Abstract:
A method and apparatus for correcting overlay errors in a lithography system. During lithographic exposure, features being exposed on the wafer need to overlay existing features on the wafer. Overlay is a critical performance parameter of lithography tools. The wafer is locally heated during exposure. Thermal expansion causes stress between the wafer and the wafer table, which will cause the wafer to slip if it exceeds the local frictional force. To increase the amount of expansion allowed before slipping occurs, the wafer chuck is uniformly expanded after the wafer has been loaded. This creates an initial stress between the wafer and the wafer table. As the wafer expands due to heating during exposure, the expansion first acts to relieve the initial stress before causing an opposite stress from thermal expansion. The wafer may be also be heated prior to attachment to the wafer chuck, creating the initial stress as the wafer cools.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 10/780,877, filed Feb. 19, 2004, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed generally to photolithography. More particularly, the present invention relates to wafer alignment in a photolithographic system. 
     2. Related Art 
     Photolithography (also called microlithography) is a semiconductor device fabrication technology. Photolithography uses radiation, such as ultraviolet or visible light, to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to implement photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) or spatial light modulator (SLM) for creating a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle or SLM with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle circuit pattern onto the wafer. 
     As semiconductor device manufacturing technology advances, there are ever increasing demands on each component of the photolithography system used to manufacture the semiconductor device. This includes increasing demands on the accuracy of the wafer alignment. A wafer is typically mounted on a wafer chuck, also referred to as a wafer table. During exposure, the features being exposed on the wafer need to overlay existing features on the wafer. To achieve overlay performance, the wafer is aligned to the wafer stage prior to exposure. Any movement of the wafer relative to the wafer stage after alignment results in overlay errors. 
     During exposure, the wafer is heated locally due to the energy transferred to the wafer from the exposure beam. This heating causes the wafer to expand. If the wafer expansion is unchecked, the expansion exceeds overlay error requirements. Clamping the wafer to the wafer chuck reduces the amount the wafer expands. The wafer chuck is typically designed to have a larger thermal mass than the wafer and is manufactured of a material which has very low thermal expansion. This results in relatively little expansion of the wafer chuck relative to the wafer. The wafer chuck is also typically designed to be much stiffer than the wafer, such that if the wafer is sufficiently clamped to the wafer chuck, the thermal expansion of the wafer is reduced. 
     If the clamping force between the wafer and the wafer chuck is not sufficient to prevent wafer expansion, the wafer can slip on the wafer chuck and larger wafer expansion will occur, resulting in larger overlay errors. 
     Slipping due to wafer expansion can be reduced by tightly clamping the wafer to the surface of the wafer chuck with a vacuum. This creates a frictional force between the wafer and the wafer chuck. However, if the wafer expansion force exceeds the frictional force, the wafer will slip, causing an overlay error. In extreme ultraviolet (“EUV”) systems, the chances of slipping increase because the environment surrounding the wafer during exposure is also a vacuum. Electrostatic clamping, which is much weaker than vacuum clamping, must thus be used in lieu of a vacuum clamp. 
     Therefore, what is needed is a system and method for reducing the effects of wafer expansion during exposure. 
     SUMMARY OF THE INVENTION 
     The present invention reduces wafer slipping by uniformly expanding the wafer chuck after the wafer has been attached. This creates an initial stress on the interface between the wafer and the wafer chuck, rather than a zero stress interface. Because the wafer chuck expands in relation to the wafer, the initial stress is opposite that caused by wafer expansion during exposure. As the wafer heats up from exposure, the initial stress will first be reduced to a zero-stress interface. Only after this point will the expansion of the wafer create an expansion stress on the interface between the wafer and wafer chuck. Ideally, the amount of heating without wafer slipping could be doubled with the present invention. 
     The wafer table expansion can be achieved in several ways. In one embodiment, a sealed circular tube, or annular ring, is attached to the circumference of the wafer chuck. The annular ring is then pressurized. The annular ring expands when pressurized, thereby expanding the wafer chuck to which it is attached. In a similar embodiment, the annular ring is not attached to the edge of the wafer chuck, but is embedded inside the wafer chuck through a groove or cavity. 
