WO1998047176A1

WO1998047176A1 - Composite ceramic dielectrics

Info

Publication number: WO1998047176A1
Application number: PCT/US1998/007054
Authority: WO
Inventors: Peter C. Smith; Randel F. Mercer; Robert M. Wood
Original assignee: The Morgan Crucible Company Plc
Priority date: 1997-04-11
Filing date: 1998-04-09
Publication date: 1998-10-22

Abstract

An electrostatic chuck (5, 7, 9) is disclosed comprising a base (14), at least one electrode in the form of an electrically conductive layer (13) on the base (14), and a dielectric layer (12, 18) overlaying the conductive layer (13), the dielectric layer (12, 18) comprising inorganic particulate material dispersed in an inorganic matrix phase, the matrix phase being a glass or glass-ceramic, and in which the dielectric layer (12, 18) has a coefficient of thermal expansion substantially matched to that of the base (14). In a second aspect an electrostatic chuck (4) is disclosed comprising an electrode in the form of an electrically conductive base (11), and a dielectric layer (10) overlaying and adhering to the conductive base (11) without any deposited intermediate interlayer, the dielectric layer (10) comprising inorganic particulate material dispersed in an inorganic matrix phase, the matrix phase being a glass or glass-ceramic, and in which the dielectric layer (10) has a coefficient of thermal expansion substantially matched to that of the base (11).

Description

COMPOSITE CERAMIC DIELECTRICS

FIELD OF THE INVENTION

The present invention relates generally to workpiece processing, and more particularly, to an electrostatic chuck for immobilizing articles, such as a semiconductor wafer. The present invention also relates to a method for making the electrostatic chuck.

BACKGROUND OF THE INVENTION

In a variety of circumstances, a workpiece must be held firmly in position throughout a multistep process. A variety of means, generally known as chucks, have been used to immobilize workpieces for processing. Three basic types of chucks include mechanical, vacuum, and electrostatic chucks.

Mechanical chucks physically engage the periphery of the workpiece by means of a mechanical arm and clamp. Although simple and inexpensive, mechanical chucks are disadvantageously associated with both contact damage and the production of contaminating debris, as well as with partial obstruction of the workpiece. Vacuum chucks have also been used in workpiece processing, and function by immobilizing a workpiece against a perforated surface behind which a vacuum is generated. Vacuum chucks are only useful in limited circumstances, however, and cannot be used in vacuum environments.

Electrostatic chucks provide a third alternative for immobilization of workpieces. Electrostatic chucks attract and hold a workpiece by Coulombic forces. In general, electrostatic chucks include a dielectric layer and a conductive layer. The opposing surfaces of the dielectric layer are in contact with the electrically conductive layer and the workpiece, respectively.

Electrostatic chucks provide a variety of advantages over their mechanical and vacuum counterparts. Compared to mechanical chucks, electrostatic chucks present a reduced risk of contact damage, contamination or surface obstruction. Unlike vacuum chucks, electrostatic chucks can be used in vacuum environments. For these reasons, and others, electrostatic chucks are commonly used in the processing of workpieces including semiconductor wafers.

A variety of materials have been proposed as dielectric layers, including both organic and inorganic materials. Organic dielectric layers are usually formed of epoxy or other plastic materials. See, e.g., U.S. 4,665,463. Inorganic materials include a variety of ceramics, including alumina. See, e.g., U.S. 5,600,530 and 5,663,865. The use of ceramics has increased with the trend toward higher workpiece processing temperatures. It has also been proposed to use glass ceramics to form dielectric layers. See WO 97/38481 and U.S. 5,207,437. The desired characteristics of an electrostatic chuck vary depending on the particular applications and the conditions of manufacture. In general, the composition of the dielectric layer is critical to chuck performance. Properties of interest include, among others, dielectric constant, electrical resistivity, thermal expansion coefficient, chemical resistance, wear resistance, fracture toughness and thermal conductivity. The ability to provide electrostatic chucks having variable properties is clearly desirable. In some cases ceramic dielectric chuck failures have been associated with difficulties in the top dielectric surface layer and its lack of thermal match with the underlying electrostatic chuck material.

Considerable effort has been directed to influencing, and particularly to increasing, the chucking force of electrostatic chucks. The force generated by a particular electrostatic chuck is largely dependent upon the characteristics of the dielectric layer.

