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

Abstract

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 in a process of moving a stage on which a sample on which the plurality of marks are formed is provided relative to an imaging optical system. The present invention provides an image acquisition device configured to acquire an image of a plurality of marks from a sample on which the plurality of marks are formed, the image acquisition device comprising: a stage on which the sample is disposed; an illumination optical system configured to illuminate the mark; an image detector configured to receive reflected light from the mark generated by the illumination and generate a mark image; an imaging optical system configured to image reflected light from the mark on the image detector; and an automatic focusing optical system configured so as to change the position of the observation region of the imaging optical system from the previous measurement position of the sample to the current measurement position by moving at least one of the stage and the imaging optical system on the X-Y plane. Detecting Z-direction position information of the surface of the sample; and a focus control unit configured so as to move an objective lens of the imaging optical system in the Z direction on the basis of the Z-direction position information during a period in which the position of an observation region of the imaging optical system is changed from a previous measurement position of the sample to a current measurement position. The distance between the surface of the sample and the objective lens is made close to a reference distance, which is the distance at which the mark is focused.

Description

Technical Field

[0001] The present invention relates to an image acquisition apparatus and method, and more specifically, to an image acquisition apparatus and method for acquiring images of multiple marks during the movement of a stage on which a sample with multiple marks is formed relative to an imaging optical system. Background Technology

[0002] Multiple patterned layers are sequentially formed on a semiconductor substrate. Furthermore, through techniques such as double patterning, the circuitry of a single layer can be divided into two patterns for formation. The desired semiconductor device can only be manufactured when these patterned layers, or multiple patterns within a single layer, are precisely formed in predetermined positions.

[0003] Therefore, to confirm whether the pattern layer is accurately aligned, overlay marks formed simultaneously with the pattern layer are used.

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

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

[0006] More specifically, Japanese Patent Publication 2020-112807 discloses a method that captures an image of overlay marks formed on a substrate, selects multiple working areas from the captured image, forms an informational signal for each selected working area, compares them, and thereby determines the relative offset between layers or patterns that are different from each other.

[0007] Figure 1 This is a plan view of an example of an engraved mark. Figure 1 The overlay mark 1 shown has four working zone groups 4, 5, 6, and 7. Furthermore, each working zone group 4, 5, 6, and 7 has two working zones arranged diagonally opposite each other. Each working zone group 4, 5, 6, and 7 is used to measure the overlay error of the pattern layer formed together with the corresponding working zone group in the X-axis or Y-axis direction. To prevent interference, the structure 2 formed with the first layer pattern and the structure 3 formed with the second layer pattern are arranged in a non-overlapping manner.

[0008] Each working area includes bars arranged at predetermined intervals from the center of overlay mark 1 to its outer contour. Therefore, using the overlay measuring device, measurements can be obtained from two working areas belonging to working area groups 4, 5, 6, and 7, respectively. Figure 2 The periodic signal shown. Figure 2 Charts, for example, can be found from Figure 1 The selected area 8 is obtained.

[0009] exist Figure 2 In the graph, the peak appears in the section with the bars. Since the bars in the previous overlay marker 1 were arranged periodically, the acquired signal also exhibited periodicity. The overlay was then measured by performing correlation analysis on two periodic signals acquired from the two selected regions 8 and 8'.

[0010] Figure 3 This is a diagram using dots to mark measurement points on a semiconductor wafer. Each dot represents an overlay mark. For example... Figure 3 As shown, multiple overlay marks are formed on a semiconductor wafer. The overlay marks are formed on the scribelane of the semiconductor wafer.

[0011] The overlay measurement apparatus moves an XY stage, on which a semiconductor wafer is fixed, so that the overlay mark to be measured is positioned below the observation area of ​​the imaging optics system of the overlay measurement apparatus. Then, an autofocus optics system focuses on the overlay mark and acquires an image of it. The focusing position can be, for example, between a previous pattern layer and the current pattern layer.

[0012] However, the focus position can vary depending on the overlay markings. This is because the warpage of the semiconductor wafer itself and the height difference of the chuck fixing the semiconductor wafer at different positions can cause different heights at different locations on the semiconductor wafer.

[0013] Therefore, each time the overlay mark image is acquired, the object being measured must be focused using the autofocus optical system. Thus, to shorten the move, acquire, and measure (MAM) time—one of the performance indicators of an overlay measuring device—it is necessary to shorten the time required for focusing using the autofocus optical system.

