Abstract:
An improved heating system for heating a semiconductor wafer during fabrication in a corrosive manufacturing environment is disclosed. The system includes a novel ceramic heater made of a layered ceramic substrate that has a plurality of heating elements and temperature sensor arrangement completely and directly embedded within the ceramic substrate of the ceramic heater. The heating elements and the temperature sensor arrangement are constructed of a molybdenum and aluminum nitride composite that provides a low temperature coefficient of resistance which improves the operating efficiency of the ceramic heater. In operation, the temperature sensor arrangement transmits temperature readings to a microprocessor capable of controlling the heating elements in such a manner as to provide a constant and uniform temperature distribution along the entire surface of the semiconductor wafer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a heating system for use in a semiconductor processing such as in a Chemical Vapor Deposition (CVD) and etching environment, and more particularly to a heating system having a ceramic heater with a plurality of individually controlled heating zones. More specifically, the present invention relates to a ceramic heater having heating elements operatively associated with one or more sensors embedded in the ceramic substrate of the ceramic heater.  
           [0003]    2. Prior Art  
           [0004]    During the process of manufacturing a semiconductor wafer in a CVD and etching process it is important that the heating system maintain a uniform temperature along the entire surface of the semiconductor wafer. If a uniform temperature is not maintained along the entire surface of the semiconductor wafer, the manufacturing process is compromised and the quality of the semiconductor wafer is diminished. However, a CVD and etching environment is a harsh chemical environment containing highly corrosive gases, such as oxygen, chlorine and fluorine series gases, that may impact and cool only one portion of the exterior surface of the semiconductor wafer, therefore making it difficult to maintain a uniform temperature along the exterior surface of the wafer by the ceramic heater.  
           [0005]    Because many ceramic heaters of the prior art normally include a single temperature sensor, such heaters cannot provide a constant and uniform heat distribution along the entire surface of the semiconductor wafer when cooler gas impact only a portion of the semiconductor wafer. Further, the highly corrosive CVD and etching environment eventually degrades and corrodes temperature sensing arrangements having multiple temperature sensors and electrical leads located along the exterior surface of the ceramic heater in order to detect temperatures along multiple zones of the ceramic heater surface. Other prior art devices teach a centrally located temperature sensor disposed inside a centralized hollow shaft of the ceramic heater in order to protect the leads and prevent corrosion of the sensor arrangement; however, this prevents accurate temperature readings along multiple portions of the semiconductor wafer surface since a single sensor can only take readings from the center portion of the ceramic heater because of the centralized location of the sensor.  
           [0006]    Accordingly, ceramic heaters for use in the manufacture of semiconductor wafers suffer from several principal drawbacks. First, such heaters are unable to provide a sensor arrangement that can take temperature readings along the peripheral portions of the ceramic heater such that a uniform and constant temperature can be maintained along the entire surface of the semiconductor wafer because of the prolonged exposure of the temperature sensors to the corrosive gases contained in a CVD and etching environment seriously degrades the sensor&#39;s capabilities over time. Second, prior art ceramic heaters have heating elements that are manufactured from refractory metal materials make the heating element of the ceramic heater inefficient and costly to operate. This inefficiency is caused by a high Temperature Coefficient of Resistance (“TCR”) exhibited by the prior art heating elements which can cause an undesirable high “rush in” current during start-up of the ceramic heater which makes power management more costly and inefficient.  
           [0007]    For example, U.S. Pat. No. 6,066,836 to Chen et al. discloses a prior art ceramic heater for use in a CVD and etching environment having one sensor to take temperature readings which are relayed to a microprocessor in order to adjust the temperature of various heating elements in the ceramic heater. Since only one sensor is utilized in the Chen et al. device it can only sense the temperature along the central portion of the semiconductor wafer and cannot maintain a uniform heat distribution along the entire surface of the semiconductor wafer. Additionally, this type of ceramic heater includes a heating element that suffers from a high TCR which causes the ceramic heater to be inefficient when operated.  
