Abstract:
The present invention provides a standardized mechanical interface (SMIF) reticle pod that is configured to provide a controlled environment for supporting a reticle wherein the controlled environment is maintained substantially free of crystal growth causing contaminants. Accordingly, there is provided a layered filter with filter elements capable of filtering particulates and adsorbing gaseous contaminants. The filter has an inwardly facing face generally planar shaped with a surface area that is substantially half or more of the area of the reticle face. The inwardly facing face is placed in close proximity to the reticle patterned surface and has an area that is a significant fraction of the reticle patterned surface area. The SMIF pod is also provided with a purge system configured to inject a very dry gas within the controlled environment to flush the controlled environment of contaminants as well as to regenerate the filter.

Description:
RELATED APPLICATION  
       [0001]     The present application claims the benefit of U.S. Provisional Application No. 60/668,189 filed Apr. 4, 2005, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to standardized mechanical interface (SMIF) substrate carriers used in semiconductor manufacturing and more particularly to transportable and shippable reticle/photomask carriers that include a filtration system to reduce chemical and particulate contaminants within the controlled environment of the carrier.  
       BACKGROUND OF THE INVENTION  
       [0003]     The processing of silicon wafers for semiconductor applications typically includes photolithography as one of the process steps. In photolithography, a wafer surface with a deposit of silicon nitride is coated over with a light-sensitive liquid polymer or photoresist and then selectively exposed to a source of radiation using a template with a desired pattern. Typically, ultraviolet light is shone through or reflected off a surface of a mask or reticle to project the desired pattern onto the photoresist covered wafer. The portion of the photoresist exposed to the light is chemically modified and remains unaffected when the wafer is subsequently subjected to a chemical media that removes the unexposed photoresist leaving the modified photoresist on the wafer in the exact shape of the pattern on the mask. The wafer is subjected to an etch process that removes the exposed portion of the nitride layer leaving a nitride pattern on the wafer in the exact design of the mask.  
         [0004]     The industry trend is towards the production of chips that are smaller and/or with a higher logic density necessitating even smaller line widths on larger wafers. Clearly, the degree of fineness to which the surface of the reticle can be patterned and the degree to which this pattern can be faithfully replicated onto the wafer surface are factors that impact the quality of the ultimate semiconductor product. The resolution with which the pattern can be reproduced on the wafer surface depends on the wavelength of ultraviolet light used to project the pattern onto the surface of the photoresist-coated wafer. State-of-the-art photolithography tools use deep ultraviolet light with wavelengths of 193 nm, which allow minimum feature sizes on the order of 100 nm. Tools currently being developed use 157 nm Extreme Ultraviolet (EUV) light to permit resolution of features at sizes below 70 nm. The reticle is a very flat glass plate that contains the patterns to be reproduced on the wafer. Typical reticle substrate material is quartz. Because of the tiny size of the critical elements of modern integrated circuits, it is essential that the operative surface of the reticle (i.e. the patterned surface) be kept free of contaminants that could either damage the surface or distort the image projected onto the photoresist layer during processing leading to a final product of unacceptable quality. Typically, the critical particle sizes are 0.1 μm and 0.03 μm for the non-patterned and patterned surfaces respectively when EUV is part of the photolithography process. Generally, the patterned surface of the reticle is coated with a thin, optically transparent film, preferably of nitrocellulose, attached to and supported by a frame, and attached to the reticle. Its purpose is to seal out contaminants and reduce printed defects potentially caused by such contamination in the image plane. However, extreme EUV utilizes reflection from the patterned surface as opposed to transmission through the reticle characteristic of deep ultraviolet light photolithography. At his time, the art does not provide pellicle materials that are transparent to EUV. Consequently, the reflective photomask (reticle) employed in EUV photolithography is susceptible to contamination and damage to a far greater degree than reticles used in conventional photolithography. This situation imposes heightened functional requirements on any reticle SMIF pod designed to store, transport and ship a reticle destined for EUV photolithography use.  
