Abstract:
A device for cleaning semiconductor wafers is provided. The device includes a carrier for holding wafers during the cleaning process. The carrier includes a frame with an open top and a plurality of carrier rods extending between opposite ends of the frame. The carrier rods have grooves that receive marginal edge portions of the wafers to retain them against movement in the carrier during cleaning and transportation. The grooves are structured to reduce the amount of contaminants remaining on the wafers after cleaning. The frame may be made substantially entirely of a polymeric material.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for carrying semiconductor wafers during the wafer cleaning process. During the semiconductor manufacturing process, the surfaces of wafers become contaminated with cutting and polishing residue, organics, metals and cleaning solution residue. Even extremely minor quantities of contaminants can negatively affect the wafer for subsequent handling steps or when it is used as a substrate for an electronic circuit. Many devices are on the market for effecting such cleaning and have been generally effective. However, as wafer specifications have become more stringent, so have the wafer cleaning requirements sometimes resulting in a lower yield rate. Further, price competition in the wafer and electronic device markets has also become more intense making manufacturing efficiencies even more important. Thus, scrap needs to be reduced as much as possible to help improve yield rate and maintain profitability. 
     A wafer can be cleaned by either or both of a spray wash and an immersion wash. While such devices and methods have been generally effective, more stringent specifications have resulted in a higher reject rate for the wafers and semiconductor devices being made from the wafers. Cleaning typically includes sequential cleaning steps, as are known in the art. Each cleaning phase is generally followed by a rinse step to remove the cleaning solution. In a sense, the rinse step is also a cleaning step whose chief purpose is to remove the cleaning solutions. Generally, the first cleaning step involves the application of a base such as ammonia followed by a water rinse step. After the first rinse step, the wafers are exposed to an acid such as hydrofluoric, fluoric or hydrochloric. If there is any ammonia carried with the wafers to the acid cleaning step, it will react with the acid and produce a salt which is a contaminant to the wafer. The acid treated wafers are rinsed again with water. After this subsequent water rinse, the wafers are exposed to isopropyl alcohol (IPA) in a vapor chamber to assist in removing the rinse water and to dry the wafers. All of the cleaning fluids must be extremely clean so as to not contaminate the wafers. Such cleaning process may be used at more than one point in the wafer manufacturing process. 
     During cleaning, wafers are carried in a carrier. The wafers are robotically placed in grooves in carrier rods in the cassette. The grooves retain the wafers in position in the cassette while exposing as much of the wafer surfaces as practicable to the cleaning and rinsing fluids. It has been found that the surfaces of the grooves will induce the formation or collection of residue from the cleaning and/or rinsing liquids on the wafers in the area where the faces of the wafer are adjacent the groove surfaces at a marginal edge of the wafer. 
     The use of grooves has been found to be particularly advantageous for holding wafers in position in the cassette. To robotically load and unload the wafers, the wafers must be accurately positioned which is accomplished by the use of narrow grooves in cassette rods. Thus, to continue to use current robotics and wafer processing equipment, narrow grooves have been preferred for accurately positioning the wafers. The grooves, although effective for use with robots, are believed to be a major contributor to the formation of the aforementioned spotting problem. Spots form on marginal edge portions of wafers where they are closely spaced from or in contact with carrier rod groove surfaces. Even though at one time the spots would have been considered minor defects, they need to be eliminated or reduced to improve the yield rate to maintain acceptable pricing and margins. It would also be desirable to continue to use cassettes with grooved rods because of their effectiveness at wafer retention, support and positioning which are also important to effective and efficient manufacturing. 
     The spots are a film or residue left when the cleaning and rinsing liquids are removed from wafers. It has been found that the incident rate of spot creation can be reduced by providing better drainage of cleaning and rinse liquids and more IPA in the area between the groove surfaces and the marginal edge portions of the wafer positioned in the carrier. This can be done while still providing the desired degree of wafer position tolerance in the cassette during cleaning. 
