Zirconia-toughened alumina, sintered ceramic bodies, and related methods

Abstract

Described is sintered zirconia-toughened alumina (ZTA), e.g., in the form of a sintered zirconia-toughened alumina body, and methods of preparing these sintered bodies, wherein the sintered ZTA includes silica to improve a machinability property of the sintered ZTA.

Description

[0001] ZIRCONIA-TOUGHENED ALUMINA, SINTERED CERAMIC BODIES, AND RELATED METHODS

[0002] FIELD

[0003] [1] The following describes sintered zirconia-toughened alumina (ZTA), e.g., in the form of a sintered zirconia- toughened alumina body, and methods of preparing these sintered bodies, wherein the sintered ZTA includes silica in an amount that improves machinability of the sintered ZTA body.

[0004] BACKGROUND

[0005] [2] Semiconductor processing equipment such as plasma etching chambers and plasma deposition chambers (referred to collectively as “plasma chambers”) generate and contain a reactive plasma atmosphere. The reactive plasma atmosphere is produced by exposing halogenbased gases to electromagnetic fields introduced into the plasma chamber interior. The plasma atmosphere must be maintained to be as free as possible from particle contaminants that cause defects in semiconductor devices. The reactive plasma atmosphere itself, however, can produce particle contaminants within the plasma chamber interior by reacting with surfaces of materials inside of the plasma chamber, causing those surfaces to degrade and shed tiny particles.

[0006] [3] One component of a plasma chamber is a window, which is a typically flat, e.g., diskshaped structure that is transparent to radio frequency (RF) and microwave energy. A window is used to allow RF or microwave energy generated by a source outside of a plasma chamber to be transmitted through the window into a plasma chamber interior. One surface of the window is exposed to plasma at the chamber interior and is made to resist degradation caused by the plasma. Examples of known plasma-resistant materials include ceramic materials such as yttrium oxide and yttrium alumina garnet (YAG), aluminum oxide, erbium oxide, variations of these, and many others.

[0007] [4] A window (also sometimes referred to as a chamber “lid”) is often a multi-layer structure that contains the plasma-resistant layer supported by a rigid ceramic support layer. In addition to being transmissive to RF and microwave energy, the ceramic support layer also exhibits physical properties such as mechanical strength, flexural strength, hardness, rigidity, corrosion resistance, high thermal conductivity, and low dielectric loss, which allow the window to function in the environment of a plasma chamber. SUMMARY

[0008] [5] Zirconia-toughened alumina (ZTA) is a material commonly used as a support layer of a multi-layer window of a plasma chamber, specifically because ZTA has high strength, “toughness,” rigidity, and hardness. But these mechanical properties, which are very useful or required for a support layer of a window, create challenges when processing the ZTA by machining techniques to produce a part that has precise dimensions as also required for a window of a plasma chamber.

[0009] [6] The support layer must be formed into a flat, typically disk-shaped body that meets precise size and shape requirements. A support layer may, for example, have a shape of a disk having a diameter from 200 to over 600 millimeters, with precise dimensions of thickness and diameter. Sintered ZTA is a common choice for the support layer but is known to have undesirable “machinability” due to its strong mechanical properties. In particular, steps of cutting and abrading sintered ZTA are very time consuming because of the hardness property. Sintered ZTA is time consuming to machine to a desired shape and rapidly wears machine tool surfaces.

[0010] [7] The following description is directed to the Applicant’s discovery that a very low amount of silica can be added to a sintered ZTA body to significantly improve machinability of the body relative to a comparable sintered ZTA body that contains no silica. Examples of these novel sintered ZTA bodies include: a first (continuous) crystalline phase that contains (comprises, consists of, or consists essentially of) alumina (AI2O3); from 5 to 35 percent by mass of a second (dispersed) crystalline phase that contains (comprises, consists of, or consists essentially of) zirconia (ZrCh); a very low amount of silica; and low levels of other materials as impurities.

[0011] [8] Additionally, while determining that the small amount of silica added to the sintered ZTA body will improve machinability, the Applicant has further determined that the small amount of silica does not substantially diminish other important properties of the sintered ZTA body. As shown quantitatively, a small amount of silica included in a sintered ZTA body can significantly improve the machinability of the body relative to a comparable “control” body that contains no added silica. The silica-containing sintered body with improved machinability can still exhibit physical properties that are comparable to properties of the control body, such as properties that are useful to use the sintered ZTA body as a component of a plasma chamber window. Examples of these properties include Vicker’s Hardness, Fracture Toughness, 4-Point Flexural Strength, Wcibull Modulus, and Thermal Conductivity.

[0012] [9] In one aspect, the disclosure relates to a sintered zirconia-toughened alumina body having a high purity, comprising: a first crystalline phase comprising AI2O3, and from 5 to 35 weight percent of a second crystalline phase comprising Z1O2, wherein the first crystalline phase is a continuous matrix and the second crystalline phase is dispersed in the continuous matrix, silica in an amount between 50 and 150 parts per million by mass, and not more than 5 parts per million by mass sodium.

[0013]

[0010] In another aspect the disclosure relates to a method of making a sintered zirconium- toughened alumina body. The method includes: preparing a powder mixture that contains: from 65 to 95 weight percent aluminum oxide particles that contain not more than 5 parts per million sodium, from 5 to 35 weight percent zirconium oxide particles, and between 50 and 1000 (e.g., from 50 to 150) ppm by mass silica; placing the powder mixture in a die; eliminating oxygen in the die; and sintering the powder mixture in the die to cause the powder mixture to form a sintered zirconium-toughened alumina body.

