Bonded wafer inspection method and device

Abstract

A bonded wafer inspection method and device for detecting a defect in a bonded wafer (BW) are disclosed. The bonded wafer (BW) to be inspected has a front face and a back face, and the method comprises the steps of (a) producing illumination light with a polarization state and a propagation direction, the illumination light having a wavelength in an infrared range at which the bonded wafer (BW) is transparent from a direction of at least one of the front face and the back face along at least a portion of the propagation direction within said bonded wafer (BW); (b) guiding said illumination light to the bonded wafer (BW) from the direction in which the bonded wafer is transparent to the illumination light, the propagation direction being substantially perpendicular to at least one of the front face and the back face; (c) making at least a first portion of the illumination light to enter the bonded wafer (BW), thereby inducing reflection of at least a portion of said first portion of the illumination light in the bonded wafer (BW) to create reflected light; (d) collecting a portion of the reflected light and filtering said collected reflected light based on the polarization state to obtain polarization-filtered reflected light; (e) guiding the polarization-filtered reflected light to a detector; (f) detecting said polarization-filtered reflected light by the detector; (g) creating an image from said detected polarization-filtered reflected light; (h) based on the image, identifying at least one region in the bonded wafer (BW) having mechanical stress induced birefringence; and (i) based on the at least one region identified, detecting the defect in the bonded wafer (BW).

Description

[0001] P141629-6133 SZT

[0002] BONDED WAFER INSPECTION METHOD AND DEVICE

[0003] This invention is related to a bonded wafer inspection method and a bonded wafer inspection device for detecting defects in bonded wafers. More specifically, the invention is related to a polarimetric method and device with improved capability for the detection and quantitative assessment of voids and other interface layer defects in bonded wafers.

[0004] The effectiveness of semiconductor devices manufactured with bonded wafer technology are greatly affected by defects located between the bonded layers, i.e. in the interface layer, such as voids or delamination. The voids are formed during the bonding process when a particle of dust or similar material or a gas bubble is trapped between the layers to be bonded thus preventing the two layers from contacting each other in a small region. With the development of semiconductor technology, the sizes of the semiconductor devices are reduced and thus defects of smaller sizes become relevant, therefore a need emerges in the industry to detect voids of smaller and smaller sizes.

[0005] Detection of voids may be done immediately after bonding or at a later stage of manufacturing, when additional layers of material are deposited onto the wafer. In either case, the bonded wafer structure may contain one or more semiconductor materials with one or more doping materials with one or more different concentrations, as well as insulators and metals. Some of these layers may be complete, covering the entire surface of the wafer or may be partial, e.g. when the layer itself is produced by lithography only on a portion of the wafer surface or after a complete layer is partially etched away to create a specific microstructure. Such layers are usually present within the bonded wafer between the wafers that were bonded together, as well as on the outer surface of the bonded wafer, on the front side, back side or both.

[0006] There are several known non-destructive methods for detecting voids in bonded wafers, including infrared (IR) microscopy, Fourier-transform infrared reflectometry, ultrasonic microscopy and polarimetry.

[0007] A couple of such polarimetric techniques are introduced in the publication of Herms et al. entitled “Materials characterization and device analysis for evaluation of semiconductor processes by highly-sophisticated photoelastic stress measurement technique” (Phys. Status Solidi C 12, No. 8, 1085-1089, 2015). The publication describes a transmission-based and a reflection-based polarimetric technique for examining the birefringence in silicon wafers induced by mechanical stress caused by through silicon vias, crystal defects and epitaxial layers. A common limitation of the afore-mentioned optical techniques is that they require light to reach the defect and after reflection thereon or transmission therethrough to reach the detector of the measuring instrument. This means that a continuous layer or other structure made of a material that is opaque in the wavelength range used for measurement, e.g. a semiconductor layer with high doping concentration or a metallic layer, may obscure the void and make its direct detection impossible. Ultrasonic techniques on the other hand suffer from lower resolution than optical techniques, especially at the full thickness of the bonded wafers. Furthermore, modern high volume manufacturing environments require throughputs of as much as 20 wafers per hour, or even more, thus limiting the applicability of the available techniques.

