Imaging stabilization apparatus and method for high-performance optical systems

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to apparatus and methods for providing image stabilization in high-performance imaging systems.

With reference toFIG. 1, there is shown an example high-performance optical system20in the form of a Wynne-Dyson type microlithographic lens. Optical system20includes a housing26with an upper surface30, a lower surface34and first and second ends38and40, respectively. Housing26supports a large concave mirror50at first end38and a refractive lens assembly56at second end40. Lens assembly56includes an upper prism surface62and a lower prism surface66. A gas-filled space76is defined by mirror50, lens assembly56and housing26. In practice, air or another gas (e.g., helium or nitrogen) is flowed through gas-filled space76. Also in practice, optical system20can have one or more gas-filled spaces76; only one is shown inFIG. 1for ease of explanation.

Optical system20is shown as part of a lithography system100having a mask104arranged adjacent upper prism surface62. Mask104includes a pattern110to be imaged. System100also includes an illumination system120arranged adjacent mask104for illuminating the mask, and a workpiece stage130arranged adjacent lower prism surface66for supporting a wafer138. Wafer138includes a photosensitive upper surface140to be exposed to an image of pattern110. An aerial image monitor system142may also be included in lithography system100at or near wafer stage130to measure both the quality of the aerial image and its precise location relative to a reference position (e.g., optical system20or wafer stage130). A main controller150is operatively connected to illumination system120, wafer stage130and aerial image monitor142, and controls the operation of lithography system100. An environmental chamber154for maintaining a thermally controlled, clean environment surrounds system100.

With reference now toFIG. 2, there is shown the image stabilization apparatus200applied to optical system20. Apparatus200includes a heating element210(shown in phantom) arranged on upper surface30of housing26. Heating element210is, for example, a flexible heater shaped to fit upper surface30of housing26. An exemplary heating element is a silicon rubber heater, such as is available from Technical Heaters, Inc., San Fernando, Calif. Heating element210needs to provide sufficient heat to housing26so that the gas in gas-filled space76is heated. In many applications, heating element210will need to provide from a few Watts up to tens of Watts of power. In an example embodiment, heating element210provides 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 elements210may arranged over different areas of housing26to facilitate the heating process.

Heating element210is electrically connected to a control unit220that controls the amount of current provided to the heating element and thus the amount of heat generated by the heating element. An indicator light226is optionally included on control unit220or elsewhere to indicate that current is flowing to heating element210(i.e., that the heating element is active). Control unit220, in an example embodiment, is electrically connected to main controller150that controls the operation of lithography system100and also the operation of apparatus200through control unit220.

With continuing reference toFIG. 2, apparatus200also includes a thermal insulating layer250arranged over at least a portion of housing26and preferably covering heating element210. Thermal insulating layer250may be, for example, a blanket of flexible insulation such as Poron®, which available from Boyd Corp, Modesto, Calif. Thermal insulating layer250is also preferably suitable for clean-room use. Thermal insulating layer250helps distribute the heat from heating element210over housing26so that the gas in gas-filled space76is uniformly heated. Insulating layer250also allows for the efficient use of heat from heating element210so that low power levels can be used.

Further optionally included in apparatus200is a gas manifold260arranged adjacent housing26near lower surface34. Gas manifold260is connected to a gas source268, such as a compressed air source or a gas cylinder. Gas manifold260is designed to flow gas269from gas source268over housing lower surface34so that heat270that otherwise might built up along the lower surface of the housing and create a temperature differential over the housing (and thus in gas-filled space76) is kept away from the housing. Gas manifold260may include, for example, an elongate hollow structure271with apertures272formed therein to distribute gas at various locations along lower surface34. In an exemplary embodiment, gas source268is an environmental chamber that surrounds lithography system100. The air is taken from environmental chamber154and provided to gas manifold260via an air pump or fan.

Gas manifold260, in an example embodiment, is also designed to flow air over one or more heat-generating elements273in lithography system100so that air heated by such elements can be dissipated rather than heating housing26.

