Abstract:
A three-step polymer removal process that reverses the conventional sequence in which polymer is removed. In the preferred embodiment of the present invention the polymer is first removed from the Gas Deposition Table, after this the polymer is stripped from the inner surface of the created contact hole.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the plasma etching of a silicon wafer in the manufacture of integrated circuits.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    As the density of circuit components contained within a semiconductor die has increased and the circuit components have decreased in size and are spaced closer together, it has become increasingly difficult to access selectively a particular region of the silicon wafer through the various layers that are typically superimposed on the surface of the silicon wafer without undesired interference with other active regions.  
           [0003]    It is especially important to have a technology that can etch openings that have essentially vertical wall, most notably when the openings are to extend deeply into the surface layers. Additionally, to tolerate some misalignment in the masks used to define such openings, it is advantageous to provide protection to regions that need isolation but that inadvertently lie partially in the path of the projected opening. To this end it is sometimes the practice to surround such regions with a layer of material that resists etching by the process being used to form the openings.  
           [0004]    Accordingly, a technology that provides the desired results will need an appropriate choice both in the materials used in the layers and the particular etching process used with the materials chosen.  
           [0005]    Dry etching, such as plasma etching and reactive ion etching, has become the technology of choice in patterning various layers that are formed over a silicon wafer as it is processed to form therein high density integrated circuit devices. This is because it is a process that not only can be highly selective in the materials it etches, but also highly anisotropic. This makes possible etching with nearly vertical sidewalls.  
           [0006]    Basically, in plasma etching as used in the manufacturing of silicon integrated devices, a silicon wafer on whose surface has been deposited various layers, is positioned on a first electrode in a chamber that also includes a second electrode spaced opposite the first. As a gaseous medium that consists of one or more gasses is flowed through the chamber, an r-f voltage, which may include components at different frequencies, is applied between the two electrodes to create a discharge that ionizes the gaseous medium and that forms a plasma that etches the wafer. By appropriate choice of the gasses of the gaseous medium and the parameters of the discharge, selective and anisotropic etching is achieved.  
           [0007]    While elaborate theories have been developed to explain the plasma process, in practice most of such processes have been developed largely by experimentation involving trial and error of the relatively poor predictability of results otherwise.  
           [0008]    Moreover, because of the number of variables involved and because most etching processes depend critically nor only on the particular materials to be etched bur also on the desired selectivity and anisotropy, such experimentation can be time consuming while success often depends on chance.  
           [0009]    The reliability of a metal interconnect is most commonly described by a lifetime experiment on a set of lines to obtain the medium time to failure. The stress experiment involves stressing the lines at high current densities and at elevated temperatures. The failure criterion is typically an electrical open for non-barrier conductors or a predetermined increase in line resistance for barrier metalization.  
           [0010]    The mean time to failure is dependent on the line geometry where this failure is directly proportional to the line width and the line thickness. Experimentally, it has been shown that the width dependence is a function of the ratio of the grain size d of the film and the width of the conductor w. As the ratio w/d decreases, the mean time to failure will increase due to the bamboo effect.  
           [0011]    [0011]FIG. 1 a  shows a non-bamboo structure  10  in a wider line, that is a thin film or a fine line that is dominated by triple points  12 .  
           [0012]    [0012]FIG. 1 b  shows a bamboo-like grain structure  20  in a sub-micron line. As the grain size increases with respect to the line width, the interconnecting points of the triple points parallel to the length of the conductor decrease and even vanish. So the probability of voids accumulating along the grain boundaries is reduced, thus retarding the electro-migration process. The final grain structure  22  of the Al film is not only a function of the metal deposition conditions but is also strongly dependent on post-metalization annealing temperatures and the texture of the underlying surface.  
           [0013]    U.S. Pat. No. 5,611,888 (Bosche et al.) shows contact hole plasma etch process.  
           [0014]    U.S. Pat. No. 5,667,631 (Holland et al.) and U.S. Pat. No. 5,607,880 (Suzuki et al.) show dry etch processes.  
           [0015]    U.S. Pat. No. 5,647,953 (Williams et al.) shows a post etch treatment to clean the etcher.  
         SUMMARY OF THE INVENTION  
         [0016]    It is the primary objective of the present invention to eliminate bamboo profile on the inner surface of semiconductor contact holes.  
           [0017]    Another objective of the present invention is to decrease the electrical resistivity for the contact holes within semiconductor devices.  
           [0018]    Yet another objective of the present invention is to prevent the negative effects of the F-ion that is released during the GDP cleaning process.  
           [0019]    Yet another objective of the present inventions is to increase the benefit of the protective polymer layer in the inside of the contact holes  
           [0020]    The present invention teaches a post etch treatment (PET) to perform the Gas Distribution Plate (GDP) polymer cleaning before the step of removing polymer in the contact holes. In a typical and previous sequence of post etch treatment (PET) procedures, the polymer in contact holes is remove first after which GDP polymer cleaning is performed. It is claimed that the polymer inside the contact holes avoids the formation of the so-called bamboo rings within the contact holes. By having the polymer present within the contact holes while the GDP polymer cleaning is performed, no bamboo rings are formed. By removing the polymer within the contact holes after GDP polymer etching, the sidewalls of the contact holes are devoid of bamboo rings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 a  shows a non-bamboo structure in a wider line,  
         [0022]    [0022]FIG. 1 b  shows a bamboo like grain structure in a sub-micron line.  
         [0023]    [0023]FIG. 2 shows the formation of bamboo structures within the inner surface of the contact holes.  
         [0024]    [0024]FIG. 3 shows the absence of the formation of bamboo rings within the inner surface of the contact holes.  
         [0025]    [0025]FIG. 4 shows a flow chart of the steps performed during polymer stripping. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    [0026]FIG. 2 shows the formation of the bamboo structures  20  within the inner surface  22  of the contact hole  24  of the semiconductor devices. The basic reason for the formation of these bamboo structures is that, during the GDP polymer etching process, F-ions are formed that can attack the inner surface of the contact holes. If however a protective coating is present that protects the inside surface of the contact holes, the F-ions can be blocked from attacking this surface.  
         [0027]    It must be understood that polymer is present at both the GDP and on the inside surface of the contact holes. The cleaning processes for removing these two different polymers are distinct and different. A choice can therefore be made as to which of the two polymers to remove first. By first removing the GDP polymers, the polymer remains in place on the inner surface of the contact holes. By first removing the GDP polymers than, even though F-ions are formed during this process, the F-ions cannot attack the inner surface of the contact holes since the polymer within these contact holes is still present.  
         [0028]    The inside surface of the area  30  within the opening  24  forms a layer of doped borophosphosilicate glass (BPSG). The top ring  26  predominantly contains platinum tetra ethyl ortho silicate (PTTEOS), the lower ring  28  predominantly contains borophospho tatra ethyl ortho silicate (BPTEOS).  
         [0029]    The post etch treatment (PET) process of first stripping the polymer from the contact holes, then cleaning the polymer of the Gas Distribution Plates (GDP) is as follows. The three processing steps are performed in the order in which they are listed.  
         [0030]    Process Step #1: Contact Hole Polymer Strip  
                                                       Etching chamber pressure:   10 to 20 milli Torr           Source or top plate power:   1000 to 1500 Watts           Bottom plate power:   175 to 225 Watts           Gas composition:   700 to 800 SCCM O 2                        
 
