Abstract:
An electrostatic chuck and a process of manufacturing an electrostatic chuck for supporting a semiconductor wafer during wafer processing and for providing a plurality of gas inlet channels extending through the chuck and through which thermal transfer gas can be supplied to the back side of the wafer to enhance the thermal transfer between the wafer and the chuck, embedding a plurality of inserts in a ceramic electrostatic chuck, each insert comprising a matrix of the ceramic of which the electrostatic chuck is made and a plurality of removable elongate members, and removing the elongate members to form a plurality of elongate holes providing the plurality of gas inlet channels.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention generally relates to electrostatic chucks for supporting a semiconductor wafer in a semiconductor wafer processing system. More particularly, the present invention relates to a process of manufacturing such an electrostatic chuck having relatively small diameter gas inlet channels that supply a heat transfer medium to the surface of the chuck. 
     2. Description of the Background Art 
     Typical semiconductor wafer processing apparatus  10  is illustrated in FIG.  1 . The apparatus  10  includes an electrostatic chuck  12  having a top wafer support surface  14  in which is embedded an electrode  16  connected to a DC/RF supply  18 . The supply  18  supplies DC voltage to the electrode  16  to provide a DC bias voltage to electrostatically retain the semiconductor wafer  20  on the top surface  14  of the electrostatic chuck  12 . The supply  18  may also provide RF current or energy to the apparatus  10  to ignite a plasma above the wafer  20 . As is known to those skilled in the semiconductor wafer processing art, the use of high plasma power improves etch rates, facilitates increased aspect ratios, and provides various other process improvements. The use of high plasma power imparts additional heat to the semiconductor wafer  20  during processing, and unless heating of the semiconductor wafer  20  is controlled, the semiconductor wafer can be so heated that the partially processed semiconductor wafer becomes damaged. To permit the use of high plasma power, it is known, as illustrated in FIG. 1, to mount the electrostatic chuck on a base  24  which is provided with a plurality of coolant receiving channels  25 . The receiving channels  25  carry suitable coolant flows for cooling the electrostatic chuck  12  which in turn cools the semiconductor wafer  20 . 
     In spite of reasonable efforts to make the bottom surface of the wafer  20 , referred to in the art as the backside of the wafer, smooth, and to make the top or support surface  14  of the electrostatic chuck  12  smooth, surface irregularities are present that result in interstitial spaces, or spacing, between the backside of the semiconductor wafer  20  and the support surface  14  of the electrostatic chuck  12 . This interstitial spacing  26  is indicated diagrammatically in FIG.  2 . Vacuum will occupy such interstitial spacing, and a vacuum, as is known, is not a good heat transfer medium for transferring heat from the wafer  20  to the electrostatic chuck  12 . It is further known in the semiconductor wafer processing art to enhance the thermal transfer between the semiconductor wafer  20  and the electrostatic chuck  12  by supplying thermal transfer gas, such as helium or argon, to the interstitial spaces  26 . The thermal transfer gas enhances the thermal transfer between the semiconductor wafer  20  and the electrostatic chuck  12 . 
     Typically, the thermal transfer gas is supplied, for example, from the helium supply  30  shown in FIG. 1 through a conduit  32  to the interstitial spaces  26  in FIG.  2 . Typically, the diameter of the thermal transfer gas conduit  32  is about 0.5-3 mm. It has been found that the diameter of the typical thermal transfer gas conduit, such as conduit  32 , is so large that the volume of the thermal transfer gas in the conduit, can become ionized upon the RF energy being applied to the apparatus  10  to ionize the etch plasma  22  (FIG. 1) and such ionization of the thermal transfer gas invites arcing. Such arcing can pit, or otherwise damage, the backside of the wafer  20  to an extent that causes wafer damage. Such arcing can also pit, and otherwise damage, the support surface  14  of the electrostatic chuck  12  to the point where the electrostatic chuck is ruined. As is also known, the ionization of a gas is a function, at least in part, of the volume of the gas, and the larger the volume of the gas, the more likely the gas is to ionize and the smaller the volume of the gas, the less likely it is to ionize. It has been found that the volume of the thermal transfer gas, such as helium contained in the typical prior art conduit, such as conduit  32  shown in FIG. 2, invites or promotes ionization of the thermal transfer gas particularly as higher plasma powers are utilized. 