     Current cassettes have frames that are typically made of fused quartz which is expensive and make the cassettes difficult to repair. Fused quartz is desirable because it is resistant to degradation by the cleaning and rinsing fluids. Many times the cassettes will use grooved carrier rods made of a polymer that is resistant to degradation by the cleaning fluids. However, polymers acceptable from a degradation standpoint have tended to be soft and not resistant to wear caused by contact with the wafers. In order to obtain acceptable life from a wear standpoint, the area that contacts the wafers was made large, making the spotting worse. Carrier rods are also lacking in structural rigidity because of the properties of the polymer and need to be reinforced to support loads. A typical carrier rod is hollow polytetrafluoroethylene with, e.g., a graphite composite rod sealed inside for structural support. However, after some use, such carrier rods tend to leak cleaning fluid which attacks the reinforcing rod and contaminate the wafers being cleaned. 
     Thus, there is a need for an improved cassette and carrier rod. The present invention provides an improved cassette and carrier rod that will improve cleaning efficiency and thereby improve the wafer production yield rate while maintaining accurate positioning of the wafers so they can be handled robotically. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention may be noted the provision of an apparatus for carrying semiconductor wafers during cleaning that will improve cleaning efficiency; the provision of such an apparatus that will accurately for robotic loading and unloading; the provision of such an apparatus that is simple in construction requiring few parts; the provision of such an apparatus that is adapted to be used in various types of cleaning apparatus; the provision of such an apparatus that can have components subject to wear replaced; the provision of such an apparatus that has a long life; the provision of such an apparatus that is economical to manufacture; the provision of such an apparatus that can be easily loaded with a plurality of semiconductors and safely transport them to and thru the cleaning apparatus; the provision of such an apparatus that can be used in both immersion and spray type washers; the provision of such an apparatus that does not require major changes to the currently used cleaning and robotic devices; and the provision of such an apparatus that can be substantially entirely made of polymeric material. 
     The present invention involves an apparatus for holding a plurality of semiconductor wafers during cleaning. The apparatus includes a frame with a bottom, an open top and a longitudinal axis. A pair of carrier rods is secured to and extends between opposite ends of the frame and are in spaced apart relation and positioned on opposite sides of the longitudinal axis and between the longitudinal axis and the frame bottom. Each carrier rod has a plurality of transverse grooves in spaced apart relation along the length of the carrier rod and opening into the interior of the frame. Each groove in one carrier rod is substantially transversely aligned with a respective groove in the other carrier rod and is adapted to receive a semiconductor wafer therein for support in a generally vertical orientation. The grooves have an open top, a bottom surface and generally opposite side surfaces each facing a respective opposite side of a wafer. The grooves retain the wafer in the generally vertical orientation. The carrier rods are transversely spaced a distance less than the diameter of the wafer. The groove side surfaces are spaced apart a distance greater than the thickness of the wafer and each groove side surface has a channel recessed therein opening into the groove. Each channel is positioned between the bottom surface and the open top of the groove forming an enlarged gap for flow of liquid out of the grooves and away from the wafers. 
     The present invention also involves the provision of an apparatus for holding a plurality of wafers during cleaning. The apparatus includes a frame made substantially entirely of polymeric material. The frame has an open top and frame end members connected in spaced apart relation defining an interior for containing a plurality of wafers. The frame also includes a plurality of elongate wafer support members extending between and secured to the end members. The support members are adapted to support and retain a plurality of wafers in spaced apart generally parallel relationship. 
