Abstract:
This invention relates to a method for manufacturing a semiconductor device having polysilicon lines with micro-roughness on the surface. The micro-rough surface of the polysilicon lines help produce smaller grain size silicide graiicide film during the formation phase to reduce the sheet resistance. The micro-rough surface of the polysilicon lines also increases the effective surface area of the silicide contacting polysilicon lines thereby reduces the overall resistance of the final gate structure after metallization.

Description:
TECHNICAL FIELD 
     This invention relates to a method for forming silicide on a polysilicon line of a semiconductor device. More particularly, this invention relates to a method for forming silicide over a controllable micro-rough surface of a polysilicon line, and a device made by the same. 
     BACKGROUND OF THE INVENTION 
     Polysilicon has become the prevalent gate material of MOS devices for over a decade. In addition to MOS gates, uses of polysilicon in MOS or other semiconductor devices have also increased prominently. However, although polysilicon provides many advantages, such as self-alignment, over metal as the gate material, its resistance level is still too high to provide interconnections among semiconductor devices. Therefore, after the formation and patterning of a polysilicon layer to form polysilicon lines, a device will typically go through, inter alia, a silicide process for forming a lower resistance silicide layer. As the size of semiconductor devices becomes smaller and smaller, how to provide good quality interconnections for devices having sub-quarter μm width of polysilicon lines becomes one of the major challenge to the current semiconductor industry. 
     Although polysilicon has a low contact resistance with silicon, it still exhibits too high a resistance to the interconnection metal materials. Creating a multi-metal stack of the polysilicon (called polycide) having a silicide, e.g., the titanium silicide (TiSi 2 ), film is one solution. The silicide film makes a low contact resistance with polysilicon and reduces the overall sheet resistance of the polycide structure. While other silicide materials have been used for reducing contact resistance with polysilicon, a titanium silicide film is the most popular one used in the semiconductor industry. 
     Growing the titanium silicide film on the polysilicon lines and on active regions of the substrate typically includes two phases. In a first phase, generally called the C49 phase formation, the titanium is first deposited on the surface of the device. The deposition of titanium (Ti) is then followed by a first rapid thermal anneal (RTA) step to cause the titanium reacted with the underlying silicon substrate or polysilicon lines and thus form the titanium silicide film. Once the titanium silicide film has been formed, those unreacted portions of the titanium are then removed, forming self-aligned silicide (Salicide) on polysilicon and active regions only. 
     In a second phase, the previously formed titanium silicide film will go through another anneal step, such as a second RTA step, to further reduce the overall sheet resistance of the newly formed titanium silicide. This second phase of transformation is commonly referred to a C54 phase. A simplified structure of a device produced after the C54 phase is illustrated in FIG.  1 . The current silicide technology, however, faces two major difficulties for future sub-quarter μm technology of silicide over polysilicon lines. First, as the width of the polysilicon lines is reduced to being in the sub-quarter μm range, it is more difficult to grow a smaller grain size titanium silicide during the C49 phase. Second, for smaller width of the polysilicon lines higher annealing temperature is required for the C54 phase transformation without resulting in TiSi 2  agglomeration. 
     Furthermore, when the width of the polysilicon lines are reduced to about 0.1 μm the formation of titanium silicide having the grain size larger than 0.1 μm during the C49 phase will suppress the grain growth of the titanium silicide during the later C54 phase. As a result, the titanium silicide will have an undesirable higher sheet resistance. To cure this problem, a much higher RTA temperature for the titanium silicide is required. The much higher RTA temperature is very undesirable because it will not only increase difficulties in manufacture, e.g., higher costs or equipment problems, but also may cause device failure due to the thermal budget limitation of the device during manufacturing, and the introduced defects such as the TiSi 2  agglomeration. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method for manufacturing a semiconductor device having low sheet resistance suicide over polysilicon lines of sub-quarter μm or even less than 0.1 μm width. In one embodiment of the invention, small indentations or scratches are formed on the surface of the polysilicon lines before the formation of the silicide. 
     The features and advantages of the present invention will be made apparent, by way of illustration and not by limitation, in the following description of embodiments illustrated in the annexed drawings. The figures show enlarged portions of certain areas to better illustrate the inventive feature and none of the figures are understood as being in scale or exact proportion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the simplified structure of a known device formed by a conventional silicide process. 
     FIG. 2 shows the structure of a device having micro-roughness on the surface of a polysilicon layer according to the present invention. 
     FIG. 3 shows the final structure after a salicide step according to the present invention. 
