1. Field of the Invention
The present invention relates to a polishing method and a polishing apparatus for polishing a surface of a substrate, such as a semiconductor wafer, into a flat mirror-like surface, and also relates to a program for controlling the polishing apparatus.
2. Description of the Related Art
A conventionally-known polishing apparatus for polishing a substrate, such as a semiconductor wafer, comprises a polishing section, a cleaning/drying section, a transfer section, a film thickness measurement device, etc. which are unified. It is common practice in such a polishing apparatus to measure a thickness of a polishing film, present in a substrate surface, prior to polishing, and again measure the thickness of the polishing film after polishing the film and cleaning and drying the substrate. This practice is to calculate the polishing performance of the polishing apparatus by comparison of thicknesses of the polishing films before and after polishing, and to feed back the data to polishing of a later substrate.
In many cases, time taken for polishing a polishing film present in a substrate surface (polishing tact time), time taken for cleaning/drying the substrate after polishing (cleaning/drying tact time), and time taken for measuring the thickness of the polishing film (film thickness measurement tact time) generally differ from one another. For example, the time taken for polishing the polishing film and cleaning/drying the substrate after polishing is 60 seconds, while the time taken for measuring the thickness of the polishing film is 90 seconds. In such a case, the time taken for measuring the thickness of the polishing film is a rate-determining factor in the conventional common polishing apparatus in which a sequence of operations for a substrate is carried out successively, and the throughput of the entire polishing apparatus is limited by the film thickness measurement time.
When the time taken for polishing a polishing film present in a substrate surface (polishing step) and cleaning/drying the substrate after polishing (cleaning/drying step) is shorter than the time taken for measuring the thickness of the polishing film, it is conventional practice to carry out the polishing step and the cleaning/drying step each for a predetermined tact time, and cause the substrate after cleaning/drying to stand by for the measurement of the thickness of the polishing film. In such an operating manner, a substrate, which is subject to the feedback control, must be one that is later than the substrate after polishing and cleaning/drying, waiting for the thickness measurement of the polishing film. That is, the substrate, which has undergone the predetermined tact time of polishing and cleaning/drying and is waiting for vacancy in a film thickness measurement device (and which has therefore been taken out of a cassette), is not subject to the feedback control. Therefore, if the substrate after drying, standing by for the thickness measurement of the polishing film, has been poorly polished, e.g., due to a change in the processing environment, the information cannot be reflected until the thickness of the polishing film of the substrate is measured by the film thickness measurement device. Accordingly, a substrate, which has entered into the polishing step during that period, may be poorly polished due to the change in the processing environment. In order to perform the feedback control early so as to early reflect a change in the polishing step in later substrates, it is necessary to shorten the film measurement tact time as much as possible.
It is also conventional practice in polishing of substrates to carry out a sequence of process steps: pre-polishing film thickness measurement step→polishing step→cleaning step→drying step→post-polishing film thickness measurement step, successively for each substrate. In this case, when the last post-polishing film thickness measurement step of the process is a rate-determining step, the upstream steps (pre-polishing film thickness measurement step, polishing step, cleaning step and drying step) must be waited for until completion of the post-polishing film thickness measurement step. Therefore, there will be a substrate which, after completion of the drying step, is standing by on a drying device and a substrate which, after completion of the cleaning step, is standing by on a cleaning device. In this case, a substrate after the polishing step cannot enter the cleaning device. Interconnects (e.g., copper interconnects) formed in a surface of a substrate, such as a semiconductor wafer, can therefore corrode after polishing, which may increase the resistance of the interconnects and provide a defective product.
Substrates, such as semiconductor wafers, are generally managed with a cassette, in which a plurality of substrates are housed, as a lot. All the process steps for all the substrates of one lot are not completed even when polishing and cleaning/drying of the last substrate of the lot is completed if the measurement of a thickness of a polishing film of the last substrate is not completed. Therefore, there is downtime of a polishing section during a lot change period from the termination of the polishing step for the last substrate of one lot to the initiation of polishing for the first substrate of the next lot. For the purposes of maintaining polishing environment, etc., polishing of a dummy wafer or the like in the polishing section is generally practiced during such downtime. However, polishing a dummy wafer or the like in every lot change period leads to an increased cost.