Abstract:
A carrier includes an enclosure portion formed substantially from polycarbonate plastic. Selected portions of the enclosure have an outer surface portion formed substantially from a plastic material having a Fire Propagation Index of not greater than 9.0 (m/s  1/2 )(kW/m) −2/3 . Suitable plastic materials include polyimide, polyether imide, polyamide imide, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyether sulphone, and polytetrafluoroethylene. A carrier enclosure according to the invention may have significant portions formed from relatively low-cost, easily formable, transparent polycarbonate. Much higher cost fire resistive polymer materials may be selectively positioned on the enclosure where necessary to impact spread of fire on the carrier and to other adjacent carriers.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Utility Patent Application Ser. No. 10/190,355 and U.S. Provisional Patent Application Serial No. 60/394,219, each of which is hereby fully incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to wafer carriers. More particularly it relates to fire retardant wafer carriers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Processing of semi-conductor wafers into finished electronic components typically requires many processing steps where the wafers must be handled and processed. The wafers are very valuable, and are extremely delicate and easily damaged by physical and electrical shocks. In addition, successful higher yield processing requires the utmost in cleanliness, freedom from particulates and other contaminants. As a result, specialized containers or carriers have been developed for use during processing, handling and transport of wafers. These containers protect the wafers from physical and electrical hazards, and are sealable to protect the wafers from contaminants. Additionally, since the processing of wafer disks is generally automated, it is necessary for disks to be precisely positioned relative to the processing equipment for the robotic removal and insertion of the wafers. A second purpose of a wafer carrier is to securely hold the wafer disks during transport.  
           [0004]    Carriers are generally configured to axially arrange the wafers or disks in article supports in the form of shelves or slots, and to support the wafers or disks by or near their peripheral edges. The wafers or disks are conventionally removable from the carriers in a radial direction upwardly or laterally. Carriers may have supplemental top covers, bottom covers, or enclosures to enclose the wafers or disks. Examples of specialized carriers and methods for forming them are disclosed in U.S. Pat. Nos. 6,439,984; 6,428,729; 6,039,186; 6,010,008; 5,485,094; 5,944,194; 4,815,601; 5,482,161; 6,070,730; 5,711,427; 5,642,813; and 3,926,305, all assigned to the owner of the present invention, and all of which are hereby fully incorporated herein by reference. For the purposes of the present application, the term “carrier” includes, but is not limited to: semiconductor wafer carriers such as H-bar wafer carriers, Front Opening Unified Pods (FOUPs), and Standard Mechanical Interface Pods (SMIFs); reticle carriers; WIP boxes, and other carriers used in the micro-electronic industry for storing, transporting, fabricating, and generally holding small electronic components such as hard drive disks and other miscellaneous mechanical devices.  
           [0005]    The semiconductor industry is in the process of evolving fabrication facilities to use wafers having a diameter of 300 mm. The wafer carriers used for 300 mm wafers are normally configured as a Front Opening Unified Pod (FOUP). Examples of FOUP wafer carriers are disclosed in U.S. Pat. Nos. 6,082,540, 6,206,196, 6,216,874 and 6,267,245, each commonly owned by the assignee of the present invention, and each of which is fully incorporated herein by reference.  
           [0006]    There are a number of material characteristics which are useful and advantageous for wafer carriers depending on the type of carrier and the particular part of the carrier at issue. Carrier materials should also have minimal outgassing of volatile components as these may leave films which also constitute a contaminant which can damage wafers and disks. The carrier materials must have adequate dimensional stability, that is rigidity, when the carrier is loaded. Dimensional stability is necessary to prevent damage to the wafers or disks and to minimize movement of the wafers or disks within the carrier. The tolerances of the slots holding wafers and disks are typically quite small and any deformation of the carrier can directly damage the highly brittle wafers or increase the abrasion and thus the particle generation when the wafers or disks are moved into, out of, or within the carrier. Dimensional stability is also extremely important when the carrier is loaded in some direction such as when the carriers are stacked during shipment or when the carriers integrate with processing equipment. The carrier material should also maintain its integrity under elevated temperatures which may be encountered during storage or cleaning. U.S. Pat. No. 5,780,127 discusses various characteristics of plastics which are pertinent to the suitability of materials for wafer carriers, and is incorporated herein by reference.  
