PROJECTION OBJECTIVE FOR MICROLITHOGRAPHY

Abstract

A projection objective for imaging a pattern arranged in an object surface of the projection objective into an image surface of the projection objective with a demagnified imaging scale has a plurality of optical elements which are arranged along an optical axis of the projection objective and are configured in such a way that a defined image field curvature of the projection objective is set in such a way that an object surface that is curved convexly with respect to the projection objective can be imaged into a planar image surface. What can be achieved given a suitable setting of the object surface curvature is that a gravitation-dictated bending of a mask does not have a disturbing effect on the imaging quality.

Claims

1. A projection objective for imaging a pattern arranged in an object surface of the projection objective into an image surface of the projection objective with a demagnified imaging scale, having: a plurality of optical elements which are arranged along an optical axis of the projection objective and are configured in such a way that a defined image field curvature of the projection objective is set in such a way that an object surface that is curved convexly with respect to the projection objective can be imaged into a planar image surface.

2. The projection objective as claimed in claim 1, the object surface being curved in such a way that an effective object surface curvature in at least one direction perpendicular to the optical axis essentially corresponds to a surface curvature which results from a gravitation-dictated mask bending of the mask.

3. The projection objective as claimed in claim 1, the projection objective being designed for use with a mask that can be moved in a scanning direction, and the object surface being curved in such a way that an object surface curvature which is scanner-integrated in the scanning direction corresponds to the surface curvature caused by gravitation-dictated bending of the mask.

4. The projection objective as claimed in claim 1, wherein at least one optical element of the projection objective bears at one nonrotationally symmetrical surface.

5. The projection objective as claimed in claim 4, the nonrotationally symmetrical surface being a toric surface designed for influencing the image field curvature.

6. The projection objective as claimed in claim 1, the projection objective essentially being corrected with regard to all field-dependent image errors, with the exception of the image field curvature.

7. The projection objective as claimed in claim 1, the projection objective essentially being corrected with regard to all image errors, with the exception of the image field curvature.

8. The projection objective as claimed in claim 1, the projection objective having an image-side numerical aperture NA>0.8.

9. The projection objective as claimed in claim 1, wherein |R.sub.p|0.1 DOF holds true, where |R.sub.p| is the magnitude of the Petzval sum and DOF is the depth of focus of the projection objective.

10. A projection objective for imaging a pattern arranged in an object surface of the projection objective into an image surface of the projection objective with a demagnified imaging scale, having: a plurality of optical elements which are arranged along an optical axis of the projection objective and are configured in such a way that a defined image field curvature of the projection objective is set in such a way that an object surface that is curved convexly with respect tp the projection objective can be imaged into a planar image surface, the object surface being curved in such a way that an effective object surface curvature in at least one direction perpendicular to the optical axis essentially corresponds to a surface curvature which results from a gravitation-dictated mask bending of the mask, and |R.sub.p|0.1 DOF holding true, where |R.sub.p| is the magnitude of the Petzval sum and DOF is the depth of focus of the projection objective.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 schematically shows, in an oblique perspective illustration, an excerpt from a microlithography projection exposure apparatus with an embodiment of a projection objective according to the invention;

[0030] FIG. 2 shows a schematic illustration of the rotationally symmetrical, curved object surface of the projection objective in FIG. 1 with a scanner slot;

[0031] FIG. 3 shows a schematic illustration of the form of the effective object surface of the projection objective which is produced as a result of scanning movement;

[0032] FIG. 4 shows a measured bending of a standard reticle in a schematic illustration;

[0033] FIG. 5 shows a schematic illustration of a bent reticle for calculating the theoretically expected mask bending; and

[0034] FIG. 6 shows a schematic illustration for quantifying the image field curvature.

DETAILED DESCRIPTION

[0035] FIG. 1 schematically shows the essential component parts of a microlithography projection exposure apparatus in the form of a wafer scanner 1 provided for the production of large-scale integrated semiconductor components by means of projection lithography. The projection exposure apparatus 1 comprises, as light source, an excimer laser (not shown) having an operating wavelength of 193 nm, other operating wavelengths, for example 157 nm or 248 nm, also being possible. A downstream illumination system 3, of which only the light exit region is shown, generates in its exit surface 4 a large, sharply delimited illumination field that is illuminated very homogeneously and is adapted to the telecentric requirements of the downstream projection objective 5. The illumination system 3 has devices for selection of the illumination mode and, in the example, can be changed over between conventional illumination with a variable degree of coherence, annular field illumination and dipole or quadrupole illumination.

