JP3089955B2 - Optical member for optical lithography and projection optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical member for optical lithography used in an optical system such as a lens, a mirror or a prism in a specific wavelength region of 400 nm or less, preferably 300 nm or less in an ultraviolet lithography technique. It relates to the projection optical system used.

[0002]

2. Description of the Related Art In recent years, VLSIs are becoming more highly integrated and more sophisticated, and in the field of logic VLSIs, system-on-chip, in which a larger system is incorporated on a chip, is in progress. Along with this, fine processing and high integration are required on a wafer of silicon or the like serving as the substrate. In an optical lithography technique for exposing and transferring a fine pattern of an integrated circuit onto a wafer such as silicon, an exposure apparatus called a stepper is used.

[0003] If DRAM is taken as an example of VLSI, LSI to V
As the capacity is expanded from LSI to 1K → 256K → 1M → 4M → 16M, the processing line width is 10μm → 2μ each.
A fine stepper of m → 1μm → 0.8μm → 0.5μm is required. For this reason, the projection lens of the stepper is required to have a high resolution and a large depth of focus. The resolution and the depth of focus depend on the wavelength of light used for exposure and the N.D. of the lens. A. (Numerical aperture).

[0004] The finer the pattern, the larger the angle of the diffracted light. A. If it is not large, the diffracted light cannot be taken. Further, the shorter the exposure wavelength λ, the smaller the intensity of the diffracted light in the same pattern. A. Will be small. The resolution and the depth of focus are expressed by the following equations. Resolution = k1.lambda. / N. A. Depth of focus = k2 · λ / N. A. ² (however, k1 and k2 are proportional constants). A. Is increased or λ is shortened. As is clear from the above equation, it is more advantageous to shorten λ in terms of depth. From such a viewpoint, the wavelength of the light source is g-line (436n
m) to i-line (365 nm), and KrF (248 nm) and A
Shortening of the wavelength to an rF (193 nm) excimer laser is being promoted.

Further, the optical system mounted on the stepper is composed of a combination of a large number of optical members such as lenses, and even if the transmittance reduction per lens is small, it is only as large as the number of lenses used. Multiplied,
To reduce the amount of light on the irradiation surface, the optical member is required to have high transmittance. Therefore, in a wavelength region shorter than 400 nm, an optical glass of a special manufacturing method is used, which takes into account a reduction in transmittance due to a shorter wavelength and a combination of optical members. Further, at a wavelength of 300 nm or less, synthetic quartz glass or C
It has been proposed to use a single crystal of fluoride such as aF ₂ (fluorite).

On the other hand, in order to realize a finer line width as a projection lens and obtain a fine and clear exposure / transfer pattern, uniformity of the refractive index is high (variation in the refractive index within the measurement area is small). It is essential to obtain an optical member.
However, due to the recent increase in the exposure area accompanying the increase in the wafer size of semiconductors, the diameter and thickness of these materials have increased, and it has become more difficult to obtain the quality.
Therefore, various attempts have been made to improve the uniformity of the refractive index of an optical member having a large diameter and a large thickness.

Conventionally, the homogeneity of the refractive index is represented by the difference between the maximum value and the minimum value of the refractive index in the measurement area (hereinafter referred to as Δn). The smaller this value is, the better the homogeneity of the optical member is. It is believed that there is. Therefore, the existing optical member called high homogeneity is manufactured for the purpose of minimizing this Δn. On the other hand, regarding the birefringence of the optical member,
It has been considered that a material having a birefringence of 5 nm / cm or less does not affect the lens performance.

However, it is generally called highly homogeneous.
n is in the order of 10 ⁻⁶ or less, and the birefringence amount is 5 nm /
In some cases, fine and clear exposure / transfer patterns could not be obtained despite the use of an optical member having a diameter of not more than 1 cm.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to introduce a concept different from the existing technology in the evaluation of birefringence of an optical member used in an optical lithography technology, and to carry out a detailed evaluation and adjustment of an optical system. The object of the present invention is to provide an optical member for photolithography and a projection optical system capable of realizing a fine and clear exposure / transfer pattern having a line width of 0.3 μm or less, for example.

