WO2010114386A1 - Thin films containing molybdenum oxide - Google Patents

Abstract

Method for depositing a thin film comprising molybdenum oxide on a surface of a substrate by ALD technique, where the method comprises the steps of: a) reacting a pulse of a halogen-free Mo-precursor with the surface; b) purging with an inert gas or pump down of reactor; c) reacting a pulse of an oxygen containing precursor with the surface; and d) purging with an inert gas or pump down of reactor; thereby obtaining a molybdenum oxide containing film. Further the present invention relates to thin films obtained by this method and use of a CoMo thin film as an ammonia decomposition catalyst.

Description

Thin Ωlms containing molybdenum oxide

The present invention relates to a method for producing thin films containing molybdenum oxide with the atomic layer deposition (ALD) technique and thin films obtained by this method.

Molybdenum oxide (MoO₃) is a transition metal investigated particularly for its electrochromic properties as described by S. S. Mahajan, S.H. Mujawar, P. S. Shinde, A.I. Inamdar, P.S. Patil, Applied Surface Science 254 (2008) 5895-5898, and for its broad industrial applications in electronic devices, but also for its interesting catalytic properties as discussed by Sobia Ashraf, Christopher S. Blackman, Geoffrey Hyett and Ivan P. Parkin, J. Mater. Chem., 2006, 16, 3575-3582. MoO₃ thin films have been deposited by many different techniques, including CVD (chemical vapour deposition) as disclosed by Sobia Ashraf et al. and PVD (physical vapour deposition) described by M. S. Burdis and J. R. Siddle, Thin Solid Films, 1994, 237, 320.

In order to be able to control a uniform growth and the thickness at the atomic scale it would be advantageous to use atomic layer deposition (ALD) technique to grow MoO₃.

Previously one has only been able to deposit Mo as metal or Mo_xN using MoCl₅ as the Mo containing precursor. Deposition of MoxN is described by L. Hiltunen, M. Leskela, M. Makela, L. Niinistδ, E. Nykanen, and P.Soininen, Thin Solid Films 166, (1988) 149 and by M. Juppo, M. Ritala, and M. Leskela, J. Electrochem. Soc. 147 (2000) 3377. Deposition of Mo is discussed by M. Juppo, M. Vehkamaki, M. Ritala, and M. Leskela, J. Vac. Sci. Technol. A 16 (1998) 2845.

Until now there has been no reported solution on how to deposit Mo-containing oxides by the ALD technique. The main reason for this is that the previously attempted Mo- precursor has been MoCl₅ which forms even more stable and volatile oxy-chlorides (MoO₂Cl₂) rather than a solid film and these species are released into the gas phase. Similar products will also be the result if other halogenides of molybdenum are used as precursor.

The application of carbonyl compounds as precursors for ALD has generally been considered too limited thermally stable to be utilised in ALD growth, as disclosed in US2009/0029036. The object of the present invention is accordingly to find a way to deposit thin films of oxides containing molybdenum by the ALD technique.

ALD = atomic layer deposition (also known as ALCVD =atomic layer chemical vapour deposition, and ALE = atomic layer epitaxy) is a thin film technique that utilizes only surface reactions, and is described in prior art, see e.g. M. Ritala, M. Leskela, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, vol. I, Academic Press, San Diego, CA, 2001, p. 103. A thin film is produced by the ALD technique by using different types of precursors. The precursors are pulsed sequentially into the reaction chamber where it reacts with a surface; each pulse is followed by a purging time with an inert gas or an evacuation of the reactor. In this way gas phase reactions are eliminated and film is constructed by precursor units in the order that they are pulsed. This technique makes it possible to change building units at the resolution of one monolayer, and therefore enables production of artificial structures of films with different types of organic and inorganic building units.

The present invention provides a method for depositing a thin film comprising molybdenum oxide on a surface of a substrate by ALD technique where the method comprises the steps of: a) reacting a pulse of a halogen- free Mo-precursor with the surface; b) purging with an inert gas or pump down of reactor; c) reacting a pulse of an oxygen containing precursor with the surface; and d) purging with an inert gas or pump down of reactor; thereby obtaining a molybdenum oxide containing film.

hi one embodiment of the present invention the halogen- free Mo-precursor is selected from Mo(CO)₆, Mo(thd)₂ (thd = 2,2,6,6-tetramethylheptane-3,5-dione), cycloheptatriene molybdenum tricarbonyl, pentamethyl-cyclopentadienylmolybdenum dicarbonyl dimer and derivates thereof.

