Abstract:
A storage and retrieval system is provided for safely and efficiently storing reticles in a clean environment. An enclosed storage unit is provided for storing the reticles, and other items such as wafers and the like, in an environment which minimizes the amount of contaminants and is suitable for use in a semiconductor fabrication clean room. A retrieval unit is provided separate from the enclosed storage unit for accessing and staging the reticles before they enter and leave the storage unit for minimizing exposure of the storage unit. The storage unit includes a movable storage matrix having a plurality of bays for storing the reticles. The movable storage matrix is selectively moved or rotated by a drive mechanism that is located external to the storage unit so that the storage unit is substantially free of contaminant generating components.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to provisional patent application Ser. No. 60/216,194 filed Jul. 6, 2000, the disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     In manufacturing integrated semiconductor circuits, many photolithography processes are performed which require repeated handling of different reticles associated with each of these processes. The reticles contain a mask of the pattern that is to be formed on the semiconductor wafer. Due to the multitude of photolithography processes, semiconductor fabrication clean rooms must store hundreds or thousands of reticles and wafers. As the most valuable component of the fabrication process, reticles are desired to have the highest degree of protection against loss, damage and contamination. If any disruptions occur in the photolithography processes, costly results follow, such as long stepper idle times, reduced productivity and missed product shipment dates. Therefore, a safe and efficient manner of storing and retrieving the reticles is desired. 
     Present storage and retrieval systems are designed to store and retrieve all of the reticles within a single storage unit. In conventional systems, catastrophic failures to the unit may at least temporarily disable the fabrication operations and potentially destroy the entire inventory of reticles stored therein. These conventional storage units include bays for storing the materials in a two dimensional linear matrix or grid type of arrangement. To store all of the necessary materials for the clean room manufacturing operations, a large storage unit is required. Because the height of the storage unit is limited by the ceiling of the clean room, the length of the unit must be sufficiently long to accommodate the needed storage bays. Therefore, the unit requires a large footprint which places undesirable constraints into the design and layout of the fabrication clean rooms. 
     A retrieval mechanism is used to retrieve the stored materials from the bays. However, because the retrieval mechanism is required to traverse great distances across the unit, difficulties arise in the repeatability of the mechanism during the retrieval process. In particular, the retrieval mechanism has difficulties in precisely traveling to each bay in the matrix. Travel imprecisions may cause the retrieval mechanism to be misaligned when the desired position in the matrix is reached which may cause damage when attempting to access the materials. As the retrieval mechanism travels to the positions at the outer edges of the grid, these travel imprecisions will be compounded and the likelihood for significant damage to the materials greatly increases. Furthermore, due to the relatively short and wide asymmetrical configuration of the units, difficulties arise in maintaining even airflow throughout these units. Specifically, because the airflow is not uniform, contaminants accumulate at the portions of the grid where the air circulation is insufficient and contaminants may be created by turbulence where the air circulation is too great. As a result, the potential damage to the stored materials increases in these areas of the unit. 
     A continuous flow of filtered air is desired over the stored materials to prevent particulates from accumulating and contaminating on their surfaces. One goal in the design of clean room equipment is to direct a supply of uniform filtered air over the stored materials. Typically, the flow direction of filtered air in a semiconductor clean room facility is vertical, whereby the filtered air enters through the ceiling, travels vertically downward, and then exits through a perforated floor. Equipment is preferred that utilizes airflow for controlling airborne contamination by exhausting at or near the floor to minimize the release of particles into the room so that the exposure risk of adjacent equipment to possible contamination or particulates from the discharged air is reduced. Most process equipment utilizes the natural vertical flow of air in the room as the primary source of clean filtered air by configuring the equipment with open or perforated tops and a venting system at the bottom for passing the filtered air therethrough. 
