Abstract:
An apparatus and method is provided for rapid thermal processing (RTP) of semiconductor wafers that compensates for variations in heat absorption characteristics of the wafers. Wafer-to-wafer temperature variation is substantially eliminated using a model of the heat absorption characteristics of different wafer types to predict a steady state temperature of a wafer undergoing processing. This prediction is used to detect potential variations in wafer temperature during the RTP process and correct for these variations by adjusting the output of the heat source.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to semiconductor manufacturing, and, in particular, to a method and apparatus to control temperature in a rapid thermal processing (RTP) system.  
         BACKGROUND OF THE INVENTION  
         [0002]    One of the procedures commonly used during the manufacture of semiconductor devices is rapid thermal processing (RTP), for example for annealing of a semiconductor wafer. Several different RTP methods have been proposed, including those utilizing a variety of sources of heat, such as arc-lamps and heating blocks.  
           [0003]    Because variations in wafer temperature during the annealing process can lead to yield problems in the resulting semiconductor devices, temperature is sought to be closely controlled and monitored during the RTP process. For example, the wafer is sought to be heated to a specified temperature for a specified time period, and the temperature conditions should not change from wafer to wafer. The specified times and temperatures often vary for the several different heating steps of a typical semiconductor production process.  
           [0004]    Semiconductor manufacturers thus are continually in search of methods to ensure precise control of wafer temperature during annealing. Conventional RTP systems typically employ a combination of techniques for controlling temperature, including specialized heat sources, reflective or absorptive heating chambers, and wafer rotation.  
           [0005]    A problem may arise in that the temperature of each wafer undergoing processing depends on the heat absorption characteristics of the wafer, which may vary significantly from wafer to wafer depending on the wafer type, films added to the wafer, and emissivity of the wafer surface. In addition, it is often difficult to directly measure the temperature of the wafer undergoing heating, due to the presence of various films and layers on the wafer and the desire to avoid invasive measurement methods. Thus, monitoring and control of wafer temperature presents a difficult problem.  
           [0006]    One popular RTP system employs a heating block or ”susceptor” of nearly constant temperature that is placed adjacent a wafer undergoing annealing. The temperature of the susceptor is closely monitored and controlled, and temperature variations in the wafer may be determined from temperature variations in the susceptor. This system offers several advantages over lamp-based systems, in that the wafer heating profile is less dependent upon wafer surface characteristics, e.g. backside films.  
           [0007]    Temperature may be tightly controlled using such a susceptor-based system, yet wafer-to-wafer temperature variations may continue to occur. When a wafer is initially placed adjacent the susceptor, the temperature of the susceptor drops as heat is initially transferred to the wafer. Over time, the wafer heats up and the temperature of the susceptor, as well as the wafer, returns to a steady-state temperature.  
           [0008]    During the initial heating of each wafer, the heat absorption characteristics of the wafer determine the rate of drop of the susceptor temperature and rate of increase of the wafer temperature. When a series of wafers is successively heated using this RTP system and each wafer has different heat absorption characteristics, wafer-to-wafer temperature variations may occur. As noted, this successive wafer-to-wafer temperature variation may result in possible yield problems for the produced semiconductor devices.  
           [0009]    Accordingly, there is a strong desire and need to produce an RTP method that substantially eliminates wafer-to-wafer temperature variations for annealing of wafers having varying heat absorption characteristics.  
         SUMMARY OF THE INVENTION  
         [0010]    An apparatus and method for rapid thermal processing (RTP) of semiconductor wafers is provided that compensates for variations in heat absorption characteristics of the wafers. The invention nearly eliminates wafer-to-wafer temperature variation by using a model of the heat absorption characteristics of different wafer types to predict a steady state temperature of a wafer undergoing processing. This prediction is used to detect potential variations in wafer temperature during the RTP process and correct for these variations by adjusting the output of the heat source.  
