Abstract:
A cathode assembly having a pedestal and a detachable susceptor. Various contact assemblies containing a canted spring are utilized to make electrical connection between the pedestal and detachable susceptor. The canted spring has coils that are tilted in one direction and joined end to end to form a doughnut shape. Such a spring creates multiple parallel self-loading electrical connections via the turns of the spring. The turns act like electrical wires to ensure reliable RF electrical energy transfer. The canted spring contact of the present invention allows for flat contact between the pedestal and the chuck.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to semiconductor wafer processing. More particularly, the invention relates to a cathode assembly with a detachable electrostatic chuck for retaining a wafer in a semiconductor wafer processing system. 
     2. Description of the Background Art 
     In semiconductor wafer processing equipment, cathode assemblies are often used as substrate supports (also known as susceptors) to retain wafers within the equipment during processing. The susceptor is typically mounted to a pedestal in a semiconductor wafer processing chamber. These susceptors find use in etching, chemical vapor deposition (CVD), physical vapor deposition (PVD) and preclean applications. In many of these applications, the susceptor contains a cathode electrode that can be biased with direct current (DC) or radio frequency (RF) voltage to enhance the process being performed in the chamber. Bias voltage is typically supplied to the cathode by an external power supply through a cable and appropriate feedthroughs. In these applications, the wafer is often subject to bombardment by energetic particles from a plasma. Such energetic bombardment heats the wafer, susceptor and pedestal to high temperatures, typically as high as 175° C. and sometimes as high as 500° C. Since excessive heating of the wafer is often undesirable, the susceptor and pedestal contain cooling mechanisms. Typically, the susceptor contains ports and surface channels for an inert backside gas that is used as a heat transfer medium between the wafer and the susceptor. Additionally, the pedestal contains a cooling plate having channels and tubes for circulating a cooling fluid such as water. The combination of backside gas and a cooling plate within the pedestal is generally sufficient to moderate the wafer temperature. 
     A particular type of susceptor is an electrostatic chuck. Electrostatic chucks secure a semiconductor wafer by creating an electrostatic attractive force between the wafer and the chuck. A voltage, applied to one or more insulated electrodes in the chuck, induces opposite polarity charges in the wafer and electrodes respectively. The opposite charges pull the wafer against the chuck, thereby retaining the wafer. For example, in a bipolar ceramic chuck, the wafer rests flush against the surface of a ceramic chuck body as chucking voltages of opposite polarity are applied to two chuck electrodes that are embedded in the chuck body. Because of the semiconductive nature of the ceramic material, charges migrate through the ceramic material and accumulate proximate contact points between the wafer and the surface of the chuck body. Consequently, the wafer is primarily retained upon the chuck by the Johnsen-Rahbek effect. Such a Johnsen-Rahbek chuck is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992 and U.S. Pat. No. 5,463,526 issued Oct. 31, 1995. 
     An improvement in the design of susceptors is a detachable electrostatic chuck such as that shown and described in commonly assigned U.S. application Ser. No. 09/071,784 filed May 1, 1998. Such a chuck is secured to a pedestal but easily removable to facilitate repair and replacement of the chuck. Such a chuck design must incorporate a releasable electrical connection between the cathode or chucking electrodes and their respective cables. 
     Prior art connections made use of “banana” plugs that have a male connector with one or more resilient contact portions that fits into a cylindrical female connector. Unfortunately, repeated connection and disconnection bends the male connectors out of alignment. Consequently, the connectors apply stress to the ceramic in which they are embedded causing cracking of the ceramic. One solution to this problem, is to use spring loaded connectors. For example, commonly assigned U.S. patent application Ser. No. 09/071,784 describes a spring loaded plate that contacts the bottom of the electrostatic chuck to achieve an RF connection. Although this type of connector works well for a DC bias, the impedance of the coil spring produces a sizable voltage drop when a high frequency (e.g. 13.56 MHz) RF voltage is applied. 
     Also, in a plasma environment, the hardware used to secure the chuck to the pedestal is exposed to attack from energetic particles within the plasma. Furthermore, a uniform chucking force depends upon a uniform distribution of contact points between the backside of the semiconductor wafer and the chuck surface. Since the contact point distribution varies from wafer to wafer, similar wafers are not chucked in the same manner. Furthermore, wafer backside materials may vary and consequently cause differences in the chucking force across the wafer as well as from wafer to wafer. As such, the magnitude of the chucking force and its uniformity depends on wafer backside morphology and wafer backside composition. 
