Autofocus device, overlay measuring device having same, and autofocus device calibration method

Abstract

The present invention provides an autofocus device comprising: a light source for emitting illumination light for automatic focus adjustment; an optical lens disposed on the path of the illumination light emitted by the light source, so as to refract the illumination light; a cylindrical lens disposed on the path of the illumination light passing through the optical lens, so as to change the illumination light into a line beam; an objective lens for focusing the line beam onto a measurement area and collecting reflected light reflected from the measurement area; a detector for receiving the reflected light and generating a signal according to the focal position of the reflected light; an actuator for moving the optical lens in the optical-axis direction; and a cylinder lens holder for rotating the cylinder lens about at least one rotational axis perpendicular to the optical axis of the cylinder lens.

Description

Autofocus device, overlay measuring device having the same, and method for calibrating the autofocus device

[0001] The present invention relates to an autofocus device, an overlay measuring device having the same, and a calibration method for the autofocus device.

[0002] Technological advancements are driving the shrinking size of semiconductor devices and increasing the density of integrated circuits. To meet these demands, various conditions must be met, and overlay tolerance is one of the key indicators.

[0003] Semiconductor devices are manufactured through numerous manufacturing processes. Forming an integrated circuit on a wafer requires numerous manufacturing processes to ensure the desired circuit structures and elements are sequentially formed at specific locations. The manufacturing process sequentially creates patterned layers on the wafer. Through these repeated stacking processes, electrically active patterns within the integrated circuit are created. If the individual structures are not aligned within the tolerances allowed by the production process, interference between the electrically active patterns can occur, potentially affecting the performance and reliability of the manufactured circuit. Overlay measurement tools are used to measure and verify alignment errors between these layers.

[0004] Common overlay metrology and methods measure and verify whether the alignment between two layers is within an acceptable tolerance. One method involves forming a structure called an overlay mark at a specific location on the substrate, photographing this structure with an optical image acquisition device, and measuring the overlay. The measurement structure is designed to measure the overlay in at least one direction among the X and Y directions for each layer. Each structure is designed to be symmetrical, and the center value between the symmetrically arranged structures is calculated and used as the representative value for that layer, and the relative difference between the representative values ​​for each layer is calculated to derive the overlay error.

[0005] Hereinafter, a method for measuring the overlay of two layers using a Box in Box (BIB) overlay mark is briefly described. First, as illustrated in FIGS. 1 and 2, a first overlay mark (OM1), which is generally box-shaped, and a second overlay mark (OM2), which is generally box-shaped and smaller than the first overlay mark (1), are formed on two consecutive layers, respectively. Then, images of the first overlay mark (OM1) and the second overlay mark (OM2) are acquired at once. Next, as illustrated in FIG. 3, a waveform representing the change in intensity by position is acquired, and the center value (C1) of the first overlay mark (OM1) and the center value (C2) of the second overlay mark (OM2) are acquired, respectively. Then, by comparing these, the overlay error between the two layers is measured.

[0006] If the height difference between the two layers is not significant, the autofocus sensor can be used to automatically adjust the focus, and then a combined image of the first overlay mark (OM1) and the second overlay mark (OM2) can be acquired simultaneously. The standard focus found using the autofocus sensor may not be the optimal focus for acquiring the combined image, but this does not pose a significant problem if the height difference between the two layers is not significant.

[0007] However, with the advancement of semiconductor processing technology, there is now a need to accurately measure overlay errors between layers with large height differences and different optical properties. This presents a problem: the difference between the standard focus obtained using an autofocus sensor and the measured focus for more accurate measurements can be significant. For example, the standard focus obtained by an autofocus sensor may be located above the current layer, while the measured focus may be located between the previous layer and the current layer.

[0008] It takes a long time to find the position of the measurement focus because it is found by comparing the contrast of the overlay mark images after acquiring multiple overlay mark images for each focus position using the optical system used to acquire the overlay mark images.

[0009] Ultimately, when measuring the overlay error between pattern layers with a large height difference, there is a problem that the overlay error cannot be accurately measured using an image acquired at a standard focus because the first overlay mark formed on the previous layer is not clear in the alignment image, and there is a problem that it takes a relatively long time to use an image acquired at a measurement focus.