     In a further aspect of the present invention, an apparatus is provided for holding a plurality of a semiconductor wafers during cleaning. The apparatus includes a frame with a bottom and an open top and a longitudinal axis. The frame includes a pair of first carrier rods secured to and extend between opposite ends of the frame. The first carrier rods are in spaced apart relation and positioned on opposite sides of the longitudinal axis of the frame and between the longitudinal axis and the frame bottom. Each first carrier rod has a plurality of transverse grooves in spaced apart relation along the length of the first carrier rod and opens into the interior of the frame. Each groove in one first carrier rod is substantially transversely aligned with a respective groove in the other said first carrier rod and adapted to receive a semiconductor wafer therein for support in a generally vertical orientation. The grooves have an open top, a bottom surface and generally opposite side surfaces each facing a respective opposite side of a wafer and retaining the wafer in the generally vertical orientation. The first carrier rods are transversely spaced a distance less than the diameter of the wafer. The grooves have a height in the range of about 1 mm thru about 3 mm. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a wafer cleaning apparatus; 
     FIG. 2 is a schematic illustration of a wafer carrier in a tank used to hold a cleaning process liquid; 
     FIG. 3 is a side elevation view of a wafer carrier containing wafers; 
     FIG. 4 is a side elevation view of a wafer carrier without wafers; 
     FIG. 5 is an end view of a wafer carrier; 
     FIG. 6 is an enlarged fragmentary view of a wafer in a groove of a carrier rod; 
     FIG. 7 is an enlarged fragmentary perspective view of a wafer retention groove in a carrier rod; 
     FIG. 8 is a side elevation view of a modified wafer carrier; 
     FIG. 9 is a plan view of the wafer carrier of FIG. 8; 
     FIG. 10 is an end view of the wafer carrier of FIG. 8 with portions broken away to show details therein; and 
     FIG. 11 is an enlarged fragmentary view of a portion of the carrier rod showing details of the wafer grooves. 
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION 
     The reference numeral  10  designates generally an apparatus for cleaning semiconductor wafers. One type of semiconductor wafer is a crystalline silicon wafer. The cleaning described herein takes place typically after the wafer is finally polished as is known in the art, and in addition, can be used at more than one point in the wafer manufacturing process to provide multiple cleanings. 
     The apparatus  10 , a wafer cleaning bench, uses a series of tanks, for example, tanks  15 ,  16 ,  17 ,  18 ,  19  (FIG. 1) for holding various cleaning and rinsing fluids preferably liquids in the tanks  15 - 18  and also vapor in the tank  19 . In a typical process, the wafers  25  are carried or transported to the apparatus  10  in a carrier  30  and sequentially placed in the tanks  15 - 19  for exposure to or application of the various fluids. 
     The tanks  15 - 19  are mounted in a housing  35  and are made of material that is inert to the chemicals contained therein. Such materials include quartz. Various feeds and outlets (not shown) are provided to convey the various cleaning and rinsing fluids to and from the tanks  15 - 19 . Robots (not shown), as are known in the art, can also be provided for moving the carrier  30  and wafers  25  therein to the various tanks  15 - 19 . Robots, not shown, are also used to load and unload the wafers  25  into and from the carrier  30  as is known in the art. 
     The carrier  30  is best seen in FIGS. 3,  4 , and  5 . The carrier  30  comprises two end walls  35 ,  36  that are generally parallel, and in use, are generally vertically disposed. The end walls  35 ,  36  each have a peripheral edge  37 ,  38  respectively and oppositely facing main surfaces  42 ,  43  and  44 ,  45  respectively. Hitches  47  are affixed to the end walls  35 ,  36  for engagement with the robot to allow the robot to pick up and move the carrier. Preferably, each end wall  35 ,  36  has a pair of feet  52 ,  54  respectively. The feet  52 ,  54  extend laterally from the respective peripheral edge  37 ,  38  and each has a bottom surface  56  and  58  respectively. The bottom surfaces  56 ,  58  define a plane and the carrier  30  rests on the bottom surfaces. The end walls  35 ,  36  are secured together in spaced apart relation with a plurality of tie bars  39 . Securement can be by any suitable means but needs to be resistant to deterioration by the cleaning and rinsing liquids used. The tie bars  39  are secured to the surfaces  43 ,  44  and are generally perpendicular thereto. The tie bars  38  and end walls  35 ,  36  form an open top frame  60  with an interior  61 . The frame  60  is adapted to support additional wafer support components. The carrier  30  has a longitudinal axis LA which for purposes of the description herein, is considered to be located at the approximate center of the wafers  25  when in position in the carrier  30 . The tie bars  39  are preferably positioned below the longitudinal axis LA. The end walls  35 ,  36  and the tie bars  39  are made of a material resistant to deterioration by the cleaning and rinsing fluids and in one form of the invention can be fused quartz. If the end walls  35 ,  36  and tie bars  38  are fused quartz, one means of securement is fusing the tie bars to the end walls. 