           [0011]    In an exemplary embodiment of the invention, the rate at which the temperature of the heating block, or susceptor, changes when the wafer is initially placed in the heating chamber is measured and this measurement is used in the prediction of the steady-state temperature of the wafer. Heating values of the susceptor may be adjusted to achieve a desired steady-state wafer temperature. Measurements and adjustments may be stored and used to update the model of wafer heat absorption characteristics for later processing of wafers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The above and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings in which:  
         [0013]    [0013]FIG. 1 illustrates a cross-sectional view of an apparatus for rapid thermal processing (RTP) in accordance with an exemplary embodiment of the invention;  
         [0014]    [0014]FIG. 2 illustrates a graph of temperature vs. time for the wafer and the susceptor shown in FIG. 1;  
         [0015]    [0015]FIG. 3 illustrates a power consumption profile for the exemplary RTP apparatus shown in FIG. 1;  
         [0016]    [0016]FIG. 4 illustrates a graph of temperature vs. time demonstrating in-situ temperature correction using the apparatus of FIG. 1;  
         [0017]    [0017]FIG. 5 illustrates a graph of temperature vs. time demonstrating wafer-to-wafer temperature correction using the apparatus of FIG. 1;  
         [0018]    [0018]FIG. 6 illustrates the steps of an exemplary embodiment of the method of the invention;  
         [0019]    [0019]FIG. 7 illustrates an exemplary controller for an RTP apparatus including a processor which employs temperature monitoring and heat source adjustment methodologies in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 1 shows an exemplary heating chamber  20  for rapid thermal processing (RTP) of a semiconductor wafer  28  in accordance with the invention. A wafer  28  undergoing processing is placed adjacent a susceptor  26  which supplies heat to the wafer  28 . The susceptor  26  is itself heated by a resistive heater  34  which is controlled by an external controller  36 . The temperature of the susceptor  26  may be monitored using a thermocouple or optical pyrometer  30 . An inner insulated wall  24  and an outer insulated wall  22  enclose the wafer  28 , susceptor  26  and resistive heater  34 , in order to maintain a nearly isothermal environment inside the heating chamber.  
         [0021]    In operation, the inside of the heating chamber  20  is maintained at a nearly constant temperature. A series of wafers is heated in succession inside the heating chamber  20 , whereby each wafer  28  is initially placed adjacent the susceptor  26 , the temperature stabilizes at a steady state value, and the wafer  28  is removed from the heating chamber  20  after a specified period of time.  
         [0022]    [0022]FIG. 2 illustrates exemplary temperature profiles  42 ,  44  of the wafer  28  and the susceptor  26 , respectively, during the RTP process. At time t 1 , the wafer  28  is placed adjacent the susceptor  26 , and at time t 2 , the wafer  28  is removed from the heating chamber  20 . The temperature profile  42  of the wafer  28  over time reflects this RTP heating process. At time t 1 , the wafer  28  is heated, eventually reaching the steady-state temperature Tss. At time t 2 , the wafer  28  is removed and its temperature eventually decreases to room temperature.  
         [0023]    During the time when the temperature of the wafer  28  is ramping up, but before the temperature reaches the steady-state value Tss, the temperature of the wafer  28  varies with the susceptor temperature according to the relation:  
         Twafer˜Tsusceptor*(1 −e   −t/τ )  
         [0024]    where τ is a time constant that is characteristic of the wafer  28 , Twafer is the temperature of the wafer  28 , Tsusceptor is the temperature of the susceptor  26 , and t is time. This relationship shows that the temperature of the wafer  28  during heating is directly proportional to the temperature of the susceptor  26 , and any variations in the temperature of the susceptor  26  indicate a proportional variation in the temperature of the wafer  28 .  
         [0025]    Referring to FIG. 2, the profile  44  of the susceptor  26  shows that the temperature of the susceptor  26  drops as the wafer  28  initially heats up. The temperature of the susceptor  26  is initially at the set point temperature Tsp. When the wafer  28  is placed in the heating chamber  20  adjacent the susceptor  26 , the temperature of the wafer  28  increases and the temperature of the susceptor  26  drops. The rate of drop in the temperature of the susceptor  26  as well as the ramp rate of the temperature of the wafer  28  depend on the heat absorption characteristics of the wafer  28 . The profile  44  shows that, as the wafer  28  continues to heat up, the temperature of the susceptor  26  stops dropping and heats up with the wafer  28 . The susceptor  26  eventually reaches the steady-state temperature Tss.  
         [0026]    As noted, the temperature of the wafer  28  may be described in terms of the temperature of the susceptor  26  during the ramp up of the temperature of the wafer  28 . The temperature profile  44  of the susceptor  26  is measured and recorded, and using the profile  44 , the temperature profile  42  of the wafer  28  may be predicted using the relation given above. Thus, the steady state temperature Tss may also be predicted.  
