Abstract:
A windowless system and apparatus are provided that prevent outgases from contaminating the projection optics of an in-vacuum lithography system. The outgassing mitigation apparatus comprises a chimney that is substantially closed at one end, a duct fluidly coupled to the chimney, and a baffle disposed within the chimney. The chimney of the outgassing mitigation apparatus is funnel shaped at the end that is substantially closed. This end of the chimney has an opening that permits a beam or bundle of light to pass through the chimney. A rotating barrier, having at least one aperture for the passage of light, can be positioned near the chimney so that the rotating barrier substantially closes an open end of the chimney except when one of the apertures of the rotating barrier is passing by the chimney. This rotating barrier can be chilled by a refrigerator unit, which is radiantly coupled to a portion of the rotating barrier. A motor is used to rotate the barrier. A light source synchronization module is used to trigger a pulsed light source while the apertures of the rotating barrier are aligned with the chimney of the outgassing mitigation apparatus. Moreover, a barrier gas system can be used to inject a barrier gas into the chimney.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to in-vacuum lithography. More particularly, it relates to photoresist outgassing in an in-vacuum lithography system. 
     2. Related Art 
     One of the many processing steps for manufacturing microelectronic circuits on a semiconductor wafer includes coating the wafer with a thin layer of photoresist and exposing the coated wafer to a source of light through a patterned mask. This process is known as lithography. The size of the microelectronic circuit features that can be produced using lithography is inversely related to the wavelength of the light used to expose the coated wafer. 
     In order to reproduce very fine microelectronic circuit features, a source of extreme ultraviolet (EUV) light, such as a laser-produced plasma (LPP) or synchrotron, must be used. Using EUV light, it is possible to reproduce microelectronic circuit features down to 0.03 micron. Because EUV light is readily absorbed by matter, EUV lithography is carried out in a vacuum. 
     One means for performing lithography is described in U.S. Pat. No. 4,408,338 to Grobman (hereinafter Grobman). Grobman describes a form of x-ray lithography known as contact or proximity printing. In contact printing, the wafer to be exposed is placed very close to the mask, and there are no reducing optics used between the mask and the wafer. The features of the mask are reproduced on the wafer without reduction. This aspect of contact printing, however, makes the masks used in contact printing systems both difficult to design and expensive to produce. Furthermore, it makes contact printing impractical for many applications such as, for example, application specific integrated circuits and systems on a chip that have very small circuit features. 
     In order to reduce the difficulty and costs associated with designing and producing masks for use in an EUV lithography system, it is highly desirable to include projection optics in an EUV lithography system between the mask and the wafer to be exposed. Projection optics can be used to reduce the size of the features reproduced on the wafer, and thereby allow masks to be used that have larger patterns. 
     It is a property of photoresist that it outgases or produces byproducts, especially when it is exposed to high energy light. These outgassed resist products are generally referred to herein as “resist gases,” “resist outgases,” or “outgases.” Among the outgases produced by photoresist are hydrocarbon molecules that can condense on the projection optics of an EUV lithography system. Condensed outgases absorb EUV light and with time significantly reduce the total reflectivity of the projection optics of an EUV lithography system. Mitigating photoresist outgassing therefore is extremely important in an in-vacuum EUV lithography system having projection optics between the mask and the wafer to be exposed. If photoresist outgasing is not controlled or mitigated in such an EUV lithography system, outgases will render the EUV lithography system useless in a very short time (i.e., in about 100 seconds). 
     In order to preclude photoresist outgased byproducts from condensing on the projection optics of an EUV lithography system, the wafer stage of a EUV lithography system must be housed in a separate chamber from the projection optics. Theoretically, the wafer stage chamber of an EUV lithography system could be connected to the projection optics chamber by a window, similar to the window of Grobman. A window would allow some light to pass from the projection optics chamber to the wafer stage chamber to expose a coated wafer while preventing photoresist outgases from entering the projection optics chamber and condensing on the projection optics. Using a window, however, would significantly lengthen the minimum time that it takes to reproduce a microelectronic circuit on a semiconductor wafer. This is due to the fact that a window, like condensed outgases, absorbs a significant amount of EUV light, thus lengthening exposure time. Even an extremely thin window would absorb too much light to work with EUV light (i.e., a window, free of outgassing contamination, would absorb more than fifty percent of the incident EUV light). It should be noted here that Grobman is able to use a window only because Grobman uses x-rays, which can penetrate the window without significant losses, to expose the wafer rather than EUV light. 
