Patent Publication Number: US-6215125-B1

Title: Method to operate GEF4 gas in hot cathode discharge ion sources

Description:
DESCRIPTION 
     1. Field of the Invention 
     The present invention relates to ion implantation, and in particular to a method for extending the lifetime of a hot cathode discharge ion source which is utilized in an ion implantation apparatus to generate source ions. 
     2. Background of the Invention 
     Ge + ion implants have been widely used in the semiconductor industry to pre-amorphize silicon wafers in order to prevent channeling effects. The demands for these pre-amorphizing implants are expected to increase greatly in future semiconductor device manufacturing. The most popular ion feed gas for Ge + beams is GeF 4 , because of its stable chemical properties and cost effectiveness. However, very short lifetimes, on the order of 12 hours or less, of the hot cathode discharge ion sources have been observed while operating with GeF 4  gas. 
     The common source failure mode is that some materials deposit on the cathode surfaces of the hot cathode discharge ion source during extended use of the ion implantation apparatus. This deposition reduces the thermionic emission rate of the source ions from the hot cathode surfaces. Consequently, the desired arc currents can not be obtained and the hot cathode discharge sources have to be replaced in order to maintain normal source operation. The short source life greatly reduces the productivity of an ion implanter. 
     The cause of the short source life in GeF 4  ion implantation is believed to be excessive, free fluorine atoms in the ion source due to the chemical dissociation of GeF 4  molecules. The arc chamber material is etched away by chemical reaction of the fluorine atoms with the material of the arc chamber. Some of the arc chamber material may eventually deposit on the hot cathode resulting in the degradation of electron emissions from the hot cathode discharge source. 
     Other implantation gases besides GeF 4  are employed in ion implantation and these other gases may cause the same shortening of the lifetime of the hot cathode discharge ion source. The term “hot cathode discharge ion source” is used herein to denote any thermionic emission element which when heated to a temperature of at least 1200° C. emits desired electrons. It is noted that the exact temperature wherein electrons are emitted from such elements is dependent on the material of the element. 
     A typical prior art ion implantation apparatus, i.e. tool, is illustrated in FIG.  1 . Specifically, the prior art ion implantation apparatus comprises an ion source chamber  10  which generates ions to be implanted into a desired substrate. The generated ions are drawn by drawing electrodes  12  and their mass is analyzed by a separating electromagnet  14 . After mass analysis, the ions are completely separated by slits  16  and the appropriate ions are accelerated by accelerators  18  to a final energy. A beam of ions is converged on the face of a sample or substrate  20  by a quadrupole lens  21  and scanned by scanning electrodes  22   a  and  22   b . Deflection electrodes  24 ,  26  and  28  are designed to deflect the ion beam in order to eliminate uncharged particles caused by collision with residual gas. 
     The ion source chamber  10  is the heart of the ion implantation tool. Five different kinds of ion source chambers are currently known including: a Freeman-type ion source chamber using thermoelectrodes; a Bernas-type ion source chamber; indirectly heated cathode type ion source; microwave type ion source chamber using magnetrons; and RF sources. It should be understood that the terms “ion source” and “hot cathode discharge ion source” are used interchangeably herein. 
     In order to better understand the present invention, a brief description of a Freeman-type ion source, a Bernas-type ion source and a microwave type ion source is given herein. The other types of ions sources mentioned hereinabove, i.e. indirectly heated cathode and RF, are not illustrated herein, but are also well known to those skilled in the art. 
     FIG. 2 is a cross-sectional view of a Freeman-type ion source chamber  10 . Specifically, in this ion source, plasma is generated by emitting thermoelectrons from a bar-shaped filament  30 , an electrical field is generated parallel to filament  30  by an electromagnet  32 , a rotating field is caused by filament current, and electrons are moved in the chamber by a reflector  34 , thereby improving the efficiency in ionization. The ions generated in the chamber pass through slit  36  and are guided in a direction perpendicular to the filament. 
     FIG. 3 is a cross-sectional view of a Bernas-type ion source chamber  10  containing molybdenum (Mo) as the main ingredient. The ion source chamber  10  includes a tungsten (W) filament  40  and its opposing electrode  44 . The ion source chamber is supplied with the desired gas from gas line  46  and emits thermoelectrons from the filament. 
     A typical microwave ion source is shown in FIG.  4 . Specifically, in this chamber  10 , plasma is generated in a discharge box  50  using a microwave caused by magnetron  52 . Since this chamber has no filaments, its lifetime is not shortened even by the use of reactive gases. However, metal as well as ions are extracted from the chamber and are attracted to the surfaces of drawing electrodes  54 ; therefore, a desired voltage cannot be applied or the metal or ions may reach a sample to contaminate it. 
     Each of the above described ion sources exhibits the problem mentioned hereinabove. Prior art solutions to the short lifetime problem exhibited by these hot cathode discharge ion sources involve either changing of the hot cathode discharge ion source itself or coating the interior walls of the ion implantation apparatus with a material that is resistant to chemical attack. The latter solution is described, for example, in U.S. Pat. No. 5,656,820 to Murakoshi, et al. 
     Despite the success of such prior art processes, there exists a need to develop a new and improved method of extending the lifetime of hot cathode discharge ion sources. Such a method is needed since the prior art solutions are either too time consuming or add additional operating costs to the overall process. The prior art solution also yields an unwanted contaminant into the substrate when implanting a BF 2  species (Nb). 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a simple, yet cost effective method for extending the lifetime of a hot cathode discharge ion source which is typically employed in the prior art to implant ions into a substrate. 
     Another object of the present invention is to provide a method which significantly reduces the time required to shut down the ion implantation apparatus to either replace the discharge source or to coat the interior walls of the apparatus thus providing improved productivity to the ion implanter operator. 
     A still further object of the present invention is to prolong the lifetime of a hot cathode discharge ion source when fluorine-containing gases such as GeF 4  are employed as the implantation, i.e. ion source, gas. 
