Overlay measuring device, focus control method for overlay measuring device for tool-guided shift measurement, and focus control program for overlay measuring device for tool-guided shift measurement

Abstract

The present invention provides a focus control method for an overlay measuring apparatus for tool-guided shift measurement, comprising the steps of: acquiring a first signal, which is a signal from an autofocus device, when the site is in focus for each site of a reference field of an unrotated reference wafer; rotating the reference wafer by 180 degrees and then acquiring a second signal, which is a signal from the autofocus device, when the site is in focus for each site of the reference field; matching the first and second signals with the respective sites and saving them; positioning a site to be measured of a wafer to be measured within the field of view of the overlay measuring apparatus; and, if the wafer to be measured is unrotated, focusing based on the first signal matched with the site to be measured, and if the wafer to be measured is rotated by 180 degrees, focusing based on the second signal matched with the site to be measured.

Description

TECHNICAL FIELD

[0001] The present invention relates to an overlay measurement apparatus, a focus control method for an overlay measurement apparatus for tool-induced shift measurement, and a focus control program for an overlay measurement apparatus for tool-induced shift measurement.

BACKGROUND ART

[0002] A plurality of pattern layers are sequentially formed on a semiconductor substrate. In some cases, circuits of one layer are divided into two patterns and formed through double patterning or the like. If these pattern layers or a plurality of patterns in one layer are accurately formed at predetermined positions, a desired semiconductor device can be manufactured.

[0003] Accordingly, overlay marks formed simultaneously with the pattern layers are used to confirm whether the pattern layers are accurately aligned.

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

[0005] Further, via the overlay measurement apparatus, images of the overlay structure of the pattern layer formed in the previous process (an image is acquired 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 value.

[0006] More specifically, Japanese Patent Publication No. 2020-112807 discloses a method for determining the relative misalignment between different layers or different patterns by capturing an image of an overlay mark formed on a substrate, selecting multiple 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 in Figure 2, the peaks are shown in the areas where the bars are placed. Since the conventional overlay mark 1 has bars placed periodically, the acquired signal also has periodicity. The overlay error is then measured through correlation analysis of the two periodic signals acquired from two selected regions 8 and 8'.

[0010] Measuring such overlay errors requires high-quality overlay measuring equipment to meet the requirements of improved lithography processes. During overlay error measurement, defects (e.g., aberrations) in the optical elements constituting the overlay measuring equipment can lead to tool-induced shift (TIS). Such optical defects in the overlay measuring equipment can cause a shift between the actual overlay error and the measured overlay error.

[0011] Tool-guided shift measurement requires repeated overlay error measurements of the wafer in both an unrotated and 180-degree rotated state. The tool-guided shift can be the sum of the overlay error values ​​in the unrotated state and the overlay error value in the 180-degree rotated state, divided by two. Ideally, the overlay error values ​​in the unrotated state and the overlay error value in the 180-degree rotated state are of the same magnitude but opposite signs, so the tool-guided shift is 0.

[0012] Overlay error measurement for tool-guided shift measurement is performed at multiple sites (overlay marks) within the reference field.

[0013] Figure 3 shows an example of a wafer. As shown in Figure 3, a semiconductor wafer contains multiple fields F. Within one field F, there are multiple (four in Figure 3) sites S1, S2, S3, and S4. Each site S1, S2, S3, and S4 has one overlay mark. The reference field is usually located in the center of the wafer. Sites that are in the same position within each field F are assigned the same site index. That is, when fields F are superimposed, overlapping sites have the same site index. The measurement sites S1, S2, S3, and S4 of the semiconductor wafer shown in Figure 3 can have site indices between 1 and 4.

[0014] To measure the overlay error for tool-guided shift measurement, an image of the overlay mark must be acquired. To acquire the image of the overlay mark, the "measurement focus" of the site must be found. The "measurement focus" is the focus for acquiring the optimal overlay mark image, and may differ from the "standard focus" found using an autofocus device. Finding the "measurement focus" is relatively time-consuming. Therefore, a method is used in which a "reference signal value," which is the signal value from the autofocus device in the "standard focus" state, is acquired in advance, and after adjusting the position of the objective lens so that the signal value from the autofocus device matches the "reference signal value," the overlay mark image is acquired.

