Patent Publication Number: US-2017352565-A1

Title: Workpiece carrier with gas pressure in inner cavities

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to prior U.S. Provisional Application Ser. No. 62/352,695 filed Jun. 21, 2016, entitled ELECTROSTATIC CHUCK WITH GAS PRESSURE APPLIED TO INNER CAVITIES by Chunlei Zhang, et al., the priority of which is hereby claimed and U.S. Provisional Application Ser. No. 62/346,764 filed Jun. 7, 2016, entitled ELECTROSTATIC CHUCK WITH GAS PRESSURE APPLIED TO INNER CAVITIES by Chunlei Zhang, et al., the priority of which is hereby claimed. 
    
    
     FIELD 
     The present description relates to workpiece carriers for semiconductor and micromechanical processing and in particular to a carrier with gas pressure in inner cavities of the carrier. 
     BACKGROUND 
     In the manufacture of semiconductor chips, a silicon wafer or other substrate is exposed to a variety of different processes in different processing chambers. The chambers may expose the wafer to a number of different chemical and physical processes whereby minute integrated circuits are created on the substrate. Layers of materials which make up the integrated circuit are created by processes including chemical vapor deposition, physical vapor deposition, epitaxial growth, and the like. Some of the layers of material are patterned using photoresist masks and wet or dry etching techniques. The substrates may be silicon, gallium arsenide, indium phosphide, glass, or other appropriate materials. 
     In these manufacturing processes, plasma may be used for depositing or etching various material layers. Plasma processing offers many advantages over thermal processing. For example, plasma enhanced chemical vapor deposition (PECVD) allows deposition processes to be performed at lower temperatures and at higher deposition rates than in analogous thermal processes. PECVD therefore allows material to be deposited at lower temperatures. 
     The processing chambers used in these processes typically include a substrate support, pedestal, or chuck disposed therein to support the substrate during processing. In some processes, the pedestal may include an embedded heater adapted to control the temperature of the substrate and/or provide elevated temperatures that may be used in the process. 
     HAR (High Aspect Ratio) plasma etch uses a significantly higher bias power to achieve bending free profiles. In order to support HAR for dielectric etching, the power may be increased to 20 KW, which brings significant impacts on an ESC (Electrostatic Chuck). Many current ESC designs cannot survive such a high voltage which comes as a direct result of a high bias power. Holes designed into an ESC may suffer in particular. Moreover, an ESC may experience bond failures in the lift pin area when excess radicals erode the bonds. Another impact is that the ESC surface temperature changes at a higher rate. The heating of the ESC surface is directly proportional to the applied RF plasma power. The heat may also be a result of bond failure. In addition bowing of the wafer carried on the ESC and the charge build up on the wafer also makes wafer de-chucking more difficult. 
     Common processes use an ESC to hold a wafer with 2 MHz 6.5 KW plasma power applied to the wafer for etching applications. High aspect ratio (e.g. 100:1) applications use much higher plasma powers. An ESC is described herein that operates with a low frequency high power plasma voltage to generate a high wafer bias. The higher power will increase failures of the ESC due to the dielectric breaking down and due to plasma ignition in gas holes that are designed into the ESC. 
     SUMMARY 
     A workpiece carrier suitable for high power processes is described. It may include a top plate to support a workpiece, a lift pin to lift a workpiece from a top plate, a lift pin hole through the top plate to contain the lift pin, and a connector to the lift pin hole to connect to a source of gas under pressure to deliver a cooling gas, for example helium, to the back side of the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which: 
         FIG. 1  is a diagram of a thermal image of an ESC during a process in a plasma processing chamber in accordance with an embodiment of the invention; 
         FIG. 2  is a top view diagram of a puck on a top plate of an ESC in accordance with an embodiment of the invention; 
         FIG. 3  is a partial cross-sectional side view diagram of an ESC with gas pressure in lift pin holes in accordance with an embodiment of the invention; and 
         FIG. 4  is a cross-sectional side view of a lift pin and lift pin hole in a top plate in accordance with an embodiment of the invention. 
         FIG. 5  is a diagram of a plasma etch system including a workpiece carrier in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The described ESC withstands high power and high bias voltages. The described inventive ESC may use a lift pin feature to deliver helium (He) for backside wafer cooling and may also control the lift pin cavity pressure. Many ESC&#39;s use a separate channel near the center of the top puck to deliver helium (He) to the backside of the wafer for cooling. The He is applied at pressure at the bottom of the ESC and is pushed up through the top plate or puck of the ESC to the space between the puck and the wafer back side. The He holes may experience arcing under high voltage (RF power). As described herein He holes in the ESC may be reduced or eliminated. Design features on the surface of the ESC are also minimized to improve temperature uniformity. This reduces local cold spots. The ESC cost is reduced and yet has improved reliability. In order to deliver He to the wafer back side, the lift pin holes may be used. These holes are open to the wafer back side to allow the lift pin to contact the wafer back side. He may be pressed though the hole around the lift pin to the space between the puck and the wafer back side. 
