Overlay Measuring Equipment

Abstract

The present invention relates to an overlay measurement apparatus. More particularly, the present invention relates to an overlay measurement apparatus capable of quickly measuring an overlay error between layers having a large height difference. The present invention provides an overlay measurement apparatus for measuring an error between a pair of first and second overlay marks formed on different layers of a wafer, the overlay measurement apparatus including an imaging system configured to acquire aligned images of the pair of the first and second overlay marks at a plurality of focal positions, and a controller communicatively coupled to the imaging system. The overlay measurement apparatus according to the present invention has an advantage that it can quickly and accurately measure an overlay error between layers having a large height difference.

Description

[Technical field]

[0001] The present invention relates to an overlay measurement apparatus, and more particularly to an overlay measurement apparatus capable of quickly measuring an overlay error between layers having a large height difference. [Background technology]

[0002] As technology advances, the size of semiconductor devices decreases, and the density of integrated circuits needs to increase. To meet these requirements, various conditions must be satisfied, among which overlay tolerance is one of the important indicators. Semiconductor devices are manufactured through a number of manufacturing processes. In order to form an integrated circuit on a wafer, many manufacturing processes must be performed to sequentially form the desired circuit structures and elements at specific locations. The manufacturing processes sequentially create patterned layers on the wafer. These repeated stacking steps create electrically active patterns in the integrated circuit. If the structures are not aligned within the tolerances allowed by the production process, interference will occur between the electrically active patterns, which can cause problems with the performance and reliability of the manufactured circuit. Overlay metrology tools are used to measure and verify the alignment errors between these layers. Common overlay measurement methods measure and verify whether the alignment between two layers is within a tolerance. One method is to form a structure called an overlay mark at a specific position on a substrate and measure the overlay by photographing this structure with an optical image acquisition device. The measurement structure is designed to measure the overlay in at least one direction, the X direction or the Y direction, for each layer. Each structure is designed as a symmetrical structure, and the center value between the structures arranged in the symmetrical direction is calculated and used as the representative value of the layer, and the relative difference between the representative values ​​of each layer is calculated to derive the overlay error. A method for measuring the overlay of two layers using a box-in-box (BIB) overlay mark is briefly described below. First, as shown in FIG. 1 and FIG. 2, a first overlay mark OM1 having a generally box shape and a second overlay mark OM2 having a generally box shape smaller than the first overlay mark OM1 are formed on two consecutive layers, respectively. Then, images of the first overlay mark OM1 and the second overlay mark OM2 are acquired at the same time. Next, as shown in FIG. 3, a waveform showing the change in intensity with respect to position is acquired to obtain the center value C1 of the first overlay mark OM1 and the center value C2 of the second overlay mark OM2, respectively. Then, by comparing these, the overlay error between the two layers is measured. When the height difference between the two layers is not large, the focus can be automatically adjusted using an autofocus sensor, and then a combined image of the first overlay mark OM1 and the second overlay mark OM2 can be acquired at once. The focus detected by the autofocus sensor may not be the optimal focus for acquiring the combined image, but this is not a big problem when the height difference between the two layers is not large. However, with the development of semiconductor process technology, it is now necessary to accurately measure the overlay error between layers having large height differences and different optical properties, and there is a problem that there may be a large difference between the standard focus acquired using an autofocus sensor and the measure focus for more accurate measurement. For example, the standard focus of the autofocus sensor is located on the top of the current layer, but the measure focus may be located between the previous layer and the current layer. In this case, if an image acquired at the standard focus is used, the first overlay mark formed on the previous layer is not clear in the alignment image, and therefore the overlay error cannot be accurately measured. To solve this problem, a method was used to measure the overlay error between two layers by obtaining a waveform showing the change in intensity of a signal with respect to position, obtained by using the first overlay mark OM1 as a focal plane, to obtain a center value C1 of the first overlay mark OM1, and obtaining a waveform showing the change in intensity of a signal with respect to position, obtained by using the second overlay mark OM2 as a focal plane, to obtain a center value C2 of the second overlay mark OM2, as shown in Figures 4 and 5. However, this method had the problem of long inspection time because it required taking two images each time to measure the overlay error. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Korean Patent Publication No. 2003-0054781 [Patent Document 2] Korean Patent No. 10-0689709 [Patent Document 3] Korean Patent No. 10-1564312 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention has been made to improve the above-mentioned problems, and has an object to provide a new overlay measurement apparatus that can quickly and accurately measure overlay errors between layers having a large height difference. [Means for solving the problem]

