Abstract:
Apparatus, reactors, and methods for heating substrates are disclosed. The apparatus comprises a stage comprising a body and a surface having an area to support a substrate, a shaft coupled to the stage, a first heating element disposed within a central region of the body of the stage, and at least second and third heating elements disposed within the body of the stage, the at least second and third heating elements each partially surrounding the first heating element and wherein the at least second and third heating elements are circumferentially adjacent to each other.

Description:
BACKGROUND  
       [0001]     Embodiments of the present invention pertain to resistive heaters, apparatus incorporating resistive heaters and methods of heating substrates such as semiconductor wafers.  
         [0002]     Resistive heaters are widely employed in the heating systems of chemical vapor deposition (CVD) systems. Temperature uniformity is an important consideration in CVD processes, and as a result, multi-zone resistive heaters have been developed to provide greater control over the heating characteristics of the heating apparatus in CVD systems. For example, U.S. Pat. No. 6,646,235 to Chen et al., the entire content of which is incorporated herein by reference, discloses a CVD resistive heater that has an inner zone and an outer zone, where the outer zone completely surrounds the inner zone. By providing these concentric zones, it is possible to compensate for the different rates of heat loss exhibited by the inner and outer regions of the heating apparatus, and so provide more uniform heating across the entire diameter of a wafer.  
         [0003]     Even slight variations in temperature uniformity across a wafer, on the order of just a few degrees Celsius, can adversely affect a CVD process. Limitations in manufacturing tolerances make it extremely difficult to make a multi-zone heater that has consistent heating power characteristics around its entire circumference. Hence, at a given radius, one region of the resistive heater may provide more or less heating power than another region at that same radius. The resulting temperature variations introduce one layer of complexity that must be controlled to insure process repeatability across multiple wafers for the same resistive heater. Moreover, putatively identical resistive heaters display different heating power characteristics amongst themselves, which introduces yet another layer of complexity that is adverse to process repeatability. In addition, the CVD chamber itself may have regions that exhibit irregularities in temperature uniformity, introducing further possible temperature irregularities.  
         [0004]     Accordingly, it would be desirable to provide a resistive heater that can provide compensation for heating irregularities to enhance process repeatability in high temperature deposition systems, such as reactors incorporating CVD chambers.  
       SUMMARY  
       [0005]     Aspects of the present invention provide methods, apparatus and systems related to resistive heaters. One aspect pertains to an apparatus that includes a stage, and a shaft coupled to the stage. The stage includes a body with a surface for supporting a wafer. At least a first heating element is disposed within a central region of the body. Additional heating elements may be provided in the central region. At least two other heating elements are disposed in the body, each partially surrounding the central region, and each circumferentially adjacent to the other. In one embodiment, only one temperature sensor, for example, a thermocouple, disposed in the central region, is used to control the heating power of all of the heating elements. In another embodiment, four heating elements are provided in the body that each partially surround the central region. In yet another embodiment, the heating element in the central region is disposed adjacent to a top side of the body, and the other heating elements are disposed adjacent to a bottom side of the body.  
         [0006]     Another aspect of the invention provides a heating system that includes a resistive heater, a temperature sensor for the resistive heater, a power supply for the resistive heater, and a control system to control the power supply. The resistive heater has a stage and a shaft coupled to the stage. The stage has a body with a surface for supporting a wafer. In one or more embodiments, a first resistive heating element is disposed within a central region of the body. At least second and third resistive heating elements are disposed in the body, each partially surrounding the central region, and each circumferentially adjacent to the other. The first, second and third heating elements provide heat to respective first, second and third zones of the stage. The power supply includes first, second and third power sources for respectively providing power to the first, second and third resistive heating elements. In one embodiment, the control system controls the first, second and third power sources according to an output from the temperature sensor and a power ratio of the power to the second and third resistive heating elements. In one embodiment, only the temperature sensor is used to measure the temperature of the resistive heater. In another embodiment, the temperature sensor is a thermocouple disposed within the central region of the body of the stage. In another embodiment, additional temperature sensors such as thermocouples may be provided for temperature control of the individual zones.  