[0008] Furthermore, quantitative assessment of the voids is not possible with most of these techniques for voids smaller than the resolution of the device, for example a void with a diameter 0.6 pm cannot be distinguished from a void with a diameter of 0.4 pm, because these are both smaller than the best possible theoretical resolution achievable by an IR microscope operating in the in the near infrared (NIR) wavelength range where silicon is transparent.

[0009] German Patent No. 19525770 C1 discloses a method and an apparatus to inspect bonded wafers by transilluminating the bonded wafer inspected with an infrared radiation that strikes major surfaces of said bonded wafer substantially perpendicularly. To calculate the probability of a bonding defect, a visual image is taken via a CCD camera by detecting light passing through the bonded wafer and assigning a gray value dependent on the intensity of the transmitted light to each pixel point of the image by means of evaluating, by e.g. a computer, said gray values for all pixels with respect to the normal grey value distribution. The slow pixel-by-pixel evaluation significantly limits the applicability of this method in modern high volume in-line manufacturing environments. A further disadvantage of said method is that it is limited to the investigation of bonded wafers built up of fully transparent wafer pairs only; it cannot provide relevant pieces of information when opaque structures are also present in the bonded wafer under study.

[0010] US Published Patent Appl. No. 2015 / 0069247 A1 , regarded as the closest prior art, discloses a method and a system for real time inspection of bonded wafers by making use of infrared light in a reflective geometry applying oblique incidence of light for the inspection and in a transmissive geometry applying perpendicular incidence of light for the inspection. According to the method, infrared plane polariscope is used to obtain an image of a bonded interface of a silicon wafer, wherein the image shows stress patterns due to birefringence caused by voids or other interface defects in the bonded wafers, and then said stress patterns are assessed with regard to their shape; a stress pattern in a form of butterfly pattern indicates a presence of an interface defect, i.e. one of a void, trapped particle or debonding region. When, however, applying imaging optics with high numerical aperture is applied, as is the case in most in-line application, due to optical aberrations and geometrical, i.e. constructional limitation for high light cone angles, oblique illumination and imaging through a thick plan-parallel sample implies a significant limitation for high optical resolution.

[0011] Voids that are obscured by structures opaque in the NIR wavelength range usually cannot be detected via IR microscopy or transmission polarimetry, because the infrared light used for these techniques cannot pass through such layers to reach the void and finally the detector. When the void to be detected is only obscured from the front or the back side, it can possibly be detected by reflection-based techniques, such as IR microscopy or reflection polarimetry, but not by transmission polarimetry. If the void is located between two obscuring structures, it cannot be efficiently detected by any method known today.

[0012] As is also known in the relevant field, optical elements, such as polarimeters, phase retarders, etc., often have minor inherent phase defects that have negligible effects on such polarimeters which image large phase differences. Typically, voids and particles basically produce limited mechanical stress in a bonded wafer and stresses produced lead to only small phase differences in the image of the silicon (Si) wafer. Thus, the minor phase pattern induced by said stresses to be measured and the optical-elements-related inherent phase retardance of the polarimeter used for the measurement becomes comparable which poses a technical problem for high-throughput (imaging) defect inspection in modern high volume in-line manufacturing environments.

[0013] The object of the present invention is to alleviate the limitations of the prior art techniques by providing a method and a device for the detection and, optionally, the quantitative assessment of voids in bonded wafers, specifically including the detection of voids obscured by structures opaque to the illuminating light, in a compact geometrical configuration in order to be easily installed into the in-line manufacturing environments, too.

[0014] An inventive idea leading to the present invention is that reflection-based polarimetric measurement of the mechanical stress induced birefringence caused in the bonded wafers by a void or other interface defect can be used for quantitative assessment of voids and other defects that are themselves smaller than the resolution limit of the imaging optical system. A further inventive idea leading to the present invention is that reflection-based polarimetric measurement of the mechanical stress induced birefringence in the bonded wafers caused by a defect, e.g. void, can be used to detect the defect even in cases where the defect itself is obscured by an opaque structure.

