JPH0787173B2 - Surface position detection method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface position detecting device for detecting a distance of an object surface from a reference surface. Such a surface position detecting device is used, for example, in the field of semiconductor manufacturing, as an automatic focus control device for an exposure device called a stepper that repeatedly reduces and projects a reticle pattern onto a semiconductor wafer surface. It is preferably used to detect the deviation of

[Prior Art] As a wafer surface position detecting method of a conventional reduced projection exposure apparatus, a method using an air microsensor, and a method of detecting a positional deviation amount of reflected light by making a light beam incident on a wafer surface in an oblique direction. A method (optical method) is known.

However, with the method using an air micro sensor, the pattern printing area cannot be measured directly, the response is slower than that of the optical method, and the distance between the air micro sensor nozzle and the wafer surface is 50%.
High precision detection is not possible unless the distance is about 60 μm.

There was a problem such as.

On the other hand, in the case of the optical method, the pattern printing part can directly measure the length and the response is fast, but the light reflected on the resist surface and the light reflected on the wafer surface interfere due to the presence of the resist applied on the wafer. As a result, there is a problem that it is difficult to detect the position with high accuracy because a detection error occurs.

[Object of the Invention] An object of the present invention is to solve the above-mentioned problems in the conventional art.
In an optical surface position detection device, a plurality of light beams having different wavelengths are incident to detect the detection light reflected on the surface to be detected and its vicinity (the resist surface and the wafer surface in the case of a semiconductor wafer coated with a photoresist). It is to improve the surface position detection accuracy based on the concept of averaging the interference action and reducing the detection error due to the interference action of the detection light.

Description of Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of an automatic focus control apparatus for a reduction projection exposure apparatus according to an embodiment of the present invention. In the figure, reference numeral 1 is a reduction projection lens, and the wafer 2 is located therebelow. The wafer 2 is mounted on a stage 3 which is vertically movable. The optical system of the automatic focus control device has a plurality of light sources 4 and 5 (only two light sources are shown as the plurality of light sources in FIG. 1 for simplicity). As the light source, lasers or LEDs having different wavelengths are used. The light beams (detection light) emitted from the light sources 4 and 5 form the same optical path by a beam splitter (or may be a half mirror) 6, pass through a lens 7, and then are reflected by a mirror 8 to be reflected at a reflection point on the wafer 2. Image on.

At this time, the incident angle of the detection light on the wafer surface is 80 ° or more, that is, the angle θ between the wafer surface and the incident light flux is 10 ° or less,
When the detection light is S-polarized with respect to the wafer, the intensity of the reflected light from the resist surface becomes dominant, and the influence of the reflected light from the wafer substrate can be reduced.

The light flux reflected by the wafer 2 is reflected by the mirror 9 and the lens
After passing through 10 and a polarizing plate 11 (or a polarization beam splitter), light is incident on a position sensor diode 12 (or a split sensor or the like) to form an image.

The polarizing plate 11 (or the polarization beam splitter) is for making the S-polarized light component of the light beam reflected by the wafer reach the position sensor diode 12, thereby further reducing the reflected component of the detection light on the wafer substrate. Is.

In this automatic focus control device, by keeping the reflection point of the light beam on the wafer surface and the incident point on the light receiving element in an image-forming relationship, the positional deviation of the wafer in the vertical direction is set as the light incident position of the light beam on the light receiving element. The focus position of the projection lens is detected and automatically controlled.

As a cause of the positional deviation detection error in the optical focus position detection device, the inclination of the wafer and the presence of the resist which is a light transmitting object coated on the wafer are considered.
In the former case, the detection error caused by the inclination of the wafer can be removed in principle by keeping the reflection point on the wafer and the incident point of the light beam on the light receiving element in an image forming relationship, as described above.

On the other hand, the presence of the latter resist appears as a shift of the center of gravity of the intensity of the light flux imaged on the light receiving element due to the interference between the reflected light on the resist surface and the reflected light on the wafer surface. . That is, it means that the detected position is different depending on the thickness of the resist or the wavelength of the light used as the detection light.