[0010] In order to solve the problem of difficulty in accurately measuring the overlay error between pattern layers with a large height difference, as shown in FIGS. 4 and 5, a double-shot method was also used in which a waveform representing the change in intensity by position of a signal obtained after focusing on a first overlay mark (OM1) is obtained to obtain the center value (C1) of the first overlay mark (OM1), and a waveform representing the change in intensity by position of a signal obtained after focusing on a second overlay mark (OM2) is obtained to obtain the center value (C2) of the second overlay mark (OM2), thereby measuring the overlay error between the two layers.

[0011] However, this method had the problem that the inspection time was longer because the measurement focus position had to be found twice since two shots had to be taken each time to measure the overlay error.

[0012] To improve these problems, Korean Patent No. 10-2524462, filed and registered by the applicant, discloses an overlay measuring device that can align the position of a standard focus with the position of a measurement focus by arranging an optical lens between a light source (light source for autofocus) of an autofocus device and an objective lens, and adjusting the position of the optical lens using an actuator (second actuator). When the position of the standard focus and the position of the measurement focus are aligned, there is an advantage in that the measurement speed is greatly increased because the position of the measurement focus does not need to be measured every time.

[0013] However, the overlay measuring device of Korean Patent No. 10-2524462 had a problem in that when the optical lens was moved to match the position of the standard focus and the position of the measurement focus, the position of the lighting light for automatic focus adjustment that illuminates the overlay mark could deviate from the correct position on the overlay mark due to a slight error of the overlay measuring device, resulting in misalignment of the lighting light.

[0014] Fig. 6 is a diagram illustrating changes in the position and size of the illumination light according to the position of the optical lens of an autofocus device. As illustrated in Fig. 6, not only the size but also the position of the illumination light (LB) changes depending on the distance between the optical lens and the light source. Ideally, even if the optical lens moves, the position of the illumination light (LB) should not deviate from the center of the overlay mark. The illumination light (LB) in Fig. 6 is illumination light that passes through the cylindrical lens and is focused in a linear shape.

[0015] Thus, if the illumination light (LB) deviates from the center of the overlay mark, the accuracy of autofocus adjustment deteriorates. In particular, when measuring overlay errors between multiple pattern layers, the position of the optical lens may vary depending on the measurement target, leading to varying degrees of illumination misalignment.

[0016] [Prior Art Literature]

[0017] Korean Patent Publication No. 2003-0054781 (July 2, 2003)

[0018] Korean Patent No. 10-0689709 (February 26, 2007)

[0019] Korean Patent No. 10-1564312 (October 23, 2015)

[0020] Korean Patent No. 10-2524462 (April 21, 2023)

[0021] The present invention is intended to improve the above-described problems, and an object of the present invention is to provide an autofocus device capable of correcting misalignment of illumination light that occurs by adjusting the position of an optical lens of the autofocus device to match a standard focus position and a measurement focus position.

[0022] In addition, it is an object to provide an overlay measuring device equipped with such an auto-focus device.

[0023] In addition, it is intended to provide a method for calibrating such an autofocus device.

[0024] In order to achieve the above-described object, the present invention provides an auto-focus device including a light source configured to emit illumination light for automatic focus control; an optical lens arranged on a path of the illumination light emitted from the light source and configured to refract the illumination light; a cylinder lens arranged on a path of the illumination light passing through the optical lens and configured to change the illumination light into a line beam; an objective lens configured to focus the line beam on a measurement area and collect reflected light reflected from the measurement area; a detector configured to receive the reflected light and generate a signal according to a focus position of the reflected light; an actuator configured to move the optical lens along an optical axis direction; and a cylinder lens holder configured to rotate the cylinder lens about at least one rotational axis orthogonal to the optical axis of the cylinder lens.

[0025] Additionally, the optical lens provides an auto-focus device that is a plano-convex lens.

[0026] Additionally, the cylinder lens holder provides an autofocus device configured to rotate the cylinder lens about a first rotation axis perpendicular to an optical axis of the cylinder lens and a second rotation axis perpendicular to the first rotation axis.

[0027] In addition, an autofocus device is provided that further includes an actuator that adjusts the distance between the measurement area and the objective lens.

[0028] In addition, an overlay mark is formed in the measurement area, and an auto-focus device is provided that is adjusted so that the line beam does not go beyond the overlay mark formed in the measurement area.

