# Patent

## Title
Illumination optical apparatus and projection exposure apparatus

## Abstract

An illumination optical apparatus includes a plurality of birefringent members made of a birefringent material and arranged in an optical path on an incidence side of an optical integrator. The members change a polarization state of illumination light such that first and second rays of the illumination light are polarized in different directions on the pupil plane. The birefringent members are arranged such that an optical path length of the first ray in the birefringent material is different from an optical path length of the second ray in the birefringent material, and are arranged so as to change the polarization state of the illumination light incident on the plurality of the birefringent members in a linear polarization state having a substantially single polarization direction such that each of the first and second rays is polarized in a substantially circumferential direction about the optical axis on the pupil plane.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a continuation of U.S. application Ser. No. 11/410,952, filed Apr. 26, 2006 (now U.S. Pat. No. 9,140,992), which is a Continuation-in-part of PCT/JP2004/015853 filed Oct. 26, 2004. This application claims the benefit of Japanese Patent Application No. 2003-367963, filed Oct. 28, 2003. The entire disclosure of each of the prior applications is hereby incorporated by reference herein in their entirety.


BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to illumination technology and exposure technology used in the lithography step for fabricating various devices, e.g., semiconductor integrated circuits (LSI and the like), image pickup devices, or liquid crystal displays and, more particularly, to illumination technology and exposure technology for illuminating a mask pattern with light in a predetermined polarization state. Furthermore, the present invention relates to device fabrication technology using the exposure technology.
Related Background Art
For forming microscopic patterns of electronic devices such as semiconductor integrated circuits or liquid crystal displays, a method adopted is to project a demagnified image of a pattern on a reticle (or a photomask or the like) as a mask on which the pattern to be formed is drawn at a proportional magnification of about 4-5 times, through a projection optical system onto a wafer (or glass plate or the like) as a substrate to be exposed (photosensitive body) to effect exposure and transfer of the image. Projection exposure apparatus used for the exposure and transfer include those of a stationary exposure type such as steppers, and those of a scanning exposure type such as scanning steppers. The resolution of the projection optical system is proportional to a value obtained by dividing an exposure wavelength by a numerical aperture (NA) of the projection optical system. The numerical aperture (NA) of the projection optical system is given by multiplying a sine (sin) of a maximum angle of incidence of illumination light for exposure onto the wafer, by a refractive index of a medium through which the light passes.
Therefore, in order to meet the demand for miniaturization of the semiconductor integrated circuits and others, the exposure wavelength of the projection exposure apparatus has been decreased toward shorter wavelengths. The mainstream exposure wavelength at present is 248 nm of KrF excimer laser, and the shorter wavelength of 193 nm of ArF excimer laser is also close to practical use. There are also proposals on the projection exposure apparatus using exposure light sources in the so-called vacuum ultraviolet region such as the F2 laser with the much shorter wavelength of 157 nm and the Ar2 laser with the wavelength of 126 nm. Since it is also possible to achieve a higher resolution by a larger numerical aperture (larger NA) of the projection optical system instead of the use of shorter wavelength, there are also attempts to develop the projection optical system with a much larger NA, and the leading NA of the projection optical system at present is approximately 0.8.
On the other hand, there are also practically available techniques to enhance the resolution of the pattern to be transferred, even with use of the same exposure wavelength and the projection optical system with the same NA, so called super resolution techniques, such as a method using a so-called phase shift reticle, and annular illumination, dipole illumination, and quadrupole illumination to control angles of incidence of the illumination light onto the reticle in a predetermined distribution.
Among those, the annular illumination is to limit the incidence angle range of illumination light onto the reticle to predetermined angles, i.e., to limit the distribution of illumination light on the pupil plane of the illumination optical system to within a predetermined annular region centered on the optical axis of the illumination optical system, thereby offering the effect of improvement in the resolution and depth of focus (e.g., reference is made to Japanese Patent Application Laid-Open No. 61-91662). On the other hand, the dipole illumination and quadrupole illumination are applied to cases where the pattern on the reticle is one with specific directionality, and are arranged to limit, as well as the incidence angle range, the direction of incidence of the illumination light to a direction suitable for the directionality of the pattern, thereby achieving great improvement in the resolution and depth of focus (e.g., reference is made to Japanese Patent Application Laid-Open No. 4-101148 or U.S. Pat. No. 6,233,041 equivalent thereto and to Japanese Patent Application Laid-Open No. 4-225357 or U.S. Pat. No. 6,211,944 equivalent thereto).
There are other proposals of attempts to optimize the polarization state of the illumination light relative to the direction of the pattern on the reticle, thereby achieving improvement in the resolution and depth of focus. This method is to convert the illumination light into linearly polarized light with the polarization direction (direction of the electric field) along a direction orthogonal to the periodic direction of the pattern, i.e., along a direction parallel to the longitudinal direction of the pattern, thereby achieving improvement in contrast and others of the transferred image (e.g., Japanese Patent Application Laid-Open No. 5-109601 and Thimothy A. Brunner, et al.: “High NA Lithographic imaging at Brewster's angle,” SPIE (USA) Vol. 4691, pp. 1-24 (2002).
Concerning the annular illumination, there are also proposals of attempts to match the polarization direction of the illumination light in an annular region in which the illumination light is distributed on the pupil plane of the illumination optical system, with the circumferential direction of the annular region, thereby achieving improvement in the resolution, contrast, etc. of the projected image.
In effecting the annular illumination by the conventional technology as described above, there was the problem of large loss in quantity of the illumination light to lower illumination efficiency if the polarization state of the illumination light was made to be linear polarization substantially matched with the circumferential direction of the annular region on the pupil plane of the illumination optical system.
Specifically, the illumination light emitted from the recently mainstream narrow-band KrF excimer laser source is uniform, linearly polarized light. If the light is kept in that polarization state and guided to the reticle, the reticle will be illuminated with the uniform, linearly polarized light, and it is thus needless to mention that it is infeasible to obtain the linearly polarized light with the polarization direction matched with the circumferential direction of the annular region on the pupil plane of the illumination optical system as described above.
Therefore, in order to realize the aforementioned polarization state, it was necessary to adopt, for example, a method of converting the linearly polarized light emitted from the light source, once into randomly polarized light and thereafter, in each part of the annular region, selecting a desired polarization component from the illumination light of random polarization, using a polarization selecting element such as a polarization filter or a polarization beam splitter. This method used only energy in the predetermined linear polarization component out of the energy of the illumination light of random polarization, i.e., only approximately half energy as the illumination light onto the reticle, and thus posed the problem of large loss in quantity of the illumination light and large loss in exposure power on the wafer in turn, resulting in reduction in processing performance (throughput) of the exposure apparatus.
Similarly, in application of multipole illumination such as the dipole illumination or quadrupole illumination, there was also the problem of reduction in illumination efficiency if the polarization of the illumination light in each dipole or quadrupole region was attempted to be set in a predetermined state on the pupil plane of the illumination optical system.
SUMMARY OF THE INVENTION
Reference symbols in parentheses attached to respective elements of the present invention below correspond to configurations of embodiments of the present invention described later. It is, however, noted that each reference symbol is only an example of an element corresponding thereto and is by no means intended to limit each element to the configurations of the embodiments.
A first aspect of the present embodiment is to provide a projection exposure apparatus for projecting a pattern image on a first object on a second object, the projection exposure apparatus comprising: a projection optical system for projecting the image of pattern on the first object on the second object; and an illumination optical system for illuminating a first object with illumination light from a light source, and comprising at least two birefringent members arranged along a traveling direction of the illumination light, wherein a direction of a fast axis of at least one birefringent member out of the birefringent members is different from a direction of a fast axis of the other birefringent member, and wherein a specific illumination beam incident in a specific incidence angle range to the first object among the illumination light generated in a substantially single polarization state from the light source is light in a polarization state consisting primarily of S-polarization.
A second aspect of the present embodiment is to provide a projection exposure apparatus for projecting an image of a pattern on a first object onto a second object, comprising: a projection optical system for projecting the image of the pattern on the first object onto the second object; and an illumination optical system for illuminating the first object with light supplied from an outside light source, the illumination optical system comprising a diffractive optical element and a birefringent member arranged in order along a traveling direction of the light.
A third aspect of the present embodiment is to provide an illumination optical apparatus for illuminating a first object with illumination light from a light source, comprising: at least two birefringent members arranged along a traveling direction of the illumination light, wherein a direction of a fast axis of at least one birefringent member out of the birefringent members is different from a direction of a fast axis of the other birefringent member, and wherein a specific illumination beam incident in a specific incidence angle range onto the first object among the illumination light in a substantially single polarization state supplied from the light source is light in a polarization state consisting primarily of S-polarization.
A fourth aspect of the present invention is to provide an illumination optical apparatus for illuminating a first object with illumination light from a light source, comprising: a diffractive optical element and a birefringent member arranged in order along a traveling direction of the illumination light.
A fifth aspect of the embodiments is to provide an exposure method using the above projection exposure apparatus according to the above embodiments.
A sixth aspect of the embodiments is to provide a method of making a device using the exposure method according to the above embodiments.
According to the present embodiments, for example, thicknesses of the birefringent members are set in their respective predetermined distributions, whereby the polarization after passage of the illumination light emitted from the light source, through the plurality of birefringent members, can be, for example, in a state consisting primarily of polarization in the circumferential direction around the optical axis in an annular region centered around the optical axis. An exit surface of the birefringent members is located, for example, at a position near the pupil plane of the illumination optical system, whereby the first object is illuminated with the illumination light (specific illumination beam) having passed through the annular region and kept in the predetermined polarization state consisting primarily of S-polarization, with little loss in quantity of light.
In this case, the apparatus may comprise a beam limiting member (9a, 9b) for limiting the illumination light incident to the first object, to the specific illumination beam. This makes the first object illuminated under the condition of almost annular illumination. When in this annular illumination the illumination light is almost S-polarization on the first object, a projected image of a line-and-space pattern arranged at a fine pitch in an arbitrary direction on the first object is formed mainly by the illumination light with the polarization direction parallel to the longitudinal direction of the line pattern and, therefore, improvement is made in imaging characteristics such as the contrast, resolution, and depth of focus.
The beam limiting member may be configured to further limit the direction of incidence of the illumination light incident to the first object, to a plurality of specific, substantially discrete directions. Since this implements illumination such as the dipole illumination or quadrupole illumination, improvement is made in imaging characteristics of a line-and-space pattern arranged at a fine pitch in a predetermined direction.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the embodiment.
Further scope of applicability of the embodiment will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a drawing showing a schematic configuration of a projection exposure apparatus, partly cut, as an example of an embodied form of the present invention, and FIG. 1A shows a liquid immersion type exposure apparatus.
FIG. 2A is a view of birefringent member 12 in FIG. 1, viewed to +Y direction,
FIG. 2B a sectional view along line AA′ in FIG. 2A.
FIG. 3A is a view of birefringent member 13 in FIG. 1, viewed to +Y direction,
FIG. 3B a sectional view along line BB′ in FIG. 3A.
FIG. 4A is a diagram showing an example of relationship between polarization phase difference ΔP1 and position X in the first birefringent member 12.
FIG. 4B is a diagram showing an example of relationship between polarization phase difference ΔP2 and position XZ in the second birefringent member 13.
FIG. 4C a drawing showing an example of polarization states of illumination light emerging from the second birefringent member 13.
FIG. 5 is a drawing showing an example of polarization states of illumination light emerging from the first birefringent member 12.
FIG. 6A is a diagram showing another example of relationship between polarization phase difference ΔP1 and position X in the first birefringent member 12.
FIG. 6B a diagram showing another example of relationship between polarization phase difference ΔP2 and position XZ in the second birefringent member 13.
FIG. 6C a drawing showing another example of polarization states of illumination light emerging from the second birefringent member 13.
FIG. 7A is a plan view showing an example of microscopic periodic pattern PX formed on reticle R of FIG. 1.
FIG. 7B a drawing showing a distribution of diffracted light formed in pupil plane 26 of the projection optical system when the pattern of FIG. 7A is illuminated under a predetermined condition.
FIG. 7C a drawing showing a condition for annular illumination for illuminating the pattern PX of FIG. 7A.
FIG. 8A is a perspective view showing a simplified relation between pupil plane 15 of illumination optical system ILS and reticle R in FIG. 1.
FIG. 8B a view of a part of FIG. 8A viewed to +Y direction.
FIG. 8C a view of part of FIG. 8A viewed to −X direction.
FIG. 9 is a drawing showing a plurality of conical prisms that can be disposed between birefringent members 12, 13 and fly's eye lens 14 in FIG. 1, for making the radius of the specific annular region variable, in an example of the embodied form of the present invention.
