CROSS-REFERENCE TO RELATED APPLICATIONS/PATENTS

This Application is related to the following Applications/Patents, all assigned to the assignee of the subject Application:

SEALED STANDARD INTERFACE APPARATUS, U.S. Pat. No. 4,674,939;

SEALABLE TRANSPORTABLE CONTAINER HAVING A PARTICLE FILTERING SYSTEM, U.S. Pat. No. 4,724,874;

CONTAINER HAVING DISPOSABLE LINERS, U.S. Pat. No. 4,739,882;

BOX DOOR ACTUATED RETAINER, U.S. Pat. No. 4,815,912;

SEALABLE TRANSPORTABLE CONTAINER HAVING IMPROVED LATCH MECHANISM, U.S. Pat. No. 4,995,430;

METHOD AND APPARATUS FOR TRANSFERRING ARTICLES BETWEEN TWO CONTROLLED ENVIRONMENTS, U.S. Pat. No. 5,169,272; and

SEALABLE TRANSPORTABLE CONTAINER HAVING IMPROVED LINER, Ser. No. 07/865,297, filed Apr. 8, 1992.

Each of these Applications/Patents is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to transportable containers for standardized mechanical interface systems for reducing particle contamination.

2. Description of the Related Art

A standardized mechanical interface (SMIF) system may be used to reduce particle fluxes onto wafers and/or reticles in semiconductor fabrication. This purpose is accomplished, in part, by mechanically ensuring that during transportation andstorage the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment. The SMIF concept isbased on the use of a small volume of controlled (with respect to motion, gas flow direction and external contaminants), particle-free gas to provide a clean environment for articles.

A SMIF system has three main components: (1) sealed pods, having a minimal volume, used for storing and transporting cassettes holding reticles or wafers; (2) enclosures placed over cassette ports and wafer processing areas of processing stationsso that the environments inside the pods and enclosures (having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette fromexternal environments. The sealed pods typically include a box, a box door and a latch for holding the box door in a closed (or sealing) position with respect to the box. The enclosures typically include a port plate, a port door and a latch forholding the port door in a closed (or sealing) position with respect to the port plate. Instead of a latch, another mechanical apparatus can be used to hold the port door in the closed position.

Wafers are stored and transported in the pods, and are transferred from a pod to a processing station in the following manner. First, a pod is placed on the interface port on top of the enclosure. Then, latches release the box door and theenclosure's port door simultaneously. The box door and the port door are opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the box and interface port doors. A mechanicalelevator lowers the two doors, with the cassette riding on top, into the enclosure covered space. A manipulator picks up the cassette and places it onto the cassette port/elevator or other receiving area of the processing station. After processing, thereverse operation takes place.

Processing stations with enclosures may have an interior region which is a controlled atmosphere; for example, controlled humidity (e.g., de-humidified), nitrogen, argon, or other appropriate gas. In situations where the processing station has acontrolled atmosphere, it is desirable to prevent contamination of the controlled atmosphere in the processing station when loading articles from the SMIF pod to the processing station. One mechanism for preventing such contamination is to use a SMIFpod having a particle filtering system as disclosed in U.S. Pat. No. 4,724,874, and to control the environment in the interior region of the SMIF pod by the removal or introduction of appropriate gases into the SMIF pod. Thus, a clean room facilityusing a SMIF pod may wish to inject a gas for example, nitrogen, into the SMIF pod to create an inert environment.

In order to maintain the inert environment inside the SMIF pod, the seal between the box and box door of a SMIF pod has been improved to reduce leakage of the gas from inside the SMIF pod. As seal technology improves, the insertion of gas intothe pod will require strict monitoring of the pressure inside the pod in order to ensure that the pressure inside the pod is the same as the pressure of the interior region of the processing station. A small difference in pressure between the interiorregion of the SMIF pod and the interior region of the processing station can create a vacuum which holds the SMIF pod box door closed after the latch has been released and the elevator assembly has lowered. In such a case, the elevator would lower theport door, and the pod door would remain closed preventing access to the interior of the pod.

