Abstract:
A method and apparatus for maintaining a purged optical gap between a pellicle and a reticle in a photolithography system is described. A porous frame between a reticle and a pellicle maintains the purged optical gap. A porous frame includes a first and a second opposing surface. The first opposing surface defines a first opening, and is configured to mate with the pellicle. The second opposing surface defines a second opening, and is configured to mate with the reticle to enclose the optical gap between the pellicle and the reticle. A gas supply interface infuses a purge gas through the porous frame and into the gap.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to photolithography systems, and more particularly, to maintaining an oxygen-purged optical path through a reticle and pellicle. 
     2. Related Art 
     In the fabrication of integrated circuits, photolithographic and projection printing techniques are used. In photolithography, an image contained on a reticle is projected onto a wafer having a photosensitive resist thereon. The reticle or mask is used to transfer a desired image onto the silicon wafer. The semiconductor wafer surface is coated with photosensitive resist so that an image is etched thereon. A pellicle may be used in combination with the reticle to protect the reticle surface from damage. The pellicle is traditionally mounted on a solid frame to the reticle. 
     Some wavelengths of light used in photolithography are sensitive to absorption by atmospheric oxygen. Hence, when such oxygen-sensitive light wavelengths are used in photolithography, they must be transmitted through an oxygen-purged atmosphere. 
     A photolithography system is typically located in a clean room environment. In some situations, the ambient atmosphere of the clean room cannot be purged of oxygen because this may cause other problems with the photolithography process. For instance, a laser interferometer used in a lithography system may be sensitive to changes in the index of refraction of the air, which may occur with a change to an oxygen-free atmosphere. Hence, the oxygen-free environment may have to be restricted to less than the entire lithography system. What is needed is a transmission medium for light wavelengths that have high absorption in an oxygen-containing environment. 
     A pellicle is generally mounted on a frame opposite a corresponding reticle. Hence, an air gap may exist between the reticle and pellicle. What is needed is a transmission medium through the reticle-to-pellicle air gap for light wavelengths that have high absorption in an oxygen-containing environment. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for a reticle with a purged pellicle-to-reticle gap. The present invention maintains a substantially oxygen-free, purge gas environment in a pellicle-to-reticle gap. The purge gas environment provides a transmission medium for light wavelengths that have high absorption in a non-purged environment. 
     In a preferred embodiment, the present invention is applied to a photolithography system. A porous frame between a reticle and a pellicle creates a gap or space between the reticle and pellicle. The porous frame may passively filter ambient air entering the gap through the porous frame to create a substantially particle-free gap. The particulate protection is required to ensure that particles do not deposit on the critical reticle surface, degrading the reticle image projected onto a semiconductor wafer surface. This includes protection during storage of the reticle and usage of the reticle in a lithographic process. 
     The passive or static porous frame acts to normalize the pressure within the reticle to pellicle gap with the external ambient air atmosphere. This normalization action effectively reduces or eliminates distortion of either the reticle and/or pellicle due to atmospheric pressure. 
     The porous frame includes a first opposing surface with a first opening. The first opposing surface is configured to mate with the pellicle. The porous frame includes a second opposing surface with a second opening. The second opposing surface is configured to mate with the reticle to enclose the optical gap between the pellicle and the reticle. 
     A purged reticle to pellicle gap may be formed by filling the gap with a purge gas that does not contain oxygen. The purge gas in the gap may be maintained dynamically by continuously infusing the purge gas. 
     A dynamic porous frame may be coupled to a purge gas supply. The purge gas supply inserts a purge gas into the gap between the reticle and pellicle through the porous frame, establishing a purge gas flow in the gap within the porous frame. 
     A vacuum source may be coupled to the dynamic porous frame to remove gas from the reticle-to-pellicle gap environment through the porous frame, further providing continuous gas flow in the reticle. 
     The purge gas flow in the gap of a dynamic porous frame may be balanced with an external atmospheric pressure to reduce or eliminate reticle or pellicle distortions. 
     The porous frame of the present invention is applicable to other environments, including other optical environments. In an example alternative optical embodiment, the porous frame can provide a purged optical path between any optical source surface and any optical target surface. The optical source surface and optical target surface may be any suitable optical surfaces known to persons skilled in the relevant art(s). 
     Further embodiments, features, and advantages of the present inventions, 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 FIGURES 
     In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the left-most digit(s) in the corresponding reference number. 
