WO1997038481A1

WO1997038481A1 - Process for providing a glass-ceramic dielectric layer on an electrically conductive substrate and electrostatic chucks made by the process

Info

Publication number: WO1997038481A1
Application number: PCT/US1997/005926
Authority: WO
Inventors: Guo-Quan Lu; Jaecheol Bang
Original assignee: Virginia Tech Intellectual Properties Inc
Current assignee: Virginia Tech Intellectual Properties Inc
Priority date: 1996-04-10
Filing date: 1997-04-10
Publication date: 1997-10-16
Anticipated expiration: 1998-10-10
Also published as: AU2451897A

Abstract

Processes for providing a durable glass-ceramic dielectric layer on an electrically conductive substrate are disclosed. Also disclosed are electrostatic chucks (1) made by the processes that include an electrode (10) coated with at least one metallic interlayer (12, 14) and a glass-ceramic layer (16), in particular, cordierite, on the at least one metallic interlayer.

Description

PROCESS FOR PROVIDING A GLASS-CERAMIC

DIELECTRIC LAYER ON AN ELECTRICALLY CONDUCTIVE

SUBSTRATE AND ELECTROSTATIC CHUCKS MADE BY THE PROCESS

FIELD OF THE INVENTION The present invention relates to a process for providing a glass-ceramic dielectric layer on an electrically conductive substrate and electrostatic chucks made by the process .

BACKGROUND Electrostatic chucks (ESCs) are gaining popularity in the semiconductor wafer processing industry because they offer a variety of advantages over mechanical or vacuum chucks. Unlike vacuum chucks, electrostatic chucks are ideally suited for use in vacuum environments. As compared with mechanical chucks, ESCs have fewer moving parts, eliminate front side coverage of the wafer, and lessen risk of particle contamination. Ideally, ESCs offer the potential for rapid and strong clamping and declamping of semiconductor wafers over a range of temperatures without bowing or warping the wafer.

Although ESCs can have multiple structural configurations, they generally require a surface dielectric layer covering an electrode. A variety of dielectric coatings have been proposed for use in ESCs, including alumina (see, e.g., U.S. Patents Nos. 5,413,360 to Atari et al . , 5,384,681 to Kitabayashi and 5,207,437 to

Barnes et al . ) , silica (see, e.g., Barnes et al. ) , silicon nitride (see, e.g., Atari et al.), tantalum oxide, diamond

(see, e.g., U.S. Patent No. 5,166,856 to Liporace) , polyimide and Teflon™. U.S. Patent No. 5,384,682 to Watanabe et al . discloses ESCs having dielectric layers of a sintered ceramic material comprising Ti0₂, Cr₂0₃ or Al₂0₃, and a silicon oxide or alkaline earth metal oxide. U.S. Patent No. 5,104,834 to Watanabe et al . discloses ESCs having dielectric layers that may comprise a major amount of alumina, 1-6 wt% alkaline earth metal oxide (e.g., MgO), 0.5-6 wt% transition metal oxide and minor amounts of Si0₂ as a sintering aid. One of the challenges in ESC manufacture is the ability to reproducibly prepare dielectric coatings on metal electrodes that are durable and perform well over a range of temperatures and electric fields. Some glass or glass-ceramic compositions are known for making coatings on metal surfaces. For example, U.S. Patent No. 3,368,712 to Sanford et al . discloses various silica-based enamel compositions for coating metal surfaces. U.S. Patent No.

3,197,291 to Michael discloses a process for applying a corrosion-proof glass-ceramic coating to a metal surface such as steel, copper, molybdenum, niobium, or tungsten.

In Michael's process, the metal surface is prepared by electroplating nickel and chromium to the metal surface prior to applying the glass-ceramic. The dielectric properties of these coatings and their compatibility in

ESCs are not disclosed. U.S. Patent No. 4,794,048 to

Oboodi et al . discloses ceramic coated metal substrates that are suitable for use as circuit board components.