In one aspect the steps a) to d) are repeated to obtain the desired thickness of the film.

hi another aspect of the present invention the method may further comprise deposition of one or more other compounds one or more times between or after the steps a) to d) are performed. In one aspect the method is performed within the temperature range between 140°C and 200°C, more preferably about 157-175°C.

In one embodiment of the invention the method further comprises the steps of e) reacting a pulse of a cobalt precursor with the surface, wherein the cobalt precursor is selected from the group comprising Co(thd)₂, Co(Cp)₂, Co(alkyl substituted Cp)₂, Co(acac)₂ and Co(PrAMD); f) purging with an inert gas or pump down of reactor; g) reacting a pulse of an oxygen containing precursor with the surface; and h) purging with an inert gas or pump down of reactor; thereby obtaining a complex Mo-Co-O containing film.

Further the steps a)-d) and e)-f) may be respectively repeated an individually selected number of times and where all the steps are repeated to grow the thin film.

Here thd stands for 2,2,6,6-tetramethylheptane-3,5-dione, Cp stands for cyclopentadienyl, acac stands for acetylacetonato and PrAMD stands for N₁N'- diisopropylacetamidinato. Alkyl substituted Cp means Cp substituded 1-5 times with C_1-6 alkyl.

In one embodiment the obtained thin film is annealed at a temperature above 400°C, preferably between 500°C and 1000°C, more preferred about 500°C-600°C.

The present invention further provides a thin film comprising MoO₃ obtained by the method according to the present invention.

The molybdenum oxides are deposited by the ALD technique using the Mo(CO)₆ precursor (molybdenum hexacarbonyl) in combination with an oxygen containing precursor. The process is performed through self limiting gas-to-surface reactions. The reaction steps of the method according to the present invention leads to the formation of MoO₃ on the surface of the substrate.

The process is repeated until desired thickness is achieved.

In one embodiment the Mo(CO)₆ precursor is introduced into the ALD reactor from an external reservoir using additional inert carrier gas flow, such as nitrogen, argon, helium or similar. The precursor was maintained at room temperature. The oxygen containing precursor according to the present invention is selected from the group consisting of O₃, H₂O, a mixture of O₃ and H₂O, plasma oxygen, N₂O, NO, NO₂ or H₂O₂.

In one aspect of the present invention O₃ is used as the oxygen containing precursors. The process does also work for deposition using solely H₂O as the oxygen containing precursor, but a notably smaller growth rate was experienced. In one embodiment of the invention a mixture of O₃ and H₂O is used as the oxygen containing precursor. The growth rate may vary dependent on the selected oxygen containing precursor.

For deposition of Mo-containing oxides that also contain other metal elements, the deposition approach has been to use individual cycles of the Mo-process as described above and similar processes for deposition of the other metal elements. In one example of deposition of a complex oxide comprising Mo and Co, the embodiment of the method according to the present invention where both O₃ and H₂O are used as oxygen precursor for deposition of Mo-O, was combined with the deposition process using Co(thd)₂ (thd = 2,2,6,6-tetramethylheptane-3,5-dione) and O₃ described by K. Klepper, O. Nilsen, H. Fjellvag, Thin Solid Films 515 (2007) 7772, for deposition of the Co-O part of the complex oxide.

The present invention will be described in more detail with reference to the enclosed figures where:

Figure 1 : Shows ALD growth rates OfMoO₃ as function of the deposition temperature. Figure 2: Shows the results of QCM measurements of the depositions of Mo-oxide.

Figure 3: Shows ALD growth rates of MoO₃ as function of the duration of the Mo- precursor pulse.

Figure 4: X-Ray diffraction pattern OfMoO₃ thin film on a substrate Si(111).

Figure 5: XRD diffractogram of the MoO₃ sample annealed at 600 ⁰C under air. Figure 6: Co content (as measured by XRF) versus fraction of Co pulses in Mo-Co-O films.

Figure 7: Shows the catalytic activity in ammonia decomposition reaction of reduced

CoMo-thin films.

Thin films containing molybdenum oxide have been deposited using a F- 120 Sat reactor (ASM Microchemistry) and Mo(CO)₆ (Molybdenum hexacarbonyls) as the metal containing precursor and a mixture of O₃ and H₂O as the oxygen containing precursor. The ozone was made by feeding 99.999% O₂ into an OT-020 ozone generator from Ozone Technology, giving an ozone concentration of 15 vol.% according to specifications. An ozone flow of ca. 500 cm³/min was used during the ozone pulses. During all depositions a background pressure of 3.5 mbar was obtained by applying a N₂ carrier gas flow of 300 cm³ min '. The carrier gas was produced in a Schmidlin UHPN3001 N₂ purifier with a claimed purity of 99.999% with regard to N₂+Ar content. The films were deposited on substrates of Si(111) single crystals which were used as obtained from the manufacturer.