         [0005]     It is well known in the art that unnecessary and unintended contact of the reticle with other surfaces during manufacturing, processing, shipping, handling, transport or storage will likely cause damage to the delicate features on the patterned surface of the reticle due to sliding friction and abrasion. Likewise, it is generally accepted by those skilled in the art that any particulate contamination of the surface of the reticle can potentially compromise the reticle to a degree sufficient -to seriously affect the end products of processes that use such a flawed reticle. In this regard, the art has developed innovative approaches to locate and support the reticle in reticle containers so as to reduce or eliminate sliding friction and consequent abrasion of the reticle and the resultant generation of contaminating particulates. In recognition of the need to maintain a controlled environment around the wafer during storage, processing and transport, the prior art has evolved approaches to isolation technology that allows for control of the environment in the immediate vicinity of a wafer by providing for a container so that it can be kept relatively free from incursion of particulate matter. Typically, containers are provided with standardized mechanical interfaces that allow automatic manipulation of the container by processing machinery. Such containers can hold photomasks of up to 200 mm and are designated standard mechanical interface pods, or SMIF-Pods. Even with such a controlled environment, migration of particulates that may be present inside the controlled environment is still possible due to pressure changes of the air trapped in the controlled environment or turbulence of the trapped air brought on by rapid movements of the container and/or by disturbing the trapped air volume. For example, thin walled SMIF pods may experience wall movement due to altitude related pressure changes causing the trapped air inside the controlled environment to be displaced. Temperature changes can set up convection currents within the container. Dimensional changes of the container and its components due to pressure fluctuations can lead to compromising the sealing between cover and door of the carrier and incursion of particulates within the carrier. Prior art approaches contemplate a breathing apparatus between the external environment and the internal controlled volume of air. The breathing apparatus provides a path for the air to flow. Prior art breathing apparatus may include a particulate filter to block the entry of particulates from the external environment into the controlled environment of the carrier.  
         [0006]     Those skilled in the art will appreciate that particulate contaminants are but one half of the equation. Equally important are gas-phase contaminants or airborne molecular contaminants (AMC) due to ambient air venting or leaking into or getting trapped in a hermetically sealed system. For example, at a suitable dew point temperature, the moisture in the air will condense out of the air and some of it may get deposited onto the reticle. Even with a perfectly sealed container, there is the possibility of air entering into the system when the reticle is removed from and replaced within the container during processing. Water vapor condensing onto the patterned surface of the reticle can interfere with the optics just as a solid particulate would. Other sources of gas-phase or vapor contamination are solvent residues resulting from reticle/pod cleaning operations during the photomask lifecycle, chemical agents. generated by out-gassing from the structural components of the carrier and chemical agents that enter into the carrier from the ambient atmosphere by breaching the hermetic sealing arrangement between the carrier shell and the carrier door. Multiple contamination species are thought to be the largest contributors to gas-phase contamination. These include NH 3  (ammonia), SO 2  (sulphur dioxide), H 2 O (moisture) and condensable organics C6-C10. Depending on the photolithography system, a photomask can be exposed to a laser light source of a wavelength that can range from 436 nm to 157 nm. Currently, 193 nm lasers are quite common. The energy of the laser can initiate chemical reactions that precipitate defect formation and propagation on the surface of the reticle. For instance, some of the chemical species are altered to form highly reactive species such as SO 4   2−  and NH 4   + . Some of these chemicals, such as acids for instance, are reactive with glass and can damage the reticle by etching it to create a haze on the patterned surface. The bases can create resist poisoning. The condensable organics can lead to SiC formation. In general, all of the contaminants can be considered to result in the same effect: crystal growth that degrades the functionality of the reticle. In this respect, the current thinking is that moisture or water is one of the key ingredients required for crystal growth. Essentially, water combines with some of the aforementioned contaminants to form the salts are generally clubbed together under the rubric of crystal growth. Prior art use of dessicants, for example, cannot ameliorate this problem because they cannot reduce the concentration of moisture to low enough levels to prevent salt (or crystal) formation. Likewise, purging a reticle carrier with clean dry air (CDA) or other dry gas may not reduce the moisture concentration to the levels required to avoid crystal growth. There is therefore a need for a contamination control mechanism at each stage of the reticle life cycle.  
