Image acquisition device, image acquisition method, and image acquisition device control program

Abstract

The present invention relates to an image acquisition apparatus and method, and more particularly to an image acquisition apparatus and method for acquiring images of multiple marks while moving a stage on which a sample with multiple marks formed on it is placed relative to an imaging optical system. The present invention provides an image acquisition device configured to acquire images of marks from a sample on which a plurality of marks are formed, comprising: a stage on which the sample is placed; an illumination optical system configured to illuminate the marks; an image detector configured to receive reflected light from the marks due to the illumination and generate a mark image; an imaging optical system configured to image the reflected light from the marks onto the image detector; an autofocus optical system configured to detect Z-direction position information of the surface of the sample while moving at least one of the stage and the imaging optical system on the XY plane and changing the position of the observation area of ​​the imaging optical system from a previous measurement position to the current measurement position of the sample; and a focus control unit configured to move the objective lens of the imaging optical system in the Z-direction based on the Z-direction position information while changing the position of the observation area of ​​the imaging optical system from a previous measurement position to the current measurement position of the sample, and to bring the distance between the surface of the sample and the objective lens closer to a reference distance which is the distance when the marks are in focus.

Description

Technical Field

[0001] The present invention relates to an image acquisition device and method, and more particularly to an image acquisition device and method for acquiring images of a plurality of marks while relatively moving a stage on which a sample with a plurality of marks formed thereon with respect to an imaging optical system.

Background Art

[0002] On a semiconductor substrate, a plurality of pattern layers are sequentially formed. Also, through double patterning or the like, a circuit of one layer may be formed by being divided into two patterns. If these pattern layers or a plurality of patterns of one layer are accurately formed at predetermined positions, a desired semiconductor element can be manufactured.

[0003] Therefore, in order to confirm whether the pattern layers are accurately aligned, an overlay mark formed simultaneously with the pattern layers is used.

[0004] The method of measuring an overlay using an overlay mark is as follows. First, on a pattern layer formed in a previous process, for example, an etching process, one structure that is part of the overlay mark is formed simultaneously with the formation of the pattern layer. Then, in a subsequent process, for example, a photolithography process, the remaining structures of the overlay mark are formed on the photoresist.

[0005] Also, through an overlay measurement device, images of the overlay structure of the pattern layer formed in the previous process (image acquisition through the photoresist layer) and the overlay structure of the photoresist layer are acquired, and the offset value between the centers of these images is measured to measure the overlay error.

[0006] More specifically, Japanese Patent Publication No. 2020-112807 discloses a method for determining relative shifts between different layers or different patterns by capturing an image of an overlay mark formed on a substrate, selecting a plurality of working zones from the captured image, forming an informational signal for each of the selected working zones, and comparing these signals.

[0007] Figure 1 is a plan view of an example of an overlay mark. The overlay mark 1 shown in Figure 1 comprises four working zone sets 4, 5, 6, and 7. Each working zone set 4, 5, 6, and 7 comprises two working zones arranged diagonally to each other. Each working zone set 4, 5, 6, and 7 is used to measure the overlay error in the X-axis or Y-axis direction of the pattern layer formed together with the working zone set. To prevent interference phenomena, structure 2 formed with the first pattern layer and structure 3 formed with the second pattern layer are arranged so as not to overlap each other.

[0008] Each working zone includes bars positioned at regular intervals from the center of overlay mark 1 to the outer edge of overlay mark 1. Therefore, using the overlay measurement device, periodic signals as shown in Figure 2 can be obtained from two working zones belonging to working zone sets 4, 5, 6, and 7, respectively. The graph in Figure 2 can be obtained, for example, from a portion of region 8 selected in Figure 1.

[0009] In the graph of Figure 2, the peaks are shown in the areas where the bars are placed. Since the conventional overlay mark 1 has periodically placed bars, the acquired signal also exhibits periodicity. The overlay is then measured through correlation analysis of two periodic signals acquired from two selected regions 8 and 8'.

[0010] Figure 3 shows measurement sites on a semiconductor wafer as dots. Each dot represents an overlay mark. As shown in Figure 3, numerous overlay marks are formed on a single semiconductor wafer. Overlay marks are formed in the scribe lanes of the semiconductor wafer.

[0011] The overlay measurement device moves an XY stage on which a semiconductor wafer is fixed, positioning the overlay mark to be measured below the observation area of ​​the imaging optical system of the overlay measurement device. Then, using an autofocus optical system, it focuses on the overlay mark and acquires an image of the overlay mark. The focus can be located, for example, between the previous pattern layer and the current pattern layer.

