ELECTROSTATIC CHUCK

Abstract: Electrostatic chucks and methods of forming the chucks are disclosed. The chucks include ceramic materials directly bonded with a layer of active braze alloy. The active alloy operates as both a conductive layer of the electrostatic chuck and a mechanism for bonding the dielectric layer to the base of the chuck. Electrical feedthroughs to the electrode may also be formed using the active alloy.

Description: ELECTROSTATIC CHUCK This invention relates to apparatus for electrostatically securing articles. In the following such apparatus will be referred to as electrostatic chucks. Fabrication of semiconductor devices is a multi-step process. During processing, chucks are employed to immobilize semiconductor wafers, despite, for example, gas pressure present at their rear, or back, faces. Commercially-available chucks for this purpose typically employ vacuum pumping, mechanical securing , or electrostatic forces to effect immobilization. Vacuum pumping is useful only in restricted instances, as it is ineffective when very low process chamber pressures are required. Mechanical clamps have been used widely in the semiconductor industry primarily because they are relatively inexpensive. Mechanical clamps physically engage the peripheries of the wafers, mcurring the risk of contact damage and shielding the clamped areas from treatment or processing. This engagement may also produce debris (i.e. particles) that reduce wafer process yields. Mechanical clamps additionally are incapable of securing central portions of the wafers, resulting in undesirable bowing of these unsupported regions. Electrostatic chucks, by contrast, use Coulombic forces to secure the entire rear face of a semiconductor wafer. As employed, these chucks are of two general types: monopolar and bi- (or multi-) polar. Monopolar electrostatic chucks, effectively functioning as one plate of a parallel-plate capacitor, include a thin dielectric layer coating and a single electrode supported by a base. A wafer, acting as the other capacitive plate, may then be placed over the dielectric. As a voltage is applied between the wafer and electrode, the wafer is attracted to the electrode and flattened against the adjacent dielectric material. Multipolar electrostatic chucks typically include two or more electrodes themselves held at different electrical potentials. As a result, no net charge transfer to the wafer need occur. Existing bipolar electrostatic chucks are relatively difficult and expensive to construct, however, and may compel adjustment or balancing of the voltages present at the electrodes during processing of the wafer. U.S. Patent No. 4,384,918 to Abe, provides equations for the attractive force generated using electrostatic chucks. For a monopolar chuck, the attractive force "F" may be calculated as follows: (1) F = 1 £ S where ε = the dielectric constant of the dielectric V = the applied voltage d = the thickness of the insulator (dielectric) material S = the area of the electrode Similarly, the attractive force for a bipolar chuck is given by: (2) F = ±As In both equations (1) and (2), the attractive force exerted on the semiconductor wafer is inversely proportional to the square of the dielectric thickness. As a consequence of these mathematical relationships, numerous efforts to decrease the thickness of the dielectric layer have been made. U.S. Patent No. 4,480,284 to Tojo, et al., mentions several such efforts, including using an adhesive agent to bond a mica, polyester, or barium titanate dielectric layer to an electrode disc and making the dielectric layer by an anodic treatment of metallic material. As discussed in the Tojo, et al. patent, however, the dielectric layers formed using these processes typically lack uniform thicknesses and flat surfaces and, in some cases, may crack over time. This unrehability is magnified when coupled with the expense associated with these complicated processes, reducing the commercial value of the processing technology. The Tojo, et al. patent itself purports to disclose a method of forming a more reliable dielectric layer for use with an electrostatic chuck. According to the patent, aluminum oxide (i.e. alumina), titanium dioxide, or barium titanate (or mixtures thereof) is flame-sprayed on an electrode plate to create the dielectric layer. Because the rough surface generated using the flame spray process lacks sufficient dielectric strength, the layer is impregnated with plastic material. To reduce warping and cracking, the dielectric layer and electrode have substantially the same coefficient of thermal expansion. The present invention aims to overcome problems in