Patent Application
20100103394
1. Field of the Invention
The present invention relates to a reflection projection optical system, an exposure apparatus, and a method of manufacturing a device. More particularly, the present invention relates to a reflection projection optical system, an exposure apparatus, and a method of manufacturing a device, which perform projection exposure of processing objects.
2. Description of the Related Art
A projection exposure apparatus that typifies semiconductor manufacturing exposure apparatuses includes a projection optical system which projects a pattern formed on a mask (reticle) onto a wafer. A resolution (a minimum feature size that can be precisely transferred) R of a projection exposure apparatus is given by:
R=k
1×(λ/NA) (1)
where λ is the wavelength of a light source, and NA is the numerical aperture of a projection optical system.
According to this principle, the shorter the wavelength or the higher the NA, the better the resolution. Only increasing the NA as before has reached a practical limit beyond which it is no longer possible to meet the recent demand for a higher resolution. Under the circumstance, shortening the wavelength is expected to improve the resolution. The current mainstream exposure light sources are a KrF excimer laser (wavelength: about 248 nm) and an ArF excimer laser (wavelength: about 193 nm). At the same time, practical application of an exposure apparatus which uses EUV light is in progress.
When the exposure light is EUV light, there exists no glass material usable for a projection optical system, and, naturally, the projection optical system can include no lens. To combat this issue, various types of reflection reduction projection optical systems including only mirrors have been proposed. For the same reason as in a projection optical system, there exists no transmissive reticle which transmits EUV light, so a reflective reticle having a reflective film pattern formed from a multi-layer film is used for EUV light.
Conventionally, for the use of a reflective reticle, a projection optical system is a non-telecentric system having an incident pupil set, for example, at a finite distance from the object plane. This is to guide light reflected by an illumination system to the projection optical system. In this case, if the relative position of the object plane in the optical axis direction shifts during scanning exposure, the magnification and distortion aberration on the image plane are prone to change, resulting in degradation in imaging performance. Japanese Patent Laid-Open Nos. 2000-100703, 2001-332489, 2003-045782, and 2003-233001 disclose conventional techniques which ensure telecentricity on the object plane side in order to reduce the adverse effect of this issue.
Conventional techniques which use a reflective reticle have the following shortcomings: telecentricity on the object side is imperfect; a given degree of freedom of the positioning of illumination system mirrors and a given imaging region are not ensured; and a high NA is not attained. For example, the arrangement described in Japanese Patent Laid-Open No. 2000-100703 includes a first concave mirror positioned adjacent to the object plane, and therefore does not secure a space wide enough to accommodate illumination system mirrors. In addition, this arrangement has imperfect telecentricity on the object side as follows. Although this arrangement attains an NA of about 0.1, any attempt to attain an NA higher than this value results in divergence of a light beam. This further deteriorates telecentricity on the object side.
Japanese Patent Laid-Open No. 2001-332489 describes an embodiment in which a reflective reticle is used. In this embodiment, an Offner type projection system is used and set to have bilateral telecentricity on both the object side and the image side, and a reticle and a wafer are symmetrically positioned to be tilted with respect to the optical axis. Unfortunately, the positioning of a reticle and a wafer described in Japanese Patent Laid-Open No. 2001-332489 prevents the object plane and the image plane from satisfying the Scheimpflug condition, so the image plane degrades in imaging performance. This makes it impossible to ensure a sufficient effective image plane width, and makes it difficult to attain a higher NA.
The arrangement described in Japanese Patent Laid-Open No. 2003-045782 has been proposed by the inventor of the present invention, and is relatively easy to ensure telecentricity on the object side. However, because the light beam height from the object plane increases or is parallel to the optical axis, it is difficult to position illumination system mirrors for illuminating a reflective reticle. The use of a reflective reticle is to virtually lowering telecentricity on the object side to some extent. A reflection projection optical system described in Japanese Patent Laid-Open No. 2003-233001 has also been proposed by the inventor of the present invention. The reflection projection optical system described in this patent reference increases the light beam height from the object plane, and therefore attains nearly perfect telecentricity on the object side, as in that described in Japanese Patent Laid-Open No. 2003-045782. This again makes it difficult to position illumination system mirrors for illuminating a reflective reticle. Furthermore, the reflection projection optical systems described in Japanese Patent Laid-Open Nos. 2003-045782 and 2003-233001 do not show details of how to position the object plane and the image plane to be tilted with respect to a plane perpendicular to the optical axis.