[0015] A yet further inventive idea leading to the present invention is that the mechanical stress birefringence induced in the bonded wafers by a void can be used to detect the void by reflection polarimetry even in cases where the void itself is obscured by an opaque structure from both sides.

[0016] A yet further inventive idea leading to the present invention is that using reflection polarimetry with an essentially perpendicular illumination of the inspected bonded wafers allows to use basically the same geometrical configuration for both the illumination and then the detection which ends up a bonded wafer inspection device of compact design.

[0017] Furthermore, application of an essentially perpendicular illumination of the inspected bonded wafers also allows a simpler correction procedure for the inherent defects of the optical elements used in a reflection polarimeter. Constructing a symmetric light path for the reflection polarimeter with its optical elements for the illumination and the detection (imaging) of a bonded wafer, which might be the case at the configuration of essentially perpendicular illumination, the variability of the inherent phase patterns of the reflection polarimeter can be reduced.

[0018] On the one hand, the above objectives have been achieved by developing the method according to claim 1 . Preferred variants of the method are set forth in claims 2-9. On the other hand, the above objectives have been achieved by developing the device according to claim 10. Preferred embodiments of the device are set out in claims 11-15.

[0019] In what follows, preferred embodiments of the method and device according to the invention are described in detail with reference to the attached drawing, wherein

[0020] - Figure 1 is a simplified illustration of a bonded wafer to be inspected by the method and device according to the invention;

[0021] - Figure 2A shows an image of an unobscured void in a bonded wafer captured by a preferred exemplary variant of the method and device according to the invention;

[0022] - Figure 2B shows a diagram of calculated Fourier component values for the defect shown in Figure 2A for different central positions;

[0023] - Figure 3A and 3B shows the same void as Figure 2A with artificially superimposed bright and dark rectangles, each partially obscuring the defect for demonstrative purposes; - Figure 3C shows a diagram of calculated Fourier component values for the defect shown in Figure 3A and 3B for different central positions and with different obstructions;

[0024] - Figure 4 is a schematic view of a preferred exemplary embodiment of the device according to the invention.

[0025] Figure 1 is a simplified illustration of a bonded wafer BW. Said bonded wafer BW usually comprises a first wafer 1 and a second wafer 2. The first wafer 1 and the second wafer 2 may be made of the same semiconductor material or different semiconductor materials, for example one of them may be silicon, while the other one may be gallium nitride. Depending on the material of the first wafer 1 and the second wafer 2, they may be bonded to each other directly or through an interspersed interface layer 3, e.g. silicon dioxide. The role of the interface layer 3 is to facilitate bonding of the first wafer 1 and the second wafer 2, especially when said wafers are made of different materials and a lattice mismatch between the two wafers would be prone to cause difficulties, e.g. delamination.

[0026] The surfaces of the first wafer 1 and the second wafer 2 that are facing each other usually contain intricate microstructures for creating microelectronic components, e.g. microchips, that includes opaque structures, such as metallic components 4, especially contact pads and wiring for establishing electric contact with said components. These metallic components 4 are opaque in the visual and NIR wavelength ranges and thus hinder direct optical observation of any defect that may occur directly above or below such a metallic component 4. Areas of the semiconductor wafers that are highly doped form similar localized opaque structures. The defects shown in Figure 1 are so-called voids that may form during the bonding process, for example when a particle prevents direct contact between the wafers to be bonded or when ambient gases are trapped in a small bubble between the two wafers to be bonded.

[0027] A first void 5a shown in Figure 1 may be directly observed by a known transmission technique or by reflection technique from above or below the bonded wafer, provided that both the first wafer 1 and the second wafer 2 are transparent at the wavelength used for the observation. Often, this is not the case when one of the wafers 1 , 2 has a high doping concentration that makes the whole wafer opaque. Then, the first void 5a would be obscured on one side by an opaque structure and, thus, could only be observed by a reflection technique from the other side of the bonded wafer BW. A second void 5b shown in Figure 1 is completely obscured from one side by a metallic component 4 and thus can only be observed by a reflection technique from merely one side of the bonded wafer BW, that is, here from a direction of the first wafer 1 . If high doping concentration of the first wafer 1 makes the first wafer 1 opaque, the second void 5b will not be observable by any known optical method.

