Abstract:
An end effector has a tower with non-stacked spatulas. Tolerance stacking is avoided by making grooves in the tower relative to a common reference surface, and mounting the spatulas in such grooves. The grooves are provided in separate planar walls of the tower. The walls intersect to enhance the structural properties of the tower. The tower has a dual-purpose clamp formed integrally with one wall for use in assembling the tower and the spatulas, and for mounting the completed end effector in a load lock. The spatula may carry a wafer during various operations, e.g., semiconductor processing, material deposition and etching systems, or in flat panel display processing systems. The carrying of the wafers is notwithstanding vibration of equipment for performing the manufacturing operations, which vibration is primarily in a range of frequencies. Each spatula is formed with a planar platform having an aperture formed therein such that the platform carrying the wafer has a resonant frequency dimensioned so that the resonant frequency while carrying the wafer is outside of the range of frequencies of the equipment vibration. Holes are provided around the aperture, and the spatula is provided with a pad for assembly with each of the holes. Each of the pads has a wafer support surface and a plurality of legs depending from the support surface. The legs are flexed to permit reception of the pad in one of the holes. Methods are disclosed for making the tower, the spatulas, and the end effector with these features.

Description:
This application is a divisional of Ser. No. 09/107,917 filed Jun. 30, 1998 U.S. Pat. No. 6,073,828. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to substrate handling, and more particularly, to towers for positioning substrates and to methods of efficiently manufacturing the towers, components of such towers, and end effectors using such towers and components. 
     2. Description of the Related Art 
     Transport chambers are generally used in conjunction with a variety of substrate processing chambers, which may include semiconductor processing systems, material deposition systems, and flat panel display processing systems. Growing demands for cleanliness and high processing precision increase the need for reduced amounts of human interaction between the processing steps. This need has been partially met by transport chambers, which operate as intermediate handling apparatus between such processing steps. 
     In the use of transport chambers, when a substrate is required for processing, a robot arm within the transport chamber may be used to retrieve a selected substrate from storage and place it into one of the multiple processing chambers. Transport of substrates among multiple storage facilities and processing chambers is typically referred to as cluster tool architecture. 
     FIGS. 1A and 1B schematically illustrate a typical cluster tool architecture. substrates  101  may be stored in a clean room  102 . The substrates  101  may be the base on which layers are deposited in semiconductor processing, or by the material deposition systems, or may be a support used in the flat panel display processing systems, for example. Such substrates are very fragile, giving rise to a need to carefully handle the substrates. The substrates  101  are commonly referred to as wafers. 
     A load lock  104  is generally coupled to the clean room  102 . In addition to being a retrieving and serving mechanism, the load lock  104  also serves as a pressure varying interface between the clean room  102  and a transport chamber  106  that interfaces with various processing chambers  108   a - 108   c . FIG. 1B shows in more detail a cassette  110  in the clean room  102  for storing the substrates  101 . The load lock  104  has a prior art end effector  112  within it. A drive assembly  114  serves to move an arm assembly  116  connected to the end effector  112 . As described below, the prior art end effector  112  is made by alternately stacking prior art spatulas  118  and spacers  120 . The load lock  104  also interfaces with the various processing chambers  108   a - 108   c  by way of a main robot arm  122  of the transport chamber  106 . 
     In use, the end effector  112  of the load lock  104  is moved through a port  124  of the clean room  102  and receives a supply of the wafers  101 . In detail, each spatula  118  receives one of the wafers  101  from the cassette  110  and supports the wafer  101  for transport. The end effector  112  is then moved out of the clean room  102  and back into the load lock  104 , where the wafers  101  are stored prior to being used for processing. Such processing is initiated by the main robot arm  122 . reaching into the load lock  104  and removing one of the wafers  101  from the supported position on the spatula  118 . 
     It may be appreciated that two wafer transfer operations are required to move the wafers  101  from the clean room  102  into a processing chamber  108 , and that each such transfer operation is to be accomplished without human intervention. For the first transfer, the spatulas  118  of the end effector  112  must be aligned with the wafers  101  contained in the cassette  110 . If not aligned, horizontal movement of the end effector  112  toward the cassette  110  may cause one or more of the spatulas  118  to move horizontally and hit one or more of the wafers  101 . Such hitting may break the wafers  101 , or otherwise damage the wafers  101 , as by scratching an upper device surface  126 , of the wafers  101 . While this type of damage to a wafer  101  is a significant cost factor in such processing, a greater cost factor results when the end effector  112  is not aligned with the main robot arm  122  in a second wafer transfer operation. For example, when the processing of the wafer  101  is substantially complete, the value of the wafer  101  includes the increased cost of the processing that has taken place since the wafer  101  left the clean room  102 . However, the first wafer transfer operation has a greater potential of damaging multiple wafers, resulting in a higher cost of production. 