[0036] In the direction of light propagation downstream of the illumination system there is arranged a device 40 (reticle stage) for holding and manipulating a mask (reticle) 6 such that the latter lies in the object surface 4 of the projection objective 5 and can be moved in a traveling direction (scanning direction) 7 (y direction) with the aid of a scanner drive 41 for scanning operation.

[0037] Downstream of the object surface 4, the curved form of which will be explained in more detail with reference to FIG. 2, there follows at a suitable distance (object-side operating distance) the reduction objective 5, which images an image of the mask, with a reduced scale of 4:1, onto a wafer 10 coated with a photoresist layer. Other reduction scales, e.g. 5:1 or 10:1 or less, are likewise possible. The wafer 10 serving as a light-sensitive substrate is arranged such that its planar substrate surface 11 with the photo-resist layer essentially coincides with the planar image plane 12 (depicted in dashed fashion) of the projection objective 5. The wafer is held by a device 50 (wafer stage) comprising a scanner drive 51 in order to move the wafer synchronously with the mask 6 parallel to the latter.

[0038] The projection objective 5 is incorporated into the wafer scanner such that its optical axis 13 is oriented vertically and thus parallel to the effective direction g of the force of gravity. The mask mount 40 is designed such that, apart from the force of gravity, no imposed forces which might lead to a deformation of the mask 6 occur at the reticle 6 placed on said mount. Outside the region through which the illumination radiation is to radiate, the transmission mask 6 is mounted on suitable bearing surfaces (or support surfaces) which are at a constructionally predetermined bearing distance from one another (cf. FIG. 5).

[0039] Between the bearing surfaces, the reticle 6 is freely suspended and is exposed to the force of gravity g, which causes a gravitation-dictated mask bending. Depending on the type of reticle and the bearing geometry, a gravitation-dictated bending is established in this case which is always present in essentially the same way as a systematic contribution and, in the case of conventional Petzval-corrected systems, would be converted into a bending of the image of the mask with the square of the imaging ratio. Given a standard size of currently used quartz glass reticles of 6 inches by 6 inches given a typical thickness of 6.35 mm, typical instances of gravitation-dictated bending may be in the range of between 300 and 400 nm depending on the bearing geometry. In the case of typical conventional systems that are optimized for imaging a planar object surface into a planar image surface, this reticle bending, given an imaging scale of 4:1, would lead to an image field curvature of the order of magnitude of between 20 and 25 nm. This indication of the image field curvature relates to the maximum excursion s of the image field IF in the image field center (at the optical axis OA) in comparison with the axial position of the image field at the edge of the image field, or to a deviation smeasured in the axially parallel directionof the curved image field from a plane IM lying perpendicular to the optical axis at the image field edge (cf. FIG. 6). This image field curvature becomes more critical the smaller the available depth of focus DOF of the projection system. Although it is possible to obtain a good compromise between sagittal and tangential image shell with the aid of manipulators by shifting lenses or displacing them in some other way, this is always accompanied by induced astigmatism on account of the Petzval condition.

[0040] These problems are avoided in the case of the embodiment of the projection objective 5 shown. The projection objective 5 is designed for imaging an object surface 4 that is curved convexly with respect to the projection objective (FIG. 2) into a planar image plane 12. Thus, in contrast to conventional systems, the mutually optically conjugate surfaces do not have the same curvature state or a corresponding curvature state transformed by way of the imaging scale, rather a curvature-altering imaging process is provided. In this case, the projection objective 5 is designed such that all image errors, with the exception of the image field curvature, are completely corrected within narrow tolerances. By contrast, the image field curvature is altered by the projection objective 5 such that a reticle 6 bent with respect to the projection objective can be sharply imaged onto a planar wafer over the entire image surface.

[0041] The gravitation-dictated bending of the reticle is cylindrical to a first approximation. A complete bias for compensation of this warpage is not possible in a rotationally symmetrical objective design. It can be approximated, however. The situation is different in the case of a scanner objective, that is to say a projection objective provided for use in a wafer scanner. On account of the scanning operation running in the y direction, a rotationally symmetrically curved object surface 4 (FIG. 2) of the projection objective is translated into an effectively cylindrical object surface 4 of the scanner system (FIG. 3). This effect results from the fact that only the slotted excerpt 10 which is depicted centrally in FIG. 2 and corresponds to the illuminated scanner slot is used for imaging. A movement of the approximately cylindrically curved region of the scanner slot in the y direction produces the cylindrically curved effective object surface 4 in FIG. 3. The curvature thereof is adapted, by the means for influencing the image field curvature that are provided within the projection objective 5, to the reticle geometry of the bent reticle such that the mask structure to be imaged essentially coincides with the effective cylindrically curved object surface 4, which is optically conjugate with respect to the image plane 12. The mask 6 bent in the direction of the projection objective can thereby be sharply imaged onto the planar wafer 10 over the entire image field diameter. A projection objective designed for an object surface which is curved convexly with respect to the projection objective and which is curved in such a way that the scanner-integrated object surface curvature corresponds to the gravitation-dictated reticle bending will accordingly on average sharply image a bent reticle into a planar image shell. As a result, the gravitation-dictated portion in the focus budget is obviated and a stabler exposure process is possible.

