Method and device for detecting cavities in bonded wafers

Abstract

In a method for detecting cavities (18) which have formed in a composite structure (10) in the region of an interface between two bonded wafers (12, 14), an optical coherence tomograph (22) generates a measuring light beam (24). A scanning device (26) deflects the measuring light beam (24) in two spatial directions (x, y) and directs it onto the composite structure (10) in such a way that the composite structure (10) is scanned successively at multiple measurement points (39). An evaluation unit (57) detects locations where cavities (18) are located from interference signals detected by the optical coherence tomograph (22), by detecting deformations of a first surface (12-1) of a wafer (12), which is opposite a second surface (12-2) of the wafer (12), which is located closer to the cavities (18) than the first surface (12-1).

Description

[0001] Method and apparatus for detecting voids in bonded wafers

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a method and a device for detecting voids that have formed in a composite structure made up of wafers in the area of ​​an interface between two bonded wafers.

[0005] 2. Description of the state of the art

[0006] Wafer bonding is a process in semiconductor and microsystems technology in which two wafers, made of the same or different materials, are joined together. In most cases, silicon wafers are used, but other materials are also employed, such as glass for the fabrication of microlens arrays.

[0007] The composite structure obtained through bonding can be used advantageously, for example, in the assembly of sensor and actuator components, for the production of substrate materials for RF components, and for the wafer-level integration of electronic, micromechanical, or optical components.

[0008] Various technologies have been developed for bonding wafers. These include processes without bonding layers and processes with bonding layers. In processes without bonding layers, the wafers are bonded directly to each other. In so-called silicon direct bonding, for example, hydrophilic and hydrophobic surfaces are brought into contact at high temperatures and high pressure. In processes with bonding layers, an alloy or adhesive causes the adhesion between the wafers.

[0009] Ideally, the two bonded wafers lie completely flush against each other across their entire surface. However, it frequently occurs that nearly disk-shaped cavities form at the interface between the wafers, with diameters ranging from approximately 20 nanometers to several millimeters. Since feature sizes in integrated semiconductors are now on the order of just a few nanometers, even very small cavities can lead to defects in electronic or optical circuits. The same applies if cavities are present in a bonding layer between the wafers.

[0010] Scanning acoustic microscopes (SAMs) are predominantly used to measure bonded wafers. These microscopes generate several thousand short ultrasound pulses per second, with durations of 20 to 100 nanoseconds and frequencies extending into the gigahertz range. These pulses are directed onto the object under investigation via an acoustic lens and a coupling medium, usually water. The ultrasound waves are reflected at the surface and internal interfaces of the object and converted back into electrical signals by an ultrasonic transducer. These signals are then evaluated by a receiver with time resolution. By mechanically moving the transducer with the acoustic lens over the object, an image can be generated that contains information for each pixel not only about the surface but also about near-surface structures below the surface.

[0011] Scanning ultrasonic microscopes of this type have also proven effective in detecting voids in bonded wafers. However, a disadvantage is that the bonded wafers must be immersed in water for measurement. Gases or gas mixtures such as air attenuate sound waves with such high frequencies too much. For immersion, the wafers must be removed from a conveyor, dried after measurement, and then returned to the conveyor, which is time-consuming and costly. The measurement itself is also time-consuming because the relatively heavy transducer must be moved across the object in a traversing device.

[0012] For these reasons, scanning ultrasound microscopy is suitable for random quality control, but not for measurement within the ongoing manufacturing process.

[0013] From US patent 2024 / 0035810 A1, a method and a device for detecting voids at the interface of bonded wafers are known, in which the wafers do not need to be immersed in water. For this purpose, individual images are successively generated in different focal planes using a Linnik interferometer and then combined to form a volumetric composite image. After each individual image is captured, the bonded wafers are moved along their thickness extent, i.e., perpendicular to the wafer surfaces. However, this method is very slow. To cover an area of ​​40 pm 2 Measuring takes hundreds of milliseconds. Accordingly, measuring 1 mm requires 2 already more than two minutes, and a complete measurement of a wafer with a diameter of 300 mm is impossible because it would take thousands of hours.