     FIG. 1 illustrates a block diagram of the relevant portion of the optical path of a conventional lithography system. 
     FIG. 2 illustrates a block diagram of the relevant portion of the optical path of a lithography system of the current invention. 
     FIG. 3 illustrates an exploded view of a reticle and pellicle assembly with porous frame, according to an embodiment of the present invention. 
     FIG. 4 illustrates operation of an exemplary embodiment of the present invention. 
     FIGS. 5 and 6 illustrate flowcharts providing operational steps for embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To more clearly delineate the present invention, an effort is made throughout the specification to adhere to the following term definitions as consistently as possible. 
     “Ambient air” means an oxygen-containing atmosphere, such as normal atmospheric air. For instance, “ambient air” may mean air in an oxygen-containing clean room atmosphere or environment. 
     “Purge gas” means a gas that does not contain oxygen, or some other undesired gas, and is used to fill a purged air gap or space. 
     FIG. 1 illustrates a relevant portion of a conventional photolithography system  100 . Conventional photolithography system  100  is located in an ambient air or gas environment. Some portions of a conventional photolithography system may not be shown in FIG. 1 for purposes of brevity, such as source optics, projection optics, etc. 
     Conventional photolithography system  100  comprises an illumination source  102 , a reticle  104 , a frame  106 , a pellicle  108 , and a semiconductor wafer  110 . 
     Illumination source  102  includes a source of radiation for exposing a surface of semiconductor wafer  110  with a pattern on reticle  104 . 
     Reticle  104  includes a mask with a pattern that is transferred to a surface of semiconductor wafer  110  by radiation from illumination source  102 . 
     Frame  106  is a conventional solid frame to which the reticle and pellicle are attached. Frame  106  comprises an air gap  112 . Air gap  112  is formed within frame  106  between reticle  104  and pellicle  108 . 
     Pellicle  108  is a clear cover for protecting reticle  104  from particulate damage. 
     Semiconductor wafer  110  is a semiconductor wafer with a surface to be exposed and etched by radiation from illumination source  102  with a pattern from reticle  104 . 
     Illumination source  102  produces radiation  114 . Radiation  114  is transmitted through reticle  104 , frame  106 , air gap  112 , and pellicle  108 , to a surface of semiconductor wafer  110 . When radiation  114  includes light wavelengths that are absorbed by oxygen, oxygen in air gap  112  may absorb at least a portion of these wavelengths, potentially preventing a sufficient amount of radiation  114  from reaching the surface of semiconductor wafer  110 . This absorption may lead to an inadequate amount of radiation transferring the pattern of reticle  104  to the surface of semiconductor wafer  110 , leading to reduced semiconductor wafer yields. 
     FIG. 2 illustrates an exemplary photolithography system  200 , according to an embodiment of the present invention. Photolithography system  200  is located in an ambient air environment. Photolithography system  200  maintains a purge gas environment between a reticle and a pellicle for transmission of light wavelengths that are sensitive to oxygen. 
     Photolithography system  200  comprises an illumination source  202 , a reticle  104 , a porous frame  206 , a pellicle  108 , and a semiconductor wafer  110 . 
     Illumination source  202  includes a source of radiation for exposing a surface of semiconductor wafer  110 . Illumination source  202  may include any applicable source of radiation suitable for exposing a semiconductor wafer surface, including a laser. Illumination source  202  transmits radiation  214 . Radiation  214  may include any type of suitable radiation, including laser light. Radiation  214  may include oxygen-sensitive light wavelengths suitable for exposing and etching a semiconductor wafer. Such light wavelengths may include 157 nm wavelength light, for example. 
     Reticle  104  receives radiation  214 . Reticle  104  includes a mask with a pattern that is transferred to a surface of semiconductor wafer  110  by radiation  214  from illumination source  202 . 
     Porous frame  206  receives radiation  214  that has passed through reticle  104 . Reticle  104  is attached to porous frame  206 . Porous frame  206  comprises a porous material that allows gas to flow through, but blocks passage of particle contaminants. 
     Pellicle  108  receives radiation  214  that has passed through porous frame  206 . Pellicle  108  is attached to porous frame  206 . Reticle  104  is in optical alignment with pellicle  108 . 
