JP2679337B2 - Illumination optical device, exposure apparatus including the same, and exposure method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an illumination optical apparatus for obtaining uniform illumination with high brightness and substantially no interference pattern using a laser light source, In particular, the present invention relates to an illumination optical apparatus or exposure apparatus and an exposure method suitable for manufacturing a semiconductor integrated circuit such as VLSI.

[Prior Art] An exposure apparatus for performing photolithography, which is used for manufacturing semiconductor integrated circuits such as VLSI, needs to perform uniform illumination with high brightness. Have often been used for such lighting. However, as the degree of integration of VLSI is further increased, photolithography in recent years is required to have higher line width accuracy than ever. Therefore, as a light source for exposure, a short-wavelength high-power laser such as an excimer laser has begun to be used instead of the above-mentioned ultra-high pressure mercury lamp.

The excimer laser light source is called a stable resonance type that emits a laser beam having a relatively low temporal and spatial coherency, and an injection that emits a laser beam having a higher temporal and spatial coherency than a stable resonance type. There are various types such as a so-called lock type or an improved version thereof. However, no matter which type of laser light source is used, spot-like illumination unevenness (interference pattern) is generated due to the interference of the laser light, although the degree of difference varies.

In order to eliminate this interference pattern, US Pat.
No. 9,508 describes a method of reducing illuminance unevenness due to an interference pattern by two-dimensionally scanning a beam with a rotating mirror or the like arranged in the optical path of an illumination system and substantially reducing the spatial coherency of the beam. Is stated.

Further, U.S. Pat.No. 4,851,978 describes illuminance unevenness due to an interference pattern generated in an illumination target surface when using an injection lock type light source that emits a laser beam having high coherency both temporally and spatially. In order to eliminate it effectively by vibrating the beam, the number of arrayed lens elements such as a fly's eye lens, etc., which is arranged in order to make the beam intensity distribution uniform in the illumination system in synchronization with the laser emission pulse, is used. It is stated that it is necessary to swing the beam two-dimensionally a corresponding number of times.

[Problems to be Solved by the Invention] However, in any case, in order to eliminate the uneven illuminance due to the interference pattern by spatially vibrating the laser beam as described above, it is possible to two-dimensionally use a vibrating mirror or the like during exposure. Since the beam has to be shaken a considerable number of times, the exposure time must be lengthened considerably,
There is a drawback that the production efficiency of semiconductor devices is significantly reduced.

Moreover, the conventional method cannot completely eliminate the illuminance unevenness due to the interference pattern even if the beam is shaken a considerable number of times. It was an obstacle.

Further, a configuration of an optical system as shown in FIG. 9 is known as a method for controlling the integrated exposure amount of the illumination target surface.
Also in this case, if the light beam from the light source has a relatively strong polarization component in a specific direction, the problem that the exposure amount control accuracy deteriorates occurs. That is, in FIG. 9, a light beam from a light source (not shown) has its beam diameter expanded by a beam expander consisting of a pair of lenses 301 and 302, and then divided into a plurality of partial light beams by a fly-eye lens 303. On the other hand, the plurality of split luminous fluxes are transmitted to the half mirror 304.
Through the condenser lens 305 and the illumination target surface 306
On the other hand, uniform illumination is achieved by this, and on the other hand, a part of the light is reflected by the half mirror 304 and the light amount detection surface 308 by another condenser lens 307.
Overlaid on top. In this way, by measuring the ratio of the amount of light on the illumination target surface 306 and the amount of light on the light amount detection surface 308 in advance, it is possible to detect the amount of light at the same time during exposure, but the half mirror 304 has a specific polarization characteristic. ,
When the polarization state of the light beam from the light source changes, the reflectance of the half mirror 304 also changes, so the illumination target surface 3
The ratio of the light intensity between 06 and the light amount detection surface 308 changes, and as a result, accurate exposure amount control cannot be performed.

An object of the present invention is to provide an illumination optical apparatus using a coherent light source such as a laser, an exposure apparatus and an exposure method including the same, and particularly, an interference pattern from an illumination target surface easily without oscillating a laser beam two-dimensionally. Illumination that can eliminate the uneven illuminance due to the light source and can always perform accurate exposure detection without changing the reflectance of the half mirror in the optical system even if the polarization state of the light beam from the light source changes. An optical device is provided.

[Means for Solving the Problems] The illumination optical device according to the invention described in claim 1 is a plurality of spatially periodically distributed from a coherent light beam emitted from a light source in a plane perpendicular to the beam optical axis. , An optical integrator forming a group of real or imaginary secondary light sources, lens means for superimposing the light flux from each secondary light source on one illumination target surface, and the inner product of the mutual polarization vectors from the coherent light beam is Two polarized light fluxes that are zero are formed, and the polarized light fluxes that are incident on the optical integrator have different polarizations so that the interference patterns of the polarized light fluxes are relatively displaced on the illumination target surface. In order to achieve the above-mentioned object, the optical integrator is provided with a rectangular quadrangle. A fly-eye lens having a plurality of positive or negative lens elements arranged in parallel with each other, and the optical means has lengths of two adjacent sides of the right-angled quadrangle a, b (where a ≦ b ), The difference θ between the two polarized light beams is θ = λ {[(n +
1/2) / a] ² + [(m + ^1/2 ) / b] ² } ^1/2 (where λ is the wavelength and n and m are integers), and the difference in declination is that of the fly-eye lens described above. It is formed at an angle of Ψ = tan ⁻¹ {[(n + 1/2) / a] / [(m + 1/2) / b]} with respect to the side of the dimension b when viewed from the optical axis direction. The optical structure parameter is defined as described above.

