Corrosion and erosion-resistant mixed oxide coatings for the protection of chemical and plasma process chamber components

Abstract

There is disclosed a method for producing corrosion and erosion-resistant mixed oxide coatings on a metal substrate, as well as a mixed oxide coating itself. A surface of the substrate metal is oxidized and converted into a first coating compound comprising a primary oxide of that metal by a plasma electrolytic oxidation (PEO) process. One or more secondary oxide compounds comprising oxides of secondary elements not present in conventional alloys of the substrate metals at significant (>2 wt %) levels are added to the first oxide coating. The source of the secondary element(s) is at least one of: i) a soluble salt of the secondary element(s) in the electrolyte; ii) an enrichment of the surface of the substrate metal with secondary element(s) prior to PEO processing; and iii) a suspension of the secondary element(s) or oxide(s) of the secondary element(s) applied to the oxide of the metal after this has been formed by the PEO process.

Claims

1. A method for producing corrosion and erosion-resistant mixed oxide coatings on a metal substrate, wherein the metal substrate comprises a substrate metal and less than 2 wt % of a secondary element chosen from transition metals, rare earth metals, lanthanoids, and combinations thereof, the method comprising: oxidizing a surface of the metal substrate by a plasma electrolytic oxidation (PEO) process in an electrolyte to form on the surface a first oxide coating comprising an oxide of the substrate metal, incorporating into the first oxide coating one or more oxides of the secondary elements to form the mixed oxide coating, wherein the oxides of the secondary elements are incorporated into the first oxide coating by at least one of: i) enriching the surface of the substrate with the secondary element(s) prior to the oxidizing step that forms the first oxide coating wherein the surface of the metal substrate is enriched with the secondary element(s) by physical or chemical vapor deposition, by sputtering, by ion implantation, by electrochemical deposition, or by hot-dipping in an alloy enriched with the secondary element(s); and ii) applying a suspension of the oxide(s) one or more of the secondary element(s) to the first oxide coating after the first oxide coating has been formed in the oxidizing step; and wherein the relative phase proportions of the oxides of the substrate metal and the oxides of the secondary elements in the mixed oxide coating vary as a function of depth from an exterior surface thereof.

2. The method according to claim 1, wherein the secondary elements are present in the mixed oxide coating in an amount greater than 3 wt %, neglecting any stoichiometric contribution from oxygen.

3. The method according to claim 1, wherein the secondary elements are present in the mixed oxide coating in an amount greater than 4 wt %, neglecting any stoichiometric contribution from oxygen.

4. The method according to claim 1, wherein the secondary elements are present in the mixed oxide coating in an amount greater than 5 wt %, neglecting any stoichiometric contribution from oxygen.

5. The method according to claim 1, wherein the PEO process is a pulsed bi-polar PEO process.

6. The method according to claim 1, wherein the substrate metal comprises aluminium or an aluminium alloy, in which case the first oxide coating comprises crystalline alumina (in either the α-Al.sub.2O.sub.3 or γ-Al.sub.2O.sub.3 form or a mixture thereof), formed from the aluminium or aluminium alloy by the PEO process.

7. The method according to claim 1, wherein the substrate metal comprises magnesium or a magnesium alloy, in which case the first oxide coating comprises crystalline MgO periclase, as formed from the magnesium by the PEO process.

8. The method according to claim 1, wherein the substrate metal comprises a magnesium-aluminium alloy, in which case the first oxide coating comprises crystalline MgAl.sub.2O.sub.4 spinel, MgO periclase, or a mixture thereof, as formed from the magnesium-aluminium alloy by the PEO process.

9. The method according to claim 1, wherein the substrate metal comprises titanium or a titanium alloy, in which case the first oxide coating comprises crystalline forms of TiO.sub.2, as formed by the PEO process.

