JP3189528B2 - X-ray projection exposure equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray projection exposure apparatus used for X-ray lithography and the like.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of semiconductor integrated circuit elements, conventional ultraviolet rays (wavelengths 193 to 436) have been used in order to improve the resolution of an optical system limited by the diffraction limit of light.
nm) instead of soft X-rays having a shorter wavelength (wavelength 5 nm).
Projection lithography technology using ２０20 nm) has been developed. An X-ray projection exposure apparatus used in this technique mainly includes an X-ray source, an illumination optical system, a mask, an imaging optical system, a wafer stage, and the like.

[0003] In the X-ray wavelength region, no transparent substance exists and the reflectance on the substance surface is very low, so that ordinary optical elements such as lenses and mirrors cannot be used. Therefore, the optical system for X-rays includes a multilayer mirror coated with a special multilayer film, an oblique incidence mirror using total reflection of X-rays, and the like. As the X-ray source, a light source capable of obtaining strong soft X-rays such as a synchrotron radiation source or a laser plasma X-ray source is used. The illumination optical system uses X
An oblique incidence mirror that reflects light using total reflection, a multilayer mirror that matches the phase of the reflected light at each interface of the multilayer film to obtain a high reflectance by the interference effect, and reflects only X-rays of a predetermined wavelength Alternatively, the mask is illuminated with an X-ray of a desired wavelength on the mask by a filter or the like that transmits light.

As the mask, a transmission type mask and a reflection type mask are known. The transmissive mask is formed by forming a pattern on a thin membrane made of a substance that transmits X-rays well by providing a substance that absorbs X-rays in a predetermined shape. On the other hand, the reflective mask is formed by forming a pattern having a low reflectance portion in a predetermined shape on a multilayer film that reflects X-rays, for example.

[0005] The pattern formed on such a mask is imaged on a wafer coated with a photoresist by a projection imaging optical system composed of a plurality of multilayer mirrors and the like, and is transferred to the resist. You. Since the X-rays are absorbed by the atmosphere and attenuated, all the optical paths are maintained at a predetermined degree of vacuum.

[0006]

SUMMARY OF THE INVENTION The conventional X as described above
In the line projection exposure apparatus, much attention has not been paid to the design of the illumination optical system. J.Vac.Sci.Technol.
B8,1509 (1990) and J.Vac.Sci.Technol.B7,1648 (1989)
Has reported an experiment of X-ray reduction projection exposure using a Schwarzschild mirror composed of two concentric spherical mirrors as a projection imaging optical system. The mask was illuminated with substantially parallel light rays emitted from a light source.

In general, the resolution of an optical system depends not only on the performance of the imaging system but also on the manner of illuminating an object (a mask in the case of lithography). That is, when an object is illuminated with parallel light (referred to as coherent illumination), as shown in FIG. 3, the transfer function (OTF) of the optical system is NA / λ (where NA is the exit-side numerical aperture of the imaging system). , Λ indicates a constant value up to a spatial frequency determined by the illumination light wavelength), but when the spatial frequency is exceeded, the value becomes 0 and the image is not resolved. On the other hand, when the object is illuminated with a light beam having a divergence angle that satisfies the entrance-side numerical aperture of the imaging system (referred to as incoherent illumination), the OTF gradually decreases as the spatial frequency increases. It does not become 0 until the spatial frequency of 2NA / λ. Accordingly, although the contrast of the image is reduced, it is possible to resolve a pattern having a higher spatial frequency in the case of incoherent illumination.

However, in the above-described conventional technique, since the mask is illuminated with substantially parallel light, there is a serious problem that the resolving power of the imaging system is limited for the above-described reason. J.Vac.Sci.Technol.B9,3184 (1991)
Describes a method of illuminating a mask by reflecting X-rays emitted from a laser plasma X-ray source with a spherical mirror coated with a multilayer film at an incident angle of about 45 degrees.
However, when a spherical mirror is used at such oblique incidence, large astigmatism occurs, so even if illumination with a certain divergence angle can be performed in one direction of the two-dimensional pattern on the mask, For a pattern in the vertical direction, parallel illumination results. Therefore, there is a problem that the resolution of the optical system is anisotropic.