According to a second aspect of the present invention, the optical means includes a deflection angle prism made of a refractive optical material inserted in the optical path.

According to a third aspect of the present invention, in the illumination optical device according to the second aspect, when the coherent light beam has a polarization characteristic, the deflection prism is arranged in an optical axis direction with respect to a polarization direction of the polarized coherent light beam. When I saw
It is made of a birefringent optical material having an optical axis forming an angle of 45 degrees.

In the invention according to claim 4, a group of a plurality of real or imaginary secondary light sources spatially periodically distributed in a plane perpendicular to the beam optical axis is formed from the coherent light beam emitted from the light source. An optical integrator having an optical integrator and a lens unit that superimposes the light flux from each secondary light source on a reticle, and in an exposure apparatus that transfers the pattern on the reticle onto a wafer, the mutual polarization vectors of the coherent light beams Two polarized light beams having an inner product of zero are formed, and the polarized light beams incident on the optical integrator have different polarizations so that an interference pattern due to each polarized light beam causes a relative displacement on the reticle. The optical integrator has an optical means for providing an angle, and the optical integrator has a positive or negative side having a rectangular-parallelepiped input / output surface. The optical means is composed of a fly-eye lens in which a plurality of lens elements are arranged in parallel. When the lengths of two adjacent sides of the right-angled quadrangle are a and b (provided that a ≦ b), the two optical elements are provided. As the deviation θ of the polarized light flux, θ = λ {[(n + 1/2) / a] ² + [(m + 1/2) / b] ² }
^1/2 (where λ is a wavelength and n and m are integers), and the deviation angle is Ψ = tan ^{− with} respect to the side of the dimension b when the fly-eye lens is viewed from the optical axis direction. ¹ {[(n + 1/2) / a] / [(m + 1/2) / b]}
It is characterized in that the optical structure parameters are determined so that the optical structure parameters are formed at an angle Ψ of.

Further, in the invention according to claim 5, a coherent light beam emitted from the light source is emitted from a plurality of real or imaginary secondary light sources spatially periodically distributed in a plane orthogonal to the beam optical axis by an optical integrator. In an exposure method of forming a group and illuminating a reticle by superimposing light fluxes from each of the secondary light sources, a pattern on the reticle is transferred onto a wafer, and inner products of mutual polarization vectors from the coherent light beams are formed. Of the two polarized light beams having a value of zero, and a step of giving different deflection angles to the two polarized light beams so that an interference pattern due to the two polarized light beams causes a relative displacement on the reticle. As the optical integrator, a plurality of positive or negative lens elements having a right-angled quadrilateral input / output surface are arranged in parallel. When the length dimensions of two adjacent sides of the right-angled quadrangle are a and b (where a ≦ b), the two polarized light beams are polarized by using the fly-eye lens described above. The angle difference θ is θ = λ {[(n + 1/2) / a] ² + [(m + 1/2)
/ b] ² } ^1/2 (where λ is the wavelength and n and m are integers),
Moreover, when the fly-eye lens is viewed from the optical axis direction, the angle Ψ formed by the deviation angle θ with respect to the side of the dimension b is Ψ = tan ⁻¹ {[(n + 1/2) / a] / [( m + 1/2) / b]}
It is characterized by satisfying.

[Operation] In the present invention, by utilizing the fact that the interference pattern on the illumination target surface has a periodic structure corresponding to the structure of the illuminance distribution uniformizing means in the illumination system, the laser beam is made orthogonal to each other, for example. The linearly or elliptically polarized beam, or the two circularly polarized beams that are reverse to each other in right and left directions are separated, and the period of the interference pattern on the irradiation target surface by each polarized beam is predetermined by N + 1/2 period (N is an integer). By shifting in the direction, the light intensity distributions of the interference patterns are complementarily smoothed by each other, and as a result, the uneven illuminance due to the interference on the illumination target surface is almost eliminated.

That is, the optical integrator is a fly-eye in which a plurality of positive or negative lens elements having a right-angled quadrangular entrance / exit surface with adjacent two sides having lengths a and b (where a ≦ b) are arranged in parallel. It is composed of a lens,
The optical structure parameters of the optical means include a deviation angle difference θ and a deviation angle difference θ between the two polarized light beams as conditions for optimizing complementary smoothing of the light intensity distribution on the illumination target surface. Direction (angle Ψ formed with respect to the side of the dimension b when the fly-eye lens is viewed from the optical axis direction)
Based on the structural parameters of the fly-eye lens arranged in the optical path and the beam wavelength λ used, θ = λ {[(n + 1/2) / a] ² + [(m + 1/2) / b ] ² } ^1/2 (where a, b (a ≦ b) are the lengths of two adjacent sides of a right-angled quadrangle that form the entrance and exit surfaces of the lens element of the fly-eye lens, λ is the beam wavelength used, and n, m is an integer), and Ψ = tan ⁻¹ {[(n + 1/2) / a] / [(m + 1/2) / b]}.