[0029] In addition, the present invention provides a calibration method for an autofocus device, including a step of checking an alignment error of the line beam according to a distance between the optical lens and the light source, and a step of adjusting a rotation angle of the cylinder lens with respect to the rotation axis based on the alignment error so that the line beam is positioned at the center of the measurement area.

[0030] In addition, an overlay mark is formed in the measurement area, and a step of checking an alignment error of the line beam is a step of obtaining an image in which the line beam and the overlay mark are displayed together, and checking an error between the center of the overlay mark and the center of the line beam. A method for calibrating an autofocus device is provided.

[0031] In addition, the present invention provides an overlay measuring device for measuring an error between a pair of first overlay marks and a second overlay mark formed on different layers formed on a wafer, the overlay measuring device including an image acquisition device configured to acquire alignment images of the pair of first overlay marks and the second overlay marks at a plurality of focus positions, and an auto-focus device for adjusting a focus of the image acquisition device, wherein the auto-focus device is the auto-focus device described above.

[0032] The autofocus device according to the present invention can compensate for errors caused by other optical elements of the autofocus device by adjusting the rotation angle of the cylinder lens. Accordingly, misalignment of the illumination light due to position adjustment of the optical lens of the autofocus device can be prevented.

[0033] Figure 1 is a plan view of an overlay mark.

[0034] Figure 2 is a side view of the overlay mark illustrated in Figure 1.

[0035] Figure 3 shows a waveform of change in intensity by position of a signal obtained from an image obtained by photographing the overlay mark illustrated in Figure 1.

[0036] FIG. 4 shows a waveform of change in intensity by position of a signal obtained from an image obtained while focusing on the first overlay mark of the overlay mark illustrated in FIG. 1.

[0037] FIG. 5 shows a waveform of change in intensity by position of a signal obtained from an image obtained while focusing on the second overlay mark of the overlay mark illustrated in FIG. 1.

[0038] Figure 6 is a drawing showing changes in the position and size of the illumination light according to the position of the optical lens of the autofocus device.

[0039] FIG. 7 is a conceptual diagram of an overlay measuring device having an auto-focus device according to one embodiment of the present invention.

[0040] Figure 8 is a drawing for explaining the movement of a line beam according to the rotation of a cylinder lens.

[0041] Figure 9 is a flowchart for explaining the operation of the overlay measuring device illustrated in Figure 7.

[0042] Fig. 10 is a drawing for explaining the step of finding the measurement focus position of Fig. 9.

[0043] Fig. 11 is a drawing for explaining the step of finding the reference signal value of Fig. 9.

[0044] Figure 12 is a drawing for explaining the change in focus position according to the position of the optical lens.

[0045] Figure 13 is a drawing for explaining the change in the reference signal value according to the position of the optical lens.

[0046] Figure 14 is a flowchart of a calibration method for an autofocus device.

[0047] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention should not be construed 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. Therefore, the shapes of elements in the drawings are exaggerated to emphasize a clearer description, and elements indicated by the same reference numerals in the drawings represent the same elements.

[0048] Figure 7 is a conceptual diagram of an overlay measuring device equipped with an auto-focus device according to one embodiment of the present invention. The overlay measuring device is a device that measures the error between a first overlay mark and a second overlay mark formed on different layers of a wafer (w).

[0049] For example, as illustrated in FIGS. 1 and 2, the first overlay mark (OM1) may be an overlay mark formed on a previous layer, and the second overlay mark (OM2) may be an overlay mark formed on a current layer. The overlay marks are formed on a scribe lane while simultaneously forming a layer for forming a semiconductor device in a die area. For example, the first overlay mark (OM1) may be formed together with an insulating film pattern, and the second overlay mark (OM2) may be formed together with a photoresist pattern formed on the insulating film pattern.

[0050] In this case, the second overlay mark (OM2) is exposed to the outside, but the first overlay mark (OM1) is covered by a photoresist layer and is made of an oxide having different optical properties from the second overlay mark (OM2) made of a photoresist material. In addition, the first overlay mark (OM1) and the second overlay mark (OM2) have different heights.

[0051] Overlay marks can be used in a variety of forms, including box-in-box (see Figure 1) and AIM (Advanced Imaging Metrology) overlay marks.

[0052] As illustrated in FIG. 7, an overlay measuring device (1) according to one embodiment of the present invention includes an imaging system (10) and a controller (20) communicatively coupled to the imaging system (10).

[0053] The imaging system (10) largely includes an image acquisition device (100) and an autofocus device (200).

