Sintered lanthanum zirconium oxide, multi-layer sintered ceramic bodies, and related methods

Abstract

Described are sintered lanthanum zirconium oxide (LZO) materials, multi-layer sintered ceramic bodies that contain sintered lanthanum zirconium oxide and a support layer, and methods of preparing the same using plasma spark sintering techniques.

Description

SINTERED LANTHANUM ZIRCONIUM OXIDE, MULTI-LAYER SINTERED CERAMIC BODIES, AND RELATED METHODSFIELD[1] The following describes sintered lanthanum zirconium oxide (LZO), multi-layer sintered ceramic bodies that contain sintered lanthanum zirconium oxide and a support layer, and methods of preparing the same using plasma spark sintering techniques.BACKGROUND[2] Semiconductor processing equipment such as plasma etching chambers and plasma deposition chambers (referred to collectively as “plasma chambers”) require a highly pure, contaminant-free plasma environment at their interiors. To achieve a contaminant-free environment these chambers use surfaces at their interiors that arc resistant to chemical degradation that would produce particle contamination that would be introduced into the plasma chamber interior.[3] Semiconductor and microelectronic device processing requires steps of processing semiconductor and microelectronic device substrate in a plasma atmosphere produced by exposing halogen-based gases to electromagnetic fields within an evacuated interior of a plasma chamber. The plasma atmosphere is used during etching or depositing materials of in-process semiconductor and microelectronic device substrates and must be substantially free of particle contamination.[4] The plasma atmosphere will degrade materials located within the plasma chamber. Contact with plasma will cause chemical degradation, erosion, and roughening of surfaces, which in turn causes materials of the surfaces to be introduced into the plasma chamber interior as contamination (e.g., particle contaminants). The particle contaminants can settle onto a substrate that is being processed within the chamber and if so will produce defects and semiconductor device yield loss.[5] To reduce particle contamination produced by the effects of plasma, surfaces within a plasma chamber interior can be made of materials that are resistant to the effects of the plasma and halogen-based process gases that are used to generate plasma, i.e., that are “plasmaresistant.” Materials such as yttrium oxide and yttrium alumina garnet (YAG) have been used asthin film coatings on interior surfaces of plasma chambers and plasma chamber components. Films or coatings have been made by several methods, including vapor deposition and aerosol or plasma spraying. Such films and coatings can exhibit performance deficiencies due to being relatively thin and having low density and significant porosity. Additionally, coatings that are applied to another material can be prone to delamination at an interface between the coating and that material, which may result in cracking and spalling and the release of particulate contamination from the surface. Coatings applied by aerosol or plasma spray techniques typically exhibit levels of porosity of between 3 percent and 50 percent, correspondingly low density, and poor adhesion between the base material and the coating material that results in flaking, exfoliation, and chamber contamination.SUMMARY[6] Continued demand exists for materials that have high resistance to plasma and halogenbased gases and therefore produce low amounts of particle contamination when used at an interior of a plasma chamber. Materials for these applications provide high resistance to plasma and chlorine and fluorine -based process gases, low dielectric loss, high thermal conductivity, and can be machined to form a solid body for use within a plasma chamber. Associated properties of these surfaces may include one or more of: high heat resistance; high purity; high density, e.g., low porosity, with any pores that are present having a small maximum size; small grain size; high mechanical strength and hardness; and a thickness that allows the surface to have an extended useful lifetime at an interior of a plasma chamber.[7] To be adapted for use in a plasma chamber, a body that contains a plasma-resistant surface may preferably have dimensions adapted for use at an interior of a plasma chamber, such as flat body having a length, width, or diameter dimension of greater than 100 millimeters (e.g., from 100 mm to 625 mm) and a thickness that allows the body to provide plasma resistance for a significant useful lifetime.[8] In some applications, a plasma-resistant surface is one layer of a multi-layer ceramic body that includes a support layer and a plasma-resistant layer. For these multi-layer ceramic bodies the individual layers can preferably have thermal expansion properties (e.g., as measured by a coefficient of thermal expansion) that arc sufficiently alike to allow the multi-layer body towithstand heating and cooling without suffering physical damage such as cracking or delamination.[9] The following describes sintered lanthanum zirconium oxide materials (“sintered lanthanum zirconium oxide” or “sintered LZO”) that may be used as a plasma-resistant material in a plasma chamber. The sintered lanthanum zirconium oxide materials can be produced using spark plasma sintering processes that have been developed to produce sintered lanthanum zirconium oxide having physical properties that include: plasma resistance, thermal resistance, high purity, high density (including low porosity and pores of a small maximum size), and small grain size (including small average grain size and a small maximum grain size). These processes can also produce lanthanum zirconium oxide materials that exhibit good mechanical strength and hardness as well as machinability. Sintered lanthanum zirconium oxide may be formed to have dimensions for use within a plasma chamber, including a circular form having a diameter of at least 100 millimeters and a thickness that allows for a significant useful lifetime when used in a plasma chamber.

[0010] In certain examples a lanthanum zirconium oxide material can be formed into a multilayer sintered ceramic body that includes a sintered lanthanum zirconium oxide layer and a support layer such as zirconia toughened alumina (ZTA). The multi-layer sintered ceramic body should have good adhesion between the layers and the different layers should be compatible for processing by a sintering process that forms the two layers of the sintered body using a single sintering step, for example the materials of the two layers should have similar coefficients of thermal expansion.