[0014] Although the above description uses an overlay measuring device as an example, in other image acquisition devices that acquire images of multiple marks formed at multiple locations of a sample, it is also advantageous to shorten the time for focusing on the marks of the object being measured.

[0015] [Existing Technical Documents]

[0016] (Patent Document 1) Japanese Patent Publication 2020-112807

[0017] (Patent Document 2) Korean Patent Publication No. 10-2001-0092740

[0018] (Patent Document 3) Korean Patent 10-2236184

[0019] (Patent Document 4) Korean Patent 10-2280137 Summary of the Invention

[0020] Technical issues

[0021] The present invention aims to address the above requirements and its purpose is to provide an image acquisition device and method that can shorten the time for focusing on the marking of the measurement object.

[0022] In addition, another objective is to provide a control program for an image acquisition device.

[0023] Technical solution

[0024] To achieve the above objective, the present invention provides an image acquisition apparatus configured to acquire an image of the marks formed on a sample. The image acquisition apparatus includes: a stage for arranging the sample; 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 sample surface during a period in which the position of the observation area of ​​the imaging optical system is changed from a previous measurement position of the sample to a current measurement position by moving at least one of the stage and the imaging optical system in the XY plane; and a focus control unit configured to, during the period in which the position of the observation area of ​​the imaging optical system is changed from a previous measurement position of the sample to the current measurement position, move the objective lens of the imaging optical system in the Z direction according to the Z-direction position information, such that the distance between the sample surface and the objective lens approaches a reference distance used as the distance for focusing on the marks.

[0025] Furthermore, in the provided image acquisition device, the autofocus optical system includes: a light source that generates illumination light to illuminate the sample; a photodetector configured to receive reflected light reflected from the sample; a light blocking wheel disposed at the front end of the photodetector and having alternating transmission areas through which the reflected light passes and blocking areas that block the reflected light along an angular direction; and a condenser lens configured to focus the reflected light toward the light blocking wheel.

[0026] Furthermore, in the provided image acquisition device, the light blocking wheel rotates at a constant speed and periodically blocks the reflected light.

[0027] Furthermore, in the provided image acquisition device, the light blocking wheel converts the reflected light into periodic discontinuous light and transmits it to the photodetector.

[0028] Furthermore, in the provided image acquisition device, the photodetector includes a first sensing unit and a second sensing unit, which are arranged side by side adjacently on the same plane orthogonal to the optical axis of the reflected light.

[0029] Furthermore, in the provided image acquisition device, the autofocus optical system is configured such that when the focal point of the reflected light passing through the condenser lens is located at the light blocking wheel, the periodic signal from the first sensing unit and the periodic signal from the second sensing unit are in phase with each other.

[0030] Furthermore, the provided image acquisition device is configured such that the optical axis of the reflected light passing through the condenser lens is located at the boundary between the first sensing unit and the second sensing unit.

[0031] Furthermore, in the provided image acquisition device, the first sensing unit and the second sensing unit are units of a dual-unit photodiode.

[0032] Furthermore, in the provided image acquisition device, the autofocus optical system is configured such that 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 through the condenser lens is located in front of the light blocking wheel is opposite in sign to 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 through the condenser lens is located behind the light blocking wheel.

[0033] Furthermore, in the provided image acquisition apparatus, the autofocus optical system is configured to: detect the Z-direction position information of the sample surface during a period in which at least one of the stage and the imaging optical system is moved slightly in the XY plane such that the mark formed at the current measurement position is located at the center of the image detector; and the focus control unit is configured to: during a period in which at least one of the stage and the imaging optical system is moved slightly in the XY plane such that the mark formed at the current measurement position is located at the center of the image detector, move the objective lens of the imaging optical system in the Z direction according to the Z-direction position information, such that the distance between the sample surface and the objective lens is close to the reference distance.

[0034] Furthermore, in the provided image acquisition device, the sample is a semiconductor wafer, and the mark is an overlay mark.

[0035] Furthermore, in the provided image acquisition device, the image acquisition device is an overlay measurement device.

[0036] Furthermore, the present invention provides an image acquisition method for acquiring images of the marks formed on a sample. The image acquisition method includes: a step of detecting Z-direction position information of the surface of the sample during a period in which the position of the observation area of ​​an imaging optical system configured to image reflected light from the marks is changed from a previous measurement position of the sample to a current measurement position; and a step of moving the objective lens of the imaging optical system in the Z-direction according to the Z-direction position information during the period in which the position of the observation area of ​​the imaging optical system is changed from the previous measurement position of the sample to the current measurement position, such that the distance between the surface of the sample and the objective lens approaches a reference distance used as the distance for focusing on the marks.

