This invention relates to the processing of wafers, particularly semiconductor wafers by the vapor deposition thereon of films under high vacuum. The invention particularly relates to the solution of the problem of the warping of wafer clamping components, particularly wafer holder clamping rings, in the course of the processing of such wafers. 
     BACKGROUND OF THE INVENTION 
     In semiconductor manufacturing processes such as, for example, the sputtering of thin films onto substrates such as those formed of silicon it is necessary to hold substrate wafers in place for processing. While held, a sequence of processes is performed on the wafer, many of which result in the application of a thin film or coating layer to the wafer. Wafers held for the application of such coatings may be held in a horizontal orientation, facing upwardly or downwardly, or may be held in a vertical orientation facing in a horizontal direction. In all such coating processes, the wafer to be coated must be retained securely to a holder in a generally stress free state. To hold the wafers in such a state and to move the wafers safely and quickly between various processors and positions, wafer holders or clamps that employ resiliently supported continuous clamping rings to urge the wafer uniformly around its edge against the wafer holder have gained wide acceptance. 
     Wafer holders that employ wafer clamping rings are exposed to a range of temperatures when holding a wafer in a vacuum chamber for processing. These rings are further exposed to deposition of the same materials that are being deposited onto the wafers. While each wafer is subjected to a single cycle in a processing chamber in which the temperatures may rise and fall, and where a film thickness measured in microns is deposited on the wafer, the holders, and particularly the clamping rings that urge the wafers against the holders, are exposed to a large number of cycles in each of which the temperatures are cycled from maximum to minimum and in the course of which multiple layers of coating material accumulate on the clamping ring surfaces. 
     Over the course of many cycles, the clamping rings are observed to warp. The warping of a clamping ring is typically a permanent deformation of the clamping ring that alters the way in which the ring contacts the wafer. When the warping becomes excessive, the ring no longer adequately clamps the wafer. As a result, wafers can move in the holder, can be unevenly subjected to clamping forces in the holder causing breakage, or are not held in the proper position. The amount of warping and the number of cycles that it takes to cause excessive warping is observed to vary with different coating materials and coating processes. When the warping becomes excessive, after a number of wafers have been processed, it is necessary to replace the clamping ring with a new ring having its original design shape that will clamp the wafer uniformly around its edge. 
     For example, in one processing sequence in which the backside of a semiconductor is coated with two deposition layers of tantalum (Ta) and one layer of gold (Au), it is found that after only several hundred wafers are processed, excessive warping of the inner diameter of the clamping ring occurs lifting the inner rim out of its normal plane and toward the deposition chamber. As a result, a chamber overhaul to replace the clamping ring is required after far fewer wafers are processed than the several thousand desired. 
     Replacement of the clamping ring is necessary to prevent damage to the wafers and results in a loss of expensive production time in the making of semiconductors. The warping problem has not been effectively solved. 
     Accordingly, there remains a need to prevent or substantially delay the onset of excessive clamp ring warping in semiconductor wafer holders used in film deposition processes. 
     SUMMARY OF THE INVENTION 
     A primary objective of the present invention is to provide a clamping ring for clamping a semiconductor wafer to a wafer holder during the deposition of film onto the wafer that will resist warping over the course of processing a large number of wafers. A particular objective of the invention is to provide a wafer clamping ring that can accumulate deposited film of substantial thickness without warping to the point that cleaning or replacement of the ring is required. A further objective of the present invention is to provide a wafer mounting ring that will return to its original geometry after being cleaned of deposits that have built up to the degree that requires the cleaning of the ring. 
     Another objective of the present invention is to provide a method and apparatus by which coatings of material, for example materials such as tantalum and gold, can be deposited on a large number of wafers sequentially held by a wafer holder that employs a wafer clamping ring in processes where the deposition process subjects the ring and deposits to wide temperature variation cycles. A particular objective of the invention is to provide such a method and apparatus in systems where the clamping ring is supported on minimal mounting points providing limited or local conduction of heat from the ring, particularly where the process is carried out in a vacuum where heat dissipation by convection is substantially absent. 