           [0008]    U.S. Patent Application Publication No. 2002/0134775 to Ohashi discloses a ceramic heater with two distinct heating zones controlled by sensors located along the outer and peripheral portions of the ceramic heater. The Ohashi sensors are located in aluminum nitride tube members formed through the ceramic substrate of the ceramic heater that still expose the sensors to the corrosive effects of gases in a CVD and etching environment. As noted above, prolonged exposure to such gases eventually degrades the performance of the sensors and prevents the ceramic heater from achieving a uniform temperature along the entire surface of the semiconductor wafer. Furthermore, such a sensor arrangement may also cause a heat sink effect that compromises the temperature uniformity of the heater due to heat loss through conduction. Also, this reference does not teach or suggest the incorporation of a heating element made from a material that exhibits a low TCR during operation.  
           [0009]    Therefore, there appears a need in the art for a ceramic heater comprising a plurality of distinct and independently controlled heating zones operatively associated with sensors that are completely and directly embedded inside the ceramic substrate of the heater in order to protect the sensor elements from the corrosive effects of gases in a CVD and etching environment. Furthermore, there also appears a need in the art for a ceramic heater that has a heating element made of material which exhibits a low TCR that is efficient to operate.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0010]    Accordingly, the primary object of the present invention is to provide a heating system including a multi-zone ceramic heater that maintains a constant and uniform temperature along the surface of a semiconductor wafer in a CVD and etching environment.  
           [0011]    Another object of the present invention is to provide a ceramic heater having a heating element made of a ceramic composite material consisting of molybdenum and aluminum nitride.  
           [0012]    Another further object of the present invention is to provide a heating element that exhibits a low temperature coefficient of resistance.  
           [0013]    Yet another object of the present invention is to provide a ceramic heater having temperature sensors and heating elements that are completely and directly embedded in the ceramic substrate of the heater.  
           [0014]    Another object of the present invention is to provide a method of manufacturing a ceramic heater that completely encases the electrical components in the ceramic substrate of the ceramic heater in order to prevent corrosion of such components in a corrosive working environment.  
           [0015]    These and other objects of the present invention are realized in the preferred embodiment of the present invention, described by way of example and not by way of limitation, which provides for a heating system with a multi-zone ceramic heater having embedded sensor and heating element arrangements that protect these elements from the corrosive CVD and etching environment as well as provide a heating element that exhibits a low temperature coefficient of resistance.  
           [0016]    In brief summary, the present invention overcomes and substantially alleviates the deficiencies in the prior art by providing a heating system with a ceramic heater having plurality of heating elements in operative association with at least one temperature sensor. Each temperature sensor is dedicated to one particular heating zone and is capable of sensing the temperature at a particular location of the heater and communicate such temperature readings to a microprocessor. The microprocessor adjusts the heat generated at a respective heating element in a particular heating zone in order to maintain a constant and uniform temperature along the wafer. For example, if one portion of the semiconductor wafer is too cool due to the impact of cool gases along a portion of the semiconductor wafer the sensor in a particular heating zone detects the cooler temperature in the ceramic heater. The sensor then transmits a signal to the microprocessor for increasing the power to heating elements of that heating zone for increasing the amount of heat applied to that particular portion of the semiconductor wafer until a uniform temperature pattern is achieved along the surface of the wafer. The multi-zone arrangement of sensors along the entire ceramic heater is made possible because the sensors and heating elements are completely and directly embedded inside the ceramic substrate during manufacture without exposing any of the electrical components to the corrosive CVD and etching environment.  
           [0017]    The ceramic heater according to the present invention is comprised of a ceramic substrate which is manufactured using a layering process wherein a series of ceramic layers are constructed from a composite material that is laminated and sintered. All of the ceramic layers are constructed of aluminum nitride with two of the ceramic layers having additional composites of molybdenum and aluminum nitride to form the sensor and heating element arrangements of the invention.  
           [0018]    The multi-zone heating element arrangement is manufactured by splicing a pre-formed resistor tape that is cut into a particular design that may be spliced into the tape made of aluminum nitride that produces the ceramic layer. Preferably, these heating elements are constructed from a molybdenum and aluminum nitride composite. The inventors have discovered that the use of a molybdenum and aluminum nitride composite material for the manufacture of heating elements produces a heating element that exhibits a significantly lower TCR than prior art heating elements. This lower TCR allows the heating elements to have a lower “rush in” current during start-up when power is first supplied to the ceramic heater, thereby increasing the heater&#39;s operational efficiency.  