[0037] Furthermore, the provided image acquisition method further includes: a step of detecting Z-direction position information of the sample surface during a period of fine adjustment of the position of the observation area of ​​the imaging optical system such that the mark formed at the current measurement position is located at the center of the image detector; and a step of moving the objective lens of the imaging optical system in the Z direction according to the Z-direction position information during the period of fine adjustment of the position of the observation area of ​​the imaging optical system such that the distance between the sample surface and the objective lens is close to the reference distance.

[0038] Furthermore, the present invention provides an image acquisition device control program stored in a storage medium for executing an image acquisition method using a computing device to acquire an image of a sample having multiple markers formed thereon. The image acquisition device control program is stored in the storage medium for performing the following steps: detecting Z-direction position information of the surface of the sample during a period when the position of the observation area of ​​an imaging optical system configured to image reflected light from the markers is changed from a previous measurement position of the sample to a current measurement position; and during the period when the position of the observation area of ​​the imaging optical system is changed from the previous measurement position of the sample to the current measurement position, moving the objective lens of the imaging optical system in the Z-direction according to the Z-direction position information, such that the distance between the surface of the sample and the objective lens approaches a reference distance used as the distance for focusing on the markers.

[0039] Furthermore, in the provided image acquisition device control program, the image acquisition device control program is an image acquisition device control program that is automatically executed when the autofocus function is executed by the computing device more than a predetermined number of times within a predetermined time interval.

[0040] Furthermore, in the provided image acquisition device control program, the image acquisition device control program is firmware installed on the fixed memory device of the computing device.

[0041] The effects of the invention

[0042] According to the image acquisition apparatus and method of the present invention, during the process of changing the observation area position of the imaging optical system from a previous measurement position to the current measurement position, the Z-direction position information of the sample surface is acquired in advance. Correspondingly, the imaging optical system is moved in advance along the Z-axis direction. Therefore, when the current measurement object mark is located in the observation area of ​​the imaging optical system, the moving distance of the objective lens of the imaging optical system used for focusing on the measurement object mark in the Z-axis direction is reduced. This shortens the time required for focusing. Attached Figure Description

[0043] Figure 1 This is a plan view of an example of an engraved mark.

[0044] Figure 2 Showing from Figure 1 The signal acquired in one working area of ​​the overprinted mark shown.

[0045] Figure 3 It is a diagram that uses dots to mark measurement points on a semiconductor wafer.

[0046] Figure 4 This is a conceptual diagram of an image acquisition device according to an embodiment of the present invention.

[0047] Figure 5 yes Figure 4 The diagram shown is a conceptual representation of an autofocus optical system.

[0048] Figure 6 It is shown Figure 5 The diagram shows the light-blocking wheel.

[0049] Figure 7 For explanation Figure 5 The diagram shows the function of the autofocus optical system.

[0050] Figure 8 This is a diagram used to illustrate micro-motion control in the Z direction.

[0051] Figure 9 This is a graph showing the change in distance between the objective lens and the sample surface over time.

[0052] Figure 10 This is a sequence diagram of an image acquisition method according to an embodiment of the present invention. Detailed Implementation

[0053] The present invention will now be described in detail with reference to the accompanying drawings. However, embodiments of the present invention can be modified in many different ways, and the scope of the present invention should not be construed as limited to the embodiments detailed below. The embodiments of the present invention are provided to provide a more complete explanation of the invention to those skilled in the art. Therefore, the shapes of elements in the figures are exaggerated for clarity, and elements labeled with the same reference numerals refer to the same elements.

[0054] Figure 4 This is a conceptual diagram of an image acquisition device according to an embodiment of the present invention. Figure 1 As shown, an image acquisition device 100 according to an 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.

[0055] The image acquisition device 100 of the present invention can be used as an overlay measurement device for measuring the interlayer overlay error of a sample S (e.g., a semiconductor wafer) having multiple overlay marks formed thereon.

[0056] The stage 10 can be moved in the X and Y directions, which are orthogonal to each other, via a horizontal drive unit. The stage 10 serves to support the sample S and move it horizontally. The stage 10 may be equipped with a vacuum chuck for fixing the sample S.