       FIG. 1  is a diagram of a thermal image of an ESC  10  during a process in a plasma processing chamber. The central spot  12  corresponds to the location of the hole for helium cooling gases and the three peripheral spots  14  correspond to the location of the lift pin holes. As shown, the three lift pin areas get hotter because the bond is eroded locally. There are issues with the wafer processes in these hot spots and the bond between the puck and the support plate is eroded around the hot spots (lift pins). Pumping He through the three lift pin holes reduces the temperature differences at these locations, and also reduces the presence of radicals near the lift pins to erode the bonding materials that hold the top plate to the rest of the ESC. 
     There is a cavity around the lift pins within the lift pin holes to allow the lift pins to move. The lift pins push the wafer off the puck when the wafer is to be moved to another position. The arcing is prevented by raising the lift pin cavity with a controlled high pressure. He is a suitable gas to be applied to the lift pin cavity because of its electrical characteristics and thermal conductivity and because, in many ESC&#39;s, it is already used against the wafer back side through He holes. The radical buildup of reaction gases in the lift pin cavity is avoided which reduces bond erosion. By filling the lift pin holes with He, there is no longer any need to pump reaction gases out of the lift pin holes. Pressure equalization is also not required for the lift pin holes which provides for further cost reduction. In addition, with this He pressure approach, the wafer will not be held to the chuck by a vacuum that is caused when the chuck cools. The helium pressure will prevent such a vacuum. This simplifies de-chucking the wafer. 
       FIG. 2  is a top view diagram of a puck  206  on a top plate of an ESC. The puck has an inner electrode  210  of  FIG. 3  to hold a wafer (not shown). The electrode is beneath a dielectric layer and is sized to be almost the same size as the wafer that it will hold. The electrode is electrically connected to a DC voltage source. 
     There is an optional central gas hole  212  and an array of lift pin holes  214 . The gas hole allows additional cooling gas to be pushed out to the space between the wafer and the puck. The lift pin holes allow lift pins to extend through the holes to push a wafer off the chuck (de-chucking) so that the wafer may be removed for other or additional processing. There may be additional holes and other structures to perform other functions. Heaters, cooling channels, plasma process structures and other components are not shown in order not to obscure the drawing figure. 
       FIG. 3  is a partial cross-sectional side view diagram of an ESC showing the top layer  208  and puck  206  of  FIG. 2 . The top plate is configured to carry a workpiece  202  such as a silicon wafer or other item. The workpiece, in this example is held by an electrostatic force generated by electrodes (not shown) in the top plate. The top plate is formed of a dielectric material such as a ceramic like aluminum nitride and is mounted to a base plate  220  using, for example, an adhesive. The base plate may be formed of any suitable material, such as aluminum, to support the top plate. The base plate may contain cooling channels  230 , wiring layers, pipes, tubes, and other structures (not shown) to support the puck and a wafer  202  that is attached to and carried by the puck. 
     The base plate is supported by a ground plate  224  that is carried by a support plate  226 . An insulation plate  222  formed of an electrical and thermal isolator such as Rexolite®, or another plastic or polystyrene, heat resistant material to isolate the base cooling plate from the lower ground and support plates. The bottom support plate provides fittings for electrical and gas connections and provides attachment points for carriers and other fittings. 
     The lift pin hole  214  extends through the top plate  208 , the base plate  220 , the insulation plate  222 , the ground plate  224  and the support plate  226  to connect to a gas line  232  that supplies gas under pressure. The gas is supplied to the gas line by a regulated cooling gas source  236  such as a tank and pump or any other type of source. The gas line supplies the gas from the gas line to each lift pin hole through a connector in the support plate for each of the lift pin holes. The connector is at the bottom of the support plate or any other suitable plate at the interface between the plate and the external environment. There may also be additional connectors for any additional gas holes. Alternatively, the support plate may use a single connector into a manifold within the support plate or another plate to supply gas to each of the lift pin holes. As mentioned above, the cooling gas may be helium, nitrogen, or any other suitable inert gas with a high thermal conductivity. A gas hole has the same or a similar appearance and the illustrated hole represents both a lift pin hole and a gas hole. 