The substrates are directly coated on at least a portion of their surfaces with a dielectric coating that is preferably a glass-ceramic based on ternary metal oxide systems containing varying amounts of MgO, Si0₂ and Al₂0₃

(e.g., cordierite) . The substrates may comprise copper, nickel, cobalt, iron and aluminum. However, Oboodi et al. does not appreciate the difficulties of directly coating such glass-ceramics on other metals having desirable properties, such as molybdenum. Moreover, although the dielectric properties and durability of these coated articles are disclosed, their compatibility in ESCs is not disclosed.

The preparation of dielectric surface coatings for ESCs is complicated by factors including: stresses and porosity that are likely to form in coatings prepared from glass or ceramic green sheets (see, for example, Bang and Lu, "Densification Kinetics of Glass Films Constrained on Rigid Substrates, " J. Mater. Res., 10(5) , pgs . 1321-1326, 1995 and Choe et al . , "Constrained-film Sintering of a Gold Circuit Paste," J. Mater. Res., 10(4) , pgs. 986-994, 1995) , and mismatch of thermal expansion coefficients (or linear coefficients of expansion) . Unlike free films, and to a lesser extent sandwiched films, the exposed dielectric films on ESCs experience stresses that may lead to cracking or peeling of the dielectric layer. Further hazards for the ESC are exposure to reactive gases during processing of the semiconductor wafers, and chemical reaction with the silicon wafer. In addition, problems may arise from crystal growth or phase changes that may occur both in firing the coating on the ESC electrode as well as during temperature cycling during use of the ESC. Similarly, repeated electric field cycling during ESC clamping and declamping operations may degrade the coating. It has been reported that grain boundaries are critical sites at the clamping surface of the ESC. See Watanabe et al. , "Electrostatic Charge Distribution in the Dielectric Layer of Alumina Electrostatic Chuck, " J. Mater. Science 29, pgs. 3510-3516, 1994. Thus, the type and distribution of grain boundaries in the dielectric coating are another important consideration in the design of the ESC coatings. In summary, the myriad of complex and interrelated factors make the task of providing dielectric coatings for ESCs both challenging and unpredictable.

SUMMARY OF THE INVENTION The present invention provides a method of making a smooth and uniform glass-ceramic coating on an electrically conductive substrate in which at least one metallic interlayer is applied to the electrically conductive substrate, a layer of precursor glass-ceramic is applied over the at least one metallic interlayer, and the resulting article is sintered to produce the glass- ceramic coating.

The present invention further provides an electrostatic chuck in which an electrically conductive electrode is coated with at least one metallic interlayer and a uniform and smooth surface layer of a glass-ceramic coating.

The present invention provides a coating that offers multiple advantageous properties for use as a smooth dielectric coating on an electrically conductive substrate, especially for use in an electrostatic chuck. These advantageous properties include: excellent dielectric properties; efficient clamping and declamping of semiconductor wafers; a very smooth surface; excellent adhesion; excellent shock and impact resistance; temperature durability without chipping or cracking; low porosity; greater thickness uniformity and range; protection of the underlying metal substrate from oxidation or chemical attack; excellent stability at high temperature and in harsh chemical environments; low cost of materials and manufacture; and high reproducibility in processing.

DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a top view of an electrostatic chuck according to the present invention.

Fig. 2 is a cross-sectional view of the electrostatic chuck of Fig. 1.

Fig. 3 is a bottom view of the electrostatic chuck of Fig. 1.

Fig. 4 is a graph showing electrical resistivity as a function of temperature for a variety of materials . DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The electrostatic chuck 1 of Figs. 1-3 has a metallic body (i.e., electrode) 10, surface grooves 3, a through hole 17, mounting screw holes 18, and knife edge

