Patent Publication Number: US-2013250471-A1

Title: Compressible conductive element for use in current-carrying structure

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The application claims priority to U.S. provisional application Ser. No. 61/614,415 filed Mar. 22, 2012, the entire content of each of which is incorporated herein by this reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to current-carrying structures and more particularly to current-carrying structures subject to high temperatures. 
     BACKGROUND 
     Current-carrying structures have been provided, for example in an electrostatic chuck formed from ceramic and having a wafer-receiving surface and an embedded electrode underlying such surface by a thin layer of the chuck. Access to the electrode is provided through a port extending through the underside of the chuck. 
     A common method for establishing electrical connection to the electrode involves placement of all or a portion of a metal pin in the port and attachment of the metal pin, using solder or a conductive adhesive, to the underside of the electrode. After such attachment, the access hole is filled with a suitable potting or encapsulating compound to isolate the thin layer of the chuck from external stresses transmitted through the electrical connection. However, owing to the significantly different thermal expansion coefficients of the assembly, that is the assembly of the chuck body, the metal connection pin and the potting compound, the thin layer of the chuck has remained a chronic source of failure. 
     Other current-carrying structures such as conductors in a wafer heater have been provided. Such electrical conductors can extend from a heating element in a disk that receives a wafer through a hollow shaft and through the chamber mount of the heater to a power source external of the wafer heater. In the case of a wafer heater made from aluminum and conductors made from nickel, the shaft can have a greater coefficient of thermal expansion than the nickel conductors and can exert tension on the conductors that in turn must be resisted by the connection of the conductors to the heating element. In the case of a wafer heater made from aluminum nitride or another ceramic material, the nickel conductors can have a greater coefficient of thermal expansion than the shaft, placing the connections between the conductors and the heating element under compression. These tensile and compressive forces on the conductors, and points of connection, may be significant causes of failure in normal operation, resulting in broken connections or actual buckling deformation of the conductors. 
     Several solutions are extant in the industry. One solution is to interpose a section of nickel wire cable in the conductor. Unfortunately, in order to have sufficient current carrying capacity, that is ampacity, such a nickel wire cable is typically quite stiff in practice. Another solution is to interpose a section of straight or convoluted nickel strip in the conductor. However, many conductors in wafer heaters are required to carry high frequency current and since the flow of radio frequency current concentrates at the surface of a conductor as its frequency increases, known as the skin effect, such a strip is often a poor radio frequency conductor as it has insufficient surface to accommodate the required radio frequency currents of a wafer heater. 
     What is needed is a flexible method of connection that is capable of significantly improved stress isolation for use in a current-carrying structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a portion of an electrostatic chuck for use in a semiconductor manufacturing process having one embodiment of a current-carrying structure. 
         FIG. 2  is a cross-sectional view of the electrostatic chuck of  FIG. 1  take along the line  2 - 2  of  FIG. 1 . 
         FIG. 3  is an isometric view of one embodiment of the electrostatic chuck of the present invention with an electrical connector coupled to the chuck. 
         FIG. 4  is a cross-sectional view of the electrostatic chuck and electrical connector of  FIG. 3  taken along the line  4 - 4  of  FIG. 3  but with the conductive element shown in plan. 
         FIG. 5  is an enlarged cross-sectional view of a portion of the electrostatic chuck and electrical connector of  FIG. 3 , with a portion of the conductive element cut away, taken along the line  5 - 5  of  FIG. 4 . 
         FIG. 6  is a cross-sectional view of a wafer heater for use in a semiconductor manufacturing process having another embodiment of a current-carrying structure. 