           [0019]    The heating elements of the present invention are controlled by a microprocessor which communicates with a plurality of sensor arrangements that are also embedded directly within the ceramic substrate of the ceramic heater during manufacture. Power is supplied to the sensor and heating element arrangements through respective conductive pathways formed through the ceramic substrate of the ceramic heater that engage lead wires fed through the hollow shaft of the heater. The temperature sensors are preferably Resistive Temperature Detecting Devices that are applied directly on the surface of the tape used to produce one of the ceramic layers using a composite material that is constructed from molybdenum and aluminum nitride. Each temperature sensor is manufactured by applying a molybdenum and aluminum nitride ink to form a specifically designed pattern on the surface of one of the ceramic layers. The ink is applied so that it communicates with a conductive pathway formed through the ceramic substrate to supply power and communicate temperature readings to the microprocessor.  
           [0020]    Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and will become apparent to those skilled in the art upon examination of the following more detail description and drawings in which like elements of the invention are similarly numbered throughout.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a perspective view of the heating system showing the ceramic heater and the microprocessor according to the present invention;  
         [0022]    [0022]FIG. 2 is a side view of the ceramic heater according to the present invention;  
         [0023]    [0023]FIG. 3 is a cross sectional view of the ceramic heater taken along line  3 - 3  of FIG. 1 according to the present invention;  
         [0024]    [0024]FIG. 4 is a top view of one of the layers of tape that comprise the ceramic heater showing the heating elements spliced into the aluminum nitride composite according to the present invention;  
         [0025]    [0025]FIG. 5 is a top view of the one of the layers of tape that comprise the ceramic heater showing the temperature sensors being applied to the layer of aluminum nitride composite according to the present invention;  
         [0026]    [0026]FIG. 6 is an exploded view of the ceramic heater depicting the series of ceramic layers that comprise the ceramic substrate according to the present invention;  
         [0027]    [0027]FIG. 7 is a table comparing the temperature coefficient of resistance of a prior art heating element against that of a heating element made of a molybdenum and aluminum nitride composite according to the present invention; and  
         [0028]    [0028]FIG. 8 is a table which shows that resistance increases with rising temperature for an embedded sensor according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring to the drawings, the preferred embodiment of the multi-zone heating system for use in a CVD and etching environment according to the present invention is illustrated and generally indicated as  10  in FIG. 1. Heating system  10  provides a means for maintaining a uniform and constant temperature along a surface of a semiconductor wafer (not shown), while protecting the electrical components of the heating system from the corrosive gases in the CVD and etching environment.  
         [0030]    Referring to FIGS. 1 and 6, ceramic heater  12  comprises a ceramic body  16  having a generally disk-like configuration attached to a hollow shaft  18 . Ceramic body  16  is made of a ceramic substrate constructed from a series of ceramic layers  26 ,  28 ,  30 ,  32 ,  34  and  36  made from an electrically conductive composite material having at least two or more heating elements  44  completely and directly embedded in the ceramic substrate with at least one temperature sensor  38  in operative association with one or more heating elements  44  through a microprocessor  14 . Ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  are laminated and sintered together during manufacture which forms a ceramic body  16  that completely and directly embeds heating elements  44  and temperature sensors  38  in the ceramic substrate of ceramic heater  12  as shall be explained in greater detail below.  
         [0031]    Referring to FIG. 1, heating elements  44  are operably controlled by a microprocessor  14 . When temperature sensors  38  sense that one area of the heater is too cold due to a temperature drop in semiconductor wafer or too warm such that a variance in temperature has occurred, the microprocessor  14  will increase or decrease power to specific heating elements  44  in order to provide and maintain a uniform temperature distribution across the entire surface of the semiconductor wafer.  
         [0032]    As illustrated in FIG. 4, maintaining a uniform temperature along the semiconductor wafer is achieved by the grouping of the heating elements  44  within discrete heating zones  28 A and  28 B along the ceramic layer  28  of body  16 . During manufacturing, heating elements  44  and temperature sensors  38  are completely embedded within the ceramic substrate by application of heating elements  44  and temperature sensors  38  on respective ceramic layers that comprise ceramic body  16 .  
