Abstract:
A plasma processing system for processing a planar workpiece is provided that has the capability of changing the rotational position of a workpiece relative to a plasma processing chamber of the system. The system workpiece transfer apparatus coupled to the reactor chambers of the system. The workpiece transfer apparatus is capable of transferring workpieces to and from each of the chambers. The system further includes a factory interface coupled to the workpiece transfer apparatus for transferring workpieces from and to a factory environment external of the plasma processing system. The factory interface includes (a) a frame defining an internal volume, (b) a rotatable and translatable arm supported on the frame within the internal volume, (c) a workpiece-handling blade attached to an outer end of the arm, and (d) a stationary workpiece-holding support bracket that facilitates rotation of a workpiece.

Description:
BACKGROUND 
       [0001]    Fabrication of photolithographic masks for use in processing of ultra large scale integrated (ULSI) semiconductor wafers requires a much higher degree of etch uniformity than semiconductor wafer processing. A single mask pattern generally occupies a four inch square area on a quartz mask. The image of the mask pattern is focused down to the area of a single die (a one inch square) on the wafer and is then stepped across the wafer, forming a single image for each die. Prior to etching the mask pattern into the quartz mask, the mask pattern is written in photoresist by a scanning electron beam, a time consuming process which renders the cost of a single mask extremely high. The mask etch process is not uniform across the surface of the mask. Moreover, the e-beam written photoresist pattern is itself non-uniform, and exhibits, in the case of 45 nm feature sizes on the wafer, as much as 2-3 nm variation in critical dimension (e.g., line width) across the entire mask. (This variation is the 3σ variance of all measured line widths, for example.) Such non-uniformities in photoresist critical dimension typically varies among different mask sources or customers. The mask etch process cannot increase this variation by more than 1 nm, so that the variation in the etched mask pattern cannot exceed 3-4 nm. These stringent requirements arise from the use of diffraction effects in the quartz mask pattern to achieve sharp images on the wafer. There is a continuous demand for improvement in the mask etch process. 
       SUMMARY OF THE INVENTION 
       [0002]    We have discovered that one way of improving uniformity of the etched patterns in the mask would be to rotate the mask before completion of a mask etch process. For example, when a particular etch step is half-completed, the etch process would be halted (plasma turned off and process gas injection halted), the mask removed from the plasma reactor chamber and rotated 180 degrees and returned back into the chamber, and the plasma etch process resumed until completed. The problem is that the mask-handling apparatus, including robotic arms and blades, are not capable of changing the rotational orientation of the mask as it is transferred from or to any of the chambers, without prohibitively expensive modifications or adaptation. That is, even though the robotic arms and blades themselves can rotate (in some cases by as much as 360 degrees), they cannot produce a net change in the mask orientation relative to any of the chambers because, as soon as the robotic blade picks up a mask, its rotational orientation relative to each of the chambers is fixed. This is because each time the blade fetches or deposits a mask out of or into any one of the chambers, it grasps the mask by the same edge always. Since each chamber typically has only a single slit valve through which the mask can be introduced into the chamber, the direction of mask ingress and egress is always the same, and therefore no net major change in mask rotational orientation is possible as long as the blade always grasps the mask by the same edge. Therefore, a rotation of the mask through 90 degrees or 180 degrees requires intervention to remove the mask from the processing system including the automatic mask-handling apparatus, followed by manual user rotation the mask. Such intervention would necessarily include suspension of the mask processing in the plasma reactor, removal of the mask from the entire system, manual rotation of the mask at some different external location, and then re-introduction of the mask back into the processing system, re-starting of the plasma process, and so forth. Such intervention can introduce productivity losses and uncertainties into the mask fabrication process, and is not practical. 
