This application claims the benefit of Application No. 99-34962, filed in Korea on Aug. 23, 1999, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming a polysilicon layer, and more particularly, to a method of forming a polysilicon layer for dual gates. 
     2. Discussion of the Related Art 
     A complementary MOSFET (CMOS) consisting of NMOS and PMOS is widely used in the integrated circuit field. As the size of devices is reduced, a p +  polysilicon gate is used for a PMOS device. Degradation of performance characteristics of devices using the p +  polysilicon gate, such as threshold voltage fluctuation and ruined reliance of gate insulating layers, has been observed. This device degradation is a result of penetration of boron into thin gate insulating layers or diffusion of boron into a channel region in a silicon substrate. 
     Referring to FIG. 1A, a gate oxide layer  2 , 80 Å-thick, is formed on an n-type silicon substrate  1 . Referring to FIG. 1B, a first polysilicon layer  3 , 1000 Å-thick, is deposited on the gate oxide layer  2  by a low-pressure CVD (LPCVD) at 625° C. Referring to FIG. 1C, a silicon nitride layer  4  is generated by nitridizing the first polysilicon layer  3  in NH 3  at 900° C. and a pressure of 120 mTorr. Next, the silicon nitride layer  4  is removed by dilute HF (not shown). Nitrogen atoms are distributed on a surface of the first polysilicon layer  3 . Referring to FIG. 1D, a second polysilicon layer  5 , 1000 Å-thick, is deposited by LPCVD at 625° C. Ion implantation of BF 2   +  on the second polysilicon layer  5  is performed with an implant dosage of 5×10 15  ions/cm 2  and an implantation energy of 50 keV. A polysilicon oxide layer  6  is formed on an upper part of the second polysilicon layer  5  by a first annealing in O 2  ambience at 800° C. for 30 minutes followed by a second annealing in N 2  ambience at 900° C. 
     A nitrogen barrier in the first polysilicon layer  3  prevents fluorine ions from spreading out and also reduces an amount of fluorine ions in the gate oxide layer  6 , thereby decreasing the penetration of boron due to fluorine ions. In a p +  polysilicon gate structure of at least two layers of polysilicon, an interface layer  31  between the first polysilicon layer  3  and the second polysilicon layer  5  or between the second polysilicon layer  5  and a third polysilicon layer (not shown) is nitridized by high temperature gas nitridization using NH 3  or N 2 O, thereby generating and accumulating numerous nitrogen atoms at the interface layer  31  as well as at other interface layers between the gate oxide layer  2  and the first polysilicon layer  3 . The nitrogen atoms are prevented from spreading out by the fluorine ions. 
     A conventional method of forming a polysilicon layer uses an active gas, such as NH 3  to form an interface layer and requires a subsidiary gas supply. When an n +  gate of NMOS in dual gates is formed, diffusion of arsenic (As) as a dopant in the interface layer is delayed, thereby decreasing doping efficiency. Thus, a previous ion-implantation is required to overcome this decreased doping efficiency. Another dopant, such as phosphorous (P), which has relatively faster diffusion and activation rates than those of As, should be implanted inside the n+ gate to the proper depth. Moreover, dopants accumulate in the interface layer  31  due to diffusion delay to reduce the activation rate, thereby increasing a shear resistance of polysilicon. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method of forming a polysilicon layer that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. An object of the present invention is to provide a method of forming a polysilicon layer which prevents boron penetration in a PMOS. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of forming a polysilicon layer includes the steps of: loading a semiconductor substrate in a CVD reactor wherein a gate insulating layer is formed on the substrate; decompressing the reactor; depositing a first polysilicon layer on the substrate by flowing an SiH 4  gas into the reactor; forming a plurality of Si—N bonds on the first polysilicon layer by maintaining atmospheric pressure of the reactor by filling the reactor with a nitrogen gas; decompressing the reactor; and depositing a second polysilicon layer on the first polysilicon layer by flowing SiH 4  gas into the reactor. 
     In another aspect of the claimed invention, a method of forming a polysilicon layer includes the steps of: loading a silicon wafer in a CVD vertical reactor, wherein a gate oxide layer is formed on the wafer and wherein the reactor maintains atmospheric pressure at 625° C. and is purged by nitrogen; decompressing the reactor to 50.5 Pa; depositing a first polysilicon layer on the wafer by flowing an SiH 4  gas into the reactor; forming a plurality of SiN bonds on the first polysilicon layer by maintaining atmospheric pressure of the reactor by filling the reactor with a nitrogen gas; decompressing the reactor to 50.5 Pa; and depositing a second polysilicon layer on the first polysilicon layer by flowing SiH 4  gas into the reactor, wherein the second polysilicon layer is formed to be thicker than the first polysilicon layer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
     FIGS. 1A to  1 D are cross-sectional views of p +  polysilicon gate fabrications according to a conventional method of forming a polysilicon layer; 
     FIG. 2 is a schematic view of a low-pressure CVD (LPCVD) vertical reactor for forming a polysilicon layer according to the invention; 
     FIGS. 3A to  3 C are cross-sectional views of P 30   polysilicon gate fabrications according to the invention of forming a polysilicon layer; and 
     FIG. 4 is a diagram of a preferred process recipe for depositing a polysilicon layer according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     Referring to FIG. 2, a silicon wafer  23  is loaded in a boat  22  surrounded by an inner tube  21  in an LPCVD vertical reactor  20 . An SiH 4  gas line  24  being diverged and an N 2  gas line  25  are connected to the inner tube  21  through a gas injection nozzle  26 . A pressure sensor  27  measures pressure inside the reactor  20 . The pressure is decompressed by a vacuum pump  29 . An automatic pressure control valve  28  located between the vacuum pump  29  and the reactor  20  controls pressure inside the reactor  20 . Remaining gases and by-products are transferred to an exhaust  30  through the vacuum pump  29 . 