20. The surface grooves 3 can be used for circulating a cooling gas.

The electrostatic chuck design in Figs. 1-3 is one embodiment of an electrostatic chuck with a glass-ceramic coating according to the invention. Multiple ESC designs can be used with a glass-ceramic coating according to the present invention. Monopolar, bipolar and multipolar DC designs as well as AC designs may be used in ESCs of the present invention. Examples of ESC designs are disclosed in U.S. Patents Nos. 4,692,836; 4,864,461; 3,253,200; 5,055,964; 5,155,652; 5,376,213; and 5,384,681, and Japanese Patent Applications Nos. 05-19462, 04-344893, 04-109715, 04-70163, 03-190023, 03-91233, 03-76850, 02-156685, 02-12817, 01-291104, 01-246032, 01-136866, 01-105678, 62-294670, 61-242322, 61-117527 and 60-298366, incorporated herein by reference along with all other references cited herein as if reproduced in full below. The electrode 10 preferably comprises molybdenum; however, the electrode 10 can comprise other materials having suitable properties. Other suitable materials include metals and metal alloys, preferably transition metals and alloys thereof, more preferably refractory metals, even more preferably metals selected from the group consisting of molybdenum, tungsten and alloys thereof.

The particular electrode material selected is guided by the desired properties of the material . The material must conduct electricity sufficiently to generate an electrostatic charge across the dielectric layer 16 and reversibly bind an object, such as a semiconductor wafer, to the dielectric layer 16. The material should be sufficiently durable to withstand, without substantial weakening, repeated exposure to typical conditions under which semiconductor device manufacturing processes are conducted (e.g., large pressure fluctuations, high temperatures and large temperature fluctuations) and to typical chemicals employed in such processes. Fig. 2 depicts a preferred embodiment of the invention having two interlayers, a lowermost interlayer 12 contacting the electrode 10, and an uppermost interlayer 14 contacting the dielectric layer 16. The at least one interlayer 12, 14 comprises metals and metal alloys, preferably transition metals and alloys thereof, more preferably metals selected from the group consisting of nickel, copper, chromium, platinum-group metals and alloys thereof.

The particular interlayer material selected is guided by the desired properties of the material and by the primary function of the interlayer 12, 14 of facilitating durable bonding of the electrode 10 to the dielectric layer 16. The material should be sufficiently durable to withstand, without substantial weakening of the material or the bonds formed therewith, repeated exposure to typical conditions under which semiconductor device manufacturing processes are conducted (e.g. , large pressure fluctuations, high temperatures and large temperature fluctuations) and to typical chemicals employed in such processes.

In preferred embodiments, the at least one interlayer 12, 14 comprises chromium or chromium alloy in contact with the dielectric layer 16. The chromium containing interlayer can be the sole interlayer, but preferably, the chromium containing interlayer is the uppermost interlayer 14 and the lowermost interlayer 12 comprises copper or nickel. It has been found that a chromium interlayer forms a surface layer of chromium oxide that facilitates bonding to the dielectric layer 16.

The total thickness of the interlayer should be at least about 2 μm, preferably at least about 5 μm, more preferably about 10 μm, with the upper limit being dictated mainly by economic considerations. Although Fig. 2 depicts each interlayer as having well-defined boundaries, in preferred embodiments, the at least one interlayer can be at least partially diffused into the underlying electrode 10. Such diffusion is accomplished by, e.g., heating an interlayer coated electrode within a temperature range of about 600°C to about 900°C for about 30 minutes to about 2 hours (the length of time necessary to accomplish diffusion being inversely proportional to the temperature at which diffusion is conducted) . The dielectric layer 16 comprises dielectric glass-ceramic. Preferably, the dielectric layer material should be sufficiently durable to withstand, without substantial weakening of the material or the bonds formed therewith, repeated exposure to typical conditions under which semiconductor device manufacturing processes are conducted (e.g., large pressure fluctuations, high temperatures and large temperature fluctuations) and to typical chemicals employed in such processes. The material should preferably have a thermal coefficient of expansion (TCE) or coefficient of linear expansion close to that of the electrode 10 and/or interlayer 12, 14, good thermal shock resistance, high mechanical strength, strong resistance to chemical agents encountered in semiconductor device manufacturing, high breakdown voltages and high resistivity at typical temperatures at which semiconductor devices are manufactured (e.g., about 25 to 500°C) .

