Abstract:
An image stabilization apparatus and method for stabilizing the imaging of a high-performance optical system prone to imaging instabilities from thermal effects. Thermal instabilities within the lens, such as convection, can result in image placement errors in a high-performance optical system. The apparatus includes a heating element arranged on the upper surface of the optical system to provide heat to one or more gas-filled spaces in the optical system. An insulating blanket covers a portion of the optical system to uniformize the heating of the optical system and increase efficiency of the apparatus. The gas in the spaces is heated so that the warmer gases reside near the upper portion of the optical system, while the cooler gases reside near the lower portion of the optical system. This creates a stable thermal environment within the lens system, thereby stabilizing the imaging. Optionally gas can be flowed over the lower surface to keep heat from heating the lower portion of the optical system.

Description:
CROSS REFERENCE 
   This application is a Divisional Application of an earlier filed application by the same title having a filing date of Aug. 6, 2001 and a Ser. No. 09/923,734 which has issued as U.S. Pat. No. 6,617,555 on Sep. 9, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to apparatus and methods for providing image stabilization in high-performance optical systems. 
   2. Description of the Prior Art 
   Many optical systems used in manufacturing today are high-performance systems in which high-resolution images are formed. The images usually need to be imaged onto a light-sensitive medium, such as a photosensitive workpiece or a detector, with great precision. 
   An example of a high-performance optical system is a microlithographic projection lens. Such lenses are used in lithography systems, which are used to fabricate semiconductor devices such as microcircuits. Microlithographic projection lenses typically are required to resolve resolution-limit “critical dimension” features on a mask. Further, the images need to be accurately located on the wafer, i.e., to within nanometers of existing features. 
   In a lithography system, the position where a mask image (referred to as the “aerial image”) is formed in the image plane can vary due to refractive index variations of the gas (e.g., air) within the spaces between the lens elements for certain types of microlithographic lenses. The index variations can be caused by lens heating, which can arise from a number of sources such as electrical and mechanical elements within the lithography system (e.g., the wafer stage linear motors). If not properly dissipated, such heat can cause a negative thermal gradient across the housing of the lens, wherein the top of the lens is cooler than the bottom of the lens. This, in turn, can cause convective heat transfer within the lens, resulting in unstable air motion within the lens and thus variations in refractive index along the optical path. If the space between lens elements is relatively large (which is the case for many catoptric and catadioptric microlithographic lens designs), such refractive index variations can significantly alter the optical path of the light rays. This, in turn, can cause slight displacements in the image at the image plane, which can lead to alignment and/or stage positioning errors, which ultimately degrade the quality of the device being fabricated. 
   SUMMARY OF THE INVENTION 
   The present invention relates to apparatus and methods for providing image stabilization in high-performance imaging systems. 
   Accordingly, a first aspect of the invention is an apparatus for providing image stabilization for a high-performance optical system. The apparatus includes a heating element arranged on an upper surface of a housing of the optical system. The heating elements provide heat to one or more gas-filled spaces between lens elements of the optical system. An insulating layer is arranged over at least a portion of the housing and facilitates the heating of gas present within the one or more spaces. The heating is performed so as to create a stable thermal environment within the optical system, wherein the warmer air resides closest to the upper portion of the housing and the cooler air resides closest to the lower portion of the housing. 
   A second aspect of the invention is that as described immediately above and further including a gas manifold is arranged adjacent the lower surface of the housing. The gas manifold is used to flow gas around the lower surface in order to prevent heating of the lower surface. The gas manifold may also be adapted to flow gas around heat-generating elements in the lithography system to prevent heat from such elements from heating the lower portion of the housing of the optical system. 
   A third aspect of the invention is a method of stabilizing the imaging of a high-performance optical system having one or more gas-filled spaces that are subject to thermal instability. The method includes heating the optical system so that gas in the one or more gas-filled spaces is heated so as to maintain a temperature differential within the one or more gas-filled spaces to form a stable thermal environment within the gas-filled spaces. In an example embodiment, the heating is performed on an upper surface of the optical system to avoid convective thermal instability in the one or more gas-filled spaces. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic side view of an example high-performance optical system in the form of a microlithographic lens as part of a lithography system, to which the present invention may be applied; 
       FIG. 2  is a schematic perspective diagram of the image stabilization apparatus of the present invention as applied to the high-performance optical system of  FIG. 1 ; 
       FIG. 3  is a schematic diagram of a substrate with first and second level exposure fields illustrating the type of alignment error between levels that can occur when there is an image placement error due to image instability from undesirable heating; 
       FIG. 4  is a plot of the stage precision test 3σ (nm) as measured for different wafers both before image stabilization (region R 1 ) and after stabilization (region R 2 ); and 
       FIG. 5  is a plot of magnification of an example microlithographic projection lens with and without the image stabilization apparatus of the present invention, the data showing no detrimental effects on the image performance of the optical system when the image stabilization apparatus is used. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates to apparatus and methods for providing image stabilization in high-performance imaging systems. 
