Patent Publication Number: US-2004040664-A1

Title: Cathode pedestal for a plasma etch reactor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims benefit of U.S. provisional patent application serial No. 60/385,753, filed Jun. 3, 2002, and U.S. provisional patent application serial No. 60/434,959, filed Dec. 19, 2002, both of which are incorporated herein by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] Embodiments of the present invention generally relate to semiconductor substrate processing equipment and, more particularly, to a pedestal typically used inside a plasma etch reactor.  
       [0004] 2. Description of the Related Art  
       [0005] Generally, a plasma etch reactor is used to process semiconductor wafers to produce microelectronic circuits. The reactor forms a plasma within a chamber containing the wafer to be processed. The plasma is formed and maintained by application of very high frequency (VHF) plasma source power coupled either inductively or capacitively into the chamber. For capacitive coupling of VHF source power into the chamber, an overhead electrode (facing the wafer) is powered by a VHF source power generator.  
       [0006] Recently, capacitively coupled plasma etch reactors have been used for dielectric etch applications at low pressures in nearly pure reactive ion etching (RIE) conditions, which required increased voltage capability (e.g., from about 4000 volts peak to peak to about 6000 volts peak to peak), creation of significant plasma at low pressures (e.g., about 30 mT), and increased efficiency of the chuck to allow the plasma to form at low pressures. Operating capacitively coupled plasma etch reactors under these conditions, however, often leads to a high voltage breakdown, high damage to the chuck, and poor etch rates, all of which may be caused by the lack of plasma density over the substrate surface. Recent investigations have discovered that the lack of plasma density was caused by a lossy transmission line that connects to the substrate.  
       [0007] Therefore, a need exists for an improved capacitively coupled plasma etch reactor that overcomes the deficiencies described above.  
       SUMMARY  
       [0008] Various embodiments of the present invention are generally directed to a plasma etch reactor. In one embodiment, the reactor includes a chamber, a pedestal disposed within the chamber, a gas distribution plate disposed within the chamber overlying the pedestal, a ring surrounding the pedestal, and an upper electrically conductive mesh layer and a lower electrically conductive mesh layer disposed within the pedestal. The ring defines a raised portion. The upper electrically conductive mesh layer is disposed substantially above the lower electrically conductive mesh layer and is substantially the same size as a substrate configured to be disposed on the pedestal. The lower electrically conductive mesh layer is substantially annular in shape and is disposed around the periphery of the upper electrically conductive mesh layer and below the raised portion of the ring.  
       [0009] In another embodiment, the reactor further includes an insulation layer disposed on the pedestal and a plurality of gas flow openings disposed through the insulation layer. At least one gas flow opening includes a porous plug disposed therein. The porous plug is configured to provide an indirect pathway for gases to flow toward an upper surface of the insulation layer.  
       [0010] In yet another embodiment, the reactor further includes at least one lift pin opening disposed through the pedestal. The at least one lift pin opening includes a lift pin disposed therein configured to lift a portion of a substrate off an upper surface of the pedestal. The at least one lift pin opening has a pressure that is substantially less than a pressure inside the chamber during a process.  
       [0011] In still another embodiment, the reactor further includes a heat exchanger disposed inside the pedestal. The heat exchanger includes a plurality of channels. Each channel defines a plurality of protrusions disposed therein. The protrusions are configured to cause turbulence to a heat exchanger fluid contained inside the channels. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
     [0013]FIG. 1 illustrates a plasma etch reactor chamber that includes various embodiments of the invention.  
     [0014]FIG. 2 illustrates in greater detail the structure of the cathode pedestal in accordance with an embodiment of the invention.  
     [0015]FIG. 3 illustrates in greater detail the configuration of the electrically conductive mesh layers in accordance with an embodiment of the invention.  
     [0016]FIG. 4 illustrates a schematic illustration of a bias tuning circuit in accordance with an embodiment of the invention.  
     [0017]FIG. 5 illustrates a dielectric sleeve surrounding the conductor in accordance with an embodiment of the invention.  
     [0018]FIG. 6 illustrates a cut-away side view of the dielectric sleeve in accordance with an embodiment of the invention.  
     [0019]FIG. 7A is a side view illustrating a version of the dielectric sleeve that is mechanically adjustable.  