     Using a window to prevent outgases from entering the projection optics chamber of an EUV lithography system also has additional drawbacks. For example, outgases would condense and buildup on the window over a short period of time. This buildup of condensed outgases would even further reduce the amount of EUV light that could pass through the window and reach a wafer. Over a short period of time (i.e., less than one hour), the buildup of condensed outgases on the window would reduce the throughput of EUV light to a point where any EUV lithography system (as compared to the x-ray system of Grobman) would be rendered useless. 
     One windowless means for controlling outgassing in an EUV lithography system is discussed in an article by Jos P. H. Benschop et al., in the September 1999 issue of Solid State Technology, titled “EUCLIDES: European EUV lithography milestones,” which is herein incorporated in its entirety by reference. In this article, the authors suggests that by connecting the projection optics chamber and the wafer stage chamber of an EUV lithography system with a tube, and injecting a gas into the connecting tube, a gas flow can be established from the tube into the wafer stage chamber that will apparently preclude photoresist outgases from entering the projection optics chamber. Apparently, this device is based on the idea that outgases will not travel against the gas flow that the authors suggest can be established from the connecting tube into the wafer stage chamber. 
     While the photoresist outgassing control means suggested by Jos P. H. Benschop et al might work in some system, it will not work in EUV lithography systems that use positional monitoring devices to keep a wafer in focus during exposure. Positional monitoring devices of the type known to those skilled in the relevant art, for example, very accurate capacitance focusing devices or gages that use changes in the capacitance of a device to detect small changes in the position of a surface near the device, must be mounted on a stable surface that is in close proximity to the wafer (i.e., these devices must be mounted on a stable surface close to the wafer so that the end of the device is firmly held within about one millimeter of the wafer). The most stable surface available for mounting positional monitoring devices is the partition located between the projection optics chamber and the wafer stage chamber, and thus the partition is the best place for mounting the positional monitoring devices. As a result, the wafer must be positioned in close proximity to the partition, and the wafer blocks the flow of gas into the wafer stage chamber from the connecting tube discussed by Benschop et al. Most if not all of the gas injected into the connecting tube discussed by Benschop et al. flows into the projection optics chamber rather than the wafer stage chamber because this flow path is the flow path of least resistance. 
     Therefore, a need exists for a photoresist outgassing mitigation device without a window that will work with any EUV lithography system, including one that uses positional monitoring devices to keep a wafer in focus during its exposure. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a photoresist outgassing mitigation system, method, and apparatus. The outgassing mitigation system and apparatus comprise a chimney that is substantially closed at one end, a duct fluidly coupled to the chimney, and a baffle disposed within the chimney. The chimney of the outgassing mitigation apparatus is funnel shaped at the end that is substantially closed. This end of the chimney has an opening that permits a beam or bundle of light to pass through the chimney. 
     In an embodiment of the present invention, a rotating mechanical barrier, having at least one aperture for the passage of light, is positioned near the chimney so that the rotating barrier substantially closes an open end of the chimney except when one of the apertures of the rotating barrier is passing by the chimney. This rotating barrier is chilled by a refrigerator unit, which is radiantly coupled to a portion of the rotating barrier. A motor having magnetic bearings is used to rotate the barrier. The magnetic bearings thermally isolate the disk from the motor. 
     In an embodiment of the present invention, a light source synchronisation module is used to trigger a pulsed light source while the apertures of the rotating barrier are aligned with the chimney of the outgassing mitigation apparatus. 
     In another embodiment of the present invention, the baffle disposed within the chimney is chilled by a cooling unit. 
     In still another embodiment of the present invention, a barrier gas system is used to inject a barrier gas into the chimney. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a diagram of an extreme ultra violet lithography system in which the present invention can be used. 
     FIG. 2 is a diagram of the projection optics of the system of FIG.  1 . 
     FIG. 3 is a top view of a portion of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 4A is a bottom view of a portion of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 4B is a side view of a portion of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 5 is a side view of a portion of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 6A is a top view of a baffle for a photoresist outgassing mitigation device according to the present invention. 