     According to the principles of the present invention, a clamping ring is formed of a material having a coefficient of thermal expansion that is approximately equal to that of the material being deposited. By “substantially equal” is meant that the coefficient of thermal expansion of the material of which the clamping ring is made is closer to the coefficient of thermal expansion of the coating material than are the coefficients of thermal expansion of alternative materials, Preferably the coefficient of thermal expansion of the material of which the clamping ring is made also has a high modulus of elasticity so that it distorts less when subjected to a given thermal stress. In addition, it is preferable that the material of which the ring is made also have a high coefficient of thermal conductivity so that temperature gradients, and thus thermal stresses, are lower. It is also preferable that the material of which the ring is made have a high yield stress at the temperatures to which the ring is to be subjected during the deposition processes performed on wafers while held by the ring so that the ring can withstand higher thermal stress before undergoing permanent or plastic deformation. 
     In accordance with the preferred embodiment of the invention, a clamping ring is provided that is formed essentially of molybdenum metal. The clamping ring is configured as an annular disc with a substantially circular opening at its center that is bounded by a continues edge that engages the entire rim of a semiconductor wafer being held in a holder for the vapor deposition of film onto the wafer. The inner edge of the opening of the annular disc is circular for most of its circumference, that is, for all of the circumference of the disc except for a cord section configured to conform to the flat orientation edge of a conventional semiconductor wafer. The inner diameter of the disc is preferably about five centimeters less than the outside diameter of the wafers being clamped. 
     Further in accordance with the preferred embodiment of the invention, a clamping ring is provided which has mounting points spaced around the disc that provide for a balanced resilient mounting of the disc to the housing of a wafer holder and presents a substantially limited portion of the disc in contact with cooler structure to which heat could flow. The ring preferably includes six spring mounting points spaced around the disc which connect to springs on which the disc is mounted. The disc preferably also supports a set of latches, preferably three in number, by which a wafer is latched to the clamping ring. 
     Preferably, a clamping ring for the deposition of a film of tantalum and gold onto wafers is provided that is formed essentially of molybdenum. Clamping rings of other materials having coefficients of thermal expansion that match that of the film being deposited may also be used with a Ta-Ta-Au film or with other film compositions. Generally, a high modulus of elasticity, high thermal conductivity and high yield strength are also preferred, and for most coating processes, non-magnetic material is also desirable. 
     With the method and clamping ring of the present invention, the number of wafers coated with a Ta-Ta-Au film, as compared to rings made of conventional materials such as stainless steel, is increased by a factor of 5 or 10 or more between overhauls of the chamber that are required to clean or replace the clamping ring due to excessive warping of the ring. Further, rings made according to the present invention will, when cleaned of the deposits after being removed due to excessive warping, return to their original flat geometries, while rings made of conventional materials such as stainless steel are found to develop a permanent deformation rendering them useless after warping, even if cleaned of the deposits. 
     These and other objectives of the present invention will be more readily apparent from the following detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a backside view of a wafer holder of a semiconductor wafer processing apparatus for application of the principles of the present invention. 
     FIG. 2 is a disassembled perspective view of the wafer holder of FIG.  1 . 
     FIG. 3 is a cross-sectional view, taken along line  3 — 3  of FIG. 1, of a latch assembly of the wafer holder of FIGS. 1 and 2 in an arrangement particularly useful for wafer backside processing. 
     FIG. 4 is a partially broken away perspective view of a semiconductor wafer processing apparatus of a type suitable for physical vapor deposition of a multiple layered coating upon a wafer, such as a Ta-Ta-Au multiple layered backside film deposition upon a semiconductor wafer. 
     FIG. 5 is a cross-sectional diagram of a sputter coating processing chamber of a semiconductor wafer processing apparatus of FIG. 4 taken along line  5 — 5  of FIG. 4 employing the wafer holder of FIGS. 1-3. 
     FIG. 6 is a graph illustrating component temperatures as a function of processing time. 
     FIG. 7 is a cross-sectional view similar to FIG. 3 illustrating a clamping ring of the holder of FIGS. 1-3 in a clean condition. 