Assume the use of a reflection projection optical system which has an incident pupil set at a finite distance from the object plane, that is, serves as a non-telecentric system. In this case, the difference in NA for each image height in the effective region may be an issue. The difference in NA for each image height in the effective region is accounted for by the fact that the angle of the principal ray as the center of a light beam with respect to the incident pupil changes for each image height. More specifically, as shown in
According to an aspect of the present invention, there is provided a reflection projection optical system which projects a pattern positioned on an object plane onto an image plane via a reflecting mirror, wherein the system has bilateral telecentricity on a side of the object plane and a side of the image plane, and has the object plane and the image plane positioned to be tilted with respect to a plane perpendicular to an optical axis of the reflection projection system so as to satisfy a Scheimpflug condition.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
A reflection projection optical system 100 and exposure apparatus 200 according to the present invention will be described below with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments, and each constituent element may be alternatively substituted by another one within the scope in which the object of the present invention is achieved. Note that the same reference numerals denote the same members throughout the drawings, and a repetitive description thereof will not be given.
Referring to
The projection optical system 100 includes six reflecting mirrors 110 to 160. The object plane MS is basically an off-axis ring-like plane, as shown in
With this arrangement, the projection optical system 100 according to the present invention forms nearly perfect bilateral telecentricity on both the object plane side and the image plane side. To position a reflective reticle on the object plane MS, the object plane MS is tilted with respect to a plane perpendicular to the optical axis OA. This arrangement successfully splits illumination light guided from an illumination system to a reflective reticle, and that reflected by the reflective reticle toward a projection optical system. This makes it possible to increase the degree of freedom of the positioning of illumination system mirrors in accordance with the setting of the tilt angle of the reflective reticle. At the same time, the image plane W is tilted with respect to a plane perpendicular to the optical axis OA, so as to satisfy the so-called Scheimpflug condition, in accordance with the tilt of the object plane MS. Hence, the projection optical system 100 according to the present invention can ensure a sufficient width of the image plane in the effective field region CR.
The projection optical system 100 according to this embodiment has an intermediate image MI of one real image, and forms the intermediate image MI twice as a whole. To implement this, both the object plane MS and image plane W are tilted counterclockwise within the paper plane with respect to a plane perpendicular to the optical axis OA. That is, in accordance with the Scheimpflug condition, the object plane MS and image plane W are tilted in the same direction if the projection optical system 100 forms an image even-numbered times as a whole, and they are tilted in opposite directions if the projection optical system 100 forms an image odd-numbered times as a whole.
Also, the projection optical system 100 according to the present invention forms nearly perfect bilateral telecentricity on both the object plane side and the image plane side. For this reason, even when the relative position between the object plane and the image plane in the optical axis direction shifts during scanning exposure using the reticle and the substrate, it is possible to satisfactorily maintain a given imaging performance by suppressing changes in magnification and distortion aberration on the image plane. It is also possible to reduce degradation in overlay performance attributed to deterioration in reticle evenness, which may be an issue encountered when the projection optical system has non-telecentricity.
The formation of telecentricity on the object side decreases the difference in NA for each image height in the effective field region, as compared to the conventional non-telecentric system. Also, the formation of telecentricity prevents the occurrence of a light amount difference attributed to the object height in a shadowy portion of the absorbing layer on the three-dimensional pattern. This is obvious because the formation of telecentricity allows the principal ray of illumination light to become incident on the reflective reticle at a predetermined angle, independently of the illumination range. In sum, since the formation of telecentricity can decrease the differences in NA and amount of light reflected by the reticle for each image height, it is possible to reduce factors of deterioration in CD uniformity in the effective field even in the conventional projection optical system.
From the foregoing, the projection optical system 100 according to the present invention increases the degree of freedom of the positioning of illumination system mirrors, can attain good CD uniformity over the entire effective field region, and can reduce degradation in overlay performance.
The tilt angle θ can satisfy:
sin−1(NAO)<θ<sin−1(NAO)+5.0 (2)
where θ (°) is the tilt angle of the object plane MS with respect to a plane perpendicular to the optical axis OA, and NAO is the numerical aperture of the projection optical system 100 on the side of the object plane MS.
