Abstract:
Improved methods for performing atomic layer deposition (ALD) are described. These improved methods provide more complete saturation of the surface reactive sites and provides more complete monolayer surface coverage at each half-cycle of the ALD process. In one embodiment, operating parameters are fixed for a given solvent based precursor. In another embodiment, one operating parameter, e.g. chamber pressure is altered during the precursor deposition to assure full surface saturation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from international Application Serial No. PCT/US2007/015917, filed 12 Jul. 2007 (published as WO 2008/010941 A2, with publication date 24 Jan. 2008), which claims priority from U.S. Application No. 60/832,209 filed 20 Jul. 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to new and useful methods for atomic layer deposition. 
       BACKGROUND OF THE INVENTION 
       [0003]    Atomic layer deposition (ALD) is an enabling technology for next generation conductor barrier layers, high-k gate dielectric layers, high-k capacitance layers, capping layers, and metallic gate electrodes in silicon wafer processes. ALD has also been applied in other electronics industries, such as flat panel display, compound semiconductor, magnetic and optical storage, solar cell, nanotechnology and nano materials. ALD is used to build ultra thin and highly conformal layers of metal, oxide, nitride, and others one monolayer at a time in a cyclic deposition process. Oxides and nitrides of many main group metal elements and transition metal elements, such as aluminum, titanium, zirconium, hafnium, and tantalum, have been produced by ALD processes using oxidation or nitridation reactions. Pure metallic layers, such as Ru, Cu, Ta, and others may also be deposited using ALD processes through reduction or combustion reactions. 
         [0004]    A typical ALD process is based on sequential applications of at least two precursors to the substrate surface with each pulse of precursor separated by a purge. Each application of a precursor is intended to result in a single monolayer of material being deposited on the surface. These monolayers are formed because of the self-terminating surface reactions between the precursors and surface. In other words, reaction between the precursor and the surface should proceed until no further surface sites are available for reaction. Excess precursor is then purged from the deposition chamber and the second precursor is introduced. Each precursor pulse and purge sequence comprises one ALD half-cycle that theoretically results in a single additional monolayer of material. Because of the self-terminating nature of the process, even if more precursor molecules arrive at the surface, no further reactions will occur. It is this self-terminating characteristic that provides for high uniformity, conformality and precise thickness control when using ALD processes. 
         [0005]    However, in practice, it has been found that ALD processes are often limited to film growth rates of half a monolayer or less. In particular, film growth rates can be influenced by the choice of precursor and by temperature and pressure limits for the selected precursor. In addition, steric hindrances from the size and shape of precursor ligands can limit the film growth rate given because of the fixed surface density of active reaction sites. These less than complete growth rates for ALD operations present production problems in wafer throughput and cost of manufacturing. In addition, sub-monolayer growth can result in island type growth and thus higher surface roughness. 
         [0006]    There remains a need in the art for improvements to ALD processes. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an ALD process that allows for thin film growth rate to be tuned to the needs of a particular deposition process by precursor composition (metal precursor concentration and solvent selection) or manipulation of process conditions (pressure, temperature). 
         [0008]    In addition, the present invention provides an ALD process that allows for thin film growth rate to be tuned during the deposition by manipulation of process conditions (e.g. pressure). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a graph plotting ALD growth rate of HfO 2  under different deposition temperature, deposition pressure and pulse length conditions. 
           [0010]      FIG. 2  is a graph plotting ALD growth rate of HfO 2  under different pressure conditions, while holding precursor concentration, delivery flow rate and deposition temperature constant. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The present invention relies on solvent based precursors. Suitable solvent based precursors are disclosed in applicants co-pending U.S. patent application Ser. No. 11/400,904, filed Apr. 10, 2006. Examples of precursor solutes that can be selected from a wide range of low vapor pressure solutes or solids as set forth in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Examples of ALD precursor solutes 
               
             
          
           
               
                   
                   
                   
                   
                 bp (° C./ 
                   
               
               
                 Name 
                 Formula 
                 MW 
                 Mp (° C.) 
                 mmHg) 
                 Density (g/mL) 
               
               
                   
               
             
          
           
               
                 Tetrakis(ethylmethylamino) 
                 Hf[N(EtMe)] 4   
                 410.9 
                 −50 
                 79/0.1 
                 1.324 
               
               
                 hafnium (TEMAH) 
               
               
                 Hafnuim (IV) Nitrate, 
                 Hf(NO 3 ) 4   
                 426.51 
                 &gt;300 
                 n/a 
               
               
                 anhydrous 
               
               
                 Hafnuim (IV) Iodide, 
                 HfI 4   
                 686.11 
                 400 
                 n/a 
                 5.6 
               
               
                 anhydrous 
                   
                   
                 (subl.) 
               