         [0007]     One of the approaches commonly employed in the art to ameliorate the effect of the chemical contaminants is periodic reticle/mask cleaning. The mean time between such cleans (MTBC) can approach, for example, approximately 8000 wafers in a 193 nm exposure tool. The threshold of the MTBC is set to prevent mean time between defects (MTBD) printed on the wafer using the reticle/mask. However, there is a limit to the number of such ‘cleans’ a reticle/mask can be subject to before resolution is degraded beyond functionality and the mask must be scrapped. In view of the above, one of skill in the art will recognize the need to ensure that the reticle environment within the carrier remains clean during storage, transportation, manipulation as well as during the standby condition when the carrier is empty of the reticle. While desirable, it is generally infeasible to construct a hermetically sealed environment that is absolutely impervious to incursion by AMCs or other contaminants. It is also infeasible to continuously purge the reticle carrier especially when the reticle and reticle carrier have to be transported or shipped.  
         [0008]     What is needed is some type of structure or device for ensuring that the incursion, concentration and rate of accumulation of AMCs within the photomask carrier is controlled to levels that preclude or significantly reduce the formation of crystalline salts so that the useful life of the photomask can be significantly extended.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a reticle/mask carrier with a controlled environment within which to house a reticle during storage, transport, processing and shipping. According to a primary embodiment of the present invention, the reticle/mask carrier is equipped with means to control the ingress and build-up of particulate and gas-phase contaminants into the controlled environment.  
         [0010]     According to one aspect of the invention, the reticle carrier is provided with a layered filter having specialized filter elements arranged to form a composite sandwich. Each filter element is associated with specialized media characterized by its ability to selectively capture at least one of the several trace impurities known to be present within or known to diffuse into the hermetically sealed space within the reticle carrier.  
         [0011]     According to another aspect of the present invention, the filter media is selected to weakly bind to the contaminants so that the contaminants could be expelled from the filter by subjecting the filter to a flow of pressurized gas thereby regenerating the filter.  
         [0012]     According to a related embodiment of the present invention, the filter is shaped, sized and located so as to present a surface on which particulates will preferentially settle instead of settling on the patterned surface of the reticle. Another aspect of the alternate embodiment contemplates a large filter, i.e. a filter with at least one major surface are that is preferably at least sixty percent of the surface area of the patterned surface, to take advantage of the diffusion length of the particulates within the container to cause the particulates to preferentially settle on the filter as opposed to settling on the patterned surface of the reticle. According to another aspect of the primary embodiment, the filter is preferably shaped and sized substantially proportionate to the reticle and preferably positioned substantially concentrically with respect to the reticle. One aspect of this embodiment at least one surface of the filter is disposed substantially to the carrier door portion with the reticle residing on the reticle supports.  
         [0013]     According to a related embodiment, the present invention provides means for limiting the exposure of the filtration medium to the exterior of the reticle pod as compared to the interior of the hermetically sealed space within the reticle carrier. One aspect of the means involves providing a perforated tray to hold the filter wherein the perforations present a restricted area through which the filter communicates with the external environment. Another aspect of the means involves providing a filter with an additional fluid impermeable layer that is provided with one way slit valves to prevent the ambient air from communicating with the hermetically sealed space within the reticle carrier. Another aspect of the means involves the provision of check valves in the purge ports to keep contaminants from entering the hermetically sealed space.  
         [0014]     According to yet another primary embodiment of the present invention, the reticle carrier is provided with a means to inject pressurized, extremely clean dry air, denominated XCDA, into the hermetically sealed space of the reticle carrier and a means to exhaust the XCDA from the sealed space. The purge gas is sufficiently pressurized to allow egress of the gas through both the filter and the exhaust means. Purging the hermetically sealed space in this manner serves to flush out contaminants as well as dehumidify the filter and the reticle carrier thereby regenerating the filter.  
         [0015]     Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a bottom perspective view of an assembly of a reticle carrier according to a primary embodiment of the present invention.  
         [0017]      FIG. 2  is an exploded perspective view of the assembly of the reticle carrier according to the primary embodiment of the present invention.  
         [0018]      FIG. 3  is a perspective view of a base portion of the reticle carrier of  FIG. 1  shown supporting a reticle.  