[0011] Previously, non-sintered lanthanum zirconium oxide (“LZO”) materials have been applied as plasma-sprayed LZO coatings for plasma chamber parts. See United States patent number 10,388,492. These coated materials have significant porosity and lack a high density that would be useful for a surface of a plasma chamber interior. According to a different previous use, Chinese patent publication CN 103803972 B describes a sintered LZO ceramic body prepared by a hot press sintering method. The ceramic body is described as being useful as a heat insulating protective material in high temperature aeronautic applications such as under hyperelevation hypersonic flight conditions, but are not described as having particularly high purity or density (e.g., greater than 95 or 98 percent of a theoretical density), low porosity or small pore size, or fine average or maximum grain size.

[0012] Also previously, sintered ceramic bodies that include a polycrystalline yttrium aluminum garnet (YAG) layer have been prepared by plasma spark sintering methods. See PCT publication WO 2022 / 133180.

[0013] Described herein are sintered lanthanum zirconium oxide materials prepared by reacting lanthanum oxide powder and zirconium oxide powder using a process of spark plasma sintering (SPS). The lanthanum oxide powder and zirconium oxide powder are of very high purity and steps of preparing and handling the powders before and during the sintering step can be controlled to achieve a desired high purity of a sintered LZO body. For example, the two powder materials can be combined into a homogenous powder mixture by milling and may optionally be calcined to reduce the impurity content of the powder mixture.

[0014] During a sintering step, a combination of process conditions that can include pressure applied to the powder mixture (“sintering pressure”), heating rate (temperature profile), maximum temperature or temperature range (“sintering temperature”), electric current passing through the die, and an amount of time that the powder mixture is held at a sintering temperature (“sintering time”) can be controlled to produce a sintered lanthanum oxide material that has a desired density (including the presence of pores and pore size), and microstructure (e.g., grain size).

[0015] Example methods of sintering a powder mixture that contains lanthanum oxide powder and zirconium oxide powder can use a sintering pressure that does not exceed 100 MPa, and a sintering temperature that does not exceed 1625 degrees Celsius. Sintering time depends on the size of the die used to hold the powder mixture, with dies of 100 mm to 150 mm in diameter having sintering times that generally do not exceed 120 minutes, 90 minutes, or 60 minutes.

[0016] Example sintered lanthanum zirconium oxide materials prepared according to methods as described can have one or a combination of: a purity of at least 99.999 percent, a density of at least 96 percent of theoretical density, porosity in a range from 0.0005 to 2 percent, pores having a maximum size of 10 or preferably 5 microns measured over a surface of the sintered lanthanum zirconium oxide, average grain size below 10 microns, and maximum grain size below 10 microns.

[0017] In one aspect, the following description relates to wintered lanthanum zirconium oxide having a purity of at least 99.999 percent and a density of density.

[0018] In another aspect, the following description relates to multi-layer sintered ceramic bodies comprising a layer of sintered lanthanum zirconium oxide and a layer of sintered zirconia toughened alumina.

[0019] In another aspect, the following description relates to a sintered lanthanum zirconium oxide and multi-layered sintered ceramic bodies as described, prepared by a method of spark plasma sintering.

[0020] In another aspect, the following description relates to a method of making sintered lanthanum zirconium oxide. The method includes; preparing a powder mixture comprising lanthanum oxide particles and zirconium oxide particles; placing the powder mixture in a graphite die; eliminating oxygen in the die; and sintering the powder mixture in the die by applying pressure to the powder mixture and passing electric current through the die to increase the temperature of the powder mixture and cause the lanthanum oxide to react with the zirconium oxide powder and form sintered lanthanum zirconium oxide.

[0021] In yet another aspect, the present description relates to a method of making a multi-layer sintered ceramic body comprising a layer of lanthanum zirconium oxide and a layer of zirconia toughened alumina. The method includes; preparing a first powder mixture comprising lanthanum oxide particles and zirconium oxide particles; preparing a second powder mixture comprising zirconia and alumina; forming a layer of the first powder mixture in a graphite die; forming a layer of the second powder mixture in the graphite die; and sintering the first powder mixture and the second powder mixture by applying pressure to the first powder mixture and the second powder mixture in the die and passing electric current through the die to increase the temperature of the first powder mixture and the temperature of the second powder mixture and cause the first powder mixture to form a layer of lanthanum zirconium oxide and to cause the second powder mixture to form a layer of zirconia toughened alumina.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic cross-sectional view of an example apparatus for preparing sintered lanthanum zirconium oxide.

[0023] Figure 2 shows coefficients of thermal expansion of lanthanum zirconium oxide and zirconia toughened alumina.

[0024] Figure 3 is a schematic cross-sectional view of an example apparatus for preparing amulti-layer sintered ceramic body comprising sintered lanthanum zirconium oxide and a support layer.

[0025] Figure 4 is a photograph of a multi-layer sintered ceramic body prepared according to the present description.

[0026] All figures are schematic and not necessarily to scale.DETAILED DESCRIPTION

[0027] Described as follows are sintered lanthanum zirconium oxide materials prepared by spark plasma sintering (“SPS”) processes whereby lanthanum oxide powder is reacted with zirconium oxide powder when forming the sintered lanthanum zirconium oxide material. The spark plasma sintering process produces sintered LZO (La2Zr2O?) that has one or a combination of: high purity, high density, low porosity (low volumetric porosity and pores of a small maximum size), and fine grain size (in terms of average grain size and maximum grain size). Example sintered LZO materials prepared as described may have one or more of: a purity of at least 99.999 percent, e.g., at least 99.9999 or 99.99999 percent; a density of at least 96, 97, 98 or 99 percent theoretical density; a porosity in a range from 0.0005 to 2 percent; maximum pores size of 5 or 10 microns, an average grain sizes of below 10 microns, and a maximum grain size of not more than 10 microns.

