Process for suppressing boron penetration in BF2 + -implanted P+ -poly-Si gate using inductively-coupled nitrogen plasma
BACKGROUND OF THE INVENTION 
     P +   -Poly -Si has been proposed and recommended as the gate material for surface-channel pMOSFETs in deep submicron CMOS (complementary metal-oxide semiconductor) structures [1]. This is because surface-channel pMOSFETs with P +   -poly-Si gates can improve short-channel and sub-threshold I-V characteristics, and better controllability of the threshold voltage compared with buried-channel pMOSFETs with n +  -poly-Si gates [2-4]. 
     However, the fast diffusion of boron in poly-Si and within the gate oxide results in the easy penetration of boron through the gate to the underlying silicon substrate [5,6]. The presence of F due to BF 2  implantation enhances the diffusion of B [7,8]. Boron penetration results in instabilities in the device performance, such as positive shifts in the threshold voltage, increases in sub-threshold swing, increases in electron trapping, decreases in low-field hole mobility, and degradation of drive current due to poly-Si depletion in pMOSFETs. 
     Many methods have been proposed to suppress boron penetration [9-19]. Following the diffusion path of boron diffusion, the first way is to retard the boron diffusion inside the poly-Si. To this end, stacked or modified structures of the poly-Si gate have been proposed [9-10]. The native oxide between the stacked layers acts as the trapping center of F, consequently retarding the enhancement of B diffusion. The stacked structure also exhibits discontinuous poly-Si grain boundaries which disrupt the diffusion path of boron. Nitridation of the stacked poly-Si interface[11], or co-implantation with N, or P is also effective to retard the boron diffusion [12-13]. Utilizing a top oxide on poly-Si to getter the F has also been reported to retard the boron diffusion [14]. 
     The second way is to establish a diffusion barrier at the interface of the poly-Si and oxide. NH 3  nitridation on the gate oxide is feasible [15]. However, the H atom residues during the NH 3  nitridation reduces the resistance to hot carrier stressing. Usually, this method needs an additional reoxidation process to reduce the H atom. Hence, the NH 3  nitridation process is too complex to implement in a real production line and a nitridation process on the oxide surface without H incorporation is worthy of development. 
     The third way to establish a diffusion barrier at the interface of the poly-Si and oxide is to build a diffusion barrier in the oxide or between the oxide and silicon substrate interface. This can be achieved by nitriding the gate oxide using N O, N 2  O oxidation/annealing[16-19]. In this invention, a novel and simple process to built the diffusion barrier between the poly-Si and oxide was developed by using inductively-coupled nitrogen plasma (ICNP). 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention is to provide a process for suppressing the penetrating effect of boron ions. 
     This object of effectively preventing penetration of boron ions at high temperature is achieved by inductively-coupled nitrogen plasma treatment, the nitrogen plasma being introduced by an inductively-coupled method using an RF generator after growth of the gate oxide layer. 
     The process of the invention thus comprises providing a nitride layer between the oxide layer and polysilicon layer, which nitride layer can act as an energy barrier which is hardly penetrable by the boron ion. The advantages of the invention process are that it is very economical--N 2  is cheaper than NH 3  and N 2  O and the system is also simpler--, the process temperature is low, the process is simple, uses low cost equipment and does not need any expensive special gas. Therefore, the practicability of the process according to the invention is very high. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the various structures made by the preferred process. 
     FIG. 2 is a diagram showing comparison of nitrogen content in the space between the oxide layer and the polysilicon substrate after various ICNP (inductively-coupled nitrogen plasma) treatments. 
     FIG. 3 is a diagram showing flatband voltages of annealed samples (at 900° C.) which have been treated at ICNP 150W for 10 min., ICNP 250W for 10 min. and without ICNP treatment. 
     FIG. 4 is a diagram showing the relationship of the ratio of low frequency capacitance to oxide layer capacitance with the gate voltage of different samples which have been treated with ICNP 150W for 10 min., ICP 250W for 10 min. and without ICNP treatment. 
     FIG. 5 is a diagram showing the relationship of the ratio of high frequency capacitance to oxide layer capacitance with the gate voltage of different samples which have been treated with ICNP 150W for 10 min., ICNP 250W for 10 min. and without ICNP treatment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred process, as illustrated in FIG. 1, is implemented as follows: 
     Substrate 1 and thick oxide 2 are formed from a silicon wafer according to conventional CMOS production methods. The silicon wafer is then washed and placed in a high temperature furnace, where an oxide layer 3 of 50-120 Å is grown by introducing oxygen at 900°-1000° C., again according to conventional production methods. Next, a nitrided barrier layer is formed by placing the silicon wafer in an ICNP system and introducing nitrogen at power of 100-300 W for 1-30 minutes at the surface of the substrate, after which one or more layers 4 of polysilicon or polycrystalline silicon of approximately 1000-3000 Å thickness are stacked, BF 2   +  ions 5 are implanted at an energy of 30-70 KeV and amount of 1×10 15  -1×10 16  atom/cm2 with layer 3 preventing penetration, and the silicon wafer is washed again and placed into a high temperature furnace again, at which time nitrogen or oxygen is introduced at 800°-1000° C. and annealed for 10-30 minutes. Subsequent steps are the same as those in conventional double gate CMOS production. 
     Referring to FIG. 2, a comparison of nitrogen contents in the space between the oxide layer formed and the polysilicon substrate after various ICNP treatments shows that after ICNP treatment, more nitrogen was stacked in the space between the oxide layer formed and the polysilicon substrate, the nitrogen content at 500W, 21, being higher than that at 250W, 22, whereas when the sample at 250W was annealed in high temperature nitrogen of 900° C., its nitrogen content, 23, became less and about the same as that of control sample, 24. Thus, this secondary-ion mass spectrum (SIMS) can demonstrate that a layer of nitrogen atoms is indeed formed at the interface between the oxide layer and the polysilicon substrate. 
     Referring to FIG. 3, the flatband voltage (vfb) of annealed (900° C.) samples which have been treated, respectively, with ICP 150W for 10 minutes, ICP 250W for 10 minutes, and without ICP, after treatment for 20 minutes, the vfb of the sample treated with ICP 150W, 31, is the lowest, while that of the sample treated with ICP 250W, 32, is the same as that of control, 33, and after annealing for 60 minutes, the vfb of the sample treated with ICP 150W, 31, is still the lowest, but in this case, the vfb of the sample treated with ICP 250W, 32, is higher than that of control, 33. 
     This result occurs because the power of ICP 250W is too strong, causing the quality of the oxide layer to be degraded so as to be unable to prevent the diffusion of boron ions. FIG. 4 shows the relationship of the ratio of low frequency capacitance to oxide layer capacitance versus gate voltage of those three samples after annealing at 900° C. for 60 minutes, while FIG. 5 shows their high frequency capacitance-voltage relationship. It can be seen that the shift of the sample treated with ICP 150W, 31, like its Vfb, is the lowest, indicating that its boron diffusion is the lowest. 
     The invention has been described in detail with reference to the above embodiments in order to illustrate the objects, features and functionalities thereof. However, a person having ordinary skill in the art will readily recognize that, according to the above description of the invention, partial variations and modifications to the embodiment can be made without departing from the scope and spirit of the invention. Accordingly, the intellectual property rights to this invention are defined only by the appended claims. 
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