[0028] A third void 5c is completely obscured from above and below, that is from both sides of the bonded wafer BW, by metallic components 4 and, thus, can be directly observed by no optical methods known today. Even indirect observation may be further hindered if one of the wafers 1 , 2 is made opaque by high doping concentration.

[0029] The method of the present invention is based on the fact that mechanical stress induced in a bonded wafer by various defects, specifically voids, to be detected can be used to detect said defects even though they are obscured or difficult to be detected due to their small sizes.

[0030] According to a first aspect of the present invention, illumination light is produced, having a predetermined polarization and having a wavelength whereat at least one of the layers of the bonded wafer, i.e. at least one of the first wafer 1 and the second wafer 2, is transparent. The preferred wavelength range for silicon wafers is between 1 pm and 1.3 pm. Instead, the preferred wavelength range for a silicon carbide wafer may be in the visual wavelength range.

[0031] The illumination light is guided onto the bonded wafer, preferably focused onto or into the bonded wafer by an objective, wherein at least a first portion of the illumination light enters the bonded wafer. At least a portion of this first portion is reflected and / or scattered on different features found within the bonded wafer. Reflection takes place, for example, on the surface of the metallic components 4 and on boundaries between two different transparent media, e.g. between silicon and silicon dioxide, silicon and gas, or silicon and vacuum. Some of the features concerned are present on purpose, e.g. the metallic pads or trench structures filled with silicon dioxide or air. Some of the features are defects, e.g. voids. The illumination light is preferably guided onto the bonded wafer substantially perpendicularly, in particular, in an angle range of ±1 ° around the axis of incidence, to the surface of the wafer. Some of the illumination light is reflected back toward the illumination direction, and can be collected, preferably by the same objective that was used to focus the light onto or into the bonded wafer. The reflected light collected by the objective is filtered based on its polarization state, preferably by being guided through a polarizing element, then onto a detector, where it is detected and an image is formed of the detected light. Forming the image may be performed point-by-point by a single element detector, line-by- line by a linear detector or all at once by a 2D detector. Preferably the polarizing element is configured so as to filter out a majority of reflected light of unaltered polarization state, but not all of it. The polarizing element filters out preferably 50-99%, more preferably 70-98%, most preferably 90-97% of reflected light having the same polarization state as the illumination light.

[0032] Filtering out most of the reflected light with unaltered polarization means that reflected light with altered polarization can be detected with much better contrast than without filtering. This effect could be maximized by configuring the polarizing element so that 100% of the reflected light with unaltered polarization is filtered out, i.e. using a so- called darkfield setup. However, the darkfield setup would make it harder to identify the position of the polarizing defect on the wafer in relation to structures that do not alter the polarization state of light reflected by them. Accordingly, preferably less than 100% of the light with unaltered polarization is filtered out, i.e. a so-called grey-field setup is used. Generally, a lower illumination intensity makes it preferable to filter out less of the unaltered polarization light, in order to provide a grey level high enough to aid navigation. For detecting smaller defects with weaker stress fields, filtering out more of the unaltered polarization light is preferred in order to increase the signal to noise ratio.

[0033] After creating an image of the polarized reflected light, regions with significant mechanical stress are identified, in particular, by means of defect-optimized neural networks or traditionally applied pattern recognition algorithms, and defects are detected on the basis of mechanical stress induced by said defects themselves. As discussed above, a defect itself may be obscured from the view of the detector by an opaque structure, but it still can be detected via the stress field created thereby.

[0034] Optionally, the method further comprises quantitative assessment of the defect on the basis of the stress field it creates. Quantitative assessment is preferably performed by identifying a center of the defect, calculating a specific Fourier component F(r) of the detected light intensity I (r, <p) for at least one radius r around the center of the defect with the following Eq. 1 formula: wherein cp is a polar angle relative to a <po reference angle, wherein the reference angle is selected as one of the inflection points of the light intensity along a circle around the center of the defect. The reference angle is determined by the configuration of polarizing elements used for polarizing the illumination light and the collected reflected light. Quantitative characterization and optional comparison of defects is based, for example, on the value of the Fourier component F(r) at the same radius or the highest Fourier component F(r) value for each defect. Naturally, other Fourier components may also be used for characterizing defects, but this Fourier component corresponds to the symmetry of the stress lobes induced by point defects and circular defects and thus fits best to characterize the stress fields of voids, which are most often approximately circular.