[0005] In order to achieve the above object, the present invention provides an apparatus for measuring an error between a pair of a first overlay mark and a second overlay mark formed on different layers of a wafer, the overlay measurement apparatus including: an imaging system configured to acquire aligned images of the pair of the first overlay mark and the second overlay mark at a plurality of focus positions; and a controller communicatively coupled to the imaging system. Here, the imaging system includes a first light source emitting a first illumination light for acquiring the alignment image, a second light source emitting a second illumination light, an objective lens for focusing the first illumination light and the second illumination light on a measurement area of ​​the wafer and collecting the first reflected light and the second reflected light reflected from the measurement area of ​​the wafer, a first detector for collecting the first reflected light emitted from the first light source and reflected from the measurement area of ​​the wafer to acquire the alignment image, a second detector for acquiring a signal according to the focal position using the second reflected light emitted from the second light source and reflected from the measurement area of ​​the wafer, and a first actuator for adjusting the distance between the measurement area of ​​the wafer and the objective lens to adjust the focal position. The controller includes a processor configured to execute instructions that cause the processor to perform the following steps: a) receiving alignment images acquired via an imaging system at a plurality of focus positions in a measurement area; b) determining a measure focus (MF) position based on a change in contrast value of the first overlay mark and a change in contrast value of the second overlay mark in the alignment images; c) acquiring a reference signal value that is a signal value from the second detector at the measure focus position; and d) measuring an overlay error based on the measurement images, which are alignment images acquired at the measure focus positions. In addition, the present invention provides an overlay measurement apparatus, wherein the controller controls the processor to perform the steps of: e) receiving a signal value acquired from the second detector in a measurement area different from the measurement area of ​​step a); f) comparing the signal value received in step e) with the reference signal value; g) controlling the first actuator to change a focus position if the signal value acquired from the second detector is different from the reference signal value; h) receiving an alignment image acquired via the imaging system if the signal value acquired from the second detector matches the reference signal value; and i) measuring an overlay error based on a measurement image, which is the alignment image received in step h). The present invention also provides an overlay measurement apparatus, wherein the imaging system further includes an optical lens disposed between the second light source and the objective lens and refracting the second illumination light emitted from the second light source, and a second actuator configured to adjust a position of the optical lens between the second light source and the objective lens, and the controller further controls the processor to, after step c), perform a step of comparing the reference signal value with a predetermined reference signal value, and, if the reference signal value is different from the predetermined reference signal value, control the second actuator to adjust the position of the optical lens to make the reference signal value match the predetermined reference signal value. The present invention also provides an overlay measurement apparatus, wherein the optical lens is a convex lens. The present invention also provides an overlay measurement apparatus, wherein step b) is a step of finding a measurement focus position from a position between the maximum value of a graph of change in contrast value due to the first overlay mark and the maximum value of a graph of change in contrast value due to the second overlay mark. The present invention also provides an overlay measurement apparatus, wherein step b) is a step of finding, as a measurement focus position, a position where a graph of change in contrast value due to the first overlay mark and a graph of change in contrast value due to the second overlay mark intersect. The present invention also provides an overlay measurement apparatus, wherein the second detector is an autofocus sensor module of a phase difference type, and the predetermined reference signal value is a value at which a phase difference measured by the autofocus sensor module is minimized. The present invention also provides an overlay measurement device, further including a hot mirror disposed between the second light source and the objective lens, for reflecting, toward the objective lens, light having a long wavelength among the second illumination light emitted from the second light source. Also, the present invention provides an overlay measurement apparatus, further comprising a cylinder lens for changing the second illumination light emitted from the second light source into a line beam. Effect of the Invention

