WO2015131981A1

WO2015131981A1 - Plasma treatment of substrates

Info

Publication number: WO2015131981A1
Application number: PCT/EP2015/000394
Authority: WO
Inventors: Pierre Descamps; Françoise MASSINES; Thomas Gaudy; Frederick Campeol; Vincent Kaiser
Original assignee: Dow Corning France; Centre National De La Recherche Scientifique (Cnrs)
Priority date: 2014-03-05
Filing date: 2015-02-21
Publication date: 2015-09-11

Abstract

Apparatus for depositing an oxide film on a substrate comprises a first chamber (13) defined by a dielectric housing (14) and having an inlet for process gas and an outlet (15), at least one first electrode (10,12) positioned between an inlet for process gas and the first chamber, means for introducing an oxide precursor into process gas flowing from the said inlet through the said first chamber (13) to the outlet (15), and means for applying a high voltage to the first electrode to generate a non-thermal equilibrium atmospheric pressure plasma in the first chamber (13); a second chamber (23) defined by a dielectric housing and having an inlet for process gas comprising oxygen and an outlet (25), at least one second electrode (21,22) positioned between the inlet for process gas comprising oxygen and the second chamber (23), the first chamber (13) and the second chamber (23) being interconnected at their outlets, and means for applying a high voltage to the second electrode to generate a non-thermal equilibrium atmospheric pressure plasma in the second chamber; and support means (37) for the substrate (35) positioned adjacent to the interconnected outlets of the first chamber and second chamber.

Description

PLASMA TREATMENT OF SUBSTRATES

BACKGROUND OF THE INVENTION

[0001] This invention relates to treating a substrate using a plasma system. In particular it relates to the deposition of an oxide film on a substrate from a non-local equilibrium atmospheric pressure plasma.

[0002] When matter is supplied with energy, it typically transforms from a solid to a liquid and, then, to a gaseous state. Continuing to supply energy causes the system to undergo yet a further change of state in which neutral atoms or molecules of the gas are broken up by energetic collisions to produce negatively charged electrons, positive or negatively charged ions and other excited species. This mix of charged and other excited particles exhibiting collective behaviour is called "plasma", the fourth state of matter. Due to their free electrical charge (free to move in response to application of a field), plasmas are highly influenced by external electromagnetic fields, which make them readily controllable. Furthermore, their high energy content/specie allows them to achieve processes which are impossible or difficult through the other states of matter, such as by liquid or gas processing.

[0003] The term "plasma" covers a wide range of systems whose density and temperature vary by many orders of magnitude. Some plasmas are very hot, for example a flame based plasma as formed by a plasma torch, and all their microscopic species (ions, electrons, etc.) are in approximate thermal equilibrium, the energy input into the system being widely distributed through atomic/molecular level collisions. Other plasmas, however, have their constituent species at widely different temperatures and are said to be in "non-local thermal equilibrium". In these non-local thermal equilibrium plasmas the free electrons are very hot with temperatures of many thousands of Kelvin (K) whilst the neutral and ionic species remain cool (temperatures orders of magnitude below those of electrons). Because the free electrons have almost negligible mass, the total system heat content is low and the plasma may operate close to room temperature thus allowing the processing of temperature sensitive materials, such as plastics or polymers, without imposing a damaging thermal burden onto the sample. However, the hot electrons create, through high energy collisions, a rich source of radicals and excited species with a high chemical potential energy capable of profound chemical and physical reactivity. It is this combination of low temperature operation plus high reactivity which makes non-thermal plasma technologically important and a very powerful tool for manufacturing and material processing, capable of achieving processes which, if achievable at all without plasma, would require very high temperatures or noxious and aggressive chemicals. [0004] WO 03/097245 and WO 03/101621 describe applying an atomised coating material onto a substrate to form a coating. The atomised coating material, upon leaving an atomizer such as an ultrasonic nozzle or a nebuliser, passes through an excited medium (plasma) to the substrate. The substrate is positioned remotely from the excited medium. The plasma is generated in a pulsed manner.

[0005] WO2006/048649 describes generating a non-equilibrium atmospheric pressure plasma incorporating an atomised surface treatment agent by applying a radio frequency high voltage to at least one electrode positioned within a dielectric housing having an inlet and an outlet while causing a process gas to flow from the inlet past the electrode to the outlet. The electrode is combined with an atomiser for the surface treatment agent within the housing. The non-equilibrium atmospheric pressure plasma extends from the electrode at least to the outlet of the housing so that a substrate placed adjacent to the outlet is in contact with the plasma, and usually extends beyond the outlet. WO2006/048650 teaches that the flame-like non-equilibrium plasma discharge, sometimes called a plasma jet, could be stabilized over considerable distances by confining it to a long length of tubing. This prevents air mixing and minimises quenching of the flame-like non-equilibrium plasma discharge. The flame-like non-equilibrium plasma discharge extends at least to the outlet, and usually beyond the outlet, of the tubing.

[0006] WO03/085693 describes an atmospheric plasma generation assembly having a reactive agent introducing means, a process gas introducing means and one or more multiple parallel electrode arrangements adapted for generating a plasma. The assembly is adapted so that the only means of exit for a process gas and atomised liquid or solid reactive agent introduced into said assembly is through the plasma region between the electrodes. The assembly is adapted to move relative to a substrate substantially adjacent to the electrodes outermost tips. Turbulence may be generated in the plasma generation assembly to ensure an even distribution of the atomised spray.