[0010]

Means for Solving the Problems The present inventors have made intensive studies on the characteristics of an optical member capable of obtaining a fine and clear exposure / transfer pattern in the photolithography technique. As a result, when the homogeneity Δn of the performance of the projection lens is almost the same, it is found that the birefringence amount and the distribution of the optical member and the distribution in the fast axis direction show a good correlation with the limit processing line width. Was. Based on this finding, it was revealed that a fine and clear exposure / transfer pattern having a line width of 0.3 μm or less can be obtained in an optical system configured using optical members having the following physical properties. .

Here, the polarization and birefringence characteristics will be described. Polarized light is a transverse wave of light with respect to an electromagnetic field, and indicates a change in an electric field observed from the traveling direction of the light. For example, linearly polarized light, circularly polarized light, and elliptically polarized light are used as terms representing the state. Expressing the same phenomenon in another way, birefringence is a phenomenon in which two refracted lights are obtained when one incident light passes through an optically anisotropic body. At this time, light having a different phase velocity (refractive index) depending on the direction of propagation in a substance is defined as an extraordinary ray, and light having a constant phase velocity regardless of the direction is defined as an ordinary ray.

Further, a refractive index ellipsoid is used as a method for indicating the distribution of the refractive index in the polarization direction. The refractive index ellipsoid indicates an amount of the refractive index for polarized light in a three-dimensional space.
The refractive index ellipsoid is generally represented by the following equation. (How to use optical parts and points to keep in mind) Tetsuo Sueda

[0013]

(Equation 1)

Here, the fast axis is defined as a direction in which the refractive index is small, that is, the direction of the minor axis of the refractive index ellipsoid. The term slow axis is used as the opposite of fast axis to indicate the direction of higher refractive index. By satisfying the following conditions of the polarization (birefringence) characteristics in addition to the homogeneity, it becomes possible to obtain a resolution close to the design performance of the projection lens.

The absolute value of the amount of birefringence is not more than 2.0 nm / cm. The distribution of the amount of birefringence has central symmetry. The fast axis direction in the index ellipsoid must be centrally symmetric. It is presumed that the high resolution can be obtained by satisfying the condition because the absolute amount of birefringence is small and the influence on the resolution is reduced due to the central symmetry of the polarization characteristic.

The absolute value of the birefringence of the optical member is 2.0 nm
/ Cm or more, the contrast of the projection optical system formed by using this decreases, the resolution deteriorates, and as a result, an exposure / transfer pattern with a line width of 0.3 μm or less cannot be obtained. Next, adjustment of the lens for minimizing the amount of birefringence at the image forming position of the projection optical system will be described.

Generally, Jones matrix calculation is used for the polarization analysis. (Basic and Methods of Spectroscopy Keiei Kudo) The Jones Matrix used in the present invention is shown below. The amplitude transmittance in the y-axis and z-axis directions was δy = δz = 1.

[0018]

(Equation 2)

An example of calculation of an electric vector in a projection lens system using this Matrix will be described.

[0020]

(Equation 3)

Here, n in the equation is the number of lenses.
Further, n, n-1... 1 are also symbols indicating each lens element (member) in the projection lens system. The lens performance was further improved by substituting the measurement results of the lens members into this calculation and assembling the projection lens with a combination of members that minimized the amount of birefringence.

As an example of this combination, consider two parallel flat plates (A, B). At each point, it is assumed that the birefringence amount is the same, the phase velocity of the extraordinary ray generated in A is positive with respect to the ordinary ray, and negative in B.
In this case, theoretically, when the perfect circularly polarized light is incident, the light beam that has passed is also completely circularly polarized light. That is, the apparent birefringence amount is equal to 0. At this time, there is also a method of using a positive uniaxial crystal (crystal or the like) or a negative uniaxial crystal (calcite or the like) in which the phase velocity of the extraordinary light is always smaller than that of the ordinary light. Actually, the number of lenses is generally ten or more, so that it is not so simple.

Similarly, with respect to the central symmetry of the optical axis of the polarization characteristic or birefringence characteristic of the lens system, the projection lens is assembled in a combination that approaches the most central symmetry according to the measurement result of each lens member.

[0024]

[Function] When the refractive index, which is a macroscopic optical property, is considered, glass is different from optical crystals and the like in a theoretically stress-free and completely uniform state. The birefringence is zero. However, such a state is practically impossible including the influence of gravity and the like.