Inequality (2) defines the relationship between the tilt angle θ of the object plane MS and the NA of the projection optical system 100 on the object side. Assume that the tilt angle e falls below sin−1(NAO) that is a lower limit. In this case, it is difficult to split light which becomes incident on the reflective reticle and that reflected by the reflective reticle. Also, it is virtually difficult to position illumination system mirrors. Assume that the tilt angle e exceeds [sin−1(NAO)+5.0] that is an upper limit. In this case, the incident angle of illumination light which becomes incident on the reflective reticle is so large that a shadow of the absorbing layer on the pattern adversely affects a projected image. This state may be undesirable also because it is difficult to optimize the thickness and transparency of the absorbing layer in reticle design. From the viewpoint of ensuring a given fidelity of a projected image and a given degree of freedom of reticle design, the tilt angle of the object plane can be minimized while taking account of the positioning of illumination system mirrors. Hence, the tilt angle θ can satisfy:
sin−1(NAO)<θ<sin−1(NAO)+3.0 (3)
In this embodiment, the first to sixth reflecting mirrors 110 to 160 each include a concave mirror or a convex mirror, which has an aspherical reflecting surface, as described above. However, at least one of the first to sixth reflecting mirrors 110 to 160 is to have an aspherical reflecting surface. Nevertheless, an aspherical reflecting mirror can be formed from the viewpoint of aberration correction. Consequently, the projection optical system 100 according to the present invention includes aspherical reflecting mirrors as many as possible, irrespective of the number of aspherical reflecting mirrors used. An aspherical shape is described by a general expression:
where Z is the coordinate in the optical axis direction, c is the curvature (the reciprocal of a radius of curvature r), h is the height from the optical axis, k is a conic constant, and A, B, C, D, E, F, G, H, and J are the fourth-, sixth-, eighth-, 10th-, 12th-, 14th-, 16-th, 18th-, and 20th-order aspherical coefficients, respectively.
In this embodiment, the six, first to sixth reflecting mirrors 110 to 160 have a Petzval sum around zero in order to even the image plane W of the projection optical system 100. That is, the sum of the refractive powers on the surfaces of reflecting mirrors can be around zero, irrespective of the number of reflecting mirrors used. In other words, the radius of curvature of the i-th mirror from the object plane MS satisfies:
where ri is the radius of curvature of the i-th mirror from the object plane MS.
The surfaces of the first to sixth reflecting mirrors 110 to 160 are coated with multi-layer films which reflect EUV light. The multi-layer film acts to allow light beams to reinforce each other. Examples of a multi-layer film applicable to the first to sixth reflecting mirrors 110 to 160 in this embodiment are an Mo/Si multi-layer film formed by alternately stacking molybdenum (Mo) layers and silicon (Si) layers on a reflecting surface, and an Mo/Be multi-layer film formed by alternately stacking Mo layers and beryllium (Be) layers on a reflecting surface. If light having a wavelength range around a wavelength of 13.4 nm is used, a reflecting mirror formed from a Mo/Si multi-layer film can obtain a reflectance of 67.5%. Also, if light having a wavelength range around a wavelength of 11.3 nm is used, a reflecting mirror formed from a Mo/Be multi-layer film can obtain a reflectance of 70.2%. However, a multi-layer film usable in the present invention is not limited to the above-mentioned materials, and a description of these materials is not intended to restrict the use of a multi-layer film having the same action and effect as them.
The experimental results of illumination using the projection optical system 100 according to this embodiment and the reflection projection optical system 300 as a comparison target will be described. The reflection projection optical system 300 has almost the same specification as that of the projection optical system 100 according to this embodiment, but has non-telecentricity on the object side.
In
an NA=0.25;
a reduction magnification=⅕×;
an arcuated image plane with an object height=130 mm to 140 mm, an image height=26 mm to 28 mm, and a width of 2.0 mm (corresponding to an arcuated slit, shown in
an overall length=1386 mm (the overall length in the optical axis direction at an object height=135 mm).
Table 1 shows the numeric values (the radius of curvature R, the surface interval D, the refractive index N, the tilt angle θ, the conic constant k, and the aspherical coefficients) of the projection optical system 100 according to this embodiment shown in
Note that the tilt angle is the angle with respect to a plane perpendicular to the optical axis.
In the projection optical system 100 shown in table 1, θ=6.0°, NAO=0.05, and sin−1(NAO)=2.866°. Since 2.866°<6.0°<2.866°+5.0°, the projection optical system shown in table 1 satisfies inequality (2).
The tilt angle θ of the reflective reticle MS can be changed by taking account of the positioning of illumination system mirrors. For example, the projection optical system also satisfies inequality (3) for 0=3.0°because 2.866°<3.0°<2.866°+3.0°. In this case, the surface interval D of the reflective reticle MS in table 1 is 803.6179804 mm.
