Patent Publication Number: US-2015060013-A1

Title: Tunable temperature controlled electrostatic chuck assembly

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate to the microelectronics manufacturing industry and more particularly to temperature-controlled pedestals for supporting a workpiece during plasma processing. 
     BACKGROUND 
     Plasma processing equipment, such as equipment designed to perform plasma etching of microelectronic devices and the like, use electrostatic chucks (ESCs) to support and hold a wafer or substrate in place during processing. Such equipment generally includes heating and/or cooling elements to adjust the temperature of the ESC, and therefore adjust the temperature of the wafer. A variety of factors result in temperature non-uniformities on the wafer, which can result in inconsistent and malfunctioning microelectronic devices. For example, in equipment where a bonding material is used to couple elements of the pedestal (e.g., heating elements, cooling elements, etc.), inconsistencies in the thickness of the bonding material and/or the chemical composition of the bonding material can result in varying thermal conductivities. The varying thermal conductivities can result in hot or cool spots on the ESC, which in turn can result in hot or cool spots on the wafer. Other causes of temperature non-uniformities can include non-uniform heating and cooling of the ESC, varying contact resistance between the wafer and the ESC, plasma load variations, and other temperature variations within the processing chamber (e.g., variations due to the location of doors to the chamber). 
     Additionally, plasma processing equipment can consume large amounts of power. Some of the power consumption by plasma processing equipment is consumed by cooling and heating mechanisms to maintain a uniform wafer temperature. 
     SUMMARY 
     One or more embodiments of the invention are directed to a temperature-controlled pedestal to support a workpiece during plasma processing. 
     In one embodiment, the pedestal includes an electrostatic chuck (ESC) having a top surface over which the workpiece is to be disposed. The pedestal includes one or more heating elements disposed under the top surface of the ESC. The pedestal includes a cooling base disposed under the ESC. The pedestal includes a plurality of compartments separated by gas seals and disposed between the cooling base and the one or more heating elements, the plurality of compartments independently controllable to different pressures. One or more controllers independently control pressure in a first of the plurality of compartments to a first pressure and in a second of the plurality of compartments to a second pressure. 
     In one embodiment, a plasma etch system includes a vacuum chamber, a gas source to supply a gas to the vacuum chamber, a pedestal, and an RF generator coupled to at least one of the vacuum chamber, gas source, or pedestal. The pedestal includes an electrostatic chuck (ESC) having a top surface over which a workpiece is to be disposed. The pedestal includes one or more heating elements disposed under the top surface of the ESC, a cooling base disposed under the ESC. The pedestal includes a plurality of compartments separated by gas seals and disposed between the cooling base and the one or more heating elements, the plurality of compartments independently controllable to different pressures. In one embodiment, the system includes one or more controllers to generate a plurality of temperature zones on the ESC surface based on maintaining a first pressure in a first of the plurality of compartments and a second pressure in a second of the plurality of compartments. The plurality of temperature zones can include an inner circular zone and an outer annular zone. In one embodiment, the system includes one or more temperature sensors disposed above the pedestal to detect a temperature of the workpiece. The one or more controllers is to maintain the first pressure in the first of the plurality of compartments and the second pressure in the second plurality of compartments based on a temperature of the workpiece determined by the one or more temperature sensors. 
    
    
     
       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 schematic of a plasma etch system including a pedestal to support a workpiece during plasma processing in accordance with an embodiment of the invention; 
         FIG. 2  is a cross-sectional diagram of a pedestal to support a workpiece during plasma processing in accordance with an embodiment of the invention; 
         FIGS. 3A ,  3 B, and  3 C are top down views of exemplary temperature zones on an electrostatic chuck (ESC) surface, in accordance with embodiments of the invention; 
         FIG. 4  is a top-down view of independently pressure-controllable compartments, in accordance with embodiments of the invention; 
         FIGS. 5A and 5B  are block diagrams illustrating systems with top-down temperature sensors, according to embodiments of the invention; 
         FIG. 6  is a flow diagram of a method of controlling the temperature of a workpiece during plasma processing according to embodiments of the invention; and 
         FIG. 7  illustrates a block diagram of an exemplary computer system for performing methods described herein, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A method, system, and pedestal to control the temperature of a workpiece during plasma processing are described. 