[0014]

[0011] In another aspect, the disclosure relates to a multi-layer sintered body that includes: a support layer comprising a sintered zirconia- toughened alumina body comprising: a first crystalline phase comprising AI2O3, and from 5 to 35 weight percent of a second crystalline phase comprising Z1O2, wherein the first crystalline phase is a continuous matrix and the second crystalline phase is dispersed in the continuous matrix, silica in an amount up to 1000 ppm by mass (e.g., up to 150 ppm by mass), and not more than 5 parts per million by mass sodium; and a plasma-resistant ceramic layer; wherein: the support layer has a thickness in a range from 5 mm to 40 mm, the plasma-resistant ceramic layer comprises yttrium oxide or yttrium aluminum garnet, and the body has a diameter of at least 200 millimeters.

[0015]

[0012] In yet another aspect the disclosure relates to a sintered zirconia-toughened alumina body that includes: a first crystalline phase comprising AI2O3, and from 5 to 35 weight percent of a second crystalline phase comprising Z1O2. wherein the first crystalline phase is a continuous matrix and the second crystalline phase is dispersed in the continuous matrix; silica in an amount between 50 and 1000 (e.g., from 50 to 125) parts per million by mass; and not more than 5 parts per million by mass sodium; wherein body has a 4-point flexural strength greater than 800 MPa measured according to ASTM C1161-13. BRIEF DESCRIPTION OF THE FIGURES

[0016]

[0013] Figure 1 shows a side view of a multi-layer ceramic body as described.

[0017]

[0014] Figures 2 shows mass loss during cutting of sample sintered ZTA bodies as described.

[0018]

[0015] Figure 3 shows CTE data of an example sintered ZTA body as described.

[0019]

[0016] Figure 4 is an image of an example sintered ZTA body in which the image was prepared by energy-dispersive X-ray spectroscopy (EDS).

[0020]

[0017] Figure 5A shows mass loss during cutting of an example sintered ZTA body as described.

[0021]

[0018] Figures 5B, 5C, and 5D show dimensional stability of a sintered ZTA body as described during post- sintering annealing.

[0022]

[0019] Figure 6 is an image of an example of ZTA doped with 1000 ppm silica in which the image was prepared by scanning transmission electron microscopy (STEM).

[0023]

[0020] Figure 7 is an image of the example of Figure 6 in which the image has been prepared by an EDS line scan across the boundary between two adjacent grains of alumina.

[0024]

[0021] Figure 8 is an image of a comparative example of undoped ZTA in which the image was prepared by scanning transmission electron microscopy (STEM).

[0025]

[0022] Figure 9 is an image of the example of Figure 8 in which the image has been prepared by an EDS line scan across the boundary between two adjacent grains of alumina.

[0026]

[0023] Figure 10 is an image of an example ZTA doped with 10,000 ppm by mass silica in which the image has been prepared by energy-dispersive X-ray spectroscopy (EDS).

[0027] DETAILED DESCRIPTION

[0028]

[0024] A sintered zirconia-toughened alumina (“ZTA” or Zr- AI2O3) body of the present description contains (comprises, consists of, or consists essentially of) alumina, zirconia, and a minor amount of silica to improve machinability of the body. Example bodies are highly pure, containing at least 99.99 or at least 99.999 weight percent alumina, zirconia, and silica, with any measurable impurities being present only at extremely low concentrations. Compared to a sintered ZTA body that does not contain any silica, a silica-containing sintered ZTA body may have better machinability, while many other physical properties (hardness, modulus) remain comparable to that of the silica-free sintered ZTA body.

[0025] As part of the sintered body, alumina is in the form of a continuous crystalline phase or “matrix,” and zirconia is in the form of a discontinuous crystalline phase dispersed within the continuous alumina matrix. The continuous crystalline alumina (AI2O3) phase or matrix is present as the major portion of the sintered ZTA body, e.g., a sintered ZTA body may contain at least 65 and up to 95 percent by weights alumina based on total weight of the sintered ZTA body. The dispersed (discontinuous) crystalline zirconia (ZrCh) phase may be present in a lower amount, e.g., a sintered ZTA body may contain from 5 to 35 percent by weight zirconia (e.g., from 20 to 35 percent by mass zirconia) based on total weight sintered ZTA. As used herein, a composition that “consists essentially of’ one or more listed ingredients includes the listed ingredients and not more than a minor amount of other materials, e.g., not more than 0.01 or 0.001 percent by weight of other materials. A sintered ZTA body that “consists essentially of’ alumina, zirconia, and silica includes the alumina, zirconia, and silica with not more than an insignificant amount of other materials, e.g., not more than 0.01 or 0.001 percent by weight of other materials such as trace levels of contaminants or impurities.

[0029]

[0026] The sintered ZTA body may have very low levels of impurities. Example sintered ZTA bodies may contain only the described alumina, zirconia, and silica, with less than 0.01 or 0.001 weight percent of any other materials (total), which would be considered impurities. Example sintered ZTA bodies may contain at least 99.9, 99.99, 99.999, or 99.9999 weight percent combined alumina, zirconia, and silica, with the silica being present in an amount below 1000, 200, or 150 ppm (mass), based on total weight sintered ZTA body.

[0030]

[0027] The sintered ZTA body is formed using a sintering process by which raw materials (e.g., a powder mixture) that include alumina powder, zirconia powder, and silica are combined and then heated to cause the alumina and zirconia to bond together without melting to form a solid sintered ZTA body. The initially-formed sintered ZTA body (“initial” body) (which may optionally be heat-treated, e.g., annealed) may be mechanically processed to refine the shape and dimensions of the initial body and form a “final” sintered ZTA body having precise shape and dimensions. Processes that may be used for refining the shape and dimensions of the initial sintered ZTA body include cutting, slicing, machining, abrading (e.g., grinding), or other mechanical processing steps.

