Abstract:
A heater assembly for an ALD or CVD reactor provides protection for an electrical conductor associated with a heating element by using a purge gas to isolate the conductor from the corrosive environment of the reactor chamber. The purge gas is introduced into a sleeve surrounding the conductor and from there is allowed to leak into the reactor chamber to be pumped out with the process gasses. This arrangement avoids the need for airtight seals at the junction of the sleeve and the heating element easing manufacturing requirements and potentially reducing component costs.

Description:
RELATED APPLICATIONS 
     This application is related to and claims the priority benefit of U.S. Provisional Application No. 60/446,892, titled “Purged ALN Heater Susceptor” filed Feb. 11, 2003 incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to the field of chemical vapor deposition and atomic layer deposition reactors and, more specifically, the heating apparatus for such reactors. 
     BACKGROUND OF THE INVENTION 
     The modern electronics we rely on today are comprised of many complex components. One such component is the microelectronic chip made up of thousands or millions of tiny transistors. These transistors are currently manufactured on semiconductor wafers by a process known as lithography. However, prior to the lithography process, the semiconductor wafer must be manufactured from a standard silicon wafer. In order to produce a semiconductor wafer, the silicon wafer must undergo a process of applying one or more layers of varying materials, usually metals, onto its surface. Two examples of such processes are CVD (Chemical Vapor Deposition) and ALD (Atomic Layer Deposition). 
     CVD is a process wherein a thin film of material is layered or deposited on varying materials, including semiconductors, insulators, and metals. The deposited film or films are formed as a result of chemical reactions between gaseous reactants at elevated temperatures within a reactor chamber. 
     ALD is another process for creating films on a surface within a heated reactor chamber. In this process, the deposition of each atomic layer of material is controlled by a pre-deposited layer of precursor. Precursors of various components are introduced alternately to produce the film on the targeted surface. As with CVD, ALD also relies on chemical agents and elevated temperatures within the reactor chamber. 
     In either process, the reactor chamber is a very harsh environment for its internal structures. The process chemicals are very toxic and very corrosive, tending to eat away any structure within the chamber. In addition to the harsh chemical environment, the temperatures can range from room temperature to 600 degrees Celsius (1,112 degrees Fahrenheit). This dramatic change in temperature creates expansion and contraction forces between components within the chamber as well as breaking down any non-metallic components, such as rubber or plastic gaskets and washers. 
     In order to produce the heat required for these processes, the reactor chamber requires heating to ensure a uniform temperature on the reactive surface. However, in order to provide power to the heating element and to monitor the temperature, an electrical connector and a thermocouple must also be routed into the harsh environment of the chamber. Because of the excessive heat and the corrosive and reactive nature of the chemicals within the chamber, the power connector and the thermocouple tend to deteriorate and fail over time. These failures result in reactor down time, damaged product and, in some cases, damage to other elements of the reactor. 
     One solution, as presented in U.S. Pat. No. 6,066,836 of Chen et al., is to feed the electrical connector and thermocouple through the inner portion of a completely sealed support shaft within the chamber. Because the electrical connector and thermocouple are sealed from the reactor and its chemical contents, the problems associated with heat and chemicals is minimized. However, this arrangement is costly to produce and costly to maintain. The upper portion of the shaft closest to the heater has to be sealed with expensive high temperature hermetic seals. Further complicating the design, in order to maintain the airtight seal, the shaft must be mounted on a flexible coupling in order to account for the thermal expansion during the process. Additionally, the high temperature seals do eventually fail in the harsh environment of the reactor chamber and the machine must be shut down and undergo repairs, each of which are very costly to the operator. 
     SUMMARY OF THE INVENTION 
     A purged heater assembly for an ALD or CVD reactor is configured to prevent process agents from damaging conductors within the reactor chamber, such as electrical power conductors and temperature sensors for a reactor heater plate. 