     Therefore, a need exists in the art for a cathode assembly with a detachable ceramic susceptor, suitable for use in an RF plasma environment, that reduces the chucking force dependence upon wafer backside morphology and composition. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by the present invention of a cathode assembly having a detachable susceptor containing a pair of coplanar embedded electrodes wherein RF and DC voltages are applied to both the electrodes. The susceptor has an interconnect electrode deposited on its surface that serves as a center tap electrode to balance positive and negative voltages applied to two embedded electrodes. Pads deposited on the susceptor surface and interconnect electrode provide a minimal contact area (MCA) structure that supports a semiconductor wafer at a predefined distance above the susceptor surface. The cathode assembly comprises a pedestal and detachable susceptor secured by an electrically floating clamp ring. A ceramic skirt protects the clamp ring against plasma damage. 
     The electrodes are releasably coupled to RF cables through spring loaded contact assemblies in the pedestal. Each contact assembly has a resilient contact element that provides multiple parallel self-loading electrical connections between a contact plate on the bottom of the susceptor and a plunger electrode in the pedestal. 
    
    
     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 partially exploded view of a cathode assembly of the present invention; 
     FIG. 2 is a partial cross sectional view of a semiconductor processing chamber that employs the cathode assembly of the present invention; 
     FIG. 3 is a plan view of a ceramic susceptor of the cathode assembly of the present invention; 
     FIG. 4 is a cross sectional view of the ceramic susceptor taken along line  4 — 4 ; 
     FIG. 5 is a partially exploded view showing the contact assembly of the cathode assembly of the present invention; and 
     FIG. 6 depicts a schematic drawing of a semiconductor wafer processing chamber that employs the cathode assembly of the present invention. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     FIG. 1 depicts a cathode assembly  100  of the present invention. The cathode assembly  100  is best understood by simultaneously referring to FIGS. 1 and 2. The cathode assembly  100  is used during plasma processing of a semiconductor wafer  101  (FIG.  2 ), e.g., etching, physical vapor deposition (PVD) and plasma cleaning. The cathode assembly  100  comprises a pedestal  110  and a susceptor  120 . A clamp ring  130  circumscribes and secures the susceptor  120  to the pedestal  110 . 
     The pedestal  110  has a basin shaped piece, known as a dog dish  111 , that serves as the basic structural support for the cathode assembly  100 . The dog dish  111  is typically made from a robust metal such as stainless steel. A flexible bellows  115 , welded to the dog dish  111 , isolates a vacuum above the pedestal  110  from an atmosphere within the pedestal  110 . The bellows  115  further serves to electrically ground the dog dish  111 . A basin shaped insulator plate  112  sits inside the dog dish  111 . The insulator plate  112  is preferably made from high strength, low dielectric constant material such as a plastic. A disk shaped cooling plate  113 , disposed within the insulator plate  112 , supports the susceptor  120 . The insulator plate  112  electrically isolates the cooling plate  113  from the dog dish  111 . An O-ring  170  disposed in an O-ring groove  171  prevents the atmosphere beneath the pedestal  110  from leaking between the dog-dish  111  and the insulator plate  112 . 
     Details of the cooling plate  113  are shown in FIG.  2 . Preferably, the cooling plate  113  is manufactured from a high thermal conductivity material such as copper. Cooling tubes  182  disposed in channels  184  carry a cooling fluid such as water or ethylene glycol. Heat from the susceptor  120  is conducted through the cooling plate and carried away by the cooling fluid. O-ring  172  disposed in an O-ring groove  173  prevents the atmosphere within the pedestal  110  from leaking between the insulator plate  112  and the cooling plate  113 . A temperature sensing device such as a thermocouple  186  can be attached to the cooling plate for measuring a temperature of the pedestal  110  and/or susceptor  120 . 