     In embodiments, an electrostatic chuck (ESC) assembly (also referred to herein as a pedestal) includes multi-zone temperature tunability. Multiple zones enable independent fine tuning of the temperature of areas on the ESC surface, which can compensate for temperature non-uniformities on the workpiece from various sources. 
     According to one embodiment, the pedestal includes variable thermal conductivity gaps to provide for the multiple temperature zones. The pedestal can include one or more heating elements which provide the heat flux to heat the ESC. According to one such embodiment, the gaps form compartments between the heating element(s) and a surface of a cooling base. The ESC surface temperature pattern can be adjusted based on, for example, the location, shape, and number of compartments, and by adjusting the thermal conductivity of the individual compartments. In one embodiment, a controller independently adjusts the gas pressure in the individual compartments to generate independent temperature zones. In one embodiment, different gases having different thermal conductivities are used to generate different temperature zones on the ESC surface. 
     In one embodiment, the pedestal includes thermal breaks to minimize cross talk and zone interaction between the individual temperature zones. In one embodiment with thermal breaks that include air gaps, the controller also controls the gas pressure in the thermal breaks. The controller can include, for example, one or more pressure control devices supplying gas to the compartments by means of a multiplexing valve array or a rotary valve assembly. Thus, according to embodiments of the invention, an ESC assembly independently controls gas pressure of multiple compartments located below the ESC and above the cooling base. Embodiments of the invention can—reduce the thermal conductance to the cooling base, thereby requiring less power to maintain any given set point. The lower power levels can result in reduced thermal non-uniformities due to, for example, heating patterns, bonding layer non-uniformities, and cooling channel patterns. 
     In the following description, numerous details are set forth, however, it will be apparent to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known methods and devices are shown in block diagram form, rather than in detail, to avoid obscuring the present invention. Reference throughout this specification to “an embodiment,” or “in one embodiment” means that a particular feature, structure, function, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention, or only one embodiment. Furthermore, the particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment may be combined with a second embodiment anywhere the two embodiments are not specifically denoted as being mutually exclusive. 
     The term “coupled” is used herein to describe functional or structural relationships between components. “Coupled” may be used to indicated that two or more elements are in either direct or indirect (with other intervening elements between them or through the medium) mechanical, acoustic, 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 and 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 mechanical components in the context of an assembly, or in the context of material layers of a micromachined stack. One layer (component) disposed over or under another layer (component) may be directly in contact with the other layer (component) or may have one or more intervening layers (components). Moreover, one layer (component) disposed between two layers (components) may be directly in contact with the two layers (components) or may have one or more intervening layers (components). In contrast, a first layer (component) “on” a second layer (component) is in direct contact with that second layer (component). 
     Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities within the computing system&#39;s registers and/or memories into other data similarly represented as physical quantities within the computing system&#39;s memories, registers or other such information storage, transmission or display devices. 
       FIG. 1  is a schematic of a plasma etch system  100  including a pedestal  142  in accordance with an embodiment of the present invention. The plasma etch system  100  may be any type of high performance etch chamber known in the art, such as, but not limited to, Enabler™, MxP®, MxP+™, Super-E™, DPS II AdvantEdge™ G3, or E-MAX® chambers manufactured by Applied Materials of CA, USA. Other commercially available etch chambers may similarly utilize the pedestals described herein. While the exemplary embodiments are described in the context of the plasma etch system  100 , the pedestal described herein is also adaptable to other processing systems used to perform any plasma fabrication process (e.g., plasma deposition systems, etc.) that places a heat load on a workpiece supported by the pedestal. 
