Abstract:
A method of manufacturing a semiconductor device is disclosed. The manufacturing method comprises heating a reactor, setting a semiconductor wafer in the reactor, supplying a reactive gas into the reactor to form a film on the semiconductor wafer or on an inner surface of the reactor, and measuring a temperature change outside the reactor and a temperature change inside the reactor to determine a thickness of the film on the semiconductor wafer or on the inner surface of the reactor on the basis of a relationship between a ratio of the temperature changes and a thickness of the film.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-093787, filed Mar. 28, 2001, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method of manufacturing a semiconductor device in which a reactive gas is thermally decomposed to form a film on a semiconductor substrate, in particular, a method of forming a film by using a CVD (chemical vapor deposition) apparatus.  
           [0004]    2. Description of the Related Art  
           [0005]    In the manufacture of semiconductor devices including ICs and LSIs, a conductive film such as a PolySi (poly silicon) film and a W (tungsten) film is formed by using LPCVD (low pressure chemical vapor deposition) method.  
           [0006]    [0006]FIG. 1 shows a schematic view of a LPCVD apparatus for forming a PolySi film.  
           [0007]    Fifty to a hundred and fifty semiconductor wafers  11  are set on a boat  12 , and introduced into a quartz tube  13 . The quartz tube  13  includes therein an inner quartz tube  113 . A heater  14  for heating the semiconductor wafers  11  is provided outside the quartz tube  13  and generates a radiant heat. The radiant heat of the heater  14  passes the wall of the outer quartz tube  13 , the wall of the inner quartz tube  113 , and then reaches and heats semiconductor wafers  11 , so that the semiconductor wafers  11  are heated to a temperature of 600° C. A temperature near the heater  14  is measured by the outside thermocouple  111  provided outside the quartz tube  13 , that is, between the heater  14  and the quartz tube  13  in detail, whereas a temperature near the semiconductor wafers  11  is measured by the inside thermocouple  112  provided inside the quartz tube  13 .  
           [0008]    The inside of the quartz tube  13  is evacuated by a pump  15  to reduce the pressure in the inside of the quartz tube  13 , and an SiH 4  gas as a film forming gas is introduced into the quartz tube  13  via a nozzle  16 , a mass flow controller  17  and a valve  18 . The gas, after having been introduced into the quartz tube  13 , passes through the inside of the inner quartz tube  113 , and then is discharged through a gap between the inner quartz tube  113  and the quartz tube  13 . The pressure inside the quartz tube  13  is maintained at 0.5 Torr by the conductance control of a main valve  110 , based on an indication value of a pressure gage  19 . The temperature of the semiconductor wafers  11  is controlled on the basis of a temperature measured by outer thermocouples  111  provided outside the quartz tube  13 , i.e., between the heater  14  and the quartz tube  13  and inner thermocouples  112  provided inside the quartz tube  13 . With the above arrangement, the SiH 4  introduced in the quartz tube  13  thermally is decomposed in the quartz tube  13 , thus depositing PolySi on the semiconductor wafers  11 . In this example, the outer thermocouples  111  are provided at three measuring points and similarly the inner thermocouples  112  are provided at three measuring points.  
           [0009]    In this time, not only the temperature of the semiconductor wafers  11  but also the temperature of the wall of the quartz tube  13  are increased, thus depositing PolySi on the inner surface of the wall of the quartz tube  13 . However, the temperature of the wall of the quartz tube  13  cannot be accurately measured because quartz is semi-transparent with respect to radiant heat of the heater  14 . For this reason, it is impossible to precisely determine how thick PolySi has been deposited on the inner surface of the wall of the quartz tube  13 . Further, PolySi film having a thickness less than 1 μm is semi-transparent and allows infrared rays to pass therethrough. Therefore, as the thickness of the PolySi film on the inner surface of the wall of the quartz tube  13  is increased, then an amount of radiant heat that is received by the semiconductor wafers  11  changes, and thus the temperature of the semiconductor wafers  11  changes accordingly. Hence, in this state, it is impossible to manufacture PolySi films of a uniform thickness.  
