1. Field of the Invention
The present invention relates to an etch amount detection method, an etching method, and an etching system.
2. Description of the Related Art
An etching process using plasma is conventionally, widely applied to semiconductor manufacturing process or an LCD substrate manufacturing process. An etching system employed for this etching process includes, for example, an upper electrode and a lower electrode provided in a treatment chamber to be parallel to each other. An object to be processed such as a semiconductor wafer is mounted on the lower electrode, a plasma is generated in an etching gas by electron discharge between the upper electrode and the lower electrode, and an etching target layer is etched using a mask layer patterned into a predetermined shape.
As one of methods for monitoring an etched state of the etching target layer, an emission spectrum analysis method is proposed. This is a method for detecting a point at which a specific wavelength is changed as an etching end point in an emission spectrum of the gas generated by the etching. With this method, an etch depth (an etch amount) of the etching target layer cannot be known.
Japanese Patent Laid-open Publication No. 2-71517, for example, discloses an etching system capable of detecting an etch amount. With this etching system, a light having a predetermined wavelength is emitted from a light source outside of a treatment chamber, and this light is irradiated onto an object to be processed through an observation window provided at an outer electrode. A part of this irradiated light is reflected by an interface between a mask layer and an etching target layer, whereas the other part of the irradiated light is reflected by a bottom of an etching target portion (an etching hole or an etching groove) of the etching target layer. An interference light (interference wave) generated by the two reflected lights is detected and photoelectrically converted into an electric signal by a detector. The resultant electric signal is amplified and digitized, and then subjected to a frequency analysis by a waveform analysis means. The etch amount is calculated based on a frequency distribution of the interference wave obtained as a result of these signal processings.
With the technique disclosed in the Japanese Patent Laid-open Publication No. 2-71517, however, the etch amount cannot be sometimes detected accurately, depending on a type of the mask layer. If the mask layer has a high absorption coefficient relative to the wavelength of the light emitted from the light source, for example, then the light emitted from the light source is not transmitted by the mask layer, and the reflected light reflected by the interface between the mask layer and the etching target layer cannot be obtained. If the reflected light reflected by the interface cannot be obtained, it is difficult to calculate the etch amount of the etching target layer.
If the reflected light reflected by the interface between the mask layer and the etching target layer cannot be obtained, attention may be paid to a reflected light reflected by an upper surface of the mask layer instead of the reflected light from the interface. If so, an interference wave between this reflected light and the reflected light reflected by the bottom of the etching target portion of the etching target layer is detected. In this case, however, it is necessary to select a constituent material of the mask layer for the following reason, in order to become larger a selected ratio between the mask layer and the etching target layer. If the mask layer is etched while the etching target layer is etched, then a height of the upper surface of the mask layer which should be a reference surface is reduced from a height when etching starts, and the etch amount of the etching target layer cannot be accurately calculated.