JPH0610695B2 - Focusing method and apparatus thereof - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention is suitable for focus detection and focusing of semiconductor masks, inspection devices for masks for reticles or printed circuit boards, dimension measuring devices, exposure devices, etc. The present invention relates to a focusing method and an apparatus therefor.

[Background of the Invention]

In a conventional focusing device of this type, as described in Japanese Patent Application Laid-Open No. 56-42205, a means for obliquely irradiating an object to be observed with an elongated light beam is used. With such focus detection means, the surface condition of the object to be observed is easily affected,
Various measures have been taken to reduce the detection accuracy. As one means for solving the problem, it is considered to arrange the slit image irradiating the object to be observed so as to intersect with the pattern direction of the IC or the like, for example, to intersect at 45 °. Further, as another means, as described in JP-A-57-53923, in order to reduce the influence of the surface roughness of the object to be observed, the incident angle irradiating the object to be observed is about 85 °. There is a means for irradiating S-polarized light to improve the refractive index n, that is, the reflectance.

However, in the above means, when the object to be observed has a pattern formed of a substance having a refractive index different from that of the substrate, for example, when a Cr pattern is drawn on a glass substrate such as a semiconductor reticle, it is appropriate. Simply irradiating the light beam obliquely at a different angle causes a great difference in reflectance between the glass surface and the Cr surface, which causes a problem. That is, in the case of the focusing device, when the reflectance is different, the output from the detector is naturally different at a strong reflection point and a weak reflection point, so the S / N at a low reflection point is poor, so the detection accuracy is low. descend. In the above-mentioned conventional techniques, the problem that the detection accuracy deteriorates due to the difference in the intensity of reflected light has not been taken into consideration.

On the other hand, when the object to be observed transmits light such as glass, there are a light flux reflected on the front surface and a light flux that enters the inside and is reflected on the lower layer (back surface) and re-emitted. When the light fluxes are combined, the detection accuracy may deteriorate.

[Object of the Invention]

In order to solve the above-mentioned problems of the prior art, the object of the present invention is to connect the surface of a mask on which a thin film circuit pattern formed of a substance having a different refractive index from that of the glass substrate is formed on the surface of a transparent glass substrate. In a mask observing method and apparatus for observing with an image optical system, the intensity of specular reflection from the surface of the glass substrate and the surface of the thin film circuit pattern is made uniform, and a simple binarization method is used for the imaging optical system. It is an object of the present invention to provide a focusing method and an apparatus therefor capable of accurately aligning the surface of a mask with a focal position.

[Outline of Invention]

In order to achieve the above object, the present invention uses an imaging optical system to form the surface of a mask on which a thin film circuit pattern formed of a substance having a different refractive index from the glass substrate is formed on the surface of a transparent glass substrate. In the mask observing method for observing, the optical axis of the imaging optical system is such that the reflectance of a P-polarized laser beam from the surface of the glass substrate and the surface of the circuit pattern is substantially the same with respect to the mask. 7 to direction
The light is projected from an oblique direction at an incident angle of 0 to 85 °, and the P-polarized laser light beam projected on the mask uniformly specularly reflects from the surface of the glass substrate of the mask and the surface of the thin film circuit pattern. The reflected light flux is imaged on at least a one-dimensional image sensor corresponding to the position of the surface of the mask in the optical axis direction of the imaging optical system, and the image signal obtained from the image sensor is binarized with a predetermined threshold value. The signal is converted into a binarized signal, the center of the binarized signal is calculated as coordinates on the image sensor, and the mask is moved in the optical axis direction of the imaging optical system based on the calculated center coordinates of the binarized signal. And the surface of the mask is adjusted to the focal position of the imaging optical system. Further, the present invention, in a mask observing device for observing the surface of a mask on which a thin film circuit pattern formed by a substance having a different refractive index from the glass substrate on the surface of a transparent glass substrate is observed by an imaging optical system, With respect to the mask, the P-polarized laser light flux is 70 to 85 ° with respect to the optical axis direction of the imaging optical system in which the reflectances from the surface of the glass substrate and the surface of the circuit pattern are substantially the same. An incident projection unit that obliquely projects at an incident angle, and a P-polarized laser light beam that is projected by the incident projection unit uniformly specularly reflects from the surface of the glass substrate of the mask and the surface of the thin film circuit pattern. Detecting optical means for forming an image of the specularly reflected light flux on the at least one-dimensional image sensor corresponding to the position of the surface of the mask in the optical axis direction of the imaging optical system, and the detecting optical means. Binary center calculating means for converting an image signal obtained from the stepped image sensor into a binary signal with a predetermined threshold value and calculating the center of the binary signal as coordinates on the image sensor; Control means for moving the mask in the optical axis direction of the image forming optical system based on the center coordinates of the binarized signal calculated by the image forming center calculating means to align the surface of the mask with the focus position of the image forming optical system. It is a focusing device characterized by comprising: Further, in the present invention, in the focusing device, the incident projection means has a slit forming optical system for forming a P-polarized laser slit light beam as a P-polarized laser light beam incident and projected on the surface of the mask, and the detection is performed. The optical means is characterized by having a one-dimensional condensing optical system for condensing the specular reflection slit light flux specularly reflected from the surface of the mask on the one-dimensional image sensor in the longitudinal direction of the specular reflection slit light flux. .