[0054] The image acquisition device (100) serves to acquire an image (hereinafter referred to as an “aligned image”) in which a first overlay mark (OM1) and a second overlay mark (OM2) are displayed together at multiple focus positions.

[0055] An image acquisition device (100) comprises an image acquisition light source (111), a first beam splitter (113), an objective lens (115), and a first actuator (117) to irradiate an image acquisition illumination light onto a measurement area of ​​a wafer (W). The measurement area is an area where a first overlay mark (OM1) and a second overlay mark (OM2) are formed in a scribe lane of the wafer (W). A single wafer (W) has a plurality of measurement areas. The overlay measurement device (1) measures an overlay error in the plurality of measurement areas.

[0056] A halogen lamp, a xenon lamp, a light-emitting diode, a supercontinent laser, etc. can be used as a light source (111) for image acquisition. The light source (111) for image acquisition can generate, for example, illumination light for image acquisition in the visible light range.

[0057] The first beam splitter (113) serves to reflect the image acquisition illumination light from the image acquisition light source (111) and guide it toward the objective lens (115). In addition, it transmits the reflected light collected by the objective lens (115).

[0058] The objective lens (115) focuses the beam reflected from the first beam splitter (113) onto the measurement area of ​​the wafer (W) and collects the reflected light reflected from the measurement area. The objective lens (115) is installed on the first actuator (Actuator, 117).

[0059] The first actuator (117) adjusts the distance between the objective lens (115) and the wafer (W) to adjust the focus position of the image acquisition device (100).

[0060] In addition, the image acquisition device (100) is equipped with a hot mirror (121), a tube lens (123), and an image detector (130) to collect reflected light from a measurement area of ​​a wafer (W) to obtain an alignment image.

[0061] The tube lens (123) serves to focus the reflected light passing through the first beam splitter (113) and the hot mirror (121) onto the image detector (130).

[0062] The hot mirror (121) reflects light having a longer wavelength than a reference wavelength and transmits light having a shorter wavelength than the reference wavelength. For example, the hot mirror (121) may reflect infrared light and transmit visible light, or reflect infrared light having a relatively longer wavelength and transmit infrared light having a relatively shorter wavelength. The hot mirror (121) may prevent long-wavelength light used in the height and tilt measurement system (200) from entering the image detector (130).

[0063] The image detector (130) may be a CCD or CMOS camera. The image acquisition device (100) acquires an alignment image using an electrical signal from the image detector (130). By analyzing this alignment image, the overlay error can be measured.

[0064] The autofocus device (200) generates a signal according to the distance between the objective lens (115) and the wafer (W). In other words, it generates a signal according to changes in the focus position of the image acquisition device (100). In addition, it can also use this signal to control the first actuator (117) to focus.

[0065] An autofocus device (200) comprises an autofocus light source (211) for irradiating autofocus illumination light onto a measurement area of ​​a wafer (W), an optical lens (212) for refracting the illumination light from the light source (211), and a second beam splitter (213). In addition, a hot mirror (121), a first beam splitter (113), and an objective lens (115) of an image acquisition device (100) are also used as part of the autofocus device (200).

[0066] A laser diode or a light-emitting diode can be used as the auto-focus light source (211). The auto-focus light source (211) generates illumination light in the infrared region. The illumination light is refracted by an optical lens (212), passes through a second beam splitter (213), and is then reflected by a hot mirror (121). A plano-convex lens can be used as the optical lens (212). When a laser is used as the illumination light, it is preferable to use a polarizing beam splitter as the second beam splitter (213). This is because it can minimize the reduction in the amount of light during reflection and transmission.

[0067] And the illumination light reflected from the hot mirror (121) passes through the first beam splitter (113) and then enters the objective lens (115). The objective lens (115) focuses the illumination light on the measurement area of ​​the wafer (W) and collects the reflected light reflected from the measurement area. The present invention can focus the illumination light for image acquisition and the illumination light for auto-focus using one objective lens (115) and collect the reflected light of these illumination lights.

[0068] In the present invention, infrared light having a different wavelength band from the image acquisition illumination light is used as the auto-focus illumination light, and the reflected light thereof is separated from each other using a hot mirror (121), thereby preventing the reflected light of the auto-focus illumination light from entering the image detector (130).

