may store in the memory 51 a value representative of the focal length of the first transducer 11, i.e. the distance between the first transducer 11 and the focal point 20. Based on this value and a pre-determined distance value H, the control unit may be configured to determine a defocus value representative of the distance O and to control the first actuator 13 based on the defocus value.

[0079] The configuration details of the control unit 43 will be explained with reference to FIG. 4. In the situation shown in FIG. 4, the focal point 20 generated by the first transducer 11 and the focal point 28 of the second transducer 27 are displaced relative to each other in the lateral direction 25 and the vertical direction 23. The highest accuracy for inspecting the object 3 is obtained when both focal points coincide. In order to make both focal points coincide, the control unit 43 can be configured to control the second actuator 15 based on the second detection signal S2 output by the second transducer 27. As mentioned above, the second actuator 15 is configured to move the first transducer 11 in the lateral direction 25 relative to the scanning head 9. The second actuator 15 can thus move the first transducer 11, and thus the focal point 20 of the first transducer 11, in the lateral direction 25 such that the displacement between the focal points 20 and 28 along the lateral direction is minimized. Thus, the control unit 43 can be configured to control the second actuator 15 such that the first transducer 11 is aligned with the second transducer 27 along the lateral direction 25. In particular, this can be achieved in that the control unit 43 is configured to control the second actuator 15 such that the strength of the second detection signal S2 is maximized. By maximizing the strength of the second detection signal S2, the displacement between the focal points 20 and 28 in the lateral direction 25 is minimized.

[0080] Additionally, the control unit 43 can be configured to control the third actuator 31 based on the second detection signal S2 to minimize the displacement between the focal points 20 and 28 in the vertical direction 23. For example, this can be achieved in that the control unit 43 is configured to control the third actuator 31 such that the intensity of the second detection signal S2 is maximized. By maximizing the intensity of the second detection signal S2, the vertical displacement between the focal points 20 and 28 is minimized.

[0081] With reference to FIG. 5, the alignment of the first transducer 11 and the second transducer 27 relative to each other and the alignment of the focal points 20 and 28 relative to each other performed by the control unit based on the second detection signal S2 will be explained in more detail.

[0082] Graph G1T shows an example representation of the second detection signal S2 in the time domain when the first transducer 11 and the second transducer 27 are not correctly aligned, i.e., when the focal points 20 and 28 never coincide. Graph G1F shows a frequency domain representation of the signal shown in graph G1T.

[0083] Graph G2T shows an exemplary representation of the second detection signal S2 in the time domain when the first transducer 11 and the second transducer 27 are normally aligned, i.e., when the focal points 20 and 28 are close to each other, e.g., 100 μm apart. Graph G2F shows a frequency domain representation of the signal shown in graph G2T.

[0084] Graph G3T shows an exemplary representation of the second detection signal S2 in the time domain when the first transducer 11 and the second transducer 27 are well aligned, i.e., when the focal points 20 and 28 are essentially coincident. Graph G3F shows a frequency domain representation of the signal shown in graph G3T.

[0085] The second detection signal S2 in the time domain (graphs G1T, G2T, G3T), in particular its maximum, intensity values ​​etc., can be used as a quantity representative of the quality of the alignment of the focal points 20 and 28. However, a quantified interpretation of the quantity can be difficult to achieve, so the signal in the time domain is best used for coarse alignment. The second detection signal S2 in the frequency domain (graphs G1F, G2F, G3F), in particular its amplitude spectrum (e.g., components from 30 to 80 MHz) and bandwidth, allow a more accurate and easier interpretation. As can be seen from graphs G1F, G2F, G3F, the bandwidth of the second detection signal S2 in the frequency domain increases as the alignment improves. Also, the amplitude of the higher frequency components (e.g., components from 30 to 80 MHz) of the second detection signal S2 in the frequency domain increases as the alignment improves. Therefore, the signal in the frequency domain is best used for fine alignment.

[0086] 1 , the control unit 43 may be further configured to control the fourth actuator 41 such that the object 3 may be variably positioned in the vertical direction 23 relative to the focal point 20. Thus, the object 3 may be displaced relative to the focal point 20 by controlling the fourth actuator 41.