               
                 Dimethylbis(t-butyl 
                 [(t-Bu)Cp] 2 HfMe 2   
                 450.96 
                 73-76 
                 n/a 
               
               
                 cyclopentadienyl hafnium(IV) 
               
               
                 Tetrakis(1-methoxy-2-methyl- 
                 Hf(O 2 C 5 H 11 )4 
                 591 
                 n/a 
                 135/0.01 
               
               
                 2-propoxide) hafnium (IV) 
               
               
                 Di(cyclopentadienyl)Hf 
                 Cp 2 HfCl 2   
                 379.58 
                 230-233 
                 n/a 
               
               
                 dichloride 
               
               
                 Hafnium tert-butoxide 
                 Hf(OC 4 H 9 ) 4   
                 470.94 
                 n/a 
                 90/5   
               
               
                 Hafnium ethoxide 
                 Hf(OC 2 H 5 ) 4   
                 358.73 
                 178-180 
                 180-200/13        
               
               
                 Aluminum i-propoxide 
                 Al(OC 3 H 7 ) 3   
                 204.25 
                 118.5 
                 140.5/8   
                 1.0346 
               
               
                 Lead t-butoxide 
                 Pb(OC(CH 3 ) 3 ) 2   
                 353.43 
               
               
                 Zirconium (IV) t-butoxide 
                 Zr(OC(CH 3 ) 3 ) 4   
                 383.68 
                   
                 90/5; 81/3 
                 0.985 
               
               
                 Titanium (IV) i-propoxide 
                 Ti(OCH(CH 3 ) 2 ) 4   
                 284.25 
                 20 
                 58/1   
                 0.955 
               
               
                 Barium t-propoxide 
                 Ba(OC 3 H 7 ) 2   
                 255.52 
                 200 (dec) 
                 n/a 
               
               
                 Strontium i-propoxide 
                 Sr(OC 3 H 7 ) 2   
                 205.8 
               
               
                 Bis(pentamethylCp) Barium 
                 Ba(C 5 Me 5 ) 2   
                 409.8 
               
               
                 Bis(tripropylCp) Strontium 
                 Sr(C 5 i-Pr 3 H 2 ) 2   
                 472.3 
               
               
                 (Trimethyl)pentamethylcyclo- 
                 Ti(C 5 Me 5 )(Me 3 ) 
                 228.22 
               
               
                 pentadienyl titanium (IV) 
               
               
                 Bis(2,2,6,6-tetramethyl-3,5- 
                 Ba(thd) 2  * 
                 503.85 
                 88 
               
               
                 heptanedionato) barium 
                 triglyme 
                 (682.08) 
               
               
                 triglyme adduct 
               
               
                 Bis(2,2,6,6-tetramethyl-3,5- 
                 Sr(thd) 2  * 
                 454.16 
                 75 
               
               
                 heptanedionato) strontium 
                 triglyme 
                 (632.39) 
               
               
                 triglyme adduct 
               
               
                 Tris(2,2,6,6-tetramethyl-3,5- 
                 Ti(thd) 3   
                 597.7 
                   
                 75/0.1 (sp) 
               
               
                 heptanedionato) titanium(III) 
               
               
                 Bis(cyclpentadinyl)Ruthenium 
                 RuCp 2   
                 231.26 
                 200 
                 80-85/0.01   
               
               
                 (II) 
               
               
                   
               
             
          
         
       
     
         [0012]    Other examples of precursor solutes include Ta(NMe 2 ) 5  and Ta(NMe 2 ) 3 (NC 9 H 11 ) that can be used as Tantalum film precursors. 
         [0013]    The selection of solvents is critical to the ALD precursor solutions. In particular, examples of solvents useful with the solutes noted above are given in Table 2. 
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                 TABLE 2 
               
             
             
               
                   
               
               
                 Examples of solvents 
               
             
          
           
               
                 Name 
                 Formula 
                 BP@760 Torr (° C.) 
               
               
                   
               
             
          
           
               
                 Dioxane 
                 C 4 H 8 O 2   
                 101 
               
               
                 Toluene 
                 C 7 H 8   
                 110.6 
               
               
                 n-butyl acetate 
                 CH 3 CO 2 (n-Bu) 
                 124-126 
               
               
                 Octane 
                 C 8 H 18   
                 125-127 
               
               
                 Ethylcyclohexane 
                 C 8 H 16   
                 132 
               
               
                 2-Methoxyethyl acetate 
                 CH 3 CO 2 (CH 2 ) 2 OCH 3   
                 145 
               
               
                 Cyclohexanone 
                 C 6 H 10 O 
                 155 
               
               
                 Propylcyclohexane 
                 C 9 H 18   
                 156 
               
               
                 2-Methoxyethyl Ether 
                 (CH 3 OCH 2 CH 2 ) 2 O 
                 162 
               
               
                 (diglyme) 
               
               
                 Butyl cyclohexane 
                 C 10 H 20   
                 178 
               
               
                   
               
             
          
         
       
     
         [0014]    Another example of a solvent useful for the present invention is 2,5-dimethyloxytetrahydrofuran. 
         [0015]    The present invention is directed to methods of using solvent based precursors, such as those noted above in order to obtain a fixed ALD thin film growth rate. The method of the present invention is described as follows.
       1. Select a metal precursor and solvent combination.   2. Dissolve the metal precursor in the solvent to a selected concentration.   3. Deliver the precursor solution to a vaporizer at a fixed flow rate.   4. Deliver the vaporized solution to a deposition chamber at a fixed temperature and pressure for a fixed length of time.   5. Purge the deposition chamber with inert gas for a fixed length of time.   6. Deliver a second precursor (such as a reactive species, e.g. oxidizer) to the deposition chamber for a fixed length of time.   7. Purge the deposition chamber with inert gas for a fixed length of time.   8. Repeat  3  through  7  above until the desired thin film thickness is achieved.       
 