         [0019]      FIG. 4  is a bottom perspective looking upward of the base portion of the reticle carrier of  FIG. 3 .  
         [0020]      FIG. 5  is a plan view of the base portion of  FIG. 3 .  
         [0021]      FIG. 6  is a side sectional view through section B-B of the base portion of  FIG. 5 .  
         [0022]      FIG. 7  is a detailed view showing the purge port of  FIG. 6 .  
         [0023]      FIG. 8  is a side sectional view through section A-A of the base portion of  FIG. 5 .  
         [0024]      FIG. 9  is still another detailed view showing the purge port of  FIG. 8 .  
         [0025]      FIG. 10  is a bottom perspective looking up of the cover portion showing the inside surface of the cover portion according to an alternate embodiment of the present invention.  
         [0026]      FIG. 11  is perspective view of an exemplary filter according to the present invention.  
         [0027]      FIG. 12  is a perspective view of a high surface area filter according to a secondary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The accompanying Figures depict embodiments of the reticle carrier of the present invention, and features and components thereof. Any references to front and back, right and left, top and bottom, upper and lower, and horizontal and vertical are intended for convenience of description, not to limit the present invention or its components to any one positional or spatial orientation. Any dimensions specified in the attached Figures and this specification may vary with a potential design and the intended use of an embodiment of the invention without departing from the scope of the invention.  
         [0029]     In  FIGS. 1-11 , there is shown a reticle carrier  100  equipped with a chemical filtration system according to a primary embodiment of the present invention. The reticle carrier  100  (alternatively referred to as a reticle container, a reticle pod, or a reticle box) generally comprises a door portion  106  (alternatively referred to as a base portion) which mates with a carrier shell  112  (alternatively referred to as a cover) to form an hermetically sealed space  118  which provides a sealed environment in which a reticle  124  may be stored and transferred. The term “reticle” in used in a broad sense to include quartz blanks, photo-masks, masks used in the semiconductor industry that are susceptible to damage from particulates and gas-phase chemical contaminants. Generally, the reticle  124  is square shaped with a first surface  126  opposite a second patterned surface  128  having a surface area  129  provided with the etched pattern as discussed above. A reticle lateral surface  130  separates the first surface  126  from the second patterned surface  128  and extends around a reticle perimeter  130 . It will be appreciated that the present invention is not limited by a particular shape of reticle  124 .  
         [0030]     The door portion  106 , best shown in  FIGS. 2, 4  and  5  includes an opposed upper door surface  136  and a lower door surface  142  separated by a lateral wall  148 . A plurality of reticle supports  154 , reticle side positioning members  160  and back positioning members  166  extend outwardly from and are disposed in spaced apart relationship adjacent an upper periphery  172  of and generally about a central portion  178  of the upper door surface  136 . The reticle supports  154  are configured to hold the reticle  124  at a predefined height  156  above upper door surface  136 . The reticle side positioning members  160  and the back positioning members  166  serve to guide manual positioning of the reticle  124  and ensure proper lateral and rearward placement of the reticle on the reticle supports  154  so that the reticle substantially occupies and its volume bounded by a reticle receiving region  168  associated with the door portion  106  and defined by the reticle supports  154 , the reticle side positioning members  160  and the back positioning members  166  as best depicted in  FIG. 3 . A Gasket  184  loops along the upper periphery  172  on the door surface  136 . Preferably, the door portion  106  and the carrier shell  112  conform to the shape of reticle  124 .  
         [0031]     Referring now to  FIGS. 2, 3  and  4 , the door portion  106  is provided with a central hole  190  extending through the door portion  106  and defined by a first opening  196  on the upper door surface  136 , a second opening  202  on the lower door surface  142  and an inside peripheral wall  208  communicating the first opening  196  with the second opening  202 . In an exemplary embodiment, illustrated in  FIGS. 2, 3  and  4 , the first and second openings  196  and  202  are substantially square shaped and are characterized by their respective first and second areas  212  and  214 . The inside peripheral wall  208  extends generally parallel to the lateral wall  148  of the door portion  106  between the first and the second Openings  196  and  202 . The inside peripheral wall  208  is configured with a peripheral shelf  220  suitable for securely supporting a filter frame  226  such that the filter frame  226  is substantially perpendicular to the first opening  196  and is located generally flush with the upper door surface  136 .  