Gas manifold260is preferably electronically connected to main controller150so that-its operation can be controlled in conjunction with the operation of lithography system100. The amount of gas flow required to maintain lower surface34of housing26at 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.1and found it to be adequate.

With continuing reference toFIG. 2, also optionally included as part of apparatus200is an array of heat sensors276arranged over housing26and connected to control unit220. Sensors276provide information about the temperature distribution over housing26, and in particular, information about thermal gradients that may be present in the housing. Some or all of heat sensors276may be in communication with gas-filled space76so that the interior temperature of gas-filled space76can be measured at different locations.

Operation of Image Stabilization Apparatus

With continuing reference toFIGS. 1 and 2, apparatus200operates as follows. First, optical system20is 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 space76due to lower surface34being at a higher temperature than upper surface30. 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 stage130. In certain lithography systems, linear motors in the workpiece stages can generate tens of Watts of heat, which rises and flows around optical system20.

With reference now toFIG. 3, there is shown a workpiece138in the form of a wafer having a number of exposure fields300formed thereon. The close-up view of one of exposure fields300shows a first exposure field300A from a first exposure level upon which has been exposed a second exposure field300B associated with a second exposure level. The relative misalignment of exposure field300A and330B appears as an overlay error or stage precision error, but may in fact be caused by the aforementioned imaging instability of optical system20.

In the present invention, the analysis of optical system20to 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 sensors276to assess the temperature distribution over housing26. The temperature distribution tolerance for housing26for maintaining stable imaging can be ascertained by making temperature measurements using sensors276and 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 system20can also identified by exposing sequential layers on a wafer in a performing a stage precision test. This is done by exposing workpiece138with a first array of exposure fields, and then without removing the workpiece from workpiece stage130, exposing the workpiece to form a second array of exposure fields overlaying the first. Misalignment between the two exposure field arrays, as illustrated inFIG. 3, can be attributed to image instability, assuming that workpiece stage130is operating properly.

Another technique that can be used to deduce imaging instabilities involves using aerial image monitor142to perform image placement measurements. This information is fed to and stored in control unit220.

With reference now toFIG. 4, there is shown measurements of the precision of workpiece stage130for performing alignment on a number of different workpieces. Region R1of the plot shows measurements taken prior to implementing an image stabilization method using the image stabilization apparatus200.

Once it is determined that the conditions that lead to imaging instability in optical system20exist, then control unit220activates heating element210by providing an electric current thereto. The flow of electric current also activates optional indicator light226, indicating that apparatus200is activated. Heat from heater element210diffuses over housing26and into gas-filled space76, thereby heating the gas within the gas-filled space. Insulating layer250insulates the portion of housing26that it covers and so assists in uniformly diffusing the heat. The heating of the gas in airspace76through upper surface30of housing26creates a stable thermal environment within optical system20, i.e., it reduces the unstable (e.g., convective) motion of gas within gas-filled space76

Optionally, control unit220also activates gas supply268to supply gas to gas manifold260to initiate the flow of gas over lower surface34of housing26. The combination of heating upper surface30with heating element210and cooling lower surface34with gas flow from gas manifold260further enhances the formation of a stable thermal environment in the gas within gas-filled space76. In addition, the flow of gas from gas manifold260may be directed over heat-generating elements273to remove heat from those elements that could otherwise heat lower surface34of housing26.

With reference again toFIG. 4, region R2of the plot includes measurements taken after the image stabilization method of the present invention was implemented using apparatus200. As can be seen by comparing region R1to region R2, activation of image stabilization apparatus200greatly reduced the variation in the stage precision test that arose from imaging instabilities.

It is important that the use of apparatus200to 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 systems20, 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 spaces76, or to changes in the internal lens mounting structure. Thus, it important that the amount of heating (and optionally, gas flow) provided to housing26not lead to an overall change in the imaging properties of optical system20(e.g., changes in the refractive index of the gas occupying gas-filled space76).

With now reference toFIG. 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 apparatus200being activated. As can be seen fromFIG. 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.