WO2000047797A1

WO2000047797A1 - Low pressure chemical vapour deposition of titanium dioxide

Info

Publication number: WO2000047797A1
Application number: PCT/GB2000/000432
Authority: WO
Inventors: Michael Leslie Hitchman; Junfu Zhao
Original assignee: University Of Strathclyde
Priority date: 1999-02-11
Filing date: 2000-02-11
Publication date: 2000-08-17
Also published as: GB9902993D0

Abstract

A method is provided whereby rutile titanium dioxide may be deposited on a substrate at relatively low temperature; that is, below 500 °C. Titanium dioxide is produced by introduction of a titanium organic precursor compound, such as tertiary titanium tetrabutoxide (TTB), together with oxygen gas and water vapour into a low pressure reaction chamber containing a substrate deposition. Preferred substrates include SnO2 coated glass, sapphire, silicon, steel or aluminium. Rutile TiO2 may be deposited using the present method at temperatures as low as 290 °C.

Description

Low Pressure Chemical Vapour Deposition of Titanium Dioxide

The present invention relates to a method for depositing rutile titanium dioxide on a substrate by chemical vapour deposition.

Titanium dioxide occurs naturally in three crystalline forms: anatase, rutile and brookite. Rutile Ti0₂ has excellent anti-reflective properties due to its high refractive index. Rutile is also a useful dielectric and has applications in the microelectronics industry, particularly in the manufacture of memory capacitors .

Various techniques have been employed to prepare rutile Ti0₂ synthetically. For example, thin films of rutile Ti0₂ have been prepared by conventional chemical vapour deposition methods at temperatures in excess of 500°C. Below this temperature, however, titanium dioxide has only been deposited as anatase or in an amorphous state.

It is among the objects of the present invention to produce thin films of rutile Ti0₂ at temperatures below

500°C.

According to the present invention, there is provided a method for the preparation of rutile Ti0₂, said method comprising: introducing a titanium organic precursor compound, a reactive gas and water vapour into a reaction chamber which is maintained at a reduced pressure; and depositing the titanium dioxide formed as a thin film on a selected substrate at a temperature below 500°C; wherein said precursor compound substantially comprises tertiary titanium tetrabutoxide. Tertiary titanium tetrabutoxide is also known as titanium tert-butoxide .

Preferably, the temperature is below about 400°C; and most preferably below about 350°C. Preferably this temperature is also above about 260°C; preferably also this temperature is above about 290°C. The substrate may be selected from polycrystalline or single crystalline materials, for example, Sn0₂ coated glass, K- glass, steel and aluminium, which materials are polycrystalline, or for example sapphire and silicon, which materials are single crystalline. K-glass is a Registered Trade Mark of Pilkington pic, and refers to glass coated first with a 57nm layer of silica, and then with a 293.5nm layer of Sn0₂. Preferably, substrates such as Sn0₂ coated glass, sapphire, silicon, steel or aluminium are used. The surface of the substrate may be heated relative to the walls of the reaction chamber so that decomposition of the precursor and the formation of thin films of Ti0₂ is confined to the substrate surface. In the present method, rutile titanium dioxide is formed at a deposition temperature below 500 °C. Preferred deposition temperatures range between 290 °C and 400 °C. For example, when substrates such as Sn0₂ coated glass, sapphire, steel or aluminium are used, the preferred deposition temperature is between 290°C and 322°C. When a substrate such as K-glass is used, the preferred rutile deposition temperature is between 322°C and 400°C.

The precursor may be vaporised prior to being introduced to the reaction chamber. Preferably, the precursor compound is vaporised and introduced into the reaction chamber with a carrier gas. Any inert gas may be used as a carrier; for example, argon or helium. Advantageously, water vapour may also be introduced in the reaction chamber with a carrier gas. It has been found that the deposition temperature plays a significant role in determining the crystal structure of the titanium dioxide deposited. The rutile phase becomes increasingly dominant as the deposition temperature is increased. For example, an anatase to rutile phase transition is observed at deposition temperatures of approximately 290 °C for a titanium dioxide film grown on Sn0₂ coated glass. Below that temperature, anatase is the dominant phase. Above that temperature, rutile becomes increasingly dominant.