     Radiation  214  is transmitted through reticle  104 , porous frame  206 , purge air gap  112 , and pellicle  108  to semiconductor wafer  110 . Semiconductor wafer  110  receives radiation  214 . Semiconductor wafer  110  comprises a surface to be exposed and etched with a pattern of reticle  104  by radiation  214  transmitted by illumination source  202 . 
     Porous frame  206  encloses air gap  112 . Air gap  112  is formed within porous frame  206  between reticle  104  and pellicle  108 . Air gap  112  may be filled with a purge gas, such as nitrogen, that does not contain oxygen, and hence does not interfere with oxygen-sensitive wavelengths of radiation  214 . Porous frame  206  further prevents particulate contamination from entering air gap  112  and damaging reticle  104 . Porous frame  206  has sufficient porosity to allow gas to pass from air gap  112  enclosed by porous frame  206  to an exterior of porous frame  206 . 
     Because porous frame  206  allows gas to flow in and out, in a static mode, porous frame  206  normalizes pressure within air gap  112  with atmospheric pressure, eliminating distortion to reticle  104  and/or pellicle  108 . 
     Lithography system  200  provides a purge gas optical path for radiation  214  from illumination source  202 . Hence, illumination source  202  may transmit oxygen-sensitive light wavelengths, without suffering from significant attenuation caused by oxygen absorption. 
     The reticle with purged pellicle-to-reticle gap of the present invention is described above in an example photolithography environment. The present invention is not limited to such an environment, and is applicable to additional photolithography environments, and non-photolithography environments. The example is presented herein for purposes of illustration, and not limitation. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the present invention. 
     Exemplary embodiments for a reticle with purged pellicle-to-reticle gap according to the present invention are described below. These embodiments are described herein for illustrative purposes, and are not limiting. The present invention is adaptable to any application requiring a reticle with purged pellicle-to-reticle gap. 
     FIG. 3 illustrates an exploded view of an exemplary purged pellicle-to-reticle gap system  300 , according to an embodiment of the present invention. Purged pellicle-to-reticle gap system  300  comprises a reticle  104 , a porous frame  206 , a pellicle  108 , an air gap  112 , a purge gas supply interface  316 , and a vacuum source interface  318 . 
     Porous frame  206  comprises a first open surface  320  and a second open surface  322  (located on opposite side of porous frame  206  from first open surface  320 , not visible in FIG.  3 ). First open surface  320  and second open surface  322  are substantially parallel to one another. Porous frame  206  is comprised of a porous filtering material. The porous filtering material of porous frame  206  allows the transmission of gases, but prevents the transmission of particles. These particles may include particles in the air, dust, particles resulting from the photolithography process, and particles resulting from other sources. In a preferred embodiment, porous frame  206  is substantially rectangular. In alternate embodiments, porous frame  206  may comprise other shapes, such as circular, elliptical, and irregular. 
     In a preferred embodiment, porous frame  206  is manufactured from one or more metals. For example, porous frame  206  may comprise iron, copper, bronze, nickel, titanium, or other metal, or any combination or alloy thereof. Porous frame  206  comprises pores formed in the metal(s) by a pore forming process. For example, porous frame  206  may be made from metal powder particles or filaments bonded at their contact points by sintering, which may create a continuous, well-defined network of pores between the particles or filaments. Sintering techniques generally weld together and grow a contact area between two or more initially distinct particles at temperatures below the melting point. Other processes for forming pores are also within the scope of the present invention. The porosity, or pore size, may be controlled by the production process, and may be determined on an application-by-application basis. For example, the porosity may be specified in microns, or in fractions of a micron. The invention, however, is not limited to these porosity values. A number of vendors can potentially supply suitable porous metals that are manufactured according to sintering and other techniques. Such vendors may include GKN Sinter Metals, in Auburn Hills, Mich., and Capstan Permaflow, Inc., in Gardena, Calif. 
     Pellicle  108  is coupled to first open surface  320  of porous frame  206 . Pellicle  108  may comprise a glass, a membrane, or other material, as would be known to persons skilled in the relevant art(s). Pellicle  108  is attached or affixed to first open surface  320  such that air gap  112  is completely enclosed at first open surface  320 . Furthermore, pellicle  108  is attached to first open surface  320  such that a substantially air tight seal is formed at the interface of pellicle  108  and first open surface  320 . Pellicle  108  and first open surface  320  are attached in a manner well known to persons skilled in the relevant art(s). For example, pellicle  108  may be glued to first open surface  320 . 