     Accordingly, there exists a need in the art for thermal transfer gas inlet channels in an electrostatic chuck that are so small that the volume of thermal transfer gas in such gas inlet channels has a very low tendency to ionize when higher plasma powers are utilized. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by an electrostatic chuck and a process of fabricating the electrostatic chuck comprising the step of embedding a plurality of inserts in a ceramic electrostatic chuck, each insert comprising a matrix of the ceramic of which the electrostatic chuck is made and a plurality of removable elongate members, and removing the elongate members to form a plurality of elongate holes that define the plurality of gas inlet channels. The removable elongate members may be etchable wires, a bundle of loosely rolled etchable metal wire mesh or a plurality of etchable lines printed on ceramic and which may be removed by chemical or thermal etching. The result is an electrostatic chuck comprising at least one insert having channels of a diameter that precludes thermal transfer gas ignition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagrammatical illustration of prior art semiconductor wafer processing apparatus; 
     FIG. 2 is an enlarged view of the encircled portion  2 — 2  in FIG. 1; 
     FIG. 3 is a side view of a ceramic body in a green state showing the embedded electrode as a solid line; 
     FIG. 4 is a diagrammatical illustration of etchable wire mesh laid out flat and having a length L; 
     FIG. 5 is an illustration of the ultimate configuration of the electrostatic chuck manufactured by the process of the present invention and having a final thickness T; 
     FIG. 6 is an illustration of a bundle of loosely rolled etchable metal wire mesh secured by a string or thread; 
     FIG. 7 is an illustration of soaking the bundle of loosely rolled etchable wire mesh shown in FIG. 6 in a slurry of ceramic material; 
     FIG. 8 is an illustration of a wire insert comprised of a matrix of the bundle of rolled etchable metal wire mesh and ceramic material; 
     FIG. 9 is a diagrammatical illustration of a ceramic body in the green state having a plurality of holes extending therethrough; 
     FIG. 10 is an illustration of the green ceramic body of FIG. 9 showing the wire insert of FIG. 8 inserted into the holes; 
     FIG. 11 is an illustration of a ceramic chuck body produced by hot pressing the green ceramic body having the wire inserts therein as shown in FIG. 9; 
     FIG. 12 is an illustration of the electrostatic chuck formed by machining the ceramic chuck body shown in FIG. 11; 
     FIG. 13 is an illustration of the electrostatic chuck manufactured by the process of the present invention showing the gas inlet channels formed therein after the metal wires of the wire inserts shown in FIG. 12 have been removed; 
     FIG. 14 is a diagrammatical view, in perspective, of a plurality of layers of fabric soaked in a slurry of ceramic material having a parallel array of etchable wires placed on each layer of fabric; 
     FIG. 15 is a diagrammatical view, in perspective, of a wire laminate in the green state provided by stacking and laminating the layers of fabric and parallel arrays of etchable wires shown in FIG. 14; 
     FIG. 16 is an illustration in of a wire insert produced by subdividing the green wire laminate shown in FIG. 14; 
     FIG. 17 is an illustration of a plurality of green tapes on which parallel arrays of etchable lines have been screen printed from paste or ink of etchable metal powder; 
     FIG. 18 is a diagrammatical view, in perspective, of a green wire laminate produced by laminating the layers of green tape and parallel arrays of etchable lines shown in FIG. 17; and 
     FIG. 19 is a diagrammatical view in perspective of an etchable line insert produced by subdividing the green wire laminate shown in FIG.  18 . 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the process of the present invention is illustrated in FIGS. 3-13. A body  35  of ceramic material in a green state (i.e., uncured) is prepared. The green ceramic body  35  is prepared in a manner known to the art, typically, by loosely compacting ceramic material in a mold to provide a cylinder or disk of relatively pliable ceramic material having somewhat the consistency of putty. An electrode  36  is suitably embedded in the green ceramic body  35 . 
     Metal wire mesh  37 , subject to etching, referred to hereinafter as etchable metal wire mesh, is shown in FIG.  4 . Etchable metal wire mesh  37  is prepared having a length L which is equal to, or at least substantially equal to, the thickness T of the ultimate electrostatic chuck provided by the process of the present invention. Such a chuck is illustrated in FIG.  5  and shown to have a thickness T. The etchable metal wire mesh  37  is rolled into a loose bundle, as shown in FIG. 6, and is secured in the bundle by a suitable string or thread  38 . As shown in FIG. 7, the loosely rolled etchable metal wire mesh bundle  37  is immersed and soaked in a slurry  40  of ceramic material which is the same as the ceramic material of which the green ceramic body  35 , shown in FIG. 3, is made. The etchable metal wire mesh  37  is soaked in the slurry  40  to cause the ceramic material in the slurry to attach or adhere to the etchable metal wire mesh so as to produce the wire insert  42  shown in FIG. 8 which is a generally cylindrical matrix of the etchable metal wire mesh  37  and the adhering ceramic material indicated diagrammatically in FIG. 8 by the irregular line  41 . 