FIG. 10 is a drawing showing an example of a polarization control optical system that can be disposed at the position of polarization controlling member 4 in FIG. 1.
FIG. 11 is a drawing showing an example of a lithography step for fabricating semiconductor devices by use of the projection exposure apparatus according to the embodiment of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An example of preferred embodiment of the present invention will be described below with reference to the drawings. The present example is an application of the present invention to a case where exposure is performed by a projection exposure apparatus of the scanning exposure type (scanning stepper) according to the step-and-scan method.
FIG. 1 is a drawing showing a schematic configuration of the projection exposure apparatus of the present example partly cut, and in this FIG. 1 the projection exposure apparatus of the present example is provided with an illumination optical system ILS and a projection optical system 25. The former illumination optical system ILS is provided with a plurality of optical members arranged along the optical axis (optical axis of illumination system) AX1, AX2, AX3 from an exposure light source 1 (light source) to a condenser lens 20 (the details of which will be described later), and illuminates an illumination field on a pattern surface (reticle surface) of reticle R as a mask under a uniform illuminance distribution with illumination light for exposure (exposure light) IL as an exposure beam from the exposure light source 1. The latter projection optical system 25 projects a demagnified image at a projection magnification M (where M is a demagnification rate, e.g., 1⁄4 or 1⁄5) of a pattern in the illumination field on the reticle R, under the illumination light into an exposure region on one shot area on a wafer W coated with a photoresist, as a substrate to be exposed (substrate) or as a photosensitive body. The reticle R and wafer W can also be regarded as a first object and as a second object, respectively. The wafer W is, for example, a substrate of disk shape with the radius of about 200-300 mm of a semiconductor (silicon or the like) or SOI (silicon on insulator) or the like. The projection optical system 25 of the present example is, for example, a dioptric system, but can also be a catadioptric system or the like.
In the description hereinafter, a coordinate system as to the projection optical system 25, reticle R, and wafer W is defined as follows in FIG. 1: the Z-axis is taken in parallel with the optical axis AX4 of the projection optical system 25, the Y-axis along the scanning direction of reticle R and wafer W (direction parallel to the plane of FIG. 1) during scanning exposure in the plane (XY plane) perpendicular to the Z-axis, and the X-axis along the non-scanning direction (direction normal to the plane of FIG. 1). In this case, the illumination field on the reticle R is a region elongated in the X-direction being the non-scanning direction, and an exposure region on the wafer W is an elongated region conjugate with the illumination field. The optical axis AX4 of the projection optical system 25 agrees with the optical axis of illumination system AX3 on the reticle R.
First, the reticle R on which a pattern to be transferred by exposure is formed, is stuck and held on a reticle stage 21, and the reticle stage 21 moves at a constant speed in the Y-direction on a reticle base 22 and finely moves in the X-direction, in the Y-direction, and in the rotational direction about the Z-axis so as to compensate for a synchronization error, to effect scanning of reticle R. The X-directional and Y-directional positions and the angle of rotation of the reticle stage 21 are measured by means of moving mirror 23 provided thereon, and laser interferometer 24. Based on measurements of the laser interferometer and control information from main control system 34, a reticle stage driving system 32 controls the position and speed of reticle stage 21 through a driving mechanism (not shown) such as a linear motor. A reticle alignment microscope (not shown) for reticle alignment is disposed above the marginal region of the reticle R.
On the other hand, the wafer W is stuck and held through a wafer holder (not shown) on a wafer stage 27, and the wafer stage 27 is mounted on a wafer base 30 so that it can move at a constant speed in the Y-direction and achieve step movement in the X-direction and in the Y-direction. The wafer stage 27 is also provided with a Z-leveling mechanism for aligning the surface of wafer W with the image plane of the projection optical system 25, based on measurements of an unrepresented autofocus sensor. The X-directional and Y-directional positions and the angle of rotation of the wafer stage 27 are measured by means of moving mirror 28 provided thereon, and laser interferometer 29. Based on measurements of the laser interferometer and control information from main control system 34, a wafer stage driving system 33 controls the position and speed of the wafer stage 27 through a driving mechanism (not shown) such as a linear motor. For wafer alignment, an alignment sensor 31 of the off-axis method and, for example, the FIA (Field Image Alignment) method for detecting positions of marks for alignment on the wafer W is disposed in the vicinity of the projection optical system 25.
Prior to exposure by the projection exposure apparatus of the present example, alignment of the reticle R is carried out with the aforementioned reticle alignment microscope, and alignment of the wafer W is carried out by detecting the positions of the positioning marks formed along with a circuit pattern in a previous exposure step on the wafer W, by means of the alignment sensor 31. After that, the apparatus repeatedly carries out the operation of driving the reticle stage 21 and wafer stage 27 in a state in which the illumination light IL illuminates the illumination field on the reticle R, to synchronously scan the reticle R and one shot area on the wafer W in the Y-direction, and the operation of terminating emission of the illumination light IL and driving the wafer stage 27 to effect step movement of the wafer W in the X-direction and in the Y-direction. A ratio of scanning speeds of the reticle stage 21 and the wafer stage 27 during the synchronous scanning is equal to a projection magnification M of the projection optical system 25, in order to keep the imaging relation between the reticle R and the wafer W through the projection optical system 25. These operations result in effecting exposure to transfer the pattern image of the reticle R into all the shot areas on the wafer W by the step-and-scan method.
Next, a configuration of the illumination optical system ILS of the present example will be described in detail. In FIG. 1, an ArF (argon fluorine) excimer laser (wavelength 193 nm) is used as the exposure light source 1 of the present example. The exposure light source 1 can also be another laser light source, e.g., a KrF (krypton fluorine) excimer laser (wavelength 248 nm), an F2 (fluorine molecule) laser (wavelength 157 nm), or a Kr2 (krypton molecule) laser (wavelength 146 nm). These laser light sources (including the exposure light source 1) are narrow-band lasers or wavelength-selected lasers, and the illumination light IL emitted from the exposure light source 1 is in a polarization state consisting primarily of linear polarization because of the narrowing of band or wavelength selection. In the description hereinafter, it is assumed that in FIG. 1 the illumination light IL immediately after emitted from the exposure light source 1 consists primarily of linearly polarized light whose polarization direction (direction of the electric field) coincides with the X-direction in FIG. 1.
The illumination light IL emitted from the exposure light source 1 travels along the optical axis of illumination system AX1 and through relay lenses 2, 3 to enter a polarization controlling member 4 (detailed later) as a polarization controlling mechanism. The illumination light IL emerging from the polarization controlling member 4 travels through a zoom optical system (5, 6) consisting of a combination of a concave lens 5 and a convex lens 6, and is then reflected by a mirror 7 for bending of optical path to enter a Diffractive Optical Element (DOE) 9a along the optical axis of illumination system AX2. The diffractive optical element 9a is comprised of a phase type diffraction grating, and the illumination light IL incident thereto travels as diffracted into predetermined directions.
As described later, a diffraction angle and direction of each diffracted light from the diffractive optical element 9a as a beam limiting member correspond to a position of the illumination light IL on the pupil plane 15 of the illumination optical system ILS and to an angle and direction of incidence of the illumination light IL to the reticle R. A plurality of diffractive optical elements, including the diffractive optical element 9a and another diffractive optical element 9b with different diffraction action, are arranged on a member 8 of turret shape. The apparatus is constructed for example as follows: the member 8 is driven by a replacing mechanism 10 under control of the main control system 34 to load the diffractive optical element 9a or the like at an arbitrary position on the member 8 to the position on the optical axis of illumination system AX2, whereby the incidence angle range and direction of the illumination light to the reticle R (or the position of the illumination light on the pupil plane 15) can be set to a desired range in accordance with the pattern of the reticle R. The incidence angle range can also be finely adjusted supplementarily by moving each of the concave lens 5 and the convex lens 6 constituting the aforementioned zoom optical system (5, 6) in the direction of the optical axis of illumination system AX1.
The illumination light (diffracted light) IL emerging from the diffractive optical element 9a travels along the optical axis of illumination system AX2 and through relay lens 11 to successively enter the first birefringent member 12 and second birefringent member 13 being the plurality of birefringent members in the present invention. The details of these birefringent members will be described later. In the present embodiment, a fly's eye lens 14 being an optical integrator (illuminance uniforming member) is disposed behind the birefringent member 13. The illumination light IL emerging from the fly's eye lens 14 travels via relay lens 16, field stop 17, and condenser lens 18 to a mirror 19 for bending of optical path, and the illumination light IL reflected thereon then travels along the optical axis of illumination system AX3 and through condenser lens 20 to illuminate the reticle R. The pattern on the reticle R illuminated in this manner is projected and transferred onto the wafer W by the projection optical system 25 as described above.
It is also possible to construct the field stop 17 as a scanning type, if necessary, and to effect scanning thereof in synchronization with the scanning of the reticle stage 21 and wafer stage 27. In this case, the field stop may be constructed of separate components of a fixed field stop and a movable field stop.
In this configuration, the exit-side surface of the fly's eye lens 14 is located near the pupil plane 15 of the illumination optical system ILS. The pupil plane 15 acts as an optical Fourier transform plane with respect to the pattern surface (reticle surface) of the reticle R through the optical members (relay lens 16, field stop 17, condenser lenses 18, 20, and mirror 19) in the illumination optical system ILS from the pupil plane 15 to the reticle R. Namely, the illumination light emerging from a point on the pupil plane 15 is converted into an approximately parallel beam to illuminate the reticle R while being incident at a predetermined incidence angle and incidence direction. The incidence angle and incidence direction are determined according to the position of the beam on the pupil plane 15.
The path bending mirrors 7, 19 are not always indispensable in terms of optical performance, but if the illumination optical system ILS is arranged on a line the total height of the exposure apparatus (the height in the Z-direction) will increase; therefore, they are arranged at appropriate positions in the illumination optical system ILS for the purpose of space saving. The optical axis of illumination system AX1 coincides with the optical axis of illumination system AX2 through reflection on the mirror 7, and the optical axis of illumination system AX2 further coincides with the optical axis of illumination system AX3 through reflection on the mirror 19.
A first example of the first and second birefringent members 12, 13 in FIG. 1 will be described below with reference to FIGS. 2 to 5.
The first birefringent member 12 is a member of disk shape made of a birefringent material such as a uniaxial crystal, and the optical axis thereof is in its in-plane direction (direction parallel to the plane normal to the optical axis of illumination system AX2). The size (diameter) in the in-plane direction of the first birefringent member 12 is larger than the beam size of the illumination light IL at the position where the birefringent member 12 is located.
FIG. 2A is a view of the birefringent member 12 in FIG. 1 viewed to the +Y direction and along the optical axis of illumination system AX2 and in the birefringent member 12, as shown in FIG. 2A, the fast axis nf, which is an axial direction to minimize the refractive index for linearly polarized light with the polarization direction parallel thereto, is directed in a direction rotated by 45° from each coordinate axis (X-axis and Z-axis) in the XZ coordinate system with the same coordinate axes as in FIG. 1. Furthermore, the slow axis ns, which is an axial direction to maximize the refractive index for linearly polarized light with the polarization direction parallel thereto, is naturally orthogonal to the fast axis nf and is also directed in a direction rotated by 45° from both of the X-axis and Z-axis.
The thickness of the first birefringent member 12 is not uniform in a plane parallel to the plane of FIG. 2A, and varies according to X-coordinates (positions in the X-direction). FIG. 2B is a sectional view of the birefringent member 12 along line AA′ in FIG. 2A and, as shown in FIG. 2B, the birefringent member 12 has such a shape that it is thin at the center (the optical axis of illumination system) and thick in the marginal region in the X-direction. On the other hand, the thickness of the first birefringent member 12 is uniform in the Z-direction in FIG. 2A and thus the birefringent member 12 is of a shape like a negative cylinder lens as a whole.
A beam passing through such a birefringent member generally has a path difference (polarization phase difference) between a linear polarization component with the polarization direction (i.e., “vibrating direction of the electric field of light,” which will also apply to the description hereinafter) coinciding with the direction of the fast axis nf, and a linear polarization component with the polarization direction coinciding with the direction of the slow axis ns. The refractive index of the birefringent member is low for linearly polarized light parallel to the fast axis nf, so that the traveling speed of the same polarized light is high. On the other hand, the refractive index of the birefringent member is high for linearly polarized light parallel to the slow axis ns, so that the traveling speed of the same polarized light is low. Therefore, there appears a path difference (polarization phase difference) between the two polarized beams. Therefore, the first birefringent member 12 functions as a first nonuniform wavelength plate in which the polarization phase difference given to transmitted light differs according to locations.
Incidentally, if the thickness of the first birefringent member 12 is optimized to make the path difference due to the birefringent member 12 equal to an integer multiple of a wavelength, the phases of the two beams cannot be substantially discriminated from each other, and a state substantially having no optical path difference can be realized. In the present example, the thickness T1 of the center part of the birefringent member 12 is set to such thickness. In the description hereinafter, as shown in FIG. 2B, the origin of the X-axis (X=0) is defined at the center of the birefringent member 12 (optical axis of illumination system).
On the other hand, the shape of the birefringent member 12 is so set that the polarization phase difference becomes 0.5 (in the unit of the wavelength of the illumination light) at positions of ±1 apart in the X-direction from the center of the first birefringent member 12 (where 1 represents a reference length and is located inside the outer diameter of the first birefringent member 12). For realizing such shape, the present example defines the thickness TA of the birefringent member 12 as the thickness represented by the following function, for the position X in the X-direction.