The dielectric layer comprises MgO, A1₂0₃, and Si0₂ as at least the majority of the layer, with additional components (e.g., boron and/or phosphorous compounds) accounting for any remaining balance of the layer. The dielectric layer preferably contains not more than a minor amount of A1₂0₃. In particular, the composition of the dielectric layer preferably comprises, in weight percent, about 20% to about 40% Si, about 5% to about 25% Al, and about 5% to about 20% Mg; more preferably about 23% to about 31% Si, about 9% to about 18% Al, and about 6% to about 15% Mg. In a preferred embodiment, the dielectric layer also comprises up to 5% B, more preferably 0.5% to 1.0% B and up to 5% P, more preferably 0.5% to 2.0% P.

In some preferred embodiments, the dielectric layer does not contain adhesion promoters such as Ni, Co, Fe, Mn, or Cr. It has been discovered that the addition of Ti0₂ may introduce dielectric properties into the film that are undesirable in some applications, thus in some preferred embodiments the films do not contain titanium. Preferably, the films do not contain lead. In other preferred embodiments, the films are essentially free of sodium or potassium, and preferably contain less than 0.5% of these elements.

The dielectric layer 16 preferably contains cordierite, more preferably at least 80% and most preferably at least 98% crystalline cordierite.

The dielectric layer 16 is preferably between about 25 to about 200 microns thick, more preferably about 75 to about 150 microns thick.

The materials of the several layers of an ESC according to the invention should be selected to provide an ESC having good stability under the conditions encountered in semiconductor device manufacture. Factors that affect the quality of bonding between the layers include the range of TCEs between the layers, the ability of adjacent layers to interdiffuse and the acid/base character of adjacent layers. Preferably, the range of TCEs between the layers should be as small as possible. Preferably, the at least one interlayer 12, 14 should be able to diffuse into the underlying electrode 10. Preferably, adjacent layers should relate to one another as acid and base.

In particularly preferred embodiments, the ESC has a molybdenum electrode 10 and a cordierite dielectric layer 16. However, cordierite does not form an acceptable film directly on a molybdenum substrate. To overcome this problem, a lowermost interlayer 12 is coated on the molybdenum electrode 10. The lowermost interlayer is preferably about 1 μm to about 10 μm thick, and more preferably about 3 μm to about 5 μm thick. Preferably, the lowermost interlayer 12 is nickel or copper because these two metals have been discovered to exhibit excellent adhesion to both molybdenum and a chromium uppermost interlayer 14 coated over the lowermost interlayer 12. The chromium layer 14 is preferably about 1 μm to about 10 μm thick and more preferably about 4 μm to about 6 μm thick. The interlayers 12, 14 are each preferably uniformly thick and should be defect-free (for example, they should not contain pin holes) .

The dielectric layer 16 of these particularly preferred ESCs contains cordierite (Al₃Mg₂ (Si₅Al) 0₁₈) . This layer must be tightly adhered to the substrate and the top surface of layer 16 must be smooth and free of chips and cracks. The terms smooth and uniform mean that cordierite coating is evenly thick, without discontinuities or lu piness. Preferably, the surface will not vary in height by more than about 5 μm and more preferably about 1 μm. While a smooth surface is important for clamping properties, the surface can have grooves or other configurations to improve chuck characteristics such as heating/cooling rate. In the electrostatic chuck illustrated in Figs. 1-3, gas can enter through the hole 17, flow into the grooves 3 and escape through the gap between the chuck surface and the semiconductor wafer. In preferred embodiments, the dielectric surface is polished by conventional means to a particularly smooth finish.

The smooth surface is also free of cracks. The dielectric layer adheres well to the at least one interlayer, which in turn adheres well to the electrode. Preferably, the coating does not lose adhesion after being dropped 1.3 meters onto a concrete floor or after cooling in liquid nitrogen followed by immersion in water at 80°C. The dielectric properties of the dielectric layer are also quite important since the electrostatic force is dependent on the dielectric constant and thickness of the layer. It is advantageous to have insulating behavior with low leakage current. In one preferred embodiment, the coating has a resistivity of at least IO¹² ohm-cm at 250°C and at least IO⁹ ohm-cm and more preferably 10¹⁰ ohm-cm at 500°C. On the other hand, a low dielectric constant is desirable for declamping. Thus, in some embodiments, it is preferred that the coating have a resistivity less than about IO¹³ ohm-cm.

