Abstract:
A method of fabricating a MOS transistor by millisecond annealing. A semiconductor substrate with a gate stack comprising a gate electrode overlying a gate dielectric layer on a top surface of a semiconductor substrate is provided. At least one implanting process is performed to form two doped regions on opposite sides of the gate electrode. Millisecond annealing activates dopants in the doped regions. The millisecond anneal includes rapid heating and rapid cooling within 1 to 50 milliseconds.

Description:
BACKGROUND  
       [0001]     The invention relates to a method of fabricating a MOS transistor, and more particularly, to a method of fabricating a MOS transistor using a millisecond anneal to activate dopants.  
         [0002]     In semiconductor fabrication, dopants are often implanted in a semiconductor wafer to control a number of electric carriers and form a plurality of conductive doped regions in the semiconductor wafer to construct electric devices. Dopants are implanted by doping processes. Typically, after doping, such as by ion implantation, thermal diffusion, or chemical evaporation, forms a doped region in a semiconductor substrate, a thermal process then repairs damage caused by the doping process and activates dopants in the substrate to reduce resistance of the doped regions.  
         [0003]     During thermal process, dopants diffuse outward from the designable doped region since the dopant concentration of the designable doped region is higher than that of the semiconductor substrate. As duration of the thermal process increases, diffusion distance of dopants, leading to obvious changes in shape, location, and dopant concentration. Some semiconductor technologies drive dopants into a semiconductor substrate deeply or enlarge the doped region in this manner.  
         [0004]     As semiconductor devices are scaled down and integration thereof increases, precise control of the doped region is needed. Thus, duration of the thermal process is reduced significantly. For example, a conventional thermal process may take 20 to 30 minutes. Current thermal process, such as rapid thermal annealing (RTA) or rapid thermal process (RTP), can typically take one minute. For laser annealing, the duration is only several nanoseconds. Since the RTP or RTA has a high temperature ramp up rate and short duration, a shallow diffusion depth can be achieved, reducing junction depth and diffusion in lateral directions and avoiding short channel effect and threshold voltage shift, associated with conventional process. However, when the device generation is nanometer scale, particularly less than 60 nm, RTA remains unable to completely satisfy the requirements of junction activation or device performance.  
         [0005]     Embodiments of a method of fabricating a MOS transistor comprise providing a substrate having a predetermined channel region in the substrate surface. A first ion implantation process is performed on the substrate surface to implant first dopants into a predetermined channel region. A gate stack comprising a gate dielectric layer and a gate electrode is formed in the predetermined channel region. A second ion implantation process is performed on the substrate to implant second dopants into two sides of the gate stack and form two doped regions adjacent to both sides of the gate stack. The doped regions serve as a source and a drain of the MOS transistor. Thereafter, a millisecond annealing process comprising rapid heating and rapid cooling is performed, both having duration of 1 to 50 milliseconds.  
         [0006]     Further provided is a method of activating dopants. A substrate having at least one doped region is provided. A millisecond annealing process comprising rapid heating and rapid cooling is then performed to activate dopants in the substrate, both having duration of 1 to 50 milliseconds. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
         [0008]      FIG. 1  and  FIG. 2  are schematic diagrams of an embodiment of a method of fabricating a MOS transistor;  
         [0009]      FIG. 3  is a schematic diagram showing the relationship between temperature and time in a thermal process used in embodiments of a method of fabricating a MOS transistor;  
         [0010]      FIG. 4  is a schematic diagram of a heating device used in the thermal process of  FIG. 3 .  
         [0011]      FIG. 5  is a schematic diagram showing the relationship between temperature and depth in the thermal process of  FIG. 3 .  
     
    
     DETAILED DESCRIPTION  
       [0012]      FIGS. 1 and 2  are schematic diagrams of embodiments of a method of fabricating a MOS transistor. As shown in  FIG. 1 , a semiconductor wafer  10  is provided, comprising a semiconductor substrate  12  with a well  13  comprising a source region  30 , a drain region  40 , and a channel region  20  disposed thereon. A first ion implantation with proper masks forms a first doped region  14 , adjusting a threshold voltage of a subsequent metal-oxide semiconductor (MOS) transistor. A gate dielectric layer  16  and a gate electrode  18  are formed on the channel region  20  of the semiconductor substrate  12  in sequence to form a gate stack. The gate dielectric layer  16 , while here silicon oxide, can comprise other dielectric materials such as silicon nitride, silicon oxynitride, high-k dielectric materials or combinations thereof. The gate electrode  18  comprises a conductive material, preferably a polysilicon or doped polysilicon layer. In another embodiment of the invention, the gate electrode  20  comprises a doped polysilicon layer with a metal gate on top. The metal gate could be formed of Ti, TiN, W, WN, or combinations thereof.  