     Reticle  104  is coupled to second open surface  322  of porous frame  206 . Reticle  104  is attached or affixed to second open surface  322  such that air gap  112  is completely enclosed at second open surface  322 . Furthermore, reticle  104  is attached to second open surface  322  such that a substantially air tight seal is formed at the interface of reticle  104  and second open surface  322 . Reticle  104  and second open surface  322  are attached in a manner well known to persons skilled in the relevant art(s). 
     Pellicle  108 , reticle  104 , and porous frame  206  combine to form a substantially air tight air gap  112 , where gases flow only through the material of porous frame  206 . In a preferred embodiment, the porous filtering material of porous frame  206  is capable of allowing transmission of a gas while simultaneously blocking the entrance of particulate contamination. 
     The “breathable” porous frame  206  assembly with reticle  104  and pellicle  108  may either be allowed to remain static (i.e. open to the surrounding environment), or be coupled to an external pressurized purge gas source as described above. Purge gas supply interface  316  interfaces porous frame  206  with a purge gas supply. Purge gas supply interface  316  connects to a first frame end surface  324  of porous frame  206 . Purge gas supply interface  316  preferably provides a purge gas from a purge gas supply to first frame end surface  324 . The purge gas infuses from purge gas supply interface  316  into air gap  112  through the pores of first frame end surface  324 . In an alternative embodiment, purge gas supply interface  316  is a first port, hole, or valve in porous frame  206  for providing purge gas through porous frame  206  and into air gap  112 . 
     Vacuum source interface  318  interfaces porous frame  206  with a vacuum source. Vacuum source interface  318  connects to a second frame end surface  326  of porous frame  206 . As shown in FIG. 3, second frame end surface  326  is located on the opposite side of porous frame  206  from first frame end surface  324  (not visible in FIG.  3 ). In alternate embodiments, second frame end surface  326  may be located on sides of porous frame  206  that are not opposite first frame end surface  324 . Vacuum source interface  318  preferably evacuates or removes the purge gas from air gap  112  through the pores of second frame end surface  326 . In an alternative embodiment, vacuum source interface  318  is a second port, hole, or valve in porous frame  206  for evacuating or removing purge gas more directly from air gap  112 . 
     In normal operation, porous frame  206  has four exposed outer surfaces: first frame end surface  324 , second frame end surface  326 , a third frame end surface  328 , and a fourth frame end surface  330  (opposite of third frame end surface  328 , not visible in FIG.  3 ). In a preferred embodiment, all exposed outer surfaces of porous frame  206  are porous, and allow gas to pass into and out from air gap  112 . In alternative embodiments, first frame end surface  324  and second frame end surface  326  are the only exposed outer surfaces of porous frame  206  that are porous. This is especially useful in dynamic uses of the present invention, allowing porous frame  206  to be coupled to a purge gas source and vacuum source at first frame end surface  324  and second frame end surface  326 , respectively, with no remaining exposed surfaces to leak gas. 
     Purge gas may enter the assembly via purge gas supply interface  316 , and be evacuated from the assembly via vacuum source interface  318  to create a continuous flow of purge gas through air gap  112 . The purge gas flow through air gap  112  is balanced to be equal to atmospheric pressure, to eliminate distortion to reticle  104  and/or pellicle  108 . 
     Exemplary embodiments of a reticle with purged pellicle-to-reticle gap of the present invention are described above. The present invention is not limited to these examples. These examples are presented herein for purposes of illustration, and not limitation. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the present invention. 
     Exemplary operational and/or structural implementations related to the structure(s), and/or embodiments described above are presented in this section. These components and methods are presented herein for purposes of illustration, and not limitation. The invention is not limited to the particular examples of components and methods described herein. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the present invention. 
     FIG. 4 illustrates operation of an exemplary embodiment of the present invention. FIG. 4 shows a porous frame reticle/pellicle assembly  404 , a purge gas supply  416 , and a vacuum source  418 . 
     In a preferred embodiment, porous frame reticle/pellicle assembly  400  comprises a reticle, a porous frame, and a pellicle, such as reticle  104 , porous frame  206 , and pellicle  108  shown in FIG.  3 . Porous reticle/pellicle assembly  400  further comprises an air gap  112 . 
