Welded susceptor assembly
FIELD OF THE INVENTION 
     This invention relates to the field of susceptor assemblies as generally used to process semiconductor substrates. 
     BACKGROUND OF THE INVENTION 
     A susceptor plate for processing semiconductor substrates (wafers) using plasma enhanced chemical vapor deposition (PECVD) and other similar processes must be in firm contact with an electrical ground to prevent warpage of the susceptor plate. When the susceptor plate ground connection is interrupted, the build-up of plasma energy causes differential thermal heating and warpage of the susceptor plate. When the susceptor warps, its edges generally rise so that the plate forms a bowl shape. The position of a substrate supported on such a warped susceptor no longer meets process criteria, and can cause wafers processed on such a susceptor to be rejected. 
     Repeated heating and cooling of clamped ground connections during normal substrate processing cycles makes it very difficult to maintain a firm ground connection because repeated expansion and contraction or breakage tends to loosen clamped pieces. The gases used in processing semiconductor wafers are often corrosive and work their way into all nooks and crannies, especially spaces between fasteners and the items being fastened, e.g. the ground connection. 
     The presence of corrosive gas and the repeated thermal cycling, differential thermal expansion, and relaxation creep of metals at elevated temperatures experienced during processing tends to degrade the electrical ground path, usually passing through an aluminum tube clamped to the back of the susceptor. Such conditions over time reduce and relax the clamping force until the electrical ground connection is no longer viable. This problem is exacerbated by the fine detail and precise assembly which is required of existing susceptor assemblies. In this day and age, the increasing demands for accuracy in semiconductor processing require that susceptor warpage be avoided so that coating and etching of semiconductor wafers be done as precisely and repeatably as possible without the susceptor warpage that occurs when the ground connection is not secure. 
     An illustration of an existing susceptor assembly in a typical processing chamber will highlight the problem. The general configuration of a vapor deposition processing chamber is shown in FIG. 1 (a susceptor assembly configuration according to the invention is pictured). A processing chamber 20 contains a susceptor assembly 31 supporting a wafer 30. Process gas flows through holes in an electrically biased gas distribution plate 22 towards the face of the wafer (substrate) 30 supported by the susceptor assembly 31. The gas distribution plate 22 is often energized by the use of RF power which causes the processed gas to form a plasma. The susceptor disk 32 in the prior art (as shown in FIGS. 2, 3 and 4) is grounded through an aluminum tube extending through a hollow passage in a ceramic susceptor arm 36. The aluminum tube is then grounded (usually by a ground strap 42) to the usually cool process chamber wall. 
     During processing, the susceptor disk 32 is heated from its backside (the bottom 28 as shown in FIG. 1) by radiant heat from heating lamps 24 shining through a sealed quartz window 26 and a ceramic backing plate 34. The susceptor temperature reaches to approximately 475 to 500 degrees Celsius. 
     An example, of such a prior an susceptor assembly is shown in FIGS. 2, 3, and 4. The susceptor assembly includes a aluminum susceptor plate 32 having a susceptor hub 68. The plate 32 is backed by a ceramic plate 34 having Swiss cheese type holes (e.g. see FIG. 5) to selectively control the susceptor plate&#39;s exposure to radiant heat from the heat lamps 24. A susceptor hub 68 integral with the susceptor plate 32 includes an alignment/grounding blade 69, a thermocouple receiving hole 59, and several holes for receiving fastening studs 73. 
     The hub 68 is supported by a ceramic susceptor arm 50. The arm 50 includes a tubular passage to protect and guide an aluminum grounding tube 80. The tube 80 extends through the passage but only a partial tube wall extension 81 extends into the hub end 51 and into contact with the bottom of the end of the susceptor hub blade 69. A thermocouple lead 40 is routed through the tube 80 and terminates with the thermocouple end 41 in the thermocouple receiving hole 59. 
