KR102750969B1 - Autofocus Device, Overlay measuring Device Equipped Therewith, and Calibration Method of the Autofocus Device - Google Patents

Abstract

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

Description

{Autofocus Device, Overlay Measuring Device Equipped Therewith, and Calibration Method of the Autofocus Device}

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

As technology advances, the size of semiconductor devices is shrinking and the density of integrated circuits is increasing. In order to meet these requirements, various conditions must be satisfied, and among them, overlay tolerance is one of the important indicators.

Semiconductor devices are manufactured through numerous manufacturing processes. In order to form an integrated circuit on a wafer, many manufacturing processes must be performed so that the desired circuit structures and elements are formed sequentially at specific locations. The manufacturing process sequentially creates patterned layers on the wafer. Through these repeated stacking processes, electrically active patterns are created in the integrated circuit. At this time, if each structure is not aligned within the tolerance range allowed by the production process, interference occurs between the electrically active patterns, and this phenomenon can cause problems in the performance and reliability of the manufactured circuit. An overlay measurement tool is used to measure and verify the alignment errors between these layers.

General overlay metrology and methods measure and verify whether the alignment between two layers is within the tolerance. One method is to form a structure called an overlay mark at a specific location on a substrate, photograph the structure with an optical image acquisition device, and measure the overlay. The structure for measurement is designed to be able to measure the overlay in at least one direction among the X direction and the Y direction for each layer. Each structure is designed as a symmetrical structure, and the center value between the structures arranged in the symmetrical direction is calculated and used as the representative value of that layer, and the relative difference between the representative values of each layer is calculated to derive the overlay error.

Hereinafter, a method for measuring an overlay of two layers using a Box in Box (BIB) overlay mark is briefly described. First, as illustrated in FIGS. 1 and 2, a first overlay mark (OM ₁ ) having a generally box shape and a second overlay mark (OM ₂ ) having a generally smaller box shape than the first overlay mark (1) are formed in two consecutive layers, respectively. Then, images of the first overlay mark (OM ₁ ) and the second overlay mark (OM ₂ ) are acquired at once. Next, as illustrated in FIG. 3, a waveform representing a change in intensity according to position is acquired, and the center value (C ₁ ) of the first overlay mark (OM ₁ ) and the center value (C ₂ ) of the second overlay mark (OM ₂ ) are acquired, respectively. Then, by comparing these, the overlay error between the two layers is measured.

If the height difference between the two layers is not large, the autofocus sensor can be used to automatically adjust the focus, and then a combined image of the first overlay mark (OM ₁ ) and the second overlay mark (OM ₂ ) can be acquired at once. The standard focus found using the autofocus sensor may not be the optimal focus for acquiring the combined image, but if the height difference between the two layers is not large, this is not a significant problem.

However, due to the advancement of semiconductor process technology, there is a problem that the difference between the standard focus obtained using the auto-focus sensor and the measurement focus for more accurate measurement may be large in the present where the height difference is large and the overlay error between layers with different optical properties needs to be accurately measured. For example, the standard focus by the auto-focus sensor may be located above the current layer, but the measurement focus may be located between the previous layer and the current layer.

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

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

In order to solve the problem of difficulty in accurately measuring the overlay error between pattern layers with a large height difference, a double-shot method was also used, in which a waveform representing the change in intensity according to the position of a signal obtained after focusing on a first overlay mark (OM ₁ ) is acquired to obtain the center value (C _{1 )} of the first overlay mark (OM ₁ ), and a waveform representing the change in intensity according to the position of a signal obtained after focusing on a second overlay mark (OM ₂ ) is acquired to obtain the center value (C ₂ ) of the second overlay mark (OM 2 ) to measure the overlay error between the two layers.

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

In order to improve these problems, Korean Patent No. 10-2524462, filed and registered by the applicant of the present invention, discloses an overlay measuring device which can match the position of a standard focus with the position of a measurement focus by arranging an optical lens between a light source (light source for autofocus) of an autofocus device and an objective lens, and adjusting the position of the optical lens using an actuator (second actuator). If the position of the standard focus and the position of the measurement focus match, there is an advantage in that the measurement speed is greatly increased because there is no need to measure the position of the measurement focus every time.