[0006] The overlay measurement apparatus according to the present invention has the advantage that it can quickly and accurately measure overlay errors between layers having large height differences. [Brief description of the drawings]

[0007] [Figure 1] FIG. 2 is a plan view of an overlay mark. [Diagram 2] FIG. 2 is a side view of the overlay mark shown in FIG. [Diagram 3] 2 shows a waveform of change in intensity of a signal according to position obtained from the first overlay mark image shown in FIG. 1. [Figure 4] 2 shows a waveform of change in intensity of a signal according to position acquired with the first overlay mark shown in FIG. 1 as a focal plane. [Diagram 5] 2 shows a waveform of change in intensity of a signal according to position acquired with the second overlay mark shown in FIG. 1 as a focal plane. [Figure 6] 1 is a conceptual diagram of an overlay measurement apparatus according to an embodiment of the present invention; [Figure 7] 7 is a flowchart for explaining the operation of the overlay measurement apparatus shown in FIG. 6. [Figure 8] FIG. 8 is a diagram for explaining the step of finding the measurement focus position in FIG. 7; [Figure 9]FIG. 8 is a diagram for explaining the step of finding the reference signal value in FIG. 7; [Figure 10] 13 is a conceptual diagram of an overlay measurement apparatus according to another embodiment of the present invention. [Figure 11] 11 is a diagram for explaining a change in focal position depending on the position of an optical lens; FIG. [Figure 12] 11 is a diagram for explaining a change in a reference signal value depending on the position of an optical lens. FIG. [Figure 13] 11 is a partial flowchart for explaining the operation of the overlay measurement apparatus shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the embodiment of the present invention can be modified into various other forms, and the scope of the present invention should not be interpreted as being limited to the embodiment described below. The embodiment of the present invention is provided to more completely explain the present invention to those skilled in the art. Therefore, the shapes of elements in the drawings are exaggerated to emphasize a clearer description, and elements represented by the same reference numerals in the drawings refer to the same elements. 6 is a conceptual diagram of an overlay measurement apparatus according to an embodiment of the present invention, which measures an error between a first overlay mark and a second overlay mark formed on different layers of a wafer w. For example, as shown in Figures 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 in a scribe lane at the same time as forming a layer for forming a semiconductor device in a die region. 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. In this case, the second overlay mark OM2 is exposed to the outside, but the first overlay mark OM1 is covered with a photoresist layer and is made of an oxide having different optical properties from the second overlay mark OM2 made of a photoresist material, and the first overlay mark OM1 and the second overlay mark OM2 have different heights. As the overlay mark, various types of overlay marks currently in use, such as a box-in-box (see FIG. 1) or an AIM (advanced imaging metrology) overlay mark, can be used. As shown in FIG. 6, an overlay measurement apparatus 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 . The imaging system 10 broadly includes an image acquisition section 100 and an autofocus section 200 . The image acquiring unit 100 serves to acquire images (hereinafter, referred to as "alignment images") in which the first overlay mark OM1 and the second overlay mark OM2 are displayed together at a plurality of focal positions. The image acquisition unit 100 includes a first light source 111, a first beam splitter 113, an objective lens 115, and a first actuator 117 to irradiate a measurement area of ​​the wafer W with a first illumination light. The measurement area is an area in which a first overlay mark OM1 and a second overlay mark OM2 are formed in a scribe lane of the wafer W. One wafer W has a plurality of measurement areas. The overlay measurement apparatus 1 measures the overlay error in the plurality of measurement areas. A halogen lamp, a xenon lamp, a light emitting diode, etc. can be used as the first light source 111. The first light source 111 generates a first illumination light in the visible light region. The first beam splitter 113 serves to reflect a part of the first illumination light emitted from the first light source 111 and guide it to the objective lens 115. In addition, the first beam splitter 113 transmits a part of the first reflected light collected by the objective lens 115. The objective lens 115 serves to focus the beam reflected from the first beam splitter 113 onto a measurement area of ​​the wafer W and to collect the beam reflected at the measurement area. The objective lens 115 is mounted on a first actuator 117. The first actuator 117 serves to adjust the distance between the objective lens 115 and the wafer W to adjust the focal position of the image acquisition unit 100 . The image acquisition unit 100 also includes a hot mirror 121, a tube lens 123, and