Therefore, the birefringence of quartz glass is caused by residual stress generated by impurities, density distribution, heat history and the like. Examples of impurities include OH, Cl, metal impurities, and dissolved gas.In the case of the direct method, OH containing several hundred ppm or more, and then Cl containing several tens of ppm are dominant from the amount of contamination. Conceivable. Other impurities are less than 50 ppb by analysis and therefore have a negligible effect on birefringence.

On the other hand, as the density distribution, the density distribution based on the thermal history is dominant. This is the direct method (Direct
Method, VAD (vapor axial deposition) method, sol-gel method, plasma burner method and the like. It is assumed that the birefringence amount and distribution are determined by such components. As means for reducing the residual stress which causes such birefringence, the following method is available.

Reduction of impurity amount and density distribution by optimizing synthesis conditions Optimization of annealing conditions Also, as a method for making the birefringence amount and its distribution and the distribution in the fast axis direction centrally symmetric, a method of synthesizing A manufacturing method that always maintains the geometric center position in each process of heat treatment for homogenization and shape change, annealing for strain removal, and mechanical processing such as cutting and rounding is required. Become.

FIG. 1 is a schematic diagram showing a procedure for manufacturing a quartz glass for lithography according to the present invention. An example of the manufacturing method will be described below. If the synthesis of the quartz glass is performed while rotating the ingot, the impurity concentration distribution, the physical property distribution, and the polarization and birefringence characteristics based on the impurity concentration distribution always become centrally symmetric. First, the obtained ingot 11 is cut into a cylindrical shape 12. Since the side surface of the cylindrical shape 12 remains as the side surface of the ingot 11, if the geometric center of the cylindrical shape 12 is obtained from the side surface, it becomes the center at the time of combining the ingot 11, that is, the center of the stress distribution. If this point is marked on a circular cut surface and the center of the subsequent cutting, rounding, or other processing is performed, the center axis of the ingot 11 and the center axis of the quartz glass member coincide, and finally the polarization characteristics and An optical member having central symmetry of birefringence characteristics can be obtained.

As mentioned above, the polarization and birefringence properties are:
The density and the like are determined by impurities and thermal history, and these can be controlled by synthesis conditions. The drive units that affect the fluctuations in the synthesis conditions, such as the gas flow rates of the raw materials, oxygen, hydrogen, and the like, the exhaust flow rates, rotation, and reduction, are configured to be controlled with high precision. In addition, using the optical axis of the laser beam as a reference axis, the furnace, the drive unit, and the burner are aligned with high accuracy.

When heat treatment such as annealing is applied, in order to maintain symmetry, it is necessary to apply heat at the center of a furnace having a cylindrical shape and a rotationally symmetric temperature distribution. This quartz glass material is preferably rotated. In the case of viscous deformation, it is necessary to pay particular attention to prevent uneven deformation. By these methods, the amount of birefringence and the polarization characteristic distribution can be adjusted to obtain a desired optical member.

Further, when processing such as rounding is performed to obtain the quartz glass member 13, the center position is marked before processing.
Perform processing so that there is no displacement. The quartz glass member 13 is further processed and polished to produce a projection lens 14. At this time, one having a uniformity Δn of 2 × 10 ⁻⁶ or less was used. FIG. 2 shows a simple schematic diagram of an excimer laser stepper. The projection lens 14 of various shapes is manufactured by the above-described steps, and the projection lens system for exposure and transfer is manufactured by combining the projection lenses 14 into a lens barrel. 24 is completed.
In this figure, 21 is an excimer laser device, 22 is an illumination system of an excimer laser stepper, 23 is a reticle, 25
Is a silicon wafer to be reduced and projected. By performing such an operation, optical performance for obtaining a fine and clear pattern in the photolithography technique can be obtained. Further, the resolution was further improved by assembling a projection lens by combining lens members so as to minimize the amount of birefringence on the image plane in consideration of the fast axis direction.

That is, as described above, according to the present invention, it is possible to obtain a fine pattern of 0.3 μm or less in the photolithography technique.