     The lift pin  216  is carried and guided through the center of the hole and extends from an actuator  234 . The lift pin assembly is used to lift and lower a workpiece or other substrate, such as a silicon wafer onto the electrostatic chuck puck  206 . The actuator may take any of a variety of different forms. In addition, the relative positions of the lift pin and actuator may be adjusted to accommodate different configurations. 
       FIG. 4  is a cross-sectional view of a lift pin and lift pin hole in a top plate. Lift pins  395  are suitable for de-chucking a substrate and are mounted in lift pin holes  314 . The lift pins overcome a vacuum and any residual electrostatic charge, through the use of physical pressure and a current sink  305 . The lift pin hole  214  is coupled to a gas line as shown in  FIG. 3 , but not shown here in order not to obscure the lift pin. The illustrated example is one configuration for a lift pin  395 , however, the lift pin may take any of a variety of other forms to suit other ESC configurations. The drawing figure is to show just one example of a lift pin for use in the example of  FIG. 3 . 
     Generally, the lift pins  395  comprise movable elongated members  310  having tips  315  suitable for lifting and lowering the substrate off the chuck. At least one lift pin  395  is capable of forming an electrically conductive path between the substrate and the current sink  305 . A voltage reducer or a current limiter may be coupled in series with the electrically conductive path of the elongated member  310 . The voltage reducer operates by reducing the voltage caused by RF currents used to form a plasma and attract the plasma to the substrate, while the current limiter operates by limiting the flow of the RF currents flowing therethrough. 
     To de-chuck a substrate held to the ESC by low frequency electrostatic residual charge, the lift pins  395  are raised and electrically contacted against the substrate. The substrate is lifted off the chuck after the residual electrostatic charge in the substrate is substantially discharged. 
     In a preferred configuration, each of the lift pins  395  have an elongated member  310  with an electrically conductive upper portion  330  that has a tip  315  suitable for lifting and lowering the substrate. A central portion  335  has a voltage reducer or a current limiter, and an electrically conductive lower portion  340  is suitable for electrical connection to the current sink  305 . The electrically conductive upper portion  330  and lower portion  340  are made from metals or other rigid conductive materials having low resistance to current flow. The upper portion  330  can also comprise a layer of a flexible material that prevents damage to the substrate when the lift pin tip  315  is pushed upwardly against the substrate. 
     In one example, the actuator  234  is a support  390 , such as a C-shaped ring around the support plate. The support may contact a plurality of lift pins  395  mounted around the support. Preferably, at least three, and more preferably four lift pins (not shown) are mounted symmetrically on the support so that the substrate  202  can be lifted off the chuck  206  by a uniformly applied pressure. Such a support may be attached to a lift bellows that can lift and lower the support, thereby lifting and lowering the lift pins  395  through the holes  314 . 
     Gas may be delivered to the back side of the wafer between the top surface of the pedestal and the wafer to improve heat convection between the wafer and the pedestal. An effective radial gas flow improves gas flow across the back side of the wafer. The gas may be pumped through a channel in the base of the pedestal assembly to the top of the pedestal. The channel may include the lift pin holes. A mass flow controller may be used to control the flow through the pedestal. In a vacuum or chemical deposition chamber, the backside gas provides a medium for heat transfer for heating and cooling of the wafer during processing. 
       FIG. 5  is a partial cross sectional view of a plasma system  100  having a pedestal  128  according to embodiments described herein. The pedestal  128  has an active cooling system which allows for active control of the temperature of a substrate positioned on the pedestal over a wide temperature range while the substrate is subjected to numerous process and chamber conditions. The plasma system  100  includes a processing chamber body  102  having sidewalls  112  and a bottom wall  116  defining a processing region  120 . 
     A pedestal, carrier, chuck or ESC  128  is disposed in the processing region  120  through a passage  122  formed in the bottom wall  116  in the system  100 . The pedestal  128  is adapted to support a substrate (not shown) on its upper surface. The substrate may be any of a variety of different workpieces for the processing applied by the chamber  100  made of any of a variety of different materials. The pedestal  128  may optionally include heating elements (not shown), for example resistive elements, to heat and control the substrate temperature at a desired process temperature. Alternatively, the pedestal  128  may be heated by a remote heating element, such as a lamp assembly. 