The optical means can be realized, for example, simply by arranging a deflection prism made of a birefringent optical material having a predetermined shape in the optical path, and substantially substantially completely illuminating the target surface without vibrating the laser beam. The interference pattern can be erased. Especially when the coherent light beam from the light source has a polarization characteristic, by using a deflection prism made of a birefringent optical crystal material having an optical axis that makes an angle of 45 degrees with respect to the polarization direction, this deflection prism Can have a function of depolarizing the light beam of the light source. That is, the light passing through the birefringent crystal gives a phase difference between the ordinary ray and the extraordinary ray of polarized light, and thus acts as a so-called wave plate. In this case, the deflection prism has a non-uniform thickness over the entire surface and the thickness varies depending on where the light passes, so the polarization state of the passing light beam differs depending on the passing location, and the polarization of the incident light beam is changed. In the case of a crystal declination prism having an optical axis that makes an angle of 45 degrees with respect to the direction (the direction of the long axis in the case of elliptically polarized light), the incident light beam is artificially depolarized.

If the coherency of the light source is quite high,
By additionally using the vibration of the laser beam by a vibrating mirror or the like, it is possible to more effectively eliminate the illuminance unevenness due to the interference on the illumination target surface with a smaller number of beam vibrations.

In order to make it easy to understand the objects, features, and emphasis points of the present invention, preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

[Embodiment] FIG. 1 shows an illumination optical apparatus according to an embodiment of the present invention applied to a projection exposure apparatus for exposing a pattern on a reticle onto a semiconductor wafer.

First, in the laser light source 1 according to the present embodiment, as schematically shown in FIG. 2, two resonator mirrors 10 and 10 'are arranged at both ends of a discharge tube 12 which causes stimulated emission, and further, an etalon. , Wavelength grating 14 such as diffraction grating or prism
This is a so-called stable resonance type spectrum narrowing band KrF excimer laser light source provided with a discharge tube 12 and a resonator mirror 10. The characteristic of the laser beam emitted from the light source 1 is that the wavelength selection element 14 is provided so that the spectrum width is narrowed (temporal coherency is improved).
Moreover, the spatial coherency is lower than that of the injection locking type. The light source in the present invention is not particularly limited, but it is not necessary to correct chromatic aberration in the projection lens PL of the exposure apparatus, and since the contrast of the interference pattern that occurs is originally low, it is easy to completely eliminate illuminance unevenness due to interference. And second
It is preferable to use a light source having a structure as shown in the figure.

Now, the laser beam LB emitted from the laser light source 1
Is incident on the cylindrical lens 11 after being bent in the optical path by the ultraviolet reflecting mirrors of M ₁ , M ₂ , M ₃ , and M ₄ , where the cross-sectional shape of the beam is shaped to make it closer to a square from a narrow rectangle. .

Thereafter, the beam LB is bent in the optical path by the ultraviolet reflection mirror M ₅ and enters the beam expander 15, where it is expanded to a predetermined beam diameter.

The expanded beam emitted from the beam expander 15 is made incident on the birefringent crystal prism 7 which constitutes the deflection prism of the optical means of the present invention. The crystal prism 7 has an apex angle of 1 ° 35 'with its ridge line flush with the crystal optical axis of the crystal prism 7.
The incident beam is separated into two kinds of polarization components whose wavefronts are inclined by about 64 ″ from each other by the crystal prism 7 and exits from the prism 7. The apex angle of the crystal prism 7 is , The laser beam, as described below
The wavelength of LB and the pitch of the fly-eye lenses 3 and 5 in the latter stage,
It is appropriately set according to the magnification by the lenses 4 and 21.

Subsequently, the two types of polarized beams are transmitted to the crystal prism 7
And the apex angle of 1 ° 50 'arranged so that the slopes face each other.
(This value is also set appropriately according to the degree of refraction of the beam at the apex angle prism 7 arranged in the preceding stage) is incident on the correcting quartz glass prism 9, and here the traveling directions of the two polarized beams Are the optical axes around the optical axis AX of the illumination system
It is corrected to make an angle of about 32 "(= 64" / 2) from AX.

After that, the two polarized beams pass through the first-stage fly-eye lens 3 to form a group of a plurality of separated secondary light sources on the emission side of the fly-eye lens 3. The light from each secondary light source is bent via a convex lens 4 by an oscillating mirror 17 (when the light source shown in FIG. 2 is used as will be described later, it may remain stationary without vibrating). , Is incident on the second-stage fly-eye lens 5 through another convex lens 21 in a superimposed manner. The superposed beam is separated into a plurality of beams by the fly-eye lens 5 to make the illuminance distribution uniform again, and then condensed again by the lens 25, and then the mirror 27.
It is bent and enters the main condenser lens 2. The beam composed of the two types of polarization components thus subjected to the illuminance uniforming action is appropriately condensed by the main condenser lens 2, and then the reticle R is uniformly irradiated. As a result, the circuit pattern on the reticle R is projected onto the wafer W by the projection lens PL, and exposure is performed.

Here, if the optical deflection means composed of the quartz prism 7 is not arranged, as described above, the fly-eye lenses 3 and 5 generate an interference pattern on the reticle R to be illuminated, and this interference pattern is generated. Is transferred together with the circuit pattern of the reticle R onto the wafer W by the projection lens PL, which hinders good pattern transfer.

In the present invention, since the coherent light beam emitted from the light source 1 is separated into two polarized beams forming a predetermined angle by the deflection angle prism (the crystal prism 7 in the embodiment), the reticle R is illuminated respectively. The polarized beam causes the interference pattern in which the light and dark periods are shifted by a half period, and the two interference patterns fill up the peaks and valleys of the light intensity distributions of each other, and as a result, the interference pattern almost disappears.