a first detector 130 to collect the first reflected light from the measurement area of ​​the wafer W to obtain an alignment image. The tube lens 123 serves to focus the first reflected light that has passed through the first beam splitter 113 and the hot mirror 121 onto the first detector 130 . The hot mirror 121 reflects ultraviolet light and transmits visible light. The hot mirror 121 prevents ultraviolet light from entering the first detector 130. The first detector 130 can be a CCD or CMOS camera. The image acquisition unit 100 acquires an alignment image using the electrical signal from the first detector 130. The alignment image can then be analyzed to measure the overlay error. The auto focus unit 200 serves to generate a signal in response to the distance between the objective lens 115 and the wafer W. That is, the auto focus unit 200 serves to generate a signal in response to a change in the focus position of the image acquisition unit 100. The autofocus unit 200 includes a second light source 211, an optical lens 212 for refracting the second illumination light from the second light source 211, and a second beam splitter 213 to irradiate the second illumination light onto a measurement area of ​​the wafer W. The autofocus unit 200 also uses the hot mirror 121, the first beam splitter 113, and the objective lens 115 of the image acquisition unit 100. The second light source 211 may be a laser diode or a light emitting diode. The second light source 211 generates a second illumination light in the infrared region. The second illumination light is refracted by the optical lens 212, passes through the second beam splitter 213, and is then reflected from the hot mirror 121. A convex lens may be used as the optical lens 212. When a laser is used as the second illumination light, it is preferable to use a polarizing beam splitter as the second beam splitter 213. This is because the reduction in the amount of light during the reflection and transmission processes can be minimized. Then, the second 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 serves to focus the second illumination light on the measurement area of ​​the wafer W and collect the second reflected light reflected from the measurement area. The present invention has an advantage in that the first illumination light and the second illumination light are focused using one objective lens 115 and the first reflected light and the second reflected light are collected. In the present invention, infrared light having a different wavelength band from that of the first illumination light is used as the second illumination light, and the second reflected light is separated from the first reflected light using a hot mirror 121, thereby preventing the second reflected light from entering the first detector 130. The second reflected light collected by the objective lens 115 passes through the first beam splitter 113 again and is then reflected by the hot mirror 121. Since infrared rays do not pass through the hot mirror 121, the second reflected light in the infrared region does not enter the first detector 130. The second reflected light reflected by the hot mirror 121 is reflected from the second beam splitter 213 to the second detector 230 . The second detector 230 may be a conventional high-speed phase difference type autofocus sensor module. For example, a phase difference type autofocus sensor module including a beam splitter, a microlens, and a pair of photodiodes may be used. The second detector 230 plays a role of generating 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 coincides with the "standard focus", the phase difference value, which is the signal value output to the second detector 230, is 0, and when it does not coincide, it may be a + or - value depending on the position of the focus. The "standard focus" is the focus of the image acquisition unit determined based on the signal of the second detector 230. The "standard focus" may coincide with a "measurement focus" suitable for acquiring an actual alignment image, but when the height difference between the first overlay mark OM1 and the second overlay mark OM2 is large, they are almost different from each other. A method of measuring the "measurement focus" will be described later. The controller 20 is communicatively coupled to the imaging system 10 via wire or wireless communication. The controller 20 includes hardware such as a processor and a memory, and software installed in the memory. The controller 20 instructs the processor to perform the steps of FIG. 7 through the instructions of the software. Fig. 7 is a flow chart for explaining the operation of the overlay measurement apparatus shown in Fig. 6. First, step S1 of receiving an alignment image will be described. In this step, the controller 20 receives alignment images acquired via the imaging system 10 at multiple focus positions. The alignment images include the first overlay mark OM1 and the second overlay mark OM2. The alignment images can be acquired while continuously changing the focus position using the first actuator 117. Next, step S2 of determining a measurement focus (MF) position will be described. The measurement focus position is a focus position used when acquiring a measurement image, which is an alignment image used for measuring an overlay error, among a plurality of alignment images. In other words, it is a focus position optimized for measuring an overlay error. The controller 20 can determine a measured focus position based on the change in contrast value of the first overlay mark OM1 and the change in contrast value of the second overlay mark OM2 of the alignment image as a function of the focus position. 