1. Field of the Invention
The present invention relates generally to a polishing apparatus which polishes a wafer, and more particularly to a polishing apparatus which is able to uniformly polish a semiconductor wafer.
2. Description of Related Art
In a process of manufacturing semiconductor chips in which circuits are formed on a semiconductor material, the surface of a semiconductor wafer is polished by a polishing apparatus. The conventional polishing apparatus includes a polishing part and a holding and pressing part, which holds and presses the semiconductor wafer against a polishing surface of the polishing part. The polishing part is provided with a polishing pad, which has the polishing surface, and a turn table, on which the polishing pad is attached.
The wafer must be polished very accurately; however, even a very slight hollow or undulation on the polishing surface can cause the unevenness of the polishing.
Japanese Patent Provisional Publication No. 8-229808 discloses a polishing apparatus, in which the wafer is stuck to a carrier via a wafer adhesive sheet, and the carrier is pressed against a polishing pad via a diaphragm via pressurized fluid to polish the wafer. The polishing apparatus is provided with a tube between a head body and a retainer ring. The amount of air supplied to the tube is adjusted to control the force of the retainer ring which presses the polishing pad in order to prevent the polishing pad from being corrugated.
This polishing apparatus cannot deform the wafer in conformity with the hollows on the polishing surface because the wafer is stuck to the carrier via the wafer adhesive sheet. For this reason, the polishing pressure applied to the wafer cannot be uniform, and it is impossible to accurately polish the wafer.
In a conventional wafer polishing apparatus, a press ring is provided at a wafer holding head in such a manner as to enclose the edge of the wafer, and the press ring and the wafer are pressed against the rotating turn table so that the wafer can be polished. In another conventional wafer polishing apparatus, a retainer ring is provided at a carrier of the wafer holding head, and the retainer ring is in contact with the circumferential surface of the wafer so that the wafer can be polished with its position being regulated.
In these wafer polishing apparatuses, the rotational force of the turn table is transmitted to the carrier via the wafer, the retainer ring and the press ring. Thereby, the carrier tends to tilt unnecessarily due to the rotational force, and then, the polishing pressure cannot be uniform, and the wafer cannot be polished uniformly as a result. Pressing the carrier to such an extent as not to tilt the carrier can eliminate the above-mentioned disadvantage. In this case, however, the polishing pressure increases, and thus, the wafer cannot be polished accurately.
Japanese Patent Provisional Publication No. 1-188265 discloses a polishing apparatus, in which the air pressure is applied to a workpiece from behind to thereby polish the workpiece while keeping the workpiece and a workpiece holder in non-contact state. The pressure for machining the workpiece is set by controlling the flow of the air supplied to the back of the workpiece by means of a flow control valve.
In this polishing apparatus, only the air layer cannot absorb the tilt of the rotary head, and the pressing force cannot be maintained constant. Thus, it is impossible to accurately polish the wafer.
Japanese Patent Provisional Publication No. 8-55826 discloses a polishing apparatus, in which a liquid such as pure water is supplied into a space between a holding head and the reverse surface of a substrate to form a fluid film between them. The holding head holds the substrate with a surface tension of the fluid film to polish the substrate which is pressed against a turn table.
This polishing apparatus uses the liquid, and thus, the density or component of slurry changes to deteriorate the flatness of the wafer and the polishing rate. If the amount of the liquid supplied is decreased or adjusted to eliminate the above-mentioned disadvantage, the liquid cannot flow uniformly on the reverse surface of the wafer. Thus, it is impossible to accurately polish the wafer.