Abstract:
Improved apparatus and methods for atomic layer deposition (ALD) are described—In particular, improved methods and apparatus for the vaporization and delivery of solution ALD precursors are provided. The present invention is particularly useful for processing lower volatile metal, metal oxide, metal nitride and other thin film precursors. The present invention uses total vaporization chambers and room temperature valve systems to generate true ALD vapor pulses while increasing utilization efficiency of the solution precursors.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to new and useful methods and apparatus for producing thin films using atomic layer deposition (ALD) processes. Methods and apparatus for the vaporization and delivery of solution ALD precursors to produce high quality thin films from a wide selection of precursors are described. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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 proceeds 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 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. 
         [0004]    The present invention relies on solvent based precursors. Examples of 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 the precursor solute 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./ 
                 Density 
               
               
                 Name 
                 Formula 
                 MW 
                 Mp (° C.) 
                 mmHg) 
                 (g/mL) 
               
               
                   
               
             
          
           
               
                 Tetrakis(ethylmethylamino)hafnium 
                 Hf[N(EtMe)] 4   
                 410.9 
                 −50 
                   79/0.1 
                 1.324 
               
               
                 (TEMAH) 
               
               
                 Hafnuim (IV) Nitrate, 
                 Hf(NO 3 ) 4   
                 426.51 
                 &gt;300 
                 n/a 
               
               
                 anhydrous 
               
               
                 Hafnuim (IV) Iodide, 
                 HfI 4   
                 686.11 
                 400 (subl.) 
                 n/a 
                 5.6 
               
               
                 anhydrous 
               
               
                 Dimethylbis(t-butyl 
                 [(t-Bu)Cp] 2 HfMe 2   
                 450.96 
                 73-76 
                 n/a 
               
               
                 cyclopentadienyl 
               
               
                 hafnium(IV) 
               
               
                 Tetrakis(1-methoxy-2- 
                 Hf(O 2 C 5 H 11 ) 4   
                 591 
                 n/a 
                   135/0.01 
               
               
                 methyl-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 i-propoxide 
                 Ba(OC 3 H 7 ) 2   
                 255.52 
                 200 ec) 
                 n/a 
               
               
                 Strontium i-propoxide 
                 Sr(OC 3 H 7 ) 2   
                 205.8 
               
               
                 Bis(pentamethylCp) 
                 Ba(C 5 Me 5 ) 2   
                 409.8 
               
               
                 Barium 
               
               
                 Bis(tripropylCp) Strontium 
                 Sr(C 5 i-Pr 3 H 2 ) 2   
                 472.3 
               
               
                 (Trimethyl)pentamethylcyclopentadienyl 
                 Ti(C 5 Me 5 )(Me 3 ) 
                 228.22 
               
               
                 titanium (IV) 
               
               
                 Bis(2,2,6,6-tetramethyl- 
                 Ba(thd) 2  * 
                 503.85 
                 88 
               
               
                 3,5-heptanedionato) barium 
                 triglyme 
                 (682.08) 
               
               
                 triglyme adduct 
               
               
                 Bis(2,2,6,6-tetramethyl- 
                 Sr(thd) 2  * triglyme 
                 454.16 
                 75 
               
               
                 3,5-heptanedionato) 
                   
                 (632.39) 
               
               
                 strontium triglyme adduct 
               
               
                 Tris(2,2,6,6-tetramethyl- 
                 Ti(thd) 3   
                 597.7 
                   
                 75/0.1 (sp) 
               
               
                 3,5-heptanedionato) 
               
               
                 titanium(III) 
               
               
                 Bis(cyclpentadinyl)Ruthenium 
                 RuCp 2   
                 231.26 
                 200 
                 80-85/0.01 
               
               
                 (II) 
               
               
                   
               
             
          
         
       
     
         [0005]    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. 
         [0006]    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. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Examples of solvents 
               
             
          
           
               