[0035] Figure 2A shows an image of an unobscured void in a bonded wafer captured by a preferred exemplary variant of the method and device according to the invention. The white circles have radii of 30, 40 and 70 pixels to indicate the outer boundary of the void itself at and of the stress field. Figure 2B shows a diagram Fourier component F(r) values calculated for the defect shown in Figure 2A for different central positions. The highest value of each curve is the amplitude of the Fourier component. It is visible from the graphs that even for positions offset from an actual center of the void by as much as half the void’s radius, a peak in the Fourier component indicates a presence of the void. This means that if a pattern characteristic of the stress lobes of an unobstructed point defect is to be detected by the Fourier components alone, it is sufficient to calculate said Fourier components for grid points that are at distances from each other equal to half the size of the smallest void to be detected.

[0036] Figure 3A and 3B respectively shows the same void as Figure 2A with an artificially superimposed dark and bright rectangle partially obscuring the defect for demonstrative purposes. Figure 3A also indicates the circles that serve as visual aids on Figure 2A. Figure 3C shows a diagram of calculated Fourier component F(r) values for the defect shown in Figure 3A and 3B for different central positions and with different obstructions. Here, the black and the white rectangles, respectively, correspond to the absorbing and reflecting obstructions. The effect of these partial obstructions can be seen on the corresponding graphs. As long as the assumed central position is correct, the amplitude of the Fourier component F(r) is still very high and, thus, a defect remains easy to be recognized. However, with incorrect central position, the calculated effect of the artifact may be larger than the effect of the stress lobes for certain radii.

[0037] The effect of the artifacts as well as the corresponding actual opaque structures that would appear on the images can be greatly reduced or maybe even completely eliminated with an appropriate correction. According to a preferred embodiment, the correction may be performed on the graph by calculating the Fourier component F(r) of the opaque structure alone and subtracting that from the Fourier component F(r) calculated for the image. According to another preferred embodiment, the opaque structures are identified on the image in a first step and then the correction is performed by applying corrected integration limits (or bounds) instead of the full circle to exclude the area covered by an opaque structure. According to yet another preferred embodiment, the opaque structures are identified on the image in a first step, and pixel values within the opaque structures are replaced with pixel values equal to an average background value, then the integration is performed for the whole range of 0 to 2TT.

[0038] Optionally, the method further comprises identifying the type of the defect on the basis of the stress field pattern said defect creates. For example, dislocations, voids and delamination defects can thus be distinguished.

[0039] According to a preferred variant of the method according to the invention, the whole area of the bonded wafer is scanned, preferably by a line scanning method, i.e. the illumination light is preferably focused onto or into the bonded wafer along a relatively thin line and, preferably, a line shaped detector, for example a CCD or InGaAs detector with 1x1024 or 1x2048 pixels is used to detect the reflected light.

[0040] According to the preferred variant of the method according to the invention, the method is performed in two stages: a preliminary stage serves for identifying areas of interest preferably by scanning the whole wafer area, which is performed quicker and with a lower resolution than a second stage, wherein the areas of interest are imaged with a resolution higher than the one used in the preliminary stage. If perpendicular illumination is used, both the illumination and the imaging can be easily adapted to the higher resolution by swapping an objective to one having a higher magnification, because in such a case the same objective functions as the objective for the illumination and the imaging.

[0041] According to a further preferred variant of the method according to the invention, the detected defects are voids, each with a size smaller than 30 pm. Such voids and their induced stress fields can be examined in detail with the present method. According to a further preferred variant of the present method, the detected defects are voids of lateral sizes smaller than 1 pm. Here, the term “lateral size” refers to the largest extension of a defect perpendicularly to the detection direction.