[0069] The reflected light of the auto-focus illumination light collected from the objective lens (115) passes through the first beam splitter (113) again and is then reflected from the hot mirror (121). Since infrared rays do not pass through the hot mirror (121), the reflected light in the infrared region does not enter the image detector (130).

[0070] The reflected light from the hot mirror (121) is reflected from the second beam splitter (213) toward the detector (230) for autofocus.

[0071] Various autofocus sensor modules can be used as the autofocus detector (230). For example, a phase-difference autofocus sensor module including a beam splitter, a microlens, and a pair of photodiodes can be used. Furthermore, a phase-difference autofocus sensor module including a focusing lens, a chopper wheel, and a bi-cell photodiode can also be used.

[0072] The auto-focus detector (230) receives reflected light and generates a signal according to the distance between the objective lens (115) and the wafer (W). For example, when the focus of the image acquisition unit matches the 'standard focus', the phase difference value, which is a signal value output to the auto-focus detector (230), may be 0, and when they do not match, the phase difference value may be + or - depending on the position of the focus. The 'standard focus' is the focus of the image acquisition device (100) determined based on the signal of the auto-focus detector (230). The 'standard focus' may match the 'measurement focus' suitable for acquiring an actual alignment image, but in most cases, they are different when the height difference between the first overlay mark (OM1) and the second overlay mark (OM2) is large. The method of measuring the 'measurement focus' will be described later.

[0073] The autofocus device (200) further includes a second actuator (217), a cylinder lens (215), and a cylinder lens holder (219).

[0074] The second actuator (217) adjusts the position of the optical lens (212) between the light source (211) for autofocus and the cylinder lens (215). The second actuator (217) moves the optical lens (212) along the optical axis.

[0075] The cylinder lens (215) is placed between the optical lens (212) and the objective lens (115). The cylinder lens (215) may have various external shapes, such as a rectangle, a square, a circle, and an ellipse. The cylinder lens (215) is a lens that focuses light on a line rather than a point. The cylinder lens (215) forms a line beam. Using a line beam has the advantage of increasing sensitivity due to optical aberration (astigmatism), thereby enabling more precise measurements. It is preferable that the line beam is formed so as not to be parallel to the lines forming the overlay mark. In addition, it is preferable that the line beam does not go beyond the overlay mark. This is because if the line beam goes beyond the overlay mark, it may be affected by structures located outside the overlay mark, such as other adjacent overlay marks.

[0076] The cylinder lens holder (219) serves to support the cylinder lens (215). In addition, the cylinder lens holder (219) also serves to rotate the cylinder lens (215) clockwise or counterclockwise about at least one rotation axis that is orthogonal to the optical axis of the cylinder lens (215). The rotation axis may include a first rotation axis and a second rotation axis that is orthogonal to the first rotation axis. When the cylinder lens (215) rotates, the line beam moves within the measurement area.

[0077] FIG. 8 is a drawing for explaining the movement of a line beam according to the rotation of a cylinder lens. Rotation of the cylinder lens (215) about the first rotation axis can move the position of the line beam (LB) illuminating the measurement area within the measurement area, for example, in a direction parallel to the first direction (a, b of FIG. 8). And rotation about the second rotation axis can move the line beam (LB) in a direction parallel to the second direction orthogonal to the first direction (c, d of FIG. 8). Therefore, the position of the line beam (LB) within the measurement area can be adjusted by adjusting the rotation angles of the cylinder lens about the first and second rotation axes.

[0078] The controller (20) is connected to the imaging system (10) in a wired or wireless manner. The controller (20) includes hardware such as a processor and memory, and software installed in the memory. The controller (20) instructs the processor to perform the steps of FIG. 9 through software commands.

[0079] Fig. 9 is a flowchart for explaining the operation of the overlay measuring device illustrated in Fig. 7. First, the step (S1) of receiving aligned images is explained.

[0080] In this step, the controller (20) receives alignment images acquired through the imaging system (10) at multiple focus positions. The alignment images include a first overlay mark (OM1) and a second overlay mark (OM2). The alignment images can be acquired by continuously changing the focus position using the first actuator (117).

[0081] Next, the step (S2) of determining the measurement focus (MF) position is described. The measurement focus position is the focus position used when acquiring a measurement image, which is an alignment image used for measuring overlay errors among multiple alignment images. In other words, it is a focus position optimized for measuring overlay errors.