     In one embodiment of the present invention, the purged heater assembly is located within a wafer processing reactor chamber and includes an electrical conductor disposed within a hollow interior of a sleeve and connected to an electrically operated heating platform. A base plate assembly is adapted to support the sleeve within the chamber so as to provide a non-airtight junction at a second end of the sleeve where it contacts the electrically operated heating platform. In another embodiment, an atmospheric support shaft may be coupled to the base plate assembly so as to support the heater assembly within the wafer processing chamber and to provide cooling lines to the base plate assembly and to provide a path to conduct heat away from the base plate assembly. 
     In one embodiment of the present invention, the electrically operated heating platform includes a heater plate that is adapted to support a wafer. Among varying embodiments, the electrically operated heating platform may include a resistive heating element and may be embedded into the surface of the heater plate or may be itself the heater plate. 
     In one embodiment of the present invention, the sleeve may be ceramic, such as aluminum oxide or aluminum nitride. In another embodiment, the base plate assembly may be metallic and the conductor may be coiled within the interior of the sleeve. In other embodiments, the base plate assembly may also include a coolant manifold for coolant flow through the base plate assembly and may include at least one purge gas manifold within the base plate assembly to provide a fluid path to the hollow interior of the sleeve. In another embodiment, a source fitting may be coupled to the sleeve and adapted to allow fluid to pass into the hollow interior of the sleeve. In yet another embodiment, a return fitting may be coupled to the sleeve and adapted to allow fluid to pass out of the hollow interior of the sleeve. 
     In one embodiment of the present invention, at least one orifice may be located at the non-airtight junction at the first end of the sleeve adapted to permit a purge gas to leak from the hollow interior of the sleeve into the wafer processing chamber. 
     In one embodiment of the present invention, the heater assembly may have an elastomeric gasket countersunk within the base plate assembly adapted to provide a seal at a second end of the sleeve. The elastomeric gasket may also provide a spring force at the second end of the sleeve to maintain contact of the first end of the sleeve with the electrically operated heating platform. In another embodiment, the seal may be an airtight seal. 
     In one embodiment of the present invention, the heater assembly includes a stabilizer countersunk flush with an upper surface of the lower base plate creating a mating surface with the insulator which is countersunk flush with the lower surface of the upper base plate. The stabilizer may be adapted to support the electrical conductor and to cooperate with a conductor seal surrounding the electrical conductor. The conductor seal is located at the mating surface to prevent fluid from the purge gas manifold from leaking along a portion of the electrical conductor, which passes through the insulator and the stabilizer. 
     In one embodiment of the present invention, a heater assembly for a wafer processing chamber includes an electrically operated heating platform supported by a pedestal having a hollow interior and a sleeve disposed within the hollow interior of the pedestal and itself having a hollow interior. An electrical conductor disposed within the hollow interior of the sleeve is connected to the electrically operated heating element. A base plate assembly adapted to support the sleeve within the hollow interior of the pedestal includes a manifold for a purge gas to be provided to the hollow interior of the sleeve during wafer processing operations. 
     In one embodiment of the present invention, the base plate assembly may be coupled to the pedestal and adapted to provide a seal at a first end of the sleeve and at a first end of the pedestal where each contacts the base plate assembly. In various embodiments, at least one of the seals at the first end of the pedestal and at the sleeve may be an airtight seal. 
     In one embodiment of the present invention, at least one orifice may be located at a non-airtight junction at the second end of the sleeve adapted to permit the purge gas to leak from the hollow interior of the sleeve into the hollow interior of the pedestal. In another embodiment, a second orifice may be located at a non-airtight junction at a second end of the pedestal adapted to permit the purge gas to leak from the hollow interior of the pedestal into the wafer processing chamber where it may be pumped out with the process gases. 
     In one embodiment of the present invention, a method to prevent process agents from damaging conductors, such as electrical and thermocouple connectors for a reactor heater plate within an ALD or CVD reactor chamber includes, delivering a purge gas within a sleeve surrounding an electrical conductor associated with an electrically operated heating assembly of a wafer processing chamber so that the purge gas envelopes the electrical conductor within the sleeve and escapes from an unsealed end thereof into the processing chamber to be pumped out with process gases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, which illustrate various embodiments of the present invention. 
         FIG. 1  illustrates a reactor having a purged heater-susceptor configured for CVD and ALD processes, according to an embodiment of the present invention. 
