This invention relates to a method of and a system for subjecting a substrate to photoresist processing and a method of evaluating a photoresist film, and more particularly to a method of evaluating a resist film made of a chemically amplified photoresist and a processing apparatus using the same.
In the photoresist processing step in the semiconductor device manufacturing process, a photoresist film is formed by applying a photoresist solution onto the surface of a substrate, such as a semiconductor wafer (hereinafter, referred to as a wafer), exposing the photoresist-coated surface through a specific pattern, and then developing the exposed surface with a developing solution. In such a series of processes, a photoresist processing system has been used.
The photoresist processing system includes processing units which separately carry out the following processes: an adhesion process to improve the fixation of photoresist, a coating process to apply a photoresist solution, a heating process to harden the photoresist film by putting the photoresist-coated substrate in an atmosphere of a specific temperature, a heating process to put the exposed substrate in an atmosphere of a specific temperature, and a developing process to supply a developing solution to the exposed substrate for development. The photoresist processing system uses a transfer mechanism, such as a transfer arm, to load and unload a wafer, or substrate, into and from each of the processing units.
A photoresist film formed on the wafer is required to have not only a desired thickness but also a good uniformity of thickness. To meet these requirements, it is necessary to measure the thickness of the photoresist film formed in the photoresist processing system. A conventional method of measuring the thickness of a photoresist film is carried out according to the following procedure.
First, a photoresist film is formed on a dummy wafer in the photoresist processing system. Thereafter, the dummy wafer is taken out of the photoresist processing system by a carrier. The thickness of the photoresist film on the dummy wafer is then measured with a thickness measuring instrument provided outside the system. The humidity in a photoresist coating unit and the rotational speed of the wafer in the photoresist coating unit in the photoresist processing system are adjusted on the basis of the result of the measurement so that the photoresist film may have a desired film thickness with good uniformity.
When the dummy wafer is taken out of the photoresist processing system and the film thickness is measured, it is necessary to stop the photoresist processing system temporarily. In addition, the thickness of the photoresist film has to be measured periodically, such as lot by lot. The temporary stop and the periodic measurement result in a decrease in the throughput. Since the environment in which the photoresist processing system resumes the operation after the measurement of the thickness of the photoresist film is not identical with the system environment at the time when the photoresist film was formed on the dummy wafer, there may be a case where the result of measuring the film thickness cannot be applied to the adjustment of the resist film forming conditions without any modification.
Use of chemically amplified photoresist is suitable for the manufacture of extra large scale integration in terms of sensitivity and resolution. When a photoresist film is made of chemically amplified photoresist, there arises a problem: the thickness of the photoresist film changes as time passes. For example, suppose one hundred wafers are coated with photoresist consecutively and after the formation of the photoresist film, the film thickness of each of the wafers is measured one by one in the order in which they have been coated with photoresist. During the time from when the film thickness of the first wafer is measured until the film thickness of the hundredth wafer is measured, the time required to measure 99 wafers has elapsed. The film thickness data varies as time passes. For example, in a case where a chemically amplified photoresist is used whose film thickness decreases by about 10 angstroms per hour with respect to the film thickness of 8000 angstroms immediately after the coating, if one hundred wafers are coated with photoresist consecutively and then the film thickness of each of the wafers is measured and it takes five hours to measure the hundred wafers, the thickness of the hundredth resist film will be thinner than that of the first resist film by about 50 angstroms.
As a consequence, even if there is nothing wrong with the coating process, the film thickness will decreases as time elapses. An abnormality in the coating process would cause the thickness of the resist film to vary, resulting in a gradual decrease in the thickness of the photoresist film subsequently formed. Because of this, an examination of the relationship between the elapsed times and the film thicknesses in the order of measuring the film thicknesses does not help make a determination whether the photoresist film is in a good condition or its film thickness has changed because an abnormality has occurred.
As described above, when a photoresist film is made of a chemically amplified resist whose film thickness changes as time elapses, it is difficult to determine from a simple measurement of the thickness of the photoresist film whether or not the film thickness itself has changed. This causes the problem of being unable to make an accurate determination whether or not there is any abnormality in the coating process or in the photoresist film.