     The cooling plate  113  is secured to the pedestal  110  by a number of (e.g. 12) bolts  160 . The bolts  160  fit through clearance holes  162  in the insulator plate  112  and thread into tapped holes  163  in the dog dish  111 . An insulating cup  164  isolates each bolt  160  from electrical contact with the cooling plate  113 . The insulating cup  164  is made from a plastic material such as Vespel®. Vespel® is a registered trademark of the Dupont Corporation of Newark, Delaware. The insulating cup  164  is received in a bore  118  in cooling plate  113 . An outer shelf  166  on the cup  164  engages an inward projecting lip  117  of the bore  118 . The insulating cup  164  has large and small central bores  167   a  and  167   b  respectively. A head  161  of the bolt  160  is received in the large central bore  167   a  and rests against an inner shelf  168  at the intersection of the large and small central bores. As each bolt  160  is tightened, the head  161  presses against the inner shelf causing the outer shelf to apply pressure to the cooling plate at the lip  117 . The pressure applied to the cooling plate  113  compresses the O-rings  170  and  172 . A dielectric cap  169 , disposed in the large central bore  167   a , covers each bolt  160  to insulate the bolt from the ceramic puck. 
     The susceptor  120  can be any type of susceptor typically used in a semiconductor processing chamber such as an electrostatic chuck, a mechanical chuck, a heater or a bias pedestal. The susceptor generally comprises a puck shaped body  121  made from a ceramic material such as aluminum nitride, boron nitride and the like. The ceramic body  121  has a support surface  123  and a peripheral flange  129 . During use, a substrate such as a semiconductor wafer  101  rests on the support surface  123 . A backside gas tube  124  and port  125  supplies backside gas that fills interstitial spaces between the wafer  101  and support surface  123  to promote thermal conductivity between the wafer and the susceptor  120 . Backside gas flows through channels  128  in the support surface  123 . The channels  128  fan out radially from the gas port  125 . The channels  128  are between approximately 1 and 2 millimeters wide and approximately 100 microns deep. An O-ring  174  disposed in an O-ring groove  175  in the cooling plate  113  prevents backside gas from leaking between the cooling plate  113  and the susceptor  120 . 
     Further details of the susceptor are depicted in FIGS. 3 and 4. In particular, one or more electrodes  315  are embedded in the ceramic body  121  of the susceptor  120 . In a preferred embodiment of the invention, the susceptor  120  is a bipolar electrostatic chuck having two chuck electrodes  315  that, together, also serve as a cathode. Specifically, the electrodes  315  comprise an inner circular electrode  315   a  and an outer ring shaped electrode  315   b  shown in phantom in FIG.  3 . 
     The support surface  123  has a number of pads  310  that support the wafer  101  (not shown) while providing minimal contact area (MCA) with the wafer backside  103 . The MCA pads  310  rise approximately 2.5 microns above the support surface  123 . The pads  310  are typically fabricated from a material different from that of the support surface  123 . Exemplary materials include titanium, titanium nitride, stainless steel and the like. Certain pads  312  are connected by an interconnect electrode  308  that lies on top of the support surface  123 . The pads  312  on top of the interconnect electrode  308  provide multiple points of contact to the wafer. Such pads are described in commonly assigned U.S. Pat. No. 5,656,093, issued Aug. 12, 1997 which is incorporated herein by reference. 
     The interconnect  308  is typically made of titanium and has a thickness of approximately 0.5 microns. The interconnect  308  is typically fabricated (in plan view) in the shape of two J-shaped hooks with their respective shanks joined back to back. Such a shape evenly distributes the connected pads  312  over the support surface  123  and ensures that at least one connected pad  312  provides a point of electrical contact between the backside  103  of the wafer  101  and the interconnect  308  if the wafer  101  is bowed. For example, if the wafer  101  is bowed higher at the center than at the rim, at least one MCA pad  312  near the rim will make contact with the backside  103  of the wafer  101 . If the wafer  101  is bowed higher at the rim than at the center, at least one MCA pad  312  near the center will make contact with the backside  103  of the wafer  101 . 
     A DC power supply  402  biases the inner electrode  315   a  with respect to the outer electrode  315   b . A center tap  404  of the power supply  402  is coupled to the interconnect electrode  308  to balance the electrostatic force applied to the wafer  101 . This type of force balancing is disclosed in commonly assigned U.S. Pat. No. 5,764,741, issued Jun. 9, 1998. Specifically, the power supply  402  biases the inner electrode  315   a  between  200  and  400  volts positively with respect to the interconnect  308 . The outer electrode is biased an equal amount negatively with respect to the interconnect  308 . An RF power supply  406  biases the embedded electrodes  315   a  and  315   b  with a signal having a frequency of approximately 13.56 MHz. The RF supply  406  is connected to the electrodes  315   a  and  315   b  through a blocking capacitor  420 . 