[0015] Traditionally, the "reference signal value" of a specific site found in the unrotated state of the wafer was used directly to measure the overlay error in the 180-degree rotated state. However, as semiconductor elements become smaller and more integrated, overlay marks become smaller, creating a problem where the area illuminated by the lighting used in the autofocus system can become even larger than the size of the overlay mark. If the lighting extends beyond the overlay mark to other surrounding overlay marks or the boundaries of scribe lines, the signal from the autofocus system is likely to be distorted. Furthermore, because the surrounding environment differs between the unrotated state and the 180-degree rotated state, the tool-guided shift value measured using the conventional method may also be distorted. Therefore, applying the tool-guided shift value measured using the conventional method to correct the measured overlay error value can result in an inaccurate overlay error value.

[0016] Figure 4 shows the overlay mark and surrounding environment in an unrotated state and a 180-degree rotated state. Figure 4(a) shows the unrotated state, and (b) shows the state after the wafer has been rotated 180 degrees. As shown in Figure 4, in the unrotated state, line L is located above the overlay mark OM, while in the 180-degree rotated state, it is located below the overlay mark OM. The laser beam LB, which illuminates the autofocus device, illuminates beyond the overlay mark OM to the surrounding line L. Therefore, reflected light from the surrounding line L is also transmitted to the autofocus device.

[0017] Figure 5 is a graph showing the distribution of the difference between site-specific overlay error values ​​for the same wafer, measured using two different overlay measurement devices with a conventional method. Figure 5 is illustrated based on a wafer with 25 sites in each field. In the graph of Figure 5, the X axis is the site index. The Y axis is the difference between the overlay error values ​​obtained for the two devices, respectively. The distribution of Y values ​​is the distribution between sites located in different fields that have the same site index. The overlay error values ​​used to illustrate Figure 5 are not general overlay error values, but rather the values ​​obtained by subtracting the overlay error value in the 180-degree rotated state from the overlay error value in the unrotated state and then dividing by 2.

[0018] In Figure 5, the sites within the box (sites 2, 3, 7, 8, and 9) are sites where the difference between the unrotated state and the 180-degree rotated state of the surrounding environment is significant. As can be seen from Figure 5, using the conventional method, the difference values ​​of the overlay error for these sites show a different distribution from that of other sites. The closer the average difference values ​​for sites with the same site index are to 0, the smaller the dispersion of difference values ​​for sites with the same site index, and the more similar the distributions of sites with different site indices are, the more accurate the measurement of the overlay error can be. [Prior art documents] [Patent Documents]

[0019] [Patent Document 1] Japanese Patent Publication No. 2020-112807 [Patent Document 2] Korean Published Patent No. 10-2001-0092740 [Patent Document 3] Korean Registered Patent No. 10-2236184 [Patent Document 4] Korean Registered Patent No. 10-2524462 [Patent Document 5] Korean Registered Patent No. 10-1869573 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0020] The present invention is made to improve the above-mentioned problems, and an object thereof is to provide an overlay measurement apparatus capable of improving distortion of tool-induced shift that occurs when the area illuminated by the illumination used in an autofocus device is larger than the size of an overlay mark.

[0021] Further, another object of the present invention is to provide a focus control method for an overlay measurement apparatus for tool-induced shift measurement, and a focus control program for an overlay measurement apparatus for tool-induced shift measurement. MEANS FOR SOLVING THE PROBLEMS

[0022] To achieve the above object, the present invention is a focus control method for an overlay measurement apparatus for tool-induced shift measurement, comprising: a step of acquiring a first signal that is a signal from the autofocus device when each site in a reference field of a reference wafer in an unrotated state is in focus; a step of, after rotating the reference wafer by 180 degrees, acquiring a second signal that is a signal from the autofocus device when each of the sites in the reference field is in focus; a step of matching and storing the first signal and the second signal with the corresponding site; a step of positioning a measurement target site of a measurement target wafer within the field of view of the overlay measurement apparatus; and a step of, if the measurement target wafer is in an unrotated state, performing focusing based on the first signal matched with the measurement target site, and if the measurement target wafer is in a 180-degree rotated state, performing focusing based on the second signal matched with the measurement target site.