TA=T1+α×(1.7×X4−0.7×X2)  (1)

In the above equation, a is a proportionality coefficient, and the value of α varies depending upon the aforementioned index difference between the fast axis and the slow axis of the birefringent material used, or the like as the thickness T1 of the center part does.
When crystalline quartz being a uniaxial crystal is used as the birefringent material making the first birefringent member 12, the refractive indices of crystalline quartz are as follows: the refractive index of 1.6638 for an ordinary ray and the refractive index of 1.6774 for an extraordinary ray in the ArF excimer laser light with the wavelength of 193 nm. Therefore, the fast axis is the polarization direction of the ordinary ray and the slow axis the polarization direction of the extraordinary ray.
The wavelengths of the ordinary ray and extraordinary ray in crystalline quartz are obtained by diving the wavelength (193 nm) in vacuum by the respective refractive indices, and are thus 116.001 nm and 115.056 nm, respectively. Therefore, a path difference of 0.945 nm is made between the two rays with every travel through one wavelength in crystalline quartz. Accordingly, after travel through 122.7 (=116.001/0.945) wavelengths, the path difference of about one wavelength is created between the two rays. However, the path difference of just one wavelength or an integral multiple of the wavelength is equivalent to substantially no path difference between the two rays. The thickness of crystalline quartz corresponding to the 122.7 wavelengths is obtained by calculation of 122.7×193/1.6638, and is equivalent to 14239 nm, i.e., 14.239 μm. Similarly, for making a path difference of a half wavelength between the ordinary ray and the extraordinary ray, the thickness of crystalline quartz can be set to a half of the above thickness, i.e., 7.12 μm.
This confirms that when the first birefringent member 12 being the first nonuniform wavelength plate is made of crystalline quartz, the thickness T1 of the center part in Eq (1) above is set to an integer multiple of 14.239 μm and the thickness at the reference position (X=1) near the marginal region is set to a thickness 7.12 μm larger than it, i.e., the aforementioned proportionality coefficient α can be set to 7.12 μm.
At this time, the polarization phase difference ΔP1 made by the first birefringent member 12 is represented as follows as a function of position X in the X-direction.

ΔP1=0.5×(1.7×X4−0.7×X2)  (2)