The crystal structure of titanium dioxide is also influenced by the nature of the substrate surface. Substrates such as Sn0₂ coated glass, sapphire, steel and aluminium exhibit a preference for the rutile phase at deposition temperatures between 290°C and 300°C. When titanium dioxide is deposited on substrates such as K- glass, copper and MgO at similar temperatures, the anatase phase is preferred, although when K-glass is used as a substrate, titanium dioxide begins to be formed as rutile above 322 °C. This suggests that the substrate surface plays an important role in the initial nucleation of titanium dioxide films at a given temperature. In accordance with a second aspect of the present invention, there is provided a substrate having rutile titanium dioxide deposited thereon, in accordance with the method of the first aspect of the present invention.

These and other aspects of the present invention will now be described, by way of example, with reference to the following drawings, in which:

Figure 1 is a block diagram of a system which may be used to prepare rutile Ti0₂ in accordance with an embodiment of the method of the present invention; Figures 2a to 2j are X-ray Diffraction patterns of titanium dioxide films prepared in Example 1,

Figures 3a to 3i show Raman Spectra of titanium dioxide films prepared in Example 1,

Figures 4a to 4h show X-ray Diffraction patterns of titanium dioxide films prepared in Example 2; and

Figures 5a to 5h show Raman Spectra of titanium dioxide films prepared in Example 2.

Reference is first made to Figure 1 which depicts a vertical low pressure chemical vapour deposition system which may be used to deposit titanium dioxide (Ti0₂) in the rutile form on a substrate. The system 10 comprises a reaction chamber 12 which contains a heated substrate 14 mounted on a susceptor 16. The reaction chamber 12 is coupled to a vacuum pump 18 which removes gaseous waste products from the reaction chamber 12. The pump also ensures that the pressure in the reaction chamber remains low, at approximately 1.0 Torr.

The system 10 also comprises a vertical bubbler 20 which is coupled to the reaction chamber 12. A titanium organic precursor compound is vaporised in the vertical bubbler 20 and introduced into the reaction chamber through a gas line 22. The gas line 22 is maintained at a temperature above the temperature of the vertical bubbler 20 to prevent the vaporised precursor compound from condensing within the gas line 22.

Also coupled to the reaction chamber 12 is a water bubbler 24. Water is vaporised in the water bubbler 24 and introduced to the upper region of the reaction chamber 12 via a gas line 26. The temperature of the gas line 26 is maintained above the temperature of the water bubbler 24.

The precursor compound and water vapour are introduced into the reaction chamber 12 with a carrier gas of, for example, argon. The carrier gas is fed into the vertical bubbler 20 and water bubbler 24 through inlets 28 and 30, respectively. The rate of flow of carrier gas through the inlets 28 and 30 are controlled independently, by respective mass flow controllers 32, 34.

The reaction chamber 12 is also coupled to a source 36 of oxygen via a gas line 38. The flow of oxygen through the chamber 12 is controlled by a mass flow controller 40.

In operation, the substrate 14 is positioned approximately 3cm from the inlet of the gas line 22 of the vaporised precursor compound. The temperature of the walls of the reaction chamber 12 are maintained at approximately 100°C, so that decomposition of the precursor compound and the formation of titanium dioxide is limited to the heated substrate 14 surface.

In the following examples, the reaction parameters which influence the crystal structure of the deposited titanium dioxide film are investigated. The films are produced using titanium tetrabutoxide (TTB) (purity >99.95%, InorgTech which has been identified as being comprised substantially of tertiary TTB (also known as titanium tert-butoxide) ) , as a precursor compound, and electronic grade argon as a carrier gas .

The deposition conditions are summarised in Table 1 below. Table 1

Example 1

In this example, titanium dioxide films are deposited on K-glass, a Sn0₂ and silica coated glass substrate at a range of temperatures (Table 2) . The samples were analysed to determine the effect of increasing deposition temperature on crystal structure. Tajble 2

X-ray Diffraction

The crystal structures of the resulting titanium dioxide films were analysed by X-ray Diffraction using Cu-

K radiation at λ=0.15405nm through a 2θ angle of 20° to

100°. All measurements were carried out using a Philips PW

1010 diffractometer operating at 40kV and 20mA with a Ni filter . The results are shown in Figures 2a to 2j .