     [0020]FIG. 7B is a side view illustrating a version having multiple sleeve sections that are each mechanically adjustable.  
     [0021]FIG. 8 illustrates a cross section view of a heat exchanger in accordance with an embodiment of the invention.  
     [0022]FIG. 9 illustrates a schematic bottom view of the heat exchanger of FIG. 8.  
     [0023]FIG. 10A illustrates a schematic top view of a channel of a heat exchanger with chevron protrusions in accordance with one embodiment of the invention.  
     [0024]FIG. 10B illustrates a schematic side view of a channel of a heat exchanger with bump protrusions in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
     [0025]FIG. 1 illustrates an example of a capacitively coupled etch reactor  100  that includes various embodiments of the invention. This illustration is based on the MxP, eMax or Super-e etch reactors available from Applied Materials. It includes a grounded vacuum chamber  32 , perhaps including liners to protect the walls. A substrate  110  is inserted into the chamber  32  through a slit valve opening  36  and placed on a cathode pedestal  105  with an electrostatic chuck  40  selectively clamping the wafer. The chuck may be powered with one or more power supplies. Fluid cooling channels may be positioned through the pedestal  105  to maintain the pedestal at reduced temperatures. A thermal transfer gas, such as helium, is supplied to openings in the upper surface of the pedestal  105 . The thermal transfer gas increases the efficiency of thermal coupling between the pedestal  105  and the wafer  34 , which is held against the pedestal  105  by the electrostatic chuck  40  or an alternatively used peripheral wafer clamp.  
     [0026] An RF power supply  200 , generally operating at 13.56 MHz, is connected to the cathode pedestal  105  and provides power for generating the plasma while also controlling the DC self-bias. Magnetic coils  44  powered by one or more current supplies surround the chamber  32  and generate a slowly rotating (on the order of seconds and typically less than 10 ms), horizontal, essentially DC magnetic field in order to increase the density of the plasma. A vacuum pump system  46  pumps the chamber  32  through an adjustable throttle valve  48 . Shields  50 ,  52  not only protect the chamber  32  and pedestal  105  but also define a baffle  54  and a pumping channel  54  connected to the throttle valve  48 .  
     [0027] Processing gases are supplied from gas sources  58 ,  60 ,  62  through respective mass flow controllers  64 ,  66 ,  68  to a gas distribution plate  125  positioned in the roof of the chamber  32  overlying the wafer  34  and separated from it across a processing region  72 . The distribution plate  125  includes a manifold  74  configured to receive the processing gases and communicate with the processing region  72  through a showerhead having a large number of distributed apertures  76  so that a more uniform flow of processing gas may be injected into the processing region  72 .  
     [0028] Other details of the reactor  100  are further described in commonly assigned U.S. Pat. No. 6,451,703, entitled “Magnetically Enhanced Plasma Etch Process Using A Heavy Fluorocarbon Etching Gas”, issued to Liu et al. and U.S. Pat. No. 6,403,491, entitled “Etch Method Using A Dielectric Etch Chamber With Expanded Process Window”, issued to Liu et al., which are both incorporated by reference herein to the extent not inconsistent with the invention. Although various embodiments of the invention will be described with reference to the above-described reactor, the embodiments of the invention may also be used in other reactors, such as one described in commonly assigned U.S. Ser. No. 10/028,922 filed Dec. 19, 2001, entitled “Plasma Reactor With Overhead RF Electrode Tuned To The Plasma With Arcing Suppression”, by Hoffman et al., which is incorporated by reference herein to the extent not inconsistent with the invention, and is commercially available as the Enabler® Reactor from Applied Materials, Inc. of Santa Clara, Calif.  