           [0010]    Conventionally, in order to avoid this problem, the PolySi deposited on the inner surface of the wall of the quartz tube  13  is removed by etching with gas or liquid of acid. Then, a PolySi film is deposited on the inner surface of the wall of the quartz tube  13  to a thickness of 0.3 μm or more, and typically to about 0.5 μm. Thereafter, a test film-formation is performed in order to confirm that the film forming conditions are proper. After that, the manufacturing process is performed. Since the conventional method needs the additional step of the PolySi film deposition, the operation rate of the manufacturing facility is low.  
           [0011]    As described above, in the conventional method of manufacturing a semiconductor device, in order to assure the stability of the film formation, there must be added a step of depositing a PolySi film having a predetermined thickness in advance in the inner surface of the wall of the quartz tube  13  so that the thickness of the PolySi film deposited in the inner surface of the wall of the quartz tube  13  will increase to 1 μm or more during the manufacture process. This additional step decreases the operation rate of the apparatus.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    According to a first aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising heating a reactor; setting a semiconductor wafer in the reactor; supplying a reactive gas into the reactor to form a film on the semiconductor wafer or on an inner surface of the reactor; and measuring a temperature change outside the reactor and a temperature change inside the reactor to determine a thickness of the film on the semiconductor wafer or on the inner surface of the reactor on the basis of a relationship between a ratio of the temperature changes and a thickness of the film.  
           [0013]    According to a second aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising comparing a temperature change outside of a reactor in which a semiconductor wafer is placed, with a temperature change inside the reactor, and determining a thickness of a film on the semiconductor wafer or on an inner surface of the reactor. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0014]    [0014]FIG. 1 is a sectional view of a low pressure CVD apparatus used in a conventional method as well as in a method according to an embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is an enlarged view of a portion of the low pressure CVD apparatus;  
         [0016]    [0016]FIG. 3 is an infrared ray transmissivity characteristic curve when a PolySi film is deposited on a quartz tube;  
         [0017]    [0017]FIG. 4A is a characteristic curve showing an outside temperature change of the quartz tube without the PolySi film;  
         [0018]    [0018]FIG. 4B is a characteristic curve showing an inside temperature change of the quartz tube without the PolySi film;  
         [0019]    [0019]FIG. 5A is a characteristic curve showing an outside temperature change of the quartz tube, when a deposition of the PolySi film has a thickness of 1000 nm;  
         [0020]    [0020]FIG. 5B is a characteristic curve showing an inside temperature change of the quartz tube, when a deposit of the PolySi film has a thickness of 1000 nm;  
         [0021]    [0021]FIG. 6 is a characteristic curve showing dependency on the PolySi film, of the quartz tube inside temperature change with respect to the outside temperature change;  
         [0022]    [0022]FIG. 7 is a characteristic curve showing dependency on the PolySi film, of the ratio of the quartz tube inside temperature change rate to the outside temperature change rate; and  
         [0023]    [0023]FIG. 8 is a characteristic curve showing an infrared ray transmissivity characteristic when Ru is deposited on a quartz tube. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. The description will be based on the use of the low pressure CVD apparatus shown in FIG. 1, and therefore, will partly repeat the description already made by reference of the low pressure CVD apparatus shown in FIG. 1.  
         [0025]    First, a first embodiment will be described with reference to FIG. 1 through FIG. 7.  
         [0026]    As shown in FIG. 1, fifty to a hundred and fifty semiconductor wafers  11  are set on a boat  12 , and introduced into a quartz tube i.e. a reactor  13 . The quartz tube  13  includes therein an inner quartz tube  113 . A heater  14  for heating the semiconductor wafers  11  is provided outside the quartz tube  13  and generates a radiant heat. The radiant heat of the heater  14  passes the wall of the outer quartz tube  13 , the wall of the inner quartz tube  113 , and then reaches and heats semiconductor wafers  11 , so that the semiconductor wafers  11  are heated to a temperature of 600° C. A temperature near the heater  14  is measured by the outside thermocouple  111  provided outside the quartz tube or reactor  13 , that is, between the heater  14  and the quartz tube  13  in detail, whereas a temperature near the semiconductor wafers  11  is measured by the inside thermocouple  112  provided inside the quartz tube or reactor  13 .  