Various efforts to increase the attractive force of an electrostatic chuck have focused on the thickness of the dielectric layer, which is inversely related to attractive force. See e.g., U.S. 5,384,682 and U.S. 5,600,530.

Other efforts to influence chucking force have focused on electrical properties such as the dielectric constant and the electrical resistivity. One approach has been to select ceramic materials having high dielectric constants and/or controlled resistivity for use in particular chucks. In high temperature applications, for example, conventional dielectric materials, such as alumina, are replaced by dielectrics having higher resistivities, such as pyrolitic boron nitride or a pyrolytic composite nitride of boron and silicon (see U.S. 5,663,865). By selecting materials based on electrical properties, however, the variety of materials that can be used to make electrostatic chucks is limited. Moreover, materials may not be available which exhibit particularly desirable electrical properties.

A second approach has been to alter, rather than substitute, a particular ceramic. Specifically, it has been proposed to alter the resitivity of alumina over a broad range by adding varying amounts a transition metal oxide, such as titania, to the dielectric during manufacturing. U.S. 5,384, 682, for example, discloses an electrostatic chuck having a dielectric layer made of a solid solution of alumina with a transition metal oxide such as titania or chromia added for adjusting the resistance of the dielectric layer. U.S. 5,384,681 discloses similar dielectric layers comprising solid solution alumina grains with variable proportions of transition metal oxide having the structure of corundum, and a glass component present in grain boundaries. U.S. No. 5,668,524 provides a further example of a ceramic dielectric containing varying amounts of atoms selected from the group consisting of Groups 2b, 4b and 6b of the periodic table. WO 97/38481 to Lu et al. discloses compositions comprising a glass ceramic for use as dielectric layers in chucks. Specific compositions comprising MgO, Al₂O₃ and SiO₂ are claimed and the examples show the use of a metallic interlayer to improve adhesion in a monopolar chuck. Some of the examples disclose composite materials comprising a cordierite glass ceramic matrix and 10% by volume of particles of aluminium oxide, boron nitride, or titanium oxide. These materials were tested as pellets for resistivity and not used as a dielectric layer in a chuck.

The structural characteristics of the dielectric layer constitute another important variable in electrostatic chuck design. The thermal coefficient of expansion (TCE) represents one important property of the dielectric layer. Specifically, the TCE of the dielectric layer must match the TCE of the base to ensure that chucks don't warp during heating and cooling, and to avoid stresses which could cause mechanical failure of the dielectric layer.

It has therefore been suggested to provide an electrostatic chuck with matched ceramic layers. See, e.g., U.S. 4,480,284. Alternatively, it has been suggested to provide an electrostatic chuck with mismatched ceramic layers, where mismatch permits the introduction of a desired residual stress into the dielectric layer. U.S. 5,600,530. Whatever the desired relationship, it is typically achieved by selecting materials on the basis of their thermal coefficient of expansion. This strategy, however, limits the variety of materials that can be used to manufacture the electrostatic chuck.

Moreover, a material characterized by a particularly desirable TCE may not be available. In some circumstances, the corrosion resistance of a particular dielectric material will also be important to the function of an electrostatic chuck. The trend toward an increase in use of corrosive process chemicals in integrated circuit manufacture has required an increase in chuck corrosion resistance. In general, electrostatic chucks having increased corrosion resistance are manufactured from dielectric materials chosen for their corrosion resistance properties. Thus, the type of materials that can be used in manufacturing dielectric layers is again limited, and material exhibiting a particularly desirable resistance may not be available.

Thus, while it is known to alter the characteristics of an electrostatic chuck for particular applications, the ability to manufacture electrostatic chucks from a wide variety of materials and the ability to tailor electrostatic chucks to exhibit properties varying over a wide range is limited.

It is therefore an object of the present invention to provide an electrostatic chuck that can be manufactured from composites comprising a wide variety of materials. It is another aspect of the present invention to provide a method of making electrostatic chucks tailored to exhibit desired properties varying over a wide range by varying the type and proportion of inorganic particulate material added to a glass or glass-ceramic matrix phase.

It is a yet another object of the present invention to provide a method of making electrostatic chucks having desired electrical properties. It is yet another object of the present invention to provide a highly reliable electrostatic chuck having a dielectric layer tailored to have a particular thermal coefficient of expansion, so that it is in slight tension, slight compression or neutral relative to the base layer or layers.