           [0007]    Visibility of wafers within closed containers is highly desirable and may be required by end users. Polycarbonate material is extensively used for the enclosure portion of carriers, because of its transparency, ease of molding, and favorable abrasion resistance, heat resistance, chemical resistance, outgassing containment, rigidity characteristics, creep reduction, fluid absorption containment, UV protection, and other performance characteristics. In addition, polycarbonate is typically dramatically less expensive than other polymers that are suitable for use in carriers, such as polyetheretherketone (PEEK).  
           [0008]    For these reasons, conventional practices typically include constructing an entire carrier enclosure of polycarbonate material. While polycarbonate has the many favorable characteristics outlined above, however, it is also a relatively flammable material that readily propagates flame. Free burn testing involving an array of only four polycarbonate FOUP wafer carriers has shown peak heat release rates of more than 1 MW. Because as many as 5,000 to 10,000 FOUP wafer carriers may be stored in a single semiconductor processing facility, there is a non-trivial risk of fire in such facilities associated with polycarbonate wafer carriers. Not only does fire in a semiconductor processing facility pose a risk of significant property loss and hazards to the life safety of occupants in the facility, even a small fire may cause extreme disruption to the semiconductor production process due to contamination from airborne particulates and combustion products. Thus, while there have been no known fire losses to date from a wafer carrier fire, the potentiality and possible consequences of such an event make improvements to fire safety associated with wafer carriers highly desirable.  
           [0009]    Wafer carriers are often stored in multi-tier storage racks known as “stockers”. In a stocker, carriers are stored side-by-side in vertically stacked tiers. These stockers are typically arranged in opposing fashion across an aisle, from which they may be accessed by robotic equipment. Each stocker may be multi-floors high and may contain hundreds of carriers. While vertical stacking offers an efficient means of storing many types of devices and materials, including wafer carriers, it is well known that stacked arrangements of flammable materials present a heightened fire protection concern. This is due to the general tendency of fire to propagate more readily vertically through buoyant motion of pyrolysis products. In addition, fixed fire sprinkler protection covering all areas of stacked material is often very difficult and expensive to achieve. As a result, in order to minimize the size and rate of fire growth in a vertical storage arrangement, one important fire protection strategy is to retard, or preferably even prevent, the vertical propagation of fire from material in one tier of storage to next tier that is immediately vertically adjacent. To the extent that propagation of a fire beyond the area of ignition can be slowed, more time is provided for detection and suppression of the fire in its incipient stages, thus minimizing damage and process disruption.  
           [0010]    Two important variables that can have a significant effect on fire propagation in vertical storage are the geometry of the stored items and the flame propagation characteristics of the material composition of the items. In common fire protection engineering practice, these variables may sometimes be altered so as to obtain an optimal result from a fire protection standpoint. Wafer carriers, however, present a unique challenge in this regard in that the requirements of the semiconductor industry for wafer carriers are very stringent and process critical. For instance, in a wafer carrier, there may be over 200 precise dimensions required to hold wafers in place repeatably, and there are also stringent material standards for mechanical strength, structural integrity, and chemical stability of the materials used in the carrier as mentioned above. Any modifications made to carriers for the purposes of fire safety must not compromise these standards. Because of these difficulties, previous efforts to improve the fire safety of wafer carriers, both existing and new, have proceeded slowly and have not produced significant changes in wafer carrier design to date.  
           [0011]    What is still needed in the industry is a relatively low cost carrier having the needed transparency, ease of molding, favorable abrasion resistance, heat resistance, chemical resistance, outgassing containment, rigidity, creep reduction, fluid absorption containment, UV protection, and fire resistance characteristics.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention substantially meets the needs of the industry for a relatively low cost carrier having performance characteristics sufficient to meet the needs of the semiconductor processing industry while also having significantly improved fire resistance charactistics. Further, the present invention meets the need for a wafer carrier that is resistant to the vertical propagation of fire, especially when multiple wafer carriers are stored in a vertical stocker arrangement. In addition, the present invention permits the use of less-expensive, more easily formable, polycarbonate plastic for the carrier, while still offering resistance to the vertical propagation of fire.  