[0014] From DE 10 2011 051 146 B3, a method and a device for detecting voids in a bonding layer between two bonded wafers are known, in which the wafers do not need to be immersed in water. The measurement is performed using an optical coherence tomograph and is performed point by point, with each measurement being carried out twice. In one of the measurements, the reference arm of the interferometer is blocked, so that the thickness of the bonding layer is measured. The other measurement is carried out without blocking, so that the distance to the surface of the bonding layer is measured. In both measurements, the bonding layer is located in the measurement plane, which is achieved by moving the wafers in the z-direction or by shifting the measurement plane. By comparing the layer thickness profile with the distance profile, it can be determined whether the voids are harmless foreign matter or problematic voids.To measure the entire interconnect layer, it is scanned in a spiral motion by rotating the composite structure consisting of the two wafers while simultaneously moving the device radially. Since both the composite structure and the device must be moved during this scanning process, the scanning speed is comparatively low, and the equipment required to generate and control the two precise movements is complex.

[0015] US Patent 2018 / 0059032 A1 also describes an interferometric measurement method that can reliably and quickly detect cavities in wafers bonded directly (i.e., without a bonding layer). The device utilizes the effect that the measuring light undergoes a phase shift when passing through a cavity, resulting in a change in contrast. In the contrast images, the cavities can be reliably distinguished from other structures on the processed wafer. This allows a wafer with a 30 cm diameter to be measured in just a few minutes without the need to immerse the wafer in water. However, this method can only detect relatively large cavities.Small cavities, whose thickness is on the order of 200 nm and thus significantly below the coherence length of the measuring light, cannot be reliably detected, since the change in refractive index in the area of ​​a cavity does not have a significant effect on the contrast if it only occurs over a fraction of the coherence length.

[0016] SUMMARY OF THE INVENTION

[0017] The object of the invention is to provide a method and a device with which very small cavities in bonded wafers can be detected so quickly that detection is possible during an ongoing manufacturing process.

[0018] Regarding the method, this problem is solved by a method for detecting voids that have formed in a composite structure in the region of an interface between two bonded wafers, wherein the method comprises the following steps: a) an optical coherence tomograph generates a measurement light beam; b) a scanning device deflects the measurement light beam in two spatial directions and directs it onto the composite structure so that the composite structure is successively scanned at several measurement points; c) an evaluation unit detects locations where voids are situated from interference signals acquired by the optical coherence tomograph by detecting deformations of a first surface of a wafer that is opposite a second surface of the wafer that is closer to the voids than the first surface.

[0019] The combination of an optical coherence tomograph with a scanning device for two-dimensional scanning, as known from DE 10 2022 104416 A1, makes it possible to scan even large bonded wafers in a short time without having to move the wafers and the optical coherence tomograph relative to each other. Immersion in water or any other liquid is also not required for interferometric measurements.

[0020] The invention is based on the understanding that deformation of a first surface of a wafer, which is directly adjacent to a cavity or separated from it only by a portion of a bonding layer, is always accompanied by a deformation of the opposite second surface, at least for commonly used wafer thicknesses. This makes it possible to detect the cavities not directly, but indirectly by measuring the second surface. Indirect measurement is advantageous because direct detection of cavities requires measuring the first surface, which is difficult to perform using an optical coherence tomograph. This is because, especially with very small cavities with thicknesses on the order of 200 nm, two closely spaced optical interfaces cannot be resolved due to the significantly longer wavelength of the measurement light.Even in a so-called thickness mode, where thicknesses are measured instead of distances, thicknesses of 200 nm cannot be measured directly.

[0021] However, it is possible to measure the distance to the second surface of the wafer on the opposite side. This exploits the property of optical coherence tomographs suitable for this purpose: while they cannot directly measure a thickness of 200 nm, they can distinguish between distances of, for example, 10.0 pm and 10.2 pm, even though this difference is also only 200 nm. Especially when the second surface of the wafer is directly adjacent to the surrounding air and faces the scanner, a strong interference signal is obtained due to the large difference in refractive index and the associated high reflectance, from which the distance can be derived with sufficient accuracy.

[0022] For a cavity to cause deformation of an adjacent wafer, the wafer must be thin, which is regularly the case with bonded wafers. Silicon wafers, for example, should not be thicker than 1 mm.

[0023] Scanning the composite structure with a measuring light beam produces an image in which the cavities hidden within the structure are visible as small structures. To automatically determine the locations and lateral dimensions of these cavities based on such an image, image processing algorithms can be used that detect the small structures and quantitatively determine their position and lateral dimensions.

[0024] The composite structure typically contains two wafers and, optionally, an intermediate bonding layer. However, the invention can also be advantageously used with composite structures containing more than two wafers. The evaluation, however, tends to become more complex with each additional wafer, as each additional optical interface leads to further interference and thus to further peaks in the Fourier spectrum that must be distinguished.