     [0029] Dual Mesh. FIG. 2 illustrates in greater detail the structure of the cathode pedestal  105 . The cathode pedestal  105  includes a metal pedestal layer  205  and an insulation layer  210 , which may be referred to as a puck. The insulation layer  210  includes an upper electrically conductive mesh layer  215  and a lower electrically conductive mesh layer  220 . The substrate  110  is generally disposed on top of the insulation layer  210 . The specific orientation of the mesh layers will be described below with reference to FIG. 3. The electrically conductive mesh layers  215 ,  220  and the metal pedestal layer  205  may be made from molybdenum and aluminum respectively. The insulation layer  210  may be made from a dielectric material, such as aluminum nitride or alumina, for example. The electrically conductive mesh layers  215 ,  220  are configured to supply the RF bias voltage to control ion bombardment energy at the surface of the substrate  110 . The electrically conductive mesh layers  215 ,  220  may also be used for electrostatically chucking and de-chucking the substrate  110 . In such a case, the electrically conductive mesh layers may be connected to a chucking power supply  140 . An example of such a power supply is disclosed in commonly assigned U.S. Pat. No. 6,005,376, issued Dec. 21, 1999, which is incorporated herein by reference. The electrically conductive mesh layers  215 ,  220  may not necessarily be grounded and consequently may have a floating electric potential or a fixed D.C. potential in accordance with conventional chucking and de-chucking operations.  
     [0030]FIG. 2 further illustrates an RF conductor  225  extending through the cathode pedestal  105 . The RF conductor  225  is electrically coupled to an RF bias generator  200  through an RF bias impedance match element  230  (shown in FIG. 1). The RF bias generator  200  is configured to apply power to the substrate  110  through the RF bias impedance match element  230  and the RF conductor  225  in a high frequency (HF) band, such as from about 2 MHz to about 13.56 MHz. The RF conductor  225  is generally insulated from grounded conductors such as the metal pedestal layer  205 . The RF conductor  225  has a top termination or bias power feed point  225   a  in electrical contact with the upper electrically conductive mesh  215 .  
     [0031]FIG. 3 illustrates in greater detail the configuration of the electrically conductive mesh layers  215 ,  220  in accordance with an embodiment of the invention. The upper electrically conductive mesh layer  215  is generally shaped like a disk and has substantially the same size as the substrate  110 . The mesh layer  215  is disposed below the substrate  110  and substantially parallel to the substrate  110 . The lower electrically conductive mesh layer  220  is substantially annular in shape, disposed generally below the upper electrically conductive mesh layer  215  and parallel to the upper electrically mesh layer  215 , and substantially proximate the periphery of the cathode pedestal  105 . The lower electrically conductive mesh layer  220  is electrically coupled to the RF conductor  225  through an electrically conductive line that runs along a diameter of the lower electrically conductive mesh layer  220 . In this manner, the lower electrically conductive mesh layer  220  is configured to supply RF power to periphery portion of the substrate  110 . Other details of the upper and lower electrically conductive mesh layers  215 ,  220  may be described in commonly assigned U.S. Pat. No. 6,232,236 entitled “Apparatus and Method for Controlling Plasma Uniformity in a Semiconductor Wafer Processing System”, issued to Shan et al., which is incorporated by reference herein to the extent not inconsistent with the invention.  
     [0032]FIG. 3 further illustrates a semiconductor ring  115  in accordance with an embodiment of the invention. The semiconductor ring  115  may also be referred to as a process kit. The lower electrically conductive mesh layer  220  is disposed below the semiconductor ring  115 . The semiconductor ring  115  defines a raised portion  118 . The lower electrically conductive mesh layer  220  in combination with the upper portion  118  are configured to shape the electric field at or near the periphery of the substrate  110 . More specifically, the combination is used to reduce the high concentration of non perpendicular field lines that are typically disposed at or near the periphery portion of the substrate  110 , causing an edge tilting effect, which causes vias to be etched in a sideway manner. By disposing the lower electrically conductive mesh layer  220  below the semiconductor ring  115  and defining the raised portion  118 , the electric field lines at or near the periphery of the substrate  110  are disposed substantially perpendicular to the substrate  110 , and thereby eliminating the edge tilting effect. In one embodiment, the raised portion  118  is about 1.5 mm to about 3 mm in height.  
     [0033] Bias Tuning Circuit. In some chambers, such as the one described in commonly assigned U.S. Ser. No. 10/028,922 filed Dec. 19, 2001, entitled “Plasma Reactor With Overhead RF Electrode Tuned To The Plasma With Arcing Suppression”, by Hoffman et al., VHF power may be applied to the gas distribution plate  125 , thereby making the gas distribution plate an electrode. The power that is applied to the gas distribution plate is commonly referred to as the “source” power as opposed to the “bias” power that is applied to the pedestal. In one embodiment, the VHF power is applied at high frequency, such as 100-200 MHz. In other embodiments, the source power frequency may be lower, e.g., 13.56 MHz or 12.56 MHz.  