The at least one interlayer is most preferably applied by electroplating; however, other conventional means of applying metal thin films to substrates, such as physical and chemical vapor deposition, are also suitable. For nickel plating, it is desirable to use a relatively low plating current density since this produces more defect-free coatings and a lower incidence of debonding from the substrate than coatings applied at a higher current density. Preferably, the current density for the nickel electroplating should be between about 50 to 220 A/m², and more preferably about 100 to 150 A/m².

For embodiments in which an uppermost interlayer is applied over at least one other interlayer, the at least one other interlayer should be annealed prior to applying chromium plating since this reduces the occurrence of debonding. Annealing must be carried out in a reducing atmosphere. If the annealing step is conducted in air, it has been discovered that acceptable coatings do not form.

In a preferred method, a cordierite precursor glass composition is formed into a slurry by mixing with organic binders, and optionally a liquid carrier media such as water or toluene. In a preferred embodiment, the precursor glass composition contains no crystalline (i.e., no calcined or presintered) refractory material. The glass and organic binder mixture can be mixed in a ball mill or other suitable apparatus. The resulting slurry may be applied to the chromium surface of the electrode by conventional techniques such as tape casting or doctor blading. Either of these techniques are convenient and economical routes to producing thick films (for example, greater than 100 micron in thickness) . After the green ceramic film has been applied to the electrode, the binder is burned out in air. The air is then replaced by a reducing atmosphere . It is important that sintering of the glass composition be conducted in a slightly reducing atmosphere, for example, about 10 vol . % hydrogen with about 90 vol . % helium. Sintering conducted in a highly reducing atmosphere, for example, about 90% hydrogen, results in a weakened layer. In order to produce a dense and well-crystallized dielectric coating, it is preferred that the sintering be conducted at a temperature greater than 900°C. The sintering time is preferably between 2 to

4 hours and more preferably about 3 hours.

EXAMPLES Measurement Techniques Thermal shock tests were carried out by a) heating a sample to 400°C and quenching the sample in cold water; and b) cooling the sample to liquid nitrogen temperatures followed by immersion in water at about 80°C. To pass this test, samples do not chip, flake or crack. Electrical resistivity was measured with a pico- amp meter (Hewlett Packard 4140B) at temperatures ranging up to 500°C. A constant voltage of 99 volts was applied to the test sample while it was being heated up to the target temperature and current readings were taken at regular intervals. Readings were also obtained during the cool-down cycle.

The microstructure and chemistry of the coatings were analyzed by a Scintag PTS x-ray diffractometer, Environmental Scanning Electron Microscope, and Electron Microprobe . Example 1

A molybdenum electrostatic chuck custom-machined from a molybdenum rod was polished with 200-grit carbide sandpaper and then ultrasonically cleaned and degreased in an acetone bath for 30 minutes. The chuck was then washed with pure acetone and dried in air. As shown in Figs. 1-2, the chuck body had surface grooves 1.5 mm wide and about 0.2 mm deep . The cleaned chuck was then electroplated in a plating bath containing 300g/l NiS0₄, 45g/l NiCl₂, and 40g/l boric acid at a pH of 2.5 to 4.0. The solution was heated at 50 to 60°C and stirred with a magnetic stirring bar. The current input was between 1 and 2 ampere (estimated current density 110 to 220 A/m²) resulting in a plating rate of about 0.25 to 0.5 micron/min. A plating thickness of about 5 μm was applied. The chuck was then annealed in an He-H₂ atmosphere at 600 to 900°C for 30 minutes with a ramping rate of 5°C/min, and cooled at a rate of about 5°C/min.

The chuck was then electroplated with chromium in a plating bath of 400g/l Cr0₃ and 4g/l H₂S0₄ at 50 to 60°C with a total current input of 21 A (estimated current density 230 mA/cm²) to result in an estimated thickness of 5 to 6 micron. The chuck was then air dried.

A silicone rubber mold was placed around the electrostatic chuck by setting the chuck face down on a molding surface and pouring molten silicone around the chuck. A slurry composition was prepared from: 100 grams of cordierite precursor glass (elemental composition in weight percent: Si0₂, 50.3; Al₂0₃, 20.9; MgO, 23.4; B₂0₃, 2.14; and P₂0₅, 3.26, or stated alternatively Al, 11.1; Si, 23.5; Mg, 14.1; B, 0.66; P, 1.42; and O, 49.2; available from Sem-Com, Inc., Toledo, OH, product code SCE-505) , 2.00 g of fish oil (Defloc Z-3 fish oil, available from Reichhold Chemical, Inc., Research Triangle Park, NC, Product Code 14424-00) , 60.3 g of toluene, and 15.0 g of ethanol. This mixture was mixed in a ceramic jar mill or plastic bottle for 48 hours. Then 11.04 g polyvinyl butyryl (Butvar B-79, available from Monsanto Chemical Co., St. Louis, MO) and 6.52 g butyl benzyl phthalate (Santicizer 160, available from Monsanto Chemical Co., St. Louis, MO) was added and milled for an additional 48 hours. The mill speed was 60 to 65 rpm with alumina cylindrical media.

The slurry was degassed in vacuum. The slurring material was placed on one side of the silicone flange and cast onto the chuck with a doctor blade to attain a green tape thickness of about 8 to 9 mil (1 mil = 25.4μm) . The slurry settled into the surface grooves to form a grooved green tape whose shape conformed to that of the underlying substrate (see Figs. 1-2) . The green tape was allowed to air dry for 24 hours. If the through hole 17 is covered during casting, it can be cleared with a pin.

The rubber mold was removed and the green tape was dried in air at 115°C for 30 minutes. The chuck was heated at a ramp rate of 1 to 2°C/min. to 450°C and held at 450 to 475°C for 3 hours in air to accomplish burnout of the binder. The furnace was then purged with He-H₂ for

1 hour and then heated at a ramp rate of 5 to 10°C/min. to

900°C. The film was sintered between 900 and 1,000°C for

3 hours in a He-H₂ (90-10) atmosphere and then cooled at a rate of 2 to 5°C/min. The resulting sintered film showed good smoothness and excellent adhesion. To achieve surface smoothness better than 1 μm, samples were polished for at least 72 hours on a Leco polishing wheel (Leco

Corp., St. Joseph, MI) at 100 rpm with a flock twill cloth (Leco Part #810-470) and 1 μm diamond compound (Leco Part

#810-869) .

Example 2

In this example, the procedure described in Example 1 was repeated except that the sintering was carried out in an argon atmosphere. The characteristics of the coating were very similar to those found in Example 1. Electrical resistivity measurements done on the sample also gave almost identical results to those from the sample in Example I (see Fig. 4) . Example 3

In this example, the procedure described in Example 1 was repeated except that in place of the electroplated chromium and nickel films was a nickel film (possibly having iron and chromium therein) about one to two microns thick deposited by ion-beam sputtering on the molybdenum substrate. The sputter-deposited nickel film was not annealed prior to tape-casting of the cordierite glass-ceramic slurry. After sintering in argon atmosphere, the ceramic film adhered strongly on the metallic substrate. Electrical resistivity measurements done on the sample showed consistently lower resistivity values than those from the sample in Example 1 (see Fig. 4) . Example 4

In this comparative example, the procedure described in Example 1 was repeated except that a molybdenum sealing glass-ceramic was used. The glass- ceramic with a product code of SCC-7 was purchased from Sem-Com, Inc. and has composition ranges by weight as follows: ZnO (<75%) , B₂0₃ (<25%) , Si0₂ (<25%) , PbO (<10%) , and CuO (<5%) . A slurry of the glass-ceramic powder was prepared following the procedure outlined above and cast on a molybdenum substrate which was electroplated with nickel and then chromium as described above. The coating was subjected to the same sintering profile and atmosphere as given above. The sintered coating was strongly bonded to the Mo substrate and had a pinkish appearance which suggests that there might be a reduction of copper oxide in the ceramic composition. Electrical resistivity measurements done on the sample showed (see Fig. 4) that the sample had much lower resistivity values compared with those of the cordierite film in Example 1 over all the temperatures tested. Example 5