     Attempts have been made to provide end effectors  112  with spatulas  118  accurately aligned with both the cassette  110  (and the wafers  101  therein) and the main robot arm  122 . One such attempt is to make a stack of alternating spatulas  118  and spacers  120  as shown in FIG.  1 C. There, bolts  132  are illustrated for squeezing the spatulas  118  and the spacers  120  together to form the end effector  112 . Referring to FIG. 1C, a desired relative positioning of the spatulas  118  is depicted by reference lines  128 . This desired relative positioning will properly align each spatula  118  with the wafers  101  that are in the cassette and with the robot arm  122  for transfer among the cassette  110 , the load lock  104 , and the transport chamber  106 . To achieve the desired relative spacing of the spatulas  118  of the end effector  112 , attempts are made to hold the thickness T of every one of the spacers  120  and every one of the spatulas  118  within a very close tolerance. For example, the same desired relative positioning is indicated in FIG. 1D by the reference lines  128 . However, the actual relative positioning (shown by reference lines  130  and  130 U) differs significantly from the desired relative positioning even though the spatulas  118  and the spacers  120  are within the desired tolerance (are in-tolerance). In this example, the significant difference is due to the thickness TT of spacers  120 TT being at the thick end of the tolerance. Such thicknesses TT are shown in FIG. 1D accumulating, and resulting in and in-tolerance spacer  120  and the in-tolerance upper spatulas  118 U being positioned above the reference lines  128  and  128 U, indicating misalignment of the spatulas  118 U. Such misalignment of the spatulas  118 U with the reference lines  128  and  128 U resulting from the accumulation of tolerances is referred to as tolerance stacking. Although not shown in FIG. 1D, such misalignment of the spatulas  118 U with the reference lines  128  may also result from the accumulation of tolerances that are at the thin end of the desired tolerance. Tolerance stacking is a significant cause of the wafer damage problem described above. 
     These misalignment problems not only cause the noted wafer damage problems, but may also result in damage to the prior art end effectors  118 . Such end effector damage may require retooling of the prior art end effector  118 , such as by shutting down the operation of the load lock  104 , removing the prior art end effector  112  and replacing any broken spatulas  118 , for example. 
     It may be appreciated that the use of the stacked spatulas  118  and the spacers  120  for the prior art end effectors  112  is dependent on the success of expensive efforts to make each of the spatulas  118  and each of the spacers  120  within very tight tolerances, e.g. plus or minus 0.0005 inches. Also, selection of spatulas  118  and spacers  120  for use in a particular end effector  112 , and other costly steps necessary to attempt to reduce tolerance stacking in stacked arrangements of spatulas  118  and spacers  120 , give rise to an unfilled need to avoid using the stacked arrangements. Further, when these expensive manufacturing efforts fail, the noted significant cost factors (e.g., damage to an unprocessed wafer  101 , or misalignment of the end effector  112 , causing damage to a wafer  101  that has been substantially completely manufactured), are but a part of the resulting costs because process shut-down and reworking of the end effectors  112  may also be required to correct the end effector misalignment. Of course, any shut down situation tends to reduce the yield or productivity of the processing and should be avoided. 
     In addition to these direct costs resulting from such misalignment problems, the risk of contamination is a factor in the prior art end effectors  112  due to the multiple separate parts that are used to make such end effectors  112 . 
     SUMMARY OF THE INVENTION 
     The present invention fills the need that is unfilled by the prior art end effectors by disclosing an end effector having a tower with non-stacked spatulas, and a method of making the tower, the spatulas and the end effector. In the described embodiments, the problem of tolerance stacking is avoided by making grooves in the tower relative to a common reference surface, and mounting the spatulas in such grooves. Also, the grooves are provided in separate walls of the tower. The walls are planar and intersect to enhance the structural properties of the tower. The tower also has a dual-purpose clamp formed integrally with one wall for use in assembling the tower and the spatulas, and for mounting the completed end effector in the load lock. 
     Advantageously, one embodiment of the present invention contemplates using the spatula for carrying a wafer during operations in semiconductor processing, or of material deposition systems, or in flat panel display processing systems. The ability to carry the wafers is notwithstanding vibration of equipment for performing the manufacturing operations. The vibration of the equipment used in such processing or systems is primarily in a range of frequencies. Each spatula is formed with a planar platform having a mounting section and a wafer carrying section. The wafer-carrying section has an aperture formed therein such that the platform carrying the wafer has a resonant frequency. In this embodiment, the aperture is dimensioned so that the resonant frequency of each unit (a spatula while carrying the wafer) is outside of the range of frequencies of the vibration of the equipment. 
     In another embodiment of the present invention, the wafer-carrying section is provided with a plurality of holes around the aperture. In conjunction with the holes, the spatula is provided with a pad for assembly with each of the holes. Each of the pads has a wafer support surface and a plurality of legs depending from the support surface. Each of the legs has a distal end provided with a retainer edge and is flexible. The legs are flexed to permit the distal ends to be received in one of the holes. Upon receipt of the distal ends in the hole the retainer edges of the distal ends retain the pad in the hole with the support surface positioned to carry the wafer. The unit defined by the platform with the pad carrying the wafer is provided with the resonant frequency. 
     In a further embodiment of the present invention, there is provided a method of making a tower for holding end effector components, such as the spatulas. The components are to be accurately held relative to each other, and the tower is provided with a column having a reference ledge that defines a common reference surface. The method includes an operation of forming a reference groove in the column, the reference groove being dimensioned to receive one of the components and defining the reference ledge at an accurate location above a base of the column. First and second additional grooves are formed in the column. The first and second additional grooves are each dimensioned to receive another one of the components, and each defines a respective first and second additional ledge. In this embodiment, the operations of forming the first and second additional ledges are performed to provide the first additional ledge spaced by a selected distance from the common reference surface defined by the reference ledge, and to provide the second additional ledge spaced from the common reference surface by a multiple of the selected distance. In this manner, the first and second additional ledges are evenly and accurately spaced from the common reference surface and from each other, which is the desired relative spacing. These forming operations avoid the tolerance stacking problems of the prior art end effectors since once the reference groove is made to establish the reference ledge with the common reference surface, each of the successive additional ledges is made with reference to the common reference surface rather than with reference to the first additional or second additional or any successive previously made additional ledge. 
