Electricity: electrical systems and devices – Electric charge generating or conducting means – Use of forces of electric charge or field
Reexamination Certificate
1998-12-02
2001-08-07
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Electric charge generating or conducting means
Use of forces of electric charge or field
C279S128000
Reexamination Certificate
active
06272002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrostatic holding apparatus, and more particularly to an electrostatic holding apparatus useful for a process performed during fabrication of semiconductor devices, liquid-crystal devices, etc., which apparatus can strongly attract and hold a conductive, semiconductive, or insulating object by means of electrostatic force at any temperature ranging from a low temperature to a high temperature, and which enables easy attachment/detachment of the object. The present invention also relates to a method of producing such an electrostatic holding apparatus.
2. Description of the Related Art
Recently, processes for fabricating semiconductor devices, liquid-crystal devices, etc.; especially processes of drying etching, ion implantation, and vapor deposition, have become more and more automated and performed as dry processes. Under such circumstances, the number of fabrication processes performed under vacuum has increased.
Meanwhile, positional accuracy at the time of patterning has become more important, since the diameters of silicon wafers and glass plates serving as substrates have increased and the degree of integration of circuits and the degree of fineness of patterns have increased. Therefore, vacuum chucks have conventionally been used to transport wafers or to attract and fix wafers. However, vacuum chucks have the following drawbacks. Vacuum chucks cannot be used under a vacuum environment because of impossibility of creating a pressure difference. Although vacuum chucks can be used under a low-vacuum environment, a wafer subjected to suction undergoes a local distortion because the wafer is subjected to suction locally, with the result that accurate positioning becomes difficult. Therefore, vacuum chucks are not suitable for recently developed processes for fabricating semiconductor devices and liquid-crystal devices.
Recently, as a device that has overcome the above-described drawbacks, there has been widely noticed and put into practical use an electrostatic holding apparatus which transports and/or attracts and fixes a wafer by means of electrostatic force. In a recent process for fabricating semiconductor devices and liquid-crystal devices, with an increase in the degree of fineness of devices, the flatness of wafers and glass plates serving as substrates has become more and more important. Therefore, employment of electrostatic holding apparatuses has been considered, with a view toward performing a straightening correction for improving the flatness of wafers and glass plates. Also, since many processes are performed in a high temperature, corrosive gas atmosphere, an electrostatic holding apparatus formed of ceramics has been developed.
Two methods for producing such a conventional ceramic electrostatic holding apparatus have been proposed. In the first method, through printing or a like process, a conductive layer is formed on a substrate (insulator layer) formed of sintered ceramics, on which an insulating dielectric layer is formed through thermal spraying. In the second method, an insulating dielectric layer (a sintered body comprising 99% alumina) is bonded onto a conductive layer through used of adhesive.
Further, in recent years, an electrostatic holding apparatus having improved durability has been proposed (see Japanese Patent No. 2129621). This electrostatic holding apparatus is produced as follows. A conductive layer is sandwiched between a green sheet for an insulating dielectric layer and a green sheet for an insulator layer, both formed through tape forming, casting, or press forming, and the layer and sheets are superposed in an unfired state so as to become united, and then fired to obtain a sintered body serving as the electrostatic holding apparatus.
However, the electrostatic holding apparatus produced in accordance with the above-described manner has problems in that the electrostatic attraction surface has distortion and/or warping that is conceivably generated due to differences in coefficient of thermal expansion among the three layers, and in that since inter-layer delamination, cracks, and the like are generated in the sintered body, a sufficient electrostatic attraction force cannot be obtained, and withstand voltage is low.
SUMMARY OF THE INVENTION
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide an electrostatic holding apparatus, in which an insulating dielectric layer, a conductive layer and an insulator layer are united while differences in coefficient of thermal expansion among these layers are minimized in order to prevent generation of inter-layer delamination, cracks, and the like, and in order to prevent generation of distortion or warping on the electrostatic attraction surface to thereby increase the electrostatic attraction force; which does not suffer deterioration in the capability of allowing removal of an object at the time of stopping application of voltage; and which is excellent in terms of withstand voltage.
Another object of the present invention is to provide a method of producing such an electrostatic holding apparatus.
To achieve the above objects, the present invention provides an electrostatic holding apparatus in which a voltage is applied to an electrode formed of a conductive layer disposed on an insulator layer and covered with an insulating dielectric layer in order to cause the insulating dielectric layer to electrostatically attract an object, wherein the electrostatic holding apparatus is formed of a sintered body in which a thermal expansion mitigating layer is disposed between the insulating dielectric layer and the conductive layer and/or between the conductive layer and the insulator layer; and the insulating dielectric layer, the conductive layer, the insulator layer, and the thermal expansion mitigating layer are superposed and press-formed in an unfired state to obtain a green body, which is then sintered to obtain the sintered body.
When the electrostatic holding apparatus is formed of a sintered body in which a thermal expansion mitigating layer is disposed between the insulating dielectric layer and the conductive layer and/or between the conductive layer and the insulator layer; and the insulating dielectric layer, the conductive layer, the insulator layer, and the thermal expansion mitigating layer are superposed and press-formed in an unfired state to obtain a green body, which is then sintered to obtain the sintered body, the differences in coefficient of thermal expansion among adjacent layers decrease because of presence of the thermal expansion mitigating layer, so that the electrostatic holding apparatus thermally expands and contracts in substantially the same manner as does an electrostatic holding apparatus formed of a single layer. Therefore, almost no distortion or warping is generated on the surface of the insulating dielectric layer serving as an electrostatic attraction surface. Further, defects such as inter-layer delamination or cracks are hardly generated within the sintered body. Accordingly, the electrostatic holding apparatus produces a strong and uniform electrostatic attraction force with enhanced responsiveness, enables easy removal of an object, and has a high withstand voltage.
Preferably, the thermal expansion mitigating layer has a coefficient of thermal expansion between that of the conductive layer and that of the insulating dielectric layer and the insulator layer and is mainly formed of a ceramic material selected from the group consisting of aluminum nitride, aluminum oxide, silicon nitride, silicon oxide, zirconium oxide, titanium oxide, sialon, boron nitride, and silicon carbide, or a mixture of two or more kinds of ceramic materials selected from the group.
When the ceramic material of the thermal expansion mitigating layer is selected from the above-described group, the thermal expansion mitigating layer can be easily formed to have a coefficient of thermal expansion between that
Kobayashi Toshimi
Mogi Hiroshi
Fleming Fritz
Hogan & Hartson LLP
Shin-Estu Chemical Co., Ltd.
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