         FIG. 2  illustrates a portion of a heater-susceptor assembly for channeling a purge gas through a sleeve within which an electrical conductor or other element is disposed, according to an embodiment of the present invention. 
         FIG. 3  illustrates a junction between a conductor-containing sleeves and a base plate assembly within a heater-susceptor assembly configured according to an embodiment of the present invention. 
         FIG. 4  illustrates a junction between a conductor-containing sleeve and a heater plate within a heater-susceptor assembly configured according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a purged heater assembly for an ALD/CVD reactor. In one embodiment, a conductor, including an electrical connector powering a heating element, is isolated from the reactor chamber within a sleeve that is disposed within a pedestal that supports the heating element. An inert purge gas is introduced within the sleeve so as to isolate the connector from the harsh environment within the reactor chamber. The purge gas is allowed to pass from the sleeve into the chamber and be evacuated therefrom with the process gases. Owing to this arrangement, the requirement to completely seal the conductor at the backside of the heater utilizing a flexible coupling and high temperature hermetic seals is obviated. Additionally, because the inert purge gas is maintained at a positive pressure with respect to the area surrounding the sleeve, the reactive chemicals within the chamber do not come in contact with the conductor or any other internal structures within the sleeve. 
       FIG. 1  illustrates a reactor  100  having a purged heater-susceptor assembly configured for CVD and/or ALD processes, according to an embodiment of the present invention. The reactor  100  has a showerhead  118  configured to deliver process gases to wafer  116  within chamber  105  through multiple nozzles. The process gas flows over and around wafer  116  and is contained within chamber  105 , until eventually it is pumped out through exit  120 . Upon exposure to the process gas and high temperature, a chemical reaction occurs, wherein the wafer  116  develops a film of material upon its top surface. 
     In order to achieve the temperatures required for these chemical reactions, the chamber  105  also contains a heater plate  114  including a heater element  112 . The wafer  116  rests upon the heater plate  114  providing a surface for heat transfer from the heater plate  114  to the top surface of wafer  116 . The heating element  112  is connected to a power source through conductor  110 . In order to monitor the temperature of the heating plate  114 , a temperature sensor  113  is coupled to the heater plate  114  and in communication with a remote temperature controller. 
     Pedestal  106  supports the heater plate  114  and encloses and protects sleeves  108  and  109  from the harsh process chemicals of chamber  105 . Sleeves  108  and  109  further enclose and protect the heating element conductor  110  and the temperature sensor  113 , respectively. The heating element conductor  110  and the temperature sensor  113 , therefore, are separated from chamber  105  and process gases by pedestal  106  and sleeves  108  and  109 , respectively. 
     The pedestal  106 , sleeve  108  and sleeve  109  each are supported by base plate assembly  104 . The base plate assembly  104  provides a sealed passage into an atmospheric support tube  102  for the heating element conductor  110  and the temperature sensor  113 . The atmospheric support tube  102  may further include cooling ducts therein to carry heat away from the base plate assembly  104 . In another embodiment, the atmospheric support tube  102  may also include passageways, such as metal or rubber tubing, to carry coolant into and out of the base plate assembly  104 . 
       FIG. 2  illustrates an exploded view of a heater assembly  200  configured according to an embodiment of the present invention. The upper portion of heater assembly  200  includes heater plate  114 . The upper surface of heater plate  114  provides a flat resting surface for the target of the deposition process, i.e., a semiconductor wafer. The top surface of such a wafer may then be exposed to the introduction of process fluids via a showerhead as discussed above. 
     Heater element  112  may be a resistive heating element disposed within heater plate  114 . Heater element  112  is electrically connected to heating element conductors  110  and  111 , which are disposed within sleeves  108  and  107 , respectively. When current flows with this arrangement, heating element  112  becomes hot and transfers heat to the heater plate  114 . The temperature sensor  113  is disposed within sleeve  109  and provides a signal path for a temperature reading of the heater plate  114 . In varying embodiments, the sleeves, such as sleeves  107 ,  108  and  109 , may be used for any type of connector or internal structure that needs to be introduced into the reactor chamber while remaining protected from the harsh chemical environment therein. 
