Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a semiconductor device manufacturing method and a semiconductor manufacturing apparatus, and more particularly, to a semiconductor device manufacturing method including a silicon nitride film manufacturing step using a thermal CVD (Chemical Vapor Deposition) method, and to a semiconductor manufacturing apparatus preferably used for the method.  
         [0003]     2. Description of the Related Art  
         [0004]     Conventionally, it is common that a silicon nitride film used in a semiconductor device is formed using mixed gas of SiH 2 Cl 2  (DCS, hereinafter) and NH 3 .  
         [0005]     According to this method, however, it is necessary to form the silicon nitride film at a temperature as high as 700° C. to 800° C. and as a result, there is a problem that impurities are adversely diffused deeply into a shallow diffused layer and a semiconductor device element can not be formed small in size. Further, there is a problem that NH 4 Cl (ammonium chloride), which is a by-product of reaction, adheres to a discharge port, this NH 4 Cl generates rust on a metal surface, and metal contamination is generated on a semiconductor wafer.  
       SUMMARY OF THE INVENTION  
       [0006]     To solve the problems, the present inventors examined forming a silicon nitride (Si 3 N 4 ) film using NH 3  and SiH 2 (NH(C 4 H 9 )) 2  (bis tertiary butyl amino silane: BTBAS, hereinafter) as raw gases. As a result, the inventors have found that the silicon nitride film can be formed at a low temperature of about 600° C. and NH 4 Cl, which is a cause of metal contamination, is not generated.  
         [0007]     However, the present inventors have found that the Si 3 N 4  film formed using BTBAS has the following drawbacks.  
         [0008]     That is, BTBAS and NH 3  introduced in a furnace are decomposed by heat, and the Si 3 N 4  film is formed not only on a wafer but also on an inner wall of a quartz reaction tube and an inner member made of quartz used in the reaction tube. The Si 3 N 4  film formed using BTBAS has a strong film stress and a film shrinkage ratio is great. A Si 3 N 4  film formed by using DCS and NH 3  and an Si 3 N 4  film formed by using BTBAS and NH 3  were compared with each other. Comparison data of a film contraction ratio in percentage is shown in  FIG. 4 , and comparison data of film stress is shown in  FIG. 5 . In  FIGS. 4 and 5 , “B” shows an Si 3 N 4  film formed by using BTBAS and NH 3 , and “D” shows an Si 3 N 4  film formed by using DCS and NH 3 . The film stress means a tensile force (film stress), and the Si 3 N 4  film formed on a reaction furnace quartz comes off by the film stress. The film is shrunk by a high temperature (about 600° C.) of the reaction furnace. Since quartz does not shrink or expand by heat and thus, distortion occurs. Therefore, when the Si 3 N 4  film becomes thick, a microcrack is generated, and particles are generated on the wafer. A thickness of an Si 3 N 4  film that may cause the microcrack is 4,000 Å.  
         [0009]     To solve the problem of the particles, it is necessary to perform maintenance in such a manner that whenever a film thickness becomes 4,000 Å, a quartz inner tube  12 , a quartz boat  14 , and a quartz cap  15  of an vertical-type LPCVD (Low Pressure Chemical Vapor Deposition) film forming apparatus  1  are disassembled, and they are subjected to wet cleaning using HF (hydrogen fluoride) to remove the Si 3 N 4  film. When one time film forming operation forms a film of 1,000 Å thickness, it is necessary to perform the maintenance every four film forming operations. Further, there is a problem that it takes 16 hours to complete the maintenance, and this is too long.  
         [0010]     Thereupon, it is a main object of the present invention to solve the problem of high frequency of maintenance when an Si 3 N 4  film is produced using BTBAS and NH 3 , and to provided a manufacturing method and a manufacturing apparatus of a silicon nitride film capable of reducing the maintenance frequency as small as possible and suppressing or preventing generation of particles.  
         [0011]     According to a first aspect of the present invention, there is provided a semiconductor device manufacturing method, comprising:  
         [0012]     a first step of forming, by a thermal chemical vapor deposition method, a silicon nitride film on an object disposed in a reaction container, with bis tertiary butyl amino silane and NH 3  flowing into the reaction container, and  
         [0013]     a second step of removing silicon nitride formed in the reaction container, with NF 3  gas flowing into the reaction container.  
         [0014]     Preferably, the semiconductor device manufacturing method according to the first aspect of the present invention further comprises the first step after the second step. That is the semiconductor device manufacturing method according to the first aspect of the present invention preferably comprises the first step, thereafter the second step and thereafter the first step again.  
         [0015]     Preferably, after repeating the first step predetermined times, the silicon nitride formed in the reaction container is removed, with NF 3  gas flowing into the reaction container.  