[0025] The scanning device preferably contains two independent mirrors or other reflective optical elements that are moved by rotating coil drives (galvanometers). Scanning devices with only one mirror that can be pivoted about two axes are also possible in principle. However, such mirrors that can be pivoted about two axes are slower and more difficult to control, which is why two single-axis pivoting mirrors are preferred.

[0026] In the simplest case, the optical coherence tomograph generates measuring light, which is split by a beam splitter into the measuring light beam and a reference light beam that propagates in a reference arm. The interference signals arise when a portion of the measuring light beam, reflected from the first surface, interferes with the reference light beam. The evaluation unit calculates distances to the first surface from the interference signals and, based on fluctuations in the measured distances, identifies the locations of the cavities.

[0027] However, such a measurement can be disrupted by vibrations caused by the moving optical element(s) of the scanning device. If optical elements involved in guiding the light in the reference arm move, for example, by 100 nm due to such device vibrations, this immediately results in a measurement error of the same order of magnitude. Therefore, in one embodiment, the interference signals are generated by the interference of a first part of the measurement light beam, reflected at the first surface, with a second part of the measurement light beam, reflected at the surface of a transparent plate located in the light path of the measurement light beam between the scanning device and the composite structure. The evaluation unit calculates the thickness of at least one medium located between the first surface and the surface of the transparent plate from the interference signals.Based on fluctuations in the measured thicknesses, the evaluation unit identifies the locations of the cavities.

[0028] Since no reference light from the reference arm contributes to the interference, the disturbances described above cannot affect the measurement inaccuracies. In this embodiment, only portions of the measuring light beam that are subject to exactly the same disturbances interfere. Any changes in path length therefore cancel each other out and consequently do not affect the measurement accuracy.

[0029] In this embodiment, the transparent plate essentially takes over the function of the reference arm.

[0030] Either the interference contributed by the surface of the plate facing away from the scanning device can be evaluated. In this case, only the medium between the composite structure and the plate, generally the surrounding air, is located between the two surfaces. Alternatively, if the interference contributed by the surface of the plate facing the scanning device is evaluated, the medium consists of both air and the plate material.

[0031] In another embodiment, the interference signals are generated when a first part of the measurement light beam, reflected from the first surface, interferes with a second part of the measurement light beam, reflected from a surface of another wafer or a bonding layer between the wafers. The evaluation unit calculates distances between the two surfaces from the interference signals and, based on fluctuations in the measured distances, identifies the locations of the voids. This variant has the advantage over the previously described embodiment that no transparent plate is required, which would complicate the measurement.

[0032] In the two embodiments described above, thicknesses are measured. However, the measured thicknesses can deviate significantly from the actual thicknesses. Since the detection of cavities depends solely on their localization, the accuracy with which the thicknesses are measured is irrelevant. If the detected thickness variations exceed predefined threshold values, it is assumed that a cavity is present at the location in question.

[0033] In embodiments where no interference is evaluated to which reference light guided in a reference arm contributes, the optical coherence tomograph can either have no reference arm or a reference arm with a switchable glare-reducing device that prevents reference light propagating in the reference arm from contributing to interference during the measurement and thus disturbing the thickness measurement.

[0034] Regarding the device, the aforementioned problem is solved by a device for detecting cavities that have formed in a composite structure in the region of an interface between two bonded wafers. The device comprises an optical coherence tomograph configured to generate a measurement light beam, and a scanning unit configured to deflect the measurement light beam in two spatial directions and direct it onto the composite structure such that the composite structure is successively scanned at several measurement points. An evaluation unit is configured to detect locations of cavities from interference signals acquired by the optical coherence tomograph by detecting deformations of a first surface of a wafer that is opposite a second surface of the wafer that is closer to the cavities than the first surface.

[0035] The advantages cited for the method according to the invention apply accordingly to the device.

[0036] In particular, the optical coherence tomograph can be configured to generate measurement light, wherein the device includes a beam splitter configured to divide the measurement light into the measurement light beam and a reference light beam propagating in a reference arm. The interference signals arise when a portion of the measurement light beam reflected from the first surface interferes with the reference light beam. The evaluation unit is configured to calculate distances to the first surface from the interference signals and to detect the locations of cavities based on fluctuations in the measured distances.

[0037] In another embodiment, the device includes a transparent plate positioned in the path of the measuring light beam between the scanning unit and the composite structure. This plate may be placed on the composite structure during the measurement. The interference signals are generated when a first part of the measuring light beam, reflected from one of the first surfaces, interferes with a second part of the measuring light beam, reflected from a surface of the transparent plate. The evaluation unit is configured to calculate the thickness of at least one medium located between the first surface and the surface of the transparent plate from the interference signals and to identify the locations of cavities below the surface from variations in the measured thicknesses.