     As depicted in FIG. 9 A plurality of holes  44  are formed extending through the green ceramic body  35  which holes may be formed suitably such as by punching. The holes  44  are formed complementary in diameter to the wire insert  42  but slightly longer in length. It will be understood that in the illustrative embodiment of the present invention the electrode  36  is comprised of molybdenum wire mesh. The molybdenum wire mesh comprises a plurality of molybdenum wires having spacing therebetween and that the plurality of holes  44  formed in the green ceramic body  35  are formed in the spaces between the molybdenum wires comprising the electrode  36 . As shown in FIG. 10, a plurality of the wire inserts  42  are inserted into the plurality of holes  44 . Although a plurality of inserts are generally used to provide sufficient gas distribution, as few as one insert can be used to provide gas to the surface of the chuck. 
     Once the inserts are in place, the green ceramic body having the wire inserts  42  inserted therein is suitably hot pressed or sintered, in a manner known to the art, at a temperature of about 1,700° C.-2,000° C., at a pressure of about 500 psi-2,000 psi, and in an atmosphere of nitrogen, to produce the ceramic body  50  shown in FIG.  11 . There will be little or no distortion of the wire inserts  42  during such hot pressing because the ceramic material comprising the wire inserts  42 , is the same ceramic material of which the green ceramic body  35  is made. 
     Thereafter, as shown in FIG. 12, the ceramic body  50  from FIG. 11 is suitably machined, in a manner known to the art, to provide the final or ultimate shape of the electrostatic chuck  52  as shown in FIG. 12 with the opposed ends of the wire inserts  42  extending upwardly from the top and downwardly from the bottom of the chuck. This upward and downward extension of the wire inserts  42  makes the metal wire mesh  37  comprising the matrices  42  accessible for removal forming or leaving holes complementary in size and shape to the metal wires comprising the metal wire mesh. As shown in FIG. 13, the holes provide the plurality of gas inlet channels  54  through which thermal transfer gas can be supplied to the back side of the wafer  20  in FIG.  1 . 
     In this embodiment of the process of the present invention, the etchable metal wire mesh  37  is molybdenum wire mesh and the ceramic material comprising the green ceramic body  35  of FIG. 3, the slurry  40  of FIG. 7, and the ultimate chuck  52  of FIG. 12, is aluminum nitride. 
     With further regard to the removal of the metal wire mesh  37  comprising the wire inserts  42  shown in FIG. 12, upon the metal wire mesh  37  being molybdenum wire mesh, ammonium ferricyanide may be used to chemically etch away the molybdenum wire mesh thereby forming or leaving holes. These holes provide the plurality of thermal transfer gas inlet channels  54  illustrated in FIG.  13 . Upon the molybdenum wires comprising the molybdenum wire mesh  37  being chemically etched away, a labyrinth of gas inlet channels will be provided corresponding to the size and shape of the molybdenum wires comprising the molybdenum wire mesh, and such gas inlet channels  54  (FIG. 13) will have the same, or at least substantially the same, diameter as the molybdenum wires comprising the molybdenum wire mesh. The diameter of such molybdenum wires is about 100 micrometers and hence the plurality of gas inlet channels  54  (FIG. 13) formed by the etching away the molybdenum wires will also have a diameter of about 100 micrometers. 
     Alternatively, the molybdenum wires comprising the molybdenum wire mesh  37  may be removed by the step of thermal etching, such as by being heated to a temperature of about 500° C. in an oxidizing atmosphere (e.g., air) to oxidize the molybdenum wires into substantially powder followed by the step of blowing the powder from the channels such as with pressurized air. 
