Overlay measurement device having optical correction system, and overlay measurement method using same

Abstract

The present invention relates to a device for measuring an overlay error of an object to be measured that comprises a thick silicon (Si) or carbon (C) layer, such as a bonding wafer or a CMOS image sensor (CIS). The present invention provides an overlay measurement device comprising: a short-wave infrared illumination optical system which includes a short-wave infrared light source, and which illuminates an overlay mark formed inside the object to be measured; an imaging optical system, which focuses reflected light from the overlay mark so as to form an overlay mark image; an image detector for acquiring the overlay mark image formed by the imaging optical system; a controller for calculating a quality index by analyzing the overlay mark image; and an optical correction system for correcting an optical aberration generated by at least one layer of the object to be measured covering the overlay mark, such that the quality index is improved.

Description

Overlay measuring device equipped with a correction optical system and an overlay measuring method using the same

[0001] The present invention relates to a short-wave infrared overlay measuring device used for measuring overlay errors of bonding wafers, CMOS image sensors (CIS), etc., and more specifically, to an overlay measuring device equipped with a correction optical system for correcting optical aberrations that occur when measuring overlay errors of bonding wafers, etc. with short-wave infrared (SWIR), and an overlay measuring method using the same.

[0002] Conventional overlay measurement devices primarily measure overlay errors using visible light or near-infrared (NIR). However, silicon (Si) or carbon (C) layers on top of bonding wafers or CMOS image sensors (CIS) absorb and scatter most of the light in these wavelength bands, significantly reducing the signal-to-noise ratio (SNR) relative to the target. Consequently, it is difficult to accurately measure overlay errors using only visible light and NIR methods.

[0003] To overcome this limitation, overlay error measurement technology utilizing short-wave infrared (SWIR) with a wavelength of 1 µm or longer is emerging. Short-wave infrared penetrates high-refractive-index materials such as silicon and carbon relatively well, allowing for effective detection of underlying overlay marks.

[0004] However, distortion or refraction of the light path caused by the high refractive indices of silicon and carbon increases optical aberrations such as spherical aberration, chromatic aberration, and distortion aberration, thereby degrading the quality of overlay mark images. Therefore, a corrective optical system is required to correct optical aberrations.

[0005] The present invention aims to solve the problems of the aforementioned prior art and to solve the following problems.

[0006] First, it is to accurately measure the overlay error of semiconductor devices, such as bonding wafers or CMOS image sensors (CIS), where a layer of high refractive index material (e.g., silicon, carbon) exists on top.

[0007] Second, it effectively corrects complex optical aberrations, such as spherical aberration, chromatic aberration, and distortion aberration caused by high-refractive-index materials, when measuring using short-wave infrared (SWIR).

[0008] Third, it is to dynamically respond to changes in the degree of aberration caused by unpredictable variables, such as variations in wafer thickness or minute differences in process conditions, in order to always maintain optimal image quality.

[0009] Fourth, the invention provides an overlay measurement device and method that maximizes the reliability and reproducibility of measurement by analyzing quality indices such as the image contrast index (CI) in real time and optimizing the optical system by feeding it back.

[0010] To achieve the above-mentioned objective, the present invention provides an overlay measuring device comprising: a short-wave infrared illumination optical system configured to illuminate an overlay mark formed inside a measuring object, the image forming optical system that collects reflected light from the overlay mark to form an overlay mark image; an image detector that acquires the overlay mark image formed by the image forming optical system; a controller that analyzes the overlay mark image to calculate a quality index; and a correction optical system that corrects optical aberrations caused by at least one layer of the measuring object covering the overlay mark so as to improve the quality index.

[0011] The above quality index may be a Contrast Index (CI) value.

[0012] In addition, the quality index may include at least one of an X-axis contrast index, a Y-axis contrast index, and an average contrast index thereof.

[0013] In addition, the present invention provides an overlay measuring device in which the short-wave infrared light source generates light in the short-wave infrared band of 900 nm to 1600 nm.