[0028] As used herein the term “spark plasma sintering” (“SPS”) refers to a method of bonding together individual particles of a powder to form a dense sintered material (a.k.a. “sintered body”) by applying pressure to the particles while the particles are heated in a die by electric current that passes through the die; the particles in the die are heated to a temperature that is sufficiently high to cause the individual particles to become bonded together by atomic diffusion at the particle surfaces, but is still below the melting point of the particles. The methods use contemporaneous application of uniaxial pressure and electric current through the die to increase the temperature of the powder particles in the die. Spark plasma sintering differs from hot press sintering, which uses an external heat source such as a furnace or resistive heating element to heat a mold that contains powder to be sintered.

[0029] A spark plasma sintering process used to prepare sintered lanthanum zirconium oxide is performed using a spark plasma sintering apparatus that includes a die that has a die interior, and a moveable surface (e.g., a surface of a “punch”) disposed within the die interior that can bemoved to apply pressure to powder contained at the die interior. One or more layers of powder mixture can be placed within the die interior and subsequently compressed within the die interior by applying pressure to the one or more powder mixture layers using the surface of the moveable punch. While a powder mixture is compressed within the die interior, heat can be generated within the die by passing electric current through the die; the heat passing through the die increases the temperature of the powder mixture while pressure is also applied to the powder mixture within the die interior. The die interior can contain the powder mixture in a vacuum or another controlled atmosphere (e.g., argon or hydrogen) to prevent unwanted reaction of the materials of the powder mixture (e.g., oxidation) to produce a sintered material having a desired high purity. The electric current may be pulsed or nonpulsed and may be alternating current (“AC”) or direct current (“DC”).

[0030] According to processes as described, the purity, preparation, and handling of the powder mixture before and during the sintering step can be controlled to achieve a desired high purity of a sintered LZO body. In an example process, a powder mixture is formed from two powders, which include lanthanum oxide (L 2Oa) powder and zirconium oxide (ZrCh) powder, in a ratio of two moles zirconium oxide to one mole lanthanum oxide. The zirconium oxide powder can be pure zirconium oxide (un stabilized) or may be a stabilized zirconium oxide powder that contains zirconium oxide and an amount of yttrium oxide (Y2O3), e.g., about 3 mole percent yttrium oxide based on total weight zirconium oxide and yttrium oxide in the zirconium oxide powder (e.g., 3- YSZ).

[0031] The powder mixture can comprise, consist essentially of, or consist of the lanthanum oxide powder and zirconium oxide powder (which may include an amount of yttrium oxide). A powder mixture that consists essentially of lanthanum oxide powder and zirconium oxide powder refers to a powder mixture that contains the lanthanum oxide powder and the zirconium oxide powder and not more than an insignificant amount of any other materials such as a sintering aid, dopant, or impurities, e.g., less than 5, 2, 1, 0.5, or 0.1 weight percent of these or any other types of materials other than the lanthanum oxide powder and the zirconium oxide powder.

[0032] The particles as a raw material are in the form of powders, meaning a collection of very fine, dry, solid, free-flowing granular particles. A powder contains particles of a relatively uniform size with a maximum particles size being on a micron scale, e.g., less than 1,000, 800, or 500 microns. The powders may be prepared based on maximum particle size by a method suchas passing a powder through a screen of a nominal mesh size less than 1 ,000, 800, or 500 microns, c.g., a number 18 mesh, a number 25 mesh, or number 40 mesh screen.

[0033] To produce sintered LZO having a very high purity, the powder mixture can be formed from lanthanum oxide and zirconium oxide having very high purities. According to example methods a lanthanum oxide powder can have a purity of at least 99.999 percent, 99.9999 percent, e.g., at least 99.99999 percent. A zirconium oxide powder (with optional yttrium oxide) can have a purity of at least 99.99 percent, e.g., at least 99.999 percent as measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

[0034] The lanthanum oxide powder and the zirconium oxide power are combined and mixed to form a homogeneous powder mixture. By one example, the powders can be mixed using a ball mill and solid milling media. Preferred types of ball mills and milling media can be those that will minimize the introduction of contaminants from the ball mill or the milling media into the powders during mixing. For example, a preferred ball mill may include an interior that is lined with a durable polymeric material that does not degrade during a milling process and is resistant to shedding particles of the polymeric material into the powder mixture. Example ball mills may be lined with a polyolefin coating such as a polyethylene or polypropylene coating. Likewise, preferred milling media (grinding balls) can be of a type that does not degrade during a milling process and is resistant to shedding of the milling media material into the powder mixture. Examples of useful or preferred milling media can be made of a ceramic material such as stabilized zirconia, so that if the zirconia milling media does shed, the shedding only adds zirconia particles, which is already present as of the powders being mixed.

[0035] Solvent can be included with the powders during a milling process. A useful solvent can be selected to be non-reactive with the powders, with an example thereof being ethanol.

[0036] Optionally after milling the powder mixture can be processed by calcining the powder mixture to remove residual solvent and potential organic impurities that may be present in the powder mixture. By a calcining step, the powder mixture is heated to a temperature that does not cause the powder particles to melt or react, to remove impurities or volatile materials within the powder mixture. A step of calcining a powder mixture that contains lanthanum oxide powder and zirconium oxide powder can be heated to a temperature in a range from 600 to 850 degrees Celsius for an amount of time that is useful to remove impurities from the powder mixture.