8, the contrast value of the first overlay mark OM1 and the contrast value of the second overlay mark OM2 change depending on the focal position (the distance between the objective lens and the wafer). Since the first overlay mark OM1 is located on the previous layer formed earlier, its contrast value has a maximum value at the focal 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, its contrast value has a maximum value at the focal position where the distance between the wafer W and the objective lens 115 is far. The measurement focus position can be found, for example, from the position between the maximum value of the contrast value change graph based on the first overlay mark OM1 and the maximum value of the contrast value change graph based on the second overlay mark OM2. For example, the position where the contrast value change graph based on the first overlay mark OM1 and the contrast value change graph based on the second overlay mark OM2 intersect can be determined as the measurement focus position. Also, the average value of the maximum value of the contrast value change graph based on the first overlay mark OM1 and the contrast value change graph based on the second overlay mark OM2 can be determined as the measurement focus position. In some cases, the measured focus position may be located outside the interval between the maximum of the contrast value variation graph due to the first overlay mark OM1 and the maximum of the contrast value variation graph due to the second overlay mark OM2. Next, step S3 of acquiring the reference signal value will be described. 9, in this step, the controller 20 obtains a reference signal value, which is a signal value from the second detector 230 at the measured focal position. If the measured focal position coincides with the standard focal position, the phase value, which is the signal value from the second detector 230, becomes 0. If the measured focal position does not coincide with the standard focal position, it has a specific phase value, which becomes the reference signal value. Next, step S4 of measuring the overlay error will be described. In this step, the overlay error is measured based on a measurement image, which is an alignment image acquired at the measurement focal position. A method of measuring the overlay error using the measurement image is a conventional technique, and therefore a detailed description thereof will be omitted. Next, step S5 of receiving a signal value acquired from a second detector in another measurement region will be described. Pairs of a first overlay mark OM1 and a second overlay mark OM2 are formed in multiple measurement areas on one wafer. Thus, the overlay measurement apparatus according to the present invention measures the overlay error in multiple measurement areas. The overlay measurement apparatus according to the present invention has an advantage that it is fast, particularly when measuring the overlay error in multiple measurement areas. At this stage, the controller 20 receives signal values ​​obtained from the second detector 230 in a measurement area different from the first measurement area in which the reference signal values ​​were obtained. Next, step S6 of comparing the signal value received in step S5 with a reference signal value will be described. In this step, the controller 20 compares the signal value received in step S5 with a reference signal value. Ideally, the signal value received in step S5 and the reference signal value would match, but since there is variation depending on the position of the wafer W, the received signal value and the reference signal value may differ from each other. Next, step S7 of changing the focal position will be described. In this step, the controller 20 controls the first actuator to change the focal position so that the signal value acquired from the second detector 230 coincides with the reference signal value. In this step, the focal position in the current measurement area, which is different from the initial measurement area, is changed to a measurement focal position, taking into account the variation in each position of the wafer W. In this manner, the present invention has the advantage that the measurement focus position is found only once in the first measurement area through a change in the contrast value of the alignment image, and then in other measurement areas, the measurement focus position can be quickly found using the signal value from the second detector 230. Next, step S8 of receiving the aligned images will be described. In this step, the controller 20 controls the imaging system 10 to acquire an alignment image, and receives the acquired alignment image. Next, a description will be given of step S9 for measuring an overlay error based on the measurement image, which is the aligned