[0007] WO2009/034012 describes a process for coating a surface, in which an atomized surface treatment agent is incorporated in a non-equilibrium atmospheric pressure plasma generated in a noble process gas or an excited and/or ionised gas stream resulting therefrom, and the surface to be treated is positioned to receive atomized surface treatment agent which has been incorporated therein, is characterized in that the particle content of the coating formed on the surface is reduced by incorporating a minor proportion of nitrogen in the process gas. [0008] WO2012/010299, WO2012/146348 and WO2013/068085 describe a process for plasma treating a substrate comprises applying a radio frequency high voltage to at least one electrode positioned within a dielectric housing having an inlet and an outlet while causing a process gas to flow from the inlet past the electrode to the outlet, thereby generating a non-equilibrium atmospheric pressure plasma. An atomised or gaseous surface treatment agent is incorporated in the non-equilibrium atmospheric pressure plasma. The substrate is positioned adjacent to the plasma outlet so that the surface is in contact with the plasma and is moved relative to the plasma outlet. The flow of process gas and the gap between the plasma outlet and the substrate are controlled so that the process gas has a turbulent flow regime within the dielectric housing.

[0009] There is industrial interest in the deposition of a dense silicon oxide film for a broad range of applications, for example for surface passivation of silicon solar cells, optical applications or for the deposition of an oxygen barrier in packaging applications. Deposition of a purely inorganic silicon oxide film requires full elimination of any carbon present.

Formation of a silicon oxide film requires a silicon compound as a starting material (a precursor of the silicon oxide). Deposition of silicon oxicarbide film starting from an organosilicon compound precursor using an atmospheric pressure plasma jet has been described in WO2012/010299, WO2012/146348 and WO2013/068085. In applications where carbon free films are required, carbon elimination from the silicon oxicarbide film is achieved by a subsequent annealing of the layer at high temperature (at least 600°C) in an oxygen rich atmosphere, as described in WO2013/180856.

[0010] A process in which oxide film coatings can be formed from an organosilicon or other organometallic precursor using an atmospheric pressure plasma jet without the use of such high temperatures would have considerable advantages. There is an advantage in reduced processing steps and reduced energy requirements. Moreover, for some applications such as deposition of an oxygen barrier in packaging applications, avoidance of high

temperatures is a mandatory requirement.

SUMMARY OF THE INVENTION

[001 1] An apparatus according to the present invention for depositing an oxide film on a substrate comprises a first chamber defined by a dielectric housing and having an inlet for process gas and an outlet, at least one first electrode positioned between an inlet for process gas and the first chamber, means for introducing an oxide precursor into process gas flowing from the said inlet through the said first chamber to the outlet, and means for applying a high voltage to the first electrode to generate a non-thermal equilibrium atmospheric pressure plasma in the first chamber; a second chamber defined by a dielectric housing and having an inlet for process gas comprising oxygen and an outlet, at least one second electrode positioned between the inlet for process gas comprising oxygen and the second chamber, the first chamber and the second chamber being interconnected at their outlets, and means for applying a high voltage to the second electrode to generate a non-thermal equilibrium atmospheric pressure plasma in the second chamber; and support means for the substrate positioned adjacent to the interconnected outlets of the first chamber and second chamber.

[0012] A process according to the present invention for depositing an oxide film on a substrate comprises flowing an oxygen-free process gas from a first inlet past at least one first electrode into a first chamber defined by a dielectric housing while applying a high voltage to the first electrode, and introducing an oxide precursor into the process gas flowing from the first inlet past the first electrode, thereby generating a non-thermal equilibrium atmospheric pressure plasma free of oxygen in the first chamber containing activated process gas species and precursor fragments resulting from interaction of the precursor with the plasma and activated process gas species; and flowing a process gas comprising oxygen from a second inlet past at least one second electrode into a second chamber defined by a dielectric housing while applying a high voltage to the second electrode, thereby generating a non-thermal equilibrium atmospheric pressure plasma in the second chamber containing oxygen activated species, the first chamber and the second chamber being interconnected at their outlets so that the said precursor fragments and the oxygen activated species interact, and supporting the substrate adjacent to the interconnected outlets of the first chamber and second chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will be described with reference to the accompanying drawings, of which Figure 1 is a diagrammatic cross section of an apparatus according to the invention for generating a non-equilibrium atmospheric pressure plasma and depositing an oxide film on a substrate from the plasma;

Figure 2 is a diagrammatic cross section of an alternative apparatus according to the invention for generating a non-equilibrium atmospheric pressure plasma and depositing an oxide film on a substrate from the plasma;

Figure 3 is a graph showing the carbon content of films deposited in Example 1 below; and

Figure 4 is a graph showing the carbon content of films deposited in Example 2 below.

DETAILED DESCRIPTION OF THE DRAWINGS

[0014] The apparatus of the invention can conveniently be configured to have an inner chamber and an outer chamber surrounding the inner chamber, although side by side chambers can be used. For example, the first chamber can be an inner chamber with the second chamber surrounding the first chamber, or the second chamber can be an inner chamber with the first chamber surrounding the second chamber. The dielectric housing forming the wall of the inner chamber can also form the inner wall of the surrounding chamber.

[0015] The inner chamber can for example be substantially cylindrical about an axis from the inlet for process gas to the substrate, with the surrounding chamber forming a collar around the inner chamber. The inner and outer chambers can be of various alternative shapes, for example they can be rectangular. Side by side chambers can for example be semicircular or rectangular chambers having a dielectric housing forming a common wall between the two chambers.