[0012] However, the focal point can vary depending on the overlay mark. This is because the height of the semiconductor wafer can differ depending on its position due to factors such as the warping of the semiconductor wafer itself and the height difference at different positions of the chucks that secure the semiconductor wafer. Therefore, it is necessary to acquire the overlay mark image each time with the overlay mark to be measured in focus using an autofocus optical system. Consequently, in order to shorten the Move, Acquire, Measure (MAM) time, which is one of the performance indicators of an overlay measurement device, it is necessary to reduce the time required to focus using an autofocus optical system.

[0013] Although the above explanation used an overlay measurement device as an example, reducing the time required to focus on the target mark is also advantageous in other image acquisition devices that acquire images of multiple marks formed at various positions on a sample. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Japanese Patent Publication No. 2020-112807 [Patent Document 2] Korean Published Patent No. 10-2001-0092740 [Patent Document 3] Korean Patent Publication No. 10-2236184 [Patent Document 4] Korean Patent Publication No. 10-2280137 [Overview of the Initiative] [Problems that the invention aims to solve]

[0015] The present invention aims to meet the above-mentioned requirements and to provide an image acquisition device and method that can shorten the time required to focus on the mark to be measured.

[0016] Furthermore, the aim is to provide an image acquisition device control program. [Means for solving the problem]

[0017] To achieve the above objective, the present invention provides an image acquisition device configured to acquire images of marks from a sample on which a plurality of marks are formed, comprising: a stage on which the sample is placed; an imaging optical system configured to image reflected light from the marks; an autofocus optical system configured to detect Z-direction position information of the surface of the sample while moving at least one of the stage and the imaging optical system on the XY plane and changing the position of the observation area of ​​the imaging optical system from a previous measurement position to the current measurement position of the sample; and a focus control unit configured to move the objective lens of the imaging optical system in the Z-direction based on the Z-direction position information while changing the position of the observation area of ​​the imaging optical system from a previous measurement position to the current measurement position of the sample, and to bring the distance between the surface of the sample and the objective lens closer to a reference distance which is the distance when the marks are in focus.

[0018] Further, the autofocus optical system includes a light source that generates irradiation light for illuminating the sample, a photodetector configured to receive reflected light reflected from the sample, an optical shutter wheel disposed in front of the photodetector and having a transmission region through which the reflected light passes and a blocking region where the reflected light is blocked, which are alternately formed along the angular direction, and a condenser lens configured to condense the reflected light toward the optical shutter wheel, and provides an image acquisition device.

[0019] Further, the optical shutter wheel rotates at a constant speed and periodically blocks the reflected light, and provides an image acquisition device.

[0020] Further, the optical shutter wheel provides an image acquisition device that converts the reflected light into periodic discontinuous light and transmits it to the photodetector.

[0021] Further, the photodetector includes a first sensing unit and a second sensing unit that are arranged adjacent to each other side by side on the same plane orthogonal to the optical axis of the reflected light, and provides an image acquisition device.

[0022] Further, when the focal point of the reflected light that has passed through the condenser lens is located on the optical shutter wheel, the autofocus optical system is configured such that the phase of the periodic signal from the first sensing unit and the phase of the periodic signal from the second sensing unit coincide with each other, and provides an image acquisition device.

[0023] Further, the optical axis of the reflected light that has passed through the condenser lens is configured to be located at the boundary between the first sensing unit and the second sensing unit, and provides an image acquisition device.

[0024] Further, the first sensing unit and the second sensing unit are cells of a bisel photodiode, and provides an image acquisition device.

[0025] Further, the autofocus optical system is configured such that the sign of the phase difference between the periodic signal from the first sensing unit and the periodic signal from the second sensing unit when the focus of the reflected light that has passed through the condenser lens is located in front of the light blocking wheel is opposite to the sign of the phase difference between the periodic signal from the first sensing unit and the periodic signal from the second sensing unit when the focus of the reflected light that has passed through the condenser lens is located behind the light blocking wheel, and provides an image acquisition device.

[0026] Further, the autofocus optical system is configured to detect the Z-direction position information of the surface of the sample while finely moving at least one of the stage and the imaging optical system on the X-Y plane so that the mark formed at the current measurement position is located at the center of the image detector. The focus control unit is configured to move the objective lens of the imaging optical system in the Z direction based on the Z-direction position information while finely moving at least one of the stage and the imaging optical system on the X-Y plane so that the mark formed at the current measurement position is located at the center of the image detector, and to approximate the distance between the surface of the sample and the objective lens to the reference distance, and provides an image acquisition device.

[0027] Further, the sample is a semiconductor wafer and the mark is an overlay mark, and provides an image acquisition device.

[0028] Further, the image acquisition device is an overlay measurement device, and provides an image acquisition device.