[0042] Figure 4 shows a simplified schematic of a bonded wafer inspection device according to the invention. Said device comprises a primary light source 11 for producing light in a predetermined wavelength range, wherein at least one layer of the bonded wafer BW to be inspected is transparent in at least a portion of said wavelength range. The primary light source is a broadband light source, e.g. an incandescent lamp, a gas discharge lamp or a laser sustained plasma light source, with an optical filter for selecting said wavelength range, or a narrow band light source, e.g. a LED or a laser, that only produces light in the predetermined wavelength range. The wavelength range may be fixed for the device, or alternatively, it may be selectable by changing the filter or by using an appropriate refractive or diffractive element, i.e. a prism or a grating, for selecting the wavelength range.

[0043] The light emitted by the primary light source 11 is conditioned by an appropriate illumination optical system 12 for conditioning the light according to the imaging method to be used. For example, if a 2D image is to be taken outright, the illumination optical system 12 may produce a relatively constant intensity illumination in a relatively large spot, or alternatively, if a point scanning or line scanning is to be performed, the illumination optical system 12 is configured to focus the light to a single small spot or line. Preferably, the illumination optical system 12 is configured to produce illumination in a line. The illumination optical system 12 comprises one or more lenses and / or one or more mirrors and optionally additional optical elements, such as an aperture or a slit, as needed to achieve the desired illumination. The device further comprises a first polarizer 13 for introducing polarization, preferably linear polarization, and a single retarder 15, preferably a quarter-wave plate that is preferably configured to produce illumination light with circular polarization. Alternatively, the retarder 15 may be a photoelastic modulator. Said first polarizer 13 may be arranged in the light path before, after or within the illumination optical system 12.

[0044] The device further comprises at least a first objective 17a for projecting the illumination light onto the bonded wafer BW. The device further comprises a wafer holder H to hold said bonded wafer BW under study. The wafer holder H is preferably movable. The wafer holder may be a stage or an edge gripping holder. The first objective 17a is held in an objective holder, preferably together with a second objective 17b and optionally with a third objective 17c, wherein the objectives 17a, 17b, 17c have different magnifications and the objective holder 16 is preferably configured to be suitable for swapping said objectives 17a, 17b, 17c in the light path, i.e. to move each objective 17a, 17b, 17c into and out of the light path, as needed. The objective holder 16 is configured e.g. as a rotary disk. With multiple objectives having different magnifications, the device according to the invention may be used in different modes: a lower magnification objective enables a lower resolution mode for quicker scanning for detecting defects, and a higher magnification objective enables a better resolution mode for examining defects in detail and for detecting defects with higher sensitivity. The wafer holder H defines a wafer plane in which a perfectly plane wafer, i.e. a bonded wafer BW that is free of any warping, would be held. The objective holder 16 is configured to hold said objective or objectives so that the optical axis each of the objectives is perpendicular to the wafer plane. Often, the bonded wafer BW is warped, and thus the optical axis may not be perfectly perpendicular to the wafer surface in every point of the wafer. This effect is usually negligible, less than 1 degree, and thus “substantially perpendicular” illumination used in the meaning of not differing from perpendicular by more than 1 degree. Optionally, the wafer holder H may comprise a tilting mechanism for adjusting for the curvature of bonded wafer BW. The advantage of using perpendicular illumination is twofold. First, the polarization of the illumination light that falls on the wafer surface perpendicularly is not altered when passing through the bonded wafer surface, only when gets reflected and when it passes through a material with birefringent property, e.g. a material that is under mechanical stress caused by a defect. Second, with perpendicular illumination, the same objective may be used for collecting reflected light which allows a compact geometrical configuration for performing the inspection, especially in the case of modern in-line manufacturing environments.

[0045] The illuminating light path and the detection light path are separated by a beamsplitter 14, that is, preferably a polarizing beamsplitter cube that allows a larger portion of the illumination light to be utilized, while introducing as little imaging errors as possible. The beamsplitter 14 is preferably arranged between the first polarizer 13 and the retarder 15. Arranging the retarder 15 right after the beamsplitter 14, that is, into that portion of the light path within the device which is common for the illumination light and the reflected light, a symmetric light path is constructed. This results in a decrease in the variability of the inherent phase patterns of the device. Thus, the correction procedure to be used with the device for defect identification of small phase retardance simplifies. Here, the correction procedure includes a subtraction of the phase pattern due to the inhomogeneous phase component of the optical elements, e.g. any of the objectives 17a, 17b, 17c of the inspection device according to the invention.