     Referring to  FIG. 1 , the plasma etch system  100  includes a vacuum chamber  105 , that is typically grounded. A workpiece  110  is loaded through an opening  115  and clamped to a pedestal  142 . The pedestal  142  can include independently controllable temperature zones as described herein. The workpiece  110  may be any conventionally employed in the plasma processing art (e.g., semiconductor wafer or other workpiece employed in plasma processing) and the present invention is not limited in this respect. The workpiece  110  is disposed on a top surface of a dielectric material  143  disposed over a cooling base assembly  210 . Process (source) gases are supplied from gas source(s)  129  through a mass flow controller  149  to the interior of the chamber  105  (e.g., via a gas showerhead). Chamber  105  is evacuated via an exhaust valve  151  connected to a high capacity vacuum pump stack  155 . 
     When plasma power is applied to the chamber  105 , a plasma is formed in a processing region over workpiece  110 . A plasma bias power  125  is coupled into the pedestal  142  to energize the plasma. The plasma bias power  125  typically has a low frequency between about 2 MHz to 60 MHz, and may be for example in the 13.56 MHz band. In the exemplary embodiment, the plasma etch system  100  includes a second plasma bias power  126  operating at about the 2 MHz band which is connected to the same RF match  127  as plasma bias power  125  and coupled to a lower electrode via a power conduit  128 . A conductor  190  provides DC voltage a ESC clamp electrode disposed in the dielectric layer  143 . A plasma source power  130  is coupled through a match (not depicted) to a plasma generating element  135  to provide high frequency source power to inductively or capacitively energize the plasma. The plasma source power  130  may have a higher frequency than the plasma bias power  125 , such as between 100 and 180 MHz, and may for example be in the 162 MHz band. 
     The temperature controller  175  is to execute temperature control algorithms and may be either software or hardware or a combination of both software and hardware. The temperature controller  175  may further comprise a component or module of the system controller  170  responsible for management of the system  100  through a central processing unit (CPU)  172 , memory  173 , and input/output (I/O) interfaces  174 . The temperature controller  175  is to output control signals affecting the rate of heat transfer between the pedestal  142  and a heat source and/or heat sink external to the plasma chamber  105 . In the exemplary embodiment, the temperature controller  175  is coupled to a first heat exchanger (HTX) or chiller  177  and a second heat exchanger or chiller  178  such that the temperature controller  175  may acquire the temperature setpoint of the HTX/chillers  177 ,  178  and temperature  176  of the pedestal, and control a heat transfer fluid flow rate through fluid conduits  141  and/or  145  in the pedestal  142 . One or more valves  185  (or other flow control devices) between the heat exchanger/chiller and fluid conduits in the pedestal may be controlled by temperature controller  175  to independently control a rate of flow of the heat transfer fluid to the plurality of fluid conduits  141 ,  145 . In the exemplary embodiment therefore, two heat transfer fluid loops are employed. Other embodiments may include one or more heat transfer loops. Any heat transfer fluid known in the art may be used. The heat transfer fluid may comprise any fluid suitable to provide adequate transfer of heat to or from the substrate. For example, the heat transfer fluid may be a gas, such as helium (He), oxygen (O 2 ), or the like. However, in the exemplary embodiment the heat transfer fluid is a liquid, such as, but not limited to, Galden®, Fluorinert®, or ethylene glycol/water. 
       FIG. 2  is a cross-sectional diagram of a pedestal to support a workpiece during plasma processing in accordance with an embodiment of the invention. The pedestal  200  includes an electrostatic chuck (ESC)  202 . The ESC  202  can be any chuck capable of holding a wafer or substrate during semiconductor processing, for example, a Johnsen Rahbek (JR) chuck, coulombic chuck, etc. ESCs can be mono-polar, bi-polar, or multi-polar. According to one embodiment, the ESC  202  includes a dielectric material over which a workpiece (not shown) is disposed. The dielectric material may be any known in the art. For example, in one embodiment, the dielectric material  143  is a ceramic (e.g., MN, Al 2 O 3 , etc.) capable of maintaining an electrostatic charge near the top surface to electrostatically clamp the workpiece during processing. In one exemplary embodiment, the ESC  202  includes a ceramic puck having at least one electrode (e.g., a mesh or grid) embedded in the ceramic to induce an electrostatic potential between a surface of the ceramic and a workpiece disposed on the surface of the ceramic when the electrode is electrified. 