[0037] The powder mixture can be sintered using a plasma sintering process to prepare sintered lanthanum zirconium oxide. The powder mixture can be placed within a die interior in a controlled atmosphere (e.g., in a vacuum or inert atmosphere to prevent the graphite die from combusting) and compressed within the die interior during sintering. The temperature of the powder mixture is increased by passing electric current through the die. During a sintering step, the pressure (“sintering pressure”), heating rate (temperature profile), maximum temperature or temperature range (“sintering temperature”), type of electric current passing through the die, and amount of time that the powder is held at a sintering temperature (“sintering time”) can be controlled to produce a sintered lanthanum zirconium oxide material that has desired density (including the presence of pores and maximum pore size), micro structure (e.g., grain size), and purity.

[0038] The pressure that is applied to the powder mixture during the sintering step can be a pressure that along with a useful sintering temperature and sintering time will produce a sintered lanthanum zirconium oxide material that has a high density, desired purity, and fine grain structure. Examples of useful sintering pressures can be up to about 100 megapascals (MPa), or up to 50 MPa, or up to 25 MPa, such as a pressure in a range from 10 to 50 MPa.

[0039] The sintering temperature is effective to cause lanthanum oxide of the lanthanum oxide powder particles to react with the zirconium oxide of the zirconium oxide particles to form lanthanum zirconium oxide. When the powder mixture is held at the sintering temperature for an effective amount of time (the “sintering time”), the particles become bonded together into solid, high density (low porosity), sintered lanthanum zirconium oxide. Examples of useful sintering temperatures may be up to 1650 degrees Celsius, e.g., up to 1600 or 1550 degrees Celsius, and useful sintering times may be less than 120 minutes, e.g., less than 90 minutes or less than 60 minutes for dies of 100 mm to 150 mm diameter or less. Differently sized dies may require different sintering times.

[0040] The type of electric current passing through the die may be pulsed, nonpulsed, alternating current (“AC”), or direct current (“DC”). Preferred current is nonpulsed direct current.

[0041] One example of a plasma sintering apparatus that is useful according to methods as described is shown at figure 1. Figure 1 schematically illustrates components of plasma sintering apparatus 100, which includes die 102, lower punch 110 and upper punch 120. Die 102 is an annular structure that defines interior 104 between the inner sidewall 106 of die 102, uppersurface of 112 of lower punch 110, and lower surface 122 of upper punch 120. The die and punch arrangement is contained within a vacuum chamber (not shown) to control an atmosphere of interior 104 during a sintering step. During sintering, oxygen can be removed from the chamber via direct removal with a vacuum pump or through displacement with a non-reactive gaseous species (e.g., helium) or a reactive gaseous species (e.g., hydrogen). Interior 104 has a volume that is adapted to receive powder mixture 130, which can be compressed under pressure applied between surfaces of upper punch 120 and lower punch 110.

[0042] Upper punch 120 and lower punch 110 are operably coupled to components of plasma sintering apparatus 100 in a manner that allows upper punch 110 and lower punch 120 to apply pressure to powder mixture 130 at interior 104. Die 102 has an inner diameter that is slightly greater than the diameters of upper punch 120 and lower punch 110, with a small gap between inner surface 106 of die 102 and the outer surfaces of upper punch 120 and lower punch 110 to allow movement of upper punch 120 and lower punch 110 within die 102.

[0043] Die 102 is made of an electrically conductive material such as graphite to allow current to be passed through die 102 to heat die 102 and powder mixture 130 contained at interior 104. Upper punch 120 and lower punch 110 are also formed of a conductive material such as a conductive graphite material. A graphite material of a punch 110, 120, or of die 102, may be of any useful type of graphite, such as isotropic graphite, reinforced graphite such as a carboncarbon composite, or a graphite material that contains fibers, particles, sheets, mesh, or laminates of other electrically conductive materials such as carbon in a matrix of an isotropic graphite material.

[0044] An arrangement of a die 102 and upper and lower punches 110 and 120 may further include additional components to facilitate a sintering process such as spacers, shims, liners, and other tool set components. Typically, additional structures such as these may also be made from a conductive material such as a graphite material but could include other materials as well.

[0045] According to certain example methods as described and as illustrated at figure 1 , a liner is placed between surfaces of a powder mixture in the die and surfaces of the punch or surfaces of the die interior. The liner can be useful to prevent the powder from sticking or adhering to a surface of the die or a punch and to allow removal of a sintered body from the die and punch following a sintering process.

[0046] A liner can be made of a highly pure conductive material such as graphite having a purity of at least 99.7, 99.8 or 99.9 percent and may be in the form of a thin film or sheet (“foil”), e.g., having a thickness of less than one millimeter, e.g., a thickness in a range from 0.13 to 0.76 millimeters. These types of graphite foil materials are available commercially with examples sold by NeoGraf Solutions, LLC under the trade name GTA Flexible Graphite.

[0047] Additionally, to form high purity sintered lanthanum zirconium oxide, a conductive liner (or “foil”) can include an inert (non-reactive) coating on a surface of the conductive liner that contacts a powder mixture that contains lanthanum oxide and zirconium oxide. The non-reactive coating can be made by coating a non-reactive species (e.g., boron nitride) or an appropriately- sized piece of bulk metal known to generally be non-reactive (e.g., molybdenum, niobium, tantalum, or tungsten) onto a conductive substrate (e.g., graphite foil). When a powder mixture contains zirconium oxide, the zirconium oxide is capable of reacting with carbon of a graphite liner during sintering to form zirconium carbide, which if formed will reduce the purity of the sintered lanthanum zirconium oxide. The non-reactive coating prevents contact between the graphite of the liner and the zirconium oxide of the powder mixture to prevent the formation of zirconium carbide during sintering.