         [0007]     Another aspect pertains to a method for providing process repeatability in resistive heating systems. A heating surface is divided into a central region and at least two outer regions, with each outer region only partially surrounding the central region. Each outer region is given a respective power ratio with respect to the central region. The temperature of the central region is measured during a heating process, and heating power is delivered to the central region according to the measured temperature. Heating power is delivered to each outer region according to the heating power delivered to the central region and the respective power ratio of each outer region. In one embodiment, a calibration procedure is performed to obtain the power ratios. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a cross-sectional view of a CVD system with a heating apparatus according to one embodiment;  
         [0009]      FIG. 2  is a top perspective view of the heating apparatus depicted in  FIG. 1 ;  
         [0010]      FIG. 3  is a bottom perspective view of the heating apparatus depicted in  FIG. 1 ;  
         [0011]      FIG. 4  is a partial cross-sectional view of the heating apparatus depicted in  FIG. 1 ;  
         [0012]      FIG. 5  illustrates a control system for the heating apparatus depicted in  FIG. 1 ; and  
         [0013]      FIG. 6  is a top perspective view of the heating apparatus according to shown in  FIG. 1  depicting a substrate disposed thereon and the heating regions of the apparatus shown in phantom. 
     
    
     DETAILED DESCRIPTION  
       [0014]     Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways.  
         [0015]      FIG. 1  presents a cross-sectional view of a CVD system  105 . A heating apparatus  10  is disposed within a reaction chamber  100  of the CVD system  105 . The reaction chamber  100  may support, for example, a CVD reaction process, an LPCVD reaction process or the like, and is defined and surrounded by chamber body  110 . The heating apparatus  10  includes a stage  20  for heating and supporting a wafer, and a shaft  30 , shown partly in cross-section, for supporting the stage  20 .  
         [0016]     As shown in  FIG. 2 , the stage  20  has a body  21  with a top surface  22 , which forms a susceptor  24  for supporting a wafer. Body  21  has a central region  41 , and outer region  49  that surrounds the central region  41 . Disposed just under the top surface  22  of central region  41  is a first resistive heater  51  that heats the central region or zone  41 . It will be appreciated that the central region or zone  41  can be heated by a single heater  51  or multiple heaters.  
         [0017]     As shown in  FIGS. 3 and 6 , the body  21  has a bottom surface  26 , to which is connected the shaft  30 . The shaft  30  is centrally mounted within the central region  41 , and has an opening  32  that extends along the longitudinal length of the shaft  30 . The outer region  49  of bottom surface  26  is divided into four substantially equal-sized zones  42 ,  43 ,  44 ,  45 . A second resistive heater  52  heats zone  42 ; a third resistive heater  53  heats zone  43 ; a fourth resistive heater  54  heats zone  44 , and a fifth resistive heater  55  heats zone  45 . Consequently, the second, third fourth and fifth resistive heaters  52 - 55  each partially surrounds the first resistive heater  51 , and the second, third, fourth and fifth resistive heaters  52 - 55  are circumferentially adjacent to each other. The second, third, fourth and fifth resistive heaters  52 - 55  are each disposed just under the bottom surface  26 . However, in an alternative embodiment, the second, third, fourth and fifth resistive heaters  52 - 55  may each be disposed just under the top surface  22 . Similarly, in an alternative embodiment the first resistive heater  51  may be disposed just under the bottom surface  26  within the central region  41 . For example, in one embodiment, the first resistive heater  51  may be disposed just under the bottom surface  26  in the central region  41 , and the second through fifth resistive heaters  52 - 55  may be disposed just under the top surface  22  in their respective zones  42 - 45  in the outer region  49 .  FIG. 6  shows the apparatus with the zones  41 - 45  shown in phantom and a substrate or wafer  301  disposed on the apparatus.  
         [0018]      FIG. 4  illustrates a cross-sectional view along line IV-IV in  FIG. 2 . The body  21  and shaft  30  may be made from any suitable material that can withstand the high temperatures and corrosive materials associated with CVD processes, such as aluminum nitride, graphite, aluminum nitride or pyrolytic boron nitride. In one or more embodiments, a dielectric material  67 , for example, pyrolytic boron nitride, is disposed across the top surface  22  to form the susceptor  24 , upon which a wafer to be processed is placed. The susceptor  24  includes lip edges  69  to ensure the wafer is held snugly and in a well-defined position on the susceptor  24  during processing. First resistive heating element  51  is disposed in the body  21 , just under the dielectric layer  69 . Third and fifth resistive heating elements  53 ,  55  are disposed in the body  21  just above the bottom surface  26 . Of course, second and fourth resistive heating elements  52 ,  54  (not shown) would be visible in a similar cross-section that is ninety degrees to line IV-IV. All of the resistive heating elements  51 - 55  may be made from any suitable material known in the art, and ideally should have thermal expansion properties that are similar to those of the body  21 . An example of a suitable material for the resistive heating elements  51 - 55  includes pyrolytic graphite. Each resistive heating element  51 - 55  has a corresponding power line  61 - 65 , running through opening  32  of shaft  30  that provides respective electrical power to the resistive heating element  51 - 55 , and thereby allows independent control of the heating power delivered to the inner region  41 , and to each of the outer region zones  42 - 45 . Of course, one or more ground lines (not shown) may be provided, also running through opening  32 , to complete the circuit of each resistive heating element  51 - 55 .  