In the above embodiment, the spatial coherency is relatively low (the contrast of the interference pattern is originally low).
The case of using the light source as shown in FIG. 2 for emitting the laser light beam has been described, but the degree of coherency varies depending on the light source, and even the light source of the same system may have a considerably high coherency depending on the manufacturer. In such a case, it is difficult to completely eliminate the interference pattern only by disposing the deflection angle prism 7, so that the vibration mirror 17 shown in FIG. 1 is vibrated during exposure to illuminate the interference pattern. It is only necessary to reciprocate on the target surface to average the illuminance and erase the interference pattern by the integration effect.
Even in such a case, by arranging the deflection angle prism, compared with the case where only the beam is oscillated,
This is very advantageous in that the uneven illuminance due to interference can be eliminated more efficiently (with a small frequency), and the production efficiency of the photolithography process is improved.

The cause of the interference pattern on the illumination target surface when the optical deflection prism is not provided will be described with reference to FIGS. 3 and 4.

Figure 3 shows a beam expander (lenses 100 and 102) that expands the beam LB from the laser light source to a certain size.
And a fly-eye lens 10 to make the illuminance distribution uniform
3 schematically shows the arrangement of the condenser lens 104 and the reticle R. FIG. 4 is a perspective view showing the structure of the fly-eye lens 103, in which a plurality of element lenses 103a are arranged at pitches a and b in the X and Y directions. Here, the pitches a and b are a ≦ b.

The parallel expanded beam emitted from the exit lens 102 of the beam expander is incident on almost the entire surface of the fly-eye lens 103. On the exit side of the fly-eye lens 103, there are as many secondary light source (focus point) groups P ₁ , P ₂ ,
P ₃ , P ₄ , ... Are formed. The light beams from each of the secondary light source groups P ₁ , P ₂ , P ₃ , P ₄ , ... Are superposed on the reticle R by the condenser lens 104 to make the illuminance uniform. However, at this time, the lights traveling from each of the secondary light source groups P ₁ , P ₂ , P ₃ , P ₄ , ... To the reticle R simultaneously interfere with each other on the reticle R to form an interference pattern.

Here, the original beam LB emitted from the laser oscillator
Is sufficiently wide (low spatial coherency), the beams from the respective secondary light source groups P ₁ , P ₂ , P ₃ , P ₄ , ... are incoherent to each other (incoherent). Therefore, a problematic interference pattern does not occur. Conversely, the original beam LB has a very narrow width (high spatial coherency)
, All the beams from the respective secondary light source groups P ₁ , P ₂ , P ₃ , P ₄ , ... Interfere with each other, and on the reticle R, a complicated interference pattern (bright and dark fringes) with high contrast is obtained. Occurs. Also, if the original beam LB has a certain size, the mutual coherence between the respective secondary light source groups becomes weaker.
Of the P ₁ , P ₂ , P ₃ , P ₄ , ..., only adjacent ones arranged at the pitches a and b interfere with each other, and as a result, the interference pattern has a sine wave shape with low contrast in the pitches a or b or both directions. The interference pattern has an intensity distribution of.

Next, the period of this sine wave will be described. In FIG. 3, the focal length of the condenser lens 104 is f to make it easier to think, and it is assumed that the secondary light sources P ₁ , P ₂ , ... And the reticle R surface are on the front and rear focal planes of the lens 104, respectively. To do. The distance between P ₁ , P ₂ , ... Is equal to the distance between the fly-eye lenses and is a in the x direction and b in the y direction. Considering only interference between adjacent light sources of the secondary light sources P ₁ , P ₂ , ..., The pitch of the interference pattern on the reticle R is λ · f / a in the x direction, where λ is the wavelength of the beam, In the y direction, λ · f / b, and the interference pattern is further reduced on the wafer surface by the reduction magnification of the projection lens. That is, the interference pattern generated on the surface to be illuminated has a periodic structure corresponding to the structural parameters of the fly-eye lens arranged for uniforming the illuminance distribution.

Therefore, assuming that the phase difference between adjacent secondary light sources shifts by π in the x or y direction, the interference pattern shifts by half a period (λ · f / 2a in the x direction and λ · f / 2b in the y direction). Then, if an interference pattern having a phase difference of π radian from the original interference pattern is superposed on the interference pattern, the interference patterns mutually fill the peaks and valleys of the illuminance distribution on the surface to be illuminated, resulting in uniform brightness. That is, if the interference patterns having the same periodicity are generated by being shifted by a half cycle and overlapped, it is possible to eliminate the illuminance unevenness due to the interference on the illumination target surface.

Now, regarding the pitch a direction, in order to shift the phase difference between adjacent secondary light sources by π radians, one of the two light beams incident on the fly-eye lens should be inclined with respect to the other. The angle θ between both beams
Is θ = 1 / 2λ / a = λ / 2a, where a is the distance between the fly-eye lenses.

Next, a method of tilting the wavefronts of the two beams by λ / 2a will be described. Note that the tilted wavefronts do not interfere with each other. If they interfere, they will cause another interference pattern. In order to satisfy this condition, for example, two kinds of polarized beams having different polarization states such as linearly polarized lights orthogonal to each other may be used.

Although various methods can be considered to make two polarized beams having wavefronts that are polarized in directions orthogonal to each other and inclined with respect to each other, the simplest and stable method is to use a prism of a subrefractive optical crystal. is there. That is, for example, if a wedge-shaped declination prism having a straight line in the same plane as the crystal optical axis of an anisotropic crystal such as quartz, MgF ₂ , ADP (NH ₄ H ₂ PO ₄ ) and calcite is used, good.