         [0032]     In one embodiment best illustrated in  FIG. 2 , the filter frame  226  can be a semi-rigid, molded plastic receptacle in which a filter  232  in accordance with the present invention may be used. The filter frame  226  is substantially hat shaped with a peripheral flange  242  (alternatively identified as a lip) circumjacent an open end  248 , a filter frame side wall  258  depending from the open end  248  and terminating at a closed end  252  to define a cavity  262  adapted to receive the filter  232 . Closed end  252  has a structure defining a plurality of perforations  264  as best illustrated in  FIG. 4 . Filter frame side wall  258  includes a shoulder  268  with a shape complementary to the peripheral shelf  220  on inside peripheral wall  208 . The filter frame is configured to be inserted through first opening  196  on upper door surface  136  and snug-fittingly received into central hole  190  for detachable mounting in door portion  106  with flange  242  resting on upper door surface  136  and shoulder  268  securely positioned on peripheral shelf  220  of inside peripheral wall  208 . In alternate embodiments, an elastomeric seal or gasket such as for example, the gasket  184  described above, can be interposed between the shoulder  268  and the peripheral shelf  220  to provide a hermetic seal between the filter frame  226  and the filter  232 .  
         [0033]     Filter  232  will next be described with reference to the illustrations of  FIGS. 2 and 12 . Filter  232  can have a variety of constructions each of which provide a fluid-permeable, clean, cost-effective, high efficiency, low-pressure drop, adsorptive composite filter such as the filters described in U.S. Pat. Nos. 7,014,693, 6,761,753, 6,610,128, and 6,447,584 the contents of which are incorporated herein by reference in their entirety.  FIGS. 2 and 12  illustrate an exemplary embodiment of filter  232 . Filter  232  is desirably a fluid-permeable filter that can include several types of adsorptive and non-adsorptive media. Adsorptive media can include, for example, chemisorptive media and physisorptive media. Non-adsorptive media may include particulate impermeable media. One of skill in the art will appreciate that the adsorptive media can be engineered with pore sizes for removal of particulate materials. Such adsorptive media is also exemplary of particulate impermeable media according to the present embodiment. Each type of media can be in separate filter elements. The term, “physisorption,” refers to a reversible adsorption process in which the adsorbate is held by weak physical forces. In contrast, the term, “chemisorption”, refers to an irreversible chemical reaction process in which chemical bonds are formed between gas or liquid molecules and a solid surface. The term “particulate impermeable” refers to the attribute of substantially filtering particulates having a size greater than a threshold size from a fluid flowing through the fluid-permeable but particulate impermeable media. Typically, the particulates may be dislodged from respective particulate impermeable media by reversing the fluid flow thereby substantially restoring the particulate filtering capacity of the media. Referring to  FIG. 2 , an exemplary filter  232  according to the present invention comprises a plurality of removable or replaceable filter elements (alternatively identified as layers, components, or laminas) arranged in parallel in a predefined series of layers. Exemplary Filter  232  includes a base layer  276  of a first particulate impermeable media, a first filter membrane  278  of a first adsorptive media, a second filter membrane  280  of a second adsorptive media and a cover layer  282  of a second particulate impermeable media with the absorptive media layers  278  and  280  being sandwiched between the particulate impermeable media base layer  276  and cover layer  282 . In the primary embodiment, the base layer  276  and the cover layer  282  may comprise, for instance, a filtering non-woven polyester, polyamide or polypropylene material or other similar materials configured for the removal of particulate materials in a fluid stream. Other particulate filter media, such as for instance, a high efficiency particulate air (HEPA) filter medium or an ultra low penetration air (ULPA) filter medium, may also be used singly or in combination without departing from the scope of the present invention. The base and cover layers  276  and  282  prevent particulate incursion into the hermetically sealed space  118  from the ambient atmosphere (from the clean room for example) as well as egress of particulates from the hermetically sealed space  118  to the ambient atmosphere. In the preferred embodiment of the present invention, the first absorptive media associated with the first filter membrane  278  is a first physisorptive media. The term, “untreated,” as used herein, means an activated carbon that has not been modified by chemical treatment to