Abstract:
A method of forming an insulating film according to the present invention reacts a nitrogen containing gas with a compound composed of silicon and chlorine under the condition that the gas flow ratio of the compound to the nitrogen containing gas is lower than {fraction (1/30)} to form a silicon nitride film. In the present invention, by forming the silicon nitride film at the gas flow ratio lower than {fraction (1/30)}, an insulating film having this silicon nitride film is improved in electric insulating property, so that a smaller leak current flows therethrough.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor device manufacturing process, and more particularly, to a method of forming an insulating film.  
           [0003]    2. Description of the Related Art  
           [0004]    For establishing a manufacturing process for next-generation semiconductor devices which entail design rules that are required to accomplish a minimum dimension of 0.14 μm or less, the diffusion of impurities must be further reduced within semiconductor substrates. For this purpose, a silicon nitride film, which is used to insulate conductors from one another, must be formed at lower temperatures. However, when a silicon nitride film is formed at temperatures decreased to as low as approximately 600° C. with conventionally used reaction gases, i.e., dichlorosilane (SiH 2 Cl 2 , hereinafter called the “DCS”) and ammonia (NH 3 ), the resulting silicon nitride film suffers from a suddenly reduced deposit rate and an insufficient throughput. To address these problems, the DCS has been replaced with hexachlorodisilane (Si 2 Cl 6 , hereinafter called the “HDC”) for forming a silicon nitride film because of its ability to deposit a film even at temperatures of approximately 600° C., as disclosed in Japanese Patent Laid-open Publication No. 343793/2002.  
           [0005]    In the following, a silicon nitride film which is formed using DCS for a reaction gas is designated by DCS-Si 3 N 4 , while a silicon nitride film which is formed using HCD for a reaction gas is designated by HCD-Si 3 N 4 .  
           [0006]    Now, DRAM (Dynamic Random Access Memory) will be described as an example of a semiconductor device which employs HCD-Si 3 N 4 .  
           [0007]    [0007]FIG. 1 is a cross-sectional view illustrating an exemplary structure of a memory cell in DRAM. It should be noted that while Si (silicon) substrate  100 , which is a semiconductor substrate, is formed with transistors each having a source electrode, a drain electrode, and the like, such transistors are omitted in the illustration because they are similar in structure to conventional ones.  
           [0008]    As illustrated in FIG. 1, plugs  120   a ,  120   b  formed in interlayer insulating film  102  are connected to source electrodes (not shown) formed within Si substrate  100 . Plug  122   a  connected to plug  120   a , and plug  122   b  connected to plug  120   b  are formed in interlayer insulating film  104  and interlayer insulating film  106 , respectively. Plugs  122   a ,  122   b  are connected to lower electrodes  124   a ,  124   b , respectively, of capacitance cylinders formed within interlayer insulating film  108 .  
           [0009]    Plugs  120   a ,  120   b ,  122   a ,  122   b  are each formed by diffusing impurities into polysilicon embedded in openings within the interlayer insulating film. Thus, lower electrodes  124   a ,  124   b  are connected to source electrodes (not shown) to provide electric conduction therebetween. In the following description, a connection comprised of plugs  120   a  and  122   a , and a connection comprised of plugs  120   b  and  122   b  are each called a “capacitive contact plug.”  
           [0010]    Bit lines  110   a ,  110   b ,  110   c  are formed within interlayer insulating film  106 . Each of bit lines  110   a ,  110   b ,  110   c  is formed of a tungsten nitride film and a tungsten film in order. Interlayer insulating films  102  to  108  are laminated in sequence, where each of interlayer insulating films  102 - 108  is made, for example, of silicon oxide film.  
           [0011]    It should be noted that since components such as the dielectric, upper electrodes, and the like formed above lower electrodes  124   a ,  124   b  of the capacitance cylinders are similar in structure to before, these components are omitted in the illustration.  
           [0012]    CAP nitride film  150  is formed on bit lines  110   a ,  110   b , and spacer nitride film  160  is formed on side walls of bit lines  110   a ,  110   b  in order to electrically insulate capacitive contact plug  130   a  from bit lines  110   a ,  110   b . Spacer nitride film  160  is formed of the aforementioned HCD-Si 3 N 4 . Spacer nitride film  160  for electrically insulating capacitive contact plug  130   b  from bit lines  110   b ,  110   c  is also formed of HCD-Si 3 N 4 .  