     Another aspect of the present invention is providing such method by forming a plurality of the first additional. grooves in the column according to the performing operation, wherein the multiple of the selected distance is increased by one for each of the plurality of the first additional grooves. 
     Still another aspect of such method contemplates having each of the grooves further define a staking section opposite to a respective one of the first and second ledges. Each groove has a given height to receive one of the components. The method further includes the operation of fabricating the column from material that is deformable by staking to reduce the given height of the grooves. 
     Yet another aspect of the present invention is a method of making an end effector for holding piece parts, such as wafers, wherein the wafers are to be accurately held relative to each other. The end effector includes a tower and a spatula, the spatula having a first edge. The method is performed by providing the tower with a plurality of grooves, each of the grooves defining a ledge and a staking portion opposite to the ledge. After making a reference groove, respective ones of the next ledges are spaced from a common reference surface by a selected distance and a different multiple of the selected distance to provide the next grooves and the next ledges without tolerance stacking. Also, the method includes inserting the first edge of the spatula into one of the grooves with the spatula on the ledge of the one groove. Staking the staking portion of the one groove is performed to hold the inserted first edge of the spatula against the ledge of the one groove. 
     A further aspect of the present invention contemplates a method of making an end effector for positioning wafers for processing, wherein the wafers are to be accurately positioned relative to each other. The end effector includes a tower and a plurality of spatulas. The method contemplates operations including making a reference groove to provide a reference ledge that defines a common reference surface, and providing a plurality of the spatulas. The tower is provided with a plurality of additional grooves, each of the additional grooves defining an additional ledge and a staking portion opposite to the additional ledge. Respective ones of the additional ledges are spaced from the common reference surface by a selected distance and by different multiples of the selected distance to accurately and uniformly space the respective additional grooves and ledges from the reference ledge without tolerance stacking. 
     Each of the grooves has a width dimensioned to loosely receive one of the spatulas. One of the spatulas is inserted into one of the grooves with the one spatula on the ledge of the one groove. By staking the staking portion of the groove to decrease the width of the one groove, the inserted spatula is held against the respective ledge. Then, the inserting and staking operations are repeated until all of the plurality of spatulas have been held in successive ones of the grooves by the respective staked staking portions. The staking operation is performed at each of a plurality of spaced locations along the staking portion. 
     In another aspect of the present invention, the inserting and staking operations define a space between each of the respective spatulas and the staking portion of a respective groove. The method further forces brazing filler into each of the spaces between each of the spatulas and the staking portions of the respective grooves. Another operation provides each of the spatulas with a free edge opposite to a respective groove. A plurality of brazing fixtures are provided with a plurality of slots, each of the slots defining an edge support. Respective ones of the edge supports are spaced from the common reference surface by the selected distance and by a different multiple of the selected distance to provide the slots and the edge supports without tolerance stacking. Upon completing all of the operations to insert the spatulas into the grooves, each respective free edge of the spatulas is inserted into a respective slot, and a spring clip is inserted into the respective slot to urge the spatula against the respective edge support. A fixtured end effector results from this method. 
     To complete the end effector, the present invention contemplates gradually preheating the fixtured end effector and dip brazing the heated fixtured end effector to secure the spatulas to the tower without tolerance stacking. 
     These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings. 
     FIG. 1A is a diagrammatic illustration of a typical prior art cluster tool architecture which illustrates how various processing chambers may be coupled to a transport chamber which operates with a load lock which receives wafers from a clean room. 
     FIG. 1B is an elevational view of a portion of the cluster tool architecture illustrating the load lock transporting a supply of wafers received from a cassette in the clean room for delivery to the transport chamber. 
     FIG. 1C is an enlarged elevational view of a prior art end effector illustrating a stack of spacers and spatulas held assembled by bolts. 
     FIG. 1D is an enlarged elevational view of the prior art end effector shown in FIG. 1C illustrating the stack of spacers and spatulas in an undesirable tolerance stacking situation. 
     FIG. 2A is a plan view of a spatula of the present invention illustrating edges positioned relative to each other at a given angle, holes for receiving wafers pads, and an aperture dimensioned for providing a selected resonant frequency. 
     FIG. 2B is a three dimensional view of a unit including a spatula, wafer pads, and a wafer on the pads. 
     FIGS. 3A and 3B are views of the wafer pad shown in FIG. 2B, illustrating legs for retaining the pads to the spatula, and in FIG. 3B illustrating the holes for receiving the wafers pads. 
     FIG. 4A is a three-dimensional view of a tower of the present invention, illustrating grooves formed in one of two walls for receiving the spatulas. 
     FIG. 4B is a plan view of the tower showing walls positioned relative to each other at an angle substantially the same as the given angle, and a clamp integral with one of the walls. 
     FIG. 4C is an elevational view of the tower illustrating how a reference groove is formed in the tower to define a common reference surface, and how each of a plurality of additional grooves is formed in the walls relative to the common reference surface to avoid tolerance stacking. 
     FIG. 5A is a plan view of the end effector of the present invention illustrating the tower assembled with one of the spatulas and a wafer supported on the spatula. 
     FIG. 5B is an enlargement of a portion of the end effector shown in FIG. 5A, illustrating the locations at which a staking operation is performed to secure a spatula to the walls of the tower. 
     FIG. 5C is an enlarged elevational view illustrating a spatula received in one of the grooves and resting on a ledge, where a space is illustrated between the upper surface of the spatula and the upper portion of the groove prior to the staking operation. 