[0087] FIG. 6 shows another example of controlling the first actuator 13 of the ultrasound microscope 1. As mentioned above, the scan head positioning device 45 is configured to move the scan head 9 in the transverse direction 25 in the transverse plane relative to the object holder 5. The control unit 43 is therefore configured to control the scan head positioning device 45 to move the scan head 9, and thus the first transducer 11, successively to a number of positions x1, x2, and x3. As shown in FIG. 6, the surface 47 of the object 3 may not be oriented parallel to the transverse direction 25. When analyzing the surface 47 of the object 3, during the scanning of the object 3, i.e. during the movement of the first transducer 11 to a number of positions x1, x2, and x3, the focal point 20 of the first transducer 11 must be controlled to coincide with the surface 47 of the object 3 in order to achieve the desired accuracy.

[0088] As an example shown in Fig. 6, the first transducer 11 is initially positioned in the lateral direction 25 at a position x1, and the first actuator 13 is controlled so that the focal point 20 coincides with the surface 47 of the object 3. When the scanning head 9 is positioned so that the first transducer 11 is positioned in the lateral direction 25 at a position x1, the control unit 43 causes the first transducer 11 to emit at least one first acoustic pulse, detect at least one second acoustic pulse, and output a first detection signal S1. The control unit 43 is further configured to store the first detection signal S1 or a value derived from the first detection signal S1 in association with the position x1 in the memory 51.

[0089] When the scan proceeds, for example by moving the scan head 9 in the lateral direction 25 to move the first transducer 11 to a position x2, the first actuator 13 can be controlled using the first detection signal stored in the memory 51 or a value derived from the first detection signal. For example, it can be assumed that when the first transducer 11 is at the position x2, the first actuator 13 must be controlled slightly differently compared to when the first transducer 11 is at the position x1. Based on the first detection signal stored in the memory 51 or a value derived from the first detection signal, a predicted value for controlling the first actuator 13 at the next position x2 can be determined by the control unit even before moving the first transducer 11 to the position x2.

[0090] After the first transducer 11 has moved to position x2, the control unit 43 causes the first transducer 11 to emit a further first acoustic pulse, detects a further second acoustic pulse, and outputs a further first detection signal S1, which detection signal or a value derived from the detection signal is stored in the memory 51 in association with position x2.

[0091] Based on the first detection signal or values ​​derived from the first detection signal stored in the memory 51, a new predicted value for controlling the first actuator 13 at the next position x3 can be determined by the control unit even before the first transducer 11 is moved to the position x3. The predicted value represents a value that approaches the actual value for controlling the first actuator 13 when the first transducer 11 is placed at the position x3. However, the predicted value is calculated in advance, i.e. before the first transducer 11 is placed at the position x3.

[0092] FIG. 7 shows a second embodiment of an ultrasonic microscope 101. The ultrasonic microscope 101 differs from the ultrasonic microscope 1 only in the configuration of the immersion device. Therefore, for the components of the ultrasonic microscope 101 that are identical to those of the first embodiment, reference is made to the description of the first embodiment. The ultrasonic microscope 101 comprises an immersion device 135. The immersion device 135 comprises a container 139 similar to the container 39 of the first embodiment. However, the container 139 is constructed and arranged such that the target area 7 is partially located within a volume defined by the container 139 and is filled with the immersion liquid 37.

[0093] The immersion device 135 further comprises a jet generating device 141 configured to generate a first continuous jet 143 of immersion liquid between the first transducer 11 and the object 3. Thus, the same effects as presented for the first embodiment are also achieved in the second embodiment.

[0094] FIG. 8 shows a third embodiment of the present invention. FIG. 8 shows an ultrasonic microscope 201 which differs from the ultrasonic microscope 1 of the first embodiment only in the configuration of the immersion device. Therefore, for the components of the ultrasonic microscope 201 which are identical to those of the first embodiment, reference is made to the description of the first embodiment. The ultrasonic microscope 201 comprises an immersion device 235 including a jet generator 241 configured to generate a first continuous jet 243 of immersion liquid between the first transducer 11 and the object 3 and to generate a second continuous jet 245 of immersion liquid between the object 3 and the second transducer 27. The immersion device 235 further comprises a container 239 configured and arranged to hold the immersion liquid 37 output by the jet generator 241. Thus, the same effects as those presented for the first embodiment are also achieved in the third embodiment.

[0095] 9 shows a perspective view of a carrier 301 for carrying an acoustic pulse transducer of an ultrasonic microscope in an immersion liquid. The carrier 301 can be used as the carrier 33 shown in FIGS. 1, 6 and 7. Thus, the carrier 301 can be configured to carry the second transducer 27 for supporting the second transducer 27 by the scan head 9.