[0038] In another embodiment, the interference signals are generated when a first part of the measurement light beam, reflected from the first surface, interferes with a second part of the measurement light beam, reflected from a surface of another wafer or a bonding layer. The evaluation unit is configured to calculate distances between the two surfaces from the interference signals and to identify the locations of the cavities based on fluctuations in the measured distances.

[0039] An optical coherence tomograph suitable for the device comprises a light source that generates measurement light and a beam splitter that divides the measurement light generated by the light source into a measurement light beam and a reference light beam. The reference light beam is guided in a reference arm, and the measurement light beam in an object arm. The reference arm includes a glare suppression device with which the propagation of the reference light beam in the reference arm can be temporarily suppressed. An optical system focuses the measurement light beam, whereby a large depth of field and thus a small numerical aperture (particularly between 0.01 and 0.1) is preferred to achieve a large axial measurement range. Furthermore, the optical coherence tomograph comprises a detector that generates the interference signals from the superposition of the reference light beam guided in the reference arm with a portion of the measurement light beam that was reflected by the composite structure.

[0040] If only thickness measurements are performed, an optical coherence tomograph without a reference arm can also be used.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. These show:

[0043] Figure 1a shows two directly bonded wafers in a perspective and not-to-scale representation;

[0044] Figure 1b shows two wafers bonded via a bonding layer in a perspective and not-to-scale representation;

[0045] Figure 2a shows a cross-section through the bonded wafers shown in Figure 1a;

[0046] Figure 2b shows a cross-section through the bonded wafers shown in Figure 1b;

[0047] Figure 3 shows a schematic representation of a measuring device according to the invention; and

[0048] Figure 4 shows important parts of a scanning device that is part of the measuring device shown in Figure 3, in a simplified perspective view;

[0049] Figure 5 shows a cross-section through a composite structure made of two directly bonded wafers to illustrate a first variant in which distances to a surface of the composite structure are measured;

[0050] Figure 6 shows a cross-section through the composite structure and a spaced-apart plate to illustrate a second variant in which the thickness between the underside of a plate and the composite structure is measured; Figure 7 shows a cross-section through the composite structure to illustrate a third variant in which the thickness of the composite structure is measured; and

[0051] Figure 8 shows a graph in which the measured thickness variations are shown during a measurement according to Figure 7.

[0052] DESCRIPTION OF PREFERRED EXAMPLES

[0053] 1. Voids in bonded wafers

[0054] Figure 1a shows a perspective view, not to scale, of a composite structure 10 consisting of a first wafer 12 and a second wafer 14. The two wafers 12 and 14 are bonded directly, i.e., without a bonding layer. Bonding can be achieved, for example, by pressing the two wafers 12 and 14 against each other, by heat treatment, or by a combination of both. The thicknesses of the wafers 12 and 14 are greatly exaggerated in Figure 1a. The diameters of the wafers 12 and 14 can be up to 30 cm, while the thickness is usually less than 1 mm. Therefore, the wafers 12 and 14 should be thought of less as plates and more as flexible sheets. In the illustrated embodiment, both wafers 12 and 14 consist of crystalline silicon.

[0055] Figure 1b shows a perspective view of a composite structure 10' based on Figure 1a, which differs from the composite structure 10 only in that the two wafers 12, 14 are not directly connected, but via a connecting layer 16, which provides the firm connection between the two wafers 12, 14.

[0056] Figure 2a shows the composite structure 10 in a cross-section, also not to scale, in which the thicknesses of the wafers 12 are again greatly exaggerated. The first wafer 12 has a first flat surface 12-1 facing away from the second wafer 14, and an opposing second flat surface 12-2 facing towards the second wafer 14. Similarly, the second wafer 14 shown below has a first flat surface 14-1 facing away from the first wafer 12, and an opposing second flat surface 14-2 facing towards the first wafer 12. The two second surfaces 12-2, 14-2 are thus in contact, while the two first surfaces 12-1, 14-1 each face outwards. During bonding, gas-filled cavities often form between the wafers 12, 14, one of which is shown exaggeratedly large in Figure 2a and labeled 18.In the region of the cavities 18, the second surfaces 12-2, 14-2 are therefore not in a plane with each other, but are spaced apart. The cavities 18 usually have an approximately disk-shaped form, but their dimensions can vary considerably. The thicknesses of the cavities (i.e., dimensions in the z-direction) can range from a few nanometers to several hundred micrometers.