   With reference to  FIG. 1 , there is shown an example high-performance optical system  20  in the form of a Wynne-Dyson type microlithographic lens. Optical system  20  includes a housing  26  with an upper surface  30 , a lower surface  34  and first and second ends  38  and  40 , respectively. Housing  26  supports a large concave mirror  50  at first end  38  and a refractive lens assembly  56  at second end  40 . Lens assembly  56  includes an upper prism surface  62  and a lower prism surface  66 . A gas-filled space  76  is defined by mirror  50 , lens assembly  56  and housing  26 . In practice, air or another gas (e.g., helium or nitrogen) is flowed through gas-filled space  76 . Also in practice, optical system  20  can have one or more gas-filled spaces  76 ; only one is shown in  FIG. 1  for ease of explanation. 
   Optical system  20  is shown as part of a lithography system  100  having a mask  104  arranged adjacent upper prism surface  62 . Mask  104  includes a pattern  110  to be imaged. System  100  also includes an illumination system  120  arranged adjacent mask  104  for illuminating the mask, and a workpiece stage  130  arranged adjacent lower prism surface  66  for supporting a wafer  138 . Wafer  138  includes a photosensitive upper surface  140  to be exposed to an image of pattern  110 . An aerial image monitor system  142  may also be included in lithography system  100  at or near wafer stage  130  to measure both the quality of the aerial image and its precise location relative to a reference position (e.g., optical system  20  or wafer stage  130 ). A main controller  150  is operatively connected to illumination system  120 , wafer stage  130  and aerial image monitor  142 , and controls the operation of lithography system  100 . An environmental chamber  154  for maintaining a thermally controlled, clean environment surrounds system  100 . 
   With reference now to  FIG. 2 , there is shown the image stabilization apparatus  200  applied to optical system  20 . Apparatus  200  includes a heating element  210  (shown in phantom) arranged on upper surface  30  of housing  26 . Heating element  210  is, for example, a flexible heater shaped to fit upper surface  30  of housing  26 . An exemplary heating element is a silicon rubber heater, such as is available from Technical Heaters, Inc., San Fernando, Calif. Heating element  210  needs to provide sufficient heat to housing  26  so that the gas in gas-filled space  76  is heated. In many applications, heating element  210  will need to provide from a few Watts up to tens of Watts of power. In an example embodiment, heating element  210  provides between 1 and 50 Watts of power. The precise amount of power that provides imaging stability without causing other imaging problems (e.g., undesirable index changes, other types of gas movements, etc.) can be determined empirically. Multiple heating elements  210  may arranged over different areas of housing  26  to facilitate the heating process. 
   Heating element  210  is electrically connected to a control unit  220  that controls the amount of current provided to the heating element and thus the amount of heat generated by the heating element. An indicator light  226  is optionally included on control unit  220  or elsewhere to indicate that current is flowing to heating element  210  (i.e., that the heating element is active). Control unit  220 , in an example embodiment, is electrically connected to main controller  150  that controls the operation of lithography system  100  and also the operation of apparatus  200  through control unit  220 . 
   With continuing reference to  FIG. 2 , apparatus  200  also includes a thermal insulating layer  250  arranged over at least a portion of housing  26  and preferably covering heating element  210 . Thermal insulating layer  250  may be, for example, a blanket of flexible insulation such as Poron®, which available from Boyd Corp, Modesto, Calif. Thermal insulating layer  250  is also preferably suitable for clean-room use. Thermal insulating layer  250  helps distribute the heat from heating element  210  over housing  26  so that the gas in gas-filled space  76  is uniformly heated. Insulating layer  250  also allows for the efficient use of heat from heating element  210  so that low power levels can be used. 
   Further optionally included in apparatus  200  is a gas manifold  260  arranged adjacent housing  26  near lower surface  34 . Gas manifold  260  is connected to a gas source  268 , such as a compressed air source or a gas cylinder. Gas manifold  260  is designed to flow gas  269  from gas source  268  over housing lower surface  34  so that heat  270  that otherwise might built up along the lower surface of the housing and create a temperature differential over the housing (and thus in gas-filled space  76 ) is kept away from the housing. Gas manifold  260  may include, for example, an elongate hollow structure  271  with apertures  272  formed therein to distribute gas at various locations along lower surface  34 . In an exemplary embodiment, gas source  268  is an environmental chamber that surrounds lithography system  100 . The air is taken from environmental chamber  154  and provided to gas manifold  260  via an air pump or fan. 
   Gas manifold  260 , in an example embodiment, is also designed to flow air over one or more heat-generating elements  273  in lithography system  100  so that air heated by such elements can be dissipated rather than heating housing  26 . 
   Gas manifold  260  is preferably electronically connected to main controller  150  so that-its operation can be controlled in conjunction with the operation of lithography system  100 . The amount of gas flow required to maintain lower surface  34  of housing  26  at a sufficiently cool temperature is readily determined empirically. In one example, the inventors used a flow of air of 600 fpm (feet per minute) around a system similar to that illustrated in FIG.  1  and found it to be adequate. 
   With continuing reference to  FIG. 2 , also optionally included as part of apparatus  200  is an array of heat sensors  276  arranged over housing  26  and connected to control unit  220 . Sensors  276  provide information about the temperature distribution over housing  26 , and in particular, information about thermal gradients that may be present in the housing. Some or all of heat sensors  276  may be in communication with gas-filled space  76  so that the interior temperature of gas-filled space  76  can be measured at different locations. 