         [0003]    According to one aspect of the present invention, a plasma processing system for processing a planar workpiece is provided that has the capability of changing the rotational position of a workpiece relative to a plasma processing chamber of the system. The system workpiece transfer apparatus coupled to the reactor chambers of the system. The workpiece transfer apparatus is capable of transferring workpieces to and from each of the chambers. The system further includes a factory interface coupled to the workpiece transfer apparatus for transferring workpieces from and to a factory environment external of the plasma processing system. The factory interface includes (a) a frame defining an internal volume, (b) a rotatable and translatable arm supported on the frame within the internal volume, (c) a workpiece-handling blade attached to an outer end of the arm, and (d) a stationary workpiece-holding support bracket. The bracket includes a support edge fastened on the frame and extending into the internal volume of the frame and defining a workpiece support plane. The bracket further includes first and second workpiece transfer edges at different portions of a periphery of the bracket. The workpiece-handling blade is capable of depositing a workpiece onto the bracket and removing a workpiece from the bracket at respective ones of the first and second workpiece transfer edges of the bracket. Rotation of the workpiece relative to the system is made possible by embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0005]      FIG. 1  is a top view of a plasma processing system having plural plasma reactor chambers and automated workpiece-handling capability. 
           [0006]      FIG. 2  is a perspective view of a factory interface in the system of  FIG. 1 . 
           [0007]      FIG. 3  is a top view corresponding to  FIG. 2 . 
           [0008]      FIG. 4  is a perspective view of mask rotation brackets in the factory interface of  FIGS. 2 and 3 . 
           [0009]      FIG. 5  is a perspective view of a typical corner post of one of the brackets of  FIG. 4 . 
           [0010]      FIG. 6  is a side view of the post of  FIG. 5  depicting capture of a mis-aligned mask. 
           [0011]      FIG. 7  is a side view of the post of  FIG. 5  depicting final alignment of the mask. 
       
    
    
       [0012]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring to  FIG. 1 , a plasma processing system  100  of the type, for example, sold under the trademark “TETRA II” by Applied Materials, Inc., of Santa Clara, Calif., can support four plasma reactor and process chambers  102 ,  104 ,  106 ,  108 . Such a system can be used to process semiconductor wafers or transparent (e.g., quartz) masks used in photolithographic processing of the wafers. The present invention primarily concerns application of such a system to mask fabrication for which uniformity and process tolerances are so much tighter. However, the invention may be adapted to wafer fabrication. In view of one application of the invention to mask fabrication, the workpiece is referred to as a “mask” throughout the description that follows, although this is not intended to limit the invention. For example, the workpiece may be either a transparent mask or a semiconductor wafer. It is to be appreciated that in other types of processing chambers (e.g., a deposition or an etching chamber), the workpiece can be a semiconductor wafer or substrate instead of a mask, and the embodiments of the present invention are similarly applicable thereto. 
         [0014]    Automatic mask transport between each of the chambers  102 - 108  and the external (factory) environment is provided. For this purpose, a mask transfer chamber  110  is in communication with the mask slit valves (not shown) of each of the chambers  102 - 108  and has a robotically controlled actuator  112  with a mask-holding blade  114  capable of performing rotational, radial and axial motion. The process chambers  102 - 108  and the mask transfer chamber  110  are maintained at vacuum pressure. In order to facilitate transfer of masks to and from an external environment which is at atmospheric pressure, a pair of load locks  116 ,  118  are provided with which the actuator/blade  112 ,  114  can transfer masks. Each load lock  116 ,  118  can be pumped down to vacuum pressure and then vented up to atmospheric pressure. 
