TECHNICAL FIELD OF THE INVENTION [00O1] The present invention pertains generally to robotic arms, and more particularly, to wrist assemblies for robotic arms of the type useful in wafer processing equipment.

BACKGROUND OF THE INVENTION [00O2] Modern semiconductor processing systems include cluster tools that integrate a number of process chambers together in order to perform several sequential processing steps without removing the substrate from the highly controlled processing environment. These chambers may include, for example, degas chambers, substrate pre¬ conditioning chambers, cooldown chambers, transfer chambers, chemical vapor deposition chambers, physical vapor deposition chambers, and etch chambers. The combination of cha.ni.bers in a cluster tool, as well as the operating conditions and parameters under which those chambers are run, are selected to fabricate specific structures using a specific process recipe and process flow. [0003] Once the cluster tool has been set up with a desired set of chambers and auxiliary equipment for performing certain process steps, the cluster tool will typically process a large number of substrates by continuously passing them, one by one, through a series of chambers or process steps. The process recipes and sequences will typically be programmed into a microprocessor controller that will direct, control and monitor the processing of each substrate through the cluster tool. Once an entire cassette of wafers has been successfully processed through the cluster tool, the cassette may be passed to yet another- cluster tool or stand alone tool, such as a chemical mecnanical polisher, for further processing. [0004J une example or a raβriσation system of the type described above is the cluster tool disclosed in U.S. 6,222,337 (Kroeker et al.) , and reproduced in FIGs. 1 and 2 herein. The magnetically coupled robot disclosed therein is equipped with robotic arms having a frog-leg type construction that are adapted to provide both radd_al and rotational movement of the robot blade within a fixed plane. The radial and rotational movements can be coordinated or combined to allow for pickup, transfer and deliver of substrates from one location within the cluster tool to another location. For example, the robotic arm may be used to move substrates from one processing chamber to an adjacent chamber. [0005] FIG. 1 is a schematic diagram of the integrated cluster tool 10 of Kroeker et al. Substrates are introduced into, and withdrawn from, the cluster tool 10 through a cassette loadlock 12. A robot 14 having a blade IL7 is located within the cluster tool 10 to transfer the substrates from one process chamber to another. These process chambers include cassette loadlock 12, degas wafer orientation chamber 20, preclean chamber 24, PVD TiN chamber 22 and cooldown chamber 26. The robot blade 17 is illustrated in the retracted position in which it can rotate freely within the chamber 18. [0006] A second robot 28 is located in transfer chamber 30 and is adapted to transfer substrates between various chambers, such as the cooldown chamber 26, preclea.ii chamber 24, CVD Al chamber (not shown) and a PVD AlCu processing chamber (not shown) . The specific configuration of chambers illustrated in FIG. 1 is designed to provide an integrated processing system capable of both CVD and PVD processes in a single cluster tool. A microprocessor controller 29 is provided to control the fabricating process sequence, conditions within the cluster tool, and the operation of the robots 14, 28. [0007] FIG. 2 is a schematic view of the magnetica.lly coupled robot of FIG. 1 shown in both the retracted! and extended positions. The robot 14 (see FIG. 1) includes a first strut 81 rigidly attached to a first magnet clamp 80 and a second strut 82 rigidly attached to a second magnet clamp 80' . A third strut 83 is attached by a pivot 84 to strut 81 and by a pivot 85 to a wafer blade 86. A fourth strut 87 is attached by a pivot 88 to strut 82 and by a pivot 89 to wafer blade 86. The structure of struts 81-83, 87 and pivots 84, 85, 88, and 89 form a "frog leg" type connection of wafer blade 86 to magnet clamps 80,80' . [0008] When magnet clamps 80,80' rotate in the same direction with the same angular velocity, then the robot also rotates about axis x in this same direction w±th the same velocity. When magnet clamps 80, 80' rotate in opposite directions with the same absolute angular velocity, then there is no rotation of assembly 14, but instead there is linear radial movement of wafer blade 86 to a position illustrated by dashed elements 81' -89'. [0009] A wafer 35 is shown being loaded on wafer blade 86 to illustrate that the wafer blade can be extended through a wafer transfer slot 810 in a wall 811 of a chamber 32 to transfer such a wafer into or out of the chamber 32. The mode in which both magnet clamps 80, 80" rotate in the same direction at the same speed can be used to rotate the robot from a position suitable for wafer exchange with one of the adjacent chambers 12, 20, 22, 24, 26 (see FIG. 1) to a position suitable for wafer exchange with another of these chambers. The mode in which both magnet clamps 80, 80" rotate with the same speed in opposite directions i-s then used to extend the wafer blade into one of these chiambers and then extract it from that chamber. Some other combination of clamp rotation can be used to extencL or retract the wafer blade as the robot is being rotated about axis x. "[00ϊ0] ' 1IO Keep waier made 86 directed radially away from the rotation axes x, an interlocking mechanism is used between the pivots or cams 85, 89 to assure an equaH and opposite angular rotation of each pivot. The interlocking mechanism may take on many designs. One possible interlocking mechanism is a pair of intermeshed geazrs 92 and 93 formed on the pivots 85 and 89. These gears are loosely meshed to minimize particulate generation by these gears. To eliminate play between these two gears because o± this loose mesh, a weak spring 94 (see FIG. 4) may be extended between a point 95 on one gear to a point 96 on the other gear such that the spring tension lightly rotates ttiese two gears in opposite directions until light contact between these gears is produced. [0011] Although robots of the type depicted in U.S. 6,222,337 (Kroeker et al. ) have many desirable features, robots of this type also have some shortcomings. Iri particular, it has been found that robots of this type often exhibit excessive wear in the wrist 85', 89' and el-bow 84', 88' joints. This problem results in excessive maintenance requirements and interruptions to the manufacturing process. There is thus a need in the art for a robotic assembly which requires less maintenance and exhibits less wear in these areas. These and other needs are met by the devices and methodologies disclosed herein and hereinafter desc-tribed.

