Patent Publication Number: US-2010124613-A1

Title: Heater integrated thermocouple

Description:
BACKGROUND 
     The present invention relates, in general, to the field of wafer processing in the semiconductor industry. More specifically, the present invention relates to the field of thermocouples for use in a reaction chamber of thermal film deposition reactors, in particular to thermocouples for use in Low Pressure Chemical Vapor Deposition (LPCVD) furnaces in semiconductor processing. 
     During the deposition process, unwanted films and particulate materials accumulate on surfaces other than the target substrate such as on functional elements. The functional elements may include a thermocouple, a quartz gas injector and the like. Unwanted layers keep depositing onto the surfaces of the functional elements during the deposition process, and due to stresses in the deposited layers, the functional elements may breakdown, resulting in improper functioning of the thermal film deposition reactor. This may result in an abort of the deposition process and increase downtime of the thermal film deposition reactor. 
     There are methods used for preventing the breakdown of the functional elements due to stresses in the deposited films. One of the existing methods involves wrapping silicon carbide sheets around the thermocouple to safeguard it from the deposition of the layers. However, this method is incompatible with NF 3  and CLF 3  cleaning. Another method adopted is to increase the thickness of the quartz sheath of the thermocouple and thereby increase the lifetime of the thermocouple. However, experiments with increased quartz sheath thickness have shown little or no success in preventing the thermocouple from breaking down. 
     Another approach to prevent breakdown of the functional elements is to reduce the stresses developed in the deposited layers. The method employs cooling and/or heating of the entire reaction chamber when the thickness of the deposited layers reaches a predetermined value. The deposited layers crack by alternate cooling and/or heating due to differences in coefficient of thermal expansion between the deposited layer and the functional elements of the reaction chamber. The cracking of the deposited layers relieves the stresses and avoids flake off of the deposited layer. The predetermined thickness is chosen small enough so that the deposited film cracks in a controlled manner without causing significant damage. However, this method involves heating and/or cooling of the entire reaction chamber and therefore there is a high risk of breakdown of other fragile parts in the reaction chamber. 
     Accordingly, there is a need for a system and method that can reduce the downtime of thermal film deposition reactors. The method and system preferably should crack the deposited layers on functional elements in a controlled manner to relieve the stresses. Further, the method and system preferably should be able to increase the lifetime of functional elements without heating and/or cooling of the entire reaction chamber. 
     SUMMARY 
     An object of the present invention is to provide a system and method for efficient processing of one or more wafers inside a reaction chamber of a thermal film deposition apparatus. 
     Another object of the present invention is to provide a system and method for preventing flake off of a layer deposited on a functional element inside the reaction chamber. 
     Yet another objective of the present invention is to provide a system and method for heating the functional elements inside the reaction chamber independently. 
     Still another object of the present invention is to provide a system and method for cracking the layer deposited on the functional element inside the reaction chamber in a controlled way to relieve the stresses. 
     To achieve the objects mentioned above, the present invention provides a system and a method that includes heating of the functional elements independently of the reaction chamber. The functional element of the present invention when integrated with a heater forms a component for use inside a reaction chamber of a thermal film deposition apparatus. The functional element is integrated with a heater for heating the functional element independently of the reaction chamber when the thickness of deposited layers on the functional element reaches a predetermined cumulative film thickness. The functional elements of the present invention are not intended to include the reaction chamber walls or the reaction chamber elements like wafers which are processed inside the reaction chamber and do not perform any function inside the reaction chamber. In various embodiments of the present invention, the functional element may be a thermocouple, a quartz gas injector and the like. Further, the functional element is heated to a predefined temperature wherein the predefined temperature is determined based on the dimensions and material of functional element, and the material of the layer deposited on the surface of the functional element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, provided to illustrate and not to limit the present invention, wherein like designations denote like elements, and in which: 
         FIG. 1  is a schematic cross sectional diagram of a reaction chamber of a thermal film deposition apparatus in which the present invention may be practiced; 
         FIG. 2  is a top cross-sectional view of a paddle thermocouple in accordance with an embodiment of the present invention; 
         FIG. 3   a  is a cross-sectional view of the paddle thermocouple taken along line A-A′ of  FIG. 2  in accordance with an embodiment of the present invention; 
         FIG. 3   b  is a perspective view of a ceramic tube in accordance with an embodiment of the present invention; and 
         FIG. 4  is a flowchart describing a method for processing semiconductor wafers in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention describes a system and a method for efficient processing of one or more wafers inside a reaction chamber of a thermal film deposition apparatus. 