It is still a further object of the present invention to produce an electrostatic chuck which does not warp or peel in use. It is still another object of the present invention to provide an electrostatic chuck with adequate electrical resistance to allow up to 2000 volt electrode charges or more in electrostatic chuck conductive layers.

It is still another object of the present invention to provide an electrostatic chuck with improved corrosion resistance.

It is still another object of the present invention to provide an electrostatic chuck with improved fracture toughness.

It is still another object of the present invention to provide an electrostatic chuck with improved wear resistance. It is still another object of the present invention to provide an electrostatic chuck with improved thermal conductivity.

It is yet another object of the present invention to provide a low cost and flexible method for making the electrostatic chuck of the present invention.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an electrostatic chuck comprising a base, at least one electrode in the form of an electrically conductive layer on the base, and a dielectric layer overlaying the conductive layer, wherein the dielectric layer comprises inorganic particulate material dispersed in an inorganic matrix phase, the matrix phase being a glass or glass-ceramic, and in which the dielectric layer has a coefficient of thermal expansion substantially matched to that of the base.

According to a second aspect of the present invention, there is provided an electrostatic chuck comprising an electrode in the form of an electrically conductive base, and a dielectric layer overlaying and adhering to the conductive base without any deposited intermediate interlayer, wherein the dielectric layer comprises inorganic particulate material dispersed in an inorganic matrix phase, the matrix phase being a glass or glass-ceramic, and in which the dielectric layer has a coefficient of thermal expansion substantially matched to that of the base. In both aspects, by "substantially matched" is meant that the coefficients of thermal expansion of the dielectric layer and the base are sufficiently closely matched that in use over a range of temperatures the dielectric layer and base remain adherent and do not separate. The coefficients of thermal expansion may be such that the dielectric layer is left in slight tension, slight compression, or neutral relative to the substrate as required by the individual use application. By "glass-ceramic" is meant a material that is wholly or, more usually, partially crystallised from a glassy precursor. The invention further provides a method of making such chucks comprising the steps of:

(a) combining one or more oxides and heating for a time and at a temperature sufficient to produce an oxide melt;

(b) converting the oxide melt to matrix particles; (c) mixing the matrix particles with inorganic particulate material to form a mixture;

(d) mixing the mixture with an organic vehicle to form a slurry;

(e) applying the slurry to the top surface of a base to form a base having a coating;

(f) heating the coated base for a time and at a temperature sufficient to remove volatile solvents and to dry the coating;

(g) heating the base having a coating for a time and at a temperature sufficient to remove remaining organic materials and to form a composite dielectric layer adherent to the base.

The characteristics of the dielectric layer can be tailored to match the thermal coefficient of expansion of the base, and tailored for other desired properties, by altering the composition, nature, and/or proportion of inorganic particulate material added to the glass or glass-ceramic matrix. Tailorable characteristics include resistivity, thermal expansion coefficient, and corrosion resistance. Electrostatic chucks tailored to a wide variety of end uses and manufacturing conditions are thereby provided. Accordingly the present invention further provides methods in which the nature and relative proportions of the matrix particles and inorganic particulate material are selected to provide desired matching of thermal coefficient of expansion of the dielectric layer to that of the base. Additionally the nature and relative proportions of the matrix particles and inorganic particulate material may also be selected to provide other desired physical or chemical properties to the dielectric layer.

The electrostatic chuck of the present invention may assume a variety of configurations. The electrostatic chuck may be uni-, bi- or multi-polar. In addition, the electrostatic chuck may comprise one or more electrical feedthroughs. The electrostatic chuck may optionally comprise an encapsulation layer.

Further features and aspects of the invention are made apparent from the claims and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional, partially schematised view of a monopolar electrostatic chuck according to one embodiment of the present invention. FIG. 2 is a cross-sectional, partially schematised view of a monopolar electrostatic chuck according to a second embodiment of the present invention.

FIG. 3 is a cross-sectional, partially schematised view of a multipolar electrostatic chuck according to a third embodiment of the present invention.