[0057] Figure 2a shows that the second surface 12-2 of the first wafer 12 deforms in the area above the cavity 18. Since the first wafer 12 is deformable but not compressible, the opposite first surface 12-1 of the first wafer 12, which points upwards in Figure 2a, bulges slightly. This will be discussed in more detail below in Section 3.

[0058] Figure 2b shows the composite structure 10' from Figure 1b in a cross-section similar to that of Figure 2a. In the illustrated embodiment, the bonding layer 16 contains a cavity 18, which leads to similar deformations of the second surface 12-2 of the first wafer 12 as in the configuration shown in Figure 2a.

[0059] Regardless of whether the composite structure contains a bonding layer or not, the second wafer 14 can deform in a similar way to the first wafer 12 as a result of the cavity 18. Since the second wafer 14 generally rests on a flat surface that offers greater resistance to such deformation than the air adjacent to the first wafer 12, this deformation is not shown in Figures 2a and 2b.

[0060] The voids 18 can disrupt the function of components manufactured from the composite structures 10, 10'. Therefore, the composite structures 10, 10' should be inspected, ideally before further complex process steps, to determine whether and, if so, where voids 18 are present. The lateral extent of the voids 18 is also frequently of interest. The location refers to the xy coordinates; the z-coordinate of a void 18, e.g., its height within the bonding layer 16, is generally not relevant. If the number and, if applicable, the size of the voids 18 exceed predefined thresholds, the composite structure 10, 10' is considered scrap or can only be used in less demanding applications.

[0061] A measuring device 20 is used to detect the cavities 18, which is explained in more detail in the following section.

[0062] 2. Structure and function of the measuring device

[0063] Figure 2 shows a schematic representation of a measuring device according to the invention, designated as 20. The measuring device 20 serves to measure the composite structure 10 made of bonded wafers 12, 14, which is supported by a holder 21. The holder 21 can, for example, be designed as a simple three-point support, as indicated in Figure 2 by supports 23. In the illustrated embodiment, the holder 21 is itself supported on a base 25. Typically, the holder 21 and the base 25 are not part of the measuring device 20. When measuring within the production process, the composite structure 10 can be fed to the measuring device 20, for example, by a conveyor system (not shown).

[0064] The measuring device 20 comprises an optical coherence tomograph 22, which generates a measuring light beam 24 and whose construction is explained in more detail below.

[0065] A scanning device, indicated by 26, deflects the measuring light beam 24 variably in two orthogonal scanning directions. For this purpose, the scanning device 26 has a first scanning mirror 28, which is rotatably mounted about a first axis of rotation 30. A second scanning mirror 32 is rotatably mounted about a second axis of rotation 34, which is oriented perpendicular to the first axis of rotation 30. The scanning mirrors 28 and 32 are driven by galvanometer drives (not shown), which are controlled by a control unit 36. Figure 4 shows the scanning device 26 with the scanning mirrors 28 and 32 (shown here as rectangular) enlarged in a perspective and highly schematic representation.

[0066] The measuring device 20 also includes an optical system 38, which is indicated in Figure 2 by three lenses L1, L2, and L3 and can be configured as an F-theta lens. In the illustrated embodiment, the optical system 38 focuses the measuring light beam 24 deflected by the scanning device 26 such that it always strikes the surface 40 of the wafer 10 facing the optical system approximately perpendicularly. The point of impact of the measuring light beam 24 defines a measuring point 39.

[0067] The optical coherence tomograph 22 includes a light source 42, a first beam splitter 44 which splits the light generated by the light source into the measuring light beam 24 and a reference light beam 46, a reference arm 48 for guiding the reference light beam 46, and an object arm 50 which uses the optical system 38 and the scanning device 26 and in which the measuring light beam 24 is guided.

[0068] During a measurement, the measuring light beam 24, propagating in the object arm 50, is focused onto the surface 12-1 of the first wafer 12 of the composite structure 10, where it is reflected predominantly specularly and to a lesser extent diffusely, and travels along the same light path back through the object arm 50 to the first beam splitter 44. There, the reflected portion of the measuring light beam 24 is superimposed with the reference light beam 46, which is guided in the reference arm 48 and reflected there by a mirror 52. Both light components are directed by a second beam splitter 54 onto a detector 56, which converts the optical interference signal into electrical interference signals.