     In one embodiment, the pedestal  200  further includes a gas distribution mechanism (not shown) to distribute gas between the top ESC surface and the workpiece. Gas between the top ESC surface and the workpiece can create pressure for thermal conduction with the workpiece. According to one embodiment, a gap for gas behind the workpiece is in the range of 10-50 μm. As described below, the gas between the ESC surface and the workpiece can be contained in compartments which are independently controllable. In one embodiment, the amount of contact between the ESC surface and the wafer can be varied to affect the temperature zones. For example, in one embodiment, the contact between the workpiece and the ESC surface is variable between 2-100%. 
     In the embodiment illustrated in  FIG. 2 , the ESC  202  is disposed above one or more heating elements  204 . The one or more heating elements  204  can include AC heating elements, DC electrodes, and/or any other type of heater element capable of providing heat flux for heating the ESC  202 . A single heating element can provide more uniform heating than multiple heating elements, according to one embodiment. The pedestal  200  also includes a cooling base  210  disposed below the ESC  202 . According to one embodiment, the cooling base  210  is capable of substantially uniform cooling of a surface of the cooling base. As mentioned above with respect to  FIG. 1 , the cooling base can include conduits/channels for a heat transfer fluid or gas, as described above. The cooling base  210  can include any number of paths, showerheads, radial pattern(s), counter flow, and/or any other cooling base features. The cooling base  210  can include a single zone (which can enable substantially uniform cooling), or multiple zones (which can enable adjustment flexibility). According to one embodiment, the cooling base  210  is made from a metal such as aluminum. 
     In one embodiment, a plurality of gaps or compartments  206  are disposed between the heater element(s)  204  and the cooling base  210 . The compartments  206  can be disposed over a top surface of the cooling base  210 , or integrated into a top section of the cooling base  210 , according to embodiments. During operation of a plasma etch system including the pedestal  200 , each of the compartments  206  typically contain a gas. For example, the compartments  206  can contain helium, hydrogen, argon, nitrogen, or any other gas with suitable thermal conductivity. As illustrated, in one embodiment, gas seals  208  between the heater element(s)  204  and the cooling base  210  separate the compartments  206 . The gas seals provide isolation between the compartments  206 . The compartments  206  have a sufficiently short height to enable changes in gas composition and/or pressure to change the thermal conductivity of the compartment. For example, the compartments can have a height of 50 μm. 
     One or more controllers  212  and  214  can independently adjust the thermal conductivity of the gas in the compartments  206  to create different temperature zones on the ESC. For example, the compartments  206  in  FIG. 2  correspond to the temperature zone pattern  300   a  of  FIG. 3A . The controller(s)  212  and  214  can adjust the thermal conductivity of the compartments  206  by adjusting, for example, the gas pressure in the compartments  206 , and/or by controlling the gas composition in the compartments  206 . In one embodiment, the controller(s)  212  and  214  independently adjust the pressure of the compartments  206  to be between 1-50 torr. In one embodiment, the controller(s)  212  and  214  independently adjust the pressure of the compartments  206  to be between 1-10 torr. Increasing the gas pressure results in a higher thermal conductivity, and lowering the gas pressure results in a lower thermal conductivity. In an embodiment with heating element(s) and variable thermal conductivity compartments, the heating element(s) can provide a constant heat source (which is substantially independent of RF power applied in the chamber) to effectively generate the different temperature zones based on the thermal conductivity of the compartments  206 . As indicated above, a pedestal with independently pressure controllable compartments such as the embodiment illustrated in  FIG. 2  can minimize power consumption. For example, the pedestal  200  in  FIG. 2  can control the pressure in one or more of the compartments  206  to a low value to minimize heat transfer. By minimizing heat transfer, pedestal  200  can operate in an ‘idle state’ and maintain the temperature of the ESC surface (and therefore workpiece) at a relatively steady temperature with minimal power consumption. 
     The thermal breaks  209  can include gaps to contain gas or a vacuum, or another thermally insulating material to reduce temperature cross talk amongst zones. As illustrated, the thermal breaks  209  are located in the ESC  202 . However, the thermal breaks  209  may be located in the section with the heating elements  204  instead of, or in addition to being located in the ESC  202 . For example, in one embodiment where the heating elements  204  and the ESC  202  are combined in a monolithic ceramic structure, the thermal breaks are located in that combined heating element and ESC structure. 