Therefore, in order to enable more accurate position detection,
It is absolutely necessary to eliminate the interference effect of the resist on the detection light.

Next, the reduction of the detection error due to the interference effect of the resist on the detection light will be described with reference to FIGS. 2 and 3.

FIG. 2 shows a light beam having a constant beam diameter on the wafer 2 coated with the resist 14 and having a uniform intensity within the beam diameter.
13 is a schematic diagram showing a state in which a light beam 15 having a distribution of different intensities within the beam diameter is formed by entering 13 in an imaged state and reflecting on the surface of the resist 14 and the surface of the wafer 2. FIG. Further, FIG. 3 is a graph showing the intensity distribution in the state where the light beam 15 reflected and formed on the surface of the resist 14 and the wafer 2 is imaged on the light receiving element by the optical system.

In FIG. 2, a light beam 13 having a constant beam diameter and a uniform intensity distribution within the beam diameter is obliquely incident on the wafer 2 coated with a resist 14. At this time, the luminous flux 13
Is divided into a component that reflects on the surface of the resist 14 and a component that passes through the resist 14 and repeats multiple reflection between the surface of the wafer 2 and the surface of the resist 14 and then goes out of the resist 14 again. To be In this way, the component reflected on the surface of the resist 14 and the component reflected on the surface of the wafer 2 are combined,
As shown in FIG. 3, a light beam 15 having different intensity distributions within the beam diameter is formed. This luminous flux 15 is the first
An image is formed on the light receiving element 12 through the mirror 9, the lens 10 and the polarizing plate 11 in the figure.

In FIG. 3, the graph shows the light intensity distribution on the light receiving element when a certain wavelength λ ₁ is used as the detection light. Also,
The graph and show the light intensity distribution on the light receiving element when different wavelengths λ ₂ and λ ₃ are used as detection light. Furthermore, the graph shows the light intensity distribution on the light receiving element when a plurality of wavelengths λ _{1 to} λ _n are used as the detection light.

The intensity distribution of the detection light on the light receiving element when a monochromatic (or quasi-monochromatic) light source is used as the detection light is shown in the graph,
, Shows a complicated intensity distribution due to the interference between the component reflected on the surface of the resist 14 and the component reflected on the surface of the wafer 2. The distribution state of this intensity varies depending on the wavelength of the detection light and the thickness of the resist 14. In addition, the center of gravity 16, 17, 18 of the intensity distribution of the detection light is output as a position signal by the light receiving element, but if the wavelength of the detection light or the thickness of the resist 14 changes, the interference state changes and the center of gravity changes.
The positions of 16, 17 and 18 also change and appear as a detection error.

By the way, this is what the present inventors have discovered after various studies, but the intensity distribution of the detection light on the light receiving element when a plurality of monochromatic (or quasi monochromatic) light sources is used as the detection light is As shown in the graph, the interference effect due to the presence of the resist is averaged by using the multi-wavelength light source, and the centroid 19 of the intensity distribution of the detection light shows a stable value regardless of the film thickness of the resist. Therefore, if the wafer surface position of the optical system is detected using such a multi-wavelength light source, it is possible to eliminate the position detection error due to the presence of the resist.

Further, as shown in FIG. 1, a plurality of light sources having different wavelengths are used, and the angle θ formed by the light incident on the wafer with the wafer is
By setting the angle to 10 ° or less and using S-polarized light for the wafer as the detection light, the light reflected on the wafer surface is reduced, and the effect of the present invention can be further enhanced.