           [0013]    In the invention, a carrier includes an enclosure portion formed substantially from polycarbonate plastic. Selected portions of the enclosure have an outer surface portion formed substantially from a plastic material having a Fire Propagation Index of not greater than 9.0 (m/s  1/2 )(kW/m) −2/3 . Suitable plastic materials include polyimide, polyether imide, polyamide imide, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyether sulphone, and polytetrafluoroethylene.  
           [0014]    An advantage of the invention is that a carrier enclosure may have significant portions formed from relatively low-cost, easily formable, transparent polycarbonate. Much higher cost fire resistive polymer materials may be selectively positioned on the enclosure where necessary to impact spread of fire on the carrier and to other adjacent carriers.  
           [0015]    Another advantage of the invention is that the fire retardancy of existing polycarbonate carriers may be improved.  
           [0016]    Accordingly, a carrier for articles according to the invention includes an enclosure having an outer surface and an article support in the enclosure. The enclosure has a first portion formed substantially from polycarbonate plastic, and a second portion formed from a fire retardant plastic material having a flame propagation index of not greater than 9.0 (m/s  1/2 )(kW/m) −2/3 . The second portion forms at least a portion of the outer surface of the enclosure, whereby the outer surface portion is relatively retardant to vertical propagation of flame. The fire retardant plastic material may be selected from the group of plastic materials consisting of polyimide, polyether imide, polyamide imide, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyether sulphone, and polytetrafluoroethylene.  
           [0017]    The invention may also include a wafer carrier having an enclosure portion formed substantially from polycarbonate plastic having at least a top, a bottom, a pair of opposing sides, a back, and an open front. The carrier further includes a door to close the open front, wherein the door has an outer surface portion formed substantially from a plastic material selected from the group of plastic materials consisting of polyimide, polyether imide, polyamide imide, polyketone, polyetherketone, polyetheretherketone, polyetherketoneketone, polyether sulphone, and polytetrafluoroethylene, and wherein the plastic material has a Fire Propagation Index of not greater than 9.0 (m/s  1/2 )(kW/m) −2/3; .  
           [0018]    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  
       [0019]    [0019]FIG. 1 is an elevation view of a plurality of wafer carriers arrayed in stockers;  
         [0020]    [0020]FIG. 2 is a perspective view of a wafer carrier;  
         [0021]    [0021]FIG. 3 is an elevation view of a wafer carrier door according to a preferred embodiment of the present invention;  
         [0022]    [0022]FIG. 4 is a perspective, partially exploded view of a wafer carrier and door according to the present invention;  
         [0023]    [0023]FIG. 5 is a perspective, partially exploded view of an alternative embodiment of a carrier according to the invention;  
         [0024]    [0024]FIG. 5 a  is a cross-sectional view of one embodiment of the invention taken at section  5   a - 5   a  of FIG. 5;  
         [0025]    [0025]FIG. 5 b  is a cross-sectional view of an alternative embodiment of the invention taken at section  5   b - 5   b  of FIG. 5; and  
         [0026]    [0026]FIG. 6 is a perspective view of a shipping box according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    The accompanying Figures depict embodiments of the wafer container 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 spacial 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.  
         [0028]    In FIG. 1, there is depicted a plurality of FOUP wafer carriers  100  arrayed in typical vertical stockers  150 . A typical semiconductor processing facility may have multiple rows of stockers  150  arranged in parallel fashion with aisles  180  between the rows as shown. Robotic handling equipment may be used in aisles  180  to transfer wafer carriers  100  to and from stockers  150 . Within each stocker  150 , wafer carriers  100  are supported side-by-side by horizontal supports  160 , forming vertically stacked tiers  162  of wafer carriers  100 . Wafer carriers  100  are normally arranged in stockers  150  so that the door of the carrier faces outward into aisle  180 .  
         [0029]    Referring to FIG. 2, a typical FOUP wafer container  100  as used in the art has an enclosure portion  102 , constructed of polycarbonate plastic, and having a top  104 , a bottom  106 , a pair of opposing sides  108  and  110 , and a back  112 . A door  114  closes the open front  116  of the enclosure portion  102 , fitting into door recess  118 . Wafer supports  122  are provided to support semi-conductor wafers within the enclosure. Kinematic coupling  124 , mounted to the exterior surface of enclosure bottom  106  is provided to facilitate automated handling of the container during use and to provide a reference datum for locating the wafers in the housing during processing. Robotic lifting flange  126  is mounted on the exterior surface of enclosure top  104  and is provided to facilitate automated handling and transport of container  100  during use.  