     The electrodes  315   a ,  315   b  and  308  are coupled to the power supplies  402  and  406  by conductive rods  415   a ,  415   b  and  408  respectively. Each electrode is electrically coupled to its respective cable by a contact plate  126  located on a bottom surface  410  of the susceptor  120 . Referring back to FIG. 2, the contact plate  126  is coupled to an electrical contact assembly  150  disposed in a bore  116  in the cooling plate  113 . The contact assembly is designed to facilitate both good electrical contact and quick removal of the susceptor  120 . Such a contact assembly is disclosed in commonly assigned U.S. patent application Ser. No. 09/126,859, filed Jul. 31, 1998 which is incorporated herein by reference. An O-ring  176 , disposed in an O-ring groove  177  in the cooling plate  113  radially adjacent the bore  116 , prevents the atmosphere within pedestal  110  from leaking between the cooling plate  113  and the susceptor  120 . 
     Each contact assembly  150  (only one of three is shown) comprises an insulating housing  152  an insulating sleeve  154 , a plunger electrode  156  and a toroidal canted spring  158 . The housing  152  and sleeve  154  circumscribe the plunger electrode  156  to electrically isolate it from the cooling plate  113 . A coiled spring  157  urges the plunger electrode towards the contact plate  126 . The canted spring  158  forms a plurality of electrically parallel self loading electrical contacts between the plunger electrode  156  to the contact plate  126  when the two are pressed together. The electrical connection between the plunger electrode  156  and the contact plate  126  has a low impedance. The low impedance reduces the RF voltage drop across the connection, thereby improving the efficiency of RF energy transfer and reducing the likelihood of arcing. 
     Further details of the electrical connection of the subject invention are depicted in FIG. 5. A resilient contact strip  155  couples each plunger electrode  156  to its respective RF cable  415   a ,  415   b  or  408  while allowing for movement of the plunger electrode. The RF cables and resilient contact strips  155  are fixed in a contact housing  502  disposed below the pedestal  110 . The contact strip is made from a highly conductive resilient material such as beryllium copper. A first end  508  of the contact strip  155  is attached to the plunger electrode by screw  159 . A second end  510  of the contact strip  155  is soldered to one of the conductive rods  415   a ,  415   b  or  408 . A cover plate  504  protects the resilient contacts  155 . The contact housing and cover plate are typically made of a dielectric material such as plastic. Conventional means such as bolts  506  secure the cover plate to the contact housing  502 . 
     Thermal contact between the cooling plate  113  and the susceptor  120  is enhanced by a foil  180  situated on top of the cooling plate  113 . The foil is made from a thermally conducting material such as copper. A plurality of bumps  181 , embossed in the foil  180 , enhance the thermal contact between the susceptor  120  and the cooling plate  113 . The bumps  181  provide a plurality of points of contact for heat transfer and close a gap  178  necessitated by the O-rings  174  and  176 . The O-rings  174  and  176 , foil  180  and canted spring  158  are compressed by a force applied to the susceptor by the clamp ring  130 . Under operating conditions, the gap  178  is evacuated and the only path for significant conductive heat transfer between the susceptor  120  and the cooling plate  113  is through the foil  180 . Note that for clarity of the invention and ease of viewing, the gap  178  and bumps  181  shown in FIG. 2 appear much larger than they actually are. 
     Referring to FIGS. 1 and 2, the clamp ring  130  engages the peripheral flange  129  of the susceptor  120  to secure the susceptor  120  to the pedestal  110 . The clamp ring  130  is secured to the insulator plate  112  without physically contacting the dog dish  111 . As such, the clamp ring  130  is electrically isolated from ground. A plurality of bolts  132  (only one shown in FIG. 1 for clarity) extend through a corresponding plurality of clearance holes  131  in a plurality of inward projecting portions  133  of the clamp ring and a corresponding plurality of clearance holes  134  in the insulator plate  112 . Cutouts  127  on the flange  129  accommodate the inward projecting portions  133 . The clearance holes  134  have counterbores  136  as seen in FIG. 2 that receive nuts  137 . The bolts  132  thread into the nuts  137 . A plurality of resilient fingers  135  extend radially inward from the clamp ring  130  between the clearance holes  131 . When the bolts  132  are tightened, the resilient fingers  135  engage the flange  129  and bend, thereby exerting a compressive force on the flange  129 . The compressive force on the flange  129  causes the susceptor  120  to compress the foil  180  against the cooling plate  113  and the cooling plate  113  against the insulator plate  112 . The bumps  181  on the foil  180  are thereby compressed ensuring good thermal contact between the cooling plate  113  and the susceptor  120 . The insulator plate  112  must be made from a material having a sufficient resilience and mechanical strength to accommodate the compressive force without cracking or permanently deforming. Suitable materials include high strength plastics such as polyetheretherketone (PEEK) and polyaryletherketone, also called PEEK™. PEEK™ is a trademark of Victrex plc, of Thornton Cleveleys, UK. 