     If more control over the airflow quality is desired within the storage chambers of the equipment, pressurized air is often provided via ductwork and filter elements in the equipment to generate filtered air closer to the materials with “point of use” filters. More specifically, some types of equipment include a subsystem or module including fans (or blowers), and filter elements. Such subsystems, known as Fan Filter Units (FFUs) provide more control of the airflow. The fans generate positive pressure that force air through the filter element material. Many FFUs have adjustments or variable controls for the blower output, which allows control over both the pressure and the velocity of the generated air. 
     FFUs are typically packaged together into a module such that the blower is placed directly behind a planar filter element and enclosed in a housing that will allow the output of the fan to exhaust only through the filter element. The FFUs are then placed into the equipment either as a top mounted unit for directing airflow downward, or a side mounted for generating horizontal airflow. The ability of the blower to uniformly generate air over a large surface area filter often causes irregularities in the airflow rate exiting the filter. In systems requiring large areas of filter coverage, multiple FFUs are typically assembled into an array for generating sufficient uniformity of the discharged air. 
     Another goal in clean room design is to ensure that uniform airflow travels through the system after leaving the surface of the filter elements. Areas in storage chambers having non-uniform, turbulent, or little or no airflow may result from chambers with asymmetric volumes, changes to the airflow direction, multiple airflow directions, and uncontrolled venting. Some known systems incorporate FFUs and regulate the exhaust rate so that a positive internal pressure with respect to the surrounding environment is developed and maintained. As a result, contamination migration into the chamber may be reduced. 
     However, these systems fail to generate uniform flow of filtered air that is required within a high aspect ratio volume of storage chambers in storage and retrieval systems. Within such storage chambers, a vertical flow direction does not prevent particle accumulation on the bottom surface of the reticles or substrates stored horizontally in a shelf of the chamber. Also, airflow traveling over the edges of the reticles often causes turbulence which may lead to contamination and damage to the reticles. 
     Furthermore, any particulate contamination that is present on the reticles in the upper chamber may become dislodged. Accordingly, a higher concentration of particulates results in the air traveling downward through the system, and the exposure of the reticles or substrates stored in the lower storage locations are subjected to a much higher risk of contamination. Multiple units of rectangular/planar FFUs mounted along the sides of the chamber may generate an inward horizontal flow of air. However, because of the proximity of the FFUs to the movable storage locations, the air tends to coalesce at the center of the chamber after flowing past the storage locations because no efficient means exists for exhausting the air exits without generating turbulence. Such a storage chamber also would require an access point to facilitate the loading and unloading of stored reticles. At such an access point, it would not be possible to place a filter element and therefore a disruption to the uniformity of airflow would result in this area. 
     It is therefore desirable to have a storage and retrieval system for reticles, wafers and similar items that safely and precisely stores and retrieves the items in a clean room environment. A system is also desired which minimizes contamination by uniformly and optimally controlling the flow of air therethrough. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a storage and retrieval system for safely and efficiently storing reticles in a clean environment. An enclosed storage unit is provided for storing the reticles, and other items such as wafers and the like requiring a clean environment, which minimizes the amount of contaminants and is suitable for use in a semiconductor fabrication clean room. A retrieval unit is also provided separate from the enclosed storage unit for accessing and staging the reticles before they enter and leave the storage unit so that exposure of the storage unit is minimized. The retrieval unit includes a reticle transfer unit for passing the reticles through an access port between the storage and retrieval units. 
     The storage unit includes a movable storage matrix having a plurality of bays for storing the reticles. Preferably, the movable storage matrix is cylindrical with the bays located about the circumference thereof. The movable storage matrix is selectively moved or rotated by a drive mechanism that is located outside of the enclosed storage unit. The drive mechanism moves or rotates the movable storage matrix so that the access port is aligned with a desired bay or column of bays. After the drive mechanism aligns the movable storage matrix and the access port, the reticle transfer unit then retrieves the desired reticles from the corresponding bay. By rotating the movable storage matrix for accessing the reticles, the distance required by the reticle transfer unit to move is greatly reduced. Thereby, the reticles can be more precisely retrieved and stored with greater repeatability so that handling damage and contamination are minimized. 