[0023] Further, the automatic focusing device is a phase difference detection automatic focusing system, and the first signal and the second signal are signals based on a phase difference. A focus control method for an overlay measurement apparatus is provided.

[0024] Further, the measurement target site is the site in the reference field of the reference wafer, or a site in a field different from the reference field. A focus control method for an overlay measurement apparatus is provided.

[0025] Further, the measurement target wafer is the reference wafer or another wafer belonging to the same lot (Lot) as the reference wafer. A focus control method for an overlay measurement apparatus is provided.

[0026] Further, whether the measurement target wafer is rotating is confirmed from the position of the flat zone (Flat Zone) or notch (Notch) of the wafer. A focus control method for an overlay measurement apparatus is provided.

[0027] Further, the first signal matched with the measurement target site is a first signal acquired from a site in the reference field that has the same in-field position as the measurement target site. A focus control method for an overlay measurement apparatus is provided.

[0028] Further, the present invention is an overlay measurement apparatus, comprising: an objective lens, a lens focus actuator configured to move the objective lens to adjust the distance between the wafer and the objective lens, an automatic focusing device that outputs a signal corresponding to the distance between the objective lens and the wafer, and a controller communicatively coupled to the lens focus actuator and the automatic focusing device. An overlay measurement apparatus is provided.

[0029] Here, the controller includes a memory in which instructions are stored and a processor configured to execute the instructions. The instructions are configured such that the processor performs the following steps: acquires a first signal, which is a signal from the autofocus device when the autofocus device is in focus on each site of the reference field of an unrotated reference wafer; rotates the reference wafer 180 degrees and then acquires a second signal, which is a signal from the autofocus device when the autofocus device is in focus on each site of the reference field; matches the first and second signals with the sites and stores them; positions the site to be measured on the wafer to be measured within the field of view of the overlay measuring device; and, if the wafer to be measured is unrotated, moves the objective lens to focus based on the first signal matched with the site to be measured; and if the wafer to be measured is rotated 180 degrees, moves the objective lens to focus based on the second signal matched with the site to be measured.

[0030] Furthermore, the present invention provides a program stored on a storage medium for executing a method for controlling the focus of an overlay measuring apparatus using a computing device, the program comprising the steps of: acquiring a first signal, which is a signal from an autofocus device when the focus is on each site of the reference field of an unrotated reference wafer; acquiring a second signal, which is a signal from the autofocus device when the focus is on each site of the reference field after rotating the reference wafer by 180 degrees; matching the first and second signals with the sites and storing them; positioning the site to be measured of the wafer to be measured within the field of view of the overlay measuring apparatus; and, if the wafer to be measured is unrotated, focusing based on the first signal matched with the site to be measured, and if the wafer to be measured is rotated by 180 degrees, focusing based on the second signal matched with the site to be measured. [Effects of the Invention]

[0031] According to the present invention, the distortion of tool guidance shift caused by the area illuminated by the lighting used in the autofocus device becoming larger than the size of the overlay mark is improved, and a more accurate tool guidance shift value can be obtained.

[0032] By correcting overlay errors using such tool-inducible shift values, it is possible to measure overlay errors more accurately even on wafers with small overlay marks. [Brief explanation of the drawing]

[0033] [Figure 1] This is a plan view of an example of an overlay mark. [Figure 2] Figure 1 shows the signal acquired from one working zone of the overlay mark. [Figure 3] This figure shows an example of a wafer. [Figure 4] This figure shows the overlay marks and surrounding environment in both the unrotated and 180-degree rotated states. [Figure 5] This graph shows the distribution of the difference between site-specific overlay error values ​​for the same wafer, measured using two different overlay measurement devices with conventional methods. [Figure 6] This is a schematic diagram of an overlay measurement device according to one embodiment of the present invention. [Figure 7] This flowchart shows a focus control method for an overlay measuring device for tool-guided shift measurement according to one embodiment of the present invention. [Figure 8] An example of a table where the first and second signals are stored is shown. [Figure 9] This graph shows the distribution of the difference between site-specific overlay error values ​​of the same wafer, measured by two different overlay measurement devices using the focus control method of the present invention. [Modes for carrying out the invention]

[0034] 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.