perform chemisorption; rather, untreated, active carbon remains as a physical, nonpolar, adsorbent. The first physisorptive filter element  278 , shown in  FIGS. 2 and 12  can include untreated, activated carbon. The carbon is porous (the specific surface area can be on the order of 1000 m2/g) and can be provided in the form of fibers or particles incorporated into a mat of polymer fibers, either woven or nonwoven. The untreated, activated carbon can be formed from a variety of sources, including coconut shell, coal, wood, pitch, and other organic sources. Further still, a sulfonated copolymer coating can be attached to the untreated, activated carbon. The medium of filter membrane  278  may include other materials such as for instance, granulated activated carbon, bead activated carbon, chemically impregnated carbon, chemically impregnated activated carbon, zeolites, cation exchange resin, anion exchange resin, cation exchange fiber anion exchange fiber, activated carbon fiber, and chemically impregnated activated carbon fiber. Physisorptive media of layer  278  specifically removes acids, organic and inorganic condensable contaminants such as C6-C10 as well as SO 2  gas. Such media are sold, for example, under the trade name Purolyte® by Purolyte Corporation. The second filter membrane  280  is a strongly acidic ion-exchange resin of a second adsorptive medium such as for instance, sulfonated divinyl benzene styrene copolymer in the form of microporous beads. The second adsorptive medium is configured to specifically capture ammonia (NH 4 ) and moisture (H 2 O) both from within the hermetically sealed space  118  and from the ambient atmosphere of the clean room. Such media are sold under the trade name, AMBERLYST® 15DRY or AMBERLYST® 35DRY, by Rohm and Haas. Catalysts with physical properties outside the ranges described above can also be used. The base layer  276  and the cover layer  282  can also serve to retain the granular or particulate media in adsorptive layers  278  and  280 . One of skill in the art will recognize that various combinations of the number of layers, the arrangement of the layers relative to each other, and media types forming the layers may be advantageously used without departing from the scope of the present invention. For example, in an alternate embodiment of the present invention chemisorptive and a physisorptive filter elements can be used. The relative thicknesses of the chemisorptive filter element and the physisorptive filter element being engineered so that the useful life of the two filter elements will be exhausted at approximately the same time in a given environment. Accordingly, a chemisorptive filter element formed of sulfonated polymer can be made thinner than a physisorptive filter element formed of untreated carbon since the physisorptive properties of the carbon will typically be exhausted more quickly than the chemisorptive properties of the acidic, sulfonated polymer. In a different embodiment, the chemisorptive and physisorptive media may be present within a single filter layer. In yet another embodiment, the multiplicity of filter elements may be sequentially supported in a frame container (not shown) to provide a multi-stage filter through which air can pass in a direction perpendicular to each of the layers. Such a multi-stage filter can be replaced as a whole after its filtration capacity is exhausted. Alternatively, filter elements having a high-surface area  338  may be formulated by forming each medium as a packed array of three dimensional cells in  FIG. 13  as disclosed in the prior art. Such a high-surface area  338  filter may be formed by pleating the medium into an accordion-like structure. A prefilter layer (not shown) of hydrophilic medium, which may be separately removable from the filter  232 , may be incorporated above the base layer  276  of the cover layer  282 .  
         [0034]     Still referring to  FIG. 2 , layers  276 ,  278 ,  280  and  282  preferably have the same shape  287 . All the layers have the same surface area  288  bounded by the perimeter  289  of the shape  287  but may have different thicknesses. In assembled condition, the several layers  276 ,  278 ,  280  and  282  are sequentially disposed within the cavity  262  of the filter frame  226  to form a multi-stage filter having a composite sandwich structure of a thickness  290 . The filter frame  226  is inserted through the first opening  196  on the upper door surface  136  and is detachably mounted into the central hole  190  with the flange  242  resting on the upper door surface  136  with the shoulder  268  being supported on the peripheral shelf  220 . In a related embodiment, the filter frame  226  and the filter  232  comprise a cartridge  270  which may be inserted into and removed from the central hole  190  on the upper door surface  136  of the door portion  106  as described in U.S. Pat. No. 6,319,297, the entire contents of which is incorporated herein by reference in its entirety.  