the above mentioned prior art and is as set out in the appended claims. Typically the invention may comprise forming an electrostatic chuck by directly bonding planar ceramic members (such as discs for example) with a layer of active brazing alloy. This single-step assembly presents numerous advantages over existing approaches, including use of commonly-available and relatively inexpensive components for the dielectric layer and remainder of the chuck. Employing similarly inexpensive pattern screen technologies for bonding patterns enhances formation of flexible geometries as well, and use of an active alloy allows electrical feedthroughs to the chuck electrode to be created. Active brazing alloys are materials which contain an active metal which on brazing reacts with ceramics to chemically bond the material to the ceramic. The term "active brazing alloys" includes non-alloyed mixtures of metals and includes pastes containing active metals. A discussion of such materials is contained in WO-A-94/03923 and such materials are commonly known in the field of bonding ceramics. In the present invention the active alloy acts as both a conductive layer (electrode) and the means by which a dielectric layer is bonded to the base of the chuck. The present invention thereby provides a relatively low cost, easily-assembled electrostatic chuck. The chuck's dielectric layer exhibits high reliability under various conditions, enhancing its commercial usefulness as a substitute for those employed in existing mono- and multipolar electrostatic chucks. The layer may also be thinned to a thickness impractical for separately fabricated dielectric layers yet acceptable to allow the generation of adequate attractive force. The present invention additionally permits greater design flexibility and formation of large-sized electrostatic chucks if desired. Because the active alloy and ceramic have different coefficients of thermal expansion, the present invention also improves the integrity of the dielectric layer by generating net compressive surface stresses in the ceramic. Depending on the active alloy selected, a sufficiently high-temperature bond may be created to allow the electrostatic chuck to be used at temperatures above those commonly utilized for chucks formed through, for example, organic adhesive methods. Appropriate patterns and thicknesses of active alloy may also enhance the uniformity of the radio frequency field in plasma-enhanced chemical vapor deposition equipment. Other features, and advantages of the present invention will be apparent from the claims with reference to the remainder of the written portion and the drawings of the application. FIG. 1 is a cross-sectional, partially schematicized view of a typical electrostatic chuck shown in connection with a sample and power source. FIG. 2 is a cross-sectional view of a monopolar electrostatic chuck of the present invention. FIG. 2 A, similar to Figure 4 of WO-A-94/03923, is a cross-sectional view of an exemplary feedthrough which may form part of the electrostatic chucks of the present invention. FIG. 3 is a cross-sectional view of a bipolar electrostatic chuck of the present invention. FIG. 4 is a cross-sectional view of a first alternate embodiment of the electrostatic chuck of FIG. 3. FIG. 5 is a cross-sectional view of a second alternate embodiment of the electrostatic chuck of FIG. 3. FIG. 1 illustrates a common monopolar electrostatic chuck 10, including electrode plate 14 and dielectric layer 18. Also shown in FIG. 1 are power source 22 and a sample 26 such as (but not limited to) a semiconductor wafer. When chuck 10 is in use, negative terminal 30 of power source 22 is electrically connected to electrode plate 14 and positive terminal 34 is electrically connected to sample 26, thereby generating an electric field between the electrode plate 14 and the sample 26. Dielectric layer 18, in turn, is polarized, with its lower surface 38 becoming positively charged and its upper surface 42 becoming negatively charged. The resulting polarization electrostatically attracts sample 26 toward electrode plate 14, securing it against dielectric layer 18. The same effect, moreover, results when the polarity is reversed. Illustrated in FIG. 2 is an exemplary monopolar electrostatic chuck 46 of the present invention. Like chuck 10, chuck 46 comprises a dielectric layer 50 and an electrode 54. Chuck 46 also includes base 58, in which may be formed feedthrough 62. Unlike chuck 10, however, electrode 54 is formed from an active braze alloy, while dielectric layer 50 and base 58 are typically ceramic materials such as (but not limited to) alumina, silicon nitride, aluminum