[0035] Figure 6 is a schematic diagram of an overlay measurement apparatus according to one embodiment of the present invention. As shown in Figure 6, the overlay measurement apparatus 100 includes a stage 10 that supports a semiconductor wafer, an illumination optical system 20 that illuminates overlay marks on the semiconductor wafer W, an imaging optical system 30 that focuses reflected light from the overlay marks to form an overlay mark image, an image detector 40 that acquires the overlay mark image formed by the imaging optical system 30, an autofocus device 50, and a controller 60.

[0036] The overlay measuring device 100 is used to measure the overlay error between multiple consecutive pattern layers.

[0037] Stage 10 can be moved in mutually orthogonal X and Y directions by a horizontal drive unit. Stage 10 supports and horizontally moves the semiconductor wafer W. Stage 10 may be equipped with a vacuum chuck for fixing the semiconductor wafer W.

[0038] The illumination optical system 20 can be configured using a variety of optical elements. For example, the illumination optical system 20 may include an illumination source 21, a beam splitter 23, and an objective lens 25. It may also include other optical elements such as lenses and apertures.

[0039] The illumination source 21 is responsible for generating illumination that lights up the overlay mark. The illumination source 21 may include a light source capable of generating light across a wide wavelength range and a variable optical filter that can adjust the wavelength range of the transmitted light. A laser diode or a light-emitting diode can be used as the light source. By combining optical filters to adjust the wavelength range of light from the light source, the illumination source 21 can generate illumination across various wavelength ranges.

[0040] The beam splitter 23 is positioned between the illumination source 21 and the objective lens 25, and its role is to transmit the illumination from the illumination source 21 to the objective lens 25.

[0041] The objective lens 25 focuses the illumination onto the measurement position on the surface of the semiconductor wafer W and collects the reflected light from the measurement position. 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 semiconductor wafer W. Focus control is performed by moving the objective lens 25 in the Z direction.

[0042] The imaging optical system 30 can be constructed using a variety of optical elements. For example, the imaging optical system 30 may include a hot or cold mirror 31 and a tube lens 33. Alternatively, the imaging optical system 30 may use the objective lens 25 and beam splitter 23 of the illumination optical system 20. It may also include other optical elements such as lenses and apertures.

[0043] The hot or cold mirror 31 prevents the illumination used in the autofocus device 50 from being directed towards the image detector 40. The hot mirror has high transmittance to short-wavelength light and reflects long-wavelength light. Conversely, the cold mirror has high transmittance to long-wavelength light and reflects short-wavelength light. If the illumination used in the autofocus device 50 has a shorter wavelength than the illumination used for image acquisition, the cold mirror is used to reflect the light from the illumination used in the autofocus device 50 back towards the autofocus device 50. Conversely, if the illumination used in the autofocus device 50 has a longer wavelength than the illumination used for image acquisition, the hot mirror is used to reflect the light from the illumination used in the autofocus device 50 back towards the autofocus device 50.

[0044] The reflected light collected by the objective lens 25 passes through the beam splitter 23 and is then focused onto the image detector 40 by the tube lens 33.

[0045] The image detector 40 receives reflected light from the overlay mark due to illumination 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.

[0046] The autofocus device 50 provides information regarding the Z-direction position of the wafer w surface necessary for focusing on the object to be measured. The autofocus device 50 provides, for example, a signal corresponding to the distance between the objective lens 25 and the surface of the wafer w. Various types of autofocus systems can be used as the autofocus device 50. For example, a phase-difference detection autofocus system, a contrast-sensing autofocus system, or a hybrid autofocus system may be used.

[0047] The phase difference detection autofocus system uses two or more sensors to sense the phase difference of light rays entering the overlay measuring device 100 from the wafer w that is the object to be photographed, and can determine the direction and distance that the objective lens 25 must move.

[0048] The contrast-sensing autofocus system analyzes the contrast between the bright and dark areas of an image to determine the focal point. The system determines that the image is in focus when the contrast is at its maximum.

[0049] The hybrid autofocus system first obtains an approximate focal length using a contrast-sensing method, and then fine-tunes the focal position using a phase-difference detection method.

