Patent Publication Number: US-5830530-A

Title: Chemical vapor deposition of tin oxide films

Description:
FIELD OF THE INVENTION 
     This invention concerns chemical vapour deposition of tin oxide. 
     BACKGROUND OF THE INVENTION 
     The metal oxide semi-conductor tin oxide (SnO 2 ) has a range of important applications, such as in transparent and conducting coatings on glass and in gas-sensing devices, especially for carbon monoxide, used in environmental and automotive applications. 
     Tin oxide gas sensors have been prepared from bulk sintered powder or from deposited thick or thin films. Thin films of tin oxide are especially suitable for gas-sensor applications, due to their better speed of response compared to thick films. Also, they can be fabricated with small dimensions on a large scale using low-cost production techniques. Also, thin films are widely compatible with existing micro-electronics technology and integrated circuits. 
     Thin films of tin oxide have been grown by a variety of techniques including reactive sputtering, spray pyrolysis and chemical vapour deposition (CVD). Of these techniques CVD has considerable potential due to its capability for large area growth, control of doping and film thickness and superior conformal step coverage. 
     A variety of precursors have been used for the deposition of tin oxide by CVD, including stannic chloride (SnCl 4 ) and organotin compounds, such as dibutyltin diacetate (Bu 2  Sn(OAc) 2 ), dimethyltin dichloride (Me 2  SnCl 2 ) and tetramethyltin (Me 4  Sn). These precursors are generally highly toxic volatile materials, which are hazardous to use, and metal oxide growth is only possible in the presence of a large excess of a separate added oxidant, such as oxygen or water. 
     For example, U.S. Pat. No. 4,865,883 (Saitoh et al) relates to a method for forming a deposited film by introducing a gaseous starting material containing tin atoms, such as SnH 4  or Sn(CH 3 ) 4 , a gaseous oxidizing agent to oxidize the starting material and an oxygen-containing gaseous compound for formation of a precursor for deposition of the film. U.S. Pat. No. 5,279,851 (Minosou et al) relates to a method of depositing a coating of stannic oxide containing potassium onto glass using the precursor, dioctyltin diacetate. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide a technique for depositing tin oxide which involves use of less hazardous source materials. 
     It has now been surprisingly found that tetraalkoxy tin compounds can be used to deposit tin oxide by chemical vapour deposition in the absence of an added oxygen source. 
     Accordingly in a first embodiment the invention provides a method of depositing tin oxide by chemical vapour deposition onto a heated substrate using a tetraalkyoxy tin compound. 
     The addition of dopants, such as platinum and palladium, to tin oxide films has been shown to improve the sensitivity and selectivity of gas sensor devices. It has further been surprisingly found that by including a β-diketonate of platinum or palladium in the chemical vapour deposition method using a tetraalkoxy tin compound can result in a platinum or palladium doped tin oxide film. 
     Accordingly in a second embodiment the invention provides a method of depositing a platinum or palladium doped tin oxide film on a heated substrate by chemical vapour deposition using a mixture of a tetraalkoxy tin compound and a platinum or palladium β-diketonate. 
     The invention further provides a substrate having a thin film of tin oxide doped with platinum deposited thereon by chemical vapour deposition from tetraalkoxy tin and a platinum β-diketonate as precursors, wherein the deposited film has reduced levels of carbon contamination. 
     The invention also provides a gas-sensor comprising a substrate having a film of platinum-doped tin oxide having low levels of carbon contamination deposited thereon produced by the method of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred tin compounds for use in the invention included branched chain tetraalkoxy tin compounds, examples of which are tetratertiarybutoxy tin and tetraisopropoxy tin. 
     Preferred platinum or palladium compounds for use in the second embodiment of the invention include platinum acetylacetonate and palladium aceytlacetonate. 
     The substrate is preferably heated in the range 250°-400° C. for chemical vapour deposition according to the invention. The organometallic precursors are preferably delivered to the substrate in an inert carrier gas, such as nitrogen. 
     In carrying out the method of the second embodiment of the invention it is preferred that a thin film of tin oxide alone be deposited on the substrate before introduction of the platinum or palladium precursor. 
    
    
     This invention will now be further described by means of the following Examples 
     EXAMPLES 1 to 8 
     Tin oxide films were deposited from tetratertiary butoxy tin at either 1 Bar (760 Torr) or 0.0013 Bar (15 Torr) in a cold-wall horizontal quartz reactor using radiant substrate heating. The Sn(OBu t ) 4  source was contained in a glass bubbler using nitrogen as carrier gas. The source and reactor inlet lines were heated during metalorganic chemical vapour deposition (MOCVD) to prevent condensation of the precursor. 
     The substrates were soda lime glass which had been cleaned using 20% nitric acid/deionised water, degreased with acetone and dried before use. 
     The growth conditions are given in Table 1 below. 
     