[0042] Taking account of the reticle bending in the design of the projection objective can also be applied, in principle, to stepper systems. In this case, it is advantageous to generate the intervention in the image field curvature with the aid of nonrotationally symmetrical, for example, toric, surfaces which may be applied on one or more lenses. Suitable aspheric forms are dependent on the bearing geometry of the reticle in this case.

[0043] In order to explain the required order of magnitude of the image shell overcorrection of the projection objective 5, FIG. 4 firstly shows, in a perspective illustration, the measured, essentially cylindrically curved profile of the surface of a standard reticle (reticle size 66 inches, thickness 6.35 mm, material: quartz glass) bent in a gravitation-dictated manner. FIG. 5 illustrates the conditions required for deriving the suitable object surface curvature. The reticle 6 bears on two bearing surfaces 70 which are at a lateral bearing distance LA from one another. The bearing distance is greater than the object field diameter F, in order that the imaging is not disturbed by the bearings. The reticle has a thickness d and is composed of a mask material having density p and modulus of elasticity E. Under the action of the force of gravity g, a reticle bending s is established which, in this illustration, is defined as the maximum excursion of the reticle in the g direction with regard to the reference plane 75 which is illustrated in dashed fashion and is defined by the bearing surface's. A circle arc 4 which is defined by the bent reticle surface and represents the optimum profile of the curved object surface for this bent reticle corresponds to said bending s. The radius of the circle arc 4 corresponds to the object surface radius of curvature OFCR of the object surface in this direction running perpendicular to the optical axis.

[0044] Given this schematic geometry of the reticle mount, the theoretically expected bending of the reticle results in accordance with:

[00001]$s=\frac{.Math..Math.g}{4.Math.E}.Math.\frac{{\mathrm{LA}}^4}{d^2}.Math.\frac{F^2}{{\mathrm{LA}}^2}$

[0045] An explanation will now be given in connection with FIG. 6 with regard to the image field curvature to which said bending leads on the image side of the projection objective and what extent of the Petzval correction is necessary for compensation of this effect. In this respect, FIG. 6 shows the image-side end of the projection objective, together with the region of the image field IF. The object-side reticle bending s is translated into an image-side image field curvature s with the square of the imaging scale in accordance with s=.sup.2.Math.s. In this case, the image field curvature is parameterized by a deviation smeasured in the axially parallel directionof the curved image field IF from a plane IM lying perpendicular to the optical axis OA at the edge of the image field IF. The edge of the image field is at a distance h (image-side image height) from the optical axis OA. In the sectional plane shown in FIG. 6, the image field IF is curved spherically to a good approximation, so that it lies on a circle arc having a radius R.sub.p. This is the Petzval radius, for which the following holds true in accordance with the circle equation for long radii to a good approximation: R.sub.p=h.sup.2/(2s). The Petzval sum 1/R.sub.p of a system which can image a mask having bending s into a planar image plane thus results from the following equation:

[00002]$1.Math.\text{/}.Math.R_p=\frac{2.Math.{}^2.Math.s}{h^{{}_2}}$

[0046] This estimation permits a corresponding bias of the image field curvature to be provided in a projection objective in order to take account of the effects of a gravitation-dictated reticle bending on the imaging quality.

[0047] A projection objective can be adapted by means of a fixedly predetermined bias to the expected bending of typical reticles. It is also possible to perform a dynamic adaptation by providing suitable manipulators within the projection system in order, upon transition to other types of reticles, by way of example, to be able to perform a changed adaptation without reconstructing the projection objective. Suitable manipulators are, in particular, devices which bring about radii changes and/or refractive index changes within the projection objective. Refractive index changes may be brought about for example by means of pressure changes and/or temperature changes in the gas in lens interspaces. Radii changes may be introduced by active optical components, e.g. by active mirrors. Heating or cooling a lens may lead to a change in refractive index and dimensioning of the lens and therefore be utilized as a manipulator.