         [0016]     Preferably, before the silicon nitride formed in the reaction container has a predetermined thickness, the silicon nitride formed in the reaction container is removed, with NF 3  gas flowing into the reaction container.  
         [0017]     Preferably, before the silicon nitride formed in the reaction container has such a thickness as to generate particles on the object, the silicon nitride formed in the reaction container is removed, with NF 3  gas flowing into the reaction container.  
         [0018]     Preferably, the reaction container itself is made of quartz and/or a member made of quartz is used in the reaction container, and before a thickness of the silicon nitride formed on the quartz is increased to such an extent as to generate particles on the object, NF 3  gas is allowed to flow into the reaction container to remove the silicon nitride formed on the quartz. In this case, it is preferable to remove the silicon nitride with NF 3  gas before the thickness of the silicon nitride becomes 4000 Å or larger.  
         [0019]     Preferably, the second step is carried out in a state where a pressure in the reaction container is set to 10 Torr or higher.  
         [0020]     Preferably, the semiconductor device manufacturing method according to the first aspect of the present invention further comprises a step of purging the reaction container using NH 3  gas at least one of before and after the first step.  
         [0021]     According to a second aspect of the present invention there is provided a semiconductor manufacturing apparatus comprising a reaction container, wherein  
         [0022]     a silicon nitride film is formed, by a thermal chemical vapor deposition method, on an object disposed in the reaction container, with bis tertiary butyl amino silane and NH 3  flowing into the reaction container, and  
         [0023]     silicon nitride formed in the reaction container is removed, with NF 3  gas flowing into the reaction container. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The above and further objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein:  
         [0025]      FIG. 1  is a schematic sectional view for explaining a vertical-type LPCVD film forming apparatus used in one embodiment of the present invention;  
         [0026]      FIG. 2  is a schematic view for explaining one example of a BTBAS supply apparatus preferably used in the film forming apparatus used in the one embodiment of the present invention;  
         [0027]      FIG. 3  is a schematic view for explaining another example of a BTBAS supply apparatus preferably used in the film forming apparatus used in the one embodiment of the present invention;  
         [0028]      FIG. 4  is a graph showing a film contraction ratio in percentage of an Si 3 N 4  film formed by using BTBAS and NH 3  as raw gases;  
         [0029]      FIG. 5  is a graph showing a film stress the Si 3 N 4  formed by using BTBAS and NH 3  as raw gases;  
         [0030]      FIG. 6  is a graph showing a etching selection ratio by NF 3  of the Si 3 N 4  film formed by using BTBAS and NH 3  as raw gases; and  
         [0031]      FIG. 7  is a graph showing a continuous film forming state wherein NF 3  cleaning is performed every 3,000 Å thickness of the Si 3 N 4  film formed by using BTBAS and NH 3  as raw gases. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     Next, one embodiment of the present invention will be explained with reference to the drawings below.  
         [0033]     Since BTBAS used in the present invention is in a liquid state at room temperature, the BTBAS is introduced into a furnace using a BTBAS supply apparatus shown in  FIGS. 2 and 3 .  
         [0034]     A BTBAS supply apparatus shown in  FIG. 2  is a combination of a thermostatic bath and gas flow rate control. A BTBAS supply apparatus shown in  FIG. 3  controls a flow rate by a combination of a liquid flow rate control and a vaporizer.  
         [0035]     Referring to  FIG. 2 , in the BTBAS supply apparatus  4 , an interior of a thermostatic bath  41  containing a BTBAS liquid raw material  42  therein is heated to about 100° C. to increase a vapor pressure of BTBAS, thereby evaporating the BTBAS. Then, the evaporated BTBAS is controlled in flow rate by a mass-flow controller  43 , and supplied, from a BTBAS supply port  44 , to a supply port  22  of a nozzle  21  of an vertical-type LPCVD (low pressure CVD) film forming apparatus shown in  FIG. 1 . In the BTBAS supply apparatus  4 , pipes from the BTBAS liquid raw material  42  to the BTBAS supply port  44  are covered with pipe heating members  45 .  
         [0036]     Referring to  FIG. 3 , in the BTBAS supply apparatus  5 , push-out gas of He or N 2  introduced from a push-out gas introducing port  53  is introduced, through a pipe  54 , into a BTBAS tank  51  containing a BTBAS liquid raw material  52  therein, thereby pushing out the BTBAS liquid raw material  52  into a pipe  55 . Then, the BTBAS liquid raw material  52  is controlled in flow rate by a liquid flow-rate control apparatus  56  and sent to a vaporizer  57 . In the vaporizer  57 , the BTBAS liquid raw material  52  is evaporated and supplied, from a BTBAS supply port  58 , to the supply port  22  of the nozzle  21  of the vertical-type LPCVD (low pressure CVD) film forming apparatus shown in  FIG. 1 . In the BTBAS supply apparatus  5 , pipes from the vaporizer  57  to the BTBAS supply port  58  are covered with pipe heating members  59 .  