     The system is designed so that the storage unit is essentially enclosed except during the storage and retrieval operations during which the storage unit is only minimally exposed. The storage unit is also designed to be substantially free of motors, moving parts, circuitry, and other contaminant generating components. For instance, features associated with the operation of the storage unit are located external to the storage unit, such as the drive mechanism for moving the movable storage matrix. By removing such components from the storage unit, these sources of contamination are reduced or even eliminated. 
     The compactness and symmetrical design of the system allows air to circulate uniformly throughout the storage unit. The air is vented from the storage unit to increase the uniformity of the airflow throughout the unit. By uniformly circulating and venting filtered air throughout the storage unit, the amount of potential contaminants exposed to the reticles are minimized throughout the system. The compact design also allows the system to utilize a small footprint so that greater flexibility is achieved in the placement of the system within the manufacturing room. 
     Other aspects, features and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood by reference to the following detailed description of the invention in conjunction with the drawings, of which: 
     FIGS.  1 ( a ) and  1 ( b ) illustrate a storage and retrieval system according to an embodiment of the present invention; 
     FIG. 2 illustrates a sectional view of the interior of a storage and retrieval system according to an embodiment of the present invention; 
     FIG. 3 illustrates a view of a support and driving mechanism for a storage and retrieval system according to an embodiment of the present invention; 
     FIG. 4 illustrates side supports for a storage and retrieval system according to an embodiment of the present invention; 
     FIG. 5 is a cut-away view of the side supports which illustrates air return panels for a storage and retrieval system according to an embodiment of the present invention; 
     FIG. 6 is a detailed illustration for portions of the reticle transfer unit according to an embodiment of the present invention; 
     FIG. 7 is a side view illustrating the storage and retrieval mechanism and the reticle garage according to an embodiment of the present invention; 
     FIG. 8 illustrates the pulley system for the reticle transfer unit according to an embodiment of the present invention; 
     FIG. 9 illustrates an air filter system for a storage and retrieval mechanism according to an embodiment of the present invention; 
     FIG. 10 is an exploded view which illustrates components of the air filter system according to an embodiment of the present invention; 
     FIG. 11 illustrates a diffuser assembly according to an embodiment of the present invention; 
     FIGS.  12 ( a ) and  12 ( b ) illustrate supports and channels for the diffuser assembly according to an embodiment of the present invention; 
     FIG. 13 illustrates a diffuser assembly in an enclosed chamber according to an embodiment of the present invention; and 
     FIG. 14 illustrates a diffuser assembly in an enclosed chamber according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS.  1 ( a ) and  1 ( b ) illustrate exemplary views of a storage and retrieval system  100  for reticles according to an embodiment of the present invention. It should be appreciated that the present storage and retrieval system  100  is also suitable for wafers and other items requiring a clean environment. The system  100  includes an enclosed storage unit  110  for storing reticles, a retrieval unit  120  for accessing the reticles, a controller  130  for controlling the operation of the system  100 , an air supply  140  for supplying filtered air to the storage unit  110 , and a staging area  150  for staging the reticles when entering or leaving the storage unit  110 . The storage unit  110 , which will be described in more detail in the figures that follow, stores the reticles in an environment which minimizes the amount contaminants by substantially separating the storage unit  110  from the rest of the system. Specifically, motors, moving parts, circuitry and the like are designed to be positioned outside of the storage unit  110  so that they are substantially eliminated from its interior. The storage unit  110  includes access panels  112  for initial loading and later servicing of its interior and at least one access panel  112  having vents  114  for venting air from the storage unit  110 . The circuitry portion  170  at the rear of the system  100  is illustrated in FIG.  1 ( b ). Rear electrical access panels  172  and  174  are provided in the circuitry portion  170 . 