Additionally, during shipping, a SMIF pod could be placed in the cargo area of an airplane. When the airplane is at an altitude, there can be an extreme pressure change. This pressure change could damage the liner of the SMIF pod or force airto leave or enter the SMIF pod through a seal, possibly damaging that seal.

One solution to these problems is to modify a SMIF pod to add an aperture with a filter. However, a SMIF pod with only an aperture and filter would not be able to maintain an inert environment isolated from the environment external to the SMIFpod. Another solution which includes actively injecting and removing gas through an outlet in the SMIF pod in order to maintain a desired pressure and clean environment can be expensive and, a small difference in pressure between the interior of theSMIF pod and the exterior of the SMIF pod could cause the SMIF pod box door to stick to the SMIF pod box.

SUMMARY OF THE INVENTION

The present invention is directed to overcome the disadvantages of the prior art.

It is, therefore, an object of the present invention to provide a sealable, transportable container which can equalize the pressure inside the container to the pressure outside the container without opening the container.

Another object of the present invention is to provide a sealable, transportable container having a breather assembly which, upon mating with a SMIF port of a processing station, allows the pressure in the interior of the container to become equalto the pressure inside the port of the processing station.

Another object of the present invention is to provide a sealable, transportable container with a breather assembly that can be maintained in an open position during shipping of the container such that pressure changes outside the container willnot damage the container.

Another object of the present invention is to provide a sealable transportable container capable of maintaining a controlled environment in the interior of the container such that when an attempt is made to open the door of the container, thedoor will not stick closed because of a vacuum effect within the container.

These and other objects of the present invention are provided by a sealable, transportable container (e.g. a SMIF pod) having a breather assembly. The container can be used as part of a SMIF system. The breather assembly has open and closedpositions. Closing the breather assembly isolates the interior region of the container from ambient atmospheric conditions. Opening the breather assembly equalizes and/or regulates pressure inside the container, allowing the pressure inside thecontainer to approach the pressure outside the container.

In one embodiment, a sealable, transportable container comprises a box having a first sealing surface and an interior region. The container further includes a box door having a second sealing surface so that the first sealing surface and thesecond sealing surface form a first seal when the box door is moved to a sealing position with respect to the box. The box door has an aperture and a valve for opening and closing the aperture. Connected to the valve is an actuator for moving the valveso that moving the actuator to a first position closes the aperture and moving the actuator to a second position opens the aperture.

The sealable, transportable container can further include a latch for holding the box door in the sealing position. The actuator can be connected to the latch or the actuator can be the latch. When the latch is in a locked position the box dooris held in the sealing position, the valve is closed and the interior region is isolated from the environment external to the container. When the latch is in an unlocked position, the valve is open and gas can flow between the interior region of thecontainer and the environment external to the container. In one embodiment, the latch includes a latch plate and a cam for moving the latch plate.

These and other objects and advantages of the invention will appear more clearly from the following description in which the preferred embodiments of the invention have been set forth in detail in conjunction with drawings.

BRIEFDESCRIPTION OF THE DRAWINGS

FIG. 1A is an isometric view of a processing station having a canopy for receiving a SMIF pod.

FIG. 1B is a cutaway side view of the processing station of FIG. 1A.

FIG. 2 is an exploded isometric view of a SMIF pod in accordance with the present invention.

FIG. 3 is an exploded isometric view of a SMIF pod and portions of a port assembly for receiving the SMIF pod.

FIG. 4 is a sectional view of a SMIF pod and a port assembly for receiving the SMIF pod.

FIG. 5 is a sectional view of the box door showing the breather assembly and the latch mechanism in a locked position.

FIG. 6 is a sectional view of the box door showing the breather assembly and the latch mechanism in an unlocked position.

FIG. 7 is a blow-up sectional view of the breather assembly of FIG. 6.