     In another embodiment, a plurality of force actuators is attached to the edge of the wafer chuck. These force actuators act on the wafer chuck to expand it. 
     The expansion of the wafer chuck can also be thermally induced. In one embodiment, a heater is directly attached to the wafer chuck. In another embodiment, a proximity heater is placed near the wafer chuck. In still another embodiment, the wafer chuck is made out of an electrically conductive material, and is connected to a power source. In a further embodiment, the wafer is heated before attachment, so that it is warmer relative to the wafer chuck. In this embodiment, when the wafer is attached to the wafer chuck, they reach a thermal equilibrium. As the wafer cools, it contracts and thus creates an initial stress opposite that of an expansion stress. The expansion from each of these embodiments results in nearly uniform expansion of the wafer, similar to an overall magnification of the wafer, and thus can be compensated for in lithographic exposure tools. 
     Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
         FIG. 1  is a flowchart of a method according to an embodiment of the present invention. 
         FIG. 2A  is an illustration of a wafer attached to a wafer chuck. 
         FIG. 2B  is an illustration of a wafer chuck expanding in relation to a wafer. 
         FIG. 2C  is an illustration of a wafer attached to an expanded wafer chuck. 
         FIG. 2D  is an illustration of a wafer expanding in relation to an expanded wafer chuck. 
         FIG. 3A  is an illustration of an expansion system according to an embodiment of the present invention. 
         FIG. 3B  is a cross-section illustration of the expansion system shown in  FIG. 3A . 
         FIG. 4A  is an illustration of another expansion system according to an embodiment of the present invention. 
         FIG. 4B  is a cross-section illustration of the expansion system shown in  FIG. 4A . 
         FIG. 5  is an illustration of another expansion system according to an embodiment of the present invention. 
         FIG. 6A  is an illustration of an expansion system using a heater according to an embodiment of the present invention. 
         FIG. 6B  is an illustration of another expansion system using a heater according to an embodiment of the present invention. 
         FIG. 6C  is an illustration of another expansion system using a heater according to an embodiment of the present invention. 
         FIG. 6D  is an illustration of another expansion system using a heater according to an embodiment of the present invention. 
     
    
    
     The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications. 
     In current lithography systems, wafer slipping is reduced by tightly clamping the wafer to the surface of a wafer chuck. One method of clamping the wafer is by creating a vacuum between the wafer and the wafer chuck. 
     This works because there is a pressure differential between the vacuum and the surrounding environment. In extreme ultra-violet (“EUV”) lithography, however, the environment surrounding the wafer during exposure is also a vacuum. This prevents using a vacuum as a clamping force. 
     Alternatively, electrostatic clamping is used to clamp the wafer to the wafer chuck. A disadvantage of typical electrostatic clamping is that the amount of force achieved with electrostatic clamping is inherently limited. 
     Electrostatic clamping is also related to the time taken to clamp and release the wafer. As a result, electrostatic clamping tends to provide between 1/10 and 1/15 the clamping force of vacuum clamping. This means that the frictional force between the wafer and the wafer chuck also decreases to 1/10 to 1/15 the frictional force in a vacuum system. 
     In most systems, the interface between the wafer and the wafer chuck is approximately a zero stress interface at the time of clamping. This means that there is no force on the interface to counteract the frictional force between the wafer and the wafer chuck. When the wafer is exposed, energy in the exposure beam heats the wafer and causes the wafer to expand. When some parts of the wafer are being exposed while others are not, the expansion causes the wafer to slip if it is not sufficiently clamped. Slipping occurs because the thermally-induced expansion stress exceeds the frictional force holding the wafer in place. This introduces error into the system. In a system where electrostatic clamping is used, the frictional force is low, and the expansion stress does not need to be very large to overcome the frictional force. 
     For EUV systems, wafer heating is likely to be higher than in non-EUV systems. This is a result of a significant amount of non-exposure energy included in the EUV exposure beam being transferred to the wafer. Most of this energy is in the form of infrared (“IR”) radiation. In some exposures, IR energy at the wafer may equal that of the energy needed to expose the wafer (also referred to as the “dose energy”). This effectively doubles the heating at the wafer compared to non-EUV systems. 