         [0035]     In the primary embodiment of the present invention best explained with reference to  FIGS. 2, 3  and  5 , the first area  212  of the first opening  196  is configured to be substantially proportional to the surface area  129  of the second patterned surface  128  of the reticle  124 . According to one aspect of the particular embodiment, the first area  212  is at least 50% of the surface area and in a further embodiment the surface area is at least sixty percent (60%) of the surface area  129  and preferably in the range of seventy-five percent (75%) to one hundred percent (100%) of the surface area  129 . In the preferred embodiment of the present invention, the first area  212  is substantially concentric with reticle receiving region  168 . Furthermore, the first opening  196  and the location of reticle supports  154  are arranged so that in a assembled configuration, i.e. when the carrier shell  112  is mated to the door portion  106  and the reticle  124  is supported on the reticle supports  154 , the filter  232  is located with the surface area  288  disposed opposite at least a portion of second patterned surface  128  within the hermetically sealed space  118  such that reticle perimeter  130  overlies perimeter  289  of surface area  288 . One of skill in the art will recognize that other operative configurations of surface area  288  and second patterned surface  128  are possible without departing from the scope of the present invention. All of the aforementioned operative configurations are selected to maximize the extent of the surface area  288  relative to the second patterned surface  128  based in part upon the dimensions of the hermetically sealed space  118 , the diffusion length generated during reticle carrier purging, reticle processing, transport, shipping and storage and other conditions the reticle  124  might encounter during its residency within the reticle carrier  100 . The surface area  288  is disposed proximate the second patterned surface  128 . By selecting the extent and location of surface area  288  in the manner of the present invention, the probability, that a particulate present within or entering the hermetically sealed space  118  will preferentially encounter and settle upon the surface  288  instead of diffusing onto the secondary patterned surface  128 , is maximized. To those skilled in the art, the extent of surface area  288  is representative of the total number of fluid passages available for entry of a fluid into the filter  232 . The term “high-surface area” associated with reference numeral  338 , on the other hand, refers to the effective surface area of the total filter media available for filtration as the fluid flows through the entire thickness  290  of the filter  232 . The effective surface area controls adsorption of gases and chemical reactions. In this regard, the filter  232  differs from the prior art SMIF pod filters in that the filter  232  of the present invention is structurally a significant component of the door portion  106  because surface  288  can extend over a substantial portion of the upper door surface  136 . Furthermore, in the assembled configuration, base layer  276  is positioned on closed end  252  so that filter  232  places the hermetically sealed space  118  in fluid communication with the ambient atmosphere outside the reticle pod  100  through the plurality of perforations  264  as best seen in  FIG. 4 .  
         [0036]     The primary embodiment of the present invention provides means for limiting the exposure of the filter media  276 ,  278 ,  280 ,  282  and other media that the filter  232  may comprise of, to the ambient atmosphere external to the reticle carrier  100 . One such means is exemplified in the illustration of  FIGS. 2 and 4 . One skilled in the art will appreciate that the extent of surface  288  of the filter  232  in fluid communication with the hermetically sealed space  118  within the reticle carrier  100  is generally maximized as explained above. However, the extent of the base layer  276  of the filter  232  in direct communication with the ambient atmosphere is much smaller due to the limited number of perforations  264  and the limited area  266  of each perforation. Another means provided in the present invention include a fluid impermeable membrane  360  adjacent closed end  252  below base layer  276 . Angularly cut criss-crossed slits (not illustrated) in the fluid impermeable membrane  360 , the slits being preferably located opposite perforations  264 , allow the fluid impermeable membrane slit portions to extend axially thus opening when the interior of the pod is pressurized in the purge mode. Generally the pod will not be subjected to a vaccum thus the slit portions will not open inwardly only outwardly effectively operating as a multiplicity of one-way valve which would open during the flow of fluid out of the reticle carrier  100 . Similar valves are described in U.S. Pat. No. 5,482,161, the entire contents of which is incorporated herein by reference. Additionally, the outer membrane  360  can be a hydrophobic membrane to prevent incursion of moisture from the ambient atmosphere into the reticle carrier  100  through the filter  232 . When the SMIF pod is pressurized providing a significant outward flow through the filter the hydrophobic characteristic may be overwhelmed and allow the exit of air or purge gas with some moisture being carried therewith. Under static minimal in and out flow through the filter, the hydrophobic effect is expected to effectively reduce the flow of moisture laden air from the exterior to the interior.  