Example of Invention

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram showing the configuration of the present embodiment. Reference numeral 1 denotes an objective lens, which is arranged to face an object to be observed, such as a mask or reticle 2, which is placed on a movably provided Z stage 12. It is installed in. 3 is a laser light source, 4 is a beam expander, 5 is a slit, 6 and 9 are first and second lenses, 7 and 8 are first and second reflecting mirrors, 10 is a position detector, for example, a linear image sensor (hereinafter Reference numeral 11 denotes a cylindrical lens. The mask or reticle contains 700 to 1000 chromium with different refractive indexes on the surface of the substrate (glass).
Is deposited to a thickness of Angstrom.

This embodiment comprises various devices as described above, and includes a laser light source 3
The laser beam emitted from the beam expander 4
And is incident on the slit 5. The slit 5
The image that has passed through passes through the first lens 6 and the first reflection mirror 7
After being projected onto the mask 2 and having its optical path bent by the reflecting mirror 7, the image is projected and imaged on the mask 2 from an oblique direction. The image is second
The optical path is bent by the reflection mirror 8, and the image is projected and imaged on the CCD 10 via the second lens 9. Since the opening of the CCD 10 is narrow, all the images of the slits 5 are compressed by using the cylindrical lens 11 so that they are projected and imaged on the pixels of the CCD 10.

Next, the incident angle for making the respective reflectances of the substance having a different refractive index from the substrate of the object to be observed uniform will be described.

Polarization must be taken into account in the reflectance at the boundary between materials having different refractive indices, for example, at the boundary between air and glass. In this case, Fresnel's equation is used for the reflectance, and the law of thin film reflection is used for the reflectance from the upper surface and the lower surface of the thin film whose thickness is at the wavelength of light.

As mentioned above, in the case of masks and reticles, chromium with a thickness of 700 to 1000 angstroms is deposited on the surface of glass, and the reflectance on the glass surface and the reflectance on the chromium surface depend on the incident angle and the wavelength of light. different.

Therefore, as an example, FIG. 2 shows a case where the above-described principle is applied to an automatic focusing system in which light is obliquely emitted with respect to the reflectance of glass and chromium. According to the known Fresnel method, the reflectance R of the glass 13 is i for the incident angle and i _{1 for} the refraction angle.
Therefore, the reflectances R _sg and R in the case of S-polarized light and P-polarized light
_pg is expressed by the following equations (1) and (2), respectively.