[0029] Furthermore, the present invention provides an image acquisition method for acquiring images of marks from a sample on which a plurality of marks are formed, comprising the steps of: detecting Z-direction position information of the surface of the sample while changing the position of the observation area of ​​an imaging optical system configured to image reflected light from the marks from a previous measurement position to the current measurement position of the sample; and moving the objective lens of the imaging optical system in the Z-direction based on the Z-direction position information while changing the position of the observation area of ​​the imaging optical system from the previous measurement position to the current measurement position of the sample, thereby bringing the distance between the surface of the sample and the objective lens closer to a reference distance, which is the distance at which the marks are in focus.

[0030] The present invention also provides an image acquisition method that further includes the steps of: detecting Z-direction position information of the sample surface while fine-tuning the position of the observation area of ​​the imaging optical system so that the mark formed at the current measurement position is located at the center of the image detector; and moving the objective lens of the imaging optical system in the Z-direction based on the Z-direction position information while fine-tuning the position of the observation area of ​​the imaging optical system so that the mark formed at the current measurement position is located at the center of the image detector, thereby bringing the distance between the sample surface and the objective lens closer to the reference distance.

[0031] Furthermore, the present invention provides an image acquisition device control program stored on a storage medium for executing an image acquisition method using a computing device to acquire images of marks from a sample on which a plurality of marks have been formed, the program to execute the following steps: detecting Z-direction position information of the surface of the sample while changing the position of the observation area of ​​an imaging optical system configured to image reflected light from the marks from the previous measurement position to the current measurement position of the sample; and moving the objective lens of the imaging optical system in the Z-direction based on the Z-direction position information while changing the position of the observation area of ​​the imaging optical system from the previous measurement position to the current measurement position of the sample, thereby bringing the distance between the surface of the sample and the objective lens closer to a reference distance, which is the distance when the marks are in focus.

[0032] Furthermore, the image acquisition device control program is provided to be automatically executed when the autofocus function is performed a predetermined number of times or more within a time interval determined by the computing device.

[0033] Furthermore, the image acquisition device control program is provided as firmware installed in the fixed storage device of the computing device. [Effects of the Invention]

[0034] The image acquisition apparatus and method according to the present invention acquires Z-direction position information of the sample surface in advance during the process of changing the observation area position of the imaging optical system from a previous measurement position to the current measurement position. As a result, the imaging optical system is moved in the Z-axis direction in advance. Therefore, when the measurement target mark is currently located in the observation area of ​​the imaging optical system, the distance that the objective lens of the imaging optical system needs to move in the Z-axis direction to focus on the measurement target mark is reduced. Thus, the time required for focusing can be reduced. [Brief explanation of the drawing]

[0035] [Figure 1]This is a plan view of an example of an overlay mark. [Figure 2] The signal obtained from one working zone of the overlay mark shown in Figure 1 is shown here. [Figure 3] This diagram shows measurement sites on a semiconductor wafer as dots. [Figure 4] This is a conceptual diagram of an image acquisition device relating to one embodiment of the present invention. [Figure 5] Figure 4 is a conceptual diagram of an autofocus optical system. [Figure 6] This figure shows the light-blocking wheel shown in Figure 5. [Figure 7] Figure 5 is a diagram illustrating the operation of the autofocus optical system shown. [Figure 8] This diagram illustrates the fine-tuning control in the Z direction. [Figure 9] This figure shows the change in the distance between the objective lens and the sample surface over time. [Figure 10] This is a flowchart of an image acquisition method related to one embodiment of the present invention. [Modes for carrying out the invention]

[0036] One embodiment of the present invention will be described in detail below with reference to the accompanying drawings. However, embodiments of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments detailed below. Embodiments of the present invention are provided to give a more complete explanation of the present invention to those who are ordinary skill in the art. Therefore, the shapes of elements in the drawings are exaggerated to emphasize a clearer explanation, and elements indicated by the same reference numerals in the drawings mean the same element.

[0037] Figure 4 is a conceptual diagram of an image acquisition device according to one embodiment of the present invention. As shown in Figure 4, the image acquisition device 100 according to one embodiment of the present invention includes a stage 10, an illumination optical system 20, an imaging optical system 30, an image detector 40, an autofocus optical system 50, and a focus control unit 60.

[0038] The image acquisition apparatus 100 according to the present invention can be used as an overlay measuring apparatus for measuring the interlayer overlay error of a sample (S, for example, a semiconductor wafer) on which multiple overlay marks are formed.

[0039] Stage 10 can be moved in mutually orthogonal X and Y directions by a horizontal drive unit. Stage 10 plays the role of supporting and horizontally moving the sample S. Stage 10 may be equipped with a vacuum chuck for fixing the sample S.

[0040] The illumination optical system 20 is responsible for illuminating the overlay marks on the surface of the sample S. The illumination optical system 20 may include, for example, an illumination source 21, a beam splitter 23, and an objective lens 25, as shown in Figure 4. The illumination source 21 may include a light source capable of generating light in a wide wavelength range and interchangeable optical filters. A laser diode or a light-emitting diode can be used as the light source. The illumination source 21 can generate illumination in various wavelength ranges by adjusting the wavelength range of light from the light source in combination with optical filters.