[0048] A non-reactive coating can be made using any material that is useful to separate the powder mixture from a conductive material (e.g., graphite) of a conductive liner and prevent reaction of a material of the conductive liner with the powder mixture. The non-reactive coating may be applied to a conducive (e.g., graphite) liner by any useful method. As a particular example, the non-reactive coating can be made of highly pure (at least 97 percent pure) boron nitride and may be applied to a conductive liner (e.g., a graphite liner) by spraying as an aerosol.

[0049] For example, as shown at figure 1, a boron nitride coated graphite foil 142 is located between upper punch 120 and powder mixture 130, and a second boron nitride coated graphite foil 142 is located between lower punch 110 and powder mixture 130. Foil 142 between lower punch 110 and powder mixture 130 is arranged with the boron nitride side 144 facing the lower surface of powder mixture 130 and the graphite side facing the upper surface of lower punch110. Similarly, foil 142 between upper punch 120 and powder mixture 130 is arranged with the boron nitride side 144 facing the upper surface of powder mixture 130 and the graphite side facing the lower surface of upper punch 120. In this arrangement, the boron nitride side of each foil 142 contacts the upper or lower surface of powder mixture 130 and prevents graphite of afoil 142 from contacting powder mixture 130, thus preventing a reaction between graphite of foil 142 and powder of the powder mixture 130, c.g., to prevent reaction of carbon of the graphite with zirconium oxide to form zirconium carbide.

[0050] A method as described can be performed using selected process features such as sintering pressure, sintering temperature, and sintering time to form sintered lanthanum zirconium oxide that has a highly useful combination of physical properties that include high purity, high density (low porosity), and a desired morphology as measured by grain size. Sintered lanthanum zirconium oxide material that has high purity, high density (including low pore content), small maximum pore size, and small grain size can be particularly useful as a thermally stable, plasmaresistant interior surface at an interior of a plasma processing apparatus, particularly of a type that performs a plasma deposition or plasma etching process involving the use of halogen-based process gases introduced into a plasma processing chamber while an RF field is applied to the process gases to generate plasma.

[0051] In these applications, to prevent localized hot spots and overheating during use, plasmaresistant interior surfaces preferably have low dielectric loss. Dielectric loss of a plasmaresistant surface may be affected by material properties that include grain size and the presence of impurities. The presence of impurities in a plasma-resistant surface may result in a higher dielectric loss. The use of highly pure powders of a powder mixture with processing and handling methods that preserve their purity as described herein can produce a sintered lanthanum oxide material that has very high purity. Accordingly, by methods as described, a sintered lanthanum zirconium oxide material may have a purity of at least 99.999 percent, e.g., at least 99.9999 percent, or 99.9999 percent as measured using ICPMS methods.

[0052] Grain size also affects properties of sintered lanthanum oxide including dielectric loss and mechanical properties such as hardness and strength. Methods as described can be used to prepare sintered lanthanum zirconium oxide that has fine grains, e.g., small grain size and a small maximum grain size. A small maximum grain size can be preferred because larger grains within a material of smaller grains can act as critical flaw locations that reduce the strength of the material.

[0053] Examples of sintered lanthanum oxide materials prepared as described can have an average grain size of less than 10 microns, e.g., less than 8 microns, such as an average grain size of from 0.4 to 6.5 microns or from 0.4 to 5 microns. These sintered lanthanum oxide materialscan also have a maximum grain size of less than 10 microns, e.g., less than 8 microns, or less than 6 or 5 microns. Grain size of sintered lanthanum zirconium oxide can be measured by known techniques, including by linear intercept grain size measurements performed in accordance with the Heyn Linear Intercept Procedure described in ASTM standard El 12-2010 “Standard Test Method for Determining Average Grain Size.”

[0054] A sintered lanthanum zirconium oxide material prepared as described can also have a high density and a high relative density, i.e., a density that is near a theoretical density. A high density can be achieved by forming lanthanum zirconium oxide that contains a low number of pores at the interior of the solid, preferably with the size of any pores that are present being small. A theoretical density of lanthanum zirconium oxide is a maximum achievable density assuming no internal pores or contaminants and has been calculated to be 6.050 grams per cubic centimeter. Example sintered lanthanum oxide materials prepared as described can have a density (“Archimedes density”) of at least 5.90 grams per cubic centimeter, e.g., at least 5.964 grams per cubic centimeter measured according to ASTM B962-17, and a relative density of at least 98, 99, or 99.5, i.e., a density that is at least 98, 99, or 99.5 percent of the theoretical density of lanthanum zirconium oxide.

[0055] With a high density, the sintered lanthanum zirconium oxide can have a low porosity, i.e., a low amount of void space at an interior or at a surface of the material. During use in a plasma chamber, pores at a surface of a lanthanum zirconium oxide material pennit reactive gas or plasma to enter the surface of the lanthanum zirconium oxide material at the pore and produce undercutting or erosion of the surface at the pore, which can cause particle shedding at the surface with the particles becoming contaminants within the chamber. As used herein a “porosity” (a.k.a. “volumetric porosity” or “void fraction”) of a porous body is a measure of the void (i.e., “empty”) space in the body as a percent of the total volume of the body and is calculated as a fraction of the volume of voids in the body over the total volume of the body. A body that has zero percent porosity is completely solid. Some amount of pores will typically be present in a sintered lanthanum oxide material, and any pores that are present can preferably have a small size and a small maximum pore size.