         [0037]     Next, the vertical-type LPCVD film forming apparatus which can preferably be used in the present embodiment will be explained with reference to  FIG. 1 .  
         [0038]     In the vertical-type LPCVD film forming apparatus  1 , a heater  13  is provided outside of a quartz reaction tube  11  so that an interior of the quartz reaction tube  11  can be heated uniformly. A quartz inner tube  12  is provided in the quartz reaction tube  11 . A quartz boat  14  is provided in the quartz inner tube  12 , and a plurality of semiconductor wafers are mounted on the quartz boat  14  and stacked in the vertical direction. The quartz boat  14  is mounted on a cap  15 . The quartz boat  14  is brought into and out from the quartz inner tube  12  by vertically moving the cap  15 . Lower portions of the quartz reaction tube  11  and the quartz inner tube  12  are opened, but they are air-tightly closed by a bottom plate  24  of the cap  15  by moving the cap  15  upward. Apparatus nozzles  18  and  21  are provided in lower portions of the quartz inner tube  12  such as to bring into communication with the quartz inner tube  12 . An upper portion of the quartz inner tube  12  is opened. A discharge port  17  is provided at a lower portion of space between the quartz inner tube  12  and the quartz reaction tube  11  so as to bring into communication with the space. The discharge port  17  is in communication with a vacuum pump (not shown) so as to evacuate the quartz reaction tube  11 . The raw gases supplied from the quartz nozzles  18  and  21  are injected from injection ports  20  and  23  into the quartz inner tube  12 . The gases then move in the quartz inner tube  12  from its lower portion to its upper portion, thereafter downwardly flows through the space between the quartz inner tube  12  and the quartz reaction tube  11 , and is discharged from the discharge port  17 .  
         [0039]     A method for forming a silicon nitride film using the vertical-type LPCVD film forming apparatus  1  will be explained next.  
         [0040]     First, the quartz boat  14  holding a large number of semiconductor wafers  16  is inserted into the quartz inner tube  12  the inside temperature of which is maintained at 600° C. or lower.  
         [0041]     Next, the quartz reaction tube  11  is evacuated from the discharge port  17  to produce a vacuum therein using a vacuum pump (not shown). In order to stabilize a temperature over the entire surface of the wafer, it is preferable to evacuate for about one hour.  
         [0042]     Next, NH 3  gas is charged from a charging port  19  of the quartz nozzle  18  to purge the inside of the quartz reaction tube  11  using NH 3  before BTBAS is charged.  
         [0043]     Then, while NH 3  gas is continuously charged from a charging port  19  of the quartz nozzle  18 , BTBAS is charged from the charging port  22  of the quartz nozzle  21 , and an Si 3 N 4  film is formed on the semiconductor wafer  16 .  
         [0044]     Next, the supply of BTBAS is stopped while keep charging the NH 3  gas from the charging port  19  of the quartz nozzle  18 , thereby purging the quartz reaction tube  11  using NH 3 .  
         [0045]     If only BTBAS is charged, a film different from the Si 3 N 4  film is formed and thus, it is preferable to purge the quartz reaction tube  11  using NH 3  before and after deposition.  
         [0046]     Next, N 2  is allowed to flow into the quartz reaction tube  11  from the quartz nozzle  18  to purge the quartz reaction tube  11  using N 2 , thereby removing NH 3  in the quartz reaction tube  11 .  
         [0047]     Then, the supply of N 2  is stopped and the quartz reaction tube  11  is evacuated to produce a vacuum therein. A set of the purge operation using N 2  and the subsequent evacuation operation in the quartz reaction tube  11  is carried out several times.  
         [0048]     Thereafter, the interior of the quartz reaction tube  11  is brought back from the vacuum state into the atmospheric pressure state. Then, the quartz boat  14  is moved down and taken out from the quartz reaction tube  11 . Then, the quartz boat  14  and the semiconductor wafers  16  are cooled down to room temperature.  
         [0049]     The above-described silicon nitride film forming method is repeated, and when a thickness of the Si 3 N 4  film formed in the quartz reaction tube  11  reached 3,000 Å, NF 3  gas is introduced into the quartz reaction tube  11  from the quartz nozzle  18 , thereby carrying out in situ cleaning of the Si 3 N 4  film.  
         [0050]     Next, this cleaning method will be explained.  