         [0019]     A thermocouple  70  may be provided to measure the temperature of the central region  41 . In one embodiment, an opening  74 , extending up from the bottom surface  26 , is used to position thermocouple  70  between the first resistive heating element  51  and resistive heating elements  52 ,  53 ,  54  and  55 , thereby thermally coupling the thermocouple  70  with the central region  41  of the body  21 . A signal line  72  may extend from the thermocouple  70  through the opening  74  of the stage  20 , and through the opening  32  of the shaft  30 , to provide temperature information about the central region  41  to a control system of the heating apparatus  10 . Of course, other temperature sensor configurations are possible. For example, an optical pyrometer may be used to measure the temperature of the central region  41 .  
         [0020]     A control system  200 , depicted in  FIG. 5 , may be used to control the heating apparatus  10 . The control system  200  may be part of the control system for the CVD system  105  depicted in  FIG. 1 , and is electrically connected to the heating apparatus  10 . Together, the heating apparatus  10  and the control system  200  form the heating system for the CVD system  105 . Numerous possibilities are available for the physical implementation of the control system  200 , and an exhaustive review of the various permutations of digital and analog circuits that may be employed to create the control system  200  is beyond the scope of this disclosure. Any suitable implementation of the control system  200  may be used, and providing a detailed control system  200  should be a routine task for one of ordinary skill in the art, after reading the following disclosure.  
         [0021]     According to one embodiment, the control system  200  includes a user input/output (I/O) system  210 , a temperature input  210 , a feedback control circuit  230  and a power supply  240 . The user I/O system  210  provides a user interface that allows a user to select a target temperature  211  for the central region  41  of the susceptor  22 , and to select second, third, fourth and fifth power ratios  212 ,  213 ,  214 ,  215  for the second third, fourth and fifth resistive heaters  52 ,  53 ,  54 ,  55 , respectively.  
         [0022]     The temperature input  220  is electrically connected to the signal line  72  of the thermocouple  70  to obtain, in real-time, the current temperature  221  of the central region  41  of the stage  20 . The temperature input  220  then passes this current temperature  221  to the feedback control circuit  230 . In a manner familiar to those in the art, the feedback control circuit  230  accepts as input the current temperature  221  and the target temperature  211  and generates a heating power control output  231 . The purpose of the heating power control output  231  is to control the power delivered to the first resistive heater  51  so that the temperature of the central region  41  as measured by the thermocouple  70  tracks as closely as possible the target temperature  211 . The feedback control circuit  230  may be designed to employ any suitable feedback control method known in the art.  
         [0023]     The power supply  240  provides the electrical power needed to individually power the resistive heating elements  51 ,  52 ,  53 ,  54 ,  55  in the heating apparatus  10 . The power supply  240  includes a first power output  241  that is electrically connected to the first power line  61  to provide electrical power for the first heating element  51 , and thus to heat the central region  41 . Similarly, the power supply  240  includes second, third, fourth and fifth power outputs  242 ,  243 ,  244  and  245 , each of which is respectively electrically connected to the second, third, fourth and fifth power lines  62 ,  63 ,  64  and  65  to heat the second, third, fourth and fifth zones  42 ,  43 ,  44  and  45 .  
         [0024]     The first power output  241  accepts as input the heating power control output  231  from the feedback control circuit  230 , which may be an analog or digital signal, and in response provides corresponding electrical power on the first power line  61 . Hence, the power provided to the first resistive heater  51  by the first power output  241  is directly related to the heating power control output  231  generated by the feedback control circuit  230 .  