[0031]

[0028] The sintered ZTA body is flat, meaning that it has dimensions of width, length, and thickness, with the length and width dimensions being significantly greater than the thickness dimension. The flat body has a shape in the length and width dimensions that may be square, rectangular, oval, or circular, and is often circular to form a flat disk-shaped body. Example thicknesses may be in a range from 5 to 40 millimeters. Example length and width dimensions or diameters may be in a range from 40 to 600 millimeters, such as up to or greater than 100, 200, 300, 400, 500, or 600 millimeters.

[0032]

[0029] Because sintered ZTA has good high hardness properties, mechanical processing of an initial sintered ZTA body to form a final sintered ZTA body is labor-intensive and time consuming. While it is possible to cut sintered ZTA using conventional cutting equipment such as a rotating saw, the process is very slow. Improving the machinability of a sintered ZTA body by adding silica reduces the amount of time needed to produce a final sintered ZTA body from an initial sintered ZTA body. A sintered ZTA body that may be machined (e.g., cut) more easily using conventional equipment, e.g., a rotating saw, bore, or other machine tool in a reduced amount of time improves the overall efficiency and throughput of processes for forming final sintered ZTA bodies. A sintered ZTA body that exhibits the improved machinability property while still maintaining other useful physical and mechanical properties is even more highly desirable.

[0033]

[0030] The silica is included in raw materials, e.g., a powder mixture, used to prepare a sintered ZTA body. The silica may be of a type and may be present in an amount that is effective to improve the machinability of the sintered ZTA body, particularly by reducing an amount of time required to cut the sintered ZTA body by a conventional cutting method, preferably without unduly affecting other physical properties of the sintered ZTA such as strength, hardness, toughness, etc. According to example sintered ZTA bodies, one or more measured properties of a sintered ZTA body that contains silica to improve machinability, e.g., one or more of a measured strength, hardness, or toughness property, can be comparable to (e.g., at least 80, 90 or 95 percent of) a similarly measured property of a comparable sintered ZTA body that does not contain any added silica.

[0034]

[0031] Silica may be combined with alumina and zirconia in a powder mixture of a sintering step in an amount that is effective to improve a machinability property of a sintered ZTA body. According to example methods, a powder mixture used to form sintered ZTA can contain alumina, zirconia, and up to 1000 pails per million (mass) silica based on total weight alumina and zirconia, e.g., from 50 to 150, 300, 500, or 1000 parts per million silica by mass. The amount of silica combined with alumina and zirconia can be an amount that limits the amount of mullite (AhC -SiC ) produced during sintering; such a sintered ZTAbody may preferably contain no mullite. Mullite can be detected and an amount of mullite in a sintered body can be measured using x-ray diffraction (XRD).

[0035]

[0032] A sintered ZTA body as described contains low levels of all other materials, i.e., impurities that may be present in a raw material, e.g., alumina, used to produce the sintered ZTA. Alumina powder is commercially available with a range of different types and amounts of impurities. Many alumina powders have significant levels of impurities such as alkaline earth metals, particularly as oxides. Examples of alumina materials with significant levels of impurities may contain up to 20 percent by weight impurities (total). The main types of chemical impurities are iron (as FeiCh), silica (SiCE), calcium (as CaO), soda (Na2O), phosphorus (as P2O5) and traces of others such as gallium (as Ga2Oa), zinc (as ZnO), and titanium (as TiCh).

[0036]

[0033] Alumina materials used to form a sintered ZTAbody can have a very low level of impurities and may contain at least 99.99, 99.999, or 99.9999 percent by weight alumina. The alumina may contain less than 10 or 5 parts per million (ppm) by mass of the following impurities measured individually: sodium (Na), potassium (K), magnesium (Mg), silicon (Si), calcium (Ca), titanium (Ti,) vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), and gallium (Ga).

[0037]

[0034] The zirconia may be pure (unstabilized) zirconia or may be yttria-stabilized zirconia, which contains zirconia and an amount of yttria in a range up to 8 mole percent yttria based on yttria- stabilized zirconia. The zirconia or yttria-stabilized zirconia used to prepare a sintered ZTAbody as described can have very high purity, may contain at least 99.99, 99.999, or 99.9999 percent by weight zirconia or zirconia and yttria.

[0038]

[0035] A zirconia-toughened alumina body can be formed using a sintering method, many of which are known. By a sintering method, a powder mixture that contains (comprises, consists of, or consists essentially of) highly pure aluminum oxide (alumina) powder, highly pure zirconium oxide (zirconia) powder, and silica is heated, optionally with pressure being applied to the powder mixture, to cause the alumina and the zirconia to become fused together without melting to form a solid, dense, low porosity sintered ZTA body.

[0036] According to example sintering processes, a powder mixture can contain from 65 to 95 percent by weight alumina and from 5 to 35 percent by weight zirconia, and a very low amount of silica as described.

[0039]

[0037] The alumina powder, zirconia powder, and silica are combined and mixed to form a homogeneous powder mixture. By one example, the powders and silica 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 powder mixture 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 zirconia, so that if the zirconia milling media does shed, the shedding only adds zirconia particles, which are already present in the powder mixture being mixed. Alumina milling media may also be used as shedding only adds alumina particles that are already present in the powder mixture. An attrition mill may be used as an alternative to a ball mill to reduce milling time.