[0096] 10 shows a cross-section of the carrier 301. The cross-section can be defined as being perpendicular to the direction in which the carrier 301 moves through the immersion liquid 37 as the scanning head 9 moves relative to the object holder 5.

[0097] The carrier 301 comprises a first part 303 that is located in the immersion liquid 37 and a second part 305 that is located outside the immersion liquid 37 .

[0098] The first part 303 comprises a solid structure 307 having openings 309 that allow the immersion liquid 37 to flow through the first part 303. A surface of the first part 303 can be characterized in terms of drag forces acting on it. For example, the total area of ​​the openings 309 in the cross section shown in FIG. 10 can be represented by a first value and the total area of ​​the solid structure 307 in the same cross section can be represented by a second value. The first part 303 of the carrier 301, and in particular the solid structure 307, can be constructed such that the ratio between the first value and the second value is greater than a certain threshold value. The greater the certain threshold value, the less drag forces act on the carrier 301 when moving through the immersion liquid 37. The certain threshold value may be greater than 5, in particular greater than 10, in particular greater than 20. Thus, the drag forces acting on the first part are minimized by the configuration of the solid structure 307.

[0099] The length L of the first portion 303 in cross section may be between 10 cm and 1 m, in particular between 30 cm and 60 cm.

[0100] Furthermore, the solid structure 307 can be shaped such that essentially no lift forces act on the solid structure 307 when the first part 303 moves through the immersion liquid 37 in a direction perpendicular to the cross section shown in Figure 10. Thus, the position of the focal point 28 of the second transducer 27 is not altered in the vertical direction 23 when the carrier 301 moves relative to the object 3 in the immersion liquid 37.

[0101] While the present disclosure has been described with respect to certain exemplary embodiments thereof, it is apparent that many alternatives, modifications, and variations will be readily apparent to those skilled in the art. Accordingly, the exemplary embodiments of the present disclosure described herein are intended to be illustrative and not limiting in any way. Various changes may be made without departing from the spirit and scope of the present disclosure, as defined in the following claims.

Claims

1. 1. An ultrasonic microscope for inspecting an object, comprising: an object holder configured to hold the object in a target area; a scan head movable relative to the area of ​​interest; a first transducer supported by the scan head; the first transducer is configured to emit a first acoustic pulse along an emission direction and to focus the first acoustic pulse at a focal point; the first transducer is further configured to detect a second acoustic pulse emanating from the object and output a first detection signal representative of the second acoustic pulse detected by the first transducer; a first actuator configured to move the first transducer relative to the scan head along a vertical direction that is essentially parallel to the emission direction; a controller configured to control a vertical movement of the first transducer relative to the scan head provided by the first actuator based on the first detection signal such that a predetermined working distance is maintained between the first transducer and a surface of the object while scanning the object; the control unit is configured to generate a control signal such that a vertical distance between the object and the first transducer is kept constant during scanning of the object; the scan head is movable in a lateral plane to scan the object; 11. An ultrasonic microscope comprising: a first transducer configured to move a first object along a first axis; a first scanning head configured to move a second scanning head along a first axis;

2. The control unit further includes: controlling emission of the first acoustic pulse by the first transducer; determining a distance value representative of a distance between the first transducer and the object based on the first detection signal; The ultrasound microscope of claim 1 , configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on the determined distance value.

3. 3. The ultrasonic microscope of claim 2, wherein the control unit is further configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator so that the distance between the first transducer and the object approaches a predetermined working distance.

4. The control unit further includes: determining a defocus value representing a distance between the focal point and a point of interest of the object based on the determined distance value; The ultrasonic microscope of claim 2 , configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on the defocus value.

5. 5. The ultrasonic microscope of claim 1, further comprising a scan head positioning device configured to move the scan head relative to the object holder in a lateral plane essentially perpendicular to the vertical direction.

6. The control unit further includes: controlling the scan head positioning device to position the scan head at a plurality of positions; each time the scan head is positioned at one of the plurality of positions, causing the first transducer to emit at least one of the first acoustic pulses, detecting at least one of the second acoustic pulses, and outputting the first detection signal; storing the first detection signal or a value derived from the first detection signal in association with the plurality of locations; 6. The ultrasonic microscope of claim 5, configured to control the vertical movement of the first transducer relative to the scan head provided by the first actuator based on a set of stored first detection signals or derived values.