[0046] A portion of the illumination light is reflected from some internal structures in the bonded wafer BW, thus reflected light is formed, that is collected by the objective, passed through the retarder 15, the beamsplitter 14, a second polarizer 18, an imaging optical system 19 to a camera 20. The second polarizer 18 is preferably configured to filter out most of the reflected light that has the same polarity as the illumination light. According to a preferred embodiment, the first polarizer 13 is configured to produce light that is polarized in the “s” direction in relation to the beamsplitter 14, while the second polarizer 18, that functions as an analyzer, filters out the “s” polarized light and only lets through light with “p” polarization, i.e. the polarizer and analyzer are arranged in a darkfield setup. If the beamsplitter 14 is a polarization beamsplitter cube, the same filtering is more or less performed by the beamsplitter too, however the first polarizer 13 and the second polarizer 18 offer a more precise control over the polarization states. The advantage of using a polarization beamsplitter cube as the beamsplitter 14 is that less of the illumination light is lost, which, for a given light source, leads to illumination of higher intensity on the surface of the sample or, alternatively, to achieve the same illumination intensity on the sample, a light source of lower power can be used. In an exemplary embodiment of the invention, the second polarizer 18 is arranged so that it is rotationally offset from the darkfield setup, to allow some of the reflected light with unaltered polarization through, i.e. to establish a so- called grey-field setup. The advantage of the grey-field setup is that structures that do not alter the polarization of the reflected light also appear in the images and, thus, can be used as reference for determining the positions of the detected defects.

[0047] The camera 20 is preferably a line camera, having a single line of detector pixels, for example a CCD or InGaAs camera with 1x1024 or 1x2048 pixels.

[0048] The device according to the present invention preferably further comprises a wafer flipper for flipping the bonded wafer BW from one side to the other to allow measurements to be performed from the front side and from the backside one after the other. The wafer flipper is a mechanical device that is suitable for grabbing the bonded wafer BW, rotating it by 180° around an axis that lies in the plane of the bonded wafer BW and releasing it. This has the advantage that when both wafers forming the bonded wafer BW are transparent at a wavelength used by the device and a defect together with its stress field is completely blocked from the view of the camera only from one side, it can be observed from the other side in the same inspection configuration. Similarly, when at least a part of a defect or its stress field has been detected from one side, the entire defect may be conveniently examined in detail from the other side.

[0049] The same reference numbers denote the same elements throughout all the Figures.

[0050] Throughout the present specification and claims, the term “layer” and “layers” are to be understood to encompass both complete layers and partial layers having predetermined structures formed therein either by partial forming of the layer e.g. by lithography or by selective etching predetermined parts of the layer. Also, the substrate of each wafer making up the bonded wafer is also considered a layer in the bonded wafer structure. The goal of the present invention is to provide an improved method and device for bonded wafer inspection, and more specifically to detect and optionally quantitatively characterize the voids and other defects by measuring the stress field of such defects generated in the material of the bonded wafer.

Claims

Claims1 . A bonded wafer inspection method for detecting a defect in a bonded wafer (BW) having a front face and a back face, wherein said method comprises:- producing illumination light with a polarization state and a propagation direction, the illumination light having a wavelength in an infrared range at which the bonded wafer (BW) is transparent from a direction of at least one of the front face and the back face along at least a portion of the propagation direction within said bonded wafer (BW),- guiding said illumination light to the bonded wafer (BW) from the direction in which the bonded wafer is transparent to the illumination light, the propagation direction being substantially perpendicular to at least one of the front face and the back face,- making at least a first portion of the illumination light to enter the bonded wafer (BW), thereby inducing reflection of at least a portion of said first portion of the illumination light in the bonded wafer (BW) to create reflected light,- collecting a portion of the reflected light and filtering said collected reflected light based on the polarization state to obtain polarization-filtered reflected light,- guiding the polarization-filtered reflected light to a detector,- detecting said polarization-filtered reflected light by the detector,- creating an image from said detected polarization-filtered reflected light,- based on the image, identifying at least one region in the bonded wafer (BW) having mechanical stress induced birefringence,- based on the at least one region identified, detecting the defect in the bonded wafer (BW).