[0040]

[0038] Solvent can be included with the powder mixture during a milling process. A useful solvent can be selected to be non-reactive with materials of the powder mixture, with an example thereof being ethanol.

[0041]

[0039] Optionally, after milling, the powder mixture can be processed by calcining 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, and that is effective to remove impurities or volatile materials from the powder mixture. A step of calcining a powder mixture as described can heat the powder mixture 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.

[0042]

[0040] Examples of useful sintering methods include spark plasma sintering and hot press sintering. As used herein the term “spark plasma sintering” (“SPS”) refers to a method of bonding together individual particles of a powder mixture to form a dense (low porosity) sintered material (a.k.a. “sintered body”) by applying pressure to the particles while the particles arc heated in a die by electric current passing through the die. The particles are heated to a temperature that is below the melting point of the particles but also sufficiently high to cause the individual particles to become bonded together by atomic diffusion at particle surfaces. A spark plasma sintering method uses contemporaneous application of uniaxial pressure and heat generated by electric current passing through the die to increase the temperature of the 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 for sintering. A hot-press may be used as an alternative to an SPS machine.

[0043]

[0041] According to example spark plasma sintering methods, the powder mixture can be placed within the interior of a graphite die in a controlled, oxygen-free atmosphere such as a vacuum or other atmosphere devoid of oxygen. The oxygen-free atmosphere is effective to prevent the graphite die from reacting or combusting. The pressure (“sintering pressure”), rate of temperature increase (temperature profile), maximum temperature or temperature range (“sintering temperature”), type of electric current passing through the die, and amount of time that the powder mixture is held at a sintering temperature (“sintering time”), are factors that can be controlled to produce a sintered ZTA body that has mechanical properties as described herein.

[0044]

[0042] Examples of useful sintering pressures can be up to about 200 megapascals (MPa), e.g., up to 100 MPa, or up to 25 or 50 MPa, such as a pressure in a range from 10 to 50 MPa. Lower pressures in a range from 10 to 50 MPa are preferred to avoid damaging the die, and more preferably from 10 to 30 MPa. Examples of useful sintering temperatures may be up to 1700 degrees Celsius, e.g., from 1400 to 1700 such as from 1450 to 1650 degrees Celsius. A useful sintering time may be up to 180 or 120 minutes, e.g., up to 90 minutes or up to 60 minutes for dies of 100 mm to 150 mm diameter or less. Differently sized dies may require different sintering times, with larger sized dies generally requiring longer sintering times.

[0045]

[0043] A sintered ZTA body may be processed after sintering by a thermal treatment step, such as by annealing in an oxygen atmosphere. A post- sintering annealing process leads to a refinement of the chemical and physical properties of a sintered ZTA body through reduction of oxygen vacancies for stoichiometric correction and reduced stress in the sintered body. The optional process step of annealing may be performed in an oxidizing atmosphere, whereby the annealing process may provide one or more of increased albedo, improved mechanical handling, and reduced porosity.

[0046]

[0044] As now identified by the Applicant, the machinability of a sintered ZTA body may be significantly improved by adding a very small amount of silica to the sintered ZTA body. By example methods, one being described in Example 1, a machinability property of a sintered ZTA body can be quantified as an amount of ceramic material that is removed from a sample body under experimentally controlled conditions. For example, a rotating cutting tool may be used to cut material from a sample body of specific dimensions at a specific cutting pressure, at a specific rotational speed, and for a specific amount of time. The cutting tool removes an amount of ceramic material from the sample, which may be measured and assessed for comparison as a loss of mass of ceramic material relative to an initial mass of the body, i.e., a “percent mass loss.” Comparing percent mass loss of a control sintered body that contains no added silica to percent mass loss of sintered ZTA bodies that contain low amounts of added silica shows that the machinability (percent mass loss that occurs under controlled test conditions) of a sintered ZTA body significantly improves by adding just a very small amount of silica to a sintered ZTA bodies.

[0047]

[0045] Using example testing parameters, a control sintered ZTA body that contains no added silica may be measured to have a percent mass loss value that is very small, such as below 0.01 or 0.005 percent. The exact measured values for different sintered ZTA bodies will differ based on testing parameters (timing, pressure, RPM of a cutting tool); instead of a specific measured percent mass loss value, a relevant comparison can be made based on relative values of mass loss of a control sample compared to a percent mass loss of a sample that includes added silica, which are substantially greater.

[0048]

[0046] Bodies that include added silica have now been shown to exhibit percent mass loss values that far exceed those of control bodies, indicating improved machinability due to the added silica. A body that contains as low as 50 ppm silica may have a percent mass loss (e.g., an average of at least 4 samples) that is at least 3, 5, 10, or 20 times the percent mass loss of a control body that contains no added silica. Figures 2A and 2B show percent mass loss data for sintered ZTA bodies that contain different amounts of added silica to improve machinability compared to a control sintered ZTA bodies that contain no added silica. These figures show significant improvement in machinability (increased percent mass loss) of sintered ZTA bodies that contain added silica at levels as low as 50 ppm (mass) relative to a control body with no added silica.

[0049]

[0047] Moreover, sintered ZTA bodies that contain added silica have also been shown to exhibit highly useful physical and mechanical properties. Table 1 shows example properties that may be achieved in sintered ZTA bodies that contain added silica to improve machinability, and that are useful for a sintered ZTA body used as a window of a plasma chamber.