         FIG. 7  is an enlarged view of a portion of the wafer heater of  FIG. 6 , taken along the line  7 - 7  of  FIG. 6  and partially cut away. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view of a portion of an electrostatic chuck  10  fabricated from any suitable dielectric material common to the art, for example aluminum nitride or alumina or another ceramic. The drawings herein are schematic and the relative size between structural elements or features described in the drawings are not necessarily to scale. The chuck includes a body  1  and an integral conductive electrode  3  underlying a chuck surface  4  on which a wafer being processed rests. In some embodiments, the conductive electrode can have a thickness ranging from 0.0005 to 0.0010 inch. It is appreciated that an electrostatic chuck can be provided with one or more conductive electrodes. For example, in one embodiment a single conductive electrode extends substantially across the entire diameter or transverse dimension of the chuck. In another embodiment, two semicircular conductive electrodes can be provided. In another embodiment, a plurality of two or more conductive electrodes can be provided in a variety of shapes and sizes which in the aggregate substantially approximate the shape and size of the chuck surface  4 . 
     Looking at a cross-section of the chuck in  FIG. 2 , the conductive electrostatic electrode  3  is disposed in very close proximity to the wafer surface  4 , that is the surface to which the wafer is electrostatically held for the purpose of subsequent processing such as lithography, deposition, etch, or ion implant. It is advantageous to have the chucking electrode close to the wafer surface  4  to maximize the capacitance between the electrode and the wafer and, as a result, maximize the chucking force, that is the attraction of the wafer to the chucking electrode  3 . The thickness of the dielectric layer or web  6  between the chucking electrode  3  and the wafer surface  4  can be of any suitable dimension. For example, a layer  6  having a thickness of 0.005 inch is typical of practical electrostatic chuck embodiments. It is customary to provide access to the underside of the electrostatic electrode  3  through the underside of the dielectric body  1  for the purpose of making electrical contact with the electrode  3 . Such access, represented here as feature  2 , can be any suitable hole, bore, aperture, opening, access port or volume. In one embodiment, the aperture or hole  2  has a diameter of approximately 0.070 inch. It is appreciated that the resulting chuck structure or layer  6  extending between the chuck surface  4  and the electrode  3  proximate to the electrical connection, for example a 0.005 inch layer or web  6  of ceramic material, is extremely fragile and subject to fracture and dielectric failure, both during manufacture of the chuck and during use of the chuck. 
     In one embodiment of the invention, a flexible or conductive element  11 , which can be referred to as a compressible or expandable element or a spring element, is provided for making electrical contact with the electrode  3  of electrostatic chuck  10  (see  FIGS. 4-5 ). Some or all of the element  11  can be disposed in the aperture or hole  2  of the chuck body  1 . In one embodiment, the element  11  is sized and shaped for placement entirely within the hole  2 . The element  11  can be directly coupled or connected to the electrode  3 , or indirectly coupled or connected to the electrode by an intermediate element (not shown). In embodiment, the element has a first end  11   a  connected to a conventional electrical terminal  12  and an opposite second end  11   b  directly connected to the electrode  3 . The electrode  3  in combination with the electrical terminal can be referred to as a current-carrying structure. 
     In one embodiment, the flexible or compressible element  11  is formed from a hollow cylindrical thin-walled body  16  that is provided with an internal cavity  17  and that includes a bellows portion or bellows  18 . In one embodiment, shown in  FIGS. 4 and 5 , the first end portion or end  11   a.  of the element is tubular for electrically joining by any suitable means, such as solder or a conductive paste or epoxy, to the electrical terminal  12 . The compressible element  11  can be made from any suitable conductive material(s) such as nickel that is gold plated so as to be corrosive resistant and suitable for use with solder when electrically connecting to other elements, such as the electrical terminal  12  and the electrode  3 . In one embodiment, the thin-walled body  16  of the compressible element  11  has a thickness ranging from 0.0005 to 0.0020 inch, although thicknesses outside of this range are permissible. 