[0057] The illumination optics system 20 illuminates the overlay marks on the surface of the sample S. For example, as... Figure 4 As shown, the illumination optical system 20 may include an illumination source 21, a beam splitter 23, and an objective lens 25. The illumination source 21 may include a light source capable of generating a wide wavelength band of light and replaceable optical filters. The light source may be a laser diode or a light-emitting diode. The illumination source 21 may be combined with optical filters to adjust the wavelength band of the light from the light source, thereby generating illumination with multiple wavelength bands.

[0058] The beam splitter 23 is positioned between the illumination source 21 and the objective lens 25, and serves to transmit the illumination from the illumination source 21 to the objective lens 25.

[0059] Objective lens 25 serves to focus illumination onto the measurement position of sample S. Objective lens 25 is mounted on lens focusing actuator 27. Lens focusing actuator 27 is used to adjust the distance between objective lens 25 and sample S so that the focal point is located at the overlay mark. Focusing control is achieved by moving objective lens 25 in the Z direction.

[0060] The imaging optics system 30 serves to image the reflected light from the overlay marks onto the image detector 40. The imaging optics system 30 may include, for example, a hot or cold mirror 31 and a barrel lens 33. Furthermore, the imaging optics system 30 utilizes the objective lens 25 and beam splitter 23 of the illumination optics system 20.

[0061] Like the stage 10, the imaging optical system 30 can also move in the mutually orthogonal X and Y directions via the horizontal drive unit. As described above, if the stage 10 is configured to move in the XY plane, the imaging optical system 30 can also be fixed. Conversely, the imaging optical system 30 can move in the X and Y directions while the stage 10 is fixed. In the following explanation, the imaging optical system 30 will be fixed, and only the stage 10 will move as an example.

[0062] The hot or cold reflector 31 serves to prevent the illumination used in the autofocus optical system 50 from being directed towards the image detector 40. The hot reflector has higher transmittance for short-wavelength light and reflects long-wavelength light. Conversely, the cold reflector has higher transmittance for long-wavelength light and reflects short-wavelength light. When the illumination used in the autofocus optical system 50 is of a shorter wavelength than the image acquisition illumination, a cold reflector is used to reflect reflected light generated by the illumination used in the autofocus optical system 50 towards the autofocus optical system 50. Conversely, when the illumination used in the autofocus optical system 50 is of a longer wavelength than the image acquisition illumination, a hot reflector is used to reflect reflected light generated by the illumination used in the autofocus optical system 50 towards the autofocus optical system 50.

[0063] Objective lens 25 collects light reflected from sample S. The light collected by objective lens 25 passes through beam splitter 23 and hot or cold reflector 31, and is then focused by lens 33 onto image detector 40.

[0064] The image detector 40 serves to receive reflected light from the overlay mark and generate an image of the overlay mark. The image detector 40 can be a CCD (Charge-Coupled Device) camera or a CMOS (Complementary Metal-Oxide Semiconductor) camera. The image detector 40 can use either an imaging element with RGB (Red, Green, Blue) color filters or a monochrome imaging element.

[0065] The autofocus optical system 50 serves to detect the Z-direction position information of the sample S surface. That is, it detects information related to the distance from the sample S surface.

[0066] Figure 5 yes Figure 4 The diagram shown is a conceptual representation of an autofocus optical system.

[0067] like Figure 5 As shown, the autofocus optical system 50 includes a light source 51, a condenser lens 55, a light-blocking wheel 56, and a photodetector 58. It may also further include an optical lens 52 for refracting the illumination light from the light source 51, a beam splitter 53, and a cylindrical lens 54.

[0068] The light source 51 can be a laser diode or a light-emitting diode. For example, the light source 51 can generate illumination light in the infrared region. After being refracted by the optical lens 52, the illumination light passes through the beam splitter 53 and the cylindrical lens 54 in sequence, and is then reflected by the reflector 31.

[0069] Optical lens 52 may be, for example, a plano-convex lens. When the illumination light is a laser, beam splitter 53 is preferably a polarizing beam splitter. This is because it minimizes the amount of light reduction during reflection and transmission.

[0070] The cylindrical lens 54 can be of various shapes, such as rectangular, square, circular, or elliptical. The cylindrical lens 54 focuses light onto a line rather than a point. It functions to form a line beam. The advantage of using a line beam is that, due to optical astigmatism, sensitivity is increased, enabling more precise measurements.

[0071] Then, the illumination light reflected by mirror 31 passes through beam splitter 33 and enters objective lens 25. Objective lens 25 serves to focus the illumination light onto the measurement area of ​​wafer W and collect the reflected light reflected in the measurement area.