[0041] The beam splitter 23 is positioned between the illumination source 21 and the objective lens 25, and its role is to transmit illumination from the illumination source 21 to the objective lens 25.

[0042] The objective lens 25 plays the role of focusing the illumination onto the measurement position of the sample S. The objective lens 25 is mounted on the lens focus actuator 27. The lens focus actuator 27 is used to adjust the distance between the objective lens 25 and the sample S so that the focus is located on the overlay mark. Focus control is performed by moving the objective lens 25 in the Z direction.

[0043] The imaging optical system 30 plays the role of imaging reflected light from the overlay mark onto the image detector 40. The imaging optical system 30 may include, for example, a hot or cold mirror 31 and a tube lens 33. The imaging optical system 30 also uses the objective lens 25 and beam splitter 23 of the illumination optical system 20.

[0044] The imaging optical system 30, like the stage 10, can be moved in the mutually orthogonal X and Y directions by a horizontal drive unit. As described above, if the stage 10 is configured to move on the XY plane, the imaging optical system 30 may be fixed. Conversely, the imaging optical system 30 may move in the X and Y directions, while the stage 10 is fixed. In the following description, the imaging optical system 30 will be fixed, and only the stage 10 will move.

[0045] The hot or cold mirror 31 serves to prevent the illumination used for the autofocus optical system 50 from being directed towards the image detector 40. A hot mirror has high transmittance to short-wavelength light and reflects long-wavelength light. Conversely, a cold mirror has high transmittance to long-wavelength light and reflects short-wavelength light. If the illumination used for the autofocus optical system 50 has a shorter wavelength than the illumination used for image acquisition, a cold mirror is used to reflect the reflected light from the illumination used for the autofocus optical system 50 back towards the autofocus optical system 50. Conversely, if the illumination used for the autofocus optical system 50 has a longer wavelength than the illumination used for image acquisition, a hot mirror is used to reflect the reflected light from the illumination used for the autofocus optical system 50 back towards the autofocus optical system 50.

[0046] The objective lens 25 collects the light reflected from the sample S. The light collected by the objective lens 25 passes through the beam splitter 23 and a hot or cold mirror 31, and is then focused by the tube lens 33 to the image detector 40.

[0047] The image detector 40 receives reflected light from the overlay mark and generates an overlay mark image. The image detector 40 may be a CCD (Charge-Coupled Device) camera or a CMOS (Complementary Metal-Oxide Semiconductor) camera. The image detector 40 may use an image sensor equipped with RGB (Red Green Blue) color filters, or it may use a monochrome image sensor.

[0048] The autofocus optical system 50 plays the role of detecting the Z-direction position information of the surface of sample S. In other words, it detects information regarding the distance to the surface of sample S.

[0049] Figure 5 is a conceptual diagram of the autofocus optical system shown in Figure 4.

[0050] As shown in Figure 5, the autofocus optical system 50 includes a light source 51, a focusing lens 55, a light-blocking wheel 56, and a photodetector 58. It may further include an optical lens 52 for refracting the light emitted from the light source 51, a beam splitter 53, and a cylindrical lens 54.

[0051] A laser diode or light-emitting diode can be used as the light source 51. The light source 51 can, for example, generate illumination light in the infrared region. The illumination light is refracted by the optical lens 52, passes through the beam splitter 53 and the cylinder lens 54, and is then reflected by the mirror 31.

[0052] For example, a planar-convex lens can be used as the optical lens 52. When a laser is used as the illumination light, it is preferable to use a polarizing beam splitter as the beam splitter 53, because it can minimize the reduction in light intensity during the reflection and transmission process.

[0053] A variety of cylindrical lenses 54 can be used, including rectangular, square, circular, and elliptical shapes. The cylindrical lens 54 is a lens that focuses light into a line, rather than a point. The cylindrical lens 54 plays the role of forming a line beam. Using a line beam has the advantage of allowing for more precise measurements because sensitivity to optical aberrations (astigmatism) increases.

[0054] Furthermore, the illumination light reflected by the mirror 31 passes through the beam splitter 33 and then enters the objective lens 25. The objective lens 25 focuses the illumination light onto the measurement area of ​​the wafer W and collects the reflected light from the measurement area.

[0055] The reflected light collected by the objective lens 25 passes through the beam splitter 23 again and is then reflected by the mirror 31. The reflected light reflected by the mirror 31 is then reflected from the beam splitter 53 towards the photodetector 58.

[0056] The focusing lens 55 plays the role of concentrating the reflected light reflected from the beam splitter 53 towards the photodetector 58 toward the light-blocking wheel 56. If the surface height of the sample S is the reference height, the focal point of the reflected light that has passed through the focusing lens 55 is located at the light-blocking wheel 56.