         [0030]    As may be seen from reference now to FIGS. 1 and 2, vertical propagation of fire within stockers  150  may be retarded between tiers  162  at the sides  108 ,  110  and back  112  of wafer carriers  100  by providing solid portions in horizontal supports  160 , blocking any vertical openings between tiers. At the front, however, doors  114  are vertically aligned, forming a pathway for vertical propagation of fire between tiers  162 .  
         [0031]    It is known in the art to classify materials according to the relative propensity of the materials to propagate fire. One such classification, considered particularly indicative of the fire propagation behavior of materials under highly radiative flame conditions prevalent in large scale fires, uses a Fire Propagation Index (FPI) value that is determined for the material. To determine the FPI of a material, the material is tested according to methods well known in the art to determine a peak chemical heat release rate per unit width (Q′ ch ), and a Thermal Response Parameter (TRP), which is calculated according to the relation: 
         
       TRP=ΔT 
       ig 
       {square root}{square root over (kρc p )} 
     
         [0032]    where ΔT ig  is the ignition temperature of the material above ambient in K°, k is the material thermal conductivity in kW/m-K°, ρ is the material density in g/m 3 , and c p  is the material specific heat in kJ/g-K°. The FPI may then be calculated according to the relation:  
       FPI   =     1000        (         (     0.42                   Q   ch   ′       )       1   /   3       TRP     )                             
 
         [0033]    Materials may be generally classified according to their FPI value. Materials having an FPI of under 7.0 (m/s  1/2 )(kW/m) −2/3  are classified as Group N-1 “Non-Propagating” materials, those having an FPI of less than 10.0 (m/s  1/2 )(kW/m) −2/3  but at least 7.0 (m/s  1/2 )(kW/m) −2/3  are classified as Group D-1 “Decelerating” materials, those having an FPI of between 10.0 (m/s  1/2 )(kW/m) −2/3  and 20.0 (m/s  1/2 )(kW/m) −2/3  are Group 2 “Non-Accelerating Propagation” materials and those having an FPI of over 20.0 (m/s  1/2 )(kW/m) −2/3  are Group 3 “Accelerating Propagation” materials.  
         [0034]    Polycarbonate plastic, as is commonly used for the enclosure and doors of wafer carriers, normally has a Fire Propagation Index (FPI) of greater than about 10.0 (m/s  1/2 )(kW/m) −2/3 , which classifies it as a Group 2 or Group 3 fire propagating material. In accordance with the invention, at least the outer surface portion  130  of door  114  of each wafer carrier is formed substantially from a Group N-1 or Group D-1 fire retardant plastic material that has an FPI of 9.0 (m/s  1/2 )(kW/m) −2/3  or less. Although any fire retardant plastic material having an appropriate FPI may be suitable for the purpose, plastics that are known to be acceptable for use in wafer carriers and that have the appropriate FPI are polyimide (PI), polyether imide (PEI), polyamide imide (PAI), polyketone (PK), polyetherketone (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyether sulphone (PES), and polytetrafluoroethylene (PTFE). The currently most preferred material is PEI having an FPI from between about 8.1 (m/s  1/2 )(kW/m) −2/3  to about 8.6 (m/s  1/2 )(kW/m) −2/3 , such as for example, Ultem 1000 made by GE Plastics, Inc. of Pittsfield, Mass.  
         [0035]    In a preferred embodiment of the invention, at least outer surface portion  130  of door  114  is formed from PEI material. It is currently most preferred that the thickness of outer surface portion  130  formed from the fire retardant plastic material be at least the typical thickness of enclosure portion  102 , which is generally about 0.3 mm. It is currently most preferred that outer surface portion  130  is the exterior panel  132  of door  114  alone as shown in FIG. 2, but may also be a separate fire-retardant layer  134  laid over exterior panel  132  of door  114  as shown in FIG. 3. Such a fire retardant layer  134  may be overmolded on exterior panel  132 , forming a thermal as well as a mechanical bond with exterior panel  132 , or may be a separate shield panel  136  as shown in FIG. 4, attached by any suitable method, including adhesives or mechanical fasteners. As an alternative, shield panel  136  may have structures allowing it to removably “snap” on and off suitable receiving structures on door  114 .  