     FIG. 5D is a view similar to FIG. 5C after the staking operation has been performed, illustrating deformation of a staking portion into contact with the upper surface of the spatula to hold the spatula against the ledge. 
     FIG. 5E is a three-dimensional view of the end effector shown in FIG. 5A, illustrating a plurality of spatulas secured to the tower and welding fixtures removably attached to the spatulas for holding the spatulas in position during brazing. 
     FIG. 5F is an elevational view illustrating fixturing of the end effector using a plurality of combs. 
     FIG. 5G is an enlarged view of a portion of FIG. 5F illustrating spring clips used in the fixturing with the comb. 
     FIG. 6 is a three dimensional view of the assembled end effector, illustrating the tower secured to the plurality of spatulas. 
     FIG. 7A is a flow chart showing the operations of one embodiment of a method of the present invention for manufacturing the tower with grooves to avoid tolerance stacking. 
     FIG. 7B is a flow chart illustrating the operations of another embodiment of a method of the present invention where a plurality of spatulas are positioned without tolerance stacking and the spatulas are secured to the tower by staking operations. 
     FIG. 7C is a flow chart illustrating the operations in the assembly of the end effector of the present invention by joining the tower with spatulas, wherein heating and brazing operations follow joining the spatulas to the tower by the staking operations. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As described above, FIGS. 1C and 1D illustrate the problem of tolerance stacking, in which there are significant differences between the desired relative positioning of exemplary prior art spatulas  118  (indicated by reference lines  128  and  128 U), and actual relative positioning of the exemplary prior art spacers  120 TT (indicated by reference lines  130  in FIG.  1 D). In the example, the significant differences are due to the thicknesses TT of spacers  120 TT being at the thick end of the desired tolerance. Such thicknesses TT are shown in FIG. 1D as accumulating, and resulting in the actual positioning (indicated by reference lines  130  and  130 U) of upper spatulas  118 U above the reference lines  128  and  128 U. The actual positioning indicates misalignment of the spatulas  118 U, as described above. It was noted that such misalignment of the spatulas  118 U with the reference lines  128  may also result from the accumulation of tolerances that are at the thin end of the desired tolerance. 
     An invention is described below for improving the efficiency of manufacture of end effectors  200  (FIG.  6 ), and of components of such end effectors (e.g., spatulas  202 ), through the implementation of ways of making grooves  204  in a tower  206  relative to a common reference surface  208  (FIGS.  2 A and  4 A). In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known manufacturing operations have not been described in detail in order not to obscure the present invention. 
     FIG. 2A is a plan view of a single spatula  202  manufactured according to one embodiment of the present invention. The spatula  202  carries a piece part, such as a wafer,  210  (FIG.  2 B), during semiconductor processing operations, or in the operation of material deposition systems or of flat panel display processing systems. The spatula  202  is machined from aluminum plate, for example, by a fine blanking technique well known to those skilled in the art. This technique defines a perimeter  212  having an edge  214  formed in many sections  216 . A first section  216 A of the edge  214  is shown intersecting a second section  216 B at an angle  218 . The angle  218  may, for example, be a right angle. Other sections  216 C and  216 D of the edge  214  extend away from the intersecting respective first and second sections  216 A and  216 B, and with the respective first and second sections  216 A and  216 B, define a mounting portion  220  of the spatula  202 . The mounting portion  220  has an upper surface  222 . Spaced sections  216 E and  216 F extend away from the mounting portion  220 . A distal edge section  216 G extends around a distal end  224 . The sections  216 E,  216 F, and  216 G define a portion  226  of the spatula  202  for carrying one of the wafers  210 . To minimize the area of a wafer  210  that is touched during such carrying, holes  228  are provided at spaced locations, such as at points defined by center lines  230 A,  230 B, and  230 C. Also, an aperture  232  is formed in the carrying portion  226  within an area defined by the holes  228 . The aperture  232  has a diameter centered on an aperture axis  234 , and each of the holes  228  has a diameter centered on a hole axis  236 . 
     FIGS. 3A and 3B illustrate a pad  238  provided for assembly with each of the holes  228 . Each of the pads  238  has a wafer support surface  240  and a plurality of legs  242  depending from the support surface  240  parallel to a leg axis  244 . The wafer support surfaces  240  of the three illustrated pads  238  cooperate to provide the minimum area of the wafer  210  that is touched during the carrying of the wafer  210 . To secure the pad  238  to the spatula  202 , distal ends  256  are positioned within the holes  228  such that surfaces  252  are caused to contract in a direction  250  while placing a holding/friction force against the inner surface of the holes  228 . To assist in inserting the pads  238  into each of the holes  228 , the distal ends  256  have bevels  258 . The distal ends  256  also have bottom surfaces  260  which are preferably contained within the holes  228  and above the level of surface  266  of the spatula  202 . 