image received in step S8. In this step, similarly to step S4, an overlay error is measured based on the measurement image. The above-mentioned steps S5 to S8 are repeatedly performed for each measurement region. In the first measurement area, a number of alignment images are obtained according to the focus position, and the measurement focus position is determined based on the change in contrast value of the first overlay mark OM1 and the change in contrast value of the second overlay mark OM2 in these alignment images, so that the measurement of the overlay error takes a long time. However, in other measurement regions, the signal value acquired from the second detector 230 can be compared with the reference signal value to control the first actuator 117, thereby quickly moving the focus to the measurement focal position, and therefore the overlay error can be quickly measured. Thus, the overlay error can be quickly measured in a plurality of measurement regions. FIG. 10 is a conceptual diagram showing an overlay measurement apparatus according to another embodiment of the present invention. The imaging system 30 of the overlay measurement apparatus 2 shown in FIG. 10 differs from the imaging system 10 of the embodiment shown in FIG. 6 only in that the autofocus unit 300 further includes a second actuator 217 and a cylinder lens 215. The second actuator 217 adjusts the position of the optical lens 212 between the second light source 211 and the objective lens 115. When the position of the optical lens 212 is adjusted, the focus position of the auto focus unit 300 is changed, as shown in Fig. 11. This causes the signal value obtained from the second detector 230 to change, which causes the reference signal value to change, as shown in Fig. 12. The cylinder lens 215 is disposed between the optical lens 212 and the objective lens 115. The cylinder lens 215 may have various shapes, such as a rectangle, a square, a circle, or an ellipse. The cylinder lens 215 is a lens that focuses light to a line, not a point. The cylinder lens 215 serves to form a line beam. The use of a line beam has the advantage that it allows for more precise measurements due to increased sensitivity to optical aberrations. The controller 40 differs from the operation of the overlay measurement apparatus shown in FIG. 7 in that it causes the processor to perform the following additional steps shown in FIG. 13 between steps S3 and S5: First, among the further steps, step S11 of comparing the reference signal value with a predetermined reference signal value will be described. In this step, the reference signal value determined in step S3 is compared with a predetermined reference signal value. For example, the reference signal value output from the second detector 230 may be a phase difference value, and the predetermined reference signal value may be 0. If the reference signal value is the same as the predetermined reference signal value, proceed to step S4. If step S11 is performed after step S4, proceed to step S5. Figure 13 shows that step S11 is performed between steps S3 and S4. If the reference signal value is different from the predetermined reference signal value, step S12 is further performed in which the second actuator 217 is controlled to adjust the position of the optical lens 212 to make the reference signal value coincide with the predetermined reference signal value. That is, in this step, the position of the optical lens 212 is adjusted so that the reference signal value in the second detector 230 coincides with the predetermined reference signal value when the focal position of the image acquisition unit 100 is adjusted to the measurement focal position. The predetermined reference signal value is a signal value that belongs to the section in which the second detector 230 has the highest accuracy. For example, if the second detector 230 is a phase difference type autofocus sensor module, the reference signal value is 0 or a value adjacent to 0. Since the accuracy of the phase difference type autofocus sensor module decreases in a section in which the phase difference is large, when the focal position of the image acquisition unit 100 is adjusted to the measurement focal position based on the signal value of the second detector 230 in a section in which the phase difference is large, an error may occur, so this embodiment is advantageous over the embodiments of FIGS. 6 and 7. The predetermined reference signal value may be a specific value, or any value within a certain range may be set as the reference signal value. For example, when the reference signal value falls within the range of 0±α where the accuracy of the second detector 230 is ensured, the value may be set as the reference signal value and the position of the optical lens may not need to be adjusted. After step S12, step S4 (or step S5) is performed. The subsequent steps are the same as those shown in FIG. The embodiments described above are merely preferred embodiments of the present invention, and the scope of the present invention is not limited to the described embodiments. Various modifications, variations, 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 also fall within the scope of the present invention. [Explanation of symbols]

