WO2000055686A1

WO2000055686A1 - Passive shaped chuck for correcting field curvature

Info

Publication number: WO2000055686A1
Application number: PCT/US1997/008588
Authority: WO
Inventors: Joseph Lyons
Original assignee: Joseph Lyons
Priority date: 1997-05-21
Filing date: 1997-05-21
Publication date: 2000-09-21

Abstract

A wafer chuck (20) used in a photolithographic apparatus having projection optics with a field curvature. A wafer chuck (20) having a curved surface (24) that matches the field curvature of the projection optics (14) holds a wafer (18) along a curved surface compensating for the field curvature. The wafer (18) being somewhat flexible is conformed to the curved surface of the wafer chuck (20) with a conventional vacuum or electrostatic holding techniques, and provides better imaging without having to compromise positioning as with a flat wafer. The present invention is particularly suitable to projection optics having a limited depth of focus in which a flat wafer is difficult to position or must be positioned with a portion of the wafer possibly not obtaining good imaging.

Description

PASSIVE SHAPED CHUCK FOR CORRECTING FIELD CURVATURE

Field of the Invention

The present invention relates generally to lithography or photolithography as used in the manufacture of semiconductor devices, and more particularly to a wafer chuck.

Background of the Invention

In the manufacture of semiconductor devices, photolithography is often used. The image of a reticle containing a circuit pattern is projected onto a semiconductor wafer with a photosensitive resist. As the desired featured sizes become smaller, in the sub-micron ranges, and as the optics become more advanced with higher numerical apertures and limited depth of focus, handling of the wafer becomes critical. Wafers are typically held on a flat chuck by a vacuum or electrostatic force. Because the wafer is not completely flat, it has often been necessary to calculate the plane of best fit for the wafer with the flat chuck holding the wafer in this plane of best fit. The wafer is then positioned in the best fit optical plane. Any deviations in the topology of the wafer from this best fit plane could create errors in imaging that may result in lower yields. One solution to the positioning of wafers is disclosed in U.S. Patent 5,094,536 entitled "Deformable Wafer Chuck" issuing to MacDonald et al on March 10, 1992, which is herein incorporated by reference. Therein disclosed is a deformable wafer chuck having a plurality of distortive actuators, such as piezoelectric crystals, which cause the wafer to be selectively deformed. This cancels the warpage of the wafer to permit reduced distortion of a projected pattern. An interferometer system is also included for activating the actuators to provide realtime correction of wafer flatness and also permit adjustment of depth of focus. While the use of the plurality of actuators helps correct for the wafer warpage or unevenness, it is a relatively complicated structure and may not completely remedy many wafer positioning problems. Therefore, there is a need for a simpler, passive structure that can position the wafer in a more desirable position or shape.

Summary of the Invention

The present invention is directed to a wafer chuck that is matched to the projection optics to optimize the exposure of the wafer and achieve or approach the theoretical depth focus for the projection optics used in a lithography or photolithography. The field curvature of the projection optics used in lithography often are not flat, but may have a slight curve. The wafer chuck is shaped such that the wafer chuck is curved to match the field curvature of the projection optics. In a lithography or photolithography tool that is not scanned, the curve is spherical. In a lithography or photolithography tool that incorporates scanning along one axis, the wafer chuck is curved along one axis forming a portion of a cylinder. The wafer, when placed on the curved chuck, will flex to match the shape of the chuck. As a result, it is possible to approach more closely the theoretical depth of focus of the projection optics in a lithography or photolithography tool.

Accordingly, it is an object of the present invention to decrease focus error due to optical field curvature.

It is another object of the present invention to increase the usable depth of field in a lithography tool.

It is an advantage of the present invention that it is relatively simple and has a passive construction.

It is a feature of the present invention that the surface of the wafer chuck is curved.

These and other objects, advantages, and features will become readily apparent in view of the following detailed description.

Brief Description of the Drawings

Fig. 1 is a block diagram illustrating the present invention.

Fig. 2 schematically illustrates the present invention.

Fig. 3 is a perspective view illustrating one embodiment of the present invention. Detailed Description of the Preferred Embodiments

Fig. 1 is a block diagram illustrating the present invention as used with a scanning optical photolithography tool. Illumination source 10 illuminates a reticle 12 containing a circuit pattern thereon. Projection optics 14, in combination with magnification correction optics 16, projects the image of a reticle 12 onto a wafer 18. Wafer 18 may be held by any conventional method, such as a vacuum or electrostatic force, placed on curved chuck 20. The curved chuck 20 has a spherical or a cylindrical curve. The curved chuck 20 is mounted on a wafer stage 22. The wafer stage 22 may have six degrees of freedom. The reticle 12 is attached to a reticle stage 11 which is controlled by controller 17 attached to wafer stage 22. The reticle 12 is then moved at a rate proportional to the movement of the wafer stage 22 resulting in exposure of the entire wafer 18.