     FIG. 6B is a rear view of a baffle for a photoresist outgassing mitigation device according to the present invention. 
     FIG. 7A is a side view of an embodiment of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 7B is a side view of an embodiment of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 7C is a top view of an embodiment of a photoresist outgassing mitigation device according to the present invention. 
     FIG. 8 is a detailed diagram of a portion of the system of FIG.  1 . 
     FIG. 9 is a diagram of a photoresist outgassing mitigation system according to the present invention. 
     FIG. 10 is a diagram of a rotating barrier of a photoresist outgassing mitigation system according to the present invention. 
     FIG. 11 is a flowchart illustrating a method for mitigating photoresist outgassing according to the present invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The system, method, and apparatus of the present invention are described with reference to the accompanying drawings, which are not drawn to scale. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears. 
     Reference will be made in detail to present embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the present embodiments, it will be understood that they are not intended to limit the invention to just these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which can be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art, upon reading this disclosure, that the present invention can be practiced without these specific details. In other instances, well-known structures and devices are not described in detail in order to avoid obscuring aspects of the present invention. 
     Environment of the Invention 
     FIG. 1 is a diagrammatic representation of an extreme ultra violet (EUV) lithography system  100 , in which the present invention can be used. Lithography system  100  comprises a vacuum chamber  102  having pressure zones  104 ,  106 , and  108 , separated by partitions  109 . Pressure zone  104  houses the optics of lithography system  100 . Pressure zone  106  houses the wafer stage of lithography system  100 . Pressure zone  108  houses the light source of lithography system  100 . Pressure zones  104  and  106  are connected by a pump-down bypass valve  110 A, and pressure zones  104  and  108  are connected by a pump-down bypass valve  110 B. Turbo-pumps  112 A and  112 B are used to evacuate pressure chamber  102  and maintain pressure zones  106  and  108  at a lower pressure than pressure zone  104 . 
     Pressure zone  108  houses an EUV light source  114 . EUV light means extreme ultraviolet radiation. In one embodiment, a wavelength in the range of 10 to 14 nanometers (nm) is used. Because EUV light is readily absorbed by matter, pressure zones  104 ,  106 , and  108  are evacuated before and/or during operation of the tool. Pressure zone  108  is maintained at about 1 mTorr, as would be apparent to a person skilled in the lithography art. The atmosphere of pressure zone  108  comprises about seventy percent helium and about thirty percent xenon. 
     EUV light source  114  is preferably a pulsed actinic light source. Other sources of EUV light can be used, however. The pulsed actinic light exits pressure zone  108  through a spectral filter  116 . The purpose of spectral filter  116  is to limit the bandwidth of the light entering pressure zone  104 , as would be apparent to a person skilled in the lithography art. About fifty percent of the incident EUV light is absorbed by spectral filter  116 , as well as virtually all other light. 
     Upon entering pressure zone  104 , the actinic light is focused onto a reticle  120  by mirrors  118 A- 118 D. Mirrors, rather than lenses, are used to focus the actinic light to prevent absorption of the actinic light. The actinic light leaving reticle  120  is focused by mirrors  122 A- 122 F onto a wafer  126  in pressure zone  106 . The actinic light exits pressure zone  104  through outgassing mitigation device  124 , according to the present invention. The structure of outgassing mitigation device  124  is further described below with regard to FIGS. 2-8. 
     Pressure zone  104  is maintained at a pressure sufficient to maintain viscous flow of the gases in pressure zone  104  between mirror  122 F and wafer  126 . This condition is met when the mean free path of the gases in pressure zone  104  is at most about one-one hundredth of the distance between mirror  122 F and wafer  126 . When this condition is satisfied, the gas molecules behave as a fluid (i.e., a continuum, wherein the gas molecules tend to push each other around and wherein collisions between the gas molecules dominate the behavior of the gases). In the preferred embodiment of lithography system  100 , a pressure of about 24 mTorr is maintained in pressure zone  104  to ensure viscous flow of the gases. If a higher pressure is maintained in pressure zone  104 , the mean free path of the gases in pressure zone  104  becomes smaller and viscosity increases, but more light is absorbed (scattered) in pressure zone  104 . 
