Abstract:
An article (e.g. a semiconductor wafer) is held in an article holder by means of a number of gas flows emitted from gas vortex chambers. Some of the gas flows act to cool an adjacent article portion more than the other gas flows. For example, some of the vortex chambers emit more gas per unit of time than the other chambers. More cooling is provided to those portions of the article which are heated more during processing. Greater temperature uniformity can be achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a division of application Ser. No. 09/877,366 filed Jun. 8, 2001, incorporated herein by reference. 

   BACKGROUND 
   The present invention relates to article holders that use gas vortices to hold an article in a desired position. 
   U.S. Pat. No. 6,168,697 issued Jan. 2, 2001 to Siniaguine et al., describes a wafer holder which emits gas vortices to hold a semiconductor wafer in a desired position while the wafer is etched with a plasma etch. The vortices have low pressure zones that hold the wafer proximate to the holder. In addition, the vortices cool the wafer. Unfortunately, the wafer cooling is not uniform. Portions of the wafer near the vortex outlets are cooled more than the rest of the wafer. The non-uniform cooling may have a negative effect on the etch uniformity, the etch rate being higher where the wafer is hotter. 
   SUMMARY 
   Some embodiments of the present invention exploit the non-uniform cooling by the vortices to compensate for other conditions that create temperature non-uniformity. Such conditions may include wafer motion, e.g. rotation. Those portions of the wafer that are farther from the rotation axis move faster, and hence are cooled more by ambient gas, than the wafer portions close to the rotation axis. In some embodiments of the invention, the vortices are arranged to provide more cooling closer to the rotation axis. For example, in some embodiments, a vortex chamber close to the rotation axis emits more gas per unit of time than a vortex chamber farther from the rotation axis. 
   The invention is not limited by the embodiments described above. The invention is not limited to plasma etches or semiconductor wafers. Other features and advantages of the invention are described below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a plasma processing system incorporating one embodiment of the present invention. 
       FIG. 2  is a cross section illustration of a portion of one embodiment of the system of FIG.  1 . 
       FIG. 3  is a perspective view of a vortex chuck according to one embodiment of the present invention. 
       FIG. 4  is a plan view of a portion of an apparatus having a holder according to one embodiment of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a plasma processing system  110  incorporating one embodiment of the present invention. Plasma source  114  generates a plasma jet  120  flowing upward towards semiconductor wafers (not shown). The wafers are positioned under wafer holders  130 . Holders  130  are attached to arms  140 A of an angle drive  140 . Drive  140  rotates the wafers around a vertical axis  140 X. Drive  140  is attached to an arm  150 A of an angle drive  150 . Drive  150  rotates the wafers around a vertical axis  150 X. When a wafer passes through the plasma, the wafer is heated. When a wafer is out of the plasma, the wafer is cooled by ambient atmosphere (e.g. air or some other ambient gas). The ambient pressure is atmospheric pressure. With the exception of the wafer holders, the system  110  can be identical to a system described in PCT publication WO 00/70659 (TruSi Technologies, LLC, 23 Nov. 2000) incorporated herein by reference, although the present invention is not limited to such embodiments. In some embodiments, drive  150  is absent, axis  140 X is stationary. In other embodiments, more than two rotational motions are provided. See for example U.S. patent application Ser. No. 09/713,137, “Plasma Processing Comprising Three Rotational Motions of an Article Being Processed”, filed by O. Siniaguine et al. on Nov. 14, 2000, incorporated herein by reference. Any number of holders  130  may be present. Plasma source  114  may or may not move during processing. 
   The wafer points farther from axis  140 X can have higher linear velocities, and be cooled more by the ambient gas, than the wafer points close to axis  140 X. The cooling non-uniformity can be especially noticeable if the wafer processing occurs at atmospheric pressure or higher pressure, but can also be noticeable at lower pressures. This non-uniformity is at least partially compensated by gas flows emitted by the wafer holders.  FIG. 2  is a cross-sectional view showing a single holder  130  holding a wafer  134 . Vortex chucks  202 . 1  through  202 . 4  are mounted in holder body  210  above the holder&#39;s bottom surface  250 . Although four chucks are shown, any number of chucks greater than one can be present. Chucks  202 . 1 ,  202 . 2  are closer to axis  140 X than chucks  202 . 3 ,  202 . 4 . Chucks  202 . 1 ,  202 . 2  are constructed to have a greater cooling effect on the wafer than chucks  202 . 3 ,  202 . 4 . 