FIG. 4 is a cross-sectional, partially schematised view of an electrostatic chuck of the present invention coated with an encapsulation layer, according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary monopolar electrostatic chuck 4 of the present invention, including an electrically conductive base 11 and a dielectric layer 10 deposited on the top surface of the electrically conductive base 11. The dielectric layer is a composite ceramic comprising a particulate phase in a glass or glass-ceramic matrix. The matrix typically has a majority oxide system formed from one or more of the following: MgO, Al₂O₃, SiO₂, CaO, Li₂O, or ZrO₂. For example, in some embodiments, the matrix is a ternary oxide system containing various amounts of Al₂O₃ and SiO₂ in combination with MgO, CaO or Li₂O. In further embodiments, the matrix also contains one or more of the following oxides: PbO, B₂O₃, Bi₂O₃ and TiO₂. Typically the matrix is an alkali metal free glass selected from the calcium aluminosilicate system optionally containing boron oxide and other oxides and comprises 20% to 90% by volume of the dielectric layer. The thermal expansion characteristics of the glass is matched to that of, typically, alumina based substrates by addition of, typically, an alumina based second phase particulate material.

The particulate phase is an inorganic particulate material and typically a ceramic, although metals (e.g. such as Kovar, niobium, molybednum or tungsten) may be of use in some applications to lower resistivity of the dielectric. "Particulate material" is used herein to include both fibres and particles of varying shape and aspect. In one embodiment of the present invention, the particulate material is selected from, although not limited to, the group consisting of aluminium oxide, silicon dioxide, magnesium oxide, mullite and similar aluminosilicate compounds. Other oxide materials such as BaTiO₃, ZrO₂, and TiO₂ may also be used, as may non-oxide materials.

It is preferable that the composite ceramic is composed of about 10% to about 60%) by volume of the inorganic particulate material. In a particularly preferred embodiment, particulate aluminium oxide comprises more than 10% by volume of the composite ceramic.

In one embodiment of the present invention, the dielectric layer ranges from about two thousandths of an inch (« 50 μm) to about twenty thousandths of an inch (« 0.5 mm) thick. Preferably, the dielectric layer ranges from about two thousandths of an inch (« 50 μm) to ten thousandths of an (« 0.25 mm) inch . As shown in FIG. 1, the conductive layer is a conductive substrate. In one embodiment of the present invention, the conductive substrate is an electrically conductive ceramic, such as silicon carbide or doped aluminium oxide. In a further embodiment of the present invention, the conductive substrate is a metallic material, selected from the group consisting of niobium, molybdenum, tungsten or Kovar. The invention is not however restricted to these materials as substrates.

Illustrated in FIG. 2 is a second embodiment of an exemplary monopolar electrostatic chuck 5 of the present invention. Unlike chuck 4, however, the conductive layer comprises an electrode 13 deposited on the top surface of a base 14. The electrode 13 is deposited on the top surface of the base 14 by any standard thick or thin film depositing technique. In one embodiment of the present invention, the electrode is deposited by physical vapour deposition. In another embodiment, the electrode is deposited by screen printing of a thick film paste. Electrode 13 materials are electrically conductive materials selected to be compatible with the base 14 and manufacturing conditions. Electrode 13 materials are typically selected from molybdenum, tungsten, niobium and platinum. Base materials are typically electrical insulators and may include, although are not limited to, alumina based materials, A1N, sapphire, mullite, and cordierite.

The electrode 13 is overlain by dielectric layer 12, which is of like material to dielectric 10 of FIG. 1 and selected to have a thermal coefficient of expansion substantially matched to that of the base 14. The dielectric layer 12 adheres to the base 14 and electrode 13. As shown in FIG. 2, the electrode 13 is electrically connected to the bottom surface of the base 14 by an electric feedthrough, as described in U.S. 5,368,220. The electrical feedthrough comprises a through hole 17 filled with a solid braze alloy 15 surrounding a conductive wire 16. Although other active alloys may also be utilised in the present invention, suitable active braze alloys are commercially available from Wesgo, Inc. under the tradenames Copper- ABA™ and Cusin- 1 -ABA™. In one embodiment of the present invention, the conductive wire 16 is selected from the group consisting of nickel and copper. The invention is not limited to this method of forming electrical connection to the electrode.

FIG. 3 illustrates an exemplary multipolar electrostatic chuck 7 of the present invention. Unlike chuck 5, the base has two or more electrodes 13 deposited thereon. The electrodes 13 are electrically connected to the bottom surface of the base by two or more electrical feedthroughs 22 and overlain by dielectric layer 18, which is of like material to dielectric 10 of FIG. 1 and selected to have a thermal coefficient of expansion substantially matched to that of the base 14. The dielectric layer 18 adheres to the base 14 and electrodes 13.