FIGS. 8A and 9A are plan views of a box door of a SMIF pod showing a latch mechanism and the bottom of the breather assembly in unlocked and locked positions, respectively;

FIGS. 8B and 9B are plan views of an alternative embodiment box door of a SMIF pod showing a latch mechanism and the bottom of the breather assembly in unlocked and locked positions, respectively.

FIGS. 10 and 11 are plan views of a two-stage rotary cam of the latch mechanism used to actuate the valve in unlocked and locked positions, respectively.

FIG. 12 is a plan view of the two-stage rotary cam.

FIG. 13 is a side view of the two-stage rotary cam.

FIG. 14 is a plan view of an interface portion of a SMIF pod box which interacts with the latch mechanism shown in FIGS. 8-9.

FIG. 15 is a partial cutaway side view of the interface portion shown in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described in the context of a system for storing and transporting semiconductor wafers and/or reticles. However, it is to be understood that a sealable, transportable container in accordance with the presentinvention may be used to store and transport many other inanimate objects such as magnetic recording disks, as well as living objects such as laboratory animals.

One example of a SMIF system is shown in FIGS. 1A and lB, which illustrate a processing station 8 having a canopy 10 which is an easily removable shield that covers the wafer handling mechanism of processing equipment 12. Equipment 12 may be,for example, a photoresist applicator, mask aligner, inspection station or any similar processing equipment. Canopy 10, which is constructed of glass or transparent plastic such as acrylic or lexan to facilitate visual inspection and/or maintenancewithin canopy 10, encloses the handling mechanism for processing equipment 12 and a holder 14, such as a wafer cassette holding wafers 16. The environment within the processing station is separately maintained and separately cleaned; therefore,equipment 12 need not be installed in a clean room.

A sealable transportable pod (or container) 18 including a box 20 having interior region 21 and a box door 32 is mounted on horizontal surface 22 of a canopy 10 of a port assembly 24. Port assembly 24 includes a port plate 26, port door 28, andan elevator assembly 30. Elevator assembly 30 transports a cassette 14, containing integrated circuit wafers 16 from interior region 21 of a box 20 onto the region beneath canopy 10.

In FIG. 1B, port door 28 and box door 32 are shown in the closed position by the dotted lines. The elevator assembly 30 includes a platform 36, a shaft engagement device 38, and a drive motor 40. Platform 36 carries port door 28, box door 32and cassette 14 in a vertical direction. Platform 36 is attached by engagement devices 38 to a vertical guide 42 of elevator assembly 30.

Typically, guide 42 includes a lead screw (not shown) and drive motor 40 drives a gear (not shown) which engages the lead screw for driving platform 36 up or down. When platform 36 is driven to the closed position, port door 28 closes the portopening in canopy 10.

In a similar manner, a manipulator assembly shown generally by numeral 44 includes a platform 46 which has an engagement means 48 for engaging vertical guide 42. Manipulator assembly 44 includes a manipulator arm 50 and engagement head 52adapted to engage cassette 14. By vertical operation of platforms 36 and 46, and by operation of manipulator assembly 44, cassette 14 is moved from its position on box door 32 to a position on equipment station 13 (as shown by the broken lines).

With reference to FIG. 2, box 20 includes a domed housing 60 and a ring-like engaging portion 62. Housing 60 and engaging portion 62 may be formed integrally, for example, by injection molding, or as separate components assembled to form box 20. A gasket 64, which provides the seal between box 20 and box door 32 resides in a gasket-retaining slot 66 in box door 32. In the preferred embodiment, a liner (not shown) is provided on door 32 and gasket 64 contacts the liner. The liner is a removableelement which may be formed of, for example, plastic materials which do not outgas or introduce particles, as described in U.S. Pat. No. 4,739,882, which is hereby incorporated by reference. The liner may also be formed of a material which providesthe capability of dissipating or preventing the formation of static charges. A latch mechanism is housed in box door 32 and protrudes from box door 32 through windows 68 to engage latch engaging surfaces 150.sub.1-8 (FIGS. 2 and 14-15) of box 20. Onthe top surface of box door 32 are cassette guides 70 and 72, which are used to position cassette 14 (FIG. 1B) on box door 32. Shown on cassette guide 72 is filter 74 which, as described below, is part of the breather assembly. Filter 74 and theaperture it covers are not required to be located on a cassette guide.