The thickness of the first birefringent member 12 is a spacing between its entrance surface 12a and exit surface 12b, and each of shapes of the entrance surface 12a and exit surface 12b may be arbitrary as long as they satisfy the aforementioned relation between thickness and X-directional position for formation of the phase difference. From the viewpoint of processing of surface shape, however, processing becomes easier if either surface is a plane, and it is thus desirable to make, for example, the exit surface 12b as a plane in practice, as shown in FIG. 2B. In this case, where the value of the thickness TA on the exit surface 12b is 0, values of thickness TA of the entrance surface 12a are equal to those of TA determined by Eq (1). It is a matter of course that the entrance surface 12a is constructed as a plane.
FIG. 4A is a diagram showing the relation between polarization phase difference ΔP1 (in the unit of the wavelength of illumination light) and position X represented by Eq (2). FIG. 5 is a drawing showing polarization states of the illumination light emerging from the first birefringent member 12 and in FIG. 5 a polarization state of illumination light distributed at each position on the XZ coordinates is indicated by a line segment, a circle, or an ellipse with the center at each position. The origins of the X-axis and Z-axis (X=0 and Z=0) in FIG. 5 are set at the center of the birefringent member 12, and the scales in the X-direction and in the Z-direction are so set that the positions of X=+1 and Z=±1 (both of which are positions the reference length apart from the origin X=0, Z=0) are located at the four corners in FIG. 5.
At a position represented by each line segment among the positions identified by the respective XZ coordinates in FIG. 5, the illumination light is in a polarization state consisting primarily of linear polarization and a direction of the line segment indicates a polarization direction thereof. At a position represented by each ellipse, the illumination light is in a polarization state consisting primarily of elliptic polarization and a direction of the major axis of the ellipse indicates a direction in which the linear polarization component in the elliptic polarization is maximum. At a position represented by each circle, the illumination light is in a polarization state consisting primarily of circular polarization.
At the positions ±1 apart in the X-direction from the center, as shown in FIG. 4(A), the first birefringent member 12 acts as a so-called half wavelength plate. Here the illumination light IL emitted from the exposure light source 1 in FIG. 1 consists primarily of the linearly polarized light polarized in the X-direction as described previously, and the half wavelength plate has the fast axis of and slow axis ns rotated 45° relative to the X-direction being the polarization direction of the incident light (or of the illumination light). Therefore, as shown in FIG. 5, the polarization state of the illumination light passing near the positions ±1 (reference length) apart in the X-direction from the center in the first birefringent member 12 is converted into the polarization state consisting primarily of linear polarization in the Z-direction by the action of the half wavelength plate.
For the illumination light passing near the positions ±0.6 apart in the X-direction from the center in the first birefringent member 12, as shown in FIG. 4A, the polarization phase difference ΔP1 is 0.25 and the first birefringent member 12 acts as a so-called quarter wavelength plate. For this reason, the illumination light passing this part is converted into a polarization state consisting primarily of circular polarization.
On the other hand, there is no path difference between the linear polarization in the direction of the fast axis nf and the linear polarization in the direction of the slow axis ns in a beam passing the center in the X-direction, and thus no conversion occurs for the polarization state of transmitted light. Therefore, a beam incident at the center in the X-direction into the birefringent member 12 emerges from the birefringent member 12 while maintaining the state consisting primarily of the linear polarization state in the X-direction. Then beams passing at positions except for the above positions of X=0, +0.6, and ±1 pass through the first birefringent member 12, in polarization states consisting primarily of elliptic polarization in different shapes according to the positions. The polarization states are as shown in FIG. 5.
In FIG. 1, the illumination light IL in the different polarization states according to the passing locations through the first birefringent member 12 is incident to the second birefringent member 13. The second birefringent member 13 is also a member of disk shape made of a birefringent material.
FIG. 3A is a view of the second birefringent member 13 in FIG. 1, viewed to the +Y direction and along the optical axis of illumination system AX2, and, as shown in FIG. 3(A), the fast axis nf of the second birefringent member 13 is set in parallel with the Z-axis of the XZ coordinate system with the same coordinate axes as those in FIG. 1 and the slow axis ns is set in parallel with the X-axis, different from the aforementioned first birefringent member 12. Concerning the second birefringent member 13, the size (diameter) in the in-plane direction thereof is larger than the beam size of the illumination light IL at the position where the second birefringent member 13 is located.
The thickness of the second birefringent member 13 is not uniform, either, and the thickness also varies according to positions in the direction of the function Z=X in the XZ coordinate system in FIG. 3(A), i.e., in the direction of line BB′ in FIG. 3A (which will be referred to hereinafter as “XZ direction”). FIG. 3(B) is a sectional view of the second birefringent member 13 along line BB′ in FIG. 3A, and, as shown in FIG. 3B, the birefringent member 13 has such a shape that it is thin at the left end (near B) and thick at the right end (near B′). On the other hand, the thickness of the second birefringent member 13 is uniform in the direction orthogonal to the XZ direction. Therefore, the second birefringent member 13 also functions as a second nonuniform wavelength plate in which the polarization phase difference given to the transmitted light differs according to locations.
In the present example the thickness TB of the second birefringent member 13 is represented by the following function, for the position XZ in the XZ direction. As shown in FIG. 3(B), the origin in the XZ direction (XZ=0) is defined at the center of the birefringent member 13 (the optical axis of illumination system) and the thickness at the center is defined by T2.

TB=T2+β×(2.5×XZ5−1.5×XZ3)  (3)

In this equation, β is a proportionality coefficient and the value of β differs depending upon the aforementioned index difference between the fast axis and the slow axis of the birefringent material used, or the like as the thickness T2 of the center part does. Here the thickness T2 of the center part is so set that the polarization phase difference ΔP2 of the second birefringent member 13 is 0.25 (in the unit of the wavelength of the illumination light), i.e., that the center part functions as a quarter wavelength plate.
The birefringent member 13 is also so set that the polarization phase differences ΔP2 at the positions +1 (reference length) and −1 apart in the XZ direction are +0.75 and −0.25, respectively. This means that differences of +0.5 and −0.5, respectively, are made between the polarization phase differences at the positions of interest and at the center.
Namely, in the second birefringent member 13 of the present example the thickness thereof is so set that the polarization phase difference ΔP2 is represented by the following equation.

ΔP2=0.25+0.5×(2.5×XZ5−1.5×XZ3)  (4)

In a case where the second birefringent member 13 is also made of crystalline quartz as in the case of the aforementioned example, the thickness T2 of the center part can be set to an (integer+1⁄4) multiple of 14.239 μm and the proportionality coefficient β to 7.12 μm. FIG. 4(B) is a drawing showing the relation between polarization phase difference ΔP2 and position XZ of Eq (4).
In FIG. 1, the second birefringent member 13 again converts the illumination light in the different polarization states according to the passing locations through the first birefringent member 12, into polarization states according to locations. FIG. 4C shows the polarization states of the illumination light IL emerging from the second birefringent member 13.
FIG. 4C is illustrated in the same manner as the aforementioned illustration method in FIG. 5, and, in FIG. 4C, a polarization state of illumination light distributed at each position on XZ coordinates is indicated by a line segment (linear polarization) or an ellipse (elliptic polarization) with the center at each position. The origins of the X-axis and Z-axis in FIG. 4C (X=0 and Z=0) are also set at the center of the birefringent member 13.
In the present embodiment, as shown in FIG. 1, the first birefringent member 12 and the second birefringent member 13 are located immediately before the fly's eye lens 14 and the exit-side surface of the fly's eye lens 14 is located near the pupil plane 15 in the illumination optical system ILS. For this reason, the first birefringent member 12 and the second birefringent member 13 are located at positions substantially equivalent to the pupil plane 15 in the illumination optical system ILS.
Therefore, the illumination light IL passing through the first birefringent member 12 and the second birefringent member 13 is incident at incidence angles and incidence directions determined according to the locations, into the reticle R. Namely, a beam distributed on the origin (the position of X=0 and Z=0) in FIG. 4C is incident normally to the reticle R, and a beam distributed at a position a predetermined distance apart from the origin is incident at an incidence angle proportional approximately to this distance, to the reticle R. The incidence direction of the beam is a direction equal to an azimuth of that point from the origin.
Exterior circle C1 and interior circle C2 shown in FIG. 4C and FIG. 5 are boundaries of a distribution of the illumination light for forming predetermined annular illumination on the reticle R. The radii of the respective circles C1, C2 are determined as follows: the radius of the exterior circle C1 is 1.15 and the radius of the interior circle C2 0.85, where the unit is the reference length used in determining the thickness shapes (thickness profiles) of the first birefringent member 12 and the second birefringent member 13. Namely, an annular ratio of the annular illumination (radius of interior circle/radius of exterior circle) is assumed to be 0.74. This is based on the assumption of so-called “3⁄4 annular illumination (interior radius:exterior radius=3:4)” used in general, but it is a matter of course that the condition for the annular illumination to which the present invention is to be applied is not limited to this.
As apparent from FIG. 4C, the illumination light emerging from the second birefringent member 13 is in polarization states consisting primarily of linear polarization with the polarization direction along the circumferential direction of a specific annular region 36, in the specific annular region 36 which is an annular region between the exterior circle C1 and the interior circle C2.
When comparing FIG. 4C with FIG. 5, the polarization states of the illumination light on the X-axis and on the Z-axis are almost equal. However, the polarization states at positions approximately 45° apart from each axis about the origin (at the upper right, upper left, lower left, and lower right positions in FIG. 4C and FIG. 5) are almost circular polarization in FIG. 5, but are linear polarization along the circumferential direction of the specific annular region in FIG. 4C, This arises from the action of the second birefringent member 13; the second birefringent member 13 functions as a quarter wavelength plate in the upper left and lower right regions in FIG. 4C and functions as a −1⁄4 wavelength plate and as a 3⁄4 wavelength plate equivalent thereto in the lower left and the upper right regions, respectively.
In the practical exposure apparatus, the actual radius of the exterior circle C1 of the specific annular region 36 is determined by the numerical aperture (NA) on the reticle R side of the projection optical system 25 in FIG. 1, the focal length of the optical system consisting of the relay lens 16 and condenser lenses 18, 20 in the illumination optical system ILS, and the value of coherence factor (illumination σ) to be set, and the radius of the interior circle C2 is a value determined further by the annular ratio to be set. It is needless to mention that for this condition for annular illumination, the thickness shapes of the first birefringent member 12 and second birefringent member 13 are determined so that the polarization directions of the illumination light distributed in the specific annular region 36 are coincident with the circumferential direction of the annular region at the respective positions.
To determine the thickness shapes of the first birefringent member 12 and the second birefringent member 13 means that the shapes are proportionally enlarged or reduced in the XZ plane and unevenness amounts thereof are kept unchanged in the Y-direction (traveling direction of light).
In the first example of the first and second birefringent members 12, 13, as described above, the polarization directions of the illumination light distributed in the specific annular region can be made coincident with the circumferential direction of the annular region at each position, with no light quantity loss of the illumination beam, by the first and second nonuniform wavelength plates. In this case, the illumination light incident through the specific annular region 36 onto the reticle R among the illumination light, i.e., the specific illumination beam incident in the specific incidence angle range to the reticle R is light in the polarization state consisting primarily of S-polarization whose polarization direction lies along the direction normal to the entrance plane. This improves the contrast, resolution, depth of focus, etc. of the transferred image, depending upon the periodicity of the pattern to be transferred, in some cases (the details of which will be described later).
Next, the second example of the first and second birefringent members 12, 13 in the illumination optical system ILS in FIG. 1 will be described with reference to FIG. 6.
In the present example the configurations of the first birefringent member 12 and the second birefringent member 13 are basically the same as those in the aforementioned first example. Namely, the first birefringent member 12 has the direction of the fast axis and the thickness shape as shown in FIG. 2A and FIG. 2B and the second birefringent member 13 has the direction of the fast axis and the thickness shape as shown in FIG. 3A and FIG. 3B. In the present example, however, the function forms for the thicknesses of the two birefringent members 12, 13 are different.
FIG. 6A, corresponding to FIG. 4A, shows a characteristic of polarization phase difference ΔP1 by the first birefringent member 12 versus X-directional position in the second example. The polarization phase difference ΔP1 in FIG. 6A is represented by the following function including a trigonometric function about position X.