[0009] OM1 1st overlay mark OM2 2nd overlay mark 1, 2 Overlay measurement device 10, 30 Imaging System 20, 40 Controller 100 Image acquisition unit 111 1st light source 115 Objective Lens 117 First Actuator 121 Hot Mirror 130 Detector No. 1 200, 300 auto focus section 211 Second light source 212 Optical Lenses 215 Cylinder Lens 217 Second Actuator 230 Second Detector

Claims

1. 1. An apparatus for measuring an error between a pair of first and second overlay marks formed on different layers of a wafer, the apparatus comprising: an imaging system configured to acquire aligned images of a pair of first and second overlay marks at a plurality of focus positions; and a controller communicatively coupled to the imaging system; The imaging system includes: a first light source that emits a first illumination light for acquiring the alignment image; a second light source that emits a second illumination light; an objective lens that focuses the first illumination light and the second illumination light on a measurement area of ​​the wafer and collects first reflected light and second reflected light reflected at the measurement area of ​​the wafer; a first detector that collects the first reflected light emitted from the first light source and reflected at a measurement area of ​​the wafer to obtain the alignment image; a second detector that obtains a signal according to the focal position by the second reflected light emitted from the second light source and reflected at a measurement area of ​​the wafer; a first actuator that adjusts a distance between a measurement area of ​​the wafer and the objective lens so as to adjust the focus position; The controller includes a processor configured to execute instructions, the instructions causing the processor to: a) receiving alignment images acquired via the imaging system at a plurality of focal positions in a measurement region; b) determining a measure focus (MF) position based on the change in contrast value of the first overlay mark and the change in contrast value of the second overlay mark of the alignment image; c) acquiring a reference signal value, which is a signal value from the second detector at the measurement focus position; d) measuring an overlay error based on a measurement image, which is an alignment image acquired at the measurement focus position.

2. The controller further comprises: e) receiving signal values ​​acquired from the second detector at a measurement area different from the measurement area of ​​step a); f) comparing the signal values ​​received in step e) with the reference signal values; g) controlling the first actuator to change a focal position if a signal value obtained from the second detector differs from the reference signal value; h) receiving an alignment image acquired via the imaging system if the signal value acquired from the second detector matches the reference signal value; 2. The overlay measurement apparatus of claim 1, further comprising: i) measuring an overlay error based on the measurement image, which is the alignment image received in step h).

3. The imaging system includes: an optical lens disposed between the second light source and the objective lens and configured to refract the second illumination light emitted from the second light source; a second actuator for adjusting a position of the optical lens between the second light source and the objective lens; The controller further comprises: After step c), comparing said reference signal value with a predetermined reference signal value; 2. The overlay measurement apparatus of claim 1, further comprising the step of: if the reference signal value is different from the predetermined reference signal value, controlling the second actuator to adjust the position of the optical lens so that the reference signal value coincides with the predetermined reference signal value.

4. The overlay measurement apparatus of claim 3 , wherein the optical lens is a convex lens.

5. The step b) comprises:

2. The overlay measurement apparatus of claim 1, further comprising a step of finding a measurement focus position from a position between a maximum value of a graph of change in contrast value due to the first overlay mark and a maximum value of a graph of change in contrast value due to the second overlay mark.

6. The step b) comprises:

6. The overlay measurement apparatus according to claim 5, further comprising a step of finding a position where a graph of change in contrast value due to the first overlay mark and a graph of change in contrast value due to the second overlay mark intersect as a measurement focus position.

7. the second detector is a phase difference type auto focus sensor module; 4. The overlay measurement apparatus of claim 3, wherein the predetermined reference signal value is a value at which a phase difference measured by the autofocus sensor module is minimized.

8. 2. The overlay measurement apparatus of claim 1, further comprising a hot mirror disposed between the second light source and the objective lens, the hot mirror reflecting light having a long wavelength among the second illumination light emitted from the second light source toward the objective lens.

9. The overlay measurement apparatus of claim 1 , further comprising a cylinder lens for changing the second illumination light emitted from the second light source into a line beam.

Images