     The susceptor hub blade 69 is aligned to a blade receiving slot 54 and secured to a web 52 in hub end 51 of the arm 50 by integral intermediate flange nuts 76 (only one is shown in FIG. 4) on the studs 73 which are carefully tightened to a predetermined torque. The end of the hub blade is shaped to match the outside of the tube extension 81. To prevent collapsing the tube extension 81 and pinching the thermocouple lead 40, a hollow half cylinder shaped mandrel 71 is located inside the tube extension 81. A clamping block 72 slips over the stud 73 ends and clamps the tube extension 81 between the mandrel 71 and the end of the of the hub blade 69 as stud nuts 74 are tighten to a specified torque in counterbore openings 75 in the bottom of the block 72. There are counterbores in the top of the block 72 which provide clearance for the middle nut flanges 76 of the studs 73, so the block 72 is secured against the flat bottom of the web 52. FIG. 4 shows the block 72 cut away to show: the extension tube 81 clamping arrangement, the middle nut flange 76 of the right stud 73 (also cut away), and the left stud end and nut 74 in the counterbore 75. 
     The hub end 51 includes a bottom opening recess 55 to receive a ceramic disk shaped circular cover 56 (FIG. 3) to shield the contents of the hub from direct exposure to radiant heating. The cover 56 is retained in the recess 55 by a ceramic pin 58 placed in two pin receiving holes 57 (FIG. 4) aligned across the recess opening 55. 
     A ceramic collar 48 surrounds the susceptor hub 68 at the top to assist in protecting the thermocouple and hub from the processing chamber environment. This pin sometimes falls out exposing the hub pieces to extreme temperatures resulting from direct exposure to radiant heating. 
     An aluminum susceptor arm end support 84 is clamped to the support end of the arm 50 to support it from the lift mechanism 38 (FIG. 2). A ground rope 42 is welded to the support end of the aluminum tube 80 and is routed into contact with the end support 84 on its way to a ground connection on the wall of the processing chamber 20. 
     The thermocouple lead 40 is routed from a threaded thermocouple receiving hole 59 in the back of the susceptor plate 32 between the mandrel 71 and the clamping block 72, through the tube 80, and through a vacuum seal in the central core of the susceptor lift mechanism 38. 
     To work properly, the above pieces must be carefully assembled. The assembly or disassembly of a large number of pieces according to a detailed assembly procedure unnecessarily complicates the configuration and increases the chance that an initially created ground connection will not be reliable after many processing cycles. When the susceptor is not grounded, the susceptor temperature builds up unevenly and warpage begins to occur. 
     When potentially thousands of wafers are processed through one processing chamber having a single susceptor assembly, even small variations in the process conditions resulting from susceptor warpage can create an operating problem. Additional monitoring (quality assurance-inspection) is needed to assure that susceptor warpage does not affect wafers being processed. 
     SUMMARY OF THE INVENTION 
     This invention provides for making a continuous uninterrupted RF ground connection by using a ground wire welded to the susceptor. The ground wire connects the susceptor to the process chamber ground (wall). The ground connection is shielded from the temperature extremes by being routed in a channel of the susceptor arm, the channel having a removable cover. The susceptor arm supporting the susceptor is a one piece ceramic material (alumina) having a low coefficient of expansion and good insulating qualities. 
     A configuration according the invention provides that an aluminum wire rope is welded to lugs (or terminals) at either of its ends. The lug at one end includes one or more side flanges or wings extending laterally from the rope. The end of the lug extends beyond the flange. The end of the lug extending beyond the flange is configured to fit tightly in a lug receiving hole in a hub on the back of the susceptor. The side flanges are welded to a back surface of the susceptor hub. The weld is located where distortion, if any, of the hub in the localized area of the weldments does not warp the dimensions of pieces supporting a wafer. The lug at the other end of the grounding rope has a cross section which is configured to pass through an opening in the susceptor arm supporting the susceptor hub. The wire rope can then be laid in the open channel of the arm and then covered as it is being routed to the ground. To assure a ground connection, a technician needs only to make one connection, i.e. secure the wire rope end to a ground terminal on the cool wall of the processing chamber. 
     A thermocouple receiving hole is located in a back surface of the susceptor to provide temperature information. The thermocouple wiring is routed from a threaded hole in the back of the susceptor through an opening in the web in the hub end of the susceptor arm and through the enclosable channel and through a seal at the center of bellows which accommodates up and down movement of the susceptor assembly. 
     The enclosable channel is covered by a channel cover which closes the channel and blocks direct radiation from the heat lamps. The cover is paddle shaped to match the configuration of the susceptor arm and slides laterally into grooves in the side of the channel to cover and enclose the contents of the channel. 