     The carrier  30  also includes a plurality of carriers rods, which as shown, include a pair of upper carrier rods  62  and a pair of lower carriers rods  65 . The rods  62 ,  65  are suitably mounted on the frame  60  preferably by mounting on and extending between the end walls  35 ,  36 . The carrier rods  62 ,  65  have round end portions  67 ,  68  respectively. As shown, the end walls  35 ,  36  each have a plurality of apertures  72 ,  73 ,  74 ,  75 . Preferably the apertures  72 - 75  are generally rectangular in shape and have a spacing between the edges defining the apertures slightly larger than the diameters of the respective round ends  67 ,  68 . The round ends  67 ,  68  are each received in a respective aperture  72 - 75  and thereby mount the carrier rods  62 ,  65  on the end walls  35 ,  36  and hence the frame  60 . By having round ends  67 ,  68 , the carrier rods  62 ,  65  can rotate about their longitudinal axes. Preferably, the carrier rods  62 ,  65  are mounted at positions below the longitudinal axis, i.e., between the longitudinal axis and the plane of the feet  52 - 55 . If one considers a point vertically above the longitudinal axis, when the feet  52 ,  54  are on a horizontal plane, as 0° (a 12 o&#39;clock position), one carrier rod  62  is positioned between about 90° and about 135°, the other carrier rod  62  is positioned between about 225° and about 270°, one carrier rod  65  is positioned between about 120° and about 170° and the other carrier rod  65  is positioned between about 190° and about 240°. Thus, one carrier rod  62  and one carrier rod  65  are one side of a vertical plane thru the longitudinal axis and the other carrier rod  62  and the other carrier rod  65  are positioned on the other side of the vertical plane thru the longitudinal axis. 
     The lower carrier rods  65  are elongate and have a plurality of wafer receiving or retaining grooves  81  spaced along the length thereof and have a spacing (pitch) therebetween (FIGS. 3,  4 ). A preferred transverse shape of the grooves  81  is shown in FIGS. 3,  4  and provides for minimal contact with the wafer  25  as is practicable with the need for reliable positioning in the grooves  81 . The preferred shape is generally V-shaped. The grooves  81  are formed by annular rings  82  that project radially from the carrier rods  65  and extend generally transversely around the periphery of the carrier rods  65 . The grooves  81  in one carrier rod  65  are generally transversely aligned with the grooves  81  in the other carrier rod  65  so that the wafers  25  extend generally transversely across the carrier  30 . The grooves  81  open at least into the interior  61  and are preferably annular extending around the entirety of the respective rod  65 . 
     The upper carrier rods  62  each have a plurality of generally transverse grooves  92  in an outer surface  93 . The grooves  92  are in spaced apart relation along the length of the carrier rod  62 . The pitch of the grooves  92  is substantially equal to the pitch of the grooves  81 . Also the grooves  92  are generally vertically or transversely aligned relative to the longitudinal axis LA and with respective grooves  81 . The grooves  92  in one carrier rod  62  at least open generally into the interior  61  and face or open generally toward a respective groove  92  in the other carrier rod  62 . Two grooves  92  and two grooves  81  form a socket for the receipt of a wafer  25  to hold it in a generally vertical orientation and transverse to the longitudinal axis LA. The carrier rods  62  are generally circular in transverse cross section and can rotate in the apertures  74 ,  75  to position the carrier rods in various rotational positions and the grooves  92  are annular, extending around the periphery of the respective carrier rod  62 . This will help place the grooves  92  in the correct orientation for receipt of the wafers  25  therein. 