[0069] In the illustrated embodiment, the optical coherence tomograph 22 is designed as an FD-OCT (FD stands for Fourier Domain). The detector 56 therefore contains a spectrometer that records the spectral intensity distribution of the interference signal. From this, an evaluation unit 57 connected to the detector 56 can calculate, in a manner known per se, the distance of an optical interface, e.g., the surface 12-1 of the first wafer 12, to the measuring device 20 (e.g., the lens L3) at the point of incidence of the measuring light beam 24. For further details on the optical coherence tomograph, reference is made to DE 10 2017 128 158 A1 (corresponding to US 2018 / 0164089 AI).

[0070] The wavelength range of the light generated by the light source 42 can be selected such that the measurement beam 24 can penetrate at least partially into the wafers 10, 12. In particular, the wavelength range lies in the infrared or near-infrared range. This corresponds to wavelengths between 950 nm and 2000 nm, with the wavelength range of 1000 nm to 1200 nm being preferred. Reflections then also occur at the surfaces 12-2, 14-1, and 14-2, which are detected by the optical coherence tomograph 22. In this case, a spectral peak is obtained for each interface, from which the distance to the respective interface can be determined.

[0071] In addition, the optical coherence tomograph 22 can be used in a thickness mode. In this case, the evaluation unit only evaluates interference signals generated by interference between measurement light components reflected at different interfaces. In thickness mode, it should be prevented that light can also propagate in the reference arm 48, as this would also interfere and thus generate unwanted noise signals. For this purpose, the reference arm 48 contains a switchable diaphragm, indicated at 59, which can be designed, for example, as a central or focal-plane shutter. When switching from distance mode to thickness mode, the switchable diaphragm 59 is automatically closed, so that no light from the reference arm 48 can contribute to the interference on the detector 56.When switching back to distance mode, the switchable dimming device 59 clears the way for the reference light beam 46 again.

[0072] In the embodiment shown in Figure 2, the measuring light 24 propagates completely in free space. In other embodiments, the light is guided partially in optical fibers. If the optical coherence tomograph 22 is housed in its own casing, it is advantageous to also guide the light between the optical coherence tomograph 22 and the scanning unit 26 in an optical fiber. The scanning unit 26 and the optical system 38 can then be housed in a measuring head that can be easily mounted in different locations.

[0073] 3. Detection of cavities

[0074] The following describes, with reference to Figures 5 to 9, various ways in which the cavities 18 in the composite structure 10 can be detected using the optical coherence tomograph 20 shown in Figures 3 and 4. a) Distance measurement In the variant shown in Figure 5, the measuring light beam 24 scans the first surface 12-1 of the first wafer 12, which faces the scanning device 26. The evaluation unit 57 analyzes interference signals that arise when a portion of the measuring light beam 24 reflected from the first surface 12-1 interferes with the reference light beam 46 guided in the reference arm 48. From these interference signals, the evaluation unit 57 calculates distances to the first surface 12-1.Based on fluctuations in the measured distances, the evaluation unit 57 detects the location of the cavity 18, since the first wafer 12 bulges above the cavity 18, and thus the measured distances a above the cavity 18 are smaller by the value Aa, as illustrated in Figure 5. Based on the measured thickness change Aa, the height of the cavity can also be roughly estimated. With sufficiently high resolution within the xy-plane, the lateral extent of the cavity 18 can also be approximately determined. b) Transparent plate.

[0075] Figure 6 shows a variant based on Figure 5 in which a plate 60, transparent to the measuring light beam 24, is arranged at a distance from the composite structure 10. The plate 60 can, for example, be made of quartz glass, which is transparent to the measuring light wavelength used in the near-infrared spectral range. The plate can, for example, be held by an external holding device or rest on spacers (not shown) that are placed on the composite structure 10.

[0076] Due to the distance between the plate 60 and the composite structure 10, the evaluation unit 57 can evaluate interference signals generated by the interference of reflections at the surface 64 of the plate 60 facing away from the scanning device 26 and the first surface 12-1 of the first plate 12. This corresponds to a measurement in the thickness mode of the optical coherence tomograph 22 mentioned above.

[0077] However, if the composite structure 10 vibrates in the variant shown in Figure 7, the distance d changes, leading to measurement errors. c) Thickness measurement without a transparent plate

[0078] Figure 7 illustrates another variant in which the interference signals evaluated by the evaluation unit 57 are generated by the interference of measurement light components corresponding to reflections at the first surface 12-1 of the first wafer 12 and the first surface 14-1 of the second wafer 14. The total thickness d of the composite structure 10 is thus measured, with the optical coherence tomograph 22 operating in the thickness mode mentioned above. Due to the bulge of the first wafer 12 in the region of the cavity 18, the evaluation unit can also determine the location of the cavity 18 in this case.