Fig. 2 more clearly illustrates the curved surface of the curved chuck 20. The wafer chuck 20 has an overall or global curved surface 24. A wafer 18 placed on the surface is held in place by a vacuum or electrostatic force, not shown, and will conform to the curved surface 24. Projection optics 14' project an image of the reticle, illustrated in Fig. 1, onto a portion of the surface of a wafer 18. The wafer 18 generally has a photosensitive resist coating which is subsequently processed, forming the circuit patterns on the wafer 18. The projection optics 14' have a curvature of field or curve of best focus 26. This curve of best focus is a characteristic of many advanced projection optics and is a lens aberration that causes a flat object surface to be imaged onto a curved surface rather than a plane. Generally, the flat reticle is imaged with projection optics 14' that have a slight field curvature. The projection optics 14' image only a portion of the wafer 18 at any one time. The wafer 18 is then either stepped or scanned across the image of the reticle projected by the projection optics 14'. The field curvature 26 of the projection optics over this local or small section is matched to or compliments the overall or global curve of the curved chuck 20.

Fig. 3 is a perspective view illustrating a curved chuck 120 attached to a wafer stage 122. The curved chuck 120 has a cylindrically curved surface 124. Wafer 18 is placed on the cylindrically curved surface 124 and is held in place by a vacuum or electrostatic force. With the use of the cylindrical curved surface 124, the wafer 18 is positioned to obtain the best focus considering the optical field curvature of the projection optics being used. The cylindrically curved surface 124 is advantageous in a scanning photolithography tool when the stage 122 is scanned in a direction indicated by arrow 128 along the longitudinal axis of the axis of rotation of the cylinder forming the cylindrically curved surface 124. While use in a scanning photolithography system only requires a cylindrical section, a spherical section will function adequately. It should be appreciated that in a scanning photolithography system the reticle is also scanned at a rate in proportion to the rate of scan of the wafer.

The operation of the present invention, with reference to Figs. 1, 2 and 3, can readily be appreciated. The curved or shaped chuck 20 or 120 has either a spherical section or a cylindrical section respectfully. The spherical section is preferably used in a stepper photolithography system that has projection optics 14 or 14' having a field curvature. Accordingly, projection optics may be used that would normally have an unacceptable degree of field curvature when used with a conventional flat chuck for holding a wafer. Even in a photolithography tool having projection optics with a sufficiently deep depth of focus to accommodate a flat chuck, the present invention could be used to optimize the imaging and reduce positioning errors.

The wafer stage 22 or 122, on which the chuck 20 or 120 is positioned, may be any wafer stage and preferably one that has six axis degrees of freedom which provides for local leveling. Accordingly, the present invention has the advantage that it results in a wafer that appears to be exposed by a photolithography tool with significantly less field curvature. While a slight degree of distortion may result due to the bending of the wafer 18, on a stepper this distortion can easily be corrected with an optical magnification adjustment, such as magnification correction optics 16. On a photolithography tool using scanning along one axis, the cylindrical curve introduces magnification distortion along one axis only which can also be corrected with magnification correction optics.

Although the preferred embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit and scope of this invention.

Claims

What is claimed is:

1. A wafer chuck for use in a photolithography tool having projection optics with field curvature comprising: a wafer chuck, said wafer chuck having a curved surface matching in at least one axis the field curvature of the projection optics.

2. A wafer chuck as in claim 1 wherein: the curved surface is spherical.

3. A wafer chuck as in claim 1 wherein: the curved surface is cylindrical.

4. A photolithography tool comprising: an illumination source; projection optics receiving electromagnetic radiation from said illumination source, said projection optics having a field curvature; a wafer chuck having a curved surface matching the field curvature in at least one axis of said projection optics; and a wafer stage attached to said wafer chuck, whereby a reticle is imaged by said projection optics onto a wafer held by said wafer chuck.

5. A scanning photolithography tool comprising: an illumination source; a reticle; a reticle stage attached to said reticle; projection optics receiving electromagnetic radiation from said illumination source, said projection optics having a field curvature; a wafer chuck having a cylindrically curved surface having a longitudinal axis matching the field curvature in at least one axis of said projection optics; a wafer stage attached to said wafer chuck, said wafer stage having six degrees of freedom; a controller coupled to said reticle stage and said wafer stage, said controller controlling a first scanning rate of movement of said reticle stage to be proportional to a second scanning rate of movement of said wafer stage, the movement of said reticle stage and said wafer stage being in a direction along an axis parallel to the longitudinal axis of the cylindrically curved surface of said wafer chuck, whereby a reticle is imaged by said projection optics onto a wafer held by said wafer chuck.