[0016] The first electrode and the second electrode can for example each be needle electrodes. The electrodes can alternatively have any other shape such as planar or concentric.

[0017] The apparatus of Figure 1 comprises two first electrodes (1 1 , 12) positioned at the entry to a first chamber (13) which is an inner chamber and forms a plasma tube defined by a dielectric housing (14) and having an outlet (15). The electrodes (1 1 , 12) are needle electrodes both having the same polarity and are connected to a suitable power supply.

[0018] If the apparatus is configured to have an inner chamber and an outer chamber surrounding the inner chamber, the apparatus may have two or more, for example at least three, electrodes having the same polarity spaced circumferentially in the surrounding chamber so that a curtain of non-thermal equilibrium atmospheric pressure plasma extends circumferentially in the surrounding chamber.

[0019] The apparatus of Figure 1 comprises four second electrodes positioned at the entry to a second chamber (23) surrounding the first chamber (13). The dielectric housing (14) of the first chamber (13) forms the inner wall of the second chamber (23) and a dielectric housing (24) forms the outer wall of the second chamber (23). The second electrodes are circumferentially spaced around the second chamber (23). Two of the second electrodes (21 and 22) are seen in Figure 1. The second chamber (23) forms a plasma tube having an outlet (25). The second electrodes (21 , 22) are needle electrodes all having the same polarity and are connected to a suitable power supply.

[0020] Although the power supply to the first and second electrodes may operate at any frequency between 0 to 14 MHz (0 MHz means direct current discharge), it is preferably a low radio frequency power supply as known for plasma generation, that is in the range 3kHz to 300kHz. The root mean square potential of the power supplied is generally in the range 1kV to 100kV, preferably between 4kV and 30kV.

[0021] The first electrode can for example be positioned within a channel through which the process gas flows. The first electrodes (1 1 , 12) are each positioned within a narrow channel (16 and 17 respectively), for example of radius 0.1 to 5mm, optionally 0.2 to 2mm, greater than the radius of the electrode, communicating with first chamber (13). Similarly the second electrode can for example be positioned within a channel through which the process gas comprising oxygen flows.

[0022] The electrodes (21 , 22) are each positioned within a narrow channel (26 and 27 respectively), for example of radius 0.1 to 5mm, optionally 0.2 to 2mm, greater than the radius of the electrode, communicating with second chamber (23). The tip of each needle electrode (1 1 , 12, 21 and 22) is positioned close to the exit of the associated channel (16, 17, 26 and 27 respectively). The ratio of length to hydraulic diameter of each channel (16, 17, 26, 27) surrounding an electrode may be at least 0:1 , optionally at least 20:1. The hydraulic diameter, D_H, is defined by the equation D_H = 4A/P, where A is the cross-sectional area of the tube or channel and P is the wetted perimeter of the cross-section. The wetted perimeter is the perimeter which is in contact with the fluid (the process gas). For example the electrode (1 1 ) can be 1 mm in diameter and positioned centrally within a channel ( 6) of length 30mm and internal diameter 2 mm. The hydraulic diameter of the channel (16) is 1 mm and the channel has a ratio of length to hydraulic diameter of 30:1. The other electrodes (12, 21 , 22) and channels (17, 26, 27) can be of the same dimensions. [0023] Oxygen-free process gas is fed to an entry chamber (19) whose outlets are the channels (16, 17) surrounding the first electrodes. The entries to channels (16, 17) thus form the inlet to first chamber (13) for oxygen-free process gas.

[0024] Process gas comprising oxygen is fed to an entry chamber (29) whose outlets are the channels (26, 27) surrounding the second electrodes. The entries to channels (26, 27) thus form the inlet to second chamber (23) for process gas comprising oxygen.

[0025] The direction of flow of process gas from the inlet channels (16, 17) of the first chamber (13) to the outlet (15) of the first chamber can be arranged to be substantially parallel to the direction of flow of process gas comprising oxygen from the inlet channels (26, 27) of the second chamber (23) to the outlet (25) of the second chamber.

[0026] Each entry chamber (19 and 29) is made of a heat resistant, electrically insulating material which is fixed in an opening in the base of a metal box. The metal box is grounded but grounding of this box is optional. The entry chambers (19, 29) can alternatively be made, of an electrically conductive material, provided that all the electrical connections are insulated from the ground, and any part in potential contact with the plasma is covered by a dielectric.

[0027] The dielectric housings (14 and 24) can be made of any dielectric material.

Experiments described below were carried out using quartz dielectric housings (14, 24) but other dielectric materials, for example glass, ceramic or plastic material can be used. Plastic materials include polyamide, polypropylene or polytetrafluoroethylene which is sold under the trade mark Teflon'. The dielectric housings (14, 24) can be formed of a composite material, for example a fiber reinforced plastic designed for high temperature resistance.