nitride, barium titanate, or calcium titanate. For example, in some embodiments of the invention consistent with FIG. 2, dielectric layer 50 and base 58 are made essentially of (i.e. 94.0-99.8%) polycrystalline alumina, and electrode 54 is formed from an active alloy consisting essentially of one of the following (in weight percent): (1) 0-97.5% Au, 0-15.5% Ni, 0-93.25% Ag, 0-35.25% Cu, 0-2.25% Ti, 0-12.5% In, 0-2% Al, 0-1% Sn, 0-3% Si, 0-0.75% Mo, 0-1.75% V (preferably 58-60% Ag, 26.25-28.25% Cu, 12-13% In, 1-1.5% Ti, provided as Incusil ABA™); (2) 62-64% Ag, 33.25-35.25% Cu, 0.75-1.25% Sn, 1.5-2.0% Ti (Cusin-1 ABA™); (3) 62-64% Ag, 34.25-36.25% Cu, 1.5-2.0% Ti (Cusin ABA™); (4) 92.25%-93.25% Ag, 4.5-5.5% Cu, 0.75-1.25% Al, 1-1.5% Ti (Silver ABA™); (5) 92.25-93.25% Cu, 1.75-2.25% Al, 2.75-3.25% Si, 2-2.5 Ti (Copper ABA™); (6) 95.9-96.9% Au, 2.5-3.5% Ni, 0.5-0.7% Ti (Gold ABA™); (7) 81-83% Au, 14.5-16.5% Ni, 0.5-1.0% Mo, 1.5-2.0% V (Nioro ABA™); (8) 64-66% Ag, 1.75-2.5% Ti, 31.5-34.25% Cu (Cusil ABA™); or (9) nominally 70% Ti, 15% Cu, 15% Ni (Ticuni ABA™). Although other active alloys may also be utilized in conjunction with the present invention, the listed alloys are commercially available from Wesgo, Inc., 477 Harbor Boulevard, Belmont, California 94002. As with the other alloys suitable for use as electrode 54, they have a sufficiently active element to react chemically with the surface of ceramic (or other) materials chosen for dielectric layer 50 and base 58. Use of such active alloys is, moreover, a significant feature of the present invention, as the alloys function as both the conductive layer (electrode 54) of chuck 46 and the means by which dielectric layer 50 and base 58 bond. One method of creating chuck 46 includes forming two ceramic discs and placing between them a sheet of an active alloy. The assembly is then brazed to bond the two ceramic discs using the active alloy. Conventional grinding techniques subsequently may be employed to reduce the thickness of one of the ceramic discs to between 0.004-0.010" (approximately 0.1- 0.25mm), a suitable thickness range for dielectric layer 50. Although, e.g., polycrystalline alumina will not normally retain its integrity at such reduced thicknesses, the active alloy uniformly holds each alumina grain in place to permit successful thinning to occur. Thus, the present invention avoids the need to use more expensive dielectric layer alternatives such as single crystal sapphire, allowing substitution of lower-cost ceramic materials instead (although sapphire may continue to be used if desired). Because the active alloy has a higher coefficient of thermal expansion than the alumina discs, brazing them also creates compressive stress in the alumina surfaces, which assist in maintaining the integrity of the layer during the grinding process. In some embodiments, the two ceramic discs are 99.5% alumina initially one-eighth inch thick (approximately 3mm) and of either one, three and one-half, or eight inch diameter, (approximately 25, 89 and 200mm respectively) and the active alloy is a sheet of Copper ABA™ approximately 0.002" thick (approximately 0.05mm). Whichever of the two discs is thinned ultimately becomes dielectric layer 50, with the other denominated base 58. As noted above, the active alloy functions as electrode 54. FIG. 2 also details feedthrough 62 which, if present, provides an electrically-conductive path through base 58 while excluding the passage of sohd, hquids, and gasses. Discussed more fully in WO-A-94/03923, feedthrough 62 may itself comprise an active alloy wire or other contact 66 intended to react chemically with (ceramic) base 58. FIG. 2A, for example, taken from the co-pending apphcation, depicts a feedthrough preform (contact 66) and article (base 58) heated for a predetermined time and temperature to achieve a brazing reaction at the interface between the inner surface of insertion hole (opening 70) of the article and the outer surface of the preform. FIGS. 3-5 illustrate exemplary bipolar electrostatic chucks 74 of the present invention. As with chuck 46, chucks 74 include dielectric layer 78 and base 82, each of which may be formed of a ceramic material. Unlike monopolar chuck 46, however, an active alloy is interposed between only portions of dielectric layer 78 and base 82, with other portions remaining unbonded. As shown in FIGS. 3-5, therefore, a non-conductive area 86 separates electrodes 90, each of which may be electrically connected to a feedthrough 94 through base 82. In FIGS. 3 and 5, area 86 is an air-filled space. Alternatively, area 86 may be filled with an organic or inorganic dielectric filler 98 such as epoxy or