[0014] In addition, the correction optical system provides an overlay measuring device comprising at least one optical element for correcting spherical aberration, chromatic aberration, distortion aberration, or a combination thereof that occurs in the overlay mark image due to at least one layer of the measurement target covering the overlay mark.

[0015] In addition, the above-described correction optical system provides an overlay measuring device comprising at least one of a lens, a filter, and a window glass.

[0016] In addition, the correction optical system provides an overlay measuring device disposed between the imaging optical system and the image detector.

[0017] In addition, the above-mentioned correction optical system can be selectively applied depending on the thickness and refractive index of the bonding wafer, and provides an overlay measuring device that is detachably mounted on the objective lens.

[0018] Additionally, the controller provides an overlay measuring device configured to analyze a mark image obtained from an image detector to calculate the quality index and to calculate the edge contrast of the overlay mark image.

[0019] In addition, the controller provides an overlay measurement device that stores the quality index as data of change over time.

[0020] In addition, the controller provides an overlay measuring device that controls the correction optical system based on the trend of change of the quality index over time.

[0021] In addition, the correction optical system provides an overlay measuring device comprising at least one optical element detachable from the objective lens.

[0022] In addition, the illumination optical system provides an overlay measuring device that performs at least one of vertical illumination and oblique illumination on the overlay mark.

[0023] In addition, the image detector provides an overlay measuring device which is a CCD or CMOS sensor having an infrared detection function.

[0024] In addition, the controller provides an overlay measurement device that verifies the reliability of the overlay measurement by comparing the quality index with a reference value stored in a reference database.

[0025] Additionally, the measurement target is a bonding wafer, and the overlay mark is formed on the lower surface of the upper wafer, the upper surface of the lower wafer, or the interface between the upper wafer and the lower wafer, thereby providing an overlay measurement device.

[0026] Additionally, the correction optical system includes a correction operating unit equipped with optical elements comprising at least one of a lens, a filter, and a window glass, and the controller provides an overlay measuring device configured to operate the correction operating unit to position one optical element selected among the optical elements on an optical path.

[0027] Additionally, the present invention provides an overlay measuring device comprising an autofocus optical system including an autofocus light source and a light detector that receives light emitted from the autofocus light source and reflected from the measurement target, and further comprising an optical filter disposed in front of the image detector that blocks light from the autofocus light source to prevent light from the autofocus light source from being detected by the image detector.

[0028] In addition, the optical filter provides an overlay measuring device disposed on the optical path between the image detector and the imaging optical system.

[0029] In addition, the controller includes a machine learning model that is pre-trained to predict an optimal optical element by receiving the quality index and process data, and provides an overlay measurement device that selects the optical element based on the prediction result of the machine learning model.

[0030] The above-described correction operating unit provides an overlay measuring device comprising: a correction wheel having a plurality of the optical elements installed along its circumference; and a motor that rotates the correction wheel according to the control of the controller to position a selected optical element on the path of the illumination.

[0031] Additionally, the correction operating unit provides an overlay measuring device comprising: a slider having a plurality of optical elements installed along a longitudinal direction; and a linear motion stage that moves the slider in a straight line according to the control of the controller to position a selected optical element on the path of the illumination.

[0032] In addition, the present invention provides an overlay measurement method using an overlay measurement device, comprising: (a) illuminating an overlay mark formed on a measurement target using a short-wave infrared light source; (b) obtaining an initial overlay mark image using light reflected from the overlay mark; (c) calculating a quality index of the overlay mark image; (d) selecting one optical element among a plurality of optical elements including at least one of a lens, a filter, and a window glass based on the quality index and placing it on an illumination light path; (e) obtaining a final overlay mark image using light reflected from the overlay mark while the selected optical element is placed, and measuring an overlay error using the same.

[0033] According to the present invention, the problem of distortion and quality degradation of overlay mark images caused by unpredictable variables, such as changes in the thickness of a bonding wafer, can be effectively solved.