     The retrieval unit  120 , which is separate from the storage unit  1101  is used to place and retrieve the reticles to and from the storage unit  110 . A reticle transfer unit and an access port (which will be shown in following figures) are included within the retrieval unit  120  for accomplishing the storage and retrieval of the reticles. The retrieval unit  120  includes a door  122  having a viewing window  123  which may be opened to access the reticles within the interior thereof. The retrieval unit  120  is enclosed so that contaminants are minimized to the extent possible in view of the accessing that is required. Moving parts, motors, and circuitry are included within the retrieving unit  120  but isolated so that contaminants are prevented from being unnecessarily introduced. The retrieval unit  120  also includes side viewing windows  124  and at least one venting panel  126 . 
     A movable arm  132  preferably attaches the controller  130  to the retrieval unit  120  so that storage and retrieval of the reticles can be viewed while operating the controller  130 . The movable arm  132  allows the controller  130  to be optimally positioned. Preferably, a computer having a sufficient microprocessor and memory for operating the control software is used as the controller. The staging area  150  is positioned proximate to the door  122  for placing the reticles on the top surface thereof. 
     The air supply  140  may be attached either to the top or side of the system  100 . The air supply  140  supplies filtered air to the storage unit  110  and will be described in more detail with reference to the following figures. In the present embodiment, the air supply  140  is attached to the top of the storage unit  110 . However, in other embodiments, the air supply  140  may be attached to the side of the storage unit  110  when the system  100  is constrained by the ceiling height in the room. 
     A cut-away view for one embodiment of the system is shown in FIG. 2. A more detailed description for the interior of the system will be provided with FIGS. 2-5. The interior of the storage unit  110  is shown to include a movable storage matrix  200 , top and bottom matrix supports  220  and  222 , and storage frame supports  230 . In the present embodiment, the movable storage matrix  200  is a cylindrical carousel including a plurality of bays  202  positioned between bay supports  204 . The bays  202  are designed to be of the appropriate dimensions for holding the desired reticles. Although the present embodiment illustrates the movable storage matrix  200  being configured as a cylindrical carousel, it is appreciated that the movable storage matrix  200  may be designed to be of any shape and configuration, such as triangular, rectangular or octangular shapes as just a few examples, so long as the movable storage matrix  200  is rotatable or movable within the storage unit  110 . The movable storage matrix  200  includes top and bottom matrix supports  220  and  222  for aligning the bays  202  and bay supports  204 . Additional spacers  206  may be placed between the bay supports  204  as necessary to maintain alignment based upon the height of the movable storage matrix  200 . 
     The bottom matrix support  220  is engaged with an engagement gear  300 . In the present embodiment, the movable gear  300  is a slightly smaller circle than the circular opening of the bottom matrix support  220  so that the gear  300  and the bottom matrix support  220  fit tightly together. To ensure that the gear  300  and the bottom matrix support  220  move together without slipping, notches  302  and  304  may be formed in the gear  300  to engage with tabs (not shown) of the bottom matrix support  220 . It is appreciated that the gear  300  and the opening in the bottom matrix support  220  may be in different shapes other than circular. 
     The gear  300  is placed on a sub-floor  225  of the storage unit  110 . The reticles stored in the bays  202  of the movable storage matrix  200  above the sub-floor  225  are separated from the movable parts, motors and circuitry below the sub-floor  225 . The gear  300  extends down underneath the sub-floor  225  and connects to a storage matrix drive motor  210  by a belt  212 . The drive motor  210  is placed away from the storage unit  110  and beneath the retrieval unit  210  in the present embodiment to reduce the possibility of contaminants being introduced into the storage unit  110 . 
     The storage frame supports  230  are attached to the corners of the sub-floor  225  so that panels  112  and other surfaces or covers can be placed therebetween to enclose the storage unit  110 . The surface of the storage unit  110  adjacent to the retrieval unit  120  includes a pair of panels that cover both sides of the surface while leaving a center column open as the access port. Each of the storage frame supports  230  includes an air return passageway  400  extending the entire length thereof. Each of the storage frame supports  230  also includes an air vent  410  slightly above the sub-floor  225 . The storage frame supports  230  include variable air return panels  500  that allow the air to escape from the interior of the storage unit  110  to the air return passageway  400 . The variable air return panels  500  are designed to allow varying amounts of air to escape so that a uniform air flow is achieved throughout the storage unit  110 . 