ΔP1=0.265×{1−cos(π×X2)}  (5)

This polarization phase difference ΔP1 can be realized by expressing the thickness TA of the first birefringent member 12 by the following function for the X-directional position X.

TA=T1+γ×{1−cos(π×X2)}  (6)

In this equation, γ represents a proportionality coefficient. In a case where the first birefringent member 12 is made of crystalline quartz, as in the first embodiment, the thickness T1 at the center can be set to an integer multiple of 14.239 μm and the proportionality coefficient γ to 3.77 μm. The value of 3.77 μm is obtained by multiplying the thickness of crystalline quartz for giving the polarization phase difference of one wavelength, 14.239 μm, by the coefficient of 0.265 in Eq (5) above.
FIG. 6B shows a characteristic of polarization phase difference ΔP2 by the second birefringent member 13 versus XZ-directional position in the second example. The polarization phase difference ΔP2 in FIG. 6(B) can be represented by the following function including a trigonometric function about position XZ.

ΔP2=0.25+0.5×sin(0.5×π×XZ3)  (7)

The polarization phase difference ΔP2 can be realized by expressing the thickness TB of the second birefringent member 13 by the following function for the position XZ in the XZ direction.

TB=T2+δ×sin(0.5×π×XZ3)  (8)

In this equation δ is a proportionality coefficient. When the second birefringent member 13 is made of crystalline quartz, the thickness T2 at the center can be set to an (integer+1⁄4) multiple of 14.239 and the proportionality coefficient δ to 7.12 μm.
In the present example the first birefringent member 12 and the second birefringent member 13 also function as first and second nonuniform wavelength plates, respectively, in which the polarization phase difference given to the transmitted light differs according to locations. Then the linearly polarized light incident in a polarized state in the X-direction into the first birefringent member 12 is converted into the polarization distribution shown in FIG. 6C to emerge from the second birefringent member 13.
As seen from comparison between FIG. 6C and FIG. 4C, the first birefringent member 12 and second birefringent member 13 of the present second example can make the polarization states of illumination light distributed in the specific annular region 36 between the exterior circle C1 and the interior circle C2, closer to linear polarization parallel to the circumferential direction of the annular region 36 than those in the first example. The reason for it is that the first birefringent member 12 and second birefringent member 13 of the present second example adopt the thickness shapes (i.e., surface shapes) determined by the high-order functions of trigonometric functions and thus they enable higher-accuracy polarization control.
However, since the first birefringent member 12 and second birefringent member 13 in the first example are represented by the functions of at most order 5, they offer the advantage that processing is easy and production cost is low, though they are slightly inferior in the polarization control performance.
In order to further reduce the production cost of the first and second birefringent members 12, 13, it is also possible, for example, to adopt a configuration wherein the surface shape of the first birefringent member 12 is a cylindrical surface (surface of a circular cross section in the X-direction) and wherein the surface shape of the second birefringent member 13 is a tapered surface (inclined plane). The polarization control performance in this case is worse than in the first embodiment, but satisfactory effect can be achieved thereby depending upon use of the projection exposure apparatus. Therefore, it can realize a high-performance exposure apparatus while achieving the reduction of production cost.
The configuration wherein the surface shape of the second birefringent member 13 is the tapered surface means that the polarization phase difference of a beam passing through the second birefringent member 13 is defined in a linear form (linear function) according to locations in the plane of the second birefringent member 13.
Incidentally, the shapes of the first birefringent member 12 and the second birefringent member 13 in FIG. 1 are not limited to the shapes shown in the above first and second examples, but may be any shapes that can make the polarization state of transmitted light in the specific annular region coincide with the circumferential direction in each part.
For example, the shapes of the first birefringent member 12 and second birefringent member 13 may be stepwise shapes with stepped shape changes at predetermined positions, instead of the shapes represented by the aforementioned continuous and differentiable continuous functions. Such stepwise shapes can be formed suitably by etching, instead of mechanical or mechanochemical polishing.
In order to implement the polarization states as described above, in the case where the illumination light is such that the polarization state of the beam incident to the first birefringent member 12 is the single polarization state consisting primarily of linear polarization, the first birefringent member 12 is preferably one that gives the polarization phase difference with 2-fold rotation symmetry around the optical axis of illumination system AX2. It is a matter of course that this embraces the nonuniform wavelength plate having the thickness of an even function in the X-direction and the constant thickness in the Y-direction, as shown in the above-described first and second examples.
The second birefringent member 13 is desirably the nonuniform wavelength plate that gives the polarization phase difference with 1-fold rotation symmetry about the optical axis of the illumination system AX2. The 1-fold rotation symmetry refers to a state in which the distribution of polarization phase differences is approximately symmetric with respect to one axis out of two axes orthogonal to the optical axis of illumination system AX2 and approximately antisymmetric with respect to the other axis. The antisymmetry generally refers to a function that provides equal absolute values but opposite signs with inversion of a coordinate axis, but the antisymmetry herein also embraces functions obtained by adding an offset of a constant to general antisymmetric functions. It is needless to mention that this encompasses the nonuniform wavelength plate having the thickness determined by an odd function with an offset in the XZ-direction and the constant thickness in the direction orthogonal thereto, as shown in the above-described first and second examples.
In the present embodiment, particularly, it is important to set the illumination light distributed in the aforementioned specific annular region to the predetermined polarization state; therefore, it is obvious as to the shapes of the first birefringent member 12 and the second birefringent member 13 that no particular problem will arise even if the shapes in the portions not corresponding to the foregoing specific annular region do not satisfy the above conditions.
The number of first birefringent member 12 and second birefringent member 13, and the directions of the fast axes thereof are not limited to those described in the above first and second examples, either. Specifically, three or more birefringent members may be arranged in series along the traveling direction of the illumination light (along the optical axis of illumination system AX2), and the rotational relation around the optical axis AX2 between the directions of the fast axes is not limited to 45°, either. In the case where three or more birefringent members are arranged in series along the traveling direction of the illumination light, a potential configuration is such that the direction of the fast axis of at least one birefringent member out of the plurality of birefringent members is different from the directions of the fast axes of the other birefringent members, in order to convert the polarization state of the illumination light into linear polarization nearly parallel to the circumferential direction in at least a partial region of the aforementioned specific annular region and, desirably, in the almost entire circumferential region.
Similarly, the materials of the birefringent members 12, 13 and others are not limited to crystalline quartz described above, either, but other birefringent materials are also applicable. It is also possible to use the intrinsic birefringence of fluorite to form the birefringent members. A material originally having no birefringence, e.g., synthetic quartz, comes to have the birefringent property when subjected to stress or the like. It can also be used for the birefringent members 12, 13 and others.
Furthermore, the birefringent members 12, 13 can also be made using a composite material obtained by bonding a material with birefringence onto a transparent substrate without birefringence. In this case, the aforementioned thicknesses are, of course, thicknesses of the material with birefringence. The bonding herein may be implemented not only by mechanical joining such as adhesion or press, but also by a method of forming a thin film with birefringence on the transparent substrate by means such as vapor deposition or the like. The thickness shapes and others of the first birefringent member 12 and second birefringent member 13 described in the above first and second examples vary depending upon the magnitude of birefringence of the material used, but, even in cases where materials except for crystalline quartz are used, the aforementioned shape determining method can also be applied and the shapes are determined thereby, of course.
The advantage of the illumination light in the annular illumination as described above, in which the polarization state of the illumination light distributed in the annular region is coincident with the circumferential direction of the annular region, will be described briefly with reference to FIGS. 7 and 8.
FIG. 7A shows an example of fine periodic pattern PX formed on the reticle R in FIG. 1. The periodic pattern PX is a pattern with periodicity in the X-direction in the same XYZ coordinate system as in FIG. 1, and the pitch PT thereof is 140 nm as a converted value in terms of the scale on the wafer W in consideration of the projection magnification of the projection optical system 25 in FIG. 1. FIG. 7B shows a distribution of diffracted light formed in the pupil plane 26 (cf. FIG. 1) of the projection optical system 25 with the wafer-side numerical aperture (NA) of 0.90 when this pattern is illuminated by annular illumination with the coherence factor (illumination a) of 0.9 and the annular ratio of 0.74, using the illumination light having the wavelength of 193 nm.
FIG. 7C is a drawing showing the condition for annular illumination for illuminating the pattern PX, and the pattern PX is illuminated with the illumination light from the annular region IL0 satisfying the above condition for annular illumination, in the pupil plane 15 of the illumination optical system ILS in FIG. 1. The whole of 0-order diffracted light DO in FIG. 7B from the periodic pattern PX is distributed in the pupil plane 26 and passes through the projection optical system 25 to reach the wafer W, but the 1-order diffracted light D1R and D1L can be transmitted only in part through the pupil plane 26 and the projection optical system 25. An image of the pattern PX on the reticle R is formed as an interference fringe pattern between the 0-order diffracted light DO and the 1-order diffracted light D1R, D1L on the wafer W, but an interference fringe pattern is formed only by a pair of 0-order diffracted light and 1-order diffracted light generated from the illumination light emitted from the same position on the pupil plane 15 of the illumination optical system ILS.
The 1-order diffracted light D1L located at the left end of the pupil plane 26 in FIG. 7B is to be paired with the part of the 0-order diffracted light DO located at the right end, and those diffracted light beams originate in the illumination light from the right-end partial region ILR in the annular region IL0 in FIG. 7C. On the other hand, the 1-order diffracted light D1R located at the right end of the pupil plane 26 in FIG. 7B is to be paired with the part of the 0-order diffracted light DO located at the left end, and those diffracted light beams originate in the illumination light from the left-end partial region ILL in the annular region IL0 in FIG. 7C.
Namely, on the occasion of exposure of the pattern PX with the fine pitch in the X-direction, beams contributing to imaging of the pattern PX among the illumination light emitted from the annular region IL0 on the pupil plane 15 of the illumination optical system ILS are limited to those in the partial region ILR and partial region ILL, and the illumination light emitted from the other regions in the annular region IL0 is illumination light not contributing to imaging of the pattern PX.
Incidentally, aforementioned Non-patent Document 1 (Thimothy A. Brunner, et al.: “High NA Lithographic imaging at Brewster's angle,” SPIE Vol. 4691, pp. 1-24 (2002) and others report that on the occasion of exposure of a pattern with periodicity in the X-direction and with the longitudinal direction along the Y-direction like the pattern PX, the contrast of its projected image is improved by illumination with linear polarization having the polarization direction along the Y-direction on the reticle R.
Therefore, it is effective in improvement in the contrast of the projected image of the pattern PX and in improvement in the resolution and depth of focus in turn, to convert the illumination light distributed in the partial region ILR and in the partial region ILL in FIG. 7(C), into linearly polarized light polarized in the PR direction and the PL direction (corresponding to the Y-direction on the reticle R in consideration of the action of mirror 19 in FIG. 1) parallel to the Z-direction in FIG. 7C.
When the reticle pattern is a periodic pattern with a fine pitch in the Y-direction resulting from 90° rotation of the pattern PX of FIG. 7A, the diffracted light distribution is also one rotated 90° from that shown in FIG. 7B. As a result, the partial regions through which the illumination light contributing to image formation of the periodic pattern passes are also located at the positions resulting from 90° rotation of the partial region ILR and partial region ILL shown in FIG. 7C (i.e., at the upper end and at the lower end in FIG. 7C) and the preferred polarization state is linear polarization with the polarization direction coincident with the X-direction. From the above, it is effective to use the illumination light in the polarization state as shown in FIG. 8, on the occasion of exposure of the reticle R including a pattern PX with fine periodicity in the X-direction and a pattern PY with fine periodicity in the Y-direction.
FIG. 8A is a perspective view showing the simplified relation between the pupil plane 15 of the illumination optical system ILS and the reticle R in FIG. 1, without illustration of the relay lens 16, condenser lenses 18, 20, and others in FIG. 1. As described above, the illumination light distributed in the annular region IL0 in FIG. 8A is desirably linear polarization in the Y-direction (the depth direction in the plane of FIG. 8A) at the ends ILL, ILR in the X-direction, in order to improve the imaging performance of the pattern PX with periodicity in the X-direction, and it is desirably linear polarization in the X-direction at the ends ILU, ILD in the Y-direction, in order to improve the imaging performance of the pattern PY with periodicity in the Y-direction. Namely, it is desirable to use linear polarization with the polarization direction approximately coincident with the circumferential direction of the annular region IL0.