     [0034]FIG. 4 is a schematic illustration of a circuit, which includes the overhead electrode  125 , the RF bias applied through the cathode pedestal  105  and the elements of the cathode pedestal  105 . FIG. 5 illustrates a top plan view of the substrate  110 , the termination or feed point  225   a,  and the RF conductor  225 . The RF return path provided by the cathode pedestal  105  consists of two portions in the plane of the substrate  110 , namely a radially inner portion  530  centered about and extending outwardly from the feed point  225   a  and the radially outer annular portion  535 . The RF return paths provided by the two portions  530 ,  535  are different, and therefore the two portions  530 ,  535  present different impedances to the VHF power radiated by the overhead electrode  125 .  
     [0035] The primary RF return path  545  is provided by the conductive mesh layers  215 ,  220 , which are coupled through the cathode pedestal  105  and the RF conductor  225 . The RF return path  540  passing through the outer annular portion  535  is dominated by reactive coupling through the substrate  110  and across the conductive mesh layers  215 ,  220  to the cathode pedestal  105 . In contrast, the RF return path  545  through the inner portion  530  is dominated by the reactive impedance of the feed point  225   a.  As a result, the two RF return paths often cause non-uniform coupling to RF power if the impedance is not uniform across the substrate  110 .  
     [0036] Since the two RF return paths are physically different, they tend to offer different impedances to the VHF power radiated by the overhead electrode  125 . Such differences may cause non-uniformities in radial distribution across the substrate surface of impedance to the VHF power, rendering source power coupling to the plasma nonuniform and giving rise to nonuniform radial distribution of plasma ion density near the surface of the substrate  110 . This in turn can cause processing non-uniformities that unduly narrow the process window. Accordingly, the reactor  100  may include certain features that adjust the feed point impedance presented by the RF conductor  225  to the VHF power, thereby enabling a more uniform radial distribution of impedance across the substrate surface and a more uniform coupling of VHF power across the substrate surface.  
     [0037] A principal purpose of this adjustment in the feed point impedance is to bring the impedance at the feed point  225   a  to at least nearly zero at the source power frequency (i.e., the VHF frequency of the overhead electrode  125  from about 100 MHz to about 200 MHz). As a result of this adjustment, the RF current return path is dominated by the conductive mesh layers  215 ,  220  through the RF conductor  225  while minimizing the current through the cathode pedestal  105 . Consequently, the impedances of the regions  530  and  535  can be made to be at least substantially the same.  
     [0038] In order to adjust the feed point impedance, a dielectric cylindrical sleeve  550  surrounds the RF conductor  225 . The axial length and the dielectric constant of the material constituting the sleeve  550  determine the feed point impedance presented by the RF conductor  225  to the VHF power. In one example, the length and dielectric constant of the sleeve  550  is selected to bring the feed point impedance to nearly zero at the VHF source power frequency (e.g., about 100-200 MHz). In a working example, the feed point impedance without the sleeve  550  was (0.9+j41.8) ohms and with the sleeve  550  was nearly a short circuit at (0.8+j0.3) ohms. The impedance presented by the outer region  535  surrounding the feed point  225   a  is nearly a short at the corresponding frequency (due mainly to the presence of the conductive mesh layers  215 ,  220 ). Therefore, in the latter example the sleeve  550  may bring the feed point impedance at the source power frequency to a value closer to that of the surrounding region. Here, the impedance of the region surrounding the feed point is determined mainly by the conductive mesh layers  215 ,  220 .  
     [0039] The sleeve  550  may also include features facilitating the foregoing improvement in VHF power distribution while simultaneously solving a separate problem, namely improving the uniformity in the electric field created by the RF bias power (at 13.56 MHz for example) applied to the substrate  110  by the RF conductor  225 . The problem is how to adjust radial distribution of VHF power coupling for maximum uniformity of plasma ion density while simultaneously adjusting the HF bias power electric field distribution across the wafer surface for maximum uniformity. Maximum uniformity would be attained if the feed point impedance at the HF bias power frequency were brought nearer to that of the surrounding region  535  dominated by the conductive mesh layers  215 ,  220  (without altering the feed point impedance at the VHF source power frequency). This problem is solved by dividing the sleeve  550  along its cylindrical axis into plural cylindrical sections, and adjusting or selecting the length and dielectric constant of each section independently. This provides several independent variables that may be exploited to permit matching the feed point impedance to that of the surrounding region at both the bias frequency (e.g., about 13.56 MHz) and at the source frequency (e.g., about 100-200 MHz) simultaneously.  