Figure 2a is an X-ray Diffraction pattern of the Sn0₂ coated K-glass substrate. The polycrystalline Sn0₂ coating shows a preferred orientation in the (200) plane, as indicated by the strong characteristic peak at 2θ = 37.85°.

Figure 2b shows the X-ray diffraction pattern of Sample 1 (T_d 193°) . There is no obvious peak which can be identified with a Ti0₂ crystal structure.

Figure 2c shows an X-ray diffraction pattern for Sample 2 (T_d 215°) . Some anatase peaks appear at 2θ angles of 25.4° (101), 37.7°(004), 48.5° (200), 54.95°(211) and

70.15° (220). Substrate peaks, however, are still visible.

The patterns for Samples 3 (T_d 236°C) and 4 (T_d 257°C) are shown in Figures 2d and 2e, respectively. Anatase peaks are observed in both samples, at 2Θ angles of 25.3° (101), 37.75°(004), 48.0° (200), 54.9°(211) and 70.2°(220) for Sample 3; and 25.5° (101), 37.9° (004), 48.1° (200), 55.15° (211) and 70.25° (220) for Sample 4. The relative intensities of these diffraction peaks differ from those reported in the literature. This suggests that the deposited anatase phase is polycrystalline with randomly oriented grains .

Figure 2f is an X-ray pattern for Sample 5 (T_d 268°C) . Titanium dioxide is deposited as a mixture of anatase and rutile, but the anatase phase is dominant with peaks arising from the (101), (004), (200), (211) and (210) anatase planes. A strong rutile peak is observed at 2θ = 39.4°, assigned to the (200) plane. Other weak rutile peaks are observed at 2θ = 44.3° (210), 62.5° (002), 65.1° (221), 82.3°(321), 85.45°(400), and 95.85° (312) .

A significant change is observed with the pattern for Sample 6 (T_d 279°C) (Figure 2g) . Only two weak anatase peaks are observed at 37.85° (004) and 55.0° (221). The strong peaks at 2θ values of 39.25° and 64.0° arise from the (200) and (310) rutile planes. Other rutile peaks are observed at 2θ = 36.25° (101), 44.1° (210) and 84.55° (400).

Figure 2h shows a pattern for Sample 7 (T_d 290°C) . The rutile phase appears to be increasingly dominant as the temperature is increased.

Figures 2i and 2j are X-ray patterns for titanium dioxide films formed at 300°C and 322°C, respectively. The patterns show that the rutile phase is dominant, with strong peaks arising from 2θ values of 36.3° (101), 39.4°

(200) and 84.4 °(400) for Sample 8; and at 2θ values of

36.15° (101), 39.3° (200) and 84.4° (400) for Sample 9.

These results show conclusively that rutile may be prepared by low pressure chemical deposition at temperatures below 500°C. In particular, the samples prepared above 300°C (Samples 8 and 9) are composed exclusively of the rutile phase. Raman Spectroscopy The crystal structures of Samples 1 to 9 were also analysed by Raman Spectroscopy. All measurements were carried out using a 514.5nm argon laser produced by a Renishaw Raman Spectrometer.

Literature values for lattice vibrations of the anatase phase are 194 cm^"1, 397 cm^"1, 515 cm^"1 and 637 cm^"1. Typical rutile vibrations occur at 144 cm^"1, 235 cm^"1, 448 cm^"1 and 612 cm^"1.

The Raman spectra for Samples 1 to 9 are shown in Figures 3a to 3i. Figure 3a shows that no crystalline structure peaks can be detected by Raman scattering for a film deposited at 193 °C. At a deposition temperature of 215 °C (Figure 3b) , two peaks are observed at 514 cm^"1 and 640 cm^"1. These correspond to the anatase phase. Figures 3c and 3d show Raman spectra for Samples 3 and

4, prepared at deposition temperatures of 236 and 257°C, respectively. Peaks at 392 cm^"1, 514 cm^"1 and 637 cm^"1, corresponding to the anatase phase are observed.

Samples 5 and 6, prepared at 268°C and 279°C, respectively, are composed of a mixture of the anatase and rutile phase. As shown in Figures 3e and 3f, anatase peaks are observed at 395 cm^"1 and 635 cm^"1, and rutile peaks are observed at 441 cm^"1 and 519 cm^"1.