[0056] Example sintered lanthanum zirconium oxide materials prepared as described can have a porosity, measured as a percentage of an area of a surface of the material, that is in a range from 0.0005 to 2 percent, e.g., from 0.001 to 1.5 percent, or from 0.01 to 0.5 percent measured usingSEM. These sintered lanthanum zirconium oxide materials may have a maximum pore size of not greater than 10 microns, e.g., from 1 to 5 microns. Porosity and pore size of sintered lanthanum zirconium oxide can be measured using known techniques, including by preparing an image of a surface of the sintered lanthanum zirconium oxide using a scanning electron microscope (e.g., at 5000 x magnification), and analyzing the images using ImageJ software (ImageJ has been developed at the National Institute of Health (NIH), USA, and is a Java-based public domain image processing and analysis program for image processing of scientific multidimensional images).

[0057] Methods as described can also be useful to prepare multi-layer ceramic bodies that includes a layer of sintered lanthanum zirconium oxide as described (including with properties as described) in combination with a support layer. A support layer can be made of ceramic material such a material that includes a combination of alumina and zirconia, e.g., zirconia toughened alumina (ZTA). The support layer can be formed by sintering the support layer using a plasma spark sintering technique whereby the sintered lanthanum zirconium oxide layer and the support layer are sintered simultaneously to form a multi-layer sintered ceramic body. The process of forming the multi-layer sintered ceramic body by sintering must be effective to form a multilayer sintered ceramic body that exhibits useful structural and mechanical properties such as strength and hardness with good adhesion at an interface between the layers, an absence of cracking, an absence of unwanted materials or impurities formed by reaction of the materials of different layers during sintering, and high purity, density, and fine grains, etc., of the sintered lanthanum zirconium oxide layer.

[0058] An example support layer that includes zirconia toughened alumina can contain alumina and zirconia, with the zirconia being stabilized or partially stabilized zirconia. Examples of useful zirconia toughened alumina can contain from 10 to 40 weight percent zirconia, e.g., from 20 to 25 weight percent zirconia, with the balance being alumina and not more than one weight percent impurities.

[0059] Preferably, the coefficient of thermal expansion (CTE) of a support layer such as ZTA can be approximately the same as the coefficient of thermal expansion of the lanthanum zirconium oxide, to allow the two layers of the multi-layer sintered ceramic body to be processed by sintering and cooled without the two layers becoming physically stressed or damaged upon cooling, e.g., fractured, cracked, or delaminated due to different rates of thermal contractionduring cooling. A coefficient of thermal expansion of lanthanum zirconium oxide is between 7.5 and 10 x 10‘6 / dcgrcc K (or parts per million, ppm) over a temperature range from 25 to 1200 degrees Celsius. A coefficient of thermal expansion of zirconia toughened alumina is between 6 and 10 x 10‘6 / degree K (or parts per million, ppm) over a temperature range from 25 to 1200 degrees Celsius. Figure 2 shows a comparison of CTEs of LZO and ZTA that contains 22 weight percent zirconia. The CTE values were measured using a vertical dilatometer from Linseis (L75 Platinum Series model).

[0060] Example multi-layer sintered ceramic bodies as described can include a sintered lanthanum zirconium oxide layer and a zirconia toughened alumina layer that have a maximum difference in CTE (i.e., a difference in CTE of the two materials measured at the same temperature) of no more than 1, 2, or 3 x 10’6 / degree Celsius (ppm), over the temperature range from 25 degrees Celsius to 1200 degrees Celsius as measured in accordance with ASTM E228- 17. Figure 2 shows (at a temperature of approximately 375K) a maximum CTE difference between LZO and ZTA of less than 2 ppm but greater than 1 ppm over this temperature range, for ZTA that contains 22 weight percent zirconia. In preferred examples of multi-layered sinter bodies as described, the ZTA can have a CTE that is even more closely matched to the CTE of the LZO, to prevent cracking or other physical damage to a multi-layer sintered body that may occur during cooling after sintering. Zirconia toughened alumina having a CTE that differs from the CTE of LZO by a maximum of less than 1 ppm across this temperature range may contain more than 22 weight percent zirconia, e.g., up to or greater than 30, 35, 38, or 40 percent zirconia.

[0061] Figure 3 shows a plasma sintering apparatus that is useful according to methods as described to form a multi-layer sintered ceramic body that includes a sintered lanthanum zirconium oxide layer and a ceramic support layer, e.g., a zirconia toughened alumina layer. Figure 3 schematically illustrates components of plasma sintering apparatus 100 as in figure 1, adapted to form a multi-layer sintered ceramic body that includes a lanthanum zirconium oxide layer and a support layer. Plasma sintering apparatus 100 of figure 3 contains powder mixture 1 that contains a combination of lanthanum oxide and zirconia oxide, and powder mixture 2 that contains a combination of zirconia and alumina. During a sintering step using conditions as described, heat and pressure are applied simultaneously to both of powder mixture 1 and powder mixture 2 contained in die 102. Pressure is applied to the powder mixtures to cause the powdermixtures to become sintered together, with the two powder mixtures becoming bonded together to form a multi-layer sintered ceramic body that includes a sintered lanthanum zirconium oxide layer (from powder mixture 1) and a support layer, e.g., zirconia toughened alumina from powder mixture 2. A boron nitride coated graphite foil 142 is located between upper punch 120 and powder mixture 130, with the boron nitride side 144 facing the upper surface of powder mixture 130 and the graphite side facing the lower surface of upper punch 120. A non-coated graphite foil 146 is located between the upper surface of lower punch 110 and the lower surface of powder mixture 130.EXAMPLESExample 1