The film is grown through alternating pulses of these precursors separated by purging by inert gas. The process is repeated until the desired film thickness is achieved. The process proved to give a growth rate of ca. 0.07 nm/cycle in the temperature range 157 — 175 °C. Above this temperature the precursor decomposes and provides uncontrolled growth. Below this range the reactivity OfMo(CO)₆ is too low for formation of film. This is illustrated on figure 1 showing the average growth rate over 1000 cycles at different temperatures.

This example applies Mo(CO)₆ as the Mo-containing precursor. This will react through self- limiting gas-to-surface reactions with an oxidizing gas such as O₃ and form MoO₃.

The oxygen containing gas as a parameter has also been studied at 167°C by QCM (quarts crystal microbalance). In a QCM measurement, the changes in measured frequency are correlated linearly to the changes of the mass of the sensor. In that way, it is possible to visualise the mass of what is deposited on the sensor.

In this study, several ALD depositions have been done at 167°C varying the oxygen containing gas pulse. The pulsing sequence was 1.4 s OfMo(CO)₆ followed by 0.6 s of purge before a pulse of 5.0 s of oxygen containing gas (water, ozone and a mix of water and ozone have been tested) followed by a purge time of 3.0 s. The result is illustrated on figure 2. The change in frequency as measured during the deposition of Mo-oxide shows that there is very little deposition of Mo-oxide when water is used as the only oxygen containing gas (curve 1); however, there are clear depositions when ozone (curve 2) or a combination of ozone and water (curve 3) is used as the oxygen containing gas. The detected increase in mass of deposited Mo-oxide is largest for the mix of water and ozone as the oxygen containing gas. Curve 4 and 5 show the use of H₂O and O₃ in sequence instead of as a mixture. Little difference is observed if water (curve 4) is used first and ozone thereafter or the opposite (curve 5). The effect of the precursor pulse parameter on the growth has been studied at 167⁰C and is shown in figure 3. Films have been deposited on substrates of Si(111) using 1000 cycles of a pulsing scheme: 0.8, 1.4, or 3.0 s pulse Of (Mo(CO)₆) followed by 1.1 s purge and 5.0 s of a mix of O₃ and H₂O as the oxygen containing gas, followed by a purge of 3.0 s. Almost the same thicknesses was obtained for the 3 different pulses. This result is illustrated on figure 3, which shows that we have an ALD growth of MoO₃ at 167°C.

The composition of one the films obtained at 167°C have been measured by X-ray photoelectron spectroscopy (XPS) recorded on a Kratos Axis Ultra Instrument, a conventional Al Ka anode was used at 15 kV and 10 mA as the source of X-ray radiation. The pressure in analysis chamber during the analyses was around 5*10^"9 Torr. The energy scale was calibrated by adjusting the C(Is) binding energy for the omnipresent air-borne hydrocarbons at 284.6 eV. The obtained graphs fitted perfectly well with the theoretical graphs OfMoO₃ proving the films to be of near stoichiometric MoO₃.

Thin film thicknesses have been measured by X-ray reflectometry (XRR) using a Siemens D5000 X-ray diffractometer equipped with a Gδbel-mirror which provides parallel Cu Ka radiation. The setup was also used to measure conventional X-ray diffraction (XRD) in reflection mode. X-ray diffraction analysis proved the films to be as deposited films to be amorphous for depositions in the range 152 to 173 ⁰C when deposited on Si(111) using 1000 cycles and a mix of water and ozone as the oxygen source.

Figure 4 is an example of the results obtained. Figure 4 shows the X-Ray diffraction pattern obtained for the film obtain at the following pulsing scheme 0.8 s Mo(CO)₆ followed by a purge of 1.1 s and 5.0 s of a mixture of ozone and water followed by a purge of 3.0 s. The deposition temperature was 167 ⁰C, a total of 1000 cycles was used and the substrate was Si(111). Shown in grey is the theoretical diffraction peaks of any crystalline Mo-oxide film and in black is the experimental diffraction graph obtained. The diffraction peak at 28° belongs to the substrate Si(111), and one can easily see that the Mo-oxide film is amorphous.