     In one configuration, the hub end of the ceramic arm includes a keyhole shaped slot which receives a susceptor hub alignment blade. The blade aligns the susceptor to the arm. A circle end of the keyhole slot allows the grounding rope (now integrally connected to the susceptor hub) with a keyhole shaped end lug to pass through the slotted opening during assembly. 
     The metallic susceptor, bellows, and sealing sleeve at the bellows end of the susceptor arm are all secured to the ceramic susceptor arm by shoulder bolts mounted to split conical spring washers which secure the pieces together special without special torquing requirements and which are able to maintain pressure while accommodating relatively large dimensional changes. 
     To increase corrosion resistance to the corrosive environment of the processing chamber, the shoulder screws should be electropolished. The surfaces of aluminum pieces (except those across which electrical conductivity is required) should be anodized to minimize deleterious effects. Stainless steel and Inconel pieces should be finished with a nickel sulfumate plating. 
     The configuration as described according to the invention provides a susceptor assembly configuration which assures that the susceptor will be grounded at all times. It also and minimizes the dependence on a technician&#39;s assembly technique when assembling components and thereby eliminates the problems of the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section of a processing chamber showing a susceptor assembly in context; 
     FIG. 2 shows a cross section of a prior art susceptor assembly; 
     FIG. 3 shows a close up of the hub end of the susceptor assembly of FIG. 2; 
     FIG. 4 shows a cut away perspective view of the hub end of the susceptor of FIG. 2; 
     FIG. 5 shows an exploded view of the susceptor assembly according to the invention; 
     FIG. 6 shows a perspective view of the susceptor arm and its channel cover piece according to the invention; 
     FIG. 7 shows an assembly view of the ground wire assembly in relation to a susceptor hub according to the invention; 
     FIG. 8 shows a cross-section through the center of the susceptor of hub of FIG. 7 taken at 8--8. 
     FIG. 9 shows a top view of the hub of FIG. 8 viewed from 9--9; 
     FIG. 10 shows a top view of a susceptor arm according to the invention; 
     FIG. 11 shows a cross-sectional view of FIGS. 10 and 12 taken at 11--11; 
     FIG. 12 shows a bottom view of a susceptor arm according to the invention; 
     FIG. 13 shows a cross-sectional view of FIG. 11 taken at 13--13; 
     FIG. 14 shows a partial cross-section of a close-up view of a susceptor assembly according to the invention; 
     FIG. 15 shows the cross-sectional of FIG. 14 taken at 15--15; 
     FIG. 16 shows a shoulder screw; 
     FIG. 17 shows a cross-section of a slotted conical spring washer of FIG. 18 taken at 17--17; and 
     FIG. 18 shows a top view of a conical spring washer. 
    
    
     DETAILED DESCRIPTION 
     As illustrated in FIG. 1, susceptor assemblies 31 are commonly used in processing chambers 20 to support a wafer 30 opposite a gas distribution plate 22. This susceptor assembly 31 moves up and down to receive and present the wafer 30 to and from lift fingers (not shown) which lift the wafer 30 from a robot blade (not shown) that transfers the wafer into and out of the processing chamber 20. The up and down movement of the susceptor assembly 31 is controlled by a bellows assembly 38 which has at its core a rigid, hollow tube (pipe) 198 connected at its bottom to a susceptor lift mechanism (not shown) which raises and lowers the susceptor assembly 31 within the process chamber according to process requirements. 
     The susceptor disk 32, as illustrated in FIGS. 1 and 5, has a bottom covered with a perforated ceramic insulating disk 34 patterned like Swiss cheese. The susceptor disk 32 and the insulating disk 34 are cantilevered over the quartz window 26, by a susceptor arm (36, 131). Radiant heat lamps 24 enclosed by a reflector/shield 28 heat the bottom side of susceptor 32 and the insulating disk 34. The temperature of the susceptor is raised and maintained in close proximity to wafer processing temperatures, thereby preventing or minimizing differential stresses and strains associated with changing temperatures. 
     The susceptor disk 32 and insulating disk 34 are supported by a one piece susceptor arm 131. The susceptor arm is made of ceramic material (preferably alumina) and includes a channel 155 along its length to carry a grounding conductor (a wire rope 123) and a thermocouple lead 178. A set of grooves 163 in the bottom sides of the channel 155 slidably receive a channel cover 166 to block radiation from the radiant heat lamps 24 from directly shining on the thermocouple and grounding wiring (FIG. 6). 