Astigmatism can be removed by using an elliptical mirror instead of a spherical mirror, but only in a very narrow range on the optical axis. Since an elliptical mirror has large aberrations off-axis, it is difficult to perform uniform illumination. In lithography, it is necessary to illuminate a certain size range on a mask, and thus there is a problem that appropriate illumination cannot be performed with such an illumination system.

[0010] In addition, the illumination by such a single multilayer mirror has a problem that the optical axis is largely bent, so that the arrangement of the optical system is complicated and adjustment is difficult. Further, since the elliptical mirror has an aspherical shape, its processing is extremely difficult as compared with a spherical mirror. For this reason, there is a problem that current processing technology cannot realize processing accuracy sufficient for use at the wavelength of X-rays.

SUMMARY OF THE INVENTION The present invention has been made in view of such a conventional problem, and an illumination optical system capable of performing incoherent illumination of a mask and thereby maximizing the performance of an imaging system. An object of the present invention is to provide an X-ray projection exposure apparatus having:

[0012]

Therefore, the present invention firstly provides "at least an X-ray source, an illumination optical system for irradiating the mask with X-rays emitted from the X-ray source, and an illumination optical system formed on the mask. A projection optical system for projecting and forming an image of the pattern on a wafer, and at least one of the mirrors constituting the illumination optical system in the X-ray projection exposure apparatus has a spherical surface. A spherical mirror formed by forming a reflective multilayer film, wherein the spherical mirror is provided at a position where X-rays incident on the spherical mirror are substantially perpendicularly incident, and the mask is arranged near a converging point of the spherical mirror. An X-ray projection exposure apparatus, wherein the spherical mirror has a radius of curvature and an outer diameter such that the divergence angle of the reflected X-ray focused upward substantially matches the numerical aperture on the incident side of the projection optical system. 1) ”.

The present invention also provides a second aspect of the invention wherein the X-ray source is a synchrotron radiation light source, and the illumination optical system is formed by forming an X-ray reflection multilayer film on at least a plane substrate. At a position where the first plane mirror reflects the X-rays emitted from the radiation light source in a direction substantially perpendicular to the orbital plane of the electron beam of the radiation light source. Wherein the spherical mirror is provided with the first
And the second plane mirror reflects the X-rays reflected by the spherical mirror in a direction substantially parallel to the emitted light from the radiation light source. The X-ray projection exposure apparatus according to claim 1, wherein the X-ray projection exposure apparatus is provided at a position where the exposure is performed.

[0014]

Hereinafter, an X-ray projection exposure apparatus according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the X-ray projection exposure apparatus shown in the drawings. In the illumination optical system of the X-ray projection exposure apparatus according to the present invention, for example, as shown in FIG. 2, a spherical multilayer mirror (spherical mirror) 1 having a multilayer substrate that reflects X-rays formed on a spherical substrate. Is used. The spherical mirror 1 is provided at a position where the X-rays incident on the spherical mirror 1 are incident substantially perpendicularly.

By the way, when a light beam is reflected by a spherical mirror at oblique incidence, there are two focal points in the plane of incidence and in a plane perpendicular to the plane of incidence. That is, astigmatism occurs. However, when a light ray is reflected by a spherical mirror at substantially normal incidence, the focal point in the plane of incidence substantially coincides with the focal point in a plane perpendicular to the plane of incidence, and the radius of curvature of the spherical surface is r. Focus 2 at a distance of f = r / 2. That is, astigmatism does not occur.