     The atmosphere of pressure zone  104  comprises hydrogen gas supplied by photoresist outgassing. Cryopump  113  is used for selectively condensing gases in pressure zone  104  that are heaver than hydrogen gas. Makeup gas that is needed to maintain a pressure differential between pressure zones  104  and  106  is introduced into pressure zone  104  through a gas port (not shown). The flow of gases from pressure zone  104  into pressure zone  106  is further discussed below with respect to FIG.  2 . 
     Pressure zone  106  houses the wafer stage of lithography system  100 . Wafer  126  is held on a rigid wafer plate or chuck  128  that is connected to a step and scan device  130 . Wafer  126  is kept in the focal plane of lithography system  100  during scanning using positional monitoring devices (not shown), for example, capacitance focusing devices of the type known to those skilled in the relevant art. 
     In the preferred embodiment of lithography system  100 , a pressure of about 10 mTorr is maintained in pressure zone  106 . The atmosphere of pressure zone  106  comprises hydrogen gas, carbon dioxide, and other hydrocarbon molecules, which are supplied by photoresist outgassing. 
     Preferred Embodiment of a Photoresist Outgassing Mitigation Apparatus 
     FIG. 2 is a detailed diagram of the projection optics of EUV lithography system  100 . FIG. 2 shows the path of travel of EUV light from reticle  120  into outgassing mitigation device  124 . Mirrors  122 A-F are located in pressure zone  104  of vacuum chamber  102  (not shown). Partition  109  separates pressure zone  104  from pressure zone  106 . 
     As can be seen in FIG. 2, outgassing mitigation device  124  couples to partition  109 . For the embodiment shown, outgassing mitigation device  124  preferably has two ducts  204  through which gases from pressure zone  104  can pass to reach pressure zone  106 . In other embodiments, however, outgassing mitigation device  124  can have only one duct  204 . (One duct would be sufficient to permit the gases to reach pressure zone  106  for the embodiment shown, however, two ducts are used in the embodiment in order to avoid the supporting structure (not shown) for mirror  11   8 D.) Because pressure zone  106  is maintained at a lower pressure than pressure zone  104 , gases in pressure zone  104  naturally flow from pressure zone  104  through outgassing mitigation device  124  into pressure zone  106 . The length of ducts  204  are long enough to discharge the flow of gases away from wafer stage equipment, such as step and scan device  130 , located in pressure zone  106 . The details of outgassing mitigation device  124  are further illustrated in FIGS. 3-7. 
     As illustrated in FIG. 3, the two ducts  204  of outgassing mitigation device  124  are connected by a section of duct  302  to form a single piece of duct work. A chimney  304  is also connected to the section of duct  302 . Chimney  304  has an opening  306  through which EUV light can pass. The shape of opening  306  is preferably matched to the shape of the cross section of the EUV light beam or bundle that passes through chimney  304 . The section of duct  302  also has an opening  308  to permit gases in chimney  304  to enter the duct work, thus enabling chimney  304  to serve as an inlet opening for outgassing mitigation device  124 . 
     FIG. 4A shows the underside of outgassing mitigation device  124 . As can be seen in FIG. 4A, chimney  304  has a funnel-shaped section  402 . Funnel-shaped section  402  preferably passes through an opening in partition  109 , as shown in FIG. 4B, and extends through partition  109  so that opening  306  is in close proximity to wafer  126  while it is being exposed. The size of opening  306  limits the number of resist outgassing molecules that can migrate from pressure zone  106  into pressure zone  104  through chimney  304 . 
     FIG. 5 illustrates the flow of gases through outgassing mitigation device  124 . As shown in FIG. 5, gases can enter outgassing mitigation device  124  either from pressure zone  104  or pressure zone  106 . Gases enter outgassing mitigation device  124  from pressure zone  106  through opening  306 . 
     As described above, pressure zone  104  is maintained at a higher pressure than pressure zone  106 . Thus, there is a natural flow of gases from pressure zone  104  through chimney  304  to pressure zone  106 . Because opening  306  is small in size and is in close proximity to wafer  126 , gas flow from pressure zone  104  through opening  306  to pressure zone  106  is restricted. The flow path of least resistance for gases flowing from pressure zone  104  to pressure zone  106  is through the two ducts  204  of outgassing mitigation device  124 . Gases flowing through ducts  204  are discharged into pressure zone  106  through openings  502  at the ends of ducts  204 . Openings  502  serve as discharge openings for outgassing mitigation device  124 . 