     FIG. 3  is a semi-transparent, perspective view of a chuck  202 . 1 . Chuck  202 . 2  is identical, and chucks  202 . 3 ,  202 . 4  are similar except as described below. Chuck  202 . 1  is described in U.S. patent application Ser. No. 09/633,086, entitled “Non-Contact Workpiece Holder Using Vortex Chuck with Central Gas Flow”, filed Aug. 4, 2000 by S. Kao and incorporated herein by reference. Other kinds of chucks can also be used. Chuck  202 . 1  of  FIG. 3  includes a body  310  surrounding a vortex chamber cavity  320 . The cavity is shown as being cylindrical, but a hemispherical cavity or a cavity having another shape might also be employed. 
   A tangential inlet passage  330  conducts a gas flow into vortex chamber  320 . This gas flow has a tangential component substantially parallel to a horizontal surface of wafer  134 . This component creates a vortex in chamber  320 . Multiple tangential passages are provided in some embodiments. 
   Chucks  202 . 3 ,  202 . 4  have the same construction, but their passages  330  are more narrow. In some embodiments, each passage  330  is a cylindrical bore having a diameter of 0.016 inches for chucks  202 . 1 ,  202 . 2 , but only 0.010 inches for chucks  202 . 3 ,  202 . 4 . These dimensions are illustrative and not limiting. 
   As shown in  FIG. 2 , holder body  210  has a cavity  420  that is pressurized via a gas passage  428  in arm  140 A and a gas inlet passage  430  in body  210 . Gas (e.g. air or nitrogen) in cavity  420  is under positive pressure. The gas in cavity  420  is optionally a temperature-controlled gas, typically cooler than the wafer when the wafer has been heated by the plasma. The pressurized gas in cavity  420  flows out of this cavity and into chambers  320  via passages  330 . The gas flow from each tangential passage  330  creates a vortex having a low pressure region near the center of the corresponding chamber  320 . Consequently, the wafer  134  is drawn to the surface  250  of holder  130 . At the same time, gas escaping through outlets of chambers  320  creates a gas cushion between the wafer and the holder that prevents the wafer from contacting the holder&#39;s surface  250 . 
   The gas flow from each of chucks  202 . 1 ,  202 . 2  is larger than the gas flow from each of chucks  202 . 3 ,  202 . 4  due to the different dimensions of the passages  330 . In some embodiments, the four chucks  202  are identical except for the dimensions of the passages  330 . Larger gas flows which exit the chucks  202 . 1 ,  202 . 2  result in the wafer portion  134 A facing these chucks to be cooled more than the wafer portion  134 B facing the chucks  202 . 3 ,  202 . 4 . 
   The chuck of  FIG. 3  includes a gas inlet passage  440  which directs a gas flow perpendicular to surface  250 . In some embodiments, passage  440  is at some other angle to surface  250 , and passage  440  may or may not be directed along a center axis of cavity  320 . (The invention is not limited to cavities  320  having a center axis.) Multiple passages  440  can be provided. Gas flowing through each passage  440  increases the pressure at the center of the vortex and leads to a more uniform pressure profile across the chuck&#39;s outlet, as explained in the aforementioned U.S. patent application Ser. No. 09/633,086. In some embodiments, passages  440  are provided in chucks  220 . 1 ,  220 . 2  but not in chucks  202 . 3 ,  202 . 4 . The gas flowing through passages  440  increases the total gas outflow from chucks  202 . 1 ,  202 . 2  and thus enhances the cooling of wafer portion  134 A. Also, in some embodiments, if passages  440  are not provided, the wider passages  330  in chucks  202 . 1 ,  202 . 2  cause the pressure at the center of the vortices in chucks  202 . 1 ,  202 . 2  to be lower than in chucks  202 . 3 ,  202 . 4 , resulting in the wafer portion  134 A being drawn closer to the holder surface  250  than the wafer portion  134 B. Passages  440  can be used to make the spacing between the wafer and the holder more uniform. 