     These factors result in a situation where there is 1/10 to 1/15 the resistance to slipping as compared to a vacuum system, and twice the heating. Wafer slipping thus becomes much more likely, causing the dose limit determined by overlay to be small in comparison with the dose allowed for non-EUV systems. 
       FIG. 1  is a flowchart of a method  100  according to an embodiment of the present invention. Method  100  allows application of a larger dose energy before wafer slipping becomes a threat. Although the present invention will be described herein with reference to EUV systems using electrostatic clamping, one skilled in the art will recognize that the present invention may also be used in non-EUV systems and/or lithography systems using clamping methods other than electrostatic clamping. 
     In step  102 , a wafer is attached to a wafer chuck in a lithography system. In one embodiment, the wafer is attached using electrostatic clamping. In another embodiment, the wafer is attached using vacuum clamping. 
     In step  104 , the wafer chuck is uniformly expanded. This creates an initial stress on the interface between the wafer and the wafer chuck. During exposure, due to heat transfer, the size of a wafer increases with respect to the wafer chuck. By expanding the wafer chuck prior to exposure, the size of the wafer is effectively decreased with respect to the chuck. Therefore, the initial stress caused by wafer chuck expansion is opposite the stress caused by wafer expansion. The initial stress may be almost equal to the frictional force between the wafer and the wafer chuck. In this embodiment, additional stress would overcome the frictional force, and cause the wafer to slip prior to exposure. However, by keeping the initial stress just below the magnitude of the frictional force, slippage is prevented. 
     In step  106 , the wafer is aligned to the wafer stage of a lithography system. This alignment centers the wafer in an exposure beam path, and ensures proper focus and alignment of a lithography pattern during exposure. 
     In step  108 , the wafer is exposed, causing the wafer to expand. Because the expansion stress is opposite that of the initial stress, expanding the wafer first acts to relieve the initial stress. Only after relieving the initial stress is a new expansion stress created as a result of the wafer getting larger with respect to the wafer chuck. 
     If the initial stress is almost equal to the frictional force between the wafer and the wafer chuck, the exposure dose may be almost doubled compared to a system having no initial stress. 
       FIGS. 2A to 2D  illustrate the succession of method  100 .  FIG. 2A  illustrates a wafer  202  attached to a wafer chuck  204  (not to scale). Wafer  202  is attached via a clamping method, such as vacuum clamping or, in a preferred embodiment, electrostatic clamping. 
       FIG. 2B  illustrates the expansion of wafer chuck  204 , as occurs in step  104 . Wafer chuck  204  expands uniformly in all directions, as indicated by arrows  206 . During expansion, the size of wafer chuck  204  increases with respect to wafer  202 . 
       FIG. 2C  illustrates a configuration of wafer  202  and wafer chuck  204  immediately before wafer  202  is exposed. Because of the wafer chuck expansion, there is an initial stress between wafer  202  and wafer chuck  204 . Because wafer chuck  204  attempts to stretch wafer  202  as wafer chuck  204  expands, the initial stress on wafer  202  may be referred to as an outward force. 
       FIG. 2D  illustrates the expansion of wafer  202  due to heating during exposure. As wafer  202  increases in size, the outward force is relieved. At some point during the wafer expansion, wafer  202  reaches a point where there is zero stress between wafer  202  and wafer chuck  204 . If wafer  202  continues expanding past this point, an inward force is created on wafer  202 . As long as the magnitude of this inward force does not exceed that of the frictional force between wafer  202  and wafer chuck  204 , wafer  202  will not slip. Wafer chuck  204  also expands due to heating during exposure. The expansion of wafer chuck  204  lessens the expansion rate of wafer  202  relative to wafer chuck  204 . Thus, the magnitude increase of the inward force created on wafer  202  is also lessened. 
     In a preferred embodiment, wafer  202  is a round wafer. In this embodiment, wafer  202  expands uniformly while heating. Since the expansion is uniform, there is no imbalance of the inward force on wafer  202 , and the likelihood of slipping is lessened. Further, if the wafer expands uniformly, the exposure pattern can be magnified to compensate for the change in size. Compensation would be difficult if sections of the wafer expanded non-uniformly. 