     In the manufacture of one embodiment of the spatula  202  of the present invention, the following is recognized. Vibrations are created during semiconductor processing operations, or in the operation of material deposition systems or of flat panel display processing systems, the equipment (not shown) used for such operations or in such systems. The vibrations of the equipment are primarily in a range of frequencies, such as 35 cps to 37 cps. In such manufacture it is also recognized that a vibration unit  268  is formed by one of the spatulas  202 , the three pads  238 , and one of the wafers  210  carried by the three pads  238 . Such a unit  268  is shown in FIG. 2B, and it is further recognized that the unit  268  will have a resonant frequency. The spatula  202 , and the associated pads  238 , must nonetheless carry the wafer  210  in such a manner that any vibration of the unit  268  will not cause the wafer  210  to move in response to the vibrations (e.g., walk) off the pads  238 . To achieve this result, once the range of frequencies of such equipment is known, the aperture  232  is dimensioned so that the resonant frequency of such unit  268  will be out of this range. In this manner, the amplitude of the vibration of the unit  268  will be reduced, which tends to avoid walking of the wafers  210  off the pads  238 . The dimensioning of the aperture  232  may, for example, use a selected diameter for a circular aperture  232 , or the aperture  232  may have any other non-circular shape designed to achieve the desired resonant frequency of the unit  268 . For determining the resonant frequency, in one embodiment, an accelerometer can be mounted on spatula  202 , tapping the spatula, and recording the signal from the accelerometer. In this manner, the proper size and shape of aperture  232  can be predicted using finite element analysis. Once the shape and size of the aperture  232  have been selected, one of the spatulas  202  is formed with that shape and size aperture  232 , and the pads  238  are assembled with the spatula  202 . The unit  268 , with a typical wafer  210  on the three pads  238 , is mounted to a tower  206  in the manner described below. The tower  206  is mounted to a vibration table (not shown). The table vibrates the tower  206  and the unit  268  to determine that the resonant frequency of the unit  268  is out of this range. The shape and size of the aperture  232  may be adjusted as necessary to achieve the desired resonant frequency of the unit  268 , which is outside of the range of vibration of the equipment. 
     In one embodiment of the spatula  202 , the spatula  202  may be fabricated from plate aluminum, such as that meeting the standard 6061-T4 specification, for example, such that the spatula  202  is planar. Such plate aluminum may, for example, have a thickness of about 0.150 inches plus or minus 0.001 inch. Further, the carrying portion  226  may be stress relieved prior to final machining. In such embodiment, exemplary dimensions of the spatula  202  include an overall length of about eleven inches, a length of the carrying portion  226  of about 7.6 inches, and a width of the mounting portion  220  of from about 3.1 inches to about 1.6 inches. Radii of the sections  216 C and  216 P having curved portions may include a radius R 1  of about 1.5 inches, and a radius R 2  of about 0.75 inches; whereas a proximal end  270  may have a radius R 3  of about 1.0 inch. Corners  272  of the spatula  202  may, for example, be arcuate having a radius R 4  of about 0.3 inches. Also, the holes  228 A and  228 B may be located about 0.3 inches from the distal end  224 . The hole  228 A may be located about 0.3 inches from the section  216 F, and the hole  228 B may be located about 4.3 inches from the section  216 F. The third hole  228 C may be located about 6.06 inches from the distal end  224 , whereas the aperture  232  may be about 1.46 inches from the distal end  224 . The third hole  228 C may be aligned with the aperture  232  at about 2.3 inches from the section  216 F. 
     FIG. 4A illustrates the three-dimensional aspects of the tower  206  of the present invention, showing the tower  206  in a vertical position for holding components, such as the spatulas  202  shown in FIGS. 2A and 2B. The tower  206  holds the spatulas  202  accurately relative to each other, which is in the desired relative positions described above. The tower  206  may include a column  274 , or other vertical member, having a plurality of the grooves  204  formed therein. Each of the grooves  204  is dimensioned to receive one of the spatulas  202  and defines a ledge  276 . Thus, the plurality of grooves  204  define a plurality of ledges  276  along the column  274 . The column  274  has a base  278  provided with a surface, referred to as an initial reference surface  280 , which defines the location of a common reference groove  204 R. The common reference groove  204 R has a reference ledge  276 R which defines a common reference surface  282  from which the desired relative positioning of additional ones of the grooves  204 A described above is determined. 
     FIG. 4B is a plan view of the column  274  shown in FIG. 4A, illustrating the column  274  including first and second walls  284 C and  284 F, respectively, which extend at a selected angle  286  relative to each other. For example, the selected angle  286  of the walls  284 C and  284 F may be a right angle relative to each other, and such angle  286  should correspond to the angle  218  at which the first and second sections  216 A and  216 B of the edge  214  of the spatula  202  are positioned relative to each other. It may be understood that for spatulas  202  having first and second sections  216 A and  216 B positioned at a different angle  218  relative to each other, the walls  284 C and  284 F of the column  274  will be at a selected angle  286  corresponding to that different angle  218 . 
     The walls  284 C and  284 F are shown having flat opposite sides such that each of the walls  284 C and  284 F is planar. The second wall  284 C of the walls  284  is shown formed integrally with a device  288  for holding the tower  206  to a post  290  (FIG. 5E) or other support which may be provided in the manufacture or use of the end effector  200 . The device  288  may be referred to as a clamp in that a cylindrical portion  292  of the device  288  is connected to the wall  284 C and extends circularly to an opening  294 . The opening  294  defines opposed flanges  296  of the cylindrical portion  292 . There is a gap  298  between the opposed flanges  296  to allow the diameter  300  of the cylindrical portion  292  to be adjusted. For example, with the gap  298  wide, the cylindrical portion  292  may be placed, on the post  290 . Then, the gap  298  may be made smaller by drawing the flanges  296  closer to each other. Holes  302  are provided in the flanges  296  and fasteners  304  are inserted in the holes  302  to tighten the flanges  296  on the post  290  to secure the column  274  in a desired place. The first wall  284 F is thus free in that it is spaced from the clamp  288 . However, because of the selected angle  286  between the walls  284 F and  284 C, when the clamp  288  is secured to the post  290 , both walls  284 C and  284 F remain in a stable vertical position for holding the spatulas  202  accurately and horizontally. 