     FIG. 7A is a cross-sectional view similar to FIG. 7 illustrating the condition of a clamping ring made of stainless steel following the accumulation of a film of about 0.040 inches of tantalum. 
     FIG. 7B is a cross-sectional view similar to FIG. 7A illustrating the condition of a clamping ring made of molybdenum following the accumulation of a film of about 0.040 inches of tantalum. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One form of wafer holder for use in physical vapor deposition systems is described in the commonly assigned and copending U.S. patent application entitled Wafer Processing Apparatus with Low Particle Generating Wafer Clamp, Ser. No. 09/183,503, filed Oct. 30, 1998, now U.S. Pat. No. 6,143,147, and hereby expressly incorporated by reference herein. An example of such a wafer holder is the wafer holder  10  illustrated in FIGS. 1-3. 
     FIG. 1 illustrates wafer holder  10  holding a semiconductor wafer  17  and viewed from the side of the wafer  17  that is opposite the surface of the wafer  17  that is to be processed. The holder  10  includes an annular housing  11  to which a wafer mounting ring  12  is resiliently attached by an array of springs that include “a set of three equally angularly spaced leaf springs  13 , at holes  13   a  by screws  13   b , and a set of three equally angularly spaced conical coil springs  14 , at holes  14   a  by screws  14   b , arranged so that the leaf and coil” springs alternate around the holder  10  and provide a balance mounting force between the ring  12  and the housing  11 . The springs  13  and  14  function to uniformly urge the mounting ring  12  toward and against the holder  11 . 
     Rotatably mounted to the mounting ring  11  at equally spaced angular intervals around the holder  10  are three latch assemblies  20 . The latch assemblies  20  are configured to rotate through approximately 90° between latched positions, as illustrated in FIG. 1, and unlatched positions in which the latch assemblies  20  are rotated 90° from their orientations illustrated in FIG. 1, as is the latch assembly  20  illustrated in FIG.  2 . The latch assemblies  20  are recessed into cutouts  21  in the housing  11 . 
     The three latches are moved in unison between their latched and unlatched positions by a latch actuator mechanism (not shown) such as the mechanisms more particularly described in the copending and commonly assigned U.S. patent application Ser. No. 08/827,690, filed Apr. 10, 1997, now U.S. Pat. No. 5,820,329 hereby expressly incorporated by reference herein, and in U.S. Pat. No. 4,915,564 referred to below in connection with FIGS. 4 and 5. 
     An example of a wafer handling and holding mechanism for the processing of wafers in a vertical orientation is described and illustrated in commonly assigned U.S. Pat. No. 4,915,564, hereby expressly incorporated by reference herein. In the apparatus of U.S. Pat. No. 4,915,564, individual wafers are gripped on their back surfaces by vacuum chucks on a transfer arm and transferred, device side first, through the door of a loadlock chamber of a processing apparatus. In the apparatus described in U.S. Pat. No. 4,915,564, unlike with the clamping ring  12  described above which bears against the entire edge of the wafer  17 , the transfer arm moves the wafer against discrete tabs on a clamping ring carried by the wafer holder positioned in the loadlock of the machine. A plurality of latches, three in number and spaced at even intervals around the periphery of the holder, move behind the wafer to clamp the wafer between the latches and the clamping ring tabs. 
     In processing apparatus  100 , the holder  10  is mounted to a carrier or index plate  103  that rotates to sequentially move each holder  10  among the stations  111 - 115  of the processing machine  100 . The holder  10  is held to the index plate by one fixed pin carried by the plate and two spring-loaded pins  22  carried by the housing  11  of the wafer holder  10 , as illustrated in perspective in FIG.  2 . The fixed pin of the plate is received by a notch  23  in the housing  11  while the spring loaded pins are received by radial recesses in the edge of an opening of the index plate  103 . 