     Generally, the air return panels  500  that allow less air to escape are positioned near the bottom of the storage unit  110  while ones of the air return panels  500  that allow more air to escape are positioned near the top of the storage unit  110 . In the present embodiment of FIG. 5, the air return ducts are configured with a plurality of perforated air return panels  502 ,  504 ,  506 ,  508 ,  510 , and  512  in varying percentages of return area openings ranging from a 5% return area opening at the bottom air return panel  502  to a 50% return area opening at the top most air return panel  512 . The purpose of the air return panels  502 ,  504 ,  506 ,  508 ,  510 , and  512  is to regulate the flow rate into the air return passageway  400  and to compensate for frictional loss and hydrostatic pressure differences that would otherwise effect the uniformity of the airflow rate entering each of the air return panels as a function of the elevations within the storage unit  110 . The net result is to establish a uniformly horizontal flow pattern within the entire elevation of the storage unit  110 . 
     Because the percent open area of perforations required is dependent on the specific geometry of the storage unit  110 , the cross section of the air return passageway  400 , and the total volume flow rate, other percent open area values, the quantity of the air panels, and the rate of change of perforations can therefore be envisioned. For instance, in one exemplary embodiment, the variable air return panels are configured to include the air return panel  502  having a 5% air return opening, the air return panel  504  having a 10% air return opening, the air return panel  506  having a 20% air return opening, the air return panel  508  having a 30% air return opening, the air return panel  510  having a 40% air return opening, and the air return panel  512  having a 50% air return opening. It is appreciated that the return area openings are not proportional to the amount of air entering the respective air return panels. The purpose is to establish and maintain a uniform amount (volume flow rate CFM) entering the duct from top to bottom. Without the aid of the restricted perforation rate, the frictional losses associated with pipe flow and the varying flow velocity of the air within the air return passageway  400  (the velocity increases towards the base), the air would tend to follow the path of least resistance. In this case, a large proportion of the air would enter the bottom most air return panel, which is closest to the external exhaust port of the duct, and therefore prevent uniform horizontal flow rate from the air supply  140  to the air return passageway  400  as a function of elevation. The air return panels configuration according to the embodiments of the present invention is directed to maintaining a uniform horizontal flow pattern within the storage unit  110 . 
     A cut-away view of the retrieval unit  120  is also illustrated in FIG.  2 . The retrieval unit  120  includes two frame supports  240  at the surface adjacent to the storage unit  110  and two frame supports  242  at the opposite front surface of the retrieval unit  120 . Panels, doors, vents and windows  122 ,  123 ,  124 ,  126  and  128  are placed between these supports  240  and  242  to enclose the retrieval unit  120 . A moveable reticle transfer support  250  is positioned within guides  252  of frame supports  240  so that the reticle transfer support  250  may move vertically along the length of the frame supports  240  in response to the controller  130 . 
     A reticle transfer unit  260  is connected within the retrieval unit  120  in a manner which allows horizontal, vertical, backward and forward movements in response to commands from the controller  130 . The reticle transfer unit  260  includes a reticle garage  262  attached to a support housing  264 . The reticle garage  262  is used to deliver and retrieve the reticles to and from the movable storage matrix  200  by being slidably engaged in a track  266  of the support housing  264  to move towards and away from the storage unit  110 . The support housing  264  is slidably engaged in a track  254  of the module support  250  which allows horizontal movement of the reticle transfer unit  260 . As a result, the reticle transfer unit  260  is able to precisely move to any coordinate along the front surface of the storage unit  110  so that desired reticles can be accessed from the appropriate bays  202 . 