10. The method according to claim 1, wherein the surface of the metal substrate comprises predominantly aluminium, magnesium, titanium, zirconium, hafnium, tantalum, yttrium, or any other metal which can be subjected to plasma electrolytic oxidation to yield a crystalline oxide coating.

11. The method according to claim 1, wherein the PEO process is further performed in an electrolyte comprising at least one soluble salt of yttrium to provide a source of yttrium for formation of yttria as one of the secondary oxides.

12. The method according to claim 1, wherein the surface of the metal substrate is enriched with the secondary element(s) prior to the PEO processing.

13. The method according to claim 1, wherein the secondary oxide is incorporated into the mixed oxide coating by a secondary, separate step of applying a suspension of the secondary oxide to a surface of the first oxide coating.

14. The method according to claim 13, further comprising an electrophoretic step to promote deposition and incorporation of the suspension onto and into the first oxide coating.

15. The method according to claim 1, further comprising a thermal curing step.

16. The method according to claim 1, wherein a region at or close to the exterior surface of the mixed oxide coating is richer in oxides of the secondary elements than a bulk of the coating.

17. The method according to claim 1, wherein a predetermined exterior surface thickness of the mixed oxide coating is removed subsequent to its formation to adjust a phase proportion of at least the secondary oxide at the exterior surface of the mixed oxide coating.

18. The method according to claim 17, wherein the exterior surface thickness of the mixed oxide coating is removed by at least one of polishing, abrading and ablating.

19. The method according to claim 1, wherein the transition metal is chosen from scandium, zirconium and manganese; the rare earth metal is chosen from tantalum and hafnium; and the lanthanoid is chosen from erbium, dysprosium, lanthanum and cerium.

Description

DETAILED DESCRIPTION

(1) The Keronite plasma electrolytic oxidation process (WO99/31303, WO03/083181, U.S. Pat. No. 6,896,785) has been recognised as a means of enhancing the erosion resistance of aluminium surfaces—most notably in the form of disposable aluminium liners for plasma process chambers in WO2007/092611, which exploits the ability of the PEO process to form the α-Al.sub.2O.sub.3 “corundum” crystalline phase of aluminium oxide for resistance to fluorine-based plasmas.

(2) In the present invention, new combinations of mixed oxides have been developed for improved plasma erosion resistance. These include, for example, the afore-mentioned alumina as a primary oxide, but with the enhancement of the addition of yttria. Sufficient longevity is achieved with the present invention for it to be possible to dispense with the use of liners altogether, and to rely on a PEO coating alone, directly applied to plasma chamber components.

(3) The degree of process control offered by the Keronite PEO process makes it possible to generate the crystalline oxides of whatever substrate (or ‘parent’) metal is used. That is to say that on aluminium alloys, it will produce crystalline phases of Al.sub.2O.sub.3, (including the α-Al.sub.2O.sub.3 phase already successfully exploited in WO2007/092611), but also wider possibilities such as pure γ-Al.sub.2O.sub.3, whilst on magnesium alloys, it is possible to form crystalline MgO periclase, and on titanium alloys, anatase or rutile TiO.sub.2 can be formed. Each of these phases offers improved erosion protection under the conditions of certain reactive plasma environments.

(4) In addition to the pure crystalline oxide phases of the substrate metals described above, it is also known that the addition to the coating of other crystalline oxide phases such as Y.sub.2O.sub.3, Er.sub.2O.sub.3 or Dy.sub.2O.sub.3 would be preferable for improved plasma resistance and for coating longevity in certain reactive plasma environments. Clearly, some modification of the coating process is necessary to incorporate oxide phases that are not present in the parent/substrate metal into the oxide coating.

(5) As discussed in the background, in WO2007/092611, the incorporation of nanopowders as suspension in the PEO process electrolyte was described as a possible means of achieving this, but that approach has proven to be of limited commercial use. Powders and colloids are typically poorly dispersed within the electrolyte, giving rise to non-uniform coatings. Only very low levels of incorporation are generally observed, since the pressure waves resulting from microplasma development tend to physically displace any suspended solids away from the surface. The use of suspensions also results in unacceptably high levels of erosion in the necessary recirculation systems for cooling and agitating electrolyte. The present invention addresses this by incorporating yttrium (or other similar elements known to produce beneficial oxides) into the PEO coating by other means.