     The present invention increases lifetime of the functional elements which are exposed to film deposition processing conditions by relieving stresses in the deposited layers. This results in preventing breakdown of the functional elements. During the deposition process, unwanted layers keep on depositing on the surfaces of the functional elements. As the deposition keeps on increasing on the surfaces of the functional elements, stresses are developed and may result in breakdown of the functional elements. The present invention involves integrating the functional element with a heater to form a component for use inside a reaction chamber of a thermal film deposition apparatus. The functional element performs specific functions inside a reaction chamber of the thermal film deposition apparatus such as measuring temperature conditions, passing process gas and the like. Examples of functional elements may include a thermocouple, a quartz gas injector and the like. The functional elements of the present invention are not intended to include the reaction chamber walls or the reaction chamber elements like wafers which are processed inside the reaction chamber and do not perform any function inside the reaction chamber. The heater heats the functional element to a predefined temperature independently of the reaction chamber when the deposited layers achieve a predetermined thickness. The alternate heating and cooling of the functional element develops cracks in the deposited layers which relieve the stresses and therefore prevent the breakdown of the functional element. 
       FIG. 1  is a cross-sectional schematic diagram of a reaction chamber of a thermal film deposition apparatus in which the present invention may be practiced. Reaction chamber  100  includes a process tube  102 , an inner tube  104 , flanges  106 , a heating element  108  comprising a first heater (not shown) and an isolation mantle  110  surrounding the first heater, a door  112 , a pedestal  114 , a boat  116 , spike thermocouples  118   a ,  118   b ,  118   c ,  118   d  and  118   e , a paddle thermocouple  120  and a plurality of substrates  122 . Thermocouples  118   a ,  118   b ,  118   c ,  118   d  and  118   e  are hereinafter referred to as spike thermocouples  118 . 
     Process tube  102  and inner tube  104  rest on flanges  106 . The assembly of process tube  102  and inner tube  104  is placed inside heating element  108 . The outer surface of heating element  108  is covered with isolation mantle  110 . The first heater (not shown) is provided in the space between process tube  102  and isolation mantle  110 . The lower end of reaction chamber  100  is provided with door  112  on which pedestal  114  is supported. Boat  116  is supported on the upper surface of pedestal  114  for accommodating a plurality of substrates  122  in a horizontal position. The plurality of substrates  122  are arranged in a vertically spaced manner. 
     Spike thermocouples  118  and paddle thermocouple  120  are used to measure the temperature. Spike thermocouples  118  measure the temperature at the outside of process tube  102  in their respective heating zones. Paddle thermocouple  120  is positioned inside the process tube for measuring the temperature inside the process tube during the processing of substrates  122 . 
     Hot gases are passed inside reaction chamber  100  for the deposition of a film on the semiconductor wafers. The temperature of reaction chamber  100  is maintained at a desired wafer processing temperature by using the first heater. The hot gases deposit on the semiconductor wafers, decompose and form a solid film. However, the hot gases are also deposited on paddle thermocouple  120 , present inside reaction chamber  100 . The hot gases decompose and form a layer on paddle thermocouple  120 . The layer formed on paddle thermocouple  120  keeps on increasing in thickness during the deposition process and stresses are developed in the deposited layer. The intensity of the stresses increases with increasing thickness of the deposited layer. The intensity of stresses varies with the materials of the deposited layers. The intensity of stresses is higher in the case of polysilicon and silicon nitride layers than other layers, and therefore, makes paddle thermocouple  120  more susceptible to failure when used in such process chambers. 
     The thickness of the layer deposited on paddle thermocouple  120  can easily be derived from the cumulative thickness deposited on semiconductor wafers in consecutive runs. In accordance with various embodiments of the present invention, paddle thermocouple  120  is integrated with a second heater, described in  FIG. 2 , to heat paddle thermocouple  120  to a predefined temperature independently of the reaction chamber  100  when the deposited layers achieve a predetermined thickness. 
     The second heater heats paddle thermocouple  120  cyclically to a temperature above the wafer processing temperature. The temperature of reaction chamber  100  is maintained equal to or below the wafer processing temperature during heating of paddle thermocouple  120 . The second heater is fired using an electric current to a temperature above the wafer processing temperature. Preferably, paddle thermocouple  120  is heated to a temperature which is 50-200° C. above the maximum process temperature, and more preferably to a temperature range of 100-200° C. above the maximum process temperature. 