     FIG. 4C is an elevational view of the column  274  showing the grooves  204  formed in the walls  284 C and  284 F. With the walls  284 C and  284 F intersecting along a line  306  (shown as vertical in FIGS.  4 A and  4 C), it is to be understood that each particular one of the grooves  204  extends horizontally across the line  306  so that each groove  204  extends continuously along the complete extent of the respective first and second walls  284 F and  284 C. 
     Each one of the grooves  204  defines one of the ledges  276 , and a staking portion  308  opposite to the ledge  276 . There is a space  310  defined by each of the grooves  204 , an under surface  312  of each staking portion  308 , and an inner end  314  of each groove  204 . The space  310  has a dimension S large enough to receive the thickness of one of the spatulas  202 . FIG. 4C also shows the initial reference surface  280  defined by the base  278 . The walls  284 F and  284 C and the clamp  288  extend vertically upwardly from the initial reference surface  280 . 
     One embodiment of a method of the present invention relates to making the tower  206  for holding the components (e.g., the spatulas  202 ) of the end effector  200 , wherein the spatulas  202  are to be accurately held relative to each other. This embodiment is described in FIG.  7 A. Referring to FIG. 7A, this embodiment of the method includes an operation M 501  of forming the initial reference groove  204 R in the walls  284 . The initial reference groove  204 R is made by measuring from the initial reference surface  280  a distance equal to the thickness (or height) of the base  278 . At that distance, the reference groove  204 R is formed, as by grinding, for example. The reference groove  204 R defines the ledge  276 , which is referred to as the common reference ledge  276 R. The common reference ledge  276 R provides the common reference surface  282  for making additional ones of the grooves  204 A and their respective additional ledges  276 A. 
     Still referring to FIG. 7A, this embodiment of the method includes a further operation M 502  of forming a first additional groove  204  in the walls  284 . The first additional groove  204  is dimensioned to receive another one of the spatulas  202  and defines a first additional ledge  276 A 1 . As shown in FIG. 4C, the distance from the common reference surface  282  to any one of the additional ledges  276 A is either a specified amount, referred to as D (e.g., for the first additional ledge  276 A 1 ), or a multiple of D (e.g., for the remainder of the additional ledges  276 A). 
     Still referring to FIG. 7A, this embodiment of the method includes a further operation M 503  of forming a second additional groove  204 A 2  in the walls  284 . The second additional groove  204 A 2  is also dimensioned to receive another one of the spatulas  202  and defines a second additional ledge  276 A 2 . 
     It may be understood that this embodiment of the method includes performing each of the additional groove forming operations M 502  and M 503  to provide the first additional ledge  276 A 1  spaced by the selected distance D from the common reference surface  282  and to provide the second additional ledge  276 A 2  spaced from the common reference surface  282  by a multiple of the selected distance D. As a result, the first additional ledge  276 A 1  and the second additional ledge  276 A 2  are evenly and accurately spaced from the common reference surface  282  and from each other. 
     This embodiment of the method may be continued by performing an operation M 504  of forming a plurality of the second additional grooves  204 A in the walls  284  as described above (e.g., additional grooves  204 A 3  to  204 AN, where N exceeds 3). In this situation, the multiple of the selected distance D is increased by one for each of the plurality of second additional grooves  204 A. 
     In more detail, and still referring to FIG. 4C, this embodiment of the method provides the first additional ledge  276 A 1  of the additional ledges  276 A spaced by the selected distance D from the common reference surface  282  defined by the reference ledge  276 R. Also, a plurality of the successive additional ledges  276 A 2  through  276 A 12  are, for example, shown spaced from the common reference surface  282  by a uniformly increasing multiple of the selected distance D. In this manner, the plurality of additional ledges  276 A 2  through  276 A 12  are evenly and accurately spaced from the common reference surface  282  at which the reference ledge  276 R is located, and from each other. The amount of the uniformly increasing multiple of the selected distance D may be 1, for example, so that the distance of the first additional ledge  276 A 1  from the common reference surface  282  is D, and the distance of the second additional ledge A 2  from the common reference surface is 2 times D, and the distance of the third additional ledge  276 A 3  from the common reference surface  282  is 3 times D, and the distance of the ledge  276 A 12  from the common reference surface  282  is 12 times D, for example. 
     The number of grooves  204  to be provided in any particular wall  284  depends on the number of spatulas  202  which need to be used to carry all of the wafers  210  contained in any given one of the cassettes  110 . In one embodiment of the present invention, up to twenty-five grooves  204  may be provided in the walls  284 . It may be appreciated that the advantages of the present invention become more significant with increases in the number of wafers  210  to be carried. In more detail, because there is no tolerance stacking of the additional ledges  276 A formed in the walls  284 , only one tolerance is involved between any given additional ledge  276 A and the common reference surface  282 . In contrast, as is clear from the above description of 
     FIG. 1D, with each increase in the number of wafers  210  to be carried by the prior art end effectors  112 , each prior art spacer  120  and each prior art spatula  118  presents another opportunity for introducing an increase in the amount of error in the actual relative positioning of the spatulas  118  as compared to the desired relative positioning. 