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

Fig. 6 is a drawing showing the change in the position and size of the illumination light according to the position of the optical lens of the autofocus device. As shown in Fig. 6, not only the size but also the position of the illumination light (LB) changes according to the distance between the optical lens and the light source. Ideally, even if the optical lens moves, the position of the illumination light (LB) should not deviate from the center of the overlay mark. The illumination light (LB) of Fig. 6 is illumination light focused in a line shape by passing through the cylinder lens.

In this way, if the illumination light (LB) deviates from the center of the overlay mark, there is a problem that the accuracy of auto-focus adjustment is reduced. In particular, when the overlay error between multiple pattern layers must be measured, there was a problem that the position of the optical lens may vary depending on the measurement target, and thus the degree of misalignment of the illumination light may vary.

Korean Patent Publication No. 2003-0054781 (July 2, 2003) Korean Patent Registration No. 10-0689709 (2007.2.26) Korean Patent Registration No. 10-1564312 (2015.10.23) Korean Patent Registration No. 10-2524462 (April 21, 2023)

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

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

In addition, it aims to provide a method for calibrating such autofocus devices.

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

Additionally, the optical lens provides an auto-focus device which is a plano-convex lens.

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

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

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

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

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

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

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

Figure 1 is a plan view of the overlay mark.
Figure 2 is a side view of the overlay mark illustrated in Figure 1.
Figure 3 shows a waveform of change in intensity at each position of a signal obtained from an image obtained by photographing the overlay mark illustrated in Figure 1.
FIG. 4 shows a waveform of change in intensity according to position of a signal acquired from an image obtained while focusing on the first overlay mark of the overlay mark illustrated in FIG. 1.
Figure 5 shows a waveform of change in intensity according to position of a signal acquired from an image obtained while focusing on the second overlay mark of the overlay mark illustrated in Figure 1.
Figure 6 is a drawing showing the change in position and size of illumination light according to the position of the optical lens of the autofocus device.
FIG. 7 is a conceptual diagram of an overlay measuring device having an auto-focus device according to an embodiment of the present invention.
Figure 8 is a drawing for explaining the movement of a line beam according to the rotation of a cylinder lens.
Figure 9 is a flowchart for explaining the operation of the overlay measuring device illustrated in Figure 7.
Fig. 10 is a drawing for explaining the step of finding the measurement focus position of Fig. 9.
Figure 11 is a drawing for explaining the steps of finding the reference signal value of Figure 9.
Figure 12 is a drawing for explaining the change in focus position according to the position of the optical lens.
Figure 13 is a drawing for explaining the change in the reference signal value according to the position of the optical lens.
Figure 14 is a flow chart of a calibration method for an autofocus device.

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art. Therefore, the shapes of elements in the drawings are exaggerated to emphasize a clearer description, and elements indicated by the same symbols in the drawings mean the same elements.

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

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

In this case, the second overlay mark (OM ₂ ) is exposed to the outside, but the first overlay mark (OM ₁ ) is covered by a photoresist layer and is made of an oxide having different optical properties from the second overlay mark (OM ₂ ) made of a photoresist material. In addition, the first overlay mark (OM ₁ ) and the second overlay mark (OM ₂ ) have different heights.

Overlay marks can use a variety of currently used overlay marks, such as box-in-box (see Figure 1) and Advanced Imaging Metrology (AIM) overlay marks.

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

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

The image acquisition device (100) serves to acquire an image (hereinafter referred to as an 'alignment image') in which a first overlay mark (OM ₁ ) and a second overlay mark (OM ₂ ) are displayed together at multiple focus positions.