FIG. 3 is an exploded view of container 18 and port assembly 24. Port door 28 has an actuating mechanism 80 with pins 82. Pins 82 fit inside holes in a cam mechanism 116 (described below). Actuating mechanism 80 rotates, thus, actuating cam116 which translates the motion to a latch (described below) and a valve in the breather assembly (described below). Such motion translated to the latch releases the box door 32 from box 20, allowing the wafers to be removed.

FIG. 4 shows container 18 mated to port assembly 24 of processing equipment 12. Container 18 is designed for sealably mating with the port assembly 24. Box 20 has first and second box top sealing surfaces 91 and 92, respectively. Box door 32has a first box door sealing surface 93 for forming a first seal (sealably mating) with first box top sealing surface 91 when gasket 64 is compressed. Port plate 26 has first and second port plate sealing surfaces 95 and 96, respectively. First portplate sealing surface 95 sealably mates with second box top sealing surface 92, forming a second seal with gasket 102.

Port door 28 has a first port door sealing surface 97 which sealably mates with second port plate sealing surface 96 forming a third seal with gasket 104. Port door 28 has a second port door sealing surface 94 which sealably mates with secondbox door sealing surface 98 to form a fourth seal.

FIGS. 5-7 show the preferred embodiment breather assembly housed within box door 32. On top of cassette guide 72 is a filter 74 covering an aperture 110. Beneath aperture 110 is a valve 112 which serves to open and close aperture 110. Valve112 is normally held in a closed position (FIG. 5) by springs 114. Springs 114 allow valve 112 to be pivoted so that aperture 110 can be opened (FIG. 6); however, when the pivoting force is removed from valve 112, springs 114 force valve 112 to closeaperture 110. The force that opens valve 112 is provided by latch plate 118 (or other actuator) which is controlled by cam 116 (described below). When latch plate 118 is in the locked position (FIG. 5), latch plate 118 does not exert a force on valve112. When latch plate 118 is in the unlocked position (FIG. 6), latch plate 118 pushes against valve 112 causing valve 112 to pivot.

During the normal operation of a SMIF system, a container is placed on a port. The port unlocks the latch which causes valve 112 to open aperture 110 and allow gas to flow between the interior region of the container 21 and the interior regionof the port.

The pivoting of valve 112 opens aperture 110 such that when the pressure inside container 20 is greater than the pressure external to container 20 air passes from interior region 21 of container 18 through filter 74, through aperture 110, throughbox door upper aperture 126 (see FIG. 7), through latch plate aperture 127 (see FIG. 7), and through box door lower aperture 128. If the container is mated to a port, gas exiting lower aperture 128 would flow into the interior region of the port. Should the pressure in container 20 be less than the pressure outside box door 32, then air or other gases would travel in a path reverse to that described above. Various alternatives are available that route the air through other paths; for example,the air can circulate within box door 32 and exit out apertures in the side of box door 32, air can travel through a conduit or air can be channeled to an outlet in a manner similar to that described in U.S. Pat. No. 5,169,272, incorporated herein byreference. The discussion of the flow of gas also includes the flow of liquids.