           [0013]    Now, a method of forming spacer nitride film  160  will be described in brief. After bit line  110  and CAP nitride film  150  are laminated in order to form a laminate, HCD-Si 3 N 4  is formed. Subsequently, HCD-Si 3 N 4  is anisotropically etched to form spacer nitride film  160  on side walls of the laminate comprised of bit line  1   10  and CAP nitride film  150 , as illustrated in FIG. 1.  
           [0014]    Next, a method of forming HCD-Si 3 N 4  will be described in detail.  
           [0015]    HCD-Si 3 N 4  is formed on the surface of a semiconductor substrate by supplying HCD and NH 3  at a gas flow ratio HCD/NH 3  equal to {fraction (1/30)} (HCD gas flow rate:30 cc/min, NH 3  gas flow rate:900 cc/min) into a reaction tube, which has been decompressed by a CVD (Chemical Vapor Deposition) system, for reaction. In this event, HCD-Si 3 N 4  exhibits a deposit rate which is equivalent to that exhibited by DCS-Si 3 N 4  formed at 760° C., even if HCD-Si 3 N 4  is formed at relatively low temperatures of approximately 600° C., lower than 700° C., from which it is appreciated that HCD-Si 3 N 4  excels in productivity.  
           [0016]    Conventionally, a single-wafer CVD system has been utilized to form a silicon nitride film on semiconductor substrates one by one in order to reduce the amount of heat treatment applied to semiconductor devices. Even in comparison with DCS-Si 3 N 4  formed by the single-wafer CVD system, it has been confirmed that HCD-Si 3 N 4  is advantageous in step coverage and pattern density dependence.  
           [0017]    However, when HCD-Si 3 N 4  formed by the foregoing method was used for a spacer nitride film for bit lines in DRAM, faults were found in a reliability test. An investigation on an estimated cause revealed that a leak current between a capacitive contact plug and a bit line was larger than when DCS-Si 3 N 4  was used for the spacer nitride film. Accordingly, a leak current characteristic was confirmed for HCD-Si 3 N 4 , showing a leak current of approximately 3 E−4 [A/cm 2 ] at an electric field strength of 4 [MV/cm], which is larger approximately by three orders of magnitude than a leak current value of DCS-Si 3 N 4  which was 2 E−7 [A/cm 2 ]. Thus, it was clarified that HCD-Si 3 N 4  was inferior to DCS-Si 3 N 4  in leak current characteristic.  
         SUMMARY OF THE INVENTION  
         [0018]    It is an object of the present invention to provide a method of forming an insulating film which excels more in the leak current characteristic than before.  
           [0019]    The method of forming an insulating film according to the present invention reacts a nitrogen containing gas with a compound composed of silicon and chlorine under a condition that the gas flow ratio of the compound to the nitrogen containing gas is lower than {fraction (1/30)} to form a silicon nitride film. In the present invention, by forming the silicon nitride film at the gas flow ratio lower than {fraction (1/30)}, an insulating film having this silicon nitride film is improved in electric insulating property, so that a smaller leak current flows therethrough.  
           [0020]    In this event, when the gas flow ratio is chosen in a range of {fraction (1/100)} to {fraction (1/150)}, the resulting silicon nitride film is further improved in insulating property, so that a further reduced leak current flows through an insulating film having this silicon nitride film.  
           [0021]    Also, when the nitrogen containing gas is reacted with the compound at a temperature in a range of 400 to 700° C., a less amount of heat treatment is applied to semiconductor devices than before.  
           [0022]    In conclusion, the method of forming an insulating film according to the present invention is capable of forming a high-quality silicon nitride film which has an improved film quality, and excels in leak current characteristic. Moreover, even at a processing temperature in a range of 400 to 700° C., the silicon nitride film can be formed at a throughput maintained sufficiently high, the semiconductor substrate is applied with a reduced amount of heat treatment. This can prevent the diffusion of impurities within the semiconductor substrate and increase the integration degree of the semiconductor device.  