[0082] The controller (20) can determine the measurement focus position based on the change in the contrast value of the first overlay mark (OM1) of the aligned images according to the focus position and the change in the contrast value of the second overlay mark (OM2).

[0083] The contrast value of the first overlay mark (OM1) and the contrast value of the second overlay mark (OM2) change depending on the focus position (distance between the objective lens and the wafer). Since the first overlay mark (OM1) is located on the previously formed previous layer, the contrast value has a maximum value at the focus position where the distance between the wafer (W) and the objective lens (115) is close. Since the second overlay mark (OM2) is located on the current layer formed on the previous layer, the contrast value has a maximum value at the focus position where the distance between the wafer (W) and the objective lens (115) is far.

[0084] As illustrated in FIG. 10, the measurement focus position can be found, for example, at a position between the maximum value of the graph of change in contrast value (G1) by the first overlay mark (OM1) and the maximum value of the graph of change in contrast value (G2) by the second overlay mark (OM2). For example, the position where the graph of change in contrast value (G1) by the first overlay mark (OM1) and the graph of change in contrast value (G2) by the second overlay mark (OM2) intersect can be determined as the measurement focus position. In addition, the focus position having the average value of the maximum value of the graph of change in contrast value (G1) by the first overlay mark (OM1) and the maximum value of the graph of change in contrast value (G1) by the second overlay mark (OM2) can also be determined as the measurement focus position.

[0085] In some cases, the measurement focus position may be located outside the interval between the maximum value of the graph of change in contrast value (G1) by the first overlay mark (OM1) and the maximum value of the graph of change in contrast value (G2) by the second overlay mark (OM2).

[0086] Next, the step (S3) of obtaining a reference signal value is described.

[0087] As illustrated in Fig. 11, in this step, the controller (20) obtains a reference signal value, which is a signal value from the auto-focus detector (230) at the measurement focus position. If the measurement focus position matches the standard focus position, the phase value, which is a signal value from the auto-focus detector (230), will be 0. If they do not match, it will have a specific phase value, and this value will become the reference signal value.

[0088] Next, the step (S4) of comparing the reference signal value with the optimal reference signal value is described.

[0089] In this step, the reference signal value confirmed in step S3 is compared with the optimal reference signal value. For example, the reference signal value output from the autofocus detector (230) may be a phase difference value, and the optimal reference signal value may be 0.

[0090] Next, a step (S5) for matching the reference signal value and the optimal reference signal value by changing the distance between the optical lens and the light source is described.

[0091] In this step, the second actuator (217) is controlled to adjust the position of the optical lens (212), thereby changing the gap between the optical lens (212) and the light source (211) to match the reference signal value with the optimal reference signal value.

[0092] As illustrated in Fig. 12, when the position of the optical lens (212) is adjusted, the focus position of the auto-focus device (200) changes. Then, the reference signal value obtained from the auto-focus detector (230) also changes.

[0093] When the position of the optical lens (212) is appropriately moved, the reference signal value changes from the reference signal value (Vp) before the movement to the optimal reference signal value (Vo), as shown in Fig. 13. At the same time, the standard focus position also changes from the standard focus position (Pp) before the movement to a new standard focus position (Po) that matches the measured focus position.

[0094] That is, in this step, when the focus position of the image acquisition device (100) is adjusted to the measurement focus position, the gap between the optical lens (212) and the light source (211) is adjusted so that the reference signal value in the auto-focus detector (230) matches the optimal reference signal value. The optimal reference signal value is a signal value belonging to the section in which the auto-focus detector (230) has the highest accuracy. For example, if the auto-focus detector (230) is an auto-focus sensor module of a phase difference type, the reference signal value is 0 or a value close to 0. The accuracy of the auto-focus sensor module of a phase difference type may decrease in a section in which the phase difference is large.

[0095] The predetermined optimal reference signal value may be a specific value, but any value within a certain range may also be set as the reference signal value. For example, if the reference signal value falls within the range of 0±α, which ensures the accuracy of the autofocus detector (230), that value may be used as the reference signal value and the position of the optical lens (212) may not be adjusted.

[0096] If the reference signal value is the same as the predetermined optimal reference signal value as compared in step S4, this step (S5) can be omitted.

[0097] Next, the step (S6) of measuring the overlay error is described.