[0079] Advantageously, only those measured values ​​that lie within a range around the expected value of the total thickness are evaluated. If the evaluation unit 57 contains an adjustable frequency filter for the thickness signal, disturbances caused by other interferences (e.g., generated by reflections at the second surface 14-2 of the second wafer 14) can be suppressed. The series of measured values ​​is filtered with a high-pass frequency filter so that slow changes in value resulting from large-area deformation or thickness variation of the wafer are filtered out. To suppress noise, a threshold filter is also used so that only deviations above the noise level are taken into account.

[0080] Alternatively, the distance between the two surfaces 12-1 and 12-2 or 14-2 of the first wafer 12, and thus its thickness d', can also be measured. The optical coherence tomograph 22 is then also in thickness mode.

[0081] Strictly speaking, the thickness d' is constant and does not vary even within a cavity 18, as can be seen in Figure 7. However, it is not possible to measure the distance between surface 12-1 and surface 12-2 in isolation, since surfaces 12-2 and 14-2 cannot be resolved separately. Outside the cavities 18, this is because both surfaces 12-2 and 14-2 coincide. Within the cavities 18, surfaces 12-2 and 14-2 are indeed slightly separated. However, the distance is so small that surfaces 12-2 and 14-2 cannot be resolved by the optical coherence tomograph.

[0082] However, a measurement of the thickness d' leads to a broadened peak in the Fourier spectrum, which allows a conclusion to be drawn about a cavity 18 located there.

[0083] In principle, it is also possible to measure other distances in thickness mode, e.g., between surfaces 14-1 and 14-2 or 12-2. As mentioned above, the lower surface 14-1 of the second wafer 14 may also be bulged in the area of ​​the cavity 18, resulting in conditions similar to those described above for the thickness d' measurement. However, due to the unavoidable light absorption in wafers 12 and 14, the reflections on the surfaces further away from the scanning device 26 are somewhat weaker, which is why measurements using the surfaces closer to the scanning device 26 are preferred.

[0084] Whether the total thickness d of the composite structure 10 or the thickness d' of the first wafer 12 is measured can depend on the reflectance of the surfaces in question. If, for example, the two wafers 12, 14 are made of the same material, the reflections at the interface between the two wafers 12, 14 may be very weak, which generally leads to less reliable measurement results due to the low signal-to-noise ratio.

[0085] Compared to the variants shown in Figure 6, the thickness measurement illustrated in Figure 7 has the advantage that no transparent plate 60 is required.

[0086] The four variants described above (a) to (d) are also applicable if a connecting layer 16 is arranged between the two wafers 12, 14, as shown in Figures 1b and 2b. Its surfaces can also define the media whose thickness is measured by the optical coherence tomograph 20. The crucial point is that the evaluated interference signals must originate solely from interferences to which the first surface 12-1 contributes, but not the second surface 12-2.

[0087] To better visualize the cavities 18, the average thickness of the structure in question can be subtracted from the measured thickness d or d'. In such a representation, the bulge of the first wafer 12 above the cavity 18 is more clearly visible.

[0088] Alternatively or additionally, the quality of the reflected signals can also be considered by the evaluation unit 57. The quality can be defined, for example, by first identifying the largest coefficients in the discrete Fourier spectrum. The corresponding value, along with the values ​​of the two adjacent coefficients, is summed and divided by the sum of the values ​​of all other coefficients. It has been shown that the quality of the reflected signals, as defined in this way, changes at locations where cavities are present. Therefore, this quality can be used as additional supplementary information to locate the cavities.

[0089] 4. Measurement Results Figure 8 shows a graph illustrating the thickness variations during your measurement in a section of approximately 14 mm x 14 mm. The different gray values ​​correspond to different deviations from an average thickness. The local thickness variations, recognizable as dark spots in the graph, are located at points where a cavity 18 is situated. The lateral extent of the cavities 18 can be estimated based on the size of the spots.

Claims

PATENT CLAIMS 1. Method for detecting cavities (18) that have formed in a composite structure (10) in the region of an interface between two bonded wafers (12, 14), comprising the following steps: a) an optical coherence tomograph (22) generates a measuring light beam (24); b) a scanning device (26) deflects the measuring light beam (24) in two spatial directions (x, y) and directs it onto the composite structure (10) such that the composite structure (10) is scanned successively at several measuring points (39); c) An evaluation unit (57) detects locations where cavities (18) are located from interference signals detected by the optical coherence tomograph (22) by detecting deformations of a first surface (12-1) of a wafer (12) opposite a second surface (12-2) of the wafer (12) that is closer to the cavities (18) than the first surface (12-1).