Illustrated in FIG. 4 is an exemplary multipolar electrostatic chuck 9 of the present invention. Unlike chuck 7, at least a portion of the surface of chuck 9 is coated with an encapsulation layer 23. As shown in FIG.4 the encapsulation material covers and protects the dielectric layer 18. Suitable encapsulation layer materials may include, but are not limited to, fluorinated polymers, aluminium oxide, aluminium nitride, aluminium fluoride, diamond, diamond-like materials, magnesium oxide, magnesium fluoride, calcium oxide or calcium fluoride. One method of creating the electrostatic chuck of the present invention includes forming a composite ceramic dielectric layer comprising a glass or glass-ceramic matrix reinforced with particulate inorganic materials by the following process. The matrix is formed by combining desired oxides and melting, for example by glass batching techniques, to ensure a homogenous mixture. Conventional quenching, fritting and/or grinding techniques may be employed to convert the glass melt to useful glass particles. In one embodiment, the glass particles have particles size of about 1 to about 40 microns (μm). Standard mixing techniques are then used to combine the desired inorganic particulate material with the glass particles to form a mixture. For example, glass powders and particulate ceramics can be mixed by rolling in a j ar with the addition of mixing media. The inorganic particulate material is preferably ceramic. The inorganic particulate material may be in the form of fibres. In one embodiment of the present invention, the inorganic material is aluminium oxide in the form of particles similar in size to the glass particles. A homogenous slurry is then produced by mixing the mixture of glass powder and inorganic particulate material with an organic vehicle. The organic vehicle employed is not critical, and may be chosen on the basis of desirable slurry properties.

The slurry is then applied to the conductive layer using conventional techniques for applying a suspension to the surface of a solid. For example, the slurry can be applied by screen printing, doctor blading, spray coating, spin coating or dip coating. In one embodiment, the slurry is silk screened onto the conductive layer. The slurry coated conductive layer is then heated at a low temperature to remove volatile solvents and to dry the coating. Temperatures may vary, typically being less than 200°C. This coating process may be repeated more than once, to develop a ceramic composite dielectric having a layered structure. The composition of these layers may be the same or different.

The resultant product is then heated to an elevated temperature, typically more than 600°C, to remove remaining organic materials and to sinter the dielectric layer to form a composite dielectric layer adherent to the conductive layer. This high temperature heating process can be carried out in an oxidising, reducing or inert atmosphere, depending on the chuck materials and the desired final properties. After high temperature heating, additional composite dielectric layers may be added if desired. The composition of these layers may be the same or different.

Conventional techniques, such as grinding or polishing, may then be used to impart desired features to the dielectric surface, such as flatness, roughness or gas distribution channels.

In an alternative embodiment of the present invention, at least a portion of the chuck outer surface may be coated with an encapsulation coating. The encapsulation coating is typically formed, for example, of fluorinated polymers, Al₂O₃, A1N, A1F₃, diamond, diamond-like materials, MgO, MgF₂, CaO and CaF₂.

EXAMPLES

EXAMPLE 1

Over 150 insulating ceramic substrates 1" wide x 5" long x 0.03" thick (« 25.4mm x 127mm x 0.76mm) were coated with various ceramic composite compositions to evaluate the ability to adjust the thermal expansion match between the composite ceramic coating and the substrate. The thermal expansion behaviour of the ceramic composite coated substrates was shown to be dependent on the type of inorganic particulate material and the loading of the inorganic particulate material; additionally, the thermal expansion behaviour was also influenced by the composition of the matrix phase and on the processing parameters such as heating rate, cooling rate, soak temperature, soak time and firing atmosphere. These are parameters that have to be determined experimentally to match a coating to a base and will vary from system to system. The relative thermal expansion match between the composite coating and the substrate was determined by measuring the amount of deflection (radius of curvature) experienced by the coated substrate. Table 1 shows exemplar data gathered for polycrystalline alumina bases coated with a ceramic composite having a glass- ceramic matrix composed substantially of 28-30% A1₂0₃, 32-34% CaO, 36-39% SiO₂ and 0-1% MgO by weight with the indicated loading of Al₂O₃ particles. Larger curvature radii indicate closer thermal expansion match between substrate and coating. The tendency of the substrates was to arch upwards on the dielectric side. The results, as exemplified in Table 1, demonstrate that the thermal expansion coefficient of the ceramic dielectric can be adjusted by the addition of varying amounts of inorganic material.

