1. Field of the Invention
The present invention relates to a developing unit for developing a latent image transferred onto a photo-resist film of a semiconductor substrate by an exposing unit.
2. Description of Related Art
FIG. 1A and FIG. 1B are a schematic cross sectional diagram and an operational line diagram respectively of a conventional developing unit for explaining a developing unit. The conventional developing unit of this type has a rotary chuck 7 for holding and rotating a wafer 13 which is a semiconductor substrate, a developing solution nozzle 5 for dripping a developing solution on the wafer 13, a rinse solution nozzle 6 for dripping a rinse solution for rinsing the front surface of the wafer 13, and a cup 8 for collecting and discharging the developing solution and the rinse solution that have been scattered around by the rotation of the wafer 13, as shown in FIG. 1A, for example. A photosensitive resin film 14 is formed on the wafer 13. A numeral 15 is a developing solution layer. At the bottom of the cup 8, a nozzle 10 for supplying a rinse solution on a back surface of the wafer 13 and a nozzle 11 for blowing an air to the back surface of the wafer 13 are provided. A pipe 12 for discharging the wasted liquid from the cup 8. Also, a projection 9 is formed on the bottom surface of the cup 8 around the chuck 7.
For developing a latent image of the photo-sensitive resin film 14 of the wafer 13 by this developing unit, the wafer 13 is fitted to the rotary chuck 7 at first, as shown in FIG. 1B, and then an operation called a pre-spin is carried out. This operation is as follows. The wafer 13 is rotated for about 2 seconds at a speed of about 100 rotations per every second, for example. Thereafter, an operation of dripping a developing solution, called a pre-wetting, is carried out. During this operation, the rotation number of the wafer 13 is lowered at stages and the dripped developing solution extends over the whole front surface of the wafer 13 to form a developing solution layer 15 on it.
Next, in order to secure a wetting of the developing solution on the photo-sensitive resin, the wafer 13 is rotated at a very slow speed so that the heaped-up developing solution layer 15 fits well with the photo-sensitive resin film 14 and air bubbles disappear at the same time. Then, the rotation is stopped and a developing operation is carried out by the developing solution. The developing operation is carried out in a state that the wafer 13 is stopped for about 75 seconds, for example. When the developing operation has been completed, the wafer 13 is rotated again and, at the same time, the rinse solution is ejected from both the rinse solution nozzle 6 and a nozzle 10 at the rear surface side of the wafer 13 so that the developing solution on the front surface of the wafer 13 is washed out and the developing solution traveled to the rear side of the wafer 13 is also washed out to the outside of the wafer 13. After carrying out the washing by this rinse solution, the wafer 13 is rotated at a high speed so that the water content adhered to the front surface of the wafer is dispersed to the outside by the centrifugal force and the front surface of the wafer 13 is dried.
At the same time as the drying of the front surface of the wafer 13, dry air is also blown to the rear surface of the wafer 13 through a gap between a projection section 9 and the wafer 13 from a nozzle 11 provided at the rear surface side, to thereby dry the rear surface of the wafer 13. The developing solution and the rinsing solution within the cup 8 are discharged to the outside of unit through a waste solution discharging pipe 12. As explained above, by providing the nozzle 10 for ejecting the rinsing solution and the nozzle for blowing dry air at the rear surface side of the wafer 13, the remaining developing solution traveled to the rear surface of the wafer 13 has been removed and pollution of the wafer 13 due to the remaining developing solution has been eliminated, to thereby stabilize the quality.
Further, a method for preventing a developing solution from traveling to the rear surface of the wafer without a provision of a cleaning mechanism at the rear surface side has also been disclosed in the Japanese Patent Application Laid-open Publication No. 63-6843 (the Japanese Patent Application Publication No. 3-34207). According to this method, an end section of a cylindrical body almost concentric with the center of the rotation of a substrate is opposed to a peripheral section of the rear surface of the substrate with a minute gap formed therebetween, and the developing solution traveling from the peripheral section of the substrate is held in the gap section by the capillary effect so that the developing solution is inhibited from further traveling inside to the rear surface of the substrate.
According to the above-described prior-art developing unit having the cleaning and drying means at the rear surface side of the substrate, it is difficult to keep always constant the thickness of the developing solution which is dripped and heaped up on the surface of the wafer. Therefore, the dimensions of a latent image pattern fluctuate to be either large or small each time when the image is developed. Sometimes, there may occur such a problem that since the thickness of the heaped-up developing solution is so small that the image is developed incompletely, a so-called dropped-out phenomenon. Further, when the developing solution is heaped up to a very large thickness due to an excessive dripping of the developing solution, the developing solution drops from the peripheral edge of the wafer by the oscillation of the wafer at the time of the rotation of the wafer and the dropped developing solution travels to the rear surface so that this can not be removed even by the cleaning means, which results in a serious defect in quality.
On the other hand, according to the above-described method for providing a cylindrical body having a gap between the rear surface of the substrate and the cylindrical body, since the developing solution having traveled to the rear surface of the substrate at an outer side of the cylindrical body is kept adhered to the rear surface, it is difficult to completely remove this remaining developing solution by simply ejecting a washing solution afterwards. Further, it is impossible to remove the developing solution which entered into the gap between the substrate and the cylinder. Furthermore, since the cited reference does not have means for making constant the heaped-up thickness of the developing solution, there is a risk of an occurrence of the above-described problems.
Further, a developing method which eliminates the variation in the heaped-up thickness of the developing solution has been disclosed in the Japanese Patent Application Laid-open Publication No. 56-9742 (the Japanese Patent Application Publication No. 60-53307).
According to this method, a ring is disposed around a substrate with a gap formed between the surface of the substrate and the ring, and a developing solution is dripped inside the ring to thereby eliminate variations in the heaped-up thickness of the developing solution at the center section and the peripheral section of the substrate. However, although variations in the heaped-up thickness within the substrate surface can be eliminated by this method, it is difficult to obtain an always constant heaped-up thickness.
Particularly, when the substrate is rotated in order to fit the developing solution with a photo-sensitive resin like the conventional unit, a friction is generated between the fixed ring and the developing solution in contact with this ring, and thus there is a risk of an occurrence of variations in the surface of the solution resulting in variations in the heaped-up thickness of the developing solution.