[0072] The reflected light collected by objective lens 25 passes through beam splitter 23 and is reflected by mirror 31. The reflected light reflected by mirror 31 is then reflected by beam splitter 53 to the photodetector 58.

[0073] The condenser lens 55 serves to concentrate the reflected light from the beam splitter 53 to the photodetector 58 onto the light-blocking wheel 56. When the height of the sample S surface is the reference height, the focal point of the reflected light through the condenser lens 55 is located on the blocking wheel 56.

[0074] Figure 6 It is shown Figure 5 The diagram shows a light-blocking wheel. (See diagram for reference.) Figure 6 As shown, the light-blocking wheel 56 has alternating transmission areas 561 for reflected light and blocking areas 563 for blocking reflected light along the angular direction. The blocking areas 563 can reflect or absorb the reflected light.

[0075] The light-blocking wheel 56 rotates at a constant speed and periodically blocks the reflected light. The rotation axis of the light-blocking wheel 56 can be parallel to the optical axis of the reflected light RL. The light-blocking wheel 56 serves to convert the reflected light into periodic discontinuous light and transmit it to the photodetector 58.

[0076] The photodetector 58 is configured to receive reflected light and generate two distinguishable electrical signals. When the blocking area 563 and the transmitting area 561 are arranged at a predetermined angular interval, a periodic electrical signal with a shape close to a square wave or a sine wave is generated. By generating this periodic signal, it is easy and accurate to determine whether the two electrical signals are consistent. The periodic waveform does not necessarily have to be a square wave or a sine wave. When the blocking area 563 and the transmitting area 561 are not arranged at a predetermined angular interval, a periodic electrical signal based on the 360-degree rotation of the light blocking wheel 56 can be generated.

[0077] The photodetector 58 includes a first sensing part 581 and a second sensing part 582, which are arranged side by side adjacent to each other on the same plane orthogonal to the optical axis of the reflected light.

[0078] 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.

[0079] The first sensing unit 581 and the second sensing unit 583 can employ various types of optical sensors. For example, photodiodes, PSD (Position Sensitive Device) sensors, CMOS (Complementary Metal Oxide Semiconductor) sensors, CCD (Charge-Coupled Device) sensors, etc., can be used. Alternatively, two separate regions of a single CCD or CMOS sensor can be used as the first sensing unit 581 and the second sensing unit 583.

[0080] Alternatively, the photodetector 58 can also be a dual-unit photodiode. In this case, the two units of the dual-unit photodiode can be 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.

[0081] The following is for reference. Figure 7 The function of the autofocus optical system 50 described above will be explained. Figure 7 (a) shows the case where the focal point F of the reflected light RL is located at the light blocking wheel 56. Figure 7 (b) shows the case where the sample surface 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 where the sample surface is raised and the focal point F of the reflected light RL is located behind the light blocking wheel 56.

[0082] like Figure 7 As shown in (a), when the focal point F of the reflected light RL is located at the light blocking wheel 56, the periodic signal S1 from the first sensing unit 581 and the periodic signal S2 from the second sensing unit 583 are in phase with each other.

[0083] However, as Figure 7 As shown in (b) and (c), when the focal point F of the reflected light RL is not located on the light blocking wheel 56, a phase difference is generated between the periodic signal S1 from the first sensing unit 581 and the periodic signal S2 from the second sensing unit 583.

[0084] More specifically, such as Figure 7 As shown in (b), when the focal point F of the reflected light RL is located in front of the light blocking wheel 56, the reflected light RL1 incident on the first sensing unit 581 will first meet the transmission area 561. Therefore, the pulse of the periodic signal S1 from the first sensing unit 581 is generated before the pulse of the periodic signal S2 from the second sensing unit 583.

[0085] On the contrary, such as Figure 7 As shown in (c), when the focal point F of the reflected light RL is located behind the light blocking wheel 56, the reflected light RL2 incident on the second sensing unit 583 will first meet the transmission area 561. Therefore, the pulse of the periodic signal S2 from the second sensing unit 583 is generated before the pulse of the periodic signal S1 from the first sensing unit 581.

[0086] The autofocus optical system 50 can detect the Z-direction position information of the sample S surface by measuring 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.

[0087] The focusing control unit 60 functions to move the objective lens 25 of the imaging optical system 30 in the Z direction during the period when the sample S is moved in the XY plane by the stage 10, so as to approach the distance for focusing on the mark formed at the current measurement position. The focusing control unit 60 moves the objective lens 25 in the Z direction according to the Z direction position information received from the autofocus optical system 50.