     The pedestal  128  is coupled by a shaft  126  to a power outlet or power box  103 , which may include a drive system that controls the elevation and movement of the pedestal  128  within the processing region  120 . The shaft  126  also contains electrical power interfaces to provide electrical power to the pedestal  128 . The power box  103  also includes interfaces for electrical power and temperature indicators, such as a thermocouple interface. The shaft  126  also includes a base assembly  129  adapted to detachably couple to the power box  103 . A circumferential ring  135  is shown above the power box  103 . In one embodiment, the circumferential ring  135  is a shoulder adapted as a mechanical stop or land configured to provide a mechanical interface between the base assembly  129  and the upper surface of the power box  103 . 
     A rod  130  is disposed through a passage  124  formed in the bottom wall  116  and is used to activate substrate lift pins  161  disposed through the pedestal  128 . The substrate lift pins  161  lift the workpiece off the pedestal top surface to allow the workpiece to be removed and taken in and out of the chamber, typically using a robot (not shown) through a substrate transfer port  160 . 
     A chamber lid  104  is coupled to a top portion of the chamber body  102 . The lid  104  accommodates one or more gas distribution systems  108  coupled thereto. The gas distribution system  108  includes a gas inlet passage  140  which delivers reactant and cleaning gases through a showerhead assembly  142  into the processing region  120 B. The showerhead assembly  142  includes an annular base plate  148  having a blocker plate  144  disposed intermediate to a faceplate  146 . 
     A radio frequency (RF) source  165  is coupled to the showerhead assembly  142 . The RF source  165  powers the showerhead assembly  142  to facilitate generation of plasma between the faceplate  146  of the showerhead assembly  142  and the heated pedestal  128 . In one embodiment, the RF source  165  may be a high frequency radio frequency (HFRF) power source, such as a 13.56 MHz RF generator. In another embodiment, RF source  165  may include a HFRF power source and a low frequency radio frequency (LFRF) power source, such as a 300 kHz RF generator. Alternatively, the RF source may be coupled to other portions of the processing chamber body  102 , such as the pedestal  128 , to facilitate plasma generation. A dielectric isolator  158  is disposed between the lid  104  and showerhead assembly  142  to prevent conducting RF power to the lid  104 . A shadow ring  106  may be disposed on the periphery of the pedestal  128  that engages the substrate at a desired elevation of the pedestal  128 . 
     Optionally, a cooling channel  147  is formed in the annular base plate  148  of the gas distribution system  108  to cool the annular base plate  148  during operation. A heat transfer fluid, such as water, ethylene glycol, a gas, or the like, may be circulated through the cooling channel  147  such that the base plate  148  is maintained at a predefined temperature. 
     A chamber liner assembly  127  is disposed within the processing region  120  in very close proximity to the sidewalls  101 ,  112  of the chamber body  102  to prevent exposure of the sidewalls  101 ,  112  to the processing environment within the processing region  120 . The liner assembly  127  includes a circumferential pumping cavity  125  that is coupled to a pumping system  164  configured to exhaust gases and byproducts from the processing region  120  and control the pressure within the processing region  120 . A plurality of exhaust ports  131  may be formed on the chamber liner assembly  127 . The exhaust ports  131  are configured to allow the flow of gases from the processing region  120  to the circumferential pumping cavity  125  in a manner that promotes processing within the system  100 . 
     A system controller  170  is coupled to a variety of different systems to control a fabrication process in the chamber. The controller  170  may include a temperature controller  175  to execute temperature control algorithms (e.g., temperature feedback control) and may be either software or hardware or a combination of both software and hardware. The system controller  170  also includes a central processing unit  172 , memory  173  and input/output interface  174 . The temperature controller receives a temperature reading  143  from a sensor (not shown) on the pedestal. The temperature sensor may be proximate a coolant channel, proximate the wafer, or placed in the dielectric material of the pedestal. The temperature controller  175  uses the sensed temperature or temperatures to output control signals affecting the rate of heat transfer between the pedestal assembly  142  and a heat source and/or heat sink external to the plasma chamber  105 , such as a heat exchanger  177 . 
     The system may also include a controlled heat transfer fluid loop  141  with flow controlled based on the temperature feedback loop. In the example embodiment, the temperature controller  175  is coupled to a heat exchanger (HTX)/chiller  177 . Heat transfer fluid flows through a valve (not shown) at a rate controlled by the valve through the heat transfer fluid loop  141 . The valve may be incorporate into the heat exchanger or into a pump inside or outside of the heat exchanger to control the flow rate of the thermal fluid. The heat transfer fluid flows through conduits in the pedestal assembly  142  and then returns to the HTX  177 . The temperature of the heat transfer fluid is increased or decreased by the HTX and then the fluid is returned through the loop back to the pedestal assembly. 