     The wafer mounting ring  12  has an opening  24  therein that is slightly smaller than a wafer  17  that is to be held the holder  10  for processing, though preferably of the same shape as the wafer. The opening  24  is circular around most of its circumference with a flat side  29  which conforms to the orientation flat edge of an industry standard wafer. Accordingly, the opening  24  is substantially circular. For clamping a 150 mm wafer, the diameter of the opening  24  is approximately 145 mm. The ring  12  has an inner annular surface  25  against which the edge of the wafer  17  rests when it is being held by the holder  10 . The housing  11  has an opening  15  in its center that is larger than the wafer  17  that is to be held in the holder  10  for processing. Such a wafer  17  is inserted by a wafer transfer arm to which the wafer  17  is held by a chuck, for example a vacuum chuck, through the opening  15  until its outer edge lies in contact with the surface  25 . The movement of the wafer  17  against the surface  25  may move the ring  12  slightly away from the housing  11  against the force exerted by the springs  13  and  14 . Preferably, the force exerted by the inner edge of the ring  24  on the outer edge of the wafer  17  is about 12 pounds. 
     When moved by the actuators to their latched positions, as illustrated in more detail in FIG. 3, the latch assemblies  20  clamp the edge of the wafer  17  against the annular surface  25  around the opening  24  in the wafer holding ring  12 . Each latch assembly  20  is pivotally mounted to the ring  12  at a mounting post  30  fixed to the ring  12 . The latch assembly  20  includes a non-metallic latch body  31  pivotally mounted on the post  30  through a tungsten carbide ball bearing  32 , and is biased against the mounting ring  12  by a conical spring  33  that surrounds the mounting post  30 . The latch bodies  31  each have a pair of actuator pin receiving slots therein (not shown), equally spaced from the mounting post  30 , to receive actuator pins of the actuator mechanism on a transfer arm (not shown) situated outside of the housing  101  when the holder is being loaded or unloaded at the loadlock  111 . 
     At opposite ends of the latch body  31  are rotatably mounted a pair of non-metallic rollers, including a front roller  35  and a back roller  36 , both rotatable about an axis  68  that is parallel to the mounting ring  12  and intersects the centerline of the mounting post  30  approximately at a right angle. The back roller or rear roller  36  has a roller diameter  37  on which the roller  36  rolls in a circle  38  around the mounting post  30  as the latch assembly is pivoted by the latch actuator. The front roller  35  also has a roller diameter  39  that is of the same diameter as the roller diameter of the back roller  36 . The roller diameter  37  of the back roller  36  is spaced on the body  31  at the same distance from the mounting post  30  as the roller diameter  39  of the front roller  35 , so that the roller diameters  37 , 39  of the rollers  35 , 36  move on the same circle  38  on the back surface of the mounting ring  12 . 
     The front roller  35  of each latch assembly  20  has a gripping periphery  40  on the outer end of the roller  35  spaced farther from the mounting post  30  than the rolling peripheries  37 , 39  of the rollers  36 , 35 . The gripping periphery  40  is of smaller diameter than the rolling peripheries  37 , 39  and, as a result, does not contact the back surface of the mounting ring  12  when the latch body  31  is rotated by the actuators to the latched position. The gripping periphery  40  is dimensioned to contact the back side of the wafer  17  and latch the wafer  17  between the gripping periphery  40  of the front roller  35  and the annular surface  25  on the mounting ring  12 , as illustrated in FIG.  3 . The gripping periphery  40  of the front roller  35  is an outwardly flared conical surface that is tapered so that only the outer edge contacts the wafer  17  regardless of the thickness of the wafer  17 . The inner and outer edges of the gripping periphery  40  are rounded. 
     A set of four detents is provided around the circles  38  on the back side of the mounting ring  12 . Three of these detents, including a back detent  42  and a pair of side detents  43 , are the same depth and size, and of the same spherical shape and diameter as the rolling peripheries  37 , 39  of the back and front rollers  36 , 35 . The fourth detent is a front detent  44 , which is of the same spherical shape and diameter as the rolling peripheries  37 , 39 , but is of greater depth and size. As such, the rolling periphery  39  of the front roller  35  only fully seats in the front detent  44  if there is no wafer  17  on the mounting surface  25  of the mounting ring  12 . If a wafer  17  is present on the surface  25 , the gripping periphery  40  of the front roller  35  of the latches rolls onto the back surface of the wafer  17  and prevents the rolling periphery  39  from dropping into the detent  44  when the front roller  35  is centered on the detent  40 . 