     The grooves  92  are defined by two generally parallel side surfaces  94 ,  95  and a bottom surface  96  (FIGS. 6,  7 ). The side surfaces  94 ,  95  can diverge slightly, while still being generally parallel, from one another or tapered from the bottom surface  96  at an angle B for each surface from a groove center line up to about 5° for a total divergence angle of twice that. The grooves  92  each have a width W in the range of about 1 mm thru about 1.75 mm and preferably in the range of about 1 thru about 1.5 mm. The groove width is in the range of about 0.4 mm thru about 1.1 mm and preferably 0.5 mm thru about 0.8 mm larger than the thickness of the wafer  25  providing space between at least one side surface  94 ,  95  and the wafer  25 . To hold the wafer  25  steady and in position, the grooves  92  need to be narrow but still wide enough for easy insertion of a wafer  25 . Because this spacing is small, it is difficult to drain or remove the cleaning and rinsing liquids from between the wafer  25  and the surfaces  94 ,  95 . Also, some capillary action can occur in narrow gaps making liquid extraction or drainage difficult. A tapered lead-in area  98  opens from each groove  92  to facilitate insertion of the wafers  25  into the grooves. 
     Channels  97  are provided for improved circulation of the liquid(s) in the grooves  92  and around the wafer  25  (FIGS. 6,  7 ). Further, it has been found that the provision of a channel  97  in each of the surfaces  94 ,  95  that the liquid(s) can be more effectively removed or drained away from the wafers  25 . Also, the channels  97  improve a subsequently applied liquid removing the prior applied liquid, e.g., a rinse liquid removing a cleaning liquid and a cleaning liquid removing a prior used rinse liquid improving both cleaning and rinsing. The channels  97  preferably have both ends thereof open to the exterior of the grooves  92  and the carrier rod  62 . The channels  97  are positioned generally centrally between the bottom surface  96  and apexes  99 ,  100  of the surfaces  94 ,  95  respectively. The channels  97  preferably have a height H that is in the range of about 25% thru about 75% of the height H 1  of the groove  92  at its apex, i. e., the groove&#39;s maximum height. H 1  is in the range of about 1 mm thru about 3 mm and preferably in the range of about 1.75 mm thru about 2.25 mm. The channels  97  are generally semicircular in transverse cross section and have a maximum depth D in the range of about 1.5 mm thru about 2.5 mm and preferably about 2 mm thru about 2.5 mm. The channels  97  provide an enlarged width to a portion of the grooves  92  and an enlarged flow path for liquids improving their flow into and out of the grooves  92 . In a preferred form of the invention, as best seen in FIG. 7, the channels  97  extend generally perpendicular to a radial line from the longitudinal axis of the carrier  30  to the lengthwise midpoint of a respective channel  97 . Thus, the channels  97  extend in a direction generally parallel to a tangent to the edges of the wafers  25  at their midpoints in the grooves  92 . 
     The carrier rods  62 ,  65  are preferably made from a polymeric material such as polytetrafluoroethylene (PTFE). In a preferred embodiment, the polymeric material has hardness in excess of about 60 based on the Shore D scale (DIN 53 505) and a modulus of elasticity (in tension) in excess of about 575 N/mm 2  and preferably greater than about 600 N/mm 2  (DIN 53 457). A preferred PTFE is Hostaflon TFM 1700. 
     A 70 percent improvement in spot reduction was achieved using carrier rods having the channels  97  than with similar carrier rods without channels  97 . 
     The process of cleaning wafers  25  will now be generally described. Wafers  25  are placed in the carrier  30  robotically such as with a Fortrend robot. The wafers  25  are placed in the grooves  81 ,  92  of the carrier rods  65 ,  62  respectively. The carrier  30  is then sequentially moved from one of the tanks  15 - 19  to the next tank for the various cleaning and rinsing steps. The tank  15  contains a base such as ammonia and after application of the base to the wafers  25  for cleaning, the carrier  30  and wafers  25  which are then moved to the tank  16  that contains a rinse liquid like water. The rinse liquid is applied to remove the remaining base cleaning liquid. After rinsing, the carrier  30  and wafers  25  are then drained of the rinse liquid. The carrier  30  and wafers  25  are then moved to the tank  17  for exposure to an acid. Acid is applied to the wafers  25  for further cleaning. The carrier  30  and wafers  25  are then moved to the tank  18 . The carrier  30  and wafers  25  are then exposed again to a rinse liquid. The rinse liquid, such as water, is applied to the wafers  25  and carrier  30  in the tank  18 . The thus rinsed wafers  25  and carrier  30 , after draining, are exposed to heated solvent vapor, such as isopropyl alcohol (IPA), in the tank  19  for removal of the rinse water and are exposed to heat to assist in drying the wafers. The vapor is also dried for removal from the wafers  25 . It is to be understood that the liquids can be applied by immersion as described above and can also be applied by spraying or a combination of immersion and spraying. The thus cleaned wafers  25  are removed from the carrier  30  robotically, as with a Fortrend robot, and sent for manufacture of semiconductor devices. The various cleaning and rinsing liquids flow through the channels  97  to improve contact with the wafers  25  in the grooves  92  and removal of the prior used liquids. The cleaning process removes substantially all of the cleaning and rinsing liquids as well as the contaminants. 