     Although the flow of gases from pressure zone  104  through opening  306  into pressure zone  106  is restricted, resist outgases produced during the exposure of a wafer do flow through opening  306  into chimney  304  of outgassing mitigation device  124 . Depending on the momentum of these outgases, they can either be carried by the stream of gases entering chimney  304  from pressure zone  104  into the ducts  204  of outgassing mitigation device  124 , in which case they are discharged into pressure zone  106  as described above, or they can continue through chimney  304  and enter pressure zone  104 . To reduce the momentum of the resist gases entering chimney  304  through opening  306 , a baffle  602  (see FIG. 6A) is inserted into chimney  304 . 
     FIG. 6A shows a preferred embodiment of baffle  602 . Baffle  602  comprises a plurality of baffle plates  604 , which have openings  606  to permit EUV light to pass through the baffle. The size and shape of openings  606  are preferably matched to the cross section of the EUV light beam or bundle used to expose wafer  126 . Baffle  602  precludes a significant portion of the hydrocarbon resist gases, which enter chimney  304  through opening  306 , from entering pressure zone  104 . 
     Outgases generated at the surface of wafer  126  leave the surface of wafer  126  in all directions, as shown in an embodiment  700  of the present invention in FIG.  7 A. While some of these outgases leave the wafer&#39;s surface at an angle substantially normal to the wafer&#39;s surface, many do not. The outgases that leave the surface of wafer  126  at an angle significantly different than normal are prevented from entering pressure zone  104  by partition  109 . The outgases that leave the surface of wafer  126  at some angle other than substantially normal, and that enter chimney  304  through opening  306 , are intercepted by baffle  602 . Even outgases that leave the surface of wafer  126  at an angle normal to the wafer will have difficulty making their way into pressure zone  104  because they will collide with other gas molecules in chimney  304  and exchange their momentum, as shown in FIG.  7 A. Collisions  701 , in FIG. 7A, are example locations where two gas molecules exchanged their momentum. 
     As illustrated in FIG. 6B, baffle  602  has at least one opening  608  that allows gases to flow from chimney  304  of outgassing mitigation device  124  into the ducts  204  of outgassing mitigation device  124 . Openings  608  align with opening  308 , which is best seen in FIG.  7 A. 
     In an embodiment of the present invention, the diameter of ducts  204  increase from about  10  millimeters near section of duct  302  to about  100  millimeters near openings  502  at the ends of ducts  204 , as illustrated in FIG.  7 C. The expanding diameter of ducts  204  ensures that the walls of ducts  204  help move the gas molecules towards opening  502  and into pressure zone  106 . Because the mean free path of the gases is no longer small compared to the distances between the walls of ducts  204 , the flow of the gases within ducts  204  is not viscous. A gas molecule within duct  204  is nearly as likely to collide with a wall of duct  204  as it is to collide with another gas molecule. Thus, wall collisions are a significant factor in the movement of the gases within ducts  204 . As would be know to persons skilled in the relevant arts, a molecule making diffuse collisions with a wall is scattered in a direction independent of its original path of travel, and its momentum is statistically the same before and after the collision unless the wall is chilled. A molecule making a diffuse collision with a wall is emitted from the wall at an angle θ with a probability that is proportional to the cosine of the angle from the normal to the wall. The most probable angle of emission is zero degrees from the normal to the wall. By using ducts that have expanding diameters, the normals to the walls of the a duct always point towards an opening  502  and make it likely that a gas molecule that collides with a wall will be emitted in a direction towards an opening  502 . As shown in FIG. 5, openings  502  discharge into pressure zone  106 . 
     In a preferred embodiment of the present invention, baffle  602  is chilled. Chilling baffle  602  reduces the likelihood that an outgas molecule will strike baffle  602  and bounce off. As would be known to persons skilled in the relevant arts, molecules that collide with a chilled surface tend to condense or adsorb to the surface. Chilling baffle  602  ensures that when an outgas molecule strikes baffle  602 , a significant portion of the molecule&#39;s momentum or energy will transferred to the baffle, and as a result the molecule will be prevented from going further into pressure zone  104 . As described above, changed momentum outgas molecules are carried from chimney  304  into the ducts  204  of outgassing mitigation device  124  and discharged into pressure zone  106 . 