     In one embodiment, the bellows portion  18  is comprised of at least one flexible element  21  and in one embodiment a plurality of spaced-apart flexible elements  21  arranged transversely of the longitudinal axis  24  of the body  16 . In one embodiment, each of the one or more flexible elements is formed from a first or top wall  22  and a second or bottom wall  23  that in one embodiment can each be planar and extend substantially perpendicular to the longitudinal axis  24  of the body  16 . The circular outer periphery of the walls  22  and  23  can be joined together by a circular outer wall  26  that can be semi-circular in cross section so as to provide the flexible element  21  with a rounded outer periphery. The circular inner periphery of the bottom wall  23  of one flexible element  21  and the top wall of the adjoining flexible element  21  can be joined together by a circular inner wall  27  that can be semi-circular in cross section. In this manner, the outer wall of the bellows portion  18  has a serpentine configuration when viewed in plan from the side, as shown in  FIGS. 4-5 . In one embodiment, the bottom of the compressible element  11  is formed from a planar bottom wall  31 , which wall  31  can be the bottom-most wall of the bellows portion  18  and in this regard can be the bottom wall  23  of the bottom flexible element  21  of the bellows  18 . In one embodiment, the bottom wall  31  of the element can have a diameter and area approximating the diameter and area of the exposed portion of the electrode  3  at the base of aperture or access port  2 , and in one embodiment the diameter of the bottom wall  31  is substantially equal to the diameter of the flexible elements  21  and thus bellows portion  18 . 
     Although it is appreciated that the compressible element  11  can be of any suitable size, in one embodiment the compressible element  11  can have a height of approximately 0.075 inch, and top end portion or end  11   a  can have an external diameter of approximately 0.028 inch. The bellows portion  18  can have a height of approximately 0.050 inch. Each of the flexible elements  21  can have an outer diameter at outer wall  26  of approximately 0.055 inch, an inner diameter at the outside of inner wall  27  of approximately 0.026 inch and a distance between the top of first wall  22  and the bottom of second wall  23  of approximately 0.004 inch. In one embodiment, the bellows portion  18  is sized and shaped for placement entirely within the access port or hole  2 . In one embodiment, the compressible element has a k or spring value of 0.2 grams per millimeter, and the bellows portion  18  and the flexible elements  21  thereof are compressible along the longitudinal axis  24  of the element  11  a distance of as much as 0.020 inch. In one embodiment, the compressible element is installed so that the bellows portion  18  is compressed 0.005 inch from its free length, or uncompressed state, during installation to allow the bellows portion, and thus the compressible element  11 , to be in a “dynamic range” wherein it may extend or compress with relatively equal k values. 
     The body  16  can be formed or made in any suitable manner and in one embodiment is electroformed. For example, the nickel or other base material of the body  16  can be plated onto a conductive mandrel, which can be made from aluminum or any other suitable material. After formation, the mandrel can be dissolved in a suitable known process using acid or another suitable corrosive material. The optional gold layer is plated onto the nickel base layer of the body  16 , either before or after dissolution of the mandrel. 
     The compressible element  11  can be secured to the electrical connector  12  and to the electrode  3  by any suitable manner such as solder or a conductive adhesive or epoxy. In one embodiment, the top end portion  11   a  of the element  11  can be joined to the bottom of the connector  12  by means of soldering, and the electrical connector  12  may be glued or potted to the ceramic electrostatic chuck body  1 . The bottom end portion  11   b  of the element  11  can be joined to the exposed surface of the electrode  3  within hole  2  by solder. Solder connections to the compressible element  11  provide a strong electrical connection between the element  11  and the adjoined connector  12  and electrode  3 . 
     The relatively large planar bottom wall  31  of the element  11  serves to provide a large connection surface between the compressible element  11  and the electrode  3 . The diameter of bottom wall  31  closely approximates the diameter of the bottom of aperture or hole  2  and thus the diameter of the portion of the electrode  3  exposed by the hole  2 . The relatively large contact or engagement area of the bottom wall  31  of the bellows portion  18  with the chuck electrode  3  provides a relatively large support structure and surface for the relatively thin portion of the electrode  3  and the underlying layer  6  exposed at the bottom of aperture  2 . 