     [0040]FIG. 6 illustrates sleeve  550  divided into three sections, namely a top section  552 , a middle section  554  and a bottom section  556 , in accordance with an embodiment of the invention. The top section  552  may be made from polytetraflouroethylene and about three inches in length, the middle section  554  may be made from alumina and about four inches in length, and the bottom section  556  may be made from polytetraflouroethylene and about three inches in length. The length and dielectric constant of the sleeve top section  552  may be selected and fixed to optimize the HF bias power distribution exclusively. The lengths and dielectric constants of the remaining sleeve sections  554 ,  556  may then be selected to optimize VHF source power distribution by the overhead electrode while leaving the HF bias power distribution optimized.  
     [0041]FIG. 7A illustrates how the sleeve  550  may be assembled to be adjustable during use. An external control knob  560  is provided on the reactor to turn a screw  565  threadably engaged with a sleeve support  570  coupled to the bottom of the sleeve  550 . As the knob  560  is rotated, the sleeve support  570  travels axially along the axis of the threaded screw  565 , forcing the entire sleeve  550  to travel in the same direction (either up or down) within a sleeve guide  558 . The knob  560  permits the user to adjust the feed point impedance by moving the sleeve  550  up or down along the RF conductor  225  during (or shortly before) operation of the reactor. The sleeve support  570  may move the entire sleeve  550  (for example, all three sections  552 ,  554 ,  556  as a unit together). Or, the sleeve support  570  may be coupled to only one or two of the three sections  552 ,  554 ,  556  so that only one or two of the three sections is moved by rotating the knob  560 . FIG. 7B illustrates that three knobs  560   a,    560   b,    560   c  may separately engage three sleeves supports  570   a,    570   b,    570   c.  The three sleeve supports  570   a,    570   b,    570   c  are individually connected to respective ones of the three sleeve sections  552 ,  554 ,  556  so that the positions of each of the sleeve sections  552 ,  554 ,  556  are separately determined within the sleeve guide  558   a  by the three knobs  560   a,    560   b,    560   c.  Other details of the bias tuning circuit as described with reference to FIGS.  4 - 7 B are described in commonly assigned U.S. Ser. No. 10/235,988, filed Sep. 4, 2002 and entitled “Capacitively Coupled Plasma Reactor With Uniform Radial Distribution of Plasma”, by Yang et al., which,is incorporated by reference herein to the extent not inconsistent with the invention.  
     [0042] Porous Plugs. Referring back to FIG. 2, the cathode pedestal  105  in accordance with an embodiment of the invention is illustrated. The cathode pedestal  105  includes a plurality of gas flow openings  202  disposed through the insulation layer  210  at or around the periphery of the cathode pedestal  105 . Each opening includes a porous plug  212 . The openings  202  combined with the porous plugs  212  contained therein are configured to permit gas (such as, helium or argon) flow from cooling gas sources (not shown) to the upper surface of the cathode pedestal  105 . The porous plugs  212  may be made from a dielectric, such as alumina having a porosity ranging from about 10% in volume to about 60% in volume, with interconnected openings that form continuous passageways through the dielectric material. The porous plugs  212  may also be made from a material selected from a group consisting of ceramic compositions, engineering thermoplastics, thermosetting resins, filled engineering thermoplastics, filled thermosetting resins, and combinations thereof. When the porous plugs  212  are formed using traditional molding and sintering methods, the particles used in the molding or sintering are of the same order of magnitude in size as the porosity and are bonded in a substantially random orientation,,producing passageways that avoid the straight line of sight configuration. In this manner, arcing or glow discharge occurring within the openings  202  may be minimized and uniform electric field from the grounded pedestal to the plasma may be generated. Other details of the porous plugs are described in commonly assigned U.S. Pat. No. 5,720,818, entitled “Conduits For Flow Of Heat Transfer Fluid To The Surface Of An Electrostatic Chuck”, issued to Donde et al., which is incorporated by reference herein to the extent not inconsistent with the invention.  