     The HTX includes a heater  186  to heat the heat transfer fluid and thereby heat the substrate. The heater may be formed using resistive coils around a pipe within the heat exchanger or with a heat exchanger in which a heated fluid conducts heat through an exchanger to a conduit containing the thermal fluid. The HTX also includes a cooler  188  which draws heat from the thermal fluid. This may be done using a radiator to dump heat into the ambient air or into a coolant fluid or in any of a variety of other ways. The heater and the cooler may be combined so that a temperature controlled fluid is first heated or cooled and then the heat of the control fluid is exchanged with that of the thermal fluid in the heat transfer fluid loop. 
     The valve (or other flow control devices) between the HTX  177  and fluid conduits in the pedestal assembly  142  may be controlled by the temperature controller  175  to control a rate of flow of the heat transfer fluid to the fluid loop. The temperature controller  175 , the temperature sensor, and the valve may be combined in order to simplify construction and operation. In embodiments, the heat exchanger senses the temperature of the heat transfer fluid after it returns from the fluid conduit and either heats or cools the heat transfer fluid based on the temperature of the fluid and the desired temperature for the operational state of the chamber  102 . 
     Electric heaters (not shown) may also be used in the pedestal assembly to apply heat to the pedestal assembly. The electric heaters, typically in the form of resistive elements are coupled to a power supply  179  that is controlled by the temperature control system  175  to energize the heater elements to obtain a desired temperature. 
     The heat transfer fluid may be a liquid, such as, but not limited to deionized water/ethylene glycol, a fluorinated coolant such as Fluorinert® from 3M or Galden® from Solvay Solexis, Inc. or any other suitable dielectric fluid such as those containing perfluorinated inert polyethers. While the present description describes the pedestal in the context of a PECVD processing chamber, the pedestal described herein may be used in a variety of different chambers and for a variety of different processes. 
     A backside gas source  178  such as a pressurized gas supply or a pump and gas reservoir are coupled to the chuck assembly  142  through a mass flow meter  185  or other type of valve. The backside gas may be helium, argon, or any gas that provides heat convection and/or cooling between the wafer and the puck without affecting the processes of the chamber. The gas source pumps gas through a gas outlet of the pedestal assembly described in more detail above through lift pin holes and any gas holes to the back side of the wafer under the control of the system controller  170  to which the system is connected. 
     The processing system  100  may also include other systems, not specifically shown in  FIG. 1 , such as plasma sources, vacuum pump systems, access doors, micromachining, laser systems, and automated handling systems, inter alia. The illustrated chamber is provided as an example and any of a variety of other chambers may be used with the present invention, depending on the nature of the workpiece and desired processes. The described pedestal and thermal fluid control system may be adapted for use with different physical chambers and processes. 
     As used in this description and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. 
     The terms “coupled” and “connected,” along with their derivatives, may be used herein to describe functional or structural relationships between components. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other. “Coupled” my be used to indicate that two or more elements are in either direct or indirect (with other intervening elements between them) physical, optical, or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g., as in a cause an effect relationship). 
     The terms “over,” “under,” “between,” and “on” as used herein refer to a relative position of one component or material layer with respect to other components or layers where such physical relationships are noteworthy. For example in the context of material layers, one layer disposed over or under another layer may be directly in contact with the other layer or may have one or more intervening layers. Moreover, one layer disposed between two layers may be directly in contact with the two layers or may have one or more intervening layers. In contrast, a first layer “on” a second layer is in direct contact with that second layer. Similar distinctions are to be made in the context of component assemblies. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, while flow diagrams in the figures show a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is not required (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.). Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     Examples of different embodiments of the gas pressure ESC include an ESC that uses a lift pin feature to deliver a cooling gas, for example helium, for backside wafer cooling. 
     Embodiments include the design above wherein the helium is applied at pressure at the bottom of the ESC and is pushed up through the top plate or puck of the ESC to the space between the puck and the back side of the wafer through the lift pin holes. 
     Embodiments include the design above wherein helium is pressed though the lift pin hole around the lift pin to the space between the puck and the wafer back side 
     Embodiments include the design above wherein the helium holes in the ESC are reduced or eliminated by using lift pin holes to apply the cooling gas. 
     Embodiments include the design above in which the lift pin cavity pressure is controlled, for example using an external regulated helium pump. 
     Embodiments include the design above in which the lift pin holes are filled with helium. 
     Embodiments include means for performing any of the functions or operations of the design above. 
     Embodiments include a method for processing a workpiece using an electrostatic chuck with a top plate and lift pins to lift the workpiece off the top plate, the method including conveying a cooling gas through lift pin holes to the back side of the wafer.