[0028] As an electric potential is applied to the first and second electrodes, an electric field is generated around the tips of the electrodes which accelerates charged particles in the gas forming a plasma. A non-thermal equilibrium atmospheric pressure plasma derived from oxygen-free process gas containing oxygen-free process gas activated species is formed in first chamber (13). A non-thermal equilibrium atmospheric pressure plasma containing oxygen activated species is formed in the second chamber (23). The sharp point at the tips of the electrodes aids the process, as the electric field density is inversely proportional to the radius of curvature of the electrode. Needle electrodes (such as 1 , 2, 2 , 22) possess the benefit of creating a gas breakdown using a lower voltage source because of the enhanced electric field at the sharp extremity of the needles. [0029] The power supply to the first and second electrodes is a low frequency power supply as known for plasma generation, that is in the range 3kHz to 300kHz. Our most preferred range is the very low frequency (VLF) 3kHz - 30 kHz band, although the low frequency (LF) 30kHz - 300 kHz range can also be used successfully. One suitable power supply is the Haiden Laboratories Inc. PHF-2K unit which is a bipolar pulse wave, high frequency and high voltage generator. It has a faster rise and fall time (< 3 με) than conventional sine wave high frequency power supplies. Therefore, it offers better ion generation and greater process efficiency. The frequency of the power supply of this unit is also variable (1 - 100 kHz) to match the plasma system. An alternative suitable power supply is an electronic ozone transformer such as that sold under the reference ETI1 10101 by the company Plasma Technics Inc. It works at fixed frequency and delivers a maximum power of 100 Watt with a working frequency of 20 kHz

[0030] The means for introducing the oxide precursor into the process gas can for example be an atomiser, or a bubbler in which process gas is bubbled through a bath of oxide precursor to form a flow of process gas containing precursor, or a flash evaporation device. The apparatus can if desired comprise at least two first electrodes having the same polarity surrounding the means for introducing the oxide precursor into the process gas.

[0031] An atomiser (31 ) having an inlet (32) for oxide precursor is situated adjacent to the first electrode channels (16, 17) and has atomising means (not shown) and an outlet (33) feeding oxide precursor to the inner chamber (13). The oxide precursor is decomposed by the plasma in first chamber (13), forming a non-thermal equilibrium atmospheric pressure plasma free of oxygen containing derived from the precursor fragments resulting from interaction of the precursor with process gas activated species.

[0032] The atomiser (31) preferably uses a gas to atomise the surface treatment agent. For example the process gas used for generating the plasma is used as the atomizing gas to atomise the surface treatment agent. The atomizer (31 ) can for example be a pneumatic nebuliser, particularly a parallel path nebuliser such as that sold by Burgener Research Inc. of Mississauga, Ontario, Canada, under the trade mark Ari Mist HP, or that described in US Patent 6634572. The atomizer can alternatively be an ultrasonic atomizer in which a pump is used to transport the liquid surface treatment agent into an ultrasonic nozzle and subsequently it forms a liquid film onto an atomising surface. Ultrasonic sound waves cause standing waves to be formed in the liquid film, which result in droplets being formed. The atomiser preferably produces drop sizes of from 1 to 100μΐη, more preferably from 1 to 50μήΐ. Suitable atomisers for use in the present invention include ultrasonic nozzles from Sono-Tek Corporation, Milton, New York, USA. Alternative atomisers may include for example electrospray techniques, methods of generating a very fine liquid aerosol through electrostatic charging. The most common electrospray apparatus employs a sharply pointed hollow metal tube, with liquid pumped through the tube. A high-voltage power supply is connected to the outlet of the tube. When the power supply is turned on and adjusted for the proper voltage, the liquid being pumped through the tube transforms into a fine continuous mist of droplets. Inkjet technology can also be used to generate liquid droplets without the need of a carrier gas, using thermal, piezoelectric, electrostatic and acoustic methods.

[0033] While it is preferred that the atomiser (31 ) is mounted within the housing (14), an external atomiser can be used. This can for example feed an inlet tube having an outlet in similar position to outlet (33) of atomizer (31 ). Alternatively the surface treatment agent, for example in a gaseous state, can be incorporated in the flow of process gas entering chamber (19) either from the channels (16, 17) or through a tube positioned at the location of the atomizer. For example the means for introducing the oxide precursor into the process gas can be a bubbler in which process gas is bubbled through a bath of oxide precursor before being fed to entry chamber (19).

[0034] The means for introducing the oxide precursor into the process gas can alternatively be a flash evaporation device.

[0035] In a further alternative the electrode can be combined with the atomizer in such a way that the atomizer acts as the electrode. For example, if a parallel path atomizer is made of conductive material, the entire atomizer device can be used as an electrode. Alternatively a conductive component such as a needle can be incorporated into a non-conductive atomizer to form the combined electrode-atomiser system

[0036] The substrate (35) to be treated is positioned across the outlet (15) of the first plasma tube chamber (13) and the outlet (25) of the second plasma tube chamber (23). The substrate (35) is laid on a support (37, 38). The substrate (35) is arranged to be movable relative to the plasma tube outlets (15, 25). The support can for example be a dielectric layer (37) covering a metal supporting plate (38). The dielectric layer (37) is optional. The metal plate (38) as shown is grounded but grounding of this plate is optional. If the metal plate (38) is not grounded, this may contribute to the reduction of arcing onto a conductive substrate, for example a silicon wafer.

[0037] The outlet for process gas from the apparatus can be a gap between the outer wall of the surrounding chamber and the supported substrate, and the outlet for process gas from the inner chamber (which is also an interconnected outlet for process gas from the surrounding chamber) can be a gap between the wall of the inner chamber and the supported substrate, so that process gas flowing through outlet of the inner chamber flows outwardly through the surrounding chamber to the outlet of the apparatus. In such an arrangement process gas and/or plasma flowing from the inner chamber is forced to cross and interact with process gas and/or plasma flowing from the surrounding chamber. The two flows combine to form a wall jet flowing through the outlet gap between the outer wall of the surrounding chamber and the supported substrate. More specifically, if as in Figure 1 the first chamber (13) is an inner chamber with the second chamber (23) surrounding the first chamber, process gas and/or plasma containing activated process gas species and precursor fragments resulting from precursor interaction with the plasma flowing from the inner chamber (13) is forced to interact with process gas and/or plasma containing oxygen activated species flowing from the surrounding chamber (23). If the second chamber is an inner chamber with the first chamber surrounding the second chamber, process gas and/or plasma containing oxygen activated species flowing from the inner chamber is forced to interact with process gas and/or plasma containing precursor fragments flowing from the surrounding chamber.

