1. Field of the Invention
The present invention relates to a substrate cleaning apparatus and a substrate cleaning method that clean a substrate, in particular, a semiconductor wafer.
2. Background of the Invention
Typically, a resist on a semiconductor wafer substrate is removed (cleaned off) by immersing the semiconductor wafer in a chemical (resist remover) in a remover tank of a substrate cleaning apparatus.
In a conventional cleaning technique, for example, an SPM (sulfuric acid-hydrogen peroxide mixture) resist removing process, a mixture (SPM cleaning fluid) containing sulfuric acid and hydrogen peroxide in a certain proportion is first produced. Typically, to improve the resist removing capability, the SPM cleaning fluid serving as a remover is kept at a high temperature equal to or higher than 100 degrees Celsius, for example. Generally, hydrogen peroxide (H2O2) thermally decomposes into water (H2O) and oxygen (O2). In addition, hydrogen peroxide has a boiling point of 150 degrees Celsius and evaporates when heated to such a high temperature. Hydrogen peroxide tends to thermally decompose and evaporate at high temperatures in this way, so that the oxidation capability (resist removing capability) of the SPM cleaning fluid is lowered on the time passes.
According to an existing measure against this, before introduction of a semiconductor wafer or the concentration of the SPM cleaning fluid is managed, a predetermined amount of hydrogen peroxide solution is additionally injected into an inner tank (wafer process tank) or an outer tank (overflow receiving tank) of a process tank at desired intervals, thereby maintaining the oxidation capability of the SPM cleaning fluid.
A method of managing the hydrogen peroxide concentration uses a concentration monitor. However, since the concentration has to be always kept constant, the amount of usage of the hydrogen peroxide solution and sulfuric acid increases substantially.
According to the method in which the hydrogen peroxide solution is introduced before introduction of the semiconductor wafer described above, it takes long before the hydrogen peroxide solution is sufficiently mixed with the circulating mixture to form a uniform SPM mixture containing the hydrogen peroxide solution and sulfuric acid.
In the case where the hydrogen peroxide solution is introduced into the inner tank as described above, the hydrogen peroxide solution having a low specific gravity cannot be effectively mixed with sulfuric acid and overflows into the outer tank, and thus, a desired resist removing capability cannot be obtained.
Similarly, in the case where the hydrogen peroxide solution is introduced into the outer tank, since the hydrogen peroxide solution is not effectively mixed, the hydrogen peroxide solution having a low specific gravity is not mixed with the sulfuric acid and remains separated in the upper part of the outer tank. Thus, it takes a predetermined time before a uniform mixture of sulfuric acid and the hydrogen peroxide solution is formed, or it is difficult to feed all the introduced hydrogen peroxide solution to a circulation piping because some of the hydrogen peroxide solution is discharged from the overflow pipe rather than being fed to the circulation piping.
Furthermore, for example, a conventional substrate cleaning apparatus using the SPM cleaning fluid has a process tank that holds a mixture containing a hydrogen peroxide solution and sulfuric acid and is used for cleaning a substrate immersed in the mixture, circulation piping that extends between a primary side of the process tank on which the mixture is injected into the process tank and a secondary side of the process tank on which the mixture is discharged from the process tank and has a pump for causing circulation of the mixture, a filter disposed in the circulation piping for removing particles in the mixture, a chemical tank for additionally injecting a mixture of a hydrogen peroxide solution and sulfuric acid into the process tank, and a heater for heating the mixture in the chemical tank (see Japanese Patent Laid-Open No. 6-342780, for example).
However, in the conventional substrate cleaning apparatus, the mixture is heated by the heater to a temperature higher than a temperature required in the process tank before the mixture is injected into the process tank. Thus, the SPM cleaning fluid heated to the high temperature thermally decomposes and evaporates, so that the hydrogen peroxide concentration of the SPM cleaning fluid decreases. As a result, a desired SPM cleaning fluid cannot be obtained in the process tank.
As described above, the conventional technique described above has a problem that the SPM cleaning fluid cannot have a required resist removing capability, and the amount of the hydrogen peroxide solution additionally injected into the SPM cleaning fluid cannot be optimized.