JPH02157608A - Focusing method - Google Patents

Abstract

PURPOSE:To easily perform focusing in an optical axis direction by placing a sample for detecting a focal point which has a transparent film with specified thickness instead of a sample and detecting the focusing position based on the interference of the reflected light of a projected light beam. CONSTITUTION:When the sample having a fine pattern such as an IC, etc., is irradiated with the linear light beam through an objective lens for illuminating so as to detect the image of the fine pattern through the objective lens for detecting, the focusing of each objective lens is performed to the sample. At such a time, the sample 26 for focusing which has the transparent film 26a with the specified thickness (d) is placed at the position of the sample instead of the sample and it is irradiated with the beam 41 which is the linear cutting light beam. When the focal point of the beam 41 agrees with the part of the transparent thin film 26a, detecting light which enters the objective lens for detecting which detects the reflected light becomes dark by the effect of the interference in the transparent thin film 26a. Therefore, the focusing position is found by detecting a point where the detected light quantity is minimum.

Description

[Detailed Description of the Invention] [Summary] Regarding a focus adjustment method, the object of the present invention is to provide a focus adjustment method that can easily focus in the optical axis direction. A linear light beam is irradiated through the light beam, and an image of a fine pattern produced by the light beam is detected through the detection objective lens.In addition, when adjusting the focus of each objective lens on the sample, the position of the sample and the illumination In a focus adjustment method that changes at least one of the optical axis direction of an objective lens and the optical axis direction of a detection-side objective lens, the focus detection method includes a transparent film of a predetermined thickness at the position of the sample in place of the sample. A sample for detection is placed, and at least one of the illumination objective lens or the detection objective lens is moved in the optical axis direction based on the interference of the reflected light of the light beam irradiated onto the focus detection sample. Configure to adjust focus on.

[Industrial application field]

The present invention relates to a focus adjustment method, and more particularly to a focus adjustment method applied to a pattern inspection device or the like that detects the three-dimensional shape of a fine pattern using a line-shaped light beam.

In the field of semiconductor technology, miniaturization of LSI wiring patterns,
With the increase in density, there is an increasing demand for inspecting the three-dimensional shape of conductor wiring. In particular, the shape of IC conductors is becoming submicron or smaller. In such fields, visual inspection by humans is no longer possible, and automation and faster inspections are desired. Therefore, it has been considered to perform visual inspection using a light beam, but focusing is important because of the fine pattern.

[Conventional technology]

Conventionally, in order to inspect fine patterns of ICs, LSIs, etc., pattern inspection equipment has been used that uses a spot-shaped light beam to form an image on the object to be inspected and scans it two-dimensionally for inspection. .

However, this device has the disadvantage that it requires scanning a minute light spot in both the X-axis and Y-axis directions, and that it takes a long time to inspect a wide area.

Therefore, in order to eliminate the above-mentioned drawbacks, the applicant of the present invention has previously filed an application for an apparatus based on the principle of the optical cutting method, as shown in FIG. In the figure, a light beam 2 from a laser 1 is made incident on an objective lens 4 through a linear slit 3 to form a thin line of light 7 on a target pattern (corresponding to a sample) 6 on a sample stage 5. The target pattern 6 illuminated by this line light 7 is imaged onto a TV camera 9 using a detection-side objective lens 8 . Then, as shown in an enlarged view in FIG. 6, a detection signal 10 having a width W and a height h' is obtained from a fine target pattern 6 having a width W and a height H. From this, the three-dimensional shape of the target pattern 6 is determined by calculation.

[Problem to be solved by the invention]

However, in the focus adjustment method used in the KW according to the above-mentioned prior application, when focusing is performed as shown in FIG. 7, the movable parts necessary for focus adjustment are in three directions, that is, the direction t! in which the objective lens 4 is moved. There are three directions: i, the direction 10 in which the objective lens 8 is moved, and the direction Z in which the sample stage 5 is moved, but there was a problem that the focusing technology was not sufficient and the focus adjustment could not be done smoothly. .