[0050] The controller 60 is installed to communicate with the lens focus actuator 27, the image detector 40, and the autofocus device 50 by wire or wireless means. The controller 60 may include hardware such as a processor, memory, ROM, storage devices such as hard disks and SSDs, and wired or wireless communication devices. It may also include programs such as firmware and software installed on storage media such as memory, ROM, and storage devices.

[0051] The controller 60 can be a computing device such as an MCU (Microcontroller unit), desktop computer, laptop computer, smartphone, or smartpad.

[0052] The controller 60 can cause the processor to perform the steps shown in Figure 7 via program instructions.

[0053] Figure 7 is a flowchart showing a focus control method for an overlay measuring device for tool-guided shift measurement according to one embodiment of the present invention.

[0054] As shown in Figure 7, the controller 60 performs step (S1) of acquiring a first signal, which is a signal from the autofocus device when the autofocus device is in focus on each site of the reference field of the unrotated reference wafer.

[0055] A reference wafer is one of several wafers belonging to the same lot. A reference wafer contains multiple fields. Each field contains multiple sites (e.g., 25). Overlay marks are located at these sites. The reference field is typically located in the center of the wafer.

[0056] In this step, a first signal is acquired for each site within the reference field, which is the signal from the autofocus device 50 when the overlay mark of that site is in focus. If a phase difference detection autofocus system is used as the autofocus device 50, the phase value output from the phase difference detection autofocus system while aligned with the "measurement focus," which is the focus applied when acquiring the overlay mark image for actual overlay error measurement, can become the first signal.

[0057] In a typical imaging system, the focus found via the autofocus device 50 becomes the "measurement focus." However, since overlay marks include overlay structures formed with different pattern layers, the "measurement focus" can be located between the overlay structures formed on the previous pattern layer and the overlay structures formed on the current pattern layer. In particular, when there is a large difference in height between the overlay structures, the difference between the focus found via the autofocus device 50 and the "measurement focus" can be large. The "measurement focus" can be found by evaluating the quality of the acquired overlay mark image using various quality indicators.

[0058] If a phase-difference detection autofocus system is used as the autofocus device 50, in a normal imaging system, focus is determined when the phase difference is zero. However, in the case of overlay marks, focus ("measured focus") may be achieved when the phase difference is not zero.

[0059] After acquiring the first signal from one site, a pattern recognition unit (PRU) is used to position the next site in the center of the field of view of the overlay measurement device 100. Then, the first signal from that site is acquired in the same manner.

[0060] This process is repeated until a first signal is obtained at all measurement sites within the reference field. These first signals may differ from site to site due to differences in the type and height of the pattern layer on which the overlay marks are formed.

[0061] Next, the controller 60 rotates the reference wafer by 180 degrees and then performs step (S2) to acquire a second signal from the autofocus device 50, which is the signal when the autofocus device is in focus on each site of the reference field.

[0062] In this step, the stage on which the reference wafer is placed is first rotated 180 degrees.

[0063] Then, for each site in the reference field used in step S1, a second signal is obtained, which is the signal from the autofocus device 50 when the overlay mark of that site is in focus.

[0064] Ideally, a second signal for an overlay mark at the same site would match the first signal. However, in practice, the second and first signals may differ from each other. For example, if the size of the laser line, which is the illumination used in the autofocus device 50, is larger than the size of the overlay mark, the laser may irradiate not only the overlay mark at the site being measured but also other surrounding overlay marks or patterns. Reflected light from these surrounding marks and patterns can affect the signal of the autofocus device 50. The degree of such an effect may vary depending on the wafer rotation angle.

[0065] For sites located in the center of the reference field, the first and second signals may be similar. However, for sites where the environment around the overlay mark changes significantly depending on the rotation angle, such as sites at the top or bottom of the reference field, the difference between the second and first signals may be very large.

[0066] This step is performed in the same manner as in step S1.

[0067] Next, the controller 60 performs the step (S3) of matching the first signal and the second signal with the site and saving them.

[0068] For example, as shown in Figure 8, the information may be stored in the memory of the controller 60 or other storage device in the form of a table containing the recipe ID, overlay mark ID, site index, first signal, second signal, etc. The table stores the first and second signals for all sites in the reference field.