     FIGS. 6-8 illustrate the reticle transfer unit  260  and its relation to the system in more detail. A covered entry port  610 , a transfer block  620 , and a body  630  are included as part of the reticle garage  262 . In storing or retrieving the reticles, an operator places or recovers the desired reticle from the body  630 . When storing, the desired reticle is placed on the body  630 , then the reticle contacts the transfer block  620  and slides along the body  630  through the entry port  610  and then grippers  622  extending from the transfer block  620  are used to place the reticle into the desired bay  202  of the movable storage matrix  200 . When retrieving, the grippers  622  of the transfer block  620  access the reticle from the desired bay  202  and through the entry port  610  onto the body  630 , and then the reticle slides along the transfer body  630  by contact from the transfer block  620  for recovery by an operator. A tracking mechanism  800  is used to move the reticle transfer unit  260  along the frame supports  240 . The tracking mechanism  800  includes lower and upper housings  810  and  812  for a pulley  820  and wheels  820  for guiding the lower housing  810  along the frame supports  240 . 
     FIGS. 9 and 10 illustrate an embodiment of the air supply  140 . In this embodiment, the air supply  140  includes a side mounted filter motor  910  and the air filter unit  920 . The air filter unit  920  is placed directly above the movable storage matrix  200  on top of the storage unit  110  and is connected to the filter motor  910 . The filter unit  920  includes a filter  922  that extends through the center of the movable storage matrix  200 , a vault roof  924 , and a filter mount  926  for connecting the filter unit  920  to the storage unit  110 . A tensioning rod  930  extends through the center of the filter  922  and is connected to an impeller  940 . The impeller  940  includes impeller housings  942  and  943  and an impeller mount  945 . A pressure sensor  950  for sensing the pressure within the storage unit  110  is located on the vault roof  924 . A top  960  and a finger guard  962  cover the top of the air filter unit  920 . 
     FIG. 11 illustrates an embodiment of the present invention directed to an air diffuser assembly  1100  for generating a uniform flow of filtered air within a storage matrix. The diffuser assembly  1100  is centrally placed within the storage matrix and includes one or more tube shaped filter elements  1120  and  1122 , an air source  1110  attached at the top of the diffuser assembly  1100 , and an end cap  1130  attached at the bottom of the diffuser assembly  1100 . The tube shaped filter elements  1120  and  1122  are preferably of a cylindrical shape. However, filter elements constructed on a plurality of planar sides (facets) are another example of the filter elements  1120  and  1122  that may be used. 
     The filter elements  1120  and  1122  are made of a material which sufficiently restricts airflow to require a high differential pressure (the pressure P 2  in storage zones  1140  and  1142  being much less than the pressure P 1  within the diffuser assembly  1100 ) for enabling airflow through the filter elements. More particularly, the material may be homogenous to the degree that when exposed to a sufficiently high differential pressure (P 1 &gt;&gt;P 2 ), the flow of air exiting the entire surface of the filter elements  1120  and  1122  achieves the desired uniformity. A fan or blower  1112  is mounted directly at the top of the diffuser assembly  1100  to provide the source of pressurized air. Alternatively, a fan or blower may be mounted remotely and connected using a duct system (not shown). 
     The back pressure created by the filter elements  1120  and  1122  counteracts the residual downward flow velocity and entrance velocity effects as the air enters the diffuser assembly  1100 . Thereby, the construction of a diffuser assembly with a high aspect ratio (length/diameter) may be constructed without affecting the uniformity of flow exiting therefrom. Because the internal static pressure inside the diffuser assembly  1100  is equal throughout, the motive force for generating the exit flow will also be uniform, and a uniform airflow across the entire surface of the diffuser assembly  1100  results as shown by the arrows in FIG.  11 . When the surrounding environment is either open, or is in a symmetrically vented chamber, the present diffuser assembly  1100  is sufficient to generate a uniform flow of air. In the case where cylindrically shaped filter elements  1120  and  1122  are installed with a vertical axis of symmetry, the airflow is horizontal, and exits radially at a uniform rate regardless of the elevation along the surface of the diffuser assembly  1100 . 