     In operation and use, the stress that may be transmitted to the thin ceramic web or layer  6  from either the electrical connector  12  or from dissimilar expansion caused by intervening potting materials serving to join the connector  12  to the chuck body  1  in the access annulus  2 , or from the dissimilar expansion between the access annulus  2  and the compressible element  11 , is reduced or minimized. In addition, the relatively large contact area between the compressible element  11  and the electrode, provided by the relatively large-diameter bottom wall  31  of the bellows portion  18  that approximates the exposed surface and area of the electrode  3  at the bottom of aperture or access annulus  2 , inhibits the formation of concentrated loads in the relatively thin laminate structure formed by the electrode  3  and the layer  6  by, among other things, providing structural support to such thin laminate structure, distributing any axial load provided by the compressible element  11  evenly across such laminate structure or a combination of the foregoing. 
     It is appreciated that other embodiments of the compressible element  11  of the present invention can be provided. For example, the compressible element  11  can include a suitable spring such as a spiral spring (not shown), either separate from bellows portion  18  and thus alone or in combination with bellows portion  18  or another compressible structure. Such a spring can be made from any suitable conductive material such as beryllium copper. The spring can be joined at a first end to the electrical connector  12  and at a second end to the electrode  3 , either directly or indirectly, by any suitable means such as a conductor paste or epoxy or solder. 
     It is additionally appreciated that the invention may have applications beyond electrostatic chucks. For example, a compressible element of the invention similar to element  11  or otherwise can be provided in any current-carrying structure. In one embodiment, the current-carrying structure can be a wire or cable, and a compressible element of a suitable type, for example as disclosed herein and including for example a bellows portion similar to bellows portion  18 , can be spliced into or formed as part of the current-carrying structure. In one embodiment, the compressible element can join together first and second portions of the current-carrying structure. Such compressible element can be open at both ends, for example bottom wall  31  of the bellows portion can be removed or an opening can otherwise be provided in the base of the bellows portion of such compressible element. In one embodiment, the current-carrying structure can include a portion with a substantially planar surface, such as an electrode similar to electrode  3  having a planar surface, and the bellows or compressible portion of the compressible element can be joined to such planar surface of the portion in any suitable manner, for example as discussed above. In each instance, the compressible element can serve to reduce stress concentrations due to disparate coefficients of thermal materials in the first and second portions of the current-carrying structure, the surrounding or related structure, or both. 
     One embodiment of a current-carrying structure is illustrated in  FIGS. 6-7 . Wafer heater  41  therein includes a body  42  that can be formed from one or more elements. In one embodiment, body  42  includes a hollow shaft  43  formed integral with at least part of a chuck or disk  44 , such as a first or bottom portion  44   a  of the disk. The disk  44  includes a second or top portion  44   b,  which is secured to the bottom portion  44   a  in any suitable manner and has a top planar surface  46  for receiving a wafer (not shown) to be treated. A chamber mount  47  is included in body  42  of the wafer heater at the base of the shaft  43  and a suitable seal (not shown), such as an O-ring, can be provided between the chamber mount  47  and the shaft  43  to provide a hermetic seal between the chamber mount and the shaft  43  and another suitable seal (not shown), such as an O-ring, can be provided between the chamber mount  47  and the bottom of the process chamber (not shown) to provide a hermetic seal between the chamber mount and the chamber. Such seals serve to inhibit process gas from leaking outside the chamber, either via the interior or central passageway  48  of the shaft  43  or directly, and in this manner the wafer heater  41  is a hermetic assembly. 