     These as well as other objects and advantages can be achieved in the present invention by introducing a nitrogen-containing gas, as a co-bleed gas, into an ion source chamber containing at least an implantation gas and a hot cathode discharge ion source. The method of the present invention is particularly applicable for use in ion implantation apparatuses wherein highly fluorinated gases such as GeF 4  are employed as the implantation gas. The term “highly fluorinated” is used herein to denote a gaseous compound which contains more than a single molecule of fluorine. It has been observed that a 50 to about 120 hour improvement in the lifetime of the hot cathode ion source can be obtained when a nitrogen-containing gas is used in conjunction with GeF 4  source gas. Similar improvements are expected to be observed with other implantation gases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a typically prior art ion implantation apparatus that can be employed in the present invention. 
     FIG. 2 is a cross-sectional view showing the various components of a prior art Freeman-type ion source. 
     FIG. 3 is a cross-sectional view showing the various components of a prior art Bernas-type ion source. 
     FIG. 4 is a cross-sectional view showing the various components of a prior art microwave ion source. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention, which provides a method for extending, i.e. prolonging, the lifetime of a hot cathode discharge ion source used in ion implantation, will now be described in greater detail with reference to the accompanying drawings wherein like reference numerals are used for describing like and corresponding elements and/or components of the drawings. It is noted that the present invention is not limited to the use of any one type of ion implantation apparatus or hot cathode discharge ion source. Instead, the method of the present invention is applicable for use with the ion implantation apparatus shown in FIG. 1 as well as any other type of ion implantation apparatus now known to those skilled in the art or those that will be developed in the future. 
     Additionally, the method of the present invention can be used with any type of hot cathode discharge ion source including, but not limited to: the Freeman-type ion source as previously described and shown in FIG. 2, the Bernas-type ion source as previously described and shown in FIG. 3; the microwave ion source as previously described and shown in FIG. 4; an indirectly heated cathode type ion source; and a RF ion source. 
     According to the method of the present invention, extended lifetime of the hot cathode discharge ion source can be obtained by introducing a nitrogen-containing gas as a co-bleed gas into an ion source chamber which contains at least a hot cathode discharge ion source and an implantation gas. 
     The term “co-bleed” is used herein to denote that the nitrogen-containing gas and the implantation gas are introduced into the ion source chamber of the ion implantation apparatus at substantially the same time. The aforementioned gas co-bleed is maintained throughout the entire ion implantation process and the implantation process is operated using conventional ion implantation conditions that are well known to those skilled in the art. 
     Suitable nitrogen-containing gases that can be employed in the present invention include, but are not limited to: nitrogen, air (dry or wet), NF 3,  NO, N 2 O, NO 3 , N 2 O 3 , NO 3 F, NOBr, NOF, NO 2 F and mixtures thereof. Of these nitrogen-containing gases, nitrogen gas is highly preferred in the present invention. 
     In accordance with one preferred embodiment of the present invention, the concentration of the co-bleed gases is from about 20 to about 80 parts of the ion implantation gas to about 80 to about 20 parts of nitrogen-containing gas. More preferably, the concentration of the co-bleed gases is from about 30 to about 50 parts of the ion implantation gas to about 70 to about 50 parts of nitrogen-containing gas. The flow rate of the co-bleed gases is controlled by conventional gas flow meters or other means well known to those skilled in the art. 
     The nitrogen-containing gas employed in the present invention can be a high purity or low purity gas. When a high purity nitrogen-containing gas is employed, the purity of the nitrogen-containing gas is greater than about 50%. More preferably the nitrogen-containing gas employed in the present invention has a purity of from about 90 to about 100%. The nitrogen-containing gas may have the desired purity or it can be purified to a predetermined level by utilizing gas purification techniques, including scrubbers, well known to those skilled in the art. 
     Suitable ion implantation gases, i.e. source gases, that can be employed in conjunction with the nitrogen-containing co-bleed include, but are not limited to: fluorinated gases such GeF 4 , SiF 4 , Si 2 F 6 , SF 6 , S 2 F 6  and SF 4  as well as other gases such as AsH 4  and PH 3 . A highly preferred ion implantation gas that can be used in conjunction with a nitrogen-containing gas is GeF 4 . 
     It is again emphasized that the use of the present invention, i.e. co-bleed gases, extends, i.e. prolongs, the lifetime of currently used hot cathode discharge ion source to times heretofore unobtainable in the prior art. For example, in current ion implantation technology which does not employ the method of the present invention, the lifetime of a molybdenum hot cathode discharge ion source is from about 12 to about 30 hours when operating with GeF 4 . By employing the method of the present invention, the Mo hot cathode discharge ion source&#39;s lifetime improves to about 80 to about 150 hrs. Such an improvement, which is on the order of 400 to about 750%, represents a significant advance in the ion implantation industry since it reduces the shut-down time one would require to repair the tool. Moreover, by employing the method of the present invention, the ion source exhibits a very stable lifetime performance as compared to an ion source which is not treated with the co-bleed gases. 
     The method of the present invention is suitable for use in a wide range of applications wherein ion implantation is required. The method of the present invention is however extremely applicable for use in the semiconductor industry to provide a semiconductor wafer, chip or substrate with source/drain regions or to pre-amorphize the semiconductor wafer of substrate. 
     The following example is given to illustrate the scope of the present invention. Because this example is given for illustrative purposes only, the invention embodied herein should not be limited thereto. 
     EXAMPLE 
     In this example, the effects of using nitrogen as a co-bleed gas were investigated using GeF 4  as the source gas and comparison was made to systems wherein no nitrogen co-bleed was employed. For this investigation, a Bernas-type ion source and an indirectly heated cathode (ELS) ion source were used. The ratio of co-bleed gases used in these experiments were 3 parts N 2  to 2 parts Ge. The hot cathode ion sources were run using conventional conditions well known for each type of ion source. 
     The results of these experiments are shown in Table 1. Specifically, the data clearly shows that the use of the co-bleed of nitrogen and GeF 4  significantly extends the lifetime of the hot cathode ion source as compared with experiments performed using only GeF 4 . In all cases, a significant improvement in the lifetime of the hot cathode ion source was observed when nitrogen was used in conjunction with GeF 4 . 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Test 
                   