The projection optical system satisfies inequality (2) for 0=7.0° because 2.866°<7.0°<2.866°+5.0°. In this case, the surface interval of the reflective reticle MS in table 1 is 794.1322645 mm.
Assume that the projection optical system 100 has perfect telecentricity on the image side. Then, a tangent function which describes the telecentricity characteristic on the object side has a maximum value of 0.000342 at an object height of 130 mm, and has a minimum value of 0.000022 at an object height of 140 mm. This means that the projection optical system 100 has nearly perfect telecentricity on the object side.
Assuming that deterioration in evenness of the reflective reticle MS is 100 nm, the tangent value for a projected image at an object height of 130 mm is 0.000342. Moreover, since a reduction magnification=⅕×, an image shift on the image plane is 0.00684 nm. This means that the adverse effect that the image shift inflicts on the overlay performance is negligible.
An NA variable aperture ST is positioned on the second reflecting mirror 120, and defines all light beams involved in imaging. Table 2 shows the NA on the image plane corresponding to each object height at this time.
NA(meri) is the meridional NA, NA(sagi) is the sagittal NA, the ΔNA image height difference is the maximum difference in NA for all image heights, and ΔNA(m-s) is the difference between the meridional and sagittal NAs at each image height. Table 2 reveals that the difference in NA between image heights is small.
The reflection projection optical system 300 shown in
an NA=0.25;
a reduction magnification=⅕×;
an arcuated image plane with an object height=130 mm to 140 mm, an image height=26 mm to 28 mm, and a width of 2.0 mm (corresponding to an arcuated slit, shown in
an overall length=1400 mm.
The reflection projection optical system 300 has almost the same specification as that of the projection optical system 100 according to this embodiment shown in
Table 3 shows the numeric values (the radius of curvature R, the surface interval D, the refractive index N, the tilt angle θ, the conic constant k, and the aspherical coefficients) of the projection optical system 300 shown in
Note that the tilt angle is the angle with respect to a plane perpendicular to the optical axis.
Assume that the projection optical system 300 has perfect telecentricity on the image side. Then, a tangent function which describes the telecentricity characteristic on the object side has a minimum value of 0.099558 at an object height of 130 mm, and has a maximum value of 0.107101 at an object height of 140 mm. This means that the projection optical system 300 has non-telecentricity on the object side. The angle of the principal ray with respect to the reflective reticle MS is assumed to be nearly 6° that is equal to that of the projection optical system 100 according to this embodiment for the sake of comparison.
Assuming that deterioration in evenness of the reflective reticle MS is 100 nm, the tangent value for a projected image at an object height of 140 mm is 0.107101. Moreover, since a reduction magnification=⅕×, an image shift on the image plane is 2.14 nm. This means that the adverse effect that the image shift inflicts on the overlay performance is too large to ignore.
An NA variable aperture ST is positioned on the second reflecting mirror 320, and defines all light beams involved in imaging. Table 4 shows the NA on the image plane corresponding to each object height at this time.
When compared to table 2 according to this embodiment, the ΔNA image height differences in meridional and sagittal NAs are 4.4 times and 5.7 times, respectively, and the maximum value of ΔNA(m-s) is 1.5 times. Hence, the projection optical system 100 according to this embodiment can reduce the difference in NA in the effective field region.
As described above, the projection optical system 100 according to this embodiment serves as an EUV imaging system which increases the degree of freedom of the positioning of illumination system mirrors, can attain good CD uniformity over the entire effective field region, and reduces degradation in overlay performance.
An exposure apparatus 200 to which a reflection projection optical system 100 according to the present invention is applied will be described below with reference to
The exposure apparatus 200 includes an illumination system 210, a reticle MS, a reticle stage 220 which mounts the reticle MS, the reflection projection optical system 100, a substrate (wafer) W as a processing object, a substrate stage 230 which mounts the substrate W, and a controller 240. The controller 240 is controllably connected to the illumination system 210, reticle stage 220, and substrate stage 230.
Although not shown in
The illumination system 210 illuminates the reticle MS with arcuated EUV light (wavelength: 13.4 nm etc.) corresponding to the arcuated effective field region of the reflection projection optical system 100. The illumination system 210 includes a light source and illumination optical system (neither is shown). The principal ray of the illumination light becomes incident on the reticle MS at an incident angle of 6.0° with respect to a normal to the reflecting surface of the reticle MS positioned to be tilted at 6.0° counterclockwise within the paper plane with respect to a plane (not shown) perpendicular to an optical axis OA of the reflection projection optical system 100. Note that the light source and illumination optical system which constitute the illumination system 210 can take any forms known to those skilled in the art, and a detailed description thereof will not be given in this specification. For example, the illumination optical system includes a condensing optical system, optical integrator, aperture stop, and blade, and can adopt any technique which can be assumed and achieved by those skilled in the art.