                 Name 
                 Formula 
                 BP@760Torr (° 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) 
               
               
                 Butylcyclohexane 
                 C 10 H 20   
                 178 
               
               
                   
               
             
          
         
       
     
         [0007]    Another example of a solvent useful for the present invention is 2,5-dimethyloxytetrahydrofuran. 
         [0008]    By using solvent based precursors for ALD, it is possible to use less volatile precursors in any physical form. Further, because dilute precursors are used, chemical utilization efficiency is improved. The copending application noted above also discloses two vaporization/delivery modes; i.e. constant pumping speed and constant pressure mode in the vaporizer. In constant pumping speed mode, room temperature gas swing systems are used to pulse hot vapor to the deposition chamber and during pulse off time, the vapor is diverted downstream of the deposition chamber. In constant pressure mode, high temperature pressure gauges and valves are required. 
         [0009]    Therefore, there remains a need in the art to further improve chemical utilization efficiency of solvent based precursors. 
       SUMMARY OF INVENTION 
       [0010]    The present invention provides improved methods and apparatus for the vaporization and delivery of solution ALD precursors. The present invention is particularly useful for processing lower volatile metal, metal oxide, metal nitride and other thin film precursors. The present invention uses total vaporization chambers and room temperature valve systems to generate true ALD vapor pulses. Utilization efficiency of the solution precursors is enhanced according to the present invention by combining liquid dosing with vapor phase pulse schemes. The result is high quality ALD thin films that can be deposited from a wide selection of precursors. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a schematic drawing of an apparatus according to one embodiment of the present invention. 
           [0012]      FIG. 2  is a schematic drawing of an apparatus according to another embodiment of the present invention. 
           [0013]      FIG. 3  is a schematic drawing of an apparatus according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention is directed to methods and apparatus that utilize a combination of liquid and vapor phases that are modulated or pulsed to delivery precise ALD doses of the solution precursors. The present invention provides a number of advantages over methods and apparatus known in the prior art. 
         [0015]    In particular, precursor consumption can be reduced by up to 90% over the previously mentioned copending application (e.g. 1 sec liquid pulse over 10 sec ALD cycle time). Further, a smaller form factor for the solution source container (e.g. standard 1 liter dip tube electro-polished stainless steel containers) is possible which in turn allow for a smaller form factor vaporizer and a smaller overall tool footprint. In addition, room temperature valve systems can still be used, but diversion of the solution liquid precursor during ALD vapor pulse off period is not longer necessary as will be explained in greater detail below. 
         [0016]      FIGS. 1 ,  2  and  3  are all schematic views of apparatus according to the present invention. In each case, the apparatus comprises a number of parts: i.e. a solution source; liquid metering or flow movement means; liquid modulation means; a vaporizer; a valve system; and an ALD deposition chamber. Throughout the drawings, like reference numbers are used for like parts. 
         [0017]    In particular, in  FIGS. 1 ,  2  and  3 , the apparatus comprises a solution source vessel  10  in fluid connection with a vaporizer  30  via a pump  20  and valve  90 . The vaporizer  30  may include a checker valve or injector nozzle  35  ( FIG. 3 ). Also connected to the vaporizer  30  is a purge gas source (not shown) through a mass flow controller  85  and valve  91 . The vaporizer is further connected to a deposition chamber  40 , or to system pump  70  through valve  97 . The mass flow controller  85  can also provide purge gas to the deposition chamber  40  through valve  92 . A separate purge gas source (not shown) can also provide purge gas to the deposition chamber  40  through a mass flow controller  80  and valve  93 . As shown in  FIGS. 1 and 3  a gas source vessel  50  provides the gas to the deposition chamber  40  through a regulator  60  and a series of valves  94 ,  95  and  96 . Alternatively, gas can be sent to the system pump  70  through valve  98 . The deposition chamber  40  is also connected to the system pump  70 . In a separate embodiment as shown in  FIG. 2 , a liquid reactant vessel  55  provides liquid reactant to the deposition chamber  70  through valve  96  or to the system pump  70  through valve  98 . In this embodiment, the purge gas is sent through the mass flow controller  80  and valve  95  into the liquid reactant vessel  55 . 
         [0018]    The solution source may be stored in the solution source vessel  10  at room temperature. This solution source comprises an ALD precursor dissolved in one or more solvents. The precursor can be a solid, a liquid, or a gas. A large number of precursors can be used, including those with low volatility, and a wide range of boiling and melting temperatures. The precursors can be metal organics or inorganics and can be suitable for building part of a metal, oxide, nitride, or other type of thin film using an ALD process. The solvent should have a dissolving power for the chosen precursor of greater than 1 molar, greater than 0.01 molar or greater than 0.1 molar. In addition, the solvent should have physical and chemical properties similar to those of the precursor and be selected to have compatible vaporization properties with the precursor to assure full vaporization without generating residue. The solution source vessel  10  preferable has a dip tube for liquid delivery, a pressurization gas port and a solution recharge port. 