[0088] Even if the overlay marks have been focused at a previous measurement position, they may not be focused at the current measurement position. This may be due to issues with the surface accuracy of the stage 10 or errors caused by the thickness of the sample S. Furthermore, when the sample S is a semiconductor wafer, the vacuum chuck supporting the semiconductor wafer may be bent, and the semiconductor wafer itself may also be slightly bent.

[0089] 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 micro-motion control in the Z direction during the period when the position of the observation area of ​​the imaging optical system 30 changes from the previous measurement position to the current measurement position.

[0090] Figure 8 This is a diagram used to illustrate micro-motion control in the Z direction. Figure 8 (a) is a diagram used to illustrate conventional control methods. Figure 8 (b) is a diagram illustrating the control method of the present invention.

[0091] like Figure 8 As shown in (a), conventionally, with the previous measurement position S1 as a reference and the Z-direction position of the objective lens 25 fixed, the position of the observation area of ​​the imaging optical system 30 is directly changed to the current measurement 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.

[0092] Then, at the current measurement position S2, using the Z-direction position information from the autofocus optical system 50, the objective lens 25 is moved in the Z-direction to focus.

[0093] like Figure 8 As shown in (a), when the height difference between the previous measurement position S1 and the current measurement position S2 is large, the objective lens 25 needs to move a longer distance in the Z direction in order to achieve focus.

[0094] like Figure 8 As shown in (b), in this invention, the position of the observation area of ​​the imaging optical system 30 is changed toward the current measurement position S2, with the previous measurement position S1 as a reference and the Z-direction position of the objective lens 25 fixed.

[0095] When the objective lens 25 approaches the current measurement position S2 to a certain extent, the focusing control unit 60 performs micro-motion control in the Z direction based on the Z-direction position information received from the autofocus optical system 50, for example, every approximately 15 ms. That is, based on the height change of the sample S surface, the objective lens 25 is micro-motion controlled in the Z direction to adjust the distance between the sample S surface and the objective lens 25 to be as close as possible to the reference distance used for focusing.

[0096] Then, when the current measurement position S2 is reached, the objective lens 25 is moved in the Z direction to focus by using the Z-direction position information from the autofocus optical system 50.

[0097] In this invention, since the objective lens 25 is moved to near the focusing position before reaching the current measurement position S2, the distance the objective lens 25 needs to move during focusing is shortened. Therefore, compared with conventional methods, the move, acquire, and measure (MAM) time is reduced. For example, the MAM time can be reduced from 100ms to 90ms, a reduction of approximately 10%.

[0098] Alternatively, while maintaining the overall MAM time, the stability of the measurement can be improved by increasing the "in-position time" or "time in position." The "in-position time" refers to the time during which the distance between the objective lens 25 and the sample S surface can be considered the focusing distance, taking into account the vibration of the overlay measurement device 100. That is, it refers to the time during which the distance between the objective lens 25 and the sample S surface remains within an allowable range centered on the reference distance used as the focusing distance. The allowable range is determined based on the degree of vibration of the overlay measurement device 100.

[0099] Figure 9 These are graphs showing the change in distance between the objective lens 25 and the sample surface over time. Graph (a) shows the distance change corresponding to the control method of the present invention, and graph (b) shows the distance change in a conventional control method.

[0100] like Figure 9 As shown, according to the control method of the present invention, the "time spent in the measurement position" is longer due to the rapid arrival within the "in-position band". By increasing the number of "in-position checks" within the guaranteed "time spent in the measurement position", measurements can be performed in a more stable state.

[0101] Furthermore, the focusing control unit 60 can also perform micro-motion control in the Z direction during the period when the sample S is moved slightly in the X and Y directions by the stage 10, in a manner that the mark of the current measurement position S2 is located at the center of the image detector 40 after the objective lens 25 reaches the current measurement position S2.

[0102] That is, during the period when the sample S is moved in such a way that the mark formed on the current measurement position S2 is located at the center of the image detector 40, after detecting the Z-direction position information of the sample S surface, during the process of moving the sample S in such a way that the mark formed on the current measurement position S2 is located at the center of the image detector 40, the objective lens 25 of the imaging optical system 30 is moved in the Z direction according to the Z-direction position information, so that the distance between the sample S surface and the objective lens 25 is close to the reference distance.