Furthermore, in a case where the reticle R includes not only the patterns in the X-direction and in the Y-direction but also patterns in intermediate directions (45° and 135° directions), it is desirable to use linear polarization with the polarization direction perfectly coincident with the circumferential direction of the annular region, taking orientations of these patterns into consideration as well.
In passing, the above-described polarization states do not always realize effective polarization states for the patterns perpendicular to the patterns with orientations suitable for the polarization states of the respective portions in the annular region IL0. For example, the illumination light polarized in the X-direction from the partial region ILU is not in a preferred polarization state for imaging of the pattern PX with the periodicity in the X-direction and with the longitudinal direction along the Y-direction. As apparent from FIG. 7C showing the light sources contributing to the imaging of the pattern with the fine pitch in the X-direction, however, the partial region ILU corresponding to the upper end of the annular region IL0 in FIG. 7C is primarily the light source not contributing to the imaging of the pattern with the fine pitch in the X-direction at all and, therefore, no matter which polarization state is taken by the partial region ILU, the polarization state thereof will never cause degradation of the imaging characteristics.
As shown in FIG. 8A, the linear polarization almost coincident with the circumferential direction of the annular region IL0 on the pupil plane 15 of the illumination optical system ILS is incident as so-called S-polarization to the reticle R. The S-polarization refers to linear polarization with the polarization direction perpendicular to a plane of incidence in which a beam is incident to an object (which is a plane including a normal to the object, and the beam). Namely, the illumination light ILL 1 from the partial region ILL consisting of linearly polarized light in the direction coincident with the circumferential direction of the annular region IL0 is incident as S-polarization with the polarization direction EF1 normal to the plane of incidence (the plane of FIG. 8B), as shown in FIG. 8B, to the reticle R. The illumination light ILD1 on the similar partial region ILD is also incident as S-polarization with the polarization direction EF2 normal to the plane of incidence (the plane of FIG. 8C), as shown in FIG. 8C, to the reticle R.
Naturally, the illumination light from the partial regions ILR, ILU at the positions symmetric with the foregoing partial regions ILL, ILD with respect to the optical axis AX41 of the illumination optical system is also incident as S-polarization to the reticle R by virtue of symmetry, because each illumination light on the partial region ILR, ILU has the polarization direction coincident with the circumferential direction of the annular region IL0. It is the general property of the annular illumination that angles of incidence of the illumination light distributed on the annular region IL0, to the reticle R are in a predetermined angular region with the center at an angle φ from the optical axis AX41 of the illumination optical system (i.e., a normal to the reticle R). A beam incident at the angles of incidence to the reticle R will be referred to hereinafter as “specific illumination beam.” The angle φ and angular range can be determined based on the wavelength of the illumination light, the pitch of the pattern to be transferred, on the reticle R, and so on.
Incidentally, the foregoing first and second birefringent members 12, 13 convert the polarization state of the illumination light distributed in the specific annular region between the predetermined outside radius (exterior circle C1) and inside radius (interior circle C2) determined from the shapes peculiar to the members, into the polarization state consisting primarily of linear polarization parallel to the circumferential direction of the specific annular region, and it is not easy to change the radii (C2, C1).
In the case where there arises a need for changing the desired annular region, for example, on the basis of the pitch of the pattern to be transferred, on the reticle R, as described above, a desirable configuration is such that a plurality of conical prisms 41, 42 of a zoom type are disposed between the first and second birefringent members 12, 13 and the optical integrator such as the fly's eye lens 14 in FIG. 1, as shown in FIG. 9, to make the radii of the foregoing specific annular region variable. In FIG. 9, the plurality of conical prisms of the zoom type are a concave conical prism 41 with a concave conical surface 41b and a convex conical prism 42 with a convex conical surface 42a, which are arranged with a spacing DD variable between them and along the optical axis of illumination system AX2.
In this case, the illumination light distributed in the specific annular region with the center at an average radius RI after passage through the first and second birefringent members 12, 13 is enlarged to a radius RO on the entrance plane of the fly's eye lens 14 and on the pupil plane 15 of the illumination optical system being the exit plane of the fly's eye lens 14, by the zoom type conical prisms 41, 42. This radius RO can be enlarged by increasing the spacing DD between the two conical prisms 41, 42 and can be reduced by decreasing the spacing DD.
This makes it feasible to form the specific annular region as a distribution of the illumination light consisting of linear polarization parallel to the circumferential direction, with arbitrary radii on the pupil plane 15 of the illumination optical system, and to change the illumination condition for annular illumination in accordance with the pattern on the reticle R to be transferred.
It is a matter of course that a zoom optical system can be used instead of the foregoing zoom type conical prisms 41, 42.
Incidentally, the above embodiment was described on the premise that the illumination light quantity distribution formed on the pupil plane 15 of the illumination optical system ILS in FIG. 1 is the annular region, i.e., that it is applied to the annular illumination, but the illumination condition that can be realized by the projection exposure apparatus of FIG. 1 is not always limited to the annular illumination. Namely, since the birefringent members 12, 13 of FIG. 1 and the zoom type conical prisms 41, 42 of FIG. 9 are configured to set the polarization state of the illumination light distributed in the specific annular region in the pupil plane 15 of the illumination optical system to the desired polarization state, it is needless to mention that even in the case where the distribution of the illumination light is further limited to within a specific partial region in the specific annular region, i.e., where the distribution of the illumination light is limited, for example, to the partial regions ILL, ILR in FIG. 7C, they can convert the illumination light distributed in the partial regions, into illumination light consisting primarily of linear polarization with the polarization direction parallel to the circumferential direction of the specific annular region.
In order to condense the illumination light only into the further specific regions in the specific annular region in this manner, the diffractive optical element 9a in FIG. 1 is replaced with such a diffractive optical member that the diffracted light (illumination light) generated from the other diffractive optical element is concentrated in the further specific discrete regions in the specific annular region on the first birefringent member 12 and on the second birefringent member 13. The locations where the illumination light is condensed are, for example, two locations of the partial regions ILL, ILR in FIG. 7C, but are not limited to this example. The illumination light may be condensed at arbitrary locations in the specific annular region, and the number of locations may be four. The locations may be selected according to the shape of the pattern as an exposed object on the reticle R.
In the case where the illumination light is condensed in the further specific regions in the specific annular region as described above, it is also possible to use, instead of the zoom type conical prisms 41, 42, an optical member group as a combination of a convex polyhedron prism and a concave polyhedron prism of pyramid shape or the like with a variable spacing similarly.
Since the illumination light distributed in the regions except for these specific regions is not suitable for exposure of the pattern as the exposed object, the light quantity distribution thereof is preferably substantially 0 in certain cases. On the other hand, manufacturing error or the like of the diffractive optical element 9a and others could produce diffracted light (hereinafter referred to as “error light”) in directions except for the desired directions from the diffractive optical element 9a and others and cause the illumination light to be distributed in the regions except for the above partial regions. It is thus also possible, for example, to adopt a configuration wherein a stop is disposed on the entrance surface side or on the exit surface side of the fly's eye lens 14 in FIG. 1 to block the error light. This makes the illumination light quantity distributions in the plurality of specific regions perfectly discrete. However, there are cases where another pattern exists except for the pattern as the exposed object on the reticle R and where the error light is effective to imaging of the pattern except for the object. Therefore, the illumination light quantity distribution in the region except for the specific regions does not always have to be 0 in such cases.
Incidentally, with attention to incidence of the illumination light to the reticle R, the limitation of the distribution of the illumination light quantity on the pupil plane 15 to within the further specific regions in the specific annular region results in further limiting the incidence directions thereof to only the aforementioned plurality of substantially discrete directions, in addition to the restriction on the range of incidence angles by the annular illumination. Naturally, in the case where the present invention is applied to the annular illumination, it is also possible to adopt the configuration wherein a stop is disposed on the entrance surface side or on the exit surface side of the fly's eye lens 14 to block the error light distributed in the regions except for the specific annular region.
The above embodiment was arranged to use the fly's eye lens 14 as an optical integrator, but it is also possible to use an internal reflection type integrator (e.g., glass rod) as an optical integrator. In this case, the exit plane of the glass rod is not located on the pupil plane 14 of the illumination optical system, but is located on a plane conjugate with the reticle R.
In the above embodiment the laser light source as the exposure light source 1 was arranged to emit the linearly polarized light polarized in the X-direction, but the laser light source, depending upon its type, can emit linearly polarized light polarized in the Z-direction in FIG. 1, or a beam in another polarization state. In the case where the exposure light source 1 in FIG. 1 emits light linearly polarized in the Y-direction, i.e., light linearly polarized in the Z-direction at the positions of the birefringent members 12, 13, the birefringent members 12, 13 described in the above first and second examples are rotated 90° about the optical axis of illumination system AX2, whereby the illumination light can be obtained in much the same polarization state as the polarization state shown in FIG. 4C and FIG. 6C (precisely, illumination light in a state resulting from 90° rotation of the state shown in the two figures).
Alternatively, the polarization controlling member 4 (polarization controlling mechanism) in FIG. 1 may be used to convert the Y-directional linear polarization emitted from the exposure light source 1, into X-directional linear polarization. This polarization controlling member 4 can be readily substantialized by a so-called half wavelength plate. In cases where the exposure light source 1 emits light of circular polarization or elliptic polarization, it can also be converted into the desired Z-directional linear polarization, similarly using a half wavelength plate or a quarter wavelength plate as the polarization controlling member 4.
It is, however, noted that the polarization controlling member 4 is not necessarily able to convert a beam in an arbitrary polarization state emitted from the exposure light source 1, into the Z-directional polarization without loss in light quantity. Therefore, the exposure light source 1 needs to generate a beam in a single polarization state (beam that can be converted into linear polarization without loss in light quantity by a wavelength plate or the like), such as linear polarization, circular polarization, or elliptic polarization. However, where the intensity of the beam except for the aforementioned single polarization state is not so high relative to the total intensity of the illumination light, the adverse effect of the beam except for the single polarization state is not so significant on the imaging characteristics, and thus the beam emitted from the exposure light source 1 may contain the beam except for the single polarization state to some extent (e.g., approximately 20% or less of the total light quantity).
When consideration is given to operating circumstances of the projection exposure apparatus of the above embodiment, it is not always the best to set the polarization state of the illumination light so that the illumination light distributed in the specific annular region is linear polarization approximately parallel to the circumferential direction of the annular region or so that the specific illumination light is incident as S-polarization to the reticle R. Namely, there is a case where it is preferable to adopt normal illumination (illumination with a circular illumination light quantity distribution on the pupil plane 15 of the illumination optical system) instead of the annular region, depending upon the pattern on the reticle R to be exposed. In this case, it is sometimes preferable not to use the illumination light in the polarization state in the above embodiment.
For making the apparatus compatible with such operating conditions as well, the polarization controlling member 4 in FIG. 1 may be implemented by adopting an element or optical system capable of converting the polarization state of the beam emitted from the light source such as a laser, into random polarization or the like according to need. This can be realized, for example, by two polarization beam splitters 4b, 4c or the like as shown in FIG. 10.
FIG. 10 shows a polarization control optical system which can be placed at the position of the polarization controlling member 4 in FIG. 1. In this FIG. 10, for example, an illumination beam IL0 (corresponding to the illumination light IL in FIG. 1) consisting of linearly polarized light is incident to a rotatable wavelength plate 4a consisting of a half wavelength plate or quarter wavelength plate. This converts the illumination beam IL0 into an illumination beam IL1 of linear polarization inclined at 45° from the plane of FIG. 10, or of circular polarization, and the first polarization beam splitter 4b divides the illumination beam IL1 into a beam IL2 of P-polarization component and a beam IL3 of S-polarization component with respect to its dividing surface. One beam IL2 travels straight upward through the prism 4b in FIG. 10, and the other beam IL3 is reflected to the right in FIG. 10.