     In a preferred embodiment, porous frame reticle/pellicle assembly  404  maintains mechanical particulate control on a critical surface of the reticle, while allowing a continuous purge gas or air environment flow in air gap  112 . Furthermore, porous reticle/pellicle assembly  400  normalizes the pressure within air gap  112 , effectively eliminating distortion of either the reticle or pellicle due to atmospheric pressure changes. 
     In embodiments, the porous filtering material of porous frame  206  is capable of allowing transmission of a gas while simultaneously blocking the entrance of particulate contamination. This “breathable” porous frame reticle/pellicle assembly  400  may be allowed to remain static (i.e. open to the surrounding environment). In a static embodiment, porous frame reticle/pellicle assembly  400  is not coupled to a purge gas supply  416  or a vacuum source  418 . Ambient air may be allowed to enter air gap  112  through porous frame reticle/pellicle assembly  400 , as in example ambient air flow paths  420 . However, in a preferred embodiment described below, a continuous flow of purge gas is injected into air gap  112  to prevent ambient air from entering air gap  112 . 
     Porous frame reticle/pellicle assembly  400  may also operate in a dynamic environment. In a dynamic embodiment, porous frame reticle/pellicle assembly  400  may be coupled to a purge gas supply  416 . Purge gas supply  416  supplies a purge gas through a porous frame of porous frame reticle/pellicle assembly  400  to air gap  112 . The purge gas entering air gap  112  is shown as inserted purge gas flow  422 . Suitable gas supply systems for purge gas supply  416  are well known in the art. 
     Furthermore in a dynamic embodiment, porous frame reticle/pellicle assembly  400  may be coupled to a vacuum source  418 . Vacuum source  418  removes purge gas and/or ambient environment gas (if present) from air gap  112  through a porous frame of porous frame reticle/pellicle assembly  400 . Purge gas being removed from air gap  112  is shown as removed gas flow  424 . Suitable vacuum systems for use as vacuum source  418  are well known in the art. 
     Flowcharts are provided that detail operational steps of an example embodiment of the present invention. The steps provided do not necessarily have to occur in the order shown, as will be apparent to persons skilled in the relevant art(s) based on the teachings herein. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the discussion contained herein. These steps are described in detail below. 
     FIG. 5 illustrates a flowchart providing operational steps for an embodiment of the present invention. A process  500  shown in FIG. 5 begins with step  502 . In step  502 , an air gap is formed within a porous frame between a reticle and pellicle. In step  504 , a purge gas is inserted into the air gap through the porous frame. 
     FIG. 6 illustrates a flowchart providing exemplary detailed operational steps for step  502  of FIG.  5 . In step  602 , a pellicle is attached to a first open surface of a porous frame. In step  604 , a reticle is attached to a second open surface of a porous frame to form an air gap within the porous frame between the reticle and the pellicle. 
     Process  500  of FIG. 5 may further include a step where the inserted purge gas is filtered by the porous frame. 
     Step  504  may include a step where a purge gas is inserted into the air gap through an end surface of the porous frame. 
     Process  500  may further include a step where the purge gas is removed from the air gap. This step may include a step where the purge gas is removed from the air gap through a frame end surface of the porous frame. 
     Process  500  may further include a step where a purge gas pressure in the air gap is balanced with an ambient environment air pressure. 
     In an alternative embodiment, a purge gas pressure in air gap  112  is maintained to exceed an ambient environment air pressure. By allowing the purge gas pressure in air gap  112  to exceed the ambient environment air pressure, a substantially oxygen-purged air gap  112  may be maintained. A purge gas supply  416  inserts a purge gas into air gap  112 . The purge gas is inserted at a rate such that the purge gas pressure in air gap  112  exceeds the ambient environment air pressure, and hence, the purge gas will leak out of air gap  112  through porous frame  206 . The purge gas is inserted at a rate slowly enough so as not to cause substantial distortion to reticle  104  and/or pellicle  208 . The purge gas leaking out of air gap  112  through porous frame  206  substantially impedes the ability of ambient air to leak into air gap  112  through porous frame  206 . In this alternative embodiment, a vacuum source  418  is not needed to remove the purge gas, because the purge gas leaks out of air gap  112  through porous frame  206 . 
     Additional steps or enhancements to the above processes and steps which may become known to persons skilled in the relevant art(s) from the teachings herein are also encompassed by the present invention. 
     CONCLUSION 
     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.