         [0015]    The system of  FIG. 1  further includes a factory interface  120  shown in  FIGS. 2 ,  3  and  4 . The factory interface  120  is at atmospheric pressure and can support as many as six loaders  122 , although typically only three are used in mask fabrication. Each one of the loaders  122  holds one or more masks to be taken into the processing system  100  or removed from it. The factory interface  120  can accept masks from or provide masks to the load locks  116 ,  118 . For this purpose, the factory interface  120  includes a robotically controlled actuator  124  with a mask-holding blade  126 . The mask-holding blade  126  can lift a mask from one of the loaders  122  and deposit it in one of the load locks  116 ,  118 , or lift a mask from one of the load locks  116 ,  118  and deposit it in one of the loaders  122 . The robotically controller actuator  124  can translate the blade  126  along a rail  130  extending the length of the factory interface  120  parallel to its major or “X” axis. The actuator  124  can elevate or lower the blade  126  using a lifter  132 . The actuator  124  can rotate and radially extend the blade  126  using an arm  134  mounted to the lifter  132  at a first rotational joint  136  and mounted to the blade  126  at a second rotational joint  138 . The width and depth of the factory interface  120  is sufficient to permit the blade  126  to rotate at least 180 degrees without interference. In all the foregoing mask transport steps or operations, the placement by the robotic blades  114  and  126  of the mask in a loader  122  or in a load lock  116  or  118  or on a wafer support pedestal in a chamber must be consistent and accurate to within a few micrometers to avoid distortion of the mask pattern. Thus, the system is vulnerable to mask-placement variations that may occur initially when the masks are first placed in a loader  122  prior to introduction to the factor interface  120 . 
         [0016]    Even though the robotic arms and blades  114 ,  126  can rotate (in some cases by as much as 360 degrees), they cannot produce a net change in the mask orientation relative to any of the chambers  102 - 108  because, as soon as the robotic blade picks up a mask, its rotational orientation relative to each of the chambers is fixed. This is because each time the blade (e.g.,  126 ) fetches or deposits a mask out of or into any one of the chambers  102 - 108 , it grasps the mask by the same edge always. Since each chamber  102 - 108  typically has only a single slit valve through which the mask can be introduced into the chamber, the direction of mask ingress and egress is always the same, and therefore no net major change in mask rotational orientation is possible as long as the blade always grasps the mask by the same edge. Therefore, a rotation of the mask through 90 degrees or 180 degrees requires intervention to remove the mask from the processing system  100  including the automatic mask-handling apparatus, followed by manual user rotation the mask. 
         [0017]    The embodiments of the present invention transform the system of  FIG. 1  into one capable of performing fully automatic rotation of a mask by either (or both) 90 degrees or 180 degrees. The embodiments of the present invention also provide the system with the capability of automatically correcting mask placement variations or errors introduced prior to introduction of the mask(s) to the factory interface  120 . The embodiments of the present invention accomplish all these features without any costly modification to the system of  FIG. 1 . 
         [0018]    A mask bracket  140  for 180 degree mask rotation is visible in  FIGS. 1-3  but is best shown in  FIG. 4 . The bracket  140  is supported on and cantilevered from an internal frame  142  in the interior of the factory interface  120 . The bracket  140  consists of a pair of main rails  144  and cross rails  146 . The main and cross rails  144 ,  146  together define an imaginary support plane. The bracket  140  has four posts  148  extending from the main rails  144  in a direction perpendicular to the support plane, which direction shall be referred to herein as “vertical” for the sake of simplicity but without limiting potential applications of the invention. In one embodiment, this vertical direction is parallel to the direction of gravity, for the sake of convenience, as will be explained below. The four posts  148  define a rectangle or square (depending upon the required mask shape) whose four corners coincide with the posts  148 . An individual post  148  is illustrated in the enlarged view of  FIG. 5 . Each of the posts  148  has a pair of internal corner surfaces  148 - 1 ,  148 - 2  that are generally orthogonal to one another and form a right angled corner when viewed from the top along the vertical direction. Each of these wall surfaces  148 - 1 ,  148 - 2  is slanted from the top down to the bottom toward the interior of the rectangle formed by the four posts  148 . This slant is relative to the vertical direction referred to above. The wall surfaces  148 - 1 ,  148 - 2  form a right-angle corner into which a corner of a rectangular mask can fit, in the manner depicted in  FIG. 4 . The four posts  148  are spaced apart so that the four corners of the mask  150  nest firmly into the four corners of the posts  148  when the mask rests on the bottom floor  148 - 3  bounded by the corner surfaces  148 - 1 ,  148 - 2 . The force of gravity pulls the mask  150  down to the floor  148 - 3  of each post  148  as it slides down the slanted corner surfaces  148 - 1 ,  148 - 2 . The mask rests on the robot blade  126  until the bottom surface  148 - 3  of the post  148  is reached. This keeps the vertical speed down and ensures a smooth handoff. Gravity keeps the mask on the blade  126  while the angled surfaces  148 - 2 ,  148 - 3  nudge it sideways. 