     The thermal breaks  209  can include a sealing band to contain a gas or vacuum of the thermal break. In one embodiment where the thermal breaks  209  include an air gap to contain a gas, the controller(s)  212  and  214  can further adjust the thermal conductivity of the thermal breaks  209 . For example, in one embodiment, the thermal breaks  209  separating the independently pressure-controlled compartments  206  are controllable to different pressures. In one embodiment, the gas composition in the thermal breaks  209  is controllable. The controller(s)  212  and  214  control the gas pressure and/or gas composition in the thermal breaks  209  via valves and tubes fed through the gas seal regions  208 , in accordance with an embodiment. According to one embodiment, the thermal breaks  209  are approximately 50 μm thick. In other embodiments, the thermal breaks  209  can have a thickness greater or less than 50 μm that minimizes thermal cross talk. The illustrated thermal breaks  209  have the same width, however, other embodiments can include thermal breaks having different dimensions. Additionally, the thermal breaks  209  are illustrated as having the same width as the gas seals  208 , however, the thermal breaks  209  and gas seals  208  may have different dimensions. The thermal breaks  209  and/or any other compartment walls can be polyimide (PI), a ceramic, or any other suitable material. 
     In one embodiment, the pedestal can include an additional plurality of compartments (not shown) disposed above the ESC which are independently controllable to different thermal conductivities. In embodiments with an additional plurality of compartments above the ESC, the thermal conductivity can be adjusted in the same ways as described with respect to the compartments  206  between the heating elements(s)  204  and the cooling base  210 . 
     As mentioned briefly above, the ESC  202 , the one or more heating elements  204 , and the plurality of compartments  206  can be comprised in a monolithic body (e.g., integrated into the bulk ceramic of the ESC  202 ). Alternatively, the heating element(s)  204  and/or the compartments can be manufactured separately and integrated by, for example, bonding, mechanical clamping, or other means of coupling the heating element(s)  204  and compartments  206  with the ESC  202 . In one embodiment, the compartments  206  are comprised in the cooling base  210 . 
     Returning to the controller(s)  212  and  214 , the gas pressure and/or composition can be controlled by a standard control device supplying gas to the compartments  206 . For example, the pedestal  200  can include a dedicated pressure controller per zone, a multiplexing valve array, a rotary valve assembly, or some combination thereof. The embodiment illustrated in  FIG. 2  includes the gas pressure controller  212 , which includes a multiple valve array. A multiplexer  214  determines which valves to open (and therefore, which compartment(s)  206  to adjust) based on one or more ‘select’ inputs (not shown). As described above, the controller(s)  212  and  214  can independently adjust the thermal conductance of each of the compartments  206 , which generates temperature zones on the ESC surface. 
     Thus, in one embodiment, one or more controllers generate a plurality of temperature zones on the ESC surface based on maintaining different pressures in different compartments and/or thermal breaks. The temperature zones are also affected by heating by the heating element(s), the configuration of the compartments, and cooling by the cooling base of the interface between the cooling base and the plurality of compartments. The temperature zones can enable compensation for temperature non-uniformities to keep a workpiece at a uniform temperature, or can enable different temperature zones on the workpiece (e.g., heating the edge of the workpiece to a different temperature than the center to compensate for chemistry differences). 
       FIGS. 3A-3C  are top-down views of exemplary temperature zones on an electrostatic chuck (ESC) surface, in accordance with embodiments of the invention.  FIGS. 3A ,  3 B, and  3 C illustrate three different configurations of temperature zones, although any configuration and number of zones are possible. A greater number of zones permits finer temperature control of the workpiece. 