         [0033]    Referring to FIG. 6, the ceramic body is constructed of ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  which are laminated and then sintered together during manufacture. It should be noted that any number of ceramic layers could be used to construct ceramic heater  12  and still fall within the scope of the invention. In the preferred embodiment, thick tapes are used to manufacture the ceramic layers  28  and  32  having heating elements  44  and temperature sensors  38  which are interposed between the other ceramic layers  26 ,  30 ,  34  and  36  formed from thick tapes.  
         [0034]    The tapes used to make ceramic layers  26 ,  30 ,  34  and  36  are constructed from aluminum nitride, while the tapes for ceramic layers  28  and  32  are also formed from the same aluminum nitride, but are used to form the base for heating elements  44  and temperature sensors  38 , respectively. Ceramic layer  28  further includes an additional composite material spliced with the aluminum nitride of the thick tape to form at least two separate heating zones  28 A and  28 B made preferably from a molybdenum and aluminum nitride composite. Further, ceramic layer  32  includes a molybdenum and aluminum nitride ink that is applied directly to the surface of the ceramic layer  32  which will form the temperature sensors  38 .  
         [0035]    As noted above, ceramic layer  28  forms a multi-zone heating element arrangement made by splicing a molybdenum and aluminum nitride composite that will function as heating elements  44  with a tape made from an aluminum nitride composite. Preferably, the volume percentage ratio of molybdenum to aluminum nitride in the composite material can be in a range of 20%-55% molybdenum to 80%-45% aluminum nitride. This heating element composite is cut out from the composite tape to produce at least two separate heating elements  44  and spliced into the tape made from the aluminum nitride to form separate heating zones. As shown in FIG. 4, the splicing of the pre-formed cut-out into tape of ceramic layer  28  results in layer  28  being divided into two separate heating zones,  28 A and  28 B. Each heating zone  28 A and  28 B includes at least one separate and distinct heating element  44  that may be operated to provide and maintain a uniform temperature along the surface of the semiconductor wafer.  
         [0036]    The inventors have also discovered that the construction of heating elements  44  from a molybdenum and aluminum nitride composite allows such elements  44  to have a lower TCR (i.e. 0.0015/° C.) than heating elements  44  made from refractory metal such as molybdenum which has a higher TCR of 0.00435/° C. This lower TCR for heating elements  44  made from a molybdenum and aluminum nitrate composite produces a low “rush in” current when the ceramic heater  12  is first activated during start up, thereby making it more energy efficient to operate.  
         [0037]    As noted above, heating elements  44  are operatively associated with at least one temperature sensor  38 , respectively, which are located along ceramic layer  32  as shown in FIG. 5. Ceramic layer  32  is made from the same aluminum nitride composite that is utilized in ceramic layers  26 ,  30 ,  34 , and  36  with the exception that a composite of molybdenum and aluminum nitride ink is applied to the surface of the aluminum nitride tape layer during manufacture to form temperature sensors  38 . The ratio of molybdenum in temperature sensors  38  can be from 20%-100% molybdenum with the remainder being aluminum nitride. Most preferably, as noted above, this composite of molybdenum and aluminum nitride is in the form of a liquid ink mixture which is applied directly to the surface of the tape for ceramic layer  32  using methods known in the art. Therefore, when the ink is first applied to tape of ceramic layer  32  it will infiltrate a small portion of the aluminum nitride tape and the proportion of aluminum nitride in the ink will increase within the overall molybdenum and aluminum nitride composite that forms temperature sensor  38 . Accordingly, even if 100% molybdenum is first applied, the temperature sensor will contain a small proportion of aluminum nitride derived from aluminum nitride infiltrating from the tape.  
         [0038]    Referring to FIG. 3, temperature sensors  38  and heating elements  44  are connected to conductive pathways  40  and  41 , respectively. Preferably, conductive pathways  40  and  41  are formed by boring axial pathways through ceramic body  16  that terminates adjacent to hollow shaft  18  for each heating element  44  and temperature sensor  38  of ceramic heater  12 . The axial pathways  40  and  41  are then completely back filled with a slug or powder of conductive material made from a molybdenum and aluminum nitride composite. Preferably, the conductive material has a ratio of molybdenum to aluminum nitride that is equal to the same ratio of molybdenum and aluminum nitride used in the construction of heating elements  44 .  