     Referring to FIGS. 14-16, an alternate process embodiment for providing a plurality of wire inserts in accordance with the present invention is illustrated. A plurality of layers of fabric,  61  . . .  69 , are shown in FIG. 14, which layers of fabric have been soaked in a slurry of ceramic material such as the slurry  40  shown in FIG.  7 . This slurry is a slurry of the same ceramic material of which the green chuck body  35  shown in FIG. 5 is made. The ceramic material is, for example, aluminum nitride. The layers of fabric  61  . . .  69  may be, for example, cheese cloth. A parallel array of wires subject to etching, which wires are referred to hereinafter as etchable wires, such as the parallel array of etchable wires, indicated by the bracket  70  in FIG. 14, is placed on each of the slurry soaked fabrics  61  . . .  69 . The slurry is dried to cause the dried slurry to hold or maintain the arrays of etchable wires  70  in place on the layers of fabric. Such etchable wires will have a length equal to, or at least substantially equal to, the thickness T (FIG. 5) of the final or ultimate electrostatic chuck formed by the process of the present invention. Thereafter, the layers of fabric having the parallel arrays of etchable wires thereon are stacked, with the wires oriented in the same direction, and laminated to produce the green wire laminate  72  shown in FIG.  15 . Thereafter, the green wire laminate  72  is subdivided into subparts to provide a plurality of generally rectangular wire inserts such as the wire inserts  74  shown in FIG.  16 . Accordingly, the wire inserts  74  comprise a matrix of etchable wires and ceramic material. Thereafter, a plurality of such wire inserts  74  are inserted into the holes  44  formed in a green chuck body, such as the green chuck body  42  shown in FIG. 9; however, in this embodiment the holes  44  formed in the green chuck body will be rectangular so as to be complementary in size and shape to the rectangular wire insert  74 . Thereafter, the same processing steps described above with regard to the wire insert  42  shown in FIG.  8  and the processing steps illustrated in FIGS. 9-13, are practiced utilizing the wire inserts  74 . More particularly, in the final step, the etchable metal wires comprising the wire insert  40  upon being etched away, either by chemical etching or thermal etching and blowing as described above with regard to the etchable metal wires comprising the wire mesh  37  of FIG. 4, a plurality of gas inlet channels will be formed complementary in size and shape to the etchable metal wires comprising the insert  74 . In this embodiment, the etchable wires comprising the wire insert  74  are molybdenum wires having a diameter of about 100 micrometers and hence the thermal transfer gas inlet channels provided in the electrostatic chuck by the etching away of such molybdenum wires will have a diameter of about 100 micrometers. In this embodiment the etchable molybdenum wires are straight and hence the gas inlet channels provided upon removal of these wires will be substantially vertical and due to the hot pressing step described with regard to FIG. 11 above, the vertical gas inlet channels may be somewhat bent. 
     Another alternate embodiment of the invention provides a plurality of inserts is illustrated in FIGS. 17-19. In this embodiment, a plurality of layers of green tapes  81 ,  82  and  83  shown in FIG. 17 are made from a casting of ceramic powder and one or more polymeric binders with the ceramic powder being powder of the same ceramic material of which the green ceramic body  35  as shown in FIG. 3 is made. A paste or ink of etchable metal powder, i.e., metal powder subject to etching, such as molybdenum powder is prepared. A parallel array of etchable lines from such paste or ink is screen printed onto each of the green tapes  81 ,  82  and  83  with such parallel array of etchable lines being indicated by the bracket  85  in FIG.  17 . It has been found that in formulating the ink or paste made from molybdenum powder that the addition of very small amounts of palladium powder, e.g. 0.1-1.0%, or of nickel powder, e.g. 5%-10%, aids in the hot pressing of the molybdenum ink or paste into solid etchable lines similar to wire. Such molybdenum etchable lines, in the preferred embodiment have a width of about 100 micrometers, a thickness of about 100 micrometers, and a spacing therebetween of about 100 micrometers. Thereafter, layers of green tape  81 - 83  with the etchable lines  85  screen printed thereon were stacked with the etchable lines oriented in the same direction and laminated, to produce the green wire laminate  86  shown in FIG.  18 . Thereafter, the green wire laminate  86  is suitably subdivided to provide a plurality of rectangular inserts such as the line insert  88  shown in FIG. 19 which is a matrix of etchable lines and ceramic material. Thereafter, a plurality of such line inserts  88  are inserted into the holes  44  formed in the green chuck body  35  shown in FIG. 9 and, in this embodiment, the holes  44  will be complementary in size and shape to the  85  lines within the rectangular line inserts  88 . Thereafter, the same processing step described above with regard to the wire insert  42  shown in FIG.  8  and the processing steps illustrated in FIGS. 9 and 13 are practiced utilizing the inserts  88 . More particularly, in the final step, the etchable metal lines that were screen printed on the green tapes  61 - 69  shown in FIG. 14 are etched away, either by chemical etching or by thermal etching and blowing as described above with regard to the etchable metal wires comprising the wire mesh  37  of FIG. 4. A plurality of gas inlet channels will be formed complementary in size and shape to such etchable metal lines and will have a cross-section substantially equal to 100 micrometers. In the above-described hot pressing of the green ceramic body  35  (FIG. 10) into the ceramic body  50  (FIG.  11 ), the green ceramic body shrinks about 50% along its thickness and the same occurs with the line inserts  88 . Hence, the length L 2  (FIG. 19) is chosen to be about equal to twice the thickness T of the final or ultimate electrostatic chuck made by the process of the present invention. Further, the hot pressing of the line inserts  88  with the green ceramic body  35  (FIG. 10) hot presses or sinters the etchable lines  85  into substantially wires. 
     The thermal transfer gas inlet channels provided by the process embodiment s described above will provide gas inlet channels having a diameter, or cross-section, of about 100 micrometers, and hence the volume of thermal transfer gas, such as helium or argon, supplied through these gas inlet channels will be of such small volume that the transfer gas will not have a tendency to become ionized and the above-described prior art arcing problem will be overcome. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detailed herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.