     Cyclic and independent heating of paddle thermocouple  120  causes cracking of the deposited layers due to differences in coefficient of thermal expansion between the materials of paddle thermocouple  120  and the deposited layer. The stresses are relieved from the deposited layer when the deposited layer cracks. Paddle thermocouple  120  is allowed to cool down after the heating of paddle thermocouple  120  is complete until paddle thermocouple  120  reaches a temperature equal to reaction chamber  100  again. In accordance with another embodiment of the present invention, the deposited layer may be cracked by cyclic cooling or alternate cooling and heating of paddle thermocouple  120 . Cyclic cooling or alternate cooling and heating of paddle thermocouple  120  causes cracking of the deposited layers due to differences in coefficient of thermal expansion between the materials of paddle thermocouple  120  and the deposited layer. 
       FIG. 2  is a top cross sectional view of the paddle thermocouple  120  in accordance with an embodiment of the present invention. 
     Paddle thermocouple  120  includes a ceramic tube  202 , a plurality of channels  204 , a second heater  206  integrated with paddle thermocouple  120 , a protective sheath  208  around heater  206  and a deposited layer  210 . 
     Paddle thermocouple  120  has at least one pair of wires (not shown) which form a temperature measuring junction. Ceramic tube  202  comprises plurality of channels  204  which run axially along the length of paddle thermocouple  120 . A pair of thermocouple wires is situated inside a pair of adjacent channels  204 . Each of the thermocouple wires is situated in a separate channel of the pair of adjacent channels  204 . Recesses are provided in the outside of ceramic tube  202  at various heights to open up two adjacent channels  204  to connect the pair of thermocouple wires to form a thermocouple junction. Ceramic tube  202  may be made of Al 2 O 3  and more preferably ultra pure Al 2 O 3 . 
     Second heater  206  is provided outside ceramic tube  202 . Second heater  206  comprises one or more heating wire loops that extend in a direction parallel to the length of paddle thermocouple  120 . The one or more heating wire loops of second heater  206  are meant for heating paddle thermocouple  120  and are different from the thermocouple wires described above. In an embodiment of the present invention, the one or more heating wire loops are made of NICROTHAL™ alloy from Kanthal. In an embodiment of the present invention the one or more heating wire loops are wound around ceramic tube  202  from a bottom end to a top end and then from the top end to the bottom end through one of channels  204 . In an embodiment of the present invention, two channels  204  may be provided in ceramic tube  202  to accommodate the one or more heating wire loops. 
     In another embodiment of the present invention, two grooves may be provided in the outer surface of ceramic tube  202  for receiving the one or more heating wire loops. The grooves are preferably provided along the axial direction of ceramic tube  202 . In various embodiments of the present invention, the one or more heating wire loops may be wound on a separate tube which is mounted to paddle thermocouple  120  by inserting the separate tube in the grooves of paddle thermocouple  120  or in channels  204 . 
     Hot gases are passed inside reaction chamber  100  for the deposition of a film on the semiconductor wafers. The hot gases deposit on the semiconductor wafers and solidify. However, the hot gases are also deposited on paddle thermocouple  120 , present inside reaction chamber  100 . The hot gases solidify and form deposited layer  210  on paddle thermocouple  120 . Deposited layer  210  formed on paddle thermocouple  120  keeps on increasing in thickness during the deposition process and stresses are developed in the deposited layer. 
       FIG. 3   a  is a cross-sectional view of the paddle thermocouple taken along line A-A′ of  FIG. 2  in accordance with an embodiment of the present invention. Paddle thermocouple  120  comprises ceramic tube  202 , plurality of channels  204 , protective sheath  208  and deposited layer  210 . 
     Plurality of channels  204  run axially along the length of paddle thermocouple  120  and are used for accommodating thermocouple wires and the one or more heating wire loops of second heater  206  (shown in  FIG. 3   b ). 
       FIG. 3   b  is a perspective view of a ceramic tube  202  in accordance with an embodiment of the present invention. In accordance with an embodiment of the present invention, the one or more heating wire loops are wound around ceramic tube  202 . In accordance with another embodiment of the present invention, the one heating wire may be wound around the outer surface of ceramic tube  202  and the other heating wire may be situated in one of the plurality of channels  204 . 