7. a second transducer supported by the scan head such that the region of interest is located between the first and second transducers; 7. An ultrasonic microscope as described in any one of claims 1 to 6, wherein the second transducer is configured to detect the second acoustic pulse emerging from the object and output a second detection signal representative of the second acoustic pulse detected by the second transducer.

8. a second actuator configured to move the first transducer relative to the scan head along a lateral direction essentially perpendicular to the vertical direction; The ultrasonic microscope of claim 7 , wherein the control unit is further configured to control lateral movement of the first transducer relative to the scan head provided by the second actuator based on the second detection signal.

9. 9. The ultrasonic microscope of claim 8, wherein the control unit is configured to control the lateral movement of the first transducer relative to the scan head provided by the second actuator such that the first transducer is aligned with the second transducer along the lateral direction.

10. 10. The ultrasonic microscope of claim 9, wherein the control unit is configured to control the lateral movement of the first transducer relative to the scan head provided by the second actuator so as to maximize the strength of the second detection signal.

11. 11. The ultrasonic microscope of claim 9 or claim 10, wherein the control unit is configured to calculate a frequency domain representation of the second detection signal and to control the lateral movement of the first transducer relative to the scanning head provided by the second actuator based on the frequency domain representation of the second detection signal.

12. 12. The ultrasonic microscope of claim 7, further comprising a third actuator configured to move the second transducer along the vertical direction relative to the scan head.

13. The ultrasonic microscope of claim 12 , wherein the control unit is further configured to control vertical movement of the second transducer relative to the scan head provided by the third actuator based on the first detection signal.

14. the controller is configured to calculate, based on the first detection signal, a control signal for controlling the vertical movement of the first transducer relative to the scan head provided by the first actuator; The ultrasonic microscope of claim 13 , wherein the control unit is further configured to control vertical movement of the second transducer relative to the scan head provided by the third actuator based on the control signal.

15. 15. The ultrasonic microscope of claim 12, wherein the control unit is further configured to control vertical movement of the second transducer relative to the scan head provided by the third actuator based on the second detection signal.

16. 16. The ultrasonic microscope of claim 15, wherein the control unit is configured to control the vertical movement of the second transducer relative to the scan head provided by the third actuator so as to maximize the strength of the second detection signal.

17. 17. The ultrasonic microscope of claim 15 or 16, wherein the control unit is configured to calculate a frequency domain representation of the second detection signal and to control the vertical movement of the second transducer relative to the scan head provided by the third actuator based on the frequency domain representation of the second detection signal.

18. a fourth actuator configured to move the object holder along the vertical direction relative to the scan head; 18. The ultrasonic microscope of claim 1, wherein the control unit is further configured to control the fourth actuator such that the object can be variably positioned relative to the focal point in the vertical direction.

19. The method of claim 19, further comprising: a second transducer supported by the scan head such that the region of interest is located between the first and second transducers; the second transducer is configured to detect the second acoustic pulse emanating from the object and output a second detection signal representative of the second acoustic pulse detected by the second transducer; a second actuator configured to move the first transducer relative to the scan head along a lateral direction essentially perpendicular to the vertical direction; the controller is further configured to control lateral movement of the first transducer relative to the scan head provided by the second actuator based on the second detection signal; the scan head positioning device is configured to displace the scan head up to a first displacement value; the second actuator is configured to displace the first transducer relative to the scan head by up to a second displacement value; 6. The ultrasonic microscope of claim 5, wherein the ratio between the first displacement value and the second displacement value is at least 10, in particular at least 100 or at least 1000.

20. 20. The ultrasonic microscope of claim 1, wherein the first transducer is positioned above the region of interest.

21. the first transducer is configured to emit the first acoustic pulse such that the first acoustic pulse has an acoustic frequency above a threshold frequency; 21. The ultrasonic microscope of claim 1, wherein the threshold frequency is selected from a group of a plurality of threshold frequencies, the group of a plurality of threshold frequencies consisting of 10 MHz and 15 MHz.

22. 1. A method of operating an ultrasonic microscope for inspecting an object, comprising:

22. A method according to claim 1, wherein the ultrasonic microscope is constructed or operated in accordance with any one of claims 1 to 21.

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