     When the holder  10  is used to process the back sides of wafers  17 , the front side of the wafer  17 , which may have partially formed devices on the side thereof, is facing a backplane  50 . To prevent contact between the device side of such a wafer  17  and the backplane  50 , a pair of abutting contact surfaces  47 , 48  is provided, as illustrated in FIG.  3 . Preferably, one surface is carried on a stop  52  extending from, for either the mounting ring  12  or the backplane  50 , to hold the mounting ring  12  away from the backplane  50  and allow a gap  54  to be maintained between the wafer  17  and the backplane  50 . 
     A processing apparatus  100  of the type referred to above is illustrated diagrammatically in FIG.  4 . The apparatus  100  has a vacuum tight housing  101  which encloses a plenum chamber  102  in which rotates a circular indexing plate  103 . The index plate  103  preferably has five openings  104  therein, spaced at equal 72° angular increments around a central axis  105  on which the plate  103  rotates. In each of the openings is a resiliently mounted annular support ring  107  in the center of which is mounted one of the wafer holders  10 . 
     The housing  101  includes five stations  111 - 115 , each also positioned at equal 72° angular intervals around the axis  105 . These stations include a load lock station  111  having a load lock door  110  through which wafers  17  are loaded and unloaded into holders  10  on the plate  103  when the holder is sealed in a load lock chamber at the station  111 . The stations also include four other processing chambers, one at each of the other stations  112 - 115 , such as, for example, a sputter etch chamber  112 , and three sputter coating chambers  113 - 115 . 
     The chambers  113 - 114  may include, for example, two tantalum deposition chambers  113  and  114  and one gold deposition chamber  115 . This configuration is used to deposit a gold layer on the backside of wafers  17  when loaded backside first through the loadlock  111 . Such wafers  17  will also subjected in other processes to processing on the frontsides thereof to coat and etch features of semiconductor devices. The tantalum-tantalum-gold (Ta-Ta-Au) process is described herein as an example of one process which particularly benefits from the present invention. Other processes and coatings will also benefit from the present invention. 
     The configuration of sputtering chambers  113 - 115  of the apparatus  100  is illustrated in FIG. 5 which shows sputter coating chamber  113  in cross-section. Chamber  113  includes a sealed processing chamber  120  formed of the plenum chamber  102  by the clamping of the annular ring  106  that is at the station  113  between a moveable chamber closure  121  and a portion  122  of the housing  101 . This clamping of the ring  106  positions the holder  10  that is supported in the opening of the ring  106  to bring a wafer  17  held in the holder  10  into position for processing spaced from and parallel to a sputtering target  124  of coating material. For Ta-Ta-Au deposition, the coating material of which the target is made is tantalum. The target  124  is mounted in a cathode assembly  125  that is in turn mounted to a cathode housing  126  which seals an opening in the chamber wall  101  at the station  113 . Once the wafer  17  is positioned in the chamber  113 , backplane  50  is moved into position adjacent the opposite side of the wafer  17  from that being coated with sputtering material. 
     In the processing of the wafer  17 , a plasma is generated in the vacuum of the chamber  120  and material is sputtered from the target  124  by bombardment of the target  124  with ions of gas from the plasma in the space of the chamber  120 . The atoms and particles of tantalum sputtered from the target  124  move across the space of the chamber  120  and coat the wafer  17  on the holder  10 . Components of the holder  10  are shielded from the deposition of sputtered coating material by a shield  130  which is attached to the housing portion  122  and surrounds the holder  10 . The clamping ring  12  is, however, located on the side of the wafer  17  to be processed and faces the target  124 . Part of the ring  12  is covered by the shield  130 . However, the inner rim  18  of the ring  12  extends beyond the inner rim of the shield  130  and is thus vulnerable to deposition of coating material from the target  124 . As each ring  12  is indexed through all of the chambers  113 - 115  during the depositions of a each of a large number of wafers, the cumulative thickness of coating that builds up on the inner rim of the ring  12  may be equivalent to the thicknesses of coatings deposited on hundreds, and perhaps thousands, of wafers. This coating buildup  19  on the ring  12  is illustrated as coating buildup  19  in FIG.  3 . 