2. Method according to claim 1, wherein the optical coherence tomograph (20) generates measuring light which is split by means of a beam splitter into the measuring light beam (24) and a reference light beam (46) which propagates in a reference arm (48), the interference signals are generated by a part of the measuring light beam (24) which is reflected at the first surface (12-1) interfering with the reference light beam (46), and the evaluation unit (57) calculates distances (a) to the first surface (12-1) from the interference signals and detects the locations of the cavities (18) on the basis of fluctuations in the measured distances (a).

3. Method according to claim 1, wherein the interference signals are generated by a first part of the measuring light beam (24), which is reflected at the first surface (12-1), interfering with a second part of the measuring light beam (24), which is reflected at a surface (62; 64) of a transparent plate (60) which is arranged in the light path of the measuring light beam (24) between the scanning device (26) and the composite structure (10), and the evaluation unit (57) calculates thicknesses (d) of at least one medium located between the first surface (12-1) and the surface (62; 64) of the plate (60) from the interference signals, and detects the locations of the cavities (18) on the basis of fluctuations in the measured thicknesses (d).

4. Method according to claim 1, wherein the interference signals are generated by a first part of the measuring light beam (24), which is reflected at the first surface (12-1), interfering with a second part of the measuring light beam (24), which is reflected at a surface (14-1; 14-2) of another wafer (14) or a connecting layer (16) between the wafers (12, 14), and the evaluation unit (57) calculates thicknesses (d; d') between the two surfaces (12-1, 14-1, 14-2) from the interference signals and detects the locations of the cavities (18) on the basis of fluctuations in the measured thickness (d, d') distances.

5. Method according to one of claims 3 or 4, wherein the optical coherence tomograph (20) either has no reference arm or has a reference arm (48) with a switchable dimming device (59) which prevents the reference light beam (46) propagating in the reference arm (48) from contributing to interference during the measurement.

6. Device for detecting cavities (18) that have formed in a composite structure (10) in the region of an interface between two bonded wafers (12, 14), comprising: a) an optical coherence tomograph (22) configured to generate a measuring light beam (24), b) a scanning device (26) configured to deflect the measuring light beam (24) in two spatial directions (x, y) and to direct it onto the composite structure (10) such that the composite structure (10) is successively scanned at several measuring points (39); c) an evaluation unit (57) configured to detect locations where cavities (18) are located from interference signals acquired by the optical coherence tomograph (22) by detecting deformations of a first surface (12-1) of a wafer (12) that is opposite a second surface (12-2) of the wafer (12) that is closer to the cavities (18) than the first surface (12-1).

7. Device according to claim 6, wherein the optical coherence tomograph (20) is configured to generate measuring light (24), and wherein the device (14) has a beam splitter (44) configured to split the measuring light into the measuring light beam (24) and a reference light beam (46) which propagates in a reference arm (48), the interference signals being generated by a part of the measuring light beam (24) reflected at the first surface (12-1) interfering with the reference light beam (46), and wherein the evaluation unit (57) is configured to calculate distances (a) to the first surface (12-1) from the interference signals and to detect the locations of the cavities (18) on the basis of fluctuations in the measured distances (a).

8. Device according to claim 6, comprising a transparent plate (60) arranged in the light path of the measuring light beam (24) between the scanning device (269) and the composite structure (10), wherein the interference signals are generated by a first part of the measuring light beam (24), which is reflected at one of the first surfaces (12-1), interfering with a second part of the measuring light beam (24), which is reflected at a surface (62; 64) of the transparent plate (60), and wherein the evaluation unit (57) is designed to calculate thicknesses (d) of at least one medium located between the first surface (12-1) and the surface (62; 64) of the transparent plate (60) from the interference signals, and to identify the locations of the cavities (18) from fluctuations in the measured thicknesses (d).

9. Device according to claim 6, wherein the interference signals are generated by a first part of the measuring light beam (24), which is reflected at the first surface (12-1), interfering with a second part of the measuring light beam (24), which is reflected at a surface (14-1, 14-2) of another wafer (14) or a compound layer (16), and wherein the evaluation unit (57) is configured to calculate thicknesses between the two surfaces (12-1, 14-1, 14-2) from the interference signals and to detect the locations of the cavities (18) on the basis of fluctuations in the measured thicknesses.

10. Device according to one of claims 6 to 9, wherein the optical coherence tomograph (20) either has no reference arm or has a reference arm (48) with a switchable glare control device (59) which prevents the reference light beam (46) propagating in the reference arm (48) from contributing to interference during the measurement.

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