         [0013]     As shown in  FIG. 2 , a second ion implantation process is performed by using the gate electrode  18  as a mask to form two first light doped drain (LDD) regions  28  adjacent to the gate oxide  16 . Then, two first spacers  25  are formed adjacent to both sidewalls of the gate electrode  18  and the gate oxide  16 . A third ion implantation process is performed by using the gate electrode  18  and the first spacers  25  as masks to form two second LDD regions  32  adjacent to the first LDD regions  28 . Two second spacers  26  are then formed adjacent to the first spacers  25 . A fourth ion implantation process is then performed to form heavily doped regions  22  and  24  served as source and drain respectively.  
         [0014]     Dosage and operating parameters of the these ion implantation processes, obvious to one skilled in the art and not directly related to key features of the invention, are not described in detail.  
         [0015]     Thermal process is performed to activate dopants in the semiconductor substrate  12  to form the source/drain and to adjust the threshold voltage of the MOS transistor.  FIG. 3  is a schematic diagram showing the relationship between temperature and time in a thermal process used in embodiments of a method of fabricating a MOS transistor. As shown, the semiconductor substrate  12  surface has an initial temperature T 0  at time t 0 , having undergone prior heating process ramping up at 100 to 200° C. per second. The semiconductor substrate  12  surface reaches a first temperature T 1  at time t 1 . Millisecond annealing is then performed, comprising rapid heating and rapid cooling. In rapid heating, the top surface of the semiconductor substrate  12  is heated at a ramping up rate exceeding 200° C. per second, preferably exceeding 50° C. per millisecond. The top surface of the semiconductor substrate  12  reaches a second temperature T 2  at time t 2 . In rapid cooling, the top surface of the semiconductor substrate  12  is cooled at a ramping down rate exceeding 20° C. per millisecond. The top surface of the semiconductor substrate  12  reaches a third temperature T 3  at time t 3 . An additional cooling process may be further performed to cool the surface of the semiconductor substrate  12  to a fourth temperature T 4 .  
         [0016]     The initial temperature T 0  and the fourth temperature T 4  may both be room temperature, the first temperature T 1  about 500 to 800° C., the second temperature T 2  about 800 to 1500° C., and the third temperature T 3  about 500 to 800° C. The rapid heating step has duration (t 2 −t 1 ) of 1 to 50 milliseconds and the rapid cooling step has duration (t 3 −t 2 ) of 1 to 50 milliseconds.  
         [0017]      FIG. 4  is a schematic diagram of an embodiment of a heating device  100  used in the thermal process of  FIG. 3 . As shown, the heating device  100  comprises a container for a semiconductor wafer  10 , a first heating source  110  above the container, and a second heating source beneath the container. The first heating source  110  and the second heating source are both arc lamps. The second heating source may comprise an argon lamp or a xenon lamp radiating light with a wavelength less than 1200 nm, the wavelength of the light absorbed by silicon.  
         [0018]     When the semiconductor wafer  10  is placed in the heating device  100 , the first heating source  110  heats the semiconductor wafer  10  from a bottom surface  10   b  thereof to temperature T 1  at time t 1 .  
         [0019]     At the millisecond annealing process, the second heating source  120  heats a top surface  10   a  of the semiconductor wafer  10 , comprising the semiconductor substrate  12  having doped regions. The top surface  10   a  of the semiconductor wafer  10  is rapidly heated to temperature T 2  at time t 2 . The second heating source  120  is turned off immediately and the top surface  10   a  rapidly cooled to the third temperature T 3  at time t 3 , providing sufficient energy to activate dopants in the semiconductor substrate  12 . The first heating source  110  is turned off to gradually cool the semiconductor wafer  10  to the fourth temperature T 4  at time t 4 .  
         [0020]      FIG. 5  is a schematic diagram showing the relationship between temperature and depth in a thermal process of  FIG. 3 . As shown, although the top surface  10   a  of the semiconductor wafer  10  is heated to the second temperature T 2  in the millisecond annealing process, only a portion of the semiconductor wafer  10  is heated to the second temperature T 2 . Most of the semiconductor wafer  10  is still at first temperature T 1 .  
         [0021]     In comparison with the related art, the invention utilizes a millisecond annealing process to activate the dopants. Since the duration of the annealing process is less than 10 2  milliseconds, dopant diffusion into the substrate  12  can be avoided and the thermal budget of the thermal process can be reduced effectively. In addition, the extremely high temperature gradient has great ability of activating the dopants, leading to reduce sheet resistance thereby. Furthermore, according to the excellent activating ability of the present invention, the dopant concentration of the ion implantation processes can be far reduced while the conductivity in the doped regions remain the same or even better. It leads to shallow doped regions and less damage during the ion implantation processes, reducing junction leakage of a MOS transistor and threshold voltage roll-off performance, and improving stability and reliability of semiconductor devices.  
         [0022]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto.