Abstract:
A selfaligned process for a flash memory comprises applying a solution with a high etch selectivity to etch the sidewall of the tungsten silicide in the gate structure of the flash memory during a clean process before forming a spacer for the gate structure. This process prevents the gate structure from degradation caused by thermal stress.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a semiconductor process, and more particularly to, a selfaligned process for a flash memory.  
         BACKGROUND OF THE INVENTION  
         [0002]    In a complex integrated circuit (IC), the shrinkage of the devices thereof makes the design more difficult. Process such as selfalignment and other techniques are used for the desired designs.  
           [0003]    [0003]FIG. 1 shows a cross-sectional view of a gate structure  10  in a typical flash memory, in which a tunnel oxide  14  is formed on a substrate  12  with a floating gate polysilicon layer  16  thereon, and an oxide-nitride-oxide (ONO) layer  18  is further formed on the polysilicon layer  16  with a control gate polysilicon layer  20  thereon. Moreover, a control gate tungsten silicide layer  22  and a hard mask layer  24  are formed on the polysilicon layer  20 , and a source  30  and a drain  32  are formed on the substrate  12 . In the formation of the gate structure  10 , deposition and etching processes are used to obtain a gate stack on the substrate  12 . Then selfaligned process with the gate stack as a mask is used to form the source  30  and drain  32 . After forming the spacers  26  and  28 , a selfaligned process is further used to form source and drain contacts. Prior arts are proposed for such selfaligned processes, such as in U.S. Pat. Nos. 5,907,781 and 6,444,530 issued to Chen et al.  
           [0004]    However, due to the tungsten silicide layer sandwiched in the gate structure, the critical dimension (CD) will be enlarged and the distances between the gate and the contact windows of the source and drain will be shortened by the thermal expansion of the tungsten silicide layer resulted from the thermal stress when the crystal structure of the tungsten silicide is transferred from tetragon cubic to hexagon cubic in the subsequent thermal process, such as oxidation and annealing. In addition, the breakdown voltage of the structure is lower for the shortage of spaces therebetween. Further, the re-growth of the tungsten silicide grain squeezes each other and causes the sidewall of the gate structure rough and uneven, and as a result, the local electric filed effect is enhanced and induces unpredicted discharge at sharp corners to damage the gate structure and shorten the lifetime of the flash memory. It is therefore desired a selfaligned process to obtain gate structure having a flat sidewall for flash memories.  
         SUMMARY OF THE INVENTION  
         [0005]    An object of the present invention is to provide a selfaligned process to reach a smoothed sidewall of the gate for a flash memory.  
           [0006]    Another object of the present invention is to provide a selfaligned process to enhance the voltage endurance of the gate structure for a flash memory.  
           [0007]    In a selfaligned process for a flash memory, according to the present invention, a first polysilicon layer, ONO layer, second polysilicon layer, tungsten silicide layer and hard mask layer are deposited on a tunnel oxide layer and etched to form a gate structure. Then the sidewall of the tungsten silicide layer is cleaned by a solution having a high etch selectivity to tungsten silicide after the formation of source and drain with the gate structure as a mask and before annealing process. Spacer is further formed on the sidewall of the gate structure, and selfaligned contact window process is subsequently preceded. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 is the cross-sectional view of the gate structure in a typical flash memory;  
         [0010]    [0010]FIG. 2 is the cross-sectional view after forming a gate stack and source/drain;  
         [0011]    [0011]FIG. 3 is the cross sectional view after forming a spacer on the sidewall of the gate structure;  
         [0012]    [0012]FIG. 4 is an illustration of a conventional gate structure after deformation caused by thermal expansion of the tungsten silicide in the gate structure;  
         [0013]    [0013]FIG. 5 is a microscope photograph of a conventional gate structure; and  
         [0014]    [0014]FIG. 6 is a microscope photograph of the gate structure produced by the inventive process.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    An embodiment according to the present invention to illustrate a selfaligned process for a flash memory is shown in FIG. 2 to FIG. 3. FIG. 2 is the cross-sectional view after a gate stack  10  and a source  30 /drain  32  are formed, in which a tunnel oxide layer  14  is formed on a substrate  12  with a polysilicon layer  16  thereon, an ONO layer  18  is formed on the polysilicon layer  16  with another polysilicon layer  20  thereon, and a tungsten silicide layer  22  is formed on the second polysilicon layer  20  with a hard mask layer  24  thereon. After forming the gate stack  10 , the source  30  and drain  32  are formed on the substrate  12  with the gate structure  10  as the mask in the first selfaligned process. Then a solution with a high etch selectivity to tungsten silicide is used to clean the sidewall of the tungsten silicide layer  22  in the gate structure  10 . Preferably, SC-1 is used for the solution in this clean process. SC-1 is an alkaline peroxide solution that composes of five parts of deionized water, one part of 30% hydrogen peroxide, and one part of 29% ammonia. After this step, the sidewall of the tungsten silicide layer  22  is etched for the control of the critical dimension of the tungsten silicide layer  22 . When the high etching selectivity solution is used to clean the sidewall of the tungsten silicide layer  22 , the tungsten silicide layer  22  has a faster etching rate than other layers, and thus the sidewall of the tungsten silicide layer  22  is etched to form recess on its sidewall.  