Here, a deflection prism using quartz (uniaxial crystal) will be described with reference to FIG.

In FIG. 5, (a), (b) and (c) are optical axes 108a, 108b and 108c and ridgelines 107a, 107b and 10 of the deflection prism.
7c is an example of a deflection prism having the function of separating the incident beam into two linearly polarized beams that are orthogonal to each other and emitting the same on the same plane as 7c. On the other hand, (d) shows the optical axis 10
8d is not on the same plane as ridge 107d,
It is not possible to separate into two mutually orthogonal linearly polarized light components, but it is separated into two clockwise and counterclockwise circularly polarized light components with mutually opposite directions, and there is a predetermined declination angle difference between them, and they are emitted. It is an example of a deflection prism having a function to do.

In the present invention, these declination prisms may have a function of separating the incident light into two polarized light beams which do not interfere with each other and differentiating the emission angles of both polarized light beams by a predetermined angle, and generally, Two linearly polarized lights whose polarization directions are orthogonal to each other are preferable, but two elliptically polarized lights whose major axes are orthogonal to each other and two circularly polarized lights whose rotation directions are opposite to each other. Good. That is, two polarized light beams emitted from the polarizing prism in the present invention are two polarized light components having substantially equal light intensities and causing no interference with each other, and generally, the inner product of the polarization vectors of the two polarized light components is zero. Such that the polarization vector of the two polarization components is (A _x · A _y ),
When the component is expressed as (B _x · B _y ), there are two polarization components that satisfy A _x ^* · B _x + A _x · B _x ^* + A _y ^* · B _y + A _y · B _y ^* = 0 ( * Represents a complex conjugate).

Optical polarization elements that satisfy this condition include those that split into two linearly polarized light shown in FIGS. 5 (a), (b), and (c), and those shown in FIG. 5 (d). For example, two circularly polarized lights can be separated, and two elliptically polarized lights orthogonal to each other such as two elliptically polarized lights that can be separated into two light beams that do not interfere in intensity are also available. Incidentally, as a device for separating two circularly polarized lights in opposite directions, it is also possible to use an element in which a right crystal and a left crystal are combined, which is known as a cornu pseudodepolarizer. The light beam that has passed through the prism is split into two light beams, a right-handed circularly polarized light and a left-handed circularly polarized light, and there is a difference in declination between the two light beams. It can contribute to the reduction.

These elements are generally used as depolarization elements, but as described above, by appropriately setting the difference in the angle of deviation between two polarizations that are orthogonal to each other or rotate in opposite directions, The generation of interference fringes due to a wavefront division type optical integrator such as a lens is reduced to enable uniform illumination. The light passing through the birefringent crystal gives a phase difference between the ordinary ray and the extraordinary ray of polarized light, and thus acts as a so-called wave plate. The deflection prism in this case is
Since the thickness is not uniform over the entire surface and the thickness varies depending on the place where light passes, the polarization state of the passing light beam differs depending on the passing place, and the incident light beam is artificially depolarized. become. Of course, in this case, if the incident light beam is linearly polarized light, the crystal optical axis of the deflection prism should be at an angle of 45 degrees with respect to the polarization direction. It goes without saying that it is necessary to make the crystal optical axis of the deflection prism at an angle of 45 degrees with respect to the major axis direction of. As described above, when the incident light beam is artificially depolarized, the accuracy of the exposure amount control by the exposure amount detection system including the half mirror as shown in FIG. 9 depends on the polarization characteristic of the light beam from the light source. Will not be affected by changes in.

For example, in a deflection prism as shown in FIGS. 5 (a), (b), and (c), synthetic light of lights having different polarization directions, or linearly polarized light having an angle of about 45 ° with the direction of the crystal optical axis, Alternatively, when elliptically polarized light having a principal axis of 45 ° or circularly polarized light is incident, the beam passing through the deflection prism is refracted in two directions as shown in FIG.

The refraction angle at this time is (n ₀ − for ordinary light: light whose polarization direction (vibration direction) is perpendicular to the optical axis).
1) α, for e-polarized light (extraordinary ray: light polarized (vibrated) in the direction parallel to the optical axis), it is (n _e -1) α.
Where n ₀ is the refractive index of o-polarized light (n ₀ = 1.6032), and n _e is e
The refractive index of polarized light (n _e = 1.6144), α is the apex angle of the deflection prism. That is, the fifth line whose ridge is in the same plane as the crystal optical axis
The wavefronts of the 0-polarized light and the e-polarized light separated by the birefringent crystal deflection prism as shown in FIGS. (A), (b) and (c) form an angle of (n _e −n ₀ ) α, And do not interfere with each other.

Next, how to obtain the apex angle of the deflection prism will be described.

The condition for shifting the cycle of the interference pattern generated by each of the o-polarized light and the e-polarized light by a half cycle on the illumination target surface is given by the following expression (1) (however, only the pitch a direction is considered).

(N _e −n ₀ ) α = λ / 2a (1) In the formula (1), if the wavelength λ = 248 nm and the distance a = b = 4 mm between the element lenses forming the fly-eye lens, then λ / 2a = (248 × 10 ^-9 ) / (2 × 4 × 10 ^-3 ) = 31 μrad≈6.4 ″, so α = (λ / 2a) / (n _e −n ₀ ) = (31 × 10 ^-6 ) / (1.6144-1.6032) ∴α ≒ 2.768mrad ≒ 0.1586 ° ≒ 9.5 'Therefore, it is sufficient to use a crystal deflection angle prism with an apex angle of 9.5'.