       FIG. 3A  is an illustration of an embodiment of a system of the present invention. An annular tube  302  is attached to the outside of wafer chuck  204 . In one embodiment, annular tube  302  is a metal tube. In another embodiment, annular tube  302  is manufactured from a plastic. Annular tube  302  includes a cavity  306 . Cavity  306  can be filled with either liquid or gas. When annular tube  302  is pressurized, it expands. Since annular tube  302  is attached to the edge of wafer chuck  204 , wafer chuck  204  uniformly expands with it.  FIG. 3B  is a cross-section of the illustration in  FIG. 3A , taken at line  304 . As shown, annular tube  302  is attached to the edge of wafer chuck  204 . 
       FIG. 4A  is an illustration of another embodiment of the present invention. Similar to the above embodiment, an annular ring  402  having cavity  406  is attached to wafer chuck  204 . In this embodiment, annular ring  402  is attached inside a cavity or groove in wafer chuck  204 . Because annular ring  402  is embedded into wafer chuck  204 , there is a lesser chance of annular ring  402  detaching from wafer chuck  204 . In addition, non-uniformities caused by materials used to attach the annular ring to the edge of wafer chuck  204  are avoided.  FIG. 4B  is a cross-section of the illustration in  FIG. 4A , taken at line  404 . In one embodiment, annular ring  402  is fully enclosed in the structure of wafer  204 , as shown in  FIG. 4B . In another embodiment, annular ring  402  is only partially embedded in wafer chuck  204 , as in a groove. 
       FIG. 5  is an illustration of another embodiment of the present invention. In this embodiment, a plurality of force actuators  502  are attached on one end to a fixed support  504 , and on the other end to wafer chuck  204 . The actual number of force actuators  502  used is variable. In one embodiment, force actuators  502  are distributed evenly and symmetrically around wafer chuck  204 . When force actuators  502  are activated, they pull on wafer chuck  204 . In this manner, force actuators  502  act together to exert a uniform force on wafer chuck  204  that is outward with respect to wafer chuck  204 . This outward force causes uniform expansion of wafer chuck  204 . 
       FIGS. 6A through 6D  illustrate various embodiments of the present invention in which wafer chuck  204  is expanded through heating. In the embodiment of  FIG. 6A , a contact heater  602  directly heats wafer chuck  204 . As the temperature of wafer chuck  204  increases, wafer chuck  204  expands. Contact heater  602  may be as large as needed in comparison to wafer chuck  204  to cause uniform heating and expansion throughout wafer chuck  204 . Although this embodiment provides direct heat flow, the attachment of heater  602  may inhibit the expansion of wafer chuck  204 . 
       FIG. 6B  illustrates another embodiment of the present invention. In this embodiment, a proximity heater  604  is placed near a surface of wafer chuck  204 . Proximity heater  604  may heat via thermal or electromagnetic radiation. Since proximity heater  604  does not come in contact with wafer chuck  204 , it does not inhibit expansion of wafer chuck  204 . As shown in  FIG. 6C , proximity heater  604  may vary in size and heat distribution as needed for uniform expansion of wafer chuck  204 . 
       FIG. 6D  illustrates another embodiment of the present invention in which a heater is used. In this embodiment, wafer chuck  204  is manufactured from an electrically conductive material. A power source  606  is connected to wafer chuck  204  by leads  608  and  610 . As power passes through wafer chuck  204 , it heats up and expands. Leads  608  and  610  may be flexible so as to allow the free expansion of wafer chuck  204 . Power source  606  may be a variable power source. 
     In a further embodiment, wafer  202  is heated before being attached to wafer chuck  204 , so that it is warmer than wafer chuck  204 . In this embodiment, when wafer  202  is attached to wafer chuck  204 , they reach thermal equilibrium. As wafer  202  cools, it contracts and thus creates an initial stress opposite that of an expansion force. 
     In each of the heating embodiments, a temperature sensor can be mounted on wafer stage  204  to monitor the expansion. A control circuit may be attached to the heater to precisely control or adjust the heating process. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.