An image acquisition device (100) comprises an image acquisition light source (111), a first beam splitter (113), an objective lens (115), and a first actuator (117) to irradiate an image acquisition illumination light onto a measurement area of a wafer (W). The measurement area is an area where a first overlay mark (OM ₁ ) and a second overlay mark (OM ₂ ) are formed in a scribe lane of the wafer (W). A plurality of measurement areas exist in one wafer (W). The overlay measurement device (1) measures an overlay error in the plurality of measurement areas.

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

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

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

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

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

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

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

The image detector (130) may be a CCD or CMOS camera. The image acquisition device (100) acquires an alignment image using an electric signal from the image detector (130). Then, by analyzing this alignment image, the overlay error can be measured.

The autofocus device (200) plays a role of generating a signal according to the distance between the objective lens (115) and the wafer (W). In other words, it plays a role of generating a signal according to the change in the focus position of the image acquisition device (100). In addition, it can also play a role of controlling the first actuator (117) using this signal to focus.

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

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

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

In the present invention, infrared rays having a different wavelength band from the image acquisition illumination light are used as the auto-focus illumination light, and the reflected lights thereof are separated from each other using a hot mirror (121), thereby preventing the reflected light of the auto-focus illumination light from being incident on the image detector (130).

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

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

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

The auto-focus detector (230) receives reflected light and generates a signal according to the distance between the objective lens (115) and the wafer (W). For example, when the focus of the image acquisition unit matches the 'standard focus', the phase difference value, which is a signal value output to the auto-focus detector (230), may be 0, and when they do not match, the value may be + or - depending on the position of the focus. The 'standard focus' is the focus of the image acquisition device (100) determined based on the signal of the auto-focus detector (230). The 'standard focus' may match the 'measurement focus' suitable for obtaining an actual alignment image, but in most cases, they are different from each other when the height difference between the first overlay mark (OM ₁ ) and the second overlay mark (OM ₂ ) is large. The measuring method of the 'measurement focus' will be described later.

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

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

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

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

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

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

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

In this step, the controller (20) receives alignment images acquired through the imaging system (10) at multiple focus positions. The alignment images include a first overlay mark (OM ₁ ) and a second overlay mark (OM ₂ ). The alignment images can be acquired by continuously changing the focus position using the first actuator (117).

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

The controller (20) can determine the measurement focus position based on the change in the contrast value of the first overlay mark (OM ₁ ) of the alignment images according to the focus position and the change in the contrast value of the second overlay mark (OM ₂ ).

The contrast value of the first overlay mark (OM ₁ ) and the contrast value of the second overlay mark (OM ₂ ) change according to the focus position (the distance between the objective lens and the wafer). Since the first overlay mark (OM ₁ ) is located on the previous layer formed earlier, the contrast value has a maximum value at the focus position where the distance between the wafer (W) and the objective lens (115) is close. Since the second overlay mark (OM ₂ ) is located on the current layer formed on the previous layer, the contrast value has a maximum value at the focus position where the distance between the wafer (W) and the objective lens (115) is far.

As illustrated in FIG. 10, the measurement focus position can be found, for example, at a position between the maximum value of the change graph (G ₁ ) of the contrast value by the first overlay mark (OM ₁ ) and the maximum value of the change graph (G ₂ ) of the contrast value by the second overlay mark (OM ₂ ). For example, the position where the change graph (G ₁ ) of the contrast value by the first overlay mark (OM ₁ ) and the change graph (G ₂ ) of the contrast value by the second overlay mark (OM ₂ ) intersect can be determined as the measurement focus position. Additionally, the focus position having the average value of the maximum value of the change graph (G ₁ ) of the contrast value by the first overlay mark (OM ₁ ) and the maximum value of the change graph (G ₁ ) of the contrast value by the second overlay mark (OM ₂ ) can also be determined as the measurement focus position.

In some cases, the measurement focus position may be located outside the interval between the maximum value of the graph of change in contrast value (G ₁ ) by the first overlay mark (OM ₁ ) and the maximum value of the graph of change in contrast value (G ₂ ) by the second overlay mark (OM ₂ ).