[0050] Table 1

[0051]

[0048] Figure 1 shows example multi-layer ceramic body 98, useful as a window of a plasma chamber. Multi-layer body 98 includes plasma-resistant layer 100 in combination with support layer 102 in the form of sintered ZTA as described. Plasma-resistant layer 100 may be a ceramic layer that is known to exhibit plasma resistance along with other useful properties of a window for use in a plasma chamber. Example ceramic layers may be made from yttria (yttrium oxide), yttrium alumina garnet (YAG, Y3AI5O12), alumina (aluminum oxide), , YAM

[0052] (Y4AI2O9 (2Y2O3AI2O3) (yttrium aluminum monoclinic), YAP(YA1C>3 (Y2O3.A12O )(yttria aluminum perovskite), quartz (SiCE), fluorinated materials such as yttrium trifluoride (YF3) and yttrium oxyfluoride (YOF), erbia (erbium oxide, E CE), erbium aluminum perovskite (ErAP, ErAlO q, erbium aluminum garnet (ErAG, E AEO^), as well as other rare earth oxides or rare earth elements.

[0053]

[0049] Plasma- resistant layer 100 may be prepared and incorporated into multi-layer ceramic body 98 by any method, such as by a coating or deposition method by which the plasma-resistant layer is applied to a surface of support layer 102 after preparing the support layer. Alternately, multi-layer ceramic body may be formed by a sintering method, e.g., a co-sintering method by which multi-layer body 98 is formed by a single sintering step to include both sintered support layer 102 and sintered plasma-resistant layer 100.

[0054]

[0050] To provide high mechanical strength and rigidity, the thickness d2 of support layer 102 is preferably greater than the thickness dl of plasma- resistant layer 100. The thickness dl of plasma-resistant layer 100 may be any thickness that provides desired plasma-resistant functionality, e.g., from 0.01 to 10 millimeters, or from 1 to 8 millimeters (mm). A thickness dl of plasma-resistant layer 100 may be as desired and can depend on a method used to form the plasma-resistant layer, with thicknesses that are outside of these exemplary ranges also be useful. Example thicknesses d2 of support layer 102 may be from 5 or 9.5 to 40 millimeters, for example from 30 to 40 mm, e.g., 35.5 mm. Multilayer ceramic body 98 may have a total thickness (dl + d2) in a range from about 10 to about 50 millimeters, e.g., from 32 to 48 mm. In certain embodiments, it may be desirable to minimize the thickness of plasma-resistant layer 100, and as such the multi-layer ceramic body 98 may be optionally machined after sintering and / or after a post-heat treatment (e.g., annealing) to reduce the thickness dl of layer 100 to modify electrical, thermal or other properties of multi-layer sintered body 98 or a device formed therefrom.

[0055]

[0051] To form a multi-layer sintered body 98 to include both support layer 102 and plasmaresistant layer 100 by a single sintering step, a sintering method as described herein may be used, e.g., a spark plasma sintering method. A layer of ZTA powder is formed in a die, and a second layer of plasma-resistant ceramic powder such as yttria oxide or yttria alumina garnet is formed in the die in contact with the layer of ZTA powder. The two layers of powder are sintered together in the die to form a multi-layer sintered body that includes two integrated layers of the co-sintered powders: the support layer (sintered ZTA) formed from the layer of ZTA powder, and the plasma-resistant layer formed from the layer of plasma-resistant ceramic powder.

[0056]

[0052] A sintered ZTA body or a multi-layer sintered body that includes a ZTA support layer and a plasma-resistant layer may be processed after sintering by thermal treatment, such as by annealing in an oxygen atmosphere. The optional post-sintering annealing step may be performed in an oxidizing atmosphere to may provide increased albedo, improved mechanical handling, reduced porosity, and improve coloration of a plasma-resistant layer.

[0057]

[0053] During a post-sintering annealing step, a sintered ZTA body will preferably exhibit stable physical and dimensional properties such as a low change in density, thickness, and width of the body due to the annealing process. Figures 5A, 5B, and 5C show that sintered ZTA bodies that contain added silica in an amount to improve machinability of the body also exhibit good dimensional stability compared to a control sintered ZTA body that contains no added silica.

[0058] EXAMPLES

[0059] Example 1

[0060]

[0054] Sample ceramic disks of sintered ZTA were prepared and tested for machinability based on the ability to remove material from a sample disk by cutting. Four sintered ZTA disks (40 millimeter diameter) were prepared as described, each containing silica as indicated. Each disk was prepared by sintering 20 g of a ZTA powder mixture having a ratio of 3.44 alumina to zirconia by weight, in which the mixture was wet milled with alumina milling media. During spark plasma sintering, the powder mixture was heated to 1600 degrees Celsius under 25 MPa pressure and held at 1600 degrees Celsius for 5 minutes, followed by cooling. Each disk was thermally annealed at 3 °C / min to 1400 °C and held at 1400 °C for 8 hours. Disks 1 through 4 included the following amounts of silica:

[0061] 1. 100 ppm (by mass) silica

[0062] 2. 500 ppm (by mass) silica

[0063] 3. 1000 ppm (by mass) silica

[0064] 4. 2000 ppm (by mass) silica

[0065]

[0055] Before cutting, each disk was ground so that the thickness of each disk was equal (approximately 1.95 mm in thickness) and the mass each of disk was recorded (each disk massed approximately 10.5 g + 0.25 g). Thereafter, a blade of a rotating saw was applied to each disk for one minute at 1000 rpm using the same cutting force each time. The saw used was a model PICO-155S commercially available from Pace Technologies in Tucson, Arizona.