         [0024]    In accordance with the present invention, specific film growth rates can be achieved by establishing particular operation parameters for the precursor/solvent combination. For example, Table 3 shows parameters that can be varied depending on the precursor/solvent combination, as long as they are kept within ranges where ALD growth occurs. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Range 
               
               
                   
                   
               
             
             
               
                   
                 Metal precursor 
                 Solid or liquid 
               
               
                   
                 Solvent 
                 Non reacting solvent 
               
               
                   
                 Metal precursor concentration 
                 0.01-10 Molar 
               
               
                   
                 Flow Rate of the precursor solution 
                 0.01-10000 uL/min liquid 
               
               
                   
                 Deposition temperature 
                 100-600 C. 
               
               
                   
                 Deposition Pressure 
                 0.1-10 Torr 
               
               
                   
                   
               
             
          
         
       
     
         [0025]      FIG. 1  shows some experimental results in accordance with the present invention. In particular,  FIG. 1  shows ALD film growth rates for a HfO2 thin film using a solvent-based precursor. The precursor solution consisted of 0.2M ((t-Bu)Cp) 2 HfMe 2  in n-Octane and was delivered to a vaporizer at a flow rate of 1-4 ul/min. Three different deposition conditions were tried, i.e. deposition temperature 230° C. and deposition pressure 0.8 Torr; deposition temperature 270° C. and deposition pressure 7 Torr; deposition temperature 290° C. and deposition pressure 4 Torr. Results of these experiments are shown in Table 4. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Deposition Temperature 
                 Deposition Pressure 
                 Thin Film Growth Rate 
               
               
                 (° C.) 
                 (Torr) 
                 (A/cycle) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 230 
                 0.8 
                 0.7 
               
               
                 270 
                 7 
                 1.5 
               
               
                 290 
                 4 
                 1.6 
               
               
                   
               
             
          
         
       
     
         [0026]    It can be seen from  FIG. 1  that substrate saturation is reached at a metal precursor pulse width of about 1 s. Further increases in metal precursor pulse width did not alter the growth rates, thus establishing that this was true ALD behavior. Further, this experiment showed that different self-limiting growth rates can be achieved by using different combinations of temperature and pressure. In comparison, ALD growth rates using conventional methods and conventional precursors are always less than one monolayer per cycle. Therefore, the present invention provides a method of obtaining higher ALD growth rates that those that can be achieved by conventional ALD methods. This advantage may at least in part be caused by the solvent assisting the substrate absorption of the metal precursor molecules and helping to remove precursor ligands from the substrate surface. 
         [0027]    The present invention also provides a method of performing variable growth rates of an ALD film by adjusting one or more operation parameters; e,g, temperature or pressure during deposition. It is preferred according to the present invention to change deposition pressure during an ALD deposition process. In one example, the growth rate of ALD thin films can be altered during deposition by the following method.
       1. Select a metal precursor and solvent combination.   2. Dissolve the metal precursor in the solvent to a selected concentration.   3. Deliver the precursor solution to a vaporizer at a fixed flow rate.   4. Deliver the vaporized solution to a deposition chamber at a fixed temperature for a fixed length of time.   5. Alter the pressure (increase or decrease) of the deposition chamber to change the thin film growth rate.   6. Purge the deposition chamber with inert gas for a fixed length of time.   7. Deliver a second precursor (such as a reactive species, e.g. oxidizer) to the deposition chamber for a fixed length of time.   8. Purge the deposition chamber with inert gas for a fixed length of time.   9. Repeat  3  through  7  above until the desired thin film thickness is achieved.       
 
         [0037]      FIG. 2  is a graph plotting ALD growth rates at different deposition pressures when precursor concentration, delivery flow rate, and deposition temperature are held constant. In particular, for the plot shown in  FIG. 2 , precursor concentration was set at 0.15M, delivery flow rate was set at 2 uL/min, and deposition temperature was set at 230° C. It can be seen in  FIG. 2  that changes to the pressure result in significant changes to the thin film growth rate. 
         [0038]    It is believed that the advantages of the present invention are provided at least in part because within certain ranges, the solvent partial pressure in the deposition chamber forms a temporary surface layer that does not react with surface reactive sites chemically. The solvent also acts to carry the precursor to the surface and helps remove ligand fragments from the deposition surface, thus opening up free reaction sites for more complete saturation and reaction with the precursor molecules. The total pressure in the deposition chamber can be varied from 0.1 to 50 Torr. The preferred deposition pressure is between 1 and 15 Torr. 
         [0039]    It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.