     FIGS. 8-11 illustrate a modified version of a wafer carrier. The modified carrier  105  has end walls  120 ,  121 . Upper and lower carrier rods  122 ,  124  respectively extend between and are secured to the end walls  120 ,  121  to form an open top, open bottom frame  126  similar to the frame  60 . The end walls have feet  127 ,  128  similar to the feet  52 ,  54  for the carrier  105  to rest on. It is preferred that the frame  126 , including the carrier rods  122 ,  124  and end walls  120 ,  121 , be made of polymeric or plastic material such as PTFE that is resistant to degradation by the cleaning and rinse fluids and wear from contact with the wafers  25  as described above. The various parts of the frame  126  can be molded in final form, machined or partially molded and then machined to final form. 
     Lower carrier rods  124 , FIGS. 8,  9 , have grooves  137  similar to the grooves  81  in shape, generally V-shaped transversely. The transverse shape of the carrier rods  124  is generally rectangular (FIG. 10) and the grooves  137  are on one side of each of the rods. The rods  124  have opposite ends  138 ,  139  that are suitably secured to the walls  120 ,  121  respectively. In a preferred embodiment, the ends  138 ,  139  are received in similarly sized and shaped sockets  141 ,  142  respectively. Preferably the ends  138 ,  139  and sockets  141 ,  142  are generally rectangular to prevent rotation of the rods  124 . Mechanical fasteners  144  extend through apertures  143  in the walls  120 ,  121  and into threaded bores  145  in the ends of the rods  135  and when tightened, removably secure the rods to the walls  120 ,  121 . 
     A pair of upper carrier rods  122  is mounted on the frame  126  and are similar to the carrier rods  63  in wafer support and retention function. The transverse shape of the rods  122  is preferably generally rectangular. The rods  122  have a plurality of grooves  146  in spaced apart relation along the length of the rod  122 . The spacing or pitch of the grooves  146  and  137  are substantially the same. Each of the grooves  146  in one rod  122  are aligned transversely with a respective groove  146  in the other rod  122  and each of the these pairs of aligned grooves  146  are in transverse alignment with a respective pair of transversely aligned grooves  137  in the rods  124  forming sockets for receiving wafers  25  therein. 
     The grooves  146  in each rod  122  are formed by a plurality of teeth  147  projecting from one side of the rod. The grooves  146  are defined by a pair of generally parallel side walls  150 ,  151  and a bottom surface  153 . A tapered lead in section  157  opens from each groove  146  to facilitate insertion of the wafers  25  into the grooves  146 . It is preferred that the width W 1  (FIG. 11) of the grooves  146  be in the range of about 1 mm thru about 1.75 mm and preferably in the range of about 1 thru about 1.5 mm. The groove width is in the range of about 0.4 mm thru about 1.1 mm and preferably 0.5 mm thru about 0.8 mm larger than the thickness of the wafer  25  to be retained therein. Wafers  25  are typically on the order of about 0.50 mm thru about 0.80 mm thick. The grooves have a height H 2  (FIG. 11) in the range of about 1 mm thru about 3 mm and preferably in the range of about 1.75 mm thru about 2.25 mm and a length L 2  (FIG. 10) in the range of about 2 mm thru about 6 mm and preferably in the range of about 3.5 mm thru about 6.5 mm. 