[0057] Figure 6 shows the light-blocking wheel shown in Figure 5. As shown in Figure 6, the light-blocking wheel 56 has alternating transparent regions 561 through which reflected light is transmitted and blocking regions 563 through which reflected light is blocked, along the angular direction. The blocking regions 563 can reflect or absorb reflected light.

[0058] The light-blocking wheel 56 rotates at a constant speed, periodically blocking the reflected light. The axis of rotation of the light-blocking wheel 56 can be aligned with the optical axis of the reflected light RL at a distance from it. The light-blocking wheel 56 plays a role in converting the reflected light into periodic discontinuous light and transmitting it to the photodetector 58.

[0059] The photodetector 58 is configured to receive reflected light and generate two distinct electrical signals. When the blocking region 563 and the transmission region 561 are positioned at a fixed angular interval, a periodic electrical signal in the form of a square wave or something close to a sine wave is generated. By creating such a periodic signal, it is possible to easily and accurately determine whether the two electrical signals match or not. The periodic waveform does not necessarily have to be a square wave or a sine wave; if the blocking region 563 and the transmission region 561 are not positioned at a fixed angular interval, a periodic electrical signal can be generated based on the 360-degree rotation of the light blocking wheel 56.

[0060] The photodetector 58 comprises a first sensing unit 581 and a second sensing unit 583, which are arranged adjacent to each other on the same plane perpendicular to the optical axis of the reflected light.

[0061] The first sensing unit 581 and the second sensing unit 583 generate electrical signals independently of each other. Preferably, the first sensing unit 581 and the second sensing unit 583 are configured to generate the same electrical signal when the same light is incident on them.

[0062] Various types of optical sensors can be used as the first sensing unit 581 and the second sensing unit 583. For example, photodiodes, PSD (Position Sensitive Device) sensors, CMOS (Complementary Metal Oxide Semiconductor) sensors, and CCD (Charge-Coupled Device) sensors can be used. Alternatively, two separate areas of a single CCD or CMOS sensor can be used as the first sensing unit 581 and the second sensing unit 583.

[0063] Alternatively, the photodetector 58 may be a bicell photodiode. In this case, the two cells of the bicell photodiode can become a first sensing unit 581 and a second sensing unit 583. The first sensing unit 581 and the second sensing unit 583 can each generate an electrical signal proportional to the intensity of the reflected light incident on them.

[0064] The operation of the autofocus optical system 50 with the above configuration will be explained below using Figure 7. Figure 7(a) shows the case when the focal point F of the reflected light RL is located on the light-blocking wheel 56. Figure 7(b) shows the case when the surface of the sample is lowered and the focal point F of the reflected light RL is located in front of the light-blocking wheel 56. Figure 7(c) shows the case when the surface of the sample is higher and the focal point F of the reflected light RL is located behind the light-blocking wheel 56.

[0065] As shown in Figure 7(a), when the focal point F of the reflected light RL is located at the light-blocking wheel 56, the phase of the periodic signal S1 from the first sensing unit 581 and the phase of the periodic signal S2 from the second sensing unit 583 coincide with each other.

[0066] However, as shown in Figures 7(b) and 7(c), when the focal point F of the reflected light RL is not located on the light-blocking wheel 56, a phase difference occurs between the periodic signal S1 from the first sensing unit 581 and the periodic signal S2 from the second sensing unit 583.

[0067] More specifically, as shown in Figure 7(b), when the focal point F of the reflected light RL is in front of the light-blocking wheel 56, the reflected light RL1 incident on the first sensing unit 581 first intersects with the transmission region 561. Therefore, the pulses of the periodic signal S1 from the first sensing unit 581 occur before the pulses of the periodic signal S2 from the second sensing unit 583.

[0068] Conversely, as shown in Figure 7(c), when the focal point F of the reflected light RL is behind the light-blocking wheel 56, the reflected light RL2 incident on the second sensing unit 583 first intersects with the transmission region 561. Therefore, the pulses of the periodic signal S2 from the second sensing unit 583 occur before the pulses of the periodic signal S1 from the first sensing unit 581.

[0069] The autofocus optical system 50 can detect the Z-direction position information of the surface of the sample S using the magnitude and sign of the phase difference between the periodic signal S1 from the first sensing unit 581 and the periodic signal S2 from the second sensing unit 583.

[0070] The focus control unit 60 moves the objective lens 25 of the imaging optical system 30 in the Z direction while the stage 10 moves the sample (S) on the XY plane, bringing it closer to a distance where it can focus on the mark formed at the current measurement position. The focus control unit 60 moves the objective lens 25 in the Z direction based on the Z direction position information received from the autofocus optical system 50.