         [0039]    Connected to each of the conductive pathways  40  and  41  at its terminal point are a pair of lead wires  42  which are operatively associated with microprocessor  14  and a power source (not shown). Preferably, microprocessor  14  provides a means for receiving signals from temperature sensors  38  which is then converted to temperature readings based on a predetermined resistance vs. temperature calibration curve and automatically adjusts the power applied to respective heating elements  44  in heating zones  28 A and  28 B in order to maintain a constant and uniform temperature along the semiconductor wafer.  
         [0040]    As noted above, ceramic heater  12  is manufactured by the creation of a tape for each ceramic layer  26 ,  28 ,  30 ,  32 ,  34 , and  36  and the modification of the tapes for ceramic layers  28  and  32  to include heating elements  44  and temperature sensors  38 , respectively. The initial process to construct tapes for ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  is similar for each tape. An aluminum nitride powder is mixed with organic binders and solvents, such as acrylic and toluene, to form a slurry. The slurry is cast by a doctor blade over a plastic carrier to allow the slurry to dry and form a tape.  
         [0041]    As further noted above, ceramic layer  28  is manufactured from two separate tapes being cut and then spliced together to form heating elements  44 . The first tape is comprised of aluminum nitride which is manufactured in an identical manner to the tapes used to produce ceramic layers  26 ,  30 ,  32 ,  34 , and  36 . The second tape is made from a molybdenum and aluminum nitride composite material and serves as the heating element portion of the ceramic layer  28 . The volume percentage ratio of molybdenum is 20%-45% with the remainder of the composite being comprised of aluminum nitride. Once the two tapes for ceramic heater  28  have been created they are spliced together to form one continuous tape. The heating element portion now spliced within tape of ceramic layer  28  may comprise two or more separate and distinct heating zones made from a molybdenum and aluminum nitride composite which form heating elements  44  of ceramic heater  12 . Preferably, at least two distinct heating elements  44  are spliced into the tape to allow for at least two individually controlled heating zones  28 A and  28 B, although the present invention contemplates that more than two heating zones may be produced.  
         [0042]    The splicing process is achieved by removing a set portion of the aluminum nitride from the tape and replacing it with an identical set portion cut from a tape containing a electrically conductive molybdenum and aluminum nitride composite that serves as heating element  44 . Preferably, each respective tape may be cut by a laser, dye cut, or other known methods in the art. After the two tapes are cut each set portion should have a shape that is converse to the other. As shown in FIG. 4 the shapes are generally circular in the preferred embodiment, however any suitable configuration, such as angular, square or any combination of the above, is felt to fall within the scope of the present invention. When the set portions are spliced together each will interlock with one another to form a flat, continuous tape for ceramic layer  28  with heating elements  44  embedded within the ceramic substrate of ceramic heater  12 . Referring to FIG. 7, heating elements  44  made in accordance with the present invention would have a much lower TCR than has been found in heating elements constructed from traditional refractory metals of the prior art. For example, a heating element constructed from only Molybdenum has a TCR as indicated on FIG. 7 of 0.00435/° C., while heating elements  44  made from a composite of molybdenum and aluminum nitride in accordance with the present invention have a TCR of 0.0001/° C. which is significantly lower than prior art devices.  
         [0043]    The temperature sensors  38  applied to ceramic layer  32  are formed from an ink which is a liquid mixture of molybdenum combined with aluminum nitride. The molybdenum and aluminum nitride ink is applied to the tape of ceramic layer  32  with a specific pattern and allowed to dry, thereby creating at least two temperature sensors  38 . In the preferred embodiment, the number of temperature sensors  38  will be equal to at least the number of heating zones as each temperature sensor  38  will be operatively associated with at least one respective heating element  44  through microprocessor  14  as shall be explained in greater detail below. Referring to FIG. 8, temperature sensors  38  are preferably Resistive Temperature Detecting (RTD) devices that exhibit a level of resistance that is dependent upon the temperature.  
         [0044]    Once tape of ceramic layer  32  has the molybdenum and aluminum nitride ink applied thereto and the tape of ceramic layer  28  is fabricated with heating elements  44  spliced therein, the next step is to assemble the tapes of ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  to form unitary ceramic body  16 .  