     Once the air exits the diffuser assembly  1100 , the uniformity of airflow is desired to be maintained. Initially, the air exiting the diffuser assembly  1100  travels radially outward, and as the air travel increases along the radius, the velocity decreases at a corresponding rate. To maintain an optimum airflow rate over the stored reticles, the storage matrix includes a plurality of storage columns  1211 - 1221  having wedged shaped cross sections as illustrated in FIGS.  12 ( a ) and  12 ( b ). The angle of the wedge shaped cross-sections of the storage columns  1211 - 1221  is such that the side channels  1230 - 1240 , where the airflow passes and the stored reticles are supported, are parallel. Therefore, as the filtered air passes through the storage locations, no further reduction in the airflow rate occurs, and the stored reticles are exposed to a uniform airflow rate across their entire top and bottom surfaces irrespective of the radial distance from the surface of the filter elements  1120  and  1122 . 
     For cases in which the diffuser assembly  1100  is in an enclosed chamber  1300 , the method for removal of the air also affects the resulting uniformity of the airflow within the chamber  1300 . As previously discussed, it is preferable to vent semiconductor equipment at or near the floor. Although the diffuser assembly  1100  generates uniform airflow, if one or more vents  1310  and  1312  are placed near the bottom of the chamber  1300 , the airflow, once exiting the surface of the diffuser assembly  1100 , tends to follow the path of least resistance. Accordingly, air exiting the bottom of the diffuser assembly  1100  has a shorter path to travel for exit from the chamber  1300  based on its proximity to the vents  1310  and  1312  and tends to continue traveling horizontally. However, air exiting towards the top of the diffuser assembly  1100  has a longer distance to travel and tends to develop a downward flow component shortly after exiting the diffuser assembly  1100  as shown by the arrows. 
     To counteract this tendency of the airflow to become disturbed by the asymmetric venting, a system of air returns is provided in an embodiment of the present invention as illustrated in FIG.  14 . In this embodiment, air is collected in a uniform manner and conveyed to exhaust points while maintaining the uniformity of airflow within the chamber  1400  prior to entering air returns  1410  and  1412 . The air returns  1410  and  1412  are hollow chambers (ducts) located around the central diffuser assembly  1100  and storage zones  1140  and  1142  to form an outer enclosure of the storage matrix. 
     The surface of the air returns facing the storage matrix are perforated, in one embodiment of the present invention, through the use of multiple perforated panels, to allow the air within the chamber  1400  to enter therethrough. The perforation schedule for panels of the air returns  1410  and  1412 , commonly expressed in percent open area, is non-uniform as a function of the elevation within the chamber  1400 . A smaller perforation schedule is used near the bottom of the chamber  1400  to add a restriction to the airflow (resistance to the airflow) and is gradually increased to a larger perforation schedule near the top of the chamber  1400  to encourage airflow. As a result, the effects of both the travel distance, and frictional flow losses that result are counteracted in the air returns  1410  and  1412 . 
     With the proper perforation schedule applied to the air returns  1410  and  1412  as a function of elevation within the chamber  1400 , the direction and uniformity of airflow at the exit time from the surface of the diffuser assembly  1100  until entering the air returns  1410  and  1412  are maintained. While the air traveling within the air returns  1410  and  1412  is no longer uniform, the performance of the charter  1400  at maintaining a uniform rate of filtered air across the stored reticles is no longer affected since the airflow has already passed over the stored reticles. The particular perforation schedule required to counteract the effects of the asymmetric exhaust is dependent on the specific geometry of the chamber, aspect ratio, air return cross section, and the particular airflow rate desired and can be determined through the application of known flow analysis equations, computer modeling or experimentation. 
     Adjustable exhaust ports  1420  and  1422  may be used in combination with the air returns  1410  and  1412  to allow for additional control of the airflow rates within each of the air returns  1410  and  1412  so that a positive pressure with respect to the clean room is maintained. The adjustable exhaust ports  1420  and  1422  may also counteract any negative effects as a result of the environment immediately surrounding the storage and retrieval system such as adjacent equipment or walls that may effect the air exhaust. 
     It will be apparent to those skilled in the art that other modifications to and variations of the above-described techniques are possible without departing from the inventive concepts disclosed herein. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.