[0071] Even if the overlay mark was focused at the previous measurement location, it is not necessarily true that it will be focused at the current measurement location. This could be due to issues with the surface accuracy of Stage 10 or errors due to the thickness of Sample S. Furthermore, if Sample S is a semiconductor wafer, the vacuum chuck supporting the semiconductor wafer may be flexed, and the semiconductor wafer itself may also be slightly flexed.

[0072] In order to solve the problem that the focus at the previous measurement position is different from the focus at the current measurement position, the focus control unit 60 performs fine movement control in the Z direction while the position of the observation area of ​​the imaging optical system 30 is changed from the previous measurement position to the current measurement position.

[0073] Figure 8 is a diagram illustrating fine-motion control in the Z direction. Figure 8(a) is a diagram illustrating a conventional control method, and Figure 8(b) is a diagram illustrating a control method according to the present invention.

[0074] As shown in Figure 8(a), conventionally, the Z-direction position of the objective lens 25 is fixed with respect to the previously measured position S1, and the position of the observation area of ​​the imaging optical system 30 is changed to the current measured position S2. The position of the observation area of ​​the imaging optical system 30 can be changed by moving the stage 10 and / or the imaging optical system 30.

[0075] Then, at the current measurement position S2, the objective lens 25 is moved in the Z direction using the Z-direction position information from the autofocus optical system 50 to achieve focus.

[0076] As shown in Figure 8(a), when the height difference between the previous measurement position S1 and the current measurement position S2 is relatively large, the distance that the objective lens 25 must move in the Z direction to focus becomes longer.

[0077] As shown in Figure 8(b), in the present invention as well, once the position of the objective lens 25 in the Z direction is fixed with respect to the previously measured position S1, the position of the observation area of ​​the imaging optical system 30 is changed toward the current measured position S2.

[0078] Then, when the system approaches the current measurement position S2 to a certain extent, the focus control unit 60 finely adjusts the objective lens 25 in the Z direction once every approximately 15 ms based on the Z-direction position information received from the autofocus optical system 50. In other words, by finely adjusting the objective lens 25 in the Z direction in accordance with the height change of the surface of the sample S, the system adjusts the distance between the surface of the sample S and the objective lens 25 to be as close as possible to the reference distance, which is the distance when the lens is in focus.

[0079] Furthermore, upon reaching the current measurement position S2, the objective lens 25 is moved in the Z direction using the Z-direction position information from the autofocus optical system 50 to focus.

[0080] In this invention, the objective lens 25 is moved in advance to a position near the focal point before reaching the current measurement position S2. Therefore, the distance that the objective lens 25 must move during the focusing process is shortened. As a result, the Move, Acquire, Measure (MAM) time is reduced compared to conventional methods. For example, the MAM time can be reduced by about 10%, from 100ms to 90ms.

[0081] Alternatively, the stability of the measurement can be improved by increasing the "in-position time (time in position)" while maintaining the overall MAM time. "In-position time" refers to the time during which, considering the vibration of the overlay measuring device 100, the distance between the objective lens 25 and the surface of the sample S can be considered to be the focal distance. In other words, it refers to the time within the "in-position band," which has a tolerance range centered on a reference distance where the distance between the objective lens 25 and the surface of the sample S is the focal distance. The tolerance range is determined according to the degree of vibration of the overlay measuring device 100.

[0082] Figure 9 shows the change in distance between the objective lens 25 and the surface of the sample over time. Graph (a) shows the change in distance according to the control method of the present invention, and graph (b) shows the change in distance according to a conventional control method.

[0083] As shown in Figure 9, according to the control method of the present invention, the measurement position band is reached quickly, resulting in a longer "time spent at the measurement position". During the secured "time spent at the measurement position", the number of "in-position checks" can be increased to perform measurements in a more stable state.

[0084] Furthermore, after the objective lens 25 reaches the current measurement position S2, the focus control unit 60 can perform fine movement control in the Z direction in the same manner while the stage 10 is used to make fine movements of the sample S in the X and Y directions so that the mark at the current measurement position S2 is located in the center of the image detector 40.

[0085] In other words, while moving the sample S so that the mark formed at the current measurement position S2 is located in the center of the image detector 40, the Z-direction position information of the surface of the sample S is detected. Then, while moving the sample S so that the mark formed at the current measurement position S2 is located in the center of the image detector 40, the objective lens 25 of the imaging optical system 30 is moved in the Z-direction based on the Z-direction position information to bring the distance between the surface of the sample S and the objective lens 25 closer to the reference distance.

[0086] The focus control unit 60 can include hardware such as a processor, memory, fixed memory (ROM), storage devices such as hard disks and SSDs, a wired or wireless communication device used to receive Z-direction position information from the autofocus optical system 50 and transmit control signals to control the objective lens 25, and a microcontroller unit (MCU) containing programs such as firmware and software installed on a storage medium such as memory, fixed memory, or storage device, as well as computing devices such as desktop computers, laptop computers, smartphones, and smartpads.