           [0023]    The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a cross-sectional view illustrating an exemplary structure of a memory cell in DRAM;  
         [0025]    [0025]FIG. 2 is a block diagram illustrating an exemplary configuration of a vapor-phase growth system for forming a silicon nitride film; and  
         [0026]    [0026]FIG. 3 is a graph showing the dependence on electric field strength of a leak current which flows through a silicon nitride film. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    To begin with, description will be made on a vapor-phase growth system for use with a method of forming an insulating film according to the present invention. FIG. 2 is a block diagram illustrating an exemplary configuration of a vapor-phase growth system for forming a silicon nitride film. Assume in the following description that semiconductor substrates include not only a substrate made of Si and or the like but also a substrate such as a Si substrate which has been formed with semiconductor elements, interlayer insulating films, and the like.  
         [0028]    The vapor phase growth system illustrated in FIG. 2 is a batch-type low pressure CVD system which is capable of forming silicon nitride films on a plurality of semiconductor substrates at one time. The illustrated vapor phase growth system comprises processing furnace  12  for forming a nitride film on a semiconductor substrate; gas conduits  16  for introducing reaction gases  14  into processing furnace  12  for forming the nitride film; mass flow controllers (MFC)  18  each for controlling the flow rate of associated reaction gas  14 ; vacuum pump  20  for exhausting gases in processing furnace  12 ; and a controller (not shown) for controlling the flow rates of various reaction gases, as well as the temperature and pressure within the processing furnace  12 .  
         [0029]    Processing furnace  12  comprises lid  12   a  for isolating the interior of processing furnace  12  from external air; a heater for uniformly maintaining the interior of processing furnace  12  at a predetermined temperature; a temperature sensor for monitoring the temperature within processing furnace  12 ; and a pressure sensor for monitoring the pressure within processing furnace  12 . A transport robot is also provided for carrying wafer board  26 , which is loaded with a plurality of semiconductor substrates, into processing chamber  12  and removing wafer board  26  from processing chamber  12 . This transport robot comprises a position sensor for monitoring the presence or absence of a cassette, the position of wafer board  26 , and the like. The transport robot carries unprocessed semiconductor substrates on wafer board  26  from a cassette yard, not shown, and returns processed semiconductor substrates from wafer board  26  to the cassette.  
         [0030]    The controller comprises a CPU (Central Processing Unit) for executing predetermined processing in accordance with a program, and a memory for storing the program. The controller is connected to control signal lines for sending control signals to the heater, MFC  18 , exhaust pump  20 , and transport robot, and to monitor signal lines for receiving signals from a variety of sensors. The controller controls the respective components through the control signal lines and monitor signal lines, and executes processing in accordance with processing conditions previously registered by the operator to form a nitride film on each semiconductor substrate.  
         [0031]    Next, description will be made on an experiment which was made for evaluating the quality of silicon nitride films which were formed under different conditions from before, including a different gas flow ratio HCD/NH 3 .  
         [0032]    TEG (Test Element Group) used in the experiment has two flat conductors in a predetermined pattern, and a silicon nitride film sandwiched between the two conductors for measuring a leak current through the silicon nitride film. Films were formed commonly under the same conditions except for the gas flow ratio HCD/HN 3 . Respective samples were manufactured in the following procedure at four different gas flow ratios HCD/HN 3  of 1:50, 1:100, 1:120, and 1:150.  
         [0033]    In the vapor phase growth system illustrated in FIG. 2, processing furnace  12  is heated by the heater to maintain the interior of processing furnace  12  at a predetermined temperature in a range of 400 to 700° C. Then, semiconductor substrates placed on wafer board  26  are carried into processing furnace  12 . Next, lid  12   a  is closed to hermetically seal processing furnace  12  from which air is exhausted by a vacuum pump to decompress the interior of processing furnace  12  at a predetermined pressure in a range of 13.3 to 266 Pa (0.1 to 2.0 Torr). Subsequently, HCD and NH 3  are supplied to processing furnace  12  at a predetermined gas flow ratio, for example, {fraction (1/100)} (HCD gas flow rate:15 cc/min, NH 3  gas flow rate:1,500 cc/min) to form a silicon nitride film on a semiconductor substrate. In this way, samples are fabricated. Likewise, respective samples were fabricated at each of different gas flow ratios in the foregoing procedure.  