     The carrier rods  122  are suitably mounted on the end walls  120 ,  121 . As best seen in FIG. 10, a carrier rod  122  has at least one flat surface  159  adjacent each of the opposite ends  160  forming a shank  161 . Each of the end walls  120 ,  121  has a plurality of sockets  162  recessed in the surfaces  163  facing one another and opening into the interior  165  of the carrier  105 . The sockets  162  each have a flat surface  166 . The sockets  162  and the shanks  161  are similarly sized and shaped and the shanks  161  are each received in a respective socket  162 . The carrier rods  122  are prevented from rotation in the sockets  162  by the interengagement of the flats  159 ,  166 . The rods  122  are releasably secured to the end walls and retained in the sockets  162  preferably by mechanical fasteners  168  that extend through apertures (not shown) through the walls and are threadably engaged in threaded bores (not shown) extending longitudinally into the ends  160 . The fasteners  168  removably secure the rods  122  to the end walls  120 ,  121 . Tie bars, such as those described above, are not required for the carrier  105  because the rods  122 ,  124  function as tie bars. It is preferred that the fasteners  144 ,  168  be made from the above described polymeric material. The carrier  105  is thus of a knock down type. It is contemplated that the carrier  105  could be an integral structure that, e.g., could be made by molding the carrier or by fusing the component parts of the carrier together. 
     The rods  122 ,  124  are preferably of generally uniform or homogeneous construction, i.e., they do not use the customary supplemental reinforcing rod therein to provide the required rigidity. The rods  122 ,  124  are thus substantially entirely made of the polymeric material and are characterized by an absence of a supplemental reinforcing member. By using polymeric material, as described above, resistance to degradation and wear and the required strength can be provided while eliminating a source of contamination. This reduces contamination potential while providing a reduced cost carrier that can easily be repaired particularly by replacing the rods  122  which have the higher wear areas in the grooves  146 . 
     In the carrier  105 , it is preferred that the rods  122 ,  124  be located with one rod  122  and one rod  124  on one side of the longitudinal axis LA 1  and the other rods  122 ,  124  on the other side of the longitudinal axis. In the preferred embodiment, one rod  122  is located between about 90° and about 135°, the other rod  122  is located between about 225° and about 270°, one rod  124  is located between about 120° and about 170° and the other rod is located between about 190° and about 240° relative to 0° as described above. 
     The carrier  105  is adapted to be moved robotically. In that regard, means is provided for the robot (not shown) to engage for releasable attachment to the carrier  105  to enable the robot to lift and move the carrier. As seen in FIGS. 8,  10 , the end walls  120 ,  121  have exterior surfaces  170 ,  171  respectively which face in opposite directions and out of the interior  165 . At least one hitch  173  is provided for each end wall  120 ,  121 . It is preferred that each hitch  173  be located below the center of gravity of the carrier  105  when loaded with wafers  25 . It is also preferred that the hitch  173  on at least one end wall  120 ,  121  be structured such that it will be in engagement with the robot pick up arm on both sides of the center of gravity of the loaded carrier to prevent rotation of the carrier when lifted. In the illustrated embodiment, the hitches  173  extend outward from the surfaces  164 ,  165  and are the same on both ends of the carrier  105  so either end may be picked up by either robot arm, i.e. the carrier is symmetrical for the purpose of robotic pick up. As seen in FIG. 10, the hitches  173  each include a pair of hooks  175  secured to each of the end walls  120 ,  121  and extend from the surfaces  164 ,  165 . One hook  175  on each end wall  120 ,  121  is positioned on one side of the center of gravity and the other hook of the pair of hooks is positioned on the other side of the center of gravity. For convenience of robot operation, a recess  177  is provided adjacent each hitch  175 . The recesses  177  provide clearance for the robot arm. Although the hitches  175  are illustrated as comprising hooks, other hitches could be used. For example, a single hitch that extends laterally from both sides of the center of gravity could be used. It is also to be understood that the hitches  173  could also be positioned above the center of gravity. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.