In this comparative example, the procedure described in Example 1 was repeated except that a lead- based glass-ceramic was used. The glass-ceramic with a product code of L-10032 was purchased from Sem-Com Inc. and has composition ranges by weight as follows: PbO (<40%) , B₂0₃ (<40%) , ZnO (<30%) , and BaO (<1%) . A slurry of the glass-ceramic powder was prepared following the procedure outlined above and cast on a molybdenum substrate which was electroplated with nickel and then chromium as described above . After the sample was sintered following the procedure detailed in Example 1, the coating appeared black with a rough and porous surface like that of sandpaper. Example 6

In this comparative example, the procedure described above was repeated except that a stainless steel substrate (SS-304, from Williams and Company, Inc., Charleston, WV) was used in place of a molybdenum substrate. The stainless steel substrate was electroplated with nickel and chromium as described above. A green tape was cast onto the surface as described above and sintered; however, sintering did not result in an acceptable film. Instead, the sintered film exhibited multiple cracks and the pieces flew off from the stainless steel substrate. Examples 7-10

In these examples, the resistivities of sintered pellet samples (not on a substrate) with four different compositions were measured in the temperature range from room temperature to 500°C. The compositions were: cordierite glass-ceramic (SCE-505) + 10 vol . % of Al₂0₃ (from Alpha/Aesar, Ward Hill, MA) ; cordierite glass- ceramic (SCE-505) + 10 vol .% of BN (from Advanced Ceramics Corp., Cleveland, OH) ; cordierite glass-ceramic (SCE-505) + 10 vol.% of Ti0₂ (from Alpha/Aesar, Ward Hill, MA) ; and SCE-5 from Sem-Com, Inc. with composition ranges by weight: Si0₂ (_<50%) , Al₂0₃ (<25%) , CaO (<25%) , and MgO (<25%) . The pellet samples in Examples 7-10 were prepared according to the procedure outlined below:

1. Cordierite glass powder and the ceramic additive (10% by volume) of a total weight of 50 grams were mixed and milled for 24 hours in a 250-ml Nalgene plastic bottle containing about 50 grams of cylindrical alumina media. No organic binder was added before or after mixing; the pellet only consisted of the glass and ceramic additive; 2. A small amount of the mixed powder (about 0.8 grams) was weighed and pressed in a cylindrical steel die with a 1/2-inch diameter cavity. A load of 5000 pounds was applied for 5 minutes before the pressure was released. A pellet was then forced out of the die cavity; 3. Steps 1 and 2 were followed to make all the other pellet samples;

4. The pellets were sintered in a programmable muffle furnace (Fisher Model 495A, Fisher Scientific) in air and using the same firing, time-temperature schedule as for the sample in Example 1.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It therefore should be understood that this invention is not unduly limited to the illustrative embodiments set forth above, but is to be controlled by the limitations set forth in the claims and equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. An electrostatic chuck comprising a dielectric layer, wherein said dielectric layer comprises MgO, Al₂0₃, and Si0₂ as at least a majority of said dielectric layer, and said Al₂0₃ constitutes a minority of said dielectric layer.

2. The electrostatic chuck according to claim 1, wherein said dielectric layer comprises cordierite.

3. The electrostatic chuck according to claim 1, wherein said dielectric layer is on at least one interlayer and said at least one interlayer is on an electrode.

4. The electrostatic chuck according to claim 3, wherein said at least one interlayer includes a chromium- containing interlayer contacting said dielectric layer.

5. The electrostatic chuck according to claim 3, wherein said dielectric layer is on an uppermost interlayer comprising chromium, said uppermost interlayer is on a lowermost interlayer comprising at least one of nickel and copper, and said lowermost interlayer is on said electrode.