     In the processing of a wafer  17  in the apparatus  100 , a wafer  17  is etched in the etch chamber  112 , then coated with two depositions of tantalum, one in chamber  113  and one in chamber  114 , then is coated with one deposition of gold in chamber  115 , then is unloaded from the holder in the loadlock chamber  111  whereupon a new wafer  17  in loaded into the same holder  20 . The cycle time required to load, process and unload a single wafer in the apparatus  100  is about five minutes, one minute in each chamber which includes about 20 seconds to index the plate  103  to move the holders  10  from chamber to chamber. During the course of this process, the retaining ring  12  is subjected to peak temperatures eventually reaching about 490° F. (254° C.). 
     Process temperatures of the wafers  17  vary, for example, from chamber to chamber, from 365° F. (185° C.) in the etch chamber  112  to about 527° F. (275° C.) in the gold deposition chamber  115 . Many components in the chamber are water cooled to about room temperature of about 68° F. (20° C.). The annular housing  11  of the holder  10 , to which the springs  13 , 14  that support the ring  24  connect, ranges from about 68° F. (20° C.) to about 104° F. (40° C.). The temperatures on the shield  130  and the components of the wafer holder  10  increase from minimums when the first wafer  17  is introduced into the machine  100  for processing and increase to steady state levels after about  18  wafers or more are processed. FIG. 6 graphically represents the temperatures on the inner and outer edges of the shield  130 , the inner and outer edges of the clamping ring  12  and the annular housing  11  of the holder  10 . 
     Holders  10  have been customarily formed with the housing  11  thereof made of aluminum and the clamping ring made of stainless steel, typically SS- 316  stainless steel. Heat buildup occurs on chamber components such as the clamping ring  12 , particularly on the deposition surfaces around the inner edge  18  of the ring  12 . Heat flow from the ring  12  is limited in the vacuum of the chamber  120  to radiation and conduction at the contact surfaces through which the ring  12  is mounted to the housing  11 , at the springs  13  and  14 . The heating of the clamp ring  12  has been found to subject the ring  12  to sufficient thermal stresses when used in Ta-Ta-Au deposition processes to cause permanent deformation of the clamp ring  12 . This deformation has been found to become excessive, rendering the ring  12  useless after the processing of a few hundred wafers, and requiring a replacement of the ring  12  with a new, clean, undeformed ring. Run through the same number of thermal cycles without subjecting the ring  12  to the Ta-Ta-Au deposition is found not to subject the ring  12  to thermal stresses that are sufficient to permanently deform the ring  12 . 
     FIG. 7 illustrates the cross-section of a clean and unstressed clamp ring  12  of SS- 316  stainless steel showing an essentially planar annular ring with its inner edge  18  free of deposited coating material. FIG. 7A shows the cross-section of the same ring  12  after being subjected to a number of deposition cycles that elevated to ring  12  to the steady state temperatures and deposited a layer  19  of tantalum 0.040 inches thick on the inner edge  18  of the ring  12 , with the ring  12  then cooled to room temperature. It is found in tests that the inner edge  18  of the clamp ring  12  warps toward the chamber  120  by an average amount of 0.0413 inches relative to outer edge  118  of the clamp ring  12 . It is concurrently found that the tantalum layer  19  is in compression while most of the clamping ring  12  is in tension. 