[0087] The storage medium of the focus control unit 60 has an image acquisition device control program according to one embodiment of the present invention installed on it. The focus control unit 60 uses the instruction words of the image acquisition device control program to instruct the processor to perform Z-direction fine-motion control of the objective lens 25 based on the Z-direction position information.

[0088] The image acquisition device control program may be executed manually, or it may be executed automatically if the autofocus function is executed a predetermined number of times consecutively within a certain time interval, for example, five times. This is to prevent the image acquisition device control program from being executed automatically unnecessarily, for example, when the autofocus function is used for troubleshooting.

[0089] The computing device can also be used to move Stage 10. Moving Stage 10 can be performed, for example, by converting the target position coordinates to the stage coordinates and transmitting control data to Stage 10.

[0090] The operation of the image acquisition device 100 described above will be explained below with reference to Figure 10. Figure 10 is a flowchart of an image acquisition method according to one embodiment of the present invention.

[0091] As shown in Figure 10, an image acquisition method according to one embodiment of the present invention includes the step (S1) of detecting Z-direction position information of the surface of the sample S while changing the position of the observation area of ​​the imaging optical system 30 from the previously measured position of the sample S to the current measured position.

[0092] In this step, the stage 10 is moved to change the position of the observation area of ​​the imaging optical system 30 from the previous measurement position of the sample S to the current measurement position, while the autofocus optical system 50 is used to detect the Z-direction position information of the surface of the sample S at regular time intervals.

[0093] Next, while the position of the observation area of ​​the imaging optical system 30 is changed from the previous measurement position of the sample S to the current measurement position, a step (S2) is performed in which the objective lens 25 of the imaging optical system 30 is moved in the Z direction based on the Z direction position information, bringing the distance between the surface of the sample S and the objective lens 25 closer to the reference distance.

[0094] Next, the autofocus step (S3) is performed.

[0095] In this step, the autofocus optical system 50 is used to focus on the measurement target mark formed at the current measurement position. Because the reference distance was brought close in the previous step (S2), in this step the objective lens 25 can be moved slightly in the Z direction to focus on the mark.

[0096] Next, step (S4) is performed to detect the Z-direction position information of the sample S surface while the position of the observation area of ​​the imaging optical system 30 is finely adjusted so that the mark formed at the current measurement position is located in the center of the image detector 40.

[0097] When the current measurement position mark enters the observation area of ​​the imaging optical system 30, the position of the mark is confirmed by the image detector 40. Then, while slightly moving the position of the observation area of ​​the imaging optical system 30 in the X and Y directions so that the mark is located in the center of the image detector 40, the Z-direction position information of the surface of the sample S is detected.

[0098] Next, while the position of the observation area of ​​the imaging optical system 30 is finely adjusted so that the mark formed at the current measurement position is located in the center of the image detector 40, the objective lens 25 of the imaging optical system 30 is moved in the Z direction based on the Z direction position information, bringing the distance between the surface of the sample S and the objective lens 25 closer to the reference distance (S5).

[0099] Next, the autofocus step (S6) is performed.

[0100] In this step, the autofocus optical system 50 is used to focus on the measurement target mark at the current position. Then, the mark image used for measurement is acquired.

[0101] The embodiments described above merely illustrate preferred embodiments of the present invention, and the scope of the present invention is not limited to the described embodiments. Various modifications, alterations, or substitutions can be made by those skilled in the art within the scope of the technical idea and claims of the present invention, and these embodiments should be understood to also fall within the scope of the present invention. [Explanation of Symbols]

[0102] S Sample 10 stages 20 Illumination optical system 30 Imaging Optical System 40 Image detectors 50 Autofocus optical system 60 Focus control unit

Claims

1. An image acquisition device configured to acquire images of marks from a sample on which multiple marks have been formed, A stage on which the aforementioned sample is placed, An imaging optical system configured to form an image of the reflected light from the aforementioned mark, An image detector configured to receive the reflected light and generate a mark image, An autofocus optical system configured to detect the Z-direction position information of the surface of the sample while moving at least one of the stage and the imaging optical system on the X-Y plane and changing the position of the observation area of ​​the imaging optical system from the previous measurement position of the sample to the current measurement position, An image acquisition device comprising: a focus control unit configured to move the objective lens of the imaging optical system in the Z direction based on the Z direction position information while changing the position of the observation area of ​​the imaging optical system from the previous measurement position of the sample to the current measurement position, thereby bringing the distance between the surface of the sample and the objective lens closer to a reference distance which is the distance when the mark is in focus.