     In a preferred embodiment  750  of the present invention, shown in FIG. 7B, heat is removed from baffle  602  using a heat conducting rod or heat pipe  702  and a cooling unit  710 . Baffle  602  is preferably supported by heat pipe  702  so that baffle  602  is thermally isolated from chimney  304  without the need for insulating spacers. Alternatively, baffle  602  can be thermally insulated from chimney  304  using, for example, rubber spacers. Heat pipe  702  passes through holes  704  in section of duct  302  and is coupled to baffle  602 . Concentric circular flanges  706  form a labyrinth seal that limits the number of gas molecules that migrate from section of duct  302  through holes  704  into pressure zone  104 . Other methods for chilling baffle  602  will be apparent to a person skilled in the relevant art given the discussion herein. 
     As shown in FIG. 7B, embodiment  750  of the present invention also comprises a heater  720  and a temperature sensor  730  that regulate the temperature of outgassing mitigation device  124  (but not baffle  602 ). In the embodiment  750 , heater  720  is coupled to section of duct  302  of outgassing mitigation device  124 . Temperature sensor  730  is coupled to chimney  304  of outgassing mitigation device  124 . Temperature sensor  730  monitors the temperature of outgassing mitigation device  124 , and provides an input to a control module (not shown). Heater  720  is turned-on and turned-off as needed by the control module in order to maintain the temperature of outgassing mitigation device  124  at a predetermined value. 
     Maintaining the temperature of outgassing mitigation device  124  at a predetermined value is an important feature of the present invention in some embodiments. For example, in some embodiments of the present invention, it may be necessary to locate outgassing mitigation device  124  in close proximity to a mirror. In such embodiments, outgassing mitigation device  124  (but not baffle  602 ) is preferably maintained at a temperature substantially equal to the temperature at which the mirror in close proximity was manufactured and tested. For example, FIG. 8 illustrates an embodiment wherein chimney  304  is located in a cutout portion of mirror  122 E. In this embodiment, if mirror  122 E was manufactured and tested at 20° C., then outgassing mitigation device  124  should be maintained at about 20° C. By maintaining the temperature of outgassing mitigation device  124  at about 20° C., chimney  304  acts as a thermal shield between mirror  122 E and chilled baffle  602 , thereby preventing any temperature induced distortion of mirror  122 E, caused by chilled baffle  602 , that might adversely affect the performance of the projection optics. Also shown in FIG. 8 is hole  704  through which heat pipe  702  passes. Embodiments of the present invention that do not have a chilled baffle  602  do not have a hole  704 . 
     Preferred Embodiment of a Photoresist Outgassing Mitigation System 
     Outgassing mitigation device  124  prevents a significant number of outgas molecules from entering pressure zone  104  and potentially contaminating the projection optics of EUW lithography system  100 . It does not, however, preclude every outgas molecule from entering pressure zone  104 . As described above, some outgas molecules that leave wafer  126  at an angle substantially normal to the surface of wafer  126  can travel through opening  306 , and the openings  606  of baffle  602 , and enter pressure zone  104 . In the preferred embodiment of the present invention, therefore, two additional elements are included to prevent these molecules from entering pressure zone  104 . These elements are illustrated in FIG.  9 . 
     FIG. 9 shows an outgassing mitigation system  900  according to the present invention. In addition to outgassing mitigation device  124 , described above, outgassing mitigation system  900  comprises a mechanical barrier  902  and an optional barrier gas system  911 . 
     Barrier  902  is configured to cover and substantially close chimney  304 . As used herein, substantially close means that barrier  902  is located within a few millimeters of chimney  304  in order to block outgas molecules exiting chimney  304  towards pressure zone  104 . In a preferred embodiment of the present invention, barrier  902  is located at a distance of about one millimeter from chimney  304 . Outgas molecules in chimney  304 , for example hydrocarbon molecules, that are traveling out of chimney  304  toward pressure zone  104  are stopped from going further into pressure zone  104  because they strike barrier  902 , which is located in the molecule&#39;s path of travel. 