         [0051]     First, the quartz boat  14  holding no semiconductor wafer  16  is inserted into the quartz inner tube  12  the inside temperature of which is maintained at 600° C.  
         [0052]     Next, the quartz reaction tube  11  is evacuated from the discharge port  17  to produce a vacuum therein using the vacuum pump (not shown).  
         [0053]     Then, NF 3  gas is charged from the charging port  19  of the quartz nozzle  18  at a flow rate of 500 sccm, the quartz reaction tube  11  is evacuated to produce a vacuum therein from the discharge port  17  using the vacuum pump (not shown), a pressure in the quartz reaction tube  11  is maintained at 10 Torr or higher, and the interior of the quartz reaction tube  11  is cleaned.  
         [0054]     Then, the supply of NF 3  gas is stopped, the quartz reaction tube  11  is evacuated to provide a vacuum therein from the discharge port  17  using the vacuum pump (not shown), and residue NF 3  gas is discharged.  
         [0055]     Next, N 2  is allowed to flow into the quartz reaction tube  11  from the quartz nozzle  18  to purge the quartz reaction tube  11  using N 2  to remove NF 3  in the quartz reaction tube  11 .  
         [0056]     Then, the quartz reaction tube  11  is evacuated to produce a vacuum therein from the discharge port  17  using the vacuum pump (not shown). The evacuation operation and the purge operation using N 2  are carried out several times.  
         [0057]     Thereafter, the interior of the quartz reaction tube  11  is brought back from the vacuum state into the atmospheric pressure state. Then, the quartz boat  14  is moved down and taken out from the quartz reaction tube  11 .  
         [0058]     At the time of cleaning using NF 3 , when the Si 3 N 4  film is etched, the quartz is also adversely etched at the same time. Therefore, important is condition in which the Si 3 N 4  film is largely etched, and the quartz (SiO 2 ) is etched as little as possible.  
         [0059]      FIG. 6  shows a relation between a pressure and an etching selection ratio. In this figure, the horizontal axis shows a pressure in the quartz reaction tube  11 , and the vertical axis shows a ratio of an etching rate (ER (SiN)) of the Si 3 N 4  film to an etching rate (ER (SiO 2 )) of the quartz. Referring to  FIG. 6 , it can be found that as the pressure becomes higher, the etching selection ratio is increased, and the quartz (SiO 2 ) becomes less prone to be etched. For these reason, it is preferable to set the pressure to 10 Torr or higher. Further, by further increasing the pressure, the etching selection ratio becomes more excellent, and the etching rate is also enhanced and thus, the etching time can be shortened. For example, although the etching time is about 30 minutes when the pressure is set to 10 Torr, when the pressure is set to 70 Torr, almost the same etching can be carried out for about 15 minutes.  
         [0060]     By carrying out the NF 3  cleaning whenever the thickness of the formed Si 3 N 4  film reaches 3000 Å, it is possible to form particle-free Si 3 N 4  films 100 times continuously in a maintenance-free manner.  FIG. 7  shows data. In  FIG. 7 , the horizontal axis shows the number of film forming operations, a blank exists every three times operation. The blank shows the NF 3  cleaning operation. The vertical axis shows the number of foreign particles of 0.18μ or greater particle size on the wafer. The cleaning operation using NF 3  gas was carried out in such a manner that NF 3  gas was charged into the quartz reaction tube  11  at a flow rate of 500 scam, the quartz reaction tube  11  was evacuated to produce a vacuum therein, the pressure in the quartz reaction tube  11  was maintained at 10 Torr (1,300 Pa), a temperature therein was set to about 600° C., and the cleaning operation was carried out for 30 minutes. In  FIG. 7 , “top” means a 115th wafer from the bottom, “cnt” means 66th wafer from the bottom, and “bot” means a 16th wafer from the bottom, when 125 wafers were processed.  
         [0061]     Time required for carrying out the NF 3  cleaning operation once is 2.5 hours (it takes 30 minutes to flow NF 3  gas, and the remaining time are required for bringing up the boat and evacuating to produce a vacuum and the like), and there is a merit if compared with 16 hours required for conventional maintenance.  
         [0062]     As described above, according to the preferred embodiment of the present invention, when Si 3 N 4  films are formed using BTBAS and NH 3 , it is possible to reduce the frequency of maintenance as small as possible and to suppress or prevent the generation of particles.  
         [0063]     The entire disclosure of Japanese Patent Application No. 11-333129 filed on Nov. 24, 1999 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.  
         [0064]     Although various exemplary embodiments have been shown and described, the invention is not limited to the embodiments shown. Therefore, the scope of the invention is intended to be limited solely by the scope of the claims that follow.

Technology Category: 4