   Operation of Image Stabilization Apparatus 
   With continuing reference to  FIGS. 1 and 2 , apparatus  200  operates as follows. First, optical system  20  is analyzed to assess whether the imaging is unstable due to the aforementioned thermal effects that can arise within the optical system. The thermal effect that is of the greatest concern in the present invention is thermal convection in gas-filled space  76  due to lower surface  34  being at a higher temperature than upper surface  30 . The heat that causes this type of temperature differential may arise from several sources, but is most likely to come from the motors that are used to drive workpiece stage  130 . In certain lithography systems, linear motors in the workpiece stages can generate tens of Watts of heat, which rises and flows around optical system  20 . 
   With reference now to  FIG. 3 , there is shown a workpiece  138  in the form of a wafer having a number of exposure fields  300  formed thereon. The close-up view of one of exposure fields  300  shows a first exposure field  300 A from a first exposure level upon which has been exposed a second exposure field  300 B associated with a second exposure level. The relative misalignment of exposure field  300 A and  330 B appears as an overlay error or stage precision error, but may in fact be caused by the aforementioned imaging instability of optical system  20 . 
   In the present invention, the analysis of optical system  20  to assess whether imaging instabilities are due to thermal effects can be accomplished in number of ways. In an example embodiment, data can be taken from sensors  276  to assess the temperature distribution over housing  26 . The temperature distribution tolerance for housing  26  for maintaining stable imaging can be ascertained by making temperature measurements using sensors  276  and correlating the measurements to errors in image placement that are known to be from the above-described thermal effects. 
   Image placement errors due to thermal effects in optical system  20  can also identified by exposing sequential layers on a wafer in a performing a stage precision test. This is done by exposing workpiece  138  with a first array of exposure fields, and then without removing the workpiece from workpiece stage  130 , exposing the workpiece to form a second array of exposure fields overlaying the first. Misalignment between the two exposure field arrays, as illustrated in  FIG. 3 , can be attributed to image instability, assuming that workpiece stage  130  is operating properly. 
   Another technique that can be used to deduce imaging instabilities involves using aerial image monitor  142  to perform image placement measurements. This information is fed to and stored in control unit  220 . 
   With reference now to  FIG. 4 , there is shown measurements of the precision of workpiece stage  130  for performing alignment on a number of different workpieces. Region R 1  of the plot shows measurements taken prior to implementing an image stabilization method using the image stabilization apparatus  200 . 
   Once it is determined that the conditions that lead to imaging instability in optical system  20  exist, then control unit  220  activates heating element  210  by providing an electric current thereto. The flow of electric current also activates optional indicator light  226 , indicating that apparatus  200  is activated. Heat from heater element  210  diffuses over housing  26  and into gas-filled space  76 , thereby heating the gas within the gas-filled space. Insulating layer  250  insulates the portion of housing  26  that it covers and so assists in uniformly diffusing the heat. The heating of the gas in airspace  76  through upper surface  30  of housing  26  creates a stable thermal environment within optical system  20 , i.e., it reduces the unstable (e.g., convective) motion of gas within gas-filled space  76   
   Optionally, control unit  220  also activates gas supply  268  to supply gas to gas manifold  260  to initiate the flow of gas over lower surface  34  of housing  26 . The combination of heating upper surface  30  with heating element  210  and cooling lower surface  34  with gas flow from gas manifold  260  further enhances the formation of a stable thermal environment in the gas within gas-filled space  76 . In addition, the flow of gas from gas manifold  260  may be directed over heat-generating elements  273  to remove heat from those elements that could otherwise heat lower surface  34  of housing  26 . 
   With reference again to  FIG. 4 , region R 2  of the plot includes measurements taken after the image stabilization method of the present invention was implemented using apparatus  200 . As can be seen by comparing region R 1  to region R 2 , activation of image stabilization apparatus  200  greatly reduced the variation in the stage precision test that arose from imaging instabilities. 
   It is important that the use of apparatus  200  to fix one type of an imaging instability problem not lead to the introduction of another type of imaging instability. For example, many high-performance optical systems  20 , and particularly microlithographic lenses, need to have stable magnification to within a few parts per million (ppm). The magnification of a high-performance optical system can be sensitive to small changes in the refractive index in the gas-filled spaces  76 , or to changes in the internal lens mounting structure. Thus, it important that the amount of heating (and optionally, gas flow) provided to housing  26  not lead to an overall change in the imaging properties of optical system  20  (e.g., changes in the refractive index of the gas occupying gas-filled space  76 ). 
   With now reference to  FIG. 5 , there is shown a plot of the magnification (in ppm) for X and Y magnification of a high-performance optical system taken with and without apparatus  200  being activated. As can be seen from  FIG. 5 , optical stabilization is achieved without introducing significant magnification changes or other detrimental imaging effects As mentioned above, the amount of heat (or heat plus gas flow) required to stabilize the imaging without introducing other imaging problems may need to be determined empirically, and will likely vary between optical systems. 
   The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claims.