         [0027]    The inside of the quartz tube  13  is evacuated by a pump  15  to reduce the pressure in the inside of the quartz tube  13 , and an SiH 4  gas as a film forming gas is introduced into the quartz tube  13  via a nozzle  16 , a mass flow controller  17  and a valve  18 . The gas, after having been introduced into the quartz tube  13 , passes through the inside of the inner quartz tube  113 , and then is discharged through a gap between the inner quartz tube  113  and the quartz tube  13 . The pressure inside the quartz tube  13  is maintained at 0.5 Torr by the conductance control of a main valve  110 , based on an indication value of a pressure gage  19 . The temperature of the semiconductor wafers  11  is controlled on the basis of a temperature measured by outer thermocouples  111  provided outside the quartz tube  13 , i.e., between the heater  14  and the quartz tube  13  and inner thermocouples  112  provided inside the quartz tube  13 . In this embodiment, the outer thermocouples  111  are provided at three measuring points and similarly the inner thermocouples  112  are provided at three measuring points. With the above arrangement, the SiH 4  introduced in the quartz tube  13  thermally is decomposed in the quartz tube  13 , thus depositing PolySi on the semiconductor wafers  11 .  
         [0028]    [0028]FIG. 2 is an enlarged view of a portion of the low pressure CVD apparatus shown in FIG. 1 and is used for discussing the temperatures outside and inside of the quartz tube or reactor  13 . For the sake of describing how the present embodiment can be applied in the low pressure CVD apparatus shown in FIG. 1, on a simple model, the arrangement from the heater  14  to the semiconductor wafers  11  is taken out and shown in FIG. 2, as the portion of the low pressure CVD apparatus. Also for simplifying the description with reference to FIG. 2, the inner quartz tube  113  is omitted in FIG. 2.  
         [0029]    In the structure shown in FIG. 2, radiant heat of the heater  14  passes the wall of the quartz tube  13  and a PolySi film  24  deposited on the inner surface of the wall of the quartz tube  13 , and then reaches and heats semiconductor wafers  11 .  
         [0030]    A temperature near the heater  14  is measured by the outside thermocouple  111  provided outside the quartz tube or reactor  13 , whereas a temperature near the semiconductor wafers  11  is measured by the inside thermocouple  112  provided inside the quartz tube or reactor  13 .  
         [0031]    The radiant heat of the heater  14  is partially reflected on the outer surface of the wall of the quartz tube  13 , and partially absorbed by the PolySi film  24  deposited on the inner surface of the wall of the quartz tube  13 . Thus, an amount of the heat reaching the semiconductor wafers  11  is decreased.  
         [0032]    [0032]FIG. 3 shows a calculation result indicating how the trarismissivity of infrared ray changes in accordance with the thickness of the PolySi film  24 , when the quartz tube  13  has a thickness of 1 cm.  
         [0033]    The transmissivity of infrared ray decreases abruptly until the thickness of the PolySi film  24  increases to 100 nm. As the thickness of the PolySi film  24  further increases, the transmissivity of infrared ray continues decreasing in a vibratory pattern due to interference. When the film thickness is 1000 nm or more, the transmissivity of infrared ray is 0.1 or less, with a greater amount of the heat being reflected or absorbed by the PolySi film  24 .  
         [0034]    Actually, since the temperature of the PolySi film  24  gradually increases, the semiconductor wafers  11  are heated by the secondary radiant heat from the PolySi film  24 . However, there is a time lag between the temperature increase of the PolySi film  24  and the temperature increase of the semiconductor wafers  11  due to the secondary radiated heat from the PolySi film  24 . Thus, when the temperature of the heater  14  is increased abruptly, the temperature of the semiconductor wafers  11  does not follow the abrupt increase of the temperature of the heater  14 . Accordingly, the temperature change rate (temperature rising rate or temperature falling rate) near the semiconductor wafers  11  is small, as compared to the temperature change rate near the heater  14 . In particular, the larger the temperature change rate near the heater  14  is, the larger the difference between these two temperature change rates is.  