         [0027]    In accordance with the invention, each wafer  28  of a series of wafers is successively heated in the heating chamber  20 . The controller  26  measures the temperature of the susceptor  26  periodically during the heating period and develops a temperature profile of the susceptor  26  over time for the series of wafers.  
         [0028]    In addition, the temperature profiles  42 ,  44  of the susceptor  26  and the wafer  28 , respectively, may be used to determine a mathematical model of the heating behavior of the susceptor  26  and the wafer  28 . The mathematical model consists of stored parameters characteristic of a particular type of wafer  28 , including, for example, values of the heating time constant, heating temperature profiles for several different set point temperatures, etc. The mathematical model parametrically describes the heating behavior of several different types of wafers that are heated in the heating chamber  20 . Using the parameters of the mathematical model, the temperature profile  42 , and hence the steady state temperature Tss, of a typical wafer  28  undergoing processing may be predicted.  
         [0029]    [0029]FIG. 3 illustrates the amount of power consumed for a given set point temperature Tsp. Set point temperature Tsp indicates the heating value provided by the resistive heater  34  and the susceptor  26 , and may be adjusted by a controller  36  (FIG. 1) in accordance with the invention. The power profile  52  shows that as Tsp increases, the amount of power consumed also increases. At higher Tsp, even small changes in the set point temperature Tsp may result in large changes in power consumption. By employing the processing methodologies of the invention, the controller  36  may adjust power consumption by anticipating and efficiently adjusting for the heat absorption behavior of differing wafer types. Other means of heating the susceptor may be used in accordance with the invention, such as by lamps, laser or microwave energy. A relationship between power and set point temperature Tsp, similar to that shown in FIG. 3, may be determined for each means of heating.  
         [0030]    Adjustments to the set point temperature Tsp may be made by the controller  36  in accordance with the invention. The set point temperature Tsp may be adjusted in order to achieve a desired steady state temperature Tss of the wafer  28 . One way in which the invention may achieve the desired Tss is via adjustment of the set point temperature Tsp during the initial heating of the wafer. Using the thermocouple or optical pyrometer  30 , the controller can measure the temperature and/or the rate of change of temperature of the susceptor  26 . When the wafer  28  is initially heated, the temperature of the susceptor  26  changes as noted, and the rate of change of temperature of the susceptor  26  may be measured and used by the controller  36 , in conjunction with the mathematical model of heating behavior noted above, to predict the eventual steady state temperature for the current set point temperature. Using the predicted steady state temperature, the controller  36  may perform an “in-situ” adjustment of the set point temperature Tsp as illustrated in FIG. 4.  
         [0031]    [0031]FIG. 4 illustrates an exemplary “in-situ” adjustment of the set point temperature by the controller  36  to achieve a desired steady state temperature Tss in accordance with the invention. Referring to FIG. 4, the wafer  28  is placed adjacent the susceptor  26  at time t 1  and removed at time t 2 . Between t 1  and t 2 , the wafer  28  should be heated to the desired steady state temperature Tss (desired) as shown by the temperature profile  42 ″, and then cools after removal at time t 2 . During the time period between t 1  and ta, the controller  36  is able to assess the temperature profile  44 ′, e.g., the rate of change of temperature of the susceptor  26  (this rate of change of temperature is represented by the slope of the curve  44 ′).  
         [0032]    From the assessed temperature profile  44 ′ between t 1  and ta, the controller  36  may predict, in conjunction with the mathematical model, the steady state temperature that would result if the current heating value were to remain constant. If the predicted value of the steady state temperature is within an acceptable tolerance of the desired steady state temperature Tss, then no adjustments of the heating value may need to be made by the controller  36 . Otherwise, the controller  36  may adjust the heating value of the resistive heater  34  until the predicted value of the steady temperature comes within an acceptable tolerance of the desired value.  
         [0033]    In the exemplary profile of FIG. 4, the initial temperature profile  44 ′ between t 1  and ta indicates that the wafer  28  would achieve the predicted steady state temperature Tss (predicted)  42 ′ if the current heating value were to remain constant. The controller  36  uses the mathematical model and the initial temperature profile  44 ′ to perform an “in-situ” adjustment of the heating value such that the wafer  28  instead achieves the desired steady state temperature Tss (desired)  42 ″.  