6. The electrostatic chuck according to claim 3, wherein said electrode comprises a refractory metal .

7. The electrostatic chuck according to claim 3, wherein said electrode comprises at least one metal selected from the group consisting of molybdenum, tungsten and alloys thereof .

8. The electrostatic chuck according to claim 4, wherein said electrode comprises a refractory metal.

9. The electrostatic chuck according to claim 4, wherein said electrode comprises at least one metal selected from the group consisting of molybdenum, tungsten and alloys thereof .

10. The electrostatic chuck according to claim 5, wherein said electrode comprises a refractory metal.

11. The electrostatic chuck according to claim 5, wherein said electrode comprises at least one metal selected from the group consisting of molybdenum, tungsten and alloys thereof .

12. The electrostatic chuck according to claim 1, wherein said dielectric layer is about 25 μm to about 200 μm thick.

13. The electrostatic chuck according to claim 1, wherein said dielectric layer has a resistivity of at least IO¹² ohm-cm at 250°C.

14. The electrostatic chuck according to claim 1, wherein said dielectric layer does not contain titanium.

15. The electrostatic chuck according to claim 3, wherein a portion of said at least one interlayer is interdiffused with said electrode.

16. The electrostatic chuck according to claim 4, wherein said lowermost interlayer and said uppermost interlayer are each about 1 μm to about 10 μm thick.

17. A process for producing the electrostatic chuck according to claim 3, comprising: providing an electrode comprising an electrically conductive substrate; coating on at least one surface of said electrode at least one interlayer comprising a metal; coating on said at least one interlayer a dielectric layer comprising MgO, Al₂0₃, and Si0₂ as at least a majority of said dielectric layer, wherein said Al₂0₃ constitutes a minority of said dielectric layer; and sintering said coated electrode to produce said electrostatic chuck.

18. The process according to claim 17, wherein said sintering is accomplished in a reducing atmosphere of less than about 10 vol .% hydrogen and at a temperature of at least about 900°C.

19. The process according to claim 17, wherein said at least one interlayer is partially diffused into said electrode prior to coating with said dielectric layer by heating the interlayer coated electrode at about 600°C to about 950°C for about 0.5 to about 2 hours.

20. The process according to claim 17, wherein said electrode comprises at least one metal selected from the group consisting of molybdenum, titanium and alloys thereof, said at least one interlayer comprises an uppermost interlayer comprising chromium and a lowermost interlayer comprising at least one metal selected from the group consisting of copper and nickel, and said dielectric layer comprises cordierite.

21. A process for providing a dielectric layer on an electrically conductive substrate, said process comprising: providing said electrically conductive substrate; coating on at least one surface of said electrically conductive substrate at least one interlayer comprising a metal; coating on said at least one interlayer a dielectric layer comprising MgO, Al₂0₃, and Si0₂ as at least a majority of said dielectric layer, wherein said

Al₂0₃ constitutes a minority of said dielectric layer; and sintering said dielectric layer coated substrate to fix said dielectric layer on said electrically conductive substrate, wherein said at least one interlayer adheres to said dielectric layer and to said electrically conductive substrate more durably than said dielectric layer and said electrically conductive substrate adhere directly to each other in the absence of said at least one interlayer.

22. The process according to claim 21, wherein said sintering is accomplished in a reducing atmosphere of less than about 10 vol . % hydrogen and at a temperature of at least about 900°C.

23. The process according to claim 21, wherein said at least one interlayer is partially diffused into said electrically conductive substrate prior to coating said at least one interlayer with said dielectric layer by heating the interlayer coated electrically conductive substrate at about 600°C to about 950°C for about 0.5 to about 2 hours.

24. The process according to claim 21, wherein said electrically conductive substrate comprises at least one metal selected from the group consisting of molybdenum, titanium and alloys thereof, said at least one interlayer comprises an uppermost interlayer comprising chromium and a lowermost interlayer comprising at least one metal selected from the group consisting of copper and nickel, and said dielectric layer comprises cordierite.