2. The aforementioned autofocus optical system is A light source that generates illumination light to light the aforementioned sample, A photodetector configured to receive reflected light reflected from the aforementioned sample, A light-blocking wheel is positioned in front of the photodetector, and has alternating transparent regions through which the reflected light is transmitted and blocking regions that block the reflected light, along the angular direction. The image acquisition apparatus according to claim 1, comprising a focusing lens configured to focus the reflected light toward the light-blocking wheel.

3. The image acquisition apparatus according to claim 2, wherein the light-blocking wheel rotates at a constant speed and periodically blocks the reflected light.

4. The image acquisition apparatus according to claim 2, wherein the light-blocking wheel converts the reflected light into periodic discontinuous light and transmits it to the photodetector.

5. The image acquisition device according to claim 2, wherein the photodetector comprises a first sensing unit and a second sensing unit arranged adjacent to each other on the same plane perpendicular to the optical axis of the reflected light.

6. The aforementioned autofocus optical system is The image acquisition device according to claim 5, wherein when the focal point of the reflected light that has passed through the condensing lens is located at the light-blocking wheel, the phase of the periodic signal from the first sensing unit and the phase of the periodic signal from the second sensing unit coincide with each other.

7. The image acquisition device according to claim 5, wherein the optical axis of the reflected light that has passed through the focusing lens is configured to be located at the boundary between the first sensing unit and the second sensing unit.

8. The image acquisition apparatus according to claim 5, wherein the first sensing unit and the second sensing unit are cells of a bicell photodiode.

9. The aforementioned autofocus optical system is The image acquisition apparatus according to claim 5, configured such that the signs of the phase difference between the periodic signal from the first sensing unit and the periodic signal from the second sensing unit when the focal point of the reflected light that has passed through the focusing lens is located in front of the light-blocking wheel, and the signs of the phase difference between the periodic signal from the first sensing unit and the periodic signal from the second sensing unit when the focal point of the reflected light that has passed through the focusing lens is located behind the light-blocking wheel, are opposite to each other.

10. The aforementioned autofocus optical system is The system is configured to detect the Z-direction position information of the sample surface while at least one of the stage and the imaging optical system is moved in a minute manner on the X-Y plane so that the mark formed at the current measurement position is located at the center of the image detector. The focus control unit, The image acquisition apparatus according to claim 1, wherein, while at least one of the stage and the imaging optical system is moved in the X-Y plane in a minute manner so that the mark formed at the current measurement position is located at the center of the image detector, the objective lens of the imaging optical system is moved in the Z direction based on the Z direction position information to bring the distance between the surface of the sample and the objective lens closer to the reference distance.

11. The image acquisition apparatus according to claim 1, wherein the sample is a semiconductor wafer and the mark is an overlay mark.

12. The image acquisition device according to claim 1, wherein the image acquisition device is an overlay measurement device.

13. An image acquisition method for acquiring images of marks from a sample on which multiple marks have been formed, The steps include detecting the Z-direction position information of the surface of the sample while changing the position of the observation area of ​​an imaging optical system configured to image reflected light from the mark onto an image detector from the previous measurement position of the sample to the current measurement position, An image acquisition method comprising the step of moving the objective lens of the imaging optical system in the Z direction based on the Z direction position information while changing the position of the observation area of ​​the imaging optical system from the previous measurement position of the sample to the current measurement position, thereby bringing the distance between the surface of the sample and the objective lens closer to a reference distance which is the distance when the mark is in focus.

14. The steps include detecting the Z-direction position information of the sample surface while fine-tuning the position of the observation area of ​​the imaging optical system so that the mark formed at the current measurement position is located at the center of the image detector, The image acquisition method according to claim 13, further comprising the step of moving the objective lens of the imaging optical system in the Z direction based on the Z direction position information, while fine-tuning the position of the observation area of ​​the imaging optical system so that the mark formed at the current measurement position is located at the center of the image detector, thereby bringing the distance between the surface of the sample and the objective lens closer to the reference distance.

15. A program stored on a storage medium for causing a computing device to execute an image acquisition method for acquiring images of marks from a sample on which multiple marks have been formed, The steps include detecting the Z-direction position information of the surface of the sample while changing the position of the observation area of ​​the imaging optical system, which is configured to image reflected light from the mark, from the previous measurement position of the sample to the current measurement position, An image acquisition device control program stored on a storage medium, for the purpose of causing the objective lens of the imaging optical system to move in the Z direction based on the Z direction position information while changing the position of the observation area of ​​the imaging optical system from the previous measurement position of the sample to the current measurement position, thereby bringing the distance between the surface of the sample and the objective lens closer to a reference distance, which is the distance when the mark is in focus.

16. The image acquisition device control program according to claim 15, wherein the image acquisition device control program is automatically executed when the autofocus function is performed a predetermined number of times or more within a time interval determined by the computing device.

17. The image acquisition device control program according to claim 15, wherein the image acquisition device control program is firmware installed in the fixed storage device of the computing device.

Images