[0038] In an arrangement comprising side by side chambers, for example semicircular or rectangular chambers, having a dielectric housing forming a common wall between the two chambers, the outlet for process gas from the apparatus can be a gap between the outer wall (the dielectric housing other than the common wall) of one chamber and the supported substrate, and the outlet for process gas from the other chamber (which is also an

interconnected outlet) can be a gap between the common wall and the supported substrate, with the outer wall of said other chamber having only a minimum gap from the supported substrate. The flows of process gas and/or plasma are thus forced to interact as they flow towards the outlet gap between the outer wall of one chamber and the supported substrate.

[0039] The outlet for process gas from the apparatus of Figure 1 is a gap (40) between the outer wall (24) of the surrounding chamber (23) and the supported substrate (35). The outlet for process gas from the inner chamber (13) is a gap (30) between the wall (14) of the inner chamber and the supported substrate (35). The gap (30) between the wall (14) of the inner chamber (13) and the supported substrate (35) also forms a possible outlet from the second chamber (23), so that the first chamber (13) and the second chamber (23) are

interconnected at their outlets.

[0040] When an electric potential is applied to the first electrodes (1 1 , 12), oxygen-free process gas is fed through channels (16, 17) and precursor is introduced through atomiser (31), a non-thermal equilibrium atmospheric pressure plasma derived from oxygen-free process gas is formed in first chamber (13). The atmospheric pressure plasma derived from oxygen-free process gas contains activated process gas species and precursor fragments resulting from interaction of the precursor with activated process gas species. This atmospheric pressure plasma and/or process gas containing precursor fragments flows through the gap (30) between the wall (14) of the inner chamber and the supported substrate (35). When an electric potential is applied to the second electrodes (21 , 22) and process gas comprising oxygen is fed through channels (26, 27) a non-thermal equilibrium

atmospheric pressure plasma containing oxygen activated species is formed in the second chamber (23). The plasma and/or process gas containing precursor fragments contacts atmospheric pressure plasma containing oxygen activated species and/or process gas containing plasma-activated oxygen species in the region of the gap (30) and intermingles with the plasma-activated oxygen species as it flows from the outlet gap (30) to the outlet gap (40).

[0041] The oxide precursor can be a liquid compound of a metal or metalloid whose oxide is required to be deposited on the substrate. The oxide precursor can for example be an organometallic compound. One example of an oxide often required as a coating on a substrate is silicon oxide. The oxide precursor can for example be an organosilicon compound such as tetraethyl orthosilicate Si(OC₂H₅)₄ and tetramethylcyclotetrasiloxane (CH₃(H)SiO)₄. Using the process and apparatus of this invention, a dense, carbon free silicon oxide film can be deposited on the substrate (35) from such an organosilicon compound as oxide precursor. The oxide film is deposited on that part of the substrate (35) which is at the time of deposition in contact with the first chamber ( 3) or the second chamber (23).

[0042] Other organometallic compounds suitable for forming oxide coatings by the process of the invention include metal alkoxides such as titanates, tin alkoxides, zirconates, alkoxides of germanium and erbium, alkoxides of aluminium, alkoxides of zinc or alkoxides of indium and/or tin. Organic compounds of aluminium can be used to deposit alumina coatings on substrates, and a mixture of indium and tin alkoxides can be used to deposit a transparent conductive indium tin oxide coating film.

[0043] The process gas which is fed through the channels (16, 17) to the first chamber (13) can for example be an inert gas such as argon, helium, nitrogen or a mixture of two or all of these. When the oxide precursor is introduced through an atomiser (31 ) or a gas flow bubbler, the carrier gas for the precursor can also be an inert gas such as argon, helium, nitrogen or a mixture of two or all of these and may conveniently be the same as the process gas which is fed through the channels (16, 17). [0044] The process gas comprising oxygen which is fed through the channels (26, 27) to the first chamber (23) can be oxygen but is typically a mixture of an inert gas with oxygen. The inert gas can for example be argon, helium, nitrogen or a mixture of two or all of these. One example of a process gas comprising oxygen is a mixture of 90 to 99.5% by volume helium or argon with 0.5 to 10% oxygen.

[0045] The velocity of the oxygen-free process gas flowing past the first electrodes (1 1 , 12) through channels (16, 17) can for example be at least 3.5 m/s, such as 3.5 m/s up to 70 or 100 m/s, usually at least 5 m/s up to 50 m/s, particularly 10 m/s up to 30 m/s. Similarly the velocity of the process gas comprising oxygen flowing past the second electrodes (21 , 22) through channels (26, 27) can for example be at least 3.5 m/s, such as 3.5 m/s up to 70 or 100 m/s, usually at least 5 m/s up to 50 m/s, particularly 10 m/s up to 30 m/s. If the atomiser (31 ) uses process gas such as helium as the atomizing gas to atomise the oxide precursor, the velocity of the process gas which is injected into the first chamber through the atomiser (31 ) can be greater than 100 m/s, for example be up to 1000 or 1500 m/s.