     In one embodiment, the heater plate  114  is made of aluminum nitride. In varying embodiments, the heater element  112  may be embedded within or on the surface of heater plate  114 . In another embodiment, the heater plate  114  is the heater element  112 . 
     Pedestal  106  is the first layer of protection for heating element conductor  110 , heating element conductor  111  and temperature sensor  113  from the highly corrosive and reactive process gases within the chamber. The pedestal  106 , according to one embodiment of the present invention, is made from a good insulator, such as aluminum oxide or another ceramic material, so that the temperature transfer is minimized from the heater plate  114  down to the base plate assembly  104 . In one embodiment of the present invention, the base plate assembly  104  may include two pieces, an upper base plate  204  and a lower base plate  205 , each of which will be discussed in further detail below. In another embodiment, the pedestal  106  is made of aluminum oxide and has a thin coating of aluminum nitride so that it maintains the low temperature transfer characteristic of aluminum oxide but also has corrosion resistant properties of aluminum nitride. 
     The pedestal  106  is coupled to the heater plate  114  by an upper flange  210  and upper flange fasteners  211 . To accommodate the heat from the heater plate  114 , the upper flange fasteners  211  may be made of a corrosion resistant and/or insulating ceramic material, such as aluminum oxide or aluminum nitride. A lower flange  212  and fasteners  213  couple the lower end of pedestal  106  to the lower base plate  205 . Because the lower end of the pedestal is cooler than the upper end, the lower flange fasteners  213  may be made of a metal, such as aluminum or nickel. In one embodiment, the upper flange fasteners  211  and the lower flange fasteners  213  have shoulders so that the upper flange  210  and lower flange  212  of pedestal  106  are held loosely against their mating surfaces, heater plate  114  and lower base plate  205 , respectively. This arrangement prevents excessive stress due to thermal expansion mismatches during the ALD/CVD deposition process. 
     Enclosed within the pedestal  106  is the second layer of protection for heating element conductor  111 , heating element conductor  110 , and temperature sensor  113 ; sleeve  107 , sleeve  108  and sleeve  109 , respectively. The upper portions of sleeves  107 ,  108  and  109  abut against the bottom surface of heater plate  114  and will be discussed further below. The lower portion of sleeves  107 ,  108  and  109  are supported and sealed within upper base plate  204  in part by gaskets  202 , which will be discussed in greater detail below. 
     In one embodiment of the present invention, the sleeves  107 ,  108  and  109  are made of a ceramic, such as aluminum oxide or another ceramic material, and the heating element conductors  110 , and  111  may be made of any conducting material, such as nickel. Temperature sensor  113  may be a conventional thermocouple, though in other embodiments other forms of temperature sensors may be used. Additionally, a conductor, such as heating element conductor  110  may be coiled (at least partially) within a sleeve, such as sleeve  108 , so that the conductor may accommodate a change in distance between the upper base plate  204  and the heater plate  114  as a result of thermal expansion during ALD/CVD processing. 
       FIG. 3  illustrates a junction  300  between the sleeve  108 , the pedestal  106 , and the base plate assembly  104 . This is an example of the type of junction that may be used at the interface between the sleeves and the base plate assembly, but other configurations of junctions may also be used. What is important is that the junctions provide means for delivering the purge gas from a source to the interior of the sleeve, so as to envelope the conductor or other element therein. In the present example, this means exists in the form of the manifold  302  for channeling a fluid (e.g., a purge gas) through the sleeve  108 . and  10 . The junctions between sleeves  107  and  109  may be similar to that of junction  300  and have been omitted for clarity. 
     In the illustrated example, base plate assembly  104  includes two pieces, a lower base plate  205  coupled to an upper base plate  204 . In other arrangements the base plate assembly may have more or fewer components. Among varying embodiments, the upper base plate  204  and the lower base plate  205  may be made of the same material or of differing materials. For example, the upper base plate  204  may be made of one type of metal, such as aluminum, while the lower base plate  205  may be made of another type of metal, such as an aluminum alloy or titanium. In other embodiments, to prevent heat damage to the gaskets and seals, at least one of the upper base plate  204  and/or the lower base plate  205  may have a cooling manifold therein for circulating a coolant fluid to carry heat away from the base plate assembly  104  during the deposition process. 