     Energetic ions from a plasma can damage the clamp ring  130 , bolts  132 , and susceptor  120 . These parts of the cathode assembly  100  are circumscribed by a skirt  140  to protect said parts from exposure to plasma. The skirt  140  is made from a ceramic material such as alumina. The skirt  140  is in the shape of a ring with a horizontal roof  141  that transitions into a downward projecting sidewall  142 . The sidewall  142  protects the clamp ring  130  and flange  129  of the susceptor  120  against plasma attack from the side. The sidewall  142  has an extension  143  that partially covers the dog-dish  111  thereby protecting any gap between the pedestal  110  and the susceptor  120 . The roof  141  protects the clamp ring  130  and bolts  132  against plasma attack from above and from the side. A number of (e.g. four) retaining pins  144  are received in a corresponding number of holes  145  in the roof  141  of the skirt  140 . The retaining pins  144  restrict horizontal motion the wafer  101 . Alignment of the level of support surface  123  with the horizontal roof  141  of the skirt  140  is very critical. The support surface must be aligned to prevent plasma attack of an overhanging portion of the wafer  101 . Alignment can be adjusted by, for example, custom made spacers that rest on the flange  129 , shims under the skirt  140  or a screw mechanism. Ideally the support surface  123  and the horizontal roof  141  of the skirt  140  are coplanar. 
     The cathode  100  assembly is situated within a process chamber  600  schematically depicted in FIG.  6 . The process chamber  600  is, for example, a Preclean II/e chamber manufactured by Applied Materials of Santa Clara, Calif. The susceptor  120  supports the wafer  101  during plasma cleaning. The process chamber  600  generally comprises a set of walls  602 , a floor  604  and a domed lid  606  that define a volume  607 . The chamber walls  602  and floor  604  are typically made of aluminum. The lid  606  is typically made of quartz. An exhaust system  609  controls pressure in the volume  607 . The cathode assembly  100  is connected to the floor  604  by the bellows  115 . An induction coil  608 , supported by the chamber wall  602 , surrounds the lid  606 . A coil power supply  610  provides RF power to the coil  608  within the chamber  600 . The coil power supply  610  provides approximately 100 to 600 watts at a frequency of between approximately 400 kilohertz (KHz) and 5 megahertz (MHz). An RF shield  605  surrounds the induction coil and prevents RF radiation from escaping into the environment outside the chamber  600 . 
     A DC voltage, from the DC power supply  402  energizes the chuck electrodes  315   a  and  315   b  causing an electrostatic attraction between the wafer  101  and the susceptor  120 . Backside gas from a source  616  flows through the tube  124  and the port  125 . The backside gas fills interstitial spaces  618  between the wafer backside  103  and the chuck surface  123 . The backside gas promotes heat transfer between the wafer  101  and the chuck  120 . The foil  180  promotes heat transfer between the susceptor  120  and the cooling plate  113 . A fluid, flowing in the cooling tubes  182 , exchanges heat between the cooling plate  113  and the environment outside the chamber. 
     A process gas source  611  supplies an inert gas, such as argon, to the chamber. The coil  608  is energized to ionize the inert gas to form a plasma  612 . A grounded, cylindrical aluminum shield  614  confines the plasma  612  to a region above the cathode assembly  101 . Ions from the plasma  612  bombard the surface of the wafer  101 . The cathode RF power supply  406  provides approximately 100 to 600 watts of power at a frequency of between approximately 1 and 100 MHz, preferably 13.56 MHz, to the cathode electrodes  315   a  and  315   b  in the susceptor. The power applied to the cathode controls the etch rate and the power applied to the coil controls the wafer bias. Optionally, the cathode and coil power together can be used to control the wafer bias. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.