         [0036]    Alternatively, the outer fire resistant layer can be a thin film that has been insert molded on the forward facing surface of the front door. A suitable method of film insert molding is disclosed in U.S. patent application Ser. No. 10/304,775, entitled “SEMICONDUCTOR COMPONENT HANDLING DEVICE HAVING A PERFORMANCE FILM”, commonly owned by the owners of the present invention and hereby fully incorporated herein by reference. Co-pending U.S. patent application Ser. No. 09/317,989 owned by the present applicant discloses the use of overmolding to manufacture carriers and components and is also herein incorporated by reference. Other portions of door  114 , such as the chassis  140 , latching components, and inner surface  142  may also be formed from the same fire retardant material used for outer surface portion  130 , and this may serve to improve the overall fire retardancy of wafer carrier  100 .  
         [0037]    Existing wafer carriers with polycarbonate outer surfaces may be retrofitted using the apparatus and methods of the present invention. Such a retrofit may be accomplished in the case of a FOUP by replacing the polycarbonate door with a door  114  manufactured according to the present invention, or by overlaying exterior panel  132  with shield panel  136  as described above. Such a shield panel may be a flexible sheet material suitably adhered to existing door structure.  
         [0038]    It will be appreciated that the materials and methods of the present invention could be applied to other surfaces on a FOUP and to any other type of carrier. Thus, for example, if fire retardant surfaces are made necessary by openings in horizontal supports  162  of stocker  150 , sides  108 ,  110  and/or back  112  of a FOUP could be made with an outer surface of fire retardant plastic having the appropriate FPI value as described hereinabove.  
         [0039]    As depicted in FIG. 5 for example, a SMIF pod carrier  200  has a base portion  202  and a cover portion  220 , with sides  222 , front  224 , and back  228 . Cover portion  220  engages base portion  202  at recessed region  282 , with the bottom periphery of cover portion  220  fitting aroung and covering periphery  280  of base  202 . A article support in the form of H-bar wafer carrier  260  having wafer shelves  262  is engaged with base  202  inside the enclosure. In accordance with the invention, selected portions  230  of the outer surface  232  of cover portion  220  may be made from a polymer material having an FPI of 9.0 (m/s  1/2 )(kW/m) −2/3  or less. At selected portions  230 , cover portion  220  may be entirely formed from the fire resistant material as depicted in FIG. 5 a.  In this embodiment, a first portion  234  of the enclosure is formed from polycarbonate, and a second portion  236  is formed from the fire retardant polymer material. The first and second portions may be molded together using conventional molding techniques. Alternatively, a layer of the fire resistant material  238  may be applied at selected portions  230  over a layer  240  of lower cost polymer such as polycarbonate as depicted in FIG. 5 b,  by any suitable method, such as film insert molding, overmolding, or welding as described above. Again, it is anticipated that layer  238  should be at least about 0.3 mm in thickness for best fire retardancy results. Alternatively, separate shield panels may be affixed over a polycarbonate cover portion  220  by welding, fasteners, or adhesive as described above.  
         [0040]    In a shipping container  300  embodiment as depicted in FIG. 6, cover portion  302  may have selected portions  304  of the outer surface  306  made from a polymer material having an FPI of 9.0 (m/s  1/2 )(kW/m) −2/3  or less. Again at selected portions  304 , cover portion  302  may be entirely formed from the fire resistant material or a layer of the fire resistant material may be applied over a layer of lower cost polymer such as polycarbonate by film insert molding, overmolding, or welding as described above. It is anticipated that it may be especially advantageous to form overhanging lip portions  310  from fire retardant plastic material. Hot pyrolysis products from a burning base portion  312  of the container  300  will be deflected outward by overhanging lip portions  310 , away from vertical surfaces  314 , thereby inhibiting vertical propagation of fire. In addition, as base portion  312  burns and melts, cover portion  302  may settle downward, tending to suppress the fire underneath.  
         [0041]    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 given.