     In one embodiment of the tower  206 , the tower  206  may be fabricated from 6061-T4 aluminum alloy. Such aluminum alloy may have a thickness of about 0.25 inches. Further, the tower  206  may be stress relieved prior to final machining. In such embodiment, exemplary dimensions of the tower  206  include the following. There may be provided an overall height of about 5.3 inches, a length of the wall  284 C of about 3.5 inches, a length of the wall  284 F of about 1.8 inches, and a distance of about 0.975 inches from the outside of the wall  284 C to the centerline  316  of the clamp  288 . The clamp  288  may be about four inches high, for example. The diameter of an outer wall  318  of the clamp  288  may be 0.875 inches, and the diameter of an inner wall  320  of the clamp  288  may be 0.625 inches, for example. The centerline  316  of the clamp  288  may be about 3.65 inches from the wall  284 F. The grooves  204  may be 0.125 inches deep (perpendicular to the plane of a wall  284 ) for example. The height of each of the grooves  204  may be 0.153 inches plus or minus 0.001 inch. In this manner, the spatulas  202  having the above identified exemplary thicknesses of about 0.150 inches may be received in the grooves  204 . 
     The common reference ledge  276 R may be spaced from the initial reference surface  280  by 0.25 inches plus or minus 0.001 inch. The distance D from the common reference ledge  276 R to the first additional ledge  276 A 1  may be 0.3937 inches plus or minus 0.0020 inches. As described above, the distance from the common reference ledge  276 R to the second additional ledge  276 A 2  may be 2 times 0.3937 inches (or 0.6874 inches) plus or minus 0.0020 inches. Thus, 2 is the multiple. As described above, there is a uniform increase in the value of the multiple from one additional ledge  276 A to another additional ledge  276 A. For example, the third additional ledge  276 A 3  is made with reference to 3 times 0.3937 inches (plus or minus 0.0020 inches) measured from the common reference surface  282  defined by the reference ledge  276 R. Similarly, the fourth additional ledge  276 A 4  is made with reference to 4 times 0.3937 inches (plus or minus 0.0020 inches) measured from the common reference surface  282  defined by the common reference ledge  276 R. It may be understood then that if there are N additional grooves  204 , the Nth additional groove  204  will be made with reference to N times 0.3937 inches (plus or minus 0.0020 inches) measured from the common reference surface  282 . 
     FIG. 5A is a plan view of the end effector  200  of the present invention illustrating the tower  206  assembled with one of the spatulas  202 . For illustration purposes, the center of a wafer  210  is shown being concentric with the center of aperture  232 . Although any size or shape substrate may be carried by the spatulas of the end effector  200 , preferably circular-type wafers, such as a 300 mm (11.811 inch) wafer is carried by each of the spatulas  202 . The first and second sections  216 A and  216   b  of the edge  214  of the spatula  202  are shown conforming to the shape of the inner ends  314  (FIG. 5A) of the grooves  204 . FIG. 5B shows an enlargement of a portion of the assembled tower  206  and the spatula  202  and illustrates a plurality of locations (each indicated by a short line  322 ) at which a staking operation is performed. FIG. 5C is a cross section illustrating the process of assembly of the tower  206  with a spatula  202 . The spatula  202  is illustrated in the groove  204 . FIG. 5D shows the result of performing the staking operation to secure the spatula  202  in one of the grooves  204 . FIG. 5C illustrates one of the three locations  322  (shown in FIG. 5B) as having the staking operation performed. It may be understood that the one embodiment of the method of the present invention may include an initial additional operation M 500  of fabricating the column  274  from material that is deformable by staking to reduce the height  324  of the space  310  of the grooves  204 . This operation M 500  is achieved by using the plate aluminum for the tower  206  as described above. 
     Referring again to FIG. 5C, one of the grooves  204  is shown defining the staking portion  308 , which is above the ledge  276  of the groove  204 . Because of the height  324  of the space  310  of the groove  204 , prior to performing a staking operation, there is the space  326  between the upper surface  222  of the spatula  202  and the under surface  312  of the groove  204 . FIG. 5D is similar to FIG. 5C, but differs in that FIG. 5D illustrates the staking portion  308  after the staking operation has been performed by using a staking tool  328 . The staking tool  328  shown in FIG. 5D is used to deform the staking portion  308  to define a tab  330  that is formed to press (as viewed in FIG. 5D) against the upper surface  222  of the spatula  202  that is in the groove  204 . The tab  330  urges the spatula  202  down (as viewed in FIG. 5D) against the ledge  276 . The strength of the staking portion  308  is such that the tab  330  remains in the position shown in FIG. 5D so as to hold the spatula  202  against the ledge  276 . 
     Referring to FIGS. 4B,  5 C,  5 D,  5 E and  7 B, another embodiment of the method of the present is illustrated. An operation M 520  is for providing the tower  206  with a plurality of the grooves  204 . Each of the grooves  204  defines one of the ledges  276  and the staking portion  308  opposite to the ledge  276 . Respective ones of the ledges  276  are spaced from the common reference surface  282  by the selected distance D, for example, and a multiple of the selected distance D to provide the grooves  204  and the ledges  276  without tolerance stacking. An operation M 521  is for inserting the first edge  334  of the spatula  202  into one of the grooves  204  with the spatula  202  on the ledge  276  of the one groove  204 . An operation M 522  is for staking the staking portion  308  of the one groove  204  to hold the inserted first edge  334  of the spatula  202  against the ledge  276  of the one groove  204 . 