     In a preferred embodiment of the present invention, barrier  902  is a rotating disk that is radiantly chilled by a refrigerator unit  906 . The rotating disk is supported by magnetic bearings (not shown) that preclude heat transfer between the disk and the motor&#39;s stator (not shown). When an outgas molecule passes through baffle  602  and strikes barrier  902 , its momentum is reduced. Because barrier  902  is chilled, outgas molecules that strike barrier  902  tend to condense on barrier  902 . Molecules that condense on barrier  902  are precluded from going further into pressure zone  104  and condensing on the projection optics of EUV lithography system  100 . 
     As illustrated in FIG. 10, barrier  902  has at least one aperture  1002  that permits EUV light to pass through barrier  902  as it rotates, and thus expose wafer  126 . As shown in FIG. 9, a motor  904  is used to rotate barrier  902  so that apertures  1002  periodically pass over chimney  304 . Motor  904  is preferably a motor having oil-free magnetic bearings in order to reduce the potential for contaminating the projection optics, and to make it easy to keep the disk very cold because no heat is conducted across the magnetic bearing. As apertures  1002  pass over chimney  304 , a sensor  907  senses an aperture position indicator  908  and sends a (e.g., electromagnetic) signal via a communications link  909  to an EUV light source synchronization module  910 . Position indicator  908  can be anything that will be sensed by sensor  907 . For example, position indicator  908  can be a metal pickup device, a reflective tape, or a hole that permits the passage of electromagnetic radiation which is detected by sensor  907 . Other position indication means, such as mechanical apparatus, that can be used will be apparent to a person skilled in the relevant arts given the discussion herein. 
     In the preferred embodiment of the present invention, whenever EUV light source synchronization module  910  receives a signal from sensor  907 , it triggers EUV light source  114 . Because EUV light source  114  is triggered only when an aperture  1002  is over chimney  304 , barrier  902  does not act as a barrier to the EUV light. 
     Whenever wafer  126  is exposed to the EUV light, outgases are produced. Before the hydrocarbon outgas molecules that are produced by the EUV light exposure can enter opening  306  of outgassing mitigation device  124  and pass through baffle  602 , barrier  902  is rotated so that aperture  1002  has moved beyond the opening of chimney  304 . In this manner, any hydrocarbon outgas molecules that are able to pass through baffle  602  are stopped by barrier  902  and precluded from going further into pressure zone  104 , where they might possibly contaminate the projection optics of EUV lithography system  100 . 
     Another element of outgassing mitigation system  900  that prevents hydrocarbon outgas molecules from reaching pressure zone  104  is optional barrier gas system  911 . Barrier gas system  911  injects a heavy gas, for example argon, into the funnel section  402  of chimney  304 . The heavy gas then acts as a barrier to the hydrocarbon outgas molecules that enter chimney  304  through opening  306 . As the hydrocarbon outgas molecules collide with the heavy barrier gas molecules, the outgas molecules exchange their momentum with the heavy barrier gas molecules. This randomizes their direction of travel but does not slow them down. (In a gas, kinetic energy and temperature are equivalent. Thus, the only way to slow down the average molecule is to cool the gas. This cannot be done by random collisions with another gas.) The randomized outgas molecules are then carried away from chimney  304  by the natural flow of the gases passing from chimney  304  through the ducts  204  of outgassing mitigation device  124  into pressure zone  106 . 
     As can be seen in FIG. 9, the shape of funnel section  402  of outgassing mitigation device  124  permits one or more capacitive focusing devices  916  to be mounted close to the exposure area of wafer  126 . The proximity of wafer  126  to opening  306  of outgassing mitigation device  124  restricts the flow of gases trying to exit chimney  304  through opening  306 , thereby ensuring that gases flow through ducts  204  to reach pressure zone  106 . The lengths of ducts  204  are such that the discharge openings  502  of ducts  204  are never blocked by wafer stage equipment, e.g., chuck  128  or step and scan device  130 . 
     Preferred Method for Mitigating Photoresist Outgassing in an In-Vacuum Photolithography System 
     FIG. 11 is a flowchart that illustrates a preferred method  1100  for mitigating photoresist outgassing in an in-vacuum lithography system, according to the present invention. Method  1100  can be implemented using the outgassing mitigation device and system embodiments described above. In order to more clearly describe method  1100 , method  1100  will be described using example EUV lithography system  100 . As would be apparent to a person skilled in the relevant art, however, method  1100  can be implemented in other lithography systems in addition to example EUV lithography system  100 . 