                       TABLE 1                                                     
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Growth conditions to be used to deposit SnO.sub.2  from                   
Sn(OBu.sup.t).sub.4                                                       
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(a) Cell pressure = 1 Bar (760 Torr)                                      
Sn(OBu.sup.t).sub.4  source temperature                                   
                        80° C.                                     
Reactor inlet line temperature                                            
                        100° C.                                    
N.sub.2  carrier gas flow                                                 
                        75 SCCM                                           
Total N.sub.2  flow     175 SCCM                                          
Substrate               Soda lime glass                                   
Deposition temperature  250-400° C.                                
(b) Cell pressure = 0.0013 Bar (15 Torr)                                  
Sn(OBu.sup.t).sub.4  source temperature                                   
                        45° C.                                     
Reactor inlet line temperature                                            
                        70° C.                                     
N.sub.2  carrier gas flow                                                 
                        25 SCCM                                           
Substrate               Soda lime glass                                   
Deposition temperature  250-400° C.                                
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     Thin films of tin oxide were deposited using Sn(OBu t ) 4 , in the absence of added oxidant, over the temperature range 250°-400° C. at both atmospheric pressure and at a low pressure. The atomic compositions of the films deposited were determined by Auger electron specotrscopy (AES) and the results are shown in Table 2 below 
     
                       TABLE 2                                                     
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Auger electron spectral analysis of tin oxide films                       
grown on glass substrate using Sn(OBu.sup.t).sub.4 .sup.(a)               
Example Substrate  Atomic composition (atom %)                            
                                     O/Sn                                 
No.     temperature                                                       
                   Sn         O        Ratio                              
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1       250        35.8       64.2     1.8                                
2       300        38.5       61.5     1.6                                
3       350        49.6       60.4     1.5                                
4       400        38.5       61.1     1.6                                
5       250        35.0       65.0     1.9                                
6       300        37.4       62.6     1.7                                
7       250        35.6       64.4     1.8                                
 8.     400        34.7       65.3     1.9                                
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 .sup.(a) Film Nos. 1-4 grown at 1 Bar (760 Torr), films 5-8 frown at     
 0.0013 Bar (15 Torr).                                                    
 
    
     This data indicates that the tin oxide films are non-stoichiometric and oxygen deficient with O/Sn ratios in the range of 1.5 to 1.9. The AES data further shows that carbon is substantially absent from the tin oxide films which indicates that a facile route exists for the removal of the organic radical of Sn(OBu t ) 4  as a volatile and non-reactive reaction product. 
     The tin oxide films were generally dark and slightly absorbing, possibly due to the presence of excess zinc, and analysis by X-ray diffraction indicated that the films were amorphous. Field electron microscopy (FEM) data for a film grown at 300° C. showed a granular surface morphology with a grain size &lt;0.1 um. 
     EXAMPLES 9 to 11 
     Platinum containing tin oxide films were deposited by MOCVD at 0.0013 Bar (15 Torr) onto glass substrates using tetratertiarybutoxy tin and platinum acetyl acetonate with nitrogen as carrier gas. Prior to the introduction of platinum acetyl acetonate vapour, a thin layer of tin oxide was deposited using Sn(OBu t ) 4  alone as precursor. The growth conditions used are shown in Table 3 below. 
     
                       TABLE 3                                                     
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Growth conditions used to deposit Pt-containing                           
Sn(OBu.sup.t).sub.4  and Pt(acac).sub.2 .sup.(a)                          
           Example 9                                                      
                   Example 10 Example 11                                  
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N.sub.2  flow through                                                     
             15        50         100                                     
Sn(OBu.sup.t).sub.4 (SCCM)                                                
N.sub.2  flow through                                                     
             50        50         50                                      
Pt(acac).sub.2 (SCCM)                                                     
SnO.sub.2  deposition time                                                
             23        10         20                                      
(min).sup.(B)                                                             
Pt/SnO.sub.2 27eposition                                                  
                       110        40                                      
time (min)                                                                
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 .sup.(a) Cell pressure = 0.0013 Bar (15 Torr); &#39;Sn(OBu.sup.t).sub.4      
 source temperature = 50° C.; Pt(acac).sub.2  source temperature = 
 100° C.; Reactor inlet lines = 120° C.                     
 .sup.(b) Deposition time of initial SnO.sub.2  layer prior to introductio
 of PT(acac).sub.2  vapour.                                               
 
    
     AES data for SnO 2  films containing Pt are given in Table 4 below. 
     
                       TABLE 4                                                     
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AES data Pt-containing SnO.sub.2  films deposited on glass                
at 400° C. from Sn(OBu.sup.t).sub.4  and Pt(acac).sub.2            
Example  Etch time  Atomic composition (atom %)                           
No.      (secs)     Sn     O       Pt   C                                 
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 9       360        35.9   60.9    3.2  0.0                               
10        60        42.8   51.4    5.8  0.0                               
         120        42.1   45.9    12.0 0.0                               
         180        44.2   32.6    23.2 0.0                               
11        60        41.1   54.0    4.9  0.0                               
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     AES data for SnO 2  films containing Pt are given in Table 4. The AES data indicate that the Pt(acac) 2  prescursor has allowed the introduction of platinum upto 23.2 atom %. Such levels are useful in gas sensor applications. 
     It is significant that carbon is generally absent from the SnO 2  films containing Pt. This is a surprising result in view of the heavy carbon contamination (up to 50 wt %) previously observed in Pt films deposited by vacuum CVD using Pt(acac) 2 . This advantageous result may prove to be extremely significant in commercial development of Pt/SnO 2  sensor materials, as Pt(acac) 2  is a readily available and relatively inexpensive Pt source. 
     In summary, the precursor Sn(OBu t ) 4  has proved to be a useful precursor for the deposition of SnO 2  by MOCVD at 250°-400° C., in the absence of an added oxidant. Furthermore, Pt(acac) 2  has been used successfully to introduce useful levels of Pt into SnO 2  films with little or no incorporation of carbon.