[0062] Two powder components, lanthanum oxide (LazO ) and zirconium oxide (ZrCL), were ball milled in ethanol for approximately 22 hours. The mixture was then rotovapped to remove the ethanol. The powder mixture was then pressed via spark plasma sintering (SPS) inside a graphite die. Both a 40 mm die and a 4-inch die were used to produce sintered lanthanum zirconium oxide (LaaZraOv). Boron nitride coated graphite foil was placed next to the powder mixture with the boron nitride coating facing the powder mixture to prevent the reaction of zirconium oxide with the graphite foil, which would form zirconium carbide. The resulting sintered lanthanum zirconium oxide materials formed using each of the 40 mm die and the 4- inch die were confirmed to be 100 percent LaaZnO? via XRD phase analysis and the density of the sintered lanthanum zirconium oxide exceeded 99.5 percent of theoretical density.Example 2

[0063] (A) Two separate layers of two different powder mixtures were placed in a die for plasma spark sintering. One powder mixture was a combination of lanthanum oxide powder and zirconium oxide powder to react to form sintered lanthanum zirconium oxide, and the second powder mixture was a combination of alumina and zirconia to form zirconia toughened alumina. The two powder mixtures were placed as two separate layers in a 40 mm die and the powder mixtures were sintered at a sintering temperature of 1625 degrees Celsius using a sintering pressure of 25 MPa. A boron nitride-coated graphite foil was placed between the lanthanumoxide and zirconium oxide powder mixture and the die, with the boron nitride side of the foil facing the powder mixture. A graphite foil was placed between the alumina and zirconia powder mixture and the die. At this temperature and pressure the LZO powder mixture reacted with the ZTA powder mixture and the materials melted due to the sintering temperature being too high.

[0064] (B) In a similar process using the same die and the same two powder mixtures a sintering temperature of 1550 degrees Celsius was used. The powder mixtures were held at the sintering temperature for 30 minutes at a sintering pressure of 25 MPa. At this sintering temperature the powder mixtures did not react together and the process successfully produced a two-layer sintered ceramic body comprising a sintered lanthanum zirconium oxide layer and a zirconia toughened alumina layer.

[0065] (C) Using a larger diameter die (4 inch diameter or about 100 mm), a first powder layer of 202 grams of a powder mixture of lanthanum oxide and zirconium oxide was placed in the die, and a second powder layer of 404 grams of powder mixture of zirconia and alumina was placed over the first powder layer. The second powder contained about 22 percent by weight zirconia with the balance being alumina and small amounts of impurities. A boron nitride-coated graphite foil was placed between the lanthanum oxide and zirconium oxide powder mixture and the die, with the boron nitride side of the foil facing the powder mixture. A graphite foil was placed between the alumina and zirconia powder mixture and the die. The two powder layers were sintered at a sintering temperature of 1500 degrees Celsius for a sintering time of 45 minutes, at a sintering pressure of 15 MPa. This process successfully produced a two-layer sintered ceramic body having a sintered lanthanum zirconium oxide layer having a 96.3 percent relative density, and a zirconia toughened alumina layer having 94.6 percent relative density, as determined by dimensional analysis. Figure 4 is a photo of the produced 4-inch multi-layer sintered ceramic body 150 having sintered lanthanum zirconium oxide layer 152 bonded to zirconia toughened alumina layer 154. Body 150 shown at figure 4 includes cracking at the upper surface of sintered lanthanum zirconium oxide layer 152.Example 3 (prophetic)

[0066] The sintered multi-layer body of Example 2(C) was formed using ZTA that contained 22 weight percent zirconia, and the multi-layer sintered body that was formed contained cracks due to the difference in CTEs of the ZTA and the LZO.

[0067] Without having yet confirmed the following experimentally, the Applicant would expect that a process similar to that of Example 2(C) could be performed using ZTA with a higher zirconia content, e.g., up to or greater than 30, 35, 38, or 40 percent zirconia, that would have a CTE that more closely matches the CTE of the LZO, e.g., that has a maximum difference from the CTE of the LZO that is not greater than 1 ppm over the temperature range from 25 degrees Celsius to 1200 degrees Celsius. The Applicant expects that the process of Example 2(C) would be used to prepare a 100 millimeter diameter multi-layer sintered body having a layer of ZTA and a layer of LZO that does not contain cracks.

[0068] More specifically, using a larger diameter die (4 inch diameter or about 100 mm), a first powder layer of 202 grams of a powder mixture of lanthanum oxide and zirconium oxide would be placed in the die, and a second powder layer of 404 grams of powder mixture of zirconia and alumina would be placed over the first powder layer. The powder mixture of the second powder layer would contain about from about 30 to 40 percent by weight zirconia with the balance being alumina and small amounts of impurities. The amount of zirconia within this range would be selected to avoid cracking of the multi-layer sintered body during cooling after a sintering step.

[0069] A boron nitride-coated graphite foil would be placed between the lanthanum oxide and zirconium oxide powder mixture and the die, and the boron nitride side of the foil would face the powder mixture. A graphite foil would be placed between the alumina and zirconia powder mixture and the die. The two powder layers would be sintered at conditions to produce a multilayer sintered ceramic body, such as a sintering temperature of 1500 degrees Celsius for a sintering time of 45 minutes, at a sintering pressure of 15 MPa. The process would be expected to produce a two-layer sintered ceramic body having a sintered lanthanum zirconium oxide layer having a relative density of at least 96 percent, and a zirconia toughened alumina layer having a relative density of at least 94 percent. The process would be expected to produce a 100 millimeter diameter multi-layer sintered body that does not include cracking caused during cooling of the body after sintering.

