As to the plasma treatment apparatus, the electron beam lithography apparatus, the ion implantation apparatus or the like that is used for the manufacturing process of a semiconductor device, or the ion doping apparatus or the like that is used for manufacturing a liquid crystal panel, it is required to securely hold a semiconductor wafer or a glass substrate that is an object to be treated without damaging the semiconductor wafer or the glass substrate. In particular, since contamination of the semiconductor wafer or the glass substrate to be treated should be controlled strictly these days, most of the systems for clamping the semiconductor wafer or the like mechanically as conventional techniques are being replaced with the electrostatic chuck systems that utilize an electrostatic attraction force.
The electrostatic chuck includes a lower insulation layer, an electrode layer and a surface insulation dielectric layer, which are formed on a metal substrate. The surface insulation dielectric layer constitutes an attracting surface for holding the semiconductor wafer or the glass substrate. Then, a high voltage potential is applied externally to the electrode layer via power feeding terminals disposed in through holes formed through the upper and lower surfaces of the metal substrate. Thus, Coulomb force or Johnsen-Rahbeck force is generated between charges distributed on the surface of the surface insulation dielectric layer (i.e., the attracting surface) and the charges polarized and induced by the object to be treated on the attracting surface. Otherwise, a gradient force is generated by the electrostatic field, so that the semiconductor wafer or the like as the object to be treated is attracted and held.
When etching treatment is performed on the semiconductor wafer by using a plasma apparatus, for instance, the temperature of the semiconductor wafer will increase up to approximately 200° C. to 400° C. Therefore, in order to cool down the wafer that is being treated to an appropriate temperature, a refrigerant is made to flow in a conduit line provided inside the metal substrate so that the temperature increase of the wafer is prevented. However, the surface insulation dielectric layer side of the electrostatic chuck is exposed to the high temperature while the temperature on the metal substrate side is kept substantially to be the temperature of the refrigerant. Therefore, a temperature gradient occurs between them. For instance, a temperature gradient of a few hundred degrees at highest occurs between the surface insulation dielectric layer and the lower insulation layer. In addition, as a matter of course, a temperature gradient of a few hundred degrees at highest occurs in the electrostatic chuck itself between the time when the apparatus is operating and the time when the apparatus is stopped.
If a stress such as a heat cycle is applied to the electrostatic chuck, various problems may occur in particular with respect to a power feeding structure for supplying a voltage to the electrode layer. More specifically, since an electric conductor such as the power feeding terminal or the electrode layer has a coefficient of thermal expansion different from that of an insulator such as the lower insulation layer and the surface insulation dielectric layer, a crack may occur easily in the region around the power feeding terminal where the electric conductor and the insulator contact with each other and are structurally complicated. This crack may be a factor causing such problem as a local deterioration in temperature characteristics of the electrostatic chuck or particle generation or the like.
FIGS. 4(a) and 4(b) illustrate a conventional example of the power feeding structure of the electrostatic chuck. A through hole 7 is formed in a metal substrate 1, and a power feeding terminal 3 is disposed in the through hole 7 via an insulation holding member 2. As illustrated in FIG. 4(b), a tip of this power feeding terminal 3 contacts with an electrode layer 5, so that a voltage supplied to the metal substrate 1 from the lower surface side is supplied to the electrode layer 3. Here, the crack as described above is likely to occur at an edge of the portion where the tip of the power feeding terminal 3 contacts with the electrode layer 5 (crack 8a), for instance. In addition, the crack is likely to occur also at a portion where the power feeding terminal 3, the insulation holding member 2, and a lower insulation layer 4 contact with each other (crack 8b).
Therefore, some methods are proposed for reducing the influence of the thermal load that is applied to the electrostatic chuck. For instance, a proposed method includes brazing a power feeding terminal for feeding power to an electrode layer disposed in a ceramic substrate, and providing a hollow portion on an end surface of the power feeding terminal so that a stress relieving member having a coefficient of thermal expansion that is substantially the same as that of the ceramic substrate is inserted in the hollow portion (see Patent Document 1). In addition, another proposed method includes disposing a power feeding terminal in a through hole formed in a substrate made of a metal and ceramic composite material via a casing portion made of a ceramic so that an end surface of the power feeding terminal is flush with the upper surface of the substrate, masking the end surface of the power feeding terminal, forming an insulation layer by thermal spraying treatment, removing the masking so as to expose the end surface of the power feeding terminal, and performing thermal spraying of a metal material so as to form an electrode layer (see Patent Document 2). Further, still another proposed method includes forming a through hole formed through an inner electrode layer of a ceramic substrate from the lower side, forming a metalized layer on the inner wall of the through hole, and brazing a power feeding terminal to be fixed in the through hole (see Patent Document 3).
However, if the power feeding terminal is fixed by brazing as described in Patent Documents 1 and 3, a thermal load is applied to a brazing material itself so that the problem may be more complicated. In addition, the brazing work itself is a manual work and is not completely reliable. On the other hand, according to the method of adjusting the end surface of the power feeding terminal disposed in the substrate to be flush with the upper surface of the substrate, and forming the electrode layer by the thermal spraying treatment so that the end surface of the power feeding terminal contacts with the electrode layer as described in Patent Document 2, the work efficiency may be improved. However, it is necessary to improve the reliability more with respect to the thermal load. In other words, since the power feeding terminal contacts with the electrode layer by surface contact, there is still a problem remaining in terms of reliability when a thermal load is applied.    Patent Document 1: JP 11-074336 A    Patent Document 2: JP 2003-179127 A    Patent Document 3: JP 10-189696 A