[0034] In particular, by checking quality indices such as the Contrast Index (CI) in real time and selectively placing optimal correction optical elements in the optical path based on the results, consistent and highly reliable measurement results can be obtained even under various process conditions.

[0035] FIG. 1 is a schematic diagram of an overlay measuring device according to an embodiment of the present invention.

[0036] Figure 2 shows an example of a correction optical system.

[0037] Figure 3 shows the correction operating unit illustrated in Figure 2.

[0038] Figure 4 is a drawing illustrating another example of a correction operating unit shown in Figure 2.

[0039] FIG. 5 is a flowchart of an overlay measurement method according to an embodiment of the present invention.

[0040] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, embodiments of the present invention may be modified in various different forms, and the scope of the present invention should not be interpreted as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art. Accordingly, the shapes of elements in the drawings are exaggerated to emphasize clearer explanations, and elements indicated by the same reference numeral in the drawings represent the same elements.

[0041] FIG. 1 is a schematic diagram of an overlay measuring device according to an embodiment of the present invention. The overlay measuring device (100) illustrated in FIG. 1 is provided for illustrative purposes, and the overlay measuring device may be configured in various ways.

[0042] As illustrated in FIG. 1, the overlay measuring device (100) of the present embodiment includes an illumination optical system (10) that illuminates an overlay mark on a measurement target such as a bonding wafer (W), an imaging optical system (20) that collects reflected light from the overlay mark to form an overlay mark image, an image detector (30) that acquires the overlay mark image formed by the imaging optical system (20), a correction optical system (40), and a controller (49) (see FIG. 2). It also includes a high-precision stage (16) for precisely moving the measurement target in the X, Y, Z, and θ directions. Additionally, it may further include an autofocus optical system (50) and an optical filter (60).

[0043] The measurement target has an overlay mark covered by a thick silicon (Si) or carbon (C) layer, such as a bonding wafer (W) or a CMOS image sensor (CIS).

[0044] An overlay mark on the bonding wafer (W) may be formed on the lower surface of the upper wafer and the upper surface of the lower wafer, or at the interface or near the interface between the upper wafer and the lower wafer. The overlay mark may be formed within the focus allowance range of the imaging optical system (20).

[0045] The overlay measuring device (100) can process the acquired overlay mark image to measure, for example, the overlay between the upper wafer and the lower wafer.

[0046] The illumination optical system (10) can be constructed using various optical elements. For example, the illumination optical system (10) may include a short-wave infrared light source (11), a beam splitter (13), and an objective lens (15). Additionally, it may include other optical elements such as lenses, apertures (18), and filters (19). The illumination optical system (10) can perform at least one of vertical illumination and oblique illumination on an overlay mark.

[0047] A short-wave infrared light source (11) generates light in the short-wave infrared band of about 900 nm to 1600 nm. This light passes through a part of the measurement target covering the overlay mark (e.g., the silicon layer of the upper wafer of the bonding wafer) to illuminate the overlay mark.

[0048] As a short-wave infrared light source (11), a laser diode, a combination of a tungsten-halogen lamp and a band-pass filter, a combination of a superconnium light source and a band-pass filter, etc. can be used.

[0049] The beam splitter (13) is positioned between the short-wave infrared light source (11) and the objective lens (15) and serves to transmit light to the objective lens (15).

[0050] The objective lens (15) serves to concentrate light onto a measurement position on the surface of the semiconductor wafer (W) and to collect reflected light from the measurement position. The objective lens (15) is installed in a lens focus actuator (17). The lens focus actuator (17) is used to adjust the distance between the objective lens (15) and the semiconductor wafer (W).

[0051] The imaging optical system (20) can be constructed using various optical elements. For example, the imaging optical system (20) may include a tube lens (21). Additionally, the imaging optical system (20) may use the objective lens (15) and beam splitter (13) of the illumination optical system (10). Additionally, it may include other optical elements such as lenses or apertures. The imaging optical system (20) may be configured to allow variable adjustment of the numerical aperture (NA).