alumina-filled organometalhc suspensions injected through port 102 in base 82 (FIG. 4). Peripheral areas 106 between dielectric layer 78 and base 82 may also contain filler 98 (FIGS. 4-5). Those skilled in the art will recognize that using pattern screen technologies for bonding patterns and an active alloy paste permits formation of other geometries, including those for multipolar electrostatic chucks. As described below, six pole chucks have been formed. The following examples, although not intended to limit the scope of the appended claims, provide additional information concerning the present invention: EXAMPLE 1 Two 3.5" diameter (approximately 89mm), 0.25" thick (approximately 6mm) essentially polycrystalline alumina wafers were bonded with a layer of an active braze alloy paste composed substantially of, by weight, 92.25-93.25% Cu, 1.75-2.25% Al, 2.75-3.25% Si, and 2-2.5% Ti (Copper ABA™). The alloy layer was 0.003" thick (approximately 0.076 mm), and the thickness of the upper ceramic layer was thereafter thinned to 0.004" (approximately 0.1 mm) by grinding. When evaluated as part of an electrostatic chuck under tests to 1 kV, the assembly performed well. EXAMPLE 2 An 8" diameter (approximately 200 mm), 0.375" thick (approximately 9.5 mm) essentially polycrystalline alumina disc was bonded to a similar disc using a 0.003" thick (approximately 0.076 mm) sheet (foil) of Copper ABA™. The top surface was thereafter thinned to 0.007" thick (approximately 0.18 mm) and the assembly evaluated as part of an electrostatic chuck. The chuck not only performed well at tests to 1.5 kV, but, because of the integrity of the dielectric layer, exhibited resistance greater than 1011 ohms at that voltage as well. EXAMPLE 3 Two rectangular plates of 0.125" thick (approximately 3.2 mm) essentially polycrystalline alumina were bonded using a pattern-screened paste composed substantially of, by weight, 62-64% Ag, 34.25-36.25 Cu, and 1.5-2.0 Ti (Cusin ABA™). After bonding, the dielectric layer was thinned successfully to a thickness ranging between 0.020" (approximately 0.5 mm) and 0.002" (approximately 0.05mm). Unbrazed areas were supported with non-conductive polymerizable material, and feedthroughs were produced with active braze alloy wire melted into holes in the ceramic material. A rear face pigtail of copper also was used. EXAMPLE 4 Two 5.2" diameter (approximately 132 mm), 0.300" thick (approximately 7.6mm) essentially polycrystalline alumina discs were bonded with Cusin- 1 ABA™ paste applied in a star-shaped pattern that defined six separate electrode areas. Each electrode was attached to a nickel wire contact using active braze alloy. The active dielectric surface of the chuck was finished to a thickness of 0.009" (approximately 0.23mm), and the thickness of the overall assembly was 0.125" (approximately 3.2 mm). Organic dielectric infiltrants were used in between electrode patterned areas. The chuck clamped a sihcon wafer effectively at voltages from 500-1000 V without failure. EXAMPLE 5 A 5.9" diameter (approximately 150mm) 0.089" thick (approximately 2.3mm) ceramic electrostatic chuck was produced using screened Cusin ABA™ braze alloy to bond two ceramic members initially each 0.125" thick (approximately 3.2 mm). The alloy produced a bipolar electrode having a concentric pattern. These electrodes were connected to the rear face of the chuck with active alloy feedthroughs that passed through the ceramic to nickel-copper alloy termination pins. Organic infiltrants sealed the edges. The dielectric surface of the chuck was 0.006" thick (approximately 1.5mm)and grooved to allow passage of gasses in use. EXAMPLE 6 A 2.5" diameter (approximately 63.5 mm) monopolar electrostatic chuck was produced with an active braze alloy paste feedthrough contact through the backside ceramic and a 0.006" thick (approximately 0.15mm) top dielectric layer. An inorganic dielectric sealant was used in areas where the braze alloy electrode was not placed, perrmtting the chuck to operate to temperatures of 600°C. The initial thicknesses of the ceramic members from which the chuck was formed were 0.4" (approximately 10.2mm) and 0.125" (approximately 3.2mm), respectively, and the sealant comprised commercial inorganic hquids (sihca and phosphate bonds) mixed with a very fine alumina powder to create a thermoset ceramic cement. The foregoing description has related to provision of electrostatic chucks and has discussed the semiconductor fabrication industry. It will be clear to the person skilled in the art that the invention is not limited to this industry and is applicable wherever apparatus is required to electrostatically secure articles.