Table 1 : Exemplar data showing base deflection as a function of composite inorganic material loading. Typical processing procedure for these samples and the following Examples comprise the steps of:- manufacture of the slurry as discussed above application of the slurry to the substrate drying the slurry coated substrate at 100°C in air heating the dried slurry coated substrate at a ramp rate of 3°C/min to 1200°C holding at 1200°C for 20 minutes cooling to room temperature at a ramp rate of 3°C/min. Such a process ramp rate is slow enough to at least partially crystallise the matrix but, if desired, a soak step may be used to extend or complete crystallisation.

The processing steps for any given combination of dielectric layer and substrate will vary according to the nature of the materials used.

EXAMPLE 2

Differential Thermal Analysis (DTA) studies were carried out on various matrix compositions and the influence of particulate additives was also evaluated using DTA. Exemplar DTA data for a glass matrix composed substantially of 19-21% Al₂O₃, 41- 43% CaO, 36-39% SiO₂ and 0-1% MgO by weight, showed no significant crystallisation on cooling from 1385°C. The addition of various particulate inorganics to the matrix phase did influence crystallisation, however; exemplar DTA data showed that the addition of 10-20% Al₂O₃ particles caused significant crystallisation to occur on cooling from 1385°C. This material therefore acts as a glass on its own but as a glass-ceramic in the presence of the alumina particles.

EXAMPLE 3 A simple monopolar 2" x 2" («51mm x 51mm) electrostatic chuck test piece was manufactured by depositing a ceramic composite dielectric directly onto a conductive substrate. The top face of a 0.04" (« 1.02mm) thick niobium substrate was covered with a ceramic composite dielectric containing Al₂O₃ particles, 10-20% by volume, in a glass-ceramic matrix composed substantially of 21-23% Al₂O₃, 39-41% CaO, 36-39% SiO₂ and 0-1% MgO by weight. The coated substrate was fired at over 1100°C in an inert atmosphere furnace and the coating was observed to adhere well to the substrate due, in part, to the good thermal expansion match between the conductive substrate and the composite.

EXAMPLE 4 A 12" (» 305mm) diameter ceramic electrostatic chuck was manufactured for operating temperatures exceeding 300°C. A multipolar refractory metal electrode pattern was deposited on the top face of a 0.3" (« 7.6mm) thick polycrystalline alumina base. The electrodes were connected to the bottom face of the alumina base with active braze alloy feedthroughs that passed through the alumina base and bonded to nickel alloy termination pins. The electrostatic chuck dielectric layer was produced by coating the patterned top face of the alumina base with 0.008" (« 0.2mm) of ceramic composite consisting of Al₂O₃ particles, 25-30% by volume, in a glass-ceramic matrix composed substantially of 28-30% Al₂O₃, 32-34% CaO, 36-39% SiO₂ and 0-1% MgO by weight. The composite dielectric thermal expansion coefficient had been tailored to be slightly lower than the base thermal expansion coefficient, thus generating a net compressive stress in the dielectric to improve integrity. The composite dielectric adhered well to the base and electrode pattern during thermal cycling and was successfully diamond ground in a post-firing operation. Additionally, the completed electrostatic chuck exhibited good wafer clamping with applied voltage potentials under 1KV and the dielectric material was found to have an acceptable resistivity and dielectric strength at use temperatures over 300°C. When the coating system using 30 weight percent aluminium oxide in a carrier was used electrical tests showed good electrical resistance at 1500 volts charge potential.

EXAMPLE 5

An 8" («203mm) diameter ceramic electrostatic chuck was manufactured for use under 100°C. A multipolar refractory metal electrode pattern was deposited on the top face of a 0.1" («2.5mm) thick polycrystalline alumina base. The electrodes were connected to the bottom face of the alumina base with active braze alloy feedthroughs that passed through the alumina base and bonded to nickel alloy termination pins. The electrostatic chuck dielectric layer was produced by coating the patterned top face of the alumina base with 0.01" («0.25mm) of ceramic composite consisting of Al₂O₃ particles, 25-30% by volume and MgO particles, 10-15% by volume, in a glass-ceramic matrix composed substantially of 28-30% Al₂O₃, 32-34% CaO, 36-39% SiO₂ and 0- 1 % MgO by weight. The chuck exhibited good wafer clamping with applied voltage potentials under 1KV and the dielectric material was found to have an acceptable resistivity and dielectric strength at use temperatures under 100°C.