In the present invention, since the spherical mirror 1 is used in a substantially perpendicular incidence arrangement and the mask 9 is arranged in the vicinity of the converging point, the pattern on the mask has a cone-shaped isotropic spread angle. It is illuminated by rays that converge with light.
Further, the radius of curvature and the outer diameter of the spherical mirror 1 are set such that the spread angle θ of the light beam at this time becomes equal to the numerical aperture on the incident side of the imaging optical system.

That is, according to the present invention, since the condition of incoherent illumination is satisfied, the resolving power of the imaging optical system can be maximized. Further, since off-axis aberrations are smaller than in the case of using an elliptical mirror, a wider range can be uniformly illuminated. Further, in the present invention (claim 2), for example, as shown in FIG. 1, an X-ray from a radiation light source is first converted into a first planar multilayer mirror (planar mirror).
By 3, the traveling direction is bent upward (or downward) at a substantially right angle, and this is reflected almost vertically by the spherical mirror 1, and then bent at a substantially right angle by the second plane multilayer mirror (plane mirror) 4. The mask 9 is illuminated by returning to the horizontal direction.

That is, according to the present invention (claim 2), the optical axis of the illuminating light (X-ray) can be made coincident with the optical axis of the X-ray emitted from the radiation light source. The optical system may be arranged on this axis, and the arrangement of the entire optical system is simplified and the adjustment is facilitated. Note that X from the synchrotron radiation light source
The lines are polarized in the plane of the trajectory of the electron beam in the electron storage ring, and generally the multilayer mirror is perpendicular to the plane of incidence (the plane containing the normal of the incident ray and the reflective surface) at angles of incidence near 45 °. It reflects only X-rays polarized in different directions. Therefore, in general, the direction in which the X-rays are bent at a right angle by the plane multilayer mirror is not the horizontal direction but the vertical direction. (The orbital plane of the electron beam of the radiation light source is generally in a horizontal plane.) Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to Examples.

[0019]

FIG. 1 is an arrangement diagram of an optical system of an X-ray reduction projection exposure apparatus according to the present invention. As a light source, a radiation light beam emitted from a deflection magnet (not shown) provided in the electron storage ring 6 of the radiation light source was used. The size of the light emitting point of the light source is about 2 mm in diameter, and the divergence angle is ± 1 mrad in all directions.

Since the emitted light beam has a continuous spectrum from X-rays to visible light, first, a 1 μm thick B
The light was transmitted through an e (beryllium) filter 7 to remove a long-wavelength component equal to or more than ultraviolet rays and a short-wavelength component equal to or less than 112 °, which is the K absorption edge of Be, by the filter. Next, the emitted light beam was reflected by the horizontal deflection mirror 8 having an incident angle of 86 degrees. This mirror is made of synthetic quartz processed into a smooth plane and totally reflects X-rays. At this time, short-wavelength X-rays having a small total reflection critical angle are removed without being reflected. The filter 7 and the deflecting mirror 8 are for extracting only soft X-rays having a wavelength of around 134 ° used for exposure in this way.

The following three multilayer mirrors constitute the illumination optical system 5. First, a radiated light beam is bent upward by a first plane multilayer mirror (plane mirror) 3 having an incident angle of 46 degrees, and is reflected almost vertically by a spherical multilayer mirror (spherical mirror) 1 having an incident angle of 2 degrees. The emitted light beam was bent in the horizontal direction by the second plane multilayer mirror (plane mirror) 4 having an incident angle of 46 degrees.

Each mirror is arranged so that the optical axis of the radiation light beam incident on the first plane mirror 3 and the optical axis of the beam reflected by the second plane mirror 4 coincide. With such an arrangement, the optical axis of the imaging optical system (here, Schwarzschild mirror) 10 is adjusted in advance so as to coincide with the optical axis of the radiation light beam bent by the horizontal deflection mirror 8. Thereafter, the illumination optical system 5 can be adjusted to coincide with the optical axis.