[0052] The reflected light collected from the objective lens (15) passes through the beam splitter (13) and is then focused on the image detector (30) by the tube lens (21).

[0053] The image detector (30) receives reflected light from the overlay mark by illumination and generates an overlay mark image. The image detector (30) may be a CCD sensor or a CMOS sensor with an infrared detection function.

[0054] The correction optical system (40) serves to correct optical aberrations such as spherical aberration, chromatic aberration, distortion aberration, or a combination thereof caused by distortion or refraction of the light path by a layer (e.g., the upper wafer of the bonding wafer) made of a material with a high refractive index such as silicon and carbon covering the overlay mark. The correction optical system (40) may also perform the role of dynamically correcting optical aberrations through real-time feedback.

[0055] The correction optical system (40) is placed in the optical path between the imaging optical system (20) and the image detector (30). For example, it can be placed between the bonding wafer (W) to be measured and the objective lens (15).

[0056] The correction optical system (40) may include at least one optical element among a lens, a filter, and a window glass. Among these optical elements, an optical element selected according to the thickness and refractive index of the object being measured is placed in the optical path between the imaging optical system (20) and the image detector (30). Additionally, this optical element may be configured to be attached to the objective lens (15) automatically or manually. Furthermore, the correction optical system (40) may further include a variable focus lens that automatically adjusts the focus in response to changes in the refractive index of the object being measured.

[0057] FIG. 2 shows an example of a correction optical system, and FIG. 3 shows a correction operating unit illustrated in FIG. 2.

[0058] As shown in FIG. 2, the correction optical system (40) includes a correction operating unit (41) equipped with optical elements (43) and a motor (45) that drives the correction operating unit (41).

[0059] As illustrated in FIGS. 2 and 3, the correction operating unit (41) includes optical elements (43), such as a lens (43a), a filter (43b), and a window glass (43c). A motor (45) serves to rotate a correction wheel (42) that supports these optical elements (43). The motor (45) may rotate the correction wheel (42) directly, or, as illustrated in FIG. 2, may rotate the correction wheel (42) through gears (47, 48).

[0060] The lens (43a) primarily contributes to the correction of spherical aberration and coma aberration. In addition to simple convex / concave lenses, an aspheric lens can be used to effectively eliminate spherical aberration, or an achromatic lens or apochromatic lens made of low-dispersion glass can be used to compensate for differences in refractive index depending on wavelength. These can be configured as a single lens or as a doublet or triplet lens combining two or three lenses.

[0061] The filter (43b) contributes to chromatic aberration correction and signal-to-noise ratio (SNR) improvement. When the spectrum of the short-wave infrared light source (11) has a specific bandwidth, the filter (43b) is used to finely adjust the center wavelength or further narrow the bandwidth to minimize wavelength-dependent refractive index differences (i.e., chromatic aberration). Additionally, it effectively removes stray light from the autofocus optical system (50) or external lighting, thereby reducing image noise.

[0062] The window glass (43c) serves to correct the focal position by adjusting the optical path length (OPL). Light passing through the silicon layer causes the optical path to shorten due to the difference in refractive index, resulting in a shift in focus. This is counteracted by inserting a window glass (e.g., quartz (Fused Silica), calcium fluoride (CaF2), etc.) that provides the opposite effect. At this time, the window glass may be provided with several pieces of different thicknesses (e.g., 0.5 mm, 1.0 mm, 1.5 mm) to respond precisely to changes in the thickness of the silicon layer.

[0063] The degree of freedom for correction can be further increased by configuring a composite optical element that performs two or more functions simultaneously, such as combining a lens and a filter in one location or placing a window glass with a specific coating.

[0064] The controller (49) analyzes the overlay mark image from the image detector (30) and calculates a quality index. Additionally, it may additionally perform the role of controlling the correction optical system (40) based on the calculated quality index.