[0046] The flow rate of the oxygen-free process gas flowing through the channels (16, 17) past the electrodes (1 1 , 12) can for example be at least 0.5 litres/minute (Urn) and below 10 l/m, for example 0.5 l/m up to 3l/m, such as 0.5 l/m up to 2 l/m. The flow rate of the process gas comprising oxygen flowing past the second electrodes (21 , 22) through channels (26, 27) can for example be at least 0.5 l/m and below 10 l/m, for example up to 3l/m or 2 l/m. The flow rate of process gas used as atomising gas through the atomiser (31 ) can for example be at least 0.5 l/m and can be up to 2 or 2.5 l/m.

[0047] The flow rate of oxide precursor through the atomiser can for example be in the range 0.2 μΙ/min up to 1 or 2 μΙ/min. The molar ratio of oxygen fed to the second chamber (23) to oxide precursor fed to the atomiser (31 ) can for example be in the range 60: 1 or 100: 1 up to 5000: 1 or 15000: 1 , optionally 800: 1 up to 3000: 1 .

[0048] An alternative apparatus is shown in Figure 2. Components which are the same as those of Figure 1 are numbered similarly. The apparatus of Figure 2 has only two second electrodes (21 and 22) positioned diametrically opposed in the outer (second) chamber (23). The second electrodes (21 and 22) are not surrounded by channels but simply protrude by 1 cm into the second chamber (23). Process gas comprising oxygen is fed to the second chamber (23) by lateral pipes (28, 28a) having inlets facing the tips of the electrodes (21 and 22 respectively). A non-thermal equilibrium atmospheric pressure plasma containing oxygen activated species is formed in the second chamber (23), and interacts with the non-thermal equilibrium atmospheric pressure plasma containing precursor fragments generated in the first chamber (13), as described above.

[0049] We have found that generating a non-thermal equilibrium atmospheric pressure plasma containing oxygen activated species in a second chamber (23), and causing the nonlocal thermal equilibrium atmospheric pressure plasma containing activated species and precursor fragments generated in the first chamber (13) to interact with the oxygen activated species, produces a better oxide film than including oxygen in the process gas fed to the first chamber or injecting oxygen into the first chamber. The oxide film produced has a higher density, is free from powder and contains little or no carbon remaining from the organosilicon oxide precursor. When oxygen is included in the process gas fed to a single plasma chamber or is injected into a single plasma chamber, oxygen activated species can react with the precursor fragments resulting from interaction of the precursor with activated process gas species close to the point at which precursor is injected, leading to oxide formation in the gas phase which forms particles detected as a powdery film on the substrate. Moreover activated oxygen species reacting at the top of the reactor with the precursor fragments to create powder are deactivated and are no longer available at substrate level to contribute to carbon elimination from the film on the substrate.

[0050] The oxide film can be deposited on the substrate at a temperature below 200°C, usually below 100°C and deposition of the oxide film may not expose the substrate to temperatures significantly above ambient temperature.

[0051] The process of the invention, using an organosilicon compound or other

organometallic compound as oxide precursor, can form a dense oxide film without needing a high temperature annealing step after deposition of the film.

[0052] The process of the invention is suitable for the deposition of a dense silicon oxide film on a substrate for a broad range of applications such as surface passivation of silicon solar cells, optical applications or for the deposition of an oxygen barrier in packaging applications. More generally the process is suitable for coating electronic equipment including textile and fabric based electronics printed circuit boards, displays including flexible displays, and electronic components such as semiconductor wafers, resistors, diodes, capacitors, transistors, light emitting diodes (LEDs), laser diodes, integrated circuits, (ICs), IC die, IC chips, memory devices, logic devices, connectors, keypads, or fuel cells. Optical

components such as lenses including contact lenses can be coated.

[0053] The oxide film deposited is generally dense and carbon-free and does not need further annealing. For uses such as surface passivation of silicon solar cells and deposition on other electronic equipment a further heat treatment can be applied if desired after deposition of the oxide film; compared to known processes less heat treatment will be required to achieve an equal density of the oxide film. For applications such as packaging, where heat treatment could damage the packaging or the goods packaged, the oxide film deposited by the process of the invention at ambient temperature will generally be sufficiently dense and robust to need no heat treatment.

[0054] The invention is illustrated by the following Examples.

Examples 1 to 3

[0055] Silicon oxide films were deposited on a silicon wafer substrate at ambient

temperature using the apparatus of Figure 2, and in a comparative example using the apparatus described in Figure 1 and Example 1 of WO 2012/010299.

[0056] In the comparative example, two 1 mm diameter needle electrodes are surrounded by dielectric creating 2mm diameter channels feeding process gas to a quartz dielectric housing of diameter 18mm and length 75mm. The gap between the quartz housing and the silicon wafer substrate was 1 mm. The process gas used was a mixture of argon with oxygen (to promote oxide formation) and was fed at a flow comprised between 0 and 10 litres/minute. The oxide precursor tetramethylcyclotetrasiloxane was fed through the atomiser at a flow rate between 1 and 5pl/min. The ratio of oxygen to argon in the process gas was kept constant equal to 98:2 by volume. By varying both the flow of oxygen containing gas and the precursor flow, we varied the molar ratio of oxygen to tetramethylcyclotetrasiloxane between 0 and 440:1. A stable non-equilibrium atmospheric pressure plasma was formed inside the dielectric housing and a low porosity film was formed on the silicon wafer substrate for molar ratio of oxygen to tetramethylcyclotetrasiloxane < 100.

