Generally, a silicon-based semiconductor layer, such as a silicon oxide film and a polycrystalline silicon layer, needs to be etched while fabricating MOS structures, such as memories or logics, on a substrate to be processed, e.g., a semiconductor wafer (hereinafter, referred to as “wafer”). For instance, a layer structure, in which a resist film is formed as a mask on a silicon oxide film on a base silicon film, is etched to form a fine contact hole through the silicon oxide film. (See, e.g., Japanese Patent Laid-open Publication Nos. 2000-91317 and H9-27471 and U.S. Pat. No. 6,081,334).
In a conventional plasma processing method, CF-based gas, such as CF4 and C2F6, fed as a processing gas into a processing chamber of a plasma processing apparatus is converted into plasma to be used to etch a silicon oxide film formed on an object to be processed in the processing chamber by the plasma. In this case, in order to increase an etching rate, a main etching process is performed until a base silicon film underlying the silicon oxide film is about to be exposed, and the remaining portion of the silicon oxide film is subjected to an over-etching process. In the main etching process, because the base silicon film is not exposed, the etching is conducted at a high etching rate, and in the over-etching process, the etching is performed under a process condition wherein the selectivity against the base silicon film is high so as to prevent the base silicon film from being eroded.
As described above, there has been disclosed the technique wherein the main etching process is shifted to the over-etching process on the basis of the exposure of the base layer. For example, in Japanese Patent Laid-open Publication No. H9-27471, the main etching process is conducted for a predetermined time period that is estimated in advance by measuring the etching time needed to reach the stage wherein the base layer is about to be exposed, and then the over-etching process is carried out.
Further, in U.S. Pat. No. 6,081,334, the main etching process is conducted until the base layer is about to be exposed and then the over etching process begins to be performed, wherein the transition time is determined by monitoring the thickness of a polysilicon layer to be processed by irradiating light to a wafer and then detecting the light reflected from the wafer.
However, in case of etching the silicon oxide film by using the resist film as the mask as described above, the thickness of the resist film is rapidly reduced after the base silicon film starts being exposed. The reason for this is considered as follows.
When the silicon oxide film is plasma-etched by using the CF-based processing gas, a reaction product, such as CFx, is generated to be attached to a surface of the resist film, wherein since the reaction product acts as a protective layer of the resist film, plasma resistance of the resist film is increased. But, after the base silicon film starts being exposed, the generation of the reaction product is reduced.
Furthermore, the effect of a reaction product suppressing gas, such as O2, contained in the processing gas is increased in removing the reaction product attached to the resist film. Consequently, the resist film itself is etched.
Accordingly, in a process wherein the base layer is exposed such as the over-etching process, the thickness of the resist film is rapidly reduced such that there occurs an area where the resist film is eliminated in the course of the etching process, thereby undesirably etching a portion which should not be etched.
Therefore, since it is highly desirable to prevent the thickness of the resist film from being reduced during the etching process, it is preferable that the etching process be conducted while paying more attention to the thickness of the resist film instead of the base layer. That is, preferably, the main etching process is shifted to the over-etching process based on the thickness of the resist film, and the over-etching process is then conducted under a process condition that the thickness of the resist film is not reduced.
However, although there has been disclosed a method of shifting the main etching process to the over-etching process on the basis of the base layer, there has not been suggested a method of shifting the main etching process to the over-etching process while the thickness of the resist film being monitored.