     [0043] Pumped Lift Pins. FIG. 2 further illustrates one of a plurality of lift pin openings  206  having a lift pin  216  in each opening  206 . The lift pin openings  206  are disposed through the cathode pedestal  105  to allow the lift pins  216  to pass therethrough to lift the substrate  110  off the upper surface of the cathode pedestal  105  once the power has been turned off and the clamping force terminated. During operation of the chamber, the pressure in the gas flow openings  202  generally ranges from about 5 to about 40 T, while the chamber operating pressure ranges from about 10 to about 500 mT. Some of the cooling gases flowing through the gas flow openings  202  often leak into the lift pins openings  206 , which may cause arcing (which may be referred to as back side arcing) during operation of the chamber. In accordance with an embodiment of the invention, the lift pin openings  206  are configured to be pumped with vacuum. In this manner, the pressure inside the lift pin openings  206  may be reduced, thereby reducing the likelihood for arcing to occur within the lift pin openings  206 . The lift pin openings  206  may be pumped with vacuum such that the pressure inside the openings  206  is less than the chamber operating pressure. The lift pin openings  206  may be pumped by either the chamber vacuum pump  46 , or a separate pump. As such, backside cooling gas is constantly evacuated from the openings  206  and does not accumulate at a pressure that facilitates arcing during chamber operation.  
     [0044] Optimization of Insulation Layer. It has recently been observed that operating the chamber at low pressures (e.g., from about 0.1 mT to about 50 mT) generally leads to minimal or no plasma ion density near the surface of the substrate  110 . A determination was made that the lack of plasma ion density near the surface of the substrate  110  is caused by a high power loss from the RF bias generator  200  to the substrate  110 . More specifically, most of the power loss occurs in the insulation layer  210 . Thus, it can be deduced that the lack of plasma ion density near the surface of the substrate  110  is caused by lack of power to the substrate  110 . One solution to minimize power loss in the insulation layer  210  is to increase the thickness of the insulation layer  210 . In one embodiment, the thickness of the insulation layer  210  is increased by about two fold, e.g., about 25-30 mm thick. By increasing the thickness of the insulation layer  210  to about 25-30 mm, the plasma conductance inside the chamber falls into a range from about 0.001+j0.01 to about 0.004+j0.02. Further, by increasing the thickness of the insulation layer  210  by about two fold, the shunt capacitance (stray resonance) coupling to ground is reduced by about 50% and the power loss that occurs in the insulation layer  210  is minimized, thereby increasing the amount of power applied to the substrate  110 . As the amount of power transferred from the RF bias generator  200  to the substrate  110  increases, the voltage capability and power capability of the RF bias generator  200  also increases. An increased power capability in turn leads to an increase in etch rate. For example, for a 300 mm substrate, the voltage capability at low pressures (e.g., from about 10-50 mT) may be increased to about 7500 volts peak to peak and the power capability may be increased to about 6000 watts.  
     [0045] Heat Exchanger. FIG. 2 further illustrates a heat exchanger  222  in accordance with an embodiment of the invention. The heat exchanger  222  is configured to provide a uniform temperature distribution across the cathode pedestal  105 . In one embodiment, the heat exchanger  222  is defined within the metal pedestal layer  205 . The heat exchanger  222  may also be defined within the insulation layer  210 . The heat exchanger  222  defines a plurality of channels  232  configured to circulate heat transfer fluid to remove heat from the cathode pedestal  105 . The heat exchanger  222  is connected to a chiller equipment  250  that supplies the heat transfer fluid to the heat exchanger. The chiller equipment may include a pump to circulate the heat exchanger fluid through the channels  232 . As the heat transfer fluid is circulated through the channels  232 , the heat from the cathode pedestal  105  is absorbed by the heat transfer fluid. After circulating the heat transfer fluid through the channels  232 , the heated heat transfer fluid is returned to the chiller equipment for further processing or recirculation.  