The beam IL2 traveling straight is then incident to the polarization beam splitter 4c and, because of the polarization characteristics thereof, the beam IL2 travels straight in the polarization beam splitter 4c and then travels as a beam IL4 upward in FIG. 10. On the other hand, the reflected beam IL3 is reflected by mirrors 4d, 4e and then enters the polarization beam splitter 4c to be reflected again therein. The reflected beam IL3 merges with the beam IL4 traveling straight. At this time, where DL represents each of the spacings between the polarization beam splitters 4b, 4c and the mirrors 4d, 4e, there is the path length difference of 2×DL between the two merging beams IL3, IL4. When this path length difference of 2×DL is set longer than the coherent length of the illumination beam, there is no coherence between the two beams, so that the merged light can be substantially of random polarization.
When this polarization control optical system is loaded in the illumination optical system ILS in FIG. 1, the illumination light IL transmitted thereby is always of random polarization and could be a hindrance to implementation of the polarization state in the above embodiment. However, this hindrance will not be caused in principle in the optical system shown in FIG. 10 because the rotatable wavelength plate 4a can be rotated to convert the polarization state of the illumination light IL1 transmitted by the rotatable wavelength plate 4a, into linear polarization the whole of which passes through the first beam splitter 4b. However, some loss of light quantity is essentially inevitable due to absorption in the polarization beam splitters 4b, 4c, reflection loss on the mirrors 4d, 4e, and so on, and it is thus optional to provide a mechanism for retracting the beam splitters 4b, 4c and rotatable wavelength plate 4a to outside the optical path of the illumination optical system when there is no need for converting the illumination light into random polarization.
Incidentally, without use of such polarization beam splitters, the following simple method can also offer an effect similar to that by the random polarization illumination. This can be implemented as follows: the polarization state of the illumination light IL incident to the first birefringent member 12 in FIG. 1 is set to polarization in the direction 45° apart from the X-direction and Z-direction in FIG. 1, whereby the illumination light distributed in the specific annular region is converted into approximately circular polarization. Therefore, in cases where the projection exposure apparatus of the present embodiment is used in applications in which the circular polarization can be deemed approximately as random polarization, i.e., in applications in which the imaging performance required is relatively not so high, it is also feasible to achieve the effect similar to that by the random polarization illumination, by a configuration wherein the polarization controlling member 4 in FIG. 1 is constructed, for example, of a half wavelength plate whereby the polarization state of the illumination light incident to the first birefringent member 12 is set to polarization in the direction inclined at 45° from the X-axis and Z-axis as described above. It is also feasible to achieve the effect similar to that by the random polarization illumination, by a similar configuration wherein the polarization controlling member 4 is constructed, for example, of a quarter wavelength plate whereby the polarization state of the illumination light incident to the first birefringent member 12 is set to circular polarization.
Alternatively, it is also feasible to achieve the effect similar to that by the random polarization illumination, by a configuration wherein the relation between the two birefringent members 12, 13 and the direction of linear polarization of the illumination light is rotated, for example, by 45° by a rotating mechanism 101 capable of wholly rotating the first birefringent member 12 and the second birefringent member 13 in FIG. 1 about the optical axis of illumination system AX2 being the optical axis of the illumination optical system ILS.
Incidentally, in the case of the normal illumination, there are also cases wherein the polarization state thereof is preferably set to linear polarization in a predetermined direction. For making the projection exposure apparatus of the above embodiment compatible with this illumination condition, the apparatus is provided with a rotating mechanism 101 capable of wholly rotating each of the birefringent members, such as the first birefringent member 12 and the second birefringent member 13 in FIG. 1, independently about the optical axis of illumination system AX2, and the direction of rotation of each birefringent member is set so that the fast axis (or slow axis) of each birefringent member is in parallel with the direction of linear polarization of the illumination light. In this case, the illumination light travels through each birefringent member, without being affected by the converting action of the polarization state at all, and emerges as keeping the linear polarization at the time of incidence.
For setting the polarization in the linear polarization state in the one predetermined direction, it can also be implemented by wholly retracting the first birefringent member 12 and the second birefringent member 13 and others to outside the optical path of the illumination optical system. Namely, the setting of the linear polarization state in the one predetermined direction may be implemented by providing a replacing mechanism 102 and replacing the birefringent members and others all together thereby. When the apparatus is provided with the replacing mechanism 102, it is also possible to adopt a configuration wherein the replacing mechanism 102 is arranged to set plural sets of birefringent member groups therein and wherein they can be replaceably arranged on the position on the optical axis of illumination system AX2. In this case, it is a matter of course that each birefringent member group is preferably provided with the characteristics of converting the illumination light into linear polarization along the circumferential direction of the specific annular region, in the specific annular region with the outside radius and inside radius different among the groups.
Incidentally, a preferred case to use the illumination light of linear polarization in the one predetermined direction as described above is, for example, exposure of a phase shift reticle of a spatial frequency modulation type with a pattern aligned along a direction. In this case, in order to further improve the resolution and depth of focus of the pattern to be transferred by exposure, the coherence factor (σ value) of the illumination light is preferably not more than about 0.4.
When consideration is given again to the action of the birefringent members according to the present invention (first birefringent member 12 and second birefringent member 13) with reference to FIG. 4C and FIG. 6C, it is apparent from the two figures that the first example (FIG. 4C) and the second example (FIG. 6C) of the first birefringent member 12 and the second birefringent member 13 cause little influence on the polarization state of the illumination light distributed inside a circle (not shown) with the center on the optical axis of the illumination optical system (X=0, Z=0) and with the radius approximately equal to half of the radius of the exterior circle C1 of the specific annular region.
Supposing the radius of the exterior circle C1 is equivalent, for example, to 0.9 as illumination a (σ value), the first birefringent member 12 and second birefringent member 13 emit the incident linear polarization in the X-direction, while keeping it almost in the original polarization state, within the range of the illumination beam of illumination σ=0.45. When linear polarization in the Z-direction (Z-polarization) is made incident to the first birefringent member 12, the polarization state of the illumination beam of approximately the above illumination σ=0.45 can be Z-polarization in the beam emitted from the second birefringent member 13.
Therefore, when the birefringent members as in the first and second examples (the first birefringent member 12 and second birefringent member 13) are used, the aforementioned polarization controlling member 4 or the like is used to switch the polarization direction of the incident light to the birefringent members, without retracting them to outside the optical path of the illumination optical system, whereby it becomes feasible to realize the illumination light being the illumination beam with the illumination σ of not more than about 0.4 and being light polarized in the X-direction or in the Z-direction (polarization in the X-direction or in the Y-direction, respectively, on the reticle R in FIG. 1), suitable for illumination onto the spatial frequency modulation type phase shift reticle.
In this case, it is also a matter of course that, in order to limit the illumination σ to about 0.4, it is preferable to use such a diffractive optical element 9a that the direction characteristic of the generated diffracted light is an angular distribution corresponding thereto. This permits the apparatus to form illumination beams in a variety of practical polarization states without provision of the whole replacing mechanism, which is also the advantage of the present invention.
Next, an example of production steps of semiconductor devices using the projection exposure apparatus of the above embodiment will be described with reference to FIG. 11.
FIG. 11 shows an example of production steps of semiconductor devices and in this FIG. 11, a wafer W is first made of a silicon semiconductor or the like. Thereafter, a photoresist is applied onto the wafer W (step S10) and in the next step S12, a reticle (assumed to be R1) is loaded on the reticle stage of the projection exposure apparatus of the above embodiment (FIG. 1), the wafer W is loaded on the wafer stage, and a pattern on the reticle R1 (indicated by symbol A) is transferred (to effect exposure) into all shot areas SE on the wafer W by the scanning exposure method. On this occasion double exposure is carried out according to need. The wafer W is, for example, a wafer with the diameter of 300 mm (12-inch wafer), and the size of each shot area SE is, for example, a rectangular region with the width of 25 mm in the non-scanning direction and the width of 33 mm in the scanning direction. In the next step S14, development, etching, ion implantation, and so on are carried out to form a predetermined pattern in each shot area SE on the wafer W.
In the next step S16, a photoresist is applied onto the wafer W and in step S18 thereafter, a reticle (assumed to be R2) is loaded on the reticle stage of the projection exposure apparatus of the above embodiment (FIG. 1), the wafer W is loaded on the wafer stage, and a pattern on the reticle R2 (indicated by symbol B) is transferred (to effect exposure) into each shot area SE on the wafer W by the scanning exposure method. In next step S20, development, etching, ion implantation, and so on of the wafer W are carried out to form a predetermined pattern in each shot area on the wafer W.
The above exposure step to pattern forming step (step S16 to step S20) are repeated the number of times necessary for production of desired semiconductor devices. Then semiconductor devices SP as products are fabricated through a dicing step (step S22) of separating chips CP on the wafer W from each other, a bonding step, and a packaging step and others (step S24).
Since the device fabrication method of the present example involves carrying out the exposure by the projection exposure apparatus of the above embodiment, the exposure step enables the reticle to be illuminated with the illumination light (exposure beam) in the predetermined polarization state with increased efficiency of utilization thereof. Therefore, the resolution and others are improved for periodic patterns with a fine pitch or the like, so that higher-integration and higher-performance semiconductor integrated circuits can be fabricated at low cost and at high throughput.
The projection exposure apparatus of the above embodiment can be produced as follows: the illumination optical system and projection optical system composed of a plurality of lenses are incorporated in the main body of the exposure apparatus, optical adjustment is carried out for the optics, the reticle stage and wafer stage comprised of a number of mechanical parts are attached to the main body of the exposure apparatus, wires and tubes are connected thereto, and overall conditioning processes (electric adjustment, confirmation of operation, etc.) are further carried out. The production of the projection exposure apparatus is preferably carried out in a clean room in which the temperature, cleanliness, etc. are controlled.
The present invention is applicable not only to the projection exposure apparatus of the scanning exposure type, but also to the projection exposure apparatus of the full exposure type such as steppers. The magnification of the projection optical system used may be a demagnification rate, a 1:1 magnification, or an enlargement magnification. Furthermore, the present invention is also applicable, for example, to the liquid immersion type exposure apparatus as disclosed in International Publication (WO) 99/49504 or the like. As shown in FIG. 1A, a liquid LQ is provided between the projection optic system 25 and the wafer W in the liquid immersion type exposure apparatus.
The usage of the projection exposure apparatus of the present invention is not limited to the exposure apparatus for fabrication of semiconductor devices, but it is also commonly applicable, for example, to exposure apparatus for display devices such as liquid crystal display devices formed on rectangular glass plates, or plasma displays, and to exposure apparatus for fabricating various devices such as image pickup devices (CCDs or the like), micromachines, thin film magnetic heads, and DNA chips. Furthermore, the present invention is also applicable to the exposure step (exposure apparatus) in production of masks (photomasks including X-ray masks, reticles, etc.) with mask patterns for various devices by the photolithography step.
It is needless to mention that the illumination optical system (2-20) in the projection exposure apparatus in the aforementioned embodiment is also applicable to the illumination optical apparatus for illuminating the first object such as the reticle R.
It is a matter of course that the present invention is not limited to the above embodiment and can be modified in a variety of configurations without departing from the spirit and scope of the present invention. The entire disclosure of Japanese Patent Application No. 2003-367963 filed Oct. 28, 2003, including the specification, scope of claims, drawings, and abstract is incorporated by reference herein in its entirety.
The device fabrication method of the present invention enables enhancement of utilization efficiency of the exposure beam (illumination light) and permits a predetermined pattern to be formed with high accuracy. Therefore, it permits various devices such as semiconductor integrated circuits to be fabricated with high accuracy and high processing performance (throughput).
From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