SUMMARY OF THE INVENTION [0012] In one aspect, a robotic arm assembly is provided which comprises a hub, a first arm segment attached to the hub, and a second arm segment attached to the first arm segment (e.g., by way of a pin or other suitable means) such that the second arm segment can rotate at least partially about its longitudinal axis. The robotic arm assembly, which preferably has a frog-leg design, may further comprise a third arm segment which is pivotally connected to the second arm segment, an end effector (to which the thirrd arm segment may be attached) , a fourth arm segment attached to the hub, and a fifth arm segment attached to the fourth arm segment such that the fifth arm segment can rotate at least partially about' its longitudinal axis. The robotic arm assembly may further comprise a sixth arm segment which is pivotally connected to the fifth arm segment and whicb. is also connected to the end effector. Preferably, the third and sixth arm segments are attached to opposing sides of the end effector, preferably by way of wrist assemblies. [0013] In another aspect, a robotic arm assembly is provided which comprises a hub, a lower arm attached to said hub, a forearm pivotally attached to said lower arm, a.nd an end effector attached to said forearm, wherein said lower arm comprises a first segment which is rotatably connected to a second segment. [0014] In still another aspect, a robotic arm assemblv is provided herein which comprises a hub, a first arm segment attached to said hub, and a second arm segment attached to said first arm segment such that said second arm segment can move with respect to said first arm segment in such a way as to relieve stress on the arm. [0015] One skilled in the art will appreciate that the various aspects of the present disclosure may be used in various combinations and sub-combinations, and each of those combinations and sub-combinations is to be treated as if specifically set forth herein. BRIEF DESCRIPTION OF THξ DRAWINGS [0016] For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numerals indicate like features and wherein: [0017] FIG. 1 is an illustration of a cluster tool equipped with a robotic wafer handling system; [0018] FIG. 2 is an illustration of the arm assembly of the robot depicted in FIG. 1, and illustrates the retracted and extended positions of the arm assembly; [0019] FIG. 3 is an illustration of the wrist assembly of the robot depicted in FIG. 1; [0020] FIG. 4 is an illustration of a prior art robotic arm assembly and illustrates the retracted and extended positions of the arm assembly; and [0021] FIG. 5 is an illustration of one embodiment of a robotic arm made in accordance with the teachings herein.

DETAILED DESCRIPTION OF THE INVENTION [0022] The aforementioned needs are met by the devices and methodologies disclosed herein. In particular, after careful investigation, it has now been found that, in conventional robotic arms of the type illustrated in FIGs . 1-4, the hub assembly can move out of concentricity -with its piece parts and force the lower arm to roll away froτn the rotating hub axis. For example, in some known robotic arm configurations, the hub to which the arm is attached contains three concentric rings. The top and bottom, rings in this configuration house bearings and are attached, to the arm, and the middle ring houses one or more rare earth magnets for the magnetic coupling drive. In use, these rings can deviate from concentricity, thus causing tltαe aforementioned roll. [00231 'In"a" "frog-Teg" construction such as that depicted in FIGs. 1-4, this roll is transferred along the beam of the lower arm such that the arm is now out of parallelism, with the second half of the frog arm. This condition induces stress within the wrist, elbow and hub assemblies, caiαsing premature wear and adding abnormal motions in the z- direction (the direction perpendicular to the plane in which the arms extend and retract) as the arm is in motion. The devices and methodologies disclosed herein provide a means for compensating for this roll, thus eliminating such premature wear and allowing the robotic arm to operate properly. [0024] FIG. 5 illustrates one non-limiting embodiment of the lower portion of a robotic arm made in accordance with the teachings herein. Some of the details of the robotic arm have been eliminated for simplicity of illustration. The robotic arm 101 comprises a hub 103, a first segment 105 which is attached to one or more rotating rings or columns in the hub, and a second segment 107 which is attachedl to the first segment by way of a pin 109. [0025] The first 105 and second 107 segments are fast ened together with a set of bolts 111 which mate with a set of threaded apertures (not shown) provided in the second element 107. The throughput for the bolts in the first segment is sufficiently larger than the bolt itself siαch that the first element can rotate slightly around the axis of the pin 109 when the bolt is sufficiently loosened and when the arm is subjected to roll. Preferably, this rotation is within the range of +2° which, though smalLl, is sufficient to relieve the stress that would otherwise be placed on the wrist and elbow assemblies. Hence, the two- part construction of the lower arm depicted in FIG. 5 allows a rotation to occur to compensate for the out-of-axis roll and to keep the entire arm balanced. IUU26J While FIG. 5 illustrates one particular means by which roll (and the accompanying stress) may be compensated for, one skilled in the art will appreciate that this objective may be achieved through a number of different means. For example, the first and second segments in FIG. 5 could be connected across a bearing assembly whicHa permits limited rotation of these segments with respect to each other. It will thus be appreciated that these varrious means are within the scope of the present invention. Tlαus, although particular embodiments of the devices and methodologies disclosed herein have been describee! in detail, it should be understood that various changes, substitutions and alterations can be made hereto ^ithout departing from the spirit and scope of the invention.