The present invention relates to an apparatus for subjecting a target object, such as a semiconductor wafer or LCD substrate, to a process, such as etching or deposition, using plasma generated by means of radio frequency (RF) discharge, and in particular to an apparatus which is capable of performing a stable process while suppressing excessive dissociation of a process gas.
In processes of manufacturing semiconductors, various kinds of plasma processes, such as etching, are performed on a target object, such as a semiconductor wafer, in a plasma atmosphere, while plasma is generated in a process chamber. In recent years, as patterns given to the target object have been increasingly miniaturized, demands have come for a plasma process to be performed with a higher accuracy under a design rule of sub-quarter micron.
Along with the demand for miniaturizing semiconductor devices at a high degree in a horizontal direction, it has become necessary for the devices to have a three-dimensional shape. For example, the structure of capacitor devices expands in vertical directions to obtain an electrostatic capacitance of a certain level, while the structure of wirings increases its vertical size to decrease wiring resistance. For this reason, portions to be etched in their manufacturing process becomes deeper, so that a ratio of the depth of the to-be-etched portions relative to the thickness of a photoresist grows higher. In such a case, etching selectivity of a material of the to-be-etched portions relative to the photoresist needs to be high in order to avoid a dimensional shift due to retreat of the photoresist.
Taking account of this demand, plasma sources for generating a high density plasma have been developed. In recent years, it has been attempted to etch, for example, an oxide film on a semiconductor wafer at a high selectivity, using a high density plasma generated by a system of, e.g., the helicon wave type, ECR (Electron Cyclotron Resonance) type, TCP (transformer Coupled Plasma) type, or the ICP (Inductively Coupled Plasma) type.
A photoresist used with I lines in the ultraviolet domain has a good etching selectivity of about 7 to 8 for SiO.sub.2 relative to the resist, in plasma of an ordinary density, such as about 1.times.10.sup.11 /cm.sup.3. However, in a high density plasma generated by, e.g., the ICP type, the etching selectivity is decreased down to a value of 3 to 4. The photoresist, thus does not have a sufficient durability relative to a high density plasma.
On the basis of the demand for high miniaturization, it has also been proposed to expose a photoresist to ultraviolet radiation, such as Kr excimer laser, having a wavelength shorter than that of the I lines. As compared to the photoresist used with the I lines, the photoresist used with the excimer laser has a lower durability against plasma, in order to suppress the polymerized degree of polymers in accordance with a shorter wavelength. As a result, the latter photoresist presents a lower etching selectivity of SiO.sub.2 to this photoresist.
Further, in a high density plasma generated by the TCP type, an equilibrium state is gradually settled by repeating a cycle in which radicals produced by dissociation are deposited on the inner wall of a process chamber and the deposited radicals are returned into plasma. With this high density plasma, as an etching process is repeated, the deposition on the inner wall grows thicker than that at the beginning, and the radical composition in the plasma is changed. The change of the radical composition brings about a difficulty in obtaining a good reproductivity of etching properties. For this reason, a countermeasure of, e.g., heating the inner wall of the process chamber is carried out in order to form a stable state on the surface of the inner wall, but it is hardly controlled to form the stable state.