                   
                   
               
               
                 # 
                 No N 2  Co-Bleed 
                 No N 2 /Arc V &gt; 93V 
                 N 2  Co-Bleed (3:2 ratio) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 In-directly Heated Cathode Source (ELS) Life Time Data 
               
            
           
           
               
               
               
               
            
               
                 1 
                 12 Hrs 
                 N/A 
                 136 Hrs 
               
               
                 2 
                 18 Hrs 
                 N/A 
                 141 Hrs 
               
               
                 3 
                 16 Hrs 
                 N/A 
                 144 Hrs 
               
               
                 4 
                 20 Hrs 
                 N/A 
                 148 Hrs 
               
               
                 5 
                 20 Hrs 
                 N/A 
                 140 Hrs 
               
               
                   
               
            
           
           
               
            
               
                 The in-directly heated (ELS) source life tests were run using 100% 
               
               
                 Ge beams 
               
            
           
           
               
            
               
                 Bernas (IAS) Source Life Data 
               
            
           
           
               
               
               
               
            
               
                 1 
                 12 Hrs 
                 24 Hrs 
                 40 Hrs 
               
               
                 2 
                 15 Hrs 
                 20 Hrs 
                 38 Hrs 
               
               
                 3 
                 20 Hrs 
                 30 Hrs 
                 N/A 
               
               
                 4 
                 16 Hrs 
                 27 Hrs 
                 N/A 
               
               
                   
               
               
                 Bernas Source Life testing was done with 6 Hrs of Ge operation then switch PH 3  then repeat.  
               
               
                 The first two tests did not fail, test had to stop at 40 and 38 hours respectively because of other priorities.  
               
            
           
         
       
     
     While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made without departing from the spirit and scope of the present invention.