[0033]

[Comparative Example] As lens members, homogeneity Δn ≦ 2 × 10 ⁻⁶ ,
In addition, a projection lens for a KrF excimer laser stepper was made of quartz glass satisfying the specification of birefringence ≦ 5 nm / cm. The obtained resolution (L / S) is equal to the design resolution (L
/ S) was 0.5 μm with respect to 0.25 μm. It was found that design performance could not be obtained only by selecting materials based on such specifications.

L / S is an abbreviation of line and space and is a numerical value generally used as an index for performance evaluation of semiconductor manufacturing. The homogeneity was measured by an oil-on-plate method using a He-Ne laser interferometer, and the birefringence was measured by a rotating analyzer method.

[0035]

Embodiment 1 Quartz glass satisfying specifications of Δn ≦ 2 × 10 ^{−6 of} the lens member, birefringence ≦ 2 nm / cm, and birefringence and polarization characteristics of central symmetry for a KrF excimer laser stepper. A projection lens was made. The obtained resolution (L / S) was 0.3 μm against the design resolution (L / S) of 0.25 μm. By sorting the members according to this specification, performance close to the specification was obtained.

The homogeneity was measured by an oil-on-plate method using a He-Ne laser interferometer, and the birefringence was measured by a phase modulation method. The phase modulation method has a sensitivity approximately two orders of magnitude higher than that of the rotary analyzer method, and facilitates confirmation in the fast axis direction. (Etsuhiro Mochida: Optical Technology Contact vol.27.N
o. 3 (1989)) The quartz glass used at this time had an internal transmittance of 10 mm at 365 nm, 248 nm and 193 nm exceeding 99.9%.

Further, after ¹⁰⁶ pulse irradiation with KrF excimer laser at 400 mJ / cm ² · pulse, 10 mm internal transmittance at 248nm was greater than 99.9%. Furthermore, after ¹⁰⁶ pulse irradiation at 100 mJ / cm @ 2 · pulse was confirmed ArF excimer laser characteristics, 10 mm internal transmittance at 193nm and it was confirmed that more than 99.9%. By using a lens design for an ArF excimer laser, this material can be used for an ArF excimer stepper.

This quartz glass has a hydrogen concentration of 5 × 10 ¹⁷ / cm
^{It is 3} or more, and the central part has a higher hydrogen concentration than the peripheral part. This projection lens can be used for a 256 MB VLSI production line.

[0039]

Embodiment 2 Lens member characteristics Δn ≦ 2 × 10 ⁻⁶ , and
A projection lens for a KrF excimer laser stepper was made of quartz glass satisfying the specification that the amount of birefringence ≦ 2 nm / cm and the birefringence and polarization characteristics are center symmetric. Further, at the time of assembling the lens members, the members used were adjusted so as to minimize the amount of birefringence on the imaging surface. The obtained resolution (L / S) is equal to the design resolution (L / S).
S) It was 0.25 μm against 0.25 μm.

The homogeneity was measured by an oil-on-plate method using a He-Ne laser interferometer, and the birefringence was measured by a phase modulation method. The quartz glass used at this time was 365 nm, 2
At 48 nm and 193 nm, the internal transmittance at 10 mm exceeded 99.9%. In addition, a KrF excimer laser was supplied at 400 mJ / cm ²
After 10 ⁶ pulses of irradiation at 10 Hz, the 10 mm internal transmission at 248 nm was greater than 99.9%.

[0041] In addition, after 10 ⁶ pulse irradiation at 100mJ / cm2 · pulse After a review of our ArF excimer laser characteristics, 193nm
It was confirmed that the 10 mm internal transmittance of the sample exceeded 99.9%. By using a lens design for an ArF excimer laser, this material can be used for an ArF excimer stepper. This quartz glass has a hydrogen concentration of 5 × 10
It is more than ¹⁷ / cm ³ , and the central part has a higher hydrogen concentration than the peripheral part.

This projection lens can be used for a fine VLSI production line of 256 MB or more. In the above embodiments, the projection lens using quartz glass as a material has been described in detail, but the present invention is not limited to this, and optical members other than quartz glass, for example, those using fluorite as a material are also applicable. The present invention can be applied to optical members other than lenses, such as mirrors and prisms.

[0043]

According to the present invention, it is possible to provide an optical member and a projection optical system capable of obtaining optical performance close to the design resolution at the time of designing a lens. The optical member and the projection optical system according to the present invention have an i-Line, ArF · K of 400 nm or less.
Applicable for rF excimer laser stepper.