     Method  1100  starts with step  1102 . In step  1102 , vacuum chamber  102  of lithography system  100  is separated into two pressure zones  104  and  106  using a partition  109 . Pressure zones  104  and  106  are both capable of being evacuated in order to establish a vacuum. 
     In step  1104 , the projection optics of lithography system  100  are located within pressure zone  104  and the wafer stage of lithography system  100  is located in pressure zone  106 . The projection optics are placed in a separate pressure zone from the wafer stage in order to limit the number of photoresist outgases that can come into contact with the projection optics. 
     In step  1106 , an outgassing mitigation device  124  is coupled to partition  109 . Outgassing mitigation device  124  has a chimney  304  and a baffle  602 . The purpose of outgassing mitigation device  124  is to control the flow of gases between pressure zones  104  and  106  while letting actinic light through. How outgassing mitigation device  124  controls the flow of gases is described above. 
     In step  1108 , a pressure differential is created between pressure zone  104  and  106  in order to establish a viscous flow of gases between pressure zones  104  and  106 . To create the pressure differential, pressure zones  104  and  106  are evacuated, but the pressure in pressure zone  106  is evacuated to a pressure lower than the pressure of pressure zone  104 . Pressure zone  104  is maintained at a pressure sufficient to maintain a viscous flow of the gases in pressure zone  104  between mirror  122 F and wafer  126 . In an embodiment of the present invention, a pressure of about 24 mTorr is maintained in pressure zone  104  during the lithography process in order to ensure viscous flow of the gases. A pressure of about 10 mTorr is maintained in pressure zone  106 . 
     In step  1110 , a rotating barrier is used to block outgas molecules traveling from chimney  304  towards pressure zone  104 . Whenever a wafer  126  is exposed to the EUV light, outgases are produced. Before the outgas molecules, for example hydrocarbon outgas molecules, that are produced by the EUV light exposure can enter an opening  306  of outgassing mitigation device  124  and pass through baffle  602 , barrier  902  is rotated so that an aperture  1002  has moved beyond the opening of chimney  304 . In this manner, any hydrocarbon outgas molecules that are able to pass through baffle  602  are stopped by barrier  902  and precluded from entering pressure zone  104 , where they might possibly contaminate the projection optics of lithography system  100 . 
     In an optional step  1112  of method  1100 , barrier  902  is radiantly chilled by a refrigerator unit  906 . When an outgas molecule passes through baffle  602  and strikes barrier  902 , its momentum is reduced. Because barrier  902  is chilled, outgas molecules that strikes barrier  902  tend to condense on barrier  902 . Molecules that condense on barrier  902  are precluded from going further into pressure zone  104  and condensing on the projection optics of EUV lithography system  100 . 
     In an optional step  1114  of method  1100 , baffle  602  is chilled. Chilling baffle  602  reduces the likelihood that an outgas molecule will strike baffle  602  and bounce off without transferring a significant amount of its momentum to baffle  602 . 
     In an optional step  1114  of method I 100 , a barrier gas is injected into a funnel-shaped section of chimney  304 . In this step, a barrier gas system  911  injects a heavy gas, for example argon, into a funnel section  402  of chimney  304 . The heavy gas then acts as a barrier to outgas molecules that enter chimney  304  through opening  306 . As the outgas molecules collide with the heavy barrier gas molecules, the outgas molecules exchange their momentum with the heavy barrier gas molecules. This randomizes the direction of travel of the outgas molecules and enables them to be carried away from chimney  304  by the natural flow of the gases passing from chimney  304  through ducts  204  of outgassing mitigation device  124  into pressure zone  106 . 
     How to implement each of the steps of method  1100  is further explained above with regard to FIGS. 1-10. As would be apparent to a person skilled in the relevant art given the discussion herein, embodiments of the present invention other than those used to describe how to implement steps  1102 - 1116  can also be used to implement method  1100 , without departing from the spirit and scope of the present invention. 
     Conclusion 
     Various embodiments of the present invention have been described above, which can be used to mitigate outgassing in an EUV in-vacuum lithography system. It should be understood that these embodiments have been presented by way of example only, and not limitation. It will be understood by those skilled in the relevant arts that various changes in form and details of the embodiments described above may be made without departing from the spirit and scope of the present invention as defined in the claims. 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.