[0098] In this step, the overlay error is measured based on the measurement image, which is an alignment image acquired at the standard focus position. Since the measurement focus position and the standard focus position are identical through step S5, the overlay error can be measured based on the alignment image acquired at the standard focus position. Since the method of measuring the overlay error using the measurement image is a conventional technique, a detailed description is omitted.

[0099] In other measurement areas, the overlay error can also be measured based on an alignment image acquired at a standard focus position that can be quickly found using an autofocus device (200).

[0100] A pair of first overlay marks (OM1) and second overlay marks (OM2) are formed in multiple measurement areas on a single wafer. Therefore, the overlay measurement device (1) according to the present invention measures overlay errors in multiple measurement areas. The overlay measurement device according to the present invention has the advantage of being fast, particularly when measuring overlay errors in multiple measurement areas.

[0101] In this way, in the present invention, after finding the measurement focus position through the change in the contrast value of the alignment images in the initial measurement area, the position of the optical lens (212) can be adjusted to match the measurement focus position with the standard focus position. In other measurement areas, the alignment image can be acquired at the standard focus position that can be quickly found using the autofocus device (200). Therefore, there is the advantage of faster measurement speed.

[0102] Hereinafter, with reference to FIG. 14, a method for calibrating an autofocus device (200) will be described. Calibration of the autofocus device (200) may be performed prior to measuring an overlay error using the overlay measuring device (1) described above. For example, calibration of the autofocus device (200) may be performed during the manufacturing process of the overlay measuring device (1). Additionally, calibration of the autofocus device (200) may be performed again after using the overlay measuring device (1) for a certain period of time.

[0103] Calibration of the autofocus device (200) means adjusting the rotation angle of the cylinder lens (215) so that the line beam is positioned at the center of the measurement area, even if the position of the optical lens (212) is adjusted in the above-described step S5.

[0104] As illustrated in FIG. 14, the calibration method of the autofocus device (200) includes a step (S11) of checking the alignment error of the line beam according to the distance between the optical lens and the light source, and a step (S12) of adjusting the rotation angle of the cylinder lens with respect to the rotation axis using the cylinder lens holder based on the alignment error so that the line beam is positioned at the center of the measurement area.

[0105] First, the step (S11) of checking the alignment error of the line beam according to the distance between the optical lens and the light source is described.

[0106] In this step, the second actuator (217) is controlled to change the distance between the optical lens (212) and the light source (211), and the alignment error of the line beam according to the distance is checked. For example, the difference between the X, Y coordinate values ​​of the center of the overlay mark formed in the measurement area and the X, Y coordinate values ​​of the center of the line beam can be measured as the alignment error. The alignment error of the line beam can be measured by changing the distance between the optical lens (212) and the light source (211) within a set range.

[0107] Ideally, changing the distance between the optical lens (212) and the light source (211) would not change the center position of the line beam illuminating the measurement area, but only change its size.

[0108] However, as shown in Fig. 6, when the distance between the optical lens (212) and the light source (211) is changed due to various errors in the optical elements of the overlay measuring device (1), the center position of the line beam also changes.

[0109] It is desirable that the center of the line beam coincides with the center of the measurement area, and it is desirable that the line beam does not extend beyond the measurement area.

[0110] The alignment error of the line beam can be measured by acquiring an overlay mark image on which the line beam is displayed using an image detector (130) and then analyzing the image. For example, the difference between the center of the overlay mark and the center of the line beam can be measured as the alignment error.

[0111] When acquiring an overlay mark image with a line beam displayed, the light source (111) for image acquisition is turned off or covered, and the light source (211) for auto focus is used to illuminate the overlay mark. Then, the hot mirror (121) is removed. When the hot mirror (121) is installed, the reflected light of the illumination light from the light source (211) for auto focus is blocked by the hot mirror (121) and does not reach the image detector (130).

[0112] Next, a step (S12) of adjusting the rotation angle of the cylinder lens about the rotation axis using the cylinder lens holder based on the alignment error so that the line beam is positioned at the center of the measurement area is described.

[0113] In this step, the rotation angle of the cylinder lens (215) is adjusted to offset errors caused by the optical elements constituting the autofocus device (200).

[0114] The rotation axis may be a rotation axis orthogonal to the optical axis of the cylinder lens (215). The rotation axis may include a first rotation axis and a second rotation axis orthogonal to the first rotation axis.

