1. Field of the Invention
The present invention relates to a method for measuring a physical quantity of a measurement object in a substrate processing apparatus, and more particularly, to a physical quantity measuring method utilizing, for measurement, interference of irradiation light and reflected light, and a storage medium storing a program for implementing the measuring method.
2. Description of the Related Art
A substrate processing apparatus for plasma-processing a substrate such as a semiconductor device wafer has strongly been demanded to be capable of contactlessly measuring the temperature of a wafer housed in a processing chamber of the substrate processing apparatus. As a contactless thermometer, a radiation thermometer has been known that measures the temperature of a measurement object based on infrared radiation emitted from the measurement object. However, the substrate is mainly made of silicon whose infrared emission characteristic changes at a temperature of about 600 degrees Celsius, and therefore, the substrate temperature cannot be measured with accuracy, especially at low temperatures, by the radiation thermometer.
A temperature measurement apparatus has therefore recently been developed, in which the thickness of a substrate is measured contactlessly and converted into a corresponding temperature for measurement of substrate temperature. This temperature measurement apparatus utilizes a phenomenon that light irradiated from a low-coherence light source to a substrate is reflected by both the rear-side and front-side surfaces of the substrate. Specifically, in the temperature measurement apparatus, light irradiated from a single low-coherence light source is divided into measurement light and reference light. The measurement light is irradiated onto the substrate, and reflected light from the rear-side surface of the substrate and reflected light from the front-side surface thereof are received. On the other hand, the reference light is irradiated onto a reference mirror that is movable parallel to the direction in which the reference light is irradiated, and reflected light from the reference mirror is received. On the basis of reference mirror positions in each of which interference occurs between the reflected light from either surface of the substrate and that from the reference mirror, the light path length of the measurement light and each reflected light is calculated. Then, positions of the front-side and rear-side surfaces of the substrate are determined based on the calculated optical path lengths, and the substrate thickness is calculated from the determined positions of the substrate surfaces. Further, the calculated substrate thickness is converted into a corresponding substrate temperature (refer to Japanese Laid-open Patent Publication (Kokai) No. 2003-307458.
In the above temperature measurement apparatus, the measurement light and the reference light are respectively guided to the substrate and the reference mirror using optical fibers. In many cases, the optical fiber for guiding the reference light is disposed in a closed space isolated from the ambient environment of the substrate processing apparatus, whereas the optical fiber for guiding the measurement light is disposed to be exposed to the ambient environment.
In recent substrate plasma processing, the temperature in the processing chamber of the substrate processing apparatus is often positively changed, and therefore, the optical fiber for guiding the measurement light can increase in length with elapse of time by being heated by heat released from the substrate processing apparatus. Since the optical path length is affected by a change in optical fiber length, it is difficult to accurately measure the optical path length of the measurement light and that of the reflected light thereof when there is a change in temperature of the optical fiber for guiding the measurement light, which causes a change in optical fiber length. The above poses a problem that it is impossible to achieve an accurate measurement of a predetermined physical quantity (thickness, temperature, refractive index, or the like) of the substrate based on the optical path length.