Filter 74 can be used to prevent contaminants from entering container 20 or from leaving container 20. The preferred embodiment filter is a continuous membrane of expanded polytetrafluoroethylene (PTFE) laminated to a 100% polyester spun bondednon-woven backer. Alternative materials to PTFE are polypropylene (PP), cellulose acetate, cellulose nitrate, UHMW polyethylene, high density polyethylene (HDPE), low density polyethylene (LDPE), very low density polyethylene (VLDPE), polyvinylidenefluoride (PVDF), polysulfone (PSO), ethylene vinyl acetate (EVA), polystyrene (PS), PFA, and nylon 66. Alternatives for the backer are polyester, polycarbonate, polypropylene, acetate, acrylar, kimdura, polystyrene, primalyn UPHC, Tyvek, vinyl andaluminum foil. Alternatives for an adhesive are pressure sensitive acrylic adhesive no. 200, rubber or repositionable wet surface acrylic. Additionally, any other filters known in the art could be used or filters known in other arts can be used, forexample, a high efficiency disk drive breather falter, syringe filter, gas filter, submicron filters with electropolished and welded 316SS housings, pleated media filters, cartridge filters, and in-line filters.

The preferred embodiment valve would be made of polycarbonate. The latch plate would preferably be made of aluminum. In the preferred embodiment, valve 112 would fit entirely within box door 32 (e.g., not stick out the bottom of box door 32) sothat an operator is not likely to accidentally open valve 112. Furthermore, the breather assembly-latch mechanism is designed so that upon unlocking box door 32, the pressure in interior region 21 of box 20 is equalized very quickly to the pressure inthe processing station so that box door 32 does not remain closed because of a vacuum effect.

In one alternative, it would be possible to fit a plug into box door aperture 128 to force valve 112 to remain in the opened position. This plug could have a hole in it so that while valve 112 is in an open position, air could flow betweeninterior region 21 and an external environment. This embodiment could be used for containers shipped by airplane. Other alternatives include actuating the valve separately from the latch plate or separately from the latch, and even placing the breatherassembly directly on box 20. Filter 74 could also be used as part of a liner. When the filter is used in conjunction with the liner, the filter could be part of the liner, the filter could be the liner, the filter could be below the liner or the filtercould be above the liner. Alternatively, there can be a breather assembly with no filter. In yet another embodiment, the breather assembly can be manually operated, or automatically operated in some other fashion.

As stated above, cam 116 and latch plate 118 control movement of valve 112. FIGS. 8-15 further explain the cam and latch mechanisms. The two-stage rotary cam latch mechanism of the present invention (preferred embodiment depicted in FIGS. 8Aand 9A) includes latch plates 118, a cam mechanism 116 pivotally mounted on box door 32, and latch plate supports 135.sub.1-6. The operation of latch mechanism is a two-stage operation. In the first stage of operation during locking, cam mechanism 116routes causing latch plates 118 to slide linearly. In the second stage, cam mechanism 116 rotates causing latch plates 118 to pivot on latch plate supports 135.sub.1-6. The breather assembly of the present invention can be used with a one stage cam orother type of cam.

Each latch plate 118 has at least one box-engaging portion. In the preferred embodiment of the present invention, each of the two latch plates 118 has two latch arms 142,144 and 146,148, respectively. Each latch arm has two latch fingers142.sub.1-2, 144.sub.1-2, 146.sub.1-2, and 148.sub.1-2 which engage respective ones of latch engaging surfaces 150.sub.1-8 of box 20 (see FIG. 14).

The first stage of the latching operation involves moving latch plates 118 from the retracted (or disengaged or unlocked) position (FIG. 8A) where latch plates 118 are entirely contained within box door 32 to the extended (or engaging or locked)position (FIG. 9A) where latch fingers 142.sub.1-2, 144.sub.1-2, 146.sub.1-2 and 148.sub.1-2 extend out of box door 32 and are adjacent to latch engaging surfaces 150.sub.1-8 of box 20. In the first stage when latch plates 118 move from the retractedposition to the extended position in a plane parallel to the plane of box door 32, latch fingers do not contact latch engaging surfaces 150.sub.1-8. Accordingly, there are no particles created by scraping or rubbing of the latch fingers against latchengaging surfaces 150.sub.1-8.