(6) These include the use of certain soluble compounds which are compatible with the PEO electrolyte systems. For instance, yttrium nitrate may be added to a typical KOH based electrolyte. Other suitable soluble compounds include (but are not limited to) phosphates, halides, carbonates, sulphates, sulfonates, sulphides, perchlorates, and a number of organic compounds such as acetates, acetylacetonates, isopropoxides, methoxyethoxides, ethylhexanoates, napthanates, napthoates, and pentanedionates.

(7) The yttrium (or complexes of yttrium) is drawn to the metal surface by electrophoresis and are dissociated and fused into the alumina coating by local plasma discharges.

(8) A second means of incorporating yttria into the coating is to perform a secondary, separate step of applying an yttria suspension (such as an yttria sol), such that a layer of yttria may be deposited on the rough outer surface of the PEO coating, and/or into the pore structure presented by the PEO coating. This exploits the range of fine scale, surface connected porosity presented by typical PEO coatings [“Porosity in plasma electrolytic oxide coatings”, J. A. Curran and T. W. Clyne, Acta Materialia v. 54, p 1985-1993 http://dx.doi.org/10.1016/j.actamat.2005.12.029]. The yttria suspension may be deposited by processes such as spraying, but is preferably achieved by a dip (immersion) process. A low voltage (<100V) electrophoretic process may then additionally be used to promoted deposition of the yttria, and to promote incorporation of the yttria onto and into the pre-existing PEO coating. After applying yttria via such a secondary process step, the yttria sol may be physically stabilised by a subsequent step of thermal curing. For instance, in a preferred aspect, the component may be held at 120° C. for 30 minutes.

(9) A third means of incorporating yttria into the PEO coating is to enrich the substrate metal surface with yttrium prior to the PEO processing. This may be achieved by additions of yttrium to a bulk alloy during manufacture, or by such surface treatment processes as physical or chemical vapour deposition, by sputtering, by ion implantation, by electrochemical deposition, or by hot-dipping in an yttrium-enriched alloy.

(10) The various means of adding yttria to the PEO coating described above allow various concentration profiles of yttria to be achieved through the thickness of the coating. For instance the surface may be yttria-rich relative to the bulk of the oxide coating—as would typically result from the incorporation of soluble yttrium compounds into the PEO process electrolyte, and particularly the method involving secondary deposition of yttria into and onto the porous structure of a pre-existing PEO coating. Alternatively, the region of coating near the substrate interface may be enriched relative to the bulk—this may be achieved by PEO processing metals that have been surface-enriched with yttrium.

Example 1: Yttria Enhancement of a-Al2O3 Coating for Fluorine-Etch Resistance

(11) Components for the reaction chamber for a standard Si etch process involving SF.sub.6 gas are to be made from aluminium alloy AA6082. The benchmark material for surface etch rate in this application is hard anodised aluminium. Under the accelerated test parameters used to evaluate this product, this hard anodising is etched at approximately 85 nm/min.

(12) Representative test pieces were processed in an aqueous electrolyte comprising 2 g/l KOH, with the components exposed to positive potentials at 480V and with 70 microsecond pulse duration, and negative pulses of 400V with 1000 microsecond duration

(13) The selection of a 480V limit for the positive potential again ensures that microdischarges are constrained to 15 mA peak power and that the duration of the discharges is constrained to ˜50 microseconds. Processing under these conditions for 20 minutes results in a 35±3 μm coating that is at least 93% crystalline (by volume), and consists of a mixture of α-Al.sub.2O.sub.3 and γ-Al.sub.2O.sub.3.