In order to obtain crystalline films, films were annealed for 15 minutes under air at 200⁰C, 300⁰C, 400 ⁰C, 500 ⁰C and 600 ⁰C. Annealing at 200 ⁰C, 300 ⁰C and 400 ⁰C did not result in crystallisation of the annealed Mo-oxide films, whereas annealing at 500 ⁰C and above resulted in a crystalline film.

Figure 5 shows the diffractogram of the resulting crystallisation of a film obtained after annealing at 600 °C in air for 15 min. The crystalline phase is identified as the orthorhombic α-MoO₃ phase (Inorganic File, Plate 18-1418, Mineral Power Diffraction File, 1986). The doted lines show the theoretical diffraction peaks which are not possible to observe due to too low intensity from the diffracting film.

In a further embodiment the present invention relates to the deposition of complex oxide thin films comprising molybdenum. This embodiment is illustrated by the deposition of Co-Mo-oxide with different compositions.

For all the depositions the following parameters were applied: The Mo(CO)₆ was maintained at room temperature and pulsed using 1.4 s followed by 1.1 s of purge (N₂). The oxygen containing gas was a mixture of H₂O and O₃ employed with a pulse of 5.0 s followed by a 1.5 s of purge.

The Co(thd)₂ was sublimed at 115⁰C and delivered by a pulse of 1.5 s followed by a purge of 0.3 s. Thereafter a pulse of 0.5 s of O₃ was introduced as the oxygen containing gas, followed by a purge of 1.5 s. The substrate was Si (111).

Two different numbers of cycles where tested for each pulse ratio: 500 cycles and 1500 cycles. The compositions of the obtained films were studied by XRF. Table 1 shows the obtained composition with different numbers of pulses and different number of cycles.

Table 1:

Table 1 clearly shows that films with different content are obtain with the different pulsing patterns.

Figure 6 shows the Co content (as measured by XRF) versus fraction of Co pulses in Mo-Co-O films grown on Si(111) at 165 ⁰C. The curve is calculated using eq. 1 and the coefficients UQ₀ = 5, U_M0 ⁼ 1. The precursor systems used are Mo(CO)₆ + (O₃/H₂O) and Co(thd)₂ + O₃ at 167 ⁰C.

A person skilled in the art would easily recognise that other types of complex oxides comprising molybdenum can be obtained by substituting some or all the Co precursor with other metal precursors.

Examples of possible interesting complex oxides include: Fe₂MoO₄, Al₂[MoO₄J₃, BaMoO₄, CaMoO₄, Cu₃Mo₂O₉, CuMoO₄, Eu₂Mo₃Oi₂, La₂MoO₆, Li₂MoO₄, MgMoO₄, MnMoO₄, Nd₂Mo₂O₇, MoNiO₄, MoSb₂O₆, V₂MoO₈, WMo₄Oi₄, Y₂Mo₂O₇, Yb₂Mo₂O₇, Zn₂Mo₃O₈, and Zr[MoO₄J₂.

To obtain these complex oxides precursors for the following elements would be utilized: Fe, Al, Ba, Ca, Cu, Eu, La, Li, Mg, Mn, Nd, Ni, Sb, V, W, Y, Zn, and Zr. Application example

Commercialization of proton-exchange membrane fuel cell can be realized only on the efficient production of CO-free hydrogen. Even a ppm level concentration of CO in the hydrogen feed is a serious threat on the efficiency of the fuel cell. The conventional process for the production of hydrogen is always associated with large amounts of CO which requires an additional process for the removal of CO in the hydrogen stream. Ammonia decomposition receives recent interest owing to its efficiency in producing CO-free hydrogen. Thus, an on-board generation of CO-free hydrogen can be realized with this process. Developing a new catalytic system for the decomposition of ammonia at lower temperatures is much appreciated. In this work, we have studied the efficiency of the CoMo-thin film catalysts in the ammonia decomposition reaction.

Catalytic testing: The ammonia decomposition reaction was carried out on a CoMo-thin film catalyst coated on quartz substrate. The catalyst coated quartz substrate was cut into small pieces that could fit in the quartz reactor with an inner diameter 0.3 cm. Prior to catalytic testing the thin film catalyst was reduced under the flow of pure hydrogen at 600 ⁰C for 2 h. The temperature was raised from RT to 600 ⁰C at a heating rate of 5 °C/min. The flow of hydrogen is 10 ml/min. After reduction, the reactor was cooled down to RT (5 °C/min) with continuous Ar flush (25 ml/min). Temperature programmed ammonia decomposition over the thin film catalyst was done by flowing NH₃ (25 ml/min) and Ar (25 ml/min) from RT to 900 ⁰C at a heating rate of 5 °C/min. The effluent gas was analyzed using a micro GC (Model: Agilent 3000 Micro GC) equipped with Molecular sieve column/PLOTU pre-column (Ar carrier gas) for detecting N₂ and H₂ and PLOTU column/PLOTQ pre-column (He carrier gas) for detecting NH₃. The columns were equipped with TCD detectors.