     FIG. 4 shows the enlarge portion of the silicide over the polysilicon gate according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 2, an electrical isolation step is first performed on a semiconductor wafer  1  to isolate a device  10  from other devices of an integrated circuit. The isolation step may be accomplished by any suitable technology, such as the shallow trench isolation (STI), etch and back fill trench, or the LOCOS technology, trench being preferred over LOCOS. Isolation regions  2   a  and  2   b  are thus formed as shown in FIG.  2 . After that, a gate oxide film  4  and then a polysilicon layer  6  are subsequently formed on the substrate. The polysilicon layer may be deposited by any suitable conventional technology, such as CVD or LPCVD. Typically, the polysilicon layer has a thickness of about 2000 Å-3000 Å, i.e., about 0.2 μm-0.3 μm. 
     After the polysilicon layer  6  is formed, a step for forming a micro-rough top surface of the polysilicon layer is carried out. The formation of a micro-rough top surface of the polysilicon layer  6  is accomplished by creating intentional scratches produced by chemical mechanical polishing (CMP). A CMP step is carried out of a selected size with microparticles in the slurry which will leave small indentations or scratches in the polysilicon. A slurry solution having particles with very small size, preferably smaller than 0.1 μm in dimension, is used to create this micro-roughness on the surface of the polysilicon layer  6 . Trench isolation is preferred (though not required) because the top of the polysilicon will be more planar prior to the CMP step. Thereafter, a cleaning step is followed to clean wafers to a required defect control level. 
     The size of the particles is selected to be somewhat less than polysilicon gate width. For a minimum dimension technology of 0.15 μm for a polysilicon line width, the particles would have a maximum dimension of 0.1 μm, or in one embodiment, 0.075 μm or less. In one embodiment, the particle size in the slurry is selected to be less than one-half the polysilicon line width in which the micro-rough surface is to be formed. The polysilicon layer  6  and the thin oxide film  4  then go through a mask and etch step to form a polysilicon gate  8  for the device  10 , as shown in FIG.  3 . Thereafter, conventional steps for defining active regions are followed. Referring to FIG. 3, these steps may include lightly doped areas ( 12   a  and  12   b ) implantation, spacers ( 14   a  and  14   b ) formation, and source ( 16 )/drain ( 18 ) regions definition. 
     After the actives regions  16  and  18  are formed, the C49 phase of silicidization follows. First, a refractory metal film, e.g., a titanium or titanium/titanium nitride (Ti/TiN) stack film, is deposited on the wafer  1 . The deposition may be accomplished by any suitable method, such as CVD, LPCVD, sputtering, electrolytic plating, or electroless plating, etc. The device  10  then goes through a RTA process to cause the refractory metal film react with the underlying polysilicon gate  8  and the wafer  1 , where the active regions  16  and  18  are located, in order to form a silicide film  20 . Those unreacted portions, i.e., those portions of the refractory metal not in contact with the polysilicon gate  8  or the source and drain regions  16  and  18 , of the refractory metal are then removed. Thereafter, the C54 transformation phase of the silicide film  20  may follow in the same or later anneal steps. According to the present invention, it is possible that the C54 phase of transformation may not be necessary and thus further annealing is not carried out while still obtaining a desired low resistance of the silicide film. Alternatively, a lower annealing temperature may be carried out during the C54 phase. The final structure of the device according to the present invention is illustrated in FIG.  3 . 
     FIG. 4 shows an enlarged portion of the polysilicon gate according to the present invention. The micro-rough surface of the polysilicon gate according to the present invention provides an enlarged effective surface area of the silicide film, as compared to those without a micro-rough surface. Such an enlarged effective surface area of the silicide will lower the sheet resistance of the polycide gate  8  of the device  10 . Furthermore, the formation of the micro-rough surface of the polysilicon gate  8  will cause smaller polysilicon grain size due to amorphozation of small particles on the polysilicon surface and therefore will prevent TiSi 2  lateral growth during the C49 phase. Therefore, the TiSi 2  agglomeration problem during the C54 phase will then be eased. As a result, the present invention has advantages of providing a low sheet resistance polycide gate structure, which could be as narrow as less than 0.1 μm, of a semiconductor device. In addition, the present invention permits lower RTA temperature for both the C49 and C54 phases, as compared to the conventional method which requires undesirable higher RTA temperature when the dimension of the polysilicon lines are reduced to sub-quarter μm range. It may be possible to avoid the anneal steps needed to reach the C54 phase in one embodiment or, on the other hand, reach this phase at a lower temperature or with the same anneal step as the formation of the C49 phase. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.