[0065] The controller (49) may include hardware such as a processor, memory, a storage device such as a hard disk or SSD, a wired or wireless communication device used to receive a signal from an image detector (30) and to transmit a control signal to control a correction optical system (40) (e.g., a motor (45) in FIG. 2).

[0066] Additionally, the controller (49) may include a program such as firmware or software installed on a storage medium such as memory, fixed memory device, or storage device.

[0067] A computing device such as an MCU (Micro controller unit), desktop computer, laptop computer, smartphone, or smart pad can be used as the controller (49).

[0068] The overlay measuring device (100) may include a user interface coupled to communicate with a controller (49). The user interface may include known input devices. For example, it may include a keyboard, a touchscreen, etc. Additionally, the user interface may include a display.

[0069] The user can input instructions for selecting an optical element based on information displayed through the display using an input device. The controller (49) may also control the correction optical system (40) according to the input instructions.

[0070] The controller (49) can store the quality index as data of change over time. The controller (49) can control the correction optical system (40) based on the trend of change of the quality index over time.

[0071] The controller (49) can verify the reliability of the overlay measurement by comparing the quality index with the reference value stored in the reference database.

[0072] The controller (49) can verify the correction status by comparing the difference in the quality index according to whether the correction optical system (40) is corrected.

[0073] For example, the Contrast Index (CI) value can be used as a quality index.

[0074] The controller (49) can receive a signal from the image detector (30) and calculate a Contrast Index (CI) value. That is, it can acquire an overlay mark image from the image detector (30), analyze it, and calculate the Contrast Index. The Contrast Index represents the difference in brightness between the brightest part and the darkest part on the overlay mark image.

[0075] The contrast index can be calculated by subtracting the brightness value of the darkest part from the brightness value of the brightest part (e.g., the gray value). Alternatively, a normalized value obtained by dividing this by the sum of the brightness values ​​of the brightest and darkest parts can be used as the contrast index. The normalized contrast index has a value between 1 and 0. 0 indicates no contrast, while 1 indicates maximum contrast.

[0076] The X-axis contrast index, the Y-axis contrast index, and their average contrast index may also be used as contrast indices. The X-axis contrast index is obtained by calculating the brightness difference between the brightest and darkest points of the overlay mark image on the X-axis baseline, while the Y-axis contrast index is obtained by calculating the brightness difference along the Y-axis. The average contrast index represents the average value of these.

[0077] The controller (49) analyzes the overlay mark image obtained from the image detector (30) to calculate the quality index and can calculate the edge contrast of the overlay mark image.

[0078] Referring to FIG. 2, the control of the correction optical system of the controller (49) is described.

[0079] The controller (49) operates the correction operating unit (41) to position one selected optical element (43) among the optical elements (43) on the path of illumination (PL).

[0080] The controller (49) may also automatically select an optical element (43) based on machine learning. The machine learning model receives real-time data such as a contrast index, silicon thickness, and process conditions, predicts the contrast index improvement effect based on the performance of each optical element (43) learned from past measurement logs, selects the optical element (43) with the maximum predicted value, and places it on the path (PL) of the light.

[0081] Figure 4 is a drawing illustrating another example of a correction operating unit shown in Figure 2.

[0082] FIG. 4 illustrates a slider (142), which is a key component used in a linear motion type correction operating unit, as an alternative embodiment to the rotary type correction operating unit (41) shown in FIG. 3. The correction optical system of the present invention may be implemented by adopting this linear motion type.

[0083] Referring to FIG. 4, the slider (142) is an elongated rod or a square plate-shaped member, and a plurality of optical elements (143) are installed or fixed at equal intervals along its length. The optical elements (143) installed therein have the same type and function as the previously described lens (43a), filter (43b), window glass (43c), and combinations thereof.

[0084] These sliders (142) are driven by a high-precision linear motion stage. The linear motion stage moves the sliders (142) precisely in a straight line according to commands from the controller (49). Specific driving methods may include a ball screw stage, a linear motor stage, etc.

[0085] For accurate position control, the stage may be equipped with a high-resolution position sensor, such as a linear encoder, capable of determining the current position of the slider (142) at the nanometer level.

[0086] The autofocus optical system (50) includes an autofocus light source (51) that generates autofocus illumination, a beam splitter (52), a mirror (53), and a light detector (55). It also includes lenses (56, 57, 58).

[0087] A laser diode or a light-emitting diode may be used as the light source (51) for autofocus. Illuminating light from the light source (51) passes through a beam splitter (52) and is reflected by a mirror (53). Illuminating light reflected from the mirror (53) passes through a beam splitter (13) and is incident on an objective lens (15). The objective lens (15) concentrates the illuminating light into a measurement area of ​​the wafer (W) and collects the reflected light from the measurement area. The reflected light collected by the objective lens (15) passes through the beam splitter (13) again and is reflected by the mirror (53). The reflected light reflected from the mirror (53) is reflected from the beam splitter (52) toward a photodetector (55). The photodetector (55) generates an electrical signal according to the Z-direction position on the surface of the wafer (W).

[0088] The optical filter (60) serves to prevent light used in the autofocus optical system (50) from entering the image detector (30). If light used in the autofocus optical system (50) enters the image detector (30), it can cause unwanted noise.

[0089] The optical filter (60) may be, for example, an optical filter (60) that transmits only light from a short-wave infrared light source (11) of an illumination optical system (10).

[0090] The optical filter (60) can be placed in front of the image detector (30), for example, on the optical path between the image detector (30) and the imaging optical system (20).

[0091] Hereinafter, with reference to FIG. 5, an overlay measurement method using the overlay measurement device (100) described above will be explained in more detail step by step. This method can be programmed by a controller (49) and executed sequentially or intelligently.

[0092] First, the step (S1) of illuminating the overlay mark is described.

[0093] When the measurement process begins, the wafer (W) is first moved by the high-precision stage (16) so that the overlay mark to be measured is positioned within the field of view of the objective lens (15). At the same time or immediately before, the autofocus optical system (50) operates to precisely detect the Z-axis height of the wafer (W) surface and drives the lens focus actuator (17) to adjust the distance between the objective lens (15) and the wafer (W) to an optimal focus state.

[0094] Subsequently, according to the control signal of the controller (49), the short-wave infrared light source (11) is activated to emit light in the short-wave infrared (SWIR) band. This light passes through various optical elements (e.g., aperture (18), filter (19)) of the illumination optical system (10) to optimize the illumination conditions, then is reflected from the beam splitter (13) and directed toward the objective lens (15). The objective lens (15) focuses this light onto an overlay mark area on the wafer (W) to illuminate it uniformly. At this stage, the correction optical system (40) is not yet activated.

[0095] Next, the step (S2) of acquiring the initial overlay mark image is described.

[0096] The short-wave infrared light reflected from the overlay mark is collected again through the objective lens (15). This reflected light passes through the beam splitter (13), passes through the tube lens (21) of the imaging optical system (20), and reaches the sensor surface of the image detector (30) to form an image of the overlay mark.

[0097] The image acquired at this stage corresponds to a 'pre-correction image' or 'diagnostic image'. If the silicon layer of the wafer (W) is thick and optical aberrations are severe, it is highly likely that the image will be of degraded quality, appearing blurry as if out of focus (spherical aberration), with marks appearing curved (distortion aberration) or showing color bleeding (chromatic aberration).

[0098] Next, the step (S3) for calculating the quality index is described.

[0099] The controller (49) receives the image before correction from the image detector (30) in the form of digital data and analyzes it through a built-in image processing algorithm. The goal of this step is to convert the degree of image quality degradation into a quantitative 'quality index'.

[0100] The controller (49) recognizes an overlay mark area within the image and extracts a pixel brightness value (gray level) profile along the line portion of the mark. It finds the brightest value (Imax) and the darkest value (Imin) from this profile and calculates the contrast index (CI). For example, the normalized CI value can be calculated using the formula (Imax-Imin) / (Imax+Imin).

[0101] If necessary, the controller (49) may calculate additional indices, such as an asymmetry index (AI), to evaluate the symmetry of the mark pattern. These quality indices serve as important indicators that objectively represent the image quality in the current optical system state.

[0102] Next, a step (S4) of selecting one optical element among a plurality of optical elements and placing it on the illumination light path is described.

[0103] The controller (49) controls the correction optical system (40) based on the quality index calculated in step S3 to select the optimal optical element (43).

[0104] First, if the calculated quality index is greater than or equal to a preset 'acceptable threshold' (i.e., if the image quality is good), the controller (49) can proceed directly to the final measurement without a separate correction process. This has the effect of shortening the measurement time by omitting unnecessary optimization processes.

[0105] If the quality index is below the threshold, the controller (49) can execute an 'Optimization Loop'.

[0106] For example, the controller (49) can drive the motor (45) to rotate the correction wheel (42) to the next position, thereby placing the first correction optical element (e.g., lens 43a) on the optical path (PL).

[0107] In this state, the image is acquired again as in step S2, and the quality index is calculated as in step S3.

[0108] Compare the calculated quality index with the previous value and temporarily store the higher value.

[0109] The above processes are repeated for all optical elements (or candidate elements specified in the recipe). Once the evaluation of all candidate elements is complete, the controller (49) finally selects the optical element that recorded the highest quality index value as the 'optimal optical element'. Then, the correction optical system (40) is controlled to accurately position the selected optimal optical element on the optical path (PL).

[0110] If the controller (49) is equipped with a machine learning model, the controller (49) may immediately predict and place the optimal optical element by inputting the quality index of the initial image and process data into the model without this iterative search process.

[0111] Next, the step (S5) of acquiring the final overlay mark image and measuring the overlay error is described.

[0112] With the optimal optical elements positioned in the optical path, the overlay measurement device finally acquires an overlay mark image. The 'post-correction image' acquired at this stage has optical aberrations effectively corrected, resulting in a high-quality image that is significantly sharper and less distorted compared to the pre-correction image.

[0113] The controller (49) uses this high-quality corrected image to finally calculate the overlay error. The center coordinates of the upper mark and the lower mark are precisely calculated using an image processing algorithm, and the difference in the X and Y directions between the two coordinates is calculated to obtain the final overlay error value.

[0114] Through such a series of methods, the present invention can provide overlay measurement results that are insensitive to process changes and always highly reliable.

[0115] Although preferred embodiments of the present invention have been illustrated and described above, the present invention is not limited to the specific embodiments described above. Various modifications are possible by those skilled in the art without departing from the essence of the invention as claimed in the claims, and such modifications should not be understood individually from the technical spirit or perspective of the present invention.

[0116] [Explanation of the symbol]