     An embedded heating element  51  is included in disk  44 , for example between the top portion  44   b  and the bottom portion  44   a  of the disk  44 . The heating element can be of any conventional type, and in one embodiment can be a circular ring made from any suitable resistive material such as metal which underlies at least a portion of the disk surface  46 . The heating or heater element  51  can have a first end portion  51   a,  for example near the central portion of the disk  44 , which is electrically coupled to a first current-carrying structure or conductor  52  and a second end portion  51   b,  for example near the central portion of a second current-carrying structure or conductor  53 , which is electrically coupled to a second conductor  53 . Each of the conductors can extend through the interior or central passageway or bore  48  of the shaft and through respective bores in the chamber mount so as to have respective first end portions  52   a,    53   a  accessible exterior or outside the wafer heater  41  for the purpose of external connection to a power supply (not shown). The first and second conductors  52 , 53  have respective second end portions  52   b,    53   b  electrically coupled or secured to respective first and second end portions  51   a,    51   b  of the heating element  51  by any suitable means such as brazing. In practice, the conductors  52  and  53  can be fixed to the chamber mount  47  so that external forces created by handling, shipping, or simply making electrical connection to the conductors are not transmitted through the conductors to the point of attachment of the conductors, that is second end portions  52   b  and  53   b,  to the heating element  51 . 
     The wafer heater  41 , including shall  43 , disk  44  and chamber mount  47  thereof, can be fabricated from the same material, for the purpose of making a unitized assembly, or from different materials. Although any suitable materials can be used, particularly suitable materials are compatible with typical gasses, such as fluorine, used in semiconductor manufacturing processes. In one embodiment, the entire wafer heater  41  can be made from aluminum. In another embodiment, the chamber mount  47  can be made from a suitable metal and the remainder of the wafer heater  41  can be made from a suitable ceramic such as aluminum nitride. The conductors  52  and  53  can be made from any suitable electrically conductive material, and in one embodiment are fabricated from a material, such as nickel, that is resistive to oxidation in air at the elevated temperatures at which wafer heaters operate. The material of the conductors  52  and  53  typically has a coefficient of thermal expansion that is different than the coefficient of thermal expansion of the materials of the shaft  42 , disk  44  and chamber mount  47 . 
     In one embodiment of the invention, a flexible element  56 , which can be referred to as a compressible or expandable element or a spring element, is interposed or provided in each of the first and second conductors  52  and  53  in interior  48  of the shaft  43  (see  FIGS. 6-7 ). The element  56  can be directly coupled or connected to the conductor, or indirectly coupled or connected to the conductor. In one embodiment, each of the compressible elements has a first end  56   a  directly connected to the upper portion of the respective conductor  52  or  53  and an opposite second end  56   b  directly connected to the lower portion of the respective conductor  52  or  53 . 
     In one embodiment, each of the compressible or conductive elements  56  is substantially similar to compressible element  11  described above and like reference numerals have been used to describe like components of compressible elements  56  and  11 . In that regard, each of the compressible elements  56  is formed from a hollow cylindrical thin-walled body  16  that is provided with an internal cavity  17  and that includes a bellows portion or bellows  18 . In one embodiment, shown in  FIGS. 6 and 7 , the first end portion or end  56   a  of each element is tubular for electrically joining by any suitable means, such as solder or brazing, to the upper portion of the respective conductor  52  or  53 , and the second end portion or end  56   b  of each element is tubular for electrically joining by any suitable means, such as solder or brazing, to the lower portion of the respective conductor  52  or  53 . In one embodiment, the internal bore of each end portion  56   a  and  56   b  is approximately equal to the external diameter of the respective conductor  52  or  53  and the upper and lower portions of the conductor seat within the respective end portion  56   a  and  56   b  of the compressible element  56 . In one embodiment, the thin-walled body  16  of the compressible element  56  has a thickness ranging from 0.002 to 0.025 inch, although thicknesses outside of this range are permissible. Relatively large thicknesses of the compressible elements  56  can be required when relatively large currents are required to be carried by the compressible elements. For example, a thickness of approximately 0.012 inch would accommodate a current of approximately 20 amperes to be carried by the compressible element. 