The reticle MS is a reflective reticle. The reticle MS has a circuit pattern (or an image) to be transferred formed on it, is supported by the reticle stage 220, and is driven within the X-Y plane while maintaining a tilt angle of 6.0°. Diffracted light generated by the reticle MS is projected onto the substrate W upon being reflected by the reflection projection optical system 100. The reticle MS and substrate W are positioned in an optically conjugate relationship. Since the exposure apparatus 200 is of the step & scan scheme, it reduces and projects the pattern of the reticle MS onto the substrate W by scanning them.
The reticle stage 220 supports the reticle MS and is connected to a moving mechanism (not shown). The reticle stage 220 can take any form known to those skilled in the art. The moving mechanism (not shown) includes, for example, a linear motor, and can move the reticle MS by driving the reticle stage 220 in at least the Y direction under the control of the controller 240. The exposure apparatus 200 scans the reticle MS and substrate W while the controller 240 synchronizes them.
The reflection projection optical system 100 is a catoptric system, which reduces and projects the pattern on the surface of the reticle MS onto the image plane. The reflection projection optical system 100 can take any of the above-mentioned forms, and a detailed description thereof will not be given. Although the reflection projection optical system 100 shown in
Although the substrate W is a wafer in this embodiment, it includes a wide variety of processing objects such as a liquid crystal substrate. The substrate W is coated with a photoresist. The photoresist coating step includes a preprocess, a process of coating the substrate with an adhesion enhancing agent, a process of coating the substrate with a photoresist, and a pre-baking process. The preprocess includes, for example, cleaning and drying. The process of coating the substrate with an adhesion enhancing agent is a surface modification process for enhancing the adhesion between a photoresist and the underlying layer, that is, a process of imparting a hydrophobic property to the substrate by coating it with a surfactant. In this process, the substrate is coated with an organic film such as HMDS (hexamethyldisilazane) or steamed. The pre-baking is a baking process but serves to more softly bake the photoresist than in baking after developing the photoresist. In this process, the solvent is removed.
The substrate stage 230 supports the substrate W. The substrate stage 230 moves the substrate W as a processing object in the X, Y, and Z directions using, for example, a linear motor. The reticle MS and substrate W are synchronously scanned under the control of the controller 240. Also, the reticle stage 220 and substrate stage 230 are driven at a constant speed ratio while a laser interferometer, for example, monitors their positions. Although the optical axis OA and the X direction are parallel in
The controller 240 includes a CPU and memory (neither is shown) and controls the operation of the exposure apparatus 200. The controller 240 is electrically connected to the illumination system 210, the reticle stage 220 (i.e., a moving mechanism (not shown) for the reticle stage 220), and the substrate stage 230 (i.e., a moving mechanism (not shown) for the substrate stage 230). The CPU includes all processors such as an MPU irrespective of their names, and controls the operation of each unit. The memory includes a ROM and RAM, and stores firmware which operates the exposure apparatus 200.
In exposure, EUV light emitted by the illumination system 210 illuminates the reticle MS to form an image of the pattern on the surface of the reticle MS on the surface of the substrate W. In this embodiment, the image plane has an arcuated (ring-like) shape, and the entire surface of the substrate W is exposed by scanning the reticle MS and substrate W at a speed ratio equal to the reduction magnification ratio.
An exemplary method of manufacturing devices such as a semiconductor integrated circuit device and a liquid crystal display device using the exposure apparatus 200 will be explained next.
The devices are manufactured by a step of transferring by exposure a pattern formed on a reticle onto a substrate, a step of developing the exposed substrate, and other subsequent steps of processing the developed substrate. The other subsequent steps include, for example, etching, resist removal, dicing, bonding, and packaging steps.
Although embodiments of the present invention have been explained above, the present invention is not limited to these embodiments as a matter of course, and various modifications and changes can be made without departing from the spirit and scope of the present invention. For example, the number of times of imaging using the reflection projection optical system 100 may be one (see
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2008-277376, filed Oct. 28, 2008, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-277376 | Oct 2008 | JP | national |