         [0019]    In one embodiment, the four posts  148  reduce or even eliminate an initial misalignment problem sometimes seen in aligning a mask while the mask is being held in one of the loaders  122 . This misalignment may arise from a very slight rotation or translation of the mask within the loader  122  from its proper location. As described above, the corner surfaces  148 - 1 ,  148 - 2  are tilted inwardly from top to bottom at slight angles relative to the true vertical direction, as shown in  FIG. 5 . In this way, the tops of the four posts  148  define a rectangular area that is slightly larger than the mask  150  and the bottoms of the four posts  148  define a slightly smaller rectangular area that corresponds precisely with the area of the mask  150 . This feature permits a slightly misplaced mask, as it is lowered onto the four posts  148 , to be captured by the top of the interior surfaces of the posts  148 , as depicted in  FIG. 6 . Once this occurs, gravity gently impels the edge of the mask  150  down the sloping interior post surfaces until it contacts the bottom surface  148 - 3 , as depicted in  FIG. 7 . At this point, all four corners of the mask have settled to the corner floor  148 - 3  of each of the four posts  148 , and the mask  150  is perfectly aligned. 
         [0020]    The bracket  140  is a 180 degree rotation bracket and is used by the robotic actuator/blade  124 ,  126  to automatically rotate the mask  150  using the following procedure: the mask  150 , which may have been previously removed from one of the chambers  102 - 108  and placed in the load lock  116 , is lifted out of the load lock  116  by the blade  126  and taken into the factory interface  120 . At this point, the position of the blade  126  corresponds to its depiction in solid line in  FIG. 2 . The blade  126 , while holding the mask  150 , is rotated through a 90 degree angle from a direction facing the loader  116  to direction along the major or “X” axis of the factory interface  120 . The blade  126  then translates along the X axis toward a first side  140   a  ( FIG. 4 ) of the bracket  140 , e.g., from left to right in the view of  FIG. 1 . The blade  126  drops the mask  150  onto the bracket  140 , with the mask corners aligned with the four posts  148  of the bracket  140 . The blade  126  is then retracted away from the bracket and lowered below the level of the bracket  140 , and is translated further along the X axis in the same direction until it is past the bracket  140 . The blade  140  is then rotated until it points in the opposite direction along the X axis. At this point, the position of the blade  126  corresponds to its depiction in dashed line in  FIG. 2 . The blade  126  is then elevated to a level at which it can capture the mask  150  that is currently sitting on the bracket  140 . The blade  126  is translated in the opposite direction (e.g., from right to left in the view of  FIG. 1 ) along the X axis toward the opposite side  140   b  ( FIG. 4 ) of the bracket  140  to pick up the mask  150 . The blade  126  therefore picks up the mask  150  on the opposite side from which it had deposited the mask on the bracket  140 , and delivers the mask  150  to the load lock  116 . This completes a 180 degree rotation of the mask  150  from its initial orientation. From the load lock  116 , the mask  150  is taken by the load lock&#39;s robotically actuated blade  114  to one of the chambers  102 - 108  for completion of the etch process. 