In the multilayer mirror of the illumination optical system 5, X
Mo (molybdenum) / S is used for the multilayer film for reflecting lines.
An i (silicon) multilayer film was used. The cycle length of the multilayer film is set to a central wavelength of 134 ° according to the incident angle of each mirror.
Was optimized to reflect X-rays. In this embodiment, the radius of curvature of the spherical mirror 1 is 3.6 m, and its focal length f is 1.8 m. Since the distance a from the emission point of the emitted light source to the spherical mirror 1 is 13 m, the emitted light beam reflected by the spherical multilayer mirror 1 is focused at a distance b = 2.1 m ahead determined by the following equation (1).

1 / a + 1 / b = 1 / f (1) At this light-converging point position, a magnification m = 0.0 determined by the following equation (2).
A light source image reduced at 16 is formed. m = b / a (2) Further, the divergence angle of the radiated light beam focused at this position is determined by the following equation (3), where the divergence angle from the light source is θ _1.
Given by ₂ .

Sin θ ₂ = sin θ ₁ / m (3) Since the divergence angle θ ₁ of the X-ray from the radiation light source is ₁ mrad, sin θ ₂ is 0.00625. The transmission type X-ray mask 9 was arranged at the position of the image of the light source thus formed. This is achieved by forming a 0.2 μm thick Au on a 0.1 μm thick SiN (silicon nitride) membrane.
(Gold) pattern is formed.

The X-ray transmitted through the mask 9 is reduced by 1 /
Wafer 1 by 32 Schwarzschild mirrors 10
1 is formed. A photoresist sensitive to X-rays is applied to the wafer 11, and the pattern on the mask 9 is reduced to 1/32 and transferred to a resist pattern. The Schwarzschild mirror is an imaging optical system composed of two concentric spherical surfaces, and a Mo / Si multilayer film is formed on a reflection surface thereof. The numerical aperture (NA) of the Schwarzschild mirror 10 is 0.2 on the wafer side, and λ / 2
The resolution at the diffraction limit determined by NA is 0.03 μm. The numerical aperture on the mask side of the Schwarzschild mirror 10 is 0.2 / 3.
Since 2 = 0.00625, the divergence angle of the X-ray for illuminating the mask 9 by the illumination optical system 5 matches this value, and the condition of incoherent illumination is satisfied.

An exposure experiment was performed using such an X-ray reduction projection exposure apparatus. PMMA (polymethyl methacrylate) was used for the resist. When exposure was performed using a 1.6 μm line-and-space pattern mask, a 0.05 μm line-and-space resist pattern with dimensions close to the diffraction limit was obtained for patterns in any direction.

[0028]

Comparative Example For comparison, an exposure experiment using a simplified illumination optical system as shown in FIG. 4 was performed. Here, the horizontal deflecting mirror 18 has a gentle spherical surface with a radius of curvature of 36 m, and the mask 9 is focused on a focal point in the incident surface (here, a horizontal plane).
Is installed. The incident angle of the emitted light beam on the horizontal deflection mirror 18 is 86 degrees, which is the same as in the embodiment. When the spherical mirror is used in the oblique incidence arrangement, large astigmatism is generated. In this case, the rays in the horizontal plane are converged on the mask 9 and the divergence angle coincides with the numerical aperture on the mask side of the Schwarzschild mirror 10 and satisfies the condition of incoherent illumination. Since the mask 9 is illuminated in parallel, coherent illumination is obtained.

When a similar exposure experiment was performed in such an arrangement, a vertical line-and-space pattern was obtained.
Resolution was possible down to 0.05 μm, but 0.15 μm was resolved but 0.1 μm was not resolved in the horizontal line and space pattern. From the above results, it has been found that in the X-ray reduction projection exposure apparatus, it is important to match the numerical aperture of the illumination optical system with the numerical aperture on the incident side of the imaging optical system in order to obtain a diffraction-limited resolution.