     [0046]FIG. 8 illustrates a cross section view of a heat exchanger  222  in accordance with an embodiment of the invention. The heat exchanger  222  defines channels  232  that have protrusions disposed along the wetted surfaces of the channels  232 . The protrusions are configured to bring about turbulence to the heat exchanger fluid. The turbulence in the heat exchanger fluid causes more of the heat exchanger fluid to contact the hot walls of the heat exchanger  222 , which in turn result in a more efficient heat exchanger. The protrusions may also be configured to increase the surface area of the wetted area in contact with the metal pedestal layer  205 . In this manner, the protrusions may be used to locally adjust the thermal resistance between the substrate  110  and the heat exchanger  222 . The protrusions may be in the form of fins, bumps, chevrons, spines, or helical structures. As illustrated in FIG. 8, the channels  232  define a plurality of fins  242  on the inside portion (i.e., the wetted area) of the channels  232 . For example, in a metal pedestal layer that is about 2 inch thick, each fin  242  may be about {fraction (1/16)} inch wide and about ⅜ inch high. The taller the fins, the more wetted area is in contact with the metal pedestal layer  205 , from which heat is transferred. The fins  242  generally have more wetted area in contact with the metal pedestal layer  205  than other type of protrusions. Consequently, the fins  242  are configured to remove more heat from the metal pedestal layer  205  than other type of protrusions, since the amount of heat removed is directly proportional to the amount of wetted area in contact with the metal pedestal layer  205 . Generally, the fins  242  are used as protrusions in thicker metal pedestal layers, such as about 1.5 inch or greater. Other forms of protrusions, such as chevrons  1010  (shown in FIG. 10A) and bumps  1020  (shown in FIG. 10B), are generally used in thinner metal pedestal layers, such as less than about 1 inch. If the chevrons  1010  are used as the protrusions, the pointed portions of the chevrons  1010  are disposed in an upstream direction to project the most turbulence. The height of each chevron may be about 10% to about 15% of the depth of the channel  232 .  
     [0047]FIG. 9 illustrates a schematic bottom view of the heat exchanger  222  of FIG. 8. The heat exchanger  222  includes an input conduit  910  and an output conduit  920  connected to the input conduit  910 . The heat exchanger fluid is received at the input conduit  910  and is transferred to the chiller equipment through the output conduit  920 . Consequently, the heat exchanger fluid contained in the input conduit  910  is generally cooler than the heat exchanger fluid contained in the output conduit  920 . In one embodiment, the position of the input conduit and the position of the output conduit are reversed. The channels  232  are configured such that the input conduit  910  is positioned substantially adjacent the output conduit  920 . In this manner, the thermal resistance between the input conduit  910  and the output conduit  920  remains substantially constant, thereby keeping temperature non-uniformity between the input conduit  910  and the output conduit  920  to a minimal. In one embodiment, the input conduit  910  is connected to the output conduit  920  at a location at which the temperature of the heat exchanger fluid in the input conduit  910  is about the same as the temperature of the heat exchanger fluid in the output conduit  920 . The input conduit  910  and the output conduit  920  may be configured in a spiral formation in order to minimize the number of sharp turns and to increase the number of loops formed by the input conduit  910  and the output conduit  920 . Furthermore, the input conduit  910  and the output conduit may be configured such that the heat exchanger fluid inside the input conduit  910  and the output conduit  920  travel in opposite directions and be alternated in a radial fashion, thereby averaging the temperature of the heat exchanger fluid across the channels  232 . In accordance with yet another embodiment, the input conduit  910  and the output conduit  920  are substantially in the same plane.  
     [0048] As mentioned above, the heat exchanger fluid is pumped into the heat exchanger  222  to remove heat from the substrate  110 . Depending upon the substrate process temperature and the amount of heat flowing from the substrate  110  to the cathode pedestal  105 , the temperature of the heat exchanger fluid may be below the freezing point of water, such as from about −20 degrees Celsius to about −10 degrees Celsius. If water is used as the heat exchanger fluid, anti-freeze chemicals, such as ethylene glycol or salts, may be added to the water. Non-water based fluids (such as, the fluorinated Galden HT-110, HT-135, and HT-200) may also be used as the heat exchanger fluid.  
     [0049] By using the various embodiments of the heat exchanger  222  described above, the substrate  110  may be cooled in a uniform manner and the temperature difference between the substrate  110  and the heat exchanger  222  may be kept at a minimum, e.g., less than about 5 degrees Celsius at 2000 Watts thermal load for a 300 mm substrate. Although the heat exchanger  222  has been described with reference to cooling the substrate  110 , the heat exchanger  222  may also be used to heat the cathode pedestal  105 .  
     [0050] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.