[0066]

[0056] The mass of each disk was weighed after a disk was cut to determine the amount of mass cut away by the saw in one minute. After each disk was cut, the saw blade was dressed to ensure the sharpness of the blade was approximately the same for each trial. The results (see Figure2) show an improvement in machinability (greater mass loss during cutting) that increases with the amount of silica. Three trials were performed for each disk and the results averaged over the three trials. The results are shown in Figure 2.

[0057] The data in Figure 2 shows a significant improvement in machinability based on this cutting test by including extremely low amounts of silica in a sintered ZTA body, c.g., up to 1000 ppm, including mass loss values of at least about 0.2 or about 0.25 percent using less than 200 or 150 ppm silica, which are substantially greater than, i.e., multiple times greater than, a control sintered ZTA body with no added silica.

[0067] Example 2

[0068]

[0058] These amounts of silica in the sintered ZTA body improved machinability but did not unduly affect mechanical properties of a sintered ZTA body. Separate sample bodies were prepared and tested for strength, dielectric properties, and hardness and toughness, and CTE.

[0069]

[0059] The added silica has an acceptable effect on the strength of the ZTA. ZTA devoid of silica has a beam strength of 956.37 ± 159.71 MPa. Silica doped with 1000 ppm silica has a beam strength of 907.76 ± 164.27 MPa. The strength of the ZTA was determined using a 4-point flexural strength method in accordance with ASTM C1161-13. Strength data gathered in accordance with the foregoing method was further used to derive a Weibull modulus for undoped ZTA and ZTA doped with 1000 ppm SiCF. As is well-known in the field of materials science, the Weibull modulus is a dimensionless parameter describing the variability in strength of brittle materials, such as ceramics. A greater Weibull modulus indicates a narrower distribution of strengths, indicating a more homogeneous material, which is preferable.

[0070] Beam Strength

[0071] Weibull Modulus

[0072]

[0060] The dielectric constant and dielectric loss (“loss”) of the ZTA were measured using the split-post-dielectric-resonator (SPDR) method (see Janezic et al, Split-Post and Split-Cylinder Resonator Techniques: A Comparison of Complex Permittivity Measurement of Dielectric Substrates, J of Microelectronics and Electronic Packaging (2009) 6, 97-100). For the measurement, a model VNA P9372A and QWED SPDR fixture available from Keysight Technologies, Inc. of Santa Rosa, California, are connected to one another by a microwave coaxial cable. ZTA test samples 40 mm in diameter were sintered and ground to a thickness of 0.8 + 0.1 mm. The samples are oxidized at high temperature to remove any organic compound that could impact electrical properties. The samples were each tested and dielectric constant and dielectric loss were measured as shown in the table below. The hardness of the ZTA was measured in accordance with the Vickers indentation method as provided in ASTM Cl 327. Toughness was measured using an indentation method based on the Anstis technique first set forth by G.R. Anstis (see Anstis et al., Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness, I, Crack Measurements, J. Am Ceramic Soc. Vol 64 No 9). The brittleness index was measured as described by the paper by Lawn et al., identified below. The foregoing properties were affected in acceptable amounts, as shown by the following data.

[0073] Dielectric data for silica doped ZTA at 100, 500, 1000, 2000 ppm SiCE.

[0074] Hardness and Toughness Data for doped ZTA

[0075] Brittleness index definition taken from

[0076] LAWN, B.R. and MARSHALL, D.B. (1979), Hardness, Toughness, and Brittleness: An Indentation Analysis. Journal of the American Ceramic Society, 62: 347-350. https: / / doi.Org / 10.llll / j.1151-2916.1979.tbl9075.x

[0061] CTE data for 1000 ppm SiCh-containing ZTA is shown in Figure 3. The data was collected using a dilatometer to make the measurements. The dilatometer used was a model L75 vertical dilatometer available from Linseis Inc. of Robbinsville, New Jersey, USA.

[0077]

[0062] At least 2000 ppm (by mass) silica can be included in sintered ZTA without causing silica rich-areas in the sintered ZTA. At 2000 ppm silica, energy-dispersive X-ray spectroscopy (EDS) shows the presence of silica-rich areas (see Figure 4). In particular, the brighter areas indicate the presence of silica rich areas. Below 2000 ppm silica, EDS does not indicate silica rich areas. Silica rich areas in the ZTA introduce flaws in the microstructure that could serve as fracture points as silica does not have the strength of ZTA. Further, silica is prone to erosion in a plasma reaction chamber and therefore silica rich areas are not desirable. While ZTA is intended as a support layer 102 as illustrated in Figure 1 and is shielded by a plasma-resistant layer 102, there is typically a hole formed through the center of the multilayer ceramic 98. The walls of the hole expose a portion of the ZTA to the environment of the plasma reaction chamber and thus it is disadvantageous to have silicon rich areas present in the ZTA that are prone to erosion. Finally, the dielectric loss at 2000 ppm silica begins to increase more rapidly, further indicating that doping with silica at 2000 ppm is the preferable upper limit.

[0078] Example 3

[0079]

[0063] Another set of five sample 40 mm disks were pressed using 30 grams of ZTA powder for each disk and respectively doped with the following amounts of silica by weight: 50, 100, 500, 750, and 1000 ppm. The disks were pressed in an SPS machine at a temperature of 1600 °C and a pressure of 25 MPa. Each disk was ground to a thickness of 3.115 ± 0.007 mm. A coupon was formed from each disk by making two parallel cuts to a width of 20.858 ± 0.027 mm.