EXAMPLE 6 An 8" («203mm) diameter ceramic electrostatic chuck was manufactured for use under 100°C. A multipolar refractory metal electrode pattern was deposited on the top face of a 0.1" («2.5mm) thick polycrystalline alumina base. The electrodes were connected to the bottom face of the alumina base with active braze alloy feedthroughs that passed through the alumina base and bonded to nickel alloy termination pins. The electrostatic chuck dielectric layer was produced by coating the patterned top face of the alumina base with 0.004" («0.1mm) of ceramic composite consisting of Al₂O₃ particles, 10-20% by volume, in a glass-ceramic matrix composed substantially of 21- 23% Al₂O₃, 39-41% CaO, 36-39% SiO₂ and 0-1% MgO by weight. The chuck exhibited good wafer clamping with applied voltage potentials under 1KV and the dielectric material was found to have an acceptable resistivity and dielectric strength at use temperatures under 100°C but not tested to higher temperatures.

EXAMPLE 7 An 8" («203mm) diameter ceramic electrostatic chuck was manufactured for use over 300°C. A multipolar refractory metal electrode pattern was deposited on the top face of a 0.3" («7.6mm) thick polycrystalline alumina base. The electrodes were connected to the bottom face of the alumina base with active braze alloy feedthroughs that passed through the alumina base and bonded to ceramic insulated, refractory metal connectors. The electrostatic chuck dielectric layer was produced by coating the patterned top face of the alumina substrate with 0.006" («0.15mm) of ceramic composite consisting of Al₂O₃ particles, 10-20% by volume, in a glass-ceramic matrix composed substantially of 21-23% Al₂O₃, 39-41% CaO, 36-39% SiO₂ and 0-1% MgO by weight. The chuck exhibited good wafer clamping with applied voltage potentials under 1KV and the dielectric material was found to have an acceptable resistivity and dielectric strength at use temperatures over 300°C.

EXAMPLE 8

A 1.5" x 3" («38mm x 76mm) ceramic electrostatic chuck was manufactured for use over 300°C in a corrosive environment. A multipolar refractory metal electrode pattern was deposited on the top face of a 0.1" («2.5mm) thick polycrystalline alumina base. The electrodes were connected to the rear face of the alumina base with active braze alloy feedthroughs that passed through the alumina base and bonded to nickel alloy termination pins. The electrostatic chuck dielectric layer was produced by coating the patterned top face of the alumina base with 0.005" («0.13mm) of ceramic composite consisting of, by volume, 10-20% Al₂O₃, particles in a glass-ceramic matrix composed substantially of 21-23% Al₂O₃, 39-41% CaO, 36-39% SiO₂ and 0-1% MgO by weight. The electrostatic chuck was then physical vapour deposition coated with a 0.5 micron (μm) thick layer of high purity aluminium oxide to act as an encapsulant. The 0.5 micron (μm) aluminium oxide layer adhered well during temperature cycling due to the excellent thermal expansion match with both the composite dielectric and the polycrystalline alumina base.

EXAMPLE 9 99.5% alumina coupons with a wall thickness of approximately 20 thousandths of an inch («0.5mm) were made. A glass frit was produced having the primary constituents calcium oxide, aluminium oxide and silicon oxide. The frit was compounded with a thick film paste vehicle and silk screen printed onto the coupons. While the glass bonded well at temperatures up to 600°C the thermal expansion was an unacceptable fit to that of the alumina causing the coupon to bow. The addition of 30 weight percent aluminium oxide particles showed a significant decrease in bowing and additions up to 60% further decreased the bowing. Subsequent experiments using fused silica additives further decreased the thermal expansion of the glass system allowing for a wide variety of thermal expansion coefficients to be achieved. Materials using fused silica additives may be of use in combination with A1N substrates.

EXAMPLE 10

Approximately 100 electrostatic chuck test specimens were created using two parallel electrodes with suitable feedthroughs. Coatings were applied in various thicknesses from two thousandths of an inch («0.05mm) to 20 thousandths of an inch («0.5mm) with firing temperatures ranging between 1200°C and 1550°C. Good performance was found at around 1300°C firing temperature in coating thicknesses of from two thousandths of an inch («0.05mm) to ten thousandths of an inch («0.25mm). Good electrical standoff to 2500 volts was demonstrated. Base ceramic specimens using alumina and aluminium nitride were tested. Other likely compatible bases include cordierite, mullite and other aluminosilicate compositions.