         [0034]    Next, description will be made on the result of the experiment showing the leak current characteristic exhibited by each of the fabricated samples. Specifically, a leak current was measured as flowing through the silicon nitride film of each sample, while the silicon nitride film was applied with a voltage to generate an electric field strength which was varied from 0 to −5 [MV/cm].  
         [0035]    [0035]FIG. 3 is a graph showing the dependence on the electric field strength of the leak current flowing through the silicon nitride film, where the horizontal axis represents the electric field strength, and the vertical axis represents the leak current. It should be noted that the leak currents were evaluated in a range of −3 to −5 [MV/cm] of electric field strength because the leak currents were smaller than 1 E−7 [A/cm 2 ] in a range of 0 to 3 [MV/cm] of electric field strength and were therefore more susceptible to noise.  
         [0036]    As shown in FIG. 3, crosses, triangles, squares, and rhombuses were plotted to indicate leak currents associated with samples having the silicon nitride films formed at the gas flow ratios 1:50, 1:100, 1:120, and 1:150, respectively. For a comparison with the result of each sample, black circles are plotted to indicate a leak current associated with a conventional silicon nitride film formed at a gas flow ratio HCD/NH 3  of 1:30, and white circles are also plotted to indicate a leak current associated with conventional DCS-Si 3 N 4 . Any of the samples tends to have a leak current which increases as the absolute value of the electric field strength is larger.  
         [0037]    The leak currents of the respective samples are compared with one another at the electric field strength of −4 [MV/cm]. The conventional silicon nitride film formed at the gas flow ratio HCD/NH 3  equal to 1:30 exhibits a leak current of approximately 3 E−4 [A/cm 2 ]. As the gas flow. rate of ammonia is increased, the leak current decreases. Specifically, a sample fabricated at HCD/NH 3  equal to 1:100 exhibits a leak current reduced to approximately 2 E−6 [A/cm 2 ], and a sample fabricated at HCD/NH 3  equal to 1:150 exhibits a leak current reduced to approximately 1 E−6 [A/cm 2 ]. It can be seen from the graph of FIG. 3 that the leak current decreases as the gas flow rate of ammonia increases, to improve the film quality of the HCD-Si 3 N 4 .  
         [0038]    The gas flow ratio HCD/NH 3  is more preferably in a range of {fraction (1/100)} to {fraction (1/150)} in which the leak current is reduced to approximately 2 E−6 [A/cm 2 ] when the absolute value of the electric field strength is 4 [MV/cm].  
         [0039]    In this embodiment, as the gas flow ratio HCD/NH 3  is chosen to be {fraction (1/30)} or less for forming a silicon nitride film in the foregoing manner, the resulting silicon nitride film is improved in electric insulating property, causing a smaller leak current to flow through the silicon nitride film than before. Also, as the gas flow ratio HCD/NH 3  is chosen to be {fraction (1/100)} or less, the resulting silicon nitride film is further improved in film quality, thus making it possible to form a high-quality silicon nitride film which further excels in leak current characteristic. Thus, an insulating film having the silicon nitride film according to this embodiment is improved in electric insulating property, and passes a smaller leak current therethrough than before.  
         [0040]    Moreover, even at a processing temperature in a range of 400 to 700° C., the silicon nitride film can be formed at a throughput maintained sufficiently high, the semiconductor substrate is applied with a reduced amount of heat treatment. This can prevent the diffusion of impurities within the semiconductor substrate and increase the integration degree of the semiconductor device.  
         [0041]    When a compound of silicon and chlorine is designated by SixCly, hexachlorodisilane used in the foregoing embodiment is represented by (x, y)=(2, 6). However, other values than (2, 6) may be employed for (x, y).  
         [0042]    Also, while an ammonia gas is used for the formation of the silicon nitride film in the foregoing embodiment, any other gas may be used as long as it contains nitrogen.  
         [0043]    While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.