## Claims

1. An illumination optical apparatus which illuminates an object with illumination light through a pupil plane of the illumination optical apparatus, the illumination optical apparatus comprising:
an optical integrator arranged in an optical path of the illumination light; and
a plurality of birefringent members made of a birefringent material, which are arranged in the optical path on an incidence side of the optical integrator in order along an optical axis of the illumination optical apparatus so as to change a polarization state of the illumination light such that first and second rays of the illumination light are polarized in different directions on the pupil plane, the first ray being a ray which passes through a first region on the pupil plane, the second ray being a ray which passes through a second region on the pupil plane, the first and second regions being different from each other and away from the optical axis,
wherein the plurality of birefringent members are arranged such that a total length of the plurality of birefringent members in an optical path of the first ray is different from a total length of the plurality of birefringent members in an optical path of the second ray, and are arranged so as to change the polarization state of the illumination light incident on the plurality of the birefringent members in a linear polarization state having a substantially single polarization direction such that each of the first and second rays is polarized in a substantially circumferential direction about the optical axis on the pupil plane.

2. The illumination optical apparatus according to claim 1, wherein the plurality of birefringent members are arranged so as to change the polarization state of the illumination light such that a polarization direction of the first ray in the first region is a first polarization direction and a polarization direction of the second ray in the second region is a second polarization direction, the first and second polarization directions being along the circumferential direction and being different from each other.

3. The illumination optical apparatus according to claim 2, wherein the first and second directions are perpendicular to each other.

4. The illumination optical apparatus according to claim 1, wherein the apparatus is arranged so as to irradiate the illumination light from the pupil plane onto the object in a polarization state in which a principal component is S-polarized light.

5. The illumination optical apparatus according to claim 1, wherein the plurality of birefringent members include a first birefringent member and a second birefringent member, the second birefringent member being arranged in the optical path between the first birefringent member and the optical integrator.

6. The illumination optical apparatus according to claim 5, wherein the plurality of birefringent members include no optical element in the optical path between the first birefringent member and the second birefringent member.

7. The illumination optical apparatus according to claim 5, wherein a direction of a fast axis of the birefringent material of the first birefringent member is different from a direction of a fast axis of the birefringent material of the second birefringent member.

8. The illumination optical apparatus according to claim 5, wherein at least one of the first and second birefringent members is removable relative to the optical path.

9. The illumination optical apparatus according to claim 5, wherein at least one of the first and second birefringent members is replaceable.

10. The illumination optical apparatus according to claim 5, wherein at least one of the first and second birefringent members is rotatable about the optical axis.

11. The illumination optical apparatus according to claim 1, further comprising a polarization controlling member arranged in the optical path on an incidence side of the plurality of birefringent members so as to change a polarization state of the illumination light from a first polarization state including a substantially single polarization state into a second polarization state different from the first polarization state.

12. The illumination optical apparatus according to claim 11, wherein a principal component in the second polarization state is the linear polarization state having the substantially single polarization direction.

13. The illumination optical apparatus according to claim 11, wherein the polarization controlling member is arranged to switch a polarization direction of a principal component in the second polarization state between a first direction and a second direction.

14. The illumination optical apparatus according to claim 11, wherein the principal component in the first polarization state is linearly polarized light, circularly polarized light or elliptically polarized light.

15. The illumination optical apparatus according to claim 11, wherein the polarization controlling member comprises at least one of a half wavelength plate and a quarter wavelength plate.

16. The illumination optical apparatus according to claim 1, wherein the optical integrator is a fly's eye lens.

17. The illumination optical apparatus according to claim 1, wherein an exit surface of the optical integrator is arranged at a position equivalent to the pupil plane.

18. The illumination optical apparatus according to claim 1, wherein an exit surface of the optical integrator is arranged so as to be coincident with an optical Fourier transform plane with respect to a plane on which the object is placed.

19. An exposure apparatus which exposes a substrate to light passing through a pattern, the exposure apparatus comprising:
a stage which holds the substrate,
the illumination optical apparatus according to claim 1 which illuminates the pattern with the light; and
a projection optical system which projects an image of the pattern illuminated with the light from the illumination optical apparatus onto the substrate held by the stage.

20. The exposure apparatus according to claim 19, wherein the exposure apparatus is a liquid immersion exposure apparatus.

21. A device manufacturing method, comprising:
transferring a pattern to a substrate by use of the exposure apparatus according to claim 19; and
developing the substrate to which the pattern has been transferred.

22. An exposure method which exposes a substrate to light passing through a pattern, the exposure method comprising:
holding the substrate by a stage;
illuminating the pattern with the light by use of the illumination optical apparatus according to claim 1; and
projecting an image of the pattern illuminated with the light onto the substrate held by the stage.

23. The exposure method according to claim 22, wherein the substrate is exposed with the light through liquid.

24. A device manufacturing method, comprising:
transferring a pattern to a substrate by use of the exposure method according to claim 22; and
developing the substrate to which the pattern has been transferred.