[0117] W: Bonding wafer

[0118] PL: Path of illumination

[0119] 100: Overlay measuring device

[0120] 10: Illumination optical system

[0121] 11: Shortwave infrared light source

[0122] 15: Objective lens

[0123] 20: Imaging optical system

[0124] 30: Image detector

[0125] 40: Correction optical system

[0126] 41: Correction operating unit

[0127] 43: Optical element

[0128] 43a: Lens

[0129] 43b: Filter

[0130] 43c: Window glass

[0131] 49: Controller

[0132] 50: Optical focus for autofocus

Claims

As an overlay measuring device, A short-wave infrared illumination optical system comprising a short-wave infrared light source configured to illuminate an overlay mark formed inside a measurement target, and An imaging optical system that collects reflected light from the above-mentioned overlay mark to form an overlay mark image, and An image detector that acquires the overlay mark image formed by the above-mentioned imaging optical system, and A controller that calculates a quality index by analyzing the above overlay mark image, and It includes a correction optical system that corrects optical aberrations caused by at least one layer of the measurement target covering the overlay mark so as to improve the quality index, The correction optical system comprises at least one optical element among a lens, a filter, and a window glass, and the optical element is selected according to the thickness and refractive index of the measurement target and is an overlay measuring device disposed between the imaging optical system and the image detector. In paragraph 1, The above quality index is an overlay measuring device in which the Contrast Index (CI) value is a quality index. In paragraph 1, The quality index above is an overlay measuring device comprising at least one of an X-axis direction contrast index, a Y-axis direction contrast index, and an average contrast index thereof. In paragraph 1, The above short-wave infrared light source is an overlay measuring device that generates light in the short-wave infrared band of 900 nm to 1600 nm. In paragraph 1, The correction optical system above is an overlay measuring device comprising at least one optical element for correcting spherical aberration, chromatic aberration, distortion aberration, or a combination thereof occurring in the overlay mark image due to at least one layer of the measurement target covering the overlay mark. In paragraph 1, The controller analyzes a mark image obtained from the image detector to calculate the quality index and is configured to calculate the edge contrast of the overlay mark image. In paragraph 1, The above controller is an overlay measurement device that stores the quality index as data of change over time. In Paragraph 7, The above controller is an overlay measuring device that controls the above correction optical system based on the trend of change of the quality index over time. In paragraph 1, The above correction optical system is an overlay measuring device comprising at least one optical element detachable from an objective lens. In paragraph 1, The above-described illumination optical system is an overlay measuring device that performs at least one of vertical illumination and oblique illumination on the above-described overlay mark. In paragraph 1, The above image detector is an overlay measuring device that is a CCD or CMOS sensor having an infrared detection function. In paragraph 1, The above controller is an overlay measurement device that verifies the reliability of the overlay measurement by comparing the quality index with a reference value stored in a reference database. In paragraph 1, The above measurement target is a bonding wafer, and The above overlay mark is an overlay measuring device formed on the lower surface of the upper wafer, the upper surface of the lower wafer, or the interface between the upper wafer and the lower wafer. In paragraph 1, The above-described correction optical system includes a correction operating unit equipped with optical elements comprising at least one of a lens, a filter, and a window glass, and The above controller is configured to operate the correction operating unit to position one optical element selected among the optical elements on the optical path, thereby forming an overlay measuring device. In paragraph 1, An autofocus optical system comprising an autofocus light source and a light detector that receives light emitted from the autofocus light source and reflected from the measurement target, and An overlay measuring device further comprising an optical filter disposed in front of the image detector and blocking light from the autofocus light source to prevent light from the autofocus light source from being detected by the image detector. In paragraph 15, The above optical filter is, An overlay measuring device positioned on the optical path between the image detector and the imaging optical system. In Paragraph 14, The above controller is, An overlay measuring device that includes a machine learning model pre-trained to predict an optimal optical element by receiving the above quality index and process data, and selects the optical element based on the prediction result of the machine learning model. In Paragraph 14, The above-mentioned correction operating unit is, A correction wheel having a plurality of the above-mentioned optical elements installed along the circumference; and An overlay measuring device comprising a motor that rotates the correction wheel according to the control of the above controller to position a selected optical element on the path of the illumination. In Paragraph 14, The above-mentioned correction operating unit is, A slider having a plurality of the above-mentioned optical elements installed along the longitudinal direction; and An overlay measuring device comprising a linear motion stage that moves the slider in a straight line according to the control of the controller to position a selected optical element on the path of the illumination. As an overlay measurement method using the overlay measurement device of claim 1, (a) A step of illuminating an overlay mark formed on a measurement target using a short-wave infrared light source; (b) a step of acquiring an initial overlay mark image using light reflected from the overlay mark; (c) a step of calculating the quality index of the overlay mark image above; (d) a step of selecting one optical element from a plurality of optical elements, including at least one of a lens, a filter, and a window glass, based on the quality index above, and placing it on an illumination light path; (e) A method for measuring an overlay that includes the step of obtaining a final overlay mark image using light reflected from the overlay mark while the selected optical element is positioned, and measuring the overlay error using the image.

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