     In one embodiment of the present invention, sandwiched between the lower base plate  205  and the upper base plate  204  is gasket  306 . Gasket  306  prevents fluid from leaking between the mating surfaces of upper base plate  204  and lower base plate  205  at the outside circumference of upper base plate  204 . 
     In one embodiment of the present invention, sandwiched between the upper base plate  204  and lower base plate  205 , is an insulator  316  and its corresponding gasket, insulator seal  310 . The insulator  316  prevents any conducting portion of heating element conductor  110  or conductor sheath  318  from shorting to the base plate assembly  104  and may be made of any material, such as a ceramic, that will prevent conductivity between the heating element conductor  110  (and/or conductor sheath  318 ) and the base plate assembly  104 . Insulator seal  310  prevents fluid leaks between the upper base plate  204  and the upper portion of insulator  316 . Stabilizer  314  may be set into the lower base plate  205 , wherein its upper surface is mated to the lower surface of insulator  316  within upper base plate  204 . A conductor seal  312  is sandwiched between the insulator  316  and the stabilizer  314  to prevent fluid leaks between the lower portions of insulator  316  and the upper portions of stabilizer  314  along the conductor sheath  318 . In another embodiment, a portion of the heating element conductor  110  or the sheath  118  may not conduct electricity and the insulator  316  and/or stabilizer  314  may be made from a conducting material, such as aluminum. Insulator  316  may include a receptacle  328 , such as a threaded hole, for coupling a device to the insulator  316  for its insertion/removal. 
     In one embodiment of the present invention, a fluid, such as an inert purge gas (e.g., argon) is fed into manifold  302  near the base of sleeve  108 . The purge gas is allowed to pass into the hollow body of sleeve  108 , surrounding the conductor or other element disposed therein, but is discouraged from leaking into the interior of pedestal  106  by the partial seal of gasket  202 . Note that the seal provided by gasket  202  may or may not be an airtight seal. The purge gas then leaks out of the top portion of sleeve  108  and into the inner portion of the pedestal  106 , as further discussed below. In various embodiments, the fluid within the manifold has a differential pressure in relation to the chamber of the reactor ranging from 5 to 150 torr. Among various other embodiments, the manifold may be of various shapes and sizes and includes at one plug  304  to prevent the fluid within manifold from leaking directly into the interior of pedestal  106 . 
     In another embodiment, a fluid connector is connected directly to the lower portions of sleeve  108  and a purge gas is forced up the sleeve and into the inner portion of the pedestal  106 . In yet other embodiments, a second fluid connector, orifice or orifices may be located at or near the top portion of sleeve  108  that permits the gas to escape or leak into the pedestal  106 . 
     The sleeve gasket  202  forms a seal between upper base plate  204  and the inner portions of the sleeve  108 , such that the purge gas is contained within the sleeve at its junction with the base plate  205 . Additionally, the sleeve gasket  202  provides an elastic force that keeps the upper portion of the sleeve  108  abutted against the heater plate  114 , as illustrated with respect to  FIG. 4 . The sleeve gasket  202  may be of any shape or any material known to have elastomeric properties, such as rubber or silicon, so as to provide at least a partial fluid seal and an elastic force to the lower portion of sleeve  108 . Among various embodiments, the size and shape of the sleeve gasket  202  may be varied in order to control the flow of the purge gas into the inner portions of sleeve  108  and the degree of seal between the sleeve  108 , upper base plate  204 , and the pedestal  106 . 
     In one embodiment of the present invention, the pedestal  106  encloses upper base plate  204  and is coupled to the lower base plate  205  at lower flange  212  by fasteners  213 . Sandwiched between the lower base plate  204  and the lower flange  212  is pedestal gasket  326 . Pedestal gasket  326  provides a seal between the junction of the upper base plate  204 , the lower base plate  205  and the bottom surface of pedestal  106 , such that fluid may not significantly leak past the lower flange  212  and into the chamber of the reactor. In another embodiment, the seal created by the union of the components and pedestal gasket  326  may be an airtight seal. 