[0057] In Examples 1 to 3 according to the present invention, two 1 mm diameter needle electrodes (11 , 12) are surrounded by dielectric creating 2mm diameter channels (16, 17) feeding process gas to an inner quartz dielectric housing (14) of diameter 18mm and length 75mm. The process gas fed through channels (16, 17) was a mixture of argon flowing at 2.5 standard litres/minute (slm) and nitrogen flowing at 0.3 slm. The oxide precursor

tetramethylcyclotetrasiloxane was fed through the atomiser (31 ). A stable non-equilibrium atmospheric pressure plasma was formed inside the dielectric housing (14) in each of Examples 1 to 3.

[0058] The apparatus in the Examples according to the present invention also comprised two 1 mm diameter needle second electrodes (21 , 22) extending 1cm. into an outer chamber (23) defined by dielectric housing (14) and an outer wall (24). The chamber (23) surrounds the first chamber (13). The second electrodes (21 , 22) are diametrically opposed in the second chamber (23). Process gas comprising a mixture of 90% by volume argon with 10% oxygen was fed through inlet pipes (28, 28a) to second chamber (23) at 4 litres/minute. A stable non-equilibrium atmospheric pressure plasma containing oxygen activated species was formed in the second chamber (23). The gap (30) between the quartz housing (13) and the silicon wafer substrate (35) was 1 mm, and the gap (40) between the outer quartz housing (24) and the silicon wafer substrate (35) was also 1 mm.

[0059] The oxide precursor tetramethylcyclotetrasiloxane was fed through the atomiser (31 ) at 15 μΙ/min in Example 1 , at 10 μΙ/min in Example 2 and at 3 μΙ/min in Example 3. The molar ratio of oxygen fed to the four second channels (26, 27) to

tetramethylcyclotetrasiloxane fed through the atomiser (31 ) was 280:1 in Example 1 , 420:1 in Example 2 and 1400:1 in Example 3. A smooth, low porosity film free of silica particles was formed on the silicon wafer substrate in each Example.

[0060] For both the comparative example and the Examples according to the present invention the composition of the film deposited was measured by infrared spectroscopy with Fourier transform (FTIR), in particular by measuring the ratio between the SiC peak around 1200cm^"1 and the Si-O-Si peak at 050cm^"1. This ratio is plotted in Figure 3. The x-axis in Figure 3 shows the molar ratio of oxygen to tetramethylcyclotetrasiloxane fed to the apparatus. The y-axis shows the carbon content of the film deposited as calculated from the FTIR results. The diamonds show the results of the comparative experiments using the process of WO 2012/010299. The squares show the results obtained in Examples 1 to 3 of the present invention.

[0061] Figure 3 shows that with the apparatus of WO 2012/010299, there is a sharp decrease of the carbon content of the film deposited when adding small amounts of oxygen. Then, we observe a plateau where any further oxygen addition has no additional impact on the oxygen content in the film. We observe powder formation in the dielectric tube for these conditions, active oxygen species being consumed for forming silica particles. Using the apparatus of the present invention, the carbon content of the deposited film is decreased by at least a factor of 2. Films deposited in Examples 1 to 3 were smooth and free of silica particles.

Example 4

[0062] A silicon oxide film was deposited on a silicon wafer substrate using the apparatus of the present invention as shown in Figure 2 and having the dimensions described in Example 1. The process gas fed through channels (16, 17) was a mixture of argon flowing at 2.5 standard litres/minute (slm) and nitrogen flowing at 0.3 slm. The process gas fed to second chamber (23) through inlet pipes (25, 28) was a mixture of 90% by volume argon with 10% oxygen and was fed at 4 litres/minute. The oxide precursor tetraethyl orthosilicate (TEOS) was fed through the atomiser (31 ) at 15 μΙ/min. The molar ratio of oxygen fed to the four second channels (26, 27) to TEOS fed through the atomiser (31) was 250:1. Stable non- equilibrium atmospheric pressure plasmas were formed inside the dielectric housing (14) and in the second chamber (23). A smooth, low porosity film free of silica particles was formed on the silicon wafer substrate (35).

[0063] Comparative experiments were carried out using the apparatus described in Figure 1 and Example 1 of WO 20 2/010299, having the dimensions described above. The process gas fed to the channels surrounding the needle electrodes was a mixture of argon and oxygen. The oxide precursor tetraethyl orthosilicate was fed through the atomiser (31) at 15 μΙ/min in most experiments with the flow of Ar/0₂ gas mixture being varied between 0 and 10 liters/min. The ratio of oxygen to argon in the process gas is kept constant equal to 98:2 by volume, equivalent to a molar ratio of oxygen to tetraethyl orthosilicate between 0 and 600:1. In one experiment the feed rate of tetraethyl orthosilicate was reduced to 3 μΙ/min, equivalent to a molar ratio of oxygen to TEOS of 3000:1. A stable non-equilibrium atmospheric pressure plasma was formed inside the dielectric housing and a low porosity film was formed on the silicon wafer substrate in each experiment having molar ratio of oxygen to TEOS <100.

[0064] The composition of each film deposited was calculated by FTIR as described above. The results are shown in Figure 4, in which the x-axis shows the molar ratio of oxygen to tetraethyl orthosilicate fed to the apparatus and the y-axis shows the carbon content of the film deposited. The diamonds show the results of the comparative experiments using the process of WO 2012/010299. The square shows the result obtained in Example 4.