That is, when performing the above-mentioned focusing, the surface of the sample stage 5 must be optically flat (without unevenness), but when the objective lens 4 and the objective lens 8 with a large numerical aperture are used, Its depth of focus is very shallow.

Therefore, the flatness of the sample surface must be as small as possible in order to obtain accurate focusing. For example, in the example of the prior application mentioned above, the depth of focus is about 1 μm, and the flatness of about 1/10 μm is used for alignment.

Further, the following problems are related to focus adjustment in each direction.

(1) Focus adjustment in the Z-axis direction Adjustment in the Z-axis direction can be easily performed by taking advantage of the fact that the line image on the sample surface is at its narrowest (focus position). All that is required is to measure the profile, find its half-width, and move the stage (sample stand) in the Z-axis direction so that the half-width is the minimum value.

(n) Focus adjustment in optical axis directions Li and Lo However, if the above method is used for optical axis directions Li and Lo, it is impossible to make the distance between the two objective lenses 4 and 8 twice the focal point. However, if the focus is coincidentally on the sample on the sample stage 5 as shown in Fig. 8 (a) (f represents the focus), it is good, but as shown in Fig. 8 (b) When focusing at a virtual image position that is off the sample, for example, scattering from the sample surface or edges has an adverse effect, making it impossible to accurately detect a pattern.

However, in practice, it is difficult to detect whether the focus is on the sample surface or not.

In particular, in the case of a submicron-order sample, the surface has irregularities of less than 1/10 of the used wavelength, resulting in specular reflection, making it easier to detect a virtual image.

Therefore, an object of the present invention is to provide a focus adjustment method that allows easy focusing in the optical axis direction.

[Means to solve the problem]

In order to achieve the above object, the focus adjustment method according to the present invention irradiates a sample having a fine pattern with a linear light beam through an illumination objective lens, and images the fine pattern by the light beam through a detection objective lens. When adjusting the focus of each objective lens on the sample, focus adjustment changes at least one of the position of the sample, the optical axis direction of the illumination objective lens, and the optical axis direction of the detection side objective lens. In the method, a focus detection sample having a transparent film of a predetermined thickness is placed in place of the sample at the position of the sample, and the focus detection sample is detected based on interference of reflected light of a light beam irradiated to the focus detection sample. At least one of the illumination objective lens and the detection objective lens is moved in the direction of its optical axis to adjust the focus on the sample.

[Effect]

In the present invention, a focus detection sample having a transparent film of a predetermined thickness is placed at the position of the sample to be inspected, and the focus detection sample is illuminated based on the interference of reflected light of a light beam irradiated onto the focus detection sample. One of the objective lenses for use or the objective lenses for detection
The focal point is adjusted by moving one or more in the direction of the optical axis. In this case, the transparent film of the focus detection sample makes the light beam almost parallel at the focal position, and when it deviates from the focus, the optical path length differs depending on the position of the light beam. Therefore, due to interference, the characteristics of the detected light amount of the reflected light (for example, minimum and The in-focus position can be detected by finding the point).

Therefore, it becomes possible to easily focus the optical axis direction regardless of the specular reflection of the sample.

〔Example〕

Hereinafter, the present invention will be explained based on the drawings.

1 to 4 are diagrams showing an embodiment of the focus adjustment method according to the present invention, and in particular are examples in which the present invention is applied to a pattern inspection apparatus for inspecting fine patterns such as RC.

First, the configuration will be explained. FIG. 1 is a diagram showing the configuration of a focus detection system in a light cutting type pattern inspection apparatus, and in this figure, 20 is a laser.

A light beam (laser light) 21 from a laser 20 is reflected by a galvanometer mirror 22, and then enters an illumination objective lens 24 through a linear slit 23, and then passes through a sample stage 2.
The image is formed on a focus sample (focus detection sample) 26 on the sample 5 . As the slit 23, for example, one that forms a slit-shaped light beam with a narrow width and a length about the same as the entrance pupil is used. A lens with a large numerical aperture is used as the illumination objective lens 24, and the illumination objective and lens 24 are movable in the optical axis direction 1i. The sample stage 25 is 2 as in the first example.
It is movable in the axial direction, and focus adjustment in the z-axis direction is performed in the same manner as in the prior application.