Also, in the case of reflection at the chromium (C _r) 14 shown in FIG. 3, left the incident angle to the upper surface of the C _r film to i _0, the lower surface of the medium of the refraction angle i _2, C _gamma film (glass) 13 The turning angle is i ₃ , C _r (14)
Assuming that the reflectance of the upper surface of the film is r and the reflectance of the lower surface is r ′, the P and S components of r and r ° of the reflectance can be calculated by the following (3) (5) according to the known thin film reflection method. ) And (4) and (6).

Then, when the reflectance of the upper surface of the _Cr (14) film is r and the reflectance of the lower surface is r ', the intensity I ^r of the reflected light of the thin film is Therefore, the above r _p , r _s , and
r _'p, r' and substituting _s, P, the intensity of the reflected light of each of the case of the S-polarized light will as the following equation (7) (8).

However, The glass used for the LSI photomask and reticle has a refractive index of 1.4 to 1.6 in the case of visible light, and chromium has a refractive index of 1.5 to 3.0. He-N with a wavelength of 633 nm as the light source
When the laser light of e is used, the refractive index of glass is about 1.5 and the refractive index of chromium is about 3.0.

Substituting the above-mentioned refractive index into the equations (1), (2), (7), and (8) to obtain the respective reflectances and illustrating them, a curve as shown in FIG. 4 is obtained.
The curve is the result when the surface is in an ideal state, but actually it does not necessarily result as shown in FIG. 4 because it differs from the ideal value due to the surface state of glass and chromium. However, focusing on the reflectance due to P-polarized light with respect to the glass and the chrome film, it is found that there is an incident angle at which the reflectance is the same.

FIG. 5 shows the measurement results of the reflectance of glass and chromium when an actual reticle is used. The curve in the figure is the result of the experiment with P-polarized light. Although the curve is slightly different from the curved surface of P-polarized light of the theoretical value shown in FIG.
It can be easily understood that the reflectances at about 80 ° are the same.

This is not limited to chromium, and in the case of a substance having a refractive index larger than glass, when the incident angle is set to 70 ° or more, the difference in reflectance is about ± 20%, which is suitable for focus detection.

CC when focusing by the incident angle (70 ° or more)
The output of D10 is as shown in FIG. in this case,
Focusing of the mask inspection apparatus is performed by detecting a pattern signal to obtain a focus position, and positioning the mask 2 at the position by the Z stage 12.

That is, CC of the position of the mask 2 when the resolution is the best
It is stored at the address of the slit image 5'on D 10. Then, the Z stage 12 is moved up and down so that the slit image 5'formed on the CCD 10 is projected and imaged on the newly set mask 2, and the slit image 5'is imaged on the CCD 10 at the focus position of the objective lens 1. Align 2 (see Figure 1).

The position of the slit image 5'is CC as shown in FIG.
A threshold value is set for the output of D 10, and CC corresponding to the threshold value is set.
The pixel 13 of D 10 is obtained, and the median value of the pixel 13 is set at the position of the slit image 5 '.

FIG. 7 shows an example of a slit image 5 ″ which is reflected on the back surface of the mask 2 when transmitting light. The optical axis 16 of the reflection on the back surface of the mask 2 is defined by the incident angle i and the mask 2. When the thickness t is determined, the distance from the refractive index of the mask 2 is away from the optical axis 15 of the surface reflection of the mask 2 by a distance l. Therefore, if the width of the slit 5 is reduced by about 1/3 of l, the linear image sensor CCD The output on 10 is separated.

Moreover, since the shift directions of the optical axes 15 and 16 are also determined, the front surface reflection and the back surface reflection of the mask 2 can be easily distinguished. For example, as shown in FIG. 8 (a), each slit image 5 ', 5 "of front surface reflection and rear surface reflection is formed on the pixel 13 of the CCD 10.
When the image is formed, the median value m of the front surface reflections shown in FIG. 7B is the reflection output of the slit image 5'of the surface reflections. Since it is the reflected output of the image 5 ″, focusing may be performed using the median value m.