         [0025]     The second power output  242  accepts as input the heating power control output  231  from the feedback control circuit  230 , and also the second heater power ratio  212  from the user I/O circuit  210 . In response, the second power output  242  provides electrical power on the second power line  62  such that the ratio of electrical power on the first power line  61  to that on the second power line  62  equals the second heater power ratio  212 . Hence, the power provided to the second resistive heater  52  by the second power output  242  equals the electrical power provided on the first power line  61  multiplied by (or divided by) the second heater power ratio  212 . Similarly, the power provided to the third, fourth and fifth resistive heaters  53 ,  54  and  55  by the third, fourth and fifth power outputs  243 ,  244  and  245  equals the electrical power provided on the first power line  61  multiplied by (or divided by) the third, fourth or fifth heater power ratios  213 ,  214  and  215 , respectively. As a result, individual control of the heating power provided to the zones  42 ,  43 ,  44 ,  45  with respect to the power provided to the central region  41  is possible by respectively adjusting the power ratios  212 ,  213 ,  214 ,  215 , and hence variations in the heating characteristics of the zones  42 ,  43 ,  44  and  45  may be individually compensated for with respect to each other and the central region  41 . Of course, other designs for the power supply  240  are possible; whatever design may be chosen, the power supply  240  should individually control the heating power of each outer region zone  42 - 45  based upon the power supplied to the central region  41  and the respective power ratio  212 - 215  of that outer region zone  42 - 45 .  
         [0026]     By dividing the outer region  49  of the stage  20  into a multiplicity of zones  42 - 45  that surround the central region  41 , and by further providing each of these outer region zones  42 - 45  a respective heater power ratio  212 - 215  with respect to the heating power provided to the central region  41 , the instant heating system makes it possible to provide compensation for variations in the heating characteristics of different heating apparatuses  10 , and to further provide compensation for variations in the heating characteristics of the CVD chamber  100  itself. By providing appropriate values for the heater power ratios  212 - 215 , a consistent heating pattern may be provided across the susceptor  24 , which should enhance process repeatability.  
         [0027]     A calibration procedure may be performed for an individual heating apparatus  10  within a particular CVD chamber  100  to determine the appropriate heater power ratios  212 - 215  at any desired target temperature  211 . With reference to  FIGS. 1-6 , one possible method for doing this is to initially set all heater power ratios  212 - 215  to default values, such as 1.0, or values obtained from an earlier calibration step. Then, a test wafer  301  may be placed onto susceptor  24  of heating apparatus  10 , and the central region  41  may be heated to the desired target temperature  211 . Subsequently, individual temperature measurements may be made in each of the outer region zones  42 - 45  on the wafer  301 , for example by the use of thermocouples attached to each zone  42 - 45 , or with one or more pyrometers. By way of the user I/O circuit  210 , the heater power ratios  212 - 215  may then be adjusted, while the feedback control circuit  230  keeps the central region  41  at the target temperature  211 , until the entire wafer  301  achieves a heating pattern that is as optimal as possible for the desired process. The resulting heater power ratios  212 - 215  may subsequently be used in production runs at that target temperature  211 .  
         [0028]     Of course, the heater power ratios  212 - 215  need not be constant values. On the contrary, the heater power ratios  212 - 215  may vary as functions of the target temperature  211 , and consequently, an entire calibration procedure may involve a series of individual calibration steps at predetermined temperatures to obtain sets of heater power ratios  212 - 215  at each of these predetermined temperatures. Interpolation may then be used to determine heater power ratios  212 - 215  at target temperatures  211  that are between the predetermined temperatures.  
         [0029]     It will be appreciated that the control system for controlling the heating apparatus  10  may comprise a plurality a temperature sensors. Each temperature sensor may measure the temperature of a single region or zone of the stage. The temperature sensors may include thermocouples, pyrometers or other suitable temperature sensing devices. Combinations of different types of temperature sensors may be used as well.  
         [0030]     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method, apparatus and system of the present invention without departing from the spirit and scope of the invention. For example, the outer region of the body of the stage may be divided not into only four zones, but into any number of zones greater than one. In certain embodiments, each of these zones would be provided its respective heating power ratio. Also, the resistive heater zones may overlap with each other. The various heating elements may be on the top surface, bottom surface or embedded in the body of the stage. Zonal temperature measurement may be provided by utilizing multiple temperature measurement devices (thermocouple, pyrometer, etc). Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.