Results and Discussion: Three types of CoMo-thin film catalysts with varying thickness and composition deposited over quartz substrate were employed for catalyst testing. The conditions of the deposition and the properties of the different films employed in catalytic testing were given in Table 2. Table 2

The activities of these catalysts in ammonia decomposition reaction were compared in figure 7. Quartz substrate itself was found to be active in the decomposition OfNH₃ however, only at high temperature regime. The T_onSet for quartz substrate was found to be at around 506 ⁰C. A complete decomposition OfNH₃ was not achieved in this case even at temperatures of about 900 ⁰C. The sample MDl 118 showed better activity than quartz. The T_onSet for MDl 118 sample was observed at around 447 ⁰C. A complete decomposition (Tf_inai) of NH3 was achieved at temperatures of about 847 ⁰C. This shows that the CoMo-thin film catalyst is active in ammonia decomposition reaction. A reduction in the T_onSet and Tf_inai value was achieved by slight increase in the Co concentration (sample MDl 119; T_onset = 410 ⁰C and T_finai = 826 ⁰C). However, by increasing the concentration of Mo present in the thin film catalyst much efficient activity in the lower temperature regime was obtained (sample MDl 120; T_onset = 464 ⁰C and Tfjnai = 775 ⁰C). Thus, we prove that CoMo-thin film catalyst produced by ALD method is active for ammonia decomposition reaction. Further, we suggest that Mo favours low temperature NH₃ activation.

Conclusion: Mo enriched CoMo-thin films deposited by Atomic Layer Deposition technique renders after reduction catalysts that are active in the decomposition of ammonia to produce CO free hydrogen.

Claims

C l a i m s

1.

Method for depositing a thin film comprising molybdenum oxide on a surface of a substrate by ALD technique characterised in that the method comprises the steps of: a) reacting a pulse of a halogen- free Mo-precursor with the surface; b) purging with an inert gas or pump down of reactor; c) reacting a pulse of an oxygen containing precursor with the surface; and d) purging with an inert gas or pump down of reactor; thereby obtaining a molybdenum oxide containing film.

2.

Method according to claim 1, where the halogen- free Mo-precursor is selected from Mo(CO)₆, Mo(thd)₂, cycloheptatriene molybdenum tricarbonyl, pentamethyl- cyclopentadienyl-molybdenum dicarbonyl dimer and derivates thereof.

3.

Method according to claim 1 or 2, where the steps a) to d) are repeated to obtain the desired thickness of the film.

4.

Method according to claim 1 or 2, where the oxygen containing precursor is selected from the group consisting of O₃, H₂O, a mixture of O₃ and H₂O, plasma oxygen, N₂O,

NO₅ NO₂ Or H₂O₂.

5.

Method according to claim 4, where the oxygen containing precursor is a mixture of O₃ and H₂O.

6.

Method according to claim 1 or 2, wherein the method is performed within the temperature range between 140°C and 200°C, more preferably about 157-175°C.

7. Method according to claim 3, wherein the method further comprises deposition of one or more other compounds one or more times between or after the steps a) to d) are performed.

8.

Method according to claim 1 or 2, wherein the method further comprises the steps of e) reacting a pulse of a cobalt precursor with the surface, wherein the cobalt precursor is selected from the group comprising Co(thd)₂, Co(Cp)₂,

Co(substituted Cp)₂, Co(acac)₂ and Co(PrAMD); f) purging with an inert gas or pump down of reactor; g) reacting a pulse of an oxygen containing precursor with the surface; and h) purging with an inert gas or pump down of reactor; thereby obtaining a complex Mo-Co-O containing film.

9.

Method according to claim 8, where the steps a)-d) and e)-h) are respectively repeated an individually selected number of times and where all the steps are repeated to grow the thin film.

10.

Method according to any one of the previous claims, where the obtained thin film is annealed at a temperature above 400⁰C, preferably between 500°C and 1000°C, more preferred about 500°C-800°C, even more preferably between 500°C and 600°C.

11.

Thin film comprising MoO₃ obtained by the method according to any one of the claims

1-10.

12.

Use of a CoMo thin film prepared by reduction a complex Co-Mo-O film prepared by

ALD as an ammonia decomposition catalyst.