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

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

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

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

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

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

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

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

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

The predetermined optimal reference signal value may be a specific value, but any value within a certain range may be set as the reference signal value. For example, if the reference signal value is within the range of 0±α where the accuracy of the auto-focus detector (230) is secured, that value may be used as the reference signal value and the position of the optical lens (212) may not be adjusted.

If the reference signal value compared at step S4 is equal to the predetermined optimal reference signal value, this step (S5) can be omitted.

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

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

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

A pair of first overlay marks (OM ₁ ) and second overlay marks (OM ₂ ) are formed in a plurality of measurement areas on a single wafer. Therefore, the overlay measuring device (1) according to the present invention measures the overlay error in a plurality of measurement areas. The overlay measuring device according to the present invention has the advantage of being fast, particularly when measuring the overlay error in a plurality of measurement areas.

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

Hereinafter, with reference to FIG. 14, a method for calibrating an auto-focus device (200) will be described. Calibration of the auto-focus device (200) may be performed before measuring an overlay error using the overlay measuring device (1) described above. For example, calibration of the auto-focus device (200) may be performed at the stage of manufacturing the overlay measuring device (1). In addition, calibration of the auto-focus device (200) may be performed again after using the overlay measuring device (1) for a certain period of time.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

OM ₁ : 1st overlay mark
OM ₂ : Second overlay mark
1: Overlay measuring device
20: Controller
100: Image acquisition device
111: First light source
115: Objective lens
117: Actuator 1
121: Hot Mirror
130: Detector 1
200: Autofocus device
211: Second Light Source
212: Optical Lens
215: Cylinder lens
219: Cylinder Lens Holder
217: 2nd actuator
230: 2nd detector

Claims (8)

A light source configured to emit illuminating light for automatic focus adjustment,
An optical lens arranged on the path of the illumination light emitted from the light source and configured to refract the illumination light;
A cylinder lens arranged on the path of the illumination light passing through the optical lens and configured to change the illumination light into a line beam;
An objective lens configured to focus the above line beam on a measurement area and collect reflected light reflected from the measurement area,
A detector configured to receive the reflected light and generate a signal according to the focus position of the reflected light;
An actuator configured to move the optical lens along the optical axis,
An autofocus device comprising a cylinder lens holder configured to rotate the cylinder lens about at least one rotational axis orthogonal to an optical axis of the cylinder lens. In the first paragraph,
An autofocus device wherein the optical lens is a plano-convex lens. In the first paragraph,
An autofocus device wherein the cylinder lens holder is configured to rotate the cylinder lens about a first rotation axis orthogonal to an optical axis of the cylinder lens and a second rotation axis orthogonal to the first rotation axis. In the first paragraph,
An autofocus device further comprising an actuator for adjusting the distance between the measurement area and the objective lens. In the first paragraph,
An overlay mark is formed in the above measurement area,
An auto-focus device in which the above line beam is adjusted so as not to go beyond the overlay mark formed in the above measurement area. As a method for correcting the autofocus device of the first paragraph,
A step of checking the alignment error of the line beam according to the distance between the optical lens and the light source,
A calibration method for an autofocus device, comprising the step of adjusting a rotation angle of the cylinder lens about the rotation axis based on the alignment error so that the line beam is positioned at the center of the measurement area. In Article 6,
An overlay mark is formed in the above measurement area,
The step of checking the alignment error of the above line beam is:
A method for calibrating an autofocus device, the method comprising the steps of: acquiring an image in which the line beam and the overlay mark are displayed together, and checking an error between the center of the overlay mark and the center of the line beam. An overlay measuring device for measuring an error between a pair of first overlay marks and second overlay marks respectively formed on different layers formed on a wafer,
An image acquisition device configured to acquire alignment images of a first overlay mark and a second overlay mark formed in pairs at a plurality of focus positions, and an auto-focus device for adjusting a focus of the image acquisition device,
The above auto-focus device is an overlay measuring device which is an auto-focus device of the first clause.

Images