The averaging of the interference effect can also be achieved by mixing the light of the plurality of wavelengths and irradiating the wafers at the same time, and detecting the reflected light. However, in the present invention, each of these light (first and (Second light) is detected independently (first and second reflected light) by, for example, irradiating in time division (sequential),
This is performed by processing the obtained plurality of detection signals (first and second signals). When detecting light of multiple wavelengths reflected by the wafer at the same time with a light receiving element, if the intensity of light of a certain wavelength is higher than that of light of other wavelengths, the light of the wavelength with a high intensity is detected as the surface position detection result. However, the averaging of the interference effect is weakened and an error is likely to occur. However, as in the present invention, light of each wavelength is independently detected by the light receiving element and a plurality of obtained signals are calculated. If the processing is performed, such a problem does not occur, and the surface position can be detected by effectively averaging the interference effect. Further, if the light of each wavelength is irradiated in a time-division manner as in the present invention,
Since a common light receiving element can be provided for the light of each wavelength, the detection system can be simplified and the detection accuracy can be improved. Furthermore, according to the method of the present invention, it is possible to process even the deviation of the irradiation optical path for each light due to an adjustment error or the like. This is to measure the light receiving position corresponding to each light source while making each light source emit light one by one using a reference wafer to which no resist is applied in advance, and the actual wafer surface position is synchronized with the light emission of each light source. The light receiving position may be measured as the reference or reference position. Such processing is performed by a central processing unit (CP) such as a microprocessor.
By using U), it can be easily carried out by those skilled in the art.

In the above embodiment, an LED, a laser or the like can be used as the light source. Further, as the light receiving element, a photoelectric conversion element having a position detecting ability such as a position sensor detector (PSD), a split sensor detector, and a CCD can be used.

In particular, when a position sensor detector is used for the light receiving element, a reference signal detection system as shown in FIG. 4 should be used in order to eliminate the detection error of the position signal due to the time change of the reference point in the position sensor detector. Conceivable.

In the figure, the luminous flux emitted from the reference light source 20 passes through a lens 21 and a beam splitter (or half mirror) 22.
After the direction is changed by, the image is formed on the position sensor detector 12.

Here, in this device, when assembling and adjusting, the reference light source
The reference light from 20 is made incident on the position sensor detector 12, and the light fluxes from the plurality of light sources 4, 5 are made incident on the position sensor detector 12 sequentially through the reference wafer, and the positions corresponding to the incident positions of these three lights are made. The output of the sensor detector 12 is obtained as a reference position signal.
However, here, the optical axes of the optical system are set so that all three incident positions coincide with the reference point (point at which the output becomes zero) in the position sensor detector 12, that is, all the reference position signals become zero. Also, it is assumed that the reference point of the position sensor detector 12 is adjusted.

Next, consider the case where the wafer 2 is actually aligned with the focal plane of the projection lens 1 in the apparatus shown in FIG. At this time, the position sensor detector 12 is
Even if the reference point changes with time Δ, before the detection light for the wafer 2 is detected, first, the reference light source 20 is caused to emit light and the position signal thereof is detected, so that the reference point of the position sensor detector 12 is changed. The change Δ with time can be measured.

Then, it is obtained by referring to the reference position signal (here, zero) obtained at the time of adjustment. The position signal S indicating the center of gravity of the intensity of each detection light with respect to the wafer 2 is sequentially detected. At this time, the time interval Δ for measuring the reference point with respect to time and the time interval for measuring the position signal S are set to such a small time that the reference point of the position sensor detector 12 does not change with time. This position signal S
Is the change in the reference point of the position sensor detector 12 with time Δ
Therefore, it is possible to detect the position with high accuracy by subtracting the position signal Δ when the reference light source 20 is made to emit light. Therefore, prior to the focusing of the wafer 2, the wafer is positioned at the focal position of the projection lens 1 by evaluating the pattern images printed at different vertical wafer positions in advance, and the light sources 4 and 5 and the reference light source 20 Position signal values a ₁ and b ₁ for each light
And δ ₁ , and based on these values, the focal plane position f = [(a ₁ + b ₁ ) / 2] −δ ₁ is obtained. When focusing the wafer 2, the position signal values a ₂ , b ₂ and δ ₂ for each light are similarly obtained for the wafer 2, and the surface position w = [(a ₂ + B ₂ ) / 2]-
The vertical position of the wafer 2 is adjusted so that δ ₂ matches the position f of the focal plane. As a result, highly accurate focusing can be performed without including a change with time of the reference point of the position sensor detector 12.