         [0021]    90 degree rotation of the mask  150  can be performed using another bracket  160  also shown in  FIG. 4 . The 90 degree rotation bracket  160  is supported on the internal frame  142  in the interior of the factory interface  120 . The bracket  160  consists of a pair of main rails  244 , cross rails  246  which together with the main rails  244  define a support plane. Four posts  248 , similar to the posts  148  of the 180 degree rotation bracket  140 , extend from the main rails  244  in a direction perpendicular to the support plane. Each of the posts  248  has a pair of mutually orthogonal walls  248 - 1 ,  248 - 2  that are perpendicular to the support plane and form a corner into which a corner of a mask can nest. The four posts  248  are spaced apart so that the four corners of the mask  150  nest firmly into the four corners of the posts  248 . The four posts define a rectangle oriented at a 45 degree angle relative to the major or X axis of the factory interface  120 . Moreover, the bracket  160  has one corner that abuts a boundary of the factor interface  120  so as to provide a maximum distance to the opposite boundary of the factory interface, in order to provide maximum room for an approach by the robot blade  126 , as will be described below. In one, the posts  248  extend vertically up so that the force of gravity pulls the mask directly down to the floor  248 - 3  of each post  248 . The mutually orthogonal walls  248 - 1 ,  248 - 2  have interior surfaces which are tilted inwardly from top to bottom at slight angles relative to the true vertical direction. This feature permits a slightly misplaced mask, as it is lowered onto the four posts  248 , to be captured by the top of the interior surfaces of the posts  248 . Once this occurs, gravity gently impels the edge of the mask  150  edge down the sloping interior post surfaces until it contacts the bottom surface  248 - 3 , at which point the mask  150  is perfectly aligned. 
         [0022]    The bracket  160  is a 90 degree rotation bracket and is used by the robotic actuator/blade  124 ,  126  to automatically rotate the mask  150  by 90 degrees using the following procedure in which the mask is (a) deposited on the bracket  160  by the blade  126  approaching a first side  160   a  of the bracket and then (b) removed from the bracket by the blade  126  approaching a second side  160   b  of the bracket that is adjacent and orthogonal to the first side  160   a . The procedure is carried out as follows: the mask  150 , which may have been previously removed from one of the chambers  102 - 108  and placed in the load lock  116 , is lifted out of the load lock  116  by the blade  126  and taken into the factory interface  120 . The blade  126  is rotated from a direction facing the loader  116  to a direction oriented 45 degrees relative to the major axis X of the factory interface  120 , so as to face the first side  160   a  of the bracket  160 . The blade  126  then translates along the X axis toward the first bracket side  160   a  (e.g., from left to right in the view of  FIG. 1 ). The blade  126  drops the mask  150  onto the bracket  160 , with the mask corners aligned with the four posts  248  of the bracket  160 . The blade  126  is then retracted away from the bracket  160  and lowered below the level of the bracket  160 , and is then translated further along the X axis in the same direction until it is past the bracket  160 . The blade  126  is then rotated 90 degrees, e.g., until it points in the opposite direction at a 45 degree angle relative to the X axis, so that it faces the second (adjacent) side  160   b  of the bracket  160 . The blade  126  is then elevated to a level at which it can capture the mask  150  sitting on the bracket  160 . The blade  126  is then translated in the opposite direction (e.g., from right to left in the view of  FIG. 1 ) along the X axis toward the adjacent side  160   b  of the bracket  160  to pick up the mask  150 . The blade  126  therefore picks up the mask  150  on the adjacent side from which it had deposited the mask on the bracket  140 , and delivers the mask  150  to the load lock  116 . This completes a 90 degree rotation of the mask  150  from its initial orientation. From the load lock  116 , the mask  150  is taken by the load lock&#39;s robotically actuated blade  114  to one of the chambers  102 - 108  for completion of the etch process. 
         [0023]    A camera  180  may be provided beneath the mask  150  on the bracket  140  (and/or on the bracket  160 ). The camera  180  is adapted to sense a legend written on a deposited photoresist film on the mask  150  or etched on the mask  150 . A processor associated with the camera  180  is programmed to perform optical character recognition to generate a string of characters in text form representing the printed legend. This string of characters may represent a process sequence for the particular mask, which information is forwarded to a process controller governing the system  100 . This information may define which one of the chambers  102 - 108  to place the mask  150  and which process is to be performed, for example. 
         [0024]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.