[0080]

[0064] Thereafter, machinability testing was performed on each coupon using a Dremel cutting tool mounted on a rail to apply an equal amount of cutting / grinding pressure to each coupon for 20 seconds. The weight of each coupon was measured before and after the cutting / grinding to determine the average percent of mass loss by each coupon over three trials The results are shown in Figure 2B.

[0081]

[0065] The data at Figure 5A show a significant improvement in machinability based on cutting tests by including extremely low amounts of silica in a sintered ZTA body, e.g., from 50 ppm up to 1000 ppm. Resultant mass loss values are at least about 0.25, about 0.3, or about 0.35 percent using less than 200 or 150 ppm silica, which are substantially greater than, i.e., multiple times greater than, a control sintered ZTA body with no added silica.

[0082]

[0066] The density of each Example 3 coupon was determined by the Archimedes method in accordance with ASTM B962-17, a thickness measurement was performed on each coupon, and a width measurement was performed on each coupon. These coupons were then placed on inert platforms in a lab furnace and thermally annealed at 3 °C / min to 1400 °C and held at 1400 °C for 8 hours. Following thermal annealing, the density, thickness, and width measurements were repeated and the results shown in Figures 5B, 5C, and 5D.

[0083] Example 4

[0084]

[0067] Magnesia (MgO) and Yttria (Y2O3) also exhibit desirable brittleness index measurements, indicating improved machinability as shown by the table above.

[0085]

[0068] Each material that contains a metal oxide shows a lower brittleness index, which indicates improved machinability. Both the cutting test and the brittleness index can indicate useful or improved machinability of sintered ZTA. The measured values of brittleness index and the results of the cutting test do not necessarily correspond, but the cutting test is believed to be the better indicator.

[0086]

[0069] Figure 6 is an image of an example of ZTA doped with 1000 ppm by mass silica. The image was prepared by scanning transmission electron microscopy (STEM) at 18,000 times magnification (19k magnification). At that magnification, individual grains in the example of ZTA are visible. The rectangular area indicated by reference numeral 10 in Figure 6 encloses an area spanning two adjacent grains of alumina. An EDS line scan was performed across this area along a path over the boundary between the grains. Figure 7 shows the results of the EDS line scan. As shown in Figure 7, there is very little silicon present within the grains, no more than 0.20 atomic percent. As the line scan approaches the boundary between the grains of alumina, the amount of silicon increases rapidly to a peak between 1.1 and 1.2 atomic percent. Thereafter, the amount of silicon decreases rapidly as the line scan passes beyond boundary between the grains of alumina, to an amount of less than 0.2 atomic percent. That is, the amount of silicon at the boundary between the grains of alumina is between five and six times greater than the silicon present within the grains.

[0070] Figure 8 is an image of a comparative example of undoped ZTA. The image was prepared by STEM at 100,000 times magnification (100k magnification). The rectangular area indicated by reference numeral 12 in Figure 8 encloses an area spanning two adjacent grains of alumina. An EDS line scan was performed across this area along a path over the boundary between the grains. Figure 9 shows the results of the EDS line scan. As shown in Figure 9, there is much less silicon present in the grain boundary between alumina grains compared to the doped ZTA. With the undoped ZTA, the amount of silicon present in the boundary between alumina grains peaks between 0.4 and 0.5 atomic percent, at approximately 0.43 atomic percent. That is less than half of the peak amount of silicon present at the boundary between adjacent alumina grains for the doped ZTA example of Figure 6. In Figures 6 and 7, a greater amount of silicon is present at the grain boundary than zirconium, aluminum, oxygen and yttrium atoms. In comparison with Figure 8, the undoped ZTA example, there are more zirconium and yttrium atoms present at the boundary between alumina grains than silicon atoms (see Figure 9).

[0087]

[0071] Figures 6 and 7 indicate that the silica added as dopant to the ZTA tends to accumulate at grain boundaries and thereby does not significantly affect other properties of the ZTA while increasing the machinability of the material. While not being bound by a particular theory of how the silica increases machinability, it may be that the silica accumulation at grain boundaries promotes cleavage between grains during machining, thereby increasing machinability.

[0088]

[0072] The images in Figures 6 and 8 were analyzed to estimate the thickness of the grain boundary between alumina grains. In the doped silica example of Figure 6, the average grain thickness was determined to be 2.03 nm + a standard deviation of 0.17 nm. In contrast, the average grain boundary thickness of Figure 8 for the undoped silica example was determined to be 1.47 nm + a standard deviation of 0.15 nm. The average was determined by measuring the thickness at twenty-five different locations along the grain boundary in each example and taking the average thereof. As will be appreciated, the silica doped example is approximately 38% thicker, which further indicates that the silica accumulates at the grain boundaries in the doped example, and not within grains. Thicker grain boundaries are generally not preferred because thicker grain boundaries are more vulnerable to erosion in a plasma reaction chamber than thinner grain boundaries. However, the increased boundary thickness is acceptable with 1000 ppm by mass silica doping. While 2000 ppm by mass silica doping is the preferably upper limit, the increased grain boundary thickness indicates that a more preferably upper limit is between 1000 ppm and 2000 ppm by mass, and even more preferably not more than 1000 ppm by mass.