In the above example, the case where the deviation prism is arranged on the incident side of the fly-eye lens has been described, but the deviation prism in the present invention does not necessarily have to be arranged on the incident side of the optical integrator. That is, since the cycle of the interference pattern is uniquely determined by the wavelength λ and the pitches a and b of the fly-eye lens as described above, if the apex angle of the deflection angle prism is appropriately selected, the deflection angle prism can be placed anywhere in the optical path. May be.

Here, when two sets of fly-eye lenses are used to further improve the uniformity of illumination by the laser beam (first
How to determine the apex angle of the deflection prism (corresponding to the case of the embodiment in the drawing) will be described with reference to FIG.

First, when placing a crystal deflection prism in front of the second-stage fly-eye lens 205 (on the incident side), use a crystal deflection prism with an apex angle of 9.5 '(λ = 248 nm, a = 4 mm) as described above. You can use it.

On the other hand, as shown in FIG. 7, when a deflection prism is placed in front of the first-stage fly-eye lens 203, the wavefront inclination is such that the first-stage fly-eye lens 203 and the next convex lens 204
It becomes smaller by the reciprocal of the magnification. Therefore, the apex angle of the deflection prism must be increased by this magnification.
Generally, since this magnification is about 10 times, the apex angle of the quartz deflection prism becomes 9.5 '× 10 = 95' = 1 ° 35 ', and the separation angle between the o-polarized light and the e-polarized light becomes about 64 ".

At this time, the direction of travel of the entire beam is
About 95 '× (1.6
-1) = 57 ', so as shown in FIG. 7, for example, an optical path correction prism 209 made of synthetic quartz (refractive index of about 1.51, apex angle of 1
It is advisable to arrange (.degree.50 ') side by side with the deflection prism 207 as necessary and correct the beam traveling directions so that they are inclined at equal angles to both sides of the optical path, as shown in FIG. In this way, the bisector of the angle between the o-polarized light and the e-polarized light that has passed through the correction prism 209 matches the traveling direction of the beam before entering the deflection prism 207.
Although the correction prism 209 is a synthetic silica correction prism in FIG. 8, it may be replaced with a birefringent deflection prism whose optical axis is orthogonal to that of the deflection prism 207.

Further, although the case where two sets of fly-eye lenses are used has been described in the above embodiment, the present invention is not limited to this, and when the optical magnification is set between the fly-eye lens and the deflection prism, the deflection angle is similarly changed. It goes without saying that the prism angle must be increased by the magnification.

As described above, it is apparent that the deflecting prism made of the birefringent optical crystal material has two effects, that is, the effect of reducing the coherence and the effect of depolarizing, and they are compatible with each other.

In the above description, the direction of the declination by the declination prism has been described only for one pitch a direction of the lens elements of the fly-eye lens, but the same is true for the pitch b direction. The declination angle will be directed to the composite direction according to the size. In the case of using a two-dimensional array fly-eye lens and a laser beam having a polarization characteristic, a method of setting a difference in deviation angle between two polarized light beams in the diagonal direction of the exit surface of the lens element of the fly-eye lens is described below. To do.

FIG. 10 is a schematic view of an overlap of a fly-eye lens 401 as an optical integrator and a deviation prism 402 in the illumination optical device according to the present invention, as seen from the optical axis direction. This fly-eye lens 401 has a configuration in which a plurality of lens elements having a rectangular entrance / exit surface are bundled, and the length of the side of the rectangle of the entrance / exit surface of each lens element is xy shown in FIG. On the Cartesian coordinates, the short side along the x-axis is a and the long side along the y-axis is b, and the diagonal line of this rectangle forms an angle Ψ ′ with the y-axis.

FIG. 11 shows the relationship between the polarization direction 403a of the incident laser beam viewed from the optical axis direction, the target declination direction 403b of the emitted light by the deflection prism 402, and the crystal optical axis direction 403c of the deflection prism 402. It is shown in the same xy coordinates as the figure.

When the coherence of the incident laser beam is strong in both the x direction and the y direction and a lattice-shaped interference fringe is generated on the surface to be illuminated, as shown in FIG. 11, the target declination direction 403b is the x axis. y
Deflection prism 40 with no alignment to either of the axes
It is necessary that the lattice-shaped interference fringes formed by the two separated polarized beams emitted from 2 are displaced from each other on the illumination target surface in both the x-axis direction and the y-axis direction.
If the coherence of the incident laser beam is strong in either the x-axis or the y-axis, in principle, the target declination direction 403b may be determined with respect to the axial direction in which the coherence is strong. To the target declination direction 403b
Is effective so that it does not match both the x-axis and the y-axis, and the resulting misalignment of the interference fringes on the illumination target surface compensates for the peaks and valleys of each other's light intensity distribution. However, there is no change in contributing to making the illuminance uniform.

In this way, in order to prevent the target declination direction 403b from matching both the x-axis and the y-axis, the crystal optical axis direction 403c of the declination prism 402 is, for example, 45 with respect to the polarization direction 403a of the incident laser beam. It is necessary to make an angle of degrees.
Needless to say, when the incident laser beam is elliptically polarized, the polarization direction 403a means the direction of the major axis of the ellipse.