At deposition temperatures of 290 °C (Sample 7) , rutile peaks are observed at 448cm^"1 and 607cm^"1. The peaks increase in intensity as the deposition temperature increases. Samples 8 and 9, prepared at deposition temperatures of 300°C and 322 °C, respectively, show two strong rutile vibrations at 445 cm^"1 and 610 cm^"1. A very weak anatase vibration is also observed at 519 cm^"1.

The results of the Raman analysis are consistent with the results obtained by X-ray diffraction. Example 2

In this example, titanium dioxide is deposited on a range of substrates to determine how the nature of the substrate affects the crystal structure of the titanium dioxide formed.

The titanium dioxide films analysed in this example are listed in Table 3 below.

Table 3

X-ray Diffraction

The crystal structures of the Samples 10 to 17 were analysed using the diffractometer of Example 1. The resulting patterns of the samples are shown in Figures 4a to 4h. The bottom trace of Figure 4a shows an X-ray diffraction pattern for K-glass. Unlike Sn0₂ coated glass, K-glass is unoriented and no characteristic peaks are observed. The top trace of Figure 4a shows an X-ray diffraction pattern for titanium dioxide deposited on glass at 268 °C (Sample 10) . The peaks arise from the anatase (101) (200) (211) and (220) planes. Weak signals corresponding to the (112) (312) and (321) planes are also observed. The effect of changing the substrate from Sn0₂ coated glass to K-glass can be seen by comparing the pattern of Figure 2f with the top trace of Figure 4a. The strong anatase (004) peak in Figure 2f is replaced by a weak peak corresponding to the anatase (112) plane. A further difference is that no rutile peaks are observed when titanium dioxide is deposited on K-glass at 268°C.

The anatase phase remains the dominant phase at deposition temperatures of 290° (Sample 11, Figure 4b) and 305°C (Sample 12, Figure 4c) . As the deposition temperature is increased to 322 °C in Sample 13, however, rutile peaks are observed at 2θ angles of 27.2° (110), 39.3° (200), 41.7° (111) and 84.2° (400) (Figure 4d) .

The results indicate that higher deposition temperatures are required to deposit rutile on un-oriented glass than on oriented Sn0₂-coated glass.

Figure 4e is an X-ray diffraction pattern for titanium dioxide deposited on a sapphire substrate at 300°C (Sample 14) . The pattern shows strong rutile peaks at 2θ values of 39.25° (200) and 84.2°(400). Other weak rutile signals are observed at 36.05° (101), 41.3° (111) and 54.35° (211). A broad rutile (110) peak is observed at 27.6°.

Figure 4f compares an X-ray diffraction pattern of a steel substrate (bottom trace) with an X-ray pattern for Sample 15 (top trace) . The sample is prepared using a steel substrate at a deposition temperature of 298°C. Titanium dioxide is deposited as rutile and a strong rutile peak is observed at 39.0°C (200). A signal from either the steel substrate or from a rutile phase is also observed at 44.0°. Figure 4g compares an X-ray pattern for aluminium (bottom trace) with an X-ray pattern for titanium dioxide deposited on aluminium at 298°C (Sample 16) . Rutile appears to be the dominant phase, with peaks appearing at 2θ values of 36.25° (101) , 39.3°(200), 65.25°(221) and 48.1° (400) .

Figure 4h shows an X-ray pattern of titanium dioxide deposited on magnesium oxide at a deposition temperature of 298°. The spectrum shows anatase peaks at 2θ values of 47.9° (200), 54.95° (211), 70.1°(220) and 95.05°(321). Anatase is the dominant phase in this sample. However, the rutile phase is also present and a rutile peak at 27.2° (110) is observed. Raman Spectroscopy The crystal structures of samples 10, and 12 to 17 were also analysed by Raman Spectroscopy. The resulting spectra are shown in Figures 5a to g. Figure 5h is a Raman spectrum of a sample prepared using a copper substrate at a deposition temperature of 294°C. Figures 5a to c show Raman spectra for films deposited on glass at 268°C, 305°C and 322°C, respectively. It is clear from Figures 5a and 5b that titanium dioxide is deposited as anatase at temperatures of 268°C and 305°C

(Samples 10 and 12). At 322°C (Sample 13, Figure 5c), however, titanium dioxide is deposited as a mixture of mainly rutile with some anatase.