[0069] Next, the controller 60 performs step (S4) of positioning the measurement target site of the wafer to be measured within the field of view of the overlay measurement device 100.

[0070] In this step, the controller 60 can move the wafer by moving the stage 10 that supports the wafer to be measured in a planar direction. The wafer to be measured may be a reference wafer, or another wafer belonging to the same lot as the reference wafer.

[0071] This step may include a global alignment step and a site alignment step. The controller 60 uses a pattern recognition unit to ensure that the overlay marks of the site to be measured are located within the field of view of the overlay measuring device 100.

[0072] The measurement target site may be the reference field or another field site of the reference wafer used when acquiring the first and second signals. Alternatively, it may be a field site of another wafer belonging to the same lot manufactured in the same process.

[0073] If the site index is the same site, the first and second signals will be nearly identical even if the site belongs to a different field on the same wafer or a field on a different wafer from the same lot. Therefore, the first and second signal information obtained from the reference field can be applied to sites in other fields on the same wafer and fields on other wafers from the same lot.

[0074] Next, the controller 60 performs the following steps: if the wafer to be measured is not rotating, it moves the objective lens 25 to focus based on the first signal matched with the measurement site; if the wafer to be measured is rotated 180 degrees, it moves the objective lens to focus based on the second signal matched with the measurement site (S5).

[0075] The controller 60 determines whether the wafer to be measured, for which tool-guided shift measurement is required, is in an unrotated state or rotated 180 degrees. The controller 60 can determine the rotation state of the wafer to be measured by, for example, using a proximity sensor to confirm the position of the flat zone or notch of the wafer to be measured.

[0076] If the wafer to be measured is not rotated, the controller 60 moves the objective lens 25 to focus using a first signal matched with a site having the same site index as the wafer to be measured stored in the table. If the wafer to be measured is rotated 180 degrees, the controller moves the objective lens 25 to focus using a second signal stored in the table.

[0077] The controller 60 can move the objective lens 25 to focus until the signal output from the autofocus device 50 matches the first signal (or the second signal if it is rotated 180 degrees). The controller 60 can adjust the position of the objective lens 25 in the Z-axis direction by controlling the lens focus actuator 27.

[0078] Once the objective lens 25 has finished moving, the illumination optical system 20 is used to illuminate the measurement target site, and the reflected light from the overlay mark is collected by the imaging optical system 30 to form an overlay mark image on the image detector 40, thereby acquiring the overlay mark image.

[0079] The X-direction overlay error and Y-direction overlay error are measured using the overlay mark image acquired in the unrotated state. Then, the X-direction overlay error and Y-direction overlay error are measured again using the overlay mark image acquired in the same way at the same site in a 180-degree rotated state.

[0080] The X-direction tool-guided shift of the overlay mark (site) in question may be, for example, the average of the sum of the X-direction overlay error in the unrotated state and the X-direction overlay error in the 180-degree rotated state. Similarly, the Y-direction tool-guided shift may be the average of the sum of the Y-direction overlay error in the unrotated state and the Y-direction overlay error in the 180-degree rotated state. Ideally, both the X-direction and Y-direction tool-guided shifts should be 0.

[0081] X-direction tool-guided shift and Y-direction tool-guided shift can be used to correct overlay error values ​​measured at the site.

[0082] Figure 9 is a graph showing the distribution of the difference between site-specific overlay error values ​​of the same wafer, measured by two different overlay measuring devices using the focus control method of the present invention. In the graph of Figure 9, the X axis represents the site index, and the Y axis represents the difference in overlay error values ​​between the overlay measuring devices. Similar to Figure 5, it shows the distribution of overlay error values ​​measured at sites with the same site index among multiple fields of a single wafer.

[0083] As shown in Figure 9, when the focus control method of the present invention is applied, the difference in overlay error values ​​between sites with different site indices of the device shows a similar distribution. In other words, site-specific deviations caused by illumination distortion of the autofocus device are reduced.