     As can be seen in FIG. 5 and 7, the susceptor assembly 30 includes a susceptor disk 32 (the same part as prior art susceptor plates or disks 32) preferably of aluminum is integral with a susceptor hub 94 with susceptor attachment holes 106, 108 and susceptor lift pin holes 90. The susceptor attachment holes 106, 108 have threads which are lined with helicoils which have been nickel sulfumate plated with 0.0003-0.0005&#34; thick plating for superior corrosion resistance. 
     The ground rope assembly 121 as shown in FIG. 7 includes a wire rope or cable 123 preferably made of aluminum. It is attached to a double winged terminal 125 at the susceptor end of the wire rope and to a key shaped lug terminal 129 at the process chamber end of the ground rope 123. To assure a tight electrical bond between the ground rope 123 and the double winged terminal 125 and keyhole shaped lug 129, each double winged terminal 125 and lug 129 are welded to the wire rope at the distal ends of the connection between the wire rope and the terminal and lug. The weld connecting the wire rope 123 to the double winged terminal 125 is at the outside end of the terminal and is designated 126. Similarly the weld at the outside end of the lug terminal 129 is identified by the number 130. Both welds connect the circumference of the cylindrical part of the terminal or lug to the circumference of the aluminum rope. This welding method minimizes the amount of distortion and differential thermal stress generated between pieces. When welding is done using electron beam techniques the amount of heat input and warpage is negligible. The double winged terminal 125 and lug 129 are first welded to the wire rope 123 and then the double winged terminal 125 is welded to the susceptor as will be described below, all prior to other assembly. The steps of a method for securing a ground connection for a susceptor are clear by following the assembly of the described pieces. 
     A close-up view of the susceptor hub 94 is shown in FIG. 7. A susceptor hub 94 includes a cylindrical susceptor hub body 100 descending from the bottom surface of the susceptor 32. The hub body 100 has flat 112 on one side to allow space for passage of the thermocouple lead through the web 147 of the hub end 133. The susceptor hub body 100 includes and end support surface 101 having attachment holes 106 and 108 therein equipped with helicoil inserts, as previously discussed. The susceptor hub body 100 also includes a ledge 104 which contains a hole 98 for receiving the welded end 126 of the double winged terminal 125 so that the cylinder of the terminal fits tightly within the receiving hole 98. Cross sectional and top views of the configuration of the terminal receiving hole 98 can be seen in FIGS. 8 and 9. In the case where the fit between the cylinder of the double winged terminal 125 and the ground wire receiving hole 98 is extremely tight, a release channel 99, (a half cylindrical groove in the side of the hole 98 located at 45° to the blade 110 axis) allows air to escape while the terminal with wings 125 is pushed into position. The double winged terminal 125 and hole 98 are configured so that the end of the terminal does not touch the bottom of the hole 98, and so that the movement of the terminal 125 into the hole 98 is stopped by the wings which extend out from the side of the terminal 125. The wings then rest on the wing ledge 104 of the hub body 100. Once the terminal 125 is in position, electron beam welding is performed at holes 127 in each wing. The welding is done within the holes to connect the terminal wings to the susceptor. When using electron beam welding the amount of heat input is very small and the body of the hub 100 easily dissipates the heat without warpage or distortion of the mating pieces. The terminal 125, the lug 129, susceptor, and wire rope are all made of aluminum preferable types 1100, 1100, 1100, and 1350 respectively. 
     A susceptor hub alignment blade 110 extends from the hub body 100 to align the susceptor arm 131. 
     During assembly the key (keyhole) shaped lug 129 and the attached wire rope pass through the perforated backing plate 34 and through a lug matching keyhole shaped slot of a web 147 located in the hub end 133 of the susceptor arm 131 so that the web 147 supports the susceptor hub at its support face 101. The keyhole shaped slot 143 (FIGS. 10, 11, 12,) allows the lug 129 to freely pass through to be routed down the channel 155 of the arm 131 to be grounded at the process chamber wall. 
     The susceptor 32 is held to the susceptor arm 131 by the two shoulder screws 188 with associated split conical spring washers 186. The shoulder bolt 188 and the conical spring washers 186 can be seen in FIGS. 16, 17, and 18. Slotted conical spring washers are described in U.S. Pat. application Ser. No. 08/094,674, filed Jul. 20, 1993, by Fodor et al. The inner diameter (I.D.) 220 of the conical spring washer closely matches the diameter of the shank 213 of the shoulder bolt 188 such that the head 214 of the shoulder bolt 188 will not pass through the inner diameter 220. The outer diameter (O.D.) 221 closely matches and/or is slightly smaller than the diameter of the counterbores 149 and 151 in the web 147. The shoulder bolts are completely threaded into susceptor attachment holes 108, 106 until the bolt is tight against the shoulder 212 between the shank 213 and the threads 211. The bottom of the screw head 214 is then resting on top of the thickness 216 of the conical spring flange and compresses the washer 186 until it is nearly flat (slightly more than the material thickness 217). As the washer 186 is being compressed, the washer gap 219 attempts to open to relieve the pressure. However, because the washer is located in a counterbore it cannot spread out, but is held tightly to elastically bend the washer ring. The use of a slot in a spring washer in this configuration increases the effective elastic range and life available when compared to using a conventional spring washer. For corrosion resistance and high strength the shoulder bolt 188 and conical spring washer 186 are constructed of Inconel X-750. The shoulder screw is electropolished with 0.0002-0.0003 (0.005 mm) of material removed while the conical spring washer is sulfumate nickel electroplated with a 0.0003-0.0005&#34; (0.010 mm) thick plating. 
     FIG. 13 provides a cross sectional end view taken at the center of the hub end 133 of the susceptor arm 131 facing the bellows end 135 of the susceptor arm. Counterbores 149 and 151 at the bottom of the screw holes 148 and 150 are sized to receive appropriately sized split conical spring washers 186 through which the shoulder screws 188 are screwed into the helicoils in the hub holes 108, 106 to thereby secure the susceptor disk 32 and hub 94 to the hub end 133 of the susceptor arm 131. The edge of the counterbores 149 and 151 at their edges open on the keyhole slot 143. A flange 145 extends up from the web 147 to act as a cowling surrounding the hub 94 of the susceptor. 
     When the susceptor disk 32 and hub 94 is attached to the hub end 133 of the susceptor arm 131 the cross-section seen in FIG. 13 changes to that seen in FIG. 15. The hub body 100 support face 101 contacts the top of the web 147 of the hub end and the susceptor hub alignment blade 110 fits into the keyhole shaped slot 143. The shoulder bolts 188 pass through the slotted conical washers 186 and tightly hold the susceptor 25 to the susceptor arm 131. 
     The thermocouple lead 178 connects to a thermocouple junction 176 which is located in a susceptor thermocouple hole 92 (FIGS. 8 and 14) in the back side of the susceptor disk 32. This susceptor thermocouple hole 92 is preferably threaded to receive a threaded end of the thermocouple lead 178. FIG. 14, shows an example of the routing of the ground wire assembly 121 and the thermocouple lead 178, which passes through the narrow end of the slot 143 at the other end of the susceptor hub line blade 110. The thermocouple lead 178 is looped within the hub end 133 to avoid bending the lead sharply and thereby enhancing its reliability at elevated temperatures as it is routed through the channel 155. 
     The susceptor arm 131 is configured to route the grounding rope 123 and the thermocouple lead 178 through the channel passage 155. Bevels 157, 159 in the ends of the channel passage 155, as shown in FIG. 11 minimize sharp corners which may damage the cables and provide a smooth transition for the wiring to enter and leave the channel 155. 
     In FIGS. 1, 10, and 11, a throat depression 172 of the arm 131 can be seen. This depression 172 provides a larger clearance between the edge of the susceptor disk 32 and the arm to reduce any conductive effects and sparking which might be induced. 