         [0034]    Another way in which the controller  36  may achieve a desired temperature of the wafer  28  is through adjustment of the set point temperature Tsp between wafer heating cycles. The controller  36  may store temperature and heating information measured throughout the heating cycle of individual wafers, and use the stored information to improve the heating of later-processed wafers.  
         [0035]    For example, FIG. 5 shows exemplary temperature profiles  42 ′″,  44 ′″ of a wafer  28  and susceptor  26 , respectively, after an adjustment between heating cycles. To account for a variation in heat absorption characteristics between first and second wafers, the set point temperature Tsp may be adjusted by the controller  36 . In accordance with the invention, the controller  36  may be notified of a change in wafer heat absorption characteristics required for the next heating cycle, and the controller  36  may adjust the set point temperature accordingly. As a result, the susceptor  26  may follow the temperature profile  44 ′″, whereby the wafer  28  ramps up to a steady state temperature Tss (desired) as shown. The wafer  28  is placed adjacent the susceptor  26  at time t 1  and removed at time t 2 . Between t 1  and t 2 , the wafer  28  is heated to the desired steady state temperature Tss (desired) as shown by the temperature profile  42 ′″, and then cools after removal at time t 2 .  
         [0036]    [0036]FIG. 6 illustrates an exemplary embodiment of the method of the invention, showing how an exemplary controller  36  (FIG. 1) may achieve a desired steady state temperature Tss using both “in situ” set point adjustments and adjustments between wafer heating cycles. The wafer heating cycle begins by heating the susceptor  26  to the set point temperature Tsp using resistive heater  34  (method segment  62 ). If the previous wafer exhibited different heat absorption characteristics from the next wafer, then the controller  36  may make an adjustment to the set point temperature Tsp at this point. The wafer  28  is placed adjacent the susceptor  28  (method segment  64 ), and the wafer  28  is heated. As the wafer  28  is heated, the temperature of the susceptor  26  is repeatedly measured (method segment  66 ). Using the repeated temperature measurements, the rate of change of susceptor temperature may be determined (method segment  68 ). This rate may be used as noted, in conjunction with the mathematical model of wafer heating behavior, to predict a temperature profile of the wafer, including the steady state temperature that would result if the heating value were to remain constant (method segment  70 ). From the difference between the predicted and desired values of steady state temperature, a change in set point temperature Tsp may be determined (method segment  72 ). The controller  36  may then adjust the set point temperature Tsp to achieve the desired steady state temperature of the wafer  28  (method segment  74 ). The method flow may then return to method segment  66  to repeat the segments of measuring, predicting and adjusting, until the wafer  28  has been heated at the desired steady state temperature for the desired period of time (method segment  78 ). The wafer  28  is then removed (method segment  76 ).  
         [0037]    [0037]FIG. 7 illustrates an exemplary processor system that may be connected to or included in the controller  36  in accordance with the invention. Referring to FIG. 7, the processor system, which may be a computer system  100 , for example, generally comprises a central processing unit (CPU)  102 , for example, a microprocessor, that communicates with one or more input/output (I/O) devices  112 ,  114 ,  116  over a system bus  122 . The input/output devices  112 ,  114 ,  116  may include connections to the resistive heater  34  and the thermocouple or optical pyrometer  30 . The computer system  100  also includes random access memory (RAM)  118 , a read only memory (ROM)  120  and may also include peripheral devices such as a floppy disk drive  104 , a hard drive  106 , a display  108  and a compact disk (CD) ROM drive  110  which also communicate with the processor  102  over the bus  122 . The mathematical model of the heat absorption characteristics of the wafers may be stored and/or accessed using any or all of these peripheral devices  104 ,  106 ,  110 ,  118 ,  120 . In conjunction with the input/output devices  112 ,  114 ,  116 , e.g., the thermocouple or optical pyrometer  30  and the resistive heater  34 , the processor  102  may implement the algorithms and processing methodologies described above with reference to FIGS.  1 - 6 . It should be noted that FIG. 7 is merely representative of one of many different types of architectures of the controller  36  which may employ the invention.  
         [0038]    While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.