The use of the composite material dielectric described above allows the production of thin, cheap, electrostatic chucks that avoid the need for the expensive hot pressing operations required in the manufacture of conventional ceramic chucks, and that have good electrical and abrasion resistant properties.

Claims

CLAIMS:

What is claimed is: 1. An electrostatic chuck (5,7,9) comprising a base (14), at least one electrode in the form of an electrically conductive layer (13) on the base (14), and a dielectric layer (12,18) overlaying the conductive layer (13), wherein the dielectric layer (12,18) comprises inorganic particulate material dispersed in an inorganic matrix phase, the matrix phase being a glass or glass-ceramic, and in which the dielectric layer (12, 18) has a coefficient of thermal expansion substantially matched to that of the base (14).

2. An electrostatic chuck as claimed in claim 1 in which the chuck is a bipolar chuck.

3. An electrostatic chuck (4) comprising an electrode in the form of an electrically conductive base (11), and a dielectric layer (10) overlaying and adhering to the conductive base (11) without any deposited intermediate interlayer, wherein the dielectric layer (10) comprises inorganic particulate material dispersed in an inorganic matrix phase, the matrix phase being a glass or glass-ceramic, and in which the dielectric layer (10) has a coefficient of thermal expansion substantially matched to that of the base (11).

4. An electrostatic chuck as claimed in any preceding claim in which the inorganic particulate material is a material selected from the group consisting of particles and fibres.

5. An electrostatic chuck as claimed in any preceding claim in which the inorganic particulate material comprises from about 10 to about 60 percent of the dielectric layer.

6. An electrostatic chuck as claimed in any preceding claim in which the inorganic particulate material comprises one or more materials selected from the group consisting of Al₂O₃, SiO₂, MgO, BaTiO₃, ZrO₂, TiO₂, cordierite or mullite.

7. An electrostatic chuck as claimed in any preceding claim in which the dielectric layer comprises no less than 10% aluminium oxide as the inorganic particulate material.

8. An electrostatic chuck as claimed in any preceding claim in which the matrix phase comprises one or more metal oxides selected from the group consisting of MgO, Al₂O₃, SiO₂, CaO, Li₂O or ZrO₂.

9. An electrostatic chuck as claimed in claim 8 in which the matrix phase consists primarily of aluminium oxide, silicon dioxide, and calcium oxide.

10. An electrostatic chuck as claimed in claim 9 in which the matrix phase comprises additionally boron oxide.

11. An electrostatic chuck as claimed in any preceding claim in which the dielectric layer is from about 0.002 to about 0.01 of an inch (┬½0.05 to 0.25mm) thick.

12. An electrostatic chuck as claimed in any preceding claim, further comprising an encapsulation layer (23) surrounding at least a portion of the outer surface of the chuck.

13. An electrostatic chuck as claimed in claim 12, in which the encapsulation layer comprises a material selected from the group consisting of fluorinated polymers, aluminium oxide, aluminium nitride, aluminium fluoride, diamond, diamond- like materials, magnesium oxide, magnesium fluoride, calcium oxide or calcium fluoride.

14. A process of manufacturing an electrostatic chuck, comprising the steps of: (a) combining one or more oxides and heating for a time and at a temperature sufficient to produce an oxide melt; (b) converting the oxide melt to matrix particles; (c) mixing the matrix particles with inorganic particulate material to form a mixture; (d) mixing the mixture with an organic vehicle to form a slurry; (e) applying the slurry to the top surface of a base to form a base having a coating; (f) heating the coated base for a time and at a temperature sufficient to remove volatile solvents and to dry the coating; (g) heating the base having a coating for a time and at a temperature sufficient to remove remaining organic materials and to form a composite dielectric layer adherent to the base.

15. A process of manufacturing an electrostatic chuck as claimed in claim 14 in which the nature and relative proportions of the matrix particles and inorganic particulate material are selected to provide desired matching of thermal coefficient of expansion of the dielectric layer to that of the base.

16. A process of manufacturing an electrostatic chuck as claimed in claim 15 in which the nature and relative proportions of the matrix particles and inorganic particulate material are also selected to provide other desired physical or chemical properties to the dielectric layer.