[0089]

[0073] Example 5

[0090]

[0074] Example 5 is ZTA doped with 10,000 ppm by mass silica. This example was prepared by SPS in a graphite die of approximately 101.6 mm internal diameter (4 inches). A powder mixture of zirconia and alumina weighing 300 g total and doped with 10,000 ppm silica was placed in the die. The die was inserted into the furnace chamber of an SPS machine and heated to a target temperature of 1500 degrees C under 15 MPa pressure and held at the target temperature for 30 minutes, allowed to cool and the sintered material removed from the die. Figure 10 is an image of Example 5 (the sintered material) in which the image has been repared by energy-dispersive X-ray spectroscopy (EDS). In contrast to Figure 4, bright areas are distributed throughout the image in Figure 10, indicating the presence of numerous silica rich areas. The numerous silica rich areas in Figure 10, i.e., the bright areas, have the possibility of introducing many flaws into the microstructure that could serve as fracture points since silica is not as strong as ZTA. While 10,000 ppm silica by mass may improve the machinability of ZTA further, the numerous silica rich areas make Example 5 a non-preferred example of ZTA for applications requiring high strength, such as a support layer 102 as shown in Figure 1. In addition, as previously noted, silica is prone to erosion in a plasma reaction chamber and thus silica rich areas are disadvantageous. Example 5 is therefore described as a comparative example for when ZTA has been doped with too much silica.

[0091]

[0075] The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limiting to the precise form or example disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from experimentation. Embodiments and examples were chosen and described in order to explain the principles and practical application in various embodiments and with various modifications as are suited to particular uses. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

Claims

Claims:

1. A sintered zirconia- toughened alumina body having a high purity, comprising: a first crystalline phase comprising AI2O3, and from 5 to 35 weight percent of a second crystalline phase comprising Z1O2. wherein the first crystalline phase is a continuous matrix and the second crystalline phase is dispersed in the continuous matrix, silica in an amount between 50 and 150 parts per million by mass, and not more than 5 parts per million by mass sodium.

2. The zirconia-toughened alumina body of claim 1, comprising from 20 to 35 weight percent zirconia.

3. The zirconia-toughened alumina body of claim 1 or 2, having improved machinability compared to zirconia-toughened alumina that is the same but does not contain silica.

4. The zirconia-toughened alumina body of claim 1 or 2, having a strength, a hardness, or a toughness that is at least 90 percent of a strength, hardness, or toughness of zirconia-toughened alumina that is the same but does not contain the silica.

5. The zirconia-toughened alumina body of claim 1 or 2, having a 4-point flexural strength of at least 800 measured according to ASTM C1161-13.

6. The zirconia-toughened alumina body of claim 5, having one or more of: a Weibull Modulus of at least 4 measured according to ASTM Cl 161- 13, a Vicker’s Hardness of at least 15 GPa measured according to ASTM C1161, and a thermal conductivity of at least 20 W / m-K measured according to ASTM E228- 17.

7. A multi-layer sintered body comprising: a support layer comprising a sintered zirconia-toughened alumina body of claim 1, and a plasma-resistant ceramic layer.

8. A multi-layer sintered body of claim 7, wherein: the support layer has a thickness in a range from 5 mm to 40 millimeters, the plasma-resistant ceramic layer comprises yttrium oxide or YAG (Y3A15O12), aluminum oxide, YAM (Y4AI2O9 (2Y2O3AI2O3) (yttrium aluminum monoclinic), YAP YAIO3 (Y2O3.AI2O3) (yttria aluminum perovskite), quartz (SiO2), yttrium trifluoride (YF3), yttrium oxyfluoride (YOF), erbia (erbia oxide), erbium aluminum perovskite (ErAP, ErAlCh), or erbium aluminum garnet (ErAG, EnAhOn), and the body has a diameter of at least 200 millimeters.

9. A plasma chamber that includes a multi-layer sintered body of claim 7 or 8 as a window.

10. A method of making a sintered zirconium-toughened alumina body, the method comprising: preparing a powder mixture comprising: from 65 to 95 weight percent aluminum oxide particles that contain not more than 5 parts per million sodium, from 5 to 35 weight percent zirconium oxide particles, and between 50 and 150 ppm by mass silica; placing the powder mixture in a die, eliminating oxygen in the die, and sintering the powder mixture in the die to cause the powder mixture to form a sintered zirconium-toughened alumina body.

11. The method of claim 10, comprising, after sintering, heat treating the body in an oxygencontaining atmosphere.

12. The method of claim 10 or 11, comprising 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.

13. A multi-layer sintered body comprising: a support layer comprising a sintered zirconia-toughened alumina body comprising: a first crystalline phase comprising AI2O3, and from 5 to 35 weight percent of a second crystalline phase comprising ZrCh, wherein the first crystalline phase is a continuous matrix and the second crystalline phase is dispersed in the continuous matrix, oxide selected silica in an amount up to 150 ppm by mass, and not more than 50 parts per million by mass sodium; and a plasma-resistant ceramic layer, wherein: the support layer has a thickness in a range from 5 mm to 40 mm, the plasma-resistant ceramic layer comprises yttrium oxide or yttrium aluminum garnet, and the body has a diameter of at least 200 millimeters.

14. A plasma chamber that includes a multi-layer sintered body of claim 13 as a window.

15. A sintered zirconia- toughened alumina body comprising: a first crystalline phase comprising AI2O3, and from 5 to 35 weight percent of a second crystalline phase comprising ZrCh, wherein the first crystalline phase is a continuous matrix and the second crystalline phase is dispersed in the continuous matrix, silica in an amount between 50 and 125 parts per million by mass, and not more than 5 parts per million by mass sodium, wherein body has a 4-point flexural strength greater than 800 MPa measured according to ASTM C1161-13.

16. The zirconia-toughened alumina of claim 15, having one or more of: a Weibull Modulus of at least 4 measured according to ASTM Cl 161-13, a Vicker’s Hardness of at least 15 GPa measured according to ASTM C1161, and a thermal conductivity of at least 20 W / m-K measured according to ASTM E228-17.

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