According to these inventions, the performance of an optical lithography apparatus can be improved and stabilized. When the optical member of the present invention is used for photolithography technology, exposure and transfer are performed using light in a specific wavelength region of 400 nm or less, and laser light such as He-Ne (632.8 nm) is used. Can also be used for wafer alignment.

[Brief description of the drawings]

FIG. 1 is a schematic view of a procedure for manufacturing a quartz glass for lithography according to the present invention.

FIG. 2 is a schematic view of a lithography apparatus incorporating a projection lens manufactured using the optical member according to the present invention.

Continuation of the front page (56) References JP-A-6-16449 (JP, A) JP-A-5-58668 (JP, A) JP-A-3-109223 (JP, A) JP-A-3-88743 (JP) JP-A-6-234531 (JP, A) JP-A-6-234530 (JP, A) JP-A-6-308717 (JP, A) JP-A-7-187684 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 1/00-1/12 G02B 3/00-3/14 C30B 1/00-35/00 C01F 1/00-17/00 C03C 1/00- 14/00 C03B 1/00-17/06 C03F 7/20-7/24 H01L 21/027

Claims (9)

(57) [Claims] 1. An optical member for photolithography used in a specific wavelength region of 400 nm or less, in which the refractive index homogeneity is high.
Δn is 2 × 10 ⁻⁶ or less, and the absolute value of birefringence is 2.0 nm
/ Cm or less, the birefringence amount distribution has a central symmetry. 2. An optical member for photolithography used in a specific wavelength region of 400 nm or less, wherein a refractive index uniformity is obtained.
Δn is 2 × 10 ⁻⁶ or less, and the absolute value of birefringence is 2.0 nm
/ Cm or less, wherein the fast axis direction of the refractive index ellipsoid has central symmetry. 3. The optical member according to claim 1, wherein a 10 mm internal transmittance at 365 nm, 248 nm, and 193 nm exceeds 99.9%. 4. The optical member according to claim 1, wherein a KrF excimer laser is applied at 400 mJ / cm ^2.
-An optical member characterized by having a 10 mm internal transmittance at 248 nm exceeding 99.9% after irradiation with 10 ⁶ pulses. 5. The optical member according to claim 1, wherein the ArF excimer laser is supplied at 100 mJ / cm ^2.
-An optical member characterized by having a 10 mm internal transmittance at 193 nm of more than 99.9% after irradiation with 10 ⁶ pulses. 6. The optical member according to claim 1, wherein the hydrogen concentration is 5 × 10 ¹⁷ / cm ³ or more, and the central portion is made of quartz glass having a higher hydrogen concentration than the peripheral portion. An optical member characterized by the above-mentioned. 7. In a projection optical system for photolithography used in a specific wavelength band of 400 nm or less and constituted by a combination of a large number of optical members, the homogeneity Δn of the refractive index of each optical member is 2 × 10 ^{−. 6} or less, projection
The absolute value of the birefringence amount at the imaging position of the optical system is 2 nm /
cm or less of the birefringence of each optical member.
A projection optical system for optical lithography characterized by combining distributions . 8. In a projection optical system for optical lithography used in a specific wavelength band of 400 nm or less and constituted by a combination of a large number of optical members, the uniformity Δn of the refractive index of each optical member is 2 × 10 ^{−. 6} or less, projection
The fast axis direction of the index ellipsoid at the imaging position of the optical system is
Each optical member so that it is centrally symmetric with respect to the optical axis
A projection optical system for optical lithography, wherein the fast axis directions of the refractive index ellipsoids are combined . 9. In a projection optical system for optical lithography used in a specific wavelength band of 400 nm or less and constituted by a combination of a large number of optical members, the homogeneity Δn of the refractive index of each optical member is 2 × 10 ^{−. 6} or less, projection
The absolute value of the birefringence amount at the imaging position of the optical system is 2 nm /
cm or less and the fast axis direction of the refractive index ellipsoid is relative to the optical axis.
Birefringence of each optical member so that
Distribution and fast axis this <br/> and photolithography projection optical system characterized by a combination of direction of the refractive index ellipsoid.