     The deformation mechanism is thought to be due at least in part to the differences in the thermal expansion coefficients between the deposited tantalum layer  19  and the SS- 316  stainless steel of which the ring  12  is made. The coefficient of thermal expansion for tantalum is 3.6×10 −6  in/in-° F. while the coefficient of thermal expansion for SS- 316  is 9.6×10 −6  in/in-° F. In that the tantalum deposition occurs when the temperature of the ring  12  is elevated at between 460° F. and 480° F., the layer  19  is essentially stress free as long as it is hot, but during the cooling down of the ring  12  and layer  19 , the SS- 316  of the ring  12  contracts much more than the tantalum layer  19 , putting the layer  19  in compression. This results in the production of a circumferential force in the tantalum that produces a resultant force vector F c  in the tantalum layer  19  and a resultant force vector F r  in the steel ring  12  that are displaced by a moment arm L, producing a bending moment about a circumferential axis around the ring  12  that bends the inner edge  18  of the ring  12  into the chamber  120 . 
     According to the principles of the present invention, the retaining ring  12  is formed of a material having a coefficient of thermal expansion that is closer to that of tantalum than is the commonly used stainless steel material. The material, which is preferably non-magnetic, may include materials such as titanium or molybdenum, with molybdenum being preferred. Titanium has a coefficient of thermal expansion of titanium is 5.0×10 −6  in/in-° F. and the coefficient of thermal expansion for molybdenum is 3.0×10 −6  in/in-° F. With the SS- 316  replaced by these materials, a similar test for the warping of the ring  12  with a 0.040 inch layer  19  of tantalum produces a deflection of the inner edge of the ring  12  relative to the outer edge of 0.022 for a titanium ring and −0.005 for a molybdenum ring, compared to the 0.0413 inches for an SS- 316  stainless steel ring ad described above. The deformation of the molybdenum ring  12  is illustrated in FIG.  7 B. 
     Selection of the material of which the clamping ring  12  should be made by selecting a material having a coefficient of thermal expansion that is closer to that of the coating material than is the coeffecient of thermal expansion of alternative clamping rings that experience premature excessive warping. In addition, a material having a high coefficient of thermal conductivity is preferred. For example, the thermal conductivity of molybdenum is 84.5 Btu-ft/hr-ft 2 -° F. while that of SS- 316  is 9.4 Btu-thr-ft 2 -° F. A high modulus of elasticity than that of the material that experiences the undesirable warping is also desirable in that the same thermal stresses will produce less deformation. For example, the modulus of elasticity of molybdenum is 47×10 6  psi while that of SS- 316  is 28×10 6  psi. 
     it is found that with the ring of SS- 316  stainless steel, deposition of a  40  mil coating under the conditions described above not only produces a permanently deformed coated ring  12  but produces a ring  12  that retains permanent deformation after the coating is cleaned from the ring. With the molybdenum clamping ring  12 , removal of the tantalum coating from the ring  12  leaves a molybdenum ring that returns to its original shape. Thus, under the circumstances, the molybdenum ring does not experience thermal stresses that exceed the yield stress of the material so that the ring does not itself experience plastic deformation. Accordingly, such molybdenum ring is reusable where the stainless steel ring is not. 
     Those skilled in the art will appreciate that the applications of the present invention herein are varied, and that the invention is described in preferred embodiments Accordingly, additions and modifications can be made without departing from the principles of the invention.
Warping of a clamping ring, by which a series of semiconductor wafers is held to a wafer holder for vapor deposition of coatings onto the wafers, is retarded by providing a clamping ring formed of the material having a coefficient of thermal expansion that is approximately the same as or close to that of the coating material being deposited onto the wafers. Preferably also, the material of which the ring is formed has a high modulus of elasticity, high thermal conductivity and a high yield strength. For the deposition of tantalum and gold, which is useful for providing backside thermal conductivity on semiconductor wafers, a clamping ring of molybdenum is preferred. The onset of excessive warping is delayed by replacing clamping rings with clamping rings formed of a material having a thermal expansion coefficient closer to that of the material to be deposited, and preferably having the other preferred properties. Preferably, the clamping ring is one having a generally circular opening that is slightly smaller than the wafers to be clamped and that has a flat edge on the inner edge of the ring corresponding to the orientation flat found on the outer edge of an industry standard wafer, so that the ring engages the wafer during clamping around the entire outer rim of the wafer. A ring having a small number of discrete mounting points for spring attachment to the holder, and having a set of latches connected thereto, is preferred.