When a crystal deflection prism is used as the deflection prism 402 as in the example described in FIG. 6, the difference between the refractive index n _e for e-polarized light and the refractive index n _o for o-polarized light is n _e −n _o = 0.01
Assuming that the angle is 12, the angle difference between the o-polarization and the e-polarization declination required on the incident surface of the fly-eye lens 401 is a minute angle, and for each x direction: θ _x = λ (n + 1 / 2) / a y direction: It can be expressed as θ _y = λ (m + 1/2) / b (where n and m are integers). Therefore, in general, the combined angle difference θ in the x and y directions is θ = λ {[(n + 1/2) / a] ² + [(m + ^1/2 ) / b] ² } ^1/2 (where λ is the incident laser beam Of the long side of the dimension b when the fly-eye lens 402 is viewed from the optical axis direction, the deviation angle direction is Ψ = tan ⁻¹ {[(( n + 1/2) / a] / [(m + 1/2) / b]}.

In the above, the method of choosing n and m is arbitrary, for example, n
= Considering the case of m = 0, theta = lambda become ^{{[1 / a] 2 +} [1 / b] 2} 1/2 / 2 Ψ = tan -1 (b / a). Since Ψ = tan ⁻¹ (b / a), the Ψ ′ shown in FIG. 10 has a relationship of Ψ + Ψ ′ = 90.

Therefore, when the angle formed by the diagonal direction of the lens element of the fly-eye lens 401 and the y-axis is Ψ ′, the deviation direction 403a by the deviation prism 402 is an angle Ψ with respect to the y-axis, that is, Ψ with respect to the x-axis. It makes an angle of ′.

However, when the coherence of the incident laser beam is strong only in one of the x and y directions and the other is low and the interference fringes on the illumination target surface are only vertical stripes or horizontal stripes, the As described above, the declination may be given only to the direction having strong coherence.

The fly-eye lens 401 exemplifies the one in which the entrance / exit surface of the lens element is rectangular, but a fly-eye lens having a hexagonal entrance / exit surface shape can also be used. It will be described below with reference to the drawings.

FIG. 12 is a schematic view of a fly-eye lens formed by bundling hexagonal prismatic lens elements as seen from the optical axis direction. When such a fly-eye lens is used, interference fringes in three directions (u, v, w) having an angle difference of 120 degrees are mainly observed on the illumination target surface. The interference fringes in the u direction are, for example, the secondary light source a
-A ₁ or a-a ₁ ′, and v-direction interference fringes are generated by, for example, secondary light sources a-a ₂ or a-a ₂ ′ interference, and w-direction interference fringes. Is, for example, the secondary light source a−
generated by interference between between a ₃ or a-a ₃ '. In this case, in order to remove the interference fringes, it is necessary that all the interference fringes in the three directions be shifted from each other by (n + 1/2) cycles. For that purpose, between aa ₁ and aa ₂ And a-a ₃
The wavefront must be tilted so that the luminous fluxes between and have a phase difference of π, and mathematically no such solution exists. Therefore, when a fly-eye lens composed of hexagonal prism lens elements is used, interference fringes cannot be completely removed by the birefringent deflection prism. Therefore, rather than a complete removal, when considered in the perspective of uniformity of the light intensity distribution on the illumination target surface, a-a between _1, between a-a ₂ and between a-a ₃ are all 2 [pi / 3 A configuration is considered in which there is a phase difference, or among interference fringes in three directions, interference fringes are allowed to occur in one direction and interference fringes in the remaining two directions are removed. The latter is, for example, between a-a ₂ and a-
The structure is such that there is a phase difference of π between a ₃ . In an excimer laser or the like, the coherence in the vertical direction and the coherence in the horizontal direction are often different in the beam cross section, so it is effective to match the direction with low coherence with the allowable direction.

It goes without saying that the embodiments described above are merely examples of the present invention, and the present invention can be variously modified within the scope of the claims.

[Effects of the Invention] As described above, in the present invention, the interference pattern appearing on the illumination target surface has a periodic structure due to the structural parameters of the optical integrator in the illumination optical system, and the By separating the beam into two polarized light beams that do not interfere with each other and giving them a predetermined declination, the interference patterns of both polarized light beams on the illumination target surface are shifted by a half cycle. In principle, a movable element such as an oscillating mirror for oscillating the beam spatially is provided by arranging an optical means such as a deflection prism having a structural parameter specified by the structure and the wavelength used in the optical path in the optical path. When there is no need, it is possible to eliminate uneven illuminance due to the interference pattern on the illumination target surface, and when the coherence of the light source is strong. Even when a vibrating mirror or the like is used together, it is possible to more effectively eliminate the uneven illuminance due to interference on the illumination target surface with a small number of vibrations.