         [0019]    As shown in  FIGS. 1 ,  2  and  3 , solution is moved out of solution source vessel  10  using a pump  20 . This pump  20  may take several different forms. Preferably, the pump  20  takes the form of a calibrated capillary line and solution is moved into the capillary line by pressure applied through the gas port of the solution source vessel using an inert gas. The inert gas preferably is provided in the range of 0 to 50 psig. Alternatively, the pump  20  may be a liquid mass flow controller, a liquid pump or a syringe pump. In accordance with the present invention the solution is moved at room temperature without vaporization, decomposition, or separation. 
         [0020]    The amount of solution provided to the vaporizer  30  is modulated or controlled by valve  90  which is an on/off switching valve. The dose of solution provided to the vaporizer  30  is selected according to the amount needed for the ALD vapor pulse and is controlled to avoid excessive solution precursor loss during the ALD vapor pulse off period. The valve  90  can be an ALD two port valve or alternatively can have a pre-determined liquid storage volume; e.g. HPLC type multi-port valve with capillary storage tubes. As shown in  FIG. 3 , the solution dose can be further separated and controlled by a checker valve or injection nozzle  35  to ensure the solution dose remains liquid phase before entering the vaporizer  30 . The modulation system, e.g. valve  90  should be thermally isolated by the use of thermal insulator conduit; e.g. ceramic feed through pipes. 
         [0021]    The vaporizer  30  includes a solution dose inlet, a hot inert gas inlet and a hot vapor outlet. The vaporizer preferably includes an internal and an external energy source to ensure full vaporization of the solution precursor dose without causing separation and decomposition. In operation, the solution dose enters the vaporizer  30  and is flashed into vapor phase under reduced pressure and hot vaporization chamber. The partial pressure of the precursor should be maintained under the saturation pressure for the precursor compound at the vaporizer  30  operation temperature. Following a controlled time delay, a controlled amount of inert gas is provided from inert gas source using mass flow controller  85  and valve  91 . This inert gas carries the vaporized precursor out of the vaporizer  30  through the hot vapor outlet. The present invention assures that the hot vapor is in uniform gas phase at the desired concentration. The inert gas is preferably is preheated, such as by heat exchange with the external energy source for the vaporizer  30 . The inert gas is preferably injected around the solution dose inlet to create a stream of jets. The internal and external energy sources for the vaporizer  30  can be electrically heated surfaces. As shown in  FIGS. 1 ,  2  and  3  the solution dose inlet and hot inert gas inlet are located near the top of the vaporizer  30 , while the hot vapor outlet is near but above the bottom of the vaporizer  30 . To provide further purification to the vapor, an inert filter medium can be used in the hot vapor outlet. 
         [0022]    The valve system for the apparatus according to the present invention utilizes a number of different valve types. In particular, valves  90 ,  91  and  95  are ALD valves, valves  92 ,  93 ,  94 ,  97  and  98  are metering valves and valve  96  is an on/off valve. One advantage of the present invention is that all of the valves used are room temperature liquid or gas valves. This allows the gas valves to be switched on and off with fast response time. It should be noted that while  FIGS. 1 ,  2  and  3  all show two separate mass flow controllers for the inert gas, it would be possible to combine these into a single unit with appropriate valves and control. The other reactant provided from either gas source vessel  50  or from liquid reactant vessel  55  can be in gaseous or liquid form; e.g. oxygen, air, ammonia, ozone, water, hydrogen, plasma forms of the preceding, etc. 
         [0023]    The ALD deposition chamber  40  can be constructed for a single wafer or a batch of wafers. Typical operating conditions for the deposition chamber  40  are pressure from 0.1 to 50 Torr and independent substrate heaters from 50° C. to 800° C. It is preferable that the conduits extending between the vaporizer  30  and deposition chamber  40  include heating means so the hot vapor can be maintained at or above the temperature of the vaporizer  30 . The hot vapor can be delivered into the deposition chamber by simple flowing inlet or shower head. It is also preferable to operate the deposition chamber  40  at a lower pressure and than the vaporizer  30  and at a temperature lower than the vaporization temperature. In accordance with the present invention, the hot vapor precursor is directed to the substrate with minimum loss to the deposition chamber  30  walls. 
         [0024]    The operation of the apparatus according to the present invention can be described as follows.
       With ALD valves  90 ,  91  and  95  switched off, the system is purged using inert gas flowing through valves  92 ,  93 . This purge can continue for 0.1 to 50 seconds.   ALD valve  95  is opened to deliver reactant either from gas source vessel  50  ( FIGS. 1 and 3 ) or liquid reactant vessel  55  ( FIG. 2 ) to the deposition chamber. This delivery can continue for 0.1 to 50 seconds.   ALD valve  95  is closed, (ALD valves  90  and  91  remain closed) and the system is again purges with inert gas for 0.1 to 50 seconds.   ALD (or liquid valve)  90  is opened to deliver a solution precursor dose to the vaporizer  30 . This delivery can extend for 0.1 to 50 seconds.   ALD valve  90  is closed, (valves  91  and  95  remain closed) and the system is purged with inert gas for 0.1 to 50 seconds.   ALD valve  91  is opened to deliver hot inert gas to the vaporizer  30  and thereby create the hot vapor precursor dose which is delivered to the deposition chamber  40 . This delivery can also run from 0.1 to 50 seconds.       
 
