CHEMICAL VAPOUR DEPOSITION

Abstract: A method of depositing a tin oxide film onto a heated substrate by chemical vapour deposition using a tetraalkyoxy tin compound and further, a method for doping said film with platinum or palladium using a beta -diketonate precursor thereof.

Description: TITLE: Chemical Vapour Deposition DESCRIPTION This invention concerns chemical vapour deposition of tin oxide. The metal oxide semi-conductor tin oxide (Sn0 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 (SnCl4) and organotin compounds, such as dibutyltin diacetate (Bu 2 Sn(OAc)2) - dimethyltin dichloride (Mβ2SnCl2) 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. 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 /2 -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 L> -diketonate. The invention further provides a substrate having a thin film of tin oxide deposited thereon by chemical vapour deposition using a tetraalkoxy tin precursor. The invention yet further provides a substrate having a thin film of tin oxide doped with platinum or palladium deposited thereon by chemical vapour deposition from tetraalkoxy tin and a platinum or palladium#-diketonate as precursors. The invention also provides a gas-sensor comprising a substrate of the invention. 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 760 or 15 Torr in a cold-wall horizontal quartz reactor using radiant substrate heating. The Sn(0Bu 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 Growth conditions to be used to deposit Snθ2 from Sn(OBu t ) 4 (a) Cell pressure = 760 Torr source temperature 80°C Reactor inlet line temperature 100°C N2 carrier gas flow 75SCCM Total N 2 flow 175SCCM Substrate Soda lime glass Deposition temperature 250-400°C (b) Cell pressure = 15 Torr Sn(OBu t ) 4 source temperature 45°C Reactor inlet line temperature 70°C N2 carrier gas flow 25SCCM Substrate Soda lime glass Deposition temperature 250-400°C 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 Auger electron spectral analysis of tin oxide films grown on glass substrate using Sn(0Bu t ) 4 ^ a) Example Substrate Atomic composition (atom %) O/Sn No. temperature Sn .0 Ratio 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 (a) Film Nos. 1-4 grown at 760 Torr, films 5-8 frown at 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(0Bu 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 <0.1um. Examples 9 to 11 Platinum containing tin oxide films were deposited by MOCVD at 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(0Bu*-) 4 alone as precursor. The growth conditions used are shown in Table 3 below. TABLE 3 Growth conditions used to deposit Pt-containing Sn(0Bu t ) 4 and Pt(acac) 2 ^ a ^ Example 9 Example 10 Example 11 N 2 flow through Sn(OBu t ) 4 (SCCM) 15 50 100 2 flow through Pt(acac) 2 (SCCM) 50 50 50 Sn0 2 deposition time (min) (B ) 23 10 20 Pt/Sn0 2 deposition time (min) 27 110 40 (a) Cell pressure = 15 Torr;'Sn(OBu*-) 4 source temperature = 50°C;Pt(acac) 2 source temperature = 100°C; Reactor inlet lines = 120°C (b) Deposition time of initial Sn0 2 layer prior to introduction of Pt(acac) 2 vapour. AES data for Sn0 2 films containing Pt are given in Table 4 below. TABLE 4 AES data Pt-containing Sn0 2 films deposited on glass at 400°C from Sn(OBu t ) and Pt(acac) 2 Example Etch time Atomic composition (atom %) No. (sees) Sn 0 Pt C 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 1111 6600 41.1 54.0 4.9 0.0 AES data for Snθ2 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 Snθ2 films containing Pt. This is a surprising result in view of the heavy carbon contamination (up to 50wt%) 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/Sn0 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 Sn0 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 Snθ2 films with little or no incorporation of carbon.

Claims: CLAIMS 1. A method of depositing a tin oxide film on to a heated substrate by chemical vapour deposition using a tin oxide precursor, wherein the precursor is a tetraalkyoxy tin compound. 2. A method as claimed in claim 1, wherein the tetraalkyoxy tin precursor is a branched chain tetraalkyoxy tin compound. 3. A method as claimed in claim 2, wherein the precursor is tetratertiarybutoxy tin or tetraisopropoxy tin. 4. A method as claimed in claim 1, 2 or 3, wherein a dopant is included in the tin oxide film. 5. A method as claimed in claim 4, wherein the dopant is platinum or palladium. 6. A method as claimed in claim 5, wherein a platinum or palladium ^-diketonate precursor is included in the method to result in the platinum or palladium doped tin oxide film. 7. A method as claimed in claim 6, wherein the platinum or palladium ^ p-diketonate is platinum acetylacetonate or palladium acetylacetonate respectively. 8. A method as claimed in any one of the preceding claims, wherein the substrate is heated in the range 250-400°C for chemical vapour deposition. 9. A method as claimed in any one of the preceding claims, wherein the organometallic precursors are delivered to the substrate in an inert carrier gas. 10. A method as claimed in claim 9, wherein the inert carrier gas is nitrogen. 11. A method of depositing a platinum or palladium doped tin oxide film on to a heated substrate by a method as claimed in any one of claims 5 to 10. 12. A method as claimed in claim 11, wherein a thin layer of tin oxide alone is deposited on the substrate before introduction of the platinum or palladium precursor. 13. A method of depositing a tin oxide film on a heated substrate substantially as hereinbefore described and with reference to Examples 1 to 8 and 9 to 11. 14. A substrate having a thin film of tin oxide deposited thereon by a method as claimed in any one of claims 1 to 13. 15. A gas sensor comprising a substrate having a thin film of tin oxide deposited thereon by a method as claimed in any one of claims 1 to 13.

Inventors: JONES ANTHONY COPELAND, JONES, ANTHONY, COPELAND
Applicants: EPICHEM LTD, JONES ANTHONY COPELAND, EPICHEM LIMITED, JONES, ANTHONY, COPELAND
Application year: 1995
Main classification: C23C16
IPC codes: C03C17, C23C16