Claims: CLAIMS An electrostatic chuck comprising: a. first and second members; and b. an electrode comprising a metal interposed between and chemically adhering to at least portions of the first and second members. An electrostatic chuck as claimed in claim 1 further comprising means, extending through the second member, for electrically connecting the electrode to a power supply. An electrostatic chuck as claimed in claim 1 or claim 2 further comprising a non-conductive material interposed between portions of the first and second non-metallic members not bonded by the metal. 4. An electrostatic chuck as claimed in any preceding claim in which the first and second members are ceramics and the metal is formed from an active alloy used to braze the first and second members together. 5. An electrostatic chuck as claimed in claim 4 as dependent on claim 2 in which the electrically connecting means comprises an opening defined by the second member and active alloy extending through and bonded to the opening. An electrostatic chuck as claimed in claim 3 in which the non-conductive material is selected from the group consisting of epoxy and alumina-filled organometallic suspensions. An electrostatic chuck as claimed in any preceding claim and comprising: a. a base part; b. a dielectric layer; and c. one or more electrodes interposed between the base part and the dielectric layer and operably connectable in use to a voltage source; and in which d. the base part and the dielectric layer are formed of ceramic; and e. the one or more electrodes are formed of an active brazing alloy bonding the dielectric layer to the base part. 8. An electrostatic chuck as claimed in claim 7 in which the dielectric layer has a thickness of 0.020" or less. 9. An electrostatic chuck as claimed in claim 8 in which the dielectric layer has a thickness in the range 0.004-0.010". 10. An electrostatic chuck as claimed in of claims 7 to 9 and useable at temperatures of 600°C. 11. An electrostatic chuck as claimed in any of claims 5 to 10 as dependent on claim 4 in which the active alloy has a higher coefficient of thermal expansion than that of the first member. 12. An electrostatic chuck as claimed in any preceding claim in which at least one of the first and second members is made essentially of alumina. 13. An electrostatic chuck as claimed in any of claims 5 to 12 as dependent on claim 4 in which the active alloy consists essentially of non-precious metals. 14. An electrostatic chuck as claimed in claim 3 in which the first ceramic member defines a port through which the non-conductive material is injected. 15. An electrostatic chuck as claimed in any preceding claim in which at least one of the first and second ceramic members comprises material selected from the group consisting of sihcon nitride, aluminum nitride, barium titanate, calcium titanate, and sapphire. 16. A method of securing a semiconductor wafer for processing, comprising the use of an electrostatic chuck as claimed in any preceding claim. 17. A method of forming an electrostatic chuck comprising the steps of: a. chemically bonding at least portions of first and second ceramic members with a metal; and b. thereafter reducing the thickness of the first ceramic member. 18. A method as claimed in claim 17 in which the step of bonding first and second ceramic members comprises bonding at least portions of the first and second ceramic members with an active alloy.

Inventors: SMITH PETER CLARK, SMITH, PETER, CLARK
Applicants: MORGAN CRUCIBLE CO, THE MORGAN CRUCIBLE COMPANY PLC
Application year: 1995
Main classification: H01L21
IPC codes: B25J15, H01L21