[0065] In the comparative experiments shown in Figure 4, we observe a decrease of carbon content when increasing the oxygen to precursor ratio. The carbon content being at least 6 times smaller using tetraethyl orthosilicate than using tetramethylcyclotetrasiloxane as in Figure 2, but the carbon content again decreases to reach a plateau, not allowing obtaining a carbon free film. In Example 4 using the apparatus of the present invention, we observe that even for a rather low oxygen to precursor ratio, we deposit smooth and powder free film not containing any measurable carbon.

Claims

1. Apparatus for depositing an oxide film on a substrate, said apparatus comprising a first chamber defined by a dielectric housing and having an inlet for process gas and an outlet, at least one first electrode positioned between an inlet for process gas and the first chamber, means for introducing an oxide precursor into process gas flowing from the said inlet through the said first chamber to the outlet, and means for applying a high voltage to the first electrode to generate a non-thermal equilibrium atmospheric pressure plasma in the first chamber; a second chamber defined by a dielectric housing and having an inlet for process gas comprising oxygen and an outlet, at least one second electrode positioned between the inlet for process gas comprising oxygen and the second chamber, the first chamber and the second chamber being interconnected at their outlets, and means for applying a high voltage to the second electrode to generate a non-thermal equilibrium atmospheric pressure plasma in the second chamber; and support means for the substrate positioned adjacent to the interconnected outlets of the first chamber and second chamber.

2. Apparatus according to Claim 1 , the first chamber and the second chamber being interconnected at their outlets such that the gas of one chamber contacts and intermingles with the other chamber before leaving the reactor.

3. Apparatus according to Claim 1 , wherein the first chamber is an inner chamber and the second chamber surrounds the first chamber.

4. Apparatus according to Claim , wherein the second chamber is an inner chamber and the first chamber surrounds the second chamber.

5. Apparatus according to any of Claims 2 to 4, wherein the outlet for process gas from the apparatus is a gap between the outer wall of the surrounding chamber and the supported substrate, and the outlet for process gas from the inner chamber is a gap between the wall of the inner chamber and the supported substrate, so that process gas flowing through outlet of the inner chamber flows outwardly through the surrounding chamber to the outlet of the apparatus.

6. Apparatus according to any of Claims 2 to 5 having at least two electrodes having the same polarity spaced circumferentially in the surrounding chamber so that a curtain of non-thermal equilibrium atmospheric pressure plasma extends circumferentially in the surrounding chamber.

7. Apparatus according to any of Claims 1 to 6, wherein the first electrode and the

second electrode are needle electrodes.

8. Apparatus according to Claim 7, wherein the first electrode is positioned within a channel through which the process gas flows and the second electrode is positioned within a channel through which the process gas comprising oxygen flows, each channel having a ratio of length to hydraulic diameter of at least 10: 1.

9. Apparatus according to any of Claims 1 to 8, wherein the means for introducing the oxide precursor into the process gas is an atomiser.

10. Apparatus according to any of Claims 1 to 8, wherein the means for introducing the oxide precursor into the process gas is a bubbler.

11. Apparatus according to any of Claims 1 to 8, wherein the means for introducing the oxide precursor into the process gas is a flash evaporation device.

12. Apparatus according to any of Claims 1 to 1 1 , comprising at least two first electrodes having the same polarity surrounding the means for introducing the oxide precursor into the process gas.

13. A process for depositing an oxide film on a substrate, comprising flowing an oxygen-free process gas from a first inlet past at least one first electrode into a first chamber defined by a dielectric housing while applying a high voltage to the first electrode, and introducing an oxide precursor into the process gas flowing from the first inlet past the first electrode, thereby generating a non-thermal equilibrium atmospheric pressure plasma free of oxygen in the first chamber containing activated process gas species and precursor fragments resulting from interaction of the precursor with the plasma and activated process gas species; and flowing a process gas comprising oxygen from a second inlet past at least one second electrode into a second chamber defined by a dielectric housing while applying a high voltage to the second electrode, thereby generating a non-thermal equilibrium atmospheric pressure plasma in the second chamber containing oxygen activated species, the first chamber and the second chamber being interconnected at their outlets so that the said precursor fragments and the oxygen activated species interact, and supporting the substrate adjacent to the interconnected outlets of the first chamber and second chamber.

14. A process according to Claim 13 conducted using the apparatus of any of Claims 1 to 12.

15. A process according to Claim 13 or Claim 14 wherein the process gas flowed to the first inlet is selected from helium, argon and/or nitrogen and the process gas flowed to the second inlet comprises oxygen and an inert gas selected from helium, argon and/or nitrogen.

16. A process according to any of Claims 13 to 15 for depositing a silicon oxide film on the substrate, wherein the oxide precursor is an organosilicon compound.

17. A process according to Claim 16, wherein the oxide precursor is

tetramethylcyclotetrasiloxane. 8. A process according to Claim 16 or Claim 17 wherein the molar ratio of oxygen fed to the second chamber to oxide precursor introduced into the process gas flowing from the first inlet is in the range 60:1 to 15000:1.

19. A process according to any of Claims 16 to 8 wherein a dense carbon-free silicon oxide film is deposited on the substrate.

20. A process according to any of Claims 16 to 19 wherein the silicon oxide film is

deposited on the substrate at a temperature below 100°C.