[0115] As illustrated in Fig. 8, when the cylinder lens (215) is rotated clockwise about the first rotation axis, the line beam (LB) can move to the left (a in Fig. 8). And when the cylinder lens (215) is rotated counterclockwise about the first rotation axis, the line beam (LB) can move to the right (a in Fig. 8).

[0116] Additionally, when the cylinder lens (215) is rotated clockwise about the second rotation axis, the line beam (LB) can move downward (c in FIG. 8). And when the cylinder lens (215) is rotated counterclockwise about the second rotation axis, the line beam (LB) can move upward (d in FIG. 8).

[0117] The direction of movement of the line beam (LB) according to the rotation of the rotation axis may vary depending on the arrangement of other optical elements such as a beam splitter.

[0118] By appropriately adjusting the rotation angle of the cylinder lens (215) with respect to the first and second rotation axes, the line beam (LB) and the overlay mark can be aligned regardless of the distance between the optical lens (212) and the light source (211) within a set range.

[0119] In this way, when the step (S5) of matching the reference signal value and the optimal reference signal value by changing the distance between the optical lens (212) and the light source (211) described above while the auto focus device (200) is corrected is performed, the problem of the line beam (LB) and the overlay mark being misaligned does not occur during the process.

[0120] The embodiments described above merely describe preferred embodiments of the present invention, and the scope of the present invention is not limited to the described embodiments, and various changes, modifications, or substitutions may be made by those skilled in the art within the technical spirit and scope of the claims of the present invention, and it should be understood that such embodiments fall within the scope of the present invention.

[0121] [Explanation of symbols]

[0122] OM1: First overlay mark

[0123] OM2: Second overlay mark

[0124] 1: Overlay measuring device

[0125] 20: Controller

[0126] 100: Image acquisition device

[0127] 111: First light source

[0128] 115: Objective lens

[0129] 117: First actuator

[0130] 121: Hot Mirror

[0131] 130: Detector 1

[0132] 200: Autofocus device

[0133] 211: Second Light Source

[0134] 212: Optical lens

[0135] 215: Cylinder lens

[0136] 219: Cylinder lens holder

[0137] 217: Second actuator

[0138] 230: Second detector

Claims

1. A light source configured to emit illumination light for automatic focus adjustment, An optical lens arranged on the path of the illumination light emitted from the light source and configured to refract the illumination light; A cylinder lens arranged on the path of the illumination light passing through the optical lens and configured to change the illumination light into a line beam; An objective lens configured to focus the above line beam on a measurement area and collect reflected light reflected from the measurement area, A detector configured to receive the reflected light and generate a signal according to the focus position of the reflected light; An actuator configured to move the optical lens along the optical axis, An autofocus device comprising a cylinder lens holder configured to rotate the cylinder lens about at least one rotational axis orthogonal to an optical axis of the cylinder lens.

2. In paragraph 1, An autofocus device wherein the optical lens is a plano-convex lens.

3. In paragraph 1, An autofocus device wherein the cylinder lens holder is configured to rotate the cylinder lens about a first rotation axis orthogonal to an optical axis of the cylinder lens and a second rotation axis orthogonal to the first rotation axis.

4. In paragraph 1, An autofocus device further comprising an actuator for adjusting the distance between the measurement area and the objective lens.

5. In paragraph 1, An overlay mark is formed in the above measurement area, An auto-focus device in which the above line beam is adjusted so as not to go beyond the overlay mark formed in the above measurement area.

6. A method for correcting the auto focus device of paragraph 1, A step of checking the alignment error of the line beam according to the distance between the optical lens and the light source, A calibration method for an autofocus device, comprising the step of adjusting a rotation angle of the cylinder lens about the rotation axis based on the alignment error so that the line beam is positioned at the center of the measurement area.

7. In paragraph 6, An overlay mark is formed in the above measurement area, The step of checking the alignment error of the above line beam is: A method for calibrating an autofocus device, the method comprising the steps of: acquiring an image in which the line beam and the overlay mark are displayed together, and checking an error between the center of the overlay mark and the center of the line beam.

8. An overlay measuring device for measuring an error between a pair of first overlay marks and second overlay marks formed on different layers formed on a wafer, An image acquisition device configured to acquire alignment images of a first overlay mark and a second overlay mark formed in pairs at a plurality of focus positions, and an auto-focus device for adjusting a focus of the image acquisition device, The above auto-focus device is an overlay measuring device which is an auto-focus device of the first clause.

Images