If such an illumination optical device is used in, for example, an exposure apparatus for manufacturing an integrated circuit, it is possible to form a finer circuit without lowering the production efficiency, which is very useful.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an optical configuration of a projection exposure apparatus according to an embodiment of the present invention, FIG. 2 is a schematic diagram showing a configuration of a light source in the embodiment of FIG. 1, and FIG. FIG. 4 is a perspective view of a fly-eye lens, and FIGS. 5 (a), (b), (c), and (d) show various examples of birefringent crystal prisms. FIG. 6 is a perspective view, FIG. 6 is an optical path diagram showing separation of beams by a birefringent crystal prism, FIG. 7 is an optical path diagram of a main part when two sets of fly-eye lenses are used, and FIG. FIG. 9 is an optical path diagram for explaining the correction of the traveling direction of the beam after passing through the crystalline crystal prism, FIG. 9 is an optical path diagram showing the main part of the optical system when the exposure amount is controlled, and FIG. 10 is the illumination according to the present invention. The overlap of the fly-eye lens and the deflection prism as an optical integrator within the optical device Fig. 11 is a schematic view seen from the axial direction. Fig. 11 shows the relationship between the polarization direction of the incident laser beam seen from the optical axis direction, the target declination direction of the emitted light by the deflection prism, and the crystal optical axis direction of the deflection prism. FIG. 12 is a schematic view of a fly-eye lens formed by bundling hexagonal prismatic lens elements viewed from the optical axis direction. (Explanation of symbols of main parts) 1 ... laser light source, 3,5,103,203,303,401 ... fly-eye lens (optical integrator), 4,21,25,104,204,305 ... lens (lens means), 7,107,207,402 ... declination prism (optical means) , 9,109 ... Correction prism, R ... Reticle (illumination target surface), PL ... Projection lens, W ... Wafer, 17 ... Vibration mirror.

Claims (5)

(57) [Claims] 1. An optical integrator that forms a group of a plurality of real or imaginary secondary light sources spatially periodically distributed in a plane perpendicular to the beam optical axis from a coherent light beam emitted from a light source, and Lens means for superimposing the light flux from the secondary light source on one illumination target surface and two polarized light fluxes from the coherent light beam such that the inner product of their polarization vectors becomes zero and the interference by each polarized light flux In an illumination optical device including an optical means that gives different polarization angles to each of the polarized light beams incident on the optical integrator so that a pattern is relatively displaced on the illumination target surface, the optical integrator is , Consisting of a fly-eye lens with multiple positive or negative lens elements arranged in parallel with a right-angled quadrilateral input / output surface. When the lengths of two adjacent sides of the right-angled quadrangle are a and b (where a ≦ b), the optical means defines θ = as a difference in deviation angle between the two polarized light beams. λ {[(n + 1/2) / a] ² + [(m + ^1/2 ) / b] ² } ^1/2 (where λ is the wavelength and n and m are integers), and the declination difference is the fly. An angle Ψ of Ψ = tan ⁻¹ {[(n + 1/2) / a] / [(m + 1/2) / b]} with respect to the side of the dimension b when the eye lens is viewed from the optical axis direction. An illuminating optical device characterized in that the optical structure parameters are determined so as to be formed. 2. The illumination optical apparatus according to claim 1, wherein the optical means includes a deflection prism made of a birefringent optical material inserted in an optical path. 3. The coherent light beam has a polarization characteristic, and the deflection prism has an optical axis forming an angle of 45 degrees with respect to the polarization direction of the polarized coherent light beam when viewed from the optical axis direction. The illumination optical device according to claim 2, which is made of a birefringent optical material. 4. An optical integrator which forms a group of a plurality of real or imaginary secondary light sources spatially periodically distributed in a plane perpendicular to the beam optical axis from a coherent light beam emitted from the light source, In an exposure apparatus having lens means for superimposing a light beam from a secondary light source on a reticle and transferring the pattern on the reticle onto a wafer, the inner products of mutual polarization vectors from the coherent light beams become zero. An optical means that forms two different polarized light beams and gives different polarization angles to the polarized light beams incident on the optical integrator so that an interference pattern due to each polarized light beam causes a relative displacement on the reticle. The optical integrator has a plurality of positive or negative lens elements having a right-angled quadrilateral input / output surface. The optical means is composed of fly-eye lenses arranged in rows, and the optical means is such that when the lengths of two adjacent sides of the right-angled quadrangle are a and b (where a ≦ b), the polarization of the two polarized light beams is deviated. As the angle difference θ, θ = λ {[(n + 1/2) / a] ² + [(m + ^1/2 ) / b] ² } ^1/2 (where λ is the wavelength and n and m are integers), Further, when the deviation angle is viewed from the optical axis direction of the fly-eye lens, Ψ = tan ⁻¹ {[(n + 1/2) / a] / [(m + 1/2) The exposure apparatus is characterized in that the optical structure parameters are determined so as to be formed at an angle ψ of / b]}. 5. A coherent light beam emitted from a light source is formed by an optical integrator into a group of a plurality of real or imaginary secondary light sources spatially and periodically distributed in a plane perpendicular to the beam optical axis, In an exposure method for transferring a pattern on the reticle onto a wafer by illuminating a reticle by superimposing light fluxes from the respective secondary light sources, an inner product of polarization vectors of the coherent light beams becomes zero. Forming the two polarized light beams, and providing the two polarized light beams with different deviation angles so that the interference pattern by the two polarized light beams causes a relative displacement on the reticle, As an integrator, a fly eye lens in which a plurality of positive or negative lens elements having a right-angled quadrilateral input / output surface are arranged in parallel. And the two polarized light beams have a difference in deviation angle between the two polarized light beams when the lengths of two adjacent sides of the right-angled quadrangle are a and b (where a ≦ b), respectively. θ satisfies θ = λ {[(n + 1/2) / a] ² + [(m + ^1/2 ) / b] ² } ^1/2 (where λ is the wavelength and n and m are integers), and When the fly-eye lens is viewed from the optical axis direction, the angle Ψ formed by the deviation angle θ with respect to the side of the dimension b is Ψ = tan ⁻¹ {[(n + 1/2) / a] / [(m + 1 / 2) / b]} is satisfied.