       FIG. 3A  illustrates a top-down view of an ESC surface  300   a  with two different temperature zones: an internal or inner zone  302  and an external or outer zone  304 . In this illustrated embodiment, the inner zone  302  is circular (i.e., substantially circular), and the outer zone  304  is annular (i.e., substantially annular).  FIG. 3B  illustrates a top-down view of an ESC surface  300   b  with three temperature zones. In addition to an inner zone  302  and an outer zone  305 , the ESC surface  300   b  has a middle annular temperature zone in between the outer annular temperature zone and the inner circular zone. In addition to different zones at different radii, the ESC surface can have temperature zones azimuthally. For example,  FIG. 3C  illustrates a top-down view of an ESC surface  300   c  with six different temperature zones. Like the ESC surface  300   b  in  FIG. 3B , the ESC surface  300   c  includes an inner zone  302  and a middle zone  303 . Additionally, the embodiment illustrated in  FIG. 3C  includes an outer substantially annular zone which is azimuthally divided into smaller sub-zones  307 . A configuration with the outer zones  307  could be beneficial for systems having chambers with asymmetries (e.g., a door on one side, but not on another side). Azimuthally segmented annular temperature zones at the edge of an ESC (such as zones  307 ) can enable compensation for temperature non-uniformities caused by such chamber features. Other configurations with azimuthally segmented zones are also possible. For example, one embodiment includes an inner circular zone such as  302  of  FIG. 3A , and an outer annular zone  304  which is azimuthally segmented. Embodiments can include any number of azimuthally segmented sub-zones (e.g., 2, 3, 4, 5, or more than 5 azimuthally segmented sub-zones). 
       FIG. 4  is a top-down view of independently pressure-controllable compartments, in accordance with embodiments of the invention. As illustrated in  FIG. 4 , according to one embodiment, the compartments  402  and  407  are patterned. 
     According to one embodiment, a bottom or top surface of a compartment is patterned. The pattern can include multiple protrusions or high points (also known as mesas), which can be flat, rounded, or in another shape. For example, the compartments  402  and  407  are patterned with protrusions  404 . The protrusions can be, for example, 10-50 μm high. In one embodiment, the protrusions have a diameter of 0.55 mm. In another embodiment, the protrusions have a diameter of 0.5-1 mm. Other embodiments can include other diameters and heights suitable for the size of the gas compartments. 
     In one embodiment, a method of making the protrusions includes applying a mask to the surface to be patterned, and patterning or embossing the surface via bead blasting, sand blasting, etching, or any other process capable of generating the desired pattern. In another embodiment, the plurality of protrusions are generated via deposition of materials on the compartment surface. A compartment which comprises a gap with a patterned surface has the advantage of being easy to manufacture while at the same time permitting a very small gap for efficient and effective heat transfer. However, other embodiment may include compartments which are not patterned, or have different patterns than the pattern depicted in  FIG. 4 . 
       FIGS. 5A and 5B  are block diagrams illustrating systems with top-down temperature sensors, according to embodiments of the invention. Traditional temperature measurement techniques include embedding probes in the ESC or other parts of the pedestal. Such traditional temperature probes include, for example, resistance temperature detectors (RTD), thermocouples, Fluoroptic thermometers, infrared (IR) sensors, or any other temperature probes capable of being integrated into the pedestal. Such sensors measure the temperature of the pedestal (e.g., the temperature of the ESC), and therefore only indirectly measure the temperature of the wafer or substrate being processed. A temperature difference typically exists between the ESC and the wafer, and therefore measurements of the ESC temperature may not accurately reflect the temperature of the wafer. Additionally, including sensors within the ESC or other parts of the pedestal can cause additional temperature non-uniformities due to, for example, holes or gaps to accommodate the sensors and/or cables. 
     As illustrated in  FIGS. 5A and 5B , a top down measurement system can detect the temperature of the wafer rather than (or in addition to) relying on indirect temperature measurements of the ESC. In one embodiment, a plasma processing system includes one or more temperature sensors disposed above the pedestal to detect a temperature of the workpiece. The system  500   a  of  FIG. 5A  is a block diagram illustrating a single overhead temperature sensor  504  capable of detecting the temperature of a workpiece  502  and/or ESC over which the workpiece is disposed. The system  500   b  of  FIG. 5B  is a block diagram illustrating multiple overhead temperature sensors  504 . Although systems  500   a  and  500   b  illustrate overhead temperature sensors, other embodiments can include side temperature sensors in addition to, or in place of, overhead temperature sensors. Similar to overhead temperature sensors, side temperature sensors can measure the temperature of the wafer directly. In one embodiment, the number of overhead and/or side temperature sensors is equal to the number of temperature zones. Other embodiments can include more or fewer temperature sensors than temperature zones. 