     Further operations may be taken to complete the tower  206 , as by the following. The insertion operation M 521  may be performed one-by-one starting from the bottom of the base  278  and first inserting a spatula  202  in the reference groove  204 R. One of the tabs  330  shown in FIG. 5D is formed. Then, the next upward spatula  202  is inserted into the next upward groove  204 , which defines the first additional ledge  276 A 1 . Another one of the tabs  330  shown in FIG. 5D is formed. This series of insertion operation M 521  and staking operation M 522  may be repeated until all of the spatulas  202  have been inserted into all of the grooves  204  and all of the staking portions  308  have been staked. 
     FIG. 7C describes another embodiment of a method of the present invention. In an operation M 530 , pre-cleaning of all of the components of the end effector  200  and of the parts for brazing (e.g., the brazing fixture  346  and the clips  354  is performed. The pre-cleaning is in an acid solution. Then such components and parts are rinsed in water. Then, an operation M 531  may be performed to mount a spatula  202  to a tower  206  one-at-a-time. Each spatula  202  is staked to the tower  206 . 
     Having inserted all the spatulas  202  into all of the grooves  204 , and having staked all of the staking portions  308 , it may be understood that except at the locations  322  at which the staking operation has been performed, the grooves  204  that have received the spatulas  202  still have the space  326  between the under surface  312  and the upper surface  222  of the spatula  202  as shown in FIG.  5 C. 
     M 532  is the next operation of this embodiment of the method of the present invention, in which a dip brazing filler  336  (FIG. 5C) is applied into each of the spaces  326  between the upper surfaces  222  and the under surfaces  312 . To apply the brazing filler  336 , the filler  336  is provided in an injector  338 , which may be syringe-like having a long hollow needle  340 . As described in FIG. 7C, and with reference to FIG. 5E, the needle  340  is inserted into a space  342  between one pair  344  of the spatulas  202 . The needle  340  extends to the space  326  shown in FIG.  5 C. The injector is then operated to discharge the brazing filler  336  into the space  326  along the entire extent of one of the grooves  204 . This process is repeated with the next space  342  between the next pair  344  of spatulas  202  until all of the spaces  326  in all of the grooves  204  have been filled with the brazing filler  336 . 
     At this juncture, the tower  206  is in the condition shown in FIG. 5F, except for four brazing fixtures (or combs)  346  that are shown in FIG.  5 F. In detail, the mounting portion  220  of each of the spatulas  202  is held in place in the respective groove  204  by the three tabs  330  (FIG.  5 B), such that the carrying portion  226  is cantilevered from the mounting portion  220 . Also, the brazing filler  336  is in all of the spaces  326 . 
     FIG. 5E illustrates the four brazing fixtures  346 , with three shown located adjacent to the carrying portion  226  and one located adjacent to the mounting portion  220 . Referring to FIG. 5F, each of the brazing fixtures  346  is provided with a plurality of slots  348 . Each of the slots  348  defines an edge support  350  on which one of the spatulas  202  rests. The slots  348  are formed in the brazing fixtures  346  in the same manner as the grooves  204  are formed. As a result, after a reference slot  348 R is formed and defines a common fixture reference surface  352 , additional slots  348  are formed so that respective ones of the edge supports  350  are spaced from the common fixture reference surface  352  by the same selected distance D and the same uniformly increased multiple of the selected distance D to provide the slots  348  and the edge supports  350  without tolerance stacking. 
     In an operation M 533 , the brazing fixtures (or combs  346 ) are used to fixture the tower  206 , which is by applying the combs  346  to hold the spatulas  202  in place. The fixturing supports the carrying portion  226  and the mounting portion  220  of the each of the spatulas  202  for a brazing operation. For this purpose, operation M 533  may include positioning the brazing fixtures as shown in FIG. 5E adjacent to the respective carrying portion  226  and to the mounting portion  220 . Operation M 533  may also include inserting the edges of these carrying portions  226  and mounting portions  220  of all of the spatulas  202  into a respective one of the slots  348  of the comb  346 . 
     FIGS. 5F and 5G illustrate a final aspect of operation M 533  of this fixturing, which is to insert a spring clip  354  into a space  356  between the upper surface  222  of the spatula and an under surface  358  of the slot  348  of the comb  346 . The clip  354  is a U-shaped resilient member having legs  360  self-biased apart. As shown in the enlarged FIG. 5G, the legs  360  of the clip  354  has been pressed together as it was inserted into the space  356 , which holds the spatulas  202  in the slots  348 . The result of this fixturing process is referred to as a fixtured end effector  362 , and is as shown in FIG.  5 E. 
     The fixtured end effector  362  is then processed in a further operation M 534 , in which there is gradual pre-heating of the fixtured end effector  362 . The preheating operation M 534  is a standard operation in dip brazing, such that one skilled in the art will understand that the pre-heating is typically performed in an oven (not shown) to increase the temperature of the fixtured end effector  362  to 1000 degrees F. As further illustrated in FIG. 7C, as operation M 535 , in a standard manner the pre-heated fixtured end effector  362  is then immersed in a molten lithium bath (not shown) having a temperature of 1100 degrees F. to activate the brazing filler  336 . In the immersion, the aluminum material from which the end effector  200  is fabricated changes from a T 4  condition to a T 0  condition and becomes softer. In another operation M 536  illustrated in FIG. 7C, the brazed end effector  200  is cooled in air at room temperature for about twelve hours. In an operation M 537  the combs  346  and the clips  354  are removed from the cool end effector  200 . In a final operation M 538 , the end effector  200  is cleaned in a standard post-brazing operation by using an acid solution and a final water rinse. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. In addition, although the preferred materials used to make the end effector  200  is plate aluminum and stainless steel as described above, any other suitable material, such as steel, etc., may be substituted therefor. Therefore, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.