     In accordance with another embodiment of the present invention, a ceramic tube of smaller diameter may be used for winding the one or more heating wire loops outside of ceramic tube  202 , keeping the inner and outer diameter of protective sheath  208  the same. 
       FIG. 4  is a flowchart describing a method for processing semiconductor wafers in accordance with an embodiment of the present invention. 
     The process starts at step  402 . At step  404 , the one or more semiconductor wafers are loaded inside reaction chamber  100 . At step  406 , a film is deposited on the semiconductor wafers. Hot gases are passed inside reaction chamber  100  to deposit a film on the one or more semiconductor wafers. 
     At step  408 , the semiconductor wafers are unloaded from reaction chamber  100  after a film is deposited on the one or more semiconductor wafers. The deposition process ends at step  410  when the deposition of film on the one or more wafers is complete. In the semiconductor industry, multiple wafers are processed continuously and therefore, wafer processing is resumed at step  402 . 
     During the deposition of films on the one or more semiconductor wafers, hot gases also get deposited on the functional elements such as on paddle thermocouple  120  and form deposited layer  210 . The thickness of deposited layer  210  formed on paddle thermocouple  120  keeps on increasing during the deposition process, and stresses are developed in the deposited layer. The intensity of the stresses increases with the increase in the thickness of deposited layer  210 . 
     The cumulative thickness of deposited layer  210  on paddle thermocouple  120  is compared at step  412  with a predefined thickness to determine if deposited layer  210  has achieved a predefined thickness. The cumulative thickness is determined by measuring the thicknesses of films on semiconductor wafers in consecutive runs since the last thermocouple sheath replacement or thermocouple heating cycle. If deposited layer  210  on paddle thermocouple  120  has not achieved the predefined thickness, the wafer processing process is resumed from step  402 . However, if deposited layer  210  on paddle thermocouple  120  has achieved the predefined thickness, the reaction chamber is put in a stand-by mode and paddle thermocouple  120  is heated to a predefined temperature at step  414 . The predefined temperature is determined based upon the material and dimensions of paddle thermocouple  120 , and the material of the deposited layer. The predefined temperature is higher than the wafer processing temperature. Preferably, paddle thermocouple  120  is heated to a temperature which is 50-200° C. above the maximum process temperature, and more preferably to a temperature range of 100-200° C. above the maximum process temperature. 
     In accordance with an embodiment of the present invention, paddle thermocouple  120  is integrated with a heater. The heater is used to heat the paddle thermocouple  120  independent of the reaction chamber. The heater comprises one or more wire loops and has been described in detail in conjunction with  FIGS. 2 and 3 . 
     Cyclic and independent heating of paddle thermocouple  120  causes cracking of deposited layer  210  due to differences in coefficient of thermal expansion between the materials of the sheath  208  of paddle thermocouple  120  and deposited layer  210 . The stresses from deposited layer  210  are relieved when the deposited layer cracks. After the heating of the paddle thermocouple is complete, paddle thermocouple  120  is allowed to cool down until the paddle thermocouple  120  reaches the temperature equal to the reaction chamber again. In accordance with another embodiment of the present invention, deposited layer  210  may be cracked by cyclic cooling or alternate cooling and heating of the paddle thermocouple  120 . 
     The system and method described above relieves the stresses from the layer that gets deposited on the surface of the paddle thermocouple and therefore, prevents flake off of the layer. This enables effective processing of semiconductor wafers and increase the lifetime of the paddle thermocouple. The heating of the paddle thermocouple is conducted independent of the heating of the reaction chamber, and causes cracking of the deposited layer only, and thus avoids any damage of the thermocouple or other fragile parts inside the reaction chamber. This increases the life-time of the other fragile parts because these fragile parts are not heated along with the thermocouple. Furthermore, regular examination and cracking of the deposited layer avoids abrupt shutting down of the reaction chamber due to uncontrolled breakage of the thermocouple and other fragile parts of the reaction chamber. This increases efficiency of the reaction chamber. 
     The present invention has been described in connection with the paddle thermocouple for the purpose of illustration and description purpose only. However, it is not intended to limit the scope of the present invention as the present invention may be embodied using other functional elements such as quartz injectors and the like. 
     While various embodiments of the present invention have been illustrated and described, it will be clear that the present invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the present invention, as described in the claims.