Figure 5d shows a Raman spectrum of titanium dioxide deposited on sapphire at 294 °C, rather than the 300°C of sample 14 (Figure 4e) . A pure rutile phase is observed with characteristic vibrations at 449cm^"1, 515cm^"1 and 607cm^"1.

Figures 5e and 5f show Raman spectra for films deposited on steel and aluminium respectively. Rutile is the dominant phase .

Figures 5g and 5h show Raman spectra for films deposited on MgO and copper respectively. The anatase phase is dominant.

These results indicate that the nature of the substrate has a significant effect on the crystal structure of the titanium dioxide films. When titanium dioxide is deposited at approximately 290°C to 300°C on substrates such as Sn0₂ coated glass, sapphire, steel and aluminium, the rutile phase is dominant. In contrast, when titanium dioxide is deposited on glass, copper and MgO at similar temperatures, the anatase phase is preferred. Also, a phase transition from anatase to rutile is observed at 290 °C for a film grown on Sn0₂ coated glass. In contrast, when glass is used as a substrate, the same phase transition occurs at 322 °C. This suggests that the substrate surface plays an important role in the initial nucleation of titanium dioxide films at a given temperature. Conclusions from Examples 1 and 2 The Examples above show that reaction conditions have a significant effect on the crystal structure of titanium dioxide formed by LPCVD of TTB at 1.0 Torr.

By depositing titanium dioxide on a substrate of Sn0₂ coated glass at temperatures in the range of 290 °C and 322 °C, a rutile titanium dioxide film is formed with preferred orientations of (200) and (400) . At deposition temperatures between 236°C and 257°C, an anatase film of titanium dioxide is formed with oriented faces of (101)

(112) (200) , (211) and (220) . At 268°C, a mixture of rutile and anatase is formed.

At deposition temperatures between 215°C to 236°C titanium dioxide deposited on Sn0₂ coated glass undergoes an amorphous to anatase phase transition. At deposition temperatures between 257°C and 290°C, an anatase to rutile transition is observed.

The nature of the substrate also has a significant effect on the crystal structure of the titanium dioxide films . The orientations of the films depend on the orientations of the substrate surface. Rutile Ti0₂ films can be selectively formed on Sn0₂ coated glass, sapphire, steel and aluminium substrates at deposition temperatures of approximately 300°C. and aluminium substrates at deposition temperatures of approximately 300 °C.

Claims

1. A method for the preparation of rutile Ti0₂, the method comprising: introducing a titanium organic precursor compound, a reactive gas and water vapour into a reaction chamber which is maintained at a reduced pressure; and depositing the titanium dioxide formed as a thin film on a selected substrate at a temperature below 500°C; wherein said precursor compound substantially comprises tertiary titanium tetrabutoxide.

2. The method of claim 1 wherein said temperature is below about 400°C.

3. The method of claim 2 wherein said temperature is below about 350°C.

4. The method of any preceding claim wherein said temperature is above about 260°C.

5. The method of claim 4 wherein said temperature is above 290°C.

6. The method of any preceding claim wherein said substrate is a polycrystalline material.

7. The method of claim 6 wherein said substrate is selected from: Sn0₂ coated glass, K-glass, steel, and aluminium.

8. The method of any of claims 1 to 5 wherein said substrate is a single crystalline material.

9. The method of claim 8 wherein said substrate is selected from: sapphire or silicon.

10. The method of any preceding claim wherein the surface of said substrate is heated relative to the reaction chamber.

11. The method of any preceding claim wherein said precursor is vaporised prior to being introduced into the reaction chamber.

12. The method of claim 11 wherein said vaporised precursor is introduced into the reaction chamber with a carrier gas .

13. The method of claim 12 wherein said carrier gas is an inert gas .

14. The method of claim 13 wherein said carrier gas comprises argon or helium.

15. The method of any preceding claim wherein the water vapour is introduced into the reaction chamber with a carrier gas .

16. The method of any preceding claim wherein the reaction chamber is maintained at a pressure of around 1 Torr.

17. The method of any preceding claim wherein the reactive gas comprises oxygen.

18. A substrate having a thin film of titanium dioxide in its rutile form deposited thereon in accordance with the method of claim 1.