In this embodiment, a transmission type X-ray mask is used. However, the present invention is not limited to this, and a similar effect can be obtained when a reflection type X-ray mask is used. Needless to say,

[0031]

As described above, according to the present invention, the pattern on the mask can be illuminated with X-rays having an isotropic divergence angle equal to the entrance-side numerical aperture of the imaging optical system. It is possible to obtain a diffraction-limited resolution of the image optical system. Further, compared with the case where an aspherical mirror such as an elliptical mirror is used, since a spherical mirror is used in the present invention, a mirror having sufficient shape accuracy even in the X-ray wavelength range can be easily manufactured by the conventional technology. I can do it. Also, since the aberration does not rapidly expand off-axis unlike an elliptical mirror, a relatively large illumination area can be obtained.

Further, according to the present invention (claim 2), the illumination optical system composed of at least three multilayer mirrors has the same optical axis on the incident side and the output side, so that the entire optical system has The arrangement is simplified, and the position adjustment of the optical system is facilitated.
Further, in X-ray reduction projection exposure using radiation light as an X-ray light source, a number of beam lines are radially provided from an electron storage ring of the radiation light, and an exposure apparatus can be provided for each beam line. . Therefore, a large number of exposure apparatuses can be installed by one electron storage ring, and an expensive radiation light source can be effectively used.

[Brief description of the drawings]

FIG. 1 is an optical system layout diagram of an X-ray reduction projection exposure apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of reflection at normal incidence by a spherical multilayer mirror according to the present invention.

FIG. 3 is a diagram showing a difference between OTFs for incoherent illumination and coherent illumination.

FIG. 4 shows X by a simplified illumination system as a comparative example of the present invention.
FIG. 2 is an optical system layout diagram of the line reduction projection exposure apparatus.

[Description of Signs of Main Parts]

DESCRIPTION OF SYMBOLS 1 ... Spherical multilayer mirror (spherical mirror) 2 ... Focus of spherical mirror 3 ... 1st planar multilayer mirror (plane mirror) 4 ... 2nd planar multilayer mirror (plane mirror) 5: Illumination optical system 6: Electron storage ring of synchrotron radiation light source 7: Be filter 8, 18, horizontal mirror 9: Mask 10: Schwarzschild mirror (an example of an imaging optical system) 11) Wafer and above

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-6231 (JP, A) JP-A-64-10625 (JP, A) JP-A-2-174111 (JP, A) JP-A-1- 307221 (JP, A) JP-A-4-121697 (JP, A) JP-A-3-29312 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/027 G03F 7 / 20 503

Claims (2)

(57) [Claims] At least an X-ray source, an illumination optical system for irradiating a mask with X-rays emitted from the X-ray source, and a projection for projecting an image of a pattern formed on the mask onto a wafer An X-ray projection exposure apparatus comprising: an optical system, wherein at least one of the mirrors constituting the illumination optical system is a spherical mirror formed by forming an X-ray reflective multilayer film on a substrate surface having a spherical shape. The spherical mirror is provided at a position where the incident X-rays incident on the spherical mirror are substantially perpendicularly incident, and the divergence angle of the reflected X-ray focused on a mask arranged near the converging point of the spherical mirror is as described above. An X-ray projection exposure apparatus, wherein the spherical mirror has a radius of curvature and an outer diameter that substantially match the numerical aperture on the incident side of the projection optical system. 2. The X-ray source is a synchrotron radiation light source, and the illumination optical system includes at least first and second plane mirrors each having an X-ray reflection multilayer film formed on a plane substrate, and the spherical mirror. And wherein the first plane mirror is provided at a position where the first plane mirror reflects the X-rays emitted from the radiation light source in a direction substantially perpendicular to the orbital plane of the electron beam of the radiation light source, and the spherical mirror is The second plane mirror is provided at a position where the X-rays reflected by the first plane mirror are substantially vertically reflected, and the second plane mirror makes the X-rays reflected by the spherical mirror substantially parallel to light emitted from the radiation light source. 2. The X-ray projection exposure apparatus according to claim 1, wherein the X-ray projection exposure apparatus is provided at a position reflecting in the direction.