[0084] 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 are possible by those skilled in the art within the technical spirit 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]

[0085] w wafer 10 stages 20 Illumination optical system 30 Imaging Optical System 40 Image detectors 50 autofocus device 60 Controllers

Claims

1. A method for controlling the focus of an overlay measuring apparatus for tool-guided shift measurement, comprising the steps of: acquiring a first signal, which is a signal from an autofocus device, when the focus is on each site of a reference field of an unrotated reference wafer, wherein the illumination of the autofocus device is illumination that is long enough to avoid the overlay marks formed on the site; The step of rotating the reference wafer by 180 degrees, and then acquiring a second signal from the autofocus device when the autofocus device is in focus on each of the sites in the reference field, wherein the illumination of the autofocus device is illumination that is long enough to avoid the overlay marks formed on the sites, The steps include matching the first signal and the second signal with the site and saving them, The steps include positioning the measurement target site of the wafer to be measured within the field of view of the overlay measurement device, A method for controlling the focus of an overlay measuring apparatus, comprising the steps of: focusing based on a first signal matched with the measurement site if the wafer to be measured is not rotated; and focusing based on a second signal matched with the measurement site if the wafer to be measured is rotated 180 degrees.

2. The autofocus device is a phase difference detection autofocus system, and the first signal and the second signal are signals based on a phase difference, the focus control method for an overlay measuring device according to claim 1.

3. The focus control method for an overlay measuring apparatus according to claim 1, wherein the measurement target site is the reference field of the reference wafer, or a site in a field different from the reference field.

4. The focus control method for an overlay measuring apparatus according to claim 1, wherein the wafer to be measured is the reference wafer, or another wafer belonging to the same lot as the reference wafer.

5. The focus control method for an overlay measuring apparatus according to claim 1, wherein whether or not the wafer to be measured rotates is confirmed from the position of the flat zone or notch of the wafer.

6. The focus control method for an overlay measuring device according to claim 1, wherein the first signal matched with the site to be measured is a first signal obtained from a site in the reference field that is at the same position in the field as the site to be measured.

7. An overlay measuring device, The objective lens, A lens focus actuator configured to adjust the distance between the wafer and the objective lens by moving the objective lens, An autofocus device that outputs a signal corresponding to the distance between the objective lens and the wafer, The system includes a controller that is communicatively coupled to the lens focus actuator and the autofocus device, The controller includes a memory in which instructions are stored, and a processor configured to execute the instructions, wherein the instructions are executed by the processor, A step of acquiring a first signal, which is a signal from the autofocus device when the autofocus device is in focus on each site of the reference field of an unrotated reference wafer, wherein the illumination of the autofocus device is illumination that is long enough to avoid the overlay marks formed on the site, The step of rotating the reference wafer by 180 degrees, and then acquiring a second signal from the autofocus device when the autofocus device is in focus on each of the sites in the reference field, wherein the illumination of the autofocus device is illumination that is long enough to avoid the overlay marks formed on the sites, The steps include matching the first signal and the second signal with the site and saving them, The steps include positioning the measurement target site of the wafer to be measured within the field of view of the overlay measurement device, An overlay measuring apparatus that, if the wafer to be measured is not rotating, moves the objective lens to focus based on the first signal matched with the site to be measured, and if the wafer to be measured is rotated 180 degrees, moves the objective lens to focus based on the second signal matched with the site to be measured.

8. A program stored on a storage medium for executing a method of controlling the focus of an overlay measuring device using a computing device, A step of acquiring a first signal, which is a signal from an autofocus device when the autofocus device is in focus on each site of the reference field of an unrotated reference wafer, wherein the illumination of the autofocus device is illumination that is long enough to avoid the overlay marks formed on the site, The step of rotating the reference wafer by 180 degrees, and then acquiring a second signal from the autofocus device when the autofocus device is in focus on each of the sites in the reference field, wherein the illumination of the autofocus device is illumination that is long enough to avoid the overlay marks formed on the sites, The steps include matching the first signal and the second signal with the site and saving them, The steps include positioning the measurement target site of the wafer to be measured within the field of view of the overlay measurement device, A program for controlling the focus of an overlay measuring apparatus, which performs the steps of: focusing based on a first signal matched with the measurement site if the wafer to be measured is not rotated; and focusing based on a second signal matched with the measurement site if the wafer to be measured is rotated 180 degrees.

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