         [0031]    Process Step #2: GDP Cleaning  
                                                       Etching chamber pressure:   70 to 90 milli Torr           Source or top plate power:   1500 to 2000 Watts           Bottom plate power:   0 Watts           Gas composition:   1250 to 1750 SCCM O 2                        
 
         [0032]    Process Step #3: GDP Cleaning  
                                                       Etching chamber pressure:   70 to 90 milli Torr           Source or top plate power:   800 to 1200 Watts           Bottom plate power:   0 Watts           Gas composition:   1350 to 1650 SCCM O 2                                    
 
         [0033]    [0033]FIG. 3 shows the inner surface  2  of the contact hole  24  if the Gas Distribution Plates (GDP) polymer cleaning is performed before the cleaning of the polymer from the contact holes.  
         [0034]    The protective effect of the polymer layer within the inner surface of the contact holes is obvious, the F-ions (formed during the first cleaning process of removing the polymer form the GDP) have not been able to attack the inner walls of the contact holes. This leaves a smooth surface of the contact hole after the polymer is removed from the inner surface of the hole. No bamboo structure is visible.  
         [0035]    [0035]FIG. 4 shows a flowchart of the steps performed during polymer stripping, that is a method for removing or etching polymer depositions in a plasma process chamber wherein substrates are processed. Step 1 is the positioning of the wafer in a plasma reactor that includes a chamber within which there can be created a radio frequency discharge and wherein pressure can be adjusted. Step 2 shows the flowing through the chamber of a gaseous medium that when subjected to the radio frequency discharge generates a plasma that includes reactive ions. Step 3 shows the adjusting of the first pressure level within the chamber. Step 4 is the adjusting of the power applied to the top plate in the chamber. Step 5 is the adjusting of the power applied to the bottom plate in the chamber. Step 6 is the adjusting the composition of the plasma cleaning gas while step 7 shows the performing of the plasma cleaning step by activating the cleaning gas and forming a plasma cleaning gas, contacting the gas distribution plate of the chamber or the inside walls of the contact holes with the plasma cleaning gas for a time sufficient to remove polymer residues from the gas distribution plate. The steps of setting the pressure, adjusting the power applied to the top and bottom plate of the reaction chamber and adjusting the composition of the plasma cleaning gas within the reaction chamber are specific and unique for a particular polymer cleaning process. These steps therefore must be specified for each specific polymer cleaning procedure.  
         [0036]    The post etch treatment (PET) process of first cleaning the polymer of the GDP, then stripping the polymer from the contact holes is as follows. The three processing steps are performed in the order in which they are listed. This is the preferred cleaning process within the scope of the present invention.  
         [0037]    Process Step #1: GDP Cleaning  
                                                       Etching chamber pressure:   250 to 350 milli Torr           Source or top plate power:   1500 to 2000 Watts           Bottom plate power:   0 Watts           Gas composition:   1250 to 1750 SCCM O 2                        
 
         [0038]    Process Step #2: Contact Hole First Polymer Strip  
                                                       Etching chamber pressure:   15 to 20 milli Torr           Source or top plate power:   1000 to 1500 Watts           Bottom plate power:   175 to 225 Watts           Gas composition:   700 to 800 SCCM O 2                        
 
         [0039]    Process Step #3: Contact Hole Second Polymer Strip  
                                                       Etching chamber pressure:   10 to 20 milli Torr           Source or top plate power   1000 to 1500 Watts           Bottom plate power:   0 Watts           Gas composition:   700 to 800 SCCM O 2                        
 
         [0040]    It can be appreciated that the specific embodiment described is merely illustrative of the basic principles involved and that various modifications can be made hereto by those skilled in the art without departing from the spirit of the present invention. Thus it is apparent that has been provided, in accordance with the present invention, a multi-step etching process.  
         [0041]    Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.