         [0016]    After the above clean process is finished, rapid thermal processing (RTP) is preceded in an atmosphere containing oxygen radical in a chamber so as to activate the gate, source and drain structures and form an oxide layer at the polysilicon  16  outskirt of the floating gate polysilicon  16  to prevent current leakage. Due to the thermal treatment using rapid thermal treatment in an atmosphere with oxygen radical, surface reaction is the main reaction mechanism of the thermal oxidation in this atmosphere and thus the tungsten silicide layer  22  is kept smooth on its surface and not easy to expand. When using the rapid thermal process in an atmosphere with oxygen free radical, hydrogen and oxygen are additionally pumped into the chamber at a low pressure from about 5 torrs to 50 torrs.  
         [0017]    After the above annealing process, the crystal structure of the tungsten silicide layer  22  is transferred from tetragon cubic crystal to hexagon cubic crystal. SiN or SiO2 is deposited and etched to form spacers  26  and  28  on the sidewalls of the gate structure  10 , as shown in FIG. 3. Since the tungsten silicide layer  22  is previously cleaned with a high etch selectivity solution, gaps  34  and  36  are formed between the tungsten silicide layer  22  and spacers  26  and  29  from the recesses, and thereby increasing the distance between the tungsten silicide layer  22  and spacers  26  and  28 . When the tungsten silicide layer  22  expands due to the thermal stress in the subsequent thermal process, no squeezing is happened to damage the gate structure  10 . As a result, the surface of the tungsten silicide layer  22  and gate structure  10  is kept smooth, and the distances between the tungsten silicide layer  22  and contact windows will not be shortened. Poor performance such as increasing in local electric field and decreasing in breakage voltage won&#39;t happen.  
         [0018]    [0018]FIG. 4 is an illustration of the deformation of a conventional gate structure due to the thermal expansion of the tungsten silicide for comparison with the resultant structure formed by the inventive process. In a conventional selfaligned process, the tungsten silicide layer in a gate structure will expand due to thermal stress in any subsequent thermal process. Since there is no excess space in the gate structure for the expanded volume of the tungsten silicide layer caused by thermal stress, the grains inside the tungsten silicide layer will push each other and increases the critical dimension of the tungsten silicide layer, and the breakage voltage between the tungsten silicide layer and contact windows will be lowered. In contrast, the inventive selfaligned process generates a buffer gaps  34  and  36  between the tungsten silicide layer  22  and spacers  26  and  28 . When the tungsten silicide layer  22  expands due to thermal stress, gaps  34  and  36  buffer the expansion of the tungsten silicide layer  22  and thereby do not affect the structure inside the tungsten silicide layer  22 .  
         [0019]    [0019]FIG. 5 is a microscope photograph of a conventional gate structure. The squeezing of the tungsten silicide layer due to the thermal expansion caused by thermal stress can be seen thereof, and the surface of the sidewall is very rough. FIG. 6 is a microscope photograph of the gate structure produced by the inventive process, in which the tungsten silicide layer is not squeezed due to the expansion caused by thermal stress, and the surface of the sidewall is very smooth. Comparing FIGS. 5 and 6, the selfaligned process proposed in the present invention has obviously improved the disadvantages of the prior gate structure.  
         [0020]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.