         [0035]    [0035]FIGS. 4A and 4B show experimental results obtained by using the low pressure CVD apparatus, for a case where there was formed no PolySi film on the inner surface of the wall of the quartz tube  13 , and electric power input to the heater  14  was increased at a time point of 20 seconds for two seconds and then decreased to a normal level. FIG. 4A shows a temperature (° C.) indicated by the outer thermocouple  111  provided outside the quartz tube  13  with time (second), i.e., an outer temperature with time, and FIG. 4B shows a temperature (° C.) indicated by the inner thermocouple  112  provided inside the quartz tube  13  with time (second), i.e., an inner temperature with time. It was determined from the experimentation that the temperature change measured by the inside thermocouple  112  was almost the same as the temperature change measured by the outside thermocouple  111 .  
         [0036]    [0036]FIGS. 5A and 5B show experimental results obtained by using the low pressure CVD apparatus, for a case where there was a PolySi film  24  of a thickness of 1000 nm on the inner surface of the wall of the quartz tube  13 , and electric power input to the heater  14  was increased at a time point of 20 seconds for two seconds and then decreased to a normal level. The temperature change rate at this time was made about 100° C./min, which caused a large change of temperature measured by the outside thermocouple  111 , on the other hand, a small change of temperature measured by the inside thermocouple  112 . This phenomenon is because the radiant heat of the heater  14  was shielded by the PolySi film  24 . Results of such experiments were summarized as shown in FIG. 6. It is understood that greater the temperature change rate on the outside of the quartz tube  13 , the greater the dependency of the inside temperature change rate, on the thickness of the PolySi film  24 . FIG. 7 shows a relationship between the ratio of the inside temperature change rate to the outside temperature change rate and the thickness of the PolySi film  24 , when the outside temperature change rate is 100° C./min. It is understood that the result was similar to the result shown in FIG. 3. This relationship can be obtained in advance, and by using this relationship the thickness of the PolySi film  24  can be obtained if the thickness of the PolySi film  24  is 1000 nm or less, by measuring a response of the inside temperature, i.e., the inside temperature change rate in a case where the outside temperature is changed.  
         [0037]    Furthermore, by obtaining in advance a relationship between the thickness of the PolySi film deposited on the inner surface of the wall of the quartz tube  13  and the thickness of the PolySi film deposited on the semiconductor wafers, it becomes possible to determine the thickness of the PolySi film deposited on the semiconductor wafers on the basis of the thickness relationship. Furthermore, when the thickness relationship is used for the semiconductor wafers in the manufacturing process, the film thickness deposited on the semiconductor wafers in the manufacturing process can also be determined, and thus the film thickness can be used as a monitor in the manufacturing process. Accordingly, it is possible to perform the manufacturing process without the before-hand formation of the additional Poly Si film which is required in the conventional manufacturing method. Hence, the operation rate of the low pressure CVD apparatus is improved.  
         [0038]    In this embodiment, description has been made for the inside thermocouple  112  as being exposed within the quartz tube  13 . In another case, the inside thermocouple  112  may be inserted into an elongated narrow quartz tube. In the case, the thermocouple  111  is essentially sandwiched between the two layers or walls of quartz tube  13 . Thus, the transmissivity in this case can be obtained by multiplying the transmissivity values of the two layers of the quartz tube  13 . For the arrangement as shown in FIG. 1, in which the quartz tube is a double-layer tube comprising the outer quartz tube  13  and the inner quartz tube  113 , it may be interpreted that the thickness of the PolySi film is about three times the thickness of the PolySi film in a case where the arrangement is a single tube structure. In other words, in the double-layer tube arrangement, the PolySi film is deposited not only on the inner surface of the wall of the outer quartz tube  13  but also on the inner and outer surfaces of the wall of the inner quartz tube  113 , and accordingly, the thickness of the PolySi film is about three times the thickness of the PolySi film of the single tube structure.  
         [0039]    If the response of the inside thermocouple  112  is inadequately small in the arrangement shown in FIG. 1, then the inside thermocouple  112  may be moved to between the outer quartz tube  13  and the inner quartz tube  113 . Furthermore, the above description was made for a specific temperature of 600° C. However, the description holds true for other temperatures. Thus, if there are obtained in advance the relationship between the temperature change rate inside the quartz tube  13  and the temperature change rate outside the quartz tube  13  and the relationship between the PolySi film deposited on the inner surface of the wall of the quartz tube  13  and the PolySi film deposited on the semiconductor wafers  11 , it is possible to obtain the PolySi film thickness at a predetermined heat treatment temperature from the temperature change rate at the heat treatment. Further, in the present embodiment, the temperature change rate was measured during the film formation. However, the thickness of the deposited PolySi film can also be obtained by performing the same procedure after the film formation.  
         [0040]    Next, reference will be made to FIG. 1, FIG. 2 and FIG. 7, to describe a second embodiment of the present invention.  
         [0041]    With the first embodiment described above, a relationship between a ratio of the temperature change rate inside the quartz tube  13  to the temperature change rate inside the quartz tube  13  and the thickness of the PolySi film  24  deposited on the inner surface of the wall  23  of the quartz tube  13  is obtained in advance of the manufacturing process. Moreover, another relationship between the thickness of the PolySi film  24  deposited on the inner surface of the wall  23  of the quartz tube  13  and the thickness of the PolySi film deposited on the semiconductor wafers is obtained in advance. Using said another relationship, a thickness of a PolySi film formed on the semiconductor wafers in the device manufacturing process is obtained.  
         [0042]    In the present embodiment, description will be made for a film formation started from a state in which no PolySi film  24  is formed on the inner surface of the wall of the quartz tube  13 .  
         [0043]    The ratio between the temperature change rate outside the quartz tube  13  and the temperature change rate inside the quartz tube  13  has a vibratory pattern as shown in FIG. 7, and thus, the PolySi film thickness at a given ratio cannot be absolutely determined. However, by starting the film formation from a state in which no PolySi film  24  is formed, and by periodically measuring the inside temperature change with respect to the outside temperature change, it becomes possible to determine how thick a PolySi film has been formed. Specifically, since the PolySi film  24  is deposited at a rate of 5 nm through 10 nm per minute, thus, by measuring the inside temperature change with respect to the outside temperature change every one minute, it becomes possible to determine the film thickness change at the step of about 10 nm, if there are obtained in advance the relationship of the temperature change rate inside the quartz tube  13  to the temperature change rate inside the quartz tube  13  and the relationship between the thickness of the PolySi film on the inner surface of the wall of the quartz tube  13  and the thickness of the PolySi film on the semiconductor wafers  11 .  
         [0044]    The above information is entered into a factory information system to determine the thickness of the PolySi film  24  by the system. Since by this system treatment the PolySi film thickness can be absolutely determined, then it is possible to judge whether a desired film thickness has been reached by the system. It is also possible to generate a stop signal at the system to stop the film formation at the film forming apparatus.  
         [0045]    Steps for this procedure will be described with reference to FIG. 1. First, in order to initialize the low pressure CVD apparatus, semiconductor wafers which have undergone the preceding cycle of the film formation process are taken out, and then a boat  12  having no semiconductor wafers placed thereon is introduced into the quartz tube  13 . ClF 3  gas is introduced at a rate of 2 SLM into the quartz tube  13  the inside of which is under the condition of 600° C. and 1 Torr to etch with the gas the PolySi film deposited on the inner surface of the wall  23  of the quartz tube  13  and the PolySi film deposited on the boat  12 . After removing the PolySi films, the inside of the quartz tube  13  is purged out, then fed with N 2  gas to bring the inside back to the atmospheric pressure, and the boat  12  is taken out from the quartz tube  13 . Thereafter, semiconductor wafers for products are set on the boat  12 , and then introduced into the quartz tube  13 . After that, the inside of the quartz tube  13  is low-pressurized to reduce the pressure therein and then the film formation is performed. The ratio of the inside temperature change rate with respect to the outside temperature change rate is measured every one minute from the start of the film formation, and the information is sent on line from the low pressure CVD apparatus to the factory computer system and stored therein.  