(14) Under accelerated etch rate test conditions, the etch rate of this coating was approximately 35 nm/min: just over twice as etch resistant as the incumbent hard anodising technology.

(15) Samples were also produced under the same conditions, but with the addition of 1 g/l of yttrium nitrate to the electrolyte. This resulted in incorporation of 12 atomic % Y into the coating (as evaluated by surface energy dispersive spectroscopy), in the form of Y.sub.2O.sub.3-concentrated near the outer surface.

(16) Under the accelerated etch rate test conditions, the etch rate of this yttria-enhance coating was <10 nm/min—demonstrating a substantial improvement over the standard crystalline alumina coating.

Example 2: Periclase Coating for Resistance to Fluorine-Based Plasmas, and Enhancement by Electrophoretic Deposition of Yttria

(17) Components for the reaction chamber for a standard Si etch process involving SF.sub.6 gas are to be made from magnesium alloy AZ91D. The benchmark material for surface etch rate in this application is hard anodised aluminium. Under the accelerated test parameters used to evaluate this product, this hard anodising is etched at approximately 85 nm/min.

(18) The magnesium components are coated using an aqueous electrolyte comprising 0.02M sodium orthophosphate, with 480V positive pulses of 80 microsecond duration at 1 kHz frequency. The ˜25 μm thick coating resulting from 8 minutes of processing is 74% crystalline in the periclase phase of MgO. The etch rate of this coating was approximately 20 nm/min under the accelerated test conditions—making it four times superior to the incumbent hard anodising technology in terms of plasma etch resistance.

(19) Further samples were processed as per the above conditions, but with subsequent immersion in an yttria sol. Electrophoretic deposition of yttria onto the surface was promoted using a 50V potential, for 10 minutes. Samples were then heat treated at 130° C. for 30 minutes. This resulted in a surface enriched with yttria (to ˜35 atomic % Y, as evaluated by surface energy dispersive spectroscopy), in the form of Y.sub.2O.sub.3, again concentrated on the outer surface of the coating.

(20) Under the accelerated etch rate test conditions, the etch rate of this yttria-enhance coating was ˜10 nm/min—demonstrating a substantial improvement over the standard crystalline alumina coating, and also further enhancement of the periclase coating's performance by the addition of the yttria secondary oxide.

Example 3: Chlorine Plasma-Resistant Coating for AA7075, Comprising Alumina and Silica

(21) Components for a GaN processing chamber are to be manufactured from aluminium. The more aggressive etch processes include exposure to chlorine based plasma at high bias. To test surface erosion resistance, a mixture of BCl.sub.3 and Cl.sub.2 at flow rates of 10 and 90 standard cubic centimeters per minute respectively, at a pressure of 15 mT is used under a high DC bias of ˜500V. Under such test conditions, typical etch rates for aluminium surfaces are of the order of 125 nm/min.

(22) For protection against this plasma etching, chamber components consisting of 7075 aluminium were processed in an aqueous electrolyte comprising 2 g/l KOH, and 10 g/l Na.sub.2SiO.sub.3 with the components exposed to positive potentials at 490V and with 80 microsecond pulse duration, and negative pulses of 400V with 1000 microsecond duration

(23) The selection of a 490V limit for the positive potential ensures that microdischarges are constrained to 15 mA peak power. This results in more intense injection of energy into the oxide coating, and in turn, the greatest degree of phase transformation from amorphous oxides (the product of conventional anodising) to crystalline oxide. The duration of the resulting discharges is also intrinsically constrained to ˜50 microseconds.

(24) Processing under these conditions for 20 minutes results in a 48±4 μm coating consists of a mixture of γ-Al.sub.2O.sub.3 and SiO.sub.2.

(25) Under the etch conditions described above, this coating shows negligible etch (<10 nm) even after 300 minutes of exposure. It is thus vastly superior to conventional anodising.

(26) Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(27) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

(28) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.