     As shown in FIGS. 6, 11, and 14, the cover receiving grooves 163 inside of the susceptor support arm channel 155 extend parallel to the throat of the support arm 131 and around the perimeter of the hub end 133 of the arm to the end of the greater than half-circle flange 153 on the bottom side of the hub end 133. This flange 153 can be seen in FIG. 11, to extend slightly past the center line of circular hub toward its distal end (FIG. 6) so that when the throat 170 of the channel cover 166 is introduced into the slot 163, it slides towards the bellow end 135 of the arm 131 until the edge of the circular end 168 of the cover 166 mates with the slot 163 in the flange 153. When the cover 166 is secured in the slot 163, the flange 153 holds the channel cover circular end 168 against an end flange 154 of the bottom of the hub end 133 of the susceptor arm 131. The channel cover 166 is sized to fully cover the channel 155 in the support arm but, as shown in FIG. 14, leaves open the cavity 137 of the bellows end 135 of the support arm. The wire rope grounding cable 123 passes through the open space at the bottom of the channel and a screw secures its lug 129 to the chamber at a location remote from the heated susceptor. 
     As illustrated in FIG. 14, outside the beam of the heating lamps, a ground rope assembly 121 connected to the back of the susceptor disk 32 and hub 94 is connected to the wall of the processing chamber 20 by a lug terminal 129 by a tightly secured screw. Since the wall of the processing chamber is relatively cool compared to process conditions the effects of thermal cycling are not as severe and therefore do not cause loosening of the connection. The thermocouple lead 178 is routed to the center of a bellows 181 via a wire guide groove 161 in the bottom of a cavity 137 at a bellows end 135 of the susceptor arm 131. 
     The thermocouple lead 178 is potted into and through a thermocouple potting plug 180 which is surrounded by a vacuum seal. The vacuum seal includes a flanged sleeve 182 mounted inside the pipe 198 inside the bellows 181. As can be seen in the exploded view of FIG. 5 and the cross-section of FIG. 14, the flanged sleeve 182 has an enlarged ring around the bottom end of the tube to present a flat surface to a flat washer 190, O-ring 192, flat washer 194 sandwich. When the potting adapter 180 is in place and a vacuum is applied in the processing chamber, the O-ring 192 is urged by outside ambient pressure to seal between the thermocouple potting adapter 180 and the inside of the rigid pipe 198. A spring 196 assists in the sealing. A ledge around inside of the pipe 198 supports the bottom end of the spring 196. 
     As shown in FIG. 5, the flange of the flanged seal 182 includes two tapped holes 184 for attachment to the channel end 135 of the susceptor arm 131. As can be seen in the cross-section of FIG. 14, the upper flange of the pipe 198 includes a recess to accommodate and envelop the flange of the sleeve 182 without interference. The flanged sleeve 182 is preferably constructed of 304 L stainless steel and is finished with a nickel sulfumate plating 0.0003-0.0005 (0.01 mm) thick. This plating prevents corrosion and extends the life of the assembly. The bellows are also nickel sulfumate plated to provide similar corrosion resistance and extend the life of the unit in the corrosive processing environment. 
     As can be seen in the FIGS. 5, 6, 10, and 12, two shoulder bolts 188 having slotted conical spring washers 186 slide in through holes 207 in a sleeve connection hole pattern 139 in the center of the bellows end 135 of the susceptor arm 131 and are threaded into the tapped holes 184 in the flange of the sleeve 182 to attach it to the susceptor arm 131. The through holes 207 have a counterbore 208 on their top side. Similarly, four through holes 204 in a bellows connecting hole pattern 141 include counterbores 205 around the holes as seen from the top side of the susceptor arm in FIG. 10. 
     Slotted conical spring washers 186 are placed in the counterbores 205, 208 (each conical spring washer 186 with its related should bolt 188 being sized according to its function). 
     The pieces are assembled as follows. The wire rope assembly, already welded to the susceptor is routed through the hub end 133 of the susceptor arm keyhole shaped slot 143. The thermocouple junction 176 is threaded into the thermocouple hole 92. The thermocouple lead 178 passes through the other end of the keyhole shaped slot 143. The susceptor hub 94 is then fastened to the ceramic arm 131 using split conical spring washers 186 held in position by shoulder screws 188. The ground rope 123 and thermocouple wire are routed through the arm channel 155. The thermocouple is routed and sealed at the center of the bellows lift mechanism 38 which is attached to the arm 131 by split conical spring washers 186 and shoulder screws. The end lug 129 of the ground rope 123 is securely attached to a ground terminal. The channel cover 166 is then slid into place. 
     While the invention has been described in regards to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.