1. Field of the Invention
The present invention relates to a method and an apparatus, both for ashing a photoresist layer or the like formed on a semiconductor wafer, by applying ozone to the layer.
2. Description of the Prior Art
The fine and complex pattern of a semiconductor IC is formed by etching a substrate formed on a semiconductor wafer, by using a photoresist layer of a high-molecular weight organic composition as an etching mask. The photoresist layer must be removed from the surface of the wafer after the etching. One of the methods for removing the photoresist layer is an ashing process. This process is used not only to remove a photoresist layer from a semiconductor wafer and clean silicon wafers and etching masks, but to remove ink or residual solvents. It is, therefore, an effective process for achieving a dry-etching required in manufacturing semiconductor devices.
Various ashing apparatuses for removing photoresist layers are known. Of these, the one using oxygen plasma is most commonly used. This ashing apparatus has an ashing chamber. It removes a photoresist layer from the surface of a semiconductor wafer in the following manner. First, the wafer is placed within the ashing chamber. Then, oxygen gas is introduced into the chamber. A high-frequency electric field is applied to the gas, thereby forming plasma. The oxygen atom radical of the plasma oxidizes the photoresist layer, thus decomposing the layer into carbon dioxide, carbon, monoxide and water, thereby removing the photoresist layer from the wafer.
However, the ashing apparatus using oxygen plasma has a drawback. Ions or electrons are present in the plasma and accelerated by the electric field. These ions or electrons are inevitably applied to the semiconductor wafer, and do damage to the wafer.
Another ashing apparatus is known which can remove a photoresist layer from a semiconductor wafer. This apparatus applies ultraviolet rays to the wafer, generating an oxygen atom radical. Hence, the apparatus can process a batch of semiconductor wafers. Certainly, the apparatus does no damage to wafers, since it uses no plasma. But its ashing rate is rather low--50 to 150 nm/min. Therefore, much time is required to remove a photoresist layer from a semiconductor wafer. In view of this, the apparatus is considered to be unfit for processing semiconductor wafers of a large diameter, one by one.
The applicants have developed an ashing technique which can remove photoresist layers from semiconductor wafers at high speed without applying ultraviolet rays to the wafers. This technique is disclosed in U.S. Patent 4,341,592. As is described in the U.S. patent, the technique uses a gas-supplying plate having a plurality of openings. The plate is positioned close to a semiconductor wafer, and an ashing gas is made to flow through the openings to the surface of the wafer. When an ashing gas containing ozone is used, the ozone must be decomposed when applied to the surface of the wafer which is maintained at high temperature, thereby to achieve a successful ashing. Therefore, in the technique, the gas-supplying plate must be cooled to prevent the ozone from being decomposed as it flows through the openings. The technique, however, has drawbacks, too. First, a great amount of deposit is formed on that surface of the wafer which opposes the gas-applying plate. This deposit is a source of dust, which will reduce the cleanness of the room in which semiconductor wafers are processed to make ICs of a high integration density. Secondly, since the gas-applying plate is closed to the wafer, the ashing gas flowing through the openings is not applied evenly onto the entire surface of the wafer. Consequently, the wafer cannot be uniformly ashed.