Claims

Claims:

1. Sintered lanthanum zirconium oxide having a purity of at least 99.999 percent as measured using ICPMS and a density of at least 96 percent of theoretical density, wherein the pores have a maximum size of 5 microns measured over a surface of the lanthanum zirconium oxide.

2. The lanthanum zirconium oxide of claim 1 having an average grain size below 10 microns or having a maximum grain size below 10 microns.

3. The lanthanum zirconium oxide of any of claims 1 or 2 having a density of at least 5.964 grams per cubic centimeter measured according to ASTM B962-17 or having a coefficient of thermal expansion in a range from 7.5xl0‘6to I Ox I O‘fi / dcgrccs Celsius over a temperature range from 25 degrees Celsius to 1200 degrees Celsius.

4. A multi-layer sintered ceramic body comprising a layer of sintered lanthanum zirconium oxide of anyone of claims 1 to 3 and a layer of zirconia toughened alumina comprising from 20 to 25 weight percent zirconia and not more than 1 weight percent impurities in which a difference in the coefficient of thermal expansion from one layer to the other is no more than 3 x 10‘6 / degree Celsius over a temperature range from 25 degrees Celsius to 1200 degrees Celsius as measured in accordance with ASTM E228-17.

5. A method of making sintered lanthanum zirconium oxide according to anyone of claims 1 to 3, the method comprising: preparing a powder mixture comprising lanthanum oxide particles and zirconium oxide particles, placing the powder mixture in a graphite die, eliminating oxygen in the die, sintering the powder mixture in the die by applying pressure to the powder mixture and passing electric current through the die to increase the temperature of the powdermixture and cause the lanthanum oxide to react with the zirconium oxide powder and form sintered lanthanum zirconium oxide, comprising placing boron nitride coated graphite between a surface of the graphite die and the powder mixture, with the boron nitride coating contacting the powder mixture.

6. The method of claim 5, wherein the powder mixture comprises: lanthanum oxide particles have a purity of at least 99.99999 percent, and zirconium oxide particles have a purity of at least 99.99 percent, as measured using ICPMS, and preparing the powder mixture comprises: combining solvent with the powder mixture, and milling the powder mixture in an interior of a cylindrical ball mill container, the interior having a polymeric liner, and using stabilized zirconia milling media.

7. The method of claim 5 or 6, comprising at least one of the following steps of removing the solvent from the powder mixture by evaporation; after removing the solvent by evaporation, calcining the first powder mixture, wherein temperature of the powder mixture during calcining does not exceed 850 degrees Celsius.

8. The method of any of claims 5 through 7, wherein the electric current is non-pulsed DC current.

9. The method of any of claims 5 through 8, comprising sintering the powder mixture at a sintering pressure that does not exceed 100 MPa, and sintering the powder mixture at a sintering temperature that does not exceed 1625 degrees Celsius; preferably comprising sintering the powder mixture for a sintering time of less than 90 minutes, preferably comprising sintering the powder mixture at a sintering pressure that does not exceed 50 MPa, preferably comprising sintering the powder mixture at a sintering temperature that does not exceed 1550 degrees Celsius.

10. The method of any of claims 5 through 9, wherein the lanthanum zirconium oxide has one or more of: pores having a maximum size of 5 microns measured over a surface of the lanthanum zirconium oxide, an average grain size below 10 microns, and a maximum grain size below 10 microns.

11. A method of making a multi-layer sintered ceramic body comprising a layer of lanthanum zirconium oxide and a layer of zirconia toughened alumina, the method comprising: preparing a first powder mixture comprising lanthanum oxide particles and zirconium oxide particles, preparing a second powder mixture comprising zirconia and alumina, forming a layer of the first powder mixture in a graphite die, forming a layer of the second powder mixture in the graphite die, sintering the first powder mixture and the second powder mixture by applying pressure to the first powder mixture and the second powder mixture in the die and passing electric current through the die to increase the temperature of the first powder mixture and the temperature of the second powder mixture and cause the first powder mixture to form a layer of lanthanum zirconium oxide and to cause the second powder mixture to form a layer of zirconia toughened alumina, wherein preferably the first powder mixture comprises: lanthanum oxide particles having a purity of at least 99.9999 percent, and zirconium oxide particles having a purity of at least 99.99 percent, as measured using ICPMS.

12. The method of claim 11, wherein the second powder mixture comprises from 20 to 25 weight percent zirconia with the balance being alumina and not more than one weight percent impurities, or wherein preparing the first powder mixture comprises: combining solvent with the first powder mixture, and milling the powder mixture in an interior of a cylindrical ball mill container,the interior having a polymeric liner, and using stabilized zirconia milling media.

13. The method of claim 12, comprising after removing the solvent by evaporation, calcining the first powder mixture, wherein a temperature of the first powder mixture during calcining does not exceed 850 degrees Celsius; or placing boron nitride coated graphite between a surface of the graphite die and the first powder mixture, with the boron nitride coating contacting the first powder mixture; or comprising sintering the powder mixtures at a sintering pressure that does not exceed 100 MPa, and sintering the powder mixtures at a sintering temperature that does not exceed 1625 degrees Celsius; or sintering the powder mixtures for a sintering time of less than 60 minutes; or sintering the powder mixtures at a sintering pressure that does not exceed 50 MPa; or sintering the powder mixture at a sintering temperature that does not exceed 1550 degrees Celsius.

14. The method of any of claims 11 through 13, wherein the lanthanum zirconium oxide has one or more of: pores having a maximum size of 5 microns measured over a surface of the lanthanum zirconium oxide, an average grain size below 10 microns, and a maximum grain size below 10 microns.

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