The focus sample 26 has a transparent film having a predetermined thickness d (determined by the formula 0 described below), and is placed in advance at the position of the sample to be inspected prior to pattern inspection of the sample, and is After alignment is completed, the focusing sample 26 is removed and the sample to be inspected is placed. As the focusing sample 26, a transparent thin film 26a having a thickness d determined from the wavelength of the light beam 21 used, the angle of incidence, and the refractive index is formed on a reflective object 26b such as a wafer, for example. (See Figure 3).

After the reflected light from the focusing sample 26 passes through the detection objective lens 27, it is directed to the detector 2 by the imaging lens 28.
Imaged at 9. As the detector 29, for example, a CCD
Alternatively, a TV camera is used. Detection objective lens 2
7 is also movable in the optical axis direction 1o. Detector 29
The detected image is A/D converted by the A/D converter 30 and input to the CPU 31.

The CPU 31 calculates processing values necessary for autofocusing the illumination objective lens 24 and the detection objective lens 27 on the focus sample 26, and outputs the calculation results to the focus controller 32. The focus controller 32 moves the illumination objective lens 24 or the detection objective lens 27 in the optical axis direction 1i based on a command from the CPU 31.
to automatically detect and adjust the focus position.

In the above configuration, the light beam entering the focusing sample 26 from the illumination objective lens 24 has an incident angle α=α=
When the angle is 45°, it is shown as shown in FIG. In FIG. 2, 41 is the beam (especially its shape) that irradiates the sample surface, 42 is the surface when it is in focus (hereinafter referred to as just focus surface), and 43 is the surface when it is out of focus ( (hereinafter referred to as the focus outer surface). Further, thick arrows in the figure indicate the normal direction of the wavefront within the beam 41, and thin arrows indicate the direction of reflection on each surface. When the light is focused using the illumination objective lens 24 with a large numerical aperture, the wavefront of the reflected light 44 temporarily becomes parallel at just focus 42, as shown in FIG. On the other hand, at the focus plane 43, the wavefront of the reflected light 45 is disturbed and does not become parallel.

In the present invention, just focus 42 and focus plane 4
This is based on the principle that the optical path length of the reflected light differs between the two types as described above.

That is, FIGS. 3(a) and 3(b) show the focusing sample 26.
FIG. 4 is a diagram showing an optical path when light is incident on the lens, of which (a) shows the case when the image is just in focus, and (b) shows the case when the image is out of focus.

The light beam 4 is applied to the transparent thin film 26a of the focusing sample 26.
1 is incident, the beam 41 becomes almost parallel light at the focal position as shown in FIG.
The optical path length of the reflected light 44 on the focusing sample 26 is the same. However, if it is out of focus, as shown in Figure 3(b)
As shown in FIG. 2, the optical path length of the reflected light 45 differs depending on the position of the beam 41. Here, it is assumed that the film thickness d of the transparent P# film 26a is set to a value expressed by the following formula (■■).

θ=sin −' (n −5in α)...
...■ However, α: Incident angle θ: Refraction angle n: Refractive index λ: Light source wavelength N: Q, 1. 2. ... At this time, if the focus of the beam 41, which is the cutting light, matches the part of the transparent thin film 26a of the focusing sample 26 (that is, the object to be focused on), then the reflected light is detected. The detection light entering the detection objective lens 27 passes through the transparent thin film 26
It becomes dark due to the influence of interference at a. This is transparent thin film 2
This is because, on the surface of 6a, the incident and reflected optical path lengths become equal and cancel each other out due to the phase difference. For example, if the optical path length in the focusing sample 26 is l, the path length β is approximately equal, and the reflected light becomes dark when there is interference. In other words, the focusing sample 26 with a film thickness of d was installed to create such an interference effect. On the other hand, when the focus does not match, the angle of the incident light varies, so the relationship is related to the optical path length β, and in the short portion @, the influence of interference is small compared to the focal position, and the amount of detected light increases. Therefore, the in-focus position can be found by detecting the point where the amount of detected light is minimum.