         [0031]    The above steps are repeated to build up successive ALD layers. 
         [0032]    In accordance with the present invention, the time delay between the solution pulse and the hot vapor pulse; i.e. the third purge stage, is adjusted to minimize precursor vapor loss through the system pump  70 . More particularly, in operation, inert gas via either mass flow controller  80  and valve  93 , or mass flow controller  85  and valve  92  continues to flow through the system even when the ALD valves  90 ,  91  or  95  are open. When the ALD valves  90 ,  91  and  95  are closed, this inert gas creates a diffusion barrier that blocks the precursor vapor coming from the vaporizer  30  and diverts any excess precursor vapor to the system pump  70 . However when an ALD valve is open, for example, when ALD valve  91  is opened, the hot precursor vapor created is carried out of the vaporizer  30  at a pressure sufficient to overcome the diffusion barrier pressure and allow the precursor vapor to enter the deposition chamber  40 . The pressure of the diffusion barrier is determined by the vacuum setting of the deposition chamber  40 . 
         [0033]    While the above describes an ALD process using to precursor sources, additional sources can be installed. For example, to produce mixed component ALD films, e.g. HfAlOx, a separate solution precursor source for both Hafnium and Aluminum can be included in the system along with the reactant source. Although mixing two or more precursors together in one solution source is possible, providing separate solution sources gives greater flexibility in composition control. 
         [0034]    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.