         [0045]    Referring to FIG. 6, the tapes of ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  are assembled with each tape being stacked one on top of the other in successive order. After the tapes have been properly stacked conductive pathways  40  and  41  are formed for each heating element  44  and temperature sensor  38 , respectively, as shown in FIG. 3, for providing power and permitting operative association between the heating elements  44  and temperature sensor  38  through microprocessor  14 . Conductive pathway  40  is formed for each heating element  44  by making a hole through the tape of ceramic layers  26  and  28 . After the hole has been formed it is back filled with either a plug or powder made from a molybdenum and aluminum nitride composite similar to the composite used to make heating element  44 . This backfill material forms conductive pathway  40  which links heating element  44  to lead wires  42  disposed inside hollow shaft  18 . In similar fashion, conductive pathway  41  is formed by making a hole through the tape of ceramic layers  26 ,  28 ,  30  and  32  for each temperature sensor  38  which is also backfilled with a molybdenum and aluminum nitride composite material such that a conductive path is established between each temperature sensor  38  and lead wires  42 . The ratio of molybdenum to aluminum nitride for the composite that forms conductive pathways  40  and  41  should be equal to that used to form heating elements  44  in the preferred embodiment of the invention.  
         [0046]    After a suitable number of conductive pathways  40  and  41  are created, the tapes of ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  are laminated and sintered to create ceramic body  16 . The lamination is accomplished by placing all of the stacked tapes in a conventional mold and then applying traditional lamination techniques to the surface of the tapes. Next, the ceramic layers  26 ,  28 ,  30 ,  32 ,  34 , and  36  are sintered together at a temperature range of between 1700 to 1900 degrees centigrade under a pressure of between 100 to 3000 pounds per square inch. It should be noted that although specific ranges are given, any range of temperatures and pressures that would effectively sinter tapes together to form a ceramic substrate would fall within the scope of the present invention. Any known means of sintering the plates together, such as a hot isostatic press, can be used and still fall within the scope of the present invention.  
         [0047]    After the sintering process is completed lead wires  42 , preferably constructed of nickel, are brazed to each conductive pathway  40  and  41  and are then fed through hollow shaft  18  which is attached to ceramic body  16  by means well known in the art. The free ends of the lead wires  42  are then connected to microprocessor  14  which operatively associates the microprocessor  14  with heating elements  44  and temperature sensors  38  as well as a power source.  
         [0048]    In operation, heating system  10  is capable of providing a constant and uniform temperature along the surface of the semiconductor wafer because of the multi-zone heating arrangement due to heating elements  44  and temperature sensors  38  being embedded along all portions of ceramic body  16 . When a semiconductor wafer is first placed upon ceramic heater  12  certain portions of the wafer may become cooler than other portions due to the impact of cooler gases upon only a portion of the semiconductor wafer.  
         [0049]    To correct this temperature variance across the surface of the semiconductor wafer, microprocessor  14  receives temperature readings from various temperature sensors  38  through a specifically designed wafer (not shown) with a calibrated curve that transforms a resistance signal into a temperature reading signal. The temperature sensor  38  then transmits a signal through lead wire  42  to respective heating elements  44  located in zone  28 A (or  28 B). A respective heating element  44  would then have its power adjusted by microprocessor  14  so that more heat is provided to a particular heating zone than was being applied to the other heating zone. This additional heating warms the area of the semiconductor wafer that was too cool due to the impact of cool gasses. Additional heat to that particular heating zone would continue until the temperature readings being transmitted from both heating zones  28 A and  28 B were substantially equal. Due to this balancing of temperatures between heating zones  28 A and  28 B the entire surface of the semiconductor wafer would remain at a constant temperature. Moreover, since heating elements  44  and temperature sensors  38  are completely and directly embedded within the ceramic substrate of body  16  they would not be subject to the corrosive environment of a CVD and etching environment.  
         [0050]    It should be understood from the foregoing that, while particular embodiments of the invention have been illustrated and described, various modifications can be made thereto without departing from the spirit and scope of the invention. Therefore, it is not intended that the invention be limited by the specification; instead, the scope of the present invention is intended to be limited only by the appended claims.