1. Field of the Invention
The present invention relates to an electrostatic chuck for holding an object such as a semiconductor wafer by electrostatic attraction and to a production method therefor. More particularly, the invention relates to an electrostatic chuck exhibiting a great attractive force even in a high temperature environment and to a production method therefor.
2. Description of the Prior Art
In semiconductor-related production processes, electrostatic chucks are conventionally employed for holding a semiconductor wafer in a reaction process chamber with a high level of accuracy in semiconductor processing apparatuses such as film forming apparatuses for forming a thin film on a semiconductor wafer by PVD, CVD, sputtering and the like and dry etching apparatuses for microprocessing a wafer.
In such a semiconductor processing apparatus, highly corrosive halogen gases such as fluorine- and chlorine-containing gases are employed for deposition, etching and cleaning, and a plasma is often generated. Since the electrostatic chucks are used under such severe conditions, the electrodes thereof need to be coated with a material highly resistant to the plasma and the corrosive halogen gases. To this end, electrostatic chucks have been proposed which include an electrode such as plate or film embedded in a ceramic plate.
The electrostatic attractive force of the electrostatic chucks which attracts an object such as a semiconductor wafer includes the Coulomb force which arises due to dielectric polarization and the Johnson-Rahbeck force which arises due to a small leak current. It is desirable to utilize the Johnson-Rahbeck effect which provides a greater attractive force.
Conventional electrostatic chucks utilizing the Johnson-Rahbeck effect are constructed such that a ceramic plate is comprised of a low resistant alumina ceramic containing a transition metal (Japanese Unexamined Patent Publication No. 62-94953 (1987)), and such that a ceramic plate includes two types of ceramic portions bonded to each other, one of which is comprised of a low resistant ceramic containing an impurity and the other of which is comprised of a high resistant ceramic containing no impurity (Japanese Unexamined Patent Publication No. 4-3956 (1992)).
To establish the Johnson-Rahbeck effect, a ceramic portion present between an electrode and an attractive surface of an electrostatic chuck needs to have a volume resistivity of not lower than 1.times.10.sup.9 .OMEGA.cm and lower than 1.times.10.sup.11 .OMEGA.cm in a process temperature range.
However, the conventional electrostatic chucks are intended to be used in a room temperature environment. Therefore, when such a chuck is used in a high temperature environment at 250 to 450.degree. C. in the semiconductor processing apparatus, the resistivity of the ceramic portion present between the electrode and the attractive surface is lower than that in a room temperature, so that the leak current is increased. When the object to be held by the chuck is a semiconductor wafer, a minute circuit on the wafer may suffer from dielectric breakdown.
The amount of the transition metal or the impurity to be added to the ceramic may be adjusted to control the resistivity of the ceramic portion for utilization of the Johnson-Rahbeck effect in the high temperature environment. However, the resistivity of the ceramic portion is likely to vary, so that it is difficult to prepare a ceramic plate having a resistivity of not lower than 1.times.10.sup.9 .OMEGA.cm and lower than 1.times.10.sup.11 .OMEGA.cm in a temperature range of 250 to 450.degree. C.
The electrostatic chuck which comprises a ceramic plate entirely composed of a low resistivity ceramic as disclosed in Japanese Unexamined Patent Publication No. 62-94953 suffers from considerable reduction in the resistivity of the entire ceramic plate in a high temperature environment. This makes it impossible to ensure sufficient insulation in the case where an electrode for plasma generation is fixed to the under surface of the electrostatic chuck.
In the case of the electrostatic chuck which comprises a ceramic plate including a low resistivity ceramic portion and a high resistivity ceramic portion bonded to each other as disclosed in Japanese Unexamined Patent Publication No. 4-3956, sinter-bonding of the ceramic portions is difficult. These ceramic portions are preferably bonded to each other by sintering to ensure a satisfactory corrosive and plasma resistance. However, it is difficult to sinter-bond the ceramic portions for preparation of the ceramic plate because the ceramic portions, even if composed of the same principal ceramic material, have different thermal expansion coefficients and shrinkage degrees at firing thereof due to the presence of the impurity.