The present invention relates to an electrostatic chuck or, more particularly, relates to an electrostatic chuck used for holding and transporting, for example, a semiconductor silicon wafer, liquid crystal display panel or other articles under processing in the manufacturing process of electronic devices by means of the electrostatic attractive force.
It is a remarkable trend in recent years in the manufacturing process, in particular, of electronic devices such as semiconductor devices based on silicon wafers, liquid crystal display panels and the like that various processing steps such as etching, ion-implantation and the like are conducted under dry conditions since high productivity by automatization can be obtained by these dry processes easily as compared with traditional wet processes. One of the remarkable characteristics in these dry processes is that the treatment of the articles is performed in an atmosphere of high vacuum. Further, devices having a larger and larger size and finer and finer patterning with high precision for an increased density of circuit integration are required in the modern electronic industry.
Needless to say, the material under processing, e.g., semiconductor silicon wafers, glass substrate plates and the like, in such a processing treatment must be held or transported by some chucking means. One of the conventional chucking devices used for such a purpose is a vacuum chuck using a chucking plate having perforations and connected to an evacuation means so as to attract the work piece by the sucking action. In principle, such a vacuum chuck cannot be used in a vacuum atmosphere because of the absence of any pressure difference between the surfaces of the work poece. Even as a chucking means used under normal pressure, vacuum chucks have a problem that the attractive force by suction is localized on and around the perforations in the chucking plate so that a local strain is caused in the work piece under working chucked thereby resulting in a decreased accuracy of alignment of the work piece for processing. Accordingly, vacuum chucks are considered not to be suitable for use in the manufacturing process of electronic devices due to this serious problem.
In place of the above mentioned vacuum chucks, electrostatic chucks, in which the work piece is held to the chucking plate by means of the electrostatic attractive force, are highlighted as a promising chucking means for holding and transporting various kinds of work pieces. Conventional electrostatic chucks typically have a structure in which metal electrodes connected to a direct-current power source are integrally embedded within a body of a dielectric material at such a depth near to the chucking surface or metal electrodes are sandwiched with two plates of a rubber or a synthetic resin such as polyimide as a dielectric material and they are integrally bonded together by use of an adhesive.
The attractive force F of such an electrostatic chuck is expressed by the equation EQU F=k.multidot..epsilon..multidot.s.multidot.E.sup.2 /t.sup.2,(1)
in which .epsilon. is the dielectic constant of the dielectric material, s is the area of the attracting surface, E is the voltage applied to the electrodes, t is the thickness of the layer of the insulating dielectric material and k is a constant. Accordingly, the attractive force is increased in proportion to the dielectric constant .epsilon. of the dielectric layer so that it is preferable that the dielectric layer is formed from a ceramic material having a large dielectric constant. A ceramic layer can be provided on the internal electrode layer either by bonding a sintered ceramic sheet prepared beforehand to the electrode layer or by the method of flame spraying or plasma spraying of a ceramic powder. A problem in such a ceramic layer is that the ceramic material is more or less porous containing many pores formed by degassing in the course of the sintering process or by entrainment of gases in the course of the flame spraying. When the ceramic layer contains a large volume of pores, the effective dielectric constant of the ceramic layer is necessarily decreased in addition to the problems that the mechanical strengths of the ceramic layer are decreased so that the thickness t of the ceramic layer cannot be decreased as desired and the accuracy and smoothness of surface finishing would be greatly affected thereby.
In this regard, Japanese Patent Kokai 58-137536 proposes an improvement according to which pores of the dielectric layer of electrostatic chucks are filled with a polymeric material such as epoxy resins, fluorocarbon resins and the like. This method is indeed effective to some extent insofar as the electrostatic chuck is used under relatively mild conditions. Such a resin-impregnated ceramic material, however, is no longer a satisfactory material when the electrostatic chuck is used at an extremely low temperature or at a very high temperature sometimes exceeding 300.degree. C. due to the possible thermal degradation of the polymeric material or when the electrostatic chuck is used in an atmosphere of a reactive or corrosive gas such as halogens, high-concentration oxygen gas and the like as is frequently used in the process of dry etching or in an atmosphere of plasma because these polymeric materials can hardly withstand such an environmental condition.