       FIG. 4  illustrates an example of a junction  400  between the sleeve  108  and the heater plate  114  within a heater-susceptor assembly configured according to an embodiment of the present invention. For simplicity, sleeves  107  and  109  enclosing heating element conductor  111  and temperature sensor  113 , respectively, are not shown, as sleeves  107  and  109  may abut against the heater plate  114  in a fashion similar to sleeve  108 . 
     The heating element conductor  110  is connected to the heater element  112  within heater plate  114 . The connection  402  may be countersunk within the heater plate  114  and provides the contact between the heating element conductor  110  the embedded heater element  112 . In another embodiment, the connection may be located at the bottom surface of heater plate  114 , and the connector  402  makes its connection by direct surface contact against the conductor  110 . 
     As discussed above, the top portion of sleeve  108  is forced to abut against heater plate  114  as a result of the use of elastomeric gaskets at the junction of sleeve  108  and base plate assembly  104 . In one embodiment of the present invention, a sleeve receiving area  403  is countersunk into the underside of heater plate  114  so as to provide a channeled recess for the top of the sleeve  108 . In another embodiment, the sleeve  108  abuts flush against the underside of heater plate  114 . 
     Regardless of how the sleeve  108  abuts against heater plate  114 , the junction is not airtight with respect to the interior of pedestal  106  and therefore does not require any type of gasket or seal. Consequently, during ALD/CVD deposition process, when a purge gas is introduced into the lower end of sleeve  108  it will leak out of the top end thereof and into the interior of pedestal  106 . 
     Thus, in addition to the heating element conductor  110  being physically isolated from the corrosive process gases by pedestal  106  and the sleeve  108 , it is also chemically isolated therefrom by the inert purge gas within sleeve  108 . The purge gas is further allowed to leak out of pedestal  106  into chamber  105  (below the surface of the wafer) where it is pumped out with the process gas. This positive flow of purge gas prevents, or at least minimizes, process gas flow into the pedestal  106  and sleeve  108  thereby isolating the heating element conductor or other elements and/or electrical connector from the corrosive environment of the reactor chamber. 
     In one embodiment, the mating surface of the sleeve  108  and the heater plate  114  include at least one orifice  404  for the inert gas to leak into the inner portion of pedestal  106 . In other embodiments, the surface of heater plate  114  covered by the upper portion of the sleeve  108  may have channels or veins that allow for the gas to escape or leak into the pedestal  106 . In yet other embodiments, a fluid connector and/or orifice or orifices may be located at or near the top portion of sleeve  108  that allow for the gas to escape or leak into the pedestal  106 . 
     As discussed earlier, pedestal  106  is coupled to the heater plate  114  by its upper flange  210  and fasteners  211  and is the first layer of protection for heating element conductor  110  from the highly corrosive and reactive process gases within the chamber  105 . Although affixed with fasteners  211 , the junction between pedestal  106  and heater plate  114  is not airtight. Rather, the junction is configured to let the purge gas from sleeve  108  leak into the chamber. In one embodiment of the present invention, the pedestal  106  includes at least one orifice  406  that allows for the purge gas to leak into the chamber. The orifice  406  may be embedded into the heater plate  114 , which may include channels leading to the exterior of pedestal  106 . In another embodiment a fluid connector and/or orifice or orifices may be located on pedestal  106  that allow for the gas to escape or leak into the chamber. 
     In sum, due to the flow of the inert purge gas through the sleeves  108  and  109 , pedestal  106  and into the chamber  105 , damage to the heating element conductors  110 ,  111  and temperature sensor  113  is greatly reduced if not eliminated. Because high temperature hermetic seals are not required to create an airtight seal for either the pedestal  106  or the sleeves, costs are minimized in both the manufacturing of the reactor and repair associated with the inevitable failure of such seals. 
     In the foregoing specification, the invention has been described with reference to specific embodiments. It will, however, be evident that various modifications and changes can be made without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.