     In one such embodiment, temperature sensors  504  include infrared (IR) cameras. In one embodiment, temperature sensors  504  include a scanning laser system. Other embodiments can include any other type of temperature sensor capable of measuring temperature at a distance from the wafer. Systems with such overhead and/or side temperature sensors can collect data from multiple sensors, and process the collected data to generate an image of the wafer. As mentioned above, the temperature sensor(s)  504  can be used with standard temperature probes which measure the ESC temperature (e.g., temperature sensors embedded in the pedestal). The image of the wafer—and/or other temperature data collected from embedded probes—can be used to determine temperature variations on the wafer, and determine how to independently control the temperature zones on the ESC surface. For example, temperature data collected from overhead temperature sensors  504  can be used by the controllers  212  and  214  of  FIG. 2  to control the thermal conductivities of the compartments  206 . According to one embodiment, a controller can select one or more areas of the workpiece to take temperature measurements from to use for controlling the temperature zones of the ESC surface. For example, the temperature at the edge of the wafer, the center of the wafer, and/or the temperature at some other area(s) of the wafer can be used to determine adjustments to the temperature zones of the ESC surface. 
       FIG. 6  is a flow diagram of a method of controlling the temperature of a workpiece during plasma processing according to embodiments of the invention. The method  600  begins at operation  602  with heating an electrostatic chuck (ESC). For example, the method  600  can include heating the ESC  202  of  FIG. 2  with heating element(s)  204 . Heating the ESC can also involve heating due to processing a workpiece disposed over the ESC (e.g., due to plasma etch processing). At operation  604 , the method involves cooling the ESC with a cooling base. The cooling base is disposed under and set apart from the ESC by compartments. For example, the cooling base  210  of  FIG. 2  can provide a heat sink for the ESC  202 . The cooling base  210  in  FIG. 2  illustrates an embodiment where the cooling base is separated from the ESC by compartments and/or thermal breaks. At operation  606 , one or more controllers independently control the thermal conductivity of the compartments between the ESC and the cooling base. For example, the controllers  212  and  214  can independently control the gas composition and pressure of the compartments  206  to generate temperature zones on the ESC surface. The generated temperature zones can enable fine control of the temperature of a workpiece held by the ESC during processing. 
       FIG. 7  illustrates a block diagram of an exemplary computer system for performing methods described herein, in accordance with an embodiment of the present invention. The exemplary computer system  700  includes a processor  702 , a main memory  704  (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), a static memory  706  (e.g., flash memory, static random access memory (SRAM), etc.), and a secondary memory  718  (e.g., a data storage device), which communicate with each other via a bus  730 . 
     Processor  702  represents one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processor  702  may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, etc. Processor  702  may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processor  702  is configured to execute the processing logic  726  for performing the operations and steps discussed herein. 
     The computer system  700  may further include a network interface device  708 . The computer system  700  also may include a video display unit  710  (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device  712  (e.g., a keyboard), a cursor control device  714  (e.g., a mouse), and a signal generation device  716  (e.g., a speaker). 
     The secondary memory  718  may include a machine-accessible storage medium (or more specifically a computer-readable storage medium)  731  on which is stored one or more sets of instructions (e.g., software  722 ) embodying any one or more of the methodologies or functions described herein. The software  722  may also reside, completely or at least partially, within the main memory  704  and/or within the processor  702  during execution thereof by the computer system  700 , the main memory  704  and the processor  702  also constituting machine-readable storage media. The software  722  may further be transmitted or received over a network  720  via the network interface device  708 . 
     While the machine-accessible storage medium  731  is shown in an exemplary embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable storage medium” shall also be taken to include any medium that is capable of storing or encoding a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present invention. The term “machine-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, optical and magnetic media, and other non-transitory machine-readable storage medium. 
     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 necessarily 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.