     Referring to FIG. 3A, a gate oxide layer  62 , 80 Å-thick, is formed on an n-type silicon substrate  61 . Referring to FIG. 3B, a polysilicon layer  70 , 2000 Å thick, is deposited on the gate oxide layer  62  by LPCVD at 625° C. A process recipe of depositing polysilicon in the reactor  20  is specifically explained as follows. 
     The silicon substrate  61  on which the gate oxide layer  62  is formed is loaded in the reactor  20  at 625° C. A first polysilicon layer  70 , of 300, 500, or 1000 Å thickness, is deposited under vacuum ambience wherein a reactant gas of SiH 4  flows into the reactor  20  which maintains a pressure of 50.5 Pa with a deposition rate of 100 Å/min. After the first polysilicon layer  70  has been deposited, the reactor  20  is purged by an N 2  gas under vacuum ambience. After purging any remaining gases in the reactor  20 , the reactor  20  is increased to atmospheric pressure once a flow rate of the N 2  gas is increased to a maximum of 10 liters/min. The reactor  20  is at 625° C. and a process pressure is maintained at atmospheric pressure for a duration of ten minutes. The reactor  20  is vacuum pumped to a vacuum. 
     A second polysilicon layer  70   a  over 1000 Å thick is deposited on the first polysilicon layer  70  under vacuum ambience wherein the SiH 4  gas flows into the reactor  20  at the deposition rate of 100 Å/min. for ten minutes at 625° C. and 50.5 Pa. Deposition is not complete until a total thickness of both the first polysilicon layer  70  and the second polysilicon layer  70   a  is 2000 Å. 
     During the duration of ten minutes that the N 2  gas in the reactor  20  is at atmospheric pressure, strong Si—N bonds  68  are formed at an interface between the first polysilicon layer  70  and the second polysilicon layer  70   a.  Nitrogen density at the interface is approximately 5×10 18  atoms/cm 3 , but when formed with N 2  gas ambience in a vacuum, the nitrogen density is approximately 1×10 18  atoms/cm 3 . 
     Referring to FIG. 3C, BF 2   +  ions are implanted into the second polysilicon layer  70   a  with an implantation dosage of 5×10 15  ions/cm 2  and an implantation energy of 50 keV. A polysilicon oxide layer  76  is formed on an upper surface of the second polysilicon layer  70   a  by a first annealing in an atmosphere of O 2  at 800° C. for 30 minutes followed by a second annealing in an atmosphere of N 2  at 900° C. 
     In the explanation above, the Si—N bonds formed at the interface of the first polysilicon layer  70  and the second polysilicon layer  70   a  prohibit fluorine ions from spreading out and reduce the amount of fluorine ions in the gate oxide layer  62 , thus suppressing the effect of boron penetration. After the deposition of the first polysilicon layer under low pressure, the claimed invention generates the strong Si—N bonds  68  which become a diffusion barrier at the interface not by unloading the substrate  61  from the reactor but by maintaining inside the reactor  20  under N 2  ambience at atmospheric pressure for ten minutes with the LPCVD equipment. 
     Referring to FIG. 4, a vertical axis represents pressure in the reactor  20  and a horizontal axis represents intervals of time. In the present invention, as shown in FIG. 4, surface temperature is constant. 
     During a first time interval  100 , a silicon wafer is loaded in a reactor while the reactor is purged by inactive N 2  gas. During a second time interval  101 , the reactor continues to be purged by the inactive N 2  gas until a vacuum in the reactor is achieved. During a third time interval  102 , when pressure in the reactor is maintained at 50.5 Pa, a reactant SiH 4  gas flows into the reactor at a flow rate of approximately 473 sccm with a deposition rate of 100 Å/min., thereby forming a first polysilicon layer at a thickness of 300, 500, or 1000 Å. 
     During a fourth time interval  103 , the reactor continues to be pumped out by vacuum purging until a vacuum in the reactor is achieved. During a fifth time interval  104 , the reactor is purged by the inactive N 2  gas to 50.5 Pa. During a sixth time interval  105 , pressure in the reactor is brought to atmospheric pressure by filling an active N 2  gas back into the reactor. During a duration period of ten minutes, strong Si—N bonds are formed at the interface with a nitrogen density of 5×10 18  atoms/cm 3 . During a seventh time interval  106 , the reactor is purged by the inactive N 2  gas until a vacuum is achieved. 
     During an eighth time interval  107 , when pressure in the reactor is maintained at 50.5 Pa, the reactant SiH 4  gas flows into the reactor at a flow rate of approximately 473 sccm with a deposition rate of 100 Å/min., thereby depositing a second polysilicon layer. The thickness of the second polysilicon layer  70   a  is greater than or equal to that of the first polysilicon layer  70 . During a ninth time interval  108 , the reactor is purged until a vacuum is achieved. During a tenth time interval  109 , the reactor is purged by the inactive N 2  gas until the reactor reaches 50.5 Pa. During an eleventh time interval  110 , purging the reactor by the inactive N 2  gas continues until pressure in the reactor reaches atmospheric pressure. The wafer  61  is then unloaded. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method of fabricating a polysilicon layer of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
A method of forming a polysilicon layer includes the steps of: loading a semiconductor substrate in a CVD reactor wherein a gate insulating layer is formed on the substrate; decompressing the reactor; depositing a first polysilicon layer on the substrate by flowing an SiH 4 gas into the reactor; forming a plurality of Si--N bonds on the first polysilicon layer by maintaining atmospheric pressure of the reactor by filling the reactor with nitrogen gas; decompressing the reactor; and depositing a second polysilicon layer on the first polysilicon layer by flowing SiH 4 gas into the reactor.