The second stage of the latching operation involves substantially vertical motion of latch fingers 142.sub.1-2, 144.sub.1-2, 146.sub.1-2 and 148.sub.1-2. "Substantially vertical motion" refers to motion in a direction perpendicular to the planeof box door 32, and to the plane of motion of latch plates 118 during the first stage of the latching operation. The motion of latch fingers 142.sub.1-2, 144.sub.1-2, 146.sub.1-2 and 148.sub.1-2 during the second stage engages latch fingers 142.sub.1-2,144.sub.1-2, 146.sub.1-2 and 148.sub.1-2 with respective ones of latch engaging surfaces 150.sub.1-8, and creates a latching and/or clamping force which sealably mates first sealing surface 91 of box 20 with second sealing surface 93 of box door 32.

Looking at FIGS. 8A and 9A, the bottom of valve 112 and aperture 127 can be seen. When the latch mechanism is in the unlocked position (FIG. 8A) the latch plate 118, at the edge of aperture 127, is exerting a force on valve 112 causing valve 112to pivot. Thus, the bottom of valve 112 appears on an angle. This situation is analogous to the drawing of FIG. 6.

When the latch mechanism is in the locked position (FIG. 9A), latch plates 118 are positioned such that the latch plates 118 are not providing any force on valve 112 and thus springs 114 hold valve 112 in a closed position. This situation isanalogous to the drawing of FIG. 5.

An alternative embodiment is shown in FIGS. 8B and 9B. Rather than have an aperture in latch plate 118B, the edge of latch plates 118B exerts a force on valve 112B. Thus, some of the gas may be deflected by the latch plate and thus circulatethrough cavity 154B. FIG. 8B shows the alternative embodiment in the unlocked position and, FIG. 9B shows the alternative embodiment in a locked position.

In operation, both embodiments work fairly similar. When the latch plates are in an unlocked position a force is exerted on valve 112, opening aperture 110. When the latch plates are in a locked position, the latch does not exert a force onvalve 112, and aperture 110 is closed.

The details of the construction and operation of the preferred embodiment cam mechanism 116 will be described with reference to FIGS. 5, 6, 10-13. FIG. 10 can be correlated with FIG. 8A. FIG. 11 can be correlated with FIG. 9A.

Cam mechanism 116 is a two-stage cam mounted on a pivot post 160 (FIGS. 5, 6 and 13). Pivot post 160 is attached to box door 32. First and second holes 162 and 164 (FIG. 13) in cam mechanism 116 engage pins 82 and 83 of actuating mechanism 80(FIG. 3).

The two stages of operation of cam mechanism 116 and latch plates 118 occur as follows. In the first stage of the latching operation, cam lobes 170 and 172 engage surfaces 174 and 176 (FIGS. 10-11), respectively of latch plates 118. As cammechanism 116 rotates through an angle of approximately 40.degree. from an unlatched to an intermediate position, cam lobes 170 and 172 cause latch plates 116 to move from the retracted to the extended position. In order to maintain latch plates 116 ina plane parallel to the plane of box door 32 during the sliding operation of the first stage of the latching operation, latch springs (not shown) can be attached to latch plates 116. When latch plates 118 are in the extended position, valve 112 is inthe closed position such that aperture 110 is closed.

Risers 180 and 182 (FIGS. 5, 6, 12 and 13) are provided on roller surface 184 of cam mechanism 116. As cam mechanism 116 rotates beyond the first 40.degree. of motion from the intermediate position to a latched position, linear motion of latchplates 118 ceases and rollers 190 and 192 begin riding up respective ones of risers 180 and 182. As rollers 190 and 192 ride up risers 180 and 182, latch plates 118 pivot about an axis defined by the contact points of latch plates 118 and latch platesupports 135.sub.1-6. The pivoting motion of latch plates 118 causes latch fingers 142.sub.1-2, 144.sub.1-2, 146.sub.1-2, and 148.sub.1-2 to move substantially vertically without scraping against fingers latch surfaces 150.sub.1-8. Experimental resultshave shown that latch fingers 142.sub.1-2, 144.sub.1-2, 146.sub.1-2 and 148.sub.1-2 undergo horizontal motion of only approximately 0.001". This minimal amount of horizontal motion minimizes the generation of particles by scraping.