2. The bonded wafer inspection method according to claim 1 , wherein the illumination light is guided to a structure in the bonded wafer (BW) being opaque at said wavelength of the illumination light, said structure obscuring the defect to be detected.

3. The bonded wafer inspection method according to claim 1 or 2, further comprising a step of quantitative assessment of said defect.

4. The bonded wafer inspection method according to claim 3, wherein the quantitative assessment comprises- identifying a center of the defect,- calculating a specific Fourier component F(r) of a light intensity I (r, (p) detected by said detector for at least a radius r around the center of the defect via the formula ofwherein cp is a polar angle relative to a <po reference angle, wherein the reference angle is selected as one of the inflection points of the light intensity I (r, <p) along a circle around the center of the defect.

5. The bonded wafer inspection method according to any of the preceding claims, wherein the type of defect is identified on the basis of the detected mechanical stress caused pattern.

6. The bonded wafer inspection method according to any of the preceding claims, wherein the bonded wafer (BW) is imaged over its whole surface.

7. The bonded wafer inspection method according to any of the preceding claims, wherein the collection of the reflected light is done in a line scanning arrangement.

8. The bonded wafer inspection method according to any of the preceding claims, wherein said illumination light is guided onto an area of interest of the bonded wafer (BW) identified previously, and wherein collecting of light from the area of interest is performed with an objective having higher magnification than an objective used to identify said area of interest of the bonded wafer (BW) previously.

9. The bonded wafer inspection method according to any of the preceding claims, wherein the defect to be detected is a void with a lateral size of less than 30 pm.

10. A bonded wafer inspection device for detecting a defect in a bonded wafer (BW), said device comprises:- a wafer holder (H) for holding the bonded wafer (BW) to be inspected,- a primary light source (11) for producing illumination light with a propagation direction in a predetermined infrared wavelength range, and- a detector, preferably in the form of a camera (20), for detecting light reflected by the bonded wafer (BW),- an illumination light path between the primary light source (11) and the wafer holder (H) to guide the illumination light to the bonded wafer (BW), and- a detection light path between the wafer holder (H) and the detector to guide reflected light to the detector; the device further comprises:in the illumination light path, in the propagation direction of the illumination light, one after the other- an illumination optical system (12) for conditioning the illumination light of the primary light source (11),- a first polarizer (13) for producing the illumination light with a predetermined polarization state,- a beamsplitter (14),- a retarder (15),- a first objective (17a); and in the detection light path, in a propagation direction of the reflected light, one after the other- said first objective (17a),- said retarder (15),- said beamsplitter (14),- a second polarizer (18) for filtering out, at least partially, reflected light of the predetermined polarization state, and- an imaging optical system (19), wherein the beamsplitter (14) is arranged so as to guide the illumination light to the bonded wafer (BW) essentially perpendicularly and to guide the reflected light to the detector.

11. The bonded wafer inspection device according to claim 10, wherein the retarder (15) is a quarter-wave plate.

12. The bonded wafer inspection device according to claim 10 or 11 , wherein the beamsplitter (14) is a polarizing beamsplitter cube.

13. The bonded wafer inspection device according to any one of claims 10 to 12, wherein the device further comprises at least one further objective (17b, 17c) and a mechanism configured to move the first objective (17a) and the at least one further objective (17b, 17c) into and out of the illumination light path.

14. The bonded wafer inspection device according to any one of claims 10 to 13, wherein the second polarizer (18) is arranged rotationally offset relative to the first polarizer (13) to avoid complete filtering out reflected light of the predetermined polarization state.

15. The bonded wafer inspection device according to any one of claims 10 to 14, further comprising a wafer flipper for flipping the bonded wafer (BW) to an opposite side thereof.

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