     In one embodiment, the bellows portion  18  is comprised of a plurality of spaced-apart flexible elements  21  arranged transversely of the longitudinal axis  24  of the body  16 . In one embodiment, each of the flexible elements  21  is formed from a first or top wall  22  and a second or bottom wall  23  that in one embodiment can each be planar and extend substantially perpendicular to the longitudinal axis  24  of the body  16 . The circular outer periphery of the walls  22  and  23  can be joined together by a circular outer wall  26  that can be semi-circular in cross section so as to provide the flexible element  21  with a rounded outer periphery. The circular inner periphery of the bottom wall  23  of one flexible element  21  and the top wall of the adjoining flexible element  21  can be joined together by a circular inner wall  27  that can be semi-circular in cross section. In this manner, the outer wall of the bellows portion  18  has a serpentine configuration when viewed in plan from the side, as shown in  FIGS. 6-7 . 
     Although it is appreciated that the compressible element  56  can be of any suitable size, in one embodiment the compressible element  56  can have a height of approximately one inch, and each of top end portion or end  56   a  and bottom end portion or end  56   b  can have an external diameter of approximately 0.155 inch. The bellows portion  18  can have a height of approximately 0.60 inch. Each of the flexible elements  21  can have an outer diameter at outer wall  26  of approximately 0.5 inch, an inner diameter at the outside of inner wall  27  of approximately 0.2 inch and a distance between the top of first wall  22  and the bottom of second wall  23  of approximately 0.1 inch. In one embodiment, the compressible element  56  can have a k or spring value ranging from 500 to 3000 grams per millimeter, and the bellows portion  18  and the flexible elements  21  thereof are compressible along the longitudinal axis  24  of the element  56  a distance of as much as 0.13 inch. In one embodiment, each of the compressible elements is installed so that the bellows portion  18  is compressed 0.04 inch from its free length, or uncompressed state, during installation to allow the bellows portion, and thus the compressible element  56 , to be in a “dynamic range” wherein it may extend or compress with relatively equal k values. 
     The compressible elements  56  can be made from any suitable conductive material such as any of the materials discussed above with respect to compressible element  11 . The compressible elements  56  can be made in any suitable manner, for example as discussed above with respect to compressible element  11 , and in one embodiment is hydroformed. In one suitable hydroforming procedure, a simple cylinder of the desired material of the compressible element, such as nickel, is expanded by high pressure fluid to conform the cylinder to the inside of a suitable die utilized to shape the exterior of the compressible element. After such formation, the die can be split to permit removal of the now-formed compressible element. Another suitable method for manufacturing the compressible elements  56  is welding. In one embodiment of such a welding procedure, adjacent top and bottom walls  22 ,  23  of the bellows  18  are welded together at circular outer wall  26  and at circular inner wall  27  to create the hermetic convolutions of the bellows  18 . As part of any of such manufacturing techniques, the nickel formation material of the compressible element  56  can be plated, for example with gold, in the manner and for the reasons discussed above. 
     In operation, the temperature of the wafer heater  41  and body  42  thereof may be between 400° C. and 800° C., depending upon the process and the materials of construction. Wafer heaters made from aluminum often operate to temperatures of approximately 500° C., while wafer heaters made from aluminum nitride often operate to temperatures of approximately 800° C. 
     The compressible element  56  interposed in each of the metal conductors  52  and  53  can accommodate the dissimilar expansion of the assembly of the body  42  and conductors  52  and  53 , for example as a result of the disparate coefficients of thermal expansion of the materials of the body  42  and the material of the conductors  52  and  53 , while at the same time providing a generous surface area for radio frequency current as appropriate to the process. The invention discloses the use of a flexible metal bellows  18 , which can be fabricated from nickel for both oxidation resistance and ease of joining to nickel conductors  52  and  53 . The bellows  18  can have a diameter that is larger than the diameter of the respective conductor  52  or  53  and thus provide greater surface area, and therefore good radio frequency current conductivity, than that of the conductor  52  or  53  itself, while offering extraordinary axial flexibility to the conductor assembly.