In the unlatching, operation rollers 190 and 192 ride back down risers 180 and 182 as cam mechanism 116 is rotated in the opposite direction. Springs (not shown) provide the biasing force to keep rollers 190 and 192 in contact with risers 180and 182. After rollers 190 and 192 come in contact with roller surface 184, cam lobes 170 and 172 engage cam surfaces 174 and 176 of latch plates 118 and move latch plates 118 from the extended position to the retracted position (FIG. 10). Rollers canalso be replaced with a slide mechanism, plastic bump or some other serf lubricated material.

When latch plates 118 and cam mechanism 116 are in the latched position (FIG. 11), cam lobes 170 and 172 are aligned with the axis of rotation of cam mechanism 116. Accordingly, any force tending to move latch plates 118 from the extended to theretracted position is transmitted along cam lobes 170 and 172 through the rotational axis of cam mechanism 116 without creating any torque which would rotate cam mechanism 116 from the latched position. Thus, cam lobes 170 and 172 physically maintainlatch plates 118 in the extended position both during the second stage of the latching operation and when the latch mechanism is in the latched orientation, preventing the latch from releasing under the application of linear impacts.

The second stage of the latching operation provides a strong clamping force to retain box door 32 in a sealing position with respect to box 20. This seal allows interior region 21 to be evacuated and back-filled with an inert gas. The clampingforce is approximately one (1) pound per linear inch. This clamping force is regulated by a slight bowing or spring action of latch plates 118. This bowing is designed into latch plates 118 which are formed of 0.125-inch thick aluminum alloy, forexample, 7075-T6. Although the clamping force is useful to maintain a strong seal, the breather assembly will still function in an alternate embodiment where the latch plates move horizontally but not vertically.

In order to prevent localized bending or deflection of box door 32, due to the application of the clamping force, the clamping loads are distributed. One aspect to the distribution of the clamping force is the use of multiple latch arms for eachlatch plate. Latch arms 142, 144, 146 and 148 are positioned at points which are half the distance from the center to the end of the edge of the box door. For a box door having a width W, latch arms are a distance approximately W/2 from each other anda distance approximately W/4 from the edge of the box door.

To strengthen the latch engaging areas of box 20, which can be formed of plastic, posts 200 are provided between each pair of latch engaging surfaces 150.sub.1-8 (FIGS. 14-15). To accommodate posts 200, each latch arm 142, 144, 146 and 148 haslatch fingers 142.sub.1-2, 144.sub.1-2, 146.sub.1-2 and 148.sub.1-8 separated by a notch. Posts 200 serve to decrease the unsupported length of latch engaging surfaces 150.sub.1-8 by a factor of approximately one third and to increase the shear area bya factor of 2.

The entire latch mechanism and breather assembly is contained in box door 32 which facilitates cleaning of box 20. Tampering with latch mechanism 80 is virtually eliminated due to the fact that outside forces intended to move latch plates 118from the extended to the retracted position do not rotate cam mechanism 116, and cam mechanism 116 is contained within cavity 154 of box door 32. Unauthorized access would require rotating cam mechanism 116 by inserting an implement into cam holes 162and 164. Accordingly, access can be limited by the use of an interlock system which must be activated, and/or mechanical keys which must be inserted before rotation of cam mechanism 116 is possible.

Further information about the cam and latch mechanisms can be found in U.S. Pat. No. 4,995,430, which is incorporated by reference. The use of the breather assembly is not limited to use in conjunction with the disclosed latch. One skilled inthe art could use the breather assembly with various other actuating devices. For example, the breather assembly could be used with another type of latch or another type of locking device. The breather assembly could be actuated directly by pins 82, avacuum port on the port door or by a magnetic actuator (e.g. using Hall effect) on the port.

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the above teaching. The preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in theart to best utilize the invention in various embodiments and with various modifications as are suited to particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

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