   In some embodiments, none of the chucks is provided with passages  440 . In other embodiments, all of the chucks are provided with passages  440 . Chucks  202 . 1 ,  202 . 2  may or may not have wider passages  440  than chucks  202 . 3 ,  202 . 4 . In some embodiments, chucks  202 . 1 ,  202 . 2 ,  202 . 3 ,  202 . 4  have identical passages  330  but chucks  202 . 1 ,  202 . 2  have wider passages  440  than chucks  202 . 3 ,  202 . 4 , or chucks  202 . 3 ,  202 . 4  do not have passages  440 . Other embodiments have other differences in the geometry of passages  330 ,  440  and chambers  320  between the chucks  202 . 1 ,  202 . 2  on the one hand and chucks  202 . 3 ,  202 . 4  on the other hand. For example, the chambers  320  of chucks  202 . 1 ,  202 . 2  may have a larger diameter than in chucks  202 . 3 ,  202 . 4 , or the cavities  320  or passages  330 ,  440  of chucks  202 . 1 ,  202 . 2  may have smoother inner surfaces, or their passages  330  or  440  may be shorter or greater in number. 
   In some embodiments, chucks  202 . 1 ,  202 . 2  are not identical to each other, with the chuck  202 . 1  cooling the wafer more than chuck  202 . 2 . For example, chuck  202 . 1  may have a larger combined cross-sectional area of passage  330  or  440  than chuck  202 . 2 , and/or a passage  440  can be provided in chuck  202 . 1  but not in chuck  202 . 2 . In some embodiments, chuck  202 . 3  cools the wafer more than chuck  202 . 4 . In some embodiments, chuck  202 . 2  is identical to chucks  202 . 3 ,  202 . 4 ; additional cooling close to axis  140 X is provided by chuck  202 . 1  but not chuck  202 . 2 . In other embodiments, chucks  202 . 1 ,  202 . 2 ,  202 . 3  are identical, but chuck  202 . 4  cools the wafer less than each of the other three chucks. In other embodiments, no two chucks are identical to each other, with the chucks closer to axis  140 X cooling the wafer more than the chucks farther from axis  140 X (i.e. the gas that flows out of chucks  202  and holds the wafer near the holder). 
   In some embodiments, separate gas sources supply gas to different chucks  202 . The gas flowing through the chucks close to axis  140 X is colder, and/or is supplied under more pressure, than the gas flowing through the other chucks. All of the chucks may or may not be identical to each other. 
   In some embodiments, the chuck density is higher closer to axis  140 X. The chucks and the pressures and temperatures of different gas flows may or may not be identical. 
     FIG. 4  is a schematic top view of one embodiment of the invention.  16  vortex chucks  202  are positioned along the periphery of wafer holder  130 , and four chucks  202  are closer to the middle of the holder. This chuck positioning is shown in the aforementioned U.S. patent application Ser. No. 09/633,086. In the embodiment of  FIG. 4 , the eight peripheral chucks closest to the axis  140 X, shown to the right of line  460 , have larger passages  330  than the remaining chucks. The eight peripheral chucks to the right of line  460  are provided with passages  440 , and the remaining chucks are not. The surface  250  of holder  130  has two portions  250 A,  250 B having equal areas. The portion  250 A is closer to axis  140 X than the portion  250 B. In some embodiments, the portion  250 A contains more vortex chuck outlets than the portion  250 B, and/or the portion  250 A has a larger percentage of its area occupied by the vortex chuck outlets than the portion  250 B. 
   U.S. patent application Ser. No. 09/456,135, which is incorporated herein by reference, further describes examples of holders including multiple vortex chucks. Such holders can be modified to provide more cooling to slower moving portions of the wafer as described herein. 
   The above embodiments illustrate but do not limit the invention. The invention is not limited by particular dimensions, chuck positioning, or to wafers undergoing a rotational motion. The wafers can be positioned above the vortex chucks or in some other orientation. In some embodiments, the plasma footprint on the wafer is smaller than the wafer, in other embodiments the plasma footprint on the wafer is at least as large as the wafer. In  FIG. 2 , the distance between each chuck  202  and the axis  140 X does not change during processing, but this does not have to be the case since in some embodiments, the holder motion can include a translational motion relative to axis  140 X. In some embodiments, a wafer or a holder or both can rotate through an axis passing through the wafer or the holder, and more cooling can be provided closer to this axis. In  FIGS. 2 and 3 , chuck bodies  310  may or may not be integral with holder body  210 , and each of bodies  210 ,  310  may or may not be of an integral construction. The invention is not limited to plasma processing or to semiconductor wafers. The invention is applicable to holders that hold panels for flat panel displays or other kinds of articles. The invention covers holders that use gas vortices in combination with other means to hold an article. For example, a holder may use gas vortices in combination with an electrostatic mechanism to hold an article. The invention is defined by the appended claims.