[0103] The focusing control unit 60 can be a computing device such as an MCU (Micro Controller Unit), desktop computer, laptop computer, smartphone, or smart tablet, including: hardware, which has a processor, memory, fixed memory (ROM), hard disk or SSD storage device, and wired or wireless communication device for receiving Z-direction position information from the autofocus optical system 50 and transmitting control signals for controlling the objective lens 25; and firmware or software programs, which are stored on storage media such as memory, fixed memory, or storage device.

[0104] The focusing control unit 60 has an image acquisition device control program according to an embodiment of the present invention stored on its storage medium. The focusing control unit 60 instructs the processor to perform Z-direction micro-motion control of the objective lens 25 based on the Z-direction position information via instructions from the image acquisition device control program.

[0105] The image acquisition device control program can be executed manually or automatically after the autofocus function has been executed a predetermined number of times within a certain time interval, such as 5 times. This is to prevent the image acquisition device control program from automatically executing in unnecessary situations, such as when using the autofocus function for faulty shooting.

[0106] The computing device can also be used for movement of the stage 10. Movement of the stage 10 can be performed, for example, by converting the position coordinates of the target into stage coordinates and transmitting control data to the stage 10.

[0107] The following is for reference. Figure 10 The function of the image acquisition device 100 described above will be explained. Figure 10 This is a sequence diagram of an image acquisition method according to an embodiment of the present invention.

[0108] like Figure 10 As shown, an image acquisition method according to an embodiment of the present invention includes step S1, which is to detect the Z-direction position information of the surface of the sample S during the period when 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.

[0109] In this step, during the process of moving the stage 10 and changing 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, the autofocus optical system 50 detects the Z-direction position information of the sample S surface at specified time intervals.

[0110] Next, step S2 is performed, which is to move the objective lens 25 of the imaging optical system 30 in the Z direction according to the Z-direction position information during the period when 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, so that the distance between the surface of the sample S and the objective lens 25 is close to the reference distance.

[0111] Next, proceed to step S3, which involves performing autofocus.

[0112] In this step, the autofocus optical system 50 is used to focus on the measurement object mark formed at the current measurement position. Since the distance between the sample surface and the objective lens has been pre-set to be close to the reference distance in the previous step S2, the objective lens 25 can be slightly moved in the Z direction to focus on the mark in this step.

[0113] Next, step S4 is performed, which involves finely adjusting the position of the observation area of ​​the imaging optical system 30 in such a way that the mark formed at the current measurement position is located at the center of the image detector 40, while detecting the Z-direction position information of the sample S surface.

[0114] When the marker at the current measurement position enters the observation area of ​​the imaging optical system 30, after confirming the position of the marker in the image detector 40, the position information of the Z direction of the sample S surface is detected during the process of slightly moving the position of the observation area of ​​the imaging optical system 30 in the X and Y directions so that the marker is located in the center of the image detector 40.

[0115] Next, step S5 is performed, which involves finely adjusting the position of the observation area of ​​the imaging optical system 30 so that the mark formed at the current measurement position is located at the center of the image detector 40. Based on the Z-direction position information, the objective lens 25 of the imaging optical system 30 is moved in the Z-direction so that the distance between the sample S surface and the objective lens 25 is close to the reference distance.

[0116] Next, proceed to step S6, which involves performing autofocus.

[0117] In this step, the autofocus optical system 50 is used to focus on the measurement object marker at the current location. Then, an image of the marker that will be used in the measurement is acquired.

[0118] The embodiments described above are merely illustrative of preferred embodiments of the present invention, and the scope of the present invention is not limited to the described embodiments; those skilled in the art will be able to make various changes, modifications or substitutions within the scope of the technical concept and claims of the present invention, and such embodiments should be understood to fall within the scope of the present invention.

[0119] Figure Labels

[0120] S: Sample, 10: Stage, 20: Illumination optical system, 30: Imaging optical system, 40: Image detector, 50: Autofocus optical system, 60: Focus control unit.

Claims

1. An image acquisition apparatus configured to acquire an image of the markers from a sample having a plurality of markers formed thereon, the image acquisition apparatus characterized in that it comprises: A platform for configuring the sample; An imaging optical system configured to image reflected light from the mark; An image detector configured to receive the reflected light and generate a marked image; An autofocus optical system configured to detect the Z-direction position information of the sample's surface during the period when the position of the observation area of ​​the imaging optical system is changed from the previous measurement position of the sample to the current measurement position by moving at least one of the stage and the imaging optical system in the XY plane. as well as The focusing control unit is configured to move the objective lens of the imaging optical system in the Z direction according to the Z-direction position information during the period when the position of the observation area of ​​the imaging optical system is changed from the previous measurement position of the sample to the current measurement position, so that the distance between the surface of the sample and the objective lens is close to the reference distance used as the distance when focusing on the mark.