〔The invention's effect〕

According to the present invention, a mask observing method for observing the surface of a transparent glass substrate on which a thin film circuit pattern formed of a substance having a refractive index different from that of the glass substrate is formed by an imaging optical system, and In the apparatus, the surface of the mask is accurately positioned on the focus position of the imaging optical system by a simple binarization method in which the regular reflection light intensity from the surface of the glass substrate and the surface of the thin film circuit pattern is made uniform. The surface of the mask can be observed in a good resolution state by the imaging optical system, and highly accurate mask inspection, measurement, or exposure by the mask can be realized.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of an embodiment of the focusing device of the present invention, FIGS. 2 and 3 are diagrams for explaining the reflection of glass and chromium films, and FIG. 4 is a diagram of glass and chromium. FIG. 5 is a diagram showing reflectance, FIG. 5 is a diagram showing reflectance in an actual mask, FIG. 6 is a diagram showing output of a slit image projected on a light receiving element, and FIG. 7 is a diagram showing back surface reflection of glass. FIG. 8 is a diagram showing the output of the slit image projected on the light receiving element of FIG. 7. 2 ... Mask 3 ... Laser light source 5 ... Slit 6,9 ... First and second lenses 7,8 ... Reflecting mirror 10 ... Position detector

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location G03F 1/08 S 7369-2H H01L 21/027 7352-4M H01L 21/30 311 N

Claims (3)

[Claims] 1. A mask observing method for observing a surface of a mask, on a surface of a transparent glass substrate, on which a thin film circuit pattern formed of a substance having a refractive index different from that of the glass substrate is formed, by an imaging optical system. With respect to the mask, the P-polarized laser light flux is 70 to 85 ° with respect to the optical axis direction of the imaging optical system in which the reflectances from the surface of the glass substrate and the surface of the circuit pattern are substantially the same. The specularly reflected light flux which is projected obliquely at an incident angle from the surface of the glass base material of the mask and the surface of the thin film circuit pattern by the projected and projected P-polarized laser light flux is masked. An image is formed on at least a one-dimensional image sensor corresponding to a position of the surface of the image forming optical system in the optical axis direction, and an image signal obtained from the image sensor is binarized by a predetermined threshold value. And the center of the binarized signal is calculated as coordinates on the image sensor, and the mask is moved in the optical axis direction of the imaging optical system based on the calculated center coordinates of the binarized signal. A focusing method, characterized in that the surface of the mask is adjusted to the focal position of the imaging optical system. 2. A mask observing apparatus for observing the surface of a mask, which has a thin film circuit pattern formed of a substance having a refractive index different from that of the glass substrate, on the surface of a transparent glass substrate by an imaging optical system. With respect to the mask, the P-polarized laser light flux is 70 to 85 ° with respect to the optical axis direction of the imaging optical system in which the reflectances from the surface of the glass substrate and the surface of the circuit pattern are substantially the same. An incident projection means for obliquely projecting an incident angle at an incident angle, and a P-polarized laser beam projected by the incident projection means to uniformly project from the surface of the glass substrate of the mask and the surface of the thin film circuit pattern. Detection optical means for forming an image of the specularly reflected light flux that is specularly reflected on at least a one-dimensional image sensor corresponding to the position of the surface of the mask in the optical axis direction of the imaging optical system; It converts the image signal obtained from the image sensor of the optical means into a binary signal by a predetermined threshold the two
Binarization center calculation means for calculating the center of the binarized signal as coordinates on the image sensor, and the mask image formation based on the center coordinates of the binarized signal calculated by the binarized center calculation means. A focusing device comprising: a focusing control unit that moves the optical system in the optical axis direction to bring the surface of the mask into a focal position of the imaging optical system. 3. The incident projection means has a slit forming optical system for forming a P-polarized laser slit light flux as a P-polarized laser light flux which is incident and projected on the surface of the mask, and the detection optical means includes a mask. The one-dimensional condensing optical system for condensing the specular reflection slit light flux specularly reflected from the surface on the one-dimensional image sensor in the longitudinal direction of the specular reflection slit light flux. The focusing device according to the paragraph.