         [0037]     According to the primary embodiment of the present invention, the concentration of moisture within the hermetically sealed space  118  is preferably maintained at concentration levels approaching a few parts per billion (ppb). Using prior art approaches, such as dessicants for example, moisture concentrations within the hermetically sealed space  118  can be controlled only to within a few parts per million (ppm). The level of humidity control achieved by coupling reticle pod  100  to a purging system which periodically flows a very dry gas, such as for example dry nitrogen gas or dry argon, through the hermetically sealed space  118 . Referring now to FIGS.  2 ,  6 - 9  there is illustrated the construction of a reticle structure  100  equipped for coupling to a purging system (not shown) according to a primary embodiment of the present invention. As seen in  FIG. 2 , upper periphery  172  of door portion  106  is configured with injector and extractor ports  306  and  312  extending through door portion  106  between upper door surface  136  and lower door surface  142  in a direction generally parallel to lateral wall  148  of door portion  106 . Injector port  306  and extractor port  312  are configured to coaxially receive an injector fitting  318  and extractor fitting  324 . Injector and extractor fittings  318 ,  324  may be threadably coupled to injector and extractor ports  306  and  312  respectively. Other connection means may also be used without departing from the scope of the invention. Injector fitting  318  is detachably coupled to gas inlet line (not illustrated). Extractor fitting  324  is detachably coupled to a gas removal line (not illustrated) which may in turn be connected to a gas evacuation means (not illustrated). Each of the injector fitting  318  and extractor fitting  32  is equipped with a check valve  330  configured to allow a unidirectional flow past and prevent ingress or egress of gaseous or particulate contaminants into the hermetically sealed space  118  when the system is not in use. Diaphragm valves with slits such as those described in U.S. Pat. No. 5,482,161 referenced above may also be employed in conjunction with or without the check valves  330 . This is a mechanical means for limiting the exposure of the filter media  276 ,  278 ,  280 ,  282  and other media that the filter  232  may comprise of, to the ambient atmosphere external to the reticle carrier  100 . One of skill in the art will recognize that injecting a very dry purge gas, for example dry nitrogen gas and dry argon gas, under pressure into the hermetically sealed space  118  will cause at least a portion of the purge gas to egress through the filter  232  and out into the ambient atmosphere through the closed end  252 . An apparatus and method of purging the reticle carrier  100  is described in U.S. Pat. No. 5,988,233 and U.S. Pat. No. 5,810,062, the entire contents of the two patents being incorporated herein by reference in their entirety. In an alternate embodiment, the extractor fitting  324  is replaced by an injector fitting  318  coupled to the gas inlet. In this configuration, the hermetically sealed space  118  is pressurized by the purge gas flowing into it through the injector fittings  318 . The purge gas exits the hermetically sealed space  118  through the filter  232 . Generally, purging the hermetically sealed space  118  removes trace contaminants by entraining them in the gas flow. Purging with dry gas also dehumidifies the filter  232 . Purging under pressure may dislodge and thus remove particulates and other contaminants that may be weakly bonded to the physisorptive media filter elements and the filter elements that specifically filter particulates. In effect, purging regenerates filter  232  by replenishing its capacity to adsorb contaminants. One of skill in the art will appreciate that the capacity of the filter  232  of the present invention may also be replenished by replacing the depleted filter  232 .  
         [0038]     Of course, many alternative embodiments of the present environmental control for a SMIF reticle pod are possible and are within the scope of the invention, as will be appreciated by those of skill in the art. Such embodiments would include, but are not limited to, varying the numbers and locations of the layers comprising the filter, varying the location of the filter, varying the area of the filter, using several smaller filters and using a plurality of purge ports.  
         [0039]     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the invention. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples described herein.