         [0046]    In the system, the thickness of the film formed by deposition on the semiconductor wafers is calculated to determine the thickness of the film, on the basis of a pre-obtained database concerning the relationship between the inside temperature change rate with respect to the outside temperature change rate and the relationship between the thickness of the PolySi film on the inner surface of the wall of the quartz tube  13  and the thickness of the PolySi film on the semiconductor wafers. As the target film thickness is approached, the system calculates how many seconds are remaining till the target thickness is reached, and determines the timing to stop the film forming process. Upon the determined timing, the system sends a film formation stop signal to the manufacturing apparatus, so that the manufacturing apparatus stops the film formation operation. Then, a new cycle of the process begins with the etching of the deposited PolySi with ClF 3  gas and then performs the film formation. By repeating this cycle of film formation and etching, the thickness of the PolySi film formed on the semiconductor wafers is controlled.  
         [0047]    Next, a third embodiment of the present invention will be described with reference to FIG. 7.  
         [0048]    In the above-described embodiments, description was made primarily for a formation of the PolySi film  24 . With the present embodiment, description will be made for an application of the PolySi film  24  to the etching.  
         [0049]    The PolySi film  24  is thinned by etching, and as understood from FIG. 7, when the thickness is 1000 nm or less, it is possible to determine the thickness of the PolySi film  24  (Horizontal Axis) on the quartz tube, from the ratio of the inner temperature change rate with respect to the outer temperature change rate (Vertical Axis). Therefore, during the process of etching the PolySi film  24  on the inner surface of the wall of the quartz tube  13  with the ClF 3  gas to thin the film thickness, it is possible to monitor the thickness of the PolySi film  24  on the inner surface of the wall of the quartz tube  13  and to judge whether or not the etching has ended. Furthermore, as described in the second embodiment, it is possible for the factory computer system to determine the thickness of the PolySi film  24  and thus determine the film thickness when the etching is started. Thus, it is also possible to define the film thickness to be etched. When the system determines that the etching has ended, the system issues an etching stop instruction after an over-etching time.  
         [0050]    Next, a fourth embodiment of the present invention will be described with reference to FIG. 8.  
         [0051]    With the embodiments above described, the methods primarily work on the PolySi film. However, those methods described above can be applied not only to the PolySi film but also to any semi-transparent film. In the present embodiment, consideration will be made to LPCVD for Ru. Ru is a metal, which is given attention as a material of a capacitor electrode of DRAMs. FIG. 8 shows an infrared ray transmissivity characteristic of Ru deposited on an inner surface of a wall of a quartz tube. In FIG. 8, the vertical axis represents the transmissivity whereas the horizontal axis represents the thickness of the Ru film (nm) formed by deposition. Since Ru is a metal, infrared ray can no longer pass through the Ru film when the film thickness increases to 150 nm or more. It is possible to determine the thickness up to 150 nm of the Ru film deposited on the inner surface of the wall the quartz tube, based on the ratio of the inner temperature change rate with respect to the outer temperature change rate. Furthermore, when the Ru film is etched with a gas, it is possible to determine the timing when the etching has ended, as is the case for the PolySi film. Thus, the ratio of the inner temperature change rate with respect to the outer temperature change rate can be used as a monitor to determine the end point of the etching.  
         [0052]    As has been described above, the manufacturing method of each of the embodiments includes measuring a abrupt change of temperature of the heater provided outside the quartz tube and a temperature inside the quartz tube to attain a relationship between the temperature changes, so that the thickness of the film deposited on the inner surface of the wall of the quartz tube is monitored in situ. Therefore, it has become possible to eliminate the additional step of depositing the PolySi film on the inner surface of the wall of the quartz tube in advance of the device manufacturing process, and to improve stability in the film formation.