2. The image acquisition device according to claim 1, characterized in that, The autofocus optical system includes: A light source that generates illumination light that shines on the sample; A photodetector configured to receive reflected light from the sample; A light-blocking wheel, disposed at the front end of the photodetector, and having alternating transmission areas for the reflected light and blocking areas for the reflected light along an angular direction; and A focusing lens is configured to focus the reflected light toward the light-blocking wheel.

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

4. The image acquisition device according to claim 2, characterized in that, 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, characterized in that, The photodetector includes a first sensing part and a second sensing part, which are arranged side by side adjacently on the same plane orthogonal to the optical axis of the reflected light.

6. The image acquisition device according to claim 5, characterized in that, The autofocus optical system is configured such that when the focal point of the reflected light passing through the condenser lens is located at the light blocking wheel, the periodic signal from the first sensing unit and the periodic signal from the second sensing unit are in phase with each other.

7. The image acquisition device according to claim 5, characterized in that, The optical axis of the reflected light passing through the condenser lens is located at the boundary between the first sensing unit and the second sensing unit.

8. The image acquisition device according to claim 5, characterized in that, The first sensing unit and the second sensing unit are units of a dual-unit photodiode.

9. The image acquisition device according to claim 5, characterized in that, The autofocus optical system is configured such that 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 through the condenser lens is in front of the light blocking wheel is opposite in sign to 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 through the condenser lens is behind the light blocking wheel.

10. The image acquisition device according to claim 1, characterized in that, The autofocus optical system is configured to detect the Z-direction position information of the sample's surface during a period in which at least one of the stage and the imaging optical system moves slightly in the XY plane such that the mark formed at the current measurement position is located at the center of the image detector. The focusing control unit is configured to: during the period when at least one of the stage and the imaging optical system is moved slightly in the XY plane in such a way 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 according to the Z direction position information, such that the distance between the surface of the sample and the objective lens is close to the reference distance.

11. The image acquisition device according to claim 1, characterized in that, The sample is a semiconductor wafer, and the mark is an overlay mark.

12. The image acquisition device according to claim 1, characterized in that, The image acquisition device is an overlay measurement device.

13. An image acquisition method for acquiring an image of the markers from a sample having multiple markers, the image acquisition method being characterized by comprising: The step of detecting the Z-direction position information of the surface of the sample during the period when the position of the observation area of ​​the imaging optical system configured to image the reflected light from the mark on the image detector is changed from the previous measurement position of the sample to the current measurement position; as well as During the period of 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, the objective lens of the imaging optical system is moved in the Z direction according to the Z direction position information, so that the distance between the surface of the sample and the objective lens is close to the reference distance used as the distance when focusing on the mark.

14. The image acquisition method according to claim 13, characterized in that, Also includes: The step of detecting the Z-direction position information of the sample surface during the fine adjustment of the position of the observation area of ​​the imaging optical system in such a way that the mark formed at the current measurement position is located at the center of the image detector; as well as During the process of finely adjusting 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 objective lens of the imaging optical system is moved in the Z direction according to the Z-direction position information so that the distance between the surface of the sample and the objective lens is close to the reference distance.

15. An image acquisition device control program, stored in a storage medium for executing an image acquisition method using a computing device to acquire an image of a sample having a plurality of markers, characterized in that... The image acquisition device control program is stored in the storage medium to perform the following steps: The step of detecting the Z-direction position information of the sample's surface during the period when the position of the observation area of ​​the imaging optical system configured to image the reflected light from the marked point is changed from the previous measurement position of the sample to the current measurement position; and During the period of 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, the objective lens of the imaging optical system is moved in the Z direction according to the Z direction position information, so that the distance between the surface of the sample and the objective lens is close to the reference distance used as the distance when focusing on the mark.

16. The image acquisition device control program according to claim 15, characterized in that, The image acquisition device control program is an image acquisition device control program that is automatically executed when the autofocus function is executed by the computing device more than a predetermined number of times within a predetermined time interval.

17. The image acquisition device control program according to claim 15, characterized in that, The image acquisition device control program is firmware installed on the fixed memory device of the computing device.

Images