In this embodiment, the amount of detected light is detected by the detector 29, and the minimum point is determined by the CPU 31 based on the detection result. At this time, the minimum point is the in-focus position, and this is shown by the characteristics shown in FIG. Then, while monitoring the amount of reflected light by the CPU 31, the detection objective lens 27 is moved in the optical axis direction 1o based on the output of the focus controller 32, and when the minimum point of the light amount is found, this becomes the in-focus position. Moreover, since this operation is performed automatically, focus adjustment can be achieved extremely easily. After automatically performing good focus adjustment in this way, place the fine pattern of the IC to be inspected on the sample stage 25 instead of the focus sample 26, and use the light as in the previous application. A cutting pattern inspection is performed.

In addition, when autofocusing, the illumination objective lens 2
4 may be moved in the optical axis direction 1i.

Furthermore, the present invention is applicable not only to the slit-shaped light beam as in the above-mentioned embodiments, but also to spot-shaped light beams, such as those cited as conventional examples. In short, it is sufficient if a plurality of objective lenses are used to form an image on the sample surface, and the effects of the present invention are particularly noticeable when the depth of focus is extremely small, such as about 1 μm. .

Furthermore, when the present invention is applied to automatic inspection of IC patterns, it is possible to achieve further automation and speed up of inspection.

〔Effect of the invention〕

According to the present invention, even when the depth of focus is very shallow, it is possible to easily focus the objective lens in the optical axis direction.

[Brief explanation of the drawing]

1 to 4 are diagrams showing an embodiment of a pattern inspection device to which the focus adjustment method according to the present invention is applied, FIG. 1 is a configuration diagram showing its focus detection system, and FIG. 2 is a diagram showing the incident light. Figure 3 is a diagram showing the state of the beam, Figure 3 is a diagram showing the optical path of the reflected light, Figure 4 is a diagram showing the relationship between the amount of detected light and the focus position, and Figures 5 to 8 are patterns related to the earlier application. FIG. 5 is a diagram showing the inspection device; FIG. 5 is a configuration diagram thereof; FIG. 6 is an enlarged view of the main part including the target pattern; FIG. 7 is a diagram showing a focus adjustment method; and FIG. 8 is an optical path related to the focus. FIG. 20...Laser, 21...Light beam, 22...Galvano mirror 23...Slit, 24...Illumination objective lens, 25 ...Sample stand, 26...Specimen for focusing, 26a...Transparent thin film, 26b...Reflecting object, 27...Objective for detection Lens, 28... Imaging lens, 29... Detector, 30... A/D converter, 31... CPU. 32...Focus controller, 41...
...beam, 42...just focus, 43...
・Focus plane, 44.45...Reflected light. FIG. 2 shows the optical path of reflected light in one embodiment. FIG. 3 shows the relationship between the detected light amount and the focus position in one embodiment. Focus position distance in the optical axis direction. Figure 4: Configuration diagram of the earlier application Figure 7: An enlarged view of the main part including the target pattern of the earlier application Figure 8: Explanation of the optical path regarding the focus of the earlier application

Claims (1)

[Scope of Claims] A linear light beam is irradiated onto a sample having a fine pattern through an illumination objective lens, and an image of the fine pattern caused by the light beam is detected through a detection side objective lens. In a focus adjustment method that changes at least one of the position of the sample, the optical axis direction of the illumination objective lens, and the optical axis direction of the detection side objective lens when adjusting the focus of each objective lens with respect to the sample, the position of the sample A focus detection sample having a transparent film of a predetermined thickness is placed in place of the sample, and the illumination objective lens or the detection side is 1. A focus adjustment method, characterized in that the focus on a sample is adjusted by moving at least one of the objective lenses in the direction of its optical axis.