Further, at this time, by providing the stopper 23 at a position displaced from the principal point on the light-receiving element side of the detection-side lens system 10 to the light-receiving element side by the focal length of the lens system 10, the resist surface on the patterned wafer is removed. The higher-order diffracted light component of the reflected light that is reflected shall be cut. As a result, even when the position detection is performed on the patterned wafer, it is not necessary to change the optical center of gravity of the detection light due to the higher-order diffracted light component. That is, even for patterned wafers,
Highly accurate position detection becomes possible.

[Effects of the Invention] As described above, according to the present invention, a plurality of lights having different wavelengths are used, and the surface position detection signals by these lights are detected independently for each wavelength and the signals are used as the basis. Since the surface position is calculated on the wafer surface, the detection error due to the interference of the detection light when there are multiple reflection surfaces for the detection light flux such as the wafer surface coated with resist is reduced, and the wafer is accurately and reproducibly. The surface position of can be detected.

Further, the present invention is particularly effective for focusing the reduction projection lens of the reduction projection exposure apparatus which requires highly accurate focus detection. In this case, high-precision focus detection can be performed, so that a high-resolution pattern can be formed, and there is an effect that a circuit with a higher degree of integration can be created.

[Brief description of drawings]

FIG. 1 is a block diagram showing an automatic focus control device according to an embodiment of the present invention, and FIG. 2 is a resist surface when a light beam is incident and imaged on a resist-coated wafer at a low angle. FIG. 3 is a cross-sectional view showing the reflection state of the light beam on the wafer surface. FIG. 3 shows the distribution of the intensity of the detected light when the light beam reflected on the resist-coated wafer enters the light receiving element and forms an image. FIG. 4 is a configuration diagram showing an automatic focus control device according to another embodiment of the present invention in which a position sensor detector is used as a light receiving element and a reference signal detection system is provided. 1 ... Reduction projection lens, 2 ... Wafer, 3 ... Stage, 4,5,20 ... Laser (or LED), 6 ... Beam splitter (or half mirror), 7,10,21 ... Lens system , 8,9 …… Mirror, 11, 22 …… Polarizer (or polarization beam splitter), 12 …… Position sensor diode (or split sensor diode or CCD), 13…
… Light flux (incident light flux), 14 …… resist, 15 …… light flux (reflected light flux), 16,17,18,19 …… centroid of intensity distribution, 23 …… stopper.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyoshi Kubo Inventor Hiroyoshi Kubo 53 Imaiue-cho, Nakahara-ku, Kawasaki-shi, Kanagawa Canon Inc. Kosugi Plant (56) Reference JP-A-57-60205 (JP, A) JP-A-SHO 59-54908 (JP, A) JP-A-59-79527 (JP, A) JP-A-60-80223 (JP, A)

Claims (1)

[Claims] 1. A wafer to be measured coated with a resist is irradiated with light, and the reflected light reflected by the wafer is incident on the light receiving surface of the photoelectric conversion means, and a signal from the photoelectric conversion means corresponding to the incident position. In the method for detecting the surface position in the vertical direction of the wafer based on, the reference light is made incident on the light receiving surface to obtain a first reference signal corresponding to the incident position, Sequentially reflecting the second light on the reference plate and making the light incident on the light receiving surface to obtain second and third reference signals corresponding to these incident positions, and the reference light on the light receiving surface. The first measurement signal corresponding to the incident position, the first and second lights are sequentially reflected on the wafer to be measured and incident on the light receiving surface, and the incident positions thereof are measured. Corresponds to The second
And a step of obtaining a third measurement signal, and the first reference signal and the first measurement signal, taking into consideration the change with time of the incident position corresponding to the second and third reference signals, And a second reference signal and a step of obtaining a surface position of the wafer to be measured based on the second and third measurement signals.